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Structural geology of North Americ

3 T1S3 00717572 M ™

Structural Geology

North America

HARPER'S GEOSCIENCE SERIES

CAREY CRONEIS, Editor

Structural provinces of North America, shown to the edge
of the continental shelf.

Digitized by the Internet Archive

in 2012 with funding from

LYRASIS Members and Sloan Foundation

http://archive.org/details/structuralOOeard

Structural Geology

of
North America

SECOND EDITION

A. J. EARDLEY

Professor of~(5eology and Dean

College of Mines and Mineral Industries

University of Utah

HARPER & ROW, PUBLISHERS, NEW YORK AND EVANSTON

STRUCTURAL GEOLOGY OF NORTH AMERICA, Second Edition

Printed in the United States of America

, IT

All rights reserved. No part of the book may be used or reproduced
in any manner whatsoever without written permission except in the case
of brief quotations embodied in critical articles and reviews. For infor-
mation address Harper & Row, Publishers, Incorporated
49 East 33rd Street, New York 16, N. Y.

l-M

Library of Congress catalog card number: 62-17482

3. RESUMl OF STRUCTURAL GEOLOGY OF NORTH AMERICA

Major Tectonic Divisions

CONTENTS

EDITOR'S INTRODUCTION
PREFACE TO THE FIRST EDITION
PREFACE TO THE SECOND EDITION

1. INTRODUCTION

Purpose of Book Method of Presentation Kinds of Illustrations
Maps for Collateral Use Authority for Stratigraphic Correlations
Exercises

2. STRUCTURAL TERMINOLOGY

Need of Standard Terms for Regional Structures Meaning and
Choice of Terms for This Book Terms for Structural Disturbances
Classification Used for Crustal Disturbances

xiii
xv

4. PRECAMBRIAN TECTONIC PROVINCES 22

Distribution of Precambrian Rocks Canadian Shield Arctic Stable
Region Precambrian Provinces of the United States

5. CENTRAL STABLE REGION OF THE UNITED STATES 37

General Characteristics Pre-Devonian Basins Transcontinental Arch
Eastern Interior Basins and Arches Northwestern Interior Basins and
Arches

6. PALEOZOIC CORDILLERAN GEOSYNCLINE 63

Divisions and their Characteristics Basins and Uplifts of the Western
United States and Southern British Columbia Eugeosyncline in
Southeastern Alaska, Northern British Columbia, and the Yukon Sum-
mary of Orogenic History

7. APPALACHIAN MOUNTAINS 91

Major Structural Divisions Relations to Geomorphic Provinces

8. SOUTHERN AND CENTRAL APPALACHIANS 97

Extent and Divisions Major Elements of Stratigraphy Folded and
Thrust-Faulted Appalachian Mountains Blue Ridge Province Pied-
mont Province Summary of Orogenic History

9. EASTERN TRIASSIC BASINS 128

Distribution of Basins Nature of Triassic Rocks Structure of Basins
Origin of Basins Late Triassic Phase (Palisades Orogeny)

10. ATLANTIC COASTAL PLAIN AND ADJACENT OCEAN BASIN 135

Extent and Character of Sediments Stratigraphy Structure Con-
stitution of Continental Shelf and Adjacent Atlantic Ocean Crust

11. NEW ENGLAND APPALACHIAN SYSTEMS 154

Divisions of New England Appalachians Hudson Valley-Lake Cham-
plain Region Central and Eastern New England Carboniferous
Basins

CONTENTS

12. MARITIME APPALACHIANS 189

Definition Geomorphic Provinces Stratigraphy Igneous Rocks
Structures Tectonic History

13. NEWFOUNDLAND APPALACHIANS 203

Physical Divisions Stratigraphy Intrusions Major Structural Divi-
sions and Their Characteristics Tectonic History Major Tectonic Re-
lations of Greater Acadia

14. OUACHITA, MARATHON, AND COAHUILA SYSTEMS 223

Ouachita System Marathon System Coahuila System

15. WICHITA AND ANCESTRAL ROCKIES SYSTEMS

AND THE TEXAS FORELAND 237

Wichita System Texas Foreland Ancestral Rockies System

16. THE LATE PALEOZOIC ZONES OF FAULTING AND
CRYPTOVOLCANIC OR METEORITE IMPACT STRUCTURES 253

Foreland Arcuate Fault Zone Lake Superior Fault Zone Cryptovol-
canic or Meteorite Impact Structures

17. MESOZOIC SYSTEMS ALONG THE PACIFIC 260

Western Nevada Northwestern Nevada Central and Northern
California Oregon Southern California Nevadan Orogeny An-
cestral Coast Range System Columbia System

18. ROCKY MOUNTAINS IN MESOZOIC TIME 291

Triassic Geography Early Jurassic Geography Early and Mid-Cre-
taceous Orogeny

19. LATE CRETACEOUS AND EARLY TERTIARY ROCKY MOUNTAIN
SYSTEMS-THE LARAMIDE OROGENY 295

Definition of Laramide Orogeny Belts of Deformation Relation of
Belts of Deformation to Crustal Constitution

20. CANADIAN AND MONTANA ROCKIES 302

Major Systems of Canadian Cordillera Divisions of Canadian and

Montana Rockies Mountain Belt Foothill Belt Age of Thrusting
The Rocky Mountain Trench

21. IDAHO BATHOLITH AND THE OSBURN FAULT ZONE 319

Extent Composition Age Conclusions

22. CENTRAL ROCKIES 327

Spatial Relations Orogenic Deposits Southwestern Montana
Southeastern Idaho and Western Wyoming Wasatch Area of Utah
Central Utah Southwestern Utah Western Utah Southern Nevada

23. CENTRAL MONTANA ROCKIES 351

General Features Central Zone of Uplifts Zones of En Echelon
Faults Stages of Orogeny Igneous Centers Structures of the
Northern Great Plains

24. WYOMING ROCKIES 361

General Characteristics Teton— Gros Ventre— Wind River Element
Beartooth Range Owl Creek and Washakie Mountains Heart
Mountain and Related Features Absaroka Range and Yellowstone
Park Big Horn Range and Big Horn Basin Black Hills and Powder
River Basin Sweetwater Range Wind River Basin Hanna Basin
Late Tertiary Downfaulting of Sweetwater Range Laramide Pattern
and Cenozoic Stages in the Sweetwater Range Region Rawlins Up-
lift Washakie Basin Green River Basin Uinta Mountains Rock
Springs Uplift Laramie Range and Basin and Medicine Bow Range
Hartville Uplift Regional Uplift in Late Cenozoic

25. COLORADO AND NEW MEXICO ROCKIES 389

Extent of Laramide Deformation Colorado Rockies New Mexico
Rockies Central New Mexico Porphyry Belt Guadalupe and Mara-
thon Uplifts

26. COLORADO PLATEAU 407

General Geology Asymmetrical Arches and Basins Salt Anticlines
Laccolithic Mountains Upheaval Dome Volcanic Fields High Pla-
teaus of Utah Age of Uplifts and Volcanism Epeirogenic Move-
ments and Isostatic and Seismic Considerations

CONTENTS

27. SOUTHERN ARIZONA ROCKIES 426

Physiographic Characteristics and Divisions Paleozoic and Meso-
zoic Basins Use of Terms, Laramide and Nevadan Orogenies Mes-
ozoic and Cenozoic Geology of Southeastern Arizona Mesozoic and
Cenozoic Geology of Southern Arizona Geology of West-Central
Arizona Nevadan Orogeny (?) Igneous Cycles and Mineraliza-
tion Tertiary Normal Faulting Conclusions Regarding Tectonic
History

28. ROCKIES OF NORTHERN MEXICO 440

Mexican Geosyncline Sonoran Region El Paso— Rio Grande Thrust
Belt Plateau Central and Sierra Madre Oriental Parras Synclin-
orium Orogenic History Foothill Belt

29. COAST RANGES OF THE PACIFIC AND THE SAN ANDREAS
FAULT SYSTEM 452

Major Divisions Central Coast Ranges of California Southern Coast
or Transverse Ranges of California Northern Coast Ranges of Cali-
fornia San Andreas Fault System Coast Ranges of Oregon and
Washington

30. BAJA CALIFORNIA AND SONORA SYSTEMS 480

Baja California Gulf of California Sierra Madre Occidental

31. MIDDLE AND LATE CENOZOIC SYSTEMS

OF THE CENTRAL CORDILLERA 493

General Divisions and Their Characteristics Basin and Range System
Late Cenozoic Trenches of the Rocky Mountains Geophysical Evi-
dence Exploring Tensional Tectonism in Western North America
Seismic Velocity Layers in the Eastern Great Basin

32. PACIFIC SUBMARINE PROVINCES 515

Discovery of Strong Submarine Relief Submarine Provinces Aleu-
tian Trench Bering Sea Floor Pacific Floor off Mexico and Central
America Fracture Zones Deep Sea Provinces Hawaiian Ridge
Mid-Pacific Mountains Circum-Pacific Tectonics

33. IGNEOUS AND TECTONIC PROVINCES

OF THE WESTERN CORDILLERA 532

Objectives Concept of Igneous Provinces

34. IGNEOUS AND TECTONIC PROVINCES

IN SOUTH AMERICA 537

Chile and Argentina Peru, Bolivia, Ecuador, and Columbia Post-
Batholithic Belt Parana Basin Basalt Field

35. IGNEOUS AND TECTONIC PROVINCES IN MEXICO 549

Geosyncline Batholithic Belt of the First Cycle Post-Batholithic
Volcanism Batholithic Belt of the Second Cycle Metamorphic and
Intrusive Belt Relation to Depressed Belts

36. IGNEOUS PROVINCES IN WESTERN UNITED STATES 553

Eugeosynclinal Province Batholithic Province Post-Batholithic Prov-
inces of the Batholithic Belt Provinces of the Miogeosyncline and
Shelf Relation of Tectonic to Igneous Provinces Distribution of
Primary Magmas

37. IGNEOUS AND TECTONIC PROVINCES

OF WESTERN CANADA 583

Geosyncline Orogenies Beltian Geanticline Batholithic Province
Post-Batholithic Volcanism Relation of Volcanism to Tectonic Prov-
inces

38. SPATIAL RELATIONS OF MAJOR TECTONO-IGNEOUS

ELEMENTS AND ORIGIN OF MAGMAS 588

Relation of Batholithic Belt to Eugeosyncline Previous Orogeny in
Eugeosyncline Relation of Post-Batholithic Compressional Orogeny
to Geosyncline and Shelf Relation of Post-Batholithic Volcanics to
Batholithic Belt Relation of Post-Batholithic Volcanic Fields to Strato-
volcanoes Post-Batholithic Volcanics to Trenches Relation of Anti-
clinoria to Other Elements Origin of Magmas Techtono-lgneous
Provinces and Deep-Seated Earthquakes Crustal Tension and Mag-
matism

Vlll

CONTENTS

39. ALASKA AND THE YUKON 605

Geomorphic Provinces of Alaska Paleozoic Geosyncline and Related
Orogeny Triassic and Jurassic Geanticline and Adjacent Basins
Cretaceous Basins and Geanticlines Mesozoic and Cenozoic Oro-
genies Tertiary Volcanic Rocks Aleutian Volcanic Belt Siberian
Tectonic Connections Yukon Territory and the District of Mackenzie
Cenozoic Trenches and Faults

40. CANADIAN ARCTIC 633

Geography and Geologic Provinces of the Arctic Archipelago Low-
lands and Plateaus Fold Belts— The Innuitian Region Arctic Coastal
Plain Correlation with Alaska and the Yukon Pleistocene Epeirog-
eny and Climatic Changes Orogenic Belts of Greenland Arctic
Ocean Basin

41. GULF COASTAL PLAIN

General Characteristics Structural Geology Igneous Rocks Tam-
pico Region, Mexico Florida Platform Crustal Structure of Gulf
of Mexico

650

42. ANTILLEAN-CARIBBEAN REGION 670

Geographic Provinces Greater Antilles Lesser Antilles Puerto
Rico Trench and Gravity Anomalies Caribbean Region and Seismic
Profiles Origin of the Caribbean Basins, Trenches, and Rises Pos-
tulated Eastward Shift of Caribbean Block

43. SOUTHERN MEXICO AND CENTRAL AMERICA 696

Major Geologic Divisions Crystalline Belt Permian Fold Belt Late
Cretaceous and Early Tertiary Fold Belt Southern Gulf Coastal
Plain Yucatan Peninsula Volcanic Fields and Faulting Isthmian
Volcanic Link Relation to Greater Antilles Mammalian Fossil Record
and Land Connections

BIBLIOGRAPHY 709

INDEX 739

COLOR PLATES

The signature of color plates follows page 14.

Plate 1. Precambrian Orogenic Belts

Plate 2. Cambrian Tectonic Map

Plate 3. Ordovician Tectonic Map

Plate 4. Silurian Tectonic Map

Plate 5. Devonian Tectonic Map

Plate 6. Mississippian Tectonic Map

Plate 7. Pennsylvanian Tectonic Map

Plate 8. Permian Tectonic Map

Plate 9. Triassic Tectonic Map

Plate 10. Jurassic Tectonic Map

Plate 11. Early Cretaceous Tectonic Map

Plate 12. Late Cretaceous Tectonic Map

Plate 13. Tectonic Map of the Cretaceous-Tertiary Transition

Plate 14. Early Tertiary Tectonic Map

Plate 15. Late Tertiary and Quaternary Tectonic Map

EDITOR'S INTRODUCTION

A. J. Eardley's Structural Geology of North America has, since its pub-
lication in 1951, become something of a landmark in the geological litera-
ture of the New World. This is demonstrated by the broad base of its
foreign sales and the fact that, at home and abroad, the volume has re-
ceived heavy use by stratigraphers, geophysicists and other specialists,
as well as by the structural geologists for whom it was written. Moreover,
although originally conceived as a textbook for advanced undergraduates,
Structural Geology soon became a handy and valued general source book
for nonacademic professional and economic geologists.

Dr. Eardley, however, has always considered that his magnum opus
was somewhat out of date even before the first edition was put through
the publishing mill. Accordingly, immediately after the book was issued,
he set about the onerous task of revising it. For a full decade now he has
devoted a considerable amount of his time and efforts to the current re-
vision. The self-imposed "labor of Hercules" has been particularly frus-
trating and time consuming because during the fifties numerous basic
concepts of structural geology have undergone radical change. Thus,
fondly held theories of less than ten years ago are now either discarded

or seriously challenged. In addition, a vast quantity of new field data hai
been accumulating so rapidly that revisions can scarcely keep up with
the scientific progress.

Dr. Eardley has taken full cognizance of the rapidK evolving theo-
retical concepts, as well as of the flood of new information. As a result
this edition of Structural Geology is far from being a reprint — in many
chapters it is so extensively revised as to be essentiall) a new volume.
But in addition, much of the best of the first edition reinainv .iud thus
it is likely that this volume will continue to be the standard text and
reference work in a subdiscipline of geologv that is of prime significance
in the proper understanding of all other phases of the subject

The structural evolution of a continent! Relatively few scientific writers
have painted on such a broad canvas as Dr. Eardley. Hi' is something of
a rarity even among such artists, for he not only works with a broad
brush but also takes pains to fill in the details.

The geological fraternity has been indebted to Dr. Eardley for an ex-
cellent compendium on structural geology, and that indebtedness is now-
increased through an exceptional initial task that has become even better
done in its redoing.

( ' u.i ■> (. I iNl is

Rice University
June, 1962

PREFACE

TO THE FIRST EDITION

This book is addressed especially to advanced undergraduates in geol-
ogy. I doubt that it could have been written on a more elementary level
and still presume to use the common terminology of the numerous source
publications and the language of the professional geologists. In fact, some
instructors may consider the book too advanced for undergraduates. I
have endeavored, however, to take such measures as will make it under-
standable to the student who has had basic courses in mineralogy,
lithology, and structural geology. It will be well if he has had a course
in stratigraphy in which correlation problems have been discussed and
in which some attention has been given to the sedimentary environments
and sources.

The reader's attention is lost most frequently by the use of unfamiliar
formational, fossil, and geographic names. Generally I have not used

formational names in the text but, instead, have referred to the dep<
by period, epoch, or stage, and have listed the formational names in
charts. This has the advantage of easing the reading of the text and still
making the student aware of the many formations in the various parts ot
the country. At the same time it sets the stage for meaningful stratigraphic
studies in other courses.

I have discussed stratigraphic correlations only where necessary, anil
have relied on the latest authoritative correlations in the literati.:
graphic names have been treated with care, and I believe all that have
been mentioned are on accompanying maps and figures, or on other well-
known maps which are referred to as the occasion arises. Where petro-
graphic research has been referred to, I have attempted to discuss it in
such terms that the student with a knowledge of the common roik names
will understand.

Several professors who teach structural geology have expressed to me
the need for a text that treats structural geology from a regional point of
view, hut I doubt it the present volume is what they want, or that it can
be used as a substitute for the standard textbooks on principles. It may
be that in those departments where structural geology is taught as a senior
course, the hook could be used, and principles could he developed col-

xui

XIV

PREFACE TO THE FIRST EDITION

laterally. I think, however, that principles will suffer this way. I have
the book in mind for an advanced course in regional or structural geology.

I hope also that the book will prove attractive to professional geologists,
because some of the maps and ideas about the many fascinating problems
of continental growth may be new to them. I also trust that they will not
hesitate to set me right about any errors I have made.

Parts of the North American continent are so well known that it did
not seem worth-while to do more than describe them briefly and sum-
marize the conclusions that have been so well presented by others. In
certain areas, however, I had to marshal the evidence and present it in
some detail in order to sustain an original interpretation. For this reason,
all parts of the continent may not seem equally treated. I had to bear in
mind the professional geologist as a reader when drawing original con-
clusions.

A series of paleogeologic maps and paleotectonic maps is included in the
book. These, I hope, will be referred to repeatedly. They differ decidedly
from the familiar paleogeographic map, and for structural studies are
much more illuminating. As geologic studies progress, the maps will un-
doubtedly bear correction, but I have been impressed repeatedly with the
adequacy of our knowledge to date in establishing many important rela-
tionships.

left

Where possible I have referred to late summary reports, and have left
the reader to go to these, if he wishes all the original references. Where
good summary reports are lacking, I have referred to the basic investi-
gations. Our literature bearing on die structural development of the con-
tinent is so extensive that I have been continuously beset by the fear that
I have missed an important reference, especially for those regions with
which I am least familiar.

The research and writing of this book was done at the University of
Michigan, where the geologic library is extensive, the departmental facil-
ities are all that were needed, the time to do research work was abundant,
and my former associates on the staff were most helpful and congenial. I
remain very appreciative of these facilities and opportunities at the Uni-
versity of Michigan.

Miss Dolores Marsik has helped over several years as typist, and Dr.
Ruth Bastanchury Boeckerman has assisted in editorial work and has
done the final typing. Mr. Derwin Bell assisted in the drafting of many
of the figures and plates.

A. J. Eardley

January, 1951

PREFACE

TO THE SECOND EDITION

The second edition is an extensively revised version of the first. Seven
new chapters have been added, one on the Precambrian orogenic belts
and six on the igneous provinces of the western cordillera. Igneous rocks
are accorded a more significant place here than in the first edition. South-
ern Mexico and Central America are treated in a separate chapter as is
also the Canadian Arctic. The colored maps of the summary in Chapter 3
have been extensively revised, and several new ones are included.

Better index maps have been added throughout and an attempt has
been made to produce an understandable text independent of outside
sources of information. However, such maps as the geologic and tectonic
maps of the United States and Canada and the several state maps will
be indispensable for instructional purposes and should be available to
the student or professional geologist reading the book.

The second edition marks a time of major transition in structural geol-
ogy. In the past geologists have seen evidence in nearly every mountain
system of crustal compression, but now a number of authorities postulate
earth expansion, differential uplift, and crustal tension. The folds and
thrust sheets are being interpreted as gravity slide phenomena from re-
gions of marked uplift. Vertical movements along with distention and
wrenching are considered to be the primary aspects of crustal deforma-
tion— not horizontal compression.

The writer sees much in favor of the hypothesis of primary vertical
movements and has perhaps accorded it greater attention than some will
like. However, he has also attempted to present the geology of the several
provinces as the authorities have depicted them. Certain sections of the
book, therefore, reflect the orthodox concepts of compression, win
other parts will seem to emphasize primary vertical movements with sec-
ondary folding and thrusting. It will take another ten years to resoK e the
irregularities and to warrant the preparation of a more definitive third
edition.

A. J. Eardli v

June, 1962

Structural Geology

North America

described. Theories of diastrophism thai have been proposed for certain

structural systems are summarized, and current concept! of mmintalii
building and continental development .ire presented where approprj

INTRODUCTION

PURPOSE OF BOOK

The purpose of the book is to describe the structural evolution of the
North American continent. The chapters concern the formation and con-
stitution of the mountain systems, basins, arches, and volcanic archi-
pelagos; the beveling of the highlands; and the filling of the basins. In
short, they treat of the procession of deformational and sedimentary
events. Not only does the book seek to chronicle the crustal unrest of
the continent, but it also tries to summarize the supporting evidence.

The igneous provinces and their relation to the tectonic provinces are
treated. The advances in geophysics in deciphering deep crustal structure
are referred to, and the constitution of the crust in several regions is

METHOD OF PRESENTATION

The structural history of the continent is one both of time and of geo-
graphic position. The major scheme of organization of the book could,
therefore, follow one or the other. For instance, if organized on a time
basis, all the structural events over the whole continent would be re-
viewed period by period. If on a geographic basis, the structural history
of each major province would be followed from the beginnil .'en-

zoic time to the present. Neither course when rigidly pursued worked out
well, but if the chapter headings are scanned, it will he apparent that l
phasis in organization has been placed on geographic position.

The necessity of treating a succession of deformational events in a cer-
tain province without serious interruption early became plain, and it
decided that the great mountain systems whose histories run through
several periods of time must be treated as units. The growth of the con-
tinent in its several provinces has been described first during the Paleo-
zoic, and then, in general, the great structural units of the MesozotC and
Cenozoic have been considered. In the resume of the structural evolution
of the continent, Chapter 3, the paleogeologic and paleotectonic maps are
presented, and there the development, period by period, is reviewed.

KINDS OF ILLUSTRATIONS

Considerable effort has been made to illustrate every important point
developed in the text. Maps, cross sections, and block diagrams are used.
Photographs have little value because the structural features described
are usually immensely larger than photographs reveal. If the reader de-
sires to know the nature of the topographv, other books with a wealth of
photographs should be referred to, such as Fenneman's Physiography of
the United States, Lobeck's Gcomorplwlogy. Hinds's Geomorphology, and
Atwood's Physiography of North America.

STRUCTURAL GEOLOGY OF NORTH AMERICA

MAPS FOR COLLATERAL USE

The book is not intended to stand entirely alone. The reader or in-
structor should have the following maps for ready reference, preferably
mounted and hanging on the wall at short range.

The Geologic Map of the United States, 1932 edition

The Geologic Map of Canada, 1957 edition

The Geologic Map of North America, 1946 edition

The Tectonic Map of the United States, 1944 edition

Landforms of the United States, 1939. Map by Erwin Raisz

The Tectonic Map of Canada, 1950

The Geologic Map of South America, 1950

These maps will be referred to repeatedly. Although the book contains
many illustrations, it does not reproduce the features of the above maps,
and if they are not consulted when referred to, the continuity will be
interrupted, the evidence not clearly understood, and perhaps the con-
clusions not appreciated or properly evaluated.

AUTHORITY FOR STRATIGRAPHIC CORRELATIONS

Most field work in structural geology is based on previous paleontologic
and stratigraphic work. A report on the structural geology of an area is
not considered worth while unless the formations are dated. The principal
method of dating is by the fossils present, and therefore, the structural
geologist is dependent upon the paleontologist, except in Precambrian
terranes. It is conceivable, but not probable, that a sequence of deforma-
tional events could be worked out in a local area without reference to
fossils or to nearby stratigraphic columns, but to date the events and to
relate them to others in widely separated areas is generally impossible
without fossils.

A series of articles has appeared in the last few years in the Bulletins
of the Geological Society of America that summarize the formational cor-
relations throughout North America for each geologic period. They have
been prepared by the Committee on Stratigraphy of the National Re-
search Council, and are taken in this book as authority in relating the

numerous orogenic episodes throughout the continent. They are as
follows :

Chart No.

1. Cambrian formations of North America, Howell et al., Bull. Geol. Soc.
Am., vol. 55, pp. 993-1004, 1944.

2. Ordovician formations of North America, W. H. Twenhofel et al., Bull.
Geol. Soc. Am., vol. 65, No. 3, 1954.

3. Silurian formations of North America, C. K. Swartz et al., Bull. Geol. Soc.
Am., vol. 53, pp. 533-538, 1942.

4. Devonian formations of North America, G. Arthur Cooper et al., Bull.
Geol. Soc. Am., vol. 53, pp. 1729-1794, 1942.

5. Mississippian formations of North America, J. Marvin Weller et al., Bull.
Geol. Soc. Am., vol. 59, pp. 91-196, 1948.

6. Pennsylvania formations of North America, R. C. Moore et al., Bull. Geol.
Soc. Am., vol. 55, pp. 657-706, 1944.

7. Permian formations of North America, A. A. Baker et al., Bull. Geol. Soc.
Am., vol. 71, pp. 1763-1801, 1960.

8. Cretaceous formations of the western interior of the United States, Bull.
Geol. Soc. Am., vol. 63, pp. 1011-1044, 1952.

9. Cretaceous formations of the Greater Antilles, Central America and Mexico,
R. W. Imlay, Bull. Geol. Soc. Am., vol. 55, pp. 1005-1046, 1944.

10. Marine Cenozoic formations of western North America, C. E. Weaver
et al, Bull. Geol. Soc. Am., vol. 55, pp. 569-598, 1944.

11. Cenozoic formations of the Atlantic and Gulf Coastal Plain and Caribbean
Region, C. Wythe Cooke et al., Bull. Geol. Soc. Am., vol. 54, pp. 1713-
1724, 1943.

Additional correlations charts

Thickness and general character of the Cretaceous deposits in the western
interior of the United States, Preliminary Map No. 10, J. B. Reeside, Jr., U.S.
Geol. Survey, Oil and Gas Investigations, 1944.

Nomenclature and correlation of the North American Continental Tertiary,
H. E. Wood, 2nd, et al., Bull. Geol. Soc. Am., vol. 52, pp. 1-48, 1941.

Paleotectonic maps of the Jurassic system, U.S. Geol. Survey, Miscellaneous
Geological Investigations, Map 1-175, 1956.

Paleotectonic maps of the Triassic system, U.S. Geol. Survey, Miscellaneous
Geological Investigations, Map 1-300, 1959.

EXERCISES

Four types of assignments and exercises are feasible. The first is the
reading and reporting of original articles in the literature. It is hoped that

INTRODUCTION

all articles of outstanding importance are referred to in the text. All publi-
cations referred to are listed in the bibliographic index. For emphasis
on local areas of interest, die instructor can assign additional publica-
tions.

The second type of exercise is the detailing of stratigraphic successions
in the different basins and mountain systems. This in itself would consti-
tute an extensive course in stratigraphy, but perhaps for local interest,
certain stratigraphic details can be fitted into the structural picture.

The third type of exercise is the assembling from the book of all the
structural events that occurred nation-wide for each of the periods. Since
the book is organized chiefly on a geographic or provincial basis, it will

be an excellent review to cut across provinces on a time basis and sum-
marize the events over the entire continent for each period. 'Ih<
paleogeologic and paleotec tonic maps and the- bri< i discussion that
companies them in Chapter 3 already do this, hut no part of the text is
devoted in detail to it.

The fourth type of exercise is the tracing of the geologic history of a
county or a state. The commonest types of reports are those that de-
scribe the geology of an area with political boundaries, and it will ser\e
the student as a good lesson to write a history of such a region. He will
have to draw his information from several structural provinces and will
find his organization, if complete, both long and complex.

STRUCTURAL TERMINOLOGY

NEED OF STANDARD TERMS FOR REGIONAL STRUCTURES

The posthumous work of Schuchert (1943) is an example of the ir-
regular use of names for the large structural features of the United States.
He speaks of the Cincinnati anticline and the Cincinnati geanticline, evi-
dently interchangeably, and the Nashville dome in the same sense as the
Cincinnati anticline. McFarlan (1943), in his book on the geology of
Kentucky, defines the Cincinnati arch as a major structure which includes
the Jessamine dome and the Nashville dome, but in several places he
refers to the arch as a dome. In Colorado the Ancestral Rockies are com-
monly called highlands and geanticlines, in New Mexico they are land-

masses, in Texas they are uplifts and arches. The buried Nemaha
"Mountains" in Oklahoma and Kansas have been called a ridge. There are
a number of other terms for which no standard structural meaning has
evolved. The professional geologist may not experience any difficulty or
inconvenience in this loose and local application of names for the large
structural features of the earth's crust, but for the student it is confusing.
I have felt impelled to define and classify for his sake, because the book is
addressed to him. In so doing, however, I feel at many turns there will
be objections, largely on the grounds of provincial usage.

In view of the undesirability of multiplying technical words, it seems
necessary to assign specific meanings to common words in their several
fields of usage. For instance, the word system when used in stratigraphy
denotes the rocks formed during a period of geologic time; when used
geographically it generally signifies a group of ranges with unifying char-
acteristics; and when used structurally it indicates a group of related
joints, faults, dikes, or the like. It is probably better to give a word such
as system several meanings rather than use a new word, or a less common
and, perchance, a less appropriate one. The commonest usage of a term
should weigh heavily in formulating a definition for it.

MEANING AND CHOICE OF TERMS FOR THIS BOOK

Arch and Dome

From 1891 to 1903 Foerste spoke of the Cincinnati uplift as an anti-
cline, then in 1904 as a geanticline, and Schuchert continued the use of
these two terms apparently interchangeably. The first mention of the
terms arch and dome for the structure has not been located in the litera-
ture, but since 1900 they have been used very commonly and usually
synonymously. They are the terms used both provincially and nationally
most frequently today. McFarlan ( 1943 ) has distinguished the two in the
sense that the Cincinnati arch is an elongate structure and includes two
dome-shaped uplifts on it, the Jessamine dome and the Nashville dome,
separated by a sag or saddle. Tennesseans will probably not accept the
subordination of their Nashville dome to a division of the Cincinnati arch,
but the principle of the distinction of arch and dome is appealing. Since

STRUCTURAL TERMINOLOGY

the Cincinnati and Nashville structures are the earliest of the broad,
gentle uplifts studied in the United States, they probably should be
taken as types, and definitions should be fashioned after their character-
istics. At the completion of the present study of the uplifts and depres-
sions of the central stable region of the United States, nothing undesirable
is recognized in taking the Cincinnati and Nashville structures as types
for the United States, if a little latitude in characteristics is tolerated. The
terms in this report will be used as follows:

An arch is a gentle, broad uplift with an evident width of 25 to 200
miles and a length conspicuously greater than the width. The structural
relief may amount to 10,000 feet or more between a bed at the top of the
arch and one of similar age at the bottom of the adjacent basin, but the
dip of the beds will generally not exceed 100 feet per mile. The struc-
tural relief may have been acquired in part by subsidence of the adjacent
basins at a greater rate than the arch area, so that the arch may actually
only at times have been an emergent landmass.

A dome is a gentle, round or elliptical uplift of arch proportions. It
usually occurs along an arch and expands the arch locally. This regional
structural meaning of dome must be distinguished from the usage in con-
nection with igneous rock masses (Rice, 1940) and from the much
smaller oil- and gas-producing structures such as salt domes or plugs.

Swell

Schuchert ( 1923 ) used the term swell to mean all large, domed areas
within the nuclear part of the continent. Rucher ( 1933 ) defined a swell as
"an essentially equidimensional uplift without connotation of size or
origin." In discussing the structures of the United States the terms arch
and dome are sufficient for all broad gentle uplifts, to which the term
swell would generally apply, and therefore it has not been necessary to
use swell in the following pages, and no attempt to define it further will
be made here.

Uplift and Upwarp

Uplift and upwarp are used for a wide variety of structural elevations,
and, therefore, should be reserved as noncommittal terms in regard to

size, shape, internal structure, and origin. If it is desired to distinguish
the two, uplift might be conceived as implying both small and lai
round and elongate elevations, with sharp and gentle variations; whan
upwarp would imply simply broad and gentle archings. Nfo precedent

can be cited for this distinction, but a perusal of the literature leaves me
with the impression that this is the most general usage, l'rovmc iallv, how-
ever, uplift may mean a rather definite type of structure. I will use the
terms only in case I am in doubt about the nature of a structural el
tion, or desire to use them as synonyms of structures being discussed in
order to eliminate repetition.

Basin

Rucher (1933) uses the term basin in a structural sense to mean any
essentially equidimensional depression without connoting size or origin,
and then gives the Michigan basin as an example. Swell is his antithetical
structure of basin. Since the drill in several places has extensively ex-
plored the subsurface distribution of the stratified rocks of the continent.
a number of downwarps have become firmly entrenched as basins in the
literature. Some embrace more than a large state, and some are of county
size. Some are fairly elongate, and most all have axial directions. Some
are troughlike or furrowlike. It has not proved disturbing in compiling
the present review to have basin used in this loose sense, and I believe the
variations in meaning will be evident to the student, so there is little urge
to attach limitations to the term. The word basin is applied a thousand
times each day by petroleum geologists in many variations of meaning,
and it would appear unwise to attempt standardization.

Coal basins have not proved to be the same as oil basins or water
basins in several places, and also the extent of the commercial materials
has not coincided with the greatest thickness of the strata and. therefore,
the greatest depression. It seems to me that the major geological features
should govern the choice of a geographic name, rather than the distribu-
tion of an economic deposit of little relative volume.

The site of maximum subsidence during an epoch, period, or era may
not coincide with that of a later one, and some confusion has resulted in
the meaning of the term basin in certain areas. This is particularly true

STRUCTURAL GEOLOGY OF NORTH AMERICA

on tectonic maps which attempt to show all structures evolved through
three eras. I have found it desirable to think of certain basins in a re-
stricted time as well as restricted geographic aspect, and to prepare ac-
cordingly the tectonic maps that accompany this book.

Geosyncline
According to Kay ( 1951 ) :

The term geosyncline should be restricted to a surface of regional extent sub-
siding through a long time while contained sedimentary and volcanic rocks are
accumulating; great thickness of these rocks is almost invariably the evidence
of subsidence, but not a necessary requisite. Geosynclines are prevalently
linear, but non-linear depressions can have properties that are essentially
geosynclinal.

Classifications of geosynclines are discussed by Kay, who takes the
position that all basins having a thick sequence of sediments are one kind
or another of geosyncline. However, only two geosynclinal terms will be
used in this text, namely, miogeosyncline and eugeosyncline, which are
the large linear basins along the margins of North America.

Miogeosyncline

A miogeosyncline is part of the great linear border geosyncline. It lies
between the shelf regions of the stable interior of the continent and the
outer part of the geosyncline. Its sediments are dominantly sandstone,
shale, chert, limestone, and dolomite, almost free of volcanic rock.

Eugeosyncline

An eugeosyncline is the outer part of the border geosyncline and is
characterized by an abundance of volcanic rock. In addition there is much
graywacke, arkose, dark shale, and chert. The strata are generally altered
by low-grade metamorphism.

Landmass

Landmass has no specific structural meaning unless used locally as in
the Ancestral Rockies of New Mexico, for instance, where an ancient
range is referred to as the Pedernal Landmass. The term usually con-

notes a land area whose elevation, climate, and life are the special object
of study through the intermediary of the sediments derived from it, or
whose changing shore fines form the basis of some paleogeographic study.
The term does not usually imply size, relief, or origin, and no specific
attributes will be affixed to it in this book.

Highland

In Colorado, two principal uplifts dominated the structural evolution
of the area in late Paleozoic time, and they have been referred to by
most writers as highlands. They are about 50 miles wide and 200 miles
long and structurally were rather abrupt, asymmetrical anticlines which
may have been faulted in part along their steep flanks. Except in appli-
cation to the Colorado uplifts, the term is used very broadly in the United
States, and no one to my knowledge has attempted to define it; nor is it
necessary here to do so. It does not seem consistent, however, to say a
certain highland was a Zotu-lying area, but the statement may appro-
priately be made of a landmass.

Ridge

The buried Nemaha uplift of Oklahoma, Kansas, and Nebraska is gen-
erally spoken of as the Nemaha Mountains, but the term Nemaha ridge
has also been used, with the implication that ridge has a certain structural
significance. The use is almost unique to this area, as far as I know. A
ridge, topographically, is generally less than 5 miles long, and its use
structurally for the Nemaha Mountains, 200 miles long, is somewhat mis-
leading. It is not necessary to use the term in the present review.

The term is used in oceanography to depict very large linear relief
features on the ocean floor, such as the Mid-Atlantic Ridge (also Rise)
or the Beata Ridge in the Caribbean Sea.

Geanticline

The term geanticline was proposed by Dana in 1873 ( Schuchert, 1923 )
for "the upward bendings in the oscillations of the earth's crust — the
geanticlinal waves or anticlinoria." According to Schuchert, Dana's typi-
cal example was the Cincinnati arch, though later on, Dana also included

STRUCTURAL TERMINOLOGY

far greater, even continental arching. Schuchert generally recognized
geanticlines and geosynclines as "complementary structures," but called
the land that divided the Cordilleran geosyncline during Mesozoic time
into an eastern geosyncline and a western, the greatest of North American
geanticlines.

Although Schuchert attempted to clarify Dana's most confused defini-
tion, he introduced contradictory thoughts, and therefore did not clarify
the meaning of the term. Others have confused the meaning still more.
According to Willis ( 1934 ) , "a geanticline is a very large elevation of the
earth's surface. The rocks of the geanticline may not be folded — may not
even be stratified — and the anticlinal significance is lost." Lahee ( 1941 )
states that a "geanticline is a very extensive uplift, generally anticlinal in
nature (also called a regional anticline)." He gives as examples the "Ar-
buckle Mountain Uplift and the Central Mineral Region of Texas," which
are two greatly different kinds of tectonic elements. According to Nevin
( 1942 ) a geanticline is a "great upwarp . . . whose dimensions are meas-
ured in hundreds of miles. . . . The Ozark Mountains and the Arbuckle
Uplift are true geanticlines." These, again, are dissimilar structures. Bill-
ings (1942) defines a geanticline as "the counterpart of a geosyncline, (it)
is an area from which the sediments are derived. The geanticline that lay
southeast of the Appalachian geosyncline is known as Appalachia." In
the Dictionary of Geologic Terms (Rice, 1940) a geanticline is "a
large, broad, and usually very gentle anticline, commonly many miles in
width."

Most of these definitions are widely divergent, and the examples are
structures of contrasting size, composition, history, and relation to the
central stable interior of the continent. Some of the definitions are synony-
mous with terms already defined, such as arch, dome, and landmass.

The confusion in American literature is paralleled by the European.
Brouwer (1925) of Holland says that a geanticline is a major uplift of
island arc size, complementary to the geosyncline. Collet ( 1927 ) , follow-
ing Argand (1916), defines a geanticline as an anticlinal ridge that ap-
pears on the bottom of a geosyncline and expresses itself as a land barrier
between the seas of the geosyncline. It is at first a long, narrow anticline
of considerable size and later evolves into a great nappe. Whether the de-

velopment of a nappe is necessary to demonstrate a true geanticline in not
stated or implied. Most Alpine geologists, it is my impression, follow tin-
usage of Collet.

King (1937) exemplifies the Alpine usage in his treatise of die evolu-
tion of the Marathon system. A structure in west-central Nevada tint
rose out of the Paleozoic Cordilleran geosyncline is i ailed a geantu line In
Nolan (1928). I have decided to follow the specific usage of Collet, King,
and Nolan and will denote a geanticline as a large, elongate, anticlinal
fold that develops in the sediments of a geosyncline. It is not a 'geanticline
if an uplift in the foreland or shelf area. Two or more geanticlines ma)
develop at the same time or following each other in a great geos\ ncline.
After the early anticlinal uplift, the great fold usually becomes a complex
anticlinorium, several imbricate thrust sheets, or a nappe. It may be
largely submarine, and suffer little erosion.

Range

The synonymous use of the terms highland, landmass, mountains, up-
lift, arch, and geanticline, all to describe uplifts of the Ancestral Rockies
and Wichita systems with fairly similar size and shape, poses a difficult
problem, especially because of the provincial nature of the usage. It is so
commonplace to say Electra arch and Uncompahgre highland that a
change of name of one or both is not easily accepted by all. For the sake
of the student who is trying to get an understanding of the rather com-
plex, regional, structural relations of the continent, uniformity of meaning
is desirable. Many geologists long since out of school recognize the need.
It is a matter of clear composition.

Nearly all the structural features to which the names highland, land-
mass, uplift, arch, and geanticline in the Ancestral Rockies and Wichita
systems have been attached are the size of a range like the Bighorn, the
Uinta, or the Selkirk ranges. It seems, therefore, that the word range
would be very expressive of the sharp and linear, now buried or nearly
buried, late Paleozoic uplifts of the interior part of the United States.

Geographers have agreed on the usage of range and mountain svstem
as follows: A range is a mountain mass within limits the size of the Big-
horns or the Selkirks, and a number of these ranges with certain unifying

STRUCTURAL GEOLOGY OF NORTH AMERICA

features in the region constitute a mountain system. In working over
the structural features of North America I find that the divisions of the
major structural provinces can fittingly be called ranges and that many of
the major provinces themselves, systems. Range, therefore, will be used
to denote a sharp uplift about 10 to 75 miles wide and 50 to 200 miles
long. Commonly the structure is an asymmetrical anticline. In some, the
steep flank has broken into a high-angle fault or a thrust. Others may
consist of several folds or even thrust slices. Probably all ranges that were
eventually buried suffered considerable erosion beforehand.

Platform and Shelf

The terms platform and shelf in a structural sense are logically used
by King ( 1942a ) in the Permian area of west Texas and southeastern New
Mexico. There previously existing range-sized uplifts were buried, and
as sediments continued to accumulate, the adjacent basins were depressed
more than the old uplifts, so that although sediments accumulated on
the uplifts themselves, broad anticlinal structures developed over them.
These are called platforms. Beyond the basins, shallow seas existed, but
the crust subsided much more slowly there than in the basins, and a
much thinner deposit of sediment accumulated. These are called shelves.
A platform is similar to a shelf in regard to thickness of sediments on it,
but much more restricted in size and bounded on the two sides by ba-
sins. This is the sense in which the terms will be used in the following
pages.

Welt and Furrow

Bucher (1933) defines welt and furrow as crustal elevations and de-
pressions that show a distinct linear development. No special size or
origin is implied. A welt may be as large as a great deformed geosyncline;
viz., note Bucher's reference to Hobbs's phrase, "the gigantic welt of the
Himalayas." In Bucher's analysis of the deformation of the crust on a
world-wide scale, he needed these noncommittal terms, but in the present
attempt to picture the structural evolution of the North American conti-
nent, the names do not seem necessary, and they will not be used.

Hinterland and Foreland

Hinterland and foreland are terms introduced by the European ge-
ologists to distinguish the landmass or resistant elements of the earth's
crust on either side of an orogenic belt. In the Alps great, intricately
folded masses of sediments of the geosyncline, plus injected rock, moved
northward many miles. The north stable land toward which they were
moved is called the foreland, and the landmass south of the geosyncline
is called the hinterland. In the main, the great thrust sheets of the Ap-
palachian and Rocky Mountain orogenic belt have overridden toward the
interior stable part of the continent, and this ( at least the parts adjacent
to the orogenic belts) has generally been called the foreland. The land-
masses or borderlands on the oceanward side have been referred to as the
hinterlands. It is apparent that confusion must arise in the use of the
terms when some thrust sheets have overridden toward the oceans and
when, perhaps, no great, stable borderland existed. Some geologists also
contended that outward from the continent is the foreland. As for usage
in this book, foreland will mean the part of the stable interior adjacent to
a marginal orogenic belt, and lands oceanward of a marginal trough of
sedimentation, created by previous orogeny and from which sediments
were derived will be called the hinterland.

TERMS FOR STRUCTURAL DISTURBANCES

Revolution and Synonyms

The term revolution is deeply intrenched in geologic literature, al-
though a number of authors, both here and abroad, have avoided its use,
and one has recommended its abandonment (Spieker, 1946).

Schuchert's (1924) definition of a revolution is more complete than any
found, and characterizes many usages of the term.

Near the close of the eras . . . occur the most extensive times of mountain
making, . . . These times of major diastrophism are the critical periods or
revolutions in the history of the earth, and they divide, as it were, the book
of geologic time into chapters. The critical periods are marked by the fol-
lowing features:

STRUCTURAL TERMINOLOGY

1. By wide-spread deformation of the earth's crust, transmitted from place
to place. This leads to the elevation of many and widely separated mountain
ranges, . . . Each revolution ... is named after one of the prominent moun-
tain ranges formed at the time designated, for example, Laramide and Ap-
palachian revolutions.

2. By wide-spread changes in the physical geography . . .

3. By marked and wide-spread destruction of the previously dominant,
prosperous, and highly specialized organic types.

4. By marked evolution of new, dominant, organic types out of the small-
sized and less specialized stocks, and by the development of hordes of new
species.

With revolutions reserved to close eras, Schuchert used the term dis-
turbance to terminate periods. Thus the crustal movements at the close of
the Devonian period in New England and Acadia would be called the
Acadian ( Schickshockian ) disturbance.

In light of recent research, certain disturbances are known to have oc-
curred within periods, and three (Taconic, Acadian, Nevadan) are equal
or exceed in size and certainly exceed in intensity the Appalachian (as
orthodoxly known) and the Laramide revolutions. In the Alps, the di-
astrophic history is followed from the middle Carboniferous to the close
of the Oligocene, and it seems difficult to apply the term revolution in
Schuchert's sense. The great paroxysms in which the nappes were formed
occurred in middle Oligocene time, and to these and all other deforma-
tions of early Tertiary time, Argand ( 1916) applies the name Alpine cycle.
Thus he speaks as follows: "The regime of deformation of Asia during
the Alpine cycle, . . . etc." (1922). He refers to the Hercynian cycle
and the Caledonian cycle, apparently in the same general way as others
do with the words orogeny, revolution, disturbance, and phase.

Rucher ( 1933) adapts the term revolution to his own nomenclature and
theory by the following: ". . . the juxtaposition of the high welt and the
deep sediment-filled furrow leads to the violent deformation traditionally
known as 'revolutions.' "

Refore deciding what terms or classification to use in this book, a few
other words need to be discussed. The terms orogeny and epeirogeny,
according to Gilbert (1890) are processes of deformation. He defined
orogeny as the process of mountain building, and epeirogeny as the
process of continental displacement to form the large swells and basins.

The two processes collectively he called diastrophism. Orogenic struc-
tures, according to Stille (1924) are visible to the eye, such is faults
folds, and thrusts; whereas epeirogenic structures arc so gentle thai dips
are scarcely noticeable, and are due to broad warping. The usage in
America today is fairly uniform in the respect that orogenic movement is
of the nature of folding, thrusting, and block faulting or rifting and for
the most part takes place in the geosynclinal belts. Epeirogenic move-
ment is vertical, of gentle nature, and affects regional parts of the trust.
The arches, domes, and large basins of the central stable region of the
continent are examples of epeirogenic movements, and the interruption of
cycles of erosion in the deformed geosynclinal belts by elevation is an ex-
ample of epeirogenic movements in the marginal and older orogenic belts.
It is in this sense that the terms will be used in this book.

A point that is confusing is the interchangeable use in our literature of
orogeny and revolution. It would seem from Gilbert's early usage that
orogeny is a process, and to say Appalacliian orogeny would be to focus
attention on the processes of deformation in the geosyncline — to em-
phasize the mechanical relations. On the other hand, to say Appalachian
revolution would be to broaden one's vista structurallv to the events in
the hinterland and the foreland as well as in the geosyncline, and to in-
clude the climates and changes in the organic world. Current usage of the
term orogeny is also often synonymous simply with crustal disturbance.
Angular unconformities and coarse, thick, basal conglomerates are com-
monly the evidence of orogenies, and the orogenies are given names such
as the Diablan, Santa Lucian, and early Laramide. Refore deciding on
definite usages of the terms, it is best to consider their time and geo-
graphic limits.

Phase

The term phase has been used structurally as well as stratigraphicallv.
In nearly all structural uses it is a division, either spatial or time, of a
revolution. For instance, Argand (1922) in explaining his tectonic map of
Asia says, ". . . we have concluded . . . that a classification of the ele-
ments (shows) only the age of the principal folding . . . neglecting the
phases but retaining the orogenic cycles." And again. ". . . all the pli

STRUCTURAL GEOLOGY OF NORTH AMERICA

of all the orogenic cycles that have affected each part of the country, etc."
Collet (1927) uses the word phase as a tectonic unit of the Middle
Oligocene orogenic paroxysms of the Alps, viz., the St. Bernard phase, the
Dent Blanc phase, the Monte Rosa phase, the phase of Adriatic sub-
sidence, and the phase Insurbrienne. This usage emphasizes the mass and
spatial aspect because all the nappes mentioned evolved within a short
time — a succession of events is not implied. On the other hand, van
Waterschoot van der Gracht ( 1931 ) uses the term more in a time aspect
in describing the structural relations in the Mid-Continent area, for he
designates the successive episodes of disturbance as the early Wichita
phase, the late Wichita phase (early Pennsylvanian ) , and the Arbuckle
phase (late Pennsylvanian).

Others terms such as epoch, stage, and impulse, have been used but to
a lesser extent than phase.

CLASSIFICATION USED FOR CRUSTAL DISTURBANCES

Revolution

If revolutions are chapters of diastrophism in earth history, it is clear
that they have both time and spatial aspects. To say they terminate the
great eras of time reflects the state of advancement of the science 45
year ago. Most of the time divisions were originally set apart by uncon-
formities, and early became more or less fixed by the fossil content of
formations between the unconformities at the type localities. Since then,
evidence of many new and important disturbances has been discovered
within the periods and eras thus set apart. Crustal deformation has come
to be known not as a repetition of pulsations that occurred precisely at
the close of periods and eras, but as developmental sequences of deforma-
tional events which frequently occurred over protracted periods of time
with shifting scenes of activity.

A revolution will be considered to encompass the deformational events
of the hinterland, the geosyncline, and the foreland, and to include both
orogenic and epeirogenic processes. Setting time limits is an arbitrary pro-
cedure, and in doing so one must be mindful of usage which will help
determine the best limits of the revolution in question.

System

The major structural divisions of revolutions will be called systems. A
system is thus primarily a spatial division and is determined by a unity
of the structural features in it, such as the folds and thrusts of a geo-
syncline in contrast to the basins, shelves, and arches of the foreland, or
by isolation of a somewhat similar structural assemblage from another
by younger overlapping deposits, such as separate the Ouachita Moun-
tains from the Marathon Mountains.

As far as noted, systems have been named after the outstanding range
or geographic feature in the division. This precedent will be followed
structurally where possible, but some exceptions seem necessary. For
instance, in organizing the structures of the central stable region of the
United States the area proved so large that no one geographic name
seemed suitable for the greatest arch, so its outstanding structural char-
acter was used, namely, the Transcontinental Arch.

Phase

Each system has its developmental history, and the structural events
of this history will be called phases. Although the types and extent of the
structures developed will be considered part of the phase, emphasis is
laid on the time aspect. It may be necessary to consider as phases two
contemporaneously evolving parts of a system, but in organizing structural
elements of the continent I have not run into this difficulty.

In the Alps, the phases have been given geographic names, and the
practice was followed in this country by Van der Gracht (1931), who
discussed the Wichita and Arbuckle phases of the Wichita system. Since
in this book the emphasis will be placed on time, I have concluded that
time names will be most meaningful. For instance, if it is written, the
early Pennsylvanian phase of the Ancestral Rockies system, the student
cannot miss the intended meaning; but if the Wichita phase of the An-
cestral Rockies appears, the student may be confused if he has not read
the chapter on the Wichita system.

Time names can be inappropriate only where the time of the disturb-
ance is not accurately known and future research shows the designation

STRUCTURAL TERMINOLOGY

wrong. A geographic name avoids this difficulty, it is true, but for the
most part stratigraphy has advanced to the point, in the United States at
least, that the times of the deformation are fairly accurately known and
not likely to be changed much in the future. The advantage to the student
weighs so heavily against the possible chance of error that time names for
the phases will be used.

Most of the chapters deal with systems and their phases. Such organiza-
tion seems adequate to explain the structural evolution of the continent.
Originally it was planned to organize the book according to revolutions,
but setting limits led to many difficulties, and the idea was abandoned.
As a result, the concept of revolution is not emphasized.

Orogeny

With the decision reached to divide the great deformational belts into
mountain systems, and to treat the several episodes of deformation of each

system as phases, the proper usage of the term orogeny teemed d-
each phase is an orogeny. Thus we speak of the "I. .it.- ( retaceoui and
Early Tertiary Rocky Mountain systems," and for one ol th<
the Central Rockies, we note its episodes of deformation, namely, the
Montana phase, the Paleocene phase, and the Eocene phase. These pb
are commonly the orogenies, which respectively would be the earl)
Laramide orogeny, middle Laramide orogeny, and late 1 •aramide OXOgi
See table of contents for the various orogenies recognized in North
America.

An orogeny should be given a geographic name, like a formation, and
if the time of deformation is found to be earlier or later than previously
recognized on the basis of later research, then the name remains the same,
but a somewhat different age is assigned it.

An orogeny should not be limited to a phase of folding and thrusting,
but should include all forms of diastrophism, according to Billings ( 1960).

RESUME OF STRUCTURAL
GEOLOGY OF NORTH AMERICA

MAJOR TECTONIC DIVISIONS

Canadian Shield

The Canadian Shield has been the great stable portion of the North
American continent since Proterozoic time. It consists of Precambrian
rock except along the southern margin of Hudson Bay, where Ordovician,
Silurian, and Devonian strata, about 1000 feet thick, occur and probably
continue northward under much of the bay. Small outliers of Paleozoic
strata, fossil affinities, and the absence of shore facies in many places
indicate that the Paleozoic formations were once much more widespread

over the shield than now, and that they have been stripped off by a long
interval of erosion during the Mesozoic and Cenozoic eras.
Hudson Bay is an epeiric sea of fairly modern time.

Central Stable Region

The Central Stable Begion consists of a foundation of Precambrian
crystalline rock, which is a continuation of the Canadian Shield south-
ward and westward, and a veneer of sedimentary rock. The veneer varies
greatly in thickness from place to place, and several broad basins, arches,
and domes are present. A number of unconformities attest the rise of
the arches and their erosion, and of great transgressions and overlaps.
For the most part the strata have only gentle dips, and aside from the
slow and prolonged vertical movements that created the basins, arches,
and domes, the geologic province properly deserves the name, the Central
Stable Begion. It and the Canadian Shield compose the great stable in-
terior of the continent.

The arches and basins developed chiefly in the Paleozoic era, but later,
during the Mesozoic and Tertiary, vast amounts of clastic sediments from
the evolving Cordilleran mountain systems were spread eastward over
the Paleozoic strata beyond the Missouri Biver as far as Lake Superior.

In the southwestern corner of the Central Stable Begion a system of
ranges was elevated in Pennsylvanian time, and then during the late
Pennsylvanian, Permian, and Mesozoic it was largely buried. The ranges
are known as the Ancestral Bockies in Colorado and New Mexico, and
as the Wichita Mountain system in Kansas, Oklahoma, and Texas. The
Late Cretaceous and Early Tertiary Laramide structures were partly
superposed on the Ancestral Bockies in Colorado and New Mexico.

Orogenic Belts of the Atlantic Margin

The Paleozoic orogenic belts bound effectively the southern, as well as
the eastern, margin of the continent. The major belt is known as the
Appalachian, and it consists of an inner folded and thrust-faulted division
from Alabama to New York, and a metamorphosed and intruded division
from Alabama to Newfoundland. One major orogeny occurred in the

RESUME OF STRUCTURAL GEOLOGY OF NORTH AMERICA

I .

inner belt, and this in late Paleozoic time. Several orogenies beset the
outer belt: the earliest one of significance occurred at the close of the
Ordovician, the major one during the Late Devonian and the last one in
Pennsylvanian and Permian time. The Carboniferous orogenic belt in
the outer crystalline division is recognized on the north along the eastern
margin of New England, the Maritime Provinces, and Newfoundland.

Volcanic rocks and great batholiths are important components of the
crystalline division of the Appalachian orogenic belt, but the inner folded
and thrust-faulted belt is comparatively free of them. Roth divisions are
made up of very thick sedimentary sequences which are characterized
as geosynclinal, in contrast to generally thinner sequences in the Central
Stable Region.

The orogenic belt bordering the southern margin of the stable interior
is mostly concealed by overlapping coastal plain deposits. Where exposed,
as in the Ouachita Mountains of Arkansas and eastern Oklahoma, the
Arbuckle Mountains of south central Oklahoma, and the Marathon Moun-
tains of western Texas, it is a folded and thrust-faulted complex, some-
what similar to the inner Appalachian division. The crystalline division,
if it parallels the inner noncrystalline division, is nowhere exposed, but
deep wells through the coastal plain deposits have penetrated low-grade
metamorphic rocks.

Orogenic Belts of the Pacific Margin

The great complex of orogenic belts along the Pacific margin of the
continent has evolved through a very long time. The oldest strata recog-
nized from their fossils are Ordovician, and deformed strata of Pleistocene
age mark the belt in places from Mexico to Alaska. In Paleozoic time, the
Pacific margin of the continent was a volcanic archipelago in outward
appearance and internally a belt of deformation and intrusion. The
Permian, Triassic, and Early and Middle Jurassic were times of excessive
volcanism, and represent a continuation of essentially the same Paleozoic
conditions well into the Mesozoic. In Late Jurassic and early Late
Cretaceous time, intense folding and batholithic intrusions (Nevadan
orogeny) occurred which are now characteristic of large parts of the

Coast Range of British Columbia, the rangei along the International
border in British Columbia, Washington, and Idaho, the Klamath Moun-
tains of southwestern Oregon and northern California, the Sierra Nevadi
of California, and the Sierra of Baja California. The same Nevadan ele-
ments may also continue into southern Mexico and eastward through
Central America.

Following the orogeny, in California at least, a new trough of accumu-
lation and a new volcanic archipelago formed outside the Nevadan belt,
and a complex history of deformation and sedimentation tarries down
through the Cretaceous and Tertiary to the present, to result in the Coast
Ranges of Washington, Oregon, and California.

Orogenic Belts of the Rocky Mountains

During the complex and long orogenic history of the Pacific margin,
the adjacent zone inward was one of gentle subsidence and sediment
accumulation, comparatively free of volcanic materials, during the
Paleozoic. By Triassic time, the troughs of deposition along the Pacific
had become effectively separated by a medial, linear uplift from those
in the Rocky Mountain area, and in the Mesozoic much coarse debris
came from the uplift or geanticline and filled the basins in eastern British
Columbia, western Alberta, Idaho, western Wyoming, central Utah, and
southern Nevada. Orogeny from place to place along the eastern margin
of the geanticline cast several floods of conglomerate eastward during
the Cretaceous.

The Paleozoic and all the Mesozoic sediments except the Upper
Cretaceous of the Rocky Mountains may be divided into thick geosyn-
clinal facies on the west and fairly thin shelf facies on the east. The line
dividing the two lies approximately along the west side of the Colorado
Plateau and thence runs northward through western Wyoming and
Montana to western Alberta. The shelf facies were part of the Central
Stable Region until the Late Cretaceous and Early Tertiary ( Laramide )
orogeny in whose belts both geosynclinal and shelf facies were deformed.
The western division of the Laramide belt (in the miogeosyncline) is
characterized by folds, thrusts, and numerous small stocks. The eastern

STRUCTURAL GEOLOGY OF NORTH AMERICA

Laramide division extended through the shelf region of central and
eastern Wyoming, central Colorado, eastern Utah, and central New
Mexico, and is characterized by large, elliptical uplifts.

The Laramide belt of orogeny extends southward through Mexico,
where thick sediments of the Mexican geosyncline of Upper Jurassic and
Cretaceous age are fairly tightly folded. The same belt of orogeny is
believed to veer eastward through Central America.

Following well after the Nevadan and Laramide orogenies of western
North America, an episode of high-angle faulting occurred, that created
the Great Basin physiographic province and gave sharp definition to
many of its ranges and to those of central and western Mexico. The
high-angle faults were superposed on both the Nevadan and Laramide
belts; most of them are Late Tertiary in age and some are still active. A
long zone of the faults extends northward from central Utah to British
Columbia and probably beyond to Yukon Territory to form a belt of
trenches with local relief of 3000 to 5000 feet. The faults cut the older
folds and thrusts both discordantly and concordantly, and the activating
forces appear deep-seated.

Coastal Plains

Following the Appalachian orogeny in Triassic time, the outer meta-
morphosed division was broken by a belt of high-angle faults that has
been traced discontinuously from South Carolina to the Bay of Fundy,
between New Brunswick and Nova Scotia. Long and narrow downfaulted
basins trapped thick series of generally red elastics. The Triassic lowland
of Maryland, New Jersey, and Pennsylvania, and the central lowland of
Connecticut are the best known of the basins.

The eastern extent or breadth of the Appalachian orogenic system
and the nature and condition of the crust that lay east of it are not
known, but the continental margin had begun to subside, at least by
Early Cretaceous time, if not before. The peneplained surface on the
crystalline rocks has been traced eastward under a Cretaceous and
Tertiary sedimentary cover to a depth of 10,000 feet, which is near the
margin of the present continental shelf. Most sedimentary units of the
cover dip gently and thicken like a wedge oceanward as far as they have

been traced by deep drilling and by seismic traverses. The zone of
Cretaceous and Tertiary overlap on the older rocks of the eastern con-
tinental margin is known as the Atlantic Coastal Plain, but because the
same sediments continue out beyond the present ephemeral shore line,
the submerged part belongs to the same province. Coastal plain sedi-
ments are known to exist in Georges Bank off Rhode Island and prob-
ably make up part of, or all, the shallow continental shelf to and
including the Banks of Newfoundland.

The Gulf Coastal Plain is continuous with the Atlantic Coastal Plain,
and counting its shallowly submerged portions, it nearly encloses the
Gulf of Mexico. The oldest known sediments of its marginal areas are
Permian. The Mississippi, Rio Grande, and other rivers draining the
interior of the continent have deposited a great weight of sediments at
their mouths and the crust has subsided along the Texas, Louisiana, and
Mississippi coast to the extent of 25,000 to 30,000 feet.

Deep drilling in Florida and the Bahamas indicates that the coastal
plain province extends southeastward almost to the orogenic belt of
Cuba and Hispaniola.

Canadian Arctic

The Precambrian rocks of the Canadian Shield are overlapped on the
north by nearly flat-lying sedimentary strata of Paleozoic age. Basins
and arches are recognized in this province as in the Central Stable
Region of the United States. North of the arches and basins is a fold
belt developed in geosynclinal sediments. The fold belt extends across
northern Greenland, northern Ellesmere Island and farther to the south-
west through other islands of the Arctic Archipelago. Folding first oc-
curred in pre-Pennsylvanian time. After erosion a voluminous sequence of
Pennsylvanian to Tertiary sediments accumulated, and then these were
somewhat folded in Tertiary time. A narrow Tertiary coastal plain is
terminated on the north by the Arctic Ocean basin.

Alaska

Alaska continues the broad and complex western cordillera across to
Asia, and has had basically the same history but with variations and
singular details.

Meaning of Colors on Tecfonic Maps
(Plates 2-15)

BLUE Denotes regions of accumulations of sediments. Contours indicate thickness of sediments and
thus, approximately, the amount of subsidence. Thickness figures indicate thousands of feet.

GREEN Denotes ocean basins; i.e., regions underlain by oceanic crust.

ORANGE Denotes significant deformation. Where sediments have been deformed during the period in
which they were deposited such has been printed on the blue.

RED Denotes belts of batholithic intrusion and appreciable metamorphism on all Plates except
1, 14, and 15. On Plate 1 various intensities of red plus orange and yellow denote orogenic
belts of different ages. On Plates 14 and 15, red denotes igneous rock, chiefly volcanic.

YELLOW Denotes regions of comparative stability of the earth's crust. It includes on some maps regions
of broad and gentle uplift (Plate 1 excepted).

PLATE 1
Precambrian Orogenic Belts

Position of belts older than the Beltian is determined principally by absolute
isotope ages. A, P, and G are dates of Algoman, Penokean, and Grenville
orogenies, respectively.

PLATE 2
Cambrian Tectonic Map

Upper Cambrian seas were probably more widespread in the Transcontinental
Arch region than shown; the strata have been eroded away there. The
Cambrian beds of eastern Newfoundland, although evidently in the Acadian
trough, are mostly miogeosynclinal in lithology.

PLATE 3
Ordovician Tectonic Map

Westward thrusting occurred at the close of the Ordovician in eastern New
York against the Adirondack dome. Some ultrabasic intrusions may have been
emplaced in the Maritime Provinces and Newfoundland at the close of the
Ordovician. W. A. Waverly arch of Early Ordovician time.

PLATE 4
Silurian Tectonic Map

The Atlantic Ocean and Gulf of Mexico are left uncolored because accumu-
lating evidence suggests that North America was once attached to and part
of a single great continent which cracked and drifted apart. The spreading
apart is presumed to have brought these ocean basins into existence, starting
in late Paleozoic time.

PLATE 5
Devonian Tectonic Map

The eugeosynclinal regions in Acadian orogenic belt preceded the orogeny;
their sediments were intensely deformed and invaded by the large batholiths,
and hence are not shown in blue.

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PLATE 6

Mississippian Tectonic Map

Stanley, Jack Fork, and Johns Valley elastics of Ouachita Mountains are
shown as a Mississippian basin. They may be in part Pennsylvanian. LS means
La Salle anticlinal belt. It rose gently, was eroded, and buried before the
Mississippian period ended. Orange here indicates areas of orogeny, signifi-
cant uplift, or mountains of an immediately prior orogeny. In the Antler
orogenic belt both sedimentation and orogeny occurred.

PLATE 7
Pennsylvania/} Tectonic Map

Uplifts shown by dotted lines were mostly buried by end of Pennsylvanian.
The Baja California block lay several hundred miles to the southeast. A,
Marathon basin; B, Fort North basin; C, Ouachita basin; D, Southern Ap-
palachian basin; E, Central Appalachian basin; F, Diablo Range; G, Pecos
Range; H, Pedernal Range; I, Zuni uplift; J, Circle Cliffs uplift; K, Emery up-
lift; L, Oquirrh basin; M, Central Colorado basin; N, Wood River basin;
P, Ardmore basin; T, Matador Range; W, Amarillo-Wichita Range.

PLATE 8
Permian Tectonic Map

Orange color over the Marathon, Ouachita, and Appalachian Mountains indi-
cates the site of an orogenic belt and mountains of the previous Pennsylvanian
period. Uplift probably occurred there during the Permian. U.R., Uncom-
pahgre Range; C.R., Colorado Range; F, Florida uplift; O.B., Oquirrh basin;
A.B., Anadarko basin; C.B.P., Central basin platform.

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PLATE 9

Triassic Tectonic Map

Baja California block lay several hundred miles to the southeast.

PLATE 10
Jurassic Tectonic Map

Baja California block lay several hundred miles to the southeast.

PLATE 11
Early Cretaceous Tectonic Map

PLATE 12

Late Cretaceous Tectonic Map

Only Dakota and Colorado deposits in Rocky Mountains are represented.
Montana time is shown on Plate 13. Main batholiths of Nevadan orogenic
belt were intruded in very early Late Cretaceous time.

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PLATE 13
Tectonic Map of the Cretaceous-Tertiary Transition

Thickness of latest Cretaceous and Early Tertiary deposits in Rocky Mountain
basins not shown. For detail see Figs. 22.4, 22.5, and 22.6. The crypto-
volcanic structure in Iowa is Late Cretaceous or Early Tertiary; the others are
not dated but are presumed to be of the same age.

PLATE 14
Early Tertiary Tectonic Map

Laramide uplifts and basins not shown except for Green River Lake in Utah,
Colorado, and Wyoming, and Uinta Range, although thick Eocene deposits
accumulated in most of them. The Rocky Mountain front is a result of previous
orogeny. The volcanics are Eocene, Oligocene, and in places Miocene in age;
most Miocene volcanics, however, are shown on Plate 15. Numerous volcanic
cones, not shown, were built in the eastern Pacific and the Gulf of California.

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PLATE 15
Late Tertiary and Quaternary Tectonic Map

Red, besides denoting volcanic rocks, shows laccolithic clusters in the Colorado
Plateau. Numerous centers of volcanism throughout the Basin and Range
province are not shown. The blue color extends to lines of maximum trans-
gression of seas during the time represented by the map. Hudson Bay and
St. Lawrence submergence pre-dates the post-glacial uplift. The submerged
coastland of British Columbia has been uplifted 600 feet in post-glacial time.
See Fig. 31.25 for regional vertical movements of the western Cordillera in
late Cenozoic time.

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RESUME OF STRUCTURAL GEOLOGY OF NORTH AMERICA

Central America

Southern Mexico, Guatemala, Honduras, El Salvador, and Nicaragua
contain a belt of metamorphic rocks which sweeps from southwestern
Mexico in an easterly direction to the Caribbean Sea. A belt of deformed
Permian strata with Permian ( ? ) granitic and ultrabasic intrusives makes
up part of the crystalline complex. A fold belt of Jurassic and Cretaceous
strata borders the crystalline belt on the north.

The major geologic feature of southern Mexico and Central America
is an extensive accumulation of Tertiary volcanic rocks which masks
much of the underlying older rocks mentioned above. All the Isthmus of
Costa Rica and Panama is made up of an igneous complex, mostly
Tertiary, or of sediments derived from the volcanics.

Antillean Region

The Greater Antilles, composed of Cuba, Hispaniola, Puerto Rico, and
the Virgin Islands, have a late Mesozoic and Ccnozoic history. Thick
limestones made up a northern facies in Jurassic and Cretaceous times
and a volcanic assemblage a southern facies. Folding, thrusting, and in-
1 trusions followed. Tertiary time saw extensive flooding and reuplift of
the islands but not much deformation of the strata.

The Lesser Antilles or Caribbees are a Cenozoic volcanic arc developed
on the oceanic crust.

Precambrian (Plate 1)

Absolute age determinations on Precambrian rocks are now sufficiently
numerous so that divisions of different ages are becoming defined. The
ages denote the time of origin of the mineral of which the analysis was
made, and this denotes the time of an igneous intrusion or of an episode
of metamorphism. In other words, the ages appear to indicate belts of
orogeny. They define a continent made up of a rather small central
region of greatest age, and belts on the northwest and southeast of
progressively younger age. The strange aspect of the belts older than
800 million years is that they project out to the Pacific Ocean basin, as
if the continent at this time continued to the southwest farther than its

present boundary. The Beltian basin or geosyncline, about S00 million

years old, lies unconformably across the older belts, and apparently, for
the first time marked a direction subparalle] with the existing mar

Subsequently, all orogenesis occurred in belts conformable to the present
margin.

Cambrian (Plate 2)

Cambrian seas and sediments defined major tectonic divisions of the
continent which lasted until the end of the Paleozoic era. The Canadian
Shield of Precambrian rocks formed the central and northeastern part
of the continent, and it probably was a vast region of low relief. By way
of an extension to the southwest, the Transcontinental Arch, the United
States was divided into western and eastern seaways, and a svinmetrial
arrangement of shelves, miogeosynclines, and eugeosvnclines resulted.
No Cambrian strata are known along the western margin or the eastern
margin south of Maine, and the conditions in these regions in Cambrian
times are not well known. The hypothesis that North America was at
this time part of a much larger continent, which cracked and spread
apart, seems to help most in understanding the paleotectonic elements.
Southern Europe, Africa, and South America are postulated by some to
have lain close together, and hence, it is suggested that the Atlantic-
Ocean and Gulf of Mexico, with associated coastal plains or continental
shelves, did not exist at this time.

Ordovician (Plate 3)

The broad Williston basin became well-defined during the Ordovician

and a narrow basin of diick carbonate sediments formed in Oklahoma.
and extended to the shallow Colorado sag nearlv across the Transconti-
nental Arch. Extensive regions of the Canadian Shield were invaded by
epeiric seas. The margins of the continent are still problematical. Flat-
lying, unmetamorphosed strata in northern Florida south ot the eastern
orogenic belt seem to require continental connections where now is the
Atlantic Ocean. The Taconic orogeny of folding and thrusting occurred
in eastern New York, Vermont, and southeastern Quebec.

STRUCTURAL GEOLOGY OF NORTH AMERICA

Silurian (Plate 4)

The Transcontinental Arch became very wide and well-defined. The
Michigan Basin, in which extensive deposits of salines accumulated, and
one in Pennsylvania and West Virginia took lasting form. The Ozark
dome and Texas arch became prominent.

Devonian (Plate 5)

During Devonian time, the Transcontinental Arch rose, but was only
gently emergent; and strata previously deposited across its site were
removed by the close of the period, except for the Colorado sag where
100 to 200 feet of beds remained. In Canada, the great arch bifurcated
into a broad arch west of Hudson Bay and another one east of Hudson
Bay, but this condition probably did not arise until the close of the
Devonian. During the Devonian the arches were at least partly sub-
mergent, because Manitoba fossil faunas are very similar to those of
Michigan and the Hudson Bay region.

Transverse arches also developed. The Ellis-Ozark uplift extended
from Kansas around the south end of the Illinois-Indiana-Kentucky basin
to the Nashville dome, and thence northward to the Cincinnati dome.
The Illinois-Indiana-Kentucky basin, with the intervening Kankakee arch
or area of much less subsidence came into prominence for the first time.

A basin of subsidence centered in Pennsylvania during the Devonian,
and sediments were supplied from the eastern Taconic orogenic belt
which was being elevated adjacent to the basin and undergoing unrest
premonitory to the Acadian orogeny. The basin sank mostly in late
Devonian time, and its dominantly clastic and subaerial sediments coarsen
toward the east. The western and northern marine facies constitute the
classic Devonian section of the continent.

A Devonian trough extended northward through New England, the
Maritime Provinces, and Newfoundland, in which much volcanic material
was deposited along with various clastic sediments. In New England sedi-
mentation was mostly east of the main Taconic belt, but in Quebec it
occurred directly on the eroded Taconic structures. The entire region,
beginning perhaps in mid-Devonian time in places, gradually became

involved in the great Acadian orogeny. The strata were folded, intruded,
metamorphosed, and thrust-faulted to form a complex of dominantly
crystalline rock. The Acadian belt extended southward through the
Crystalline Piedmont of the eastern United States, where numerous
large batholiths were emplaced and considerable metamorphism occurred.

At the close of Devonian time a belt of orogeny, the Antler, formed j'
in the central part of the western geosyncline. The belt continued active
by way of folding and thrusting through Pennsylvanian, Permian, and
Mesozoic time with a number of phases of orogeny fairly accurately
documented. It effectively separated the miogeosyncline on the east from
the eugeosyncline on the west.

Devonian and Silurian strata have been identified on the Pacific margin
of the continent in the Klamath Mountains, and therefore it is concluded
that the continental margin was then about where it is now.

Mississippian (Plate 6)

Mississippian seas were widespread and in the Rocky Mountain region
a small basin subsided 10,000 feet along the Idaho-Montana boundary. A
long eastward-extending basin sank through central Montana, and is
known as the Big Snowy. A broad eugeosyncline of poorly known limits
extended through northern California, southern Oregon, and north-
western Nevada west of the Antler orogenic belt. The amount of subsid-
ence is unknown. The Antler oros;enic belt in central Nevada was
marked by major thrusting and complex folding.

The Transcontinental Arch sagged gently through its central area and
was covered, but by the close of Early Pennsylvanian time it had risen
enough to have suffered erosion, and the Precambrian was again exposed.
The Texas arch was covered in central Texas and the Ozark-Nashville
arch was severed from the Transcontinental Arch.

In latest Mississippian or earliest Pennsylvanian time, a deep and prob-
ably large basin sank rapidly in eastern Texas, southern Oklahoma, and
western Louisiana, and received about 17,000 feet of clastic sediments.

The La Salle anticlinal belt first began to rise at the close of the
Mississippian and continued to grow during the Pennsylvanian. It split
the Illinois-Indiana-Kentucky basin in two parts.

RESUME OF STRUCTURAL CKOLOCY OK NOHTII AMKHK.\

The Ozark uplift developed into a broad, continuous arch with the
Nashville and Cincinnati arches, and the northern arms of the Cincinnati
arch, the Kankakee and Findlay arches, became well established. Gentle
, erosion probably occurred throughout this system of arches.

Subsidence continued in the Appalachian trough area, and a maximum
of 4000 feet of sandstones, shales, and limestones accumulated.

In the Maritime Provinces and Newfoundland, a basin sank within the
older Taconic and Acadian orogenic belts, and received about 5000 feet
of clastic sediments, presumably from a rising orogenic belt on the
i east.

Pennsylvanian (Plate 7)

3 The south-central part of the continent was one of considerable and
widespread unrest in Early Pennsylvanian time, and a number of ranges
and basins were formed. The Wichita Mountain system of Oklahoma and
northern Texas was uplifted together with the Ancestral Rockies of New
Mexico and Colorado. The Pecos and Diablo ranges in west Texas ap-
peared. The long, narrow Nemaha Range rose sharply, and at the same

i time basins on the east sank. The previously formed La Salle anticlinal

, belt was mostly buried.

The trough of the deep basin in eastern Texas of latest Mississippian
and earliest Pennsylvanian time shifted northward to central Arkansas,

, and over 10,000 feet of sediments accumulated there.

The Arkansas basin was probably continuous with one in the southern
Appalachians, where 10,000 feet of sediments, mostly clastic, accumu-
lated. Such a thick and clastic deposit undoubtedly means vigorous uplift
immediately on the southeast and south. The area of deposition in the
southern Appalachians in Early Pennsylvanian time shifted to the
central Appalachians in Late Pennsylvanian time, and somewhat more
that 3000 feet of coal-bearing strata were deposited there. Although de-
position had proceeded at variable rates here and there during the
Paleozoic in the southern and central Appalachians, which lay to the
west of the Taconic orogenic belt, it is generally stated that more than

j 30,000 feet of sediments had accumulated. In Late Pennsylvanian time
or possibly in Early Permian time, the thick succession of strata from

Oklahoma and Arkansas to Pennsylvania suffered folding and thrusting

toward the continental interior, and the Ouachita Mountains and ( lassical
Appalachian Mountains (Valley and Ridge Province) WOt brought
into being.

The Marathon orogeny of west Texas occurred in Late Pennsylvania!!
time and several thrust sheets moved northward toward the shelf. The
Arbuckle Mountain system was formed hy considerable folding and
thrusting of the sediments of the Ardmore basin, and the structures
were appressed tightly against the early Wichita Range.

In New England, the Maritime Provinces, and Newfoundland, sub-
sidence followed somewhat the same pattern as that of the Mississippian.
and coarse red Pennsylvanian elastics rest there on the 'laconic and
Acadian complex, and also in places in angular unconformity on the
Mississippian.

Extensive subsidence occurred during the Pennsylvanian in the Cordil-
leran geosyncline with the deposition of more sand than in any time
since the Cambrian. A local basin in west-central Utah subsided greatly
and was filled in one place with about 25,000 feet of beds. The Antler
orogenic belt dominated the sedimentary conditions east of it, and coarse
elastics were spread there. Allochthonous masses were translated 25 to 75
miles eastward.

The volcanic orogenic archipelago persisted along the west margin of
the continent, and was the source of volcanic contributions to the sedi-
ments of the adjacent seas, and the cause of unconformities and low-
grade metamorphism in the deposits.

Ry Late Pennsylvanian time the Transcontinental Arch was almost
entirely overlapped and buried, and the Early Pennsylvanian uplifts of
Kansas, Oklahoma, and Texas were covered. Only the Ancestral Rockies
in Utah, Colorado, and New Mexico remained as islands above the ac-
cumulating sediments.

Permian (Plate 8)

An eugeosyncline of deep and broad proportions developed in
Permian time along the Pacific and was filled largely with volcanic ma-
terials. The Permian was a time of most extensive volcanism, and the site

STRUCTURAL GEOLOGY OF NORTH AMERICA

of maximum subsidence and fill later became the locale of the great
Nevadan batholiths.

Orogeny continued in central Nevada, and a small deep trough in
western Utah filled with sandstone, shale, and limestone. Extensive
shelf seas stretched eastward and southward.

The Colorado and Uncompahgre ranges of the Ancestral Rockies
remained as islands in the surrounding seas.

The previously compressed Marathons were elevated epeirogenically,
and in front of them several basins sank to considerable depth. The
platforms of little subsidence between were the sites of the previous
Pecos and Diablo ranges. The Anadarko basin also subsided appreciably.

The Carboniferous basins and adjacent areas of New England were
intensely deformed either in Late Pennsylvanian or Permian time, and
in places intruded by granitic batholiths. The deformation is not defi-
nitely dated, but presumably it occurred after the beds of the Permian
basin of Pennsylvania and West Virginia had been deposited. It seems
probable, also, that folding in Pennsylvania and West Virginia occurred
at this time.

The crustal movements and spread of seas in Late Pennsylvanian
and Permian time profoundly altered the geologic outcrop pattern of the
continent. The greatest change comes from the extensive overlap of the
pre-middle Pennsylvanian structures by the Upper Pennsylvanian and
Permian sediments. All the Transcontinental Arch southwest of Wis-
consin was buried, the structures of Kansas and parts of the Ozarks dome,
the Wichita and Arbuckle mountain systems, and the, ranges of west
Texas vanished beneath the deposits. Only the Colorado and Uncom-
pahgre ranges of the Ancestral Rockies remained visible, not because of
renewed uplift, but because of considerable relief inherent from their
original development.

Triassic (Plate 9)

Eugeosynclinal conditions continued in the west with extensive vol-
canic accumulations. Crustal unrest continued in central and western
Nevada. In northern Utah a basin subsided and collected 8000 feet of

carbonates and elastics. Eastward the Triassic deposits are largely con-
tinental and red. An emergent corridor connected the Canadian Shield
with northern Mexico and southern Arizona.

The Colorado and Uncompahgre ranges still stood as islands in the
surrounding deposits.

The Marathon-Ouachita orogenic belt of earlier development was still j|
mountainous and had a broad piedmont generally free of deposits. The
mountainous belt may have risen gently as its rocks were eroded and
carried away, but orogeny there had ended.

Within the metamorphic and igneous core of the Paleozoic orogenic
belts of the Atlantic margin, a zone of high-angle faults dropped basins
and raised blocks of mountainous proportions. Volcanism was a prom-
inent accompaniment of the faulting. The basins were the site of ac-
cumulation of thick, red, clastic sediments which were mostly derived
from the uplifted, adjacent blocks. The basins are narrow and long, and
because of their fault origin, their size was probably not much larger
originally than now. The faulting and igneous activity ran its course in
Late Triassic time, and the orogeny is known as the Palisades.

Jurassic (Plate 10)

The Cordilleran geanticline developed in Early Jurassic time and
separated a western trough effectively from an eastern. The western
again was one of extreme subsidence, and about 30,000 feet of volcanics,
black shale, and other sediments accumulated in it. Central Nevada con-
tinued to experience orogeny, and thrusting of large proportions oc-
curred. Late Jurassic was also a time of considerable batholithic intrusions
in central and northern California and possibly western Nevada.

The eastern trough was generally the site of marine transgression and
deposition, but the Jurassic deposits are less extensive than the earlier
Permian, Triassic, and the later Upper Jurassic and Cretaceous. The
Jurassic overlap on the Paleozoic strata of Montana, Alberta, and Saskatch-
ewan, particularly on the Mississippian, is striking. The Mexican geosyn-
cline began to form. It was separated on the north by a peninsula, the
Coahuila, from the seas of the Gulf of Mexico. The wide basin of the

RESUME OF STRUCTURAL GEOLOGY OF NORTH AMERICA

Gulf of Mexico had come into existence. Much salt was precipitated in an
evaporite sequence in the Mexican geosyncline as well as along the
northern part of the Gulf in Louisiana and Texas.
The interior of the continent was extensively emergent.

Early Cretaceous (Plate 11)

The Cordilleran geanticline widened and stretched from Rritish Colum-
1 bia to Mexico, and from eastern California to central Utah. A broad
branch extended across Arizona and central New Mexico into Texas.
'Further deformation is noted in northwestern Nevada.

Basins sank greatly on the west in California, Oregon, and Washington.
Volcanism continued there from previous times. The geanticline was
flanked on the east by a trough of sedimentation from Alberta to northern
Utah into which clastic sediments were shed. The Ancestral Rockies
were buried save for a small island in central Colorado. The Mexican
' geosyncline enlarged and sank over 15,000 feet. It received considerable
volcanic material from the west. The seas spread over the Coahuila penin-
sula to make it a platform, and were more extensive now over the
southern and western part of the country than at any previous Mesozoic
time.

The Rocky Mountain sea merged with the Gulf of Mexico, and the
Gulf Coastal Plain sediments accumulated to an appreciable extent. Only
the northern part of Florida was emergent. It was otherwise a platform
and with the Bahama platform made up a large region of carbonate
deposition and slow subsidence. It bordered on the south with a volcanic
belt in Cuba where a carbonate facies on the north grades into a
volcanic facies on the south.

Late Cretaceous (Plate 12)

The Late Cretaceous was a time of widespread and intensive crustal
unrest along the western margin of the continent. The climatic phase
of the Nevadan orogeny occurred at the very beginning of Late Cre-
taceous time when most of the batholiths that characterize the belt were
intruded. Narrow basins subsided to considerable depths on the west

margin of the belt where again volcanoes contributed some material.

The Nevadan belt of orogeny became part of the broad Cordilleran
geanticline, along whose eastern margin strong uplift with thrusting
occurred. Floods of coarse conglomerate were poured into an adja
trough in Utah and western Wyoming, and in places thrust sheets over-
rode the elastics.

The Late Cretaceous seas and deposits were even more widespread
over the Rocky Mountains and Great Plains states than those of the
Early Cretaceous, and the deposits were much thicker. East of the deep
trough in central Utah, only thin deposits had previously accumulated
under shelf sea conditions. Now sediments in excess of 5000 feet thick
collected over a wide area of the shelf.

The Mexican geosyncline had shrunk and changed decidedly from its
Early Cretaceous form. A trough extending from southeastern Arizona
into northern Mexico contains much coarse conglomerate and volcanic
material. South of the Coahuila platform a deep east-west trough, the
Parras, sank and received over 15,000 Eeet of sediments, mostly lime-
stones and shales.

Florida sank progressively through Late Cretaceous time, tilting south-
ward to a trough that centered in Cuba where some 10,000 feet of
carbonaceous sediments accumulated. As the carbonates thin northward
through Florida, they change into argillaceous and arenaceous facies. Tne
Atlantic margin of the continent was widely invaded, and a wedge of
sediments that thickens seaward was deposited. The sediments overlap
the Lower Cretaceous strata in most places.

Cretaceous-Tertiary Transition (Plate 13)

During the latest Cretaceous (Montanan) and earliest Tertiary (Paleo-
cene) the main structures of the Rocky Mountains of Canada and the
United States came into existence, and Plate 13 has been prepared prin-
cipally to show these features. The crustal unrest is known as the
Laramide orogeny. The Cordilleran geanticline was broadly deformed
with its eastern margin and the adjacent basin deposits of the Triassic,
Jurassic, and Cretaceous folded and thrust-faulted. Major overtlrrust

STRUCTURAL GEOLOGY OF NORTH AMERICA

sheets rode eastward from the Yukon to southern Utah, and repeated
floods of coarse elastics occurred in this marginal belt. Several phases of
deformation are documented in most places.

East of the thrust belt including the large region from Montana to
southern New Mexico and generally in the shelf region of sedimentation
anticlinal uplifts, mostly elliptical in ground plan and asymmetrical in
cross section rose in latest Cretaceous and Paleocene time. They are 75
to 150 miles long and 20 to 50 miles wide. Where the uplift has been
great enough to result in erosion exposing the Precambrian rocks, thrust-
faulting has occurred on the steep margin. The elliptical uplifts compose
the major mountain ranges of the region. Retween are intermountain
valleys where, particularly in Wyoming and Montana, considerable
amounts of Early Tertiary continental-type sediments were caught.

The western or Pacific margin of the geanticline continued to shed
sediments to the adjacent basins, and no strong disturbance is indicated.
The San Andreas fault had probably come into existence and the west-
lying block at this time was lodged several hundred miles to the south,
but now had started to shift northwestward along the fault, as indicated
by the arrows.

Major deformation of previously deposited Jurassic and Cretaceous
sediments occurred in the Greater Antilles with northward thrusting
in Cuba.

Early Tertiary (Plate 14)

The most conspicuous and probably most significant feature of Early
Tertiary time in the western cordillera was magmatic activity, especially
volcanic. As can be seen from the map that the Great Rasin region of
Nevada, western Utah, and central and southern Arizona, together with
the vast region of western Mexico, was mostly covered with volcanic
materials. Southern Idaho was also extensively covered. Significant al-
though scattered fields occur in New Mexico, Colorado, and Montana.
The central Cordillera of Canada developed a large field. Several hun-
dred small stocks also were intruded in the Great Rasin, southern Ari-
zona, and northern Mexico. All this activity followed the Nevadan and
Laramide orogenies and, in places at least, marked the beginning of

block faulting and rifting that dominated the Late Tertiary activities.

A eugeosyncline formed in Oregon and Washington, which is made ;
up of a very thick mass of sediments and volcanics. Deep but restricted
basins between uplifts developed in central and southern California.
The San Andreas fault was very active and the west block moved north-
ward, but was still considerably south of its present position. The
Atlantic and Gulf of Mexico continental margins continued to subside
during the Tertiary, but only in one or two places, particularly the
Mississippi embayment, did the Tertiary beds overreach the Upper
Cretaceous deposits. The Cretaceous and Tertiary sediments form the
present Gulf Coastal and Atlantic Coastal Plains.

As the Atlantic margin of the continent subsided in Late Cretaceous
and Tertiary time, the Appalachian orogenic belt arched gently, and
successive erosion surfaces record the epeirogenic uplift.

The Greater Antilles sank and appeared as a belt of islands around
which Tertiary sediments accumulated. Florida and the Rahama plat-
forms also continued to sink and to be built up by carbonate sediments.

Late Tertiary and Quaternary (Plate 15)

Volcanism continued prominent in the Late Tertiary with basalt fissure
eruptions in Washington and Oregon building the Columbia River field.
To the south in southern Oregon and Idaho another extensive basalt field
formed chiefly from vent eruptions. The west margin of these two large
basalt fields has been built especially high by additional volcanoes to
form the Cascade Range. A row of majestic stratovolcanoes of Quaternary
age dominates the Cascades and extends into southern Rritish Columbia
beyond the basalt fields. The Cascade volcanics are chiefly andesite.

Rlock faulting of major proportions spread from the Sierra Nevada of
California to the Wasatch Mountains of Utah. It also extended through
southern Arizona and southward along the west coast of Sonora, Mexico.
An arm of the faulting extended northward through eastern Idaho,
western Wyoming, and western Montana to the Rocky Mountain Trench
of Rritish Columbia. The block, trench, or rift faulting is believed to be
of tensional origin and to penetrate deeply into the crust.

The San Andreas fault block moved northward to its present position

RESUME OF STRUCTURAL GEOLOGY OF NORTH AMERICA

■'

and drifted apart from the continent at the south end to form the Gulf
of Raja California which is floored by oceanic crust. Deep but local
basins sank in southern California.

The Colorado Plateau block was uplifted with associated subsidence
on the south and west. Several laccolithic groups were intruded into the
Plateau strata, and several volcanic piles accumulated around the southern
and eastern margins. In central Wyoming certain blocks were depressed
along normal faults, particularly the Laramide Sweetwater Range. The
Great Plains came into existence by uplift progressively greater toward the
west. The Laramide Rockies were also uplifted, starting a new erosion
cycle.

The marginal areas of the Gulf Coastal Plain continued to subside

greatly under a heavy load of deltaic sediments. An area in northwestern

Florida became emergent. The Atlantic Coastal Plain south of Ixmg
Island gradually rose and the sea retreated, but north of Long Island
submergence and overlap of the sea occurred. The submergence has
also been effective in Quaternary time southward where emergence

has occurred previously.

Broad arching in the Appalachian region continued.

The Canadian Shield had been depressed under the weight of the
ice sheets but in post-glacial time has lifted progressively to the north.
The tilting starts at the hinge line shown on Plate 15 and amounts to
700 feet along the northern shores of Lake Superior, and possibly 900
feet along the eastern side of James Bay at the south end of Hudson Bay.

PRECAM BRIAN
TECTONIC PROVINCES

DISTRIBUTION OF PRECAMBRIAN ROCKS

The continent of North America is made up in a broad way of a stable
interior and surrounding belts of deformed, intruded, and metamorphosed
rocks. The stable interior has been free of orogeny since a time in the
late Precambrian, or approximately for the last billion years. Before that
time, however, a number of intense and widespread orogenies occurred.

The Canadian Shield is the greatest expanse of Precambrian rock
exposures. These same rocks are blanketed by Paleozoic, Mesozoic, and
Cenozoic strata over most of western Canada and the United States; only

in areas of local uplift or doming have the old rocks been exposed.
The Crystalline Piedmont of the Atlantic margin of the continent con-
tains much rock of Precambrian age, and the western Cordillera exposes
the ancient rocks of several ages and complex relations in a number
of places.

CANADIAN SHIELD

Physiography

The Canadian Shield is characterized by a vast expanse of Precambrian
rock. Its upland surfaces are uniform in height over large areas and,
although now dissected, represent an old age erosion surface as large
as. any in existence today. The extensive surface rises 1000 to 2000 feet
above sea level north of the St. Lawrence River and Lake Superior.
Around Hudson Bay, especially on the south and west, is a wide lowland
that ranges from sea level to 500 feet in elevation. In northern Labrador
along the coast just southeast of Ungava Bay, the surface rises to 5000
feet and is extensively dissected. Hudson Bay is a great modern epeiric
sea; it is a marine invasion from the north due to gentle subsidence in j
the heart of the shield in pre-Pleistocene or early Pleistocene time. The I
ice caps imposed such a weight on the shield in and around Hudson j
Bay that the area sank over a thousand feet in addition to the previous
subsidence, and then with the melting of the ice it has risen about 900
feet.

Post-Proterozoic History

Paleozoic strata lap upon the shield from the Canadian plains on the
west, and from the southwest in Saskatchewan and Manitoba. In
northern Minnesota the Precambrian rocks lie exposed and extend south-
ward into Wisconsin and eastward into northern Michigan. Paleozoic
rocks continue to overlap the Precambrian across southern Ontario and
Quebec to the Frontenac axis, where the Precambrian extends southeast-
ward and forms the Adirondack dome in New York. See the Geologic
Map of North America. For the most part, the Paleozoic rocks that skirt

PRECAMBRIAN TECTONIC PROVINCES

ithe shield are Devonian and Silurian, and are chemical deposits or fine
elastics. Along the southern margin of Hudson Bay is a fairly large area
pf flat-lying Devonian, Silurian, and Ordovician sedimentary rocks, and
:rom fossil studies it seems probable that the Manitoba, Hudson Bay, and
Michigan Devonian deposits were once continuous (G. M. Ehlers, per-
sonal communication). The thickness of the Devonian and Silurian south
af Hudson Bay is at least 1000 feet, but their extension northward under
;he Bay's waters is not known. It can easily be imagined that they are
continuous to Coats and Mansel islands at the entrance of Hudson Bay
ind thence to the nearly horizontal Paleozoic strata of Southampton
island and the Arctic Archipelago. If continuous, one wonders if some-
where in that large area the beds are not thick and form a trough or
basin, perhaps similar to the Michigan basin. In fact, basins and arches
have been recognized in the far north, and are described in Chapter 40.

It has been thought until lately that the Canadian Shield was com-
paratively free of epeiric seas in the past; but now, by the discovery
pf a number of small erosional remnants of Paleozoic strata far within
'the crystalline rocks (W. Sinclair and J. Tuzo Wilson, personal com-
jmunications ) , it is believed that large areas were blanketed by sediments.
Perhaps very little escaped submergence. What seems more important is
'that no orogenic belts developed across it during all of post-Proterozoic
time. The same is true with some exceptions of the stable region of the
United States.

In the iron ore belt of central Labrador (the Redmond iron deposit)
downfaulting of a trench occurred in early Late Cretaceous time, and in
jit various argillites and ferric concretionary deposits accumulated. The
Redmond deposit is in a basin 1 mile long, 1000 feet wide, and 600 feet
deep. Abundant plant fossils in certain beds serve to date the deposits
|and the faulting. The extent of the Cretaceous faulting is not known
!(R. A. Blais, 1959).

From simple map examination, it looks probable that Greenland was
part of the Canadian Shield until Cretaceous time when, perhaps, a
Cretaceous trough extended as far north as Disco Island. Greenland was
further severed from the shield either by Tertiary downfaulting or by
drifting apart. See Chapter 40.

Geologic Provinces

The Canadian Shield until recently has been difficult of access, and
this with the extensive "bush" cover has made geologic exploration
pensive of energy and slow. The advent of airplanes and aerial photos
has hastened the work immensely, and a beginning has now been made
in analyzing the composition of the great Precambrian shield. But the
time has not yet arrived, according to M. E. Wilson, when the vast region
can be broken down into divisions with confidence. He draws approxi-
mate boundaries between five provinces (see map, Fig. 4.1), namely,
the Western or Churchill, the Ungava, the Arctic Island, the Greenland,
and the St. Lawrence. The last is divided into subprovinces, the North-

Fig. 4.1. Geologic provinces of the Canadian Shield best suited at present for individual forma-
tional names. After M. E. Wilson, 1958.

STRUCTURAL GEOLOGY OF NORTH AMERICA

Erm
(HfYeen)

Penod-Syilem

Major Sequence

Formation

Orogeny

Intrusive Rocka

Ptlcoxoic

C»mbnmn

Grenville
. Penokean

. Algoman
-Lauxentiaa

(0* b,y.)
(11 bjr.)

lati mtawa

Keweenawan

Hinckley sandstone
Fond du Lac sandstone

North Shore volcanic Undivided
group

Duluth complex, sills at DulurJi, Beaver Bay

complex, Logan intrusive*

Puckwunge

Sioux quartxite (?)

= Thomson
Gunflint

Granite: St. Cloud Red, Rockville (?) granite at

Granite Falls, Bellingham (?)
Gneiss: McGralh. Montevideo (?)

(1.7 bj.)

Haiooiu

Animikie froup

Virginia argHlile = Rove
Biwabik iron-formation =

Tonalites: St Cloud Gray. Warman, Hillman.
Freedbem, Montevideo

MwMlr Preewrabrita

Pokegama quartxile

Granite: Gold Island, Giants Range, Sacred

«•» bjr.)
(fbjr.)

Early Pircambriao

Tuxttskaraiao

OnUnui

Knife Lake group
Keevatin group

Undivided

Soudan iron- formation

Ely greenstone

Gneiss: Giants Range, Vermilion, Morton
Saganaga granite, Grassy Island lonalile (?)

Coulchiching (?)

Undivided

Older rocka

Fig. 4.2. Stratigraphic succession and geochronology of the Precambrian of Minnesota. Repro-
duced from Goldich ef a/., 1961.

west, the Southern, the Timiskaming, and the Grenville. The provinces
and the subprovinces thus defined represent natural divisions and the
limits to which attempts should be made to correlate rock units. Wilson
recommends separate names for formations, series, or intrusive bodies
within each of these divisions at least for the present.

Geologists in recent culminating studies in the iron and copper region
of Lake Superior recognize a threefold division of the rocks ( Grout et ah,
1951, James, 1955, and Goldich et ah, 1961). The Precambrian of Minne-
sota is classified by Goldich et ah after many radioactivity age determina-
tions, as shown in Fig. 4.2.

Previously, in 1934, a committee of the Royal Society of Canada on
stratigraphical nomenclature had recommended that Precambrian time
be divided into two eras, Archean and Proterozoic, and since then this
classification has been used on most geologic maps issued by the
Geological Survey of Canada. M. E. Wilson in 1958 contends that the dual
classification is still the best and includes the Middle Precambrian rocks
of Grout and James in the Proterozoic. In all provinces of the Canadian
Shield a profound unconformity is known, by reference to which the
rocks can be divided into two great groups (M. E. Wilson, 1958). The
standard of reference, for instance, is the rock succession on Lake Timis-

kaming where the Huronian (Cobalt series) rests "with great uncon-
formity" on granite — the Laurentian.

The Archean rocks consist of clastic sediments and various volcanic
rocks conformably interbedded, and even though extremely old they are
so little affected by metamorphism in places that original sedimentary
structures are clearly visible. In many other places they are meta-
morphosed to various degrees. According to Pettijohn (1943) one may
study the bedding in certain argillites in the finest detail, and the asso-
ciated volcanics show pillow structures, amygdules, spherulitic structures,
the same as in lavas of much later geologic time. Metamorphism is mainly
of the low-grade variety, and orogeny has left the very ancient rocks of
many areas untouched. Recognizing the near-absence of metamorphism
in places, however, it must also be understood that enormous volumes of
intrusive igneous rocks occur, and estimates have been made that these
intrusive rocks constitute as much as 80% of the shield. The great bulk
of these are granites of various types, with relatively small but important
amounts of basic rocks such as gabbro, norite, and peridotite. Need-
less to say, much metamorphism has occurred and gneisses and schists
(migmatites) are extensively developed.

The Archean sediments of the southern Canadian Shield are mainly gray-
wacke. Much conglomerate, a litde slate, and still less iron-bearing formation
are also present. Excessive thickness, especially of the conglomerates, abundance
of graded bedding, rarity of cross-bedding and absence of ripple mark, the
graywacke nature of the arenaceous beds, the absence of true quartzites and
limestones, and the scarcity of normal argillaceous sediments, and the associa-
tion with greenstones and tuffs are all the earmarks of a geosynclinal facies of
sedimentation (Pettijohn, 1943).

In particular, these types characterize the eugeosyncline, and since they
are repeated in later Precambrian rock series, it is little wonder that
confusion in correlations has resulted.

In eastern Ontario and adjacent parts of Quebec the oldest rocks are
sedimentary gneisses associated with great thicknesses of crystalline
limestone and a little basic metavolcanics. These rocks are termed the
Grenville series. They appear to have been originally shales, sandstones,
limestones, and some lavas, but owing to the intense metamorphism, they
are now biotite schists and sillimanite-garnet gneisses, vitreous quartzite,
and crystalline limestones.

PRECAMBRIAN TECTONIC PROVINCES

In southern Ontario, particularly in Hastings County a younger series,
the Hastings, overlies the Grenville with erosional unconformity but,
apparently with little structural discordance. The series consists of
gray, blue-weathering limestone interstratified with argillite, except near
the base where beds of congolmerate interstratified with argillite, buff-
weathering dolomite, graywacke, and mica schist occur. Both Grenville
and Hastings rocks are intruded by a group of gabbros, anorthosites,
pyroxene diorites, and pyroxene syenites. Later still are dikes, sills, and
batholiths of granite and syenite, and their gneissic equivalents.

The Grenville subprovince is believed to be separated from the Timis-
kaming subprovince by a great fault called the "Lake Mistassini-Lake
Huron fault" by M. E. Wilson (1956) and the Grenville front or fault
zone" on the Tectonic Map of Canada (1950). The fault marks a zone
of considerable disturbance, and in the Lake Mistassini area it seems
evident that the Grenville rocks have been thrust over those of the Timis-
kaming subprovince. The theoretical fault lies under lakes and glacial
deposits for most of its length, and considerable controversy centers
about it.

For further discussion of the many rock units already described over
the vast Canadian Shield read M. E. Wilson 1956 and 1958, and Harrison,
1957. A recent symposium publication, "The Grenville Problem," pub-
lished by the University of Toronto Press, presents a fascinating picture
of the many problems involved.

Tectonic Provinces

With the advent of physiochemical age determinations (about 1931)
Imuch new light has been shed on the relative ages of rocks in the
Canadian Shield. The ages are actually for minerals occurring in igneous
rocks or in reconstituted rocks, metamorphosed during an orogeny; the
| original age of the graywacke, shale or lava is not determined but rather
the age of the orogeny. Therefore, with the absolute age determinations
has come an increased attention to orogenic belts, and certain geologists
have postulated a division of the Canadian Shield into tectonic provinces
or orogenic belts, in place of the "geological provinces." See Fig. 4.3.

The oldest orogeny in Minnesota is called the Laurentian by Goldich
et al. (1961), but this he regards as an early phase of folding to the

Fig. 4.3. Precambrian orogenic belts of North America defined by isotope oges.

STRUCTURAL GEOLOGY OF NORTH AMERICA

greater Algoman orogeny ( see Fig. 4.2 ) . The latter occurred about 2500
m.y. ago, although the very ancient dates range from 2200 to 2600 m.y.
The name Algoman is here used for the belt of ancient dates through the
southern part of the Canadian Shield. It has been variously called the
Keewatin and Superior by other writers.

The Algoman and the Slave (also called Yellowknif e ) provinces are
the oldest known and possibly parts of the original nucleus of the con-
tinent. They have a high ratio of lavas to sediments which are of the
graywacke facies, presumably deposited in geosynclinal basins. The
Churchill province is considered an orogenic belt by which the two
nuclei were welded together (J. Tuzo Wilson, 1949, 1954). See also
Farquhar and Russell (1957) and Lowdon (1960).

A belt of Huronian rocks extending from Minnesota through Wisconsin
into Michigan and lying south of the main Algoman belt has ages of about
1700 m.y. Goldich et al. ( 1961 ) call it the Penokean orogenic belt, and
the name has been applied in this book to adjacent regions on the south-
west in the United States and on the northwest in Canada.

The Grenville subprovince of M. E. Wilson approximately is postulated
as an orogenic belt about 1000 m.y. old. Its deformed front borders
directly on the Algoman province. Southeast of the Grenville belt are
the Taconic and Acadian orogenic belts, about 400 and 300 m.y. old,
respectively.

Eighty-three isotopic age analyses on biotite, K-feldspar, and whole-
rock samples from forty-five localities, using both K-Ar and Rb-Sr
methods have been made on igneous rocks and a few metasediments in
the Sudbury-Blind River area of the Grenville belt by Fairbairn et al.
(1960).

The numbers obtained, forming an almost continuous age spectrum from
1.0 b.y. to 2.2 b.y., are correlative with widespread and repeated diastrophism
in the region. Whole-rock analyses of igneous material, where available, show
higher ages than coexisting minerals in most examples, and there is reason to
believe that these are close approximations to the true age. There is consider-
able evidence by both K-Ar and Rb-Sr methods, of orogenic events at ap-
proximately 1.0 b.y., 1.2 b.y., and 1.6 b.y.

The oldest igneous rock found thus far is the Copper Cliff "rhyolite" (2200
m.y.), which intrudes the basal section of a thick series of conformable

metasediments and volcanics southeast of Sudbury. At Quirke Lake granite in
the basement, uncomformably beneath U-bearing pebble beds, is 2050 m.y.
old. As the time of uranium mineralization in these Huronian sediments is
placed at 1700 m.y., and gabbro which intrudes them may possibly be older
than 1800 m.y., their deposition must have been in the age bracket 1800-2050
m.y.

ARCTIC STABLE REGION

South of the orogenic belt of northern Greenland and Ellesmere Island
and north of the Precambrian Canadian Shield is a stable region com-
posed of a Precambrian crystalline basement with a veneer of nearly
horizontal Paleozoic sedimentary rocks. It includes most of the Arctic
islands, and the shallow sea-covered areas between. See the Geologic
Map of North America or the Geologic Map of Canada. The Precambrian
rocks of the shield extend northward into Baffin and Devon islands, and
exposed extensively in Melville and Boothia peninsulas, but the Paleozoic
blanket indicates that much, if not all, of the Arctic islands region
(also called Arctic Archipelago) and the northern part of the Canadian
Shield were submerged at times during the Paleozoic. The part south
of the fold belt ( Chapter 35) has suffered only gentle vertical movements
since the Proterozoic, and is therefore part of the great stable interior of
the continent. The Precambrian crystalline rocks extend southward into
the United States under a veneer of Paleozoic sedimentary rocks com-
monly called the Central Stable Region. It seems appropriate, therefore,
to speak of the similar northern geologic province as the Arctic Stable
Region.

PRECAMBRIAN PROVINCES OF THE UNITED STATES

Isotope Age Determinations

Recent age determinations fall into a pattern that marks successive
orogenic belts in the central, southern, and western states of the United
States, and these are shown in Fig. 4.3. The ages pertain to rocks gen-
erally called Archean or basement complex. In Arizona, Utah, Idaho, and
Montana, younger and much less metamorphosed strata rest unconform-

PRECAMBRIAN TECTONIC PROVINCES

ably on the crystalline basement, and are variously called Algonkian,
Proterozoic, Beltian, or Upper Precambrian. These are shown on the map
iby the dotted lines. Extending southwestward from the western part
of Lake Superior is another belt of late Precambrian rocks, namely the
Keweenawan Series with its included large gabbro sills. Beneath the
Paleozoic and Mesozoic sedimentary cover of Texas and southeastern
New Mexico still other young Precambrian sediments, volcanics, and
gabbro sheets have been recognized, resting on an older granitic terrane.

Algoman Oogenic Belt

The ages thus far published for north-central Wyoming and south-
central Montana are very old (2500 to 2760 m.y.) and stand apart from
other ages in the Rocky Mountains (Aldrich et al., 1957; Gast and Long,
1957; Hayden and Wehrenberg, 1959 ) . An absolute age determination in
southeastern Manitoba between Winnipeg River and Johnston Lake indi-
cates that a plutonic and metamorphic cycle occurred 2650 =■= 100 m.y.
ago (Eckelmann and Gast, 1957). These ages are 400 to 500 m.y. older
than those recorded for the "Superior" Province in Canada, but even so
iare much closer to it than to those of the adjacent younger orogenic
;belt, and hence are regarded related.

Penokean Orogenic Belt

A number of isotope ages to date seem to establish an orogenic belt
;of intermediate age between the very old Algoman and the younger
Mazatzal. These are in the range of 1600 to 1750 m.y. See Fig. 4.3. The
belt contains a mixture of the old dates and the younger, and this is
taken to mean that the younger orogeny was superposed on the older. The
analyses are so few to date that the northern limit of the belt is poorly
defined, and not much reliance for tectonic interpretive purposes can
yet be placed on the distribution. The southern limit is somewhat better
defined, with none of the older dates in the general field of the 1250
to 1450 m.y. dates.

On the basis of the geology of the rocks of southwestern Montana the
two ages are understandable. Perhaps even more ages within the belt will
be recognized. A brief description of the recognized units is as follows:

(1) The oldest units underlie the Cherry Creek Group and Include
of the Poiin' Group as well as other pre-Cherry Creek rocks which probably
an- not time equivalents ol the Pony. Main types present are bioriti
gneiss, granite gneiss, injection gneiss, and amphibole gneiss. 1 The CI*
Creek Group consists of mctascdiincnts including marble, quaitzite, micai i
schists, sillimanitc schist, handed ironstones with Intercalated layers of
amphibole gneiss, and amphibolite representing metamorphosed mafic silk and
flows. (3) A number of post-Cherry Creek intrusives, all of which show -
ing degrees of metamorphism, include, among others, the Dillon granite gn<
widespread in Beaverhead and Madison counties, the granite of the [ardine
district, and the Pinto metadiorite in the Little licit Mountains. (4) Wider)
distributed bodies of unmetamorphosed peridotite and associated ultrainafu
rocks have as their largest representatives the Stillwater Complex. (5) Post-
Stillwater intrusives are represented mainly by the granite of the Beartooth
Range. (6) Numerous and widespread diabase dikes that cut all these older
units but do not extend into Beltian rocks (Ileinrich, 1953).

The crystalline basement of the Beartooth Range from what is known
consists of schists and gneisses, possiblv the Cherrv (.'reck. On the north-
east is the Stillwater ultramafic complex which has been intruded into a
series of dense gray hornfels, an iron formation, and light-colored quartz-
ites. It may be part of the Cherrv Creek group. A light gray gneissoid
biotite granite cuts the ultramafic complex. At Cook City two granites are
recognized (Parsons and Bryden, 1952).

The roof of a granitic batholith is exposed in the Teton Range of
western Wyoming. The deep canyons that dissect the range show gigantic
zenoliths and an irregular roof of gneiss and schist.

Mazatzal Orogenic Belt

Distribution of Dates. A good scatter of age determinations has been
made in the Rockies from the Black Hills to Arizona and southern
Nevada and defines a belt of rather consistent age between 1300 and
1400 m.y. old. A low age is given for the Front Range of central Colorado
of 1100 m.y., a high age for the Black Hills of South Dakota of 1600 m.v.,
and a high age of 1590 m.y. for the Central Wasatch Mountains in Utah,
Other than these three, ten other ages fall fairly close to 1350 m.y.

No orogenic belt or province in the Canadian Shield has yielded such
ages. The 1350-m.y.-old belt of the western United States appears to

STRUCTURAL GEOLOGY OF NORTH AMERICA

PINE RIDGE

CHISTOPHER
MOUNTAIN RIDGE

CHEDISKI RIDOE

REDWALL LM (M)

MARTIN FM. (D)

_^V / J «-7- //^ MAKTIN I.. 101 ■<-.■ X • ,>^\V\\V ^-TN.

■/■ //;wA.:^-KSff!h»J*2*!?ss:wiJ5^.v-\. "MMSgV. TROY >A«I>?Z" OUARTZITE

70 MILES APP.-

Fig. 4.4. Restored section across the northern part of Mazatzal Land. After Huddle and Dob-
rovolny, 1950.

project into the Grenville belt or wedge out to the northeast in the Great
Lakes region.

Arizona. The 1350-m.y. orogenic belt is here called the Mazatzal from
relations in central Arizona (Fig. 4.4). A correlation of the Precambrian
rocks of Arizona by Anderson (1951) is given in Table 4.1, and in it will
be seen that the Mazatzal quartzite is regarded as the youngest of a
group of old rock units, mostly schists. E. D. Wilson (1939) showed that

Table 4.1. Correlation of Precambrian Rocks of Arizona (C. A. Anderson, 1951)

Grand

Canyon

(Noble and

Hunter

1917; Darton,

1925)

Bradshaw Mtns.

(Lindgren,

1926)

Mazatzal Mtns.

(E. D. Wilson,

1939)

Globe

(Ransome,

1903)

Younger
Precambrian

Grand

Canyon

Series

Chuar group
Unkar group

Unconformity

Apache group

Apache
group

Orogeny— Intrusion of granitoid magmas

Older
Precambrian

Vishnu schist

Yavapai schist

Mazatzal quartzite
Maverick shale
Deadman quartzite

Alder series

Red Rock rhyolite

Yaeger greenstone

Pinal schist

granite, and thus dated the orogeny and intrusions as post-Mazatzal.
the Mazatzal quartzite was folded and faulted prior to the intrusion of
He named the orogeny the Mazatzal revolution, and this event now seems j
to be dated by the new isotope age determinations, and therefore is ap-
plied to the entire belt up through Colorado, Wyoming, and South
Dakota.

It should be noted that the Vishnu schist is 25,000 feet thick where ex-
posed in the Grand Canyon of the Colorado, and was originally fine-
grained argillaceous sandstones and sandy shales. A sequence of basaltic
lavas and tuffs is now represented by amphibolites in which relict pillow
and anygdaloidal structures prove the volcanic character. The Vishnu
schist is intruded by plutonic rocks that range from quartz diorite to
granite. In fact, granite is more widespread in outcrop in Arizona than the
host rocks, and therefore the Mazatzal orogeny must be considered, there
at least, to be identified with great batholithic intrusions of fairly acidic
rock.

Colorado. The largest exposure of basement crystalline rocks in the
Rockies is in the core of the Front Range of Colorado. It consists essen-
tially of granite, schist, and gneiss (Lovering and Goddard, 1950).

The oldest rocks in the Front Range are the schists and gneisses of the
Idaho Springs formation, which are highly metamorphosed sedimentary rocks
of early pre-Cambrian age. The thickness is approximately 20,000 feet. The
hornblende schist and gneiss of the Swandyke hornblende gneiss is overlain
by a series of quartzites and quartz pebble conglomerates at least 14,000 feet
thick. These formations are all cut by an extensive series of granite intrusives,
the oldest of which is a quartz monzonite gneiss. It occurs chiefly in small stocks
peripheral to granite batholiths or as a lit-par-lit injection of the older schists
and gneisses. Gneissic granite, gneissic aplite, and gneissic diorite are found
in abundant but small masses within the metamorphic terrain and are believed
to be related to nearby granite batholiths of different ages.

The earliest of the batholithic granites is the Boulder Creek granite; it is
common in stocks and small batholiths in the central part of the Front Range.
Its dark-gray color and faindy banded appearance distinguish it from the
pink coarse-grained Pikes Peak granite, which is somewhat younger and forms
the extensive batholith of the southern part of the Front Range. The appearance
and age relations of the Pikes Peak granite are the same as those of the Sherman
granite exposed in the large batholith extending from the northern part of
the Front Range well into Wyoming. Small batholiths and stocks of the younger
fine-grained to medium-grained light pinkish-gray Silver Plume granite are

PRECAMBRIAN TECTONIC PROVINCES

widely distributed, and locally have been given different names. Lead-uranium
ratios indicate that the age of the Pikes Peak granite is approximately 1 billion
years and that of the Silver Plume granite approximately 940 million years
(Lovering and Goddard, 1950).

The lead-uranium ratio age determinations for the granites are younger
than those yielded elsewhere by the potassium-argon and rubidium-
strontium methods, and it seems probable that these will be recognized
as too young and replaced by new age determinations.

Utah. The Precambrian rock succession in central Utah is shown in
the correlation chart of Fig. 4.5. The Farmington Canyon complex is the
basement rock and consists of gneisses, schists, and granulites, about
20,000 feet thick, once a stratified sequence of arkose, calcareous shale,
impure dolomitic and tuffaceous beds, and very pure quartz sandstone.
Metamorphism is of the lower amphibolite facies and therefore medium-
grade (Larson, 1957; Bell, 1951). The metamorphism is dated as 1590
m.y. (Gast and Long, 1957).

Another sequence of beds, the Willow Creek and Harrison, seems to
be of intermediate age, and it is not clear yet whether they were in-
volved in the Mazatzal orogeny. The Farmington Canyon complex is
overlain unconformably by the Big Cottonwood quartzite and argillite
series and did not participate in the metamorphism of the older gneisses
and schists.

The Big Cottonwood and Uinta series are generally correlated with the
Belt series of western Montana which is very thick and widespread. These
will be referred to under the next heading.

Beltian Orogenic Belt

A major trough or geosyncline of sediments and volcanic rocks of post-
Mazatzal age, yet pre-Paleozoic age, extends north and south from the
Mexican border through Arizona, Utah, Idaho, western Montana, eastern
Washington, western Alberta, and eastern British Columbia to the
Yukon, and possibly into Alaska. Its stratigraphy is complex, and much
remains to be discovered and worked out. Two major divisions appear
to stand out, namely, a lower one, the Beltian, and an upper one, which is
typified by a thick and well-described succession in the western Purcell

Range (Reesor, 1957) and in northeastern Washington (Park and Can*
non, 1943). It has been referred to as the Upper Purcell b) 1
(1957) and also as the Lipalian series by Gussow ( L957). In northern
Utah, it may find representation in the Mineral Fork tillite and Mutual
formation (Crittenden et al., 1952).

Angular unconformities have been recognized in a number of places up
and down the trough between the Beltian and Metaline sequences and
between them and the overlying Cambrian. In central Arizona Mazatzal
Mountains) the Apache (Beltian) group is tilted, beveled, and overlain
by the Cambrian. In the Grand Canyon of northern Arizona, the Grand
Canyon series (Beltian) group is tilted, faulted, beveled, and overlain by
the Cambrian. In north-central Utah 12,000 to 15,000 feet of the Big
Cottonwood series (Beltian) and the Mutual formation are cut out
beneath the basal Cambrian angular unconformity.

In western Montana and southeastern British Columbia Deiss (1935)
believes the Beltian strata were strongly uplifted, tilted, mildly folded,
and eroded before the Cambrian beds were laid down. In the Purcell
Range Cambrian beds lie across various Purcell formations (Beltian)
through a stratigraphic interval of 8000 feet, and although the dis-
cordance is generally slight, in one place it is 90 degrees (White, 1959).
Large sills and dikes are present in this region and probably accompanied
the orogeny. Campbell (1959) recognizes an unconfromity between
Middle Cambrian and Beltian strata in northwestern Montana and north-
ern Idaho in which up to 18,000 feet of Beltian is missing.

Stimulated by a paper by Weiss (1959) the writer has prepared a
cross section from northeastern Washington across southern British-
Columbia to Waterton, Alberta, showing postulated conditions at the
beginning of Middle Cambrian time (Fig. 4.6). The Beltian correlatives
would be the Deer Trail, Priest River, and Lower Purcell groups. The
LTpper Purcell group would include the Monk, Three Sisters, and Horse-
thief Creek association, the Huckleberry, Leola, Irene, and Purcell vol-
canics, and the basal Huckleberry, Shedroof, and Toby conglomerates. It
may be seen that the Upper Purcell group rests unconformly on the
Beltian and is introduced by a thick and widespread conglomerate. This
unconformity is taken specifically to mark the orogeny of the Beltian

STRUCTURAL GEOLOGY OF NORTH AMERICA

PROMONTORY
RANGE (Olsen)

WESTERN
UINTA WITS.

ILARSEN)
€ QTZ.

RED PINE
SHALE

\4_

UTAH

Fig. 4.5. Correlation chart of Precambrian formations in northern Utah. After Larson, 1957.

orogenic belt, and its extent is assumed to be approximately that of the
Beltian trough. The conglomerates appear to have come from the west,
and if so, the orogeny was most severe along the western margin of the
trough.

The age of the Beltian orogeny cannot be accurataely fixed with exist-

ing data. A sample of illite from a shale in the Siyeh formation in Glacier
National Park (Goldich et al, 1959) yielded a date of 740 m.y. by the
potassium-argon method and 780 m.y. by the strontium-rubidium method.
Goldich et al. reason that this age is not a time of metamorphism but
more probably marks the time of deposition. The Siyeh formation is near

PRECAMBRIAN TECTONIC PROVINCES

TURTLE
LAKE
QUAD.

MAGNESITE
BELT

METALINE
QUAD.

LITTLE
SALMO

PURCELL RANGE CRANBROOK GALTON

WESTERN DIVIDE RANGE

WATERTON

MIDDLE CAMBRIAN

PEND OREILLE
METALINE LS. GR.

LARDEAU SER

LOWER CAMBRIAN:::: :•■••••■. •.••.•; •.••.;••.■:•.•

; •.AppYQUARTZITE'-.':-

MAITLANO PHYLLITE LAIB GR
•QUARTZ IT E R

"UPPER PURCELL s M F PPARD--KJNJ' L_A
^PURCELL LAVA

10,000

■ 20,000

130,000
VERTICAL SCALE
IN FEET

Fig. 4.6. Suggested correlation of Precambrian formations of southern British Columbia and northeastern
Washington, after Reesor (1957) and Weiss (1959), restored to Middle Cambrian time.

the top of the Belt series, and the Beltian orogeny occurred soon after its
deposition, so the date is about as good for the time of deposition as for
the metamorphism, if any, or orogeny.

In conflict with the illite date we note that samples of uraninite in a
vein system in the Coeur d'Alene district of Idaho that cuts folded meta-
sedimentary rocks of the St. Regis formation of the Belt series have
yielded a date of approximately 1190 m.y. (Eckelmann and Kulp, 1957).

Although different laboratories have confirmed this date, Wehrenberg
(personal communication) thinks there is still justification to question its
validity in dating the age of the strata and their folding. The St. Regis
is about three-quarters of the way up from the lowermost beds of the
Belt exposed. From samples of galena in the same mine Farquhar and
Cummings (1954) give the age as 1030 =*= 290 m.y.

It is clear from Fig. 4.3 that the Belt sediments and correlatives lie in

STRUCTURAL GEOLOGY OF NORTH AMERICA

a great elongate basin generally north-south and parallel to the Pacific
margin of the continent, and that the basin is discordant with the older
orogenic belts, across which it lies. This, if true, is of great significance.
It suggests that following the Mazatzal orogeny that a major part of the
western margin of the continent was removed, because the older belts of
orogeny now extend at nearly right angles to the continental margin.
It also suggests that in Beltian time the processes of sedimentation and
orogeny first became established along and parallel to the present con-
tinental margin.

The discordant relation of the Beltian trough and orogenic belt to the
older belts emphasizes the concern that must be attached to the uraninite
date. It is almost as old as the Mazatzal orogeny, and presumably should
be separated from it by considerable time.

Not only is the Beltian orogenic belt discordant with the Mazatzal
orogenic belt, but also are the Antler and Shuswap belt and Nevadan belt
which lie west and parallel with the Beltian ( see Chapters 6 and 17 ) . If
the theory is held that the nucleus of the continent has been added to by
successively younger orogenic belts, then some major change occurred
to the southwest margin of the North American continent in Beltian or
pre-Beltian time. Perhaps a major part of the southwest margin as it ex-
isted in pre-Beltian time is missing, but no plausible theory of translation
or foundering has been thought of to restore the missing part. It is con-
ceivable that a major change occurred in the constitution and assembly
of the continents in the interval of time immediately preceding the
Beltian.

Purcell Orogenic Belt

Following the Beltian orogeny in the southern British Columbia and
northeastern Washington region a thick conglomerate was deposited, and
then extensive volcanic rocks were spread all the way from the Columbia
River in Washington to Waterton, Alberta. These were followed by sand-
stones and argillites, particularly in a main trough in the Purcell Range
area. After this depositional and volcanic cycle another disturbance oc-
curred in which, in the Purcell Divide area, the entire series was removed
together with a considerable thickness of the underlying Belt series

(Weiss, 1959). This unconformity attests the removal of a greater thick-
ness of strata than the one at the base of the Shedroof-Toby con-
glomerates, according to Weiss. See Fig. 4.6.

The overlying Lower Cambrian quartzite appears to have been derived
from the west, like the basal Huckleberry-Shedroof-Toby conglomerate,
and, if so, indicates that the major axis of orogeny lay to the west. The
zone from the Purcell Range to the front of the present Rockies was a
broad geanticline across which the Early Cambrian seas failed to spread.
The Middle Cambrian seas, however, probably transgressed much of the
geanticlinal area (Campbell, 1959).

The orogeny of post-Monk and Three Sisters age, yet of pre-Early
Cambrian age, will here be called the Purcell.

In dealing with Precambrian formations distant correlations are gener-
ally questionable, and this is especially so when assuming that the
Mineral Fork tillite and Mutual strata of northern Utah are equivalent
to the Upper Purcell group. If valid, however, an orogeny can be said
to have occurred after the close of Mutual time and before the late
Lower Cambrian sands were spread across the beveled edges of these
formations as well as those of the Big Cottonwood series. It is not clear
how discordant the tillite and Mutual are to the underlying Big Cotton-
wood strata because of limited exposures, but Crittenden et al. (1952)
note that the tillite occupies broad smooth-bottomed basins scooped out
of the upper part of the Big Cottonwood series.

Both the Beltian and Purcell orogenies may be combined in one angular
unconformity in the Grand Canyon of the Colorado in northern
Arizona. It is evident that information on the extent of the Beltian and
Purcell orogenies in scanty and that the pronouncements of the preceding
paragraphs are postulates of fairly tenuous nature.

Keweenawan Belt

The Keweenawan series of the Lake Superior region is the youngest
of the Precambrian rocks there and is well known because of the great
value of its copper mineralization. An imposing sill dated 1100 m.y. by
Goldich (personal communication) and believed to be part of the Ke-
weenawan series, crops out along the northwest shore of Lake Superior.

PRECAMBRIAN TECTONIC PROVINCES

It is called the Duluth gabbro. Three divisions of the Keweenawan are
recognized, namely, a lower clastic sequence 1400 feet thick, then a
thick unit of basic amydaloidal lava flows interbeddcd with sandstones
and conglomerates, and at the top a continental clastic sequence possibly
reaching a thickness of 25,000 feet in the center of the basin of ac-
cumulation. The widespread extent of the flows and the paucity of ash
suggest that the flows issued from a system of fissures rather than central
vents. Associated with the flows and intruded into them are numerous
dikes and sills, dominantly basic. The most prominent sill is the Duluth
gabbro.

The thick upper Keweenawan elastics consist of red feldspathic shaly
sandstones at the base and these grade upward into arkosic and quartzose
sandstones. They accumulated as the basin foundered in response, pre-
sumably, to the extrusion of the large volume of volcanics. Highlands
existed on both sides of the basin (Hamblin and Horner, 1961).

Several large faults break the Keweenawan series. The Douglas and
Keweenawan are found on opposite sides of the synclinal or basin axis
with thrusting away from the axis. See map, Fig. 4.3 and cross sections
of Fig. 4.7. Vertical displacements up to 4 miles are indicated by the cross
sections. The North Shore fault, postulated from physiographic data
solely (principally from the straight shorelines) was not detected by
gravity surveys, but the surveys do not rule out its existence. If it is a
reality, it may be a normal fault and of later age than the reverse faults.
The orogeny of post-Keweenawan time consisting of volcanism and fault-
ing has been called the Killarnean and is dated at about 950 m.y. ( Fair-
bairn et al, 1960).

The sills and volcanic rocks are strongly reflected by positive gravity
anomalies, and the deep basins of clastic rocks by negative anomalies.
Thiel (1956) has recognized this fact and traced the Keweenawan series
under the Paleozoic sedimentary rock cover by means of these strong
anomalies southwestward to the Salina basin of Kansas. The positive
feature has an average width of 30 miles and an amplitude of 100 miligals
above the regional gravity value. For the greater part of its length it is
flanked on both sides by gravity lows. The igneous rock masses are
responsible for the gravity highs and the clastic-filled basins, the lows.

CLASTICS

RED-ROCK
PHASE

DULUTH GABBRO

-I

20 MILES
Fig. 4.7. Keweenawan orogenic belt. Sections in the Duluth area after Thiel, 1956.

The Keweenawan belt projects toward the volcanic and gabbroic
terranes of Oklahoma and Texas, and perhaps these are part of the same
tectono-igneous belt. No strong gravity anomalies are known between
Kansas and Texas, but the grain of gravity contours (Lyons, 1950) is
southwesterly, and thus the belt may be marked by sedimentary rocks
and an absence of volcanic in this region.

The Precambrian rocks of the Wichita Mountains of Oklahoma rep-
resent the upper granitic part of a large gabbroic lopolith which

STRUCTURAL GEOLOGY OF NORTH AMERICA

CENTRAL TEXAS

NORTH TEXAS

TEXAS PANHANDLE

WICHITA MOUNTAINS-
BURIED AMARILLO
MOUNTAINS

ARBUCKI.E MOUNTAINS

VAN HORN AREA

WEST MARGIN OF TEXAS

CRATON ; FRANKLIN

MOUNTAINS; SOUTHEAST

NEW MEXICO

LATE
PRECAMBRIAN

sedimentary rocks
(Van Horn sandstone)

SWISHER GABBROIC

TERRANE
emplacement of gabbro
(lopolith?) ; contact
metamorphism of sedi-
mentary rocks

WICHITA IGNEOUS
PROVINCE

gabbro-granite (670m.y.)
intrusion; contact meta-
morphism of sedimentary
rocks (Meers quartzite)

local orogeny — cata-
clastic metamorphism;
diorite intrusion

subsidence; sedimentary
rocks (carbonate rocks
and siltstones)

sedimentary rocks
Meers quartzite)

sedimentary rocks
(Allamoore and Hazel
formations)

PANHANDLE
VOLCANIC TERRANE
lavas, tuffs, shallow
intrusives — mostly
rhyolite

rhyolite intrusions?
(East and West Timbered
Hills porphyries)

rhyolite intrusions

rhyolite intrusions and
extrusions

FISHER
METASEDIMENTARY

TERRANE
regional metamorphism
of sedimentary rocks

RED RIVER
MOBILE BELT
regional metamorphism
of sedimentary rocks;
intrusion

synorogenic? granite
intrusions

VAN HORN
MOBILE BELT
regional metamorphism
(Carrizo Mountain
group pre-rhyolite)

regional metamorphism
of sedimentary rocks
(Lanoria quartzite?)

MIDDLE
PRECAMBRIAN

TEXAS CRATON
granitic intrusions
(about 1000 m.y.)

Texas era ton to south

TEXAS CRATON
granitic intrusions

Texas craton to north
and northeast

Texas craton to east

regional metamorphism
and intrusion (Valley
Spring gneiss, Pack-
saddle schist, older
gneissic meta-igneous
rocks.

Fig. 4.8. Tentative correlation of Precambrian rocks and structural events in Texas, southern Oklahoma,
and southeast New Mexico. Reproduced from Flawn, 1956.

Hamilton (1956c) thinks might correlate with the Duluth gabbro.

The lithologies and age relations recognized by Flawn (1956) of the
Texas Precambrian rocks leave considerable to be desired for a conclusive
tie with the Keweenawan belt. The volcanics are mostly rhyolite and
not basic varieties as in the Keweenawan series, and orogeny including
acidic intrusions and some metamorphism appears to be indicated. This
is not characteristic of the Keweenawan belt.

J. Tuzo Wilson (1956) has suggested that the sediments of the Ke-
weenawan, Huronian, and Mistassini groups along the Grenville front

in Ontario and Quebec have been derived from the Grenville orogenic
belt, and that a secondary mountain belt has resulted by their deforma-
tion at a later time. The Huronian rocks in Minnesota, Wisconsin, and
Michigan have a much wider distribution than the Keweenawan series
with its flanking faults, and are not so clearly a narrow belt as the
Keweenawan. The writer sees in the Keweenawan belt one somewhat
like the Triassic basins of the Piedmont crystalline province of the greater
Appalachian mountain systems. See Chapter 9. These are long narrow
fault-formed basins filled with thick sections of continental clastic sedi-

PRECAMBRIAN TECTONIC PROVINCES

PRECAMBRIAN
STRUCTURAL
TRENDS

MINERAL
PROVIN

DATE
CES

Fig. 4.9. Precambrian structural trends (left map) and mineral date provinces (right map) of North
America. Reproduced from Gastil, 1960.

ments and basic flows, sills, and dikes. The basalts have been described
as tholeiitic in both belts (Turner and Verhoogen, 1951). The signifi-
cance of tholeiitic basalt is discussed in Chapter 33, and the occurrence is
believed to be evidence that the belts formed under similar tectonic
settings. Both are on the inside (toward the continent) of master orogenic
belts involving extensive metamorphism and great batholithic intrusions.
According to this interpretation the Keweenawan belt should mark ap-
proximately the inner front of the Grenville orogenic belt or province.
Regarding the succession in Texas, it is possible that the Swisher

gabbroic terrane and parts of the Wichita igneous terrane arc Keweena-
wan equivalents, and that the metasedimentary and volcanic (rhyolite)
terranes are Huronian or somewhat older than the Keweenawan.

Texas Precambrian Rocks

In Texas and southeastern New Mexico a subsurface study of well
samples penetrating the Precambrian has enabled Flawn (1956"> to
delimit several rock assemblages, which he calls terranes I Fig. Is . The
basement rock is a granite dated about 100 m.y. old, and this is overlain

STRUCTURAL GEOLOGY OF NORTH AMERICA

apparently unconformably by metasedimentary and volcanic rocks, and in
one place by a gabbro sheet (?). The granitic intrusion therefore, cor-
related in age with the Grenville and Piedmont orogenies, and the meta-
sediments and volcanics presumably with the Keweenawan series of the
Lake Superior region. The Mazatzal orogenic belt appears to separate the
Texas Precambrian assemblage from the Grenville, and hence, the most
natural tectonic tie of the Texas assemblage appears to be with the Pied-
mont (Fig. 4.3).

Crystalline Piedmont

A broad belt of crystalline rocks extends from Alabama and Georgia
northeastward along the Atlantic margin of the continent to New Jersey,
and its relation to the Appalachian Mountains will be explained in some
detail in Chapters 8 and 9. In summary, its rocks are now believed to be

Precambrian and early Paleozoic in age, and to have been metamorphosed
and intruded particularly during the Taconic and Acadian orogenies of
Late Ordovician and Late Devonian ages, respectively. Age determina-
tions on the rocks of the Piedmont indicate two ages, namely, an older
one of Grenville age and a younger one of Paleozoic age. In fact, in one
sample the zircon grains yielded an age of 1050 m.y., and the feldspars
an age of 300 =*= m.y. (Wetherill et al., 1959). It is reasoned that this
means an early orogeny in which the zircons were created, and a late
orogeny in which the feldspars were formed but the zircons of the
early orogeny left unaltered.

The distribution of dates so far published is shown on Fig. 4.3 and a
comprehensive compilation and interpretation of Precambrian trends and
orogenic belts of North America by Gastil (1960) is reproduced in Fig.
4.9.

CENTRAL STABLE REGION
OF THE UNITED STATES

other structures of the Central Stable Region, with few <-\( eptions, formed
during the Paleozoic era, and many of them yield < \ idenoe ol a prolonged
history of development.

Up to Pennsylvania!! time, there was a certain bilateral symmetry to
the stable region, with a great medial transcontinental arch, and basins
and smaller arches on either side. An approximate parallelism of a s<n<s
of arches with the Ouachita and Appalachian orogenic belts was existent
and is still apparent today.

During Mississippian, Pennsylvanian, and Permian time, great overlaps
on some of the arches occurred. Others were either not completely buried
or have since been partially exhumed by erosion. In some areas the
Triassic overlapped on the Central Stable Region beyond the limits of the
Permian, and especially in late Cretaceous time did epeiric seas exten-
sively invade the region of arches and basins.

The large arches and basins are rippled and checked with numerous
folds and faults; and these, with the unconformities created by the great
overlaps, constitute immensely valuable structures for oil and gas accumu-
lation. The strata also contain great coal deposits and numerous other
nonmetallic mineral resources. Each basin and each arch will, therefore,
be considered separately. The geologic and tectonic maps of Chapter
3 will be especially helpful in relating the diastrophic histories of the
various major structures, and should be referred to repeatedly.

GENERAL CHARACTERISTICS

The Central Stable Region of the United States is made up of a founda-
tion of Precambrian crystalline rock previously described, with a veneer
of sedimentary rock. The veneer varies greatly in thickness from place
1 to place. For the most part, the Central Stable Region has suffered vertical
movements, and broad basins and arches have formed. Some of the
basins have more than 10,000 feet of strata in them, and in the cores of
some of the arches the Precambrian crystalline rock is exposed. Some of
the arches and sharper uplifts are not expressed in the surficial layers and
have been revealed only by drilling operations. The arches, basins, and

PRE-DEVONIAN BASINS

The basins of greatest extent and deepest subsidence in early Paleozoic
time were the geosynclines along the western and eastern margins of the
continent. Each constitutes an important part of our continent and will
be discussed in separate chapters: the Paleozoic Cordilleras geosyncline
in Chapter 6, and the Appalachian geosyncline in Chapters 7. 8, 11. 12. and
13. Refer to the map of Plate 2, Chapter 3, in the following paragraphs.

The Appalachian geosyncline subsided most in West Virginia, Virginia,
Tennessee, and Alabama. In a small area across the border of Virginia
and Tennessee, sediments accumulated to a thickness in excess of 25.(KX)
feet during Cambrian, Ordovician, and Silurian time. A distinct sag in the

STRUCTURAL GEOLOGY OF NORTH AMERICA

form of an embayment from Texas into southern Oklahoma resulted in
the local accumulation of more than 6000 feet of strata, the chief forma-
tion of which was the Arbuckle limestone. Another embayment possibly
extended to the western Texas region, where later the Pecos Range de-
veloped. The pre-Devonian sediments are thin in the Marathon and
Ouachita systems as compared with the Appalachian system.

A rather deep basin formed in Michigan, Indiana, and Illinois in pre-
Devonian time, approximately parallel with the Appalachian geosyncline.
Its largest and deepest part is the present Michigan basin.

The great western geosyncline of early Paleozoic time extended from
Alaska to southern California. It sank 15,000 to 20,000 feet across Nevada,
and at the Nevada-California boundary it contained over 20,000 feet of
beds (Nolan, 1943). No information is available farther southwest in
California because of the extensive Mesozoic and Cenozoic cover, in-
tensive metamorphism, and widespread Jurassic intrusions. The south
termination of the geosyncline shown on the map is, therefore, hypo-
thetical. The inner trough of the geosyncline becomes progressively deeper
to the southwest and undeniably heads into the later Jurassic orogenic
belt, which with still younger tectonic elements determines the margin of
the continent today.

TRANSCONTINENTAL ARCH

General Features

During Devonian and Mississippian time the great Central Stable
Region of North America consisted of three major divisions, a central
northeast-southwest-trending Transcontinental Arch, and large basins,
shelves, and arches and domes of various sizes on each side (Plates 3 and
5). The arch had three peninsular extensions to the southeast, one into
Kansas and Missouri, the Ellis and Chautauqua arches and the Ozark
dome; one into Wisconsin, the Wisconsin dome; and possibly one into
Texas. It is also known to have sagged below sea level in two places
where thin lower Paleozoic sediments were deposited, one in Colorado
and one in Arizona. Until the rise of the ranges of the Ancestral Rockies
and the Wichita systems, the Transcontinental Arch and its flanking

basins dominated the landscape. The Transcontinental Arch may have
bifurcated north of Lake Superior, with one arm extending northward
on the west side of the Hudson Bay basin, and the other extending first
eastward and then northward along the east side of the basin. This sup-
position is based on present Precambrian exposures, but paleontological
evidence and newly found erosional outliers suggest that much of the
area of the arms may have been submerged in early Paleozoic time.

Northeast of Colorado

The arch in Nebraska, South Dakota, and Minnesota was recognized
by Schuchert and called Souxia. It was later clearly depicted by Levorsen
(1931, PI. 1), and then still later mapped by Ballard (1942). The
boundaries of the formations shown on the geologic maps of the close
of the Devonian and the close of the Mississippian are those preserved
under the extensively overlapping Pennsylvanian strata (Plate 7) which
covered most of the arch. Ballard has gathered together the available well
records of the area and believes enough data is at hand to establish defi-
nitely the existence of the arch and fairly well the formational contacts
on either side of it.

The arch was referred to as the continental backbone by Keith (1928)
in his notable paper on "Structural symmetry of North America," and
later, also, by Levorsen. The name implies that it was a strong, resistant,
centrally located tectonic element with flanking basins and marginal oro-
genic belts in bilateral symmetry. With the exception of the peninsulas
and sags previously mentioned, the bilateral symmetry of the United
States part of the continent in a northeast direction was pronounced until
the Pennsylvanian transgression. The building of the Ancestral Rockies
altered conspicuously the aspect of the Transcontinental Arch, and then
the late Mesozoic and early Cenozoic mountain building disturbances
left the southwest half unrecognizable on a geologic map of the present
time.

The Transcontinental Arch appears very dominant on a pre-Pennsyl-
vanian geologic map, but this appearance should not be misinterpreted.
During the Devonian and Mississippian, the arch was very low-lying and
furnished chiefly chemical sediments to its flanking basins (Weller, 1931).

CENTRAL STABLE REGION OF THE UNITED STATES

The "backbone" was also not very strong in resisting deformation. In its
southwestern part, as previously mentioned, it was the site of Pennsvlva-
nian and Cretaceous-Tertiary mountain building, and its other parts have
been almost completely covered by Pennsylvanian, Permian, Mesozoic,
jand Cenozoic strata, in places of considerable thickness.

Wisconsin Dome

The area of central Wisconsin was probably uplifted several times in
the Paleozoic, but evidence both for time and spatial relations is scarce
and, therefore, all the geologic boundaries cannot be definitely fixed.
The isopach maps of the Ninth Annual Field Conference of the Kansas
Geological Society have been used as the chief source of information in

i making the interpretations shown on the maps of this book. The isopach
maps generally show the existing thickness of the various formations or
groups, and their compilers say that the original thickness and extent
over the Wisconsin dome area is not certain. However, some of the
formations thicken basinward under cover of protecting formations, and

! such contacts can be projected and the limits before burial located ap-
proximately.

Two pre-Devonian times of significant uplift are recognized; the first
preceded the deposition of the St. Peter formation in Early Ordovician
time, and the second followed the deposition of the Silurian beds. During

; the second uplift, an arch was formed that extended southeastward from
Wisconsin into Illinois, almost to the city of Kankakee (Fig. 220, Ninth
Annual Field Conference, Kansas Geological Society).

By the close of Mississippian time, a pronounced dome had appeared
(Plate 6). A strip of Cambrian sediments extending southwest from the
Keweenaw peninsula of Michigan indicates that the dome was separated
from the Transcontinental Arch by a fairly broad, gentle syncline. A
broad, noselike uplift extended southeastward from the Wisconsin dome
in approximately the position of the post-Silurian arch and connected with
the Kankakee arch of Illinois and Indiana (Plate 6). How far the
Mississippian sediments spread over the dome area is not ascertainable,
but following the late Mississippian uplift they were eroded back appre-
ciably.

Colorado and Arizona

The rise of the Ancestral Rockies in late Mississippian and Pennsylva-
nia!) time destroyed the Transcontinental Arch in Colorado. The pre-
Pennsylvanian sediments present are very thin, and cover the arch
throng!) central Colorado in a /one 100 miles wide. The zone was i
dently the site of a gentle sag in the arch norma] to its length, and as
Burbank's (1933) map shows, it lines up almost precisely with the
Wichita trough that others have shown in Oklahoma and Kans.is. It
seems, therefore, that the Wichita trough extended northwestward toward
the Colorado sag, and not in the direction of the Amarillo Mountains in
the Panhandle of Texas as has been suggested by some writers.

Arizona was mostly above water during the early Paleozoic (Stoyanow,
1942). The Mazatzal orogeny of Precambrian time (sec previous dis-
cussion in this chapter) produced a chain of mountains that extended
from southwestern Arizona to southwestern Colorado with subparalli 1
folds and thrust faults trending northeastward (Huddle and Dobrovolny,
1950).

The orogeny and associated intrusions took place after the Mazatzal quartzite
was deposited. The mountains subsequently were well worn down by erosion,
but the very resistant Mazatzal quartzite formed ridges along the core of the
old mountain chain. The ridges served to separate the basins in which the
rocks of the Apache and Unkar groups were deposited. . . . Both were con-
siderably eroded before the Troy quartzite and Tapeats sandstone of Cambrian
age were deposited. . . . After the deposition of the Cambrian sandstones.
Mazatzal land probably was up-arched slightly and eroded, because the
Martin formation in central Arizona rests on a surface of some relief. Then-
are neither Ordovician nor Silurian rocks in central Arizona, and probably
there never have been any. Cambrian rocks may have extended through the
Mogollon sag, and a considerable thickness of them may have been removed
from Mazatzal land during the long erosional interval between the retreat oi
the Late Cambrian seas and the spread of the Late Devonian sets. The
gradual burial of the mountains and Mazatzal land before Pennsylvanian tunc
is summarized diagrammatically in Fig. 4.4. Because the Martin formation
was not deeplv eroded prior to the deposition of the RedwaD limestone, prob-
ably no diastrophic disturbances of Mazatzal land occurred at the close of
the Devonian. After the Mississippian limestone was laid down, however,
Mazatzal land again was uparched. as shown by the great erosional reduction

STRUCTURAL GEOLOGY OF NORTH AMERICA

of the Redwall limestone on Mazatzal land and the related increase in the
thickness of the red residual member of the Naco formation nearby (Huddle
and Dobrovolny, 1950).

EASTERN INTERIOR BASINS AND ARCHES

General Features

Three basins of subsidence and sedimentation had become clearly
established by late Devonian time southeast of the Transcontinental Arch,
namely, the Michigan basin, the Illinois-Indiana-Kentucky basin (East-
ern Interior basin), and the West Virginia-Pennsylvanian basin (Appala-
chian basin). In Pennsylvanian time a fourth became defined, which is

Fig. 5.1. Basins southeast of the Transcontinental Arch showing areas of sand accumulation
early Pennsylvanian time and the direction of stream transport. After Potter and Siever, 1956.

called the Western Interior basin as a coal province, and the Forest City
basin as an oil province. See map, Fig. 5.1, and Plate 7. The Western
Interior and Eastern Interior basins were first so labeled when studied
as coal basins of Pennsylvanian age, and although the nomenclature is
not consistent with the state name applied to the Michigan basin, also a
coal basin, it is generally retained and used today.

Appalachian Basin

The history of the Appalachian basin is recounted in Chapter 7 in
connection with the Appalachian Mountains. As shown on the map, Fig.
5.1, it lies between the Valley and Ridge Province of the Appalachians
and the Cincinnati arch, but in its development its deepest part lay in
the mountainous belt; the eastern half of the basin became involved in
folding and thrusting in late Paleozoic time leaving the western half
relatively undeformed and what is now called the Appalachian basin.
See Figs. 8.11 and 8.12. It is filled with a remarkable succession of
miogeosynclinal and shelf strata ranging in age from Cambrian to
Permian.

Michigan Basin

In pre-Devonian time, the Michigan and Illinois-Indiana-Kentucky
basins were continuous; but beginning in the Devonian, the Kankakee
arch began to form, and the two basins became increasingly individual-
istic thereafter. The Michigan basin today is circumscribed by the
Great Lakes depressions on the west, north, and east, and by the Cin-
cinnati dome on the south. It consists of a sequence of beds representative
of all periods of the Paleozoic, cast in saucer fashion, each one of which
is smaller than the preceding on which it rests. The youngest strata are
thin and patchy red beds of either Upper Pennsylvanian or Permian age.
All Paleozoic strata are overlain and nearly completely blanketed by a
layer of glacial drift which ranges in thickness from a few feet to 1200
feet. As the basin subsided through the Paleozoic, its crystalline pre-
cambrian floor acquired the configuration shown in Figs. 5.2 and 5.3.
The total thickness of sediments in the basin is about 14,000 feet ( Cohee,
1948).

CENTRAL STABLE REGION OF THE UNITED STATES

The major unconformity in the Paleozoic sequence is at the base of the
St. Peter sandstone and the Trenton and Black River limestones. See
Figs. 5.4 and 5.5. The St. Peter sandstone is late Lower Ordovician, and
marks the time of uplift and erosion. When traced eastward from Indiana
to Ohio and northeastward into Ontario in well logs, the Lower and
Middle Ordovician formations rest successively across the several forma-
tions of the Upper Cambrian, and finally come to rest directly on the
Precambrian crystallines of the Canadian Shield. Through western
Ontario, the Cambrian beds are absent.

Significant units in the Michigan basin are the evaporite series of the
Silurian and Devonian. A number of beds of salt are present throughout
much of the basin and southwestern Ontario which in places may aggre-
gate over 2000 feet in thickness. Porous dolomites in these evaporite series
are reservoir rocks for oil and gas, and many oil fields have been developed
in the basin. Very gentle folds or "highs" ripple the basin beds and
take an irregular northwest-southeast direction. They have served to trap
the oil (Fig. 5.4).

In the Straits of Mackinac region, the most prominent outcrops are a
limestone breccia. It is noted for its resistance to erosion and forms the
scenic pillars and cliffs of the region. The map of Fig. 5.6 shows its known
distribution.

The columns of breccia, according to Landes ( 1945 ) , may range up to
1500 feet in vertical dimension. The solution of Silurian salt has resulted
in subsidence and roof collapse, and the breccias are the result. Certain
blocks can be shown to have fallen or settled 600 feet. The formations
involved and the nature of the breccias are illustrated in the cross section
of Fig. 5.7. Supporting the salt solution and collapse theory is the map
showing the abrupt thinning of the Salina salt in the Mackinac Straits
region (Fig. 5.8). The solution of salt and the collapse of the overlying
layers of limestone and dolomite took place chiefly in pre-Dundee time
( Middle Devonian ) , but even now some leaching may be occurring.

Great Lakes Depressions

The Salina salt emerges from the basin in a horseshoe-shaped pattern
that corresponds closely with Lake Michigan and Lake Huron. The out-

ILLINOIS

Fig. 5.2. Configuration of the Precambrian floor in the Michigan basin and adjoining areas.
Contours in thousands of feet. After Cohee, 1948.

CAMBRIAN 8 ORDOVICIAhT

Fig. 5.3. Cross section of the Michigan basin, after American Association of Petroleum Geolo-
gists, 1954, Geo/ogic Cross Section of Paleozoic Rocks, central Mississippi to northern Michigan.
The Cambrian is mostly a sandstone and shale sequence; the Black River through Traverse a

limestone, dolomite, and evaporite sequence, the Antrim through Michigan a shale and sand-
stone sequence.

CENTRAL STABLE REGION OF THE UNITED STATES

TRENTON AND BLACK RIVER LIMESTONES

0 » 30 tfo ^j,

XATION OF SECTION

Fig. 5.4. Cross section from Illinois to western Ontario showing the unconformity at the base of
the St. Peter sandstone and the Trenton and Black River limestones. Top of Trenton is taken as

horizontal datum. Younger formations and present structures not shown. By George Cohee,
U.S. Geological Survey.

crop then swings eastward through the basins of Lake Erie and Lake
Ontario. The salt would emerge mostly under water, and since the
aggregate thickness of salt beds that once may have cropped out was
several hundred feet, it has been suggested ( Newcombe, 1933 ) that the
depressions of the Great Lakes (excepting Superior) may be due to salt
solution and consequent subsidence. The basins do not correspond to

faults or folds, and were probably existent long before the Pleistocene
ice lobes occupied them. The theory of salt solution seems the most logi-
cal explanation yet advanced.

The Lake Superior depression is north of the belt of salt outcrop and is
mostly in Precambrian rocks. The northwest shore may correspond to a
fault, and the lake bottom topography suggests fault scarps. Because the

STRUCTURAL GEOLOGY OF NORTH AMERICA

Fig. 5.5. Thickness of Upper Cambrian and Lower Ordovician Rocks in the Michigan Basin.
After Cohee, 1948.

SURFACE EVIDENCE OF COLLAPSE
x SUBSURFACE EVIDENCE OF COLLAPSE
o WELLS WITH NO EVIDENCE OF COLLAPSE

Fig. 5.6. Map of Mackinac Straits areas showing zone of collapse and exposures of Mackinac
breccia. Reproduced from Landes, 1945.

faults have been regarded as either Precambrian or late Paleozoic in age,
they are very ancient, and any scarps would be erosional features of the
fault-line variety. Previous conjecture places the Grenville front in the
position of the lake, and later subsidence along this zone may have oc-
curred to form the lake basin. It must be conceded, however, that the
origin of the Lake Superior basin, over 1000 feet deep in places, has not
yet been worked out satisfactorily.

Eastern Interior Basin

The Eastern Interior or Illinois-Indiana-Kentucky basin is deepest in
Wayne, White, and Hamilton counties where the base of the Mississippian

CENTRAL STABLE REGION OF THE UNITED STATES

H /AG ARAN

Fig. 5.7. Hypothetical section of the Mackinac Straits region showing collapse formations above the
Niagara limestone, and the breccia chimneys and stacks. Reproduced from Landes, 1945.

tOo

Fig. 5.8. Isopach map showing aggregate thickness of Salina salt. Reproduced from Landes, 1945.

shales is 4800 feet below sea level. As previously explained, the Eastern
Interior basin was part of a depression that included the Michigan in pre-
Devonian time, but from then on the two basins sank separately, leaving
the Kankakee arch between.

The La Salle anticlinal belt (see Fig. 5.9) is a row of anticlines
arranged en echelon, and it extends over 200 miles from north central to
southeastern Illinois. The north end of the en echelon belt may be con-
nected with the east-west trending Savanna-Sabula anticline ( Eckblaw,
personal communication), which extends into eastern Iowa. The south
end may merge with the Wabash River anticline. The La Salle anticlinal
belt formed chiefly during the Pennsylvanian period and divided the pre-
Pennsylvanian Illinios-Indiana-Kentucky basin into two parts, the larger
and western of which is generally known as the Illinois basin. The Oak-
land anticline borders the La Salle closely on the east.

The first deformation took place in post-Chester, pre-Pennsylvanian
time (Fig. 5.10). Further deformation continued during the Pennsylva-
nian progressively southward. In La Salle and Douglass counties at the
north end, the early movements were the greatest, and the crest of the
anticline was elevated 900 to 1400 feet above the adjacent basins. In
Lawrence and Wabash counties to the south, the greatest movements
occurred within the Pennsylvanian. Since the Pennsylvanian beds are

STRUCTURAL GEOLOGY OF NORTH AMERICA

LAKE

MICHIGAN

^^LLE OUTG$",

MILES

slightly folded over the anticline, it is possible that some movement
occurred after they were deposited as well as during the time of de-
position.

At the beginning of the Pennsylvanian there was a regional southwest
slope furrowed by numerous subparallel valleys as deep as 200 feet. An
eastward slope prevailed along the western border of the basin with

Fig. 5.9 Structure contour map of Eastern Interior basin. Contours on Illinois Coal No. 2, in
hundreds of feet. After Wanless, 1955. Oak. A., Oakland anticline; M-S Syn., Marshall-Sidell
syncline; D.M., Duquoin monocline; R.C.F., Roush Creek fault zone.

Fig. 5.10. Cross section of the Illinois basin, after American Association of Petroleum Geologists
1954, Geo/ogic Cross Section of Paleozoic Rocks, central Mississippi to northern Michigan. The
Eau Claire and older beds of the Cambrian are sandstone and shaly sandstone; from the upper
part of the Eau Claire through the Ordovician, Silurian, Devonian, and the Mississippian to the
Upper Mississippian Chester series the sequence is dominantly limestone and dolomite with much
chert. The St. Peter is conspicuous sandstone in the Ordovician, and the Osage of the Mississippian
has considerable sandstone and shale toward the La Salle anticlinal belt. The Chester and Penn-
sylvanian strata are sandstone and shale with several thin limestone beds, and coal in the
Pennsylvania.

STRUCTURAL GEOLOGY OF NORTH AMERICA

smaller valleys. The geologic map at this time would have appeared as in
Fig. 5.11, when formations from the Middle Ordovician St. Peter sand-
stone to the Upper Mississippian Kinkaid limestone cropped out. The
southeast border of the basin sank progressively and resulted in a regular
increase in thickness of the uppermost Pennsylvanian strata in that direc-
tion (Wanless, 1955).

The assemblage of faults in southern Illinois, shown on the map of
Fig. 5.9, consists principally of the following trends: (1) the east- west
trend of the Rough Creek-Shawneetown system which extends west into
Illinois as the Cottage Grove and associated faults; (2) a prominent
northeast-southwest system of faults which is dominant in the fluorspar
district; and (3) the Wabash Valley fault system with north-northeast
trend, a few of which cross and offset the Rough Creek fault. These
faults are post-Pennsylvanian in age and the maximum throw is about
800 feet along the Rough Creek fault.

Studies of crossbedding and stratigraphic relations indicate that the
late Mississippian Chester sands as well as those of the Pennsylvanian
came mostly from the northeast (Potter et al., 1958, Potter and Siever,
1956, and Wanless, 1955), and some were probably carried by streams
from the site of the Michigan basin across the site of the previous Kanka-
kee arch. A minor amount of sand came from the Transcontinental Arch.
See Fig. 5.1.

Nashville Dome

The Nashville dome is at present the site of a topographic basin, with
surrounding escarpments of successively younger rocks. Ordovician strata
are the oldest rocks exposed in the core, and the escarpments are in the
overlapping Mississippian and Pennsylvanian formations. The dome ex-
perienced several movements in pre-Chattanooga (early Mississippian)
time, synchronous with those of the hinterland of the Appalachian geo-
syncline, according to Wilson ( 1935 ) . The dome was below sea level dur-
ing several epochs of various lengths of time, and during other times the
central part was above sea level but probably so slightly emergent that
little erosion occurred. The structure is a broad, gentle arch, less because
of uplift than because of greater subsidence of the adjacent basins. Its

domal structure was acquired by gentle sags between it and the Ozark
dome on the west (Wilson, 1939) and the Cincinnati dome on the north
(MacFarlan, 1943). See cross section of Fig. 5.12.

The first major uplift in which considerable truncation of the beds oc-
curred was in late Devonian time. The Chattanooga shale rests on the
Trenton (Ordovician), showing that about 500 feet of beds had been
eroded away in the central part of the dome consequent to this pre-
Mississippian doming (Wilson and Born, 1943).

The second major uplift was in late Mississippian and early Pennsyl-
vanian time, when its associated domes, the Ozark and Cincinnati, were
also elevated (Plate 5). The Chattanooga shale was domed gently, pro-
ducing regional dips of 16 feet per mile on the flanks, and along the axis,
both northeast and southwest, of about 8 feet per mile. A structural re-
lief of 700 feet was acquired by the dome above the saddle separating it
from the Cincinnati dome on the north. The structural relief of the dome
over the flanking basins was at least twice as much (Wilson and Spain,
1936).

Detailed structure contour maps reveal many local irregularities in the
Nashville dome. A conspicuous "grain" to the northwest is noted by Wil-
son and Born (1943), and axes of folds may be drawn in a few places. A
structure contour map of the Pencil Cave ( Ordovician ) formation shows
the grain equally as well as one drawn on the Chattanooga shale ( Missis-
sippian), but the local structures are not closely superposed. It may,
therefore, be inferred that part of them originated in pre-Chattanooga
time, and part in post-Chattanooga.

Cincinnati Dome

The Cincinnati dome is much like the Nashville dome, and is separated
from it by a shallow structural saddle. Several writers refer to the two
domes together as the Cincinnati arch, with the central part of the
northern structure, the Jessamine dome, and the Nashville dome as ele-
ments of it. The Cincinnati dome splits into two branches on the north,
one extending to the west-northwest and the other to the north-northeast,
which are known, respectively, as the Kankakee arch and the Findlay
arch.

CENTRAL STABLE REGION OF THE UNITED STATES

CENTRAL BASIN

CUMBERLAND
PLATEAU

Fig. 5.12. Section across the Nashville dome, after C. W. Wilson, Jr., 1935. 1, Lower and Middle Devonian;
2, Decatur; 3, Lobelville; 4, Beach River, Bob, and Dizon; 5, Lego, Waldron, Laurel, and Osgood; 6, Brass-
field; 7, Richmond.

The Cincinnati dome probably had an early Paleozoic history much like
the Nashville dome, but the first elevation in which appreciable erosion
occurred preceded slightly the one in the Nashville dome. MacFarlan
(1943) shows that the Middle Devonian (Boyle) limestone overlaps suc-
cessively older formations toward the center of the dome where it rests
on the Ordovician ( Richmond and Maysville ) . The Lower Mississippian
shale (Ohio shale, probably the Chattanooga equivalent) has been found
to "cut out" the Boyle limestone in a few places, and therefore locally
some late Devonian movement has been suggested.

Preceding the Mid-Devonian uplift of the Cincinnati dome and about
100 miles east of it, arose the Waverly arch in Early Ordovician time. It
has a structural relief of 750 feet ( Woodward, 1961 ) .

The Pennsylvanian-Mississippian contact is one of marked dis-

conformity and one of considerable relief as shown in a number of
Pottsville-filled valleys. The post-Mississippian uplift represented by the
unconformity was much broader than the doming of Middle Devonian
time. Compare Plates 5 and 6. It is generally regarded that after the late
Mississippian arching, the Cincinnati dome was submerged, and that
Pennsylvanian beds from the Appalachian region spread westward across
it so that the Appalachian and central interior coal fields were connected.
Several of the conglomerates, fireclays, and limestones have been corre-
lated across the dome. See Plate 7.

In order to produce the present distribution of the Pennsylvanian strata,
still another broad, gentle arching is required in post-Pennsvlvanian time.
This is shown on the tectonic map of Plate 8.

Some faults cut the dome, and these will be described later as part of

STRUCTURAL GEOLOGY OF NORTH AMERICA

a large fault zone that extends across several states. Local structures are
not as well mapped as in the Nashville dome; but as far as known, the
perceptible northwest "grain" does not exist. Instead, one or two "highs"
have been described on the eastern flank of the dome that trend parallel
with the main axis. It may be that with better contouring, a northwest
direction of local structures will be noted.

Kankakee Arch

The Kankakee arch, as defined by Ekblaw (1938), is the northwest
branch of the Cincinnati dome, and passes in a northwest direction across
Indiana and Illinois, connecting with the Wisconsin dome. Kankakee is
preferred to Wabash, a name sometimes used. The earliest significant
uplift preceded the deposition of the St. Peter sandstone, as in the Wis-
consin dome. The St. Peter sandstone rests on Cambrian beds at Oregon,
Illinois, indicating arching above sea level and removal of 500 to 600 feet
of rock in this early movement. The Cambrian and Prairie du Chien ( pre-
St. Peter) beds are believed to be about 4000 feet thick, both on the
Kankakee arch and in the Illinois basin, and therefore the arch was evi-
dently an area of subsidence just as much as the basin until Early Ordo-
vician time.

Oil wells show that the structural relief at present, if measured on the
top of the Trenton limestone, is about 6000 or more feet in relation to
the Illinois basin and 10,000 feet in relation to the Michigan basin. As the
Trenton is above the St. Peter, the arch has acquired this much additional
structural relief since the pre-St. Peter uplift. It is clear that the large part
of this structural relief is a result of subsidence of the basins on either
side of the arch, and that the upward movements of the arch itself, suffi-
cient to cause it to be eroded, contributed only in small part to the relief.
See Figs. 5.4 and 5.5 for pre-St. Peter structural relations.

The only reflection of the Middle and Late Devonian uplifts of the
nearby Cincinnati and Nashville domes is the conspicuous thinning of one
of the zones of the Traverse group in the Michigan basin toward the
arch (Cohee, personal communication). The greater subsidence of the
basin area than the arch area, as indicated by this zone in the Traverse,
occurred in late mid-Devonian. The basin had previously sunk rapidly,
and a thick evaporite series was deposited during the Silurian and pre-

Traverse Devonian. These thick salt, gypsum, limestone, and dolomite
beds are represented by thinner nonevaporite series in the Illinois basin,
and hence the structural relief of the arch is not so great to the southwest
as to the northeast.

The early Mississippian seas probably spread over the arch even though
Lower Mississippian rocks are not there today. This is concluded because
the beds do not display any characteristics of overlap on a land area. The
Upper Mississippian (Chester) beds of Illinois are not represented in
the Michigan basin, nor anywhere north of the Kankakee arch, and it
therefore seems that in late Mississippian time the arch and the area to
the northeast were gently emergent, and from this region and still farther
north the Chester sands were derived. Since the present structure dis-
plays the geologic pattern of a broad anticline with Silurian rocks in the
core and Devonian and Mississippian successively away on either side, it
follows that in addition to regional uplift over the Great Lakes region in
late Mississippian time there must also have been local uplift along the
arch. This movement occurred at the same time as the one described in
the Cincinnati dome with which the Kankakee arch merges.

The deposition of Pennsylvanian sediments across the Cincinnati dome
on a surface of appreciable relief corresponds to the well-known Pennsyl-
vanian overlap in Illinois south of the Kankakee arch and over the La
Salle anticlinal belt. The Upper Mississippian and pre-Pennsylvanian up-
lift along the arch was probably a movement of only a few hundred feet.
Again it was the appreciable subsidence of the adjacent basins that con-
tributed most to the arch structure.

Recause the Pennsylvanian strata were gently arched and eroded back
from the Cincinnati arch, a post-Pennsylvanian uplift of gentle but broad
dimensions is indicated. It appears that the uplift spread northward so as
to embrace the Kankakee arch, the Wisconsin dome, the Michigan basin,
and the southern part of the Canadian shield.

In summary, the Kankakee arch acquired its structural relief chiefly by
greater subsidence of the basins on its sides than by actual uplift. It was
lifted out of water in early Ordovician time, and in one place it suffered
600 feet of erosion. Again it rose out of water in late Mississippian time,
and finally participated in a regional uplift of the Great Lakes region in
the late Pennsylvanian.

CENTRAL STABLE REGION OF THE UNITED STATES

A sag between Peru and Logansport across the arch is called the
Logansport sag, and many other minor irregularities make up the oil field
structures in the area.

Findlay Arch

The Findlay arch is the right arm of the Cincinnati dome, and extends
north-northeastward into the peninsular area of Ontario and thence to
the Canadian Shield (Plate 5). It is similar in size and relief to the
Kankakee arch and, since the early Ordovician uplift, it has had a similar
history (Cohee, personal communication). It was not an area where
thick pre-St. Peter sediments accumulated, and may actually have been
a low ridge of Precambrian rock at the beginning of Cambrian deposition
(Cohee, personal communication).

The uplift along the Findlay arch was localized and of somewhat
greater magnitude than along the Kankakee arch (Cohee, personal com-
munication ) . The cross section, Fig. 5.4, shows the base of the Black River
and the progressive overlap northward to the Precambrian crystallines
of southeastern Ontario.

The names Lima axis and Sandusky arch (Phinney, 1891), Algonquin
axis (Kay, 1942), and Cataract axis have been used for all or part of the
arch, but Findlay arch is preferred by Ekblaw ( 1938 ) and others. A sag

in the axis near Chatham, as contoured by Cohee (personal communica-
tion), reflects movements at the same time approximately as those in the
arch. The cross structure is called the Chatham sag.

Arches of Central Kansas

The geologic map of mid-Pennsylvanian time (Plate 7) shows the
superposition of one arch over another in central Kansas, with axes trend-
ing in slightly different directions. At the close of the Devonian, a broad
arch, for which the name Ellis is reserved (Moore and Jewett, 1942),
rose (Plate 5) and was eroded so that the Lower Ordovician Arbuckle
limestone was exposed in the core. The Mississippian seas then lapped
onto the Ellis arch and perhaps covered it. Post-Mississippian arching
in a somewhat more northerly direction and in a narrower zone resulted in
the erosion of the Mississippian strata and the exposing of the strata in
the Ellis arch again. This new uplift is called the central Kansas arch.
However, the local folds that developed parallel with the major axis of
the Ellis arch trend obliquely across the core of the central Kansas arch.
Examine the cross section of Fig. 5.13 and the map of Fig. 14.1.

The Ellis arch continued eastward as the Chautauqua to the Ozark
dome. The Chautauqua connection existed only at the close of the De-
vonian.

ELLIS ARCH (PRE-MI5SI3SIPPIAN) AND CENTRAL KANSAS
ARCH (POST MISSISSIPPIAN)

NEMAHA
RANGE

MILC3

Feci

BOURBON ARCH

Fig. 5.13. Section along central Kansas arch, Nemaha Range, and Bourbon arch, taken from cross section
by Betty Kellett (1932). Line of cross section shown on map of Fig. 14.1.

STRUCTURAL GEOLOGY OF NORTH AMERICA

Nemaha Range

A very sharp uplift, the Nemaha Range, trends south-southwest from
Omaha through southeastern Nebraska across Kansas into northern Okla-
homa, but is now buried. See Figs. 5.13 and 14.1 and Plate 6. It came
into mountainous relief during early Pennsylvanian time, because the
Mississippian strata are tilted up and truncated along its sides. Uplift
and dissection were sufficient to expose Precambrian crystalline rocks in
the core before burial. See cross section of Fig. 5.14. Structural relief is
3600 feet in the central part of the range, and the east flank is so steep
and straight that a block-fault movement has been visualized ( Lee et al.,
1946). The range was eroded rapidly so that the Pennsylvanian strata,
partly derived from the range itself, encroached on its flanks and, to-
gether with much exotic material perhaps in part from the early Ouachi-
tas, finally buried the range. The present depth of the "granite" at the
Kansas and Nebraska line is only about 400 feet (500 feet above sea
level), but at the Kansas and Oklahoma line, it is over 3000 feet below
the surface (2500 feet below sea level).

The Nemaha Range contrasts strongly with the central Kansas arch in
relief and symmetry. The Nemaha Range has 3600 feet of relief, whereas
the arch has 1500. The range has a very steep eastern front and gentle
back slope, whereas the arch is symmetrical and gentle. The nearly north-
south trend of the Nemaha Range is unlike the northwest trend of the
broad, gentle arches, and this sets it apart from the arches as a different
structural type. It resembles the Colorado Range of the Ancestral Rockies,
and therefore the characterization of it as a range is more appro-
priate than as an arch, anticline, or ridge, as it has variously been
called.

Bourbon Arch

Slightly north of the site of the previous Chautauqua arch, a later but
narrower one rose in early Pennsylvania time. It was probably a shallow
platform between the Forest City basin on the north and the Cherokee
basin on the south (Moore and Jewett, 1942). See Fig. 5.13.

Ozark Dome

At present, the Ozark dome is a broad, nearly circular area of Cambrian
and Ordovician limestones, surrounded by escarpments of Mississippian
limestone. In the east central part, knobs of pre-Cambrian crystalline
rocks project through the Cambrian and Ordovician strata to the surface.
The crystalline outcrops occur in southeastern Missouri, the area of the
St. Francis Mountains, and the strata dip everywhere away from them
(Croneis, 1930). The dome itself spreads over two-thirds of the state,
and also into northern Arkansas where the Boston Mountains make up
the southern flank. The Precambrian surface had considerable relief, and
the younger strata were deposited on it with initial dips in places up to
30 degrees (Bridge, 1930).

The first major unconformity in the Paleozoic succession around the
Ozark dome, especially on the west side in the Forest City basin, is at the
base of the St. Peter sandstone. Lee et al. (1946) summarize the subsur-
face geology in maps and cross sections and show that subsidence took
place in the Ozark region in pre-St. Peter time, while upwarping took
place in southeastern Nebraska and northeastern Kansas (the Nebraska
arch). The structural relief between basin and uplift was about 2000
feet.

With the coming of St. Peter time, the crustal movements were re-
versed, and the Ozark basin now started to rise as the Nebraska arch
started to subside. At the end of Silurian time, widespread erosion oc-
curred, with the greatest amount around the rising Ozark dome. The
Devonian strata not only rest on the truncated Silurian and older rocks
around the dome, but themselves in turn are truncated and covered by
the Mississippian strata.

The Mississippian overlap is most extensive and very well known from
many well records on the west side of the dome. Consult the geologic
map of the close of the Devonian, Plate 5, and cross sections of Figs. 5.15
and 5.16. The unconformity indicates that the dome was again uplifted in
late Devonian time and considerably eroded. The pre-Mississippian geo-
logic maps of the region (Moore and Jewett, 1942; Wrather, 1933; Lee
et al., 1946), together with surface outcrops, indicate that the Ellis arch,

NORTHERN PART OF SALINA BASIN

Fig. 5.14. Cross section of the Nemaha Range and Forest City basin. Reproduced from Wallace Lee er o/.,
1946. Note the several unconformities.

ST. FRANCOIS MOUNTAINS

HORIZONTAL SCALE

Fig. 5.15. Taken from Geologic Map of Missouri (1939). Section runs from Mt. Vernon to Perry-
ville. Mo. The wedge of Pennsylvanian strata at the left is added to show the Mississippian

and Pennsylvanian unconformity as it occurs about 50 miles north of the line of cross section.
Section B-B' on map of Fig. 14.1.

Fig.

Fi9-

5.16.
14.1.

Section across northeastern Oklahoma. Taken from White (1926, PI. 1). Section C-C on map of

CENTRAL STABLE REGION OF THE UNITED STATES

the Chautauqua arch, and the Ozark dome made up one continuous
broad arch which left the Transcontinental Arch at right angles and
veered eastward in southern Missouri.

The dome was uplifted again slightly in the late Mississippian (Plate
6). This time the movement was not in company with the Ellis and
Chautauqua arches, but apparently with the Hunton arch to the south-
west in Oklahoma (Dott, 1934). The great Pennsylvanian transgression
nearly, if not entirely, covered the dome (Plate 6), and no recurrences
of uplift during the Pennsylvanian or Permian have been described. The
Devonian and Mississippian uplifts left the dome wrinkled with very
gentle narrow folds that trend in a northwest direction.

The Arkansas Valley lies south of the Ozark dome and north of the
complexly folded and thrust-faulted Ouachita Mountains. It is a structural
basin as well as valley, and will be described in Chapter 14 under the
heading, "Ouachita System."

Cambridge Arch

A number of wells which have penetrated "granite" have been drilled
through the Pennsylvanian formations in a line running northwesterly
across Nebraska (Ballard, 1942). Isopach maps along this row of wells
suggest that the central Kansas arch, well known from many wells,
continues northwestward to the Black Hills and beyond to the south-
eastern corner of Montana. The arch across Nebraska is known as the
Cambridge arch (Plate 7). Geologic contacts determined from both sur-
face and subsurface data, however, do not reveal the arch, because it lies
mostly within the Precambrian rocks of the larger Transcontinental Arch
(Plates 4 and 5). No wells have yet been drilled to the Precambrian
northeast of the Cambridge-central Kansas arch, and therefore the bound-
aries of the pre-Pennsylvanian formations along the Transcontinental Arch
may have to be shifted considerably at a later date.

NORTHWESTERN INTERIOR BASINS AND ARCHES
Williston and Alberta Basins

The Williston basin was first thought of as a gentle Tertiary downwarp
in western North Dakota and eastern Montana, and was named after

the town of Williston, N.D., on the Missouri River. Cretaceous strata
were known to underlie the Tertiary and these to cover Paleozoic rocks
of the extensive region of South and North Dakota, Montana, south-
western Manitoba, and southern Saskatchewan. With the discovery of
commercial oil in 1951 in North Dakota, the term Williston basin became
applied to the Paleozoic strata more particularly than to the Tertiary
or Mesozoic, and with the drilling of many holes the distribution of
formations and systems has become well known. Isopach maps of the
several systems important in the Williston basin are shown in Figs. 5.17,
5.18, and 5.19.

A vast region in Alberta, western Saskatchewan, northeastern British
Columbia, and the Mackenzie area of the Northwest Territories is a
continuation of the Paleozoic sequence of the Williston basin, and the
accompanying maps show the close relationship of the geology of the
two regions, although they are generally treated separately in oil field
parlance. The term "Alberta shelf" has been applied to the Paleozoic
sedimentary province under the Great Plains of western Canada, because
it is a shallowing shelf region to the Cordilleran geosyncline or Alberta
trough on the west for most of the systems (Webb, 1954). It is also
commonly referred to as the Alberta basin as a region for oil exploration
and structurally as the Alberta syncline. During the Devonian period a
broad basin did develop (see Fig. 5.17C), but otherwise the region can
more properly be called a shelf. The syncline developed as the result of
Cretaceous and Tertiary subsidence, mountain building on the west, and
sedimentation, but the synclinal axis is not reflected under the Great
Plains in the thicknesses of any of the pre-Cretaceous systems.

The Cambrian strata are dominantly clastic with a sandstone generally
at the base and a sequence of green and maroon shales and light gray
calcareous siltstones and fine-grained sandstones above. These beds were
deposited unconformably on a Precambrian terrane as the seas invaded
the shield region from the west and southwest (Fig. 5.17A).

The Ordovician beds are extensive under the Williston basin but
generally absent on the Alberta plains. The outcrops in Manitoba contain
a 50- to 100-foot basal, white quartz sandstone with interbedded shales

STRUCTURAL GEOLOGY OF NORTH AMERICA

B. ORDOVICIAN

JL*T

MAN.

C DEVONIAN
"EVAPORITE BASIN

Fig. 5.17. Thickness map of Williston and Alberta basins: Cambrian, Ordovician and Devonian.
Cambrian, after Webb (1954) and Sloss (1950); Ordovician, after Webb (1954) and Sloss (1950);

and then a sequence of 400 feet of limestones and dolomites. The car-
bonates are the chief rocks encountered in wells; the basal elastics appear
to wedge out to the northwest (Webb, 1954).

The Silurian is represented in east-central Alberta Plains by an evapo-
rite sequence and is generally included with beds which may be Middle
Devonian. The Silurian and Middle (?) Devonian beds are the Elk
Point formation of the stratigraphic chart, Figs. 5.20 and 5.21, and con-
tain a composite salt thickness of 1200 feet in 1700 feet of beds. The Silu-
rian is present in Manitoba, North Dakota, much of Saskatchewan and
northern Montana, but with the Ordovician, is absent in the Sweetgrass
arch region. It consists of light yellowish gray and yellowish orange, finely
crystalline to dense dolomite (the Interlake group).

The Upper Devonian strata in western Canada are much more wide-

Devonian with evaporite region, after Webb (1954), Sloss (1950), and Baillie (1955).

spread than the Middle, and the original extent was still greater. Post-
Paleozoic erosion has removed the beds over considerable areas. The
Upper Devonian is characterized by thick deposits of limestones, dolo-
mites, shales, and evaporites. It marks a time of limestone reef growth
on widespread banks with numerous local bioherm and biostrom deposits
and abrupt facies, changes, all holding large oil reserves.

The Devonian succession of the Williston basin is shown on the chart
of Fig. 5.21, and its distribution in Fig. 5.17C. It is divided into four
major lithologic units, which in ascending order are, Elk Point group,
Manitoba group, Saskatchewan group, and Qu'Appele group. The lower
three are chiefly carbonates but the upper is composed of red shales
and siltstones. An extensive evaporite sequence occurs in the lower Elk
Point group and also in the Manitoba group. In north-central Montana

CENTRAL STABLE REGION OF THE UNITED STATES

Fig. 5.18. Thickness map of the Williston and Alberta basins: Lower and Upper Mississippian
and Pennsylvanian. Lower Mississippian (Kinderhookian, Osagian, and Meramecian series), after

a third evaporite sequence occurs at a still higher stratigraphic position,
in the top of the Jefferson.

The Mississippian beds which rest on an erosion surface on the De-
vonian are marked at the base by black shale in the Williston basin. The
Mississippian is more restricted in the Alberta region, but the beds
possibly extended east at the time of deposition as far as the present
margin of the Canadian Shield. The beds in Alberta start with a lower
dark gray calcareous shale or dark brown-gray argillaceous limestone
with fine-grained sandstone beds in the south. The upper beds are buff
crystalline to dense limestones. The succession in the Williston basin
beginning with the Kinderhookian and Osagian strata is largely lime-
stone. These beds make up the Lodgepole and Mission Canyon forma-
tions. The Meramecian is dominated by dolomites which compose the

Webb (1954) and Sloss (1950); Upper Mississippian (Chesterian), after Sloss (1950); Pennsyl-
vanian, includes Permian in Canada, after Webb (1954) and Sloss (1950).

Charles formation. See Fig. 5.22. The Charles contains considerable
thicknesses of evaporites. See map, Fig. 5.18D.

The Upper Mississippian or Chester beds lie in an east-west basin
through central Montana, called the Big Snowy. The eastern part of this
basin, however, is in the general region of the Williston basin and hence
it is considered part of the Williston. The strata are dominantly clastic
in contrast to the chemical precipitates of the Lower Mississippian and
compose the Kibbey, Otter, and Heath formations. Also part of the
overlying Amsden formation is Chester in age.

The Alberta shelf region was emergent and suffered long-continued
erosion during the Pennsylvanian. In the front ranges of the Rockies,
however, a thin sequence of sandy dolomites and quartzitic and cherty
sandstones are Pennsylvanian and Permian in age, and are known as

STRUCTURAL GEOLOGY OF NORTH AMERICA

t- PRESENT PRECAMBRIAN SURFACE

Fig. 5.19. Thickness maps of Williston and Alberta basins: Jurassic and Lower Cretaceous. Also
contour map on Precambrian surface. Jurassic, after Webb (1954), Francis (1957), and Peterson

the Rocky Mountain formation. Farther north in adjacent parts of Yukon
and Northwest Territories equivalent sandstones with a chert member at
the top attain a maximum thickness of 1200 feet. The erosion surface
on the Mississippian in the Peace River region has local sharp relief, and
beds believed to be Pennsylvanian and Permian cover the surface and
range up to 500 feet thick.

In the Montana and South Dakota area (see map, Fig. 5.18F) elastics
predominate over non-elastics, and clean quartzose sandstones are the
rule, making up the Quadrant sandstone in central and western Montana
and the Tensleep sandstone over the Wyoming shelf. In the southern
part of the Williston basin a wedge of Pennsylvanian is preserved, and
consists of dolomite interbedded with sandstone, red shale, and evapo-
rites.

(1957); Lower Cretaceous, after Webb (1954) and Reeside (1944); Precambrian surface, from
Tectonic Map of Canada (1954) and Moss (1936).

Triassic time was marked by widespread emergence, but in the Peace
River Country a thick sequence of marine elastics, impure limestones and
anhydrite accumulated. Thicknesses up to 3000 have been measured in
the adjacent Rockies.

A group of red beds has been charted across part of the Williston
basin by Ziegler ( 1956 ) . The beds lie between the Permian Minnekahta
limestone and the Piper beds of the Jurassic. See map, Fig. 5.22. A lower
shale and siltstone unit is thought to correlate with the Spearfish red beds
of the Black Hills which are Triassic, and three overlying units, a salt, a
siltstone and sandstone, and an upper salt are thought to be lower
Jurassic but may also be Triassic.

The Jurassic beds in Alberta have about the same distribution as the
Triassic except for a wider transgression in the southern Foothill belt

CENTRAL STABLE REGION OF THE UNITED STATES

TERTIARY

CRETACEOUS

JURASSIC

TRIASSIC

PERMO-PENNSYL

MISSISSIPPIAN

DEVONIAN

ORDOVICIAN

PRECAMBRIAN

Pliocene

Miocene

Oligocene

Eocene

Paleocene

■!■■*■ *»^ ^i H'Nw^t ^jlllMW^I%,UWll^'

Upper

Lower

Upper

Middle

Lower

Upper

Middle

Lower

inn

Upper
Lower

Upper

Middle
Lower

Upper

Middle

Lower

Upper

Middle

Lower

Upper

Middle

Lower

Front Range
and Foothills

Paskapoo

Edmonton
Belly River

Wapiabi

Bighorn (Cardium)

Blackstone

Blairmore

Upper Kootenay

Lower Kootenay
Fernie

Spray River

3SC L-l— L-l— l-i.-l.

Rocky Mountain

■^I^O^^^-l^^^.i^^U^M-^1'

Rundle
Banff

Exshaw
Palliser
Fairholme
Ghost River ?

[Ghost River ?]

Cathedral

Late Proterozoic
sediments

Central and
Southern Plains

Cypress Hills
Swift Current
Ravenscrag

I \ i i_[.i

Edmonton
Bearpaw
Pale Beds
Foremost
Pakowki
Milk River

Lea
Park

Alberta (Colorado)

Blackleaf (Viking)
member (Bow Id.)
Blairmore

Ellis group

Madison group

Exshaw

Wabamun (Potiatch)

Wmterburn (Jefferaon)

Woodbend

Beaverhill (Waterways)

Elk Point (upper)
(part)

Elk Point

(lower)
(part)

Upper Cambrian

Southeastern
Plains

Wood Mountain

Turtle Mountain

J>i i!«m'h L iJ ■ L i»k J ■

Boissevain
Riding Mountain

Boyne

Morden

Favel

Ashville

Swan River

Jl.l.l.1.1 1

Morrison ?
Sundance
Gypsum Springs
(Amaranth ? )

Charles
Madison
Kinderhook

Exshaw

Lyleton

Jefferson

Manitoban
Winnipegosan
Elm Point

group

Stonewall

Stony Mountain
Red River
Winnipeg

Chiefly Archean Intrusives and
met amorphics

Fig. 5.20. Generalized correlation chart of western Canada basin, southern part, after Webb,
1954.

Fig. 5.21. Devonian correlation chart of the Williston and Alberta basins. Reproduced from
Boillie, 1955.

near Calgary. There, a fairly thick succession representing Lower, Mid-
dle, and Upper Jurassic occurs. Over the Sweetgrass arch (Fig.
5.19G) only a thin marine sequence of shales and sandstones of Middle
and Upper Jurassic beds is present. These rest on an irregularly eroded
surface of the Mississippian. Peterson (1957) traces the depositional
history of western Montana, and for the intermittently positive area
where thinning and overlap occurred he uses the term Belt Island, but
explains that it was rarely emergent and then only in small areas. It had
been emergent in early Jurassic time and probably furnished some of
the clastic material for the adjacent Middle Jurassic formations. See
chart, Fig. 5.23. Another area that tended toward shoal conditions during
parts of mid- and late Jurassic time, although not emergent, was the
Sheridan arch. Middle and Upper Jurassic beds are widespread over the
Williston basin and define it in about the position of the older Mis-
sissippian basin but centered somewhat south of the Devonian basin.

Fig 5.22. Distribution of formational outcrops before Mesozoic strata were deposited unconform-
ably over the Paleozoic strata. The hachured line indicates extent of Triassic (?) red bed deposi-

tion (from Ziegler, 1956). Jurassic sediments spread over almost entire area. Map reproduced
from Francis (1956).

CENTRAL STABLE REGION OF THE UNITED STATES

By Jurassic time the rise of the Cordilleran geanticline had become ex-
tensive (see Plate 10 of Chapter 3), and considerable sediment was
shed from it eastward to the subsiding areas of accumulation. Part of
the geanticline became engrossed in major mountain building in Early
Cretaceous time, and this, The Nevadan orogeny, resulted, in British
Columbia, in the uplifting, disruption, and widespread intrusion of the
sedimentary rocks of the Cordilleran geosyncline. A new restricted trough
or longitudinal basin formed, as shown in Figs. 5.19 and 5.20, in about
the position of the present Canadian Rockies. The Nevadan Orogeny
engrossed the Selkirk Range on the west as well as a vast region west-
ward to the continental margin. The earliest Lower Cretaceous sediments
deposited were a thick coal-bearing series, the Kootenay formation, and
then after a brief erosion interval elastics of the Blairmore formation
spread eastward over the Kootenay and extensively over the Alberta shelf
region. See Fig. 5.19H. The coarse elastics along the foothills and front
ranges of the Rocky Mountains and maximum thickness there indicate
that the rise of the mountain belt on the west was rapid, and that it was
suffering active erosion.

The distribution of Upper Cretaceous sediments is about that of the
Lower Cretaceous and follows about the same pattern of thickening
westward into the trough. The Upper Cretaceous are much thicker than
the lower Cretaceous in the Williston basin and attain thicknesses of
4000 feet in eastern Montana and the western part of the Dakotas. The
Upper Cretaceous beds reflect the growth of the later Laramide Rockies
and become involved themselves in deformation. They, with a central
blanket of Tertiary beds, have been deposited and gently folded adjacent
to the major belt of mountain building on the west to form the Alberta
syncline.

A contour map of the pre-Paleozoic surface reflects the summation of
all subsidences and uplifts in the Alberta-Williston region, and it will be
seen (Fig. 5.191) that the center of the Williston basin is about at the
international boundary and the North Dakota-Montana line. All told, it
now holds over 7000 feet of sediment. Its position and extent are some-
what modified by the central Montana and Black Hills uplifts of Late
Cretaceous age. The Sweetgrass arch is a strong element of 4000 feet

Fig. 5.23. Jurassic correlation chart of the Williston basin and adjacent areas. Reproduced from
Peterson, 1957.

relief. The Alberta basin centers between Peace River and Edmonton,
and contains there in front of the disturbed belt over 10,000 feet of
sediments. Within the disturbed belt the thickness is much greater, and
had the Precambrian surface not been broken and deformed in the
Nevadan and Laramide orogenies it would lie very deep, indeed.

Utah-Wyoming Shelf

The Williston basin and its relation to the Alberta shelf has already
been described. Southward through central and eastern Wyoming and
the Colorado Plateau of Colorado and Utah relatively thin layers of
Cambrian, Ordovician, Devonian, Mississippian, Pennsylvanian, and
Permian strata occur. They represent the transition from the geosyncline
on the west to the Transcontinental Arch on the east. The influence of
the Ancestral Rockies and other land movements in Carboniferous time

STRUCTURAL GEOLOGY OF NORTH AMERICA

SOUTHEASTERN
IDAHO

WIND RIVER
MTNS.

DEVONIAN

NORTHERN
BLACK HILLS

I
WHITEWOOD DOLry
GREEN SHALE ft L
SCOLITHUS SS-7/

Stratigraphic diagram showing the relations of Cambrian and Ordo-
vician rocks between southeastern Idaho and the northern Black Hills.

Fig. 5.24. Relation of shelf in Wyoming to Transcontinental Arch and Cordilleran geosyncline. Repro-
duced from Thomas, 1949.

on the sites of deposition is shown on Plate 7, and Fig. 6.7. Figure 5.24
is a cross section to illustrate the shelf and its relation to the Trans-
continental Arch and the Cordilleran geosyncline.
The formations of the Wyoming and Montana part of the Utah-

Wyoming shelf differ somewhat from those of the Utah and Arizona
part. The formations have been the object of numerous stratigraphic
studies because of their importance as oil and gas producers. See cor-
relation charts listed in Chapter 1.

PALEOZOIC

CORDILLERAN GEOSYNCLINE

DIVISIONS AND THEIR CHARACTERISTICS

Schuchert is probably more responsible than anyone else for the use
of the expression Cordilleran geosyncline in describing the basins of
accumulation of sediments along the western margin of the continent.
He also used the term Rocky Mountain geosyncline. During the Mesozoic,
his "Cordilleran intermontane geanticline" split the overall broad and
irregular basins into two longitudinal divisions, but before the geanticline
became pronounced, the divisions were already evident by the nature of
their sediments, the western being an eugeosynclinal assemblage and the
eastern a miogeosynclinal. The eugeosyncline extended from mid-Nevada

to the Pacific Coast, and the miogeosyncline from mid-Nevada to central
Utah (Fig. 6.1). The miogeosyncline is much better known than the
eugeosyncline. The basins of sedimentation and geography shifted some-
what from one period to another, but the broad overall relations remained
fairly constant. The change from the thick sedimentary sequence of the
miogeosyncline to the thin sediments of the shelf has been called the
Wasatch line (Kay, 1951), and for all Paleozoic periods except Silurian
the change is fairly abrupt and in much the same position. The broad
divisions as outlined were probably first recognized by Stille (1941) and
later elaborated on by Kay (1942, 1951, 1960) and Eardley (1947).

The eugeosyncline probably sank more and received a greater thickness
of sediments than the miogeosyncline, but the extent of sediments in
both was great. The major difference lies in the character of the sedi-
ments. The eugeosyncline received a dominant amount of volcanic
material and graywacke, whereas the miogeosyncline was filled with
sandstones, quartzites, shales, limestones, and dolomites. The volcanic
material in the eugeosyncline is in several forms: flows, volcanic con-
glomerates, and various pyroclastics. The volcanics and graywackes occur
in every stratigraphic system from Upper Cambrian to Cretaceous. The
Permian especially was a time of excessive volcanism, and the volcanics
of that period have been traced from California and western Nevada to
Alaska (Wheeler, 1939; White, 1959). In the Humboldt Range of north-
western Nevada, over 10,000 feet of Permian strata, largely volcanic, have
been identified.

Roberts et al. (1958) estimate that the miogeosynclinal strata in east-
ern Nevada and western Utah above the thick basal quartzite of the
Cambrian consist of 60 per cent limestone, 30 percent dolomite, 8 percent
shale, and 2 percent quartzite. They estimate that the eugeosvnclinal
strata, on the other hand, in the Sonoma Range and vicinity consist of
20-40 percent shale, 10-30 percent sandstone, graywacke, and quartzite,
up to 30 percent of chert, with shale partings, and up to 30 percent of
volcanic and pyroclastic rocks.

The units are characteristically lenticular, and thin or thicken abrupdy
parallel with and normal to the geosynclinal trend. Limestone, generally shah'
or sandy, locally forms thin, discontinuous layers. The shale units are commonlv

STRUCTURAL GEOLOGY OF NORTH AMERICA

Fig. 6.1. Major Paleozoic tectonic elements of western United States. The eugeosynclinal
bondary was farther east than shown in Permian time. The Wasatch line through southern
Nevada has been called the Las Vegas line (Welch, 1959).

sandy and few are calcareous. The quartzites are generally nearly pure, but
the sandstones are either graywackes or feldspathic sandstones. The chert units,
partly of volcanic derivation, range from a few inches to several hundred feet
thick; individual chert layers are lenticular and range from a fraction of an
inch to 3 feet. They are separated by shaly partings which are also lenticular;
laterally, chert units grade into siliceous shale units with subordinate chert.
The volcanic rocks are largely andesitic or basaltic pillow lavas and pyroclastics
that accumulated mainly in a marine environment; most are highly albitic.
Siliceous pyroclastic rocks locally form thick sections. The volcanic rocks are
highly lenticular, and probably formed around many source centers (Roberts
etal, 1958).

Another characteristic of the sediments of the eugeosyncline is their
metamorphism. The thick sequences, especially in the Sierra Nevada,
Klamath Mountains, western British Columbia, and southeastern Alaska,
are made up of phyllites; slates; argillites; quartz, chlorite, hornblende,
and calcareous schists; hornblende gneiss; recrystallized chert; marble;
meta-conglomerate; meta-andesite; and various metamorphosed pyro-
clastics. Still another characteristic is the presence of great intrusive
bodies of later age, and the metamorphism of the sediments about the
intrusions.

The sediments of the miogeosyncline, on the other hand, are not
metamorphosed. Many of the sands are cemented with silica and termed
quartzite, but little dynamic metamorphism incident to Paleozoic,
Mesozoic, or Tertiary orogeny has occurred.

The medial belt in central Nevada contains transitional types of the
two environments, and became not only a geanticline but a belt of
orogeny in late Devonian time. The western eugeosynclinal strata were
thrust many miles eastward to rest on miogeosynclinal strata of strikingly
different lithology.

BASINS AND UPLIFTS OF THE WESTERN UNITED STATES
AND SOUTHERN BRITISH COLUMBIA

Cambrian Basins

The miogeosyncline is noted for its Cambrian sections (Fig. 6.2). At
one locality in southern Nevada and California 17,000 feet of Lower,
Middle, and Upper Cambrian beds have been measured.

PALEOZOIC CORDILLERAN GEOSYNCLINE

The oldest Cambrian rocks over much of eastern and southern Nevada
and southwestern Utah is the Prospect Mountain quartzite, which may
be over 5000 feet thick in places. The Osgood Mountain quartzite in
north-central Nevada, the equivalent of the Prospect Mountain, may be
as much as 10,000 feet thick. Overlying the quartzite are shale, dolomite,
and limestone formations of uniform and wide occurrence. Stratigraphic
sections from the eugeosyncline to the miogeosyncline of north-central
Nevada are shown in Fig. 6.9, and of the miogeosyncline of western and
northern Utah in Figs. 6.9 and 6.10.

In southeastern British Columbia is another succession of Cambrian
strata which totals about 10,000 feet in maximum thickness. From the
Burgess shale of this succession Wolcott took an amazing assortment of
fossils and greatly enriched our knowledge of life at the beginning of
Paleozoic time. Lower Cambrian beds are absent at the international
boundary, but further north in the Mount Robson vicinity they are
present and consist of 3900 feet of quartzitic sandstone, siliceous shale,
and limestone. Upper Cambrian strata are restricted and consist mostly
of limestone ( Lord et al., 1947 ) .

Another thick Cambrian sequence is known in northeastern Washington
where at least 12,000 feet of beds dated by fossils occur. The Gypsy
quartzite lies at the base; over this is the Maitlen phyllite, and over this
the Metaline limestone (Park and Cannon, 1938; Campbell, 1947). The
assemblage is miogeosynclinal in aspect and contains elements of the
same fauna as the miogeosyncline of western Utah and eastern Nevada
( Wm. Lee Stokes, personal communication).

Representative of the eugeosynclinal assemblage in Cambrian time is
the Scott Canyon formation in Battle Mountain. It is composed of green-
stone, chert, and some shale, and is about 5000 feet thick ( Roberts et al.,
1958).

Lower and Middle Cambrian sediments are just about entirely re-
stricted to the geosyncline, but Upper Cambrian strata are spread widely
over the Central Stable Region of the United States as far as Wisconsin
and Ohio. Here they are overlapped by Ordovician sediments which
extend to the north and northeast over the Precambrian rocks of the
Canadian Shield.

CAMBRIAN

Fig. 6.2. Thickness and paleographic map of the Cambrian.

STRUCTURAL GEOLOGY OF NORTH AMERICA

ORDOVICIAN

Fig. 6.3. Thickness and paleographic map of the Ordivician.

In Chapter 4 the Precambrian Mazatzal and Reltian orogenic belts have
been described. Although the Beltian trough of sedimentation and later
belt of orogeny marked the first tectonic development parallel with the
present Pacific margin of the continent, the older Mazatzal belt seems to
have made an impress on the Paleozoic geosynclinal basins. The Trans-
continental Arch, which reflects the Mazatzal orogenic belt, borders
directly on the arch in southeastern Utah, Arizona, and Colorado, and
the two have the same trend to the southwest. See maps, Figs. 6.1 to
6.5

An uplift here called the Raft River geanticline, is identified in north-
western Utah (Stokes, 1952; Felix, 1956) and southwestern Montana
( Scholten, 1957) on the south and north sides of the Snake River volcanic
field respectively (see Fig. 6.11). Its extent northwestward cannot be told
because of the cover of Tertiary volcanic rocks and the intrusion of the
great Idaho batholiths, but in the interpretation rendered on Fig. 6.2 it
appears as a geanticlinal uplift between the eugeosyncline basin in north-
ern Nevada and the miogeosyncline of Utah. An unconformity in the
Upper Cambrian detected in the South Stansbury Mountains (Rigby,
(1958) with 700 feet of beds removed may be a lateral affect of the Raft
River geanticline (see Fig. 6.10). The erosion surface lies beneath the
Cole Canyon dolomite.

Still farther north in northwestern Montana, northern Idaho, and
British Columbia is an extensive region of Precambrian strata, the Belt
series, and this is here interpreted to have been a fairly persistent struc-
tural feature from Cambrian time on. Evidence cannot be sited for
shoreline deposits and overlapping relations, but this is mostly due to
the extensive batholithic intrusions and metamorphism. Early geologists
considered the Beltian terrane the shore of an extensive, west-lying land
which they called Cascadia, but later ones have considered the Paleozoic
strata, beginning with Middle Cambrian, to have been deposited across
and then eroded away incident to the emergence of the modern gean-
ticline in Cretaceous and Tertiary times. Sloss ( 1950 ) however, suggests
a small uplift there, and his interpretation is reflected on the maps of the
Williston basin, Figs. 5.17, 5.18, and 5.19. The writer takes the view that
it has been a significant feature from Cambrian time on ( see Chapter 33 ) .

PALEOZOIC CORDILLERAN GEOSYNCLINE

No Cambrian or Ordovician fossils have been found in northern Cali-
fornia, Oregon, and all Washington except the northeast corner. The
lack of information about the western margin of the continent in Cam-
brian time, and in Ordovician as well, is disappointing. The oldest fossils
yet discovered along the Pacific margin in the United States and British
Columbia are Silurian. These have been found in the Klamath Mountains
by Wells (1956). Three metamorphic series underly the fossiliferous
Devonian strata there, according to Hinds (1939), and one or more of
these might be Ordovician and Cambrian. See Fig. 6.3. In southeastern
Alaska Buddington reports Ordovician fossils, but no Cambrian. In con-
clusion it may be assumed that the entire region west of central Nevada
was eugeosynclinal from Ordovician time to the close of the Paleozoic.

Ordovician Basins

A broad Ordovician basin exists in western Utah and Nevada with
miogeosynclinal type sediments in the eastern and eugeosynclinal type in
the western part ( see Fig. 6.3 ) . The formations and their lithologies are
shown in Fig. 6.9, which is a section across central Nevada and marks the
change from the eugeosyncline to the miogeosyncline. The miogeo-
synclinal sediments of western Utah are reviewed by Hintze ( 1951 ) and
summarized in the table of Fig. 6.12.

Another basin, which was narrower and completely miogeosynclinal in
character (Fig. 6.12, Logan area), existed in southeastern Idaho and
northern Utah. For a review of the stratigraphy see Ross ( 1953 ) . In
both basins the rocks are dominantly limestones and dolomites, but con-
spicuous quartzite formations exist in each. The Swan Peak quartzite
of southeastern Idaho and northern Utah is about 500 feet thick, and the
Eureka quartzite and the Swan Peak quartzite of western Utah and
eastern Nevada are nearly 800 feet thick together. The Eureka quartzite,
537 feet thick at Ibex, Utah, overlies an 85-foot dolomite member, and
this overlies the Swan Peak quartzite, 249 feet thick. The dolomite mem-
ber wedges out east of Ibex, and there the upper quartzite rests directly
on the lower. The absence or near absence of these sandstones together
with a thinner Ordovician section in Utah southwest of Great Salt Lake
indicates an uplift there which Webb (1958) has defined and named the

Fig. 6.4. Thickness and paleogeographic map of the Silurian.

STRUCTURAL GEOLOGY OF NORTH AMERICA

Tooele arch. The arch and erosion is pre-Fish Haven (see Fig. 6.12).

A deep and evidently narrow trough of Ordovician sediments exists in
the Canadian Rockies of western Alberta and eastern Rritish Columbia.
It is interpreted to lie east of the Reltian geanticline and to be separated
by it from the basin of northeastern Washington containing the Ordo-
vician Ledbetter slate, also of miogeosynclinal type. The Ordovician strata
of the Canadian Rockies consist of 3000 to 7000 feet of limestone, shale,
and slate beds with fossils representing a range from Lower to Upper
in different places ( Lord et al., 1947 ) .

According to Roberts et al. (1958):

Rocks of Ordovician age that belong to the western assemblage (eugeosyn-
cline) are widely exposed throughout north-central Nevada. They underlie
large areas in the Sulphur Spring Range, Roberts Mountains, Tuscarora Moun-
tains, Cortez Mountains, northern Shoshone Range, Toyabe Range, Batde Moun-
tain, and the Sonoma Range. So far as known they are allochthonous.

Merriam and Anderson (1942, p. 1694) used the name Vinini formation
for rocks of Ordovician age of the western assemblage in the Roberts Moun-
tains. They divided the formation into two units. The lower part of the Vinini,
Early Ordovician in age, consists of quartzite, limestone, and calcareous sand-
stone, and silty and shaly sediments with minor amounts of andesitic lava flows
and tuffs; perhaps the relatively abundant limestone here suggests an approach
to the transitional assemblage. The upper part of the Vinini, of Middle
Ordovician age, is composed of bedded chert and black organic shale, clearly
of normal western lithologic type.

The most complete stratigraphic section of the Vinini formation thus far
seen is in the Tuscarora Mountains, northern Eureka County, about 5 miles north
of U.S. Highway 40. Strata of Early, Middle, and probably late Ordovician
age are present; no detailed measurements were made, but it is estimated that
the section is at least 7,000 feet thick.

In the Shoshone Range, Battle Mountain, and Sonoma Range the proportion
of massive quartzite, chert, and volcanic material in the Ordovician rocks of
the western assemblage is larger than in the Vinini formation. These rocks were
named the Valmy formation in Battle Mountain (Roberts, 1949, 1951) where
they have been subdivided into two members. The lower part of the Valmy
consists mainly of rather pure, generally light-colored quartzite, dark gray and
greenish chert, some gray to black siliceous shale, and a significant amount
of greenstone. The upper member consists principally of dark thin-bedded
chert interbedded with dark shale and a little greenstone. The base of the
Valmy is concealed but at least 4,000 feet is present. The upper beds of the
Valmy are highly contorted, but are estimated to be 3,000 or more feet thick.
[Refer also to Ross (1961).]

In the shelf region the Transcontinental Arch was nearly completely
emergent, or at least no Ordovician strata occur on it under a Devonian
and Mississippian cover, except for the Colorado sag. This embayment
probably did not extend all the way through to the western geosyncline
or the Williston basin because in the eastern Uinta Mountains of Utah
the Mississippian beds (possibly Devonian) rest directly on the Cam-
brian.

The ancestral Sweetgrass arch was broadly emergent and well-defined.

Silurian Basins

The Silurian seas were more restricted than any others in Paleozoic
time. The Laketown dolomite of northern Utah and southeastern Utah
has been traced widely over western Utah and is the sole representative
of the Silurian thus far recognized there. In eastern and central Nevada
the Roberts Mountain formation and overlying Lone Mountain dolomite
correlate with the Laketown. The entire section is carbonate rock, and
over half of it is dolomite (see Figs. 6.9 and 6.12).

Silurian rocks of eugeosynclinal aspect appear to be widespread in
north-central Nevada, but because they resemble the Ordovician units
they may not have been recognized in mapping.

On the east side of Pine Valley about 8 miles south of Carlin, unnamed
black shale and tawny to buff tuffaceous shale and calcareous shale have
yielded Monograptus determined by R. J. Ross, Jr., to be of Silurian age.
The thickness of these beds is not known.

Black shale containing Monograptus is reported by C. W. Merriam (oral
communication) from the vicinity of McClusky Pass in the northern part of
the Simpson Park Mountains. C. A. Nelson (oral communication) also reports
Monograptus in shale on the east side of Pine Valley near Mineral Hill. On
the west side of the Tuscarora Mountains in the valley of Mary's Creek,
graptolites that according to R. J. Ross, Jr., have affinities with Silurian forms
were collected by Roberts in 1954. Silurian strata (R. J. Ross, Jr.), including
about 4000 feet of sandstone, arkose, shale, and a little chert, from part of
the overriding plate of the Roberts Mountains thrust in the northern Shoshone
Range and in the Cortez Mountains.

The beds containing graptolites of Silurian age are on the whole less cherty,
and contain more calcareous shale and limestone layers than the Vinini and
Valmy formations. On the other hand, the Silurian beds of the western
assemblage appear much less calcareous than the Silurian of the transitional

PALEOZOIC CORDILLERAN GEOSYNCLINE

'assemblage. The western rocks contain some siliceous pyroclastics, which have
not been recognized in the other assemblages (Roberts et al., 1958).

Silurian strata have been recognized in the northern Klamath Moun-
tains by Wells et al. (1951), and rest on highly foliated schists which
may be metamorphosed Ordovician and Cambrian or Precambrian in
age. The Silurian beds had formerly been considered Devonian, but
patches of Devonian limestone of undetermined stratigraphic relations
crop out nearby. The sequence of units now recognized by Wells and co-
workers is as shown in Fig. 6.13, and is compared with the assemblage
of rock units in the southern Klamath Mountains. Since no Ordovician
or Cambrian beds are yet known west of north-central Nevada, the
possibility of correlating the Salmon and Abrams schists with the Ordo-
vician and Cambrian is suggestive. The Copley and Chanchellula are
questionably correlated with the "Silurian strata" of the northern Klamath
Mountains.

Devonian Basins

The Devonian basins are in much the same pattern as the Ordovician
although the strata are not so thick. The Transcontinental Arch in Utah
and Arizona was more widely covered, however (Fig. 6.5).

Although Devonian strata are found nearly everywhere west of the
Transcontinental Arch ( Rrooks and Andrichuk, 1953 ) , they are over 1000
feet thick only in the western part of the general Rocky Mountain area.
In the Roberts Range, Nevada, Merriam ( 1940 ) has described 4465 feet
of Devonian beds, and at nearby Eureka he has found 4000 to 5000 feet
of them. They are composed chiefly on limestones and dolomites, their
fossil content indicates a rather complete section, and the broad trough
in which they accumulated subsided during most of Devonian time. ( See
Fig. 6.9.)

Devonian rocks of the Sulphur Spring and Pinyon ranges have been recently
described by Carlisle and others, who showed that northward from the
Roberts Mountains the Nevada-Devils Gate sequence thickens, becomes more
dolomitic, and less fossiliferous. The sequence contains vitreous quartzite units
as much as 400 feet thick that grade into carbonate quartz arenites and thus
resembles the Devonian section near Eureka more than the section at Lone
Mountain.

DEVONIAN

Fig. 6.5. Thickness and paleogeographic map of the Devonian. Antler orogenic belt, Sta
bury anticline, and Beaverhead dome made appearance in Late Devonian. Most of sedime
are Middle Devonian.

ns-

nts

STRUCTURAL GEOLOGY OF NORTH AMERICA

Devonian rocks of the western assemblage appear to be widespread through-
out north-central Nevada, but are most abundant from the Shoshone Range
eastward. These lack the basic volcanic flows and pyroclastics characteristic of
Cambrian and Ordovician rocks of the western assemblage, but locally contain
silicic pyroclastics, much chert and shale, and a litde calcareous shale.

In Slaven Canyon in the Shoshone Range and elsewhere in the Mt. Lewis
Quadrangle, there are at least 4,000 feet of strata composed dominantly of dark
gray to black chert with some dark shale, a little sandstone, and very small
amounts of limestone. These have yielded ostracods and conodonts of Middle
Devonian age. Similar rocks on and south of Bald Mountain, in the northern
Toyabe Range southwest of Cortez, are probably correlative.

Tuffaceous shale and calcareous shale on the east side of Pine Valley about
8 miles south of Carlin have also yielded conodonts of Devonian age. These
rocks are associated with Silurian and Ordovician rocks in the upper plate of
the Roberts Mountains thrust (Roberts et al., 1958).

In the southern Klamath Mountains siliceous black shales and slates
containing thin beds of sandstone and fossiliferous limestone, now largely
recrystalized, make up the Kennett formation of Devonian age. It crops
out in two restricted belts, and rests unconformably on the older rocks.
Devonian strata are not known in the Sierra Nevada or Coast Ranges
south of the Klamath Mountains in California.

Late Devonian Orogeny

Toward the end of the Devonian period, according to Nolan ( 1943 ) , a
geanticline began to rise in central Nevada, approximately along the
transition zone of eugeosynclinal and miogeosinclinal sediments. See Fig.
6.5. The uplift divided the geosyncline into a western and an eastern
trough, and the distribution of Devonian sediments is reflected in two
ways, viz., by the almost complete removal of the earlier Devonian
deposits along the axis of the arch, and by an eastward shift to the vicinity
of Eureka, Nevada, of the zone of maximum sedimentation. The geanti-
cline was later named the Manhattan (Eardley, 1947). Since then a large
amount of significant field work has been done and the geanticline has
come to be recognized as a belt of major orogeny, and has been called
the Antler orogenic belt ( Roberts et al., 1958 ) .

At the close of the Devonian, fundamental changes took place along
the western part of the area of predominantly carbonate deposition

(miogeosyncline). The carbonate rocks were folded and overridden by
the Roberts Mountains thrust plate that brought clastic and volcanic
rocks of equivalent age but different facies from the west or northwest.
Clastic rocks eroded from the rising upland in the west marked the end
of the broad geosyncline in north-central Nevada as it had existed earlier,
and introduced a change to narrow straits and embayments in the
orogenic belt during the remainder of the Paleozoic. The clastic rocks
do not resemble the assemblages laid down in the geosyncline during
early and middle Paleozoic, but overlap all of them. On the west, over-
lapping rocks rest with angular unconformity on rocks of the western
and transitional assemblages; in the Carlin area, west of Elko, the un-
conformity is much less marked; and on the east, the discordance fades
out and the overlapping rocks interfinger with the eastern assemblage
rocks and grade eastward into the carbonate section of late Paleozoic
age of eastern Nevada and western Utah. Examine Figs. 6.9, 6.14, and
6.15.

In latest Devonian or earliest Mississippian time a sharp anticline rose
in the site of the Stansbury Range of west-central Utah. It was eroded
down to the Cambrian before early Mississippian seas covered it (see
Fig. 6.12). Coarse slide debris accumulated on its northwest flank, and
sand dunes were blown northward for several miles to build a sandstone
unit several hundred feet thick. The angular unconformity and the com-
pleteness of the anticline, about 30 miles long and 5 miles wide as mapped
by Rigby ( 1958 ) , are particularly impressive.

No Devonian strata are known in the Raft River Mountains of north-
western Utah; only Pennsylvanian strata in fault contact with Precam-
brian rocks have been mapped, and the Devonian relations have not
been specifically deciphered (Felix, 1956). Small remnants of al-
lochthonous Paleozoic (?) strata occur on the Precambrian rocks, and
the possibility exists that this area may be a continuation of the Stans-
bury anticline and a belt where orogeny was more severe than to the
south. The belt may join the Antler orogenic belt to the northwest. See
Fig. 6.5. More details of the Antler orogenic belt will be given in the
discussion of the Mississippian, Pennsylvanian, and Permian strata.

PALEOZOIC CORDILLERAN GEOSYNCLINE

Mississippian Basins

Major miogeosynclinal deposits extend from the Big Snowy basin of

: Montana in a fairly narrow trough southward through eastern Idaho

into Utah and then southwesterly into southern Nevada. The greatest

thickness is reached in the Lemhi and Lost River ranges of Idaho (Figs.

'6.6 and 6.11).

Characteristic formations of the trough are shown in Fig. 6.16. In
summary of the strata of the eastern trough it may be said that they
consist mostly of limestones, but that the limestones grade into a thick
shale (now argillite) section in Idaho, which may savor of the eugeosyn-
cline. Also the Manning Canyon shale of western Utah is thick (1100
feet) and marks the transition from the Mississippian to the Pennsyl-
jjvanian. For references see Scholten (1957), Morris (1957), and Gilluly
•i(1932).

The change from shelf to miogeosyncline is shown in Figs. 6.11 and
6.17. The Raft River geanticline just southwest of the Montana-Idaho
^boundary is well illustrated in Fig. 6.11.

Antler Orogenic and Post-Orogenic Stratigraphy

Coarse elastics in places 10,000 feet thick were spread eastward and
westward from the Antler orogenic belt, and overlap the pre-existing
:3ugeosynclinal, transitional, and miogeosynclinal assemblages. According
to Roberts et al, (1958):

The lithologic character of the overlap assemblage is variable from place to
ilace, and different names have been applied to correlative beds. In the east,
he Eureka-Carlin sequence includes the Chainman shale, Diamond Peak
formation, Ely limestone, Carbon Ridge, and Garden Valley formations of the
iureka area, and correlative formations in the Carlin area. In the west, the
Antler sequence includes the Battle formation, Highway limestone, Ander Peak
iimestone, and Edna Mountain formation. Because of local variations in source
ireas, in conditions of deposition, and subsequent history of these rocks, it is
mpossible to make precise correlations of the units in the different sequences.
Regional lithologic similarities indicate, however, that similar environmental
jjonditions prevailed over broad areas. The Havallah formation of the Sonoma
md East ranges was probably laid down 50-100 miles west of the orogenic
|)elt and was thrust eastward into juxtaposition with the Antler sequence during
vlesozoic orogeny. It therefore has had a somewhat different history and is

MISSISSIPPIAN

Fig. 6.6. Thickness and paleographic map of the Mississippian. A-S.G. AR. is Apishapo-
Sierra Grande arch. Uncompahgre and Colorado uplifts first became emergent in latest Missis-
sippian, and developed into major ranges in Early Pennsvlvanian.

STRUCTURAL GEOLOGY OF NORTH AMERICA

not strictly comparable with the approximately contemporaneous Eureka-
Carlin and Antler sequences.

The basal sediments of the overlap assemblage differ in age throughout
north-central Nevada. In the Eureka area the intertonguing Chainman shale
and Diamond Peak formation of Late Mississippian age are the earliest orogenic
sediments recognized. In the Carlin area the Tonka formation of Dott (1955,
pp. 2222-33) and correlative units farther southeast in Pine Valley mapped by
J. Fred Smith and Keith Ketner included Lower Mississippian clastic beds
that overlap the upper plate of the Roberts Mountains thrust fault, indicating
that the thrust reached the Carlin area during Late Devonian or Early
Mississippian time.

Orogenic movements continued along the belt in Pennsylvanian and
Permian time, and also throughout the Mesozoic. Examine the structure
cross sections of Chapter 17, Figs. 17.3-17.6.

Walter Sadlick and F. E. Schaeffer (personal communication) recog-
nize an angular unconformity at the base of the Chainman formation in
western Utah and are calling the disturbance represented by it the
Wendover phase of the Antler orogeny. They are of the opinion that
this time (early Valmeyer of the early Mississippian) marks the begin-
ning of the Antler orogeny. They recognize beveled folds covered by the
Chainman, and the axes of the folds trend to the northwest.

Klamath Mountains and Sierra Nevada

The Mississippian is made up of two formations in the Klamath Moun-
tains, the Bragdon and the Baird (Fig. 6.13). They are probably the most
widespread formations in the region. The Bragdon is chiefly shale and
slate, generally gray, in contrast to the black shale and slate of the older
Kennett formation of Devonian age. Some sandstones are conglomeratic
near the base and contain fragments of both the Kennett and Copley
formations. Within the Redding quadrangle, a volcanic sequence called
the Bass Mountain basalt is present. The Bragdon may exceed 6000 feet
in thickness in places. The Bass Mountain volcanic sequence contains
many tuff beds. Its position on Bass Mountain, according to Hinds ( 1939),
is in the lower part of the Bragdon formation.

The Baird formation consists largely of sandstone and tuff, but the
upper part has calcareous and siliceous slates. It is about 700 feet thick
and apparently rests conformably on the Bragdon (Hinds, 1939).

In the northern Sierra Nevada, the metamorphic Calaveras formation
of Carboniferous age is widespread. It consists chiefly of black phyllite
with subordinate fine-grained quartzite, limestone, and chert. Associated
and in part interbedded with the formation are green amphibolite schists
of contemporaneous age. From fossils, found chiefly in the limestone, the
Calaveras formation is known to be at least in part of Carboniferous
age, but parts of it as mapped may be Devonian and Triassic. Because
of the metamorphosed condition of the rocks in which the fossils are
found, it has been difficult for paleontologists to determine to what part
of the Carboniferous the faunas belong. Groups of Calaveras fossils from
the Taylorsville region are more closely related to the Baird, now recog-
nized as Mississippian, than to the McCloud limestone, now believed to
be Permian.

The amphibolite schists were originally fine pyroclastics (Knopf, 1929).
The bedded rocks are most abundant in the northern Sierra Nevada,
but southward become increasingly metamorphosed, and progressively
greater areas are occupied by granitic intrusives. In the Tehachapi Moun-
tains and the southern Coast Ranges, pre-granitic rocks are present, but
highly altered.

A thick sedimentary deposit, now schist, in southern California, has
yielded Mississippian fossils ( Larsen, 1948 ) . The sequence appears to be
miogeosynclinal in type and at the same time seemingly out of place in
the geosynclinal setting.

Pennsylvanian Basins

Of the miogeosyncline the Oquirrh basin is the most striking feature
of Pennsylvanian and Permian time. It appears to have been a sharp and
small basin in which over 15,000 feet of strata accumulated. The thickest
section is in the Provo part of the Wasatch Mountains of central Utah
where Baker ( 1947) reports 26,000 feet of beds. The upper 9800 feet is of
Permian age. A short distance to the southeast 20,000 feet of beds have
been estimated in the Mt. Nebo district (Eardley, 1934), and in the
range to the west, the Stansbury, 15,000 feet (Rigby, 1958). The basin
has been contoured with a northwest trend and an abrupt northeast
margin (Stokes and Heylmun, 1958). This permits the interpretation

PALEOZOIC CORDILLERAN GEOSYNCLINE

: -

that the Uncompahgre Range of the Ancestral Rockies (Chapter 15)
extends through in subdued form to a small uplift in northwestern Utah.
The sharp margin was not a fault scarp, however, because no coarse
flanking debris is known as in Paradox basin. The conspicuous change
from shelf to basin is illustrated in Fig. 6.17. The basin was filled, at least
on the north by progressive overlap from south to north, with the oldest
Pennsylvanian Morrowan sediments on the Manning Canyon shale on
the south and with Atokan, Desmoinesian, and Missourian successively
deposited on the shale to the north (Rigby, 1958). Limestone and sand-
j stone are the principal lithologies in the thick succession, and cyclical
■j sediments dominate the Desmoinesian section in the Stansbury Moun-
tains. Quartzite and sandstone dominate over limestone in the Missourian
and Virgilian section.
A deep and evidently large basin developed in Idaho in which the
i Wood River formation accumulated possibly 12,000 feet thick. The forma-
' tion extends westward from the Lost River Range an unknown distance.
i The shelf deposits in southwestern Montana are represented by the
Quadrant quartzite which attains a maximum thickness of 2600 feet
(Scholten, 1957). The Wood River contains fusilinids of Desmoinesian,
i Virgilian, and Wolfcampian ages (Rostwick, 1955), and therefore was
deposited simultaneously with the upper part of the Oquirrh formation.

The basal Wood River consists of several hundred feet of conglom-
erates, consisting of angular to well-rounded chert and quartzite pebbles.
Dark arenaceous limestone beds overlie the conglomerate, and then the
rest of the formation, which is the bulk of it, is a monotonous sequence
of calcareous sandstones and sandy limestones. Recrystallization and re-
placement are common. The sandstones are mostly made up of quartz
jgrains with 5 percent or less of feldspar, moscovite, magnetite, and
Jzircon. The formation is characterized as miogeosynclinal by Rostwick.
Although the sandstones may resemble those of the Quadrant to the
'east, it is difficult to see how the conglomerate could have been derived
rom an eastern source and transported over the region of sand deposition.
jft seems more logical to think of the chert and quartzite pebbles coming
rom the west, and thus the inference is drawn that the Antler orogenic
'Selt extended from Nevada northward through central Idaho, and

was the source of the conglomerate and, possibly, of much of the sand.

The relation of Pennsylvanian rocks to the Antler orogenic belt is
diagrammed in Figs. 6.14 and 6.15.

In Nevada the Pennsylvanian rocks, like the underlying Mississippian
are particularly thick east of the orogenic belt, but not quite so coarse.

Basal beds in the overlap assemblage near the orogenic belt, expecially in
Mississippian and Early Pennsylvanian, are usually coarse conglomerates which
grade laterally into finer conglomerates and sands, then into silt, clays, and
limestone. These clastic beds may be terrestrial locally within the belt, but
they are mainly marine adjacent to it. The belt may have been largely sub-
merged at times, for widespread marine limestone units interfinger with the
elastics. The lenticularity of the overlap sediments as a whole suggests deposi-
tion in several separate basins, possibly in a series of straits separated by
peninsulas and islands. The presence of coarse elastics throughout much of
the Pennsylvanian indicates continued orogenic activity from time to time,
perhaps continuing into the Permian (Roberts et al., 1958).

Volcanoes were active west of the orogenic belt as attested by the
presence of volcanic materials particularly in the Pumpernickel and
Havallah formations. These deposits are believed by Roberts et al. to
have been moved as an allochthonous mass a number of miles from the
vicinity of the Nevada-California border eastward to the west side of
the orogenic belt, because they have no lithic counterparts nearby. The
Calaveras beds in the Sierra Nevada appear to have been metamorphosed
more than associated Jurassic beds (refer to Chapter 17), and since no
Pennsylvanian rocks have been recognized in the Sierra Nevada or
Klamath Mountains, an episode of low-grade dynamic metamorphism
has been postulated in Pennsylvanian time. Accordingly on the map of
Fig. 6.7 an orogenic belt is shown in the California region.

A thick quartzite formation overlies a Mississippian schist in southern
California and is here placed in the Pennsylvanian although no fossils
have been found in it (Larsen, 1948).

Permian Basins

The Permian was a time of extensive volcanism in the west, and various
kinds of volcanic rocks were spread from the Klamath Mountains on the
Pacific coast to central Nevada. The sequence is 5000 feet deep at
Rlairsden in the Sierra Nevada and thickens eastward to 12,000 feet in

STRUCTURAL GEOLOGY OF NORTH AMERICA

PENNSYLVANIAN

Fig. 6.7. Thickness and paleogeographic map of the Pennsylvanian.

the Humboldt Range, Nevada ( Nolan, 1943 ) . Northwestward into central
Idaho, it thins to about 4000 feet. See Figs. 6.8 and 6.13.

In the Klamath Mountains the Nosoni formation occurs and is com-
posed of basaltic agglomerates, lithic crystal tuffs, flows of andesite and
of olivine basalt, dark brown, fossiliferous, shaly limestone, and dark
gray to brown tuffaceous shales and slates. The maximum thickness
measured by Hinds is 1200 feet. It is considered to be upper Lower
Permian (Wheeler, 1933).

The Nosoni rests, probably unconformably (Hinds, 1939), on the Mc-
Cloud limestone which is highly fossiliferous. It was probably a massive
cherty limestone, but now owing to the Jurassic intrusions it is mostly
metamorphosed in various degrees to marble. It reaches a maximum
thickness of 2000 feet. Its fossils were first thought to represent a Penn-
sylvanian age, but a recent study by Wheeler ( Hinds, 1939 ) shows them
to be Lower Permian.

The McCloud limestone overlies the Mississippian Baird formation dis-
conformably, so it appears that most of the Pennsylvanian was a time of
emergence.

Central and Eastern Oregon. A heterogeneous group of east-west
trending ranges and dissected lava plateaus known collectively as the
Blue Mountains uplift or the Blue Mountains-Ochoco Mountains uplift
(Waters, 1933) extends from central to eastern Oregon. The ranges are
formed of Paleozoic and Mesozoic sediments and lavas and Mesozoic
plutons, and the complex protrudes island-fashion through the Columbia
River lava fields. The oldest beds that crop out are Lower Carboniferous
limestones and calcareous sandstones (Merriam and Berthiaume, 1943).
See Fig. 6.18. About 1000 feet of them are exposed, and they are called
the Coffee Creek formation. No volcanic materials have been noted.

Overlying the Coffee Creek formation is the Spotted Ridge formation.
The exact contact relations have not been observed, but if an unconform-
ity does exist, it is probably not angular and does not represent much of a
time break. The Spotted Ridge consists of plant-bearing sandstones and
mudstones, conglomerates containing diorite, andesite, and dacite
boulders, and bedded chert. It may be 1500 feet thick. The plants are
believed to be Lower Pennsylvanian.

PALEOZOIC CORDILLERAN GEOSYNCLINE

The Coffee Creek and Spotted Ridge formations are reported as in-
tensely folded, but no mention is made of metamorphism (Merriam and
Berthiaume, 1943). They lie in a tectonic belt of deformed strata in which
' the rocks on the west ( Klamaths ) and on the east ( Baker area ) are meta-
morphosed, and it is puzzling that these also are not metamorphosed.

The Spotted Ridge is overlain by the Coyote Butte formation. A slight
i angular unconformity separates the two. The Coyote Butte is made up
almost entirely of massive limestones. Some chert pebble conglomerates
i are present near the base. The age is probably Lower Permian.

A prominent angular unconformity exists between the Paleozoic beds
i of central Oregon and the overlying Triassic conglomerates which attain
1 a thickness of 4000 feet.

In the Baker quadrangle of eastern Oregon, Gilluly (1937b) described
a formation, the Burnt River schist, which, chiefly because of greater
metamorphism than that of known Carboniferous rocks nearby, he cau-
tiously treats as older. The rock varieties are greenstone schists, quartz
] schist, conglomerate schist, limestone, slate, and quartzite, and make up a
series at least 5000 feet thick, maybe several times as much. The various
1 types mentioned grade into each other.

Gilluly visualizes the origin of the strata as follows:

. . . pyroclastic material was added in amounts varying from time to time
^to a basin of sedimentation to which at some times sand and at others clay,
widi some carbonates, were being supplied. When volcanic contributions were
small, the deposits were such as have yielded the quartzites and carbonaceous
Ulates now found, but when the volcanic material increased relative to the
} normal terrigenous sediment the deposits were such as have yielded the inter-
mediate rocks. At times such floods of volcanic material were contributed that
practically unmixed tuff was formed.

The Burnt River schist has lithologic similarities with the Calaveras for-
mation, but differs, it seems, in generally having greater metamorphism
md an absence of chert. The Burnt River appears from published de-
scriptions to be surprisingly similar to the Salmon schist of the Klamaths,
which is probably pre-Silurian. See Figs. 6.17 and 6.18.
j Above the Burnt River schist is the Elkhorn Ridge argillite about 5000
eet thick. It is probably the most widespread of the pre-Tertiary forma-
ions and is a thick series of argillite, tuff, and chert with subordinate

C / / »6— / I ^ -" PHOSPHATE ■

PERMIAN

Fig. 6.8. Thickness and paleogeographic map of the Permian. S.G. and L.A. ARCH means Sierra
Grande and Las Animas arch, which rose at end of Permian.

STRUCTURAL GEOLOGY OF NORTH AMERICA

WESTERN ASSEMBLAGE TRANSITIONAL ASSEMBLAGE
3 4 12

"Hi

EASTERN ASSEMBLAGE
5 6

liOGEOSYNCUNe

INFERRED FORI OF GEOSYNCUNE BEFORE SHORTENING IN ANTLER OROGENY

Fig. 6.9. Stratigraphic sections of pre-Late Mississippian rocks in north-central Nevada. Repro-
duced from Roberts ef a/., 1958. 1, Hot Springs Range; 2, Osgood Mountains; 3, Battle Moun-
tain; 4, northern Shoshone Range; 5, Cortez Mountains; 6. Roberts Mountains; 7, Eureka.

limestone and greenstone masses. A number of large intrusive bodies
have been noted in the east-west belt of argillite; and these, together
with the overlapping Cenozoic rocks, effectively prevent the recognition
of contacts and the determination of stratigraphic relationships. The beds
are Pennsylvanian in age, because of Fusulina fossils found in the lime-
stones. Reds younger than Pennsylvanian may have been included in the
formation as mapped (Gilluly, 1937).

The whole formation is provisionally considered marine. The tuffaceous
argillite, the tuff, and the tuffaceous limestone all clearly attest notable
pyroclastic contributions to the formation, and it is highly probable that
cherts so numerous and thick as those in this formation may be considered
evidence of igneous contribution also.

The association of limestone with volcanic materials may have no
genetic significance, but a dependency is suspected because volcanism

might have raised the temperature of the sea and hence decreased the
solubility of the lime (Gilluly, 1937).

The Clover Creek greenstone overlies the Elkridge argillite and con-
sists of altered volcanic flows and pyroclastic rocks, with subordinate con-
glomerate, limestone, chert, and argillite. It is known to extend as far
eastward as the Snake River Canyon, and is therefore probably the same
as the "Permian volcanics" of several areas in eastern Idaho. It is at least
4000 feet thick (Gilluly, 1937).

The effusive rocks in order of abundance are quartz keratophyre ( lava-
bearing albite), quartz keratophyre tuff, and meta-andesite. Fossils col-
lected from the formation betray a Permian age.

The marine limestone and associated fossiliferous tuffs demonstrate a
marine origin for part of the formation, at least. The type of albitization
which most of the volcanic rocks have undergone is common in demon-
strably submarine volcanic rocks, and the association here with marine
limestone suggests rather strongly that the Clover Creek greenstone is in
large part of submarine origin.

Northern Washington and Southern British Columbia. Where the
Okanogan River crosses the international boundary, extensive areas of
pre-Tertiary rocks are found. The pre-intrusives (pre- Jurassic) meta-
morphic rocks are called the Anarchist series; they crop out in the
Okanogan Range adjacent to the Okanogan Valley on the west and exten-
sively in the Okanogan highlands on the east. According to Krauskopf
( 1939) neither the top nor the bottom of the Anarchist series has been
found, but at least 10,000 feet of beds exist. They can be divided rather
vaguely into three divisions. The lower consists chiefly of gray to jet black
phyllites with some interbedded quartzite and a little chlorite schist; the
middle consists of limestone, massive quartzite, graywacke, conglomerate,
some phyllite, and to the north and east of much greenstone; the upper
consists for the most part of greenstone with some interbedded phyllite
and quartzite. The albite in the greenstones of the upper division suggests
a correlation with the keratophyres of eastern Oregon.

Regional metamorphism has converted the original sedimentary and
volcanic rocks to a typical chlorite zone assemblage. Near some of the
plutonic bodies higher-grade contact metamorphism has been superim-

PALEOZOIC CORDILLERAN GEOSYNCLINE

HOUSE RANGE

Wheeier and Steele 1951
Benliey 1958

SHEEPROCK MTS.

Cohenour 1957

EAST TINTIC MTS.

Morns 1957
Benlley 1953

OQUIRRH MTS.

Gilluly 1932

Bissell ond R1o,py(notes)

S.STANSBURY
MOUNTAINS

N STANSBURY
MOUNTAINS

LAKESIDE MTS.

Young 1955

PROMONTORY MTS. LOGAN AREA

Oiton 1957 Win, ara 1946

Bunder bera^

Cole Conyon

Ophir Group

Dunderbefq^

Cote Conyon

Lynch

Worm Creek mem

Bnghom

Morjurn

S«osev

Condor

Fig. 6.10. Cambrian formations of central and northern Utah. Reproduced from Rigby, 1958.

.posed, yielding biotite and amphibolite schists and diopside rocks ( Kraus-
kopf, 1939).

A few fossils establish a marine origin for part of the series at least,
and a late Paleozoic age.

In Stevens County in northeastern Washington, Weaver ( 1920 ) de-

scribed the Stevens series, a group of metamorphic rocks with the great
thickness reportedly of 42,900 feet. It consists of quartzites, argillites,
phyllites, dolomitic limestones, and schists. It is believed to be in part of
Carboniferous age, but the lower parts are undoubtedly older. Bancroft
(1914) had previously found fragmentary plant fossils which appeared

STRUCTURAL GEOLOGY OF NORTH AMERICA

0 10 20 3.0

MILES

TEMDOr
RANGE

B»e of Btot

Severol hundred feel of or -

c Qillaceous rocks In this oreo>

may be L.-M. Mississippi

litf.-l P*nnJ

x k x x x x :

K/'^lOjrj-Dj Jeffenonft Grand View doll.
- w.$llurion[| "" |S| taketown dolomite

I I On

M-UCoir.tirioii[lA\Ntf ,
E3»ou

Precombnon

SOturdoy Mountain dol.
Kinmhinic quartxitt
notheod tandttont to Pilgrim dol.
fttlt quarliife

Llil^jJPBim Pre-Beltion crystalline: rock*

0 feet

1000

2000

3000

4000

5000

-$000

-7000

-SOOO

9000

io.ooq

Fig. 6.11. Geosyncline, geanticline, and shelf of southwestern Montana and adjacent Idaho.
After Scholten, 1957.

to be Carboniferous. The Carboniferous part of the Stevens series is prob-
ably equivalent to part of the Anarchist series on the west and to the Pend
Oreille group (Daly, 1912) on the northeast along the 49th parallel. The
Pend Oreille is also considered in part Carboniferous. It and equivalents
rest on the immensely thick Beltian strata of Proterozoic age which form
a north-south belt in northern Idaho, western Montana, and British Co-
lumbia east of Kootenay Lake.

The lower part of the Stevens series was later divided into a number of
formations by Park and Cannon ( 1943 ) after Cambrian, Ordovician, and
Devonian fossils had been found. Their section is as follows:

Formation

Thickness, Feet

Limestone (Devonian)

Ledbetter slate (Ordovician)

Metaline limestone (Middle Cambrian)

Maitlen phyllite (Lower or Middle Cambrian)

Gypsy quartzite (Lower or Middle Cambrian)

Monk formation (Cambrian?)

Unconformity
Leola volcanics (Precambrian)
Shedroof conglomerate (precambrian)

Unconformity
Priest River group (Precambrian)

700
2500
3000

3000 plus
5300-8500
3800

5000 plus
5000 plus

The Cambrian formations of Park and Cannon have been traced to
northeastern Stevens County by Campbell (1947), where diagnostic
early Middle Cambrian fossils were found.

Cache Creek Sequence of British Columbia. Upper Permian sediments
are widespread and very thick over much of British Columbia ( Fig. 6.8 ) .
A number of formations and groups have been collected under the gen-
eral term Cache Creek sequence by White ( 1959 ) . Cherts are very abun-
dant in several forms, as well as interbedded andesite and basalt flows
and related pyroclastics. Limestone units range from thin intercalated
laminae to massive beds thousands of feet thick. In places the Cache
Creek beds have been involved in sharp folding and metamorphism, in-
cident to later orogenies, but where their relations to older beds have

; i

PALEOZOIC CORDILLERAN GEOSYNCLINE

been clearly noted, they generally rest in angular unconformity on de-
formed, metamorphosed, and intruded rocks.

The zone of maximum subsidence extends through the center of British
Columbia with reported thickness ranging from 10,000 feet at the south-
ern border to 24,000 feet in the northern part of the province.

The Cache Creek strata have yielded Upper Permian fossils in a num-
ber of places but lower beds in the sequence may be Carboniferous.

Shuswap Terrane and Orogeny

A large complex of metamorphosed rocks in southern British Columbia
is known as the Shuswap terrane. Its location is shown on the map of
Fig. 17.14. The metamorphism has long been attributed to Mesozoic
batholithic processes, but now certain positive information indicates that
extensive parts were metamorphosed in Pre-Cache Creek time. An au-
thoritative summary of the nature of the Shuswap terrane by Cairnes
(1939) is quoted below in which he leans toward metamorphism in
Mesozoic time but recognizes that early metamorphism may have oc-
curred.

The rocks of this Shuswap terrane are a metamorphic complex, and their
transformation is attributed to processes connected with Mesozoic batholithic
intrusions, of which those of the Nelson batholith of the West Kootenay region
have played a principal part. The nature of these processes is, however, not
entirely clear, though certain probable conditions may be surmised from the
available evidence. On the one hand it is apparent that, in part and over large
areas, the Nelson batholith, together with other adjacent or comagmatic in-
trusives, has been emplaced to the accompaniment of much deformation in the
invaded formations. On the other hand it seems equally plain that, within the
broad areas occupied by much of the Shuswap terrane, the mechanics of batho-

j lithic intrusion have been of a quite different sort. There is little evidence here
of those pronounced deformations with which batholithic invasion is so gen-
erally associated in mountainous regions; nor of that abrupt shouldering aside of
formations flanking the irruptive mass which elsewhere characterizes the in-
vaded strata bordering the Nelson batholith. On the contrary, batholithic inva-
sion within the Shuswap terrane has apparendy progressed under conditions of

h comparative stability by a process or processes of gradual soaking of the super-
incumbent rocks with tenuous and mobile products from the underlying magma
reservoir. The nature of these products can perhaps best be judged from the
occurrence of abundant bodies of pegmatitic granite throughout the Shuswap
terrane; from the many associated aplitic dykes; and from the aplitic injection
material that is such an important constituent of the gneissic members of tire

Shuswap complex. The fact, too, that large areas of massive granite contain
many bodies of pegmatitic granite of precisely the same mineral composition as
the granite, and show every textural gradation into these pegmatitic bodies, is
further indication of the character and composition of the magmatic products
effecting the transformations in the Shuswap terrane. These products are be-
lieved to have been essentially of the nature of pegmatitic and aplitic differenti-
ates, high in volatile constituents and extremely mobile. The principal processes
have seemed to involve a gradual upward seepage of this material, infiltration
along bedding planes, replacement or partial replacement of intervening rock
matter, and the growth, in situ, of perhaps much of the pegmatitic granite. In
places the continued supply of magmatic material resulted in the complete con-
version of large bodies of the original strata into massive granitoid rock which,
under the conditions of transformation, became partly plastic or molten and,
where subjected to local stresses, behaved much as a normal intrusive rock in
its contact relations with adjoining rock masses.

An important fact in the history of the Shuswap rocks, and one that has been
stressed adequately by Daly, is the great depth at which their transformation
has been achieved. Unquestionably the Shuswap terrane at that time was
deeply buried, and unquestionably the temperatures within the zone of trans-
formation were extremely high and long sustained. That this zone lay, in part
and at times, within the zone of plastic flow is indicated in many places by
numerous local sigmoid folds in which the Shuswap gneisses are involved. That
temperatures within the metamorphic zone were high is indicated alone by the
abundant and widespread occurrence of pegmatitic bodies everywhere within
the terrane. That this condition of deep burial may, as Daly points out, afford
an explanation of why the Shuswap terrane as a whole has escaped the severe
deformations effecting more superficial formations (such as are now found bor-
dering the Shuswap area), must be kept in mind in any interpretation of the
origin and mode of formation of these rocks. That conditions implied by depth
of burial would be most effective on the stratigraphicallv oldest formations is
evident from the fact that for any sizable area of Shuswap rocks it is the oldest
formations, or basal strata, the alteration of which has been most complete.
Thus it is quite probable that within the principal area of the Shuswap terrane.
as about Shuswap Lake, die formations principally effected are, as suggested
also by the general structural trend of their foliation, of pre-Cambrian (Beltian?'*
age. In other areas, however, it is known that metamorphism lias extended up-
ward to include late Paleozoic and probably Triassic formations, but that the
effects of this metamorphism have been less intense as, in general, the depth of
burial has decreased.

Since 1950 evidence has been accumulating that points to the conclu-
sion, if not the fact, that the Permian strata rest unconformably on the
Shuswap and are not affected by the same orogenic and intrusive activity.
Reesor (1957) summarizes recent opinion as follows:

STRUCTURAL GEOLOGY OF NORTH AMERICA

SHEEPROCK MTS.
Cohenour 1957

EASTTINTIC MTS.

Morris 1957

S.STANSBURY MTS.

Teicherl 1958
(modified)

N. STANSBURT MTS.

Arnold 1956
(modified)

LAKESIDE MTS.

Young 1955

PROMONTORY MTS.

Olson 1957

LOGAN AREA

Williams 1957

Fig. 6.12. Ordovician, Silurian,
Devonian, and Mississippian forma-
tions of central and northern Utah.
Reproduced from Rigby, 1958.

No reasonable doubt exists that rocks of the Cache Creek (Permian and
possibly in part Carboniferous) lie with profound unconformity over rocks of
the Shuswap terrain. Basal conglomerates of the Cache Creek contain boulders
of metamorphosed Shuswap rocks. Thus metamorphism and deformation of
the Shuswap rocks took place before the Permian.

White (1959) sites a striking example of the basal Permian uncon-
formity in the Cariboo district. There, the Cariboo group of Early Cam-
brian age is closely folded into synclinoria and anticlinoria, and clastic

members are regionally metamorphosed to the chlorite-muscovite grade.
The Slide Mountain group of Permian age of entirely different lithology
unconformably overlies the Cariboo group. It is mildly folded and not
metamorphosed. Because of the clear-cut relationship here, White pro-
poses the name, Cariboo orogeny, and includes all deformational events
from Early Ordovician to Pennsylvanian in it that occurred throughout
the entire Canadian Cordillera.

Attention on previous pages to Devonian, Mississippian, and Pennsyl-

PALEOZOIC CORDILLERAN GEOSYNCLINE

vanian data in the western United States from which the maps of Figs.
6.5-6.7 were constructed lead to the suggestion that the Antler orogenic
belt of Nevada extends northward through western Idaho and eastern
Oregon and Washington to the Shuswap terrane of southern British
Columbia. If so, we would infer that the Shuswap orogeny is of the same
age as the Antler; that is, it started in Late Devonian and continued vigor-
ously through the Mississippian and early Pennsylvanian. The Shuswap
is marked by considerable metamorphism and perhaps batholithic intru-
sion and related processes, whereas the Antler belt is marked especially
by great thrust sheets.

