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Full text of "The Bedrock geology of Massachusetts"

The Bedrock Geology of 
Massachusetts 




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The Bedrock Geology of 
Massachusetts 

NORMAN L. HATCH, JR., Editor 

E. Stratigraphy of the Milford-Dedham Zone, Eastern 
Massachusetts: An Avalonian Terrane 

By RICHARD GOLDSMITH 

F. Stratigraphy of the Nashoba Zone, Eastern Massachusetts: An 

Enigmatic Terrane 

By RICHARD GOLDSMITH 

G. Stratigraphy of the Merrimack Belt, Central Massachusetts 
By PETER ROBINSON and RICHARD GOLDSMITH 

H. Structural and Metamorphic History of Eastern Massachusetts 
By RICHARD GOLDSMITH 

I. Intrusive Rocks of Eastern Massachusetts 

By DAVID R. WONES and RICHARD GOLDSMITH 

J. Radiometric Ages of Rocks in Massachusetts 

By ROBERT E. ZARTMAN and RICHARD F. MARVIN 

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1366-E-J 



Chapters E-J are issued as a single volume 
and are not available separately 



FftAiu 

OOCUMENTS Exi-F')iTWC Wttiw? 
80V«NMENT DOCUMENTS DEHARTMENI 

FEQ92 




UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1991 



U.S. DEPARTMENT OF THE INTERIOR 
MANUEL LUJAN, Jr., Secretary 

U.S. GEOLOGICAL SURVEY 
Dallas L. Peck, Director 



Any use of trade, product, or firm names in this publication is for 

descriptive purposes only and does not imply endorsement by the 

U.S. Government 



Library of Congress Cataloging in Publication Data 

The bedrock geology of Massachusetts. 

(U.S. Geological Survey professional paper ; 1366-E-J) 

Bibliography: p. 

Supt. of Docs, no.: I 19.16:1366E^I 

1. Geology— Massachusetts. I. Hatch, Norman L., Jr. II. Series: Geological Survey professional paper ; 1366-E-J. 

QE123.B44 1988b 557.44 87-600472 

For sale by the Books and Open-File Reports Section, U.S. Geological Survey, 
Federal Center, Box 25425, Denver, CO 80225 



Editor's Preface to Chapters E through J 



This Professional Paper was planned as a companion to 
the bedrock geologic map of Massachusetts (Zen and 
others, 1983; hereafter referred to as the State bedrock 
map). It is being published as lettered chapters of 
Professional Paper 1366, six of which are included in this 
volume, and four of which were published in a single 
volume as chapters A-D (Hatch, 1988). Compilation of 
the geology for the State bedrock map was completed in 
1980. Some of the chapters in this Professional Paper 
reflect field or laboratory data that were gleaned as much 
as 6 years later. Each chapter was prepared, however, 
with the objective of explaining and further describing 
the geology as portrayed on the State bedrock map. In 
some instances, information and interpretations devel- 
oped since 1980 have caused chapter authors to suggest 
revisions that they would make to the map if they were 
able to redraw it, but in each case these suggested 
revisions are discussed in the context of the map as it was 
published. 

The previous State bedrock map (which also showed 
the geology of Rhode Island) was published in 1917 by 
Benjamin K. Emerson as U.S. Geological Survey Bulle- 
tin 597. (The publication date of Bulletin 597 is 1917. 
Some confusion arises from the fact that the bedrock map 
of the two States, which is included in the pocket of the 
Bulletin, bears the date of 1916.) All who were involved 
in the preparation of the new bedrock map, particularly 
those responsible for the parts of the State in which 
Emerson himself had done the original field work, feel a 
great deal of respect for Professor Emerson and his 
remarkably perceptive and thorough understanding and 
portrayal of the geology. Although the new map is very 
different from Emerson's in many aspects, particularly 
with regard to the interpretation of the geologic history, 
the basic distribution of map units is remarkably similar. 

The State bedrock map and this report are direct 
outgrowths of a cooperative geologic mapping program 
between the U.S. Geological Survey and the Common- 
wealth of Massachusetts, which was begun in 1938. They 
also include the results of more than 25 years of mapping 
and topical studies by faculty and students at the Uni- 
versity of Massachusetts at Amherst and at many other 
colleges and universities. 

The subdivision of the material in this Professional 
Paper into the constituent chapters is based on the 



grouping of the 343 individual lithic units on the State 
bedrock map into the 8 lithotectonic packages discussed 
by Hatch and others (1984). The temporal and geo- 
graphic distributions of these eight packages are indi- 
cated on figures 1 and 2. Also indicated on the figures are 
the geographic and geologic coverages of the chapters 
included in this volume. In this packaging scheme, the 
older, primarily pre-Silurian, rocks of the State are 
grouped into five "zones" whose exposed and buried 
parts completely cover the State. From west to east, 
these zones are the Taconic-Berkshire, the Rowe- 
Hawley, the Bronson Hill, the Nashoba, and the Milford- 
Dedham. Their mutual boundaries are, or could reason- 
ably be interpreted to be, faults. Overlying and 
overlapping the zones in the central part of the State are 
the Connecticut Valley and Merrimack "belts" of prima- 
rily Silurian and Devonian strata. Their mutual boundary 
is somewhat arbitrarily taken to be the east contact of 
the easternmost exposed Silurian Clough Quartzite. 
Finally, the Mesozoic "basins" unconformably overlie the 
Connecticut Valley belt. 

For some packages, all aspects of the geology are 
treated in the same chapter. For others, aspects such as 
the structure, metamorphism, and tectonics are dis- 
cussed separately from stratigraphy and lithology. These 
differences in treatment resulted from peculiarities of 
the geology and the preferences of the individual 
authors. Many of the plutonic rocks of the State are 
described and discussed in chapter I. 

Many of the lithologic subdivisions of formal units on 
the State bedrock map have not been given formal 
names. In order to avoid potentially cumbersome discus- 
sions of such things as "the thick-bedded micaceous 
quartzite and mica schist unit of the XYZ Formation," 
many chapter authors have chosen to refer to such units 
simply by their map symbols. Thus the micaceous quartz- 
ite, quartz-mica-garnet schist, and calc-silicate unit of 
the Devonian Goshen Formation may be referred to 
simply by its map symbol "Dgq," but in a context where 
the reader will be easily guided to the correct unit. 

The terms "granulite" and "granofels" have been used 
rather arbitrarily and interchangeably throughout this 
Professional Paper, although on the State bedrock map 
the term "granofels" was used exclusively. Both terms 
are used to describe a metamorphic rock composed 



PREFACE 



predominantly of even-sized, interlocking granular min- 
erals; no implication as to the grade of metamorphism is 
intended by either term. The choice of words merely 
reflects individual author preference, and we hope that 
no confusion to the reader will result from the unre- 
strained use of two words for the same kind of rock. 

This volume contains six chapters that deal primarily 
with terranes of eastern Massachusetts, terranes that 
were amalgamated and accreted to North America dur- 
ing the Paleozoic. Chapter E, on the stratigraphy of the 
Milford-Dedham zone, deals with the stratified rocks of 
the easternmost part of the State, a terrane considered 
to represent part of Avalonia. Chapter F deals with the 
stratified rocks of the Nashoba zone, a "suspect terrane" 
west of the Milford-Dedham zone, that is bounded by the 
Bloody Bluff fault on the east and the Clinton-Newbury 



fault on the west. Chapter G describes the stratigraphy 
of the Silurian and Devonian rocks of the Merrimack belt, 
which adjoins and overlies the Nashoba zone on the west. 
Chapter H discusses the structure and metamorphism of 
the rocks of the Milford-Dedham and Nashoba zones and 
of the easternmost part of the Merrimack belt. Chapter 
I describes and discusses the critically distinctive intru- 
sive rocks of the eastern part of the State. Finally, 
Chapter J tabulates all of the isotopic ages on the rocks 
of the whole State available as of 1986. Most of these ages 
are from the abundant plutonic rocks of the eastern part 
of the State. 

We would herein like to acknowledge the invaluable 
contributions to this Professional Paper of two key 
people. Jewel Dickson did the cartographic work on the 
majority of the illustrations, the principal exceptions 




MIDDLE 
PROTEROZOIC 



Figure 1.— Diagram simplified from the "Correlation of Map Units" on the State bedrock map 
showing the eight lithotectonic packages into which the rock units have been divided. Also 
indicated are the letter designation(s) of the chapter(s) in this volume covering various aspects of 
the geology. Chapter J deals with rocks from the entire State and thus is not shown on the figure. 
Modified from Hatch and others (1984, fig. 1). 



PREFACE 



Chapter G 
MERRIMACK BELT ■ 




TACONIC- 
BERKSHIRE ZONE 



Nantucket 



Figure 2. — Map of Massachusetts showing the geographic distribution of the eight lithotectonic packages into which the rock units of the State 
have been grouped and the letter designation(s) of the chapter(s) in this volume in which aspects of the geology are discussed. Chapter J deals 
with rocks from the entire State and thus is not shown on the figure. Modified from Hatch and and others (1984, fig. 2). 



being those prepared by author Peter Robinson. Kath- 
leen Krafft Gohn suffered bravely over the years with 
the editor and authors of this Professional Paper as its 
technical editor. 



REFERENCES CITED 

Emerson, B.K., 1917, Geology of Massachusetts and Rhode Island: 
U.S. Geological Survey Bulletin 597, 289 p. 



Hatch, N.L., Jr., editor, 1988, The bedrock geology of Massachusetts: 
U.S. Geological Survey Professional Paper 1366- A-D, variously 



Hatch, N.L., Jr., Zen, E-an, Goldsmith, Richard, Ratcliffe, N.M., 
Robinson, Peter, and Wones, D.R., 1984, Lithotectonic assem- 
blages as portrayed on the new bedrock geologic map of Massa- 
chusetts: American Journal of Science, v. 284, p. 1026-1034. 

Zen, E-an, editor, Goldsmith, Richard, Ratcliffe, N.M., Robinson, 
Peter, and Stanley, R.S., compilers, 1983, Bedrock geologic map 
of Massachusetts: Reston, Va., U.S. Geological Survey, 3 sheets, 
scale 1:250,000. 



VOLUME CONTENTS 



[Letters designate chapte: 



Editor's Preface to Chapters E through J, by Norman L. Hatch, Jr. 

(E) Stratigraphy of the Milford-Dedham zone, eastern Massachusetts: An Avalonian terrane, by 

Richard Goldsmith 

(F) Stratigraphy of the Nashoba zone, eastern Massachusetts: An enigmatic terrane, by Richard 

Goldsmith 

(G) Stratigraphy of the Merrimack belt, central Massachusetts, by Peter Robinson and Richard 

Goldsmith 
(H) Structural and metamorphic history of eastern Massachusetts, by Richard Goldsmith 
(I) Intrusive rocks of eastern Massachusetts, by David R. Wones and Richard Goldsmith 
(J) Radiometric ages of rocks in Massachusetts, by Robert E. Zartman and Richard F. Marvin 



The following chapters of Professional Paper 1366 were published in a single volume in 1988: 

(A) The pre-Silurian geology of the Rowe-Hawley zone, by Rolfe S. Stanley and Norman L. 

Hatch, Jr. 

(B) Stratigraphy of the Connecticut Valley belt, by Norman L. Hatch, Jr., Peter Robinson, and 

Rolfe S. Stanley 

(C) Post-Taconian structural geology of the Rowe-Hawley zone and the Connecticut Valley belt 

west of the Mesozoic basins, by Norman L. Hatch, Jr., and Rolfe S. Stanley 

(D) The Whately thrust: A structural solution to the stratigraphic dilemma of the Erving Forma- 

tion, by Peter Robinson, Norman L. Hatch, Jr., and Rolfe S. Stanley 



Digitized by the Internet Archive 

in 2010 with funding from 

Boston Public Library 



http://www.archive.org/details/bedrockgeologyofOOhatc 



Stratigraphy of the 
Milford-Dedham Zone, 
Eastern Massachusetts: 
An Avalonian Terrane 

By RICHARD GOLDSMITH 

With a section on MESOZOIC AND TERTIARY STRATIGRAPHY OF 
CAPE COD AND THE NEARBY ISLANDS 

By E.G.A. WEED 

THE BEDROCK GEOLOGY OF MASSACHUSETTS 

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1366-E 



CONTENTS 



Page 

Abstract El 

Introduction 2 

Metasedimentary and metavolcanic rocks older than the 

Proterozoic Z Rhode Island and Dedhara batholiths 2 

Rocks possibly older than the quartzitic assemblage 4 

Quartzitic assemblage 4 

Westboro Formation (Zw) 4 

Plainfield Formation (Zp) 5 

Blackstone Group: Mica schist and phyllite (Zbs) and 

Quinnville Quartzite (Zbq) 5 

Paleoenvironment of deposition of the quartzitic 

assemblage 13 

Metavolcanic assemblage 13 

Metamorphosed mafic to felsic flow, volcaniclastic, 

and hypabyssal intrusive rocks (Zv) 17 

Blackstone Group: Greenstone and amphibolite 

(Hunting Hill Greenstone) (Zbv) 20 

Metamorphosed felsic volcanic rock (Z vf) 20 

Correlation of the mafic metavolcanic rocks (Zv) and 

the Hunting Hill Greenstone (Zbv) 20 

Metamorphic rocks of southeastern Massachusetts 21 

Gneiss and schist near New Bedford (Zgs) 21 

Biotite gneiss near New Bedford (Zgn) 21 

Correlation 22 

Age of the prebatholithic rocks 23 

Proterozoic Z rocks younger than or equivalent to the 

southeastern Massachusetts batholith 23 

Mattapan and Lynn Volcanic Complexes (Zm, DZ1) 24 

Felsic and mafic rocks southwest of the Boston basin (Zfm) .. 26 

Age of the Mattapan and Lynn Volcanic Complexes 26 

Boston Bay Group 28 

Paleoenvironment 29 

Age 29 

Cambrian strata 30 

Hoppin Formation (Ch) 30 

Weymouth Formation and Braintree Argillite (Cbw) 32 

Green Lodge Formation of Rhodes and Graves (1931) (€g)... 32 

Paleoenvironment 33 

Newbury Volcanic Complex (Silurian and Devonian) 33 



Pennsylvanian strata E33 

Bellingham Conglomerate (PZb) 34 

Pondville Conglomerate (Pp), Wamsutta Formation 
(Pw, Pwv). Rhode Island Formation (Pr, Pre), and 

Dighton Conglomerate (Pd) 34 

Paleogeography and age 37 

Triassic and Jurassic rocks 37 

Red arkosic conglomerate, sandstone, and siltstone ("fie).... 37 

Stratigraphic problems 37 

Felsic volcanic rocks 38 

Lynn Volcanic Complex 39 

Boston Bay Group 39 

Bellingham Conglomerate 39 

Brighton Melaphyre 39 

Sequence in the Burlington area 40 

Mafic metavolcanic rocks and the Marlboro Formation 40 

Newbury Volcanic Complex 40 

The stratigraphic record in the Milford-Dedham zone 40 

Regional relations in southeastern New England 42 

Quartzitic assemblage 44 

Volcanic-plutonic complex in eastern Massachusetts and 

correlative rocks 44 

Rhode Island and southeastern Massachusetts batholiths 

and related granitoids 45 

Mattapan Volcanic Complex and equivalent rocks 45 

Boston Bay Group and equivalent rocks 45 

Younger units 45 

The Milford-Dedham zone in the Caledonides— Correlation 45 

Mesozoic and Tertiary stratigraphy of Cape Cod and the 

nearby islands, by E.G.A. Weed 46 

Introduction 46 

Geologic setting 47 

Description of post-Paleozoic units 47 

Triassic and Jurassic basalt (J"fib) 47 

Triassic and Jurassic sediments and volcanic rocks (J"fi) 50 

Cretaceous sediments (K) 50 

Tertiary sediments (T) 55 

Stratigraphic history 57 

References cited 57 



ILLUSTRATIONS 



Figure 1. Sketch map of the major features of the geology in the Milford-Dedham zone in eastern Massachusetts E3 

2. Correlation diagram of stratigraphic units in the Milford-Dedham zone 6 



[V 



CONTENTS 



Page 

3-5. Maps of: 

3. Stratigraphic units north and northwest of the Boston basin E9 

4. Stratigraphic units west and southwest of the Boston basin 10 

5. Stratigraphic units in southeastern Massachusetts and adjacent Rhode Island 11 

6. Correlation chart of stratified rocks older than Proterozoic Z granitoids in eastern Massachusetts and northern 

Rhode Island 12 

7. Correlation chart of stratified rocks older than Proterozoic Z granitoids in southeastern Massachusetts and southern 

Rhode Island 13 

8. Map of stratigraphic units in and around the Boston basin 18 

9. Stratigraphic section and lithologic description of the Boston Bay Group 25 

10. Map of stratigraphic units in and around the Narragansett basin 27 

11. Geologic map and measured sections on the east side of Hoppin Hill, Attleboro, Mass 31 

12. Measured section of Lower and Middle Cambrian strata of the Hoppin Formation at the north end of Hoppin Hill 

Reservoir 32 

13. Map showing distribution of major groups of rocks in the Milford-Dedham zone, Massachusetts, Rhode Island, and 

Connecticut 42 

14. Correlation diagram of some Proterozoic Z units in the Milford-Dedham zone in southeastern Connecticut, eastern 

Massachusetts, and Rhode Island 43 

15. Map showing locations of outcrop and auger- and core-drilling sites that provide information on the pre-Mesozoic 

basement and Mesozoic and Tertiary deposits in the area of Cape Cod and the nearby islands, Massachusetts 48 

16. Diagram showing correlation of the Cretaceous section in bore hole USGS 6001, Nantucket, with the exposed Cretaceous 

section in New Jersey 56 



TABLES 



Table 1. 
2. 



Sedimentary basins in the Milford-Dedham zone, Massachusetts E4 

Nomenclature of stratified metamorphic rocks older than the Proterozoic Z Rhode Island and southeastern Massachusetts 

batholiths 8 

Descriptions from previously published works of stratigraphic units in eastern Massachusetts and northern Rhode Island 

older than the Proterozoic Z Rhode Island and southeastern Massachusetts batholiths 14 

Stratigraphic units of the Boston basin and vicinity 23 

Description of stratigraphic units in the Narragansett and Norfolk basins 35 

Stratigraphic record in the Milford-Dedham zone 41 

Location and identification of outcrops and auger- and core-drilling sites in the area of Cape Cod and the nearby islands, 

Massachusetts 49 

Description of materials encountered in outcrops and in drilled holes in the area of Cape Cod and the nearby islands, 

Massachusetts 51 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, 
EASTERN MASSACHUSETTS: AN AVALONIAN TERRANE 



By Richard Goldsmith 



The sedimentary and volcanic rocks and their metamorphic equiva- 
lents in the Milford-Dedham zone include rocks of Proterozoic Y? and 
Proterozoic Z, Proterozoic Z, Cambrian, Silurian and Devonian, Penn- 
sylvanian, Triassic-Jurassic, and Cretaceous and Tertiary ages. Rocks 
older than Proterozoic Z batholithic rocks are divided into two major 
sequences. A sequence of quartzitic rocks includes the Westboro 
Formation, the Plainfield Formation, and mica schist and phyllite and 
Quinnville Quartzite of the lower part of the Blackstone Group. These 
formations represent shelf-edge sedimentation associated with minor 
volcanism. A group of largely mafic metavolcanic rocks younger than 
the quartzitic group, and probably arc related, includes metamor- 
phosed mafic to felsic flow, volcaniclastic, and hypabyssal intrusive 
rocks, which in the literature of northeastern Massachusetts have been 
mapped as Middlesex Fells Volcanic Complex, Cherry Hill Formation 
and associated units, and Marlboro Formation. Included within this 
group in northern Rhode Island and adjacent Massachusetts are 
greenstone and amphibolite equivalent to the Hunting Hill Greenstone 
of the upper part of the Blackstone Group. In southeastern Massachu- 
setts, near New Bedford, rocks older than Proterozoic Z batholithic 
rocks include gneiss and schist and biotite gneiss. These rocks were 
derived from metamorphosed clastic, volcaniclastic, and felsic to inter- 
mediate volcanic material. The correlation of these rocks to the 
quartzitic or mafic volcanic assemblages is uncertain. 

Proterozoic Z rocks equivalent to or younger than the batholithic 
rocks are located primarily in and around the Boston basin. The 
Mattapan and Lynn Volcanic Complexes, primarily felsic volcanic 
rocks, may be in part coeval with younger phases of the batholithic 
rocks. Above the Mattapan and Lynn is the Boston Bay Group, a 
largely sedimentary, probably turbidite sequence of conglomerate, 
sandstone, and argillite consisting of the Roxbury Conglomerate and 
the Cambridge Argillite. Within the Roxbury Conglomerate, in the 
lower part of the group, are horizons of mafic volcanic rock, the 
Brighton Melaphyre. The Cambridge Argillite contains Proterozoic Z 
to Cambrian(?) acritarchs. 

Lower and Middle Cambrian strata containing fossil assemblages 
typical of the Acado-Baltic province overlie the Proterozoic Z rocks 
around the Boston basin (Weymouth Formation, Braintree Argillite) 
and in the northwest corner of the Narragansett basin, near North 
Attleboro and West Wrentham (Hoppin Formation). The Lower Cam- 
brian consists of basal quartzite, overlain by slaty phyllite, limey 
phyllite, and limestone; the Middle Cambrian consists primarily of 



Manuscript approved for publication November 16, 1987. 



argillite and slate. The Upper Cambrian is represented by only one 
locality near Dedham, where the Green Lodge Formation of Rhodes 
and Graves consists of quartzite and phyllite. 

Silurian and Devonian strata are confined to two narrow fault- 
bounded basins in northeastern Massachusetts, near Newbury and 
near Middleton. These strata, the Newbury Volcanic Complex, consist 
in the lower part primarily of volcanic rocks, including felsic and mafic 
flows and volcaniclastic material, and in the upper part largely of 
mudstone and siltstone and subordinate volcanic detritus. Fossil fauna, 
also of Acado-Baltic affinity, range from latest Silurian (Pridolian) to 
earliest Devonian (Gedinnian). 

Pennsylvanian strata occupy one large basin (Narragansett basin) 
and two smaller basins (Norfolk and Bellingham (Woonsocket) basins). 
The Pennsylvanian strata are primarily continental sandstone, shale, 
and conglomerate (Pondville Conglomerate, Wamsutta Formation, 
Rhode Island Formation, and Dighton Conglomerate). The Wamsutta 
and Rhode Island Formations contain plant fossils, and the Rhode 
Island Formation is coal bearing. The strata in the Bellingham basin 
are nonfossiliferous, and their age is in question. The Wamsutta 
Formation contains basalt and felsic volcanic rocks in the northwestern 
part of the Narragansett basin and the southwestern part of the 
Norfolk basin. The Dighton represents channel deposits high in the 
sequence. 

Red arkosic conglomerate, sandstone, and siltstone of Late Triassic 
to Early Jurassic age occupy a small fault-bounded basin, the Middleton 
basin, in northeastern Massachusetts. A Triassic-Jurassic basin lies in 
the subsurface beneath Coastal Plain sediments on Nantucket Island 
and Martha's Vineyard and beneath Nantucket Sound. The basin 
contains Triassic-Jurassic basalt, which was identified in a deep drill 
hole on Nantucket, and overlying sedimentary strata known only 
through seismic profiles. 

Cretaceous strata of the Atlantic Coastal Plain crop out and are 
identified in drill holes on Nantucket and Martha's Vineyard. Tertiary 
strata are identified in drill holes and sparse outcrops on the islands, on 
Cape Cod, and in the Marshfield area on the mainland. Most material 
containing Tertiary spores or pollen, however, seems to have been 
disturbed and is in places physically mixed with, or reworked into, 
Pleistocene materials. 

The stratigraphic record indicates four major episodes. A Protero- 
zoic episode involves several phases: (1) arc margin and volcanic-arc 
accumulation in a compressional phase (Marlboro Formation; metamor- 
phosed mafic to felsic flow, volcaniclastic, and hypabyssal intrusive 
rocks), (2) felsic volcanism and plutonism (Mattapan and Lynn Volcanic 
Complexes; Dedham Granite), (3) uplift and erosion, and (4) flysch to 



E2 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



molasse deposition and mafic volcanism in an extensional regime 
(Boston Bay Group). A second episode began with encroachment of 
Cambrian seas and shelf-deposition of Cambrian sediments (Hoppin 
Formation, Weymouth Formation, and Braintree Argillite) on a pas- 
sive continental margin and lasted with little deposition into the middle 
Paleozoic. Tectonic conditions were primarily extensional and were 
characterized by static plutonism, largely alkalic, from Ordovician into 
Devonian time, but mafic and felsic arc volcanism and marine sedimen- 
tation occurred locally(?) in the Late Silurian and Early Devonian 
(Newbury Volcanic Complex). In the Pennsylvanian, continental sedi- 
mentary deposits and minor volcanic material (Pondville Conglomer- 
ate, Wamsutta Formation, volcanic rocks in the Wamsutta Formation, 
Rhode Island Formation, Dighton Conglomerate) accumulated in a 
shallow basin or basins on the Proterozoic and the older Paleozoic 
rocks. An episode of sedimentation and volcanism associated with 
rifting and the opening of the present Atlantic Ocean occurred during 
Triassic and Jurassic time. A new episode has begun with onlap of 
Cretaceous and Tertiary coastal plain sediments. 

The stratigraphy within the Milford-Dedham terrane is in many 
respects similar to that in the Avalon terrane of southeastern New- 
foundland and can be correlated with that of similar terranes in the 
Piedmont of the southern Appalachians, in the Maritime Provinces of 
Canada, in Wales, in England, and in northwest Africa. 



INTRODUCTION 

The Milford-Dedham zone is defined as encompassing 
those rocks and sediments lying east and southeast of the 
Bloody Bluff-Lake Char fault system. The zone includes 
Cape Cod and the outer islands and extends eastward an 
unknown distance beneath the Atlantic Ocean. The zone 
is further characterized in that it contains intrusive 
granites of Proterozoic Z age, in contrast to the adjacent 
Nashoba zone, which lacks such granites. Rocks of the 
zone continue southward into Rhode Island and adjacent 
eastern Connecticut. 

The Milford-Dedham zone contains two extensive 
batholithic masses of granitoid rock of Proterozoic Z age, 
one in western Rhode Island and in the Milford antiform 
of adjacent Massachusetts (hereinafter called the Rhode 
Island batholith) and the other in eastern and southeast- 
ern Massachusetts comprising the Dedham and equiva- 
lent granites (hereinafter called the southeastern Massa- 
chusetts batholith) (fig. 1). Older metasedimentary, 
metavolcanic, and metaplutonic rocks are preserved in 
discontinuous belts on the flanks and in septa or roof 
pendants. Plutons of gabbro to granite, the latter primar- 
ily alkalic, of Ordovician to Devonian age intrude the 
older rocks. Superimposed on the Proterozoic rocks are 
structural and stratigraphic basins containing unmeta- 
morphosed to variably metamorphosed sedimentary and 
volcanic rocks ranging in age from latest Proterozoic to 
Triassic and Jurassic (table 1). Coastal plain deposits of 
Cretaceous and Tertiary age overlap the bedrock on the 
southeast. These and their substrata are described later 
in this chapter. The deposits of Pleistocene age are not 
shown on the State bedrock map (Zen and others, 1983; 



hereinafter referred to as the State bedrock map) and are 
not discussed in this chapter. 

The State bedrock map contains 36 named units of 
sedimentary and volcanic rocks and their metamorphic 
equivalents in the Milford-Dedham zone (fig. 2). This 
number is condensed from a larger number of units that 
have been described and named by many different people 
working in different areas and at different times in 
eastern Massachusetts. In addition, the terrane is bro- 
ken up by many faults so that many units are exposed 
only in one or two fault blocks. The result is that the 
geology as presented in this chapter tends to be frag- 
mented. To remedy this, an attempt has been made to 
group units systematically within the limits of available 
fossil control and isotopic data. 

This chapter summarizes descriptions of stratigraphic 
units partly described elsewhere, presents more detail in 
areas where new observations have been made, and 
concentrates discussion on controversial points of inter- 
pretation and on observations that are important in 
developing a regional synthesis. The plutonic rocks 
within the zone are described in another chapter (Wones 
and Goldsmith, this vol., chap. I). The structure is 
described in chapter H (Goldsmith, this vol.). The 
descriptions and discussion to follow are arranged primar- 
ily by relative age and only secondarily by structural 
block or basin. The data in this chapter are drawn from 
a number of sources used in the preparation of the State 
bedrock map and listed thereon. Nomenclature used 
consists of existing names and names modified for use on 
the State bedrock map (table 2; fig. 2; Goldsmith and 
others, 1982c). Significant exposures of rock units in the 
Milford-Dedham zone are described in guidebooks of the 
New England Intercollegiate Geological Conference pub- 
lished over a long span of years. Some of the strati- 
graphic units in the Greater Boston area can be seen by 
following a field guide prepared by Skehan (1979). 



METASEDIMENTARY AND METAVOLCANIC 
ROCKS OLDER THAN THE PROTEROZOIC Z 
RHODE ISLAND AND DEDHAM BATHOLITHS 

Metasedimentary and metavolcanic rocks older than 
the Proterozoic Z granitoids of the Rhode Island and 
Dedham batholiths are found (1) in a metavolcanic- 
metaplutonic complex primarily in northeastern Massa- 
chusetts (fig. 3), (2) on the flanks of and as inclusions in 
the Rhode Island batholith (fig. 4), and (3) as septa in the 
southeastern Massachusetts batholith in the New Bed- 
ford area (fig. 5). Most of the lower part of the sequence 
of stratified rocks in and around the batholiths in Mas- 
sachusetts consists of an assemblage of metasedimentary 
formations characterized by significant amounts of 



STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS 



E3 



CNewbury basin 



NEW HA MPSHI RE 
"MASSACHUSETTS 




EXPLANATION 

Paleozoic plutons 

Paleozoic and Proterozoic Z sedimentary- 
volcanic basins 

Proterozoic plutonic, metaplutonic, meta- 
volcanic, and metasedimentary rocks, 
including areas of the Rhode Island and 
southeastern Massachusetts batholiths 

Contact 
— Fault — Dashed where approximate, dotted 
where concealed 

Limit of Rhode island and southeastern 

Massachusetts batholiths not 
otherwise defined 



ATLANTIC OCEAN 



40 KILOMETERS 



Figure 1. — Major features of the geology in the Milford-Dedham zone in eastern Massachusetts. 



quartzite— the Westboro Formation (Zw), the Plainfield 
Formation (Zp), and the lower part of the Blackstone 
Group (Zbs, Zbq, Zb). Most of the upper part is an 
assemblage of formations characterized by significant 
amounts of mafic metavolcanic rocks— metamorphosed 
mafic to felsic flow, volcaniclastic, and hypabyssal intru- 
sive rocks (Zv); metamorphosed felsic volcanic rocks 
(Zvf); and greenstone and amphibolite (Zbv) of the 
Blackstone Group. The two assemblages are gradational 
through interlayering. Gneiss and schist near New Bed- 



ford (Zgs) and biotite gneiss near New Bedford (Zgn) are 
Proterozoic Z in age, but their correlation with the 
assemblages mentioned above is uncertain. In a few 
places, metasedimentary strata apparently lie below the 
quartzitic assemblage, and, at others, metasedimentary 
strata apparently lie above the mafic metavolcanic 
assemblage. These strata, however, are thin and hence 
are included in one or another of the larger units on the 
State bedrock map. The correlation of these older strat- 
ified rocks is shown in figures 6 and 7, and descriptions 



E4 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



Table 1.— Sedimentary basins in the Milford-Dedham zone, Massachusetts 



Basin 



Lithotype 



Fossil type 



Metamorphism 



Age of strata 



Middleton Fluviatile conglomerate, arkose, 

and shale. 

Norfolk Fluviatile conglomerate, sandstone, 

and siltstone; minor volcanic com- 
ponent. 

Narragansett Fluviatile and deltaic conglomerate, 
sandstone, and coal. Minor mafic 
and felsic volcanic rocks in north- 
west part. 

Newbury Mafic and felsic volcanic flows and 

tuffs; marine siltstone and mud- 
stone. 

Bellingham Fluviatile(?) and marine(?) conglom- 

erate, sandstone, and siltstone; 
minor felsic volcanic rocks. 

Boston Marine sandstone, siltstone, shale, 

calcareous shale, and limestone. 
Fluviatile and marine conglomerate, 
sandstone, siltstone, shale, tillite; 
mafic and felsic volcanic rocks. 



Plant None 

Plant Diagenetic to zeolite facies(?) 

(well-developed cleavage in 
slaty beds). 

Plant Diagenetic to greenschist 

facies (mainly polydeformed 
poor to well-developed cleav- 
age). 

Shelly fauna None 



None 



Greenschist facies (foliated) 



Shelly fauna Diagenetic 
Acritarchs 



Late Triassic and Early Juras- 
sic. 

Late Mississippian(?) to Pennsyl- 
vanian. 

Late Mississippian to Pennsylva- 
nian. 



Late Silurian and Early Devo- 
nian. 

Proterozoic Z or Pennsylvanian. 



Early and Middle Cambrian. 



Diagenetic (incipient cleavage) Proterozoic Z to Early 
Cambrian. 



from previously published works are summarized in 
table 3. 



ROCKS POSSIBLY OLDER THAN THE QUARTZITIC 
ASSEMBLAGE 

Bell and Alvord (1976) described "unnamed stratified 
remnants" consisting of fine-grained gneisses and 
quartzite from a hill in the western part of Saugus (fig. 3, 
table 3). These remnants form a band of poorly exposed 
slablike xenoliths, lying apparently conformably below 
thick-bedded quartzite assigned to the Westboro Forma- 
tion cropping out 0.5 km to the north, and a zone of 
scattered inclusions in the Dedham Granite near Lynn. 
These rocks occupy a limited area and have been included 
in the Westboro Formation on the State bedrock map. 



QUARTZITIC ASSEMBLAGE 

WESTBORO FORMATION (Zw) 

The Westboro Formation (Zw), the Westboro Quartz- 
ite of Perry and Emerson (1903, p. 155), is primarily a 
quartzitic unit consisting of thick- to thin-bedded ortho- 
quartzite and subordinate mica schist, calc-silicate rock, 
amphibolite, and biotite gneiss and schist, the latter 
commonly quartzitic. Although the rock sequences of 
Nelson (1974) and of Bell and Alvord (1976) (table 3) 
cannot be correlated precisely, their descriptions are 
similar and typify the formation in eastern Massachu- 
setts. Hepburn and DiNitto's (1978) description of the 
Westboro is of rocks closest to Emerson's type area near 



Westboro. They proposed that the type area of Westboro 
be exposures in 200 m of cuts on F495 north of Mt. Nebo, 
south of the Southboro-Westboro town line. 

West and north of Boston, disconnected masses of 
quartzite and associated rock in Proterozoic Z mafic 
plutonic and volcanic rocks are shown on the State 
bedrock map as Westboro, although they are not in a 
continuous belt with the Westboro of Nelson (1974) or of 
Hepburn and DiNitto (1978). Other isolated masses of 
quartzite, such as those mapped by Castle (1964) in the 
Reading area, are also assigned to the Westboro. Bell 
and Alvord (1976) mapped a thin-bedded quartzite pri- 
marily in their Burlington Formation (table 3), in a group 
of formations near Burlington that lie between the 
Bloody Bluff fault and their Middlesex Fells Volcanic 
Complex. Although this unit lies 2,300 m above the 
Westboro on their section (table 3), and they cited 
indirect evidence that the group lies unconformably on 
the Middlesex Fells, the Burlington has been shown 
arbitrarily as Westboro on the State bedrock map to 
indicate its quartzitic composition and to decrease the 
number of small units on the map. The remainder of this 
group (Bell and Alvord's Greenleaf Mountain Formation 
and unnamed gneiss and quartzite; see table 3) has been 
included in the metamorphosed mafic to volcanic rocks 
(Zv) of Proterozoic Z age because these units contain 
metavolcanic rock. I now believe that the three units 
should be assigned to a separate metamorphic suite 
above the main sequence of metavolcanic rocks (Zv, Zvf). 

Some small masses of quartzite reported in the litera- 
ture have not been shown on the State bedrock map. 
Emerson's (1917) map of Massachusetts shows small 
masses of quartzite in the Essex and Ipswich areas, 



STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS 



E5 



which later geologists mapping in these areas have not 
recognized. Emerson included many small areas of 
quartzite, in addition to the larger ones shown as West- 
boro on the State bedrock map, in his Marlboro Forma- 
tion (a term now confined to the Nashoba zone for a 
sequence characterized by amphibolites) because of their 
intercalation in the Milford-Dedham zone with metavol- 
canic rocks that he mapped as Marlboro but that are now 
assigned to the mafic and felsic metavolcanic rocks (Zv, 
Zvf). Sears (1905, p. 110) mentioned that a well in 
quartzite at Lynnfield penetrated white limestone inter- 
stratified with a light-blue slate and quartzite. A descrip- 
tion of the quartzite given by Sears indicates that the 
rock had been crushed and recrystallized. It may have 
been a silicified zone. 

The Westboro in the Framingham area (figs. 4, 8) and 
to the northeast is overlain by the assemblage of meta- 
morphosed mafic and felsic volcanic rocks (Zv, Zvf) 
(tables 2, 3). South of Westborough, the Westboro is 
truncated by the Bloody Bluff-Lake Char fault system. 
The Westboro is intruded by Proterozoic Z batholithic 
rocks. 

Nelson (1974) placed a metasedimentary-metavolcanic 
unit of gneiss, schist, and quartzite he named the Rice 
Gneiss (table 3) below thick-bedded quartzite of the 
Westboro in the Natick quadrangle. The Rice Gneiss 
could be the equivalent of the unnamed stratified rem- 
nants of Bell and Alvord on the basis of their descrip- 
tions. The gneiss and calc-silicate rocks described as Rice 
Gneiss by Volckmann (1977) in the Holliston area (fig. 4; 
table 3), however, do not resemble the description of 
Rice Gneiss by Nelson (1974). Rocks similar to the Rice 
Gneiss have not been identified elsewhere in the region, 
although they may form part of the Blackstone Group in 
northern Rhode Island. The Rice Gneiss may be equiv- 
alent to gneiss and schist lying below the Plainfield 
Formation in the Lyme Dome of southeastern Connect- 
icut (Lundgren, 1967, p. 14). However, that is a long- 
range correlation for such a small unit; furthermore, the 
complicated fold pattern and lack of topping information 
in southeastern Connecticut render questionable the 
proper stratigraphic assignment of the inner rocks of the 
Lyme Dome. Because the Rice Gneiss is not character- 
istically quartzitic but predominantly feldspathic, and 
because it lies in a different fault block from the West- 
boro, it was included in the comprehensive unit of 
metavolcanic rocks (Zv) of Proterozoic Z age on the State 
bedrock map. I now believe that assignment to the 
Westboro or as a separate unit below the Westboro 
would be preferable. 

The Westboro is equivalent to the Plainfield Forma- 
tion of eastern Connecticut because it lies in the same 
strike belt, and the Westboro is believed to be equivalent 
to the Quinnville Quartzite (Zbq) and mica schist and 



phyllite (Zbs) of the Blackstone Group. Both have similar 
stratigraphic positions below mafic metavolcanic rocks— 
greenstone and amphibolite (Zbv) in northern Rhode 
Island and metamorphosed mafic and felsic volcanic 
rocks (Zv, Zvf) in eastern Massachusetts. The boundary 
between the Westboro and the Blackstone is drawn 
arbitrarily on the State bedrock map on the basis of 
proximity of the isolated exposures of the two to their 
respective type areas. No area of continuous exposure 
exists between the two units. 

PLAINFIELD FORMATION (Zp) 

The Plainfield Formation (Zp) extends into Massachu- 
setts in the Webster-Oxford area (fig. 4) from eastern 
Connecticut and western Rhode Island along the west 
flank of the Rhode Island batholith. Elongate lenses of 
quartzite mapped as Westboro on the State bedrock 
map, trending northeast through Grafton towards West- 
borough, are on strike with the Plainfield in the Webster- 
Oxford area. 

The lithology of the Plainfield, as described by Gold- 
smith (1966, 1976) in the New London area, southeastern 
Connecticut, and by Harwood and Goldsmith (1971) and 
Dixon (1974) in eastern Connecticut, and its structural 
and stratigraphic position indicate that the Plainfield and 
Westboro are equivalent formations. For example, Nel- 
son's sequence (1974; as given in table 3) approximates 
the three-part division of the Plainfield Formation sug- 
gested by Goldsmith (1976) in the New London area. 
However, this division of the Plainfield does not seem to 
carry northward into the Thompson quadrangle in north- 
eastern Connecticut and northwestern Rhode Island 
(Dixon, 1974). 

The Plainfield is truncated along most of the 
Connecticut-Rhode Island border by the Lake Char 
fault, but, in the New London area, the Plainfield is 
overlain by a suite of largely mafic metavolcanic rocks 
called the Waterford Group (Goldsmith, 1980). These 
relations are similar to those of the Westboro Formation 
(Zw) and overlying metavolcanic rocks (Zv) in Massachu- 
setts. The base of the Plainfield is not known, but 
possibly gneiss and schist in the center of the Lyme 
Dome, referred to earlier, lie below the Plainfield. 

The Plainfield is believed to be equivalent to the 
quartzite (Zbq) and schist (Zbs) in the Blackstone Group 
because of its similar lithology and structural relations 
with the plutonic rocks of the Rhode Island batholith. 

BLACKSTONE GROUP: MICA SCHIST AND PHYLLITE (Zbs) 
AND QUINNVILLE QUARTZITE (Zbq) 

The Blackstone Group, the Blackstone Series of Wood- 
worth (in Shaler and others, 1899, p. 106; Quinn and 
others, 1948, 1949), was named for exposures along the 



E6 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



STRATIFIED ROCKS 



JUa 



Newbury 
basin 


DSn 


DSnr 


DSnu 


DSna 
DSnl 



€g\ /'*? 






INTRUSIVE ROCKS 



Alkalic granitic 
rocks 



l \irjS^!-^!\<\,\ 



Alkalic granitic 

and 
gabbroic rocks 






Batholithic rocks 



r- i- i f. > ' 

H -t vi < < < -j 


— «^ 1 


< J 


\, -J •? v -i * 


V A 



EZpE 2wE Z6q: 



BUI 



Mafic rocks 



9 s Zgn 



AGE 



Tertiary 



Lower 
Jurassic 



Upper 
Triassic 



Penn- 
sylvanian 



Ordovician 



Cambrian 

to 

Proterozoic Z 



Proterozoic Z 



Figure 2.— Stratigraphic units in the Milford-Dedham zone. 



STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS 



E7 



EXPLANATION 



Jl 

JTJb 

"Re 

Pd 

Pr 

Pre 

Pw 

Pwv 

Pp 

PZb 

ftZc 

ftZr 

RZrb 

DSnr 

DSn 

DSnu 

DSna 

DSnl 

DZI 

€g 

Cbw 

€h 

Zm 

Zfm 

Zp 

Zw 

Zbv 
Zbq 

Zb 
Zbs 

Zv 
Zvf 
Zgs 
Zgn 



□ 



Tertiary sediments 

Cretaceous sediments 

Sedimentary and volcanic rocks 

Altered amygdaloidal basalt 

Red arkosic conglomerate, sandstone, and siltstone 

Dighton Conglomerate 

Rhode Island Formation 

Conglomerate, sandstone, and graywacke in the Rhode Island Formation 

Wamsutta Formation 

Rhyolite and mafic volcanic rocks in the Wamsutta Formation 

Pondville Conglomerate 

Bellingham Conglomerate 

Cambridge Argillite 

Roxbury Conglomerate 

Melaphyre in the Roxbury Conglomerate 

Newbury Volcanic Complex 

Micrographic rhyolite 

Undivided sedimentary and volcanic rocks 

Upper member 

Porphyritic andesite 

Lower member 
Lynn Volcanic Complex 

Green Lodge Formation of Rhodes and Graves (1931) 
Braintree Argillite and Weymouth Formation 
Hoppin Formation 
Martapan Volcanic Complex 
Felsic and mafic volcanic rocks 
Plainfield Formation 
Westboro Formation 
Blackstone Group 

Greenstone and amphibolite — Includes Hunting Hill Greenstone 

Quinnville Quartzite 

Undivided 

Mica schist and phyllite— Includes Mussey Brook Schist and Sneech Pond Schist 
Metamorphosed mafic to felsic flow, volcaniclasric, and hypabyssal intrusive rocks 
Metamorphosed felsic volcanic rocks 
Gneiss and schist near New Bedford 
Biotite gneiss near New Bedford 
Marks a fossil -bearing unit 
Intrusive rocks 
Diabase dikes and sills 
Massive quartz and silicified rock 

Alkalic granitic rocks 

Includes Peabody Granite (Dpgr); Wenham Monzonite (Dwm); Cherry Hill 
Granite (Dcygr); granite of Rattlesnake Hill pluton (Drgr) 

Alkalic granitic and gabbroic rocks 

Includes Blue Hills Granite Porphyry (SObgr); Cape Ann Complex (SOcgr, 
SOcsm, SOcb}; Quincy Granite (SOqgr); alkalic granite in Franklin (DOgr); 
Nahant Gabbro (Ongb) 

Serpentinite 

Batholithic rocks 

Includes biotite granite (Zgr), Milford Granite (Zmgr, Zmgd); Hope Valley AJaskite 
Gneiss (Zhg); alaskite (Zagr); Sciruate Granite Gneiss (Zsg); Esmond Granite 
(Zegr); Topsfield Granodiorite (Ztgd); Grant Mills Granodiorite (Zgmgd); 
Dedham Granite (Zdgr, Zdngr); Westwood Granite (Zwgr); fine-grained granite 
and granite porphyry (fgr); granite of the Fall River pluton (Zfgr); Ponaganset 
Gneiss (Zpg); porphyritic granite (Zpgr); granite, gneiss, and schist, undivided (Zgg) 

Mafic rocks 

Includes diorite at Rowley (Zrdi); diorite (Zdi), diorite and gabbro (Zdigb), 
gabbro (Zgb); Sharon Syenite (Zssy) 



Figure 2.— Continued. 



Blackstone River in northern Rhode Island. The Black- 
stone as defined by Quinn and others (1948; table 3) 
consists of three lower, largely metasedimentary units, 
the Mussey Brook Schist (Zbs), Quinnville Quartzite 



(Zbq), and Sneech Pond Schist (Zbs), and an upper, 
largely metavolcanic unit, the Hunting Hill Greenstone 
(Zbv). Only the lower metasedimentary part of the 
Blackstone Group is correlated with the Westboro and 



E8 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



Table 2.— Nomenclature of stratified metamorphic rocks older than the Proterozoic Z Rhode Island and southeastern Massachusetts batholiths 

(stratigraphic order only approximate) 



Used on State bedrock map 



Previously published nomenclature 



Metamorphosed mafic to felsic flow, volcaniclastic, and hypabyssal 
intrusive rocks (Zv). Metamorphosed felsic volcanic rocks (Zvf). 



Blackstone Group (Zb): 
Greenstone and amphibolite (Zbv) 
Mica schist and phyllite (Zbs) 
Quinnville Quartzite (Zbq) 
Mica schist and phyllite (Zbs) 

Westboro Formation (Zw) 

Plainfield Formation (Zp) 

Westboro Formation (Zw) 

Gneiss and schist near New Bedford (Zgs) 

Biotite gneiss near New Bedford (Zgn) 



Burlington Formation 

Greenleaf Mountain Formation 

Unnamed gneiss and quartzite 

Claypit Hill Formation 

Cherry Brook Formation 

Kendall Green Formation 

Middlesex Fells Volcanic Complex 

Amphibolite, mixed rocks, banded volcanic rocks, biotite-hornblende 

schist (Volckmann, 1977) 
Rice Gneiss 

Blackstone Group: 
Hunting Hill Greenstone 
Sneech Pond Schist 
Quinnville Quartzite 
Mussey Brook Schist 

Westboro Formation 

Plainfield Formation* 

Unnamed stratified remnants (Bell and Alvord, 1976) 

(new unit) 

(new unit) 



"The Plainfield extends into the Webster-Oxford area of Massachusetts from eastern Connecticut and western Rhode Island and is equivalent to the Westboro in Massachusetts. 



Plainfield Formations (fig. 6). The base of the Blackstone 
Group is not known. 

The Quinnville Quartzite (Zbq) (table 3) was earlier 
called Westboro Quartzite by Emerson (1917) and 
included an Albion Schist Member (of the Quinnville). 
Other schist in the area distant from the quartzite beds 
and an interbedded marble were included by him in the 
Marlboro Formation, as was the greenstone. Quinn 
(1971) thought the quartzite was not equivalent to the 
Westboro in the type area; he named the quartzite 
Quinnville and schists above and below it Sneech Pond 
and Mussey Brook, respectively. The schists (table 3) 
contain similar assemblages of rock, and recent mapping 
by Drier and Mosher (1981) in the Blackstone River area 
northwest of Pawtucket (fig. 4) indicates no basis for 
distinguishing the two schist units. Possibly the Quinn- 
ville is a quartzitic lens in the sequence, or possibly the 
schists are repeated by folding. The schist units, includ- 
ing the marble beds, are combined into a unit of mica 
schist and phyllite (Zbs) on the State bedrock map. This 
unit and the Quinnville Quartzite (Zbq) are shown only in 
their type areas. Outside those areas, the Blackstone 
Group consists of rock believed to be equivalent to the 
Quinnville Quartzite and the mica schist and phyllite but 
which cannot be distinguished separately, and the Black- 
stone is therefore shown as undivided (Zb) on the State 
bedrock map. The Blackstone also includes, in places, 
layers, dikes, and sills of greenstone or amphibolite 
believed to be equivalent to the greenstone and amphib- 



olite of the Hunting Hill Greenstone (Zbv), which is also 
shown only in its type area. The amphibolite layers, 
where present outside the type area, are usually inter- 
layered with quartzite or schist and are not volumetri- 
cally large enough to show at the scale of the map. 