The term Shuswap orogeny as here used will denote tectonic events
in the Shuswap terrane region that occurred during the same time
approximately as those of the Antler belt, and the term Cariboo orogeny
as proposed by White will be considered to have wider and longer conno-
tation.

White (1959) summarizes information which suggests that the Shu-
swap belt extends into northern British Columbia and the Yukon Terri-
tory, and when so conceived the Antler and Shuswap orogenic belt is
continuous from southern California to Alaska.

EUGEOSYNCLINE IN SOUTHEASTERN ALASKA, NORTHERN BRITISH
COLUMBIA AND THE YUKON

Southeastern Alaska. The Paleozoic rocks in southeastern Alaska from
54° 30' to 60° N. Lat. are of geosynclinal thickness and make up a
number of formations of Ordovician, Silurian, Devonian, Mississippian,
Pennsylvanian, and Permian ages (Buddington and Chapin, 1929). The
stratigraphic succession is given in the table on p. 82.

One of the commonest types of rock is andesite in various forms. It
occurs in at least seven formations of Permian age to Ordovician, and
perhaps older. Many of the volcanic rocks are now greenstone schist.
Pillow lava is abundant in the Lower and Middle Ordovician, Silurian,
Middle and Upper Devonian, Lower Permian, and Upper Triassic.

The other predominant rock types are sheared graywacke, slate, and
phyllite. The vast amount of greenish graywacke with associated slate is

SOUTHERN KLAMATH MOUNTAINS

NORTHERN KLAMATH MTS .

REDDING QUAD.
(HINDS ET AL. )

WEAVERVTLLE QUAD.
(HINDS ET AJ.. )

WELLS EJ. AL.

U. TRIASSIC

APPLEGATE GROUP, METAVOLCANICS
AND METASEDIMENTS. FORMERLY
CALLED DEVONIAN OR CARBONIFEROUS

PERMIAN

NOSONI VOLCANICS_

McCLOUD LIMESTONE

MISSISSIPPIAN

BAIRD FM.

DRAGDON FM. WITH BASS
MOUNTAIN BASALT

BRAGDON FM.

DEVONIAN

KENNETT FM.

DEVONIAN LIMESTONE PATCHES

SILURIAN

COPLEY META-ANDESITE
(POSSIBLY SILURIAN)

COPLEY META-ANDESITE
CHANCHELLULA FM.
(POSSIBLY SILURIAN)

SALMON SCHIST
ABRAMS SCHIST

SILURIAN STRATA (NOT NAMED)

PRE -SILURIAN

HIGHLY FOLIATED SCHIST

Fig. 6.13. Correlation of Paleozoic formations in Klamath Mountains.

the most striking feature of the stratigraphic sequence of southeastern
Alaska. Graywacke is found in every system of the Paleozoic and Meso-
zoic, and in many places it is difficult or impossible to tell one graywacke
unit from another.

Limestone forms a very considerable part of each Paleozoic formation
except the Ordovician. The thickest unit is in the Upper Silurian and is a
very high calcite variety. Some limestone carries considerable chert.

Beds of cobble and boulder conglomerate form conspicuous and thick
members of the Silurian and Devonian formations. A peculiar but com-
mon form is composed of andesite and limestone pebbles and cobbles in
a tuffaceous matrix. The same lithology is found in the Middle Devonian.
Coarse conglomerate beds occur at the base of the Devonian.

Another characteristic lithology in the Paleozoic systems in southeastern
Alaska is coarse, waterworn intraformational limestone conglomerate.
Beds occur in the Silurian, Devonian, Permian, and Triassic formations,
and in all of them the cobbles of limestone carry the same fauna as the
formation in which the conglomerate occurs. Buddington believes the
intraformational conglomerates originated from crustal movements ac-
companying the volcanic activity during these periods.

Black slate and argillite are widely distributed, and thin-layered black
chert several hundred feet thick occurs in the Ordovician and Missis-

STRUCTURAL GEOLOGY OF NORTH AMERICA

Series

Character

Thickness, Feet

Series

Character

Thickness, Feet

Unconformity
Andesitic rocks, including breccia, with limestone matrix
and lava flows (in part with pillow structure), locally
interbedded with slate and other sediments 1400 plus

Unconformity
Conglomerate, sandstone, and limestone; in the
Ketchikan district includes considerable black slate
in upper part 1600 plus or minus

Unconformity

Thick-bedded limestone; with common to abundant in-
tercalated layers of white chert 1000

Conglomerate, limestone, sandstone, andesitic and ba-
saltic lava, tuff, and locally rhyolitic volcanic rocks 3000 plus or minus

Unconformity
White massive limestone 100 plus

Interbedded coarsely crystalline limestone and black
chert, overlain by interlayered dense gray quartzite
and cherty limestone; sparse conglomerate 1000

Basalt, andesite (in part pillow lava), tuff, limestone,

sandstone, slate, and conglomerate 1000

Unconformity (?)
Limestone 600 plus

Andesitic green to gray tuff (locally cherty) and gray-
wacke, with locally fine, conglomeratic layers, inter-
calated limestone, and a minor amount of andesitic
lava and breccia 2400 plus

Andesitic lava (in part pillow lava), breccia, tuff, con-
glomerate and locally rhyolitic lava 2000

Interbedded limestone, slate, chert, andesitic lava,
breccia, tuff, and locally conglomerate

Conglomerate and graywacke-like sandstone, with lo-
cally interbedded limestone 2000

Unconformity

Green-gray graywacke with sparse conglomerate beds.
Interbedded red, green-gray, and gray graywacke,
like sandstone with small amount of shale 5000 plus

Green-gray shale with intercalated red beds and thin-
layered fine-grained gray sandstone, shale, and
dense limestone 500 plus

Jurassic
Upper Triassic
Permian

Pennsylvanian (?)
Mississippian

Upper Devonian
Middle Devonian

Silurian

Predominantly thick-bedded dense limestone; interca-
lated with thick beds of coarse conglomerate, thin-
layered limestone, nodular and shaly argillaceous
limestone and sandstone Ls, 3000; Congl. 1500 4500 plus or minus

Andesite (in part pillow lava) and andesite porphyry
lava; conglomerate; with some associated gray-
wacke, tuff, breccia, and limestone 3000 plus or minus

Unconformity (?)
Indurated graywacke with associated black slate and

sparse conglomerate and limy sediments ?

Unconformity (?)

Indurated graywacke with associated black slate and
sparse conglomerate and limy beds; locally andesitic
pillow-lava and volcanic rocks ?

Thin-layered black chert with black graptolitic slate part-
ings, graywacke, and locally andesitic volcanic rocks ?

Greenstone schist with intercalated or interbedded

limestone ?

Limestone ?

Schist with beds of limestone and slate ?

Schist ?

Middle
Ordovician

Lower Ordovician

Probably pre-
Ordovician to
Devonian Wales
group (meta-
morphic rocks)

sippian formations. Thick-bedded chert and cherty tuff occur in the Mid-
dle Devonian, and white chert is common in the Upper Permian.

Schists and gneisses are also common, and are the result principally of
permeating hot solutions attendant upon the emplacement and solidifica-
tion of the vast volume of magma in addition to orogenic stresses (Bud-
dington and Chapin, 1929).

Northern British Columbia and the Yukon. The Geologic Map of
Canada summarizes what is known of the distribution of Paleozoic rocks
in northern British Columbia and the Yukon. Great areas are still marked
"Paleozoic, mainly sedimentary rocks," but other large areas are labeled
"Carboniferous and Permian sedimentary rocks." Geology and Economic
Minerals of Canada, 1947, summarizes the distribution as follows:

During the Carboniferous and Permian periods apparendy nearly the
whole of the Western Cordilleran region (west of the Rocky Mountain trench)
lay beneath the sea, and great thicknesses of sedimentary and volcanic material

HAVALLAH SEQUENCE

EUREKA-CARLIN SEQUENCE

ANTLER SEQUENCE

SONOMA RANGE
Kolpoto formation

Hovoiloh formation

CARLIN AREA

(Modified after

Dott, 1955.

Fails, I960)

EUREKA AREA

(After Nolon, 1956, fig I,

and pp 56 to 68)

Pumpernickel

formotion

IMississippian

or older)

\ EDNA

MOUNTAIN

BATTLE MOUNTAIN

Middi"e —- ^'"<"Hoo^

Pennsylvanion ~ -—

Highwoy limestone

Preble formotion
(Combr ion)

„ Woo"*0'

rn«£2ei— - —

■ — Bottle formotion

' Volmy formotion
(Ordovicion)

ks?5*??2S2e:

K^%Vtbuntain

i mw

™ Winnemucco '~J ft///.
Lovelocks Eferg/afef

s. tr* La s//w

City

Monhotian

!... '■ Sonomo Range

140*
2. Edno Mountoln

| 3. Bottle Mountain

i 4. Corlin orea

! 5 Eureko orea

Pioche

100 Miles \
p >

-Of-

Bosins of deposition of overlap sequences

K^Vj

EXPLANATION

feists!

(Section Is 75

miles south of

Carlln)

r6000 fttt

Chert

Shole

Greenstone

Sandy limestone

sre^i

Sondstone

Intermediote ond

siliceous volcanic

rocks

Cherty limestone

Conglomerate

-4000

2000

Vertical scoi«

Fig. 6.14. Detail of Mississippian, Pennsylvanian, and Permian formations involved in Antler orogeny
of north-central Nevada. Reproduced from Roberts ef a/., 1958.

STRUCTURAL GEOLOGY OF NORTH AMERICA

EUGEOSYNCLINE

MIOGEOSYNCLINE

• OLfCAMPlAK

MISSISSIP
AND OLD

ORDOVICIAN

NORTH-CENTRAL
NEVADA
LAND AXIS

»00 f
i too

IOOO

- soo

PROBABLE NORTH EDGE
OF LATE PENN. EMERGE
AREA

Fig. 6

strata
1955.

15. Antler orogenic belt of central Nevada showing Mississlppian, Pennsylvanian, and Permian
restored to early Wolfcampian time. Section extends from Winnemucca to Elko. Reproduced from Dott,

accumulated. Wide areas of almost unexplored country in eastern Yukon are
presumed to be underlain chiefly by Paleozoic strata, but may also contain
rocks of Mesozoic and Precambrian age. In northern British Columbia, where
exposed strata are thought to represent much of Paleozoic time, no important
disturbance has been recognized. In a number of localities sedimentation and
volcanism probably proceeded more or less continuously from late Paleozoic
into early Mesozoic time, but in places an interval of uplift and erosion without
marked tilting or folding may have intervened.

A report on the Cassiar Mountains, Finley River district between lati-
tudes 56 and 58, and longitudes of 124 and 126, by Dolmage (1928) de-
scribes a series of metamorphosed rocks of Carboniferous age. They are
"green ash rocks pressed and altered into schists, interbedded with layers
of graywacke, felsite, halle-flinta, serpentine, and argillite." Along Takla
and Stuart lakes and vicinity the series is made up of limestones, argillites,
cherty quartzities, green schists, slates, volcanic flows, tuffs and breccias,
and narrow bands of dolomite. Fusulina and other Carboniferous fossils
have been found in some of these beds.

Underlying the Carboniferous series, great belts of schist and quartzite

occur. Quartz mica schist constitutes about three-fourths of the whole.
In many places, the schist grades into quartzite, both of which were de-
rived undoubtedly from siliceous sediments (Dolmage, 1928). Such rocks
as these are widespread and have been correlated with the Shuswap
terrane of southern Rritish Columbia, which now as previously explained,
is believed to be made up of rocks of several Paleozoic periods as well as
Precambrian. Also some coarse quartzites, quartz pebble conglomerates,
and limestones have been likened to the Cambrian strata of the southern
Canadian Rockies, previously described.

The areas of such rocks are shown on the map of Fig. 33.12. A great
medial area of Proterozoic (Beltian?) rocks separates the western areas
of Carboniferous rocks from the eastern Paleozoic rocks, but whether or
not this was a highland in Paleozoic time is unknown.

SUMMARY OF OROGENIC HISTORY

The maps, Figs. 6.1 to 6.8, are fairly expressive of our present know-
ledge and postulates of the evolution of the western margin of the con-

PALEOZOIC CORDILLERAN GEOSYNCLINE

tinent during Paleozoic time. That the western margin has a belt of major
orogeny with associated intrusive and extrusive igneous activity and
metamorphism needs no longer to be defended. At the time of writing
of the first edition of this book the profession was just accepting the view
and abandoning the older one of a small continental borderland, now
partly submerged beneath the Pacific Ocean.

It may be stated that we have no information on conditions in Cam-
brian time west of northwestern Nevada. Cambrian strata are recognized
farther south in California in the Death Valley region, but these lie on
the projection of the eastern miogeosynclinal assemblage. Ordovician
rocks, like the Cambrian, are not known for sure west of northwestern
Nevada. In southeastern Alaska, however, they have been identified very
close to the Pacific margin of the continent, and are part of an extensive
eugeosynclinal assemblage. Silurian rocks have now been recognized near
the Pacific in the Klamath Mountains, but the paleogeography of the entire
region from northwestern Nevada to the Pacific is practically unknown.
The presence of Silurian strata in the Klamath Mountains and sequences
under them which might be Ordovician and Cambrian lead to the con-
clusion that the western margin of the continent as early as Cambrian
time was about where it now is; and that the continent has not grown
appreciably since.

We must also postulate several phases of major orogeny together with
the accumulation of eugeosynclinal sequences in adjacent and associ-
ated basins or troughs in early Paleozoic times along the western margin
of the continent. The transitional zones of the eugeosynclinal and miogeo-
synclinal assemblages are now fairly well positioned, and the basins of
the miogeosyncline are beginning to take on specific shape and distribu-
tion in light of our present knowledge. Geanticlines, the Beltian and Raft
River, are postulated, and the Tooele arch seems clear. These add com-
plexity to what was previously considered a simple broad basin.

A major and unsolved problem is the relation of the southwesterly
trending Paleozoic tectonic elements in southern Nevada, Arizona, and
California to the continental margin — they are distinctly discordant
rather than approximately concordant or unilateral. The problem has been
discussed in Chapter 5.

BAYHORSE QUAD.
IDAHO

LOST RIVER RANGE
IDAHO

TENDOY RANGE
S. W. MONT.

NORTHEASTERN
UTAH

OQIIRRH MTS.,
CENTRAL UTAH

AMSDLN FM.»

MANNING CAN. SH.»

MANNING CAN. SH.'

"BRAZER"*

BIG SNOWY GR.

GREAT BLUE LS.
HUMBUG FM.

GREAT BLUE LS .
HUMBUG FM.

MILLIGEN

DESERET LS.

ARGILLITE

z
o
to

MISSION CAN. LS.

MILLIGEN
ARGILLITE

a
3

MADISON LS.

LODGEPOLE LS .

SAPPINGTON FM.+

LEATHAM

PARTLY L. PENNSYLVANIA
+ OCCURS EASTWARD AS PART OF SHELF SEQUENCE

Fig 6.16. Correlation of Mississippian formations of southwestern Montana, eastern Idaho and
northern Utah.

A major orogenic belt began to develop in central Nevada in late
Devonian time, and through several phases of folding and thrusting con-
tinued development through the rest of the Paleozoic. The belt is pro-
jected northward through eastern Oregon and Washington into southern
British Columbia in Mississippian and Pennsylvanian time to the Shuswap
orogenic belt in British Columbia. Another orogenic belt lay to the west
in Pennsylvanian and Permian time, and it seems to have been separated
from the central Nevada belt by a basin of sedimentation. The entire
region including both belts and the intervening basin become involved in
orogeny, volcanism, and intrusive activity thereafter, starting in Permian
time.

Shifting basins and the appearance of uplifts of several kinds add com-
plexity to the miogeosyncline and its relation to the shelf in the late
Paleozoic.

The Canadian cordillera is not as wide as that of the western United
States, and perhaps its development is more regular. From what is known
it appears that a geanticline of Beltian strata developed early in Paleozoic
time and separated a western eugeocynclinal trough of sedimentation
from an eastern miogeosynclinal trough. The eugeosynclinal region was
subjected to repeated orogeny, metamorphism, and igneous activity. In
this connection it is pertinent to review Buddington's observations in
southeastern Alaska.

SILVER IS. RANGE
SCHAEFFER, PERSONAL
COMMUNICATION

OQUIRRH

STANSBURY MTS.
RIGBY.I958

BASIN

WASATCH MTS.

PROVO SECTION

BAKER, 1947

DUCHESNE RIVER
UTAH

SHELF

SOLS CANYON

10 MILES SW MANILA, UTAH

RANGELY FIELD
W. COLORADO

Fig. 6.17. Mississippian, Pennsylvanian, and Permian formations in Utah showing change from the shelf
assemblage to the miogeosyncline assemblage. Sections of shelf were furnished by Walter Sadlick, who also
assisted in the general correlations.

PALEOZOIC CORDILLERAN GEOSYNCLINE

NORTHWEST SECTION

NORTHEAST SECTION

GRINDSTONE- TWELVEMILE CREEKS AREA, CENTRAL OREGON (MERR1AM AND BERTHIAUME)

Cc, Coffee Creek fm. (Lower Carb.) Cs, Spotted Ridge fm. (Penn.) Pc, Coyote BuTte fm. (Perm.)

I MILE

^^K^-mmMM^WMSm,

MlBl

cm$m*m

NORTH-SOUTH SECTION NEAR BAKER, OREGON (GILLULY)

brs, Burnt River schist; Ce, Elkhorn Ridge argillite (Penn. ?) Ccg, Clover Creek greenstone (Perm.)
qd, biotite-quartz dionte; sg, silicified gabbro; mg, metagcbbro; gb, gabbro

, 5 Ml LELS

Fig. 6.18. Cross sections in central and eastern Oregon.

The Silurian graywackes in general of southeastern Alaska are com-
posed of particles of rock similar to the kinds that form the pebbles and
cobbles in the conglomerates with which they are interbedded, and in
addition, of a considerable percentage of plagioclase, potassic feldspar,
and quartz grains. The conglomerates are largely made of andesite
pebbles and boulders, but slate, diorite, rhyolite, and limestone pebbles
are abundant, if not dominant, in some conglomerates. One specimen of
graywacke of Devonian or Silurian age, for example, consisted of particles
of andesite, felsite, plagioclase, granophyre, quartz, spherulitic rhyolite,
and orthoclase, with a chloritic and slightly calcareous groundmass.

The association of the graywackes and conglomerates that Buddington
describes is very revealing of their origin. The conglomerates in them-
I selves are indicative of a volcanic archipelago and deserve further men-
tion. The following is a reume of the Silurian conglomerates according to
Buddington. Varieties of conglomerates are as follows:

1. A conglomerate composed almost wholly of well-rounded andesite or an-
desite porphyry cobbles and boulders; the matrix may be calcareous, and
lenses of limestones are intercalated but limestone cobbles are sparse.

2. A conglomerate composed almost wholly of limestone cobbles or boulders
in a limestone or andesitic tufflike matrix; this type is rare, but beds 100
feet thick have been noted.

3. Peculiar conglomerates intermediate between 1 and 2, consisting of pebbles
and cobbles of andesite and limestone in a greenish tufflike matrix.

4. A homogeneous-appearing rock composed of fragments of andesite in a
matrix of the same material; the structure is that of a conglomerate or
water-wom breccia.

The limestone fragments are usually of a dense-textured limestone
typical of the Silurian, and many carry fossils of Silurian age. The fossils
are the same as from the overlying limestone. It is, therefore, believed
that the limestone conglomerates are intraformational and that the lime-
stone fragments are of practically the same age as the volcanic fragments.
Vertical movements of the sea bottom, perhaps local, must have accom-

STRUCTURAL GEOLOGY OF NORTH AMERICA

Fig. 6.19. Map showing coincidence of
Permian volcanic trough (stippled margins)
and zone of Sierran intrustives (lines). Dots
indicate location of Carboniferous, Perm-
ian, and Triassic areas referred to the
text. Pennsylvanian and Permian basins
combined isopached. Zone of Sierran in-
trusives includes nearly all satellites and
palingenetic areas.

panied the volcanism and resulted in contemporaneus erosion and sub-
marine slumping of slightly compacted fine lime mud. A part of the
volcanic material, at least, must have been erupted from central vol-
canoes, which were built up above the surface of the ocean and were
thus subjected to erosion.

Although recognizing unsolved elements in the problem of the origin
of the graywackes, conglomerates, and limy argillaceous beds, Budding-
ton visualizes a sedimentary environment as follows: the great lens-
shaped beds of conglomerate may be local deposits made by torrential
streams, and the graywacke may be in part the more finely comminuted
peripheral marine equivalent. The calcareous shale and argillaceous limy
beds which are locally intercalcated with the clean, thick-bedded lime-
stone may be in part the more distant offshore equivalent of the con-
glomerate and graywacke.

The limestone is in part dense white on fresh surfaces, and massive
with only rare, if any, evidence of stratification. Beds as thick as 2000 feet
have been observed. In part it is interbedded with thin-layered limestone,
nodular and shaly limestone, calcareous shaly argillite, dense platy si-
liceous layers, green-gray shale, and sparse buff-weathering sandstone.
The massive limestone seems to be due to rapid deposition, and where
clean the site of accumulation was sufficiently distant from land so not to
have received any clastic material. Volcanic activity has been thought of
as contributing to the deposition of the limestone, through the activity of
magmatic waters or meteoric waters draining from a volcanic terrane or
by the warming of the marine water, but the chemistry and oceanography
of the problem have not been worked out.

Schofield ( 1941 ) discussed the problem of granitoid pebbles and
cobbles in the conglomerates of several periods, especially the Triassic.
Buddington refers to them also. In one locality, the Britannia map area
of British Columbia, an arkose is described as composed of irregular
grains of quartz, plagioclase, orthoclase, and sericite schist. The lack of
rounding of the grains, the freshness of the plagioclase, and the consider-
able thickness of the unstratified beds, prove that the material accumu-
lated rapidly and was transported only a short distance from a source of
granitoid plutonic rocks. Buddington failed to trace the granitoid elastics

PALEOZOIC CORDILLERAN GEOSYNCLINE

to their source, despite the fact that their size and abundance indicated
to him a nearby local origin. It seems necessary, he believes, to assume
that granitoid intrusions existed in a land that formerly stood to the
west where only the Pacific Ocean now lies.

Krynine (1941) has studied the tectonic significance of arkoses, and
concludes that they are deposited when a granitoid terrane has just been
uplifted and is being vigorously dissected. They are related to the de-
formed geosyncline into which granitoid rocks have been intruded. The
plutons have become exposed by erosion of the mountains created by
the orogeny, and then uplifted in a further stage of deformation, and
vigorously eroded.

Granite plutons are seldom exposed in arcs of small volcanic islands.
We must look to the larger islands of an archipelago for the source of
granitoid conglomerates and arkoses. The geologic map of the Japanese
Archipelago, Fig. 6.20, shows extensive areas of granitic intrusions and
Precambrian gneisses which could furnish the necessary material. The
major archipelago like the Japanese has had a long orogenic history and
is composed not only of rocks that will make graywackes but also arkoses.
Such a one seems to have been the sourceland of the sediments of the

■ western part of the Cordilleran geosyncline.

Great beds of chert are present in the sediments of the volcanic archi-
pelago. Extensive beds of chert and cherty limestone are present in the

1 miogeosyncline as well as in the inland basins and shelfs of the main-
land, and so the factors governing the precipitation of the silica are

j probably several. Its transportation in solution in marine currents may re-

1 suit in precipitation a great distance from its source. I find it easy to be-
lieve that a large part of the silica originated in the volcanic activity of
the archipelago, that some of it was carried by currents across the seas
between the archipelago and the mainland free from the area of deposi-

I Fig. 6.20. Generalized geologic map of the Japanese Archipelago and the easten part of Asia.
Isobaths are in meters. Coarsely stippled areas are those chiefly of sedimentary rocks but with
large areas of Archean gneiss and schist and some smaller areas of intrusive and extrusive rock.

; Finely stippled areas denote alluvium. Hachured areas are those of plutonic rocks, chiefly granite
and granadiorite, but with considerable areas of Archean gneiss and schist and some sedimentary
rocks. Solid black areas are andesite. Horizontally ruled is basalt and verticlly ruled is trachyte.

300 MILES

STRUCTURAL GEOLOGY OF NORTH AMERICA

tion of volcanic material, and that it was precipitated copiously in the
shallow seas of the eastern trough and mainland shelf where, from place
to place and time to time, clay, lime mud, or sand were accumulating.
Perhaps the tectonic conditions of the eugeosyncline of the western
margin of North America can be visualized better if reference is made to
the Japanese Archipelago (see Fig. 6.20). It is believed that there a
fairly good example and close parallel of conditions exists now as ex-
isted in times past along the west coast of North America.

In the first place the scale and shape of the arcuate features are the
same. In the second place, the geology of the Japanese Archipelago is
somewhat the same as that postulated for the sourceland of the sediments
of the Pacific trough of the Paleozoic Cordilleran geosyncline. The most
abundant rocks mapped in the Japanese Archipelago are as follows:
andesite, granite, syenite, schistose granite, gneiss, schist, slate, chert,
sandstone, limestone, diorite, pyroxenite, amphibolite, gabbro, and
trachyte, in approximate descending order of abundance.

APPALACHIAN MOUNTAINS

MAJOR STRUCTURAL DIVISIONS

The index map of Fig. 7.1 shows the structural divisions of the east-
ern margin of the continent from New York to Alabama. The interior
stable region of the continent is represented by the Appalachian plateaus
province, where the strata are nearly horizontal and dissected by an
elaborate arborescent drainage system. In southern New York, central
Pennsylvania, and northern West Virginia, the strata are cast into very
gentle folds which are the site of extensive gas and oil fields. The folding
is so gentle that the drainage is little affected, and the arborescent plateau
type exists, scarcely distinguishable from the region farther west.

The folded and thrust-faulted province represents the Appalachian
Mountains proper. It is the well-known region of flat-topped, parallel, or
subparallel ridges and valleys that are carved out of anticlines, synclines,
and thrust sheets. The drainage pattern is rectangular (trellis), and stands
conspicuously apart from the arborescent pattern of the Appalachian
plateaus. The strata are of Paleozoic age in both provinces but thicken
from the shelf along the western margin of the plateaus to the geosyn-
cline in the eastern part of the plateaus and in the folded and thrust-
faulted belt. See Fig. 7.2.

The Blue Ridge province is made up of Cambrian and Late and prob-
ably Early Precambrian metamorphic and igneous rocks, which are older
than those of the Appalachians to the west, and are more or less meta-
morphosed. It is widest in the south, and highest in the Great Smoky
Mountains of Tennessee and North Carolina (Fig. 7.1). It dies out in
southern Pennsylvania only to take up again in eastern Pennsylvania,
New Jersey, and New York. The Blue Ridge province is generally one of
conspicuous relief east of the Great Valley of the folded Appalachians
and west of the crystalline Piedmont. The Piedmont province is broad and
generally of low relief. Its rocks are not well exposed and, as yet,
thoroughly known in only a few places. They are chiefly metamorphosed
Precambrian and Paleozoic sediments and volcanics, and Paleozoic
plutons, a number of which are of batholithic proportions.

Several long, narrow basins of Triassic sediments rest unconformably
on the older rocks of the Piedmont, and in one place on the Blue Ridge
belt. They are down-faulted troughs, all apparently part of a major fault
or rift zone. The Triassic sediments are mostly red standstones and shales,
and are cut by numerous large dikes and sills of diabase, also of Triassic
age.

The Atlantic Coastal Plain is a continuation of the Gulf Coastal Plain,
and is made up of Cretaceous and Tertiary sediments that rest uncon-
formably on the older rocks of all the structural systems of the Appala-
chian Mountains. They overlap the Triassic deposits slightly in New
Jersey. They dip gently seaward and probably extend out under water

Fig. 7.1. Index map of the structural systems of the eastern margin of the continent.

APPALACHIAN MOUNTAINS

CUMBERLAND
PLATEAU

VALLEY AND RIDGE PROVINCE

BLUE RIDGE PROVINCE

INNER PIEDMONT

GREAT SMOKY MOUNTAINS

BREVARD BELT

ASHEVILLE I , i

-•■.'■Vv,;^'->/---::;--^:-".---: ■;'..■:■■'■ sv.;:' s'Vm^^Sg

INNER PIEDMONT

KINGS MTN. BELT

OUTER

CHARLOTTE BELT

PIEDMONT

COASTAL PLAIN

■ CAROLINA SLATE BELT.

SHELBY

Jj MILES

Fig. 7.2.. Cross section of Appalachian system from Cumberland Plateau to Atlantic Coastal Plain,
from King, 1955 and 1959. Section B-B', Fig. 7.1 "Es, Triassic Newark group; PM, Mississippian
and Pennsylvanian rocks; SO, Middle and Upper Ordovician and Silurian rocks; OC, Cambrian
and Lower Ordovician rocks; Cc, basal Cambrian Chilhowee group; pCO2, Great Smoky con-

glomerate and related rocks; pCo', Hiwassee slate and Snobird fm.; pCs, gneiss and schist
(mainly Carolina and Roan gneisses); pCg, Cranberry and Max Patch granites; vol, slate,
tuff, rhyolite and andesite flows and breccia interbedded; gr2, massive granites; gr', foliated
granites; di, diorite and gaboro; gd, granite-diorite injection complex; gn, gneiss and schist.

in the Atlantic Ocean to the margin of the continental shelf, so that the
province geologically should be considered to include the continental
shelf. It is clear that coastal plain sediments are being deposited today.
In addition to the great longitudinal structural divisions of the Atlantic
margin of the continent just described, a traverse division is also com-
monly made, and the terms central Appalachians and southern Appala-
chians are used. Generally, the three structural systems, the folded and
faulted Appalachians, the Blue Ridge, and the Piedmont provinces in the
states of Alabama, Georgia, Tennessee, North Carolina, and Virginia
are included in the southern Appalachian region, and the same three
divisions in northern West Virginia, Maryland, Pennsylvania, and New
Jersey are included in the central Appalachian region, although some
authors call the whole structural complex south of New York the
southern Appalachians.

RELATIONS TO GEOMORPHIC PROVINCES

Appalachian Plateaus Province

The structural divisions or systems are in large part reflected in the
geomorphic provinces and, therefore, except for minor variations, their
boundaries are the same. See Fig. 7.3. The Appalachian plateaus province
includes two main plateaus, the Cumberland on the south, and the
Allegheny on the north. The province is one of mature or submature
dissection, and stands throughout four-fifths of its periphery higher than
its neighbors; and parts of it are properly called mountains. The province
is a broad, gentle, synclinal basin, whose youngest rocks are the Dunkard
group or "upper barren measures" (Permian). They are mainly a thick
mass of red shale and sandstone, and occupy a belt extending southwest
from near Pittsburgh to near Huntington, West Virginia. Cropping out

STRUCTURAL GEOLOGY OF NORTH AMERICA
APPALACHIAN PLATEAU

Hudson, R.

Fig. 7.3. Block diagram of the geomorphic provinces of the central Appalachians and the
Atlantic Coastal Plain, reproduced from Johnson, Bascom, and Sharp, 1933. M, Manhattan;
Sb, Stroudsburg; P. Pottsville; R, Reading; Hb, Harrisburg; CI, Carlisle G, Gettysburg; Ch,

around it in successive elliptical zones are the Monongahela ("upper
productive"), Conemaugh ("lower barren"), Allegheny ("lower produc-
tive"), and finally the fairly thin Pottsville. Most of the limestone and
the best coal beds are in the Monongahela formation.

Chambersburg; Mr, Mercersburg; H, Hagerstown; HF, Harpers Ferry; F, Frederick; Rv, Rock-
ville; Wash, Washington; Bal, Baltimore; Phil, Philadelphia; Tr, Trenton.

The Allegheny plateau is continuous with the Cumberland plateau, and
any boundary is arbitrary. The southern plateau is somewhat less dis-
sected, and the nearly flat-lying strata are largely the sandstones, shales,
and basal conglomerates of the Pottsville formation.

APPALACHIAN MOUNTAINS

... in southern Ohio the Mississippian rocks on the western margin of the
Allegheny Plateau form cuestas rising to the full height of the plateau. The
prominence of these cuestas diminishes toward the south, but they continue
to form a narrow belt included in the plateau as far as latitude 37° 30', beyond
which the Mississippian rocks (all except the uppermost) spread widely to the
west at a lower level and belong to a different province. Farther south the strong
conglomerates or sandstones at the base of the Pottsville (Rockcastle group)
underlie and support the margin of the plateau. All beds here dip slighdy to
the east, and the strong basal formations are to some extent stripped, leaving
at places a decided eastward dip slope. As the stripped belt widens toward
the south, and the province narrows, the entire width of the Cumberland
Plateau in Tennessee and Alabama comes to be on the strong formations here
| known as Walden and Lookout sandstones.

For nearly 200 miles along the median line of the province in Tennessee and
Alabama, runs the straight Sequatchie anticline, broken on the west by a thrust
fault. If left uneroded, it would form a range of mountains, as it still does at
its northern end where the Crab Orchard Mountains are in line with the perfect
anticlinal valley which marks the rest of the uplift. Like the more extensive
and complex Allegheny and Cumberland Mountains, this anticline represents
the propagation into the plateau of the compressive stress by which the Valley
and Ridge province was folded. Parallel to this feature, and 15 miles to the
east is the similar Wills Creek anticline, marked by the valley west of Lookout
Mountain (Fenneman, 1937).

Valley and Ridge Province

The folded and thrust-faulted Appalachian structural system is the
geomorphic Valley and Ridge province, which as already stated consists
of parallel or subparallel ridges and valleys of 1000 to 2000 feet local
relief. It has been spoken of as the newer Appalachians in contradistinc-
tion to the older Appalachians which would include the Blue Ridge and
| Piedmont provinces.

The Valley and Ridge province can readily be divided longitudinally into a
northwestern section, in which high ridges alternate with valleys of moderate
width (the "Valley and Ridge" section), and a broad southeastern lowland
section (the "Great Valley"). This division is more or less apparent throughout
the length of the province.

Except for a short distance in New York, the entire northwestern boundary
of the province is an erosional escarpment formed on gendy dipping or horizon-

i tal sediments of the Appalachian Plateau. From southern Pennsylvania to
Alabama, the southeastern boundary is formed by the resistant rocks of the

1 Blue Ridge, towering above the Great Valley. This boundary is erosional in
origin, weaker Paleozoic sediments having been stripped from the Precambrian

surface (in some places from resistant Cambrian quartzites) on which till
were deposited. In other localities the contact of weak Paleozoic sediments with
resistant crystalline rocks takes place along a low-angle thrust fault, and erosion
has lowered the sediments northwest of the Fracture plane.

The rocks of the province are Paleozoic sediments ranging in age from Cam-
brian to Pennsylvanian. Their resistance to erosion varies gready and has a very
important effect upon the topograph}'. The broad low land composing the Great
Valley is due to the weakness of the Cambro-Ordovician limestones (Kittatinnv
and other formations) and Ordovician shales (Martinsburg). The ridges of the
Valley and Ridge belt are composed of very resistant middle and upper Paleo-
zoic sandstones and conglomerates, particularly the Tuscarora quartzite and
conglomerate (Silurian), the Pocono sandstone (Mississippian), and the Potts-
ville conglomerate (Pennsylvanian).

At the end of Paleozoic time the sediments in the Newer Appalachian
province were subjected to strong pressure from the southeast and folded into
great anticlines and synclines, in places overturned toward the northwest.
Reverse faults were also commonly developed in the zone of greatest pressure,
the horizontal attitude of the beds was scarcely disturbed. The region of
undisturbed rocks today forms the Appalachian Plateau; the folded area has
become the Newer Appalachians. In the latter province the structural trends
are northeasterly, and owing to the remarkable development of subsequent
streams the topographic features trend in the same direction (Fenneman, l937).

Blue Ridge Province

The Blue Ridge province rises in southern Pennsylvania as the Carlisle
prong and continues southwestward in accordance with the general trend
of the Appalachian systems to northern Georgia. It stands conspicuously
above the Great Valley section of the Valley and Ridge province on the
northwest and the much lower Piedmont province on the southeast. The
province takes its name from the Blue Ridge in Virginia, which is a rela-
tively narrow mountainous ridge that extends from the Potomac River
200 miles southwestward to Roanoke. It has an altitude of about 1000 feet
near the Potomac, but attains an elevation of more than 1000 feet to the
southwest. Southwest of Roanoke, the Blue Ridge province is a rolling
plateau, about 10 to 65 miles wide ami with an average elevation of
3000 feet. Its bounding escarpments are 1000 to 2000 feet high. This part
of the province includes the Great Smokies which are the highest land
east of the Rockies. Mount Rogers, near the northwestern escarpment
in Virginia has an altitude of 5719 feet, and Mount Mitchell in North
Carolina has an elevation of 6711 feet.

STRUCTURAL GEOLOGY OF NORTH AMERICA

The Blue Ridge geomorphic province terminates southward in northern
Georgia, just north of Gainsville, where the Piedmont and the Valley
and Ridge provinces seem to close around the Great Smokies. The Blue
Ridge structural belt, however, extends on southward into Alabama,
where it is buried by the coastal plain sediments; but because it has
been eroded down to the level of the Piedmont, it is generally included
in the Piedmont province by the geomorphologists.

The Piedmont province emerges from the Triassic lowlands in New
Jersey, where it is known as the Trenton prong (see Fig. 7.3), and extends
southwestward to Alabama. It is only a few miles wide in Pennsylvania,
Maryland, and northern Virginia, but widens conspicuously to about 170
miles in North Carolina, from which place southwestward it continues
wide. The surface of the Piedmont rises gradually westward to the foot
of the Blue Ridge, where it reaches an altitude of 500 feet at the north
and 1500 feet at the south. It is a vast plain along the horizon, but is
maturely dissected to a local relief of a few hundred feet in places.

Numerous hills and ridges rise as monadnocks 200 to 1000 feet above
the general plains surface, and are more numerous near the Blue Ridge
escarpment.

The rocks of the Piedmont province are mostly granites, gneisses, and
schists, with some belts of marble and quartzite, partly of Paleozoic age
but also in part of Precambrian age. A belt of basic rocks containing
talc and soapstone is found near the western border. Several elongate
basins of Upper Triassic sandstones and shales, cut by diabase dikes and
sills, are found in the province. The Richmond basin contains coal,
which was the first mined in North America in about 1750.

The Piedmont crystallines are overlapped on the east by the Cretaceous
and Tertiary sediments, and the boundary of the two provinces is called
the fall zone. Baltimore, Washington, Fredericksburg, Richmond, Peters-
burg, and other cities are located along it, and also mark approximately
the points to which the tide extends up the estuaries.

SOUTHERN AND
CENTRAL APPALACHIANS

EXTENT AND DIVISIONS

The southern and central Appalachians extend from Alabama to New
York and the Hudson River, and include the area shown on the index
map of Fig. 7.1. They will be treated under their three longitudinal divi-
sions, the folded and thrust-faulted Appalachian Mountains province,
the Blue Ridge Cambrian and Precambrian province, and the Piedmont
crystalline province. The use of the words southern and central implies
that a northern division is also recognized, but this is referred to as the
New England province. New Brunswick and Nova Scotia will be in-
cluded in the northern division because of their close geological relation
to New England.

MAJOR ELEMENTS OF STRATIGRAPHY

Appalachian Geosyncline

From the time tiiat James Hall contributed voluminously to geologic-
literature (1840 to 1860) to about 1920, the following views were widely
held regarding the Appalachian geosyncline. It extended from New-
foundland to Alabama and beyond, over 3000 miles; subsided most in the
site of the present Valley and Ridge province and the eastern side of the
Allegheny synclinorium, where more than 30,000 feet of sediments ac-
cumulated in places; shallow shelf seas extended inland from the geosyn-
cline over the Central Stable Region; and a great borderland, Appala-
chia, lay along its southeast side, from which came much of the sediment
that filled the subsiding trough.

Failure to appreciate facies changes and the absence of detailed
mapping, especially in the Blue Ridge and Piedmont provinces, militated
against a correct understanding of the tectonic development of the region.
It appears now that the Blue Ridge province marks approximately the
boundary between a west-lying miogeosyncline and an east-lying eugeo-
syncline in Cambrian time, but in post-Cambrian Paleozoic time the
Blue Ridge and Piedmont were generally emergent. The concept of a
borderland that extended beyond the present continental shelf into the
Atlantic ocean is discredited.

Because of the metamorphosed nature of the strata in the Piedmont
and the almost complete failure to find fossils in them, the work of
unraveling their stratigraphy and structure has been slow. The stratig-
raphy of the Valley and Ridge province, however, has received a gnat
deal of attention. It will be seen that geosynclinal subsidence in the site
of the Appalachians and the plateaus shifted from time to time and
place to place so that a strict coincidence of structural divisions and the
sedimentary provinces does not exist. In a broad way, however, the
western half of the miogeosyncline is undeformed or cast only into very
gentle folds — it is structurally the Allegheny svnclinorium and physio-
graphically the Plateaus province — whereas the eastern half of the mio-
geosyncline is the folded and thrust-fanlted province.

STRUCTURAL GEOLOGY OF NORTH AMERICA

East-central Ten-
nessee (Chilhowee
Mountain)

Northeastern Ten-
nessee (Johnson,
Carter and Unicoi
Counties)

Northern Virginia
(Elkton and Har-
pers Ferry areas)

Shady dolomite
(in Miller Cove)

Shady dolomite

Tomstown dolomite

—

Hesse quartzite

Erwin
quartzite

Antietam

Murray shale

quartzite

Nebo quartzite

Nichols shale

Hampton shale

Harpers shale

Cochran
conglomerate

Unicoi formation
(with basalt flows
1000-1500 feet
below top)

Weverton
quartzite

Loudoun formation
(with tuffaceous
slate and rare
flows)

CIS

<1)

Ocoee series

Volcanics of Mt.
Rogers area

Cranberry granite

Injection complex

Fig. 8.1.
1949.

Formations of the Chilhowee group in Tennessee and Virginia. From P. B. King,

Major Sedimentary Divisions of the Miogeosyncline

Lower Cambrian Marine Clastics. The oldest beds of the Cambrian,
referred to as basal Cambrian, are conglomerates, arkoses, and shales,
that pass upward into quartzites. They make up the Chilhowee group
(Fig. 8.1) and attain a thickness of 5000 to 6000 feet. Tentative correla-
tions with metamorphic units of the Piedmont suggest that these strata
of the miogeosyncline grade southeasterly into eugeosynclinal facies
in the manner illustrated in Fig. 8.2.

The basal Chilhowee beds rest in places unconformably on the vol-
canics and greenstones of the Ocoee series, and hence are believed to be

part of the Lower Cambrian sequence. They are limited to a trough
which runs the length of the central and southern Appalachians and
are absent over the foreland or shelf region.

Cambrian and Lower Ordovician Carbonates. The miogeosyncline
with its clastic deposits from Alabama to Pennsylvania became one
dominantly of limestone and dolomite deposition. Some 9000 feet of
carbonates representing the remainder of the Lower Cambrian, the entire
Middle and Upper Cambrian, and the Lower Ordovician accumulated
to a fairly uniform thickness up and down the entire trough. In the
southern and northern ends of the geosyncline carbonate deposition
continued into Middle Ordovician time. A correlation chart of the im-
portant formations of this period is given in Fig. 8.3. The carbonates
grade into shale facies toward the northwest side of the miogeosyncline
and the shelf in the manner illustrated in Fig. 8.4.

The basal Cambrian clastics and the succeeding thick carbonate
sequence are typically miogeosynclinal and correspond in distribution
approximately with the later orogenic belts of the Blue Ridge and
Valley and Ridge provinces (King, 1959). The clastics were derived from
an emergent stable interior, and the carbonates were deposited on a
broad continental shelf, evidently without off-lying tectonic lands or a
volcanic archipelago. The eugeosynclinal equivalents of the carbonates,
if ever deposited, are not yet clearly recognized in the Piedmont.

Middle Ordovician Clastic Wedge. The regimen of erosion and sedi-
mentation characterized by an emergent interior and a gently sub-
merging continental border gave way abruptly in Middle Ordovician
time to a reversed situation in which an uplifted borderland now
furnished the sediments to a subsiding inside basin. The sediments were
mostly clastic (Fig. 8.5), and the main source was in western Virginia,
western North Carolina, and eastern Tennessee. A great fan of sedi-
ments is visualized to have apexed in this region in about the Great
Smoky Mountains area and extended radially to the west, northwest,
and north (P. B. King, 1959). See Fig. 8.31. It spread considerably
beyond the later deformed belt of the Valley and Ridge province, and
unlike the Cambrian and Lower Ordovician sediments was not confined
to an elongate basin parallel with the continental margin. The wedge

SOUTHERN AND CENTRAL APPALACHIANS

Fig. 8.2. Stratigrauhic relations of Late Precambrian and Early Cambrian formations in Blue Ridge
of Virginia. After Bloomer and Werner, 1955.

or fan was about 8000 feet thick near its apex but thinned toward its
edges. Beds representing the Middle Ordovician, as well as the Upper,
are only 500 feet thick to the southwest in Alabama, and are carbonates.
Likewise to the northeast in Pennsylvania Middle Ordovician beds are
carbonates and only 700 feet thick.

Late Ordavician-Devonian Clastic Wedge. Apexing in east-central
Pennsylvania is another great wedge of clastic sediments which began
to accumulate in Late Ordovician time and continued through the
Silurian and Devonian. The greatest subsidence and sediment accumula-
tion occurred during the Late Devonian, which deposit is commonly
referred to as the Catskill delta. It has a maximum thickness of over
8000 feet. Isopach maps of the Late Ordovician and Silurian deposits are
shown in Fig. 8.6, and detail of facies relations in Fig. 8.7. A cross
section of the Devonian wedge is given in Fig. 8.8, and a map of the
deposit in Fig. 8.9. Further detail on the stratigraphy may be found in
publications by Willard (1936).

Mississippian Deposits. In eastern Tennessee in the Great Valley
a sheet of black shale may be seen transgressing across the Silurian strata
and on the southeast side of the Valley to be resting on Middle Ordovician
rocks. See Fig. 8.5. It is known as the Chattanooga shale and probably
ranges in age from latest Devonian to earliest Mississippian (Rodgers,

1953). It thickens northeastward and eastward to a maximum of 400 feet
at Cumberland gap. The Chattanooga shale is extremely widespread in
the Nashville and Cincinnati arch areas and represents a marine facies
of the upper continental beds of the Catskill delta.

The Mississippian above the Chattanooga in eastern Tennessee may
have attained a maximum thickness of 6000 feet at the time of deposi-
tion near the Blue Ridge source region, but is generally much thinner
than this in sections now preserved. It consists of three units each ex-
hibiting a parallel gradation from finer, thinner, and less detrital —
more carbonate sediments on the northwest side of the Great Valley to
coarser, thicker, and more detrital sediments on the southeast side.

In Alabama, the thin Mississippian limestones of the foreland change
toward the southeast into 5000 feet of sandstones and shales. In north-
ern Virginia, Maryland, and Pennsylvania, the lower 1000 to 2000 feet
of the Mississippian is shale and sandstone, the middle formations are
limestone with a maximum thickness of 4000 to 5000 feet, and the upper
formations are calcareous shale, red mudrock, and red and gray sand-
stones. The Pocono and similar sandstones of the lower division are thick
bedded and conglomeratic. The thickening of most all units of the
Mississippian from the western shelf to the eastern geosynclinal trough is
conspicuous, and the coarsest material occurs where the section is thickest.

100

STRUCTURAL GEOLOGY OF NORTH AMERICA

TIME SCALE

CENTRAL
PENNSYLVANIA

SUSQUE-
HANNA-
NEW RIVER

NEW RIVER-
TENNESSEE

TENNESSEE-
ALABAMA

z
<
a
<
z
<
u

LARKE

BEEKMAN-
T0WN

CHEPULTEPEC

E
■

tt
in
<L

MADISON
TREMPEALEAU

o
a
r*-

o
o

a>
■a

4
(9

MINES
150-200

ORE HILL
STACY

C0N0C0-
CHEAGUE
1600-2000

COPPER RIDGE
1200-2800

BIBB

250-500
KETONA

400-600
BRIERFIELD

1500

FRANCONIA

DRESBACH

WARRIOR
1250

NOLICHUCKY
400-750+

4
O

Z>
4

4
Z
O

NOLICHUCKY

r
<

4
U

_l
o

MARJUM

ELBROOK
1800-3000

K
HI
3£

4
Z
O

MARYVILLE
150-750

MARYVILLE

WHEELER

PLEASANT HILL
600

ROGERSVILLE
70-250

ROGERSVILLE

SWASEY

DOME

RUTLEDCE
200-500

RUTLEDGE

OPHIR

HOWELL

Fig. 8.3. Middle and Upper Cambrian formations of central and southern Appalachians. After
Resser, 1938.

The Mississippian trough coincides with the Valley and Ridge province
and does not reflect the great westward bulging wedges of the Ordovician
and Devonian. See Plate 6. It is probable that the Mississippian seas
shored at about the Rlue Ridge.

Mississippian rocks may never have been deposited in the northern
part of the geosyncline in southeastern and eastern New York. The
coarsest beds in eastern Pennsylvania were deposited nearest the high-

lands that formed in New England in the Devonian, and with reduction
of the highlands the earlier Mississippian elastics were succeeded by
calcareous sediments (Kay, 1942).

Pennsylvanian Clastics. The Pennsylvanian strata are distinctly clastic,
both in the shelf and the geosynclinal areas. They are the great coal-
bearing formations of the Allegheny Plateaus and Valley and Ridge
provinces. A cross section from Virginia to Illinois that does not contain
the present structural details is shown in Fig. 8.10. The trough is deep-
est in Alabama, where a maximum of 10,000 feet of strata — all Pottsville
— is known. The Pottsville thins gradually northeastward until in Pennsyl-
vania it is only 200 to 400 feet thick. As the Pottsville thins, younger
Pennsylvanian formations appear, and in West Virginia and Pennsylvania
the Allegheny formation is 300 feet thick, the Conemaugh 600 feet, and
the Monongahela with the extremely valuable Pittsburgh coal at the base,
250 feet. The maximum thickness of the Upper Pennsylvanian is estimated
to be 3000 feet.

The 10,000 feet of Pottsville beds in Alabama in the Coosa coal field
area is rather restricted in east-west distribution because of the nearness
of the Nashville arch to the Blue Ridge, but probably the original dis-
tribution was in the form of a wedge which spread westward over the
site of the arch. This is the representation of King, 1959.

Permian System. Overlying the Monongahela formation in an oval
area in West Virginia and Ohio, entirely in the Plateau province, is the
Dunkard group or "upper barren measures" of Permian age. It is com-
posed of shale, partly red, and sandstone with thin coal beds. Its maxi-
mum thickness is about 1500 feet.

FOLDED AND THRUST-FAULTED APPALACHIAN MOUNTAINS

Salients and Recesses

When viewed as a whole, the folded and thrust-faulted belt of the
central and southern Appalachians consists of two major salients and
three recesses. These are terms used by Keith (1923) in his well-known
"Outlines of Appalachian structure." The salients are the arclike portions
of the belt that are convex inland, and the recesses are the arclike portions

SOUTHERN AND CENTRAL APPALACHIANS

101

NORTHWESTERN
FACIES

CENTRAL
FACIES

SOUTHEASTERN
FACIES

SOUTHWEST
VIRGINIA

Copper Ridge dolomite

dolomite

Conococheogue limestone

-2000

_ LJ

— '— '—^ *— Pumpkin Valley shale znz^z

-1500

1000 <
o
if)

-500

<
o

UJ
>

Rome

formation

Fig. 8.4. Middle and Upper Cambrian sedimentary rocks of eastern Tennessee and southwestern Virginia.
After Rogers, 1953.

that are convex toward the ocean. The southern salient is principally in
Tennessee and southeastern Kentucky (see Tectonic Map of the United
States), and the northern salient is in central Pennsylvania. They are
about 400 miles apart. Keith points out two other salients in the northern
Appalachians which will be described later.

Structural Characteristics in Alabama, Georgia, and Tennessee

If the Tectonic Map of the United States is studied, it will be seen that
the southern half of the Valley and Ridge province is characterized by
thrust faults, whereas the northern half is chiefly one of long parallel
anticlines and synclines. In the southern part, the thrust sheets are
stacked in imbricate fashion on top of each other, and in eastern Ten-
nessee a succession of nine such sheets has been mapped. Some of the
thrust sheets carry almost the entire Paleozoic succession; others du-
plicate the lower Paleozoic succession only. Precambrian rocks have

nowhere in the belt been exposed as the result of thrusting and erosion.

The belt is made up almost entirely of thrust sheets in Tennessee, but
southward, especially along the northwest margin, the beds are cast into a
long anticline (Sequatchie) and syncline (Coalburg), which extend from
central Tennessee almost to the Cretaceous cover in Alabama. Also along
the southeast side of the belt in northwestern Georgia, a number of folds
are evident. They occur in a conspicuous embayment of the Blue Ridge
front.

The nature of the thrusts and folds is illustrated in sections 1 to 4 and
8 to 12 of Figs. 8.11 to 8.17. The location of the sections is given on the
index map of Fig. 7.1. Most all the thrust sheets have moved toward the
stable interior of the continent; only a few exceptions are known. One of
these is illustrated in section 2, Fig. 8.12.

The Rome sheet was thrust forward at least 10 miles and then folded
into anticlines and synclines. See section 3, Fig. 8.12. Some of the folds

102

STRUCTURAL GEOLOGY OF NORTH AMERICA

MAYNARDVILLE QUADRANGLE

KNOXVILLE QUADRANGLE

Northwest of
Wallens Ridge

Southeast of
Wallens Ridge

Northwest of
Bays Mountain

Chilhowee
Mountain

■1-yzv-v":"---* Chattanooga" shale \ -_-_j-l= ~r-_ -r'-~- - - -~~ - = ~ ~ ~~~ ~~~~ """ ""

S^^agjb»^£^evrer"j jjiii ~£?&S^%£

•.!.-.:.v.-:.'-..fpllico sandstone"; •„•:

2 *

33
<:

4.t;

*■»

5 Co

Conglomerate locally

Fig. 8.5. Stratigraphic diagram of Middle and Upper Ordovician and Silurian rocks of Valley and Ridge
province of eastern Tennessee. After P. B. King, 1950a.

of the strata below the thrust sheet are in the same position as those of
the thrust sheet, but in detail the contacts are discontinuous against the
thrust, and in other areas a complete lack of coincidence occurs. This
suggests three episodes of compressional orogeny, perhaps almost in
continuous succession: first the folding and erosion of the strata in front
of the thrust and, perhaps, the early development of the thrust itself;
then the movement of the great sheet out over the folded and eroded
terrane; and third, further folding, involving both the thrust sheet and
the underlying strata. Immediate waste products of the folds and thrusts
which have been overridden and preserved, or which partially bury the

structures, are not apparent. Such waste products in the form of coarse
piedmont elastics are present in some of the Rocky Mountain thrusts and
serve to date the various stages of deformation. Regarding the Rome
thrust, however, all three closely related episodes of deformation are
younger than the Lower Pennsylvanian Pottsville, which is involved in
the deformation.

Another conspicuous structural division of the Valley and Ridge
province of the southern Appalachians is the zone of shallow, flat thrust
sheets, like the Rome, along its eastern margin. These are largely part
of the Rlue Ridge province, and involve Cambrian and Ordovician strata,

SOUTHERN AND CENTRAL APPALACHIANS

103

but in part are in the Great Valley. Modern interpretations show a num-
ber of fensters and klippes. See sections 4, 10, and 12 of Figs. 8.13, 8.16,
and 8.17, respectively.

In northeastern Tennessee and southeastern Kentucky, the Appalachian
front is characterized by an unusual thrust. Elsewhere the Appalachian
front is one of fairly sharp folds that start abruptly from the flat-lying
plateaus sediments. As seen in Figs. 8.14 and 8.15, an extensive block of
the flat plateau strata has been torn loose and thrust, with only gentle
deformation, toward the stable interior. The great, basal fault is known
as the Pine Mountain and the two lateral tears as the Jackson and Russell
Fork. Although the large mass is a thrust sheet, the strata from Pine
Mountain to Cumberland Mountain are so flat that an arborescent drain-
age has developed and the region is considered geomorphically part of
the plateaus province. The thrust mass is known as the Cumberland block
and is 125 miles long and 25 miles wide. Its displacement has been cal-
culated as 5.8 miles (Miller and Fuller, 1947). Along the Powell Valley
anticline in the thrust sheet, erosion has cut several small fensters, and
the Rose Hill oil field has been developed in the underlying beds with
production from the Moccasin limestone.

Structural Characteristics in the Virginias, Maryland, and Pennsylvania

The southern part of the Appalachian belt, characterized by thrust-
ing, is narrow; but toward the north in west-central Virginia a number
of folds begin to show and the belt broadens. Sharp asymmetrical folds
and mild metamorphism characterize the Great Valley, strong upright
! folds the main Valley and Ridge province, and very gentle folds, a
western belt. See index map, Fig. 7.1. The folds of the westernmost zone
are so gentle that the region is considered part of the Plateaus province,
and the Appalachian structural front here is regarded as the western
boundary of the zone of sharp folds. The plateaus generally stand in relief
above the valleys and ridges of the strongly folded belt, and the eastward-
facing escarpment is called the Allegheny front, which is a geomorphic
feature, whereas the Appalachian front is a structural feature.

The chief faults are the Pulaski and North Mountain overthrusts. They
may be parts of one great thrust which extends from southern Penn-

f ^'

!4c

>r~ 7SO — "~

-^-

I \ P /

V * '

/s^

\ /

A j

/ "

i \

\\ \ '

Fig. 8.6. Basins of deposition in middle and late Ordovician time and in Silurian time in the
Pennsylvania-New York region. After Kay, 1942.

104

STRUCTURAL GEOLOGY OF NORTH AMERICA

sylvania to northeastern Tennessee, over 500 miles long. Rack of the
Pulaski thrust front are several fensters, as illustrated in sections 12 and
13 of Fig. 8.17. See also Fig. 8.16. Sections 14, 16, and 17 of Figs. 8.18 and
8.19 also illustrate the thrusts of the central and eastern parts of the belt.

The nature of the strong folds is illustrated in sections 15, 16, 18-21,
and 24 of Figs. 8.17 to 8.20. Most of the folds are asymmetrical and
steepest on the northwest flank. According to the orthodox view, this
marks active pressure from the southeast, as do almost all the thrusts.

SILURIAN

The folds of this region are some of the best known in North American
geology, and some are markedly long and regular. See the Tectonic Map
of the United States. Keith ( 1923 ) points out that the troughs of the folds
extend downward to almost a common level, whereas the anticlines
extend upward to variable elevations. Some of the anticlines are over-
turned and have broken into thrust faults. Most of the more eastern
thrust sheets have extensively flat or folded lower surfaces.

The faults die out in southern Pennsylvania, and from there northwest-

6ASS ISLAND

L. KEVSER 7

10 N T A R I 0
I
CAPE HORD OWEN SOUND

NEW YORK

!
GUELF HAMILTON LAKE ERIE

CINCIN N ATI A N

PENNSYLVANIA
I
ALLEGHENY FRONT

HARRISBURG

30 MILES .

Fig. 8.7. Late Ordovician and Silurian stratigraphy of Pennsylvania, western New York, and western
Ontario. After Kay, 1942.

SOUTHERN AND CENTRAL APPALACHIANS

105

ERIE, PA. PA

N.Y.

WARREN, PA. PORTAGE, N.Y. NAPLE5

WATKINS GLEN BINGHAMPTON

CAT5KILL MT5.

Hamilton gr.
' ' h

Onondaga /j /*

0r/sX

any

/) snoK an

Fig. 8.8. Upper cross section, the great Catskill delta from Erie, Pa., to the Catskill Mountains,
N.Y. After Schuchert, 1924.

Black is black shale and white is conglomerate, sandstone, shale, and calcareous shale. The
elastics are dominantly red and generally coarsest in the eastern part. Vertical scale much ex-

ward almost the entire belt is one of anticlines and synclines. See section
29, Fig. 8.20. They veer markedly eastward in central and eastern Penn-
sylvania, and by southern New York both the gently folded belt and most
of the strongly folded belt die out. The folds, if projected, would run
into the Adirondack uplift and the lower Hudson Valley. A narrow eastern
zone of the folded and thrust-faulted Appalachians, which is intimately
connected with the Rlue Ridge province, extends up the Hudson Valley. It
seems very crowded between the Adirondacks and the New England
metamorphic masses. See section 30, Fig. 8.21.

As far as the folded and thrust-faulted Appalachians are concerned,
and aside from the narrow belt up the Hudson, it can be said that they
begin in southern New York in gentle folds and become stronger south-

Onona'aga J

Or/5Honyi fte/a"., r Decker

aggerated. Thickness may be judged by reference to the isopach map of Fig. 8.9.

Lower cross section, the Catskill Mountains and Hudson Valley north of Kingston, N. Y., after
Chadwick and Kay, 1933. It shows the present eastern erosional termination of the Catskill
delta, and presents the relations concerned with the problem of the source highlands.

ward. Thrust faults appear and become the dominant structure in the
southern Appalachians. Also, in general, it can be said that the intensity'
of deformation across the belt becomes greater toward the southeast,
and in the Great Valley and at the Blue Ridge front it is the greatest.

Regarding metamorphism, Keith (1923) pointed out long ago that a
distinct change in constitution of the strata occurs along the eastern
margin of the Valley and Ridge province in the Great Valley, and in the
adjacent Blue Ridge. Shales have taken on a slatv character, limestones
and dolomites are somewhat marmorized, and sandstones are quartzitic.
The slate belt of northeastern Pennsylvania and southeastern New York
in the tightly appressed and narrow belt of deformation east of the Blue
Ridge is well known. The change from bituminous to semibituminous to

MICH OHIO

Fig. 8.9. Restored section of the Paleozoic rocks across the Allegheny basin and Cincinnati arch.
The line of cross section is shown on the inset map, but it continues across Ohio to the southern
Michigan line. After Tafferty, 1941, personal communication. The inset map shows the great

Catskill delta and is taken from Barrell, in Schuchert's Historical Geology, 1924. The heavy lines
are isopachs in feet.

SOUTHERN AND CENTRAL APPALACHIANS

107

anthracite coal eastward through Pennsylvania has been emphasized re-
peatedly as a demonstration of greater intensity of deformation from west
to east. Although the coals have been metamorphosed within the belt
west of the Great Valley, the associated shales, sandstones, and carbonates
have not been much altered. Some doubts exist that the devolatilization
is entirely a result of folding, because of anomalies in the relations,
especially in West Virginia. Farther south the Knoxville, Tennessee,
"marble" in the highly thrust-faulted belt is a slightly recrystallized rock.

StCllON I

I L.LIN0I5

OHIO

W. VIRGINIA

o
z
— o
r z
o o
2£

VIRGIL

MISSOURI

°E3 MOINES

^""MSAS

QokeritpO-"-

_CjL>Lt°2

__ >

:ig. 8.10. Correlation and relative thickness diagram of Pennsylvania strata from West Virginia
o Illinois. After committee report, Chart No. 6 G.S.A., Vol. 55, 1944. The Cincinnati arch and
)ther structures are not shown, nor is the section restored to any one time. The dashed lines are
he various coal beds.

Cby
CrCpv/ £01. -. COcr

-"--S&

r^t^;

RED

M7N.

Dc,C(p,CwJ.g Ch

0e'i?m

1 CwrCsr Cpv

Fig. 8.11. Cross section (No. 1 of index map, Fig. 7.1) of the Bessemer and Vandiver quad-
rangles, Alabama, after Butts, 1927. Cr, Rome fm.; COK, Ketona dol.; COcr, Copper Ridge dol.;
COc, Chepultepec dol.; Olv, Longview Is.; On, Odenville, Newala, Lenoir and Mosheim Iss.;
Oa, Athens sh.; Ol, Little Oak Is.; Dc Chattanooga sh. and Frog Mtn. ss.; Cfp, Fort Payne
chert; Cf, Floyd sh.; Cpw, Parkwood sh. and ss.; Cs, Cpv, Cpi, Cwr, Pottsville ss., sh., congl.,
and coal beds.

The change in constitution of the rock along the east side of the
Great Valley is taken as a good boundary between the Valley and Ridge
and Blue Ridge provinces by King (1950a).

Intrusive igneous rocks are almost entirely absent in the Valley and
Ridge province, and hence no metamorphism incident to heat and
volatiles is known.

BLUE RIDGE PROVINCE

Divisions

In the Blue Ridge and Piedmont provinces, we are confronted with a
geology mostly of metamorphic and igneous rocks, only in part studied

108

STRUCTURAL GEOLOGY OF NORTH AMERICA

€c

GAYLOR
RIDGE

5r 5c

SIMS
MTN.

ROME THRUST

HORSE
MTN.

SECTION 3

SECTION 2

Fig. 8.12. Upper cross section (No. 3 of index map, Fig. 7.1) of Rome quadrangle, Georgia and
Alabama. After Hayes, 1902.

Lower cross section (No. 2 of index map) of Birmingham quadrangle, Alabama. After Butts,
1910.

Cr, Rome fm.; €c, Conasauga Is.; Cbr, Beaver Is.; €Ok and £sk, Knox dolomite; Oc, Chicka-
mauga Is.; Sc. Clinton ss. and sh.; Sr, Rockwood fm.; Da, Armuchee chert; Cfp, Fort Payne chert;
Ch, Floyd ss. member; Cb, Bangor Is.; Cp, Pennington sh.; Cby and Cpv, Pottsville gr.

and understood, and with an extensive literature that reveals a striking
evolution of interpretation.

The Blue Ridge province embraces two rather distinct tectonic ele-

ments, about coincident with the geomorphic divisions. Northeastward
from the vicinity of French Broad River in eastern Tennessee and western
North Carolina, the Blue Ridge is narrow, whereas southwestward, it is
broad and more complex. See Fig. 8.22.

Stratigraphy and Structure— Potomac to the French Broad River

The northeastern division, where most typically developed in northern
Virginia, is composed of a core of older Precambrian crystalline base-
ment rocks which are overlain and flanked by a considerable body of
later Precambrian metavolcanics and metasediments (Catoctin green-
stone and related units), and by Lower Cambrian clastic rocks
(Chilhowee group). This segment is known as the Blue Ridge-Catoctin
Mountain anticlinorium.

The structure and stratigraphy of the north end of the Blue Ridge belt
of Fig. 8.22 across Catoctin Mountain and South Mountain is shown in
Fig. 8.23. This sction is north of Harpers Ferry. Just south of the city
the structure across Short Hill and the Blue Ridge is given in Fig. 8.24.
Farther south in the Elkton area of Virginia a section on the west side of

GREAT

VALLEY

BLUE

RIDGE

Pu
Pine Mountoin foult

MDc DSu

Fig. 8.13. Cross section of folded and thrust faulted Appalachians in eastern Tennessee. After
Rodgers, 1953. (Section 4.) Pu, Pennsylvanian rocks; Mp, Pennington formation; Mn, Newman
limestone; Mg, Grainger formation; MDu, Lower Mississippian and Upper Devonian rocks (Fort
Payne, Grainger, and Chattanooga formations); MDc, Chattanooga shale; MDs, Basal Missis-
sippian and Devonian shale; DSu, Lower Devonian and Silurian rocks (Hancock limestone, Rock-
wood formation, and Clinch sandstone); Os, Sequatchie formation; Oj, Juanita formation; Ouc,
Upper part of Chickamauga limestone, including Reedsville shale; Omb, Martinsburg shale;
Olmc, Lower and Middle parts of Chickamauga limestone, undivided; Ob, Bays formation; Oo.

Ottosee shale; Oh, Holston formation, Ol, Lenoir limestone; Oa, Athens shale; Osv, Sevier
shale; OCk, Knox dolomite or group, undivided; Cc, Conasauga shale or group, undivided; Ccu,
Upper Cambrian part of Conasauga group; €hk, Monaker dolomite; Cr, Rome formation; Ss,
Shady dolomite; Ce, Erwin formation and equivalent rocks (Hesse sandstone, Murray shale, and
Nebo sandstone); Che, Hesse sandstone; Cnb, Nebo sandstone; Ch, Hampton formation; Cni,
Nichols shale; €u; Unicoi formation; €ch, Cochrane conglomerate; ocu, Ocoee series, undivided;
ocss, Sandsuck shale; ocsb, Snowbird formation; pCc, Precambrian crystalline complex.

;

SOUTHERN AND CENTRAL APPALACHIANS

109

:ig. 8.14. Major structural features of the Cumberland overthrust block (upper map). Area
)f fensters ruled and shown in more detail in smaller map (lower). The area of fensters is now
an oil field and is known as the Rose Hill district. Reproduced from Miller and Fuller, 1947.

the anticlinorial belt is as shown in Fig. 8.25. These recent interpretations
of the structure show no important thrusts along the inner side of the
Blue Ridge belt, but rather folded normal sequences. Southward, espe-
cially south of the James River, reverse faults are numerous and thrusting
Jbecomes dominant, as will be seen in the following discussion of the
Great Smokies.

The old Precambrian crystalline complex is composed of granite,
granodiorite, and gneiss. Cutting through it are basic dikes believed to
be feeders of the overlying basaltic Catoctin greenstone. The whole
Catoctin mass has undergone low-grade metamorphism, and a slaty or

schistose cleavage pervades it, which dips southeastward as shown in tin-
sections just referred to. A distinct lineation occurs along the general
boundary of the Precambrian and Paleozoic rocks, and in northern Vir-
ginia, Maryland, and Pennsylvania, the cleavage in which the lineation
lies extends into the Beekmantown beds according to Cloos ( 1957 ) and
into the Martinsburg shale according to Nickelsen (1956). See Fig. 8.24.
Lineation and cleavage is limited to the Precambrian from Roanoke
southwestward where thrusting has brought the basement rocks into
abrupt contact with the unaltered Paleozoic rocks.

The shear type of deformation accompanied by thickening along the
fold axes and thinning along the flanks is most characteristic of the Blue
Ridge, and sets it apart from the Valley and Ridge structures.

Only one deformation has been detected from the lineation north of
the Potomac River in the South Mountain anticlinorium. Since the Pre-
cambrian Catoctin greenstone as well as the Cambro-Ordovician lime-
stones and shales of the Great Valley are affected and since the lineation
is remarkably regular along the Blue Ridge from Pennsylvania to the
French Broad River in North Carolina, Cloos ( 1957) thinks that this one
deformation is post-Ordovician, and therefore either Taconian or Acadian
in age. These orogenies will be described presently.

Great Smoky Mountains

South of the French Broad River the Blue Ridge belt loses its weltlike
form, and a broad, high, and geologically complex terrane sets in. Along
the Tennessee-North Carolina boundary between the cities of Knoxville
and Ashville are the Great Smoky Mountains where 16 peaks rise above
6000 feet. The general expansion of the Blue Ridge in this region is shown
on Fig. 8.22, and a geological map by P. B. King is presented in Fig.
8.26. A small-scale cross section is part of Fig. 7.2, and a more detailed
section is given in Fig. 8.27. Most of the Great Smokies is a thrust com-
plex of the Ocoee Late Precambrian series.

This is a body of terrigenous clastic sedimentary rocks, which has minor
intercalations of limestone and dolomite but no volcanic components or known
fossils. The series is probably 30,000 feet or more thick. It lies unconforniahly
on a basement of earlier Precambrian granitic and gneissic rocks, and on the

MIDDLESBORO SYNCLINE

-* ROSE HILL DISTRICT---

CUMBERLAND MOUNTAIN
MONOCLINE

POWELL

VALLEY ANTICLINE

■X.
<r
o

o
o

>. Z

KEN!
CUMB
VIRGI

IT
O

" I _J

< CO

Z W

>
cc

o w
_i Q

SE.

Fig. 8.15. Section across Cumberland overthrust block along line A-A' of Fig. 8.14. Length
of section, 27 miles. Displacement along Pine Mountain fault, 5.8 miles. Reproduced from

Miller and Fuller, 1947. Section line A-A', is line 8 on index map of Fig. 7.1.

110

Fig. 8.16. Geologic map and section of the area from Bristol, Va., to Mountain City, Tenn. Re-
produced from Butts ef a/., 1933. Section 10 of index map, Fig. 7.1. Oa, Athens shale; Olm,
Lenoir, and Mosheim limestones; Ob, Beekmantown dolomite (Nittany and post-Nittany); Cc, Cono-

cocheague limestone; Cn, Nolichucky shale; Chk, Honaker dolomite; Cr, Rome formation; Cs,
Shady dolomite; Ce, Erwin quartzite; Cq, Cambrian quartzite and shale, undifferentiated; gr,
Precambrian granite.

Ill

LITTLE
MTN.

WARM

SPRINGS

MTN.

LITTLE
MARE
MTM

Offset

MILL
MTN.

SECTION 15

SECTION 13

READ MTN
FENSTER

^/SjAj _ fht

< Pa/aski thrust e$ * ^ . A$>

& V

SECTION 12

Fig. 8.17. Sections in the folded and thrust-faulted Appalachians of western Virginia, after
Butts ef a/., 1933. Section 15 is from Warm Springs to Goshen; Section 13 is through Hollins

College; and Section 12 is through Newport and Christiansburg. See index map. Fig. 7.1.

112

SOUTHERN AND CENTRAL APPALACHIANS

113

JP MONTEREY
<?^° MTN.

fY £&_

5 to

SECTION 16

Dph

5HENAND0AH

MTN.

Dch

Dck

/O MIL £5

BLUE

RIDGE

V C

i>>

BRUSHY
MTN.

5cc

SECTION 14

BIG LITTLE

HOUSE HOUSE"

MTN. MTN.

Sec

Oc Om

Fig. 8.18. Section in the folded and thrust-faulted Appalachians of west-central Virginia, after Butts et a/.,
1933. Section 14 runs through Lexington and Section 16 from the West Virginia line to Waynesboro and
Afton. See index map, Fig. 7.1.

northwest side of the mountains it is overlain by the Cochran formation, or basal
unit of the Chilhowee group, which is of Cambrian and Precambrian (?) age.
South of the mountains it is overlain by rocks of the Murphy marble belt; here,
the top of the Ocoee is placed tentatively at the base of the Nantahala slate.
The Ocoee series is divisible into three broad units of regional extent and
contrasting lithologic character, which are herewith designated groups and
named the Snowbird group, the Great Smoky group, and the Walden Creek
group. The groups consist of local intergrading and intertonguing formations
and have complex stratigraphic and structural relations. The Ocoee series is
split by major thrust faults into three sequences, a southern, central, and
northern, none of which contains more than two groups of the series (King
etal.,1958).

The west front of the Great Smokies and the Blue Ridge belt south-
westwardly is characterized by great folded thrusts, described in part
under the previous Valley and Ridge province. Where the overridden
rocks are exposed as re-entrants or windows and composed of limestone
or dolomite, they form "coves" or valleys lying within the mountains of

3'MENANDOAM VALLEY

o^cy-c/ ooob' .-..osX.

T)m

MASSANUTTEN

. Dohl

MOUNTAIN

-V\ 1

BURKETOWN

0a Oh

.5" M/lES

Fig. 8.19. Sections in the folded and thrust-faulted Appalachians of Virginia. After Butts et at.,
1933. Section 17 is about 10 miles north of Staunton, and Section 18 is 10 miles north of
Shenandoah Caverns. See index map, Fig. 7.1.

114

STRUCTURAL GEOLOGY OF NORTH AMERICA

ALLEGHENY PLATEAU

Fr°si^urgJMd^'coa/ measures "

ALLEGHENY
FRONT

\3I

Corriganville

DEVILS
BACKBONE
0/~! WILLS
JO' MOUNTAIN

IRON ORE
RIDGE

Flintstone

RAGGED
MOUNTAIN
POLISH
MOUNTAIN

SIDELING
HILL

•pg j Roconoss.
"***■ """rboniferous)

CACAPON
_-.,-,.,.,... MOUNTAIN F

RIDGE ANTICLINE t"

oei C/osely folded Hanco^

U. MILES

Fig. 8.20. Section from Allegheny plateau to Cacapon Mountain anticline at Hancock, Md. Reproduced
from Butts et a/., 1933. Section 21 of index map, Fig. 7.1.

granitic or clastic rocks. One of the windows, such as that containing
Grandfather Mountain, North Carolina, lies 35 miles southeast of the
northwest boundary of the Rlue Ridge province (King, 1951).

PIEDMONT PROVINCE

Principal Foliate Rocks

The most voluminous rocks in the crystalline Piedmont of Penn-
sylvania, Maryland, and Virginia is a great schist series which exhibits
in part high-grade metamorphism (Jonas, 1932). It contains in places
various metavolcanics. In Cecil and Harford counties, Maryland, a
volcanic sequence well described by Marshall (1937) shows considerable
variation, grading from massive amygdaloids and even-textured volcanics

through schistose amygdaloids to fine-grained hornblende schists which
in places are indistinguishable from sheared gabbro except by micro-
scopic examination. Several bodies of mylonitized granite in the schist
series have been recognized.

The most troublesome and yet unsolved problem is the age of the
schist. It has been correlated with the Glenarm series of Pennsylvania
and Maryland, and to this most authorities agree; but the age of the
Glenarm is not yet known. It is generally believed to be late Precambrian
or early Paleozoic.

A few anticlines and domes of older rocks, the Raltimore gneiss, are
found within the schist series, in Maryland and Pennsylvania, and pre-
sumably others occur in Virginia.

The Piedmont and eastern part of the Blue Ridge in North and South

SOUTHERN AND CENTRAL APPALACHIANS

115

Carolina consists of a complex of contorted gneisses, containing granite
plutons and satellitic offshoots, swarms of small ultrabasic intrusives, and
narrow zones of metasedimentary rocks. The boundary of the Piedmont
and Rlue Ridge provinces is here indistinct on the basis of bedrock
geology. The dominant unit of this complex is the Carolina gneiss. It
consists of quartz, feldspar, mica, gneiss, and hornblende gneiss and these
are considered to be originally sedimentary and volcanic rocks but altered
incident to the batholithic intrusions. King (1951) points out that no
clear break exists between the gneiss complex and the Ocoee and Tal-
ladega series in the Great Smokies to the northwest, and a considerable

part of it may be a highly altered phase of these late Precambrian
geosynclinal deposits.

Metamorphism

As noted by King (1951, 1959) the metamorphism increases progres-
sively southeastward from the Great Valley across the Blue Ridge, into
the Piedmont province, and climaxes with the development of silimanite
in the central part of the Piedmont between the Brevard and Kin-js
Mountain belts (see Fig. 7.2). Southeast of the silimanite zone the meta-
morphism is less intense. The belt of decreased metamorphism is marked

T r i a s s i c Lowland province

HookMfn. Wafchun? Mb. Palisades
Schooley p e n e p I

Manha tt a n prong
New England Upland prov.

Ma nhattan island

Coastal Plain province
Long Island

/./:> -,V^ .C . R Y $ T A L- LI N *E .. R '0 C K vS£.W-«^}£/r>^

Miles

KITTATINNY MOUNTAIN
St

GODFREY RIDGE

Brodhead

SECTION 30

J M.le

Fig. 8.21. Upper section across Triassic basin and Manhattan Island to Long Island, N. Y. Re-
produced from Johnson ef a/., 1933. No. 31, Fig. 7.1. Lower section through Delaware Water
Gap. Johnson et a/., 1933, after Willard. Om, Martinsburg shale; St. Tuscarora ss.; Scl,

Clinton ss.; Shf, High Falls sh.; Spi, Paxono Island sh.; Dhb, Helderberg Is.; Do, Onondago ss.;
De, Esopuc grit; Don, Onondago Is.; Dm, Marcellus shale; Dh, Hamilton ss. No. 26, Fig. 7.1.

YOUNGER THAN
CHILHOWEE

Poleozoic

CHILHOWEE
GROUP
Lower

Combnan

MURPHY MARBLE CATOCTIN

BELT GREENSTONE

Age Lote

OCOEE SERIES
Pre-Catoctin,

CRYSTALLINE
COMPLEX

Early

V GRANITE INTRUDED

\ INTO OCOEE SERIES

Myc luic r r c ^u iui*i in, t_uiiy

uncertain Precambrian Late Precambnon Precombnan

Paleozoic ?

Lynchburg James R
0

100

\'\ Washing-

Fig. 8.22. The Blue Ridge province from Georgia to Pennsylvania showing principally the
Lower Cambrian clastic group (Chilhowee) and the Late Precambrian Catoctin greenstone and
Ocoee series The Catoctin greenstone includes volcanics and sediments of Mt. Rogers area.

After P. B. King (1949). E.R., Elk Ridge; S.M., South Mountain; S.H., Short Hill; I.M., Iron
Mountain

Catoc+in Mta

10 MILES

Fig. 8.23. Change from open to close folding along east side of Great Valley, in vicinity
of South Mountain, Md. After P. B. King, 1950a. pCv, volcanic rocks; Cc, Lower Cambrian Chil-

howee gr.; COl, Cambrian and Ordovician Is., dol., and some sh.; Ordovician shade.
See Fig. 8.22.

SOUTHERN AND CENTRAL APPALACHIANS
Osp Om\ Osp

Oc V
OpsUsP QrrV

117

Orr

MMk

Os ».

0€c

Os Orr

0€c

mm«

a\\\

0€c 0€c

GREAT VALLEY, MARYLAND

AFTER SANDO, 1957

i—

2
i

3 Miles

BLUE RIDGE

BLUE RIDGE, LOUDOUN COUNTY, VIRGINIA

SHORT HILL
€wu €wm €wl

AFTER NICKELSEN,I956

Fig. 8.24. The Great Valley (Shenandoah) and Blue Ridge in Maryland and northern Virginia.
For location see Fig. 8.22. Om, Martinsburg shale; Oc, Chambersburg limestone; Osp, St. Paul
group (limestone); Ops, Pinesburg Station dolomite; Orr, Rockdale Run formation; Os, Stonehenge

chiefly by the Carolina slate belt, which extends from Virginia through
the Carolinas into Georgia (see Fig. 7.1). Its rocks are slates, graywackes,
pyroclastics, and lavas, which are only moderately folded or meta-
morphosed except near some granitic body. Still farther southwest in

limestone; OCs, Conococheague formation; Cc, Tomstown formation; Ca, Antietam quartzite; €h.
Harpers formation; €wu, €wm, and Cwl, Weverton quartzite; CI, Loudon formation; pCc, Catoctin
metabasalt; p€sr, Swift Run phyllite; pCg, gneissic basement.

southwestern Georgia and southeastern Alabama is the smaller Pine
Mountain belt of quartzite, marble, and schist. The age of the rorks
of both the Carolina slate belt and Pine Mountain belt is unknown, but
recent workers are inclined to think they may be early Paleozoic and

118

STRUCTURAL GEOLOGY OF NORTH AMERICA

Cambrian , •'' ,.

aol v /s / Ch.lho^ee gr

Chilhowee gr

I oudoun fm
c°TocTin ~~ '9reer>srone

Fig. 8.25. Blue Ridge near Elkton, Va. After P. B. King, 1950b.

somewhat metamorphosed during the later Taconian or Acadian

Batholiths

A number of plutons, most of batholithic proportions, occur in the
Piedmont province. Their distribution is shown on the Tectonic Map of
the United States, on the Geologic Map of the United States, and on the
Geologic Map of North America. Major differences in distribution appear
on the three maps; the later one shows a far less extent of the plutons in
South Carolina and Georgia than the earlier one. According to Keith
(1923) most of the plutons are granite and are little deformed or non-
deformed. According to Jonas (1932) the Petersburg granite of Virginia
is not deformed; it cuts across older structures without disturbing them
and enters the rock by replacing those already there.

The plutons are known today, however, to be both concordant and
discordant. The former are foliated, and in the older reports are con-
sidered early Precambrian. The more or less discordant plutons are the
massive ones, and according to the older reports ( Keith, 1923, and others)
are of late Paleozoic age and associated in time with the folding and
thrusting of the Valley and Ridge province. The separation into two
vastly different time groups is now held to be unwarranted for two
reasons: (1) A similar complex is well-worked out in New England
(Chapter 11), and on the basis of fossils and stratigraphic succession the
intrusions range in age from Late Ordovician to Carboniferous; (2)
isotope age determinations now date the intrusions as Paleozoic. It seems
probable that the metamorphism of the Blue Ridge and crystalline Pied-
mont developed progressively during Paleozoic time as a result of
orogeny, possibly several phases of orogeny. The silimanite schist and

gneiss zone of the inner Piedmont evolved as a result of the invasion
of the vast granitic plutons.

Structure of the Piedmont

From within the central metamorphic and plutonic belt northwestward
to the Great Valley nearly all the faults, folds, and cleavage are steeply
inclined but have a northwestward asymmetry; i.e., the fault planes, fold
axial planes, and cleavage planes dip to the southeast. Toward the
Coastal Plain a tendency is noted for the opposite asymmetry. The
northwest asymmetry of the inner zone (Fig. 7.2) is more one of folia-
tion than major displacement along a few discrete faults, with relatively
slight movement along an infinite succession of foliation planes ( Bloomer,
1950).

-J L

Fig. 8.26. Geologic map of Greaf Smoky AAountains and vicinity. After King ef a/., 1958. A,
Early Precambrian granitic and gneissic rocks; b,c,d,e, groups of the Ocoee series (later
Precambrian); P. Chiihowee group (Cambrian and Precambrian(?)); h, Mississippian, Ordovician,
and Cambrian rocks.

SOUTHERN AND CENTRAL APPALACHIANS

119

ENGLISH MTN

Chilhowee gr
v\ ^ Cochron SonasucA

GftEAT SMOKY PITS (COST END)

SECTION 5A

Fig. 8.27. Northeast part of Great Smoky Mountains and adjacent foothills on north.
After P. B. King, 1950a. The Great Smoky conglomerate, the Nantahala slate, the Pigeon
siltstone, and the Sandsuck shale, are part of the Ocoee series (Late Precambrian) which
forms most of the Great Smokies. The Cochran conglomerate is basal Cambrian. For location
see Fig. 8.22.

Infolded Belts of Metasedimentary Rocks

Besides the gneisses, the metamorphic and plutonic belt contains other
metamorphic rocks that are clearly of sedimentary origin. These characteristi-
cally form narrow belts or bands of considerable linear extent. The principal
belts of metasedimentary rocks are:

1. The Arvonia slate belt, near the James River, and the Quantico slate belt,
near the Potomac River, in Virginia. These are synclines of fossiliferous Ordovi-
cian rocks, lying uncomformably on older schists and granites.

2. A belt of quartzite, schist, and marble in North and South Carolina, which
has been mapped by Keith (1931) in the Kings Mountain area. Further details
have been given by Kesler (1944), whose interpretations differ from those of
Keith.

3. The Brevard schist belt [Figs. 7.1 and 8.30], which is by far the longest,
and extends from central North Carolina through South Carolina, Georgia, and
Alabama to the Gulf Coastal Plain. Jonas (1932) states that similar rocks con-
tinue northeastward from central North Carolina into southern Virginia. The
rocks of the Brevard belt consist of contorted dark slates and schists, with
lenses of limestones, apparently of a somewhat lower grade of metamorphism
than the rocks which flank them on either side.

4. The Murphy marble belt of western North Carolina and Northwest
'■ Georgia (Fig. 8.22), has many features similar to the others just described, but

differs in that it is not flanked by crystalline rocks, but by altered sedimentary
i rocks of the Ocoee series.

No fossils have been found in the belts south of Virginia and the age of the
! rocks which compose them is unknown. They have been variously assigned to

the Paleozoic and the Precambrian (King, 1950a).

Carolina Slate Belt

In the southeast part of the Piedmont province, highly metamorphosed
rocks give place to less metamorphosed sedimentary and volcanic rocks
which make up the Carolina slate belt (Fig. 7.1). Granite intrusions are

present, but they appear to be small and widely scattered and also cross-
cutting rather than concordant. The most extensive rock unit is the
"volcanic series." It is composed of flows, breccias, and bedded tuffs of
volcanic origin with some interbedded slates and sandstones. To the
southwest in southwestern Georgia and northeastern Alabama is the
shorter and narrower Pine Mountain belt. It is composed of quartzite,
marble, and schist clearly of sedimentary origin and intruded bv a
gneissic granite. The beds are broadly rather than steeply folded. The
age of both the rocks of the Carolina slate belt and the Pine Mountain
belt is uncertain; they have been assigned to both the Precambrian and
Paleozoic.

Paleozoic of Florida

Within the area embracing northern Florida and adjacent parts of
southern Alabama and Georgia, recent drilling has shown that the Meso-
zoic rocks are underlain by volcanic rocks and by sedimentary rocks of
Paleozoic age (Applin, 1949).

In the Ocala uplift, pre-Mesozoic rocks are reached in places at depths
of less than 4000 feet, but elsewhere they may lie as much as 10,000
feet below the surface. Penetration of the pre-Mesozoic rocks has not
been sufficient to establish a sequence; in other words, different rock
types have been found in different wells, but have not been found in
superposition.

The sedimentary rocks are mainly sandstones and shales. Some of the
sandstones contain worm tubes of Scolithus type, not unlike those found
in the older Paleozoic rocks of the Appalachians; others contain large
quantities of detrital mica. The shales are gray, black, and even red.
Graptolites have been found in places, as well as various other fossils.
The only Paleozoic systems whose existence has been definitely proved
paleontologically are the Ordovician and Silurian, although others might
be present. The volcanics may be related to the "volcanic series" of the
southeast part of the Piedmont area, but like this series, their age has
not been established.

Well cores show that these rocks are little deformed. Metamorphic
effects, such as cleavage and recrystallization, are lacking. Bedding dips

120

STRUCTURAL GEOLOGY OF NORTH AMERICA

Fig. 8.28. Serpentine belt of the Appalachians. By H. H. Hess, Princeton University; and pub-
lished with his permission. Circles represent known bodies of serpentine.

at low angles; in some places the strata are flat, and the maximum in-
clination is 25° to 30°. Drilling is too widely spaced to permit determina-
tion of more than the gross structural pattern. As the rocks have been
encountered over an extensive area, even these low dips would be suffi-
cient to account for a sedimentary and volcanic sequence of considerable
thickness.

These discoveries are of great interest, as they show that southeast of
the Appalachian system there is a foreland or shelf of little deformed
rocks, just as there is northwest of it.

Ultrabasic Intrusives

Hess ( 1937a ) has charted the serpentinized ultramafic intrusives of the
Appalachians and finds they form a narrow belt lengthwise of the Pied-
mont crystalline province through New England to Quebec City, thence
through the Taconic and Acadian belt of Quebec to the Gaspe Peninsula,
and again in a belt through Newfoundland. See the map of Fig. 8.28. In
his work in the Greater and Lesser Antilles, he has concluded on the basis
of considerable evidence (see Chapter 42) that the serpentines occur in
the arcuate, highly deformed, orogenic belt, and as a conclusion, that in
certain ancient orogenic belts, now obscured by metamorphism and
blanketing deposits, they can be taken to indicate the position of the
zone of maximum orogeny. The serpentinites are chiefly associated with
the Taconian orogenic belt in New England and the Maritime Provinces,
and they are strong evidence, it seems to Hess, that the core of the
Taconian orogeny stretched through the crystalline Piedmont of the
southern and central Appalachians.

Resides the granite plutons, the metamorphic and plutonic belt con-
tains a group of intrusives of ultrabasic composition — peridotites, dunites,
pyroxenites, and others, now in part altered to serpentine. Unlike the
granites, they mostly occupy small areas, but in many places they form
well-defined zones, indicating that they were intruded under the in-
fluence of some sort of tectonic control. The most prominent zone lies
toward the northwest edge of the metamorphic and plutonic belt, in the
southeast part of the Rlue Ridge province of western North Carolina; it
continues northeastward into Virginia, and southwestward into Georgia.

SOUTHERN AND CENTRAL APPALACHIANS

121

j Fig. 8.29. Map of part of the Blue Ridge and Piedmont provinces of western North Carolina,
| showing the distribution of ultrabasic igneous rocks. After P. B. King, 1950a. Stippled areas
I are those of sedimentary rocks, mainly Paleozoic, but including the Ocoee series, probably Pre-

Other less well-defined groups of intrusives occur toward the southwest

edge of the metamorphic and plutonic belt, as near the inner margin of
1 the Coastal Plain in Georgia. The age of the ultrabasic intrusives in the

southern Appalachians is unknown. Pratt and Lewis, on very tenuous
f evidence, conclude that they are of older Paleozoic age (King, 1950b).

See Fig. 8.29.

J Crystallines of Maryland and Southern Pennsylvania

The Piedmont of Maryland and southern Pennsylvania merits special
j attention because of the considerable detailed work done there by Ernst

Cambrian, and Brevard schist of unknown age. Blank areas east of, and within, stippled area
are those of gneiss and schist with bodies of intrusive granite. Small black areas are ultra-
basic igneous rocks. Heavy lines are faults. After P. B. King, 1950a.

Cloos, students, and colleagues. Cloos (1953) has divided the region into
twelve divisions or zones, the first being the Coastal Plain. See map,
Fig. 8.30. The second division is the belt of most intense metamorphism
of the Piedmont province (Wasserburg ct ol, 1957) and contains a
number of gneiss domes. Six of these are in the vicinity of Baltimore and
their cores are made up of gneiss and migmatite (Baltimore gneiss)
which are mantled by the metasedimcnts of the Glenarm series (Tilton
et at, 1958). The lowest formation of the Glenarm is the Setters quartzitc,
the next above the Cockeysville marble, and the last the Wissahickon

Fig. 8.30. Tectonic map of Maryland and southern Pennsylvania. Reproduced from Cloos, 1953.

SOUTHERN AND CENTRAL APPALACHIANS

123

schist. Granitic stocks and pegmatite dikes cut the domes and meta-
sedimentary mantle.

Foliation in the Baltimore gneiss parallels the contact with the mantle
and arches over the domes in asymmetrical form with the steep flanks
to the southeast. Lineation appears like raindrops running off an umbrella
(Cloos, 1953).

The Baltimore gneiss has been considered Precambrian in age and
possibly as old as any rock in the Piedmont. The Glenarm sediments are
thought to have been deposited in late Precambrian or early Paleozoic
time unconformably upon the gneiss. It is clear, however, that the same
degree of metamorphism pervades the overlying Glenarm rocks as the
Baltimore gneiss, and since the foliation of one parallels the other, it
has been assumed that metamorphism and doming of the mantle has
obliterated the original basement structures and produced a new con-
cordant foliation.

If the unconformity existed, two periods of tectonism are implied, one prior
to Glenarm sedimentation and another following it. If the unconformity did
not exist, a single period of deformation, metamorphism, and injection can
explain observed relationships. All previous investigators of the domes favor
existence of the unconformity, but conclusive proof is lacking (Tilton et al.,
1958).

The age of the post-Glenarm tectonism is generally considered
Taconian or Acadian. Evidence bearing on this conclusion will be pre-
sented later. The age of the Precambrian tectonism will also be taken
up later.

The third division of the Piedmont shown on Fig. 8.26 consists mostly
of the Glenarm series with generally horizontal fold axes. Foliation is
vertical on the east border and is inclined to the southeast on the west
border. The fold axial planes dip to the southeast also. Metamorphism
lessens toward the west, and mica schists become phyllites; amphibolites
become epidote- and chlorite-rich greenschists.

The rocks of this zone have not yet been correlated with the fossil-
bearing early Paleozoic strata west of the Martic line. It is possible,
according to Cloos, that the Cambro-Ordovician limestones of the
westerly zone (6) are facies of the once sandy rocks of the Glenarm.

Zone four encompasses the Sugarloaf structure which, as shown by
the closure of the bedding, is an anticlinal dome. The western limb is
overturned. Cleavage confirms the domal structure. The rocks arc in
the chlorite and greenschist facies of zone three. The local phyllites are
correlated with the Cambrian Harpers phyllite, and the quartzites which
are below the Harpers are most likely the Lower Cambrian Weverton
quartzite (Scotford, 1951).

The Martic line is called division five. It was first recognized as an
overthrust in which the then presumed older YVissahickon schist was
thrust westward over the presumed younger Paleozoic strata, and all
rocks southeast of the "fault" were regarded as Precambrian and north-
west of it as Paleozoic. Careful work has shown that the line is not a
discrete plane of major displacement, but that in most places complicated
conditions pertain (Cloos and Heitanen, 1941). It was also presumed
that the Martic "thrust" is a boundary between highly metamorphosed
schists and little metamorphosed Paleozoic strata. Cloos and Heitanen
have demonstrated that metamorphism is not restricted to the YVis-
sahickon schist but that all rocks including the Cambrian Antietam schist,
Vintage dolomite, and Ordovician Conestoga limestone show the same
intensity of metamorphism. At one place the sequence is repeated five
times, where the Conestoga is capped by the YVissahickon schist, which
in several ways is similar to the Antietam. At another place the Antietam
schist almost meets a spur of YVissahickon.

Along the Martic line the fold axes are horizontal or plunge predominantly
to the southwest. All folds are overturned southward. Flow cleavage is an
axial plane cleavage and dips to the north. Bedding is intensely crumpled and
at manv localities is entirely obscured by later cleavage. Since all members of
the sequence are thin and underlie large areas, it can safely be assumed that
bedding is roughly parallel to the boundary planes and thus largely conform-
able in all members of the sequence (Cloos and Heitanen. 194H.

Zone six consists mainly of Cambro-Ordovician limestones which are
strongly cleaved and overturned to the west. The zone is covered in large
part with the Triassic deposits (division seven).

Zone eight is the Blue Ridge belt previously described, and cleavage
and lineation extend northwestward to the position labeled "tectonite

124

STRUCTURAL GEOLOGY OF NORTH AMERICA

front." From this line westward the sedimentary rocks are non-tectonites.
The other zones have been described in previous parts of this chapter.

Age Determinations by Radioactivity

The first isotope age determinations on the minerals of the crystalline
Piedmont were published in 1941 (Goodman and Evans), and since then
methods and calculations have been refined, new methods developed,
and a fair number of presumably reliable dates have been determined.

Two groups of ages are now fairly well established, namely, one
ranging from 1000 to 1100 m.y. and one ranging from 250 to 390 m.y.
References to all significant dates may be found in recent publications by
Tilton et al. (1958), Hurley et al. (1958, 1959), and Carr and Kulp
( 1957 ) . An abstract by Kulp et al. ( 1957 ) is significant for ages in the
southern Piedmont.

The older ages (1000-1100 m.y.) come principally from zircon sub-

U238 U235 Pb207 , Th232

jected to , , , and analyses.

Pb206 Pb207 Pb206 Pb208

Three of the mantled domes in the Baltimore area (zircon from the
Baltimore gneiss ) , two gneisses from Bear Mountain, New York, a gneiss
from Shenandoah National Park, Virginia, and two gneisses from
Hibernia. New York, were sampled and the zircons run. The results range
from 1030 to 1170. Rubidium-strontium age measurements were also
made on microcline from the three Baltimore gneiss domes and a value
is fixed for one at 1200 plus 100 or minus 200 m.y. and for another at
about 1040 m.y. It is concluded by Tilton et al. ( 1958 ) that the zircon
and microcline ages record a 1000-1100-m.y. crystallization in the Pied-
mont.

Now, from the same specimens of Baltimore gneiss from which the

iv i_ • j i.. • i. K40 j Rb87
zircon and microcline ages were obtained, biotite by and —

5 y A40 Sr87

analyses gave ages of 305-339 m.y. For the older and younger dates of
the same rock two interpretations can be thought of:

(1) The gneiss was crystallized or recrystallized 1000-1100 m.y. ago (2)
The gneiss was originally a clastic sediment metamorphosed 300-350 m.y. ago,

and the zircon and microcline were relict detrital grains eroded from a terrain
1000-1100 m.y. old. The first interpretation is favored, chiefly because of the
non-clastic character of the microcline grains. Their irregular shapes, with deli-
cate projections and interlocking contacts with other minerals, were clearly
formed during crystallization of the gneiss. Possible detrital origin for the
zircon cannot be excluded, although if this were the case a greater age than
that of the microcline might be expected. It is concluded that the microcline
and the zircon probably record a 1000-1100 m.y. crystallization in the Balti-
more gneiss, while biotite records a second crystallization 300-350 m.y. ago. It
should be noted that these conclusions allow either a sedimentary or igneous
origin for the gneiss (Tilton et al., 1958).

Kulp et al. (1957) report a granite from eastern Georgia about 250
m.y. old. They also give "apparent ages" of 320-370 m.y. for the "meta-
morphic series" in western Virginia and North Carolina as well as the
pegmatite swarms in the Spruce Pine and Bryson City districts of North
Carolina.

In New England a number of radioactivity age measurements have
been made on plutons where the intrusive relations to well-dated fos-
siliferous strata are visible, and it is concluded that the Devonian period
began approximately 400 m.y. ago and ended slightly less than 250 m.y.
ago. These data will be presented in Chapter 11 on New England. It
appears, therefore, that the recrystallization and plutonism (tectonism)
in the Piedmont province ran its course during the Devonian period. This
is younger than the Taconian orogeny of New England and the Maritime
provinces which, from angular unconformities, is dated as late Ordo-
vician. The Acadian orogeny is generally regarded as having occurred
during the upper half of Devonian time, so the dates over 300 m.y. seem
too old for it, unless extended by definition.

SUMMARY OF OROGENIC HISTORY

The major lines of evidence of orogeny in the Appalachian mountain
system come from the sedimentary domains, the structures and structural
relations, metamorphism, plutonism, and isotope age determinations.
These have all been reviewed, and now may be integrated and the fol-
lowing conclusions reached.

1. An orogeny occurred along the Atlantic margin of the United
States south of New York City in which previously existing rocks were

Fig. 8.31. Regimen of Appalachian sedi-
mentation in the early Paleozoic. Partly after
P. B. King, 1959.

:\A:v.:

APPALACHIAN
: STRUCTURAL
FRONT

■ORDOVICIAN CLASTICS

*2 / NW EDGE

\^ / OF BlUE
\ ' RIDGE BELT

SILURIAN AND OEVONIAN CLASTICS

..APPALACHIAN
j STRUCTURAL
FRONT

/ NW EDGE
/ OF BLUE
' RIDGE BELT

Fig. 8.32. Regimen of Appalachian sedi-
mentation during Middle and Late Paleozoic.
Partly after P. B. King, 1959.

SOUTHERN AND CENTRAL APPALACHIANS

127

recrystallized 1000-1100 m.y. ago. This correlates in time with the Gren-
ville orogeny of Ontario and Quebec.

2. The continental margin was subparallel with the present, but may
have been extended by a continental shelf and slope type of deposit in
times following the Grenville orogeny, particularly in Late Precambrian
and Early Cambrian time. This was the time of accumulation of the
Ocoee series and the Chilhowee group.

3. The Atlantic margin of the continent was beset with deformation
beginning in the last part of early Ordovician time, and the previous
region of sedimentation now was elevated and became the source land
of sediments to the west. See Fig. 8.31. A great fan or wedge of clastic
sediment was spread northwesterly from the Great Smoky region during
the Middle Ordovician and another one in Late Ordovician time in New
England. The crustal deformation must have been mostly elevatory at
this time because the metamorphic and plutonic activity occurred some-
what later. The New England clastic wedge records part of the Taconian
orogeny as defined, but no name has been proposed for the Middle
Ordovician uplift.

4. Clastic sedimentation on a large scale shifted during Silurian
and Devonian time to New York, Pennsylvania, and West Virginia, and
another great fan of sediments was deposited there, also derived from
uplifted lands on the east. See Fig. 8.32. The Silurian and Lower Devo-
nian elastics were not very thick, about 5000 feet, but then a flood of
sediments reached 10,000 feet in thickness in late Devonian time. Strong
compression and plutonic tectonism started in early Devonian time, ac-
cording to the isotope age measurements, but evidently high mountains
were not created until the beginning of the Late Devonian.

Figure 8.33 is an idealized section of the southern Appalachian system
and illustrates the central belt of most profound Devonian metamorphism
and plutonism. This, when much eroded, became the crystalline Pied-

CUMBERLAND
PLATEAU

BlUC RIDGC PICDM0NT PROVINCt

VALLCY 1 RIDCt ..tferomorphic f flutomc belt

\ V. 1 ill Coro/mo i/ofe

PROVINCE

He/t

ATLANTIC COAJT-
al plain

Fig. 8.33. Idealized cross section of the southern Appalachian Mountains system. After P. B.
King, 1950a.

mont. The age of the Carolina slate belt sediments is unknown but evi-
dently older than the Devonian tectonism. It may be speculated that they
were a collateral eastern deposit of the Middle Devonian clastic wedge
on the west of a medial uplift, but their age must be known first before
they can be correctly fitted into the picture.

5. Uplift of the orogenic belt was general along its entire length
during the Mississippian and sediments were carried westward and added
to the miogeosyncline. However, in Early Pennsylvania!! time uplift was
particularly great in the southern Piedmont and another thick wedge
accumulated on the west. Later, sedimentation shifted to the West
Virginia and New York and considerable clastic material of continental
environment accumulated during Late Pennsylvanian and Permian time.

6. The eastern part of the miogeosyncline including the thickest parts
of the clastic wedges and the eastern part of the great Cambro-Ordovician
carbonate sequence was then compressed and cast into folds and thrusts
as exemplified in the Valley and Ridge province of Fig. 8.33. The de-
formation is generally referred to as the Appalachian orogeny. It may
have started in Mid- or Late Pennsylvanian time in the south but farther
north Valley and Ridge deformation could not have occurred until the
close of Pennsylvanian time, and it may not have happened until near
the close of the Permian.

site of a large Triassic basin, and under the Bay of Fundy and along its
east shore in Nova Scotia another such basin exists. See Plate 9.

The Triassic areas are generally sites of lowlands because the basin
beds have yielded to erosion more than the adjacent crystallines. The
Triassic lowlands is the physiographic name generally given to the Penn-
sylvania-New Jersey basin. The lowlands are marked, however, by ridges
of trap rock that stand rather prominently above the lowland plain.

EASTERN TRIASSIC BASINS

DISTRIBUTION OF BASINS

A series of long, narrow basins of Triassic deposits occurs along the
eastern margin of the continent. It will be seen by reference to the Geo-
logic Map of the United States or Geologic Map of North America that
the basins start at the north boundary of South Carolina in the Piedmont
crystalline province and extend through North Carolina, Virginia, Mary-
land, Pennsylvania, and New Jersey to the lower Hudson River Valley in
New York. The basin in Pennsylvania and New Jersey is the largest of any
in the United States, and for a distance between the Carlisle prong and
Reading prong of the Blue Ridge element, it borders on the Ridge and
Valley province.

The Connecticut River Valley in Connecticut and Massachusetts is the

NATURE OF TRIASSIC ROCKS

General Character

The Triassic sedimentary rocks of the eastern basins are chiefly clastic
and dominantly red. Fanglomerates, conglomerates, sandstones, arkoses,
siltstones, shales, and argillites are the common sedimentary types. Much
basic magma has invaded the sediments and now exists as thick sills and
long dikes of diabase. Basalt flows from the same magma are also inter-
calated in the shales and sandstones. The intrusive rocks have commonly
altered the red sediments to blue or gray along the contacts in zones 50
to several hundred feet thick.

New Jersey-Pennsylvania-Maryland-Virgina Basin

Newark Group. The sediments of the New Jersey-Pennsylvania-Mary-
land-Virginia basin are known as the Newark group. The basin has a
maximum width of 30 miles and is over 300 miles long. Part of it is shown
in Fig. 9.1. The Newark group has been classified in three formations, the
Stockton, Lockatong, and Brunswick, the last-named being the youngest.
These subdivisions are clearly separable along the Delaware River and
northeastward in New Jersey, where they were first established and
named.

The Stockton formation in general comprises arkosic sandstone with
some red-brown sandstone and red shale, in irregular succession and pre-
senting many local variations in stratigraphy. It lies unconformably on
Paleozoic and pre-Paleozoic crystalline rocks. The sandstones are in places
cross-bedded, and the finer-grained rocks exhibit ripple marks, mud
cracks, and raindrop impressions, which indicate shallow-water conditions

128

EASTERN TRIASSIC RASINS

129

during deposition. The arkose, a sandstone containing more or less feld-
spar or kaolin derived from granite or gneiss, indicates proximity at the
time of deposition to a shore of Precambrian crystalline rocks.

The Lockatong formation consists chiefly of dark-colored fine-grained
hard and compact argillaceous rocks. Some beds are massive, and others
are flaggy. They show mud cracks and other evidences of shallow-water
deposition, but their materials are clay and very fine sand, some of the
beds also contain carbonaceous material.

The Rrunswick formation, in its typical development, consists mainly of
a great thickness of soft red shale with local and thin layers of sandstone.
Northward and westward the sandstone increases in amount and coarse-
ness. It overlaps irregularly older Traissic formations and Paleozoic and
pre-Paleozoic formations.

The three formations are not sharply separated by abrupt changes of
material, but usually merge into one another through beds of passage
which appear to vary somewhat in thickness and possibly also in strati-
graphic position in different areas.

The thickness of the Stockton is estimated to range from 1000 to 3000
feet, the Lockatong from 1500 to 3000 feet, and the Brunswick from 12,000
to 16,000 feet. The total thickness of the Newark group as generally men-
tioned is about 20,000 feet, but figures up to 35,000 feet have been pro-
posed. This great amount is computed by the dip angle and the distance
across dip of the homoclinal beds, but several writers have suggested the
possibility of duplication of certain beds by faulting, and hence that the
figure may be excessive. Stose and Stose ( 1944) suggest that the beds over-
lapped from east to west in somewhat the manner shown in Fig. 9.2 and
that therefore the combined thickness of all the beds will not be found in
any one place. It cannot be doubted, however, that the long, narrow
troughs containing the Triassic sediments are very deep, undoubtedly
over 10,000 feet, and probably 20,000 in places.

The age of the Newark group is probably Upper Triassic, but the high-
est beds may be lowermost Jurassic. According to Bascom and Stose
(1938),

A comparison of fossil plants, crustaceans, and vertebrates of the Newark
with simliar forms of the Jura and Trias of Europe establishes a correspondence

» 2* milCS

Fig. 9.1. Triassic basin in western New Jersey, Pennsylvania, and Maryland. Stippled area,
Triassic sedimentary rocks; solid black areas and heavy black lines, Triassic diabase sills and
dikes; light black lines, faults. Reproduced from Stose and Stose, 1944.

within general limits, but a correlation of exact horizons is not practicable.

The Newark strata did not share in the folding that occurred at the end of
Carboniferous time and therefore must be of later date; they are, however,
clearly older than the lowest Cretaceous formations, which overlap them un-
conformably. They are thus separated from earlier and later deposits by inter-
vals of upheaval and erosion of unknown duration, but their position in geo-
logic history cannot be determined more closely than by the general correlation
of fossils above indicated.

Igneous Rocks. The map of Fig. 9.1 shows the distribution of outcrop-
ping sills, lava flows, and dikes in the Newark group and in adjacent
rocks of the Piedmont. The sills and flows are confined to the Triassic
basin, but some of the dikes cross out into the older rocks of the Piedmont
and persist for many miles. The Conshohocken and Downington dikes

130

STRUCTURAL GEOLOGY OF NORTH AMERICA

NW.

SE.

Fig. 9.2. Origin of the Newark Triassic basin. Reproduced from Stose and Stose, 1944. The
sediments are postulated to have first been derived almost entirely from the east. After intrusion
of the diabase dikes and sills and renewed faulting, much fanglomerate was washed in from
the west.

are 60 to 70 miles long, and the Safe Harbor dike extends an equal length
before it is covered by the Cretaceous of the Coastal Plain.

The largest sill in the southern part of the Newark basin is the Gettys-
burg, which is 1800 feet thick. Farther northeast in New Jersey four
great sheets of trap rock occur and form the Watchung Mountains which
are more prominent that the ridges of the great Gettysburg sill. The low-
est of the four sheets is intrusive and in places reaches a thickness of 1000
feet. It forms the Palisades of the Hudson. See section 31 of Fig. 8.21.
Above the Palisades sill and separated from it and each other by several
hundred feet of intervening Triassic shales are three extensive (buried)
basalt flows which, from bottom to top, are 650, 850, and 350 feet thick.

The dikes are believed to follow tension cracks which in places become
faults and offset the Triassic beds. According to Stose and Stose ( 1944)
the dikes and the normal faults that the dikes follow represent major lines
of Triassic fractures. They cut across older structural lines, which are
nearly at right angles to them. Many of the diabase dikes originate in or
join the diabase sills which are most abundant along the northwestern
part of the basin. See map, Fig. 9.1.

The diabase sills, with which many of the dikes connect at their northwestern
ends, coalesce to form extensive intrusive bodies in the northwestern part of
the Triassic area of Pennsylvania. The larger sills are the Haycock, Ziegler, Saint
Peters, Yorkhaven, and Gettysburg. They parallel the strike of the sedimentary
rocks for long distances, and then the intrusive body cuts across the strike at
right angles. Most of these crosscutting bodies extend to the northwestern edge
of the Triassic basin where they terminate against the faults that form the
boundary of the basin. Each of these intrusive bodies, therefore, has the form
of a great tilted trough bounded on the southeast side by the west-dipping sills
and at the ends by the crosscutting bodies and open at the west.

The fissures through which the diabase entered the Triassic rocks are be-
lieved to lie near the northwest edge of the basin where the greatest amount of
progressive sinking and faulting occurred during Triassic deposition. The
rising magma broke through the Triassic beds near the vents in the form of
crosscutting bodies, and injected the beds to the southeast in the form of sills.
The magma extended still farther southeastward as dikes that followed vertical
fractures in the Triassic sedimentary rocks and continued into the older under-
lying rocks southeast of the limits of the basin of Triassic sedimentation. Some
of these dikes in the area southeast of the Triassic outcrops may have been
feeders of large diabase bodies in Triassic sedimentary rocks that are now re-

EASTERN TRIASSIC BASINS

131

moved by erosion, but the evidence is not available to support such a view
(Stose and Stose, 1944).

Border Conglomerate. Along the northwest border of the Triassic
basin occur deposits of fanglomerate, generally called conglomerate and
breccia. They make up the "Border conglomerates." In width of exposure
they range from less than half a mile to about 8 miles and lie in discon-
tinuous patches along the Precambrian and lower Paleozoic rocks of the
northwestern border. The largest area is south of Reading, Pennsylvania,
which extends across the Gettysburg (Brunswick) formation to the New
Oxford (Stockton). Most of the gravel fragments were derived from
Lower Paleozoic limestones, dolomites, sandstones, and quartzites, but
some came from beds as high as the Devonian, and some are Precambrian
rocks. In one place Triassic basalt forms boulders and cobbles in the
fanglomerate (Carlston, 1946).

The Border conglomerate is for the most part of Brunswick age, and as
depicted in certain cross sections is the top and youngest layer of the
Triassic group. It seems to lie unconformably across the older Triassic
beds in places, and in others rests directly on the pre-Triassic. On the
other hand, the conglomerate beds pass into sandstones and shales and
are undoubtedly mostly a northwestward marginal facies of the Bruns-
wick. Even Border conglomerate wedges have been observed in the Stock-
ton and Lockatong, and although the conglomerate is chiefly of Bruns-
wick age, local bodies of it may be of any age within the Newark group
(McLaughlin, 1931, 1958).

Although the Border conglomerates clearly betray a northwest origin,
most of the material washed into the Triassic basin is thought to come
from the southeast. The reason, according to Stose and Bascom ( 1929 )
lies in the composition of the basin beds. The "poorly assorted arkosic
grits, containing feldspar and mica derived from disintegrating granitic
rocks" were exposed, they believe, only in the land southeast of the basin.
Except for a stretch of about 75 miles in southeastern Pennsylvania to
which Stose and Bascom refer specifically, the Triassic basins in the Pied-
mont are bordered on both sides by crystalline rocks that could have sup-
plied feldspar and mica, but the Paleozoic pebbles in the border con-

glomerate indicate that little Precambrian was exposed on the northwest
at the time of Triassic deposition in the southeastern Pennsylvania area.

Deep River Basin

The Deep River basin is in North Carolina and is generally regarded
as made up of the Cumnock basin on the southwest and the Durham
basin on the northeast. The southwestern basin is noted for its Triassic
coal. The deposits in these basins are much like those of the New ark basin
with an abundance of gray arkosic beds lensing into red sandstones and
shales and gray to buff sandstones. Locally thin carbonaceous shale beds
occur. Conglomerates, fanglomerates, and in places landslide breccias
mark the border zones, but here, unlike in the Newark basin, both bor-
ders are marked by the coarse deposits. Thin conglomerates with an abun-
dance of quartz pebbles occur also in the central areas (Prouty, 1931).

The torrential fanglomerates are more voluminous along the eastern
margin of the basin than the west, which shows that the eastern margin
was the steeper and that an area of land existed there as well as on the
west.

Connecticut Valley Basin

The Triassic sedimentary rocks of the Connecticut Valley are all clastic
and, if anything, coarser than those in New Jersey, Pennsylvania, and
Maryland. Red colors dominate, and they are also interlayered with trap-
rock sheets. The basin is bordered in part on both sides by faults, and is
thus a graben; but the eastern fault is by far the greatest and is known as
the Great Fault. All beds dip generally eastward into it, as the beds dip
generally westward into the border fault of the Newark basin. See map
of Fig. 9.3. The Great Fault has a throw estimated variously between
17,000 and 35,000 feet, but the basin beds and floor have not been re-
garded in the same way as Bascom and Stose conceived the structure of
the Newark basin. As diagrammed in the cross sections of Fig. 9.4 the
throw would be of the great magnitude mentioned, but if diagrammed as
it is in Fig. 9.2, the displacement would be much less.

According to Krynine (1941a) the wedge of sediments is built of coa-

132

STRUCTURAL GEOLOGY OF NORTH AMERICA

EXPLANATION

id shale

of mi/rsion

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SPRINGFIELD SECTION

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HARTFORD SECTION

/D°/eoJ:o

s °i& metamorphose**/

Fig. 9.3. Triassic basin of Connecticut and Massachusetts. Reproduced from Longwell, 1933.

Fig. 9.4. Generalized east-west sections across the Triassic basin in Connecticut and Massa-
chusetts. Somewhat modified after Longwell, 1933.

lescing alluvial fans that radiate westward from the Great Fault and thin
from 16,000 to 1500 feet in some 32 miles.

The stratigraphic units are (1) Lower: New Haven arkose, up to 8550 feet,
relatively coarse fluvial gray and pink arkoses, conglomerates, red feldspathic
sandstones, and subordinate red siltstones and shales; (2) Middle: Meriden
formation, up to 2800 feet, fine-grained lacustrine and paludial variegated and
dark siltstones, shales, limestones, light feldspathic sandstones, subordinate
coarse elastics, and three basaltic lava flows; (3) Upper: Pordand formation,
up to 4000 feet, fluvial deposit similar to New Haven.

Conglomerates form 10%, sandstones 64%, siltstones and shales 25%, red
color is present in 52%. Near the Great Fault sediments pass into fanglomerates.

Two main groups of alluvial fans are present: Central Connecticut (indicolite
and little epidote) and Southern Connecticut (no indicolite, much epidote).
Almost all the sedimentary detritus is derived from a source area only 5 to 10
miles wide east of the steep, moderately high Great Fault, whose recurrent
rejuvenation controlled sedimentation.

Four formations have been mapped on the state geologic map of Mas-
sachusetts (1916), but their distribution as continuous-layered units could
hardly be shown on cross sections. The central part of the basin at the

EASTERN TRIASSIC RASINS

1 ,,

surface is marked by the Chicopee shale; this is bordered on both sides
and the north by the Longmeadow sandstone, and this in turn by the
Sugarloaf Arkose. Along the east side is a coarse border aggregate called
the Mount Toby conglomerate. These formations are clearly facies and
grade into each other or are interdigitated. The Mount Toby conglomer-
ate is a fanglomerate in large part and an actual talus in others. There
can be little doubt about its relation to a great border fault; but in places
bedrock crops out surounded by conglomerate, and the position of the
fault is obscure.

Intercalated in the elastics and grouped close together in their cen-
tral part are three lava sheets of diabase. The middle one, the Holyoke
diabase, is the thickest and in places reaches 400 feet. Between it and the
upper are sandstones that contain large and small reptile tracks which are
very well known. Shortly after the third lava outpouring, an explosive
eruption took place; and fragments and dust of diabase were spread over
a large area to form the Granby tuff. Over the tuff was spread rusty sand
in which most of the tracks have been preserved. In the southern part of
the basin "dolerite" sheets have been intruded. Dikes are few.

Here as in the other Triassic basins, normal faults cut and displace the
beds and volcanic sheets. See sections, Fig. 9.4.

The red color and salt crystal impressions have led a number of writers
to envision a semiarid climate; but Krynine, on the other hand, contends
that the flora and swamps suggests a precipitation of about 50 inches a
year and a temperature of 70° to 80° F. Fresh arkoses and fanglomerates
can easily form under tropical humid climate in regions of steep topogra-
phy. Desiccation marks indicate alternating dry and wet seasons.

STRUCTURE OF BASINS

All the Triassic basins in the eastern United States are bordered on one
side or the other by major normal faults. A great fault, although irregular
and with branches and perhaps steps borders the Newark basin on the
west. The Deep River basin has a major fault on each side. The Con-
necticut Valley Triassic is bordered on the east by a major fault, also of
a complex nature. The long and very narrow basin that stretches from

North Carolina into Virginia is bordered on the west by a fault. The sev-
eral other small and detached basins are shown with faults on either the
east or west sides on the Geologic Map of the United States.

Associated with all the great border faults and perhaps due to them is
a general dip of the beds and sills toward them. See cross sections of Fi'_is.
9.2 and 9.3. The dips range from 5 to 50 degrees and are more generally
10 to 20 degrees. The Triassic beds are not folded as the underlying
Paleozoics and metamorphics, upon whose beveled edges they rest un-
conformably.

Strike faults within the sediments are known, somewhat parallel with
the border faults, and many transverse faults cut and offset the beds and
sills. In places the transverse faults terminate against normal strike faults
and produce a rhombic pattern. Some of the transverse normal faults have
been traced out into the folded and thrust-faulted Paleozoic rocks which
they also offset.

The normal faults within the basin cut the Triassic sediments and sills,
yet some of the dikes associated with the sills follow cross faults. It is gen-
erally concluded that the faulting is later than most of the beds, but before
the end of the period of volcanic activity, so that most of the sills are cut
by the faults, yet some dikes were injected immediately into the fractures
when they formed.

ORIGIN OF BASINS

The Triassic basins of the Piedmont province and of the Connecticut
Valley have a similar history. The troughs in which the sediments were
deposited are due mainly to downfaulting with a major fault or chain of
faults on either the outer or inner side. The trough block rotated by set-
tling most adjacent to the border fault. The border faulting is conceived
as a fairly continuous process during which the sediments accumulated in
the basins as they were progressively deepened. Stose and Bascom ( 1929)
represent sedimentation in the Newark basin to have started considerably
before the border faulting began (see Fig. 9.2); then with the onset of
faulting the previously deposited beds which came from the southeast
were tilted, and the site of later sedimentation, with continued faulting.

134

STRUCTURAL GEOLOGY OF NORTH AMERICA

shifted more toward the northwest. Also, with the onset of faulting, fan-
glomerates were washed in by torrential streams from the uplifted block.
In the Deep River basin, with faulting on both sides, the fanglomerates
came from both directions. If Triassic sedimentation started before fault-
ing, it may have been due to one of two causes : ( 1 ) a broad syncline may
have developed which later broke into faults on one or both sides, or (2)
a change may have occurred from a warm humid climate in whch red
soils were developed on the surrounding lands to an arid or semiarid one
in which salt crystals developed in the sediments from time to time, and in
which torrential floods were common.

The throw of the border faults according to the cross sections of Fig. 9.4
would equal the total thickness of the basin sediments, and therefore,
would be of the magnitude of 20,000 feet. This is twice as much as postu-
lated or computed for any other post-Proterozoic normal fault in North
America, and leads one to regard the large figure critically. Stose and
Bascom ( 1929 ) compute the throw at 6000 feet in the southeastern Penn-
sylvania area by means of their postulated origin of the Newark basin.

The nature of the faults of the Triassic basins, both in vertical and
horizontal position and movement, and the general plan of the entire zone
of faults from the Carolinas to Nova Scotia reminded Bain ( 1941 ) of the
Rift Valleys of Africa, and he considers them a rift zone.

LATE TRIASSIC PHASE (PALISADES OROGENY)

The onset of faulting that formed the troughs in which the Triassic sedi-
ments accumulated marked the beginning of the Palisades orogeny. It
started in late Triassic time and probably ran its course before the end of
the period. After the border faults had become major faults and great

thicknesses of sediments had accumulated, vast amounts of basic magma
entered the basins, chiefly along the border faults, and spread into the
sediments as numerous sills, some exceedingly thick, and as great dikes.
In places the dikes cut long distances into the country rock. Great amounts
of magma reached the surface as basalt flows, which were immediately
buried by the accumulating sediments. Accompanying the igneous activity
was an additional episode of faulting. Both strike and transverse parallel
faults provided avenues of ingress of the magma, and continued faulting
broke and offset some of the sills as well as the sediments. The great
border faults undoubtedly also continued active in places, dropping the
basins farther and inviting new floods of fanglomerate.

The entire activity from the inception of the border faulting through
the intrusive and extrusive activity and additional faulting seems to have
been fairly continuous and hence not separable into early and late phases.
It will all be recognized here as the late Triassic phase, or the Palisades
orogeny.

The faulting and dike intrusions spread into rocks adjacent to the
Triassic basins, and it is clear that at the time of maximum accumulation
the sediments were much more extensive than now. Their beds are bev-
eled on the sides opposite the border faults, and the fanglomerates still
bury in places the fault scarps and spread considerable distances over the
upthrown blocks. Whitcomb (1942) considers the Spitzenberg conglomer-
ate as a Triassic outlier 20 miles north of the present margin of the New-
ark basin. The now separate basins may easily have been confluent in
places, but such cannot be proved, it seems. It is also possible that, while
the Palisades orogeny was taking place in the Piedmont and folded Appa-
lachians, the continental margin lay 100 to 200 miles eastward and Triassic
sediments were accumulating there.

How toward the Atlantic. The Virginia, Delaware, Maryland, and New
Jersey section of the Coastal Plain is one of great estuaries in which tide
waters reach across the plain to the Piedmont. These are regarded as
drowned river valleys.

The Coastal Plain as a geologic unit extends out into the Atlantic Ocean
and forms the broad and well-known continental shelf there. Off Cap<
Hatteras, the shelf is only 30 miles wide, but both northeastward and
southwestward from the cape it broadens. Off New England, it is over
250 miles wide. See the Tectonic Map of the United States and Fig. 7.1 of
this book.

The Atlantic Coastal Plain is continuous with the Gulf Coastal Plain,
which is described in Chapter 41. Florida has been included in the Gulf
Coastal Plain, so will not be treated here.

ATLANTIC COASTAL PLAIN
AND ADJACENT OCEAN BASIN

EXTENT AND CHARACTER OF SEDIMENTS

The Atlantic Coastal Plain is underlain by poorly consolidated Quater-
nary, Tertiary, and Cretaceous sediments that dip gently seaward. The
Cretaceous sediments form a narrow inland belt of outcrop, and the Ceno-
zoic sediments a broad outer belt. In places, the Cenozoic sediments over-
lap the Cretaceous entirely and rest on the crystalline rocks of the Pied-
mont. See the Geologic and Tectonic maps of the United States. The
surface is nearly a plain, as the term coastal plain implies. The interrup-
tions to the plain are low, inland-facing questas and, in places, slightly
intrenched streams that cross the Cretaceous and Tertiary rocks as they

STRATIGRAPHY

The stratigraphy of the Atlantic Coastal Plain is illustrated by a chart,
Fig. 10.1 and five cross sections, viz., numbers 32, 33, 34, 35, and 36 of
Figs. 10.2, 10.3, and 10.4. Refer to the index map, Fig. 7.1, for the position
of the sections. Three of the sections across the Coastal Plain and two of
them run lengthwise of it.

The chief elements of the stratigraphy are the Upper Cretaceous, Eo-
cene, and Miocene. Lower Cretaceous beds have been noted in the north-
ern half of the Coastal Plain, and Oligocene beds in the southern part
(South Carolina and Georgia). A thin Quaternary cover is fairly extensive
in the area between Chesapeake and Delaware bays and in North Caro-
lina. For details of the stratigraphy, see Richards (1945, 1947).

A well in Maryland penetrated 169 feet of dark red, argillaceous sand-
stone, apparently of Triassic age. See section 36, Fig. 10.4.

STRUCTURE

Coastal Plain

Regional Dip. With few exceptions, the beds dip gently toward the
Atlantic. The crystalline floor upon which the sediments rest dips the