The undivided Blackstone Group (Zb) extends along 
the Blackstone River into Massachusetts in the towns of 
Blackstone, Milford, and Uxbridge (figs. 4, 8). It is also 
present to the north in the Franklin and Medway areas, 
where it appears as disconnected segments of phyllite 
(see below) and locally quartzite in the plutonic rocks. 
West of Blackstone, the Blackstone Group is difficult to 
divide because outcrops are poor and quartzite and 
amphibolite are extensively interlay ered. A large area of 
poorly exposed amphibolite, hornblende gneiss, quartz- 
ite, and minor rusty- weathering feldspathic mica-quartz 
schist occupies a belt from the town of Blackstone north 
through Hopedale and Milford, mainly west of the Mill 
River. A similar belt of Blackstone (Zb) extends south 
from Nipmuck Pond, southwest of Mendon. The Black- 
stone in both these areas lies largely in valleys. In places 
sheets of Milford Granite have intruded the Blackstone, 
producing layered migmatite as on Bear Hill, 1 km 
southeast of Milford. Southeast of East Douglas, quartz- 
ite is exposed in a large area of undivided Blackstone, 
which has the form of a large fold nose. Rocks included in 
the undivided Blackstone Group (Zb) include amphibolite 
interlay ered with quartzite mapped by Shaw (1967) in 
the Milford area and by McKniff (1964) in the Blackstone 



STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS 

71°15' 71°00' 



E9 

70°45' 



Newburyport 



Georgetown^ 



EXPLANATION 

Contact — Dotted where concealed 
Fault — Dotted where concealed 




.-<^ 






10 KILOMETERS 



**■ 



_y<f7\* * /, ,* Danvers » ^Vf* £ " # t £££ip^> ^^ s /i' 



Boston basin 



MASSACHUSETTS 
BAY 



^WaJtharrM 
:? v vfj Weston. 



FIGURE 3. — Stratigraphic units in the Milford-Dedham zone north and northwest of the Boston basin. Unit designations as on figure 2. 



area, small areas of amphibolite containing epidote pods 
and stringers, and epidote-biotite-hornblende gneiss in 
the Franklin and Wrentham areas. The rocks mapped by 
Shaw and McKniff were considered by them to be 
correlative with the Hunting Hill Greenstone. 

East of the Bellingham basin (also called the Woon- 
socket basin), the Blackstone Group (Zb) mostly lacks 



quartzite and consists mainly of phyllite and schist. 
Gray-green laminated phyllite and schist, locally 
ankeritic, are conspicuous. Also present are epidote- 
chlorite-biotite schist and small masses of metadiorite. 
Some of the schists are feldspathic and are subporphy- 
ritic metavolcanic rocks. A conglomerate containing len- 
ticular quartzite pebbles and cut by thin sills and dikes of 



E10 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 




Figure 4. — Stratigraphic units in the Milford-Dedham zone west and southwest of the Boston basin. Unit designations as on figure 2. 



greenstone crops out on Bound Road, just south of the 
Massachusetts-Rhode Island State line, west of Woon- 
socket. A similar conglomerate and clean quartzite crop 
out sporadically in a zone extending from this area north 
to Framingham and Natick. 



The boundary between rocks mapped as Blackstone 
Group and rocks mapped as Westboro is arbitrarily 
drawn as mentioned above, on the basis of proximity to 
their respective type areas. For example, small areas of 
quartzite near Walpole, Mass. (figs. 4, 8), have been 



STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS 



Ell 



EXPLANATION 

■ Contact 




Buzzards Bay 



Figure 5. — Stratigraphic units in the Milford-Dedham zone in the New Bedford area, southeastern Massachusetts and 
adjacent Rhode Island. Unit designations as on figure 2. 



E12 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



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STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS 



E13 



Newport area, Rhode Island 


Sakonnet River area, 
Rhode Island 


New Bedford area, Massachusetts 


Rast and Skehan (1981) 


Quinn (1971) 


Quinn (1971) 


Geologic map of Massachusetts 
(Zen and others, 1983) 


Price Neck 
Formation 


Volcanic tuff, 

conglomerate, 

and quartzite 

of Newport 

vicinity 


Mica-chlorite schist 

of Sakonnet; 

chlorite-biotite schist 

of Tiverton; mica 

schist of Bristol 

r 

, 


Gneiss and schist 

near New Bedford 

(Zgs) 


Newport 
Formation 


Slate and quartzite 
of Newport vicinity 


Biotite gneiss 

near New Bedford 

(possibly a facies 

of above) (Zgn) 



Figure 7. —Stratified rocks older than Proterozoic Z granitoids in southeastern Massachusetts and southern Rhode Island. 



mapped as Westboro Quartzite by Volckmann (1977) but 
are shown as undivided Blackstone Group (Zb) on the 
State bedrock map because they are near the area of 
Blackstone to the south. Outcrops of quartzite near 
Hopkinton, Mass. (fig. 4), adjacent to large areas of 
Westboro Formation in the Framingham area, are 
shown as Westboro Formation. 

Small areas consisting largely of phyllite near Franklin 
and Medway have been assigned to the Blackstone 
Group. Tan, crinkled phyllite and phyllitic metawacke 
crop out west of a sliver of Dedham Granite on Pond 
Street, Norfolk, 1.5 km west of Pondville in the middle of 
the Norfolk basin. The Wamsutta Formation of Lower to 
Middle Pennsylvanian age to the west is unmetamor- 
phosed. Greenish-gray, calcite-bearing, locally pyritic 
phyllite and metasandstone are exposed in roadcuts on 
F495 at the Maple Street overpass, 2 km northeast of 
Bellingham. Gray felsite exposed in these cuts is proba- 
bly a continuation of an extensive area of felsite to the 
south on the flanks of the Bellingham basin. Phyllite 
crops out on the banks of the Charles River south of the 
Lincoln Street Bridge at Medway. 

The correlation of the phyllite in the Franklin and 
Medway areas is uncertain. On the one hand, its compo- 
sition and grade of metamorphism are similar to those of 
pelite in the type area of the Blackstone Group in Rhode 
Island and its degree of metamorphism contrasts with 
the lack of metamorphism in the nearby Wamsutta 
Formation. Thus I have shown it as Blackstone Group 
(Zbs) on the State bedrock map. On the other hand, its 
association with felsic volcanic rocks (Zfm; fig. 2), which 



may be equivalent to the younger Mattapan Volcanic 
Complex (Zm), suggests that it might be a sedimentary 
part of that complex in a more metamorphosed regime 
than the Boston area. 

PALEOENVIRONMENT OF DEPOSITION OF THE 
QUARTZITIC ASSEMBLAGE 

The paleoenvironment of deposition of the quartzitic 
assemblage was probably the flank of a volcanic arc. 
Although the Plainfield and Westboro contain calc- 
silicate layers, and the Blackstone contains marble 
locally in northern Rhode Island, no carbonates of the 
shelf or platform type exist in the sequence. On the 
contrary, the Plainfield, Westboro, and lower part of the 
Blackstone are in sequence below, and are interlayered 
with, rocks of primarily mafic volcanic derivation (table 
3). This association suggests that the formations of the 
quartzitic assemblage were deposited on the margins of a 
probably mature volcanic arc, possibly of the continental- 
margin type. Zircons from the Westboro indicate a much 
older source than the rocks now present in the Dedham- 
Milford zone, and the zircons were probably derived from 
continental crust (Olszewski, 1980). 



META VOLCANIC ASSEMBLAGE 

In eastern Massachusetts, the assemblage of primarily 
mafic metamorphosed volcanic rocks (Zv, Zvf, Zbv) is 
associated temporally and spatially with gabbro, diorite, 
and syenite (Zgb, Zdigb, Zssy); together, these units are 



E14 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



Table 3. — Descriptions from previously published works of stratigraphic units in eastern Massachusetts and northern Rhode Island older than 

the Proterozoic Z Rhode Island and southeastern Massachusetts batholiths 

[Areas described roughly follow a southwest-trending line, from around Boston, Mass., to northern Rhode Island] 



Symbol 
on 

State 
bedrock 

map 



Stratigraphic unit 



Description 



Metamorphic 
facies 



Northeastern Massachusetts (Bell and Alvord, 1976) 



Zw 



Zw 



Burlington Formation 



Greenleaf Mountain 
Formation. 

Unnamed gneiss and 
quartzite. 



Middlesex Fells 
Volcanic Complex. 



Zw 



Westboro Formation, 
upper part. 



Westboro Formation, 
lower part. 



Unnamed stratified 
remnants. 



Fine-grained, randomly interlayered impure 
quartzite, quartz-feldspar gneiss, mica- 
quartz feldspar gneiss, amphibolite, 
metawacke. Metaconglomerate at top. Lay- 
ers a few to several centimeters thick. [Age 
uncertain, Proterozoic Z to Ordovician.]. 

Fine-grained, thinly laminated, dark-green 
amphibolite and minor pale-green calc- 
silicate rock. Layers 1 cm thick. 

Light-colored, fine-grained quartzite, 
feldspar-quartz gneiss, biotite gneiss and 
hornblende gneiss as xenoliths in Cape Ann 
Complex. Bedding obscure. 

Fine-grained, dark-gray to black, foliated, 
thickly layered to massive amphibolite, and 
hornblende gneiss, pods and lentils of epi- 
dote common, locally pillowed; laminated 
amphibolite, rare quartzite and calc-silicate 
rock and light-gray metadacite. Tuff pre- 
dominates in upper part, flows in lower 
part. An apparently conformable contact 
with the Westboro Formation is exposed in 
two places north of Boston. 

Fine-grained, white to pale-gray quartzite 
and pale-green hornblende gneiss. Layers a 
few centimeters to 15 m thick. 

At top, interlayered fine-grained quartzite, 
argillite, slate, and quartz-bearing calc- 
silicate rock in beds a few centimeters to 3 
m thick; in middle, massive white to pale- 
gray fine-grained quartzite in lenticular 
masses 5-60 m long; at bottom, massive 
quartzite, interbedded quartzite and mica- 
ceous quartzite. 3-5 m of reddish-brown 
biotite quartzite near basal contact. 

Very fine grained, gray biotite gneiss in slab- 
like xenoliths. Fine-grained quartzite, feld- 
spathic quartzite, laminated light-gray 
gneiss containing minute biotite flakes. 
Forms inclusions in Dedham Granite. 



Shallow marine to littoral 
sands, fine-grained mafic 
tuff, and volcaniclastic 
detritus. Metaconglomer- 
ate is a channel deposit, 
possibly unconformable 
on rest of unit. 

Fine-grained marine tuff; 
some layers contain car- 
bonate. 

Epiclastic sediment and 
tuff. 



Mafic flows and tuffs in 
marine environment. 
Amygdular and pillow 
structures evident. 



Shallow marine and littoral 
sand, silt, and mud, 
partly calcareous, and 
airborne tuff. 

Shallow marine and littoral 
sands, silt, and mud, 
partly calcareous. 



Near-shore marine sand 
and volcaniclastic detri- 
tus. 



300 m Amphibolite. 

225 m Amphibolite. 

<250 m Amphibolite. 

1,500 m Greenschist. 



Amphibolite. 



Amphibolite. 



150 m for bio- Obscured by 
tite gneiss; intrusion, 

thickness of 
quartzite 
not known. 



Framingham and Natick area, Massachusetts (Nelson, 1974) 



Claypit Hill Formation Mostly dark-gray and greenish-gray, fine- 
grained hornblende gneiss interlayered with 
two-mica gneiss locally containing garnet 
and sillimanite; biotite-hornblende gneiss, 
biotite gneiss, and thin beds of amphibolite. 
Layers 8 cm to 0.5 m thick. Unconform- 
able^) on Cherry Brook Formation. 



Not given 



Amphibolite 
(sillimanite- 
muscovite). 



STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS 



E15 



Table 3. 



-Descriptions from previously published works of stratigraphic units in eastern Massachusetts and northern Rhode Island older than 
the Proterozoic Z Rhode Island and southeastern Massachusetts batholiths — Continued 



Symbol 
on 

State 
bedrock 

map 



Stratigraphic unit 



Description 



Metamorphic 
facies 



Framingham and Natick area, Massachusetts (Nelson, 1974)— Continued 



Cherry Brook Forma- 
tion, upper part. 



Cherry Brook Forma- 
tion, lower part. 



Kendall Green Forma- 
tion. 



Westboro Quartzite 



Zv 



Rice Gneiss 



Predominantly amphibolite and minor interca 
lated biotite schist and quartzite. Amphibo- 
lite is fine to coarse grained, equigranular, 
thin to thick layered, locally massive, 
amygdaloidal, and pillowed. 

Light- to pinkish-gray, massive felsic crystal Crystal tuff 
tuff and a few thin beds of medium-grained 
schist. 

Very fine grained, light-tan, laminated felsic Felsic tuff 
tuff interlayered with dark-greenish-gray, 
fine-grained tuff and discontinuous layers of 
quartzite. [A zone of strong shearing. Mylo- 
nite according to Castle and others (1976). 
Woburn Formation of LaForge (1932).]. 

Upper part, light-gray, thick to massively 
bedded quartzite, a few thin layers of bio- 
tite gneiss near base. Middle part, dark- to 
medium-gray, fine-grained biotite and horn 
blende gneiss and schist; relatively pure 
quartzite, feldspathic quartzite, and 
amphibolite; beds 3 cm to 1.5 m thick. 
Lower part, light-gray, thick-bedded 
quartzite in beds as much as 5 m thick and 
minor interbedded biotite gneiss. 

Medium- to dark-gray, fine- to medium- 
grained, equigranular to inequigranular bio- 
tite gneiss and schist, two-mica schist, and 
thin beds of quartzite and feldspathic 
quartzite. 



Basaltic and minor andesitic 900-> 1,200 m 
tuffs and flows. 



Volcaniclastic detritus and 
beach sand. 



980 m 



Amphibolite. 

Amphibolite. 
Not given. 

Amphibolite. 



Beach sand and tuffaceous 
detritus. 



Amphibolite. 



Marlboro area, Massachusetts (Hepburn and DiNitto, 1978) 



Zw 



Westboro Formation Light- to dark-gray, tan, and pinkish-gray Not given 

feldspathic quartzite, orthoquartzite, and 
micaceous quartzite; massive with thin 
micaceous partings to well bedded in beds a 
few centimeters to 0.5 m thick. Gray 
quartz-rich mica schist. Calc-silicate quartz- 
ite and granofels in layers and lenses as 
much as 0.5 m thick. Light-gray muscovite- 
quartz schist and dark-gray rusty- 
weathering biotite-quartz-feldspar schist. 
Amphibolite and biotite amphibolite in beds 
as much as 1 m thick. 

Westboro Formation, Dark-gray, fine- to medium-grained biotite Not given 

(lower) biotite schist schist and interlayered minor, thin, impure 
member. quartzite. Subordinate biotite amphibolite, 

amphibolite, quartz-rich biotite schist, and 
biotite-plagioclase gneiss. 



Not given 



Amphibolite. 



Not given 



Amphibolite. 



E16 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



Table 3. — Descriptions from previously published works of stratigraphic units in eastern Massachusetts and northern Rhode Island older than 

the Proterozoic Z Rhode Island and southeastern Massachusetts batholiths— Continued 

[Areas described roughly follow a southwest-trending line, from around Boston, Mass., to northern Rhode Island] 

Symbol 



State Stratigraphic unit 

bedrock 
map 



Description 



Metamorphic 



Holliston and Medfield areas, Massachusetts (Volckmann, 1977) 



Zv Cherry Brook Forma- 

tion. 



Zv Amphibolite 



None Mixed rocks 



Zv Banded volcanic rocks 



Interlayered blastoporphyritie to equigranular 
amphibolite, hornblende-biotite gneiss and 
hornblende-biotite-quartz gneiss; contains 
relic clasts and bombs as much as 2.5 m in 
diameter; layers a few centimeters to 10 m 
thick. These rocks overlie the Westboro 
Quartzite. [Appears to be equivalent to 
only the upper part of the Cherry Brook 
Formation of Nelson (1974).]. 

Amphibolite and hornblende gneiss containing Not given 
thin layers of alternating felsic and mafic 
constituents; epidote porphyroblasts com- 
mon; local intercalations of microcline-rich 
quartz-feldspar rock. [Probably equivalent 
to part of Cherry Brook Formation of Nel- 
son (1974).]. 

Interlayered metamorphosed blastoporphy- 
ritie andesitic tuff, biotite-epidote- 
plagioclase schist, chlorite schist, and mas- 
sive amphibolite. 



Chaotically bedded volcanic 
flows, breccias, and tuffs. 



Amphibolite. 



Assorted volcanic and vol- 
caniclastic rocks. 



Not given 



2,700 m 



None Biotite-hornblende 
schist. 

Zw Westboro Quartzite 



Zv Rice Gneiss 



Biotite-hornblende gneiss in alternate thin (1 Volcanic rocks 
mm to 2 cm) layers of mafic and felsic min- 
erals; interleaved with light-colored biotite 
gneiss, feldspathic quartzite, and massive 
black porphyritic rock containing andesine 
laths as much as 2 cm in length. [Possibly 
equivalent to part of Cherry Brook Forma- 
tion of Nelson (1974).]. 

Biotite-hornblende (-epidote) schist and Not given 

gneiss, biotite gneiss; indistinctly layered. 
[Stratigraphic correlation uncertain.]. 

Massive to thick-bedded orthoquartzite, minor Not given 
interbedded quartz-muscovite schist, and 
biotite-hornblende gneiss. Intruded by 
hornblende gabbro and Milford Granite. 

Interlayered light- to medium-gray and Not given 

greenish-gray biotite gneiss and schist 
showing a swirled foliation, quartz-feldspar 
gneiss, epidote-biotite schist, and calc- 
silicate gneiss. Intruded by diorite and Mil- 
ford Granite. 



Amphibolite. 



Amphibolite 
partly ret- 
rograded to 
greenschist 
facies. 

Amphibolite. 



750 m 



Amphibolite. 
Amphibolite. 

Amphibolite. 



Northern Rhode Island (Quinn, 1971) 



Zb Blackstone Series 

Zbv Hunting Hill Green- 
stone. 



Dark-green, fine-grained greenstone, locally 
pillowed; contains knots and veins of epi- 
dote; local pyroclastic texture. Rare serpen- 
tine and steatite. [Drier and Mosher (1981) 
recognized thin quartzite (chert?) and vol- 
caniclastic layers as well as flows.]. 



Basaltic and minor andesitic 
flows and tuffs. Intrusive 
dikes and sills. 



-1,200(?) m Greenschist 
near Paw- 
tucket, 
R.I.; 

amphibolite 
in the Mil- 
ford, 
Mass., 
area. 



STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS 



E17 



Table 3. — Descriptions from previously published works of stratigraphic units in eastern Massachusetts and northern Rhode Island older than 
the Proterozoic Z Rhode Island and southeastern Massachusetts batholiths — Continued 



Symbol 
on 

State 
bedrock 

map 



Stratigraphic unit 



Description 



Metamorphic 
facies 



Northern Rhode Island (Quinn, 1971)— Continued 



Zbs 



Sneeeh Pond Schist 



Zbq Quinnville Quartzite 



Zbs Mussey Brook Schist 



Greenish-gray, fine-grained, thin-bedded 
chlorite-quartz schist, thin-bedded quartz- 
ite, marble, greenstone, amphibolite schist, 
feldspathic mica schist, and serpentine. 

Light-gray, clean, massive, medium-grained 
quartzite and minor thin beds of light-gray 
to greenish-gray quartz-mica schist. 

Same as Sneeeh Pond Schist. [Drier and 
Mosher (1981) recognized only one schist 
unit in the Blackstone. The Mussey Brook 
is, then, equivalent to the Sneeeh Pond 
Schist.]. 



Marine sand and silt, and 


2,600 m 


Amphibolite 


mafic and intermediate 




and green 


volcanic and volcaniclastic 




schist. 


material. 






Mature quartz sand and 


1,100 m 


Greenschist. 


interbedded silt and clay. 






Same as Sneeeh Pond 


Same as 


Same as 


Schist. 


Sneeeh 


Sneeeh 




Pond Schist. 


Pond 

Schist. 



here called the mafic volcanic-plutonic complex in eastern 
Massachusetts. This complex was subsequently intruded 
by the Dedham Granite (Zdgr) and related Proterozoic Z 
granitoids. The metavolcanic rocks have been given 
different names in different areas (table 3), but they 
appear in general to belong to one episode of Proterozoic 
Z volcanism and plutonism. 

The stratigraphic positions of many of the assemblages 
of metavolcanic rocks in eastern Massachusetts are dif- 
ficult to determine because of the complex fault pattern, 
which has broken the terrane into numerous wedges and 
blocks; the lack of topping evidence; and extensive 
igneous intrusion. On the State bedrock map, the named 
and unnamed metavolcanic rocks in eastern Massachu- 
setts (table 2) are combined into a single unit of meta- 
morphosed mafic and felsic flow, volcaniclastic, and 
hypabyssal intrusive rocks (Zv). Mappable areas of met- 
amorphosed felsic rock (Zvf) that appear to be older than 
the Dedham and Milford Granites are identified by a 
separate symbol. Greenstone and amphibolite (Hunting 
Hill Greenstone) (Zbv) of the Blackstone Group in Rhode 
Island are considered to be the equivalent of the mafic 
metavolcanic unit in Massachusetts. 

Some of the rocks in the metavolcanic assemblage 
were included in the Marlboro Formation by Emerson 
(1917). The name Marlboro is not now applicable east of 
the Bloody Bluff fault. The mafic volcanic-plutonic com- 
plex of eastern Massachusetts is intruded by Proterozoic 
Z granitoids and lies east of the Lake Char-Bloody Bluff 
fault zone, whereas the Marlboro Formation is intruded 
only by Paleozoic granitoids and lies west of the Lake 
Char-Bloody Bluff fault zone. 



METAMORPHOSED MAFIC TO FELSIC FLOW, 
VOLCANICLASTIC, AND HYPABYSSAL INTRUSIVE ROCKS 

(Zv) 

The metamorphosed mafic to felsic flow, volcaniclastic, 
and hypabyssal intrusive rocks (Zv) (referred to hereaf- 
ter as mafic metavolcanic rocks) as shown on the State 
bedrock map include primarily the Middlesex Fells Vol- 
canic Complex (Bell and Alvord, 1976) and the Cherry 
Brook Formation (Nelson, 1974) (table 3, fig. 6). The 
mafic metavolcanic rocks include also the Claypit Hill 
Formation and unnamed metavolcanic rocks mapped by 
Nelson (1975a,b) in the Natick and Framingham areas 
and by Volckmann (1977) in the Medfield and Holliston 
areas; biotite-hornblende schist southwest of Holliston 
(Volckmann, 1977); and the unnamed gneiss and quartz- 
ite and Greenleaf Mountain Formation mapped by Bell 
and Alvord (1976) in the Burlington area. Widely scat- 
tered small masses of metavolcanic rock north and east of 
the Narragansett basin, including the greenstone at 
North Plympton, are also included in the mafic metavol- 
canic unit. 

North and northeast of Boston in the Salem and 
Danvers area (fig. 3), small masses of the mafic metavol- 
canic rocks (Zv)— amphibolite, chlorite schist, and augen 
gneiss— mapped by Toulmin (1964) as Marlboro Forma- 
tion are engulfed in gabbro and diorite (Zdigb), primarily 
Salem Gabbro-Diorite (Toulmin, 1964; Wones and Gold- 
smith, this vol., chap. I). Near the Bloody Bluff fault in 
the Reading area and northeast in Georgetown, Ipswich, 
and Rowley are more extensive areas of mafic metavol- 
canic rock (Zv), primarily amphibolite, and less mafic 
plutonic rock. Those in the Reading area were mapped as 



E18 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 




ebw 

Massachusetts 
Bay 



I* Hingham r- 1 
North Scituate 



Fault — Dotted where concealed 



Figure 8. — Stratigraphic units in and around the Boston basin. M, Milton. Unit designations as on figure 2. Jd shown on islands east of Boston 

intrudes FiZe. 



Marlboro Formation (Emerson, 1917; Castle, 1964). 
Those in the Georgetown area were included in the 
Middlesex Fells Volcanic Complex in the regional syn- 
thesis by Bell and Alvord (1976). Parts of this general 
area, particularly the Danvers area, where plutonic and 
volcanic rocks are mixed could have been more appropri- 
ately shown on the State bedrock map as an undifferen- 
tiated volcanic-plutonic complex (volcanic-plutonic com- 
plex in eastern Massachusetts). 

The Kendall Green Formation as used by Nelson (1974; 
table 3) is included in the mafic metavolcanic rocks (Zv) 
on the State bedrock map. The protolith of the Kendall 
Green Formation (Kendall Green slate of Hobbs, 1899; 
Woburn Formation of LaForge, 1932; fig. 6) has been the 
subject of several interpretations in recent years. Nelson 



(1974) considered that the Kendall Green represented 
thinly layered metamorphosed felsic and mafic tuffs. Bell 
and Alvord (1976) considered the rock to be a protoclastic 
phase of the Ordovician and Silurian Cape Ann Complex. 
Castle and others (1976) considered the rock to be a 
mylonite in the Bloody Bluff fault zone. I think all three 
interpretations are partly correct. The rock was origi- 
nally either layered felsic and mafic tuffs or sheets of 
felsic rock intruded into the mafic pile, which have been 
smeared out into thinner layers and laminae within the 
Bloody Bluff and other fault zones in the Weston- 
Waltham area west of Boston. Corresponding rock in a 
similar stratigraphic position can be seen in southeastern 
Connecticut, where thinly interlayered fine-grained 
Hope Valley Alaskite Gneiss and (hornblende)-biotite- 



STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS 



E19 



quartz-plagioclase rock have been milled down in a splay 
of the Honey Hill fault zone to a fine-grained blastomy- 
lonite consisting of alternating thin dark and light layers, 
lenses, and discontinuous laminae. This blastomylonite 
lies near the base of a predominantly amphibolitic part of 
the metavolcanic-plutonic Waterford Group (Goldsmith, 
1980). I consider the Kendall Green Formation of Nelson 
part of the mafic metavolcanic assemblage (Zv) rather 
than a valid formation; it is merely a zone rendered 
distinctive through sheetlike intrusion and cataclasis. In 
eastern Connecticut and adjacent Rhode Island and 
Massachusetts, felsic layers may be intrusions of Hope 
Valley or may be felsic volcanic rock that is part of the 
carapace for the Sterling Plutonic Group and Milford 
Granite of the Rhode Island batholith. In the Framing- 
ham area, mappable zones of felsic volcanic rock (Zvf) 
interpreted in the latter way are shown on the State 
bedrock map. 

The Claypit Hill Formation and the group of three 
units of Bell and Alvord (1976) above their Middlesex 
Fells Volcanic Complex (fig. 6) in the Burlington area are 
less certainly part of the mafic metavolcanic assemblage 
(Zv) but have been included in it on the State bedrock 
map. Nelson (1974) suggested that the Claypit Hill is 
unconformable on the Cherry Brook Formation, on the 
basis that attitudes of layering in the Claypit Hill were 
discordant to those in the Cherry Brook, but he did not 
discount the possibility that the contact is a fault. The 
Claypit Hill resembles the Marlboro Formation in the 
Nashoba zone to the northwest (Goldsmith, this vol., 
chap. F), and it may be a slice of Marlboro caught in the 
Bloody Bluff fault zone. The Claypit Hill, according to 
Nelson's map (1975b), is intruded by Proterozoic Z 
granite, however, placing it clearly within the Milford- 
Dedham zone. 

Two of the three formations exposed in the Greenleaf 
Mountain area, Burlington (fig. 3; Bell and Alvord, 1976; 
table 3), are included in the mafic metavolcanic assem- 
blage (Zv). These are an unnamed gneiss and quartzite 
and the Greenleaf Mountain Formation. The third for- 
mation, the Burlington Formation, has been assigned to 
the quartzitic assemblage on the State bedrock map. 
These formations form a lens between the Bloody Bluff 
fault on the west and outcrops of the Middlesex Fells 
Volcanic Complex on the east. These formations rest 
unconformably on the eroded surface of the Middlesex 
Fells Volcanic Complex although no actual contact has 
been seen (Bell and Alvord, 1976, p. 202-203). They are 
assigned to the mafic metavolcanic assemblage because 
they are near the metavolcanic rocks of the Middlesex 
Fells Volcanic Complex and because they contain biotite 
gneiss and amphibolite as major components. However, 
as discussed in the section on the quartzitic assemblage, 
the three units in the Burlington area should be assigned 



to a separate suite of their own. They may actually lie in 
a fault sliver of the Bloody Bluff fault system. In contrast 
to the Claypit Hill Formation and Middlesex Fells Vol- 
canic Complex, these formations are intruded not by the 
Proterozoic Z Dedham Granite but only by Paleozoic 
alkalic granite, and therefore they could be younger than 
Proterozoic Z. I did not attempt to correlate the Claypit 
Hill with these units (fig. 6), and their assignment to the 
mafic metavolcanic assemblage (Zv) on the State bedrock 
map is arbitrary. 

Massive greenstone is exposed on the edge of the 
Narragansett basin west of Plymouth, at North Plymp- 
ton (fig. 5). The rock is greenish gray and contains a 
few scattered, small, greenish-white phenocrysts. It 
has a weak but measurable cleavage. In thin section, the 
rock has a relict trachytic or felty texture in a flow 
pattern and contains scattered larger saussuritized pla- 
gioclase grains. Mineral constituents are light-green to 
yellowish-green amphibole, locally in aggregates as 
much as 2 mm in diameter, clinozoisite, olive-green 
chlorite, yellow epidote, and fine-grained accessory 
albite, oriented tabular flakes of leucoxene, and white 
mica. Quartz is dispersed but also forms a few aggre- 
gates that contain rutile. The greenschist-facies meta- 
morphism of this rock is in contrast to the lack of 
metamorphism and cleavage in the adjacent Rhode 
Island Formation. The relationship can be seen in expo- 
sures on a farm 0.8 km northeast of North Plympton 
(Lyons, 1977, Plympton quadrangle). 

Woodworth (in Shaler and others, 1899, p. 116) con- 
sidered the greenstone near North Plympton to be of 
Pennsylvanian age on the basis of comparison with a 
similar rock cutting fine-grained "granitite" south of the 
main exposure of the felsite north of Plympton, and thus 
he considered it to be younger than the southeastern 
Massachusetts batholith. He correlated the greenstone 
with the felsite at Attleboro in the Wamsutta Formation 
(Pwv). However, the greenstone at North Plympton is 
altered, as are the plutonic rocks in the area, and has a 
cleavage and is thus unlike the adjacent Middle to Upper 
Pennsylvanian Rhode Island Formation. The rock is 
either pre-Dedham (and its equivalents), and part of the 
volcanic-plutonic complex in eastern Massachusetts, or it 
is para- and post-Dedham and correctable with the 
Mattapan Volcanic Complex. On the State bedrock map 
it is mapped as metamorphosed mafic metavolcanic rock 
(Zv). 

The greenstone at North Plympton is somewhat simi- 
lar to a flow-banded light-greenish-gray porphyry in 
ledges 1.3 km south of Rock. This porphyry contains 
saussuritized plagioclase phenocrysts as much as 1 cm in 
longest dimension and twinned green hornblende, locally 
clustered, in a matrix of chlorite, quartz, potassium 
feldspar, epidote, and rare biotite and sphene. Cleavage 



E20 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



is not apparent in this rock. Its contact with adjacent 
granite is not exposed. This porphyry, not shown on the 
State bedrock map because of its small size, is considered 
to be the same age as the greenstone at North Plympton 
because it has the same greenschist-facies metamor- 
phism. 

BLACKSTONE GROUP: GREENSTONE AND AMPHIBOLITE 
(HUNTING HILL GREENSTONE) (Zbv) 

The Hunting Hill Greenstone as described by Quinn 
(Quinn and others, 1948; summarized in Quinn, 1971; 
table 3) is quite similar to the Cherry Brook Formation 
(Nelson, 1974) and Middlesex Fells Volcanic Complex 
(Bell and Alvord, 1976) and is in a similar stratigraphic 
position. In the type area in northern Rhode Island, 
northwest of Pawtucket, the Hunting Hill forms massive 
ledges, in some of which pillow structure can be dis- 
cerned. The Hunting Hill Greenstone also forms sills and 
dikes in the mica schist and phyllite unit (Zbs) (Sneech 
Pond and Mussey Brook Schists) in northeastern Rhode 
Island (Rutherford and Carroll, 1981). 

The term "greenstone and amphibolite" (Zbv) used on 
the State bedrock map was originally meant to include 
not only the Hunting Hill Greenstone in the type area but 
also amphibolite and other metavolcanic or metavolcani- 
clastic rocks of middle to high metamorphic grade in 
areas west and northwest of the Blackstone type area. 
However, the amphibolites in these areas could not be 
readily separated on the scale of the map from the rest of 
the Blackstone Group and so were included in the 
undivided Blackstone Group (Zb) rather than being 
mapped separately. Some of these rocks have been 
mentioned above in the description of the lower part of 
the Blackstone Group. 

METAMORPHOSED FELSIC VOLCANIC ROCK (Zvf) 

Metamorphosed felsic volcanic rock (Zvf) forms a 
subordinate part of the metamorphosed mafic and felsic 
volcanic rock assemblage (Zv). Mappable units of felsic 
metavolcanic rock are shown only in the Framingham 
area (figs. 4, 8) on the State bedrock map. The Cherry 
Brook Formation, as described by Nelson (1974; table 3), 
has a lower felsic part consisting of a metamorphosed 
massive crystal tuff containing a few layers of biotite 
schist. This felsic part was not recognized by Bell and 
Alvord to the northeast in the Medford area, where 
pillow lavas of the Middlesex Fells Volcanic Complex 
rest directly on quartzite (Bell and Alvord, 1976, p. 198), 
nor was an equivalent zone of felsic rock or interlayered 
felsic and mafic rock described by Quinn (1971) below the 
Hunting Hill Greenstone in the Blackstone Group. How- 
ever, the felsic part of the Cherry Brook may be correl- 
ative with felsic layers described by Volckmann (1977) in 



the units lying below the Cherry Brook Formation (table 
3). The Cherry Brook studied by Volckmann is equiva- 
lent to only the upper part of the Cherry Brook studied 
by Nelson. However, Nelson's crystal tuff is similar in 
composition and texture to, although finer grained than, 
mafic-mineral-poor felsic rocks north and east of the 
Milford Granite that Volckmann (1977) mapped as Mil- 
ford Granite. I have shown the felsic rocks of Volckmann 
as Hope Valley Alaskite Gneiss on the State bedrock 
map. I could not separate out the felsic rocks of the 
Cherry Brook from the rest of the Cherry Brook for 
compilation as metamorphosed felsic volcanic rocks on 
the State bedrock map, but my reconnaissance in the 
Framingham area in preparation for compiling the State 
bedrock map convinced me that the northern belts of 
rock that Nelson (1975b) mapped as Milford Granite 
would be better shown as metamorphosed felsic volcanic 
rock (Zvf) because of their fine-grained texture and 
mafic-mineral-poor composition. 

CORRELATION OF THE MAFIC METAVOLCANIC ROCKS 
(Zv) AND THE HUNTING HILL GREENSTONE (Zbv) 

The Middlesex Fells Volcanic Complex, Cherry Brook 
Formation, Claypit Hill Formation, and Rice Gneiss 
have been included in the metamorphosed mafic and 
felsic volcanic rocks (Zv) on the State bedrock map. Bell 
and Alvord's (1976, p. 199-200) Middlesex Fells Volcanic 
Complex encompassed the rocks mapped by Nelson as 
Cherry Brook and Claypit Hill Formations as well as the 
mafic rocks in the upper part of the Blackstone Group 
(Bell and Alvord, 1976, p. 199-200) shown on the State 
bedrock map as greenstone and amphibolite (Hunting 
Hill Greenstone (Zbv)). The descriptions of the Middle- 
sex Fells Volcanic Complex in the type area by Bell and 
Alvord (1976) and of the Hunting Hill Greenstone by 
Quinn and others (1948) are similar (table 3), and the 
units occupy a similar stratigraphic position. I agree with 
the correlation of these units. 

I used the term "metamorphosed mafic to felsic flow, 
and volcaniclastic and hypabyssal intrusive rocks" (Zv) 
on the State bedrock map instead of Middlesex Fells 
Volcanic Complex because the type area of the Middlesex 
Fells is in a fault-bounded block unique in containing the 
Lynn Volcanic Complex (DZ1), and I was wary of extend- 
ing the usage to the amphibolites and gneisses outside 
the block to the northeast and north, as well as to units 
which Nelson had already mapped in the Framingham 
area. Furthermore, the Middlesex Fells contains no 
felsic rocks like those in the Cherry Brook Formation. 
Bell and Alvord's (1976) usage of Middlesex Fells Volca- 
nic Complex in a broad sense may be correct, but I 
thought it better to use an informal lithologic name on 
the State bedrock map pending further work. 



STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS 



E21 



METAMORPHIC ROCKS OF SOUTHEASTERN 
MASSACHUSETTS 

Layered schist and gneiss form arcuate and linear 
septa in the plutonic rocks of the southeastern Massa- 
chusetts batholith southeast of the Narragansett basin 
(fig. 5). Two types of rock are present, thinly layered 
gneiss and schist (gneiss and schist near New Bedford, 
Zgs, table 2) and thickly layered biotite gneiss (biotite 
gneiss near New Bedford, Zgn, table 2). They are 
exposed primarily in the New Bedford-Fall River area, 
but the poorly exposed terrane to the east in Plymouth 
and Barnstable Counties probably contains similar rocks. 
A deep drill hole near Harwich on Cape Cod (see fig. 15) 
encountered thinly layered phyllitic schist and subordi- 
nate limestone (Koteff and Cotton, 1962). The gneiss and 
schist near New Bedford (Zgs) and the biotite gneiss 
near New Bedford (Zgn) are described below in more 
detail than are rocks in other areas because no previous 
descriptions of them exist. 

GNEISS AND SCHIST NEAR NEW BEDFORD (Zgs) 

The most extensive exposures of the gneiss and schist 
near New Bedford (Zgs) are in cuts on 1-195 near New 
Bedford (fig. 5). Elsewhere only a few exposures were 
seen in road reconnaissance. The belt of gneiss and schist 
shown on the State bedrock map extending from Hixville 
to Brayleys is based on the trend of a valley, the 
magnetic pattern (U.S. Geological Survey, 1971a), and 
the strike of schist in one outcrop. Similarly, the belt 
trending southwest from the south part of Dartmouth 
across the East Branch of the Westport River is based on 
the strike of one outcrop and the magnetic pattern (U. S. 
Geological Survey, 1971b). 

The gneiss and schist are thin-bedded to laminated, 
medium-gray to dark-greenish-gray rocks differing in 
proportions of feldspar, quartz, hornblende, biotite, and 
epidote; they locally contain muscovite. Amphibolite is 
rare. Rocks exposed on 1-195 west of New Bedford 
consist of garnet-biotite-quartz schist containing small 
calc-silicate pods and randomly oriented elongate black 
poikiloblastic hornblende, muscovite-green-biotite- 
plagioclase-quartz schist, locally containing pyrite, and 
layers of laminated epidote-biotite-two-feldspar schist 
or gneiss containing green biotite, clusters of green 
biotite and epidote, quartz, calcic (An 20 _ 2 9) oligoclase, 
minor hornblende, sphene, allanite, and microcline. Sim- 
ilar rocks are exposed in other places. Microcline-bearing 
phases tend to be more abundant near granitic contacts. 
The microcline in some of these layers is poikiloblastic 
and aggregated with plagioclase and quartz forming 
light-colored knots. Some layers are of coarse two- 
feldspar-quartz augen gneiss containing green biotite, 
accessory epidote, opaque minerals, and sphene. Others 



are a splintery-weathering, gray, sugary-textured 
quartz-biotite-feldspar gneiss. A laminated, greenish- 
gray schist near Center Village, Westport, contains 
plagioclase, biotite, calcite, epidote, allanite, opaque 
minerals, and rare scattered aggregates of quartz. 
Flanking the diorite at Acushnet is somewhat granitoid, 
medium-gray to dark-gray gneiss consisting of clusters of 
epidote, blue-green hornblende, and green biotite in a 
matrix of plagioclase and rare quartz. A lighter colored 
phase lacks hornblende and contains a minor amount of 
potassium feldspar. In a nearby quarry, a streaked and 
spotted rock flanking the diorite contains elongate pale- 
green hornblende prisms in a matrix of greenish-brown 
biotite locally clustered with fine-grained hornblende and 
epidote, saussuritized plagioclase, and quartz. In most of 
the rocks described above, the plagioclase is poorly 
twinned. 

A septum of epidote-biotite schist containing quartz, 
calcic oligoclase, and accessory apatite and sphene is 
exposed near the Route 24—1-195 interchange at Fall 
River. Inclusions of similar rock are abundant in the 
band of porphyritic granite in that area. Similar schist 
forms inclusions in the porphyritic granite along Long 
Pond, north of Deans Point (fig. 5). 

The textures and composition of the rocks suggest that 
their protolith was intermediate to felsic volcaniclastic 
rocks, primarily tuffaceous sediments and tuffs, depos- 
ited in an aqueous environment. 

BIOTITE GNEISS NEAR NEW BEDFORD (Zgn) 

Layered granitoid biotite gneiss (Zgn) forms a tightly 
arcuate band south of New Bedford along the north shore 
of Buzzards Bay (fig. 5). It is flanked on either side by 
gneissic alaskite. The biotite gneiss is best exposed at 
Fort Phoenix Park, Fairhaven, east of New Bedford (fig. 
5). Other exposures are at Wilbur Point and on U.S. 
Route 6, Mattapoisett, 200 m west of its junction with 
Mattapoisett Neck Road, Marion quadrangle. 

The gneiss is thickly layered, gray, and relatively 
homogeneous in texture. Layers differ in proportions of 
feldspars and biotite; some layers are quite poor in mafic 
minerals. Typical layers are flecked with biotite and 
magnetite, giving the rock a salt-and-pepper appearance. 
Foliation is pervasive and approximately parallel to 
layering. It is marked by uniform orientation of the 
disseminated biotite flakes. Some dark-gray layers, how- 
ever, contain elongate clots of biotite, sphene, and allan- 
ite. Some small pegmatitic streaks and patches contain 
only magnetite. Mineralogy is simple. The gneiss con- 
sists of plagioclase, quartz, biotite, microcline, and acces- 
sory sphene and allanite. The exposure at Mattapoisett 
contains about 20 percent biotite and augen of flesh- 
colored potassium feldspar as much as 1 cm long. The 



E22 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



biotite gneiss is not well exposed in the hinge of the arc 
in Dartmouth. Exposures at Bayview, Dartmouth, con- 
sist of lineated and foliated inequigranular biotite gneiss 
containing flesh-colored feldspar 2.5-5 mm in diameter. 
Pegmatite masses in these outcrops contain muscovite as 
well as biotite, quartz, and two feldspars. Exposures in a 
playground off Front Street in southern New Bedford on 
strike with the Fort Phoenix exposures are of relatively 
massive granitoid biotite gneiss in which pegmatite 
masses have large books of biotite. Granitoid phases at 
Fort Phoenix approach in composition and aspect 
gneissic phases of the granite of the Fall River pluton. 
The biotite gneiss, however, is distinguished by its 
layered character and greater amount of biotite and 
magnetite. 

The protolith of the biotite gneiss was probably a 
sequence of layered intermediate to felsic volcanic rocks, 
more likely flows and massive tuffs rather than waterlain 
volcaniclastic materials. Alternatively, the gneiss could 
represent metamorphosed sheeted intrusions. 

Contact relations with the granite of the Fall River 
pluton are not clear. The biotite gneiss appears to be 
conformable with the adjacent gneissic alaskite and is in 
fact interlayered with it. On the one hand, both may be 
part of a plutonic or volcanic assemblage older than the 
granite of the Fall River pluton. On the other hand, both 
could be phases of the southeastern Massachusetts bath- 
olith but metamorphosed and deformed under higher 
pressure-temperature conditions than the batholith to 
the north and west. 

CORRELATION 

The gneiss and schist (Zgs) and the biotite gneiss (Zgn) 
near New Bedford are not well enough exposed to 
provide a good basis for correlation with rock units of 
apparently similar age in adjacent Rhode Island (fig. 7). 
However, on the basis of similar composition and bed- 
ding characteristics, they are probably equivalent to 
mica-chlorite schist and chlorite-biotite schist of Pollock 
(1964, p. D2) exposed at several places along the east 
side of the Sakonnet River in the Tiverton area. The 
mineralogy of the schist as described by Pollock (1964, p. 
D2-D3) is similar to that in layers in the gneiss and schist 
in the New Bedford area: "Chief constituents are quartz, 
biotite, and chlorite; muscovite, epidote, calcite, and 
hornblende are locally abundant. Minor and accessory 
constituents are albite, microcline, microperthite, zircon, 
apatite, sphene, magnetite, ilmenite, and garnet." Quinn 
(1971), following Pollock (1964), described the mica- 
chlorite schist on the Sakonnet River south of Tiverton 
near Browns Point (fig. 5) as light-gray to greenish-gray, 
thin-bedded mica-chlorite schist containing thin beds of 
marble and quartzite. The mica-chlorite schist is 



intruded by the Bulgarmarsh Granite, which is here 
considered a phase of the Fall River pluton. Quinn (1971) 
described the chlorite-biotite schist mapped by Pollock 
(1964) near Tiverton Four Corners (fig. 5) as green to 
gray, fine-grained, poorly foliated chlorite-biotite schist 
containing subordinate layers of amphibolite, epidote- 
hornblende schist, and quartzite. Quinn (1971) equated a 
pink- to light-gray, thin-bedded, lineated muscovite- 
quartz-biotite schist at Bristol Neck, Rhode Island, 
northwest of Tiverton, with the rocks in the Sakonnet 
River area. The rocks in the Sakonnet River and Bristol 
areas described above are in the greenschist facies to 
epidote-amphibolite facies of metamorphism. 

Not seen in the New Bedford area were the thin beds 
of limestone in the mica-chlorite schist near Browns 
Point, nor the quartzite at Tiverton Four Corners, nor a 
volcanic breccia that Pollock mapped on Gould Island in 
the Sakonnet River (fig. 5). Pollock tentatively corre- 
lated these rocks with the Blackstone Group, but Quinn 
(1971, p. 54) did not venture such a correlation. The mica 
schist at Bristol Neck, R.I. (Quinn, 1971, p. 23), resem- 
bles some of the gneiss and schist near New Bedford. It 
is reasonable to correlate the rocks mostly east of the 
Sakonnet River in Rhode Island, mentioned above, with 
Proterozoic Z volcanic and sedimentary rocks near New- 
port on Aquidneck and Conanicut Islands described by 
Kay and Chappie (1976), and more recently by Rast and 
Skehan (1981), because they are both intruded by Prot- 
erozoic Z granites. Rast and Skehan (1981) assigned the 
volcanic and sedimentary rocks to two formations. The 
lower, the Newport Formation, consists of graded gray- 
wacke, siltstone and pelite, and minor conglomerate, 
felsic volcanic rock, quartzite, calc-silicate rock, dolo- 
mite, and diamictite containing fragments of quartzite, 
calc-silicate rock, dolomite, and serpentine. They inter- 
preted the formation as turbidite deposits that included 
minor felsic volcanic material. The upper formation, the 
Price Neck Formation, consists primarily of volcano- 
genie materials: agglomerate breccias (in part lahars), 
conglomerate, coarse tuff, thinly bedded tuff and lami- 
nated sediments; graded-bedded tuff, siltstone and slate; 
and possible rhyolite flows. Both formations are in the 
lower greenschist facies of metamorphism. 

Correlation of the metavolcanic and metasedimentary 
rocks in the New Bedford area (Zgs, Zgn) with either the 
mafic metavolcanic rocks (Zv) or the quartzitic assem- 
blage (Zp, Zw, Zbs, and Zbq) north and west of the 
Narragansett basin is uncertain. Mafic metavolcanic 
rocks are generally lacking in the New Bedford area of 
Massachusetts and the Sakonnet River area and Aquid- 
neck and Conanicut Islands in Rhode Island. Volcanic 
rocks, where they do exist, tend to be felsic. A more 
likely correlation would be with the mica schist and 
phyllite (Zbs) of the lower part of the Blackstone Group 



STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS 



E23 



Table 4. — Stratigraphic units of the Boston basin and vicinity 
[Descriptions and thicknesses adapted from Rehmer and Roy (1976)] 



Unit on State bedrock map 


Formation 


Preferred age 


Description 


Thickness 

(meters) 


Braintree Argillite and Wey- 


Braintree Argillite 


Middle Cambrian 


Argillite and slate 


Not given. 


mouth Formation (Cbw). 


Weymouth Formation 


Early Cambrian 
Unconformity (?) 


Argillite, slate, and limestone 


Not given. 


Boston Bay Group 


Boston Bay Group 








Cambridge Argillite (frZc) 


Cambridge Argillite 


Proterozoic Z and 


Argillite, slate, sandstone, 


>2,300 (from tun- 






Early Cambrian(?). 


and quartzite. 


nel data). 


Roxbury Conglomerate 


Roxbury Conglomerate 


Proterozoic Z 






(ftZr). 


Squantum Member 




Diamictite 


20-180. 




Dorchester Member 




Argillite and sandstone 


360. 




Brookline Member 




Conglomerate, argillite, sand- 


150-1,300 (thins 


Melaphyre in the Roxbury 






stone, and melaphyre 


abruptly to 


Conglomerate (ftZrb). 




Unconformity 


(Brighton Melaphyre). 


south). 


Mattapan Volcanic Com- 


Mattapan Volcanic Com- 


Proterozoic Z 


Felsite, tuff, melaphyre, and 


0-1,000? 


plex (Zm); Lynn Volcanic 


plex (and Lynn Vol- 




argillite. 




Complex (DZ1). 


canic Complex). 


Disconformity(?) 






Dedham Granite (Zdgr, 


Dedham Granite 


Proterozoic Z 


Granite and granodiorite 


Not applicable. 


Zdngr). 










Metamorphosed mafic and 


Volcanic-plutonic complex 


Proterozoic Z 


Mafic plutonic and volcanic 


Not applicable. 


felsic volcanic rocks (Zv); 


in eastern Massachu- 




rocks. 




diorite and gabbro 


setts. 








(Zdigb). 











in northern Rhode Island or possibly the lower part of 
the Cherry Brook Formation (included in Zv) in the 
Framingham area. On the other hand, the gneiss and 
schist (Zgs) and the biotite gneiss (Zgn) near New 
Bedford and the rocks in the Sakonnet River area, Rhode 
Island, are not correlatable directly with either of the 
major groups of Proterozoic Z, prebatholithic rocks north 
of the Narragansett basin. 