135

Macrofauna

Microfauna

New Jersey

Europe

Texas

Epochs
.and

Groups

Per-
iods

No fossils known
from outcrops

Beacon Hill

Cohonsey

Well preserved Mollusks
neor Shiloh, equivalent to
Calvert of Maryland

Neritic founo from wells

Poorly preserved Mollusks

Large neritic founa in wells
only

Kirkwood
Shark River

Auversion

Lutetian

Arenaceous species in outcrop

Monosauan

Cuisian

Bryozoo abundont
in South

Lorge well preserved founo
with mony plonktonic

Ypresian

Vincentown

Sparnacian

Cuculloea.Oleneothyns,
Gryphoeo Founo

\ Cuculloea Fauna

Diagnostic Danian ond Thone-
~~~X tion Species /

Thanetian

Danian

Lucino Fauna

Well preserved neritic founa.
Latest Marine Cretaceous NA

Red Bank

Cucullaea Fauna

Large, well preserved
neritic founo

Navesink

Maestrichtian

Lucina Fauna

Mt Laurel

Wenonah

Cucullaea Fauna

Well preserved neritic fauna
southern port of stole

Marshalltown

No fossils known
from outcrops

Englishtown

Companion

Lucina Fauna

Chiefly Arenaceous species in
outcrop.

Woodbury

Cucullaea Fauna

Arenaceous species.

Merchantville

Lucina Fauna

(Extensive Flora)

Brockish-woter and
Monne Mollusks
( Extensive Flora )

Magothy

Santonian

Coniacian

Turonian

Arenaceous species in
surfoce exposures;

Calcareous forms downdip
in wells

Raritan

Cenomanian

Claiborne

Wilcox
?

Midway

Navarro

Taylor

Austin

Eagle Ford

Woodbine

Plio.

Pol

2 °-

o o
o tj5

fr Glouconitic

_i_ Calcareous

Fig. 10.1. Cretaceous and Tertiary formations in the Coastal Plain of New Jersey. Reproduced from
Dorf and Fox, 1957.

ATLANTIC COASTAL PLAIN AND ADJACENT OCEAN BASIN

137

greatest amount, because most all the formations thicken seaward, and
each successively higher sedimentary surface dips somewhat less than the
"basement" floor. From a number of deep wells that have penetrated the
crystallines, the ancient surface can be contoured as shown in Fig. 10.6.
Its gentlest slope is in North Carolina, where a dip of 10 to 14 feet per
mile exists from the inner margin ( fall line ) to the coast in the southeast-
ern part of the state.

It then breaks seaward into a steeper slope of 122 to 124 feet per mile
(Berry, 1948). Two deep wells in northern Maryland demonstrate an off-
shore dip there of about 100 feet per mile (Balsley et al., 1946), and a
uniform slope is indicated. The two slopes in North Carolina are taken
to mean two peneplains by Berry ( 1948 ) , but their local development is
puzzling if this theory is true.

Unconformities. The great unconformity at the base of the Cretaceous
has already been implied in the discussion of the slope of the surface of
the crystallines. This ancient erosion surface, buried by the Cretaceous
sediments, has been called the fall zone peneplain. See block diagrams
2 and 3 of Fig. 10.8. Since an outer and sharper slope has recently been
defined, the ancient surfaces appear more complicated. It will be dis-
cussed further when the continental shelf is considered.

The Lower Cretaceous beds do not crop out anywhere along the At-
lantic Coastal Plain; they form a subsurface wedge between the crystalline
floor and the Upper Cretaceous. The dashed lines of Fig. 10.6 show the
extent and thickness variations of the Lower Cretaceous. It will be seen
that the wedge corresponds in position approximately to the outer steeper
slope of the crystalline floor. The isopachs should be related to those of
Plate 11 which depicts the distribution of Lower Cretaceous strata in the
Gulf Coastal Plain and the Caribbean regions. Not enough is known of the
Lower Cretaceous and Upper Cretaceous contact to decipher the rela-
tions. The Lower Cretaceous Potomac formation is regarded as nonma-
rine, and the overlying Tuscaloosa as marine (Richards, 1945).

According to Richards' ( 1945 ) correlations the Eocene bevels the Up-
per Cretaceous beds near Asbury, New Jersey ( section 32, Fig. 10.2 ) and
rests on the Lower Cretaceous in parts of Virginia ( section 33, Fig. 10.3 ) .

Fig. 10.2. Cross sections of the Atlantic Coastal Plain, after Richards, 1945. Section 32, from
Allentown, N. J., to Asbury Park, N. J. Section 34, from Goldsboro, N. C, to Cape lookout,
N. C See index map, Fig. 7.1, for location of sections.

138

STRUCTURAL GEOLOGY OF NORTH AMERICA

SECTION )S

■£

■SJ

Fig. 10.3. Crojs sections of the Atlantic Coastal Plain, after Richards, 1945. Section 33,
Richmond, Va., to Norfolk, Va. Section 35, Wilmington, N. C, to Parris Island, S. C. See
index map. Fig. 7.1.

SECTION J6

Fig. 10.4. Section of the Atlantic Coastal Plain from Virginia to Long Island, N. Y., after
Richards, 1947.

Evidently, therefore, an unconformity of considerable magnitude exists
between the Tertiary and Cretaceous systems.

The absence of Oligocene beds, except in the south, suggests an un-
conformity between the Miocene and Eocene. In most of Richards' sec-
tions, however, the Miocene seems conformable on the Eocene. One ex-
ception is noted near Summerville, South Carolina. A break, however,
occurs between Lower and Upper Miocene in the area between Norfolk,
Virginia and Wilmington, North Carolina, where the Yorktown-Duplin
formation rests across the entire Lower Miocene, Eocene, and most of
the Upper Cretaceous succession. The Geologic Map of the United States
shows very clearly the unconformity between the Yorktown beds and the
entire Upper Cretaceous, Eocene, and Lower Miocene succession in the
region adjoining the states of North and South Carolina. Inspection of the i
map also reveals an unconformity between the Pliocene beds and older
ones in this region.

Cape Fear Arch. The most conspicuous feature of the Coastal Plain
is the Cape Fear arch of North and South Carolina. See index map, Fig.
7.1, and the Geologic and Tectonic Map of the United States. Structure
contours on the top of the Cretaceous bulge outward at this place and
reveal a very broad nose on the regional seaward dip, so the structure is

ATLANTIC COASTAL PLAIN AND ADJACENT OCEAN BASIN

139

not truly an arch as defined in Chapter 2. The Eocene and Miocene con-
tacts with the Cretaceous also reflect the broad nose. The unconformities
around the Cape Fear arch indicate the principal times of uplift and ero-
sion to have been at the close of the Cretaceous and again at the close of
the early Miocene.

In the New Jersey region Dorf and Fox ( 1957 ) recognize eight trans-
gressive-regressive cycles of sedimentation in the history of the Coastal
Plain from Raritan ( Upper Cretaceous ) to Cohansey ( close of Miocene )
time (Fig. 10.1). If these prove to be of local extent, then it would be
concluded that the continental margin pulsed up and down locally this
many times, but if the cycles are found to be widespread and recorded
in the Gulf Coastal Plain sediments, then eustatic changes in sea level
would be the more probable cause. The subject will be considered in
Chapter 41 on the Gulf Coastal Plain.

known outcrops and well records; and two submarine traverses were run
across the continental shelf, one from Woods Hole southward, and one
from Cape Henry, Virginia, eastward (section 37 of index map. Fig. 7.1
The Cape Henry section is the most significant. Many reflection surfaces
were recorded in the sediments above the crystalline floor, and two par-
ticularly strong ones were measured by refraction beyond the present
shore line. See Fig. 10.5. The seismic data on the crystalline floor are in
fair agreement with the deep-well records and indicate that at a point
60 miles at sea off Cape Henry the basement would be 12,000 feet deep.
The significance of the other two surfaces is not altogether clear. Miller
( 1937 ) suggests that the "unconsolidated" zone consists of Cenozoic and
Cretaceous, and the "semiconsolidated" zone consists of Jurassic and
Triassic. The surface separating the two is known as the M /one to the
geophysicists, and this has later been considered as a reflection horizon

CONSTITUTION OF CONTINENTAL SHELF AND
ADJACENT ATLANTIC OCEAN CRUST

Composition of Basement

As a result of seismic refraction studies in the Atlantic Coastal Plain
between Virginia and New Jersey, Ewing et al. ( 1939 ) believe that the
^ocks of the crystalline Piedmont, as known in the exposed belt, are also
^present in the basement complex below the Cretaceous. They recognize
|the Petersburg granite and the Wissahickon schist into which the granite
ijis intrusive, under the unconsolidated sediments east of Petersburg, Vir-
ginia, and think they can trace the belts northward through Maryland,
^Delaware, and New Jersey. It would appear, they say, that the Peters-
burg granite is a correlative of, or is continuous with, the late Devonian
granites of Connecticut and Rhode Island.

Deposits of Continental Shelf

■

The continental shelf off the Atlantic Coastal Plain has been investi-
gated geophysically in the past 12 years, and some interesting results
have been obtained (Ewing et al., 1937, 1940). Several seismic traverses
were run across the Coastal Plain in order to check the seismic data with

WOODS HOLE SECTION

CONTINENTAL SHELF

sea level

c«

STATUTE

^cojsolk)

Sf*'C>NS0L

-■"^PL OATEI)

t**F

StlUT H

PAF A L L

CAPE

HENRY SECTION

V- CONTINENTAL

■HELP

*•.■ m '

o— c

•"•CONSOLIOHTio L_____-o— — -° A

MM'

C R

VST

* "•'■<»,.

^"co,

sO(_l|>ATED

V
\

«:■:.*

r*-*

•oeo'

. ..

■ 300'

INI

STA

U TE

M 1 I

E S

EAST

PET : RS B J RC

;

• 0

r^^o

•

Fig. 10.5. Seismic traverses on the Atlantic Coastal Plain and continental shelf, after Ewing.
Crary, and Rutherford, 1937. Small circles represent elevations determined by the refraction
seismograph. The Cape Henry section is section 37 of the index map of Fig. 7.1. The Woods
Hole section runs southward from Woods Hole, Mass. The M Zone is probably a horizon
within the Upper Cretaceous.

140

STRUCTURAL GEOLOGY OF NORTH AMERICA

in the Upper Cretaceous ( Ewing et al., 1939 ) . Richards thinks the M zone
in the Upper Cretaceous is the contact between the Magothy and Mata-
wan or Magothy and Merchantville formations. See section 32, Fig. 10.2.
If so, about 700 feet of Upper Cretaceous strata, which generally underlie
the Magothy, and 1000 feet or more of Lower Cretaceous would be in the
semiconsolidated layer. In southeastern Virginia the Eocene rests on
the Lower Cretaceous, and the M zone is probably absent; but perhaps
seaward the Upper Cretaceous comes in again, and the zone is present.

Contour of Crystalline Basement Surface

In a paper of 1950, Ewing et al., report on profiles off Cape May, New
York, and Woods Hole, and concluded that the Precambrian surface does
not slope constantly toward the Atlantic Ocean basin floor but has a pro-
nounced reversal of dip at a depth of 16,000 feet before the margin of the
shelf is reached off Long Island and Delaware Ray. Structure contours
on the surface are shown on Fig. 10.6. Farther south off the Cape Fear
arch the slope of the crystalline floor reflects the arch nearly to the mar-
gin of the shelf (Richards, 1945, 1947; Rerry, 1948; Hersey et al, 1959).
The surface is lost seaward over the Rlake Plateau, where no seismic
record of it or deeper boundaries of velocity layers were obtained. See
Fig. 10.7. The strike of the surface veers westward in South Carolina
and northern Florida. Near Jackson, Florida, the surface dips steeply
southward and is lost at a depth of 19,000 feet. The basement contours
here are distinctly discordant to contours drawn on the top of the
Cretaceous (Fig. 10.6).

In the shelf profiles off Long Island and Delaware Ray the unconsoli-
dated sediments thicken gradually outward under the shelf. In the upper
section of Fig. 10.7 Heezen et al. (1959) show a ridge of basement rock
at the shelf margin and then an abrupt fall-off apparently of fault nature.
Oceanward is a second basin in which the unconsolidated and consoli-
dated sediments attain a maximum thickness of 33,000 feet ( 10.3 kilome-
ters). The unconsolidated layer thins over the deep Atlantic floor to about
2 kilometers, but becomes much thicker again on the approaches to the
Rermuda Rise and Mid-Oceanic Ridge.

The seismic profiles across the Atlantic Coastal Plain and continental
shelf to date have been summarized by Drake et al. (1960), and these

writers point out that a ridge of basement rock near the edge of the shelf
is a common feature. It separates two sedimentary troughs, one under the
shelf, and another in deeper water under the shelf slope and rise. The
ridge and basins can be seen in the upper section of Fig. 10.7 and section
A-A' of Fig. 11.34. The sediments in the inner or shelf trough have been
drilled in several places along the Atlantic Coastal Plain and are mostly
shallow water sands, silts, and clays. Cores of the upper part of the sedi-
ments of the outer trough have revealed features attributed to slump-
ing, sliding, and turbidity currents, and are in part similar to graywackes.
Drake et al. point out that the size of the troughs and the thickness and
character of sediments in them are similar to the early Paleozoic troughs
of the Appalachians as restored by Kay (1951) and that here is a good
representation of the miogeosyncline (inner trough) and eugeosyncline
(outer trough). Compare with Figs. 11.17, 6.6, and 6.15. Evidence of past
volcanism in the outer trough is present in the form of partially buried
seamounts with large magnetic anomalies.

The eugeosyncline, according to the above view, develops largely on
the oceanic crust, and represents, when uplifted, an accretion to the
continent. The theory appears very attractive when related to the Paleo-
zoic Cordilleran geosyncline.

Submarine Canyons. Comprehensive submarine surveys of the whole
of the continental shelf and slope of the northeastern United States have
been made since 1930, using the most advanced methods, and the results
were published in 1939. Chart 1 of the publication, "Atlantic submarine
valleys of the United States and the Congo submarine valley" (Veatch
and Smith, 1939) is a composite of all the modern work from Cape
Hatteras to Georges Rank. The same results are presented in reduced
scale and somewhat simplified on the Tectonic Map of the United
States.

The shelf is a fairly smooth plain and a continuation of the emerged
Coastal Plain. The most prominent feature is the Hudson channel, which
is entrenched 50 to 150 feet in the shelf from the mouth of the Hudson
River to near the edge of the shelf. South of the channel are many shallow
depressions and low, irregular ridges trending generally parallel with the
shore. They have been likened to bars and lagoons. Northeast of the chan-
nel, the shelf is a regular oceanward slope, perhaps rilled with many

[Fig. 10.6. Structure of Atlantic Coastal Plain and adjacent ocean crust from Cape Canaveral
to Cape Cod. Thin contour lines are on the ocean floor and are in fathoms. The following
contours are in feet. Heavy continuous lines are structure contours on the top of the Pre-

cambrian crystalline basement. Heavy dashed lines are on the base of velocity layers interpreted
to be sediments. Dotted contours are on top of Cretaceous. Compiled from Ewing ef a/., 1950
and Hersey ef a/., 1959.

142

STRUCTURAL GEOLOGY OF NORTH AMERICA

FROM NEW YORK SOUTHEASTERLY NEARLY TO BERMUDA PEDESTAL

400

500 600

KILOMETERS

700

800

FROM SOUTH OF CAPE CANAVERAL EASTERLY TO BLAKE BAHAMA BASIN, THEN NORTHEASTERLY

1200

200 KILOMETERS 400

500

700

1100

Fig. 10.7. Sections of the crust of the Atlantic Coastal Plain and adjacent ocean.

shallow valleys. The shelf breaks abruptly at about the 600-foot depth to
a steeper slope, known as the "slope" which carries down to 8000 feet and
more below sea level in approximately 50 miles. In a few sections, the
slope is as steep as 700 feet per mile (732 degrees).

The slope is riven by two kinds of dip-slope features; canyons that ex-
tend headward into the shelf 10 to 30 miles from the outer margin, and
numerous deep parallel rills that are limited entirely to the slope. The
bottoms of the submarine canyons range from 2000 to 3700 feet below
the floor of the shelf at the outer margin. Those south of the submarine

Hudson channel and canyon generally lose their identity on the slope,
merging with the many rill-like canyons or not being larger than the
canyons limited to the slope. The Hudson canyon and others that indent
the shelf to the eastward along Georges Rank more clearly retain their
identity down the slope. Only one submarine canyon in this section of
continental shelf can be related with any assurance to a major river on
land. This singular relation is the Hudson, whose channel from New York
has been mapped about 100 statute miles across the shelf to the head of
the deep shelf-indenting and slope canyon.

Rejuvenated Appalachians in post-Newark time

The Fall Zone peneplain

Encroachment of Cretaceous sea and deposition of Coastal Plain beds

Fig. 10.8. Early stages in Appalachian epeirogeny. Reproduced from Johnson, 1931. Diagrams 1, 2,
and 3 from top to bottom.

143

Arching of Fall Zone peneplain and its Coastal Plain cover; regional superposition of southeastward-flowing streams

The Schooley peneplain

Arching of Schooley peneplain

Fig. 10.9. Tertiary stages in Appalachian epeirogeny. Reproduced from Johnson, 1931. Diagrams 4,
5, and 6 from top to bottom.

144

Dissection of Schooley peneplain and development of Harrisburg peneplain on belts of nonresistant rock

Uplift and dissection of Harrisburg peneplain and development of Somerville peneplain on the weakest rock belts

Allegheny Front* — *
APPALACHIAN PLATEAU — ♦

Ridge and Valley belt

NEWER APPALACHIANS

-Great Valley-#-Readingprong-»TriasLovvl'd-»- Piedmont ,0-

-*— OLDER APPALACHIANS ^-*COASTAL PLAIN

Uplift and dissection of Somerville peneplain to give present conditions

Fig. 10.10. Late Tertiary and Quaternary stages of epeirogeny and erosion in the Appalachians.
Reproduced from Johnson, 1931. Diagrams 7, 8, and 9 from top to bottom.

145

146

STRUCTURAL GEOLOGY OF NORTH AMERICA

80* 75'

Fig. 10.11. Physiographic provinces, Atlantic Ocean. Reproduced from Heezen et a/., 1959.

The Atlantic continental shelf is most probably constructional and due
to sedimentation influenced by fluctuating sea level during the Pleisto-
cene. Although certain early writers during a vigorous controvery ( 1930-
1940) contended that the canyons are due to subaerial erosion, and
therefore that the Atlantic coast has subsided 5000-10,000 feet subse-
quently, the theory is generally held today that the canyons are due to

submarine slumping, mud flows, and turbidity currents. See discussion
in Chapter 32 of submarine canyons off the California coast.

Appalachian Epeirogeny

Following the Appalachian orogeny in the late Paleozoic and the
Palisades orogeny in the late Triassic, a long period of erosion set in and

ATLANTIC COASTAL PLAIN AND ADJACENT OCEAN BASIN

147

lasted during all of the Jurassic. By the beginning of Cretaceous time, an
extensive and very subdued surface across the folded and thrust-faulted
Appalachians, and across the Blue Ridge, the Triassic basins, and the
Piedmont had formed. This is known as the fall zone peneplain. Study
diagrams 1 and 2 of Fig. 10.8. The entire area as far westward as the
plateaus province, according to Johnson ( 1931 ) , was then invaded by
shallow epicontinental seas, and in them Cretaceous sediments were de-
posited (diagram 3). From subsurface studies of the Coastal Plain sedi-
ments, it has been shown that the Lower Cretaceous is entirely buried by
the Upper, and it appears that the extensive overlap that Johnson visual-
izes occurred in Upper Cretaceous time. Others admit that the Cretaceous
extended farther inland than the present erosional margin but do not be-
lieve that it extended beyond the Blue Ridge. Johnson and later Strahler
(1945) believe the overlap necessary to explain the stream pattern of the
Ridge and Valley province.

The fall zone peneplain was then arched broadly with the crest in the
Ridge and Valley and Blue Ridge provinces and the flanks far westward in
the plateaus and far eastward in the site of the present Coastal Plain and
continental shelf. Another episode of base-leveling followed, which, like
the previous one, established an extensively subdued surface, but lower
and younger. This is known as the Schooley peneplain. See diagrams 4
and 5 of Fig. 10.9. The only remnant of the fall zone peneplain is that
buried beneath the Cretaceous sediments of the Coastal Plain. The
Schooley surface is now generally recognized in remnants as the highest
flat tops of ridges in the Appalachian region.

Broad arching again occurred, and the Schooley peneplain was dis-
sected in the manner represented in diagrams 6 of Fig. 10.9 and 7 of Fig.
10.10. A few master streams persisted across the folds and thrusts, while
many subsequent streams etched out the resistant formations to produce
the first appearance of flat-topped, subparallel, ridges and valleys. The
new base level below the flat-topped ridges is known as the Harrisburg
peneplain. See diagram 7 of Fig. 10.10. Still third and fourth stages of
arching are recognized in the dissection of the Harrisburg peneplain and
the establishment of the lower Somerville surface, and the dissection of the
Somerville to the present stream bottoms. See diagrams 8 and 9 of Fig.

10.10. An extensive literature may be found on the geomoiphology <>i the
Appalachians, and most premises and conclusions of the above summary
of Johnson's work have been contested. Most authorities recognize the
vertical uplift, but some contend that a symmetrical arching did not occur.
It may also be argued that the arching was a slow, continuous pro<
and not one of four stages with interims of standstill.

Physiographic Provinces of North Atlantic Floor

Echo sound tracts of fifty expeditions in the North Atlantic including
over 200,000 miles by vessels of the Lamont Geological Observatory with
the Luskin precision depth recorder were compiled by Heezen ct at
(1959), and a physiographic relief map of the ocean floor was prepared.
From it the physiographic provinces are resolved as shown on the map of
Fig. 10.11. Profiles to accompany the map are reproduced in Fig. 10.12.
There are three major divisions, each with its subdivisions as follows:

Continental Margin
Category I

Continental Shelf

Epicontinental Seas

Marginal Plateaus
Category II

Continental Slope

Marginal Escarpments

Landward Slopes of Trenches
Category HI

Continental Rise

Marginal Trench-Onter Ridge Complex

Marginal Rasin-Outer Ridge Complex
Ocean Rasin Floor
Abyssal floor

Abyssal Plains

Abyssal Hills

Abyssal Gaps and Mid-Ocean Canyons
Oceanic Rises
Seamount Groups
Mid-Oceanic Ridge
Crest Provinces

Rift Valley

Rift Mountains

High Fractured Plateau

148

STRUCTURAL GEOLOGY OF NORTH AMERICA

kiA R 7 HA S VMe YARD

Azores

Soo Miguel

■*—» sZ'JSfi" emum*

moo nmous

Fig. 10.12. Relief profiles across the Atlantic. Reproduced from Heezen ef a/., 1959. Letters a to q indicate
where sounding profiles of different cruises were joined.

Flank Provinces
Upper Step
Middle Step
Lower Step

For further information the reader is referred to the work of Heezen
et ah, Geological Society of America Special Papers 65. A tectonic map
to supplement the publication is yet to appear, but the gross details as
now conceived by Heezen and colleagues of Lamont Geological Labora-
tory are portrayed in the cross section of Fig. 10.13.

Blake Plateau, Blake Bahama Basin, and Outer Ridge

As shown on Fig. 10.11 the continental shelf breaks into two steps
south of Cape Hatteras, and the lower step is known as the Blake Plateau.

East of the Blake Plateau is the Blake Bahama basin, and east and north
of it is the low Outer Ridge. The Outer Ridge swings northwestward at
29° N. Lat., 73° W. Long., and heads toward the Cape Fear arch to
merge with the Blake Plateau. Details are given on Fig. 10.6. The outer
escarpment of the Blake Plateau is probably a fault scarp, according to
Heezen et al. ( 1959 ) . See lower diagram of Fig. 10.7.

A seismic refraction survey of part of the Blake Plateau was made by
Hersey et al. ( 1959 ) , and the principal profiles are shown on Fig. 10.6.
The same letter designations are retained for the profiles as in the original
article. The purpose of the study was to determine the relation of the
Plateau crust to the continental crust on one side and to the oceanic crust
on the other. Four characteristic profile sections are shown in Figs. 10.14
and 10.15.

MID- ATLANTIC RISE

AFRICA

Fig. 10.13. Crustal structure across North Atlantic. After Heezen ef al., 1959, with minor changes taken
from new section furnished by Tharp and Heezen.

KILOMETERS

Fig. 10.14. Crustal structure sections C-C and E-E' of Fig. 10.6. After Hersey et al., 1959. Numbers are
velocities per second of the various layers. Stippled layers are interpreted as unconsolidated and con-
solidated sediments.

150

STRUCTURAL GEOLOGY OF NORTH AMERICA

1.83)

1.83,

1.83 k

1.83, 2.77?

WATER

1.74:-. v;: I.83-V-'?-. :"i". .

■••.'• 2.6I.V

315 2.78

J-?

. I.83-.
4.11

3.84 3 87 3.80

Ir ^_

4.89

4.45 '4.6

/■" 5.5

55 62Z

552

4 •

1 — -
\

*•* *~

— b.85

639 S, /
• X /

6.21

12
14

ToV

MANTLE

KILOMETERS

200

300

100 KILOMETERS

Fig. 10.15. Crustal structure sections D-D' and G— G' of Fig. 10.6. After Hersey ef a/., 1959. Numbers are
velocities in kilometers per second. Stippled layers are interpreted as unconsolidated and consolidated
sediments.

The results on the continental shelf are correlated with adjacent continental
geology. The deepest horizon traced along the shelf is interpreted as granitic
basement, which has compressional velocities of 5.82-6.1 km/sec. At the
southern extremity it is at a depth of 6 km., shoals to 0.86 km. near Cape
Fear, and deepens north of Cape Hatteras to more than 3 km. North of
Charleston, South Carolina, there is excellent depth correlation with granitic
basement in coastal wells; to the south all deep wells are inland. Age correla-
tions are based on well data near the coast, which indicate to us that most of
the observed section is Cretaceous.

On the Blake Plateau, several layers (1.83-4.5 km/sec.) are interpreted
as sedimentary. A 5.5-km/sec. layer is found only south of a line from 30°30'
N., 78°W. to Cape Canaveral. Velocities higher than 5.5 km/sec. have been
measured on six profiles on the Blake Plateau. The 5.5-km/sec. layer and a
6.2-km/sec. layer appear to form a positive feature to the south of the above-
mentioned line [indicated as fault on Fig. 10.6]. Higher velocities, 8.0 km/sec,
and 7.28 and 7.3 km/sec, which are probably not the same horizon, are

found at markedly different depths. Possibly these represent the M layer and
ultrabasic material, depending on relations not now known.

[The Outer Ridge along section G— C] is underlain by thick low-velocity
layers (1.83-2.96 km/sec), interpreted as sediments, and higher-velocity
layers which form a distinct linear structure having the same general trend as
the ridge. At its northwestern end this trend treminates against a thick
lower-velocity section interpreted as a sediment-fuled trough (Hersey et ah,
1959, p. 1).

An attempt is made on Fig. 10.6 to contour the base of the interpreted
sedimentary layers (velocities less than 4.5 km/sec) from the profiles of
Hersey et al. The results are to be taken simply as pictorial. There seems
little doubt, however, that a major fault transects the Blake Plateau, but
of a date preceding the deposition of the upper two velocity layers of sedi-
ments, because they bury the escarpment. This fault, extended south-

ATLANTIC COASTAL PLAIN AND ADJACENT OCEAN BASIN

151

easterly, probably forms the south boundary of the Outer Ridge, described
above by Hersey et al., but if so, it does not show in section H-H'. The
Blake Plateau south of the fault, at any rate, stands 15,000^30,000 feet
above the block on the north, in reference to the base of the interpreted
sedimentaries. The deeply filled block extends northward at least to
32° N. Lat.

Regarding the origin of the Outer Ridge, Hersey et al. point out that
two velocity layers appear there that are unusual, namely the 5.20-5.67-
km/sec layer and the 7.21-7.73-km/sec layer. The 5.2-km/sec layer is
interposed between the sedimentary layers and the basaltic "oceanic
layer" (6.5 =*= km/sec), and the 7.5-km/sec layer interposed between the
oceanic layer and the mantle. Since profile D-D' shows a 5.22-5.52-
km/sec layer under the Blake Plateau the rock represented by this
velocity range is probably not unique to the Outer Ridge. The 7.5-km/sec
layer, however, seems more restricted to the Ridge, but it, nevertheless, is
known to extend as far north as the northern end of profile H-H'.

The 5.2-km/sec layer is regarded as a mass of extruded volcanic mate-
rial, lighter and more porous than the basaltic "oceanic crust" layer, and
the 7.5-km/sec layer is taken to be a mixture of mantle rock with the
oceanic crust, probably by intrusion of peridotitic magma into basalt, in
the manner postulated for the Mid-Atlantic Ridge (Fig. 10.13).

Hersey et al. (1959) speculate that the ultrabasic intrusions fed the
volcanic extrusions, then at the surface, and that the two are comple-
mentary. Another theory might be one in which basalt is formed by
partial melting of the mantle, with the basalt rising to concentrate in mesh
fashion in the upper part of the mantle. This basalt could then rise in
fissures and vents through the oceanic crust to eruption at the surface.
See Chapter 33 on igneous rock provinces.

Mid-Atlantic Ridge

Topography. The Mid-Atlantic Ridge is a broad arch or swell that
occupies approximately the center third of the ocean (Figs. 10.11 and
10.12). The higher and central part is less than 1600 fathoms below sea
level, and the flanks fall between 1600 and 2500 fathoms. The Ridge is
very rough as the profiles indicate, and the most striking feature is a
deep notch or cleft in the crest of the arch, called the Rift Valley. On an

average profile the floor of the valley lies at about 20CK) fathoms below sea
level, whereas the adjacent peaks average about 1000 fathoms. The re-
lief from floor to adjacent peaks ranges from 700 to 2100 fathoms. The
width of the valley between crests of the adjacent peaks ranges between
15 and 30 miles; at an elevation of 500 fathoms above its floor the width
is from 5 to 22 miles (Heezen et al., 1959).

On either side of the Rift Valley are terranes of sharp and strong relief
called the Rift Mountains. Immediately adjacent to the central Rift
Valley are the High Fractured Plateaus with local relief of 400 fathoms
and ranges 8 to 20 miles apart. Flanking the High Fractured Plateaus
is a succession of provinces known as the Upper Step, Middle Step, and
the Lower Step. The topography here likewise is rough with local relief
of 200 fathoms. Peaks over 200 fathoms high occur at about the fre-
quency of 7 per each 100 miles. The steps appear to be separated from
each other by scarps of considerable length.

Seismicity. The High Fractured Plateaus and Rift Valley make up a
zone of considerable seismicity. See Fig. 10.16. Another zone extends from
the Rift Valley through the Azores eastward to Gibraltar.

Sediments. Photos taken on the sides of seamounts in the Rift Moun-
tains show scour and ripple marks indicating deep-ocean currents. Cores
taken in intermontane basins show interlayering as turbidity current
deposits.

Rocks. The lithology of the Mid-Atlanic Ridge is known from three
sources: (1) rocks dredged from the sea floor, (2) detrital rock frag-
ments found in sediment cores, and (3) rocks exposed on the islands of
the Ridge. These all point to olivine gabbro, serpentine, basalt, and dia-
base as the predominating rock types. One limestone sample probably
of Tertiary age was collected from the Rift Valley at about 30° N. Lat.
(Heezen et al, 1959).

Crustal Structure. Seismic refraction records have been obtained in
about twenty places on the Mid-Atlantic Ridge, and the following layer-
ing is reported (Heezen et al, 1959). See Fig. 10.13.

. . . the average crustal structure of the crest provinces and Upper Step
consists of 0.4 km of low-velocity sediment and 2.8 km of rock with a velocity
of 5.1 km/sec. overlying a substratum in which the velocity is 7.3 km sec.
The thickness of the layer of low-velocity sediment varies considerably From

152

STRUCTURAL GEOLOGY OF NORTH AMERICA

^e^J

*n" — ^~ '/• ■••• :

* *• •> •• •

Fig. 10.16. Earthquake epicenters, North Atlantic. Reproduced from Heezen ef a/., 1959.

place to place. In the crest provinces the 5.1 km/sec layer is commonly
exposed. In the flank provinces appreciable thicknesses (to 1 km) of sediment
have been measured.

Under the abyssal floor of the ocean the low velocity sediment layer
is underlain by a 6.7-km/sec layer, and this by a 8.1-km/sec layer. The
lower is considered the mantle of peridotite and the overlying layer a

gabbroic or firm basalt layer. Under the Ridge neither of these two are
present but instead layers of 5.1-km/sec and 7.3-km/sec.

Ewing and Ewing (in press) suggest that this intermediate velocity
(7.3 km/sec.) is the result of a physical mixture of oceanic crustal rocks and
mande rocks. To explain such large-scale mixing they propose that extensive
vulcanism and intrusion along the Mid-Adantic Ridge have produced an

ATLANTIC COASTAL PLAIN AND ADJACENT OCEAN BASIN

153

intermingling of the crustal and mantle rocks, and that this was associated with
convection cells in the deep mantle which supply large quantities of basaltic
magma and produce extensional forces on the crust and upper mantie ( Heezen
et d., 1959).

In a paper (in press) Heezen and Ewing compare in detail the topography
and seismicity of the African rift valleys and the Rift Valley of the Mid-Adantic
Ridge. Their conclusion is that the two areas are of basically the same structure,
and in fact both form parts of the same continuous structural feature. Since

the African rift valleys seem clearly to be the result of normal faulting resulting
from extension of the crust, Heezen and Ewing conclude that the topo£raphy
of the Mid-Adantic Ridge is largely the result of normal faulting. Whether
the forces are the result of horizontal extension or vertical uplift remains the
most important unsolved problem in connection with the origin of the con-
tinental as well as the sub-oceanic rift-valley systems. Hess ( 1954) has proposed
a mechanism relating suboceanic uplift to expansion due to serpentization
of the upper mande (Heezen et al., 1959).

11.

HUDSON VALLEY LAKE CHAMPLAIN REGION

Relief Features

The relief features of the Taconic erogenic system stretch along the
general Hudson Valley, Lake Champlain lowlands, and St. Lawrence
Valley. In addition to hills and ridges within the lowland, it is convenient
under this heading to discuss the Hudson highland and Catskill and
Adirondack Mountains on the west, the Laurentian highlands on the
northwest, and the Taconic and Green Mountains on the east. The
Taconic orogeny culminated in late Ordovician time, and most of the
structures of the Hudson and Lake Champlain valleys and of the ranges
along its eastern margin are Taconic. The Catskills and Adirondacks,
however, are part of the stable interior. See index map, Fig. 11.1 and
geologic map, Fig. 11.2.

NEW ENGLAND
APPALACHIAN SYSTEMS

DIVISIONS OF NEW ENGLAND APPALACHIANS

The New England Appalachian systems will be divided for purposes of
discussion into a western belt and an eastern. The western belt includes
those structures in and on either side of the Hudson Valley and Lake
Champlain lowlands, and the eastern belt includes a north-south zone
through central and eastern Vermont, New Hampshire, and Maine. The
western zone is essentially the core of the Late Ordovician Taconic
orogeny and the eastern the site of the Late Devonian Acadian orogeny.
A third division may be recognized through Rhode Island and Massa-
chusetts on the far east where Carboniferous basins and related igneous
activity indicate a still later orogenic belt.

Catskill Mountains

The Catskill Mountains are west of the Hudson River and about 100
miles north of the city of New York. See geomorphic diagrams of Figs.
11.3 and 11.4. They are a dissected plateau with highest summit levels
about 5000 feet above sea level and local relief of over 3000 feet. They
were the site of pioneer geologic studies in North America, and in them
the stratigraphic sequence of the Silurian and Devonian systems was early
established. The Catskills proper consist of nearly flat-lying beds, gently
inclined toward the west, and as such are part of the Appalachian
Plateaus geomorphic province. The most widespread rocks are the
Devonian. Along the east margin and in the adjacent Hudson Valley,
the strata, especially the Cambrian and Ordovician, are highly deformed;
and the Devonian and Silurian beds rest on their beveled edges. The
classic angular unconformity between the Ordovician and Silurian beds,
which here marks the Taconic orogeny, is displayed along the southeast
margin of the Catskills. See the Geological Map of the United States.
Also, the system of folded and thrust-faulted Appalachians of the south
narrows here into a belt a few miles wide, and some of its late Paleozoic
structures may here be impressed on the strata and in part superposed

154

Fig. 11.1. Principal physical features of New England and the Maritime Provinces. M. H. means
Montarigian Hills.

Fig. 11.2. Generalized geologic map of New England. Reproduced from Billings, 1956.

Fig. 11.3. Block diagram of lower Hudson River region by Raisz. Reproduced from /nfernaf.
Geo/. Congr. Guidebook 1, 1933, Eastern New York and Western New England.

Fig. 11.4. Block diagram of lower Hudson River region. Joins opposite figure on north.

NEW ENGLAND APPALACHIAN SYSTEMS

157

on the older structures of the Taconic orogeny. The section along the
Catskill aqueduct, Fig. 11.5, gives a good idea of the composition and
structure of the Catskills and adjacent Hudson Valley.

The regional stratigraphy including the Catskills has been presented in
Chapter 8 on the southern and central Appalachians. See Figs. 8.10 to
8.12.

Regarding the structural history, Chadwick and Kay (1933) say the
following:

There is evidence in the region of at least two periods of deformation. In
several exposures, Ordovician beds lie in close contact with angular uncon-
formity beneath the basal Silurian sediments. Formations as young as Middle
Devonian have been folded and affected by faults of low angle showing relative
overthrust from the east.

The first of these deformations is definitely assigned to the Taconian dis-
turbance, for which this is the classical area of study. The later deformation may
have been produced either in the Acadian disturbance at the end of the
Devonian or in the Appalachian revolution, or in both. Inasmuch as late
Paleozoic rocks are not present in the disturbed areas, it is not possible to date
the movements precisely. The tectonic movements that produced the coarse
clastic Upper Devonian sediments to the west may have been accompanied by
this folding and faulting; if so, the structures are Acadian. On the other hand,
the structures are similar to those formed farther to the southwest and north-
east in the Appalachian revolution, and it is probable that some of the effects
were produced at that time.

Erosion has been dominant in the region since the end of Paleozoic time.
Remnants of a peneplain may be preserved in the accordant summits of the
higher peaks in the western part of the region, of which Plateau Mountain is
typical. The high areas that bear these remnants seem to stand above an erosion
level represented by the open upper valleys of the Catskills and by the beveled
surface of the Helderberg Plateau, to the north, seen from Windham Notch.
This lower level lies 2,500 feet (750 meters) below the supposed summit
peneplain and has been correlated by some geologists with the Schooley pene-
plain of Pennsylvania, by others with the Harrisburg peneplain, of later Tertiary
age. Further elevation and subsequent erosion produced a peneplain that bevels
the weaker folded rocks in the Hudson Paver Valley west of the river. This later
Tertiary surface is 1,500 feet (450 meters) below the last and has been called
the Albany peneplain. More recent movements have elevated this surface a few
hundred feet (100 meters or more) above present base-level, permitting the
excavation of valleys in the weakest rock belts. Thus erosion has brought about
the removal of a great mass of later Paleozoic sediments through several cycles
of erosion with intervening uplifts, exposing early Paleozoic rocks in the eastern
part of the region.

Adirondack Mountains

The Adirondack Mountains constitute a nearly circular uplift about 150
miles across, which extends from Lake Ontario on the west to Lake Cham-
plain on the east, and from the Mohawk Valley on the south to the St.
Lawrence lowland on the north. The northwestern part of the Adiron-
dacks is a rolling upland of gentle relief and a mean altitude of about
1000 feet above sea level, whereas the southeastern part is a rugged moun-
tain mass, individual ridges of which reach 3000 above the valley
floors, and the highest peak, Mount Marcy, stands 5344 feet above the sea.

The Adirondacks consist mainly of Precambrian rocks. These are sur-
rounded by gently upturned Cambro-Ordovician sediments, except near
Kingston, Ontario, along the St. Lawrence, where a neck of the Pre-
cambrian rocks connects with the Precambrian of the Canadian Shield
( the Frontenac axis ) and along Lake Champlain where highly deformed
strata of the Taconic system bound the dome.

According to Balk ( Longwell, 1933 ) :

The unconformity between pre-Cambrian and Paleozoic rocks is exposed in
numerous places, although in the southeast the primary relations are somewhat
blurred by post-Ordovician faults along which the Adirondacks have been
elevated with reference to the surrounding younger rocks. One of these faults
passes through Saratoga; another one forms the escarpment northwest of town
and is followed by the road from Saratoga to Glens Falls for many miles.
Escarpments near Lakes George and Champlain are due to additional border
faults along the eastern margin of the Adirondacks.

The pre-Cambrian sedimentary rocks of the Adirondacks appear to be
identical with rocks of the same general age in the Provinces of Quebec and
Ontario, so that the whole region is to be regarded as an outlier of die Canadian
shield.

Sedimentation in and around the Adirondack region in Cambrian and
Ordovician time is illustrated in the paleograpbic maps of Fig. 11.6
and by the cross section of Fig. 11.7. The Adirondack dome persisted with
some irregularities as an area of gentle uplift during the Cambrian and
Ordovician, and by late Cincinnatian time a broad domal structure was in
existence. Then the Taconic orogeny occurred along the east side and
following the orogeny closely the dome was broken by block faults. Figure
11.13 is a cross section that restores the Adirondack uplift and adjacent

158

STRUCTURAL GEOLOGY OF NORTH AMERICA

Fig. 11.5. Cross section along the Catskill aqueduct. Reproduced from Geological Society of America
Guidebook of Excursions, 1948.

areas to this time. The distribution of faults and the Taconic front are
shown in Fig. 11.12 in relation to the Lower Ordovician facies.

Lower Hudson Valley Crystallines

Definition. The block diagrams of Figs. 7.3 and 11.3 show the lower
Hudson Valley area to be made up of the Triassic basin sediments and
sills, and the New England upland. The following paragraphs concern
the New England upland thus designated, but the term is general for
much of New England, and more specific names have been given to the
features of the lower Hudson Valley area. The Reading prong of Penn-
sylvania and the New Jersey highland merge on the northeast with the
Hudson highland, whose upland surface is about 1000 feet above sea
level. The Hudson River cuts a fairly narrow valley without flood plain
through the highland between Newburgh and Peekskill. See map of Fig.
11.3. The Hudson highland continues northeastward into Connecticut as
the Housatonic highland.

Lower Hudson Valley. From Peekskill to Manhattan Island, the Hud-
son is bounded on the west by the Triassic rocks, mostly thick diabase sills
that form the Palisades of the Hudson, and on the east by rounded hills
of a metamorphic and plutonic complex. The rocks along the route from
New York City to Peekskill consist of gneisses intruded and injected by
granite with infolded belts of limestone and schist. See cross section of
Fig. 11.8. The major structural axes trend north-northeast and are strongly
reflected in the general arrangement of ridges and valleys. Along the
lower part of the river in the vicinity of Yonkers, the structures trend
about N. 20° E. and are parallel with the river, but a few miles above

Yonkers they strike more easterly, whereas the course of the river is nearly
due north.

Hudson and Housatonic Highlands. Balk (1937) and Barth (1937)
have made a thorough study of the Hudson and Housatonic highlands
and adjacent areas, and report a complex of Precambrian crystalline
rocks and a series of three sedimentary formations of Cambrian and
Ordovician age. The highlands themselves are formed of a complex of
gneisses of granitic and syenitic composition. Associated are injection
gneisses as well as narrow tracts of amphibolite, marble, and other highly
metamorphic rocks. Along the northwestern border of the highlands,
medium- to coarse-grained granites and granite gneisses are fairly abun-
dant.

The Paleozoic strata are described by Balk (1937) as follows:

The oldest Paleozoic rock is a pink or white quartzite (Poughquag quartzite)
that rests unconformably upon the various pre-Cambrian rocks. At the base, a
conglomerate may be present, though rarely more than a few feet thick. Quartz
pebbles, about an inch across, and an occasional black chert fragment, are the
most abundant constituents. Fossils of Lower Cambrian age have been described
from several localities in southeastern New York.

The quartzite is succeeded by a sequence of carbonate rocks to which, in
the Poughkeepsie area, the name, Wappinger terrane, has been applied. As
elsewhere in the Appalachian region, the rocks include members of Cambrian
and Ordovician age, but Quaternary deposits obscure so much of the bedrock
that no complete section is available. Fossils ranging from Lower Cambrian
to Middle Ordovician have been reported from various localities, but it is
believed that there are several disconformities within the terrane. The thickness
of the series is difficult to estimate, but may well exceed 1,000 feet.

A series of slates and similar rocks, resting on the carbonate rocks, is called
the Hudson River pelite. Fossils of Middle Ordovician age have been found in

NEW ENGLAND APPALACHIAN SYSTEMS

L50

the western portion of Poughkeepsie quadrangle, but farther east, cleavage
seems to have destroyed them. Hudson River slates of black, gray, greenish,
and red color are known; commonly, argillaceous layers are interbedded with
thousands of thin, fine-grained sandy layers, or aphanitic cherty beds that
weather whitish. Scattered through the series are hundreds of lenses of sand-
stone, or quartzite, conglomerate, and graywacke, and quartz veins penetrate
the rock in almost every outcrop. On account of the intricate folding, and
absence of continuous exposures, the thickness of the Hudson River series is
unknown, but it may exceed that of the carbonate rocks below.

Balk's interpretation of the structure of the region may best be under-
stood by the study of the lower cross section of Fig. 11.9. Of first im-
portance is the unconformity at the base of the Poughquag quartzite
which clearly reveals the Precambrian age of the gneiss and granite com-
plex of the Hudson and Housatonic highlands.

The highlands are regarded as uplifted blocks. As the uplift occurred,
the Paleozoic succession along the west side was tilted westward, and in
addition was broken by a number of faults, most of which are thrusts of
medium to steep southeasterly dip. Thrust faults are also recognized along
the east flank of the northeast end of the Hudson highland. That the
Precambrian highlands are uplifted masses is shown by the general basin
distribution of the youngest rocks, the Hudson River pelites, in the middle
of the intervening areas, and then the next older rocks, the Wappinger
limestone and Poughquag quartzite next to the gneiss.

Between the Hudson and Housatonic highlands is a Paleozoic area
which is regarded as a faulted syncline. It has the special significance of
affording a connection between the known Cambrian and Ordovician
strata on the west of the highlands to unfossiliferous and more meta-
morphosed strata on the east, and it is here that Balk and Barth have
demonstrated the progressive metamorphism of the Hudson River slates
and phyllites to schist and even injection gneisses, and the increase in
marmorization of the carbonates.

The general basin structure of the strata between the masses of Pre-
cambrian gneisses is greatly marred and distorted by normal and thrust
faults which have cut the quartzite for miles along the gneiss borders,
and at many places have brought the limestone to the level of the pelite.
Most of the faults strike north-northeast or north-south; hence, the rock

CAMBRIAN 5ERIES

CANADIAN SERIES

CHAZYAN SERIES

LOWEST MOHAWKIAN (PAMELIA) LIME-
STONE ON PRE- MOHAWKIAN GEOLOGY

Fig. 11.6. Cambrian and Ordovician paleogeography of the New York and St. Lawrence
region, after Kay, 1942. The ruled areas represent the spread of deposits, and the Taconic
allochthone as postulated in Figs. 11.12 and 11.13 is shown in both present ^left) and
original (right) position.

160

STRUCTURAL GEOLOGY OF NORTH AMERICA

units are arranged in belts of north-southerly trend. The horizontal forces
that caused the thrusts are also believed to have cast the sedimentary
rocks into folds which are overturned to the west. The folds, however,
are very small ones in otherwise gently downfolded beds.

Cleavage pervades the crenulated sediments widely. It is everywhere
parallel to the axial planes of the crenulations, and is best developed in
the slate phases northeast of Poughkeepsie.

The metamorphic rocks of the lower Hudson River Valley have been
regarded as Precambrian, but in light of Ralk's and Barth's work it seems
probable that only the Fordham gneisses is Precambrian and that the
Inwood limestone is equivalent to the Wappinger limestone and the Man-
hattan schist to the Hudson River pelite, both of Cambro-Ordovician age.
Refer to cross sections of Figs. 11.5 and 11.8. For discussion of the prob-

DENKIARK

WEST CANADA
CREEK

lem see Balk, 1937. Paige (1956) has correlated undoubted Cambro-
Ordovician rocks west of the Hudson River near Peekskill with the
Inwood marble and Manhattan schist east of the river.

Potassium-argon age determinations on the micas of the Manhattan
schist, the Inwood marble, the Fordham gneiss, some discordant pegma-
tites, and a diorite were made by Long and Kulp (1958). An average age
for the generation of the micas of the post-Fordham gneiss formations is
given as 366 =*= 9 m.y., which they say may tentatively be correlated
with the Late Ordovician Taconic orogeny. Very recent interpretations
by Hurley et al. ( 1959 ) indicate that this absolute age may be post-Early
Devonian, and in connection with orogeny in New Hampshire their work
will be referred to again.

Biotite from the Fordham gneiss is slightly older; the "apparent age"

MOHAWK VALLEY

WELL5

LAKE CHAMPLAIN CANAJOHARIE

Zone

/} m /o/ ex og f* a jo f </sjtz-T~z. am r> / ' e. jt / c a <y / '/\s _r~ _

CAnAO,AN 'S^^^~~^T 8=7 h

^oJvv^ — £HO/7EHflM * f H

ADIRONDACK AYIS 0*p

a
o

vo
hi

Fig. 11.7. Restored section of pro-Middle Trenton formations across
York, after Kay, 1942.

NEW ENGLAND APPALACHIAN SYSTEMS

161

Fig. 11.8. Cross section along Kenisco bypass tunnel of the Delaware aqueduct. Kenisco Dam is
just east of Croton Lake in the lower Hudson Valley. Reproduced from Geological Sociefy of America
Guidebook of Excursions, 1948.

of two samples is given as 400 and 440 m.y. The authors suggest that the
Fordham gneiss being demonstrably older and probably the Precambrian
basement did not lose all its argon during the 365 m.y. recrystallization
I process, and hence its micas yield somewhat older dates. It will be re-
called that zircons from the Baltimore gneiss of the crystalline Piedmont
yielded ages of about 1100 m.y., whereas the micas from the same rock
gave ages of 300 to 350 m.y.

The age of the sediments themselves is not indicated by the isotope
age determinations but, at least, the time of the last major orogeny and
metamorphism is sufficiently young so that the sediments could well be
Cambro-Ordovician.

Green Mountains

The Hudson and Housatonic highlands, if followed northerly, lead
to the Taconic Mountains and northeasterly to the "western highland" of
Connecticut and Massachusetts, of which the Berkshires are a part. See

Precambrian area in western Massachusetts, Figs. 11.2 and 11.9. East
of die western highland is the Triassic lowland. The Berkshire Mountains
extend to the Green Mountains at about the Massachusetts and Vermont
border, and the Green Mountains continue northward through central
Vermont to Quebec. See Cady, 1960. The Taconic Range extends
northerly along the New York-Vermont border to about central western
Vermont, and between it and the Berksire-Green Mountain element is
the "marble belt." For the broad relations of these geologic units see the
tectonic map, Fig. 11.10. The Green Mountains are comparable in eleva-
tion with the Adirondacks which lie across the Lake Champlain Valley,
but the other highlands and ranges are comparatively low.

The core of the southern end of the Green Mountains is made up of
granites and gneisses of Precambrian age. These ancient rocks are over-
lapped on the flanks by quartizites of lowest Cambrian age. See lower
cross section of Fig. 11.11. The northern part of the range is a gneiss and
schist anticlinorium which plunges northerly, and although somewhat like

162

STRUCTURAL GEOLOGY OF NORTH AMERICA

TACONIC RANGE EAST OF TROY

GREEN MTS.

TACONIC RANGE NEAR CHATHAM

Chatham
thrust

HUDSON HIGHLAND

p€g

EAST OF POUGHKEEPSIE
Ohr

HOUS ATONIC
Ohr

HIGHLAND
p€g

Fig. 11.9. Cross sections of central and southern Taconic Range. Section east of Troy, N. Y., after
Balk, 1953. p€g, Precambrian gneiss; Cc, Lower Cambrian Cheshire quartzite; CO, Cambro-Ordo-
vician limestone and dolomite; Oa, gray, purple, and black slate and quartz-chlorite schist.

Section near Chatham, N. Y., after Craddock, 1957. €s, green slate with interbedded gray-

wacke and quartzite; Oc, carbonate rock; Ode, green shale; Ons, red shale member; Onm,
Mount Merino dark shale wtih interbedded chert; Ona, Austin Glen graywacke and dark shale.
Section east of Poughkeepsie, N. Y., after Balk, 1937; pCg, Precambrian gneiss; €Ow,
Wappinger dolomitic limestone; Ohr, Hudson River pel lite, phyllite, and schist.

the southern core is believed by Cady ( 1945 ) to be part of the Taconic
allochthone. See map of Fig. 11.10. In its east flank the Green Mountain
anticlinorium contains a discontinuous belt of ultrabasic intrusives which
are associated with volcanics including pillow basalt.

Taconic Mountains

The Taconic Mountains are a low range of hills composed mostly of
argillaceous rocks such as phyllite, slate, and shale. This clastic sequence
is surrounded in the adjacent lowlands by rocks, chiefly carbonates. In the
Taconic sequence, as it is called, there is one thin quartzite formation
and one very thin limestone which together form perhaps 5 percent
of the section. There are three slate formations of Middle Ordovician age

and six of Lower Cambrian. No Middle or Upper Cambrian is present
and no Lower Ordovician. The Lower Cambrian of the Taconic Range
lies beside the Lower Cambrian of the valleys and the two groups have
no features in common except that of age (Keith, in Longwell, 1933).
Similarly, most of the Ordovician of the mountains differs from the Ordo-
vician of the surrounding valleys. These relations have led through a long
controversy to the interpretation of the Taconic clastic sequence as a
klippe, which represents an eastern trough facies that has been thrust
westward 30 to 50 miles or more on a western trough sequence. It is part
of the Taconic allochthone. The carbonates of the western trough sup-
posedly are the autochthone. See cross section D-D', Fig. 11.11. The
details and relations will be taken up later.

Fig. 11.10. Tectonic and palinspastic maps of the
Taconic system in eastern New York, western New
England, and southern Quebec, after Cady, 1945.
The palinspastic map attempts to restore the thrust
slices to their approximate position before they
were moved westward. Since the Devonian strata
were deposited after the Taconic orogeny, they
were not displaced by it and do not participate
in the restoration. S.L. means slice or thrust sheet
and the abbreviations in the tectonic map may be
identified by comparison with the palinspastic
map.

HINESBUBG SYNCLINORIUM
CHAMPLAIN THRUST HINESBURG THRUST A>

CHAMPLAIN THRUST
r ORWELL TH. /

MIDDLEBURY SYNCLINORIUM

0 o

0 £i€m£dh€f^-1 B'

ORWELL THRUST

OrcJov/cion

SUDBURY THRUST

MIDDLEBURY SYNCLINORIUM

O 0 €d £w €m £dh _£c

5 MILES

TACONIC MTS.

GREEN MTS.

p£q

•"gv-''-''7:r~rv?-N-€f-q _^^=^££ll Osc _£4 £q^-rr^rr^r^r-r^^€q

€Q €d cis

-TACONIC THRU5T

Fig. 11.11. Cross sections of the Taconic system of western Vermont, A-A', B-B', and C-C,
after Cady, 1945. Refer to map of Fig. 11.16. £?md, Mendon series; Cc, Cheshire quartzite;
Cdh, Dunham dolomite; £p, Perker shale; Cm, Monkton quartzite; Cw, Winooski dolomite; €d,
Danby formation; O, several Ordovician formations; Obm, Bascom formation.

Cross section of Taconic and Green Mountains along Vermont-Massachusetts border and into

/O MILES

eastern New York, D-D', after Knopf and Prindle in Longwell, 1923. pCq, granite gneiss; €q,
quartzite, including phyllite and conglomerate; Cd, dolomite; €rg, graywacke; Cs, black shale;
Olm, limestone and marble; Osc, black shale, red shale, and chert; gph, Cambrian (?) green ,
phyllite; as, Cambrian (?) albite schist.

NEW ENGLAND APPALACHIAN SYSTEMS

165

The manner of thrusting, as conceived by Kay, in map view is graphi-
cally illustrated in Fig. 11.12, and in cross section in Fig. 11.13.

Two cross sections of the central and southern Taconic Range are pre-
sented in Fig. 11.9 and should be referred to in the following discussion
against the klippe hypothesis.

In a study of the Taconic Range west of Troy, Ralk ( 1953 ) recognizes
thrusting and an eastern allochthonous sequence and a western autoch-
thonous sequence, but concludes that dense vegetation cover and much
drift leave so few outcrops that the existence of a great Taconic klippe
cannot be proved or disproved. In a study farther south near Pough-
keepsie (1937) he believes there is little evidence to support the thrust
and klippe hypothesis.

Thrust sheets and klippen are postulated because of anomalous stratigraphic
successions, not otherwise explainable; or because of structure anomalies not
understandable from other points of view; or because the klippen, although
closely related to rocks nearer the root zones, were obviously out of their
proper geologic setting; or on the evidence of intensely crushed subhorizontal
zones of deformation; or on the evidence of exposed soles. None of these
criteria appears to be fully applicable here. There is no proof of an anomalous
stratigraphic succession in the gap of Wingdale; the deformation of the
supposed thrust sheet of pelite is, to all appearances, synchronous with that
of the autochthonous formations; the gneiss of the supposed klippen is known
to underlie the sedimentary rocks a few thousand feet below the surface; no
crush horizons, or exposures of indubitable soles, have been observed (Balk,
1937).

Craddock (1957) also concludes against the major klippe hypothesis
(middle cross section of Fig. 11.9) in a study of the southern cud ol tin-
Taconic Range. He says:

Fig. 11.12. Distribution of Canadian (Lower Ordovician) facies in New England and New
York, after Kay, 1942. The map on left shows the present distribution as a result of the
Taconic (post-Ordovician) thrusting, and the map on right shows the inferred distribution
before thrusting (a palinspastic map). Vertically ruled sediments are carbonates; horizontally
dashed sediments are shales.

GREEN

MT5. CONNECTICUT

/? D/ROND^CM AXIS
Fig. 11.13. Section of Adirondack dome and Taconic system restored to early Silurian time (after Kay, 1942).

AXIS

BELT

166

STRUCTURAL GEOLOGY OF NORTH AMERICA

< >

_i —

m or

uj oc

Q. CD

o- Z

z
UJ <x

_l —

Q CD

"J CD

STANDARD

RICHMOND

MAYSVILLE

EDEN

GLOUCESTER

COLLINGWOOD

COBOURG

SHERMAN FALL

ROCKLAND

CHAUMONT

LOWVILLE

PAMELIA

VALCOUR

V///////A

? ?. — ? —

MIOOLEBURY LS.
8EL0ENS FM

CROWN POINT

DAY POINT

/////////

SMITHVILLE

COTTER- POWELL

ROUB1DOUX

GASCONADE

TREMPEALEAU

FRANCONIA

DRESBACH

MARJUM

WHEELER

SWASEY

DOME

HOWELL

SPENCE

LANGSTON

ROME

SHADY

ERWIN

HAMPTON

UNICOI

WEST-CENTRAL
VERMONT

HORTONVILLE SL
GLEN FALLS LS.

ORWELL LS.

BURCHAROS

BRIDPORT OOL ..

BASCOM FM.

CUTTING OOL.

SHELBURN.MARBIE
CLARENDON SPRINGS

-■? ? ?

WINOOSKI DOL.

MONKTON OT2ITE.

DUNHAM DOL.

CHESHIRE OTZITE

-? ? ? —

"MENDON SERIES"

"MT HOLLY SERIES"

NORTHWESTERN
VERMONT

TstanbridgeI

L si- J

[mystic congl]
?

grandge sl.
corliss congl.
highgate sl.

ROCKLEOGE CONGL
HUNGERFORO SL.
SAXE BROOK DOL.
SKEELS CORNERS F
MILL RIVER CONGL.
ST. ALBANA SL.

RUGG BROOK FM.
PARKER SL.
DUNHAM OOL.
GILMAN OTZITE.

WEST SUTTON SL
WHITE BROOK OOL.
PINNACLE GRAYWCK.
CALL MILL SL.
TIBBIT HILL SCHT.

EASTERN
NEW YORK

INDIAN LADDER BED

ISLE LAMOTTE LS
AMSTERDAM LS.

CHAUMONT LS

LOWVILLE LS.

VALCOUR LS
CROWN PT. LS.
OAY POINT LS.

BEEK.E
CASSIN FM I SEEK.

BEEK. 01 a 02 ?-
BEEK.C *

— ■ TRIBES HILL

BEEK B WHITEHALL

LITTLE FALLS

DOL.

THERESA FM.

NEW YORK
QUEBEC

SNAKE HILL SH. ?

NORMANSKILL SH

OEEPSKILL SH.

SCHAGHTICOKE

SILLERY SL. ?

? ?

SCHODACK SH.B LS.

BOMOSEEN GRIT

NASSAU BEOS

Fig. 11.14. Stratigraphic correlations in west-central Vermont and adjoining areas, after Cady,
1945.

Evidence for the existence of the "Taconic klippe" was not found in mapping
this quadrangle. Analysis of the development of the klippe hypothesis indicates
that it is based principally upon stratigraphic considerations; available struc-
tural evidence weighs against this interpretation. While the klippe hypothesis
seems to explain well the relations at the north end of the Taconic Range,
the problem of adequately defining the boundaries of this 'Tdippe" causes
serious doubt about its existence.

An alternative interpretation of the regional relations is suggested, involving
unconformities and facies changes in a single indigenous sequence. Trentonian
rocks lie unconformably on rocks as old as Precambrian from Vermont to
Pennsylvania and pass indiscriminately in and out of the "Taconic klippe."
The Normanskill and Deepkill rocks (mainly shale) are interpreted as passing
transitionally into limestone to the west. The Deepkill is believed to rest
unconformably on rocks of Early Cambrian to middle Canadian age. Middle
Canadian formations in the kinderhook quadrangle are carbonate rocks and
appear to rest unconformably upon Lower Cambrian slates; their striking
similarity to equivalent rocks in the near-by "autochthonous" series suggests
they have not been displaced any great distance. The lower Cambrian is a
thick series of argillite, graywacke, and quartzite with some thin carbonate
rocks near the top. The thick, lower part of this series is considered a southward
continuation of the Mendon Series of Vermont. The upper strata are interpreted
as the offshore equivalents of shallow-water quartzites and carbonate rocks
deposited marginal to an eastern welt in later Early Cambrian time.

Lake Champlain and St. Lawrence Lowlands

The Champlain Valley lies partly in New England. In the largest view
it is bounded on the east by the Green Mountains and on the west by the
Adirondack Mountains, and at the south it is split by a minor group of
mountains, the Taconic Range. A large part of the valley is occupied by
Lake Champlain, the surface of which is 100 feet (30 meters) above sea
level and the bottom is below sea level. The valley passes northward into
Canada and curves northeastward, merging into the St. Lawrence Valley
(Keith, in Longwell, 1933).

The valley is divided by the Taconic Mountains into a western part
which is continuous with the Hudson River Valley, and an eastern part
which extends along the eastern side of the range nearly to Long Island
Sound. This eastern part of the valley is known as the Rutland Valley in
Vermont and the Stockbridge Valley in Massachusetts.

The St. Lawrence lowlands are of two divisions separated by the fault
known as Logan's line ( Fig. 12.2 ) . Southeast of the fault is the deformed

(

NEW ENGLAND APPALACHIAN SYSTEMS

187

SOUTH

NORTH

Fig. 11.15. North-south section in north-
western Vermont of the Cambrian and
Lower Ordovician formations, restored to
early Ordovician. Reproduced from Shaw,

1958.

OUNHAM DOLOMITE
MIOOLEBURY SYNCLINORIUM

FRANKLIN BASIN

OUARTZITE

DOLOMITE

SLATE

LIMESTONE CONGL.

Taconic belt and northwest of it is the undeformed shelt sediments which
lay onto the Canadian Shield.

Stratigraphij. The stratigraphic columns presented in Fig. 11.14 are
by Cady ( 1945) and represent a long endeavor by numerous geologists to
unravel the succession and to correlate the different formations in the
region. As previously noted, it appears that two lower Paleozoic succes-
sions of approximately equivalent age exist within the same area, and
1 the tendency of most workers is to regard the argillaceous sequence as an
allochthone from the east now reposing on a calcareous western sequence.
The Cambro-Ordovician limestones and dolomites grade westward into
foreland sandstones of the Adirondack area, and, it is believed, eastward
into shales of geosynclinal thickness. Cambrian and early Ordovician
sandstone tongues extend far to the east. The geosynclinal trough mi-
grated westward later in Ordovician time and resulted in the deposition
of a shale facies over, and in places uncomformably on, the calcareous
and sandy succession. This was the occasion of the Vermontian disturb-
ance (Kay, 1942).

Kay's (1942) map of Fig. 11.11 restores the distribution of Lower

Ordovician strata in the region. He names the eastern trough in which the
shale facies was deposited, the Magog; a postulated barrier to the west.
the Quebec; and the shallower trough in which the carbonates were de-
posited, the Champlain.

In the St. Alban's area of northwesternmost Vermont, north of Cady's
mapping, Shaw (1958) reports some unexpected facies changes in the
Cambrian and Lower Ordovician along the structural strike. These are
illustrated in Fig. 11.15. A northern basin, the Franklin, was partially
restricted from a southern bv an east-west high, the Milton, and streams
carried considerable clastic material into it from the Adirondack and
Laurentian land area. Throughout Cambrian and Early Ordovician times
the basin was one of considerable crustal unrest, as evidenced by the
several unconformities.

Structure. Although considerable doubt exists about the Taconic
klippe hypothesis south of Albany, there seems Little question in the
minds of those who have worked in the Lake Champlain lowlands about
the reality of major cast to west thrusting.

A number of thrusts other than the great Taconic thrust, but of the

168

STRUCTURAL GEOLOGY OF NORTH AMERICA

GREEN

M O U N T A I N

Fig. 11.16. Tectonic map of west-central Vermont, after Cady,
1945. Ruled areas are Ordovician strata in the synclinoria.
Faults with knobbed bars are normal faults.

same orogeny, have been mapped. All are interpreted as having moved
from east to west. The chief ones of these are the Champlain and Hines-
burg-Oak Hill. The Champlain thrust trends parallel to Lake Champlain
and extends from a point near the south end of the lake northward about
60 miles to the Canadian border. About 3 miles north of the border, near
the village of Rosenberg, it becomes obscure in a shale terrane (Cady,
1945). See Fig. 11.16. The thrust at the north end is known as the Rosen-
berg slice (sheet). According to Cady, near the south end:

At Snake Mountain Lower Cambrian beds of the mountain proper are
thrust westward across and beyond Upper Cambrian and Beekmantown rocks
of the Orwell thrust plate onto the Middle Trenton limestones and shales next
west and structurally continuous with those found along the lake; the Champlain
thrust apparently truncates the Orwell thrust.

The Hinesburg-Oak Hill thrust complex floors a tectonic unit east of
the Champlain thrust and it in turn is bounded on the east by the Green
Mountains. The southern part is called the Hinesburg thrust, and the
northern the Oak Hill. The Oak Hill thrust sheet passes beneath the
Hinesburg.

The rocks of both the Hinesburg and Oak Hill thrust slices grade eastward
into the schist and gneiss terrane of the Green Mountains. Both of these slices,
so far as they have been delineated, apparendy have undergone considerable
displacement, as evidenced by the depth of erosional re-entrants and by the
outlying position of klippes. The Hinesburg and Oak Hill thrusts form the
eastern boundary of the Rosenberg slice.

^ The rather highly deformed quartzose slates, phyllites, and graywackes east
of the Hinesburg thrust, a short distance north and east of Hinesburg village,
lie with angular discordance across the east limb of the Hinesburg synclinorium,
where the thrust plane truncates minor folds which are made up of beds from
Lower Cambrian to Beekmantown age. The thrust plane has not been observed
at any point, but the depth of the re-entrants suggests that it dips at a very
low angle to the east. Non-quartzose black slates and phyllites crop out west
of the quartzose rocks along the thrust front in St. George and Williston town-
ships. These latter Upper Cambrian argillaceous rocks comprise the Muddy
Brook thrust slice, which was apparently dragged up along the sole of the
Hinesburg thrust. These same slates and Upper Cambrian sandy dolomites
crop out in the re-entrant west of Williston village. Northwest of Williston
village the quartzose rocks are thrust over a closely folded syncline of the
Oak Hills slice. In this syncline are formations from Lower Cambrian to prob-
ably Upper Cambrian age.

In general, the rocks east of the Oak Hill thrust are less deformed and less
uniform in appearance than those east of the Hinesburg thrust. The lower
Cambrian Dunham dolomite is everywhere recognizable, and at many places
along the thrust front, where structures involving the Dunham are truncated
at erosional re-entrants or at klippes such as Cobble Hill in Milton township, it
locates the fault. Where argillaceous rocks are near the contact, the fault is
much more difficult to locate, inasmuch as the eastern exposures of the
Rosenberg slice are in a predominandy argillaceous terrane (Cady, 1945).

Two synclinoria lie on a common north-south axis and are separated
by the Monkton cross anticline. See Fig. 11.16. They are bounded on the
west by the Adirondack dome and Champlain thrust and on the east by
the Hinesburg-Oak Hill thrust and the Green Mountains.

The southern synclinorium, known as the Middleburg, makes up the

NEW ENGLAND APPALACHIAN SYSTEMS

169

structure of the area between Snake Mountain on its west limb and the
Green Mountain front on its east limb. The center of the synclinorium is
covered by the great Taconic klippe south of the latitude of Brandon.
The east limb may be traced fairly continuously into the marble belt south
of this latitude (Cady, 1945). The west limb loses its identity in an area
of high angle faults southwest of Orwell. The nature of the numerous
small folds of the synclorium are best shown in the cross sections B and
C of Fig. 11.11.

The northern synclinorium, known as the Hinesburg, composes the
structure of most of the area between Lake Champlain and the Green
Mountain front. See section A, Fig. 11.11. Most of the east limb is covered
by the Hinesburg-Oak Hill thrust slices. The Hinesburg synclinorium is
not so symmetrical as the Middleburg synclinorium, and the folding is
limited to the development of a series of moderately broad basin struc-
tures (Cady, 1945).

The normal faults of the Adirondacks have already been described.
The eastern border of the crystalline mass is formed in part by these
faults, and they seem to be genetically related to the uplift of the dome.
They do not intersect the major thrusts of the Lake Champlain region,
but they parallel the Orwell and Champlain thrusts, and for a distance
the bends in the normal faults coincide with bends in the thrust fronts. It
lis suggested (Cady, 1945) that the thrust fronts may have retreated by

erosion eastward after they were trimmed by the normal faults, and thus
the parallelism has resulted.

Tectonic History

Champlain and Magog Troughs. In 1923 on the occasion of his presi-
dential address on North American geosynclines, Schuchert postulated a
western trough, the St. Lawrence, through the Lake Champlain and St.
Lawrence region, a medial divide or geanticline, and then an eastern
trough, the Acadian, principally through Nova Scotia and New Bruns-
wick. The geanticline included the Green Mountains of Vermont and
the White Mountains of New Hampshire and Maine. The rocks of these
mountains were then regarded as Precambrian. Since then several groups
of fossils have been found, and most of the metamorphosed sediments
of Schuchert's geanticline have turned out to be Lower and Middle
Paleozoic in age. Still two troughs seem necessary, but the eastern one
must have occupied approximately the site of Schuchert's geanticline. It
has been called the Magog eugeosyncline by Kay ( 1942), and the western
has been called the Champlain miogeosyncline. The Magog is character-
ized by shales, cherts, and various volcanics, the western by carbonates.
Until Mid-Ordovician time, the separation of the two troughs was prob-
ably a matter of facies, but then a land barrier called Vermontia rose
within the western part of the Magog trough and caused the deposition of
elastics over the carbonates of the western trough. See Fig. 11.17. Later in

WEST ERA!
NX

SITE OF UPPER DEVONIAN "DELTA"

MIOGEOSYNCLINE
Lower Devonion'} S-)

, VT i \SOUTH
NYMUASS. VT.W.H N.H\MAINE GULF OF MAINE
-+- ' --LATER ACADIAN BELT- *•

ATLANTIC

VERMONTIA
GEANTICLINE . EUGEOSYNCLINE

Lower Devonion';

APPALACHIA

Fig. 11.17. Basins of deposition across New England just prior to Acadian orogeny. Compiled
from Kay (1951), Billings (1956), and other sources. Vermontia had risen in Mid-Ordovician
time and evidently was considerably wider than present dimensions indicate to supply
the voluminous elastics to the miogeosyncline in Mid- and Late Ordovician time. Vermontia as

---0 _---'
Grenville orogenic complex

shown was also essentially the site of the Taconic orogeny at the close of Ordovician time. The
eugeosyncline was the site of much volcanism, and Vermontia the site of ultramafic intrusions, cm,
Cincinnatian; moh, Mohawkian; and ch-can, Chazyan and Canadian. The region of Vermontia in
places probably received Silurian and Devonian sediments, so its history and nature is complex.

170

STRUCTURAL GEOLOGY OF NORTH AMERICA

NORTHWESTERN
VERMONT

Montpelier Quad.
Cady, 1956

CENTRAL AND EAST
CENTRAL VERMONT

White and Jahns, 1950

WESTERN, CENTRAL

AND NORTHERN

NEW HAMPSHIRE

Billings, 1956

SOUTHEASTERN
NEW HAMPSHIRE

Billings, 1956

LOWER
DEVONIAN

SILURIAN

?Meeting House slate
Gile Mountain fm.

Littleton fm.
15,000'+

Littleton fm.

15,000'±

Waits River fin.
Northfield slate

"Standi fig Pond vols.
Waits River fm.

Northfield slate

Fitch, fm.
0-769'

Berwick fm.
10,000'+

Eliot fm.
6,500'i

ORDOVICIAN

Shaw Mtn. fm.

Serpentine, talc-
carbonate rock,
and steatite

Shaw Mountain fm.
Ultramafic rocks

Clough quartzite
0-1200'

Partridge fm.
0-2000'

Ammonoosuc vols.
2000-5000'

Kittery quartzite
1,500'±

Rye fm.

2,000'i

Moretown fm.
Stowe fm.

Cram Hill fm.

Arenites of the Brain-
tree-Northf ield Range

Albee fm.
5000'

Orfordville fm.
3500-4000'

CAMBRIAN

Ottauquechee fm.
Camels Hump gr.

Ottauquechee phyllite

Pinney Hollow schist

Quartzose schist,
quartzite, dolomite,
and conglomerate

PRECAMBRIAN

(To the southwest)

Fig. 11.18. Correlation chart of pre-Acadian Paleozoic formations across Vermont and New
Hampshire. The Standing Pond volcanics and Meeting House slate are listed by Billings for
westernmost New Hampshire in the stratigraphic order shown but not included by Cady for
Vermont. The total Vermont section is immensely thick.

Ordovician time, another uplift, the Oswegan disturbance, occurred and
spread westward past the Adirondack axis into the Allegheny basin.

Taconic Orogeny. At the close of the Ordovician period the major
Taconic orogeny occurred, and the argillaceous rocks of the Magog trough
were thrust far westward. The Quebec barrier and eastern part of the
Champlain trough were concealed by it. The amount of horizontal dis-
placement probably exceeded 40 miles (Kay, 1942).

The thrust sediments are in tectonic contact on Queenston shale in south-
eastern Quebec, and the autochthonous Cincinnatian has been folded con-
siderably. The overthrust rocks are overlain at Becraft Mountain, New York,
and in the Catskill Front by latest Silurian Manlius limestone. Thus, there is
direct evidence that the principal lateral movements were pre-Manlius and
post-Queenston. Folds in autochthonous Ordovician are truncated by the

Shawangunk and Tuscarora quartzites of the earliest Silurian in southeastern
New York and Pennsylvania; if the folding accompanied Taconic thrusting, the
revolution is pre-Silurian.

The front of the thrust sheet is not very high. Middle Ordovician sediments
are preserved near to the westernmost remnant of the sheet and probably never
were buried deeply. On Anticosti Island in the Gulf of St. Lawrence, there is
essentially continuous section of Cincinnatian and early Silurian calcareous shale
and limestone in the Champlain belt within 50 miles of the overthrust rocks
of Gaspe; the allochthone was beneath the sea or not high enough to produce
significant detritus after the revolution. Though the quantity of Silurian terrig-
enous sediments is distinctly smaller than that of the Ordovician, . . . this
reflects repeated uplift and continued presence of Vermontian highlands in
later Ordovician, in contrast to progressive reduction of the transposed Taconia
in the Silurian. The greatest quantity of eroded material was laid down in the
latitude of Pennsylvania, as shown by isopachs; that the greatest elevation was
there is also shown by the coarser texture of the sediments. The lateral move-
ment of the allochthone may have been as great or even greater in Quebec, but
Vermontia and its transposed descendant, Taconia, were more continually high
farther south.

Acadian Orogeny. The next great influx of clastic sediments was in the
Middle Devonian, and the sediments generally coarsen upward and east- j
ward. They came from rising highlands on the east. The elevation termi-
nated in the Acadian orogeny which was followed by the deposition of "
Mississippian elastics to the west of the orogenic belt.

The Acadian belt is known best in New Hampshire and the Maritime
Provinces and will be described later, but it is possible that it spread
westward to the Hudson Valley and Lake Champlain lowlands and im-
pressed additional folds on the Taconic structures. It is possible, also,
that the later structures are Appalachian in age.

Unsolved Problems. The above summary of the history of the Taconic
system savors of those who postulate the great Taconic allochthone, and
this is the general opinion of those who have worked in northern Massa-
chusetts, Vermont, and eastern New York. Yet Balk and Craddock in very
thorough work, at the south end of the Taconic klippe where the great
thrust and its roots should be found, do not find evidence of it, and
they do not believe the thrust theory necessary to explain the facies and
metamorphism there. Similarly, the roots of the thrust are not yet estab-
lished at all well in the Green Mountains.

tchq

NEW ENGLAND APPALACHIAN SYSTEMS

HYDE PARK QUADRANGLE, NW VERMONT

€chq £0 Oi

171

MONTPELIER QUADRANGLE, NW VERMONT

Os o,sgo

Om 5i"!J"^ 5*

Fig. 11.19. Cross sections of northwestern Vermont in Green Mountains. Hyde Park section
;after Albee, 1957. Montpelier quadrangle after Cady, 1956. €ch, Camels Hump group; Cchg,
.albite and tremolite greenstone; Co, Ottanquechee fm.; Os, Stowe fm.; Osga, middle unit of

CENTRAL AND EASTERN NEW ENGLAND

Definition

The Acadian orogeny of Late Devonian time affected much of New
jEngland and the Maritime Provinces, and undoubtedly spread southward
through the Piedmont crystalline province of the Atlantic margin. It
treated a mountain system that was superposed in part on the earlier
Taconic system. Where best known and perhaps best displayed in New
Hampshire, New Brunswick, and Nova Scotia, it is an irregular north-
south belt east of the Taconic system, but its western limit is as yet poorly
defined.

The region here discussed lies east of the crest of the Green and Berk-
shire Mountains and includes the New England seaboard lowland, the
New England upland and the White Mountains in the United States and
Canada. See map of Fig. 11.1 and 11.2. The seaboard lowland extends
along the Atlantic coast as a narrow zone from Rhode Island to the
border of Maine and New Brunswick.

Os consisting of greenstone and amphibolite; Om, Moretown fm.; Omsp, carbonaceous and
slate member; Ssm, Shaw Mountain fm.; Sn, Northfield slate; Swr, Waits River fm.; Da,
Adamant granite.

Stratigraphy and Structure of Vermont

An immensely thick section of stratified rocks exists in northwestern,
central, and east-central Vermont, probably reaching a thickness of 100.-
000 feet (White and Jahns, 1950). The strata except some lamprophyre
dikes are folded and metamorphosed sedimentary and volcanic rocks. A
number of units, members or formations of volcanic rock throughout the
section from Cambrian to Lower Devonian attest the eugeosynclinal
nature of the deposits. See correlation chart of Fig. 11.18.

Northwestern Green Mountains

Two quadrangles, the Hyde Park and Montpelier, have been mapped
by Albee (1957) and Cady (1956), and depict the structure and stratig-
raphy near the north end of the Green Mountains a few miles east of the
crest. The sections of Fig. 11.19 show the thick succession of folded beds
from Cambrian to Devonian.

The axis of the Green Mountain anticlinorium trends north-northeast across
the northwest corner of the Hyde Park quadrangle. This anticlinorium. which

172

STRUCTURAL GEOLOGY OF NORTH AMERICA

is the principal structural feature of the bedrock of Vermont, extends north-
northeast from the Massachusetts- Vermont border the full length of the state
and about 50 miles into Quebec, a total distance of about 210 miles. The
stratigraphic sequence and lithologic character of the rocks on the west limb
of the anticlinorium are different from those on the east limb, and a generally
accepted correlation of the two is not yet possible. In the Hyde Park quad-
rangle, and in the Montpelier quadrangle (Cady, 1956), which borders on
the south edge of the Hyde Park quadrangle, the general eastward dip of
the rocks is interrupted by a group of anticlines whose axes parallel the axis
of the Green Mountain anticlinorium. [See Fig. 11.18.]

The bedrock of the quadrangle (s) comprises chiefly metamorphosed sedi-
mentary and volcanic rocks, principally schist, phyllite, slate, granulite,
quartzite, greenstone, amphibolite, crystalline limestone, and conglomerate,
that range in age from Cambrian probably to Devonian. Intrusive igneous
rocks, some of which are metamorphosed, underlie less than 1 percent of
the area and comprise serpentinite and its derivatives (talc-carbonate rock
and steatite ) , granite, and diabase that range in age from Ordovician probably
to Mississippian.

All the rocks in this erea except the lamprophyre dikes have been affected
by regional metamorphism. In this area, chlorite, garnet, and kyanite have
been interpreted as successively general indicators of increasing metamorphic
grade in the schists. Similarly, chlorite, actinolite, and hornblende are indicators
in the greenstone and amphibolite. Most of the Hyde Park quadrangle is in
the chlorite zone of metamorphism (Cady, 1956).

Bodies of serpentinite or its alteration products, talc carbonate rock and
steatite, are numerous, having been noted in fifteen places by Albee and
in five by Cady. They occur chiefly in the Stowe formation.

The serpentinite (or its derivatives) forms tabular, lenticular, or pod-
shaped masses that strike north-northeast and dip steeply, parallel with the
schistosity and commonly also with the bedding of the enclosing rocks. The
serpentinite is dark green to dark greenish black on the fresh surface but
weathers to a characteristic pale greenish-white or light-buff rind traversed
by a reticulate system of sharply cut lines; it is composed almost entirely of
the mineral serpentine, probably of the antigorite variety. The talc-carbonate
rock is mottled greenish gray and weathers brown; it is composed of the
minerals talc, magnesite, and locally small amounts of dolomite. The steatite
ranges from white to green and greenish gray and weathers grayish tan; it
is composed of the mineral talc (Albee, 1957).

Thick sills of granite invade the Waits River formation of the Mont-
pelier quadrangle, and have generated cordierite and diopside as contact
metamorphic effects. These sills are probably a late element of the

Acadian folding which took place in Mid- and Late Devonian time
(Cady, 1956).
The minor folds do not accord with the major folds.

The axes of most of the minor folds and granular quartz columns, as well
as the intersections of fold bands and of slip-cleavage lamellae with bedding,
are nearly vertical. This attitude implies that most of these minor structural
features were not produced by shearing movements in a nearly east-west
oriented vertical plane, such as were evidently responsible for the gently
plunging structures of the Green Mountain anticlinorium. Instead they were
probably either formed before folding of the anticlinorium by shearing move-
ments in a north-south vertical plane, or after folding and tilting of the limbs
of the anticlinorium by shearing movements in a north-south vertical plane,
or after folding and tilting of the limbs of the anticlinorium to near vertical,
by shearing movements in a horizontal plane. The pattern of movement of
these minor folds is uniform over rather wide areas; thus most of the folds in
the fold bands in the Moretown formation southeast of the Worcester Mountains
are dextral in plan (see White and Jahns, 1950, p. 197, for usage of terms
"dextral" and "sinistral"), and it appears that the rocks to the east have moved i
south relative to those to the west. This relationship is well shown at the
previously cited exposures of the Moretown formation in Middlesex Gorge
(Albee, 1957).

Central and East-Central Vermont

The outcrop pattern of three key formations in central and eastern
Vermont is broadly shown on the map of Fig. 11.20, and the stratigraphic
succession in Fig. 11.18. According to White and Jahns:

The formations of central and east-central Vermont are exposed as a series
of parallel belts that strike nearly north. Most of the rocks dip steeply, and
many are overturned. With one possible exception, there seem to be no
major repetitions within the sequence, and the order of formations from
west to east appears to be the same as the order of their deposition. The
formations are dominantly schist or phyllite, with varying proportions of
arenaceous material. One thin formation, the Shaw Mountain, contains quartz
conglomerate, calcareous tuff, and crinoidal limestone. The third-from-highest
formation, the Waits River, is very thick and contains a large proportion of
calcareous beds. The distance from the base of the lowest formation to
the top of the highest, measured normal to bedding, is more than 100,000
feet; this large apparent thickness is believed to be not very much greater
than the original thickness.

The metasediments have been intruded by granitic dikes and plutons

afic dikes, and small ultramafic plutons.

^Yn

NEW ENGLAND APPALACHIAN SYSTEMS

173

Two principal stages of deformation are distinguished. During the earlier
stage the rocks were folded, and a schistosity was developed nearly parallel to
bedding. Throughout the area the minor folds of this stage indicate a consistent
upward movement of rocks on the east with respect to those on the west. The
folds plunge at low to moderately steep angles, typically northward.

Phenomena associated with the later stage of deformation decrease in
intensity both eastward and westward from the belt underlain by the calcareous
Waits River formation. At a distance from this formation, the rocks have
prominent slip cleavage, and the earlier schistosity is folded. The minor folds
plunge moderately to steeply northward on the western side of the area and
^ore gendy northward on the eastern. As the Waits River formation is
approached, slip cleavage passes gradually into a schistosity that obliterates
the earlier schistosity, and the intensity of later folding increases. In both
the eastern and the western parts of the area the later minor folds indicate
"that the rocks of the Waits River formation have moved upward with respect
| to the formations on either side.

1 The central part of the belt underlain by the Waits River formation is marked
(lby a huge arch, 10—20 miles across, whose axis is more or less parallel to the
1 belt and plunges gently northward. This is shown to be an arch, not in bedding,
)!but in the later schistosity and in the axial planes of large isoclinal folds that
1 were formed during the later stage of deformation. The axial planes of three
1 of these large isoclinal folds can be correlated across the crest of the cleavage
arch at Strafford Village.

Western, Central, and Northern New Hampshire

i Stratigraphy. A series of metasedimentary and metavolcanic rocks in
1 western, central, and northern New Hampshire ranges in age from Ordo-
vician (?) to Lower Devonian and has an aggregate thickness of 16,000
feet. See Fig. 11.18. Figure 11.21 is a columnar section of the Littleton-
,i Moosilauke area in the White Mountains of west central New Hampshire.
The stratified rocks fall into six major units. The Albee, Ammonoosuc,
and Partridge formations are of pre-Silurian, probably Upper Ordovician
age, the unconformably overlying beds are the Clough conglomerate and
J Fitch formation of Silurian age, and the Littleton formation is of Lower
; Devonian age. The Albee was originally a shale and sandstone formation,
land although no fossils have been found in it, it appears to be above the
i fossilif erous Middle Ordovician of Vermont (Billings, 1937).

The Ammonoosuc volcanics consist principally of soda-rhyolite, soda-
rhyolite volcanic conglomerate, meta-andesite porphyry breccia, and slate
and impure quartzite. The Partridge formation is largely a black slate. In

WHITE MOUNTAIN
I PLUTONIC-VOLCANIC SERIES

aJV

t''"j;J3 NE* HAMPSHIRE _ fiU£B££.

\""t'-\ PLUTONIC SERIES /" "V VT (V- S

);-/J PROBABLY OF NEW HAMPSHIRE
PLUTONIC SERIES^

OLIVERIAN PLUTONIC
SERIES FORMING CORES
OF D

Fig. 11.20. Major structures of eastern Vermont and New Hampshire. After White and
Jahns (1950) and Billings (1956). The narrow Connecticut Valley synclinorium lies between the
Northey Hill and Ammonoosuc thrusts and is not labeled on the map. Osp, Standing Pond
volcanics; On, Northfield slate; Oo, Ottauquechee phyllite.

UTHOLOGY IN LOW-GRADE
ZONE (EPIZONE)

ICKNESS
N FEET

INTRUSIVE ROCKS

GLACIAL TILL 8. OUTWASH, ALLUVIUM

»LATE AND SANDSTONE, WITH VOL-
CANIC MEMBER CONSISTING OF
CHLORITE SCHIST AND META-
BASALT (Olc), SODA-RHYOLITE
VOLCANIC CONGLOMERATE (Dive),
AND SODA-RHYOLITE, SODA-
RHYOLITE TOFF AND BRECCIA ,
AND SODA-TRACHYTE (Dlr).

LIMESTONE, MARBLE, DOLOMITIC
SLATE, ARENACEOUS DOLOMITE,
CALCAREOUS SLATE, ARENACEOUS
LIMESTONE, CALCAREOUS SAND-
STONE, IMPURE QUARTZ I TE, ARKOSE,
QUARTZ CONGLOMERATE, AND
GRAY SLATE.

5000

WHITE MOUNTAIN
MAGMA SERIES (wm)'

NEW HAMPSHIRE
MAGMA SERIES (nh)

OLIVERIAN MAGMA
SERIES (ol)

SODA
AND

-RHYOLITE DIKES
SILLS (sr)

QUARTZ CONGLOMERATE AND
QUARTZ I TE.

BLACK SLATE, WITH THIN-BEDDED
QUARTZITE, SLATE, AND VOLCANIC
MATERIAL AT BASE.

SODA-RHYOLITE TUFF, BRECCIA,
AND VOLCANIC CONGLOMERATE;
CHLORITE SCHIST; CHLORITE-
EPIDOTE SCHIST; SLATE AND
IMPURE QUARTZITE.

QUARTZITE, ARGILLACEOUS
QUARTZITE , GREEN SLATE, AND
BLACK SLATE.

2000 k

4000 *

HIGHLANDCROFT
MAGMA SERIES (h)

Fig. 11.21. Columnar section of the Littleton Moosilauke area. Reproduced from Billings, 1937. In addi-
tion to the sequence and character of the sedimentary and volcanic rocks, the time of intrusion of igneous
rocks is shown.

NEW ENGLAND APPALACHIAN SYSTEMS

175

places at the base, black slate and fine-grained, light quartzite alternate
in beds a quarter of an inch thick.

The Clough conglomerate is one of the best key horizons in western
New Hampshire, and although thin, it is resistant and exceptionally well
represented in outcrops. It apparently continues southward to the Massa-
chusetts boundary. Its outcrops are generally white cliffs. The pebbles in
the conglomerate are chiefly vein quartz, but some are quartzite, jasper,
greenstone, or soda-rhyolite. In places only a few pebbles are present; in
others they constitute over 60 percent of the rock (Billings, 1937). The
matrix is pure or slightly impure quartzite.

The Clough conglomerate directly underlies the Fitch formation which
carries middle Silurian fossils. Moreover, the two formations are closely related
in age, for a few beds of quartz conglomerate are found in the Fitch. The
Clough conglomerate, however, is separated from the underlying strata by an
unconformity. It is apparent that the formation is either middle or lower
Silurian. In many respects the Clough is similar to the Shawangunk conglomer-
ate of New York, although the former is thinner and purer. The Clough under-
lies fossiliferous middle Silurian, and the Shawangunk carries middle Silurian
fossils in its upper part. The two are closely related, if not identical, in age
(Billings, 1937).

The Fitch formation in its least altered form consists of white to buff
marble; gray limestone and marble; buff dolomitic slate; buff to brown
arenaceous dolomitic limestone; gray calcareous slate ("trilobite slate" of
earlier workers); white to gray arenaceous limestone and calcareous,
arkosic conglomerate; gray impure quartzite; white to gray arkose; white
quartz conglomerate; and gray slate. Fossils have been found at two
localities in the Fitch formation southeast of the Ammonoosuc thrust, and
are recognized as of Middle Silurian (Niagaran age).

The Littleton formation of Lower Devonian age consists in its least
metamorphosed condition chiefly of slate and sandstone, with subordinate
amounts of soda-rhyolite conglomerate, tuff and breccia, and some green-
stone.

Formations older than those listed in the chart of Fig. 11.21 are known.
The Orfordville formation, first recognized in west central New Hamp-
shire (Kruger, 1946) underlies the Albee formation, and the Waits River
formation first found in central Vermont, underlies the Orfordville ( Cur-

rier and Jahns, 1941). The base of the Waits River is 2000 feet abo'
crinoidal limestone which appears to be Middle Ordovician. If so, both
the Waits River and Orfordville are Middle Ordovician or vounger. The
Orfordville formation was originally' a shale with very thin beds of sand-
stone, and the Waits River a calcareous shale and limestone formation.

Structure

General Statement. In Massachusetts and southern New Hampshire
the structures trend northerly; in northern New Hampshire they veer
northeasterly. A succession of anticlinoria and synclinoria make up the
major elements of the structure. See Figs. 11.20 and 11.26. Proceeding
eastward from the great monocline of central and eastern Vermont three
thrust faults occur, and between the middle ( Ammonoosuc ) and eastern
( Northey Hill ) is the Connecticut Valley synclinorium. This lies approxi-
mately astride the boundary line of Vermont and New Hampshire. Next
east is the Bronson Hill anticline, the Merrimack synclinorium and in
southeastern New Hampshire the Rockingham anticlinorium. The Coos
anticlinorium is in the northern part of the state and lies between the
Monroe and Ammonoosuc thrusts.

The older plutonic series, especially the Oliverian and New Hampshire
series, participate in the northerly and northeasternly trend. This may
be seen by the Oliverian series making up the cores of the domes along
the Bronson anticline, and by the foliated Mt. Clough and Cardigan
plutons of the New Hampshire series striking along the western flank of
the Merrimack synclinorium.

Bronson Hill Anticline. The Bronson Hill anticline extends from
Massachusetts to Maine, a length of 150 miles. It ranges from 6 to 16
miles wide. The core is composed of the Ammonoosuc volcanics and the
Oliverian plutons with the Clough, Fitch, and Littleton formations on
both flanks.

Rockingham Anticlinorium. The Rockingham anticlinorium. lies in
southeastern New Hampshire, between the Atlantic Ocean and the Fitch-
burg pluton. The individual folds of the anticlinorium are, from south-
east to northwest, the Rye anticline, the Great Bay (Eliot) syncline. and
the Exeter anticline (largely occupied by the Exeter pluton).

176

STRUCTURAL GEOLOGY OF NORTH AMERICA

Merrimack Synclinorium. East of the Bronson Hill anticline and
northwest of the Rockingham anticlinorium is a large area of Littleton
formation, all in the sillimanite zone of metamorphism. Inasmuch as this
band of the Littleton formation is bordered on either side by older strata,
it must occupy a synclinorium. This structural feature is called the Mer-
rimack synclinorium, because much of it is drained by the Merrimack
River and its tributaries.

Throughout much of western New Hampshire the western limb of the
Merrimack synclinorium is invaded by large bodies of the New Hamp-
shire plutonic series. These relations are well shown on sections A-A' and
B-B'-B" of Fig. 11.26.

Thrust Faults. The Ammonoosuc thrust is marked generally by Am-
monoosuc volcanics being thrust over the Littleton formation with a
stratigraphic displacement of 7000 feet. The fault dips from 32 to 50
degrees westerly. It is younger than the regional metamorphism.

The Northey Hill thrust predates the metamorphism because there is
no break in grade of metamorphism across it. This feature renders recog-
nition of the fault a little difficult, yet mapping shows the Littleton forma-
tion lies in contact with several different formations along it, and a
maximum stratigraphic displacement of 12,000 feet may be measured. A
steep dip characterizes much of its length, and this is believed due to
later deformation.

The Monroe thrust is about as long as the Ammonoosuc (85 miles).
It is nearly vertical throughout most of its length, but in places dips
southeasterly. It is mostly older than the regional metamorphism, but later
deformation steepened it and also caused some renewed movements
along it.

Magma Series

Plutonic rocks are abundant and varied in form and composition. Four
magma series have been worked out (Billings, 1937). The oldest is known
as the Highlandcroft magma series and is probably of late Ordovician
age. See chart, Fig. 11.21 and map, Fig. 11.20. Some time after the Lower
Devonian, probably in Mid- and Late Devonian time, other large

quantities of magma invaded the region. The Oliverian magma
series preceded the folding and was followed by the New Hampshire
magma series, the earlier members of which were contemporaneous with
the main period of folding, and the later members of which were slightly
younger than the folding. The White Mountain magma series is the
youngest of the plutonic rocks, and it appears less extensive than the
others. It is probably early Mississippian in age (Billings, 1945).

The Highlandcroft magma series is represented by the Highlandcroft
granodiorite and small bodies of diorite, quartz diorite, and quartz
monzonite. The Oliverian magma series is represented by the pink Owls
Head granite in the Littleton area and by other units in the Rumney, Mt.
Cube, and Mascoma quadrangles. Many sills in the Ammonoosuc vol-
canics are of this series.

The White Mountain magma series is characterized by ring-dikes,
stocks, a batholith, and by eruptive differentiates. According to Billings,
1945:

Much of the magma of the White Mountain magma series was erupted on
the surfaces to from the Moat volcanics. Tuffs, breccias, and lavas, composed
chiefly of rhyolite, andesite, and basalt, but also including some trachyte, are
typical. Rhyolite is by far the most common; trachyte is rare.

The intrusive rocks range in composition from gabbro to granite, and a great
variety of intermediate types are developed. Chapman and Williams, in a
careful, detailed study, have shown that the mafic rocks are the oldest and the
felsic are the youngest. They have also determined the areal extent of the
plutonic rocks and calculated the percentage of each compared to the whole
magma series. The order of intrusion, from oldest to youngest, and the percent-
age of each as exposed at the surface, are gabbro, norite, diorite, and quartz
diorite (0.5 per cent); monzodiorite and monzonite (1.5 per cent); syenite, in-
cluding some nepheline-sodalite syenite (9 per cent); quartz syenite (10 per
cent); granite and granite porphyry (79 per cent). Although the rocks in gen-
eral became more siliceous as differentiation progressed, this is not true in
detail. Especially important is the fact that the Albany quartz syenite is younger
than the granite porphyry. This is significant in considering the tectonic evolu-
tion of the area.

Chapman and Williams have also shown that fractional crystallization con-
trolled the evolution of the series, but that abyssal assimilation played an im-
portant role.

The Moat volcanics, in large part contemporaneous with the granite por-
phyry, are older than the Albany type of quartz syenite, but their age relative
to the more mafic plutonic rocks is uncertain.

NEW ENGLAND APPALACHIAN SYSTEMS

177

Metamorphism

All the sedimentary and metamorphic rocks have been deformed and
metamorphosed to various degrees. The metamorphism increases gen-
erally to the southeast, and three zones have been recognized by Billings,
namely, the low-grade, the middle-grade, and the high-grade. See map
of Fig. 11.22.

The distinction between the zones is based primarily on their mineralogy.
The low-grade zone is characterized by chlorite, epidote, albite, sericite, and
dolomite; the middle-grade zone, by staurolite, garnet, hornblende, actinolite,
diopside, biotite, and intermediate and calcic plagioclase. The mineralogical
contrast between these two zones is striking. The high-grade zone differs from
the middle-grade zone chiefly in that sillimanite is present and staurolite is
absent or is in small crystals. Thus, if aluminous sediments are not present, it
is difficult or impossible to distinguish the middle-grade and the high-grade
zones on mineralogical criteria alone. In general, the high-grade rocks are
coarser than the middle-grade, but this criterion is difficult to apply, and,
wherever the rocks might belong to either of the two higher zones, they have
been assigned to the middle-grade zone.

The change in the degree of metamorphism in a southeasterly direction is
readily apparent. The cumulative effect of these changes is so great that, for a
long time, rocks now known to belong to the same formations were believed to
be of very different ages. Whereas, northwest of the Ammonoosuc thrust the
rocks are dominandy sandstone, slate, calcareous slate, dolomitic slate, rhyo-
lite tuff, and greenstone, composed of such minerals as sericite, chlorite, albite,
dolomite, calcite, quartz, and epidote, to the southeast the rocks are mica schist,
calcite-biotite schist, actinolite-diopside granulite, biotite gneiss, and amphibo-
lite, composed of such minerals as biotite, garnet (almandite), staurolite, silli-
manite, actinolite, diopside, hornblende, calcite, quartz, and calcic plagioclase.
Moreover, there is a general coarsening in grain. These changes clearly repre-
sent progressive metamorphism toward the southeast, for the new rocks are
farther and farther removed mineralogically from the original rocks from which
they were derived.

A number of the intrusive rocks are older than the regional metamor-
phism and were affected to different degrees. The Highlandcroft grano-
diorite was in the zone of low-grade metamorphism, and its original
andesine plagioclase has been replaced by albite-oligoclase, epidote, and
sericite. Green biotite, which is found in places as a shell around the horn-
blende, is of metamorphic origin. The Moulton diorite has been subjected
to low-grade metamorphism, and its original condition is much altered.

7I-4S"

Scale of Miles

LEGEND

DRIFT AND
ALLUVIUM

WHITE MOUNTAIN
MAGMA SERIES?

NEW HAMPSHIRE
MAGMA SERIES

OLIVERIAN
MAGMA SERIES

HIGHLANDCROFT
MAGMA SERIES

METAMORPHIC
ZONES

HIGH-GRADE ZONE
WHERE SILLIMANITE
HAS RETROGRESSED
TO STAUROLITE

HIGH-GRADE ZONE
(KATAZONE)

middle-grace zone
(mesozone)

TRANSITION
BETWEEN LOW-
GRADE ANO MIDDLE-
GRADE ZONES

LOW-GRADE ZONE
(EPIZONE)

FOSSIL LOCALITIES
SOUTHEAST OF
AMMONOOSUC THRUS1

Fig. 11.22. Metamorphic zones in the Littleton-Moosilauke area. Metamorphism is progressive
toward the southeast. Reproduced from Billings, 1937.

178

STRUCTURAL GEOLOGY OF NORTH AMERICA

Rasic dikes and sills have attained equilibrium under the new metamor-
phic conditions.

Rillings regards the main alteration to have occurred after the Northey
Hill thrust and during the intrusions of the New Hampshire magma
series. Then the Ammonoosuc thrust brought different metamorphic
zones into sharp contact with each other. Also in certain places retro-
grade metamorphism set in with the formation of much chlorite.

The cause of the metamorphism is apparently the intrusions of the vari-
ous plutons of the New Hampshire magma series. Northwest of the
Ammonoosuc thrust where metamorphism is least, the intrusions of the
New Hampshire magma series are absent except that a few small bodies
of the Bethlehem gneiss and Kinsman quartz monzonite appear. Billings
points out that, as intrusions are common eastward to the Maine border,
and as the sedimentary rocks almost invariably are recrystallized to high-
grade metamorphic rocks, there must be a causal connection between the
increase in metamorphism and these intrusions. Not only is there a gen-
eral increase in the intensity of metamorphism toward the area where
igneous intrusions are most abundant, but there is an increase locally
toward individual bodies. Such high-grade zones surrounding intrusive
masses are not well defined in the map of Fig. 11.22, but it is suggested
that the contact metamorphic zones vary in width greatly, and that cer-
tain zones betray the presence of unexposed plutons.

Mechanics of Instrusion

Introduction. The post-tectonic White Mountain magma series is
characterized by ring-dikes, stocks, and a batholith (Billings, 1945). The
ring-dikes, most of which range in composition from monzonite to quartz
syenite, intruded arcurate and circular vertical fracture zones by piece-
meal stoping and related mechanisms. Cauldron subsidence, although
associated with some ring-dikes, is not essential for their intrusion. The
stocks of the White Mountain magma series were emplaced by under-
ground cauldron subsidence.

The New Hampshire magma series, emplaced during the Acadian
^orogeny, occurs chiefly as great sheets, lenses, and stocks, forcefully
injected into the older formations.

Ring-Dikes. Altogether, 36 ring-dikes associated with the White
Mountain magma series have been discovered in New Hampshire. A ring-
dike complex is a structural unit containing one or more ring-dikes. Ac-
cording to Billings ( 1945 ) :

There are five ring-dikes at Mt. Tripyramid, four each in the Pliny region and
the Franconia quadrangle, and six in the Belknap Mountains, although the six
separate intrusions could be considered to belong to two composite ring-dikes.
Ring-dikes have also been described from adjacent areas in Quebec and Maine.

Complete ring-dikes that encompass 360 degrees are rare, but the ring-dike
of the Ossipee Mountains and some of those on Mt. Tripyramid are of this type.
Most ring-dikes are arcuate in plan and those in New Hampshire encompass,
on the average, 170 degrees of the total possible 360 degrees. The average
radius of ring-dikes in New Hampshire, measured from the outer margin of the
ring-dike to its center of curvature, is three miles. A ring-dike composed of
Albany quartz syenite in the Franconia quadrangle has a radius of 9.2 miles
and is one of the largest known anywhere in the world. The smallest ring-dike
in New Hampshire, with a radius of only 0.8 mile, is on Mt. Tripyramid. The
average width of ring-dikes in New Hampshire is 1900 feet. The arcuate body
of amphibole granite in the southern part of the Franconia quadrangle is 14,000
feet wide, but this may not be a true ring-dike.

Inside some of the ring-dikes are accumulations of extrusive rocks,
known as the Moat volcanics. They are never found outside the ring-dike.
The volcanics also have the same composition as the ring-dike within
which they have subsided.

The Moat volcanics are at least 10,000 feet thick and rest with pronounced
angular unconformity on the older metamorphic rocks of the Litdeton forma-
tion and the plutonic rocks of the New Hampshire magma series. It is almost
always impossible to determine the attitude of the Moat volcanics, because
many of the pyroclastic rocks and lavas are devoid of bedding and flow struc-
ture. Available data indicate, however, that near the ring-dikes the volcanics
are essentially vertical, but toward the center of the complex the dips become
progressively less [Fig. 11.23].

Unfortunately, precise data concerning the amount of subsidence are diffi-
cult to obtain in New Hampshire. The key horizon used for such studies is the
base of the Moat volcanics. It is apparent from Fig. 11.23 that the center of the
subsided block has settled 10,000 feet relative to the margins of the block near
the ring-dike. Moreover, the edge of the subsiding block just inside the ring-dike
has apparendy settled at least 5,000 feet relative to the rocks some distance out-
side of the ring-dike. Therefore, the center of the subsided block has dropped
at least 15,000 feet relative to the rocks some distance outside of the ring-dike.

NEW ENGLAND APPALACHIAN SYSTEMS

L79

Mt. Faraway

S.Nickerson Mtn.

I Albany

I quartz syenite

Moat volcanics

"iyZ'-tfy, Winnipesaukee
?/?t'/;'4 quartz diorite

Scale In Miles

Scale in Kilometers

Fig. 11.23. Section through the Ossipee Mountains, N. H. Reproduced from Billings, 1945, after
Kingsley.

It is apparent that the intrusion of some ring-dikes is associated with the sub-
sidence of a central block. It does not follow, however, that all ring-dikes are
associated with central subsidence.

Billings (1945) believes, because the ring-dikes are vertical in New
Hampshire, that their intrusion was controlled by an annular vertical
fracture zone, the width of which was comparable to the width of the
ring-dike. Such a fracture zone would be susceptible to piecemeal stoping.
Various combinations of the annular or partially annular fracture zone ■
with sagging or doming are shown in Fig. 11.24.

Stocks. For most of the stocks there are few data to indicate whether
they are concordant or discordant because many of them have been in-
truded into areas already occupied by relatively massive or weakly foli-
ated older plutonic rocks. The Mt. Ascutney stock has been shown to cut
discordantly across the steeply dipping older strata, and the lineation and
fold axes of the older strata have not been modified by the intrusion
(Chapman and Chapman, 1940). A process of underground cauldron sub-
sidence, whereby large blocks with outward-dipping walls approximately
the size of the present stocks sank, is visualized, and is illustrated in Fig.
11.25. The activity occurred in the last stages of the evolution of the
White Mountain magma series. The remarkable uniformity of the White
Mountain magma series through New Hampshire suggests that a single
reservoir underlay much of the state (Billings, 1945).

Plutons of Forceful Injection. Many plutons belonging especially to
the New Hampshire magma series have been emplaced bv forceful in-

Fig. 11.24. Origin of ring-dikes. Reproduced from Billings, 1945. Broken line is present erosion
surface.

180

STRUCTURAL GEOLOGY OF NORTH AMERICA

Fig. 11.25. Evolution of the syenite-granite stock of Ascutney Mountain, Vt. Reproduced from
Chapman and Chapman, 1940.

jection. Notable of these are the Kinsman quartz monzonite and the
Bethlehem gneiss.

According to Billings (1945):

The Mt. Clough pluton, composed of Bethlehem gneiss, is undoubtedly the
longest intrusion in New Hampshire. The main body extends southward for 90
miles from the northern part of the Franconia quadrangle to the south end of
the Lovewell Mountain quadrangle, which is beyond the limits of Fig. 11.20.
The width ranges from half a mile to 7 miles. In the Moosilauke quadrangle the
contacts are essentially vertical and the pluton is a vertical sheet. Further south,
however, the contacts dip to the east and along the eastern border of the Mas-
coma quadrangle and the western border of the Cardigan quadrangle, the upper
and lower contacts dip 30 degrees east. Here the pluton is a huge sheet inclined
to the east [Fig. 11.26].

A series of plutons composed of Kinsman quartz monzonite lie east of the
Mt. Clough pluton. The most northerly of these, which may be called the Kins-
man pluton ... is a gigantic lens, essentially vertical in the surrounding schists.

In the western part of New Hampshire, some ten miles east of the Connecti-
cut River, the crest of a major anticline is occupied by a series of "domes." In
their essential features these domes, nine of which have been mapped, are re-
markably similar. A central oval-shaped core of plutonic rocks, ranging in com-

position from granodiorite through quartz monzonite to granite, has a foliation
that dips outward. The plutonic rocks, overlain by Ordovician (?), Silurian,
and Devonian strata, include the Ordovician (?) rocks in many localities and
the Silurian rocks in at least one locality. The upper contact of the plutonic
rocks is at essentially the same stratigraphic horizon in all the domes, approxi-
mately 500 feet below the top of the Ammonoosuc volcanics, but ranges from
the top to an horizon 1,000 feet below the top. The overlying formations like-
wise participate in the domical structure.

Originally considered to be laccoliths or "bottomless" plugs that had bowed
up their roof, it is possible that they all belong to a single great concordant
sheet, originally horizontal, that has been buckled up during orogeny.

Tectonic History

Ordovician Sedimentation. The oldest rocks known so far in the eu-
geosyncline of New Hampshire are Middle Ordovician limestone, cal-
careous shale, and shale, 7000 to 8000 feet thick. Over these accumulated
the Upper Ordovician Ammonoosuc volcanics, about 4000 feet thick, and
over the volcanics another 500 to 2000 feet of shale.

Taconic Orogeny. Near the close of Ordovician time the previously
deposited sediments and volcanics were mildly folded and eroded. The
disturbance here probably marks the subdued effects of die Taconic
orogeny of the Hudson-Champlain region farther west.

Silurian and Lower Devonian Sedimentation. In a Middle Silurian
sea that moved in from the southwest, conglomerates and sands of the
Clough formation and the dolomitic sandstones and shales of the Fitch
formation, not over 800 feet thick, were deposited. Late Silurian history
is obscure, but during early Devonian time about 10,000 feet of sand-
stone, shale, and volcanic materials accumulated. See upper left section
of Fig. 11.27.

Acadian Orogeny. During Mid- or Late Devonian, die strata were
caught in a major orogeny. Even before the deformation, or at least in
its early stages, successive injections of the Oliverian magma series
formed a great sheet in the Ammonoosuc volcanics, later to be domed
in several places along the western margin of New Hampshire. The
Ordovician, Silurian, and Devonian strata were thrown into a series of
anticlinoria and synclinoria whose axes trend north and northeast, and
countless minor folds were impressed upon the larger. Also the Northey

NEW ENGLAND APPALACHIAN SYSTEMS

1S1

•COOS ANTICLINORIUM-

BRONSON

«*— HILL

ANTICLINE

Om3<C> 5 fog 5jLD_Djf0b£?k£^^

Monroe Ammonoosuc Northey Hill
thrust thrust thrust

MERRIMACK SYN CLI NORIUM-

WHITE MOUNTAIN BATHOLITH

CO ,9£ P'q mo syj^ C9 PV.

irfMWTi

CONNECTICUT

■*VALLEY->-«-BRONSON HILL ANTICLINE-
SYNCLINORIUM

UNITY DOME "*" MT CLOUGH^
PLUTON

CARDIGAN PLUTON

MERRIMACK SYNCLINORIUM

big Dl gqg

Osp 0 D, 0 D 5i bg PI /Jlk kqtn Pig Dl big Dl big Dl big Dl gqg

Mn,+ h,a„ 1-1,1 1 5d

Northey Hill
thrust (overturned)

FITCHBURG PLUTON-

949 Dl

PAWTUCKAWAY
RING-DIKE COMPLEX

■ m, nn „S0,
d id/ Q<W i \

ROCKINGHAM

EXETER
PLUTON

ANTICLINORIUM

GREAT BAY
SYNCLINE

_!3 MILES

RYE ANT. BAY OF MAINE
50rv .'5o""vSOr«

349 Dl 949 d ,d/ gg^Tx 30 /di v'\ 30 50rv .-jy^sOrv

Fig. 11.26. Cross section of New Hampshire. After Billings, 1956. Section A-A' is across northern part of
state and B-B'-B" across southern part. Refer to map, Fig. 11.20.

Hill thrust occurred. Schistosity parallel to the bedding formed during
the earlier stages of the folding, and fracture cleavage, essentially parallel
to the axial planes of the minor folds, formed during the later stages.
The rocks were subjected to low-grade metamorphism northwest of the
Ammonoosuc thrust, and to medium and high-grade alteration southeast
of it. The main metamorphism occurred after the Northey Hill thrust
and during the intrusions of the New Hampshire magma series which
were chiefly responsible for the medium- and high-grade metamorphism.
See third section in Fig. 11.27.

Succeeding the metamorphism was the Ammonoosuc thrusting and,
following this, some normal faulting. Then the Moat volcanics were
erupted, and the plutons of the White Mountain magma series wore
emplaced to complete the bedrock complex. This may have occurred in
Mississippian time. Examine the last four diagrams of Fig. 11.27.

Isotope Ages and the Acadian Orogeny

It is becoming evident that die Devonian period began almost 400
m.y. ago, and that our previous estimates that designate this age for

182

STRUCTURAL GEOLOGY OF NORTH AMERICA

^\f r f"i'W»TC'

«V

jVlVj

'OJn

'./--■■■

Oiil -".■-

£n0 OF LOwca PE.ONU

LATE OEvOniam? IfftCTS Of COMTEMPORAWEOwS E«03iON OM.TTEO

£ABLT C*«&ON(fE«0OS?

Fig. 11.27. Evolution of the Franconia quadrangle terrane, White Mountains, N. H. Reproduced
from Williams and Billings, 1938. Oal, Albee formation; Oam, Ammonoosuc volcanics; Sc, Clough
conglomerate; Sf, Fitch formation; Dl, Littleton formation; bg, Bethlehem gneiss; kqm, Kinsman
quartz monzonite; mv, Moat volcanics; ml, Mt. Lafayette granite porphyry; mq, Mt. Garfield
prophyritic quartz syenite; eg, Conway granite and Mt. Osceola granite. Bethlehem and Kins-
man belong to the New Hampshire magma series.

the Late or Mid-Ordovician must be revised. Hurley et al. ( 1959 ) report
the age of a quartz monzonite stock in northwestern Maine which in-
trudes well-documented, fossiliferous, Lower Devonian slate as 360 m.y.
The metamorphism of the beds is believed to have occurred along with
the intrusion.

Therefore, the Oriskany sedimentation took place prior to this time. This is
in agreement with findings of Fairbairn in Nova Scotia where sediments of
similar age have been intruded by granitic rocks . . . (Hurley et al. 1959).

Ages in the 320-380 m.y. range category have generally been correlated
with the Taconic orogeny, but if they indicate Acadian orogeny, then
we must conclude that nearly all the metamorphism and most of the
plutonic activity is Acadian in New England and the crystalline Pied-
mont.

CARBONIFEROUS BASINS

Location

Emerson in 1917 recognized five major Carboniferous basins and a
number of minor ones in eastern Massachusetts, southeastern New Hamp-
shire, and Rhode Island, and they are shown on the Geological Map of
the U.S. ( 1932) accordingly. The new geological map of New Hampshire
by Billings (1956), however, recognizes the "Carboniferous" basins of
Emerson in New Hampshire as Devonian and older, and therefore it
appears that only two major basins are now to be considered, the Nar-
ragansett and the Boston. Two smaller basins in northern Rhode Island
also are definitely demonstrated, and they will be referred to as the
Woonsocket basins, following Emerson. The above basins are shown
on the map of Fig. 11.28.

The Carboniferous stratified rocks are in the slope from the New
England upland to the Seaboard lowland and in the lowland itself.

Narragansett Basin

The generalized stratigraphy of the three basins shown on the map of
Fig. 11.28 is illustrated on the correlation chart of Fig. 11.29. The igneous
intrusive rocks are also shown. It will be noted that the basement com-
plex consists of metamorphosed Precambrian sediments and intrusives
and various Acadian intrusives. Some fossiliferous Lower Cambrian beds
are known in eastern Massachusetts (Chute, 1950).

According to Emerson (1917) the strata of the Narragansett basin
are in large part coarse elastics with an aggregate thickness of 12,000
feet. At the base is the Pondville quartz conglomerate, which is a coarse,
white, granitic waste or arkose 100 feet thick. Above the Pondville is the
Wamsutta group of dominantly red conglomerates, sandstones, shales,
slates, and felsite flows, breccias, and conglomerates, some 1000 feet
thick. Above these strata are the thick Rhode Island coal measures that
include dominantly dark gray conglomerate, pebbly sandstone, sandstone
and gray wa eke, shale, and coal beds. They contain the Odontopteris
flora and insect beds, and are about 10,000 feet thick. Above the coal
measures is the Dighton conglomerate of the northern field and the

NEW ENGLAND APPALACHIAN SYSTEMS

183

Purgatory conglomerate of the southern field. The basin beds become
metamorphosed to slates and quartzites to the south and the pebbles of
the conglomerates are elongated and indented. They are regarded as
Carboniferous in age and probably Pennsylvanian.

Recent detailed work by Richmond (1952) Quinn et al. (1949), Quinn
(1951, 1952), Nichols (1956), Quinn and Springer (1954), and Chute
(1950) is responsible for the correlation chart (Fig. 11.29), and the
following generalizations. The succession of formations given by Emer-
son is not found in any one quadrangle. The unconformity at the base of
the Pennsylvanian beds in the Narragansett basin is striking, and is
shown by the near right angle discordance of the contacts of older forma-
tions with the Pennsylvanian, and by the discordance in outcrop of bed-
ding and schistosity. Three episodes of metamorphism may be detected
(Quinn, 1952). The beds of the Blackstone series were first moderately
affected — sandstone to quartzite, mudstones to amphibolite schist. The
later Esmond granite is mildly metamorphosed as are the volcanics of
the East Greenwich group. Since the East Greenwich beds contain peb-

Fig. 11.28. Carboniferous basins of Rhode Island and Massachusetts.

NARRAGANSETT DASIN

W00NS0CKET BASINS

BOSTON BASIN

Sedimentary

Igneous

Sedimentary

Igneous

Sed lmentary

Igneous

TRIASSIC

Diabase d ikes

Diorite and
diabase dikes

PERMIAN*?
PENNSYLVANIAN

Dighton congl.
Rhode Island fm.
Wamsutta gr.
Pondville congl.

Narragansett Pier
granite b peg.

Pegmatite and
aplite

Bell ingham
congl .

Cambridge

slate
J fSquantun
u tillite

Dorchester
£ . slate
a Brook line
X cgl. &

£ I vols.

MISSISSIPPI.**

East Greenwich

group
(granite ff vols . )

tiuincy granite

DEVONIAN

OR
EARLIER

Esmond granite
?Scituate granite

Fine grained

gran .
Esmond granite
Metadiabase

dikes
Scituate gr. gn.

Fossil iferous
Lower CaAbnan

Dedhaa

granodionte
Salem gabbro-

d lorite
Volcanic rocks

PRECAMBRIAN

Amp

JL IT

■a

hibolite
hist
Sneech Pond

schist
Westboro

qtz.
Mussey Brook
. schist

Blackstone

ser,

Woonasqua-
tucket fm.

Absalona fm.

Sipsachuck
gneiss

Porphyritic
metadiorite

Fig. 11.29. Some sedimentary and igneous rocks of Rhode Island and Massachusetts.

184

STRUCTURAL GEOLOGY OF NORTH AMERICA

wq,m

SOUTHERN

apg

WOONSOCKET

wq,m

eg fg eg

ngn

wqm

NARRAGANSETT BASIN, PAWTUCKET QUAD., R.I.

eg __q.d h-^-L..llg_ wq.a hg Cgp hg dd gg C u

mb wq wq as ss

NARRAGANSETT PIER
sga qbs Pr'lS

QUAD., R.I.

Pris

MILES

P P

npg

Fig. 11.30. Cross sections of Woonsocket and Narragansett basins. Top section after Richmond,
1952. ngn, Nipsachuck gneiss; apg, Absalona fm.; wqm, Woonsasquatucket fm.; pmd, metadiorite;
Sg, Scituate granite gneiss; eg, Esmond granite; fg, fine-grained granite; Pb, Bellingham con-
glomerate.

Middle section after Quinn ef a/., 1949. mb, Mussey Brook schist; wa, Westboro quartzite; wqa,

bles of the Esmond granite, their metamorphism was later than that of
the Blackstone series. The later intrusives rocks of the East Greenwich
group are essentially unmetamorphosed. The Pennsylvanian rocks are
folded and fault-tilted, and schistosity is widespread. It is commonly
not parallel to the bedding, and chloritoid, garnet, amphibole, biotite,
and muscovite are developed.

Albion schist member; ss, Sneech Pond schist; hg, Hunting Hill greenstone; gg, Grant Mills
granodiorite; Cqp, Quiney granite; Cu, Carboniferous undifferentiated; dd, diabase dike.

Lower section after Nichols, 1956. sgg, Scituate granite gneiss; qbs, Blackstone quartz-biotite
schist; Pris, Rhode Island formation; npg, Narragansett Pier granite; p, pegmatite.

In the Narragansett Pier quadrangle a reddish, massive to gneissic
granite is clearly intrusive into the Pennsylvanian beds. It has been
named the Narragansett Pier granite by Nichols ( 1956 ) . A cross section
is shown in Fig. 11.30. Elsewhere granites intrusive into the Penn-
sylvanian beds have been reported but the modern mapping casts doubt
on such relations.

NEW ENGLAND APPALACHIAN SYSTEMS

185

Woonsocket Basins

A section across the southern of the two small basins, here called the
Woonsocket, is given in Fig. 11.30. The western margin of the Penn-
sylvanian basin dips steeply, although it is a sedimentary contact. The
east margin is a high-angle normal fault contact ( Richmond, 1952 ) . The
Bellingham conglomerate which fills the small basins generally dips east-
ward although it has many small and closely spaced folds. The west
margin is a sedimentary overlap. The conglomerate pebbles are stretched
in the plane of schistosity and the long axes point down dip. The matrix
in places is a mica or chlorite schist which tends to enwrap the pebbles.
The conglomerate in the southern basin is more sandy and less meta-
morphosed, and contains beds of graywacke, biotite-sericite schist, dark
phyllite, and slate.

Boston Basin

The strata of the Boston basin comprise the Roxbury conglomerate
below, and the Cambridge slate or argillite above. The Roxbury lies un-
conformably on the Dedham granodiorite of Precambrian (?) age, and
is possibly Pennsylvanian and probably Permian in age, according to
Billings et al. (1939). The conglomerate is over 3500 feet thick, and the
slate about 3500 feet; both constitute the Boston Bay group. Part of the
I Roxbury conglomerate is volcanic and part sedimentary. The volcanic
! rocks include not only effusive lavas but also thick beds of tuff, ag-
glomerate, volcanic breccia, and conglomerate.

The Roxbury conglomerate above most of the volcanics is described by
Emerson as consisting of the Brookline conglomerate at the base, the
Dorchester slate in the middle, and the Squantum tillite at the top. Ac-
cording to La Forge the threefold division does not persist throughout
the area occupied by the formation with sufficient definiteness to warrant
mapping the members separately. In some areas, beds like the Dorchester
! slate are intercalated in most of the formation below the tillite. The
Brookline conglomerate is massive, coarse, and in some areas 1200 feet
thick. It contains cobbles and boulders, many of which are of the under-
lying Dedham granodiorite or of the volcanic complex. The slate mem-
ber is red and purple, and in one place possibly 2000 feet thick. Much of

Northern

Soufhrrn

• Roxbury conglomerate -

Dec/horri orono -
O/orite

Cambridge*^ „ ' • /

argillite x /roxbury cong/omerore

Decfharn grono&iorite
zooo rSET

Fig. 11.31. Cross sections of Boston basin. Upper section from northwest to southeast across entire
basin. Cr, Roxbury conglomerate; Cc, Cambridge slate; blank, pre-Carboniferous, mainly igneous
(LaForge, 1932).

Middle section across Nantasket area. Section about 4000 feet in length. After Billings, Loomis,
and Stewart, 1939.

Lower section across the Hingham area. After Billings et al., 1939.

it is reworked basaltic and andesitic tuff, and layers of purple sandstone
and grit are common. The Squantum tillite is exposed in many places in
the southern part of the Boston basin, and is about 600 feet thick. It
possesses many characteristics of glacial drift and is generally believed to
have been deposited by local mountain glaciers.

The various lithologic types of the Roxbury conglomerate interfinger in
a complex fashion in the Nantasket area, according to Billings ( 1939 ) ,
and the formation consists of numerous lenses of sedimentary and vol-
canic materials overlapping one another. See cross sections, Fig. 11.31.

The Cambridge slate, over the Roxbury conglomerate, underlies nearly
all the northern part of the Boston basin and occupies several long belts
in the southern part. The rock is practically nowhere a true slate, but it
generally has a dominant cleavage parallel with the bedding. It has vari-

186

STRUCTURAL GEOLOGY OF NORTH AMERICA

Fig. 11.32. Gulf of Maine and contnental shelf off Nova Scotia showing location of seismic
profiles and Triassic basin in Bay of Fundy.

ously been called a pelite, shale, argillite, and slate. It contains some
quartzite beds.

Dott ( 1961 ) believes the Roston Ray group may be mid-Paleozoic and
not Carboniferous, and also that the Squantum is not a tillite but rather
an orogenic clastic interfingered in the other lithologies.

Gulf of Maine

Cenozoic and Cretaceous Geology. The continental shelf extends east-
ward from Nantucket and Cape Cod, and a broad peninsula-like platform

under less than 500 feet of water, bounded1 on the north by the Gulf of
Maine and on the south by the deep Atlantic, is known as Georges
Rank (Fig. 11.32). The Atlantic margin of the bank is trenched by deep
submarine canyons, and from their walls have been dredged rock samples
carrying both Tertiary and Upper Cretaceous fossils. Fragments of a
coarse sandstone, Lower Monmouth or Upper Matawan (both Upper

MT. KATAHDIN

LATE PLIOCENE

MODERN WAVE-SMOOTHED BANK

Gulf of Maine

GEORGt'J BAr.

Po/eozo/l ~ , ~ r"/- -V-'J~ ~

"'-• v rocki

OP <-\U]=.

Fig. 11.33. Evolution of the Gulf of Maine and Georges Bank, generalized after a
chart exhibited at the Geological Society of America meetings, 1948, by G. H. Chadwick and
with his permission. Vertical scale greatly exaggerated.

Halifax

ANOMALOUS LAYER
7.07 AT 40,000"

5000

10,000

15,000'

20,000'

25,000'

B-»-"«"r-

Fig. 11.34. Seismic profiles of Gulf of Maine and continental shelf off Nova Scotia. See Fig. 11.32 for
location of profiles. After Drake ef a/., 1954, and Officer and Ewing, 1954.

188

STRUCTURAL GEOLOGY OF NORTH AMERICA

Cretaceous); of a glauconitic greensand, Navarro (equivalent of Mon-
mouth); of an indurated green silt not older than Miocene; and of an
impure glauconitic sandstone, late Tertiary in age, were broken from
the walls of newly charted canyons cutting the southern margin of
Georges Bank (Stetson, 1936). The thickness of the Tertiary sediments
cannot exceed 1500 feet, and the top of the Upper Cretaceous ranges
between 1450 and 1800 feet below sea level. Glacial drift and recent
material mantle the gentler slopes, but in several places the older forma-
tions crop out on the steeper slopes. It is clear, therefore, that the Atlantic
Coastal Plain, made up of Tertiary and Cretaceous sediments, continues
eastward from the New York region and forms Georges Bank.

George H. Chadwick has prepared sections across the Gulf of Maine
and Georges Bank showing the composition and evolution of the sub-
merged coastal plain, and he has given permission to reproduce them,
although they have not been published. See Fig. 11.33. These sections
integrate the erosional surfaces, the sediments of Georges Bank as recog-
nized by Stetson, an extension of the Nova Scotian Triassic trough,
eustatic changes in sea level, and two stages of glaciation. The Lower
Cretaceous Potomac is a projection from the New Jersey coastal plain
and has not been sampled by the dredge.

Seismic Profiles. Refraction profiles have been run by Drake et al.
(1954) and Officer and Ewing (1954) of the Gulf of Maine and con-
tinental shelf off Nova Scotia. These support the conclusions of Chad-
wick and Stetson as far as the unconsolidated and semiconsolidated sedi-
ments go (Cenozoic and Cretaceous). Compare Figs. 11.32 and 11.34,
B-B'. It will be seen that the unconsolidated sediments are very thin

over the Gulf of Maine and only thicken under the shelf slope.

The Triassic trough sediments are believed to exist, as Chadwick
pictured them, on the basis of the layer that yielded the 3.7-4.02-km/sec
velocities (Drake et al., 1954).

The crystalline basement appears complicated by layers with lower
than normal velocities. The 4.6-km/sec velocity layer south of Yarmouth
(section C-C), the 4.52-5. 13-km/sec layer under the shelp slope and
rise off Nova Scotia (sectionA-A'), and the 5.11-4.78-km/sec layer in
the same place off Georges Bank (section B-B') are the cases in point.
They have been interpreted by Drake et al. to be part of the crystalline
basement on the grounds that Katz et al. (1953) found two layers in
Maine, recording at Falmouth, with a velocity of 5.34 km/sec for the
upper and 6.24 km/sec for the lower. These are both somewhat higher
than the presumed equivalents under the Gulf of Maine. In a study of
the Outer Ridge and Blake-Bahama basin (reviewed in Chapter 10) a
5.2-km/sec velocity layer on a =•= 6.5-km/sec velocity layer was theorized
to be a mass of extruded basalt on the typical "oceanic basalt" layer. The
Gulf of Maine "anomalous layer" has velocities somewhat slower than the
'Volcanic" layer under the Outer Ridge, and also lies on the crystalline
basement—not on the ocean basalt layer. It would appear, therefore, that
the anomalous layer is part of the Paleozoic complex of New England. It
could be a mildly metamorphosed Carboniferous basin type of deposit,
or conceivably a Mississippian (?) volcanic accumulation.

The floor of the continental shelf and shelf slope sediments off Nova
Scotia and Georges Bank show a depression or trench similar to that off
New Jersey. Refer to Fig. 10.6.

12.

MARITIME APPALACHIANS

DEFINITION

The Maritime Appalachians will here include the Paleozoic mountain

systems of Nova Scotia, New Brunswick, Prince Edward Island, and the

, continuation of the structural elements of New York, Vermont, and New

Hampshire in Quebec. The folded and thrust-faulted chains south of the

St. Lawrence River extend northeastward into the Gaspe Peninsula, and

j all are intrinsically part of the Maritime geologic province. See index map

1 of Fig. 11.1. The Maritime Appalachians, as here defined, are also known

as the Appalachian-Acadian region ( Alcock, 1947 ) and together with

New England and Newfoundland, as Greater Acadia ( Schuchert and

Dunbar, 1934).

GEOMORPHIC PROVINCES

General Characteristics

The Martime Appalachians are made up of dissected uplands and
broad lowlands. The shoreline is notably long and irregular, with many
deep embayments. It is a fine example of a ria coast in which the linear
structural elements run out under the sea. Figure 12.1 shows the physical
divisions of New Brunswick and Nova Scotia which correspond to the
following descriptions by Alcock (1947).

Nova Scotia

Nova Scotia is made up of five upland and as many lowland areas. The
former comprise: (1) the large Southern Upland, which embraces the southern
and central part of the peninsula and slopes from elevations of about 600 feet
southeastward towards the Atlantic Ocean and also southwestward towards the
Gulf of Maine; (2) North Mountain, a narrow, flat-topped belt, averaging about
550 feet high, that extends along the southeast side of the Bay of Fundy from
Cape Blomidon in Minas Basin southwest for 120 miles to Brier Island; (3)
the Cobequid Mountains, lying north of Minas Basin and stretching for 75
miles across Cumberland County from the head of the Bay of Fundy almost
to Northumberland Strait; this region shows broad, rounded summits blending
to form a somewhat rolling surface with an average elevation of a little more
than 900 feet; (4) the highlands of eastern Pictou and Antigonish counties
between New Glasgow and Antigonish and stretching northeastward to Cape
George; in the southern part the average elevation is about 800 feet, but near
Arisaig it is more nearly 900 feet; (5) the upland belts and northern tableland
is the largest of these areas and presents an even flat-topped surface about
1,200 feet high.

The lowlands are underlain by less resistant rocks, such as sandstone, shales,
limestone, and gypsum and show a considerable diversity of elevation and form.
They comprise: (6) the Annapolis Cornwallis Valley, a long trough-like depres-
sion lying between the steep, straight wall of North Mountain and Colchester
counties surrounding Minas Basin on the north, east, and south, and merging
into Cornwallis Valley on the west; (7) the lowlands of Hants and Colchester
counties surrounding Minas Basin on the north, east, and south, and merging
into Cornwallis Valley on the west; (8) the Cumberland-Pictou area occupying
all that part of the isthmus of Chignecto lying north and east of Cobequid
Mountains; (9) the lowland of Antigonish and Guysborough counties, which
lies south and east of the highlands extending towards Cape George: and
(10) the lowlands of Cape Breton Island, areas lying between the upland belts
and occupied by undulating country of landlocked lakes.

189

Fig. 12.1. Physical divisions of the Maritime Provinces, New Brunswick, Nova Scotia, and Prince
Edward Island. Reproduced from Alcock, 1947.

MARITIME APPALACHIANS

191

New Brunswick and Prince Edward Island

New Brunswick falls naturally into four major topographic divisions whose
boundaries, however, in most places are not sharply defined. The first, which
may be regarded as the main axis of the province, is known as the Central High-
lands, an upland region developed largely on resistant granitic, volcanic, and
metamorphic rocks. It trends northeast through the central part of the province
and is made up of ridges and hills, most of which have flat summits. Its eleva-
vation varies considerably, but much of it has an average height of about 1,000
feet. The highest part is where the tributaries of Miramichi, Nipisiguit, and
Tobique Rivers take their rise. Here broad summits have a general elevation of
about 2,200 feet, with some ridges and peaks rising to still greater heights. For
example, Mount Carleton, the highest point in the province, has an elevation of
2,690 feet.

To the northwest of the Central Highlands is a second division, which may
be termed the Northern Upland. It stands at an elevation of 800 to 1,000 feet
above sea level and is developed on folded Paleozoic strata. The upland pre-
sents a remarkably uniform, flat-topped surface whose regularity is broken only
by a few peaks and ridges rising slighdy above the general level and by valleys
such as those of the St. John and the Restigouche, which are deeply entrenched
in it. The Stewart highway from Campbellton to St. Leonard crosses this belt.
The third division, the Eastern Plain, lies to the east of the Central High-
lands, and makes up almost one-half of the province. It is a region of low relief,
rarely more than 600 feet high, sloping gendy to the Gulf of St. Lawrence. Its
underlying rocks are mosdy flat or gendy dipping Carboniferous sediments.
Prince Edward Island may be regarded as an outlier of this division, and the
Cumberland-Pictou lowland area of Nova Scotia is continuous with it.

The fourth division, termed the Southern Highlands, lies along the Bay of
Fundy. It is mainly an upland belt of ridges of which the most important is the
II flat-topped Caledonis Mountain belt of Albert, Kings, and St. John counties,
'J which reaches a maximum elevation of 1,350 feet southeast of Markhamville.
j To the southwest, in Charlotte county, the belt merges into the Central High-
I lands. The region shows considerable topographic diversity and a great variety
of rock types. The ridges are composed mainly of hard volcanic and intrusive
, rocks, whereas minor lowland areas within the belt have been carved from
- weaker strata.

i Quebec

In Quebec the Appalachian region is bordered on the northwest by the St.
ji Lawrence Lowlands into which it merges imperceptibly. In fact, considered
| from the point of view of topography, the lowland belt overlaps the Appalachian
geological region. To the southwest the upland region includes three parallel
groups of ridges and isolated hills and mountains. These are highest in the south,
and decrease in elevation towards the northeast. The highest point is Round
Top on Sutton Mountain, elevation 3,175 feet, near the Vermont border.

The most easterly of the three belts is known as the Megantic anticline. It
forms part of the International Boundary, and to the northeast passes into
Maine. To the west the Stoke Mountain anticline extends as far as Lake St.
Francis, where it loses its identity. Still farther west, a little beyond Lake Mem-
phremagog, the third range, the Sutton Mountain anticline, is a continuation
of the Green Mountains of Vermont. Between the anticlinal ranges the country
varies from 900 to 1,000 feet in elevation, presenting in places a remarkably
level surface. To the northeast, it continues as an upland belt of ridges and roll-
ing country cut across by deep valleys such as those of the St. Francis and
Chaudiere. It decreases in elevation to a point about opposite Quebec City, but
farther northeast it rises again and in the central part of the Gaspe Peninsula
becomes the Shickshock Mountains, with elevations ranging up to more than
4,200 feet. The individual members of this range show broad flat summits and
the range is bordered both to the north and south by another flat-topped upland
at a lower level into which the present river valleys are deeply incised. On the
north side of the Shickshock the descent to the lower upland is for the most part
abrupt; on the south it is more gradual. The lower surface slopes off both to the
north and to the south, and to the southwest merges with the Northern Upland
of New Brunswick.

STRATIGRAPHY

Introduction

The Maritime Appalachians are a continuation of the New England
Appalachians and present much the same geology. See geologic map of
Fig. 12.2. They are composed mostly of Paleozoic rocks, both sedimentary
and igneous, but some older Precambrian and some younger Triassic
rocks are also present. The chart of Fig. 12.3 correlates the principal for-
mations of Nova Scotia, New Brunswick, and Quebec, and may be re-
ferred to in the following brief enumeration of the stratigraphic systems.
Several groups such as the Green Head, the George River, and the Cold-
brook are known definitely to be Precambrian, and others such as the
Meguma and Macquereau are regarded as Precambrian but on less satis-
factory evidence. They may be Cambrian. Certain granite intrusions of
the southern highlands of New Brunswick and in Cape Breton Island are
also probably Precambrian, but absolute proof of this has not been estab-
lished. Other belts of rock shown on early maps as Precambrian arc now
either definitely known or else inferred to be of Paleozoic age ( Alcock,
1947).

■•'■'■

SCALE OF MILES
O SO lOO

Fig. 12.2. Geologic map of the Maritime Provinces and Quebec. Reproduced from Alcock, 1947.

MARITIME APPALACHIANS

193

Cambrian System

Alcock (1947) reports that Cambrian rocks are found in southeastern
Quebec, in Gaspe Peninsula, in southern New Brunswick, and in Cape
Breton Island, Nova Scotia. In southeastern Quebec most of the rocks of
this age are metamorphosed to a greater or less degree, and some are
highly schistose. In the Oak Hill region near the Vermont border a series
of Lower Cambrian strata 3000 to 4000 feet thick consist of slate, quartz-
ite, dolomite, graywacke, and sericite schist. Rocks presumably of Cam-
brian age of the Thetford-Beauceville region, known as the Caldwell
group, consist of nearly pure quartzites, slates, and pillow lavas of basaltic
composition. A Cambrian seaway and trough of deposition probably
extended from the Lake Champlain region to Quebec City and hence to
Gaspe where some hard, gray limestone and ribboned, shaly limestone
of late Cambrian age occur.

At St. John, southern New Brunswick, strata from Lower Cambrian to
Lower Ordovician crop out, and these are known collectively as the St.
John group. It consists of quartzites, limestones, and black shales. Similar
beds occur on Cape Breton Island. They range in age from Lower to
Upper Cambrian and consist of gray and black shales and slates with
some quartzite and conglomerate, red sandstone and red and gray argillite
carrying hematite, and greenish gray and reddish gray argillites.

Ordovician System
According to Alcock (1947):

In the Appalachian belt of Quebec, strata of Lower, Middle, and Upper
! Ordovician age are known, but in most places fossils are not sufficiently well
preserved to permit an exact age determination. In the long belt from the Ver-
mont border to the east end of Gaspe the deformed Ordovician strata were
formerly referred to as the "Quebec group." This term had first been applied
by Logan in 1860 to beds at Quebec City that had been thrust against and
over the younger strata of Middle Ordovician age. Later the term became a con-
venient one to include all those early rocks whose exact age was unknown.

In Nova Scotia, Ordovician rocks are known to occur in the Pictou-Antigonish
upland. They comprise metamorphosed sedimentary, volcanic, and intrusive
varieties. The Browns Mountain group, consisting of argillites, slates and gray-
wacke, is regarded on the evidence of a few fossil linguloids, as of Lower Ordo-
vician age. Locally associated with the sediments are interbedded volcanic flows

Era

Period

Epoch

Nova Scotia

New Brunswick

Quebec

Mesozoic

Triassic

Annapolis

Ouaco; Lepreau

Permian

Carboniferous

Pennsylvania

Pictou; Morien;

Stellarton
Cumberland
Riversdale

Clifton \

Lancaster f

>Petit-
i codiac

Mispek

Bon a venture

Mississipian

Canso

Windsor
Horton

Hopewell
Windsor

Moncton

Albert

Memramcook

Paleozoic

Devonian

Upper Devonian

Perry

Escuminac
Fleurant
Pirate Cove

Middle Devonian

Gaspe

Gaspe; Malbaie;

Heppel

Lower Devonian

McAdom Lake; Tor-
brook; Knoydart

Dalhousie

Grand Greve
Bon Ami

St. Albans; Dalhousie;
Lake Aylmer

Silurian

Arisaig; Kentville

Chaleur Bay; Mas-
carene

Chaleur Bay

Ordovician

Upper Ordovician

Matapedia

Matapedia; Paboi;
White Head

Middle Ordovician

Malignant Cove;
Stewart Brook

Browns Mountain
Boisdale

Tetagouche

Pohenagamooke;
Mictaw; Quebec
City; Beaucevitle;
Farnham; St. Fran-
cis

Lower Ordovician

Saint John

Levis
Sillery

Cambrian

Murphy Creek; Cald-
well; Sutton; L'Met

ProTerozolc

Meguma (Gold-
bearing)

Coldbrook

Macquereau; Tibbit
Hill

Archean

George River

Green Head

Fig. 12.3. Correlation chart of the principal formations of Nova Scotia, New Brunswick, and
Quebec. Reproduced from Alcock, 1947.

and tuffs, and cutting them is a stock of granite and dvkes and stocks of rh\ o-
lite and quartz porphyry. In the Arisaig region, strata of this group are overlain
by coarse conglomerate, and grit of the Malignant Cove formation, which is
believed to be of Middle Ordovician age. In the Pictou region purplish red. ar-
kosic conglomerate, purplish gray, arkosic grit, and purplish red argillite form
what is known as the Stewart Brook formation, which is probably correlative
with the Malignant Cove.

In New Brunswick, rocks of Middle Ordovician age occur near Bathurst.
Stretching to the southwest is a wide belt of sedimentary rocks, with, in places,

194

STRUCTURAL GEOLOGY OF NORTH AMERICA

associated volcanic varieties. Much of this complex may be of Ordovician age.
In the southwestern part of the province the Charlotte group is probably of Or-
dovician age. It is made up of two divisions, one known as the Dark Argillite,
the other as the Pale Argillite. The former lies unconformably below strata of
Silurian age and is composed of argillite, slate, quartzite, mica schist, gneiss,
and minor amounts of volcanic rocks. It is intruded by masses of granite and
gabbro. The Pale Argillite consists of argillite, sandstone, arkose, slate, and
mica schist. In the St. Stephen area the beds are apparently comformable with
and grade into those of Dark Argillite. On early maps the Pale Argillite was
classed as Devonian on account of the reported finding on Cox Brook, a tributary
of Magaguadavic River, of a Lepidodendron-like form. Later work has failed
to find any fossils whatever in these rocks.

In the Thetford area, the Quebec group (Sillery and Levis) consists of black
slates with a basal conglomerate and some interbedded impure quartzite or
graywacke, overlying unconformably the Cambrian Caldwell group. In the
Beauceville region volcanic tuffs and flows are interbedded with the sediments,
and in places the series is so altered that it is difficult to distinguish the volcanic
from the sedimentary members. Still farther southwest, near Phillipsburg in the
Lake Champlain region, a thick series of fossiliferous Beekmantown sediments
consisting of shales and limestones overlies Upper Cambrian beds and is fol-
lowed by strata of Chazy of Middle Ordovician age.

To the northeast of Levis, rocks consisting of red, green, gray, and black
slates, quartzites, and conglomerates form a belt in places 20 miles wide. These
beds have been correlated with the Sillery, but both younger and older strata
may be included. An interesting feature in these rocks is the presences of belts
of limestone conglomerates. These occur at various horizons in both the Sillery
and the Levis, forming bands from about a foot to more than 100 feet in thick-
ness. The pebbles and boulders consist of gray limestone, and weigh from less
than an ounce to many tons. Similar limestone conglomerates are found in New-
foundland to the northeast and Vermont to the southwest. They have been in-
terpreted as the result of local slipping and breaking up of limestone along the
sea bottom by earthquakes in a zone where faulting was prevalent. Another
feature of the Sillery is the occurrence of belts of quartzite, locally called the
Kamouraska formation. These belts are lenticular but extensive, and their thick-
ness varies greatly.

Interbedded arkose and volcanic rocks of Ordovician age are known
in the Shickshock Mountains; and dark shales, limestones, conglomerates,
argillites, quartzose sandstone, and volcanic flows and tuffs occur to the
south on both sides of Chaleur Bay.

Silurian System

The best Silurian section in Nova Scotia is at Arisaig where 3800 feet of
highly fossiliferous sandstones and shales occur. The series is overlain by

Lower Devonian beds, and it overlies a flow of rhyolite probably of
Lower Ordovician age. The faunas can be correlated better with British
than with American; even the resemblances with the Chaleur Bay Silurian
faunas are slight.

On the north side of Chaleur Bay is probably the thickest marine
Middle Silurian succession in North America. At the top of the sequence
are volcanic flows interbedded with sediments, chiefly elastics, and flows
are present also in other formations farther down in the succession. A
total of 8427 feet or more of sedimentary rocks and 4626 feet of volcanic
rocks are present in the Black Cape area.

In southern New Brunswick, on the Bay of Fundy, great quantities of
volcanic rocks, chiefly rhyolites and andesites, are interbedded with sedi-
ments. At Oak Bay a basal Silurian coarse conglomerate rests unconform-
ably on the dark argillite of the Charlotte group of Ordovician age. The
belt is a continuation of one extending from the Eastport area of Maine,
where a number of formations of Middle and Upper Silurian age occur.

Devonian System

Rocks of Lower Devonian age occur in Quebec, New Brunswick, and
Nova Scotia. Sedimentation at this time was accompanied by widespread
volcanism, and at the close of the epoch the main phase of the Acadian
orogeny took place. In the Middle Devonian, great thicknesses of clastic
sediments accumulated in the Gaspe Peninsula, and in Upper Devonian
time sedimentation progressed locally in the Chaleur Bay and Bay of
Fundy regions ( Alcock, 1947 ) .

A well-known Lower Devonian section is at the eastern end of Gaspe
Peninsula, where about 2000 feet of limestone and limy shale beds have
been described. Within central Gaspe Lower Devonian shales and lime-
stones, associated with thick deposits of volcanic rocks, are widespread.
At the west end of the peninsula, shales and argillaceous limestones of the
same age are 2200 feet thick.

The Lower Devonian rocks at Dalhousie consist of highly fossiliferous
marine sediments, volcanic flows, and tuffs, dikes, and volcanic rocks.
The principal Nova Scotian Lower Devonian section is southwest of
Arisaig, where fine-grained, red, arenaceous slates and gray sandstones

MARITIME APPALACHIANS

195

1000 feet thick, and apparently of continental origin, overlie with marked
erosional unconformity Silurian strata of the Arisaig series.
. Much of the interior of Gaspe is underlain by sandstones, conglom-
erates, and arenaceous shales varying in color from green and drab to red.
The type locality is on Gaspe Bay where a section 7000 feet thick rests on
the Lower Devonian limestones.

Upper Devonian beds are present on the north side of Chaleur Bay in a
three-unit sequence. The lower formation consists of pebble conglom-
erates and sandstones and 450 feet of coffee-colored shale. The middle
formation consists of a coarse pebble-and-boulder conglomerate with
gray matrix. It is only 45 feet thick. The upper formation is 385 feet
thick and consists of gray shales and shaly sandstones.

On the western side of Passamaquoddy Bay, in the St. Andrews region of
' New Brunswick, near the Maine border, on the opposite side of the bay on
Mascarene Peninsula, at Black Harbour south of St. George, and on some of
the adjacent islands are areas underlain by beds of red sandstone and conglom-
erate that are correlated with the Perry conglomerate of Maine.

The beds lie for the most part with low dips and in gende folds. In places

they rest unconformably on the Silurian rocks, and in places are in fault contact

i against them. The conglomerates contain boulders of the Silurian and pre-

Silurian rocks and of the St. George granite intrusive rocks. On Hill Island two

basic amygdaloidal lava flows are interbedded with the red sediments, and simi-

; lar volcanic rock shows on Howard Island. Locally the beds are cut by dark

j dykes. Similar dykes and flows are associated with the conglomerate beds at St.

j Andrews (Alcock, 1947).

Carboniferous System

Carboniferous strata underlie extensive areas of New Brunswick and
i Nova Scotia. They also underlie all of Prince Edward Island and the Mag-
dalen Islands in the Gulf of St. Lawrence, and they crop out along the
north shore of Chaleur Bay. They represent Mississippian, Pennsylvanian,
and possibly part of Permian time, and are the source of coal, oil, gas, and
gypsum in New Brunswick and Nova Scotia. They are generally softer
i and more susceptible to erosion than the older Paleozoics and form the
lowlands. The lowlands of the geomorphic map of Fig. 12.1 are, therefore,
I about coincident with the Carboniferous beds. See also the Geologic Map
of North America.

Fig. 12.4. Correlation chart of the Carboniferous formations of New Brunswick and Nova
Scotia. Reproduced from Alcock, 1947.

196

STRUCTURAL GEOLOGY OF NORTH AMERICA

The Carboniferous strata make up extremely thick sequences, are
dominantly conglomerates, sandstones, and shales; they contain several
angular unconformities, and are particularly instructive of crustal unrest
and of the tectonic history of the region. The correlation chart of Fig. 12.4
gives the important formations of the Carboniferous rocks in the Maritime
Provinces. From it some idea of the numerous units, large thicknesses,
and unconformities can be gained. The sedimentary and tectonic history
is even more detailed than the chart indicates. For instance, the Missis-
sippian strata of Nova Scotia belong to two groups, the Horton and the
Windsor; and along the lower part of the Avon River, the Horton group

... is made up of two formations, a lower known as the Horton Bluff, con-
sisting of some 3,400 feet of dark shale, sandstone, and conglomerate, and an
upper, the Cheverie, made up of 600 feet of red and grey shales, sandstone and
arkose. The Horton Bluff formation rests unconformably on pre-Carboniferous
metamorphic and igneous rocks; it contains plant remains, buried forests, and
soils, and has a fauna of ostracod, crustaceans, and fish remains. The Cheverie
rests with an angular unconformity on the Horton Bluff and is succeeded, also
unconformably, by the Windsor group of marine sediments. The latter comprise
limestone, gypsum, shale, sandstone and limestone conglomerate, the whole
having a thickness of about 1,550 feet. The limestone members are rich in fos-
sils and have yielded one hundred and twenty-seven species, chiefly molluscs
and brachiopods.

The Mississippian rocks extend eastward through the lowland belt to the
Strait of Canso, and also occupy much of the lowlands of the southwestern part
of Cape Breton Island. In the Lake Ainslie area, the Horton group includes
about 6,000 feet of conformable, dominantly clastic sediments containing a
meagre flora and fauna. They are intruded by diabase dykes and sills. The suc-
ceeding Windsor beds have here a thickness of about 2,000 feet. In the Arisaig
region, diabase and basalt dykes and stocks intrude red conglomerate, sand-
stone, and sandy shale of the Mississippian McAras Brook formation, but are
apparently older than the limestone of the Ardness formation of Mississippian
age (Alcock, 1947).

The Pennsylvanian rocks of Nova Scotia are wholly nonmarine, as far
as known, are dominantly clastic and red, and contain locally beds of coal
and thin limestones. Pennsylvanian rocks also cover much of the plain of
eastern New Brunswick, being made up of red and gray shales, sand-
stones, grits, and conglomerates.

The north shore of Chaleur Bay is bordered for considerable distances by red
clastic beds of the Bonaventure formation, which takes its name from the Bona-

venture Island at Perce. The strata consists chiefly of coarse conglomerates and
sandstones, with associated red shales, shaly sandstones, and locally limestone.
The beds for the most part lie horizontally, but are locally tilted and in places
faulted.

For relations along the north shore of Chaleur Bay see Fig. 12.5.

Magdalen Islands are composed of folded sedimentary and volcanic
rocks of Mississippian age, surrounded by flat-lying beds of red sandstone
of probable Pennsylvanian age.

Triassic System

Red sandstones, shales, and conglomerates of Triassic age occur in the
Bay of Fundy region. They are most extensive on the southeast side of
the bay, where a belt stretches along the entire length of the bay and
borders both sides of Minas Basin. See Fig. 11.31. They rest uncon-
formably on various Paleozoic and Precambrian formations and are
capped by about 1000 feet of basaltic lavas that form the North Moun-
tain upland. On the northwest side of the Bay of Fundy, patches of
similar red conglomerate and sandstone occur. The beds of all these
patches dip to the northwest and are in fault contact with the older
formations. It has been concluded that they are deposits in a down-
faulted basin similar to those of the Triassic red beds in the Connecticut
and New Jersey basins. This is the northeasternmost of the known Trias-
sic fault basins in the Appalachian mountain systems. It is believed to
extend under the Gulf of Maine nearly to Boston. See Fig. 11.31.

IGNEOUS ROCKS

Extrusive Rocks

Interbedded volcanic rocks of various kinds have already been men-
tioned in the account of the stratigraphy. They are known in the Cam-
brian and Lower Ordovician of the Thetford-Beauceville region of Que-
bec, in the Middle Ordovician in the central Shickshock Mountains, and
on the south side of Chaleur Bay. They are also known in the Middle
Silurian in various places on the Gaspe Peninsula on the north side of
Chaleur Bay, along the New Brunswick side of the Bay of Fundy, and

DETAILS OF

ESCUMINAC BAY SECTION

GASPE PENIN. QUEBEC

> Red shale and conglomerate

SCALE OP MILES
10 0 10 20 30

G S.C.

Fig. 12.5. Diagrammatic section along the north shore of Chaleur Bay. Reproduced from Alcock, 1947.
Gaspe sandstone is middle Devonian.

198

STRUCTURAL GEOLOGY OF NORTH AMERICA

in the Eastport area of Maine. The Silurian outpourings were especially
voluminous and, where identified, are chiefly andesites and basalts, al-
though acidic varieties have been noted. Volcanism was again widespread
and voluminous in the Devonian. Lower Devonian volcanics are known
in the Gaspe Peninsula, in northern New Brunswick, and in the Lake
Ainslie area of Nova Scotia; Upper Devonian lavas have been noted in
the St. Andrews region of New Brunswick near the Maine border. The
Devonian volcanics are mostly andesites.

The Carboniferous was unremitting in volcanic activity, and consid-
erable amounts of lavas and tuffs were extruded. The Mississippian
rocks of the Magdalen Islands contain basaltic lavas and fragmentals, and
those of the Hampstead area of New Brunswick contain rhyolite. Penn-
sylvanian rocks in the St. John region of the Bay of Fundy contain ex-
trusive and intrusive rocks, and the Bonaventure formation along the
north shore of Chaleur Bay contains amygdaloidal basalt flows.

Lavas, chiefly andesitic and basaltic, and graywackes and arkoses with
sandstones, shales, and limestones compose a stratified assemblage typical
of the eugeosyncline of Kay (1951).

Intrusive Rocks

Intrusive rocks are widespread in Nova Scotia and New Brunswick.
They are granites and associated differentiates, that accompanied the
Acadian orogeny at the close of Lower Devonian time. The granites are
exposed over much of the southern upland of Nova Scotia, and the central
highlands of New Brunswick.

A belt of ultrabasic plutons, now largely serpentinized, extends through
the Quebec Appalachians from Vermont to Gaspe, and their intrusion is
thought to have accompanied the Taconic orogeny. See Fig. 8.29.

Many dikes and sills are mentioned in the literature, and these prob-
ably relate to the volcanic series.

A group of eight small intrusions in southern Quebec form the Mon-
terigian Hills. The most westerly is Mount Royal at Montreal. Except for
one, they lie along a curved line that extends easterly from Montreal. Five
of them rise well over 1000 feet above the surrounding plain; the others to
heights of 600 to 700 feet. The five westerly ones intrude the flat-lying

beds in front of Logan's line, and the three easterly ones cut the folded
and faulted Paleozoics east of the line. According to Caley ( 1947 ) :

Brome and Shefford Mountains are thought to be unroofed laccoliths, or per-
haps parts of a single laccolith still covered by sedimentary strata in the 2% mile
interval of lower land between the hills. The remaining hills appear to be vol-
canic necks with nearly vertical walls.

The age of the intrusions is Devonian or younger. Evidence for this, in addi-
tion to that supplied by the St. Helen Island breccia, is afforded by Yamaska,
Shefford, and Brome Mountains, which lie within the folded Appalachian re-
gion. The intrusive masses show no effects of deformation, and hence must
have been intruded after the last folding that affected this region in Middle
Devonian time. It has also been noted that in the Monterigian intrusive rocks
pleochroic haloes surrounding crystals of zircon and titanite are invariably
poorly developed and immature. In this they resemble those in Tertiary intru-
sive rocks, whereas in certain Devonian granites haloes are numerous and prom-
inent. The suggestion has, therefore, been advanced that the igneous rocks of
the Monterigian Hills may be as young as Tertiary.

STRUCTURES

Unconformities

The Paleozoic section is replete with unconformities and conglomerates
which indicate intermittent orogeny from place to place over a long
time.

A coarse conglomerate of Lower Cambrian age containing large gra-
nitic boulders rests on rocks of the same material as the boulders near
St. John, New Brunswick. Lower Ordovician black slates with a basal
conglomerate and some interbedded impure quartzite or graywacke
overlie unconformably the Caldwell group of the Cambrian in the Thet-
ford area of southern Quebec. Limestone conglomerates of Lower
Ordovician age occur in places from Vermont through Quebec to New-
foundland and have been interpreted as the result of local slipping and
breaking up of limestones, just deposited, along the sea bottom by earth-
quakes in a zone of crustal deformation.

In Nova Scotia, a coarse conglomerate and grit of Middle Ordovician
age overlies beds of Lower Ordovician age. On the north side of Chaleur
Bay, coarse conglomerates of Middle Ordovician age made up largely of
the Proterozoic (?) Macquereau rocks, rest on the Macquereau. In the

MARITIME APPALACHIANS

199

same general area is a broad belt of Upper Ordovician conglomerate and
grit about 2000 feet thick. Its relations to underlying beds are not noted.

The Arisaig Silurian series of Nova Scotia contains conglomerates and
rests on Lower Ordovician volcanics. At Oak Bay in southern New Bruns-
wick, the base of the Silurian succession is a coarse conglomerate which
rests unconformably on the dark argillite of Ordovician age.

Lower Devonian red slates and gray sandstones southwest of Arisaig
overlie with a marked unconformity Silurian strata. Arkoses and con-
glomerates occur in the Lower Devonian of Cape Breton Island. Much of
the interior of the Gaspe Peninsula is underlain by Middle Devonian
sandstones, conglomerates, and arenaceous shales. The change from lime-
stones of the Lower Devonian to elastics of the Middle Devonian is gen-
erally regarded here as marking the principal phase of the Acadian
orogeny. In the zinc and lead district of Berry Mountain and Brandy
Brooks, the limestones are cut and mineralized by granitic and syenitic
intrusive rocks, but not the overlying sandstones.

Upper Devonian beds on the north side of Chaleur Bay consist at
the base of about 600 feet of coarse conglomerates and sandstone. These
have been cast into a broad syncline, eroded, and are unconformably
overlain by the Pennsylvanian Bonaventure conglomerate. More con-
glomerates of the Late Devonian age occur in New Brunswick near
the Maine border; they are correlated with the Perry conglomerate of
Maine. These beds are seen to rest unconformably on the Silurian rocks,
and they contain boulders of the Silurian and pre-Silurian rocks of the
St. George granitic intrusives.

The Carboniferous sediments rest everywhere, it is believed, in marked
angular unconformity on older rocks, which range from Precambrian
to Late Devonian in age. They are thousands of feet thick and contain
great quantities of coarse elastics, particularly the Pennsylvanian. In
Nova Scotia, the Horton Bluff elastics at the base of the Mississippian
rest unconformably on pre-Carboniferous metamorphics and igneous
rocks, and are in turn separated by an angular unconformity from the
overlying Cheverie, also of Mississippian age.

Mississippian limestones and volcanics are folded, eroded, and over-
lain unconformably by Pennsylvanian (?) strata on the Magdalen Islands.

Gussow's (1953) studies in New Brunswick lead to the conclusion that
the Lower Mississippian strata rest unconformably on the older Acadian
complex, and in turn are overlain unconformably by the Upper Missis-
sippian strata. The Upper Mississippian strata were in turn strongly
folded and faulted, eroded, and overlain unconformably by the Penn-
sylvanian elastics. The structure imposed on the Mississippian strata,
both during and at the close of the period, is typically Appalachian-type
folding and thrust faulting. The Pennsylvanian strata have not been dis-
turbed to any extent since deposition and are essentially flat. The great
amount of conglomerate attests the vigorous elevation of sizable high-
lands immediately before and during deposition.

Folds and Thrusts

All pre-Carboniferous rocks are considerably deformed and in part
metamorphosed. In places, the Carboniferous strata are also deformed.
The chief structures are folds. Some thrusts have been observed and
mapped, particularly in New Brunswick, but for the most part mapping
has not been sufficiently detailed to bring out the existence of long and
great thrust sheets. The linear elements in the compressional structures
trend generally northeastward in continuation of those of New England.
The folds and thrusts of the Taconic and Acadian systems of New York
and Vermont pass into southern Quebec, and the Taconic front reaches
the St. Lawrence at Quebec City. There the Quebec formation carries
Trenton fossils, and consists of limestone and shale and thin beds of lime-
stone conglomerate. See Fig. 12.6. Its beds have been altered and cleaved.
Beds of the older Levis formation have been thrust from the southeast
against the Quebec City, whereas on the northeast the Quebec City is
thrust against and over younger Upper Ordovician beds, the Utica-
Lorraine. The Utica-Lorraine in turn is in contact with the Precambrian
of the Canadian Shield. Resting horizontally and free from disturbance,
directly on the Precambrian, are Trenton limestones unlike the beds of
the Quebec City formation of similar age. This boundary between the
highly deformed and the undeformed strata has long been known as
Logan's line or Logan's fault (see map, Fig. 12.2). From Quebec City
the line runs under the waters of the St. Lawrence, and sweeps in a

200

STRUCTURAL GEOLOGY OF NORTH AMERICA

North

trent<5n

XXX

xxx

XXX

xxx
xxx

V X X

PRECAMBRIAN

XX X X X X X

XXX X X x X

X X X X X X X

Fig. 12.6. Section across the St. Lawrence at Quebec City. Reproduced from Alcock, 1947.

smooth curve easterly to the tip of Gaspe, passing between Anticosti
Island and the peninsula. Where information is available, the faults in
the great arcuate zone of deformation south of the St. Lawrence are
known to be overthrusts from the southeast. The rocks of this belt,
particularly those of Gaspe, can be divided into four main assemblages
according to the number of orogenies by which they have been affected.
The metamorphic rocks of the Macquereau group were deformed by a
late Proterozoic to early Cambrian orogeny; the Upper Cambrian and
Ordovician rocks were deformed by the Taconic orogeny; the Silurian

and Devonian rocks were affected by the Acadian orogeny; the Car-
boniferous is comparatively little disturbed (Alcock, 1935). Figure 12.5
illustrates the structures in a small way.

Ry reference to the Geologic Map of Canada, it will be seen that the
lower and outer part of Nova Scotia is made up of Precambrian rock, as
well as a belt through St. John, New Rrunswick. These were not immune
to Paleozoic orogeny, however, because overlying and marginal Paleozoic
strata are much deformed and the Precambrian rocks are intruded by
many plutons of Paleozoic age.

^Qc ^/i

-?-''

\ \5Wff )gfly

--?_

tf* tf &

77777/ T-^ —

• — • • 7^

SJ^P^^??^

5" MILES

\ Fig. 12.7. Cross sections in the Maritime Provinces. Upper two sections are near Port Daniel
Bay or the south coast of Gaspe Peninsula. After Northrop, 1939. €?m, Macquerean metaclastics,
either Cambrian or Precambrian; Om, Mictaw elastics; Sc, Clemville formation; Sac, Anse Cascon
formation; Slv, LaVieille formation; Sg, Gascons formation; Sb, Bouleaux formation; Swp, West
Point formation; Cb, Bonaventure formation. All Silurian formations are Middle Silurian in age.

Lower section is of the St. John area, New Brunswick. After Hayes and Howell, 1937. pCc,
Cold Brook volcanics; pCst, St. Martin volcanics, conglomerates and intrusives; Ce, Etcheminian
sandstone; Ch, Hanfordian formation; CI, Loch Lomond slate; Cj, Johanian sandstone; Ck, Kenne-
becasis conglomerate; Cw, Windsor limestone; Chr, Red Head conglomerate; Ct, Tynemouth
Creek conglomerate; Trq, Quaco elastics.

202

STRUCTURAL GEOLOGY OF NORTH AMERICA

The Arisaig region of Nova Scotia was affected by folding and in-
trusives at the close of Lower Ordovician and probably again at the close
of the period, when the Taconic orogeny spread over much of the Mari-
time Provinces. Numerous plutons, mostly of Middle Devonian age, were
emplaced in the Nova Scotian Precambrian and in the pre-Devonian
strata of central New Brunswick as previously described. Similar in-
trusions occurred in the Gaspe Peninsula. The strata of New Brunswick
and Nova Scotia were cast into northeasterly trending folds at this time,
which probably paralleled former structures. Figure 12.7 shows the folds
and faults of the St. John area in New Brunswick.

Normal faults are shown in a number of cross sections in the literature
but are not much discussed. They are evidently later than the compres-
sional orogenies or due to late adjustments of the individual orogenies.
Some may be related to the Triassic basin faults and some even to
Tertiary faulting.

TECTONIC HISTORY

Most writers emphasize two great orogenies in the Maritime Provinces,
the Taconic at the close of the Ordovician and the Acadian or Shicksho-
kian that ran its course through middle and late Devonian time. If the
geologist is not influenced unduly by the interpretations and conclusions
of numerous writers and considers only the numerous coarse conglom-
erates, unconformities, and volcanic series without previous impressions,
it would be natural to conclude that a long succession of compressional
impulses with accompanying intermittent volcanic and magmatic ac-
tivity affected the Maritime Provinces. At intervals from Proterozoic to

late Triassic time, vigorous deformation occurred from place to place.
It does not seem altogether sound to the writer to conclude that two
orogenies stand apart as clear-cut and distinct. Perhaps orogenic events
reached maxima, and these maxima are to be considered the Taconic and
Acadian orogenies. The great angular unconformity at the base of the
Carboniferous emphasizes the superior nature of the orogenic phases
that preceded the Mississippian.

The Mississippian beds are folded in places, and so are the Penn-
sylvanian, but the phases of Carboniferous orogeny are not so severe as
the earlier ones. Over the New Brunswick lowlands the beds are fairly
flat. Bordering highlands were intermittently and sharply elevated, how-
ever, throughout the Mississippian and Pennsylvanian to supply the great
amounts of coarse elastics that make up the thick deposits. One of these
source areas probably was the Precambrian area of Nova Scotia; another
possibly lay to the northeast along the St. Lawrence.

During the succession of orogenies that beset the Maritime Provinces,
several ranges were undoubtedly elevated and several troughs of deposi-
tion undoubtedly sank, and this activity was accompanied by voluminous
volcanism. With the sea extensively invading the cordillera, a condi-
tion is visualized much like the partially submerged Andean system of
southern Chile, Patagonia, and Tierra del Fuego. The changing geo-
graphic scene during the Paleozoic has not been set down on maps —
perhaps the geological information is not yet sufficient to perform such a
snythesis.

The fronts of the orogenic belts, however, seem clear by now, and after
the geology of Newfoundland has been presented, an attempt will be
made to relate the orogenic belts of Greater Acadia.

13.

NEWFOUNDLAND
APPALACHIANS

PHYSICAL DIVISIONS

Newfoundland may be divided into upland and lowland. Examine the
map of Fig. 13.1. The upland over large areas has remarkably little relief,
and generally breaks off in steep slopes to the lowland. Most lowland
areas are on weak rocks, and a number of the steep slopes between up-
land and lowland are known to be fault-line scarps; others are thought to
I be. An article of Twenhofel and MacClintock (1940) discusses the physi-
ography of Newfoundland and is the basis for the following review.

The highest part of the upland is the Long Range topographic feature
along the west margin of Newfoundland. It has been referred to as a

Fig. 13.1. Physical divisions of Newfoundland. After A. K. Snelgrove, Newfoundland Geological
Survey. The horizontally ruled areas are upland and the unruled areas, lowland. Small num-
bered uplands are 1, Hare Bay serpentine hills; 2, Highlands of St. John; 3, Indian Head Range.
The lowlands take appropriate names such as West Coast Lowland; Grand Lake-White Bay basin;
Notre Dame Bay basin; Bay d' Espoir basin; Trinity Bay basin; and Conception Bay basin.

203

204

STRUCTURAL GEOLOGY OF NORTH AMERICA

plateau because of a fairly flat top. Actually only remnants of a high, flat
surface exist, and these are about 2000 feet above sea level. High valleys
of late mature aspect range in elevation from 1300 to 1700 feet and are
correlated with the highest surface in the central plateau at 1400 to 1600
feet. This same surface declines to about 1000 feet in the Baie d'Espoir
region, and 700 to 800 feet at St. Johns. A third surface in the western
Long Range is at 500 to 1000 feet above sea level. In the central plateau
this surface is believed to mark the mature upland of 500 to 1000 feet at
Grand Lake — White Bay basin, and the 200-foot level at Notre Dame Bay
and the lower Exploits basin, and the 350- to 400-foot level at St. Johns.
The three surfaces, or peneplains, are regarded as sloping to the east and
representing corresponding tilt of the island in that direction. The pene-
plains were developed through fluvial erosion, not marine; and as in the
southern and central Appalachians were eroded, it is believed, in Tertiary
time. Perhaps the highest Long Range peneplain formed in the late
Cretaceous.

The Anguille Mountains have an upland surface much like that of Long
Range. The highest points are at about 1800 feet above sea level. The
Serpentine Range includes the Lewis Hills and Blow-me-down Moun-
tains south of the Bay of Islands, Arm Mountain on the north side of the
bay, the St. Gregory highland on the north entrance of the bay, Table
Mountain on the south side of Bonne Bay, and Lookout Hills on the
south entrance of Bonne Bay. These several relief features are parts of a
basic intrusive complex. Lewis Hills have a remarkably flat surface at
about 2300 feet above sea level, a well-preserved, mature surface at 1300
to 1700 feet, and a surface shown by upland valleys at 700 to over 1100
feet.

STRATIGRAPHY

Introduction

The stratigraphy of western Newfoundland was first summarized by
Schuchert and Dunbar (1934). The report also reviews the stratified units
of the rest of the island in the light of information up to 1934. Several
Bulletins and Information Circulars of the Newfoundland Geological

Survey under the direction of A. K. Snelgrove contain additional informa-
tion; and a few journal articles by Twenhofel (1947), Twenhofel and
Shrock (1937), Dorf and Cooper (1943), Kindle and Whittington (1958),
and others present new stratigraphic and paleontologic information.

The island has been divided into four sections in the chart of Fig. 13.2
for the purpose of listing representative sequences. A fifth section is
added on the west for the coast of Labrador, and still a sixth for the
Canadian Shield. The chart attempts to summarize not only the stratified
sections, but also the tectonic history. It can be referred to later when the
structure and tectonic history of the island are discussed. The sections
from west to east may represent the major stratigraphic provinces, since
they are taken across the strike of the linear structural elements. The
Notre Dame Bay section in the north-central part of the island and the
Fortune Bay section in the south-southeastern part of the island may be
parts of a common central province, the details of which are not yet
known.

Cambrian System

In western Newfoundland limestones, dolomites, siltstones, and shales
predominate and build up a sequence 3000 to 3500 feet thick. Along the
west coast for a distance of 800 miles, and especially at Cows Head
(between Bonne Bay and St. John Bay, Fig. 12.1) a succession of lime-
stone conglomerates interbedded in shales and limestone, about 1000
feet thick represent Middle Cambrian to Middle Ordovician time ( Kindle
and Whittington, 1958). The conglomerates consist of small, flat chips,
angular to subangular boulders, and scattered large blocks up to 600
feet in length. The matrix is a mudstone. The fragments came from a
source area where calcarenites, oolites, calcilutites, and dense fine-
grained, varicolored porcellanous limestones, in places with shale part-
ings, were accumulating. The boulders have fossils of the same age as
the matrix. These observations lead Kindle and Whittington to con-
clude that the conglomerates are not thrust breccias but intraforma-
tional units in a flysch sequence. The source direction could not be deter-
mined.

The Burin Peninsula has Cambrian beds of carbonates, shale, and

NEWFOUNDLAND APPALACHIANS

205

sandstone plus manganiferous shales and limestones, and about 1000 feet
of sandstone and shale in the Conception Bay area. In the Rencontre
East area of Fortune Bay, a section of Lower Cambrian or Proterozoic
rocks is composed of more than 6000 feet of conglomerate, sandstone,
arkose, limestone, and shale. So far, no volcanic rocks have been recog-
nized in the Cambrian in Newfoundland.

In the Conception Bay area, it is interesting to note the occurrence of
a great volcanic series, the Avalon, that underlies the Cambrian. Within
the Avalon volcanic series at least three Precambrian epochs of sedi-
mentation and volcanism are recognized, and each was terminated by
folding, uplift, and erosion. The last disturbance probably preceded
the deposition of the Cambrian only a short time, and the whole of the
Avalon peninsula probably sank thereafter and was covered by the
Cambrian sediments.

The fossils of all Cambrian sections have European affinities.

Ordovician System

The Ordovician strata of western Newfoundland consist of a lower
sequence of 6700 feet of sandstones, shales, limestones, and dolomites,
and an upper sequence, some 5000 to 10,000 feet thick, of dark and
variegated shales and sandstones with minor amounts of conglomerate,
arkose, and limestone. Some lava flows, agglomerate, and ash beds have
also been noted in the upper or Humber Arm series. These are the first
evidence of volcanism in western Newfoundland, and they were prob-
ably extruded near the close of the Ordovician.

The two thick sequences are separated by a disturbance that involved
considerable faulting and erosion. The lower is massive and more compe-
tent; the upper is generally thin-bedded and incompetent. It is much dis-
torted in nearly all outcrops. The volcanics in the upper sequence
probably preceded ultramafic serpentine intrusions that penetrate the
beds extensively.

The Ordovician in the Notre Dame Bay and in Fortune Bay areas is re-
plete with volcanics. The sequences are very thick and generally associ-
ated with elastics containing the impure varieties of sandstone — arkose
and graywacke. Only at the base of the Ordovician section, in the Fortune

CANAOIAN
SHIELD

COAST OF
LABRADOR

WESTERN
NEW FOUNDLAN0

NOTRE 0»ME
BAY AREA

FORTUNE BAT AND
BURIN PENINSULA

AVALON
PENINSULA

_ 1

z
-z.

FOLDING, THRUSTING

FAULTING

BARACHOIS SER.
COWL, SS, SH.
COAL, 3,000'

EROSION

SHARP UPLIFT

UPLIFT

EROSION

CODROY SERIES «
CONGL , SS, SH, LS
GYPSUM

ANGUILLE SER
CONGL , SS, SH

SPRINGDALE GROUP

RED CLASriCS,

VOLCANICS

EROSION

z
<
z

SHARP UPLIFT

DISTURBANCE
INTRUSIONS

SHARP UPLIFT

BATHOLITHIC
INTRUSIONS

UPLIFT

EROSION

CLAM BANK SERIES

COARSE CONGL, SS.,

1,700'

? ? ?

FOLDING, FAULTING
INTRUSIONS

ORE AT BAY OE

L'EAU CONGL

3,000'

z
<

K
in

<
a.

SHARP UPLIFT

FOLDING, EROSION f

FOLDING, EROSION'

SHARP UPLIFT

EROSION

SILURIAN IN WHITE
BAY. CLASTIC, 2g00'

SILURIAN IN NOTRE

DAME BAT, CLASTIC

2,000'

RENCONTRE FM
OTZ, GRAYWACKE,
VOLCANICS,

3J00'

ULTRABASIC
INTRUSIONS

DISTURBANCE ?

>
O

o
a
o

GREEN SS.

o
It

HUMBER ARM SERIES
WITH RED CLIFF VOL

\ CROSS POND
VOLCANICS

SNOOKS ARM VOLCAN
TABLE HEAD SERIES
SH

LATE STCEORCE SER
SLATES, VOLCANICS

MOORING COVE
VOLCANICS, 1,500'
ANDERSON COVE
SLATES 1,500

BELL BAY
VOLCANICS
13.000'*

POOLS COVE CONGL
5.000'

WABANA SH. HEMA-
TITE 3.000'
BELL ISLE 6.000'
CLARENVILLE SH , SS

DISTURBANCE

BAY DEST LS.
2.000'

z
<

■
2
<

LABRADOR SER.
SS,LS., 470'

7
7

DISTURBANCE AND
EROSION

JOHANNIAN 500'
MANUELS SH. 300'

HANFOROIAN SH . LS

KELLIGRE WS SH
LONG POND SH
CHAM8ERLAINS BR SH

EROSION

LABRADOR SERIES
SS.LS.OTZ, 2.600'

HANFOROIAN SH.. LS.

EROSION

BRIGUS CLASTICS
AND LS.

ETCHEMINIAN SERIES
CONGL. SS.SH.LS, 200'

o
o

o
a.

UPLIFT, EROSION

UPLIFT. EROSION

FOLDING, INTRUSION

FOLDING. INTRUSIONS

z
o

c
a

EROSION

BAY O'ESPOIR
SERIES. I5.0OO'
GRAYWACKE. BASAL

VOLCANICS
HARBOUR MAIN VOL-
CANICS OF BURI H °

AVALON VOLCANIC
SERIES I5.OO0'
TWO OROGENIES
WITHIN SERIES

Fig. 13.2. Representative sections and crustal disturbances of Newfoundland. Compiled from
various reports mentioned in the text and with the aid of Daniel A. Bradley, University of
Michigan. The age of the folding, faulting and intrusions of the Notre Dame Bay area as in-
dicated between the Silurian and Mississippian beds is doubtful; they may be Acadian rather
than Caledonian.

206

STRUCTURAL GEOLOGY OF NORTH AMERICA

Bay area, is a nonvolcanic series present. There about 2000 feet of lime-
stone occur.

In Conception Bay on the east the volcanics are absent, or if deposited,
have been eroded away. The Belle Isle and Wabana formations, the latter
carrying sedimentary iron-ore beds, are chiefly sandstones and shales,
about 9000 feet thick.

The thick Ordovician sections in central Newfoundland with their
abundant volcanics resemble the Ordovician Ammonoosuc volcanics of
New Hampshire more than any strata of similar age in the Maritime
Provinces.

Silurian System

Strata of Silurian age are not known in either the western or eastern
divisions of Newfoundland, but in the central belt various elastics are
fairly voluminous. In the White Bay and Notre Dame Bay areas over
2000 feet of Silurian sandstones and shales have been noted. In the For-
tune Bay area, the Rencontre formation consists of quartzite, graywacke,
and volcanics, about 3500 feet thick. Other sequences in the central divi-
sion may prove to be Silurian.

Devonian System

The Clam Bank series along the western shore of St. George peninsula
is a coarse, red conglomerate, with intercalated masses of soft, coarse
brown sandstone and shaly sandstone of early Devonian age. The well-
rounded and polished pebbles in the conglomerates are of many kinds and
range up to 4 inches in diameter. The beds resemble the Triassic sedi-
ments of the Connecticut Valley. In places they appear nearly flat, but in
others they are on end. They indicate a sharp uplift immediately preced-
ing and collateral with their deposition, and their deformation indicates a
following orogeny.

In the Fortune Bay area, the Great Bay de l'Eau conglomerate is 3000
feet thick, and is also believed to be early Devonian.

Early Devonian plant impressions were discovered in the La Poile Bay
area of southeastern Newfoundland east of Long Range in 1940 (Dorf
and Cooper, 1943) in the Bay du Nord series which, because of its meta-

morphosed character, had previously been thought of as Precambrian.
The fossils occur in a grayish-black slate which is associated with gray-
wacke and conglomerate. Much of the central plateau is metamorphic
and igneous rock, and a belt of schistose character flanks Long Range
on the east. The early Devonian fossils occurring in rocks of this terrane
open up the possibility that much of the stratified altered rock, previously
called Precambrian, is Paleozoic; and that the numerous and large cross-
cutting plutons are Acadian in age. Recognizing the well-established
Acadian orogenic history in the Maritime Provinces and in New England,
which includes much metamorphism and plutonic activity, a number of
modern investigators are classifying the stratified, altered, lithologic units
as Paleozoic rather than Precambrian. It seems probable that much of
central Newfoundland will prove to be underlain by Paleozoic rocks. The
recent Geologic Map of North America shows most of it as Ordovician
strata and Devonian intrusives. Undoubtedly more Paleozoic systems will
be recognized in this complex in future investigations.

Mississippian System

Mississippian rocks are present abundantly in the St. George Bay area
and in the White Bay — Grand Lake lowland. They are also known at
Cape Rouge and Groais Island, and in part of the Notre Dame Bay area.
The chart of Fig. 13.3 correlates the Mississippian formations of these
areas. They are chiefly elastics. The St. George Bay series contains in
addition some evaporites, and the Notre Dame Bay area, some volcanics.
Up to 3500 feet of beds have been noted in these sections.

ST GEORGES
SAT AREA

WOOOT POINT SS
WOOOT COVE SH.LS
BLACK POINT LS.
COOROT SH , GYP
SNAKE BRIGHT SH.
CAPE ANCUILLE SS

DEER LAKE

UPPER GRAY

AND

LOWER RED

SHALES

WHITE BAY

SPEAR POINT SS.SH

CAPE ROUGE -
GROAIS ISLAND

CAPE ROUGE SH.,
SS.. SILTSTONE

PILIER CONGL..SS

REO INDIAN
LAKE

SHALE, CONGL. AND
LIMESTONE

NOTRE DAME
BAY

SPRINGOALE GRP.

REO CLASTICS,

VOLCANICS

Fig. 13.3. Mississippian formations of Newfoundland, after Betz, 1948. All are regarded as
Lower Mississippian.

NEWFOUNDLAND APPALACHIANS

207

Pennsylvanian System

A body of coarse elastics, the Barachois series, rests on the Mississip-
pian Codroy series in the St. George Bay area. It consists of 5000 or more
feet of coarse conglomerate, sandstone, arkose, and shale, with some thin
coal beds, presumably all continental, and indicates a new sharp uplift
nearby. No other Pennsylvanian strata are known in Newfoundland.

INTRUSIONS

Serpentine Belts

Two belts of ultrabasic plutons occur in Newfoundland. They are
known as the eastern and the western serpentine belts. Not only serpen-
tine but also chromite are common associates of the basic intrusions
(Snelgrove, 1934). The principal rocks are peridotite, pyroxenite, and
gabbro. See map, Fig. 13.4.

The eastern serpentine belt extends from Carmanville to the head-
waters of the Gander Biver. Serpentine masses are exposed intermittently
over 120 miles in a general northeast-southwest direction. According to
Snelgrove:

This part of the island is relatively low-lying and is characterized by undu-
lating topography. The ultrabasic rocks of this belt, in contrast with those on
the west coast, are only partly exhumed by erosion and consequently lack any
striking topographic expression. The serpentines form low, bare ridges, with
few prominent peaks or knolls.

At the north tip, it has an outcrop width of one-half mile, and dips westward.
Highly serpentinized dunite is confined to a band varying from one hundred to
five hundred feet in width, flanked by pyroxenite. The serpentine band forms
small prominences. The country rocks beneath the intrusives are chloritized vol-
canics, locally fragmental, underlain by micaceous black slate and quartzite.
Above the ultrabasic rocks are black slate, gray quartzitic sandstones, and con-
glomerate. These sedimentary and volcanic rocks are probably of early Paleo-
zoic age; they appear to have been intruded conformably by the plutonic rocks.

The section of the belt exposed near the headwaters of the Gander River,
central Newfoundland, consists of serpentinized dunite with lenticular segre-
gations of medium-grained to pegmatitic pyroxenite. Its width was not deter-
mined. Structurally, the intrusion appears to be nearly vertical; it is invaded by
a granite batholith lying to the south and east.

The Western Serpentine Belt consists of a series of four main intrusions,
which seem to have been injected concordantly at different horizons into a

Fig. 13.4. Ultramafic plutons of Newfoundland. Reproduced from Snelgrove, 1938.

folded sedimentary and volcanic series (Humber Arm series), probably of
upper Ordovician age, which underlies this part of the lowland of the west
coast of Newfoundland.

South of Bay of Islands, the eastern section of this belt, as exposed in Blow-
me-down Mountain, is a pseudo-stratified complex and is composed ol a wide
zone of various types of peridotites at the base, succeeded by more siliceous
rocks toward the top. Both the intrusives and the country rocks of sandstones,
slates, argillites, and lavas have a general westward dip near Blow-me-down
Mountain. In the section south of Bay of Islands, a lopolithic structure is indi-
cated. Five miles to the east of the southernmost intrusive ol the western belt
is a satellitic serpentine mass containing an asbestos prospect. The structural

208

STRUCTURAL GEOLOGY OF NORTH AMERICA

relations of the mass are unknown. A smaller satellite some 1,000 feet thick and
well-differentiated occurs in Lark Mountain, south of the mouth of Bay of
Islands.

North of Bay of Islands, also, the basal portion of the ultrabasic rocks com-
posing the serpentine belt is composed of a wide zone of peridotites which dip
westward.

Since no igneous rocks are known to cut the Carboniferous of western New-
foundland, the intrusives are referred to either the Taconic (late Ordovician)
or the Acadian (late Devonian) orogeny.

The western serpentine belt extends adjacent to the west coast from
Port au Port Ray to Ronne Ray and forms the flat-topped Serpentine
Range, previously mentioned, with summit elevations around 2000 feet.

Other areas of serpentine not included in the eastern and western belts
are on the east side of the northern peninsula at Hare Ray, and at Raie
Verte and Ming's Right. At Hare Ray considerable thicknesses of perido-
tites have an eastward dip and, with the enclosing sediments, form the
eastern limb of the northern peninsula anticline.

At Raie Verte, the formation of that name, which consists of greenstone
and greenstone schist with minor amounts of graywacke, tuff, agglom-
erate, lava, slate, ferruginous chert, sandstone, and marble, has been in-
truded by large, dominantly concordant bodies of ultramafic rock and
gabbro (Watson, 1943). Much of the ultramafic rock has undergone in-
tense serpentinization and steatitization. The gabbro has suffered saus-
suritization, uralitization, silicification, carbonatization, and alteration to
zoisite-quartz and zoisite-prehnite rock. Granite, quartz-porphyry, and
quartz-diorite intrusions occur in the Raie Verte formation. Adjacent to
the latter, the greenstone and gabbro have been metamorphosed to the
amphibolite fades. Small sills and dikes of mafic gabbro, porphyrite,
diorite, and kersantite were observed in the area.

The above areas of ultramafic rocks are shown on the map of Fig. 13.4.
These occurrences in Newfoundland are considered part of a major ser-
pentine belt from Georgia through the crystalline piedmont belt to the
Hudson Valley and through the Taconic system to the St. Lawrence and
the Gaspe Peninsula. They have been compiled by Hess, and his map is
reproduced in Fig. 8.29. Hess has developed the theory that serpentine
plutons occur in linear arrangement and mark the heart of the belts of

great compressional deformation, especially of the volcanic arc type. If
the linear belt of ultramafic plutons be interpreted in this way, we have
to deal with additional evidence of a great orogenic belt, and can point to
its core of greatest deformation.

In the St. Lawrence-Gaspe belt, most of the serpentinized plutons are
Taconic in age, but some may be Devonian. About the same can be said
of their age in Newfoundland. Their age is not known in the crystalline
piedmont, but it is inviting to think of the entire serpentine belt as one
of the manifestations of the great Taconic orogeny.

Granitic Plutons

Many large discordant granitic to dioritic plutons, some of batholithic
proportions, occur in the central part of Newfoundland between the Pre-
cambrian of Long Range and the Precambrian of Avalon peninsula.
Some lie within the Precambrian areas also. For the most part they have
not yet been mapped and differentiated. They are now regarded as prob-
ably Acadian in age, since one has been found intruding the early
Devonian beds of the La Poile Ray area and another one cuts the De-
vonian beds of the Fortune Ray area. Some may be late Silurian (Cale-
donian ) ; most are known to cut the Ordovician strata, and pebbles of the
granite are found in a Mississippian conglomerate.

Instructive examples are the Ray du Nord granodiorite and Ackley
granite batholiths of the Fortune Ray and Rurin peninsula region. See
map, Fig. 13.5. According to White (1940):

The (Ackley) batholith intrudes the northwest limb of a large syncline, the
major structure of the Fortune Bay synclinorium. The invaded rocks are largely
the Ordovician (?) Belle Bay volcanics, and to a lesser extent, tuffaceous slates
conformably overlying the volcanics, and Cambrian quartzites. The mapped
extent of the batholith is over 160 square miles, but this is probably less than
half of the total. The long axis of the intrusion is oriented approximately north-
east, parallel to the dominant regional structural trends. The dip of the contact,
where it could be determined, is 25° to 45° outward from the batholith.

The topography of the batholith is of low relief, with elevations averaging
about 750 feet, in contrast to the higher elevations and considerably greater
local relief of the volcanics to the south.

The intrusion consists mainly of granite ("white granite") and alaskite ("red-
granite"), with the latter the more abundant, in the southern part of the batho-

NEWFOUNDLAND APPALACHIANS

209

lith. These two phases are generally gradational, but sharp contacts and local
cross-cutting relationships have been observed.

Basic and intermediate rocks are completely absent, although early phases of
the differentiation series may be represented by the nearby Bay du Nord batho-
lith.

The Bay du Nord and Ackley batholiths are in turn cut by the Belle-
orum granite, which is known to intrude the Great Bay de l'Eau con-
glomerate of Devonian age (D. A. Bradley, personal communication).
The three plutons are regarded by Bradley as closely related genetically.

Composite batholiths have been noted in the St. Lawrence area of the
Burin peninsula where the Lawn (?) metagabbro, possibly of Taconic
age, is succeeded by the St. Lawrence granite of Acadian age (Van Al-
stine, 1948); in the Trinity Bay area where the Powder Horn diorite is
intruded by the Northern Bight granite (Hayes and Bose, 1948); and in
j the Notre Dame Bay area where a pink granite batholith with satellites in
the Hodges Hills vicinity intrudes a gray hornblende diorite. The latter
diorite has gabbro facies and exhibits all the characters of xenolithic
assimilation (John J. Hayes, personal communication).

MAJOR STRUCTURAL DIVISIONS AND THEIR CHARACTERISTICS

Tectonic Map

The tectonic map of Fig. 13.6 is an attempt to classify the major struc-
tural divisions of Newfoundland, and to show some of the important fold
axes and faults of the large island. It is based chiefly on Snelgrove's Geo-
logic Map of Newfoundland (1938) and on additions that he has made
on a copy loaned to the writer. The faults and folds of the Notre Dame
Bay area were taken from a work map of John J. Hayes.

Considerable field work has been done that is not yet in print; much
of the central plateau has never been seen by geologists; and areas of
crystalline rock are now being considered more as Acadian orogenic com-
plex rather than Precambrian. These factors lead to an almost hopeless
task of bringing the geologic map up to date and making it tolerably
correct, even if generalized. As a substitute, a generalized tectonic map
was constructed (Fig. 13.6) that divides Newfoundland into four major

Fig. 13.5. Geologic map of Recontre Bay area. Reproduced from White, 1940.

geologic zones, each with distinguishing characteristics. In addition, the
Carboniferous basins, basic plutons, principal fold axes and faults, and
Cambrian outcrops, as far as known, are shown. Each zone will be de-
scribed separately.

Principal Structural Directions

Overall, the fold axes, the faults, and the foliation take a north-north-
easterly direction; but upon closer observation, some structures trend
more easterly, especially in the Notre Dame Bay area. The stratigraphic
and structural composition is much like that of the Maritime Provinces
and New England, and undoubtedly Newfoundland is part of the great
Appalachian Mountain systems.

Relation to Physiographic Provinces

The Long Bange highland of the physiographic map, Fig. 13.1, is
coincident with the crystalline Precambrian (?) rocks of zone 1 of the
tectonic map, Fig. 13.6; the serpentine plutons are generally strong relief

210

STRUCTURAL GEOLOGY OF NORTH AMERICA

THRUST FAULT

SAW TEETH ON SIDE
OF UPPER PLATE

s 1 MAJOR ANTICLINE OR
r ANTICLINORIUM

/K MINOR FOLD AXIS

w' HIGH ANGLE FAULTS

^^r SERPENT I NIZEO
I NTRUSIONS

GABBRO OR PERIOOTITE

>'>'' CARBONIFEROUS 3ASI N

Fig. 13.6. Tectonic map of Newfoundland, taken mostly from Snelgrove's Geo/ogic Map of New-
foundland, Newfoundland Geological Survey. Interpretations assisted by J. J. Hayes, D. Bradley,
and Joe Kerr. Zone I consists of schists, gneisses, and intrusives, believed to be chiefly Pre-
cambrian, which in part may be metamorphosed volcanics. It was actively deformed during
Taconic and Acadian orogenies. Zone II is the Paleozoic orogenic belt of Ordovician, Silurian,
and Devonian metasediments, metavolcanics, and batholiths. It may contain both older and
younger rocks, but in exposure they are of minor importance. Zone III is a Paleozoic orogenic
belt, but in addition to the rocks of zone II it contains major Precambrian linear elements. Zone

features; and the Carboniferous areas are for the most part lowlands;
but the uplands and lowlands east of these do not clearly indicate in-
dividualized geologic provinces, as far as known.

Characteristics of Tectonic Zones

Zone One. Zone one is the Long Range highland, and it consists chiefly
of schists and gneisses similar to those of the nearby Canadian Shield of
Labrador. At the south and between La Poile Ray and Cape Ray, how-
ever, part of the rocks may be metamorphosed Paleozoic. George Phair
has mapped the coast from La Poile Ray westward, according to Joe Kerr
(personal communication), and finds at the bay a fossiliferous Lower
Devonian formation with the argillaceous members slaty and sharply
folded. As the upturned succession is traversed westward, it becomes phyl-
litic and finally schistose. No contacts could be found between the
Devonian slates and the phyllites, and the schists, previously called Pre-
cambrian. Phair visualizes the southern end of the Long Range as an anti-
clinorium of isoclinal folds, pitching north-northeastward, and with
increasing metamorphism toward the core; perhaps Precambrian rock is
exposed in the core, but contact relations are not evident to prove it.

At the north end of the range and along its flanks at intervals — Ray of
Islands area on the west and White Ray on the east — Cambrian beds rest
on the schists and gneisses, and hence demonstrate the Precambrian age
there of the foliate rocks.

Zone Two. Zone two east of Long Range appears to be basically the
Acadian orogenic complex. It is made up principally of the great Ordo-
vician and Silurian volcanic sequences and numerous great batholiths,
presumably of Caledonian or Acadian age. The stratified sequences are
much folded and generally subject to low-grade metamorphism. Some
Precambrian rocks may exist, but this possibility seems less as work

IV consists principally of Precambrian sediments and volcanics with small infolded or faulted
basins of Cambrian and Ordovician strata. The zone is generally much less deformed than the
others. Carboniferous basins are stippled and postdate the major orogeny, but were affected by
Appalachian faulting. Black areas with smooth borders are serpentinized intrusions, and black
areas with hachured edges are gabbros and peridotites. Numbers 1 to 1 1 are lines of cross
sections.

NEWFOUNDLAND APPALACHIANS

211

v; l

\ /

CODROY \ ■•'•• \ , lj . . "-*,„■> , ••— V* * V

/ingu/7/e se

/ MILE

SECTION I

Codroy ser/es

STORMY PT.

CODROY RIV.

\\\» Baracho/5 ser/es

BAY ST. GEORGE

SECTION 2

WEST

COAST

L O W L A N

Carboniferous

LONG RANGE

'7W \/\'\/WW\/\/\ /

W.WPR E- CAM BRIAN s L>
\\\/\ ir \"V\ jwwuw"/ \

/\ /\ / \/\/W \/\/\/\/

■^ M I L E3

WHITE 6 AY

O/c/er Pa/e ozo/ cs \ <5//ur/ar? x" M /js / ss/pp/on

— /V/A £''5 i Quartz porphyry, tn/cf- Pcr/eozo/c

SECTION 3

Fig. 13.7. Representative cross sections of Newfoundland. Section 1 after Hayes and Johnson,
1938; section 2 after Betz, 1943; section 3 after Betz, 1948.

progresses. Much of the region is unknown. Several serpentinized ultra-
| mafic intrusions occur in a line southwest of Carmanville.

Zone two west of the Long Range Mountains consists of folded and
faulted Cambrian, Ordovician, and Devonian strata, with the Ordovician
\ thickest but with volcanic rocks present in only one formation. It repre-
sents the front of the Taconic and Acadian systems. It contains the major

Carboniferous basin and the principal belt of serpentine intrusions.

The Long Range has been elevated in a steep reverse fault against the
Carboniferous basin. See section 2, Fig. 13.7. Section 1 shows the faulted
and folded nature of the Carboniferous rocks themselves. They are gen-
erally far less folded, however, than the underlying Ordovician. Folded
Carboniferous is also shown in section 4B resting unconformably on the

212

STRUCTURAL GEOLOGY OF NORTH AMERICA

Tab/e. Head fc.p 5f- Georpp

V77

/'///

7 ' / &rff'''//

WJJJ ) ! } I ' Si'M '

5ECTI0N 4A

/<$/ 5f. George ser/es
Tab/e /ieod /s.

Carbon/ ferous

SECTION 43

C. FOX PEN.
CONCtiE HSR.

M u m b e r Arm s e. r / e s - Or do v / c / a n

Tab/e. Head
As.

S A7/L £S

<5t. George ser/es
O r do v/ c /on

GR0AI5 15.

GULL 15.

Ordov. ' M/Js/js/pp/'an
5ECTI0N 5

/^re - Ca/rrbr/on ?

Fig. 13.8. Representative cross sections of Newfoundland. Section 4A and 4B after Walthier,
1949; section 5 after H. Johnson, 1941.

Humber Arm series of the Ordovician.

The upfaulting of Long Range on the west started in early Mississip-
pian time and resulted in the deposition on the downfaulted block of the
coarse Anguille series. Movement continued during the deposition of the
entire Mississippian and Pennsylvanian sequence, or at least recurred
after the Mississippian sediments were deposited, because the Precam-
brian is now in fault contact with the Mississippian. Faulting recurred
after the Pennsylvanian Barachois beds were deposited.

The structure along the east flank of the Long Range uplift is illustrated

in sections 3 and 5, Figs. 13.7 and 13.8. High-angle thrust faulting seems
the dominant structure, but probably a large syncline or synclinorium
exists between the mainland and Groais Island. Groais and Bell islands
are presumably Precambrian schists and gneisses, and hence are believed
to mark an anticlinal fold.

Representative of the folding and faulting in the Notre Dame Bay area
are sections 6 and 7 of Fig. 13.9. Through the islands and headlands of
Notre Dame Bay area, a system of faults with an east-west bearing oc-
curs. Those shown on the tectonic map were taken from a compilation

NEWFOUNDLAND APPALACHIANS

2J3

RED CLIFF
POND

SECTION 6

6 5

I I I

SNOOKS
ARM

Iwrrtf

SECTION

TOMMYS

ARM SOPS ARM

SHOAL
ARM

BEAVER
7 BIGHT

WILD BIGHT

ROTI

15LE AU
N^BOIS

////ft

BAIE
I/D'ESPOIR

5ECTION 8

Schist

•5 Safes, phy///fes, qucrrfz/tes, groywocAes

\ i s / k~/ ri
/\ '/ n/w w '

/www C7

Devon /on (?) gron/f'e

Bo/e cf ' £spo/r ser/e5

4 MILES

Fig. 13.9. Representative cross sections of Newfoundland. Section 6 after Snelgrove, 1931; 1
to 5 make up the Snooks Arm series of Ordovician age. 1, andesite pillow lava; 2, andesite; 3,
rhyolite; 4, pyroclastics; 5, slates, argillite, sandstone, chert. Nos. 6 to 8 are post-Ordovician.
6, gabbro; 7, diabase and basalt; 8, Burtons Pond granite porphyry. Section 7 after Espenshade,

by J. J. Hayes. Some of the northeast are probably horizontal shears, and
the main east-west faults are high-angle ones with movement in the ver-
tical direction. The fold axes trend acute to the major faults, and to put
them in the same mechanical frame as the folds seems impossible. The
folds appear to the writer to be Acadian, and the faults more likely to be
associated with the faulting of the Carboniferous basins and later than
with the Acadian folding.

Zone Three. Zone three is much like zone two but includes several
Precambrian linear masses. These may be upfaulted blocks or cores of

1937. 1, pillow basalts; 5, andesites; 4, shales and sandstone; 3, coarse, massive sandstone; 6,
argillaceous graywacke and chert; 2, shales, tuffs and cherts; 7, gabbro. All units are probably
Ordovician. Section 8 is after Jewell, 1939.

anticlinoria. Cross sections 8, Fig. 13.9 and 9A and 9B, Fig. 13.10, are rep-
resentative of the structure. They show especially the trans gressive grani-
toid intrusions. The Precambrian rocks that appear in zone three are
sediments and volcanics, and are considered later than the schists and
gneisses of Long Range.

Zone Four. Zone four is predominantly a late Precambrian sedimen-
tary and volcanic series, with infolded or downfaulted Cambrian and
Ordovician sediments in several places. On Belle Isle of Conception Bay,
Ordovician sediments occur which carry iron ore. See map and sections,

214

STRUCTURAL GEOLOGY OF NORTH AMERICA

Harbor Ma/r?

Litf/e. Lawn 5/?., groyivacAe,
arg/'//fte - Orc/ov/c/0/7 (?)

3er/e3

SECTION 9A

Granrfe - Qevon/cm(?)
4 rrrL.ES

\\ // f/ ?/ l/tf/e. low/'? fm

/Jy'y/J'/J,t/f//;*/ffIli/rtli h" >///■■ ///■/■■/ //■/^r>/ •< » » \ » V\ \ v * » » v * *• v • .» • y > ' / / '

Harbor Ma/n £3 r /get 5 S3.,^h.,_

vo/can/c jer/es to. -L. Combr/0/7

SECTION 9B

Sarin series -bo^o/fic
/ Qva^ y- ^ea/n7e.t7t3 - Orcfov/c/'an (?)

Oc £ec

5MITH
•Tb £.f*2 £b 50UND RANDOM 15.

SECTION 10

Fig. 13.10. Representative sections of Newfoundland. Sections 9A and 9B after Van Alstine, 1947; section
10 after Hayes and Rose, 1948; pCm, Musgravetown granite; pCcp, Connecting Point granite; £r, Randon
quartzite; €b, Brigus conglomerate, quartzite, shale; Gee, Elliott Cove shale; Oc, Clarenville shale, sandstone.

(locality 11) Fig. 13.11. The Cambrian and Ordovician sediments have
largely escaped metamorphism. Along the west side of Trinity Bay (sec-
tion 10) the Cambrian and Ordovician sediments are rather tightly
folded, whereas to the east in Avalon peninsula, the Paleozoic beds are

less folded and chiefly faulted. The impression is conveyed that zone two
east of the Long Range Mountains suffered the most intense deformation,
and that zones one and three, although deformed and intruded exten-
sively, are marginal; and that the eastern part of zone four escaped the

NEWFOUNDLAND APPALACHIANS

215

Fig. 13.11. Map and sections of Conception Bay and the Wabana iron ore deposits. Repro-
duced from Hayes, 1931.

sharp folding and metamorphism common to zones one, two, and three
but was faulted and elevated in post-Ordovician time, probably in the
Appalachian orogeny.

TECTONIC HISTORY

Early Cambrian Phase

By reference again to the chart of Fig. 13.2, the numerous disturbances
and orogenies that characterized the Appalachian systems in Newfound-
land can be reviewed. Nine orogenic phases are fairly clear. When cor-
relations are more precise, this number may be increased.

It is evident from the angular unconformity at the base of the Cambrian
and the coarse, basal elastics that an orogeny immediately preceded or
accompanied the early Cambrian sedimentation. This is noted in the
west along the coast of Labrador and the western lowland of Newfound-
land, and in the east from the Bay d'Espoir to the Avalon peninsula. In
the east the orogenic phase is the last of three or more that accompanied
the deposition of a great Precambrian volcanic series. It is not yet pos-
sible to define the distribution of land and sea in the orogenic belt in
Cambrian time. For that matter, the same can be said of the belt in all
pre-Carboniferous time. Volcanic activity was pronounced in eastern
Newfoundland in the Proterozoic but abated everywhere, it seems, during
the Cambrian period.

After the early Cambrian Brigus and Eteheminian elastics had been de-
posited in the Fortune Bay and Trinity Bay areas respectively, a slight
disturbance occurred which resulted in uplift and erosion before the next
Lower Cambrian Hanfordian beds were deposited.

Late Cambrian Phase

In the Burin peninsula, a disturbance occurred in late Cambrian time in
which the Middle Cambrian beds were tilted and somewhat eroded be-
fore covered with the Ordovician strata (Van Alstine, 194S). Deposition
of carbonates occurred apparentlv undisturbed on the east and on the
west while the uplift was taking place.

216

STRUCTURAL GEOLOGY OF NORTH AMERICA

Early Orodovician Phase

With the beginning of Ordovician time, western Newfoundland started
to sink more rapidly and became the site of deposition of a thick clastic
series, and later of considerable limestone and dolomite. The central
area around Fortune Ray received much limestone, at least in places. East
Newfoundland also sank considerably and received over 6000 feet of fine
elastics and carbonates. It seems necessary to picture the western New-
foundland Lower Ordovician elastics coming from the Canadian Shield
where a rather sharp uplift set in (see Plates 2 and 3), but the source of
the shales in eastern Newfoundland is not clear.

After early Ordovician time, the whole central part of Newfoundland
became a site of profound volcanic activity, much of it submarine, with
the passive emission of flows; but there was also abundant pyroclastic
activity, probably both submarine and subaerial. The Ordovician must
also have been a time of tumultuous crustal activity in the volcanic zone
because various elastics, such as graywacke, conglomerate, sandstone,
and shale, are commonly interbedded in the volcanics, or mixed with tuf-
faceous material, and they necessarily must have come from nearby up-
lifts. Chert and carbonate were also deposited, which with the above
lithologies are the common associates of volcanic orogenic belts. In places
upward of 20,000 feet of volcanics and sediments accumulated.

Andesites are the most common of eruptive rocks in the orogenic belts,
but in the Relle Ray volcanic series of Fortune Ray, about 13,000 feet
thick, most of the volcanics are rhyolite (D. A. Bradley, personal com-
munication). This is indeed a great outpouring of rhyolite in an orogenic
belt. Hobbs ( 1944) has found that andesites are the first eruptives in new
orogenic belts in the southwest Pacific, but after a period of growth, other
less basic forms appear, with rhyolite one of the late entrants. Since vol-
canic activity continued long after the Belle Bay rhyolites in central New-
foundland, it appears that new volcanic cycles followed the early Ordo-

vician one.

Late Ordovician Phase (Taconic Orogeny)

The Taconic orogeny is generally held to have been pronounced in
Newfoundland, not because of a great angular unconformity between

Ordovician and Silurian rocks, but first, because the Ordovician sequences
are more metamorphosed than the younger ones ( Schuchert and Dunbar,
1934); second, because the Silurian has much conglomerate in it; and
third, because the Taconic orogeny of the Gaspe and Maritime Provinces
could not very well end abruptly without extension into Newfoundland.
Silurian beds are relatively not very abundant in Newfoundland, and
good exposures of their contact with the Ordovician sequences have so
far escaped detection. Twenhofel and Shrock wrote in 1937 that so far as
known there is no angular unconformity between the Ordovician and
Silurian systems. However, White ( 1940 and Ph.D. thesis, Princeton,
1939) recognized evidence of the Taconic orogeny in the Rencontre East
area of Fortune Ray, where the Long Harbour volcanics of Ordovician
age were folded and extensively eroded, he believes, before the Silurian
Rencontre series was deposited.

The contention that the Ordovician sequences are more metamorphosed
than younger ones is correct only in so far as the "younger ones" are
Carboniferous sequences or, perhaps in a few places, Devonian. Some of
the granitic batholiths are now known to be Acadian, and most of the
metamorphism may be incident to them, in which both Silurian and
certain Devonian strata are altered as much as the Ordovician. Aside from
the Rencontre East area, it is difficult to find tangible evidence of a sharp
orogeny in Newfoundland at the close of the Ordovician. The Silurian se-
ries, with its volcanics and elastics, resembles the Ordovician of central
Newfoundland, and it seems more logical to regard the central belt as one
of continuing, but intermittent, orogenic and volcanic activity into and
through the Silurian.

The ultrabasic intrusions of western Newfoundland are regarded as Late
Ordovician mostly by relation to those of the Gaspe and Quebec Taconic
belt (Snelgrove, 1934). Some of the ultrabasic plutons are known to in-
trude the Ordovician volcanic series and are therefore not older than the
Taconic.

Late Silurian Phase (Caledonian Orogeny)

The Clam Bank conglomerate of western Newfoundland and the Great
Bay de l'Eau conglomerate of Fortune Bay, both of early Devonian age,

NEWFOUNDLAND APPALACHIANS

217

indicate sharp uplift nearby, and the influx of much coarse clastic mate-
rial. Since Devonian plant fossils have been found in schistose strata in
the La Poile Bay area, it now seems probable that considerable of the
metamorphic rocks of central Newfoundland, aside from the batholiths,
will prove to be Devonian, and therefore a site of deposition during part
of Devonian time, at least. The sources of the Lower Devonian conglom-
erates and sandstones must have been along the Labrador coast on the
west and in an uplift through the Avalon peninsula on the east.

A Caledonian orogeny in the White Bay and Notre Dame Bay region
has been suggested by Heyl ( 1937a ) in view of the lithologic similarity of
the Devonian and Mississippian there, in contrast to the Silurian and older
rocks. Also, the amount of deformaton of the Devonian and Carbonifer-
ous is less than that of the older beds. Schuchert and Dunbar ( 1934) note
that the Devonian sediments in the St. George Bay area are not strongly
deformed, except along Appalachian phase faults; they are apparently no
more disturbed than the Mississippian strata, and much less disturbed
than the Ordovician Humber Arm series.

If an orogeny occurred in the White Bay and Notre Dame Bay region,
it is not unlikely that intrusive activity accompanied the deformation.
Some of the plutons of that region may, therefore, be Caledonian. They
may also have come in during the Devonian or at its close (Acadian).
Composite relations undoubtedly exist (Hayes, personal communication).

Late Devonian Phase (Acadian Orogeny)

Like the Taconic orogeny the Acadian is also illusive. Mississippian
elastics in themselves indicate sharp uplift nearby, and are generally be-
lieved to rest in angular relation on much deformed Ordovician strata
in western Newfoundland and in the White Bay and Notre Dame Bay
area, although the contact is seen in only a few places. The Mississippian
strata have suffered little metamorphism, however, and this sets them off
strikingly from the older deformed and altered rocks. Nowhere in New-
foundland has an angular unconformity yet been recorded between the
Mississippian and Devonian systems. Nevertheless, all workers in New-
foundland are aware of profound folding, batholithic intrusions, volcan-
ism, and metamoq:>hism that occurred sometime between the Ordovician

and Mississippian; and since in two places the batholiths are found in-
truding the Lower Devonian series, it seems probable that many plutons,
similar in composition, are of the same age. The Acadian orogeny, pro-
ceeding through the late Devonian and into early Mississippian in the
Maritime Provinces and New England, was one of superior and wide-
spread proportions, and it is highly unlikely that Newfoundland, with its
similar geosynclinal assemblages and lying in the projection of the great
belt of orogeny, could have escaped it.

Mississippian Phase

The desposition of the Anguille conglomerates in the St. George Bay
area attended the upfaulting of the Long Range mass, and the same ac-
tivity is probably indicated by the Pilier conglomerate at Groais Island.
The Springdale elastics in the Notre Dame Bay area, if correctly dated, in-
dicate orogeny nearby.

Early Pennsylvanian Phase

The coarse and thick Barachois series of the St. George Bay area rests
conformably on the Lower Mississippian Codroy formation, but the
abrupt change from fine-grained, mottled red and green sandstones of the
Codroy to the coarse, red, feldspathic sandstone of the Barachois is strik-
ing. The influx of coarse red elastics signifies another sharp uplift, proba-
bly in the Labrador coast area.

No other Pennsylvanian rocks are known in Newfoundland, and hence
nothing is known of the early Pennsylvanian disturbance outside the St.
George Bay area.

Post-Early Pennsylvanian Phase (Appalachian Orogeny)

The major fault zone that extends from the southwestern coast of New-
foundland in a northeasterly direction to Grand Lake, White Bay, and up
the east coast of the northern peninsula postdates the youngest sediments
of Newfoundland. These are the Barachois series of lower or middle Potts-
ville (Early Pennsylvanian) age. Relief features and escarpments in other
parts of the island trend northeasterly and parallel the western fault zone.
These in part may also be due to faults of the same phase. Betz ( 1943 )

218

STRUCTURAL GEOLOGY OF NORTH AMERICA

suggests that the orogeny is an extension of the Appalachian orogenic belt
of the Canadian and United States Appalachians.

Volcanic activity had died out by the Pennsylvanian after very little in
the Mississippian, and no Carboniferous intrusions have yet been noted.
The post-Barachois faulting and thrusting mark, as far as known, the last
compressional deformation in the Appalachian mountain systems of New-
foundland.

Post-Appalachian History

No Triassic fault basin sediments are known as in Nova Scotia and New
England, and no coastal plain sediments of Cretaceous or Tertiary age oc-
cur above water on Newfoundland. Without these signs of submergence,
it is concluded that the island has been mostly above sea level since the
Appalachian orogeny, and has been a site of erosion. It undoubtedly has
had broad connections with the Maritime Provinces and the Gaspe in the
Mesozoic. Likewise, the region of its northeastward projection into the
Atlantic must have been extensively emergent in times past.

The broad banks off Newfoundland continue the continental shelf from
Nova Scotia, and as late Cretaceous fossils have been dredged off Nova
Scotia (see Chapter 10), one could assume the same fossil-bearing beds
will be found under the Banks of Newfoundland. An enticing experiment
would be the drilling of a deep well on Sable Island.

Twenhofel and MacClintock ( 1940 ) have described three fluvial erosion
surfaces in Newfoundland in much the same aspects as in the central
Appalachians, and hence assign a similar history of Cenozoic epeirogenic
uplift to the island. The major difference is that the Maritime Provinces
and Newfoundland have not emerged as much as the Appalachians south
of New York City. If they should rise another 1000 feet, then much of the
continental shelf would be land and probably a large bordering coastal
plain with Cretaceous and Tertiary sediments would appear.

Cabot Strait Fault (?) and Seismic Profile

The Tectonic Map of Canada (1950) shows a fault along Cabot Strait
between Nova Scotia and Newfoundland, with the implication that it is a
transcurrent fault offsetting the structural elements of the two provinces.

Fig. 13.12. Paleozoic orogenic belts of Greater Acadia. In addition to the Taconic, Acadian, and
Appalachian orogenies there were several others in various places that are not represented. The
post-Silurian Caledonian orogeny was pronounced in Newfoundland and Nova Scotia. A mid-
Ordovician Vermontian is known in the Vermont-Gaspe region.

NEWFOUNDLAND APPALACHIANS

219

Reference to the map of Fig. 13.12 will indicate the position of the pre-
sumed fault. The structural front passes between Anticosti Island and the
Gaspe Peninsula and between Labrador and the Northern Peninsula of
Newfoundland under the Straight of Belle Isle (Figs. 13.1 and 13.6). Since
the front is entirely submerged, its position as shown on Fig. 13.12 is only
a guess. Nevertheless, the conclusion must be drawn that a deep recess in
the structural front exists between the Champlain-Gaspe salient and the
Newfoundland salient. Perhaps this is the result of horizontal offset along
a transcurrent fault.

The submarine trough of the Gulf of St. Lawrence extends out under
Cabot Straight to the edge of the continental shelf. See Fig. 13.12. It has
a depth of over 600 feet for a distance of 750 miles, and from a point
midway south of Anticosti Island to the shelf rim is over 1200 feet deep.
At two places it is 1800 feet deep, and has a closed basin in this area
about 150 miles long below the 1320-foot contour. One large tributary of
the trough extends up toward the Straight of Belle Isle, and another ex-
tends along the north side of Anticosti Island.

Six seismic profiles were shot on the extensive banks off Nova Scotia
and Newfoundland by Press and Beckmann ( 1954), and a combination of
three of them across the outer end of the Cabot Straight trough is shown
in Fig. 13.13. The position of the section is indicated on Fig. 13.12.

The seismic section indicates for one thing that the trough is erosional
into the unconsolidated sediment layer, and this is the conclusion that
Shepard (1930) reached. From a study of the shape of the submarine
valley he concluded that it was first a subaerial stream valley and then
was modified by glaciers flowing seaward along it. Glacial striations and
roches moutonees on the southern tip of Newfoundland and on St. Paul
Island off the north end of Nova Scotia demonstrate the past ice flow.
The present depth of the trough is no greater than fiords elsewhere. The
trough walls do not resemble fault scarps — they are straight segments
with hanging valleys.

In interpreting the seismic section, Press and Beckmann say that it sup-
ports the thesis that the trough is of fault origin, yet at the same time say
that the faulting occurred during the deposition of the sediments of the
3.80-km/sec layer. They regard the 3.8-km/sec layer under the north side

Banquereau
Bank

Cabot Strait Trough

St Pierre

Bank

1 70

2 94

r . ■.-,•.

— 10 ooo-

'•.'•"■ .'•"•*• 3.0 V-V

v.:

— "' ' " ""* TIT

4.6

3.80

1
I
/

3.30

15,000-

MB****"^

/
/

20.000'

— 25,000'

— 30.OO0-

e S3

Fig. 13.13. Seismic profile across Cabot Strait, Nova Scotia, and Newfoundland. See section
line A-A', Fig. 13.12. Figures are velocities in km/sec.

of the trough (Fig. 13.13) as demonstrating the faulting. It is possible
that the wedge shape of this layer does indicate faulting, but not in post-
unconsolidated sediment time. The 3.80-km/sec layer is logically inter-
preted as consolidated sediment. Consolidated sedimentary rocks would
be either Triassic red-beds or Carboniferous of the nature of the basin
sediments of southwestern Newfoundland, and faults of this age are long
since dead, according to the history of the Piedmont and Greater Acadia.

Mild earthquake activity is cited as evidence for the fault origin of the
Cabot Strait trough. Two earthquakes whose epicenters were on the shelf
slope immediately off the trough mouth have caused submarine land-
slides and numerous Trans-Atlantic cable breaks. Shepard questions the
presumed connection between these earthquakes and continuing displace-
ment along faults causing the trough.

Both sides of the modern trough are about the same, yet the seismic
profile indicates the possibility of a fault on one side only. The conclusion
is reached that in Carboniferous or Triassic time a trough formed, pos-
sibly bv downfaulting, but that since then no further movement has
occurred.

Now, to the original question; could the structural elements of the
Maritime Provinces and Newfoundland be offset appreciably by horizontal
motion along a transcurrent fault? The seismic profiles have demonstrated
the possibility of a late Paleozoic or Triassic fault along the outer stretch
of the Cabot Strait trough. If this fault is part of the Triassic fault system,
it would probably be one of vertical displacement. If like the fault that

220

STRUCTURAL GEOLOGY OF NORTH AMERICA

bounds the east side of the Carboniferous basin of the St. Georges Ray
area of southwestern Newfoundland, it would also be one of vertical dis-
placement. The wedge of sediments of the 3.80-km/sec layer suggest a
vertical fault. The major structural elements from Newfoundland to Nova
Scotia may be drawn across to Nova Scotia with reasonable continuity
and without a horizontal offset, as shown in Fig. 13.12. Although none of
these is compelling evidence against horizontal movement, they lead the
writer to conclude that considerable transcurrent movement has not
occurred.

MAJOR TECTONIC RELATIONS OF GREATER ACADIA

Definition

Greater Acadia has been defined by Schuchert and Dunbar ( 1934 ) as
the combined regions of New England, the Maritime Provinces, the St.
Lawrence-Gaspe area of Quebec, and Newfoundland. Much of the area is
now covered by shallow waters, and from an historical point of view
Greater Acadia includes all the lands of the past in the great geosynclinal
and orogenic belt seaward to the continental shelf slope.

Major Geocynclinal Characteristics

Numerous series of beds in Greater Acadia have thicknesses of 5000 to
15,000 feet, and the total thickness in places ranges up to 100,000 feet.
Thick and coarse elastics in every stratigraphic system of the Paleozoic
and numerous unconformities within and between systems attest long-
continued crustal unrest in the geonsyncline and at times in belts
adjacent to it. A dominant lithology of the materials in the geosyncline
is volcanic rocks of all descriptions. They consist chiefly of andesites and
basalts, but other varieties, especially rhyolites, are by no means absent.
A very thick accumulation of Ordovician rhyolite marks the central part
of the geosyncline in Newfoundland. The volcanics occur as flows, in
large part submarine, and as various pyroclastics. They are especially
concentrated in the medial part of the geosyncline, if the Precambrian
rocks of Nova Scotia and the Avalon peninsula of Newfoundland mark
the site of the outer or southeastern portion. The inner belt of the Taconic

Mountains-Lake Champlain-St. Lawrence-Gaspe region was compara-
tively free of volcanics until late Ordovician and Silurian time when the
igneous activity spread to the Gaspe Peninsula and to western New-
foundland in the western belt. Aside from Devonian volcanic activity
in the Gaspe Peninsula the western belt was again free of volcanism
after Silurian time. Eruptive activity had died out in all Newfoundland
by late Mississippian time but not in the Maritime Provinces and in the
eastern part of New England. Volcanism continued exceedingly active
there in places, and was accompanied and followed in the Carboniferous
basins of New England by intrusive activity.

Batholiths

The central zone of the geosyncline, along with tumultuous volcanic ac-
tivity, was the site of great batholithic intrusions. Where better known, as
in New Hampshire, four magma series are recognized, the first about of
Taconic age and the other three of Acadian, which there started in mid-
Devonian and lasted probably until early Mississippian. Of the three Aca-
dian magma series, the first preceded the major compressional orogeny,
the second was synorogenic, and the third followed the orogeny.

As studies progress in the Maritime Provinces and in Newfoundland, it
is becoming clearer that most of the dioritic to granitic batholiths there
are Acadian also. The batholiths are not limited to the medial volcanic
zone of the geosyncline but some have intruded the inner, less volcanic
complement of geosynclinal sediments and others in great volume,
the outer zone, now mostly of Precambrian rocks.

Metamorphism

A striking character of the stratified rocks of the geosyncline of Greater
Acadia is their metamorphism. Where distant from the batholiths they
are generally slates, phyllites, argillites, quartzites, and metavolcanics.
Where close to the altering influence of the intrusions they are schistose
and gneissic. The very-low-grade and low-grade metamorphism is more
characteristic of the inner belt, and also the outer where Paleozoic sedi-
ments are preserved, as in the Conception Ray area of Newfoundland.
Medium-grade metamorphism is more characteristic of the central belt.

NEWFOUNDLAND APPALACHIANS

221

Ultramafic Intrusions

A zone of serpentinized ultramafic intrusions extends from Georgia
through the crystalline Piedmont to New York City, and from New York
northward through the Taconic system to the St. Lawrence and Gaspe.
From there it is believed to continue through western Newfound-
land.

Fronts of Successive Orogenies

An attempt was made by Schuchert in his early paleogeographic maps
and later by Schuchert and Dunbar ( 1934 ) to show the major structural
elements of Greater Acadia. They postulated a western trough of sedimen-
tation, the St. Lawrence geosyncline; a central land barrier, the New
Brunswick geanticline; and eastern trough of sedimentation, the Acadian
geosyncline; and beyond this, a borderland, Novascotica. As described
on previous pages, the "New Brunswick geanticline" has been found to
be approximately the heart of the geosyncline — a site of such sedimenta-
tion and prodigious igneous and orogenic activity. Crustal movements
within the orogenic belt were numerous, and the island barriers and pen-
insulas were too many and transitory to be charted satisfactorily with
present knowledge.

Kay (1947) has illustrated the Taconic, Acadian, and Appalachian
orogenic systems of Greater Acadia to have been formed by deforma-
tion of the sediments of the eugeosyncline. This great sedimentary
province includes the volcanic assemblages of sediments, the batholiths
and serpentinites, in contrast to the relatively igneous-rock-free inner mio-
geosyncline typified by the sediments of the Bidge and Valley province.
It is clear that the belts of deformation of the eugeosyncline impinge on
the Canadian Shield in the Greater Acadia region, and that the belt of
deformation of the inner miogeosyncline terminates approximately at the
Adirondacks.

Some progress can be made toward an understanding of the spatial
relations of Greater Acadia if the distribution of the orogenic belts is
charted, rather than the poorly documented and transitory shore lines.
The fronts of the Taconic, Acadian, and Appalachian orogenic belts are

known in places with considerable precision and in others only approxi-
mately. Figure 13.12 shows these fronts, as well as the zones of superposi-
tion of one belt over the other. Evidence of the locations for the most part
has already been presented, and when composed for the entire Greater
Acadia, yields the picture recorded on the map. In the lower left-hand
corner, the northern end of the Appalachian folded and thrust-faulted belt
of the Valley and Bidge province is seen. The Taconic front then faces the
shield (with its thin sedimentary veneer). At Quebec City on the St. Law-
rence, the front of the Acadian orogenic belt impinges on the shield, and
as far as known from Quebec City to the tip of Gaspe and beyond, the
Taconic and Acadian belts are superposed. The two belts in the Gaspe
Peninsula swing eastward, and even somewhat southward of east, and
project in that direction into the Gulf of St. Lawrence.

Where next observable in southwestern Newfoundland, the front of the
Appalachian belt faces the shield, and is impressed on all older belts. It,
therefore, appears that from Vermont northeastward successively younger
orogenic belts overlap inward and front on the Canadian Shield. The
equivalent of the Bidge and Valley folded and thrust-faulted province
does not exist north of the Catskills. In Keith's terminology the Taconic
and Acadian orogenic systems compose a pronounced "salient" toward
the shield in the Vermont-St. Lawrence-Gaspe region.

The map also shows linear Precambrian masses that were uplifted dur-
ing the Appalachian orogeny and, if once covered by Paleozoic strata,
were later subject to erosion and stripped of their mantle. The Long
Bange Mountains element of western Newfoundland is fairly definitelv
of this origin. It seems to find continuation in northern Nova Scotia, in
Precambrian exposures on the western side of the Bav of Fundv, and
perhaps even in Precambrian rocks in the Boston basin region. Pre-
cambrian rock forms most of the Avalon peninsula of Newfoundland
and also crops out in several places west of the peninsula. It has not been
proved that this region is one of late Paleozoic uplift, but only inferred
because of the numerous escarpments and shore fines that parallel the
known Appalachian elements of western Newfoundland, and the faults of
Conception Bay which resemble those of the western Carboniferous
basins. It ties in well with the extensive Precambrian area of eastern

222

STRUCTURAL GEOLOGY OF NORTH AMERICA

Nova Scotia in relation to the Appalachian front, and in having a
similar thick Proterozoic volcanic sequence of rocks. The zone marks
the site of a great Proterozoic trough in which volcanic rocks accumu-
lated voluminously and were frequently deformed. The Avalon peninsula
contains no sedimentary rocks younger than early Ordovician and may
have been an area of erosion since then. The Great Ray de 1'Eau con-
glomerate suggests a sharp uplift of eastern Newfoundland in late
Silurian or early Devonian time, and the region was probably affected

by the Acadian movements and intrusions. The Precambrian of Nova
Scotia contains numerous batholiths, presumably of Acadian age. It is
entirely possible that the outer Precambrian uplift is one that dates back
to mid-Paleozoic time and is complex.

The presence of the geanticline of Precambrian rocks along the outer
exposed margin of Greater Acadia is rather significant in demonstrating
that the continent has not been added to appreciably, or has not grown
seaward much, since Proterozoic time.

14.

OUACHITA, MARATHON,
AND COAHUILA SYSTEMS

OUACHITA SYSTEM

Location and Topography

The Ouachita Mountains occupy a belt 50 to 60 miles broad and
200 miles long in southeastern Oklahoma and western Arkansas. See
maps, Figs. 14.1 and 14.2. They are somewhat like the Appalachians in
topographic appearance, although not generally so high. Their level-
topped subparallel east-west ridges reflect structure and dissection of
erosion surfaces. The ridges rise scarcely 250 feet above the valley west
of Little Rock but gradually increase in height toward the Oklahoma-
Arkansas border, where the highest point is 2900 feet above sea level

and nearly 2000 feet above the valley floors. Their eastern, western,
and southern margins are blanketed by the Gulf Coastal Plain sediments.

Stratigraphy

The oldest rocks of the Ouachita Mountains are Cambrian, and these
are exposed in the central anticlinorium. The section of the anticlinorium
or "core area" of southeastern Oklahoma in McCurtain County as
measured by Pitt (1955) is as follows:

Bigfork chert

Womble shale

66+ ft

Mazarn shale

600 ft

Crystal Mountain sandstone

50-100 ft

Collier shale

180 ft

Lukfata sandstone

150 + ft

Northwestward each thrust sheet has elements of its stratigraphy, and
these are given by Hendricks ( 1943 ) in Fig. 14.3.

The Arkansas novaculite is a conspicuous formation of the pre-
Mississippian sequence. It has a counterpart in the Marathon uplift of
west Texas, the Caballos chert, but is not present in the southern Appa-
lachians. The Bigfork chert, Pinetop chert, and Woodford chert, as well
as the siliceous nature of the limestones and shales indicate that a
dominant characteristic of these formations is silica. Pitt (1955) thinks
that much of the silica is secondary, having been introduced by ground-
water after extensive fracturing.

The combined thickness of the Cambrian, Ordovician, Silurian, and
Devonian rocks is hardly 3000 feet, and they are regarded as a shelf or
platform type of deposit, although the high silica content is unusual in
such a setting. The Mississippian and Pennsylvania!) strata are almost
entirely clastic — shale and sandstone — and are very thick. A measure-
ment of 18,950 feet for the Ouachita Mountains sequence of Stanley,
Jackfork, and Johns Valley formations is given by Cline and Moretti
(1956), and 17,000 feet for the foredeep sequence of Atoka (Hendricks
et al, 1936).

The terms Ouachita facies and Arbuckle facies have been widely used
to compare or contrast the sequences of the Ouachita Mountains and

223

'♦ H»ROE«AN {MS IN : V ?»i^ ' ^ °" < V^

ij -, >~svi — I ■•"•/• % ;.% ^v ,\\Xv

vaiiYmrn • ' .1, / 5 i

^■-Wsr,-- *\ v' I)

*-*■.*■ i % i .'///■■ <W

12 ..-

<■

s /;

Fig. 14.1. Composite map of the tectonic
features developed in the late Paleozoic in
the Mid-Continent region. Taken from R. E.
King et al. (1942), Moore and Jewett
(1942), and other publications. In Kansas
the dotted names designate the older fea-
tures. A. A., Arbuckle anticline.

OUACHITA, MARATHON, AND COAHUILA SYSTEMS

225

-^r

PRE-MISSISSIPPIAN
FORMATIONS

r^L.

Fig. 14.2. Generalized structure map of the Ouachita and Arbuckle Mountains. MC, Magnet Cove.

the Arbuckle Mountains. The Ouachita facies is characterized by an
abundance of silica in the pre-Mississippian formations and by the very
thick Carboniferous clastic sequences. Also it appears that incipient
metamorphism is included by some as a mark of the facies. This is all
a misuse of the term facies as defined, but for local paleogeologic studies
it is convenient, if properly understood.

Structure

The Ouachita Mountains may be divided into a western division, re-
plete with thrust faults, and an eastern division, intensely folded but
not appreciably faulted.

According to Miser (1929) there are five thrust sheets in the Okla-
homa Ouachitas (see cross section D-D', Figs. 14.1 and 14.4), but in
light of Hendricks' additional work there are four "independent"

thrusts. They are, from northwest to southeast: (1) the Choctaw fault,
(2) the Pine Mountain fault, (3) the Ti Valley fault, and (4) the Wind-
ingstair fault. See Fig. 14.5. Each sheet has been thrust from south to
north and has been broken by numerous smaller, high-angle reverse
faults that presumably join the main thrusts at depth. Minor cross faults
are numerous, and larger cross faults are present in several settings. The
stratigraphy of each thrust sheet is somewhat different and is sum-
marized in Fig. 14.3.

In front of the thrust sheets is the Arkansas Valley basin whose beds
have been cast into open folds which gradually decrease in intensity
toward the north. These folds partake of some of the characteristics (4
both its bounding provinces, the beds on the south being rather close!)
folded near the Ouachitas but progressively more open farther north
toward the Ozark dome. Normal faults on the north side of the valley

COAL BASIN

BLOCK S.E.

of the

CHOCTAW FAULT

BLOCK S.E.

of the

KATY CLUB FAULT

BLOCK S.E.
of the
PINE MTN. FAULT

BLOCK S.E.
of the
Tl VALLEY FAULT

UJ
U.

z
o

<
O

z
<

z
z

McAlester sh.
Hartshorne s s.
Atoka fm.
Wapanucka Is.

Springer fm.

Atoka fm.
Wapanucka Is.

Springer fm.

Atoka fm.

Chickachoc

chert
Springer fm.

Atoka fm.
Springer fm.

Atoka fm.
Johns Valley sh.

Jackfork ss.
Stanley sh.

CO
CO

Caney sh.
Sycamore Is.

Caney sh.

Caney sh.
Sycamore Is. (?)

DEVON-
IAN ?

Woodford chert

Bois d'Arc Is.
Haragar. sh.

Woodford chert

Pinetop chert
Unnamed Is.

Arkansas
novaculite

DEVON-
IAN

o
a.
o

z
o

1-
t-

z
<
S

Henryhouse sh.
Chimneyhill Is.

Missouri
Mountain sh.

Z
<

>
o
o

K
O

Sylvan sh.
Fernvale Is.
Viola Is.
Simpson group

Arhii^klA nrnnn

Polk Creek sh.

Bigfork chert
Womble sh.

CAM^~
BRlAf*

Reagan ss.

Fig. 14.3. Sequence of strata characteristic of each of the structural blocks of the Black Knob
Ridge area of the western end of the Ouachita Mountains. After Hendricks, 1943. Katy Club

fault is a minor shear along the line of cross section in Fig. 14.5. The Stanley shale is now
considered Upper Mississippian.

OUACHITA, MARATHON, AND COAHUILA SYSTEMS

227

ARKANSAS VALLEY

COASTAL PLAIN OUACHITA MOUNTAINS

window .Jockfork ,' . WINDOW „

BOSTON MOUNTAINS

-Cambrian 'S'S'->, '-/c/\'\>

)°

Mississippion

Formations in Ooachitas
Atoka -formation
Johns Volley shale
dock fork sandstone
Stanley shale.
Coney shale
Arkansas novacu/ite
Missouri Mountain slate
Blaylock sandstone Silurian

Polk Creek shale
Big fork chert

Womble shale (Stringtownj \ Ordovician
Bloke ly sandstone
Mazarn shale i

Crystal Mountain sondstoneX Cambrian
Collier shale J

Formations in Arkansas Valley
Boggy shale
5a van a sandstone
McAlester shale
Hartshorne sandstone
Atoka formation -9,ooo'
thick in southern part

L Pennsylvanian

Vertical t horizontal scale in miles

Fig. 14.4. North-south cross section through Ouachita Mountains and Arkansas Valley. Section D-D', Fig.
14.1. Somewhat idealized from Miser, 1929, and Hendricks et a/., 1936.

are common (Croneis, 1930). Their south sides are generally down,
thereby augmenting the basin structure.

The thrust faults appear to die out eastward into Arkansas where a
fold complex indicates also considerable compression. See Fig. 14.6.
An anticlinorium is the dominant structure in the approximate center of
the exposed fold belt. The minor folds on the major anticline are sharp
and mostly asymmetrically inclined northward. Two large anticlines with
amplitudes of 7000-10,000 feet dominate the belt north of the intricately
folded anticlinorium. Precambrian rock is nowhere exposed in the
Ouachitas — a condition similar to that in the Valley and Ridge province
of the Appalachians.

In Arkansas it is not clear just where the line should be drawn
separating the folds of the Arkansas Valley basin and those of the
Ouachitas. The Choctaw thrust is considered the northern boundary of

the Ouachitas in Oklahoma. Numerous folds in the Arkansas Valley
basin sediments are conspicuous on the Tectonic Map of Oklalioma
(Arbenz, 1956).

The turn of the thrusts of the west end of the Ouachitas to the south
is very conspicuous. The number of thrust slices increases also, and it
appears that the strata were more crowded here than elsewhere. The
junction with the Arbuckles is unfortunately covered by the Cretaceous
sediments, but a number of wells and some geophysical work help to
explain the obscure relationship. The strike of the structures and trend
of the Arbuckles is nearly at right angles to the southward veering
Ouachita structures, and the formations are in part conspicuously differ-
ent. The problem of the relation of the Arbuckles to the Ouachitas will
be taken up later.

No rocks or structural elements resembling the Rlue Ridge or the

228

STRUCTURAL GEOLOGY OF NORTH AMERICA

Fig. 14.5. Cross section of the Black Knob Ridge area of the western end of the Ouachita Mountains.
After Hendricks, 1943. Formations may be identified by reference to chart, Fig. 14.3.

crystalline Piedmont are exposed on the south flank of the folded and
thrust-faulted Ouachitas. These tectonic units have been looked for in
numerous wells which have penetrated the Cretaceous and Jurassic cover,
but the wells are apparently not sufficiently far enough down dip and
seaward to discern the units.

/Aetamorphism

The pre-Mississippian formations of the central anticlinorium or
"core" of the Ouachita Mountains in both Oklahoma and Arkansas are
slightly metamorphosed. The shales are dynamically altered to argillites,
meta-argillites, and in places to phyllites (Goldstein and Reno, 1952;
Flawn, personal communication and 1956). The novaculite and chert
units are most metamorphosed at the eastern end of the anticlinorium
near Little Rock and at the southwestern end in McCurtain County,

Oklahoma (Miser, 1943). In McCurtain County the fissility of the
Cambro-Ordovician shales is parallel or subparallel with the bedding
(Pitt, 1955). The small folds around the central core are overturned
southward and slaty cleavage has developed which dips generally steeply
north.

The position of the Ouachita front under the Cretaceous and Tertiary
cover is recognized on the basis of metamorphism and high dips in
contrast to the lack of metamorphism and very low dips of the beds
of the foreland. See Fig. 14.6. The siliceous nature of the Devonian to
Cambrian rocks of the Ouachitas is an additional guide.

Structural Problems

The Geological Map of Oklahoma (Miser, 1954) shows the Hendricks
version of the multiple thrust structure as well as two windows, the

OUACHITA, MARATHON, AND COAHUILA SYSTEMS

22')

Fig. 14.6. Cross section of Ouachita Mountains in Arkansas. After cross section on Geo/ogic Mop
of Arkansas, 1929. Gc, Collier shale; Owe, Womble shale, Blakely sandstone, Mazarn shale, and

Crystal Mountain sandstone; DSO, Arkansas novaculite, etc; Cs, Stanley shale; Cj, Jackfork sand-
stone; Ca, Atoka formation; Csh, Savanna, Paris, Fort Smith, Spadra, and Hartshorne formations.

Potato Hills and the McCurtain County core area (also called the
Choctaw anticlinorium ) . These have been reproduced in Fig. 14.2.
Hendricks' synthesis of the thrust structure involves translation of rocks
considerable distances, a seeming requisite of the Ouachita overthrusting
of the Arbuckles. See Figs. 14.1 and 14.6. Hendricks postulates that a
deep-seated thrust plane exists, the Powers, along which rocks of
"Arbuckle facies" were thrust southeastward, and then, slightly later, the
strata involving the thick Carboniferous clastic sequences were thrust
northward to rest as allochthonous sheets on a foreign ( Arbuckle) founda-
tion.

The Tectonic Map of Oklahoma (Arbenz, 1956) shows the thrust
complex of the Geologic Map including Potato Hills window, but not
the core window. The core area was remapped and reported on by Pitt
in 1955, and he concluded that a normal sequence of formations exists
on and around a rather simple dome — that no klippe is indicated; the
previous need for a fault was due to erroneous reading of bedding and
an inadequate understanding of the stratigraphic succession.

In 1957 Misch and Oles took issue with Hendricks on the basis of their
own detailed mapping of the Ouachitas. They concur with Pitt on the
structure of the "core" and also recognize no window in the Potato
Hills. They conclude that Potato Hills is an anticlinorium of closely
spaced, steep, and partly overturned folds.

The overturning is both to north and, against the direction of the supposed
overthrusting, to south. Some overturned anticlinal limbs have ruptured, and
steep reverse faults have developed. Some of these faults yield to the north;
others yield to the south. All of these reverse faults die out along the strike,
generally in the steep limbs of anticlines.

The Arkansas anticlinorium displays the same fold pattern as that seen in

the Potato Hills. Steep northward and southward overturning of folds are about
equal. The greatest stratigraphic and structural depth is exposed in the core
of the western part of the anticlinorium (south of Mt. Ida), and there is the
same continuous change in tectonic style as that found in the core of the
Choctaw anticlinorium.

Misch and Oles contend that the mapped overthrusts, both major and
minor, are partly steep reverse faults and partly no faults at all. The large
exotic boulders of Arbuckle rocks in the Johns Valley shale are considered
evidence of thrusting by Hendricks, but Misch and Oles believe they are
of "deposition origin" — apparently not associated with an advancing
thrust front.

Misch and Oles also believe that the differences between the "Ouachita
facies" and the "Arbuckle facies" have been overemphasized.

Some units are indentical, as for example, the upper Arkansas novaculite of
the Ouachitas and the Woodford chert of the Arbuckle region. Others differ
relatively litde, as the Bigfork "chert" and the major part of the Viola lime-
stone, or the Stanley shale and the Caney shale. Others differ more strongly,
as the Ouachita Mountains correlatives of the Simpson group. And some units
differ very strongly, as the Missouri Mountain shale and the lower Hunton
limestone. However, contrasted facies are not disconnected as the hypothesis
of overthrusting requires. Most of the contrasted facies have transitional re-
lationships. Some of the transitions are very gradual; others are pronounced
and also have been accentuated by the intense shortening resulting from folding
and faulting. None of these changes, however, exceeds those often encountered
in adjacent and connecting basins, or different parts of the same basin. More-
over, the fact is often overlooked that there are marked facies changes within
the Arbuckle region itself, as well as within the Ouachita Mountains.

For a review of the problems in the Ouachita Mountains see Tomlin-
son (1959).

230

STRUCTURAL GEOLOGY OF NORTH AMERICA

Phases of Ouachita System

Early Mississippian Phase. Elevations precursory to the late Paleozoic
orogeny seem to be indicated by an unconformity between the Arkansas
novaculite (Devonian) and the Upper Mississippian elastics (Chaney
shale). Chert conglomerates rest on the novaculite in the Potato Hills
section of the Ouachitas and they are found at the base of the Stanley
shale (Lower Pennsylvanian) in southern outcrops. In addition to this
suggested late Mississippian disturbance, the rise in the foreland of the
Ellis-Chautauqua-Ozark arch in late Devonian time may be mentioned.

Late Mississippian Phase. The deposition of more than 17,000 feet of
clastic sediments of the Stanley, Jackfork, and Johns Valley formations
all within a very short time indicates a great and sudden uplift nearby,
which undoubtedly was one of active orogeny because a sedimentary
mass of the character and quantity noted requires actively rising moun-
tain chains. The elastics were deposited in a foredeep.

Whereas van der Gracht and others before him postulated the orogeny
in the hinterland to the south, Hendricks (1943) believes that early
thrust sheets came from the north and pushed southward to form a land-
mass. The Stanley, Jackfork, and Johns Valley shales were deposited
in a basin to the south of this landmass, and the thrusting culminated in
Johns Valley time. The Atoka sediments were then spread thickly over the
sites of both facies. Van der Gracht believes the Atoka came from a
southern highland; Hendricks does not comment on the source. The Atoka
sediments reflect the second pulsation this time in the Early Pennsyl-
vanian.

In eastern Texas, a foreland basin to the southward-trending chains
of the hinterland came into existence, and in the basin the Strawn and
Millsap formations were deposited, having been derived from an eastern
source.

Mid-Pennsylvanian (?) Phase. The age of the major deformation of
the Ouachitas is believed by several authors to have occurred in post-
Atoka and pre-Boggy time. According to Fitts (1950);

The unconformity at the base of the Boggy formation is the largest within
the Pennsylvanian of Oklahoma and is probably the most widespread. Along the
line of outcrop, it is progressively underlain by Pennsylvanian beds from

Savanna to Atoka, locally in the Tri-State area upon Mississippian and in wes-
tern areas of Oklahoma all formations down to granite.

The top of the Boggy is marked by another unconformity, this one of more
importance locally and to the westward in the Seminole region. The section of
beds above this unconformity is generally devoid of any angular discordance
and for the first time can be seen a relationship which will persist through the
rest of the Pennsylvanian and lower Permian; i.e., predominandy limestone in
the north grading to shales and elastics in the central to coarser elastics and red
beds as the Arbuckle Mountains are approached.

The deformation of the Arbuckles in the Mid-Pennsylvanian influenced
the development of the red-bed facies in the upper Cisco and Lower
Permian, but later in Permian time much clastic material in the Wichita
system came from an eastern source (Cheney, 1929).

Drilling operations have penetrated a formation, the Morehouse,
under the coastal plain sediments, in northern Louisiana, which contains
"late Paleozoic fossils" (Imlay and Williams, 1942). Its areal relations
have been worked out for a limited distance in southern Arkansas and
also, to some extent, its stratigraphic relations (Philpott and Hazzard,
1949; Fisher et al, 1949). See Fig. 14.6. It occurs above the Eagle Mills
formation and below the Louann salt and Werner formation. (Philpott
and Hazzard, 1949). According to the usage of Imlay and Williams, the
Louann sail and Werner formation make up the Eagle Mills. At any
rate, the Eagle Mills seems to overlie the folded Ouachita facies uncon-
formably, and if such is the case, the Ouachita thrusting predates the
Eagle Mills and Morehouse. When their age eventually is fixed, the age
of the Ouachita thrusting possibly will be fixed more definitely than is
now possible.

Connection of Ouachitas and Appalachians

Spatial Relations. The relation of the Ouachita system to the Appa-
lachian is hidden by the Cretaceous and Tertiary rocks of the Mississippi
embayment, but they have been traced by deep wells to within 60 miles
of each other. See map, Fig. 14.7. Both are strongly folded and faulted,
and in both there has been thrusting toward the central stable region of
the continent. In both areas there is a thick development of Early Pennsyl-

OUACHITA, MARATHON, AND COAHUILA SYSTEMS

231

Fig. 14.7. Relation of Ouachita Mountains to southern Appalachians under the Coastal Plain
cover. The pre-Upper Cretaceous geology of Arkansas and Louisiana is by Fisher, Kirkland, and
Burroughs (1949). Fredericksburg, Paluxy, Mooringsport, Ferry Lake anhydrite, Lower Glen
Rose, and Hosston formations are Lower Cretaceous; the Smackover and Cotton Valley are
Upper Jurassic; the Eagle Mills is possibly Lower Jurassic (King, 1950a) or Permian (Philpott and
Hazzard, 1949).

vanian clastic rocks derived from the hinterlands. See paleotectonic
maps, Plates 5 and 6.

According to King ( 1950a ) :

The sequence of Paleozoic deposits in the Ouachita Mountains resembles
that in the Valley and Ridge province in that it is composite, the older part
indicating quiet deposition, and the younger part deposition during a time of
considerable crustal mobility. It differs in that the boundary between the older
and younger parts is post-Devonian rather than Middle Ordovician as in the
Valley and Ridge province, so that there is no representation of the Taconian
orogeny. Moreover, the deposits of the older part are black graptolite shales,
bedded cherts, novaculites, and fine sandstones, rather than carbonates, and
hence are of "eugeosynclinal" facies, as contrasted with the "miogeosynclinal"
facies of the Valley and Ridge province to the east, and of the Arbuckle and
Wichita Mountains farther west in Oklahoma. Deposits of the younger part,
laid down under conditions of greater crustal mobility, are of early Pennsyl-
vanian (Springer) age, and probably formed in response to the Wichita period
of orogeny. They are similar to the thick late Mississippian and early Pennsyl-
vanian deposits of the Valley and Ridge province in Alabama. The deposits of

the Ouachita geosynclinal were remarkably persistent in character, for nearly
the same units are present in the extension of the system in the Marathon
region, Texas, many hundreds of miles to the southwest.

The Appalachian folds have been traced as far southwest as Marengo Counts .
Alabama, on line of strike from the exposed structures of the Valley and Ridge
province in the Birmingham district, where wells have encountered Ordovician
limestones and dolomites directly beneath the Mesozoic.

The Ouachita folds have been traced southeastward from their outcrops in
the Ouachita Mountains, across the Mississippi Embayment and into central
Mississippi. Here, the boundary between Paleozoic rocks of Ouachita facies and
the foreland rock trends southeastward. That this is likewise the strike of the
folding is suggested by the fact that folds in the adjacent Black Warrior Basin
trend southeast. In Newton and Neshoba Counties, Mississippi, near the bound-
ary between the Ouachita area and the foreland, wells have encountered Ordo-
vician limestones and dolomites below the Mesozoic. These are of Appalachian
or Arbuckle facies, rather than Ouachita facies, which indicates the existence
of an intermediate slice between the Ouachita folds and the foreland.

The Appalachian and Ouachita systems have thus been traced by drilling to
within about 60 miles of each other, and they seem to be approaching at an
acute angle. Southward, they pass beneath the thick Jurassic and Lower Cre-
taceous deposits of the Gulf Coastal Plain, so that their point of junction is
beyond the reach of the drill.

Connection of Ouachitas and Marathons

The Ouachita thrust sheets not only overlie the east end of the Wichita
system, but continue southward under the Cretaceous rocks of the Gulf
Coastal Plain. If not the thrust sheets, the deformed strata of the orogenic
belt wrap around the Llano uplift of Texas and connect with the Mara-
thon Mountains to the west. Miser and Sellards (1931) have traced the
Ouachita front under the Cretaceous strata by means of well records south
to the Llano uplift, and Sellards ( 1931 ) has traced the geosynclinal rocks
westward from the uplift to the Marathon exposures, also by means of
well records. Flawn (personal communication and 1956) more recently
has mapped this front in considerable detail.

MARATHON SYSTEM

Location and Principal Structures

Paleozoic formations appear in the Marathon basin of trans-Pecos
Texas, and there reveal another great orogenic system. The Marathon

232

STRUCTURAL GEOLOGY OF NORTH AMERICA

I 1 oermun 1---; -I :. 7 ";j cretaceous \lll^ TvoL«mcs [

*•* TERTIARY INTRUSIVES »""■

PRE-PERMIAN PALEOZOIC STRATA
FOLDED AND FAULTED

PENNSYLVANIAN THRUSTS

CONTOURS BASE COMANCHE SER.
? MILES

Fig. 14.8. Structure map of the Marathon uplift. After King, 1937. Black Peak thrust is post-
Cretaceous. A number of Cretaceous outliers in the Paleozoic area not shown.

region lies on the edge of the Mexican highlands physiographic province
which merges with the Great Plains on the east. Structurally, the region
is a broad dome of Cretaceous rocks, from whose central part the Creta-
ceous cover has been stripped away, leaving an area of low country in
the center, the Marathon basin. See Figs. 14.1 and 14.8 and cross-section
M-M'-M", Fig. 14.9. Here, strongly folded Paleozoic rocks are exposed.
The Paleozoic rocks in the basin, and in the Glass Mountains which flank
it on the northwest, have a thickness of 21,000 feet. The greater part of
them was laid down in a subsiding trough commonly referred to as the
Llanorian geosyncline. The oldest rocks are Upper Cambrian sandstones
and shales, whose base is not exposed. Overlying them are 2000 feet of
Ordovician rocks composed of shaly limestone and shale, with some beds
of chert. The Ordovician is overlain by the Caballos novaculite, possibly
of Devonian age, which reaches 600 feet in thickness. The Caballos no-
vaculite is over-lain by a great series of clastic rocks of Pennsylvanian age,
as much as 12,000 feet thick in the southeastern part of the area but
much thinner in the northwest.

Llanoria and the Llanorian Geosyncline

The belt of folded sedimentary rocks of the Ouachita Mountains ex-
tends around the Llano uplift to the Marathon region and thence south-
westward across the Solitario near the Rio Grande and on into Mexico.
See Plate 8. The early Paleozoic trough lay about 100 miles north of the
present mountains (Rarton, 1945). See Fig. 14.10. In Permian time, a
trough of geosynclinal proportions existed in Coahuila, 200 miles south of
the Solitario.

Pre-Carboniferous sediments of the Marathon and Solitario uplifts,
like those of the Ouachitas, are rather thin and include much clastic ma-
terial. They are composed of sandstones, conglomerates, boulder beds of
debated origin, and impure limestone with much shale, chert, and, con-
spicuously, novaculite. Some of the sediments evidently accumulated at
no great distance from shore; others such as the shales may have been
carried much farther away from their source. In the foreland areas of
both the Ouachitas and Marathons, the sediments are mostly limestones.
It is generally concluded that the early Paleozoic sediments came from a

OUACHITA, MARATHON, AND COAHUILA SYSTEMS

rcd,-? ct j

GLASS MOUNTAINS i
I
I

MARATHON ANTICLINORIUM

DUOOU

T CREEK THRuTF g^-Tgfg ..€.0^1

233

J^' DAGGER FLAT ANTICLINORIUM

ARDEN DRAW
vx\ '^> \\. THRUST v

Ct Dc r ->\ \ jQ\ ^ Cd

HELL'S

^ V.

€d-J

HALF ACRE V ^\\ ^ J Kf
>T * x> "^i

TRES HERMANAS MTN.
Kt

Vet'

—I

HORIZONTAL AND VERTICAL SCALE IN MILES

| Fig. 14.9. Cross section of Marathon uplift and Permian basin. Taken from King (1937, PI. 23,

1 section B-B'). Cd, Dogger Flat sandstone (Cambrian); O, Maravillas chert, Woods Hollow shale,

Fort Pena formation, Alsate shale, and Marathon limestone (Ordovician); De, Caballos novaculite

(Devonian ?); Ct, Tesnus formation, Cd, Dimple limestone, Ch, Hamond formation, and Cg,

landmass to the south or southeast and accumulated in a sea whose shore

lines moved back and forth, but the propriety of calling the basin of depo-

I sition of that time a geosyncline with only 1500 to 3100 feet of sediment

i has been questioned (Sellards and Baker, 1934). Deep wells have enabled

Barton ( 1945) to diagram the extent of the pre-Pennsylvanian deposits

Gaptank formation (Pennsylvanian); Cwc, Wolfcamp formation, CI, Leonard formation, Cw,
Wood formation, and Cc, Capitan limestone (Permian); lb, Besset conglomerate (Triassic ?); Kt,
Trinity group, and Kf, Fredericksburg group (Lower Cretaceous).

with more detail than heretofore. See Fig. 14.10. He shows that the axis
of the basin was considerably north of the later Pennsylvanian, and also
that the basin was too shallow to deserve the name geosyncline.

In both of the regions, the deposits of Carboniferous time attained a
great thickness, possibly over 20,000 feet in the Ouacbitas and 12,000 or

App. loo Miles ■

SITE OF LATER
'MARATHON MOUNTAINS

LL ANOBIA

M/ss/ss ippign hj

Woodford sh.

Tpyrpyr_..

\ AA /_\ A A S^STT^Ty^^'^6
a A a a A a A a a A aaa/

\aaaaa/\aaaaaaa"aa~aaaa/ \ a/ "\~a~a~7 \A7 \~7 <7 \~/\~ www \";\"/\"a"/\"/ "/ \~V\7 \i \i \7 v> ~i \/~/\/\/\/ \i \7 \~> \7 \> \7 \7 \~ \7 \7 v/ \~> \~ \~ \~ \~ \

A A A A A /_\ A AM /

v A A A '_> A A <_> <_y_>

A A A A A A A A A /
\ A A A A A A A A A
A A A A / \ A A AA /
\ A A A AA A A A A
A A A. A A A A A A /
\ /_\ A A A A A A A A
/ \ A A / \ A A A A A /
\ A A A A A A A A A
A A/ \A A A Aa A/

Fig. 14.10. Pre-Pennsylvanian basin of deposition in the region of the Marathon Mountains,
after Barton, 1945. The section extends approximately north-south through the Marathons and

into the Delaware basin, and restores the strata diagrammatically to their pre-Pennsylvanian
condition.

234

STRUCTURAL GEOLOGY OF NORTH AMERICA

more feet in the Marathons. The trough in which these Carboniferous
sediments accumulated appears to have extended uninterrupted from the
Ouachita to the Marathon and Solitario regions. This Pennsylvanian
trough is referred to as the Llanorian geosyncline. The Fort Worth
( Strawn ) and Kerr basins seem to be expansions of the geosyncline over
the margin of the foreland.

The land area of Llanoria, southeast of the Llanorian geosyncline, ap-
pears to have been composed largely of crystalline rocks and probably
stood as a highland or mountain area during a large part of Paleozoic
time. For the most part, the former highland is now buried beneath
Cretaceous and younger strata, and the hypothesis of its former existence
is based largely on evidence supplied by the composition of the Paleozoic
sediments in the geosyncline (Miser, 1929; King, 1937).

Both Pennsylvanian clastic and Devonian cherty formations thicken
southeastward across the Llanorian geosyncline in the Marathons; lime-
stones are replaced by shales or cherts; and the clastic deposits contain
grains of schistose or granitic rocks, pebbles of vein quartz, and cobbles
of igneous rocks. The distance south at which the land lay during Paleo-
zoic time is unknown, but it may have been 100 or more miles away.
Examine Fig. 14.10.

Phases of Marathon System

Early Pennsylvanian Phase. The lowest of the Pennsylvanian forma-
tions, the Tesnus, was deposited in the Llanorian geosyncline, probably in
early Pennsylvanian time (King, 1937). It is a great mass of inter-
bedded sandstone and shale in thin and thick beds, nearly barren of fos-
sils. In the southeastern part of the basin it exceeds 6500 feet in thickness,
and it is predominantly sandstone with many arkose layers and several
prominent massive layers of white quartzite. In the northwestern part of
the basin, it is about 300 feet thick and is nearly all black shale with a few
sandstone beds. The Tesnus, the Dimple limestone, and the lower part
of the Raymond formation make up the flysch facies — a European term
to signify sediments deposited during the time of a rising hinterland and a
sinking geosyncline. The Dimple limestone is over 1000 feet thick in the
Marathon basin, and thins southward. The Ilaymond formation is a mass
of arkosic sandstones and shales 3000 feet thick.

Overthrusting in the southern part of the Marathon area began at this
stage, as is suggested by a remarkable layer of mudstone in the upper
part of the Haymond, in which are embedded large blocks of older rocks.
The blocks are believed to have been derived from the erosion of ad-
vancing thrust sheets and to mark the first strong compression in the
region (King, 1937).

Late Pennsylvanian Phase. The uppermost Pennsylvanian formation,
the Gaptank (Upper Pennsylvanian in age), consists of conglomerate and
sandstone derived from the erosion of rising folds. The strong deformation
to which the Paleozoic rocks of the Marathon basin have been subjected
apparently culminated after the deposition of this Upper Pennsylvanian
formation. The Permian rocks of the Glass Mountains to the northwest
rest, at least in places, with great angular unconformity on the disturbed
older beds. See section M-M'-M", Fig. 14.9. The structural features con-
sist of close folds that trend northeast and are overturned to the north-
west, and several thrust faults. The faulting culminated on the northwest
in the nearly flat-lying Dugout overthrust, with a known displacement of
more than six miles. Farther southeast the other thrusts have miles of dis-
placement and some are folded and therefore older than the frontal fault
(P. B. King, 1937).

COAHUILA SYSTEM

Known Geologic History

Exposures of Late Pennsylvanian (?) and Permian rock in the south-
western part of the Mexican state of Coahuila, some 250 miles south of the
Marathon region of Texas, are believed to reveal a continuation of the
Llanorian geosyncline and the approximate position of the west margin of
Llanoria. In the Acatita-Las Delicias area, according to Kelly (1936) and
R. E. King et al. (1944), a series of sediments and interstratified igneous
rocks over 10,000 feet thick was deposited in a subsiding trough. The
sediments came from the landmass of Llanoria, and the lava flows, sills,
fragmental igneous material, and graywacke came from the west. The
volcanics are rhyolite, andesite, and basalt flows and tuffs.

Late Pennsylvanian ( ? ) limestones, possibly in part of reef origin, were
deposited simultaneously with products of volcanic activity. Coarse

OUACHITA, MARATHON, AND COAHUILA SYSTEMS

235

W-E SECTION PASSING 2 KM. NORTH OF LA DIFUNTA
K

POST-PERMIAN PRE-CRETA.
GRANITE INTRUSION

CRETACEOUS

POST- PERMIAN ' PRE-'WeTACEOUS GRANITE INTRUSION

WSW-ENE SECTION PASSING I KM. NORTH OF THE NORIA DE MALASCACHAS

MILES

! Fig. 14.11. Cross sections near Las Delicias, Coahuila, Mexico. The Permian strata consist of
1 interbedded conglomerate, graywacke, sandstone, shale, limestone, and intermediate and basic

, detritus from these and older rocks accumulated in the western part of

1 the area either contemporaneously with the reefs or as a clastic wedge

i on the flank of an early Permian uplift. The coarseness and unsorted char-

j acter of the boulder conglomerates indicate that the boulders must have

] been transported by unusual processes. During the remainder of Permian

time, the geosyncline received deposits of clay from Llanoria on the east,

flows of lava from fissures in the basin to the west, and volcanic detritus

derived from the reworking of pyroclastic deposits and possibly by action

of waves on the lava flows.

lavas. The graywacke and lava make up about 60 percent of the sequence. After R. E. King
ef al., 1944.

At some time beween Late Permian and Late Jurassic, the Pennsvl-
vanian (?) and Permian rocks were intensely folded and overthrust. See
cross section, Fig. 14.11. If the deformation took place in Late Permian
time, it was the last phase of orogeny affecting the sediments of the Paleo-
zoic geosyncline. Possibly it occurred in Early Jurassic or Triassic time, but
not as late as the Nevadan disturbance (Late Jurassic), because the
Upper Jurassic Oxfordian sediments rest unconformably upon the trun-
cated Permian. The geosyncline is shown as deformed in Late Permian
time on the tectonic map of Plate 8. See Chapter 17 and Fig. 17.9.

236

STRUCTURAL GEOLOGY OF NORTH AMERICA

The folded rocks were intruded by batholiths of granite and grano-
diorite before Oxfordian time.

Structural Trends

The dominant strike of the beds in the northern part of the main Per-
mian area is N. 35° to N. 50° E. The strike of secondary cleavage is
N. 75° E. and may indicate the trend of the axis of the folds. In the south-
ern part of the Permian area the strike of the beds swings sharply to
S. 40° E. R. E. King et al. (1944) suggest that this may mean that the
Las Delicias area is a salient part of a mountain arc in the Paleozoic struc-
ture which possibly controlled the outline of the Coahuila peninsula of
Upper Jurassic and Lower Cretaceous time. Post-Cretaceous folds in con-
tinuation of this S. 40° E. trend may have been controlled by Permian
folds, and thus indicate the trend of the older structures. King's sug-
gestion is illustrated on the tectonic map.

Relation to Marathon System

On previous pages it has been explained that the folding and thrusting

in the Marathons reached a climax in late Pennsylvanian time. See Plate 7.
Thereafter the compressed structures were deformed only by epeirogenic
uplift. The Pennsylvanian and older rocks were deeply eroded, and even-
tually Permian deposits overlapped them progressively southward. The
tectonic map of the Permian shows the previously deformed belt as one
of epeirogenic uplift. The Permian Delaware and Marfa basins were con-
tinuous with the Coahuila Permian basin; but while saline residues were
being deposited in the northern basins, waters of normal salinity persisted
in the south basin and probably replenished the evaporating waters to the
north. After the Permian deposition in both the north and south basins,
the folding and intrusions of the Coahuila area occurred.

The Coahuila structures have commonly been tied to the Marathon
belt, which lies 250 miles to the north. The Permian volcanics and the
post-folding granitic intrusions present characteristics foreign to the
Ouachitas and Marathons in late Paleozoic time, and the writer is inclined
to favor a connection with the early Nevadan belt of western Nevada and
California, where the same characteristics hold. This correlation, however,
presents problems in working out logical map relations.

15.

WICHITA AND ANCESTRAL

ROCKIES SYSTEMS

AND THE TEXAS FORELAND

WICHITA SYSTEM

i
Ranges and Basins of the System

Wichita Mountains. The Wichita Mountains in southwestern Okla-
homa rise 1100 feet above the plains and 2480 feet above sea level. The
hills are chiefly granite surrounded and embayed with nearly horizontal
Permian strata. Outcrops of Arbuckle limestone of the same facies as in
the Arbuckle Mountains are numerous, especially along the north side;
and others on the soutii side and within the hills indicate that three en

Fig. 15.1. Configuration of the Precambrian surface in Oklahoma and Texas. After Flawn 1956
and others. Numbers on contours are in thousands of feet below sea level.

echelon anticlines are present, with granite in the cores, Arbuckle lime-
stone on the flanks, and both overlapped unconformably by the Permian
strata. The relief of the buried Precambrian surface is shown in Fig. 15.1.
and a cross section at the eastern end of tire range reveals the structure
( Fig. 15.2 ) . Intricate folding is described by Hayes ( 1952 ) .

Amarillo Range. The Amarillo Range in the Texas Panhandle is a
series of buried hills without surface expression, and is cored by Pre-
cambrian crystalline rocks. The buried hills are known to extend 125

237

238

WICHITA

&*^<f\i W ~i \i w \i \i w ~i w w w w w W ~i \< ~i \i \i\i \i\i \ i\ / w/T i \i w w \"
_\ /W\7\7 \"/ \i\l\l~t\i ~>\ PRE- CAMBRIAN/) ~y N~ ~, v") x~ /> Q s~y s"/// WW WW

7 w w \7 w w \-/ w ~/ w w w w w \~/ w w "/ w CV w w w w \~/ C7 w w w /" / w w w w \

\/\'\/\/s / w w "' w w wO w w w w w w www \-/ \-/_x_/ V-/_N_/ v_/ v,_'l/ ' "O"7 x~' x_/ X"Z?

/ w \ if W W \ I WW W W W W W W W W W W WWW W W \ /WWW W W w \ / W W \ I \
\l\ l\ '_~s W W W W \t W w w w W W W W W W W W W s I W W W W W W W W \-/ ~l \l

I \i\ I \ I \i\ 1^ l\ l\i w W W W W W W W W w W W \~/ "/ ~/ W ~' w w w W_\_/ W ~l w \"
W W \V W W W W W W W W \7 W \7 W \ /_~'_W W W w W W W W W W WWW W W \_/ w

/ \ / \ / \ / \ / \ I ~i \ I s / \ / \ / \-/ \ / \ / \ / \ / \ / \ / W w w W W W \ / ~l w w \~/ \-/ w W W w ^

STRUCTURAL GEOLOGY OF NORTH AMERICA

MOUNTAINS
-Cp.

ANADARKO BASIN

HORIZONTAL AND VERTICAL SCALE

J MILES

Fig. 15.2. Section through Wichita Mountains and Anadarko basin. Compiled from Taff (1904) and Millison
and Reed (1939). Os, Simpson group; Dv, Viola limestone; Osh, Silvan and Hunton formations; Mw, Wood-
ford formation.

AMARILLO DISTRICT

ANADARKO BASIN

BUSH DOME

JOHN RAY DOME

MILES

LEILA DOMfr

HORIZONTAL SCALE

Fig. 15.3. Sections J-J' and H-H' of Fig. 14.1 across the buried Amarillo Range. Taken from Cotner and
Crum, 1933.

miles east-west across the Panhandle. See Fig. 15.1. The highest peaks
reached by the drill are about 1300 feet above sea level and 2000 feet
below the surface. Some of the granite peaks are overlain directly by
the Permian beds, but others are covered with the Pennsylvanian. See
cross sections, Fig. 15.3. En echelon faults bound some hills and help
produce ridges in en echelon arrangement. The Armillo Range and the
Wichitas are continuous as shown in Fig. 15.4.

Las Animas Arch. The Amarillo Range probably extended to south-
eastern Colorado and northeastern New Mexico, where it joined other
ranges and an arch known as the Las Animas (Maher, 1946). See the
tectonic map of the Early Pennsylvanian and Fig. 15.5. The Precambrian
rocks may have been exposed above sea level in Early and Mid-Pennsyl-
vanian time along the Las Animas arch, but the thinning of the Pennsyl-
vanian and Permian strata over the arch is chiefly due to subsidence of

WICHITA AND ANCESTRAL ROCKIES SYSTEMS AND THE TEXAS FORELAND

239

the crust on either side at a more rapid rate than the arch itself. A
structural relief of 3000 to 4000 feet appears to have formed during
these times. Still further arching occurred in post-Paleozoic time, accentu-
ating the structural relief.

Muenster anticline. The Muenster anticline or arch is the south-
eastern end of the Amarillo Wichita uplift. See Fig. 15.4. Like the
Amarillo Range it is completely buried and was rangelike at the time
of uplift, during the Pennsylvanian. Altogether the Amarillo-Wichita-
Muenster alignment makes up an uplift with a Precambrian core and
flanking truncated Lower and Middle Paleozoic strata 350 miles long.