AGE OF THE PREBATHOLITHIC ROCKS 

A minimum age for the metamorphie rocks older than 
the Rhode Island and southeastern Massachusetts bath- 
oliths is established by the Proterozoic Z age (595-630 
Ma) of the plutonic rocks (Dedham Granite, Milford 
Granite) intrusive into them (Goldsmith, 1980; Zartman 
and Naylor, 1984; Zartman and Marvin, this vol., chap. 
J, table 1). A maximum age is indicated by a 1,500-Ma 
U-Pb age on detrital zircon from the Westboro Forma- 
tion (Olszewski, 1980). It may be, however, that some 
quartzites now mapped as Westboro or Plainfield and 
some felsic volcanic rocks now included in the mafic 
metavolcanic rocks are actually younger than or equiva- 



lent to the batholithic rocks. Some of these particular 
problems will be addressed in a later section of this 
chapter, "Stratigraphic problems." 



PROTEROZOIC Z ROCKS YOUNGER THAN OR 

EQUIVALENT TO THE SOUTHEASTERN 

MASSACHUSETTS BATHOLITH 

Unmetamorphosed to weakly metamorphosed volcanic 
and sedimentary rocks of known or of possible Protero- 
zoic Z to early Paleozoic age occupy and flank structural 
and stratigraphic basins in the Proterozoic Z crystalline 
complex of eastern Massachusetts and Rhode Island. 
Proterozoic Z volcanic rocks (Zm, ftZrb) and Proterozoic 
Z to Cambrian sedimentary rocks (ftZr, ftZc) have been 
identified in and around the Boston basin (fig. 8). Similar 
rocks flanking the Norfolk basin (Pwv), and in the 
Bellingham basin (PZb, Zm), may be similar in age to 
those in the Boston basin. In the Boston basin (table 4), 
the basal Mattapan Volcanic Complex has been shown to 
be of Proterozoic Z age (602 ±3 Ma; Kaye and Zartman, 
1980) and in places to intrude and to lie unconformably 
above the Dedham Granite (LaForge, 1932, p. 31). 



E24 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



Metasedimentary and metavolcanic rocks of the Boston 
Bay Group, formerly thought to be of Silurian to Devo- 
nian or Pennsylvanian age (Billings, 1979), overlie the 
Mattapan and are now considered to be of Proterozoic Z 
to Early Cambrian(?) age (Skehan, 1979; Kaye and 
Zartman, 1980; Skehan and Murray, 1980a; Billings, 
1982; Lenk and others, 1982). The volcanic rocks will be 
described first. 



MATTAPAN AND LYNN VOLCANIC COMPLEXES (Zm, DZ1) 

The volcanic rocks in the Boston basin area younger 
than the Dedham Granite include an earlier, primarily 
felsic suite (Mattapan (Zm) and Lynn (DZ1) Volcanic 
Complexes) and a later mafic suite (Brighton Melaphyre 
(ftZrb) of the Boston Bay Group) (table 4). The Mattapan 
is distributed in the west and southwest part of the 
Boston basin and beyond, and to the south in the Blue 
Hills (Chute, 1969; fig. 8); the Lynn is confined to a block 
north of the Boston basin between the northern border 
fault and the Walden Pond Fault of Bell and Alvord 
(1976, fig. 1). The Mattapan and Lynn Volcanic Com- 
plexes have similar lithologies. According to Kaye (1980), 
they consist largely of rhyolite and rhyodacite flows, in 
part porphyritic; welded ash flows; vitric, lithic, and 
lapilli tuffs; flow breccias; breccia pipes; and extrusion 
domes. LaForge (1932, p. 30-33) mentioned the presence 
of andesitic and basaltic rocks, some of them amygdaloi- 
dal. However, he may have been referring to the older 
Middlesex Fells Volcanic Complex cropping out in the 
area of the Lynn Volcanic Complex or to the younger 
Brighton Melaphyre. Rhyolites and rhyodacites in the 
Mattapan are thinner and less varied in composition and 
texture than those in the Lynn, and volcanic breccias are 
largely absent in the Lynn. In composition, the volcanic 
rocks tend to be sodic rather than potassic (Chute, 1966). 
The Mattapan contains thick zones of interbedded meta- 
sedimentary rocks ranging from laminated argillite with 
fine-scale graded bedding (Lyons and Goldsmith, 1983) 
to volcanic conglomerate. Some of the latter have been 
confused with the Roxbury Conglomerate, as noted by 
LaForge (1932, p. 34-35). A pinkish-red to red-maroon 
conglomerate, formerly considered to be Roxbury Con- 
glomerate, containing mainly granite and rhyolite frag- 
ments interfingers with welded ash-flow tuff in the 
northwestern part of the Medfield quadrangle and is part 
of the Mattapan (Volckmann, 1977). Other kinds of 
volcanic rock have been noted elsewhere. Nelson (1974) 
described a laharic unit and andesitic rocks in the Mat- 
tapan in the Natick quadrangle. Volckmann (1977) 
described a quartz-latite crystal-vitric tuff in the Med- 
field quadrangle. The volcanic rocks in the Blue Hills are 



masses of red, pink, purple, brown, and gray altered and 
devitrified rhyolitic flows, ash flows, and breccias (apo- 
rhyolite of Emerson, 1917). 

The relative stratigraphic position of the Mattapan and 
Lynn is fairly clear. The Mattapan and Lynn are 
reported to lie nonconformably on the Dedham Granite 
and the volcanic-plutonic complex in eastern Massachu- 
setts. LaForge (1932, p. 31) cited exposures near Med- 
ford and in the Saugus area where the Lynn overlies 
weathered plutonic rocks; it consists in one place of a 
basal arkose and in another of an agglomerate containing 
disintegrated granite. Bell (1976, p. 289) cited a location 
in the Saugus area where the Lynn overlies both the 
Middlesex Fells Volcanic Complex and diorite intruded 
into it. Mattapan dikes and stocks cut the Dedham 
Granite and other basement rocks (see Billings, 1976a, p. 
8; Kaye and Zartman, 1980, p. 258; Chute, 1966, p. B27). 
On the other hand, Zarrow (1978, cited by Naylor, 1981) 
described a mass of Dedham Granite at Pine Hill, Med- 
ford, that contains inclusions of Lynn. Kaye (Kaye and 
Zartman, 1980, p. 258) described two places where the 
Dedham grades upward through finer grained phases 
including granophyre into rhyolite and aphanitic rock 
resembling rhyolite. Chute (1966, p. B15) noted several 
exposures southwest of Dedham where a fine-grained 
phase of the Westwood Granite (Zwgr), which intrudes 
the Dedham Granite (Zdgr), resembles phases of the 
Mattapan. Thus it would seem that some of the Mattapan 
is or could be penecontemporaneous with younger phases 
of the Proterozoic Z Dedham batholith. In support of this 
contention, no cobbles of Westwood Granite have been 
observed in the Roxbury Conglomerate that overlies the 
Mattapan, so that it is possible that the Westwood is an 
intrusive equivalent of the extrusive Mattapan Volcanic 
Complex and was not exposed to erosion at the time of 
deposition of the Roxbury. Radiometric ages, discussed 
below in the section on ages of the Mattapan and Lynn 
Volcanic Complexes, permit the possibility of equivalent 
ages for the Mattapan and the Westwood. 

The Mattapan is overlain by the Roxbury Conglomer- 
ate of the Boston Bay Group in what was described by 
LaForge (1932, p. 34) as a fairly continuous horizon. 
Billings (1929, p. 104) noted an angular relation between 
the Mattapan and the overlying Roxbury north of the 
Neponset River in Hyde Park and Mattapan, although 
elsewhere he observed that the contact seems to be 
conformable. The Roxbury Conglomerate contains peb- 
bles and cobbles of the volcanic rocks. LaForge (1932, p. 
34-35) cautioned against possible confusion in identifying 
Roxbury Conglomerate because of the similarity of some 
of its layers containing the volcanic material to some of 
the sedimentary layers interbedded in the volcanic rocks 
of the Mattapan. However, the Boston Bay Group in 



STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS 



E25 



THICKNESS 

(IN METERS) 

8000 




Dedham Granite 



Modified from Billings (1976a. figure 4) 



5000 10,000 15.000 20,000 FEET 



1000 2000 3000 4000 5000 METERS 



EXPLANATION 

(Descriptions modified from Rehmer and Roy (1976)) 

CAMBRIDGE ARGILLITE — Fine-grained argillite. mostly argillaceous, some siltstone and tuff; typically 90 percent argillite, 
10 percent slightly calcareous feldspathic sandstone and quartzite: fossil acritarchs Common pinch-and-swell bedding and 
small-scale crossbedding, oscillation and interference ripple marks, scour marks, graded bedding, slump structures and 
contorted zones, and load casts Rhythmic bedding in one-half of the formation, beds 12-7 6 cm thick; pinch-and-swell in 
beds 7.5 mm thick Color mostly gray to the north. 60 percent reddish to purplish gray and 40 percent gray or greenish gray 
to the south. Minimum thickness probably 2.300 m 



ROXBURY CONGLOMERATE-Squantum Member: Diamictite, 50 to 63 percent matrix of silt and clay, locally sandy; 
some thin sandstone and argillite layers Pebble to boulder clasts: subrounded to angular, rarely striated, some faceted; 
mostly quartzite and granite, some felsite and argillite; average size 7 to 15 cm, common large size 1 m, rarely 6 m; some 
large argillite clasts bent and deformed Bedding obscure or unstratified Dropstones. slump structures, and contorted zone 
in argillite layers Thickness 20 to 180 m Dorchester Member: Approximately 60 percent argillite. 25 percent sandstone. 
15 percent conglomerate. 1 percent tuff. Sand fine- to medium-grained, feldspathic; quartz grains rounded Pebbles 
mostly quartzite. some granite, no argillite clasts: average maximum pebble size 14 cm. Crossbedding and ripple marks 
common Bedding absent or indistinct in sandstone beds, distinct in argillite Thickness 180-500 m. mostly about 350 m. 
Brookline Member: 43 to 60 percent conglomerate. 20 to 55 percent argillite. 2 to 20 percent sandstone. Interbedded 
mafic volcanic rocks (Brighton Melaphyre) in lower part. Clasts well rounded, generally 2 5 to 7 6 cm. average large size 
10 cm. a few 30 cm; mostly quartzite. granite, felsite. lesser melaphyre (Bnghton Melaphyre) and argillite. clasts in basal 
beds larger and typically of underlying formation. Oscillation ripple marks, current lamination and crossbedding, graded 
bedding Bedding mostly lacking or obscure, rarely well stratified: some sand and shale partings and lenses. Thickness 
150-1.300 m; thins rapidly to south 

Approximate location of fossiliferous horizon (Billings. 1982. p 912) 

Figure 9. — Stratigraphic section and lithologic description of the Boston Bay Group. 



general overlies the Mattapan as shown by Billings 
(1976a, b). The Lynn has no known overlying strata. 

The thickness of the Mattapan is indicated on table 4 
and figure 9. It is thickest on the central anticline, 
Boston (1,000 m on projection from surface exposure), 
600 m in Hyde Park, near Milton (M, fig. 8), and 760 m in 
Natick (Billings, 1979, p. A17), but it is missing to the 
southeast in Hingham and Nantasket where the Rox- 
bury lies directly on the Dedham Granite (K.G. Bell, 
written commun., 1976; Billings, 1979). Its extent and 
thickness to the north under the Charles River syncline 
between Boston and Medford are not known. 

Metavolcanic rocks that resemble the Mattapan in the 
Blue Hills, south of Boston, are assigned on the State 
bedrock map to the Mattapan. These rocks were consid- 



ered for many years to be either Silurian and Devonian 
(LaForge, 1932; Naylor and Sayer, 1976) or Carbonifer- 
ous (Emerson, 1917; Billings, 1929). These metavolcanic 
rocks are chemically and mineralogically distinct from 
the nearby Blue Hills Granite Porphyry (SObgr), which 
is comagmatic with the Quincy Granite (SOqgr) (Sayer, 
1974). Most geologists agree that the volcanic rocks in 
the Blue Hills are older than the Quincy Granite and the 
Blue Hills Granite Porphyry (Naylor, 1981), which Bill- 
ings (1982) postulated to be an Ordovician caldera com- 
plex. Chute (1966) could find no difference between the 
volcanic rocks (Zm) in the Blue Hills and the Mattapan 
Volcanic Complex in the type area. We have followed 
Chute in assigning these rocks to the Mattapan Volcanic 
Complex. 



E26 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



FELSIC AND MAFIC ROCKS SOUTHWEST OF THE BOSTON 
BASIN (Zfm) 

Felsic and minor mafic volcanic rocks flanking other 
basins southwest of the Boston basin are correlated or 
could be correlated with the Mattapan Volcanic Com- 
plex. These rocks lie in a structural belt that also 
encompasses the Bellingham and Norfolk basins (Emer- 
son, 1917; LaForge, 1932). Modern mapping has carried 
the Mattapan as far south as Medfield (Volckmann, 
1977). Similar felsic volcanic rocks not previously 
mapped as Mattapan lie on the flanks of the Bellingham 
and Norfolk basins. 

In or on the flanks of the Bellingham basin, a felsite 
porphyry (Zfm, mislabeled Zm on the State bedrock 
map) crops out in a series of exposures east of Maple 
Street, about 1 km east of Bellingham. The felsite 
porphyry contains quartz "eyes" and has a measurable 
cleavage. Igneous texture is still recognizable in thin 
section. The porphyry consists of perthite, quartz, both 
interstitial and in phenocrysts, tabular zoned plagioclase, 
and interstitial white mica, quartz, and an opaque min- 
eral. The felsite is also exposed on 1^95 to the north. 

Felsic and minor mafic volcanic rocks exposed west of 
Lake Pearl, Wrentham, on the west flank of the Norfolk 
basin are possibly also equivalent to the Mattapan Vol- 
canic Complex but are shown on the State bedrock map 
as volcanic rocks in the Wamsutta Formation (Pwv). At 
one stage in the compilation of the State bedrock map, 
these rocks were shown as being equivalent to the 
Mattapan, as is the porphyry near Bellingham (Zfm). 
However, the proximity of these rocks to similar felsite 
at Diamond Hill, R.I., to the south, mapped by Quinn 
(1971) as Pennsylvanian, made an assignment to a Penn- 
sylvanian age preferable. These rocks will be described 
in the section on the Pennsylvanian rocks, under the 
heading "Rhyolite and mafic volcanic rocks in the Wam- 
sutta Formation." 

Two small exposures of rhyolite were mapped by 
Chute (1965) north of Plymouth in the Duxbury quad- 
rangle at Green Harbor, Duxbury (GH, fig. 10), and at 
Cripple Rocks (CR) in Kingstown Bay. Their age is not 
known, and they are not shown on the State bedrock map 
but are included in the unit granite, gneiss, and schist, 
undivided (Zgg) underlying the poorly exposed southeast 
corner of the State. They are possibly correlative with 
the Mattapan. The granite, gneiss, and schist unit (Zgg) 
is discussed in more detail in Wones and Goldsmith (this 
vol., chap. I). 



AGE OF THE MATTAPAN AND LYNN VOLCANIC 
COMPLEXES 

The Mattapan Volcanic Complex was for many years 
considered to be Pennsylvanian in age, through correla- 



tion with the volcanic rocks in the Narragansett basin, or 
Silurian and Devonian, through correlation with the 
Newbury Volcanic Complex. Recently, however, Zart- 
man (Kaye and Zartman, 1980) reported a U-Th-Pb 
isotopic age on zircons from the Mattapan of 602 ±3 Ma. 
Although Billings (1979) questioned the reliability of 
zircon ages from volcanic rocks, particularly those from 
vents, the discovery of Proterozoic Z acritarchs in the 
overlying Cambridge Argillite (Lenk and others, 1982) 
indicates that the zircon age is appropriate. 

The age of the Lynn Volcanic Complex, like that of the 
Mattapan, has been equivocal. Emerson (1917) consid- 
ered the Lynn, as well as the Mattapan, to be Carbonif- 
erous. LaForge (1932) correlated the Mattapan and 
Lynn with the Newbury Volcanic Complex; Naylor and 
Sayer (1976) suggested a similar correlation. Billings 
(1979, 1982) accepted a Proterozoic Z age for the Matta- 
pan but preferred to correlate the Lynn with the New- 
bury because of its proximity and lithologic similarity to 
the Newbury. The argument based on the proximity of 
the Lynn to the Newbury is not strong, because the 
Lynn is geographically and structurally much closer to 
the Mattapan. The closest Newbury lies in a tectonic 
wedge 13 km to the north, whereas projection of the 
Mattapan northward beneath the Boston Bay Group in 
the Charles River syncline to the northern border fault of 
the basin places the Mattapan at most a few kilometers 
from the Lynn. The throw on the northern border fault 
probably is not great (Billings, 1976b, p. 41); my recon- 
struction suggests a throw of 1-1.5 km on the border 
fault using top of basement (Dedham Granite) as an 
approximate horizon. This amount of offset would place 
the two volcanic complexes within a short distance of 
each other. Zarrow (1978, cited in Naylor, 1981) has 
shown that the rare-earth-element pattern in one of the 
units in the Lynn is identical to that in part of the 
Mattapan. I think it reasonable to assume that the Lynn 
and the Mattapan are parts of the same volcanic complex 
and that the Lynn is also Proterozoic Z in age. However, 
in view of the uncertainties in the correlation at the time 
the State bedrock map was prepared, the Lynn is shown 
on the map as Lower Devonian, Silurian, or Proterozoic 
Z. 

Although much of the Mattapan and Lynn is younger 
than the Dedham Granite, some evidence suggests a 
genetic relation between the Dedham and the felsic 
volcanic rocks. Kaye (Kaye and Zartman, 1980, p. 258) 
described the gradation from granite to rhyolite seen in 
places and pointed out that Crosby (1880, 1893, 1900) saw 
field evidence that convinced him that the rhyolitic 
volcanic rocks were genetically related to the granites. 
Zarrow (1978, cited in Naylor, 1981) found rock consid- 
ered to be Dedham containing inclusions of Lynn and 
apparently truncating mappable members of the Lynn. 



STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS 



E27 




Figure 10. — Stratigraphic units in and around the Narragansett basin. WW, West Wrentham. Unit designations as on figure 2. 



The identity of granitic rocks intrusive into the Lynn 
might be questioned, as Bell (1976) mentioned granite 
dikes of the Paleozoic Cape Ann Complex in a block of 
rock containing the Lynn. Agreement exists, however, 
that the intrusive rock is Dedham (G.R. Robinson, Jr., 
oral commun., 1980). If so, it is more likely to be a 
younger phase of the Dedham batholith. A similarity in 
trace-element content between the Lynn and rock 
mapped by Zarrow as Dedham in this area suggests 
consanguinity (Zarrow, 1978, cited in Naylor, 1981). The 
observation that the Mattapan and Lynn are both older 
and younger than the Dedham is resolved if a genetic 
affinity does exist between the felsic volcanic rocks and 
the Dedham and if we recognize that the Dedham 



consists of several phases. More than one kind of granite 
is observed in the Dedham batholith of southeastern 
Massachusetts, where type Dedham Granite is found. 
The Westwood Granite (Zwgr), also found in the Dedham 
batholith, is younger than the Dedham (Chute, 1966). A 
slightly more mafic granite north of Boston mapped as 
Dedham (Zdngr) is another phase. Kaye (Kaye and 
Zartman, 1980, p. 259; Kaye, 1980) recognized at least 
four pulses of intrusion in the block containing the Lynn 
Volcanic Complex. 

The isotopic age for Dedham Granite as a whole is 
630±15 Ma (Zartman and Marvin, this vol., chap. J, table 
1). This is probably a maximum age; because multiple 
intrusions exist, the zircon age of 602 Ma for the Matta- 



E28 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



pan given by Kaye and Zartman (1980) is quite within 
reason. It is well below the approximate date of the base 
of the Cambrian at about 570 Ma. Ample time is available 
for granite plutonism, accumulation of felsic volcanic 
rocks, and deposition of the Boston Bay Group before 
commencement of Cambrian deposition. The Mattapan 
and Lynn quite probably represent a period of felsic 
volcanism that started during at least the late stages 
of emplacement of the southeastern Massachusetts 
batholith. 



BOSTON BAY GROUP 

The primarily sedimentary rocks of the Boston Bay 
Group (table 4) comprise the Roxbury Conglomerate 
(ftZr), which consists of the Brookline, Dorchester, and 
Squantum Members and a volcanic unit in the Brookline, 
the Brighton Melaphyre (FfeZrb), and the Cambridge 
Argillite (RzZc). The Boston Bay Group overlies the 
Mattapan Volcanic Complex disconformably (LaForge, 
1932; Billings, 1929). The most detailed and comprehen- 
sive studies of the Boston Bay Group have been made by 
M.P. Billings, by his associates and students over many 
years, and by LaForge (1932). Early studies of the 
Boston Bay Group were made by W.O. Crosby in the late 
1800's. More recent work has been done by Kaye (Kaye 
and Zartman, 1980; Kaye, 1980). Bell (written commun., 
1976) has mapped marginal areas of the basin. On the 
State bedrock map, the three members of the Roxbury 
Conglomerate (ftZr) are not shown separately. Only the 
Brighton Melaphyre unit (frZrb) is shown within the 
Roxbury Conglomerate. The stratigraphy is summarized 
in table 4. Bedding characteristics and other distinguish- 
ing features of the units of the Boston Bay Group are 
shown in figure 9. 

The Roxbury Conglomerate (ftZr) forms the base of 
the Boston Bay Group. Both LaForge (1932) and Billings 
(1929; 1976a,b; 1982) considered the base of the Roxbury 
to be an unconformity or disconformity. The Roxbury 
clearly lies nonconformably on the Dedham Granite near 
Hull, and the base of the Roxbury can be traced above 
the Mattapan Volcanic Complex as a continuous horizon, 
according to LaForge (1932). Emerson (1917) and Bill- 
ings (1929, 1976a) separated the Roxbury into three 
clearly defined members (table 4). LaForge (1932), how- 
ever, claimed that the threefold division did not persist 
throughout the area and that the middle Dorchester 
Member had no clear base. Since then, Billings and 
Tierney (1964) found evidence that the Roxbury interfin- 
gers with the lower part of the overlying Cambridge 
Argillite in the northern part of the Boston basin, 
indicating that the Squantum Member pinches out and 
that the Dorchester has no clearly defined top in this part 
of the basin. Conglomerate in the Brookline Member 



contains clasts of Dedham Granite (Zdgr), quartzite, and 
volcanic rock from the underlying Mattapan Volcanic 
Complex (Zm). The source of the quartzite clasts in the 
Roxbury Conglomerate is most likely the Westboro 
Formation. If the Dedham Granite was exposed to 
erosion to provide the clasts of granite, then the West- 
boro was most likely exposed also. The quartzite clasts in 
the Roxbury do not contain traces of fossils as do the 
quartzite pebbles in the Pennsylvanian Purgatory Con- 
glomerate in Rhode Island (Shaler and others, 1899). The 
Dorchester Member consists of interbedded argillite and 
sandstone forming an intermediate unit between the 
primarily conglomeratic Brookline Member and the over- 
lying Cambridge Argillite. The uppermost Squantum 
Member of the Roxbury is a distinctive diamictite, best 
exposed north of Quincy; its origin has been subject 
to differing interpretations, most being that the Squan- 
tum Member is a tillite (Cameron, 1979). It is not 
everywhere a diamictite, however. In Brighton (Newton 
area) and Hingham, the Squantum Member is a very 
coarse conglomerate (Billings, 1976a, p. 10). It may not 
be as continuous as indicated by Billings (Kaye, 1980) 
and, as mentioned above, appears to pinch out in the 
northern part of the basin. Most recently, Caldwell 
(1981) presented arguments indicating that the Boston 
Bay Group as a whole, including the Squantum, is unlike 
Pleistocene glacial deposits in that deltaic deposits are 
lacking, sand-size fractions are rare, the varvelike Cam- 
bridge Argillite is too thick to be a glacial-lake deposit, 
and evidence of multiple episodes of glaciation is lacking. 

Within the lowermost Brookline Member of the Rox- 
bury Conglomerate are mafic volcanic rocks, the Brigh- 
ton Melaphyre (ftZrb). The Brighton Melaphyre consists 
primarily of quartz keratophyre, keratophyre, and spi- 
lite (Kaye, 1980). These are dark-gray to dark-greenish- 
gray and reddish-gray aphanitic rocks. The spilites form 
flows, pillow lavas, feeder pipes and vents, and pyroclas- 
tic rocks. The keratophyres form massive flows, brec- 
cias, pillow lavas, and laminated de vitrified palagonite 
tuff. Altered mafic dikes in the Mattapan are considered 
to be part of the Brighton (LaForge, 1932, p. 42). Nelson 
(1975a) described the Brighton Melaphyre in the Natick 
quadrangle as consisting of bluish- to dark-greenish-gray 
basaltic and andesitic flows and tuffs and minor interbed- 
ded, very fine grained ash and slate. Bouchard (1979) 
described the Brighton in the Newton area as a complex 
composed primarily of varicolored mafic basaltic extru- 
sive rocks and subordinate andesitic to rhyolitic lavas 
and tuffs; lahars, mudflows, breccias, and agglomerate 
are also present. Some of these rocks are aphanitic, 
others are amygdaloidal, others are lapilli rich. 