Criner Hills. The Criner Hills are a complexly faulted horst con-
sisting largely of Arbuckle limestone which is exposed at the surface
and is flanked by Pennsylvanian and Permian strata. The horst is the east
end of an anticline off the Amarillo-Wichita uplift. See Figs. 15.4 and
' 15.6.

Matador Arch. The Matador arch as here defined is made up of a

narrow series of east-west-trending buried granite hills which extend

from the New Mexico line across the Llano Estacado to Wichita Falls

and beyond, a length of some 300 miles. If the overlying Cretaceous

and late Paleozoic deposits were removed, the uplift would be found

j to consist of scattered peaks rising above an upland. Strong faults and

folds trend obliquely across the uplift in a northwest direction, and these

; have produced an en echelon character to the topography (the buried

J peaks) and to the "highs" of the overlying formations. The Upper

! Pennsylvanian rests directly on the Precambrian in some localities.

Parts or all of the Matador arch have variously been called the Red

River uplift, the Electra arch, and the Matador arch. The term Matador

arch appears to be gaining general acceptance. The string of small

uplifts produced islands in the Pennsylvanian seas and because of this

j the feature has also been called the Matador archipelago.

Palo Duro and Hardeman Basins. The general depression between

j the Amarillo-Wichita uplift and the Matador arch is filled with Pennsyl-

! vanian and Permian sediments, and has a western and an eastern divi-

j sion, as may be seen on Fig. 15.4. The western is the Palo Duro basin and

the eastern the Hardeman basin. Various names have been used for the

Fig. 15.4. Generalized paleogeologic map, Texas and Oklahoma, of pre-Pennsylvanian rocks.
Black is sub-Pennsylvanian and Permian outcrop of Cambrian, Ordovician, Silurian, ond De-
vonian formation. Hachured area is Precambrian. Mississippian outcrops not shown. After Totten
(1956), Flawn (1956), and others. The Pennsylvanian and Permian cover has been eroded away
in places in the Wichita and Arbuckle and in the Llano uplift. Doming of the Marathon uplift
is post-Cretaceous. H.A., Hunton arch; A. A., Arbuckle anticline; C.H., Criner Hills anticline; M.A.,
Muenster anticline; Cent. Bas. Pf., Central Basin platform; O.C.A., Oklahoma City anticline.

features of this region as drilling has progressed and the geology become
better understood.

Arbuckle Mountains. Topographically the Arbuckle Mountains are
the hills between Davis and Ardmore, and are the surface expression of

INDEX MAP SHOWING LINE OF SECTION

30 M.les

Horizontal scale
SEA LEVEL DATUM

GROUPS AND FORMATIONS

Nippewalla group

Sumner, Chase, and
Council Grove groups

Admire shale

Wabaunsee group

Shawnee, Douglas and
Pedee groups

Lansing, Kansas City,
and Bronson groups

Marmaton group and
Cherokee shale

Ste. Genevieve Is.

Louis Is.

Spergen ond Worsow Iss

Keokuk and Burlington ls».

Gilmore City Is.

Misener sd."

Simpson (T) oroup

Arbuckle Is.

GUADALUPIAN

LEONARDIAN

and
WOLFCAMPIAN

DESMOINESIAN

MERAMECIAN

KINDERHOOKIAN

BEDS OF TRENTON AGE

BEDS OF BLACK RIVER
AND CHAZYAN AGE

BEDS OF BEEKMANTOWN-
IAN AND ST. CROIXIAN
AGE

as
a:<

Fig. 15.5. Correlation of Paleozoic rocks across the Las Animas arch in Baca, Las Animas, and Otero
Counties, Colorado. Stratigraphic classification on right is mainly after State Geological Survey of Kansas;
that on left follows common usage in Colorado. From Maher, 1946.

WICHITA AND ANCESTRAL ROCKIES SYSTEMS AND THE TEXAS FORELAND

241

Cp Pontotoc for. (Permian)

Ch Hoxbor group (Missouri •* Virgil)

Cd Dcese group ( Des Moines)

Cdh Dornick Hills group (U. Morrows Lamp.)

CS Springer group ( L. Morrow)

|>|w Woodford (Mississippion)

Mc Coney shale (Mississippion)

Osh Sylvan and hunton'

Ov Viola limestone JOrdovician

03 Simpson group

OCo Arbuckle limestone (Ord i-Comh)

Cr Reagon sandstone (Cambnon)

p-G Pre -Cambrian crystallines

/ w w w \7 w

Fig. 15.6. Cross sections through the Ardmore basin, Arbuckle Mountains, and Hunton arch, compiled
from Dott (1934), Tomlinson (1929), and Moore ei al. (1944).

a large, complex anticline. In the core of the anticline, two prominent
peaks of Precambrian porphyry ( the Timbered Hills) rise 700 feet above
the valley of the Washita River and 1400 feet above sea level. Geologically
the term Arbuckle Mountians applies also to the hilly area to the north
and east in which lower Paleozoic rocks crop out and where structural
features of mountain proportions are located. A thick sequence of rocks
from Precambrian to Late Pennsylvanian is exposed in the range. See
cross sections, Fig. 15.6.

The regional structure of the Arbuckle Mountains is a series of much-
faulted subparallel folds trending northwest and southeast. They are
shown on the map of Fig. 14.2, where it will be seen from north to south

the several divisions are as follows; Lawrence uplift, Franks graben,
Hunton anticline, Mill Creek syncline, and Tishomingo anticline. On
Fig. 15.4 the Hunton anticline, Franks graben, and Lawrence uplift are
combined under the general term, Hunton arch. The Arbuckle anticline
is next south of the Tishomingo anticline but offset to the west. The
Washita syncline and fault zone separate the Tishomingo anticline from
the Arbuckle anticline. The structures are compressional in nature, and
especially in the Arbuckle anticline and south-lving Ardmore basin
thrust faults and tight folds are pictured by Dott ( 1934 ) and confirmed
by Swesnik and Green (1950). The overriding is northward. Study
sections in Fig. 15.6.

242

STRUCTURAL GEOLOGY OF NORTH AMERICA

Ardmore Basin. The Ardmore basin is a folded and faulted basin
between the Arbuckle anticline (Mountains) and Criner Hills. It con-
tains a thick and deeply depressed Pennsylvanian sequence of clastic
sediments, overlying a rather thick Cambro-Ordovician carbonate se-
quence with unconformable relations attesting two principal times of
orogeny. These will be outlined presently.

Over 30,000 feet of Paleozoic sediments are involved, about 13,000 feet
of which are Pennsylvanian and include the Springer, Dornick Hills,
Deese, Hoxbar, and Pontotoc formations, from oldest to youngest. Most
of the pre-Pennsylvanian beds are limestone, and the Pennsylvanian are
mostly sandstone and shale. The Ardmore basin is considered a foredeep
by van der Gracht, north of the Wichita and Criner Hills anticlinorium.
At the time of deposition of the beds, the basin spread over the site of
the present Arbuckle Mountains as well as the present Ardmore syncline,
and extended to the Hunton-Tishomingo landmass (Dott, 1934).

Anadarko Basin. North of the Wichitas is the extensive Anadarko
basin. It occupies the greater part of western Oklahoma. Its axis runs
west-northwest and approximately parallels the Wichita-Amarillo up-
lift. The Permian beds thicken to 4500 feet just 25 miles north of the near-
est granite outcrop. The thickness of the Pennsylvanian in the center of the
basin is unknown but may be rather great, notably in the eastern part,
and may be an extension of the Ardmore basin. Becker (1930) cal-
culates the highest part of the Wichita anticlorinium to have been
elevated structurally about 19,000 feet above the axis of the Anadarko
"foredeep."

The Ardmore trough trends into the Anadarko basin under the blanket
of Permian red-beds and Cretaceous. It is not known how much Pennsyl-
vanian subsidence occurred in the Anadarko basin, but it is clear that
most of the subsidence in the Ardmore basin is Pennsylvanian, and at
least 4500 feet of subsidence in the Anadarko is Permian.

Paleogeology of the Wichita-Ouachita Region

The history of sedimentation in Oklahoma is in two distinct divisions,
both in time and space. An uplift and peninsula through Texas from
Mid-Ordovician to Mid-Mississippian separated the West Texas basin

Fig. 15.7. Mid-Ordovician to Early Mississippian tectonic features of Texas and adjacent areas.
After Adams, 1954.

from the Oklahoma basin. See Fig. 15.7. The Cambro-Ordovician
Arbuckle limestone sea spread across the arch in platform fashion where
about 1000 feet of carbonates accumulated, but in the Oklahoma basin
on the northeast 4000 to 6000 feet accumulated. On top of these
deposits, while the Texas arch was emergent, an additional 3000 feet of
sediments were deposited in Late Ordovician, Silurian, and Early
Devonian time. These were also mostly carbonates. To the north in
Kansas the equivalent strata are only about 1000 feet thick. The region

WICHITA AND ANCESTRAL ROCKIES SYSTEMS AND THE TEXAS FORELAND

243

of subsidence, defined by the pre-Mississippian strata, the Oklahoma
basin, extended west-northwesterly toward the Colorado sag in central
Colorado. The core area of the Ouachitas received about 3000 feet of
sediments during this time, so the axis of the Oklahoma basin appears
to have lain in the northern part of the Ouachitas and under the
Arkansas Valley, and to have extended eastward to a connection with
the Appalachian geosyncline. See Plates 2, 3, and 4.

In Late Mississippian time and during the Pennsylvanian a new
regimen of sedimentation dominated the region, and over the carbonates
and cherts great volumes of shales and sandstones were deposited. The
basin of sharp subsidence and accumulation followed mainly the belt of
later orogeny of the Ouachita system. The site of the present Ouachita
Mountains, the Arkansas Valley and the Wort Worth basin marked the
region of heavy deposition, but a spur of this arcuate basin projected off
to the west through the Ardmore and Anadarko basins where at least
10,000 feet of clastic sediments accumulated. See Plate 8 of the Early
Pennsylvanian. The history of Pennsylvanian sedimentation is complex
because of deformational impulses from time to time and place to place.
These will be discussed under the next heading.

Phases of Orogeny

Late Mississippian Phase. The great flood of Stanley, Jackfork, and
Johns Valley elastics in the Ouachitas and the equivalent Springer group
in the Ardmore basin reflect major uplift and associated deformation.
This was a belt in the hinterland, toward the Gulf of Mexico, most prob-
ably, because there is no plausible source area to the north.

Early Pennsylvanian Phase. The first disturbance within the basin of
accumulation is detected in a post-Springer and pre-Dornick Hills or
Deese unconformity, in the Criner Hills-Ardmore basin area. See Fig.
15.6. This probably marked the beginning of rise of the entire Amarillo-
Wichita element (Swesnik and Green, 1950). See tectonic correlation
chart, Fig. 15.8.

The Ardmore basin then proceeded to sink and received an addi-
tional 17,000 feet of sediments making up the Dornik Hills, Deese, and
Hoxbar groups.

Late Pennsylvanian Phase. By McAlester time (late Lampasas),
the crest of the Hunton-Tishomingo uplift had been eroded to the
Hunton limestone, while the northeastern flank was being submerged by
encroaching seas. Erosion of the crest, due to intermittent uplifts, had
exposed the Viola limestone. Then followed a rather extensive sub-
mergence, and by Missouri time the entire northwest end of the Hunton-
Tishomingo landmass had been covered. The Ardmore basin, received
sediments from the Hunton-Tishomingo and Wichita land areas as well
as the previously elevated Ouachita Mountains. The basin spread over
the site of the present Arbuckle Mountains. A series of rocks was de-
posited in this basin that differs in fades and sequence from the material
that was being laid down simultaneously in the McAlester basin north-
east of the Hunton-Tishomingo land mass. The two basins were probably
never connected.

Late in Hoxbar time, compressive forces from the southwest re-
juvenated the older folds of the Wichita uplift, and the entire element
was moved northward toward the Hunton-Tishomingo buttress. The
Amarillo Mountains were also rejuvenated, and the erosional detritus of
the "granite wash," was formed. The sediments of the Ardmore-Arbuckle
basin were greatly compressed, and the Arbuckle anticline originated.
As the forces continued to move the southern elements northward, the
eastern part of the Wichita system was thrust still farther north, ap-
parently moving as a pivot, with the west end of the Amarillo Mountains
remaining about stationary. The thrusting at the eastern end resulted in
minor folds, first in the Arbuckle anticline and later in the Hunton-
Tishomingo arch. Most of these minor folds became asymmetrical, and
many were finally overturned toward the northeast. In the final stages
of the movement, the major anticlines broke on their overturned axes,
finally developing into thrusts and overriding the adjacent synclines.

Thirteen small erratic masses have been found toward the west
extension of the Mill Creek syncline (Lehman, 1945). The erratics are
remnants of an extensive thrust sheet which overrode at this place the
truncated edges of the Simpson group in post-Hoxbar and pre-Pontotoc
time.

In a detailed study of a small area in the Arbuckle anticline Dun-

Permian

Virgil

Missouri

Des Moines

Atoka

Morrow- Springer

Mississippian

Devonian

Silurian

Ordovician

Cambrian

QUACHITA-
McALESTER
PROVINCE

ARBUCKLE-

HUNTON ARCH

PROVINCE

CRINER.
WICHITA
PROVINCE

ANADARKO-

ARDMORE

BASIN

NORTH & CENTRAL
OKLA. PLATFORM
& NEMAHA RANGE

OZARK
PROVINCE

TENTATIVE CORRELATION OF MAIN TECTONIC MOVEMENTS IN OKLAHOMA

Down warps of
basins and sediment
accumulation

Orogenic pulses

Epeirogenic pulses
with unconformities

Positive behavior

Fig. 15.8. Graphic representation
of phases of deformation in Mid-
Continent region. Reproduced from
Tectonic Map of Oklahoma, 1956.

WICHITA AND ANCESTRAL ROCKIES SYSTEMS AND THE TEXAS FORELAND

245

ham (1955) finds evidence by way of conglomerates, unconformities,
and fault offset of fold axes that the deformation there began in Deese
( Mid-Pennsylvanian) time, culminated in late Pennsylvanian, and con-
tinued on into early Permian by tilting the Lower Pontotoc conglomerate
beds up to 40 degrees.

The Arbuckle anticline was thrust far northeast of its original position
and overrode a considerable distance onto the Hunton-Tishomingo uplift.
The magnitude of the overthrusting decreased a great deal within a short
distance from southeast to northwest where crustal shortening was taken
up mainly by complex folding. It probably follows that the thrust along
its strike continued for a considerable distance southeast under what
is now the Ouachita Mountains. The Tishomingo anticline was shoved
northward in an overthrust second in magnitude only to the one in
the southeast end of the Arbuckle anticline, and overrode the syncline
to the north. The Franks graben, Wapanucka syncline, and other minor
folds and thrusts were formed in the Hunton arch.

Tomlinson ( 1929 ) has estimated the amount of crustal shortening in
the Ardmore basin as 16 miles; and Dott (1934), whose theory of
structural evolution the above summary depicts, suggests in his illustra-
tion (Fig. 15.6) a net shortening at right angles to the trend of the struc-
tures, in late Pennsylvanian time only, of several scores of miles. It is,
therefore, probably incorrect to show the positions of structural elements
as they existed in times past in the places where the features now re-
pose, but so many uncertainties attend the construction of palinspastic
maps (Kay, 1945) in this region that it seems best for present purposes
to crown the elements so as to conform to their present geographic
positions. Such has been done on the tectonic maps of Plates 6, 7, and
8.

During the great late Pennsylvanian phase, marine deposits of Canyon
age and older were highly tilted on the flanks of the Arbuckle Mountains,
and during the following Cisco time erosion removed a sedimentary
mantle probably 3 miles thick, and cut into granite. The granite thus
removed was distributed in beds of Wo If camp age over wide areas. North
central Texas was affected to some extent at this time, as shown by
thinning over the Matador arch.

TEXAS FORELAND

Definition

The Texas Foreland, as here defined is the fairly undeformed portion
of the earth's crust north and west of the Ouachita-Marathon orogenic
belt, south of the Wichita system, and west of the Laramide cordillera.
See Fig. 14.1. It is characterized by broad arches, basins, platforms, and
shelves. It appears to be a small part of the Central Stable Region cut
off by the Wichita system. In reference to the Precambrian rock area
Flawn (1959) has designated large parts of it as the Texas craton. Con-
siderable igneous activity and probably deformation occurred in Pre-
cambrian time, but from the beginning of the Paleozoic era to the
present it has been a fairly stable region with practically no igneous
activity.

For purposes of discussion the Texas Foreland may be considered to
have two divisions, the Central Texas and the West Texas-New Mexico.

Central Texas

Texas Arch. During Cambrian and Early Ordovician time Texas was
mostly a shelf region of carbonate deposition. The carbonate deposit
known as the El Paso limestone in New Mexico, the Ellenburger in West
Texas and the Arbuckle in Oklahoma and adjacent north Texas,
thickens southeasterly from a thin layer on the northwest to a massive
deposit over 2000 feet thick at the edge of what may have been the
continental shelf at the time. The Oklahoma basin lay to the north and
the West Texas basin to the west. In about Mid-Ordovician time a
broad and gently emergent peninsula extending southeastward through
Texas rose (Adams, 1954). See the map of Fig. 15.7 and stratigraphic
column of Fig. 15.9.

The subsurface outcrops as indicated on the map are interpreted to
be depositional edges, with the peninsula, as large as Florida, emergent
throughout the long period of time. The deposits in general gradually
encroached on the peninsula; the Simpson, Viola, Montova. Sylvan, and
Hunter being Upper Ordovician and Silurian, and the Woodford De-
vonian. The lithologies are remarkably similar on either side of the arch.

246

STRUCTURAL GEOLOGY OF NORTH AMERICA

STRATIGRA PHY

UJ
H
(/}

>-
W

=--
<

<
>

-1
>-

H
PL.

SERIES

STAGE

GROUP

FORMATION

MEMBER

CISCO

THRIFTY
GRAHAM

CANYON

CADDO CREEK
BRAD

GRAFORD
WHITT

Home Creek Ls
Colony Creek Sh
Ranger Ls
Placid Sh
Winchell
Cedarton Sh
Adams Branch
Upper Brownwood Sh
Palo Pinto
Keechi Creek
Salesville

Lake Pinto Ss

a
OS

i=
O
—

STRAWN

East Mountain Sh

Capps Ls

LONE CAMP

Garner

Brazos River cong.
Mingus Sh
Thurber Coal

MILLSAP LAKE

Grindstone Creek

Goen Ls
Santo Ss
Buck Creek Ss

Lazy Bend
(Restricted )

Brannon Bridge Ls
Hill Creek

LAMPASAS

KICKAPOO
CREEK

u »

£ o

H fl

J tfc.

O 3

§•§

Rayville
Parks
Caddo Pool

Kickapoo Falls Ls
Dickerson Sh

ATOKA

Smithwick

Lower "Caddo Ls"

Lake Ss pay

a
o

p <-
5 o

co 3
o

Big Saline
Upper Marble Falls

Brister
Lemons Bluff

Gibbons cong.

MORROW

Comyn of Subsurface
Lower Marble Falls

Aylor
Sloan

SPRINGER

Fig. 15.9. Pennsylvanian stratigraphy of the Llano uplift. After Cheney and Goss, 1952.

Concho Arch. In late Mississippian time the orogeny of the hinter-
land of the Ouachitas resulted in the depression and fill of the Fort
Worth and Kerr basins marginal to the later belt of compression. This
resulted in the development of a broad, pronounced asymmetrical arch
involving the previous formations and the top of the Precambrian. The
situation is illustrated in the lower cross section of Fig. 15.10. Subsidence
continued through the Atoka and Kickapoo elastics (subdivisions of the
Lampasas according to Cheney and Goss (1952). The asymmetrical arch
is called the Concho. With the deposition of the thick Permian sedi-
ments of the Midland basin (second cross section, Fig. 15.10) the arch
becomes a very strong and large feature. The present contour of the

Precambrian surface reflects the arch essentially as it was at the close
of Permian time. See Fig. 15.1. It pitches gradually to the north-north-
west and reaches to the Matador arch.

Bend Axis. The Permian and Upper Pennsylvanian beds overlap the
Concho arch from the west in the manner illustrated in Fig. 15.11. As far
as these beds are concerned an axis of down tilting to the west is in-
volved, and this has been called the Bend axis or arch. There may be
an arch in the Upper Pennsylvanian beds but probably not in the
Permian.

Llano Uplift. The southeast end of the Concho arch was so high
that all beds were stripped off down to the Precambrian before the in-
vasion of the Cretaceous seas, which spread a cover of coastal plain
sediments widely over the south and east flanks of the arch. These sedi-
ments have since been mostly removed from the Precambrian and a
domal area known as the Llano uplift results. This is the most prominent
feature evident on the geologic map of Central Texas.

The Pennsylvanian history of the site of the Llano uplift is somewhat
more involved than the cross-sectional representation of the Concho
arch in Fig. 15.10. According to Cheney and Goss (1952):

Mississippian outcrops in the Llano region transgress the truncated Ordo-
vician Ellenburger group. Drilling has shown an increasing loss of section west
of the Llano uplift so that, as a result of both erosion and non-deposition,
Upper Pennsylvanian (Canyon) marine sediments locally overlap Cambrian
rocks in and near northeast Menard County. Farther west and northwest,
Middle Pennsylvanian beds rest on truncated Mississippian and Ordovician
or older rocks in a large region, heretofore called the Concho arch, where
local as well as regional tectonic features had developed mainly along trends
varying from north-northeast to northwest. Thin Middle Pennsylvanian marine
sediments of the Lampasas and Strawn series deposited across this base-levelled
region are chiefly limestones and shales of the platform type in contrast to
thick basinal type deposits on the east and south.

A system of large faults extending northward from the present Llano
uplift into die Fort Worth basin developed during very late Lampasas
time. The faults, well known from surface mapping in the Llano uplift,
have now been followed by geophysical work and drilling for more than
100 miles northward into the Forth Worth basin. Some of the faults have
displaced upper Lampasas and older beds as much as 1100 feet in the

WICHITA AND ANCESTRAL ROCKIES SYSTEMS AND THE TEXAS FORELAND

247

WESTERN EPEIRIC SEA

CRETACEOUS LAND

GULF COASTAL PLAIN

MIDLAND BASIN

CONCHO ARCH

OUACHITA SYSTEM

PENN.

o a s-7 €

WEST TEXAS BASIN

CONCHO ARCH

(Mid-Pennsylvonian)

FORT WORTH BASIN

Fig. 15.10. Evolution of central Texas as idealized
the Ouachita orogenic belt.

Strawn basin. The faults in the Llano uplift have formed narrow grabens,
and the three most prominent horsts over which the later strata are
flexed are called, from west to east, the Richland Springs, San Saba,
and Lampasas "axes." The Richland Springs axis forms the southern part
of the present Rend arch. See map, Fig. 14.1.

The time of deformation of the Ouachita orogenic belt is believed
to be post-Kickapoo. As cited in the treatment of the Ouachita Moun-
tains a major unconformity across the Atoka, McAlester, Hartshorne, and

along an eastwest section from the Midland basin to

Savanna beneath the Roggy shale in the west end of the Arkansas Valley
is believed to mark the time of main deformation in the Ouachitas. This
accords with Cheney and Goss's interpretation of the Pennsylvanian
around the north and east sides of the Llano uplift.

West Texas-New Mexico Region

During Permian time, the foreland area in front of the Marathons was
divided into a number of irregularly shaped provinces which received

248

STRUCTURAL GEOLOGY OF NORTH AMERICA

different types of deposits and which were probably tectonically unlike.
Refer to Figs. 14.1 and 15.12. Some were basin areas, like the Delaware
basin in which a total of 10,000 feet of sediments accumulated. Others
were shelf areas. Akin to the shelves were several narrow masses, or
platforms, lying between the basins. The basins were areas of greater
subsidence; the platforms and shelves, areas of lesser subsidence. The
Central Rasin Platform was covered with 2000-4000 feet of sediments,
as were also the shelf areas. The provinces appear to have been inherited
from the pre- Wolfcamp foreland features, and each platform is underlain
by one of the more important pre-Wolfcamp uplifts. The Permian tectonic
features may have been formed during a time of dominant crustal ten-
sion, following the pre-Wolfcamp time of dominant crustal compression
(King, 1937). The basins were centers of accumulations of clastic rocks,
first black shales and later sandstones, and the total thickness of beds
deposited in them was greater than elsewhere. Limestone tended to form
over all the higher standing areas. Landward, because of climatic condi-
tions that favored evaporation, evaporites were laid down in the fringing
seas. On the margins of these seas, red-beds were deposited which were
derived from the bordering lands.

The subsidence in Permian time that led to the burial of the Penn-
sylvanian ranges also resulted in the burial of the Matador and Amarillc—
Wichita ranges to the north, and the northern part at least of the folded
and thrust Marathons. Much of the sediment in the extensive Permian
basin came from the Ouachitas which were actively being elevated at
this time. Some debris from the Marathons reached surprising distances
northward. The subsidence was regional in aspect and accentuated the
Concho arch.

Extending across the larger features of the Marathon foreland and im-
parting a distinctive grain to their surfaces are numerous minor tectonic
features in which the linear element dominates. These include the
flexures in the Guadalupe Mountains region, the minor folds on which

Fig. 15.11. Concho arch, Bend axis, and Llano uplift, after Cheney and Goss, 1952. Forma-
tional contacts generalized. Heavy contours are isopachs on the Paleozoic interval below the
Strawn formation and illustrate the nature of the resulting Concho arch in Mid-Pennsylvanian
time.

WICHITA AND ANCESTRAL ROCKIES SYSTEMS AND THE TEXAS FORELAND

249

Delaware Basin

Central

Basin

Platform

Miolano

Basin

l-^I.-TriaVsic;

Eastern Platform

CRtTACtOua?

5a/odo
Votes
5even Rivera
Queen

Croyburg

3 an Andre 3

Clear Fork - Wich/to
£/o7fcomp

^ffcon,p

PfNNSYLVANIAN -------^

-'-'_5trown

SITE OF PECOS RANGE, ELEVATED IN EARLY AND
IN LATE rENNSTLVANIAN

.Scale

Fig. 15.12. Principal stratigraphic units and structural features of the South Permian basin of New Mexico
and Texas. Line of cross section shown on map, Fig. 14.1. Taken from Plate 2, King ef a/., 1942.

many of the oil fields are located, and various faults. Some were formed
in pre-Wolfcamp time, but most of them were formed during the
Permian. Some suffered movement again in the Cenozoic. They may be
grouped into four systems, viz., northwest trending, northeast trending,
north-northwest trending, and east-west trending; but much yet remains
to be learned regarding the systems because those mentioned may not
be natural units and may include some unrelated features. The systems
apparently include features of several different ages, as well as features
that were formed during several periods of movement.

ANCESTRAL ROCKIES SYSTEM

Major Structural Features

A group of imposing uplifts in Colorado and New Mexico of Penn-
sylvanian age, the Ancestral Rockies, has long been known. Considering

the far greater length and breadth of the modern Cordillera known as
the Rocky Mountains, the Ancestral Rockies only partially deserve their
name. Deep basins are associated with the uplifts, and collectively repre-
sent a rather important orogenic system in the foreland. The Ancestral
Rockies are separated from the Wichita system and the Texas foreland
chiefly for purposes of discussion, but probably are continuous with and
intimately related to them.

By reference to the map of Fig. 6.7 the several uplifts of the Ancestral
Rockies and the adjacent basins may be seen. Two of these, the Un-
compahgre and Colorado, were particularly bold and high. The Colorado
Range is frequently referred to as the Front Range highland. The
Pedernal uplift is not yet very well defined, but seems to be an emergent
area in east-central New Mexico which connects southward with the
Diablo uplift. The Zuni uplift, like the Pedernal, seems to have been wide
and not particularly high.

250

STRUCTURAL GEOLOGY OF NORTH AMERICA

I"---;'- j) 'to BEOS | ^BLICK !
U'-'--]'""<rOBITC || | ||MtuTI

J LIMESTONE »N0 DOLOMITE

UNCOMPAHGRE
UPLIFT

Fig. 15.13. Pennsylvanian deposits of the Paradox basin. After Herman and Barkell, 1957.

Pre-Pennsylvanian Setting

The total thickness of the Paleozoic formations present in central
Colorado by the end of Lower Mississippian (Leadville) time was only
1000 feet. In southwestern Colorado, only 400-500 feet existed, and in the
northern part of the Front Range, they were still thinner. Since the fairly
pure Mississippian limestones occur in areas close to the Pennsylvanian
ranges and no lithologic changes are evident in the limestones as the
ranges are approached, the Mississippian seas probably spread over the
sites of the highlands (Lovering, 1933).

Evidence of thinning, probably by erosion, is evident when isopachs
are worked out, and it appears that some of the ranges first began to be
expressed in latest Mississippian time, as illustrated in Fig. 6.6. The New
Mexico arch of Mississippian age exposed Precambrian rock over much
of central New Mexico.

Uncompahgre and Colorado Ranges

The Uncompahgre and Colorado ranges were flanked by basins as
indicated on the map of Fig. 6.7; the Paradox, the Central Colorado, and
the Denver. The extreme and abrupt facies changes of sediments de-
posited against their flanks is the evidence of the sharp uplifts. One of
the flanking basins, the Paradox, is illustrated in Fig. 15.13. The south-

west margin of the Uncompahgre Range was a fault scarp, and the thin
pre-Pennsylvanian sedimentary veneer was soon stripped from the rising
block, with the Precambrian crystallines furnishing flood deposits of
arkose to the adjacent subsiding basin. During part of Pennsylvanian
time evaporite conditions prevailed and four evaporite sequences —
cyclothems — resulted (Herman and Rarkell, 1957). This part of the
Hermosa formation is the Paradox facies or member.

The Molas is Atoka in age and the Hermosa spans the Des Moines,
Missouri, and Virgil. The Cutler extends on into the Permian. The time
of the most vigorous uplift, then, is clearly Atokan through to the begin-
ning of the Permian.

The Pennsylvanian and Permian sediments overlap the gently beveled
edges of the older Paleozoic rocks and rest on Precambrian crystallines
in the cores of the old ranges ( Lovering, 1933; Rurbank, 1933; Glockzin
and Roy, 1945). See Fig. 15.14. The crystalline rock was the source of
many of the Pennsylvanian and Permian strata which are commonly
coarse and arkosic near the old landmasses. For instance, Rrill (1944)
describes the sediments of the central Colorado basin as consisting mostly
of red and gray arkoses, arkosic conglomerates, sandstones, siltstones, and
gypsum which thicken to 13,000 feet in the deepest part of the basin.
Lateral variations are abrupt and extreme. During the most active time
of uplift of the adjacent ranges, the coarse elastics were deposited as
deltas along the margins of the trough, and the fine-grained sediments
were carried into the center. Identical mineral assemblages in the elastics
on both sides of the basin indicate that the exposed bedrock of both the
Uncompahgre and the Colorado Range was much the same.

Pedemal Uplift

The Pedernal landmass, named by Thompson (1942) from the Pedemal
Hills, is a large north-south-trending range in east central New Mexico,
about midway between the Rio Grande and the Pecos rivers. Red shales,
sandstones, variegated shales, and limestones of Permian age rest directly
on igneous and metamorphic rocks of Precambrian age in an area ex-
tending from the eastern side of the Sacramento Mountains, Otero

WICHITA AND ANCESTRAL ROCKIES SYSTEMS AND THE TEXAS FORELAND

251

WEST

cast

f\/w\/\ / \ / \

7 \~/\ /\/\ / \/

V \"/ \"/ \"/ \~/ \~/ \"/ O \"7 \ / w \~ \~/ T/ ~/ ~/ ~/ \ / \ / \7 w w w "/ \7 \~/ \"/ w ~/ w y~/ \~/ \ / \~/ \ / w \ / \ / \ / \7 \ / s / -. / \ / \ / \ / \/ \ / \~/ \ / \ / \~ w s / \ / \ ^ \ / \ / \ / s / \~ \ /

Fig. 15.14. Idealized section after Lovering (1929) of the Colorado Range near the close of
Pierre time (Upper Cretaceous) and before deformation during the Laramide revolution. Kp,
Pierre sh.; Kn, Niobrara Is.; Kb, Benton sh.; Kd, Dakota Si.; Jm, Morrison fm.; Je, Entrada ss.; Js,
Sundance fm.; Cp, Belden sh.; Maroon fm. (Des Moines) and State Bridge siltstone (Permian);

County, apparently continuously to northern Torrance County. Very
coarse conglomerates with cobbles of quartzite and other metamorphic
rocks are present in the Pennsylvanian rocks in the Sacramento Moun-
tains on the west side of the uplift. From these and other similar data,
Thompson concludes that the Pedernal Range was in existence from
early Pennsylvanian time until well after the beginning of Permian time.

The Colorado Range probably extended southward into New Mexico

through Colfax and Mora counties. Very thick sections of Pennsylvanian

j rocks of Des Moines age or older crop out on the eastern edge of the

| Sangre de Cristo Range from the region of Pecos River almost to the

! Colorado border. These rocks include arkoses, arkosic conglomerates and

I sandstones, and black shales.

1 Zuni Uplift

In northwestern New Mexico and northeastern Arizona evidence of

Pennsylvanian uplift is noted in the modern Zuni and Defiance ranges.

With additional subsurface evidence from drilling the configuration of

; the range appears to be like that illustrated in Fig. 6.7. Red sandstones

and shales identified as the Permian Ajo formation rest on Precambrian

Cml, Leadville Is. (Mississippian ?); DO, Devonian and Ordovician formations; Cs, Sawatch
quartzite; Cpf and Cpl, Fountain fm. and Lykins fm. (Pennsylvanian and Permian ? red arkosic
ss. and congl.); Cmm, Millsap fm. (Mississippian); Ofhm, Fremont Is. (Ordovician); Cq, Cambrian
quartzite. Vertical scale exaggerated and relative thicknesses of formations only approximate.

crystalline rock in the Zuni Range, and the Permian Moenkopi formation
rests on the crystallines in the Defiance Range. The old uplift is desig-
nated both as the Zuni and Defiance, but Zuni seems to be preferred.

Florida Uplift

In the Florida Mountains of southwestern New Mexico, Permian lime-
stone rests on Ordovician limestone; and a short distance to the north
in the Cooks Range, the Permian rests on Mississippian limestone. The
absence of Pennsylvanian strata is due to a Pennsylvanian or post-Penn-
sylvanian disturbance, probably at the same time as those in the south
end of the Hueco Mountains and the Diablo Mountains near El Paso.
The direction in which the elevated land trends is believed to be south-
easterly. It will be called the Florida Range.

Burial of the Ancestral Rockies

During Triassic, Jurassic, and Cretaceous time the Ancestral Rockies
were gradually buried by accumulating sediments. Immediately around
them were their own waste products, but marine epeiric seas brought
carbonates and fine elastics from adjacent regions, and these sediments

252 STRUCTURAL GEOLOGY OF NORTH AMERICA

helped in the burial process. Jurassic desert conditions brought great the early Upper Cretaceous before the last peaks were drowned. See

volumes of wind-transported sand from the western Cordilleran geanti- Fig. 15.14. Ry this time the early manifestations of crustal unrest in the

cline. Rocky Mountains are evident, and the old buried ranges with over-

The Zuni and Pedernal uplifts were buried by the Permian deposits, tying sediments were considerably deformed. An array of new super-

the Uncompahgre lasted in a number of small islands until late Jurassic posed structures and ranges developed, which will be reviewed in later

before final burial, and the Colorado Range lasted until Pierre time in chapters.

16.

THE LATE PALEOZOIC
ZONES OF FAULTING
AND CRYPTOVOLCANIC
OR METEORITE
IMPACT STRUCTURES

FORELAND ARCUATE FAULT ZONE

An arcuate zone of faults extends from the Llano dome in Texas north-
ward to Oklahoma, northeastward to the Ozark dome and eastward across
Kentucky to West Virginia. The faults are subparallel with the zone for
the most part, but some are divergent, especially two long faults in Mis-
souri which strike northwestward directly athwart the zone. See Tectonic
Map of the United States, 1944. The zone crosses domes and basins alike,
and therefore does not seem to be controlled by them. On the other hand,
the fault zone wraps around the Ouachita arc of the marginal orogenic
belt of the continent, and although the fault zone has a lesser curvature

than the Ouachita arc and departs from it a considerable distance on the
north, the subparallelism may mean that a genetic relationship exists.

Most of the faults are known to have originated in Pennsylvanian time
or immediately thereafter. A few others are post-Devonian. Thus the time
relation as well as the spatial indicates that the zone of faults is a single
tectonic element.

The faults in the Llano uplift are known from surface mapping. Oil
wells and geophysical prospecting have extended the known length of
some of the faults more than 100 miles to the north-northeast into the
Strawn basin (Cheney, 1940). The faults are probably of the high-angle,
normal variety, and have blocked out narrow grabens and horsts. The
high blocks have been named from west to east, the Richland Springs,
Pontotoc, San Saba, and Lampasas axes. According to geophysical work
in the Fort Worth basin, some of these faults have displaced the Smith-
wick formation 1100 feet, so the movement occurred in post-Smithwick
(Early Pennsylvanian) time. However, beds only slightly younger than
Smithwick, namely middle Strawn, are only slightly disturbed along a
major fault near Regency in the Colorado River area, and the faults are
not known in still younger Pennsylvanian beds. Cheney, therefore, con-
cludes that the faulting in the Llano dome and Strawn basin occurred
in early Pennsylvanian time.

The Stonewall fault in the Hunton arch area of southern Oklahoma is
said to have occurred in about middle Strawn time and to have a displace-
ment of about 3500 feet (Morgan, 1924), but from Dott's (1934) discus-
sion the fault may be one of the Arbuckle group of thrust faults and not a
part of the arcuate fault zone.

A group of faults in the northeastern corner of Oklahoma bound six
small crustal blocks, each about 6 miles wide (Wilson, 1937). The faults
trend in general northeastward, and some have been traced for 50 miles.
One block is tilted to the north; two are tilted to the south, and the re-
maining three are about level. Their throws range from 90 to 600 feet, but
these figures apply only to surface offsets. The faults, as well as the folds
in this region, become more pronounced in the older underlying forma-
tions. Where it is possible to trace the stratigraphy at depth by oil wells,
the structural relief decreases upward through the conformable sand-

253

254

STRUCTURAL GEOLOGY OF NORTH AMERICA

stones and shales of the Atoka, Hartshorne, McAlester, Savanna, and
Boggy formations. Bloesch (1919) believed that this decrease is due to
recurring deformation during the deposition of the above formations,
which are of the late Early Pennsylvanian age.

North of the six fault blocks and parallel with them is the hundred-
mile-long Seneca fault. It extends into the southwest corner of Missouri.
Surface evidence for the fault is not conclusive everywhere along the
structure, and it may be a syncline rather than a fault in several places
(Weidman, 1939).

Several rows of small faults are well known in Creek and Osage coun-
ties, Oklahoma, just west of the previously mentioned Seneca fault. The
individual faults are arranged en echelon and trend northwest. The rows
trend nearly north and make an angle of about 45 degrees with the in-
dividual faults. On Plate 8 the rows are indicated by dashed lines. The
largest stratigraphic throw at the surface is about 130 feet, and the great-
est length is 3/4 miles; yet the length of one of the rows is 80 miles. They
are all normal faults.

Fath (1920) analyzed the en echelon faults as follows. The Precam-
brian crystalline rocks were cut by a system of faults before the Paleozoic
veneer of sediments was laid down. In fact, peneplanation had removed
most of the relief incident to the faulting before the Paleozoic beds started
to accumulate. Beginning in Early Pennsylvanian time, the faults in the
basement complex again became active, this time, however, with hori-
zontal (strike-slip) movement. The overlying veneer was shear-strained
along the underlying faults and broke in rows of small faults arranged
en echelon. The east side of each fault in the Precambrian moved north-
ward. The movement recurred several times during the Pennsylvanian, so
that the throw of the faults is greater at depth. Some rows of en echelon
faults may not even show in the Pennsylvanian beds at the surface today,
and others are reflected in small asymmetrical anticlinal flexures over the
faults. Several such rows of anticlines farther west in Oklahoma and north
in Kansas may belong to the same system.

The postulate that the great arcuate fault zone is related to the Ouach-
ita lobe of the marginal orogenic belt is supported by Fath's mechanical
analysis. As would be expected in this theory, the foreland block directly

in front of the lobe would be moved horizontally ahead of its left-hand
neighbor, which is the direction of shear indicated by the en echelon
faults.

In eastern Missouri, two stages of faulting are recognized (Weller and
St. Clair, 1928), one in late Devonian and one in post-Mississippian. The
faults form a complex system, and the total displacement in the fault zone
ranges up to 1200 feet.

The eastern Missouri faults continue eastward across the southern tip
of Illinois into Kentucky, where a region of widespread and intensive
faulting exists. Along the north side of this complex of faults is the Shaw-
neetown fault of southern Illinois, and its eastward continuation, the
Rough Creek fault zone of Kentucky. The Shawneetown-Rough Creek
fault zone is really an uplift that varies in structural relief and detail from
place to place (McFarlan, 1943). Most characteristic of the uplift is its
anticlinal structure. At places, a series of anticlines is developed, in part
asymmetrical to the north and broken to form reverse faults. Normal faults
arranged en echelon are also present. The structural relief of the uplift
ranges from a few hundred feet to 2500 feet, and Mississippian beds in
places are brought into outcrop. The complex structural zone divides the
Pennsylvanian coal basin into a northern and a southern division.

Just south of the Shawneetown-Rough Creek structure in western Ken-
tucky is a cluster of high-angle faults, the main ones of which trend north-
east and east and have displacements up to 1500 feet. They are joined by
smaller cross faults. The area is semicircular, about 60 miles in diameter,
and is the most intensely faulted area in the interior lowlands. Along with
the faults, peridotite dikes and highly commercial veins of fluorspar occur.
The faults are post-Pennsylvanian and pre-Cretaceous.

The Rough Creek fault zone is continued after a gap of a few miles to
central and eastern Kentucky by the Kentucky River fault and its asso-
ciates, the West Hickman fault, the Irvine-Point Creek fault, and other
smaller ones (McFarlan, 1943). Maximum displacement on the Kentucky
River fault is 600 feet. Some suggestion of pre-Pennsylvanian movement
and progressive movement has been made, but McFarlan believes the
faulting occurred in post-Pennsylvanian time.

LATE PALEOZOIC ZONES OF FAULTING AND CRYPTOVOLCANIC OR METEORITE IMPACT STRUCTURES

255

LAKE SUPERIOR FAULT ZONE

The Tectonic Map of the United States shows a group of long and
subparallel faults extending from the Lake Superior region southwest-
ward into Wisconsin and Minnesota. The Keweenawan fault is probably
the best known. It runs lengthwise and approximately in the center of
the Keweenawan peninsula of Michigan and separates the copper-bearing
Keweenawan volcanic series from the Cambrian (?) sandstones. The fault
is clearly a thrust in one exposure near Houghton, but probably a fairly
high-angle one, with the Keweenawan series displaced southwestward
over the Cambrian (?) sandstone.

North of the Keweenawan fault, the volcanic series is downfolded into
a broad syncline with dips on the southeast flank of about 30 degrees.
The beds rise and crop out on Isle Royal in Lake Superior. A fault which
cuts the north flank of the syncline has been postulated just north of Isle
Royal. This northern fault has been connected with the Douglas fault,
which runs almost east-west south of Superior, Wisconsin, and which,
according to Thwaites (1912, 1935) dips 38 to 60 degrees southward. He
believes the south block has been thrust northward 6 to 12 miles. The
Douglas fault swings southward after entering Minnesota, and there it
has been studied by geophysical means. Near Pine City, the fault is
believed to be nearly vertical, with the east side upthrown about 9000
feet (Welch, 1941).

The great syncline between the oppositely dipping Keweenawan and
Douglas faults is thought by Thwaites to contain numerous minor folds
in Wisconsin and hence to be a synclinorium. The structure is discussed
in Chapter 4 and illustrated in Figs. 4.3 and 4.7. He also states that part
of the displacement could have occurred in late Keweenawan time, but
that part of it probably occurred later. The complementary relation of
the Keweenawan and Douglas faults suggests they are of the same age.
The syncline in the Keweenawan peninsula region appears to have sub-
sided partly at the time the volcanic flows and conglomerates were ac-
cumulating, according to Broderick (personal communication), but
considerable faulting undoubtedly occurred later. Thwaites ( 1943 ) agrees
in substance with this view.

A fault along the north coast of Lake Superior has been surmised,
chiefly on physiographic evidence (Martin, 1908), but this is not sup-
ported by gravity surveys.

Ten miles southeast of the Keweenawan fault in Michigan, two hills,
Limestone Mountain and Sherman Hill, are made up of a basal sandstone
and overlying dolomites and limestones. The sandstone, according to
Case and Robinson (1915) is either Cambrian or Lower Ordovician, and
the limestones and dolomites are Ordovician, Silurian, and Devonian.
According to Thwaites (1943) the sandstone is Upper Keweenawan, and
the dolomites and limestones are Trenton-Black River. The strata are
cut by small faults and, along the east side, exhibit dips up to 55 degrees.
A major fault may exist along the east margin, and the high dips may be
drag along the fault which would be approximately parallel with the
Keweenawan. The Ordovician beds in Limestone Mountain are 80 miles
from the nearest Ordovician outcrops; and the Devonian, if present, 150
miles from the nearest Devonian outcrops.

Dating the faults in Limestone Mountain and Sherman Hill is difficult
because of lack of agreement on the age of the sandstones associated with
them (Cambrian or Precambrian ) , and the extensive swamp and drift
cover that prevents working out the geologic relations. Opinion seems
to favor an early episode of subsidence in which the Keweenawan basins
were formed, and a later episode of faulting in which the Paleozoic rocks
were affected.

The disposition of an immense amount of material that came from
the truncation of thousands of feet of Keweenawan strata along the
Douglas and Keweenawan faults poses another problem. If most of the
movement were Precambrian, representative deposits possibly should
occur in the Cambrian, but the orogenic waste products do not seem
to make up any of the Paleozoic rocks nearby in Wisconsin or Michi-
gan.

If all but a small part of the faulting were Precambrian and associated
with the downfaulting of a basin in which the Keweenawan series ac-
cumulated, and if the Keweenawan series is 1100 m.y. old as recounted
in Chapter 4, then during the next 500 m.y. before the basal Cambrian
sands were spread across the region, all relief could have disappeared.

256

STRUCTURAL GEOLOGY OF NORTH AMERICA

and the Cambrian sediments need not necessarily contain the Kewee-
nawan lithologies.

CRYPTOVOLCANIC OR METEORITE IMPACT STRUCTURES

Definition

Eight small circular structures, one of known volcanic origin, and the
others supposedly of volcanic origin, have been mapped in the Central
Stable Region of the United States, and possibly a ninth one in the
Colorado Plateau of Utah. See map of Fig. 16.1. They are described by
Bucher (1933) as characterized by a nearly circular outline, a central
uplift with intense structural derangement, and a marginal, ring-shaped

Fig. 16.1. Cryptovolcanic or meteorite impact structures in the United States. 1, Jeptha Knob,
Shelby County, Ky.; 2, Serpent Mount structure, Adams and Highland counties, O.; 3, Flynn
Creek disturbance, Tenn.; 4, Wells Creek basin, Houston and Stewart counties, Tenn.; 5,
Decaturville structure, Camden and Maclede counties, Mo.; 6, Kentland structure, Newton
County, Ind.; 7, Magnet Cove, Hot Springs County, Ark.; 8, Upheaval dome, San Juan County,
Ut. After Bucher, 1933. 9, Manson, la.

Fig. 16.2. Geologic map of the Wells Creek basin, Tenn. Reproduced from Bucher, 1933. 1,
Wells limestone (L. Ordovician); 2, mid-Ordovician limestone; 3, Hermitage formation (mid-
Ordovician); 4, Silurian and Devonian formations; 5, Lower Mississippian formations; 6, War-
saw limestone (mid-Mississippian); 7, St. Louis limestone (mid-Mississippian); 8, alluvium.

LATE PALEOZOIC ZONES OF FAULTING AND CRYPTOVOLCANIC OR METEORITE IMPACT STRUCTURES

257

depression with irregular and local faulting. Including the marginal ring,
they range in diameter from 2 to 8 miles. The inner intensely deranged
core may be only part of a mile across in some, but in others up to 2
miles.

The faults make both an approximate concentric pattern and a radial
pattern. In some, the radial pattern is resolved strongly into a northwest-
southeast direction. Examine the representative illustrations of Figs. 16.2
and 16.3.

i Distribution

The map of Fig. 16.1 shows the distribution of the cryptovolcanic
structures. Numbers 1 to 6, and 9 are in the general arch and dome area
of the central Mississippi Valley. Numbers 1 to 4 are in the Cincinnati
and Nashville domes, number 5 in the Ozark dome, and number 6 in the
| Kankakee arch. They avoid the Illinois basin fairly well. Number 7 is
I in the orogenic belt of the Ouachita Mountains, but it is dissimilar to
the rest in having igneous rocks exposed in the core, in being free of
faults, and in being the site of considerable mineralization. See Fig. 14.2.
Number 8 is in the Colorado Plateau and is complexly associated with
salt dome upheaval.

Origin and Age

No volcanic rocks are associated, at least at the surface, with the small
circular structures of the Central Stable Region, yet their circular shape,
their upheaved, broken, and in places brecciated condition, and the

J presence of a number of dikes cutting the near horizontal Paleozoic rocks
in surrounding areas, lead Rucher to imagine an explosive volcanic

i origin.

These cryptovolcanic structures are thought to be the result of a sudden lib-
eration of pent-up volcanic gases, which had accumulated near the surface, the
; explosion having been too weak to produce a shallow crater such as formed in
] the Ries Basin, southern Germany (Bucher, 1933).

These unique structures in the United States have been eroded more
than those of Tertiary age in Germany, and so Rucher regards them as
older and of probable late Paleozoic or Mesozoic age.

Fig. 16.3. Structure contour map of Serpent Mound, O. The length of each square is about
2200 feet. Reproduced from Bucher, 1933.

Recently a new cryptovolcanic (?) structure has been found near
Manson, Iowa. It is number 9 on Fig. 16.1. Unlike the others it has a
Precambrian crystalline core about lM square miles in area which lay
unknown because of a cover of glacial drift until discovered by core
drilling (Hoppin and Dryden, 1958). In this area a thin Paleozoic veneer
of sedimentary rocks plus a cover of Cretaceous shale is the normal
expectation under the drift. The contact of the crystalline rock with the
surrounding sedimentary rocks dips outward 350 feet per mile to the

R. 17 W.

t- If L. 1 ft

-, V r v B

Fig. 16.4. Geologic map of Magnet Cove, Arkansas. Reproduced from Bucher, 1933; after Landes, 1931.
1, Pleistocene (T, tufa); 2, sandstone and shale (Mississippian); 3, novaculite (Devonian); 4, metamorphosed
sandstone and shale; 5, metamorphosed limestone; 6, igneous rocks (M, magnetite).

LATE PALEOZOIC ZONES OF FAULTING AND CRYPTOVOLCANIC OR METEORITE IMPACT STRUCTURES

256

southeast and 290 feet per mile to the west. The relief of this rather
sharp, small dome is at least 1500 feet. Surrounding the crystalline core
is a "disturbed area" 20 miles in diameter in which the sedimentary rocks
are severely deformed. Mississippian limestone and Lower ( ? ) Cretaceous
shale have been sampled in drill cores in the disturbed area, and there-
fore the structure was formed in post-Early Cretaceous time. The upper
200 feet of the Precambrian rock beneath the drift is badly shattered.

The writers believe that the crystalline rock was forced upward into
the limestone and shale, and in the process was badly shattered. The
mechanism responsible is postulated to be a hidden igneous intrusion.

Magnet Cove

The Magnet Cove structure is included by Rucher in his resume of
cryptovolcanic structures, although it consists of an elliptical intrusive
complex about 3 miles across and is within the compressional structures
of the Ouachita Mountains. See map of Fig. 16.4. The igneous rocks are
alkaline, and for the most part belong to the nephelite-syenite group
(Landes, 1931). The peripheral intrusives, which are more resistant to
erosion than those toward the center of the complex, form a circular ring.
Metamorphosed sandstone and shale border the intrusions in places.

Some time after the folding of the Ouachitas, a stock of highly alkalic
magma was intruded into the Paleozoic rocks, and then either by differ-
entiation or through separate intrusions several rock types were formed
and an unusual suite of minerals was emplaced. Compounds of titanium
are especially abundant.

Upheaval Dome

The Upheaval dome is in the flat-lying red-beds of the Colorado
Plateau and is sharply conical with a surrounding ring-like syncline. From

the axis of the syncline on one side to the axis of the syncline on tin-
other is only 2 miles, and the diameter of the entire affected area is 3
miles. The White Rim sandstone member of the Cutler formation appears
as huge, up-ended blocks the size of a house in the highly disturbed
central area (McKnight, 1940), and the cliff-making Wingate sandstone
rings a spectacular crater about a mile in diameter.

Roth aeromagnetic and gravity surveys have been made of the area.
The magnetic contours resolve two strong and symmetrical highs, one
directly over the Upheaval dome and the other about 7 miles to the
southeast. The gravity survey also indicates two structures in about the
same places but not so distinctly. Joesting and Plouff (1958) conclude
that the broad magnetic and gravity highs each require the rise of a
mass of Precambrian crystalline rock about 5 miles in diameter 2000
feet above its normal position. Salt flow emphasized the one dome
( Upheaval ) but failed to materialize for some unknown reason in the
other. Lastly, because the gravity anomalies are not entirely satisfied by
the salt plug, a small igneous intrusion into the salt of the dome is postu-
lated. The process took place in several steps from Permian to the
Miocene. Refer to "salt anticlines" in Chapter 26 on the Colorado Plateau.

Meteorite Impact Origin

With the space age has come increased interest in terrestrial meteorite
impact craters, and Dietz (1960) has called attention to this theory of
origin, especially for such structures as Serpent Mound (Fig. 16.3) and
the Wells Creek basin (Fig. 16.2). Evidence for the impact theory comes
from the presence of shatter cones (small percussion fractures in conical
shape) and coesite powder, a high pressure crystalline form of silica,
supposedly generated at the time of impact. According to some geologists,
the theory is gaining much favor.

17.

MESOZOIC SYSTEMS
ALONG THE PACIFIC

WESTERN NEVADA

Central and western Nevada and all California were involved in
orogeny during the Mesozoic era, and the index map, Fig. 17.1, shows
the chief areas and features with which the following discussion is con-
cerned. The map also extends eastward to central Utah where late
Mesozoic disturbances occurred. These will be discussed in Chapter 18.

A trough of geosynclinal proportions centered in western Nevada in
Triassic and early Jurassic time. It has already been referred to in con-
nection with the Permian and Mesozoic geanticline in central Nevada.
See tectonic maps, Plates 8, 9, and 10. In general, its axis probably lay
slightly east of the axis of the Permian trough. In the Hawthorne and

Tonapah quadrangles, Nevada, it sank and received a total thickness of
sediments of about 30,000 feet (Muller and Ferguson, 1936). The sedi-
ments are predominantly marine elastics, cherts, and limestones with a
considerable proportion of more or less altered pyroclastic rocks and
lavas in the lower and upper parts of the section.

The table, Fig. 17.2, shows the sequence of Mesozoic formation there
and elsewhere in western Nevada, California, and southern Oregon.
The Lower Triassic Candelaria formation rests with marked erosional
unconformity on the thin Permian sandstones and grits and in places on
the beveled Ordovician strata. A slight disturbance, therefore, affected
the area in late Permian time and probably reflects greater orogeny in the
westward-lying orogenic belt. During the deposition of the Candelaria
formation, the area of sedimentation as well as the western highland were
comparatively quiet, and shales, sandy shales, sandstones, some of tuf-
faceous aspect, and scattered, thin layers of limestone were deposited.
Then marked volcanism and orogeny occurred to the west in middle
Triassic time, and over 12,000 feet of strata, chiefly pyroclastics and lavas,
accumulated. This group of rocks is known as the Excelsior formation.
The lavas range in composition from andesite through quartz latite to
rhyolite. Alteration, principally epidotization and chloritization, has af-
fected the formation over wide areas. Volcanic breccias, especially those
containing altered andesite fragments, are abundant; and in some sections
they exceed the effusive rocks in amount. In the Pilot and Excelsior
ranges, a considerable thickness, estimated to exceed 8000 feet, consists
of massively bedded chert. Examination under the microscope shows this
rock to be an extremely fine-grained water-laid tuff, cemented and largely
replaced by cryptocrystalline quartz. Interbedded with the chert are dark
tuffaceous slate, a little impure sandstone, and some lava and breccia.

The volcanics were then subjected to erosion for a time but not much
disturbed before the thick Upper Triassic sequence accumulated. Dark
limestone and dolomite predominate, but siliceous argillite, argillite,
calcareous shale, shale, and chert pebble conglomerates are not uncom-
mon. These beds are known as the Luning formation. Above the limestone
and dolomite sequence are 420 feet of purple to black shale and dark
brown limestone, known as the Gabbs formation. The Gabbs is conforma-

260

MESOZOIC SYSTEMS ALONG THE PACIFIC

261

Fig. 17.1. Index map showing significant features
and localities of Mesozoic orogeny in the western
Cordillera. G.P., Grants Pass quadrangle; Med., Med-
ford quadrangle; Winn., Winnemucca quadrangle;
Gol., Golconda quadrangle; Tobin, Mt. Tobin quad-
rangle; Moses, Mt. Moses quadrangle; Gun. P., Gun-
nison Plateau.

TERTIARY

WINN

GOL.

TOBIN

MOSES

OLCimcs

IDAHO

~ NEVADA I UTAH

SIERRA NEVADA AND
KLAMATH BATHOLITHS
AND SATELLITES

UINTA

t -* *

"\ » \ t— •

CANYON, ;
RANGE ,' I

IRON
SPRINGS
OISTRICJ--^

UTAH _
~ARlZQN*

ble with underlying and overlying formations. Deposition was continuous
from Triassic to Jurassic time, while the western orogenic belt remained
fairly quiet. Its relief was evidently low, and volcanism is not recorded in
the shales, limestones, and sandstones of the Sunrise formation which
were deposited in the trough.

At this stage in early Jurassic time, the sediments of the trough were
sharply folded ( Ferguson and Muller, 1937 ) . The most intense deforma-
tion was approximately coextensive with the area of deposition of the
Upper Triassic deposits. The orogeny began apparently with the forma-
tion of a marginal trough at the border of the geosyncline. In the trough,

the Dunlap formation of Early Jurassic age was deposited. It consists
dominantly of fanglomerate, conglomerate, and sandstone with an upper
volcanic member of andesitic, quartz-latitic, and rhyolitic composition.
The fanglomerates and conglomerates were derived chiefly from the
limestones of the Luning formation and only locally from the great Ex-
celsior volcanic series. The Dunlap has been observed resting on upturned
cherts of the Excelsior formation with an angular discordance of 90 de-
grees, and also to be truncating folds of the Luning limestones. The
Dunlap is characteristically an orogenic deposit, and Ferguson and Muller
(1937) recognize a continuation of deformation during its deposition.

262

STRUCTURAL GEOLOGY OF NORTH AMERICA

These movements were the beginning of thrusting, at least in the area of
former deposition. Later compression resulted in thrusting on a large
scale, and the earlier structures were greatly complicated. The thrusting
postdates the Dunlap Lower Jurassic formation, and precedes the in-

EUROPEAN
STAGES

MAESTR1CHTIAN

CENOMANIAN

HAUTERIVIAN

VALANGINIAN

BERRlASIAN

(TITHONIAN)
PORTLANDIAN

KIMMERIDGIAN

WEST SIDE
SACRAMENTO
VALLEY AND
COAST RANGES

MORENO
FM

Ws<"

MORSE-
TOWN

U.S KNOKVILLE

EAST-
CENTRAL
CALIFORNIA

MARIPOSA
SLATE

AMADOR
GR

EASTERN

CALIFORNIA

WESTERN

NEVADA

EXCELSIOR

(VOLCANICS)

CANOELARIA

SOUTH-
WESTERN
OREGON

GALICE FM

ROGUE FM.

DOTHAN FM

NORTH-
EASTERN
CALIFORNIA

TTi'm'rTTfm.rr

IINCHMANSS

BICKNELL SS

UJ-IJJJJl

VOLCANICS

MORMON SS.

THOMPSON LS
FANT ANDESITE

HARGRAVE SS

LOWER UPPER

PLATE PLATE

FACIES FACIES

RASPBERRY F

WINNEMUCCA

DUN GLEN F

NATCHEZ
PASS FM

PRIDA FM

CANE SPRING

FAVRET FM

Fig. 17.2. Principal Mesozoic formations of California and western Nevada. West side, Sacra-
mento Valley and Coast Ranges, taken from Irwin (1957) and Briggs (1953). Potassium argon
dating of Nevadan orogeny by Evernden et al., 1957. Jurassic correlations from McKee ef al.,
1956. Triassic of eastern Nevada from Ferguson and Muller (1937), of west-central Nevada
(Mount Tobin Quadrangle) by Muller et al. (1951), and of southwestern Oregon by Wells (1956).

- s^-r^ZLlW^&'^-J*!* TH" tH*J<<» ■><>*

_jas__

^S^'Ji==J-.=-S^'^

-7%-^ «* Jd. "Rid ^g-pSmygT**. /M,J L ?^SS

• ••••• •.■•■•'-> - - . i - <• ° - J_S i ~ - 1

^<7> i i 1 1 1 ' T~tsftii I i /s»J?* <W , ■ i» nnrTT~i i i i i 1 1 1 nTi Tii ft ■ ■ , *■ ■

•' * ' " \/ //" * * i • i • ' • *

Fig. 17.3. Cross sections in the Hawthorne and Tonopah quadrangles, reproduced from Ferguson
and Muller, 1949. Top section, north of Garfield Flat showing relation of Dunlap formation to
Luning and Excelsior formations. Middle section, south of Sunrise Flat, Gabbs Valley Range,
showing thrusting and later normal faulting. Bottom section, south of Redlich siding, showing
relations of Ordovician and Permian and the Excelsior formation. Symbols, top section: Jdf,
Dunlap fanglomerate and congl.; Jdg, Dunlap vols.; Jds, Dunlap ss.; Jdl, Dunlap Is.; Ilu,
Luning upper Is.; lis, Luning slate; Teg, Excelsior vols.; Tec, Excelsior chert and tuff. Middle
section; Jdv, Dundap vols.; Jds, Dunlap ss.; Jdc, Dunlap congl.; lid, Luning dol.; Jdt, thrust
cong. Bottom section: le, Excelsior vols.; 1c, Candelaria formation; Pc, Permian congl.; Os,
Ordovician slate and tuff.

trusion of the Sierra Nevada batholith, whose satellites are present in the
western part of the sediments of the Triassic and Jurassic trough.

The thrusting in general was easterly along the eastern margin of the
trough and southerly along the southern border.

Representative sections from the Hawthorne and Tonopah quadrangles
are reproduced in Fig. 17.3, and the evolution of the complex thrust
structure in Dunlap and post-Dunlap time is shown in Fig. 17.4.

MESOZOIC SYSTEMS ALONG THE PACIFIC

263

In the Winnemucca, Golconda, Mt. Tobin, and Mt. Moses quadrangles
of west-central Nevada (column 7, Fig. 17.2; area denoted as W-G— T-M
on Fig. 17.7) the late Paleozoic Antler orogeny is strikingly displayed, as
well as strong orogeny in mid-Permian time. Volcanism in late Permian

^OUNLAP
FOR.

[Jdv - voiconics and sediments

Jdt • Conglomerates ond fonglomerate

I Jds • Sondstone

STAGE J

SUNRISE AND „ ,

GABBS FOR. Js " Limestone ond sholt

i*ptlu -Massive limestone and dolomite
Tils -Shale with conglomerate lenses
XI - Thin oedded limestone
>ec-Chert
,tteg-Greenstone ond breccia

EXCELSIOR
FOR.

yn^^" " frj

rW^N

)' MM

p$~2

^Sff^^^

/jfcss

Z.^^9r

/*'t»^

j&y

W^Mi^S

b^ HI'/

Fig. 17.4. Development of complex structure in the northwestern port of Pilot Mountains, Haw-

' thorne and Tonopah quadrangles, Nev. From Plate 3, Ferguson and Muller, 1949. Stage 1,

* folding near margin of Luning embcyment and deposition of conglomerate and fanglomerate of

the Dunlap formation. Stage 2, development of Mac thrust. Deposition of coarse material and

folding of Mac thrust. Stage 3, further folding with development of Spearfish thrust. Movement

toward the trough was along an erosion surface cut on the upper plate of the Mac thrust.

I, Stage 4, development of five other thrusts and intricate folds. The relative length of the four

diagrams indicates the postulated shortening of the stratigraphic section involving the Triassic

and Jurassic sediments.

QOLOONOA QUAO)

Fig. 17.5. Map showing inferred extent of Tobin and Golconda thrusts. Reproduced from
Ferguson et al., 1951.

264

STRUCTURAL GEOLOGY OF NORTH AMERICA

TOBIN THRUST.

_TpBIN_THRUST
"ftnp

CpPh

GOLCONDA

THRUST
C

'Cp

SECTION IN MOUNT TOBIN QUADRANGLE <>

I MILES

SECTION IN WINNEMUCCA QUADRANGLE

Tidg pk

Fig. 17.6. Representative cross sections of northwest central Nevada. Top section shows the
Tobin thrust of Late Jurassic age and the angular unconformity between the Permian Koipato
and Havallah formations. Middle section shows the Dewitt thrust of late Mississippian or Early
Pennsylvanian age and the associated angular unconformity between the Pennsylvanian Battle

and, again, mild orogeny at the end of the Permian is noted. See top
and middle sections of Fig. 17.6.

Large-scale thrusting occurred in the late Jurassic probably correspond-
ing in time to the major deformation in the Hawthorne and Tonopaw
quadrangles. Considering the time of intense deformation of the Mariposa
slate in eastern California, to be discussed immediately, the orogeny is
thought to have culminated in Kimmeridgian time of the Late Jurassic.
The distribution of the major thrusts of this age, the Tobin and Golconda,
is shown in Fig. 17.6. The two may actually be one and the same. At
least, the horizontal translation has been so great that two suites of
formations of different facies probably deposited an appreciable distance
apart, have been brought into juxtaposition. In the four quadrangles the
upper thrust plate covers an area extending 50 miles from north to
south and 40 miles from east to west. The Permian formations are

Mountain formation and the Ordovician Comus formation. Lower section shows the succession of
thrusts; first the Thomas, then the Sonoma, and then the Clear Creek, all of Late Jurassic age.
The Tobin thrust nearby cuts the Clear Creek thrust.

common to both plates. The direction of relative movement of the upper
plate is uncertain. In the Sonoma Range a succession of four thrusts, all
occurring in the Late (?) Jurassic orogeny, is recognized, and the three
lower ones moved from east to west. It seems possible that the Tobin
thrust plate could have moved toward the north (Ferguson et al., 1951).
See lower section of Fig. 17.6.

NORTHWESTERN NEVADA

Lower and probably Upper Cretaceous rocks have been found in north-
western Nevada, and these record a continuation of deformational phases
beyond the Late Jurassic Tobin and Golconda thrusting. According to
Willden (1958):

MESOZOIC SYSTEMS ALONG THE PACIFIC

263

u u
a. <

3 U

Danian

Maestrichtian

Senonian

Turonian

Cenomanian

95-101-

Albian

Apt i an

Barremian
Hauterivian
Valanginian
Berriasian

133

Portlandian
(Tithonian)

Kimmeridgian
Oxford ian
Callovian

Bathonian
Bajocian

Lias

17S

Rhaetian

Norian

Karnian

Ladinian

Anisian

Scythian

■185-200

Ochoa
Guadalupe
Leonard
Wolfe amp
210-

Virgil
Missouri
Des Moines
Lamp ass as
Morrow

MISSISSIPPIAN

Folding and faulting in northwestern Nevada.

Thrusting of Permian volcanics over King Lear and

Pansy Lee elastics .
Deposition of Pansy Lee conglomerate in north—

western Nevada.
Santa Lucian phase in Central Coast Ranges.
Intrusion of great batholiths of Sierra Nevada

and Coast Ranges.

r Folding and erosion of King Lear fm.
I

y Uplift and deposition of King Lear fm. in north-
west Nevada.

Subsidence of Luning Embayment.

Strong, local orogeny and volcanism; folding and
thrusting in Hawthorne and Tonopaw Quadrangles.

.*- Mild, local disturbance resulting in angular
unconformity.

4-"

Mild orogeny in central and western Nevada
resulting in unconformity.

*— Volcanism, extensive. Folding in central Oregon*
•*- Orogeny in Western Nevada: Golconda thrust

Strong orogeny, folding and thrusting in central
and western Nevada. Sharp folding and low-
grade metamorphism of Calaveras fm. in eastern
California possibly at this time.

Continued orogeny probably in several phases.

Beginning of geanticlinal uplift in central
Nevada, and compressional orogeny in part.

« v

« c

> o

O 111

z o

jss

■o So

ffl c

> o

c BO

z o

I u

■o o

Intrusion of batholiths in southern Klamath Mts.

and northwestern foothills of Sierra Nevada.
•Strong orogeny; Tobin and related thrusts of

VM5-T-M Quadrangles. Mariposa slate of eastern

California isoclinally folded with resulting

low-grade metamorphism.
Volcanism and local folding and thrust-faulting

during deposition of Dunlap fm. in Hawthorne

and Tonopah Quadrangles.

O M

c O

O Ih

en o

Fig. 17.7. Sequence of disturbances in central and western Nevada and California from the

| central Coast Ranges to the Sierra Nevada. Numbers are absolute ages in terms of millions of

years and in part are modifications of the Holmes time scale as proposed by Curtis ef a/., 1958.

\n\ vnWklamath

1 M^— wxMTS.

I Z \

/ o s

-I

^>1'34.4

HR7

<?v

^
kv

k *

131.5 812

o^ :

Fig. 17.8. Location and age of
granitic rocks in California
dated by potassium-argon

method. Stippled areas are
granitic plutons. After Curtis
et al., 1958.

rY0SEMITE

^9 plutons ranging
trom 83.3 to
953

SOUTHERN
vCALIFORNIi

THOLIT/

A formation of Early Cretaceous age composed of locally derived clastic-
rocks, including pebble to boulder conglomerate, siltstone, coarse graywacke.
and finely crystalline limestone is exposed at several places in the central and
northern part of the Jackson Mountains, Humboldt County, Nevada. This for-
mation (King Lear) was folded and at two places probably completelv eroded
before deposition of the next younger unit — a pebble conglomerate com-
posed of exotic pebbles of chert and quartzite derived from rocks of early
Paleozoic age. This younger pebble conglomerate (Pansy Lee) may be of Late
Cretaceous or early Tertiary age and may be equivalent to rocks of similar
stratigraphic position and lithologic character exposed over a considerable area
of eastern Nevada and western Utah. Both of these coarse clastic formations
have been overridden by a thrust sheet of Permian or older volcanic rocks. The
dimensions of the thrust sheet are not known exactly but remnants arc exposed
over a 25-mile-long segment of the range. Upper Tertiary volcanic rocks ex-
posed in the range are not involved in the thrusting.

The Cretaceous and younger rocks of the Jackson Mountains record a long
period of orogenic unrest that included: (1) uplift of the source area of and
deposition of the Lower Cretaceous rocks; (2) folding and beveling by erosion:
(3) deposition of the exotic-pebble conglomerate; (4) thrusting of the Per-

266

STRUCTURAL GEOLOGY OF NORTH AMERICA

mian or older volcanic rocks over the two coarse clastic formations; and (5)
later folding, faulting, and erosion providing the present outline of the range.

These relations record orogeny of Laramide age and undoubtedly mean
that the Laramide belt of the Central Rockies ( Chapter 22) spread west-
ward over most of Nevada. The folding of the King Lear elastics before
the deposition of the Pansy Lee conglomerate records deformation prob-
ably in Early Cretaceous time, and this has been labeled the mid-
Nevadan orogeny on Fig. 17.7.

CENTRAL AND NORTHERN CALIFORNIA

In the California Sierra Nevada region, Taliaferro (1942) summarizes
an eastern belt of Triassic and Jurassic rocks and a western belt of Juras-
sic rocks. The two were probably continuous, but due to the Nevadan
orogeny (to be described immediately) a dividing mass 25 to 50 miles
wide of Calaveras rocks and granite of the Sierra Nevada batholith exists.
The eastern belt consists of discontinuous areas of Upper Triassic and
Jurassic sediments and volcanics. Doubtless these formed a continuous
belt at one time, but as they lie in the region of maximum plu tonic in-
vasion and maximum Tertiary uplift, they have been obliterated or re-
moved by erosion in many places. Near the northern end of the Sierra
Nevada, the Milton formation represents the Triassic and Jurassic rocks,
and where not engulfed by the plutons or removed by erosion it lies in a
broad, steep-limbed syncline, practically free from minor crumbling,
thrusting, and overturning (Taliaferro, 1942). The conglomerates contain
abundant debris of the Paleozoic rocks (Calaveras) and thicken and
coarsen westward. It seems clear, therefore, that the Milton was derived
from the west.

The best-exposed and most complete section of the east belt is on the
north fork of the American River in Placer County. On the west limb of
the syncline, basic and intermediate volcanics and radiolarian cherts, 200
feet thick, are followed by 12,800 feet of conglomerates, sandstones, hard
slaty shales, and fine-grained andesitic tuffs. The center of the syncline is
occupied by 9500 feet of intermediate and basic flows, agglomerates, and

tuffs. Only about 900 feet of sediments and tuffs lie below the volcanics
on the east limb of the syncline, the lower part having been obliterated by
batholithic intrusions. Well-preserved Upper Triassic fossils are found
at and near the base of the sediments on the west limb of the syncline,
Lower Jurassic fossils 2500 feet above the base, and Middle Jurassic fos-
sils 9500 feet above the base; no fossils have been found in the upper
13,000 feet of the sediments and volcanics. The section is well exposed
and no unconformities or disconformities have been observed. Possibly
part of the upper 13,000 feet is equivalent to the Mariposa slate of the
western belt. The upper volcanics are possibly equivalent to the extensive
Logtown Ridge volcanics lying between Amador and Mariposa west
of the Mother Lode. No evidence supports the idea that the Milton of
the eastern belt was separated from the Mariposa and Logtown Ridge of
the western belt either by deposition in separate basins or by a period of
batholithic intrusion and orogeny (Taliaferro, 1942). See column 2,
Fig. 17.2.

In comparing the sediments of the eastern belt of the Sierras with those
of the trough of western Nevada, it appears that Lower and Middle
Triassic sediments were deposited in the central part of the trough which
lay in western Nevada, and then Upper Triassic sediments overlapped on
highlands both westward and eastward. See Fig. 17.8. Great subsidence
occurred in early Middle, and early Late Jurassic time; the center of the
Jurassic trough migrated west of that of the Triassic trough; and over-
lap on the western volcanic orogenic belt was extensive.

The western belt is made up of the Amador group and the Mariposa
slates in the Sierra Nevada and northwestward in Oregon, of the Dothan
and Galice. The Amador and Dothan are probably Middle Jurassic in
age, with their upper beds containing Late Jurassic fossils. The Mariposa
and Galice are early Late Jurassic. The great bulk of the Amador consists
of volcanics and elastics, but red and green radiolarian cherts and dense,
unfossiliferous limestones are common. On the Cosumnes River, 1200
feet of conglomerates and sandstones are at the base of the Amador. On
the Merced River, radiolarian cherts, tuffs, and shales are over 1500 feet
thick, and these overlie about 1400 feet of pillow basalts. The entire
Amador group ranges in thickness from 5000 to 15,000 feet, and usually

MESOZOIC SYSTEMS ALONG THE PACIFIC

267

grades upward into the Mariposa (Taliaferro, 1942), but between the
Merced and Mariposa rivers, conglomerates are at the base of the
Mariposa. The pebbles are presumably from the underlying Amador. See
column 4, Fig. 17.2.

The Mariposa formation consists of black slate and graywacke, with
which greenstone is closely associated (Knopf, 1929). Conglomerate
occurs locally, and sericite schist and limestone in a very few places. The
greenstone, because of its intimate interbedding with the normal sedi-
mentary rocks, is in many places an inseparable part of the formation, and
locally predominates in volume. The conglomerate contains a variety of
rocks, namely: quartz keratophyre (submarine lava flow origin), quartz-
ite, chert, quartz, aplite, and biotite granophyre. The last two point to
plutonic intrusions older than those of the Sierra Nevada ( Knopf, 1929 ) .
The graywacke contains grains of quartz, plagioclase, slate, quartzite, and
keratophyre (?). On the one hand they grade into slate and graywacke
slate, and on the other, by the presence of augite, into augite tuff. The
greenstones were principally augite basalt breccias, tuffs, and lavas,
now somewhat metamorphosed (Knopf, 1929). It appears that some of
jthe volcanics included by Knopf in the Mariposa are what Taliaferro
places in the Amador.

j The great thickness of volcanics is a striking feature of practically all
Jurassic units in California and southwestern Oregon. The volcanic rocks
range from rhyolite to basalt, but augite andesites predominate. Practi-
pally all, if not all, are submarine, as they are interbedded with marine
Isediments (Taliaferro, 1942). Pyroclastics predominate over flows. Cen-
ters of volcanism have been recognized in the form of necks, both breccia
sand solid, and great accumulations of flows, tuffs, and very coarse brec-

cias.

I Intrusions of peridotite and dunite, now largely serpentinized, together

with their closely associated gabbroic and diabasic differentiates, are com-
mon in the Jurassic of California and southwestern Oregon. They occur as

r>ills, dikes, plugs, and large masses of undetermined form. The great ma-
jority were intruded prior to folding of the Jurassic sediments and before
the Sierra Nevadan batholith was emplaced. The basic intrusions of the
Mother Lode were serpentinized immediately after their emplacement

(Knopf, 1929). They were slightly metamorphosed by the folding, and
greatly altered at the contacts of the granodiorite plutons.

OREGON

In central Oregon, a fairly complete Jurassic section has been described
by Lupher ( 1941 ) . He sets apart ten formations which range in age from
Early to Late Jurassic, perhaps to Early Cretaceous, and altogedier are
over 11,000 feet thick. These beds show only a succession of gentle emer-
gent and submergent movements. The lithology is in conspicuous contrast
to that of the Jurassic of the Sierra Nevada in lacking volcanics and having
only minor amounts of coarse elastics. It is nearly all sandstone and shale,
and in part it is very fossiliferous.

The Oregon Jurassic rests with marked angular discordance on a base-
ment of highly folded Upper Triassic and Mississippian rocks. Some of the
beds called Upper Triassic may be Lower Jurassic, because a sequence
of shales, sandstones, and conglomerates, many thousands of feet thick,
overlies the fossiliferous Upper Triassic but underlies the great unconform-
ity. The folds in the Jurassic beds trend at divergent angles from those
of the Upper Triassic, and basic plutons now largely altered to serpentine
invade the Upper Triassic but not the Jurassic. It is, therefore, apparent
that an orogeny of considerable proportions is indicated. It will be re-
called that a similar unconformity separates two formations of Early Juras-
sic age in western Nevada, and it is evident, therefore, that the two may
be the same, perhaps with slightly different ages. It seems necessary, in
order to account for the different lithologies of the Jurassic beds of central
Oregon and those of the Sierra Nevada, to separate the central Oregon
beds from the volcanic belt by a nonvolcanic highland or Piedmont. See
the tectonic maps, Plates 11 and 12. The pebbles are cherts and lime-
stones, evidently from Paleozoic formations (Lupher, 1941).

SOUTHERN CALIFORNIA

Larsen (1948) has summarized the geology of the region southeast of
Los Angeles in southern California, especially in relation to the great

268

STRUCTURAL GEOLOGY OF NORTH AMERICA

Nevadan intrusions there; and he also reviews the southern continuation
of the batholithic province into Baja California. A group of slates and
argilhtes, with some quartzites, lie west of the main batholith and form
most of the Santa Ana Mountains. Triassic fossils have been collected there
in several places. Somewhat more metamorphosed remnants of these
rocks occur within the batholith. The group is known as the Bedford Can-
yon formation, and about 20,000 feet of beds are exposed in the Santa Ana
Mountains. Parts of the formation may be older than Triassic and parts
may be Jurassic. The uniform argillaceous lithology is a dominant char-
acter.

A group of volcanic beds, mostly mildly metamorphosed, andesitic
agglomerates, overlies the Bedford Canyon formation unconformably.
The extrusives have been called the Santiago Peak volcanics (Larsen,
1948), and they are probably many thousands of feet thick. They are
older than the batholithic intrusions and, therefore, are probably Jurassic
in age.

Along the east side of the main batholithic region are coarsely crystal-
line schists, all of which contain much quartz. Interbeds of limestone
have yielded Mississippian fossils (Larsen, 1948). A quartzite sequence
with interbedded, coarse, mica schist is also thought to be Carboniferous.
It is some 12,000 feet thick. Larsen believes that the Paleozoic sediments
were metamorphosed and intruded by granite rocks before the deposition
of the Triassic rocks, and that this older metamorphism was more intense
than the later metamorphism of the Triassic rocks.

NEVADAN OROGENY

History of Concept

The literature, up to the last few years, suggests that the Late Jurassic
folding and thrusting was followed very shortly by the great batholithic
intrusions, and that the two events occurred between the Kimmeridgian
and Portlandian. See Figs. 17.2 and 17.7. Recent isotope age determina-
tions have demonstrated fairly conclusively, however, that the intrusions
are mid- or early Late Cretaceous in age. Also new fossil finds have re-
sulted in a revision of concepts of the Upper Jurassic and Lower Creta-

ceous stratigraphy which is not incompatible with a Mid-Cretaceous age
of the batholiths.

Additional sampling and potassium-argon age determinations by Curtis
et al. (1958) indicate that granitic rocks along the northwest foothills of
the Sierra Nevada and in the southern Klamath Mountains are Tithonian
( Portlandian ) in age, as the early geologists had concluded. Furthermore,
they found that several plutons in the Central Coast Ranges are early
Late Cretaceous (about Cenomanian to Senonian), the same age as the
plutons of Yosemite National Park. The various potassium-argon ages
to date in California are shown in Fig. 17.8. Curtis et al. conclude that the
bulk of the great batholiths of California are of the later date, but that
some are late Jurassic, and, as the earlier writers concluded, are closely
associated with the post-Kimmeridgian folding and thrusting.

The term Nevadan orogeny has been used to denote those tectonic
events that occurred in the general region of the Sierra Nevada in a rather
limited interval of time between the Kimmeridgian and Portlandian. The
great batholiths are indelibly impressed in the literature as an outstand-
ing characteristic of the orogeny, so now with die recognition that the
main batholiths are much younger we are faced with a redefinition of the
term, Nevadan orogeny. It is here proposed to call those disturbances
and intrusions in Late Jurassic time (post-Bathonian ) the early Nevadan
orogeny, those of Early Cretaceous time the mid-Nevadan orogeny, and
those of Mid- and early Late Cretaceous time the late Nevadan orogeny
(see Fig. 17.7).

General Characteristics

The Jurassic and pre-Jurassic rocks thus far described were severely
folded and thrust-faulted in the Sierra Nevada, and then invaded by
granitic magma. The maximum deformation seems to have been con-
centrated along what is now the western slopes of the Sierra Nevada in
the zone of the western belt of Jurassic deposits. Overturned folds, some
of great amplitude, great thrusts, such as the Mother Lode zone, and mild
dynamic metamorphism were widespread. The eastern belt of Triassic and
Jurassic rocks, near the present crest of the Sierra Nevada, is strongly
folded, but less dynamically metamorphosed. The eastern belt of Triassic

MESOZOIC SYSTEMS ALONG THE PACIFIC

269

and Jurassic rocks continued southward into southern California, but is
much obscured there by Tertiary lavas and late Cenozoic faulting. See
Kg. 17.9.

Central and Northern California

At the north end, in the Taylorsville region, the Paleozoic rocks are
thrust eastward over the Jurassic, overturning them toward the east, just
the opposite of the thrusting along the Mother Lode on the west flank
jof the Sierra Nevada. It will be recalled that the Late ( ? ) Jurassic thrust-
''ing in western Nevada was both toward die east and west and locally
(probably southward and northward. In the Grass Valley area of the
northern Sierra Nevada, Johnston (1940) finds the rocks to have been
compressed into northwest-trending isoclinal folds. The metamorphism
swas of the feeblest kind. The Mariposa was compacted and cemented,
land some of the andesitic rocks acquired schistocity; but the chemical
sand physical changes were much less severe than those imposed upon
the Calaveras formation in late Paleozoic orogeny.

Regarding the post-Mariposa plutons, Knopf (1929) says that in the
'Mother Lode belt the oldest of these rocks there are peridotites which,
•soon after intrusion, were transformed into serpentines. Smaller masses
■of gabbro and hornblendite were then intruded into the peridotite, to
iwhich they seem to have a predilection. The peridotite, gabbro, and horn-
blende appear to represent the "basic prelude" to tremendous intrusions
jof granodiorite that form the bulk of the present Sierra Nevada.

The granodiorite is uniform in texture and composition, and contains
Jbasic clots which are very common in the high Sierra. Quartz diorite
porphyry is intrusive into the Mother Lode belt south of Placerville. It
[grades into the granodiorite and has exerted no perceptible contact meta-
morphism. Knopf believes that the granodiorite ascended to a high level
"in the earth's crust in the gold belt area. Dikes and small intrusive masses
?lof a white rock composed almost entirely of albite complete the intrusive
licycle. Allied varieties of the granodiorite are quartz-monzonite, granite,
'and alaskite. The Mariposa is affected by contact metamorphism as much
as a mile away from the granodiorite contact.

In the northern Sierra Nevada, Johnston (1940) finds essentially the

CALIF. I NEV

NARROW I

VOLCANIC CALIFORNIA TROUGH

ARCHIPELAGO i

ASUNCION GR

WAIN 6ATH0LITHIC INTRUSIONS
(NEVADAN OROGENY) AND OEPOSITION DVRINt
AL61AN AND CENOMANlAN TIME

POST-KIMMERIOGIAN FOLD1NO AND THRUSTING
DEPOSITION FROM LATE JURASSIC IT1THON1AN)
THROUGH LATE EARLT CRETACEOUS
(APTIAN) IN NEW WESTERN TROUGH

I
i
i

6ATH0LITHIC INTRUSIONS IN TlTHONlAN

DEPOSITION THROUGH EARLT LATE JURASSIC
(KIHMERIDGIAN) WITH DISTURBANCE IN
EARLY JURASSIC

OEPOSITION THROUGH TRIASSIC WITH
DISTURBANCE 8ETWEEN MIDDLE AND
UPPER TRIASSIC

Fig. 17.9. Evolution of the Sierra Nevada through Mesozoic time. C is Calaveras formation of
late Mississippian (?) age; F is Franciscan group; K, Knoxville formation; P, Pashenta formation;
and H, Horsetown formation.

same batholitic cycle as Knopf does to the south in the Mother Lode,
namely, an intrusive succession of ultrabasic rocks, gabbro, diabase,
granodiorite, granite, and aplite. Granodiorite was intruded in tremendous
batholithic masses that now form the backbone of the high Sierra. On the
western slope, smaller masses of granodiorite are satellitic to the main
mass. The earlier formations were shoved aside and possibly in part as-
similated, and contact metamorphic zones were developed in the sedi-
mentary rocks. From the last emanations of the granitic intrusions were
formed die gold quartz veins of the Sierra Nevada.

In the southern part of the Sierra Nevada, Mayo ( 1941 ) in reviewing
the work of others and himself, finds that hornblende gabbro and horn-
blende diorite were forerunners to the main granitic intrusions. These

270

MT. McGEE

STRUCTURAL GEOLOGY OF NORTH AMERICA

MT. EMERSON

ZONE OF GRANODIORITE

NORTH PALISADE

DEVIL'S CRAGS

^•r. ■■'-..:

5 BASIC COMPLEX

TABLE MTN

LOOKOUT

"1 — ^T~— — ■

Gr 5 Gr 3 Di Gr

COYOTE RIDGE

ROUND MTN.

. ' ' T.i.'l ■■!■■■•' ■ jl -

5 3 Di

Gr Gr

SCALE IN MILES

5 6r 5

Fig. 17.10. Structure sections across southern Sierra Nevada Mountains. Upper section is north
of Connecting Link; second section is south of it. Lower two sections are across the northern part
of Coyote Salient, s, septum; Gr, granitic rocks; Di, Diorite and gabbro. After Mayo, 1941.

basic intrusions now appear widely distributed as dark zones, strips, and
masses of various shapes and simulate the remnants of the metamorphic
rocks. According to Mayo, the bulk of the Sierra Nevada core ranges in
composition from granodiorite to granite, with quartz monzonite predomi-
nating. All members of the intrusive sequence are penetrated by dikes of
aplite and pegmatite. Some basic dikes were late comers also.

The groups of intrusions are separated at many places by long, narrow
strips and by local broad areas of metamorphic rocks. The metamorphic
rocks are divisible into two groups: an older series of metasediments of
probable Paleozoic age, and a series of metavolcanics, part of which
Knopf has assigned to the Triassic.

The metamorphic rocks are remnants of septa (Fig. 17.10) that divided
the intrusions to unknown depths. During the earliest recorded deforma-

tion, the original bedding and other layered structures were thrown into
a series of closely appressed, nearly vertical-sided, isoclinal folds. Cleavage
developed approximately parallel to the axial planes of the folds, and
was followed by many small shears and a few upthrusts. Linear structures
that vary greatly in pitch were formed in the planes of cleavage, bedding,
shears, and upthrusts. These metamorphic rock structures are separated
from the intrusions by contacts that are usually very steep and sharp.
Gradational contacts are suggested in a few places.

Within the granitic rocks, a parallel arrangement of inclusions, min-
erals, and schlieren reveals layered and linear traces of flow that are as-
signed to the plastic stage of intrusion. These structures of the plastic
stage, by grading into fractures, locally record the stage of transition.
The stage of transition was followed by the solid stage, when adjustments
resulted in fracturing.

In the Huntington Lake area of the western slope of the central Sierra
Nevada, Hamilton (1956a) has concluded that the crystalline rocks there
consist of ten separate, sharply bounded, plutons which range in size
from one square mile, approximately, to several hundred square miles.
Only small parts of this area consist of metamorphic rocks. See Fig. 17.11.

The granite rocks range from alaskite to quartz diorite, but it is impor-
tant to note that a rock type does not constitute a separate intrusion, but
rather, each intrusion may be made up of two or more rock types. Two of
the plutons range from quartz diorite through granodiorite to quartz
monzonite. In another, the content of ferromagnesian minerals varies from
2 to 19 percent. The abundance of ferromagnesian minerals and of the
dark inclusions are closely parallel. The inclusions are xenolithic, and
some and possibly most of the mafic minerals are products of assimilation
of metamorphic rocks. Most of the granite rocks are believed to have
formed from the upward intrusion of mobile materials.

The western group, consisting of the Tamarack Creek, Huntington
Lake, Sheepthief Creek, and Kaiser Peak plutons, is considered the older,
and eastern group, consisting of the Mt. Givens, Red Lake, Rodeo
Meadow, Dinkey Lake, Coyote Creek, and Helms Creek plutons, the
younger.

The relative aerial abundances of the rock types are as follows:

MESOZOIC SYSTEMS ALONG THE PACIFIC

271

alaskite

granite

quartz monzonite

granodiorite

quartz diorite

5 percent

4 percent

47 percent

33 percent

11 percent

This confirms Mayo's observation that quartz monzonite is the most
voluminous rock type in the Sierra Nevada where studied petrographi-
cally.

Age of the Batholiths

The first determination of the age of the Sierra Nevada batholith by
isotope methods was made by Larson et al. in 1954. Lead-alpha activity
ratios were determined on the accessory minerals zircon, monozite, and
xenotime. Seven samples yielded an average age of 100 m.y. Twenty-five
samples were run from the batholith of southern California, and these
gave an average age of 105 m.y. (Larson et al., 1954).

A few years later samples were taken by Evernden et al. ( 1957 ) from
eight individual intrusions in the Yosemite Canyon area of the Sierra
Nevada batholithic complex, plus a pegmatite of one of the plutons and
their ages determined by the potassium-argon method. The major plutonic
bodies had been mapped by Calkins (1930) and Rose (1957) who had
established for the most part the relative ages of the intrusions on con-
vincing field evidence. From youngest to oldest the seven plutons are
named as follows: Johnson granite porphyry, Cathedral Peak granite,
Half Done quartz monzonite, Sentinel granodiorite, El Capitan granite,
Gateway granodiorite, and Arch Rock granite. The Hoffman quartz mon-
zonite, which is noted to have intrusive relations to the Cathedral Peak
granite, was also sampled. The experimental age determinations agreed
perfectly with the relative ages determined by geological field relations.
The youngest, the Johnson granite porphyry, yielded a date of 82.4
( ± 1-2%) m.y., and the oldest, the Arch Rock granite, 95.3 ( ± 1-2%). The
authors from theoretical considerations regard these ages as slightly
younger than the true absolute ages of the plutons, but believe any change
made ultimately will be in the order of a few percent at most.

A pegmatite in the Hoffman pluton (83.3^1-2% m.y.) yielded an age

of 76.9 m.y., and the range from this youngest rock to the oldest is there-
fore approximately 18 m.y. This intrusive activity would have occurred
according to Curtis et al. (1958) during the Cenomanian, Turonian, and
Senonian (see Fig. 17.7) epochs.

If the series of nine plutons, including a late pegmatite, were intruded
during an interval of 18 m.y., a separate intrusion approximately each
2 m.y. would have been emplaced. Evernden et al. (1957) review the
field evidence to the effect that most of these intrusions were almost com-
pletely crystallized at the time die succeeding pluton was emplaced,
and thus conclude that crystallization of each would require somewhat
less than 2 m.y.

Fig. 17.11. Plutons and rock types of the Huntington Lake area; Sierra Nevada batholithic com-
plex. Direction of pattern lines has no significance.

272

STRUCTURAL GEOLOGY OF NORTH AMERICA

The granitic rocks were exposed by erosion at the time the Turonian
sediments were deposited (see Figs. 17.7 and 17.8) and hence, only a
short time separated the last intrusion from its exposing. It may thus be
assumed that uplift and erosion kept close pace with granitic emplace-
ment. From this Evernden et al. deduce that the space for the batholiths
was produced by the elevation of the roof slowly and by small increments,
and that the overlying sedimentary rocks were stripped by erosion as
rapidly as they rose.

ANCESTRAL COAST RANGE SYSTEM

Franciscan Basin

Following the post-Kimmeridgian folding and thrusting (Fig. 17.9)
a trough or basin sank on the west in California, and in it an exceedingly
thick sequence of sediments accumulated. These are those of the Fran-
ciscan group and equivalents (Fig. 17.2). West of this trough lay a
sourceland of sediments, viewed as a narrow volcanic archipelago by
Taliaferro ( 1942 ) . The strata known as Franciscan crop out in the Coast
Ranges and the Shasta and Upper Cretaceous strata occur in the Sacra-
mento Valley. The thickness of the Franciscan is about 35,000 feet. That
of the Shasta series is about 10,000 feet and of the Upper Cretaceous on
the west side of Sacramento Valley is 15,000 feet.

According to Irwin ( 1957 ) :

The Franciscan group consists dominandy of detrital sedimentary rocks
with interbedded chemical sedimentary and volcanic rocks. The detrital
rocks are chiefly sandstones of the graywacke type, with interbedded shale
and conglomerate. Reliable criteria have not yet been described for dis-
tinguishing, either in hand specimen or under the microscope, between
detrital rocks of the Franciscan group and those of the Sacramento Valley
sequence. The most obvious and significant difference between the lithologic
character of the Franciscan group and that of the Sacramento Valley sequence
is the presence and local abundance of interbedded volcanic rocks and as-
sociated chemical sedimentary rocks in the Franciscan. The chemical sedimen-
tary rocks include rhythmically thin-bedded chert, and, much less abundandy,
a distinctive foraminiferal limestone. In addition, the Franciscan group includes
small areas of glaucophane schists. In some areas, strata of the Franciscan group
have been metamorphosed to slates and phyllites.

The Franciscan group has been intruded by mafic and serpentinized ultra-

mafic rocks, and has been highly faulted and pervasively sheared. The general
appearance of the Franciscan terrane, because of the net effect of the lithologic
heterogeneity and complex structural deformity, is in striking contrast to areas
underlain by strata of the Sacramento Valley sequence.

The Knoxville formation as exposed along the west side of Sacramento Valley
between Wilbur Springs and Paskenta is perhaps 10,000 feet in average thick-
ness. The base is unknown, as along most of the valley the lowest exposed beds
are in fault contact with the belt of ultramafic rock. The Knoxville formation is
generally considered to consist typically of a thick section of thin-bedded shales
with small lenses of limestone, but interbedded sandstones and conglomerates
are locally abundant. Fossils indicate that it is Late Jurassic (Tithonian) in
age. One of its most characteristic and abundant fossils is Aucella piochii Gabb.

The contact between the Knoxville formation and overlying Shasta series is
marked by a fairly abrupt and complete change in fauna, and at many places
by beds of conglomerate. Here, as well as at other places, the concept of a
"basal conglomerate" has much influenced the subdivision of the Sacramento
Valley sequence. Along much of Sacramento Valley the transition from one
unit to the other is one of nearly continuous deposition and, judged from broad
structural conformity, was accomplished with litde disturbance.

The strata referred to the Shasta series have a higher ratio of sandstone to
shale than has the Knoxville formation.

Upper Cretaceous strata along the west side of Sacramento Valley consist
of sandstones and shales and are about 15,000 feet in average thickness. They
represent only the lower part of the Upper Cretaceous section of northern
California.

Mid-Cretaceous Phase (Mid-Cretaceous Orogeny)

In many places in the Coast Ranges there is either a definite discon-
formity or a strong unconformity or overlap between the Shasta series
and the Upper Cretaceous strata. Especially in the Santa Lucian Range,
there is evidence of deep erosion and overlap. Along the crests of some
of the folds produced during this disturbance, the Lower Cretaceous
and Upper Jurassic beds were removed, so that the Shasta trough was
lifted in subparallel fragments. Other parts of the Shasta beds were
little affected. The orogeny represented by the unconformity has been
called the Mid-Cretaceous by Taliaferro (1943b).

Mid-Upper Cretaceous Phase (Santa Lucian Orogeny)

The Upper Cretaceous strata in the Coast Ranges are divisible into
two groups, the Pacheco and the Asuncion, which together make up
the Chico (Taliaferro, 1943b). See Fig. 17.2. The Pacheco consists in

Fig. 17.12. Evolution of structure along cross section through south
central part of Adelaida quadrangle, Calif., showing relations between
various units of Cretaceous and relations to older and younger rocks.

1. Structure as it exists at present.

2. Structure along same section during deposition of Middle Miocene.

3. During deposition of Asuncion, late Upper Cretaceous.

4. During deposition of Jack Creek formation, early Upper Creta-
ceous.

(After Taliaferro, 1944.)

Middle Miocene
Rhyolite flows and shallow

Middle Miocene
Siliceous shales, marls,

cherts and limestones.

Unconformity

•^Y^A Late Upper Cretace
rJ^^Asuncon Group

Unconformity

Lower Cretaceous
MarmoleJO Formation.
^J-^j basal breccia. Kmb

^SjZI Middle Miocene k. I Lower Miocene

^■%{ Analcile Diabase- sills and | ^| Voqueros sandstone

dikes.

I i i i 9 1

. Early Upper Cr«taceou»
.'-,^ic,'; Jack Creek Formation

Jurasilc
„ JffcJ Francican- Knoivlll*
d.fferent.aUd

Horizontal Scale and Vertical Scale The Same

274

STRUCTURAL GEOLOGY OF NORTH AMERICA

the central Coast Ranges of 7000 to 8000 feet of gray sandy shales, silts,
sandstones, and conglomerates. If it was not removed by erosion before
the Asuncion group was deposited, it rests on the erosion surface that
followed the Diablan orogeny. The Pacheco sediments may be less widely
distributed in the central Coast Ranges than the Asuncion but probably
more widely in the northern Coast Ranges (Taliaferro, 1943b).

The Pacheco and Asuncion groups are separated by an unconformity
which in places in the central Coast Ranges is as angular as 80 degrees.
See Fig. 17.12. The disturbance represented by this unconformity has
been named the Santa Lucian by Taliaferro ( 1943b). Where the Asuncion
laps over older rocks than the Pacheco, which it does in a number of
places, it is difficult if not impossible to distinguish the two disturbances
— in fact, to recognize that more than one disturbance has occurred ( Tal-
iaferro, 1943b).

The Santa Lucian orogeny was strongest in the Santa Lucia Range and
died out eastward. During the orogeny, the Gabilan mesa rose for the first
time (Taliaferro, 1944). This has been called the Diablo uplift by Reed
( 1933 ) . Another land projection into the general north-south trough lay
to the south and has been called Catalina. At the north various authors
have recognized the Klamath Island or Klamathonia. All three are here
treated as peninsulas, branching off the volcanic archipelago, which as
a whole has been called Pacifica. See the tectonic map of the Late Cre-
taceous, Plate 12.

As with other diastrophisms in California, the Santa Lucian appears to
have taken but a relatively short time. Although there was deep erosion
and widespread stripping, subsidence again took place, and the sea spread
rather rapidly over an area of considerable relief. The latest Upper Cre-
taceous, the Asuncion, is the most widespread Cretaceous unit in the Coast
Ranges. The Asuncion is predominantly coarse grained, being made up
of arkosic sandstone and coarse conglomerates perhaps 10,000 feet thick.
Fine sediments increase eastward. Franciscan debris increases toward the
west. Near the present coast, the basal conglomerates contain large angu-
lar to subrounded blocks of Franciscan chert, basalt, diabase, and sand-
stone, as well as well-rounded pebbles, cobbles, and boulders of the
ancient crystalline complex (Sur series and Santa Lucia granodiorite).

To the east in what is now the great valley of California, the Upper
Cretaceous deposits have been divided into twelve foraminiferal zones,
and these grouped into seven stages (Goudkoff, 1945). During the first
three stages, the sea was transgressive eastward on the early Sierra Ne-
vadan landmass, and reached a maximum distance at the end of the third
stage except in the most northerly part. Near the end of the Upper Cre-
taceous (beginning of seventh stage) a low land barrier just south of
Stockton divided the region into two basins. The extent of the barrier
westward into the site of deposition of the Chico strata has not been
worked out. During the earlier stages, the sediments came from the west
as pointed out by Taliaferro, but in the later stages considerable material
came from the east according to Goudkoff, and some from the northwest.
The eastern source suggests slight uplift in the closing phase of the
Cretaceous in the Sierra Nevadan landmass.

Evidence of igneous activity is present in many formations of the Meso-
zoic and Cenozoic in the central Coast Ranges of California, and Talia-
ferro, emphasizes the fact that volcanism was nearly continuous in one
place or another nearby during these eras.

COLUMBIA SYSTEM

Extent

The term Columbia system will here be used to signify the mountains
and troughs of the Mesozoic era in British Columbia, southeastern Alaska,
the Yukon, Washington, western Idaho, and eastern Oregon. It is defined
approximately by the extent of the Triassic and Jurassic troughs and the
volcanic archipelago on the west that supplied much of the material to
the troughs. In many respects it is a parallel, if not a continuation, of the
great Sierra Nevada and Ancestral Coast Range systems of the United
States. See tectonic maps, Plates 10, 11, and 12, and Fig. 17.13.

Triassic and Early Jurassic Phase

In the southern interior of British Columbia and more particularly
southward from Kamloops Lake, strata, presumably of Triassic age, are
widely displayed. See map, Fig. 17.14. This assemblage is generally re-

MESOZOIC SYSTEMS ALONG THE PACIFIC

275

ferred to as the Nicola series and consists largely of volcanic intrusives
and effusives, tuffs, and agglomerates with argillites and limestone at
several horizons. The total thickness in places is 10,000 to 15,000 feet, but
the uppermost part may be of early Jurassic age. In the vicinity of Kam-
loops, the Triassic beds with a basal conglomerate rest without angular
unconformity on Carboniferous beds, and possibly the same general re-
lation holds elsewhere. The Triassic strata occur also west of the Fraser
River, and consist of dark green massive andesite and basalt, chert, argil-
lite, limestone, and tuffaceous shales (Cairnes, 1936). Volcanic rocks
with minor amounts of sediments occupy a great part of Vancouver Island
and of Queen Charlotte Islands. See map, Fig. 17.18. The bedded char-
acter of the fragmental volcanic rocks, the presence of a few limestone
members which in some places are several thousand feet thick, and the
marine fossils found in tuffs and other sediments indicate that the gen-
eral assemblage is marine in origin. It seems probable that the beds formed
in a sea which, like that of Carboniferous time, extended over the greater
part, if not all, of the Canadian Cordilleran region.

In Queen Charlotte Islands, a thick clastic series with some pyroclastic
material ranges in age from Late Triassic to Jurassic and grades upwards
into a volcanic assemblage of tuffs and effusives, 5000 feet or more
thick. The tuffs are fossiliferous, evidently were laid down in the sea, and
are of Jurassic and, presumably, Middle Jurassic age. The thick assem-
blage of Triassic volcanics and sediments which is widely displayed over
Vancouver Island and known as the Vancouver group also may be suc-
ceeded by beds of Jurassic age. Strata, resembling the Vancouver group
and related series but in places much metamorphosed, occur at intervals
along the mainland coast and as included masses in the granitic rocks of
the Coast Range. Along the eastern side of the Coast Range, within the
basins of Nass and Skeena rivers, is a thick assemblage of sedimentary and
volcanic rocks known as the Hazelton group and which, as indicated by
an imperfectly known flora and fauna, is of Jurassic, possibly Mid- Jurassic,
age. The proportions of sedimentary and volcanic material composing the
Hazelton group varies from district to district with some indication that
the nonvolcanic sedimentary rocks become more and more preponderant
as the formation is followed eastward from the margin of the Coast

Fig. 17.13. Batholiths of the North
American Cordillera. Heavy dashed
lines indicate axes of anticlinoria, syn-
clinoria, belts, and general trends of
the late Jurassic and early Cretaceous
phase. The dashed line north of the
Nelson batholith is an anticlinorium in
Proterozoic strata and may be Lara-
mide in age.

276

STRUCTURAL GEOLOGY OF NORTH AMERICA

SHUSWAP TERRANE

Fig. 17.14. Generalized distribution of major rock units of southern British Columbia. After
Smith and Stevenson, 1955. The Shuswap terrane has some Paleozoic and Belt outcrops and the
Belt terrane has some small patches of Paleozoic. All are invaded by great plutons, not shown,

Range. The strata are, in part at least, marine; but the presence of plant
remains and other features indicates that the sea was shallow and that
possibly some parts of the assemblage may be nonmarine. In southern
British Columbia, various assemblages of sediments or of sedimentary and
volcanic strata are known, and others are believed to be of Jurassic age,
as in the Kootcnay Lake district where fossiliferous Jurassic beds rest on
Paleozoic strata. In general, the Jurassic beds appear to be as widely, or
even more widely, distributed than the Triassic; and as yet there is no
evidence of any interval of orogenic movements that separated the two

except the Upper Cretaceous of Vancouver Island and the strata east of the Rocky Mountain
trench.

periods. Thick assemblages of sediments and pyroclastics, as well as great
volumes of extrusive and intrusive volcanic strata, occur in northern Brit-
ish Columbia and southern Yukon and apparently correspond to the
Hazelton group in the south. For more detail, see Canadian Geological
Survey, Economic Geology Series, No. 1, 1957.

Daly (1912) describes Triassic strata in northwestern Washington that
have a thickness between 3000 and 7000 feet. They are principally dark
gray to black argillite, in part bituminous, generally associated with bands
of gray to greenish gray sandstone and grit and in a few places with fine

MESOZOIC SYSTEMS ALONG THE PACIFIC

277

conglomerate. The gritty beds are charged commonly with small angular
fragments of black argillite. All the coarser types are decidedly feld-
spathic. Some of these sediments could probably be called graywacke.

In southeastern Alaska, Buddington and Chapin (1929) have noted
numerous outcrops of Triassic rocks and others that may be Triassic. All
the strata that carry fossils are Upper Triassic, and they seem to be
divisible into three units, one consisting of sediments and the other two
of volcanic rocks with a little intercalated sedimentary material. The
volcanic formations are differentiated from volcanic formations of other
periods on the basis of their faunas. Their character and structural rela-
tions over wide areas are insufficiently known, and their lithology is too
similar, to separate them otherwise. They comprise green andesitic flows,
breccia, and tuffs. The lava predominantly shows pillow structure but is
in part amygdaloidal and in part polygonally jointed. Much of the breccia
has a limestone matrix and is in considerable part the result of primary
disaggregation of the radial-jointed pillows. The basal part of the volcanic
rocks on Kuiu Island consists of interbedded limestone and green
andesitic tuff and lava with local conglomeratic beds. On Kupreanof
Island, the volcanic formation has a bed of conglomerate 150 to 200 feet
thick in local areas at its base. The basal Triassic conglomerates and the
unconformable relations to the Paleozoics have been discussed previously
in the section on the late Permian or early Triassic orogenic phase.

On Kupreanof Island and the islands southeast of Kake, Upper Triassic
sediments overlie the upper limestone division of the Permian and are
overlain in apparent conformity by volcanic rocks of late Triassic age.
Locally there is a thick bed of coarse conglomerate of the Upper Triassic
volcanic rocks. On the northeast side of Kuiu Island, however, the Upper
Triassic volcanic rocks overlie the lower division of the Permian without
any apparent angular unconformity. The volcanic rocks of Kuiu Island
also carry a different fauna from those on Kupreanof Island. Uncon-
formities are indicated, therefore, not only at the base of the Upper
Triassic, but within it (Buddington and Chapin, 1929).

The Triassic occurrences in British Columbia and Washington are so
little known that unconformities within the beds assigned to this period,
if they exist, are not known. It is of interest, however, to recall the un-

conformities below and above the Upper Triassic beds of western Nevada,
and to note the same position of breaks in southeastern Alaska.

Another series of beds that was intruded by the great Coast Range
batholith in southeastern Alaska has been assigned questionably to the
Jurassic. Some of these beds may be Lower Cretaceous and some Triassic
and Paleozoic. They have been divided into two groups for mapping pur-
poses, namely, a predominantly sedimentary facies consisting of gray-
wacke, black slate, and conglomerate; and a predominantly volcanic
facies consisting of schistose greenstone made up of breccia, flows and
tuffs, and black slate and graywacke.

These questionable Jurassic and Lower Cretaceous rocks are believed
to overlie the Paleozoic and Triassic formations unconformablv. A pro-
nounced angular unconformity separates Jurassic from Devonian forma-
tions at the north end of Kupreanof Island, but where the Jurassic rests
on the Triassic the break is more in the nature of a disconformity ( Bud-
dington and Chapin, 1929).

The Jurassic ( or Lower Cretaceous ) slate and graywacke appear to be
much less metamoq)hosed than the older Mesozoic formations, but this is
certainly in part due to their character. In all the formations, the
argillaceous beds are most resistant to recrystallization, and their abun-
dance in this series gives the Jurassic formations a misleading appearance
of minor metamorphism. The pebbles and cobbles of the intercalated
conglomerates are very markedly flattened as a result of very strong
pressure.

To summarize, map, Fig. 17.14 may be referred to again. Triassic and
Jurassic strata were deposited east of the present Canadian Rockies, and
this basin of deposition was separated from a broad region of deposition
by a Mesozoic geanticline which now is displayed chiefly as the Beltian
terrane. The sediments of the eastern basin are miogeosynclinal and shelf
types, whereas the sediments west of the geanticline are eugeosynclinal.
and may have accumulated in several deep troughs. Due to the great
batholiths that occupy much of the region of southern British Columbia
it is apparently impossible to recognize the original extent of the basins or
their number. Triassic and Jurassic strata occur in places from Kootenay
lake westward to the Pacific Ocean. Since thev are laden with volcanic

278

STRUCTURAL GEOLOGY OF NORTH AMERICA

SCALE

IN MILES
10

Fig. 17.15. Diagrammatic east-west section through the Okanogan composite batholith. s, schists
and associated Paleozoic rocks; Cr, Pasayton lower Cretaceous arkose sandstones; la, Chopaka
peridotite; lb, Basic complex; 2, Ashnola gabbro; 3a, Remmel batholith, western phase; 3b,
Remmel batholith, eastern phase; 3c, Osoyoos batholith; 4, Kruger alkaline body; 5, Similkameen

materials a west-lying volcanic archipelago and orogenic belt may be
postulated similar to that of California and western Nevada, but possibly
the regions of sedimentation were complicated by geanticlines which
also were sites of volcanic activity.

Cretaceous strata have a more restricted distribution. Lower Cretaceous
deposits of interior basin type occur in the Rockies and eastward. A
narrow trough of them is recognized west and south of Kamloops Lake in
mid-interior and east of the great Coast Range batholith. A small deposit
occurs on the west coast of Vancouver Island. It would appear that the
Belt geanticline had widened westward from Jurassic time, and perhaps
another broad geanticline existed in the site of the Coast Range batholith
and Vancouver Island.

In Late Cretaceous time the entire interior from the Rocky Mountains
to the Strait of Georgia had become emergent and only flanking deposits
accumulated. This was undoubtedly a consequence of the great batho-
lithic intrusions and previous compressional orogeny of the broad cordil-
leran region.

Nevadan Orogeny

Batholiths of the International Border. An almost continuous succes-
sion of batholiths stretches more than 350 miles along the international

batholith; 6a, Cathedral batholith, older phase; 6b, Park granite stock; 7, Cathedral batholith,
younger phase.

The components of the batholith are numbered in order of intrusion. Vertical scale is exag-
gerated twice the horizontal. After Daly, 1912.

border between Washington and British Columbia. These have been de-
scribed by Daly (1912), Smith and Calkins (1904, 1906), and Smith
(1904), and later studies have been made by Waters, Krauskopf, Camp-
bell, and Pardee (see references in Waters and Krauskopf, 1941). It is
highly probable that the plutons form the basement southward under vast
areas of lavas of the Columbia Plateau because granitic rocks appear in
the Ochoco-Blue Mountains uplift of central and eastern Oregon midway
to the Klamaths and Sierra Nevada (Waters, 1933) and also southeast-
ward under more lavas through the Thatuna batholith to the great Idaho
batholith.

Intrusions of peridotite, gabbro, and diorite are associated with the
prevailing granodiorite of the great batholiths. Quartz monzonite, quartz
diorite, and granite are locally widespread. In certain areas where the
succession of intrusions has been worked out, it is a cycle similar to that of
the Sierra Nevada, viz., first the smaller bodies of ultrabasic rock, then
gabbros and diorites, and finally the great granitoid bodies. Pegmatites are
rare in the batholiths along the border in Washington, but aplite masses
locally of almost batholithic proportions crosscut the earlier intrusions.
The borders of the batholiths commonly show discordant relations to the
country rock, and extensive masses of contact breccia are found along the
intrusive margins (Waters, 1933). Some of the best examples of dis-

MESOZOIC SYSTEMS ALONG THE PACIFIC

279

SCALE IN MILES
I 2 3

; Fig. 17.16. Cross sections in the Similkameen District, British Columbia, Latitude 49, Longitude
| 120. 1, Vaseaux fm., paragneiss, schist, quartzite; 3, Koban group, schist, greenstone; 6, Barslow
| fm., argillite; 8, Shoemaker fm., chert, some tuff, greenstone; 9, Old Tom fm., greenstone, basalt
j flows, sills, bosses, some diorite; 10, altered rocks of dioritic composition; 11a, Osoyoos grano-
f diorite; lib, Fairview granodiorite; 12a, hornblendite; 12b, pyroxenite; 14a, Kruger syenite;

rcordant batholiths, as well as some of the most conclusive evidence of
Utoping, may be found in the northern Cascades (Daly, 1912). See Figs.
$7.15 and 17.16.

The igneous bodies generally designated by the names Osoyoos and
. Colville batholiths (see map, Fig. 17.13) are really complex associations of
| eight plutons. Contact metamorphism is intense near some but almost
i absent near others ( Krauskopf , 1941 ) . Detail along the border of the Col-
j ville batholith has been worked out by Waters and Krauskopf ( 1941 ) . See

14b, Oliver syenite; 15, granodiorite; 16a, Oliver granite; 17, Springbrook fm., conglomerate,
some sandstone and shale; 18, maroon fm., basaltic lava, some breccia, tuff, conglomerate.

1 and 3, Carboniferous. 6, 8, and 9, Triassic or older. 11 and 12, Jurassic (?). 14, 15, and
16, Jurassic and (or) younger. 17 and 18, Eocene. After Daly, 1912.

Fig. 17.17. The batholith is a complex plutonic mass that intrudes folded
and dynamometamorphosed sedimentary and volcanic rocks of late Paleo-
zoic and Triassic age. Along the sharply discordant contact, the wall rocks
are much fractured and granulated, but contact metamorphism is slight or
absent. The batholith is remarkably heterogeneous, both structurally and
petrographically. A central mass of structureless granodiorite grades out-
ward into a belt of foliated igneous rock which commonly shows intricate
swirling of the foliation. The swirled rocks grade into a peripheral belt of

280

STRUCTURAL GEOLOGY OF NORTH AMERICA

Fig. 17.17. Structural map of part of the border of the Colville batholith. Reproduced from
Waters and Krauskopf, 1941.

variable but well-foliated migmatitic gneisses ( magmatic injection and re-
placement) characterized by severe granulation of the constituent min-
erals. Over broad zones this rock is a mylonite (crushed and rolled-out
streaky powder); locally recrystallization has produced types resembling

metamorphic granulites. That the crushing was protoclastic (localized
along the contact) and not due to regional metamorphism following the
solidification of the batholith, is proved by the relations with the wall
rocks and by the widespread cementation of the broken materials by films
and stringers of undeformed quartz and microcline.

Along the contact between the approximately contemporaneous Oso-
yoos and Colville batholiths occurs a narrow belt of heterogeneous syenite
with highly complicated internal structure. This is believed to be a hybrid
rock formed by the action of magmas and emanations from both batho-
liths upon a thin septum of wall rock.

The Coast Range Batholith and Related Structures. The Coast Range
batholith extends for more than 1100 miles from Fraser River in British
Columbia northwestward into Yukon Territory. See Figs. 17.13 and 17.18.
From Vancouver to Skagway on the mainland, the batholith forms the
backbone of the Coast Range and is exposed either at the shore fine or a
short distance inland. Outlying dikes, stocks, and batholiths believed to
be of the same general period of intrusion as the main batholith and
genetically allied to it are found locally on Vancouver Island and the
Queen Charlotte Islands, and abundantly throughout most of the Alex-
ander Archipelago. The Coast Range batholith is the largest on the North
American continent, aside possibly from certain ones of Precambrian age.

It is widest south of Skeena River in British Columbia, where it reaches
110 miles east and west. In southeastern Alaska, it is 35 to 60 miles wide.

Buddington ( Buddington and Chapin, 1929 ) discusses the batholith in
southeastern Alaska in respect to six approximately parallel belts, namely,
the border zone east of the Coast Range batholith, the Coast Range batho-
lith of the mainland, the Wrangell-Revillagigedo metamorphic belt, the
Prince of Wales-Chicagof belt, the Kuiu-Heceta belt, and the Dall-
Baranof belt. See map, Fig. 17.19.

The eastern border zone is conspicuous for its absence of contact
metamorphism on a regional scale. Even local metamorphism is meager.
The border zone rocks are closely folded; argillaceous rocks have been
changed to slaty types, and locally andesitic volcanic rocks to greenstone;
but there is practically no phyllite and no crystalline schist away from the
immediate contact of the intrusive bodies.

MESOZOIC SYSTEMS ALONG THE PACIFIC

281

«r—

Fig. 17.18. Geomorphic divisions of British Columbia and southeastern Alaska showing the divi-
sions of the western, or Pacific mountain, belt of the Cordilleran region. To bring the fiord
system into prominence the sea within the 100-fathom line is shown in solid black. Within the

Some stocks or batholiths within the eastern border zone are imper-
fectly known. Between the Skeena and Nass rivers the border of the main
batholith is irregular with apophyses and outlying stocks of granodiorite.
Between Nass River and the Portland Canal, the border is fairly straight.
In the Hyder district, a mass of hornblende granodiorite has been called
the Texas batholith (Buddington and Chapin, 1929). It is probably ad-
jacent to the main Coast Range batholith and is cut by many dikes of
younger quartz monzonite and granodiorite. It is locally intensely crushed,
evidently from the thrust of the younger intruding magma. Although defi-
nitely older than the intrusions that cut it, the Texas batholith is probably
post- Jurassic, because it is similar to a nearby quartz diorite at the head of
Hastings Arm and Observatory Inlet which is intrusive into the "Bear
River series" and Nass argillite of Jurassic age.

Intrusive bodies are also known in the Atlin and Whitehorse districts,

black area are many basins which are deeper than 100 fathoms, especially in the fiord
channels. After Peacock, 1935.

but for the most part the vast region northwest of the Hyder district is
unknown.

The great Coast Range batholith itself is inadequately known, but Bud-
dington's description (Buddington and Chapin, 1929) for the section
between the Portland Canal and the Stikine River is illuminating. The
southwest border facies in a belt 5 to 15 miles wide, has the average com-
position of a granodiorite, and is composed predominantly of granodiorite,
quartz monzonite, and quartz diorite; the eastern border facies, 10 to 15
miles wide, is quartz monzonite. Dolmage ( 1923 ) reports that the more
silicic variations lie in the center of the batholith south of Portland Canal
as Buddington finds to the north, but that there are exceptions. The
changes from one type of rock to another appear to take place rather
abruptly, but no evidence of brecciation of one variant by another has
been seen, except in the small masses of gabbroic and ultrabasic rocks.

282

STRUCTURAL GEOLOGY OF NORTH AMERICA

Fig. 17.19. Tectonic map of southeastern Alaska and adjacent parts of Canada. After Bud-
dington and Chapin, 1929. Stippled areas are related bodies of the Coast Range batholith. The
solid heavy lines are the axes of anticlinoria and synclinoria that were formed at approximately

Buddington believes the variations are closely related to interlocking bath-
oliths which together build what is known as the Coast Range batholith.
The great batholith is paralleled on the west by a belt of injection
gneiss, crystalline schist, and phyllite intruded by abundant batholiths,
stocks, sheets, and dikes, believed to be outlying masses genetically associ-
ated with the main batholith. See cross sections, Fig. 17.20 This zone as

the same time that the batholith and its satellites were intruded. The dashed heavy lines are the
axes of broad Tertiary arches and sags, and also in part of folds associated with the intrusions.

previously mentioned is the Wrangell-Revillagigedo belt of metamorphic
rocks. It is narrow and loses its individuality at the north but widens and
is very well defined toward the south. Near the mouth of Gastineau Chan-
nel and west of Thomas Bay this belt has a width of about 13 miles;
opposite the mouth of Stikine River, about 25 miles; and at the soudi end
of Revillagigedo Island, about 35 miles. This composite belt of sedimen-

MESOZOIC SYSTEMS ALONG THE PACIFIC

2&3

tary and intrusive rocks appears to swing from a northwest strike north-
west of the Cleveland peninsula to a north-south strike south of the penin-
sula. This is not due to a change from the prevailing northwest strike of
the beds but to a farther west penetration of the intrusive masses at the
south end. The differences between the smaller intrusive masses in the
metamorphic belt and the quartz diorite of the western border of the
batholith are slight.

West of the metamorphic belt is the Prince of Wales-Chicagof belt in
which intrusive masses are common but less quartzose, and the country

'rock consists predominantly of slate, limestone, graywacke, greenstone,

I and dynamically metamorphosed schistose rocks with locally some crystal-
line schist and marble. The metamorphism is much less advanced than in

| the Wrangell-Revillagigedo belt, though locally adjacent to large igneous
bodies it may be intense. The belt is 40 miles wide on the north, but not
much more than 5 miles wide on Kupreanof Island; it widens on Etolin
Island, and is about 25 miles wide through Prince of Wales Island. The
intrusive rocks of this belt differ, in general, from those to the east in that
they are predominantly diorite, rather than quartz diorite, and that dif-

| ferentiates of highly contrasted composition are more adundant.

The Kuiu-Heceta belt is next west and is characterized by the least
metamorphism of any of the belts, by the fewest intrusives, and as a re-

| suit, by the best preserved and oldest fossils in its strata. The belt includes

i. the western fringe of the north half of Prince of Wales Island, the north
end of Dall Island, San Fernando, Heceta, Tuxekan, Kosciusko, and Kuiu
islands, Kupreanof Island with the exception of the Lindenberg peninsula,

i and the southwestern part of Admiralty Island.

The Dall-Baranof belt is the westernmost of the six belts of the great

i batholith with its satellites, and is characterized again by numerous stocks
and batholiths. It includes Dall, Forrester, Suemez, Baker, Lulu, Noyes,

i Warren, Coronation, and Baranof islands. The intrusive rocks on the aver-
age are more silicic and carry less of ferromagnesian minerals than the
average of the Prince of Wales-Chicagof belt. Quartz diorite and, to a

| lesser extent, granodiorite predominate.

In the main batholith, the rocks are prevailingly gneissoid. The banded

i character is most accentuated near the borders of the batholith or near

inclusions within the batholiths. Local schistose zones are found along in-
tensely sheared narrow bands. The gneissic structure is for the most part
interpreted by Buddington as primary, but still the batholith was stressed
considerably after its complete solidification. Yielding occurred by mash-
ing along local belts or zones, which may be of considerable width and
great length (75 miles or more), or by intensive shearing along narrow
zones, or by slipping along many planes of various orientation throughout
a zone. A belt of highly mashed rock 15 miles wide is crossed by Stikine
River from the head of Little Canvon to and below Flood Glacier.

In some places west of the main batholith, extensive belts, including
intrusive igneous stocks, dikes, and sills, constitute a local shear zone or
zone of close folding; the larger masses of igneous rocks may show con-
siderable mashing, and the thin sills may be closely folded together with
the schists.

After reviewing pertinent studies on the succession of the intrusions
that make up the great batholith all the way from Vancouver to Cross
Sound, Buddington draws the following conclusions:

West of the main batholith a group of ultrabasic intrusive rocks is
present in considerable volume. These include hornblendite, pyroxenite,
dunite, peridotite, and intermediate variants; they are older than the
more silicic-alkalic types.

Diorite and gabbro-diorite occur both as discrete stocks and batholiths,
and also to a minor extent as marginal variations of quartz diorite and
granodiorite. Locally gabbro-diorite and augite gabbro are the marginal
phases of diorite. The gabbro-diorite and gabbro are locally intruded by
diorite, quartz diorite, and more silicic-alkalic types.

Granodiorite in stocks and small batholiths mav show marginal variants
of quartz diorite, gabbro-diorite, diorite, monzodlorite, and very rarely of
syenite. A decrease in potassic feldspar and quartz locally on the margins
is a common feature.

Granite is the youngest of the major members of the plutonic complex,
and is uniformly found with intrusive contacts through the older mem-
bers.

Buddington also point out that west of the Sierra Nevada batholith in
the Coast Ranges of California there is a similar older group of ultrabasic

Se J level

Warren
Island
qd
Sea level-^tsfcrTUT?

JGzsJie varx>r Passage

Screen Islands

^*i Wrangell bland
dt Kgr qd *gs son 3d sph ph sph qd sph qdsphjn

G ravin a . sy nclinoriuW N _ : ' '" •'■* -•' '-.-"'W^ ****•" ^ AJ ;j|NJ!|glN^^

B' Wrangell-Revillagi^edo beltof metamorphic rocks

Kehii Stra.it

Tba

Dunca/z
CaiiaL

i level

Kuiu Island
Orfs

|ft /T<?/oa Straits

* Cmc .Cpl CPC\ *d V*

SI Da Dgt J Cpc ;T»cl Da- Cpl ;Cpl Cpl "Kay Ksg ;

Dgt

Kupreanof Island
Cpl "feci
■fiav ;"6av/ Dgt Da Dsa

Kjjs

FredericTt Sontu/

5 O

i i I ' ' '

MESOZOIC SYSTEMS ALONG THE PACIFIC

285

intrusive rocks and a younger group of more silicic-alkalic intrusive rocks.

Structure of Southeastern Alaska. Most pervading of any structural
feature is the isoclinal folding. In a belt 15 to 30 miles wide adjacent to
the west side of the Coast Range batholith, the isoclinal folds are slightly
or markedly overturned toward the southwest. See Fig. 17.20. In general
the axes or axial planes of the isoclinal folds escape detection, and uni-
form dips occur through wide intervals. In the Wrangell-Revillagigedo
belt of metamorphic rocks near the batholith, the dip of the schist and
gneiss is 60 to 90 degrees northeast. In the outer part of the belt, the dip
is 30 to 50 degrees northeast. On the east side and at the northeast end
of Prince of Wales Island, the isoclinal folds are overturned toward the
northeast (Buddington and Chapin, 1929).

Out of the numerous isoclinal and close folds, a number of anticlinoria
and synclinoria may be recognized. Two dominate the structure of the
Alexander peninsula, namely: the Junean synclinorium next west to the
great batholith, and west of the synclinorium the Prince of Wales-Kuiu
anticlinorium. Both synclinorium and anticlinorium seem to be divided
into branching or parallel synclinoria. The axes of all the major fold com-
plexes are shown on the map of Fig. 17.19.

The formations exposed in the Prince of Wales-Kuiu anticlinorium are
almost exclusively Paleozoic, and make up a belt 40 to 50 miles wide. In
the trough of the synclinorium the Upper Jurassic and Lower Cretaceous
formations are exposed. The Keku-Gravina synclinorium is dominantly a
shallow downwarp of Tertiary formations. They rest unconformably on
Mesozoic formations which in general are folded into the great Junean
synclinorium.

On the west flank of the Prince of Wales-Kuiu anticlinorium in the
Sitka district is the Sitka Mesozoic belt. A western belt of Tertiary sedi-
ments and volcanics is exposed along the shore north of Cross Sound, and
the southern half of Kruzof Island on the north side of Sitka Sound is
composed of Quaternary volcanics (Mt. Edgecombe).

Through Frederick Sound is an axis of cross folding. Jurassic beds are
exposed at intervals along the south side of Admiralty Island and form a
considerable part of the coastline. They appear to have been folded along
two axes — the usual north-northwest axis and another about parallel to
Frederick Sound — that is, east-west. The gently dipping Tertiary lava
beds about Frederick Sound seem to represent a broadly folded anticline
from the center of which they have been eroded away. The axis of the
anticline strikes northeast, approximately in the direction of the Mesozoic
cross fold, and Buddington suggests that the forces in Tertiary time were
oriented almost at right angles to those that effected the folding in pre-
Tertiary time.

Structure of the Island Ranges of British Columbia. For a clear dis-
cussion of the topographic elements of British Columbia refer to Peacock
(1935). Under the present heading, the area west of the Coast Range
batholith in British Columbia is signified. No summary treatment of the
folds and faults of this great island region has been written such as Bud-
dington and Chapin's account of southeastern Alaska, although numerous
reports of specific areas are available. They are chiefly Suminanj Reports
of the Geological Survey of Canada. Even if possible, it does not seem
feasible for the present writer to attempt a synthesis, but in general it
appears that the same type of structure as in southeastern Alaska con-

Fig. 17.20. Structure section in southeastern Alaska, after Buddington and Chapin, 1929. A,
across Gravina and part of Revillagigedo Islands, showing Triassic beds thrust over Devonian.

I B and B', continuous section from Iphigenia Bay to the mainland. C, across Kuin and Kupreanof
Islands to the mainland. D and D', continuous section along the south side of Frederick sound
to the mainland. Upper Jurassic of Lower Cretaceous intrusives: dt, diorite; md, monzodiorite;
qd, quartz diorite. Metamorphic rocks, probably Ordovician to Jurassic or later, Wrangell-
Revillagigedo belt: sgp, schistose greenstone and green phyllite; ph, phyllite; sph, md, crystalline

| schist and phyllite with beds of marble; gn, layered gneisses. Lower and Middle Ordovician:
Ogs, indurated graywacke with slate, andesitic volcanics, chert, conglomerate, and limestone.
Silurian: Sar, andesitic volcanics and conglomerate; SI, limestone, with thick conglomerate,
sandy beds or argillaceous beds (Sc); Sgr, predominant graywacke. Middle Devonian: Dsa,
slate, limestone and chert with interbedded andesitic volcanics; Da, andesitic lava, breccia and

conglomerate with limestone cobbles; Dgt, predominantly graywacke and tuffaceous beds; Dsr,
sediments, including graywacke, conglomerate, slate, limestone and chert, with associated
volcanics. Mississippian: Cmc, chert, quartzite and limestone. Permian: Cpc, conglomerate, lime-
stone, sandstone, andesitic and basaltic volcanics; Cpl, limestone with white chert layers.
Triassic: Icl, conglomerate, sandstone, and limestone; lis, slate with sandstone in upper part;
Trav, andesitic volcanics, including breccias and lava flows locally interbedded with sediments.
Jurassic or Cretaceous: Kgs, graywacke, slate, and conglomerate with tuff and limestone; Kgr,
greenstone volcanics. Lower Cretaceous: Ksg, slate and graywacke with chert nodules, impure
limestone, and conglomerate. Eocene: Tra, rhyolite and andesite volcanics, conglomerate, and
dacite porphyry sills; Tba, basaltic and andesitic lava with some breccia and conglomerate.
Quaternary: Qb, basalt and tuff.

286

STRUCTURAL GEOLOGY OF NORTH AMERICA

tinues southeastward through the Island Ranges of British Columbia. The
isoclinal folds, the prevailing northwestern strikes, and the steep north-
eastern dips are similar. The belt of metamorphic rocks that flanks the
western margin of the batholith seems to continue a considerable distance
southward into British Columbia. The synclinoria and anticlinoria do not
seem to have been worked out, but perhaps the variety or number of such
features is not exposed or not existent.

The Keku-Gravina synclinorium in Mesozoic rocks extends southeast-
ward across the international border to Pitt Island, but seems to end in
the great batholith before reaching Douglas Channel. The Dall-Long
Island and Dolomi-Sulzer anticlinoria in the Paleozoic strata either die
out southeastward or are covered by the waters of Hecate Strait, because
on the west is a wide Mesozoic belt of Queen Charlotte Islands and an-
couver Island. Lacking information, it can only be assumed that this belt
is a broad synclinorium. It seems to correlate with the Sitka Mesozoic belt
200 miles to the northwest, but if so it must bulge westward around the
Paleozoic anticlinoria of Dall and Prince of Wales islands.

There seems to be plenty of room for an anticlinorium and another
synclinorium under Hecate Strait and Queen Charlotte Sound.

The Paleozoic belt striking nearly east-west on the southern end of
Vancouver Island may mark another anticlinorium outside the Queen
Charlotte- Vancouver Mesozoic belt.

Concordant Fracture System. The pattern of the great batholiths of
the Upper Jurassic and Lower Cretaceous of western Mexico, the United
States, Canada, and southeastern Alaska remind Peacock ( 1935) of the
arc-and-cusp plan of the circum-Pacific orogenic belts. This feature has
already been referred to. Since the batholithic rocks show little evidence
of deformation, the curved plan appears to have originated during the
emplacement of the igneous rocks and during the preceding orogenic
events. The grain of the coastland, in British Columbia and southeastern
Alaska, as defined by the folds and foliation, is longitudinal to the arcs,
and therefore is intimately associated with the arcs in origin. Deformation
during Cenozoic time has had little effect on the Mesozoic pattern.

Peacock (1935) recognizes the fiords and straight stretches of coastline
to be the result of erosion controlled by a fracture system composed of
two elements, viz., a concordant one and a discordant one in relation to

the arc. The concordant system is composed of fractures parallel to the
grain and normal to it, and the discordant of a north-south and east-west
system. See Fig. 17.21 and compare with Fig. 17.18. The first was formed
shortly after the solidification of the batholith; the second at the close of
the Cretaceous. Dikes and mineralized veins follow the transverse frac-
tures of the concordant system. Because of the fissure type of vein fill, the
transverse fractures appear to be of tensional origin (Balk, 1937) and
associated with the batholith. The main faults thus far recognized along
which the fiords have been eroded are the Lynn Canal and Chatham
Strait, but these apparently belong to the younger discordant system.

Peacock's (1935) analysis of the mechanics of the great fracture system
is as follows:

If the coastiand be regarded as a tabular body of rigid material undergoing
deformation by dominating horizontal forces acting from the northeast, causing
differential horizontal displacement toward the southwest, with the develop-
ment of an arc bent away from the dominant pressure, then tensile stresses, as in
a bent beam, would develop in the advanced part of the arc. These stresses
would be relieved by tension fractures running normal to the directions of
maximum tensile stress and, therefore, transversely to the grain or radially to
the arc. It is not to be expected that such tension fractures should follow
strictly radial directions. Although generally transverse, such fractures might
deviate considerably from directly transverse courses, because of irregularities
in the mechanical strength of the region; they might also change abruptiy from
the transverse to the longitudinal direction, the weak direction of the grain, and
thus develop a cranked course with rectangular elbows.

With the mode of deformation suggested, shearing stresses would also be set
up along vertical planes parallel to the grain, and these would be relieved by
longitudinal shear fractures of the shear type.

If formed in the manner oudined, the transverse fractures would be open
fractures, and when mineralized they would appear as fissure veins. The longi-
tudinal fractures would be closed fractures along which some horizontal differ-
ential movement would occur to relieve the shearing stresses. Mineralization of
such ruptures would result in mineralized shear-zones such as Schofield has
found usually to lie in the longitudinal direction. Both sets of fractures would
provide ready-made planes of faulting when subsequent crustal unrest affected
the region and caused differential movement between the already separated
blocks.

It is also possible that thrust faults would form. If relief or elongation is
easiest in the vertical direction, then shear planes would form which strike
longitudinally and have dip-slip movement. Buddington mentions shear
zones or faults with strikes of N 38° W. to N 60° W.

MESOZOIC SYSTEMS ALONG THE PACIFIC

2S7

Fig. 17.21. Sculpture pattern of the coastland of British Columbia and southeastern Alaska
(obtained by drawing straight lines along all nearly straight fiord reaches, lake shores, stream
courses, and portions of coastline. The complex pattern resolves itself into a concordant pattern

consisting of lines running parallel and at right angles to the curving longitudinal grain, and
a discordant pattern composed of lines lying north-south and east-west obliquely to the grain.
All four directions are prominent directions of jointing. After Peacock, 1935.

Age of the Batholiths. The following summary of the age of the great
-batholiths is taken from Buddington and Chapin (1929).

The age of the Mesozoic intrusive rocks has not been definitely determined.
To the northeast, on the east side of the batholith in the Whitehorse district,
{Yukon Territory, the intrusive rocks are reported by Cockfield to cut rocks of
(Middle Jurassic age and, therefore, to be probably of Upper Jurassic age or
jlater. Hanson reports that on the east side of the batholith, in British Columbia,
between Skeena River and Steward, the Coast Range batholith intrudes the
Hazelton group (Jurassic) but does not intrude the Skeena (Lower Cretaceous)
Iseries. He says: "It is, therefore, probably mainly of Upper Jurassic age, but
parts of the batholith may be of later age." Dolmage, in describing the Tatla-
BeUa Coola area, writes: "In Taseko Lake district, what appears to be the main
Coast Range batholith cuts a thick series of coarse fragmental volcanic rocks in
iwhich the writer found plant remains, determined by E. W. Berry to be of
'Cretaceous age. . . . This evidence proves that this part at least of the batho-
lith is younger than the lowest Cretaceous, and the evidence found in Tadavoko
Lake, Taseko Lake, and Bridge River districts strongly suggests that much of
the eastern part of the batholith is of post-basal Lower Cretaceous." Cairnes

suggests that at the southeastern part of the batholith, on the eastern border,
there are intrusions of two ages. Masses of intrusive rocks that cut probable
Jurassic beds are reported by him to be overlain unconformably by beds of
Lower Cretaceous age, and the Lower Cretaceous beds are in turn cut by in-
trusions of pre-Tertiary age. On Vancouver Island the Mesozoic intrusive rocks
are known definitely to be older than Upper Cretaceous.

In southeastern Alaska all the intrusive rocks classed as Mesozoic are defi-
nitely known to be older than the Eocene. On Chicagof Island intrusions of the
Coast Range type are proved by Overbeck to cut fossiliferous beds ol Upper
Jurassic age. The writer is convinced that on Admiralty Island intrusions of the
Coast Range type cut beds which, where not metamorphosed, carry the fossil
Aucella crassicollis and which are therefore probablv of Lower Cretaceous age.
At the head of Portland Canal there is positive evidence of two epochs of in-
trusion; the older batholith cuts beds of the Hazelton series (Jurassic) and is in
turn intruded by the quartz monzonite of the Coast Range batholith.

It is evident that for the most part the youngest beds with which the Meso-
zoic intrusive rocks are found in contact are of Middle or Upper Jurassic age;
at a number of localities intrusive rocks of the Coast Range tvpe cut Lower
Cretaceous formations; there were at least two epochs of intrusion; and the
Mesozoic intrusive rocks are all older than the Upper Cretaceous. So far as

288

STRUCTURAL GEOLOGY OF NORTH AMERICA

southeastern Alaska is concerned, the writer is aware of no evidence to disprove
the assumption that all the Mesozoic intrusive rocks may be of Lower Cretace-
ous age, but the data given for adjacent territory suggest that they may be in
part of Upper Jurassic and in part of Lower Cretaceous age.

Follinsbee et al. (1957) report a potassium-argon age of the Coast
Range batholith near Vancouver of 105 m.y. This would be at least mid-
way up in the Lower Cretaceous, and corresponds well with the strati-
graphic evidence above.

Relation of Batholitlis to Folding. Regarding the relation of folding
and intrusion, Ruddington states:

There is a most pronounced increase in the degree of crumpling, plication,
foliation, and isoclinal folding as the border of the batholith is approached from
the west, suggesting that the batholith has exerted a tremendous thrust. The
manner in which the batholith has peeled off great slabs of schist constitutes
further evidence. On the other hand, in the vicinity of the adjacent oudying
stocks, sintering and compacting of the phyllite and slate as the contact is
approached indicates that the cleavage and foliation are in part older than the
intrusion.

The data are inadequate for a solution of the problem. But if we assume that
the intrusion of the batholith took place within the same general period as the
Jurassic or Cretaceous folding, then it is probable that at least two factors were
involved — an increased local intensity in the dynamic metamorphism above the
location of the rising magma and a thrust exerted by the magma itself during
its emplacement at horizons equivalent to those now exposed. Under the same
stress and with other conditions the same, rocks will be much more highly
deformed under higher temperature. Thus it might be that though stresses of
essentially similar orders of magnitude affected beds both far to the east and
far to the west of the present highly folded zones, the beds to the east and
west, relatively much cooler, yielded by close folding and development of cleav-
age, whereas those in the intensely folded zone, at a higher temperature due to
the rising magma with its advance wave of escaping highly heated vapors,
yielded far more extensively. A preliminary foliated or cleaved character had
thus already been induced before the arrival of the magma, which accentuated
the dynamic effects by its own thrusting pressure and aided recrystallization by
heat, vapors, and solutions.

Another important factor appears to have been the structural relations which
the invaded formations bore to the magma. For example, where they were in
steeply dipping attitudes above the rising magma, conditions for penetration by
magmatic solutions and vapors were favorable and metamorphism was corre-
spondingly facilitated; such seems to have been the condition in the belt ad-
jacent to the southern part of the batholith in southeastern Alaska. Where the
contact plunges steeply the transfer of solutions and vapors was markedly

Fig. 17.22. Belts of the Laramide orogeny in the Rocky Mountains and the folded Upper
Cretaceous trough of Oregon, Washington, and British Columbia.

MESOZOIC SYSTEMS ALONG THE PACIFIC

289

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Fig. 17.23. Cross sections of the east coast of the southern part of Graham Island of the
Queen Charlotte Islands, British Columbia. After MacKenzie, 1916. Jl, Maude fm. (banded
argillites and tuffs. Lower Jurassic or Triassic ?); J2, Yakoun fm. (basal agglomerates and minor
flows. Middle Jurassic); K2, Kano quartz diorite; Kl, Haida fm. (sandstone, shale, and coal.

obstructed by the relatively much greater impermeability across the foliated
surfaces and that portion highly metamorphosed above and adjacent to the
batholith must lie below the present topographic surface, deeper down on its
flank.

The gneissic structure of the batholith suggests that the magma moved up-
ward along planes dipping steeply to the northeast and that the maximum
effect of its thrust was directed against the adjoining formations on the south-
west. The country rock was probably irregularly domed up to a considerable
extent by the invading magma, was fractured, faulted, and stoped to some ex-
tent, and was thrust aside to a very considerable degree. There is abundant
evidence in residual structures and in the composition of the resulting rocks
J that, locally, narrow belts of sediment were wholly incorporated in the magma
through a process of reactive replacement, but this was probably not the major
factor in the process of emplacement of the batholith.

Upper Cretaceous); K2, Honna fm. (conglomerate and sanastone. Upper Cretaceous); K3,
Skidegate fm. (sandstone and shale. Upper Cretaceous); T2, Skonun fm. (sandstone, shale, and
conglomerate. Lower Pliocene ?); T3, Masset volcanics (basalt flows and agglomerates. Pliocene ?).

Idaho Batholith. The Idaho batholith is part of the Nevadan orogenic
belt, but at the same time it is closely associated with the Laramide
Rockies whose building occurred at a slightly later time. Because of the
complex geology around it the great pluton is treated separately in
Chapter 21.

Late Cretaceous Phase

The only Upper Cretaceous deposits of the Columbia system are con-
fined at present to a narrow belt along the northeast coasts of Vancouver
and Graham Island in the Queen Charlotte group. Although the belt is
narrow, the sediments have a thickness of 10,000 feet (Gunning, 1932).

290

STRUCTURAL GEOLOGY OF NORTH AMERICA

The belt of Vancouver Island may be projected south-southeastward to a
deposit of very thick Upper Cretaceous strata in Washington (see maps,
Figs. 17.13 and 17.22). If the two were connected, as seems possible, then
a narrow but deep trough formed in this area after the orogeny and
batholithic intrusions of the late Jurassic and early Cretaceous phase. The
trough was continuous to Graham Island and, if farther, then it must
now be west of any land in southeastern Alaska and, therefore, in the
continental shelf.

Near the base of the series on Vancouver Island, coarse conglomerate
is found. It contains angular to subangular pebbles and boulders, on the
average 2 inches in diameter but varying greatly in dimensions, of vol-
canic rocks, granodiorite, argillite, and quartzite. This conglomerate prob-
ably indicates the existence of a closely adjacent highland being actively
elevated while the trough sank. The Upper Cretaceous strata of Van-
couver Island occur in an open basin and dip about 15 degrees toward
the center. The gentle folding occurred before the intrusion of dikes and
stocks which are believed to be Eocene or Oligocene in age (Gunning,
1930).

Farther north on Graham Island, the Upper Cretaceous strata have
been folded somewhat more intensely. See cross sections of Fig. 17.23. It

is evident that a very late Cretaceous or early Eocene episode of folding
affected the thick sediments of the narrow Upper Cretaceous trough.
Since the history of the Eocene in nearby Washington and Oregon is
chiefly one of trough subsidence, it seems best to assign the disturbance
to the later Upper Cretaceous and to relate it provisionally to the Santa
Lucian orogeny of the central Coast Ranges of California.

Peacock (1935) imagines that the Upper Cretaceous of Vancouver and
Graham islands was once more widespread than now, and that by the
close of the Cretaceous wide arms of the sea washed the margins of
remnants of the once great mountain system reduced to insignificant
relief. Because no Upper Cretaceous rocks are known in southeastern
Alaska, it is concluded that the region there was land for the rest of
the Cretaceous. Not until Eocene time did any significant subsidence oc-
cur.

It will be recalled that the Coast Ranges of California are composed
mostly of the trough sediments, and the Island Ranges of southern British
Columbia are only in small part latest Jurassic and Cretaceous; they are
mostly the Nevadan complex. In southeastern Alaska, an offshore belt of
post-batholithic Cretaceous strata may exist, however, but submerged
beneath the continental shelf. See Fig. 17.21.

18.

The Triassic sediments were spread in the shape of a wing of a butterflv
over the Rocky Mountain states (Plate 9). The site of greatest subsidence
was along the east margin of the former Paleozoic geosyncline, or along
the Wasatch line, where marine waters entered and considerable limestone
was deposited, such as the ammonite bearing Thaynes formation of north-
ern Utah and southeastern Idaho. The geanticline which started to emerge
in Permian time became more pronounced in Triassic time, but still a
wide shallow connection existed with the Pacific. It seems also that a
southwestern passage to the Pacific existed. East of the marine deposits
the sediments are mostly of flood-plain origin and are deep red. They
are now known to extend northeasterly over part of the Williston basin.
They overlapped the edges of the Ancestral Rockies but did not bury
them completely.

ROCKY MOUNTAINS
IN MESOZOIC TIME

TRIASSIC GEOGRAPHY

The seaways that had existed in the Paleozoic miogeosyncline were

i considerably changed during Mesozoic time, and a wide belt of land

'gradually rose in the middle of the old geosyncline to separate two troughs

of sedimentation. The western trough as recounted in Chapter 17 was

filled with more than 30,000 feet of interbedded sediments and volcanics

and was subjected to repeated orogeny. The eastern trough was filled with

marine and nonmarine beds with only a trace of volcanic material. The

| eastern was fairly stable with disturbance reaching orogenic proportions

only in the late Mesozoic along the border of the geanticline.

EARLY JURASSIC GEOGRAPHY

The folio of the U.S. Geological Survey, Paleotectonic Maps of the
Jurassic System is taken here as a guide and should be referred to for
details of the distribution, thickness, and lithology of the several time divi-
sions of the system in the United States (McKee et al., 1956). Four major
units are recognized which from oldest to youngest are labeled A, B, C,
and D. The two oldest which include strata of Lias, Bajocian, Bathonian,
and Callovian ages are represented in Plate 12. They include the well-
known continental sandstone formations, Nugget, Navajo, and Kayenta,
and the marine limestone and shale formations, the Twin Creek, Gypsum
Springs, Lower Sundance, Sawtooth, Carmel, etc.

The Cordilleran geanticline became continuous by Early Jurassic time
and joined with a large emergent area of the southwestern states and
Mexico.

In Middle Jurassic time an irregular island in western Montana was
uplifted. It is known as the Sweetgrass arch and stretched from Great
Falls northward to the Canadian border. About and over it unconformities
occur which involve the Jurassic Sawtooth, Rierdon, Swift, and Morrison
formations and Early Cretaceous Kootenai formation. See Fig. 18.1. The
lowest of the formations, the Sawtooth, is sandstone, siltstone, sandy lime-

291

292

STRUCTURAL GEOLOGY OF NORTH AMERICA

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stone, sandy oolite, and shale. The medial Rierdon is largely limy shale
and nodular limestone. The upper Swift is dark noncalcareous shale and
flaggy glauconitic sandstone. The Morrison consists of fine-grained green-
ish gray clay shale and fresh-water limestone. From the structural point of
view, it is significant that the Swift overlies the other formations uncon-
formably (see cross section, Fig. 18.1), and indicates that the Sweetgrass
arch rose in early Late Jurassic time in about the same position and
with the same detail as that produced by later Laramide movement. The
arch consisted of two domes, a northern and a southern; the southern one
was the site of greatest uplift and erosion. It is also believed that the
southern dome rose gently and remained above sea level during the
deposition of the Sawtooth and Rierdon formations, just preceding early
Upper Jurassic uplift.

The Morrison was deposited conformably on the Swift; following
Morrison time, the arch was again elevated slightly, and several low
anticlines and synclines were formed. Subsequent erosion removed part of
the Jurassic beds from the crests of the anticlines. Erosion was most pro-
nounced on an anticline extending approximately north-south through
the north dome (Kevin-Sunburst). The Morrison, the Swift, and part of
the Rierdon were removed along the crest of the anticline and westward
for an unknown distance. Over this area the earliest Kootenai sands and
gravels (Cut Bank sandstone) were deposited, while the area east and
south continued to undergo erosion. It was not until Sunburst time (a
sandstone member of the Kootenai) that the entire area received sedi-
mentation.

By the close of Mid-Jurassic time (Callovian) the Sweetgrass arch
had spread as a land area of very low relief to include central and western
Montana and northwestern Wyoming. See Plate 12.

The Ancestral Rockies had been overlapped still more in Early and Mid-
Jurassic time, and during the Late Jurassic were entirely buried save for a
few peaks in the Front Range of Colorado.

Fig. 18.1. The Sweetgrass arch in Jurassic time, after Cobban, 1945. Left, isopach map of
the Sawtooth formation. Right, isopach map of the Rierdon formation. Ruled areas were
exposed Paleozoic strata just before deposition of the Swift formation. The crest lines of the
Kevin-Sunburst dome and the south "arch" are those of the present, and together they make
up the Sweetgrass arch.

ROCKY MOUNTAINS IN MESOZOIC TIME

293

EARLY AND MID-CRETACEOUS OROGENY

The Fernie strata in Alberta appear to grade into the overlying Koote-
nay formation of Lower Cretaceous age. The Kootenay consists of
alternating sandstone and dark shale with many coal beds, perhaps all of
nonmarine origin, and decreases in thickness from west to east. Its greatest
thickness is 5000 feet. The presence of thick sandstone and conglomerate
beds in the Kootenay is indicative of further uplift of the land to the west,
and the presence of granite pebbles in the conglomerates indicates that
erosion and differential movement by Kootenay time had so far proceeded
as to lead to the uncovering of deep-seated plutonic masses.

Near the south end of the Wasatch Mountains in the Cedar Hills, Gun-
nison plateau, and Sanpete Valley are immense, coarse deposits probably
of early Late Cretaceous age. They make up the basal part of the Indianola
group (Spieker, 1946). See Fig. 22.16 and Chapter 22. Since the con-
glomerates, sandstones, and shales, together with some higher fossiliferous
marine beds, are a lithologic unit, Spieker believes that all the deposit is a
consecutive response to an uplift, and therefore that the orogeny occurred
at the beginning of late Cretaceous time. Because of the thick deposit of
Mid-Cretaceous age in the Cedar Hills, and the information obtained
there about the disturbance, the orogeny will be called after them, namely,
the Cedar Hills orogeny.

The elastics of the Indianola group are coarsest toward the west in the
Cedar Hills (Schoff, 1937) and the Gunnison plateau. They grade east-
ward into the Mancos shale at the east front of the Wasatch plateau. The
greatest thickness known is 15,000 feet in the Cedar Hills. The belt of
intense deformation lay west of the Cedar Hills, because in the Cedar
Hills the conglomerates rest conformably upon the underlying Upper
Jurassic shales (Spieker, 1946).

The belt of the Cedar Hills orogeny must have extended from southern
Nevada northward through Utah to eastern Idaho. In southern Nevada,
the Overton fanglomerate in the Muddy Mountains is of early late Cre-
taceous age (Hewett, 1931), and rests in angular unconformity on
folded and thrust-faulted Mesozoic rocks, the youngest of which are Juras-
sic in age (Longwell, 1928, 1936). The Overton fanglomerate and the

angular unconformity are believed to mark the Cedar Hills orogeny in
southern Nevada, and the inference has been made that the belt of
orogeny extended continuously between southern Nevada and central
Utah.

North of the Cedar Hills in north-central Utah, a coarse conglomerate,
the Kelvin, is probably the equivalent of the lower Indianola conglom-
erates. It is about 200 feet thick and grades eastward into finer sediments.
The uplift lay immediately west of the present Wasatch Mountains, and
Permian cherts and Pennsylvanian quartzites in the uplift furnished most
of the pebbles of the conglomerate. The site of conglomerate accumula-
tion became a trough of subsidence, and in the Colorado epoch of late
Cretaceous time, over 5000 feet of strata collected in it. Volcanoes nearby
emitted dust which collected as tuff in the lower part of the sequence
(the Aspen formation); then sandstones with numerous oysters and
fresh-water shales and sandstones with coal seams accumulated alternately.
Several conglomerates in the Colorado series mark continued unrest to
the west. See the paleotectonic map, Plate 12.

Mansfield (1927) believes that the Lower Cretaceous Gannett group
in southeastern Idaho, with its several coarse sandstones and conglom-
erates, signifies a sharp uplift in the land to the west as a reflection of the
intense Nevadan orogeny still farther west. Probably this uplift in the
Utah trough area was a forerunner to the main orogeny which resulted
in the deposition of about 3000 feet of coarse debris of early Late Cre-
taceous age, the Wayan formation (Read and Brown, 1937) unconform-
ably on the Gannett.

About 200 miles northwest of southeastern Idaho in southwestern Mon-
tana, a Cretaceous sequence is present, but has not yet been well worked
out. In places below beds of Aspen ( Colorado epoch ) aspect, and above
the Lower Cretaceous Kootenai elastics is a pebble and cobble con-
glomerate. Although these beds may be part of the Kootenai, fossil evi-
dence is lacking and they may be early Late Cretaceous. If so, the belt
of Mid-Cretaceous orogeny may have extended northward to western
Montana.

Since no Jurassic and Cretaceous beds were deposited in the Mesozoic
geanticlinal area along the east side of which the Cedar Hills orogeny oc-

294

STRUCTURAL GEOLOGY OF NORTH AMERICA

curred, the folds and faults there are all in Paleozoic strata, and therefore
the date of the folding cannot be fixed except as post- Paleozoic. The main
orogenic events in the eastern trough came in Late Cretaceous time and
during the Paleocene and Eocene, and therefore the folds and faults in
the Paleozoic strata of the geanticline immediately to the west have gen-
erally been accredited to these later orogenies. However, the work of
Nolan ( 1935 ) in the Gold Hill mining district of western Utah is espe-
cially significant in making clear the complexity of deformation in the
geanticlinal area. There the structural history is characterized by at least
four and possibly five phases of folding and faulting, each phase com-
posed of an initial stage in which compressive forces were active and a
final stage in which normal faulting was dominant. The first two phases
predate the Eocene by a long interval of erosion and are regarded pro-
visionally as Cretaceous by Nolan. It is probable that they are related to

the Nevadan and post-Nevadan Cretaceous disturbances to the west (see
chart, Fig. 17.2 and 17.7) and to the sinking of the Utah trough during
the time that the Indianola, Kelvin, Aspen, and Frontier and other
formations were deposited in it. The map, Plate 10, shows the crust
intensely affected in the Sierra Nevada region in late Upper Jurassic time,
while the area on the east was only epeirogenically uplifted. During
Cretaceous time, the reverse seems to have been true. The Sierra Nevada
region was one of gentle emergence, and the eastern part was probably
orogenically deformed.

In conclusion, there is no evidence to preclude the generalization that
the most intense disturbance in the landmass just west of the trough was
localized opposite the area of greatest subsidence, which also coincides
with the central part of the arcuate pattern. See especially Plates 11 and
12.

19.

LATE CRETACEOUS
AND EARLY TERTIARY
ROCKY MOUNTAIN SYSTEMS-
THE LARAMIDE OROGENY

DEFINITION OF LARAMIDE OROGENY

Geologists to date in the Rocky Mountains have discovered a succession
of dynamic events through late Mesozoic and Tertiary time. At first, a
single, rather violent orogeny was visualized, but now numerous uncon-
formities, coarse conglomerates, and structural relations attest a condition
of unrest in the general Rocky Mountain region from middle Mesozoic
to the present. The single and intense orogeny in the Rocky Mountains
; which they visualized was called the Laramide Revolution, and this was
supposed to have occurred precisely at the close of Cretaceous time, or
at the beginning of Tertiary time.

Some geologists advocate dropping the term Laramide because of
the many recognized deformational pulses and the current concept that
crustal deformation is continuous. Limits cannot logically be set, they
contend. The writer believes, however, that since the usage is so deeph
ingrained in the literature that it is better to try to define the term ar-
bitrarily, and furthermore, finds the attempt helpful and not confusing.
For the purposes of this book the following nomenclature will be used:

Orogenic events during Eocene time — Late Laramide
Orogenic events during Paleocene time — Mid-Laramide
Orogenic events during Montana time — Early Laramide

Any orogenic phases older than Montana or younger than Eocene
will not be called Laramide, and, where desirable, new orogenies will
be defined. The Cedar Hills orogeny of central Utah of Colorado age
falls in this category. The disturbances in late Mesozoic time were gener-
ally precursory to the climatic ones of the very Late Cretaceous or the
Early Tertiary.

BELTS OF DEFORMATION

Major Divisions

The map, Fig. 19.1, has been prepared to show the mountain systems
of the Laramide orogenic belts. Two major divisions of the systems have
been pointed out in the literature, namely, a western composed of ranges
formed of the thick sediments of the Paleozoic and Mesozoic troughs, and
an eastern composed of ranges and intermontane valleys formed of the
shelf sediments and the crystalline basement complex (Fig. 19.2). The
generalization needs scrutiny from both a spatial and time aspect, else
a number of misconceptions will arise. This will be done in the following
several chapters.

General Characteristics

Thrust faults and folds are the most characteristic structures of the
Laramide Rocky Mountains. In the eastern division great asymmetrical
anticlinical ranges dominate. Those that have been uplifted so much

295

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LATE CRETACEOUS AND EARLY TERTIARY ROCKY MOUNTAIN SYSTEMS— THE LARAMIDE OROGENY

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that Precambrian cores show extensively are marked by thrusts on the
steep flank (Fig. 19.2). Such structures are here interpreted as primary
upthrusts and gravity slide phenomena.

Many of the thrust faults of the western division are low angle and
define sheets that have moved horizontally considerable distances. Some
of the thrust sheets are folded by later movements. The thrust sheets were
rather thin and escaped regional metamorphism. In fact, the rocks
involved in the Laramide orogeny are characterized by an absence of
metamorphism, except perhaps some of the deeper Proterozoic strata.
This distinguishes them from the rocks of the Nevadan orogeny. It will
be recalled that the Nevadan in the batholithic belt is characterized by
isoclinal, nearly vertical folds, as well as flow cleavage. Isoclinal folding
is rare in the Laramide Rockies. The Nevadan is characterized by great
batholiths. Aside from the Idaho batholith, the Laramide orogenic belt
has few plutons large enough to be called batholiths; its intrusions are
mostly stocks, but the stocks exist in considerable number. The Nevadan
developed in sediments of the eugeosyncline, the Laramide in sediments
of the miogeosyncline and shelf.

Canadian and Montana Rockies

The Canadian and Montana Rockies consist of a mainland assemblage
of geosynclinal sediments of late Proterozoic, Paleozoic, and Mesozoic
age, cast into a great imbricate series of thrust sheets. The Proterozoic
rocks of western Montana form an extraordinarily thick group of clastic
sediments, known as the Relt series. Originally clays, sands, and marls,
diey have been metamorphosed to argillites, quartzites, and impure sid-
eritic marbles and limestones. They are at least 50,000 feet thick near
Missoula. The Paleozoic rocks are dominantly limestones, and are nearly
7000 feet thick. The Madison limestone of Mississippian age is about 2000
feet thick, and forms steep cliffs and canyon walls in many of the ranges
southeast of Missoula. The Mesozoic rocks are about 7000 feet thick and
are dominantly shales, with some limestone, sandstone, and conglomerate.
Consult tectonic and geologic maps of the Permian, Triassic, Jurassic, and
Cretaceous, Plates 8 to 12 for information on the deposition and distribu-
tion of the various stratigraphic systems, and Chapters 5 and 6 for isopachs.

The Reltian has not been reported to be as thick in Idaho and Utah as
in western Montana, and north of the border it also seems to be thinner.
There, it crops out almost entirely west of the Rocky Mountain trench and
leaves the main Canadian Rockies to be composed mostly of Paleozoic
strata. The Cambrian thickens to over 15,000 feet along the Alberta-
British Columbia boundary, and most of the scenic ranges there are
sculptured in it.

Very few intrusions occur east of the Rocky Mountain trench north of
the Idaho and Boulder batholiths. Large sheets of diorite and gabbro
split the Beltian rocks in places, and one near the Canadian border is
identified as a lava flow and is called the Purcell lava. The sills and flows
have been deformed with the Beltian strata.

The igneous intrusions are very abundant and voluminous in west-
central Montana, and are composed chiefly of quartz monzonite and
diorite.

Central Rockies

The central Rockies consist of folded and thrust-faulted Paleozoic-
strata in their western part and Proterozoic, Paleozoic, and Mesozoic
along their eastern margin. The Mesozoic sediments were especially thick
in places, and a number of episodes of compression occurred from mid
Cretaceous to early Oligocene. The structures of some of the episodes
trend discordantly to those of others. Thick, coarse conglomerates
mark the orogenies and, being deformed themselves, add to the com-
plexity.

A review of the tectonic maps of the Permian, Triassic, Jurassic, and
Cretaceous, Plates 8 to 12, will impress one with the fact that the Laramide
trough zone of orogeny, especially in Utah, embraced parts of two major
elements, the Cordilleran intermontane geanticline and the Mesozoic
trough. The sinking of the Permian trough in Utah started a series of
subsidences that followed generally one on top of the odier until the
Laramide orogeny. The total accumulation of sediments of the Permian
and Mesozoic, therefore, has been isopached, and the basin is charted on a
map so as to compare with the Laramide deformational belt. The map,
Fig. 19.3, shows the extent to which the Laramide belt cut into the

298

STRUCTURAL GEOLOGY OF NORTH AMERICA

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LATE CRETACEOUS AND EARLY TERTIARY ROCKY MOUNTAIN SYSTEMS— THE LARAMIDE OROGENY

299

Permian and Mesozoic trough sediments and the extent to which it over-
lapped the Paleozoic sediments of the geanticlinal area.

Precambrian rocks are not exposed at the surface in many ranges, but
from central Utah northward to western Montana, those of Proterozoic
(Reltian) age become increasingly widespread. In western Montana,
most of the Laramide Rockies are in the Beltian strata, and this zone
extends to the northwest in eastern British Columbia. The crystalline
complex, supposedly everywhere older than the Beltian, is exposed in
the trough zone only in the Wasatch and Raft River Mountains. In the
shelf ranges the opposite is true; the crystalline complex is exposed in
many of the cores of the ranges. The extent of the Beltian trough, as
well as can be determined, is shown in comparison with the Laramide
belts of deformation in Fig. 19.3 and will be referred to later.

Thrust faults dominate the structure in the trough zone. The overriding
sheets of greatest displacement and shallowest dip moved mostly east-
ward, but several thrusts, especially in Montana and Canada, have moved
westward.

Southern Arizona Rockies

The southern Arizona Rockies consist largely of Precambrian rocks
of several ages, both igneous and sedimentary. The ancient rocks were
veneered with a thin Paleozoic cover, and in places with thin Triassic
and Jurassic strata. The Cretaceous Mexican geosyncline extended north-
westward into the southernmost part of Arizona, and its strata are there
thrown into folds and thrust sheets. Part of the Cretaceous accumulations
were lavas. See the Paleotectonic maps for the details of the setting for
the Laramide orogeny. Intrusive rocks of Laramide age are abundant, and
they are associated with valuable ore deposits. A succession of volcanic
episodes spread through the Tertiary, and probably some are early enough
to be considered Laramide.

\[ Wyoming Rockies

The Wyoming Rockies consist in part of the shelf facies of Paleozoic,
1 Triassic, Jurassic, and Lower Cretaceous rocks, in part of thick clastic

deposits of Late Cretaceous age and in part of Beltian (?) and pre-
Beltian crystalline rocks. The most conspicuous ranges are sculptured out
of great asymmetrical anticlines in which the Precambrian rocks .in-
exposed in the cores. The Black Hills, Big Horn, Laramie, and Wind
River ranges are eroded in such folds. The anticlines are asymmetrical
to the extent of overturning and thrusting in places, and began to rise
in Late Cretaceous time, while the broad basins between sank and
received thousands of feet of sediments. Examine the paleotectonic map
of the Late Cretaceous. The thick Upper Cretaceous sediments are
generally involved in late phases of the compressional orogeny. Paleocene
and Eocene sediments have accumulated in the basins to considerable
thicknesses in places, and certain phases of deformation marginal to the
basins have deformed them also.

The northwest corner of the state of Wyoming became the site of
considerable volcanic activity in middle and late Eocene time, and the
pyroclastics and lavas of the Absaroka Range and Yellowstone Park were
mostly exuded at that time.

Central Montana Rockies

The central Montana Rockies consist of a general east-west assemblage
of monoclinal flexures, domes, and belts of en echelon faults. Numerous
bodies of igneous rocks consisting of stocks, laccoliths, radiate dike sys-
tems, and various extrusions lie in a belt approximately transverse to the
sedimentary rock structures.

During the Beltian epoch of the Proterozoic, a trough extended east-
ward into central Montana and may have predetermined the location of
the central Montana Laramide structures. See map, Fig. 19.4. Also, in
Mississippian times a broad east-west basin through central Montana sub-
sided and received over 2000 feet of beds, appreciably more than on
either side. See paleotectonic map, Plate 5. This basin, like the Beltian.
may have helped to determine the position that the later Laramide struc-
tures took, or else the coincidence in space of all three means that some
deep-seated influence has been at work repeatedly from Beltian times to
Laramide.

300

STRUCTURAL GEOLOGY OF NORTH AMERICA

Fig. 19.4. Beltian trough (vertically ruled) and the belts of the Laramide orogeny (white).

Colorado and New Mexico Rockies

The Ancestral Rockies of Pennsylvanian and Permian age were gradually
buried by Triassic, Jurassic, and Cretaceous sediments, and it was upon
this crustal make-up that the Laramide belt of deformation was super-
posed in Colorado and New Mexico. The back of the ancestral Colorado
Range was broken, and two modern ranges were created, both with
subparallel elements such that the older range seemed to have exerted
some control over the younger. The western half of the ancestral range
with a thin sedimentary veneer on the Precambrian crystallines developed
a number of thrust sheets. A transverse prophyry belt carries numerous
stocks and much ore.

The Laramide belt of deformation in New Mexico was a narrow one
through the central part of the state, and aside from the common north-
south orientation of both the Ancestral and Laramide Rockies, their
relation seems to be a matter of chance, viz., the younger ranges rose in
part where the older ones stood and in part in the sites of the older basins.

The Laramide structures are large asymmetrical anticlines like those in
Wyoming, with gravity slide thrusting on the steep flanks. Graben or
rift faulting broke through the Laramide uplifts in a north-south zone in
Late Cenozoic time. Much Tertiary volcanism occurred in Colorado and
New Mexico.

Rockies of Northeastern Mexico

The Laramide system of northeastern Mexico includes the El Paso-Rio
Grande thrust belt, the Sierra Madre Oriental, the Sabinas foothill belt,
and the Parras synclinorium. The Pennsylvanian Marathon and Coal-
huila systems, the Permian ranges, platforms, basins, and shelves of the
Marathon foreland, and the late Mesozoic Mexican geosyncline are the
foundational elements upon which the Laramide structures were super-
posed. The strata of the Mexican geosyncline were generally closely
folded lengthwise of the basin, the thin veneer on the old Coalhuila system
— the Coalhuila peninsula — was domed broadly and locally flexed, and
the basin beds along the east side of the peninsula were folded and thrust
eastward. The Parras trough at the south end of the peninsula was in-
tensely compressed from south to north, and tight east-west folds and

LATE CRETACEOUS AND EARLY TERTIARY ROCKY MOUNTAIN SYSTEMS— THE LARAMIDE OROGENY

301

some thrusts were formed. The Pennsylvanian and Permian Marathon
and Coalhuila systems are, therefore, thought to have extended consider-
able control over the later Laramide structures.

Sonoran Rockies

, The Sonoran Rockies include three geomorphic provinces, namely
from east to west, the Sierra Madre Occidental, the parallel ranges and
valleys, and the Sonoran desert. From coarse conglomerates along the

i western margin of the Mexican geosyncline, it is clear that the orogenic
belt in western Sonora continued active, and at least twice in late Cre-
taceous time rose sharply and crowded the sediments eastward. The early
Laramide structures thus created are obscure, first for lack of field work,
and second because other younger orogenies have been superposed, and

i much Tertiary lava covers them.

: The Paleozoic and Mesozoic sediments are of a mainland assemblage,
as far as known, and the volcanic assemblage of the Pacific border systems
is absent. On the other hand, the Permian beds of the Coahuila peninsula,

,a considerable distance to the east, have much volcanic material.

Colorado Plateau

The Colorado Plateau is a rudely circular and lesser deformed part of
the crust within the broad zone of Laramide orogeny. A sedimentary
Veneer of about 6000 to 10,000 feet overlies a Precambrian basement,
jmd over much of the Plateau the strata are nearly flat. Several large
jnonoclinal flexures of Laramide age break the monotony of the flat-
lying beds.

The monoclines are the steep flanks of asymmetrical anticlines in size
mich like those of Wyoming, Colorado, and New Mexico, but with about
lalf as much vertical uplift. Consequently, it is believed, no thrusts have

developed through gravity gliding as in the ranges where the Precambrian
cores are so broadly exposed, and topographic relief is so much greater.
In post-Laramide time, the Colorado Plateau became the site of con-
siderable intrusive and extrusive igneous activity, but it must not be
inferred that the igneous activity was confined to the Plateau. It was
equally pronounced in the more severely deformed belts to the east and
west. The Plateau includes part of the Ancestral Rockies in its eastern
part and contains along its western edge some Reltian (?) strata, but
most of the Precambrian is a pre-Beltian crystalline complex.

RELATION OF BELTS OF DEFORMATION TO CRUSTAL CONSTITUTION

The outer ranges of the Rockies were all developed in the shelf zone
of the westward-lying Paleozoic Cordilleran geosyncline, and shelf con-
ditions of deposition continued through the Triassic and Jurassic. But with
the coming of Cretaceous time, rather thick masses of sediments accumu-
lated locally over the former shelf, particularly in Upper Cretaceous
basins incident to the early uplift of the Wyoming, Colorado, and New
Mexico Laramide ranges.

The belt of deformation in the shelf extends into the region of the
Devonian Transcontinental Arch without effect. The thick Proterozoic
metasediments, perhaps all of Beltian age, are shown as well as possible
in relation to the Laramide belts of deformation in Fig. 19.4, and do
striking coincidence is noted, except locally, perchance in the Uinta
Range and the mountains of central Montana. If the relations as depicted
are correct, then only one conclusion seems warranted, namely, that the
belts of deformation are due to deep-seated causes, not influenced par-
ticularly by deeply filled troughs or basins, nor by the crystalline base-
ment with a thin veneer of sediments.

20.

CANADIAN AND
MONTANA ROCKIES

MAJOR SYSTEMS OF CANADIAN CORDILLERA

The Geological Survey of Canada classifies their great Cordillera into
the western and eastern regions, and the western region is further sub-
divided into the western system of coastal ranges and the interior system
of plateaus and ranges. The eastern Cordilleran region is spoken of as
the eastern system. Examine map, Fig. 20.1.

Western System

The western system, which here will include the coast ranges and is-
lands of southeastern Alaska, is basically Nevadan in its geological com-
plexity, and its Mesozoic history has already been described. Its Tertiary

INDEX
WESTERN SYSTEM

1 . Queen Charlotte Mlns.

2. Vancouver Island Mlns.

3. St. Elias Mtns.

4. Coast Mtns.

5. Cascade Mtns.

INTERIOR SYSTEM

6. Yukon Plateau

7. Ogilvie Mtns.

8. Selwyn Mtns.

9. Pelly Mtns.

10. Liard Plain and Hyland

Plateau

11. Cassiar Mtns.

12. Omineca Mtns.

13. Stikine Plateau

14. Skeena Mtns.

15. Nass Basin

16. Hazellon Mtns.

17. Interior Plateau

18. Cariboo Mtns.

19. Monashee Mtns.

20. Selkirk Mtns.

21. Purcell Mtns.

CORDILLERAN REGION

Fig. 20.1. Physiographic divisions of ihe Canadian Cordillera. Reproduced from Bostock
ef a/., 1957.

302

CANADIAN AND MONTANA ROCKIES

303

history will be considered in a later chapter. Quoting from Lord et al.
(1947):

The western system includes the St. Elias, Coast, Cascade, and Vancouver
Island Mountains. The St. Elias Mountains occupy an area in the extreme north-
west corner of British Columbia and adjacent southwestern Yukon. They are
the highest in Canada, extremely rugged, and in large part covered by an ice-
field. The elevation of Mount Logan, the highest peak in Canada, is 19,850 feet,
and other peaks exceed 15,000 feet.

The Coast Mountains occupy a belt 100 miles wide and 1,000 miles long, and
border the Pacific coast from Yukon southeast almost to the International Bound-
ary at the 49th parallel. They rise abrupdy from the sea, and towards the axis
of the range are characterized by an almost unbroken succession of bare, rugged
peaks and saw-toothed ridges rising to elevations from 7,000 to more than
13,000 feet. Alpine glaciers and icefields are common, and in a few places in
the northern half of the range valley glaciers extend to sea-level. The range is
crossed by a number of deep river valleys, and its western margin is pene-
trated by numerous, long, narrow fiords continued inland by deep U-shaped
valleys.

The Cascade Mountains project into Canada from the State of Washington
and are more than 100 miles wide where they cross the border. They lie on the
east side of lower Fraser River Valley, which separates them from the Coast
Mountains, and extends as far north as Thompson River. Many of the higher
peaks and ridges near the International Boundary attain elevations between
7,000 and 8,500 feet; and they are fully as rugged as those of the adjacent
Coast Mountains, and, like them, hold many alpine glaciers.

Mountains occupy most of Vancouver Island and culminate, in the central
part, in peaks 5,000 to 7,000 feet or more above sea-level. The western side of
the island, like the western side of the Coast Mountains, is characterized by an
intricate set of fiords and by heavily timbered rocky slopes that rise abrupdy
'• from the sea to heights of several thousand feet. A lowland as much as 10 miles
wide borders the east coast.

Central System

The central system, like the western, is for the most part fundamentally
Nevadan in its geology. Its present geomorphic characteristics are ade-
quately described by Lord et al. ( 1947 ) . Referring again to the map of
Fig. 20.1 and quoting from them:

The central system, composed of dissected plateaux and scattered mountain

ranges, occupies a belt that averages more than 200 miles wide and extends

,i southeast from the Alaska Boundary at Yukon River to the southern boundary

of British Columbia at Okanagan River. In Yukon it includes the Yukon Plateau

and Ogilvie, Selwyn, Pelly, and other mountains. In British Columbia north of

latitude 54 and 55 degrees it includes Cassiar and Omineca Mountains, Babine
and Bulkley Mountains, and Stikine Plateau. In the southern part of the prov-
ince, it comprises the Interior Plateau and Cariboo, Monashee, Selkirk, and
Purcell Mountains.

Yukon Plateau in Canada includes much of the drainage basin of Yukon
River and, commencing in northern British Columbia near Adin and Tcslin
Lakes, extends northwestward through Yukon and thence westward into Alaska
It has been deeply dissected by a drainage system whose main channels are
several thousand feet deep, and the once gently rolling upland has been broken
into a series of high, flat-topped hills and ridges. Ogilvie and Selwyn Mountains
border it on the north and northeast respectively, and to the southeast the
plateau ends against Pelly Mountains.

Little is known about Ogilvie and Selwyn Mountains. The former, with bor-
dering peaks as high as 7,000 feet, extend easterly from the Alaska boundary,
near latitude 65 degrees, for 150 miles. There they join Selwyn Mountains,
which form the northeast rim of the Yukon Plateau and stretch nearly 400 miles
southeasterly to end in low country east of Frances River near latitude 61
degrees. Selwyn Mountains rise from the Plateau along an irregular front, and
are broken into groups of mountains by broad valleys and other depressions.
Probably a few peaks are more than 10,000 feet above sea-level, and many rise
to elevations in excess of 7,000 feet. Selwyn Mountains are bordered on the
northeast by the Mackenzie Mountains of the eastern physiographic subprov-
ince.

Pelly Mountains form a triangular area in the southern part of the Yukon
Plateau, with corners near Teslin Lake, Frances Lake, and Pelly River at longi-
tude 135 degrees. They include Glenlyon, Pelly, and Big Salmon Ranges, and
rise from adjacent plateau areas through border areas characterized by long,
smooth-topped spurs and dissected tablelands. The highest peaks of the main
unit, the rugged Pelly Range, may be more than 8,000 feet above sea-level, and
hold a few small alpine glaciers.

Cassiar and Omineca Mountains constitute a continuous belt stretching 450
miles northwesterly from near Takla Lake into Yukon, and extending 50 to 75
miles west from Finlay and Parsnip Rivers. These mountains comprise a great
number of ranges separated by broad, transverse and longitudinal vallej s se\ -
eral thousand feet deep. The higher peaks and ridges range in elevation from
6,000 feet to more than 8,000 feet. Permanent ice is confined to rather small.
scattered, alpine glaciers.

Babine and Bulkley Mountains and their northerly extensions occupy an area
of more than 20,000 square miles, bounded on the east by Cassiar and Omineca
Mountains, on the south by the Interior Plateau, on the west bv the Coast
Mountains, and on the north bv Stikine Plateau. Bulklev and Babine Moun-
tains lie on either side of the northwesterly trending Bulkley-upper Skeena
Valley. They comprise many individual mountains or mountain groups isolated
by wide low areas or great valleys. Most peaks are highly dissected, and some
rise more than 7,500 feet above the valleys.

Stikine Plateau occupies much of the drainage basin of Stikine River east of

304

STRUCTURAL GEOLOGY OF NORTH AMERICA

the Coast Mountains: on the north it joins Yukon Plateau between Atlin and
Teslin Lakes, and elsewhere is bounded by the northerly extensions of Babine
and Bulkley Mountains or by Omineca and Cassiar Mountains. Its gently undu-
lating surface averages 4,000 feet or more above sea-level, and is dissected into
a number of smaller plateaux by the larger stream and river valleys.

The Interior Plateau stretches from Bulkley, Babine, and Omineca Moun-
tains approximately 500 miles southeasterly to the International Boundary. At
its north end it extends from the Coast Mountains 200 miles east to the Rocky
Mountains. Toward the south it becomes progressively restricted by the Cascade
Mountains, on the west, and by Cariboo and Monashee Mountains, on the east,
and at the Boundary near Okanagan and Ketde Bivers is less than 50 miles
wide. This great plateau region, with a general elevation of 3,000 to 4,000 feet
is composed of a succession of plateau surfaces interrupted by the deeply cut
valleys of a drainage system whose main channels lie 1,000 feet or more below
the remnants of the upland surface.

Cariboo, Monashee, and Purcell Mountains form a mountain group within a
triangular area between the Interior Plateau on the west and the Bocky Moun-
tain Trench on the east; the apex is in the big bend of Fraser Biver, and the
base at the International Boundary. The various members of the group are sepa-
rated by deep valleys or trenches trending northward and northwestward. Sel-
kirk Mountains are exceedingly rugged, with summits rising to elevations of
11,000 feet and more above sea-level.

Eastern System

The eastern system, or the Canadian and Montana Rockies, is the sub-
ject of the present chapter because it is basically Laramide in origin. It

. . . includes Bichardson, Mackenzie, Franklin, and Bocky Mountains, and
intervening plateau and plain areas.

In British Columbia the eastern and central systems are separated by the
Bocky Mountain Trench, a great trough that extends northwesterly from the
International Boundary nearly to the southern boundary of Yukon, and includes
aligned parts of Kootenay, Columbia, Fraser, Parsnip, and Finlay Bivers. The
boundary between these systems is less well defined beyond the northern end
of the trench; it enters Yukon near longitude 126 degrees, extends northerly
into Northwest Territories, and swings to the northwest between Selwyn Moun-
tains on the southwest and Mackenzie Mountains on the northeast to re-enter
Yukon near latitude 65 degrees, and thence proceeds northwesterly on a sinuous
course to pass west of Bichardson Mountains and enter Alaska near latitude 69
degrees.

Bichardson Mountains form a straight wall 175 miles long extending northerly
from Peel Biver near longitude 136 degrees nearly to the Arctic coastal plain
west of Mackenzie Biver delta. In the north they are more than 40 miles wide,
and contain rugged, northerly trending asymmetrical ridges with peaks rising to

heights of 5,000 feet or more. Throughout most of their length, however, they
comprise a much narrower belt of steep-sided ridges, the flat tops of which lie
mainly below 4,000 feet. No cirques or other evidence of alpine glaciation has
been found in aerial photographs of even the highest peaks.

Mackenzie Mountains occupy a broad crescentic area, convex towards the
northeast, stretching 425 miles southeasterly from south of Peel Biver near
longitude 134 degrees nearly to Liard Biver at latitude 61 degrees. Their maxi-
mum width exceeds 100 miles. They are distinguished from Selwyn Mountains,
which adjoin them on the southwest, not by any abrupt topographic boundary,
but by absence of intrusions, conspicuous stratification, and more youthful to-
pography. On the north and northeast they rise abrupdy from the Mackenzie
Biver lowland. In the main they comprise a compact mass of conspicuously
layered, northwesterly trending ridges topped by peaks that commonly rise to
elevations of more than 7,000 feet, and in some places are reported to exceed
elevations of 9,000 to 10,000 feet. Small alpine glaciers are widespread. The
Canyon Banges, which form their northeastern front and occupy a belt up to 40
miles wide, include more subdued mountains and high plateau areas traversed
by deeply incised river valleys.

Peel Plateau is a great triangular terrace occupying the angle between the
east front of Bichardson Mountains and the north front of Mackenzie Moun-
tains. Its northeastern edge is in part a scarp rising 200 to 1,000 feet above the
Plains region. The major rivers traversing the plateau, such as the Peel and
Arctic Bed, are deeply entrenched in the otherwise rather flat, glaciated upland
surface.

Throughout most of their length Franklin Mountains lie a short distance east
of and parallel with Mackenzie River. They extend from Fort Good Hope more
than 400 miles southeasterly to the mouth of South Nahanni Biver and average
less than 30 miles wide. They include, from north to south, Norman, Franklin,
Camsell, and Nahanni Banges, each comprising a number of parallel north to
northwesterly trending ridges. In places they reach heights of 5,000 feet.

The Bocky Mountains form the eastern front of the Cordilleran region in
British Columbia. Here they rise sharply from the comparatively flat Plains
region, through a Foothills belt, to peaks reaching elevations of 10,000 to nearly
13,000 feet. These mountains, with their eastern foothills, have a maximum
width of about 100 miles, and extend from the International Boundary at longi-
tude 114 degrees 850 miles northwesterly to Liard Biver. At their northwest
end, they are separated from Selwyn and Mackenzie Mountains by a distance
of more than 1O0 miles. They have been carved from a thick series of sedimen-
tary strata of rather simple structure, and the resulting layering, visible from
great distances, at once distinguishes them from most other mountains in British
Columbia. They consist of a series of overlapping ranges that trend northwest
and, on the whole, have precipitous eastern faces and much less steep western
slopes. Individual ranges are broken or terminated by deep cross-valleys, and
the whole mountain mass is crossed by several deep depressions having com-
paratively low heights at the divides (Lord et al., 1947).

CANADIAN AND MONTANA ROCKIES

305

DIVISIONS OF CANADIAN AND MONTANA ROCKIES

The two divisions of the Canadian and Montana Rockies generally
recognized are the mountain belt and the foothill belt. The latter is com-
monly referred to simply as the foothills. The two divisions are shown on
the map of Fig. 20.2. The western limit of the mountain belt of Laramide
age in Canada is recognized by some as the remarkably regular depression
called the Rocky Mountain trench. It extends from Liard River southeast
for 800 miles to Flathead Valley in Montana. This serves as a convenient
physiographic boundary of the Canadian Rockies, but as a structural
boundary it is not secure. The Rocky Mountain trench may be traced
southward to Kalispell in Montana, but from this point southward three
great valleys exist, any one of which might be chosen for the trench.
Clapp ( 1932 ) believes all the ranges of western Montana should be in-
cluded in the Rocky Mountain system because they are alike structurally
and stratigraphically. There, even more so than in Canada, the relation of
the Nevadan and Laramide orogenic belts and the position of their bound-
ary, if a common one, are little known and still speculative.

MOUNTAIN BELT

The mountain belt is made up of imposing front ranges such as the
Lewis Range (Glacier Park) and the Canadian Rockies of the Ranff and
Jasper areas, as well as many large ranges to the west. All ranges trend
approximately parallel with each other in a north-northwest direction,
except from the Idaho batholith southward, where later deformation has
imposed a topography discordantly in places across the Laramide struc-
tural trends. Compare Fig. 20.2 of this book with Raisz' Landforms Map
(1939).

The mountain belt may be divided into two parts according to the

Fig. 20.2. Tectonic map of the Canadian and Montana Rockies showing their major divisions,
the chief faults, the intrusions, and the lines of cross sections. Cross-ruled area is, with minor
exceptions, folded and faulted Beltian (Proterozoic) rocks. Vertically ruled orea is folded and
faulted Paleozoic and Mesozoic rocks. The horizontally dashed area is the folded and faulted
Mesozoic foothills belt. Horizontally ruled area is Beltian or underlain by Beltian but not part
of the folded and thrust belt. Structures associated with the intrusions not shown. The arrows
through the faults indicate the direction of movement of the overriding sheets.

306

STRUCTURAL GEOLOGY OF NORTH AMERICA

ROCKY MOUNTAINS WEST OF BANFF

DOC-TOOTH

MOUNTAINS
? ...

'^2 .!.*-*' ' +*

f^ '•.'••.'•.'-"--"-"

ROCKY MTN TRENCH VAN HORNE MOUNTAINS

MC G1LLIVRAY RANGE

ROCKY MOUNTAINS ALONG THE 49TH PARALLEL

ROCKY MOUNTAIN TRENCH GALTON RANGE

MACOONALD RANGE

SCALE IN MILES

Fig. 20.3. Sections through the Canadian Rockies from the Rocky Mountain Trench to the Plains.

Section B— B' after Evans, 1932. 1, Beltian (?); 2, Cambrian; 3, Ordovician and Silurian; 4,
Devonian and Carboniferous; 5, Mesozoic; 6, Edmonton (Montanan); 7, Paskapoo (late Paleocene).

Section K-K' after Daly, 1912. 1, Waterton dolomite; 2, Altyn limestone (1 and 2 Beltian);
3, Appekunny argillite; 3a, Hefty sandstone; 3b, MacDonald argillite; 4, Grinnell argillite; 4a,

formations involved. One part is made up almost entirely of the forma-
tions of the great Beltian group, and the other of Paleozoic and Mesozoic
formations. The two parts are designated by cross ruling (Beltian) and
vertical ruling (Paleozoic and Mesozoic) on the map, Fig. 20.2. The
Beltian division lies to the west except at the international boundary,
where it extends to the east front of the mountain belt and adjoins the
foothills belt, thus dividing the Paleozoic and Mesozoic division into a
northern (Canadian) and southern (Montana) segment.

The stratigraphy and structure of the mountain belt are shown in a

Wigwam sandstone and argillite (3, 3a, 3b, 4, and 4a Lower Cambrian); 5, Siyeh limestone; 6,
Purcell lava; 7, Sheppard dolomite; 7a, Gateway argillite; 8, Kintla argillite; 8a, Phillips argillite
and quartzite; 8b, Roosville argillite (5, 6, 7, 7a, 8, 8a, and 8b Middle Cambrian); 9, Mississip-
pian and Devonian limestone; 10, Kishenehn clays (Miocene).

series of cross sections in Figs. 20.3 to 20.7. Section B-B', K-K', G-G',
and I— I' are especially intended to typify the structures in southern Can-
ada and in Montana.

A great thrust fault is the dominant feature along the eastern margin of
the mountain belt. At the east base of the Lewis and Lewis and Clark
ranges it is called the Lewis thrust and has been extended southward 150
miles in Montana to the Lombard thrust (Clapp, 1932) and northward
from the international border at least 150 miles (Calgary Sheet, of the
Canadian Geological Survey, Alberta, 1928). The great fault has several

CANADIAN AND MONTANA ROCKIES

.307

branches as can be seen on the map, Fig. 20.2, and it is not obvious every-
where which should carry the name. It is a low-angle thrust. See sections
G— C, I-F and K-K'. In general, metamorphosed rocks of Beltian age
have been thrust up and over shales and sandstones of Mesozoic age. The
Lewis thrust in places is complex, consisting of several closely spaced
parallel faults with considerable drag folding. In other places the fault is
a single fracture, and the rocks on either side have not been greatly dis-

turbed. For example, on the north side of Cut Bank Creek Valley, the
Altyn limestone of Beltian age appears to rest almost conformably upon
relatively uncrushed carbonaceous shale of the Colorado formation of
middle Cretaceous age.

The fault in Glacier National Park and southern Alberta is very well
known from the writings of Willis ( 1902), Campbell ( 1914 ) , Daly ( 1912) ,
Clapp (1932), and Billings (1938). Here the fault has a lower' dip than

10,000' -,
JeoJeve.1

SCALE tN MILES

i—

—i

p io

Fig. 20.4. Cross sections in northwestern Montana showing Lewis thrust and related structures.
Al to A8, the Beltian formations; Al, Prichard argillite; A2, Altyn siliceous limestone; A3,
Appekunny quartzite and argillite; A4, Grinnell argillite; A5, Newland limestone and argillite;

A6, Spokane argillite and quartzite; A7, Helena argillaceous limestone; A8, Missoula group,
chiefly argillites, quartzites, and sandstones; PI, Lower Paleozoic formations; P2, Upper Paleozoic
formations; Kl, Lower, Middle and Upper Cretaceous. After Clapp, 1932.

308

STRUCTURAL GEOLOGY OF NORTH AMERICA

* V"

3,000
Sea level

Fig. 20.5. A, section through Canadian Rockies at Mountain Park, Alberta, after MacKay,
1929. Section is north of limits of index map, Fig. 20.2. la, lb, and lc, Devonian; 2a, Banff
shale (Mississippian); 2b, Rundle limestone (Mississippian ?); 3, Rocky Mountain quartzite (Pennsyl-
vanian ?); 4, Spray River formation (Triassic); 5, Fernie shale (Jurassic); 6, Nikanassin shale
and sandstone; 7, Cadomin conglomerate; 8, Luscar shale and sandstone; 9, Mountain Park
sandstone, shale, and conglomerate (Lower Cretaceous); 10, Blackstone shale and sandstone
(Colorado). The thrust under Cheviot Mtn. cuts the Big Horn and Wapiabi formation of Colorado
age and the Brazean formation of Montana age, all younger than the Blackstone.

C, section near Jumpingsound Creek, Alberta, after Hume, 1932. 1, Kootenay sandstone and
shale; 2, Blairmore sandstone and shale (1 and 2, Lower Cretaceous); 3, Lower Alberta shale;

in most places — 7 degrees — and the fault surface has either been warped
or was uneven when formed. Two conspicuous klippen composed of
Reltian rocks on Mesozoic shales are known as Chief Mountain and
Divide Mountain, and near the headwaters of Ole Creek in the southern
part of the park there is a window of Mesozoic shales entirely surrounded
by Reltian rocks. South of Glacier National Park, the dip of the fault is
generally steeper, but beyond Fiord Creek, 75 miles south of the park,
the fault flattens out, and another window 3 miles long and half a mile
wide appears. Southeastward, still, it becomes steeper, and eventually it

4, Cardium sandstone and conglomerate; 5, Upper Alberta shale (3, 4, and 5. Colorado);

6, Belly River sandstone and shale; 7, Bearpaw shale; 8, Edmonton sandstone and shale (6 and

7, Montana; 8, Montana ?); 9, Paskapoo sandstone and shale (late Paleocene).

D, section through Turner Valley structure, Alberta, after Hume, 1931. 1, Paleozoic limestone;
2, Fernie shale (Jurassic); 3, Kootenay sandstone and shale; 4, Blairmore sandstone and shale
(3 and 4, Lower Cretaceous); 5, Lower Alberta shale; 6, Cardium sandstone (5 and 6, Colorado);
7. Upper Alberta shale (Colorado and Montana); 8, Belly River shale and sandstone (Montana);
9, Edmonton sandstone and shale (Montana ?); 10, Paskapoo sandstone and shale (late
Paleocene).

is believed to join the Lombard thrust which has a dip of 40 degrees to
the west and northwest.

Ross and Rezak (1959) conclude that the horizontal displacement of
the sheet was at least 15 miles, probably 35 miles, and possibly more.
They note the absence of erosional debris or an irregular land surface
over which some geologists had suggested the sheet rode, and postulate
the fault surface to be a shear.

Whereas some thrusts and thrust complexes clearly exhibit character-
istics of gravity down-slope transport, it is difficult for the writer to con-

CANADIAN AND MONTANA ROCKIES

309

ceive of thrusts such as the Lewis to originate in any other way than by
compression of a considerable thickness of the crust. The concept of
compression is deeply ingrained in the literature of the Rocky Mountains,
and the representations in this and following chapters reflect these views.
They are challenged only if recent workers have taken a different view or
if the writer feels strongly in favor of gravity induced movements.

West of the Lewis thrust and between the two tear faults that bound
the Reltian segment is a broad syncline in the Reltian strata. See section
I-I', Fig. 20.4. West of the syncline, or on its west flank, a number of fairly
high-angle faults that dip eastward repeat the formations ( Clapp, 1932 ) .
The beds dip eastward at angles ranging from 20 to 50 degrees, and the
faults dip more steeply than the beds, generally at angles of 60 to 80 de-
grees. These western faults, together with the eastern, form a set of huge
downward-pointing wedges. Clapp estimates the amount of throw of the
western high-angle thrusts to be from about 10,000 to 30,000 feet.

Each of the western faults follows closely the west base of one of the
ranges which appear to have been uplifted along the faults. The faults
have been named for their respective ranges (Clapp, 1932).

In addition to the eastern and western thrusts, there are steeply dip-
ping transverse faults of both reverse and normal categories. They have
displacements up to 10,000 feet. The transverse faults are most abundant
in the southern part of the area north and northeast of the batholiths. One
fault of a singular category has been mapped that parallels the beds but
dips at a very low angle and is normal (Clapp, 1932).

Clapp relates the various groups of faults in the following way:

It appears as if the forces causing faulting acted from the southwest, first
uplifting and folding the rocks, then breaking them along the longitudinal
(western) thrust faults. Later, the deformation continued to such an extent that
relief from the stresses came by overthrusting (to the east). The two sets of
transverse faults seem to be a still later effect of the continued pressure from the
southwest, and consequent elongation to the northwest and southeast. ... As
the compressive forces acting from the southeast lessened, normal strike faults
with low dip relieved the vertical pressures resulting from the great height of
the uplifted rocks.

Normal faults, probably of late Cenozoic age, are also present and will
be discussed under a later heading. Most of this faulting appears to have
taken place along the much earlier longitudinal thrust faults.

In Canada, at least from Jasper at latitude 53° N southward to the bor-
der, the eastward and westward thrusts are found much in the same rela-
tions as in northwestern Montana. See section B-B', Fi<4. 20.3. The strata
instead of being mostly Beltian are mostly Cambrian, which in a broad
way are synclinal, although a distinct and great anticline occurs west of
Banff along the British Columbia-Alberta border.

A difference from the Montana division, however, is the nature of the
western boundary. In Montana, the Beltian rocks continue westward un-
der the entire terrane until the great intrusions make their appearance.
In Canada, the belt of longitudinal thrust faults in Paleozoic formations
is bounded on the west by the Rocky Mountain trench.

The belt of great imbricate thrust faults may be traced northward to
Mountain Park (section A-A', Fig. 20.5) and from there to ranges west of
Fort Nelson. The Alaska Highway west of Fort Nelson, between miles
380 and 497, crosses two ranges, the Sentinel on the west and the Stone on
the east. Here various rocks from Precambrian to Jurassic are exposed.
The section, according to Laudon and Chronic (1949), is:

Triassic strata

Black shale and black limestone 500 feet

Unconformity
Mississippian strata

Kindle formation: gray, silty limestone and chert 300—400

Unconformity
Devonian strata

Ft. Creek shale: black, pyritic shale 800

Ramparts limestone: massive, tan, gray, and black 4imestone 1500

Muncho limestone: gray and black limestone 600

Unconformity
McConnell limestone: gray and black limestone 680

Unconformity
Silurian strata, entirely Niagaran in age

Roninng limestone: gray and black, cherty, dolomitic limestone 1200

Cambrian (?) strata

MacDougal sandstone: tan sandstone "thin"

Precambrian rocks

Quartzite, slate, marble, and schist, intruded by basic igneous rocks

Corbin

RC-Alta. Boundary

Crowsnest Mta

Sentinel i

-iio

Sentinel

Blairmore

Hillcrest

B.C.-Alta. Boundary

.Crowsnest

Mtn.

Blairmore

Hillcrest

Bellevue

and

Byron Creek

| Passburg

JO 8

Geological Survey, Canada

Fig. 20.6. Section E— E' from Corbin to Burmis before and after the Laramide orogeny, after MacKay,
1932. (See Fig. 20.2 for the line of section.) 2, Devonian; 3 and 4, Mississippian; 5, Pennsylvanian;
6, Triassic; 7, Jurassic; 8 and 9, Lower Cretaceous; 10, 11, and 12, Upper Cretaceous.

CANADIAN AND MONTANA ROCKIES

311

Lew/ 5^ thrust Folded and faulted foothi// be/t of Cretaceous and Tert/ory

~$^^Z~ r\ j. /o ~-t ~j formations

rau^ — ■ Pet a/ is not mapped

* F1

Oweetgrass '

arch — >-

KJs Kc

Kmb

at,

_H<L

J CIs

Kk Kc Kv

SCALE IN MILES
5

Fig. 20.7. Cross sections of Foothill structure in northwestern Montana. Upper section, F— F',
after Stebinger, 1916. Lower two sections, H— H' and J— J', after Stebinger, 1918. KTsm,
St. Mary River formation; Kh, Horsethief sandstone; Kb, Bearpaw shale; Ktm, Two Medicine

The structure of the two ranges is one of folding and thrust faulting
typical of the Canadian Rockies farther south. The cross section of Fig.
20.10 illustrates the structure along the highway from miles 375 to 443.

See Figs. 37.1 and 39.14 and related text for brief discussion of the
Mackenzie, Franklin, and Richardson Mountains in the far north.

FOOTHILL BELT

Sections R-R', C-C, and D-D', Figs. 20.3 and 20.5, are typical of the
folded and faulted foothill belt in Canada. Sections F-F', H-H', and G-C,
Figs. 20.4 and 20.7 are examples of the structure of the foothills in north-
western Montana. The belt ranges in width from 5 to 25 miles and extends
from north central Montana ( southwest of Cutbank, Fig. 20.2 ) northwest-

formation,- Kmb, Bearpaw and Two Medicine undifferentiated; Kv, Virgelle sandstone; Kc and
Keb, Colorado shale; KK, Kootenai formation; KJs shales and sandstones undifferentiated,
belonging to Colorado, Kootenai, and Ellis (Upper Jurassic).

ward to at least the 54th parallel, a distance of 500 miles or more. The foot-
hills preserve remnants of early erosion surfaces, and are topographically
low and related more to the Great Plains than to the mountains, but inter-
nally their structure is complex and reveals a great deal of compressional
deformation. They are composed for the most part of the Cretaceous
shales which have been easily eroded. The prevalence of the weak shales
probably explains the reduction of the belt to one dominated bv low,
graded slopes. Only in the cores of a few anticlines are Paleozoic beds ex-
posed (section H-H', Fig. 20.7). The most common conception is that
small reverse faults of a few hundred feet displacement are numerous,
and that these terminate downward in major low-angle thrusts (Hume,
1926, 1931; Goodman, 1932). The anticlines and svnclines that exist

312

STRUCTURAL GEOLOGY OF NORTH AMERICA

5 6 7

ELK CREEK SECTION

Faults

Horizontal and Vertical Scales

Fig. 20.8. Geologic sections along Elk Creek and Cardinal River in the southern part of the Cardinal
district. After Hake et a/., 1942. See map. Fig. 20.9.

between the reverse faults are generally overturned toward the east and,
in harmony with the faults, represent eastward movement of the thrust
sheets.

In Montana, much of the foothill belt is covered with glacial drift, and

the structures there, especially just south of the border, are not well
known. See section F-F', Fig. 20.7. Farther south, folding and overturn-
ing to the east seems to dominate reverse faulting (sections H-H' and
J-J' Fig. 20.7).

Fig. 20.9. Fault pattern of Cardinal district. After Hake ef a/., 1942.

314

STRUCTURAL GEOLOGY OF NORTH AMERICA

SENTINEL RANGE

Or Dm Dpic ^ &r /~\

5T0NE RANGE

A// J/ PauL

IO MILES

Fig. 20.10. Cross section along Alaska Highway, west of Fort Nelson, in northeastern British Columbia,
between miles 375 and 443. After Loudon and Chronic, 1949.

North of Calgary, Alberta, and east of Jasper National Park in the
Upper Brazeau River foothill area, Hake et al. (1942) have mapped a
group of low-angle thrusts. The thrusts divide the Upper Cretaceous
sediments into thin sheets that have been strongly folded. See sections
of Fig. 20.8 and map, Fig. 20.9). The thrusts are considered noteworthy
( 1 ) because they are developed in weak beds, and the fault planes lie at
an angle to the bedding so that the sheets themselves are not competent to
have transmitted the thrust which caused the displacement; (2) because
the faults bear an exceptionally systematic relation to the bedding. The in-
vestigators believe these faults developed in an asymmetrical syncline,
and the faulting and the attendant crumpling relieved stresses which in
other folds and in other stratigraphic sections are relieved by bedding-
plane slippage. The thrusts are thought to be confined to the Mesozoic
section and the major syncline from which they developed, and to die
out completely with depth without producing any dislocation of the
Paleozoic rocks.

It is evident that this concept is at variance with the more commonly
portrayed one of many small high-angle reverse faults meeting a major
low-angle thrust at depth, but the authors think that their theory may
have widespread application in the foothill belt. This is confirmed by
Scott (1954), who describes much the same structure as Hake et al.
(1942) and repeats that it is found in other parts of the foothill belt
besides the Brazeau and Cardium areas. He thinks that two distinct

episodes of compression occurred, first thrusting of the thin sheets, and
second, folding of the thrust sheets.

AGE OF THRUSTING

The Cretaceous and Paleocene formations of the Rockies, foothills, and
plains of Alberta preserve the record of orogeny in the region to the west.
See correlation charts, Figs. 20.11 and 20.12. Warren (1938) has sum-
marized the evidence, and Fig. 20.13 is an attempt to show in diagram
what he has said in words. Incorporated in the diagram are also Evans'
ideas of the origin of the Rocky Mountain trench, and in addition, the
concept of post-Laramide graben-type faulting.

The Kootenay and Blairmore of Early Cretaceous age thicken west-
ward, and conglomerates become abundant. A basal conglomerate of the
Blairmore is believed to represent the first pronounced uplift to the west.
The formations are exposed in the Canadian Rockies in Elk River at
Crowsnest Pass (see Fig. 20.8), and some of the pebbles and boulders of
the conglomerates are medium- to fine-grained granite and granite por-
phyry that could only come from the Selkirks (Evans, 1932). This seems
adequate evidence to date the first uplift of the Selkirks and to indicate
that the Rockies had not yet come into existence but were a site of de-
position. Since the Blairmore is Aptian and Albian, it is evident that the
Selkirks first rose in latest Jurassic or earliest Cretaceous time. The Lower

CANADIAN AND MONTANA ROCKIES

315

Cretaceous sediments have been charted for the entire western part of
the continent on the tectonic map, Plate 11.

The Kootenay and Blairmore formations are continental in origin, and
reflect an uplift of the region to the west and a source of abundant feld-
spathic sediments. The next formation, the Colorado or Alberta shale, is
marine and represents a marine invasion. See chart, Fig. 20.11. The Belly
River that followed the Colorado is continental and resembles the Blair-
more. It reflects renewed uplift on the west. A local sea invaded the
southern foothills belt from the south, and in it the Bearpaw shale was
deposited. A continuation of uplift in the Selkirks resulted in the deposi-
tion of the continental Edmonton. Then a period of erosion occurred that
represents the Lance, early Paleocene, and middle Paleocene (Russell,
1932); and following it, the upper Paleocene Paskapoo sandstone and
shale were deposited. In the foothills and plains no angularity between
the Edmonton and Paskapoo has been noted.

All the foregoing Cretaceous formations and also the uppermost Paleo-
cene beds are folded and faulted in the foothills (Russell, 1932), and
therefore the main deformation of the frontal Canadian Rockies occurred
in post-Paleocene time.

A second but milder orogeny is noted by Bostock et al. ( 1957 ) :

In the Flathead Valley, west of Clark Range, the Kishenehn formation of
very late Eocene or very early Oligocene (Upper Duchesnean) age uneon-
formably overlies early Mesozoic strata which are involved in the structures
of the southern Rocky Mountains. The strata are gently folded, dipping mainly
about 30 degrees northeast. These observations indicate two phases of deforma-
tion, the first, pre-Kishenehn and post-early Mesozoic, probably post-Paleocene
in age, during which the main orogenic movements took place, and the second,
post-Kishenehn in age, during which the Kishenehn beds were tilted. Con-
glomerates of the Kishenehn carry pebbles of Proterozoic rocks indicating that
these rocks were exposed to adjacent ranges following the first phase of the
deformation.

Further evidence on the age of the uplifts associated with the orogenic
movements in the southern Rocky Mountains is found in conglomerates on the
Plains of southern Saskatchewan. The products of erosion from the uplift of
the southern Rocky Mountains during two phases of the deformation are
thought to be represented by gravels in the Cypress Hills region which carry
pebbles of the distinctive Proterozoic rocks and bracket in age the time of
deposition of the Kishenehn formation. Mammalian fossils in the Swift Current
Creek beds are Uintan (late Eocene) and those in the Cypress Hills formation

are of Chadronian (early Oligocene) age. In summary, the first and main de-
formation in the southern Rocky Mountains took place in the interval between
the Paleocene and the late Eocene. Uplift and erosion occurred in the late
Eocene (Uintan), followed by relative quiescence during the deposition of

CANADIAN ROCKIES
HIGMWOOD-ELBOW AREA

FOOTHILLS
TURNER
VALLEY

PLAINS

Fig. 20.11. Cretaceous formations south of Calgary, Alberta. Lithology is sandstone and shale
except where conglomerates and limestones are indicated. The Colorado and Bearpaw sedi-
ments are marine, the rest brackish and fresh water. After Thompson and Axford (1953).

316

STRUCTURAL GEOLOGY OF NORTH AMERICA

Fig. 20.12. Correlation of uppermost Cretaceous and Paleocene formations of southwestern
Alberta. Reproduced from Tozer (1953).

the Kishenehn beds Duchesnean. Moderate deformation and renewed uplift
took place in early Oligocene (Chadronian) time.

The Eocene age of the Lewis thrusting has been demonstrated fairly
well by MacKenzie (1922) from an Eocene formation in the Flathead
Valley, back of the Clark Range. Alden (1932) also believes the Lewis
thrust occurred in Eocene time.

Evans (1932) presents two arguments to support an earlier age of the
Selkirk system, west of the trench. The great Rocky Mountain trench
and the structures of the Rocky Mountains trend parallel with each other,

but the mountains west of the trench, viz., the Purcell Range, the Selkirk
Range, and the mountains west of Columbia Lakes, trend nearly north
at an acute angle to the trench, and are truncated by it. From these
relations it seems that the trench is associated with the building of the
Rockies which were formed later than the Selkirks.

Secondly, the Selkirk system contains many great intrusions; the Rockies
only a few smaller ones. See Fig. 37.1. The intrusions have been related
to the Coast Range batholith, and considerable evidence in Chapters 21
and 37 has been summarized that shows they are probably of Early or
Mid-Cretaceous age.

The structural discordance and the great intrusions of the Selkirk sys-
tem fit into the sedimentary record very well, and all three together
demonstrate a fairly substantial case for the Early Cretaceous age of the

SELKIRK SYSTEM

NORTHERN ROCKY MOUNTAIN TftOUSN

•LSERTt ShELF

SELKIRK SYSYEM

CANADIAN ROCKY MOUNTAIN SYSTEM

FOOTHILLS

SELKIRK SYSTEM TRENCH CANAOlAN ROCKY MOUNTAIN SYSTEM FOOTHILLS PLAINS

Fig. 20.13. Evolution of the eastern margin of the Selkirks and the Canadian Rockies.
Idealized sections incorporating parts of sections B— B' and E— E'. 1, Beltian; 2, Paleozoic,
Triassic, and Jurassic; 3, Kootenay; 4, Blairmore; 5, Colorado; 6, Belly River; 7, Edmonton;
8, Paskapoo; 9, Lower Oligocene conglomerate. Upper diagram, growth of Selkirks during
Cretaceous time and subsidence of northern Rock Mountain trough. Middle diagram, Laramide
orogeny during the Eocene and the deposition of the Lower Oligocene conglomerate. Lower
diagram, erosion of Rocky Mountain trench and Lower Oligocene conglomerate producing
present aspect.

CANADIAN AND MONTANA ROCKIES

317

mountains west of the trench and an Eocene age for the Rockies east of
i it. The Rocky Mountain trench is probably still younger and of mid- or
late Cenozoic age.

THE ROCKY MOUNTAIN TRENCH

In British Columbia. As previously indicated, a deep, wide valley
separates the opposing Canadian Rockies on the east from the Selkirk
system of ranges on the west in southern British Columbia. The Dogtooth,
Purcell, and McGillivray ranges ( see sections B-B' and K-K', Fig. 20.3 )
are parts of the Selkirk system that flanks the valley on the west, and the

; Van Horn, Brisco, and Galton ranges are examples of the Rocky Mountain
system on the east. The great valley is so regular and continuous that it

: was called the Rocky Mountain trench by Daly.

It does not have a continuous downhill gradient, but within the trench
are low divides that separate courses of several great rivers. The Ketchika
River drains the trench northward from latitude 58° into the Liard River.

i South of latitude 58°, the Finlay River drains the trench into the Peace
River which flows eastward through great canyons in the Rockies. The

! Parsnip River is a tributary of the Peace that extends southward nearly

, to the 54th parallel. The Frazer River occupies the trench from 54 to 53
N. Lat, and then the Columbia and its tributaries flow in the trench

: nearby to the international border.

Except for about 60 miles between the big bend of the Frazer River
and latitude 55° the trench is sharply or fairly sharply defined from
Kalispell, Montana, to beyond latitude 58°, nearly to the Yukon border, a
distance of over 900 miles.

The Rocky Mountain trench lies at the boundary approximately be-
tween the Nevadan and Laramide orogenic belts. According to Bostock
etal. (1957):

Throughout most of its length it forms the approximate boundary between
intensely deformed, altered, and intruded rocks characteristic of the western
Cordillera, and the moderately deformed and comparatively unmetamorphosed
strata that typify the eastern Cordillera. However, the trench does not every-
where coincide with this geological boundary; in several places it obliquely
transects structures on both sides and, south of about latitude 50 degrees, the
geological boundary lies east of the trench. North of this latitude the trench
is known in several places to be the locus of extensive faulting and may have

^BRITISH COLUMBI
IDAHO

CRETACEOUS (')

I 1 I BATHOUTHS '• \ N

DEVONIAN AND LATER '• 'I

PRE ■ DEVONIAN

WINDERMERE

4 TOBY HORSC THltF C8£l K

PURCELL (BELT)
UPPER

KlTCMtNSIt. S/V£W WALLACC.
DUTCH C««. MISSOULA. £TC.

10WE

m
m

LOWER

CfteSTON. BAVALUCTC

ALDMDGt, PRITCHARO

FAULT cW-w.

Fig. 20.14. Geology on either side of the southern part of the Rocky Mountain trench.
Reproduced from Leech, 1959. Stippled zone is trench.

318

STRUCTURAL GEOLOGY OF NORTH AMERICA

Fig. 20.15. Sketch of east face of Rocky Mountain trench at latitude 49°08'. Made from
photograph by Leech, 1959.

been an active feature since the oldest Cordilleran disturbances. From latitude
50° to 51° 30' or beyond, several longitudinal faults pass into the trench at small
angles. Westerly dipping thrust faults cut the rocks of the Dogtooth Mountains
into slices, and such a fault or fault zone is assumed to underlie the floor of
the trench for many miles. Easterly dipping faults east of this part of the
trench have been interpreted as underthrusts. Some long straight steep faults,
such as the Redwall, may be dominandy strike-slip faults. North of Finlay Forks,
rocks of the Sifton formation that floor the trench have been tilted and cut into
long narrow slices by closely spaced faults that strike parallel with the trench.
The fault slices transect, at a small angle, the strike of the Sifton strata and
that of structures immediately east of the trench.

The long trench has been little studied until recently when a report on
its nature immediately north and south of the international border
has appeared (Leech, 1959). Previously it had been postulated to be the
result of erosion following Cretaceous thrusting and folding (section B-B',
Fig. 20.3 ) , or to be due to normal downfaulting, either of late Laramide
or late Cenozoic age (Section K-K', Fig. 20.3). In the first edition of this
book it was postulated to be due to late Cenozoic graben-type faulting or
rifting, and part of a great belt that extends from southwestern Utah
to the Yukon.

Figure 20.14 is a map reproduced from Leech (1959) which shows
the complex Laramide and Nevadan ( ? ) structures on either side of the
trench. The following points are mostly by Leech:

1. The trench in the Cranbrook area is particularly sinuous in contrast to its
linear extent farther north.

2. It is asymmetrical, with the east flank high and of youthful fault-scarp
topography (Fig. 20.15).

3. It contains outcrops of Paleozoic and Belt strata on its floor and does not
appear to be as heavily alluviated as are some of the trenches further south
in the United States.

4. It is probably of block fault origin but bounding normal faults are not
everywhere apparent, especially in the sinuous section.

5. The postulated bounding normal faults are commonly disposed acutely to
the older thrust faults.

6. Since the same formations appear on either side of the trench in this
southern region, it is evident that here the rift is not an exact boundary be-
tween the Nevadan orogenic province on the west and the Laramide on the
east.

In the Yukon. The trench loses its identity north of latitude 59°.
The division between Nevadan and Laramide provinces also is difficult to
identify, but probably swings northerly to he east of the Selwyn Moun-
tains.

About 100 miles northwest of Watson Lake on the Yukon-British Co-
lumbia border a remarkably strait valley, the Tintina, extends for about
400 miles northwesterly to the Alaskan border. See Fig. 39.1. Although
not connected with the Rocky Mountain Trench it is in alignment with it,
and a Tertiary filled valley at Watson Lake helps bridge the gap. Ac-
cording to Bostock et al. (1957);

Major faults mark the course of the valley near Ross River Post and in the
Glenlyon and southwest Mayo area, and major geological boundaries coincide
with it in other places. Early Tertiary beds, only gently warped, outcrop at
intervals along the valley floor, proving its early development as a physio-
graphic feature.

Shakwak Valley, another long straight lineament, extends from the Alaska
boundary southeast through Kluane Lake almost to latitude 60 degrees. Through
most of its length it forms a major geological boundary and is believed to mark
a great fault zone. Evidence of recent movement is found in unconsolidated
deposits along the valley floor. Southwest of Shakwak Valley in the Kluane area,
a zone of overthrust faults is believed to form, with the Shakwak Valley fault,
a graben structure enclosing upper Paleozoic to Tertiary rocks.

21.

of the batholith are blanketed by Tertiary strata, mainly Miocene vol-
canic rocks, and it is generally recognized that the batholith is much
larger than that exposed and shown on maps.

IDAHO BATHOLITH AND
THE OSBURN FAULT ZONE

EXTENT

The Idaho batholith extends from the vicinity of Boise northward
through the center of Idaho into Montana, and has an area of over 16,000
square miles. Plutons of batholithic dimensions occur in the narrow north-
ern end of Idaho, as if to link the main Idaho batholith to the Loon Lake,
Colville, and Nelson batholiths. See Fig. 17.13. Smaller batholiths and
stocks also occur nearby in western Montana, namely, the Boulder bath-
olith near Butte, the Philipsburg stocks near Philipsburg, and other un-
studied and unnamed batholiths to the south and southeast.

Most of the western, all of the southern, and part of the eastern borders

COMPOSITION

Composition of Main Mass

According to Boss (1928), the Idaho batholith is composed mainly of
quartz monzonite, although marginal fades are commonly granodiorite.
In northern Idaho north from Pend Oreille Lake large plutons are com-
posed of granodiorite, quartz-monzonite, and granite, and are regarded
sufficiently similar to the Idaho batholith to be connected with it geneti-
cally and to bridge it to the Nelson batholith in British Columbia ( Boss,
1928).

Later Anderson (1942) found the marginal faeies to have been diorite
originally, with only minor amounts of quartz, and that it was subse-
quently altered by widespread rising solutions rich in silica. Much quartz
was added, and generally also smaller amounts of potash, feldspar, biotite.
and sphene. Where little or no potash feldspar was added, as along the
northwest margin of the batholith, a quartz-rich diorite (tonalite) was
produced; where considerable amounts of potash feldspar were added, as
along the south and southwest margin, granodiorite formed. The inner
faeies, upon consolidation, was less calcic than the marginal, and origi-
nally ranged from a diorite to granodiorite, with oligoclase rather than
andesine. Postconsolidation emanations added considerable silica and pot-
ash and increased slightly the amount of biotite. Considerable added pot-
ash feldspar changed the rock to a quartz monzonite; less, to a grano-
diorite. Locally, enough potash was added to form granite; in places, a
muscovite granite.

Younger Intrusives

The great batholith is now known to be composite and to contain plu-
tons younger than the main mass of quartz monzonite or granodiorite. See
the Geologic Map of Idaho bv Boss and Forrester, 1947. Boss ( 1935) has
described an intrusion in the Casto district that cuts the Laramide struc-

319

320

STRUCTURAL GEOLOGY OF NORTH AMERICA

tures and perhaps even Miocene (?) beds. It consists of a pink granite
to a quartz monzonite. The map of Fig. 21.2 shows this intrusion, as well
as others of similar age and relation to the main batholith. The data were
taken from the Tectonic Map of the United States. Ross also mentions
pink granites in the northwest corner of Idaho and in Rritish Columbia
that probably cut Miocene (?) strata, and which he believes are distinctly
younger than the Nelson batholith. These pink granites lead the writer
to think of a pink granite in southwestern Montana which proved to be
Precambrian in a thrust sheet which was later cut and displaced against
Miocene ( ? ) basin beds by a high-angle fault, as if in intrusive contact
with them.

Anderson ( 1948 ) describes two areas of younger intrusives within
the main batholith and says there are "many others." The younger in-
trusives are of two sets, one believed to have been emplaced at the close
of the Laramide orogeny and the other in mid-Tertiary time. The early
Tertiary magma was chiefly noritic, and the mid-Tertiary injections range
in composition from dacite to rhyolite, with quartz monzonite porphyry
and rhyolite porphyry most abundant. The Tertiary plutons within the
main batholith are small and elongated. One, however, Anderson de-
scribes as 8 miles long and %0 to 1% miles wide. They invade fault and
shear zones, the main ones of which extend in a northeast direction.

Again in 1952 Anderson cites evidence that discrete masses of the
granitic rock were emplaced under deep-seated conditions and others at
much shallower depths. The deep-seated plutons include one that evolved
while the major orogeny was taking place, and another which came in
during the later, less intense stages of deformation. The shallower intru-
sions are those of Laramide and later age.

Border Zones

The Thatuna pluton, a satellite on the west, is principally granodiorite
but grades into adamellite, tonalite, and granite. The Beltian strata which
the Thatuna batholith intrudes are variably affected. In extremely fine-
grained types, the contact is sharp and follows joint planes; but the con-
tact with the granular quartzite is gradational through several hundred
yards. By increase in feldspar the quartzite grades into igneous rock. To

the southeast of the Thatuna pluton, thin layers of pegmatite and aplite
are interlayered with paragneiss and diopside quartzite to form an exten-
sive mass of gneiss. The belt of gneiss is 12 miles wide in Latah County,
and extends for 15 miles at least into Clearwater County, where it borders
the Idaho batholith. An extension of the Idaho batholith is believed to
underlie the metamorphic belt (Tullis, 1944).

The Bitterrot Range of Idaho and Montana is largely a zone of gneiss
and schist that borders the Idaho batholith on the northeast corner. It is
a migmatite of the intrusion, according to Langton ( 1935 ) , but according
to Sydney Groff of the Montana Bureau of Mines and Geology (personal
communication) it is a Precambrian terrane.

AGE

Consanguinity

The age of the great Idaho batholith is an important problem in the
tectonic setting and, at the same time, a matter of controversy. The prob-
lem seems to be resolved into an issue between a Nevadan and a Lara-
mide age.

The principal argument advanced for the Nevadan age of the Idaho
batholith is its lithologic similarity to the batholiths of the Nevadan orog-
eny, specifically to the Nelson batholith (Ross, 1928). Since all the batho-
liths exhibit many variations in the granitoid series, generally, from diorite
to granite, it does not seem possible to correlate them closely in age on the
basis of lithologic similarity. It must be granted, however, that the granit-
oid character, together with great size and clustered grouping, seems to
relate them to a common great orogenic belt and batholithic cycle. Noth-
ing similar to the Idaho batholith occurs elsewhere in the Laramide oro-
genic belt.

Intrusive Relations

Near its southeastern end the batholith intrudes a thick series of Paleo-
zoic strata. In this vicinity, isolated granitic masses similar and probably
satellitic to the batholith cut Paleozoic strata as young as Pennsylvanian.
Farther north the bordering formations are mostly Proterozoic ( Beltian )

IDAHO BATHOLITH AND THE OSBURN FAULT ZONE

321

quartzites and slates. Still farther northeast in Montana the batholith is
believed to be bordered chiefly by Beltian rocks, although little is known
geologically of this region. On its west side, the bordering formations, in
addition to the extensive Tertiary volcanics of later origin, include pre-
Tertiary sedimentary and volcanic strata which are intruded by it. The
pre-Tertiary rocks along the part of the western boundary north of Sal-
mon River are mostly so metamorphosed as to make correlation doubtful,
but along Snake River there are considerable thicknesses of Permian strata
and some Triassic beds, both of which include volcanics. Small granite
masses, presumably satellites, cut the Permian strata. The Thatuna
batholith is one of these (see Fig. 17.13).

In numerous places, Tertiary strata, mainly Miocene (?) volcanic rocks,
rest on the eroded surface of the batholith; and it is clear that much of the
eastern part of the batholith now exposed was laid bare by erosion prior
to the Tertiary volcanism. Some of the volcanic flows resting on the batho-
lith may be as old as Oligocene ( Ross, 1928 ) .

Satellites (?)

The numerous plutons east of the Idaho batholith in western Montana,
such as the Philipsburg (Calkins, 1915), Boulder (Knopf, 1913), and
Marysville (Barrell, 1907), intrude either Cretaceous formations or older
Mesozoic formations that were folded and thrust following the deposition
of the Upper Cretaceous beds. The intrusions are distinctly discordant
with the folds and thrusts and, as far as known, were all emplaced after
the Laramide thrusting. They constitute a middle or late phase of the
Laramide orogeny.

If the assumption is correct that the main batholith was intruded at the
same time as its smaller eastern neighbors, then the great pluton must be
Laramide in age and not Early Cretaceous or Late Jurassic (Nevadan).
In further consideration of this line of evidence, it may be seen (Fig. 21.2
and Tectonic Map of the United States, 1944) that the Philipsburg thrust
is truncated by the main eastward-extending appendage of the Idaho
batholith. But this appendage is represented on the new Tectonic Map of
the United States as a separate intrusion of later age than the main igne-
ous mass. The representation comes of necessity when the main mass is

shown as Nevadan. Details are not known, because the appendage has not
yet been described in print.

The Casto intrusion is exposed along the axis of a broad anticline and
involves both Permian (?) and Miocene (?) strata (Ross, 1935). Injec-
tions of pink granite into the Miocene (?) beds indicate the age of the
pluton to be Miocene (?), according to Ross; but then, the exact age of
the Tertiary beds is not known. Ross mentions other pink granites in the
northwest corner of Idaho and in British Columbia that probably cut Mio-
cene (?) strata and are distinctly younger than the Nelson batholith. The
Nelson is believed to be earlier than Late Cretaceous because pebbles of
its granite are found in the Blairmore conglomerate of Late Cretaceous
age. The pink granites appear to be the youngest of the plutons, even con-
siderably younger than the Boulder batholith (Ross, 1928).

Setting in Laramide Tectonic Plan

Figure 21.1 has been prepared to show in a broad way the relation of
the Idaho batholith to the Nevadan and Laramide orogenic belts. In brief,
the batholith is located at the junction of two arcuate segments of the
Laramide belt, one extending from Canada into Montana on the north,
and the other extending from Utah through Wyoming and southwestern
Idaho on the south. A third major structural element, the zone of thrusting
of the shelf ranges, converges here also. The converging of the three large
elements of the Laramide orogeny at about the position of the Idaho
batholith may be genetically significant.

The dominant trend of the fold axes and thrusts about the batholith, as
shown in Fig. 21.1, is a generalization of the detail shown in Fig. 21.2. The
latter map was compiled from the Tectonic Map of the United States.
with faults of post-Laramide age (as well as known) deleted and with
additional fold axes and also some fault detail from the new Geologic Map
of Montana (1945) added. The conclusion reached by inspection of the
detailed map is that the intrusions are markedly discordant locally, but
in a broad way the structures of the sedimentary rocks wrap concordantly
around the east and north end of the main batholith. This may mean either
that the batholith was already there and served as a buttress around
which the Laramide structures were wrapped, or that in the process of

322

STRUCTURAL GEOLOGY OF NORTH AMERICA

intrusion it shouldered aside the adjacent surficial crust and formed the
Laramide structures. In the first case the discordant structures would
have to be due to later intrusions.

Lewis and Clark "Line"

About 30 miles north of the north end of the Idaho batholith is a zone
of large high-angle faults which trends slightly north of west. The chief
ones are called the Hope, Osburn, Rurnt Cabin, Placer Creek, and St. Joe,
and the whole zone referred to as the Lewis and Clark "fine" (Wallace
et ah, 1960). They dominate zones of complexly fractured rock and are
the chief localizers of ore in northern Idaho.

The Hope is the most northerly of these great earth fractures. It has a more
northwesterly trend than the others, averaging N. 55°-60° W., and dips steeply
southwest. It closely parallels the lower course of the Clark Fork of the Colum-
bia River for about 65 miles, then extends through the north arm of Pend Oreille
Lake and through a notch across the Selkirk Mountains, giving it a total length
of not less than 95 miles. It has an impressive vertical component of movement
and stratigraphic throw, but the horizontal component is about 12 miles, the
northeast side having been displaced southeast relative to the southwest side.
Along the fault zone are many associated fault fractures — low-angle thrust,
high-angle reverse, high-angle normal, and two sets of strike-slip faults — all re-
lated to the Hope and resulting from the tensional and compressional com-
ponents of the horizontal shearing stresses which produce the Hope. The
faulting, intrusion, and mineralization are closely related events and are re-
garded as products of the Laramide orogeny.

The Osburn fault of the Coeur d'Alene district is of even greater magnitude
than the Hope and has been mapped for 90 miles east-southeast of Coeur
d'Alene Lake. Its length is probably much greater, for its course approximately
coincides with an old valley extending from Spokane, Washington, to Deer
Lodge, Montana, a distance of 300 miles. Its course is N. 70°-80° W. and its
dip is steeply south. It also has many associated faults of variable magnitude,
some of which are mineralized. Igneous intrusion and mineralization in the
Coeur d'Alene district are largely localized along the course of the Osburn
fault (Anderson, 1948).

Fig. 21.1. The relation of the Idaho batholith to the Nevadan and Laramide orogenic belts. The
Nevadan belt is white and the Laramide belt is dotted and lined. The bold lines in the Laramide
belt are axes of prominent folds, thrust faults, and major trends. The Nevadan and Laramide
belts overlap; in fact, the geosynclinal division of the Laramide belt was strongly deformed in
places in Early and Mid-Cretaceous time.

IDAHO BATHOLITH AND THE OSBURN FAULT ZONE

According to Wallace et al. ( 1960 ) pronounced strike slip is indicated
by the following features:

(a) the offset of large upwarped blocks more or less delineated by areas of
outcrop of the Prichard formation, the oldest unit of the Belt series; (b) the
offset of major folds and faults, and the dissimilarity of structural features adja-
cent to one another on opposite sides of the fault; (c) large-scale drag features;
(d) offset of the same sense along parallel or subparallel faults; and (e) the
position of major mining areas on opposite sides of the Osburn fault and the
pattern of ore and gangue-mineral distribution within the areas. A maximum
of about 16 miles of right-lateral strike slip is indicated on the segment of the
Osburn fault east of the Dobson Pass fault and about 12 miles displacement in
the same sense is indicated west of the Dobson Pass fault. The difference in
displacement on these two segments is believed to be principally the result of
contemporaneous dip slip on the Dobson Pass fault, which has effectively
lengthened the block north of the Osburn fault relative to the block south.
A few miles east of the area shown in Fig. 21.3, in the vicinity of Superior,
Mont., the cumulative lateral movement in the Osburn and the related Boyd
Mountain fault, as shown by stratigraphic displacement, appears to be ap-
proximately 16 miles, which strongly corroborates the suggested displacement
on the Osburn fault in the Coeur d'Alene district.

The age of the Osburn fault is known only within broad limits. It cuts
rocks of the Belt Series of Precambrian age and is capped by flows of Colum-
bia River basalt of middle Miocene age. The probably contemporaneous Dob-
son Pass fault cuts the Gem stocks, which have been dated as about 100 million
years old. Other geologic evidence indicates that a lineament in the general
position of the Lewis and Clark line may have been in existence since early
Precambrian time.

Ages obtained from uraninite from the Sunshine mine indicate that uranium
mineralization occurred about 1,250 million years ago. Thus tight folds, such
as the Big Creek anticline (Fig. 21.3), that are cut by the uraninite veins,
must have been developed before that time. In contrast, the principal ore-
bearing veins are younger than the Gem stocks of about 100-million-year age.

The same authors outline the history of development of the structural
complex as follows:

During an early stage of deformation (Fig. 21. 3A), the principal folds were
developed and overturned to the northeast, and reverse faults that strike north-
west and dip southwest were formed. A large domelike structure, the Moon
Creek-Pine Creek upwarp, was formed west of the reverse faults.

( Fig. 21.2. Detail of the belt of Laramide orogeny and the Idaho batholith. Both major fold
f axes and thrust faults of the Laramide orogeny shown by lines. The main batholith is stippled,

and the plutons of known Laramide age are black. Compare with Tectonic Map of the United

Sfates, 1945.

SCALE IM Hltei

ijo ite jop

At ItHTA
iOmTAMA '

324

STRUCTURAL GEOLOGY OF NORTH AMERICA

EXPLANATION

Monzonite stocks

Areas of outcrop of
Prichard forma-
tion, represent-
ing positions of
large upwarps

Movement patterns

10 MILES

Accompanying a major reorientation of the stress system, the axes of the
folds began to bow (Fig. 21. 3B), the southern part of the region moved
relatively westward, and incipient strike-slip faults developed. The Mill Creek
and Deadman syncline was separated from the Granite Peak syncline and
wrapped around the truncated end of the Granite Peak syncline. The northern
flank of the Lookout-Boyd Mountain anticline was sliced off by one of the
antecedent fractures of the Osburn fault.

Monzonite stocks intruded the structural knot thus produced (Fig. 21.3C),
and the principal period of ore deposition followed. Most of the veins are
included in spatial groups that define distinct linear belts trending slighdy
more northwesterly than the Osburn fault system. The concentration in such
belts of veins, which are subparallel but differ in size and orientation, suggests
that linear feeders for the mineralizing solutions existed at depth, although no
through-going structural elements reflect these feeders in the upper crust.

After the principal period of ore deposition, strike-slip movement along the
ancestral Osburn zone of weakness became more through-going than previously,
and apparently deep-seated stresses were accommodated at this time by dis-
placement on relatively few faults, most of which were in or parallel to this
zone. The Osburn fault offset the major folds and early reverse faults, and
separated the northern segment of the ore-bearing area from that to the south.
The Thompson Pass fault also offset the major folds, and the Placer Creek fault
offset the Pine Creek anticline and vein system. The Dobson Pass fault came
into existence concurrently with the Osburn fault. The small stocks a few miles
west of the Dobson Pass fault may represent cupolas displaced from the main
part of the Gem stocks by dip slip on the Dobson Pass fault.

Some of the early-formed tight folds and strike-slip faults were flexed as later
rotational stresses were accommodated along newly developed slip planes. Thus,
the east end of the Savenac syncline and the adjacent north branch of the
Osburn fault were sharply bent and later movement was "short-circuited" along
the south segment of the fault. Likewise, the Polaris fault may have accommo-
dated strike-slip deformation after the Placer Creek fault buckled.

Late normal faults, some resulting from the final stages of strike-slip deforma-
tion, and others possibly of Quaternary age (Pardee, 1950), have affected the
area.

The fault and fold pattern of the map of Fig. 21.2 suggests immediately
that the Idaho batholith has moved eastward as a rigid mass, and that
the thrusts along its east side are a direct compressional result. But this
idea seems incorrect when it is realized that the strike-slip movement on
the Osburn fault zone was in the wrong direction.

Fig. 21.3. Stages in the development of the Osburn fault zone in the Coeur d'Alene district,
Idaho. Reproduced from Wallace ef a/., 1960.

IDAHO BATHOLITH AND THE OSBURN FAULT ZONE

325

Setting in Nevadan Tectonic Plan

It is clear that two great arcuate segments of the Nevadan orogenic
belt converge in eastern Idaho (refer again to Fig. 17.13), and just a
little south of this junction is the Idaho batholith. The same relation to the
Laramide orogenic belt has already been pointed out, although the
Nevadan segments are convex westward and the Laramide are convex
eastward. The Nevadan segments are also curved more and meet at a
more acute angle than those of the Laramide. As previously suggested, the
junction area of such arcuate segments of a great orogenic belt may be a
favorable place for the rise of great batholiths, but it is difficult even to
; guess why.

The somewhat similar relation of both Nevadan and Laramide belts to
the Idaho batholith does not help in restricting or narrowing down the age

I of the pluton.

Relation to Tertiary Sediments

A fruitful field of research on the age of the Idaho batholith seems to lie
in Paleocene and Eocene conglomerates to the east. Certain voluminous
I conglomerates in northwestern Wyoming are composed of Beltian quartz-
ite boulders and pebbles which are foreign to the formations of the areas
: in which they occur. Their only source seems to be the Beltian strata that
: crop out along the eastern edge of the batholith in Idaho and western
Montana. See the Geologic Map of the United States. Also, Ross ( 1928 )
points out that the Idaho batholith was intruded extensively in the Beltian
strata, and that a roof of Beltian rocks, fully a mile thick, has been largely
removed. In fact, it was removed before the Oligocene and Miocene
lavas and sediments accumulated. The connection between the doming
of the quartzites, their erosion, and the formation of extensive con-
glomerate deposits nearby seems obvious; the dating of the intrusions by
the conglomerates seems a certain procedure. But the extent and age of
the various conglomerates east of the batholith are only fragmentarily
known, and some of the conglomerates may be made up of boulders that
had already composed a former conglomerate. As far as known, the
nearest coarse deposit is the Lima conglomerate in southwestern Montana

which is Paleocene in age (Scholten et al, 1955). The oldest of the
extensive conglomerates of the Yellowstone-Gros Ventre-Wind River
region is late Paleocene in age, and its boulders have been transported a
great distance because of the near-perfect rounding of them. This frag-
ment of information suggests very Late Cretaceous or early Paleocene
age, again, for the Idaho batholith.

Isotope Age Determinations

The absolute age of the Idaho batholith has recently, and with reason-
able assurance, been determined by Larson et al. (1954) by lead-alpha
activity ratios on the accessory minerals, zircon, monozite, and xenotime.
Five analyses yield an average age of 103 m.y. Similar determinations
on 7 samples from the Sierra Nevada averaged 100 m.y., and 25 samples
from the batholith of southern California gave an age of 105 m.y. Accord-
ingly, it may be concluded that the Idaho batholith is very nearly the
same age as the Sierra Nevada. Also, a potassium-argon age determination
on the Coast Range batholith near Vancouver by Follinsbee et al. ( 1957 )
is reported as 105 m.y., again approximately the same. A few years after
Larson et al. samples were taken by Evernden et al. (1957) from 8 individ-
ual intrusions in the Sierra Nevada whose age relations had been
determined geologically. The samples were run by the potassium-argon
method and the ages reported range from 76.9 m.y. for the youngest to
95.3 m.y. for the oldest. These ages are a little under the true absolute
age, but not more than a few percent, according to the authors. It may
follow that when potassium-argon age determinations are made of the
Idaho batholith that they will prove appreciably lower than those of
the lead-alpha activity ratio method. Since the Idaho batholith is probably
composite, the relation of age determinations by different methods is a
bit uncertain, especially since the sequence of intrusions in the Sierra
Nevada ranges through 18 m.y.

According to the Holmes B time scale the intrusions dated by the potas-
sium-argon method in the Sierra Nevada range through the Albian
(uppermost Lower Cretaceous) and the Cenomanian (lowermost Upper

326

STRUCTURAL GEOLOGY OF NORTH AMERICA

Cretaceous ) ( Evernden et al., 1957 ) . Presumably this should be the tenta-
tive geologic age assigned to the Idaho batholith. As far as the writer can
see there is nothing inconsistent geologically with such a conclusion.

CONCLUSIONS

The Idaho batholith is composite, with some of the smaller parts and
satellites of Late Cretaceous and early Tertiary age and some as young
as Miocene. The Batholith occurs at the junction area of great arcuate
segments of both the Laramide and Nevadan orogenic belts. It is similar
in size and composition to the batholiths of the Nevadan orogeny and
entirely dissimilar to the plutons of the Laramide belts.

Having intruded the Permian volcanic sequence along its western
margin, it lies partly in the Pacific eugeosynclinal province. Its eastern
part intrudes miogeosynclinal sediments of the Rocky Mountan type.
It is strikingly discordant with the Laramide structures locally, but over-
all a fairly clear concordance prevails. This and extensive Paleocene con-
glomerates to the east, derived, presumably, from the roof rock of the
batholith, are the best evidence for a Cretaceous age. Isotope age deter-
minations indicate a Mid-Cretaceous date for the main and early com-
ponents of the great granitic mass. After cooling it formed a buttress
against which the Laramide folds and thrusts developed. Still later,
younger intrusions cut discordantly through older plutons and the
Laramide structures.

22.

CENTRAL ROCKIES

SPATIAL RELATIONS

The system of Laramide mountains referred to here under the heading
"Central Rockies" includes the ranges that formed from the geosynclinal
sediments of southwestern Montana, eastern Idaho, western Wyoming,
central and western Utah, and eastern and southern Nevada. The belt
starts at the Idaho batholith and extends southeastward to the Snake River
lava plains where it is covered by late Tertiary and Pleistocene lavas and
alluvium. See the Geologic Map of the United States and Fig 22.1. Emerg-
ing from beneath the lavas, it continues southeastward to the Snake River

and Hoback ranges of western Wyoming, where it turns southward and
extends into northern Utah and to the junction of the east-west-trending
Uinta Range. In Utah and Nevada, the Middle and Late Tertiary block
faulting has modified somewhat the topographic features resulting from
the Laramide orogeny; but it is clear that a belt of complex Laramide
thrusting and folding continues on south of the Uinta junction into south-
western Utah and southern Nevada.

The eastern border of the Central Rockies system is sharply defined,
whereas the western is indefinite. The eastern margin is made up in part
of the Paleozoic strata, in part of the Mesozoic strata and the orogenic de-
posits of the Cretaceous and the Early Tertiary; but westward only the
Paleozoic and some Triassic rocks of the Cordilleran geanticline are
involved. Examine the paleotectonic maps of the late Paleozoic and the
Mesozoic. Recause rocks younger than Paleozoic are almost entirely absent
in the western part of the Central Rockies, it is generally impossible to
date accurately the phases there or to distinguish the Laramide structures
from those of the Cedar Hills, Antler, and Nevadan orogenies. Most
probably, the Laramide structures were superposed on the Antler and
Nevadan in a medial zone, but details are not known. The map, Fig. 21.1,
shows the relation of the orogenic belts to the Laramide as well as possible
with existing data.

The Uinta Mountains are a great flat-crested anticlinal uplift and, as
far as Paleozoic and Mesozoic strata are concerned, are part of the shelf
province. Physiographically, they separate the Colorado Plateau from
the great ranges and intermontane basins of Wyoming, and are more
closely related to the shelf ranges of Wyoming than to the Colorado
Plateau. They are definitely not similar in structure to the Central Rockies,
and generally they have thinner formations. Therefore, they are not in-
cluded in them.

Aside from the Uinta re-entrant in the eastern margin of the central
Rockies, the great mountain system is one of approximate arcuate pattern
with a radius of curvature of about 450 miles. In it, probably all major
overriding thrust sheets have moved continentalward, or toward the con-
vex side of the arc, viz., northeastward and eastward.

327

328

STRUCTURAL GEOLOGY OF NORTH AMERICA

-1 '

Fig. 22.1. Index map of Central Rockies. Lines of cross sections are indicated by numbers.
Intrusive igneous bodies are indicated by dotted lines.

OROGENIC DEPOSITS

A number of coarse conglomerates and thick sequences of sandstone and
shale mark the eastern border of the central Rockies, and in connection
with thrust faults and unconformities define a succession of orogenic
phases. The various formations with which we are mostly concerned from
southwestern Montana to southwestern Utah are shown in the correlation
chart of Fig. 22.2.

The coarse conglomerates have generally been taken to record the
chief phases of mountain building immediately to the west, but thick
sequences of sandstone, siltstone, and shale may be equally significant.
The conglomerates record settings where a mountain front rose pre-
cipitously from a plain, as might have been the case of a vigorously
advancing thrust front. Rut a 10,000-foot section of sandstone, siltstone,
and shale of limited time range also records a substantial uplift in the
hinterland, possibly less vigorous but sustained, and without an immedi-
ately nearby thrust front.

The example of the clastic deposits of Colorado time may be considered
( see Fig. 22.3 ) . The lower 3,000 feet of the Indianola group in the Cedar
Hills is coarse conglomerate, but eastward and upward it becomes more
sandy and shaly. The thick conglomerate has been considered to mark
the Cedar Hills orogeny (Chapter 18). An associated thrust sheet rode
over part of the conglomerate in the Canyon Range but finally the thrust
front was buried by the last of the coarse deposits. Now, going north
to the Evanston area an accumulation of more than 8000 feet of sandstone
and shale occurs. Conglomerates are insignificant, yet the volume of
sediments appears almost as much as in the Indianola area, and the
adjacent uplift, therefore, almost as significant.

Where a thrust sheet overlies a coarse conglomerate two orogenic
phases might be interpreted; the first to form the conglomerate and the
second by the riding of the thrust sheet over the deposit. However, the
conglomerate exposed may be simply an early part of the orogenic deposit
which was overridden as the thrust sheet advanced, in which case the
conglomerate and thrust are manifestations of the same orogeny. Local
settings have to be studied individually, and isopach maps such as shown

CENTRAL ROCKIES

329

SW MONTANA

LIVINGSTON

MT. LEIDY

HOBACK BASIN

EVANS TON

COALVILLE

STRAWBERRY

WASATCH
PLATEAU

SW UTAH

PLEISTOCENE

Glacial deps.

PLIOCENE

Gravels on
intermed. surface

Bivouac
Teewinot

Camp Davis

Huntsville
f angl.

Rhyolite flows
and pyroclaatics

MIOCENE

Medicine Lodge
Blacktail Deer Cr.

Bozeman
Lake

Colter

Bishop cgl.

Traychyte flows

Page Ranch (vol3. )

OLIGOCENE

Muddy Cr.
Cook Ranch

beds

Wiggins vol.

Gray Gulch vols

Quichapa (vols.)
Isom (vols. )

EOCENE

Sage Creek

■7

Aycross
Wind River
Indian Meadows

Pass Pk cgl.

Fowkes tuff

Knight

Fowkes tuff
Knight

Park City vols.
Uinta

Crazy Hollow

•>
Green River

Colton

Needles Ra
Gray Clare

;e( voLs . )

PALEOCENE

Beaverhead cgl.

Pinyon cgl

Hoback

Almy
Evanston

Current Cr.

Flagstaff

U. North Horn

Red Clarori

DANIAN

c
C

•o

Livingston

Harebell cgl.
Meeteetse

Adaville

Ii
Mesaverde

L. North Horn

ICaiparowits

MAESTRICHTIAN

a >

-H
C <D

Eagle

Lenticular
sequence

Echo Can. cgl.

Price River cgl.

Wahweap

SENONIAN

<0 >

U H

Coaly sequence

Hilliard

Wanship

Blackhawk

Strait Cliffs

•o
a
c

(H
O

TURONIAN

.«- >>

•H .O
T3 3

C o!

3 w

Colorado

Bacon Ridge
Cody

Frontier

Mancos

Star Point

Indianola
gr . ( cgl . )

Tropic
Dakot ,i

M
c c

O -H

u u
m a

CENOMANIAN

ALBIAN

3
0

u o
4> u

s a

0 -U

Aspen

Mowry

Aspen
Bear River

Aspen
Kelvin

Mowry

APTIAN
NEOCOMIAN

Kootenai

Thermopolis
Cloverly and

Gannett
gr

Gannett
gr.

Cedar
Mountain

PORTLAND IAN

Morrison

Cloverly ?

Morrison

Fig. 22.2. Correlation of Cretaceous and Cenozoic formations along the east front of the Central Rockies.

in Figs. 22.3 to 22.6 compiled in order to understand the situation bet-
ter.

Although the standard time divisions need not have any bearing on

I nature's orogenic phases in any particular region they seem to reflect

the rhythms or cycles in the Central Rockies. Five main pulses of what are

conventionally called compressional orogeny are indicated, namely, Early

i Cretaceous, Colorado, Montana, Paleocene, and Eocene. The coarse and

: thick clastic deposits and shifting sites of activity provide the basis for

the recognition of the five main phases. After Eocene time volcanism

and large-scale normal faulting were widespread and dominant (Fig.

22.7).

SOUTHWESTERN MONTANA

Early Cretaceous Phase

Ry reference to the paleotectonic maps of Chapter 3, it will be seen
that the Paleozoic formations thicken westward into the geosyncline from
about Dillon (see Fig. 22.3), and thin to shelf aspects eastward. As an
example the Pennsylvanian Quadrant sandstone is nearly 3000 feet thick
in the thrust sheet west of Lima, but a few miles to the northeast it is
only 400 to 500 feet thick. The shore line of the Triassic and Jurassic
formations lay approximately along the Idaho-Montana border west of
Lima, but a deep trough failed to develop immediately on the east of the

330

STRUCTURAL GEOLOGY OF NORTH AMERICA

Fig. 22.3. Orogeny and sedimentation in Colorado time in the Central Rockies.

Cordilleran geanticline, as it did in Idaho and Utah, and the sediments of
these two periods are only about 2000 feet thick altogether. They thin
eastward. Some gentle epeiric movements occurred in the shelf in Jurassic
time, as outlined in Chapter 18.

In early Cretaceous time the western geanticline was raised sharply,
and the Kootenai conglomerate and arkosic sandstone were washed
eastward. It is not thick in most places, but very persistent in western
Montana. No structures have been segregated from the structural com-
plex of the west-lying ranges that were formed during the uplift respon-
sible for the Kootenai conglomerate. See paleotectonic map, Plate 15.

A second conglomerate, the Dakota or basal Colorado, is much like the
Kootenai; it is perhaps not so uniform in distribution, but it is taken to
represent another uplift of the eastern margin of the geanticline.

The radioactivity dates of the Idaho batholith appear to show it a little
later than the Dakota conglomerates. At least the first flood of Reltian
quartzite boulders so far identified appeared in late Montana time, and
these may have arrived at their present destination sometime after the
doming of the Reltian strata, consequent to the intrusion of the batholith.
The conglomerates in question make up the Harebell formation of the
northern Jackson Hole country. These will be considered in a later para-
graph.

Montana Phase

Little can be said about events in Montana time in southwestern Mon-
tana except that about 5000 feet of sandstone, siltstone, and shale accumu-
lated. These sediments make up an undifferentiated series near Monida,
and they undoubtedly attest uplift to the west. See Fig. 22.4.

Paleocene Phase (Mid-Laramide)

In Paleocene time a broad arch of about the size and shape of that of
the Rig Horn Mountains rose, and extended in a northeast direction ( Fig.
22.5). Its southeast flank in part was marked by a thrust fault; its north-
west flank was a fairly gentle flexure where observed. The Reaverhead
conglomerate seems to be localized around this great arch and to be

CENTRAL ROCKIES

.3-31

made up in large part from Paleozoic limestones and quartzitic sand-
stones derived from the arch, but in places Beltian boulders are present.
These may have come from the west, or from a pre-existing conglomerate
not yet found in place. Another uplift west of Yellowstone Park may have
appeared at this time, but no conglomerate around it is noted, so the time
of the appearance of the uplift and the exposure there of the Precambrian
rocks is not yet clear.

The steeply upturned beds and the overriding Precambrian sheet of the
southeast flank of the main arch from Lima to Virginia City now stand as
the Snowcrest and Green Horn ranges. Part of the northwest flank may
be seen in the Blacktail Range southeast of Dillon ( Scholten et al, 1955 ) .

The Beaverhead conglomerate is believed to be Paleocene ( Lowell and
Klepper, 1953). Soon after it was deposited, it was upturned along the
Snowcrest Range, and perhaps gently folded in other places.

Early Eocene (?) Phase

We find in southwestern Montana two systems of compressional struc-
tures nearly at right angles to each other. The northwesterly trending one
is clearly the later (see Fig. 22.6). It is characterized by numerous
thrust sheets, some of which override the Beaverhead conglomerate or
have carried the conglomerate on their backs in the horizontal movement.
See cross section, Fig. 21.8, which runs nearly north-south, just south of
Lima. Two folds of the earlier northeasterly trending disturbance are
impressed as sharp cross folds in the frontal thrust sheet.

The belt of thrusting of southwestern Montana is undoubtedly a con-
tinuation of the one of western Wyoming and eastern Idaho under the
Snake River volcanic field, as illustrated in Fig. 22.1. As far as known,
all thrusts moved toward the northeast. One or two brought the Pre-
cambrian crystalline rocks to exposure, but now are dismembered by
erosion into klippen and fensters. They involved the Paleozoic rocks of
geosynclinal character, and along the eastern front of the thrust belt the
Mesozoic rocks occur and are deformed.

The belt of thrusting on the northeast, between Virginia City and
Bozeman, involved the thin-shelf sediments, and in the uplifting that
accompanied each thrust sheet, much of the Paleozoic and Mesozoic

Fig. 22.4.

over Adavil

Orogeny and sedimentation in Montana time in the Central Rockies. Absaroka is thrust

7X7

Fig. 22.5. Orogeny and sedimentation in Paleocene time in Central Rockies. Lower part of
Evanston formation is latest Cretaceous.

Fig. 22.6. Orogeny and sedimentation in Eocene time in the Central Rockies. Heart Mountain
and South Fork thrusts shown at place where they originated. H.M. is Heart Mountain, a glide
block of the Heart Mountain thrust.

CENTRAL ROCKIES

333

veneer was removed, and the crystalline rocks were exposed. The thrust
sheets dip fairly steeply in this belt both northeasterly and southwesterly.

Numerous folds in the Paleozoic and Mesozoic strata developed at the
same time as the thrusting. The Reaverhead conglomerate was much
eroded after the thrusting and folding, and a singular remnant, the Sphinx
conglomerate, now holds up the highest peak in the Madison Range,
Sphinx Mountain, northwest of the northwest corner of Wyoming.

Several porphyry stocks were intruded immediately after the thrusting
along the Idaho-Montana border in the Nicholai and Cabin thrust sheets
(Fig. 22.6), and it is probable that a good deal of the intrusive and min-
eralizing activity in the Melrose, Rutte, and Philipsburg areas, immedi-
ately to die north, occurred at this time.

Late Eocene to Early Miocene Phase

Following the main thrusting in southwestern Montana, a long episode
of erosion, with possibly some additional crustal movements, changed the
topography to an almost unrecognizable extent. The arches and thrust
sheets that had brought Precambrian rock to exposure were irregularly

: reduced, and perhaps broadly downfolded in places. Instead of concen-
trating their attack on the sedimentary rocks, the erosional processes cut

; great intermontane valleys through the Precambrian crystalline rocks as
well, with only local structural control.

Then, in late Eocene time, volcanism broke out in nearby regions, and
focused in Yellowstone Park and the Absaroka Range (Fig. 22.8). Vol-
canism of superior magnitude also broke out in the Coast Range region
of Oregon and Washington at this time. It resulted in the damming of
drainage ways and in abundant ash and dust falls. The regimen of erosion
changed to one of alluviation in the great intermontane valleys, and the
heavy deposition of the Sage Creek formation (late Eocene) resulted in
southwestern Montana. Other formations of equivalent age were laid
down in the basins elsewhere over a wide region.

Local deformation and erosion in early Oligocene time are noted by an
unconformity between the Sage Creek beds and those that overlie it.
Volcanism continued nearby, and the deposition of the Cook Ranch beds

i in middle Oligocene time on the Sage Creek beds resulted.

'ir

'^Sr-

IGN1MB8IT£$

C 0 LORADO

PLATEAU

Fig. 22.7. Orogeny, sedimentation, and volcanism in late Cenozoic time in the Central Rockies.
The Great Basin is brought into existence by block faulting and becomes a region of considerable
sedimentation (the Salt Lake group) on the downfaulted blocks. The east end of the Uinta Moun-
tains sank along the axial area and the Browns Park formation was deposited in the depression.
Volcanism starting in late Eocene and running through the Cenozoic was widespread. Much of the
Tertiary volcanic rocks in the Geat Basin are buried by later alluvium.

334

STRUCTURAL GEOLOGY OF NORTH AMERICA

Cm Mkfo*f

dlc-

)n<i

d9e

LIMA PKS.

IDAHO I MONTANA

\XMS W

10, ooo'-

Fig. 22.8. Cross section of thrusts in southwestern Montana and adjacent Idaho, after Drexler,
McUsic, and Kildal, Master's theses, University of Michigan. Cm, Madison formation; Ct, Tensleep
sandstone; Cp, Phosphoria formation; Rd, Dinwoody formation, 'Rw, Woodside formation; 'St,

Miocene-Pliocene Phase

A fairly extensive episode of erosion followed the deposition of the
Cook Ranch beds, and in the Blacktail Range southeast of Dillon, tilting
and the overlap of younger beds seem to indicate the inception of block
faulting. This would have occurred in latest Oligocene or earliest Miocene
time. Then volcanism broke out anew at the north end of Blacktail Range
and extensively in the Snake River Valley, Yellowstone Park, and the Co-
lumbia Plateau. Deposition of lower Miocene Blacktail Deer Creek beds
and associated basalts, tuffs, and agglomerates resulted in the Upper Sage
Creek area, along the northwest flank of the Snowcrest Range, and in the
Ruby Reservoir basin.

Then followed erosion to an extensive surface of moderate relief. In
places the pre-Sage Creek surface may have been reexhumed and become
coextensive with this new post-Blacktail surface, which is present now in
summit areas of the Blacktail Range. There, lower Miocene basalts and
tuffaceous beds are beveled.

An episode of block faulting is clearly recorded in the Ruby Reservoir
basin following the deposition of the Blacktail beds, and then in the down-
faulted basin, the upper Miocene and lower Pliocene Madison Valley beds
accumulated.

Seo /eve/

Thaynes formation; Js, Sawtooth formation; Jr, Rierdon formation; Jm, Morrison formation; Kk,
Kootenay formation; Kbr, Bear River formation; Ka, Aspen formation; Tc, Paleocene (?) conglomer-
ate; Tvr, Rhyolite flows. Section 1, Index map, Fig. 22.1.

Pliocene and Quaternary Faulting and Erosion

Regional uplift, in places possibly accompanied by more block faulting,
and the erosion of extensive pediments followed. The pediments on the
northwest side of Snowcrest Range are the most extensively and perfectly
developed. The pediments on basin beds of the back valleys in Beaver-
head Range (graben valleys) are of this age. In valleys like Beaverhead
River, Blacktail Creek, and Sweetwater, downfaulting was so extensive
that alluvial aprons were deposited along the base of the fault scarps.

A third episode of block faulting resulted in alluviation in places, and in
others of gentle uplift and dissection of the pediments. Two episodes of
glaciation in the Beaverhead Range are recorded, one probably occurring
before dissection of the pediments, and one afterward.

Block faulting at the front of the Tendoy and Madison ranges has con-
tinued in modern times.

SOUTHEASTERN IDAHO AND WESTERN WYOMING

Latest Jurassic and Early Cretaceous Phase

Like the southwestern Montana Rockies, those of southeastern Idaho
and western Wyoming contain Paleozoic formations of geosynclinal thick-
nesses on the west, of shelf thicknesses on the east, and of marginal ge-

V SWAN VALLEY

Cp^ld

Neeley Bojin

Fig. 22.9. Cross sections of the northern central Rockies from the Caribou Range in Idaho east-
ward to the Hoback basin (north end of the Green River basin) in Wyoming. See section 2, Fig.
22.1. The sections are not continuous but each is staggered southward somewhat from west to
east. (Upper section adapted from R. Enyert's thesis; middle section adapted from K. Keenmon's
thesis; lower section adapted from Jack St. John's and Alex Ross' thesis, all of the University
of Michigan.) The Ferry Peak, Absaroka and Darby thrusts are Paleocene in age; the Cabin
thrust is late lower Eocene (post-Hoback fm.); and the Grizzly thrust is late middle Eocene
(post-Pass Peak congl.). Also a cross section from Hoback Range to the Gros Ventre Range across

the Hoback basin, which is the north end of the Green River basin. Cf, Flathead quartzite; Cgv,
Gros Ventre formation; Cb, Boysen formation; Ob, Bighorn dolomite; Dd, Darby formation; Cbm
Brazer and Madison limestones; Ca, Amsden formation; Ct, Tensleep sandstone; Cw, Wells
(Amsden and Tensleep); Cp, Phosphoria fm; Id, Dinwoody and Woodside; Tic, Ankareh; Jn,
Nugget sandstone; Jtc, Gypsum Spring and Twin Creek; Js, Preuss and Stump; Kg, Gannett
group; Kbr, Bear River; Ka, Aspen; Th, Hoback fm.; Tp, Pass Peak congl.; Tc and Ted, Camp
Davis fm.; Tla, lower andesite, Tua, upper andesite, Ts, silt of Camp Davis; Kf, Frontier fm.

336

STRUCTURAL GEOLOGY OF NORTH AMERICA

anticlinal deposts of Mesozoic age in their central and eastern parts.
Refer again to the paleotectonic maps of Chapter 3.

The Ephraim conglomerate marks the first vigorous uplift of the geanti-
cline to the west, and the age of the conglomerate, according to Mansfield
(1927), is Early Cretaceous, but according to W. L. Stokes (personal
communication) may be latest Jurassic. Somewhat later, but still in early
Cretaceous time, the Bechler conglomerate was washed eastward from
the westward-lying geanticline.

Colorado Phase

The orogenic deposits of the Colorado phase ( Fig. 22.3 north and north-
east of Jackson are the Frontier formation, Cody shale, Bacon Ridge
sandstone, and the Coaly Sequence (Love, 1956a,b). They make up a
series of clastic deposits about 5000 feet thick. East of Evanston the
Frontier formation and Hilliard shale are about 9000 feet thick. These
deposits undoubtedly attest the rise of adjacent land on the west, but for
most of the length of the deformed belt it is impossible to identify any
structures there that were formed at this time. The Taylor and Ogden
thrusts predate the Willard thrusting, which is probably Montana in age,
so they may be structures formed as the west-lying land was elevated.

Montana Phase (Early Laramide)

The deformed belt of western Wyoming and southeastern Idaho is
noted for a number of thrust faults, the main ones of which are shown on
Fig. 22.4. They have all moved eastward, or at the north end of the belt
northeastward, and in places a number of sheets are stacked on each
other in imbricate fashion. These probably formed during late Montana or
early Paleocene time.

The Bannock thrust was first detailed by Mansfield (1927) as shown in
Fig. 22.10. A sheet of wide proportions was postulated to have moved
eastward over 40 miles and to have been folded and eroded such that a
large window occurs in it. Later work by geologists of Standard Oil