The Brighton is younger than the Mattapan and 
intrudes it. Possibly some of the Brighton is contempo- 
raneous with part of the Mattapan. The Brighton appar- 



STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS 



E29 



ently rises no higher in the section than the Brookline 
Member of the Roxbury, although tuffaceous beds are 
noted in the Dorchester Member and in the Cambridge 
Argillite (fig. 9). 

The sandy horizons in the Cambridge Argillite (ftZc) 
are in places quartzite. The most prominent of these are 
the Milton quartzite unit of Billings (1976a), in the 
southern part of the basin, and the Tufts Quartzite 
Member, described by Billings (1929) and LaForge 
(1932), in the northern part of the basin. Billings (1976a, 
p. 12) projected the Milton quartzite unit to lie about 820 
m above the Squantum Member of the Roxbury Con- 
glomerate in east Milton. He projected the Tufts Quartz- 
ite Member to lie 2,280 m below the Squantum Member 
(Billings, 1976b, p. 35), which places it quite low in the 
section (fig. 9). Red sandstone and sandy argillite shown 
by Kaye (1980) in the Chelsea and Revere area and in the 
Milton-Quincy area intertongue with green argillite of 
the Cambridge. Kaye (1980) claimed that the red beds lie 
above the cleaner quartzites such as the Tufts Member 
and Milton unit. The possible significance of these 
quartzites in the otherwise turbidite-type bedding of the 
Cambridge is discussed in the following section. 



PALEOENVIRONMENT 

The descriptions of the formations of the Boston Bay 
Group given by Rehmer and Roy (1976; fig. 9) indicate 
that the group was deposited in a fairly low-energy 
environment in a marine or lacustrine basin. The Rox- 
bury Conglomerate represents deposits proximal to a 
volcanic source area in an alluvial fan-delta complex. 
Laminar graded beds of fine-grained sand and silt are 
interstratified with the coarser clastic beds and the 
volcanic material. The Cambridge Argillite represents 
primarily distal deposits of mud and silt characterized by 
rhythmic bedding but containing wedges of sand from 
near-shore sources intertonguing with the silt and mud 
as water levels shifted. It is not certain whether there is 
more than one quartzite horizon and whether the quartz- 
ite represents shoal areas during Cambridge deposition 
or marine regression followed by transgression in the 
Cambrian. The presence of acritarchs (Lenk and others, 
1982) suggests that the basin was marine, possibly 
protected in some way. The bedding style in the Cam- 
bridge suggests turbidite deposition. The Squantum 
Member "tillite" could be more aptly considered in this 
environment to be a submarine-landslide deposit, per- 
haps derived from glacial deposits, as suggested by 
Rehmer and Hepburn (1974), or from other sediments, in 
accordance with the doubts of Caldwell (1981) concerning 
the glacial origin of the Boston Bay Group. Laminar 
graded beds like those in the Cambridge are present in 



the Mattapan Volcanic Complex (Lyons and Goldsmith, 
1983), indicating a long-standing basin in which the 
environment of deposition changed from primarily volca- 
nic (Mattapan) to primarily proximal turbidite (Roxbury) 
to primarily distal turbidite (Cambridge). 

The total thickness of the Boston Bay Group increases 
from south to north (Billings, 1976a; fig. 9). The Brook- 
line Member is thickest over the central anticline and 
thins to the south. These relations indicate that the 
material was derived from a southern and southwestern 
source and that the basin of deposition deepened to the 
north and northeast. The center of the basin subsided 
more rapidly than the margins (Billings, 1976a). The 
Squantum Member, however, appears to maintain a 
fairly uniform thickness (fig. 9), suggesting a cessation of 
subsidence toward the end of Roxbury deposition. 



The age of the Boston Bay Group has long been a 
matter of controversy. Suggested ages have ranged from 
Primordial (Cambrian) (Crosby, 1880) to Pennsylvanian 
(Crosby, 1900; Emerson, 1917), on the basis of lithologic 
correlation with the strata in the Narragansett basin and 
now discounted plant fossils; the most recent suggestion 
is Proterozoic Z (Kaye and Zartman, 1980; Lenk and 
others, 1982). Kaye and Zartman concluded that the 
Boston Bay Group lay below the fossiliferous Cambrian 
strata located on the margin of the basin, on the basis of 
an interpretation of field relations between the Cambrian 
strata and the strata within the basin. This interpreta- 
tion has been recently supported by the identification of 
acritarchs in the Cambridge Argillite, including a diag- 
nostic species that ranges in age from Proterozoic Z to 
Early Cambrian but that is most abundant in Proterozoic 
Z time (Lenk and others, 1982). Billings (1982, p. 912) 
estimated that the horizon containing the acritarchs lies 
about 3,300 m below the highest beds of the Cambridge, 
as deduced from his tunnel investigations in the Boston 
area, and about 2,500 m above the lowest beds (fig. 9). 
No acritarchs or other microfossils were found by Lenk 
and others in the known Cambrian rocks adjacent to the 
basin (P.K. Strother, oral commun., 1982). No reliable 
diagnostic fossils other than these have been found in the 
rocks of the Boston Bay Group (Lyons and Goldsmith, 
1983). The Proterozoic Z age indicated by the acritarchs 
is supported on several other counts: none of the plant 
fossils so numerous in the strata of the Narragansett 
basin have been found; Kaye (1980) showed that the 
Quincy Granite of Late Ordovician to Early Silurian age 
contains argillite inclusions that are on strike with Cam- 
bridge Argillite; and the primarily marine stratigraphy is 
not similar to the terrestrial stratigraphy in the Nar- 



E30 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



ragansett basin (Mutch, 1968). The basal Cambrian 
strata in eastern Massachusetts are quite unlike the 
Roxbury Conglomerate. 

Existing evidence thus indicates a certainly Protero- 
zoic Z to possibly Early Cambrian age for the Boston Bay 
Group. How much of the Boston Bay Group may be 
Cambrian is uncertain. David D. Ashenden (written 
commun., 1980) of the Metropolitan District Commis- 
sion, who has studied in detail cores of both the Cam- 
bridge Argillite in the Roslindale syncline and the Brain- 
tree Argillite (Cbw on the State bedrock map) at the Old 
Quincy Reservoir, Braintree, concluded that the two 
rocks are identical. C.A. Kaye (oral commun., 1979) 
believed that quartzites like the Tufts Quartzite and the 
Milton quartzite unit as used by Billings (1976a) and 
overlying red beds may be basal Cambrian and that 
overlying red sandstone and sandy argillite are Cam- 
brian strata. These red beds interfinger with green and 
red argillite mapped as Cambridge in the northern and 
southern parts of the basin. However, to date, Cambrian 
strata have been found only on or beyond the margins of 
the Boston basin. 



CAMBRIAN STRATA 

Fossiliferous rocks of Cambrian age in eastern Massa- 
chusetts are found in and around the Boston basin, at 
Hoppin Hill in North Attleboro, and north of Diamond 
Hill in West Wrentham. The largest area of fossiliferous 
Cambrian is south of the Boston basin in the Braintree 
and Weymouth areas (fig. 8) where the Weymouth 
Formation and Braintree Argillite contain Lower and 
Middle Cambrian fossils, respectively. The many studies 
of fossils from these formations have been summarized 
by Theokritoff (1968). Mr. G. Stinson Lord of Quincy has 
made extensive fossil collections in these formations. 
Lower Cambrian fossils have been found on the north 
side of the Boston basin at Nahant (Foerste, 1889) and at 
Revere Beach by C.A. Kaye (oral commun., 1981). The 
Lower and Middle Cambrian Hoppin Formation at Attle- 
boro and West Wrentham (WW, fig. 10) has been 
described by Foerste (in Shaler and others, 1899), Shaw 
(1950, 1961), and, more recently, Anstey (1979) and 
Landing and Brett (1982). Fossils reported as Upper 
Cambrian by Rhodes and Graves (1931) from the Green 
Lodge Formation near Dedham, Mass., cannot be reli- 
ably assigned to that period according to Shaw (1961, p. 
436). The Braintree Argillite and the Weymouth Forma- 
tion have been combined into a single unit (Cbw) on the 
State bedrock map because of their small areal distribu- 
tion. 

Rocks of Cambrian age may be more widely distrib- 
uted than has been mapped. C.A. Kaye (oral commun., 



1979) believed that some of the red sandstones in the 
Cambridge Argillite at the northern part of the Boston 
basin could be Cambrian rather than Proterozoic Z in 
age. Chute (1964) described beds resembling Cambrian 
strata in the Wamsutta Formation in the Norfolk basin, 
and J. P. Schafer (oral commun., 1982) has observed 
limestone-bearing beds in the Wamsutta near outcrops of 
Dedham Granite in the Attleboro area southeast of 
Hoppin Hill that might be Cambrian strata. Cambrian 
strata have been found to be much more extensive than 
previously thought in southern Rhode Island (Skehan 
and others, 1981), although, as Skehan and others 
pointed out, Dale (1885a,b) early differentiated what are 
known to be Cambrian strata from the Pennsylvanian 
strata in Rhode Island. 

The Cambrian fossil assemblages in eastern Massachu- 
setts and Rhode Island are all of the Acado-Baltic 
province (Theokritoff, 1968) and compare favorably with 
assemblages from southeastern Newfoundland, Eng- 
land, and Morocco (Landing and Brett, 1982; Skehan and 
others, 1978). 

HOPPIN FORMATION (€h) 

A sequence of strata containing Lower Cambrian 
fauna at Hoppin Hill, North Attleboro (fig. 10), was 
named the Hoppin Slate by Foerste (in Shaler and 
others, 1899) and more recently named the Hoppin 
Formation (Goldsmith and others, 1982a) because the 
sequence contains rocks other than slate. The Hoppin 
Formation (Ch) consists primarily of green and red slaty 
shale, locally containing calcareous nodules, and lenses 
and layers of red argillaceous limestone. At the base it 
contains arkosic quartzite, which is locally conglomer- 
atic, and sandstone. The nonconformity at the base of the 
Hoppin Formation is clearly exposed at Hoppin Hill 
(Dowse, 1950). Anstey (1979) studied the Hoppin For- 
mation in detail and measured several sections in and 
around the Hoppin Hill Reservoir (fig. 11). The section 
shown in figure 12, measured by me before I was aware 
of Anstey's work, duplicates section A of Anstey. On the 
east side of Hoppin Hill, a basal quartzite and grit unit 
containing quartz and feldspar grains from the underly- 
ing Dedham Granite rests nonconformably on the Ded- 
ham. Obscure crossbedding indicates the beds top away 
from the granite. The quartzite (10-15 m thick), which is 
conglomeratic in places, is interbedded with and passes 
upward into arenaceous slate, argillaceous siltstone, and 
dark-green slate containing thin sandy horizons above 
scour-fill channels (18-30 m). This unit is succeeded by 
red argillaceous limestone, containing fossil fragments 
(biomicrite, biomicrudite) dominated by Volborthellids, 
hyolithids, and trilobites, interbedded with red slate 
(34^44 m). This limestone and slate unit is overlain by a 



STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS 



E31 




A' 



1000-meter Universal Transverse Mercator 
grid ticks, zone 19. shown for reference 



z^-— \3 



EXPLANATION 

Limestone (Chi) 



|335] Slate (€hs) 
| .-.->/- I Quartzite (€hq) 



B 



Line of measured section 

Contact — Dotted where projected 

Fault 

Bedrock exposures 



pp 

Pondville 
Conglomerate 



€h 

Hoppin 

Formation 



Zdgr 
Dedham 
Granite 



Figure 11.— Geologic map and measured sections on the east side of Hoppin Hill, Attleboro, Mass. 



thick nonfossiliferous section of red and green slate 
(more than 186 m). The top of the sequence is overlain in 
angular unconformity by the Pondville Conglomerate of 
Early Pennsylvanian age. Measured sections indicate the 
exposed maximum thickness of the Hoppin Formation is 
approximately 244 m. Skehan (1969, p. 798) believed the 
Hoppin Hill section is similar in lithology to the Manuels 
Brook section in the Avalon terrane of southeastern 



Newfoundland described by Walcott (1890; cited by 
Hutchinson, 1962). 

The Hoppin Formation at West Wrentham is poorly 
exposed in a hillock beneath a powerline east of Cumber- 
land Street, just north of the Rhode Island State line. 
Here, red and green slate and red argillaceous limestone 
containing fossil fragments similar to those at Hoppin 
Hill can be found. A small outcrop of quartz on the 



E32 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



Thickness 
(meters) 



Hoppin Hill Reservoir 
Covered 



(Float of red slaty shale and limestone) 



underlain by green slaty shale 



(Float of red slaty shale) 



Red slaty shale containing calcareous 



/ lenses; scattered 



Red shaly limestone 



quartzite and quartz wacke. fissile, brown-weathering 



: wacke and quartzite 



Quartz wacke and quartzite, fissile, brc 



^ > Pebble conglomerate and pebbly quartzite 



Quartzite 



(Float of pebbly quartzite and pebble conglomerate) 



Crossbedded quartzite, pebbly quartzite, and pebble conglomerate 



(Float of quartzite and pebbly quartzite 
(Float of granite in 71-m interval) 



Dedham Granite (Zdgr) 






Figure 12.— Measured section of Lower and Middle Cambrian strata 
of the Hoppin Formation at the north end of Hoppin Hill Reservoir. 



southwest side of the hillock may be vein quartz, related 
to the massive vein quartz at Diamond Hill immediately 
to the south, rather than the basal quartzite. An inclu- 
sion of "slate" in ledges of Dedham Granite adjacent to 
the west mentioned by Foerste (in Shaler and others, 
1899, p. 393) is not the Hoppin Formation strata but is a 
chloritoid-bearing phyllite (E-an Zen, oral commun., 
1979) belonging to the Blackstone Group, which crops out 
0.7 km to the southwest. The contact with the Dedham to 
the west is probably a fault. To the north and east of the 
hillock is the Wamsutta Formation, and to the south are 
the felsic volcanic rocks of Diamond Hill. 



WEYMOUTH FORMATION AND BRAINTREE ARGILLITE 

(Cbw) 

The Weymouth Formation (Cbw) at Weymouth con- 
sists primarily of greenish-gray and dark-red slate con- 
taining calcareous nodules, and subordinate beds and 
lenses of argillaceous limestone. It was locally converted 
to hornfels by the intrusion of Quincy Granite. At Nahant 



(fig. 8), the Weymouth includes a layer of greenish-white 
limestone. Kaye (oral commun., 1981; Kaye, 1980) found 
exposures of Weymouth Formation at Revere Beach 
following a storm that scoured sand from the beach, 
which resembled rock in the exposures at Nahant. Clark 
(1923, p. 475) reported a concentration of fossiliferous 
pebbles within a mile of the south end of the beach. 
Fossils in the Weymouth are largely conoidal forms but 
include some olenellids. The lithology of the Weymouth is 
similar to that of the Hoppin Formation at Hoppin Hill, 
except that the lower quartzitic part of the section has 
not been recognized. A complete section of the Wey- 
mouth Formation has been nowhere described. The base 
of the Weymouth is not exposed in the Boston area, and 
its thickness can only be inferred to be at least 100 m 
(LaForge, 1932, p. 20). 

The Braintree Argillite (Cbw) is present only south of 
the Boston basin in and around the Quincy Granite. The 
formation consists of noncalcareous green to dark-gray 
or black massive slate or argillite. Adjacent to the Quincy 
Granite it is a greenish-gray hornfels with indistinct 
bedding. Chute (1969) described the Braintree in the 
Blue Hills quadrangle as a dark-gray slate containing 
thin beds of light- and medium-gray siltstone. The slate 
is cyclically thinly bedded. LaForge (1932) estimated the 
thickness to be at least 1,000 ft (300 m). Its contact with 
the underlying Weymouth Formation is nowhere 
exposed, nor is the top of the formation known. No 
complete section has been described. The faunal assem- 
blage in the Braintree is characterized by Middle Cam- 
brian representatives of Paradoxides. At Conanicut 
Island in southern Rhode Island, Skehan and others 
(1978, 1981) described a Middle Cambrian sequence 350 
m thick of siltstone and phyllite in which the beds in the 
lower part coarsen upward, indicating deposition in 
shallow water, and the beds in the upper part are cyclic 
and fine grained, indicating deposition in quieter water. 
The lower part of the sequence contains a trilobite fauna 
characterized by Badelusia and Paradoxides. The upper 
part is nonfossiliferous. The lithologies described there 
appear to be similar to those of the Braintree. 



GREEN LODGE FORMATION OF RHODES AND GRAVES 

(1931) (€g) 

Poorly preserved brachiopod impressions in a quartz- 
ite (Cg) in the Dedham area were reported by Rhodes 
and Graves (1931). The fossiliferous quartzite is in a 
sequence of light-gray quartzite overlain by dark-gray 
phyllite containing siltstone laminae and thin layers of 
siltstone containing limonite-bearing pits. These rocks 
were exposed in a hill south of Route 128 near the 
Westwood-Dedham town line. Rhodes and Graves (1931) 
named the sequence the Green Lodge Formation and 



STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS 



E33 



assigned it to the Upper Cambrian on the basis of the 
fossil impressions. Shaw (1961, p. 436), however, exam- 
ined the fossils from the Green Lodge and said that they 
were too poorly preserved for reliable assignment. The 
Green Lodge therefore is shown as questionable Upper 
Cambrian on the State bedrock map. The outcrops, also 
seen by Loughlin (1911; Loughlin and Hechinger, 1914) 
and by Chute (1964), are no longer accessible because of 
construction. Chute estimated that about 150 m of phyl- 
lite is present overlying an undeterminable thickness of 
quartzite. Rhodes and Graves estimated the thickness of 
the Green Lodge to be not less than 300 m. Chute (1964) 
mapped a locality of similar phyllite on the main Amtrak 
railroad line in southern Norwood but did not correlate 
this phyllite with the Green Lodge. 

The Green Lodge is most likely the source of the 
fossiliferous quartzite pebbles found in the Pennsylva- 
nian Purgatory Conglomerate in the Narragansett basin 
in Rhode Island and also in the glacial drift on Cape Cod 
and the offshore islands (Kaye, 1983b). Samples of 
quartzite containing impressions of fossils gathered from 
outcrops of the Green Lodge shown to me by C.A. Kaye 
are similar to pebbles he has collected from the drift. The 
Green Lodge then may be more extensive beneath the 
glacial drift than it appears; it is shown on the State 
bedrock map as covering a larger area than originally 
shown by Rhodes and Graves (1931) and Chute (1964). 

PALEOENVIRONMENT 

The Cambrian strata in eastern Massachusetts repre- 
sent a marine littoral to shelf sequence of sediments. 
Anstey (1979) described the Lower to Middle Cambrian 
Hoppin Formation as a deepening-upward transgressive 
marine sequence. The basal quartzite in the Hoppin Hill 
area is a littoral deposit lying nonconformably on a 
Proterozoic Z granitic terrane. The calcareous fossilifer- 
ous beds represent a near-shore shoal deposit, and the 
overlying nonfossiliferous slates in the upper part of the 
sequence represent detrital muds washed further off- 
shore. The lithologically similar Weymouth Formation 
around the Boston basin can be interpreted as represent- 
ing the same sedimentary sequence. The cyclically bed- 
ded fossiliferous silt and shale of the Middle Cambrian 
Braintree Argillite were deposited in relatively shallow 
water but in a low-energy environment. The Middle 
Cambrian at Conanicut Island, R.I., described by Ske- 
han and others (1978, 1981) represents a transgressive 
sequence. A minor regression may have occurred 
between Early and Middle Cambrian time. If the quartz- 
ite of the Green Lodge is indeed Upper Cambrian, then 
a return to littoral conditions occurred in Late Cambrian 
time. The paleoenvironment of the Cambrian thus seems 
to be one of low tectonic activity in which crustal 



movements were epeirogenic. The volcanic activity of 
Proterozoic Z time had ceased. The Proterozoic Z bath- 
oliths were exposed and were being eroded on a surface 
of low relief flanked by a transgressing shallow sea. 

NEWBURY VOLCANIC COMPLEX (SILURIAN 
AND DEVONIAN) 

Sedimentary and volcanic rocks of known Silurian and 
Devonian age in eastern Massachusetts are confined to 
two wedge-shaped and lenticular fault-bounded basins 
aligned between Newburyport and Middleton-Topsfield 
(fig. 3). The Newbury Volcanic Complex (DSn) was 
carefully described by Shride (1976) in the Newburyport 
and Rowley area and by Toulmin (1964) in the Middleton- 
Topsfield area. The information below is condensed from 
Shride's description. 

The lower part of the Newbury Volcanic Complex 
(DSnl, DSna), 2,900 m thick, is composed largely of 
volcanic materials consisting of basalt flows; flow-banded 
rhyolite, vitrophyre, and ash-flow tuff; and porphyritic 
andesite flows, breccias, and tuff, partly waterlain. The 
upper part of the complex (DSnu), 1,500 m thick, consists 
of shallow marine siliceous siltstone, red sandy mudstone 
containing some volcanic detritus, and calcareous mud- 
stone. The porphyritic andesite (DSna) contains a shelly 
fauna indicating a latest Silurian age (Pridolian). The 
calcareous mudstone of the upper part (DSnu) contains 
ostracodes having a greater range than the andesite 
fauna, and it could be as young as earliest Devonian 
(Gedinnian). Lenticular masses of micrographic rhyolite 
(DSnr) 100-600 m thick intrude the sequence subparallel 
to the strata at various horizons. Neither the top nor the 
bottom of the complex is known. The rocks have under- 
gone no more than diagenesis and propylitization, as 
indicated by the descriptions by Shride (1976). Primary 
textures are well preserved. 

The Newbury Volcanic Complex can be correlated 
with rocks along the Appalachian trend to the northeast, 
but no Upper Silurian and Lower Devonian rocks of this 
sort are known to the southwest. The Newbury is 
equivalent to the Leighton Formation of the Pembroke 
Group in the Eastport area, Maine, on the basis of 
correlation of the shelly fauna. Faunal assemblages of the 
Leighton are of Acado-Baltic affinity. The Ames Knob 
Formation and the Thorofare Andesite of the Penobscot 
region, Maine, are also temporal equivalents. On the 
basis of similar lithologies, the Newbury has been 
equated in the past with the Mattapan and Lynn Volcanic 
Complexes in the Boston area, but the Mattapan and the 
Lynn now are believed to be Proterozoic Z in age. 

PENNSYLVANIAN STRATA 

Fossiliferous rocks of Pennsylvanian age in the 
Milford-Dedham zone in Massachusetts occupy the 



E34 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



Norfolk and Narragansett basins (figs. 1, 8, 10). These 
rocks consist of the Pondville Conglomerate (Pp), Wam- 
sutta Formation (Pw, Pwv), Rhode Island Formation 
(Pr, Pre), and Dighton Conglomerate (Pd). The non- 
fossiliferous Bellingham Conglomerate (PZb) occupies 
the Bellingham basin. The Bellingham is usually corre- 
lated with the Pennsylvanian strata in the Narragansett 
and Norfolk basins, but it may instead be of Proterozoic 
Z age. The only other known fossiliferous rocks of 
Pennsylvanian age in Massachusetts are the Coal Mine 
Brook Formation (Pcm) at Worcester on the east edge of 
the Merrimack synclinorium (Goldsmith and others, 
1982a). The Coal Mine Brook Formation is discussed in 
the chapter on the Merrimack belt (Robinson and Gold- 
smith, this vol., chap. G). 

BELLINGHAM CONGLOMERATE (PZb) 

The Bellingham Conglomerate (PZb) (Mansfield, 1906; 
Hall, 1963; Quinn, 1971) is primarily a conglomerate and 
lithic graywacke and is confined to the Bellingham basin. 
The dominant lithology, conglomerate, consists of 
quartzite and granite pebbles and cobbles, flattened in 
varying degrees, set in a green and greenish-gray mica- 
ceous matrix. Interbedded with the conglomerate and 
lithic graywacke is chlorite phyllite composed of musco- 
vite, quartz, chlorite, zoisite, magnetite, and locally 
chloritoid (Warren and Powers, 1914). An outcrop of 
Bellingham Conglomerate near the intersection of Black- 
stone Street and River Street in Woonsocket (fig. 4) 
consists of green conglomerate containing pebbles of 
granite in a sandy matrix, green sandstone, and dark- 
green phyllite containing a lens of gray limestone. The 
Bellingham Conglomerate contains pebbles of quartzite 
from the adjacent Blackstone Group and of Milford 
Granite containing its typical blue quartz; it is therefore 
locally derived. An instructive exposure lies on the east 
side of Woonsocket Hill, southeast of Woonsocket. Here 
cliffs of steeply dipping, thin-bedded, white to gray 
quartzite of the Blackstone Group stand above green 
schistose conglomerate, containing many flattened white 
to gray quartzite pebbles, and interbedded green calcar- 
eous quartz schist. The contact is probably a fault here, 
but the source of the quartz pebbles is obvious. It is 
difficult to distinguish schists of the Blackstone Group 
from those of the Bellingham in this area because of the 
low-grade metamorphism of the Blackstone. Warren and 
Powers (1914) claimed the distinction could be made in 
that schists of the Blackstone contain knots of epidote, 
whereas schists of the Bellingham do not. This criterion 
is not always reliable, and it is possible, if not probable, 
that some of the low-grade Blackstone Group mapped in 
the Blackstone River valley northwest of Woonsocket is 
part of the Bellingham. 