Company of California and the U.S. Geological Survey indicates that
several imbricate thrust sheets are involved and that the interpretation
of one single sheet is not correct.

The Absaroka thrust has been traced the entire length of the belt and
is an integral part of the frontal structure of the central and southern
parts. To the north it runs back of the Darby thrust, presumably of the
same age. Also on the north end a complex of thrust sheets, one par-
ticularly of considerable extent, the St. John, overrides the Absaroka in
the Snake River Range. It may belong to the Paleocene or Eocene phase
of deformation.

The youngest strata involved in the Bannock thrusting are Lower
Cretaceous Gannett. The Frontier formation of Colorado age is deformed
within the Absaroka and Darby sheets. The Absaroka overrides the Ada-
ville beds near Kemmerer.

The foredeep beds deposited during the Montana epoch attained a
thickness of over 7000 feet in the Jackson area, and their deposition
climaxed in the Harebell conglomerate of Beltian boulders, cobbles and
pebbles ( Love, 1956a ) . At the extreme southern end of the belt the Echo
Canyon conglomerate and related deposits accumulated at about the same
time (Williams and Madsen, 1959). The manner and route of long-
distance transit of the Beltian cobbles of the Harebell conglomerate
from closest Beltian outcrops 200 miles to the northwest are a mys-
tery.

Since the Adaville is overridden by the Absaroka thrust, the de-
formation, at least here along the front of the belt of deformation,
carried on into late Montana time and possibly into early Paleocene.
The Paleocene Hoback formation was deposited in a foredeep (see
map, Fig. 22.5), and it is possible that the foredeep occurred in
response to the thrusting, and that the thrusting is therefore related to
the Hoback formation rather than to the late Montana sediments. The
thrusting in the southern end of the belt is pre-Knight, and Veatch ( 1907)
had presumed it to be post-Almy, but a recent revision of the stratig-
raphy and mapping in the Fossil basin (Tracy and Oriel, 1959) shows
the thrusting there to be pre-Evanston. The lower part of the Evanston
is latest Cretaceous, and hence the thrusting is Late Cretaceous. De-
formation in and around Fossil basin continued through the Paleocene,
however, as indicated by the conglomerates and unconformities in the
Evanston and Almy.

■

CENTRAL ROCKIES

337

BEAR RIVER
VALLEY Cb

Eighteen miles north of Montpelier, Idaho Section 3

Snowdrift Mtn^ & CR0W CREEK VALLtY

5.0OO'

Sect /on 4

NOUNAN VALLEY
Til "*<v

v5eve^7 miles north of Montpelier, Idaho

Js Jp

Fig. 22.10. Cross sections of the Central Rockies in southeastern Idaho, after Mansfield, 1927.
Cq, Brigham quartzite; Ogc, Garden City Is.; D, Devonian Three Forks or Jefferson Is.; Cm,
Madison Is.; Cb, Brazer Is.; Cw, Wells quartzite; Cpa and Cpb, Phosphoria fm.; "Ew, Woodside

The Rannock and Willard thrusts are presumed to have formed in
Montana time, the same as the Absaroka, but they might be older.

Figure 22.11 shows thrusting during the deposition of the Echo
Canyon conglomerate, but this is an inferred structure.

Paleocene Phase (Mid-Laramide)

Figure 22.5 illustrates deposits and uplifts along the east front of the
i Central Rockies in Paleocene time. The major sediment accumulation was
the continental Hoback formation made up of about 15,000 feet of sand-
stone, siltstone, and shale. A few thin limestone and conglomerate beds
jare also present (Dorr, 1958).

Sedimentation was very rapid, probably beginning and accelerating in
Torrejonian, culminating during late Torrejonian, then decelerating during
Tiffanian, Clarkforkian, and Graybullian times prior to a late phase of orogeny.
,Sediment was derived locally from western, mid-Laramide highlands which
began to rise in the early Torrejonian; the uplift culminated between the middle
and end of the Torrejonian. Orogenic phases were relatively brief but intense.
The area of deposition was much lower, forested, temperate, humid, locally

Miles

sh.; lit, Thaynes Group; 'Sty, Timothy sandstone; "Eh, Higham grit; 16, Deadman Is.; Jn,
Nugget ss.; Jtc, Twin Creek fm.; Jkb, Beckwith fm.; Jp, Preuss ss.; Js, Stump ss.; Kge, Ephram
conglomerate of the Gannett group.

swampy with some lakes, and largely inhabited by a forest-dwelling mam-
malian fauna (Door, 1958).

The Cliff Creek thrust sheet (Jackson thrust of Fig. 22.9) overrides
the Hoback formation. It is overlapped by the Eocene Pass Peak forma-
tion, so probably is a last phase of the deformation during the Paleocene
which resulted in the deposition of the Hoback beds.

The Uinta uplift appeared first in Paleocene time. The Currant Creek
conglomerate, which had previously been related to the Montana Price
River of central Utah, is now regarded as Paleocene by Bissell (1959). It
rests unconformably on older strata, and postdates the Deer Creek-
Strawberry thrusts.

The linear uplift and basin (Fossil basin) east and north of Evanston
are developments during latest Montana-Paleocene time.

Eocene Phase (Late Laramide)

At the north end of the deformed belt of western Wyoming the Pass
Peak conglomerate was deposited on the Hoback formation and older

338

STRUCTURAL GEOLOGY OF NORTH AMERICA

WASATCH MTS.

ECHO CANYON

EVANSTON

OYSTER RIDGE

Green River fm.

Adaville fm.

Lozeart ss.--

Hill i ard sh.

j-Oyster Ridge ss.

Frontier

Bear River fm.

Aspen sh.

Gannett gr.

M. Eocene
"L. Eocene

Montana

Colorado
Dakota

L.Cret.

- U. Jurassic

Fig. 22.11. Idealized cross section showing relations of formations from Wasatch Mountains to
Oyster Ridge, Wyo., restored to the close of Green River deposition. Folding of Knight near

beds, and apparently immediately overridden by rising thrust sheets
both on the east and west (Fig. 22.6). The Wind River Range rose along
a high-angle thrust which cuts the conglomerate, and new thrusts broke
out in the Hoback Range on the west (Grizzly and Cabin thrusts, Fig.
22.9). The Cliff Creek thrust sheet is overridden by these later slices.

The Knight formation was spread widely over erosion-beveled strata
in the Evanston-Salt Lake City area, and then was itself folded in large
open folds with amplitude of several thousand feet and fold widths of
5 to 15 miles. The frontal Wasatch Range north of Salt Lake City first
came into existence at this time. The folding was accompanied by
longitudinal normal faulting ( Eardley, 1944 ) , and this may have marked
the inception of Basin and Range faulting.

In very late Eocene time the Park City volcanic field was formed, and
the related Fowkes tuff accumulated in erosional valleys in the Knight
conglomerate and older formations.

The Uinta Mountains had their chief growth in late Eocene time
(Bridger, Uinta, and Duchesne River time).

Late Cenozoic Phases

The chief orogenic activity in late Cenozoic time was block or rift
faulting. A belt of trenches, horsts, and tilted blocks formed from

Morrison (?) fm.

Wasatch Mountains followed, then erosion, then deposition of Norwood tuff (early Oligocene),
the Basin and Range type faulting. Section 5, Fig. 22.1.

northwestern Arizona through western Wyoming and southeastern Idaho
to British Columbia, and ranges and valleys came into existence such as
shown on Fig. 22.7.

At the junction of the fold belt with the Wind River-Gros Ventre
uplift overthrusting occurred in early Pliocene time (Love, 1956b). This
late thrusting is unique in the Rocky Mountains and most probably
does not represent a part of an extensive compressional belt. We have to
deal with the deposition of the Camp Davis conglomerate, overthrusting
on the conglomerate, and normal block faulting all in a very short time.
The thrusts are also not traceable for any appreciable distance. These
observations lead the writer to the conclusion that the thrusting is a
gravity slide phenomenon associated with uplift. The events, structures,
and deposits would be interpreted as follows. Normal faulting of vigor-
ous nature started, and on the down-thrown block the conglomerate
accumulated to a thickness of 2000 to 3000 feet. Then gravity gliding
of large masses from the upthrown blocks occurred down over the con-
glomerate in places. Deposition of conglomerate continued around the
slide masses, and with continued normal faulting the slide masses across
the fault were cut and offset, and the uplifted parts removed by erosion.
As the fault pattern is studied it seems to fit best, if not require, this
interpretation.

CENTRAL ROCKIES

a39

Fig. 22.12. Cross section lengthwise of the Wasatch Mountains east of Salt Lake City. After Granger,
Sharp, and Crittenden, unpublished map. Section 12, Fig. 22.1.

BASIN AND RANGE
FAULT 5

Cgb

'C/f-i

'Cgb>

o a u i r r h

MOU NTAIN5

OPHIR ANTICLINE

Ch.

. x s^ — r-

€1

Tm-

2? ^

— >-E

BINGHAM 5YNCLINE
77

3\\\

Clf

2 3

, Mil F«. -;,.,/-,,,./

j)eo ii>ve.l

Fig. 22.13. Cross section of the Oquirrh Mountains. After Gilluly, 1932. Section 13, Fig. 22.1.

WASATCH AREA OF UTAH

Colorado Phase (Cedar Hills Orogeny)

The foredeep basin east of Evanston continued to the southwest, and

;in the Wasatch area east of Salt Lake City the Frontier and Wanship

''formations were deposited in it, making up a sequence of sandstones,

shales, and coal beds about 7000 feet thick. See Fig. 22.3. A conglom-

erate about 50 feet thick forms the lower part of the Wanship which
rests unconformably on the Frontier and older beds.

Although the degree of discordance is very slight and difficult of recognition
in the Coalville area, the unconformity is very pronounced locally and attains
90° of discordance at the head of Dry Creek about 2 miles east of Rockport
Reservoir. The angular unconformity is at the base of the conglomerate that is
near the middle of the sequence in the Coalville area which has heretofore
been regarded as Frontier. At the Dry Canyon locality the conglomerate con-

340

STRUCTURAL GEOLOGY OF NORTH AMERICA

^
?*

FRONTAL WASATCH MTS.
Syrie swell

f>£p

OGDEN VALLEY

Huntsvi//e sag

S<zo /eve/

">**Z£

""0,

» -

7
9 +

■War0

Eyrie Pk

Ogden Can.

Tay/or Can.

»^-

ugaen can. r adtyJ&fg?

GREAT SALT LAKE VALLEY

Qqdt

^/~'»-V\V\vv ' Wasatch fault g'

FRONTAL WASATCH MT3.

Morgan Valley

DURST MOUNTAIN
TK contact to sooth

InSh.— /'„sh

'iv 'ix Is On 'i*1 'T" "x '

COALVILLE! ANTICLINE

Fig. 22.14. Cross sections of the north-central Wasatch Mountains, after Eardley, 1944. A— A',
an east-west section just south of Ogden Canyon extending from the Great Salt Lake to Ogden
Valley and beyond (section 6). B— B', a north-south section in the range from the north to the
south side of Ogden Canyon. The section terminates in the Great Salt Lake Valley because the
mountain front veers to the east of this place (section 7). This shows the crossfolding of the
Taylor and Ogden thrust sheets. C— C, an east-west section from the Great Salt Lake Valley to
Durst Mountain about midway between Salt Lake City and Ogden (section 9). Formation; pCf,

Farmington Canyon complex; pCp, Proterozoic strata; Ct, Tintic quartzite; Cos, Ophir shale; €1,
€lu, €11, Cambrian limestone, upper and lower divisions; Drf, Three Forks (?) formation; Mn,
Madison limestone; Mb, Brazer formation; Pm, Morgan sandstone; Pw, Weber quartzite; tw,
Woodside shale; Tit, Thaynes limestone; Tia, Ankareh formation; Jn, Nugget sandstone; Je,
Entrada sandstone; Jte, Twin Creek formation; Kk, Kelvin formation; Ka, Aspen fm.; Kf, Frontier
formation; Kw, Wanship fm.; Tk, Knight formation; Tn, Norwood tuff (latest Oligocene, now same
as Fowkes).

tains boulders at least as old as the Gardison (Madison) Formation. Between
Dry Creek and Crandall Canyons the conglomerate rests variously upon late
Jurassic, Early Cretaceous and older late Cretaceous strata and lies undisturbed
across two post-Carlile faults. Thus, a marked though perhaps localized tectonic
disturbance is indicated (Williams and Madsen, 1959).

The faulting and folding of post-Frontier and pre-Wanship time are
part of the Cedar Hills orogeny which centered farther south. At first
they were thought to mark an early uplift of the northwest end of the
Uinta Mountains, but when one isopachs the formations of Colorado age

CENTRAL ROCKIES

341

13000-
IIOOO
(0000
9000
8000

rooo

6000
5000
4000'
3000
2000
1000'
SEA LE.VEL-1

4-»

3
<
u.

4-*

>
O

Big Baldy Mtn

n
in

Q
Z
Id

1-
u
111
I/)

UJ
0.

<
i-
to

It
I

North Fork Ridge
SECTION

c
m
O

«''"~"N-J?

i»

A-A'

«-

Bonneville

^^ipPo~Z^~-^~

PPo ^* —

r^b

^PPo~~~~^__

a
o

It
CD

shoreline

Fob

PMm

PFo

,,,ii

BO^1.

^vjii.

(.O^1

SECTION BIG BALDY TO DEER CREEK

Fig. 22.15. Cross section of Wasatch Mountains east of Provo. Reproduced from Baker, 1959.

and considers their lithology, this surmise seems doubtful — no uplift
in the site of the Uinta Mountains appears to have existed at this time.
In the Ogden segment of the Wasatch Mountains the Taylor and
Ogden thrusts were formed as indicated in section A-A' of Fig. 2.14,
sometime preceding the Willard thrust. The Willard thrust is considered
Montana in age, so therefore the Taylor and Ogden thrusts are probably
Colorado. They were then cross folded with axes trending east-west,
as shown in section B-R'. The cross section may also be interpreted to
mean that the beds were folded before the thrusting. Since the cross-
folding involves a different framework of stresses it seems probable that
, the two were formed some time apart. However, for the present they
will both be considered to have developed during the Colorado epoch.

Montana Phase (Early Laramide)

The deposition of the Echo Canyon conglomerate (Williams and
Madsen, 1959) and a related sequence east of Henefer (Eardley, 1944)
over 8000 feet thick marks a major phase of orogeny immediately to the
west. It began in latest Colorado time and ran its course well into the

Montana epoch. The Willard thrust seems to have formed at this time as
well as the major Charleston-Deer Creek-Strawberry-Nebo line of
thrusts. Figure 22.15 by Baker (1959) shows the extensive, flat-bottomed
thrust sheets of the Provo section of the Wasatch Mountains, and Fig.
22.16, the Nebo thrust at the south end of the Wasatch Mountains.
The interpretation rendered in Fig. 22.4 suggests a great gravity slide
for the thrust salient. It may be noted that the sheet is made up largely
of the thick Oquirrh formation of Pennsylvanian age, described in
Chapter 6.

The broad folds of the Oquirrh and Great Rasin ranges nearby, as
well as the Sheeprock thrust (Cohenour, 1957) possibly formed in
Montana time. See Fig. 22.13.

Paleocene Phase (Mid-Laramide)

The Currant Creek conglomerate (Fig. 22.5) rests unconformably
across the beveled edges of older formations at the southwest end of the
Uinta Mountains and although not definitely dated paleontologically is
called Paleocene by Rissell (1959). It marks the first rise of the Uinta

342

STRUCTURAL GEOLOGY OF NORTH AMERICA

Fig. 22.16. Diagram of the southern Wasatch Mountains showing the Nebo thrust and the later
normal block faulting. After Eardley, 1933. Section 14, Fig. 22.1.

uplift. It is not clear what relation the Evanston formation on the north
bears to the early Uinta uplift. The Cottonwood dome and east-west
folding north of it also probably formed at this time. See Fig. 22.12. In
north-south cross section the Cottonwood intrusion appears to have had
some effect on the doming, but it is quite discordant eastward and may
be later. It is discussed further in Chapter 36.

Eocene Phase (Late Laramide)

The Knight conglomerate of early Eocene age was spread widely
over deformed and eroded strata of the east front of the central Rockies
and marks another superior uplift on the west. See Fig. 22.6. A large
fresh-water lake formed on either side of the early Uinta uplift in which
the Green River formation was deposited under quiet conditions and at
about 1000 feet above sea level. The principal and sharp Uinta uplift
then occurred with high-angle border faults, and the uplift was recorded
by the deposition of the overlapping Uinta, Duchesne River, and Bridger
elastics.

The Knight conglomerate in the Salt Lake-Evanston area was then
cast into broad folds extending north-south, as shown in section C-C of
Fig. 22.14.

CENTRAL UTAH

The stratigraphic relations of central Utah as worked out by Spieker
(1946) are idealized in Fig. 22.17. This is the physiographic region of
High Plateaus of Utah, described colorfully 80 years ago by Dutton.
The formations are flat-lying and conformable but faulted on the eastern
flank of the Wasatch Plateau (upper cross section of Fig. 22.18) which
stands above the Colorado Plateau to the east, but on the west flank
and adjacent San Pete Valley the structural relations are very complex.
A monoclinal flexure is prominent on the west slope and is interpreted
by Spieker as having been originally a truncated anticline following the
deposition of the Price River conglomerates and the North Horn forma-
tion. The Flagstaff limestone was then deposited over the truncated
anticline and then flexed downward to the west at a later time. Uncon-
formities exist at the base of the Price River, North Horn, Flagstaff,
Colton, Green River, and younger formations, and Spieker and students
have postulated numerous orogenies extending from the Colorado epoch
through Paleocene, Eocene, and later time. Stokes (1952), however, has
pointed out the similarity between this structural complex and the salt
anticlines in the Colorado Plateau, and contends that the thick Arapien
shale of Jurassic age with its salt and gypsum beds has moved upward
in an anticlinal core, has suffered extensive erosion, has permitted over-
lying beds to sag or collapse, and possibly has moved upward again on
several occasions. The localized, numerous unconformities are thus
explained. Hardy ( 1952) has shown that the central core of Arapien
shale is a tight anticline, so the shale cannot be said to have flowed
upward like a viscous intrusive salt body.

Farther west the remarkable Canyon Range thrust ( Chistiansen,
1952) occurs and is shown in the third from the top section of Fig.
22.18. According to Christiansen a first episode of thrusting preceded the
deposition of the Indianola conglomerate. The sheet probably came from
the west but its roots are not evident. A second episode followed the
deposition of the conglomerate.

An extensive volcanic field was built in the central part of the High
Plateaus province of Utah, after the main phases of Laramide orogeny.

CENTRAL ROCKIES

343

Cedar Hills Orogeny

!Fig. 22.17. Stratigraphic and structural relations in central Utah, in the Wasatch Plateau and
adjacent areas. Section 15, Fig. 22.1. After Spieker (1946) except for the designation of the
JCedar Hills orogeny. It is an idealized stratigraphic and structural diagram that extends from the

Volcanism may have started in late Eocene time but the main eruptions
appear to be Oligocene. The volcanic rocks are elaborated on in Chap-
iter 36.

SOUTHWESTERN UTAH

The western margin of Colorado Plateau in southwestern Utah con-
jsists of a series of great steps eroded in the sedimentary rocks descending
southward. These steps are cut transversely by a few long, northerly

Gunnison Plataau on the west to the Green River in the Colorado Plateau. Kt, Tuscher fm.;
Kmb, Buck tongue.

trending faults of Mid- and Late Tertiary age (Fig. 22.19). Before the
faulting the western margin of the Plateau had been moderately folded,
which is a transition zone into a western belt of strong folding and
thrust faulting. The latter two belts are shown in Fig. 22.20. Following
the folding and thrusting the Claron conglomerate of Eocene (?) age
was spread over the beveled edges of the older formations. Mackin
(1960) views the Claron deposition to have culminated in an extensive
plain over much of southwestern Utah, upon which the later voluminous
ignimbrites spread. These volcanics are discussed in Chapter 36.

344

STRUCTURAL GEOLOGY OF NORTH AMERICA

PLATEAU

SAN PETE VALLEY

CASTLE VAL.
Ksp

GUNNISON

LONG RIDGE

Fig. 22.18. Section from the Wasatch Plateau, west-
ward through the Gunnison Plateau, Canyon Range
(section 16, Fig. 22.1); Wasatch Plateau after Spieker,
1946; Gunnison Plateau after Spieker (1949); Canyon
Range after Christiansen (1952); House and Con-
fusion Range after unpublished map by Quigley er a/.
Tf, Flagstaff; Tnh, North Horn; Kpr, Price River; Kc,
Castle Gate member; Kbh, Blackhawk; Ksp, Star Point;
Ks, San Pete; Kav, Allen Valley; Kf, Funk Valley;
Ksx, Sixmile Canyon; Tc, Colton; Tgr, Green River;
Teh, Crazy Hollow; Tfc, Fool Creek (Oligocene; Ja,
Arapien sh.; Jm, Morrison (?); CIs, Cambrian Is.; Co,
Ophir sh.; Ct, Tintic quartzite.

CONFU SION

RANGE

Cross sections of the Pine Valley Mountains and Hurricane Fault zone
in the southwestern corner of Utah are shown in Fig. 22.21. This area
is in the belt of moderate folding. We should note ( 1 ) that the porphyry
pluton of the Pine Valley Mountains was intruded as a laccolith between
the Claron conglomerate and the overlying ignimbrites in about mid-
Tertiary time; (2) the Claron lies across a truncated fold in which

the latest Cretaceous Kaiparowits formation is involved; and (3) the
Hurricane fault developed after the ignimbrites were spread over the
country. The evolution as conceived by Cook (1957) is represented in
Fig. 22.22.

The Hurricane fault is particularly impressive because of the deep
red color of some of the formations, the faulted black basalt flows, and

CENTRAL ROCKIES

345

N
k

Intrusions
Volcanic eoneT"

Iron Springs -/district j

/o/u /

/ / /

/ QC«darCI»y

Fig. 22.19. Structural features of southwestern Utah. Reproduced from Cook, 1957.

the sparsity of soil and vegetation. The most recent displacement, so
strikingly evidenced by the offset horizontal basalt remnants, repre-
sents the lesser of two periods of faulting separated by a long intererosion
cycle.

In Fig. 22.19 it will be seen that two structural trends intersect. One
which developed during the mid-Laramide orogeny along the boundary
between shelf and geosyncline trends northeasterly. It is marked by
folds, faults, and aligned intrusions. The second is a more or less north-
south group of faults and belongs the Tertiary Basin and Range system.
All the igneous masses in the Iron Springs district are intruded along a
single horizon in the Jurassic Carmel formation (Mackin, 1947).

WESTERN UTAH

Western Utah and eastern Nevada are characterized by approximately
north-south trending ranges separated by alluviated basins and generally
exhibit features of Tertiary block faulting. This is the Great Basin of
the geographer or the Basin and Range province of the geologist. The
older internal structure of the ranges is commonlv discordant with the
bounding block faults, and is one of strong folding and thrusting.

In the Gold Hill district of western Utah, a complex of thrust and
normal faults provides a record of prolonged orogeny. See Fig. 22.23.
No accurately dated Cretaceous or Tertiary beds are present, and hence
the times of orogeny are not known. Nolan ( 1935) believes the succes-
sion of deformational events to have spanned the Cretaceous-Tertiary
boundary.

In the East Tintic Mountains of Utah (Eureka district) north-trending
anticlines and synclines are superposed on a broad east-trending uplift
(Morris, 1957). Overthrust faults are closely related to the folds, and
cross sections (Fig. 22.24) suggest that the thrusts developed originally
as bedding plane faults which later cut the beds as the folds were intensi-
fied and overturned. Some of the thrusts are themselves folded.

West of the East Tintic Mountains are the Sheeprock Mountains
in which an 11,000-foot thick sequence of late Precambrian meta-
sediments is extensively exposed (Cohenour, 1959). A tillite similar to
that in the Wasatch Mountains is a prominent part. All formations —
Precambrian and Paleozoic — are strongly folded and broken bv thrust
faults (Fig. 22.25). The major thrusts are the Sheeprock and Pole Canyon.

346

STRUCTURAL GEOLOGY OF NORTH AMERICA

v;:;\::*.".:\:\v.;.;:^^ claron formation :v:v/;Xv/VV"v:V\Vv!v>V:'::. *:'::::::

V'IRON SPRINGS FM. JJ.T"
U.

V CARMEL FM.y — ■; —

i i i i. i J ■ ' ' ' ' ! ' ',

-"•'•.•'•• -NAVAJO SS.

Fig. 22.20. Diagrammatic cross section of southwestern Utah restored to the time before eruptive activity
started. After Mackin, 1960. Section 20, Fig. 22.11.

The complex structure is believed by Cohenour to have evolved as
follows: (a) an early folding (monocline) predates the thrusting and
may be of the Cedar Hills orogeny, (b) Overturning and thrusting east-
ward— the Sheeprock thrust formed as a wide flat sheet, now dissected
so that several windows appear through it. (c) Southwesterly thrusting
in which the Pole Canyon and Lion Hill thrusts formed, (d) Intrusion
of the West Tintic monzonite pluton probably in Eocene time, (e)
After extensive erosion basin and range faulting started and the Sheep-
rock granite was intruded which has been dated as Miocene. Erosion,
volcanism, and renewed block faulting ensued.

In a notable study of central-northeast Nevada and adjacent parts of
Utah Misch (1960) sees eastward thrusting of the decollement type in
nearly every range. He regards it as large scale. As a reasonable work-
ing hypothesis, the individually exposed decollements are considered
to have moved on a regionally large plane. The main thrusting was mid-
Mesozoic and predated the Laramide movements of central Utah.

SOUTHERN NEVADA

A well-known group of major thrusts occurs in southern Nevada, near
the eastern margin of the Basin and Range province. Examine the Tec-
tonic Map of the United States. The group comprises eight or more thrusts
with variable, but commonly low, westerly dips. The absence of basement

rocks in the overriding block, together with the observed changes in dip,
suggests that the thrusts pass downward into a nearly horizontal sole
(Hewett, 1931; Longwell, 1928). Hewett (1931) regards the thrusts
in the Goodsprings area as being successively younger westward, and
cites evidence indicative of an erosion interval between them. The belt of
thrusting has been traced 100 miles north-south in this region.

Longwell has reported several times on the geology of southernmost
Nevada, particularly on the Muddy Mountains, and his latest diagnosis
of the complex structure there is as follows. Figures 22.26 and 22.27
should be referred to.

Instead of a single large thrust in the Muddy Mountain area, Nevada, as
reported from earlier field study, the writer distinguishes two superposed thrusts
which may represent distinct orogenic episodes separated by a considerable
time interval. Both thrusts "root" to the west. The structurally lower thrust (for
which the name Muddy Mountain thrust is retained) is the more extensive; as
reported previously, it has brought Paleozoic carbonate formations over Jurassic
sandstone, with the heave-component of slip at least 15 miles. The higher
thrust (here called the Glendale thrust) has heave-displacement of at least 5
miles. Together with associated smaller thrusts, it involves formations of early
Upper Cretaceous age, as well as thick piedmont deposits that may be consid-
erably younger. "Orogenic deposits" several thousand feet thick were laid down
in front of the Glendale thrust as it advanced.

Conglomerate at the base of the Upper Cretaceous section, containing boul-
ders and cobbles derived from resistant units in older systems as low as the
Permian, indicates earlier strong deformation not far west of the Muddy Moun-
tain area. This earlier orogenic episode may have included development of the

Ob — Bosolt cinder cones and flows

Ti — Intrusive porphyry

Tvp— Pyroclasticj: breccia, tuff-breccia, etc.

Tvt — Oulchapa group ignimbritee

Tc — Claron tormotion

Kk — Koiparowits formolion

Ksw— Straight Cliffs a Wahweap sondstonss

Kf— Tropic formation
Kd- Dakota (?) sondttone
Je— ' Entrada formation
Jc — Carmel formation
Jn —Navajo sandstone

5000' 10000'

SCALE

15000'

Sea Level

WNW

ESE

Pine Volley Mtns

Qb — Basalt flows

Ti — Intrusive porphyry

Tc — Claron formation

Kk — Koiparowits formation

Ksw- Straight Cliffs and Wahweap sandstones

Kt —Tropic formation

Kd — Dakota(T) sandstone

Je — Entrada formation

Jc — Carmel formation

Jn — Navajo sandstone

SCALE

*Rm — Moenkcpi formation
Ck — Kaibob limestone

Csc-Coconino(?) sandstone

Fig. 22.21. Sections across Pine Valley Mountains and Hurricane Fault. From Cook, 1957. Section 21, Fig.

348

STRUCTURAL GEOLOGY OF NORTH AMERICA

KAIPAROWITS FORMATION- Kk

STRAIGHT CUFFS AND WAHWEAP SANDSTONES Ksw

" DlKOTflW- 3ANOSTo>JE "™ AND TROPTc- FORMATION — ~ K Jt "

CARMCL FORMATION— je

C N T H *<M — &Of>waTION- — +

NAVftJO SANDSTONE — Jo

I. CLOSE OF KAIPAROWITS TIME . BEFORE FOLDINS

BEDOED PYROCLASTICS AND FLOWS-
WELDED TUFFS— Eorly TtTfiory

2. EARLY OR MID- TERTIARY, AFTER BEVELING OF LANAMIOE FOLDS AND
DEPOSITION OF CLARON FM. ANO VOLCANICS

3. MID- TERTIARY!?), INTRUSION, FOLOIN0, DEVELOPMENT OF HURRICANE FAULT

ptNE VALLEY

INTRUSION

1—

•vTrrs^, Ti.

Ob,

He,

~^tt

^ 1

.1 ""^"^V

4. PRESENT. DISPLACEMENT AND TILTING ALONS MINOR FAULTS OF THE
HURRICANE FAULT ZONE NOT SHOWN

Fig. 22.22. Evolution of Pine Valley Mountains and Hurricane fault across Ash Creek Valley.
Reproduced from Cook, 1957. Csc, Coconino ss.; Pk, Kaibab Is.; "6m, Moenkopi fm.; "Rs,
Shinarump congl.; 1c, Cfiinle sh.; Jn, Navajo ss.; Jc, Carmel fm.; Je, Entrada ss.; Kdt, Dakota ss.
and Tropic sh.; Ksw, Strait Cliffs and Wahweap ss.; Kaiparowits fm.; Tc, Claron fm.; Qb,
Quaternary basalt.

Muddy Mountain and other large thrusts in the region which are not known to
involve formations younger than Jurassic. Therefore the earlier orogeny can
now be dated merely as post-Jurassic and pre-Upper Cretaceous.

Overturned and faulted folds associated with the Glendale thrust rival in
complexity some structural features of the Swiss Alps. Important transverse
faults with large strike-slip component pose problems of origin; the largest of
these displaces the Muddy Mountain thrust plate as much as 2 miles vertically
and may be genetically related to the Glendale thrust. Numerous normal faults,
variously oriented, bear witness to movements ranging in date from the Glen-
dale thrusting episode to late Cenozoic time (Longwell, 1949).

For another more recent summary treatment of southern Nevada see
Longwell (1952a, b).

MOSTLY HIGH
ANGLE FAULTS

Fig. 22.23. Faults in the northern part of the Gold Hill Mining District, Ut. After Nolan, 1935.
Section 23 of Fig. 22.1.

A
eooo'-

North Tintic Anticline

Hannifin PeaK

Tintic Syncline

6000'-
4000'-
2000' ■

Pinyon Peok

Cm |

Cm Jj Um

.* "^Inferred position and relotions
of Allen's Ranch thrust fault

-»ood

eood

-*ood

20O0'

Fig. 22.24. Cross section of the East Tintic Range. By Morris, Disbrow, Lovering, and Proctor. Reproduced
from Morris, 1957. Section 19, Fig. 22.1.

LION HILL THRUST
Cw,

BUTCH PEAK

pCsIs

p€slq

/-vV';4il \0 - - - _ _ - Tar _ .

lo?^^^^-= pC" QTp9,

FAULT

OTpj

/_\ /_\ /> /
rwwO

"/ "/ w \"

V/\/\,\AT/T'.— .— »

'S'S'S'S'Z'. Tm"

"-'- f„-/,-„'V.-.,-..--5»'-C,,"M'"'f POSITION OF SHEEPROCK THRUST^
' -. - .-^— T.^—. - -\"yt- — ^ PRIOR TO INTRUSION

Fig. 22.25. Cross section of the Sheeprock Mountains. After Cohenour, 1959. Section 18, Fig. 22.1.

G/endale thrust fragment

Ths Tmc

iooo' datum

PK * -ftm ' T?m KTof

G fen dole thrust

WiJ/owTank thrust KTof
J a ~) Kwt

Fig. 22.26. West to east sections through northern Muddy Mountains, after Longwell, 1949,
showing Glendale and Willow Tank thrust sheets. Glendale thrust sheet is made up of Cambrian
to Pennsylvanian strata. Prb, Permian red beds; Pk, Permian Kaibab Is.; "Em, Moenkopi fm.; lis,

Shinarump congl.; Tic, Chinle fm.; Ja, Aztec ss.; Kwt, Willow Tank congl.; Kb, baseline ss.; Ktof,
Overton fm.; Ths, Horse Spring fm.; Tmc, Miocene (?) Muddy Creek fm. Section 20 of Fig. 22.2.

Early Paleozo/c

Muddy Mountain
thrust^

M/55/55ipp/'an
f- Pennsy/i/onian

? Qa

~"~-r--"_~ ^" --'**" ~J7 Jurassii

J" ////«

Tertiary beds

WHEELER

NV
JOHNNIE

Wheeler Pass
Johnnie
thrust

Fig. 22.27. Generalized cross sections of the Muddy Mountain thrust (upper diagram) and
Wheeler Pass-Johnnie thrust (lower diagram). In the upper diagram, section 22 of Fig. 22.1,
the Paleozoic strata are limestone and dolomite, and the Jurassic strata are thick-bedded sand-
stone. In the lower diagram about 21,000 feet of strata are shown in the thrust sheet, but 8000

Mile 5

more of younger strata were probably affected by the thrust. The Johnnie and Wheeler Pass
area is 60 to 80 miles west of the Muddy Mountains, section 23 of Fig. 22.1. Both diagrams after
Longwell, 1945.

CENTRAL MONTANA ROCKIES

GENERAL FEATURES

The structures included under the name, Central Montana Rockies, are
those in Montana east of the Canadian and Montana Rockies and north
of the Wyoming Rockies. The boundary southeasterly of the Foothill belt
(see Fig. 23.1) is not clearlly defined and is drawn chiefly for the pur-
pose of discussion. The transition from the Cordilleran geosyncline to
the shelf is approximately along the west side of the Foothill belt and
along the east side of the central Rockies, so that the central Montana
Rockies are developed from the shelf. In Chapter 5 it will be recalled
that the Big Snowy basin formed in an east-west direction through
central Montana in Mississippian and early Pennsylvanian time and

merged into the Williston basin in eastern Montana, the Dakotas and
southern Saskatchewan and Manitoba. Also an arm of the Beltian basis
extended eastward through the Little Belt Mountains.

In the Laramide orogeny a major zone of domes and monoclinal flex-
ures formed in an east-west direction approximately in the site of the
older Big Snowy basin, and in addition, six smaller, subcircular moun-
tain groups evolved about igneous centers. Some striking en echelon fault
zones also developed. The mountain groups rise imposingly 2(K)0 to
5000 feet from the plains. The primary cause of the entire assemblage
of mountain groups is probably magmatic. Major intrusions into the
Precambrian rocks domed up the Paleozoic and Mesozoic veneer, and in
places central vents and associated dike swarms broke through to the
surface or fed sill and laccolithic intrusions into the Cretaceous strata
a short distance below the surface. A certain amount of horizontally
acting crustal stress was relieved at about the same time as the intru-
sions. This is attested by the en echelon fault zones. The magmatic
theory is elaborated upon in Chapters 19 and 36 which discuss the
igneous provinces of the western United States.

CENTRAL ZONE OF UPLIFTS

The central zone of uplifts extends from the Little Belt Mountains on
the west to the Porcupine dome on the east. The Big Snowy Mountains
comprise a prominent and perhaps the best-known dome. See Figs. 23.2
and 23.3. The Madison limestone has proved very resistent to erosion
and forms the surface rock of a large central area. The Big Snowy
dome has a length of about 25 miles and a width of 12 miles; the
structural relief is 12,000 feet on the steep southern flank.

The dome of the Little Belt Mountains is much larger in horizontal
dimensions than the Big Snowy, but not so complete. Its southwestern
half is composed of Beltian strata intruded by granodioritic batholiths
and the domal structure obliterated by sharp folds and thrust faults. Its
northeasterly flank is gentle and stretches beyond the Highwood vol-
canic group to the Blood Creek syncline with a structural relief of
11,000 feet.

351

Fig. 23.1. Mountain basins and uplifts of Montana. See Fig. 23.2 for structural details of central
Montana. The Sweetgrass arch includes the Kevin-Sunburst dome and the South arch. The Genou

trend is a relief feature of the Precambrian surface which was buried by onlapping Cambrian
and Devonian strata (Alpha, 1955).

Fig. 23.2. Structure contour map of central Montana. Reproduced from Reeves, 1930.

354

STRUCTURAL GEOLOGY OF NORTH AMERICA

South to Eost

Tort Union

formotion

(Tlul

Tongue River member [Ttr>

lebo shole member (TD

Tuilock member (Tt)

St Mory River
for motion

(Ksm)

Hell Creek formotion (Knc)

Niobroro shole
Coriile shole

y "Bovdoin sond'^f
S "pniiiips sond" ~%

Greenhorn limestone (Kg)

Frontier formotion (KO

Belle Fourche shole (Kbf)

Mowry shole (Km)

[1 ThermopoliS Mu0t)y ss, memDer
i >. shole (Kt)

Newcostle sondstone (Kn)
ond Skull Creek shole ik5)

-CONTOURED HORIZON •

<^GreyDuii sondstone member J>
\ of Cloverly formotion (Kor) ^

Foil River ss = 1st Cot Cr sond
so-colled "Dohoto sond"

Kootenoi ond Cloverly formotions (Kk)

X^'Mou

Fuson shale
IKIul

,. .Kk-Cb) ^UJI

Cut Bonk sond ^

I™. (KK-sh) , J Pryor cgl memDer \

V Sunburst sond" \ } of Cloverly fm, (Kpn ^

Lokota sandstone

IKI)

UHCOHFOftUI

Morrison formotion (Jit

Swift formotion (j$«)

Sundance formation us)

Piper formotion (jp)

-^UNCOHFOHMITT-

Fig. 23.3. Stratigraphic diagram of Cretaceous and Paleocene formations of Montana. Repro-
duced from Dobbin and Erdman, 1955.

Most of the anticlines and domes of the central zone of uplifts are
asymmetric and can be considered as flexures. The Cat Creek anticline
which extends eastward from the Judith Mountains, is a good example.
Thorn (1923) considers the flexures due to draping of the flexible sur-
ficial strata over faulted blocks of the more brittle Precambrian rocks
below. See lower diagram of Fig. 23.3.

Central and eastern Montana and northern Wyoming are the sites of a
remarkable succession of formations that bridge the Cretaceous and Terti-
ary periods. The position of the boundary of the two stratigraphic sys-
tems has been a matter of lively argument and study for many years. The
chart of Fig. 23.4 shows the age assignments of the various formations
on the Structure Contour Map of the Montana Plains by Dobbin and
Erdman ( 1955 ) . The youngest formation generally reported upturned
in the flexing is the Fort Union, which is regarded as Paleocene. It is
known that the Big Horn Mountains arch had already risen and been
stripped to the Precambrian by Fort Union time, and that further uplift
occurred soon afterward. Since the Big Horn arch extends into south-
central Montana, in the proximity of the east-west flexures and related
domes just to the north, it is possible that the central Montana structures
came into existence during the same period, viz., immediate pre-Fort
Union and again in post-Fort Union.

The laccoliths, dikes, and stocks are nearly all in Cretaceous strata; only
in the Bearpaw Mountains has a deposit as young as the Teritary in asso-
ciation with the volcanics been reported. The oldest volcanics there rest
mostly on the Fort Union formation, but in places a cobble layer inter-
venes which may be Eocene or even Oligocene in age. At least the oldest
extrusions, which are older than the stocks of the area, are younger than
the cobble layer, and therefore at least lower Eocene in age, and perhaps
younger (Pecora, 1941).

ZONES OF EN ECHELON FAULTS

Characteristics and Structural Relations

Three zones of en echelon faults occur in central and south-central
Montana. The Cat Creek fault zone is at the north, the Lake basin fault
zone in the middle, and the Nye-Bowler zone on the south, just north of

CENTRAL MONTANA ROCKIES

355

BIG 5N0WY MOUNTAINS

5ca/e in miles

36 mile 5

BIG 5N0WY MT5.

SHAWMUT
ANTICLINE

CRAZY
5YNCL

-30,000
- 15,000

0 feet

Fig. 23.4. Upper diagram is section across the Big Snowy Mountains, after Reeves, 1931. Lower diagram
illustrates the relation of flexures in the surficial sedimentary rocks to deep-seated faults in the Musselshell
Valley region of Montana, after Thorn, 1923.

the Reartooth uplift. See Figs. 23.1 and 23.2. The Cat Creek anticline
(monoclinal flexure) extends eastward from the Judith Mountains, and
the strata of the steep flank are broken by a series of small faults. It ap-
pears that the north block, the Rlood Creek syncline, moved slightly
westward in the downward movement.

The Lake basin fault zone extends east and west of Rillings, is the larg-
est of the three, and has a length of over 100 miles. On the west end it
cuts the south flank of the Rig Coulee-Hailstone dome, and on the east
end it cuts the northward dipping strata from the Rig Horn uplift. For the
most part, the southeast side of each fault is downthrown, but there are
many exceptions (Hancock, 1918). Along some of the faults, the direction

of throw changes from one end to the other. In any event, the throw is
small. The fault planes are generally inclined 30 to 80 degrees. It would
seem that the zone of faults came into existence after the dome and flexure
that it cuts and that a deep-seated fault with horizontal movement was the
cause. The surficial strata over the horizontallv displaced blocks broke in
numerous small tensional faults oriented obliquely to the master fault
beneath ( Chamberlin, 1919 ) . As with the Cat Creek zone, the north block
moved westward.

The Nye-Rowler zone consists of a series of anticlines, domes and half
domes in perfect alignment for 56 miles, extending from the Reartooth
Mountain front to the Pryor Mountains. Dips on the south limb are

356

STRUCTURAL GEOLOGY OF NORTH AMERICA

steeper than on the north. Cutting the anticlines are two principle sets of
faults, viz., faults parallel with the strike of the zone on the anticlinal
axes, and faults in en echelon arrangement diagonally across the zone.
There are also a number of feather faults that terminate at the longi-
tudinal faults, together with two downdropped fault blocks. It has also
been noted that the formations vary in thickness across the zone or
"lineament," as Wilson (1936) calls it. The Lance formation and Lebo
member of the Fort Union thicken abruptly south of the axis of the fold.
This is taken to mean the beginning of flexing or faulting during the
deposition of the uppermost Cretaceous and Paleocene beds.

Centers of volcanism are also aligned with the Nye-Bowler lineament.
The laccoliths of Limestone Butte and Round Mountain were intruded
along the westward projection of the belt. Also, the intrusions of Green
Mountain and Squaw Peak in the McLeod area came up along the linea-
ment (Wilson, 1936).

The assemblage and relation of all the structural features of the Nye-
Bowler lineament have led Wilson (1936) to conclude that they are the
surface expression of a single deep-seated fault, along which both vertical
and horizontal movement took place. The south block both sank and
moved eastward. This horizontal movement is in the same direction as
that of the blocks north and south of the Lake basin and Cat Creek fault

zones.

STAGES OF OROGENY

The first perceptible stage of the Laramide orogeny in south-central
Montana was marked by slightly coarser sediments in the Judith River
formation and by the eruption of volcanoes that may have heralded the
early rise of the Beartooth block. The earliest movement on the basement
fault of the Nye-Bowler lineament also occurred, and dikes were intruded
and agglomerate (the lower part of the Livingston formation) piled up
along the fault trace. The second stage, according to Wilson, lasted
through the deposition of the Bearpaw, Lennep, Colgate, and Lance for-
mations and the Lebo member of the Fort Union. The Nye-Bowler flexure
first appeared in the area of deposition of the Upper Cretaceous sediments

just mentioned, and they accumulated thinly over the crest and thickly
on the depressed area to the south. The en echelon faults of normal dis-
placement also were formed at this time. Volcanism continued from the
first stage all through the second.

The third stage was principally that of uplift of the Beartooth Moun-
tains, and with the orogeny the Nye-Bowler monocline was compressed
and arched, with the formation of its individual domes. Its normal faults
in part became reverse ones in face of the compression; more horizontal
movement occurred, and the feather faults came into existence. Erosion
was actively attacking the rising Beartooths, and the waste products were
spread out to the east as the Tongue River sandstones and shales. By the
time of the next uplift of the Beartooth block, which would be the fourth
stage, the Precambrian crystalline rocks in it had been exposed and the
beds of flexure had been considerably truncated. The following wave
of debris from the Beartooths spread over the truncated structures of the
basin. All this deposit, the "Wasatch sandstones and conglomerates," has
subsequently been eroded away save for a downfaulted and protected
block. The block faulting marks the last and fifth stage of the Laramide
orogeny in the region. This last stage should possibly be considered post-
Laramide.

According to a recent study by McMannis (1958) three major spread-
ings of andesitic debris into the Crazy Mountain basin from the south-
west occurred, each one reaching further than the other. These were in
Judith River, Lennep, and Lebo times. The Lennep and Lebo "pulses"
of McMannis make up the second stage of Wilson above reviewed. The
Lebo volcanism and uplift constituted the culmination of the Laramide
orogeny, according to McMannis, and this occurred in Paleocene time.

IGNEOUS CENTERS

Distribution and General Structure

Six igneous mountain clusters of the Central Montana Rockies may be
recognized as follows, beginning on the northwest: the Sweetgrass Hills,
the Bearpaw Mountains, the Little Rocky Mountains, the Highwood
Mountains (Fig. 23.1), the Moccasin and Judith Mountains (Fig. 23.2),

CENTRAL MONTANA ROCKIES

357

and the Crazy Mountains. Those where local domes have been created,
presumably by laccolithic intrusions in the Cretaceous strata, are Sweet-
grass Hills, Rearpaw, Little Rocky, Moccasin and Judith Mountains. The
other two groups, the Highwood and Crazy Mountains, are character-
ized by remarkable radiate dike swarms and not by domal uplifts. The
Highwood Mountains are on the north flank of the Little Relt Mountains,
and the Crazy Mountains are directly in the lowest part of the Crazy
Mountains basin (Structure Contour Map of the Montana Plains).

Bearpaw Mountains

The Rearpaw Mountains are made up of two large volcanic fields with
a central strip, 2 to 8 miles wide, of deformed and metamorphosed sedi-
mentary rocks, known as the Rearpaw Mountains structural arch. It
trends N 60° to 80° E as does an accompanying swarm of thousands
of dikes (Pecora, 1957). The oldest formation involved is the Madison,
and the youngest the Judith River of Late Cretaceous age.

The arch was first developed as a prevolcanic structure and continued
to develop throughout the magmatic history. Vertical uplift of 5000 to
7000 feet is demonstrable, with block faulting in prevolcanic time per-
j mitting a good part of the uplift in places.

The great abundance of Precambrian basement inclusions in the rocks of
latitic composition represents transportation vertically of at least 2 miles
through the Paleozoic and younger formations and at least 4 miles if the
1 volcanic pile is also pierced (which is 10,000 to 15,000 feet in maximum thick-
j ness). The extensive distribution of the inclusion-bearing felsic rocks over
I 1600 square miles of the Bearpaw Mountain uplift area and the absence of
quartzite fragments representative of the Belt series are significant relation-
ships that may indicate either an angular uncomformity and the removal of
the late Precambrian rocks of the Belt series before deposition in the early
Paleozoic sea in this region or a development of the felsic magma very deep in
the basement itself (Pecora, 1957).

The volcanic activity ran its course during middle and late Eocene,
and radiogenic ages of zircons in a syenite are reported to be about 40
to 60 m.y. Post-volcanic faulting and intrusions have disturbed the orig-
inal attitude of much of the layering of the volcanic pile.

A great variety of mafic subsilicic-alkalic to felsic silicic-alkalic rocks
occur, with the mafic rocks exceeding the felsic in volume.

An extensive skirt of small thrust faults flanks on the south the Bi
paw igneous centers and domal uplift, and is regarded by Reeves
as a gravity slide phenomenon down slope from the uplift.

Little Rocky Mountains

The Little Rocky Mountains are a singular structural type. They lie-
apart — somewhat north — of the other uplifts and domes of central Mon-
tana, and are erosional features of a subcircular dome, about 20 miles
in diameter, which embraces more than 50 faulted subordinate domes.
See Fig. 23.5. Alkalic igneous rocks of Tertiary age, mainly in the form
of sills, have intruded the Cambrian strata but are not known to have
intruded sedimentary rocks younger than Cambrian. All their contact-,
with post-Cambrian rocks appear to be fault contacts, and indicate that
the igneous rocks, after having consolidated at and near the base of the
Cambrian, were deformed, broken, and faulted by upward pressure,
probably due to an underlying, rising magma ( Knechtel, 1944 ) .

The subordinate domes on the large subcircular dome of the Little
Rocky Mountains were formed by bodies of igneous rock which were
punched upward into the sedimentary rocks. They range in diameter
from 13i to 3M miles. Each is typically subcircular or subelliptical in
plan and normally includes a hinged block that has been raised like a
trap door (Knechtel, 1944). See cross section of Fig. 23.5.

Relation of Igneous Activity to Tilted Fault Blocks

The Little Rocky, Moccasin, and Judith Mountains, the domes of the
Rig Relt, and those southeast of the Little Relt Mountains, and possibly
the Porcupine dome are variations of laccolithic uplifts. The Highwood
and Crazy Mountains are the products largely of extrusive activity, but
stocks, numerous dikes, and laccoliths are present.

In spite of the flexures and deep-seated faults beneath them, the sedi-
mentary beds were almost horizontal in most places at the time oi
igneous activity. The belt of vigorous Laramide deformation lay to the
west. The laccolithic intrusions especially domed up the beds, but in the
two mountain groups where extrusive rocks are most abundant, the
strata seem little deformed below the volcanics.

358

STRUCTURAL GEOLOGY OF NORTH AMERICA

EXPLANATION

BEARPAW SHALE
(UPPER CRETACEOUS!

JUDITH RIVER FORMATION
(UPPER CRETACEOUS)

CLAGGETT SHALE, EAGLE SANDSTONE AND

COLORADO GROUP

(UPPER CRETACEOUS)

KOOTENAI ANO ELLIS FORMATIONS
( LOWER CRETACEOUS AND JURASSIC)

MISSION CANTON LIMESTONE TO FLATHEAO SANDSTONE
(CARBONIFEROUS TO CAMBRIAN)

METAMORPHIC ROCKS (PRECAMBRIAN)
AND ALKALIC IGNEOUS ROCKS (TERTIART)

NORMAL FAULT
U, UPTHROWN SIDE 0, DOWNTHROWN SIDE

Thrust fault
t, overthrust side

Fig. 23.5. Geologic map of Little Rocky Mountains, Montana. Reproduced from Knechtel, 1944.

An approximate parallelism of the volcanic groups with the major
faults and flexures of central Montana has been pointed out by Thom
(1923), but it is evident from inspection of the Tectonic Map of the
United States that the major faults are clearly not the loci of the mag-
matic activity. However, the subparallel alignment and contemporaneity
of origin lead Thom to view all the faults, flexures, and igneous rocks as
tied to the regional deforming forces of the Laramide orogeny.

Petrology

The igneous rocks range from rhyolites to basalts in one category and
from shonkinites through nepheline syenites to syenites — rocks that are
rich in potash and soda and almost devoid of plagioclase — in another.
The rocks of each mountain group fall into one or more eruptive stages;
and the rocks of each stage have peculiar mineral and chemical features,
although they commonly range from highly mafic to highly felsic. Each

CENTRAL MONTANA ROCKIES

359

stage is separated from the other by intervals during which few or no
eruptions occurred, but instead, extensive erosion. Chapter 33 deals
with the origin of the igneous rocks in this province and should be re-
ferred to for a discussion of the igneous and tectonic provinces of the
western United States.

In each of the stages a rock near the mafic end is believed to repre-
sent the primary magma. This rock ranges from an ordinary basalt to
orthoclase basalt to plagioclase shonkinite to shonkinite rich in potash
and lacking plagioclase. The gradational character of the eruptive stages
and their close association in time and space indicate a common origin
(Larsen, 1940). Two periods of magmatic differentiation are required:
first, a deep-seated differentiation to yield the primary magmas of the
individual eruptive stages, and second, a shallower differentiation of the
primary magmas which were probably derived from a basaltic magma by
the removal of crystals of calcic plagioclase and hypersthene in depth.
| The relative flatness of the sedimentary rocks into which and through
I which the magmas have moved indicates that the magmas have not been
i disturbed by orogenic forces; therefore they could have differentiated
I j during the long, quiet interval which seems necessary. The second period
of magmatic differentiation by crystal settling was characterized, in most
stages, by assimilation of siliceous material. The amount of assimilated
material was especially large in the Crazy and Little Relt Mountains
where syenites were followed by granites.

The Shonkin Sag laccolith, one of nine in the Highwood Mountains, is
worth special mention. It has long been held as a classic example of
magmatic differentiation in place, but the theory has been questioned
and one of multiple intrusions proposed (Rarksdale, 1937). More re-
cently, Hurlbut and Griggs ( 1939 ) contend that the first theory has the
greatest merit. In describing the laccoliths of the Highwood Mountains
they point out, first, that they are broad, sill-like bodies and not the
domed-shaped ones that Gilbert ( 1877 ) pictured in the Henry Mountains
of Utah, and second, that the peripheral contacts are not simple wedges
of intrusive rock, but a complex of multiple sills, crumpled strata, and
small normal and reverse faults. Examine Fig. 23.6.

0 1000 200O FT.

Sh __,c_ K^x.

T\ I TOmiiimniimn,,,,,,;,,,

lUIIUUJHJ

-r

'. Ro?f

of Sandstone • •

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»rt.;

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0

ion

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el

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ble Shonk,nit«)(
of Sandstone

/ J

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nq.no,

Floor

Fig. 23.6. Upper section: the Shonkin Sag laccolith, K is Cretaceous sandstone, Sh is shonkinite,
Tr is transition rock, and Sy is syenite. After Hurlbut, 1939.

Middle section: detail of eastern termination of Shonkin Sag laccolith, K, Cretaceous strata,
Sh is shonkinite, Sy is syenite, Ph is phonolite, Nos. 1 to 5 are sills of shonkinite porphyry. After
Hurlbut, 1939.

Lower section: diagrammatic section of Boxelder laccolith. After Pecora, 1941.

The main body of the Shonkin Sag laccolith is made up of three hori-
zontal layers, an upper one and a lower one of shonkinite, and an inter-
mediate one of syenite. This is true of all the laccoliths in the group; the
larger the pluton, the greater the amount of syenite. According to the

360

STRUCTURAL GEOLOGY OF NORTH AMERICA

theory of separate injections, the syenite magma was injected into a
partially solidified shonkinite; but according to the theory of magmatic
differentiation in place, the syenite was formed by the settling of heavy
minerals out of the shonkinite magma and the rising of leucite crystals.
The minor injections of the shonkinite in the syenite at the lower contact
of the syenite are explained as due to surges of magma incident to
deformation of the magma chamber.

The Boxelder laccolith of the Bearpaw Mountains is also an instruc-
tive example of differentiation in place (Pecora, 1941) and the lower
cross section of Fig. 23.6 has been prepared to show the relations.

STRUCTURES OF THE NORTHERN GREAT PLAINS

East of the zone of flexures and domes and north of the Black Hills
is a long, asymmetrical, gentle fold known both as the Cedar Creek anti-
cline and the Baker-Glendive anticline. See Fig. 23.1. Between it and

the Porcupine dome is the shallow Sheep Mountain syncline. All are
Laramide structures. They are so gentle, however, that they hardly
deserve inclusion in any belt of Laramide orogeny. The very low Bow-
doin dome northeast of the Bearpaw Mountains is in the same class.
The Cedar Creek anticline has produced commercial gas from the Upper
Cretaceous strata in several local domes along it, and deep wells have
shown the presence of the Lower and Upper Mississippian strata there,
and consequently the extension eastward of the Big Snowy trough (De
Wolf and West, 1939). One reached the Precambrian at a depth of
9680 feet, having passed through 3920 feet of Upper Cretaceous strata,
220 feet of Lower Cretaceous, 1450 feet of Jurassic and Triassic, and
4090 feet of Poleozoic (Seager, 1942). Oil was found in a local dome,
the Pine field, on the anticline in 1952 in Ordovician and Silurian strata.
Several other small anticlines and domes in the setting of the major
structures previously described, have been drilled and produce oil. The
Charles evaporite sequence is a prominent productive zone.

WYOMING ROCKIES

GENERAL CHARACTERISTICS

The topographic features of Wyoming are for the most part large,
northwest-trending ranges and large intermontane basins. Study Fig. 24.1.
Of the ranges, die most imposing are the Beartooth, Absaroka, Wind
River, and Big Horn. Numerous peaks in these ranges reach elevations
above 12,000 feet and stand 5000 to 7000 feet above the basin floors.
Other ranges, now not so high and partly buried by Tertiary sediments,
were undoubtedly once very high and are equally important structural
elements. The Wyoming structural system is defined for convenience as
extending slightly beyond the borders of the state. The Pryor Mountains

at the north end of the Big Horn and the Beartooth Range extend into
southern Montana; the Black Hills lie mostly in western South Dakota,
and the Uinta Range mostly in Utah. On the other hand, the Colorado
Rockies extend into southeastern Wyoming by way of the Laramie,
Medicine Bow, and Park ranges. Certainly the Colorado and Wyoming
rockies are closely related, and any separation structurally is arbitrary
and for the sake of organization.

The Wyoming Rockies have been referred to as the outer ranges or
shelf ranges of the Rocky Mountain Cordillera, in contrast to the inner or
geosynclinal. This point has been discussed in the introduction to the
general subject of the Late Cretaceous and Early Tertiary Rocky Moun-
tain systems, Chapter 19. By inspection of the paleotectonic maps of the
Paleozoic and Mesozoic eras, it will be apparent that the area of the outer
ranges was generally one of shelf seas except in Late Cretaceous time,
when in certain basins of Wyoming and Colorado over 10,000 feet of
strata accumulated.

In addition to a rather thin veneer of Paleozoic, Triassic, and Jurassic
sediments the ranges have extensive, oval-shaped cores of Precambrian
rock, and for the most part are asymmetrical uplifts either in the form of
large anticlines or great tilted fault blocks. The Absaroka Range is an
exception because it is composed chiefly of pyroclastics and volcanic flows
of a date later than most of the other mountain building. The Absarokas
are connected with and closely related to the volcanic plateau of Yellow-
stone Park.

TETON-GROS VENTRE-WIND RIVER ELEMENT

The Teton, Gros Ventre, and Wind River ranges are in general align-
ment and extend from the Idaho line south of Yellowstone Park southeast-
ward for 150 miles. They are of great height and beauty, and support a
number of small glaciers. The Grand Teton is 13,747 feet high, and Gan-
nett Peak in the Wind River Range is 13,785 feet high. These are tin-
highest peaks in Wyoming. All three ranges have Precambrian crystal-
line cores and fairly simple structure along dieir northeastern flank, such
as characterizes the great anticlinal arches of the Big Horn and Black

361

mLLISTON—
BASIN

TERTIARY
VOLCANIC ROCKS

«S

TERTIARY
INTRUSIVES

EARLY TERTIARY
BASIN SEDIMENTS

PALEOZOIO AND
MESOZOIC SEDIMENT-
ARY ROCKS

."i'-".".-»/J

PRECAMBRIAN
ROCKS

Fig. 24.1. Index map of Wyoming. Certain small Tertiary basins not shown.

WYOMING ROCKIES

3&3

TETON RANGE

Crancf Teton
El / 3,7*7

WIND RIVER RANGE
Wmcf River Ph.

f/- CO JS,O0O

Fig. 24.2. Cross sections of the Teton and Wind River Ranges. The west slope of the Tetons is
after Horberg (1938), the Blacktail Butte and Gros Ventre geology after Foster (1946), and the
eastern slope of the Wind Rivers is after Branson and Branson (1941). Other parts are by the
author. Ob, Big Horn dolomite; Dd, Darby formation; Cbm, Brazer and Madison limestones; Cta,

i Hills. Along their southwestern flank, however, steep upturning and over-
j thrusting is the rule. The Wind River Range is separated from the Gros
Ventre by a broad sag or saddle in which most of the Paleozoic and
jMesozoic formations are preserved and in which folds and faults of con-
siderable magnitude occur (Richmond, 1945). The Gros Ventre Range is
separated from the Tetons by a broad and picturesque valley, Jackson
Hole, which trends north and south. The depression is due mainly to late
Cenozoic block faulting, and the Laramide structural setting between the
two ranges is not known. The post-Laramide faulting has been discussed
in Chapter 22, and will be mentioned again in Chapter 30.

A cross section into the Gros Ventre Range from the facing Hoback
Range has already been presented (Fig. 22.9), and the structural relations
of the two ranges discussed. Other sections of the Tetons, Gros Ventre,

Tensleep and Amsden; Cp, Phosphoria formation; Tic, Chugwater formation; Jn, Nugget sand-
stone; Jtc, Gypsum Spring and Twin Creek formations; Tp, Pass Peak (middle Eocene); Tc, Camp
Davis (uppermost Miocene); Tea, andesites of Camp Davis formation.

and Wind River ranges are given in Fig. 24.2, which bv inspection should
explain the broad features of each.

The southwest flank of the Wind River uplift has been traversed seis-
mically by Berg and Wasson (1960), and they report a thrust that dips
as low as 18 degrees and carries under the range about 8 miles. The
amount of vertical uplift in the Wind River Mountains is in excess of
35,000 feet.

BEARTOOTH RANGE

The Beartooth Range extends from southern Montana into northern
Wyoming. Its northeast front is uplifted and generally overthrust north-
eastward, whereas the southwest front of the Wind River Range is ap-
parently overthrust southwestward. A number of porphyry intrusions are

Canyon Mountain

Trail Creek- Canyon Mountain Area, south of Livingston. After Skeels, 1939

I \ l\ l\ l\ t \ /~v '/ \ / \ / \ /_\ /
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i , .~~. .-, .~. ."/ .— . r. .~. .-. ~.

Beartooth front between Stillwater and Bou/der rivers.
After Foote in Rouse, Hess, Foote, Vhay, Wilson, 1937.

Scale of above sections

l\ /\ l\ l\ l\ I \l\l\ I

\t \i w \l \i \l W \l\ l\i\ l

Ob, Dj, Dtf

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Fast front of Be or tooth Range . After
Perry in B ucher, Thorn, y Chamber/in, 1934.

I 2 J

Miles

Be or tooth Range Bighorn Basin

Pa/eocene v Focene

Pry or Mts. Dry Head Bighorn Range Powder River Basin
Basin

> Miles After Thorn, 1953.

Idealized section from the Bsortooth front to the northern end of the Bighorn Range.

Fig. 24.3. Cross sections of the front of the Beartooth Range and adjacent basins.

WYOMING ROCKIES

in close proximity to the Beartooth thrust. They were intruded before
the thrusting took place and have been cut and displaced by the fault
or tears associated with it. See Lodgepole intrusive, Fig. 24.3. The in-
trusions are in the form of small sills principally in the Cambrian strata,
nearly horizontal sheetlike masses not far below the Cambrian strata in
the Precambrian and laccoliths. The latter are found near the mountain
front where the Nye-Bowler lineament is closest.

The northwest end of the Beartooth Range and hills in the vicinity of
Livingston, Montana, are structurally complex. The northward flowing
Yellowstone River bounds the range on the west, but extending north-
westward beyond are low mountains that link with the Bridger Range.
The northeast front of the Beartooth Range is generally bounded by a
low-angle thrust dipping into the range, and the thrust sheet has moved
northeastward. In the Livingston area, however, several thrust sheets
from in front of the main mountain block have moved southward and
have been resisted by a corner of the "North Snowy block" (Lammers,
1937). See upper cross section, Fig. 24.3. The thrusting may have been
preceded by a stage of folding and erosion which could correspond with
the post-Lance and pre-Fort Union unconformity (Skeels, 1939). The
thrusting itself may correspond to the post-Fort Union and pre- Wasatch
unconformity in the Livingston basin. See discussion of the Beartooth
thrust in Chapter 23.

Foose ( 1960 ) has treated the Beartooth Bange as a rectangular block
j primarily elevated above adjacent basins and secondarily affected in
places by horizontal transport of its marginal rock masses. At the north-
|east (Bear Lodge) corner the vertical structural relief is 15,000-20,000
feet, and in the absence of confinement, he concludes that the mountain
mass has moved outward on the adjacent basin as much as 10,000 feet.
The movement was facilitated by such secondary structures as bent high-
angle faults, tear faults, and imbricate thrusts.

OWL CREEK AND WASHAKIE MOUNTAINS

Rattlesnake Mountain west of Cody and other smaller topographic fea-
tures continue the Beartootii uplift southward, but on the west great

accumulations of volcanics compose the mountain mass and extend south-
ward for about 50 miles, where the Owl Creek Mountains appe.tr. The
volcanics spread northwestward, over considerable areas of sedimentary
rock, and lay up on the southwest flank of the Beartooths. They form the
Absaroka Range (lower section in Fig. 24.4). From under the volcanics a
large asymmetrical anticline, the Owl Creek Mountains, appears, which
extends generally eastward, and in places at least, is overthrust south
ward. See upper section in Fig. 24.4. The large anticline is broken b)
many faults and rendered further complex by small folds (Fanshaw,
1939). The shelf facies of Paleozoic, Triassic, and Jurassic rocks is essen-
tially the same here as in the Big Horn and Wind River ranges. It is
probable that the structures of the Owl Creek Mountains extend north-
westward under the Absaroka volcanics so as to lie west of Rattlesnake
Mountain and the Beartooth plateau, but the volcanics cover most of die
area and little is known of the underlying rocks or structure.

Wise (1961) recognizes a primary vertical uplift of about 20,000 feet
of the Owl Creek block, then gravity sliding of Mississippian strata away
from the crest of uplift, where kegstone-like graben exist. The sliding
toward the Wind Biver basin was unrestrained, and the lower extremities
of the thrust sheets are much brecciated and grade into conglomerate
lenses of the Eocene Wind River formation.

South of the Owl Creek Mountains and between it and the Wind River
Range is the Wind River basin, which contains an instructive sequence of
orogenic sediments. They are tabulated in Fig. 24.5. At the west end ot
the Owl Creeks and at the south end of the Absarokas is die Washaki.
Range, which has been studied in considerable detail by Love I 1939). His
account is representative of the Laramide history of the Wind River basin
and adjoining ranges and is abstracted with minor changes as follows

The Owl Creek Mountains and the Washakie Range were folded and prob-
ably faulted at the close of Lance time and before the beginning of Fort Union
deposition in the area to the northeast; the granitic core of the Washakie Range
was exposed and being eroded when the upper part of the Fort Union forma-
tion was being deposited in the southwestern portion of the Big Horn basin: at
the close of Fort Union time there was additional folding and probably faulting
along the margins of the Owl Creek and Washakie ranges; the Pinyon con-
glomerate was deposited in the northwestern part of the Wind River basin dur-

WIND RIVER BASIN
Wind River ?

Boys en fault
I

OWL CREEK MOUNTAINS

BIGHORN BASIN
Pho^s/ohor'/a y S2eo'_

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ABSAROKA MOUNTAINS

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Miles

Fig. 24.4. Upper section through the Owl Creek mountains near the Wind River Canyon. After
Fanshawe, 1939. Lower section along the southwest side of Clark Fork Valley in the Absaroka
Mountains. After Rouse, 1937. pC, Precambrian crystallines; Cf, Flathead quartzite; Cg, Gallatin

shale; Dj, Jefferson and Big Horn; Dtf, Three Forks; Cm, Madison; Ku, Upper Cretaceous un
differentiated.

WYOMING ROCKIES

367

■ing Fort Union time; and a well-defined syncline north of the Washakie Range
,:was drained by streams flowing from the west across the present site of the
.Absaroka Range.

v The second pulsation of the Laramide orogeny came at the close of Fort
-Union time. The intensity and extent of folding and faulting are not known.
-"During this and the preceding movement, the major structures of the Washakie
Range developed.

Then followed the deposition of 1000 feet or more of early lower Eocene

rocks (Indian Meadows) on a surface of high relief. The third pulsation of the

Laramide orogeny is believed to have occurred at this time, and the klippen

south of Coulee Mesa may be remnants of a thrust sheet pushed southward into

.the basin.

The fourth pulsation of the Laramide orogeny caused gentle folding along
the northeastern flank of the Wind River Mountains. The early lower Eocene
strata were eroded in places, and a broad southeastward-trending valley was
.'formed between the Wind River and Washakie ranges.

Following this cycle of erosion, 500 feet of late lower Eocene rocks (Wind
River) were deposited in this valley.

The fifth pulsation of the Laramide orogeny caused folding and thrust fault-
ring along the center of the syncline between the Washakie and Wind River
ranges. This was followed by the deposition of 1000 feet of middle Eocene
rocks (Aycross) and the beginning of active Cenozoic volcanism in the general
Absarokan region. Acidic and andesitic volcanic and pyroclastic rocks dominate.
The sixth pulsation of the Laramide orogeny resulted in gende localized fold-
ing and some erosion after the close of middle Eocene time.

Deposition of 3000 feet of Oligocene (?) pyroclastic rocks (Wiggins), in-
trusion of plugs, extrusion of flows, and climax of Cenozoic volcanism. Acidic
andesites dominate. Washakie and Owl Creek ranges were completely buried;
jWind River and Righorn basins were filled; Wind River and Righorn ranges
were partially buried.

The eighth pulsation of the Laramide orogeny caused folding in localized
! areas, recurrent uplift along parts of the buried Washakie Range, and erosion.
Intrusion of dacite plugs and extrusion of flows followed.

The cross section of Fig. 24.6 shows some of the above relations.

HEART MOUNTAIN AND RELATED FEATURES

South of the Precambrian mass of the Beartooth Range and lying
along the east front of the Absaroka Mountains are a number of relatively
smaller features made up dominantly of Paleozoic limestones and dolo-
mites. See Fig. 24.7. The anticlines known as Pat O'Hara and Rattlesnake

SOUTHERN WIND

RIVER BASIN

AND

BEAVER DIVIDE

DUNCAN
AREA

BIGHORN
BASIN

BRIOGEft
BASIN

WASHAKIE
BASIN

UINTA

BASIN

wtogins

formation
1

a
o

Beds «ith
lo»er Brule

fauna

a.
o

Beds with
Ghadron
-> fauno

or uj
O z

o °
O -1

Beds with

Duchesne Riv.

fauna

Lopo-M

HtjIftlOy

■

z
o

•3

Beds with
Uinta

fauna

Tepee Troll

formation

iT>#mbt'

Woshofci*

fofmoton

Gfonger

Wogon-
hountt

lo"T»oi>on

a
a

Green Cove

formotion of

Wood

Aycross
formotion

Tolmon
formotion

Twin Bulles

member

G'ten Ri.er
fofmo'ton

B
A

Blocks For*
member ^

etn Rivtr fn

Cathadroi Bluffs
-j tongue of
^"^---^Wosatch

Hiowatha W«ftibif
Wotaich

E
cr
S

Beds with

Lost Cabin

founo

Wind River
formotion

Beds with

Lost Cabin

founo

Tipton longut of
Gr«tn R.vir fm

Beds with
Lysife founo

Beds with
Lysile founo

Wotaich

Indian Meadows
formotion

Beds with

Grey Bull

founo

'- c- Ci

Fig. 24.5. Correlation chart of. the lower Tertiary formations of south-central Wyoming and the
Uinta basin in Utah. After Tourtelot and Nace, 1946.

Mountains, and Logan Mountain and Sheep Mountain are prominent.
Two remnants of Paleozoic strata, Heart Mountain and McCulloch Peaks,
consisting of large, irregularly disposed blocks rest on the Eocene Will-
wood formation. Heart Mountain is in the Rig Horn basin at least 12 miles
east of any possible root area, and McCulloch Peaks is over 2S miles.
How these relatively small masses got where they are has proved a real
mystery, and considerable has been written about them. Pierce ( 1941 and
1957) summarized the previous views and presented his own interpro-

368

STRUCTURAL GEOLOGY OF NORTH AMERICA

Td = Pojt-Olig. Dacrte
To=Oligocene? Pyroclostics
Tuo = U. Eocene & Ol.g?
Tue = U. Eocene Pyroclostics
Tme = Middle Eocene
Tim = L & M. Eocene
Tule = Upper L. Eocene
Tlte = Lower L Eocene
Ku = Upper Get. Und.v,
Kf=Fronher

Klmr=Therniopoli* & Mowry
Jm = Momjon
Js = Sundonce
Rc = Chugwater
Ct=Tenjleep
Cm=Madison
€u = Cambrian Undiv.
pre-€=pre- Cambrian
Pk=Paleozoic "Klippen"

SECTION 1

-^ Wiggins NE

1 1,000

T±~^~\- 10,000

^^T^X^^Ty

9,000

8,000

- 7,000

Fig. 24.6. Cross sections of the Washakie Range at the south end of the Absaroka Mountains. Reproduced
from Love, 1939.

tation to the effect that the Paleozoic remnants of Heart Mountain,
McCulloch Peaks, Logan Mountain, and Sheep Mountain are detached,
gravity slide blocks as represented on Fig 24.7. He recognizes a second
and immediately older thrust, the South Fork (mapped also as detached

slide remnants) and the Logan and Sheep Mountain remnants to have
slid on top of the South Fork remnants.

In broad outline the Heart Mountain fault of Wyoming is a nearly horizontal
thrust whose overriding sheet was derived from a source without any known

WYOMING ROCKIES

369

roots, and whose frontal part has ridden across a former land surface. The
suggestion is here made that this thrust and the near-hy South Fork thrust are
detachment thrusts or decollements, that is, they are sheets of sedimentary
rocks which have broken loose along a basal shearing plane, have moved long
distances probably by gravitational gliding, and have been deformed inde-
pendendy from the rocks below the fault plane.

The present remnants of the Heart Mountain thrust sheet include more than
50 separate blocks which range in size from a few hundred feet to 5 miles across
and which are scattered over a triangular area 30 miles wide and 60 miles
long. The rock formations represented in the thrust blocks comprise a very
limited stratigraphic range, none being older than the Bighorn dolomite (Ordo-
vician) and none younger than the Madison limestone (Mississippian) . The
maximum stratigraphic thickness of the formations involved is 1,800 feet, but
these include the most competent group of beds in the sedimentary sequence
in this area.

In the northwestern part of its known extent the Heart Mountain thrust plane
follows the bedding of the rocks and lies at the base of the massive and resistant
Bighorn dolomite and above the underlying Grove Creek formation (a thin
unit at the top of the Cambrian sequence). Near the center of the area here
described this bedding thrust plane changes abruptly to a shear plane that
cuts stratigraphically upward across the Bighorn and younger formations; the
thrust plane then passes southeastward onto and across a former land surface.
The present thrust remnants on this surface are separated blocks that rest on
rocks ranging in age from Paleozoic to Tertiary. See Fig. 24.8.

In the area of the bedding thrust the displaced sheet was broken into
numerous blocks which became detached from one another by movement,
with large spaces or gaps separating them. Thus by tectonic denudation the
thrust plane was exposed at the surface. Associated with the events accompany-
I ing the thrusting was the rapid formation of a stream channel deposit, here
named the Crandall conglomerate. Next there followed the deposition of the
"early basic breccia." This blanket of volcanic rock, which is now in the
process of being eroded, has preserved much of the geologic record pertaining
to the development of the Heart Mountain thrust since middle Eocene time.

Pierce ( 1960 ) has more recently recognized the break-away point of the
detached slide blocks.

NATURE OF BEDDING AND SHEAR THRUST

Mcculloch
peaks

Fig. 24.7. Origin of Heart Mountain thrust, after Pierce, 1957. Dotted area is postulated area
of bedding plane gliding of extensive sheet; zone marked shear thrust is where glide sheet cut
across beds, and the area to east is where detached blocks glided 20 and 40 miles to form
Heart Mountain and McCulloch peaks. South Fork thrust is older than Heart Mountain thrust,
and Logan Mountain and Sheep Mountain are detached remnants of Heart Mountain thrust rest-
ing on South Fork sheet.

ABSAROKA RANGE AND YELLOWSTONE PARK

Breccia Series of the Absaroka Range

The Absaroka Range and Yellowstone Park comprise a large volcanic
i area which is made up of pyroclastic rocks and lavas. Two groups have

been distinguished, each of which is composed of a lower acid breccia,
a middle basic breccia, and an upper series of basalt sheets. Altogether,
they are known as the breccia series. See columnar sections in the chart
of Fig. 24.9. The early acid breccia was probably erupted just before the
Heart Mountain thrust occurred, and the succeeding breccias and flows
accumulated on a rugged surface, the local relief of which ranged from

370

STRUCTURAL GEOLOGY OF NORTH AMERICA

Thrust or glide surface follows bedding

Glide surface
across beds

Glide surface here
former land surface

CRANDALL CR.

WINDY DETACHED
M™: / TV

SLIDE BLOCKS

rifiKA

• % /*/ W »/ »

'MMM'tf"','ii

'tp£

HEART MTN. THRUST (GLIDE)
^ S U_R_FA_CE _ _J*^

HEART
Tw. MTN-

Fig. 24.8. Generalized section along Clarks Fork to Heart Mountain showing nature of thrust
(glide) surface and detached glide blocks. After Pierce, 1957. Tv, mostly early basic breccia
(middle Eocene); Tc, Crandall conglomerate (early (?) Eocene); Tw, Willwood fm. (early Eocene);

1000 to 4000 feet. The maximum thickness of the series in any continuous
section is 6500 feet (Rouse, 1937).

The pyroclastic rocks were erupted through hundreds of small vents
and from a few volcanoes of moderate size. The basalt sheets are all
fissure eruptions.

The breccia series have been divided into formations better suited for
mapping purposes by Hay (1954) and Wilson (1959), and the succession
of formations thus established is given under the column, Wood River,
in Fig. 24.9.

BIG HORN
BASIN

ABSAROKAS
(OLD TERMINOLOGY)

ABSAROKAS
(WOOD RIVER)

WASHAKIE
MTS.

MIOCENE

Rhyolite

8 Granodiorite

m Andesite

*j Dacite

h Rhyodacite

OLIGOCENE

Late basalt flows
Late basic breccia
Late acid breccia

Wiggins fm.
(volcanics)

u
o
M

UPPER

Tepee Trail

fm.

Ay cross fm.

MIDDLE

Tat. man fm.

Early basalt flows
Early basic
breccia

Pitchfork fm.
(80% andesite)

LOWER

Willwood fm.

Early acid breccia

Willwood fm.

Wind River fm.
Indian Meadows

PALEOCENE

■ ■

Polecat
Bench fm.

* Crandall conglomerate of Heart Mountain region is Lower Eocene.

Fig. 24.9. Tertiary formation of Big Horn basin and Absaroka Mountains.

Pal

Mes

Mes, Mesozoic and Paleocene strata; Pal, Permian to Ordovician strata. The Heart Mountain
thrust blocks are from the Madison, Threeforks, Jefferson, and Big Horn formation. Vertical
scale in thousands of feet.

Plutons of the Absaroka Range

In the northern Absaroka Range stocks, laccoliths, plugs, cone sheets,
and radial dike systems occur and are closely related to the volcanic
centers. The magma of the radial dikes moved horizontally outward.
The rocks in general show a normal differentiation series from Olivine
gabbro and basalt through diorite and andesite to sodic syenite and
trachyte (Parsons, 1939).

A number of intermediate felsic stocks are known in the central area
of the southern Absarokas, and these occur in striking alignment. The
zone may have served first for the breaking through of the volcanic con-
duits and then later for the stocks that cut the breccias ( Rouse, 1937 ) .

The kinds of post- Wiggins intrusive rocks which Wilson ( 1959 ) mapped
in the Wood River area are listed in Fig. 24.9.

Breccia Series of Yellowstone Park

The general region of Yellowstone Park was a basin in the time of
accumulation of the first volcanics, the early acid breccias. Although
partly surrounded by higher topographic features, it was a rugged surface
like that of the early Absaroka Range but somewhat lower. It is estimated
that in places about 1000 feet of the early acid breccias accumulated
( Howard, 1937 ) . Then the voluminous early basic breccias were erupted.
These include breccias, agglomerates, tuffs, and flows of a more basic
character with basalt predominating. They reach a maximum thickness
of 4000 feet and have a wide distribution from the Absaroka Range
through northern Yellowstone Park to the Gallatin Range in Montana

WYOMING ROCKIES

371

northwest of the park. The Washburn Range within the park is formed
entirely of the early basic breccias.

A trachyte was possibly extruded next, approximately along the course
of the Yellowstone River. Then over the early basic breccias, but nowhere
over the trachyte, were poured out a great series of basalt flows 1200 feet
thick. These basalts form many of the higher flat-top summits in the
northern part of the Absaroka Range, but are overlain by later deposits to
the south. They are distributed widely in eastern Yellowstone Park, and
they are an important horizon marker because they separate the early
breccias from the later.

Renewed explosive activity resulted in the accumulation of the late acid
breccias previously mentioned. They are limited chiefly to those portions
of the Absaroka Range that lie within the Park and extend westward to
Yellowstone Lake.

A period of erosion evidently followed, and the late basic breccias 2500
feet thick were deposited over an irregidar surface. They form extensive
plateau areas in southeastern Yellowstone Park and make up chiefly the
southern half of the Absaroka Range but are exposed sparingly over the
late acid breccias. Where the late acid breccias are absent, the late basic
breccias rest directly on the early basalt sheets. The eruption of more
basalt flows closed the period of late basic breccia volcanic activity.

The last of the breccia series, which includes the early and late breccias
and the basalts, was an andesite outpouring in the southeastern part of
the park. It is now preserved in the higher peaks there.

The following is Howard's summary ( 1937 ) of the post-breccia history
of Yellowstone Park.

Post-Breccia Faulting. Study of the Washburn Range indicates that the
next event of major importance was extensive faulting of the great series of
volcanic rocks previously described. It appears to have been this faulting that
gave the Washburn Range a relief so great that it could not be buried by the
later rhyolite floods. Presumably, other inequalities of the old basin floor are
attributable to faulting at this period, but only where the later rhyolite failed to
bury the inequalities, or where post-rhyolite erosion has later uncovered them,
1J can the evidence of faulting be studied.

Post-Breccia Erosion. The faulting of the great masses of pre-rhyolite vol-
canic formations was associated with a long period of erosion, sufficiently im-
portant to deserve special mention. Locally, at least, a gently rolling topography

was developed on the breccias within the park. Thus, the erosion contact be-
tween the breccias and the overlying rhyolite, where exposed for a distance of
8 miles in the walls of Yellowstone Canyon, from Broad Creek almost to Tower
Creek, is gendy undulating. Elsewhere, the relief is much greater, but how
much of it is due to faulting is unknown. Presumably, the faulting took place
progressively over a considerable period, and erosion must have accompanied
the movements. Whether sufficient erosion preceded the faulting to produce a
faint relief, which was then locally intensified by uplift, or whether strong relief
due to early faulting was not effaced by the erosion that elsewhere produced a
gently rolling topography, is not clear from the evidence obtained.

After the faulting and erosion, the Yellowstone basin, its dissected sides and
floor now composed partly of pre-Tertiary rocks of all kinds and, partly, of
Tertiary volcanic rocks, received the floods of late Tertiary rhyolite lavas. The
rhyolite floods were locally preceded by basaltic extrusions.

Early Canyon Basalts. Basalts are found locally under the rhyolite, below
the level of the surrounding breccias, and indicate a period of eruption later
than that represented by the basalt that closed the period of breccia accumula-
tion. The early Canyon Basalts were probably poured out after the faulting of
the breccias and after the extended erosion period associated with that faulting.
They are exposed in patches along Yellowstone Canyon and in the canyon of
Gardiner River, in the northwestern part of the park.

Rhyolite Floods. There now occurred one of the most remarkable events in
the history of Yellowstone Park, for enormous floods of rhyolite lava filled the
lowlands of the earlier landscape to depths of a thousand feet or more, swept
around the Washburn Range and other highland areas, which stood as islands
in the lava sea, lapped against the foothills of the encircling ranges, and con-
tinued an unknown distance to the west, where the mountain rim is lacking.
Today, the lava plateau terminates a short distance outside the Park in a steep
scarp of uncertain origin, which drops sharply to the lower Snake River Plains.

Certain basalts in the northern part of the Park, at the edge of the basin, may
have been extruded during pauses in the extrusion of the rhyolite.

Post-Rhyolite Faulting. Following die extrusion of the rhyolite, the broad,
level plateau surface was broken by block-faulting, perhaps a result of setding
in response to the withdrawal of the vast quantities of magma from below.
Many of the lake and hot-spring basins, and many of the "topographic fault
blocks" visible on the contour maps, may have been formed at this time.

Post-Rhyolite Erosion. There next ensued a period of erosion, the extent of
which remains an unsolved problem. The crispness of most of the block units
in the topography, however, suggests slight denudation of the park area as a
whole, but a few deep valleys, such as the Lamar Valley in the north, may have
been eroded. The carving of the Grand Canyon of the Yellowstone River may
have begun at this time or during the first half of the Pleistocene. Its present
depth, however, was attained during the late Pleistocene. The scarp th.it limits
the rhvolite plateau to the west was presumably fashioned at this time, for its

372

STRUCTURAL GEOLOGY OF NORTH AMERICA

base is submerged by basalts, which are probably the equivalents of the Late
Canyon Basalts.

Late Canyon Basalts. Erosion was then followed by another period of basalt
extrusion, these basalts being the most recent flows of the Park. They are
found largely in the northern part of the Park, in Lamar, Yellowstone, and
Gardiner valleys, and at a few places on the broad interstream uplands. Other
patches are preserved on the uplands in west-central Yellowstone. The basalts
of the Snake River Plains, which crowd against the western scarp of the
rhyolite plateau, are probably of the same general age.

BIG HORN RANGE AND BIG HORN BASIN
Divisions of Big Horn Range

Overall the Rig Horn Range is a great anticlinal fold, steep to over-
turned to overthrust on the east, and gently dipping on the west. Examine
Fig. 24.1. The range is arcuate in plan view and terminates in the Pryor
Mountains on the northwest and the Owl Creek Range on the southwest.
The Precambrian crystalline rocks on which the Paleozoic strata rest are
exposed in three areas in the core of the range, and serve as natural
divisions.

At three points near the center of curvature in the central division along
the east front of the range, blocks of the range, including the crystallines,
have been thrust out upon the Cretaceous strata. The main block is clearly
bounded by tear faults.

On cross sections published by Hoppin (1961), overturning, thrusting,
and a detached slide mass are shown. It appears evident that sharp uplift
and upturning of the beds are the primary movements and then that
secondary gravity movements have resulted in downhill overturning of
the beds, tear faults, and small-scale thrusting.

As the axis of the great fold in the central division is traced northward,
it plunges, and the dips on the northeast flank become gentle. Reyond, in
the northern division, the asymmetry is reversed, and the crystalline rocks
are exposed close to the southwestern flank. Here steep dips, overturning,
and even thrusting to the southwest occur. The northern division is further
distinguished by sharp flexures which trend northwestward, northward,
and eastward.

In the southern section, the trend of the mountain axis curves from a
north-south direction to a southwesterly one; but in spite of this change,

the smaller structures within the range maintain the northwesterly direc-
tions that dominate the northern division. The marginal folds and faults
trend dominantly to the northwest, and the dips are steeper on the south-
west sides of these small folds (Rucher, 1934).

The Tensleep fault cuts across the Rig Horn Range from the town of
Tensleep on the west to the Horn on the east, and separates the central
from the southern divisions. As the range was uplifted and the central
division developed asymmetrically eastward and the southern asymmetri-
cally westward, the Tensleep fault came into existence (Wilson, 1938;
Demorest, 1941 ) . Relations are complex along the fault, but they point to
a downthrow on the south side.

Laramide History

The Laramide history of the Rig Horn Range has been summarized by
R. P. Sharp ( personal communication ) for the writer. According to him, a
series of coarse to bouldery fans ( the Kingsbury conglomerate ) composed
primarily of Precambrian debris, were built up along the east base of
the central Rig Horn Mountains in Early Tertiary time. This was the
section of greatest uplift, and the fan debris was presumably coarser and
thicker here than elsewhere. Subsequently, the Paleozoic beds of the
mountain front were thrust eastward against and over the gravel, and
erosion during the remainder of the Cenozoic has gradually etched out
the thickest and coarsest parts of the fan deposits so that they form
prominent ridges in the present landscape. At least three periods of Lara-
mide uplift of the range are indicated: (a) An uplift which produced the
Kingsbury conglomerate. Faulting probably occurred during this uplift.
( b ) A second uplif t, also probably attended by faulting, which deformed
the Kingsbury and produced the coarse granite-boulder gravel. This uplift
may possibly have been accompanied or closely followed by alpine glacia-
tion. ( c ) A third, postgravel, uplift marked by thrust faulting toward the
east in the central segment of the range.

Pryor Mountains

Northwest of the Rig Horns the Paleozoic strata rise once more by means
of two pairs of flexures to form the Pryor Mountains. One pair trends

WYOMING ROCKIES

373

| east-west, and the other north-northwest to divide the uplift into four
units. In three of these units, the beds rise toward a high point near the
northeast, beyond which they drop off abruptly. Most of the flexures have
ruptured to produce faults of moderate displacement. Thom ( 1923 ) and
later Blackstone ( 1940 ) have concluded that the faults curve under the
uplifted blocks at depth and have resulted from horizontal compression.
The pliable sedimentary veneer flexes first over the scarp of the crystalline

4 rocks and later, when displacement becomes sufficient, it breaks to reveal

i the deep-seated fault. See Fig. 24.10.

Big Horn Basin

The Big Horn basin is underlain in its deepest parts by 2500 to 3200 feet
of Paleozoic strata, by about 1500 feet of Triassic and Jurassic strata, by

I 7000 to 9000 feet of Cretaceous strata, and in the central and western
parts by several thousand feet of Paleocene and Eocene strata. For a re-

| view of the formations, see Wyoming Geological Society Seventh Annual
Field Conference Guidebook, 1952. As indicated in earlier parts of this
book, the Wyoming region, including the Big Horn basin, was a shelf
area of sedimentation until Cretaceous times, when considerable sub-
sidence occurred adjacent on the east to the active Cordilleran geanticlinal
area that extended through Utah and eastern Idaho. See the paleotectonic
maps of the Early and Late Cretaceous.

With the elevation of the ranges surrounding the Big Horn Basin, its
sediments were thrown into many folds, all trending in a northwest direc-
tion. The Early Tertiary strata probably obscure many folds in the central
part of the basin, for the anticlines and synclines are known only in a
broad marginal belt. Those on the east side have steep flanks facing the
Big Horn Mountains. The major anticline on the west side, the Rattle-
snake Mountain, is asymmetrical toward the west, but the smaller folds
do not have any regular symmetry. Some have steeper flanks toward the
basin, some are about symmetrical, and some are dome shaped. The anti-
clines and domes are nearly all oil or gas producing. Two of the anticlines,
especially, are cut by numerous, small, high-angle faults in a transverse
direction. These are the Elk Basin and Garland anticlines in the northern
part of the basin. The deepest part of the Big Horn basin, according to

5
4000J

4O00J

^~-,5'',"«!»"ir>','^v'J •

A->,.Tiv.-r.^.:,V//,/,/

m m m m

PRE- CAMBRIAN ORDOVICIAN MISS. MISS." PENN-

CAMBRIAN DEADWOOD BIGHORN MADISON PENN. PERM

GRANITE AMSDEN TENSLEEP

8. SCHIST , EMBAR

* — ' / \ — "■iff*' J* ^^^=1=rr^=iri^

TRIASSIC JURASSIC CRETACEOUS
CHUG- SUNDANCE CLOVERLY
WATER MORRISON

IOOO' 2000'

P-C

Fig. 24.10. Cross sections of the frontal faults of the tilted blocks of the Pryor Mountains. The
lower diagrams, A to E, illustrate the theory of origin. Taken from Blackstone, 1940.

374

STRUCTURAL GEOLOGY OF NORTH AMERICA

geophysical prospecting, is somewhat west of the geographical center.

The folds of the Rig Horn Rasin, according to Fanshawe ( 1947 ) , are
due to an interplay of two forces. The Precambrian basement was faulted
as it adjusted to Laramide mountain building on both sides, and the
Paleozoic and Mesozoic strata flexed over the fault scarps. Also, as the
sides of the basins were upturned, the upper beds of the basin were
crowded and buckles developed.

Map, Fig. 22.4, shows the Rig Horn Rasin to have come into existence
in Montana time, and Van Houten (1952) notes that Precambrian rock
had been exposed in places in the surrounding ranges by late Paleocene
time (Fig. 22.5). Sandstone, mudstone, and coal beds accumulated to a
thickness of 7000 feet just east of the Reartooth front during Paleocene
time.

The early Eocene Willwood formation overlies older beds unconform-
ably at the margin of the basin, and this time is taken as one important
deformation of the surrounding uplifts. As previously noted, the detached
blocks of the Heart Mountain thrust (?) rest on the Willwood. See Fig.
22.6. The Willwood is spread widely over the Rig Horn basin.

Middle Eocene time saw the accumulation of the Tatman formation,
which is almost free of volcanic debris except at Lysite Mountain at the
south end of the basin. West of the southern half of the Rig Horn basin
the Tatman is overlain by more than 1000 feet of volcanic debris of the
Absaroka Range. Remnants of the volcanics are noted elsewhere, and it
is postulated by Van Houten (1952) and Love (1956a,b) that sedimenta-
tion continued after Tatman time.

Ry late Eocene time the Rig Horn basin had sunk relative to the uplifts
on either side about 17,000 feet. About 9000 feet of the depression had
been filled. Yet all the while, Mackin (1947) and Van Houten (1952)
contend, the climate had not been changed, and the orogenic debris
accumulated in a warm, humid lowland near sea level. In middle Ceno-
zoic time gradual regional uplift occurred. Pediments were widely cut in
the uplands and the lowlands were broadly alluviated, producing an
extensive graded surface. Ry late Cenozoic time further regional uplift and
increased aridity initiated the present cycle of erosion, and the graded
surface was widely dissected.

Intrusive Rocks

A belt of Laramide intrusions extends across the Rlack Hills about at
the north end of the exposed Precambrian core. Most have been con-
sidered laccoliths or modified laccoliths, such as Ragged top laccolith
(Fig. 24.11), Rear Rutte, Deadman Mountain, Cook Mountain, White-
wood Peak, Rlack Ruttes, and Devils Tower (Robinson, 1956). Within
the Precambrian basement complex the intrusions are chiefly sills and
dikes, and by charting the base of the Cambrian sandstone Noble et al.
(1949) have shown that the sedimentary rocks overlying the Precambrian
have been domed in two places notably, and believe that intrusive stocks
are the cause. Some of the so-called laccoliths are believed to be stocks.

BLACK HILLS AND POWDER RIVER BASIN

General Characteristics of Black Hills

The Rlack Hills rise island-like several thousand feet above the sur-
rounding Great Plains in western South Dakota and northeastern Wyo-
ming. They are the easternmost of the outer ranges of the Rockies, and
in point of Laramide structure involving the sedimentary rocks, perhaps
the simplest. Their ridges, peaks, and valleys are the erosional remains of
a broad dome, some 120 miles long and 60 miles wide. A Precambrian
core of crystalline rocks trends nearly north-south and is flanked by up-
turned and truncated Paleozoic and Mesozoic strata. The broad anticline
trends and pitches northwestward beyond the crystalline area. The east
flank is fairly steep, with dips up to 45 degrees and more; the broad top
is fairly flat; and the west flank is fairly gentle, with dips of a few to 20
degrees. Four geomorphic units are distinct, namely, the central Pre-
cambrian core of fairly rugged mountains, a plateau area in the west cen-
tral part that is formed of Paleozoic limestones not yet stripped from the
Precambrian rocks, a remarkably continuous strike valley around the
Hills eroded in Jurassic and Triassic strata between the inner Paleozoic
formations and the outer Cretaceous sandstones (Fig. 24.11), and a bold,
inward-facing hogback held up by the Fall River and Lakota Cretaceous
sandstones. The strike valley is known as the Red Valley from the red

H/gh eros/o/? surface

RED VALLEY

HOGBACK

High terrace

Fig. 24.11. Generalized cross section of the east front of the Black Hills just south of Rapid
City, and cross section of the Ragged Top laccolith. Lower diagram after O'Harra, 1933. Cd,
Deadwood fm.; OW, Whitewood Is.; Ce, Mississippian Englewood Is.; Cp, Mississippian Pahasapa
Is.; Cm, Pennsylvanian Minnelusa ss.; Pm and Po, Permian Opeche fm. and Minnekahta Is.; "is,

Spearfish fm.; Js, Sundance fm.; Ju, Unkapapa ss.; Jm, Morrison fm.; Kl, Lakota ss.; Kf, Fuson
sh.; Kfr, Fall River ss.; Kgr, Graneros sh.; Kgl, Greenhorn Is.; Kc, Carlile sh.; Kn, Niobrara sh.;
Kp, Pierre sh.

376

STRUCTURAL GEOLOGY OF NORTH AMERICA

Triassic Spearfish shales that principally underlie it, and also as the Race-
track (Darton and Paige, 1925; O'Harra, 1933).

The Precambrian rocks consist of highly folded schists intricately in-
vaded in the southern hills by large and small masses of granite. Laramide
plutons intrude the Precambrian in the northern part, where the great
Homestake gold mine is located, and, as some believe, are responsible for
the ore deposits in large part.

General Characteristics of Powder River Basin

West of the Rlack Hills and between them and the Rig Horn Range is
the broad Powder River basin, floored by the Cretaceous, Paleocene, and
Eocene beds. The Early Tertiary deposits are 10,000 feet thick in the
deepest part of the basin, as indicated by seismic prospecting, and over
most of the basin no reversals of dip, i.e., gentle anticlines or synclines,
have been found. Only along the east flank of the Big Horns do any folds
occur. Consult U.S. Geological Survey Preliminary Map 33. The very
productive Salt Creek anticline is at the southern end of this belt. Darton
estimates the strata were uplifted 9000 feet in the Black Hills, so the
structural relief between the bottom of the Powder River basin and the
top of the Black Hills is in the neighborhood of 20,000 feet.

Age of Uplift

The age of the uplift can be only approximated, because no Paleocene
or Eocene overlaps exist. Those deposits of Laramide age that might have
been in part derived from the Black Hills are now in surrounding areas
fairly distant from the uplift and separated from it by a wide Cretaceous
belt of outcrop. The doming could have started in latest Cretaceous
time, with the deposition of the Fox Hills and Lance beds in the Powder
River basin and around the north and northeast ends; and the distribu-
tion of the Fort Union and Wasatch beds, partly around the uplift and
especially to the northeast, seem to indicate that the uplift had occurred
and was furnishing some of the sediments that were accumulating.

Post-Laramide History

By early Oligocene time, erosion had trenched the uplift almost as
deeply as now, and a mountain and valley surface of at least 1500 feet

relief existed. Then the regimen of erosion changed to one of aggradation
coincident with the change through central Wyoming and the Great
Plains, and even in the early, deep valleys of the Black Hills, lower Oligo-
cene beds began to accumulate ( Darton and Paige, 1925 ) . Deposition in
these mountainous valleys lagged until middle Oligocene, whereas it was
taking place in early Oligocene on the Great Plains ( Fillman, 1929 ) . The
sediments may have reached such a thickness that all but the highest
features of the range were buried, judging from the elevation of the
White River beds to the east of the uplift, but if so they have since been
removed within the hills except in small, protected patches. With the
renewal of erosion, a drainage pattern, in details slightly at variance with
the old, has failed to clean out all the Oligocene deposits, and has left
them in places, forming low ridges and also extending down nearly to
present valley bottoms. The surface upon which the middle Oligocene
deposits accumulated in the hills has been called the Mountain Meadow
(Fillman, 1929).

The Great Plains on the east of the Black Hills are covered by several
formations ranging in age from lower Oligocene to Pliocene, and within
this group are several disconformities. Some geologists have related the
disconformities to uplifts in the Black Hills, but Mackin ( 1947 ) believes
that the dominant form was a graded surface — erosional in the uplift and
depositional on the peripheral regions. With regional uplift, in mid-Ter-
tiary time, and associated change in climate to aridity, the graded surface
was dissected to produce the landforms of today.

SWEETWATER RANGE

Extending westward from the north side of the Hanna basin to the
southeast end of the Wind River Range is a series of hills, most of which
are islands of Precambrian rock in Miocene and Oligocene sediments. Suf-
ficient Paleozoic and Mesozoic strata are also exposed to indicate that
the Precambrian islands demarcate the position of the core of a former
great range, extending in general in an east-west direction through central
Wyoming (Fig. 24.12). It probably was traversed obliquely by several
sharp folds that trended in a northwest direction and which cast the bor-

WYOMING ROCKIES

377

SE END
WIND RIVER
RANGE

TERTIARY INTRUSIVES

FOLDED AND FAULTED PALEO-
ZOIC AND MESOZOIC STRATA

': X

HANNA BASIN

Jj5 PRECAMBRIAN CORE EXPOSED
--''I BEFORE BURIAL

RAWLINS

"n*

Fig. 24.12. Laramide Sweetwater Range and Late Tertiary normal faults. Somewhat after Black-
stone, 1951. Thick hachured lines are the Late Tertiary faults. All others are Laramide. The range
': subsided and was covered by Mid-Eocene, Oligocene, and Miocene sediments and volcanics.
Later erosion has exposed many peaks of the old range which are called the Granite Mountains.
The Hanna and Wind River basins contain thick Early Tertiary deposits.

ders of the Precambrian core into a jagged pattern with a decided north-
west fabric. It is clear that the range was elevated in the general Laramide
revolution, and that the sedimentary veneer and probably much Precam-
brian rock was removed before the range started to sag. It was a singular
phenomenon in Wyoming and Colorado, because all other Laramide
ranges have remained as strong relief features until today, but similar to
a Laramide uplift in southwestern Montana. By the time of maximum
volcanism in the Absarokas, and at the time the Great Plains became a site
of sedimentation, the Sweetwater Range, although still possessing sharp
relief, had sunk to such an extent that it was being covered by shales, tuffs,

and sands. This was in Oligocene time. A few remnants of Miocene beds
suggest that deposition continued beyond mid-Tertiary time, and cer-
tainly the entire range was buried. Then erosion set in, and many of the
granite peaks and flanking sedimentary ridges of the old range were re-
exposed. The stream pattern, as established on the Oligocene and Miocene
beds, became superposed on the Precambrian, Paleozoic, and Mesozoic
rocks, and the several examples of gorges through the islands are thus
explained. The history of burial is detailed under the headings, Hanna
Basin and Wind River Basin.

The Sweetwater Range first rose in Fort Union ( Paleocene ) time and,
immediately afterward, was thoroughly eroded during early Eocene, was
partly buried by the Wind River beds, and then sank appreciably in late
Eocene time.

The islands are in three rows today, the northern reflecting several
northwestward-trending anticlines and synclines in the Paleozoic and
Mesozoic rocks of the north flank of the range (see Tectonic Mop of the
United States), the central all in the Precambrian core, and the southern
revealing southward overthrustins; of the Precambrian rocks over the
sedimentaries.

WIND RIVER BASIN

The Wind River basin rests between the Wind River and Sweetwater
ranges on the southwest and south, and the Absaroka, Owl Creek, and
Big Horn ranges on the north. The basin is sometimes construed physio-
graphically to cover the former site of the Sweetwater Range because of
the low relief there.

Details of the basin are best known from the work of Love ( 1939) at
the west end, Tourtelot and Nace ( 1946 ) at the northeast side, and Van
Houten (1957) on the south side. Love's work has already been sum-
marized in connection with the Absaroka Range. The Tertian* formations
of the basin are shown in the chart of Fig. 24.5. They range in age from
Paleocene to Oligocene, and in parts of the basin they may be over 10,000
feet thick. The basin is asymmetrical with the axis near the north margin.
The two chief structural variations from gentle basinward dips in the

378

STRUCTURAL GEOLOGY OF NORTH AMERICA

eastern part are the Cedar Ridge fault and the McComb anticline. The
fault trends northwestward, and the northeast side is down about 1000
feet. It cuts the youngest rocks in the area, and is therefore post-Oligo-
cene.

The McComb anticline is a complex structure and is associated with
southward thrusting of Copper Mountain. Thrusting is also indicated in
connection with the Cedar Ridge fault. According to Tourtelot and Nace:

At the west end of Cedar Ridge, and on the south side of the Cedar Ridge
fault, over 1,000 feet of Upper Cretaceous beds, nearly vertical or slighdy over-
turned away from the Big Horn Mountains, are overlain by a thick sequence of
boulder beds in the Lysite member of the Wind River formation. The over-
turning of the Upper Cretaceous beds may be explained by the passage of a
thrust sheet of older rocks from the north over them, or by the presence of a
thrust sheet just to the north that did not override the Upper Cretaceous rocks
but strongly deformed the beds beyond the point of its farthest advance. In
addition, as Love points out, there is not enough room for a normal section
between the southward-dipping Paleozoic formations and the overturned Upper
Cretaceous beds standing about a mile to the south. It is believed that the
boulder beds in Cedar Ridge were derived from a thrust sheet which moved
southward from the Big Horn Mountains. Knight has postulated a similar origin
for boulder beds of this type in the Crooks Mountain area, where the sole of the
thrust mass, from which the boulder beds were derived, is exposed. If the
boulder beds in the Wind River formation on Cedar Ridge were deposited
as erosion products of a thrust sheet, the thrusting must have occurred in
Wasatchian (early Eocene) time. These Wasatchian and also younger rocks
were cut by the Cedar Ridge fault during or after Oligocene time.

The sequence of major diastrophic events that affected rocks in the north-
eastern part of the Wind River Basin is summarized as follows:

1. Mountain building during or at the end of late Cretaceous time.

2. Thrust faulting from the north in Wasatchian time along the southern
margin of the Big Horn Mountains and the south side of the Owl Creek Moun-
tains.

3. Localized gentle folding after the close of Bridgerian time along the south-
ern margin of the Big Horn Mountains.

4. Normal faulting during or after Oligocene time along the south end of the
Big Horn Mountains and the south side of the Owl Creek Mountains.

The Cenozoic history of the north flank of the Sweetwater Range and
the south flank of the Wind River basin is portrayed in a series of block
diagrams by S. H. Knight, reproduced in Fig. 24.13.

HANNA BASIN

The Hanna basin is bounded on the west by the Rawlins uplift, the
north by the Sweetwater uplift, the south by the Medicine Row Range,
but on the east it merges with the northwest end of the Laramie basin.
Retween the Laramie basin and the Hanna basin is the Carbon basin,
through which the two were once continuous but are now separated by
Laramide anticlines. The Saddleback Hills anticline separates the Carbon
basin from the Hanna, and the Medicine Row and associated anticlines
separate the Carbon from the Laramie. These anticlines are rather sharp
and extend northerly from the broad north end of the Medicine Bow
Range. The Hanna basin is fairly circular and, although not so large as
the other basins of Wyoming, it carries a very thick succession of beds.
Paleozoic, Mesozoic, and lower Tertiary formations are over 35,000 feet
thick, with Upper Cretaceous, Paleocene, and Eocene accounting for
most of the accumulation. The succession is very important because it re-
cords better than elsewhere the several episodes of deformation in this
part of Wyoming. The formations listed by Dobbin, Bowen, and Hoots
( 1929 ) are as follows :

North Park fm. (Miocene?)

Unconformity
Hanna fm. (early Eocene)

Unconformity
Ferris fm. (lower part is uppermost Cretaceous)
Medicine Bow fm. (uppermost Cretaceous)
Lewis sh. ^

Mesaverde fm.
Steele sh.

Niobrara fm. I Upper
Carlile sh. Cretaceous

Frontier fm.
Mowry sh.
Thermopolis sh.

Cloverly fm. (Lower Cretaceous)
Morrison fm. (Upper? Jurassic)
Chugwater (Triassic)
Embar(?) fm. (Permian)
Tensleep (Pennsylvanian)
Probably pre-Pennsylvanian beds

0-400 feet

7000

6500

4000-6200

3300

2200-2700

4000-5000

700

400

725

120

180

128

350

1300

150

250

North flank
Sweetwater Arch

East end
Wind River Basin

l Paleocene (Fort Union) time. Uplift of the Sweetwater arch, erosion of the crest of the arch and
deposition (Fort Union) in the Wind River Basin to the north.

4. Oligocene (late Chadronion) time. Following the cessation of volcanic activity in mid-Eocene time
erosion removed much of the volcanic ejecto during lote Eocene time. In eorly Oligocene time the region
was buried for the most part or entirely by tuffs. It is believed that these tuffs come from a remote
source, possibly from the Yellowstone Plateau -Absoraka Mountains orea.

fe) e^on°^B^in^in„d.Rrer)iIi!ne- l,l Devel°Pmen' ofVe WusMaulted' RaWesnake anticline.

SJr^nirsn'.nrsssseTS." ,he eore of ,he on,iciine- ond (3) deposi,ion <* "™

gmmmmimsiwi=

<l PRE-CAMBRIAN (->>!'> I I

5. Mid -Miocene (Heminafordian) Jime. Erosion during late Oligocene (post-Chodronian) time and
probobly early Miocene (Arikareean) time removed much of the Oligocene deposits and further re-
duced the remnants of the Rottlesnoke ejecta. The region was again buried under deposits of
volcanic ashes, sandstones, coliches and conglomerates, which were laid down during mid-Miocem
time. Agoin these deposits came from a remote source.

/IN.// v. --i /
— \ \ / \ _.

r/-\/-.\^_~- > — \ ir/>

'f\'\ PRE-CAMBRIAN O-'vi

6. Recent time. Post mid-Miocene movements coused the collapse of the Sweetwater arch
ft?n,30n,^La ^°e T°J "S1'!',"1 ?lioc.ene °"d pleis'ocene erosion has removed much
Miocene rocks, reduced the Rattlesnake ejecta to necks and modified the older rocks

although
of the

Fig. 24.13. Idealized evolution of north flank, Sweetwater arch. Reproduced from Knight, 1954.

380
Powder River Basin

Ko5 /^Crok.Co.Ch.^Cq

STRUCTURAL GEOLOGY OF NORTH AMERICA

Hartville: Uplift
Gq t Ch

^^»!i.™i!:!';!?i!il!i'Vi:i';iiiiii;i'!i;i;ii\i!

m ■ i ■ \ u i ' ) >'i i itTi i ■' i s \ w ( i'ii tP i i :'■ i i'i i -i i nTTTM

nwiiMiTTrrnti^

Agr Cq+Ch

Aw 1 Ta Agr i

High Plains
Tci

2 3 4 5 M ,

i Miles

Hanna Basin
Th

5ADDLEBACK H1LL5
ANTICLINE

Carbon

Basin
.Th

KTmb
Miles

Fig. 24.14. Upper diagram, cross section of the Hartville uplift from the Powder River basin
southeastward. Aw, Algonkian Walen group; Agr, granite intrusive into Walen group; Cg,
Guernsey fm.; Ch, Hartville fm.; Co, Opeche red ss.; Cmk, Minnekahta Is.; Jm, Spearfish (?),
Sundance, and Morrison fms.; Kd, Dakota ss.; Kgs, Graneros sh.; Ta, Miocene Arikaree fm. After

The lower cross section of Fig. 24.14 shows the unconformity at the base
of the Hanna formation, which, according to Dobbin et al. ( 1929 ) :

. . . occupies the central portion of the Hanna and Carbon Basins and con-
tains most of the coal mines in this area. It rests uncomfortably on the Ferris
formation and transgresses across all underlying formations at least down to the
Cloverly and possibly down to the granite. It consists of alternating conglomer-
ate, sandstone, shale, and coal beds, and its base is marked by a thick con-
glomeratic sandstone and locally by massive conglomerate.

The pebbles of the conglomerate are abundantly of Precambrian
derivation, and formations from Tensleep to Mesaverde are also repre-
sented. According to the map and cross sections of Dobbin's report, the
principal folds of the basin were formed in post-Ferris and pre-Hanna
time, and then accentuated in post-Hanna time when the Hanna forma-
tion was appreciably folded. Rut during the first episode of folding, the
Medicine Row Range was vigorously uplifted and considerably eroded to
furnish much of the debris for the basin. The Medicine Row Range had

W. S. T. Smith, 1903. Lower diagram, cross section from the Hanna basin to the Carbon basin,
after Dobbin, Bowen, and Hoots, 1929. Ks, Steel sh.; Kmv, Mesaverde fm.; Kl, Lewis sh.; Ktmb,
Medicine Bow fm.; Tf, Ferris fm.; Th, Hanna fm.

been gently uplifted in Pierre time, and the Precambrian may have been
cut into at that early date, but certainly it was widely exposed after the
second uplift and during the deposition of the Hanna formation.

The Cenozoic history of the north flank of the Hanna basin and south
flank of the Sweetwater Range, together with the thrust structure, is
vividly shown in block diagrams by S. H. Knight and reproduced in Fig.
24.15. His comments are are follows:

The upper diagram represents an early stage in the deformation of the
Basin and depicts conditions as they are believed to have existed during early
Paleocene time. Erosion has breached the Paleozoic and Mesozoic rock succes-
sions and Precambrian rocks are exposed along the crest of the rising Sweet-
water Arch. Rock debris derived from the entire succession is accumulating
on the shallow Basin floor. Just when, in terms of the local sequence of forma-
tions, the central portion of the Sweetwater Arch rose above base level and
erosion began to feed debris into the Basin is a question which still remains
to be answered. The writer subscribes to the concept that the Sweetwater Arch
and the Medicine Bow Mountains may have risen as islands out of the

WYOMING ROCKIES

381

Cretaceous Sea and that some of the last of the marine deposits, such as the
basal Medicine Bow and Lewis, and even possibly the Mesaverde, may be
locally derived.

Diagram No. 2, represents conditions as they may have existed during the
deposition of the late Lower Eocene (Wind River?). The Paleocene-Eocene (?)
succession has been steeply upturned adjacent to the highlands and the large
thrust fault has brought the Precambrian in contact with these rocks and they
have been truncated by erosion in the vicinity of the highlands. The coarse
conglomerates, derived chiefly from the Precambrian, lie with marked angular
discordance upon older rocks along the margin of the Basin. These late
Lower Eocene (Wind River?) rocks become finer textured and the pronounced
angular discordance between them and the underlying rocks disappears as
they are traced basinward.

Following the deposition of the Eocene (Wind River?) rocks the region
was subjected to moderate folding.

It is apparent that if any Oligocene rocks were laid down in the Basin they
were largely or entirely removed before the deposition of mid-Miocene rocks.
To the south mid-Miocene rocks rest unconformably upon Cretaceous and
older rocks. Following the deposition of the mid-Miocene (Browns Park) the
region was subjected to considerable disturbance. A notable feature of this
disturbance was wide-spread normal faulting. Available evidence indicates that
this disturbance took place in the late Miocene time. It is believed that the
region suffered rather extensive uplift during this disturbance. It is probable
that the numerous normal faults common to the Basin were formed at this time.
Regional evidence indicates that the area was blanketed with sediments during
early Pliocene time. The question of the time of the superposition of the North
Platte River across the Basin and elsewhere has interested the writer for many
years. Until evidence to the contrary is forthcoming, it is concluded that the
present course of the North Platte River was established upon the Lower
Pliocene surface following regional uplift with some tilting. This uplift began
the present cycle of erosion.

Sweetwater Arch
*

Hanna Basin

Fiqureno.1 Early Paleocene

Fiqure ho. Z Eocene (Wind River f)

Medicine Bow River

Fiquneno.3 Recent Front face of diagram
alonq north-south line one mile east of TroublesomecreeK.

LATE TERTIARY DOWNFAULTING OF SWEETWATER RANGE

Along the north and south margins of the exposed Precambrian core
of the Laramide Sweetwater Range normal faults of Late Tertiary age
have been recognized which have resulted in the downward displace-
ment of the core area some 2500 to 3000 feet ( Blackstone, 1951 ) . See
Fig. 24.12. Near the volcanic necks of the Rattlesnake anticline the normal
faults can be dated as post-middle Miocene.

Since the Late Tertiary downfaulting follows approximately the same
pattern as the late Eocene, Oligocene, and early Miocene sagging or

Trace of thrust

Preb'

'■• ,'" Paleocene- Early Eocene ?^« '^^-:- -

<Gret)

Fiqure no.4 Recent Front face of diaqram
alonq north-soutn line midway between Troublesome CrocK
and Austin CreeK. M.-R for Miocene -Pliocene.

Fig. 24.15. Development of the north flank of the Hanna Basin. Reproduced from Knight, 1951.

382

STRUCTURAL GEOLOGY OF NORTH AMERICA

downwarping, we may assume that the normal faults are the late result
of the subsiding process. In the early stages gentle flexing must have oc-
curred, but later ruptures broke along the sides of the subsiding block.

Tertiary faults have been recognized in several places in central Wyo-
ming, but as yet the extent of the system has not been very well delimited.
The pattern of subsidence is outlined under the next heading.

LARAMIDE PATTERN AND CENOZOIC STAGES IN
THE SWEETWATER RANGE REGION

It is evident by inspection of the Geological Map of Wyoming, from
which Fig. 24.12 is taken, that the structures trend in two directions,
northwesterly and westerly. From this it may be concluded that two
phases of Laramide orogeny occurred. The relations of Casper Mountain
to the Emigrant Gap anticline might be taken to indicate that the north-
westerly trending structures are the older, and that the east-west trending
structures have been superposed on the northwesterly. However, on
stratigraphic grounds Love ( 1954 ) lists the following succession of events :

1. At the close of Cretaceous the broad Sweetwater Range arch rose.

2. The arch continued to rise during the Paleocene.

3. At the close of the earliest Eocene the thrusting along the south flank
of the Sweetwater Range occurred.

4. At the close of Oligocene time the gentle northwesterly trending folds
of central Wyoming developed. The settling of the Sweetwater Range
was taking place.

5. In post-middle Pliocene and pre-Pleistocene time:

large-scale block faults developed in many parts of Wyoming; the floor of
Jackson Hole dropped several thousand feet; the southern end of the Wind
River Mountains collapsed; the central arch of the Granite Mountains
(Sweetwater Ridge) dropped several thousand feet; local areas west of the
east margins of the Sierra Madre, Medicine Bow, and Laramie mountains
were down-dropped; part of the Rawlins uplift collapsed and a broad west-
trending anticline formed south of Rawlins; a large area southeast of the
Hartville uplift was downfaulted; the southern end of the Big Horn Moun-
tains probably collapsed at this time (Love, 1954).

RAWLINS UPLIFT

The Rawlins uplift (see Tectonic Map of the United States) is one of
fairly sharp and high structural relief. It stands perhaps 40,000 feet above
the crystalline floor of the Hanna basin. It is complicated by folds and
thrust faults, and one of the thrusts has brought Precambrian rock in
contact with the Mesaverde. This thrust is located just west of the city of
Rawlins, and the movement of the overriding sheet is toward the west
and southwest. The uplift dips generally northward, toward the Sweet-
water uplift, which has been thrust southward against and over it.

WASHAKIE BASIN

Bradley (1945) has reported on the Washakie basin, and his summary
is as follows:

The synclinal structure of the Washakie Basin has given rise to a bold,
outward-facing, encircling escarpment, developed on beds in the Green River
formation that are more resistant to erosion than other beds in the section.
Along the northern margin of the basin this rim rises 600 to 700 feet above the
country to the north, and is known as Laney Rim. Southward the escarpment
increases in height, and locally on each side of the basin its crest stands about
1,200 feet above the surrounding terrane. Along the southwestern margin of the
basin the rim is known as the Kinney Rim. The southward facing escarpment at
the southern margin of the basin is broken by stream valleys at many places,
and is generally lower. Near the head of Powder Wash, just south of the
Wyoming-Colorado boundary line, where the escarpment is low, there is a nar-
row, southward extension of the Basin, which is clearly shown by the outcrop
pattern of the Green River formation. This panhandle is expressed topographi-
cally by a pair of outward-facing escarpments that rise to an altitude of more
than 8,000 feet at Lookout Mountain.

The rocks in the Washakie Basin are divided into four main units, from bot-
tom to top, the Wasatch, Green River, and Bridger formations of Eocene age,
and the Browns Park formation of probable Miocene age. In the broadest and
simplest terms, the Green River formation is a huge lens of relatively fine
grained fluviatile sandy mudstone that formerly filled a huge intermontane
basin far larger than the Washakie Basin. The mudstone is divided into two
formations: (1) the Wasatch formation, below the lens of Green River forma-
tion, and (2) the Bridger formation above. The sedimentary history of the in-
termontane basin was complicated, however, by changes in the level of the
lake, which resulted in an intertonguing relationship between the Wasatch and

WYOMING ROCKIES

3S3

the overlying Green River as shown in the generalized columnar section
[Fig. 24.5].

The Washakie Basin is a shallow syncline lying on the east side of the Rock
Springs uplift. Along the north and east sides of the basin, the dip of the beds
ranges from 3° to 5° toward the center, whereas along the west and southwest
sides it ranges from 8° to 12°. In the large central area of the basin the rocks lie
nearly flat. This essentially uniform synclinal structure is broken only along the
Wyoming-Colorado line, where a west-trending fault zone forms the southern
boundary of the basin. This zone, which appears to be an eastward extension of
the structural lines in the Hiawatha gas field, is described in an earlier report.
The faults of this zone fall into two broad groups, according to their general
direction of strike. The faults of the dominant group strike generally westward,
whereas, those of the other group strike more nearly northward or northwest-
ward. The faults of the first group are, in general, the older as they are cut by
faults of the second group. Moreover, the faults of the second group cut beds of
the Browns Park formation, whereas the other faults in most places do not.
However, some of the larger east-west faults were apparendy active during the
I second stage of faulting, because locally, as along Cherokee Ridge, they also cut
the Browns Park formation.

The rocks in the fault zone are folded into several synclines, the axes of
which are parallel to the dominant, westward-striking faults. Most of the folds
are rather gentle, having dips that range from 3° to 7°, though locally, as in the
vicinity of Baggs, the beds dip as much as 16°.

GREEN RIVER BASIN
':

The Green River basin (also referred to as the Bridger basin) is
bounded on the south by the great Uinta anticline, on the west by the
central Rockies of western Wyoming, on the east by the Rock Springs
uplift, and on the north and northeast by the Wind River and Gros Ventre
ranges. The extreme northern end of the basin is a wedge between the
southwestward thrust Gros Ventre Range and the eastward thrust Hoback
Range, and is drained by the Hoback River, a tributary of the Snake
River, and hence locally known as the Hoback basin. The Green River
drains the rest of the Green River basin. Refer to Wyoming Geological
Association Guidebook, Tenth Ann. Field Conference, 1955.

The evolution of the Green River basin has been depicted in Chapter
22. See Figs. 22.4 to 22.6 and 24.16. The relation of the Uinta anticline to
the basin is shown in Fig. 24.17. From the diagram it may be seen that
considerable arching and erosion preceded the deposition of the Green

Fig. 24.16. Isopach map of
Mesaverde group, Wyoming,
Utah, and Colorado. Courtesy
John Burger. The site of the
present Rock Springs uplift was
a basin in Mesaverde time.

River formation, but from other locations along the north flank of the
Uintas the major and sharp rise, involving high-angle thrusting, followed
the Green River. Regarding the graded surfaces exhibited on the north
flank of the Uinta Mountains and extending far out into the Green River
Basin Bradley says:

Long, narrow remnants of four old erosion surfaces slope gently northward
from the north flank of the Uinta Range and truncate the upturned edges of
hard and soft beds. The Gilbert Peak erosion surface, which is the highest and
oldest of these surfaces, once extended from the crest of the range at an altitude
of about 13,000 feet to the center of the Green River Basin. Because un-
disturbed remnants of this surface have gradients ranging from about 400 feet
to the mile near the crest of the range to 55 feet to the mile 35 miles out in the
basin, because island mounts of limestone rise rather abruptly from it. and be-
cause it apparendy never had a soil mantle but is covered in most places by
conglomerate, this surface is interpreted as a pediment formed in a semiarid or
arid climate. At the time the Gilbert Peak surface was cut the Green River
Basin was filled to a greater depth than now with Eocene sedimentary rocks.
The Gilbert Peak erosion surface truncated these rocks at very low angles and
extended northward across them as a continuous plain. On this plain the master

384

STRUCTURAL GEOLOGY OF NORTH AMERICA

N.50*E

5,000'

Fig. 24.17. Cross section of the north flank of the Uinta Mountains after Bradley (1936), show-
ing remnants of the Gilbert Peak surface projected laterally to the plane of the section. Note the
even truncation of both hard and soft strata. pC Uinta Mountain group; Cu, Carboniferous un-

differentiated; J— "5 Jurassic and Triassic undifferentiated; Ku, Cretaceous undifferentiated; Tgs,
Green River fm.; Tb, Bridger fm.; Tbc, Bishop congl.

stream of the basin apparently flowed eastward to join the ancestral Platte or
some similar river that drained into the Gulf of Mexico.

The Bishop conglomerate, which covers much of the Gilbert Peak surface,
is coarse-grained and very poorly sorted and fills the deepest concavity in the
profile of the pediment, where it is about 200 feet thick. The same streams
that cut the Gilbert Peak pediment deposited the Bishop conglomerate, be-
cause their transporting capacity changed in response to a climatic shift toward
still greater aridity. This climatic change, though critical, probably was not
great.

No fossils have been found in the Bishop conglomerate, but the Gilbert Peak
surface truncates the latest Eocene rocks and yet is distinctly older than the
Browns Park formation (late Miocene or early Pliocene). Hence the Gilbert
Peak surface and the Bishop conglomerate are either Miocene or Oligocene. I
believe that the Gilbert Peak surface is probably correlative with Blackwelder's
Wind River peneplain, near the top of the Wind River Range.

About 400 to 500 feet below the remnants of the Gilbert Peak surface these
same streams later cut the less extensive Bear Mountain erosion surface. The
characteristics of the Bear Mountain surface are so nearly identical with those
of the Gilbert Peak surface that it is regarded as a pediment formed under arid
conditions probably closely similar to those which prevailed while the Gilbert
Peak surface was being cut. Correlated with the Bear Mountain surface are
two large, rather smooth-floored valleys, the Browns Park Vafley and Summit
Valley. These valleys are in the eastern part of the Uinta Range and are each
roughly parallel to the range axis. The floor of the Browns Park Valley descends
eastward and passes beneath the Browns Park formation, which is of upper
Miocene or lower Pliocene age. As there is no indication that the deposition of
the Browns Park formation did not follow immediately the completion of the
Bear Mountain surface, that surface is probably also of essentially this geologic
age.

After the deposition of the Browns Park formation the east end of the Uinta
Mountain arch collapsed by block faulting, . . . and . . . apparently lowered

the stream flowing along the ancient Browns Park Valley (on the depositional
surface of the Browns Park formation) enough for one of its tributaries, which
has already cut through the divide on the north side of the valley, to be re-
juvenated and thus to extend its course headward so far northward in the soft
Tertiary rocks that it finally captured the ancient master stream of the Green
River Basin. When this river, the new Green River, first entered the Browns
Park Valley it flowed on the uppermost beds of the Browns Park formation,
following the ancient Browns Park stream eastward beyond the east end of the
range. But soon thereafter it was captured by Lodore Branch, a tributary to the
ancestral Cascade Creek, which drained Summit Valley, and so came to flow
along the present site of Lodore Canyon.

UINTA MOUNTAINS

The Uinta Mountains are eroded from a flat- topped anticlinal uplift,
the major details in cross section of which are shown in Fig. 24.18. The
thrust faulting along the north flank is post-Green River formation and its
character suggests horizontal spilling or mass flowage of the margin of the
uplift toward the Green River basin as a late or secondary effect of the
primary vertical uplift. The vertical uplift of anticlinal crest over basin
trough exceeds 32,000 feet.

ROCK SPRINGS UPLIFT

Separating the Green River basin on the west and the Washakie basin
on the east is the Rock Springs uplift, a 40-mile-long, doubly plunging,
north-south-trending anticline. The resistant sandstones hold up hog-

WYOMING ROCKIES

backs on the west side with dips of 30 degrees, and cuestas on the east
side where dips do not exceed 10 degrees. The principal ridge-making
sandstone is the Mesaverde, which stands 1000 feet high in places and
surrounds the elliptical Baxter basin in the center.

The evolution of die Rock Springs uplift is shown in Figs. 22.3 to 22.6
and 24.16.

The Leucite Hills at the nordi end of the Rock Springs uplift are
remnants of cinder cones and lava sheets. The lavas cap hills of sedi-
mentary rock that now stand 800 to 1200 feet above the plains and pre-
serve remnants of an old, subdued erosion surface (Rich, 1910). This may
be equivalent to the Gilbert Peak surface of the Uintas.

The Rock Springs uplift was formed mainly after the Upper Eocene
sediments had accumulated. This seems evident because the sediments
do not coarsen appreciably toward the uplift. It may have started to rise
. in late Washakie time incident to the bold arching of the Uintas in mod-
ern form, because the upper part of the Washakie formation in the
'Washakie basin is not present in the Green River basin; but the main
^elevation of the Rock Springs uplift was post- Washakie.

In the Gilbert Peak erosion cycle of the Uinta Mountains the Rock
Springs uplift was beveled, so the folding predated the erosion cycle
which terminated in Miocene time.

EAST END. UINTA MTS

LARAMIE RANGE AND BASIN AND MEDICINE BOW RANGE

The Laramie basin is, in general, a northward-plunging syncline be-
tween the Laramie Range on the east and the Medicine Bow Range on
the west. The ranges on either side are formed of Precambrian crystalline
rocks, and about 8000 feet of Carboniferous and Mesozoic beds overlie the
crystallines in the basin. The sediments are preponderantly shaly. Along
the west side of the basin are four anticlines in en echelon arrangement,
with exposed Precambrian cores; and they are known from south to
nordi as Bull, Ring, Jelm, and Sheep mountains. High-angle thrust faults
occur on one or both sides of these anticlines.

Thrust faulting was the chief activity in Laramide times, with the
sides of the basin generally bounded by thrusts dipping under the moun-

Fig. 24.18. Cross sections of the eastern and central parts of the Uinta uplift. After Ritzma, 1959.

tains. The Medicine Bow Mountains are thrust moderately eastward over
the western margin of the Laramie basin (Beckwith, 1938, 1942), and
the Front Range crystallines at the southern end of the basin are thrust
westward over the sedimentaries. See cross sections of Fig. 24.19. Tear
faults and faults that turn into stratification faults at depth without pass-
ing into the Precambrian have been described. Younger beds have been
thrust over older in places, and at the very south end of the valley, where
the thrust sheet from the west is opposed to the thrust sheet from the
east, the basin is not as wide as the amount of movement on the thrust
surface. From this, Beckwith (1942) concludes that the fronts of the
thrust sheets were eroded back sufficiently fast as they advanced so that
they did not meet head-on.

The date or phases of deformation cannot be directly determined in
the Laramie basin except that they occurred in the interval post-Mesa-
verde and pre-Oligocene. From reference to the orogenies in nearby
Hanna basin and North Park, Beckwith infers that arching of the Medi-
cine Bow and Park ranges started in Late Cretaceous time, while the Cre-
taceous seas still persisted a short distance from the present mountains.
Folding and thrusting occurred during early Eocene and then again some
compression shortly afterward cast the lower Eocene beds into folds. The
evidence is set forth as follows by Beckwith:

The folded sediments in the upper Laramie River Valley are about 8000 feet
thick. Farther north the Mesaverde is succeeded conformably bv 3000 feet of

386

Medicine Bow

Range

7,500

5,000

2,500-

STRUCTURAL GEOLOGY OF NORTH AMERICA

Sheep Mtn.

7,500'

5,000

2,500'

Green Ridge

2,500

Fig. 24.19. Cross sections along the east front of the Medicine Bow Range and the west side
of the Laramie basin, after Beckwith, 1938 and 1942. Cfn, Fountain fm., Cc, Casper fm.; Cs,
Satanka sh.; Cf, Forelle Is.; He, Chugwater fm.; T\, Jelm fm.; Js, Sundance fm.; Jm, Morrison

Medicine Bow
Range

A pt

7,500'

5,000- ~\yt

7,500'

IV ■ 5,000'

fm.; Kd, Dakota group; Kb, Bsnton group;
Kl, Lewis sh.; Twr, White River group.

■ 2,500'

Kn, Miobara fm.; Ks, Steel sh.; Km, Mesaverde fm.;

marine Lewis shale and several thousand feet of grits, standstones, carbonaceous
shales, and coals constituting the Medicine Bow formation. The Medicine Bow
is overlain unconformably by the Hanna formation. At the Citizen's Coal Mine,
5 miles north of Sheep Mountain, the lower beds of the Medicine Bow contain
conglomerates with pebbles of Dakota sandstone and Mowry shale several
inches across. Thousands of feet of marine beds must therefore have been
stripped from the adjacent rising arch by early Medicine Bow time. A similar
conclusion is reached for the region to the south.

Lovering (1935) states:

Before the end of Pierre time, the central part of the Front Range highland
was pushed above the level of the sea, and recently deposited shales were ex-
posed to erosion. They were reworked into the upper part of the marine Creta-
ceous, and the Dakota sandstone was also exposed and reworked in many
places and was probably the source of much of the sandy material found in the
Fox Hills sandstone.

WYOMING ROCKIES

387

Earliest known locally-
derived conglomerate

■

Figure No. 3, LATE LOWER EOCENE TIME

^0^^aI^>:v/:v^^\^kv

Figure No. I, LATE UPPER EOCENE TIME

Figure No. 3, PRESENT TIME

Fig. 24.20. Idealized evolution of Medicine Bow Mountains, Wyo. Reproduced from Knight, 1953.

At the north end of the Laramie basin is a belt of folds arranged en
echelon. The belt is on the easterly projection of the Sweetwater uplift,
and the direction of the fold axes in the en echelon belt is northeasterly.
Jurassic and Triassic rock is exposed in the cores of the anticlines, and on
the north they are blanketed with the Oligocene White River beds.

The evolution of the Medicine Row Mountains in Cenozoic time is
shown in Fig. 24.20.

HARTVILLE UPLIFT

The Hartville uplift is a northeast arm of the Laramie Range, and
connects it effectively with the Rlack Hills uplift. The Laramie Range is
a broad, flat-topped anticline, or uplifted plateau with bounding mono-
clinal flexures, and so also is the Hartville uplift, if viewed from the south
end of the Powder River basin to the Great Plains. See cross section of
Fig. 24.14. The Hartville uplift is broadest at its junction with the Laramie

388

STRUCTURAL GEOLOGY OF NORTH AMERICA

Range but narrows toward the Rlack Hills. The east-bounding monocline
has a structural relief of about 9000 feet within a few miles, and the
one on the northwest drops the strata as abruptly and about an equal
amount.

The top of the Hartville uplift is about 2000 feet below that of the
Laramie uplift. Its sedimentary veneer has been stripped off only in a
narrow zone along the axis; from this it is deduced that the uplift was
never as high as the Laramie or Sweetwater uplifts, and that its relatively
lower position today is not due principally to late Laramide subsidence
like that of the Sweetwater, but to the fact that it was never elevated
high enough in the face of much higher elevations nearby to have suf-
fered as much erosion. It was largely buried by the White River beds in
lower Oligocene time, and in the later cycles of erosion it has been partly
exhumed.

REGIONAL UPLIFT IN LATE CENOZOIC

The Eocene deposits of Wyoming and adjacent areas accumulated in
swamps, on flood plains, and in fresh-water lakes. The flora and fauna
indicate a warm, rain-forest climate, and the elevation above sea level
at which the sediments were laid down is generally considered to have
been not in excess of 1000 feet. Today they occur at about 7000 feet,
especially in the Green River, Wind River, and Big Horn basins. The
Great Plains adjacent to the Rockies contain Early Tertiary sediments
deposited at low levels, but which now stand in places as high as 5000
feet. It is patent that uplift on a very broad scale has occurred. We must
be aware of the history of subsidence and sedimentation in central
Wyoming, the eastern end of the Uintas, and elsewhere in mid and late
Eocene, Oligocene, and early Miocene time, and post-Miocene normal
faulting resulting in further subsidence. However, in the broad picture
from the Northwest Territories of Canada, southward through Alberta,

and the outer Rockies of Montana, Wyoming, Colorado, and New Mexico,
the dominant late Cenozoic activity was uplift, and in an amount from
2000 to 6000 feet. The southern part of the Colorado Plateau was up-
lifted perhaps 8000 feet.

Most of the literature concerning the erosional and depositional activity
during the building of the Laramide Rockies and the later regional uplift
depicts a history as follows. Immediately after the Laramide ranges were
uplifted, extensive erosion surfaces were developed — in places several,
one below the other — indicating times of crustal stability separated by
uplift and dissection. The erosion surfaces in places can be traced out
and are said to level the basin fill deposits. Then, with the regional uplift,
just mentioned, the erosion surfaces were greatly dissected and produced
our present topography.

Mackin, Van Houten, and others more recently view the history as fol-
lows. Erosion affected the uplifts immediately as they appeared from the
Cretaceous seas and removed sediments to the intervening basins. The end
result of the erosional and transportational processes was a vast graded
surface, in part erosional and in part depositional. With the building of
great volcanic piles in the Yellowstone and Absaroka region in mid-
and late Eocene and Oligocene time the streams draining eastward, per-
haps fanlike from the volcanic field, were overloaded with fine debris, and
according to Love (1956b), the intermontane basins of Wyoming were
so filled that the large bordering ranges were almost submerged. Volcanic
activity broke out in other areas, and as pointed out some areas subsided,
so that sedimentation was of irregular thickness in places and continued
where subsidence continued. But in late Miocene or early Pliocene time
most of the graded surface was uplifted, an arid climate resulted, and a
regimen of erosion started. This has continued in most places until
today. With the coming of the arid and semiarid climate the grassy plains
came into existence, and many animals evolved and adapted to a life on
the open prairie.

COLORADO AND

NEW MEXICO ROCKIES

COLORADO ROCKIES

Geography

Figure 25.2 shows the principal features of geologic interest in Colorado,
and on it the above-mentioned belt of Laramide deformation may be
identified. The Front Range is the largest and highest of any in the
Rockies of the western United States. The most rugged mountains are
in the central part with a number of peaks exceeding 14,000 feet in ele\ a-
tion. The western flank slopes steeply away from the crest of the range,
but the eastern slope is characterized by broad, dissected, benchlike
erosion surfaces that descend in steps to the Great Plains.

A series of valleys or basins occupy a central position to the flanking
ranges on east and west, namely, North Park, Middle Park, South Park,
and Huerfano Park. The San Luis Valley lies west of the Sangre de Cristo
Range and continues the basins in offset fashion into New Mexico.

The Colorado Plateau extends across western Colorado to the Park
and Sawatch ranges and is generally considered to include the Piceanoe
basin, the White River uplift, and the Uneompahgre uplift. An extensive
volcanic field obscures much of the Laramide geology between the Needle
Mountains of the San Juans and the Sawatch and Sangre de Cristo
ranges.

The Denver, Trinidad, and Raton basins on the east of the Colorado
Rockies are Laramide downwarps.

JEXTENT OF LARAMIDE DEFORMATION

Most geologists regard the high relief features in central Colorado,
composed principally of the Park, Front, Sawatch, Wet, and Sangre de

rCristo ranges, as the Laramide structures of the state. The belt is about
80 miles wide and extends in a north-south direction. See Fig. 25.1. It
continues southward to southern New Mexico where it joins the Laramide
belts of Mexico and southern Arizona.

An arm extends southwestward from the Sawatch Range to and includ-
ing the San Juan Mountains.

Relation of Laramide Rockies to Ancestral Rockies

The Colorado Range of the Ancestral Rockies has been described on
previous pages. It was gradually overlapped from the east and west and
nearly buried by late Cretaceous time. During the Laramide unrest
the modern Front Range rose approximately along the eastern half of the
ancestral range, and the basins of the Middle and North parks and the
Park Range appeared approximately along the western half. See Figs.
25.3 and 25.4

The ancestral Central Colorado basin with its thick fill of Pennsylvania!!
and Permian red beds and evaporites, and also an appreciable thickness

389

390

STRUCTURAL GEOLOGY OF NORTH AMERICA

Fig. 25.1. Index map for cross sections of Colorado and New Mexico. Refer to Raisz' Landforms
Map of the United States for location of ranges, mountains, and valleys. Stippled area is zone of
marked laramide disturbance.

of later Cretaceous shales and sandstones, was considerably deformed in
Laramide times. The evaporites contributed to great distortion of the
beds in die Eagle area (Fig. 25.4), and the White River uplift or arch as
a continuation of the Uinta uplift became the largest structure in the
ancestral basin.

Central Parks

Although Middle Park is a well-defined drainage basin, it is more
complex topographically and structurally dian the other parks. North
and South parks have little, or only moderate, internal topographic relief;
structurally they are broad, open synclines, marginally faulted, particu-
larly along the eastern sides. Middle Park is also generally synclinal, but
the structural and topographic continuity is disturbed by many projecting
mountain spurs characterized by overthrust faulting. Northward trending
spurs of the Front Range, comprising the Williams River Mountains and
the Vasquez Mountains, extend into the southern part of the park as far
as the Colorado River. The northern part is largely occupied by spurs
projecting southward from the Never Summer Mountains and from
the unnamed ridge followed by the continental divide between Middle
and North parks.

Front Range

The Front Range is mostly an expanse of Precambrian rock with up-
turned or overthrust sedimentary rocks on east and west flanks. The Pre-
cambrian rocks have been reviewed in Chapter 4. According to Lovering
and Goddard (1950):

The western side of the Front Range is marked by a series of great over-
thrust faults that formed at this time from the southern end of the South Park
as far north as the Wyoming line. The displacement on the Williams Range
thrust fault north of Breckenridge is more than 4/2 miles, and the movement
on the Never Summer thrust north of Branby is more than 6M miles. The
eastern side of the Front Range was subjected to much less severe deformation
but was the locus of many echelon northwesterly folds and persistent steep
northwesterly faults. Its structure is dominantly that of a steep monoclinal
fold, though locally, as at Colorado Springs and Boulder, some thrusting has
taken place.

Fig. 25.2. Index map of central
and western Colorado.

CENOZOIC
EXTRUSIVES

TERTIARY
1NTRUSIVES

SEDIMENTARY ROCKS
UNDIFFERENTIATED

PRECAM8RIAN
ROCKS

392

STRUCTURAL GEOLOGY OF NORTH AMERICA

scale: in miles

Fig. 25.3. Upper section generalized across the Front Range of Colorado and three of the back
ranges, from Boulder to State Bridge. pC, Precambrian undifferentiated; Cf, Fountain formation;
CI, Lyons formation; f Lylcins formation; lis, State Bridge siltstone; Cm, maroon formation; Jm,
Morrison formation; Kd, Dakota sandstone; Kp, Benton, Niobrara, and Pierre formations; Kl,
Laramie formation; Kde, Denver formation; Kmv, Mesa Verde formation; Ti, Tertiary intrusives;

The period of overthrusting was followed by northeasterly and east-north-
easterly faulting on a large scale throughout the mineral belt during and after
the intrusion of the porphyritic rocks that dot it. Many of the mineral deposits
are localized at the intersection of easterly and northeasterly faults with the
earlier persistent northwesterly faults where they cross the mineral belt.

Faults formed after the Laramide revolution are comparatively local and
largely confined to Miocene volcanic areas and Tertiary basins close to the
mountain front.

A group of northeast-trending faults is confined mostly to the western
part of the range and seems to mark the western limit of the mineral belt.
They appear to be steep and are marked by gougy shear zones 10 to 600
feet wide. The largest of the group is the Moffat Tunnel fault (Lovering
and Goddard, 1938a), which is intermittently exposed on the surface for

Tnp, North Park fm.; Tmv, Miocene volcanics. Section A of Fig. 25.1.

Lower section is a detail of the Front Range near Montezuma, after Lovering, 1935. Ais, Idaho
Springs formation; As, Swandyke hornblende gneiss; Asp, Silver Plume granite; Cm, maroon
formation; Jm, Morrison formation; Kd, Dakota sandstone, Kb, Benton shale; Kn, Niobrara forma-
tion; Kp, Pierre shale; Tqm, Tertiary intrusives. Section B of Fig. 25.1.

a distance of more than 25 miles and forms a wide zone of "heavy
ground" 1000 feet wide in the Moffat Tunnel, 2000 feet below the out-
crop. It passes through Berthoud Pass, where the zone of fractured rock
is 200 feet wide, and through Loveland Pass, where it is less than 50 feet
wide. The badly broken rock is apparently responsible for these depres-
sions.

The Rocky Mountains through central Colorado have generally been
regarded as a belt of horizontal compression, but they can also be in-
terpreted as a group of closely packed uplifts with the Front Range
longer and wider than any of the others in the entire province. The an-
cestral Colorado Range was an uplift almost as large as the entire cluster
of ranges in the central Colorado belt, but the later Laramide uplifts de-

COLORADO AND NEW MEXICO ROCKIES

393

PICEANCE BASIN

Fig. 25.4. Section along the Colorado River from the Gore Range to Rifle and the Piceance
basin. Tgr, Green River fm.; Tw, Wasatch fm.; Kwf, Williams Fork fm.; Ki, lies fm.; Kmc, Mancos
sh.; Kb, Benton gr.; Kd, Dakota ss.; Jm, Morrison fm.; Je, Entrada ss.; \ Chinle sh.; Cpp,
Permian and Pennsylvanian; CD, Miss, and Dev.; OC, Ordovician and Cambrian. After Bench
et a/., 1948.

veloped as independent units, not much controlled by the ancestral uplift.
The flanks of the central Colorado ranges are replete with thrusts, and
apparently reflect the superior uplift of the Front Range.

Harms (1961) has studied the sandstone dikes of the eastern margin
of the Front Range south of Denver and presents a convincing case for
granite tectonics there. Large Laramide faults place Precambrian rocks in
contact with sediments as young as Tertiary in age. The stratigraphic
displacement in places is 15,000 feet and the structural relief from 15,000
to 25,000 feet. He concludes that the stress distribution causing the in-
fection of the sandstone dikes was governed by dip-slip movement along
,.steeply westward dipping, convex upward fault surfaces, and that, there-
[fore, the major structures outlining the flank of the range are high-angle
reverse faults which steepen with depth.

Uplift of the Front Range began in middle Pierre time while the Denver
jbasin was sill being downwarped, and from that time till well into the Paleocene
,the central part of the range moved upward at an ever increasing rate. Parts
of it rose above the ocean during Fox Hills time, and at the beginning of
'Denver time large areas were shedding pre-Cambrian debris to the east and
west. Intense folding and faulting occurred at the edges of the basins of
.deposition where the troughs merged with the old positive element about the
[end of Denver time and outlined the Front Range as it now is (Lovering and
Goddard, 1950).

>i
North Park Thrusts

J An unusual example of thrusting in the general Front Range region,
and as indicated previously one that represents considerable horizontal

movement, is at Cameron Pass. Here the Never Summer Range borders
on the southern part of the Medicine Row Mountains adjacent to North
Park. A tear fault extending along the Middle Fork of Michigan Creel
separates two patterns of thrusting (Gorton, 1953). See map of Fig. 25.5.
Although they developed simultaneously, each produced its own struc-
tures. The block on the north exhibits two thrusts, as shown in the upper
section of Fig. 25.6, whereas the block on the south is interpreted to have
one thrust. All thrusts have been folded, and the sequence of events
appears to Gorton as follows:

1. Folding, probably in Late Cretaceous.

2. Thrusting, post-Middle Park and pre-North Park.

3. Open folding. The quartz monzonite stock was emplaced during this
stage or immediately afterward.

The Renton Gulch thrust klippe and the downfolded Never Summer
slice would be interpreted by some geologists as detached gravity slide
blocks, and the amount of compressional orogeny minimized. The writer
is inclined to view vertical uplift as of paramount importance with mar-

Fig. 25 5. Thrusts of the Cameron
Pass area, Never Summer Range,
Colorado. After Gorton, 1953.

394

STRUCTURAL GEOLOGY OF NORTH AMERICA

,gRgfJ3«£t

Fig. 25.6. Cameron Pass cross sections. Upper section north of tear fault; lower section south
of tear fault. Refer to Fig. 25.5. After Gorton, 1953. Tc, Chugwater fm.; Jm, Morrison and
Entrada; Kcl, Dakota ss.; Ks, Benton sh.; Kn, Niobrara Is.; Kp, Pierre sh.

ginal gravity flow movements auxiliary to the vertical in the Colorado
and New Mexico Rockies. See discussion under New Mexico Rockies.

A transverse fault, called the Independence Mountain, is prominent at
the north end of North Park. It trends N. 65° W., dips at a low angle
northward, and has been mapped for 20 miles (Rlackstone and de la
Montagne, I960). Precambrian gneisses have been thrust southward over
all formations from Triassic Chugwater to Paleocene Coalmont. The
overlapping nature of the Coalmont indicates that it was derived from
previous uplift, and some subthrust folds suggest previous folding also.
The thrust is of Eocene age and is offset by post-late Miocene normal
faults. Isolated Precambrian rocks resting on the Coalmont formation
south of and at lower elevations than the trace of the thrust are con-
sidered by Blackstone and de la Montagne to have reached their position
by gravity sliding, but they do not propose gravity sliding for the main
thrust sheet.

Transverse Porphyry Belt

Many small instrusive bodies in Colorado will be noted on the map of
Fig. 25.2. They are largely concentrated in a narrow diamond-shaped
belt that extends from the southwest corner of Colorado, even from the
adjacent states of New Mexico, Arizona, and Utah, northeastward through
the San Juan Mountains and the Sawatch Range to the Front Range, and

across it to Boulder City. They compose the so-called porphyry belt, and
most of the state's mineral deposits are localized in it. The belt trends
nearly normal to the belt of thrusting. See Fig. 25.1. All the small stocks
are Laramide in age and their intrusion generally accompanied the moun-
tain building.
According to Lovering and Goddard ( 1938b ) :

The late Cretaceous and early Eocene (Laramide) igneous rocks or "porphy-
ries" of the mineral belt are readily distinguished from all but a very few of
the pre-Cambrian rocks, and many different varieties are so distinctive in
appearance as to justify correlation between districts separated by several
miles. See Fig. 25.7. These igneous rocks are commonly medium- to fine-
grained and nearly all are prophyritic. Some of the early rocks of mafic or
intermediate character are holocrystalline, and thin dikes of widely differing
composition have felsitic to glassy textures. These intrusives show a wide range
in chemical and mineralogic composition, and include dikes as mafic as limburg-
ite, as silicic as alaskite, and as alkalic as aegirite syenite. Most of the intrusive
rocks are intermediate or siliceous porphyries whose compositions range from
hornblende diorite or biotite-quartz monzonite.

The mineralization followed the intrusion of the Eocene porphyry. The
extensive lead-silver deposits of Leadville, the iron-zinc deposits of Gillman,
the molybdenite deposits of Climax, the lead-silver deposits of Montezuma.
Silver Plume, and Georgetown, the gold deposits of Gilpin County and Central
City, and the tungsten deposits of Nederland were formed at this time. Although
most of the mineralization in the San Juan Mountains occurred in Miocene
time, some deposits of Eocene age were formed near the centers of early
Tertiary intrusion in that region.

Sangre De Cristo Range

The Wet Mountains are almost separated from the Front Range by the
Canyon City embayment, but in the early history of the Ancestral Rockies
and the later history of the Laramide orogeny, the two were closely as
sociated. The eastern border of the Wet Mountains is characterized by
Laramide overturning or overthrusting of the Carboniferous and Meso
zoic formations toward the east. The western border is one of sharply up
turned beds and steep faults.

West of the Front Range in the area of overlap on the ancestral Colo-
rado Range and near the center of the Carboniferous basin, sharp folds
and several large thrust faults have resulted in a chain of ranges, prin-
cipal of which are the Sawatch and Sangre de Cristo. They extend to the

COLORADO AND NEW MEXICO ROCKIES

395

east front of the Rocky Mountains south of the Wet Mountains. See Fig.
25.2. The Sangre de Cristo Range extends southward into New Mexico
110 miles and has a regular arclike form, convex toward the east. Its width
is small, ranging from 10 to 20 miles. Together with the San Luis Valley
on the west and the Wet Mountain Valley and Huerfano Park on the
east, it represents a prong of the Laramide belt. Complex folds and
overthrusts dominate the belt. The folds in the northern part of the range
consist of a major central anticline bounded on the west by a much com-
pressed, overturned, and faulted syncline, and on the east by a more
open syncline. The major anticline is complicated by the presence of a
metamorphic core and by several small intrusive bodies along or near
its axis. The overthrusts are best preserved in the Huerfano Park region,
where several imbricate thrust sheets were thrown into steeply inclined
^positions by the compressional forces (Burbank and Goddard, 1937).
jSee Fig. 25.8.

The upper section of Fig. 25.9 represents a supposed early stage of com-
pression and overthrusting prior to the upthrust of the Precambrian rocks
and the downfaulting of the San Luis Valley; the lower section, the
^structures afterward. The anticline is viewed as an injective mass due to
'considerable mobility of the shaly beds of the Lower Pennsylvanian
Which were overlain by a great thickness of less mobile conglomerates.
When compressed, the shales flowed into the core. The belt of plastic
deformation is limited to an area just east of the arc-shaped bend in the
thrust zone.

The belt of thrusting along the east side of the Sangre de Cristo, op-
posite Huerfano Park, is illustrated in Fig. 25.8. The principal thrusting
/Dccurred after the deposition of the Poison Canyon formation (lowermost
Eocene or Paleocene) and before the deposition of the Chuchara forma-
ion (middle ? Eocene). See stratigraphic chart of Fig. 25.10. Both the
Ohuchara and overlying Huerfano formations are affected by the thrust-
ng; but Burbank and Goddard believe the thrusting, although con-
inuing into Eocene time, was of declining intensity.

West of, and inside the arc of dirusting, are two elongate masses of
3recambrian rock which were buried or were much lower in elevation
han now during the thrusting, but which were later elevated as blocks

bounded by high-angle faults. Accompanying the uplifting was consider-
able plastic deformation of the adjacent shales. The maximum vertical
uplift is estimated as 2 to 3 miles (Burbank and Goddard, 1937), and
most of it occurred in post-Huerfano (late ? Eocene) time.

After the Precambrian "massifs" were uplifted, they were broken by
tensional faults, and in part settled so much as to be covered by effusions

LEADVILLE

ALMA

SOUTH PARK

MONTEZUMA

AND
ARGENTINE

CENTRAL CITY

AND
IDAHO 5PRING5

TUNGSTEN
BELT

Tungsten ore
Limburgite

Tellurtde ore — Telluride ore

,Bio ti te lati te — Intrusion breccio

Biotite monzonite

Pynte gold ore
Bostonite

Rhvo/ite agqlom/ Alkali syenite-- Hornblende

Lead -silver ore

1 Contact met dep

monzonite

I Rh 0i,r Felsite

I I [Sadie monzomte i Puorn Monzomte- Quartz Monzonre
I J '{Lincoln porphyry

.Monzonite

Lead- silver-, ,'< ', r-w

qold ores i ! i Intermediate atz
i j i monzonite ,

Late white .' ' j
porphyry

Lincoln
porphyry

[Dacite I

'[Hornblende diorite

i i
i ,

I i

Johnson Gulch >
porphyry /

Diorite J W* diorite

__,- Diabase

Ffhyo/ite

Monzoniti
Quartz rr,

White porphyry

Monzonite
Quartz monzonite

Fig. 25.7. Laramide folding, faulting, and emplacement of igneous rocks and ore deposits in
the Front Range mineral belt, Colorado. Abbreviated after Lovering and Goddard (1938b). The
dashed lines connect intrusions and ore deposits of similar kind. Equivalent age is portrayed by
similar horizontal position. From southwest to northeast the intrusions and ore deposits become
generally progressively younger.

''■■»» »

.w— — T

• ! SoUwU imMm

■

• ••••

•«•

COLORADO AND NEW MEXICO ROCKIES

Wet Mtns.

^■^Wet Mtn Valley

> j '''iSTTtiri

I i It

5Miles

Fig. 25.9. Generalized and diagrammatic sections across the north central part of the Sangre
de Cristo Mountains. See section C, Fig. 25.1.

Section A constructed chiefly from a traverse across the range near Crestone. Symbols are
generalized: solid black pattern (base) represents lower Paleozoic sandstones and limestones;
overlying crinkled line pattern, Lower Pennsylvanian beds involved in zones of shearing and
plastic deformation; lighter line and conglomerate pattern, higher Pennsylvanian and Permian
formations; the filling of the San Luis Valley depression is Late Tertiary and Recent alluvium,
and older lavas and gravels; intrusive bodies are entirely hypothetical but are at positions
corresponding to similar bodies exposed in other parts of the range.

Section B is a diagrammatic representation of the same section, based upon the hypothesis of
an earlier phase of lower-angle overthrusts, which are presumed to be responsible for the greater
part of the shallower tangential deformation in the marginal belt. From Fig. 1, Burbank and
Goddard, 1937.

of lava. This is a post-Laramide tectonic event, and it is believed to have
started not sooner than late Oligocene.

The structure of the Sangre de Cristo Rrange southward in New
Mexico is less complicated, and the mountain front resembles that of the
Colorado Front Range (Smith and Ray, 1941).

NEW MEXICO ROCKIES

Geography

The Rio Grande flows southward through central New Mexico from
Colorado to El Paso, Texas, a distance of 450 miles. It occupies a series

PERIOD

PLIOCENE OR

PLEISTOCENE/

EOCENE

UPPER
CRETACEOUS

LOWER CRET.

UP. JURASSIC

FORMATION

Terrace gravels

Huerfano fm.

Cucharafm.

Poison Canyon fm.

Possibly including
beds equivalent
to f/oton form-
ation.

Vermejo fm.

Trinidad ss.

Pierre shaJe

Apishapash

Timpas Is.

Carlile sh.

Greenhorn Is.
Grejieros sh.

°5a

.Dakota. _ss.

Purgatoire fm.

Morrison fm.

----^--i-H I60-2Z5

*E3tofrPS3F-£3-

THICKNESS
IN FEET

E30O-35O0

300-500

2000-3500

0-4-50

^^3^ 1800-2000

==^S=S1 450-500

I80-2OO

no -180

30- 4-0

350-4-00

GENERAL CHARACTER

Moris, cloys, soft sandstones
ond shales, predominantly
red, but in port gray, yellow
green and purple.

White on 3 pink so-idstone wrth
thin layers of snole; surface
cavernous.

Arkosic sandstone ond flna
conglomerate, with thin beds
ofyeilow cloy; loner beds
weother poieye/iow.

P.rl zho^. I J'i arty friotle sor^rsto^

o"d cool in alternate layers.

Massive soodstom.sholy in lower po t

Yellowish pray to dork grey
shales, witn zone of impure
limestone concretions.

Bluish gray shales ot base,
grading upward through papery
shales to sandy shc/es

Gfwbh *h;te b cc ■ a^'c~>TZvs j r

Dark gray shole, coppec 'by y&.'cuish ss.

Thin-beetled dove colored limestone.

Gwtobhdt sh ¥t/UiumuttHjus v ~~ '-~<g

Dense, finegrained sandstone.
Coarse, yellowish gray ss., overtoin

by th.>n bed of gray shale .

w< te •

QrrJV She
pii and <

. e scrdste~*f p ii ond yneen Stales
org' hard fine a Tzineo' li —cstone .

Fig. 25.10. Mesozoic and Tertiary formations of Huerfano Park and vicinity, Colorado. From
Plate 3, Burbank and Goddard, 1937.

COLORADO AND NEW MEXICO ROCKIES

399

of depressions between bordering ranges and plateaus or platforms. The
basins are known collectively as the Rio Grande depression. See Fig. 25.11
for names and locations of the principal physiographic features. The
ranges are the result of Laramide deformation first, and later graben
faulting which has resulted in the general depression. This north-south
belt is generally recognized as the southward continuation of the Rockies
from Colorado, as well as a graben or rift belt of late Cenozoic
age. The Colorado Plateau lies on the west and the Great Plains on the
east.

The basins of the Rio Grande depression have an approximate en
echelon arrangement, and the principal ones are as follows beginning on
the north in southern Colorado: 1, San Luis; 2, Espafiola; 3, Belen-Al-
buquerque; 4, San Marcial; 5, Engle; and 6, Palomas. The northern end
of the Belen-Albuquerque basin is called the Santo Domino basin. The
Estancia basin is separated from the Glorieta Mesa by normal faults, and
although shallow and irregularly alluviated, it is probably part of the
rift belt. The Jornado del Muerto basin, however, is not part of the rift
belt, but mainly a Laramide downwarp between two Laramide uplifts.
The Tularosa Valley is a downfaulted basin, and although not part of
the Rio Grande depression, is associated with it tectonically, and is part
of the general rift belt.

Major Laramide Structures

If the map of Fig. 25.11, and particularly the Geologic Map of New
Mexico ( 1928 ) are studied, a major anticlinal uplift is suggested by the
geology of the Sacramento Mountains front, the Oscura Mountains, and
the San Andres Mountains. The Tularosa Valley, which is mostly a down-
faulted basin, appears to have formed essentially in the core of the large
uplift. The uplift evidently had folds within it because several small
islands of Pennsylvanian strata appear in the alluvium well out in the
basin, and therefore, one cannot assume that erosion had stripped the
central part of the broad uplift everywhere to the Precambrian before
the graben faulting occurred. See cross sections S and T of Fig. 25.16.
The postulated uplift is labeled the San Andres in Fig. 25.12.

It has been assumed by some geologists that the westward tilted strata

Fig. 25.12. Laramide uplifts and late Cenozoic belts of rifting around the Colorado Plateau.
The uplifts with extensively exposed Precambrian cores are shown by double line; those with
Paleozoic core principally, by single line. Refer to Fig. 25.8 for New Mexico, and Fig. 25.2 for
Colorado. Rift valleys stippled.

SIERRA DE
LOS VALLE5

Kma ^ Jt_Jg>

SIERRA NACIMIENTO

Cm_9 Cmq.

^^fy^W^^^^^'PfSf'Mjf'"'^''c^^_

Cmg>

SSPANOLA VALLEY
IS

MESA CMIVATO

Quaternary hosa/t

Mesa Verde fm

Mane OS sh.

Dakota J3.

-r^

pre- Dakota fms.

T^ V / " V Jl] P\ continued

I below

iT>

(,o\o

CEJA DEL P1I0 PUERCO

SIERRA LADRON

Fig. 25.13. H, section across the northern Sierra Nacimiento. After Renick, 1931.

I, section across the southern Sierra Nacimiento. After Renick, 1931. Cmg,
Magdalena group; Ca, Abo sandstone; Cc, Chupadera formation; Teh, Chinle
formation; Jw, Wingate sandstone; Jt, Todilto format/on; Kms, Morrison formation;
Kd, Dakota sandstone; Km, Mancos shale; Kmv, Mesa Verde formation; Qv, Qauter-
nary volcanoes of the Sierra de los Valles.

J, section across Espanola Valley. After Denny, 1940b. Ts, Santa Fe formation.

K, section from Mesa Chivato to Ceia del Rio Puerco. B, Boundary between
Colorado Plateau and Basin and Range provinces. After Bryan and McCann, 1938.

M, section across the Lucero uplift. After Kelley and Wood, 1946. Cs, Sandra
formation; Cml, Gray Mesa member; Cma, Atrasado member; Cmr, Red Tanks
member of the Madera limestone; Pa, Abo formation; Pym, Meseta Blanca member;
Pyl, Los Vallos member of Yeso formation; Psg, Pse, Psl, members of San Andres
formation. C is Pennsylvanian, P is Permian. Ti, Tertiary intrusives; Tv, Tertiary
extrusives; Tv, Tertiary agglomerate; Ts, Miocene Santa Fe formation.

N, section through Sierra Ladron and the southern end of the Sucero uplift, here
a structural basin. After Kelley and Wood, 1946. Cs, Sandra formation; Cml, Gray
Mesa member; Cma, Atrasado member; Cmr, Red Tanks member of the Madera
Limestone; Pa, Abo formation; Pym, Meseta Blanca member; Pyl, Los Vallos member
of Yeso formation, P is Permian. Ti, Tertiary intrusives; Tv, Tertiary agglomerate;
Ts, Miocene Santa Fe formation.

O, section of the San Acacia area. Tv, Tertiary volcanic flows and tuffs; Tp,
Popotosa formation; Ts, Santa Fe formation. After Denny, 1940a.

SCALE IN MILtS

COLORADO AND NEW MEXICO ROCKIES

401

in the San Andres Mountains and the eastward tilted strata in the Sac-
ramento front are due to the block faulting, and hence that the ranges
are entirely late Cenozoic or Rasin and Range in age. If late Cenozoic
sediments are found to rest on Mesozoic beds on the floor of the graben,
then this is the proper interpretation, but if the basin fill rests on Pale-
ozoic and Precambrian beds, which seems to be the case, then the
uplift is much older than the faulting, and would be considered Lara-
mide.

Another fairly evident large uplift is indicated by the geology of the
Ladron and Lucero Mountains, Puerco Platform, and Nacimiento Moun-
tains on the west, and the Los Pinos, Manzano, Manzanita, and Sandia
Mountains on the east. Precambrian rock appears to have been ex-
tensively exposed in the core before graben faulting of the Relen-Al-
,buquerque basin. The uplift is called the Sandia, in Fig. 25.12 after the
imposing Sandia Mountains.

, A thrust fault of Laramide age runs along the west side of the Naci-
miento mountains, and it has resulted in the Precambrian crystallines
.resting on the Cretaceous (Renick, 1931; Wood and Northrop, 1946). Ex-
famine cross sections H and I of Fig. 25.13. The thrust is of fairly high
^ angle along most of its length, but at the north end it has several im-
bricate slices that dip at low angles. The maximum stratigraphic throw
;(is 3500 feet (Wright, 1946).

A thrust that flanks the east side of the south end of the Sierra Naci-
miento is shown by Renick ( 1931 ) but not by Wood and Northrop ( 1946 ) .
See section I, Fig. 25.13. The elevated block between the two opposing
thrusts contains Paleozoic strata that are somewhat folded and faulted.
Northward, the general structure of the range is an asymmetrical faulted
uplift, the west flank being composed of steeply upturned and overthrust
beds.

A broad, faulted monocline with downthrow on the east (see Fig. 25.14)
leads southward from the Sierra Nacimiento about 45 miles to the Lucero
uplift, where again thrusting has been recorded. The thrust on the east
front of the Lucero uplift extends from the Ladron Mountains northward
30 miles to Carrizo Arroyo, where it dies out in a tear fault (Kelley and

Fig. 25.14. Faulting of the monocline between the Sierra Nacimiento and the Sierra Lucero.
After Hunt, 1938. Ku, Cretaceous shale and standstone; Jm, Jurassic standstone.

Wood, 1946). The thrust dips westward at angles ranging from high to
low with the west side, the Lucero uplift, overthrust eastward. The
stratigraphic displacement ranges from 1000 to 4000 feet.

The connecting monocline and its faults may have come into existence
later than the Nacimiento and Lucero thrusts (Wright, 1946), but defi-
nitely before Miocene time (pre-Santa Fe formation, Upper Miocene).

Disconformities between the Ojo Alamo sandstone and the Torrejon
formation, and between the Torrejon and the Wasatch, point to the be-
ginnings of uplift in latest Cretaceous and early Paleocene time ( Reeside,
1924). Probablv the main uplift and thrusting occurred at the beginning
of Eocene time, preceding the deposition of the Eocene Wasatch, and
continued for some time during its deposition.

From the above it is evident that the postulated large Sandia uplift was
not a simple anticline, but that it had small thrust structures within it,
such as the Lucero, and possibly folds. Also the development probably
proceeded in phases from latest Cretaceous into the Tertiary.

402

STRUCTURAL GEOLOGY OF NORTH AMERICA

The Sangre de Cristo Range in New Mexico is less complicated than in
Colorado. The eastern front resembles that of the Front Range of Colo-
rado (Smith and Ray, 1941), and the flat-lying sedimentary formations
of the plains are abruptly upturned along the mountain front, and the
Dakota sandstone makes prominent ridges. See section E of Fig. 25.15.
A normal fault follows the contact between the sedimentary strata and
the Precambrian core of the range for more than 7 miles.

Still farther south, the structure becomes a broad arch out of which
Glorieta Mesa is now eroded. See sections F and G of Fig. 25.15. The
east flank is fairly sharply flexed at the north end, but toward the south
through Cuervo Butte the arch is broad and regular. The low Pedernal
Range, once one of the Ancestral Rockies, is covered in the Glorieta Mesa
area; the overlapping of the Pennsylvanian strata on its north end is pic-
tured in the lowest cross section. The faults along the west side of the
Glorieta Mesa are younger than the arch and flexures, and are classed
as Basin and Range. They probably resulted in the valley fill of the
Estancia basin.

The extent of the original Sangre de Cristo uplift in Laramide times is
difficult to decipher because of the graben faulting and the extensive
volcanism. The rendition of it in Fig. 25.12 is very approximate.

Still another Laramide uplift, the Sierra, appears to have formed in
southwestern New Mexico. Throughout its extent chiefly Precambrian
and Cambrian rocks are exposed, but through it the major Rio Grande
depression now exists. On the west it probably became part of the ex-
tensive Laramide uplift southwest of the Mogollon Rim, but the region
is so extensively covered with Tertiary volcanics that the relations cannot
be well established.

The uplifts of the Colorado Plateau and the central Laramide belt of
Colorado and New Mexico as depicted in Fig 25.12 all seem to be re-
lated tectonically, and their origin by vertical uplift is emphasized. Most
geologists who have mapped in the Laramide belt of Colorado and New
Mexico have considered the thrust faults to indicate compressional orog-
eny, and especially intense compression in Colorado. The Williams Range,
Gore Range, and Never Summer Range thrusts in Colorado are probably
the most impressive, but these have in no respect the stratigraphic throw
of the thrust sheets of the central Rockies.

Vertical uplift of the magnitude of 2 or 3 miles is indicated by the
Front Range, and where the vertical movement has been abrupt along
one flank or the other, and steep fronts of imposing elevation have formed,
the mechanical elements for major gravity slide blocks are set up. The
uplifts of the Colorado Plateau where the Precambrian rocks are not ex-
tensively exposed represent a less amount of vertical movement, and it is
noted that thrusts have not formed on their margins. It seems logical to
the writer, therefore, to regard the thrusts of the Laramide Rockies of
Colorado and New Mexico as gravity slide phenomena.

In Chapter 33 on the igneous provinces of the western part of the con-
tinent the theory is advanced that the uplift of the Colorado Plateau and
other adjacent areas in the Rocky Mountains in Laramide time and after-
ward was due to expansion of a column of the mantle underneath, and
that this expansion was at least partly due to its partially melting. Also
considerable magma made its way up to the crystalline complex, and
there spread out in megasills to elevate the crust above as great blisters.
These are the uplifts shown in Fig. 25.12. The concept is illustrated in
Fig. 36.4. Lagging somewhat after the intrusion of the megasills came
the near-surface and surficial igneous activity, so widespread throughout
the Plateau and marginal areas.

Rio Grande Rift Belt

Kelley (1952) has reviewed the Rio Grande Rift Belt very well, and
summarizes his conclusions as follows:

In about middle Tertiary time volcanic activity that extruded rhyolitic to
andesitic rocks developed on an enormous scale. These eruptions, together
with their great outwash of alluvial material, accumulated to thicknesses of
several thousand feet. The volcanic suites occur mostly in the western half
of the Rocky Mountain belt and in the adjacent Colorado Plateau; but locally,
as in the Raton, Cerrillos-South Mountain, and Sierra Blanca areas, the erup-
tions developed along the Great Plains border. Nevertheless, the uplifts border-
ing the east side of the depression are notably lacking in this suite of rocks.
Littie or no sharp folding or overthrusting accompanied the volcanic episode.
High-angle faulting, however, appears to have accompanied and followed
the great igneous activity. In several places there appears to have been two
or three distinct volcanic stages separated by intervals of tectonic disturbance
and erosion. Although local basins of accumulation appear to have developed
during this epoch of Tertiary deposition and deformation, the areas of accumu-

3ANGRE DE
RANGE

CRI3T0

RATON BASIN
TV-

SIERRA GRANDE ARCM

GLO RIETA

MESA

Chupoaero frFT
/Ibo -55

J2&kofo

/ S/ \s \/\/

E5TANCIA
VALLEY

CUERVO
BUTTE!

ChuPadzra fm.

' v \ /\ / \ / s / \ y\/\/\/\/

■/ \ / \ / \ ' » " ' » ' '
/www w-~ PROBABLE
>ww NORTM END 0
V/> PEDERNAL RA

PIEDRA
PINTADA

5CALE IN MILES

Fig. 25.15. Upper section from Sangre de Cristo Range eastward across the Raton basin and
Sierra Grande arch. Modified after Darton, 1928. Cmg, Magdalena group; "6s, red shale and
sandstone; Jm, Morrison formation; Kpr, Purgatoire formation; Kt, Trinidad sandstone; Kv,
Vermejo formation; Tr, Raton formation.

Lower sections across the north and south ends of Glorieta Mesa, south of the Sangre de
Cristo Range, f, Triassic red shale and Jurassic, Wingate, Kayenta, and Morrison. Modified
after Darton, 1928. The fault on the west side of Glorieta Mesa is probably Basin and Range
in age, and younger than the Laramide.

404

STRUCTURAL GEOLOGY OF NORTH AMERICA

fXopifon reef

Guoaalupe series
Dell Canyon;? Cherry Canyon-; Brushy Ca

^-Wolfcomp series

MERRA CADALLO

FLORIDA MOUNTAINS

V NORTHERN QUITMAN MOUNTAINS, TCXAJ „0C.Y\^ ,0^ *0

lation appear to have been rather wide, and the troughlike aspects of the
later Rio Grande depression were not yet developed. In wide areas, the middle
Tertiary flows and pyroclastic and volcanic alluvial beds lie with only slight
unconformity or discordance upon the earlier non -volcanic sediments. The
intense fracture belt and prominent tilted blocks which are so characteristic
of the Rio Grande depression and adjoining uplifts are later features.

The development of the Rio Grande structural belt probably began in
late Miocene time and culminated . . . toward the end of Pliocene time. With
the development of the linked en echelon basins the Santa Fe sediments,
which are the characterizing feature of the Rio Grande depression began to

Fig. 25.16. Cross sections of south-central New Mexico and the trans-Pecos of Texas. See index
map, Fig. 25.1.

P, section across Chupadera Mesa. After Darton, 1928. Cm, Magdalena group; Ca, Abo sandstone;
Cc, Chupadera formation; "is, Dockum group; Kd, Dakota (?) sandstone; Km, Mancos (?) shale.

O, section across Sierra Blanca system. After Darton, 1928. Cm, Magdalena group; Ca, Abo
sandstone; Cc, Chupadera formation; Is, Dockum group; Kd, Dakota (?) sandstone; Km,
Mancos (?) shale.

R, section across the Zuni Mountains and the Cebolleta Mesa. Ca, Abo sandstone; Cc,
Chupadera formation; 'Em, Moencopi formation; "lis, Shinaru conglomerate; "lis, Chinle shale;
Jw, Wingate sandstone; Jt, Todilto sandstone; Jn, Navajo sandstone; Km, Mancos shale; Kmv,
Mesa Verde formation; Tb, basalt. After Darton, 1928.

S, section across the Sacramento Mountains. Cm, Magdalena group; Ca, Abo sandstone; Cc,
Chupadera formation. After Darton, 1928.

T, section from Guadalupe Point (El Capitan) eastward across Culbertson County, Tex. The
Delaware basin lies east of the section. After King, 1942a.

U, section across the Sierra Caballo. After Darton, 1928. C, Bliss sandstone; O, El Paso
limestone and Montoya limestone; S, Fusselman limestone; D, Percha shale; M, Lake Valley
(?) limestone; Cm, Magdalena group; Ca, Abo sandstone; Cc, Chupadera formation; K, Creta-
ceous beds.

V, section across Cooks Range. After Darton, 1928. C and O, Bliss sandstone, El Paso
Montoya, and Fusselman limestone; D, Percha shale; C, Lake Valley limestone, Magdalena forma-
tion; Pg, Gym limestone; \ Lobo formation; K, Sartan sandstone.

W, sections across the Florida Mountains. After Darton, 1928. C and O, Bliss sandstone
and El Paso; Om, Montoya, and Fusselman limestones; Pg, Gym limestone; II, Lobo formation;
Tag, Tertiary agglomerate.

X, north-south section across the San Juan basin. After Darton, 1928. C, Pennsylvanian and
Permian; 1, Triassic; J, Jurassic; Kmc, Mancos shale; Kpl, Point Lookout sandstone; Kmf, Menefee
formation; Kch, Cliff House sandstone; Kle, Lewis shale; Kpc, Pictured Cliffs sandstone; Kfd,
Fruitland formation; Kk, Kirtland shale containing Farmington sandstone member, Kkf; Kmd,
McDermot formation; Koa, Ojo Alamo sandstone; Tan, Animas formation; Tpt, Torrejon and
Puerco formations, Paleocene; Tw, Wasatch formation, Eocene.

Y, sections across the northern Quitman Mountains, Texas. After Huffington, 1943. Pbbr, Briggs
formation (Permian); Jmu, Malone formation (Upper Jurassic); the following formations are
all Lower Cretaceous; Kt, Torcer formation; Ky, Yucca formation; Kb, Bluff formation; Kc, Cox
formation; Kf, Finlay formation; Kes, Espy formation; Kc, Etholen formation; T-Q, Tertiary and
Quaternary alluvium.

COLORADO AND NEW MEXICO ROCKIES

405

form. The Santa Fe has been assigned to ages that range from late Miocene
to Pleistocene. In its typical development it is an alluvial-fan deposit of a
characteristic pinkish or light-tan color. Although it is locally grayish, it
i generally stands in fairly marked contrast to the somber brown, purplish-
brown, or grayish-white of the middle Tertiary sediments upon which it often
rests. The Santa Fe is typically a relatively non-volcanic sediment, but in
imany places, especially along the west side of the depression, its coarse frag-
iments may be almost exclusively volcanic, but even in these places the
characteristic pinkish color is evident in the clay and sand beds. The Santa Fe
i in large part reflects the rocks which were at the surface in the adjoining
uplifts, and the superposition of its local members commonly roughtly reflects,
iin reverse order, the stratigraphic superposition of the adjoining areas. In
many places where the adjoining uplift consisted of carbonate rocks such as
the Magdalena, San Andres, or lower Paleozoic formations, the adjacent Santa
Fe is largely a calcirudite fanglomerate. Elsewhere playa and lake deposits
form a large part of the Santa Fe. Pyroclastic breccia and tuff may be abundant
'in the Santa Fe, and this is especially true around the Jemez uplift. Basaltic
flows are almost a characteristic of the Santa Fe, and are intercalated sparingly
throughout the section.
j

CENTRAL NEW MEXICO PORPHYRY BELT

A zone of Laramide intrusions extends from Rlack Mountain in the
Sangre de Cristo Range southward through central New Mexico into
Mexico. It coincides with the belt of faulting and uplifts in the central
part of New Mexico. The intrusions take the form of stocks, laccoliths,
dikes, and sheets. The largest stocks are in the Sandia and Ortiz Moun-
tains just southwest of Santa Fe, and in the Sierra Blanca, Capitan, and
Gallinas Mountains, just north of the Sacramento uplift. Also, the Organ
Mountains northwest of El Paso are mostly of intrusive rock. These plu-
jtons are clearly intrusive into the Paleozoic strata, and in places into the
Upper Cretaceous, and all are believed to be Laramide. Like the uplifts,
most of them have not been accurately dated because the youngest rocks
intruded are commonly Paleozoic. See Chapter 36 for further discussion
of the igneous rocks.

GUADALUPE AND MARATHON UPLIFTS

An arm of the Sacramento uplift extends southeastward to the Texas
boundary, where the Guadalupe Mountains compose themselves and ex-

tend south-southeastward about 80 miles to the Davis Mountains volcanic-
area. The Guadalupe Mountains separate the salt basin on the west from
the Delaware basin on the east. See cross sections S and T of Fig. 25.16.
Like the other uplifts, they are an asymmetrical structure, an eastward-
tilted block with a complex zone of high-angle faults along the west side.
Examine the Tectonic Map of the United States.

The Davis Mountain volcanic area is in effect a large structural basin.
Along its northeast border is a belt of long, fairly gentle anticlines and
synclines that lead to the Marathon uplift, in whose core the late Paleozoic
compressional structures known as the Marathon Mountains are exposed.

The Marathon uplift is dome-shaped and surrounded by Cretaceous
beds. The exposed core is not centrally located in the Cretaceous dome; it
is mostly in the western half. According to King ( 1937 ) , the base of the
Lower Cretaceous strata 4 miles north and south is 5500 feet below the
base in the Glass Mountains that form the north rim. The dip of the Cre-
taceous beds to the east is about 100 feet per mile.

The western margin of the Marathon dome is formed by the Del Norte
and Santiago mountains, which are eroded out of a sharp monocline or
anticline. The fold is overturned toward the west and broken in most
places by an eastward-dipping thrust that has raised Paleozoic and Lower
Cretaceous rocks on the east against younger Cretaceous beds on the west
(King, 1937). This narrow belt of compressional deformation seems to
be alone in the belt of domes and basins of central New Mexico and the
Trans-Pecos region of Texas. About 75 miles to the southeast, the Sierra
Madre Oriental of Coahuila, Mexico, which is a system of folded and
thrust structures, continues in the strike of the Santiago thrust, and may be
a continuation of it. The Sierra Madre Oriental, however, seems more
related to the Laramide folding and thrust belt of southern Arizona,
southwestern New Mexico, and the Quitman Mountains region of Texas-
Mexican border, southeast of El Paso and northwest of the Davis Moun-
tains volcanic area. See the map of the Laramide orogenic belts. Fig. 19.1.

The Marathon dome is younger than the Lower and Upper Cretaceous
beds, but only part of the elevatory movement preceded the effusion of
the Davis Mountain volcanics, which are Eocene and Oligocene in age.
In one place, the lavas overlie Upper Cretaceous strata, and in another

406

STRUCTURAL GEOLOGY OF NORTH AMERICA

they rest on the Lower Cretaceous. Also, the lavas do not dip away from
the dome as steeply as die underlying Cretaceous beds, but since the
lavas have been tilted, part of the doming followed their outpouring.
Furthermore, King (1937) points out, an anticline that has folded the
Permian and Cretaceous rocks in the Glass Mountains extends north-

westward into the Davis Mountains, where it involves the Tertiary lavas.
The folds are definitely older than the normal faults that break the strata
in the Glass, Del Norte, and Santiago Mountains, and there offset the
lavas. The normal faulting appears to have taken place soon after the
lavas were poured out, possibly in Oligocene or Miocene time.

26.

COLORADO PLATEAU

GENERAL GEOLOGY

The Colorado Plateau is one of the world's show places, not only for
the tourist but for the geologist. Badlands, high escarpments, and deep
gorges leave few geologic secrets covered if they are searched for. Early
geologists such as Gilbert, Powell, and Dutton, who first explored the
Plateau geologically, made it classical territory. Their line drawings of the
physiographic features and their diagrams of the structures still stand as
masterpieces. The contributions in the years since these early investiga-
tions have been on the details.

A bird's-eye view of part of the province may be obtained from a
stereogram of Gilbert's (1877), reproduced here in Fig. 26.2. Pictures of

the Grand Canyon, Zion National Park, the natural bridges of San Juan
County, Cedar Breaks, and Bryce National Monument are commonplace
and serve to identify small spectacular parts of the great region. The
major structures of the plateau are shown in the index map of Fig. 26.1.

The paleotectonic and paleogeologic maps of this book show clearly
that the Colorado Plateau was a shelf area adjacent to the westward lying
Cordilleran geosyncline. Parts of Arizona were Precambrian terrane until
the Pennsylvanian. From Mississippian times to the close of the Creta-
ceous, shallow seas covered the entire province, with the exception of the
Uncompahgre and Zuni ranges of the Ancestral Bockies, which were
finally buried in Triassic time.

The Paleozoic section of the Grand Canyon is given in Fig. 26.4, and a
number of Mesozoic and late Paleozoic sections of various parts of the
Colorado Plateau are reproduced in Figs. 26.5 and 26.6. Beferences to de-
tailed stratigraphic studies in the plateau may be obtained from the two
figures.

The paleogeographic and tectonic development of the region in
Paleozoic time has been treated in Chapters 6 and 15.

In brief, the pre-Laramide history of the Plateau is as follows. When
the Cambrian seas invaded the area, a relief in places, at least, of about
800 feet existed (Sharp, 1940), and the surface was entirely buried by the
Cambrian sediments. In general, the absence of Ordovician and Silurian
strata throughout the Plateau, with only a disconformity marking their
place, indicates either gently emergent conditions during these periods
or that, toward the close of the Silurian, the region became emergent,
and any beds that were deposited during the interval were removed. The
Mississippian was one of limestone deposition, but the Pennsylvanian was
one of considerable crustal unrest with the building of the Ancestral
Bockies and the subsidence of the Paradox basin. The western margin
of the Plateau was the transition from shelf to miogeosyncline in Paleozoic
time and later in Cretaceous and early Tertiary time the site of mountain
building and accumulation of tiiick orogenic deposits. See Chapter 22 on
the Central Bockies.

Although horizontal strata dominate the landscape, several monoclines
were formed which are the steep flanks of large asymmetrical anticlines,
some 30 miles across and 100 miles or more long.

407

408

STRUCTURAL GEOLOGY OF NORTH AMERICA

. IDAHO

UTAH ' |

Fig. 26.1. Major geologic features of Colorado Plateau. Black areas are lava fields; stippled
areas are Early Tertiary sediments; horizontally dashed areas are Cretaceous sediments; and
cross-ruled areas are laccolithic mountains. S.R.S., San Rafael Swell; C.C.U., Circle Cliffs uplift;
S.L.C., Salt Lake City; P., Provo; F., Filmore; B., Beaver; Fl., Flagstaff; PC, Prescott. Lava fields
in the Great Basin not shown, especially in the St. George area.

Another significant type of structure in the Colorado Plateau is the
laccolithic mountain. There are several clusters of laccolithic intrusions,
and these have produced the mountains known as the Henry, La Sal,
Abajo (or Blue), La Plata, Ute, and Carrizo. See index map, Fig. 26.1
and Fig. 33.7. Also, another high mountain, Navajo, is probably a lac-
colithic structure. There are several major volcanic fields, one in the High
Plateaus of Utah; one just south of the Grand Canyon in Arizona, the
San Francisco Mountains; and one in eastern Arizona and western New
Mexico, the Datil field. The flexures, laccoliths, volcanic fields, and other
features are described in the following pages.

ASYMMETRICAL ARCHES AND BASINS

In a very broad way the Monument uplift, with Permian beds exten-
sively exposed in the core, is the center of the Plateau, and is nearly sur-
rounded by Cretaceous and Tertiary basins. Auxiliary uplifts break the
continuity of the surrounding basins or render the general pattern ir-
regular (Figs. 26.1 and 19.2). The San Rafael Swell, the Circle Cliffs
uplift, and the Uncompahgre uplift lie just inside the Cretaceous and
Tertiary basins on the west, north, and northeast, and the Kaibab (Fig.
26.8) and Defiance (Fig. 26.9) uplifts break the continuity of the Cre-
taceous basins on the south and southwest. The uplifts in Utah and the
Kaibab in Arizona are characterized by a sharp monoclinal flexure on the
east side, broad tops, and gently dipping west flanks (Fig. 26.7). In the
high, desert climate of the Plateau these monoclines are grand features
of the scenery. The arches are all believed to be of Laramide age, and
are crossed indiscriminately by the master streams that drain the Plateau.
The streams are, therefore, either superposed or antecedent. The greatest
scenic spectacle in the Plateau, in the minds of many people, is the gorge
of the Colorado River (Grand Canyon) across the Kaibab uplift. The
river here has cut through the entire Paleozoic section and also well into
the Precambrian.

The Cretaceous and Tertiary basins, aside from the Black Mesa of
Arizona, are bounded on the outside by major Laramide uplifts, and the
deepest parts or troughs of the basins he close to these uplifted and
mountainous areas. The Tertiary section in the Uinta Basin is 10,000

^ V?ii..A>H:

J iiv-^CV

^^M — -

! '■ T

)

0 '« %

r ...

-^V ! « U j < V**-*»J Phoiographotf fron. amodc

Trin ngtil-tnon by A H
Thompson.

Sc.L ofMiltv

STERKO G R A \1
HkISRY MOI M AINS
WATERPOCKET FOLD

o »

; t

Fig. 26.2. Waterpocket monocline, Henry Mountains, and parts of the Kaiparowits and Aquarius
plateaus and the Colorado River, all in Utah. Reproduced from Gilbert, 1877.

Fig. 26.3. Waterpocket monocline and Henry Mountains as if no erosion had occurred since
the folding and intrusions. The same area is depicted structurally in this illustration as physio-
graphically in the opposite illustration. Reproduced from Gilbert, 1877.

409

Fig. 26.4. Generalized columnar section of rocks forming the walls of the Grand Canyon of the Colorado.
After Noble, 1924.

410

Wasatch 0 formation

Generalized section from

southwestern part of Kaipo-

rowits Plateau to upper

part of Marble Gorge

Top of plateau

pahota(7)aar>d6ton<».?5-B0rt

Wahweap
sandstone
L200-i,290fe*t

Stra.ghtCl.ffs
sandstone
950-1,000 feet

Tropic shale'
630 feet

Entrad* sandstone ,205 feet

Wingate sandstone
450 feel

Chinle formation
850-980 feet

Shin a rump congl ,0-1 15 feet

Coconino sa ndstone. 0-9 3 Teet

— — Supai formation

D
O

a'
u
O

_ — —

SSI

Kaiparowiti
formation
2,00 01 feet

£==3

----

Wahweep
sandstone
1,300 feet

^_"^L_-

Straight Cliff*
sandstone
L250 feet

Tropic shale
640 feet

^Dakota r) sands ton*F5D - feet

b«

» . A!? '

Morrison format.on. 300 *ft.

In. J

: . _ — :

5ummervilt#form«t.oi\t00ift

Errtrada sandstone
650 feet

Carmel formation. 170 feet

i. -i - -

vajo sandetona
3O0!fe«t expoaad

Generalized section of lower
San Juan River Canyon

Generalized section of south-
eartern and north-central
_, parts of Koiporowits
'' Plateau along Last Chance
Creek to Table Cliff Plateau

EXPLANATION

Morrison formation, 250 i feet

Shale and aandstoneJ*0-2?0rt

Navajo sandston
675 feet

YodirtoWforoation.lOOtft

Chinle formation
1,000 feet

Shinarumo congl.,Q-2?0n

MoenUopi formation
840-1,010 feet

Goodridge formation
1.338 feet

Limestone Limestone con- Dolomite
tfl.n.ngnodules
of chert or flint

gSSS

■■■ •■■ - -

= '~-~ a

Shale Sandy shale Conglomerate Gypoum

Generalized lection ot rocks
in »ovthwe«tern Colorodo

Generalized lecfion of Orel*

Cliffs. Woterpocket Fold,

ond Henry Mountains

Unconformity

Fig. 26.5. Paleozoic and Mesozoic formations of the Kaiparowits Plateau and lower San Juan
River Canyon. After Gregory and Moore, 1931.

Fig. 26.6. Paleozoic and Mesozoic formations of Circle Cliffs uplift, Henry Mountains, San
Rafael swell, Wasatch plateau, and southwestern Colorado. After Gregory and Moore, 1931.

411

/ ////^yr^vm^^NTA'.- BASIN,

Fig. 26.7. Tectonic map of central part of Colorado Plateau. Compiled by Shoemaker, 1954.

412

TERTIARY SEDIMENTS

DIATREMES

TERTIARY VOLCANIC ROCKS

LACCOLITHS ANO SILLS

UPPER CRETACEOUS TUFF

INTRUSIVE MASSES
OF SALT ANO GYPSUM

THICKENED ROLLS OF SALT ANO
GYPSUM NOT INTRUSIVE BEYOND
NORMAL STRATIGRAPHIC POSITIONS

STRUCTURE CONTOURS ON TOP
CHINLE

INTERVAL 500 FEET

COLORADO PLATEAU

413

GRAND CANYON

COCONINO PLATEAU

COCONINO

PLATEAU

CNANOVif* SCCTION

Of thc cast kaibab monocline

GRAND CANYON "" 'A,SAB

ocscit view sccrioii

UONOCLIHC

CCDAR HCSA
CCOM K£Si

2 ~^-—

Or ~r~^_
Cbo "l^^
cT^^-h

Pre- C p^

sccno*

M4R8LC

-^^^^:r^""-z~-^> ::.'-"-"'--

;:::::r::-"---::-":iI-irrr"-----r"-Vc>yv____

■ T^nt^-—

■ ~ Cf_

■ ' Ct

— ■♦■!- ~

Pre-C

— ; — ^^H-A- '!-,

===== -^-V-^^^^^uy

COCONINO PLATEAU

SXIfcNER RiDCf.

CHANOVICW SCCTION Of THC
CAST HA IB A a UONOCLINC

UPPER 8ASIH

WATCHUX) HILL SCCTION of
ThC CAST KAJBAB UONOCLPlC

Fig. 26.8. Cross sections from north to sou'h of the Kaibab uplift and East Kaibab monocline. After Baben-
roth and Strahler, 1945. Cr, Redwall Is.; Cba, Bright Angel sh.; Ct, Tapeats ss.

feet thick (Fig. 26.10). A great Eocene fresh-water lake was impounded
by the crustal movements around the Uinta arch, and in it the petro-
liferous strata of the Green River formations were depoisted. The Upper
Cretaceous and Lower Tertiary strata of die High Plateaus are very thick
(Chapter 22), and the Cretaceous of the San Juan basin is about 10,000
feet in maximum thickness (Fig. 26.11).

The major monoclines displace the strata vertically 5000 to 8000 feet.
They die out gradually at the ends, and curve in toward the uplift. These
features have been taken to mean that the Precambrian basement was
upfaulted and that the flexible sedimentary beds were draped over the
faults ( Baker, 1935 ) . In places, the faults break through to die surface.
Small faults are numerous in the uplifts and basins.

4000'

About 90 miles

After Onion. 1925

Cross section of Mogollon Rim, illustrating overlap of Upper Cretaceous formations southward onto
Permian formations.

6oocy-

4000'-
2000'

J L

j L

10 Miles

Modified after Gregory, 1917

Te, late Tertiary lavas; T, Chuska sandstone; Kmv, Mesaverde group; Kmc, Mancos shale; Jm, Morrison
formation and San Rafael group; Jgc, Glen Canyon group; "fi, Triassic formations; P, Permian formations

Cross section of the Defiance uplifts.

WSW.

^«*fc2#S&* .^tfom***

S^SS^3^i2^8'^^=?rJ=^ zuNIJ^°_^™*s .^r^__

ENE.

Recent ;. Old lava

Todilto limestone
gate sandstone
le formation
Moenkopi formation
■Yeso and San Andres
After Darton, 1922 formations

HORIZONTAL SCALE
0 5

10 MILES

VERTICAL SCALE
5,000 0 5,000
l i i i i ■ '

10,000 FEET

Section across the Zuni Mountain upwarp, New Mexico.
Fig. 26.9. Some uplifts of the Colorado Plateau. Reproduced from Hunt, 1956. Chuska ss. is Pliocene in age.

COLORADO PLATEAU

415

STRUCTURAL CROSS-SECTION OF THE UINTA BASIN, UTAH

ELEVATION 10.000 FEET

HORIZONTAL SCALE IN MILES

HYPOTHETICAL DIAGRAM OF UINTA MOUNTAIN-BASIN RELATIONSHIPS
AT DEPTH

----------

HORIZONTAL SCALE IN MILES

Fig. 26.10. Section of the Uinta basin approximately through Vernal and southward along the Green
River. Compiled by Orlo Childs and P. T. Walton.

SALT ANTICLINES

A zone of flexures and faults extends in a northwest-southeast direction
from east-central Utah into west-central Colorado. They consist basically
of eight elongate anticlines variously modified by collapse folds and
normal faults. Their position and orientation are shown in Figs. 26.7 and
26.12. Between the anticlines are simple and gentle synclines. Stokes
(1948) observes that their strike is common with that of the Uncom-

pahgre Range of the Ancestral Rockies and suggests that this relation
points to a system of deep-seated breaks in the basement rocks. The prob-
lem is very complex, however, because the faults and folds (the Meander
anticline) have a northeast direction, and the great monoclinal flexures
a northerly trend.

It is fairly certain that the structures are closely connected with salt
flowage and solution. At certain times and places, the salt and gypsum
have acted as ordinary rock and have suffered the same deformation as

I s.

Modified after Reeside. 1924

Tw, Wasatch formation; Tn. Nacimiento formation (Puerco and Torreion faunal zones)-. Tan, Animas formation; Koa; Ojo
Alamo sandstone; Kmd, McDermott formation; Kkf, Fruitland formation and Kirtland shale; Kpc, Pictured Cliffs
sandstone; Kle, Lewis shale; Km, Mesaverde group; Kmc, Mancos shale

Diagrammatic section across the San Juan basin from Durango, Col., to Pueblo Bonito, N. Mex.

SW.

spoo'

Horizontal scale

After Darton. 1922

-j ■ ■

K, Cretaceous formations; Kmv, Mesaverde group; Kms, Mancos shale; Kd, Dakota sandstone;
Jn, Navajo sandstone; ^w, Wlngate sandstone; Tic. Chinle formation; lis. Shinarump
conglomerate; 'Rm, Moenkopl formation; PAL, Upper Paleozoic rocks

Sections across the Gallup-Zuni basin, New Mexico. A, near Zuni; B, near Atarque.

Fig. 26.11. Some basins of the Colorado Plateau. Reproduced from Hunt, 1956. For the San Juan basin
see also Fig. 25.13.

COLORADO PLATEAU

417

contiguous strata, but at other times and places the saline beds have be-
haved independently and, through plastic flow and solution, have pro-
duced large-scale structures in the overlying beds that have no expression
in the underlying strata.
Taking the Gypsum Valley anticline as an example, Stokes ( 1948 ) re-

5T A GE I

jTAGE 2

stage: 3

5 TA6E A

Fig. 26.12. Sketch map of eastern Utah and western Colorado showing principal structures due
to salt flowage and solution. Reproduced from Stokes, 1948.

Fig. 26.13. Development of the salt anticlines in the Colorado plateau. Reproduced from Stokes,
1948. Vertical scale exaggerated. C, Hermosa formation; P, Permian formations; \, Triassic
formations; Jgc, Glen Canyon group; Jsr, San Rafael group; Jm-Kd, Morrison to Dakota forma-
tions; Km, Mancos shale; Kmv, Mesaverde group.

cords the following development (see series of cross sections in Fig. 26.13).

1. Deposition of salt and gypsum in Paradox formation of late Penn-
sylvanian age. Deposition of covering Hermosa limestone beds.

2. At end of Pennsylvanian or in early Permian time, the salt pushed
upward and domed the Hermosa; the Hermosa was eroded and the
salt exposed.

3. The late Permian Rico and Cutler formations were deposited around
the dome or anticline.

4. The salt dome was eroded nearly to a peneplain by late Triassic, and
the Triassic and early Jurassic formations overlapped across the edges
of older formations around the dome.

5. The late Jurassic sediments practically submerged the salt mass, and
then the late Cretoceous formations were deposited undisturbed o\ er
the site of the salt mass. These five steps are all recorded in the first
cross section of Fig. 26.13.

418

STRUCTURAL GEOLOGY OF NORTH AMERICA

MILES

Fig. 26.14. Ground plan and cross sections of intrusive bodies of Mt. Ellen, Henry Mountains,
Utah. 1, Corrall Ridge laccolith; 2, Durfey Butte laccolith; 3, South Creek Ridge laccolith; 4,
Slate Creek laccolith; 5, Cooper Ridge laccolith; 6, Granite Ridges laccolith; 7, Butler Wash
laccolith; 8, Mt. Ellen stock; 9, shatter zone; 10, Ragged Mountain bysmalith. Cross sections il-
lustrate relation of a laccolith, such as No. 2, to the sedimentary rocks, and also of a bysmalith,
such as No. 10. The top of the stock has been cut off to show the shatter zone and hard rock.
The lower cross sections are somewhat enlarged views of the igneous bodies 2 and 10, and
show the relations to the sedimentary beds. After Hunt, 1954.

6. In the early Tertiary, the entire section above the salt was arched up,
and the salt intruded slightly but not more than through the late
Jurassic formations.

7. The entire Colorado Plateau was elevated in mid-Tertiary time, and
coincident with the uplift the laccolithic intrusions occurred which are
described under the next heading. As a result of the uplift, ground-
water commenced to circulate more freely, the salt masses were sub-

jected to solution, and collapse of adjacent and overlying strata
occurred.
8. With the uplift of the plateau, several partial cycles of erosion have
followed, and the salt anticlines have been excavated and in places a
gypsum residue over the salt exposed. See last diagram of Fig. 26.13.

LACCOLITHIC MOUNTAINS

The Henry Mountains were first described by Gilbert ( 1877 ) , and with
his publication they became classical for the laccolithic type of mountain.
He pictured the laccolith as a tack- or mushroom-shaped, concordant plu-
ton — as a centrally thickened sill which has arched the beds above it and
has been fed through a conduit from below.

Hunt (1954) has restudied the Henry Mountains in detail and shows
them to be concordant tongues extending outward from a central, trunk-
like stock. They are like semicircular, conical fungus growths on tree
trunks. The largest cluster is illustrated in Fig. 26.14. Ninety-five percent
of the intrusive rock is diorite porphyry, and the rest is monzonite
porphyry. According to Hunt,

The Henry Mountains are located in the structural basin that is one of the
major folds of the Colorado Plateau [Fig. 26.15]. The basin is the antithesis of
the adjoining Circle Cliffs uplift and San Rafael Swell, being of the same size
and form, only inverted. The basin is sharply asymmetric and its trough is
crowded against the steep west flank; the deepest part is 8,500 feet structurally
lower than the neighboring uplifts.

Each of the divisions of the Henry Mountains is a structural dome several
miles in diameter and a few thousand feet high. In general, the domes have
smooth flanks, but on most of them are superimposed many small anticlinal
noses that were produced by the laccoliths. At the center of each of the domes
is a stock, around which the laccoliths are clustered. The stocks are cross-cutting
intrusions, mostly surrounded by a shatter zone, which consists of highly
indurated sedimentary rocks irregularly intruded by innumerable dikes, sills,
and irregular masses of porphyry.

The dome of Mount Ellen, the largest dome in the Henry Mountains, has a
broad, plateau-like upper surface that is marked with many small anticlinal
folds. On this mountain the laccoliths were injected in all directions from the
stock. The Hillers dome is the highest and steepest of the domes and the anti-
clinal folds over the laccoliths around it are restricted to the north and northeast

420

STRUCTURAL GEOLOGY OF NORTH AMERICA

sides. Mount Pennell is similar to Mount Hillers, only smaller. The Holmes
dome is broken by faults, and its top is wrinkled with some minor anticlinal
noses. The Ellsworth dome has no anticlinal folds to mar its symmetry, although
it is broken by a few faults.

Several lines of evidence indicate that the laccoliths were injected radially
from the stocks: (1) the laccoliths are tongue-shaped in plan and make a
radial pattern around the stocks; (2) dike-like ridges on the roofs of the lac-
coliths trend away from the stocks; (3) the laccoliths are bulged linearly and
the axes of the bulges radiate from the stocks.

Coherence and competency of the invaded rocks appears to have been an
important factor controlling the stratigraphic distribution of the laccoliths. Pre-
Jurassic formations, estimated to be about 5,000 feet thick, consist of well-
bedded, relatively coherent, alternating thin competent and incompetent units
in which very few laccoliths were injected. Overlying this is 1,200 feet of
competent and highly coherent sandstone of the Glen Canyon group (Wingate,
Kayenta and Navajo formations), containing even fewer laccoliths than the
underlying zone. Overlying this sandstone is the San Rafael group and lower
half of the Morrison formation of Jurassic age, aggregating a thickness of about
1,000 feet, and consisting of incoherent, incompetent, poorly-bedded rocks and
interbedded competent layers. About 15 per cent of the total volume of the
laccoliths is in these formations. The sequence including the upper half of the
Morrison and the Cretaceous formations, aggregating a thickness of about 2500-
3000 feet, consists largely of incoherent, incompetent shale in very thick units
separated by thin competent layers. By far the greatest number of laccoliths,
and at least 70 per cent by volume, are in this zone. Through this zone the
laccoliths are concentrated along the thin competent layers.

Because the stratigraphy and structure of the Colorado Plateau is fairly
uniform, the similarity in form of intrusion, geologic structure, and igneous
rock types at the several laccolith mountains in the Plateau reflect close simi-
larity of the igneous processes involved. The mountains are believed to repre-
sent a series of examples of one igneous process that was arrested at various
stages of completion.

Navajo Mountain, a structural dome containing no exposed igneous core,
represents the least advanced stage of the process. Mount Holmes, whose dome
is slightly higher than Navajo Mountain and whose center contains a small
stock from which a moderate number of dikes, sills, and very small laccoliths
radiate, represents the next more advanced stage. The process is still farther
advanced at Mount Ellsworth, where the doming is steeper and higher than at
Mount Holmes and where a moderate size stock surrounded by a shatter zone
occupies the center of the dome and abundant dikes and sills intrude the flanks.
On Mounts Pennell and Hillers which illustrate the next most advanced stage,
the doming is much steeper than on Mount Ellsworth, the stocks at the centers
of the domes are much larger, the flanks of the domes contain abundant dikes
and sills and in addition, to the north and northeast, huge linear, tongue-like
laccoliths were injected. The dome of Mount Ellen covers a much greater area

than the domes of the other mountains, and the laccoliths radiate in all direc-
tions from the stock (Hunt, 1954).

The La Sal Mountains are made up of three separate masses both
topographically and geologically, and each mass consists of a stock and
a cluster of radiating concordant intrusions into the sediments (lacco-
liths). According to Hunt (1958):

The intrusions are in the midst of a series of salt anticlines and synclines
whose axes trend northwest. Although the folding and attendant faulting in
the area around the La Sal Mountains are chiefly the result of late Late Cre-
taceous or early Tertiary deformation, the structural history is complicated
because there has been repeated plastic deformation of the salt beds and the
strata arched over them. These structures antedate the intrusions and are
not believed to be causally related to them.

North La Sal Mountain is located on an anticline, South La Sal Mountain
is in a faulted syncline, and Middle La Sal Mountain is in an area of gende
homoclinal dips between these two structures. The North La Sal Mountain
forms a dome 10 miles long and 5 miles wide, and the uplift on it exceeds
6,000 feet. This dome is greatly elongated northwesterly, parallel to the axis
of the anticline in which it is located. The Middle Mountain dome is nearly
circular in plan, about 5 miles in diameter, and about 3,500 feet high. At
Mountain dome is 6 miles long, 4 miles wide, and about 6,000 feet high. At
the center of each of these domes is a stock, and radiating from each stock
are laccoliths. The domes are attributed to the physical injection of the stocks.
In the North and South La Sal Mountains the laccoliths spread in the salt
beds of late Paleozoic age; in the Middle La Sal Mountain the laccoliths spread
in shale of late Cretaceous age.

The petrology of the laccolithic groups is discussed in Chapter 33, but
of interest here is the conclusion reached by Hunt that in the closing
stages of fusion, crystallization, and intrusion of the North Mountain
stock four pipelike masses of explosion breccias formed as diatremes were
blasted through the arched roof.

The Abajo Mountains consist of two unequal parts, the smaller and
northern division being an isolated dome known as Shay Mountain. It is
believed to be underlain by a stock. The southern and main division con-
sists of two parts, East Mountain and West Mountain, where central
stocks are exposed with surrounding shatter zones and clusters of radi-
ating laccoliths. A fourth stock is postulated although not exposed, and
altogether around the four stocks 31 laccolithic intrusions have been

COLORADO PLATEAU

421

mapped (Witkind, 1958). They are mostly intrusive into the Morrison,
Burro Canyon, Dakota, and Mancos formations.

UPHEAVAL DOME

Upheaval dome is a small circular structure of most peculiar and spec-
tacular nature. It is described by McKnight (1940) as follows:

The Upheaval dome . . . lies about 4 miles east of the Green River at the
head of a short canyon (Upheaval Canyon) that cuts through the Wingate
cliff (Fig. 26.7). It is circular in ground plan and consists of a conical dome
surrounded by a ringlike syncline. The diameter of the dome, measured through

• center from the axis of the syncline on one side to the axis on the other, is 2
miles. The outer flank of the syncline is uniformly half a mile wide, making
the complete diameter of the affected area 3 miles. Outside of the very sharply
defined line along which the strata dip in abrupdy toward the syncline, the
regional low dip to the north has been undisturbed. The inward dip on the
outer flank of the syncline is generally between 15° and 30°; the outward

i dip on the central dome ranges between 30° and 90°, though generally between
40° and 60°.

, Upheaval dome is considered by McKnight to be a salt dome rather

/than a cryptovolcano, because of the occurrence of thick salt beds at

moderate depth, and because deformation apparently took place slowly.

Gravity and aeromagnetic surveys related to the geology have led

Joesting and Plouff (1958) to the following conclusions:

1. Uplift totalling some 2,000 feet of comparatively dense, magnetic base-
iment rock at Upheaval Dome and Grays Pasture. The uplift took place before
!the deposition of the White Rim sandstone member of the Cutler formation of
Permian age, and may have coincided with tectonic activity during Pennsyl-
vanian and Permian times in other parts of the Paradox basin.
, 2. Formation of a salt dome centered at the present Upheaval Dome,
possibly controlled by the basement uplift. Plastic flow of salt continued,
probably intermittently, until late Triassic (Wingate time).
< 3. Further doming due to salt flow, possibly in Tertiary time, coincided
with renewed flow of salt in the nearby salt anticlines. The rim syncline formed
during this period as a result of thinning of salt around the dome and subsidence
of everlying beds.

4. Intrusion of igneous rock into the salt dome, probably coincident with the
late Tertiary igneous activity in other parts of the Colorado Plateau (Hunt,
1956, pp. 42-53). The igneous intrusion was comparatively small. It did not
displace all of the salt in the core of Upheaval Dome, but it may have been
responsible for some of the additional upward movement.

VOLCANIC FIELDS

Peripheral Fields

Several volcanic fields lie around the periphery of the Colorado Plateau:
the High Plateaus field in southwestern Utah, the Unikaret or Mt. Trum-
bull field of northeastern Arizona, the San Francisco field of north-
central Arizona, the Datil field of southeastern Arizona and southwestern
New Mexico, the Mt. Taylor and Jemez fields of northwestern New-
Mexico, the San Juan field of southwestern Colorado, and several small
fields in western Colorado. See particularly Fig. 33.7. These are all de-
scribed in Chapter 33.

Hopi Buttes and Navajo Volcanic Fields

Scores of volcanic necks, dikes, and lava-capped mesas rise from the
high plateau of northeast Arizona and the adjacent parts of Utah and New-
Mexico. These are the remnants of a volcanic field that formerlv covered
many thousands of square miles. Erosion has so far dissected this field
that the original cones have disappeared, the sheets of lava have been dis-
membered, and the old volcanic conduits now rise as giant towers, re-
vealing their inner structure with singular clarity (Williams, 1936). The
largest volcanic cluster is the Hopi Buttes, an isolated field 35 to 40 miles
on a side (Fig. 33.7).

The surface on which the flows of the Hopi Buttes area were erupted
was one of low relief and is now nowhere far from an elevation of 6000
feet. According to Williams (1936):

Such valleys as existed on this old surface must have been choked by showers
of ash during the opening stages of volcanic activity. The streams were re-
peatedly dammed, forming playas and ponds, which seem to have been united
ultimately into a lake of great extent. This may be spoken of as Hopi Lake. Its
deposits stretch as far north as Jedito Wash and south to the vicinity of the Five
Buttes, a distance of almost 35 miles; in an east-west direction they are trace-
able for 50 miles, from near Ganado to Dilkon.

Although the original cones have long since been removed, there is no
difficulty in recognizing where they once existed. Erosion, acting rapidly on the
surrounding sediments, has left the crater- and conduit-fillings as conspicuous
towers, the summits of which cannot be more than a few hundred feet below
the tops of the former cones. It is not surprising, therefore, that the intrusive
rocks are indistinguishable from the surface flows.

422

STRUCTURAL GEOLOGY OF NORTH AMERICA

At least 30 volcanic necks have been recognized. A few are isolated, but most
occur in clusters or are aligned in a northwest-southeast direction, parallel to
the adjacent dikes. The typical neck is circular in plan, though some are oval
and merge gradually with dikes.

The typical Hopi vent was opened by the explosive drilling of a cylindrical
pipe, and doubdess a pyroclastic cone or maar-like depression was formed
about the orifice. Subsequendy, upwelling lava filled the crater and finally
spilled over the rim in broad floods. The evidence of this history is abundant.
There is hardly a neck without a jacket of inwardly dipping pyroclastic debris,
made up of lava and sedimentary fragments than range in size from the finest
dust to blocks many yards in diameter. Normally, the dip of these ejecta in-
creases both upward and inward. Inbedded friction breccias bordering the
necks are extremely rare, and in general the enclosing sandstones and shales
remain undisturbed.

Hack ( 1942 ) has found that the explosive pipes or diatremes are nu-
merous, and that most of them occur in a dense cluster within an area of
800 square miles. He writes as follows:

In general the diameter of the vents decreases as erosion increases. In areas
where dissection has been slight, at levels above the Hopi Buttes surface, ex-
plosion pits are generally 3000 to 4000 feet in diameter. In many places, the
initial explosion pit is overlain by domes of lava which have pushed outward,
spilling over the sides, crumpling and pushing out the underlying and border-
ing tuffs and sediments. In a few places these lava eruptions were of sufficient
duration to form rather continuous flows. The more deeply dissected diatremes
range in diameter from 500 feet to 2000 or 3000 feet. In general the material in
them is less well bedded, and, if pyroclastic, is coarser.

The volcanism occurred in Pliocene time, because vertebrate bones of
that age have been found in the Hopi lake beds ( Bidahochi formation ) .

The igneous rocks of the Navajo region differ from those of the Hopi
Buttes because they contain a paucity of lava flows. Also, according to
Williams (1936):

. . . most of the Navajo volcanic necks are made up predominandy, not of
columnar lava, but of coarse tuff-breccia and are crowded with fragments of
plutonic rock, chiefly of granitic type. Petrographically, also, the two provinces
differ markedly; in the Hopi Buttes, monchiquitic rocks are typical, while in
the Navajo region these are far subordinate to minettes. Probably the Navajo
vents were more explosive. Indeed, many of them can never have erupted lavas.
How closely they resemble the well-known diatremes of the Schwabian Alb,
the Bhongebirge, and Central Scotland will be apparent from what follows.
Like those explosive vents, many of the Navajo volcanoes seem to be scattered

at random, without regard to pre-existing structures. None is located on a fault.

Monument Valley, with its fantastic, castellated crags, is carved from the De
Chelly sandstones and the Moenkopi shales that occupy the broad and gently
rippled top of a domical uplift, bordered on the south and east by the sharp
monocline of Comb Ridge and on the west by less-prominent folds that traverse
the Rainbow Plateau. On the summit of the upwarp, dips of more than 30
degrees are rare, but in the flanking folds they may reach 60 degrees. Many
intrusions are to be found along the Comb Ridge monocline, extending from
the village of Kayenta in an arc to the San Juan River. These tend to follow a
strong system of joints, approximately normal to the trend of the monocline. In
Monument Valley itself there is much less regularity in the trend and distribu-
tion of the intrusions.

All the necks rise boldly from the surrounding sediments, despite the fact
that they consist almost entirely of tuff-breccias. The few thin dikes which cut
the breccias are not responsible for this resistance to erosion; it results, rather,
from the compactness of the neck fillings, for the fine tuffaceous matrix has
been indurated by hot solutions, and much of it is cemented by calcite. The
absence of strong joints, such as cut the adjacent sandstones, is, doubdess,
another contributing factor.

Shiprock, Agathla, and Alhambra Rock are some of the well-known
necks referred to by Williams.

Two examples of cauldron subsidence in the Navajo region have been
described (Williams, 1936), one at Buell Park, north of Fort Defiance,
near the Arizona-New Mexico line, and one at Indian Wells in he Hopi
Buttes. The first is about 2/2 miles in diameter and collapsed at least 1000
feet. The second is J£ by K mile in diameter, and it collapsed perhaps
100 or 200 feet.

HIGH PLATEAUS OF UTAH

The High Plateaus of Utah are generally considered a subprovince of
the Colorado Plateaus physiographic province. They lie along the western
margin of the Colorado Plateau as shown in Fig. 26.1. The individual
relief features are indicated in Fig. 26.16, and bold escarpments 2000 to
6000 feet high between valley floor and Plateau top are common.

The High Plateaus span the transition from Paleozoic miogeosyncline
to shelf, and also contain the thick clastic deposits of the Cretaceous and
early Tertiary described in Chapter 22 on the Central Rockies. The thick
Jurassic evaporite trough also lies within them. In other words, the

COLORADO PLATEAU

423

Wasatch line is contained within the High Plateaus. The dotted line of
Fig. 26.1 represents the boundary of the Laramide Colorado Plateau and
Laramide Central Rockies as designated by Hunt (1956), and separates
the comparatively flat beds of Colorado Plateau aspect from the deformed
strata of the Central Rockies. However, from the southern Wasatch Moun-
tains southward a gently deformed belt separates the highly deformed
strata from the little deformed, so the boundary is not a sharp one. Also
confusing the problem of boundary is the highly deformed zone in San
Pete Valley between the Wasatch and Gunnison plateaus which to the
writer and others seems to be a salt anticline structure. If this is con-
sidered a local structure of the Colorado Plateau and not one of regional
orogenic significance, then the dividing line would lie in or west of the
Gunnison Plateau.

The main structural exhibit in the High Plateaus of Utah is a normal
fault zone of trenches and uplifted blocks. The faults, as well as known,
are shown on Fig. 26.16. The large mid-Tertiary volcanic field occupies
a central position in the High Plateaus and has been broken and offset
by the faults, the same as the older sedimentary rocks. The faults are thus
late Tertiary. In part they have continued active through the Quaternary
because of recent earthquakes and fresh scarps in places. See Chapter 31.

The High Plateaus belong to the fiasin and Range system if only the
late Cenozoic normal faults are considered, but most geologists have
given attention more to the Laramide structures in designating a bound-
ary.

AGE OF UPLIFTS AND VOLCANISM

Near the close of the Cretaceous period the region probably was low
(Hunt, 1956). In Late Cretaceous or early Eocene time occurred the de-
formation that resulted in the anticlinal uplifts such as the San Rafael
Swell, Circle Cliffs uplift, and Henry Mountains structural basin. Evidence
for this date of the folding is found in the St. George basin (Gardner,

1941), the vicinity of Escalante (Gregory and Moore, 1931), and the

Fig. 26.16. High Plateaus of Utah. Dotted line represents western margin of Colorado Plateau
'according to Hunt, 1956.

424

STRUCTURAL GEOLOGY OF NORTH AMERICA

north end of the VVaterpocket fold (Dutton, 1880). At each of these
places, Eocene strata lie across the eroded edges of the older folded rocks.
It seems probable that the other large, northerly trending folds of the
plateau, like the Kaibab uplift, the Defiance anticline (see Fig. 26.1), and
the Monument upwarp also were formed at this time. As a first result of
this folding, the structural uplifts became topographically high areas, and
the structural basins became the sites of deposition of sediments eroded
from the highland. San Juan basin in New Mexico and Uinta basin in
Utah still preserve their basin sediments; from the other basins such fill
as was deposited has been removed.

Following the monoclinal flexing and the creation of the uplifts and
basins in Laramide times, the Plateau was broadly uplifted. The history
of epeirogenic uplift, superposition of the major streams across the struc-
tures, and the gorge cycle of erosion are not yet well known, although
considerable has been written about these fascinating subjects. Longwell
(1946) reviews the problem of dating these events, and concludes pro-
visionally that the major uplift occurred in the Pliocene. See also Hunt
(1956).

The intrusion of the laccolithic clusters and the main phases of the ex-
trusive activity in the volcanic fields probably occurred in the Pliocene.
The activity can be dated accurately only in the Hopi Ruttes area, but be-
cause of the similar setting of all the volcanic fields in the Colorado Pla-
teau, it is safest to assume this age for all until proved otherwise. The
volcanic activity continued in several stages down to very recent times,
both north and south of the Grand Canyon and in the Datil field.

Regarding the age of the laccolithic intrusions, Hunt (1956) observes:

The drainage on the laccolithic mountains is consequent and some of the
main tributaries of the Colorado River appear to have been shifted mono-
clinally as a result of the doming by the intrusions. For example, the Fremont
River swings in a wide arc around the north side of the Henry Mountains and
follows the trough between the mountains and the San Rafael Swell. The
Dolores River swings in a similar arc around the north side of La Sal Mountains
and follows the trough between them and the Uncompaghre Uplift. The head-
ward part of Glen Canyon has avoided the domes at the two southern Henry
Mountains; the lower part of San Juan River and the adjoining section of the
Colorado River have avoided the dome at Navajo Mountain. The adjustment

of the drainage to the intrusive structures stands in striking contrast to the lack
of adjustment of the drainage to the orogenic structures.

This adjustment would have developed if doming of the laccolithic moun-
tains had dammed earlier stream courses, forcing streams like the Fremont
and Dolores into new courses. Inasmuch as both streams now follow the struc-
turally lowest course possible, they may have been flowing across Tertiary
basin sediments when the intrusions occurred, and their courses shifted mono-
clinally off the domed areas even though the doming progressed slowly.

EPEIROGENIC MOVEMENTS AND ISOSTATIC AND
SEISMIC CONSIDERATIONS

The summation of movement in the Colorado Plateau in late Creta-
ceous, Paleocene, and Eocene time was downward around its borders, save
for the south and southwest parts. The thick deposits in the High Plateaus
of Utah, the Uinta and Piceance basins, and the San Juan basin attest to
subsidence there, leaving the central and southern parts positive. The
general relations may also be viewed as a northward down-tilting. The
occurrence of gravels containing Precambrian quartzite on the Mogollon
Rim ( Fig. 26.9 ) suggests that this part of the Plateau was low-lying and
was receiving debris from mountains to the south (Hunt, 1956). The
several asymmetrical uplifts within the Plateau formed at this time also.

After the development of the marginal basins the entire Plateau was
uplifted as a block between 6000 and 8000 feet. The uplift of the southern
part was probably greater than the northern, and the impressive Mogollon
Rim and Mountain Region of central Arizona was brought into existence.
The epeirogenic rise is thought to be contemporaneous with the late
Tertiary and Quaternary block faulting to the west with the creation of
the Basin and Range province. The magmatic activity represented by the
laccolithic clusters, the Hopi and Navajo volcanic fields, and the large
marginal fields was also contemporaneous with the broad uplift.

For the Colorado Plateau mass of the earth's crust to stand 6000 to 8000
feet above sea level it has generally been concluded from isostatic con-
siderations that roots of "granitic" crust several times as thick must ex-
tend downward into the subcrust. However, Tatel and Tuve (1955)
believe they recognize the Moho discontinuity under the Colorado Plateau
at about 30 kilometers. If so, and thus in the absence of roots, some phe-

COLORADO PLATEAU L25

nomenon in the mantle must be sought to explain the uplift. Expansion turn to the partial melting hypothesis as the principal cause of widespread

of the underlying column of the mantle, either due to phase changes or uplift. The subject is elaborated on in the final pages of Chapter 36. The

partial melting, or to both, seems an immediate recourse. Since magmatic asymmetrical uplifts may be great blisters incident to an early stage of

activity is widespread in and around the Plateau, the writer is prone to megasill intrusion in the crystalline basement

27.

SOUTHERN ARIZONA ROCKIES

PHYSIOGRAPHIC CHARACTERISTICS AND DIVISIONS

Arizona is divided into three physiographic units : the Colorado Plateau
on the northeast, the Sonoran Desert region on the southwest, and the
Mountain Region between them. (See index map, Fig. 27.1.) The Moun-
tain Region and Sonoran Desert have been included in the Basin and
Range physiographic province (Butler and Wilson, 1938).

The Mountain Region (also referred to as the Mexican highland) forms
a belt 60 to 100 miles wide and contains most of the large ore deposits of
the state. It is characterized by many short ranges nearly parallel to each
other and to the margin of the Plateau. The individual ranges are sepa-
rated by valleys deeply filled with fluviatile and lacustrine deposits which

are now generally being eroded to widespread pediments capped by
veneers of gravel.

The southern margin of the Colorado Plateau is usually taken as the
erosional escarpment of the Kaibab limestone and Coconina sandstone,
called the Mogollon Rim, but lower Paleozoic beds extend southward in
certain summit areas well within the Mountain Region. The Mogollon
Rim is at an altitude of 8000 feet or slightly less, and Phoenix near the
junction of the Mountain Region and the Sonoran Desert is 1100 feet
above sea level. The elevation of Tucson is 2372 feet and Bisbee in the
Mountain belt is 5490 feet. The Sonoran Desert (also called the desert
region) is characterized by a great preponderance of broad desert plains
over mountain ranges. The ranges are relatively short and far apart, and
generally have lower elevations than those of the Mountain Region.

According to Ransome ( 1933 ) :

Probably the most impressive feature of the landscape, to one who sees it
for the first time, is the sharp contrast between steep and rugged mountains
and wide expanses of desert plain. It is true that the plains merge impercep-
tibly with smooth, evenly graded alluvial slopes, which may attain considerable
altitude where they meet the mountain fronts, but the presence of these great
ramps of detritus scarcely detracts from the general striking contrast between
mountain and lowland. Such topography is, of course, characteristic of most
mountainous desert regions, in which the greater part of the debris washed
from the mountains is deposited in the adjacent valleys, these gaining in extent
and becoming more plainlike as the minor eminences are worm down and
buried.

The surface forms of the Sonoran Desert may be classified into three
groups, according to Gilluly (1937c), which are:

... (1) The mountains, commonly rugged and steep-sided, with either bare
rock at the surface or only a thin cover of talus; (2) the pediments, smooth
carved-rock plains that generally border the mountains and are strewn with a
thin but discontinuous mantle of gravel; and (3) the bajadas, smoothly rounded
alluvial aprons that slope forward into the axes of the "valleys." Of these, the
mountains and pediments are chiefly carved by erosion; the bajadas are chiefly
depositional.

A glance at the Geologic Map of the United States will show that the
ranges of the southern California and Arizona and southwestern New
Mexico are smaller, more irregular in shape, less linear and parallel, and

426

SOUTHERN ARIZONA ROCKIES

427

> »

Chloride-

>S^

\.x

Jeromer •.

*>*.

s :;

v Pre'seott •«;_

2
p

5.S

(

Miami 5tx Globe

\x* R°y

Superior

, Moqollon ,.•

1 x -="

<*
*

Ajo

Silver Bell
X

•J.

OTUCSON

' Twin Buitev ■<«.

jX Empire MtJ -,
\^ %#k* Tombstone*
<*"^^ \ BisbeeX \_

■.Hanover
X

/ Santa Rita
X
Tyrone

X Lordsburq

..J

X Cananea

I Fig. 27.1. Index map of Arizona showing the central mountain region and the desert region
: (Sonoran Desert). Both mountains and desert regions are part of the Basin and Range physio-
graphic province. The chief mining districts are also shown together with mountains for which
1 cross sections are given in Figs. 27.6, 27.7, and 27.9.

separated by relatively wider basins than those of western Utah and
-Nevada. Hence, the inclusion of the Sonoran Desert of Arizona in the
Rasin and Range province from a structural point of view must be made
with reservations. The crisp boundaries imparted to ranges by block

faulting are generally absent, and if the region is one of extensive block
faulting, then the faults are older than those in Utah and Nevada, and
erosion has beaten the fault scarps back considerable distances to form
broad flanking pediments. The desert floors are in need of gravity surveys
to delimit buried faults and their patterns, if such exist. Some reports
refer to late Tertiary normal faults as Rasin and Range faults, but other
reports do not use the term Rasin and Range.

PALEOZOIC AND MESOZOIC BASINS

The paleotectonic maps of Figs. 5.1 through 5.8 show that the Trans-
continental Arch dominated most of Arizona in Paleozoic time. Except in
the southeastern corner of the state (Figs. 27.2 and 27.3), the deposits are
thin or absent, and it does not seem possible that they could have affected
in any major way the pattern of later Mesozoic and Tertiary structures.

Triassic and Jurassic sedimentary rocks are absent in the Mountain
Region and Sonoran Desert and hence events which may have occurred
during these times cannot be accurately dated stratigraphically. Lower
Cretaceous rocks are present in southeastern Arizona as part of the Mexi-
can geosyncline, and most igneous and deformational events there can be
dated either as pre-Lower Cretaceous or post-Lower Cretaceous. Late
Cretaceous strata also occur in places there, and assist further in dating of
events. However, over most of the Sonoran Desert and Mountain Region,
outside of this southeastern part of the state, the ages of rock masses and
structures are poorly known. The history is eventful, and the sequence
of events can be established but few of them accurately dated.

The Gila conglomerate whose oldest fossils to date are early Pliocene
(Anderson et al., 1958), but which for the most part probably is late
Pliocene (Knechtel, 1936), is widespread, and serves as an upper dating
plane. Events between the Cretaceous ( generally Lower Cretaceous ) and
the Pliocene must be spaced or interpolated according to the best judg-
ment of the researchers concerned.

USE OF TERMS, LARAMIDE AND NEVADAN OROGENIES

Recause of the inability of geologists to date accurately events in
southern Arizona during the Mesozoic and Cenozoic, the terms Nevadan

428

STRUCTURAL GEOLOGY OF NORTH AMERICA

ARTILLERY MTN
LASKEY a WEBBER

AJO DISTRICT

eiLLULr

CHRISTMAS

DISTRICT

C.R. ROSS

TUCSON MTS.
BROWN

DRAGOON MTS.
GILLULY

SANTA RITA
DISTRICT, Nil.
PAIGE a SPENCER

QUATERNARY

BASALT t

FAULTING

BASALT

BASALT NEAR 8I5BEE

BASALT

UPPER
TERTIARY

CONGLOMERATE

BATAMOTE
ANOESITE

GILA CONGL

GILA(f) CONCL.

%
kl

BASALT AMD
FANGLOMERATE

CHILD S
LATITE

BLOCK
FAULTING

DANIELS
CONGLOMERATE,

PEARCE VOLS.
S 0 VOLCANICS

SEDIMENTS
AND
VOLCANICS

RMYOLITE A MO
ANOESITE

SNEAO
ANOESITE

AJO VOLCANICS

BLOCK
FAULTING j

LOWER
TERTIARY

LAVAS

LOCOMOTIVE
FANGLOMERATE

ft
Q

CONGL. ARKOSE.
SS. SH. LS ,

CORNELIA
OTZ MONZONITE

WHITE TAIL CONGL.

INTRUSIONS

INTRUSIONS AND
ANOESITE BRECCIA

UPPER
CRETACEOUS

UPLIFT ,

. INTRUSIONS '^

K. FOLDING s^

VOLCANICS AND

SEDIMENTS

THRUSTING

THRUSTING ,

LIMES
MIMOR
or SMA

TONE.
AMOUNT

IE AND

A MOLE FM.

BRONCO VOLCANICS
UPPER CRET NEARBY

COLORADO SH.
BEARTOOTH OTZ.

RECREATION FM.

BROAD FOLDING

LOWER
CRETACEOUS

CONCENTRATOR
VOLCANICS

UPLIFT

- VOLCANICS,
CHIEFLY
ANOESITE

CINTURA FM.
MURAL LS
MO RITA FM.
GLANCE CONGL.

PENN.
MISS
DEV.

SEDS.
SEDS.
SEDS

JURASSIC
MD

TRIASSIC

CHICO SHUNIE

OUARTZ

MONZONITE

UPLIFT

INTRUSIONS

PALEOZOIC

HORNFELS

PERM. SEDS.
PENN. SEDS.

MISS. SEDS.

DEV. SEDS.

CAMB. SEDS.

\ FOLDING /
PERM SEDS
PENN. SEDS.
MISS SEDS.
DEV. SEDS.

CAMB. SEDS

PERM. SEDS.
PENN. SEDS. .
MISS. SEDS.
DEV. SEDS.

o. a S. SEDS.
CAMB. SEDS.

IM PART META-
MORPHOSED

DIABASE .
CAMB. SEDS.

PRECAMBRIAN

PRESENT

CARDIGAN GNEISS

PINAL SCHIST

Fig. 27.2. Comparative histories of districts in the Mountain Region and Sonoran Desert of
Arizona. The question marks indicate uncertainty of age assignments, but the sequence of
events is fairly secure.

orogeny, Laramide orogeny, and Basin and Range orogeny are not used
by authors of some of the most authoritative works. Where used in this
chapter the attempt is made to make clear the uncertainties involved.

MESOZOIC AND CENOZOIC GEOLOGY OF SOUTHEASTERN ARIZONA

The ranges of southeastern Arizona (Fig. 27.4) contain Paleozoic strata
representative of the Cambrian, Devonian, Mississippian, Pennsylvanian,
and Permian periods, and also a thick succession of Lower and Upper

Cretaceous formations. These either do not occur or occur in limited or
altered form in other parts of the Mountain and Desert regions, and hence,
the nature of crustal deformation and igneous activity is better recorded
and deciphered in southeastern Arizona than in the south-central or
southwestern part of the state. It is best, therefore, to refer to this region
first for an understanding of the Mesozoic and Cenozoic geology before
turning to the other areas.

The Paleozoic section including Permian beds described for the Bisbee
district in Fig. 27.3, is characteristic of southeastern Arizona and adjacent
New Mexico. These beds were fairly sharply folded some time after the
Permian and before the deposition of the overlying Lower Cretaceous beds
( Bisbee group ) . Examples of the folds, thrusts, and unconformable rela-
tions are shown in Fig. 27.5 and 27.6. After the folding and faulting and
before the deposition of the basal Glance conglomerate of the Bisbee
group a number of plutons were intruded including the Gleeson quartz
monzonite, the Copper Belle monzonite porphyry, the Turquoise granite,
the Juniper Flat granite, and the Cochise Peak quartz monzonite of the
Dragoon and Mule mountains (Gilluly, 1956).

The Mexican geosyncline extended nordiwestward into southeastern
Arizona and southwestern New Mexico, and in the Bisbee district the
basal beds of Comanche age are conglomerates (Glance conglomerate)
which range up to 500 feet thick. Overlying sandstones, shales, and lime-
stones attain great thickness, estimates of which range from 5000 to 18,000
feet. Fossils collected from the Mural limestone, about 2500 feet above the
base of the Comanche series, are Trinity in age. At a number of other
localities in southeastern Arizona, such as Tombstone and in the Hua-
chuca, Patagonia, Oro Blanco, Baboquivari, Sierrita, Tucson, Santa Rita,
Empire, and Whetstone Mountains, masses of sediments, presumable
Comanchean, rest unconf ormably upon Paleozoic or older rocks ( Ransome,
1933). See the cross sections of Fig. 27.6. In places a deeply dissected
surface was buried by the Lower Cretaceous sediments. The intrusive
rocks had been exposed in this erosion cycle (Gilluly, 1956).

If the correlations of the table, Fig. 27.2 are correct, then the Lower
Cretaceous Mexican geosyncline was continued to the northwest chiefly
by a volcanic fill. The volcanic series of the Tucson Mountains ( Brown,

430

STRUCTURAL GEOLOGY OF NORTH AMERICA

shows them to have once covered the entire area. At all places observed,
there is a marked unconformity at the base ( Ransome, 1933 ) .

Recause the Upper Cretaceous beds, where identified, rest in angular
unconformity on all the older rocks, it is concluded that crustal movements
of some proportions occurred about at the close of the Early Cretaceous.
Reeside ( 1944 ) shows central and southern Arizona out of water during
all but the last of Late Cretaceous time ( Fox Hills and Lance time ) , when
a trough in the site of the earlier one formed and sank at least 7500 feet.
It seems possible that the sediments could come from the geanticlinal area
on the southwest, but the spread of data does not preclude the existence
of land areas to the northeast of the trough. Ross ( 1925 ) believes a shore
line was immediately west of the Christmas-Ray-Miami districts, and
this is shown on the Late Cretaceous paleotectonic map as a narrow vol-
canic peninsula extending southeastward from the main geanticlinal area.
A northwest trend to the structures of central and southern Arizona had
thus become established.

n*tn,

MARBLE QUARRY SYNCLINE
PIPe

Fig. 27.4. Mountains of southeastern Arizona.

1939 ) composed of rhyolite, andesite, tuff, and arkose is considered Lower
Cretaceous here; some of the andesites of the Christmas area ( Ross, 1925 )
may be Lower Cretaceous, and the Concentrator volcanics of the Ajo
district (Gilluly, 1946) are of the same age evidently. These are chiefly
andesites and keratophyres. It was a belt of andesitic eruptions, chiefly,
both of the explosive and passive kinds of activity.

Upper Cretaceous strata have not been recognized in as many places
in southeastern Arizona and southwestern New Mexico as the Lower
Cretaceous, nor are they as thick where known; but Reeside's map ( 1944)

Fig. 27.5. Section through Chiricahua Mountains showing Apache Pass fault (1) and Fort Bowie
thrust (2), after Sabins, 1957. Compare with Fig. 27.8.

Section in the Mule Mountains showing angular unconformity between Lower Cretaceous
Morita fm (Km) and Paleozoic formations; also post-Lower Cretaceous thrusting. After Gilluly,
1956.

Tortilla Range
Cpc<u Q9 ^_^__dgjl5

Gila River
09

Dripping Spring Range _ . ., „

Dripping Spring Volley

T,P.d Ctq q«

finite

Ct»' '"'" Tgd

Mescol Range
€p»

/.0m^€,«j:<«>

ftarroome , 1/5 6 5 Prof Paper us
RAY AND MIAMI DI5TRICTS

1^'-~-

"Op^c

P3^

^"F--:-'-tr>. ^

ft •

.Tea /tve/J

TUCSON MOUNTAIN5

j Miles W H Brown, 6 5 A Bull SO

Pa.

PrT

EMPIRE MOUNTAINS

GromTe ancf quarfz porphyry

ft A Wiljon, Jour Ceo/ vol 4?

Santa Rita Mountains

Huerfano Butte

__€qc

Cell
"/wagb

€1c/-^i

Ksh
^^?T7^^^r^ K,h

3ea level

Patagonia Mountains
Tr IT Ksh qrp _D\ Qgi

Miles Jhracler, 1/565 Bull SBZ

SANTA RITA AND PATAGONIA MOUNTAINS

KmKc Ofd

rtanoome, U.5G5 PP 31

MULE MOUNTAINS, BI5BETE DI5TRICT

Fig. 27.6. Cross sections in southeastern Arizona. Symbols for Ray and Miami districts; €ps.
Pioneer sh.; €ds, Dripping Spring quartzite; Cm, Mescal Is.; Ctq, Troy quartzite; Dm, Martin Is.;
Ct, Tornado Is; Qg, Gila conglomerate, db, Mesozoic (?) diabase; Tgd, granodiorite; Tpd,
quartz diorite. Tucson Mountains; Cpl, Permian Is; Kv, Cretaceous volcanic rocks; Ka, Amole
arkose; Ti, acid dikes and volcanic necks; Tua, upper andesite; Teg, conglomerate; Tcr, Cat Mt.
rhyolite; KTa, Amole latite; Ts, spherulitic rhyolite; Qa, alluvium and talus. Santa Rita Moun-

tains, €pc, quartzite, and congl.; Dl, Devonian Is.; Cml, light Is.; Cdl, dark Is.; Ksh, chiefly Mesozoic
but may include some Precambrian; qm, quartz monzonite; grp, granite porphyry, agh, alaskite
granite porphyry; Tr, rhyolite. Mule Mountains; ps, Pinal schist; €b, Bolsa quartzite; Co, Albrigo
Is.; Dm, Martin Is; Ce, Escabrosa Is.; Cn, Naco Is.; Kg, Glance congl.; Kmr, Morita fm.; Km,
Mural Is.; Kc, Cintura fm.; Qfd, fluviatile deposits.

EXPLANATION

Contact, approximately located

Fault, showing dip

Dished where approximate// located;

dotted where conceited

Strike and dip of beds

Strike and dip of overturned beds

Strike of vertical beds

R.25E-

Alluvium

Fault breccia

TKsl

Sugarloaf quartz latite
member

I.ZC

1 > ■

5 ,' ■ •''

Gleeson quartz Copper Belle monzonite
monzonite porphyry

vvPCe-xv

Earp formation

Horquilla limestone

Esc fibrosa limestone

IDmj
Martin limestone

Abrigo limestone

v,pcp,y,

Pin»l Khist

o o

2000 Fa«t

.£»"••

., d J£~- "p€d

A Q QT M^WtVwm -"

•TKsl

Geology by J Gilluly and R. S. C«nnon, Jr., 1938

Fig. 27.7. Relationships in the thrust breccia east of Gleeson, Dragoon Mountains. Reproduced from Gil-
luly, 1956.

SOUTHERN ARIZONA ROCKIES

■}>i

In the northern Dragoon Mountains andesite and quartz latite were
erupted on an erosion surface of mild relief developed after the deposition
of the Risbee sediments. These volcanics may be Late Cretaceous in age;
at least they preceded the strong thrusting.

The most profound deformation of the area took place after the Bronco vol-
canics and Sugarloaf quartz latite were erupted. This involved great thrust
faults of northerly to northwesterly trend in the Dragoon Mountains and the
overturning of a section of the Bisbee formation fully 3 miles thick along the
eastern flank of this range. A gigantic breccia of fragments of nearly every
older formation exists in the Courdand and Gleeson areas. Refer to Fig. 26.6.
It suggests that the major fault was, in this section of its exposed course, advanc-
ing over the surface, producing the breccia by attrition of the overriding
thrust plate. Minor thrust fragments of this age are found in the Tombstone
Hills. . . .

The Stronghold granite is younger than the thrusting and has domed the
thrust sheets slighdy. This doming does not appear, however, to account for
the emplacement of the granite, which is clearly transgressive. In the Tomb-
stone Hills the Schieffelin granodiorite seems also to be younger than all
important compressional stresses, as is the Uncle Sam porphyry.

The sequence and pattern of Laramide (?) events in the Chiricahua
Mountains is instructive. The northwest course of the main structures
dominates the geologic map, whereas the giant thrust breccia in the
Dragoon Mountains leaves the trends uncertainly recognized there. Ac-
cording to Sabins (refer to Fig. 27.8),

During the major post-Comanche to pre-Pliocene orogeny, strong southerly
to southwesterly horizontal compression caused the following tectonic sequence.
The autochthonous rocks along the northeast front of the range were overridden
from the southwest by the first thrust sheet. Strike-slip displacement along the
Emigrant fault cut the autochthonous block and the overlying thrust sheet,
which was separated into the Fort Bowie plate and the Wood Mountain
plate. The Fort Bowie plate was later folded to form the Marble Quarry
syncline and was truncated by the younger Fort Apache reverse fault. Finally,
the Whitetail plate overrode the Fort Apache fault block.

Volcanic extrusions of approximately mid-Tertiary age then accumu-
lated on an erosion surface on the deformed strata. These were faulted
and tilted, possibly just prior to the deposition of the Gila conglomerate
in Pliocene time. Not only was the faulting the cause of the deposition
of the Gila conglomerate, but also probably the modern ranges were

blocked out by it at this time. Some faulting continued after the Gila
conglomerate accumulated.

The age of the thrusting cannot be more accurately placed than in the
Late Cretaceous (probably very Late Cretaceous) or Early to Mid-Tei
tiary, but when it and the stocks of post-thrusting age are compared to a
similar sequence of events in Utah, Nevada, Colorado, and central New
Mexico, one may logically point to a Laramide age. The folding of the
Upper Cretaceous beds of the Mexican geosyncline in Coahuila, Chi-
huahua, and Sonora is generally referred to as Laramide, and this broad
region continues the folding and thrusting of southeastern Arizona and
southwestern New Mexico southward.

The picture described in the above paragraphs of the structure of the
ranges of southeastern Arizona is clarified by Jones ( 1961 ) , who recog-
nizes most of the ranges to be complex anticlines with Precambrian or
Triassic-Jurassic granite in their cores. The structural relief of some of
the uplifts is 25,000 feet. Some began to rise in Mesozoic time and con-
tinued intermittently through at least the Miocene. High angle reverse
faults define the flanks, and appreciably downslope mass movement has
occurred to form the low-angle thrusts.

MESOZOIC AND CENOZOIC GEOLOGY
OF SOUTHERN ARIZONA

The geology of the Ajo mining district of south-central Arizona in the
Sonoran desert is well known from the work of Gilluly (1946), and prob-
ably is representative of the geology of this general region. The main rock
units and events are listed in Fig. 27.2. A cross section is shown in Fig.
27.9. The only fossils found in the entire district are in blocks of lime-
stone in the Locomotive fanglomerate, presumably of about middle
Tertiary age. The fossihferous boulders are referable to the Devonian,
Mississippian, and Pennsylvanian, and hence when the fanglomerate was
being deposited outcrops of beds of these Paleozoic ages were probably
nearby in upland areas. The rocks and events, although their sequence
is relatively well established, are not dated by stratigraphic methods,
and the ages assigned are very tentative.

434

STRUCTURAL GEOLOGY OF NORTH AMERICA

Jt'lS

Fig. 27.8. Thrust plates of the Cochise
Head and Vanar quadrangles, Chiricahua
Mountains. Reproduced from Sabins, 1957.
Compare with Fig. 27.5.

Of particular importance in regional tectonics are the ages of the
Chico Shunie quartz monzonite and the Cornelia quartz monzonite. The
Chioo Shunie precedes the Concentrator volcanics and the Cornelia
follows. The sequence is similar to the one in the Dragoon and Mule

Mountains of southeastern Arizona, except for the absence of the Bisbee
sediments, which may be represented in part by the Concentrator vol-
canics. Gilluly assigns the Chico Shunie intrusion questionably to the
Mesozoic and the Cornelia intrusion to the early Teritary. Speculatively,

Manganese Mesa

Artillery PH.

B ooo'

■3ea /eve/

One Mile

Rawhide fit hor/>o

Artillery fits.

fhrujt

looo'

ARTILLERY MT5. MANGANESE REGION

Chico 5hunie Hills Copper Canyon

dT/?/ /CO Mo unto in
Tb

Tdc

jooo'

Tsa

J<?o /eve/

Tvb

New

Corn el /a
Mine

Tern

Tov

AJO MINING DISTRICT

r jooo

-5<?c /eve/

j iV/'/eJ

Fig. 27.9. Cross sections of the Artillery Mountains, after Lasky and Webber, 1944, and the
Little Ajo Mountains, after Gilluly, 1937a.

Formations in the Artillery Mts.; pT, Paleozoic Is., sh., and qutz., and Precambrian granite,
gneiss, and schist; Tec, lower Eocene (?) congl., ss., and sh.; Tmv, Miocene (?) volcanic rocks;
Tpf, lower Pliocene (?) alluvial fan and playa deposits containing manganiferous beds; Tb, lower
Pliocene (?) basalt; Tpc, upper Pliocene (?) basalt; Qb, Early Pleistocene (?) basalt.

Formations in the Ajo mining district; p€g, Cardigan gneiss; Msc, Chico Shunie quartz mon-
zonite; Kcv, Concentrator volcanics; Ted, Cornelia quartz monzonite, Tlf, early Tertiary Loco-
motive fanglomerate; Tav, middle (?) Tertiary Ajo volcanics; Tsa, middle (?) Tertiary Sneed
andesite; Tdc, middle (?) Tertiary Daniels conglomerate; Tel, Pliocene (?) Childs latite; Tvb,
Pliocene andesite breccia; Tb, Pliocene andesite flows; Tbi, intrusive basalt.

436

STRUCTURAL GEOLOGY OF NORTH AMERICA

the Chico Shunie could correlate in age with the Juniper Flat granite
and other related plutons, and the Cornelia with the Stronghold granite
and Schieffelin granodiorite.

GEOLOGY OF WEST-CENTRAL ARIZONA

The Artillery Mountains near the west end of the mountain region
has been studied by Lasky and Webber (1944), and the rock units and
succession of events there are probably representative of the general
area. The stratigraphy is briefly described below, and the relation of
rock units to the structural events is shown in Figs. 27.2 and 27.9.

Of particular interest are two thrust sheets, one of which occurs in the
Artillery Mountains and one in the Chemehuevis Mountains just south-
east of Needles (see Tectonic Map of the United States). Both sheets
overrode toward the Colorado Plateau and are probably Laramide in
age.

The Laramide orogeny seems to have been in two phases, first an up-
lift that furnished the lower Eocene (?) conglomerate, arkose, sandstone,
shale, and limestone beds to a northwestward-trending trough, and then
the thrusting that brought the Precambrian and Paleozoic (?) sedimen-
tary rocks on top of the lower Eocene (?) beds. These lower Eocene beds
have a structural setting similar to the conglomerates, arkosic sandstones,
and claystones of the New Water Mountains farther south.

The Artillery Mountains thrust is overlapped by the Miocene (?) vol-
canics. Minor normal faulting then formed a graben in which lower Plio-
cene ( ? ) sediments and an overlying basalt accumulated. After the graben
basin had become integrated into a regional drainage system, the upper
Pliocene (?) conglomerate was deposited. The Pliocene (?) rocks were
then folded into a shallow composite syncline that parallels the northwest-
ward trend of the basin, and that now occupies the valley between the
Artillery and Rawhide Mountains.

The folded rocks along either side of the valley, together with the over-
lying Pleistocene (?) basalt, are broken by northwestward-trending nor-
mal faults, which presumably are the effect of renewed movement along
older fault zones. See cross sections of Fig. 27.9.

Rock Units of the Artillery Mountains Manganese Area

Thickness, Feet

0-350 plus

0-250 plus

Recent:

Talus deposits, and gravel and sand along the present drainage.

Erosional Unconformity
Later Pleistocene:

Pediment gravel and valley fill.

Angular Unconformity
Earlier Pleistocene (?):

Massive, fine-grained to vesicular glassy basalt.

Angular Unconformity
Upper Pliocene (?):

Largely light to dark red, poorly sorted conglomerate with dis-
continuous bedding. Includes a prominent basalt member in the
southwestern part of the area.

Erosional Unconformity
Lower Pliocene (?):

Massive aphanitic vesicular basalt

Alluvial fan and playa deposits— fan-glomerate, conglomerate, sand-
stone, siltstone, mudstone, clay, and limestone; in part gypsiferous.
The principal manganese-bearing formation.

Angular Unconformity
Miocene (?):

Tuffs, breccias, and flows, rhyolitic to andesitic.

Angular Unconformity
Lower Eocene (?):

Conglomerate, arkose, sandstone, shale, limestone, and a little
clay, with some tuff and a widespread basalt member; in large
part highly indurated.

Angular Unconformity
Paleozoic (?):

Limestone with minor quantities of shale and quartzite in part
metamorphosed.

Angular Unconformity

Precambrian:

Granite, gneiss, microbreccia, and subordinate schist, including
some monzonitic rock in the Rawhide and Buckskin Mountains that
may be of post-Cambrian age.

0-2000 plus or minus

0-1500 plus or minus

1800 plus

2500 plus

SOUTHERN ARIZONA ROCKIES

437

NEVADAN OROGENY (?)

Post-Permian and pre-Lower Cretaceous folding is recorded in south-
eastern Arizona and to the southeast in Coahuila (see Chapter 14). In
both places granitic magmas have intruded the folded strata and were
exposed by erosion before the Lower Cretaceous beds were deposited.
To the west at Ajo, folding ( ? ) and metamorphism preceded the intrusion
of the Chico Shunie quartz monzonite. These events seem to correlate
with the pre-Lower Cretaceous orogeny in southeastern Arizona. Farther
to the northwest in the Artillery Mountains Paleozoic limestone, shale,
and sandstone were in part metamorphosed before the Tertiary, at least.
No intrusions of possible Mesozoic age are noted there, however. Then
in central and western Nevada a long succession of deformational events
are documented from late Devonian to the close of Jurassic time. In the
Kimmeridgian (latest Jurassic) considerable volumes of granitic magma
invaded the folded and thrust-faulted strata. This Late Jurassic orogeny
has been classed as early Nevadan in Chapter 17.

It will be recalled (Chapter 14) that the Coahuila peninsula rose in
Kimmeridgian time, and that the Mexican geosyncline took form to the
west during the Late Jurassic. It received sediments from a geanticlinal
area on the west as well as from the Coahuila peninsula. The rise of the
peninsula and geanticline may indicate that both were orogenic belts.
This was a time of thrusting and intrusions in central and western
Nevada. These coincident relations support the thesis that southern
Arizona was a belt of orogeny in the Late Jurassic and that the belt
existed as a branch from the main belt in Nevada which continued south-
ward into Sonora, Mexico, in the region of the geanticline that lay west
of the Mexican geosyncline. See tectonic map of Plate 10.

IGNEOUS CYCLES AND MINERALIZATION

Igneous rocks possibly of Palezoic age have been described in two

places. According to Ettlinger (1928), a diabase in the mountain region

of central Arizona is intrusive as multiple sills in Cambrian strata but not

; in any younger Paleozoic strata and, therefore, may be pre-Devonian.

The diabase extends over 1600 square miles, and the combined thickness
of the sills in places approaches a mile. Others have suggested a post-
Permian and Cretaceous age. It predates the mineralization of the region.
Gilluly ( 1946) regards a hornfels in the Ajo district of the desert region as
possibly Paleozoic. It, however, is of andesitic and rhyolitic derivation,
unlike the composition of diabase.

Rutler and Wilson ( 1938 ) list the Juniper Flat stock of the Bisbee dis-
trict as post-Paleozoic and pre-Cretaceous, and suggest that the activity
may be Nevadan in age. Ransome considered the Sacramento stock at
Bisbee and associated metallization also as pre-Cretaceous. As already
noted, Gilluly identifies several plutons in the Dragoon Mountains as
post-Permian pre-Lower Cretaceous.

A large number of stocks that range from granite to diorite occur in the
mountain and desert regions of Arizona, and all are probably vounger
than the Kaibab (Permian) limestone. Of late years they have been con-
sidered Laramide in age, mainly upon the argument that they are similar
in lithology and their structural setting is similar to other known Lara-
mide intrusions of the southwest. Definite proof of Late Cretaceous age
is probably not obtainable for most of them.

A group of the stocks in central Arizona are considered to be cupolas
of a major underlying parent pluton called the central Arizona batholith
(Ettlinger, 1928).

In general, the stocks have little plan or pattern in their distribution.
In the Superior-Miami-Globe and Morenci-Metcalf districts, however,
the intrusive bodies have a general northeastward direction across the
mountain region. Likewise, the ore-bearing fissures in these and other
districts strike northeastward (Butler, 1938). The mining districts shown
on the index map, Fig. 27.2, are a pretty good clue of the distribution
of the stocks. Many of the ore deposits of Arizona are due to the
mineralizing activity of the magmas of these stocks, especially in the
central and southeastern part of the state.

Lava outpourings are very extensive in Arizona and New Mexico, and
some are shown on the index map of Fig. 27.2. Refer also to the Geologic
and Tectonic maps of the United States. In part they are younger than the
monzonitic intrusions, but in part thev are possibly contemporaneous or

438

STRUCTURAL GEOLOGY OF NORTH AMERICA

even older. The most extensive and probably thickest is the Datil field,
which is described in Chapter 36. In the mountain region of Ari-
zona, the Laramide ( ? ) stocks had been exposed by erosion and then were
covered unconformably by the lavas. The Tertiary in general was a time
of prolonged volcanic activity from place to place, the lavas were more
acid than those of the Cretaceous, and widespread block faulting was
prevalent. See charts, Figs. 27.2 and 27.3.

A third group of mineral deposits is associated with the Tertiary lavas.
The districts that belong to this class are listed in the following table. The
ore deposits are in the form of fault veins that cut the lavas. The veins are
generally crustified, shallow in depth, and contain adularia. Gold is the
chief ore mineral.

Age of Ore Deposits in Arizona and New Mexico

Precambrian

Nevadan

Laramide

Late Tertiary

Jerome-Prescott

Bisbee?

Magma

Mogollon

Pecos

Patagonia?

Globe

Steeple Rock

Red Bed

Miami

Lordsbury?

copper

Ray

Stanley Butte

deposits?

Christmas
Morenci
Tombstone?
Twin Buttes
Magdalena
Santa Rita-Fierro
Pinos Altos
Tyrone
85 Mine?

Oatman?
Ajo?
Silver Bell?

Silver-manganese

metalliza-

tion below sha

beds in

southwest New

Mexico;

Silver City, Cooks

Peak,

Kingston, etc.

TERTIARY NORMAL FAULTING

Everywhere, it seems, in the mountain and desert regions of Arizona,
high-angle faults cut and offset the bedrock. They trend in many direc-

tions. They both predate and postdate the Gila conglomerate of Pliocene- ]
Pleistocene age; some predate the Laramide orogeny, some are part of it,
but the majority postdate it, and are Middle and Late Tertiary.

Either because of block faulting, regional warping, or both, the central
and southern part of Arizona became an area of aggradation in late Ter-
tiary time, and stream and lake sediments, in places 10,000 feet
thick, accumulated in the lower areas. The deposits, though given various
names in several local areas, are best known as the Gila conglomerate.
Mild volcanic activity accompanied the sedimentation, and lava flows are
locally present in and on top of the formation. Relative uplift of the ranges
and subsidence of the intermontane trough areas continued intermittently
into Quaternary time, and the Gila formation is tilted, faulted, and locally
folded. Where uplifted, it is trenched, and the material eroded from it
and other sources has been deposited as a relatively thin veneer of Qua-
ternary terrace and stream alluvium.

Tertiary volcanic rocks are nearly everywhere, and in one place or an-
other represent continuing volcanic activity down to the time of the In-
dians. A resume of the Tertiary volcanic activity throughout the Tertiary
and Quaternary in the mountain and desert regions of Arizona and in the
Colorado Plateau from the point of view of age, distribution, and com-
position is a very inviting study.

In general, the ranges trend northerly in southeastern Arizona and
northwesterly in the central and southwestern part. These directions are
probably due to the major Late Tertiary faults. Considerable time has
elapsed since the last major movements, because extensive pediments
have formed across many faults and true fault scarps are few.

CONCLUSIONS REGARDING TECTONIC HISTORY

In southern Arizona sometime during the Triassic and Jurassic, the
Paleozoic and Precambrian rocks were folded, intruded by granitic stocks
or small batholiths, and deeply eroded. The orogeny is tentatively corre-
lated with the early Nevadan of central and western Nevada and
California, and the belt of orogeny is recognized to extend from Arizona
to south-central Coahuila in the site of the Late Jurassic Coahuila penin-

SOUTHERN ARIZONA ROCKIES

439

sula. It is regarded as a branch of the main orogenic belt of Nevada
which continued southward into Mexico west of the Mexican geocyn-
cline.

The Mexican geosyncline transgressed northward in Early Cretaceous
time, and considerable thicknesses of Lower Cretaceous elastics were
spread over southeastern Arizona. The geanticline on the west was a site
of much volcanism, and the volcanic materials contributed to the sedi-
ments of the geosyncline. The Concentrator volcanics of south-central
Arizona suggest that the volcanic belt continued northwestward into
Arizona.

Generally mild deformation followed the Early Cretaecous volcanism

and sedimentation, and then over most of the southeastern Arizona Upper
Cretaceous strata and volcanics were spread. In Early (?) Tertiary time
as part of the Laramide orogeny uplifts with attendant folding and thrust-
ing in the Mountain Region and Sonoran Desert occurred. This was
followed immediately by the intrusion of granitic stocks.

Mid-Tertiary time was marked by a varied volcanic activity from place
to place, and finally in late Tertiary time block faulting of regional
character occurred and caused the widespread deposition of the Gila con-
glomerate. Some faulting continued in places during the Quaternary,
but this was a time chiefly of the development of extensive pediments
around the desert ranges.

28.

ROCKIES OF NORTHERN MEXICO

MEXICAN GEOSYNCLINE

The Tectonic Map of Northern Mexico by Philip B. King (1947) is re-
produed in Fig. 28.1, and on it the belt of Laramide folds can be seen
extending southward from New Mexico and Texas into central and east-
ern Mexico. Preceding the Laramide orogeny and in about the same
region a major basin subsided and received a thick complement of sedi-
ments. It is known as the Mexican geosyncline. See Plates 10, 11, and 12
for its limits.

Late Jurassic History

In the northern part of the geosyncline in Late Jurassic time, sediments
2600 to 4800 feet thick accumulated, while in the southern part at least

5000 feet of beds were deposited. At the beginning of Late Jurassic time,
the subsidence and marine invasion was limited to the southern part,
where 2000 feet of dark marine clay, lime mud, and sand were deposited.
The southward-lying land was evidently not high, but it was stable and
a humid climate prevailed (Imlay, 1943). After this stage, the first
widespread marine transgression occurred (Devesian stage), and thick
salt and anhydrite beds, associated with red clays, sands, and gravels,
were deposited. The salt facies was deposited in northern Central
America, southern Mexico, and the southern United States. A thick red-
bed facies, at least partly of continental origin, was formed throughout
much of northern and eastern Mexico at apparently the same time as the
salt facies to the south and north. Both facies transgressed a peneplained
surface.

The thickness and extent of the salt layers suggest that the entire Gulf
of Mexico was a salt-depositing basin completely enclosed except for a
relatively narrow, shallow strait that probably connected with the Atlantic
Ocean.

The material composing the red-beds was probably derived from the
Central Stable Begion on the north, where older red-beds cropped out,
and from the geanticlinal areas in western and southern Mexico (Imlay,
1943). Further sinking of the geosyncline brought on normal marine con-
ditions; and lime, clay, and silt were deposited on top of the salt and red-
beds. Still later in the late Jurassic time ( Kimmeridgian ) , more red-beds
with anhydrite were deposited in the central parts of the geosyncline.

At this stage in late Jurassic time an uplift formed the Coahuila penin-
sula extending southward across western Coahuila and eastern Chihua-
hua as far as the Parras basin of southern Coahuila (Kellum, 1944). See
map of Late Cretaceous, Plate 12. Coarse elastics that were deposited
marginal to it on three sides during latest Jurassic as well as Early Cre-
taceous time attest a fairly high topography to the peninsula.

In the description of the Coahuila system, an orogenic belt was de-
scribed, part of which, at least, occupied the position of the later Coa-
huila peninsula. It appears that the folded and faulted Permian beds
were first intruded, then eroded, and then epeirogenically uplifted before
the Upper Jurassic beds were deposited in the area. See Plate 13. The
belt of volcanoes on the west that supplied much of the Permian sedi-

440

UNITED STATES DEPARTMENT OF THE INTERIOR
GEOLOGICAL SURVEY

UNIVERSIOAO NACIONAL AUTONOMA DE MEXICO
INSTITUTO DE GEOLOGIA, GEOFISICA Y GEODESIA

Scdimantify rocks

Sedimentary rocks

OomtnantJy not metamorphosed
Oomlnantty metamorphosed

Pre-Mesojotc rocks

CSD

Faults

hactuirvd oa downtfvwo MM

Volcanoes
Ve*> acova. oc *

Structure contours
( Wore or ta .

IOOO /Ml. ;l

( iMf'uii j peninsula

Ifti.m iti l.\i;uui,r
Airiuf oJcW Mmozdjc ohm

Fig. 28.1. Tectonic map of northern Mexico. Reproduced from P. B. King, 1942b.

442

STRUCTURAL GEOLOGY OF NORTH AMERICA

ments sank, and the area became the Mexican Upper Jurassic and Creta-
ceous geosyncline.

Northeast of the Coahuila peninsula, there is evidence of a narrow
promontory, called the Oriental geanticline by Imlay. The structure for
the most part was barely emergent.

Early Cretaceous History

During Early Cretaceous time, the Mexican geosyncline sank over
12,000 feet and received its greatest load of sediments. The contours of
the tectonic map, Plate 11, were drawn on the basis of thicknesses given
by Imlay (1944), and in conformity with the Coahuila peninsula and the
Orental geanticline as outlined by Kellum and Imlay. The Coahuila pen-
insula was overlapped considerably, and by Aptian time (middle Early
Cretaceous) it was completely submerged and for the rest of Creta-
ceous time was a platform on which 1500 feet of lagunal deposits ac-
cumlated.

The land along the western margin of the Mexican geosyncline, at
least in the northern part, had probably suffered intense deformation dur-
ing the late Early Cretaceous. Over 10,000 feet of beds of Aptian age lie
along its eastern margin in central Sonora. The outcrops farthest west
consist of andesite flows, tuffs, and agglomerates, but this volcanic facies
is replaced to the east by a marine limestone, shale, and sandstone facies.
According to R. E. King (1939), there is evidence of great oscillations in
level of the sea, with repetition of cycles of marine and continental de-
posits.

In northern Sonora, thick coarse conglomerates of the same age as the
volcanics of central Sonora occur. They increase in thickness southeast-
ward from Risbee, Arizona, and are at least 5000 feet thick only 30
miles to the south. The boulders are in part large and angular and, to-
gether with the large volume, show that the sea was bordered by steep
shores, and that the southwestern landmass was suffering active defor-
mation. The deposition and causative orogeny was rapid, because the
time represented by the conglomerates and related sediments is but a
fraction of Cretaceous time (Imlay, 1939). The presence of finer clastic
sediments, as well as coal, higher in the section indicates times of

lowered lands, broader littoral zones, and marginal swamps. The litho- |
graphic character of the Risbee group shows that the landmass to the |
south was composed of Precambrian gneisses and schists, and Paleozoic
quartzites and limestones similar to those outcropping at present in the
Risbee district and locally in northern Sonora (Imlay, 1939).

Still farther west than Sonora in Raja California, Lower Cretaceous
rocks have been identified, and their nature is significant regarding the
belt of orogeny west of the Mexican geosyncline. They occur along the
west shore of the northern part of the peninsula and consist of conglom-
erates, quartzites, tuffs, and agglomerates, with thick lava flows inter-
bedded. They are cut by dikes and large stocks. In some localities, the
intrusive rocks predominate over the sediments and pyroclastics, and in
places there is much metamorphism. Unaltered, or but little altered, sand-
stones and shales appear in places, and limestone also occurs. The meta-
morphosed Cretaceous rocks may be equivalent to schists and coarse,
massive, white granite that are widely distributed southward down the
peninsula. The granitoid rocks that intruded the Lower Cretaceous beds
are probably early Late Cretaceous in age. Chapter 30 is devoted to the
geology of Raja California and Sonora, and should be consulted for
further details.

The composition of the Lower Cretaceous rocks is that of the volcanic
archipelago type, and indicates an associated orogenic belt. They appear
to be separated from the deposits of the Mexican geosyncline by the Oc-
cidental geanticline, but not enough is known of the distribution and litho-
logic variations of the Lower Cretaceous strata in this region to dem-
onstrate the interpretations. Refer further to Chapter 30.

Late Cretaceous History

Upper Cretaceous deposits are widely distributed from the Santa Ana
Mountains southeast of Los Angeles (Woodford, 1939) throughout the
length of Raja California to Todos Santos. They are separated from the
strata of Early Cretaceous age by a period of intrusion and metamor-
phism, and were themselves faulted and in places folded before the
Cenozoic sediments were laid down. For further details, see Chapter
30.

ROCKIES OF NORTHERN MEXICO

443

NORTH

SIERRA OE LA PAILA

SOUTH

MELCHOR OCAMPO

COAHUILA
PLATFORM

^^rz jl°- - *-- 2

Fig. 28.2. Parras trough at close of Cretaceous sedimentation. After Imlay, 1939.

The exogenic belt of western Sonora continued active during Late
Cretaceous time and crowded the Mexican geosyncline eastward. At two
different times, the land rose sharply, and thousands of feet of sediment
were deposited along the western edge of the trough (now in eastern
Chihuahua); first shale and sandstone, and later conglomerate. In north-
ern Zacatecas, southeastward from Sonora but still along the western
margin of the geosyncline, thousands of feet of tuffaceous beds were de-
posited.

An east-west trough subsided over 15,000 feet in southern California.
Figure 28.2 shows the sediments in it and the Coahuila platform to the
north. The depression has been named the Parras trough from the present
Parras basin in which the Upper Cretaceous sequence crops out (Imlay,
1944).

SONORAN REGION

Very little is known about the Paleozoic history of Mexico. The Per-
mian of the Mexican state of Coahuila has been treated in Chapter 14.
On the west side of the country in Sonora, additional Permian beds have
been noted (R. E. King, 1939). They consist of limestones with abundant
crinoid stems and fusulinids. Reefs of massive limestone about 1500 feet
thick grade laterally into lesser thicknesses of well-bedded, darker lime-
stone. Permian strata may have occurred, originally, east of the central
Sonoran outcrops, because cobbles of fossiliferous Permian limestone are
found in the basal conglomerate of the Cretaceous there (R. E. King,
1939 ) . Present data are not sufficient to outline the basin in northwestern
Mexico, and the tectonic significance of the Sonoran exposures is, there-

444

STRUCTURAL GEOLOGY OF NORTH AMERICA

5 MILES

Fig. 28.3. Sections illustrating structures and stratigraphy in central Sonora, Mexico. A, Clasita
area; B, Urro Cobachi area; C, Zubiate area; D, El Trigo area; E, Arroyo Arenosa area; F,
Southern Sierra de San Javier; G, vicinity of Guamochil. pal, Paleozoic rocks; JTb, Barranca
formation; Kv, Cretaceous volcanics; Tv, early Tertiary volcanic rocks; Tbl, lower member of
Baucarit formation; Tbu, upper member of Baucarit formation; Gr, granite; Di, diorite; Sy,
syenite. After R. E. King, 1939.

fore, obscure. Not helping to clarify the obscurity is the average eastward
strike of the younger rocks. See map, Plate 8. Late Permian or early Trias-
sic folding is indicated by generally greater metamorphism and folding
in the Permian than in nearby Upper Triassic and Cretaceous rocks, as
well as by the divergent strikes. The immediate impulse is to relate the
Sonoran east-west trends to the Coahuila, but then the belt of volcanics
that supplied much material to the Coahuila Permian basin apparently
lay between, and it would therefore seem that the two did not form a
single continuous tectonic system. The peculiar thing is that the volcanic
archipelago type of sediments is on the east in this region and the inland
basin type on the west, just the opposite from the distribution to the north

in the western United States and Canada. Because the Sonoran Permian
is of the inland basin type, it might be supposed that the Pacific orogenic
belt lay considerably west. Since the peninsula of Baja California is made
up in large part of Cretaceous batholithic intrusions, evidently a continu-
ation of the great Sierra Nevada batholithic belt, and since the batholithic
belt coincides strikingly with the Permian trough in the United States,
British Columbia, and southeastern Alaska, it can also be supposed that
the Permian orogenic belt paralleled the peninsula and perhaps in part
lay west of it. It is evident that this is supposition, but possibly a reason-
able guess in the absence of factual information. The interpretation
rendered on the Permian tectonic map, Plate 8, does not produce a mean-
ingful tectonic pattern and is probably not correct, but various other
arrangements seemed even less tenable. We must await more field work
in western Mexico.

Paleozoic rocks older than Permian are rare in Mexico. In central
Sonora, Ordovician limestone, sandstone, and conglomerate have been
identified (R. E. King, 1939). See cross sections, Fig. 28.3. In southern-
most California, an outcrop of marble is Mississippian. Farther north in
the San Bernardino Mountains, the Furnace limestone may be Mississip-
pian ( Woodford, 1939 ) . It is bounded above and below by quartzite for-
mations. Very similar massive dolomitic limestones and somewhat similar
quartzites are widely distributed in the Perris and San Gabriel moun-
tains. The Arrastre quartzite of the San Bernardino Mountains underlies
the Furnace limestone, and is so far below the fossiliferous horizon that
it is probably pre-Mississippian (Woodford, 1939).

The earliest formation of the Mesozoic in Mexico is the Barranca. It is
Late Triassic and Early Jurassic in age, and crops out extensively in north-
western and central Sonora (R. E. King, 1939). There are also isolated ex-
posures in southeastern Sonora and western Chihuahua. In the ranges
bordering die Rio Yaqui, the formation is wholly of continental origin,
widi a diickness of more than 3300 feet. Three members are recognizable.
The upper and lower divisions are massive sandstones with some inter-
bedded dark shale. The middle member consists of shale and thin-bedded
sandstone with layers of coal and graphite. Farther east a short dis-