Volcanic rocks are present in a few places in the 
Bellingham. Near the edge of the basin, in a roadcut on 
new Route 146 at Premisy Hill, west of Woonsocket, a 
rhyolite porphyry sill 30 cm thick cuts actinolitic green- 
stone, schist, thin gray quartzite, and gray, streaked 
biotite schist containing epidote pods, rocks probably 
belonging to the Blackstone Group. East of Bellingham 
in the Franklin area are exposures of felsite porphyry 
described in an earlier section that may be the same age 
as the Bellingham Conglomerate. Warren and Powers 
(1914, p. 448) mentioned the presence of amygdaloids in 
the Bellingham area, but I could not locate these in my 
reconnaissance. Exposures are poor in the Bellingham 
basin north of Woonsocket, and the distribution of the 
Bellingham is mapped largely from float. 

The age of the Bellingham is conjectural. No fossils 
have been found. Customarily the rocks have been 
correlated with rocks of the Narragansett basin and thus 
labeled as Pennsylvanian in age. Rocks in some expo- 
sures, such as the one at River Street and Blackstone 
Street in Woonsocket, resemble outcrops of Roxbury 
Conglomerate seen in the Boston basin, and it is not 
unreasonable to assume that the age of the Bellingham 
conglomerate is similar to that of those rocks in the 
Boston basin as suggested by Skehan and Murray (in 
Skehan and others, 1979, p. 14-15). Some support for 
this conclusion lies in the observation that the Belling- 
ham basin is a structural trough (Goldsmith, this vol., 
chap. H) that extends southwest from the Boston basin 
and that separates primarily gneissic Proterozoic Z gran- 
itoids from altered but nongneissic Proterozoic Z granit- 
oids (Wones and Goldsmith, this vol., chap. I). In defer- 
ence to tradition, however, and because a Proterozoic Z 
age is not proven but only suspected, the Bellingham 
Conglomerate in the Bellingham basin is shown as Penn- 
sylvanian to Proterozoic Z in age on the State bedrock 
map. 



PONDVILLE CONGLOMERATE (Pp), WAMSUTTA 

FORMATION (Pw, Pwv), RHODE ISLAND FORMATION 

(Pr, Pre), AND DIGHTON CONGLOMERATE (Pd) 

The Pondville Conglomerate, Wamsutta Formation, 
Rhode Island Formation, and Dighton Conglomerate are 
the stratigraphic units of known Pennsylvanian age in 
the Norfolk and Narragansett basins in Massachusetts. 
These formations and the Purgatory Conglomerate in 
Rhode Island were referred to collectively as the Nar- 
ragansett Bay Group by Skehan and Murray (in Skehan 
and others, 1979, p. A4). The Pennsylvanian strata in the 
Narragansett and Norfolk basins have been described by 
many authors from Shaler and others (1899) through 
Quinn and Oliver (1962), Mutch (1968), and Skehan and 
Murray (1978). This work was summarized by Skehan 



STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS 



E35 



Table 5. 



-Description of stratigraphic units in the Narragansett and Norfolk basins 
[Modified slightly from Skehan and others (1979, table 1)] 



Description 



Sedimentary 

and other 

distinguishing 

features 



Approxi- 
mate 

thickness 
(meters) 



Additional references 



Purgatory Coarse-grained to very coarse grained Not given 
Conglom- conglomerate containing thin lenses of 

erate. sandstone and magnetite-rich sand- 

stone; clasts in conglomerate consist of 
several varieties of quartzite. 



30 



Dighton Gray conglomerate consisting primarily 

Conglom- of rounded quartzite cobbles to boul- 

erate. ders and containing subordinate 

rounded granite cobbles and slate peb- 
bles; very little sand matrix; lenses of 
medium-grained sandstone form less 
than 20 percent of the unit. 
Rhode Island Gray sandstone and siltstone and lesser 
Forma- amounts of gray to black shale, gray 

tion. conglomerate, and coal beds 10 m 

thick. Sandstone and conglomerate are 
quartz rich. 



Wamsutta Interbedded red coarse-grained conglom- 
Forma- erate, lithic graywacke, sandstone, 

tion. and shale; conglomerate layers less 

than 1.2 m thick contain felsite clasts; 

a few lenses of limestone, one rhyolite 

flow, and several sheets of basalt are 

present. 



Pondville At type locality (Pondville Station, 

Conglom- Mass.), interbedded red and green 

erate. slate, siltstone, arkose, and quartzite- 

pebble conglomerate; elsewhere 
includes gray to greenish-gray coarse 
conglomerate containing clasts 15-60 
cm in diameter and abundant sandy 
matrix (clasts mostly quartzite, but 
some are granite or schist) and dark- 
gray granule conglomerate containing 
pebbles of smoky quartz 5 mm in 
diameter irregularly bedded with 
sandstone and lithic graywacke. 



Sandstone lenses are 
faintly crossbedded 
and coarsen both 
upward and down- 
ward into adjacent 
conglomerate. 

Contains both fining- 
and coarsening- 
upward sequences; 
paleocurrents have 
been defined only 
locally; conglomerate 
is relatively less abun- 
dant than in Dighton 
Conglomerate. 

Crossbedding and inter- 
fingering of layers are 
characteristic. 



First-deposited beds are 
siltstone or arkosic 
sandstone, rarely con- 
glomerate; however, 
sandstone and shale of 
the Wamsutta Forma- 
tion or Rhode Island 
Formation may lie 
directly on older 
rocks. 



<3,000 



No Pennsylvanian 
flora yet known; 
distinctive early 
Paleozoic faunas 
are present in 
quartzite clasts. 

Late Pennsylvanian; 
small isolated 
amounts of alloch- 
thonous nondiag- 
nostic plant debris 
are present. 

Late and Middle 
Pennsylvanian. 



Middle and Early 
Pennsylvanian; 
partly equivalent 
to Rhode Island 
Formation as the 
red layers interfin- 
ger with gray and 
black; contains a 
few plant fossils. 

Early Pennsylvanian 



Mosher and Wood 
(1976). 



None. 



Skehan and Murray 
(1978), Lyons and 
Chase (1976). 



Lidback (1977). 



None. 



'These references l 



l addition to Quinn and Oliver (1962), Mutch (1968). and ( 



l (1971), which contain information on all these stratigraphic units. 



and Murray and Murray and Skehan (in Skehan and 
others, 1979). Detailed maps have been made by Chute 
(1950, 1966, 1969), Hartshorn (1960, 1967), Koteff (1964), 
and Lyons (1969). Lyons (1977) mapped the Massachu- 
setts part of the Narragansett basin in reconnaissance 
fashion and reviewed existing detailed work. Drilling by 
the U.S. Geological Survey (USGS) in 1977 and 1978 
showed that the Narragansett basin and its deposits 



extend to Massachusetts Bay a few kilometers south of 
Scituate. 

The units in the Narragansett and Norfolk basins are 
described in table 5. Briefly, the strata are primarily 
fluviatile and consist of sandstone, siltstone, conglomer- 
ate, shale, and coal. Volcanic rocks are present in the 
northwest part of the basin (fig. 10). Chute (1966, p. B32) 
described lenses of carbonate rock in red and green 



E36 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



shales in the Wamsutta Formation in the Norwood 
quadrangle. Limestone has been observed in rock 
mapped as Wamsutta adjacent to exposed Dedham Gran- 
ite at the Manchester Pond Reservoir, Attleboro (Scha- 
fer, oral commun., 1982). Possibly these red and green 
shales and calcareous beds are actually Cambrian in age. 
Total thickness of the strata is estimated to be 3,700 m 
(Skehan and Murray, 1978). The Rhode Island Forma- 
tion is the thickest and most extensive of the formations 
although it does not extend into the Norfolk basin. Coal 
beds are found only in the Rhode Island Formation. Only 
the Pondville Conglomerate and the Wamsutta Forma- 
tion are present in the Norfolk basin. Chute (1964, 1966, 
1969) recognized a lower boulder conglomerate member 
and an upper sandstone to pebble conglomerate member 
in the Pondville in the northeast part of the Norfolk 
basin, but such a division is not readily made to the 
southwest because of facies changes. The upper member 
grades into and interfingers with the Wamsutta Forma- 
tion; in turn, the Wamsutta interfingers with the Rhode 
Island Formation in the northwest part of the Narragan- 
sett basin. In the northern part of the Narragansett 
basin, basal beds of the Pondville, the Wamsutta, and in 
places the Rhode Island Formation rest nonconformably 
on weathered Dedham Granite. Basal beds are usually 
sandstone or arkose rather than conglomerate; little 
indication exists that the material has been transported 
far. A drill hole near Assonet at the edge of the basin 
revealed that the basal beds of the Rhode Island Forma- 
tion were a reworked regolith (J. A. Sinnott, oral com- 
mun., 1979). At the northeast end of the Norfolk basin 
(fig. 8), the Pondville overlies weathered and partly 
transported Blue Hills Granite Porphyry without clear 
definition (see Naylor and Sayer, 1976; Naylor, 1981). 

Daniels Street, between Medway and Franklin, 
crosses the top of a hill covered with float of polymict 
conglomerate containing red-stained gray and white 
quartzite cobbles and pebbles and fragments of reddish- 
purple slate. The matrix appears to be reddish-colored 
sand. No rock was seen in place. The material does not 
seem to be metamorphic. The bedrock surrounding the 
hill is granite. This material is shown on the State 
bedrock map as Wamsutta Formation (Pw). It could be 
Pondville Conglomerate or possibly Bellingham Con- 
glomerate, as it is somewhat on line with the Bellingham 
basin to the southwest; however, the rock material does 
not seem to be sufficiently metamorphosed to be the 
Bellingham. 

Rhyolite and mafic volcanic rocks in the Wamsutta 
Formation (Pwv).— A rhyolite flow and two sheets of 
basalt flanked by fossiliferous horizons lie within the 
Wamsutta Formation near Attleboro (Lyons, 1977; fig. 
10). Here and in the Norfolk basin, conglomerate in the 
Wamsutta contains many clasts of volcanic rock. Bottino 



(1963) attempted to date the rhyolite by using the 
whole-rock Rb-Sr method, but the material did not yield 
a reliable date. Differences in initial ratios of strontium 
isotopes, however, indicated that the rhyolite and the 
basalts were probably not related (Bottino, 1963). 

Northwest of Attleboro, near Grants Mill, R.I., the 
Diamond Hill Felsite as used by Skehan and Murray (in 
Skehan and others, 1979, p. A5) is overlain by the 
Wamsutta Formation at the south end and underlain by 
the Wamsutta at the north end of the Diamond Hill 
Reservoir (Quinn and others, 1948, p. 18). The Diamond 
Hill Felsite is primarily dacite, much altered and cut 
through by vein quartz; it is gray, greenish gray, and 
reddish purple, fine grained, and porphyritic (Quinn, 
1971, p. 41). Phenocrysts are quartz and altered plagio- 
clase. The rock locally shows flow structure, but much of 
it is massive. At the north end of the reservoir, I 
observed greenish-gray, gray, and reddish-purple felsite 
and agglomerate that pass downward through interbed- 
ded tuff, sandstone, slate showing cleavage, and con- 
glomerate containing white quartzite and granite cobbles 
into red and green conglomerate typical of the Wamsutta 
Formation. Some faulting has occurred near the contact 
of the Diamond Hill with the Wamsutta, but displace- 
ment appears to have been minor. The amount of vol- 
canic detritus in the Wamsutta indicates that volcanoes 
were active close to the time of its deposition. 

Volcanic rocks somewhat similar to the Diamond Hill 
Felsite crop out west of Lake Pearl, between Franklin 
and Wrentham, on the west flank of the Norfolk basin. 
These are shown as volcanic rocks in the Wamsutta 
Formation (Pwv) on the State bedrock map because of 
their proximity to Diamond Hill. They also resemble the 
Mattapan Volcanic Complex, as mentioned in that sec- 
tion of this chapter. The volcanic rocks at Lake Pearl 
consist of dark- and light-colored aphanitic felsite, 
agglomerated), and breccia. One type is a greenish-gray 
aphanitic rock containing saussuritized lath-shaped pla- 
gioclase and clots of chlorite and iron-oxide in a fine- 
grained matrix of alteration products (white mica, epi- 
dote, chlorite). The plagioclase is locally glomerophyric. 
Another type is a white-weathering, aphanitic rock con- 
taining fine-grained angular quartz, feldspar, and rock 
fragments in a fine-grained sericitic matrix. A light- 
colored felsite contains quartz "eyes" like those in the 
felsite in Franklin. A grayish-green agglomerate or 
conglomerate containing fragments of felsite porphyry 
and smaller clasts of quartz and pink feldspar in a 
greenish-gray sandy matrix exposed on the east side of a 
large quarry west of Lake Pearl may be part of the 
volcanic assemblage or it may be basal Wamsutta For- 
mation. Further west, a felsite containing pale-tan phe- 
nocrysts in a pale-red aphanitic matrix crops out west of 
Uncas Brook and east of Summer Street, 2 km south of 



STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS 



E37 



Franklin. On Summer Street, due west of Uncas Pond, a 
dike of spherulitic rhyolite containing scattered quartz 
phenocrysts cuts Dedham Granite. The rocks near and 
along Summer Street are separated from the volcanic 
rocks near Lake Pearl by the Wamsutta Formation and 
a sliver of Dedham Granite but are considered to be part 
of the volcanic assemblage. Outcrops of fine-grained 
granite (fgr) and of porphyry exposed in roadcuts north 
of Lake Pearl in the town of Norfolk and in a few cuts 
northeast of Lake Pearl near the Wrentham State School 
may also be part of the volcanic assemblage. 



PALEOGEOGRAPHY AND AGE 

Mutch (1968, p. 201-203) discussed the paleogeogra- 
phy of the Narragansett basin; he concluded that a 
highland supplying most of the detritus for the Nar- 
ragansett Bay Group of Skehan and Murray (in Skehan 
and others, 1979) existed to the west and northwest of 
the basin and that a stable source to the east supplied 
less material. Current directions measured in the Rhode 
Island Formation in the northern part of the basin 
indicate a flow from northeast to southwest. In support 
of this observation, conglomerate is more abundant on 
the west and northwest side of the basin than to the east 
and southeast. In the southern part of the basin in Rhode 
Island, on the contrary, coarse cobble to pebble conglom- 
erate in the Purgatory Conglomerate is interpreted as 
being derived from the northeast and east. The center of 
volcanism was to the northwest. The basin fill is primar- 
ily alluvial; conglomerates represent stream channel 
deposits, and coal beds indicate interchannel swamps. 

The age of the deposits ranges from Early to Late 
Pennsylvanian (table 5), although Skehan and Murray 
(1980b, p. 69) assigned the lower part of the Pondville to 
the Upper Mississippian. It is possible that lower parts of 
the sequence in the Narragansett Bay Group of Skehan 
and others (1979), below the fossiliferous horizons, may 
contain Paleozoic strata older than Mississippian or 
Pennsylvanian. However, little, if any, stratigraphic 
thickness is present in the northern part of the basin 
between the fossiliferous horizons and the Cambrian and 
older rocks, so that such strata must be thin, if they 
exist. Possibly some nonfossiliferous Cambrian beds 
have been mistaken for Pennsylvanian beds. The north- 
west part of the Narragansett basin and the southern 
part, at least, of the Norfolk basin need critical study in 
this regard. 



TRIASSIC AND JURASSIC ROCKS 

Triassic and Jurassic rocks occupy a small basin, the 
Middleton basin in Essex County ("fie), and lie in the 



subsurface beneath the Coastal Plain cover in Nantucket 
Sound and Nantucket Island. The Triassic and Jurassic 
sedimentary (J"fi) and volcanic (J"fib) rocks in the Nan- 
tucket area are described in a later section of this 
chapter. An extensive basin of Triassic and Jurassic 
rocks lies northeast of Essex County in the Gulf of Maine 
(Uchupi, 1966) and in the Bay of Fundy but is outside the 
area of the State bedrock map. In this section, only the 
rocks in the Middleton basin are described. 



RED ARKOSIC CONGLOMERATE, SANDSTONE, AND 
SILTSTONE ("Re) 

A narrow wedge of red to reddish-gray conglomerate, 
arkosic sandstone, and siltstone ("fie) lying near the 
southernmost of the Newbury basins in the towns of 
Peabody, Danvers, Middleton, and Topsfield northwest 
of Salem, Essex County (fig. 3), contains a few shale beds 
bearing plant fossils of Late Triassic or possibly Early 
Jurassic age (Kaye, 1983a). Fragments of rock of possible 
Triassic age were first recognized in the glacial drift in 
this area (Oldale, 1962). The conglomerate was later 
exposed on one edge of a quarry in northeast Peabody 
and called to the attention of C.A. Kaye by A.E. Shride. 
Plant fossils found later by Peter Robinson from a red 
shale layer in the conglomerate were examined by E.S. 
Barghoorn and found to correspond to fossils from the 
Newark Group of Late Triassic and Early Jurassic age 
(Kaye, 1983a). The true size and shape of the basin called 
the Middleton basin by Kaye (1983a) is not known. It is 
fault bounded on its southeast side. Its northwest side is 
not exposed; the strata are only exposed in the quarry 
face. Its size is inferred, from its topography and aero- 
magnetic signature, to be about 5.7 km by 0.5 km. 

The strata are poorly to well bedded. Conglomerates 
are poorly sorted and have abundant sandy matrix. The 
conglomerate contains rounded to subrounded cobbles of 
nearby granite and granodiorite and poorly rounded to 
angular fragments of red shale and sandstone. 



STRATIGRAPHIC PROBLEMS 

The descriptions of the various units in the Milford- 
Dedham zone and the discussions of their relationships 
presented in the preceding sections have revealed a 
number of problems and uncertainties, which need fur- 
ther investigation. All of these problems require field 
work, but some require an emphasis on petrologic, 
petrochemical, and isotopic methods; others an attention 
to structural features and degree and nature of meta- 
morphism; and others a more thorough stratigraphic 
analysis. 



E38 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



FELSIC VOLCANIC ROCKS 

Immediately obvious from the descriptions of the rock 
units presented earlier in this chapter is the confusion 
between two similar-appearing volcanic assemblages, 
one dated as Proterozoic Z (the Mattapan and Lynn 
Volcanic Complexes) and the other dated as Early and 
Middle Pennsylvanian (the volcanic rocks in the Wam- 
sutta Formation at Attleboro and at Diamond Hill). 
Emerson (1917) equated the Diamond Hill Felsite with 
the Mattapan, both of which were then considered to be 
Carboniferous in age. Since then, the Mattapan has been 
revealed to be Proterozoic Z in age, whereas the Dia- 
mond Hill Felsite is still considered to be Carboniferous. 
LaForge (1932), who considered the Mattapan to be 
Silurian or Devonian, recognized the problem of having 
dissimilar ages for similar rocks. He said (LaForge, 1932, 
p. 29): 

So far there is no difficulty in correlating the volcanic rocks (Mattapan 
and Lynn), but an argument of the same sort fails when applied to the 
volcanic rocks of the Attleboro district, in the Narragansett basin. 
Those rocks, which are of the same lithologic types as part of those of 
the Mattapan complex and hence might be supposed to be of the same 
age, are interstratified with sedimentary rocks that are undoubtedly 
Pennsylvanian. At what point in the chain of reasoning there is a flaw 
has not been determined***. 

The felsic volcanic rocks flanking the Norfolk basin near 
Lake Pearl and flanking the northern part of the Bell- 
ingham basin lie between the two areas, and their age 
assignment is moot. Petrologic geochemical studies such 
as Zarrow's (1978) studies on the Lynn and Mattapan 
Volcanic Complexes might be of considerable aid in 
comparing the suites, as would radiometric analyses. 
Detailed mapping in the different areas might reveal 
significant differences in the associated sedimentary 
strata and in the structural position of the rocks. At the 
present time, I am not convinced that all of the strata 
mapped as Wamsutta Formation in the northwest Nar- 
ragansett basin and southern Norfolk basin are of Penn- 
sylvanian age. Just as agglomerate of the Mattapan has 
been mistaken for Roxbury Conglomerate in the Boston 
basin, so may fragmental rocks to the south in the 
Norfolk basin and northwest part of the Narragansett 
basin have been wrongly assigned to the Carboniferous. 
In this regard, the volcanic rocks at Attleboro and at 
Diamond Hill appear only in the part of the basin where 
basement is shallow and projects in two places through 
the Pennsylvanian strata; one at Hoppin Hill and another 
to the south-southwest at Manchester Pond Reservoir, 
Attleboro (J. P. Schafer, oral commun., 1982), where a 
sliver of granite like that at Hoppin Hill is brought up 
along a reverse fault. The granite is overlain by carbon- 
ate rocks and red beds (J. P. Schafer, oral commun., 
1982) that resemble the Cambrian strata at Hoppin Hill. 
Perhaps even older strata such as those in the Boston 
basin may be faulted up in this area. However, in neither 



area are there volcanic rocks between the Pennsylvanian 
strata and the basement. The Cambrian, where known, 
rests directly on granitic basement. Possibly Precam- 
brian rocks are preserved in down-dropped blocks such 
as the Bellingham basin. The Pennsylvanian age for the 
volcanic rocks at Attleboro seems fairly firm. The most 
likely possibility is that the felsic volcanic rocks near 
Lake Pearl flanking the Norfolk basin are Pennsylvanian 
but the felsic rocks flanking the Bellingham basin, Zfm 
(Zm on map), are equivalent to the Mattapan. This is the 
interpretation used on the State bedrock map. The 
Boston basin and the Bellingham basin lie along the same 
structural grain (figs. 1, 8), whereas the Norfolk basin is 
offset south of the Boston basin. 

A less obvious problem pertaining to the felsic volcanic 
rocks in the Milford-Dedham zone is the correlation of 
the layered felsic rocks in the Natick-Framingham area 
shown as metamorphosed felsic volcanic rocks (Zvf) on 
the State bedrock map (lower part of the Cherry Brook 
Formation of Nelson) and the Mattapan Volcanic Com- 
plex in the Boston area, which lies above the Middlesex 
Fells Volcanic Complex. The felsic volcanic rocks at 
Framingham have been suggested as being the volcanic 
cover into which the Milford-Dedham granite plutons 
intruded. Most of the Mattapan, however, lies noncon- 
formably above the batholithic rocks and is intrusive into 
them, or in a few places is paracontemporaneous with 
them. Because the Proterozoic Z batholiths of eastern 
Massachusetts comprise several plutons differing in age 
and composition, the associated felsic volcanic rocks in 
different areas could be pre-, post-, or syngranite. The 
Mattapan and the felsic rocks in the Framingham area 
thus could belong to a suite of felsic volcanic rocks having 
a range of age similar to that in the granitic plutonic 
rocks. Similar relations in rocks of the same time interval 
have been described by Wood (1974) in Anglesey, Wales, 
and by Hughes and Bruckner (1971) in Newfoundland. 
Bell and Alvord (1976) did not have a zone of felsic 
metavolcanic rocks below the Middlesex Fells, and Drier 
and Mosher (1981) pointed out the lack of felsic volcanic 
rocks below the Hunting Hill Greenstone in northern 
Rhode Island. I suggest that the felsic layers in the 
Framingham area are not volcanic rocks but thin intru- 
sive sheets into the mafic volcanic suite. They are here in 
a more highly metamorphosed regime than in the Boston 
basin and in addition are close to and in a zone of ductile 
shear associated with the Bloody Bluff fault zone (Nel- 
son, 1976). In this interpretation the felsic layers are 
younger than the mafic volcanic rocks; they are more or 
less synchronous with emplacement of the Proterozoic Z 
granites and thus more or less equivalent in age to the 
Mattapan. A similar situation exists in southeastern 
Connecticut, where thin sheets of metamorphosed fine- 
grained alaskite are interlayered with metamorphosed 



STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS 



E39 



mafic volcanic rocks above thick intrusive sheets of Hope 
Valley Alaskite Gneiss. These thin sheets have been 
ductilely deformed, producing the thinly layered to lam- 
inated blastomylonites in the Honey Hill fault zone, 
referred to earlier, and alternating layers and laminae of 
felsic and mafic material. Another explanation for the 
apparent interlayering of mafic and felsic rock is that the 
mafic layers are dikes, for example dikes of Brighton 
Melaphyre, cutting the intrusive granite sheets, which 
have been subsequently brought into parallelism through 
pervasive ductile shear. One sees a suggestion of this 
along the Massachusetts Turnpike (1-90) from Natick to 
west of Framingham. Mafic dikes to the east in light- 
colored granite become increasingly sheared, recrystal- 
lized, and less discordant to foliation and eventually 
become black amphibolitic layers parallel to the foliation 
in gneissic granite. Clearly, the problem of the felsic 
volcanic rocks of eastern Massachusetts has not yet been 
resolved. 



LYNN VOLCANIC COMPLEX 

The case for a Proterozoic Z age for the Lynn Volcanic 
Complex is not entirely closed, although evidence pre- 
sented in previous pages tends to support that assign- 
ment. Additional petrologic work and detailed field stud- 
ies in the Mattapan, Lynn, and Newbury Volcanic 
Complexes might provide additional evidence for such an 
age assignment. 