tance, the formation is 4250 feet thick. In northwestern Sonora, a clastic

ROCKIES OF NORTHERN MEXICO

445

section is about 7350 feet thick. The lower part is Late Triassic, and the
upper part is Early Jurassic.

EL PASO-RIO GRANDE THRUST BELT

When the Late Cretaceous seas of the Mexican geosyncline finally
withdrew from the El Paso-Rio Grande area, the sedimentary veneer on
the Precambrian crystallines was of appreciable, although variable,
thickness. In the nortiiern Quitman Mountains, it was 10,000 feet thick.
The ancestral Diablo Range along the north side of the Rio Grande was
an area of thinning of Pennsylvanian and Permian strata ( see the paleo-
tectonic maps of Plates 7, and 8 ) , and the Coahuila platform to the south
and west was an area of marked thinning of the Lower Cretaceous
(see paleotectonic map, Plate 15).

The Laramide compressional forces then gripped the El Paso-Rio
Grande area and subjected it to intense deformation. Examine the map
of Fig. 28.1. The rocks were highly folded and thrust-faulted (Huffington,
1943 ) . Three thrusts are prominent, namely, the Devil Ridge, Red Hills,
and Quitman. See cross section Y of Fig. 25.16. They occur in the Quit-
man and Malone Mountains and in Devil Ridge and, altogether, make
a zone about 75 miles long from the Hueco basin on the north to the
Chinati Mountains on the south. Folds and thrusts are known in a
broad belt west of the Quitman Mountains in Chihuahua, Mexico. The
areas of bedrock are few in northern Chihuahua, and these are little
known geologically. Consequently, the western limit of the Laramide
Sonoran Rockies there is indefinite. They probably merged with the
Sierra Madre Occidental Rockies to form a great broad belt of defor-
mation. Much of this area is now in the Basin and Range structural prov-
ince because of the superposition of later block faults on the Laramide
folds and thrusts.

The El Paso-Rio Grande thrust belt probably extends far enough south-
eastward to merge with the Sierra Madre Oriental system of Laramide
ranges in Coahuila. Its eastern boundary is sharply defined between the
Malone Mountains and the Sierra Blanca. Folding and thrusting are
prominent in the Malone Mountains, whereas the strata in the Sierra

Blanca are but slightly folded and cut by a few small normal faults.
Igneous activity was conspicuous in the thrust belt, principally in the
Quitman Mountains. The same igneous province spread over an extensive
part of the domes and basins of the foothill province, viz., the Davis
Mountains volcanic field, the Chisos Mountains, and the Serranias de
Burro uplift. See the Tectonic Map of the United States. In the Quitman
Mountains, the folding and thrusting are followed by extensive erosion
and then eruption of a volcanic series of rhyolites, trachytes, and andes-
ites. The volcanic rocks sagged and were intruded by a ring dike of
diorite. Then an intrusion of quartz diorite followed, and afterward the
quartz monzonite Quitman pluton (Huffington, 1943). This activity, if
related to the Davis Mountains volcanics, occurred in Eocene and Oligo-
cene time.

PLATEAU CENTRAL AND SIERRA MADRE ORIENTAL

The Jurassic and Cretaceous geosyncline of Mexico and the Coahuila
peninsula have already been described. Refer particularly to the paleo-
tectonic maps of this book. During the Laramide orogeny, the thick sedi-
ments of the geosyncline were caught in compressive forces and severely
folded, but the thinly veneered peninsula was only slightly deformed. In
places, later block faults are superimposed on the Laramide folds; but
for the most part, the present-day mountains of the Sierra Madre Oriental
and the high surface of the Plateau Central are the result of a long
chronicle of erosion and alluviation. The two provinces from the eastern
half of the highlands of northern Mexico. They are closely related tectoni-
cally but are somewhat different in surface features. The Plateau Cen-
tral is an area of relatively low relief, but high altitude, and consists of
wide bolson plains from which rise mountains composed largely of the
folded sedimentary rocks. The Sierra Madre Oriental is a region of
high relief along the east side of the Plateau Central and is composed
of parallel mountain ranges also of folded sedimentary rocks.

Figure 28.4 is a typical example of the folds in the geosynclinal parts
of the province. They are tight to the point of being isoclinal, overturned,
and even fan-shaped in places, and closely packed; but in spite of the

SIERRA
CONCORDIA

SIERRA CUPIOO

SIERRA
GARAMBULLO

LEGEND

SEDIMENTARY ROCKS
RECENT | Qa [ ALLUVIUM

PLEISTOCENE

I"q"1

SIERRA
GUITARRA

SIERRA
MESQUITE
OEL SUR

SIERRA SIERRA

MESQUITE BE^RNAD0 SIER«A D|
OEL NORTE / — .. UOSPINOS

SIERRA
CANEJO

AURORA LIMESTONE
kKlpH LA PENA FORMATION
■Jkcu) CUPIOO LIMESTONE

TARAISES FORMATION

LA CASITA FORMATION
LA GLORIA FORMATION

OXFOROIAN

ICNEOUS ROCKS

CCNOZOIC ? SZ?1 PORPHYRY AND GRANITE

SIERRA
MESQUITE
DEL NORTE
Ka kc

SIERRA DE

SAN FRANCISCO

Kip

SIERRA

MESQUITE

OEL NORTE

ELEVATION ABOVE
MEAN SEA LEVEL
IN FEET

VERTICAL ANO HORIZONTAL
SCALES IDENTICAL

Fig. 28.4. Cross sections of the middle part of the Sierra de Parras, Coahuila, Mexico. Reproduced from
Imlay, 1937.

ROCKIES OF NORTHERN MEXICO

147

intense shortening, few thrust faults developed. The Sierra Madre
Oriental, west and northwest of Tampico for a distance of about 100
miles, is made up of several ranges, the Sierra Cucharras, the Sierra Tan-
chipa, and the Sierra del Abra. These also consist mostly of folds, but at
a number of places Kellum ( 1930 ) has interpreted a thrust structure
along their east front. His drawings are reproduced in Fig. 28.5. This belt
of thrusting is approximately in line with the El Paso— Rio Grande thrust
belt farther north. Several large thrusts along the west side of the Sabinas
basin seem to connect the northern and southern thrusts and to form a
belt about 800 miles long. See Tectonic Map of the United States and Fig.
28.1.

The region formerly occupied by the Coahuila peninsula is one, accord-
ing to Kellum et al. (1936a):

... of broad, gentle folding and includes the great brachyanticlines or periclinal
folds of the Sierra de la Paila, the Sierra de los Alamitos, and the Sierra de
Garcia, in the east. It also includes the Sierra del Venado, the Sierra del
Sobaco, the Sierra del Tlahualilo, the Sierra de Campana, and related ranges,
in the west. Undoubtedly, it takes in many mountain ranges lying to the north
of the western group, but these have not been studied in sufficient detail to
demonstrate the regional structural plan. The eastern group of ranges also has
never been studied in detail, but the general structure, as seen from the south
and as reported by Bose, Kane, and others who have crossed them, is a gentle
uplift. The western group is essentially the same but differs in that erosion has
progressed much further and divided the broad, gende uplift into numerous
ranges, more or less separated by valleys filled with alluvium.

These ranges are composed, in large part, of the gypsum facies in the
Cuchillo formation. This is an easily eroded unit, and, where the gypsum and
marl predominate in the section, the mountains have been cut down more
rapidly than where limestone predominates.

The structure of the Cretaceous rocks in the mountains bordering the valleys
of Las Delicias and Acatita illustrates the type of folding characteristic of the
central province ( Coahuila Peninsula ) . The major structure between the two
valleys is a broad, composite, anticlinal uplift, trending northwest-southeast
and plunging in both directions. Superimposed upon it are many sharp, per-
sistent folds, parallel to the central axis. Minor cross folds, ordinarily non-
persistent and with gende dips, appear to reflect topographic irregularities in
the basement rocks. The main axis of the major anticlinorium extends along the
western margin of the range, in its northweastem part, but to the southeast
the axis crosses the central part of the mountain area. Limited observation on
the minor anticlines southwest of this axis indicates that they tend to be asym-
metrical, with the steeper dip on the southwest. The major structure of the

Cretaceous rocks in the Sierra del Tlahualilo, west of the Acatita Valley, is .i
broad, gentle fold, almost perpendicular to tin's major trend, cross Ok- rang! —
one, at its north end; the other, about 15 miles farther south. These are believed
to reflect topographic features, or zones of displacement, in the underlying
basement rocks.

Figure 28.6 shows an example of the structure that developed over the
site of the former Coahuila peninsula.

Tamau//pas

Goaya/ejo Cc/nyOn

Southwest of
3an Lucas ranch

Tamou/zpas /s

Va//ey north of Monte. Cr/sto
ranch

ei fli°r"liX

Gomez Farias

£/ fibre, /s

Car/7 ton F'ass

Atascacfor

Theor/zed development of /I bra- Tanch//oo
moun ta/n fro n t

Fig. 28.5. Sierra Madre Oriental front west of Tampico. After Kellum, 1930.

448

STRUCTURAL GEOLOGY OF NORTH AMERICA

VALLE OE
ACATITA

SIERRA
ACATITA

ACATITA ANTICLINE

CANON
BLANCO

BLANCO ANTICLINE

SIERRA DE LAS
MARGARITAS

NORTHEAST END OF
VALLE DE LAS DELICIAS

LAS DELICIAS ANTICLINE

SIERRA
DEL VENADO

AURORA LIMESTONE
UPPER CUCHILLO

LOWER CUCHILLO

WEST ^ALLUVIUM

EAST PERMIAN?

CROSS SECTION OF MOUNTAINS BORDERING ACATITA AND LAS DELICIAS VALLEYS NEAR NORTHERN EDGE
OF ACATITA.- LAS DELICIAS AREA.

60001

SANTA ANA
PARRITAS ANTICLINE
MUCHACHO ANTICLINE SAN ANTONIO ANTICLINE

INDIDURA

TERTIARY?
CONGL.

PUERTO DE
VENTANILLAS

ACATITA ANTICLINE

SIERRA
DEL SOBACO

SIERRA
CANDELARIA

SOUTHEAST END OF SIERRA

VALLE DE LAS DELICIAS DEL VENADO

LAS DELICIAS ANTICLINE

U. 2000

SEA
LEVEL

AURORA LIMESTONE

LLUVIUM

UPPER CUCHILLO

WEST

EAST

CROSS SECTION OF MOUNTAINS BORDERING ACATITA AND LAS DELICIAS VALLEYS NEAR SOUTHERN EDGE OF ACATITA.- LAS DELICIAS AREA. ELEVATIONS
APPROXIMATE AND RELATIVE.

CONCEALED (GYPSUM,
DOLOMITE,& LIMESTONE)

GYPSIFEROUS BEDS
OF CUCHILLO

BASAL LIMESTONE

OF CUCHILLO

PERMIAN

SHALES

LEGEND

LIMESTONE

Es3

DOLOMITE

GYPSUM

WEST

EAST

PROFILE SECTION OF NORMAL FAULTING OBSERVED ON
SOUTH SIDE OF EAST-WEST SPUR OF SIERRA ACATITA
IMMEDIATELY NORTH OF LAS UVAS. LENGTH OF SECTION
ABOUT ONE-HALF OF ONE MILE. VERTICAL RELIEF ABOUT
500 FEET.

AURORA

LIMESTONE-

LIMESTONE MEMBER
IN CUCHILLO

WEST

EAST

PROFILE SECTION ON SOUTH SIDE OF EAST-WEST SPUR
OF SIERRA CANDELARIA ABOUT DUE NORTH OF EL RAYO.
LENGTH OF SECTION ABOUT THREE-QUARTERS OF ONE
MILE. HEIGHT OF SCARP ON EAST ABOUT 1500 FEET.

Fig. 28.6. Cross sections of the Acotita and Las Delicias area, Coahuila, Mexico. Reproduced from Kelly, 1036.

PARRAS SYNCLINORIUM

South of the Coahuila peninsula is a belt of sinuous folds that trends
approximately east-west. It is about 130 miles long, and tapers from a
width of 40 miles in the west to 20 miles in the east. The folds developed
out of the sediments of the Parras trough (see paleotectonic map of the

Late Cretaceous). The anticlines plunge to the west beneath the alluvial
plain of the Laguna de Mayran; they are steepest on their north flank,
and are usually overturned. Along any one occur domes and saddles, of
which the domes are more overturned (de Cserna, 1956).

At the western end of the Parras basin, the Sierra de Hispana over-
thrust occurs. See Fig. 28.7. The thrust sheet has ridden northeastward

ROCKIES OF NORTHERN MEXICO

449

LOMAS
COLORADAS Kaa

SIERRA DE SARNOSO

7000'
6000'
5000'
4000'
3000'

Fig. 28.7. Cross sections of the mountains west of the Laguna district, after Kellum, 1936. Tc, Late
Cretaceous or Early Tertiary conglomerates; Kct, Cenomanian-Turonian sh. and Is.; Kaa, Aptian-Albian Is.;
Klv, Torcer-Las Vigas series; Jrr, Red Rock series; Tig, igneous intrusives.

against the buttressing and less deformed Coahuila peninsula. To the
west of the Sierra de Hispana thrust, a high mountainous mass consists
of tight folds overturned toward the northeast. This zone has not been
traced in ranges to the northwest, but it undoubtedly continues in that
direction.

The east-west belt of folding veers south-southwesterly at Saltillo and
merges with and forms the Sierra Madre Oriental. Quoting from Kellum
et al. (1936a) again:

In this region, intensive compression has developed a series of overturned
or fan-shaped anticlines and synclines in Cretaceous and Jurassic rocks, with
enormous horizontal shortening. The axes of these folds trend, in general, east-
west and pass eastward by a rather short curve to a southeast direction. At the
west end, where they close and plunge into the Parras Basin, they become sym-
metrical, and then are overturned westward, parallel to the strike of their axes.
No important faulting has been recognized.

East of the Sierra Madre Oriental, this prominent zone of east-west folds is
present in central Tamaulipas in the San Carlos Mountains, which rise out of
the coastal plain about midway between the Cordilleran front and the Gulf
Coast. The San Carlos Mountains are a broad, arcuate geanticline [arch as de-
fined in this book], trending in a general easterly direction, with the convex
side to the south. Superimposed upon this major structure are numerous, low
flexures, parallel to it, and also a number of domes produced by igneous in-
trusions. The axes of folds in this geanticline, are not the continuation of axes in
the Sierra Madre Oriental, but are the continuation of axes which lie east of,
and parallel to, the Cordilleran front, farther northwest, and are turned east-
ward in the zone of cross-folding.

West of the Sierra Madre Oriental and south of the Parras Basin the structure
of this belt has been studied in a number of areas. In the region of Mazapil-
Concepcion del Oro, in northern Zacatecas, the mountain ranges are anticlinal
and trend generallv eastward; their structure is complicated by several faults
and by the presence of intrusive masses. The Sierra de Santa Rosa and the core
of the Sierra de la Canutillo show a slight tendencv toward fan structure. The
Concepcion del Oro anticline, a southeast continuation of the Siena de la Caja
structure, is overturned toward the northeast and is crossed bv a fault.

450

STRUCTURAL GEOLOGY OF NORTH AMERICA

SEDIMENTARY ROCKS
REVNOSA fe-^-d MtNOEZ kyXT^SAN T ELIPeEHtaMAULIPAS

□ i

IGNEOUS ROCKS

SIERRA DE SAN JOSE

TUNA MANSA

ANTICLINE

1000

2000 SOUTHWEST

1000 At

HUAHUIRAN-OJO

DE ACUA
LINE OF FOLDING

EL MULATO

Inortmeast

CERRO OIENTE

CIRRO JATERO
\

3000

,ooo SOUTHWEST

1000 C g. t ^_>,--' '.\\'-;t

CERRO CUERO DURO-

SACRAMENTO TAMAULIPECA HUAHUIRAN-OJO

TUNA MANSA 1 AURELIS STRUCTURE OE AGUA

ANTICLINE I LINE OF FOLDING I LINE OF FOLDING

CERRO CORCOVADO

CRtll L L A S

3000

looo SOUTHWEST
iooo E

NORTHEAST
.F

Fig. 28.8. Cross sections of the San Carlos Mountains, Tamaulipas, Mexico. Reproduced from Kellum, 1937.

OROGENIC HISTORY

In review of the orogenic history of northeastern Mexico, it has been
concluded that the late Paleozoic belt of folding and thrusting of the
Marathons of western Texas curved southward and probably extended
into the site of and formed the later Coahuila peninsula. From the south-
ern part of the peninsula, the late Paleozoic orogen turned eastward to

Monterey, according to Humphrey ( 1947 ) and then south-southwesterly
for a long, but unknown, distance. It seems doubtful that the orogenic
belt could have endured throughout more than early Mesozoic time,
when epeirogenic movements may have rejuvenated segments of it which
served as barriers to marine invasions from the Gulf region. The Mexican
geosyncline developed in Jurassic and Cretaceous time on the western
and southern (foreland) sides of the Paleozoic belt, but sedimentation

ROCKIES OF NORTHERN MEXICO

451

also occurred on the eastern (hinterland) side. In Late Jurassic time,
movements of the southern part of the peninsula adjacent to the geosyn-
cline are reflected in the sediments; and these in turn reflect the Nevadan
orogeny. In the Early Cretaceous, minor and local movements in the
Coahuila peninsula furnished some coarse sediments to the seas. The
peninsula was finally submerged in Aptian time but continued to act
as a relatively high and stable mass. In the Late Cretaceous, considerable
thicknesses of sediments were deposited along the southern border of
the peninsula in the Parras trough, whose position coincides with the
margin of the Late Jurassic seas.

In the Early Tertiary, the deposits adjacent to the peninsula and its east-
ward and southward extension were deformed into long narrow folds by
forces acting about normal to the western and southern border of the
buttressing mass, which itself was only slightly deformed. Along the
east side another belt of folds was formed. This belt is overthrust east-
ward along its east margin, and a foothill belt of structures was formed
in front.

FOOTHILL BELT

In places along the inner margin of the Gulf Coastal Plain and not far
east of the east front of the Sierra Madre Oriental are several low ranges.
About 60 miles southwest of Del Rio on the Rio Grande is the Serranias

del Rurro; to the southeast of this is the Sierra Lampazos; still farther to
the southeast is the Sierra de San Carlos; and then northwest of Tampico
is the Sierra Tamaulipas. These are generally broad folds and domes that
rise from the nearly horizontal beds of the Coastal Plain and expose
gently arched Lower Cretaceous limestones along their crests. Some of
these mountains, notably the Sierra de San Carlos, contain igneous pings
and laccoliths (Fig. 28.8) in part of alkalic composition. The San Carlos
Mountains also contain gentle, east-west trending folds superposed on
the dome; and these, Kellum (1936, 1937) believes, reflect the eastward
trend of the Parras basin folds. The folds, in fact, are found, although ill-
defined, still farther east in the Sierra de Cruillas in central Tamaulipas.
where they have a northeast trend, and there disappear under the
Coastal Plain.

About 60 miles west of Tampico is the Sierra del Abra, which there
forms the east front of the Sierra Madre Oriental. It is an uplift in part
of eastwardly overturned Middle and Upper Cretaceous beds (Kellum,
1930). East of the Sierra del Abra is a Cretaceous and Tertiary basin and
then the buried Tamasopo ridge about halfway between the Sierra Madre
and the coast. Kellum believes that this buried ridge has had a historv
similar to the Sierra del Abra. It is the site of the remarkable "southern"
oil fields of Tampico-Tuxpam district of Mexico. Rasic intrusions and
extrusions are present in both the Tamasopo buried ridge and the Sierra
del Abra.

COAST RANGES

OF THE PACIFIC AND THE

SAN ANDREAS FAULT SYSTEM

MAJOR DIVISIONS

A belt of mountains parallels the present coast in Washington, Oregon,
and California and, except as noted, the mountains are known as the
Coast Ranges. They are separated from more interior chains by broad
depressions, in Washington and Oregon called the Willamette-Puget
Sound depression, and in California called the Great Valley. The index
map, Fig. 29.1 shows these features. The Coast Ranges have had a pro-
longed Tertiary and Quaternary history, and their development occurred
parallel in time with the Laramide Rockies and the ranges and valleys of
the Great Basin.

sm^ \

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Fig. 29.1. Index map of the Coast Ranges and associated geological provinces of Washington,
Oregon, and California, showing the lines of cross sections and the San Andreas fault and
possible associates.

452

COAST RANGES OF THE PACIFIC AND THE SAN ANDREAS FAULT SYSTEM

The division of the Coast Ranges from San Francisco Bay southward to
Santa Maria will here be designated the Central Coast Ranges ( Fig. 29.2),
and the division north of San Francisco Bay to the Klamath Mountains
will be called the Northern Coast Ranges. A division in southern California
with pronounced east-west trends including the Santa Barbara, Ventura,
and Los Angeles districts is referred to as the Southern Coast Ranges or
Transverse Ranges. The Coast Ranges of Oregon and Washington are a
unit geologically and will be considered as a fourth division. They are
separated from the Northern Coast Ranges of California by the Klamath
Mountains, which are part of the Nevadan orogenic belt.

The Sierra Baja California of southermost California or the Peninsular
ranges, and the peninsula of Baja California is a fifth division, but will be
discussed in Chapter 30. It is a complex of Nevadan geology and later
Cretaceous and Tertiary beds affected by folding and block faulting.

Still another division, the sixth, remains to be mentioned, namely, the
submarine area south of the Transverse Ranges. This ocean bottom has
been found in recent years to be one of rugged relief, and the researchers
who have ventured a diagnosis of the topography there agree that it is
part of the continental framework. It is discussed in Chapter 32.

CENTRAL COAST RANGES OF CALIFORNIA

San Joaquin Embayment and the Diablo Uplift

In the tectonic map of the Late Cretaceous, Plate 12, it will be seen that
the uplift of Salinia separated a basin of sedimentation to the north and
south in the region of the Central Coast Ranges. The chief change that
occurred in Early Tertiary time is that Salinia altered position and size
somewhat and became the Diablo uplift; and another small uplift, the
San Rafael, came into existence just to the south. The details of these
changes are shown in the paleotectonic maps of Fig. 29.3. Also, the trough
of deposition, the San Joaquin embayment, became less constricted op-
posite the uplifts and received from 5000 to 15,000 feet of sediments in the
site of the Central Coast Ranges, and 30,000 feet in the Southern Coast
Ranges.

Under a later heading, the San Andreas fault system, it will be shown

r. ,

Fig. 29.2. Index map of the Coast
Ranges and fault systems of Cali-
fornia. Compiled from the fault map
of California (1955), from Dibblee,
unpublished maps, and other sources.

Fig. 29.3. A, map of southern California showing the distribution of Franciscan outcrops (black)
and granite of the Nevadan orogeny (hachured). After Reed and Hollister, 1936. The granitic
area marked Salina rose and became a landmass in Upper Cretaceous time, according to
Taliaferro, 1943. Compare with Plate 16.

B, map of southern California showing the Tertiary provinces and their relations to the
basement rock. See opposite map. After Reed and Hollister, 1936.

COAST RANGES OF THE PACIFIC AND THE SAN ANDREAS FAULT SYSTEM

455

that the Coast Ranges of southern California have probably shifted some
300 miles northwestward to their present position, and therefore the
paleotectonic maps of Fig. 29.3 are probably not correct. They show, how-
ever, the principal tectonic elements, and are reproduced because they
help in understanding the make-up of the region.

The evolution of the central Coast Ranges in Early Tertiary time is
idealized in Fig. 29.4, and the deposits under that part of the San Joaquin
embayment that were not deformed appreciably and later became the
San Joaquin Valley are shown in Fig. 29.5.

Early Tertiary Phase

In Chapter 17 it was pointed out that the Santa Lucian orogeny was the
last disturbance in the Cretaceous, and that following it, the widespread,
thick Asuncion group was deposited in Senonian, Maestrichtian, and
Danian time. The deposition of the Asuncion was brought to a close in the
Central Coast Ranges by uplift, tilting, and probably folding; but so little
of the Paleocene is preserved that its original extent and thickness and the
degree and extent of the post-Cretaceous disturbance are not known.
However, it is believed that the disturbance was not as severe as previous
ones, because the uppermost Cretaceous and Paleocene, where observed,
are only slightly discordant, and very little change in the character of the
sediments is noted ( Taliaferro, 1943b ) .

Taliaferro (1943a) suggests that the Paleocene represents a final stage
in the history of the late Mesozoic geosyncline, the California trough of
this book, in which the Franciscan, Knoxville, Shasta, and Chico sedi-
ments were deposited. See Fig. 29.6. The part of the trough in the site of
[ the present central Coast Ranges and along the western border of the
present San Joaquin Valley received sediments throughout the late Upper
Cretaceous, and then weak uplift, folding, and erosion occurred to the
west, while the central part of the trough was little affected. Probably
general uplift occurred, and the seas retreated; but the uplift appears to
have been quickly succeeded by downsinking, and the Paleocene sea
flooded at least parts of the Cretaceous deposits. This was the last time
that deposition took place over rather large areas of the trough. The
changes that had taken place previously were of lesser magnitude than

PACIFIC

DIABLO

5AN JOAQUIN

MOMAVIA

OCEAN

UPLIFT

ELMBAYMENT

Rosomond Basins

™ TV , ,

------.----------; ; -.V_-_-£££=,

■■■/:>:-:--,^r-

Beginning of Miocene (Monterey) Time
Tov? ^-

In late Olioocene (close of Vaqueros) time

Close of Eocene time

Close of Cretaceous (Chico) time

Fig. 29.4. Evolution of Coast Ranges and Great Valley in Early Tertiary time. Kc, Chico
formation; Te, Eocene formations; Tov, Oligocene and Vaqueros formation; Tt, Temblor forma-
toin. After Reed, 1933. Section E-E', Fig. 29.1.

those that took place after the Paleocene. The available evidence indicates
that the final fragmentation of the California trough took place in the
Eocene. Great thicknesses of Tertiarv sediments accumulated, but they
formed in comparatively narrow basins, some of which were at a marked
angle to the more extensive and enduring late Mesozoic trough.

Eocene and Oligocene strata have limited distribution in the central
Coast Ranges, and their nomenclature and correlation have been the ob-
jects of considerable discussion. Typical Tertiarv formations are listed in
the chart on page 458. Although thick lower Eocene sections occur,
they are in small, isolated localities; and more of the California Coast
Ranges were emergent than at any time during the Cretaceous and Juras-

456

STRUCTURAL GEOLOGY OF NORTH AMERICA

TEMBLOR RANGE

SIERRA NEVADA FOOTHILLS

Fig. 29.5. Generalized section across the southern
Section K-K', Fig. 29.1.

sic. The Santa Lucia Range, most of the Santa Cruz Mountains, and much
of the Diablo Range stood above sea level, as did also the central Sierra
Nevada.

The middle Eocene sea appears to have had a much wider extent and
to have flooded much of the San Joaquin embayment. The middle Eocene
formations are recognized by Taliaferro (1943b) to be the Capay, Do-
mengine, and lone. They are sandstones, shales, clays, limestones, and
coal beds; and they are unusually fine grained except at the margin of
the border lands. The lone is clearly of an eastern source, but the Middle

San Joaquin Valley. After Hill and Eckis, 1943.

Eocene along the Diablo Range contains detritus from the Franciscan
and Cretaceous strata of the ancestral Coast Ranges as well as the crys-
talline rocks of the ancestral Sierra Nevada. The Middle Eocene covered
the east flank and northern end of the Diablo Range, probably a part of
the Santa Cruz Range, and northeastern part of the Santa Lucia Range.
Minor volcanic activity can be recognized by rhyolitic and andesitic debris
in the lone of the Great Valley, supposedly of an eastern source in the
Sierras, and by bentonite in the Domengine of the Coast Ranges, sup-
posedly of a western source ( Taliaferro, 1943b ) . See accompanying chart.

Fig. 29.6. Maps of southern California showing the basins
of deposition and the land areas (cross-ruled) during the
Tertiary. After Reed (1933) and Hoots et a/. (1954). Com-
pare these maps with those of Fig. 29.3. SF, San Francisco;
ST, Stockton; S. Salinas; C, Coalinga; SM, Santa Maria; M,
Maricopa; B, Bakersfield; V, Ventura; LA, Los Angeles. If the
Coast Ranges southwest of the San Andreas fault have
moved about 200 miles to the northwest since the beginning
of Tertiary time, then progressive adjustments in its relation
to the basins and lands on the northwest must be
visualized.

PALEOCENE ANO EOCENE

MIDDLE ANO UPPER MIOCENE

'OO MH.E5

458

STRUCTURAL GEOLOGY OF NORTH AMERICA

Typical Formations

Age

Assignments

in California

Current Usage

Grouping

R. D. Reed

Upper San Pedro

Upper
Pleistocene

Pleistocene

Lower San Pedro, Saugus,

Lower

Upper

Tulare

Pleistocene

Neogene

Etchegoin, Pico, Repetto

Pliocene

Neogene

Santa Margarita, Monterey,

Upper and

Lower

Modelo, Topango, Temblor

Middle Miocene

Neogene

Vaqueros, Temblor, Pleito,

Lower Miocene

Upper

San Lorenzo, San Ramon
Kreyenhagen, Tejon, Capay,

and Oligocene
Eocene and

Paleogene
Lower

Paleogene

Domengine, Meganos, Martinez,

Paleocene

Paleogene

lone, Poway

The upper Eocene (Tejon, Markley, Kreyenhagen, Gaviota, and
Wheatland) has a more limited distribution than the Middle Eocene.
Very slight folding and faulting may have intervened, but no mountains
were built, and the same seaways as before persisted, though somewhat
restricted. The Kreyenhagen has some bentonite and vitric tuff beds, and
the Wheatland has some andesitic debris, both indicating continued small-
scale volcanic activity.

The Oligocene strata have even a more restricted distribution than the
upper Eocene, but occupy the same basins. They generally rest uncon-
formably on Eocene sediments and, in turn, are generally unconformably
overlain by the Miocene. The sediments regarded as Oligocene at present
are those of the San Lorenzo group. Volcanism occurred during the Oligo-
cene in the Mount Diablo and San Francisco Ray regions, where more
than 100 feet of rhyolite tuff occurs in the Kirker formation.

The disconformities and slight angular unconformities that are known
in the Eocene and Oligocene might indicate comparative quiet in strong
contrast to the preceding and succeeding periods. This seeming lack of
important diastrophism, however, may be more apparent than real be-
cause of lack of evidence. The Upper Jurassic and Cretaceous unconform-
ities show that the various crustal movements were strongest in the western

coastal region, the volcanic archipelago, and died out eastward. The
same may be true of the Eocene and Oligocene (Taliaferro, 1943b).

The structures formed probably represent the general effect of several
episodes of movement. Although both folding and faulting occurred, nor-
mal faulting in the Diablo Range of great magnitude predominated. It
was during the Early Tertiary phase that the uplift and westward tilting of
the Gabilan Mesa (Diablo uplift) occurred, approximately along a line
corresponding to the present position of the San Andreas fault. This north-
eastern boundary fault may be thought of as ancestral to the San Andreas
fault in the central Coast Ranges, where the two coincide. The southwest-
ern side of the uplift is irregular, with several smaller faults. See Fig.
29.4.

Late Miocene Phase

Over most of the central Coast Ranges, the Miocene began with gentle
sinking, and basins of the early Tertiary were first uniformly flooded and
then overlapped. Early in middle Miocene, the uniform and gentle
sinking gave way to sharper downwarping, and great thicknesses of sedi-
ments accumulated locally. It is believed that the movement was caused |
by compression and that the interbasin areas rose at the same time as the
basins sank. The heterogeneous pre-Tertiary basement is believed to have
precluded uniform folding throughout the Coast Ranges. An important
and rather long-enduring trough developed along the western downtilted
side of the Gabilan Mesa, west of the Santa Lucia Range. The trough east
of the range continued to sink and expand both southward and northward,
until a connection was made with the sea in the site of the present Mon-
terey Ray.

The crest of the Coalinga anticline, now composed of Franciscan, stood
above sea level throughout the Miocene.

The effect of movements during the later upper Miocene cannot be
clearly evaluated in all places, because erosion incident to later severe
deformation has removed much of the evidence. This is especially true in
the Santa Lucia Range. However, in the northern part of the Castle
Mountain Range, the nature of upper Miocene deformation is well shown.
Figure 29.7 has been prepared to illustrate the structural evolution. Santa

COAST RANGES OF THE PACIFIC AND THE SAN ANDREAS FAULT SYSTEM

45';

Tms

Tinm-

Tmm -^2

Fig. 29.7. Evolution of the Castle Mountain Range. Ideal sections showing late Upper Miocene
folding (lowest section), erosion and deposition of McLure shale (middle section, latest Upper
Miocene), and thrusting and folding in late Pliocene (upper section). The marginal thrusts of
the Castle Mountain Range developed in the sites of the Upper Miocene anticlines. More
thrusting occurred in mid-Pleistocene which is not represented. After Reed and Hollister, 1936.
Section H-H', Fig. 29.1. Tms, Santa Margarita sandstone; Tmm, MsLure shale; Tp, Purisma fm.
]

"Margarita sands 100 to 300 feet thick of late Miocene age were deposited
lover a fairly even-floored basement complex consisting chiefly of Fran-
jciscan, but with remnants of Knoxville, Shasta, and Upper Cretaceous
.sediments. After, or perhaps even during the deposition of these sands,
igentle anticlinal folding occurred along two subparallel lines 6 to 8 miles
apart, which correspond approximately to the present margins of the
range. The maximum observed dip of the flanks is 11 degrees. The two
(anticlinal ridges were planed off, perhaps as rapidly as they rose, and the
McLure shale was then deposited over the region. Where it crosses the
;two anticlines, it lies unconformably on the Santa Margarita sands and on
the Franciscan. Elsewhere, the Santa Margarita and McLure are conform-
able, and in places they appear to grade into one another. In the later

Pliocene and Pleistocene deformation, thrusts developed approximately
in the sites of the anticlines.

Late Pliocene and Mid-Pleistocene Phases

The thick accumulation of Miocene sediments was accentuated, in gen-
eral, by further deposition in the same troughs in early and middle Plio-
cene time. The gentle compressive movements, which started in the
Miocene and then relaxed for a while, surged to a peak in the late Pliocene
and again to another peak in the mid-Pleistocene. The last surge is prob-
ably still climactic.

The folds and thrust faults that are the conspicuous features of cross
sections and field observation are largely the result of these two move-
ments. Cross sections D-D', F-F', G-C, and I-I' of Fig. 29.8 are espe-
cially illustrative of the compressional deformation to which the rocks of
the Diablo uplift and the San Joaquin embayment in the central Coast
Ranges were subjected.

Opinions differ as to the relative importance of the two phases. In some
places, only one has been recognized. According to Taliaferro ( 1943b ) , the
geologists in general who have worked in the western part of the Coast
Ranges have emphasized the importance of the late Pliocene disturbance
there, and those who have worked chiefly in the eastern part have stressed
the mid-Pleistocene compression.

The regions underlain at comparatively shallow depths by crystalline
rocks, or those where the crystalline rocks were exposed, yielded by fault-
ing; and those underlain by thick sections of strata (8000-20,000 feet)
yielded by folding and thrusting ( Taliaferro, 1943b ) , except for the eastern
part of the San Joaquin embayment which was left little deformed and is
now the Great Valley. The ranges are generally bordered by thrusts, but
the individual thrusts can be traced only 20 to 25 miles. As a thrust dies
out, its place is commonly taken by one or more en echelon faults. The
thrusting is both westward and eastward, with some structural units
(ranges) being bordered by complementary inward-dipping faults. The
thrusts marginal to the structural units generally have shallower dips than
those within. The structure of the Central Coast Ranges as interpreted in
the cross sections is rather similar to that of the Montana and Alberta

Pilar citos
thruit
Ku Tm
T\/\'\ _
\/\,Bc\/N/\ j-f
>\/\/\A/\/

San Andreas
fault
Tp

Hoy war a fault
Tvol

fit Diablo

Orfigolito
thrust

3a n Joaquin
Valley

Trnz 7;

5ar> Andreas
fault

Tmcl

R A Av\ AAA />7_y/_\ A A A A A A A A A A A/_\A AAA,

Joaquin Ridge

IS?

J><7/? Joaquin l/a I ley

Qa\

— ' MILES

Fig. 29.8. Cross sections of the central Coast Ranges after Taliaferro, 1942. Refer to index
map, Fig. 29.1. Be, granite, gneiss, schist, and marble; Jf, Franciscan sediments and volcanics;
Jk, noxville shales and sandstones (Jf and Jk are Upper Jurassic); Ks, Shasta group, Lower
Cretaceous; Kp, Pacheco group; Ka, Asuncion group; Ku, undifferentiated (Kp, Ka, Ku, Upper
Cretaceous); Tmz, Martinez, Paleocene; Te, Eocene undifferentiated; Tv, Vaqueros, Lower

Miocene; Tm, Salinas shale, Temblor, etc., Middle Miocene; Tmcl, McLure shale. Upper Miocene;
Tsm, Santa Margarita, San Pablo, etc.. Upper Miocene; Tvol, volcanics, sills, dikes, Miocene; Tej,
Etchegoin, Tj, Jacalitos, Tp, Purisma, Pliocene; TQpr, Paso Robles, Santa Clara, San Benito,
Tulare, etc., Plio-Pleistocene.

COAST RANGES OF THE PACIFIC AND THE SAN ANDREAS FAULT SYSTEM

•lol

Rockies. The Coast Ranges have a more heterogeneous basement, which
has served to localize the thrusts; the strata in them are generally less
indurated; the scale is somewhat smaller; and the movement along the
thrust surfaces is generally less.

In addition to the thrusts, there are transverse faults, some of which
cut almost completely across a range. They relate to the uplift of the Santa
Lucia Range, because in its southern part, each transverse fault is down-
thrown on the south, and the range becomes progressively lower in eleva-
tion in that direction ( Taliaferro, 1943b ) .

There was little volcanism in the central Coast Ranges during the
Pleistocene as compared with the extensive and important volcanism in
the Sierra Nevada Range and in the Cascades. Olivine basalt flows and
agglomerates occur in the Santa Lucia and Diablo ranges and along the
east side of Santa Clara Valley.

The Tertiary structural history was much like that of the Late Jurassic
and Cretaceous in the following respects. The Orogeny was generally
severest westward, because the unconformities are more angular and
bring rocks of greater age differences together the farther west from the
Great Valley they are observed; and volcanism continued, with tuffs and
flows a characteristic part of middle Eocene, upper Eocene, lower Mio-
cene, middle and upper Miocene, lower and middle Pliocene, and Pleisto-
cene formations.

The mid-Pleistocene orogeny occurred farther inland (eastward) than
the late Pliocene orogeny and is a contrary note to the generalization of
increasing intensity westward. However, the two disturbances are closely
connected in time and may be part of a general wave of deformation
originating in the west and progressing eastward.

The mid-Pleistocene disturbance is associated with the final disappear-
ance of the Tertiary troughs of deposition and the foundering of consider-
able segments of the Coast Ranges into the Pacific. It is evident from
inspection of the tectonic and geologic maps that the sea has transgressed
part of the Coast Range orogenic belt; the structures are discontinuous
at the present shore line. Also, the reconstructed Tertiary uplifts and
troughs head out to sea, as if only half exposed in the Coast Ranges.
Recent detailed mapping of the ocean floor off California has revealed a

topography much like that in the Coast Ranges, and it can best be ex-
plained as the surficial expression of the long-evolving volcanic archipel-
ago of Paleozoic, Mesozoic, and Cenozoic time, with particular respect to
the late Pliocene and Pleistocene deformations. The interpretation of the
topography of the sea floor will be taken up later in a separate chapter.
The San Andreas fault, that stretches through the three divisions of the
Coast Ranges of California will be considered later.

Erosion Following Main Orogeny

Following the late Pliocene and mid-Pleistocene orogeny, which re-
sulted in rapid uplift and oversteepening of the mountain fronts, vigorous
erosion reduced the escarpments and ranges until now there is little
physiographic evidence left of individual faults, although some of them
were of several thousand feet displacement. Conspicuous features of the
rapid erosion are the landslides from the oversteepened mountain fronts.
Some were gigantic in size and took place coincident with the thrusting
and uplift of the ranges; others have occurred since. In places, there is a
definite sequence of slides observable, detected by different amounts of
dissection. They obscure the true structure of the mountain front in many
places.

Late Pleistocene and Recent Gentle Folding

In the Los Angeles, Ventura, and San Joaquin basins, gentle folds have
developed so recently that they have been little modified by erosion, and
precise elevation surveys show that movement is still going on vigorously.
The subject will be taken up at greater length under the next major head-
ing, "Southern Coast or Transverse Ranges of California."

Terraces

Terraces are numerous and well developed along the shore and in in-
terior valleys of the Coast Ranges. The marine terraces are found at
elevations up to 1500 feet, and attest the rise of the Coast Ranges in very
recent times. They are cut on the beveled edges of the folded Plio-
Pleistocene sediments, and therefore are very young. Individual terraces
are difficult if not impossible to follow from one region to another, and

462

STRUCTURAL GEOLOGY OF NORTH AMERICA

App S Miles

Fig. 29.9. Ventura basin showing conditions before Middle Pleistocene folding. Section P-P',
Fig. 29.1.

there is little definite correspondence of the various terrace levels over
wide areas. Over limited areas, there may be very definite intervals be-
tween terrace; a few miles away, the terraces may be equally well
developed; but the intervals between terraces in the two areas differ.
Furthermore, the marine terraces along the coast cannot be correlated
definitely with the terraces of the interior valleys, but there is strong evi-
dence that the coastal area has very recently been uplifted more than the
interior (Taliaferro, 1943b).

The San Francisco Bay area was probably depressed rather than up-
lifted, but it is not possible to say that the entire lowland and bay was
depressed subsequent to the folding and thrusting because it may have
been left that way as orogeny progressed ( Taliaferro, 1943b ) .

The terraces have been cited as evidence of widespread epeirogeny,
but Taliaferro thinks they may be due to gentle folding or upbowing of
the ranges.

A few but indisputable examples of tilted beaches are known, but the
structural meaning is yet obscure.

SOUTHERN COAST OR TRANSVERSE RANGES OF CALIFORNIA

Principal Structural Features

The Southern Coast Ranges trend in an east-west direction which is
transverse to that of the Central and Peninsular Ranges. See Fig. 29.2.
The relief features as well as the faults and folds are generally so oriented.

The formations and structure of the southern part of the San Joaquin
basin are shown in cross section in Fig. 29.5. A cross section of the Ven-
tura basin, restored to the time preceding the major deformation, is pre-
sented in Fig. 29.9. The cross sections L-L', M-M', and N-N', Fig. 29.10,
and O-O', Fig. 29.11, are representative of the present structure and major
groups of beds in various parts of the southern Coast Ranges.

Early Tertiary Phase

Paleocene, Eocene, Oligocene, and early Miocene times were generally
characterized by subsidence of the basins previously mentioned, but at
times during these epochs slight surges of crustal unrest are attested by
conglomerates and local small-angle unconformities. During the Eocene,
the greatest subsidence occurred, and it centered in the Ventura basin.
The Paleocene beds are generally coarse, variable in lithology, and of
restricted distribution. In most places, the contact with Cretaceous beds
is difficult to locate, and the two systems seem conformable. Aside from
the coarser aspect, the Paleocene beds are not much different from the
Cretaceous. In one locality, an angular unconformity of 30 degrees has
been noted (Reed, 1933), and it has been taken to mean gentle folding
in places at the beginning of the Tertiary.

The Eocene sediments were generally finer, and consisted of arkosic
sandstones and silty and sandy shales. They accumulated to a depth of
11,900 feet in the Ventura basin. Perhaps the total thickness there of
Paleocene and Eocene beds, the Martinez and Tejon formations, was
20,000 feet. See thickness contours of Fig. 29.6. The Eocene deposits
spread over much larger areas than the Paleocene, but the subsidence
followed the earlier troughs or defined them better. Toward the end of the
Eocene or during the Oligocene, the areas of deposition remained large,
but the facies represented became highly varied. They included the Poway

COAST RANGES OF THE PACIFIC AND THE SAN ANDREAS FAULT SYSTEM

463

Pacific
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5a n Joaquin
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Los Angeles Basin

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MILES

Fig. 29.10. Generalized cross sections of the Coast Ranges of southern California, after Reed and
Hollister, 1936. Be, granite and metamorphic rock basement; Jf, Franciscan; Ksc, Cretaceous strata;

Te, Eocene strata; To, Oligocene strata; Tm, Miocene strata; Tp, Pliocene strata; Tv, volcanics.
Sections J-J', L-L', M-M\ and N-N' of Fig. 29.1.

conglomerate, lower Sespe continental red sandstone, Coldwater and
iTejon marine sandstone and sandy shale, and the Kreyenhagen siliceous
I shale.

Oligocene and early Miocene (Fig. 29.6) time saw a great increase in

size of the land areas, considerable parts of which received thick deposits
; of red and green shales, the nonmarine part of the Sespe formation. Later
ion in Vaqueros time, the sea invaded much of the Sespe lowland. In the
jSan Joaquin embayment, the Kreyenhagen shale was deposited, and it

graded into sandstone southward. Still farther southwest, in the Santa
Barbara embayment, the Sespe red beds accumulated. Reed (1933) be-
lieves a basin had become semi-inclosed and was gradually filled with
silts and oozes of high organic content that later evolved the oil in the
Coalinga district.

Although the land areas increased in size and the seaways decreased,
the Santa Barbara trough continued strongly negative, and Miocene and
Pliocene sediments accumulated 25,000 to 30,000 feet thick in the deepest

Santa Ynez
Range

Santa Ynez
River
Santa Ynez

Unit Pine
Mt

Big Pine
.Big Pine m+

\fiiutt

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Santa Ynez
Range

Santa Ynez
River

Little Pine
Mt

Big Pine N
Mt
I

M.and U. Miocene 2 50O"

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SCALE

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mu vm waamm wzmnnm vmimmit*

Fig. 29.11. Presenf and Upper Miocene structure in Santa Ynez-Santa Barbara district. Section O-O',
Fig. 29.1. Jf, Franciscan; Kk, Knoxville; Kc, Chico, Te, Eocene formations; To, Oligocene formations; Tm,
Miocene. Reproduced from Reed and Hollister, 1936.

COAST RANGES OF THE PACIFIC AND THE SAN ANDREAS FAULT SYSTEM

465

part. The Oakridge uplift started to evolve when, at the end of Sespe time,
local subsidence ceased to be so rapid. The lower Miocene strata include
the Vaqueros sandstone and the Rincon (Temblor) clay shale. The mid-
dle Miocene has a basal limestone. Both middle and upper Miocene
contain a predominance of siliceous organic shale, including diatomite,
chert, and various other siliceous varieties.

Middle and Late Miocene Phase

The late Miocene phase (Fig. 29.6) is not well known in the southern
Coast Ranges, and the evidence that is available suggests only local, gen-
tle folding and volcanism. Near Santa Barbara, a coarse breccia of middle
late Miocene age occurs at the plunging ends of cross folds in the
east-west trending structures. Reed believes this breccia was formed dur-
ing the cross folding. In the cross section O-O', Fig. 29.11, the folding
seems to be mostly pre-middle Miocene, and the anticline grew by several
movements from Eocene to Miocene. It is probable that the Santa Barbara
district of the Santa Barbara embayment in middle Miocene time was
one of southward regional dips, fluted by a few low folds of northerly
trend. The most important of the cross folds was along the axis of the San
Rafael uplift and extended to Ventura (Reed and Hollister, 1936). East
and west of Los Angeles, in the Los Angeles basin, gentle folding oc-
curred in late Miocene time. In spite of the folding and the change to
heterogeneous facies in the late Miocene from homogeneous facies in
the middle Miocene, the boundaries of the two basins were much the
same ( Reed and Hollister, 1936 ) .

Volcanic rocks form an important constituent of the middle and upper
JMiocene along the axis of the San Rafael uplift but are not conspicuous
jelsewhere.

Pleistocene Phase

Pliocene and lower Pleistocene deposits of considerable thickness occur
only in restricted parts of the Miocene basins. See Fig. 29.6. The three
thickest deposits of Pliocene beds in southern California are found in the
Maricopa, Ventura, and Los Angeles basins. The thicknesses are very

great, possibly 10,000 to 20,000 feet in the first, 18,000 feet in the second,
and 10,000 in the third. The foraminifera in the lower Pliocene beds of
the Ventura and Los Angeles basins suggest that the sea was one to two
miles deep at the time of deposition.

Lower Pleistocene was deposited in all the Pliocene basins, but only in
the western depressions did marine beds accumulate. Eastward, the beds
are continental and are 1000 to 5000 feet thick.

The late Pliocene orogenic phase of the central Coast Ranges as de-
scribed by Taliaferro is not a "notable disturbance" in the southern Coast
Ranges, according to Reed and Hollister (1936). A disconformity is pres-
ent in the Repetto Hills, the Ventura basin, and the San Joaquin Valley
between the lower and upper Pico, but the break has not been observed
as an angular unconformity anywhere.

Along the seaward margin of the Los Angeles basin, there is a pro-
nounced angular unconformity between lower San Pedro beds of early
and middle Pleistocene age and upper San Pedro beds of late Pleisto-
cene age (Reed and Hollister, 1936). The evident folds and thrusts of
southern California can best be explained, according to Reed and Hol-
lister, as having formed approximately at this time. Examine cross sections
L-L', M-M', N-N', Fig. 29.10, and O-O', Fig. 29.11.

In the Ventura basin, about 5000 feet of lower Pleistocene beds have
been turned up so as to have dips of 30 to 90 degrees beveled by erosion
and covered by about 300 feet of upper Pleistocene fanglomerates. The
fossil Equus, cf. occidentalis, occurs both above and below the angular
conformity, apparently without change, and indicates that the structure
was formed during a very short period of time (Bailey. 1943).

The structural history of the Kettleman Hills anticline is instructive. In
it, the Tulare formation, which is lower Pleistocene and not older than
latest Miocene, is folded apparently as strongly as the underlying forma-
tions. The anticline, therefore, was formed almost entirely in post-Tulare
time. After its rise, it was eroded until several thousand feet of rock were
removed from its axial part. Toward the south end, it was reduced to a
plain which then became buried in alluvium. After this, the alluvium
was arched into a new, though gentle, fold.

466

STRUCTURAL GEOLOGY OF NORTH AMERICA

In the Los Angeles basin, a number of unconformities within the upper
Miocene and the Pliocene section indicate a succession of uplifts along
the major structural trends during these times ( Wissler, 1941 ) .

The present condition of the crust in southern California is one of
decided instability. Folding, thrusting, and high-angle faulting have not
only manifested themselves in earthquakes and buckled pipelines, cables,
and pavements; through precise surveys, the amount and rate of the move-
ments have been measured in places. Gilluly (1949) reviews these move-
ments and concludes that the present is a time of typical orogeny. The
seismicity of the western Cordillera will be considered in Chapter 31.

It is evident that the division of the structural history of southern Cali-
fornia into phases is not altogether a satisfactory treatment, because the
deformation was prolonged and shifting in time and place. Rasin sub-
sidence, sediment accumulation, the tilting and erosion of marginal beds,
and the rise and truncation of anticlines in nearby and related areas all
went on together. Very little time is represented in some of the angular
unconformities, hardly enough for a change to occur in the faunas, yet
the angular unconformities have caught and fixed the rise of landmasses
in process of movement in the same manner almost as a photograph
stops an object in motion. Perhaps the unconformities should not be con-
sidered rigidly as indicators of separate widespread impulses. In the
analyses of Paleozoic and Mesozoic orogenies in the great system of west-
ern Cordilleran troughs, the theory seems repeatedly substantiated that
deformation was almost continuous in an oceanward volcanic archipelago,
and that from time to time the compressive movements spread into the
flanking trough and deformed the sediments in it to variable intensities
and distances. These deformational waves off the main belt of constant
unrest probably constitute our orogenic impulses or phases in the Coast
Ranges.

NORTHERN COAST RANGES OF CALIFORNIA

General Features

The Northern Coast Ranges, as generally defined, extend from San Fran-
cisco Bay to Trinidad Head and perhaps beyond. They are bounded on

the east by the Sacramento Valley and on the north by the Klamath Moun-
tains. They are composed mostly of Franciscan-Knoxville strata, but other
pre-Tertiary formations may be present; and in this respect, they contrast
with the Central and Southern Coast Ranges, which in good part are made
up of Tertiary deposits.

The southern end of the Northern Coast Ranges is not greatly different
from the northern end of the Central Coast Ranges. In both the Tertiary
is prominent, but northward it is limited to a few small basins and to the
marginal areas. Most of the hills and valleys are probably underlain only
by Mesozoic rocks. The complex structure of the Mesozoic and Cenozoic
rocks, their poor outcrops in many places, and their slight economic im-
portance as yet, have contributed to a lack of detailed geologic work
except in a few areas.

Weaver ( 1949 ) has published on seven quadrangles north of San
Francisco Ray, and reports that the hills there are arranged in three
blocks, one west of the San Andreas fault, the Montara block; one east
of it and west of the Tolay fault (a northwestward extension of the
Haywards fault system), the Francisco-Marin block; and one east of the
Tolay fault, the Berkeley Hills block. Refer to Figs. 29.1 and 19.2. Each
block is tilted toward the northeast. The Franciscan group constitutes the
surface exposures in most of the intermediate block, and the eastern block
is made up of more than 30,000 feet of Jurassic to Quaternary marine and
fresh-water sediments, together with about 1200 feet of Pliocene andesites,
rhyolites, and tuffs.

These sediments probably accumulated in structural troughs whose areas and
physical environments changed greatly during the Cretaceous and Tertiary. The
lower portion consists of clay shales and subordinate amounts of sandstone and
conglomerate as much as 17,000 feet thick, containing a marine fauna of am-
monites, pelecypods, and gastropods. These rocks include the Jurassic and
Lower Cretaceous portions of the Knoxville formation and the Upper Creta-
ceous Chico. Several faunal zones may be distinguished in the Knoxville, but
the formation in the mapped area cannot be subdivided on a lithologic basis.
The chico formation consists of interbedded shales and sandstones about 7000
feet thick.

The Paleocene is represented by the Martinez formation, and the Eocene in
ascending order by the Capay shale and the Domengine and Markley sand-
stones. The formations of the Paleocene and Eocene series consist of marine

COAST RANGES OF THE PACIFIC AND THE SAN ANDREAS FAULT SYSTEM

467

sediments ranging in thickness from 2000 to 5000 feet that were deposited in
embayments far more restricted in area than the seas of the Upper Jurassic and
Cretaceous time. The marine sedimentary formations of the Oligocene and
lower part of the Miocene series occupy still more restricted areas than those of
the Paleocene and Eocene, and near Carquinez Strait are more than 5000 feet
thick. The upper Miocene sandstones of the San Pablo group are far more
widely distributed and are nearly 2500 feet thick. They are characteristically
coarse-grained and were deposited in moderately shallow water which locally
was brackish or fresh. The Pliocene rocks crop out extensively in the north-
central part of the area and consist largely of alternating flows of andesite,
basalt, dacite, and rhyolite together with associated tuffs and agglomerates,
whose total thickness is 100 to 1200 feet. In Santa Rosa and Petaluma quad-
rangles marine sandstones contain invertebrate fossils closely allied to those of
the Merced formation in San Francisco. The beds in Marin and Sonoma coun-
ties are about 250 feet thick and rest unconformably upon the Franciscan
group. Near Petaluma Valley they interfinger with tuffs (Weaver, 1949).

The Eel River embayment north of Cape Mendocino is the largest area
of Tertiary sediments, and the beds there are said to be 7000 to 11,000
feet thick and of Pliocene age. Another deposit extends along the coast at
Point Arena, where Miocene beds are several thousand feet thick. A third
deposit is near Clear Water Lake, where 4000 feet of lower Eocene beds
have been identified.

Early Pliocene Phase

In early Pliocene time before the Pliocene volcanics accumulated, the
entire area east of the San Andreas fault was folded and faulted, and then
deeply eroded. Particularly a great low-angle overthrust, the St. Johns
Mountain thrust fault, was formed at this time.

Late Pliocene and Quaternary Phases

The Pliocene volcanics were laid down on the beveled surface of the
older rocks, and later were moderately folded and broken by normal faults
and locally overturned and broken by thrust faults.

Since the Pliocene beds in the Eel River embayment (Fig. 29.2) are
folded and faulted, the northern part of the northern Coast Ranges was
deformed in late Pliocene and Pleistocene time. This phase is similar to
that in the San Francisco Bay area on the south. The main middle area

is undoubtedly structurally complex, but it seems reasonable to conclude
that it also was folded in late Pliocene and Pleistocene time, and perhaps

during earlier phases.

Late Pleistocene and Recent Movements

As in the central Coast Ranges, there have been significant elevatory
movements since the compressional deformation. The movements seem to
be vertical and horizontal along faults, and also broader elevatory and
depressional warpings.

Perhaps long before the compressional orogeny, the Klamath Moun-
tains area projected westward as a peninsula, with the flanking areas
below sea, especially on the north and west. A widespread erosion surface
is believed to have developed over the Klamaths during this time ( Fenne-
man, 1931). Then during the folding and thrusting on the north and west,
it was only elevated, the Klamaths standing like a buttress to the deform-
ing belts of Cretaceous and Tertiary strata. In relation to the trough sedi-
ments, the borders of the buttress were pushed westward up and over
them.

Broad valleys were then cut in the high Klamath surface, according to
Fenneman (1931), but not in a single uplift because the valley walls are
terraced, and locally the floors of these broad valleys are themselves fairly
widespread erosion surfaces. The highest peaks in the Klamaths rise sev-
eral thousand feet above these broad valleys. In the Coast Ranges proper,
there are remnants of erosion surfaces, but they have probably been
jostled about in fault block movements. Their age, although most prob-
ably post-folding and post-thrusting, is not clearly demonstrable nor easy
to compare with the Klamath peneplain and the broad valleys cut in it.

A great uplift affected the Klamaths and adjoining areas after the ero-
sion of the broad valleys. Deep inner valleys 1000 to 2000 feet deep were
cut and later glaciated. As the glaciation is generally recognized as Wis-
consin, it would follow that the uplift and high-erosion surface are pre-
Wisconsin in age. The uplift of the Klamaths may have been associated
with the adjacent compressional orogeny, or it may have followed closely.
At any rate, the uplift and dissection must have occurred in middle or
post-middle Pleistocene. The narrow continental shelf was added to the

468

ISLAND RANGES

STRUCTURAL GEOLOGY OF NORTH AMERICA
INTERMONT VALLEY BELT MAINLAND OR CA5CADE RANGE

5,000'

BRITISH COLUMBIA AND SOUTHEASTERN ALASKA

UPLAND SURFACE

KLAMATH MOUNTAINS AND NORTHERN CALIFORNIA

Fig. 29.12. Idealized diagrams to represent vertical movements of the crust in Pleistocene time
along the Pacific coast. The upper diagram is schematic for the coastland of British Columbia
and southeastern Alaska. It runs east-west, and the U-shaped valley is representative of the
many great fiords that trench the upland. The lower diagram is schematic for the Klamaths of
northern California and for the coastland of this area. It should be considered as a north-
south section in the Klamaths with the horizontal lines representing sea level at different times
along the coast. The horizontal lines in both diagrams represent different sea levels. Sea level 1
was the base to which the high surface in both regions was graded. Sea level 2 was the one after

land and dissected by streams flowing over it. In the lower diagram, Fig.
29.12, the horizontal datum line marked 2 indicates sea level at this time.
The uplift was probably over 2000 feet in the Klamath area. Then fol-
lowed a subsidence of over 1500 feet. Datum line 3 indicates the sea level
at this stage. The oldest beaches known in the region were established at
this time. The highest are 1500 feet above the present sea level. They
remain only in remnants today. The deep and narrow valleys cut in stage
2 were partly alluviated in stage 3. Through a succession of uplifts,
beaches were formed at successive levels down to the present, with the

the great emergence to which the deep gorges were eroded. Sea level 3 was the one after the
great submergence to which the highest beaches now remaining were eroded. Sea level 4 is the
present one after appreciable emergence. In British Columbia and southeastern Alaska this last
emergence has only recovered 600 feet of the previous 1600 feet of submergence, whereas in
nothern California the recovery has been almost complete. The original great uplift was caused
undoubtedly by deep-seated crustal disturbances, but the later submergence and emergence were
due to isostatic adjustments to the loading and unloading of the glaciers.

modern coastal plain not far above sea level as the last major beach.

Northward from the Klamaths in southern Oregon, the shore terraces
gradually disappear. The same is true southward in northern California.
The most recent submergence north of the 40th parallel can be detected
in the tidal portions of the rivers which are somewhat drowned. The
subsidence increases as far north as the Columbia.

These very considerable epeirogenic movements in late Pleistocene and
Recent time must be viewed with respect, when the offshore submarine
topography is considered, because they show how possible it is for ex-

COAST RANGES OF THE PACIFIC AND THE SAN ANDREAS FAULT SYSTEM

469

tensive parts of the continental shelves to have been emergent and how
quickly the geography can change.

SAN ANDREAS FAULT SYSTEM

Aspects of Controversy

Perhaps the most discussed and widely known structural feature of the
western United States is the San Andreas fault. See index map, Fig. 29.2,
for location. It may be traced with ease and certainty from Tomales Bay,
40 miles northwest of San Francisco, to Cajon Pass, 50 miles east of Los
Angeles. It has also been traced with a little doubt and difficulty for
some scores of miles northwest and southeast of these limits. Its total
known length is, therefore, more than 600 miles. This fault is so con-
spicuous that it was well known even before April 18, 1908. On that
date, it was the site of a violent earthquake in the vicinity of San Fran-
cisco.

There is much conflicting literature written about the age of the San
Andreas fault, its movement, and its relation to the compressional folds
and faults. Some believe it came into existence first in pre-Cretaceous
time and moved recurrently through the Cenozoic to the present. Some
view the movement to have been mostly vertical, others mostly horizontal.
The vertical movement is said to be great, around 20,000 feet by some;
and only a few feet, by others. Those who recognize horizontal movement
are divided in their opinions. Some think the movement has been a few
thousand feet, others 300 miles or more. The most perplexing problem
about the San Andreas fault in the central ranges is its setting in typical
compressional structures running parallel or at an acute angle to it. The
f great fault seems at odds with the geomorphic provinces.

Those who have studied the fault north of the Garlock fault commonly
[interpret it differently from those who have studied it southward. Dib-
blee, however, who has studied the fault system both north and south
of the Garlock fault probably more extensively than any other geologist,
sees right-lateral movement predominantly throughout the entire length
(Hill and Dibblee, 1953).

Main Faults and Relations of the System

The master fault of the system is considered the San Andreas, and the
Big Pine and Garlock faults principal conjugate sheers (Hill and Dib-
blee, 1953). See Fig. 29.2.

In the San Francisco Bay area the Hayward fault passes through
Berkeley and the site of the University of California stadium. A little to
the east is the parallel Calaveras fault. Branches of the San Andreas ex-
tend up the peninsula on the west side of the bay. No long faults have
been mapped in the northern Coast Ranges except some just north of
San Francisco Bay.

The Garlock fault is conspicuous from its position at the boundary of
a region of strong relief on the north and subdued relief on the south
in the Mojave Desert.

The San Jacinto and Elsinor faults are major ones in the Peninsular
Ranges and most probably shared the horizontal movement with the
San Andreas. In fact, most all the faults shown on the map of Fig. 29.2
are large, and probably parts of the system.

In studying displacements and ages of the faults the following rock
types, as far as manner of response to deformation, have been distin-
guished (Hill and Dibblee, 1953):

1. Sierran basement complex (pre-Cretaceous): metasedimentary and meta-
volcanic rocks, intensely deformed and widely invaded by granitic rocks. Be-
cause of physical similarity, the Santa Lucia granitics and metamorphics of the
southern Coast Ranges and the complexes of the Transverse and Peninsular
ranges belong in this group. These are relatively rigid rocks which fail locally by
fracturing and, since they or rocks like them are extensively exposed and are
presumably of state-wide occurrence at depth, their mechanical behavior is
tectonically important.

2. Franciscan basement (pre-Cretaceous): sedimentary and volcanic rocks,
regionally unmetamorphosed but highly indurated, commonlv intruded bv basic
igneous rocks which are usually altered to serpentine and have caused local
metamorphism. These rocks are exposed in large areas in the Coast Ranges; on
the northeast side of the San Andreas fault, and also on the west side of the
Nacimiento fault zone. They presumably underlie a much greater area but are
probably in turn underlain by granitic rocks. The Franciscan, unlike the granitic
basement, is typically incompetent. Although in places intensely fractured,
often before being covered by later Jurassic or Cretaceous strata, and usually in

470

STRUCTURAL GEOLOGY OF NORTH AMERICA

fault contact with the other principal rock types, its response to deformational
forces has been characterized by folding.

3. Cretaceous and Cenozoic sedimentary and volcanic formations: mainly
marine clastic sediments with local volcanics and nonmarine deposits, not
strongly lithified and of extremely variable thicknesses and facies. Deposited
in large and small basins; locally highly deformed, especially during the late
Pliocene-Pleistocene revolution in the Coast and Transverse Ranges, and in
uplifts in the Mojave Desert and Salton Sea region regions. These rocks form
a pliable mande on the above described complexes and have therefore re-
sponded to tectonic forces primarily by folding, particularly where the sedimen-
tary section is thick or where underlain by Franciscan basement.

The San Andreas fault marks such an important contact that rarely can
it be crossed, except in Recent alluvium, without passing into significantly
different rocks. It is also a steep, if not nearly vertical fault and extends
to depths of at least 10 miles, according to seismological evidence.

Evidence of Horizontal Displacement

The following evidence of horizontal movement on the San Andreas
fault is presented by Hill and Dibblee ( 1953) :

1. The trace of the San Andreas zone is typically continuous and straight.
There is evidence of recent activity along its entire course. Excepting a 30-mile
segment trending eastward in the San Emigdio Mountains, and another stretch
of similar trend 100 miles to the southeast, the zone is remarkably straight from
Point Arena southeastward nearly to Mexico. These aspects of continuity and
straightness are considered typical of strike-slip faults.

2. The San Andreas is a steep fault which transects major topographic fea-
tures but develops all along its course one or several parallel trenches, sag
ponds, low ridges, saddles, and/or scarps. Its steepness is indicated by the
straight trace, the fact that mapped fault planes are nearly vertical, and the
failure of near-by drill holes to penetrate the zone. These characteristics are
typical of strike-slip faults. The development of fresh topographic features,
many of which are in unconsolidated recent sediments, and the common lack of
appreciable vertical or consistent vertical components of offset clearly indicate
the recency of lateral movements. Seismic evidence for recent right lateral
movements on the San Andreas, as summarized by Wallace (1949), comprises
the following maximum displacements at the time of earthquakes: 30 feet (San
Emigdio Mountains, 1857), 10 feet (San Francisco area, 1868), 21 feet (San
Francisco area, 1906), and 10 feet (Salton Sea area, 1940).

3. The San Andreas fault zone ranges from a few feet to a few miles in width.
Locally a single recent trace may be irregular, with 15-degree variations in
strike within a few hundred feet, or it may disappear and be replaced, en
echelon, by another. Occasionally two or three parallel traces widen the zone

of recent traces to a maximum of about half a mile. Wider segments of the
zone consist of several faults (not necessarily active) which are usually steep
and nearly parallel to the trend of the zone. These characteristics are considered
typical of strike-slip fault zones along which recurring movements have taken
place.

4. The apparent throw is commonly reversed along the San Andreas fault
as indicated by topographic and geologic relationships. These throws are prob-
ably due to the major strike-slip component which places in juxtaposition un-
like topographic elevations and geologic sections, and thus the reversals of
dip-slip are mainly illusory.

5. Drainage lines are consistently offset in a right lateral sense. These offsets
are especially clear on the southwest side of the Temblor Range where a maxi-
mum of 3000 feet of displacement has occurred through recent movements on
the fault. Wallace (1949, p. 805) reports a probable drainage offset of VA
miles on the north side of the San Gabriel Mountains, and Allen (1946, p. 50)
reports 3800-foot offsets of drainage lines near the Gabilan Range, also in a
right lateral sense.

6. Recendy developed trenches which irend southward into the fault have
been observed in aerial reconnaissance on the southwest side of the Temblor
Range. These are oriented correctly to be tensional in origin and due to right
lateral movement on the San Andreas.

7. Locally developed west-northwest trending folds adjacent to the San
Andreas are obviously drag folds resulting from the right lateral movement on
the San Andreas. Such drag folds are expecially clear in the Salton Sea Region,
and, besides indicating the right lateral sense of movement on the fault, many
of them show by their discordance with topographic form that the fault was
active before the present physiographic features were developed.

8. Wallace (1949) reports a probable 6-mile right lateral offset of terrace
deposits on the north side of the San Gabriel Mountains, and L. F. Noble
(personal communication) describes similar late offsets in that area of several
miles.

9. Between the San Emigdio Mountains and the Temblor Range, there are
two facies of Pleistocene gravels. On the southwest side of the San Andreas,
the pebbles are granite, gneiss, quartzite, limestone, black shale, and sandstone
which undoubtedly came from the San Emigdio Mountains. On the other side
of the fault, the pebbles are almost exclusively white siliceous shale which
probably came from the Miocene shale of the Temblor Range. These two
facies are in direct contact along the San Andreas for several miles. Further-
more, the northwest end of the crystalline clast facies is about 14 miles north-
west of the crystalline rocks of the San Emigdio Mountains. These relationships,
thus indicate a right lateral displacement of approximately 10 miles on the San
Andreas fault since Pleistocene deposition in this area.

10. In the Caliente Range, marine sediments of upper and middle Miocene
age grade laterally eastward into continental red beds which strike into the
San Andreas fault, whereas strata of the same age are marine shales on the

COAST RANGES OF THE PACIFIC AND THE SAN ANDREAS FAULT SYSTEM

471

other side of the fault. This juxtaposition of unlike facies again demonstrates
substantial lateral movement. In this case the general trend of the western
margin of the continental facies in the Caliente Range is northward across the
Carrizo Plain toward the San Andreas, whereas possibly the same transition line
may be extrapolated southward from along the east side of the San Joaquin
Valley to the fault. Thus, by simple projections the right lateral offset on the
fault since the upper Miocene time would be about 65 miles, although the
probability of irregularities in trend of this facies contact precludes a strictly
quantitative solution of that cumulative shift. Note the comparable offset of
the upper Miocene "Pancho Rico"-"Santa Margarita" shale, shown in the same
figure.

11. Going back only slightly farther in the geologic record, approximately
175 miles of right lateral offset may have accumulated on the San Andreas
fault since early Miocene time. This is suggested by the unique similarities of
rock types and sequences in the San Emigdio Mountains, as described by
Wagner and Schilling (1923), and the Gabilan Range as described by Kerr
and Schenck (1925), and Allen (1946). In each of these areas, a section of
lower Miocene volcanics, red beds, and marine lower Miocene and Oligocene
strata occurs [B-B' of Fig. 29.13].

12. A similar relationship is suggested by some lithologic and faunal simi-
larities between the Eocene formations of the Temblor-San Emigdio and the
Santa Cruz Mountains which indicate the possibility of an offset of approxi-
mately 225 miles since late Eocene time [C-C of Fig. 29.13].

13. Also the southern limit of Cretaceous strata in the Temblor Range may
match with the southern limit of Cretaceous beds near Fort Ross which would
indicate an offset of approximately 320 miles [D-D' of Fig. 29.13].

These evidences of progressive movement from the Cretaceous to the
present are consistent with each other and yield a rate of 0.2 to 0.3 inch
of movement per year. However, geodetic measurements of rates since
the turn of the century are about tenfold the ones based on offsets of
rock masses (Hill and Dibblee, 1953).

Contrary to the substantial evidence of large horizontal movement
south of San Francisco, Higgins ( 1961 ) concludes that less than 15 miles
of right-lateral displacement has occurred along the San Andreas north
of San Francisco since mid-Pliocene time. During the same time the east
i side has been raised about 500 feet relative to the west side.

Big Pine and Garlock faults

Both the Big Pine and Garlock faults have left lateral movement in
contrast to the right lateral movement of the San Andreas. The one is

Fig. 29.13. Maps showing postulated strike-slip movement along the San Andreas fault. Left
map shows position of Baja California and Coast Ranges of California (shaded area) in
Cretaceous time. Right map shows the present position. D and D' were juxtaposed in Cretaceous
time; C and C in Eocene; B and B' in Oligocene and early Miocene; D and D' offset of Big Pine
fault. Hill and Dibblee, 1953; Hill, 1954.

472

STRUCTURAL GEOLOGY OF NORTH AMERICA

believed to be the offset of the other (Hill and Dibblee, 1953). The
shift to the northwest has been about 5 miles. See A-A', Fig. 29-13. If
such is true, and if the horizontal displacement along the San Andreas has
been in the order of 300 miles, then the San Andreas is much older and
had been active a long time before the Big Pine-Garlock fault came into
existence. The 5 mile offset would indicate that the age is Pleistocene.
There is no question about the recency of activity along the Big Pine and
Garlock faults, but the time of beginning may be suspect. It could be that
the Big Pine fault originated many miles to the south and by coincidence
is now about opposite the Garlock.

Hill and Dibblee believe the strain system of the Big Pine-Garlock
shear and the San Andreas shear is a conjugate or complementary one
with the south wedge moving against the north wedge. Moody and Hill
(1956) elaborate on the stress-strain relations of the San Andreas system
in which they call the strike-slip faults of large displacement "wrench
faults." They develop second and third order effects and believe they
demonstrate at least eight directions of wrench faulting and four direc-
tions of folding or thrusting possible. They conclude that dynamically
the orientation of the Garlock is not correct for the primary left lateral
direction, and it would more nearly fit a theoretical position for a second-
order left lateral fault, assuming north-south compression. The Trans-
verse Range may represent the primary fold direction, consequently
shortening the crust in this area and altering the San Andreas direction
(Moody and Hill, 1956).

Relation to Pacific Fracture Zones

Great fracture zones trend generally westward from the United States,
Mexico, and Central America across the Pacific. These are depicted in
Chapter 32, and their relation to the fault system of California is shown
in Fig. 32.15. The relation of the two systems is an enigma.

Origin of Gulf of California

Recent seismic work in the Gulf of California has shown the crust
there to be oceanic (H. W. Menard, personal communication), and conse-

quently one's impulse is to postulate drift of the peninsula away from the
mainland. Not only westward but northwestward drift compatible with
movement along the San Andreas fault must be postulated. If the Coast
Ranges oceanward of the San Andreas fault and the Peninsular Ranges
with Baja California are moved as a unit southeastward in the amount of
movement proposed by Hill and Dibblee from the Cretaceous to the
present, the Baja California is brought into a likely former position with
the mainland. The Nevadan belt of the Sierra Madre del Sur would con-
tinue in this arrangement without break into Baja California, as postulated
in Chapter 38. The restored relations are shown in Fig. 29.13.

Two difficulties appear; the long unit has to be bent slightly to make
the fit, and it has to snake around the major bend of the San Andreas
fault east of the Los Angeles Basin in making its way to the northwest.
The passage is accomplished in a more straight-away course if a good
deal of the movement occurred along the Elsinor, San Jacinto, and San
Gabriel faults. Hill and Dibblee have commented that the San Gabriel
fault may have been principally active in the past. It seems possible that
the segment of the strip now making up southern California has been
pressed somewhat against the continent since late Miocene time, and
although right lateral movement has continued along the San Andreas
fault that the folds and thrusts of the Transverse Ranges were thereby
formed. If a subcrustal convection current is carrying the strip north-
westward, then the current might have become a little deflected toward
the continent in the southern California region and the unusual complex
of structures formed there. We could imagine that the Elsinor fault first
carried the brunt of the dislocation, then the Jacinto, and finally the San
Andreas through the Salton Sea area, as the compressive component of
the carrying force increased against the continent. All these faults are
still comparatively active.

Seismicity in the Coast Ranges

Figure 29.14 shows the general seismicity of the California region. The
epicenters are scattered through the San Andreas fault zone more widely
than might be expected, yet there is a general clustering along the great
fault. The Agua Blanka fault of northern Baja California is believed to

Fig. 29.14. Earthquake shocks and faults of California and western Nevada. The faults are
those generally considered to have suffered late Pleistocene or Recent activity. Earthquakes
compiled from Byerly (1940), Gutenberg (1941), Byerly and Wilson (1936, 1937), and tables
supplied by C. F. Richter. Earthquakes above the magntiude of 5 are shown in large dots,
those below by small. In the compilations some earthquakes may have been shown twice
because of overlapping and discrepancies in location assignments.

Fig. 29.15. Index map of Washington and Oregon. Coast Ranges are vertically dashed; the
extensive volcanic fields are unruled; and the pre-Tertiary rocks, mostly Nevadan complex
are cross-ruled.

474

STRUCTURAL GEOLOGY OF NORTH AMERICA

project out to the San Clement basin and escarpment on the basis of
the epicenters and submarine topography (Allen et al., 1960).

COAST RANGES OF OREGON AND WASHINGTON

Geomorphic and Geologic Provinces of Oregon and Washington

Figure 29.15 has been prepared to show the geomorphic provinces of
Oregon and Washington, and in a broad way the geologic divisions. The
Klamath, Rlue, and Northern Cascade Mountains, and the Okanogan
Highland have been referred to in Chapters 6 and 17. They are made up
chiefly of the Nevadan complex. The trends in the Klamath Mountains
veer northeastward as they pass under the Tertiary volcanics and are
generally thought to find a continuation in the Blue Mountains. Most of
the sedimentary rocks of the Blue Mountains are unmetamorphosed, and
this is puzzling because the rocks of the Nevadan complex elsewhere are
fairly crystalline. The large Idaho batholith lies east of the Blue Moun-
tains and appears to make up a knot at the intersection of the Sierra
Nevada-Klamath-Blue arc and the British Columbian Coast Range arc
with its great batholiths. The basement geology of Oregon and Washing-
ton is thus believed to be the Nevadan complex at the junction region
of two great arcs. It evolved as a Paleozoic and early Mesozoic eugeo-
syncline. In Late Jurassic and Mid-Cretaceous time folding, meta-
morphism, and batholithic intrusions brought its history to a climax.
The Tertiary Coast Ranges and the extensive volcanic fields developed
thereafter.

As in California the Coast Ranges are bordered on the east by a
general depression, known in Oregon as the Willamette River Valley,
and in Washington as Puget Sound. The two are referred to as the Wil-
lamette-Puget depression or Willamette-Puget Sound depression. On
the east of the depression are the Cascade Mountains, made up of
volcanic rocks. They are divided into the Western Cascades and the High
Cascades as shown in Fig. 29.15, and are treated fairly extensively in
Chapter 36.

East of the Cascade Mountains and surrounding the islands of pre-
Tertiary rocks in the Blue Mountains are vast Tertiary volcanic fields.

North of the Blue Mountains and including part of them is the Columbia
River basalt field, and south of the Blues are several geomorphic province-,
all underlain by volcanics, sometimes collectively referred to as the
Malheur field. The southern lavas are generally younger than the north-
ern. The Columbia and Malheur fields are outlined in Chapter 33.

Divisions of Coast Ranges

The Coast Ranges of Oregon and Washington are a coherent unit
geologically, because their formations are probably all Tertiary and they
have been deformed as a unit. The northern end is composed of the
Olympic Mountains, a domal uplift supporting the highest peaks of the
Coast Ranges, with Mount Olympus 7954 feet above sea level. The
canyons of the Olympic Mountains have been heavily glaciated.

At the northern end of the Coast Ranges of Oregon, just south of the
Columbia River and west of the city of Portland is another uplift in
which a core of fairly old rocks (middle Eocene) relative to those of the
ranges elsewhere is exposed.

Stratigraphy

Selected sections of the Cenozoic rocks of the Coast Ranges of Oregon
and Washington are given in Fig. 29.16. They are taken from Weaver's
(1945a,b) extensive study with the western Oregon section modified
according to Baldwin ( 1959 ) and Wilkinson ( 1959 ) . The idealized cross
sections, A- A' and B-B' of Fig. 29.17, attempt to restore the deposits to
their condition before the late Miocene folding.

At the beginning of Tertiary time, according to Weaver ( 1945 ) , a vast
erosion surface existed in eastern and western Washington in the manner
of a coastal plain. It had been carved chiefly in the rocks of the Nevadan
orogenic belt. Early in the Eocene, the plain began to subside, and the
earliest deposits filled the broad valleys of the extensive erosion surface
The Swauk formation of eastern Washington may be a fresh-water de-
posit in the upper part of one of these valleys, and the Solduc formation
of the Olympic Mountains may be the marine equivalent. Both of these
formations were folded somewhat and eroded before the overlying vol-

COAST RANGES OF THE PACIFIC AND THE SAN ANDREAS FAULT SYSTEM

475

canics were poured out. These outpourings have been called the Tean-
naway volcanics in eastern Washington, the Metchosin volcanics in
western Washington, and the Tillamook and Siletz volcanics in western
Oregon.

The basal Eocene volcanics are a voluminous deposit. They originally
formed a vast lava field that extended from Vancouver Island 500 miles
southward to the Klamath Mountains and from a line considerably west
of the present coast 150 miles inland. Their minimum average thickness
was 3000 feet. According to Weaver, the volume of these volcanics was
greater than the Columbia plateau basalts. They consist mainly of an-
desitic and basaltic flows with tuffs, agglomerates, and numerous in-
trusive plugs and dikes. The latter crosscutting intrusions, Weaver
believes, were the vents of much of the volcanic material.

Ry the close of the Metchosin volcanism, a narrow north-south trough
formed with its axis in the approximate position of the present Willamette-
Puget Sound depression, and its sediments extended westward into the
site of the modern Coast Ranges. After the volcanic eruptions 8000 to
14,000 feet of sediments were deposited. They make up the Puget group
of the Seattle region, the Cowlitz formation southward in Washington,
and the Tyee sandstone and Coaledo formation in Oregon.

The basal volcanics remained emergent in a narrow peninsula that
projected southward from Vancouver, with the trough to the east. In
early Oligocene time the peninsula submerged in part, and sediments
were deposited directly on the Metchosin volcanics there; farther east
they rest on the late Eocene strata of the trough. Ry late Oligocene, the
peninsula area had sagged so much that 8000 feet of sandstone and shale
had accumulated. Again in middle Miocene time, over 4000 feet of sand-
stone and shale, the Astoria formation, were deposited in the Coast Range
area.

During Miocene times, great quantities of lavas were coming to the
surface through numerous vents and fissures, especially in the areas of
the Columbia plateau and the present Cascade Mountains. These flows
fingered out westward, but north of Portland they are particularly abun-
dant and form about 50 percent of the Astoria formation (Weaver, 1945).
See cross sections A-A' and R-R', Fig. 29.17.

o
o

UJ
2
UJ

o
o

_l
a

WESTERN
OREGON

MARINE TERRACE
OEPOSITS

DEVELOPMENT OF
MT. HOOD, ETC.

OIASTROPHISU

EMPIRE SS

OIASTROPHISM

Minimum

ASTORIA FM.

NYE SHALE

TUNNEL POINT
SS.

BASTENDORFF
SH.

COALEDO FM.

TYEE SS

WESTERN
WASHINGTON

MARINE TERRACE
OEPOSITS

GLACIAL OEPOSITS

DEVELOPMENT OF
MT. RAINIER, ETC

MONTESANO
SS.

Illlllllllllll

OIASTROPHISM
Illlllllllllll

ASTORIA FM.

UPPER
TWIN RIVER

LOWER
TWIN RIVER

lllllllllllllll

LINCOLN FM.

KEASEY SH.

COWLITZ FM.

CRESCENT FM.

TILLAMOOK 8

SILETZ RIVER, MECH0S|N

VOL. SERIES VOLCANICS

EASTERN
WASHINGTON

OLACIAL ANO
ALLUVIAL DEP

ELLENSBURG

COLUMBIA RIVER
VOLCANICS

YAKIMA BASALT

UPPER
KEECHELUS

LOWER
KEECHELUS

ROSLYN FM.

TEANNAWAY BAS

I ll I I I I I I I II I I I I
SWAUK

EASTERN
OREGON

VOLCANICS AND

ALLUVIUM

VOLCANICS

RATTLESNAKE

MASCALL

COLUMBIA
RIVER LAVAS

UPPER
JOHN DAY FM.

LOWER
JOHN DAY FM.

CLARNO

Fig. 29.16. Representative stratigraphic sections of the Tertiary in Washington and Oregon.
After Weaver, 1944.

FUTURE SITE OF
COAST RANGES

Present coast line

FUTURE SITE OF
CASCADE MT3.
Mt.

Adams

5t. Helens
i

COLUMBIA PLATEAU

Future site of Yakima and
Pasco basins

LAVA

FIELD

Future site of
Lewiston Basin

IDAHO
BATHOLITM

_\ /\ /\ /N/N /_\
VCA /N/N M /_\ /N /

\ /_\VX /N /_\ /_S /_\ /_\ /_\ /_\ /V

A >_\ /_\ /_\ /_\ /_W_\ /\ /\ /J / N /

/\/\y\/\/\/\/\/\/\/\/~/

Present coast line

FUTURE SITE JFUTURE SITE OF
COAST RANGES ' CASCADE MTS.

Klamath structure
exposed now

? i <n

MALHEUR PLATEAU

Now covered with Pliocene
and Pleistocene lavas

LAVA FIELD

Present site of
Steens Mt.

TYEE . walCA*4*05

MOOK, SILETZ RIVER AND OMP°oA V ^

puex

N E V ADAN

CO w

5CALE

MILES

I0O

Fig. 29.17. A-A', cross section through Washington and B-B', cross section through Oregon. For
positions see Fig. 29.1. They attempt to restore ideally the Eocene, Oligocene, Lower and Middle Miocene
sediments and volcanics just before the folding in the trough area of late Miocene time.

15 000 FT.
10000
-5000

150

COAST RANGES OF THE PACIFIC AND THE SAN ANDREAS FAULT SYSTEM

177

Fig. 29.18. Section R-R' west of Tacoma in King County, Washington. See index map, Fig.
29.1. After Warren ef a/., 1945a.

Late Miocene Phase

After the deposition of the Astoria formation, the trough sediments of
Washington and Oregon were subjected to compression. As far as known,
mostly open folds resulted. Perhaps in places they were compressed so
as to have steep flanks or to be overturned. Examples are given in cross
sections R-R', and S-S', Fig. 29.18. Faults are not common, and where
present have been illustrated as the normal type. The fold axes that are
known to have originated in this late Miocene phase have been assembled
on the index map of Fig. 29.15. Through Washington, according to
Weaver, they pass in a west-northwest direction. The axes that Weaver
shows are those of very broad folds defined by the Vancouver Island-San
Juan Islands-northern Cascade upwarp and the Olympic-Newcastle
Hills-Cascade upwarp, with the intervening downwarp of the Strait of
Juan de Fuca. Also, the Columbia River lavas in the western part of the
basalt basin have been deformed into several northwest-trending broad
anticlines and synclines. A map by Warren et al. (1945b) just west of
Puget Sound (locality of section R-R', Fig. 29.18) shows the folds to be
small and rather tight, and they curve sharply from a west-northwest
direction to a southerly and southwesterly one. The area covered by the
new map is so small, however, in relation to that of the state and the
broader picture, that the significance of the local variations is not known.

Section S— S' across the Coast Range From Cape Meares to Williamette River, Oregon. After
Warren et al., 1945b. The Tillamook volcanic series is probably equivalent to the Metchosin
volcanic series.

The Miocene folds of the state seem to be of low to medium intensitv and
to trend generally to the northwest.

In the Portland area of the Coast Range, the fold axes are gentle and
also extend in a northwest direction. They show a tendency to bend
southward and generally parallel the coast. Farther south in Oregon,
they parallel the coast, and some even trend to the southwest in the
northern Klamath Mountains.

The Olympic Mountains uplift is ringed by a horseshoe-shaped ex-
posure of the Metchosin volcanics with the Solduc formation underneath
and presumably forming the core. The latter is more metamorphosed than
the Metchosin and consists of phyllites and argillites. It seems to have great
thickness. However, Oligocene fossils have been found in the area of
Solduc (?) rocks, and thus the simple dome structure is doubted. Park
(1950) concludes that the uplift contains steeply dipping thrust faults,
and considerable buckling, thus reducing the previous estimates of a very
great thickness for the Solduc.

Late Pliocene and Early Pleistocene Phases

Deformation at the close of the Tertiary and in the Pleistocene through-
out the Oregon and Washington region has been of the broad arching,

478

STRUCTURAL GEOLOGY OF NORTH AMERICA

Fig. 29.19. Tectonic map of the late Pliocene and Quaternary crustal movements along
the Pacific.

sagging, and warping type, and therefore contrasts sharply with the close
folding, thrusting, and wrench faulting in the California Coast Ranges.
The main orogeny of the southern ranges as previously pointed out
occurred in late Pliocene and mid-Pleistocene time, but it mostly escaped
the Washington and Oregon ranges. On the other hand, the late Miocene
deformation seems to have been about of equal intensity both north and
south of the Klamaths.

The gentle archings have been deduced from several lines of evidence.
The first and most conspicuous is the parallelism of the three major
topographic features, namely, the Cascade Range, the Willamette-Puget
Sound depression, and the Coast Range. The two ranges are taken as
arches or broad, gentle anticlines, and the depression as an intervening
broad, gentle syncline. The second line of evidence comes from erosion
surfaces, both inland and coastal. The third concerns the glacial deposits,
which are very extensive in parts of the Puget Sound depression, and
vertical crustal movements associated with the glaciation.

According to Weaver (1931-37) the Pliocene deposits, where known,
rest unconformably upon the Miocene and are much less tilted. Thus
the late Miocene phase is dated. During the latest Miocene and Pliocene,
minor differential movements allowed the oceanic waters to transgress
easterly and cover small restricted areas on the western side of the
Olympic peninsula and in the coastal portion of southwestern Oregon.
All other areas were undergoing erosion, and it is probable that the
major channels of Puget Sound, such as Hood Canal, Admiralty Inlet,
Georgia Strait, and the Strait of Juan de Fuca, were being excavated. The
marine waters that occupy these valleys at the present time gained access
as the result of Pleistocene depression just preceding and during the
glacial epoch.

Near the close of Pliocene time the two broad anticlines and inter-
vening syncline developed and emphasized the individuality of the Coast
and Cascade ranges and the Puget trough (Weaver, 1937). See map,
Fig. 29.19. These north-south structures were probably superposed on
the Miocene northwest trending folds. The Cascade Mountains ultimately
attained their present elevation during the early Pleistocene, and upon
their surface was built a row of majestic volcanic cones such as Mount

COAST RANGES OF THE PACIFIC AND THE SAN ANDREAS FAULT SYSTEM

479

Baker, Mount Rainier, Mount Adams, Mount St. Helens, Mount Hood,
and numerous smaller cones in southern Oregon. See Chapter 33.

It seems probable that the erosion surface, developed after the late
Miocene folding, was itself gently folded, as were the rocks beneath in
the late Pliocene archings, and that it was intensely dissected where
uplifted most.

After the elevation of the erosion surface, and after its deep dissection
by the voluminous streams of the region, the ice age came on, and is
recognized in two stages. During the later advance all the valleys of both
the eastern and western slopes of the Cascade Range were filled with ice,
which moved downward to lower elevations and built terminal moraines.
The valley glaciers in northern Washington entered the Puget Sound

basin and coalesced with one another, and with the extensive piedmont
glacier that had moved southerly between Vancouver Island and the
mainland. This great ice floe broke into two tongues; one extended west-
erly through the trough of the Strait of Juan de Fuca, and the other
moved southward into the southern part of the Puget Sound basin, where
it built up a terminal moraine from the southeast corner of the Olympic
Mountains easterly to the Cascades.

After the withdrawal of the ice, the crust has risen in the Puget Sound
basin and along the coast of Washington and Oregon from 20 to over 200
feet. A most recent submergence has already been noted near the mouth
of the Columbia River, and the tidal influence extends eastward to the
Cascades.

30.

BAJA CALIFORNIA
AND SONORA SYSTEMS

BAJA CALIFORNIA

Topography

Baja California is as long as California but only a third as wide. See
maps of Figs. 28.1 and 30.1. Its northern half is mountainous with peaks
that rise to elevations of over 10,000 feet. These comprise the Peninsular
Range, which is a continuation of the ranges of southern California west
of the Salton basin. Most of the high area is granite and metamorphosed
rocks of the Nevadan type. The southern half of the peninsula is lower in
relief and for the most part is a great area of conglomerates, sandstones,
agglomerates, and lava flows of post-early or middle Miocene age. See

Fig. 30.1. Geologic and tec-
tonic map of Baja California
and the Gulf of California, after
Beal, 1948. Grm, crystalline
rocks of Nevadan complex; Km,
Lower Cretaceous San Fernando
formation; Kr, Upper Cretaceous
Rosaria formation; Tt, Paleocene
or Eocene Tapetate formation;
Tm, Oliogocene to Pliocene for-
mations.

480

BAJA CALIFORNIA AND SONORA SYSTEMS

481

LAI 31° N

Pacific Ocean

Detritus of Colorado Riv
sierra san La Providencia & possibly older sed.

pedro martir / valley of San Felipe ~~? Sa. Enterada

KmGrm ^^ \ .V^S IXOa/_ Qal Rulf of California! Ts Altar Desert >

Probably faults in gulf bottom

LAT. Z9° N.

Cedros Is.

Pacific Ocean *> Bay of Sebastian Viscaino

_ Ballenas / /7^e/ ^ ,Q G"°rd°

lb Channel Tc Gulf of California

7i^>

Grm

LAT. 2 7° N.

Pacific Ocean

Tb Qal To

I /ono cfe. /as l//rgene5 /■ Tortuga

Tt Ty Tb Tc 6uif 0f $ California

Fig. 30.2. Cross sections of Baja California, after Beal, 1948. Positions designated by latitudes
are approximate.

Grm, crystalline rocks of Nevadan orogeny, diorite varieties, schists, gneisses; Tb, Cenozoic
flows and intrusions, mostly andesite and basalt; Km, San Fernando formation of Cretaceous

metamorphics, limestone, shale, quartzite and intrusive rocks; K, Rosario Upper Cretaceous marine
sandstone and dark shales; Tt, Tepetate silts, sandstones, Paleocene to Eocene; Ty, Ysidro sand-
stone, siltstone and tuffs, Miocene; Tc, Comondu volcanics and elastics, upper Miocene; Ts,
Salada formation, Pliocene.

cross sections of Figs. 30.2 to 30.4. Inland southeastward from the Bay
of Sebastian Vizcaino is a vast desert of Quaternary alluvium about 800
feet above sea level.

The Tertiary rocks are in two narrow belts along each side of the
Nevadan core in the northern part. Just south of latitude 30°, however,
the Pacific belt of Tertiary deposition (perhaps only the conglomerates

and volcanics of the upper Miocene) extends across to the east coast.
Then from latitude 29° southward to the southern end of the peninsula,
the Nevadan rocks crop out in the bordering islands on both sides, in-
cluding the western cape region (Sierra Vizcaino). Finally, a large part
of the south end of the peninsula (Sierra Victoria) is made up of the
Nevadan complex. See the tectonic map, Fig. 30.1.

Ocean

Yellow beds^

IGNEOUS AND METAMORPHIC

GRANITE i(

Mesa^San Carlos

/iiimiTiiiiiiiiitfTlpiniimrninr~

Ocean

„„ "3B^^5ja|^B=»B§^jgS^ IGNEOUS AND METAMORPHIC g^V\/VV\A

Chico { ^

Santa Clara Desert

' ' " I ' I I I I IT

Cerro
Santiago

Cerro Angel

^fflp^iffljIsS^^ agglomerate, conglomerate
" 9 " and igneous

I—

Yellow beds
5 io

15
j MILE5

Fig. 30.3. Sections across central Baja California. After Darton, 1921.

BAJA CALIFORNIA AND SONORA SYSTEMS

483

sandstone

Agglomerate,,
conglomerate,
a no" igneous

Ocean

ii^^gS^^^^"^^^1'? — :-^y£7Iow ^ beds' y

jondstone

conQ10 _^tk jgneou*

Ocean

MILES

Fig. 30.4. Sections across parts of northern and central Baja California. After Darton, 1921.

Stratigraphy

Beal (1948) records:

The rocks of Baja California consist of ( 1 ) unaltered marine sedimentary
^ocks ranging in age from Cretaceous to Pleistocene; (2) a series of sedimentary
vrocks of probable Cretaceous age exhibiting varying degrees of alteration; (3)
^extrusive rocks, principally of andesite and basalt; and (4) intrusive rocks
consisting principally of quartz diorite and granodiorite, which have intruded
and metamorphosed older rocks, the age of which is not definitely known.
L| The Cretaceous is represented by the San Fernando and the Rosario forma-
tions. The San Fernando formation, Lower and early (?) Upper Cretaceous in
age . . . consists of a series of slates, conglomerates, quartzite, limestone, and
sandstone, with varying amounts of associated intrusive and extrusive rocks;
some parts of the series are only slightly, but others greatly metamorphosed.
The younger Rosario formation (Upper Cretaceous) is unconformable on the
,San Fernando formation. It consists of unaltered red and gray shale, brown
sandstones, and conglomerates on the Pacific Coast near Rosario. . . .

The Tertiary is divided into the following formations: the Tepetate (Paleo-
cene to Eocene in age) . . . composed generally of yellow to brown silt and

sandstone; the San Gregorio formation, Oligocene (?) to Lower (?) Miocene
in age . . . resembling in some respects the Monterey shales of California: the
Ysidro formation, late Lower or Middle Miocene, or both, in age, comprising
a lower member of shales, in part diatomaceous, and an upper member of
light-colored sandstone and shale; the Comondu formation. Upper Miocene
(?) in age, composed mainly of agglomerates, tuffs, and lavas; and the Salada
formation (Pliocene), and consisting mainly of yellow marine sandstone and
shale.

Following are the outstanding geologic features in the different districts of
Baja California:

1. The Northern district is characterized by a high, westward-sloping block
of crystalline rocks, which appear to owe their elevation to profound faulting
along the east side. The axial mountains have a granitic core, flanked on both
sides by gneisses, schists, and slates probably of pre-Crctaceous age; the bed-
rock complex is overlain on the west side by irregularly metamorphosed rocks of
Cretaceous age, which are themselves overlain by unmetamorphosed marine
sediments of Cretaceous and Tertiary ages; these rocks do not appear in any
outcrop of importance on the east side. The crystalline rocks arc prominent but
decrease in elevation as far south as the 28th parallel, die southern boundary
of the Northern district.

484

STRUCTURAL GEOLOGY OF NORTH AMERICA

2. The major feature of the Western Cape region (28° and 114°) is the
northwesterly trending Sierra Vizcaino, bordered on the north and east by De-
sierto de Santa Clara. Crystalline rocks, including small areas of the Franciscan
formation, constitute the bedrock complex of Sierra Vizcaino the islands farther
northwest. Tertiary and probably some Cretaceous sediments, dipping in gen-
eral northeastward toward the synclinal desert and southward toward the ocean,
overlie this bedrock complex. Volcanism is a major feature of the southeastern
part of this area.

3. The areal geology in the South-Central area is dominated by volcanic
rocks of Tertiary age, which obscure some of the earlier marine formations, but
where these formations are exposed, they usually occupy the axis of a syncline
and a part of the sierran area, which is anticlinal. Crystalline rocks, probably
elevated by faulting, are exposed on the southwest coast at Bahia de Magdalena,
but only small areas of these rocks, at relatively low elevations, occur along
the uplifted gulf coast of the peninsula.

4. The Southern Cape region (24° and 110°) is looked upon as a distinct
structural block and is almost entirely granitic and metamorphic rocks, although
some marine Tertiary sediments occur east of the high sierra.

5. The islands adjacent to the peninsula are composed principally of vol-
canic and granitic rocks.

Metamorphic and Intrusive Rocks

Regarding the metamorphic and intrusive rocks older than the Lower
and early ( ? ) Upper Cretaceous San Fernando formation, which itself in
places is metamorphosed, the following passages are quoted from Beal
(1948):

Lindgren (1889) states that the principal mass of the peninsula at 32° N.
Lat. is an enormous granitic plateau with minor areas of highly metamorphosed
and compressed slates, the granites appearing to be a "white hornblende granite
similar to that of the Sierra Nevada of California." Emmons and Merrill (1894)
in their examination of the area adjacent to the 30th parallel found rocks of
the same type as those mentioned by Lindgren and to the eastward found
metamorphic slates which led them to remark on the similarity of structural
conditions and lithological character of the rocks in the two areas. According to
Hirschi the granitic zone of the Northern district is flanked on the gulf
side by old crystalline schists, of a sort not observed on the Pacific side;
and, in the desert sierras west of the mouth of the Colorado, great and varied
schist zones occur, which extend southeastward along the gulf coast almost to
the 28th parallel.

The metamorphic rocks, mapped with the intrusive granitics, were observed
during this study to consist of gneisses, slates, schists, and other metamorphics;
they are exposed on both flanks of, and on, the batholith which makes up the
axis of the northern part of the peninsula and are known farther north on both

sides of the batholith in San Diego, Imperial, and Riverside counties, California.
Lindgren (1888) states in referring to the slates on the west side of the range
at the latitude of Ensenada that "one cannot fail to be impressed by the enor-
mous extent of the granite and the small area occupied by metamorphic rocks.
It seems evident that the slates are of but litde depth and everywhere are rest-
ing, as detached fragments, on the granite."

. . . Woodford and Harriss (1938), in a careful study of the granitic and
associated metamorphic rocks adjacent to 31° N. Lat., state that the crystalline
rocks consist of stocks and batholithic masses of quartz diorite. They consider
that the plutonic rocks in northwestern Baja California "are typically quartz
diorite, as contrasted to the granodiorite or quartz monzonite, which is the com-
monest rock of the Sierra Nevada." . . .

... In parts of the Western Cape region the granitic rocks are greenish-gray
diorite and pink granite, occasionally cut by large intrusions of serpentine.
According to Hanna (1927), chert, presumably of Franciscan age, occurs on
Isla San Roque, Isla Asuncion, Islas San Benitos, and possibly on Isla San Ge-
ronimo, far to the north just below the 30th parallel; he also reports (1925,
p. 268) "Franciscan cherts, sandstones, and conglomerate" on Isla Cedros. At
Punta San Hipolito (on the south coast of Western Cape region) are quartzites,
cherts, cherty limestones, and igneous rocks, which were mapped as the San
Fernando formation but may be older. In this area, as well as near Punta
Asuncion, the granites and metamorphic rocks are intruded by dikes of serpen-
tine, but they have failed to alter the near-by Eocene sandstones which usually
dip toward the crystalline rocks, indicating that the serpentine dikes are older
than the Eocene and that the Eocene has been brought to its present position
with reference to the crystalline rocks by faulting. At Punta San Pablo, about
25 kilometers northwest of Punta Asuncion (Scammon Lagoon quadrangle),
Hirschi and De Quevain (1933) state that the greenish-black rocks of the high
coast line are probably of amphibolite and gabbro (?) broadly intruded by
pegmatites, and that on the south end of the "intensively folded Paleozoic
range" of Isla Cedros they observed strongly altered, glaucophane-bearing dia-
base porphyritic dike rocks.

The same authors refer to a great peridotite intrusion at Cabo San Lazaro
and Punta Entrada (Magdalena Bay quadrangle), which is shattered, pene-
trated by east-west dioritic or dioritic porphyry dikes, and usually wholly altered
to serpentine. They refer to andesitic and basaltic rocks of Tertiary age, which
overlie the basement complex exposed along Bahia de Magdelena. Lindgren
(1889) states that Isla Santa Margarita is composed principally of crystalline
schists, with some chloritic slaty rocks and talc and serpentine. The numerous
reported and observed occurrences of rocks of Franciscan character constitute
good reason to believe that the Franciscan formation of California extends
southward as far as the Western Cape region and perhaps to Bahia de Magde-
lena.

The metamorphic rocks of the Nevadan complex of Baja California can
be judged better by comparison with their northward continuations in

BAJA CALIFORNIA AND SONORA SYSTEMS

485

California. The southern California area has been summarized in Chapter
17 to which the reader is referred for details. In brief, Larsen believes
that there are many bodies of metamorphosed rocks older than the
granitic rocks. Originally the sediments were shales, impure shales, and
sandstones.

The argillaceous metasediments are chiefly on the west side of the main
batholithic masses and within them, and they are probably mostly Triassic
in age. The quartzites and coarse sericite schists are on the east side and
are probably Carboniferous in age. A body of mildly metamorphosed vol-
canics of Early Cretaceous age makes up part of the pre-intrusive complex
on the west.

The batholiths and older metamorphic rocks are overlain by Upper
Cretaceous strata, and the date of the main intrusion is some time within
the Lower Cretaceous.

Structure

Nevadan System. The metamorphic rocks and granitic batholiths of
the Nevadan system probably make up the basement complex the entire
length of Baja California. The geologic map, Fig. 30.1, shows a single
great batholith extending halfway down the peninsula to the Desert of
Santa Clara, and numerous other granite bodies carry the Nevadan system
southeastward and end in the large batholith of the southern cape region
south of La Paz. Islands on both the east and west coasts are composed
entirely or in part of Nevadan complex. The Nevadan complex has been
described, so the following structural study will deal with the Cenozoic
folds, faults, and uplift.

Anticlines and Synclines. Beal ( 1948 ) has mapped a long, gentle anti-
jcline and an almost equally long, gentle syncline in the southern half of
the peninsula. See map, Fig. 30.1. The syncline, known as the Baja Cali-
jfornia, extends from 31° N. Lat. southward for 600 miles to the isthmus of
La Paz.

\* For the first 200 kilometers of its course it follows the Pacific Coast, first
Dffshore and then on land, with marine sediments dipping gently toward its
ixis. At 29° 30' N. Lat. near Bahia San Carlos, the syncline leaves the peninsula
and crosses Bahia Sebastian Vizcaino, enters the peninsula again in the north-
western part of Desierto de Santa Clara, and extends thence through the desert

in nearly a straight line toward the Isthmus of La Paz. From the south pari of
the desert at 27° N. Lat. most of the marine sediments dip gently toward the
axis of the syncline, but, throughout much of this segment, these sediments are
overlain and piled high with Comondu and later volcanic debris. Numerous
local folds, some gentle, but others sharp, narrow wrinkles, were Found in the
trough of this great syncline.

The anticline along the east coast is called La Gigante and extends from
Santa Rosalia southeastward about 200 miles to the Bay of La Paz.

Detailed mapping will undoubtedly show the area through which the axial
line is drawn to be generally anticlinal and will probably disclose that the
uplift is made up of several discontinuous anticlines, and faulting has been a
factor in its elevation. Over nearly the entire distance the elevation of the area
has resulted in great coastal escarpments which rise steeply for hundreds of
feet from the gulf shore or the narrow coastal plain.

Bahia Concepcion is definitely anticlinal as the Comondu rocks on both sides
dip gently in opposite directions, and the same rocks near Aguaje at the south-
ern end of the bay are folded into a well-marked anticline and several smaller
folds, with dips ranging up to 30°. North of Loreto (Comondu quadrangle) the
mountain shown as 2227 feet high has been forced up causing the Pliocene
sediments to dip away in all directions. This area lies on another anticline east
of the major uplift, but a few kilometers southwest granitic rocks are exposed
on the axis of La Gigante anticline with Comondu rocks arching over the
exposure. Southeast of Agua Verde (Santa Cruz quadrangle) the mountains
back of Punta San Marcial are distinctly anticlinal, and east of Rancho Carriza-
lito (Santa Catalina quadrangle) the Ysidro formation is arched over a small
mass of crystalline rocks.

Faults. The great eastward-facing escarpment along the Sierras Juarez
and San Pedro Martir is believed to mark a fault zone which has been
called the San Pedro Martir by Beal (S.P.M. on map of Fig. 30.1). The
scarp is likened to that of the Sierra Nevada, and the fault zone is
thought to be continuous along the east side of the Peninsula Range into
southern California. Beal (1948) judges the vertical displacement to be
about 5000 feet at the 31st parallel.

The western face of the Sierra Victoria of the southern cape region is
considerably steeper than the eastern, and is regarded by Beal as marking
a fault, called the La Paz (L.P. on map, Fig. 30.1). Submarine contours
northward in the gulf suggests a projection of the fault. Beal points out
that the Sierra Victoria trends northward obliquely across the peninsula
and stands apart as a distinct unit. It thus seems to require a structure

486

STRUCTURAL GEOLOGY OF NORTH AMERICA

such as the postulated La Paz fault, which he considers pre-Tertiary
in age.

The submarine topography of the Gulf of California will be described
immediately, and a downfaulted origin postulated. The Ceralbo fault is
the major dislocation visualized.

Darton ( 1921 ) believed the major orogeny of the peninsula in Tertiary
time resulted in the tilting of the long block upward on the east side and
the sinking of the gulf, as diagrammed in Fig. 30.5. This presumably
is the overall picture, but Real adds three other structural elements,
namely, the long, gentle folds and the diagonal La Paz fault, the major
faults indicated by the submarine topography along the west side
of the peninsula, and regional uplifts and submergences in late Cenozoic
time. The submarine topography is treated separately in Chapter 32 and
the regional vertical movements in the following tectonic history.

Tectonic History

The following resume of the tectonic history of Raja California is com-
posed of quotations from Real's (1948) memoir.

Cretaceous Phase. The earliest record of the Cretaceous in Baja California
is the San Fernando formation, which, insofar as it is known, was deposited only
on the western slopes of the peninsula. Its lower stratigraphic limit is not known.
The area over which the formation occurs probably was subjected to erosion
during a long period before the deposition of the Rosario sediments and was
extensively intruded during that time, which in places almost obliterated the
sedimentary character of the series. No intrusions of the same type were ob-
served to cut the younger Rosario formation.

The base of the Rosario formation was not seen as it probably lies under the
ocean, and the series may be much thicker than indicated by the exposures.
. . . During the deposition of the Rosario formation considerable areas of the
San Fernando formation and of the earlier metamorphic rocks stood above
water; erosion, whether shoreline or by streams, was principally in such rocks;
and parts of the Rosario formation were also above sea level while sediments of
the same series were being deposited. . . .

Following the deposition of the Rosario formation, the strata were locally
distorted, but where observed, they were not usually folded sharply nor faulted.
These structural phenomena suggest compression and folding while the sedi-
ments were but slighdy loaded and before they had been completely indurated.
. . . The diastrophic activity resulted not only in the mild folding and partial
erosion of the Rosario sediments, but also marked an important emergence ex-

tending as far south along the west coast as 28° N. Lat., because the succeeding
Eocene beds north of that parallel were laid down in a sea which transgressed
over a rugged topography in which many kinds of rocks were exposed.

Early Tertiary Phase. The Paleocene and Eocene periods were marked by
an important subsidence during which the sea, with some protruding insular
areas, covered the western flanks of the peninsula. ... It appears that the sea
may have first occupied the coastal regions of the northern part of the peninsula
from about 31° 30' N. Lat. southward to the 27th parallel. . . . The southern
margin of the sea in Baja California at that time appears to have been near the
isthmus of La Paz, and the sea may have extended across the isthmus to the
present lower gulf. . . . [See Fig. 30.5.]

The back country must have been of moderate elevation, well watered, thus
supporting large streams, and the climate was tropical as indicated by the
faunas.

An emergence near the close of Eocene time marks the beginning of a period
of erosion and local folding of the Tepetate formation. The contact between
Eocene beds and the overlying Miocene appears to be almost conformable
where observed near the axis of the Baja California syncline, but in the Western
Cape region the unconformity between the Tepetate and Miocene is more im-
portant, indicating that the earlier movement along the western marginal uplift
continued in the post-Eocene.

If the sandstones at Santa Gertrudis east of Desierto de Santa Clara prove to
be Tepetate in age, they probably represent the eastern limit of the formation
in that area. They are overlain direcdy by Upper Miocene volcanics, thus in-
dicating post-Eocene pre-Lower Miocene uplift along the axis of the peninsula
near the eastern marginal uplift.

After the deposition of the Tepetate and before Miocene time, volcanism of
some importance must have broken out, for the granitic rocks underlying the
Ysidro beds in the Southern Cape region are intruded and in places covered by
volcanic rocks; furthermore, east of San Ignacio Lagoon the basal light con-
glomerate of the Ysidro formation, resting on the Tepetate with slight un-
conformity, contains pebbles of volcanic rock.

Mid-Tertiary Phase. The depressed area (in early and middle Miocene)
probably covered the synclinal region from a point in the desert area northwest
of Purisima and southeastward to the Isthmus of La Paz; it probably was
bordered on the west by the uplifted granitic areas at Bahia de Magdelena,
which protruded as islands in the sea. The eastern extension of the marine in-
vasion may have occupied the east coast of the peninsula from Punta San
Marcial to La Paz and extended well into the adjacent gulf. . . . This possible
eastward marine transgression, insofar as known, is the first Tertiary sea to have
occupied any part of the gulf coast, except for the period during which the
Cornwallius beds were deposited. [See Fig. 30.5.]

The upper Ysidro submergence in the southern area appears to have been a
continuation of that which allowed the deposition of the lower shale member.
It was important and widespread — much more so than the preceding. The

BAJA CALIFORNIA AND SONORA SYSTEMS

481

PALEOCENE

EOCENE

OUGOCENE

PLIOCENE

Fig. 30.5. Paleogrography of Baja California during the Tertiary, after Durham and Allison, 1960. Ruled
areas denote land. The Oligocene beds of Beat are earliest Miocene on the basis of the megafauna,
according to Durham and Allison.

iPurisima region was again submerged. The San Ignacio area and probably
much of Desierto de Santa Clara, much of the Western Cape region, and part
of Isla Cedros suffered their first Miocene submergence. The northern limit of
ithis sea may have been some place north of the 28th parallel. The eastern limit
of the sea extended along the west side of the sierras, beginning not far west
of Las Tres Virgenes, and crossed the peninsula to the gulf coast near Punta
San Marcial.

Some structural considerations indicate that much of the lower gulf was
occupied by the Ysidro sea. For example, the Southern Cape region probably
was an elevated block from early Cretaceous, as it appears that in Eocene, and
probably in Cretaceous time, the Isthmus of La Paz marked the southernmost
'limit of marine invasion, and no sedimentaries are known to have been de-
tposited on it until Ysidro time.

The La Paz fault is deeply significant from the standpoint of the geologic
history of the gulf and of the peninsula. Downthrow on the west side allowed
■the deposition of Tertiary and perhaps Cretaceous beds from the Isthmus of
La Paz northwestward, and the northerly extension of the fault may have been
a factor in severing the peninsular structural block from the old land mass. . . .

After the deposition of the Ysidro formation the peninsular area was elevated;
its western margin may have been roughly coincident with the western marginal
juplift and the eastern side bounded by the ancestral gulf over a part of its

length, but parts of the near-shore insular area, from about 27° 30' X. Lat. to
La Paz west of the Ceralbo fault zone, were still a part of the land area. The
northern half, which stood above water during Ysidro time, was further ele-
vated, and areas of Ysidro sediments were elevated sufficiently to allow consid-
erable erosion, especially along the margins of the peninsular area. The syncli-
nal and some other areas appear to have suffered but minor erosion, as at many
places there is littie evidence of unconformity between the Ysidro and the
overlying Comondu rocks.

Late Tertiary Phase. Volcanism broke out in late Miocene time and, in
places, has continued down to the present. The Comondu formation of the
peninsula is thick and made up of many kinds of rocks of volcanic origin.
The northern half of the pensinsula must have been out of water, but the
southern half was largely a site of deposition. The volcanism is probably
related to the Baucarit sedimentation and volcanism of Sonora. See second
from top section in Fig. 30.6.

The Baja California syncline was gentiy depressed; the Isthmus of La Paz
elevated; the Western Cape region became a part of the peninsula, if formerly
separated from it; the northern half of the peninsula had not reached its present

488

PACIFIC PENINSULA

I -v I

STRUCTURAL GEOLOGY OF NORTH AMERICA

GULF

SONORA DESERT AND
PARALLEL RANGES

SIERRA MADRE
OCCIDENTAL

PLEISTOCENE

1 1

SALAOA FM. AND ",

COMONDU VOLV ^^-ff^

BAUCARIT SE05. * V0LCANIC3

LATE TERTIARY

i^^r

-^** ™' ' ' ''" ___— — ^C<?<'^

~^^WK^

1 i

YELLOW BCDJ OR YSIORO FM.

-? rr^Z?Z

1 1

MID-TERTIARY OROGENY^-

::■:■' -~-~-^~S*^i&~'~~>-^$~W~^

BY LATE CRETACEOUS

Fig. 30.6. Evolution of the provinces of western Mexico from late Cretaceous time to the
present. Diagrams are highly idealized.

height; faulting and folding probably occurred along the east side of the penin-
sula; and further movement may have occurred along the Ceralbo and Bellenas
fault zones and along unmapped faults in the adjacent sea bottom.

Some of the islands probably continued as independent structural units, the
upward or downward movement of one not necessarily being coincident with
or dependent upon the movement of another or with that of the peninsula as a
whole. This is indicated by steeper dips in the Comondu on many of the islands
than on adjacent parts of the peninsula, though some or much of this deforma-
tion may have occurred in the Pleistocene. The Comondu often shows more
deformation on the east coast than farther west, which suggests that the major
structural forces were more effective on that side. If true, that may have re-
sulted in the first westward tilting of the peninsula.

The Pliocene history is complex and not well understood (Real, 1948).
Pliocene deposits in places indicate sea-level deposition, but since now
observed at elevations over 1000 feet, late Pliocene or Pleistocene uplift
must be postulated. In places the uplift is believed to be post-Comondu
but pre-Pliocene.

An area of greater significance is the known marine Pliocene at Santa Rosalia,
which has been elevated to about 500 feet, possibly more. According to in-
formation from Mr. Ivan F. Wilson, the underlying Comondu formation occurs
in a series of fault blocks cut by southwesterly dipping faults. As these faults
are probably of pre-Pliocene age, the sierra southwest, composed principally of
Comondu rocks and rising to more than 5000 feet, must have reached nearly
that elevation at the time of the post-Comondu uplift and deformation, but in
pre-Pliocene time. It is doubtful if any major change in the relative elevations
of the Comondu and Salada areas has been caused by erosion and deformation.

In the diagrams of Fig. 30.6, the post-Salada and post-Raucarit disastro-
phism is indicated as due to compression, and the Gulf of California
had not yet come into existence. According to Real, however, some fault-
ing had probably occurred in mid-Tertiary time, and not all of the down-
faulting of the gulf and the uplift of the peninsula took place in the I
Pleistocene, as illustrated. It is certain, however, that a great deal of the
displacement that shaped these major elements is post-Salada.

Quaternary Phase. The submarine canyons on the continental margin
have been regarded as of subaerial erosion, and hence to represent a great
emergence, according to Real, in postfaulting time. This does not seem
necessary, however, because when once deeply submerged, the form re-
mains little changed, and the canyons may be of considerable antiquity.
Retter understood is evidence of a great Pleistocene submergence. Ter-
races and marine shells lead Real to conclude that

. . . there seems little doubt that the sea level rose at least 1600 feet, and
Wittich (1920) believed it rose to about 3000 feet. If the depression of 1600
feet was uniform throughout the full length of Baja California, the peninsula
would have been only about two-thirds its present length with a string of is-
lands extending southeastward.

Johnson (1924), in his study of the fauna and flora of Baja California, states
"For some reason the fauna and flora were subjected to a crisis during Pleisto-
cene, and all but a few vertebrates were destroyed." This wholesale destruction
might have resulted from the submergence of the peninsula, indicated by the
presence of sea shells at considerable heights.

BAJA CALIFORNIA AND SONORA SYSTEMS

4S9

The following emergence of equal magnitude may still be going on in
places.

Volcanoes were active during the Pleistocene and have continued their ac-
tivity to Recent time. Isla Tortuga is the youngest island in the gulf; it erupted
from the gulf floor about 6000 feet deep and reached an elevation above the
gulf of more than 1000 feet. Its poorly eroded surface and lack of vegetation
vouch for its youth. Las Tres Virgenes are said to have been active in historic
time. On the west slope of the sierras there are many Quaternary craters and
cones, and at San Quintin the volcanic flows, according to Woodford (1928),
may be in part historic. Cerro Prieto, near Volcano Lake (Mexicali quadrangle),
is a small perfect crater probably formed in Recent time.

Movement along older fault lines continued during the Quaternary, and
probably many new crustal breaks were initiated, whether in early or late
Pleistocene is not known, but one may assume that much of this activity oc-
curred at the time of the Middle Pleistocene revolution of California. Movement
still continues along some of the fault zones in both California and Baja Cali-
fornia. The existence of zones of faulting which border the peninsula is indi-
cated most strongly by recent phenomena, though activity along some of these
zones probably has been nearly continuous from some remote time.

Quaternary uplift has increased the height of the mountains of the peninsula,
rejuvenated streams in regions of low relief, and exposed a considerable area of
partially consolidated beach material to erosion, with the resulting development
of a coastal slope which appears from a distance to be a plane surface, but
which is really an intricate pattern of small arroyos and narrow ridges. Recent
erosion has cut deep canyons into the rocks of the peninsula and reduced the
height of its mountains, while alluvial deposition has in places half buried some
of the ranges in fans of detritus derived from them. The wind has assisted in
sculpturing some of the softer rocks in regions of rugged topography, and the
oudines of the topography are softened by the addition of aeolian material in
the broad low desert regions; giant sand dunes, or medanos, are numerous and
(cover large areas in the desert regions.

At the head of the Gulf of California the Colorado River formed an enormous
delta over which it flowed alternately into the gulf and then northward into the
Salton Sea, making what is now the Salton Basin into a fresh-water lake. The
jCoahuila Indians have handed down legends about this diversion.

GULF OF CALIFORNIA

y Shepard and Emery (1941) and Beal (1948) consider the Gulf of Cali-
fornia to be a downfaulted trough complementary to the uplift of the
peninsula of Baja California. King (1939) has suggested a relation of

the faults of adjacent Sonora to those of the Gulf. Beal has described
the submerged topography as follows:

The northern quarter of the gulf is shallow — at no place more than 600 feet
deep. The deepest parts of the gulf south of the 30th parallel appear to lie
west of its center, and thus probably before the floor was deformed by so much
faulting it simulated, in some respects, the westward-tilted block of the penin-
sula, suggesting an extension of the basin and range structure of the Sonora
area.

The east side of the gulf appears not to have been affected by faulting; the
gulf floor slopes gently from the Sonora coast to the irregular escarpments near
the center of the gulf. The most important of these is the great submarine cliff
nearly 6000 feet high between the 25th and 26th parallels. Between the 24th
and 27th parallels there are many irregularities in the submarine topography
between the Ceralbo fault zone and the east coast of the peninsula, but most of
them lie west of the Ceralbo fault zone.

Between 30 and 40 islands varying in size from Isla Angel de la Guarda,
between 75 and 80 kilometers long, to very small ones, rise above the surface
of the gulf, some to surprising elevations. Other islands such as Consag Bock,
San Pedro Martir, Ceralbo, and Santa Catalina have the appearance of wedges
uplifted from the granitic floor of the gulf or as stocks or spurs still attached to
the granitic batholith.

Much of the south half of the gulf is occupied by a remarkable depression in
the sea floor, extending 400 kilometers southeastward from a point east of Isla
Tortuga. It widens into enormous proportions at places and becomes narrow in
others, with the closing depression contour 5400 feet below sea level. This
great depression area is occupied by three separate smaller basins, the largest
and deepest (10,740 feet) of which lies in the center of the gulf between 25
and 26° N. Lat.

A distinctive depression about 250 kilometers long, the origin of which can
reasonably be assigned only to faulting, separates the Angel de la Guarda group
of islands from the peninsula. The deepest part of the trough is about 5100
feet and lies adjacent to Isla Sal si Peudas. The closing depression contour is
1200 feet below sea level, thus furnishing a long narrow basin, nearly 4000 feet
deep, which widens at its north end. A line indicating the east boundary of the
graben is called the Ballenas fault zone, the northwestward extension of which
may lie farther west than shown and join the northwestern extension of the
Ceralbo fault zone. [This fault, or fault zone, has been drawn on Fig. 30.6 as the
western boundary of the depression which the writer interprets as a graben.]

From the configuration of the gulf floor, there seems good evidence of a
fault east of the Isla Ceralbo [Fig. 30.6]. At the sea bottom, north and east of
this island, is a submerged island nearly three times as long as Ceralbo, with
its crest approximately 1000 feet below sea level and rising about 2500 feet
above its base. Topographically, the submerged island appears to ha\ e been
once a part of Isla Ceralbo, and both apparently a part of the Southern Cape

490

STRUCTURAL GEOLOGY OF NORTH AMERICA

region, but the submerged island is now separated from Ceralbo by a deep,
narrow basin with its bottom 6000 feet below sea level and a sill depth of 4800
feet. Immediately northwest of Isla Ceralbo there is a smaller submerged hill
about the same size as Ceralbo; its crest lies only 600 feet below sea level, and
it may originally have been a part of the same mountain mass. If facts can
finally be collected on the structure of the gulf floor east and northeast of the
Southern Cape region, they will probably show that the deep basin immediately
east of San Jose del Cabo has been caused by north-south faulting parallel to
the La Paz fault, and that the deep narrow basins farther north owe their origin
to northwest-southeast faulting, with the same structural trend as the gulf
trough.

In seeking for the cause of the broad, deep basins in the gulf below the 28th
parallel, one may conjecture that they are probably structurally depressed,
wedge-shaped blocks, bounded by faults. Ballenas Channel and the depression
east of Isla Ceralbo, both of which appear to be grabens, may have originated
in the same way. If they were deep troughs with open ends, instead of elongated
steep-sided basins, their unusual depths might have been attained by the erosive
action of the gulf currents. It appears that their great depth as basins, however,
can logically be explained only by assuming the basins to be the apices of
structurally depressed wedges.

Tertiary sediments in the Salton basin and farther northwest may be
very thick, and Real suggests that basement rocks under the north end of
the gulf trough may be 25,000 to 30,000 feet below sea level. It is gen-
erally recognized that the Colorado delta has contributed much toward
filling the trough and making the present floor shallower.

SIERRA MADRE OCCIDENTAL

Early Tertiary Phase

According to R. E. King (1939), the Sierra Madre Occidental takes
form south of the international boundary by the coalescing of mountain
ranges which, in southern New Mexico and Arizona, are more or less
isolated. South of the boundary, the plains between the mountains be-
come narrower, and the volcanic rocks spread out in a broad plateau.
The western edge of the plateau, at an elevation of 6000 feet or more,
breaks off toward the Gulf of California in lofty escarpments which are
trenched by most impressive gorges. West of the Sierra Madre proper,
high ranges are separated by long, narrow valleys. Still farther
west, bordering the gulf, low mountains are separated by broad

plains, as in the Basin and Range province of southwestern United States.

The three geomorphic divisions have been called, by King, the Sierra
Madre Occidental province, the province of parallel ranges and valleys,
and the Sonoran Desert province.

The Sierra Madre Occidental has generally been assumed to be a struc-
turally simple plateau of flat-lying lavas overlying a basement of sedi-
mentary rocks and ancient granites, but a reconnaissance survey by
R. E. King ( 1939 ) has added greatly to our knowledge of the region and
revealed a complex structural history. The rocks studied by King have
been much folded and faulted and are intruded by numerous plutons
of fairly large size. There are several periods of deformation, but only
those of the Tertiary can be deciphered with any assurance. Two un-
conformities in the Tertiary mark times of important mountain building.
The structural features produced by the Tertiary episodes of deformation
trend in general north-northwest, and produce a conspicuous alignment of
rock outcrops and ridges.

The effects of the Laramide revolution have already been mentioned
in connection with the Mexican geosyncline. See lower two sections of
Fig. 30.6. Early Tertiary volcanic rocks spread out over much of the sur-
face of western Sonora but reached their greatest development in the
plateau section of the Sierra Madre Occidental. See third section from
bottom of Fig. 30.6. In the plateau section, the underlying Mesozoic rocks
are probably greatly deformed, for such disturbance is evident along the
western edge of the plateau and in the few inliers within the plateau and
in the Nevadan type rocks of Baja California. The later or post-volcanic
deformations strongly expressed to the west in the parallel ranges and
Sonoran Desert have, however, scarcely affected this region. Over wide
areas, the volcanic rocks are more than 5000 feet thick and are flat or
gently tilted. They consist of flows and pyroclastics with basalts dominant
in northern Sonora (Imlay, 1939) and more acidic types most voluminous
in central Sonora (King, 1939). The volcanic layers were then uplifted
epeirogenically thousands of feet, evidently, because an erosion surface
developed to maturity on them. It is now deeply dissected by the present
cycle of erosion (King, 1939). See fourth section from bottom, Fig. 30.6.

Toward the west the plateau gradually loses its structural simplicity.

BAJA CALIFORNIA AND SONORA SYSTEMS

491

Within the barranca section (great gorges indenting the west-facing
escarpments) not only are the plateau summits largely destroyed by
erosion, but the volcanic rocks are also broken by faults that belong to
later deformational phases.

Mid-Tertiary Phase

Parallel Ranges and Valleys. After the early Tertiary eruptions, there
was a vigorous phase of mountain making that is known principally in the
province of parallel ranges and valleys and in the Sonoran Desert. Within
the plateau section of the Sierra Madre, the volcanic rocks were only
gently folded, and over wide areas they still remain nearly horizontal.
This gentle folding contrasts with the strong disturbance of the Creta-
ceous and other Mesozoic rocks, where they can be observed beneath,
and indicates that the Laramide orogeny was greater than the mid-
Tertiary in the Sierra Madre proper.

Farther west, as in the province of parallel ranges, folds and thrust
faults occur that can be assigned to the mid-Tertiary deformation, which
here exceeds the Laramide. The mountains probably began to assume
their present form at this time.

The ranges are generally bordered by faults. North of the 28th parallel,
the faults are high- and low-angle thrusts. To the south, steep normal
faults predominate. They are not all, however, of the mid-Tertiary dis-
turbance; some are late Tertiary.

Accompanying the mid-Tertiary orogeny were vast intrusions of granite
;and other plutonic rocks, which ascended through the Paleozoic and
iMesozoic rocks and, in places, penetrated the early Tertiary volcanics.
(Some of the faulting started at this time, because several of the thrust
jfaults that break the early Tertiary volcanic rocks of the barranca section
|are cut off by granite intrusions. See fourth cross section from bottom of
jFig. 30.6.

Sonoran Desert Province. North of latitude 28° 30' N., a large propor-
tion of the detached mountain ranges in the Sonoran desert province con-
sists of Paleozoic and Mesozoic sedimentary rocks. To the south in the
unmapped area that extends to the coast, they consist of volcanic rocks
jand granite. The ranges of sedimentary and volcanic rocks appear to be

only detached roof pendants in a vast granite batholith or group of
coalescing batholiths (King, 1939). They probably represent the lowest
part of the roof at the end of the period of intrusion. Nearly all the non-
granitic rock in the ranges is cut by apophyses of granite.

The granite intrusions have complicated the pregranite structure of the
sedimentary and volcanic rocks by metamorphosing and shattering them
close to the contact and by jointing them excessively for some distance
from the contact. Alternations of competent and incompetent strata, such
as are found in parts of the Paleozoic and the Jurassic Barranca formation,
shows such a confusion of dips and small faults that it is very difficult to
work out the main structural features. Only the most massive, resistant
formations, such as the Permian limestone and the upper part of the
Barranca formation, show the structure clearly; and even these only at
some distance from the nearest granite contact (King, 1939). Despite
these confusing relations, King finds some of the larger features of the
structure plain. The mountains in part are clearly upfaulted. Some still
preserve the form of tilted fault blocks, although considerably modified
by erosion. Some of the depressions are downfaulted, and some over-
thrusting is present in the Sonoran Desert.

Baja California. The bulk of the Tertiary formations in Baja California
are the result of orogeny to the east in Sonora. This is particularly true of
the "yellow beds" (Darton terminology). They are present in great
volume and coarsen eastward. From the relations that Darton depicts, the
yellow beds are the great orogenic deposit in the southern half of Baja
California, and if they are the late lower and middle Miocene Ysidro
formation of Beal, then the mid-Tertiary orogeny of King in Sonora is
probably dated by them.

In Sonora itself, the next youngest formation after the disturbance is the
Baucarit of late Pliocene or Quaternary age. It occupies the depressions
between ranges and probably was deposited some time after the orogeny.
See second section from the top of Fig. 30.6. The yellow beds were up-
turned in places, eroded, and then covered with sands, conglomerates,
agglomerates, and basalt flows. Since these capping deposits are late
Miocene (?) and Pliocene in age (Beal, 1948), it appears that the yellow
beds were deposited in a hurry and then immediately somewhat de-

492

STRUCTURAL GEOLOGY OF NORTH AMERICA

formed. Both the orogeny that resulted in their deposition and the
impulse that deformed them might, therefore, be considered parts of the
same phase until more information is available.

Late Tertiary or Early Pleistocene Phase

After the mid-Tertiary orogeny in the province of parallel ranges and
valleys and in the Sonoran Desert, the Baucarit formation was laid down
in the structural depressions between the uplifted mountain ranges. A
moderate recurrence of volcanic activity is indicated by the basalt flows in
the lower part of the formation. Interbedded with and overlying the
basalts are conglomerates which were doubtless laid down as coalescing
alluvial fans at the margins of the mountains. They contain fragments
derived from the cores of the ranges including boulders of granite. Similar
deposits overlying the yellow beds of Baja California have already been
mentioned. Beal describes two formations there, the lower Comondu vol-
canics and the overlying Salada formation, which appear similar to the
Baucarit, and correlative with it. The next structural phase postdates the
Baucarit, and is of varied aspect. There was renewed volcanic activity,
and the Baucarit formation was thrown into low folds and tilted.

Quoting from King (1939):

North of the 28th parallel, the rocks of each of the high mountain ranges,
from the crest of the Sierra Madre westward into central Sonora, were pushed
to the west on overthrust faults which partly overrode the Baucarit formation,
lying in the valleys next to the west. Some minor faults were thrust to the east.
The strong thrusting and the gende warping of this orogenic epoch suggest
that the strata of the mountains had already become so consolidated by previous

folding and igneous intrusions that they could no longer yield to lateral pressure
by folding. The greater amount of thrusting north of the 28th parallel may be
due to the greater thickness of Paleozoic and Mesozoic sedimentary rocks in
that region.

The normal faults extensively developed south of the 28th parallel and far-
ther west in central Sonora were somehow related to the thrust faults. At La
Colorada these offset the plane of an overthrust fault, but both here and to the
south they have the same north-northwest trends as the overthrusts and thus
may have taken their form from the same forces. In the province of parallel
ranges and valleys, the localization of overthrusts north of the 28th parallel and
of normal faults to the south of it suggests that the orogenic forces, although
dominandy compressional, produced local areas of tension.

During rather recent geologic time, a mature erosion surface of low relief was
developed in the lava country along the crest of the Sierra Madre. After its for-
mation, the area was gready uplifted, and streams draining to the west deeply
intrenched their courses, forming the tremendous barrancas of the western flank
of the Sierra Madre. It is not entirely certain when this uplift took place, but
the great height of the surface above low country not far to the west strongly
suggests that it was raised by faulting on the west side of the plateau. This
faulting may have been the post-Baucarit thrust faulting, or it may have been a
renewed movement at a later time along the same trends.

King cannot date the elevation of the lavas of the Sierra Madre Oc-
cidental with accuracy, but he believes the elevation was due to faulting
in post-Baucarit time. It seems probable, therefore, that the elevation
of the Sieras occurred at the same time as the sagging of the Gulf, and
that they are parts of the same fault block system. The differential move-
ment, as estimated from the bottom of the Gulf to the crest of the Sierras,
is about 12,000 feet.

The position of Baja California in the regional tectonic plan is treated
in Chapters 29 and 31.

31.

MIDDLE AND LATE
CENOZOIC SYSTEMS OF
THE CENTRAL CORDILLERA

i
l

GENERAL DIVISIONS AND THEIR CHARACTERISTICS

For structural purposes it seems best to treat the middle and late Ter-
nary mountain systems in the central part of the great Cordillera of
Morth America in three divisions, namely, the Basin and Range system
)f southern Oregon, eastern California, Nevada, western Utah, and north-
vestern Arizona; the Sonoran-Chihuahua system of desert ranges in
vestern and southern Arizona, New Mexico, and central Mexico; and
he system of great trenches in central Utah, eastern Idaho, western
Wyoming, western Montana, and British Columbia. The first two di-
osions are generally included by the physiographers in the Basin and

Range province, and the third has generally not been distinguished From
the Laramide Rockies on whose folds and thrusts its fault-made trendies
are superposed.

The Basin and Range system is one generally of north-south-trendin.:
basins and ranges, with the majority of the ranges probably blocked out
by high-angle faults. The distinctive features of the province, accordin'j;
to Fenneman (1931), are "isolated, nearly parallel mountain ranges (com-
monly fault blocks ) and intervening plains made in the main of subaerial
deposits of waste from the mountains. These deposits, although locally
absent, are often very deep and are generally unconsolidated."

The boundaries of the Basin and Range system are shown on the map
of Fig. 31.1. The Great Basin of internal drainage, the Mojave Desert,
and the Salton trough are the chief regions here included. The Basin and
Range province is bounded on the west by the Sierra Nevada, on the
east by the Wasatch Mountains and High Plateaus of Utah, and on the
north by the Malheur plateau and Snake River lava plains. The nar-
rowing south end has been arbitrarily defined by Nolan (1943) to have
the San Andreas fault on the west and the Colorado River on the east.
The physiographic section of the Great Basin, called the Sonoran Desert
by Fenneman, includes large areas on both sides of the Colorado River;
and the Basin and Range system is probably continuous across it to the
desert ranges of southern Arizona.

The desert ranges of Arizona, New Mexico, and part of the Mesa Cen-
tral of northern interior Mexico are somewhat similar to those of the
Great Basin in being rudely parallel and separated by basins filled in part
or completely by alluvium. Those in southern Arizona stretch northwest-
ward across the southern and southwestern part of the state. They con-
verge haphazardly, in the southeastern corner, with basin ranges of New
Mexico which extend northward through the central part of the state.
Together, the ranges of block-fault character of Arizona and New Mexico
extend southward into Mexico through the state of Chihuahua to Du-
rango, Coahuila, Zacatecas, and San Luis Potosi. The sources of
information on this great region are a few detailed studies of wide sep-
arated areas. Large parts of it have never been reported on, so the con-
cept is not secure that it is everywhere a mountain system whose present

493

494

STRUCTURAL GEOLOGY OF NORTH AMERICA

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form is due largely to middle and late Tertiary deformation. Literature
on the structure of the desert ranges of the western part of the Plateau
Central of Mexico is almost nonexistent.

All the great trenches of central Utah, eastern Idaho, western Wyo-
ming and Montana, and British Columbia are probably fault valleys and
of middle and late Tertiary age. They extend as a narrow belt from the
High Plateaus of northern Arizona and central Utah through the Wasatch
Mountains in Utah and northward along the boundary of Idaho and
Wyoming to the Teton Range and Jackson Hole in northwestern Wyo-
ming, thence northwestward as a wider belt through western Montana
to the great trenches of British Columbia. See the map, Fig. 31.1 for
boundaries. Much also remains here to be worked out; but sufficient is
known, it is believed, to compose these great valleys into a structural
system and to treat them collectively as such.

With few exceptions, the middle and late Tertiary high-angle faults of
the Great Basin and the folds and thrusts of southern California are
superposed on earlier Nevadan or Laramide structures.

BASIN AND RANGE SYSTEM

Evidence of Faulting

Four types of evidence have been used to prove that the individual
ranges in the Great Basin are bordered by block faults: physiographic!
evidence, stratigraphic evidence, exposure of a fault plane, and presence|
of recent fault scarps along die range fronts. As the boundary betweenl
mountain and valley blocks is commonly concealed by the alluvium ac-
cumulating in one or more closed basins, die second and third types:;
of evidence are rarely found; for most places physiographic evidence has
been called upon to determine the existence of a fault block.

Fig. 31.1 Tectonic map of the western Cordillera in late Miocene, Pliocene, and early Pleistocene T>
time. The sediments along the Pacific are late Miocene and Pliocene in age. They are horizontally
dashed. The obliquely ruled area denotes the Basin and Range and Sonoran-Chihuahua systems,
the faulting of which took place chiefly in Pliocene and early Pleistocene time, although in «i
places it started earlier and lasted longer, even to the present. The cross-ruled belt is thei i
system of great trenches. Miocene and Pliocene basin deposits are common in all three fault"
systems.

J 01

MIDDLE AND LATE CENOZOIC SYSTEMS OF THE CENTRAL CORDILLERA

495

The following kinds of physiographic evidence have been used: the
front of the range is linear and cuts indiscriminately across the rock struc-
ture; the range rises abruptly from a waste-filled valley; many steep,
narrow V-shaped ravines cleave the mountain block and open abruptly
onto the gravel fans of the valleys, and triangular facets are aligned
along the mountain front. Major valleys cutting through the ranges are
generally absent; mature topography or thin caps of volcanic rocks mark
summits or back slopes of the ranges; landslides are common along the
range fronts; hanging valleys are present on some range fronts; and the
lowest point in the adjoining valley is close to the scarp along the range
front.

Rlackwelder ( 1928 ) has reviewed these and other proposed criteria
and has pointed out that several of them are equally applicable to ex-
humed or "fault-line" scarps. He regards the following features as positive
evidence of true fault scarps: (1) lack of correlation between rock re-
sistance and surface form; (2) rift features; (3) alluvial deposits on the
down thrown block thickest near the fault line; (4) lake or sink close to
;he scarp base; (5) alluvial fans abnormally small; (6) frequent severe
nirthquakes; (7) displacement of an older topographic surface; (8) dis-
location of Recent or late Pleistocene formations; (9) basal scarplets; (10)
varped terraces in the canyons; and (11) the fault plane identified as
orming part of the scarp face. Nolan ( 1943) comments that some of these
^atures are of relatively little value because of their infrequent oc-
currence (item 10, for example) or because of the absence of adequate
Information (item 3); and others, such as item 6, are of questionable
jlependability. Other observers would probably regard additional features
s equally valid evidence.

When critically used there is little doubt that physiographic evidence
lone is adequate and diagnostic. In many places, however, use of evi-
ence of this type has resulted in a failure to distinguish between fault
carps and fault-line scarps; and there has even been a tendency to con-
sider that any elevated block with an approximate linear trend is neces-
iirily a fault block.

| Stratigraphic evidence of faulting along the borders of ranges is gen-
;ally difficult to find because valley fill commonly conceals the down-

thrown block. Stratigraphic proof of faulting has been found in the
Humboldt Lake and adjoining ranges, Nevada (Louderback, 1904); the
Lake Tahoe region, California-Nevada (Reid, 1911); the Oquirrh Range,
Utah (Gilluly, 1928b); the Warner Range, California (Russell, 1928); the
Wasatch Range, Utah (Eardley, 1934); the Deep Creek Range, Utah
( Nolan, 1935 ) ; the Roulder Dam region, Nevada ( Longwell, 1936 ) ; and
the Comstock Lode, Nevada (Gianella, 1936). In other places faulting
along the range front has been inferred from the presence of parallel
step faults within the range (Fuller and Waters, 1929).

In a few places, no evidence of faulting at the contact between the
rocks that form the ridges and the Tertiary sedimentary beds that under-
lie the valleys is apparent. Ferguson and Cathcart (1924), however, have
interpreted similar occurrences in central Nevada as the result of sedi-
mentation on the downthrown block, which overlapped the outcrop of
the fault.

Actual exposures of faults bordering the ranges have been made ac-
cessible by artificial excavations, but in a few places they have been
revealed by erosion. The W'asatch fault has been located by Pack (1926)
and Eardley (1934), several faults along the west edge of the Oquirrh
Range have been located by Gilluly (1928b), several Pliocene faults
in soudiern Nevada have been located by Longwell (1936), and addi-
tional faults in central Nevada have been located by Ferguson. In the
region studied by Longwell a considerable vertical extent of the fault
was revealed, and here at least the dip of the fault steepened upward; at
the other localities fairly steep valleyward dips prevail, ranging from
50 to 72 degrees.

Small scarps formed by recent faulting, called piedmont scarps by Gil-
bert (1928) or fan scarps by Longwell (1930), correlate closely with
the scarps bordering many of the basin ranges. This was first pointed out
by Russell ( 1884 ) , and since that time these recent scarps have been com-
monly considered to indicate the presence of persistent faults. Many of
them have been recognized throughout the Great Basin, those in the La-
hontan and Bonneville basins by Russell (1885) and Gilbert ( 1S90,
1928a); those along the Sierra Nevada by Hobbs (1910), Lawson (1912 V
and Knopf (1918); those in central Nevada by Jones (1915). Page

496

STRUCTURAL GEOLOGY OF NORTH AMERICA

OWENS
VALLEY

Fig. 31.2. Generalized diagram of part of tilted Sierra block. The great fault fractures that
separate the Sierra block from the Owens Valley block, on the east, are shown by a single line.
The height and slant of the Sierra block are much exaggerated. The streams are shown in their
characteristic arrangement, the main rivers flowing down the western slope but many of their
tributaries in directions approximately at right angles to them. No specific streams are represented.
In front is a strip of the Great Valley of California, whose thick layers of sand and silt, derived
from the elevated part of the Sierra block, bury the sunken part. At the back is a strip of Owens
Valley, veneered with a thinner layer of sediment. After Matthes, 1930.

(1935), Gianella and Callaghan (1934); those in southern Nevada by
Longwell (1930); and those in southern Oregon and northeastern Cali-
fornia by I. C. Russell (1884) and R. J. Russell (1928). In some places
these scarps have clearly been developed between the hard rocks of
the range and the gravel of the valley. Commonly, however, they are
found in the gravel some distance from the range front, and tend to be
more irregular than the front in plan. Although most of the recent scarps
he at or close to range fronts, some are also found in the intervening
valleys (Gianella, 1934; Gianella and Callaghan, 1934) and within the
mountain ranges (Callaghan and Gianella, 1935). Many of them are
accompanied by hot springs (I. C. Russell, 1884) or are coincident with
volcanic cones (Knopf, 1918).

Nature of Block Faults

The Sierra Nevada Range is a westward-tilted fault block. See Fig. 31.2.
The faults that border the east front of the range are staggered in map
plan. Along the great escarpment that faces Owens Valley there may be
a single fault, or perhaps a set of closely spaced parallel faults; but farther
north the successive offsets in the front of the range indicate the existence

otc <z

lis is I „

5* o uj<

oar

ZtU

sz
<<

111 -W

2* I

(0(9

25 bJUi

z~ (no

<-> o<

40 NEVADA! UTAH

-JO

UJ<

ICC

U/LJ

*o
<tz
z<

(nir

of? ^ J^\ ^ Z "NEVADA! UTAH -1-

200 Miles

Fig. 31.3. Diagrammatic section from central Nevada to western Utah. Reproduced from
Osmond, 1960. Solid black represents tilted Tertiary volcanic deposits. The ranges can be
interpreted as being the remnants of four large anticlines, A, and intervening synclines, B.

of discontinuous northward-trending fractures that replace one another
at intervals, thereby splintering the northwestward-trending margin o
the block on a large scale. From the neighborhood of Lake Tahoe, which
itself lies in a trough produced by the subsidence of a great splinter, Ion
lines of faulting diverge in northerly directions, each marked by ar
escarpment of its own. Northward the eastern margin of the Sierra blocl<
becomes progressively more irregular, and the displacements are distrib
uted over a belt that broadens gradually to a maximum of 50 miles
Some of the escarpments measure but a few hundred feet in height
and the highest do not exceed 2000 feet (Matthes, 1930).

Fig. 31.4. Diagrammatic sections of southwestern Utah. Section A from the Nevadan boundaf]
to the High Plateaus, and section B from the Escalante Desert to the High Plateaus. The blaci
areas represents Tertiary volcanic deposits. Reproduced from Mackin, 1960.

MIDDLE AND LATE CENOZOIC SYSTEMS OF THE CENTRAL CORDILLERA

497

Fig. 31.5. Block diagram showing nature of crustal deformation by block faulting in the Ruby-
East Humboldt Range, Nevada. The view is to the southwest. The block is about 50 miles long
(S) and 30 miles wide. (W). The diagram is approximately to scale with the maximum throw
of the faults about 6000 feet (Sharp, 1939). The faults acquired their present displacement
by four stages of movement from upper Miocene to the Pleistocene.

Studies by Hudson ( 1955 ) indicate that the uplift of the Sierra Nevada
is not due to simple tilting of a rigid block. An important zone of faulting
about midway between summit and the western edge of the range
divides the range into two blocks of deformation, and Hudson, from
gradient calculations, thinks there may be five separate bocks of adjust-
ment.

Most of the blocks throughout the Great Basin are rotated or tilted.
[Study Osmond's representation in Fig. 31.3, which includes thirteen up-
tilted blocks and probably a number of down-dropped additional blocks
ijin a distance of 200 miles. In the eastern part of the Great Basin Mackin,
Working with the ignimbrite sheets, shows a series of blocks all rotated
n the same direction. See Fig. 31.4. Some blocks, however, are horstlike,
;uch as the Ruby Mountains and East Humboldt Mountains (Fig. 31.5).
Mapping in the Wasatch Range (Eardley, 1939, 1944) indicates that
i master fault, the Wasatch fault, 115 miles long with displacement of
1000 to 6000 feet, forms the eastern limit of the faulted part of the Great
3asin. In places its displacement is distributed along step faults with
jhe west side down. It is a dip-slip normal fault and dips 50 to 70
jegrees west. A quasi en echelon pattern of smaller normal faults
preads across the thick sediments of the Pennsylvanian Oquirrh basin

of west central Utah. The Basin and Range faults are not aligned with
the Precambrian or Laramide structures. Neither have the crystalline
rocks of the northern Utah highland, the intrusions of the Cottonwood
uplift, nor the late Precambrian basins influenced perceptibly the course
or the throw of the faults. The widths of the fault blocks range from 4 to
30 miles, but a fairly uniform width of 18 to 24 miles is found in the four
major blocks of the area, the Wasatch, Oquirrh, Stansbury, and Cedar
mountain blocks. A relief of 3000 feet or more is believed to have existed
at the inception of faulting.

Age of Block Faulting

Ferguson (1926) and Ferguson and Cathcart (1924), in addition to
presenting physiographic evidence that the block faulting occurred at
different times, found that similar faults, though without present topo-
graphic expression, both preceded and followed the deposition of sedi-
ments belonging to the Esmeralda formation (late Miocene and early
Pliocene). The conclusion that these earlier faults were of the same
character as the later block faults is based on the fact that the Esmeralda,
adjacent to the pre-Esmeralda faults, is composed of material similar to
that now being deposited in the fans along range-front scarps, and
further that at least some of the topographically expressed faults have fol-
lowed the lines of these early faults. Westgate and Knopf (1932) have
also found evidence in the Boulder Dam region for block faulting that
preceded, accompanied, and followed the deposition of his Muddy Creek
formation, of questionable Pliocene age. Gianella (1936), similarly, has
distinguished two major epochs of movement at the Comstock Lode.

A typical range in the north central part of the Basin and Range
province for which the geology has been worked out is the Ruby-East
Humboldt. According to Sharp (1939) the range consists of pre-Miocene
igneous, metamorphic, and sedimentary rocks of complex structure. The
adjoining basins contain deformed beds of the upper Miocene Humboldt
formation. The boundary structure of the mountain block is well exposed
because of dissection by the through-flowing Humboldt River. See Fig.
31.6.

This range is a westward-tilted horst, bounded by normal faults which

498
W

STRUCTURAL GEOLOGY OF NORTH AMERICA

Ruby Mtns.

Lamoille Valley
Th Qa

5,000

dip 60 to 70 degrees basinward. Displacements on the east boundary-
faults have been at least twice as great ( 5500 to 6000 feet ) as on the west
boundary faults (2000 feet). The northern termination of the mountains
is due to intersection of the east and west boundary faults. The structure
of the pre-Miocene rocks is discordant with the trend and shape of the
range.

Five periods of basin-range faulting have been established: (1) late
middle or early late Miocene, displacement small (open to question);
( 2) late Miocene, during deposition of the Humboldt formation, displace-
ment larger; (3) latest Miocene to Pliocene, younger than the Humboldt
formation and older than the Pliocene (?) lava, amount of displacement
unknown; (4) Pliocene to Pleistocene, later than the Pliocene (?) lava
and extending to middle or late Pleistocene, the period of last major uplift
of the range, displacement large; (5) late Pleistocene to Recent, later than
the earliest Wisconsin, displacement small.

The history of faulting in the Sierra Nevada is fairly completely known.
Toward the end of the Eocene, volcanoes were intermittently active, and
they emitted rhyolite lava and mud that filled the existing valleys. This
volcanic activity, interspersed in an erosion cycle, continued well into
the Oligocene; at the same time die Sierras were gaining elevation by
vertical arching. The country lying to the east was warped and flexed;
low ranges came into existence, and between them were formed wide
basins in which the water collected in shallow lakes.

According to Matthes (1930), the disturbances died out at last and

were followed by a long interval of relative quiet, during which most
of the rhyolite and much other rock waste was stripped from die Sierra
region and deposited on its western border and in the basins to the east
of it. Then, presumably in the second half of the Miocene epoch, volcanic
activity and earth movements began anew on a large scale. This time,
the eruptions yielded mostly andesitic lava of brown, reddish, and grayish
colors. Down the valleys this material flowed, sheet upon sheet, obliterat-
ing the stream beds and compelling the waters to seek new paths. In the
north half of the range, the outpourings were especially frequent and
voluminous; they piled up to thicknesses of a thousand feet or more,
overwhelming all the features of the country save the higher peaks and
crests. In the soudiern parts of the range, the volcanic flows were less ex-
tensive and less thick; they filled only the bottoms of certain valleys, and
caused no notable displacements in the drainage system. Only the drain-
age basin of the Merced River, in the central part of the range, remained
free from volcanic outpourings.

The crustal movements of this epoch increased the height of the Sierra
region by several thousand feet and gave it the aspect of a mountain
range, or rather a belt of mountains, that dominated all the country round
about. Mount Lyell probably attained an altitude of about 7000 feet
Strong faulting took place along some parts of the eastern border, an
the great depression in which Lake Tahoe is situated was formed by
subsidence; the lava which dams the lake itself was not poured out, ap
parently, until after the depression was formed. The ranges and valleys

MIDDLE AND LATE CENOZOIC SYSTEMS OF THE CENTRAL CORDILLERA

499

Fault dies out

Ford Cr
Throw of foult
I600' t thickness
of alluvium

Bar nor d Cr

Parr/shCr Centerville Cr

Ward Cr * Hoi brook Cr.

Throw of iouh
looo' 1 thickness
of o//uvium

Mill Cr.

Lake Bonne-
vrlle facets.
Salt Lake
salient

Fig. 31. 7 Wasatch fault in the north central Wasatch Range and its relation to the erosion surfaces.

the Great Basin region were accentuated, in part by warping, in part by
faulting.

Next followed another long interval of repose, or relative repose, that
lasted through the entire Pliocene epoch. Only feeble eruptions took
place from time to time, and meanwhile the waters in the lava-covered
parts of the range reorganized themselves into new rivers and cut new
canyons, some of which attained depths of more than 1000 feet.

Then, at the beginning of the Quaternary period the great uplift and

tilting commenced that gave the Sierra Nevada its present great altitude.

The summit peaks were raised to almost double their previous height,

with Mount Lyell reaching more than 13,000 feet above sea level. At the

same time, fracturing and faulting took place on an enormous scale.

Owens Valley and other desert regions adjoining the range on the east

ind south subsided, or else suffered but slight uplifts as compared with

he mountain block; and so the Sierra Nevada came to stand out in its

present imposing form, with gentle westward slope, sharply defined

prest, and abrupt eastward-facing escarpment. Strangely, the volcanic

jiccompaniments of this great upheaval and inbreaking of the earth's

irust were not extensive in the immediate vicinity. Though molten ma-

erial forced its way up repeatedly through fractures in or near the

lone of faulting, and also through cracks in the Sierra block, the result-

ig volcanic cones and lava flows were insignificant compared with

hose elsewhere in the Great Basin and northward in Oregon and Wash-

lgton.

In the north-central Wasatch Mountains, the Wasatch fault broke and
Jisplaced an erosion surface of mid-Tertiary age. Most of the displace-
ment was attained by the early Pleistocene (Eardley, 1944). See Fig.

31.7. Fresh scarps in the alluvium and across terminal moraines also
attest post-Wisconsin movements.

Nolan believes that the best conclusion possible from present infor-
mation is that block faulting probably began in places in early Oligocene
time and has been more or less continuous ever since. Topographically
expressed faults, however, probably date back only to late Pliocene or
early Pleistocene, though earlier movements may have occurred along
then.

Amargosa Chaos

An immensely disordered complex occurs in the Death Valley region
which Noble (1941) has studied. See Fig. 31.8. In a centrally located
district 10 miles square, called the Virgin Spring area, he finds the prin-
cipal structure to be a flat thrust fault which originally followed approxi-
mately the contact of later Precambrian sediments and earlier Precam-
brian metamorphic rocks. On this thrust later Precambrian, Cambrian,
and Tertiary rocks have moved relatively westward for an unknown dis-
tance. The rocks of the overthrust plate are broken into innumerable
blocks and slices, which are thrust over one another to form an extremely
complex mosaic. This assemblage of blocks is named the Amargosa
chaos, and the flat fault upon which the chaos lies is named the Amargosa
thrust. The chaos is divided into the Virgin Spring, Calico, and Jubilee
facies. The Virgin Spring is characterized by blocks of late Precambrian
and Cambrian dolomite, marble, sandstone, quartzite, shale, and slate.
The Calico is made up almost wholly of Tertiary volcanic blocks, and
the Jubilee contains a much larger proportion of poorly consolidated
and broken-up material than the other two phases. The irregular blocks

500

STRUCTURAL GEOLOGY OF NORTH AMERICA

BLACK MOUNTAINS

DEATH VALLEY
FormerLohe Monty

T5^.—

ALEXANDER HILLS

?p€btn*CU Qo€n AMAR605* TH pCp Qa

— vg^qfe^g ' "■" iMin.t. 11 ., ii mm i "

Fig. 31.8 Cross section of the southern Death Valley region. After Noble (PI. 3, 1941). Tp,
Pliocene (?) fanglomerate; Tv, undifferentiated volcanic rock; £wc, Wood Canyon formation
(quartzite, shale, and fossiliferous limestone); Cs, Sterling quartzite; Cj, Johnnie formation
(quartzite, shale, and dolomite); Cn, Noonday dolomite; p€, earlier Precambrian basement
complex; p£k, Kingston Peak formation (conglomerate, quartzite, and shale); p€b. Beck Spring
dolomite; p€c, Crystal Spring formation (quartzite, shale, and dolomite).

are granite, red Tertiary conglomerate, rhyolite, rhyolite tuff, porphy-
ritic andesite, quartz latite porphyry, gypsiferous shale, fresh-water lime-
stone and fanglomerate of Tertiary age, and various Precambrian and
Cambrian rocks.

The Amargosa thrust and chaos are folded into several plunging anti-
clines of northwesterly trend, along whose crests the earlier Precambrian
rocks below the thrusts are exposed. Lying unconformably upon the
folded and eroded thrust sheets and chaos is the Funeral fanglomerate,

probably of late Pliocene age, which consists of fanglomerates and
basaltic lava flows. These rocks are deformed by folds and faults so
recent that they are still reflected in the topography. The structure of
Death Valley is thought to be a broad syncline modified by normal
faulting. The Funeral fanglomerate is downfolded into this syncline and
broken by step faults, downthrown toward the wide valley, along the
east limb. These faults are, therefore, later than the Pliocene ( ?) Funeral
fanglomerate. Very fresh scarps in Quaternary alluvium betray recent
movement on them.

There is no evidence in this region of the Nevadan orogeny found to
the west and north. There are, however, a number of large thrusts that
bring older over younger Paleozoic rocks, which may represent the
Laramide orogeny studied by Longwell (1928) and others farther east
(Noble, 1941).

The Amargosa chaos terminates on the south against the east-west
Garlock fault. Noble (1926) traced this fault eastward along the north
side of the Avawatz Mountains, where it turns southward along their
east side with reverse fault relations. Metamorphic rocks of probable
Precambrian age are thrust against Tertiary beds (Nolan, 1943). A few
miles farther east, Hewett ( 1928 ) has found remnants of a large horizon-
tal thrust extending over an area of 30 square miles, along which
Precambrian and lower Paleozoic rocks have overridden Miocene (?)
sedimentary beds. The eastward movement of the thrust sheet is esti-
mated to be at least 10 miles and may be as much as 20 or 25 miles.

The thrusting of late Tertiary age in southern California in the midst
of the Rasin and Range Province is most logically explained, it seems to
the writer, as a gravity slide phenomenon incident to vertical uplift.

LATE CENOZOIC TRENCHES OF THE ROCKY MOUNTAINS

High Plateaus of Utah

Extending from the Coconino Plateau south of the Grand Canyon of
the Colorado in Arizona northward to central Utah is a system of im-j
pressive fault scarps which bound a group of smaller plateaus and inter-

MIDDLE AND LATE CENOZOIC SYSTEMS OF THE CENTRAL CORDILLERA

501

plateau valleys along the west edge of the Colorado Plateau. North of
the state boundary, the assemblage is known as the High Plateaus of Utah,
early described by Dutton (1880), and in northern Arizona the plateaus
are known as the Kaibab, Kanab, and Shivwits.

In general, the faults and flexures block out ranges and intermontane
valleys from the horizontal sediments of the Colorado Plateau, but toward
the west the folded beds of the Laramide orogeny are involved. This is
especially true in the Wasatch Plateau of central Utah and along the
Hurricane fault of southern Utah and northern Arizona, previously de-
scribed. The map of Fig. 31.9 shows the largest faults that have been
attributed a post-Laramide age. Many small ones exist that are not shown,
and even some major ones of which the age is uncertain or which have
not been mapped as post-Laramide, may exist that are not shown. The
Hurricane fault is illustrated in Figs. 20.21 and 20.22; the Sevier and
Tushar faults, in Fig. 31.10.

A large volcanic field occurs in the central part of the High Plateaus
and connects westward with other volcanic areas of the Great Rasin.
These are discussed in Chapter 36. They were mostly erupted immedi-
ately preceding the faulting.

An indication of the complexity of the volcanism, faulting, and erosion
cycles of the region is revealed in Koons's ( 1945 ) work on the Hurricane
iand Toroweap faults just north of the Grand Canyon. The oldest erup-
tions of late Miocene or early Pliocene time preceded the earliest move-
ments along the Hurricane fault and antedated the cutting of the Grand
Canyon. They poured out on a large gently sloping pediment extending
iat least 16 miles north from the Colorado River. The main faulting then
| occurred, with displacements over 2000 feet at the Colorado River. The
stream held its course, a new and lower pediment was eroded, and the
(region was brought approximately to its present configuration, with the
Colorado River approximately as deep as now. The second eruption then
occurred; they were local, and at the Toroweap fault filled the inner
gorge to a height of 600 feet and perhaps 1200 feet. The lavas were
-entirely removed before later flows dammed the river again. These were
subsequently also nearly all eroded away. Repeated movements along the
Toroweap fault have occurred in late Pleistocene time, and in the very

Fig. 31.9. Faults of the belt of great
trenches in northern Arizona, Utah,
Wyoming, Idaho, and southwestern
Montana. Hachures are on the up-
thrown side. Only those faults are
shown that have been fairly well de-
monstrated as late Cenozoic in age.

MONTANA

502

Tusmar Plateau

STRUCTURAL GEOLOGY OF NORTH AMERICA

Sevier Plat.

Hurricane fault zone

Fig. 31.10. Upper diagram: the Tushar and Seviur faults of the High Plateaus in Utah, after
Eardley and Buetner, 1934.

Lower diagram: the Hurricane fault in Uinkaret plateau, northern Arizona, after Koons, 1945.
Im, Moencopi fm; CK, Kaibab Is.; Ct, Toroweap fm.; Ch, Hermit sh.; Cc, Supai ss.

recent past renewed volcanic activity has formed a single, small cone
and lava flow.

Wasatch Range

The late Cenozoic high-angle faulting along the west front of the
Wasatch Range and the faults of the ranges immediately westward have
already been described as part of the Rasin and Range province. The belt
of great trenches includes these faults.

Western Wyoming and Southeastern Idaho

Superposed on the Laramide structures of western Wyoming and south-
eastern Idaho are several northward-trending high-angle faults that have
helped delineate and deepen the major intermontane valleys. Since the
later structures parallel the earlier in northern Utah, southeastern Idaho,
and southwestern Wyoming, the two have not been clearly distinguised;
but toward the northern end of the belt in connection with the Snake
River, Hoback, and Teton ranges, the Laramide structures veer northwest-
ward, and the later high-angle faults cut across them at acute to right
angles. A distinctive basin fill is also a result of the faulting, and helps
distinguish the older from the younger.

A straight and youthful-appearing fault scarp occurs along the east

side of Rear Lake in northern Utah and southeastern Idaho. It is re-
sponsible for the Rear Lake depression ( Mansfield, 1927 ) .

Star Valley in western Wyoming and its northward continuation in
Grand Valley and Swan Valley between the Caribou and Snake River
ranges is blocked out on one side and in places on both sides by-
faults of late Miocene and early Pliocene age. See cross section of
Fig. 31.11.

An extensive graded surface had been eroded by middle Miocene time,
and remnants of it still exist at elevations of 8500 to 9500 feet, especially
in the Gros Ventre and Wind River ranges to the east. Rlackwelder
(1915) has called it the Union Pass surface. The main drainage lines of
the present, except where affected by later faulting, had been established
in and across the Laramide folds and thrust sheets by this time. Then the
region was broadly uplifted, the streams rejuvenated, and the surface
deeply dissected. The transverse and longitudinal canyons and valley
were eroded as deep as today and in the same position. These include the
Snake River Canyon through the Snake River Range and the Hoback
Canyon through the Hoback Range. Following the dissection of the Union
Pass surface, normal faulting occurred as depicted in the series of dia
grams of Fig. 31.12. In Grand Valley, west of the Snake River Range, the
faulting and consequent deposition occurred in two episodes, and at
unconformity was produced between two divisions of the valley fill. The
sediments more than filled the graben and accumulated on the prefault
ing surface to elevations above the fault scaqD, and the canyons tributan
to the graben that had previously been eroded in the Union Pass surface
were flooded with debris. Toward the heads of these canyons, coars<
material accumulated to elevations of 8500 feet. Volcanic activity accom
panied the deposition of the valley fill, and much tuffaceous materia
was contributed to the deposits, and some thick sills split the basin beds
Then another cycle of erosion followed, and the Rlack Rock surface wa
cut at about 7500 feet. It was also a pediment that flanked the grabei :
valley, and it beveled both the basin fill and the bedrock. The stream
were again rejuvenated, perhaps several times, and the present valley
about 1000 below the Rlack Rock surface were eroded. The old fanH

:;.'

MIDDLE AND LATE CENOZOIC SYSTEMS OF THE CENTRAL CORDILLERA

503

Fig. 31.11. Late Tertiary faulting near Alpine, Idaho, and Wyoming, and its relation to the
laramide structure. After Bayless, 19*7. Cgv, Gross Ventre formation; €b, Boysen limestone; Ob,
Bighorn dolomite; Dd, Darby formation; Cmb, Madison and Brazer limestone; Cw, Wells forma-

tion; Pp, Phosphoria formation; Trd, Dinwoody formation; Trw, Woodside formation; Trt, Thaynes
formation; Ted, Camp Davis conglomerate (upper Miocene or lower Pliocene).

carps that had been buried by the basin deposits were partly, but con-
jpicuously, exhumed below the Black Rock surface. The one along the
ivest side of Grand Valley has all the physiographic features of a youthful
jault scarp, yet is a fault-line scarp.
j Jackson Hole, between the Teton and Gros Ventre ranges, is the result

i downdropping along the Teton fault (Horberg, 1938; Love, 1956a).

"he Union Pass surface is believed to have been broken and rotated so

hat it passes below the valley fill on the Gros Ventre side, and has been
ilevated and tilted westward on the Teton side. The basin deposits, largely
conglomerates, tuffs, and lavas, may have been folded somewhat after

Reposition, but this aspect of the history is not clear.

The discordant relations of the Grand Valley, Hoback, and Teton faults
to the Laramide structures in map view are shown in Fig. 31.13.

Southwestern Montana and Central Idaho

Fresh fault scarplets occur along the west base of the Madison and
the Tendoy ranges of southwestern Montana, and major fault scarps occur
along the east faces of the Blacktail and Ruby ranges, and the northwest
face of the Bitterroot Rane;e.

The northeast face of the Lemhi Range in Idaho is thought to be. in
part at least, a fault scarp. There may be others, but these are the only
ones that the writer has seen. Although not yet studied in detail, these

504

STRUCTURAL GEOLOGY OF NORTH AMERICA

CARIBOU RANGE

SNAKE RIVER RANGE

'"o^e'af '^cilea Vr'bo'tory S~ "" "PrlfJFe o7"»c'ised tributary
Intrenched Snake River
Grand Valley before faulting in upper miocene time

p«»J!^^

First stage or faulting, volcanism and sedimentation

Second stage of faulting and culmination of deposition

JnoAe ftiver

Unio

„ Pea sorface

Fig. 31.12. Idealized diagrams showing the late Cenozoic evolution of the Grand Valley
trench in Wyoming and Idaho.

mountain fronts appear surprisingly like the classical Wasatch scarp in
Utah. Basin beds are widespread in the large intermontane valleys, and
in part were deposited before the faulting and have been displaced by
it, but in part are a direct consequence of it. In the erosion that followed
the faulting, the basin beds have been stripped away in places from
bedrock against which they had been faulted or in other places deposited,
and fault-line scarps have formed, as in Fig. 31.12. The basin beds in
which fossils have been found are upper Eocene, middle Oligocene, lower
Miocene, and uppermost Miocene or lower Pliocene, and have a large
tuffaceous and volcanic ash content, and even sills or lava flows in places.

The Tertiary history is reviewed under the heading "Southwestern Mon-
tana," in Chapter 22. See also Fig. 31.14.

Northwestern Montana, British Columbia, and the Yukon

The Rocky Mountain Trench of British Columbia is described in Chap-
ters 21 and 33. It continues the zones of great trenches to the Yukon and
probably to Alaska.

Fig. 31.13. Relation of late Tertiary faulting to the laramide elements in northwestern Wye
ming and eastern Idaho. After Bayless, 1947.

MIDDLE AND LATE CENOZOIC SYSTEMS OF THE CENTRAL CORDILLERA

505

Fig. 31.14. Cross section of the Tendoy Mountains; Cm, Madison Is.; Ca, Amsden fm.; Cq,
Quadrant quartzite; Pmp, Phosphoria fm.; "id, Dinwoody fm.; 'Rw, Woodside fm.; tt, Thaynes fm.;

Seismicity in the Trench Zone

After the past pages on the zone of great trenches that extends from
Arizona to the Yukon had been written, attention was called to the earth-
quake maps of Woolard (see Fig. 31.15). The concentration of major
shocks in the zone of trenches is striking. The coincidence not only sup-
ports the existence of the zone of faults but also indicates that a number
lof them are still active.

GEOPHYSICAL EVIDENCE

'Gravity and Seismic Surveys

The fill of the down-faulted basins in the Basin and Range provinces
llends itself to analysis particularly by gravity surveys. Since the alluvium
(has lighter density than the lithified bedrock, the magnitude of the gravity
anomaly can be related to the depth of fill, and this becomes a measure of
the magnitude of faulting. Also, faults concealed beneath the alluvium
may be detected, and new light is shed on the fault pattern. The computed
cross sections on the basis of gravity surveys have been checked by seismic
surveys across the valleys.

Recent earthquakes in the Great Basin have been studied seismically
and the results add to our concepts of Basin and Range structure.

Jst, Sawtooth fm.; Jr, Rierdon fm.; Kk, Kootenay fm.; Trr, Red Rock conglomerate (Paleocene ?);
Tbb, Muddy Creek basin beds.

Fault Patterns

Two kinds of patterns appear at present to exist. The one consists of
subparallel faults which define graben, horsts, and tilted blocks, and the
other of faults in semicircular or polygonal form which bound completely
or nearly completely downfaulted blocks. The two are illustrated in
Fig. 31.16 of the Owens Lake-Mono Lake region of California.

Mono Lake Basin

Mono Lake is in a somewhat triangular-shaped basin about 15 miles in
length at the eastern foot of the Sierra Nevada. As a result of gravity and
seismic studies Pakiser et al. (1960) conclude that nearly vertical faults
bound the triangular-shaped block, and that it has subsided 18,000 ± 5000
feet and has received about 300 ^ 100 cubic miles of light clastic sedi-
ments and volcanic material of Cenozoic age. The nature of the gravity
profile and the interpreted geologic section on the northwest side are
shown in Fig. 31.17. A section across the entire basin is given in Fig. 31.18.
It will be seen that the basin fill is divided into layered deposits, a lower
thick one of relatively high velocity (7800-10,800 feet per second) and
an upper thin one ( 2000 feet ) of low velocity ( 5500-6200 feet per second ) .
The recent deposits have not been displaced by faulting and conceal the
buried faults. The lower deposit is believed to be mostly Tertiary volcanic

.506

STRUCTURAL GEOLOGY OF NORTH AMERICA

it I *

7 *\ ,•

f v ^ o •

.••i

% •.

i—-

\z*~$t

l»

* ' •*>

'• •

•»•

•1-

— *T

material, and the cause of subsidence of the pluglike block to be due to
the relief of pressure from below by the movement away of magma in a
supporting chamber. The magma is presumed to have found escape at
the surface, but only part of the extrusives accumulated in the subsiding
basin. The magmatism and pluglike faulting are believed to be related to
the general tectonic framework of deforming forces of the Basin and
Range provinces. The nature of the relation will be considered in follow-
ing paragraphs.

Region West of Wasatch Range

Cook and Berg ( 1957 and 1961 ) report on an extensive gravity survey
in Salt Lake and Utah counties where they made 1100 observations over
an area of 5000 square miles. Steep gravity gradients reveal buried faults
unrecognized by surface geologic surfaces, and although the downfaulted
valley block between the Wasatch and the ranges on the west was known
to contain over 2000 feet of unconsolidated or semiconsolidated sediments
a deep inner trough was discerned which with a number of irregularities
extends north-south for over 100 miles. "Several large fragments . . . have
apparently dropped deeper than the other fragments, as if slipping into a
great crevasse."

Fallon-Austin Earthquake Area

A major earthquake occurred in the Dixie Valley-Fairview Valley area of
west-central Nevada in 1954, and fresh scarps were formed. Their pattern
is shown in Fig. 31.19. The faulting is most advantageous to study because
a first order triangulation net and a first order line of levels had been
established across the area before the movements. The stations were re-
occupied and the amount of vertical and horizontal movement accurately
determined. A vertical displacement of 7 feet occurred in Dixie Valley
and also 7 feet where the fault is in bedrock east of Fairview Peak. The
arrows of Fig. 31.19 indicate the horizontal extension that occurred and
which averages about 5 feet in magnitude in a northwesterly direction.

Fig. 31.15. Earthquake epicenters of the Rocky Mountain region showing coincidence of zone
of concentrated seismic activity and the belt of trenches. Taken from map compiled by G. P. j
Woolard from U.S.C. & G.S. reports.

MIDDLE AND LATE CENOZOIC SYSTEMS OF THE CENTRAL CORDILLERA

507

There was no displacement of points 40 miles west and east of the fault

zone.

A gravity profile across Fairview Valley and the interpreted geology
are shown in Fig. 31.20 (Thompson, 1959). The valley fill is about 1 mile
thick, and the topographic relief of the adjacent range is about 1 mile,
so that Thompson concludes a total cumulative vertical displacement of

EXPLANATION

MAJOR FAULT

S«ctior B-B'

Bouq

jer

._ ^Computed Bouguer gravity

750

i i i i

50,000 Feet

20,000

{2000 Ft

18,000 Ft

6 /*- -0 4 gm per cm3

Simplified configuration assumed

Mono Lake

+ 5,000 -

10,000 -

Geologic cross section

Plon viewof ossumed bosm
outline showing locotionof
profile A- A'

:ig. 31.16. Index map of Basin and Range faults in the Mono Lake— Owens Lake area im- Fig. 31.17.

nediately east of the Sierra Nevada. Reproduced from Pakiser ef at., 1960. 1960.

Mono Lake basin interpreted from gravity profile. Reproduced from Pakiser el at.,

508

STRUCTURAL GEOLOGY OF NORTH AMERICA

Fig. 31.18.
1960.

Generalized geologic section across Mono basin. Reproduced from Pakiser er a/.,

about 2 miles has occurred since the inception of faulting, which he
assumes here was in the Miocene. If the basin is bounded by normal faults
considerable distention of the crust must have occurred over the course
of movement. If the basin is bounded by faults dipping 60 degrees ( lower
diagram, Fig. 31.20), the extension normal to the strike amounts to about
a mile on each side of the basin or a total extension of 2 miles. If the faults
dip 70 degrees, the extension amounts to about VA miles.

The location of the focal depths probably reveals the depth to which
faulting extended. Two earthquakes occurred 4 minutes apart in time and
35 miles apart in distance. The southern Fairview Peak focal depth was
determined by Romney ( 1957) to be 15 kilometers below the surface, and
the northern Dixie Valley one to be 40 kilometers. Also a close correspond-
ence of dip and direction of motion at the surface was found to obtain
at the 15-kilometer focus. These points lead Romney to believe that the
fault fracture extended to a depth greater than 15 kilometers. The even
greater depth of the northern focus supports the conclusion that the entire
crust to the Moho discontinuity is possibly affected. Two possible fault
structures are shown in Fig. 31.20, with the one on the right coming
closest to fitting the facts (Thompson, 1959).

The amount and rate of distention of the entire Basin and Range prov-
ince are estimated by Thompson as follows:

The data indicate that the region of Dixie and Fairview Valleys has been
distended in a nearly east-west direction about a mile and a half. If we assume

that each of the principal basins between the Sierra Nevada and the Wasatch
Mountains has been deformed this much on the average, the total distention
amounts to 30 miles or 5 pet. And if the deformation took place in the last
15 million years, as suggested by the geologic history (deformation of Miocene-
Pliocene and younger rocks ) , the rate is 2 mi/million years or only 1 ft/century.
The rate of extension indicated by several fault movements within historic times
appears to be at least 1 ft/century. The faults lie in a north-south belt about
250 mi long. For at least this distance the data are consistent with an extension
of 1 ft or more in the last hundred years. Prehistoric Quaternary faults are also
numerous; they strongly suggest that the historic rate of deformation is not
abnormally high.

Tilted blocks, which are characteristic of large parts of the Great Basin,
may or may not be the result of extension of the crust. If they are an ex-
pression of tension then the general level of the surface is depressed and
the crust thinned. Since the Great Basin appears from other geo-
logical considerations to be a depressed region, the tilted blocks

Fig. 31.19. Horizontal movements in the
Fairview and Dixie valleys earthquake. After
Thompson, 1959.

SCALE OF MAP
IN MILES

SCALE OF VECTORS
IN FEET

MIDDLE AND LATE CENOZOIC SYSTEMS OF THE CENTRAL CORDILLERA

509

GRADIENT, ACROSS VALLEY

•170

-180

--I90 <r

■200 ffl

-4000

MANTLE

j. 31.20. Gravity profile and section across Fairview Valley. Also alternate interpretations of
;ulting of crust under extending forces. From Thompson, 1959.

will be considered tensional features as well as the graben blocks.

If the crust has been extended some 30 miles between the Sierra Nevada
and Wasatch Mountains, then our understanding of the penetration of
magma into and through it comes into better focus. In Chapter 33 it is
suggested that the large volumes of quartz monzonite magma originated
in the base of the silicic (granitic) layer of the crust at depths of 10 to 20
kilometers, and we can see that the tensional fractures illustrated by
Thompson in Fig. 31.20 would penetrate such magma chambers and
conduct the magma upward. From this point of view both the block fault-
ing and magmatism are the result of the tensional tectonism, and only
in the local examples of pluglike basin subsidence should we conclude
that the evacuation of a magma chamber is the direct cause of the
faulting.

We are led to speculate that fractures have penetrated to the basaltic
subcrust in Oregon and Washington to conduct the olivine and tholeiitic
magmas to the surface.

EXPLORING TENSIONAL TECTONISM IN WESTERN NORTH AMERICA

The theory of expansion of the Basin and Range province in late Ceno-
zoic time in the magnitude of 30 miles piques one's curiosity to consider
the entire framework of movements in western North America. The strike-
slip movement along the San Andreas system and the postulated extension
of the Basin and Range province with its components of horizontal move-
ment should be related. Figure 31.21 has been prepared to show the
directions of fault traces and the horizontal movement on the San Andreas.
Only a few of the faults of the Great Basin are shown such as to indicate
the direction of tensional forces that must be entertained.

Figure 31.22 is a diagrammatic map which resolves in bold strokes
the distention cracks and horizontal movements of the crust previously
postulated. The expansion fractures of the Basin and Range province are
distributed across the entire basin, but for purposes of illustration are
concentrated along the eastern and western margins. The width of the
lines represents the approximate amount of postulated expansion. The
main pulling away appears to have been in a west-northwesterly direction

•:\ \ ' / *

! \ / I \ '

\ )

i\ \.

\ /

■J

/ . '

° /

*> /

j j

\ \x

^/\ 1

7 0 V 1 1 *%<

V / 9/ zV a

W /X,*-— nJL_

/ #^ ^v ^^.

1 xl

7\ w * ■ \ ^^ — ^^

1 ^ ^ ^Pc

1 I « 1 "" Q ^K?

\ 7 - ^#

\ \ JS? y o

Ax\V/°

*<\B ^-Ok. X. *^M&

i TO m ^w-* ^-^.^v

I t. ^M *„ x ^T

\ ** <~>^^ z ^» \1'

\ o ^"^1 ° ^^ ^^

\\ k\ ')

\ M ■ ^>

Fig. 31.21. Framework of late Cenozoic fault systems of western United States.

Fig. 31.22. Exploring the concept of extension and drift affecting western North America.
Black lanes represent amount of expansion as if localized along a few separations. Except
for the Gulf of California the extension is distributed in a number of separations across the
entire Basin and Range province. Small arrows represent apparent vectors of movement. Large
arrows the apparent resultant direction of movement.

MIDDLE AND LATE CENOZOIC SYSTEMS OF THE CENTRAL CORDILLERA

511

with a strong northwesterly component in central California keeping the
Coast Ranges block snug against the adjacent continental mass. Perhaps
the same is true of the Rocky Mountain Trench. Some separation and also
horizontal displacement have been postulated for the Rocky Mountain
Trench. This movement is possible when the Snake River fault is con-
sidered to be one of considerable separation (Chapter 36).

The drifting away from the continent of Baja California as well as a
northwesterly gliding movement seems substantially demonstrated. See
Chapter 29.

A major strike-slip fault is postulated across south-central Arizona at
the south margin of the Colorado Plateau. Southern Arizona remained
5000-8000 feet below the Colorado Plateau after vertical adjustments
occurred in late Cenozoic time, and is generally considered to be a block-
faulted region, although not so clearly as the Great Basin in western Utah
and Nevada. A few alluvial-filled valleys parallel the grand escarpment
and support the concept of down-dropping along major faults. However,
a master horizontal couple as indicated on the map of Fig. 31.22 has not
been recognized or postulated, as far as the writer knows. This then, is a
very speculative element of the framework of movements illustrated on
the map.

The rifting of central New Mexico finds a compatible place in the frame-
work. The Sonoran-Chihuahua basin and range region is poorly under-
stood, and the illustration of considerable distention there is hardly more
than a guess.

SEISMIC VELOCITY LAYERS IN THE EASTERN GREAT BASIN
Seismic Layers

The recognition of a crustal layer with a velocity of 7.5 ± kilometers per
second in several areas of the western United States and Canada comes as
a very significant find and perhaps a key to tectonism there. The work of
Berg et al. (1960) in the eastern Great Basin, Press (1960) in the Cali-
fornia-Nevada region, and the summary article by Diment ( 1961 ) should
be referred to. The seismic velocity layers recognized to date are por-
trayed in Fig. 31.23.

Fig. 31.23. Seimic velocity layers in western
United States Velocities in kilometers per
second (1) Press, 1960; (2) Berg et al., 1960;
(3) Meyer ef al., 1960. Refer to Fig. 38.1.

£20

° 40

1- 60
a.
bj
0

CALIFORNIA

NEVADA

(1)

EASTERN

BASIN AND

RANGE

(2)

EASTERN

MONTANA

(31

6 1

5 7

6 2

6.3

y/y//y

8 1

8.1

8 0

Geologic Requirements

In attempting to interpret the constitution of the seismic layers the
following geologic requirements should be kept in mind.

1. The Great Basin has been distended about 30 miles (50 kilometers)
in the last 15 m.y. A strong horizontal coupling along the Pacific-
margin is evident, with the Pacific facing blocks moving to the north-
west.

2. The Great Basin has been elevated during the same time 1-1/2 kilo-
meters.

3. The High Plateaus of Utah and the Sierra Nevada have been elevated
2—3 kilometers during the same time.

4. The Colorado Plateau has been elevated 2-2J-2 kilometers during the
same time.

5. Silicic lavas have been poured out over most of the Great Basin in
amounts equal to a layer 1-2 kilometers thick since early Oligocene
time. This material must have come from the melting of a portion of
the silicic crystalline mantle. See Chapter 36.

6. Equal amounts of basalt (viz., the Columbia basalt field) have flowed
to the surface from a source probably immediately below the crystal-
line basement, and in the Great Basin the basalt reservoir has been
tapped from time to time during the general acidic lava eruptive
cycle.

512

STRUCTURAL GEOLOGY OF NORTH AMERICA

80* 70* 60* 50'

■60* ISO* 140" 130* 120* 110* 100* 90* 60* 70' 60* 50'

Fig. 31.24. East Pacific Rise and pattern of heat flow. Reproduced from Menard, 1960.

Interpretations

The East Pacific Rise of the ocean floor has been considered by Menard
(1960) to extend to the Gulf of California and hence under the western
part of the continent of North America appearing in the Pacific again off
Oregon, Washington, and British Columbia. See Fig. 31.24.

The puzzling slope between California and Hawaii is the west flank of the
rise. . . . Where the crest and east flank of the rise intersect Mexico are found
the plateau of Mexico, the Colorado Plateau, and the Basin Ranges comprising
a topographic bulge of the continent comparable in scale to the bulge of the
sea floor.

Cook (1961) follows Menard in projecting the East Pacific Rise under
the continent, and assigns the broad uplift to the development of the
7.4-7.7-kilometer-per-second velocity layer under it. In fact, he believes
from still incomplete data that the oceanic rises of the Pacific, Atlantic,
and Indian oceans with their accompanying rift systems and volcanism
are due to the uplift of the crust as the 7.5 layer develops. He calls it the
mantle-crust mix layer, and regards it as a change from eclogite to basalt
with attendant expansion.

The views of Menard and Cook related to the western United States
lead to many thoughts which will only be summarized here. First, the
Late Cenozoic uplift should be considered. Approximate uplift contours
are shown in Fig. 31.25. They are admittedly approximate, and in the
Great Basin represent an average of the uplift of the Tertiary deposits in
the valley blocks and the uplift of the mountain blocks. From the picture
presented the Snake River downwarp and associated Columbia basalt
region may represent a transverse break in the continuity of the 7.5-
km/sec layer from south to north. The Colorado Plateau has been uplifted
more than the Great Basin, and it has generally been considered that the
Great Basin is one of collapse or subsidence in relation to the Plateau,
although in relation to sea level, both have been uplifted. It will be very
interesting to see what the relative heat-flow measurements will indicate
as to the central part of the rise over the 7.5 layer. None has been made
yet. Cook seems to infer that the zone of Great Trenches and accompany-
ing seismicity is the central rift zone of the rise. Fig. 31.15.

In Chapter 36, the igneous rocks of the western United States are re-

MIDDLE AND LATE CENOZOIC SYSTEMS OF THE CENTRAL CORDILLERA

513

CONSTITUTION

VELOCITY

CRUST

OLD MOHO

TRANSI-
TIONAL

NEW MOHO
MANTLE

14.' jUi'I-uI'li

1 HI MOBILJZED_BASEMEN_T i_N 1 I 1 I
i BASALT_- BAS_EMENT TRANSITION^,
BASALT

liii,

LL. '_L t_i_

ft.

PARTIALLY MELTED PERIDOTITE
OR

PHASE TRANSITION
ECLOGITE TO BASALT

PERIDOTITE OR ECLOGITE

6 5r KM/SEC

7.59 KM/SEC

8.0 KM /SEC

0 KN

Fig. 31.26. Postulated constitution of velocity layers under eastern part of Great Basin.

viewed. These must certainly be considered in visualizing the constitution
of the crustal layers and the role of the 7.5 layer in tectonism. The writer's
ideas of the arrangement in the eastern part of the Great Rasin are shown
in Fig. 31.26, and are discussed as follows.

The mantle is regarded as either peridotite or eclogite. If the 7.5 layer
is a transition layer, as seems necessary from its seismic velocities, then if
the mantle is peridotite, the transition layer would be one of peridotite
and its early melt product, basalt. If eclogite, then basalt or gabbro would
result as a phase transition. In either case, Cook's name, mantle-crust mix,
would be suitable. The writer favors the peridotite-basalt mix, because he
sees in it a means of bringing molten basalt in large quantities upward

Fig. 31.25. Late Cenozoic uplift in western United States. An attempt is made to portray the
broad vertical movements of the silicic crystalline basement layer. Contours in thousands of
feet.

514

STRUCTURAL GEOLOGY OF NORTH AMERICA

to the base of the silicic crystalline basement layer. This is necessary to
feed basalt to the surface in ways listed in the geologic requirements
previously mentioned. The basalt layer is visualized as growing in thick-
ness as the molten basalt from below rises and is added to it. In case of
tensional fractures in the crust which reach downward through the base-
ment, the basalt reservoir is tapped, and fissure flows result. When eclogite
changes to solid basalt through polymorphic phase transitions, much heat
is consumed in the process, and unless considerably more is generated in
the mantle or the basalt so formed, none converts to liquid basalt.

The heat of the liquid basalt which has risen to the base of the silicic
crustal layer mobilizes, if not melts, a considerable amount of it; and it is
this silicic magma which is postulated to have erupted at the surface to
form the voluminous silicic flows of the Great Rasin and the alkalic
igneous rocks of the shelf province ( Chapter 33 ) .

Menard postulates a convection current rising under the East Pacific
Rise and flowing westward under the crust. The drag of this current

creates tensional block fault features in the central zone of uplift, it trans-
lates the adjacent crust westward, and in the region of downward plunge
of the current, compressional structures are formed. He has difficulty,
however, fitting the San Andreas fault into the convection current
hypothesis.

Reference to Figs. 31.21, 31.22, 31.25, and 32.15 should convince one
that the cause of late Cenozoic tectonism must be complex, and more is
involved than westward movement of the convection cell. In addition to
the San Andreas fault with large strike-slip movement to the northwest,
there is the Snake River fault which appears to separate the western
Cordillera into two distant segments. A drift of the crust to the northwest
with extension to the west-northwest is fairly clearly indicated. Resides
variations of convection circulation and expansion of the mantle to ac-
complish these movements of the crust, there is need to consider the (as
yet intangible) forces presumed to cause polar migration, drift, and rota-
tion of the continents. The pattern suggests such forces to the writer.

32.

PACIFIC SUBMARINE
PROVINCES

DISCOVERY OF STRONG SUBMARINE RELIEF

It was current opinion until 1925 that the ocean floors were montonous
plains. The continental shelves above the floor and the great deeps below
the floor were known, but not their details. The technique of echo sound-
ing was successfully introduced in 1925 by the U.S. Coast and Geodetic
Survey, and since then remarkable progress in mapping the floor of both
the Pacific and Atlantic oceans has been made. Many thousands of miles
of traverses have been run, and with progressively more accurate means
|af location available the contouring has become more accurate and the
topography better known. The Gulf of Alaska was explored before 1940,

and instead of a featureless floor a number of bold seamounts were dis-
covered. The most detailed early survey was off the coast of southern
California, where basins, banks, ridges, and escarpments of comparable
size to those on the adjacent land were indicated.

In addition to many seamounts in the northeastern Pacific, various
ridges, depressions, and trenches were discovered, and by 1955, the length
of sounding lines to show the extent and some of the details of these
features had reached about 80,000 miles (Menard, 1956). This work
was done chiefly aboard ships of the Navy Electronics Laboratory and
the Scripps Institution of Oceanography. Several expeditions each year
since 1955 continue to add to an ever amazing picture of the Pacific ocean
floor.

Study of the submarine topography is pertinent to an understanding of
the deformation of the oceanic crust, and most interpretations to date have
been made from the relief features. Valuable supplementary informa-
tion has come from seismic and gravitational surveys, and most recently
from extensive magnetic intensity surveys.

SUBMARINE PROVINCES

Basins, Banks, and Ridges off California

The submarine topography for 150 miles off the southern California
shore is one of basins, banks, and ridges comparable with that of the
adjacent land. Shepard ( Shepard and Emery, 1941 ) calls it the continental
borderland. See Figs. 32.1 and 32.2.

In this borderland are eleven basins which would contain large lakes
if the land became emergent. Some of them would cover 1000 square
miles and would range up to 2880 feet deep. The basins are roughly oval
and elongated northwesterly. Their walls are generally steep, long, and
straight, but are gashed by a few valleys. However, abrupt changes in
direction exist. The basin floors are very flat, and do not possess the
piedmont slopes of their land counterparts in southern California and
Nevada. The general elevation of the basins and their overflow sills be-
comes greater to the southeast (Shepard and Emery, 1941).

The elevations on the continental borderland are numerous and diverse.

515

516

STRUCTURAL GEOLOGY OF NORTH AMERICA

Fig. 32.1. Pacific submarine relief provinces off North America. After Menard, 1955.

The higher elements are comparable to the short mountain ranges of
the adjacent land. The submarine relief is also comparable in magnitude,
but not in the intricacy of detail. The San Bernardino Range rises about
9000 feet above the adjacent basins, and the San Juan seamount rises
about 10,000 feet above the adjacent ocean floor. Santa Cruz Island rises
almost 9000 feet above the floor, and Catalina Island about 6000 feet.

Some of the relief features have flat tops. The most extensive are banks
under about 3000 feet of water. Another group of flat-topped seamounts
ranges in depth from 1200 to 3480 feet.

Continental Shelf

Shelf. North of Point Conception, the basin and range type of
topography on the sea floor composes itself into a continental shelf gen-
erally not over 500 feet deep. Off central and northern California, the
shelf is about 25 miles wide, and off Oregon and Washington, somewhat
less. The borderland of southern California, after deepening southward,
shoals again and abuts against the 80-mile-wide shelf of Sebastian Viz-
caino Bay of central Baja California. From Sebastian Vizcaino Bay south-
ward, a distinct shelf and straight shelf slope extend all the way to the
southern tip of the peninsula. See map, Figs. 32.1 and 32.5.

The shelf zone continues fairly regularly along the coast of British
Columbia and southeastern Alaska to a point off Yakutat Bay, where it
turns southwestward along the Aleutian Islands and borders the Aleutian
trench. It is a submerged surface of great glacial valleys off British Colum-
bia and southeastern Alaska (see Fig. 17.18). Along the Aleutians, it is
over 100 miles wide in places, and generally less than 500 feet deep.

Longitudinal depressions just off shore in the shelf of southeastern
Alaska (off Yakutat Bay and Cross Sound) are interpreted to be due to
faulting incident to the Pleistocene uplift of the adjacent ranges (Hol-
tedahl, 1958).

Shelf Slope. From Yakutat Bay, Alaska, to Baja California, the shelf
and basin and range borderland are terminated oceanward by a slope of
great proportions. The decline where greatest extends from the brink at
500 feet to the base at 10,000 feet. In places it is sufficiently steep to be
comparable with the Sierra Nevada scarp, and hence considered b;

SEA FLOOR BATHYMETRY

OFF CALIFORNIA

CONTOUR INTERVAL 200 FATHOMS (1200 FEET)
CONTINENTAL SLOPE CONTOURS AFTER SHEPARD 8 EMERY (1941)

"Up in* in«

Fig. 32.2. Bathymetric chart of sea floor off California. Reproduced from H. W. Menard, 1955b.

518

STRUCTURAL GEOLOGY OF NORTH AMERICA

Shepard ( Shepard and Emery, 1941 ) to be a fault scarp. In other places it
is not so steep and does not appear to be due to faulting.

One of the most fascinating discoveries of echo sounding is canyons
that gash the shelf and its outer slope. Some of them are veritable gorges.
A V shape is characteristic. There are about 66 of these submarine gorges
or canyons along the California coast, and they are spaced irregularly at
distances of 10 to over 50 miles. Most of the large canyons head within
3 to 5 miles of the present shore, but a few extend to within half a mile.
Some of the smaller ones head 30 miles out. The longitudinal gradients
are high and compare closely with stream gradients whose canyons have
been cut in fault scarps. The gradients average about 4 degrees, are
steeper near their heads, and gentler in the lower reaches and the longest
canyons. The canyon bottoms are as continuous down hill as those of
typical mountain canyons, at least out to depths of 6000 to 9000 feet,
where the gentler outer slope may in places have suggestions of shallow
basins.

The depth of the canyons is variable. The long Arguello Canyon west
of the Santa Rarbara basin starts in four tributaries, each only 300 feet
deep. These shallow gorges trench the shelf slope out to where it is 3000
feet deep. Each of the tributaries is about 15 miles long. They converge
into a single canyon which, in another 15 miles, is nearly 2000 feet deep.
At about the 5000-foot depth contour the V widens, although the canyon
is over 1000 feet deep at the point. The canyon turns southward, and
may be followed down to 11,700 feet below sea level.

Another great submarine canyon, the Monterey, begins in tributaries
in the Ray of Monterey which are 2000 feet deep a mile below their heads.
The main canyon is 3000 to 4000 feet deep, and it trenches the shelf
margin as a narrow V-shaped valley to a depth of 9000 feet, where it
widens and shallows. It turns southward at this point and may be traced
clearly still deeper to 11,000 feet below sea level.

Long stretches of the outer slope of the continental shelf are not dis-
sected by submarine canyons. One stretch is north of Arguello Canyon
between latitudes 34° and 35° 40', and another is between Eel Canyon,
off Cape Mendocino, and the Columbia River. Gentle slopes are in
part characteristic of these margins, and Shepard points out that

canyons are not so common on gentle offshore slopes as on steep ones.
The continental shelf north of the Aleutian trench, quoting from
Murray (1945), is:

. . . approximately defined by the 100-fathom contour. The maximum width
of the shelf, 120 miles, is in the vicinity of Kodiak Island. To the northeast
and southwest, the shelf narrows to a few miles as it converges with the major
land features. The coast line is generally irregular and precipitous, although
there are interspersed occasional areas of low relief. Only two principal rivers,
the Susitna emptying into Cook Inlet and the Copper northwest of Cape St.
Elias, discharge sediment onto the shelf or into the inland waters.

Deep-Sea Fans. Turbidity currents debouching from the mouths of
submarine canyons have built large cone-shaped deposits called deep-
sea fans. See Fig. 32.2. Their volume is usually many times the volume
of material that could have been eroded from the canyons, so it is pre-
sumed that much sediment is contributed by shoreline processes to the
heads of the submarine canyons (H. W. Menard, Jr., 1955), which then
moves down the canyons to the fans below. The fans bury much or all
of the previous relief on the deep-sea floor and produce smooth gentle
slopes.

Origin of Submarine Canyons. The submarine canyons of the Cali-
fornia shelf were postulated to be drowned subaerial valleys, smothered
by sediment, and excavated by glacial and recent turbidity currents (Daly,
1936). Shepard (1952) contends that turbidity currents are not potent
enough to erode the canyons and suggests that drowned river valleys
have been kept permanently open by the turbidity currents during the
process of submergence. Kuenen (1953) counters that this process does
not explain all types of submarine canyons. Figure 32.3 is a reproduction
of his conception of the different kinds of submarine canyons off the Cali-
fornia coast, and he comments as follows about their origin:

Instead of assuming that drowned valleys were perpetuated by sliding and
turbidity currents, which have no ability to erode, it is suggested that the
ancient land surface was first smothered; later the poorly consolidated covering
materials were eroded during the Ice Age, and to some extent in postglacial
times to form the submarine canyons.

Some localities were particularly favorable to the generation of turbidity
currents because of incompletely buried topographic depressions, local supply

PACIFIC SUBMARINE PROVINCES

519

Fig. 32.3. Possible constitution of different submarine canyons off California. After Kuenen,
1953.

of sediment by rivers, and coastal configuration. Some narrow rocky land
valleys were thus swept out (Carmel, Scripps, etc.), but the majority of old
valleys may still lie buried in the terrace beneath sediments.

In some cases the turbidity currents only cleaned parts of the old valleys
where these happened to offer small resistance. But other parts of these valleys
did not conform to the requirement of following the present slope. Such parts
remained buried.

Elsewhere a new valley cleaned off along its wall some small part of an
ancient mountain slope, without conforming to the original drainage pattern.
This may be the case for Monterey Canyon, which has granite overlain by
sedimentary rock on one wall opposite a wall which has yielded only mud or
soft sedimentary rock; or for Dume Canyon with basalt on the east side and
mud with calcareous shale on the west.

Origin of the Continental Shelf Slope. The imposing slope has been
ascribed to faulting, and the shelf itself primarily to wave cutting
(Shepard, 1948). The Atlantic terrace, however, has been described as
developed by sedimentation and isostatic subsidence caused initially by
i the sedimentary load (Kuenen, 1950). This theory of origin is amply
attested locally, for instance, by the Mississippi delta building and con-
sequent subsidence in the Gulf of Mexico. See Chapter 36.

We have to deal primarily with the consequences of orogeny in the
marginal belts of the continent and then secondarily, with the processes of
erosion, sedimentation, and epeirogeny in explaining the existing con-
: tinental shelf and shelf slope. It is not clear yet what an orogeny such as
the folding of the strata of the Coast Ranges of Oregon and Washington
does to the continental shelf slope, or in what condition it is left, but in

any consideration, the gradation from continental crust to oceanic crust
will result isostatically in a suxficial (submarine) slope toward the ocean.
This may then be altered by erosional, depositional, and epeirogenic proc-
esses. In the previous discussions of submarine canyons and slope aprons
or fans, and in subsequent discussions of the Aleutian and Middle Amer-
ica trenches and the possible faulting off Oregon and northern California
the nature of the secondary processes is illustrated.

ALEUTIAN TRENCH

The Aleutian trench is a narrow depression in the ocean floor parallel-
ing the convex side of the Kenai and Alaska peninsulas and the Aleutian
volcanic island archipelago. See Figs. 32.1, 32.4, and 39.1. It extends from
Yakutat Bay in the Gulf of Alaska westward to Attu Island, a distance of
over 2200 statute miles. It has a maximum depth of 25,000 feet. Accord-
ing to Murray ( 1945 ) :

The vertical relationship between the crest of the conspicuous mountain
features and the floor of the trench is shown in Fig. 32.4. An approximate
difference of 28,000 feet exists throughout most of the region. The greatest
single known difference throughout the entire arc exists slightly east of the
mid-section and is centered at Unimak Island, where Shishaldin Volcano (9372
feet) rises 32,472 feet above the floor of the trench (about 110 miles dis-
tant). . . .

BERING SEA

X /

2545, /I

STATUTE MILES

f ft • KtSKA I

/ AGATTU I A^ *

■/-. I ' ^

Fig. 32.4. Profiles of the Aleutian trench in the vicinity of Attu and Agattu Islands, western end
of the Aleutians. After Murray, 1945.

520

STRUCTURAL GEOLOGY OF NORTH AMERICA

Fig. 32.5. Submarine contour map of west end of Aleutian Ridge. Reproduced from Gates
and Gibson, 1956.

The continental slope comprising the inner north side of the foredeep is
considered approximately as the area between the 100-fathom contour (50-
fathom contour frequently applicable) and the floor of the trench. It ranges
from 20 to 70 miles in width, is narrower near Cape St. Elias, and widest off
Umnak and Unalaska islands. South of Unimak Island, a pronounced widening,
herein termed the "Aleutian Bench," exists between the 2000- and 2500-fathom
contours and extends westward to Umnak Island. This bench is approximately
20 miles wide and 170 miles long. The bench lies several hundred fathoms
higher than the top of the outer seaward side of the trench.

The average slope of the north face or continental slope is 3°-4° and
terminates in depths ranging from around 2,000 to 4,000 fathoms. Steeper
slopes, however, are found in limited areas or between successive soundings.
When the slope exceeds 30°, it usually occurs near the bottom of the trench
where the profiles show an abrupt slope or escarpment as, for instance, the
apparent escarpment off Cape St. Elias.

The surveyed slopes on the north and south sides of the Umnak Island
locality differ materially with respect to relief and rate of descent. The north
side of the island is characterized by long valleys and ridges in the deeper
area. For instance, the maximum seaward distance of the 1000-fathom curve on
the north side of Umnak Island is 45 miles, whereas that on the south is barely
5 miles.

The floor of the trench, 20 to 70 miles off the edge of the continental shelf,
undulates, but steadily descends in the 1000-mile stretch from Cape St. Elias
to Umnak Island. In many profiles, the converging side slopes of the trench

meet in a narrow area defined by one or two soundings at, or close to, the base
of the continental slope.

The gentle incline of the trench terminates at about 2,000 fathoms, off
Cape St. Elias. The trench, however, continues eastward across the continental
slope and then, apparently, is continuous with a depression extending across
the continental shelf toward Yakutat Bay. The delineation of the 100-fathom
curve on the shelf here is inconclusive, as it is controlled by only a few widely
spaced soundings. A bar with depths of 8 to 16 fathoms extends entirely across
the entrance to Yakutat Bay. Depths as great as 167 fathoms, however, are
found about 4% miles inside the bay.

Detailed contouring of the west end of the Aleutian Ridge has led
Gates and Gibson ( 1956 ) to postulate that the submarine topography re-
flects the structure. The Aleutian Ridge with its islands is shown in Fig.
32.5, and the suggested structure in Fig. 32.6. The geology of the Aleutian
Islands will be discussed in Chapter 39, but suffice it to say here that
Attu, Agattu, the Semichi Islands and the southern part of Kiska lack
young stratovolcanoes and are composed of pre-middle Tertiary rocks
and subordinate amounts of upper Tertiary coarse clastic sediments and
subaerial lava flows. They owe their height to faulting and alpine char-
acter to vigorous erosion. The fault pattern of Attu and Agattu, par-
ticularly, is obvious and intricate. It has led to the interpretation of
submarine features as fault reflections.

Four principal topographic provinces are recognized: (1) The Crest of the
Aleutian Ridge contains the Aleutian Islands, the Insular Shelf at depth ranging
from present shore lines to 70 fathoms, and the Ridge Shelf at a depth of 100
to 500 fathoms, all apparentiy the result of subaerial and marine erosion since
the middle Tertiary and of glaciation in the late Pleistocene. (2) The Insular
Slopes form the sides of the Aleutian Ridge. The North Insular Slope is a long,
steep, linear scarp that probably marks a major fracture in the earth's crust.
The South Insular Slope appears to be a broad, faulted and warped arch con-
taining numerous steep-sided linear sea valleys and canyons. Many of these
traverse the south slope at an angle to the maximum regional gradient, and
several line up with observed faults on the island. These linear topographic
features probably mark fault zones. (3) The Aleutian Bench is a prominent
step in the general slope from the islands to the Aleutian Trench, and its inside
edge may be the trace of a thrust fault. (4) The arcuate Aleutian Trench has a
steep north side, a flat floor at a depth of about 4000 fathoms, and a south side
containing an en echelon topographic pattern. The Trench perhaps marks a
major thrust zone dipping north beneath the Aleutian Ridge.

A structural interpretation of the submarine topography suggests that the

PACIFIC SUBMARINE PROVINCES

521

Fig. 32.6. Postulated faults of
end of Aleutian Ridge and
trench. Reproduced from Gates
and Gibson, 1956.

j western part of the Aleutian Ridge is an arched and faulted asymmetrical
j wedge bounded by a northward-dipping normal fault on the north and by a
I northward-dipping zone of reverse faults on the south. Formation of this
1 1 wedge probably began with major uplift and faulting of the western Aleutian
.area during the middle Tertiary, and the many earthquakes and active
(volcanoes in the Aleutian arc today indicate that deformation is still continuing

;(Gate and Gibson, 1956).

The structure of the ridge as Gates and Gibson speculate is shown in
iFig. 32.7.

BERING SEA FLOOR

The Bering Sea is a closed triangular-shaped basin bounded by two
Continents and the arc of the Aleutians. About half the area is continental

shelf, and half lies at depths of 1600 to 2240 fathoms. The greater depths
are in the southwestern portion. The maximum depth recorded, 2240
fathoms, lies 45 miles northeast of Attu Island, and is approximately 2
miles above the floor of the trench on the south side of the Aleutian
Islands. See Figs. 32.1 and 39.10.

The deep division of the Bering Sea is marked by a submarine range
that takes off northward from the Aleutian arc and veers westward. It is
300 nautical miles in length, 60 miles in width, and rises in one place
12,156 feet above the bottom. It is known as the Bowers Bank Range and
supports Semisopochnoi Island and the Petrel Bank, as well as Bowers
Bank.

The Pribilof Islands emerge from the shelf of the Bering Sea. which

522

STRUCTURAL GEOLOGY OF NORTH AMERICA

Pacific Ocean Trench Bench

Aleutian Ridge

Bering Sea

Fig. 32.7. Speculative and diagrammatic cross section of western end of the Aleutian Ridge
and Aleutian trench. Reproduced from Gates and Gibson, 1956.

in large measure appears to be the great delta of the Yukon and Kuskok-
wim rivers. See Chapter 39.

PACIFIC FLOOR OFF MEXICO AND CENTRAL AMERICA

Middle America Trench

The Middle America trench is continuous at depths greater than 14,400
feet for 1260 miles, except for two submarine volcanoes which lie in the
trench. (See Figs. 32.8 to 32.10). Northwest of Acapulco the trench is
generally U-shaped in cross section, with a steeper shoreward flank and a
flat bottom suggesting sedimentary fill. Off Guatemala for a distance of
380 miles it is over 18,000 feet deep with a maximum sounding of 21,000
feet. Thence southeastward it shoals gradually to merge into the sea floor
off Costa Rica. The southeast segment is also asymmetrical in cross section,
but V-shaped with irregular bottom, in contrast to the flat bottom north-
west of Acapulco.

Along the trench as explored to date, a series of breaks in slope or
terraces suggests a downwarped or downfaulted shelf below the more
normal shallow shelf. Faulting across the shelf may have been important
south of the Isthmus of Tehuantepec ( Fisher and Shor, 1959 ) .

Fig. 32.8. Middle American trench and related features. Compiled from Fisher 1961, and Shori
and Fisher, 1961. Rows of dots are submarine canyons.

PACIFIC SUBMARINE PROVINCES

523

VERTICAL EXAGGERATION 10.

Fig. 32.9. View of Middle American trench to northwest from Gulf of Tehuantepec. Tehunan-
tepec Ridge is in left foreground. Reproduced from Fisher 1961.

Tehuantepec Ridge

A northeast-southwest trending band of ridge and trough topography,
60 miles wide, separates the 10,800-1 1,400- foot sea floor outside the trench
off southern Mexico from the 12,600-13,200-foot Guatemala basin. This
zone has been traced from several hundred miles offshore to an inter-
section with the trench near the west side of the Gulf of Tehuantepec,
and has been called the Tehuantepec Ridge (Figs. 32.8 and 32.9).

Ocean Floor and Seamounts

The ocean floor outside the trench is fairly flat except for numerous
seamounts which undoubtedly are volcanic cones. The map of Fig. 32.8
shows the distribution of the seamounts charted by Fisher and Shor
(1959) and also the volcanic cones of Recent or Pleistocene age on land
in southern Mexico and Central America as far as the writer has been able
to locate them from the literature.

The Guatemala basin, which is about 1800 feet deeper than the floor
north of the Tehuantepec Ridge, shoals to the southeast. It contains few
volcanoes whereas a row of majestic active and dormant volcanoes lies
opposite on land and stretches from southern Chiapas across Guatemala,
El Salvador, Nicaragua, and Costa Rica. Volcanism in Mexico is discussed
in Chapter 35.

As far as known the distribution of volcanoes on the ocean floor south-

west of the trench is random, although one or two rows seem apparent.
None of the seamounts has been recognized as beveled, so it is Dot
possible to infer vertical movements of the ocean floor such as in the
Mid-Pacific Mountains, described on following pages.

Crustal Structure

Three seismic refraction stations were taken along the axis of the
trench west of Acapulco and two along its axis off Guatemala and El
Salvador. Another station was shot on the shelf and one 60 miles seaward
of the trench off Guatemala. Upper mantle velocities appear on all lines
(Fisher and Shor, 1959).

Thick sediments were found in the Tres Marias basin off Manzanillo
and at the shelf station off Guatemala. On a section normal to the trench
off Guatemala, the depth below sea level to the Mohorovicic discontinuity
in the trench zone is 16 kilometers, and in the shelf area 17 kilometers.
Below the sea floor the crust thickens from 5 to 7 to 10 to 17 kilometers
along this section (Fig. 32.11).

The Mohorovicic discontinuity is deeper and the crust below the sedi-
ments thicker under the two southern stations than under the two central
trench stations. The mantle is deeper under the Tres Marias basin, where
thick sediments (1/2 kilometers) are found, than under the central
stations.

Fig. 32.10. View of southeastern end of Middle America trench. Reproduced from Fisher 1961.

524

STRUCTURAL GEOLOGY OF NORTH AMERICA

500 400

DISTANCE IN KILOMETERS
300 200

100

t SHORELINE

- -£^~ 2 4-
,^ 32

-— — -~T~<^^' 4.7
^"~-\57

^<X0WER CRUSTAL LAYER^
^\^ 69

SEDIMENTS
BASEMENT

^57
67

"-S7-
66

10
IS

. 81

VERTICAL EXAGGERATION 5X

10
15

6.2

MANTLE

Fig. 32.11. Crustal layers across Middle America trench after Shor and Fisher, 1961. Numbers
represent wave velocities in kilometers per second.

Age of Trench

The Gulf of Tehuantepec marks a major change in trench configuration
and possibly in age. Northwest of Tehuantepec the flat trench bottom
suggests a greater age than the deep V-shaped profile southeast of the
Gulf. Thicker crustal layers and a bordering volcanically active coast
also mark the younger division. The zone of ridge-and-trough topography,
the Tehuantepec Ridge, trending southwest from the point of change
may be another evidence of the division of the trench into older and
younger parts.

FRACTURE ZONES

Four great bands of linear relief features, named fracture zones (H.
W. Menard, 1955), have been discovered in the northeastern Pacific
basin. They are the Mendocino, Murray, Clarion, and Clipperton, and
are shown on the map of Fig. 32.12. A lesser zone, the Pioneer Ridge, is
labeled on Fig. 32.16. It had not been surveyed well at the time the map
of 32.2 was constructed.

The zones range from 1400 to 3300 miles long and average 60 miles
wide. The Mendocino and Murray stretch across the Pacific floor to the
Hawaiian Ridge. They follow great circle courses and are approximately
parallel. Topographic relief within the fracture zones is characterized by
large seamounts, deep narrow troughs, asymmetrical ridges, and escarp-

ments. Two escarpments are about 1 mile high and more than 1000 miles
long. See Fig. 32.2.

The Clipperton fracture zone is more varied and irregular than those
to the north (Menard and Fisher, 1958). The western half consists of
narrow ridges and low seamounts, but the eastern is dominated by an
enormous ridge, about 60 miles wide, 330 miles long, and 8000 to 10,000
feet high. A trough about 10 miles wide and a mile deeper than the
surrounding region borders the ridge. See Fig. 32.13.

The over-all easterly trend of the ridge is complicated by a southeasterly
cross trend indicated by the alignment of volcanoes, by orientation of minor
ridges on the south side of the main ridge, and by the marked change in trend
of the main ridge at its eastern end. Clipperton Island, the only feature in
the whole Clipperton fracture zone that reaches the sea surface, is one volcano

• PLEISTOCENE OR
RECENT VOLCANOES
(INCLUDING ISLANDS)

-y-GUYOTS, FORMER
VOLCANIC ISLANDS

X SEAMOUNTS, SUBMARINE
VOLCANOES NOT KNOWN
TO BE GUYOTS

Fig. 32.12. Fracture zones and seamounts of northeastern Pacific. Reproduced from H. W.
Menard, 1955b. Also volcanoes of adjacent coastland.

PACIFIC SUBMARINE PROVINCES

olio

on a cross trend. The maximum relief of the Clipperton Ridge is 18,000 feet
from Clipperton Island to the deepest spot in the trough at 2,960 fathoms.

DEEP SEA PROVINCES

Gulf of Alaska Seamount Province

The northernmost division of the northeastern Pacific basin is the Gulf
of Alaska Seamount Province (Menard and Dietz, 1951). Its northwestern
boundary is the Aleutian trench and its western the continental shelf
slope, which here is only about 8000 feet high. A rather steep apron
flattens seaward and appears to be a graded profile. The apron and
smooth deep-sea floor are interrupted by thirty-six majestic submarine
volcanoes. Eleven of these are guyots, and their flat tops indicate they
were once truncated by erosion. Most of them are now about 2500 feet
below sea level and some are much deeper, so it is concluded that a like
amount of subsidence has occurred since the truncation.

The region is seismically inactive, and the topography is old with a
thick apron of sediment evidently across the entire province. Major
subsidence of the region is postulated but some time in the geologic
past, possibly Cretaceous.

Ridge and Trough Province

The continental slope of the Ridge and Trough Province is about 132
miles high and is dissected by several well-known submarine canyons.
An apron of sediment spreads from the base of the slope off Queen
Charlotte Island in the northern part of the province, but a long, narrow,
seismically active trough lies between the apron and the base of the
slope. Evidently the top of the apron has been faulted down so recently
that sediment moving out from the continent has not yet filled the trough
to re-establish an even gradient seaward (H. W. Menard, 1955).

The sea floor presumably was block-faulted into long thin ridges which trend
northeast or north. From the ridges rise a few submarine volcanoes some of
which are only a few fathoms below the surface, but most crossings of the
ridges indicate steep-sided, low blocks, unlike volcanoes.

The long ridges roughly parallel the continental slope and guide the flow of
turbidity currents moving sediment out from the continent. One of several

leveed channels on the otherwise smooth plain at the base of the continental
slope off Oregon was traced southward for almost 200 miles. Apparcnth the
turbidity currents cannot surmount the ridges to flow west (direction of the
regional slope) but are diverted southward to a divide through which they again
flow westward or fan out to fill low spots on the downstream side of the ridges.
A few basins appear entirely ringed by high ridges so that turbidity flows
moving along the bottom cannot fill them with sediment. These basins are
thousands of feet below the level of the surrounding alluvial plains formed
by deposition from turbidity currents; their bottoms are irregular, which sug-
gests that deposition from suspension in the main mass of the ocean may
be much slower than deposition from turbidity currents moving in concentrated
clouds along the bottom.

Deep Plain

South of the Mendocino escarpment the sea floor is about half a mile
deeper than it is to the north, and it is called the Deep Plain. It is bounded
on the south by the Murray escarpment, and south of the Murray escarp-
ment the sea floor is roughly a quarter of a mile higher than it is to the
north.

The continental slope off central California forms the eastern boundary
of the Deep Plain. It drops off abruptly to a depth of more than 2 miles,
and three great deep-sea fans form an apron which grades imperceptibly
into the gently sloping Deep Plain at a depth of about 2% miles. Crossing
the fans are leveed and unleveed channels.

The Deep Plain is unique in that it appears to contain few seamounts.
Five seamounts, probably volcanoes, rise from the continental slope

vertical exaggeration io*

„^v

X...

.—■-'••"

:--"V.

rv " •

CLIPPERTON 1 • ^

. r-*"^^*

_*T"

".•■

.... ■■.-^"

•

\:rr:

V.kv.Vv:.':''

..,.***;• -^-

!-;-JT*VvX -

|p^

.. ••.'•'■

.-..-. •.::.-.. '-■.

-f*- . .

J?S. '

m^j

'■'<-

, .

- '.y^*.~

.90 ■'.'■'■

too

" t

' . . : ' joo

300 NAUTICAL MILES f *

Fig. 32.13. View to southwest toward Clipperton Island and the Clipperton Ridge. Reproduced
from Menard and Fisher, 1958.

526

STRUCTURAL GEOLOGY OF NORTH AMERICA

bordering the area, but they trend parallel to the coast and may be
genetically unrelated to the deep-sea floor. (H. W. Menard, 1955).

Baja California Seamount Province

South of the Murray fracture zone a mountainous area, studded with
volcanoes, forms the Baja California Seamount province.

The continental slope drops off abruptly to a depth of about 2 miles. It is
irregular but does not appear deeply dissected by canyons. A smooth apron a
few tens of miles wide lies at the base of the slope in some places. Off the
southern half of Baja California the continental slope drops abruptly for 2-2/2
miles into a series of long thin troughs a few hundred fathoms below the
general level of the deep-sea floor to the west. The troughs are flat-bottomed
indicating a fill of sediment.

Widespread vulcanism, particularly recent vulcanism, characterizes the
province. Guadalupe Island comprises a group of eroded Late Tertiary or
Quaternary volcanoes. Alijos Rocks are three steep-sided remnants of a large
volcanic cone. Volcanic islands are so rare in the northeastern Pacific basin
that these deserve special consideration, but the evidence supporting unusual
vulcanism comes chiefly from submarine volcanoes. Of 51 seamounts, 15 are
more than 1 mile high, and every expedition crossing the province finds new
seamounts. Seven seamounts have been surveyed, and Jasper and Henderson
have been dredged. The volcanoes are typical isolated cones with steep sides
and pointed tops. None are guyots with wide flat tops. Henderson Seamount
appears to have a flat top at 220 fathoms, but the area is only half a square
mile, and this is too small to demonstrate that a sharp peak has been planed
off. However, hundreds of pounds of coarse, basaltic gravel were dredged from
the top of this seamount, and a large fraction of subrounded and subangular
pebbles and cobbles suggests wear in the surf zone.

Contrasting strongly with the smooth floor of the Deep Plain to the north,
the Baja California Seamount province is irregular. Recorded echo soundings
show thousands of miles of jagged bottom in which the irregularities have a
relief of 100-200 fathoms. The relief must be tectonic, but it is uncertain
whether it is caused by vulcanism or faulting. The lack of a smooth blanket
of sediment suggests either that the topography was formed relatively recently
or that the rate of sedimentation is unusually slow. No large rivers carry sedi-
ment from southern California and Baja California into the ocean, and even the
limited amount introduced by intermittent small rivers is trapped in the basins
of the continental borderland or in the troughs off Baja California (Menard,
1955).

Constitution of Deep-Sea Crust

A seismic refraction survey by Raitt (1956) indicated that at a position
in the Baja California Deep-Sea Province due east of Sebastian Vizcaino

Bay (Lat. 27°24'N, Long. 121°35'W) in a depth of 4176 meters of water,
the crust had the following velocity layers:

Thickness, km
0.26
0.93
6.24

Velocity, km/sec

2.15 (Sediments)

5.88 ± 0.23 (Volcanics?)

6.96 ± 0.68 (Crust, gabbroic?)

■'-

8.41 ± 0.43 (Mantle)

Mason uses similar figures in his analysis of magnetic profiles of the
Deep-Sea Plain. See subsequent pages and Fig. 32.17.

Magnetic Intensity Surveys

Magnetic intensity surveys and contour maps have now been made of
a large region off the western United States including a portion of the
Deep Plain province and the Murray and Mendocino fracture zones
(personal communication, H. W. Menard). The results are striking and
tectonically significant.

Figure 32.14 is a sample of the magnetic intensity map and shows an
area 350-400 miles out from the shore along the Murray fracture zone.
The lines of equal magnetic intensity have been so adjusted that they
do not reflect the increase of the earth's magnetic field across the area.
The intensity highs and lows are in sharp zones about 15-25 miles wide
and extend conspicuously and rather regularly in a north-south direc-
tion. This pattern is dominant west of a less intense and more irregular
near-shore zone with a fabric to the north-northeast. Some of the strong
north-south magnetic features have been contoured for a length of 370
miles on the Deep Plain (Menard and Vacquier, 1958; Mason, 1958).

Figure 32.15 shows the topography of the ocean bottom of the same
area as Fig. 32.14. It will be seen that the Murray fracture zone is fairly
narrow here and is reflected clearly in the magnetic intensity contours. It
may also be detected that the zone is one of horizontal offset of the
intensity pattern. This is brought out forcefully if an east-west profile
curve of the anomalies field is plotted both north of the fracture zone
and south of it. If the two profiles are then moved east or west they
match well but in only one position. This is taken to mean that the

tan

■

127"

I26«

125*

127 '

12 6*

12 5 *

127

126'

125'

25°

). 32.14. Total magnetic intensity of an area off the California coast,
gammas. Reproduced from Menard and Vacquier, 1959.

Contour interval is Fig. 32.15. Generalized topography of ocean bottom of Fig. 32.14. Reproduced from Menard

and Vacquier, 1959. Contours in fathoms.

528

STRUCTURAL GEOLOGY OF NORTH AMERICA

MENDOCINO FRACTURE ZONE

Mill I '

170 M

PIONEER RIDGE-

Correlotable
magnetic
intensity
zone—*

'fracturFzone "*"97~M

Fig. 32.16. Horizontal displacements along fracture zones indicated by the offset magnetic
intensity field. Horizontal displacement along San Andreas fault also shown. After Menard (pri-
vate map). Murray fracture zone offset by Mason (1958) and Pioneer Ridge offset by Vacquier,
letter to Nature, 1959. Distances are in miles.

block of oceanic crust south of the Murray fracture zone has moved 97
statute miles westward. Likewise, the intensity pattern is offset along the
Pioneer Ridge 170 statute miles (see Fig. 32.16) with the north block
having moved west (Menard and Vacquier, 1958). The north block of
the Mendocino fracture zone has moved the astonishing distance west-
ward of 1250 kilometers, according to Vacquier et al. (1961). These con-
siderable horizontal displacements are immediately thought of in
connection with postulated strike-slip movement of the San Andreas fault,

and the relation of the several postulated movements is shown in Fig.
32.12.

The magnetic expressions of the volcanoes are puzzling. Most all yield
positive magnetic impressions in the intensity contours, but in no way are
they as striking as the relief contours of the volcanic cones would suggest.
Compare Figs. 32.14 and 32.15. They deflect the intensity contours of
the dominant linear features or are superposed on them but are not
sufficiently strong to make much of an impression. The magnetic effect
is also variable according to Menard and Vacquier, who propose the
variability to be due to the fact that some cones are built of fragmental
material of lower intensity and some of massive flows of higher inten-
sity.

The topography of the ocean floor has an irregular north-south fabric
but it is of low relief and in striking contrast to the relief of the volcanic
cones; yet its intensity contours are sharp and strong.

Regarding the cause of the anomalies it is evident that the distribution
of rocks with different magnetic intensities must match the intensity
pattern, with allowance made for depth and several magnetic factors.

In analyzing the profiles across the linear magnetic positive features
the seismic refraction data of the area were first considered (Mason,
1958). The velocities and interpreted rock layers are shown in Fig. 32.17.
The magnetic values are concluded to be compatible with those of basic
igneous rock, which is here characterized by a high susceptibility and
also a high intensity of remanent magnetization. As a consequence the
"volcanics" layer is taken as the most likely seat of the anomalies, and
calculations made to determine the depth, thickness, and lateral extent
of basic igneous rock masses to produce the observed profiles. If the
tabular mass is flat-bottomed, it would appear as in R, Fig. 32.17; if flat-
topped, as in C, to produce the anomaly shown in A.

Rut how can we manage on a sound geologic basis the elongate tablets
of basalt, diabase, or gabbro of the required shape and magnitude prop-
erly spaced and in parallel arrangement? The structure must be com-
patible with the subdued relief of the ocean floor. It should be pointed
out that the topography of the northeast part of the area of Figs. 32.14
and 32.15 is particularly smooth and appears to be a graded alluvial

PACIFIC SUBMARINE PROVINCES

52')

300r

i 200

E
3

« 100 -

5 -loo

KILOMETERS

_ 10^

^ 15

20 25

40 i\

k 45 50

MAGNETIC

PROFILE

WATER
- SEDIMENTS^

/2.I5 KM/SEC

VOLCANICS ^^

g§{

0.015 J

Ita**^. 5.39

CRUST

6 89 "

L MANTLE

8 29 •■

WATER
SEDIMENTS.

CRUST

MANTLE

Fig. 32.17. Interpretation of magnetic profile (A), with flat base of basic igneous rock at 6.3
km depth (B), and with flat top at 5 km depth (C). After Mason, 1958.

profile or continental slope apron. If such, sediment has just about buried
all previous existing relief there.

Menard (1955) thinks that the displacement along the fracture zone
took place during Cretaceous or Tertiary time and that the structures
causing the magnetic anomalies are older than the fracture zones. Pos-
sibly, therefore, the east-west fracture zones and the north-south struc-
tures are not related mechanically. Menard, Vacquier, and Mason suggest
that parallel valleys were filled or partially filled with basalt and that later
sediments were carried out by turbidity currents and by being spread
in the remaining depressions still further reduced the relief. The cause
of the parallel valleys and the nature of the eruptions is not considered,
nor the relation to the other volcanic (?) rocks of the "volcanics" layer.

More intensive seismic surveys will undoubtedly help in solving the
problem.

HAWAIIAN RIDGE

The Hawaiian Islands are peaks of a ridge or swell built by volcanic
action on the ocean floor. It has a relief from deepest ocean floor to top
of peaks of nearly 32,000 feet, is about 150 miles across in the widest part

Fig. 32.18. Generalized topography around southern end of Hawaiian Ridge showing deep
and arch, after Hamilton, 1957. Contours in fathoms.

530

STRUCTURAL GEOLOGY OF NORTH AMERICA

and trends to the northwest (E. L. Hamilton, 1957). The islands are
believed to have formed during the Tertiary with volcanic activity pro-
gressing southeastward. Present volcanic activity is confined to the island
of Hawaii, which may have had its inception as late as the Pliocene
(Stearns and Macdonald, 1946).

Submarine contouring has indicated a sag, the Hawaiian deep, adjacent
to the ridge, which in its deepest part is about 3600 feet below an outer
gentle arch. See Fig. 32.18. The bottom of the deep is above the level
of the ocean floor beyond the arch.

The peripheral deep and arch are believed by Hamilton to be due to
the loading of the earth's crust by the volcanic piles, and to consequent
downbowing and lateral bulging.

MID-PACIFIC MOUNTAINS

A submerged relief feature known as the Mid-Pacific Mountains, ex-
tends southwesterly from Hawaii. There a series of flat-topped volcanic
peaks, called guyots, are submerged 4200 to 5400 feet. The study of
dredged samples from the flat tops yielding Upper Cretaceous, Paleocene,
and Eocene foraminifera indicate that in Cretaceous time the guyots were
a chain of basaltic islands, wave-decapitated with coral-rudistid reefs
lodged on and among the erosional debris. Submergence followed to the
depths indicated (E. L. Hamilton, 1956). The recognition of broad sub-
sidence of the ocean floor in the magnitude of one mile is very significant
in understanding the processes of mountain building.

CIRCUM-PACIFIC TECTONICS

In Chapter 29 the San Andreas fault and associated structures were
depicted, and there the theories of Hill and Dibblee and of Benioff on
the mechanics of formation were outlined. It is recognized that the
major movement on the San Andreas fault has been right strike-slip move-
ment. Hill and Dibblee (1953) have suggested a horizontal displacement
of 560 kilometers.

Incident to the study of aftershock sequences Benioff ( 1957 ) recognized

-2".

Fig. 32.19. Circum-Pacific tectonics. Reproduced from Benioff, 1957.

that the extent of faulting for earthquakes where the fault is not visible
could be determined. Since the direction of slip can also be determined,
a study of Circum-Pacific earthquakes leads to the presumed discovery
that around the entire margin the slip is dextral as indicated in Fig.
32.18. Only for Antarctica are observations wanting.

Critical of Benioff's hypothesis of counterclockwise rotation of the
Pacific basin crust, Chingchang (1958) points out that the section be-

PACIFIC SUBMARINE PROVINCES

531

tween Japan and the equator is rotating clockwise. The evidence lies in
the study of several great earthquakes in the region and in the geology
of known faults in the Philippines and Japan.

Whether or not the entire Pacific is moving counterclockwise, the
problem arises along the North American margin: What is the relation of
the dextral Pacific movement to the fracture zones? In a personal com-
munication on the subject Dr. Benioff comments as follows:

I assume as a working hypothesis that the radial movements at the continental
margins are expressions of growth of the continents by accretion of material
from below by unknown processes. As the continents rise, the margins are
driven over the adjacent oceanic masses by gravity as mentioned in my paper
on the fault origin of oceanic deeps (Benioff, 1954). In general, the movement

is thus normal to the trend of the coastline. On this basis the curvature ot the
San Andreas system and the existence of the Garlock Fault are the result of
differential expansion of the continent at the Pacific margin — with the northern
portion expanding faster.

The movement along the Garlock Fault is sinestral, whereas the movement
on the Murray fracture, given by Mason's magnetic surveys, is dextral. More-
over the Mendocino fracture appears to have no expression within the conti-
nent east of the San Andreas Fault. I am inclined therefore to the opinion that
these oceanic fracture systems are unrelated to the systems shown in my figure
[Fig. 32.19, this book]. They are probably older — or at least no longer active
since they have no earthquake activity of consequence except in those portions
adjacent to the continental margins where it is probably induced bv the move-
ments going on there. It would seem to me that if the oceanic fracture svstems
were closely related to the present radial flux pattern they should be active
seismically over most of their lengths.

33.

IGNEOUS AND TECTONIC

PROVINCES OF

THE WESTERN CORDILLERA

OBJECTIVES

Volcanic rocks cover large parts of the surface of the western United
States and, by forming appreciable segments of certain sedimentary
sequences, underlie other extensive areas. The Nevadan batholiths are
possibly the most voluminous of all rock units. At least three hundred
stocks and small batholiths occur in Nevada, Utah, Arizona, Colorado,
Montana, and Idaho, and numerous laccoliths, sills, and dikes have been
described in the Colorado Plateau, Wyoming, Montana, and Colorado.
So much of our attention is focused on the sedimentary rocks that the
extensive array of igneous rocks is generally passed by with only in-

cidental reference. It is the object here first to summarize the kinds and
distribution of the igneous rocks in the western Cordillera of South and
North America, and then second, to find a relation, if any, to the tectonic
divisions.

We are always seeking an answer to the deep-seated cause of mountain
building, and since the primary magmas are generally thought to have
developed in the base of the silicic crust, in the basaltic subcrust, or in
the outer mantle shell, it is possible that a careful analysis of the distri-
bution patterns of igneous rocks and their parentage may help us under-
stand the nature of orogeny. This will be the final objective.

CONCEPT OF IGNEOUS PROVINCES

Kennedy's Associations

It has long been recognized that certain regions are characterized by a
related assemblage of extrusive and intrusive rocks, and that this assem-
blage differs from an adjacent one in dominant petrologic types, chemical
composition, and nature of extrusion or intrusion. Such a region will here
be called an igneous province. The rocks of one province may be relatively
uniform in composition such as the basaltic rocks of the Columbia River
Plateau, or they may be varied both in mineralogy and chemical composi-
tion, such as the olivine basalt-nepheline basalt-melilite basalt-trachy-
andesite-trachyte-phonolite differentiation series of the San Juan
Mountains.

In spite of the striking variation in mineral and chemical composition
in these series, it is evident that certain primary magmas are indicated from
which the series have evolved either directly by magmatic differentiation
or by differentiation along with the assimilation of certain kinds and
amounts of country rock. (See Turner and Verhoogen, 1951, for a system-
atic discussion of the process and problems.)

Professor W. Q. Kennedy of the Scottish Geological Survey postulated
in 1933 that the differentiation series and the great basalt fields come from
two basic kinds of primary magmas, namely, olivine basalt and tholeiitic
basalt. The first is characterized by appreciable olivine and augite, and
is commonly alkalic. Kennedy recognized it as the type present in the

532

IGNEOUS AND TECTONIC PROVINCES OF THE WESTERN CORDILLERA

533

oceanic volcanic outpourings and in some of the large basalt fields of the
continents. In the second, olivine is generally absent or if present, is sub-
ordinate. Pyroxene ( hypersthene ) is prominent. This is the primary basalt
of the majority of plateau or flood basalts, such as in the Columbia
River basalt field, generally in the eugeosynclinal assemblages, and to
some extent in the andesite complexes of the orogenic belts. The scheme
of magmatic descent as he gave it is as follows:

Olivine basalt
(Alkalic)

Trachyandesite

Trachyte

Phonolite

Tholeiitic basalt
(Calc-alkalic)

Andesite

Rhyolite

Kennedy also recognized a third magma association which he called

the plutonic. This igneous kindred appears to be limited to the cores of

orogenic belts, and includes all discordant and concordant batholiths,

stocks, and sheet complexes there. It also includes the minor associated

aplitic, pegmatitic, and lamprophyric intrusions. The plutonic associations

consist almost entirely of granodiorite and granite together with the small

amounts of hornblendic, basic, and ultrabasic types. The granodioritic and

granitic plutons are generally emplaced after an episode of intense com-

pressional orogeny, but some in places are known to have accompanied

:the orogeny.
i

Many of the rock types possess no effusive equivalents nor has any true
subjacent plutonic mass been found within a nonorogenic area. This latter
feature alone is sufficient evidence of some fundamental genetical distinction
between rocks of the volcanic and plutonic associations.

We know that a granitic liquid can be produced by the fractional crystalliza-
tion of basaltic magma and, within the volcanic associations, the relative pro-
portion of acid to basic rock types and the chemical composition of the former
is consistent with the view that the rhvolites, granophyres and granites of the

non-orogenic suites have been formed by high-level differentiation and frac-
tionation of a primary basaltic liquid. This mode of origin applies also to the
volcanic associations of the orogenic zones where subordinate quantities of acid
lavas are associated with the predominantly basic extrusives.

The acid rocks of the true plutonic associations, however, represent such an
enormous bulk of granitic and granodioritic material that it is impossible to
conceive of their derivation from a basaltic parent and we are forced to con-
clude that they must have formed from some primary acid magma . . .
(Kennedy, 1933).

Whereas many volcanic associations are believed to have been derived
from a basaltic magma which originates by remelting of a universal sub-
crustal basaltic layer, or by partial melting of the outer mantle, plutonic
associations are believed to originate by melting of a downfolded or
thickened part of the overlying "granitic" layer. It is commonly stated
that such thickening seems possible only where compressional orogeny
has caused the base of the silicic crust to extend down into the range of
melting.

Turner and Verhoogen's Associations

Following Kennedy, Turner and Verhoogen ( 1951 ) define a volcanic
association or kindred as one including all igneous rocks, intrusive as well
as strictly volcanic, that are genetically related to a cycle of volcanic
activity. They emphasize a classification based on oceanic and continental
distribution which is as follows:

1. Oceanic associations (for the Pacific)

a. Olivine basalt-trachyte (Intra-Pacific)

b. Andesite dacite-rhyolite of marginal island arcs ( Circum-Pacific )

2. Volcanic associations of nonorogenic continental regions

a. Olivine basalt-trachyte-phonolite association

b. Leucite basalt-potash trachybasalt-trachyte association

c. Tholeiitic basalts and equivalent quartz diabases

3. Volcanic associations of orogenic zones

a. Spilite-keratophyre association

b. Basalt-andesite-dacite-rhyolite association

The Circum-Pacific oceanic association is similar to the continental
orogenic basalt-andesite-dacite-rhyolite association. Both are dominantly

534

STRUCTURAL GEOLOGY OF NORTH AMERICA

andesites and basalts, but a clear relation to either of the parent basalt
magma types has generally not been agreed upon or established. Great
volumes of andesite are erupted in some orogenic belts with little or no
accompanying olivine basalt, and this gives rise to the belief that the
roots of the thickened "granitic" crust in orogenic belts may be melted
and in part mixed with basalt magma to form andesitic magma directly
and even rhyolitic magma at times (Waters, 1955).

Kuno (1954) reports on a volcanic zone on the Izu peninsula southwest
of Tokyo, Japan, which is a small part, but perhaps typical, of the Circum-
Pacific igneous association. Most of the lavas (basalts and andesites) are
characterized by a low MgO:FeO + Fe203 ratio and low alkalies, and
also by low normative feldspar rich in An and high normative quartz.
However, the lavas of the Omuro-yama group, a small field in the zone,
are high in the MgO:FeO + Fe203 ratio and in alkalies. Some of them
have a considerable amount of normative olivine, and most of them con-
tain resorbed xenoliths captured from a granitic rock. He concludes that
the main rocks of the zone represent various stages of fractionation of a
tholeiitic magma, but that the Omuro-yama rocks represent products of
contamination by granitic rock. The xenoliths were taken from the wall
of the magma reservoir which supplied extrusive lavas, and not from the
walls of the conduits, because in order to effect assimilation, the magma
must have been in contact with the salic plutonic rock for a considerable
time, otherwise only mechanical mixing would have taken place.

The spilites of the orogenic zones are soda-rich olivine-poor basalts,
with albite or oligoclase the sole or principal feldspar. Some of the albite
in certain flows is secondary. A keratophyre is a sodic trachyte with albite
as the principal constituent. Many spilites are pillow lavas and are inter-
bedded with marine sediments; hence probably erupted on the sea floor
as submarine flows. The spilites and keratophyres are commonly associ-
ated with normal basalts and andesites, and are typical volcanic rocks of
the eugeosyncline. Recause of this position they are particularly subject
to low-grade metamorphism and become the greenschists of the orogenic
belts. Waters (1955) regards the spilite-keratophyre association in the
Coast Ranges of Washington and Oregon as a tholeiitic province, but
Turner and Verhoogen ( 1951 ) think the chemical data yet insufficient to

establish a clear-cut relation to the tholeiitic or the olivine basalt magma
types:

The spilitic association, whatever its relation to the basaltic kindreds, is one
of striking individuality maintained in widely scattered provinces of all ages
and recognized wherever the rocks of geosynclinaal terranes have been petro-
graphically investigated (Turner and Verhoogen, 1951, p. 205).

The olivine basalt-trachyte-phonolite association is displayed in places
in the Rocky Mountains, particularly in moderately deformed belts of
Laramide orogeny. It is an extensive differentiation series ranging from
olivine basalt to basanites to trachybasalts and trachyandesites to phono-
lites. The members generally have alkaline affinities. Within a single vol-
canic episode hundreds of flows together with much pyroclastic material
may be erupted from numerous centers to form a continuous field 50 to
75 miles across. Intrusive sills, laccoliths, plugs, and dikes are a minor
part of the field. Xenoliths are commonly conspicuous in the flows and
several authors believe the original olivine basalt magma was contam-
inated by reaction solution (fusion) of the wall rock. The type of wall
rock and the amount assimilated determines to a large extent the course
of differentiation of the magma. This general association is represented
by the San Juan volcanic field (Larsen and Cross, 1956) and probably
other fields in Colorado and New Mexico.

The leucite basalt-trachybasalt and trachyte association in the western
United States is represented by the feldspathoid, alkali-rich rocks of the
Colorado Plateau, Leucite Hills and Rlack Hills in Wyoming, and the
well-studied region of central Montana (Larsen, 1940).

The association called tholeiitic basalts and equivalent quartz diabases
are the flood basalts of such volcanic fields as the Columbia River Plateau.
The most striking characters are the enormous volume, wide extent, and
uniform composition of the basalt sheets.

Tyrrell's Tectono-lgneous Cycle

Emphasizing the tectonic and time aspect of petrographic provinces
Tyrrell ( 1955 ) has proposed the following tectono-igneous cycle." It
applies to the complicated region of northwestern Europe consisting of

IGNEOUS AND TECTONIC PROVINCES OF THE WESTERN CORDILLERA

535

'three ancient orogens welded onto the Scandinavian-Raltic shield," and
particularly to the Scottish Paleozoic.

Diastrophism

Kindreds

Locus

I. Geosynclinal phase 1. Ophiolitic kindred

II. Orogenic phase (with 2. Granodiorite-andesite

two or three subphases) kindred

III. Post-orogenic phase 3. Trachybasaltic kindred

(with two subphases) 4. Quartz dolerite kindred

In orogen

In kratogen

Proposed Classification of Provinces

In the western United States, certain igneous rock associations stand
clearly apart from others. Discussions generally center about such strik-
ing igneous provinces as the Cascade Mountains, the San Juan Moun-
tains, or central Montana, yet no one has published a map of the entire
western United States on which are grouped the many volcanic fields
and plutons into igneous provinces. Several emphasize the transitional
and elusive nature of boundaries, and this is certainly realized when one
attempts to draw them. The main goal of this chapter is thwarted, how-
ever, if the petrographic provinces are not mapped and compared with
the tectonic provinces.

In struggling with the problem, difficulties in two categories arise.

First, in the provinces of extensive basalt outpourings a distinction be-

jtween rocks of the olivine basalt kindred and the tholeiitic kindred is

Commonly obscure. The problem is met with specifically in classifying

\he Malheur and Snake River basalt fields. Second, in the alkalic and

^alc-alkalic "provinces" of the Rocky Mountain states, the boundaries

)f the numerous subdivisions suggested in the literature are generally

mpossible to fix or map. Second, the main kind or kinds of rock present

s generally a characteristic feature which can be mapped objectively,

vhereas the kindred represented may be controversial.

A classification believed better suited for tectonic studies is as follows,
t will serve as a guide in the discussion of the igneous rock provinces of

the western Cordillera of the Americas, and is especially adapted to the
western United States.

A. Rasalt provinces

1. Oceanic (mostly olivine basalts)

2. Continental flood and cinder cone fields (both olivine and tholeiitic
basalts)

R. Andesite provinces

1. Eugeosynclinal (mostly tholeiitic basalts and andesites-spilites and
keratophyres characteristic )

2. Volcanic arcs

3. Orogenic belt ( post-batholithic volcanic fields)

4. Stratovolcanos of continental margin

C. Trachyte and phonolite provinces

1. Alkalic (leucite basalt-trachyte-phonolite group)

2. Calc-alkalic ( olivine basalt-phonolite association, also andesite and
rhyolite )

D. Latite-monzonite provinces

E. Rasalt-rhyolite provinces

F. Grandiorite-granite batholithic provinces

1. First cycle

2. Second cycle

The petrologic terms basalt, andesite, latite, trachyte, and phonolite
are used to denote the main type of rock of the province. In the andesite
provinces especially, differentiation products are common as well as
olivine and tholeiitic basalts. The basalt-rhyolite provinces specify those
in which the intermediate to subacid differentiates are dominant.

Evident Tectonic and Igneous Cycle

In the orogenic belts that form the margins of the continents, such as
exemplified by the Sierra Nevada of California and the Acadian belt of
New England, the main events follow a fairly consistent pattern or cycle.
The one given below is modeled after Turner and Verhoogen, (1951),
but with additions and modifications as seen necessary from a study of
the Cordillera of South and North America.

536

STRUCTURAL GEOLOGY OF NORTH AMERICA

1. Eruption of dominantly basic (spilitic, keratophyric, basaltic, and
andesitic) lavas during the eugeosynclinal phase.

2. Injection of ultrabasic and basic plutonic intrusions into the eugeo-
synclinal sediments and volcanic rocks which are almost constantly being
disturbed by episodes of folding.

3. The climactic folding and dynamic metamorphism of the eugeo-
synclinal rocks. In the South American Andean system the folding seems
to have been mostly late Paleozoic, preceding the Late Cretaceous batho-
liths by a long time. Much eugeosynclinal volcanic rock accumulated
between the metamorphism and the batholithic intrusions. In the Siena
Nevada a series of orogenic phases stretching at least from the Devonian
to the Cretaceous preceded the batholithic intrusions. In Late Jurassic
time intense folding and low-grade regional metamorphism climaxed the
train of disturbances. Ratholithic intrusions followed immediately in Late
Jurassic and again, most voluminously, in Mid-Cretaceous. In the Acadian
belt of New Hampshire, specifically the White Mountains, early folding
and thrusting resulted in regional low-grade metamorphism, then fol-
lowed the main batholithic intrusions with accompanying medium and
high-grade metamorphism, and finally a second episode of thrusting. The
three stages occurred within late Devonian time.

4. Emplacement of the great granodioritic and granitic batholiths into
the folded and metamorphosed complex. Some batholiths are involved
in the folding, but the great bulk of the plutonic rock is post-folding in
age. Each great batholith is commonly composed of a number of indi-
vidual plutons with each having a slightly different composition. They
range from diorite to granite with granodiorite the most abundant. In
the Sierra Nevada the sequence of intrusions seems to have occurred over
a period of 18 m.y.

5. An extensive episode of erosion in which the batholithic rocks are

exposed, with the development commonly of adjacent longitudinal basins
of sedimentation.

6. Renewed volcanism with the building of great lava and pyroclastit
fields, chiefly andesitic, on the folded and metamorphosed batholithic belt.
These are the Tertiary volcanic fields of the Andes and of the Sierra Madre
Occidental of Mexico, and probably the Mississippian (?) Moat vol-
canics of the White Mountains. In places latites may be very abundant.

7. Following shortly the post-batholithic eruptions and probably part of
the same renewed igneous activity are new batholithic intrusions which
in places reach up to the volcanic accumulations and intrude them. Ex-
amples are the White Mountain magma series of the White Mountains
the post-volcanic batholiths of the Cascade Range of Washington, and the
imposing belt of mid-Tertiary batholiths of western Sonora. This is
the second cycle batholithic province of the proposed classification above.

8. A late volcanic activity occurs in segments of the orogenic belt and
results in the building of a majestic row or belt of stratovolcanoes, or an
extensive field of basaltic flows and cinder cones.

In the western United States the broad Paleozoic miogeosyncline and
shelf, and the superposed Mesozoic basins and Laramide belts of deforma-
tion, are replete with igneous rocks of the trachyte, phonolite, and latite
associations. When compared with the Andean, Mexican, and Canadian
Cordillera, the wide and complex western United States Cordillera is an
exception. Tectonic provinces like the Great Basin, Colorado Plateau,
and the Wyoming and Colorado Rocky Mountains belt of orogeny are
either not developed to the north and south of the United States or are
reflected in a narrower or restricted way. It is the object of the following
pages to review the petrographic and igneous provinces of the western
Cordillera of South and North America by using the above depicted con-
cepts in an attempt better to understand the process of orogeny.

34.

IGNEOUS AND
TECTONIC PROVINCES
IN SOUTH AMERICA

CHILE AND ARGENTINA

Geosyncline

Two references are most significant for a general understanding of the
Andean geology of South America, viz., Handbook of South American
Geology, Geol. Soc. Am. Memoir 65, 1956, edited by W. F. Jenks, and
Ban der Sudamerikanischen Kordillera (Gebriiden Borntraeger, Berlin)
by Heinrich Gerth, 1955. The Cordillera of Chile and western Argentina
marks essentially the site of a previous geosyncline, particularly the eugeo-
synclinal division. (See Fig. 34.1.) Its Paleozoic history is not well known,
but as far as a great igneous and geosynclinal cycle is concerned we may

start with the late Permian and early Triassic, when continental condi-
tions probably prevailed. During this time voluminous extrusions of
keratophyre and quartz porphyry occurred.

. . . These extrusions are pierced by granites which are the intrusive phases
of the lavas. Later, the sea advanced from the west, eroding the volcanic rocks
and depositing a transgressive series which has at its base the products of the
destruction of the volcanics which in turn pass upward into shale with a
marine fauna. This transgression marks the beginning of the Andean geo-
syncline. Later the keratophyre extrusions were renewed, with a more basic
composition than previously, and flows partially filled the marine basin. Plant-
bearing shales were deposited. But all these episodes were transitory because
the ocean transgressed again during the Norian (late Triassic), with the deposi-
tion of thick layers of shale. Later, continental conditions returned, perhaps be-
cause of tectonic movements whose nature is unknown. The topograph v
formed was then destroyed during the Rhaetian (latest Triassic) when a surface
was prepared for the Liassic (early Jurassic) transgression (Cristi. 1956.
p. 197).

The Triassic volcanic rocks are over 12,000 feet thick in the Frontal
Cordillera of Mendoza but thin toward the east.

Late Triassic volcanism continued into early Jurassic time but the
distribution of the eruptives is possibly limited to southern Atacama
and northern Coquimbo.

... In the rest of Chile andesitic volcanics seem to be lacking in the Lias.
However, in the Coast Range of Aconcagua and Valparaiso, the Upper Lias
sediments contain thick keratophyre flows and tuffs. Apparently similar condi-
tions are found in the Argentinean Cordillera. It is interesting to note that this
type of extrusion is not known in the region north of Atacama; this proves that
the keratophyre extrusions, which probably began during the Lower Triassic in
an area of enormous size, become more and more restricted. At the same time
acidity of the flows diminished. This phase of volcanism ended in the late
Liassic.

During the Upper Dogger (mid-Jurassic), andesitic extrusions covered almost
all the area occupied by the western part of the geosvncline: but. at least in
the Coast Range and the central zone, it seems that before these lavas were
deposited many important tectonic movements occurred, possibly in the form
of block faulting, since in some places the deposits lie on Triassic and in others
on Liassic formations.

We know little about the mechanism of these extrusions, but judging by
some masses of andesite which pierce the Triassic or Liassic, and by the
abundance of pyroclastic materials in the series, probably they were produced
by volcanoes of the central type, which must have been elevated above the sea

537

^«

IGNEOUS AND TECTONIC PROVINCES IN SOUTH AMERICA

5-39

bottom, spreading their lavas partly as submarine and partly as terrestrial
flows. These lavas and pyroclastics repeatedly filled large areas of the basin.
They were subjected to marine erosion, and conglomerate and limestone were
deposited on the marine terraces. Finally the filling became so thick that much
of the basin acquired continental characteristics, except in the eastern zone,
where sediments continued to be deposited until early Malm (Upper Jurassic)
time (Cristi, 1956).

The earlier keratophyres gave way to Jurassic andesites which occurred
as flows, "porphyritic" tuffs and welded tuffs, and andesite breccias and
conglomerates (boulders are generally andesite porphyry). According to
C. Lomnitz of the University of Chile ( personal communication ) some of
the keratophyres mentioned in the literature are probably spilites, and
pyroclastics predominate over lavas.

Volcanism continued into earliest Cretaceous time with the accumula-
tion of andesite breccia conglomerates and red sandstones derived from
the volcanic rocks. Marine sediments are extensive along the Chilean-
Argentinian border, and it is thought that the intertonguing volcanic rocks
graded into a volcanic chain along the western coast.

The Santa Cruz basin (also called Magellan geosyncline) on the south
lacks Late Triassic volcanic rocks, but in the Jurassic intense volcanism
broke out there, and a thick series of keratophyre and andesite flows
accumulated. These are called the "Serie Porfirica." (See Fig. 34.1 and
section D-D', Fig. 34.5. )

. . . The Serie Profirica of the Magellan region has frequently been likened
to the Triassic keratophyres of Central Chile and Argentina. Although the two
series show great petrographic similarity, they are not synchronous since accord-
ing to modern studies by Argentinian geologists, the Serie Porfirica starts during
the Jurassic and ends during the Lower Cretaceous. Another important differ-
ence between the two extrusive aggregates is that the Mesozoic extrusions of
the Andean geosyncline changed from quartz-keratophyres to andesites, whereas
in the northern part of the Magellan geosyncline the acidic character was
maintained during the whole interval, and only in one place do a few un-
important andesites appear.

On the other hand, formation of both basins was preceded by extensive
keratophyre eruptives.

The rocks above the Serie Porfirica in the Magellan region consist of dark
fine-grained sediments with a phyllitic aspect and include marly clay shales and
graywackes. Radiolaria are abundant in the lower beds. A few basic dikes cut

the lower strata. In the Cordillera of the Brunswick Peninsula and Tic i ia del
Fuego this series must be several thousand meters thick (Cristi, 1956).

These seems to be little development of a miogeosynclinal division of
the geosyncline; the volcanic sequences and interbedded sediments thin
to the shelf area of the foreland with volcanic rocks making up part of
the much thinner sequences.

Batholithic and Metamorphic Belt

As seen on the map of Fig. 34.1 the entire coastal zone of Chile is made
up of a belt of metamorphic and batholithic rock. From Valparaiso south-
ward a metamorphic rock zone stretches along the shore almost to the
Straits of Magellan. The batholithic zone in this segment lies inland or
east of the metamorphic belt but gradually transgresses the metamorphic
zone and comes to the coast just north of the Straits of Magellan. As the
combined belt veers eastward through Tierr-a del Fuego, the metamorphic
belt appears on the inside or to the northeast. The reality of a great
batholith is not questioned. In some cross sections it is shown underlying
the whole Cordillera, from the Coast Range through the high Andean sys-
tem. On the Geologic Map of South America (1950) it is 75 miles wide
just east of the Gulf of Corcovado and is almost continuous from Val-
paraiso southward through Tierra del Fuego. It has thinned to about 25
miles at Valparaiso and from there extends as a narrow belt another 400
miles northward. It then becomes discontinuous and is represented by
scattered intrusions into southern Peru. From Valparaiso northward it is
intrusive into the eugeosynclinal sediments, mostly Late Triassic and
Jurassic volcanics.

The relation of the batholiths to the metamorphic rocks has not been
clearly established. In places the metamorphic rocks are intensely folded
Paleozoic strata, but the orogeny may be Permian or earlier, at least in
places, and not an immediate prelude to the later batholithic intrusions.
Extensive gneissic zones in the Coast Range near Valparaiso are probably
migmatites of the batholith.

Briiggen ( 1934 ) has very carefully analyzed the phenomena at the con-
tact between the batholith and the country rock, which is generally the
Serie Porfirica, and he has concluded that the gneissic appearance of the

540

STRUCTURAL GEOLOGY OF NORTH AMERICA

batholith in places near the contact is due to migmatization. No doubt, in
numerous places a certain amount of migmatization has taken place, but
generally the gneissic aspect consists of a folded primary structure, in-
jected by veins during the latest stages of magmatic consolidation.

Both the gneissic appearance and the migmatization are much stronger in
the Coast Range, probably because in this range the lower levels, where
magmatic phenomena could develop with greater efficiency, are more accessible
to observation. For this reason the batholith has until recendy been considered
very old. But Briiggen demonstrated through his analysis of numerous outcrops
that the intrusion cannot be older than Early Cretaceous, because the strata
of this age which are near the batholith are always affected by thermal meta-
morphism. Besides, the Paleozoic and Triassic conglomerates, which are rela-
tively abundant in Coquimbo, contain no pebbles that could have been derived
from the Andean batholith; the pebbles are aplitic granite containing albite or
microperthite. Such rocks are subordinate in the batholith, which is mostly
tonalite and granodiorite. However, in the Coast Range the first phases of the
intrusion might well have been tied to the orogenic movements which occurred
in the Jurassic (Cristi, 1956).

Although the great piles of volcanic rock were intruded, a zone of the
eugeosyncline along the east side of the batholithic zone was left free of
intrusions. The layers of volcanic rock, here many thousands of feet thick,
were tilted so as to dip eastward and from this great monocline in the
region east of Santiago the high peaks of the main Andean Cordillera
were carved.

Petrographically the main batholith ranges from granodiorite to tonalite.
Fairly extensive bodies of granite are known, especially aplitic granite.
Gabbro and hornblendite are listed as present and are said to be basic
derivatives, presumably of the parental dioritic magma. Diorite porphyry
and dikes of lamprophyre, aplite, and especially kersantite and spesartite
are mentioned. True pegmatites are uncommon (Cristi, 1956).

PERU, BOLIVIA, ECUADOR, AND COLUMBIA

Geosyncline

Geosynclinal sedimentation is known to have become established in
Middle Ordovician time in Peru approximately in a north-south basin in

the site of the present Andes. The basin extended, at least, as far west as
the present shoreline. Silurian strata have not been observed but Middle
Devonian strata are known in places. (See map, Fig. 34.5 section B-B'.)
In central Peru, Middle Pennsylvanian beds rest disconformably on the
Middle Devonian. In southern Peru, Permian beds rest disconformably on
older Paleozoic hard, micaceous shale. Again in central Peru, Permian
conglomerate and red beds rest in strong angular unconformity on older
Paleozoic rocks in various conditions of metamoq^hism. In southern
Peru, marine Permian ( and possibly Carboniferous ) beds cover contorted
and metamorphosed older formations. Mississippian of Pennsylvanian
continental deposits rest unconformably on older rocks in northwestern
and central Peru. Absence of known Upper Devonian and marine Missis-
sipian strata in Peru is a further suggestion of orogeny and uplift begin-
ning at the end of Middle Devonian time.

In central Peru late Paleozoic orogeny must have begun at the close of the
Middle Pennsylvanian. Continental elastics and volcanics of Permian age here
rest disconformably on the Pennsylvanian sequence (Jenks, 1956).

Permian time was marked in places by marine transgressions and by
the deposition of 2300 feet of "red beds and conglomerates" apparently
of continental origin. In central and southern Peru the Permian volcanic
rocks attain great thicknesses.

The close of the Paleozoic in Peru was marked by strong orogeny. Permian
granites and associated quartz veins cut thick folded and faulted Paleozoic
metamorphics in northwestern and probably in southern coastal Peru. There
was intense volcanic activity and general emergence of the Andean zone in
upper Permian time. Evidently the whole of Andean Peru was land from some
time in the Upper Permian until at least the beginning of the Upper Triassic
(Jenks, 1956).

The history thus related is responsible for the belt of metamorphic rocks
shown on the map, Fig. 34.2, which stretches from northernmost Chile to
northwestern Peru. Its present borders are probably due to later orogeny.
A similar belt of metamorphic rocks forms the Cordillera Oriental of
Ecuador. Its paragneisses and paraschists probably represent Paleozoic
and perhaps some Precambrian sediments deposited in an Andean geo-
syncline.

£ >

542

STRUCTURAL GEOLOGY OF NORTH AMERICA

MACIZOS OCCIOENTALES

REPISA OCCIDENTAL

MARGEN OCCIDENTAL

DE LA
CORDILLERA ANDINA

PARTE CENTRAL

DE LA

CORDILLERA ANDINA

cffia

rr&

Rocot intruii<o>

ESTRATIGRAFIA COMPARADA

DE LA SECCION MESOZOICA
EN EL
NOROESTE DEL PERU

Fig. 34.3. Comparative sections of the Meso-
zoic of northwestern Peru. Reproduced from
Fischer, 1956.

Middle Paleozoic to early Mesozoic disturbances may well have contributed
to the metamorphism of the present day crystalline rocks, but there are more
hiatuses than basic data . . . Some of the orthogneisses probably represent
pre-Jurassic intrusions (Jenks, 1956).

The Mesozoic history of the Andean geosyncline in Peru and Ecuador
is in general like that of Chile, and its main stratigraphic elements are
described by Fischer (1956) whose illustrations are reproduced as Figs.
34.3 and 34.4. The pre-batholithic Mesozoic volcanic rocks extend north-

ward from Chile into southern Peru but there wedge out against the
coast. They reappear in northwestern Peru and Ecuador. The map of
Fischer ( Fig. 34.4 ) and the map of the Andes of Peru, Ecuador, Columbia,
and Bolivia ( Fig. 34.2 ) , which was taken from the Geologic Map of South
America (1950), represent the distribution of the Mesozoic pre-batholithic
volcanic rocks (Jenks, 1956). The concept of a volcanic archipelago and
eugeosyncline on the ocean side (west), and then the miogeosyncline on

IGNEOUS AND TECTONIC PROVINCES IN SOUTH AMERICA

543

the mainland side is portrayed. The volcanic arc and eugeosyncline are
about 200 miles wide. In California and western Nevada the same has a
width of about 400 miles. Permian volcanic rocks are abundant in both
places.

Batholithic Belt

The batholithic belt extends up the coast from Chile to northwest Peru,
and the great plutons form much of the western slopes and foothills of
the Cordillera Occidental. In northern Peru the batholithic belt swings
northeast across Peru, following the Mesozoic geosyncline. According to
the Geologic Map of South America (1950) the plutons are separate and
small in Ecuador, and in Colombia the belt becomes so inconspicuous
1 that its identity, at least on the map, is problematical.

The rocks of the great coastal batholith of Peru are, in order of abun-
dance, granodiorite, tonalite, granite, and diorite. Others in small volume
, are quartz monzonite, monzonite, syenite, and gabbro. Wide, deeply
eroded parts of the batholith appear to be fairly homogeneous in compo-
sition, but apically truncated parts show a wide range of petrologic types
(Jenks, 1956).

The main part of the batholith was intruded, apparently, in early Upper
Cretaceous time. Lower Senonian and even Turonian strata have been
intruded and metamorphosed, but younger ones have not been affected.
Lower and Middle Cretaceous rocks in Peru contain abundant volcanics,
jbut when the batholithic intrusions ocurred, no further volcanism is
recognized through the Maestrichtian, Danian, and Paleocene to the late
Eocene. Abundant volcanic rocks appear to be largely Miocene, Pliocene,
and Quaternary in age.

Anticlinorium of Pre-Mesozoic Rocks

A belt of Paleozoic and Precambrian rocks extends from the Argentine
'border northwestward through Bolivia and lengthwise through Peru
almost to Ecuador. It is lacking for about 100 miles and then at the
southern Ecuadorian border commences again and extends through Ecua-
dor and nearly through Colombia. In disconnected areas it is present in
western Venezuela. ( See map, Fig. 34.2, and cross section, Fig. 34.5. ) In

CRETACICO: FACIES VOLCANICA

JURASICO-TRIASICO: FACIES VOLCANICA

Fig. 34.4. Idealized restoration of Mesozoic sedimentary and tectonic divisions in northwestern
Peru. Reproduced from Fischer, 1956.

TRENCH

Ku T Ku Ku J

ANDEAN COROtLLERA
Chimborozo vol. Tungurohuo vol

°M» -— _ Pa\ _Jv K

...,M?2 '

pKm

ECUADOR

After Marks, Lewis, and Tschopp, 1956

CORDILLERA OCCIDENTAL

TITICACA
TROUGH

Pichupichu vol

CORDILLERA
ORIENTAL

KRUMr.'.EL DEEP

IHvTp P 0 T O «u Hu * Kl Ku _B_-

SOUTHERN PERU

filler Jenks, 1956

SHELF SLOPE

ANDEAN CORDILLERA
QMv

CENTRAL CHILE

After Critti, 1956

SHELF SLOPE

ANDEAN CORDILLERA
prim Kl K0

200 KILOMETERS

SHELF SLOPE

'pKm Kl>K
SOUTHERN CHILE

BAY OF SEBASTIAN VIZCAINO

After Cristi, 1956

OCCIDENTAL
TV

8AJA CALIFORNIA

GULF OF CALIFORNIA

SONORAN COASTAL PLAIN "" ~ PARALLEL VALLEYS AND RIDGES
Tv J* Tv_ Jj» _ Tb j-j, Tb Ife^iUU- Tv Tb Kv Tb Tb

MEXICO

Fig. 34.5. Cross sections of the western Cordillera of South America and Mexico showing
major igneous and sedimentary units. See maps, Figs. 34.1, 34.2, and 35.1 The vertical scale
is about twice the horizontal scale but, even so, some of the thicknesses of the stratified units
are undoubtedly too great. Interpretations at depth have been made which are not shown
on original sections of authors cited.

PC, Precambrian; Pal, Paleozoic; D, Devonian; J?, Jurassic and Triassic strata; J. Jurassic

After R E King, 1939

strata; Jv, Jurassic volcanics; pKm, pre-Cretaceous strata; Klv, Lower Cretaceous volcanics; Klj
Lower Cretaceous strata; Ku, Upper Cretaceous strata; K, Cretaceous strata; Ki, Cretaceous bath-
olithic intrusives; Kv, Cretaceous volcanics; T, Tertiary strata; Tb, Tertiary basin beds; Tv
Tertiary volcanics; E, Eocene strata; QMv, Quaternary to Miocene volcanics; Ti, Tertiary in;
trusives.

IV-

lit

ISO
HI

Jff

PI!
k

IGNEOUS AND TECTONIC PROVINCES IN SOUTH AMERICA

545

Bolivia the broad Paleozoic area comprises the eastern and central Cor-
dilleras, and the strata are folded but evidently not enough to produce
much metamorphism. In Peru the belt is narrower with considerable fold-
ing, faulting, and metamorphism, and may include Precambrian rocks.
It also includes several large intrusions, one of which is mentioned as a
granite (Jenks, 1956).

In Ecuador the crystalline rocks are highly metamorphosed and form
the backbone of the Cordillera Oriental (also called Eastern Andes and
Cordillera Real). The types are orthogneiss and paragneiss, mica, garnet
schists, also amphibolitic, sericitic, talcose, and graphitic schists, phyllites,
and some quartzites and calcareous slates. Also prominent are meta-
morphosed granodiorites. Minor amounts of metamorphosed syenite and
low-quartz granite are noted. The belt of crystalline rocks is flanked on the
east by little or non-metamorphosed Paleozoic and Mesozoic strata, with
associated volcanic rocks and Mesozoic (?) granites. The complex is
thrust eastward at its eastern margin. ( See section A-A', Fig. 34.5. )

Between the Cordillera Occidental, or batholithic belt, and the Cor-
dillera Oriental, or older metamorphic belt, is the intercordilleran depres-
sion. It may be compared to a huge graben bounded on the east and west
by fault zones which dip away from the graben at fairly high angles.
Beginning in Miocene time, as far as known, the graben has had large
amounts of volcanic materials poured into it. Faulting is believed to
have continued intermittently during the accumulation of the volcanic
irocks.

As the two Cordilleras have risen relative to the intercordilleran depression,
the volcanoes in and bordering the depression have filled it with vast quantities
of predominantly andesitic pyroclastic and flow rocks. At the same time, heavy
rainfall and melt water from the snow and ice-clad heights of the Cordilleras
have eroded the depression-facing slopes and deposited the resulting clastic
sediments in the depression. The huge area flooded by this volcanic and — to a
lesser degree — clastic fill makes up by far the greater part of the intercordilleran
depression (Lewis et ah, 1956).

The prominent anticlinorium of Paleozoic and Precambrian rock is
for most of its length bounded by reverse faults, and is interpreted to be
thrust over the flanking basin sediments on the east and also over the
rocks on the west such as the intercordilleran depression in Ecuador and

the Lake Titicaca trough at the Peru-Bolivia border. These faults delimit
the raised zone of older metamorphic rocks, but do not mark the original
width of it.

In Figs. 34.3 and 34.4 highly folded Paleozoic strata are shown to
form the core of the postulated Mesozoic volcanic archipelago on the
Pacific border of the continent. This picture is built from a few small
outcrops, but nonetheless it is as logical a foundation for the Mesozoic
volcanic effusives as any. It must be concluded that the width of the belt
of Paleozoic folding and metamorphism is much wider than that exposed
in the anticlinorium.

POST-BATHOLITHIC VOLCANIC ROCKS

Age Relation to Batholithic Belt

Following the batholithic intrusions and the accompanying folding and
faulting a long cycle of erosion removed much of the roof rock and in
places cut deeply into the plutons. The surface developed over much of
the adjacent Cordillera also. Upon this extensive erosion surface new
effusives were spread. The plutonic cycle occurred in most places during
Mid- and Late Cretaceous times, and the earliest eruptives are late
Eocene, but the main volcanic activity in most places did not start until
the Miocene. There was a lapse of time, then, of about 40 m.y. between
the plutonic cycle and the beginning of the volcanic cycle.

Areas of Volcanic Rocks

The volcanic accumulations may be grouped in three divisions: (1)
between Santiago and the Straights of Magellan; (2) southern Peru,
Bolivia, northern Chile, and northwestern Argentina; (3) Ecuador and
southwestern Colombia. All are confined to the general Cordillera ex-
cept in Southern Argentina where flows occur on the foreland. Each
of the three areas support a magnificent belt of active or dormant vol-
canoes in addition to extensive volcanic fields. (See maps, Figs. 34.1
and 34.2.)

The central division is the largest in areal extent and undoubtedly the
largest in volume. It occurs around the bend of the great Cordillera from

546

STRUCTURAL GEOLOGY OF NORTH AMERICA

the northerly trends of Chile and Argentina to the northwesterly trends
of Bolivia and Peru. The southern division is notably an assemblage of
individual smaller fields.

Although the eugeosynclinal, batholithic, and fold belts are continuous
from Tierra del Fuego to Colombia, the Cenozoic volcanic fields are not.

Spatial Relations to Older Belts

In a broad way the volcanic fields lie between the Cordillera Oriental
and the Cordillera Occidental, and in part fill a graben whose sinking
between the two linear relief elements was about contemporaneous with
the eruptions. The volcanic rocks, however, spread over both adjacent
Cordillera extensively in places, and in northernmost Chile extend west-
ward across the batholithic belt to the coast. In the Puna de Atacama
region of northern Chile opposite Antofagasta, the extrusions are entirely
east of the batholithic belt and mostly on the eugeosynclinal strata free
of batholithic intrusions. They spread eastward, also, to the deformed
miogeosynclinal and shelf sediments. Possibly 800 active and inactive
cones exist in this large field and seem to be arranged in several rows. A
few scattered fields are on the Precambrian and Paleozoic anticlinorium
to the north.

In Ecuador and southwestern Colombia the main volcanic field fits
rather snugly in a graben between the batholithic belt on the west and
the anticlinorium of older metamorphosed rock on the east. Some of the
great stratovolcanoes have vents through the cordilleran rocks on either
side, however, and have built considerable volumes of ejecta on these
foundations beyond the faults that bound the graben.

The southern division of volcanic fields is generally east of the batho-
lithic belt on the batholithic-free Mesozoic volcanic rocks, and as men-
tioned, a number of isolated fields lie on the miogeosyncline and shelf
areas of the foreland. This division is singular in that the row of great
stratovolcanoes is mostly in the batholithic belt and not a part of the vol-
canic fields. At the north end of the division, the row cuts acutely into the
eugeosyncline, and several vents are offset sufficiently far east so that they
are in the miogeosyncline (see map, Fig. 34.1). South of Santiago numer-
ous volcanoes have been active in recent years. The great isolated vol-

canoes or groups of volcanoes are spaced at about 30- to 40-kilometer
intervals in this part of the zone.

Composition

The flows associated with active volcanoes in Chile are mostly basalts,
ranging from hypersthene basalt in the oldest flows to olivine basalt in the
more recent, with the exception of Calbuco, which still erupts hypersthene
basalt (Cristi, 1956, p. 213).

The volcanic fields of the foreland in Argentina are nearly all basalt
flows of Pliocene-Quaternary or Quaternary age. The Eocene and Oligo-
cene volcanic rocks in the cordilleran region are andesites and dacites,
and overlying Miocene flows are basalts.

The great Puna field consists of augite and hypersthene andesite with
the latest flows of olivine basalt. Rhyolite is also reported.

The Tertiary and Quaternary volcanic rocks of southern Peru range
in composition from basalt to rhyolite, with andesite, trachyandesite, and
trachyte very abundant. Cutting the extrusives are numerous small stocks
of diorite, monzonite, quartz monzonite, syenite, and dacite porphyry.

The volcanic rocks of Ecuador and southeastern Colombia are domi-
nantly andesitic pyroclastic and flow rocks.

It may be concluded, therefore, that andesites are the most abundant
of the Cenozoic volcanic rocks which appear within the Cordillera, with
olivine and hypersthene basalts probably next in abundance and also
usually latest in the eruptive sequence. In southern Peru the trachyte
volcanics and the monzonite and syenite stocks are unusual because of
their high alkalic content.

Relation to Graben Faulting

Both Gerth ( 1955 ) and Cristi ( 1956 ) emphasize the relation of fault-
ing to volcanism, or more generally stated, to "recent tectonic depres-
sions." In the entire Andes only where a well-developed longitudinal
valley exists do volcanoes occur. This concept relates specifically to the
rows of active and dormant stratovolcanoes. In the Ecuador division,
however, the entire field is fairly closely tied to the graben faulting which
here has been interpreted as of compressional nature.

IGNEOUS AND TECTONIC PROVINCES IN SOUTH AMERICA

547

The broad and somewhat irregular field of Southern Peru, Rolivia, and
northern Chile and Argentina is less positively tied to faults even though
the modern volcanoes seem to be. The Puna field could perhaps be
developed over a Basin and Range type of faulted terrane, judging from
the several rows of volcanic vents there.

Extending from Santiago southward for nearly 1000 miles is a depres-
sion that separates the Coast Ranges from the Andean Cordillera. This is
called El Valle Central, and is believed to be a complexly faulted graben.
The zone of active and dormant stratovolcanoes is almost exactly com-
mensurate in length with the depression, and in the central and southern
part the volcanoes follow closely the eastern side or are within the
graben. At the north end they occur about 60 miles to the east of the
graben.

The fault zones do not bear the same relation everywhere to older
tectonic units. In Ecuador the graben occurs between the batholithic
belt and the older anticlinorium. In southern Peru the fault zone is mostly
within the batholithic belt or along its east side, and in the Puna de Ata-
cama it is developed on the nonintruded eugeosynclinal strata.

The great, tilted, fault blocks that comprise the Pampean Ranges make
up a region free of volcanic rocks, and conversely, the volcanic fields of
the Argentina foreland are evidently not related to faulting.

El Valle Central is almost entirely in the batholithic zone, but prefers
the eastern side at the north end.

PARANA BASIN BASALT FIELD

The Parana basin is one of Paleozoic and Mesozoic age, developed by
subsidence of a large, approximately oval-shaped region in the Precam-
brian Brazilian shield. The known Paleozoic section consisting of strata
representing all periods except the Mississippian is at least 10,500 feet
thick, The basin is about 1200 miles long and 400 miles wide ( see map,
Fig. 34.1). Desert conditions prevailed in mid- or early Late Triassic time
and a windblown sand deposit was spread around irregularly. Then came
the eruption of great floods of basalt. Between sheet flows in places more
desert sand accumulated.

In southern Brazil these eruptive rocks are generally at least 400 in thick
and are locally as much as 800 m. In Sao Paulo, north of Parana and Rio
Grande do Sul, the flows are locally separated by lenticular layers of cross-
bedded eolian sandstone, some of which reach a thickness of 40 m. A charac-
teristic of these extrusives is the general absence of olivine. Some lava flows
are amygdaloidal, and these alternate with flows in which an irregularly de-
veloped columnar structure occurs. Pyroclastic rocks seem to be absent; the
extrusion was of the calm type of fissure eruption.

Many feeding dikes and associated sills cut the underlying formations. Al-
most all the dikes are vertical. Most of the faults that cut the underlying
formations also have steep dips. A number of fault planes, including some
along which there was movement of 50 m or more, are filled by dikes of
diabase. One of these, cut by the Santa Clara-Urubici highway on the top
of the Serra do Panelao, Municipality of Bom Retiro, Santa Catarina, is a fault
that vertically displaced the Botucatu sandstone (and apparently the basal
part of the overlying eruptives) about 95 m. The fault is occupied by a diabase
dike more than 300 m. thick (Avelino, 1956).

It may be calculated from the above figures and map extension of
the field that about 75,000 cubic miles of basalt were extruded in fissure
flows.

It is interesting to note that the Karroo system of sedimentary rocks
in South Africa was invaded in Jurassic times by a multitude of diabase
dikes and sills which crop out intermittently over an area of 1,500.000
square miles or about five times the area of the Parana basalts. The vol-
ume has not been figured, but must be as much or more than that of the
Parana basin.

One half of the large island of Tasmania was once covered bv diabase
sheets which totaled at least 30,000 cubic miles. The Columbia River field
contains about 40,000 cubic miles of basalt, also. All of these dike, sheet,
and fissure-flow diabase and basalt fields are of the tholeiitic type.

Classification of Igneous Provinces

The Parana basin field is clearly a tholeiitic basalt province, and it is
evident that large volumes of primary tholeiitic basalt magma were
generated and rose to the surface without differentiation. The origin of
such a magma is a controversial question (Turner and Verhoogen, 1951),
but it is generally agreed the source was below the silicic crust. The
problem will be taken up later.

548

STRUCTURAL GEOLOGY OF NORTH AMERICA

The voluminous volcanic rocks that accumulated prior to the batho-
lithic intrusions with their abundant andesites and keratophyres are clearly
of the eugeosynclinal andesite province, according to the writer's classifica-
tion.

All the post-batholithic volcanic rocks that occur within the orogenic belt
of the Andes are of the andesite orogenic belt province and are very similar
to the eugeosynclinal rocks except that they lack the spilites and kerato-
phyres. The rows of great stratovolcanoes which are very late in the
general Cenozoic volcanic sequence are conspicuous for their alignment
and dominant central vent character, but in terms of composition are
must like the orogenic belt andesites with which they are closely asso-
ciated. The melting of downward extended roots of mountains in the

orogenic belts has been visualized as the source of the large volumes
of andesite, but since large volumes of basalt, both olivine and tholeiitic,
are also erupted in the orogenic belt with the andesite, we must provide
for the rise into the granitic crust of such magmas from the subcrust. The
basalt is generally more prominent in the late volcanic stages than as
alternating extrusions with the andesite, and this fact should be kept in
mind. Also it should be noted that by theory the roots of orogenic belts
are thought to melt to form granodiorite and granite in great volume for
the batholithic cycle, and on the other hand, some petrologists have
postulated that roots melt in volume to provide the magma for the ande-
sitic extrusions. Since the composition of granodiorite is considerably
different, the same conditions, exactly, cannot exist for both.

35.

Volcanism broke out on the west in Sonora with thick accumulations
grading into the miogeosynclinal types on the east. The extent of vol-
canism is not well known, but altogether during Cretaceous time the
deposits probably extended to the Pacific across what is now Baja Cali-
fornia. ( See Chapters 18 and 30. ) Intense deformation of the geanticlinal
area also occurred especially in the Early Cretaceous along the northern
part, and coarse conglomerates were derived from the uplifted region, so
we cannot characterize the area west of the miogeosyncline entirely as
eugeosynclinal. Parts of it probably were eugeosynclinal, however, as indi-
cated by the San Fernando formation of the northern part of Baja Cali-
fornia. The eruptives are said to be andesite flows, tuffs, and agglomerates.
The extent of the volcanic area and geanticline is shown by the legend.
pre-batholithic volcanic rocks, on the map of Fig. 35.1.

IGNEOUS AND TECTONIC
PROVINCES IN MEXICO

3EOSYNCLINE

Very little is known of Mexico in Paleozoic time. In fact, it is not until
Late Jurassic that much can be said of paleotectonic conditions when the
Mexican geosyncline (Plate 10) began to form. It occupied central Mexico
ind extended longitudinally from Arizona to Mexico City (see Fig. 35.1).
[t is presumed to have been flanked on the north, west, and south by land
ireas, with the western land known as the Occidental geanticline. Up to
5000 feet of sediments accumulated in it, in large part an evaporite se-
quence. During Early Cretaceous time the geosyncline sank in places
12,000 feet to receive additional sediments of the miogeosynclinal type.

BATHOLITHIC BELT OF THE FIRST CYCLE

The Nevadan orogenic belt with its great granodioritic batholiths de-
veloped in the region of Baja California. This was the western margin of
the eugeosynclinal and geanticlinal belt. The Lower and Middle Creta-
ceous sediments were folded and invaded by the batholiths and deeply
eroded before the Upper Cretaceous sediments were deposited. The
plutons are of immense size but have only been studied in northwestern
Baja California, where they are typically quartz diorite. Reconnaissance
reports generally refer to "granite." The metamorphic rocks have already
been described in Chapter 30, and the belt may be summarized as typical
of the Sierra Nevada in California and a continuation of it.

POST-BATHOLITHIC VOLCANISM

Minor disturbances and general uplift of Baja California, the Gulf of
California, and adjacent Sonora followed, leaving a broad land area
in this region. New volcanic outpourings occurred in the region of
parallel ranges and valleys which are the foothills to the lofty es-
carpments of the Sierra Madre Occidental and in the Sierra Madre Occi-
dental itself. These are the volcanic rocks that build the extensive Sierra

549

>@ ACTIVE AND DOR
MANT VOLCANOES

LATE CENOZOIC
VOLCANIC ROCKS

MID -CENOZOIC
VOLCANIC ROCKS

INTRUSIVE ROCKS
MID-TERTIARY IN SONORA
LATE CRET (?) IN S M. DEL SUR
EARLY TERTIARY IN COASTAL PLAIN

FOLD AND THRUST
BELT (LARAMIDE)

PRE-BATHOLITHIC
VOLCANIC ROCKS
(CHIEFLY EARLY CRET.)

NEVADAN BATHOL1THIC
AND METAMORPHIC BELT

PALEOZOIC METAMORPHIC
ROCKS AND ASSOCIATED
INTRUSIVES

NORMAL FAULTS

<<<<<<<<

(<<<<< t

/\/\/w\/

\/\/\/\/\

i S* 5 i S

MILES

Fig. 35.1. Major tectonic and igneous units of Mexico. See Fig. 43.3 for active and dormant volcanoes.

IGNEOUS AND TECTONIC PROVINCES IN MEXICO

551

Madre Occidental proper (see Fig. 34.5, section E-E'-E"). They are
thought to be early Tertiary by King (1939) but the new geologic map
of Mexico (1956) indicates them "principally as andesites of the Oligo-
cene and rhyolites of the Miocene with corresponding pyroclastics." King
also indicates that the Tertiary volcanic rocks are more acidic and more
varied than the older beveled Cretaceous volcanic rocks upon which they
rest in places in central Sonora. They contain a considerable thickness of
rhyolite and some flows of basalt. In northeastern Sonora, Imlay ( 1939 )
notes that the lavas aggregate more than 5000 feet in thickness, and basalt
predominates toward the top of the deposits but rhyolite and andesite
are the most common. Basalt appears more abundant than in the region
studied by King. Howell Williams (personal communication) recognizes
large sheets of welded tuffs and thinks that these may be very extensive
in the Sierra Madre Occidental. Much of the volcanic rock, heretofore
called flows, at the north end of the Sierra Madre Occidental are welded
rhyolitic tuffs (Enlows, 1955). The volcanic flows appear to be the result
of fissure eruptions (King, 1939), but tuffs and pyroclastics indicate the
occurrence of central vent eruptions also. The accumulations are thickest
in the eastern Sierra Madre Occidental.

BATHOLITHIC BELT OF THE SECOND CYCLE

Along the western margin of the Sierra Madre Occidental, particularly
in the region of parallel ranges of west-central Sonora, a mid-Tertiary
|( post-volcanic) orogeny occurred, and the volcanic and older rocks were
folded in a measure exceeding the previous Laramide folding there. Ac-
companying the folding were vast intrusions of granite, diorite, and
granodiorite which ascended through the Paleozoic and Mesozoic strata
and in places penetrated the Oligocene and Miocene volcanic rocks.
Granites predominate (King, 1939). These are the black areas on the
map of Fig. 35.1 along the western margin of the Sierra Madre. In the
Sonoran Desert geomorphic province the granites are carved to broad
pediments, and the plutons are so extensive there that one may infer that
the whole region is underlain by a vast batholith or series of large related
plutons.

METAMORPHIC AND INTRUSIVE BELT

Extending across southern Mexico from Banderas Bay to the Isthmus
of Tehuantepec is a belt of metamorphic rocks and various intrusive
bodies. In width the belt extends from the coast to the Tertiary volcanic
rocks of the Mesa Central which cover it irregularly on the north (Chap-
ter 43).

Considerable parts of the belt shown on the map of Fig. 35.1 are covered
with Jurassic and Cretaceous strata as well as fields of volcanic rocks
whose age is not well known.

Although very little can be learned about the belt of orogeny, it seems
evident that a pre-Jurassic and probable late Paleozoic age for most of it
must be recognized. The Sierra Madre del Sur with its numerous post-
metamorphic intrusions is regarded as a continuation of Baja California
and therefore, of the Nevadan belt.

RELATION TO DEPRESSED BELTS

The Gulf of California is regarded as a depressed area along a zone of
faults (Shepard, 1950). The faults in places have displacements com-
parable to those along the east side of the Sierra Nevada, and if the slope
of their submarine escarpments has not been reduced since faulting, then
the fault planes dip at rather low angles, which seems unusual. It is also
observed that the San Andreas fault system extends through southern
California to the head of the Gulf of California, and thence continues
southward as the fault zone of the depressed Gulf area. Since the San
Andreas and related fractures are generally recognized as a system of
strike slip or wrench faults (Hill and Dibblee, 1953), a conflict in interpre-
tation of the nature of faulting is evident. It is postulated in Fig. 31.22
that the block of Baja California has moved northwestward about 300
miles along the San Andreas fault zone and in so doing, has pulled away
from the mainland somewhat, leaving the Gulf of California floored with
oceanic crust. There can be no doubt, however, that the Gulf is a zone
of subsidence in late Tertiary and Quaternary time. Anderson (1950)
observed the faulting on islands in the Gulf to have extended from

552

STRUCTURAL GEOLOGY OF NORTH AMERICA

Pliocene to Recent, and the zone is known to be one of modern seismic
activity.

Along the adjacent western margin of Sonora, particularly in the
province of parallel ranges and valleys, are fanglomerates with basal
basalt flows and agglomerates of late Pliocene and perhaps younger age,
the Baucarit formation (King, 1939). These have accumulated in down-
faulted intermontane depressions. The basalt flows are generally conspicu-
ous on the back slopes of tilted blocks where the overlying fanglomerates
have been eroded away. The early or mid-Tertiary eruptives of the
Sierra Madre Occidental are generally less basic.

Renewed orogenic activity resulted in overthrusting of rocks of each
of the ranges west of the Sierra Madre westward over the Baucarit beds.
This was observed north of the 28th parallel ( Guaymas ) , but south of the
parallel the faults are normal (King, 1939). In addition the western
Sonoran normal and reverse faults are of the same age approximately as
the Gulf faults and, hence, evidently belong to the same system. The
reverse faults may be gravity slide phenomena. We have to deal, then,
with a complex fault zone 150 miles wide in which submergence of the
Gulf area relative to uplift of Baja California and the Sierra Madre was
of the order of 10,000 feet.

Accompanying the faulting was the eruption of a large volcanic field on
the southern part of the peninsula of Baja California. The accumulation
is known as the Comondu formation which is made up of "many kinds" of
volcanic rocks. The volcanism occurred possibly in Miocene time, but
stratigraphically the flows seem related to the Baucarit formation of west-
ern Sonora of late Pliocene age. Comondu rocks may have been deposited
near sea level and now are at elevations of 1000 to 5000 feet, which
means adjustment of this order of magnitude along the great fault zone
in Pleistocene time ( See Chapter 30 ) .

Here, in western Mexico, the downfaulted belt has developed along the

continental side of the batholithic (Nevadan) belt, and evidently on the
Cretaceous eugeosynclinal volcanic belt. This is a normal relation in
reference to the Andean depressed belts. If the Paleozoic metamorphic
belt exists here, it is mostly under the depressed area and covered. The
second cycle batholithic belt is partly involved in the faulting, but mostly
it is along the east margin of the fault zone. The great early and mid-
Tertiary volcanic field of the Sierra Madre Occidental is east of the fault
zone and suffered uplift at the time. Within the fault zone and on the
west, on top of the first cycle batholithic belt, volcanism was recurrent.
The field is of great extent in the southern part of the peninsula. Vol-
canoes were active during the Pleistocene and have continued active to ;
the present. Isla Tortuga is a very young volcano in the Gulf, and Las
Tres Virgenes are said to have been active in historic times. Isla Coro
nada is a Pleistocene andesitic volcano. Many cones and flows on the
western slopes of Baja California exhibit features of recency (Beal,
1948).

The tectonic and petrologic relations of Baja California, the Gulf, and
adjacent Sonora are similar to those of the Andes, but south of the Gulf
the relations are less familiar. The Nevadan batholithic belt seems to be
continued by the Paleozoic metamorphic belt and Mid-Cretaceous intru-
sions. North of the Paleozoic metamorphic belt is the southern termina-
tion of the great early and mid-Tertiary volcanic field of the Sierra Madre
Occidental, and on top of these post-batholithic volcanoes and on the meta-
morphic rocks are great new piles of late Tertiary and Quaternary vol-
canics. According to Andean precedent these stratovolcanoes should be
accompanied by a fault zone. The southern limit of the Mexican Plateau
is said to be marked by a high fault scarp, but its position is not evident
on the new geologic map of Mexico. The Balsas basin province may be due!
to downfaulting, but the writer has not been able to learn anything of the
fault relations there.

cut the lavas and tuffs of the Excelsior formation consist of much altered
basic and also silicic porphyritic types. They are probably contemporane-
ous with the extrusions (Muller and Ferguson, 1936).

Following marine invasions and sharp folding and thrusting more thick
volcanic deposits occur, which are of Jurassic age. These rocks were ex-
truded during continued crustal unrest, and petrographically cannot be
distinguished from the Triassic Excelsior volcanics. For further details
refer to Chapter 6 and 17. Also examine map, Fig. 36.1 (symbol, pre-
batholithic volcanic rocks) for distribution.

IGNEOUS PROVINCES IN
WESTERN UNITED STATES

UGEOSYNCLINAL PROVINCE

The region west of the Antler orogenic belt in Nevada and California
jvas one of considerable volcanic activity in middle and late Paleozoic
ime, especially in the Permian, and a thick assemblage of strata accumu-
ited typical of the eugeosyncline. Volcanism persisted into the Mesozoic,
nd in the mid-Triassic 12,000 feet of strata, chiefly pyroclastics and lavas,
ccumulated to form the Excelsior formation. The rocks range in com-
'osition from andesite through quartz latite to rhyolite with andesite
robably predominating. Keratophyres with secondary albite have been
lentified but probably have limited distribution. Certain intrusions which

BATHOLITHIC PROVINCE

Repeated Paleozoic, Triassic, and Jurassic orogeny occurred in the
eugeosynclinal province before the deformed complex was invaded by
the great batholiths. See Figs. 17.2 and 17.7. It has been pointed out that
the Calaveras formation ( Mississippian ) is more metamoqmosed than the
Mariposa (Jurassic) in places, but it is clear that the Mariposa was
sharply folded and thrust-faulted before the granodiorite intrusions. This
has been regarded as a climactic orogeny immediately preceding the
intrusions, but in the Sierra Madre del Sur of Mexico and in the South
American Andes such an orogeny is either not evident or was of milder
intensity, and the rocks into which the batholiths were emplaced are
believed to be Paleozoic strata deformed and metamorphosed in late
Paleozoic time.

The Sierra Nevada plutonic mass is a composite of many separate
intrusions each of batholithic size. In the area of Yosemite National Turk
the individual batholiths made their ascent at about 2-million-year inter-
vals over a period of 18 million years (Evernden et a!., 1957). The
process of intrusion took place during Albian and Cenomanian time of
the middle Cretaceous. Much of the rock is of forceful intrusive nature
but considerable stoping, migmatization, and contamination of the
primary magma occurred in places.

As shown in the cross section E-E' of Fig. 34.1 the batholithic belt in
central Baja California is about 175 miles (260 kilometers) wide, and in
the California-Nevada region it has about the same width, if the satellite

553

IGNEOUS PROVINCES IN WESTERN UNITED STATES

000

plutons in western Nevada are included. The pre-Franciscan meta-
morphosed sedimentary and igneous rocks exposed in the Coast Ranges
of California seem to belong to a metamorphic belt such as was intruded
by the batholiths in Chile, and may be west of the true batholithic belt.

In Oregon, Washington, Idaho, and southern British Columbia the belt
is immensely wide — more so than at any other place. It has been pointed
out in Chapter 17 that this region marks the intersection of two great
arcuate segments of the Cordillera of western North America. The
maximum width measured from the Cascade Range to the east side of the
Idaho batholith is over 400 miles (650 kilometers). Farther north in
southeastern Alaska and adjacent British Columbia it is about 300 miles
wide, depending upon interpretations. By way of comparison, the
Andean batholithic belt ranges from 40 to 70 miles in width.

In composition the great bulk of the Sierra Nevada batholith ranges
from granodiorite to granite, with granodiorite indicated by some as the
most voluminous, but quartz monzonite by others. See Chapter 17.
Tonalite is said to be the dominant batholithic rock of southern Cali-
fornia.

POST-BATHOLITHIC PROVINCES OF THE BATHOLITHIC BELT

Cascade Volcanic Complex

Divisions. The Cascade Range is a post-batholithic volcanic complex
in Oregon and southern Washington (see map, Fig. 36.1), but in northern
[Washington and its continuation as the Coast Range of British Columbia
■lit consists of the Nevadan complex. The central and southern volcanic
,part may be classed as an andesite orogenic belt province, and is divisible
iinto the Western and the High Cascades.

Extrusive Rocks. According to Williams ( 1957) the Western Cascades:

<L . . consists of gently folded volcanic rocks ranging in age from late Eocene
*!to late Miocene. Most of the topography here is mature and there are no
traces of original volcanic forms. The High Cascades, on the other hand,
jconsist of younger volcanic rocks that are virtually undeformed; most of the
topography there is constructional and the original forms of the volcanoes, even
though modified by glaciation, are easy to visualize. Other important contrasts
distinguish the two belts. The thick volcanic accumulations of the Western
Cascades are mainly products of fissure eruptions that produced extensive

plateaus. Hence there are few eroded plugs marking the conduits of large
volcanoes; instead, eruptive fissures are marked by narrow dikes of irregular
trend. The High Cascades, on the contrary, were built almost wholly by
eruptions from central craters so that clusters of large, coalescing cones were
formed, many of which have been dissected by glaciers so as to reveal their
feeding pipes. Finally, whereas the High Cascade volcanoes grew almost
entirely by effusions of basalt and basaltic andesite, the rocks of the Western
Cascades were produced by much more varied eruptions. Moreover these older
rocks range in composition from rhvolite to basalt, and the lavas are inter-
calated with heterogeneous sheets of explosion debris, ranging from coarse
agglomerates to fine tuffs, as well as with layers of tuffaceous sediment.

The Western Cascade belt averages approximately 50 miles in width, and
the volcanic rocks are as much as 13,000 feet thick. Beneath the High Cascades,
these rocks must interfinger with equivalents of the Clarno, John Day, Colum-
bia River, and Mascall formations, which are exposed on the plateau to the
east.

The High Cascade volcanoes probably began to erupt about the beginning
of the Pliocene epoch, and almost all of them were broad shield volcanoes built
by quiet outpourings of gray olivine basalt and subordinate flows of oliviue-
bearing basaltic andesite. Explosive activity contributed little to their growth
until the final stages when the summit craters of many shields were capped by
steeper cones of fragmental ejecta. Glacial erosion has modified the shapes of
all these volcanoes; indeed, most of them have been reduced to radiating ridges
separated bv glacial cirques. The parasitic cones on their flanks have been all
but demolished. The fragmental cones on their summits have been denuded
until the more resistant fillings of their central pipes have been left standing
as gigantic monoliths, like miniature Matterhorns.

The earliest High Cascade lavas were erupted from a north-south chain of
volcanoes close to the present edge of the Western Cascades. It seems more-
over, that these volcanoes lay on or near the base of an eastward-facing erosion
scarp cut in the rocks of the Western Cascade sequence. In places, this buried
scarp was between 1,500 and 3,000 feet high, and where it was steepest and
straightest it was almost certainlv the result of faulting. As the volcanoes gained
in height and the crest of the scarp was lowered by erosion, more and more
of the High Cascade lavas were able to flow westward, inundating the scarp
and spreading beyond on to a surface of low to moderate relief cut in the older
volcanic rocks.

The bulk of the High Cascades, as noted already, consists of Pliocene and
Pleistocene olivine basalts and olivine-bearing basaltic andesites erupted from
flattish shield volcanoes, and in places discharge of similar lavas continued until
very recent times. But during the Pleistocene epoch several large, steep-sided,
composite cones of andesite and dacite were built either on the tops of the older
shields or in the depressions between them. The South Sister, for example is
made up of three parts. Its lower part is an eroded basaltic shield volcano
capped by a steeper cone composed of andesitic and dacite lavas, whereas its

556

STRUCTURAL GEOLOGY OF NORTH AMERICA

upper part is composed of two Recent lava-scoaria cones of olivine basalt, the
younger of which has a well-preserved crater that may have been active
during the present millenium.

The largest Pleistocene andesite-dacite volcano was undoubtedly Mount
Mazama, the ancestral mountain in the collapsed summit of which lies Crater
Lake. This volcano, and its parasitic cone, Mount Scott, grew to full height by
eruption of pyroxene andesites; then, in late Pleistocene time, more siliceous
andesites and dacites were discharged from vents on a semicircular fissure on
the northern slopes of the volcano, while a cluster of dacite domes rose near
its eastern base and many basaltic cinder cones were formed elsewhere on
the mountainsides.

During Pleistocene time, long flows of massive, pale-gray olivine basalt
poured down the ancestral canyons of several of the principal rivers that now
traverse the Western Cascades, such as the North Santiam, North Umpqua,
and Rogue rivers, and the North Fork of the Willamette River. These flows
did not issue from the central vents of the High Cascade volcanoes, but from
fissures near the feet of these volcanoes and others farther west. They accumu-
lated to a thickness of 1,600 feet in the ancestral canyon of the North Santiam,
to about 1,000 feet in the North Umpqua, and to lesser thicknesses in other
canyons. No doubt their eruption took place intermittently over a long span of
time.

The principal eruptions of Pliocene and early Pleistocene time were
from vents close to the crest of the range, but later eruptions are numer-
ous on the eastern flank and on the adjacent plateau farther east. One
of the most impressive recent lava fields is around and north of Relknap
and Little Relknap crater. A line of cinder cones in the northern part of
this field betrays rise of magma along a fissure. Another recent field
stretches from Rachelor Rutte through Sheridan Mountain to Lookout
Mountain. More than 15 cinder cones and lava-scoria cones lie along a
fissure system here.

A third large recent volcanic field is that around Newberry Crater
(N, Fig. 36.1) which is 40 miles east of the crest of the High Cascades.
According to Williams (1957) again:

The Newbury volcano is an approximately circular shield volcano about 20
miles in basal diameter which rises 4,000 feet above the surrounding plateau
(Williams, 1935). On top there is a caldera, 5 miles long and 4 miles wide.
The oldest visible lavas of the volcano are rhyolites exposed on the walls of
the caldera. The rhyolites are overlain by basaltic flows and fragmental ejecta
and by subordinate flows of andesite, and these in turn are capped by rhyolite

flows that aggregate 1,000 feet in thickness, forming Paulina Peak. Presumably
the volcano grew to its full height during the Pleistocene epoch; then its summi
collapsed along ring fractures, probably in consequence of drainage of tha
underlying reservoir either by subterranean migration of magma or, more likely
by copious eruptions of basalt from flank fissures. Thereafter eruptions tool-
place within and outside the caldera. No basaltic flows and only a few basaltic
cinder cones occur within the caldera, where most of the eruptions involved
discharge of rhyolite. Outside the caldera on the flanks of the Newberry shield
no less than 150 basaltic cinder cones were built and innumerable basaltic flow;
issued from them.

The row of stratovolcanoes of the High Cascades is continued north-,
ward by Glacier Peak ( G. P. ) and Mt. Raker ( R ) which are cones built
on the Nevadan batholithic complex and isolated from the main volcaniq
complex of the Cascades. Even farther north in Rritish Columbia 40 to 123,
miles north of the city of Vancouver other Pleistocene volcanic cones!i
occur. Mount Garibaldi (G on map, Fig. 37.1) has recently been described]
by Mathews ( 1958 ) . There about 6 cubic miles of lava and pyroclastics
have been erupted in good part during the Wisconsin stage of the Pleisto-
cene. The extrusives are basalt and dacite; the dacite is most voluminous,,
Andesite in minor amounts is noted. Proceeding still farther north otheiy
volcanic mountains occur which are Mt. Clayley, Meager Mountain, and
an unnamed one at 51°00'N. Lat.

These cones give the stratovolcanic row a length from Mount Shasta
on the south to Meager Mountain on the north of 750 miles. The nexj
known volcanic cone northward is Mt. Hoodoo, 400 miles from Meagei|
Mountain, but it is possible that other volcanic cones occur between;
which have not yet been discovered. The rows of stratovolcanoes of tha
South American Andes range from 650 to 900 miles long, and henceftl
are of the same order of magnitude as the Cascades volcanic row.

The volcanic rocks of the older Western Cascades are classed as
tholeiitic by Waters ( 1955 ) , and he describes them as pyroxene andesites
and basaltic andesite constituting about 75 percent of the total and tho-
leiitic basalt, hypersthene basalt, and dacite pumice accounting for most
of the rest. Some olivine basalt and rhyolte are also present.

Many of the lava flows and pyroclastic rocks contain abundant

IGNEOUS PROVINCES IN WESTERN UNITED STATES

OOi

xenoliths. Most are fragments of graywacke, silty argillites, greenstones,
and basalts. Some show little change, others have been coarsely re-
crystallized and complexly modified by the enclosing magma. The
abundance of inclusions in the andesites, and their near absence from the
Eocene and Miocene basalts are noteworthy (Waters, 1955).

Much olivine basalt was erupted in the main growth of the Pliocene-
early Pleistocene shield volcanoes and also in the late Pleistocene and
Recent fissure eruptions. It is therefore evident that tholeiitic and olivine
basalt kindreds are in close association and that magmas resulting from
certain amounts of assimilation and subsequent fractional crystallization
also played a role. After the volcanic rocks of the adjacent Coast Ranges
of Oregon and Washington have been discussed, the origin of this com-
plex suite will be considered.

Intrusive Rocks. Refore leaving the extrusive rocks of the Cascade
Range an intrusive group must be mentioned. According to Waters
(1955):

Numerous stocks and small batholiths of granodiorite and diorite cut the

volcanic rocks. The largest is the Snoqualmie batholith, a composite mass of

pyroxene quartz diorite, hornblende granodiorite, and granophyric quartz

j monzonite about 20 miles in diameter. The stocks occur in a linear belt along

the core of the range [map, Fig. 36.1].

Most of these intrusives are rather mafic augite— hornblende granodiorites
and quartz diorites. Phenocrysts of plagioclase commonly show complex oscilla-
tory zoning similar to that in the andesites. In places the Snoqualmie granodiorite
lis chilled against the enclosing andesites, but elsewhere the andesite is coarsely
recrystallized at the contact and intimately penetrated by granodiorite. Miaro-
litic cavities are common. Parts of the granodiorite are altered; ferromagnesian
JJminerals are decomposed to chlorite, the rock is cut by stringers of quartz and
epidote, plagioclase is saussuritized, and albite, quartz, and epidote form ir-
regular impregnations and replacements. These features suggest solidification
under only a thin cover.

The plutonic activity is not closely dated. The Snoqualmie batholith invades
die Guye formation which contains fossil plants originally thought to be
Miocene (Smith and Calkins, 1906) but now regarded as Eocene. The
batholith had been deroofed by erosion before the building of the Mount
Rainier stratovolcano whose basal lavas rest on grandiorites believed to be
judiers of the Snoqualmie mass. The Shellrock Mountain instrusion of the
Columbia River gorge cuts the Columbia River basalt and is overlain uncom-
ormably by Quaternary andesites.

According to the classification proposed in this chapter, the stocks
and batholiths of the Cascade Range are of the second cycle, whereas
the batholiths of the Nevadan belt are of the first cycle.

Ratholiths of two ages have recently been noted in the Vancouver area
(Mount Garibaldi map area) by Mathews. The older underlies most of
the area and is a heterogeneous assemblage of foliated and unfoliated
quartz diorites and diorites. It is overlain unconformably by mid-Upper
Cretaceous sedimentary rocks. The younger intrusive rocks consist of
two plutons, one of which is a quartz diorite and trondhjemite and the
other a quartz-rich granodiorite and quartz monzonite. Neither of the
younger batholiths are in contact with the Upper Cretaceous beds, but
they have escaped the extensive deformation which has tilted and
block-faulted the stratified rocks, and are therefore considered younger
than mid-Late Cretaceous. The potassium-argon age determination made
by Follinsbee et al. ( 1957) appears to have come from the older batholith,
for which an age of 105 m.y. is given. This is about Mid-Cretaceous and
is consistent with the age indicated by the overlying mid-Upper Creta-
ceous beds.

The younger batholiths may correlate with the Snoqualmie batholiths of
the Cascades of Washington, which according to Waters above, is post-
Eocene and possibly as young as Miocene.

Coast Ranges Spilite and Keratophyre Province

Oregon-Washington Field. According to the classification of petro-
graphic provinces proposed at the beginning of this chapter the Coast
Range spilite and keratophyre province belongs to the eugeosynclinal
class of "Andesite provinces." The western half of Oregon and Washing-
ton was a trough area of subsidence in which a great volume of volcanic
rocks accumulated in Eocene and early Oligocene time (see Chapter
29). Weaver (1945b) estimates that more lava is represented here than
the Columbia River basalt field, and Waters (1955) notes that more
than 60,000 square miles were covered by the flows, and that in the
northeastern Olympics the lavas are over 15,000 feet thick and in the
Oregon Coast Ranges in a number of sections are over 6000 feet thick.

558

STRUCTURAL GEOLOGY OF NORTH AMERICA

He estimates that the volume of Eocene basalt here is at least 40,000
cubic miles.

Petrographically the lavas are typical representatives of the tholeiitic magma
type (Kennedy, 1933). They are aphanitic rocks composed of monoclinic
pyroxene and labradorite set in a tachylitic base highly charged with magnetite
dust. Phenocrysts of augite or plagioclase appear in some flows, but the series
as a whole is characteristically nonporphyritic. Olivine is scarce or absent. Glass
commonly accounts for 20^10 per cent of the rock. Chlorophaeite is abundant
in some flows (Waters, 1955).

Waters points out that the flows at the bottom of a continuous sequence
several thousand feet thick are of the same composition as those at the
top or in the middle, and concludes that progressive differentiation had
not occurred in the deep-seated magma chamber during the process of
eruption. In contrast, the thick sills after emplacement show magmatic
differentiation, and commonly consist of granophyric gabbro grading
downward into feldspathic gabbro. The lower portions of the sills are
rhythmically banded with layers of pyroxene and feldspar.

The basalts have been described in part as spilites, and the albitization
in the Olympics has been pictured by Park ( 1944 ) as due to circulating
heated sea water through the pillowed lavas on the sea floor. Waters
( 1955) does not reject this theory but believes it is not the entire explana-
tion. He says:

Some dolerite sills, dikes, and subaerial flows are as thoroughly albitized as
the pillowed flows. Albite veins and albite overgrowths on detrital feldspars are
locally abundant in the graywackes and argillites that underlie the Olympic
flows. Furthermore, most lavas might be better described as ordinary green-
stones, zeolitized basalts, propylitized and saussuritized basalts, silicified basalts,
and chloritized basalts, instead of spilites.

The Eocene basalts are underlain by thousands of feet of graywackes,
argillites, and tuffaceous sediments. In the writer's opinion the alteration of the
lavas to "spilites" and greenstones, and the simultaneous albitization, silicifica-
tion, and chloritization of the underlying sediments and intrusive bodies have
been produced by water, alkalis, silica, and other easily removable constituents
stewed from the slowly metamorphosing root of geosynclinal sediments as it
was downbuckled to form a tectogene. Fluids expelled from this metamorphos-
ing root rose along zones of mechanical deformation altering the overlying
volcanics and sedimentary rocks. This is essentially the same conclusion reached
by Gilluly (1935) after an extensive review of the spilite-keratophyre problem.

California Field. Volcanic materials are observed in several of the
Tertiary formations of the Coast Ranges of California but by all odds
those of the Miocene are the most abundant, and are particularly well
known in the central and southern Coast Ranges. The volcanic rocks
are thickest in certain basins or around certain centers of volcanism, and
in the central Coast Ranges several thousand feet of rhyolite tuffs, augite
andesite, basalt, and olivine basalt flows occur in the San Luis Obispo—
Huasna basin. Thick sills of analcite diabase and numerous plugs of
andesite and rhyolite porphyries also occur.

In the southern end of the Santa Lucia Range there are rhyolite tuffs and
flows and sills, flows of olivine basalt, often having a well-developed pillow struc-
ture, and numerous plugs of rhyolite porphyry. Rhyolite ash, basaltic peperites,
flows of basalt and numerous sills of analcite diabase occur in the Santa Cruz
Mountains. Thin rhyolite ash, flows and breccias of basalt, and diabase sills
are present in the Berkeley Hills, but they are not thick. Basalt flows occur in
the Miocene of the Point Arena region. Aside from bentonized ash there are
few volcanics in the Miocene in the San Joaquin Valley but there are numerous
flows in the Cuyama Valley and the Carrizo Plain. There is abundant evidence
that the volcanics were largely submarine; the tuffs and ashy sediments are
often fossiliferous and the flows are generally interbedded with sediments con-
taining marine fossils. It is possible that in some instances the volcanics accumu-
lated so rapidly that local evanescent volcanic islands were built up, especially
in the immediate vicinity of vents.

No single description would fit all of the occurrences of Miocene volcanics
as the sequence and relative proportions of the various types vary somewhat.
However the usual sequence is rhyolite tuffs and flows, flows of andesite and
basalt, intrusions of sills and analcite and thomsonite diabase and intrusions of
plugs, sills and dikes of soda rhyolite and waning explosive activity.

The sills of analcite diabase are an important and widespread phase of the
Miocene volcanism. . . . Some of the thicker sills show gravitational differentia-
tion and vary from a picrite at the base to a highly feldspathic diabase at the
top. Most of them show chilled margins of analcite basalt, usually vesicular
(Taliaferro, 1943b).

In the southern Coast Ranges 2280 feet of Miocene volcanic rock is
exposed on San Miguel Island, 4700 feet on Santa Cruz Island, 8000 to '
10,000 feet in the western Santa Monica Mountains and Conejo Hills, and
at least 2000 feet in the area northeast of Glendora. Many wells have
penetrated the same volcanics in the subsurface. Shelton ( 1954) estimates I
an average thickness of 1000 feet over an area of 700 square miles for;

IGNEOUS PROVINCES IN WESTERN UNITED STATES

559

the volcanics of the southern Coast Ranges, and this would mean a volume
of approximately 140 cubic miles.

Breccias and tuff breccias are most common but massive flows and
intrusions are prominent in the Conejo Hills and Glendora areas. In the
Conejo volcanic assemblage hornblende andesites occur at the base, and
above these generally are breccias, tuffs, and flows of augite andesite.
The upper part consists of flows of hypersthene basalt and olivine basalt.
The basalts probably thicken southward in the subsurface. The intrusives
in the area are chiefly diabase and hypersthene diabase (Shelton, 1954,
1955).

The Glendora volcanics are largely andesites, but olivine basalt and
rhyolitic varieties are noted. In fourteen analyzed rocks the Si02 content
ranges from 47.23 to 75.50 percent, and the most common types contain
59 to 63 percent. Present knowledge of the province as a whole indicates
that andesites predominate among the extrusives, with basalt and dacitic
or rhyolitic rocks following in that order. The associated intrusive rocks
are dominantly basaltic or diabasic (Shelton, 1954).
Most of the volcanic rocks of this province are middle Miocene, but

i some may be slightly older. Shelton concludes that much of the lava
was poured out on the sea floor or from vents close enough so that ac-
cumulation took place under water. Source fissures or vents have not

i been recognized. The relation of volcanism to tectonism is striking in the

| Los Angeles Basin. According to Shelton (1955):

The Los Angeles basin is an area of locally derived Cenozoic sediments
II at least 25,000 feet thick, and as now exposed is a structural depression approx-
imately 60 miles long and 40 miles wide. The most pronounced cycle in its his-
jtory began in middle Miocene time and reached a climax of depth and localiza-
tion during the upper Miocene and Pliocene. The climax of Miocene volcanism
in southern California thus corresponds fairly closely with the beginning of the
period of maximum growth of the basin.

Basalt Fields of Eastern Oregon and Washington

The Blue Mountains are composed of central island-like masses of
Paleozoic and Mesozoic sedimentary rocks and intrusive masses with
flanking volcanic flows and tuffs. See Fig. 29.15. The north flank volcanics
ire older and consist at the base of the Clarno formation of late Eocene

(Duchesnean) and early Oligocene (Chadronian) age. It consists of a
thick sequence of rhyolite and basalt flows with interlayered breccias and
varicolored tufts. Local unconformities are noted. See cross sections of
Fig. 36.2. Overlying the Clarno is the John Day formation of late Oligo-
cene and early Miocene age. It consists of colorful tuffs which in places
may grade into acidic flows and breccias. Overlying the John Day is the
Columbia River basalt which is now restricted to flows of mid-Miocene
age. They are widespread in northern Oregon and southeastern Washing-
ton.

The section at Picture Gorge along the John Day River [D, Fig. 36.2] may
be considered as typical of this formation. Here it is situated between the John
Day formation and the Mascall formation. The basalt series appears to be
unconformable upon the John Day beds as shown by slight discordant relation-
ships over a wide area, but appears to be generally conformable with the
overlying Mascall formation.

The Columbia River basalt poured out upon an area of varied relief. The
basalt flows in places tend to be thick where they filled irregularities in the
surface. The basalt flows are usually more massive and less columnar than
flows high in the formation. Some flows contain appreciable amounts of
olivine and weather more rapidly than the dense basalt higher in the section.
Zeolites are particularly common in some of the basal flows, particularly in
the Monument and Ritter quadrangles.

The upper part of the Columbia River basalt characterized by "flow upon
flow" structure is by far the thicker and more widespread part of the formation.
Relatively parallel flows, commonly columnar, are visible for many miles
along the canyon walls of northeastern Oregon. The upper flows are character-
istically dark dense basalts with scoriaceous zones at the tops of each flow.
According to Waters (1955, p. 708) continuous sections of more than 5.000
feet of basalt are found in northeastern Oregon (Baldwin, 1959).

Waters also calculates that about 35,000 cubic miles of basalt are present
in the field.

The Mascall formation is largely made up of nearly white to buff
bedded tuffs. It is late Miocene in age.

Following Mascall deposition the Columbia River basalts were folded
and faulted near the Blue Mountains as shown in Fig. 29.15, and then
eroded. On the erosion surface in mid-Pliocene time the Rattlesnake
formation was spread. It consists of gravels, tuffs, and silts with a bed of
welded tuff in the upper part. Uplift and moderate folding took place

t-^^L- Q^Lf°' sp Pal,, Jrt Trs

JOHN DAY FAULT
QJSQtg Qal

Tcr

imTnTtmrrrrmmiiii jiiimimmiUUWPW^

.■■■iinni'iimr-rrrtl

Fig. 36.2. Sections showing relations of Columbia River basalt to other Tertiary formations and
to pre-Tertiary complex. A-A' and B-B' near Mitchell, Ore., on U. S. 26. C-C near John Day at
junction of U. S. 26 and 395. Reproduced from Wilkinson, 1959. D-D' is schematic of Picture
Gorge area, John Day River. Reproduced from Baldwin, 1959.

pal, metavolcanic and sedimentary rocks; sp, serpentine; pk, pre-Cretaceous rocks; Ksh, Cre-
taceous shale; Keg, Cretaceous conglomerate; Tel, Clarino lavas; Tci, Clarno intrusives; Tjd, John
Day formation; Tcr, Columbia River basalt; Trs, Rattlesnake fm.; Trt, welded tuff.

IGNEOUS PROVINCES IN WESTERN UNITED STATES

561

after the deposition of the Rattlesnake formation. The Pleistocene in
central Oregon was mostly a time of erosion.

The Rlue Mountains are flanked on the south by Mio-Pliocene volcanics
of the Payette and Owyhee formations and correlative beds. The pre-
Columbia River basalt formations are missing along the southeast side
of the Blue Mountains and the Payette, oldest in the area, is correlated
with the late Miocene Mascall on the north side. The High Lava Plains
(Fig. 29.15) south of the Blue Mountains are made up of relatively
undeformed young lava flows dotted in places by cinder cones and lava
buttes. The formations are dominantly Pliocene lavas, tuffs, and alluvium,
few of which have been formally named ( Baldwin, 1959 ) .

Basalt Kindreds. H. A. Powers of the U.S. Geological Survey has com-
mented in a letter to the writer about the problem of basalt kindreds in
the northwestern states, and has charted the chemical analyses of about
65 characteristic basalts in regard to Si02 and MgO from the Columbia
Plateau, the Snake River downwarp, the Malheur Plateau and Hawaii.
He finds such a scatter of points that the concept of a clear-cut distinc-
tion of tholeiitic and olivine basalt seems to break down. The Columbia
River basalts of Miocene age run relatively high in Si02 and low in MgO;
the Hawaiian basalts classed as tholeiitic run slightly less in Si02 and
intermediate in MgO; Hawaiian rocks classed as olivine basalts are inter-
mediate to low in SiOo and low, intermediate and high in MgO; the
Snake River Pliocene and Recent basalts run generally low in SiOL> and
intermediate in MgO; the Steens Mountain basalts in the Malheur field
run intermediate to low in SiOL» and generally low in MgO. As a result
he says:

In some provinces, there is a decided gap, or absence of rocks showing all
the intermediate stages. In such provinces there appears to be an impressive

I difference between tholeiite and olivine basalt, in the chemical sense. My
feeling is that the concept of a fundamental distinction between two kindreds
of basalts has been developed from a concentration on such single provinces,
but that the concept breaks down and is not convincing when one considers

! all the basalts that we know about from good comparable chemical analyses.

I I have plotted in different ways about a thousand reasonably good analyses
of basalts trying to establish a natural division zone, and so far have succeeded
only in showing a complete gradation — a lot of crossbreeding if there are
really two kindred.

On the other hand, he believes that perhaps a difference can be made
between flood eruptions and cinder cone or small lava dome eruptions,
and that this may reflect fundamental differences in the tectonic setting.
Such a distinction is based on the field characteristics and not on the
chemical compositions. In the Columbia River basalt field flood basalts
predominate and are presumed to have issued from fissures. Most of the
Pleistocene basalts in the Columbian River field are fissure flows also,
but some seem to be of cinder cone activity (Powers, personal communi-
cation). The Snake River and Malheur fields, on the other hand, are
mostly of the cinder cone and small lava dome type.

Snake River Basalt Field. The eastern part of the Snake River lava
plain from King Hill and Twin Falls to Yellowstone Park, a distance
of 200 miles, has been studied in considerable detail by Stearns, et al.,
(1938). They report that about 95 percent of the rock of the depression
or downwarp is the so-called Snake River basalt of Pliocene, Pleistocene,
and Recent age. Locally sedimentary lenses, closely related petrologicallv
to the flows, exist, and some of these are very fossiliferous such as the
Hagerman lake beds. In numerous places on the borders of the plain
rhyolitic flows and pyroclastics emerge from beneath the basalts. Per-
haps the rhyolites are younger and stratigraphically above the Challis
volcanics on the north border which are dominantly latite and andesite.
The Challis volcanics are regarded from fossil leaf beds as late Oligocene
or early Miocene, and ages up to early Pliocene have been assigned to the
rhyolites. At places rhyolites crop out within the basalt plain under the
basalt, and hence it is believed that the rhyolite volcanics extend widely
under the field and form the basal layer (Kirkham, 1931).

The rhyolites have been loosely referred to as the Mount Bennett
rhyolite and Owyhee rhyolite, but much of the rock is quartz latite or
even possibly andesite similar to the Challis volcanics (Stearns et al.,
1938).

Three old cones are prominent landmarks in the area between Arco
and Blackfoot, and their building seems to predate the Snake River basalt.
Big Southern Butte, about 5 miles in diameter, rises nearly 2500 feet
above the plain and is composed of basaltic and rhyolitic flows. The
main mass is a light-colored porphyritic rock containing large quartz

562

STRUCTURAL GEOLOGY OF NORTH AMERICA

crystals, and has been identified megascopically as rhyolite. The cone is
much eroded.

East Butte is made up of beds of trachyte, pumice, and obsidian, which
strike east-west and dip 30 degrees south. No vestige of a crater remains,
and it is possible that the butte is part of a tilted fault block. The third
butte, known as West or Middle Butte, lies 4 miles away. It is composed
entirely of basalt which dips 10 degrees south. If East and West Buttes
are both parts of the same tilted fault block, then interlayered trachyte
and basalt must be postulated. Whether a fault block or separate cones,
they were deeply dissected by erosion before the Snake River basalts were
spread around them. A thin section of the basalt of West Butte shows
"abundant feldspar, olivine, and pyroxene, with a little brown glass."

A number of units in the Snake River plain are younger than the
rhyolites yet older than the basalts that cover most of the plain. They are
mostly basalts and associated lake beds. The extensive Pleistocene and
Recent basalts are said by Stearns to have come from about 400 vents
in the plain. He charted the position of about 300 of them. Except for the
cluster in the Craters of the Moon National Monument and the group
north of St. Anthony, they are rather evenly distributed and neither a rift
nor fault pattern is discernible, although here and there short rows of
cones occur.

Near the north side of the Snake River Plain cinder cones 50 to 200
feet high predominate. However, over most of the plain the vents are
broad lava domes each usually about 100 feet high and the related flows
covering about 30 square miles. Only a suggestion of a crater or crater
rim is left generally when eruption ceases. The lava welled out quietly
and profusely and each vent had only one period of activity. With
activity over in one vent another one nearby seems to have formed and
poured out considerable lava.

The geology of the western part of the Snake River volcanic field has
been summarized by Kirkham ( 1931 ) . He believes that the basal layer is
a Miocene basalt and that this is very widespread. He calls it the Colum-
bia River basalt, but describes it principally as an olivine basalt which
does not correlate with the tholeiitic basalts of the Columbia River basalt
field proper. This basal unit has been eroded irregularly and its existing

thickness in outcrop ranges from 300 feet to over 1200 feet. The basal
"Columbia River basalt" occurs in three stratigraphic parts, namely,
lower and upper basalt flow units and intermediate lake beds containing
much tuff, the Payette formation.

The Owyhee rhyolite, previously mentioned, rests on the basalt, at least
in the area of southwestern Idaho south of the Snake River. Kirkham
states that the rhyolite is actually a series, and is generally made up of
basalt and andesite flows at the bottom, and above by trachyte, latite,
and rhyolite flows interbedded with ash, fresh-water limestone, clay,
shale sandstone, and conglomerate layers. He correlates the series with
the Salt Lake formation south of the Snake River plain. The distribution
and stratigraphic and petrographic relations of the "Columbia River
basalt" and Owyhee "rhyolite" seem to need much more study before
the picture can be significantly summarized.

Above the Owyhee rhyolite and Salt Lake beds is the widespread
Snake River basalt, so characteristic of the eastern part of the field pre-
viously described. The Snake River basalt flows give way to and are
covered by lake beds in western Idaho which are known as the Idaho
formation (Kirkham, 1931), but here as in the eastern part of the plain,
the Quaternary history was eventful with repeated, if scattered, construc-
tional volcanic activity, struggling against the destructional activity of the
Snake River for supremacy (Norman Anderson, personal communication).

The Snake River volcanic field together with the Malheur and Colum-
bia field constitute a unique petrographic province from the tectonic
point of view. The western part of this great field covers the Nevadan
batholithic and orogenic complex, and the eastern arm lies across the
Laramide fold and thrust belt of the central Rockies (Chapter 22). We
are accustomed to a parallel arrangement of volcanic deposits with the
orogenic belt; even if discontinuous in extent, the volcanic fields do not
take a transcurrent direction. Here, however, the eastern arm of the
Snake River field extends almost at right angles over the underlying folds
and thrust sheets of southeastern Idaho and southwestern Montana.

Malde (1959) reports a great fault zone along the northern boundary
of the Snake River Plain in the area west of Boise. Gravity, seismic, and
geologic studies indicate that at least 9000 feet of aggregate throw has

IGNEOUS PROVINCES IN WESTERN UNITED STATES

563

displaced the Plain downward relative to the highlands on the north.
At least 5000 feet of movement occurred between the early and middle
Pliocene, and progressively diminishing movements amounting to 4000
have occurred since.

The crustal break implied by the gravity measurements is possibly expressed
by a line of earthquake epicenters that extends diagonally from Puget Sound,
across the Columbia River Plateau, along the northern boundary of the
western Snake River Plain, and thence across the plain to northern Utah. In
Idaho, these earthquakes originate principally at average depths of 61 and 38
km (38 and 24 mi), the shallower earthquakes being near the base of the
crust (6). The displacement calculated from the gravity measurements there-
fore ranges from one-tenth to one-third of the local crustal thickness (Malde,
1959).

The geology of northern Utah hardly permits the extension of the fault
zone into this region. The writer believes, rather that a more logical
projection is eastward under the Snake River volcanic field to Yellow-
stone Park. It is thus shown on Figs. 31.21 and 31.22, where its tectonic
significance is discussed. It is interpreted chiefly as a zone of distention,
and if so, seems to afford a natural channelway for the lavas from the base
of the silicic crust and from the basaltic subcrust. See Fig. 31.25. The
transcurrent nature of the Snake River volcanic field is thus better under-
stood. Also, the fissure effusion of great volumes of basalt from the sub-
i crust may be accounted for.

! PROVINCES OF THE MIOGEOSYNCLINE AND SHELF

^General Characteristics

The tectonic provinces of the Rocky Mountains stand apart from the
'(Pacific marginal provinces in several respects; their mountains, plateaus,
and basins were developed by late Mesozoic and Tertiary orogeny and
epeirogeny in the Paleozoic miogeosynclinal and shelf regions and also
ton the miogeosynclinal-type sediments of various Jurassic, Cretaceous,
and Tertiary basins. Thick late Precambrian sandstone and shale se-
quences underlie part of the Paleozoic miogeosyncline and shelf areas,
and in other areas, particularly in Colorado and Arizona only a very thin
sedimentary veneer existed on the crystalline rocks of the Precambrian

basement at the time of late Cretaceous and Tertiary orogeny. Such is the
general tectonic setting for the eventful and diversified igneous history
of the Rocky Mountains which began in Cretaceous time and continued
from place to place to the present.

The igneous rocks of the Rocky Mountains, like the sedimentary rocks
and structures, stand apart fairly distinctly from those of the eugeo-
synclinal and batholithic belt to the west; in particular they are generally
more alkalic. Rasalts and andesites are present and in places abundant,
and the orogenic type basalt-andesite-dacite-rhyolite association is promi-
nent, and therefore a similarity exists with this overwhelmingly pre-
ponderant kindred of extrusive types in the Pacific marginal regions. But
where present the intermediate latitic differentiates are most abundant in
contrast to the dominant andesites of the Pacific marginal belts. The
Rocky Mountains are characterized especially by the classical kindreds
of calc-alkalic olivine basalt-trachyte-phonolite and alkalic leucite basalt-
trachybasalt-trachyte. The nepheline syenites are intrusive accompani-
ments in places. For the fractional crystallization associations an olivine
basalt is generally considered the parent magma, but assimilation or
fusion of small or appreciable amounts of calcic or alkalic country rock
such as limestone, amphibolite, granite, or mica schist by the olivine
basalt magma is postulated, or at least admitted as possible, to produce
the melts from which the high calc-alkalic or alkalic fractional crystalliza-
tion kindred resulted.

Trachyte and Phonolite Provinces

Extent of Provinces. Igneous rocks containing a high amount of either
sodium or potassium or both are characteristic of large areas in the Rocky
Mountains.

Three high alkalic kindreds are generally recognized on a world-wide
basis, the leucite basalt-trachyte, the olivine basalt-phonolitc, and the
nepheline syenite (Turner and Verhoogen, 1951). The first two are classed
as nonorogenic assemblages and the last, which is, of course, an intrusive
type is regarded as a low-temperature, high alkalic residue of an evolu-
tionary series in which volatiles played an important role. The phonolites,
trachytes, and syenites appear as minor end members of an olivine basalt

564

STRUCTURAL GEOLOGY OF NORTH AMERICA

parentage. The writer has not found it possible to chart these three
kindreds in separate provinces in the Rocky Mountains, and therefore
does not try to distinguish them. They will be referred to collectively as
the trachyte and phonolite province, (see map, Fig. 36.3). Igneous rocks
adjacent to the region of alkali-rich igneous rocks in the Rocky Mountains
are generally more calcic or do not display an excess of alkaline elements
such as to yield die feldspathoid minerals, and are grouped in calc-
alkalic subprovinces.

Colorado Plateau. The chief igneous centers in the Colorado Plateau
which belong to the high alkalic subprovince are the laccolithic groups
(Henry, La Sal, Abajo, Ute, and Carrizo Mountains), the Navajo and
Hopi Ruttes volcanic fields of northeastern Arizona; and the San Rafael
Swell. The Elkhead Mountains, White River, Grand Mesa, and Rattle-
mount Mesa fields are also of alkalic affinities and are grouped in the
Colorado Plateau for convenience sake.

In the laccolithic groups (Hunt, 1954, 1956) the first intrusions are
diorite porphyry which constitutes about 60 percent of the total volume
of igneous rock. Intrusions of monzonite porphyry follow to the extent
of about 25 percent, and then last a syenite porphyry to the extent of
about 13 percent. The last intrusion is noted only in the La Sal Moun-
tains. The rocks are high in Na20, but the ratio of K..O to CaO -f Na20
increases eastward. The earliest intrusions in each group contain about
5 to 6 cubic miles of rock. These were stiff and relatively low-temperature
magmas. The central stocks probably breached the surface and erupted
more potassic rock than contained in the intrusions.

The magmas were intruded in basins, broad domes, and benches of
the Colorado Plateau. Olivine basalt is regarded as the primary magma
which assimilated amphibolite and hornblende gneiss to yield a potash-
rich magma which then differentiated (Waters, 1955).

The Hopi Ruttes (Williams, 1936) is a volcanic field of lava-capped
mesas and many necks. Ejecta consists of limburgite (dark, glass-rich and
usually minus feldspar) and monchiquite (nepheline basalt) in sedi-

Fig. 36.3. Igneous provinces of the western United States. The numbers relate to in-
trusions listed in the table on page 574.

IGNEOUS PROVINCES IN WESTERN UNITED STATES

565

mentary matrix. Lavas are analcite basalt. Feldspar is scarce or absent
and analcite abundant. MgO, CaO, and NaaO are high; K20 is low.

The Navajo volcanic field (Williams, 1936) consists principally of a
number of necks of tuff breccia and agglomerate crowded with frag-
ments of granitic rocks. These breccias and agglomerates are high in K20
in contrast to the Hopi Ruttes rocks, and fairly low in Na20 and fairly
high in MgO and CaO, and have been called sanadine-rich trachybasalts
and leucite basalts. Williams suggests that an originally sodic ultrabasic
magma having the composition of nepheline basalt reacted with the
potash feldspar of granites in the basement and so attained the high
potassic composition which prevails in the subprovince.

In the interior of the Plateau, in the laccolithic mountains, soda greatly
exceeds potash. The same is true in the Hopi Ruttes field along the
southern edge of the Colorado Plateau, but in the intervening Navajo
field potash greatly exceeds soda.

The Elkhead Mountains of northwestern Colorado constitute a high
alkalic volcanic field. The suite is unusual with rocks containing both
olivine and quartz, a nepheline-bearing trachyte with phenocrysts of yel-
low-brown mica in a groundmass of sanadine and nepheline, and analcite
basalt without feldspar, and with dikes of soda verite, analcite syenite,
and soda syenite (Carey, 1955).

Central Wyoming. Leucite Hills are located in south-central Wyoming
on the north end of the Rock Springs uplift. They are remnants of lava
flows and cinder cones on a mid-Tertiary erosion surface, now much dis-
sected and left about 800 feet above the present valley floors. The rock is
called Wyomingite, and contains phlogopite, leucite, and diopside ( Cross,
1897).

The Rattlesnake Hills field of central Wyoming consists of three large
necks and a number of small necks and related dikes in an area of 150
square miles. The first and largest intrusions and extrusions were viscous,
acid quartz latites. Following these a series of highly alkalic trachytes,
phonolites, and vogesites were erupted. (Vogesites are lamprophyres,
' generally considered to be hypabyssal. ) The alkalic rocks are unique for
their content of the relatively rare feldspathoidal minerals, huayne, and
nosean. Although the necks are in a rather small area, the amount of

material ejected was large and certain clastic parts are believed to have
been transported 100 miles from the volcanic center. The activity is dated
as mid-Eocene (Carey, 1954). Most of the immediate ejecta has since
been eroded away, but water-transported fragments are prominent in a
middle and upper Eocene formation of die general region (Van Houten,
1955).

Black Hills. Across the north end of the Rlack Hills uplift is a row of
imposing Tertiary volcanic necks and laccoliths in Mesozoic strata known
from west to east as Devils Tower, Rear Lodge Mountain, Rear Rutte,
Inyankara Mountain, and Mineral Hill. These are composed of phonolite,
pseudoleucite porphyry, nepheline syenite, and aegerite syenite (Robin-
son, 1956).

Several of the centers of Tertiary igneous activity are domal uplifts in
the Paleozoic and Mesozoic sedimentary rocks and the underlying cause
of doming is regarded by Noble et al. (1949) as due to the intrusion of
stocks rather than laccoliths. One of the domes includes the noted Home-
stake gold mining district at Lead. It is 10 by 12 miles in size and con-
tains several rather ragged Tertiary stocks and numerous sills and dikes.
The intrusive rocks have been described as phonolite porphyry, rhyolite,
and quartz porphyry ( O'Harra, 1933 ) .

The entire domal structure of the Rlack Hills, some 50 miles by 1(K)
miles, is considered possibly due to a major Tertiary batholithic intrusion
by Noble et al., but they see no way of finding evidence of the intrusion.
The gravity picture which might help is clouded by the dominance of
gravity lows over the adjacent Cretaceous and Tertiary basins.

Central Montana. North-central Montana is characterized by a num-
ber of mountain groups, each of which owes its existence to igneous
activity, both intrusive and extrusive. The region is east of the Laramide
belt of intense compression and the magmas have penetrated nearly
horizontal sedimentary strata.

The rocks range from rhyolites to basalts in one category and from
shonkinites through nepheline syenites to syenites in another. The rocks
of the latter category are rich in potash and soda and almost devoid of
plagioclase. The rocks of each mountain group fall into one or more
eruptive stages; and the rocks of each stage have peculiar mineral and

566

STRUCTURAL GEOLOGY OF NORTH AMERICA

chemical features, although they commonly range from highly mafic to
highly felsic. Each stage is separated from the other by intervals during
which few or no eruptions occurred, but instead, extensive erosion.

In each of the stages a rock near the mafic end is believed to represent the
primary magma. This rock ranges from an ordinary basalt to orthoclase basalt
to plagioclase shonkinite to shonkinite rich in potash and lacking plagioclase.
The gradational character of the eruptive stages and their close association in
time and space indicate a common origin. Two periods of magmatic differentia-
tion are required: first, a deep-seated differentiation of a basaltic magma
from which crystals of calcic plagioclase and hypersthene were removed and
second, a shallower differentiation to form the magmas of the individual erup-
tive stages. The relative flatness of the sedimentary rocks into which and
through which the magmas have moved indicates that the magmas have not
been disturbed by orogenic forces; therefore they could have differentiated
during the long quiet interval which seems necessary. The second period of
magmatic differentiation by crystal settiing was characterized, in most stages,
by assimilation of siliceous material. The amount of assimilated material
was especially large in the Crazy and Little Belt mountains, where syenites were
followed by granites (Larsen, 1940).

The abundant flows and dikes of mafic phonolite, and flat laccoliths
and dikes of chemically equivalent shonkinite are derivatives of basic
potassic magmas. Syenite is undoubtedly a differentiate of a parent
shonkinite magma after intrusion as a sill or laccolith (Turner and Ver-
hoogen, 1951). Larsen (1940) believes essentially that all petrographic
and chemical variations within this region may be explained in terms of
magmatic differentiation from an olivine basalt. A long perod of un-
disturbed differentiation in depth is required in which settling of olivine
and diopsidic augite takes place to leave the melt enriched in IC.O. Turner
and Verhoogen ( 1951 ) would place more emphasis on reactive assimila-
tion with the granitic basement.

Summary. The province of high alkalic rocks has the following charac-
teristics:

1. The region is one of crustal stability for the most part. It was a shelf
to the west-lying miogeosyncline and part of the interior stable region in
Paleozoic time. Triassic and Jurassic deposition was thin but Cretaceous
sediments accumulated in several separate intermontane basins to a
thickness of about 5000 feet. The total section of nearly flat-lying sedi-
mentary rocks did not exceed 10,000 feet in any place, and in some areas,

as in central Colorado, only a few hundred feet of sedimentary rocks
existed at the time of igneous activity.

2. The relatively thin veneer of sedimentary rocks rest directly on
metamorphosed crystalline rocks, generally of a gneissic or schistose
character. In the region of high alkalic rocks no Beltian type rocks are
known, except in west-central Montana on the border of the alkalic
province. This feature correlates well with the common observation of
granitic, gneissic, and amphibolitic inclusions in rocks of a number of the
igneous centers, and also with the conclusion that such crystalline rocks
have been assimilated in various amounts by an olivine basalt magma.
The inference is warranted that olivine basalt underlies the "granitic"
crust directly, that the primary activity begins in the basaltic layer or sub-
crust, then proceeds to the granite crust where assimilation takes place.
With stable crustal conditions prevailing, the various alkalic rocks origi-
nate through fractional crystallization, intrusion, and further differentia-
tion.

3. This is a region of high BaO and SrO and also of the most abundant
uranium ores so far discovered in the West. Such elements may have been
derived from the assimilated Precambrian crystalline rocks and later
concentrated by differentiation. The UsOs would be further concentrated
by meteoric or epithermal processes.

4. No basalt is found in the laccolithic groups, but these igneous cen-
ters stand apart from the others in having only small volumes of intruded
magma and relatively stiff cold magmas at the time of intrusion. In the
other fields, in fact in most all volcanic fields of any size, basalt is erupted
generally either early or late in the history of the field, and therefore we
must think of a facility whereby some basalt from the subcrust makes its
way directly to the surface without an intermediate rest stage for assimila-
tion or differentiation.

5. The Rockies of Montana, Wyoming, Colorado, New Mexico, and
Utah including the Colorado Plateau, are east of the fold belt of the cen-
tral Rockies and are the result primarily of large domal uplifts with
lateral gravity slide affects in places. See Fig. 25.12. The surficial igneous
centers in the trachyte-phonolite province occur in the basins, domes, and
across monoclinal flexures, and graben. If the domal uplifts are supported

IGNEOUS PROVINCES IN WESTERN UNITED STATES

567

by downward protuberances of the granitic crust or of the basaltic sub-
crust, and if these are melted and responsible for the location of the
igneous centers, such as is generally held to be the case in the Nevadan
orogenic belt, then the upward coursing magma must have worked later-
ally considerable distances to have found outlet in the interuplift sedi-
mentary basins.

The domal uplifts are structures caused by vertical forces, and hence it
is believed that roots could not have developed; roots are the result of
horizontal compression or crustal shortening. The conclusion seems evi-
dent that the domes are themselves the result of igneous activity; they
are great blisters above giant laccoliths or thick megasills in the "granitic"
layer. The original magma in the megasills is postulated to be olivine
basalt, which while still molten, assimilated variable amounts of the
crystalline basement, and then as a secondary magma intruded through
the overlying crystalline basement and the sedimentary veneer to the
surface. In certain places like the Henry Mountains, minor amounts
worked somewhat laterally to emerge in the adjacent basin. The position
of some of the igneous rocks which have penetrated the sedimentaiy
veneer poses a problem, it must be admitted, but then, to the writer's
knowledge no attempt has been made to explain their distribution by any
other hypothesis.

The blister concept is illustrated in Fig. 36.4.

West Texas and Mexican Coastal Plain. The principal volcanic field
in the west Texas province is the Davis Mountains which extend from the
southern flank of the Delaware basin to and across the Rio Grande into
Mexico, a distance of 125 miles ( Tectonic Map of the United States, 1944).
The Chisos Mountains and the Terlingua-Solitario region to the south-
east in the Rig Rend Country, have many igneous bodies. A number of
intrusives are known in adjacent Mexico in the Sierra Madre Oriental
and Serrania del Rurro uplift. Northwest of the Davis Mountains are the
Eagle Mountains and Quitman Mountains which contain intrusive and
extrusive bodies, and north of these and east of El Paso are a group of
>mall intrusives that make up the Cornudus field. The Marathon basin
also contains a number of plugs and dikes.

An alkalic composition has been noted in many of the igneous rocks of

S"4pf

MES

ZESE

SILICIC LAYER

.BASALTIC LAYER.

""«« .«w0yii
1LLLLUJ

5 £

to g

IS Ml

BASALTIC LAYER.

Fig. 36.4. Concept of blister structure and gravity mass movements of oval-shaped uplifts in
shelf province.

west Texas but some are calc-alkalic. The overall province, however, is
generally referred to as alkalic and related to the Spanish Peaks and
central Montana provinces.

The extrusive rocks of the Davis Mountains are trachytes, phonolites
and some rhyolites. Intrusive rocks are syenite and sodic syenite porphy-
ries. Olivine basalt occurs in minor amounts. All these igneous rocks are
early Tertiary in age, but one Recent vent has been observed (King, 1937).

The igneous rocks of the Cornudus field are augite syenites and analcime
nepheline syenite.

A volcanic area in the Quitman Mountains has a ring-dike and stock
of quartz monzonite as a central feature. This locally cuts a volcanic
series which consists of lower rhyolites, intermediate trachytes, rhyolites,
latites, and andesites, and upper trachytes. The total thickness is about
3500 feet, and rhyolite appears to occur in largest amounts. Indirect
fossil evidence suggests an early Tertiary age. According to the alkali-
lime index of Peacock, the volcanics of the Quitman Mountains fall near
the boundary of the two intermediate series, alkalic-calcic and calc-
alkalic (Huffington, 1943).

568

STRUCTURAL GEOLOGY OF NORTH AMERICA

Kef

Eogleford

C Penn Tesnus

Kbu

Buda

0 Devonian Santiago and Cobollos

Kdr

Del Rio

OP Ordovicion Maravillas and under!

Krfl

Georgetown

Poleoioic undifferentiated

Ked
Kcp

Edwards
Comanche Pk.

B|B Undifferentiated igneous rock;

Kgr

Glen Rose

Tr.n.ty congl.

Glenrose, Edwo

ds, and Cretaceous undifferentiated

Fig. 36.5. The Solitario. Simplified from E. H. Sellards, W. S. Adkins, and M. B. Arick. Un-
published map from Bureau of Economic Geology, University of Texas.

The Terlingua-Solitario region is one of profuse and diversified igneous
rocks. According to Lonsdale (1940), there are several hundred masses
distinct enough to be mapped in an area of about 400 square miles. They
occur as lava flows, plugs or necks, dikes, sills, laccoliths, bysmaliths, and
possibly stocks. The largest plutons are laccoliths. Solitario is the largest
domed-shaped structure of the group and is strikingly circular. It may be
a laccolithic dome (see Fig. 36.5). The igneous rocks of the district in-
clude an analcite-bearing series which ranges from melanocratic gabbro
to syenite types. Analcite is primary, deuteric, and hydrothermal. Also

included is an intermediate trachytic and rhyolite group. Most of the
varieties are soda-rich. Lonsdale shows the igneous rocks of the Terlingua-
Solitario region to be closely related to those of the Spanish Peaks region
and also to those of north-central Montana.

The analcite-bearing rocks obviously are a related series and originated
through differentiation which preceded from melanocratic types through
labradorite-rich types to syenite (Longsdale, 1940).

Baker ( 1935 ) has suggested that the uplifted block containing the
Solitario dome is underlain by a batholith. In the adjacent sunken block
in which nearly all the analcite-bearing rocks occur it is possible that
the sinking resulted in rise of magma drawn from the lower and relatively
basic part of the batholith. The result would be not a single immediate
source of all the analcite-bearing rocks, but a number of differentiating
masses in laccoliths and other minor intrusions from which, in the total,
a relatively large number of varieties would be produced (Lonsdale,
1940). This is much the same arrangement as Larson postulates for the
calc-alkalic series of the San Juan volcanic field.

The Chisos Mountains consists of a number of sharp peaks of intrusive
and extrusive rocks. The area is referred to as an uplift, and is com-
parable to the Solitario in varieties of igneous rocks and includes alkalic
types similar to the Terlingua-Solitario district.

Alkalic rocks have been penetrated in wells drilled for oil in the
adjacent Delaware basin, but a problem exists in determining whether
these are Tertiary or Precambrian (Flawn, 1952).

The west Texas alkalic province extends southeastward well into
Mexico, for in the San Carlos Mountains an alkalic suite occurs. Kellum
( 1937 ) describes in the Sierra de San Jose division of the San Carlos
Mountains an "alkalic rock complex," a feldspathoid-bearing sill, ijolite
plugs, as well as microgranite, quartz diorite, and diorite porphyries.
There are also late basalt flows. The porphyries are probably laccoliths.
In the Sierra de Cruillas division of the San Carlos Mountains Imlay
( 1937 ) describes microgranite and sills as the most common type of
igneous rock. A vogesite sill was noted which is about 90 feet thick and
at least 232 miles long. A trachyte sill was also mapped. Basalt of alkalic
varieties occurs as a laccolith and as sills and plugs. One plug is an

IGNEOUS PROVINCES IN WESTERN UNITED STATES

569

olivine basalt, the laccolith is an hauyne basanite, and some of the sills
in one place are nepheline-hauyne basalt. The basalts were intruded con-
siderably later than the microgranites.

It is evident, in review, that the west Texas and northeastern Mexico

alkalic province contains differentiates similar to the Spanish Peaks field

of Colorado, the Rattlesnake Hills field of central Wyoming, and some of

the igneous groups of central Montana. Fairly stable crustal conditions

I obtained in most all places, an olivine basalt was the parent magma, but

I probably some assimilation of alkalic country rock occurred, and in

) places a mixing of magmas in different states of differentiation seems to

I be necessary to explain the unusual types.

- Calc-Alkalic Subprovinces

San Juan-Front Range Subprovince. The San Juan-Front Range will
here include the igneous rocks of the San Juan Mountains, and the Front
Range as well as the Spanish Peaks, Chico, and Raton basin fields ( see
map, Fig. 36.1). All the rocks of this large area have a notable calc-
i alkalic composition, range from basalt to rhyolite, and show a great vari-
ation from one flow to another.

San Juan Field. The great bulk of the San Juan Mountains volcanic
field, about 100 miles in diameter, is made up of andesitic and rhyolitic
rocks in about equal amounts. Rasalts transitional to andesites are sub-
ordinate.

In the following stratigraphic sequence (Larson and Cross, 1956) the
Miocene volcanics of the Potosi series are by far the most extensive and
aggregate between 5000 and 6000 cubic miles in total bulk.

Quaternary andesite: one small body.
Erosion to mountain topography.
Pliocene ( ?) andesite, andesite-basalt, and rhyolite.
Erosion to peneplain.
Miocene latite-andesite.
Erosion to mountain topography.

Miocene (Potosi series) andesites, quartz latites, rhyolites, and sub-
ordinate andesitic basalts; several internal erosion intervals separating

conformable sequences of lavas in which dominantly quartz-latite
lavas and tuffs are succeeded upward by dominant andesites.

Erosion to mountain topography.

Upper Cretaceous to Eocene andesite (dominant), latite, and rhyolite;
all occur locally and several internal erosion intervals can be rec-
ognized.

The volcanics lie partly on the northeast flank of a dome some 50 miles
in diameter. They spread principally across the central part of the Un-
compahgre Range of the Ancestral Rockies (Chapter 15). This range
rose in Pennsylvanian time and was gradually buried during succeeding
Permian, Triassic, and Jurassic time. In large parts of the range and the
area upon which the volcanics accumulated not more than 3000 feet of
strata existed, chiefly Cretaceous, prior to the Laramide doming. The
area was characterized by doming on the west. To the east compres-
sional deformation occurred in South Park and the Front Range ( Chapter
25). At the time of Miocene volcanism large areas had been stripped of
any sedimentary veneer, and the volcanics accumulated directly on the
Precambrian crystalline rocks. The volcanics cannot be directly related
therefore, to a basin of sedimentation, to a broad Laramide uplift, or to
a belt of strong Laramide orogeny. As for the ancestral Uncompahgre
uplift it would seem that its roots would long since have disappeared by
isostatic adjustment before Tertiary volcanism occurred. This andesite
assemblage is therefore somewhat of an anomaly but must not be neg-
lected in shaping a theory of the origin of andesitic magmas in the oro-
genic belts.

Serial derivation from basic magma by fractional crystallization was
the dominant process, but also prominent was the thorough mixing of
magmas from the same common parentage but at different stages of dif-
ferentiation. Some assimilation of country rock may also have occurred
(Larsen and Cross, 1956).

The evidence of mixing of magmas, contamination by foreign material.
resorption of hornblende and biotite, and great variation in composition from
one flow to another characterizes the San Juan volcanic pile . . . the evidence
demonstrates that magmas of chemically related but quite dissimilar composi-
tions, were generated locally within spongv subterranean chambers, and that

570

STRUCTURAL GEOLOGY OF NORTH AMERICA

in general more than one chamber was tapped during an eruption (Waters,
1955).

Although the existence of an orogenic root is questionable, Waters
suggests:

. . . [This] part of the Rocky Mountain root was undergoing renewed granitiza-
tion and anatexis, and that the volcanic rocks were fed from growing pods
filled with mixtures of magma and migma. Pluto's genetic traits can actually
be seen in the volcanic rocks! But it is not a root of argillites and graywackes
that was undergoing partial melting as in the Cascades. Instead the richness
in potash, and the abundance of biotite and hornblende in process of resorption
point to a mountain root in a much later stage of metamorphic development —
one in which the principal rocks were mica schists, amphibolites, and granodio-
rite intrusives.

Spanish Peaks Field. The igneous rocks of the Spanish Peaks area
(Knopf, 1936) consist of two central stocks of which the older is a mass
of granite porphyry. It is cut by the later pyroxene syenodiorite. A strik-
ing system of radial dikes (Chapter 25) evidently emanated from the
stocks, and they range from highly silicic to mafic varieties. The order of
intrusion is: 1, granite porphyry stock and granite porphyry dikes; 2,
granodiorite porphyry stock and biotite porphyry dikes; 3, pyroxene
syenodiorite stock; 4, microsyenodiorite; 5, teschenite, camptonite, and
shonkinite, and trachydolerite; 6, augite syenodoirite porphyry; 7, campto-
nite and biotite lamprophyres. Their origin is discussed by Waters ( 1955)
as follows:

The order of intrusion in the stocks is the order of decreasing silica content,
the reverse of the normal plutonic order. Noteworthy, too, is the great variety
and abundance of the lamprophyres. Another interesting fact is that some
of the lamprophyres are of the kind commonly considered related to calc-
alkalic masses, whereas others are of the kind believed to be genetically
related to alkalic rocks.

Without added evidence from mineral paragenesis and inclusions it would
be presumptuous to suggest that the Spanish Peak rocks may be igneous
offshoots from a zone of biotite-rich metamorphic rocks that were undergoing
partial fusion. Nevertheless, such a hypothesis, in contrast to derivation from a
parental basalt magma, better fits the reversal in the "normal" sequence of
intrusion. Also the rising temperature during anatexis, resulting ultimately in
partial fusion of hornblende and biotite, can account for the formation of the
varied suite of lamprophyres and can explain their heteromorphism (Waters,
1955).

Chico Field. In northeastern New Mexico adjacent to the Spanish
Peaks field basalt flows cover over 700 square miles. They are here col-
lectively called the Chico field. There were three periods of basalt ex-
trusion separated by active stream erosion, and all are believed to be of
Quaternary age although it is possible that the oldest is Pliocene. The
basalt extrusions are mostly fissure-type eruptions, but some necks are
noted. The extrusion loci have not been tied to post-Eocene structure.
The volcanics occur on the southeast flank of the Raton basin of Creta-
ceaus and Tertiary age. Olivine basalts predominate in all three periods.
The intermediate flows have the greatest variation and include olivine
basalt, olivine-free basalt, olivine basalt with quartz inclusions, felspathoid
basalts (tephrite?, olivine absent), and basanites (olivine present).
Dacites, andesites, soda trachytes, and phonolites in minor amounts are
also noted. All these rocks of the area probably originated from one
magma whose original composition approximated olivine basalt. The suite
is sodic alkalic (Collins, 1949; Stobbe, 1949).

The Chico field basalts are grouped in the same province with the San
Juan andesites and rhyolites because they are adjacent and have had the
same parentage, namely an olivine basalt magma. It should be noted that
the Chico basalts are distinctly in a nonorogenic region.

Front Range Igneous Rocks. The transverse porphyry or mineral belt
of Laramide intrusions and related rocks of the Front Range of Colorado
has been reviewed in Chapter 25. The succession of igneous rocks and
their chemical and mineralogical composition suggest that those of the
western slopes were derived from an augite diorite magma, and those on
the eastern slopes from an olivine basalt magma. The augite diorite
magma differentiated into a series ranging from porphyritic diorite
through porphyritic quartz monzonite to granite porphyry in compara-
tively shallow hearts. The olivine basalt magma gave rise to the differ-
entiate series: diorite, monzonite, quartz monzonite, granite, alaskite,
lead-silver ores; alkalic syenite, bostonite, pyrite gold ores; and biotite
monzonite, biotite latite, latitic intrusion breccia, gold telluride ores,
and tungsten ores. This was accomplished by withdrawal of portions of
the changing residuum of the slowly solidifying magma into shallow
reservoirs, and further differentiation by the subtractive processes of

IGNEOUS PROVINCES IN WESTERN UNITED STATES

571

crystal settling, crystal zoning, and filter pressing. The late differentiates
of the olivine basalt magma were much more alkalic than those of the
augite magma because of the initial difference in the composition of the
parent magmas (Lovering and Goddard, 1950).

The eastern slope olivine basalt differentiate series is similar to that of
the San Juan volcanic field except one is an intrusive succession and the
other mostly an extrusive. Relative volumes are unknown, but at least,
both are postulated to have come from an olivine basalt parent. Some
assimilation may have occurred in the San Juan magma reservoirs but
Lovering and Goddard, if the writer correctly understands, do not pre-
sume assimilation for the olivine basalt series of the east slopes of the
Front Range However, the augite diorite parent magma may have been
generated entirely by fusion of a crystalline basement rock.

Both magma subprovinces of the Front Range developed across the
ancestral Colorado Range of Pennsylvanian age (see Chapter 25). This
general area all through Paleozoic time had been dominantly positive
and had received only a very thin veneer of sediments on the crystalline
basement, which was broadly exposed by erosion as the Colorado Range
was uplifted. The range was gradually buried during the Mesozoic, and
Cretaceous beds were deposited on the Precambrian over wide areas of
the old range and constituted in these places the only sedimentary rock
at the time of Laramide orogeny. Again in Laramide times uplift was
prominent but large-scale overthrusting occurred, especially in the west-
ern part of the old Colorado Range, now the Vasquez Mountains, the
Williams Range, and the Gore Range, and the uplift, thrusting, and in-
trusive sequence are closely related in time.

Yellowstone Subprovince. The Yellowstone subprovince will here in-
clude the Absaroka, the Crazy Mountains, the Livingston and Adel Moun-
tain fields as well as the Yellowstone Park field. The rocks of this province
are generally calc-alkalic in mild contrast to the alkalic rocks of central
Montana, previously described and also to the andesites and quartz
latites of the Elkhorn Mountains volcanics and the Hogan formation. See
Figs. 36.1 and 36.3. Actually the differences are slight and boundaries
separating the three provinces are difficult to draw, principally because
two of the volcanic centers have episodes of alkalic rock eruption sepa-

rated by episodes of calc-alkalic rock eruptions. Superposed volcanic se-
quences are subprovinces in Larsen's nomenclature (1940).

Yellowstone Field. The eruptive rocks of Yellowstone Park range from
basalt to rhyolite, with the basalts containing calcic plagioclase, augite,
hypersthene, and olivine. The Absaroka Range has trachydolerites and
orthoclase gabbros (alkalic) as its mafic rocks, and where the age rela-
tions have been determined the older effusives are generally calc-alkalic
and the younger alkalic. The Absaroka field may therefore be placed in
either the Yellowstone calc-alkalic province or the central Montana
alkalic province.

Crazy Mountains Field. The igneous rocks of the Crazy Mountains
consist of an older calc-alkalic series of two stocks and associated dikes,
sills, and laccoliths, and a younger alkalic series, found chieflv in the
northern part of the mountains, and occurring as sills, laccoliths, and
dikes. The alkalic bodies are richer in soda than anv of the other groups
of central Montana, and have been determined as granite porphyry,
syenite, nepheline syenite, shonkinite, and lamprophyre. The older and
more calcic stocks are chiefly diorite with minor amounts of granodiorite,
gabbro, and picrite (Larsen, 1940).

Livingston Field. The Livingston formation is a series of pyroclastic
rocks several thousand feet thick which crop out on the north side of the
Beartooth Mountains. They grade laterally into the Claggett, Judith River,
Bearpaw, and Lennep formations and hence represent a center of vol-
canism that was active during most of the Montana epoch of the Upper
Cretaceous. Pyroxene andesite breccias are by far the most abundant, and
occur both above and below hornblende andesite breccias (Vhay, 1939).

Adel Mountain. A fairly large volcanic field in the southern end of the
Foothill belt of the Canadian and Montana Rockies, west of the Highwood
Mountains and on the northern end of the Big Belt Mountains may be
divided into two parts. Its southeastern part, the Adel Mountain, has been
studied by Lyons (1944). He finds that the volcanic rocks consist of po-
tash-rich basalts which were erupted on Cretaceous sediments. The
trachybasalt breccias, flows, and agglomerates are 3200 feet thick and have
been intruded by many chonoliths, sills, and dikes ranging from gabbro
to quartz monzonites. The chemical and mineralogical analyses relate

572

STRUCTURAL GEOLOGY OF NORTH AMERICA

the field to the Highwood Mountains and the alkalic province of central
Montana.

Summary. The Yellowstone subprovince as a whole is one of olivine
basalt parentage and although it is in a Laramide belt of mild to appreci-
able deformation, it is definitely not of the orogenic andesite lineage; it
belongs to the nonorogenic calc-alkalic and alkalic provinces east of the
central Rockies.

Crowsnest Volcanic Field. Still another Cretaceous volcanic field has
been described in the deformed belt (MacKenzie, 1956). It is known as
the Crowsnest volcanics and occurs about 30 miles north of the interna-
tional boundary ( see Figs. 20.2 and 20.6 ) . The volcanic deposit lies within
the mid-Cretaceous sediments; viz., the continental Blairmore (Dakota)
formation underlies it, and the marine Blackstone (Benton) overlies it.
At Coleman the lower unit consists of trachyte agglomerate beds. This is
overlain by ash beds with scattered large fragments of pyroclastics; this
in turn is overlain by water-laid ash beds rich in andesite, and this in turn
by more ash beds with varying amounts of coarse pyroclastics. Some of
the ash beds are hard resembling flows; but no actual flow rocks are
reported. Four main lithologies have been identified, namely, augite
trachyte breccia, tinguaite, andesite tuff, and phonolite tuff. The name
blairmorite has been suggested for certain analycite-rich fine-grained rocks
in the volcanics.

The rock on account of its ultra-alkaline nature, will show numerous varia-
tions in texture and in proportions of component minerals . . . (MacKenzie,
1956).

It is evident that the Crowsnest volcanics are alkalic and related to the
central Montana petrographic province rather than a field to the south in
the Foothill belt of Montana, the Dearborn River which is generally
andesitic. The Dearborn River field is described under a later heading.

The age by stratigraphic position is Mid-Cretaceous, and by potassium-
argon dating (Folinsbee et al., 1957) is 96 m.y. The Crowsnest volcanics
are slightly older than the Dearborn River Volcanics, according to strati-
graphic position, and slightly younger than the main Nevadan batholithic
intrusions to the west.

The Crowsnest volcanic field is estimated to have a maximum average

thickness of about 1000 feet and to spread over 500-600 square miles. It
contains about 50 cubic miles of rock.

Southern Colorado Plateau Basalt-Rhyolite Province

Extent. Several volcanic fields along the southern margin of the
Colorado Plateau may be conveniently grouped together because of their
close proximity, but they hardly have enough common characteristics to
justify the grouping. The questionable province starts on the east with
the Jemez field in north-central New Mexico and includes the Mount
Taylor, the Datil, the San Francisco, and the Uinkaret fields. See map, Fig.
36.1.

Cliaracteristics. The Jemez, Mt. Taylor, and San Francisco fields are
the result of large, central vent-type volcanoes. One large volcano or a
cluster of several with numerous, later, small cinder and lava cones make
up the fields. The rocks range from basalt to rhyolites and appear to repre-
sent the basalt-rhyolite differentiation suit. The source of the lavas in the
large Datil field is not recorded in the literature as far as the writer can
determine. The Uinkaret field consists of youthful small cones and basalt
flows.

Description of Fields. The Mt. Taylor field is dominated by the Mount
Taylor volcano which erupted in late Miocene time, after folding and
faulting in the district.

The volcano broke out in a syncline. The eruption, which occurred in a
fairly well defined compositional sequence, began with rhyolitic tuff. This
was followed by relatively quiet eruptions of porphyritic lavas in which two
and possibly three series are distinguishable on the basis of their content of
potash feldspar. The oldest of these is porphyritic trachyte, but the volume
is very small. The next eruption was a large volume of porphyritic latite,
interrupted, however, by at least one more flow of porphyritic trachyte. The
latite, in turn, was followed by a slightly smaller volume of porphyritic
andesite.

The total volume of the tuffs and lavas is about 12.5 cubic miles, of which
about 5 cubic miles is rhyolitic tuff, 4 cubic miles is latite, and 3.6 cubic
miles is porphyritic andesite.

The erosion surfaces that subsequendy were developed around the base of
the cone later became flooded with sheets of nonporphyritic basaltic and
andesitic lavas erupted from the scores of vents that comprise the volcanic
field. A few of the sheets were erupted prior to the latest eruptions on Mount

IGNEOUS PROVINCES IN WESTERN UNITED STATES

573

Taylor, but most of them were erupted after Mount Taylor had become
quiescent and they overlap the outer edges of the Mount Taylor cone (Hunt,
1938, p. 58).

The Mount Taylor central vent volcanics are slightly more alkalic than
the rocks of the laccolithic groups of the Colorado Plateau, and Hunt,
therefore, points out a close tie of the two.

The San Francisco volcanic field is much larger than the Mt. Taylor,
and the initial activity consisted of the eruption of about 30 cubic miles
of sheet basalt over a broad structural dome, the Coconino Plateau.
Several large vent-type volcanoes broke out; San Francisco Mountain
being built of almost 40 cubic miles of volcanic ejecta, Kendrick Peak
of more than 6 cubic miles, Rill Williams Mountain of 3 cubic miles, and
O'Leary Peak of 2 cubic miles. The five stages of eruption of San Fran-

' cisco Mountain volcano were as follows: 21 cubic miles of latitic lava,
tuff, and breccia, 13 cubic miles of pyroxene dacite lava, 0.5 cubic mile of
hornblende dacite, 0.5 cubic mile of rhyolite, and 3 cubic miles of andesite.
On a succession of erosion surfaces four separate basalt flows occurred,

I and basalt lavas and pyroclastics were extruded from about 200 small
vents. This was the last phase of activity and about 20 cubic miles of lava
was extruded. One of the cinder cones, Sunset Peak, was active 800 years
ago (Robinson, 1913).

Datil field of eastern Arizona and adjacent New Mexico, is largely
andesite, with subordinate amounts of rhyolite, rhyolite tuff, quartz latite,
and various pyroclastics consisting mostly of basalt (Sabins, 1957). The
Mogollon Mining District is within this large field, and there Ferguson
(1927) describes 8000 feet of andesite, rhyolite, rhyolite tuff, and quartz
latite. This assemblage savors of the Great Rasin latite-monzonite prov-
ince, and perhaps has some welded tuffs. Variations from mostly basalt
o mostly latite and rhyolite would appear to be dependent upon the
imount of silicic crust effected by partial melting. Refer to theory
oresented under next heading, Rasin and Range latite-monzonite province.

kisin and Range Latite-Monzonite Province

Extent and General Characteristics. Rutler ( 1920 ) summarized the
Volcanic rocks of western Utah and adjacent parts of the Rasin and Range

physiographic province as follows. They range in composition from
rhyolite to basalt, but the great bulk of the material is of intermediate
composition, including rather basic rhyolites, quartz latites, dacitcs, and
andesites. Rasalt is very subordinate in amount when compared with
the series as a whole though present in many localities and usually con-
spicuous as representing the latest volcanic outflows.

A large region in Nevada and western Utah consists dominantly of
welded tuffs of approximate quartz-latite composition. The alkalic types
of rock, such as the leucite and nepheline-bearing lavas are to the writer's
knowledge, very scarce, and have only been noted in East Fork Canyon
of the High Plateaus where Dutton described an isolated occurrence
of phonolite and in the Traverse volcanics of the Oquirrh Mountains
(Gilluly, 1932). A brief review of the Tertiary volcanic rocks in southern
Arizona indicates that they are essentially the same as in Nevada and
western Utah, and fit Rutler's general description. The intrusive rocks are
principally in the form of stocks. They have a dioritic to granitic compo-
sition, with monzonitic the most common. Like the extrusives the intru-
sives are preponderantly intermediate to acidic in composition.

Intrusive Rocks. The following is a tabulation of the intrusive rocks of
ninety-five mineral districts which Stringham has made in the course of a
systematic study of the mineralized and barren stocks of western Utah,
Nevada, Arizona, California, and New Mexico (personal communication).
About 300 plutons, mostly stocks, are shown on various maps of areas in
these states or are known from personal field work, according to Dr.
Stringham. When a district is mapped, more intrusive bodies are usually
found, so he estimates that possibly 1000 intrusions may exist. In western
Nevada most of these are probably satellites of the Sierra Nevada batho-
lith and not Laramide or later in age as elsewhere in the Great Rasin.

The intrusive bodies shown in the table on page 574 are charted on
Fig. 36.3, where it is seen that the three classifications of the tabulation
have little significance geographically. It might be concluded that western
Utah and eastern Nevada are free of intrusions as basic as diorite, but
elsewhere in the Great Rasin the three divisions are fairly well scattered.

The Tertiary intrusives of central New Mexico range from diorite to
granite, with a preponderance of monzonite and quartz monzonite ( Lind-

574

STRUCTURAL GEOLOGY OF NORTH AMERICA

Intrusive Bodies of Mineral Districts in Great Basin

Diorite and
Andesite

Pearce (1)
Superior (2)

Mogollon (29)
Steeple Rock (30)
Pinos Altos (31)

Tonopah (39)
Aurora (40)
Tuscarora (41)
Fairview (42)
Divide (43)
Virginia City (44)
Searchlight (45)
Eldorado (46)
Tenabo (47)
Lewis (48)

Granodiorite Quartz

Monzonite, Monzonite,

Latite, Dacite

Arizona

San Manuel (3)
Christmas (4)
Big Bug (5)
Johnson (6)
Miami (7)
Ajo (8)
Tombstone (9)
Silver Bell (10)
Castle Dome (11)
Courtland Gleeson (12)
Patagonia (13)
Helvitia (14)
Harshaw (15)
Bagdad (16)
Twin Buttes (17)

New Mexico

Kingston (32)
Central (33)
Magdalena (34)
Tyrone (35)
Hillsboro (36)
Chloride Flat (37)

Nevada

Tybo (49)

Copper Canyon (50)
Candelaria (51)
Gold Acres (52)
Ely (53)
Yerington (54)
Goldfield (55)
Eureka (56)
Copper Basin (57)
Austin (58)
Mill City (59)
Getchell (60)
Cherry Creek (61)

Granite, Rhyolite

Morenci (18)
Bisbee (19)
Oatman (20)
Kofa (21)
Mammoth (22)
Jerome (23)
Ray (24)
Chloride (25)
Arivaipa (26)
Congress (27)
Aqua Fria (28)

Lordsburg (38)

Pioche (62)
Goodsprings (63)
Jarbidge (64)
Bristol (65)
Silver Peak (66)
Wonder (67)
Rochester (68)
Unionville (69)
National (70)
Seven Troughs (71)
Bullfrog (72)
Delamar (73)
Hamilton (74)

Granodiorite Quartz

Diorite and

Monzonite, Monzonite,

Andesite

Latite, Dacite

Granite, Rhyolite

Nevada

Manhattan (75)
Round Mtn. (76)
Bullion (77)

California

Bodie (78)

Cerro Gordo (79)

Calico (82)

Randsburg (80)

Blind Spg. Hill (83)

Mohave (81)

Ludlow (84)
Darwin (85)

Utah

Park City (86)

Stockton (87)

Ophir (91)

San Francisco (88)

Mercur (92)

Gold Hill (89)

Bingham (93)

Lucin (90)

Marysvale (94)
Tintic (95)

gren et ah, 1910). They are intruded into Precambrian granites and schists
and all parts of the Paleozoic and Mesozoic stratigraphic sequence, and
take the form of stocks, sills, and dikes. The volcanics are basalts, an-
desites and rhyolites, with the order of eruption generally, rhyolite,
andesite (or latite), rhyolite again, and finally basalt.

Possibly the largest Laramide or Tertiary intrusive in Utah, Nevada,
Arizona, or New Mexico is that of the Sierra Blanco in Lincoln County,
New Mexico. It is probably connected underground with plutons to the
north in the Jicarilla Mountains and Gallinas Mountain and to the east in
the Capitan Mountains. The Sierra Blanco, Jicarilla, and Gallinas plutons
extend a distance north-south of 70 miles, and the Sierra Blanco pluton
itself has a maximum width of 15 miles. The Capitan pluton extends 20
miles in an east-west direction and about 5 miles in a north-south direc-
tion. The major intrusive mass, the Sierra Blanco, and the Jicarilla, have
penetrated a basin downwarp containing Cretaceous strata not much
larger than the exposed plutons. The Gallinas and Capitan plutons have

IGNEOUS PROVINCES IN WESTERN UNITED STATES

575

come up through Pennsylvanian and Permian strata which are nearly
horizontal.

Another fairly large pluton is one in the Organ Mountains, about 30
miles north of El Paso. It extends 18 miles north-south and 9 miles east-
west.

Lindgren et al. (1910) describe the large plutons mentioned above as
monzonite and quartz monzonite porphyries. They observe that the in-
trusive monzonites and effusive latites and andesites in general in the
central north-south belt of New Mexico have a fairly uniform composition
and suggest that all were derived from an intermediate magma. The last
differentiates were basalt and rhyolite, which were the last ejections.

The general problem of the nature of the primary magmas will be dis-
cussed presently, but it should be said that the fusion of a gneissic,
schistose, and granitic basement such as would produce a monzonitic
magma, would not have enough magnesium and iron in it to yield a
basaltic differentiate, especially an olivine basalt. Also the volume of basalt
in some fields is too much to have been derived from the postulated parent
primary monzonitic magma.

Extrusive Rocks. As for the extrusive rocks only limited information
is at hand. Those of central and eastern Nevada and southwestern Utah
consist of a thick older assemblage and a thinner younger group which is
approximately equal in age to the younger volcanics of western Nevada,
southern California, Arizona, and southwestern New Mexico. The older
assemblage is dominantly of the quartz latite type, and more conspicu-
ously, it consists chiefly of a great series of avalanche or welded tuff
deposits, whereas those in peripheral location are more of the basalt-
andesite-dacite-rhyolite suite.

Rrief descriptions of selected fields outside of the avalanche sub-
province follow.

In the Ajo District of south-central Arizona the lavas are basaltic,
[ andesitic, and latitic. In southern Nevada in the Goodspring's quadrangle
the extrusive rocks range from latite through andesite to basalt. At Gold-
field, Nevada, the eruptive sequence is rhyolite, latite, rhyolite, olivine
basalt, andesite, da cite, andesite, rhyolite, andesite, olivine basalt, rhyolite,
and olivine basalt. At Gold Hill on the Utah-Nevada line a normal series

of basalts, andesites, and rhyolites occurs. They are all rich in potash.
Latite Welded Tuffs Subprovince. The welded tuff subprovince is
shown on the maps of Fig. 36.1 and 36.3, and its existence has only re-
cently become clear. Mackin and Cook and associates in southwestern
Utah and several petroleum and U.S. Geological Survey geologists have
recognized the welded tuffs (ignimbrites) and something of their magni-
tude. However, the writer is especially indebted to Dr. Howel Williams
for the following resume. He was among the first to gain the conception of
the unique field and the almost unbelievable magnitude and awesomeness
of the eruptions.

Welded tuffs are formed during eruption bv distention of magma in which
the vapor tension is low. Instead of explosive eruption of vitric ashes, the
discharge is in the form of a glowing avalanche that sweeps rapidly down-
slope. The most widespread of these result from escape of foaming magma
through swarms of fissures as a mixture of incandescent spray, droplets, and
larger clots enveloped in hot, expanding gas. So mobile are these mixtures that
they spread over vast areas, down even the gentlest gradients. Other glowing
avalanches issue from the flanks of volcanic domes of Pelean type; still
others originate when foaming magma is upheaved en masse until it spills
over a crater rim and then, aided by gravity, races downward. Because these
avalanche deposits accumulate rapidly and usually to great thickness, many
remain hot for a long time, especially in their central parts. As a result, the
shards of glass, while still hot and under heavy overburden, are squeezed and
flattened, and some are buckled between phenocrysts. At the same time
pumiceous lapilli and bombs are deformed to disks, some of them paper-thin,
and all the constituents become firmly annealed. The rocks thus formed are
called welded tuffs. They have a delicate, streaky lamination deceptive!) like
the fluidal banding seen in many lavas. Besides, some of them develop
columnar and spherulitic structures as they cool, so that their resemblance
to lavas is increased. Little wonder, therefore, that welded tuffs have often been
wrongly identified. The fact is that they are now known to be of truly colossal
extent in the circum-Pacific volcanic regions, and they are undoubtedly wide-
spread elsewhere (Williams et al., 1954).

The welded tuffs according to Williams constitute 95 percent of the
older and more voluminous volcanics of the avalanche subprovince. They
probably average over 2000 feet thick, and south of Eureka, Nevada,
they are 8000 feet thick. This general area is the part of the field of maxi-
mum thickness. Since immense amounts of these easily weathered tuffs
have been removed, the original volume was undoubtedly larger than

576

STRUCTURAL GEOLOGY OF NORTH AMERICA

present thicknesses indicate. At least 30,000 cubic miles of welded tuffs
were erupted in this subprovince.

They originated in fissure eruptions, and dike feeders are the rule. No
cones, central vents, radial dike swarms, or quaquaversal dips have been
noted. Dr. Williams' ideas about die age of the welded tuff accumulations
is as follows. The rapidity of accumulation is startling because a layer
as thick as 1000 feet may have accumulated in one day. Although there
were many eruptive centers the entire explosive activity occurred un-
doubtedly in a very short time geologically, say a few thousand years.
The major activity of a certain group of fissures may have taken place in
three or four davs. This is deduced because of the absence of erosional

breaks of any kind in the succession of welded tuff flows. Soil profiles be-
tween flows were sought but not found.

A potassium-argon age determination yielded a date of 35 =■= 2 m.y., and
Professor Williams thinks this will prove to be the age of the main unit of
the thick assemblage of welded tuffs over the entire subprovince. There
are younger welded tuffs above and outside the subprovince, but these are
another matter. The age would then be early or mid-Oligocene according
to which absolute time scale is used.

According to E. F. Cook (personal communication), who has studied
the welded tuffs extensively through Nevada and western Utah and who
has attempted to correlate many measured sections, breaks in the deposi-
tional sequences occur, with flows and sediments interlayered. He believes
the eruptions were intermittent and extended through the Oligocene into
the Miocene, and suggests that the period through which the welded tuffs
were erupted was several million years long. T. B. Nolan informed the
writer that the 35 (or 34) m.y. potassium-argon date appears to conflict
with Miocene fossils in the Eureka-Austin-Winnemucca area. Since the
interest of a number of geologists and geochemists is high on the problem,
our knowledge will undoubtedly be more precise in a short while.

The surface at the time of the numerous and widespread outbreaks
seems to have been very flat, according to Dr. Williams, because wherever
the base of the series is exposed it is without relief, and since the indi-
vidual avalanche flows can be traced scores of miles, there must not have
been sizable topographic obstructions in their way. This is especially true

for the upper flows of the welded tuff sequence, according to Dr. Cook,
but he believes the early flows filled basins of appreciable relief or closure.
The bulk of the material of the flows is approximately of quartz latite
composition, and it is mostly slightly potassic with some parts rather
potassic. Some inclusions occur and these confirm the suspicion of Pro-
fessor Williams that assimilation of the crystalline basement of the silicic
crust is involved in the origin of the welded tuffs.

Eleven ignimbrites are widespread in southeastern Utah and have been
given formal stratigraphic names by Mackin (1960). He says:

The fact that the oldest of them lies unconformably across the beveled edges
of thrusts and folds involving late Cretaceous strata indicates that the beginning
of volcanic activity post-dates the Laramide orogeny. As planar units which
provide a record of Tertiary crustal movements, the ignimbrites confirm the
Gilbert theory, based originally on physiographic evidence, that block faulting
has been the characteristic type of post-Laramide deformation in the Great
Basin.

The volcanic field of southwestern Utah abounds in welded tuffs. These
and associated volcanics are described by Cook ( 1957 ) in probably the
best account of them so far. Indurated acidic pyroclastic rocks ranging
from welded tuffs to bedded tuff-breccias dominate the volcanic column
which is several thousand feet thick. The bedded tuff-breccia occurs in
beds 2 to 20 feet thick and fills depressions in a rough topography de-
veloped on folded and faulted volcanic rocks. Its tuffs are non welded but
in one locality appear to grade downward into welded tuff-breccia. Cook
concludes that the bedded tuff breccia was deposited by a series of
rapidly moving, widely spreading flows of gas ( possibly steam ) , hot water,
and solid particles, in which the temperature was below that required for
welding.

Breccia and air-fall tuff form a minor portion of the volcanic rocks of
the area. Lava flows, conspicuous locally but also of minor amount, in-
clude dacite porphyry, locally porous and fluidal, latite (?) and latite
porphyry, olive-brown to black andesite, and dark gray to black basalt.
The higher part of the Pine Valley Mountains consist of latite ( or quartz
latite ) porphyry. Except for a chilled basal phase the latite is lithologically
uniform throughout its thickness of 2000 feet, and is similar mineralogi-

IGNEOUS PROVINCES IN WESTERN UNITED STATES

577

cally and texturally to the monzonite porphyry of the intrusive laccolith
there.

The mode of origin of the extrusive rocks of the Pine Valley Mountain
appears to be related to their chemical composition. The basalts, andesites,
and latites are flows; the dacites are found both as ignimbrites and
as flows; and the rhyolitic rocks are all ignimbrites. Apparently the more
acidic magma effervesced into nuees ardentes, although some of
dacitic composition merely foamed into frothy flows; the intermediate and
basic lavas, on the other hand, welled up without violent loss of gas to
form finely vesicular flows. (Cook, 1957, p. 49).

A study by Van Houten (1956) of the Cenozoic sedimentary and re-
lated volcanic rocks of Nevada and western Utah indicates that a good
I datum for correlation is a tuffaceous unit of late Miocene and early and
mid-Pliocene age. A vitric tuff in this general bentonitic and tuffaceous
unit is prominent and widespread. It rests on somewhat older Oligocene
and Miocene (?) volcanic rocks in southern, central, and western Nevada,
as well as locally in the northeastern part of the state.

The lower volcanic units were tilted by fault block rotation and eroded
j before the widespread, late Miocene-Pliocene tuffaceous unit began to
I accumulate. During this late Miocene to mid-Pliocene interval the south-
ern Cascade andesites were accumulating as well as the younger basic
i lavas of the Sierra Nevada. The inference is natural that the volcanism
I and faulting are related, but this subject will be left until later for dis-
I cussion.

The study by Van Houten emphasizes the existence of extensive fluvi-
atile and lacustrine deposits derived largely from the eruptive centers.
The sedimentary derivatives fill the numerous intermontane valleys in
places to the depth of over 5000 feet and have been tilted in the Basin and
Range faulting to be exposed on the backs of the tilted blocks. In some
places as much as 10,000 feet of volcanic rock, including derived strati-
fied outwash and lacustrine deposits have been measured (personal com-
munication, various petroleum geologists ) . The volcanic fields, other than
the avalanche subprovince, have not been determined and circumscribed.
Those shown on the map of Fig. 33.7 are taken from the Geologic Map
-of the United States (1932), and it is presumed they are the most con-

tinuous and thus indicate the major centers of volcanism. It is evident
that this representation is likely to be changed considerably by future
work.

As a result of the extensive exploratory work for oil in Nevada, nearly
every intermontane valley is regarded as a downfaulted block, but only in
a few places have the faults been shown on maps either published or
available to the writer. The best recourse, it seems, is to show each valley
by a single fault, and this has been done in the absence of better informa-
tion. The Basin and Range fault system is undoubtedly more complicated
than shown.

Origin of the Latite Magmas. In 1932, Gilluly observed from a study
that focused in the Oquirrh Mountains that a close relation of all the
extrusions is evident, and although several of the volcanic masses have
been described as andesites, these when analyzed have the alkali ratios
characteristic of latites. Whether they contain orthoclase or not, their
chemistry justifies the inference that they all belong to the latite and
quartz latite group.

Plotting the CaO, KsO, and Na20 as ordinates and the silica as abscissas
reveals the interesting fact that the soda shows almost linear decrease with
increase of silica. The average slope of the curve of soda is almost precisely
that which would appear from the mere addition of silica to the monzonite
magma. However, the lime decreases at a rate altogether disproportionate to
the silica content, and the potash remains very nearly constant or decreases at
a much lower rate than the silica increases. This relation is close to that which
would be expected as a result of differentiation of a monzonitic magma by
fractional crystallization.

Both intrusive and effusive rocks of western Utah have CaO, Xa.O. and
K,0 proportions close to the average quartz monzonite, and hence are believed
to be of the monzonite kindred. Wherever chemical analyses are available,
it is seen that the so-called andesites are without exception so high in potassa
that they are properly classed as latites. Similarly, several so-called dacites
resemble quartz latite closely (Gilluly, 1932, pp. 66, 67).

Gilluly concluded and reaffirms the conclusion in recent correspondence
that there is no evidence here of any more basic rocks that could be
considered parental to the latite-monzonite magmas.

In Bowen's scheme of magma evolution it would be necessary to have
basaltic and andesitic rocks prior to the quartz monzonite. All of the volcanics

578

STRUCTURAL GEOLOGY OF NORTH AMERICA

and plutonics, with the trivial exception of some alkaline bodies, are either
quartz latite or quartz monzonite. In other words there is no evidence of any
magma antecedent to these. In that sense the monzonite is primary magmatic
rock.

I know of no discussions of this problem in the literature. I would be inclined
to feel that the magma is perhaps a regenerated one, produced by melting
of the deeper part of the crust (James Gilluly, personal communication, 1957).

In a detailed study of the Ringham stock Stringham ( 1953 ) finds types
other than monzonite according to mineral content, such as granite,
monzonite, diorite, syenite, and syenodiorite, with granite the dominant
variety. Chemically, however, the granite is not much different from
quartz latite or quartz monzonite. The granite and actinolite syenite ap-
pear to have originated by granitization, perhaps as fringe effects of a
central or more deep-seated magma, according to Stringham.

Western Montana and Eastern Idaho Andesite-Granodiorite Province

Peculiar Nature of Province. The term, andesite-granodiorite province,
is used for want of a better one for the assembly of igneous rocks in south-
western Montana and adjacent Idaho. The province does not fall readily
into any of the other categories outlined at the beginning of this chapter.
It seems to be a hybrid of the Nevadan batholithic province and the mon-
zonite-latite province of the Great Rasin with certain aspects of the an-
desite province added.

Several volcanic fields of Late Cretaceous age are known in the general
province of western Montana as shown on Fig. 36.1. They may be grouped
into the Livingston, the Elkhorn Mountains, and Adel Mountain fields
and the Hogan formation. The Livingston and Adel Mountain fields are
classed under the Yellowstone subprovince of calc-alkalic rocks, whereas
the Elkhorn Mountains and Hogan fields are andesites and latites, and
therefore less calc-alkalic. They belong more properly to the orogenic belt.

Elkhorn Mountains Field.

Remnants of a thick plateaulike accumulation of calc-alkaline volcanic rocks
of probable Late Cretaceous age — the Elkhorn Mountains volcanic rocks — are
exposed in an area of about 3700 square miles around the Boulder batholith
in the Elkhorn Mountains and Boulder Mountains, western Montana. The
presence of similar rocks across the Jefferson River to the south, and near Wolf

Creek to the north, suggests that the volcanic pile once covered more than
10,000 square miles.

In places these rocks rest unconformably on Paleozoic and perhaps older
rocks. Elsewhere they are gradational into underlying tuffaceous sedimentary
rocks of Late Cretaceous age, and the contact is arbitrarily placed at the base
of die lowest volcanic conglomerate, breccia, or flow.

The volcanic pile comprises three major units; maximum thickness of each
exceeds 5000 feet. The lower unit consists predominandy of dacitic, andesitic,
and basaltic fragmental rocks and autobrecciated lava flows. The middle
unit is about half quartz latite in welded tuff sheets as much as 300 feet
thick, interlayered with more calcic bedded pyroclastic rocks and autobrecciated
lava flows; it is locally unconformable on the lower unit. The upper unit
consists dominandy of reworked volcanic rocks and subordinately of fine-
grained pyroclastic rocks. A thick succession of basalt flows near Elliston,
Montana, may be equivalent to the upper part of this unit or may be younger.

The volcanic rocks were altered by and locally foundered in penecon-
temporaneous shallow-magma reservoirs. They were folded and faulted and
later invaded and thermally metamorphosed by the Boulder batholith (Klepper
and Smedes, 1959).

Regarding the intrusive rocks, Klepper et al. (1957) say:

The intrusive igneous rocks, except for a few felsite dikes of uncertain age,
are divisible into two groups, primarily on the basis of structural relations
and secondarily on the basis of composition and fabric. The older group of
dioritic and andesitic rocks were intruded in part, if not wholly, prior to the
main folding and are similar in chemical and mineralogical composition to the
Elkhorn Mountains volcanics. They were probably emplaced throughout the
period of volcanism that commenced in late Niobrara time and continued
until late Cretaceous time. The younger group consists chiefly of quartz-
bearing phanerites but includes rocks ranging from gabbro to alaskitic granite
and aplite. These rocks were emplaced after the main episode of folding and
faulting. The Boulder batholith, composed dominantly of quartz monzonite,
is the principal body of this younger group (Klepper et al., 1957).

Although Klepper and Smedes class the Elkhorn volcanics as calc-
alkalic, the rocks are less alkaline than the Livingston and Adel Mountain
volcanics and, with the associated intrusive rocks, are more closely re-
lated to the igneous rocks of the Great Rasin than to those of central
Montana.

Hogan Field. The volcanic rocks of the Hogan formation, according
to George Viele (Ph.D. thesis, University of Utah, 1960), are nearly 2500
feet thick and consist of interbedded breccia, welded tuff, volcanic-rich
graywackes, shale, black sandstone, and arkose. Andesitic and more acidic

IGNEOUS PROVINCES IN WESTERN UNITED STATES

579

eruptives provided the material for the pyroclastics. The rocks are not
rich in the alkalies and hence stand apart from the nearby Adel volcanics.
The nearby Adel volcanics, according to Viele, are probably of St. Mary
River formation age (latest Cretaceous) and extend into the Paleocene.
The Hogan volcanics are slightly older and are correlated with the upper
part of the Two Medicine formation and the lower Horsethief formation.
The Adel field has been only slightly tilted; whereas the Hogan field
beds have been involved in the folding and thrusting of the Foothill
belt.

Batholiths and Stocks. The region is noted for its large intrusives,

particularly the largest, the Boulder batholith. This cluster of intrusives

in west-central Montana and adjacent Idaho is easily the most voluminous

! anywhere in the Laramide belts. The plutons consist dominantly of quartz

| monzonites and granodiorites, although a number of variations in facies

and separate intrusions have been noted which range from gabbro to

granite. It has been pointed out by Emmons and Calkins (1912) that the

j intrusives of the west-lying Philipsburg district are less alkalic than the

i Boulder batholith and that the Boulder batholith has mild alkalic affinities

to the high alkalic rocks of the central Montana petrographic province.

\ This may mean that fusion of the crystalline basement is to be reckoned

with, and that the fused rocks become less alkalic westward.

The Boulder batholith has been studied by Profesor Knopf ( 1957 ) . He
I describes it as follows :

On the basis of a recent potassium-argon age determination, which gave
87 million years as the most probable age of the granodiorite of the Boulder
bathylith, it is concluded that the bathylith was emplaced late in Cretaceous
time.

The Boulder bathylith has hitherto been considered to be a one-magma
intrusion, but like other large plutonic masses it proves to be of composite
construction. The order of intrusion is ( 1 ) Unionville granodiorite, a basic
hypersthene-bearing granodiorite which itself has developed basic faces of
granogabbro; (2) Clancy granodiorite; (3) porphyritic granodiorite; (4)
biotite adamelite; and (5) muscovitic biotite granite. Alaskite and aplite
are abundant and were presumably (but not yet proved) developed most
abundandy during the final stages of bathylithic consolidation. The order of
emplacement of the successive intrusives is in the order of increasing silicity.

The Boulder bathylith and its satellitic stocks, have exerted extensive
contact metamorphism, both purely thermal and pyrometasomatic. Most

notably, the Helena dolomite has been transformed into aphanitic tremolite-
diopside hornfels to a maximum distance of 10,000 feet from the edge of the
bathylith. The highest rank of metamorphism attained is in sillimanite-cordi-
erite-microperthite hornfels, remarkable rocks that have formed at widely
separated localities. In places the magma has reacted with limestone xenoliths
with the result that the xenolith is surrounded by an aureole of augite granodio-
rite. In other places the evidence appears to demand that the magma in
depth had dissolved limestone. By this syntexis alkalic rocks were generated
that range from mildly alkalic, such as the Priests Pass leucomonzonite and
the syenodiorite of the large stock northwest of Helena, to strongly alkalic, as
represented by the nepheline shonkinite occurring east of Montana City.

In order to be consistent with the epoch designations for the absolute
ages of the Sierra Nevada batholiths, we must assign a mid- or early Late
Cretaceous age to the 87 m.y. date by the potassium-argon method of the
Boulder batholith. Since the batholith is composite we wonder whether
an early or late pluton in the intrusion cycle there is dated. It should be
noted also that a date of about 103 m.y. by the lead-alpha activity ratio
method is assigned to the Idaho batholith, but again, it is not known what
part of the batholithic cycle is dated. See Chapter 21. The difference in
method used also leaves the comparison uncertain. For the time being,
however, we should presume that the Boulder batholith and associates in
western Montana are slightly younger than the Idaho batholith.

Conclusions. The batholiths and stocks are fairly similar in composi-
tion to those of the Great Basin but have overtones of similarity in their
variations and size with the major batholiths of the Nevadan belt. The
volcanics are more andesitic than the dominantly latitic volcanics of the
Great Basin. It seems, therefore, that the western Montana and eastern
Idaho province displays characteristics transitional from the moderately
orogenic region of the Basin and Range province to the intensively oro-
genic region of the Pacific marginal regions.

Late Precambrian (?) Sills and Flows. Sills, flows, and some dikes of
basic rock have been noted in a number of places in the region of the
Boulder batholith and northward through the Garnet Range to Glacier
National Park. The sills are all intrusive into the upper part of the Belt
series and appear as dark beds of remarkably uniform thickness for main-
miles. They also hold remarkably well to a single stratigraphic horizon
and range up to 300 feet thick.

5 SO

STRUCTURAL GEOLOGY OF NORTH AMERICA

NEVAOAN BATHOLITHIC
BELT

GEOSYNCLINE INVOLVEO IN SHELF AREA INVOLVED IN
POST-BATHOLITHIC OROGENY POST-BATHOLITHIC OROGENY

Fig. 36.6. Relations of tectonic provinces of western United States to igneous. To identify
igneous provinces compare with Fig. 36.3.

The sill in the upper Belt series of Glacier National Park is a dark diorite
but in places is pink due to orthoclase feldspar and in other places green
from the presence of mica. Stratigraphically several hundred feet above
the sill is the Purcell lava unit 50 to 275 feet thick. It is made up of several
submarine flows with pillow structure. Hundreds of feet higher forming*
the top of the exposed Belt strata are several other flows. It is not known
whether the flow rocks are spilites and keratophyres (Dyson, 1949).

Basic sills of diabase ( ? ) occur in the Ray-Superior-Globe area of the
Mountain Region of south-central Arizona, and are the only other ones
known to the writer in the Laramide systems similar to those in the Belt
series of western Montana.

RELATION OF TECTONIC TO IGNEOUS PROVINCES

The major tectonic and igneous provinces of the western United States
are related to each other on the map of Fig. 36.6. The Nevadan batholithic
belt has about the same distribution as the previously existing eugeosyn-
cline. The Mexican geosyncline has considerable volcanic material on the
north and west in the Cretaceous sediments, but the Nevadan belt of
metamorphism and batholithic intrusions developed to the west in Baja
California. The later second cycle batholiths intruded the western flank
of the geosyncline, however, in great volume.

The batholithic belt was the site of later basaltic and andesitic eruptions
in two regions : ( 1 ) a narrow zone extending from northern California to
southern British Columbia, and (2) in central British Columbia along the
east-central part of the broad batholithic belt with the vast Coast Range
batholith entirely on the west. As in the Sierra Madre Occidental no row
of late Cenozoic stratovolcanoes occurs in the broad fields of central
British Columbia. The Cascade basalt-andesite field with its row of strato-
volcanoes correlates in north-south extent with the eastern bulge of the
batholithic belt in Oregon, Washington, Idaho, and southern British
Columbia. It does not continue southward where the batholithic belt
narrows in central and southern California. A genetic relation to the bulge
is implied.

The igneous rocks east of the batholithic belt in the miogeosyncline of
Nevada and western Utah are mostly of the monzonite-latite clan with

IGNEOUS PROVINCES IN WESTERN UNITED STATES

581

numerous stocks and widespread volcanism. The magma has generally
risen through a thick sedimentary veneer, and little basalt has emerged
at the surface. However, similar intrusions and extrusions occur in south-
ern Arizona where the sedimentary rocks are thin, so the sedimentary
veneer is not important, it seems. The latite magma is considered to be a
primary one, and its origin will be taken up on later pages.

The Laramide Rockies of central Montana, Wyoming, and Colorado,
as well as the Colorado Plateau constitute a large calc-alkalic and alkalic
province where assimilation of calcium-, sodium-, and potassium-rich
rocks in the crystalline "granitic" crust has been a prominent process. The
belt of Rockies through the shelf region seems to have affected the igneous
suites very little — they are approximately the same in the Colorado Pla-
teau, in the Wyoming and Colorado Rockies, and in the fairly stable area
east of the Rockies in Montana. Their prolonged and complicated differ-
entiation history bespeaks rather stable crustal conditions.
| The Columbian River tholeiitic flood basalts are principally of Miocene
age, center approximately in the great batholithic bulge, and have been
jfed upward through the batholithic complex (see Fig. 36.5).

The vent basalt field of the Snake River Valley and Malheur Plateau, is
■principally one of late Pliocene and Quaternary activity and occupies a
downwarp around the south side of the Idaho batholith and, very approxi-
mately, along the south side of the great bulge which seems to be con-
tinued eastward into western Montana by the large Laramide plutons
,there. It is suggested that since the basalt came directly from the subcrust,
lind the downwarp is across the Laramide trends of the central Rockies,
diat the folding and thrusting is shallow and that the downwarp is due
:o movements in the subcrust. It must be noted, however, that a large part
)f the field lies on the batholithic belt and has been fed by basalt from
he subcrust through the batholithic complex to the surface.

DISTRIBUTION OF PRIMARY MAGMAS

The map of Fig. 36.7 has been prepared to show the distribution of the
Afferent types of primary magma postulated to have given rise to the
gneous rocks now displayed at the surface.

Two principal types of primary magma are postulated, namely, the

GRANODIORITE GRANODIORITE ANDESITE
l$t CYCLE 2nd CYCLE

SPILITE- THOLEIITIC

KERATOPHYRE BASALT

— LATITE -MONZONITE

OLIVINE
BASALT

Fig. 36.7. Distribution of primary magmas. Batholiths of the second cycle occur in the Cas-
cade andesite province. The spilite-keratophyre magmas of the pre-Nevadan batholithic time
had about the same distribution as the batholiths. Andesite magma refers to the basalt-
andesite association of the orogenic belts (post-batholithic).

582

STRUCTURAL GEOLOGY OF NORTH AMERICA

basaltic and the granitic. The basaltic magma is of two classes, olivine and
tholeiitic, with transitional varieties recognized. The granitic type ranges
from tonolite to alaskite, and is considered to have originated in two
slightly different ways. The origin of the primary magmas will be con-
sidered under later headings.

It is evident that both olivine basalt and tholeiitic basalt magmas have
been conducted up through the granitic batholitic complex, and hence
both varieties in large amounts can supersede the granodioritic magma in
certain places. Smaller amounts of basaltic magma, probably all of the
olivine variety, have made their way up through the crust in the province
of the miogeosyncline or the latite-monzonite igneous province generally
as a prelude or as a closing note to the main magnetic activity.

Olivine basalt is considered the primary magma of the alkalic and
calc-alkalic provinces, although appreciable assimiliation and contamina-
tion of the magma has occurred. In a few places, considerable melting of
short-lived roots may have occurred, and here, by definition, the primary
magma would be granodioritic, quartz dioritic, or augite dioritic as locally
identified. Even here, some basalt may have been mixed in.

The andesite and spilite-keratophyre provinces probably do not mark
primary magma types. It is concluded on a later page too that they are
fractional differentiates of primary basalt, probably of tholeiitic basalt,
but because there is doubt about this conclusion they are shown separately
(Fig. 36.7). The latite-monzonite province is concluded to be a primary
magma province, although perhaps an unusal one.

IGNEOUS AND
TECTONIC PROVINCES
OF WESTERN CANADA

region subsided and great thicknesses of sedimentary and volcanic mate-
rial accumulated.

Large areas in the Yukon and Interior plateaus are underlain by
Triassic, Jurassic, and Lower Cretaceous formations made up of inter-
bedded limestone, argillite, graywacke, conglomerate, tuff, breccia, and
andesite flows. They have been invaded widely by great batholiths. Their
original extent may have been approximately that of the batholithic belt
of the map, Fig. 37.1, plus the areas shown as eugeosyncline. See also
Fig. 17.13.

Miogeosyncline

The position of the Paleozoic miogeosyncline was that of the eastern
Cordillera or more commonly referred to as the Canadian Rockies, east of
the Rocky Mountain trench. The Cambrian and Ordovician strata are here
particularly thick. The transition from the miogeosyncline to the Alberta
shelf is probably a gradual one and lies under the Alberta basin. The
miogeosyncline apparently dies out at about the Yukon Territory bound-
ary on the north, and thence northwestward the eugeosyncline is transi-
tional to the shelf. Thickness and lithologies in the Mackenzie and Selwyn
Mountains are very poorly known, and therefore, also geosynclinal and
shelf conditions cannot be very well discerned.

GEOSYNCLINE

Eugeosyncline

The eugeosynclinal division of the Cordilleran geosyncline of western
Canada and southeastern Alaska has been described in Chapter 6. Suffice
it to say here that sediments of the eugeosynclinal type occur west of the
Beltian geanticline of British Columbia. In southeastern Alaska strata of
Ordovician, Silurian, Devonian, Permian, and Triassic age are laden with
volcanics, whereas the lower and middle Paleozoic systems are not repre-
sented as far as known in the interior east of the Coast Range batholith.
During the Carboniferous and Permian periods, however, the interior

OROGENIES

The eugeosynclinal complex attests crustal unrest almost constantly
In places it was intense (see Chapter 5). Isoclinal folding with attendant
low-grade metamorphism occurred in late Jurassic or early Cretaceous
time to precede immediately the invasions of the numerous and large
batholiths.

The Laramide belt embraces the eastern Cordillera, and possibly a wide
region in Yukon Territory and in the western part of the Northwest
Territories, including the Franklin, Mackenzie, Selwyn, and Richardson
Mountains. The Mackenzie and Selwyn region is described as one of
broad folds and a subordinate amount of faulting (Lord et a!., 1947).
The folds are commonly arcuate and arranged en echelon. The Franklin

583

584

STRUCTURAL GEOLOGY OF NORTH AMERICA

| I FOLD AND THRUST

I | BELT, MOSTLY POST-

1 1 BATHOLITHiC

Mountains are probably part of this system, and the Richardson Moun-
tains are also thought to be made up of folds with a northerly trend. The
age in part from map relations appears to be pre- or Early Cretaceous, but
elsewhere to be Late Cretaceous or Early Tertiary. As described in fol-
lowing paragraphs the age of one of the batholiths in the Selwyn Moun-
tains is Mid-Cretaceous.

BELTIAN GEANTICLINE

A geanticline of Reltian strata emerges in southeastern Rritish Columbia
from the broad region of Reltian rocks in northwestern Montana, and
extends northwestward in narrowing fashion almost to Yukon Territory.
The Rocky Mountain trench lies along its east side for the most part. In
northwestern Montana the deformed Reltian is on both sides of the
trench zone, and in fact, forms the entire Rocky Mountain system there.

Rroad areas of strata, probably equivalent to the Relt series occur in
the Yukon, but since the whole region of outcrop is intruded by numer-
ous batholiths, it is regarded as part of the Nevadan orogenic belt and,
therefore, not shown as geanticlinal. The geanticline of Beltian strata
in eastern Rritish Columbia is probably one of Laramide orogeny. In
Chapter 6 the geanticline is postulated to have developed as early as
Cambrian time in the form of an arch separating the eugeosyncline from
the miogeosyncline, but the main rise, evidently, was incident to folding
and thrusting of the Laramide orogeny (see Chapter 20). The Beltian
geanticline is somewhat of a parallel to the South American geanticlines.

BATHOLITHIC PROVINCE

The batholiths of Rritish Columbia, Yukon Territory, and southeastern
Alaska are arranged in two divisions or belts. Those on the west are
described as follows by Lord et al. (1947):

k* STOCKS AND SMALL BATHOLITHS
% OUTSIDE MAIN BATHOLITHIC BELT.
MID- AND LATE CRETACEOUS

ACTIVE AND DORMANT VOLCANOES

Fig. 37.1. Major tectonic and igneous units of the Canadian Cordillera. G, Mt. Garibaldi.
Numbers such as 102 m.y. are absolute ages in millions of years determined by the potas-
sium-argon method (Follinsbee er al., 1957).

IGNEOUS AND TECTONIC PROVINCES OF WESTERN CANADA

585

The Coast batholith is the largest of the Mesozoic intrusions. It forms the

core of the Coast Range and extends northwesterly about 1,100 miles from

I the northern part of the State of Washington to Yukon. Its width averages

I more than 50 miles and locally exceeds 125 miles. Flanking it for many miles

on either side are smaller, related intrusive masses that, with the rocks of

the main batholith, comprise what is commonly known as the Coast intrusions.

1 In southern British Columbia the batholith curves towards the east and is

I linked with the presumably related Nelson batholith of Kootenay district by

other intervening intrusions. The Coast intrusions range in composition from

: granite to gabbro, but are mainly of granodiorite and quartz diorite. The

I batholith is a composite of an unknown number of phases that were emplaced

. as successive irruptions over a long period of time, and, presumably, the

numerous satellitic bodies are likewise of more than one age. The younger

I phases commonly show sharp intrusive contacts against older phases, and in

many localities that batholithic rocks cut Lower Cretaceous sediments that

contain pebbles of earlier batholithic rocks. It has been suggested that in

northern British Columbia the more acid phases are most common towards

the interior of the batholith. In the southern part of the province, however,

the eastern intrusions, such as the Nelson batholith, are more acid and contain

a greater proportion of granite than those nearer the coast.

A potassium-argon age determination of the Coast Range batholith
near Vancouver is reported by Folinsbee et al. ( 1957 ) as 105 m.y. This
i is about Mid-Cretaceous on the Holmes scale.

The eastern belt of batholiths starts southwest of the south end of the
Finlay River volcanic field at about Fort Frazer and Ruins Lake ( see
Geologic Map of Canada, 1947 ) and extends northward to the Yukon
and Alaska. In the Yukon the batholiths are so numerous that no marked
division can be noted between those of the Coast Range belt and those
of the eastern belt. Also at the south end of the eastern belt in the Burns
1 Lake region numerous small plutons bridge the Coast Range batholith
i to the eastern batholiths. Retween the south end and the northern cluster
is the Cassiar-Omineca batholith, which is over 500 miles long but rather
narrow. It is only partially explored and may not yet be completely
unroofed by erosion.

These rocks commonly grade into one another and are not known to
represent more than one continuous period of intrusion. Near Takala Lake
the batholithic rocks cut Jurassic strata of early Upper Jurassic age. They also
appear to have been the source of pebbles found in the early Upper Cretaceous

conglomerate. Thus, so far as known, the main Cassiar-Omineca batholith

is of Upper Jurassic or Lower Cretaceous age.

Intrusive, stock-like, tabular, and irregular bodies of serpentinized dunite,
peridotite, pyroxenite, and gabbro are found in southern Yukon, in Deasc Lake
and Takla areas of northern and central British Columbia, and in Bridge River.
Hope, Princeton, and other areas of the southwestern part of the province. The
largest are more than 100 square miles in area, but most of them are much
smaller. They are commonly considered to be early phases of the Mesozoic
batholithic intrusions and to be of Jurassic age (Lord et al.y 1947 .

A number of batholiths in the Selwyn-Mackenzie mountains salient
seem to lie east of the main eastern belt, and might be thought of as
Laramide. Yet a potassium-argon age determination by Folinsbee et al.
(1957) on the Itsi batholith (Fig. 37.1) is 102 m.y. or Mid-Cretaeeous.
This is about the same age as the Coast Range batholith at Vancouver.
It seems necessary to conclude that all the scattered batholiths in this
salient are of the same age until further determinations are made.

In southeastern British Columbia the Bayonne batholith immediately
east of Kootenay Lake has a potassium-argon age of 82 m.y. and is, there-
fore, about the same as the Boulder batholith of Montana to the south-
east which is 87 m.y. old ( Knopf, 1957 ) . Both would be Late Cretaceous
according to these dates and referable to the early Laramide phase of
orogeny. This batholith and the one just north of it, are therefore placed
in the Laramide belt of orogeny (see map, Fig. 37.1) and are considered
late satellites of the Nelson batholith and the Idaho batholith.

POST-BATHOLITHIC VOLCANISM

In the Canadian Cordillera crustal movements occurred during the
Tertiary from place to place along with considerable volcanism. The
Tertiary disturbances, from what is known, consisted of faulting, tilting,
and open folding. From Paleocene through Oligocene time sedimentary
and volcanic rocks accumulated in numerous small basins unconiormablv
on all older rocks. In Miocene and Pliocene time the major volcanism
broke out and several large fields of nearly flat-lying units accumulated.
The Miocene and Pliocene volcanics generally rest unconformably on the

586

STRUCTURAL GEOLOGY OF NORTH AMERICA

older Tertiary volcanics which had been tilted and eroded. Some late
Tertiary and Quaternary volcanic fields are also prominent. The volcanic
rocks throughout are basalt and andesite lavas and related pyroclastics
(Lord et al, 1947).

The major areas of Tertiary volcanic rocks are as follows. In Yukon
Territory and extending into Alaska irregular isolated areas occur which
lie mainly within two northwesterly trending belts. The easterly of the
two belts extends from near Carmacks, at the mouth of the Lewes River
to and into Alaska, and is here called the Yukon field. The westerly belt
lies along the northeast flank of the St. Elias Mountains and is here
called the St. Elias field (see map, Fig. 37.1).

In northern Rritish Columbia one area, in part of Quaternary age,
extends north from Telegraph Creek on the Stikine River for about 80
miles (the Telegraph Creek field) and another floors the Rocky Moun-
tain Trench for perhaps 150 miles along Finlay and Fox rivers ( the Fin-
lay River field). The latter consists of sediments and volcanics of late
Oligocene and early Miocene age.

The largest field is in the southern interior of Rritish Columbia where
major accumulations in places several thousand feet thick extend 350
miles in a northwesterly direction and 150 miles in a northeasterly direc-
tion. These are flat-lying and commonly referred to as the plateau basalts.
The accumulation is labeled "Plateau Volcanic Field" on the map. A
large and associated field immediately to the northwest is here called
the Fort Frazer. It consists in the Fort Frazer area of a lower series of
upper Oligocene and lower Miocene sediments and volcanics dipping at
angles up to 30 degrees and overlain unconformably by an upper series
of nearly horizontal basalt, andesites, and other volcanic rocks about
2000 feet thick.

An informative Tertiary section is found in the Okanogan Valley close
to the International Boundary. It is described as follows:

Here the Springhrook formation, perhaps of Paleocene age, and composed
of soils, alluvium, talus, stream and lake deposits, and tuff, rests on a pre-
Tertiary rock surface of steep relief. These strata accumulated in the valleys and
are overlain by and to some extent interlayered with the andesites, basalts, and

pyroclastic rocks of the Marron formation, which buried the valleys and
reached a thickness of more than 4,000 feet. The White Lake formation,
consisting mainly of lake and stream deposits with coal, was deposited on the
Marron strata from which most of their materials were derived. They are locally
as steep as 65 degrees and 4,000 feet or more thick. Their age is probably late
Eocene, but they may be somewhat younger. The White Lake strata are
overlain unconformably by beds of more gendy dipping andesitic breccia and
agglomerate, which are succeeded upwards by agglomerate and conglomerate.
The youngest conglomerate beds are horizontal and of pre-Pleistocene age
(Lord et al, 1947).

Volcanic activity occurred on a much reduced scale in the Quaternary
period. In Yukon very young lavas occur and a loose, white volcanic
ash is widespread which is at best only a few thousand years old. In the
Telegraph Creek field Hoodoo Mountain on Iskut River may still be an
active volcano. Recent lavas have been noted in several places along the
coast.

RELATION OF VOLCANISM TO TECTONIC PROVINCES

Most all post-batholithic volcanism in the Canadian Cordillera is
limited to the batholithic belt. Minor activity has occurred west of the
major Coast Range batholith in the island archipelago, but the major
activity was to the east of the island belt, and very approximately between
the western zone of batholiths and the eastern zone. The fields are dis-
continuous and the volume of extruded rock is apparently not large.
About one-tenth of the batholithic belt is covered.

The three trenches shown on the map, Fig. 37.1, are believed to be
Tertiary grabens, but little is known about them. The Finlay River
volcanic field fills the Rocky Mountain trench for a distance of about
180 miles. This is the only occurrence of volcanics in association with
the trenches whose combined length in Canada is 3000 miles. It can be
thought, therefore, that the association is accidental and not genetical. In
South America, the trenches and volcanism seem more closely associated.

The row of stratovolcanoes of the Cascades extends into southwestern
British Columbia. Pliocene, Pleistocene, and Recent activity is noted in

IGNEOUS AND TECTONIC PROVINCES OF WESTERN CANADA 587

the Telegraph Creek field, at Mount Hoodoo and Mt. Edgecomb and in Rockies. If a geanticline is present there, it is very broad and is invaded

the Wrangell field, but this is an alignment only in the broadest sense, and by so many batholiths that it is represented as part of the batholithic

separated by a 400-mile gap from the Cascade volcanoes. belt.

The Rocky Mountain trench separates the Beltian geanticline from the The batholithic belt is about 400 miles wide at the international bound-

Laramide Rockies in British Columbia. Its projection in Yukon, the ary and 300 miles wide at the Yukon-British Columbia boundary. At the

Hntina trench, separates the batholithic belt from the Laramide (?) Alaska border it is 200 miles wide. See Fig. 39.2.

38.

SPATIAL RELATIONS OF
MAJOR TECTONO-IGNEOUS
ELEMENTS AND
THE ORIGIN OF MAGMAS

PREVIOUS OROGENY IN EUGEOSYNCLINE

In all parts of the eugeosyncline of South and North America, evidence
of mid- or late Paleozoic orogeny is at hand, and especially in the Sierra
Nevada of California and the Coast Ranges of southeastern Alaska we
note a succession of orogenies of both Paleozoic and Mesozoic age. The
isoclinal folding and development of slaty cleavage in the Jurassic Mari-
posa formation previous to the intrusion of the batholiths in the Sierra
Nevada has clouded the effects of an orogeny of late Paleozoic age there,
but the importance of the older orogeny is emphasized by a study of the
eugeosyncline in South America where little note is made of the Late
Jurassic or Early Cretaceous orogeny and commonly only the older
Paleozoic one is recognized.

The Paleozoic orogeny in South America affected a belt from the
present coast to the eastern mountain ranges in places. It shows today
as the metamorphic basement of the Coast Ranges and as the meta-
morphic rocks in the anticlinoria 150 to 300 miles inland from the coast.
The anticlinoria are in the eastern part of the Mesozoic eugeosyncline
and in the western part of the miogeosyncline, and have formed during or
later than the batholithic orogeny. The great width of the belt of dynamic
metamorphism indicates orogeny of superior intensity long before the
invasion of the great batholiths. The batholiths generally were emplaced
in the oceanward margin of the metamorphic belt.

RELATION OF BATHOLITHIC BELT TO EUGEOSYNCLINE

The spatial relations of the major tectono-igneous elements of the west-
ern Cordillera of South and North America will now be summarized.
The batholithic belt coincides almost exactly with the previous eugeosyn-
cline. In places eastern segments of the eugeosyncline have not been in-
vaded by the great batholiths, and in one known place, the batholiths
have invaded the entire width of the eugeosyncline and also part of the
miogeosyncline. This is in Idaho, western Montana, and southeastern
British Columbia.

RELATION OF POST-BATHOLITHIC COMPRESSIONAL
OROGENY TO GEOSYNCLINE AND SHELF

After the main batholithic intrusions, mainly in the eugeosyncline,
strong folding and thrusting occurred in the miogeosyncline. In South
America Cretaceous and Tertiary sediments have accumulated on the
former transition region from miogeosyncline to shelf, and thus the rela-
tion to folding is obscured. Aside from the Pampean Ranges it appears
that no conspicuous orogeny has occurred in the shelf. The width of
the belt of folding ranges from 50 to 300 miles.

Post-batholithic folding in Mexico is extensive. Laramide folding from

588

SPATIAL RELATIONS OF MAJOR TECTONO-IGNEOUS ELEMENTS AND THE ORIGIN OF MAGMAS

589

the Nevadan batholiths to the east front of the Sierra Madre Oriental
forms a belt up to 450 miles wide. Its relation to Paleozoic sedimentary
basins is largely unknown, but it embraces the Cretaceous eugeosyncline
and miogeosyncline.

In the United States folding and thrusting extend through the mio-
geosyncline. See Fig. 36.6. Basins and asymmetrical anticlinal uplifts
expressive of significant vertical components of force are common in the
shelf. The maximum width of the belt of folding in the Paleozoic mio-
geosyncline is about 300 miles, and the front of the belt of deformation
in the shelf is another 400 miles farther east at its most easterly point.
As in South America rather thick Mesozoic sedimentary sequences have
accumulated along the Paleozoic miogeosyncline and shelf transition zone
and also in places over the shelf. The shelf in the United States was also
the site of building of the Ancestral Rockies in late Paleozoic time.

The belt of Laramide folding in British Columbia and Alberta is intense
and about 100 miles wide. It is confined to the Paleozoic miogeosyncline
and the western part of the Mesozoic basins over the miogeosyncline and
shelf. Farther north, the folding is less intense but has a maximum width
of about 300 miles. It spreads here mostly over a Paleozoic cratonic
■basin and an overlying Cretaceous basin, and the force component is

vertical.

I
■

RELATION OF POST-BATHOLITHIC VOLCANICS TO BATHOLITHIC BELT

In Chile south of Antofagasta and Argentina the post-batholithic vol-
canics are chiefly on the deformed eugeosyncline, miogeosyncline, and

shelf east of the batholithic belt. The maximum width of the general

belt of extrusive rocks is 300 miles. In northern Chile and southern
Teru the belt of volcanic deposits is partly on the batholithic belt, and

spans across the narrow eugeosyncline to the deformed miogeosyncline.

Only a small amount of extrusive rock is found as far east as the anti-

jclinorium. The main belt of volcanism is about 150 miles wide.

I1 The belt of volcanic deposits in Ecuador and Columbia spreads across

the boundary of the batholithic belt and the anticlinorium. This volcanic
.•field is narrow in comparison with the others, and does not exceed 50

miles unless some of the stratovolcanoes are considered part of it, in
which case the belt reaches 80 miles in width.

There are two major volcanic fields in Mexico. The larger of the two,
the Sierra Madre Occidental, rests on the deformed Cretaceous eugeo-
syncline and in places over the miogeosyncline, a considerable distance
east of the Nevadan batholithic belt. The smaller field, and the more-
recent, rests on the batholithic complex in the southern part of Baja
California. The great Sierra Madre Occidental field is 200 to 300 miles
wide. The smaller field in Baja California is 30 to 60 miles wide.

The volcanic fields in western Canada lie almost entirely within the
batholithic belt. The largest accumulations, in British Columbia, are 150
miles in width.

The volcanic fields of the western United States are broad and varied.
Where the batholithic belt is narrow in southern and central California,
volcanic deposits are few, but all through the broad miogeosyncline 150
to 300 miles wide, they are extensive. They also occur in considerable
quantity in scattered fields in the shelf to the east which in part has been
moderately deformed in post-batholithic time. Volcanic eruptions in
Colorado are 500 miles east of the miogeosyncline, and the Black Hills
igneous rocks are 350 miles east of the miogeosyncline.

In northwestern United States, where the Nevadan belt is very wide,
the great basalt fields occur. The tholeiitic (Columbia River) basalt field
is entirely on the batholithic belt and is nearly 300 miles wide. The olivine
vent basalt field is mostly on the batholithic complex but extends east-
ward over the miogeosyncline to the shelf. These two large basalt fields
are exceptional to all other fields in the western Cordillera of South
and North America, and seem related to the great batholithic bulge at the
intersection of two Nevadan orogenic arcs.

RELATION OF POST-BATHOLITHIC VOLCANIC FIELDS
TO STRATOVOLCANOES

The three rows of stratovolcanoes of the South American Cordillera
are closely related to the orogenic andesite complexes. The southern row.
south of Santiago, however, is not accompanied by voluminous fields; the

590

STRUCTURAL GEOLOGY OF NORTH AMERICA

volcanoes for the most part stand as isolated piles having been fed by
conduits through the batholithic complex and deformed eugeosyncline.

The stratovolcanoes of the southern Cascades of Oregon and Washing-
ton have been built on an older andesite complex but in the northern
Cascades of northern Washington and southwestern British Columbia
they stand as isolated cones fed by conduits through the batholithic com-
plex.

The stratovolcanoes of southern Mexico are built on an extensive older
volcanic field and are part also of extensive fields evidently as young as
the volcanoes themselves. The belt of stratovolcanoes seems to lie on
the inner margin of the metamorphic belt and also partly on the de-
formed adjacent geosyncline. See Chapter 43.

POST-BATHOLITHIC VOLCANICS TO TRENCHES

Trenches are of two kinds, the submarine deeps marginal to the con-
tinents and the fault-depressed zones generally within the batholithic
complex or separating it from the anticlinoria. The volcanics are of the
stratovolcanic and basalt-andesitic field types.

The fault-depressed trenches are the sites of both basalt-andesite com-
plexes and stratvolcanoes. The stratovolcanoes occur in both the de-
pressed blocks and on the adjacent upraised blocks. In Chile south of
Santiago the cones are chiefly on the east side of the depressed zone. In
northern Chile and southern Peru the volcanic deposits may have filled
depressed blocks, with extensively faulted regions both east and west of
the volcanic accumulations. In northern Peru, Ecuador, and southern
Columbia, the volcanic deposits have filled a long graben-like depression,
and stratovolcanoes are present within the depression and on its marginal
uplifted blocks, particularly on its eastern block.

The fault-depressed blocks of the South American Cordillera are gen-
erally described as compressional structures, bounded on one side or both
by uplifted overthrust blocks. The southern Mexico stratovolcanic
province is probably bounded on the south by a block-faulted region, but
little is known of the structures there. The great Sierra Madre Occidental
field is broken and bounded on the west by a thrust-faulted zone and
then by the major depressed zone of the Gulf of California. The de-

pressed zone here is postulated to be due to the drift of Baja California
away from the continent and to the northwest, in connection with the
San Andreas fault movements of California. A young volcanic field exists
on the west or outer side of the depressed zone.

In western Canada the andesite complex is west of the depressed zone,
here the Rocky Mountain Trench, which separates the Nevadan complex
and geanticline from the deformed miogeosyncline.

In the United States the relations are very complicated, and compari-
sons with the South American and Canadian can only be imagined.
The broad Basin and Range province would be the fault-depressed belt,
which in Nevada and Utah is superposed on the eastern side of the eugeo-
syncline and across the entire miogeosyncline. It is replete with volcanics
but not of the basalt-andesite complex, but rather of the monozonite-
latite clan. A zone of particularly conspicuous trenches (graben and
horst blocks) make up the eastern side of the Basin and Range province
and these extend northward through Idaho and western Montana into
British Columbia. Relatively minor volcanic activity is noted in the zone
of trenches from the High Plateaus field of south-central Utah to the
Finlay River field of northern British Columbia.

A spatial coincidence of the stratovolcanoes of South America to the
offshore, submarine trenches is immediately conspicuous, but in detail we
may note first, the Chilean row south of Santiago extends southward
beyond the limits of the submarine trench and second, the trench is con-
tinuous but the stratovolcanoes occur in three separate rows or seg-
ments.

The Central American Trench lies opposite the stratovolcanoes of south-
ern Mexico and also the active and dormant volcanoes of Central America,
but the trench, as an ocean-floor phenomenon, does not continue north-
ward where the major volcanic complex of Mexico occurs. The sub-
marine trench coincides well with recent volcanic activity but not with
the older activity.

The Cascade andesite complex and row of stratovolcanoes is not com-
plemented by a submarine trench. It may, therefore, be concluded that
a submarine trench is not a necessary accompaniment of a row of adjacent
stratovolcanoes; one may exist without the other, but their coincidence
spatially is more likely than not.

SPATIAL RELATIONS OF MAJOR TECTONO-IGNEOUS ELEMENTS AND THE ORIGIN OF MAGMAS

591

It may also be concluded that trenches within and east of the batho-
lithic belt are nearly everywhere present along the entire Cordillera of
North and South America, and that in places volcanism seems fairly well
localized to the trench or immediately adjacent to it. Extensive volcanism
occurs in Mexico, however, on either side of and at a considerable
distance from the depressed zone.

RELATION OF ANTICLINORIA TO OTHER ELEMENTS

Anticlinoria of Precambrian or metamorphic Paleozoic rock occur
parallel to and on the inside of the batholithic belt. These generally ele-
vated areas are encompassed in the belt of post-batholithic folding and in

; places are separated from the batholithic zone by the fault-depressed
zone. The anticlinoria range in width from 50 to 150 miles. They are

i present in the more typical South American and Canadian Cordillera but

i not present in the atypical United States Cordillera. They lie generally
east of the major volcanic fields, although some volcanics occur on them

! and even east of them.

| ORIGIN OF MAGMAS

Physical Considerations

Crustal Structure. The crust forming the continental masses according
to seismic information (Tatel and Tuve, 1955), has a general thickness of
28 to 35 kilometers, but interpretations as low as 20 kilometers in coastal
California and as great as 65 kilometers under the Sierra Nevada and 72
kilometers under the eastern Great Rasin are given. An abrupt change in
seismic velocities at the base marks the Mohorovicic discontinuity which
is believed to be world-wide.

The upper layer of the crust which has low velocity is called the granitic
crust, silicic crust, or sial, and the lower, the basaltic crust, gabbroic crust,
sima, or subcrust. Tatel and Tuve concluded that the two are probably
transitional, but others have postulated distinct layers locally of inter-
mediate velocity and of different relative thicknesses.

The silicic layer consists of the fighter rock-forming silicates and is high
in Si and Al, and the basaltic layer, as the name implies, consists of the

darker and heavier silicates and is lower in Si and Al and higher in Fe
and Mg.

The floor of the oceans, beneath a thin veneer of lava flows and un-
consolidated sediments, consists of a basaltic laver 5 to 10 kilometers
thick, which overlies the mantle. Extensive volcanic accumulations, aLso
composed mostly of basalt, rest on the basaltic crust in many places.

The outer part of the mantle down to a depth of several hundred
kilometers is crystalline. It consists mainly of dense silicates of Mg and
Fe, prominent among which is olivine, and is often referred to as peri-
dotitic (Turner and Verhoogen, 1951), but many be eclogite, a high-
density phase of gabbro (Kennedy, 1960).

Geothermol Gradient and Melting Points. Measurements in mines and
wells indicate that the earth temperature increases with depth at a rate
of approximately 30 °C per kilometer. Gradients as low as 7°C per kilo-
meter and as high as 50°C per kilometer are known but are exceptional.
According to Turner and Verhoogen ( 1951 ) the temperature at the base
of the crust, say at 40 kilometers, is 500 to 600°C, at 100 kilometers 800
to 900°C, and at 2900 kilometers 1500°C.

Magmas erupted from volcanoes have been found to have temperatures
as high as 1000 to 1200° C, and the melting temperature at the surface of
basalt of about 1000° C is in this general high-temperature range. Rut such
a temperature is not normal to the rocks at the 40-kilometer depth. Con-
sequently, according to Turner and Verhoogen, either the magma origi-
nates by fractional melting of deep-seated earth material of peridotitic
or eclogitic composition, or it is the result of melting of shallower rocks
in place under temperatures temporarily raised far above the average
temperature normally prevailing at that depth.

Temporary and local increase in temperature within the crust or outer
shell of the mantle might be developed in three ways : ( 1 ) by the blanket-
ing effect of a thick sediment-filled basin; (2) local radiogenic heat; and
(3) frictional heat due to diastrophism. Turner and Verhoogen conclude
that the blanketing effect of sediments 10 kilometers thick would result in
an increase in temperature of less than 200 or 300° C in the crustal rocks
beneath. Regarding radiogenic heat in the outer mantle shell, they be-
lieve that this could result in cyclical convective overturn, and that the
temperature of the crust immediately above would be raised appreciably

592

STRUCTURAL GEOLOGY OF NORTH AMERICA

above its normal value by conduction with each fresh overturn of the con-
vection cell. This should correlate with intermittent magma generation,
and possibly the development of the 7.5-kilometer-per-second seismic
velocity layer (Chapter 31).

Heat generated by crustal deformation has been held very significant
by some, and the epigram "Diastrophism is the mother of volcanism" is
commonly recited; yet widespread, and in places voluminous, magmatic
activity has occurred in regions of crustal stability. As concluded in the
chapters on the Rocky Mountains, igneous activity may be the cause of
the diastrophism. There must be a real tie between the batholiths of the
Nevadan belt and crustal deformation, and also between the later
andesitic-basalt eruptions and diastrophism. In reference to the great
mantle fault, shown in Fig. 38.3. Benioff suggests that considerable
heat is generated in the aftershocks which are a manifestation of creep
strain in the rocks, and that this heat may be sufficient for the apparently
related volcanic activity. He says,

A rough idea of the magnitude of energy released, say per year, by the
aftershock sequences in a region on one side of the fault can be obtained by
taking one fourth of the energy released in the same time by seismic waves
in the principal earthquakes. Thus, in the case of South America, the shallow
and intermediate earthquake sequences each liberate approximately 4 x 1021
ergs per year. Thus roughly 10J4 ergs per year is being released in the fault
blocks. The writer has no knowledge of the amount of energy per year
required to maintain the South American system of volcanoes, and consequently
it is not possible to say whether or not the energy requirements are met on
this hypothesis. Moreover there must be a large time lag between the liberation
of heat in the depths and its appearance in the form of volcano output. Thus
the present rate of volcanic energy release should be equated to a phase of
seismic-heat generation which occurred long ago, rather than to the present
rate.

The problem of the origin of magma is not one of quantity of energy
according to Turner and Verhoogen (1951):

. . . radiogenic heat in the earth seems to be ample to account for all geologic
(including igneous) phenomena, but what must still be sought is some process
which will concentrate this energy locally, and raise the temperature sufficiendy,
at points of concentration. Igneous activity itself testifies to the operation of
some such process. But its precise nature remains an unsolved problem.

Primary Magmas

Definition. Primary magma, by definition, originates by partial or
complete fusion in great volume of pre-existing rock. It is conceivable
that some igneous bodies have come from a primitive liquid still existing
from an early stage in the earth's history but no satisfactory evidence for
such has been recognized (Turner and Verhoogen, 1951). The modifica-
tion of a primary magma results in derivative magmas.

Criteria by which a primary magma may be recognized as such are somewhat
vague. Probably the most satisfactory is a pronounced tendency for the
magma to appear repeatedly throughout geologic time, in great quantities and
in extensive individual bodies (lava floods, batholiths, lopolithic sheets, etc.),
over large sectors of the earth's crust. A further criterion is predominance of
corresponding rocks within one or more rock associations, the other members of
which could have been derived from the primary magma by accepted modifying
processes — differentiation, assimilative processes, etc.

Conversely there is a tendency to regard magmas as belonging to the
derivative class when they occur habitually in small quantities, when they
are constantly found in association with a magma conventionally considered as
primary, and when derivation from the latter can be explained in terms of
accepted modifying processes (Turner and Verhoogen, 1951).

Classification. There is general agreement that two broad primary
magma families exist, namely, granitic and basaltic. By granitic is meant
the common associates, granodiorite, quartz monzonite, and granite, and
perhaps tonalite, diorite, and others closely akin which in places occur in
great volume. Extrusive andesite is regarded by Waters (1955) as a pri-
mary magma, but this is questionable. Its relation to the granitic group
will be discussed later.

The basalt family is made up of two main varieties, namely, olivine and
tholeiitic. Gradational varieties are common.

Magmas of the Alkalic Igneous Province

Prevalence of Olivine Basalt as Primary Magma. Under a previous
heading in connection with Fig. 36.6 it was concluded that the exposed
igneous rocks of the alkalic igneous province of the western United States
were derived from a primary olivine basalt magma. It was also postulated
that the surficial intrusions and extrusions come from megasills in the sur-

SPATIAL RELATIONS OF MAJOR TECTONO-IGNEOUS ELEMENTS AND THE ORIGIN OF MAGMAS

593

ficial granitic crust where various amounts of assimilation have occurred.
Of course, magmas intrusive into shallow sedimentary sections have
affected the overlying beds such as over laccoliths and bysmaliths, and
even over and around stocks in places, but these structures could not be
related to the origin of the magma.

The Laramide structures of the alkalic province appear not to have
roots as previously suggested, and one of the most intriguing geophysical
studies is the seismic charting of the velocity layers in pursuit of this prob-
lem, and also the source of magmas there.

Seismic Evidence of Crustal Structure. Tatel and Tuve (1955) report
the base of the basaltic layer ( Mohorovicic discontinuity ) under the
Colorado Plateau ( part of the alkalic province ) at the shallow depth of 30
kilometers. This was surprising because from isostatic considerations the
high plateau should have been supported by a crust some 50 to 70 kilo-
meters thick (50 to 70 kilometers to the M discontinuity). From this and
other data they conclude neither the Airy nor Pratt concepts of crustal
structure hold. Gravitv observations indicate a continent over which there

is isostatic compensation, and therefore, they conclude that the outer
mantle below the crust has density variations (see Chapter 31). Thus,
in turning to the outer mantle for causes of vertical movements, a column,
perhaps one to several hundred kilometers thick (or long) may be in-
volved, and if so, only small changes are necessary to elevate the plateau
5000 to 8000 feet.

Basaltic Magma from the Mantle. Rasalt magma can originate (1)
by fractional melting of deep-seated earth material of different composi-
tion, (2) by complete melting of a deep-seated rock of the same composi-
tion, or (3 ) by complete melting of shallower rocks temporarily raised far
above the average temperature normally prevailing at that depth. If
surface basalts come from the mantle the process has been postulated to
be one of partial melting of a basic rock of the composition of stony
meteorites ( peridotitic ) , or of melting of eclogite, a heavy crystalline rock
of basaltic composition. Also the subcrust, presumably of basaltic com-
position, might melt in places to form a basaltic magma. The primary
olivine basalt of the Laramide Rockies is believed by the writer to have
come from the mantle and the following reasons are given.

1. The seismic evidence of lavering, and the consequent interpretation
of gravity measurements indicate that rock density changes must occur
in the mantle. The Laramide structural province generally lacks roots
and is underlain by the thick 7.5 layer. Therefore, density changes in the
mantle are almost the sole explanation of isostatic adjustments. This sug-
gests that magmatic processes may be occurring there.

2. The two primary basaltic magmas, tholeiitic and olivine, are best
explained as coming from the mantle. See discussion under a later head-
ing, "Tholeiitic Magma."

3. The heat necessary for local partial melting of the upper part of the
mantle in places may adequately be provided by underlving conveetive
overturn, or possibly by generation along faults deep in the mantle.

If partial melting of spots in the outer mantle shell is postulated as the
source of primary basaltic magma, a series of consequences must be en-
visaged, which, if the theorv is correct, must fit the pattern of structures
and igneous intrusions and extrusions through time and space as well as
geophysical observations and analyses. First, there are the considerations
of expansion. Since the Colorado Plateau is some 2 kilometers above sea
level and has no roots to buoy it up, we can think of expansion of the
mantle beneath to have raised the crust. In melting, a volume increase of
11.2 percent occurs, and if 6 percent of a 300-kilometer column should
melt, the crust would be elevated 2 kilometers. There would also be ex-
pansion in the solid state of the column over the convection cell as heat
is conducted upward. The problem is complicated and will require the
attention of experts, but evidently the amount of solid expansion is con-
siderable. The concept of rise of basaltic magma from the mantle is at-
tractive because it presents a plausible theory of the origin of the basaltic
subcrust. Instead of a primitive basic differentiate of a more silicic melt
from above, the subcrust would be the result of additions through time
from below. This view would hold for the basaltic substratum under the
continents, but for the ocean floors we would need to think of early out-
pourings, and after sufficient accumulation, increasing amounts of magma
from below to build up the basaltic layer.

In connection with the arrest of basaltic magma from die mantle in the
subcrust we may think of uplifts like the Rlack Hills, Rig Horn Moun-

594

STRUCTURAL GEOLOGY OF NORTH AMERICA

tains, Uinta Mountains, and San Rafael Swell as results. These elongate
uplifts have lengths of 75 to 150 kilometers and widths of 40 to 75 kilo-
meters. Giant-sized laccolithic intrusions in the subcrust of similar hori-
zontal dimensions could have originally arched and upfaulted die struc-
tures, whose structural relief would thereafter have been augmented by
sediment transfer to adjacent basins and gravitational adjustments. A
giant-sized laccolith would need to be perhaps only 1 kilometer thick to
result in a final structural relief at the surface of perhaps 3 or 4 kilo-
meters. See Fig. 36.4.

Still other considerations of the theory of primary basalt magma gener-
ation in the outer mantle remain. They are in the fields of gravity and
seismicity. Dr. Kenneth L. Cook's reactions to the gravity problems are
as follows. If the outer shell of the mantle should expand and elevate the
crust, say of the Colorado Plateau, 2 kilometers, isostatic anomalies in
the order of 10 to 20 milligrams would probably occur, but effects of local
surficial density variations might mask the overall isostatic anomaly pic-
ture to the extent that it would be unrecognizable. The problem is fairly
complex. At least the concept of partial melting and expension of the
outer mantle under regions as large as the Colorado Plateau does not run
afoul of any gravity observations or interpretations that he could see
off hand.

Dr. Joseph W. Berg's reactions to the seismic problems are as follows.
Melting of 5 percent of a certain column of the mantle in a disperse sys-
tem of some kind would lower earth wave velocities, but not any more
than the observed range of velocities interpreted from seismic records in
the upper mantle or lower crust. As far as he could see, the concept of
partial melting of parts of the upper mantle 50 to 200 miles across is not
contrary to any seismologic analysis.

From the above considerations it is concluded that under the Laramide
systems of the alkalic igneous province olivine basalt was generated by
partial fusion of the upper mantle and rose to the subcrust where it was
intruded, probably in giant sill bodies; only minor amounts escaped up
through the silicic crust to the surface. Large bodies of the molten basalt
lay directly under the silicic crust, affected some melting and assimila-
tion, and by various routes of fractional crystallization, mixing, and sieving,

the contaminated primary magmas bore in small amounts the unusual
alkalic and calc-alkalic suites of the Colorado Plateau, Wyoming, and
Montana.

Magmas of the Nevadan Systems

The conclusion has been reached on prevous pages that the batholithic
masses of the Nevadan belt represent such an enormous bulk of quartz-
monzonitic and granodioritic material that it is impossible to conceive of
their derivation from a basaltic parent by fractional crystallization, and,
providing they were once mobile, we are forced to conclude that they rep-
resent a primary acid magma. Further, the primary magma originated by
the melting of a part of the silicic crust in a master belt of orogeny along
the continental margin. The conventional concept involves a thickened
crust whose roots melt. The thickness of the silicic layer under the Sierra
Nevada is now about 20 kilometers and about 25 kilometers in north-
central Utah, but possibly before melting and isostatic adjustments, the
crust there was much thicker. See Fig. 38.1. The basaltic subcrust seems
about as thick as the silicic crust under the Sierra Nevada, but if upward
adjustment has occurred after orogeny then the silicic crust would have
been thinned by erosion, as well as viscous flow, and this consideration
points to a previous much thicker silicic crust.

The theory of origin of primary basalt, therefore, contrasts sharply
with that of primary granodiorite; the first by partial fusion of the upper
mantle shell and upward migration to the subcrust and crust, and the
second by fusion of large masses of the lower part of the thickened silicic
crust in belts of master orogeny.

The above discussion is in the manner of those who believe that the
great batholiths were emplaced by mobile magma, but there are many
authorities who believe that the batholithic rock formed in place by a
transformation of previously existing rock. Strong evidence is presented
to support this point of view, namely that of granitization. For a review of
the evidence see Gilluly (1948). One's point of view changes radically
in considering crustal layering, roots, and intrusion space problems when
convinced that granitization is the process at hand. It will be commented
on later under the headings of andesite magmas and quartz latite magmas.

SPATIAL RELATIONS OF MAJOR TECTONO-IGNEOUS ELEMENTS AND THE ORIGIN OF MAGMAS

595

PACIFIC
OCEAN

PACIFIC OCEAN TO COLORADO PLATEAU

BASIN

AND

COAST GREAT

_RANGES VALLEY

g — _^ — — — — — _ l_'i j.» 7 -"- -i'-"- -"-'-JU-"- *— **~ D.I J l\M/3CW C 11 V

jftsi—*"7-0

40 KM-' "~-

SIERRA NEVADA

20 KM''

WASATCH
PROVINCE MOUNTAINS UINTA BASIN

-iTv r~T~rY]/y^7-

633 KM/SEC d KM " 16'TKM-^

25 KM

8 0 KM/SEC

50KM 759 KM/SEC

z»¥"km/sec

72 KM

ELKO TO PROMONTORY BLAST SITE

KILOMETERS

4200 FT. ABOVE
S.L.

25 KM

Fig. 38.1. Postulated seismic layering in relation to geologic structures at the surface of the western
Cordillera. Velocities in Wasatch and Great Basin from Berg ef al. (1960); for western Great Basin, Press
(1960) and in the Sierra Nevada, Gutenberg (1943).

Tholeiitic Magma

The occurrence of theoleiitic basalt in eugeosynclines, over a wide area
of the older Nevadan batholithic complex, in the Triassic fault basins of
the Appalachian Mountains systems, in the Parana basin of the stable
shield area of Brazil, and in the Hawaiian Islands of the Pacific basin
indicates diat no tectonic setting has a monopoly on the magma. Previous
considerations have shown that there is a fairly complete range from
olivine basalt to tholeiitic basalt, if world-wide examples are tabulated
together, but in local occurrences the spread is usually small, and separate
igneous provinces may be recognized.

Convincing petrographic evidence for the origin of basalt by partial
melting of the mantle comes from olivine-rich nodules in basalts. These
are concluded by Ross et al. ( 1954 ) to have been xenoliths derived from
the peridotitic mantle, and studies by Kuno et al. ( 1957 ) lead them to the

same theory. Such xenoliths are known from about sixty localities scat-
tered throughout the oceanic as well as continental regions. Kuno con-
cludes that most of the olivine-rich nodules occur in alkali olivine basalt
and allied rocks such as andesine andesite, nepheline or leucite basalt,
and basanite and limburgite. Only a few doubtful examples in tholeiites
are known. This distribution was first thought to signify that only the
alkali olivine basalt comes from the mantle, and that the tholeiitic origi-
nates from bodily melting of the basaltic subcrust, but when it was re-
alized that the Moho discontinuity under the Hawaiian Islands is only
5 kilometers deep, it was concluded that temperatures could not become
high enough in the basaltic subcrust to cause melting. Kuno concluded,
consequently, that both basalt types originate by partial melting of the
upper mantle.

From extensive petrographic and chemical studies, particularly in

596

STRUCTURAL GEOLOGY OF NORTH AMERICA

Hawaii, Kuno et al. (1957) conclude that neither basalt magma may be
derived from the other by fractional crystallization, or in other words,
that neither is the parent of the other. This emphasizes again the origin
by partial melting of the peridotitic mantle.

Two possible reasons may be presented for the origin first of one magma
and then of the other, or of one magma in one place, and the other in
another place. The first is based on the assumption that the mantle is
slightly heterogeneous in composition and that by partial melting of one
part an olivine-rich basalt will be produced and by partial melting of an-
other part with a slight difference in composition a tholeiitic basalt will
result. Since these variations in the mantle are not tied to the tectonic
divisions of the continental or oceanic crust in any way at present recog-
nizable, it would thus be apparent why either variety of basalt rises in
most any tectonic setting.

Kuno proposes a second possibility, namely that the mantle is of uni-
form composition and that different pressures will cause slightly different
melting of the peridotite. In the Japanese Archipelago he suggests that
the parental tholeiite magma is produced by partial melting of the peri-
dotite layer (mantle) at depths shallower than 200 kilometers, and that
the parental alkali olivine basalt magma is produced by partial melting
at depths greater than 200 kilometers (Kuno, 1959).

Basalt-Andesite Assemblages of the Eugeosyncline and Orogenic Belts

On previous pages we have seen that the igneous rocks, so abundant
in the stratified sequences of eugeosynclines are principally andesite and
tholeiitic basalt. Spilites are common, and according to Waters (1955)
they are tholeiitic basalt altered mostly by rising hydrothermal solutions
but in part by sea water in connection with submarine flows. The albite
may also be added in subsequent metamorphism, but in any case, they do
not therefore add to the problem of the origin of the association of
tholeiitic basalt and andesite. Keratophyres bear much the same relation
to andesite as the spilites do to basalt and, hence, likewise do not pose an
additional problem in the nature and origin of the primary magma. Olivine
basalt is reported in places in the eugeosyncilinal assemblage but infor-
mation on its relative volume and distribution is not well at hand; never-

theless it seems that provision should be made for its presence in the
eugeosyncline in any theory of origin devised of the igneous complex
there. Certain acid varieties are present in small amounts and are un-
doubtedly derivatives of the others.

The association of basalt and andesite in the volcanic fields of post-
batholithic age has been elaborated on in previous pages. Reference to
the cross sections of Fig. 34.5 and the map of Fig. 36.5 indicates that the
site of most extensive occurrence is on the deformed belt immediately
inside the batholithic belt toward the continent, which embraces parts
of the older eugeosyncline not metamorphosed and invaded by the batho-
liths, and most or all of the miogeosyncline. In the United States where a
belt of Laramide deformation is beyond the miogeosyncline in the shelf
region, andesites also occur. About half the bulk of the San Juan field in
Colorado is andesite, the other rhyolite, with basalt subordinate, so it is
evident that somewhat different conditions apply there. The broad Great
Rasin region of the western United States is also unusual in relation to the
general composition of the great western Cordillera of the Americas and
will need special consideration.

The Cascade volcanic complex of Oregon and Washington should be
mentioned in regard to post-batholithic activity because of significant
associations there. It, however, is not a parallel with apparent normal
conditions in the Cordillera, because it is a local field on the batholithic
belt and also probably on the oceanward side of the batholithic belt built
as part of a new continental margin. The older Cascade complex is more
variable than that of the younger stratovolcanoes and according to
Waters is a tholeiitic-andesite assemblage with some olivine basalt present
whereas the younger is an olivine basalt-basaltic andesite assemblage.

Andesite is also found on Hawaii, in an olivine basalt, ocean basin
assemblage. In connection with this occurrence and with the transitional
nature of basalt and andesite the following quotation from Williams et al.
( 1954 ) is significant.

Olivine-bearing andesites. These are widespread on oceanic volcanoes,
like those of the Hawaiian Islands, and in orogenic belts of the continents.
Indeed they probably predominate among the Tertiary and Quaternary lavas
of the Circum-Pacific belt. Many of them lie so close to the boundary between

SPATIAL RELATIONS OF MAJOR TECTONO-IGNEOUS ELEiMENTS AND THE ORIGIN OF MAGMAS

597

andesite and basalt that only chemical analyses serve adequately to classify
them; in default of analyses, these borderline lavas are sometimes spoken of
as "basaltic andesites." Olivine and labradorite may be their principal minerals,
yet their silica content and the presence of normative quartz relate them to
the andesite family.

Another variety is pyroxene andesite which according to the above
authors is especially common on large composite volcanoes in the orogenic
belts. Still others are hornblende and biotite andesites. These generally
form thick short flows, steep-sided domical protrusions, or intrusive plugs
and dikes, and are generally more siliceous and alkaline, and graded into
dacites and trachyandesites.

With the above observations about the tectonic distribution and petro-
logic relations of andesite in mind, we must recognize four possibilities
of origin: (1) a rock of andesitic composition melting completely and
furnishing a primary andesitic magma; (2) a more basic rock melting and
partially freeing a liquid of andesitic composition; (3) a granodiorite-
granite primary magma mixing with a primary basaltic magma to form
an andesitic magma; or (4) formation of an andesite by some variation
of fractional crystallization.

The fourth category has two variations according to Turner and Ver-
hoogen (1951). Ry fractional crystallization tholeiitic basalt may yield
andesite; and a primary granodiorite-granite magma may yield andesite as
a basic differentiate. The fractional crystallization of an alkali olivine
basalt, according to Kuno (1959), could not result in an andesite, but
instead various rocks like trachybasalt, nepheline basalt, trachyte, phono-
lite, or syenite would form. A calc-alkalic olivine basalt, however, can
differentiate to an andesite (Kuno, 1959). The andesites in the San Juan
field are regarded by Larson and Cross, with various mixings and con-
taminants, to have come from an olivine basalt. It is concluded that both
tholeiitic and olivine basalt can give rise by differentiation to one varietv
or another of andesite.

For the small quantities of oceanic andesite the process of fractional
crystallization from a tholeiitic basalt seems the most likely origin. This
theory necessitates the presence of tholeiitic basalt in an olivine basalt
province, but fortunately tholeiitic basalt is present in some oceanic
islands.

As to the mixing of primary granodioritic magma with primary basalt
magma to yield andesite magma, the process conceivably could occur in
the root region of the batholithic belt and would involve the rise of pri-
mary basalt from below, according to the theory proposed on previous
pages for the origin of either primary olivine or tholeiitic basalt. This
could probably produce the necessary large volumes of andesitic magma
necessary and also the transitional varieties from the two primary types.
Mixing is not possible if the batholiths form by granitization.

In connection with the concept of roots of the batholithic belt it does
not seem logical to think of them at one time melting to form a magma
of granodioritic and granitic composition and then later melting to form
one of andesitic composition. This is probably a good argument to the
effect that the batholiths formed in place, and have nothing to do with
roots. If it is accepted that the batholiths form by granitization, then it
seems possible that roots, if they exist, could melt to form the andesites. It
has been suggested, also, that the eugeosynclinal sequence of graywacke,
argillite, and basic volcanics, if melted in bulk, would form a magma of
andesitic composition. Inspection of the maps of South America, Figs.
34.1 and 34.2, will reveal that the large basalt-andesite complexes spread
about equally over the eugeosyncline and miogeosyncline, so that the
rocks in the eugeosyncline do not seem to have a direct bearing on the
origin of the andesitic magma.

Ry elimination then, and with a bias for the magmatists, we arrive at
the conclusion that the andesites are differentiation products of basaltic
magmas, which vary in composition themselves between olivine and
tholeiitic. The andesites in the alkalic and calc-alkalic provinces ( techni-
cally the shelf, partly deformed in the Laramide orogeny) are probably a
different breed from those of the deformed eugeosyncline and miogeo-
syncline, and have come about through an eventful history of mixing of
differentiating magmas, and by appreciable assimilation of high calcic and
alkalic rocks of the silicic crust. The andesites of the eugeosyncline and
post-batholithic orogenic belts are only a step away from the basalts, the
more acidic differentiates are centainly in the small minority, and the
volumes of andesites and basalts are great, and the succession of flows
and repetition in space monotonous.

598

STRUCTURAL GEOLOGY OF NORTH AMERICA

If the andesites of the basalt-andesite complexes of the eugeosynclines
and orogenic belts are differentiates of basaltic magma, then large volumes
of the rising basalt from the upper mantle are trapped or arrested in the
basaltic subcrust, where partial fractional crystallization and the develop-
ment of andesitic and basalt-andesitic magmas takes place. Then as fissures
in the overlying crust come into existence various magma pools are tapped,
which may be basaltic through transitional phases to andesitic or even
in rare instances, dacitic, and the surficial basalt-andesite complexes are
extruded. In the voluminous outpourings andesite is commonly the most
acidic rock produced, and so it would appear that the arrested bodies or
reservoirs of magma in the subcrust are sill-like and not very thick, other-
wise if in large bodies of several kilometers in vertical dimensions more
varied and more silicic magmas might result.

Why the restriction to the orogenic belts and the eugeosynclines? The
eugeosynclines are essentially orogenic belts themselves, but probably
without appreciable roots until involved in the climactic batholithic orog-
eny. The blanketing sediments of the geosyncline result in the rise of
temperature in the underlying crust and upper mantle, and hence may be
thought of as bringing on the subcrustal igneous cycle. However, basalt
has risen under the shelf of the stable region in considerable amounts
without a thick, widespread, sedimentary blanket. The miogeosyncline
developed irregularly with basins and arches, and these from previous
discussions would have resulted from expanding and contracting columns
in the mantle below without appreciable exclusion of magma. The eugeo-
syncline, on the other hand, has been built partly by volcanic activity.
This is part of the unrest of the continental margin, and for some reason
the mantle there has been, since Ordovician time at least, the site of exces-
sive heat evolution causing magmatism and surficial orogeny.

Magmas of the Latite Ignimbrite Subprovince

Petrology. The first requirement in consideration of magmas of the
latite-ignimbrite subprovince is a voluminous supply of a fairly uniform
quartz latite magma. The volume is comparable to that of the Columbia
River basalt field. The composition appears fairly uniform within the
province; according to Howcl Williams a number of rocks called andesites

and dacites are only such by certain systems of nomenclature, and are
really close to the quartz latite welded tuffs. Certain stocks are as basic
as diorite or quartz diorite, and provision for them must be made in any
theory of origin of the magmas.

Relation of Welded Tuffs to Stocks. The commonness of monzonite
and quartz monzonite stocks and their similar composition to the
quartz latites is striking. Nothing can be added to Gilluly's discussion
of the close relation of the two as reviewed on previous pages, in which
he postulated a reservoir of primary magma of quartz latite composition
from which both the intrusives and extrusives were derived without
further differentiation. Stringham's survey of the stocks of western Utah,
Nevada, southern California, Nevada, and New Mexico indicates that
some are as basic as diorite, but these are few. The quartz diorite of the
Cottonwood stock of the central Wasatch ( Fig. 38.2 ) lies close geographi-
cally to the Bingham quartz latite or granite stock and indicates the
variation in composition that can exist within a few miles. As to the dis-
tribution, the stocks are abundant in the ignimbrite subprovince but
equally abundant, evidently, outside the subprovince but within the Basin
and Range province. It is computed that one stock occurs in about every
100 square miles, on the average, and each stock has an exposed area of
about 5 square miles. From data at hand no difference in composition can
be noted in the intrusive rocks inside the subprovince from those outside,
but possibly there is a small difference which has not been detected.

Stringham (1958) has classed the stocks in two divisions, the aphanitic
matrix porphyry and granitoid. The first he regards as mobile intrusions,
but the second he believes formed by granitization. The Cottonwood
stock of Fig. 38.2, for instance, formed by granitization, and the Bingham
stock was probably intruded.

The age of the stocks is approximately the same as the welded tuffs.
Some stocks are as old as Eocene, and others as young as Miocene accord-
ing to zircon and potassium-argon age determinations, so they seem to
predate, possibly accompany, and postdate the great avalanche eruptions.
For instance, the Cottonwood (Alta) stock of the Wasatch Mountains
is late Eocene ( Crittenden et al., 1952 ) , the Sheeprock stock of the Sheep-
rock Range is middle Miocene or 15 to 17 m.y. (Cohenour, 1957), and

SPATIAL RELATIONS OF MAJOR TECTONO-IGNEOUS ELEMENTS AND THE ORIGIN OF MAGMAS

599

the Silver City stock of the East Tintic Mountains, mid-Eocene or 38 to
46.5 m.y. ( Morris, 1957 ) . No one has proposed that the stocks may have
fed the welded tuff flows because the stocks have generally been con-
sidered older than the volcanics, yet the new age determinations indicate
some may be younger. The welded tuffs are undoubtedly fissure eruptions.

It appears logical, therefore, to conclude that the ignimbrite magma
was similar to that of the stocks, at least the porphyry stocks. Elsewhere
in the general province a flow and a pyroclastic sequence of greater vari-
ability without the preponderance of welded tuffs and generally with a
greater amount of basalt ( olivine ) occur.

Only one area within the ignimbrite subprovince of which the writer is
aware has appreciable basalt. In a 6000-foot sequence of volcanics in one
of the canyons of the Pioche, Nevada, district, is an olivine basalt unit
120 feet thick. The rest of the rocks are described as rhyolite, dacite and
andesite, with the last two predominating. These are taken to be the
welded tuff sequence, but some of them could be the younger Miocene-
Pliocene volcanics.

Relation of Flows to Miogeosyncline . The latite avalanche subprovince
is entirely within the miogeosyncline, with the exception of a very small
overlap on the eugeosyncline southeast of Winnemucca. The western
limit of the avalanche flows is close to the boundary between the eugeo-
syncline and miogeosyncline. The writer is not inclined to take this dis-
tributional relation to the geosynclinal divisions as very significant, be-
cause welded tuffs occur in the eugeosyncline of western Nevada and
eastern California as part of the younger volcanics, and hence magma
of avalanche composition and propensity can form in the crust under the
eugeosynclinal strata.

Relation of Flows to Gravity Faults. The Wasatch and Cache Valley
faults extend the Basin and Range system into the trench faults of south-
eastern Idaho and westernmost Wyoming from where they continue north-
ward through Montana to the trenches of British Columbia with very
little volcanism evident. The faults of the High Plateaus of Utah extend
southward beyond the welded tuff subprovince.

Basin and Range faulting is believed to have started in about early
Oligocene time, at least in southern Nevada, and to have continued from

CUVMAIUNC B»SE»CNT

iTnrTfTfTfTfTfinnnrfrwfriiinTiiiTiTfrfffllltltfll

Fig. 38.2. Postulated origin of the monzonite-latite magmas. Section A is of the Oquirrh and
Wasatch Mountains, Utah, and is factual at the surface but interpretive at depth. Section B
represents the Laramide folding before the quartz monzonite and quartz diorite intrusions.
Section C is section B restored to pre-folding time.

place to place to the present. Since early Oligocene is the time of the major
avalanches a tie may be imagined. The flows are widely broken and tilted
by the faults. After considerable erosion they were covered by the later
volcanics and associated sediments, which in turn have been broken in
places by further faulting. The association implies a genetic relation of
the volcanics and gravity faults, but close scrutiny leaves one with the
thought that the association is not as ubiquitous as desired.

Relation of Laramide Structures to Crustal Velocity Layers. In order
to approach the problem of the origin of the latite magma, the rela-
tion of the Laramide structures to seismic velocitv layers must be

600

STRUCTURAL GEOLOGY OF NORTH AMERICA

considered. This has been done in Chapter 31 and Fig. 31.26.

If the boundary layers are fairly flat, as suspected from previous geo-
logical analysis, then widespread adjustment of the deeper crystalline
basement must be postulated incident to the folding and thrusting of the
Paleozoic and Mesozoic basin sediments above. The sections of Fig. 38.1
have been prepared to show the folding and faulting where seismic in-
formation on crustal layers is most available. Section R extends from the
blast site at the south end of the Promontory Range westward to Elko,
Nevada. Graduate theses at the University of Utah furnish stratigraphic
and structural information on Promontory Range (Richard Olsen), New-
foundland Range (R. E. Paddock), Silver Islet Range (Fred Schaeffer
and W. L. Anderson), and Pilot Range (Donald Rlue). Sharp's (1942)
mapping of the Ruby Range and Dott's ( 1955 ) work at Elko and eastward
permit the drawing of a fairly satisfactory, if somewhat generalized and
simplified, cross section. The 9-kilometer surface, if of uniform depth
across the entire section, is just about tangent to the troughs of maximum
downfolding of the Paleozoic and Proterozoic sedimentary sequences. A
more detailed and larger-scaled section is shown in Fig. 38.2 which takes
a course northeasterly across the Oquirrh Mountains, across the Jordan
(Salt Lake) Valley to the Cottonwood dome of the Wasatch Mountains,
and then northward to the major exposure of the Archean crystalline
(Farmington) complex. If the transition surface between the silicic and
the basaltic crust is as illustrated in restored sections R and C, then con-
siderable flowage must have taken place in the base of the silicic crust
during and after folding of the sediments.

Origin of Latite Magma. The chief reason for postulating the primary
nature of the latite assemblage of the Great Rasin is great volume with
only minor amounts of rocks of other composition. A few basalt flows
have been noted as part of the latite assemblage but most basalt occur-
rences are later, and were extruded in Pliocene-Pleistocene time. Andesite,
dacite, and rhyolite generally occur along with the latite, but in relatively
small amounts. A plausible theory for the origin of the magma must there-
fore account for variations as indicated by the above observations, and
even, on occasion, to explain the transit of basalt to the surface.

Two possibilities occur to the- writer: (1) the base of the silicic crust

melts in part or in bulk to form the primary magma, or ( 2 ) basaltic magma
is intruded in megasills at the base of the silicic crust at temperatures
sufficiently above the melting temperature of the silicic crustal rock to
melt an appreciable layer of it or to melt it partially in decreasing amounts
upward from the basalt sills; some mixing of the basalt with the melted
silicic crust might occur. The second theory supplies heat for the phenom-
enon and basalt, on occasion, as required. An expanding column of the
mantle to produce a surface uplift of about 2 kilometers is needed every-
where in the Rocky Mountains, Colorado Plateau, and Great Rasin region
(see Chapter 31), and a primary basalt magma is needed under most of
it, so it seems logical to start with the premise of rising basalt from the
mantle where, it has been concluded, differences in density exist. The
basalt would furnish a good part of the heat needed to raise the tempera-
ture of the base of the silicic crust to melting. The idea of partial melting
of the base of the silicic crust, especially those parts thrust slightly down-
ward during the Laramide orogeny, is attractive because, thereby, a
magma of monzonitic composition might be formed rather than one of
granodioritic composition as in the case of bulk melting of great roots.
Partial melting will not only facilitate viscous flow to level out the base of
the silicic crust (Fig. 38.2) but also will produce the great volumes of
various gneisses and schists called migmatites whose features characterize
them as transitional to igneous. A granitic or monzonitic magma would
have been squeezed out, and represent the first minerals to melt, hence
more acidic, and more basic varieties would represent the melting of a
larger percentage of a basal portion of the silicic crust nearby. The basalt
immediately beneath may be tapped by a fissure conduit from time to
time and add its conspicuously dark and perhaps unexpected presence to
the surface assemblage.

Mixing of a small amount of the silicic magma with basalt would pro-
duce an andesite, or the basalt could fractionate to an andesite. Very
little andesite is needed in this province.

The latitic magma of the ignimbrite subprovince contained sufficient
water such that effervescence of water vapor at a temperature high enough
for welding occurred. The extrusion temperatures of the tuff-breccias in
the Pine Valley Mountains is interpreted to be lower than that necessary

SPATIAL RELATIONS OF MAJOR TECTONO-IGNEOUS ELEMENTS AND THE ORIGIN OF MAGMAS

601

for welding, and perhaps the ignimbrite subprovince was determined not
only by abundant water but also by a higher than normal temperature. It
corresponds to the postulated projection of the East Pacific Rise under the
western United States (Chapter 31).

TECTONO-IGNEOUS PROVINCES AND DEEP-SEATED EARTHQUAKES

The South American Andes and adjacent shields and basins are noted
for their intermediate depth and deep-seated earthquakes. In charting the
foci Renioff ( 1954 ) was led to the conclusion that they lie along an ex-
tensive, inclined plane or surface that extends down under the Cordillera
and stable region to depths of nearly 700 kilometers. He illustrated the
earthquake foci along two sections, one across northern Chile and Argen-
tina, and another across Equador to the Guayana shield. For these sec-
tions the writer has idealized the crustal geology as shown in Fig. 38.3 in
accord with the more detailed cross sections of Fig. 34.5. It will be seen
that the volcanic fields as previously pointed out are east of the Nevadan
batholithic belt and he principally on the deformed miogeosyncline. They
occur somewhat shoreward of the break in slope of the earthquake foci
surface. Renioff postulates this surface to be a gigantic reverse fault due
to compression in the mantle. It has also been postulated that this great
fault is the region of origin of basaltic magma, especially of the alkali
olivine variety (Kuno, 1959). Here is a likely place where the partial
fusion of the upper mantle shell occurs to supply basalt and heat to the
subcrust and crust, and where consequent intrusive and extrusive mag-
matic activity takes place. Although andesites are widely recognized in
the Andes, Dr. Howel Williams informs the writer that he has a knowl-
edge in part and a strong hunch that great volumes of the volcanic piles
in South America and Mexico are of the composition called latite or quartz
latite in the discussion of the Great Rasin volcanics on previous pages.
If the base of the silicic crust is fused partially, then by postulate, more
latite than andesite would probably be extruded.

The deep-seated earthquakes in South America and Mexico are comple-
mented by a trench at the continental margin, which is presumed by
Benioff and others to be a compressional consequence of reverse move-

ment along the great fault defined by the earthquakes. Deep-seated earth-
quakes have not been recorded in the western United States or western
Canada, and no trench exists at the continental margin; yet, the other
igneous and tectonic components of the western orogenic belts are present.
It seems to the writer that the deep-seated earthquakes have been an
integral part of the western Cordillera of Canada and the United States
during most of the Tertiary, but that the fault along which they occurred
is now inactive. It may have been replaced by the East Pacific Rise and
associated expansion of the mantle.

CRUSTAL TENSION AND MAGMATISM

Previous References

The belief that the earth's crust has suffered large amounts of shortening
in the orogenic belts has been an orthodox tenet of geologists for many
years. Lately several individuals, including de Sitter (1956) and Rucher
(1956), have argued for vertical uplift with consequent gravity flow or
sliding of the surficial rocks away from the uplift to form the folds and
thrusts, and therefore, for minor amounts of, or no horizontal shortening.
Others are now contending for expansion of the earth and tension as the
primary and dominant force of crustal deformation.

In Chapters 41 to 43 the concept that the earth is expanding is men-
tioned in connection with the possible drifting apart of North and South
America. Also in Chapter 31 the Rasin and Range province was explored
relative to tension in the crust and expansion of the earth. The Mid-
Atlantic rift in Chapter 10 was treated as a tensional structure as a result
of earth expansion. It is now absorbing to speculate on magmatism in the
framework of crustal tension.

Evidence of Tension

Fissure Eruptions and Tension. The most plausible concept of the
origin of fissures through which large volumes of magma have passed to
the surface is one of tension. Fissure eruptions have always been difficult
to explain in the framework of crustal compression. Fissures through which
basic magmas in large amounts have flowed from the basaltic subcrust

HIGHLY GENERALIZED SECTION THROUGH ECUADOR

,CH BATHOLITHIC BELT VOLCANICS

«Uo»

"f»

*eD

Mrf

..x--

HIGHLY GENERALIZED SECTION THROUGH NORTHERN CHILE AND ARGENTINA

TRENCH BATHOLrTHIC BELT EU6EOSYNCLINE ^"N* VOLCANIC FIELO MIOSEOSYNCLINE

Fig. 38.3. Relation of deep-seated earthquakes, postulated fault in mantle, and crustal constitution in
the South American Andes.

SPATIAL RELATIONS OF MAJOR TECTONO-IGNEOUS ELEMENTS AND THE ORIGIN OF MAGMAS

603

to the surface cannot be explained as surficial features of the folding of
sedimentary sequences.

Basin and Range Province and Tension. In Chapter 31 it was postu-
lated that the Rasin and Range province has been distended about 30
miles since Miocene time, and the suggestion made that this process could
provide for the intrusion from great depth of much magma. The earth-
quake foci have been interpreted to mean that the great faults extend
to depths of 20-40 kilometers.

Eugeosyncline and the Tension Hypothesis

With an expanding earth and a crust cracking apart in places we may
devise a scheme of magmatism for the eugeosyncline. See Fig. 38.4A. The
crystalline crust of the continents seems to end abruptly at the ocean basins,
and in the realm of an expanding earth the continent-ocean boundary may
generally be a zone of weakness where extension will be focused. If so,
then here will be a likely site for the rise of magma from the upper
mantle. The continental margins are commonly sites of deep-seated seis-
micity as well as unusual thermal activity in the mantle. Fissure eruptions
in the eugeosyncline have been postulated.

Provision must be made for the evolution of the andesites, and if they
arise by fractional crystallization from basalt, then there must exist large
magma chambers in the subcrust where the process takes place. It would
appear that basaltic eruptions should be dominant in the early stages of
the eugeosyncline with andesites more abundant later; also undifferenti-
ated basalt could be conducted to the surface from time to time as new
fissures break through to great depths. It is not known if observations in
the eugeosynclines support the supposition that andesites become more
abundant in the later stages.

Batholithic Belts and the Tension Hypothesis

Speculating further, as the eugeosyncline develops the crust is depressed
under it, and the depression is mostly the result of removal of support
by the ejection of magma through fissures to the surface. See Fig. 38.4R
and C. Eventually, the silicic upper crust or the base of the eugeosyncline
comes into the domain of melting, and it is at this stage, with continued

PACIFIC OCEAN

DISTENSION AND
FISSURE ERUPTIONS

MELTING OF LOWER PART
OF BASALTIC CRUST

EUGEOSYNCLINE SUBSIDES
ZONE OF MELTING MOVES UP

ZONE OF DISTENSION
FRACTURES AND
BATHOLITHIC INTRUSION

Fig. 38.4. Speculations on crustal structure at the continental margin and the relation of
magmatism to the eugeosyncline and batholithic belt, if the earth should be expanding and
the crust distended.

expansion and tension that the growth of the batholiths of intermediate
and acidic composition begins. The fluidity of the basaltic melts provides
for rapid flow to the surface, but the greater viscosity of the more silicic
magmas makes for slower, more irregular intrusions, with attendant varied
intrusive relations. The space problem is largely accounted for, however,
by irregular Assuring and pulling apart of the crust (Fig. 38.4C).

Several adjacent fissures may develop, and each is invaded by the silicic
magmas, thus perhaps accounting for the great septa of metamorphosed
country rock noted in some of the batholithic belts. The batholithic belts
in some places are narrow and linear and seem to fit nicely the tension
hypothesis, but others are more irregular with the batholiths in clumps,
and therefore do not accord with the hypothesis very well.

Problems

The conventional explanation for the origin of large volumes of magma
of intermediate composition is the melting of the lower part of a thick-

604

STRUCTURAL GEOLOGY OF NORTH AMERICA

ened silicic crust; the thickening is due to compression. In the concept of
expansion and tension no thickening is possible. In fact, the conditions of
rise of the batholithic magmas may be similar to those of the quartz
latite magmas of the Basin and Range province where the lower part of
the silicic layer is believed to melt without previous thickening.

The explanation of batholithic belts based on extension fails to provide
adequately for dynamic metamorphism and isoclinal folding such as
occurs in the Sierra Nevada. Possibly a facility for such metamorphism
is present in the slices of rock that settle toward the batholith as exten-
sion occurs. See Fig. 38.4. It would appear that an extensive aureole of
thermal or hydrothermal alteration would occur around the intruding
batholith because of the fracturing. In the Andes of Peru no stage of
dynamic metamorphism is reported just prior to batholithic intrusion
(H. L. Hosmer, personal comunication), so the problem evidently does not
exist there.

Another phenomenon that occurs and for which an explanation is not
readily seen by the writer is the post-batholithic uplift. The belts of
batholithic intrusion are elevated and deeply eroded to expose the large
intrusive bodies. In the framework of crustal extension what causes the
uplift? One may counter that the Sierra Nevada block is a much later
affair and not related to the uplift immediately after intrusion. Also it may
be observed that parts of the batholithic belt of South America and North
America are fairly low-lying today and that the batholiths may have come
closer to the surface than illustrated in Fig. 38.4.

Still another problem is apparent in consideration of the Gulf of Cali-
fornia. It was speculated that the eugeosyncline forms by fissure erup-
tions as the crust is pulled apart. In Chapters 30 and 31 evidence was
presented that Baja California has been pulled apart from the mainland
of Mexico during Cenozoic time, so we should expect extensive fissure
eruptions there. The volcanism instead is concentrated on the east in the
Sierra Madre Occidental with some also on the west in Baja California.

Only a few volcanic cones exist in the Gulf itself. Possibly no large magma
chambers existed where the fractures and separation occurred.

Basic Conflicts

The foregoing discussion of the origin of the various magmas is wrought
with several conflicting concepts. In certain considerations we conjure up
a state of compression in the crust and outer mantle; in others we enter-
tain extension of the crust. Where extension, we recognize certain zones
of extension complementary to zones of compression, or we imagine
world-wide tensional strain. If tension in local zones, then we usually
think of convection circulation in the mantle; if world-wide, we say ex-
pansion of the entire earth. Some believe a little expansion has occurred,
some considerable, but considerable expansion appears impossible ( Cook
and Eardley, 1961 ) . Others recognize local or regional vertical uplift due
to changes of state in the mantle as the basic tectonic activity, without
appreciable overall earth expansion. Secondary, gravity-caused flow move-
ments on the flanks create the compressional structures. World-encircling
rises underlain by an expanded mantle-crust transition layer seem to be
a reality. And finally, there are many geologists who support the concept
of drifting and rotating continents without earth expansion. These move-
ments are commonly attended by horizontal coupling of varying magni-
tudes. Then there is the pointed conflict of granitization versus magmatic
intrusion, particularly in regard to the great batholiths of the eugeosyn-
cline. The writer finds convincing examples of each and all of the above-
mentioned theories, yet none seems adequate to explain the entire
panorama of structural and igneous observations.

It was hoped that the igneous rocks, when their origin was investigated
and related to crustal structure, would point out which of the theories
are valid, and perhaps the study has accomplished this to some small
extent, but there still remains much uncertainty.

39.

ALASKA AND THE YUKON

GEOMORPHIC PROVINCES OF ALASKA

The principal geomorphic provinces of Alaska are, from north to south,
the Arctic Coastal Plain, the Brooks Range, the Central Yukon Plateau and
Lowland, the Alaska and associated Coast Ranges, the Alaska Peninsula,
the Aleutian Archipelago, and the Alexander Archipelago. See map, Fig.
39.1. They are part and parcel of the continent's great western Cordillera.
The generalized tectonic divisions are shown in Fig. 39.2.

The Brooks Range stretches east-west across northern Alaska and in-
cludes several smaller ranges, such as the De Long, Baird, and Endicott
Mountains. They support an extensive upland erosion surface, whose
higher elevations reach from 5000 to 6000 feet above sea level. The Brooks
Range in its central portion is a sharply defined mountain mass that rises

conspicuously from the Yukon Plateau on the south and above the foot-
hills of the Coastal Plain on the north. The Colville River drains much of
the northern slopes of the range and the piedmont of the Arctic Coastal
Plain. The Brooks Range is covered for the most part with perennial
snow fields and contains a number of glaciers. The erosional features are
described as distinctly youthful, and presumably very little erosion has
occurred there since the once far greater ice fields and valley glaciers of
the Pleistocene have disappeared. The Arctic Coastal Plain from the air
appears as a bleak, flat wasteland of frozen lakes and rivers and snow-
covered flats.

The Yukon or Central Plateau in the central and upper Yukon drainage
is a broad dissected plateau bounded on the north bv the Brooks Range
and on the south by the Alaska and Coast Ranges. The two great ranges
are about 300 miles apart. The plateau loses definition in the lower Yukon
drainage, where it is characterized by the flat-topped interstream areas
separated by broad and low-lying, estuarine-like embayments. A few
minor ranges and peaks rise above the general level of the upland sur-
face. The Yukon River has eroded a meanderbelt 35 miles wide in places,
but with several narrows along its coarse. Near its mouth, a very low
alluviated portage separates it from the Kuskokwin River, and both rivers
are in the process of building large deltas in the Bering Sea.

The Central Plateau was dissected and then, during the maximum gla-
ciation, heavily alluviated, chiefly with silt. The Yukon and tributaries
have since been engaged mostly in clearing out the silt.

The Seward Peninsula is a geological entity in itself and will receive
special mention later. It is generallv included in the Central Plateau prov-
ince.

The Coast Range of southeastern Alaska and British Columbia extends
northwestward by way of the St. Elias Range, Wrangell Mountains, and
Nutzotin Range into the Alaska Range, which together form a great arc
approximately parallel to the margin of the Gulf of Alaska. The Alaska
Range continues southwestward to the Aleutian Range, which forms the
backbone of the Alaska peninsula. Mt. McKinley in the Alaska Range
(20,300 feet) is the highest mountain in Alaska. The St. Elias Range and
the Chugach Mountains support the greatest ice field in North America;
several peaks rise above 14,000 feet, including Mt. Logan, the highest at

605

Fig. 39.1. Index map of Alaska and the Yukon.

Fig. 39.2. Tectonic map of Alaska and the Yukon Territory. Laramide orogenic belt character-
ized by folds and thrust faults mostly in shelf and miogeosyncline sedimentary rocks. The
structures of the basins are Late Cretaceous or Early Tertiary; of the geanticlines are partly
of Early Cretaceous age or older, Nevadan orogenic belt characterized by numerous batho-

liths and deformed eugeosynclinal sediments. The intrusions and structures are Early and Late
Cretaceous with Early Tertiary structures in the Alaska Range area. Coast Range orogenic belt
is marked in part by Tertiary sediments and by folding and thrusting principally during the
Late Cenozoic. Interior Tertiary basins not shown.

608

STRUCTURAL GEOLOGY OF NORTH AMERICA

19,850 feet. Mt. Sanford in the Wrangell Mountains is 16,000 feet high.
The Aleutian Range has a general summit level of 3000 to 6000 feet and
includes a number of active and dormant volcanoes. The famous Valley
of Ten Thousand Smokes and the great crater of Aniakchak are situated
at the southwest end of the Aleutian Range.

An oceanward arc of ranges, more truly called coast ranges, extends
through the Cook Inlet and Prince William Sound region. The Kenai
Mountains forming the Kenai peninsula east of Cook Inlet connect with
the Chugack Range, bordering the coast, and this merges with the St.
Elias Range. Some writers have grouped the St. Elias Range entirely with
the Coast Ranges, but its geology is too little known to permit a definite
conclusion. At the west end, the Kenai Range probably is continued by
the low mountains of Kodiak Island, and thence by beveled or buried ele-
ments in the shelf between the Aleutian Islands and Aleutian trench to
Unimak Island.

The Shelikof Strait-Cook Inlet and Susitna River depression effectively
separates portions of the inner ranges from the outer. The Copper River
Valley and its tributary, the Chitina River, form another separating de-
pression. The Talkeetna Mountains break the continuity of the two
depressions and, anomalously, seem to bridge the two great mountain
systems.

PALEOZOIC GEOSYNCLINE AND RELATED OROGENY

Most of the Paleozoic rocks of Alaska are exposed in the Rrooks Range,
Seward peninsula, Central Plateau, and Alexander peninsula. The latter
has already been considered in a previous chapter. The Alaska, Nutzotin,
and Wrangell Ranges also contain Paleozoic rocks, and a nearby belt ex-
tends along part of Copper River and Chitina River valleys. The towering
mass of Mt. McKinley in the Alaska Range is eroded mostly from de-
formed Paleozoic strata.

For a detailed study of the Paleozoic rocks of Alaska, Smith's 17. S. Geo-
logical Survey Professional Paper 192 should be consulted, particularly
the large correlation chart in the pocket. The formations are well devel-
oped in the Tanana-Yukon region of the Central Plateau, and a resume of
them is given on page 609.

The igneous rocks of the Tanana-Yukon region have been summarized
by Mertie ( 1935 ) . Basic lavas of basaltic and diabasic character have
been extruded during at least five geologic epochs in the Paleozoic. The
first was in the Middle Ordovician, the second in the Middle Devonian,
and the last three during three epochs of the Carboniferous. Granular
intrusives of the same general character accompanied the extrusion of the
lavas, but the volume of such rocks is relatively small. Some rhyolite and
dacitic lavas and tuffs are found among the Carboniferous lavas, but gen-
erally speaking, lavas of acidic or intermediate character are rare. Ultra-
basic rocks were intruded during the late Devonian epoch.

The volcanism that occurred during the Carboniferous period in
Alaska, according to Mertie ( 1935 ) was greater than in any other period
and most intense in the Alaska Range. The eruption of the basic lavas was
accompanied by epeirogenic movements that persisted into the Triassic.

It is immediately clear that the above rocks represent the eugeosyn-
clinal assemblage previously recognized and described in the western
Cordillera of southeastern Alaska, British Columbia, Washington, Oregon,
California, and Nevada. The presence of the basic intrusives in the vol-
canic assemblage suggests that the belt was more the site of the archi-
pelago than an adjacent trough.

In northern Alaska, the Paleozoic rocks are mainly sandstones, shales,
and limestones, and are typical of the miogeosyncline or shelf, also previ-
ously described in the Cordillera of Canada and the United States. No
volcanic rocks have been found in the sediments. A resume, as listed in
the correlation chart of Professional Paper 192, is given on page 610.

The Upper Devonian and Mississippian rocks of a southeastern area
of the Brooks Range have recently been measured, and the section is
given in Fig. 39.3. Although about 8000 feet of strata of the two systems
are present, they are regarded as platform-type deposits and not miogeo-
synclinal by Bowsher and Dutro (1957). The massive lower and middle
members of the Kanayut conglomerate help to define a region of uplift
in the Late Devonian. See Fig. 39.12. The above strata are overlain by
variegated shale and siltstone, the Siksikpuk formation, about 350 feet
thick, which is probably Permian in age, and then the Shublik formation
of Triassic age. The Pennsylvanian is missing over all Alaska except the
northeast corner.

Selected Paleozoic Sections of Alaska

Thickness, Feet

Selected Paleozoic Sections of Alaska

Thickness, Feet

ermian
Kandik district

\ississippian
Porcupine district

Koyukuk-Melozi

district
Yukon-Tanana

district

Limestone

Conglomeratae, shale,
and sandstone

Tahkandit
limestone

Wiseman-Chandalar

district
Marshall district
Sheenjek district

Dark shale and limestone, in part same as
Calico Bluff formation

Greenstone, little rhyolite, formerly consid-
ered part of Kanuti group

Clay shale, sandstone, conglomerate. Na-
tion River formation

Lava flows and associated sediments. Ram-
part group and Circle volcanics

Limestone, shale, slate. Calico Bluff forma-
tion

Limestone beds

Undifferentiated schist, shale, chert, quartz- I
ite

Chert with minor amount of limestone and
shale, Livengood chert

Comparable with northern Alaska section

Andesite and basalt flows and tuff

Dark limestone, somewhat silicified, weath-

4-6000
5-10,000

13,000
2-4000

•

ering light, with argillaceous and arena-

ceous beds

6000 plus or minus

Quartzite, conglomerate, shale, with cherty

matrix

Chisana district

C'a):sla,ei ] Wellesley
Shale and conglomerate >

formation
Conglomerate

1-2000

)evonian

Upper

Wiseman-Alatna

Quartzite, sandstone, slate, little conglom-

district

erate, grit, limestone

Wiseman-Chandalar

Quartzite, flint, calcareous black slate, im-

district

pure limestone. Formerly part of West

Fork formation

Eagle district

Basalt lava and pyroclastics of greenstone

habit. Woodchopper volcanics

10,000 plus or minus

Middle

Porcupine district

Brown shale and basalt

Massive light gray-blue limestone. Salmon-

trout limestone

300

Kandik district

Argillite, chert, and cherty grit

Central-East

district
Wiseman-Chandalar

district
Kantishna-Nenana

district

Chisana-Tok

district
Sheenjek district

Silurian

Porcupine district

Sheenjek-Alatna
Kandik district

Preacher-Tolovana-
Hot Springs district

Ruby district
Ordovician

Porcupine district
Ruby district

Preacher district

Cambrian

Kandik district

Thin beds dark gray limestone, shale, and
chert

Lithographic limestone, dark gray crystal-
line limestone

Clay shale, siliceous slate, chert, quartzite,
sporadic limestone, and conglomerate

Slate with small amounts of limestone
Massive limestone, equivalent to part of^

Tonzona
Limy shale, more calcareous at top
Slate, argillite, graywacke, quartzite
Black conglomerate, white conglomerate,

shale and graywacke
Crystalline limestone associated with black

slate, sandy beds, somewhat schistose
Quartzite, sandstone, slaty sandstone, and

argillaceous sediments

Middle
Black fissile shale, little siliceous limestone
Buff magnesian limestone
Slate, schist, thin layered limestone; Skagit
Massive somewhat siliceous limestone; Skagit
Massive white to cream limestone
Calcareous and dolomitic limestone, some-
what recrystallized. Tolovana limestone
Lower
Undifferentiated limestone

Middle
Gray limestone
Magnesian limestone overlain by calcareous

limestone, not differentiated on map
Volcanic tuff and associated igneous rocks
Black shale, merging downwards into schist

Upper
Limestone, with dark gray to black

and chert in higher part
Limestone

Middle
Upper plate of limestone
Thin layers of slate an quartzite
Lower plate of limestone

slate

1000
500

> 10,000 plus or minus

2500 plus

3000 plus

600

5000
?

610

STRUCTURAL GEOLOGY OF NORTH AMERICA

Thickness, Feet

Mississippion

Central-Western
district

Canning district

Devonian

Colville-Noatak
district

Lisburne district
Canning district

Silurian

Noatak-Kobuk
district

Massive light-colored semicrystalline limestone, consid-
erably silicified; Lisburne limestone 4000 plus

Sandstone, shale, thin limestone, in place chert, con-
glomerate; Noatak formation thousands

Gray and black limestone, somewhat brecciated, much

silicified, equivalent of part of Lisburne limestone 3000

Black limestone, slate, shale and sandstone; age uncer-
tain; rests unconformably on highly metamorphic
schists ?

Upper
Quartzite, sandstone, slate, subordinate conglomerate,

grit, and limestone ?

Middle
Calcareous sandstones and shale ?

Black shale, slate, and subordinate sandstone; not dif-
ferentiated on map 1000 plus

Middle
Massive, somewhat siliceous limestone; Skajit limestone 6000 plus

A line separating the eugeosynclinal assemblage of sediments on the
south from the platform (shelf) or miogeosynclinal sediments on the
north follows the Yukon River approximately.

The distribution in outcrop and the geosynclinal thicknesses where
known of the Paleozoic strata indicate that the whole of Alaska was a
region of subsidence and sedimentation in the Paleozoic, and an extension
of the Cordilleran geosyncline.

An episode of granitic intrusion occurred during the early Devonian
in the North Fork of the Chandalar River along the south flank of the
Brooks Range ( Mertie, 1935 ) . This is the only Paleozoic granitic intrusive
so far identified in Alaska, and it is in the area of the mainland assem-
blage of stratified rocks. Representatives of the Lower Devonian are
absent, and the strata of Middle Devonian age rest unconformably on
those of Silurian age. The rocks below are more metamorphosed than
those above the unconformity. This evidence indicates crustal unrest
in the geosyncline, such as is found in Paleozoic beds of the Alexander

Alopah
limestone

Wochsmuth
limestone

Kayak

shale

Kanoyut
conglomerate

Unnamed
shale and
sondstone

Stuver
member

Middle

conglomerate

member

Lower
member

Feet
2000

-1500

- 1000
t 500

- 0

Verticol
scale

Fig. 39.3. Generalized section of Upper Paleozoic rocks in the Shainin Lake area. Reproduced
from Bowsher and Dutro, 1957.

Archipelago, although slightly more continentward than the trough of
accumulation of the volcanic assemblage.

Numerous other unconformities undoubtedly exist in the volcanic as-
semblage.

TRIASSIC AND JURASSIC GEANTICLINE AND ADJACENT BASINS

During the Triassic period and persisting into the Jurassic, a great ge-
anticline rose from the Paleozoic geosyncline and separated two adjacent

ALASKA AND THE YUKON

611

basins of accumulation, one on the north and one on the south. Examine
Fig. 39.4. The basin on the south collected chiefly sediments of the
eugeosynclinal assemblage, while the trough on the north received lime-
stones, sandstones, shales, and cherts of the miogeosyncline and shelf.
Fixed lines have not been drawn on maps to show this feature, but Mertie
(1930) describes it as a region of epeirogenic uplift and erosion.

The geanticline is a parallel, except in detail, of the Cordilleran geanti-
cline in Canada and the United States, already described and pictured
in the paleotectonic maps of Plates 9 to 11.

The columnal sections of Fig. 39.5 are characteristic of the Coast
Ranges. Here in southern Alaksa Mid-Jurassic time marked the begin-
ning of development of basins and separating geanticlines that persisted
through the Cretaceous.

In the Kuskokwin region Cady et al. (1955) described the Gemuk

group of siltstone and chert with local developments of basalt and
andesitic rocks to be 15,000 to 25,000 feet thick.

The Triassic and Jurassic of the northern Brooks Range and Arctic
Coastal Plain consists of two formations. The Shublik of Late Triassic
is 300 to 1000 feet thick and is composed of interbedded dense bituminous
limestone, chert, shale, siltstone, Iimonite oolite, and calcareous glauco-
nitic siltstone. It is entirely marine. The Kin<j;ak shale spans most of the
Jurassic and is about 4500 feet thick. It contains graywacke, varicolored
bedded chert, lenses of conglomerate, and coquina limestone.

CRETACEOUS BASINS AND GEANTICLINES

An examination of the geologic map of Alaska and the correlation chart
of Professional Paper 192 shows the Lower and Upper Cretaceous strata

Fig. 39.4. Idealized evolution in
| cross section of Alaska from Point
Barrow on the north to Kodiak
I Island on the south. In part after
| Cady ef al., 1955. Vertical scale
j highly exaggerated. Large dots in-
I dicate eugeosynclinal assemblage.
Blank units represent miogeosyncline
or shelf assemblage. Small dots rep-
resent elastics with dominant gray-
wacke content. Kl, Lower Cretace-
ous; K2 Upper Cretaceous. Nevadan
structures and batholiths not shown.
Mogatza arch and Kobuk basin be-
tween Yukon basin and Brooks gean-
ticline not shown.

ARCTIC
OCEAN

BARROW
GEANTICLINE

RESTORED TO LATE CRETACEOUS PRE-LARAMIDE TIME

♦ CENTRAL GEANTICLINE —

COLVILLE

BASIN

BROOKS RANGE
GEANTICLINE

YUKON
BASIN

ALEUTIAN RANGE
GEANTICLINE

KUSKOKWIM
BASIN

ALASKA RANGE
BASIN

ALEUTIAN
TRENCH

ARCTIC
OCEAN

BARROW
GEANTICLINE

RESTORED TO LATE JURASSIC TIME

CENTRAL GEANTICLINE JURASSIC AND TRIASSIC E UGEOSYNCLINE

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ALASKA AND THE YUKON

61 I

BROOKS RANGE

SOUTHERN FOOTHILLS.

NORTHERN FOOTHILLS

ARCTIC COASTAL PLAIN

5000'

-5000'-

C OLVIL LE

BASIN

BARROW

ARCH

CREEK

\SCHRADER
\ BLUFF

V^\

LOWER JURASSIC
Vpp £R~~T~RTVssic

PRECAMBRIAN (?)
AR6ILLITE

Fig. 39.6. Cross sections of Arctic Foothills, and Coastal Plain. A-A', existing section; B-B', et a/., 1951; somewhat altered after Grye et a/., 1956. Black is "inland facies"; stippled is
restored to close Cretaceous showing facies of Nunushunk and Colville groups, after Payne "coastal facies"; blank is "offshore facies." Kot, Torok fm.; Knc, Nanushuk group.

so widespread that much of Alaska must have been under water and
receiving sediments during Cretaceous times. Certainly large parts of
the Triassic and Jurassic geanticline were covered. Mertie (1930) states
that, at least at one time or another during the Cretaceous, all Alaska
was subjected to sedimentation. But, it seems evident that a number
of long linear uplifts rose and separated the basins of sedimentation in the
manner shown on Fig. 39.4. The Cretaceous sediments are everywhere
very thick and are almost entirely clastic. They probably have been
studied most under the Arctic Coastal Plain and in outcrop in the

Foothills Belt under the auspices of the U. S. Navy Department in Naval
Petroleum Reserve No. 4. Cross sections A-A' and B-B' of Fig. 39.6 show
the beds and structure there approximately as they are today and as
restored to pre-Laramide time, respectively.

The discovery of the Barrow arch of Precambrian (?) rocks under the
northern edge of the Coastal Plain was unexpected, but it points to a
positive region there, and to a northern source of sediments in mid- and
late Paleozoic times ( Dutro, 1960 ) .

In the Kuskokwim distict Cady et al. (1955) describe Upper Creta-

614

STRUCTURAL GEOLOGY OF NORTH AMERICA

ceous strata in the immense thickness of 40,000 to 65,000 feet. The rocks
are dark, interbedded shale and fine-grained graywacke. Breccia and
conglomerate facies are present in a few localities. The record of crustal
deformation and sedimentation is described as follows:

. . . Sediments were eroded from emerged areas of the geanticlines and were
carried by streams to the trough of the intervening Kuskokwim geosyncline,
where scores of thousands of feet of sediments were deposited while subsidence
continued, during latest Early Cretaceous and early and possibly middle
Late Cretaceous time. The sediments were drawn from older rocks exposed
in the geanticlines — phyllite, slate, quartzite, limestone, siltstone, chert, basalt,
and andesite.

The geanticlines, particularly the Aniak-Ruby geanticline continued to be
uplifted rapidly during at least the early part of the Late Cretaceous time,
and areas of sharp relief evidently appeared from which the older rocks were
violently eroded and subjected to disintegration almost entirely mechanical.
The disintegration products, chiefly angular silt and sand-size fragments, were
transported fairly short distances to the Kuskokwim geosyncline. The submarine
relief of the belt of the Kuskokwim geosyncline, like the subaerial relief of
the geanticlines, was continually steepened in the early Late Cretaceous epoch,
particularly along the borders of the trough. Sediments left by the streams in
this marginal area formed loose, unconsolidated deposits that were continually
and repeatedly upset by the steepening of the trough borders, and slid down
the submarine slopes of the trough. Part of the silt and sand involved in the
slides became incorporated in turbidity currents of high density and were
distributed in the otherwise unagitated water below wavebase. The sediments
of the slides and of the turbidity currents came to rest to form the interbedded
graywacke and shale of the Kuskokwim group. The graywacke beds formed at
the time of sliding, and are possibly related to turbidity currents capable of
transporting the sand-size particles. The latter settled at depths at which the
currents were checked by seawater of equal density. Shale beds were laid
down in more quiet intervals of setding. Beds of graywacke, many of which
are as much as two feet thick, were probably formed in a very short time by
this process, an instant of time in the geologic sense (Cady et al. 1955).

In the Alaskan peninsula 800 feet of Cretaceous arenaceous limestone
occur in the Herenden Bay area, and 1175 feet of marine limestone,
sandstone, shale, and conglomerate are noted on the southeast flank
of the Talkeetna Mountains (Miller, 1959). These are part of the
Matanuska basin. See Fig. 39.2.

Earliest Tertiary deposits occur in the Matanuska Valley and consist
of shale, sandstone, conglomerate, and coal. They have a maximum

thickness of 7000 feet and are tentatively assigned to the Paleocene. They
are considered by Payne (1955) to represent the closing phase of sedi-
mentation in the Matanuska basin.

MESOZOIC AND CENOZOIC OROGENIES

Belts of Orogeny

The eugeosyncline of southern Alaska during the Paleozoic and
Mesozoic eras bespeaks almost constant orogeny. The platform region
to the north was involved principally in Mid-Cretaceous and Early Terti-
ary orogeny. The southern margin of the eugeosyncline was involved
in deformation during the Cenozoic, and thus a parallel with the
western United States is at hand, for we have to deal with the Laramide
belt on the north, the Nevadan belt in the southern half, and the Coast
Range belt along the southern margin.

Nevadan Orogenic Belt

The Nevadan belt (Fig. 39.2) is characterized by Paleozoic and
Mesozoic eugeosynclinal strata, by their intense deformation and low-
grade metamorphism over large areas, by voluminous and numerous
batholithic intrusions, and by the fact that the climatic orogenic events
took place in the latter half of the Mesozoic. Three phases of Mid- and
Late Jurassic orogeny are noted by unconformities in the Jurassic
sequence in the Matanuska basin, and a fourth phase in the earliest
Cretaceous (Miller, 1959). Intrusive activity began in Mid-Jurassic
and continued through Late Jurassic in the Talkeetna geanticline. The
major and intense deformation of the Alaska and Seymore basins oc-
curred in late Early Cretaceous time (late Neocomian and Aptian). It
was accompanied by the intrusion of the major batholiths there.

Jurassic orogeny is obscure in central Alaska but strata of pre-Albian
age (late Early Cretaceous) are strongly deformed and considerably
intruded. The batholiths may be Jurassic as well as Early Cretaceous.

The Kuskokwim group of Late Cretaceous age in the Kushokwim basin
and similar strata in the Yukon basin (Fig. 39.2) were strongly folded,

ALASKA AND THE YUKON

615

IDIDAROD
FAULT

KUSKOKWIM
RIVER
t

KIOKLUK MOUNTAINS

■','///,'/,'-

HOLITNA FAULT

TAYLOR MOUNTAINS

^:c^Mmmmmm?m^nn^

«r:.Tqm;--:yN,

:-:--sooo'

K"pPCg

MILES

KT<PCg

Fig. 39.7. Cross section of central Kuskokwim region, Alaska. After Cady ef al., 1955. Th, Holkuk basalt;
Ki, Ididarod basalt; Kk, Kuskokwim group; K"fiPCj.., Gemuk group.

probably in late Paleocene time. See structure of Fig. 39.7. According to

Cady et al. (1955);
«

. . . Late in Late Cretaceous time the deposits in the geosyncline were up-
lifted slightly above sea level, and the lava flows of the iditarod basalt spread
'out over the uppermost strata of the Kuskokwim group.

The geanticlinal tracts moved closer together in earliest Tertiary time,
probably because the more rigid continental platform and Pacific Ocean floor
approached one another and decreased the width of the mobile belt. The
geosynclinal accumulations of the Kuskokwim group, which were structurally
less competent than the geanticlines, were as a result thrown into folds that
were draped around the margins of the geanticlines, and were also grouped
into rather extensive anticlinorial uplifts, such as the Gemuk anticlinorium,
;\vhich includes an upbuckled portion of the floor of the Kuskokwim geo-
syncline. Biotite basalt sills and dikes and albite rhyolite sheets, sills, and dikes,
pardy concordant with the enclosing formations, were intruded in the geo-
synclinal rocks and underlying strata near the close of folding.

Nonmarine Early Tertiary rocks, presumably of Eocene age, occur as
ierosional remnants on the Nevadan complex in several areas of central
JAlaska, particularly in the Healy basin (Fig. 39.2), north of the Alaska
JRange. They consist of claystone, sandstone, conglomerate, and lignite
'up to 5000 feet thick. Sedimentary rocks of this age are believed to have
'been deposited extensively in what are now the alluvium-floored low-
!land basins. These Eocene sediments were gently, and locally strongly,
deformed in Oligocene or early Miocene time.

Strong uplift occurred lastly at the close of the Tertiary and during the
Quaternary to produce the high mountain ranges and upland areas of
i central Alaska.

Coast Range Orogenic Belt

The Coastal Range orogenic belt as here defined is much like the
Central and Northern Coast Ranges of California, inasmuch as the bed-

rock geology is the Nevadan complex with deformed Tertiary beds
superposed.

The belt shown on Fig. 39.2 is widest in the Cook Inlet and Prince
William Sound region where it includes the Chugach and Kenai Moun-
tains. Three areas of Tertiary rocks are recognized, the Gulf of Alaska
Tertiary province, the Cook Inlet Tertiary province, and the Aniakchak
Tertiary province.

Gulf of Alaska Tertiary Province

Stratigraphy. The Yakataga basin of Tertiary deposits is an arcuate
lowland and foothills belt. The province borders the Gulf of Alaska
from the Copper River delta 300 miles southeastward to Icy Point, and
extends inland up to 40 miles to include the southern front of the Chugach
and St. Elias ranges. Although generally lowlands, the Gulf of Alaska
Tertiary province includes groups of hills and unnamed moun-
tains in the Katalla district up to 5000 feet above sea level, the Robinson
Mountains in the Yakataga district rising to 9000 feet, the Chaix and
Samovar Hills along die north margin of the Malaspina Glacier to 6000
feet, and a ridge in the Lituya district up to 3500 feet. Elevations above
1500 feet are covered by permanent snow fields and glaciers.

Typical sections of the Tertiary rocks are given in Fig. 39. S.

Three major subdivisions of Tertiary rocks are recognized on the basis of
gross lithologic characteristics and fossil evidence. These units arc believed
to correspond to major changes in the depositional environment oJ the Yakataga
geosyncline.

The oldest unit, of Eocene and possibly early Oligocene age. consists pre-
dominandy of interbedded or intertonguing nonmarine coal-bearing strata
and shallow marine or brackish water strata. Fossil plants and marine in-
vertebrates in this unit are regarded as indicating subtropical to temperate
climate on land and tropical to warm-temperate marine environment. This

KATALLA DISTRICT
West of Ragged Mountain

YAKATAGA DISTRICT

MALASPINA DISTRICT
Western part of Samovar Hills

EXPLANATION

Conglomerate

O - ~--0

Quaternary unconsolidated deposits

or >-

UJ cc

5 <

O v-

. ANGULAR UNCONFORMITY

Unfossiliferous siltstone, sandstone, and
conglomerate; 3500 feet or more;
marine and nonmarine(?)

O ANGULAR UNCONFORMITY!?!: MAY ,
BE FAULT CONTACT IN PART

Metasedimentary and metavolcanic rocks of Mesozoic age

"Conglomeratic" sandy mudstone

KATALLA DISTRICT
East of Ragged Mountain

Sandstone

Silts tone or shale

Coal

I o o o <^p 9.
t, Y A ,

Tuff, volcanic breccia, or
tuffaceous sandstone

Approximate stratigraphic position of

oil seep, show of oil in well, or

petroliferous rock

Approximate stratigraphic position of
gas seep or show of gas in well

Approximate stratigraphic position of
petroliferous rock

Quaternary unconsolidated deposits

-ANGULAR UNCONFORMITY —

y.-.xgfgg

Katalla formation; 8700+ feet; marine

■CONFORMABLE CONTACT.

Tokun formation; 2000 feet; marine

CONFORMABLE CONTACT

Kushtaka formation (predominantly
nonmarine), Stillwater formation
(marine). Stratigraphic relation not
definitely establisnea; Stillwater for-
mation believed to thin eastward,
forming a marine tongue in the coal-
bearing Kushtaka formation

■BASE NOT EXPOSED

Quaternary unconsolidated deposits; glaciers

ANGULAR UNCONFORMITY — — —

= ?>r~r=>T":

Yakataga formation; at least 10,000
feet, possibly 15,000 feet or more;
, marine

-CONFORMABLE CONTACT'

Poul Creek formation; 6100 feet; marine

■ DISCONFORMITYOI •

Kulthieth formation; 9300+ feet; pre-
dominantly nonmarine in outcrop

-BASE NOT EXPOSED

| HIGHER BEDS CONCEALED

GLACIERS

Yakataga formation (upper part);
3000+ feet; marine. Overlaps with
angular contact onto Cretaceousf?)
Yakutat formation to east, onto lower
part of Yakataga formation to west

ANGULAR UNCONFORMITY

Kulthieth formation (upper part); 2700
feet; nonmarine and marine. Over-
laps with angular contact onto Cre-
taceous(?) Yalcutat formation to east

ANGULAR UNCONFORMITY

Kulthieth formation (lower part);
2000+ feet; nonmarine and marine

BASE NOT EXPOSED -

In northwestern part of Malaspina district the Kulthieth for-
mation is underlain by 3000 feet or more of lower Tertiaryf?)
marine siltstone

LITUYA DISTRICT
Topsy Creek to LaPerouse Glacier

^-^^li^-^i

HIGHER BEDS CONCEALED BY .
GULF OF ALASKA

"Conglomeratic" sandy mudstone, sand-
stone, and siltstone; 9000 - feet;
marine

CONFORMABLE CONTACT .

Sandstone ana siltstone; 600-1500 feet;
marine

£ DISCONFORMITY

0-1000 feet; nonmarine(?)

CONTACT RELATIONSHIP NOT
OBSERVED. UNCONFORMITY!?)

Siltstone; 1200 ± feet; marine

ANGULAR UNCONFORMITY

MesoK»c(7) metasedimentary and metavolconic rocks

Fig. 39.8. Representative stratigraphic sections of the Tertiary sequences exposed
Tertiary province. Reproduced from Miller et a/., 1959.

the Gulf of Alaska.

ALASKA AND THE YUKON

617

unit includes the Kushtaka, Stillwater, and Tokun formations in the Katalla
district, and the Kultheith formation in the Yakataga and Malaspina districts.
It is not represented in the exposed Tertiary sequence of the Lituya district.

The middle unit, formed in middle Oligocene to approximately middle
Miocene time, is characterized by massive concretionary mudstone and silt-
stone, believed to have been deposited in moderately deep water, in part in a
reducing environment. Local volcanic activity is indicated by interbedded
.marine tuff and agglomerate. This unit is highly organic at some places, and
many of the known indications of petroleum in the Katalla and Yakataga
districts are associated with it. The unit includes the lower and middle parts
of the Katalla formation in the Katalla district, the Poul Creek formation in
the Yakataga district, and the basal part of the exposed Tertiary sequence in
the Lituya district. It is absent in the exposed Tertiary sequence in the
Malaspina district, where the early and late Tertiary units are in unconformable
contact.

The youngest unit, deposited during the time interval from middle or
late Miocene to late Pliocene or possibly earliest Pleistocene, consists of shallow
marine sandstone and siltstone interbedded with marine tillite ("conglomeratic"
sandy mudstone). The marine invertebrate fauna, on the whole, indicates
considerably colder water than in earlier Tertiary time, and the marine glacial
deposits indicate rigorous glaciation of adjacent land areas. This unit is
represented by the upper part of the Katalla formation in the Katalla district, by
the Yakataga formation in the Yakataga and Malaspina districts, by the upper
part of the unnamed sequence in the Lituya district, and by strata exposed on
Middleton Island, a small island in the Gulf of Alaska 80 miles southwest of
Cordova (D. J. Miller, 1959).

Structure. The structure and orogenic history of the Gulf of Alaska
Tertiary Province is described by D. J. Miller (1959) as follows (Fig.
39.9): '

In late Tertiary or early Pleistocene time the Chugach-St. Elias Mountain
chain was uplifted along an arcuate northward-dipping fault system, and
the bordering belt of Tertiary sedimentary rocks was folded and displaced
along many high-angle thrust faults. The largest of these faults, the Chugach-St.
Elias fault, has been traced along the southern front of the Chugach and
St. Elias mountains from the delta of the Copper River to Yakutat Bay, a
distance of 180 miles. This fault, which dips 30°-60°N., is estimated to have
a stratigraphic throw of not less than 10,000 feet. In the Lituya district the
Fairweather fault, lying in a great trench at the base of the Fairweather Range,
bounds the Tertiary province.

The major thrust faults and grain of folding in the Tertiary rocks in general
parallel the trend of the bordering fault system along the Chugach-St. Elias
front; the intensity of folding and magnitude of displacement along faults
increases toward the mountain front. Transverse trends in the western part

Fig. 39.9. Generalized geologic map of Gulf of Alaska Tertiary Province, after Miller et al., 1959.
Q, lowland area covered by ice or unconsolidated deposits of Quaternary age; possible underlain
by sedimentary rocks of Tertiary age. T, sedimentary rocks of Tertiary age. M, metamorphosed
sedimentary rocks and volcanic rocks of Mesozoic and older ? age.

of the Katalla district apparently are related to the northward-trending Ragged
Mountain fault that exposes the pre-Tertiarv basement rocks. In the Katalla
district the folds are typically of small amplitude, tightly compressed, and
asymmetric, the axial planes being inclined to the west or north.

In the Yakataga district three belts of differing structural pattern are re
nized: In the belt nearest the Chugach-St. Elias fault the Tertian rocks
show intense minor folding with much overturning, and are displaced along
many northward-dipping high-angle thrust faults, which in general arc .sub-
parallel to the axial planes of the folds. In the intermediate belt the folds arc
of small amplitude but relatively long, and are less tightly compressed and
more widely spaced. The belt nearest the coast is characterized by broad
synclines and narrow, tightly pinched, asymmetric, longitudinally faulted
anticlines.

In the Malaspina district, faulting and uplift predominated over Folding
during the late Cenozoic orogeny, for the youngest Tertiary strata are only
broadly folded or gentlv tilted. At least two earlier stages of deformation and
uplift within the Tertiary period are recorded bv angular unconformities
within the Kulthieth and Yakataga formations, and by overlap of the upper

618

STRUCTURAL GEOLOGY OF NORTH AMERICA

part of the Yakataga formation on early Tertiary and pre-Tertiary rocks.
Near Lituya Bay in the Lituya district the narrow belt of Tertiary rocks
is folded into a shallow syncline and a strongly asymmetric anticline. These folds
pass to the southeast into a seaward-facing homocline which, at Icy Point, is
overturned. Upper Tertiary rocks in the outlier in the northern part of the
Lituya district form a broad syncline trending northwest.

Cook Inlet Tertiary Province

The Cook Inlet Tertiary province includes the Cook Inlet lowland
and the lower part of the Susitna River valley. About 75 miles to the
east of the lower Susitna River basin and separated from it by the
Talkeetna Mountains, is the Copper River Tertiary and Quaternary basin.
The basins are floored extensively with Quaternary deposits, and these
are believed to cover Tertiary beds which crop out mostly in marginal
areas. See map, Fig. 39.1.

Stratigraphy. The chief display is a coal-bearing series of nonmarine
elastics. In the Kenai lowland the strata have been named the Kenai
formation. They consist of partly indurated sand, silt, clay with thin
conglomerate lenses and many thin beds of sub-bituminous coal or lignite,
and have a thickness of at least 4700 feet. The formation is presumed
to be Eocene and to rest unconformably on the deformed Mesozoic
rocks.

At a locality on the northwest margin of the Cook Inlet province, 900
feet of clay, sand, and gravel, presumed to be Eocene, rests with angular
unconformity on highly deformed slate and graywacke of Mesozoic age.
The unconsolidated beds are overlain, apparently conformably, by
500-1100 feet of coarse gravel, possibly of Oligocene or younger age.

Structure. The Tertiary beds of the Cook Inlet province are not as
much deformed as those of the Gulf of Alaska province. For the most
part they are nearly flat or only gently tilted or folded. In some marginal
areas dips up to 60 degrees have been observed.

Aniakchak Tertiary Province

A Tertiary area, here called the Aniakchak Tertiary province, com-
poses the southwestern half of the Alaska Peninsula. The Upper Jurassic
and Cretaceous rocks are overlain with minor unconformity by Early

Tertiary nonmarine, coal-bearing arkosic sandstones and shales and
much fragmental volcanic material interbedded with flows. These rocks
are presumed to underlie much of the Shelikof Strait depression. See Fig.
39.1.

Marine strata of Eocene, Miocene, and Pliocene (?) age are exposed
in the Herendeen Bay area and Shumagin Islands.

The Early Tertiary strata of the Alaska Peninsula are in general
gently tilted or folded. Several well-defined anticlines with flank dips of
5 to 45 degrees have been mapped. One is 30 miles long.

Laramide Oogenic Belt

The Laramide orogenic belt lies north of and adjacent to the Nevadan.
It is made up of two parts, the Foothills or gently deformed belt, and
the main or strongly deformed. In contrast to the Nevadan belt, the
Laramide involves the Paleozoic platform-type sediments, as well as
Mesozoic sediments mostly of miogeosynclinal nature. Also, intrusive
masses are few and not so large as in the Nevadan. The line drawn on
Fig. 39.2 separating the Nevadan from the Laramide was determined
mostly from the distribution of late Mesozoic intrusions, viz., most of
the intrusions lie south of the line. Included in the Laramide belt, ac-
cordingly, are the Yukon basin, Seward uplift, Hogatza arch, Kobuk
basin, Brooks Range geanticline, and the Arctic Foothills belt.

Brooks Range Geanticline. The northern limb of the Brooks Range
geanticline consists of slightly metamorphosed Devonian and Carboni-
ferous rocks. Dark clastic rocks of the Sadlerochit formation (Permian
and Early Triassic) generally overlie the lighter carbonate rocks of
the Lisburne group (Mississippian) and form conspicuous hogbacks
along the northern edge of the range. The structure of the northern
half of the geanticline is one of folds and thrusts.

The southern limb consists of early Paleozoic metamorphic rocks and
Silurian limestone. Tight folds and thrust faults toward the north repeat
the formations in numerous subparallel belts (D. J. Miller, 1959).

At least 10,000 feet, and perhaps 15,000 feet, of Devonian and Carboni-
ferous sedimentary rocks including much limestone were deposited in a
Paleozoic basin in the area of the present Brooks Range. Most of the

ALASKA AND THE YUKON

619

clastic materials were probably derived from an uplifted shield north
of the present land area, according to Miller. Permian rocks in the western
Romanzof Mountains area become coarser toward the north.

The Brooks Range geanticline began to rise in Jurassic time ( Miller,
1959). In one place mafic and ultramafic intrusions were emplaced
in Late Jurassic time. The main phase of orogeny occurred in Aptian
time (late Early Cretaceous) when the metamorphism of the rocks was
accomplished under deep burial, and an east-west structural pattern
took form. Uplift occurred throughout Late Cretaceous time and much
debris was shed to the Colville basin. A late (?) Paleocene phase of
deformation possibly resulted in the thrust faults, but these may have
formed earlier, and the east-west structural grain was intensified. Pene-
planation, and Quaternary uplift followed.

Romanzof Uplift. The Romanzof uplift appears as a northern bulge
of the Brooks Range. Fold axes plunge westward in the Canning River
area, and strata primarily of Carboniferous, Devonian, and possibly Pre-
cambrian ages are exposed. Mesozoic rocks are preserved in certain
structural depressions. The general uplift started in mid-Cretaceous, or
possibly earlier, and continued in uplift during the Tertiary.

Tigara Uplift. A small area of complexly folded and faulted rocks
pf Devonian, Carboniferous, and early Mesozoic age is exposed along
the coast line between Cape Lisburne and Point Hope, north of the
.De Long Mountains. These older rocks rise from the Southern Foothills
•belt (Index map of Fig. 39.6) and are called the Tigara uplift. It must
be a more extensive feature under the shallow water to die west.

Seward Uplift. The Seward peninsula is made up largely of deformed
Paleozoic rocks with Cretaceous intrusions and three large areas of
Tertiary volcanic rocks. The most extensive area of Ordovician rocks in
Alaska is in the western part of Seward peninsula. The rock is domi-
nantly limestone, and the beds have been cast into broad open folds
and show little effects of dynamic metamorphism. Their exact thickness
[is not known but at least 5000 exist (Smith, 1939).

There are also large thicknesses of Silurian, Devonian, and Carboni-
ferous limestones on Seward peninsula, but identities, correlations, and
thicknesses are not yet well known. Although the Ordovician strata of

the western part of the peninsula are only gently folded, tin- strata <>l
other areas are intensely deformed.

According to Payne (1955 and 1959) the dominant structural grain is
east-west and represents Early Cretaceous and possibly I. ate (post-Port-
landian) Jurassic phases of orogeny. Basic intrusions came in first and then
a number of large stocks or small batholiths of more acidic rocks. The
granitic intrusions with accompanying local metamorphism and miner-
alization occurred probably in Aptian time. The peninsula thereafter
remained mostly emergent and furnished sediments to adjacent basins,
particularly the Yukon. In early Tertiary time a second episode of
deformation produced a north to northeast grain superimposed on the
older east-west grain. Faulting was prominent.

During the Tertiary, erosion was extensive but the peninsula remained
broadly above sea level. Considerable volcanism occurred in late Cenozoic
time and resulted in blankets of extrusive rocks over die deformed Pa-
leozoic complex.

As portrayed on the map of Fig. 39.2, the Seward uplift included not
only the Seward peninsula but an approximately circular region under
the shallow water of Norton Sound and the Bering Sea. Although a
positive area in Mesozoic and Cenozoic times it is considered part of
the general Laramide belt. The phase of major deformation and intru-
sions, here as in the Brooks Range, appears to have been late Early
Cretaceous, and orogeny of this age is generally considered to be pre-
Laramide in the Rocky Mountains of the western United States. How-
ever, as previously explained, the Laramide belt is defined by physical
characteristics as well as time of orogeny and a phase of deformation
earlier than Late Cretaceous is a normal attribute of the Laramide belt.

Arctic Foothills Belt

A belt of "plateaus standing at different elevations" ( Mertie. 1930 )
lies north of the Brooks Range, and much work incident to the explora-
tion of Naval Petroleum Reserve No. 4 has established clearly that this
is a foothills belt, both topographically and structurally. It is subdivided
into two sections, the southern foothills and the northern foothills. See
Fig. 39.6.

620

STRUCTURAL GEOLOGY OF NORTH AMERICA

Southern Foothills. The southern foothills are characterized by iso-
lated, irregular hills and ridges of sandstone, limestone, and chert
which rise above low shale areas of little relief. This section has the
structural complexity of the Brooks Range but differs in being composed
of less resistant rocks, including a great thickness of shale. Ridges and
hilltops are at altitudes of 2500 to 3500 feet and rise 1000 to 2000 feet
above the surrounding plains. The southern foothills are readily travers-
able by such vehicles as the weasel but not so easily by boat, plane, or
foot. Lakes suitable for landings by small float planes ( 1 to 2 passengers )
are not abundant, and only a few lakes such as Noluk and Liberator are
suitable for larger float planes (3 to 6 passengers). The flat areas be-
tween the hills or along ridgetops are ideally suited to the use of
tracked vehicles.

Northern Foothills Section. The northern foothills section differs
from the southern section in having more regular topography, including
persistent ridges and elongate mesas that reflect a simpler structure of
Appalachian-type folds, with minor cross faults, and a few major
overthrusts. Anticlines are commonly asymmetric with steeper limbs on
the north.

TERTIARY VOLCANIC ROCKS

Volcanics of the Coast Ranges

The Wrangell Mountains (Figs. 37.1 and 39.1) consist of a major
Quaternary stratovolcanic accumulation. At least four major centers of
eruption form a cluster of majestic peaks, namely, Mt. Wrangell ( 14,000
feet), Mt. Drum (12,000 feet), Mt. Sanford (16,210 feet), and Mt.
Blackburn (16,140 feet).

Of these only the first-named has been seen "smoking." Apparently volcanism
in this region did not begin until some time after an early Tertiary plain of
erosion had been formed, uplifted, and somewhat dissected. Since that time
there has been almost unceasing volcanic activity in different parts of the
area, during which the present huge agglomeration of flows, breccias, and
tuffs has accumulated. Most of these rocks are porphyries of medium coarseness
and light or dark-gray color. In composition the usual type is a hypersthene
or hornblende andesite, but more basic or more acidic phases range from

basalt to dacite. The color of these rocks also shows a considerable variation
from the type, as brick-red, pink, lavender, brown, and greenish tones are bv
no means rare. The eastern limit of the lavas in the Copper River region that
may be correlated with the Wrangell lava is in the mountains adjacent to
Skolai Pass, where they cap many of the highland areas and unconformablv
overlie Paleozoic and younger sedimentary rocks. That the lavas in this area
are correlative with the older members of this volcanic series seems clearly
indicated by the extensive dissection they have undergone, whereby the
deep valleys of Skolai Creek and the Nizina River and Nizina Glacier have
been deeply trenched through them. None of these Tertiary-Recent lavas
shows evidence of marked deformation after they were poured out. The
thickness of the lava series differs considerably in different places, and no
measurements are available that disclose the total thickness of these beds in
the heart of the range. Partial sections have shown more than 4000 feet of
these volcanic rocks near Regal Glacier, in the Nizina Valley (Smith, 1939)

Cook Inlet-Susitna Field

Overlying the sedimentary coal-bearing and associated rocks in the
Matanuska area and extending both eastward into the Nelchina area
and northward into the Talkeetna Mountains is a series of andesitic
basalt flows with intercalated tuffs. They are nearly horizontal and at
least 1000 feet diick. They are deeply dissected and form cappings of
the highlands. In the Nelchina area certain rhyolites appear. The series
is thought to be late Eocene to Miocene in age (Smidi, 1939).

Tertiary volcanic rocks are widespread in the Nevadan belt and only
a few examples will be mentioned.

Volcanics of the Nevadan Orogenic Belt

In the Kuskokwim region the Lower (?) and Upper Cretaceous
Kuskokwim group, is overlain disconformably by the Ididarod basalt,
also of Late Cretaceous age. It is regarded as the first of a succession of
volcanic rocks deposited in a continental environment. The Getmuna
rhyolite group and the Holokuk basalt are early to mid-Cenozoic in
age, and are separated from the older rocks by an angular unconformity.
In late Cenozoic time die Waterboot basalt was erupted.

Intruded into the Kuskokwim sediments are a number of stocks of,
quartz monzonite, believed to be post-Holokuk basalt.

In the Yukon-Tanana region an older unit consists of rhyolite, dacite,

ALASKA AND THE YUKON

621

and andesite, with rhyolite die most abundant, and basalt practically
absent. They are so widespread that they must have been erupted
from several craters or from fissures. Mid-Tertiary intrusive granite
-ocks are probably later. In the younger group the acidic varieties occur,
but basalt is common. Some of the more basic members have inclusions
of ultrabasic composition. All the younger units are post-Miocene and
in part Quaternary.

Farther down the Yukon in the Chandaler Valley and in the Koyukuk
Valley volcanic rocks believed to correlate with the older unit of the
Tanana region occur. These rocks are partly in the Laramide belt.

In the lower Yukon Valley volcanic rocks undoubtedly of several ages
occur with the older presumably more acidic than the younger.

Volcanics of the Laramide Oogenic Belt

i Other than the Tertiary volcanic rock occurrences in the Yukon Valley
which are partly in the Laramide belt, the main eruptions have been in
the Seward peninsula. Three large fields are shown on the tectonic map
bf D. J. Miller (1959). One has an area considerably more than 1000
square miles. Old flows occur but the bulk of the volcanic rock is typically
;Recent. The ropy surface is preserved, Quaternary gravels are covered,
and stream drainages blocked. The sources are not evident, and perhaps
ithe flows issued from fissures. The composition is basaltic (Smith, 1939).

Aleutian volcanic belt

Kinds of Volcanoes

A great arc of volcanoes extends from Mount Spurr on Cook Inlet
along the whole Alaska peninsula and the Aleutian Archipelago. See map,
Fig. 39.10. This arc is 1500 miles long. Unfortunately, most of the
Volcanoes are situated in regions of sparse population little visited by
outsiders, and therefore their grandeur is seldom seen. The highest stand
3000 to 11,000 feet above the sea and excel in beauty many of the vene-
rated volcanoes of better-known lands. The Wrangell volcanic field and
Mt. Edgecumbe extend the belt of active or recently active volcanoes
another 1000 miles to the east and southeast.

Southward from the Mount Spurr group at the extreme northeastern
limit of southwestern Alaska, the sites of Tertiarv to Recent vok.misin
become increasingly evident until, at Mt. Veniaminof they include prac-
tically all the features of the bedrock. The lofty modern volcanoes that
overshadow all the other topographic features are dominant in almost
every landscape.

According to Coats (1950) there are at least 76 major volcanoes,
active and extinct in the arc from Mt. Spurr to Buldir Island. Of these,
36 have been active since 1760. Seventeen calderas have been recognized.
These are volcanic depressions, more or less circular, and over 1
mile in diameter. Of the 17 calderas the three largest are Fisher on
Unimak Island which measures 10x11 miles, Aniakchak, 9.7x8.4
miles, and Veniaminof, 8.4 miles.

A number of volcanic domes have also been recognized. As defined,
these are steep sided, viscous protrusions of lava forming a more or
less dome-shaped mass around the vents.

The older volcanoes of the arc seem to include both shield volcanoes,
characterized by many relatively thin flows, with a small proportion of
fragmental material, accumulated on slopes of low declivity, and stratovolcanoes
or composite cones, made up both of flows and fragmental material, the
slopes of which approach the angle of repose of the fragmental material. The
major active volcanoes of the arc are without exception composite cones
(Coates, 1950).

Petrographic Character

Smith (1939) summarizes the general petrography as follows:

The composition of the lavas has in the main been fairly comparable with
that of normal andesites, but more basic phases analogous to basalt and more
acidic phases approaching rhyolite are by no means unknown.

Coates (1950) depicts them as follows:

The volcanic rocks of the Aleutian arc range from olivine basalt to rhyolite.
They include basalts characterized by olivine and andesites without olivine, in
both of which hornblende and hypersthene occur separately or together.
Relatively high percentages of conspicuous calcic plagioclase crystals and
usually less conspicuous green augite characterize most of the rocks. Those
that are comparatively rich in silica, such as dacites and rhyolites, are much

622

STRUCTURAL GEOLOGY OF NORTH AMERICA

*&■■■

&>&■

-»v-

"- a

i/3

ATTU
AGUTTU

,8ULOlR I

-. ^

^A*.,-l^'' I •* •*« J?'

Fig. 39.10. Distribution of volcanoes in Alaska. Taken from Smith, 1939. Dots are active or recently
active volcanoes.

less plentiful; most of them are present either as small bodies of highly glassy
lava or as blankets of light-colored pumice.

Relation of Volcanism to Structure

In general, the volcanoes are superficial structures, built upon a basement
of Tertiary and older rocks that is exposed at intervals throughout the length
of the arc. The nature of the structures that have determined the position of
the eruptive centers can be determined in few places. Some writers have
thought that the line of volcanoes, because of its narrowness, represents the
trace of a great thrust plane or fault, movement on which is thought to be
responsible for the frequent earthquakes. In detail, the volcanic line does not
form a perfecdy simple arc, but consists of segments of different lengths; the
included angles between adjacent segments may be as little as 140°. Certain
volcanoes, like Bogoslof and Amak, lie some distance away from the main line,
on the concave side of the arc. In the Aniakchak region, Knappen mapped a

tension fault with an east-west trend, along or close to which several volcanic
structures are alined; he considered that the site of the eruptive center was
determined by the existence of the fault. It is probable that similar relationships
exist elsewhere in the arc and that most of the volcanoes have had their sites
determined by minor tensional fractures striking at an angle to the major
overthrust zones. The distance of a volcano from the major active zone of
movement is probably dependent upon the depth at which such a tensional
fracture, originating in and limited to an overthrust block, taps eruptible
magma (Coates, 1950).

Seismicity

The distribution of earthquake foci (Gutenberg and Richter, 1941, 1945) is
such that the epicenters of shallow earthquakes tend to be south of the chain;
those of intermediate-depth earthquakes ( deeper than 60 kilometers ) are in the
islands of north of the chain. It seems probable that the general structural picture

ALASKA AND THE YUKON

623

of the Aleution Islands, when more information is available, will resemble
that presented by Gutenberg and Richter for the structurally similar Japanese
are (1941). The distribution of deep and intermediate earthquake foci will
probably fall along an active zone or surface, which will be shown to reach the
surface of the crust along the northern slope of the Aleutian Trench and to
dip northward at a moderate angle (Coates, 1950).

Age of Aleutian Arc

The southern part of Kiska and the nearby islands of Attu, Agattu,
and the Semichis at the west end of the Aleutian arc lack the young
stratovolcanoes characteristic of the central and eastern islands. Instead,
they are composed of pre-middle Tertiary rocks and subordinate amounts
jof Upper Tertiary coarse clastic sediments and subaerial lava flows. On
jTanaga and Oglinga islands of the west-central Aleutians smoothly
rounded boulders in gravel beds on a wave cut platform appear to repre-
sent the oldest rocks of the region. The rock types are hornfels, horn-
'blende gneiss, slate, schist, granulite, granodiorite, biotite granite and
hornblende granite. The bedrock from which the boulders were derived
was not discovered. Judging from the lack of directional characters
in the granites, they are presumed to be intrusive into the other meta-
raorphic rocks.

A sequence of basalt flows, tuft-breccia, and agglomerate, intruded by
llarge masses of gabbro and small masses of rhyolite, underlies most of
the island of Adak and are known as the Finger Bay volcanics (Coates,
jl956). These rocks have generally been greatly deformed and hydro-
jthermally altered, although in no way metamorphosed like the meta-
imorphic types in the boulders, which are therefore considered older.

A third sequence of basalt flows and tuffs, gray, hard argillite, and
gray-green, coarse graywacke, seen on Attu and Shemya islands, has
'been intensely sheared and may be of intermediate age between the
boulder rocks and the Finger Bay volcanics.

A plant fossil was found in the Finger Bay volcanics and identified as

ate Paleozoic in age (Coates, 1956). Therefore, the intermediate basalts

nd graywackes and the metamorphic and granitic rocks of the boulders

ire regarded as Paleozoic. Coates regards the gneiss, schist, granulite,

granodiorite, and granite as continental types, and concludes, therefore,

that a continental land area stood nearby from which the boulders were
derived. This poses a difficult tectonic problem because the Aleutian
Islands in this segment rise from a rather narrow welt which is Hanked
on each side, most probably, by oceanic crust. It seems possible to the
writer that in the evolution of a great volcanic island arc from the oceanic
crust that deep-seated metamorphism is possible, and that granitic tvpe
magmas can originate there by fractional crystallization. These acidic
differentiates will not be large in volume such as those that arise in the
master eugeosynclinal belts of the continental margin.

Although the evidence is preponderant that the Aleutian arc as we
now see it is Cenozoic in age, we must recognize some much older
aspects in its evolution. These are certainly not clear to us in their tectonic
relations. As will be postulated under a later heading, the main tectonic
elements of continental Alaska are believed to veer northwestward to
the Anadyr Gulf and Chukotski peninsula of the Siberian mainland,
holding within the confines of the Bering Sea shelf. See Fig. 39.2.

SIBERIAN TECTONIC CONNECTIONS

Aleutian Projection

Since the structures of the Alaska Bange extend in a smooth curve into
the Aleutian Bange of the Alaska peninsula, and since the adjacent
geanticlines and basins, including the Aleutian trench, project in the
same direction, the natural inference has been that the Xevadan and
Coast Bange orogenic belts run out to sea and mostly die out abruptly
or continue as a single geanticline concealed by Tertiary volcanics. This
is the main assumption of Carey (195S) in the presentation of his theory
of the Alaskan orocline.

Anadyr-Chukotski Projection

In 1955 Payne showed on a tectonic map of Alaska the Colville basin
and Brooks Bange geanticline to project northwestward under the shallow
waters of the shelf off Siberia toward Wrangell Island, and this view is
reiterated by D. J. Miller (1959), who conceived the Seward and Tigara

624

STRUCTURAL GEOLOGY OF NORTH AMERICA

uplifts to be part of a much larger uplift embracing the eastern end of
the Chukotski peninsula.

Now, if the Geologic Map of the U.S.S.R. (1955) is consulted, the
Chukotski peninsula and adjacent areas to the west are found to be made
up of three geologic provinces, namely, ( 1 ) a deformed and considerably
intruded Cretaceous basin on the north; (2) a Tertiary Coast Range
province on the south; and (3) an intermediate Tertiary volcanic belt
in which it appears that the volcanics rest mostly on the Cretaceous com-
plex. See map, Fig. 39.2. The Cretaceous basin with its abundant Creta-
ceous volcanics and many batholiths and stocks seems similar to the
central geanticline and adjacent basins of southwestern Alaska, and if
tectonic connections on this basis are attempted several lines of evidence
support the postulate.

The Coast Range orogenic belt is adjacent on the south in Siberia as
in Alaska. St. Lawrence Island with its major intrusions appears to be
Nevadan and falls within the projected Nevadan belt. See Fig. 39.2. The
shallow water shelf of the Bering Sea will contain both belts of orogeny,
and the outer margin of the shelf lies in the line of projection. By this
theory an erosion surface of the orogenic belts would have been buried
by the deltaic deposits of the Yukon and Kuskokwim rivers. The last
evidence suggestive of the northwest bend of the Nevadan and Coast
Range belts is the bathymetry of the shelf off the southeast side of the
Alaska peninsula. If the map of Fig. 39.11 is referred to, it will be seen
that the shelf is broad off Kodiak Island and westward to Unimak Island
(Fig. 39.1), and then narrows so that hardly any shelf exists along the
volcanic islands of the archipelago. The narrowing shelf margin projects
almost exactly to the Bering Sea shelf margin, as if this is a major tectonic
line. It may thus be imagined that this line marks the swing of the Coast
Bange belt toward the northwest and Anadyr Bay.

The Seward uplift then becomes a coigne around which the Nevadan
belt wraps rather sharply.

The Aleutian Archipelago is here considered a welt or geanticline that
has developed with customary curvature, volcanism, and trench from
ocean basin crust, whereas the Nevadan and Coast Range belts are
marginal to continental crust. The archipelago and the Coast Range belt

have evolved probably simultaneously, although the archipelago is now
very active while the Coast Range belt under the Bering Sea is quiescent

Bering Land Bridge

With Nevadan, Laramide, and Coast Range belts extending from
Alaska to the Anadyr-Chukotski region of Siberia there can be little doubt
that land was continuous from one continent to the other many times
from the beginning of the Cretaceous to the present.

Hopkins ( 1959 ) reports that if sea level were lowered 120 feet, only
a channel 20 miles wide would remain. If lowered 150 to 180 feet an
intercontinental land connection would be established via St. Lawrence
Island and the Diomede Islands. If lowered 300 feet, presumably to the
level during the maximum glaciation of the Wisconsin, Alaska and Siberia
would be joined by an almost featureless plain nearly 1000 miles wide
from the shrunken Bering Sea to the shore of the Arctic Ocean.

YUKON TERRITORY AND THE DISTRICT OF MACKENZIE

Geography

The principal mountains and rivers of Yukon Territory and the ad-
jacent district of Mackenzie are shown on Fig. 39.1. The Selwyn Moun-
tains form the major drainage divide, with the several tributaries of the
Yukon River flowing to the west, and tributaries of the Mackenzie flowing
to the east and north. The long arcuate Mackenzie and Franklin Moun-
tains stand off to the northeast of the main Cordillera, with the Mackenzie
River flowing between the two ranges. Several plains and plateaus in
addition to those shown are recognized by various writers (Bostock,
1948; Martin, 1959), but the geographic nomenclature is not com-
pletely standardized.

Stratigraphy

Strata of every Paleozoic and Mesozoic system are present in the region
as well as rocks of Precambrian and Tertiary age. Dominant rock types
are as follows: Precambrian and Lower Cambrian, elastics; Middle and
Upper Cambrian, Ordovician, and Silurian, carbonates, black shales, and

Fig. 39.11. Aleutian trench and Bering Sea, showing relation of broad shelf off the Aleutian peninsula
to Alaskan-Siberian shelf. Reproduced from Murray, 1945.

626

STRUCTURAL GEOLOGY OF NORTH AMERICA

bedded cherts; Middle Devonian, carbonates and shales; Upper Devo-
nian, elastics; Mississippian, carbonates; Upper Pennsylvanian and Lower
Permian, elastics; Triassic, shales and limestones; Jurassic and Cretaceous,
elastics; and Tertiary, elastics (Hume, 1954; Martin, 1959). A cross sec-
tion restored to the time of pre-Laramide deformation from west of the
Barn Mountains to the Mackenzie delta is shown in Fig. 39.12. Several
unconformities attest several times of crustal unrest with the formation
of various basins and uplifts.

The first conspicuous disturbance occurred in the British and Barn
Mountains area, probably during late Middle Devonian or Late Devonian
time. Upper Devonian sediments derived from the uplift form a deposi-
tional body much like the Catskill delta (Martin, 1959). The area of
uplift was probably mountainous for a while. Judging from the Upper
Devonian elastics in the Brooks Range and the Barron arch under the
Coastal Plain the uplift extended westward through northern Alaska
as shown on Fig. 39.13 (Dutro, 1960).

The second conspicuous uplift occurred in Pennsylvanian time in
the Richardson Mountains area. It seems to have proceeded in two im-
pulses, one before Late Pennsylvanian time and one during the Late
Pennsylvanian. The uplift was flanked by a complementary basin on
the northwest. A Pennsylvanian basin exists also under the Arctic Foot-
hills and Coastal Plain, whereas the rest of Alaska was emergent at the
time, so that a partial and approximate view of Pennsylvanian condi-
tions is shown in Fig. 39.13.

An Upper Triassic and Jurassic basin subsided in a general north-
south direction in the Richardson Mountains area. Cretaceous beds are
absent in northwestern Yukon toward the Brooks Range, but reach con-
siderable thickness in the Mackenzie Mountains near Norman Wells.

Laramide Orogeny

The present mountains, plateaus, and plains are the aftermath of
Laramide deformation and some Cenozoic faulting, but the exact time
of disturbance or the number of phases have not been well fixed. The
Mackenzie and Franklin Mountains are foreland type, with gentle folds
the dominant structure. High-angle faults are reported in places but no
thrusts of typical Rocky Mountain fashion are known. The Franklin
Mountains are reported as narrow, flat-topped anticlines, generally
faulted on one side or both.

General structures of the Mackenzie Mountains and of other ranges
in the region are shown in Fig. 39.14. As may be seen, folds dominate
the structural types, but along the eastern side of the Richardson Moun-
tains Jeletzky ( 1961 ) has mapped a fault pattern which he describes as
follows (see Fig. 39.15):

Major faults split the area into a number of irregularly shaped and structural]}
disconnected fault blocks, which differ strongly in the degree of structural
complexity and age of their rocks.

The structure of the area contrasts strongly with that of the central parts
of Richardson Mountains, which is dominated by symmetrical, large, mostly

BARN BLOW

MOUNTAINS PASS

RICHARDSON MOUNTAINS

WEST ED6E
MACKENZIE DELTA

FAULT BELT OF LATE DEVONIAN

Fig. 39.12. Cross section from
British Mountains to Mackenzie
delta, restored to pre-Laramide time.
After Martin, 1959.

ALASKA AND THE YUKON

627

Fig. 39.13. Geosynclines, basins, and
uplifts of Paleozoic age in Alaska and
northern Canada. The fold belt of the
Arctic Archipelago involves the miogeo-
syncline. The Wollaston, Victoria Straits,
and Foxe basins are of the cratonic basin
type. Most all Alaska appears to have
been emergent in Pennsylvanian time.
Ordovician and Silurian carbonate depo-
sition was extensive from the Seward
peninsula to the Yukon. P. P. I., Prince Patrick
Island; B.I., Borden Island; A.H.I., Axel Hei-

1 berg Island; M.I., Melville Island; P.W.I.,
Prince of Wales Island; B.I., Bathurst Island.
D.I., Devon Island; M.P., Melville peninsula;

)$.!., Southampton Island.

'open folds, rather than by faults. The area appears, therefore, to form part of a
separate structural zone, which separates the central parts of Richardson

^Mountains from the essentially stable belt situated further east.

i Largest faults trend northerly and appear to be strike-slip faults. Folds are
medium to small-sized, irregularly patterned, and commonly dome-like. Larger
folds are strongly disrupted by faults and were apparently caused by an earlier
orogenic phase. Smaller folds are subordinated to and were apparently caused
by major faults. Both thev and the major faidts were, therefore, apparently
caused by a later orogenic phase.

Hauterivian, late Aptian, early Albian, and late Albian or early Cenomanian
(? at the Lower/Upper Cretaceous boundary) unconformities were observed

in the area. Late Aptian unconformity is accompanied by a 5 to 10° angular
discordance. Others are only recognizable because of smaller or larger trans-
gressive overlaps.

The above unconformities were apparently caused largely by epeirogenic
movements as no tectonic structures are known to be caused by them. The
mid-Upper Cretaceous rocks of the area were, however, constantly involved
in the major dislocations. The contemporary structures of the area were,
therefore, caused largely or entirely by the post mid-Upper Cretaceous (? earl)
Tertiary) orogenic movements.

A general description of the structures by Martin ( 1959) is informative.

628

STRUCTURAL GEOLOGY OF NORTH AMERICA

140"

138°

136"

134

MES0Z0IC SEDIMENTS
|^s$^ PLAINS

PALEOZOIC SEDIMENTS
I | MOUNTAIN

I I TERRANE

PALEOZOIC SEDIMENTS

69° fg|j

MAINLY PAL. a MES. SEDS.

ACIDIC INTRUSIVE ROCKS

MILES

68°

Fig. 39.14. Mountains and structural trends of the
lower Mackenzie River region and northern Yukon.
Map kindly supplied by P. E. Kent and W. A. C.
Russell, British Petroleum, Ltd. Heavy lines surround
mountainous areas. Dave Lord Ridge extends east-
erly from Alaskan border at 67 N. Lat. Canyon
Ranges are northwest end of Mackenzie Range.

The fault structures (of the Ogilvie Mountains, Dave Lord Ridge, and
northwestern Richardson Mountains) are of the same general type as those
that form the Rocky Mountains of western Alberta, but a preliminary examina-
tion indicates that the stratigraphic displacement caused by individual faults
is not as great as in the case of the Alberta Rockies. Faults in the Dave Lord
Ridge area are irregular in trend and displacement, and do not result in the
typical Canadian Rocky Mountain topography.

The anticline that forms the southern Richardson Mountains is of a type

similar to the Wyoming Rockies uplifts, such as the Righorn and Wind River
mountains. The Franklin Mountains structures appear to be of the same type,
but on a smaller scale.

The intrusion of the Old Crow Range batholith may have been in part re-
sponsible for Tertiary or late Mesozoic movements that took place in the Rritish
Mountains, and may have affected to some extent other structural patterns in
the region.

Uplift of the coastal area following the retreat of Pleistocene glaciers is

ALASKA AND THE YUKON

629

demonstrated by the presence of raised beaches along the Arctic Coastal Plain
and in the area west and southwest of the Mackenzie Delta.

Potassium-Argon Dates of Intrusives

The ages of several intrusives in the Yukon and the District of
Mackenzie have been determined by Baagsgaard, Folinsbee, and Lipson
(1961). The oldest date, 353 m.y., indicates an Acadian age. Two dates
of 220 and 240 m.y. suggest late Paleozoic magmatic activity. Several
dates ranging from 94 to 101 m.y. indicate intrusive activity in Mid-
Cretaceous time or during the Nevadan orogeny. Figure 39.16 shows the
position of the above intrusions and Fig. 39.17 shows the relation of
Paleozoic orogenic belts and dated intrusions around the Arctic in
Eurasia, Greenland, and northern Canada.

CENOZOIC TRENCHES AND FAULTS

Topographic Expression

The new Army Map Service Relief Quadrangles of Alaska, the Yukon,
and northern British Columbia show strikingly five major linear
topographic trends. Several smaller ones are also apparent. These linear
features consist in part of trenches and in part of bold mountain escarp-
ments, but the continuity of one with the other cannot be doubted. The
Rocky Mountain, Tintina, and Shakwak trenches mentioned in Chapter
37 are especially clear on the maps. Some have been partially described

i in the literature and mentioned on previous pages of this chapter. The
major alignments are so striking and the geomorphic provinces on either
side in places so distinct that the writer is prone to consider them major,

: if not the most important, structural features of central and southern
Alaska and adjacent regions. They are emphasized by bold lines on the

1 map of Fig. 39.2.

A Tintina-Rocky Mountain Trench Fault Zone

i It is fairly evident that the Rocky Mountain trench projects to the

; Tintina trench, and thence to the south side of the Yukon Flats in

1 Fig. 39.15. Structures of the southern Richardson Mountains. Reproduced from Jeletzky, 1961.

LEGEND
MESOZOIC

Cretaceous and Jurat*, c
I Jurassic only m
northwestern corner

-est ol Peel River
and Husky Channel and
north ol VitVekwa River)

PAL/EOZOIC

Palaeozoic tCambr>an to
Perm.an west ol Peel

River and Husk,
mostly or entirely
Devonian east thefefromt

Principal Strike ■ S 3«
{?) normal faults (known

and assumed!, arrows
indicate inferred or
assumed direction of

horizontal movement

Principal thrust faults
Iknown and assumed!

Major anticline f celled.
approximate, arrow
indicates plunge)

— i

Ma/Of synd^ne (defined,
approximate, arrow
indicates plungef

Ma/or dome
Piercement structure

Boundary between stable
and tectonicatly active
regions (approximate
assumed f

G S C

630

STRUCTURAL GEOLOGY OF NORTH AMERICA

Fig. 39.16. Potassium-argon dates in British Columbia Mackenzie District and Yukon Territory.
Map kindly supplied by R. E. Folinsbee. See Baadsgaard, Folinsbee and Upson, 1961

Alaska, and down the Yukon River, possibly to the junction with the
Tanana River. The bedrock is so much covered by alluvium from the
Tanana down the Yukon Valley that the further course of the fault zone,
if existent, is not evident.

Farewell-Shakwak Fault Zone

The most arresting alignment of valleys and mountain fronts starts on
the northwest front of the Alaska Range (Mt. McKinley) and extends
eastward as a trench through the southern part of the Alaska Range from
Cantwell to Miller's Roadhouse, thence southeastward along the north-
east front of the Mentasta and Nutzotin Mountains. It then crosses the
border, follows along the Alaska Highway to Kluane Lake and to
Dezadeash Lake where it jogs a bit to extend to the "Haines Cut-off"
valley. It follows to the Lynn Canal. A branch may go out Chatham
Straight, but the main fault appears to follow along Stephens Passage to
Wrangell. The great fault zone has been named the Farewell in the
Kuskokwim region and the Shakwak in the Yukon.

Mt. Logan Fault Zone

Not specifically pointed out in the literature as far as the writer knows
is a major narrow topographic lineament just south of the Farewell-
Shakwak zone. It is labeled "Mt. Logan Fault Zone" on the map of Fig.
39.2. Reginning on the southeast at Chatham Strait it proceeds as a
trench along Icy Straight and Glacier Ray through the Mt. St. Elias Range
to and along the Hubbard Glacier Valley. It then extends along the
Logan Glacier Valley and the Chitina River. It thence passes a little south
of the town of Chitina and down the Matanuska River Valley to
Matanuska. It continues southeastward along the base of the mountains
east of Anchorage to Kachemak Ray. This postulated fault zone is not as
smoothly curved or linear as the others.

Chugach-St. Elias and Fairweather Faults

A great fault has been described in the Tertiary province of southern
and southeastern Alaska and it is illustrated in Fig. 39.9. It is also vividly
expressed on the relief maps.

ALASKA AND THE YUKON

631

ruin Bay Fault

Along the northeast side of the Cook Inlet depression is the Bruin Bay
fault (D. J. Miller, 1959). Between it and the Mt. Logan fault is the
Hook Inlet Tertiary province, described on a previous page.

faults of the Kuskokwim Region

The Iditarod-Nixon fault, the Holitna fault, and the Farewell fault
lave been described on previous pages and are part of the great fault
'ystem.

Mature and Age of Faults

The faults of southwestern Alaska are undoubtedly high-angle faults,
Imd Cady et al. apparently sees no evidence of horizontal movement on
'he faults. An old erosion surface has been arched and faulted, and certain
;treams are regarded as antecedent to the vertical uplifts.

The Bruin fault is a major high-angle thrust that dips to the north-
vest (D. J. Miller, 1959). The Chugach-St. Elias fault is also a thrust
ivhich dips northward 30° to 60° and is estimated to have a throw of
1.0,000 feet. The 1958 movement on the Fairweather fault at the head of
Lituya Bay produced scarps in which the vertical displacement was 3/2
ieet, and the horizontal movement 21/2 feet. The southwestern down-
thrown block moved to the northwest in the horizontal movement (D. J.
(tiller, 1959).

| In general the benches suggest tension, but their straightness or
l.moothly curved nature suggests, if not demands, the interpretation of
'major horizontal displacement. The Farewell-Shakwak fault seems to
'have cut the Alaska Bange and displaced the south side westward (Mt.
McKinley block) and the north side eastward, leaving the opposite facing
'escarpments in nice alignment. These topographic features remind one
bf features of the San Andreas fault.

The pronounced and fresh-looking topographic features can only mean
jthat major movements on the faults have occurred in Late Cenozoic time.
If the faults are like the San Andreas system, then we might suspect a
beginning of displacement as far back as the Cretaceous.

The Farewell fault has been connected with the Shakwak trench fault

Fig. 39.17. Absolute dates from granites in the orogenic belts around the Arctic. Map kindly
supplied by R. E. Folinsbee. See Baadsgaard, Folinsbee and Lipson, 1960.

zone by St. Amand (1957) and the entire system, including the projection
down the Lynn Canal, called the Denali fault, for Mt. Denali, the Indian
name for Mt. McKinley. He extended the fault down the Alexander
Archipelago, along the outer side of the Queen Charlotte Islands, and
connected it with the San Andreas fault off Cape Mendocino. Pursuant
to the thesis that the Pacific block is in counterclockwise rotation, as

632

STRUCTURAL GEOLOGY OF NORTH AMERICA

presented by Benioff and depicted in Chapter 32, the Denali fault is
presumed to be of right-lateral strike-slip movement, and to have trans-
lated rocks 150 miles along the fault.

The principal evidence for right-lateral slip, other than the possibility
of the existence of a great fault with smooth arcuate curvature is that of
first motion seismograms, and these, according to St. Amand, "indicate
that the north Pacific Basin, from Baja California to the Kurile Islands at

least, is and has been for a long time, rotating counter-clockwise."

Not much can be said to resolve the problem. St. Amand's interpreta-
tion may serve to evoke careful observation of the fault features by those
who work in the field along them, and eventually it may be said either
that horizontal motion has or has not been appreciable. It may be noted
that the other great fault zones of the system were not recognized by St.
Amand.

40.

The provinces can be better understood if the entire region is assumed
to be emergent because they represent the grouping of the islands or
parts of the islands into belts or regions of common geology. See Fig.
40.2.

The sedimentary provinces (eugeosyncline, miogeosyncline, and stable
interior) have been defined fairly well, and these should be distinguished
from and related to the geologic provinces listed above. Also a younger
sedimentary basin (epigeosyncline) has been recognized reposing on
parts of the older fold belts and geosynclinal divisions. See Fig. 39.13.

CANADIAN ARCTIC

GEOGRAPHY AND GEOLOGIC PROVINCES OF THE ARCTIC ARCHIPELAGO

I
i

The Canadian Arctic Archipelago is a vast domain of islands, channels,
bays, gulfs, and peninsulas. It is approximately 1500 miles wide and
1500 miles long, and represents the partly and gently submerged northern
margin of the North American continent. The major features of its
geography and relief may be seen in Fig. 40.1.

It has been divided into four geologic provinces, namely, (1) the
northern part of the Canadian shield, (2) the Arctic Lowlands and
Plateaus, (3) the Innuitian region (fold belts), and (4) the Arctic
Coastal Plain.

LOWLANDS AND PLATEAUS

Definition

The Lowlands and Plateaus province consists of shelf sediments and
intercratonic basins, and is the northern counterpart of the Central Stable
Region of the continent. It was called the Arctic Stable Region in the
first edition of this book. It will here be considered to include the arches
of Precambrian rocks that separate some of the basins.

Areas of Precambrian Rock

The exposures of Precambrian rock of the Canadian Arctic have been
described by Fortier (1957) as follows:

Three areas of the Shield, namely, the Baffin-Ellesmere Belt, the Melville-
Southampton Belt, and the Boothia Arch, are underlain by Archean rocks and
smaller amounts of Proterozoic rocks. The other two areas, the Wellington and
Minto Arches, are underlain by Proterozoic formations.

The Baffin-Ellesmere Belt is the largest and most easterly of the Precambrian
areas. It occupies the larger part of Baffin and Bylot Islands, the eastern part
of Devon Island, and stretches about half-way along the east coast of Ellesmere
Island. The belt is composed chiefly of Archean gneisses and granitic rocks.
The structures of the gneisses are complex but a northwesterly trend is preva-
lent in southern Baffin Island. Proterozoic strata are found in northern Baffin
Island and are gendy flexed along northwesterly to northerly trending axes.
Flat-lying or gendy inclined Proterozoic strata also occur at the north end of
the belt on Ellesmere Island.

The Melville-Southampton Belt underlies almost all of Melville Peninsula and
eastern Southampton Island, continues across Fury and Hecla Strait, and ob-
viously connects with the Baffin-Ellesmere Belt. Little is known about the

633

LEGEND

MOUNTAINS

1. Baffin -Ellesmere mountains
of crystalline rocks

2. Ellesmere -Axel Heiberg
mountains of folded rocks

UPLANDS of crystalline rocks

3. Baffin uplands

4. Southampton-Melville uplands

5. Boothia -Somerset uplands

UPLANDS of folded rocks

6. Western Victoria uplands

7. Parry Islands uplands

8. Cornwallis-Grinnell uplands

9. Ellesmere -Grinnell uplands

10. Hazen Lake upland

PLATEAUX of flat-lying strata
SS3 11- Jones -Lancaster plateaux
v^ 12. Bache plateau

13. Banks-Melville plateaux

LOWLANDS and PLAINS

14. Foxe Basin lowlands

15. Southwestern Southampton
lowlands

16. Boothia -Regent lowlands

17. Eastern Victoria lowlands

18. Arctic coastal plain

REGIONS of undifferentiated lowlands,
plateaux and uplands

19. Sverdrup -Parry region

^ 20. Western Axel Heiberg region
21. Eureka -Nansen region

G. S. C

Fig. 40.1. Provisional physiographic divisions of the Arctic Archipelago. Reproduced from Fortier, 1959.

CANADIAN ARCTIC

635

Fig. 40.2. Structural stratigraphic
elements of Arctic Archipelago. Re-
produced from Thorsteinsson, 1959,
which is revised after Fortier, Mc-
Nair, and Thorsteinsson, 1954.

LEGEND

Mainly (') Mesozoic and
Cenozolc shell sediments

Unfolded miogeosynclinal
sediments, Cambrian-
Devonian

Cratonic basin sediments,
A Ordovician - Silurian

Precambrian Shield
rocks

INNUITIAN OROGENIC SYSTEM

Unclassified geosyncline
fold belts

Miogeosynclinal
fold belts

Eugeosynclinal
fold belt

Structural trend of .

fold belts ,*=^cr>

Piercement domes ••»*•

Margin of Shield >— > 4«_V

G. S. C.

geology of this area, but it is apparently underlain mainly by Archean rocks
with Protozoic strata along and north of the strait.

The Boothia Arch occupies most of Boothia Peninsula, the western part of
Somerset Island, and fringes the southern part of the western shores of Peel
[Sound. It appears to be mainly of granitic rocks and gneisses, much folded
along a northerly to northeasterly regional trend. In the northern part it is
flanked apparently by Proterozoic strata which appear to form the outer limbs
of a geanticline. This northerly structure may have been in part the effect of a
late Silurian orogeny which has affected lower Paleozoic strata adjacent to the
Precambrian formations. Basic dvkes similar to the so-called diabase dvkes so

widespread on the mainland Canadian Shield occur throughout the Precambrian
of Baffin, Devon, southern Ellesmere, Somerset, and Prescott Islands, and of
Boothia Peninsula. They are the youngest Precambrian rocks and their pre-
dominant orientation is northwesterly.

The Wellington Arch, in southern Victoria Island, is apparend) made ex-
clusively of Proterozoic rocks in obvious extension of the Proterozoic strata of
Kent Peninsula and of Bathurst Inlet on the mainland. It trends northerly
through Washburn Lake and possibly joins the Minto Arch.

The Minto Arch is much more extensive. It stretches from southern Banks
Island across the northern part of Amundsen Gulf to the west coast oi Victoria

STRUCTURAL GEOLOGY OF NORTH AMERICA

Island, between the west half of the north shore of Prince Albert Sound and the
North shore of Walker Bay. Thence it crosses Victoria Island to its northeastern
part, where it probably stretches from Richard Collinson Inlet to the west part
of Goldsmith Channel, from which it trends southerly, being possibly within
40 miles of the east coast at Greely Haven. Magnetic data suggest that the
Precambrian extends, at shallow depth beneath a thin cover of Paleozoic strata,
from the latter locality to the Precambrian of eastern Prince of Wales Island.
The rocks of the Minto Arch appear to be entirely Proterozoic and include
sedimentary strata in part intercalated with lava and sills. The strata trend
northeasterly to northerly and, over most of the belt, form undulatory folds with
gende dips, although in some areas the beds are practically flat lying. In the
south half of Wollaston Peninsula unmapped rocks of reddish colour, as ob-
served from aircraft, form many ridges of uniform elevation and oriented east
to northeast. Possibly these are Proterozoic formations similar to those of the
Minto Arch.

Basins

The basins may be divided into two kinds, those of the miogeosynclinal
sedimentary province, and those in the shield (intercratonic). Those con-
sidered miogeosynclinal are the Jones-Lancaster and Melville basins, and
those of the intercratonic type are the Wollaston, Victoria Strait, and
Foxe basins. See Figs. 39.13 and 40.2. In a version of the sedimentary
provinces by A. H. McNair these basins are considered mostly inter-
cratonic, with the miogeosyncline being restricted to the fold belts (map
supplied writer by McNair).

The Jones-Lancaster and Melville basins are separated by the Boothia
arch. According to Fortier (1957):

They extend from Banks Island to Bache Peninsula, midway along the east
coast of Ellesmere Island and accordingly, lie mainly between the outer areas
of the Shield and the Innuitian Region. Most of the strata of the Jones-Lan-
caster Basin Range in age from Cambrian to Devonian but may include rocks
of Tertiary age. Although normal faults and a few folds are present, the strata
throughout most of the basin dip gently away from the Shield areas and towards
the Innuitian Region. Thus, in the northern part of the basin on Ellesmere
Island, the regional dip is northerly, farther south it is westerly, and near the
south coast of the island it is northwesterly. On Devon Island the dip is westerly
and on Brodeur Peninsula of Baffin Island it is northwesterly. Near the In-
nuitian Region, however, at least some of the beds are flexed into folds which
are probably related to the orogenies that affected that region, but are on a
smaller scale. Such folds are found, for instance, on Somerset Island. In the
northwestern and south central parts of Somerset Island and in the northeastern

part of Prince of Wales Island, that is, on each side of the Boothia Arch, a late
Silurian or early Devonian conglomerate is made of detritus derived from the
Precambrian rocks of the arch. However, the arch is presendy separated from
the conglomerate by a wide exposure of earlier Paleozoic strata, the gende
flexure of which, at least in the east, may have been contemporaneous with
the uplift and denudation of the arch and with the deposition of the conglom-
erate. Litde is known of the Melville Basin, except for the above conglomerate,
but Silurian strata are apparendy widespread in its eastern part and Devonian
strata occur in its western part. North of the Minto Arch, the strata on north-
western Victoria Island regionally dip gendy to the northwest; on northern Banks
Island they are flexed in gentle, southerly trending folds; and on southwestern
Melville Island they are flat lying to gently flexed.

FOLD BELTS-THE INNUITIAN REGION

Nature and Distribution

A belt of strong deformation extends from North Greenland south-
westerly through the Arctic Archipelago to the Parry Islands. It consists
of folds of mid- and late Paleozoic age (pre-Middle Pennsylvanian )
developed in eugeosynclinal and miogeosynclinal strata, and structures
of late Mesozoic and Tertiary age in basin beds laid down on the older
orogenic complex. The fold belt in the miogeosynclinal strata ( Fig. 40.2 )
is divided into a western segment, the Parry Islands fold belt, and an
eastern, the Ellesmere-Greenland fold belt, by a transverse zone of
structures, the Cornwallis fold belt. The Cornwallis fold belt is a northern
continuation of the Boothia arch.

The Northern Ellesmere Island fold belt is regarded as deformed and
metamorphosed eugeosynclinal strata.

The Eureka fold belt is the northeastern part of the Sverdrup basin
which is composed of Late Pennsylvanian and younger beds laid down
on the deformed eugeosyncline and miogeosyncline.

Parry Islands Fold Belt

The Parry Islands Fold Belt includes, in its eastern part, at least 1,800 feet
of calcareous and dolomite mudstone and shale, in part silty, overlain bv 3,000
feet of further Silurian graptolitic, argillaceous and calcareous, fine-grain sand-
stone. These are conformably overlain by 1,200 feet of Silurian or Lower
Devonian calcareous and argillaceous sandstone, 800 feet of Lower Devonian
shale and these are followed by a Middle and Upper Devonian sequence similar

CANADIAN ARCTIC

63"]

to that found on southern Ellesmere Island. The shaly equivalent of the grapto-
litic rocks might occur in the unexplored southern part of Bathurst Island, as
some are known on southern Cornwallis and northern Prince of Wales Islands.
The western part of the fold belt includes over 1,000 feet of Ordovician and
possibly earlier limestone and conglomerate. In part the Ordovician, Cornwallis
formation, with over 1,500 feet of shaly limestone and dolomite, is overlain by
2,500 feet of graptolite shale of the Cape Phillips formation; in other parts are
Ordovician and Silurian black graptolitic shale, argillite, chert with minor
dolomite, in all some 3000 feet thick; still elsewhere are over 6,000 feet of
Silurian and possibly Ordovician dolomite and limestone. The Devonian in-
cludes up to 8,000 feet of marine and non-marine sandstone, siltstone, and shale;
2,500 feet of non-marine sandstone, and 4,000 feet of non-marine sandstone,
shale, and coal, with marine bands.

The Parry Islands Fold Belt was folded before the deposition of the Pennsyl-
vanian. The synclines generally have broad troughs and the anticlines have
narrow crests with the more steeply dipping strata close to the crests. Many
folds are doubly plunging but closures are still to be determined. Where the
belt abuts the transversal Cornwallis Fold Belt, deformation has resulted in
folds of various shapes and orientations, some folds are almost circular in shape,
others have curving axes, and some are aligned parallel to those of the Corn-
wallis belt (Fortier, 1959).

Ellesmere-Greenland Fold Belt

The Ellesmere-Greenland Fold Belt comprises at least 870 feet of Middle
Cambrian limestone and minor shale; 4,800 feet of limestone and impure lime-
stone with gypsiferous beds, possibly ranging from Cambrian to Middle Ordo-
vician; up to 4,400 feet of the Middle Ordovician Corwallis formation; 3,700
feet of the Ordovician to Middle Silurian Allen Bay formation; at the very
least 1,500 feet of Middle to Upper Silurian limestone, silty limestone, and dolo-
mite. The Ordovician to Upper Silurian graptolitic Cape Phillips formation, at
least 2,300 feet thick, has been located only north of Baumann Fiord and ap-
proximately halfway across the fold belt. Either in the Upper Silurian and/or
the Lower Devonian are numerous sections correlated with difficulty either be-
cause of the nature of their fauna or their unfossiliferous nature. They differen-
tially contain dolomite, sandstone, limestone, siltstone and shale in various
degrees of purity, and vary in thickness, the thicker section measuring some
4,000 feet. Marine calcareous shale and siltstone, over 1,000 feet thick, are prob-
ably Lower Devonian. The Middle Devonian includes up to 3,800 feet of lime-
stone, dolomite, and calcareous shale, in part with coral biostromes and bioherms,
overlain by a maximum of 2,900 feet of marine limestone, sandy limestone,
sandy shale, and sandstone. The Upper Devonian over 10,000 feet thick, is
largely made of non-marine sandstone and shale with thin seams of bituminous
coal.

The above formations were folded, in the southern and western parts of the
Ellesmere-Greenland Fold Belt, prior to the Pennsylvanian, but in the eastern

part of the belt they were folded only in the Tertiary, conformably with non-
marine Tertiary and possibly Upper Cretaceous sandstone, shale, and coal. In
general the lower Paleozoic miogeosyncline is the most deformed in this belt,
folds are symmetrical and asymmetrical, some are overturned, thrust faults and
high angle faults are known. The deformation has been more severe northward,
where the stratigraphy is less known and some metamorphism produced slates,
phyllites, and fine-grained schists (Fortier, 1959).

Northern Ellesmere Fold Belt

The Northern Ellesmere Fold Belt underlies the northern coastal area of
Ellesmere Island and apparently extends to northwestern Axel Heiberg Island.
The rocks comprise sedimentary and volcanic material possibly ranging from the
Precambrian to the Tertiary. A part of the belt includes gneisses and intrusions
that vary from granitic to ultrabasic. These are undated but it is probable that
they are Precambrian in age and were deformed during that time. Some volcanic
rocks are pre-Permo-Carboniferous, either Silurian or Devonian, and are ad-
joined by greywackes. These rocks and Ordovician beds are mildly metamor-
phosed but have complex structures that probably resulted from the Yariscan
orogeny. Widespread outliers of mildly folded Permo-Carboniferous strata un-
conformably overlie older rocks of more complex structures and indicate that
the Late Mesozoic and Tertiary deformation extended to those parts (Fortier,
1957).

After Fortier wrote the above paragraph, a note was published by
Blackadar (1960) on a group of gneisses and migmatites between Cape
Aldrich and Markham Inlet which he had named the Cape Columbia
group. These had been demonstrated on stratigraphic grounds to be
older than Middle Ordovician. A potassium-argon analysis was made
on a biotite-rich gneiss and an age of 545 m.y. was obtained. This is
latest Precambrian or earliest Cambrian. Blackadar concludes that the
orogeny formed a landmass from which clastic sediments in the Parry
Islands and Ellesmere Island came. By the close of the Paleozoic era the
Cape Columbia terrane had been lowered and Permian limestones were
deposited on it.

Cornwallis Fold Belt

McNair (1960) has reported as follows on the Cornwallis Fold Belt
(see Fig. 40.3):

Two sets of regional structures meet on eastern Bathurst Island. The older,
north-south trending Cornwallis folds are characterized by vertical movement

Fig. 40.3. Fold axes and faults of east-cen-
tral Bathurst Island. Kindly supplied by A.
D. McNair, Dartmouth College.

CANADIAN ARCTIC

and appear to be of cratonic origin. These range from narrow, steep-flanked
anticlines to broad synclines and anticlines. Conglomerates and two uncon-
formities indicates that the Cornwallis folds had an initial, intermittent develop-
ment extending from Middle Silurian to the Middle Devonian. During short
times of stability in the Silurian many small reefs grew along the margins of
some anticlines.

The east-west Parry Islands miogeosynclinal fold belt consists of long parallel
folds which decrease in amplitude eastward toward the Cornwallis folds. How-
ever, at several places the Cornwallis structures are relatively highly deformed
by the east-west folds. The Parry Islands belt was deformed in the latest
Devonian or during the Mississippian.

The final phase of deformation occurred as persistent north-south postoro-
genic faults. In the southeastern part of Bathurst Island the faults controlled
the emplacement of small sills, dykes and plugs of olivine basalt.

Sverdrup Basin

The Sverdrup Basin includes a voluminous sequence of Pennsylvanian to
Tertiary beds which have been mainly deformed in Tertiary time. In the best
exposed and apparendy thicker part of the basin, the units of the sequence,
although varying in thickness, appear essentially conformable. At the periphery
of the basin there are unconformities, disconformities, oversteps and some
facies developments.

In the southern and eastern peripheries of the basin, the Permo-Carboniferous
commonly includes units of limestone, units of sandstone with layers of con-
glomerates, and lesser units of shale. In the northern part of Axel Heiberg
Island, the Permo-Carboniferous includes volcanic measures. Nearby, Permian
limestone is at least 5,000 feet thick. Across the middle part of the island, the
basin contains the following units: Permian siltstone with lesser shale and silty
shale, 4,000 feet thick; Middle, Upper, and probable Lower Triassic shale
with siltstone and sandstone, 10,000 feet thick; Upper Triassic, marine, and
possibly Lower Jurassic, non-marine sandstone, shale, siltstone, with carbonace-
ous film in the upper part, up to 5,600 feet thick; Jurassic marine shale up
to 900 feet thick, non-marine sandstone and lesser marine shale up to 1,300
feet thick; Jurassic and Cretaceous shale as thick as 2,500 feet; Lower Cretace-
ous sandstone with a maximum thickness of 4,500 locally with a 200-foot
stratum of volcanic breccia, and shale in thicknesses reaching 3,000 feet; Lower
or Upper Cretaceous sandstone and shale, over 700 feet thick, locally overlain
by basalt flows up to 600 feet thick; Upper Cretaceous shale, as thick as 1200
feet and conformably overlain by Tertiary and possibly Cretaceous non-marine
siltstone, sandstone and silty shale with coal, over 8,000 feet thick. In the
Ringnes and Cornwallis Islands these or similar units down to the Upper
Triassic occur but in somewhat lesser thicknesses. Facies indicate an eastern
and southern source for most of the Mesozoic sediments. Gabbro sills and lesser
dykes are common in some units and are most numerous in the region of
Eureka Sound. They are as far widespread as from Melville Island to the region

of Baumann Fiord, from Ellef Ringnes to the northeast coasl ot KHwtiimie
Island. There is no regional metamorphism and any alteration related to these
intrusions is limited to a few feet in the country rocks.

The strata of the basin have been folded in Tertiary time. From the Ri]
Island eastward the deformation has been more marked, and the northwester!)
and northerly trending folds form the Eureka Sound Fold Belt. On the southern
part of Axel Heiberg Island many folds are of the "box" type. The regional
plunge of the folds is inward to the basin, that is towards the longitudinal axis
of the basin. This axis on the Ringnes and Axel Heiberg Islands is generally
the locus of the youngest formations. Coinciding with this axis Irom northern-
most Melville Island, across the Ringnes Island, middle Axel Heiberg Island.
the eastern part of this island along Eureka Sound to Nansen Sound and Elles-
mere Island is a zone of diapiric intrusions of gypsum of Pennsylvanian and or
Permian age. Most of these intrusions are in the crestal area ot the Tertian
folds. There is a rough parallelism between the longitudinal axis of the Sverdrup
Basin, the zone of diapiric intrusions, and the trend of the deformed lower
Paleozoic miogeosyncline. It would thus appear that the Paleozoic orogen)
had long range effects in that not only was it a factor in the formation of the
depression in which Permo-Carboniferous evaporites were eventually laid down,
but also it ultimately had some bearing on Tertian' tectonism (Fortier, 195

See the summary by Tozer (1960).

ARCTIC COASTAL PLAIN

The Arctic Coastal Plain covers the western part of Banks Island, the west-
ern and northwestern parts of Prince Patrick Island and probably extends to
the northwestern parts of Brock, Borden, and Ellef Ringnes Islands. The rocks
include Cenozoic beds unconformably covering Mesozoic strata and. south of
the Sverdrup Basin, comprise Cretaceous and possibly Jurassic formations ap-
parently overlapping Devonian strata (Fortier. 1957).

CORRELATION WITH ALASKA AND THE YUKON

Reference to Fig. 39.13 will bring to one's attention the following
possible correlations of the geologic provinces of the Arctic Archipelago
and Alaska and the Yukon. The Pennsylvanian and Permian of the
Sverdrup basin would appear to have a tie with the Late Pennsylvanian
and Permian of the basins of northeastern Alaska and northern Yukon.
The closeness of the shelf margin to the present shore leaves little room,
however, to connect them into a continuous basin. The unconformitv

640

STRUCTURAL GEOLOGY OF NORTH AMERICA

Fig. 40.4. Belts of deformation of northern North America. The fold belts of the Arctic
Archipelago are after Fortier ef a/., 1954. A, Ellesmere-Greenland fold belt; B, Eureka Sound
fold belt; C, Parry Island fold belt; C, northern Ellesmere fold belt (in eugeosynclinal sedi-
ments); D, Coastal Plain. For details of the Cordilleran belts of deformation see Fig. 39.2.

below the Late Pennsylvanian strata and the older formations is common
in both regions and draws them together in a common province.

The northern Ellesmere fold belt in rocks of eugeosynclinal character,
parts of which are probably Precambrian, may relate to the Paleozoic
positive area of northern Alaska. The latter's rocks are only known in well
cores and are identified as argillite, probably Precambrian in age. Again,
the continental shelf is fairly narrow from Ellesmere Island to Alaska,
and not much room is available under it to connect the Precambrian
terranes. Certain authors have presumed the lands to have extended
northward into areas now of deep water, and imagined subsidence in
the order of 10,000 feet to have occurred, but as we shall see, this is
probably not possible.

The Laramide structures of Alaska extend to the Arctic shoreline in
northeastern Alaska and northern Yukon, as if perhaps, they once con-
tinued northeastward under the continental shelf. See Fig. 40.4. Structures

of the same age in the Eureka Sound Fold Belt suggest that the two
may have been continuous. There seems no way, however, to prove or
demonstrate this postulate.

The subject of possible connections will be pursued farther on follow-
ing pages when the origin of the Arctic Basin is considered.

PLEISTOCENE EPEIROGENY AND CLIMATIC CHANGES

Washburn (1947) reports that Victoria Island has emerged at least
500 feet since the last glaciation, as demonstrated by raised strand lines
and marine fossils. In addition he believes the whole of the Arctic
Archipelago has suffered comparable movements. Elevated beaches are
reported by G. M. Stanley (personal communication) up to 900 feet
above sea level along the east coast by Hudson Bay.

Continental ice sheets formerly covered all Arctic Canada east of the
Cordillera except some of the western Queen Elizabeth Islands (Craig
and Fyles, 1960). The elevated strand lines represents an isostatic ad-
justment following the melting of the ice, and such emergence was
almost complete before the final eustatic rise of the sea.

Numerous Tertiary deposits have been found in the Arctic region, and
fortunately most of them carry coal beds and fossil plants. By recon-
structing the character and distribution of the Tertiary flora from the
Arctic to the temperate regions of the northern hemisphere, with par-
ticular reference to the redwood Sequoia, Chaney (1940) concluded as
follows (Fig. 40.5); the Arctic cooled gradually from late Eocene to the
Pleistocene with a slight reversal in mid-Miocene (personal communica-
tion, E. Dorf), and the temperate rain forests shifted southward across
60° of latitude incident to the cooling. He postulates that the gradual
cooling was caused by and was coincident with a gradual uplift of the
continent.

OROGENIC BELTS OF GREENLAND

Paleozoic

East Greenland north of 70° N. Lat. is marked by a belt of Caledonian
( Late Silurian and Early Devonian ) orogeny, and another belt of orogeny

CANADIAN ARCTIC

641

of the same age extends across the northern margin of the great island.
The East Greenland fold belt developed during three phases (Koch,
1961):

1. Orogeny of Silurian (?) age affected the entire east coast, with
thrusting toward the west and extensive granitization.

2. Deformation south of 76° N. Lat, in places closely related to in-
trusive granite bodies, occurred in Devonian time.

3. Subsiding basins were filled with thick deposits of molasses-type sedi-
ments in the Middle and Late Devonian, in the Carboniferous and in the
Early Permian. They attest times of nearby crustal unrest and elevation,
but the Devonian detritus was mildly deformed itself in two episodes,
one in Early Carboniferous and one in Early Permian. The entire east
coastal area was strongly affected by faulting, especially during the
Carboniferous.

Mesozoic and Tertiary

A marine transgression in Late Permian time covered large areas along
the coast, and this was followed by several Mesozoic transgressions.
Many of the old faults were reactivated in the Tertiary.

A large basalt field of Late Cretaceous and early Tertiary age occurs
in the east-central part of Greenland ( Fig. 40.5 ) and of this region Wager
(1947) writes:

Subsequent to the forging of the metamorphic complex which probably took
place in Pre-Cambrian times, the area was for long dominantly subjected to
upward movement with concomitant erosion. Towards the end of the Mesozoic
era, when next there is definite information, the area seems to have been of sub-
dued relief and near sea level. In the Kangerdlugssuaq area a local marine
transgression of approximately Senonian age produced thin sediments resting
on the metamorphic complex, and a similar and perhaps contemporaneous
marine transgression took place further south on what is now Kap Gustav Holm.
Within a short time of the maximum development of the Cretaceous trans-
gression volcanic activity broke out in the Kangerdlugssuaq region giving the
Lower Lavas and Tuffs.

The Lower Lavas and Tuffs of latest Cretaceous or very early Eocene age,
mark the beginning of intensive igneous activity in East Greenland, extending
in a N.N.E. direction over a distance of 1,200 km., from 66° to 75° N. South-
wards, the coast line has the same N.N.E. direction and there are many basic
dikes, which almost certainly form part of the same igneous episode.

The eruption of vast quantities of basalt to give the Plateau Basalt Series,

Fig. 40.5. Upper map, very generalized distribution of seas and lands of the Arctic during
Triassic and Jurassic times. The seas at any one time were not as extensive as the total dis-
tribution shown. Lower map, early Tertiary deposits of the Arctic. The dotted lines are isoflors
after Chaney, 1940, and the crosses denote Chaney's Eocene and Oligocene localities, plus a
few other localities where "Arctic Miocene" coal beds are known. The ruled area denotes
the Greenland-lceland-Scotland basalt field of early Tertiary time.

attaining in places a thickness of certainly O.'-i km. and probably a good dial
more, is the greatest igneous event in the region judging by the quantity of
magma involved. The time taken for the accumulation of the Plateau Basalt
Series can be estimated from the fossils found immediately below and above the
series as approximately equal to the duration oi the Lower Eocene, and this

642

STRUCTURAL GEOLOGY OF NORTH AMERICA

may be taken to be of the order of 5-10 million years. The fact that the sedi-
ments immediately underlying and overlying the thick Plateau Basalt Series
are both of shallow water marine origin shows that during or soon after the
extrusion of the basalts there must have been sinking of the basalt pile com-
parable in amount with its thickness.

Some basic intrusions, e.g., the Skaergaard and Kap Edvard Holm complexes
were formed during or soon after the main period of basalt outpouring. This
also seems to have been the chief period of sill intrusion although this phase
never reached large proportions.

The chief tectonic event affecting the area, namely, the elevation of what is
now the coastal mountain belt of East Greenland and the sinking of the area
which is now the Denmark Strait, took place subsequendy to the formation of
the main plateau basalts. The junction between the two areas of differential
epeirogenic movement is marked in Middle East Greenland by a flexure of the
crust. Where the flexure is intense with dips of more than 10°, a dike swarm
is developed which follows the convex part of the flexure. The intensive flexur-
ing and associated dike swarm occur along much of the Middle East Greenland
coast and, as it is likely that all the flexuring took place during the same limited
period of time, we are provided with a useful method of dating certain local
events. The coastal flexure and dike swarm almost certainly came after the
formation of the Kap Dalton sediments, which are Middle or Lower Eocene.
The main part of the inland doming of Knud Rasmussens Land is considered
to have been incidental to the general epeirogenic uplift and to have developed
at that time.

Not all of this impressive differential vertical movement is to be ascribed to
the coastal flexure stage and it is suggested that the total movement as now de-
termined by the lie of the rocks can be analysed into the following parts:

1. Early slight flexuring due to differential sinking of the lava pile as it ac-
cumulated.

2. The main epeirogenic movement and associated flexuring, with a dike
swarm where flexuring was sufficiently intense.

3. Possible later up-warping of the edge of the uplifted area as a result of
isostatic adjustments to erosion and to the development of the ice cap.

Faults have been recognized on both the west and east coasts of Green-
land. The Cape York district of northwest Greenland is especially broken
by high-angle faults (Koch, 1929), and the fiords of the west coast about
Disko and Umnak bays generally take their courses parallel to faults
(Hobbs, 1932). It is not clear, however, that these faults are to be
associated with Tertiary land movements. Koch ( 1935) believes that
strong faulting in Tertiary time may be recognized in many places
along the eastern coast, and that it is associated with the great vol-
canic activity just described. The faults have tilted a plane to the west on

Molne Land, and may be seen cutting the sediments there. Along the east
side of Hurry Inlet are Tertiary faults, and Liverpool Land was doubtless
strongly raised in Tertiary time. Although the basalts with their great
flexure are not present north and south of the middle east Greenland area,
the topography along the coast in the absence of the basalts suggest com-
parable crustal movement (Wager, 1947). The volcanics of east Green-
land, as a number of writers have proposed, must be continuous with the
basalt fields of Iceland, the Faeroes, and Scotland; but Wager does not
believe that they extend under the ice of Greenland and connect with the
basalts of the west coast.

Precambrian

An outline of the Precambrian rocks and history of east Greenland is
given below. It is after Koch ( 1961 ) .

East-Central Green/and

Eleonore Bay group
(Proterozoic)

Archean basement
Northeast Greenland

Upper

Tillite and varved strata, 200-1000 m

Dolomite and Is, 1100 m

Psammite, pelite, 3000 m
Lower

Tillite, Is, phyllite, 2600-7400 m.

Proterozoic

Hagens Fiord group, derived from Carolinidian belt
Faulting and eruption of basalts

Folding and magmatic activity (Carolinidian orogenic belt)
Basalt dikes and sills
Thule group (psammites), 3000 m

Greenlandian (semipellites), 3000 -f- m
Archean basement

ARCTIC OCEAN BASIN

Surrounding Shelves

The floor of the Arctic Ocean is about half shelf and half deep basin.
See Fig. 40.6. Off Alaska and the Canadian Arctic Archipelago the shelf

Fig. 40.6. Bathymetric chart of Arctic Ocean. Compiled from Soviet sources as of 1956 by Chief
Cartographer, Surveys and Mapping Branch, Dept. of Mines and Technical Surveys for Defense Research
Board of Canada, Ottawa, 1957.

644

STRUCTURAL GEOLOGY OF NORTH AMERICA

NORTH POLE

BASIN A

ALASKA OFF
POINT BARROW

ALLUVIATEO PLAIN CHUKCHI CAf

ALLUVIATEO PLAIN

ALPHA RANGE,

MENDELEEV RANGE.

OR CENTRAL 8ASIN RISE

LOMONOSOV

RANGE ALLUVIATED PLAIN REGION OF SEAMOUNTS HANSEN'S SILL SEA

IOOO
J 2000
"3000
3«000
E5000

/ ^"'^4*^^^^^^ ^^IYlmjfL^m^ri^''^—^

Fig. 40.7. Bathymetric profile across Arctic basin, taken in August, 1958, by SSN (571)
Nautilus. Drafted from chart kindly supplied by Dietz and Shumway. Basin A is called Beau-
fort Sea basin by Soviets, and Canada basin by Dietz and Shumway. Basin B is called

is narrow, but off Eurasia it is very broad. Spitzbergen ( Svalbard ) , Franz
Josef Land (Zemlya Frantsa Iosifa), North Land (Severnaya Zemlya),
Novaya Zemlya, New Siberian Islands ( Novosibirskiye Ostrova), and
Wrangel Island (Ostrov Vrangelya) all rise from the shallow but broad
shelf north of Siberia and Norway.

Spitzbergen was formerly believed to be tied to northern Greenland
by the Nansen sill, but recent soundings show that the sill is broken by a
transverse trench with a floor 3100 to 3900 meters deep and about 200
kilometers wide (Hope, 1959b).

Deep Basin

The deep basin is approximately triangular in shape with the base
about 1150 miles across and the side from Spitzbergen to Alaska about
1650 miles long. On the basis of post-war soundings, principally by the
Russians, the large basin is known to be divided by the Lomonosov
Range (or Ridge) which extends from the New Siberian Islands to Green-
land and Ellesmere Island, a distance of 1800 kilometers. Its peaks rise
2500 to 3000 meters above the adjacent ocean floor, and the highest
peak yet sounded is 954 meters below the ocean surface. Saddles to a
depth of 1500 meters, spurs, and steep slopes are characteristic.

On the Alaskan side of the Lomonosov Range another range was dis-
covered by the United States drifting ice station Alpha. It has pro-
visionally been called the Alpha Range by Hope (1959a). Its extent is
not known and its relief appears to be less than the Lomonosov Range.
Its apparent plateau-like top rises to 2300 meters below sea level. The

Makarov basin by Soviets and central Arctic basin by Dietz and Shumway. Basin C is called
Nansen basin by Soviets and Eurasia basin by Dietz and Shumway. Nansen's Still is called
Nansen Ridge by Dietz and Shumway.

two ranges then divide the major deep basin into three sub-basins which
have not yet been named officially. They will be referred to here as
basins A, B, and C. The scientists of the U.S.S.R. and of the United States
respectively have called them as follows; Basin A, Beaufort Sea Basin
and Canada Basin; Basin B, Makarov Basin and Central Arctic Basin;
and Basin C, Nansen Basin and Eurasia Basin (personal communica-
tion V. N. Sachs and charts prepared by Dietz and Shumway). The
Alpha Range is called the Mendeleev Range by the Russian scientists,
and on unpublished charts by Dietz and Shumway, the Central Basin
Rise.

Basin C, which lies north of the Greenland, Barents, Kara, and Laptev
seas, is the deepest of the three and has a maximum depth of over 5220
meters. Basin B on the opposite side of the Lomonosov Range, has depths
over 4000 meters. Basin A which lies north of the Chukchi and Beaufort
seas has depths up to 3820 meters.

The sonic depth profile recorded by the submarine Nautilus across the
Arctic Ocean, is summarized in Fig. 40.7. It extends from a point north
of Point Barrow directly to the North Pole and beyond to the middle of
Basin C, and thence southwesterly to Nansen's Sill between Spitzbergen
and Greenland. Its features should be noted, and in succeeding para-
graphs they will be referred to.

Seismic studies over the Alpha Range by Hunkins ( 1961 ) indicate the
boundary between the 4.7 km/sec layer and the basaltic layer at about
5 kilometers which is less than that shown by Demenitsckya. Relief of the
rise is rugged and apparently the result of block faulting. The constitution
of the crust is similar to that of the Atlantic Ocean floor.

CANADIAN ARCTIC

645

Nature of Crust under Deep Basin

A seismic surface wave of unusually large amplitude (Lg wave) was
recognized by Press and Ewing in 1952, and it was noted to have the
characteristics of traveling only over paths of continental structure. It
does not propagate across oceanic crust. Oliver et al. ( 1955 ) subsequently
studied the Lg wave paths across the Arctic region and concluded that
the Arctic basin was floored by ocean crust, and that it could not be
sunken continental crust as had been postulated by Soviet geologists
and Eardley. The subsidence theory will be considered later.

Figure 40.8 is an interpretation of the crustal constitution across the
Arctic basin from Franz Josef Land to Alaska by Demenitsckya (1958).
A thickening of the basaltic layer under both the Lomonosov and Alpha
ranges is conspicuous, as well as the existence of a 5-kilometer thick
"granitic" lens under each. The crust under the basins is typically oceanic.

Theories of the Origin of the Arctic Basin

Permanency of the Basin. In about 1860 James D. Dana began to
teach that the continents and ocean basins are permanent features of the
earth's crust. He contended that in the main the ocean basins have been
sinking and the continents rising, but several continental fragments have
subsided. Fifty years later Charles Schuchert (1916) in his studies of
paleogeography was foremost in supporting Dana. He said:

Now, however, geologists are holding more and more to the hypothesis that
the earth periodically shrinks, and each time it does so some parts or all of the

FRANZ JOSEF LOMONOSOV MENDELEEV
LAND RANGE RANGE ALASKA

"~~'Jt^ NANSEN BASIN /s^MAKAROV B^-r- rr-. rr— -^. BEAUFORT BASIN ^-rr..-- ■■^i.-'Si-

10-

GRANITE '"--^•--------■-^ *""~~11d_ GRANITE ^Z--===£ — " VLv_V__"" GRANITE

yS >V ^— ^BASALT ^S ^^^

20-

/ MANTLE \^S > ' MANTLE ^^>v

30-

^ ^V

40-

1000 KM

Fig. 40.8. Crustal structure of the Arctic basin, after Demenitsckya, 1958. Basin names
and Mendeleev Range name, have been added according to information from V. N. Sachs
personal communication. Mendeleev Range has been called the Alpha Range by Americans.

continents rise more or less; but that in the main there is subsidence of tin- ocean
bottoms equal in amount to the rising land-masses, that the water of the hydro-
sphere is constandy increasing in amount, and that even though the continent!
are in the main permanent, yet they are partially breaking down into the oceanic
basins.

Reference is made by Schuchert to the permanency of the North
Atlantic Ocean basin, but we can only presume that he considered the
Arctic Ocean basin a permanent feature; his maps are not definitive
about the Arctic.

Subsidence Theory. In the period of 1930 to 1950 and beyond, the
Russians, beginning with Shatski (Hope, 1959a) considered the Arctic
Ocean basin as a sunken region, once emergent. The sunken crust was
called the Hyperborean shield, and the depression as the Hyperborean
basin. Later, associates of Shatski referred to the shield as a massif or
platform. The sunken platform was postulated as a result of an en-
visioned belt of Mesozoic folding encircling the basin, as a once resistant
shield.

A development of the thesis that the fold belts which extend to the
Arctic Ocean cross the shelves and deep basin is shown in Fig. 40.9.
Here, Sachs et al. (Hope, 1959a) show Caledonian, Hercynian, and
Alpine fold belts extending across the deep basin, particularlv where the
Lomonosov Range and Nansen Basin (C) now exist. They postulate, of
necessity, that the fold belts have sunken to form the deep basins.

In 1948 Eardley reviewed the geology of the lands around the Arctic
Ocean basin and concurred with the Russians that the basin was a sunken
region which in Precambrian and perhaps early Paleozoic times had been
land. The broad shelves and relatively small size of the basin, the facing
Precambrian shields (Canadian, Greenland, Russian-Baltic, and Angara),
the Paleozoic orogenic belts that project to and under (?) the Arctic
Ocean (Ural-Nova Semlya, Norway and Spitzbergen, East Greenland,
Canadian Arctic Archipelago, Northland, and New Siberian) suggested
to him that the region was once land and beginning in Paleozoic time
has foundered. Paleozoic geosynclinal conditions in Alaska seemed to
require a source for some of the sediments north of land today, where
water is fairly deep. Paleozoic fossil faunas common to North America

646

STRUCTURAL GEOLOGY OF NORTH AMERICA

VZZA

PreCambrian
platforms

Caledonian
folding

Hercynian
folding

Mesozoic
folding

Alpine
folding

Seismicicy

Fig. 40.9. Postulate of fold belts across the Arctic basin, by Sachs, Belov, and Lapina (1955).
Reviewed in English by Hope (1959a). The broad black line is the great Arctic magnetic ano-
maly, and the row of dots the Lomonosov Range.

and Eurasia find an explanation in the possible shallow sea migration
routes bordering the once emergent and later subsiding regions.

Continental Drift. Although two or three geologists before him had
suggested without much documentation the concept of horizontal shifting

of continental fragments Taylor, in 1910, is generally given credit for
"specifically advocating continental drift" (Van Waterschoot van der
Gracht, 1928). Wegener first addressed the subject in 1912 but it was
not until his comprehensive study, Die Enstehung der Kontinente unci
Ozeane, was published in 1922, that the theory became of international
concern. Although many European geologists supported the concept of
continental drift in one form or another, most American geologists con-
tinued to favor the Dana-Schuchert concept of permanency of the con-
tinents and ocean basins. The theories of continental drift, however,
focused attention on the Arctic Ocean basin, and Taylor in particular
dwelt specifically on it. He postulated drift toward the equator and
away from the North Pole. Figure 40.10 illustrates the general concept
and his view of the origin of the Arctic Ocean depression as a "dis-
junctive basin." Eurasia and North America were once together over the
North Polar region as one great continent, but pulled apart leaving the
Arctic basin as one of the disjunctive depressions. Greenland was con-
sidered a fragment left between the Baffin Sea basin and the Greenland
Sea depression as Europe drifted away from North America. The ap-
proximate extent of the continental shelves and the deep basin in the
Arctic had been established by Nansen and other explorers but no detail
of the bottom topography was known at this time.

Wegener gave more attention to the southern hemisphere and
Antarctica than to the Arctic Ocean basin, and we are left to examine his
maps to discern his thoughts about the origin of the Arctic basin. The
maps show an existing ocean there, although small, before the breakup
occurred. In a major publication in 1924, however, Koppen and Wegener
show the small basin to enlarge appreciably as North America, hinging
in the North Polar region drifted westward and away from Europe.

By the time of Wegener's major publications the concept of a layered
crust had become established. The continents were made up of a silicic
and lighter upper layer, the sial, resting on a mafic and heavier layer,
the sima, and when a continent broke and its parts drifted away from
each other, it was the sial that parted and drifted over the sima, leaving
a crust made up only of the sima. This, for isostatic reasons, was also a
basin. Hence, according to Koppen and Wegener, the Arctic Ocean

CANADIAN ARCTIC

647

i Fig. 40.10. Taylor's view of continental drift toward the equator. Reproduced fron
Waterschoot van der Gracht (1928).

Van

basin is underlain by simatic crust. This is the seismic velocity layer con-
sidered today to be made up of a silicate of gabbroic composition.

In 1937 Du Toit presented a theory of origin of the Arctic and North
Atlantic basins using the concept of continental drift facilitated bv one
or two major strike slip faults. The blocks bounding the Arctic basin are
presumed to have rotated apart and the movement to have been accom-
modated by strike-slip along faults.

Fig. 40.11. Carey's (1958) concept of the origin of the Arctic basin.

Elaborating on the ideas of Du Toit, Carey | 1958, pp. 195-216) pre-
sents the following theory:

1. Scissors-like drifting apart to form the Arctic basin, hinging at a
point in south central Alaska in what the present writer recognizes as the
Nevadan otogenic belt. See Fig. 40.11.

2. The triangularly shaped basin is a tension rift with two sides being
the radii from the hinge point and the third side a "megashear" or strike-

648

STRUCTURAL GEOLOGY OF NORTH AMERICA

ASIA

Fig. 40.12. Gross elments of
orocline-sphenochasm concept of
origin of Arctic basin. Simpli-
fied by omission of transcurrent
movement. Reproduced from
Carey (1958).

slip fault ( Spitzbergen to Severnaya Zemlya ) . Such a tension rift he calls
a sphenochasm.

3. The bending of the orogenic belts of the western American Cor-
dillera in Alaska is believed to be the result of the rotation of the blocks
which opened up the "Arctic sphenochasm." The bent segment of the
orogenic belts is called the "Alaskan orocline."

4. The Lomonosov Range is believed to be a stretched-out more viscous
part of the crust across the Arctic sphenochasm. The concept will be
understood if the following model anology is considered. Quoting from
Carey (1958, p. 195):

If I break a slab of toffee which is cold and brittle except for one warm
spot, the slab will break cleanly except at the hot spot where a thread of toffee
will be drawn out across the rift. The thread will be straight or curved according
to the path of separation. If the isotherm at which fracture passes into flow is lo-
cally above the Mohorovicic discontinuity sialic material will rise into the rhom-
bochasm along with the rising mantel material and form a thread of sial across
the rhombochasm. In view of the density difference it will endure permanendy
as a submarine ridge on the ocean floor. For such threads the name nematath
[from Greek meaning, thread and stretched] is proposed. In practice I find that
such nemataths commonly join similar igneous centres across the rhombochasm,
giving support to the above hypothesis of their generation. Where the transition
from fracture to flow is below the Mohorovicic discontinuity, no nematath
results even if the isotherms are higher in some places than others.

5. The Novaya Zemlya, the Pai Khoi, and Severnaya Zemlya orogenic
arc segments are considered oroclines and related to the "Iceland
megashear." Restored to original positions they form a continuous,
smoothly arcuate, Hercynian orogenic belt from the Urals to the North
Greenland-Ellesmere orogenic belt. Norway would lie along side east
Greenland and the Caledonian belts of each become parts of one broader,
original zone. The Alaskan orocline is the hub of all movements of the
northern hemisphere.

6. Paleomagnetic polar wandering is considered in light of the Alaskan
orocline theory, and found to conform better to it than to other proposed
patterns of fragmental shifts or drifting.

An idealized bold portrayal of the fracture and drift pattern of the
Alaskan orocline and related features is shown in Fig. 40.12.

Rift Theory. Heezen, in November of 1956 (p. 1703), presented a
paper at the meetings of the Geological Society of America in which
the Mid-Atlantic rise and rift zone were postulated to extend to and
across the Arctic basin. Since then papers by Heezen and Tharp ( 1959 )
and Ewing and Heezen (1957) have appeared which elaborate more on
the concept. Figure 40.13 is a map supplied the writer by Dr. Heezen
which shows the modern seismic activity of the Arctic and a new interpre-
tation of the bottom topography of the Greenland Sea basin and the
Arctic Ocean basin on the Eurasian side of the Lomonosov Range. He
divides the deep basin (C) into two longitudinal parts with a gentle
medial rise broken by a rift valley along the active seismic zone. The rift
topography is born out by the sonic depth profile of the Nautilus (Fig.
40.7). The deep trench across Nansen's sill is along the postulated rift
zone, and supports the concept of rifting, but it also favors drifting.

The gentle rise and medial rift constitute a tectonic element com-
patible with oceanic crust, and if similar to the Mid-Atlantic rise, we
must postulate the zone to be one of volcanic activity.

Heezen postulates an expanding earth and the widening of the ocean
basins as a result. The broad Mid-Atlantic rise and medial rift have
developed progressively as the expansion occurred.

Conflicts and Problems. Assuming that the Arctic basin is underlain
by oceanic crust, which seems probable, then the postulated belts across

CANADIAN ARCTIC

it pose a problem. As far as the writer is aware no fold belts have thus
far been proved in an ocean-type crust although parallel ridges and
valleys have been taken to mean folding in one or two places. Generally,
when fairly well defined, the parallel features are asymmetrical ridges or
escarpments, and considered of fault origin. The continuity of fold belts
across the Arctic basin, therefore, is to be considered doubtful.

The premise of fold belts across the basin was one of the chief reasons
for postulating subsidence of part of a continental crust, but if the folds
are doubtful and the crust is seismically oceanic, then the subsidence
theory is improbable. A conflicting situation exists in Hakkel's publica-
tion (Hope, 1959a) in that he shows in a cross section oceanic crust,
without any indication of or provision for folding, yet on the map he
indicates a continuous fold belt across the deepest basin.

The theories of permanent ocean basins and of continental drift both
provide for oceanic crust under the Arctic basin but in both the
Lomonosov Range poses a problem. It does not appear to be of volcanic
origin from the shape given it so far by contourers. If volcanic, the sup-
porters of most any theory for the origin of the Arctic basin would find a
compatible place for it in the framework of their concepts, but the non-
volcanic nature is a real enigma. Even the stretched-out nonorogenic
thread idea of Carey is difficult to visualize without magmatism. If the
future soundings indicate that the range is volcanic, and this is possible,
then we will have been trying to solve nonexisting problems.

The rift theory, in view of the seismicity, seems attractive. It is in
harmony with oceanic crust, but as far as the writer understands it does
not present an explanation of the origin of the nonvolcanic ( ? ) Lomonosov
Range. The basins on the Alaskan side of the Range are not accounted for
in the rift theory.

Roth the subsidence theory and drift theory provide for a sourceland
of sediments north of Alaska; the theory of permanence fails in this
respect.

The orocline concept is complex but provocative and undoubtedly will
elicit a good deal of attention in the future. More and better data are

Fig. 40.13. Heezen's rift theory of the Nansen basin. Map kindly supplied by Dr. Heezen.
The dots are earthquake epicenters.

needed before further progress can be made on the origin of the Arctic
basins. The published record to the time of this writing leaves the subject
an enigma.

41.

GULF COASTAL PLAIN

GENERAL CHARACTERISTICS*

Topography

The Gulf Coastal Plain is coextensive with the Atlantic Coastal Plain
(discussed in Chapter 10) and, together, from Tampico, Mexico, to
Cape Cod, Massachusetts, they are 3000 miles long. The Gulf Coastal
Plain averages 250 miles wide, and the Mississippi embayment from the
delta to Cairo is 575 miles long. The peninsula of Florida is 400 miles
long. See Fig. 41.1. This vast plain rises very gently from the sea, and

0 For an up-to-date detailed account, see Grover Murray's Gulf Coastal Plain
(Harper & Brothers, New York, 1961).

in parts of Texas attains an elevation somewhat more than 1000 feet.
Beyond the Rio Grande in Mexico, the country that can properly be
classed as coastal plain narrows toward the south and becomes struc-
turally more complex than in the United States. At Tampico, it is very
narrow and continues so to Yucatan, where the plain broadens to include
most of the peninsula.

Geologically, the coastal plain extends out under the sea to the outer
margin of the continental shelf.

Sedimentary Rocks

The Gulf Coastal Plain is underlain by a series of sedimentary forma-
tions composed chiefly of sand, clay, marl, limestone, and chalk, with
subordinate amounts of salt, diatomaceous earth, volcanic tuff, and gravel.
The calcareous deposits are more abundant in lower formations and
along the seaward margin. The various sediments range in age from
Late Jurassic to Recent and are mainly unconsolidated, though some in-
durated layers are intercalated from place to place. All the beds are
lenticular and interfingered with others, and no two columnar sections
are similar unless close together. This diversity in succession poses a
constant problem for the stratigrapher, and microfossils have proved in-
valuable in correlation.

The Gulf Coastal Plain sediments were deposited in seas that invaded
the margin of the continent. Several rivers draining the central part of the
continent deposited vast amounts of sand, silt, and clay in the sea
as the crust along the invaded margin subsided, and large amounts of
chemical precipitates from the sea water were added. As a result, a great
wedge was built up that thickens seaward. Along the site of the present
coast of Mississippi, Louisiana, and Texas, the wedge of sediments is
estimated to range from 20,000 to 30,000 feet thick. In spite of the great
thickness, the wedge is very thin in relation to its length in cross section;
and if it is laid out to true scale, one is impressed with the very small
angle of tilt imparted to the beds by the subsiding of the land.

The stratigraphy of the central part of the Gulf Coastal Plain (Texas,
Arkansas, Louisiana, Mississippi, and Alabama) is summarized in Figs.
41.2 and 41.3.

650

Fig. 41.1. Simplified geologic map of Gulf Coast. Structures enclosed by dashed lines are mostly sub-
surface; the Peninsular arch of Florida is a notable exception. Some salt domes have surface expression.

VICKSBUKS FLEXURE FRIO FLEXURE

FAULT ZONE

Fig. 41.2. Cross sections of the Gulf Coastal Plain in Texas. Taken from Guidebook for the Joint Annual
Meeting of the A.A.P.G., S.E.P.M.. and S.E.G. in Houston, 1953. Section A-A' prepared by J. D. M.
Williamson. Section B-B' prepared by S. L. Stoneham.

GULF COASTAL PLAIN

653

Triassic sediments have not been recognized under the Coastal Plain,
but during the Jurassic the Gulf waters invaded the continent and a
succession of formations was deposited. From top to bottom they are the
Cotton Valley, Buckner, and Smackover. Underlying the Smackover are
the Werner gypsum and Louann salt formations which according to some
authors are Permian (?), as in Fig. 41.2, and according to others Jurassic
(Eagle Mills) as in Fig. 41.3. The Jurassic sediments were everywhere
overlapped by the Cretaceous. The Lower Cretaceous sea and deposits
extended across Texas and Oklahoma to connect with the vast epeiric
sea of the Great Plains and the Rocky Mountains (see Plate 11). Upper
Cretaceous seas probably spread over most of the Lower Cretaceous
deposits in Texas, but their sediments have subsequently been stripped
back so that the Lower Cretaceous now occurs farther inland (see Fig.
41.1).

The Upper Cretaceous deposits overlap the Lower in Mississippi,
Alabama, Georgia, and South Carolina (see Fig. 41.3). After Late Creta-
ceous time the seas began a persistent retreat and the younger sediments
are spread generally in successive belts toward the present Gulf of
Mexico. Exceptions may be noted in the Mississippi embayment where
on the west side the Eocene sediments overlap the Upper Cretaceous,
and in Georgia and South Carolina where the Eocene sediments reach
just beyond the Cretaceous in places and rest on the crystalline Piedmont.

The Cretaceous and Eocene seas especially extended up the Mississippi
Valley, and their sediments reflect a transverse downwarp known as the
Mississippi embayment. The evolution of the embayment is shown in four
stages in Fig. 41.4.

The Rio Grande embayment is a gentle transverse downwarp and ex-
tends approximately from Corpus Christi northwestward for 200 miles up
the Rio Grande. The axis of the downwarp lies somewhat northeast of
the present river and close to the Nueces River.

The embayment is due strictly to Eocene downwarp, as only the Eo-
cene sediments produce the embayed pattern. The Cretaceous strata
cover large areas inland and merge with the widespread deposits of the
Cretaceous seas in Mexico and the Rocky Mountains and Great Plains of
the United States. See paleotectonic maps of Plates 11 and 12. The Oligo-

Fig. 41.3. Cross section of Gulf Coastal Plain through Mississippi, after Paul Weaver, 1951.
Vertical scale is in thousands of feet.

cene, Miocene, and Pliocene deposits of the outer margin of the coastal
plain continue around the Gulf without an embayment at the Rio Grande.
A large part of the sediments of the Rio Grande area from Eocene to
Pleistocene is of deltaic origin and was carried from the interior of the
continent by rivers ancestral to the present Rio Grande, Pecos, and
Nueces (Storm, 1945).

Concept of the Gulf Coast Geosyncline

Recognizing the existence of 20,000 to 30,000 feet of sediments in the
thick part of the wedge from surface and well studies of the coastal plain
formations, and confirming the figures by geophysical data. Barton ( 1936)
realized that the floor of the wedge was at least 10,000 feet below the
floor of the Gulf of Mexico. In addition, he believed that the layer of sedi-
ments at the bottom of the Gulf of Mexico is only a few thousand feet
thick at the most; and so he depicted a great elongate downwarp which
he thought should rightfully be called a geosyncline. A number of papers
by Barton and others have established the name Gulf Coast geosyncline
firmly in the literature. The great accumulation of sediments along the
sinking continental margin, however, has not yet been deformed — it has
not been cast into folds and thrust sheets — but on the other hand, it is

654

STRUCTURAL GEOLOGY OF NORTH AMERICA

one of very gentle seaward dips, except where locally disturbed by salt
plugs, high-angle faults, and gentle warpings. A southward-migrating
trough line has been postulated such that the maximum thicknesses of
the different stratigraphic units are not superposed.

STRUCTURAL GEOLOGY

Balcones and Luling-Mexia Fault Zones

A complex assembly of faults follows approximately the border of the
Tertiary and Cretaceous formations of the Coastal Plain in Texas. See map,
Fig. 41.1. The zone of faults is located between the Edwards plateau of
comparatively flat Cretaceous strata, and the seaward dipping Tertiary
beds of the Coastal Plain. It crosses the San Marcos arch, which is a broad
southeastward-plunging nose of the Llano dome.

The zone of faults has been divided in several ways. The Tectonic Map
of the United States shows the southwestern part to be called the Bal-
cones fault zone, and the northeastern part the Luling-Mexia. Weeks
(1945) believes the Luling and Mexia are distinct and describes the three
belts as follows:

Balcones Fault Zone.

Extending through the vicinities of the towns of Georgetown, Austin, San
Marcos, New Braunfels, and some distance north of the towns of San Antonio,
Hondo, and Uvalde is the Balcones fault zone with downthrown side principally
on the southeast.

This zone of faults is located between the comparatively flat dip of the Ed-
wards Plateau and the more steeply dipping beds of the Coastal Plain, and
crosses the San Marcos arch, a broad nose which plunges southeastward from
the Llano uplift. In the vicinity of Austin, Travis County, the total throw across
the Balcones zone of faults approximates 900 feet; in northwestern Bexar
County, 1200 feet; in northeastern Uvalde County, 500 feet; and in southwest-
ern Uvalde County, 200 feet. In Kinney County the Balcones zone of faults
dies out.

Fig. 41.4 Distribution, thickness, and structure contour maps of the Mississippi embayment and
delta regions, after Murray, 1947. The black areas are the areas of outcrop, the solid lines are
structure contours, and the dotted lines are isopachs.

GULF COASTAL PLAIN

655

Luling Fault Zone.

The Luling fault zone lies coastward from the Balcones zone and is composed
principally of faults with downthrown side on the northwest. Examples of this
zone are: (1) the Staples, Larremore (along which the Larremore oil field is
located), and Lytton Springs line of faults in Guadalupe, Caldwell, and Bastrop
counties; (2) the Luling fault in Guadalupe and Caldwell counties along which
the Luling oil field is located, and which extends northeastward cross Caldwell
County and into Bastrop County; (3) the Darst Creek-Salt Flat line of faults
along which fields of these names are located in Guadalupe and Caldwell
counties; and (4) the Somerset and Alta Vista faults in Atascosa and Bexar
counties. All of these faults have considerable length. The average throw ap-
proximates 450 feet.

In Caldwell County along San Marcos River, a total throw of more than
1,500 feet is indicated on faults of the Luling zone. The faults of this zone have
the greatest throw and are most numerous from Travis and Bastrop counties
southwest through Bexar County, thus crossing the San Marcos arch.

Mexia Fault Zone.

Farther down the dip than the fault zones described above is the Mexia
zone of faults characterized by faults with downthrown side on the south-
east and by faults with downthrown side on the northwest. Both faults
commonly occur together with a graben of varying width between them. In
the Mexia area, the name Tehuacana has been given to faults on the northwest
side of the graben. The Mexia zone of faults extends from the vicinity of Mexia,
Limestone County, northeastward and eastward around Tyler basin. Faults in
southwestern Arkansas probably are a part of this zone. From Mexia southwest-
ward this zone of faults extends far into South Texas and offsets down the dip
at various points. At Mexia, Midway beds at the surface are cut by the faults; in
Lee County, Mount Selman; in Bastrop and Fayette counties, Cook Mountain
and Yegua; and in Gonzales County, Yegua and Jackson.

The zone of faults extending southwest through parts of DeWitt, Karnes,
Goliad, Bee, Live Oak, and Duval counties may be part of this zone. Repre-
sentative faults are those along which oil and gas have accumulated in northern
Bee County in the vicinity of Pettus. Considering all of these faults as belong-
ing to the Mexia zone, in Texas alone the length of the zone is over 500 miles.

In the region of Mexia many of the faults along which oil and gas production
is obtained from the Woodbine are en echelon, with the south end of the fault
at the north passing west of the north end of the fault at the south. This causes
closure in this direction. There is a structural high in the region of Mexia, and
south of this high the strike of the beds and the strike of the faults tend to con-
verge at the south end of each fault structure and tend to divirge at the north
end. This lack of effective north closure, plus absence of Woodbine sand,
probably is the reason for barren structures on the south toward the Falls
County regional low.

Minor movements may have occurred in Cretaceous time, but the main
displacements came in late Oligocene (late Catahoula) or Miocene (early
Oakville) time (Weeks, 1945). The sediments of the Catahoula and Oak-
ville reflect the movements. Because certain Pliocene beds are displaced
less than older beds, it follows that some movement in places has
occurred in post-Pliocene time.

The structure of the Coastal Plain from the Bend arch of central Texas
eastward across the fault zones to tire Sabine uplift is shown in the cross
section of Fig. 41.5.

Flexure Fault Zones

Paralleling the coast of Texas and shoreward of the Miocene boundary
(Fig. 41.1) are three flexure and fault zones. These are called flexure
zones or flexure fault zones, and they are shown in the lower cross
section of Fig. 41.2. The Gulf side is down 500 to 1500 feet, but the
unusual aspect is the reverse (?) drag aspect of the beds on the down-
thrown side. This has been interpreted as sagging or slumping of the
beds incident to the tendency of fissure opening as down-tilting of the
block toward the Gulf occurs. The faults die out upward in the Miocene
and Pliocene sediments and hence are about mid-Tertiary in age. Need-
less to say the flexure fault zones are the sites of very productive belts
of oil fields.

Tyler or East Texas Basin

East of the Balcones and Mexia fault zones is the Tyler basin, so
called on the Tectonic Map of the United States, but often named the
East Texas basin. See cross section of Fig. 41.5. It is the result of gentle
dips eastward off the Bend arch of central Texas and westward off the
Sabine uplift. It consists of a thick Tertiary and Upper Cretaceous se-
quence of beds. The Lower Cretaceous succession has not been pc ni-
trated in the deeper parts of the basin, nor has the mother salt that
has spawned a score of salt domes within the basin.

Sabine and Monroe Uplifts

A large gentle dome in easternmost Texas and northwestern Louisiana
is reflected in the surficial Tertiary strata, and is known as the Sabine

656

STRUCTURAL GEOLOGY OF NORTH AMERICA

■Jutcrop Coleman Junction Ls
PENN PERMIAN CONTACT

M-crop Range' Ume5'oneh , ,

BALCONES FAULT ZONE

EAST TEXAS BASIN

Fig. 41.5. Cross section from the Bend arch, central Texas, eastward to the Sabine uplift.

uplift. The shallow structural sag on the west is the Tyler basin, just
described. A small and shallow syncline separates the Sabine uplift
from the gentle Monroe uplift in northern Louisiana and southern Ar-
kansas. The axis of each uplift trends northwest-southeast. The doming
started in Cretaceous time. The Sabine uplift was an island at the close
of the Early Cretaceous, and the Monroe uplift was an island during
much of Late Cretaceous time. The Sabine uplift especially was effected
by upward movements in post-Claiborne (post-middle Eocene) time,
and this doming with ensuing erosion has left a core of Midway (Pa-
leocene), Wilcox (early Eocene), and Claiborne (middle Eocene) sedi-

ments surrounded by younger formations (Murray and Thomas, 1945).
See Fig. 41.6.

Jackson Dome

The Jackson dome is a sharp uplift in the subsurface in west-central
Louisiana. See Fig. 41.7. It is about 30 miles across. Local doming suffi-
cient upon erosion to expose the Upper Jurassic Cotton Valley forma-
tion occurred at the close of the Late Cretaceous. The amplitude of the
fold is about 10,000 feet, but the dips shown on Fig. 41.7 are excessive
owing to the grossly exaggerated vertical scale.

Notchitoches Ph., La.

Red River- dull Bayou

Shreveport

Pme Island Vicinity of Vivian

- 1200

<v:. .■>

ir-rA<

*•■; •••'

ft6

,\ov

zoo-

<6v ..4^ •

fm.

- 800'

- 400'

r^7T
4°V '

_Seo
level

-400'

- 300

Fig. 41.6. Cross section of the Sabine uplift approximately from north to south showing the details of
the Tertiary stratigraphy. The vertical scale and hence the structure are tremendously exaggerated.
After Murray and Thomas, 1945. Midway is Paleocene in age, Wilcox is lower Eocene, and Claiborne is
middle Eocene.

658

STRUCTURAL GEOLOGY OF NORTH AMERICA

Miles

■3000

5000

600 0

.7000

9000

Fig. 41.7. Cross section through the Jackson dome, Mississippi, taken from west to east. After McGloth-
lin, 1944.

Another structure in the Mississippi embayment is the Desha basin
north of the Monroe uplift. It is not a closed basin but opens on the
east into the broad embayment. In southwestern Alabama is the Hatche-
tigbee anticline which trends to the northwest and has surface out-
crop expression.

Salt Domes

Distribution. Semicylindrical masses of salt have thrust their way
upward in the poorly consolidated sediments of the Gulf Coastal Plain

in a variety of forms. They are known as salt domes or salt plugs,
characteristically from one-half to two miles in diameter, and are the
loci of many fine oil fields. Over 200 are now known in the Gulf Coast.
They are distributed in two general groups: (1) the coastal domes prin-
cipally through southern Texas, the Mississippi delta of Louisiana, and
the shallow offshore shelf (Figs. 41.1 and 41.8); and (2) the interior
domes. Some coastal domes also occur in northern Mexico in the Vera
Cruz-Tabasco basin. The black dots on Fig. 41.9 indicate the salt domes
discovered to date. It will be seen that the greatest number are in the

GULF COASTAL PLAIN

650

Fig. 41.8. Offshore salt domes
on the continental shelf of Louis-
iana as of March, 1958. After
Habarta, 1958.

coast belt. Those in the interior are divided into three areas, one in
the Tyler basin, one in the eastern part of the Sabine uplift, and one
in a broad zone across south central Mississippi.

Classification. Salt domes are classified in several ways. The divisions
deep, intermediate, and shallow are the most commonly mentioned.
Deep domes are considered those whose salt core tops are greater than
5000 or 6000 feet below the surface (Billings, 1942), intermediate domes
'between 6000 or 5000 and 3000 or 2000, and shallow domes, less than
3000 or 2000 feet deep. Some have reached the surface. Deep domes are
[divided into those whose salt has been reached by the drill and those
! whose salt is below any deep wells.

Another classification concerns the relation of the salt plug to the
country rock. If the salt has simply domed the overlying beds in the
manner of a concordant laccolith, the structure is called a nonpiercement
dome. If, on the other hand, the salt has penetrated through the beds,
the structure is said to be a piercement dome. Generally all domes are
now considered as piercement type, whether shallow, intermediate, or
deep-seated. Refer to Fig. 41.9 illustrating the origin of salt domes
for these types and also a number of transitional ones.

Some salt domes have mushroomed out at the top, and the cap rock
and part of the salt core is said to overhang. These horizontal expansions
or wedges have been drilled through and their presence thus demon-
strated.

irmfi/.td a/let Mu>ror 11956)

?*&>$ I STRUCTURAL ELEMENTS
A i, OF

; /GULF COASTAL PROVINCE

*• *I/IT ZONE

Fig. 41.9. Structures of the Coastal Plain around the Gulf of Mexico. Reproduced from
Atwater and Forman (1959).

STRUCTURAL GEOLOGY OF NORTH AMERICA

Salt domes also differ on the basis of their cap rock. Overlying the
massive rock salt of the core of the dome is an irregular layer or cap
of limestone, gypsum, and anhydrite. Limestone is generally at the top,
and anhydrite at the bottom of the capping layer. The cap rock of a few
domes contains immensely valuable native sulfur deposits.

The sedimentary layers over the salt domes have been domed up
gently, and the layers adjacent to the intrusive salt plugs have been
dragged upward to a greater or lesser extent. Oil is found, therefore,
over the salt plug in the domed strata, on the flanks against the plug,
under overhangs, and in associated fault traps.

Deep drilling of certain salt domes of southern Louisiana shows that
very large volumes of salt are involved, and that one structure at a
depth of 20,000 feet has an area of 200 square miles, and contains 265
cubic miles of salt above the 20,000 foot datum (Atwater and Forman,
1959). Further, the salt is intrusive like major igneous discordant
plutons; the country rock has been "replaced" rather than shoved aside.
The manner of emplacement is an enigma. Also, large masses of contorted
shale have been carried up far above their normal stratigraphic position
and look like intrusive masses themselves. The intrusive action has been
localized to one part of the large dome at one time, and then to another
part at another time.

Origin. Salt domes result from the plastic intrusion of sedimentary
rock salt into overlying beds. Rock salt under pressure deforms easily
and flows from places of greater pressure to places of lesser pressure.
Many geologic observations confirm the concept that salt flows easily
and that the associated shales are commonly intensely deformed. Ac-
cording to Nettleton (1936) it is reasonable to assume that the present
shape of the dome is due to ( 1 ) initial configuration that localized the
dome, (2) the thickness of the mother salt layer, (3) the strength or vis-
cosity of the overlying rocks, and (4) the strength or viscosity of the
salt. Figure 41.10 shows the theoretical development of salt domes under
three conditions.

A number of deep-seated salt domes are marked by faults that cut
and offset the arched beds over the salt core. Perhaps the faulting is a
general characteristic. The faults are normal and form a complex graben

through the central part of the dome. Wallace (1944) believes that the
common fault patterns are simple offsets and simple and complex
graben such as illustrated in Fig. 41.11. The first fault that occurs is
called the principal fault, which produces the simple offset. The next is
the complementary fault ( also called minor fault ) , and the next is another
minor fault. The generalized diagrams of Fig. 41.11 give the impression
that all graben cut across the domes; but as more is learned of the detail
of the deep domes, more faults are recognized, and their ground pat-
tern may be somewhat concentric in certain domes, somewhat radial
in others, and crosscutting in still others.

The intruding salt has also caused small reverse faults on the sides
of certain domes. These are significant in forming oil traps (Halbouty
and Hardin, 1954, 1956).

Wiggins Anticline and the Deep Wells

Just 50 miles north of the Gulf of Mexico in southern Mississippi a
well was drilled 20,450 feet deep in a subsurface structure called the
Wiggins anticline. It is known as the George Vasen's Fee well and was
completed in 1951 (Applin and Applin, 1953). At the total depth it
reached rock salt of pre-Smackover (Jurassic) age. Nearly 5500 feet of
consecutive cores of unmetamorphosed Jurassic strata were obtained.
From a depth of 14,670 to the bottom the formations penetrated have
been identified as follows:

Lower Cretaceous

Lower part of Hosston fm.

275

feet

Dark brownish-red shale

Upper Jurassic

Cotton Valley group

2053

feet

Mostly nonmarine or detaic

deposits in upper part; lower fourth is

marine and fossiliferous

Cotton Valley (?) group and Buckner (?) fm.

1700

feet

Nonmarine sandstone and shale

Smackover formation

105

feet

Dark sandstone, siltstone, and shale

Dips 25° to 60°

GULF COASTAL PLAIN

661

Jurassic (undifferentiated)

Smackover (?)

Limestone, dolomite, anhydrite

Pre-Smackover

Gray crystalline anhydrite and at bottom
1 foot of clear white rock salt

1620 feet
30 feet

A well near the front of the Mississippi delta penetrated to a depth of
22,570 feet and ended in Miocene strata (Paul Lyons, personal communi-
cation). In relation to the Vasen's well on the Wiggins anticline, 120
miles to the north, a marked southward dip is evident, which is re-
ported as 7°. The steepening of southward dip in the Mississippi delta is
prominent on the maps of Fig. 41.4, and for the Miocene beds a trough
axis has been discerned extending east-west through the delta.

IGNEOUS ROCKS

Moody (1949) has summarized the igneous rocks of the Gulf Coastal
Plain, both pre-Cretaceous and Cretaceous in age. The greatest concen-
tration of igneous activity centers in the Monroe uplift and Jackson
dome areas (as designated on the Tectonic Map of the United States) in
the tristate area of Arkansas, Louisiana, and Mississippi. There, in well-
drilling operations, alkaline and ultrabasic intrusive rocks have been
drilled into, and also volcanic rocks in the form of flows or sills; pyroclas-
tics are abundant, both alkaline and basic. Some of the intrusive bodies,
possibly dikes and stocks, are definitely intrusive into Upper Jurassic
strata. Some are older and believed to be related to the Triassic diabase
of the Atlantic piedmont.

Throughout the entire northern part of the Coastal Plain in the Upper
Cretaceous sediments fragments of volcanic rocks are found in associa-
tion with the common sedimentary detrital minerals.

TAMPICO REGION, MEXICO

The Tampico region has a somewhat different Cretaceous geology
from the rest of the Gulf Coast, but a similar Tertiary. Instead of an

overlap from the Gulf, the Cretaceous beds are continuous with tliose of
the interior Mexican gcosyncline and the Parras basin. The Cretaceous
beds of the geosyncline are intensely folded, and along the cast front of
the Sierra Madre Oriental they are thrust eastward in places. The zone
from the Sierra Madre front to the coast, 60 to 100 milts wide, may be
regarded as the coastal plain where the sedimentary rocks are fairly flat;
but several anticlinal mountains (or hills) formed of Cretaceous rock
interrupt the plain. The Tertiary sediments were deposited in seas that
invaded the coast from the Gulf and buried unconformably a number of
relief features.

The anticlinal or domal mountains that rise from the plain are. from
north to south, the Sierra Burro, Lomerio Peyotes, Sierra Lampazos, Sierra
San Carlos, and Sierra Tamaulipas (Muir, 1936). See maps, Fig. 42.1 and
35.1. The Sierra San Carlos has already been described in Chapter 28
and is fairly representative of the mountains east of the Sierra Madre
front. Some of the ranges have gentle dips on the flanks from 3 to 10
degrees. The Sierra Papagayos is steeply folded, with dips up to 40 de-
grees and more. The doming of the Sierra San Carlos has been accentu-
ated by the intrusion of a stock of nepheline syenite, and the folding of
the Sierra Picochos has been influenced bv intrusions (Muir, 1936).

All the ranges just mentioned in the coastal plain are parts of a contin-
uous structural element and hence related genetically. The Sierra Tamau-
lipas anticline plunges southward, and the so-called northern oil fields are
on its prolongation. See cross section of Fig. 41.12. Near the termination
of the Sierra Tamaulipas on the southern flank is an offshoot named the
Sierra de Buenavista. The Tamaulipas limestone in the core is intruded by
a laccolith. Muir concludes that the forces that produced the mountains
and oil-field structures of the coastal plain are due to vertically acting
forces, in contrast to the Sierra Madre and interior structures which are
due to horizontally acting forces.

The northern oil fields are in an area of Cretaceous rock that reaches
nearly to the coast at Tampico. Immediately south of Tampico. beds of
Eocene, Oligocene, and Miocene age lap 50 miles inland across the Cre-
taceous and bury an arcuate ridge which lies west of Tuxpam. Albian-
Cenomanian reef limestones were probably laid down on a late Aptian

CONDITION

I V

Fig. 41.10. Theoretical development of
salt domes under various conditions, after
Nettleton, 1936. Diagrams are patterned
partly after model experiments involving
viscous flow, and partly after actual ex-
amples.

FOUR

t • . . ...ZT.

4 /f=======:fc^

^//y^^aW^-^—

2 -

J ■ ■

.-,,,'/,'.',/',',, '//////////,

r/ff///// //////// .'.'.'. 7.'. '.'.'. ./"

THREE

T A

TWO

2 __^ ■ "

//////////77777777T,

1 — — - —mz=^r~—

=J-nr--=-- s a lt_=^^777////////7

7!n////m///>n7T777n/////////if///l

T A

ONE
662

SECTION

MAP

DOME WITH SIMPLE OFFSET

DOME WITH GRABEN

GENTLE DOMING
AND OIL MIGRATION

MAJOR FAULT AND SIMPLE
OFFSET

COMPLIMENTARY FAULT
AND SIMPLE GRABEN

THIRD FAULT AND COMPLEX
GRABEN

Fig. 41.11. Origin of faults over deep salt domes, after Wallace, 1944.

663

SIERRA

TAMAU LI PA S

CERRO PICACHO GUADALUPE C. SAROINAS

I YUCATE | ESLABONES I

S.J. DE LAS RUSIAS

WELL 5
CHAPAPOTE

fault

SOUTH ERN

0 I L

FIELDS

9> Av

RIO TANCOCHIN

cP^+

*> :*

JL I III Ifissa/r | | Basdlt

Fault*

<f %>*

Kilometers

Fig. 41.12. Cross sections of the coastal plain of eastern Mexico after Muir, 1936. The upper section
is the southward plunging end of the Sierra Tamaulipas, where the northern oil fields are located. The
lower section is west of Tuxpam, and runs nearly north-south, longitudinal of the structure.

664

Fig. 41.13. Cross section from southern Georgia to Key West in southernmost Florida, after Applin and
Applin, 1944.

665

666

STRUCTURAL GEOLOGY OF NORTH AMERICA

submarine limestone reef that formed a ridge. The ridge area was ele-
vated in post-Turonian time, and again repeatedly in Eocene and Oligo-
cene time. With sedimentation repeatedly burying it, a number of un-
conformities occur around and over it. The spatial aspect of the buried
ridge is difficult to fit into the structural picture ( Muir, 1936 ) .

FLORIDA PLATFORM

Sediments from Recent to Early Cretaceous age are known to overlie a
crystalline basement in Florida, and beds are probably present in places
between the Lower Cretaceous and the crystalline rocks. The thickness
of the sedimentary cover ranges from 4350 feet in southeastern Georgia to
more than 11,600 feet in the southern end of the peninsula. The earliest
tangible history of Florida is that of the Early Cretaceous, when approxi-
mately the western half of the peninsula was submerged and the eastern
half was land. A number of drill holes bear out this picture fairly well.
If an outer belt of Lower Cretaceous strata in Georgia and the Atlantic
Coastal Plain under the Upper Cretaceous and Tertiary beds is related
to the land area of Florida, a long peninsula seems to have existed then,
as now, only slightly eastward of the present.

If the isopachs of the Upper Jurassic of the Mississippi embayment
region and Arkansas and Texas are projected to Florida where problemati-
cal Upper Jurassic has been recognized in just two deep wells (Applin
and Applin, 1944), only the southern third of Florida seems to have
been under water, and the rest was land. In fact a very broad land
projection seems to have existed. See paleotectonic map of Plate 10.

The cross section and map of Fig. 41.13 show the stratigraphic and
structural relations recognized in Florida. The chief structural feature is the
Peninsular arch in the north-central part of the peninsula which first ap-
peared in the Late Cretaceous. The axis of the arch trends northwestward
and is parallel with a deep trough that centered in the Greater Antilles.
The arch is also pronounced in the Middle and Upper Eocene beds, but
with variations in detail. A flexure developed on the west flank of the
Peninsular arch has distinct outcrop expression and is properly called
the Ocala uplift, according to the Florida Geological Survey, but the

large arch itself is commonly called the Ocala.

According to Applin and Applin (1944) the chief structural features
of Florida are:

(1) An axis extending northwest from about Cape Canaveral on the east
coast of Florida to south-central Georgia, upon which are located two large
locally high areas; (2) a channel or trough extending southwestward across
Georgia through the Tallahassee area of Florida to the Gulf of Mexico, nearly at
right angles to the aforementioned axis; (3) an upwarped area in the vicinity o£
Jackson County, Florida, with dips extending away from it toward the southeast,
south, and southwest; (4) a structurally low area with an axis extending north-
west from the vicinity of Lake Okeechobee toward Tampa, approximately
parallel with the axis first mentioned; (5) a possible second north-west-trending
upwarped area at the south end of the Peninsula.

The modern peninsula of Florida is about the emergent third of a
broad platform, as may be seen in Fig. 42.1 The shelf on the west
side is 100 miles wide and ends in a very steep escarpment which carries
down to the abyssal plain of the Gulf of Mexico. This West Florida
escarpment has been thought of as a fault scarp (Jordan, 1951), but on
hand of a uniform magnetic intensity field over the escarpment and
the aseismic nature of the region, Miller and Ewing (1956) believe it is
not due to faulting but to processes of sedimentation. The constitution
of the crust under the Gulf of Mexico, and the origin of the Gulf will be
discussed under a later heading.

The shelf on the east of the peninsula of Florida is continuous with
and supports the Rahama Ranks whose geology will be discussed in
the next chapter. The great shelf region is largely one of carbonate de-
position today, and as explained, has so been in the southern half
since at least Early Cretaceous time. Accumulation has equaled subsid-
ence, and the imposing submarine escarpments may be due to the
growth of reefs, firm enough to keep the sediments from slumping down
to the abyssal plain. Some local magnetic anomalies on the West Florida
escarpment may indicate buried volcanic piles (Miller and Ewing,
1956).

A number of wells have penetrated the Mesozoic sedimentary rocks,
and maps of the surface are shown in Fig. 41.14. The contour of the
surface is that of the dominant Peninsular arch. The outcrop pattern, how-

GULF COASTAL PLAIN

CRUSTAL STRUCTURE OF GULF OF MEXICO

867

Fig. 41.14. Configuration of surface of pre-Mesozoic rocks in Florida and southern Georgia,
and distribution of pre-Mesozoic rocks. Precambrian consists of granite, diorite, and meta-
j morphic rocks; Paleozoic (?) and Precambrian (?) consist of rhyolite, tuff, and agglomerate.
After Applin, 1951.

ever, suggests a structural high offset to the southeast, with intrusive

igneous rocks, probably Pre-cambrian, exposed in the core. These are

| flanked on the northwest and southwest by volcanic rocks which may be

the equivalent of the Unicoi formation (basal Chilhowee) of the southern

' Appalachians. Then in nortiiern Florida a basin of Ordovician and

! Silurian sedimentary rocks occurs, fairly flat-lying and unmetamorphosed

(Applin, 1951). These undisturbed Paleozoic strata are southeast of the

Appalachian orogenic belt, and pose a rather mysterious problem in

tectonics and the evolution of the southeastern margin of the continent.

Geophysical Data

Refractive seismic traverses by Ewing et al. (1955) and a magnetic
intensity survey by Miller and Ewing (1956) serve as the principal evi-
dence for sediment layering and crustal structure under the Gulf of
Mexico. The seismic data are given in Fig. 41.15, and the magnetic
data have been used in constructing the geologic cross section of the
same figure. Another seismic refraction profile by Antoine ( 1959 ) across
the Colombian basin from western Cuba to Colombia continues the Gulf
of Mexico section to South America. Although the two sections are
offset from Yucatan to Cuba, the Yucatan-Cuba tectonic element may
be visualized as shown in Fig. 41.15, and the effect of a continuous section
obtained, which helps in understanding the constitution and history of
the great mediterranean region. Cuba and the Caribbean region will be
discussed in Chapter 42.

In making the geologic interpretation the rocks indicated by the
various seismic velocities are taken as follows. These are generally the
ones suggested by geophysicists in previous references on the Atlantic
continental shelf and ocean floor, and in the above articles.

1.8-3.7 km/sec

4.5-5.2 km sec

5.2-5.5 km sec
4.5—5.5 km/sec

5.6-6.1 km sec
5.8-6.1 km sec
6.5± km/sec
7.0-7.5 km, sec
8.0-8.3 km sec

Shelf of Gulf Coastal Plain

Unconsolidated and semiconsolidated sedi-
ments

Semiconsolidated and consolidated sedi-
ments

Limestone and dolomite

Extruded porous volcanic rock. Lower
values probably indicate porous rock

Intrusions in volcanic rock

Crystalline basement of continent

Gabbroic or basaltic subcrust

Transition layer, mantle to subcrust

Mantle (periodotite or eclogite)

It may be seen in Fig. 41.15 that the wedge of sediments of the Gulf
Coastal Plain thickens nearly to the shelf slope where a total thickness

668

STRUCTURAL GEOLOGY OF NORTH AMERICA

GALVESTON
SHORE

GULF COAST

OULF OF MEXICO

BASIC SEISMIC REFRACTfON DATA

CUBA YUCATAN BASIN

NICARAGUAN RISE

— «

COLOMBIAN BASIN

MEXICAN BASIN

SPECULATIVE GEOLOGICAL INTERPRETATION

YUCATAN-CUBA TECTONIC ELEMENT YUCATAN BASIN CAYMAN TRENCH NICARAGUAN RISE

COLOMBIAN BASIN COLOMBIA*

UNCONSOLI- Vl-

oateo Lr

SEDIMENTS

_-_-) SEMI- TO -1-,-! J
-li-3 CONSOLIDAT- 13 — P LIMESTONE
ED SEDS.

l-*-*-*lvOLCANICS

AUT1CAL MILES

Fig. 41.15. Crustal structure of Gulf of Mexico (Ewing ef al., 1955; Miller and Ewing, 1956) and western
Caribbean (John Antoine, 1959). The speculative geologic interpretation is slightly altered and somewhat
more detailed than given by the authors cited.

of about 45,000 feet of combined consolidated and unconsolidated
sediments appears to exist. Refractions from the base of the "consoli-
dated sediment" layer could not be obtained, and it is inferred by
Ewing et al. (1955) that either a limestone or salt layer of about 5.28
kilometers per second velocity overlies lower velocity sedimentary rocks.
The great thickness of the "consolidated" layer may be the result of con-
solidated carbonate facies, and the boundary shown on the geologic
section of Fig. 41.15 may therefore not be a systemic or time boundary.
This seems a more logical interpretation than one involving vertical move-
ments of the ocean floor. Offshore carbonate deposition in the form
of barrier reefs could have affected the semi-isolation of extensive la-
goonal seas for the precipitation of salt and gypsum. Some such retaining

form or structure is necessary to produce the Jurassic or Permian evap-
orite conditions of the Gulf Coastal Plain.

It will also be observed that the sediments near the shelf slope rest
directly on the gabbroic subcrustal layer, and that the continental crystal-
line basement layer does not make an appearance until about the shore-
line. This arrangement is concluded by Miller and Ewing (1956) to exist
because the magnetic intensity field is remarkably uniform and without
conspicuous anomalies from the basin across the shelf slope onto the
shelf.

The shelf slope has been considered to be a fault scarp and in ad-
dition to indicate that the Mexican basin is a down-faulted depression
(Gealy, 1953; Jordan, 1951; Eardley, 1954). The uniform magnetic field

GULF COASTAL PLAIN

669

across the steep slope argues against the fault theory, as does also the
lack of seismic activity there (Miller and Ewing, 1956).

Mexican Basin

The seismic data indicate that some 30,000 feet of unconsolidated and
consolidated sediment under the Mexican basin rests directly on a gab-
1 broic subcrust which in turn is about 25,000 feet (8 kilometers) thick.
This condition indicates that the Gulf of Mexico crust is of the oceanic
type, but that sediments have been accumulating in the large amounts
characteristic of continental borderlands on the gabbroic layer from at
least the beginning of Mesozoic time.

Yucatan Platform

The north side of the Yucatan peninsula or platform, the Campcche
Bank, is believed to be underlain by limestone or dolomite with only a
thin veneer of unconsolidated sediments. The velocity of 5.6 (Fig. 41.15)
is regarded by Miller and Ewing (1956) to indicate limestone, dolomite,
or crystalline basement, but the exposed geology suggests the presence of
carbonates rather than a crystalline basement. The carbonates are be-
lieved to be sufficiently lithified and strong to hold up an exceedingly
steep slope, which in turn is interpreted to be an escarpment built up
by sedimentary processes and not a fault scarp. The uniform magnetic
field over the escarpment points to the sedimentary origin.

42.

ANTILLEAN-CARIBBEAN
REGION

taken by Schuchert (1935) to mark the eastern limit of the Greater
Antilles. The Anegada Passage is the site of a submarine channel across
the Caribbean submarine ridge, and its shallowest course is over 3000 feet
deep. The arc of volcanic islands south of Anegada Passage is known
variously as the Caribbees, the Windward Islands, and the Lesser Antilles.

The Caribbean Sea, according to most maps, includes all water south of
the Greater Antilles, west of the Lesser Antilles, north of Colombia and
Venezuela, and east of Central America. The major basin is south of
shallow banks that stretch from Honduras and Nicaragua to Jamaica, and
from Jamaica to Hispaniola. It is divided into a western half, the Colom-
bian basin, and an eastern half, the Venezuelan basin, by the Beata ridge
which extends southwesterly from Hispaniola. The Tanner basin or deep
in the eastern half has a greatest known depth of 16,800 feet. The Aves
swell, marked on the north by Aves Island, separates the Venezuelan
basin from the Grenada basin, which is bounded on the east by the Carib-
bees and their supporting ridge.

North of the Rosaline and Pedro Banks and Jamaica, and south of the
Misteriosa Bank, the Caymans, and eastern Cuba is a deep, east-west-
trending basin with greatest known depth of 22,788 feet. The major basin
is called the Cayman trench, and the deep inner trough, the Bartlett. See
Fig. 41.15.

GEOGRAPHIC PROVINCES

The West Indies were discovered by Columbus when he came ashore on
the island of San Salvador. The name Antilles, which comes from the
mythical island of Antilia or Antillia, and this in turn possibly from
Atlantis, Plato's vanished land in the Atlantic, came to be applied to the
islands of the region (Schuchert, 1935). Following the general pattern
of use today, the term Greater Antilles will refer to the major islands,
Cuba, Jamaica, Hispaniola (the Dominican Republic and Haiti), Puerto
Rico (Porto Rico), the Virgin Islands, and the Bahama Islands. See map,
Fig. 42.1. Puerto Rico and the Virgin Islands are separated from the
volcanic islands on the south by the Anegada Passage, which has been

GREATER ANTILLES

Cube

Physiograpluj. Cuba is the westernmost island of the Greater Antilles.
It is 100 miles south of Florida, is 750 miles long, and has an average
width of 50 miles. The shape of Cuba as defined by the existing shorelines
would be considerably changed if the water level dropped only 50 feet.
The Isle of Pines and numerous cayos on the north and south coasts would
become part of the mainland, and the area would be increased 30 per-
cent (Palmer, 1942). Beyond the 50-foot isobath, deep water sets in almost
everywhere.

The principal geomorphic divisions are shown in the upper map of
Fig. 42.2.

670

Fig. 42.1. Map of the Gulf of
Mexico and the Caribbean Sea
regions. The lined areas are
underlain by Tertiary sedimen-
tary rocks. The sea and ocean
floors are contoured in hun-
dreds of fathoms.

iftonf.

^N - — 100 f —

GEOGRAPHICAL MAP

»i»m!}!»

METAMORPHIC ROCKS

9 NAUTICAL MILES

GEOLOGICAL MAP

Fig 42.2. Geographical and ge-
ological maps of Cuba. Geology
after Butterlin, 1956. Facies lines
from C. W. Hatten (personal
communication) and Wassail
(1957) apply to Jurassic and
Cretaceous strata.

ANTILLEAN-CARIBBEAN REGION

673

CUBA

HISPANIOLA

- PUERTO RICO

VIRGIN ISLANDS

JAMAICA

RECENT

Calcareous reefs, alluvium

Alluvium

Reefs and alluviua

Alluviua

Keefs of aeveral islands

PLEISTOCENE

JAIMANITAS reef la. (20m.)

Reef Is., nepheline basalt-, alluvium

SAN JUAN aeolian calcareous ss.

Sand and reefs

IIGLASLA, clay, •and gravel

(1Mb.)

PLIOCENE

MATANZAS Is. and marl

HI NICHE alluvium

RIVIERE GAUCHE molassr ( 500m. ) ^

i aarl and while

la.

UPPER

GUANAJIBO(T) sandy Is., clay, silt, sand

KINGSHILi. start (iSOa.)

BOWOEN gravels and aarlaa

EL ABRA clay V sand (50m.)

MORNE DELMAS basalt (400m. »
ARTIBOUITE

CROUP LAS CAHOIUS HAISSADE

congl. ,1s. , clay, lignite
gravel, clay

MIDDLE

CANIMAR marl & argillaceous Is. (45m.)

LOW LAYTON LAVAS T

d'AYMAMON la. (325". )
d' ACL ADA Is. ( 75m.)

WHITE

u '

(000a. 1

MAY PIN, yellow Is.
SEW7-ORT la.
WALDLRSTON la

LOWER

GUINES Is. (40m. ) PASO marly Is.

UPPER

COJIMAR aarl (3S*.) REAL congl.

THOMONDE clay and ss.(7S0m.)

CIBA0 aarl (230a. )
RIO GUATE- GUAJATACA detritus (120a.)
MALA GR. LARES Is. (400a.)

SAN SEBASTIAN sh., sand, gravl . ( 300a. )

JEALOUSY
GROUP

clay, gray-creen (300a.)
congloaerate O gray clay (30a.)
clay, Cray and la. (90a. +)

MOSTMLIt* la.

MIDDLE

JARUCO, atari, congl . , sand

MADAME JOIE sh.bls. LA CRETE ss.& Is.

brows rani

LOWER

TIVCUARO marl

LIMESTONE

CANAS Is., siliceous sh. (660m.)

UPPER

CONSUELO marl

JABACO marl & congl. JICOTEA marl

BNNERY Is. Dolerite 1/
(1 ,000m. ) Basalt

COAMO SPRINGS Is., tuffs (300m.)

SOMERSET la.
SWAKSICK la.

GIBRALTAR la.
SWAKSICK Is.
TROT dol. & Is.

MIDDLE

LOMA CANDELA, Is. , marl, ss., congl.

PLAISANCE Is. PERODIS tuffs, sh., U
(1,000m.) lavas (1,000m.)

RIO JUEYES ah., congl . , Is., tuffs (1050m.)

YELLOW Li. (lBOa.)

HALBLRSTADT GJLCL'P

LOWER

UNIVERSIDAD marl (130m.) TOLEDO clay
CAPDEVILA sh. 0 as. LUCERO member, ss,, congl,

ABUILLOT sh., SS., congl. (1,000m.)

C0ROZAL Is.

Serpentinixed poridotite, cal. tuffs
-Tuffs, breccias, agglomerate, sh. , la., ■*-. — ■

WAGWATER GROUP, rongl . ,
as., y sh. (460a. )

PALEO-
CENE

MADRUGA ss., clay gravel, REMEDIOS cryat-
congl. (600a. ) alline Is.

MARIG0T, congl., ss., sh. (600m.)

SEDIHESTART "" C' «*""■"«
SERIES, sh. e> la.
(2100 to 2400a.)

MAL&Oki PERILS a/4

aaitaa Vfcasalla
SUttJtRLAaD SERIES
vol. brace las r

UPPER

northern facies - Is.
HABANA southern facies - tuffs

dolerites
MACAYA marl (/ radiolarite (2,000m.)

andesitic and basaltic (3000m.)

rocks, la., Rudistid Is., andesitic lava*

LOWER

and gabbro intrusions
TUFF SERIES, tuffs, Is., narl. (8,000a)

BASAL COMPLEX, tuffs, andesite, basalt,
mica, chlorite, and calcareous schists,
amphibole

BARRANQUITAS-CAYEY sh.. Is., tuffs (900a.)

RIO DE LA PLATA, tuffs, cons;!., sh. , Is. (600a. )

T CABO ROJ0 rhyolite, granodiorite, granite

BASAL COMPLEX, schist, aarbl

a, aapbllaollt*

UPPER
JURASSIC

VINA LBS Is. ...

JAGIA schistose Is.
AZUCAR Is.

MIDDLE
JURASSIC

SANCAYETANO sh- , ss - , slate, phyllite.
Includes basal met amorphic complex

Fig. 42.3. Stratigraphy of the Greater Antilles. After Butterlin (1956) with modification of the Cuban
Jurassic sequence. See modifications in text of sections in Hispaniola and Puerto Rico.

Stratigraphy. The succession of rock units of Cuba is given in the
chart of Fig. 42.3. The oldest rocks are a metamorphic complex which
crops out in the eastern Oriente Province, in two places in central Cuba,
and on the Isla de Pinos. It is included in the middle Jurassic by Butterlin
(1956) but may be older (Taber, 1934). The complex in the Trinidad
Mountains consists of limestones and dolomites and a carbonaceous, chlor-
itic, mica schist. Quartz-garnet-mica schist and epidote and talc schists
are also noted (Hill, 1959). No fossils were found. Serpentines of two
types occur in the complex, a nodular one derived from periodotite and
a fine-grained one derived from microgabbro. The one derived from
microgabbro is older and has been affected by two movements, one pre-
serpentinization and one postserpentinization. The rocks are isoclinally
folded.

The Jurassic and Lower Cretaceous strata are irregularly treated in the
literature, but now it is believed that a continuous sequence exists from
the Middle Jurassic to the Tertiary. According to C. W. Hatten of Standard

Oil of California ( personal communication ) the northern succession from
the Vinales limestone through the Cretaceous is a carbonate facies closely
related to the Florida deposits, and the southern facies is a graywaeke-
volcanic succession. The graywacke-volcanic facies is called the clastic-
volcanic facies by Wassail ( 1957 ) and the carbonate facies the limestone
and clastic-volcanic facies. North of the carbonate facies is the exaporite
facies. These facies hold for the Upper Jurassic and Lower and Upper
Cretaceous; the sites of deposition did not shift appreciably during the
entire time. The approximate facies zones are indicated on the geologic
map of Fig. 42.2. The San Cayetano formation consists of some 35,000
feet of highly folded shales, slates, phvllites, and minor amounts of schist
The Vinales limestone consists mainly of dark gray to black, fairly thin-
bedded limestone, but it includes considerable amounts of dark shale and
chert. Its thickness from place to place has been variously estimated from
1000 to 5000 feet. It crops out chiefly in parts of western Cuba.

The lower Tertiaiy deposits are mostlv elastic and contain coarse eon-

674

STRUCTURAL GEOLOGY OF NORTH AMERICA

glomerates and sandstones. They have been involved in strong orogeny
along with the Cretaceous strata and are separated from the middle and
upper Eocene by a major unconformity.

Middle and upper Eocene are found in all the provinces of the island.
In this series occur fine conglomerates, sandstones, limestones, marls, and
chalks. The Eocene deposits indicate a progressive deepening of the
depositional area.

The Oligocene is very well represented. At least seven horizons have
been recognized in various parts of the island ranging from the lower
Oligocene to an Oligo-Miocene transitional one. It carries a large and
well-preserved fauna. The formation is predominantly lime in various
stages of induration, and coral reefs are common. The lowest known
Oligocene member is a marly shale. Roth Eocene and Oligocene contain
shale and some marl members that would afford admirable cap rock for
petroleum reservoirs.

There was a continuity of deposition from the Oligocene into the lower
Miocene. During this period Cuba was submerged except for a few
islands. The general aspect was probably not greatly different from that
of the Lesser Antilles today, that is, a series of small islands. The deposi-
tion of this period is predominantly a hard limestone which has been
named the Guines. This limestone forms an interrupted collar nearly
around the island as far east as Camaguey, and crosses the island in two
low, flat saddles, one in Matanzas and the other in western Camaguey
Province. It lies unconformably upon almost all the preceeding forma-
tions. Except where folding has subjected it to erosion, the Guines lime-
stone masks the older formations. Geological data are here dependent on
geophysics and core drilling.

Deposits of mid- and late Miocene age are limited to a few estuaries
that were inundated at the time. They are best developed around Matan-
zas Ray, Santiago de Cuba, and Manzanillo, and extend but a short dis-
tance inland from the coast. The remaining Tertiary deposits are but
small local patches along the coast.

The Pleistocene record is confined to well-developed terraces in several
parts of the island and to a few scattered unimportant deposits along the
coast.

The Upper Cretaceous and the Tertiary, except for the lower Eocene,
carry large and well-preserved faunas. These consist of Foraminifera,
Radiolaria, corals, echinoids, and mollusks. A noteworthy feature of the
Cuban fossil faunas, of both the Cretaceous and Tertiary, is that they are
definitely not North American. They are tropical faunas and form a part
of a Caribbean unit. This unit is in turn a part of the Mediterranean or
Tethyan fauna of the Old World. The Aptychus beds are a striking illus-
tration. Deposits with the same fauna, of the same lithologic aspect, at-
tributed to the same age are found in the Cape Verde Islands and in
Persia. Another equally striking illustration is that what appear to be the
same species of echinoid occur in the Eocene of both Cuba and Egypt.

Igneous Rocks. Roth intrusive and extrusive rocks occur in Cuba. The
intrusives are both acid and basic. The acid rocks for the most part occur
in the southern half of the island. This is illustrated by a large granite
intrusion that borders the Trinidad Mountains on the north and by the
granite and other acid intrusions on the southern slope of the Sierra
Maestra. These intrusions are relatively not extensive.

In contrast, the basic intrusions for the most part lie in the northern
half of the island and are by far the more prominent type. Most of the
basic rocks are serpentine. There is no agreement on the age of these
intrusions. Most of them occur in the Cretaceous terrane and appear to be
post-Cretaceous. Two are known in an Oligocene terrane and appear to
cut the limestone of that age. The very extensive intrusions of serpentine
and associated rocks in Santa Clara Province are thought to have accom-
panied the post-middle Eocene period of overthrusting.

Lower Cretaceous volcanic activity was considerable. This is evidenced
by thick series of tuffs, volcanic breccia, and flows. At least 6000 feet
accumulated in the southern part of Habana Province.

Except in Oriente Province, there is but little effusive volcanic material
in the Tertiary. In that province the middle Eocene deposits are largely
basaltic. The Sierra Maestra is composed of rocks of this material. Taber
(1934) estimates the thickness "to be over 4500 meters and possibly as
much as 6000 meters."

The upper Eocene in Oriente Province is also basaltic in part, but much
less so than the middle Eocene. In Matanzas Province there are thin beds

ANTILLEAN-CARIBBEAN REGION

675

of pumice in the upper Eocene. In Camaguey and Oriente there are a few
Late Tertiary or Pleistocene flows. Elsewhere the Tertiary is free of vol-
canic material.

Tectonic History. The Jurassic and Cretaceous tectonic history of
Cuba has been interpreted variously by different writers, but this is most
probably due to the fact that until recently the facies relationship of the
several formations has not been entertained. Unconformities and several
pre-Tertiary deformations have been postulated. According to Wassail
( 1957 ) the main deformation occurred near the close of Cretaceous time
when the southern clastic-volcanic sequence was thrust northward over
the carbonate facies and even over the southern margin of the evaporite
sequence. Later normal faulting parallel with the facies zones resulted in
the dropping of blocks of the thrust sheet into graben. The upfaulted
blocks of the thrust sheet were eroded away but the downfaulted masses
were preserved, and appear to be out of place in the northern facies
unless thrusting of great magnitude is postulated. The age of the graben
faulting is not given by Wassail, and it is not known how it fits in the
Tertiary chronology of Butterlin, given below.

The serpentine is believed to be tabular, associated with layered gabbro,
and carried northward in the thrusting. Others have proposed that the
basic plutons were intruded at about the same time as the acidic plutons.
Butterlin (1956) suggests that the intrusions occurred near the close of
Early Cretaceous time, but evidently Wassail considers the acidic plutons
much later than the basic.

Early Eocene time saw much flooding and probably the development
of deep water in the west. Close to the mountains conglomerates, sand-
stones, and shales accumulated, but at a distance marls.

Orogeny then occurred at the close of the early Eocene, probably con-
tinuing into mid-Eocene (Butterlin, 1956). The effects are most con-

Fig. 42.4. Cross sections of the Greater and Lesser Antilles. The upper two sections are in Cuba,
after Thayer and Guild, 1947. The third and fourth sections are across Antigua and St. Bartholo-
mew islands, after Christman, 1950. The fifth section is across Tobago, after Maxwell, 1948. The
schists, volcanics, and intrusive rocks are regarded by him as Cretaceous. Lowest section is a
hypothetical interpretation by Senn (1940), across the arc of the Lesser Antilles showing the
conditions of sedimentation during the Oligocene epoch. The north to south thrusting in Camaguey
is now doubted.

Woterlo,d Tuff ■Serpentine

FT TMRU3TIN6 IN ORIENTE PROVI NC E , C U 3A

THRUSTING IN CAMAOUEY CHROMITE DISTRICT, CUBA

Sw Tuffs Mainly ondesite por

jbreccia 5 ^<~>c
crgg/om.

Middle Oligocene hl

Tuffs, braccios, en,* Is fintiquo /j

' LMiiii 1 ANTIGUA

ST. BARTHOLOMEW

Parlotuv/er r~m

Main Ridae fm

Tobago vo/conic

North Coast Schist Group
TOBAGO °

1 L/ltramaf/c roc/i
2

Volcanic arc

Plutonic ore
■ith reef limestones
Volcanic dust-

_.. Intermediate basin
Caribbean Sea f j^\ with tuff deposit/on Atlantic Ocean \Borbaios

50 Km

GENERALIZED SECTION ACROSS LESSER ANTILLES
IN OLIGOCENE Tine

676

STRUCTURAL GEOLOGY OF NORTH AMERICA

spicuous in the western and central provinces. Overthrusting is described
in several places along the northern margin of the island (Fig. 42.4) and
presumably occurred at this time. All thrusting is now believed to be
toward the north. It is not known whether or not this mid-Eocene thrust-
ing of Butterlin is the same as the very Late Cretaceous thrusting of
Wassail.

Orogenic movements then probably spread to the southern part of the
Province of Oriente. ... It should be added the upper Eocene begins with
conglomerates. Recurrence of volcanic activity is shown by the presence of
tuffs and of subsidence basalt dykes, thereupon marls and limestones (San Luis
formation) forming a deposit. In the Guantanamo Basin are found thick shales
of the same period (Guantanamo formation). At the end of the middle Eocene,
in the central and western areas, the sea appears again depositing at first
conglomerates and sandstones and, afterwards, limestones and marls (Loma
Candela formation). During the upper Eocene, the sea still progresses and if
littoral series (conglomerates and marls of Jabaco formation) are found, deep
deposits prevail (marls of the Jicotea group of the Jabaco formation and pelagic
marls of the Consuelo formation (Butterlin, 1956).

Hispaniola

Physiography. Haiti and the Dominican Republic make up the island
of Hispaniola, which is the second largest of the Antilles. It is about 400
miles long and in its widest part 160 miles. The greater part of the island
is ruggedly mountainous, with three or more of the clearly defined north-
ern ranges trending N 60° W. The axial or Sierra Central reaches an alti-
tude of 10,249 feet. This is the highest peak in the West Indies.

North of Hispaniola is a narrow submarine trough with a general depth
of 12,000 feet, and beyond this is the shallow platform of the Bahama
Islands. See Fig. 42.1. Eastward the trough leads into still deeper water,
the Puerto Rico trench. South of Hispaniola the narrow shelf soon drops
off into the deep water of the Caribbean Sea. Cape Beata is a southward
projecting peninsula which continues as a submarine relief feature, the
Beata ridge, into the Caribbean basin, and divides it fairly well into
eastern and western parts.

The intermontane valleys are thought by some to be of fault origin.
This is especially true of the Cul de Sac and the Basin of Enriquillo.

Stratigraphy and Structure. The oldest rocks of Hispaniola are meta-

morphic and igneous rocks which according to the present literature make
up the axis of the Cordillera Central and a large part of the northeastern
peninsula of Samana. See Fig. 42.5. Greenstones and amphibolites also
occur in the northern part of the island and may be a part of the ancient
complex. The quartz diorite is said to be of batholithic proportions.

A new study of the complex has been made by Carl Bowin and he
reports on it in a letter to the writer as follows:

Metamorphic rocks occur in central Dominican Republic at the eastern end
of the Cordillera Central and continue westward along the northern flank of
the Cordillera Central. These metamorphic rocks are probably of early Lower
Cretaceous or pre-Cretaceous age although direct evidence as yet only proves
a pre-Tertiary age. Thus in central Dominican Bepublic the oldest rocks do not
form the core of the Cordillera Central (as would be concluded from previous
reports), but flank the high mountains on the east and north. Towards Haiti,
however, the metamorphic rocks may make up the high mountains of the
Cordillera Central.

Schistose limestone and quartz-calcite-chlorite-muscovite schists of unknown
age are found on Samana Peninsula. The foliation in these metamorphics is
reported to trend east-west. Metamorphic rocks are known in the basement
rocks that crop out near Puerto Plata on the north coast. However, the litholo-
gies present and their relations are but poorly known. Pre-Tertiary (?) argillites
are reported to occur on the south flank of the Cordillera Central and on the
south slope of the Cordillera Septentrional, but the grade of metamorphism
represented, if any, is unknown.

A large serpentinized periodotite mass occurs in the metamorphic belt in the
central part of the country. The intrusive extends northwestward from north
of Ciudad Trujillo for a distance of 95 kilometers. A few small peridotite masses
are found in the metamorphics along the north flank of the Cordillera Central.
These appear to be the westward continuation of the large peridotite mass in
central Dominican Republic. Another serpentinized peridotite intrusive, trending
N 75° W, has been traced for 80 kilometers along the north coast. A few small
bodies of peridotite occur in the eastern portion of the island.

The most detailed work on the pre-Tertiary rocks of the island of Hispaniola
has been carried out in central Dominican Bepublic. Here the metamorphic
belt trends NW-SE and consists of primarily epidote amphibolite and schistose
siricitic quartz keratophyre. The epidote ampribolites are intruded by sev-
eral plutons of leucocratic muscovite tonalite and two plutons of gabbro.
Both igneous types are probably of early Lower Cretaceous or pre-Cretaceous
age.

The amphibolitic rocks are in fault contact with Upper Cretaceous (Ceno-
manian to Maestrichtian) unmetamorphosed volcanic rocks to the west. These
Upper Cretaceous volcanic rocks make up the high mountains of the eastern

ANTILLEAN-CARIBBEAN REGION

877

<s^

-J

NAUTICAL MILES

CENOZOIC SEDIMENT- ! • ! >
ARY ROCKS

QUARTZ DIORITE, CRETACEOUS

BASAL COMPLEX. METAMORPHIC
AND IGNEOUS ROCKS

Fig. 42.5. Simplified geologic and terrane map of Hispaniola (Haiti on west and Dominican Republic on
east). Geology after Butterlin (1956) and Bowin (unpublished). Terrane from USAF Aeronautical charts.
Cretaceous rocks considered mostly Upper Cretaceous by Bowin.

Cordillera Central. They are intruded by hornblende tonalite plutons, at least
one of which is of batholithic dimensions. Several plutons and batholiths of
hornblende tonalite intrude the metamorphics along the northern flank of the
Cordillera Central. The hornblende tonalites are probably all of one general
period of intrusion. Cobbles of hornblende tonalite are found in uppermost
Upper Eocene conglomerate a short distance north of the Cordillera Central.
Thus the hornblende tonalites are considered to be of post-Campanian, pre-

Oligocene age. They are probably related to the strong late Eocene deformation
that thrust the metamorphic belt northeastward over unmetamorphosed, dated.
Lower Cretaceous to Middle Eocene, volcanic and limestone rocks.

The unmetamorphosed rocks lying to the northeast of the overthrust meta-
morphic belt in central Dominican Republic consists of Lower Cretaceous vol-
canic rocks overlain by Lower Cretaceous limestone. The Lower Cretaceous
section is unconformably overlain by Upper Cretaceous limestone followed

678

STRUCTURAL GEOLOGY OF NORTH AMERICA

apparently conformably by Paleocene, and Lower and Middle Eocene tuff with
lenses of algal limestone.

The eastern part of the Dominican Republic is composed predominandy of
fine-grained tuff and interbedded dark gray limestone. These rocks are as yet
undated, but are probably of Upper Cretaceous age. Upper Cretaceous sedi-
ments are reported from a few localities on both the north and south flanks of
the Cordillera Central.

North and south of the Cordillera Central Eocene sections are dominated by
limestone. However, in the Cordillera Central and east of it, clastic sediments
and tuffs were deposited in earliest Tertiary. Thus in the earliest Tertiary there
was a zone of volcanism and uplift in central Dominican Republic. This zone
may trend WNW into Haiti parallel to the trend of the Cordillera Central.

The Oligocene and younger sections of the Dominican Republic are domi-
nated by clastic sediments and reflect a complex history of uplift and basin
development.

According to Rutterlin (1956) the Lower Cretaceous Tuff series of
Cuba spreads eastward through Hispaniola, especially in the northern
and central regions where andesitic tuffs, basalts, and andesites accumu-
lated.

In the peninsula of southern Haiti thick pelagic limestones accumulated
whose fauna bespeaks a Senonian age. This is the Macaya formation. Thick
and widespread underwater basalt flows occurred just before and after
the limestone depositing epoch, and probably extended westward to
Jamaica.

According to Rutterlin again, sea flooding and deposition of sediments
were resumed in Paleocene time and then lasted until mid-Eocene. Con-
glomerates, sandy shales, calcareous sandstones, and clastic limestones,
the Marigot formation, started the sequence, but these give way in places
during early and mid-Eocene time to chalky limestones. In the north-
western peninsula a trough spread to Cuba, and in it thick basaltic and
andesitic tuffs accumulated which alternate with thin calcareous layers
(Perodin formation). In other areas crystalline or detrital limestones
resembling the yellow limestone of Jamaica were deposited and make up
the Plaisance and Hidalgo limestones.

In the northern and north-central regions a new disturbance set in.
Folding was accompanied by dolerite and granodiorite intrusions. It is
impossible to distinguish the folds of this orogeny from the older ones
(Rutterlin, 1956).

Limestone deposition continued until late Oligocene when renewed
orogenic movements set in to last until the Quaternary. From this time
on throughout the Tertiary flysch and molasse type sediments accumu-
lated. Tight folding seems the dominant structure with overturning both
north and south (Rutterlin, 1956).

Considerable attention has been given the longitudinal valleys or
basins between the main Sierras. Some, like Rutterlin, favor the view that
the mountains continued to rise during the late Tertiary and that a gravity
flow type of structure developed toward the basins. Woodring et al.
(1924) describe the bounding faults as overthrusts. Rich (1956) treats
the Cul de Sac as produced by recent upfaulting and upbowing of the
bounding mountain block. Rucher (1950) postulates a good deal of
strike slip along bounding faults as sympathetic fractures to eastward
movement of the great Caribbean block. Hess and Maxwell ( 1953 ) show
the southern peninsula and the Sierra de Rahoruca to have moved many
miles from a west-lying position to its present position, and hence a
wrench fault of great magnitude to lie along the south side of the Cul de
Sac and the Rasin of Enriquillo. Several have related the graben-like
depressions to the Cayman trench which projects to the Cul de Sac.

Puerto Rico

Physiography. The island of Puerto Rico is roughly rectangular and is
about 35 miles wide and 105 miles long. See Fig. 42.6. Its highest peak is
3750 feet above sea level, whereas the Puerto Rico trench immediately
north of the island is 27,972 feet deep. The absolute relief between the
two is thus 31,700 feet. The plateau-like ridge upon which Puerto Rico
occurs also supports the Virgin Islands to the east. The slope into the
trough is in the proportion of one mile vertical to thirteen horizontal. See
Fig. 42.1.

To the south of the Puerto Rico and Virgin Island platform the bottom
slopes steeply and, within 55 miles, is 17,000 feet deep. This is the site of
a submarine trench that leads northeastward to the Anegada Passage. The
bottom of the trench is generally 15,000 feet deep but rises to about 3850
feet below sea level at the summit or Passage.

To the west, Puerto Rico is separated from Hispaniola by the Mona

ANTILLEAN-CARIBBEAN REGION

879

Passage, where the water ranges from 1200 to 3760 feet in depth.

The central part of Puerto Rico is a rugged, east-west-trending moun-
tainous mass of the basement complex rocks, and averages about 2000
feet in height.

A coastal plain is particularly prominent along the north side, and a
more limited one occurs along the south side. These have been studied
in detail by Zapp et al. (1948). A rugged foothills belt flanks the south
side of the central Cordillera.

Geology. Kaye (1957) notes two major structural and stratigraphic
rock units in Puerto Rico: the older complex, ranging in known age from
Late Cretaceous to late Paleocene or early Eocene, and the middle
Tertiary sequence, ranging from late Oligocene possibly to late Miocene.
The former rocks are eugeosynclinal in character and the latter non-
volcanic, made up dominantly of calcareous marine sediments. The middle
Tertiary crops out on the north and south sides of the island and in struc-
tural troughs on the west coast. On the north coast the beds dip gently
to the north, and, except for slight terracings and a flexure at the north-
western corner of the island, are not folded. The middle Tertiary sequence
on the south side of the island is somewhat folded. Seismic-reflection
studies of the north coast indicate, however, a pronounced northward
thickening, possibly some folding, and unconformities at depth. Uncon-
formities which may be local have also been noted at several places on
the surface.

Berryhill et al. (1960) have detailed the Upper Cretaceous stratigraphy
and facies changes, which may be summarized as follows:

Rocks of Late Cretaceous age (Turonian to Maestrichitan) in Puerto Rico
are of three types: (1) primary volcanic rocks, including tuffs, tuff breccias,
and lavas; (2) intermixed pyroclastic and epiclastic rocks, including volcanic
conglomerates, volcanic sandstones, and volcanic siltstones; and (3) limestones,
most of which were formed as reefs around volcanic islands. These rocks,
which have a maximum thickness of more than 20,000 feet, crop out along the
crest and flanks of a complexly faulted, northwestward-trending anticlinorium
that forms the mountainous core of Puerto Rico.

The major aspect of the structure of the eastern part of the island is
anticlinal which Berryhill et al. believe is due to doming of the strata
during intrusion of a batholith in early Tertiary time. See map, Fig. 42.6.

s j* j j Mm

MM Ml

RAHOOIORITC

obotjl, Early

Fig. 42.6. Geologic map of Puerto Rico. Compiled from Kaye (1957), Berryhi
Glover (I960), and Mattson (1960). Hachured faults indicate graben.

Briggs and

Complex faulting that accompanied the batholithic intrusion helped to ac-
centuate the anticlinal structure hut in some places modified it. The regional
trend of the main faults and also many of the subsidiary faults is west-northwest,
but some of the subsidiary faults diverge from that general pattern.

Two faults of regional significance are recognized in eastern Puerto Rico.
One crosses die northern part of the island, and the other traverses the south-
central part.

Movement along the northern fault appears to have been largely transcurrent.
The crustal block north of the fault apparently has moved eastward relative to
the block south of the fault.

The subsidiarv, northwestward-trending faults on the north formed as tears
along the main fault. Movement along most of these subsidiary faults appears
to have been both horizontal and vertical because of rotational movement <>f
the blocks formed by the faults. Associated with the northern fault are two
grabens. ... A third, smaller graben, . . . lies south of the northern fault. The
stratigraphic displacement at the southeastern end of the largest of these three
grabens is approximately 6,000 feet.

The second regional fault, which trends west-northwestward across the
southern part of the island, appears to be in part a transcurrent fault and in
part a high-angle reverse fault which dips about 70° toward the southwest
The stratigraphic displacement along this fault, based on good stratigraphic
control is about 12.000 feet.

The pattern of faulting is related to the crude west-northwestward alignment

680

STRUCTURAL GEOLOGY OF NORTH AMERICA

of the plutonic intrusive bodies which form a belt across the island. Moreover,
the northern major fault roughly coincides with the belt of pillow lavas and
volcanic breccias that were extruded during Robles (Late Cretaceous) time.
That belt of volcanism may have been located along a regional line of weakness
and the younger plutonic intrusives may have moved upward in part along this
same general zone.

Although the general anticlinal structure of eastern Puerto Rico is probably
a result of doming by a batholith, several localized anticlines and synclines
have been formed by the movement of fault blocks (Fig. 42.6). The largest of
these secondary structures is the northeastward-plunging, faulted anticline near
the northeastern corner of the island. The Luquillo Range is the northwest limb
of this breached and faulted fold. This structure probably was formed by com-
pression from the northwest as the crustal block north of the transcurrent fault
moved eastward. Tight folding is localized near some of the faults but is not
extensively developed in eastern Puerto Rico.

Ry comparing Kaye's and Rerryhill's analysis with that of Rutterlin
( chart, Fig. 42.3 ) it will be seen that Rutterlin suggests older rocks than
they found on the island, and that late Eocene and early Oligocene forma-
tions are present whereas they indicate a hiatus for this interval.

Rutterlin also points out that broad arching with an east-west axis was
the dominant part of the mid-Miocene disturbance.

Rerryhill (1959) has elaborated on the transcurrent faulting to the
effect that the two principal faults or sets of faults divide the island into
three blocks, with the northeastern and southwestern blocks having
moved toward the southeast and the central block toward the northwest.
This is presumed to reflect eastward movement of the Caribbean block.
He assigns the major faults to an Eocene age, whereas Kaye recognized
the many "block-faults" as late Pliocene and early Pleistocene.

Isla Mona and the Mona Passage

Isla Mona, 21 square miles in area, and Isla Monito, less than one
quarter square mile, are situated in the Mona Passage between Puerto
Rico and Hispaniola. Isla Mona is a limestone tableland bounded by steep
to vertical cliffs except for a narrow coastal terrace about its southern
perimeter ( Kaye, 1959 ) . The Isla Mona limestone forms most of the mass
of both islands and is probably early or middle Miocene. Dips up to 3/2
degrees are visible in the cliffs. In places a thin cavernous lime-
stone, the Lirio, overlies the Isla Mona, and in one place a small angu-

lar unconformity is visible. The Lirio is Pliocene or Pleistocene in age.
The great purity of the Isla Mona limestone indicates that it was de-
posited in an oceanic reef environment far from land, and from this it is
deduced that the Mona Passage was in existence in Miocene time ( Kaye,
1959).

Jamaica

Physiography. Jamaica measures 144 miles from east to west and has
a greatest width of 49 miles. It is very mountainous, with about one-half
of its area 1000 feet above sea level and much of it over 2000 feet. The
principal range, called the Rlue Mountains, occupies an axial position at
the east end of the island, and has a sharp crest and numerous, generally
cloud-wrapped peaks, the highest of which is 7520 feet above sea level.
From the sea on the north, the land rises in gentle hills to the higher
country, but on the south high cliffs and abrupt precipices mark the shore-
line. See Fig. 42.7.

The relief of Jamaica is of four major types: (1) the interior mountain
ranges, constituting the nucleus of the island; (2) an elevated and dis-
sected, arched and karsted, white limestone plateau which surrounds the
interior mountains and ends abruptly toward the sea, occupying in all
fully four-fifths of the total area; (3) the coastal bluffs or back coast
border of the seaward margin of the plateau; and (4) a series of low flat
coastal plains between the sea and the back coast border (Schuchert,
1935).

Jamaica is separated from Cuba by 90 miles of water, and the marine
basin between is the Cayman trench, here everywhere more than 15,000
feet deep and directly off Cuba, 21,000 feet deep. The long and narrow
peninsula of Haiti is about 90 miles northeast of Jamaica, and the two
islands are separated by water which has a general depth of over 4000
feet. On the south side of Jamaica lies the Caribbean Sea, whose bottom
sinks to 13,800 feet. From the island to Honduras it is 900 miles, and the
intervening area is mainly shallow water. It is a broad platform on which
the Mosquito, Rosalind, and Pedro banks occur, and which drops off
steeply into the Cayman trench on the north and slopes gently into the
Caribbean on the south.

ANTILLEAN-CARIBBEAN REGION

fvSl

' ~/\>\~/\7

LOWER MIOCENE TO
LOWER EOCENE

UPPER CRETACEOUS

V - VOLCANIC ROCKS
S - SERPENTINE

GRANODIORITE

METAMORPHIC ROCKS

NAUTICAL MILES
25

Fig. 42.7. Geologic map of Jamaica. Shoreline and 2000-foot contour (clotted line) from World Aero-
nautical Chart No. 647. Geology adopted from Butterlin, 1956. Dotted contour line outlines Blue Mountains.

Geology. Jamaica, like the other islands of the Greater Antilles, has
a basal complex in part older than Late Cretaceous. Three cycles of
deposition followed the basal complex, each with a sequence of conglom-
erate, sand, shale, mudstone, calcareous shale, and limestone, and each
separated by an unconformity ( Butterlin, 1956 ) . The third cycle ended in
early Eocene with intense deformation and intrusions. Thrusting has been
noticed and is assigned to this time.

The Yellow limestone of mid-Eocene age was then deposited. White
limestone accumulation continued to mid-Miocene time when block-
faulting occurred. The faults trend generally north-northwest or north-

west, and rejuvenate in places the earlier structures. Extrusions of lavas
also occurred.

During the Pliocene, block faulting continued and raised up the cal-
careous tablelands and in places tilted them. The Cayman Islands across
the trench were possibly connected with Jamaica before the block faulting
(Butterlin, 1956).

Virgin Islands and Anegada Trough

A bank not more than 165 feet deep extends 100 miles eastward from

Puerto Rico like a crescent curving northward. From this bank rise about

682

STRUCTURAL GEOLOGY OF NORTH AMERICA

100 islands, cays, and rocks which are known as the Virgin Islands. The
bank is terminated on the south by the Anegada trough, named from the
passage, previously mentioned. Taber (1922) points out that the south
side of the trough near the island of St. Croix is a great escarpment which
descends 14,130 feet in less than 5 miles, and thus has an average slope of
30 degrees. This he regards as a fault scarp due to vertical movement. As
will be related later, Hess believes it is due chiefly to horizontal move-
ment. On the basis of biogeographic data, Schuchert thinks the Virgin
Islands were joined to St. Croix across the Anegada trough during either
the Miocene or the Pliocene, and that they have been separated due to
block faulting along the Anegada Passage in fairly recent times.

It appears that Puerto Rico, the Virgin Islands, and the island of
St. Croix developed as a unit, and that the geology of the Virgin Is-
lands, if only partially exposed, fits that of Puerto Rico. See chart, Fig.
42.3.

After the igneous activity and deformation of the older series of Puerto
Rico that is also believed to form the foundation of the Virgin Islands bank,
a mountainous upland probably existed. According to Meyerhoff (1927),
fluvial erosion reduced the mountainous upland to an imperfect peneplain
in early Eocene time. The relatively level summits of the upland of Saint
John, 1000 feet above sea level, are a remnant of the old surface. Uplift
in late Eocene time resulted in dissection of the old surface, and all but
the central cores of the present larger islands were reduced to a late
mature of old surface about 800 feet below. Only a few remnants of this
second surface have been preserved, because a second uplift in early
Oligocene time was followed by about 500 feet of downcutting.

The third cycle of erosion formed the lower peneplain of Puerto Rico,
as well as the mature to old surface which extends beneath the coastal
plain on St. Croix and Vieques, and which underlies remnants of the
coastal plain on the submarine platform. Formation of the lower peneplain
was followed by subsidence and deposition of coastal plain sediments in
the middle Tertiary, and during late Tertiary time uplift exposed the
coastal plain marls and limestones to dissection. The Tertiary deposits
collected in the entire area now constituting the submarine platforms of
the islands. Toward the close of the Tertiary, differential movement or

warping caused submergence of the eastern Puerto Rico and Virgin
Islands region, while western Puerto Rico remained elevated.

Bahama Islands

Physiography. The Bahama Islands stretch for 900 miles in a north-
west-southeast direction in front of southern Florida, Cuba, and Haiti, and
include some 29 inhabited islands, 661 keys, and 2387 rocks. The Bahamas
are all very low, flat islands and resemble most the coast and keys of
southern Florida. All the islands, keys, and rocks rise from a platform thai
is roughly triangular, with the narrow base of the triangle on the north-
west. See map of Fig. 42.1. It is bounded on the west by the Florida Chan-
nel, which separates it from Florida by a distance of 50 miles; on the south
by the Bahama Channel, which separates it from Cuba by an equal dis-
tance; and on the east by the Atlantic Ocean. The greater part of the plat-
form is covered by water only 3 or 4 fathoms deep, but in part it emerges
slighty above sea level, forming low islands. Great submarine valleys,
such as the Tongue of the Ocean, Exuma Sound, and the Providence
Channels, form deep indentations in the platform. On the east, the plat-
form drops off abruptly to oceanic depths (2600 fathoms, 15,600 feet).
The extensive shallow banks are remarkable for their white lime oozes.
See Fig. 43.4.

Submarine Canyons. The great submarine valleys, which are mani-
festly a very important character of the Bahama platform, are reviewed as
follows by Hess (1933):

(1) The longitudinal valleys have a general NW-SE trend for the greater
part of their lengths, but short steep cross valleys at right angles to this trend
connect the longitudinal valleys with the ocean.

(2) So far as the information goes, it appears that the valleys slope continu-
ously from the shallowest parts of their upper reaches (720 fathoms, 4,320 feet
below sea level) to the floor of the ocean basin proper (2,500 fathoms, 15,000
feet). The longitudinal valleys have gradients of approximately 15 to 20 feet to
the mile, and apparently have gendy sloping undulating bottoms, from 720
fathoms (4,320 feet) to about 1,000 fathoms (6,000 feet).

(3) The cross valleys have steeper gradients, 100 feet to the mile, from 1,000
fathoms (6,000 feet) to the floor of the ocean at 2,500 fathoms (15,000 feet).
They have the typical V-shaped cross profile of a youthful river valley, and
some have a distinct inner gorge near the center.

ANTILLEAN-CARIBBEAN REGION

883

(4) Where examined, the outer rims of all the valleys rise steeply, perhaps
even as vertical cliffs, from a depth of about 500 fathoms (3,000 feet) to the
edge of the platform.

(5) The Tongue of the Ocean and Exuma Sound Valleys are parallel, and
about 50 miles apart, but the Tongue of the Ocean slopes continuously north-
west from its shallowest point at a depth of about 720 fathoms (4,320 feet),
whereas Exuma Sound, starting from a similar depth, slopes continuously in
the opposite direction, southeast.

(6) Where "tributaries" meet the "main stream" they appear to do so at the
same level or "at grade," and where the valleys enter the ocean basin proper,
they do so at the same level.

Andros Island Deep Test. A deep test well was drilled on Andros
Island of the Bahamas to a depth of 14,585 feet, and enhances our knowl-
edge of this little-known region immensely. The following details were
given orally by Maria Spencer at the St. Louis meetings of the American

Log of Andros Island Deep Test

Depth, Feet

Recent and Miocene (?)

Limestone as at surface. Corals and

bryozoans

0-530

Limestone, dolomitized

530-1625

Coquina

1625-2200

Eocene

Coquina of microfossils

2200-2640

Alternating limestone and dolom

ite

2640-4640

Paleocene (?)

Dolomite, fine-grained

4640-6220

Dolomite and chalky limestone

6630-7590

Paleocene or Upper Cretaceous

Dolomite, brown

7990-8760

Upper Cretaceous

Dolomite, tan, granular

8760-9760

Dolomite, coarsely crystalline, cavernous

9760-10,035

Limestone, part brecciated, part

cha

Iky, cemented with

brown dolomite

10,036-10,660

Cavernous

10,660-10,709

Dolomite, fine-grained

10,709-11,940

Limestone, creamy white, chalky

11,940-12,480

Lower Cretaceous

Dolomite, crystalline and porous

Sunnyland zone in Florida

12,480-13,710

Bottomed

14,587

Association of Petroleum Geologists, 1949, and taken down as notes by
the writer.

Spencer commented that the Upper Cretaceous section has the same
thickness as that of Florida, but it consists of dolomite and limestone,
whereas the Florida section is nearly all limestone. The base of the Lower
Cretaceous was not reached in the Bahama test, but the 2100 feet known
consists mostly of crystalline dolomite, whereas the Florida section con-
sists of limestone, anhydrite, and dolomite.

Reef Building. Heretofore it could be said only that reef limestones ait
prominent in many places on the Bahama Islands and have been studied
below sea level. A bore hole 395 feet deep on New Providence Island
passed through Pleistocene and into Miocene reef material (Hess, 1933 ).
The calcareous material consisted mostly of calcite to a depth of 165 feet,
and below it was mostly dolomite. The porosity decreased to 5 percent at
the bottom of the hole. Hess recognizes nearly everywhere almost clifflike
dropoffs of the submarine canyon walls, ridges, and platforms, from the
surface down to a depth of 4000 feet, and believes this feature could not
be accounted for by erosion, but on the contrary to reef upbuilding. He
finds no geophysical evidence to dispute a conclusion that the reef ma-
terial may be 4000 feet thick on the Bahamas, and believes it may include
the entire Cenozoic section if not also the Upper Cretaceous. His conclu-
sion in theory if not in magnitude proved correct when the deep test
described above was drilled. It is concluded that most of the Bahama
platform area was a site of subsidence and deposition during the late
Jurassic, early Cretaceous, late Cretaceous, and parts of the Cenozoic.
The foundations of the Bahamas have been regarded as volcanic by
some; but this, in light of present stratigraphic and tectonic data, is onl)
possible below a depth of 15,000 feet.

LESSER ANTILLES

Divisions

The Lesser Antilles, also known as the Caribbees, are an island festoon
that extends from the Anegada Passage on the north 430 miles to the
Island of Grenada on the south. See maps. Fig. 42.1 and 42. S. Several

684

STRUCTURAL GEOLOGY OF NORTH AMERICA

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divisions may be recognized if the submarine features are considered.
First, on the west is the Aves Rank that ends on the north at its only
emergence, Aves Island. It is slightly convex eastward. The next division
is the Grenada basin, a broad and not very deep depression, which bounds
the island festoon of the typical Caribbees. These islands are Pliocene-
Pleistocene volcanic cones, and they form the inner or younger Caribbees.
Outside the volcanic arc and at the north are the Limestone Caribbees,
another row of islands which seem to merge with the younger volcanic
islands. Outside of the Limestone Caribbees is the deep, narrow Brownson
trough, which shallows southward and ends in the wide Tobago trough.
East of the Tobago trench is the Trinidad-Barbado element that extends
northward in the form of the Barbado submarine ridge outside the Puerto
Rico trench.

The volcanic or younger Caribbees begin on the north with Saba ( 2820
feet high) and extend in succession through St. Enstatius (1950 feet), St.
Christopher (4314 feet), Nevis (3596 feet), Redonda (1000 feet), Mont-
serrat (3002 feet), Basse Terre of Guadeloupe (4869 feet), Isles des
Saintes (1036 feet), Dominica (4747 feet), Martinique (4428 feet), St.
Lucia (3145 feet), St. Vincent (4048 feet), the Grenadines (a series of
rocky islands on a narrow bank nearly 100 miles long), and finally
Grenada (2749 feet). A considerable number of these islands have well-
preserved cones. Some volcanoes are still active, notably La Soufriere
on St. Vincent, which erupted violently in 1902 and killed 2000 people,
and Mont Pelee on Martinique.

The Limestone Caribbees are characterized by limestones and pyro-
clastics into which various hypabyssal rocks have been intruded; these are
overlain by younger marine limestones. To the Limestone Caribbees
belong Sombrero, Anguilla, St. Martin, St. Bartholomew ( Barthelemy),
Barbuda, Antigua, Grande Terre of Guadeloupe, Desirade, and Marie
Galante. Woodring (1928), Senn (1940), and Maxwell (1948) have sum-
marized the geological history of this group.

Fig. 42.8. The Lesser Antilles showing the Limestone Caribbees (also called older and outer)
the Volcanic Caribbees (also called younger and inner).

Outer Limestone Caribbees

The following summary is principally from a report by Maxwell ( 1948).
In the outermost islands of the Limestone Caribbees, Sombrero and

ANTILLEAN-CARIBBEAN REGION

685

Barbuda, only Quaternary limestone is exposed. Volcanic rocks crop out
on Anguilla, St. Martin, and St. Bartholomew. Oligocene limestone rests
unconformably on the basement rocks of Anguilla, and Eocene or Oligo-
cene limestone covers the volcanics on St. Martin. The basement of St.
Martin consists of well-stratified, strongly folded and metamorphosed tuff,
tuff-breccia, and somewhat crystalline limestone, intruded by a quartz
diorite-pyroxene diorite complex. According to G. A. F. Molengraaff
(1931) the sedimentary material of the basement may be of Cretaceous
age, but Christman (1953), as a result of work on St. Martin, St.
Bartholomew, and Antigua, states that "there is apparently no Cretaceous
basement in the Lesser Antilles." Miocene limestones are nearly horizontal

i and have been deformed to a less degree than the Oligocene and upper
Eocene.

I The oldest rocks of St. Bartholomew consist of volcanic debris and an
overlying upper Eocene limestone; both are intruded by an andesite
porphyry. Also found intruding the Eocene limestone beds are a volcanic

< agglomerate and a dacite porphyry ( Christman, 1953 ) . See cross section
of Fig. 42.4.

Antigua and Desirade likewise belong in the outer islands of the Lime-
stone Caribbees. Both have volcanic basement rocks. In Antigua, gently
1 dipping tuffs in the central plain are overlain conformably by the Antigua
j limestone of middle Oligocene age. The tuffs become coarser to the south-
iwest. At Crab Hill, Christian Valley, and St. Luke's Quarry, intrusive
andesite porphyries cut the series (Christman, 1953). See section in Fig.
,42.4. Desirade possesses a basement of intrusive granodiorite, with con-
temporaneous andesite and rhyolite flows. Miocene limestone uncon-
formably overlies the basement.

Grande Terre and Marie Galante are the southernmost, and also the
innermost, of the Limestone Caribbees. The latter is covered by a cap of
recent limestone (Woodring, 1928). Grande Terre, however, has a base-
ment of granodiorite which is overlain unconformably by lower Miocene
tuffs and limestone (Senn, 1940).

To summarize, the outer islands of the Limestone Caribbees are char-
acterized by a basement of lava flows and coarse volcanic debris of late
'Eocene age, or younger, and andesitic to dioritic rocks of post-late Eocene

age which intrude the volcanics. Oligocene and younger beds .ire mostly
limestone, and volcanic debris is fine-grained (as the Central Plain tuff ol
Antigua), where present. Apparently, these islands were active volcanic
centers in late Eocene and Oligocene time and have since received vol-
canic debris only sporadically and from a distance. They have not been
disturbed much by crustal deformation in post-Oligocene time.

Inner Volcanic Arc

The inner arc of the Lesser Antilles, stretching from Saba to Grenada
is characterized by Recent or subrecent volcanic activity. Tuffs of Oligo-
cene age on Martinique (Senn, 1940) and Carriacou (Trechman, 1935)
represents the oldest beds identified in the inner arc. Apparently volcanic
activity started here about in early Oligocene time and continued with
few interruptions to the present. As in the older (pre-Oligocene) vol-
canics of the Greater and Lesser Antilles, andesites predominate, with
basalts and dacites also present. The andesites and basalts of the more-
recent volcanoes contain hypersthene as a common constituent, whereas
the mineral seems to be extremely rare in the pre-Oligocene volcanics.
The significance of this mineralogic variation is not apparent.

The volcanoes do not rest on the crest of the swell toward the south;
there they are found some 30 to 40 miles west of the crest. At the north
end of the island arc, however, they are approximately at the crest, and it
happens that here the islands are made up largely of sedimentary rocks.

According to Hess (1938), sonic soundings show a series of peaks on
the western flank of Aves swell, parallel to tire Lesser Antilles arc and
250 kilometers west of it. Profiles across the peaks strongly suggest sub-
merged volcanoes. The lack of seismic activity along the greater part of
the Aves swell in the vicinity of the peaks suggests that if they are vol-
canoes, they are extinct.

Margarita and the Dutch Leeward Islands

The following is abstracted from Maxwell's report (1948). The north-
ern part of Margarita is composed of paraschists intruded by quartz
diorite and serpentinized peridotite. A zone of slightly metamorphosed
sediments lies south of the schist area, and unmctaniorphosed sediments of

686

STRUCTURAL GEOLOGY OF NORTH AMERICA

Late Cretaceous age are folded into a syncline along the south coast. Up-
per Miocene sediments lie unconformably on the lower Tertiary-Creta-
ceous section. In Margarita, as in Tobago, the foliation in the schist strikes
slightly north of east and dips steeply to the southeast. Locally, the peri-
dotite shows relatively low-temperature hydrothermal alteration, prob-
ably related to the diorite intrusions, though this is the only evidence
bearing on the relative age of the peridotite and diorite. The diorite has
suffered strong shearing, as in Tobago. The period of major deformation
was post-Middle Cretaceous and pre-Maestrichtian. Detrital grains of
chromite and enstatite in Middle Eocene sands prove that the ultramafic
mass had been exposed to erosion by that time and hence is pre-Middle
Eocene in age. Sometime between the middle Eocene and upper Miocene,
a period of moderate deformation folded the Cretaceous and lower
Tertiary sediments into a syncline, with its axis approximately parallel
to the foliation in the schist.

The Dutch Leeward Islands, Aruba, Curacao, and Ronaire, are vol-
canic in character, comparable with the Greater Antilles and the outer
islands of the Lesser Antilles. A deformed basement of intrusive and ex-
trusive gabbroic and dioritic rocks with intercalated radiolarian cherts,
limestone, and graywacke is present on all three islands. Quartz-augite
diorite in Curacao (G. J. H. Molengraaff, 1931) and quartz diorite in
Aruba ( Westermann, 1931) intrude the basement rocks. On Ronaire,
limestone of latest Cretaceous age unconformably overlies the basement
rocks, which likewise are believed to belong to the Upper Cretaceous
(Pijpers, 1933). On Curacao, a series of coarse detrital sediments overlies
the limestone and is folded with it. Upper Eocene limestone is not in-
volved in the folding. In the Dutch Leeward Islands then, a basement
of volcanic rocks was deformed in pre-latest Cretaceous time, uncon-
formably overlain by Upper Cretaceous-Eocene ( ? ) sediments, and again
folded prior to late Eocene.

Barbados-Trinidad Belt

Barbados. The geology of Rarbados has been discussed in detail by
Senn (1940). Clastics of early and middle Eocene age are the oldest beds
exposed. These beds were uplifted, strongly folded and thrust-faulted and

eroded, then covered by a thick series of mud flows. Upon the strongly
folded clastic sediments and mud flows were deposited the Oceanic beds,
a considerable thickness of upper Eocene chalk, radiolarian earth, and
tuff, which Senn and earlier writers interpret as a deep-sea deposit. Senn
shows that the area moved down very suddenly into a region of deeper-
water sedimentation, a circumstance explained by great downbuckling,
to be considered later in this chapter. Senn also points out that radio-
larian earth similar to that of the Oceanic formation occurs in the upper
Eocene of northern Cuba, and that the radiolarian earths of Rarbados and
Cuba probably were deposited in a late Eocene equivalent of the Puerto
Rico trench. See Fig. 42.4, bottom section.

Deformation apparently continued in Rarbados during the deposition
of the Oceanics, for these beds are also folded and faulted, though much
less so than the older formations. The Oceanic beds, in turn, were uplifted
and eroded, then submerged and covered by upper Oligocene-Miocene
marls. There is evidence of Miocene-Pliocene folding and post-Pleisto-
cene uplift and fracturing. The above review was taken from Maxwell,
1948.

Tobago. The northern part of Tobago Island is made up of isoclinally
folded schists, phyllites, predominantly metavolcanic in origin. South of
the schists lies a belt of igneous rocks, including ultramafic and dioritic
intrusives and andesitic and basaltic volcanics. A low, coral-covered plain
forms the southwest tip of the island.

At least two periods of diastrophism are indicated. The earliest, prob-
ably of Late Cretaceous age, produced the schists. Intense igneous activity
followed this diastrophic period; then the igneous rocks were themselves
strongly sheared by diastrophic movements considered to be of late Eo-
cene age.

Undeformed, fossiliferous upper Miocene-Pliocene sands and clays lie
unconformably on volcanic rocks near the present coast line, and Quater-
nary coral limestone overlaps both igneous rocks and late Tertiary sedi-
ments. The above review was taken from Maxwell, 1948.

Trinidad. The middle Eocene clastic sedimentation, the late Eocene
deformation, and the Miocene-Pliocene period of folding of Barbados are
paralleled by a similar sequence of events in Trinidad. In addition, Juras-

ANTILLEAN-CARIBBEAN REGION

687

sic and Cretaceous rocks crop out in Trinidad, giving insight into the
pre-Eocene history of the southern West Indian region. Jurassic rocks are
found only in the North Range. According to Senn ( 1940 ) , they consist
mainly of phyllites with abundant lenses and veins of quartz and inter-
bedded crystalline limestone. Presumably they are equivalent to the lower
Caribbean series of Waring (1926), which he describes as calcareous and
carbonaceous schists and quartzitic grits. Associated with the Jurassic
rocks is a younger system of less metamorphosed dark limestones, grits,
and slightly metamorphosed shales, from which Trechman ( 1935 ) col-
lected fossils of late Cretaceous age. The North Range schists have been
tightly folded, in general, showing axial-plane foliation (Waring, 1926).
They strike a few degrees north of east and are overturned toward the
north.

In a small area near the village of San Souci, igneous rock identified as
"granophyr" intrudes dark, calcareous schists of the lower Caribbean
series (Waring, 1926). Both massive and pyroclastic igneous rocks are
present, and on Manantial hill, dark, calcareous schist seems to be in-
folded into the igneous mass. A fine-grained, holocrystalline augite andes-
ite with a diabasic texture occurs in the San Souci area.

The North Range seems to have been involved in at least two periods of
folding, one in post-Jurassic and one in post-Late Cretaceous time. Igneous
rocks at San Souci were intruded and extruded between the periods of
deformation; otherwise there is no evidence of igneous activity. Quite
probably the earlier folding, involving Jurassic and possibly Lower Creta-
ceous sediments, took place in Late Cretaceous time, since this is a period
of major deformation in the Coast Range of Venezuela. A slight amount of
volcanic activity followed this deformation in Trinidad, then uppermost
Cretaceous sediments were laid down and subsequently folded, probably
during the strong middle Eocene deformation.

The North Range schists of Trinidad resemble the Tobago North Coast
schists in degree of metamorphism, in the fact that both series are iso-
clinally folded and overturned to the north, and in that both have as-
sociated younger andesitic volcanics. On the other hand, Tobago lacks
the limestone, graphitic schists, and coarse grits of the North Range; and
Trinidad has no counterpart of the metavolcanics comprising a major part

of the Tobago schists. The small amount of igneous activity in Trinidad
is likewise in marked contrast to the predominantly igneous character of
Tobago. The above review was taken from Maxwell, 1948.

Northern Venezuela. Schists identical with those of the North Range
of Trinidad appear in the Serrania de la Costa Oriental of Venezuela.
East of Caracas, fossils of probable latest Jurassic age were found in the
older beds of the Serrania de la Costa Occidental (Wolcott, 1943). Pre-
sumably the overlying schists are of Cretaceous age, as in Trinidad. The
first great deformation in northern Venezuela occurred in the Cretaceous,
probably in pre-late Senonian and certainly in pre-Maestrichtian time.
The second deformation came about at the beginning of the upper Eo-
cene, at which time granitic rocks were intruded in the Coast Range, and
the Cretaceous and Tertiary sediments were metamorphosed (Hedber'i.
1937). In the Miocene-Pliocene deformation, the Serrania del Interior was
formed, involving Cretaceous and Tertiary beds in tight folding and
southward overthrusting.

A belt of small seq^entine intrusives occurs in the Serrania de la Costa
near Caracas, and several larger peridotite bodies intrude Cretaceous
sedimentary rocks south of the Serrania del Interior. Hence the ultra -
mafics are late Mesozoic or younger in age.

PUERTO RICO TRENCH AND GRAVITY ANOMALIES

Submarine Topography

North of Hispaniola, Puerto Rico, and the Virgin Islands bank is a nar-
row trough of great depth. Its bottom exceeds 24,000 feet from the west
end of Hispaniola eastward to a point off the island of Barbuda, a distance
of about 500 miles. For a distance of 200 miles north of Puerto Rico, the
trough is over 27,000 feet deep, with a greatest recorded depth of 2S.6S0
feet. Southward from a point off Barbuda, the trough follows the arc of
the volcanic Caribbees but begins to shallow, and finally it ends in the
Tobago trough, a fairly wide basin betwen the volcanic arc on the west
and the island of Barbados on the east. The Tobago trough has a greatest
known depth of 8220 feet. Refer to map of Fig. 42.1 and cross section of
Fig. 42.11.

688

STRUCTURAL GEOLOGY OF NORTH AMERICA

The island of Barbados lies on a ridge that flanks the convex side of the
trough and that plunges northward into deep water. Southward from
Barbados, the ridge continues to Tobago, where it merges with a broad
shelf off Venezuela.

Gravity Anomalies

Since Vening Meinesz's ( 1930) discovery of the belt of high deficiencies
in gravity around the islands of the West Indies, the U.S. Navy has taken
numerous gravity readings, under the direction of several scientists, and
has demonstrated there a strip or belt of great negative anomalies. Its
position is shown on the map of Fig. 42.9, which has been compiled by
Lyons (personal communication, 1956) from all available sources. The
anomaly values along the negative strip commonly reach — 150 milligals,
with the largest over the Puerto Rico trough north of Puerto Rico of — 183
milligals. Here the axis of the negative strip is practically coincident with
the axis of the trough. The negative axis extends over the Barbados ridge,
however, as it is traced southwards, and over land in Trinidad and adja-
cent Venezuela where negative values of over —200 milligals are re-
corded. Another axial strip of high negative anomalies lies just north of
the Dutch Leeward Islands and is about coincident with the Leeward
trench (Fig. 42.1).

The anomalies are strongly positive over the Mexican, Colombian,
Venezuelan, and Yucatan basins, and also over the Cayman trench ( Fig.
42.9), which suggests that the Cayman trench is a different kind of tec-
tonic feature from the Puerto Rico trench.

Concept of the Tectogene

In order to account for the belt of strong negative anomalies, generally
parallel with the rises and troughs of the volcanic arcs but falling in-
discriminately on one and the other, Vening Meinesz (1930) concluded
that the cause was much more deep-seated than these topographic
features and due to masses of lighter density material of great volume
downfolded into the heavier subcrustal material. The great downbuckle
is illustrated in Fig. 42.10. It was named the tectogene by Kuenen
(1936). The gravity anomaly curve is also shown in Fig. 42.10; and it

may be seen that the relation of the great downfold to the surficial
features is direct, but that they are puny in relative size, and that the
position of the negative anomaly axis to them is fortuitous. The downfold
is thought by some to be driven by convection currents in the mantle,
and by others the process of downfolding is thought to stimulate convec-
tion circulation. The downfold has been illustrated in model form by
Kuenen (1936), and the driving mechanism and nature of surficial de-
formation also in model form by Griggs ( 1939 ) .

In the event that the driving mechanism slows or stops, the tectogene
will start to rise through isostatic adjustment, and two broad linear uplifts
will appear on either side of the axis of the downfold. Pursuing this
thought and mindful of the geology of the Greater and Lesser Antilles,
Hess (1933) has written as follows:

A second great deformation has occurred a considerable time after the first
one, during which the tectogene originally was developed. In the interval be-
tween the first and second great deformations, one or both of the geanticlines
on either side of the tectogene may have emerged above sea level. Erosion of
these emergent portions, plus a great contribution of volcanics from the concave
side of the arc, may deposit great thicknesses of material in "geosynclines"
within the inner geanticline, and perhaps also outside of an outer geanticline,
as well as in the central basin over the tectogene itself. This basin over the
tectogene will henceforth be called the "geotectocline" because of its different
structural behaviour and in many cases its different type of sedimentary se-
quence than that which occurs in a geosyncline as the term is generally used
today. The second deformation will deform very intensely the material of the
geotectocline. Strong folding and perhaps thrusting of the interdeformational
sediments, if deposited, will occur, and probably further upthrusting of ma-
terial originally squeezed out of the tectogene, if present, will take place. This
happens because the material in the geotectocline is pinched between a sort of
jaw-crusher as the main crust moves toward the tectogene and down over its
rolling hinges. Furthermore, the material which may be on the geanticlines or
in the adjacent geosynclines on the sides (or side) away from the geotectocline
will be carried forward toward the geotectocline. This material may then im-
pinge against the upsqueezed mass in the geotectocline. Upon coming against
this bulwark, the weak upper part may be literally scraped off the main crust as
it rides forward and down into the tectogene. This is particularly true if very in-
competent horizons, such as salt beds or argillaceous sediments are contained in
it. The result will be that the cover will be thrown into folds and perhaps de-
velop a schuppen structure as its forward progress is stopped by the bulwark
and the main crust under-rides or in reality underthrusts it.

HAYFORD BOWIE ISOSTATIC ANOMALIES T 1137 KM.
CONTOUR INTERVAL 20 MSAL

Fig. 42.9. Gravity map of the Mexican-Antillean-Caribbean region, by Paul Lyons, 1956.

690

STRUCTURAL GEOLOGY OF NORTH AMERICA

Anomaly
curve

Axis of deep

Light Upper Crust

Convection | c^d

heavy sub-
materia/

Axis of down-fold

and of negative
J strip

Fig. 42.10. The hypothetical tectogene and its relation to the axis of the deep and the negative
anomaly strip. After Hess, 1933.

Serpentinite Intrusions and the Negative Strip

Serpentinized peridotite intrusions occur all along the great negative
strip of the East Indies, and are present similarly in the West Indies. Hess
( 1937b ) has pointed out this relation, and believes they come up along
each side of the tectogene, but only in a few places are both sides exposed
and not covered by younger rocks. He believes also that they are intruded
only during the first great deformation producing the strip. It will be re-
called that most of the serpentinized ultrabasic rocks in the West Indies
previously described are part of the Late Cretaceous and Early Tertiary
orogenies. Hess believes also that the serpen tinites are a water-rich prod-

uct of partial fusion of the peridotite substratum squeezed off as a result
of the downbuckling. Conditions are favorable for its migration to the
surface along the vertical limbs of the tectogene. Any later deformation
does not produce such intrusion, because the bottom of the tectogene is
sealed by fusion, or all the low-melting constituents of the underlying
peridotitic material have been removed during the earlier cycle.

[The serpentinized intrusions] . . . thus become useful guides in the interpre-
tation of any region such as the West Indies. The serpentinites clearly indicate
the former extension of the negative strip from the west end of Haiti along
the north coast of Cuba, and thence probably to Guatemala.

CARIBBEAN REGION AND SEISMIC PROFILES
Seismic Data and General Observations

Figures 42.11 and 41.15 give the principal results of seismic exploration
of the crust in the Puerto Rico region, the Lesser Antilles, the Venezuelan
basin, and the Colombian basin. This significant work has been under
the general direction of Maurice Ewing.

It will be noted first, that the crust of the general Caribbean region is
thicker than the typical oceanic crust, but not as thick as typical continen-
tal crust; second, the Caribbean crust appears to contain no silicic
basement complex characteristic of the continents, but instead, rocks
interpreted mostly as volcanics. In short, the crust of this mediterranean
region was once typical oceanic crust but has had unusual amounts of
volcanic rocks spread widely but irregularly on it, and has suffered certain
deformation. It will also be noted that under four of the basins the crust
is thinner (Mexican, Yucatan, Cayman, and Venezuelan), but that under
the Colombian basin, and perhaps under the Puerto Rico trench, it is
thicker. The rises, ridges, and land elements are supported by thicker
crust. From general isostatic considerations this is to be expected, but the
negative anomaly belt and postulated tectogene under the Puerto Rico
trench pose a problem.

Puerto Rico Trench

Following a widespread acceptance and intensive development of the
tectogene concept, the seismic refraction survey seemed to refute the

ANTILLEAN-CARIBBEAN REGION

691

CARIBB
SEA

AN \ H

li^iO '^ /v / \ / w\ irw*~ \/\*#0 w w \""/ w w \~ w \~ w V-T^i-'^""' W W W ~/\ /W w W \ / \~V\ / WV/WW - /\7 \/\"/\ / \ / \ /\~/>>— T
MODIFIEO MANTLE MiNIMUM I "TT i ? ppi" L>"", 'J ^LCS Of ' 7 1 I '.

WANTte

•10 u

K
H

ail.- a

■ o

20 "

SEMI- TO CONSOLID-
ATED SEDIMENTS

VOLCANIC ROCKS

INTRUSIONS (?) IN
VOLCANIC ROCKS

SABBROIC LAYER

NAUTICAL MILES

Fig. 42.11. Seismic sections and speculative geologic interpretations of the Venezuelan basin
and Greater and Lesser Antilles. After Officer ef a/. (1957) and J. I. Ewing ef al. (1957). Rocks
having velocities of 4.5 km/sec, if volcanics would be porous or more silicic than basalt; those

theory in the Puerto Rico area, one of its classical localities. Ewing and
Heezen (1955), Worzel and Shurbet (1955), and Shurbet and Ewing
(1956) conclude from topographic profiles and sediment samples that
the trench has been partly filled with several thousand feet of light, un-
consolidated sediments, and that these and a thin crust can account for
the belt of negative gravity anomalies equally as well as a great downfold
or tectogene. The sediment fill may range up to 7 kilometers thick, de-
pending on the thickness of the gabbroic layer assumed. They pursue the
idea further by bringing to bear on the subject earthquake seismology,
magnetism, and seismic refractions. No refractions had been obtained
from the Moho discontinuity, or top of the mantle, under Puerto Rico or
the trench, but an excellent profile had been determined across the Cay-
man trench (Fig. 41.15), which indicates there a marked thinning of the

having velocities of 5.8 to 6.0 would be volcanic with intrusions of basic rock or mixtures of
basalt and more silicic type, or perhaps andesite. For explanation of "modified mantle" see text.
Partially modified under Puerto Rico and the Puerto Rico trench according to Talwani ef al., 1959.

crust. The Cayman trench was considered by Ewing and Heezen (1955)
the same kind of tectonic feature as the Puerto Rico trench, and hence.
the crust should be conspicuously thin under the Puerto Rico trench. The
concept of thinning under the trench is diametrically opposed to the tec-
togene concept. From gravity calculations the crust under Puerto Rico
should be about 20 to 25 kilometers thick (Worzel and Shurbet. 1955).
The gravity map of Fig. 42.9 indicates a general positive gravity area
across the Cayman trench with a negative belt on the south through the
northern part of the Nicaragua!) rise and a mild negative belt on the north
in the Bartlett deep strip. These gravity data do not agree with the com-
puted values of Ewing and Heezen (1955), from which they deduce a
conspicuous thinning of the crust. The gravity data of Fig. 42.9. are
difficult, in fact, to reconcile with the refractive seismic data across the

692

STRUCTURAL GEOLOGY OF NORTH AMERICA

Yucatan basin, Cayman trench, Nicaraguan rise, and Colombia basin. It
appears that the refractive seismic data are to be sought, and that the
gravity data are to be considered afterward in light of the seismic data.
Later refractive recordings and computations by Officer et al. (1957)
and Talwani et al. (1959) in the Puerto Rico trench suggest a thickening
of the crust there. Examine the crustal structure under the Puerto Rico
trench of Fig. 42.11, section A-A'. The gabbroic and volcanic layers seem
to be dropped down by a series of faults under the trench with the Moho
at 20 kilometers. The gabbroic layer under Puerto Rico thickens greatly,
and the Moho discontinuity reaches to a depth of about 30 kilometers
there.

Lesser Antilles and Barbados Ridge

A section through the Lesser Antilles and Rarbados Ridge is given in
R-R', Fig. 42.11.

Refore proceeding with the interpretation of the specific velocity layers
of the Lesser Antilles it will be pointed out that, in general, velocity layers
are interpreted to designate rock types as follows :

1.8-3.7 km/sec

Unconsolidated and semiconsolidate sedi-

ments

4.7-5.2 km/sec

Semiconsolidated and consolidated sedi-

ments

5.2-5.5 km/sec

Extruded porous volcanic material or lime-

stones and dolomites

5.8-6.1 km/sec

Granitic basement

6.5± km/sec

Basaltic or gabbroic subcrust

7.5± km/sec

Mixture of mantle and gabbroic subcrust

or mantle with a higher temperature

than general

In commenting on the velocity layers of Fig. 42.11 J. I. Ewing et al.
(1957) conclude that a low velocity sedimentary layer consisting of an
upper unit with velocity of about 1.7 km/sec and a lower unit of about
2.4 km/sec extends across most of the section. Underneath this is a layer
of about 4 km/sec which also extends across all the section except under
the Atlantic Rasin. It could be identified as lithified sediments or porous
volcanic rocks. The next higher velocity layer is one having velocities of

4.9 to 5.2 km/sec, and in interpreted to be intruded volcanic rocks. This
layer is lenticular in cross section and principally under the Rarbados
Ridge; it does not extend under the ridge of the Lesser Antilles. However,
in the Puerto Rico region a layer having velocities of 4.9 to 5.8 km/sec
seems similar and has been interpreted as the Cretaceous basement of
folded shales, tuffs, and agglomerates which have been extensively in-
truded. The layer has velocities up to 6.1 km/sec in places, and it is those
parts with velocities between 5.7 and 6.1 particularly that are shown in
the cross sections of this book as having considerable intrusive rock. Meta-
morphism incident to orogeny as well as intrusions may contribute to
higher velocities in the layer, such as is evident under Puerto Rico. The
most likely interpretation is that the Rarbados Ridge is a large synclinal
structure as far as the gabbroic layer is concerned and the low-velocity
layers above the volcanic lens of the Rarbados Ridge are anticlinal. The
Lesser Antillean Ridge is a large anticline, or at least a belt of uplift of
the gabbroic crustal layer. It appears to J. I. Ewing et al. (1957) that
the uplift is due to extensive intrusions from below of gabbroic material.
Some of this activity has penetrated through to the surface to form the
present volcanoes and extrusive rock. During the course of magmatic
activity differentiation to more silicic types has occurred, which are
observed at the surface.

The thick lens of volcanic ( ? ) material in the syncline under the Rar-
bados Ridge, by the same reasoning, would be the result of an older phase
of intrusive activity, but the belt has subsided incident to the new adjacent
intrusive activity. The cause of subsidence is not clear. The Rarbados Ridge
is not active volcanically or magnetically whereas the Lesser Antillean
Ridge is highly active in both respects.

Grenada Basin, Aves Swell, and Venezuelan Basin

Layers of volcanics and sediments are spread under the Grenada basin,
the Aves swell and the Venezuelan basin as illustrated in section R-B',
Fig. 42.11. The gabbroic layer becomes thinner westward of the Lesser
Antilles uplift and the overlying volcanics thicker. The Aves swell is ap-
parently an especially thick pile of volcanics, and like the Barbados ridge,
may have been the site of a previous uplift with intrusive activity.

ANTILLEAN-CARIBBEAN REGION

Colombian Basin

The Colombian basin appears to have the same volcanic layer as the
Venezuelan but the gabbroic layer is much thicker. The gabbroic layer
is especially thick under the Nicaraguan rise but thins somewhat under
the basin.

ORIGIN OF THE CARIBBEAN BASINS, TRENCHES, AND RISES

Magmatic Activity

It appears evident from the widespread occurrence of volcanic and
intrusive rocks in the islands of the Greater and Lesser Antilles, and in
the submerged areas as interpreted from the seismic layers, that the de-
velopment of the Caribbean region started with oceanic crust and pro-
ceeded to evolve by abundant and widespread magmatic activity (J. I.
Ewing et al., 1957 ) . Although previous theories have held the magmatic
activity to be secondary to tectonic forces of compression or shear, J. I.
Ewing et al. believe it may be the primary cause of the island arc struc-
ture. As suggested in Chapter 33 on the igneous provinces, the upper
mantle is believed to melt partially at times and in places, and to yield a
liquid of basaltic composition which intrudes the gabbroic layer and
adds to it. In continental areas the heat released from the basaltic intru-
sive masses may result in the melting of lower parts of the sillicic crystal-
line basement and produce large volumes of monzonitic magma, but in
the oceanic areas no silicic layer is present to be melted, and only frac-
tionation of the basaltic magma can occur to produce eruptives other than
basalt. Perhaps large volumes of the extruded rock is andesite. ( See Chap-
ter 33 on origin of andesite.) In the major basins perhaps the volcanics
are fissure flows and mostly basalt. The variable velocities will depend
on the relative amounts of basalt and andesite, on the porosity of the erup-
tives, and on the presence and volume of later intrusives into the vol-
canics.

The thickening of the gabbroic layer and the accumulation of a number
of kilometers of volcanics on the thickened basalt layer, plus intrusives
in the volcanics will make up a crust which stands higher than the
adjacent oceanic crust. Therefore, in the manner postulated by Benioff

in Chapter 32 for the orogenic belts along the Pacific margin, the higher
crust of the Venezuelan and Colombian basins will tend to flow outward
toward the lower Atlantic and Pacific crusts and override them. This ac-
counts for the compressional structures at the junction and the formation
of the complementary upfold and downfold (rise and trench) of the
Puerto Rico and Lesser Antilles arc, and also for the arcuate map pattern
of the belt of deformation (theory of J. I. Ewing et al. [ 1957] and Officer
et al. [1957] ). The overriding of thick crust on thin crust generates a shear
which dips under the thick crust to great depths (Chapter 32 and Fig.
38.3) and provides an avenue for volatiles and perhaps even magma to rise
further from the mantle. This engenders additional volcanic activity in
the uplift inside the trench.

Since continental Venezuela and Colombia stand higher than the adja-
cent basins, the tendency will be for the continental margin to move north-
ward and override the basin. The deformed belt of the Dutch Leeward
Islands and the Leeward and Los Roques trenches, together with the belt
of negative gravity anomalies, support this postulate. The Colombian
basin crust may have tended to flow westward toward the Pacific, and
the Central American trench suggests this idea (Chapter 32), but the
trench continues northwestward to southern Mexico beyond the sphere of
influence of the Colombian basin. Conditions are complex in Central
America and will be commented on later.

Mexican, Yucatan, and Cayman Depressions

It has been pointed out that M. Ewing and associates believe the Cay-
man trench and the thinned crust under it to denote a structure which
is the result of tension. In cross section the structure is like a necked por-
tion of a steel rod which has been deformed under tension. The Yucatan,
Mexican and southern part of the Colombian basin are structures which
in line of cross section (Fig. 41.15) appear to be similar to the Cayman,
but their shape in plan view must also be regarded.

The Cayman and Yucatan basins are relatively narrow and long, and
are marked by strong positive gravity fields. Tin- Mexican and Colombian
basins are broad, but also are marked by positive gravity fields. The active
seismic belt passes westward from Puerto Rico through Hispaniola to the

694

STRUCTURAL GEOLOGY OF NORTH AMERICA

Cayman trench, along the Cayman trench and to the Gulf of Honduras,
and thus it is seen that the Cayman trench of all four basins alone is
seismically active. If the other basins have a similar origin, then their
formation occurred in earlier times, and the Cayman should be considered
in process of formation today.

The Cayman trench has long been considered a down-faulted trough,
and the fault scarplike topography of the trench walls has been cited as
evidence. Also, the escarpment of the Sierra Maestra of southeastern Cuba
facing the trench, and the fault valley of the Cul de Sac of Hispaniola are
taken to mark the eastern extent of the trench faults (Taber, 1922). The
other basins have been imagined blocked out by faults (review by Eard-
ley, 1954 ) but on a tenuous basis. On any grounds, the downf aulting could
not affect the base of the crust which has moved up.

Ramifications of Tension Hypothesis

With the tension hypothesis before us several thoughts result: ( 1) How
does tension in the western Caribbean relate to the outward flow and
peripheral compression of the eastern Caribbean crust, the theory just
proposed? (2) If these basins mark lanes of thinning of the entire crust
and consequently extension, are we dealing with the drifting of South
America apart from North America? (3) If the outward flow theory
pertains to the eastern Caribbean, why has not the Gulf Coast of the
United States flowed toward the Mexican basin and caused a volcanic
archipelago and trench there? Likewise why has not the Rrazilian conti-
nental margin overridden the Atlantic Ocean crust?

Continental drift and the oceanward flow hypothesis of continental mar-
gins freshly formed by fragmentation and drifting apart seem logically
related, but a serious objection to the oceanward flow hypothesis as noted
above, may be an argument against drifting.

POSTULATED EASTWARD SHIFT OF CARIBBEAN BLOCK

In 1938 Hess presented a theory of evolution of the Antillean region
that involved eastward displacement of the Caribbean block. He regarded
major horizontal shortening in the orogenic belt of the Lesser Antilles

necessary as the crust was rolled down in the tectogene, and accordingly
imagined the Caribbean block between Jamaica, Hispaniola, and Puerto
Rico on the north and the Leeward Islands on the south to have been
translated 50-100 miles eastward, and in the course of this movement the
faults of the Cayman trench and the Anegada Passage were formed, and
were chiefly ones of horizontal movement. Hess and Maxwell in 1953
depict some changes in the original theory (Fig. 42.12), and propose
that the areas of metamorphic rock of the Greater Antilles were once
joined in a single strip before the strike-slip faulting of great magnitude
broke and displaced the strip. They write as follows:

This reconstructed strip of metamorphic rocks represents the axis of the
mid-Cretaceous down-buckle or downbulge. The tectonic axis is not the present
negative anomaly strip north of Puerto Rico and Hispaniola as previously be-
lieved. ... In all our previous analyses the structure of Puerto Rico appeared
to be anomalous. Here the folds are overturned to the north-northeast. If the
tectonic axis lay to the north, the overturning should have been southward. In
our present analysis the tectonic axis lies to the south-southwest, and the Puerto
Rican structures are then in a consistent relation to it.

A system of faults in northern Colombia and Venezuela are interpreted
as wrench faults with movement of the Caribbean block eastward a con-
siderable distance ( Rod, 1956; Alberding, 1957 ) . This supports the theory
of Hess and Maxwell.

Bucher (1952) has presented a variation to Hess's theory. He believes
that en echelon arrangement of fold axes along the coast of Venezuela on
the south and in the islands of the Greater Antilles along the north indi-
cates that the crust of the Caribbean Sea basin has moved eastward.
Crosswise of these compressional structures is a set of high-angle faults,
presumably tensional structures, which completes the picture of a shear
zone along the south and north sides of the sea basin. He says:

In the Greater Antilles, 500 miles to the north, the same combination of
features recurs, but with directions reversed. There also, straight east-west
trending coast lines are conspicuous in the shapes of the islands from Jamaica
and the Sierra Maestra of Cuba through Hispaniola and Puerto Rico. As in the
ranges that form their counterpart in the south, the axes of individual folds
trend obliquely across the ranges and shore lines. But here they trend east-
southeastward, while there they bear east-northeastward.

A complementary set of northeast-trending fractures finds conspicuous ex-

ANTILLEAN-CARIBBEAN REGION

695

Fig. 42.12. Strike-slip faulting in Greater Antilles. Hy-
pothesis of Hess and Maxwell (1953) showing presumed
positions before and after horizontal translation.

pression in the contours of the sea floor, as in the Anegada Passage . . . and
again west of St. Croix; in the Beata Ridge and its northeastward continuation
along the south coast of Santo Domingo, northeast of Cape Beata; in the
Navassa-Jamaica Passage (Bucher, 1952, p. 83).

Since the seismic data essentially preclude the existence of a major
itectogene in the Lesser Antilles, the need for large-scale eastern movement
of the Caribbean block is mostly dissipated. Also since the seismic data
of the Cayman trench indicate considerable stretching of the crust, the
fwrench fault hypothesis hardly seems compatible with so much tensional
strain. In fact, both the tectogene and wrench faults were conceived
before the seismic refraction surveys, and they do not account for the
crustal structure that the surveys reveal. The postulated wrench fault

pattern of the Colombian and Venezuelan coast is fairly impressive, but
yet some of the assumed relations are rather tenuous. In the continental
drift hypothesis South America because of its present position appears
to have moved eastward as well as southward while maintaining a north-
south orientation. If so, considerable eastward shearing could have oc-
curred along the Greater Antillean alignment. But at the same time tin-
north coast of South America should have moved eastward also relative
to the Caribbean block, and this is just opposite to the direction indicated
by the postulated wrench fault pattern.

The origin of the Gulf of Mexico and the Antillean region is still un-
known.

43.

Precambrian ages. A belt of deformed Permian strata with Permian (?)
granitic and ultrabasic intrusives makes up part of the crystalline belt
through Oaxaca, Chiapas, Guatemala, and northwestern Honduras.

A system of folds in Jurassic and Cretaceous basin-type sediments
(the Mexican geosyncline in Mexico) extends along and inside the crystal-
line belt from southern Mexico eastward through British Honduras to
the Caribbean and projects toward Cuba, the Yucatan basin, and the
Cayman trench. It is called the Late Cretaceous and Early Tertiary fold
belt on Fig. 43.1.

Facing the Gulf of Mexico is a narrow coastal plain which extends to
the broad platform of the Yucatan Peninsula and Campeche Banks.

The crystalline and fold belts and the Coastal Plain are referred to as
nuclear Central America, in contrast to the narrow volcanic province of
southern Nicaragua, Costa Bica, and Panama, which is called the Isthmian
link (Boberts and Irving, 1957).

SOUTHERN MEXICO
AND CENTRAL AMERICA

MAJOR GEOLOGIC DIVISIONS

A great Cenozoic volcanic province, or possibly a complex of three or
four volcanic provinces, extends through southern Mexico and Central
America. The role of volcanism is most important in the geologic thinking
about the region. If however, the rocks older than the Cenozoic volcanics
are considered, a significant foundation geology becomes evident. A belt of
crystalline rocks extensively overlain by volcanics comprises the south-
western and southern coast of Mexico, of southern Guatemala, most of
Nicaragua, and all of El Salvador and Honduras. See map, Fig. 43.1.
These metamorphics are assigned variously Mesozoic, Paleozoic, and

CRYSTALLINE BELT

Metamorphic rocks crop out in wide areas in the Mexican State of
Sinaloa which borders the southern part of the Gulf of California. Other
occurrences are shown inland at Parral in the State of Chihuahua. See
the new Geologic Map of Mexico (1956). All are labeled Mesozoic and
are identified as marbles and slates. The same rocks crop out on Las Tres
Marias.

Beginning at the Bahia Banderas at the west end of the Sierra Madre del
Sur (maps, Figs. 35.1 and 43.1) and extending eastward through the
Sierra to the Chiapas-Guatemala border is a metamorphic belt noted as
Paleozoic in age on the Geologic Map of Mexico. The small amount of
data available indicates that the rocks consist of gneisses and schists,
possibly of Early Paleozoic age, and greenstone conglomerates and phyl-
lites, possibly of Late Paleozoic age. A large batholith in eastern Oaxaca
and Chiapas is intrusive into the metamorphic rocks and is considered as
Paleozoic in age by Boberts and Irving (1957) and as Mid-Paleozoic by
de Cserna (1958 and 1960). The metamorphics are also intruded by
Mid-Cretaceous (Cenomanian) stocks and batholiths which are probably

696

Fig. 43.1. Tectonic map of southern Mexico and Central America. Compiled from Roberts and

Irving (1957), de Cserna (1958), Terry (1956), and Geological Map of Mexico, I. G. C. (1956).

,., For distribution of Tertiary volcanic rocks in southern Mexico see Fig. 32.8. Southern Guate-

mala, El Salvador, southern Honduras, central and southern Nicaragua, Costa Rica, and Panama
are nearly all covered by Cenozoic volcanic rocks. B.H., British Honduras.

698

STRUCTURAL GEOLOGY OF NORTH AMERICA

related to the Nevadan orogenic belt (see Chapter 35). The older intru-
sions are overlain in Chiapas by late Paleozoic sediments and locally
elsewhere in the Sierra Madre del Sur by Lower Cretaceous and Jurassic
marine strata. A few scattered observations of the direction of foliation
seem random, and so no conclusions are yet justified regarding the trends
in the metamorphic complex (de Cserna, 1958).

In northern Honduras and Nicaragua the metamorphic belt is labeled
"probably Precambrian" on the Geologic Map of Central America ( Roberts
and Irving, 1957), and it consists of undifferentiated schist, gneiss, phyl-
lite, quartzite, and marble. Sapper ( 1937 ) has emphasized that the major
structures here are pre-Permian.

PERMIAN FOLD BELT

The Geologic Map of Central America shows a belt of folded Permian
strata across central Guatemala. The Chochal limestone and Santa Rosa
limestone, conglomerate, shale, and sandstone are the formations identi-
fied. They are intruded by granite and serpentine, which may be Late
Permian or Triassic in age, and were involved in the orogeny in which
the Permian rocks were folded. The rocks of the Santa Rosa formation
become progressively more metamorphosed to the east so that shale be-
comes phyllite and schist in eastern Guatemala and in Honduras. The
Jurassic Todos Santos formation rests on the folded Permian rocks.

The crystalline complex was widely blanketed by Upper and Lower (?)
Cretaceous rocks in Honduras and Nicaragua, and these have been folded
in the Late Cretaceous and Early Tertiary orogeny.

An area of outcrop of the Santa Rosa formation with granitic intrusions
occurs north of the main Permian fold belt in British Honduras. This may
indicate that the original Permian fold belt was once wider than now, and
that the later Cretaceous fold belt largely covers it.

LATE CRETACEOUS AND EARLY TERTIARY FOLD BELT

General Characteristics of Mexican Fold Belt

The strata of the Mexican geosyncline (Chapter 28) extend into
southern Mexico and overlap broadly southward on the crystalline belt.

These sedimentary rocks are chiefly Jurassic and Lower Cretaceous car-
bonates, with some fine elastics near the base, and Upper Cretaceous
shales. The Albian and Cenomanian seas advanced widely over the
southern and southwestern crystallines and deformed Paleozoic rocks
which had previously been land and the source areas for the Jurassic
and earlies Cretaceous sediments (de Cserna, 1958). At places, however,
Lower Jurassic strata rest on the crystallines, as in Oaxaca. Laramide
compression then deformed the Jurassic and Cretaceous strata and a long
system of folds resulted. These are labeled on Fig. 43.1 the Late Creta-
ceous and Early Tertiary fold belt. For the most part the folds are asym-
metrical toward the northeast. The fold belt is broad in northern and
central Mexico (Chapter 28), but narrows southward and is marked es-
sentially in southern Mexico by the Sierra Madre Oriental (de Cserna,
1958).

Red Conglomerates in Central and Southern Mexico

A number of occurrences of Early Tertiary red conglomerates have
been noted in central and southern Mexico (Edwards, 1955). They are
particularly important in deciphering the Laramide and Tertiary history
of the fold belt. The localities where the red conglomerates are known
are shown on the map of Fig. 43.2. The three areas studied are noted on
Fig. 43.1 where they may be seen in relation to the crystalline belt and
fold belt. They are the Zacatecas, the Guanajuato, and the Taxco.

The oldest rocks in the Guanajuato City area are folded, hard, black,
thin-bedded, marine shales which now appear in places as phyllites or
schists. Small quantities of limestone, sandstone, and volcanics appear in
the series. No fossils have been found but on the basis of lithologic
similarity they have been correlated with the Upper Triassic shales at
Zacatecas City (Edwards, 1955). Six miles northwest of Guanajuato the
La Luz schist occurs which contains an amygdaloidal basalt about 1000
feet thick. A dense, dark gray limestone, possibly of early Cretaceous age,
is believed to have once covered the Triassic (?) shales, but was removed
locally before the overlying conglomerates were deposited.

After the full development of the Mexican geosyncline the main Lara-
mide orogeny occurred. Folds are the main exhibit (see Chapter 28) but
here in south-central Mexico, considerable plutonism occurred. A deeply

-1 £ AGUAS I P Q T O S I i-

r CALIENJES^

Based on map o( Mexico prepared by

American Geographical Society

L <$r \e>> ^iDALGo^-7 r1 <

Approximate scale
I

108*

104*

Fig. 43.2. Map of Mexico showing localities in which red conglomerates are known to occur. Repro-
duced from Edwards, 1955.

700

STRUCTURAL GEOLOGY OF NORTH AMERICA

GUANAJUATO
RIVER

Tpc-^ RIVER QoU

Fig. 43.3. Cross section at city of Guanajuato, Mexico, after Edwards, 1955. Qal, Alluvium; Tpc,
Pliocene conglomerate; Tmv, Young volcanic rocks, Miocene; Tomb, Bufa ss., Oligocene-Miocene;

weathered granite is intrusive into the Triassic (?) shales and schists.
Other plutons are dioritic and monzonitic in composition, and all are
believed to be pre-conglomerate by Edwards.

Uplift, faulting, erosion, and volcanism followed the folding and intru-
sions. Although the volcanics do not occur interstratified in the sequence
a great pile of them is believed to have existed nearby because derived
fragments constitute a large part of the conglomerate.

The red conglomerate at Guanajuato City is named after the city. There
it is about 5000 feet thick but thins northeastward and southwestward.
Volcanic fragments form more than half of the deposit, with granite,
diorite, limestone, and chert making up the rest. Granite fragments in-
crease upward and compose 35 percent of the mass near the top. This
indicates an increasing exposure of the granitic pluton in the source area.

The source of the conglomerate was a highland northeast of Guanajuato
City where silicic volcanics capped shales and limestones of Cretaceous
age. The highland was also an area where a granitic pluton had intruded
the Cretaceous strata (Edwards, 1955).

The Guanajuato conglomerate is late Eocene or early Oligocene in age.
Similar red conglomerates presumably of the same age occur at Zacatecas
and Taxco.

The conglomerate is overlain by the tuffaceous Rufa sandstone into
which it is transitional. The sandstone is about 50 feet thick.

The great Miocene (?) volcanic epoch followed, which was initiated
by the deposition of massive, bedded tuffs more than 1500 feet thick
in the Guanajuato area. Then followed normal faulting, which produced
the tilted block and graben structure so strongly evident today. The slip

Toe, upper part of Guanajuato conglomerate; Teoc, lower part of Guanajuato conglomerate; upper
Eocene-Lower Oligocene; MES, Mesozoic sedimentary rocks and coarse-grained silicic, intrusive rocks.

on several of the northwest striking faults is as much as 3000 feet. See
cross section of Fig. 43.3.

Fold Belt in Central America

The fold belt is shown to include gentle folds in the states of Tabasco
and Vera Cruz along the north side in a region recognized as coastal plain
by some.

As the fold belt is traced eastward into Guatemala and Rritish Hon-
duras, continental Jurassic and Lower Cretaceous beds are involved.
These beds probably covered much of the "nuclear region" of Central
America (Imlay, 1944; Roberts and Irving, 1957). Toward the close of
Early Cretaceous time subsidence and marine embayments resulted in the
deposition of limestone and dolomite approximately coextensive with the
underlying terrestrial beds. Deposition continued in most places until
Late Cretaceous when the folding occurred.

The earliest Tertiary beds are coarse clastic rocks of the Sepur formation,
whose composition shows that they were derived from a wide variety of sources
including crystalline rocks, Permian limestone and quartzite, and limestone
and volcanic rocks of Mesozoic age. The Sepur strata were probably deposited
during orogenic movements in Late Cretaceous and Eocene time. Intrusions of
granodiorite and diorite that accompanied the orogeny cut the Cretaceous rocks
throughout eastern Guatemala and Honduras. Folds, largely trending eastward,
were developed in the Cretaceous rocks and appear also to have involved rocks
as young as those of Sepur age.

The orogenic movements culminated in thrust faulting, first mapped in the
Departamento de Huehuetenango, which thrust the Permian rocks over the
Todos Santos formation and the Cretaceous limestone. The extent of the thrust-
ing is not known, and many such faults may be present in central Guatemala
(Roberts and Irving, 1957).

SOUTHERN MEXICO AND CENTRAL AMERICA

701

Two distinct phases of orogeny are represented. First, the deformation
that resulted in the deposition of the Sepur strata, and second the involve-
ment of the Sepur in folding and thrusting as noted in Guatemala in the
Departamentos de Huehuetenango, Alta, Verapaz, and Peten. The last
phase must therefore be as late as Eocene or possibly post-Eocene. Mio-
cene strata overlap the earlier Tertiary rocks on the Peten area and are
hardly deformed.

The Geologic Map of Central America (Roberts and Irving, 1957)
shows considerable Cretaceous strata lying on the crystallines of Hon-
duras and northern Nicaragua, and also that the Cretaceous strata have
been folded. The obvious fold axes are indicated on Fig. 43.1. It is there-
fore evident that the Laramide fold belt spread southward in this part
I of Central America and involved the crystalline belt somewhat.

I
SOUTHERN GULF COASTAL PLAIN

The Southern Gulf Coastal Plain is made up of marine Cenozoic sedi-
' ments and some volcanics, and it extends as shown on Fig. 43.1 along
the east side of the Sierra Madre Oriental southward through the State
of Vera Cruz and thence in a narrow belt eastward through Tabasco to
the Yucatan Peninsula. It is divided into a number of basins partly for
the convenience of petroleum exploration, and the boundary of these
basins with the fold belt is not well defined nor usually agreed upon
(Benavides, 1956; Guzman, 1959). See Fig. 41.9.

The strata of the Coastal Plain dip gently toward the Gulf of Mexico.
In the Coatzacoalcos region, or the Isthmus (of Tehuantepec) saline
basin salt intrusion structures are prominent, and to the east, gentle fold-
ing is prevalent.

YUCATAN PENINSULA

The terrane of the states of Tabasco, Campeche, and Yucatan is under-
lain by flat or very gently folded marine strata that range in age from
late Eocene to Pleistocene. On the whole, the Yucatan Peninsula consists

of lowlands under 650 feet in height. As the strata are mostly limestones,
the country is almost destitute of rivers, and the rains sink quickl) through
the soluble and karsted limestone and gather in subterranean basins. The
water table lies at various depths down to 300 feet or more beneath the
land surface.

Yucatan extends north beneath the Gulf of Mexico for at least 150 miles
as the Campeche or Yucatan Bank, and then descends abruptly into the
depths of the Gulf. Carbonaceous sediments are accumulating on the
shelf (Fig. 43.4).

VOLCANIC FIELDS AND FAULTING

Southern Mexico and Central America are particularly noted for vol-
canoes, and volcanic rocks cover extensive areas. The map, Fig. 33.6,
shows the volcanic rocks of southern Mexico, where two general ages are
recognized, the mid-Cenozoic and the late Cenozoic. The older volcanics
make up the southern end of the extensive Sierra Madre Occidental
province, but the younger eruptives are in the form of an east-west belt of
stratovolcanoes including such well-known cones as Arizaba, Popoca-
tepetl, Ixtaccihuatl, Paricutin, and Colima. The belt of stratovolcanoes
has been called the Trans-Mexican volcanic belt (de Cserna, 1958), and
from the map of Fig. 43.1 it can be seen to extend from the Bahia Ban-
deras to the Gulf of Mexico below the city of Vera Cruz.

Beginning in Chiapas, not far from the east end of the Trans-Mexico
volcanic belt is another great belt of modem volcanoes which stretches
through southern Guatemala, El Salvador, southern Nicaragua, and
western and central Costa Rica. In El Salvador major faults trend north-
westward parallel to the volcanic chain and to the coast line. Lago de
Ilopango occupies a graben that developed over a long period of time
and was partly filled by a succession of lavas and pvroclasties during its
formation. Other lake basins such as Lago de Atitlan and Lago de Ama-
titlan in Guatemala also probably formed, at least partly, by collapse
( Roberts and Irving, 1957 ) . The extrusive rocks are olivine basalt, basalt,
labradorite andesite, and in lesser distribution dacite (Weyl. 1956). A
transverse zone of faults across Honduras from the Gulf of Fonesea to the

Fig. 43.4. Sediments of the
shelves around the Gulf of Mex-
ico. Reproduced from Atwater
and Forman, 1959. Bahama
Banks are also calcareous.

SOUTHERN MEXICO AND CENTRAL AMERICA

Caribbean is shown on the Map of Mexico, and the Valle de Comayagua
is regarded as formed by block faulting. Parallel structures along the
coast of Rritish Honduras may be related to this transverse fault zone.
Uplift accompanied the volcanism over much of Central America ( Roberts
and Irving, 1957).

Reference has been made to the numerous submarine volcanic cones
on the Pacific floor adjacent to Central America, and also to the Central
American trench, in Chapter 32. See especially Fig. 32.5.

ISTHMIAN VOLCANIC LINK

Southernmost Nicaragua, Costa Rica, and Panama constitute the so-
called Isthmian link. By it the more broad and massive foundations of
North and Central America are connected to South America. The rocks of
the link are largely igneous and stratified deposits derived mostly from
igneous formations.

The Cordillera de Talmanca in central and southeastern Costa Rica
has been studied in considerable detail by Weyl ( 1956, 1957 ) and the
table of Fig. 43.5 gives the history of the isthmus there as he depicts it.
The Cordillera Central referred to in the table is just west of the Cordil-
lera Talamanca and is the eastern end of the stratovolcanic belt.

The record in the Sierra Talamanca goes back only to the Oligocene,
but as may be seen, it is one of sinking and volcanism; folding, erosion,
and deposition; intrusive activity with much metamorphism; more fold-
ing; and finally uplift, faulting and more volcanism and the building of the
modern cones.

The geology of Panama begins with a basement complex of Eocene
and possibly pre-Eocene (Terry, 1956) or Cretaceous (?) age (Wood-
ring, 1957) which crops out over half the country. It is made up pre-
dominantly of altered basic flows, agglomerates, tuffs, and diorite
intrusions. It is strongly deformed but little metamorphosed, although
argillites are known and schist float has been reported in two places.
The areas of outcrop of the basement complex are anticlines or horsts,
and their axes are shown on Fig. 43.1.

EPOCH

DEPOSITS

STRUCTURAL EVENTS

MAGMATISM

PLEISTOCENE

River gravel
and sand

Uplift, faulting,
arching

Later volcanisa
in the
Cordillera
Out r.i 1

PLIOCENE

Suretka conglomerate

Relative quiet
Folding of

MIOCENE

Molasse sediments
on mountain flank

border zone

Late orogenic
intrusive
activity

Gat un conglomerate

in the mountains

Limestone, marl

Geosynclinal

OLIGOCENE

Tuff

Chert

Sandstone

Marl

Tuff

Limestone

sinking

Beginning of

basic

volcanism

EOCENE

Unknown foundations

Fig. 43.5. History of the Cordillera de Talamanca in Costa Rica. After Weyl, 1956.

The strong deformation of the Cretaceous lavas and sediments
occurred in Late Cretaceous, Paleocene, or early Eocene time (Wood-
ring, 1957).

The basement complex is overlain on the flanks of the uplifts by
sediments and volcanics of several kinds, earning fossils in many places.
Fossil collections indicating late Eocene, Oligocene, early, mid-, and
late Miocene, Pliocene, Pleistocene, and Recent have been described
(Terry, 1956; Woodring, 1957). A section across the isthmus at the
Canal Zone is shown in Fig. 43.6.

Volcanism again reached a climax during Oligocene and early Mio-
cene time. The rocks have been identified as diorite, quartz diorite,
dacite, andesite, and basalt (Woodring, 1957).

A number of trans-isthmian faults have been postulated by Tern
(1956). These are shown on Fig. 43.1. Two of them in the Canal Zone
and one farther west have strike slip movement, and two in eastern
Panama are high-angle thrusts. The Panama Canal Zone seems to be
the most complicated area, but this may be the result of more in-
tensive field work there than elsewhere. In general it is more de-

704

STRUCTURAL GEOLOGY OF NORTH AMERICA

"wT^ — — — — ' N — — _ - _ —

Fault >\'"/N/\/», /y/w w s / \~ i \ / \ i \ / \ i

Fig. 43.6. Generalized section across Isthmus of Panama, after MacDonald, 1919. Intrusions
are granodiorite, diorite, andesite, rhyolite, and basalt. Bedded rocks: 1, Bas Obispo volcanic
breccia; 2, Las Cascades agglomerate (Oligocene); 3, Bohio conglomerate; 4, Culebra fm.; 5,
Cucuracha fm.; 6, Emperador Is.; 7, Caimito fm. (Miocene); 8, Panama fm.; 9, Gatun fm.
(Pliocene); 10, Toro Is.; 11, Pleistocene.

pressed than on either side with the basement complex showing to a less
extent. A somewhat different fault interpretation of the Canal Zone has
been rendered by Woodring (1957).

The Isthmian link has the shape of an S curve and the faulted area of
the Canal Zone is in the middle, which may indicate that deformation
has been in one direction on one side and opposite on the other (Terry,
1956).

The basement complex of Cretaceous (?) age with dioritic intrusions
has been taken to continue the Nevadan orogenic belt into South America
(Eardley, 1954). The broad nature of the platform upon which the ex-
posed narrow isthmus rests has been regarded as wide enough to contain
a major belt of deformation. Evidence to the contrary may be cited
as follows. The intrusions are probably not batholithic in size and some
may be Oligocene or Miocene in age. Also it is evident now from seismic
refraction work that modified oceanic crust lies on the Caribbean side
of the link, and true oceanic crust on the Pacific, so if the isthmus repre-

sents an orogenic belt, it has evolved from oceanic crust, and should
not be similar to the Nevadan which in general evolved from a great
eugeosynclinal complex along the margin of the continents of North
and South America.

RELATION TO GREATER ANTILLES

Projection of Crystalline and Fold Belts

The Crystalline Belt through the coastal ranges of northern Honduras
and Nicaragua passes out into the Caribbean Sea, and it is inferred
from exposures of the crystalline rocks on the Isla Roatan that the belt
continues toward Jamaica, bounding the Cayman trough on the south
(Roberts and Irving, 1957). The seismic traverse of Fig. 41.15, reveals a
layer of 5.2 to 6.1 kilometers per second, and although this has been
interpreted as a volcanic layer, the parts with higher velocities could
be the rocks of the Crystalline Belt. It seems hardly thick enough, how-
ever, to represent an old orogenic belt of metamorphic rocks. Meta-
morphic rocks of the Crystalline Belt varieties are not exposed on
Jamaica, but rather the oldest core rocks are folded volcanics of Late
Cretaceous age. It is tentatively concluded, therefore, that the Crystal-
line Belt as a crustal layer, wedges out not far east of the east coast
of Nicaragua.

The fold belt has commonly been projected north of the Yucatan Basin
to Cuba, and the Cockscomb Mountains in British Honduras project east-
ward toward the Misteriosa and Cayman banks (Roberts and Irving,
1957). If connections north or south of the Yucatan Basin ever existed
with Cuba, they must have been very transitory, according to vertebrate
paleontologists (Schuchert, 1935). Yucatan has about seventy species of
verterbrates which are of the fauna of the Atlantic neotropical realm. If
it was united with Cuba at any time during the late Cenozoic, it is in-
conceivable why tortoises, pit vipers, Opisthoglypha, and Cnemidophorus
should not have crossed over in to Cuba. Since Yucatan was beneath the
sea during most of the Cenozoic era, land connections through the penin-
sula, at least, seem impossible.

SOUTHERN MEXICO AND CENTRAL AMERICA

705

Meaning of Yucatan and Cayman Depressions

The thin crust under the Yucatan and Cayman troughs, as previously
explained, might indicate immediately to the proponent of continental
drift, that the crust has been stretched and is in the process of being
fragmented or pulled apart. Even one not ready to accept large scale
drift of the continental masses may concede some pulling apart and
thinning. If stretching and thinning is admitted as a possibility, then the
thin gabbroic layer under the Mexican basin may also represent drifting
apart there, but at a somewhat earlier time because of the thicker layers
above of consolidated and unconsolidated sediments. Right off, however,
the seismic evidence does not suggest that we are dealing with a silicic
layer in the Cayman region — simply the gabbrioc layer is being thinned.
In continental drift, if it occurs, are we dealing with the movement of the
total crust over the mantle with a new gabbroic layer forming immedi-
ately in the breach, and progressively as it widens, or does the silicic
layer slide over the gabbroic?

Postulated Wrench Faults

With the discovery of the great fracture zones in the eastern Pacific
the attempt has been made to connect one of them, the Clarion, with the
Cayman trench across Central America. Hess and Maxwell's (1953)
postulate of Fig. 42.12 shows Honduras, Nicaragua, and El Salvador to
have moved eastward over 200 miles from a former position in what
is now the Pacific Ocean, along a break that would transect Central
America from southern British Honduras to the Pacific coast at about
the Chiapas-Guatemala border. Such a break would transect the Permian
fold belt and the metamorphic belt, for which there is no existing geologic
evidence. Also the Central American trench is a late Cenozoic feature
which, because it is continuous, precludes horizontal translation of
crustal blocks across it during this time. Since the Cayman trench is a
late Cenozoic feature, the movement postulated by Hess and Maxwell
has to be late Cenozoic, which is impossible across Central America.
The Clarion fracture zone takes off northwest of Acapulco and does not
line up with a projection of the Cayman trench.

The zone of stratovolcanoes across southern Mexico might also be re-
garded as the line of horizontal movement, but no geologic relations
are known there to denote a wrench fault zone other than the aligned
vents.

MAMMALIAN FOSSIL RECORD AND LAND CONNECTIONS

The record of mammals, both existent and fossil, in North and South
America and in the isthmus itself is impressive, and speaks more posi-
tively of land connections than the physical, but still the two lines of
evidence lead to parallel and supporting conclusions.

South America may have had southern and eastern connections during
the Mesozoic, but since late Cretaceous time at least, it has been isolated
from all the rest of the world, except for occasional connections with
North America (G. W. Simpson, personal communication). This lias
established it as an ideal laboratory in experimental evolution over a
lapse of seventy million years, and the record is remarkably clear. By
reference to the chart of Fig. 43.7, it will be seen that a group of early
immigrants were left isolated and proceeded to evolve in their own way.
The connection between North and South America to permit the influx
of these early mammals evidently was the result of volcanism and the
disturbances just reviewed.

In late Eocene and early Oligocene time, shallow seas and volcanic is-
lands in the area of the isthmian uplift allowed certain forms adequately
equipped to make passage from island to island, and thus a wave of "is-
land hoppers" entered South America. After certain adjustments with the
ancient immigrants already there, the newcomers also proceeded to evolve
their own way. No continuous land of any breadth or durability was es-
tablished at this time, and the migration route served as a screen or sieve
to a host of North American forms which would have moved in under
more suitable environmental conditions.

Orogeny and extensive volcanism again convulsed the isthmian swell
in late Tertiary and Quaternary time, and at first the region seems to have
been a chain of islands permitting a second wave of "late island hoppers,"
and then a solid subaerial connection, permitting a wave of new immi-

706

STRUCTURAL GEOLOGY OF NORTH AMERICA

STRATUM 3

\LATE ISLAND\

LATE

HOPPERS | IMMIGRANTS

STRATUM 2

\0LD ISLAND

, HOPPERS

STRATUM 1 ANCIENT

IMMIGRANTSX

NORTH t SOUTH

1 ' ^~~

AMERICA -«v^
CONNECTED ^*sj

ISLAND CHAINS

NORTH t SOUTH
AMERICA DISCON-
NECTED

PLEISTOCENE

CRETACEOUS

PA LEO CENE

EOCENE

OLIGOCENE

MIOCENE

PLIOCENE

+ RECENT

Fig. 43.7. Times of migrations between North and South America. Copied from lantern slide of G. W.
Simpson, 1950 Sigma Xi lecture.

grants from North America. Some of the peculiar forms from South
America also made their way into North America.

It is possible that the Antillean arc from latest Cretaceous time on
could have been a land connection as well as the Costa Rica-Panama
isthmus, and that their histories may not have run exactly parallel. With
the possibility of two bridges between continents, ocean-to-ocean migra-
tion may have been delayed for a while in the mediterranean between

bridges, in the manner of a ship negotiating locks in a canal. Also, if the
history of the two bridges did not run parallel, then opportunities for ex-
change of land animals would be more frequent than if only one bridge
had recurring emergences and submergences. The paleontologic record
of the Greater and Lesser Antilles, however, does not indicate that the
eastern orogenic belt was of importance at any time as a land bridge be-
tween the continents.

SOUTHERN MEXICO AND CENTRAL AMERICA

to:

Permian reptiles and flora were isolated and did not migrate from one
continent to the other (Schuchert, 1935). The separation continued
through the Triassic. Little can be said of the Jurassic and Early Cre-
taceous.

At the same time as land migration routes are established between
North and South America, so are migration routes of marine invertebrates

severed between the Atlantic and Pacific. The conclusions reached In tin-
invertebrate paleontologist should therefore dovetail those of tin- verte
brate paleontologist. According to Schuchert ( L935) not all invertebrate
paleontologists agree on the relation of Atlantic or Gulf and Pacific forms,
but most evidence points to a portal in Early and Middle Tertian time,
and thus supports the mammalian record.

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INDEX

Abajo Mountains, Fig. 26.7
Absaroka Range, 369
Absaroka thrust, Fig. 22.9, 334
Acadian belt, 120
Acadian orogeny, 36, 124, 170,

180, 216
Acatita-Las Delicias area, Fig.

28.6
Adel Mountain field, 571
Adeloida quadrangle, Fig. 17.2,

272
Adirondack dome, 22
Adirondack Mountains, 157
Ajo district, Fig. 27.9
Alaska, 29, 81, 605
Alaska Peninsula, 614
Alaska Range, Fig. 39.1
Alberta basin, 55
Alberta shelf, 55
Alberta syncline, 61
Aleutian ridge, 519, 520

Aleutian trench, Fig. 39.11
Aleutian volcanic belt, Fig.

39.11, 621
Alexander Range, Fig. 17.18,

280
Algoman orogenic belt, 27
Algoman orogeny, 26
Algoman province, Fig. 43, 25
Alkalic provinces, 566
Allegheny basin, Fig. 8.9, 106
Allegheny front, 103
Allegheny Plateaus, 100
Allegheny synclinorium, 97
Alpha Range, Fig. 40.6
Amargosa chaos, 499
Amarillo Mountains, 39, 237
Ammonoosuc thrust, 176
Anadarko basin, 242
Anadyr Gulf, Fig. 39.1
Andros Island deep test, 683
Anegada Passage, 681
Ancestral Rockies, 38
Ancestral Rockies system, 249
Anguille Mountains, Fig. 13.1
Aniakchak Tertiary province,

618
Anticosti Island, 219
Antillean-Caribbean region, 670
Antler orogenic belt, Fig. 6.15,

84
Antler orogeny, 83
Antler Peak quadrangle, 263
Apache (Reltian) group, 29
Appalachia, 169
Appalachian basin, Fig. 5.1, 40
Appalachian epeirogeny, 146
Appalachian geosyncline, Fig.

5.1, 40, 97
Appalachian Mountains, Fig.

8.3, 36, 40, 91, 97
Appalachian orogeny, 37, 217
Appalachian Plateau province,

Fig. 7.1, 91, 92, 94
Appalachian salients and re-
cesses, 100
Arbuckle Mountains, Fig. 14.2,

239
Arctic Archipelago, 23
Arctic Coastal Plain, 639

Arctic foothills belt, Fig. 39.2,

619
Arctic stable region, 26
Ardmore basin, 242
Arden draw thrust, Fig. 14.8,

231
Arguello deep-sea fan, Fig. 32.2
Arizaba volcano, 701
Arizona Mountain region, 426
Arizona, southeastern, 428
Arizona, west-central, 438
Arkansas valley, 55
Artillery Mountains, Figs. 27.2,

27.9
Adantic Coastal Plain, Figs. 7.1,

7.3, 91, 94, 135
Atlantic continental shelf, 139
Atlantic ocean crust, 139
Attu Island, 519
Avalon Penninsula, Fig. 13.1,

210
Aves swell, 692
Axel Heiberg Island, Fig. 40.1

Baffin Bay, Figs. 40.1, 40.11

Baffin Island, 26

Bahama Islands, 682

Baird Mountains, Fig. 39.1

Baja California, 480

Baker, Oregon, region, 75

Baker-Glendive anticline, 360

Balcones fault zone, 654

Bannock thrust, 336

Barbados, 686, 692

Basins of Pennsylvania-New
York region, Fig. 8.6, 103

Basins and banks, southern Cali-
fornia, 515

Basin and Range system, 494

Bathhurst Island, Fig. 40.3

Batholiths of the international
border, 278

Battle Mountains, Fig. 6.9, 76

Bay of Fundy, 786

Bayonne batholith, 276

Bearpaw Mountains, Fig. 23.1,
357

Beartooth Mountains, Fig. 23.1,
24.1, 363

Beata ridge, Fig. 42.1

739

Beaverhead dome, Fig. 6.5

Beaufort Sea basin, 644

Belle Isle, Newfoundland, Fig.

13.1, 13.11
Belt Island, 59
Beltian geanticline, 584
Beltian orogenic belt, 30
Beltian sequences, 29
Beltian strata, 29, 78
Beltian trough, Fig. 19.4, 32,

299
Bend axis, 246
Bering land bridge, 624
Bering Sea, 521
Berkeley Hills block, 466
Berkshire Mountains, 161
Bermuda rise, 140
Big Belt Mountains, Fig. 23.1
Big Cottonwood series, 29
Big Pine fault, 471
Big Snowy basin, Fig. 6.6
Big Snowy Range, Fig. 23.1
Bighom basin, 372
Bighorn Range, 372
Bisbee district, 428
Bitterroot Range, 320, 503
Black Hills, Figs. 24.1, 24.11,

27, 58, 374
Black Hills igneous rocks, 565
Black Mesa basin, 408
Black Warrior basin, Fig. 7.1
Blacktail Range, Fig. 22.5
Blake Bahama basin, Fig. 10.12,

151
Blake Plateau, Fig. 10.6, 148
Blood Creek syncline, Fig. 23.1
Blue Mountains, Fig. 29.15, 74
Blue Mountains— Ochoco Moun-
tains uplift, 74
Blue Ridge front, 105
Blue Ridge Plateau, Fig. 8.13,

108
Blue Ridge province, Fig. 7.1,

92, 95, 97, 102, 107
Boktukol thrust, Fig. 14.2, 239
Boothia arch, Fig. 39.13, 635
Boothia Peninsula, 26
Boston basin, 185
Boulder batholith, 321, 579
Bourbon arch, Fig. 5.13, 51, 52

Bowdoin dome, Fig. 23.1
Bowers Bank, 521
Brazeau River area, 314
Brevard schist belt, 119
British Honduras, 70}
British Mountains, 626
Bronson Hill anticline, 175
Brooks Range, Fig. 39.1, 610,
618

Cabot Strait fault, 21 S
Cache Creek group, 79
Cache thrust, Fig. 22.9, 334
Caledonian orogeny, 216
California troueh. Fig. 17.9
Cambrian strata, 52, 55, 61, 85,

84, 102
Cambridge arch, 55
Canadian Arctic, 633
Canadian cordillera, 80. 85
Canadian Rockies, Fig. 19.1. M.

297, 302
Canadian shield, Fig. 5.1, 22.

40, 51, 65
Candclaria Hills, 48
Canyon Ranee thrust. Fin. 22.18
Cape Breton Island, Fie. 12 1.

196
Cape Fear arch, 138
Cape Henry, 139
Cape Mendocino, 467
Capistrano embayment, 439
Carihbees, 670
Carlisle prong. 107
Carolina slate belt, 119
Cascade Mountains, high. Fie.

29.15
Cascade Mountains, northern,

Fig. 29.15
Cascade Mountains, western,

Fig. 29.15
Cascade volcanic complex, 555
Cascadia, 66
Cassiar Mountains, 84
Castle Mountain Ranee. Fig.

29.7
Cat Creek fault /one. Fie. 23.1
Catalina uplift. Fie. 29.3
Catalina, 274
Cataract axis, 37

740

INDEX

Catoctin belt, 103

Catoctin Mountain, 198

Catskill delta, 99

Catskill Mountains, Fig. 8.8,

105, 154
Cayman trench, Fig. 41.15, 693,

704
Cedar Creek anticline, Fig. 23.1
Cedar Hills orogeny, 328
Cedros Island, 481
Central America, 700
Central basin platform, 248
Central Coast Ranges of Cali-
fornia, Fig. 29.1, 453
Central Colorado basin, Fig.

6.7, 250
Central Kansas arch, 37, 41
Central Montana Rockies, 351
Central New Mexico porphyry

belt, 405
Central Rockies, Fig. 19.1, 295,

298, 327
Central stable region, 26, 37, 65
Central Wasatch Mountains, 27
Chaleur Bay, 195
Champlain thrust, Fig. 11.1, 162
Champlain trough, 169
Channeled scabland, Fig. 29.15
Charleston thrust, Fig. 22.15,

336
Chautaugua arch, 38, 55
Chief Mountain, 308
Chihuahua, 440
Chile, central, Fig. 34.5
Chile, southern, Fig. 34.5
Chiricahua Mountains, Fig.

27.6, 430
Chisos Mountains, 565
Cherokee basin, 52
Cherry Creek group, 27
Chiapas, 701
Churchill province, Figs. 4.1,

4.3, 23, 25
Choctaw fault, Figs. 14.2, 14.3,

239
Chugach-St. Elias fault, Fig.

39.2, 630
Chukotski Peninsula, Fig. 39.1
Chupadero Mesa, Fig. 25.16
Chuska Mountains, Fig. 26.9

Cincinnati arch, 40, 48, 99, 106
Cincinnati dome, 48
Circle Cliffs uplift, 408
Circum-Pacific tectonics, 532
Clarion fracture zone, Fig. 32.12
Clipperton fracture zone, Fig.

32.12
Coahuila peninsula, 234, 440
Coalburg syncline, Fig. 8.12
Coast Range batholith, 275, 280
Coast Ranges of Oregon and

Washington, 474
Coast Ranges of the Pacific, 452
Coats Island, 23
Coconino Plateau, Fig. 26.8
Coeur d'Alene district, Fig. 21.3
Colima volcano, 701
Colombian basin, Fig. 41.15
Colorado Plateau, Fig. 19.1, 61,

297, 301, 407
Colorado Plateau igneous rocks,

564
Colorado Range, 250
Colorado Rockies, Fig. 19.1,

297, 300, 389
Colorado sag, Fig. 65, 39
Columbia River basalts, 559
Columbia system, 274
Colville basin, Fig. 39.6
Colville batholith, 280
Comb Ridge monocline, Fig.

26.7
Conception Bay, Fig. 13.1
Concho arch, 246
Connecticut Valley basin, 131
Continental drift, 646
Cook inlet Tertiary province,

618
Cordilleran geanticline, 330
Cordilleran geosyncline, 37, 61,

63,89
Cornwallis fold belt, 637
Cortez Mountains, Fig. 6.9, 76
Costa Rica, 701
Cottonwood dome, Fig. 22.5
Cows Head, 204
Crazy Mountains, Fig. 23.1
Crazy Mountains volcanic field,

571
Criner hills, Fig. 14.2, 239

Crowsnest volcanic field, 572
Crustal tension, 601
Cryptovolcanic structures, 256
Cuba, 670

Cumberland Mountain, 103
Cumberland overthrust, Fig.

8.15, 110
Cumberland Plateau, 93

Dagger flat anticlinorium, Fig.

14.8, 231
Darby thrust, Fig. 22.9, 334
Datil lava field, Fig. 27.1
Davis Mountains, 405, 565
Death Valley, 500
Decaturville structure, Fig. 16.1,

257
Deep River basin, 131
Deep-sea fans, 518
Deep-sea plain, Fig. 32.2
Deep-seated earthquakes, 601
Deer Creek thrust, Fig. 22.15,

336
Defiance uplift, 408
Delaware basin, 248
Delgado deep-sea fan, Fig. 32.2
DeLong Range, Fig. 39.1
Denali fault, 631
Denver basin, Fig. 25.2, 393
Deschutes-Umatilla Plateau,

Fig. 29.15
Devon Island, 26
Diablo uplift, California, 249,

453
Diamond Peak basin, Fig. 6.7
Diatremes, 422
Disco Island, 23
District of MacKenzie, 624
Dixon Entrance, Fig. 39.1
Dominican Republic, 676
Douglas fault, 255
Dragoon Mountains, Fig. 27.9
Dugout Creek thrust, Fig. 14.8
Duluth gabbro, 32
Durst thrust, Fig. 22.14, 336
Dutch Leeward Islands, 685

East Kaibab monocline, Fig.

26.8
East Pacific rise, 512

East Texas basin, 655
East Tintic Mountains, 345
Eastern Interior basin, Figs.

5.1, 5.9, 5.11, 40, 46, 47
Eastern Triassic basins, 128
Ecuador, Fig. 34.5
Eeel River embayment, 467
El Paso- Rio Grande thrust belt,

445
El Salvador, 701
Elkhead Mountains (Colorado)

volcanic field, 565
Elkhorn Mountains (Montana)

field, 578
Ellesmere, northern, fold belt,

637
Ellesmere-Greenland fold belt,

637
Ellesmere Island, 26
Ellis arch, 38, 51
Elsinor fault, 469
Empire Mountains, Fig. 27.6
Endicott Mountains, Fig. 39.1
Espana basin, Fig. 25.11
Estancia basin, Fig. 25.11
Eureka, Figs. 6.9, 22.24
Exploits basin, Fig. 13.1

Faeroes, 642

Fairweather fault, Fig. 39.2, 617

Fallon earthquake area, 506

Farewell-Shakwak fault, 633

Findlay arch, 48, 51

Finlay River volcanic field, 586

Florida Mountains, Fig. 25.16

Florida Paleozoic, 119

Florida platform, 666

Florida uplift, 251

Flynn Creek disturbance, Fig.

16.1, 257
Foothills or Foothill belt, 311
Foreland arcuate fault zone, 253
Forest City basin, Fig. 5.14, 40,

51, 53
Fort Nelson, Fig. 20.10
Fort Worth basin, Fig. 15.11,

246
Fortune Bay, Fig. 13.1
Foxe basin, 635

Fracture system, coastland Brit-
ish Columbia, Fig. 17.21

Franciscan basin, 272

Francisco-Marin block, 466

Franklin Mountains, Fig. 39.1,
626

Franks graben, Fig. 15.6, 242

Frederick sound cross folds, Fig.
17.19, 280

French Broad River, 108

Front Range, 27, 390

Front Range igneous rocks, 570

Frontenac axis, 22

Gabilan Mesa, 274

Gallop-Zuni basin, Fig. 26.11

Gannett orogeny, 293

Garlock fault, 471

Gaspe, 195

Genou trend, Fig. 23.1

George Vasen's fee well, 660

Georges Bank, 142, 186

Glacier Park, 30

Glass Mountains, Fig. 14.8, 231,

405
Glenarm series, 121
Golconda thrust, 263
Gold Hill district, Fig. 22.23,

294
Gore Range, 393
Graham Island, Fig. 17.23
Grand Canyon, 28
Grand Canyon of the Colorado,

Fig. 26.4, 408
Grand Valley, 504
Great Bahama bank, Figs. 42.1,

42.2
Great Basin, Fig. 22.7, 332, 493
Great Basin seismic layers, 511,

595
Great Smoky Mountains, 98,

109
Great Valley, Fig. 8.13, 103,

108
Greater Acadia, 220
Greater Antilles, 670
Green Mountains, 161, 171
Green River basin, 383
Greenland, 640
Grenada basin, 692

Grenville belt, province, 23, 26
Grenville orogenic belt, 34
Grenville orogeny, 36
Gros Ventre Range, 361
Guadalupe uplift, 405
Guanajuato, Fig. 43.2
Guatemala, 701
Guatemala basin, 523
Gulf of Alaska, Fig. 32.1
Gulf of California, 472, 489
Gulf Coastal Plain, 91, 650
Gulf of Maine, 186
Gulf of Mexico, 667

Half dome, 271
Haiti, 676
Hanna basin, 378
Harderman basin, 239
Harpers ferry, 108
Hartville uplift, 387
Hatchetigbee anticline, 552
Hatteras abyssal plain, Fig.

10.6, 148
Hawthorne Quadrangle, Ne-
vada, 262
; Haymond thrust, Fig. 14.8, 231
Hayward fault, Fig. 29.8, 469
Heart Mountain, 367
Heart Mountain thrust, 332
Hecate basin, Fig. 17.18
Hell's Half Acre thrust, Fig.

14.8, 231
Henry Mountains, 408, 418
High Plateaus of Utah, 422, 500
Highlandcroft magma, 176
Highwood Mountains, Fig. 23.1
Hinesburg synclinorium, Fig.

11.11, 162
Hispaniola, 676
Hoback Range and basin, Fig.

22.9
Hogan volcanic field, 578
Hogata arch, Fig. 39.2
Holbrook Range, Fig. 6.3
Hopi Buttes, 421
Hopi Buttes volcanic field, 564
Hot Springs Range, Fig. 6.9, 76
Housatonic highland, 158
Hudson Bay, 22
Hudson Bay basin, 38

Hudson Canyon, 142
Hudson highland, 158
Hudson Valley, 105, 154
Huerfano Park, Fig. 25.8
Humboldt Range, 74, 497
Huntington Lake, Fig. 17.11,

240
Hunton anticline, Fig. 15.6, 242
Hunton arch, Fig. 14.1
Hunton-Tishomingo uplift, Fig.

14.1
Huronian group, 34
Hurricane fault, Figs. 22.21,

22.22

Iceland, Fig. 40.13, 642

Idaho batholith, Fig. 19.1, 297,

319
Ididarod-Nixon fault, Fig. 39.2
Igneous provinces, concept, 532
Igneous provinces, western U.S.,

553
Ignimbrite subprovince, Fig.

36.3
Illinois basin, Fig. 5.10, 46, 50
Innuitian region, 636
Iron Springs district, Figs.

22.19, 22.20
Isthmian volcanic link, 703
Ixtaccihuatl volcano, 701

Jackson dome, 656

Jackson Hole, 503

Jackson thrust, Fig. 22.9, 334

Jamaica, 680

Jan Mayen Island, 642

Japanese archipelago, Fig. 6.20,

89, 90
Jemez Caldera, Fig. 25.11, 572
Jeptha Knob, Fig. 16.1, 256
Jessamine dome, 48
John Day basin, 559
Johnston Lake, 27
Jornado del Muerto, Fig. 25.11
Judith Mountains, Fig. 23.2

Kaibab monocline, Fig. 26.8
Kaibab uplift, 408
Kaiparowits basin, 408
Kamloops Lake, 274

INDEX

Kankakee arch, 39, 48, 50
Kansas arch, Fig. 5.15, 51
Katalla district, 615
Kenai Peninsula, 608
Kentland structure, Fig. 16.1,

257
Kerr basin, Fig. 15.11, 247
Kettleman hills, 465
Kevin-Sunburst dome, 292
Keweenawan belt, 32
Keweenawan fault, 255
Keweenawan series, 27, 32
Killarnean orogeny, 32
Klamath Mountains, Fig. 6.13,

64, 72, 81
Klamath peneplain, 467
Klamathonia, 274
Kobuk basin, Fig. 39.2
Kodiak Island, 608
Kootenay Lake, 276
Kuiu-Heceta belt, 280
Kuskokwin basin, 611

La Paz fault, Fig. 30.1, 485
La Sal Mountains, Fig. 26.7
La Salle anticlinal belt, 45, 50
Labrador, 23
Laguna district, Fig. 28.7
Lake basin fault zone, Fig. 23.1
Lake Champlain, 157, 166
Lake Superior basin, 44
Lake Superior fault zone, 255
Lampasas axis, 247
Lanoria, 232
Laramide orogeny, 295
Laramie Range and Basin, 385
Las Animas arch, 75, 238
Late Cenozoic uplift, 513
Late Devonian orogeny, 70
Latite magmas, origin, 577
Latite province, Basin and

Range, 573
Lesser Antilles, 683, 692
Leucite hills, 656
Lewis and Clark Line, 322
Lewis thrust, Figs. 20.2, 20.3,

20.7, 311
Lisbon Valley, Fig. 26.12
Little Belt Mountains, Fig. 23.1

Little Rocky Mountains, Figs.

23.1, 23.5, 357
Lituya district, 617
Livingston field, 571
Llano uplift, 246
Logan's line or fault, 198
Lombard thrust, Fig. 20.2
Lomonosov Range, Fig. 40.6
Long Range, 210
Los Angeles basin, Fig. 29.3
Lost River Range, 73
Lucero Mountains, Fig. 25.11
Luling fault zone, 6.54

Mac thrust, 263

Mackenzie Mountains, Fig.

39.1, 626

Mackinac Straits, Fig. 5.7, 41,

45
Madison Range, Fig. 22.7
Magdalen Islands, 196
Magmas, origin, 591
Magnet Cove, Figs. 14.2, 16.1,

239, 258
Magog trough, 169
Makarov basin, 644
Malaspina district, Fig. 39.8
Malheur Plateau, Fig. 29.1
Manhattan geanticline, 70
Manhattan prong, Fig. 8.21
Mansel Island, 23
Marathon anticlinorium, Fig.

14.8, 231
Marathon system, 38, 231
Mazatzal revolution, 28
Medicine Bow Range, 385
Medicine Lodge thrust, 334
Melville peninsula, 26
Mendocino escarpment, Fig.

32.2, 524

Merrimack synclinorium, 176
Metaline sequences, 29
Meteorite impact craters, 256
Mexia fault zone, 654
Mexican basin, 693
Mexican foothills belt, 451
Mexican geosyncline, 442
Mexico, crystalline belt, 696
Mexico, southern, 696
Mexico, tectonic provinces, 549

711

Mexico Igneous proi in. •
Mic bigan basin, Fig, 5 I
Mid-Atlantic ridge, Fig. 10.11,

151
Mid-Nevada eugeosyni In

6.3, 63, 64
Mid-.\Y\ ada miogeosyncline,

Fig. 6.3, 83, <)1
Mid-Pai iflc Mountains, E
Middle \iiutk .i treni h. 522
Middle Park, 390
Middleburg synclinoiium

11.11, 162
Midland basin, Fie 15.12, 2 IS
Mill Creek syiulinr. Fig. 15.6,

212
Minas basin, 196
Mint.) arch, Fig. 29.13
Mississippi embavment, Fig.

41.1
Mistassini group, 34
Misteriosa Bank. 70 1
Moab fault, Fig. 2H.12
Moat volcanics, 176
Moccasin Mountain'-. Fie. 23.2
Modoc Plateau, Fins. 29.2,

29.15
Mogollon Plateau, Fie. 26.9
Mi Hon Rim. 1-1
Mojavia, Fig. 29.3
Mona passage, 680
Mono Lake basin, 505
Monroe dirust, 176
Monroe uplift, 65S
Montana, southwestern, 329
Montana block. 466
Montana Rockies, Fig. 19.1, 297,

302, 351
Monterey deep-sea fan, Fig.

32.2
Monterigian Hills, 198
Monument uplift. 408

Mother Lode thrust. 268
Mount Edgecombe, :2s"
Mount Logan fault. Fie. 39.2,

630
Mount Taylor field. 572
Muenster anticline. 239
Murphy marble belt. Fig. 8.22

742

INDEX

Murray escarpment, Fig. 32.2,
524

Nansen's sill, 644

Narragansett basin, 182

Nashville dome, 48, 99

Navajo volcanic field, 421

Nebo thrust, Fig. 22.16, 341

Nelson batholith, 275

Nemaha Range, Fig. 5.13, 51,
52

Nevadan orogeny, Fig. 17.7, 61,
268, 278, 437

New Brunswick, 189

New England Appalachian sys-
tem, 154

New Hampshire magma series,
176

New Jersey-Pennsylvania-Mary-
land-Virginia basin, 128

New Mexico porphyry belt, 405

New Mexico Rockies, Fig. 19.1,
297, 300, 398

Newark basin, 130, 134

Newfoundland, 203

Nicaragua, 701

Nicaraguan rise, Fig. 41.15

Noonsocket basin, 185

North Park, 390

North Park thrust, 393

North shore fault, 33

Northeastern Mexico Rockies,
Fig. 19.1, 297, 300, 440

Northern California, 266

Northern Coast Ranges, 466

Northey Hill thrust, 176

Northwest subprovince, Fig.
4.1, 23

Notre Dame Bay, Fig. 13.1

Nova Scotia, 189

Nye-Bowler fault zone, Fig.
23.1

Oakland anticline, 45
Oaxaca, Mexico, Fig. 43.1
Ochoco-Blue Mountains, 278
Octavia fault, Fig. 14.2, 239
Ocula uplift, 666
Ogden thrust, Fig. 22.14, 336
Okanogan batholith, 278

Okanogan highland, Fig. 29.15,

76
Okanogan Range, 76
Okanogan Valley, 76
Oliverian magma, 176
Olivine basalt, 533
Olympic Mountains, Fig. 29.15
Ontario, 24, 41
Oquirrh basin, 72
Oquirrh Mountains, Fig. 22.13,

336
Orwell thrust, Fig. 11.11, 162
Osage County en echelon faults,

254
Osburn fault zone, Fig. 21.3,

319
Osgood Mountains, Fig. 6.9, 76
Oswegan disturbance, 170
Ouachita Mountains, 223
Ouachita orogenic belt, 37, 52
Ouachita system, 38
Owl Creek Mountains, 365
Owyhee rhyolite, 562
Owyhee upland, Fig. 29.15
Ozark basin, 52
Ozark dome, 52
Ozona platform, Fig. 15.4

Pacific fracture zones, 524
Pacific magnetic surveys, 526
Pacific submarine provinces,

515, 525
Pacifica, 274
Palisades orogeny, 134
Palo Duro basin, 239
Palomas basin, Fig. 25.11
Panama, 703
Panhandle of Texas, 39
Paradox basin, 73
Parana basin basalt field, 547
Paricutin volcano, 701
Parras basin, Fig. 28.2
Parry Islands fold belt, 636
Pecos Range, Fig. 15.12, 248
Pedernal uplift, 250
Pedro banks, Fig. 42.1
Peninsular arch, 666
Penn-Colorado synclinorium,

Fig. 14.8, 231

Pennsylvanian basin, Fig. 6.7,

74
Penokean orogenic belt, 26
Penokean province, Fig. 4.3, 25
Peridotite intrusions, Figs. 8.28,

8.29
Permian basin, 73
Peru, Fig. 34.5

Petrographic provinces, con-
cept, 532
Philipsbury batholith, 321
Piceance Creek basin, 408
Piedmont crystalline province,

Fig. 7.1, 92, 107, 114
Piedmont orogeny, 36
Pine Mountain belt, 117
Pine Mountain fault, Fig. 14.2,

239
Pine Valley Mountains, Fig.

22.21
Pioneer Ridge, 528
Plateau central, 445
Point Arena, 467
( Point ) Barrow geanticline,

Fig. 39.4, 613
Pontotoc axis, 247
Popocatepetl volcano, 701
Porcupine dome, Fig. 23.1
Porphyry belt, 394
Potatoe Hills, 229
Potomac River, 108
Powder River basin, 374
Pribilof Islands, 521
Primary magmas, 581
Prince Edward Island, 191
Prince of Wales-Chichagof belt,

280
Provo Wasatch, 341
Pryor Mountains, Fig. 23.1
Puerco platform, Fig. 25.11
Puerto Rico, 678
Puerto Rico trench, 687, 690
Pulaski thrust, Fig. 8.13
Purcell Range, 29

Queen Charlotte Islands, Figs.

17.18, 17.23, 275
Quitman Mountains, Fig. 25.16,

565

Raft River geanticline, Fig. 6.2,

66
Raft River Mountains, 70
Raton basin, Fig. 25.11
Rattlesnake Hills, 565
Rawlins uplift, 382
Ray and Miami districts, Fig.

27.6
Reading prong, Figs. 7.3, 8.30
Red River uplift, 239
Richardson Mountains, 626, 629
Richland Springs axis, 247
Ring-dikes, 178
Rio Grande depression, 399
Rio Grande embayment, Fig.

41.1
Rio Grande rift, Fig. 25.12, 402
Roberts Mountains, Fig. 6.9, 76
Rock Springs uplift, 384
Rockingham anticlinorium, 175,

181
Rocky Mountain trench, 306,

317, 586
Rome thrust, Fig. 8.12
Rosaline Banks, Fig. 42.1
Rose Hill district, 103
Rough Creek fault, 28, 254
Rough Creek-Shawneetown sys-
tem, 48
Ruby Range, 497

Sabine uplift, 655

Sacramento Mountains, Figs.

25.11, 25.16
Saint Elias Range, Figs. 39.1,

39.9
Saint John thrust, Fig. 22.9,

334
Saint Lawrence lowlands, 166
Salina basin, 33
Salinia, Fig. 29.3
Salt anticlines, 415
Salt domes, 658
Salton basin, 493
San Andreas channel, Fig. 29.6
San Andreas fault, 452, 469
San Andres Mountains, Fig.

25.11
San Carlos Mountains, Fig.

28.8, 565

San Francisco volcanic field, 573
San Jacinto fault, 469
San Joaquin embayment, 452
San Joaquin Valley, Fig. 29.5
San Juan basin, Fig. 25.11, 408
San Juan Mountains, Fig. 25.2,

408
San Juan volcanic field, 569
San Marcos arch, Fig. 41.1
San Rafael Swell, Figs. 25.12,

26.1, 408
San Saba axis, 247
Sandia Mountains, Fig. 25.11
Sandia uplift, Fig. 25.12
Sangre de Cristo Range, 394
Santa Ana Mountains, 268
Santa Barbara district, Fig.

29.11
Santa Cruz Mountains, Fig. 29.5
Santa Lucia orogeny, 272
Santa Lucia Range, Fig. 29.8
Santa Maria basin, Fig. 29.6
Santa Rita Mountains, Figs.

27.3, 27.6
Savanna— Sabula anticline, 45
Sawatch Range, Fig. 25.2
Sebastian Viscaino, 481
Seismicity in California, Fig.

29.14
Seismicity in trench zone, Fig.

31.15
Selkirk Range, 61, 319
Selwyn Mountains, Fig. 39.1
Semisopochnoi Island, 521
Seneca fault, 254
Sequatchi anticline, Fig. 7.1,

95
Serpent Mound, Fig. 16.1, 257
Serpentine intrusions, Figs.

8.28, 8.29
Sevier fault, 502
Seward Peninsula, Fig. 39.1
Shawak fault, Fig. 39.2
Shawneetown fault, 254
Shawneetown-Rough Creeks

fault zone, 254
Sheeprock Mountains, 345
Shenandoah National Park, 124
Sheridan arch, 59
Sherman Hill, Michigan, 255

INDEX

743

Shichshock Mountains, 194
Shonkin Sag laccolith, Fig. 23.6
Shuswap orogenic belt, Figs.

6.5, 6.6, 69, 79, 81
Siberian connections, 623
Sierra de Parras, Fig. 28.4
Sierra Grande arch, Figs. 6.6,

6.8, 25.16, 71, 75
Sierra Ladron, Fig. 25.13
Sierra Lucero, Fig. 25.13
Sierra Madre del Sur, Fig. 35.1
Sierra Madre Occidental, Figs.

28.1, 35.1, 445, 490
Sierra Madre Oriental, Figs.

28.1, 28.5
Sierra Nacimiento, Fig. 25.13
j Sierra Nevada batholith, 265
I Sierra Nevada block, 496
Sierra Nevada Mountains, 72
Sierra Nevada root, Fig. 38.1
Sierra Talmanca, 703
Sierra Tamaulipas, Fig. 41.12
Similkameen district, 279
Sitka belt, Fig. 17.19, 280
I Slave province, Fig. 4.3, 25
• Snake River basalts, 562
. Snake River canyon, 76
1 Snake River downwarp, Fig.

22.1, 328
Sohm abyssal plain, Fig. 10.11.

151
Solitario, Fig. 36.5
Sonoma orogeny, Fig. 17.1, 268
Sonoma Range, 264
Sonora, central, Fig. 28.3
Sonoran Desert, 426, 491
Sonoran Rockies, Fig. 19.1, 297,

300, 480
South America, igneous prov-
inces, 537
South arch, Fig. 23.1

South Park, 390
Southampton Island, 23
Southeastern Alaska, 277
Southeastern Idaho, 334
Southern Arizona Rockies, Fig.

19.1, 297, 299, 426
Southern California, 267
Southern Coast Ranges, 462
Southern Nevada, 346
Southwestern Montana, 329
Southwestern Utah, 343
Spanish Peaks, 570
Spitzbergen, Fig. 40.6
Stansbury anticline, Fig. 6.5
Stockton arch, Fig. 29.6
Stratovolcanoes, 589
Submarine canyons, Atlantic,

140
Submarine valleys, Pacific
Sudbury thrust, Fig. 11.11, 162
"Superior" province, 27
Sverdrup basin, Fig. 39.13, 639
Sweetgrass arch, Fig. 18.1, 59,

68, 291
Sweetgrass Hills, Fig. 23.1
Sweetwater Range, 376

Taconic allochthone, Fig. 11.10,

162
Taconic belt, 120
Taconic Mountains, 162
Taconic orogeny, 36, 157, 170,

180, 208, 216
Talkeetna fault, Fig. 39.2
Taxco, Fig. 43.2
Taylor thrust, Fig. 22.14, 336
Tectono-igneous cycle, 534
Tehuantepec ridge, 523
Temblor Range, Fig. 29.5
Tendoy thrust, 334
Terlingua-Solitario region, 568

Terminology, 4

Teton fault, 503

Teton Range, 27, 361

Texas arch, Fig. 15.7, 245

Texas foreland, 245

Tholeiitic basalt, 533

Ti Valley fault, Fig. 14.2, 239

Tigara uplift, Fig. 39.2

Timiskaming subprovince, Fig.
4.1, 23

Tintina fault, Fig. 39.2

Tishomingo anticline, Fig. 15.6,
242

Titicaca trough, Fig. 34.5

Tobago, 686

Tobin thrust, 263

Tonopah quadrangle, Nevada,
262

Tooele arch, 68

Tortuga Island, 481

Trachyte and phonolite prov-
inces, 563

Transcontinental Arch, 38, 48,
55, 68

Transverse Ranges, 462

Trenches, zone of great, 506

Triassic lowlands, Fig. 7.3, 94

Trinidad, 686

Tucson Mountains, Fig. 27.6

Tularosa Valley, Fig. 25.11

Tushar fault, 502

Tyler basin, 655

Uinta basin, Fig. 26.10, 408
Uinta Mountains, 384
Uinta series, 29
Uncompahgre Range, 250
Uncompahgre uplift, 408
Ungava Bay, 22
Ungava province, Fig. 4.1, 23
Upheaval dome, Fig. 16.1, 257,
421

Val Verde basin, Fig. 15.1
Valley and Ridge province, 40,

97, 108
Vancouver Island, Fig. 17.18,

274
Vellecitos channel, Fig. 29.6
Venezuela, northern, 687
Venezuelan basin, Fig. 42.11,

692
Ventura basin, Fig. 29.6
Vermont, central, 172
VermontJa geanticline, 169
Victoria Strait basin, Fig. 39.13,

635
Virgin Islands, 681
Vishnu schist, 28

Wabash River anticline, 45
Wabash Valley fault system, 48
Wasatch line, Fig. 6.1
Wasatch Mountains, Fig. 22.13,

336, 499
Wasatch Plateau, Figs. 22.17,

22.18
Washakie basin, 382
Washakie Range, 365
Waterpocket monocline, 409
Waverly arch, 49
Wellington arch, Fig. 39.13,

635
Wells Creek basin, Fig. 16.1,

257
Wendover arch, Fig. 6.6
West Texas basin, Fig. 15.7,

245
Western Canada basin, Fig.

5.20, 59
Western Interior basin. Fig. 5.1,

40
Western Wyoming, 334
White Mountain magma, 176

White Mountains, 173

White Rivet Plateau, 408

Wichita Mountains, 33, 237

Wichita system, 38.

Wiggins anticline, 660

Willamette— Puget Sound de-
pression, Fig. 29.15

Willard thrust, Fig. 22.14, 336

Williston basin, 55

Wind River basin, 377

Wind River Range, 361

Windingstair fault, Fig. 14.2,
239

Windward Islands, 683, 697

Winnemucca quadrangle, 264

Winnipeg River, 27

Wisconsin dome, 39

Wollaston basin, Fig. 39.13, 635

Wood Ri\er basin, 74

Wrangell Mountains volcanic
field, 620

Wrangell-Re\ illagiyedo belt,
280

Wyoming Rockies, Fig. 19.1,
297, 299

Wyoming shelf, 58

Yakataga basin, Fig. 39.2, 615
Yakutat Bay, 516
Yellowstone Park, 369, 571
Yosemite, Fig. 17.8, 268
Yucatan basin, Fig. 41.15
Yucatan Peninsula, 701, 705
Yucatan platform, 669
Yukon, 29, 605, 624
Yukon Plateau, Fig. 39.1

Zaccatecas, Fig. 43.2

Zuni Mountains. Figs. 25.16,

26.9
Zuni uplift, 251

\-^l

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