BOSTON BAY GROUP 

A Proterozoic Z to Early Cambrian(?) age for the 
Boston Bay Group is confirmed by the find of acritarchs 
of that age range in the Cambridge Argillite. A Pennsyl- 
vanian age for the Boston basin even before this find was 
at any rate quite convincingly ruled out by the difference 
in paleoenvironment and depositional style between the 
deposits in the Boston basin and those in the Narragan- 
sett basin (fig. 9, table 5) and the lack of plant fossils in 
the Boston basin as compared with the abundance of 
plant fossils in the Narragansett basin. However, the 
question remains as to how much of the Boston basin fill 
is Cambrian, equivalent to the Weymouth Formation 
and Braintree Argillite. The proposal by C.A. Kaye 
(1980) that the quartzites and red beds in the Cambridge 
Argillite are Cambrian needs to be explored. Are the 
Tufts Quartzite Member and Milton quartzite unit equiv- 
alent to the quartzite and feldspathic quartzite at the 
base of the Hoppin Formation, or are they merely 
deposits on shoals formed during Cambridge deposition 
and before Cambrian encroachment as Billings (1976a) 
indicated? The Cambridge and other units of the Boston 



Bay Group contain many thin to laminar graded beds, 
which should be useful in detecting an unconformity or 
disconformity. 



BELLINGHAM CONGLOMERATE 

The age of the Bellingham Conglomerate in the Bell- 
ingham basin is highly uncertain. Like the rocks in the 
Boston basin, the Bellingham lacks the plant fossils so 
abundant in the rocks of the Narragansett basin, and 
other similarities to the rocks of the Boston basin exist. 
Skehan and Murray (in Skehan and others, 1979) sug- 
gested that the Bellingham Conglomerate is correlative 
with conglomerates in the Boston basin. The structural 
alignment of the Boston basin with the Bellingham basin 
and the North Scituate basin in Rhode Island, their 
similar lithologies, and their lack of fossils certainly 
suggest that the three might be of similar age. The rocks 
in the Bellingham basin are, however, more metamor- 
phosed than are the rocks of the Boston basin. 

An ancillary problem that requires careful mapping is 
the similarity of the Bellingham Conglomerate in the 
Bellingham basin to the Blackstone Group in the Woon- 
socket area adjacent to the basin. Both are in the 
greenschist facies of metamorphism. The criterion of 
Warren and Powers (1914) involving the presence or 
absence of epidote to distinguish rocks of the Blackstone 
Group from the Bellingham Conglomerate needs to be 
reexamined, although in field reconnaissance the crite- 
rion seemed to be useful. The area of particular interest 
is that occupied by the schist of the Blackstone Group 
east of the Bellingham basin and north of the belt of 
Quinnville Quartzite. The State bedrock map probably 
shows the correct distribution of the two sets of rocks, 
but in the field it is not easy to distinguish the two nor to 
draw a line between them with confidence. 



BRIGHTON MELAPHYRE 

The Brighton Melaphyre represents an interval of 
volcanism, predominantly mafic, which is younger than 
the Mattapan and Lynn (LaForge, 1932, p. 42). As 
described, it appears to have phases similar to some in 
the Mattapan Volcanic Complex; these phases may rep- 
resent the transition from sialic Mattapan volcanism to 
the more simatic Brighton volcanism. This change in 
volcanism probably reflects a change to extensional 
tectonics associated with late Proterozoic Z rifting. We 
do not know where similar rocks of this age exist in 
eastern Massachusetts other than in the Boston area. 
The areal extent of the Brighton is very small. Possibly 
the amphibolitized dikes to the west on 1-90 described 



E40 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



above are Brighton. The dikes described as cutting 
Precambrian Z rocks in the Jamestown area in Rhode 
Island might also be of Brighton age. The temporal and 
areal extent of the Brighton needs study. 



SEQUENCE IN THE BURLINGTON AREA 

The units in the Greenleaf Mountain area, Burlington, 
described by Bell and Alvord (1976; table 3) are not 
clearly related to the other units in the Milford-Dedham 
zone. One can only speculate as to where they fit into the 
stratigraphy. As they are close to the Bloody Bluff fault 
zone, they might be a fault-bounded slice of allochtho- 
nous rock or merely a mylonitized part of the mafic 
metavolcanic assemblage. 



MAFIC METAVOLCANIC ROCKS AND THE MARLBORO 
FORMATION 

The relationship between the juxtaposed mafic volca- 
nic complex in eastern Massachusetts (Zv), east of the 
Bloody Bluff fault, and the Marlboro Formation to the 
west of it is an intriguing question. The Marlboro as 
described in chapter F (Goldsmith, this vol.) consists of 
amphibolite, hornblende gneiss, biotite-quartz-feldspar 
gneiss and granofels, calc-silicate rock and rare marble, 
rusty- weathering garnet-sillimanite schist, and garnet- 
muscovite-biotite schist. Some layers contain coticule. 
The metamorphosed mafic and felsic volcanic rocks of the 
Milford-Dedham zone (Zv; table 3) contain amphibolite, 
hornblende gneiss, and minor intercalated schist and 
quartzite, as well as felsic layers, but lack the calcareous 
rocks, the rusty schists, and the distinctive coticule- 
bearing layers of the Marlboro. The two contain some 
rocks that are similar in lithology, primarily the amphib- 
olite and hornblende-bearing rocks; this similarity 
has led previous workers to consider them to be one 
suite of rocks (Emerson, 1917). Current studies indi- 
cate that they are two different suites, but where to 
place the boundary between them in the Marlborough- 
Framingham-Concord area is in places unclear. The 
Marlboro Formation, on the whole, is a somewhat more 
varied unit. The primary distinction, as pointed out 
above in this chapter, is that the Middlesex Fells is 
intruded by Proterozoic Z granitoids, whereas the Marl- 
boro is not. However, we do not know with certainty the 
age of the formations in the Nashoba zone. If they are 
Proterozoic as they seem to be, they are perhaps a part 
of the Proterozoic volcanic-arc complex, but which accu- 
mulated in a different place than where they are at 
present and which represent a somewhat different 
facies. Stratigraphic analysis of the formations in the 
Nashoba zone (Goldsmith, this vol., chap. F) suggested 



that the center of volcanism was to the east. Possibly, 
the volcanic-plutonic complex in eastern Massachusetts 
represented the core of an arc, and the formations of the 
Nashoba zone were deposited on its flanks. At some later 
time the original continuity was disrupted and the rocks 
of the Nashoba zone were transported from their original 
position relative to the Milford-Dedham zone, to become 
affixed, probably in telescoped fashion, in their present 
position against the Milford-Dedham zone. Naylor (1976, 
p. 422) has noted the juxtaposition of two lithologically 
similar but probably temporally different terranes along 
a continuation of the Bloody Bluff fault in southern 
Connecticut. The rocks of the two zones could, however, 
have been originally quite unrelated. 

NEWBURY VOLCANIC COMPLEX 

Deposition of the Newbury Volcanic Complex does not 
readily fit into the proposed history of the Milford- 
Dedham zone. The fossil assemblages in the Newbury, 
however, indicate Acado-Baltic affinities, indicating that 
the Newbury accumulated on the same side of the 
Iapetus Ocean as did the Cambrian of the Milford- 
Dedham zone. One can only draw the conclusion that the 
Newbury did not accumulate at its present site. How- 
ever, it could possibly have accumulated on the flanks of 
the zone as a volcanic belt developed along the leading 
edge of the zone during its westward movement in the 
middle Paleozoic (Robinson and Hall, 1980). The actual 
site of the superposition of the Newbury on the crystal- 
line basement has since been obliterated by faulting. 



THE STRATIGRAPHIC RECORD IN THE 
MILFORD-DEDHAM ZONE 

The stratigraphic record in the Milford-Dedham zone 
(table 6) comprises two dissimilar cycles. A Proterozoic 
Y or Z cycle of arc-margin accumulation, volcanic arc 
accumulation, and orogenesis followed by Proterozoic Z 
plutonism, volcanism, and subsequent flysch to molasse 
deposition of turbidites during an extensional phase was 
completed by the Cambrian Period. The Cambrian initi- 
ated a new cycle, spanning the Paleozoic, beginning with 
shelf deposition and closing with deposition during rifting 
in the Mesozoic. In the earlier cycle, the felsic volcanism 
(and plutonism) represented by the Mattapan Volcanic 
Complex (and Dedham Granite) is considered to be a 
precursor to the opening of the proto- Atlantic Ocean 
(Iapetus) (Rankin, 1975), and the subsequent basin filling 
and mafic volcanism (Brighton Melaphyre) to be associ- 
ated with the actual rifting. The length of the hiatus 
between the end of development of the volcanic-plutonic 
complex in eastern Massachusetts and the beginning of 
the phase of felsic volcanism and plutonism is not known. 



STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS 



E41 



Table &. — Stratigraphic record in the Milford-Dedham zone 



Event 



Paleotectonic environment 



Representative units 



Deposition of arenite, calc-arenite, 
and pelite. A volcanic component 
increasing towards top. 

Volcanism and plutonism, predomi- 
nantly mafic to intermediate calc- 
alkalic; volcaniclastic and epiclas- 
tic sedimentation. 

Intrusion, calc-alkalic to subalkalic 
granite and associated volcanism 
and later felsic volcanism. 

Deposition of turbidite sequence 
accompanied in early stages by 
felsic to mafic volcanism. 



Deposition of arenite, pelite, and 
carbonate. 

Gabbroic to granite intrusion, typi- 
cally alkalic; associated volcanism. 



Mafic to felsic volcanism, and vol- 
caniclastic and epiclastic sedimen- 
tation. 

Deposition of conglomerate, sand, 
silt, and coal. Mafic to felsic vol- 
canism in early stage(?). 

Plutonism (in southern Rhode 

Island). Intrusion of mafic dikes 

in Boston area. 
Deposition of conglomerate, arkose, 

and mud. Extrusion of basalt as 

flows, dikes, and sills. 
Deposition of sand, silt, and clay 



Littoral to shelf deposition on unknown 
basement. Source of material a 1,500- 
Ma terrane, probably to the east. 

Island arc-continental margin(?) associ- 
ation in compressional tectonic 
regime. 

Pre-rifting intrusion and volcanism in a 
compressional to extensional tectonic 
regime. 

Post-rifting continental to marine depo- 
sition in deepening marginal basin or 
aulacogen formed in extensional tec- 
tonic regime. Highlands and volcanic 
centers to the south, southwest, and 
west(?). 

Transgressive littoral and shallow-shelf 
sequence in epeirogenic regime. 

Anorogenic intrusion in epeirogenic- 
extensional tectonic regime. 



Island-arc(?) volcanism and sedimenta- 
tion in compressional tectonic 
regime(?). 

Alluvial-fan fill in shallow basin; high- 
lands and volcanic center(?) to north- 
west. 

Pre-rifting plutonism and volcanism in 
extensional tectonic regime. 

Alluvial filling of rift basin, extrusion 

of simatic material in extensional 

environment. 
Marine transgressive and regressive 

sequences on a continental shelf in an 

epeirogenic regime. 



Middle or Late Proterozoic; 
older than 630 Ma, 
younger than 1,500 Ma. 

Middle or Late Proterozoic; 

older than 630 Ma, 

younger than 1,500 Ma. 

Possibly about 750 Ma. 
Late Proterozoic; 630 Ma 

and younger (about 600 

Ma?). 
Late Proterozoic, probably 

younger than 600 Ma. 



Early and Middle 
Cambrian. 

Ordovician to Devonian 



Late Silurian and Early 
Devonian. 

Late Mississippian and 
Pennsylvanian. 



Permian 



Triassic and Jurassic 



Cretaceous to Tertiary 



Westboro and Plainfield 
Formations; Quinnville 
Quartzite and Sneech 
Pond Schist. 

Middlesex Fells Volcanic 
Complex, Hunting Hill 
Greenstone, Salem 
Gabbro-Diorite. 

Dedham, Milford, and 
Westwood Granites, Mat- 
tapan Volcanic Complex. 

Boston Bay Group including 
Brighton Melaphyre. 



Weymouth and Hoppin 
Formations and Brain- 
tree Argillite. 

Quincy Granite, Cape Ann 
Granite, Peabody Gran- 
ite, Blue Hill Granite 
Porphyry, Nahant Gab- 
bro. 

Newbury Volcanic Com- 
plex. 

Narragansett Bay Group of 
Skehan and others (1979), 
volcanic rocks in Wam- 
sutta Formation. 

Narragansett Pier Granite. 



Middleton-basin fill; Med- 
ford dike. 

Coastal plain deposits. 



A period of uplift and erosion must have occurred before 
deposition of the Roxbury Conglomerate. A basin must 
have been available during accumulation of the Matta- 
pan, because the volcanic rocks contain intercalated 
terrestrial and waterlain beds. This suggests a marine or 
partly marine basin-and-range environment. The sand- 
stone in the upper part of the Cambridge Argillite 
indicates shoaling at that time, which may have been a 
precursor for uplift and erosion before the Cambrian 
transgression. In the later, Paleozoic cycle, however, the 
depositional record within the zone does not indicate a 
period of orogenesis of the sort recorded in the Protero- 
zoic. The tectonic processes were primarily extensional, 
as indicated by the static Paleozoic plutonism. The 



western part of the Milford-Dedham zone was involved, 
however, if only passively, in Paleozoic compressive 
tectonics (Goldsmith, this vol., chap. H). A new cycle has 
begun with the Cretaceous and Tertiary marine onlap 
onto the passive continental margin of accreted North 
America. The scheme of tectonic evolution derived is 
similar to that proposed by Williams and King (1979) for 
the Trepassy area of the Avalon Peninsula, Newfound- 
land. The alternation of compressional and extensional 
tectonics evident from the depositional history shown in 
table 6 can be translated into terms of plate-tectonic 
theory better when Massachusetts and the New England 
region are considered as a whole in another chapter of 
this volume (Goldsmith, this vol., chap. H). 



E42 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 




Figure 13. — Distribution of major groups of rocks in the Milford-Dedham zone, Massachusetts, Rhode Island, and Connecticut. 



REGIONAL RELATIONS IN SOUTHEASTERN 
NEW ENGLAND 

The relations of the Milford-Dedham zone in Massa- 
chusetts to the rest of southeastern New England are 
summarized in figures 13 and 14. Primary sources for the 
compilation were the State bedrock map, Quinn's (1971) 



paper on the bedrock geology of Rhode Island, and the 
preliminary bedrock geologic map of Connecticut (Rodg- 
ers, 1982). Structural data have been omitted except for 
the names of the bounding faults on the west. A certain 
amount of interpretation has been introduced in the 
compilation, and some of the debatable units discussed in 
this chapter have been assigned to age brackets that 



STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS 



E43 




E44 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



differ from those on the State bedrock map. Such assign- 
ments can stimulate future investigation and discussion. 
These changes are discussed below. 

QUARTZITIC ASSEMBLAGE 

The quartzitic assemblages (Zq on fig. 13) include the 
Westboro Formation (Zw), the Plainfield Formation 
(Zp), the lower part of the Blackstone Group (Zbq, Zbs), 
part of the undivided Blackstone Group (Zb) in Massa- 
chusetts and northern Rhode Island, and rocks mapped 
as Blackstone Group in southern Rhode Island. The 
undivided Blackstone in Massachusetts and northern 
Rhode Island includes some amphibolite and other met- 
avolcanic rocks, but, where quartzite is the most conspic- 
uous lithology, the undivided Blackstone has been 
included in the quartzitic assemblage, as for example in 
the town of Uxbridge, Mass. These formations do not 
contain a particular quartzite layer but represent forma- 
tions in which quartzite layers are conspicuous if not the 
predominant lithology. 

The Rice Gneiss, which has a volcanic component, has 
been included in the quartzitic assemblage in figure 13 
rather than in the overlying metavolcanic assemblage 
because the Rice lies stratigraphically below the West- 
boro (Nelson, 1974) and is in part quartzitic (table 3). 
Also, as no clear base has been observed for the quartz- 
itic units in the quartzitic assemblage elsewhere, and as 
the Rice Gneiss occupies only a small area, it is most 
conveniently placed in the quartzitic assemblage rather 
than in an isolated unit lying below the quartzitic forma- 
tions. As described by Nelson (1974), the Rice Gneiss 
contains lithologies resembling some of those in the 
Plainfield Formation. 



VOLCANIC-PLUTONIC COMPLEX IN EASTERN 
MASSACHUSETTS AND CORRELATIVE ROCKS 

The metavolcanic assemblage that lies primarily above 
the quartzitic assemblage is part of the volcanic-plutonic 
complex in eastern Massachusetts. In Connecticut it is 
called the Waterford Group (fig. 14); in Rhode Island it is 
not recognized as a unit but consists of separately 
mapped metavolcanic rocks and plutons of gabbro and 
diorite (Zgb, Zdi). Shown on figure 13 are both the 
metavolcanic component, Zev, and the plutonic compo- 
nent, Zep. Where these components are too mixed or too 
thin to be shown separately, they are shown as a single 
unit, Ze. 

In northeastern Massachusetts, Bell and Alvord (1976) 
used the term "Middlesex Fells Volcanic Complex" for 
the largely mafic metavolcanic rocks older than the 
Dedham Granite. Their term, as used, includes the 
Cherry Brook Formation, the Middlesex Fells Volcanic 



Complex in its type area, stray pieces of stratified 
metavolcanic rocks such as the unnamed units of Volck- 
mann (1977) in the Holliston area, amphibolite and augen 
gneiss mapped by Toulmin (1964) in the Salem quadran- 
gle, and mafic metavolcanic rocks mapped by Bell and 
others (1977) and Dennen (1975) in the Georgetown and 
Ipswich quadrangles, respectively. This collective use of 
the term makes it more useful than the more areally 
limited term Cherry Brook Formation, in spite of the fact 
that the term Cherry Brook Formation has priority. Bell 
and Alvord considered the Middlesex Fells Volcanic 
Complex to be the equivalent of the Hunting Hill Green- 
stone of the Blackstone Group. The metamorphosed 
mafic and felsic volcanic rock unit (Zv) used on the State 
bedrock map is equivalent to the Middlesex Fells Volca- 
nic Complex as used by Bell and Alvord (1976). On figure 
13, however, I have used volcanic and volcaniclastic 
rocks (Zev) as a collective name for the units in the 
metavolcanic assemblage that overlies the quartzitic 
assemblage in eastern Massachusetts and northern 
Rhode Island. 

Included in the metavolcanic assemblage are those 
parts of the undivided Blackstone Group (Zb) that con- 
tain prominent amphibolite, such as near Hopedale and 
Milford and near Blackstone, Mass. Also included in the 
metavolcanic assemblage are the three metamorphic 
units in the Georgiaville quadrangle in northern Rhode 
Island that Richmond (1952) named the Absalona and 
Woonasquatucket Formations and the Nipsachuck 
Gneiss. His description of these rocks suggests that they 
are largely metamorphosed felsic or intermediate volca- 
nic rocks, in part at least tuffaceous. Their exact strati- 
graphic position is uncertain (Quinn, 1971, p. 14-15), for 
they may lie above or below the Blackstone Group. 
Because they seem to lie above the quartzitic formations 
on the east flank of the Rhode Island anticlinorium 
(Goldsmith, this vol., chap. H), I have assigned these 
rocks to the metavolcanic assemblage. 

The plutonic component of the volcanic-plutonic com- 
plex in eastern Massachusetts includes all the mafic 
plutonic rocks older than the granites and granodiorites 
of the Rhode Island and southeastern Massachusetts 
batholiths and related plutons; for example, the Dedham 
Granite (Zdgr, Zdngr), Milford Granite (Zmgr), and 
Topsfield Granodiorite (Ztgd). These mafic-plutonic 
rocks consist mostly of gabbro, diorite, and syenite, such 
as the Salem Gabbro-Diorite (Zdigb), unnamed diorite 
and gabbro (Zdi, Zgb), and the Sharon Syenite (Zssy). 
These plutonic rocks are discussed in Wones and Gold- 
smith (this vol. , chap. I). North of Salem and southeast of 
the Middleton basin, the plutonic and volcanic compo- 
nents are not distinguished separately on figure 13 
because of their intermixing. 



STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS 



E45 



The Claypit Hill Formation and the formations of Bell 
and Alvord (1976) near Burlington (see table 3), which 
were grouped with the metavolcanic assemblage on the 
State bedrock map, are shown separately on figure 13 as 
a possibly younger group of metasedimentary and meta- 
volcanic rocks (Zee) lying above the metavolcanic assem- 
blage (Zev). 

In southeastern Connecticut, the Waterford Group 
(Goldsmith, 1980) is the stratigraphic equivalent of the 
volcanic-plutonic complex in eastern Massachusetts (fig. 
14). It contains both stratified metavolcanic and meta- 
sedimentary rocks and metaplutonic rocks; for example, 
the New London Gneiss, the Rope Ferry Gneiss, and 
possibly the Stonington Gneiss phase (Rodgers and oth- 
ers, 1959) of the Mamacoke Formation. The volcanic and 
plutonic rocks in this complex are not distinguished 
separately on figure 13 because of the thinness of the 
units. 

The older metamorphic rocks of southeastern Massa- 
chusetts and the rocks of similar age in the Newport and 
Sakonnet areas (fig. 7) are included in the metavolcanic 
assemblage because of their apparently large original 
volcaniclastic component, although the rocks are not 
necessarily mafic. They could, however, be all or in part 
equivalent to part of the mica schist and phyllite (Zbs) of 
the Blackstone Group in northeastern Rhode Island. 
These units in southeastern Massachusetts and southern 
Rhode Island are the gneiss and schist near New Bedford 
(Zgs), the biotite gneiss near New Bedford (Zgn), the 
Proterozoic Z strata at different places along the Sakon- 
net River (Quinn, 1971), the Jamestown Formation of 
Skehan and Murray (1980b), and the Price Neck Forma- 
tion of Rast and Skehan (1981) in the Newport area 
following the age assignments and the descriptions of 
Rast and Skehan (1981). 



RHODE ISLAND AND SOUTHEASTERN MASSACHUSETTS 
BATHOLITHS AND RELATED GRANITOIDS 

The Rhode Island and southeastern Massachusetts 
batholiths (Zgr on fig. 13) occupy much of the area of the 
Milford-Dedham zone. Units included in this assemblage 
are the Dedham Granite (Ddgr, Ddngr), Topsfield Gran- 
odiorite (Dtgd), Westwood Granite (Zwgr), Milford 
Granite (Zmgr, Zmgd), Esmond Granite (Zegr), granite 
of the Fall River pluton (Zfgr), porphyritic granite 
(Zpgr), alaskite (Zagr), the Ponagansett Gneiss (Zpg), 
Hope Valley Alaskite Gneiss (Zhg), Scituate Granite 
Gneiss of the Sterling Plutonic Group (Zsg), and biotite 
granite (Zgr). In southeastern Connecticut and southern 
Rhode Island, the units belonging to the Sterling Plu- 
tonic Group are present including the Potter Hill Granite 
Gneiss. Hermes and others (1981) have found that a large 
area formerly mapped as Scituate Granite Gneiss in 



central Rhode Island is of middle Paleozoic rather than 
Proterozoic Z age. (This area is shown on fig. 13 as Pzg.) 
Also included in Zgr are the Newport Granite of Rast 
and Skehan (1981) and the Willimantic Gneiss in east- 
central Connecticut. The Bulgarmarsh Granite is a phase 
of the granite of the Fall River pluton. 



MATTAPAN VOLCANIC COMPLEX AND EQUIVALENT 
ROCKS 

The Mattapan and Lynn Volcanic Complexes (Zm, 
DZ1), and the felsite porphyry near Bellingham (Zfm (Zm 
on the State geologic map)) are shown as Zm on figure 
13. The volcanic rocks near Lake Pearl (Pwv) are shown 
as equivalent to the Mattapan because of their lithologic 
similarity and because they lie in a structural trough 
more or less aligned with the Boston basin. The felsite at 
Diamond Hill remains assigned to the Pennsylvanian, 
although detailed mapping in this area might reveal that 
all or part of the rocks mapped as Wamsutta in this area 
are actually equivalent to part of the Roxbury Conglom- 
erate. 



BOSTON BAY GROUP AND EQUIVALENT ROCKS 

The Bellingham Conglomerate (PZb) in the Belling- 
ham and North Scituate (Rhode Island) basins is shown 
on figure 13 as equivalent in age to the Boston Bay 
Group. The Bellingham resembles the Roxbury Con- 
glomerate, and no fossils have been found in the Belling- 
ham in contrast to the highly fossiliferous Pennsylvanian 
strata in the Narragansett basin to the east. As men- 
tioned in an earlier part of this chapter, the Bellingham 
and North Scituate basins lie in a structural trough 
aligned with the Boston basin. 



YOUNGER UNITS 

The distribution of the Cambrian and younger rocks 
(€, DS, Ps, and ftg on fig. 13) has not been changed from 
the State bedrock map except that the Pennsylvanian 
volcanic rocks near Lake Pearl have been correlated with 
the Mattapan Volcanic Complex (Zm on fig. 13). The 
distribution of Cambrian strata on Conanicut Island, 
R.I., is taken from Skehan and others (1981). 



THE MILFORD-DEDHAM ZONE IN THE 
CALEDONIDES-CORRELATION 

The stratigraphic sequence in the Milford-Dedham 
zone has been correlated by numerous authors with 
sequences in the Maritime Provinces, Newfoundland, 
England, Wales, and Morocco. The term "Avalonian" 



E46 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



(Kay and Colbert, 1965) has been applied to terranes 
containing these sequences in North America. The Ava- 
lonian terranes form a southeastern platform of the 
Appalachian orogenic belt, as first pointed out by 
Williams (1964). Summary sections and their correlations 
in the different terranes, most of which cover eastern 
Massachusetts, have been presented by Weeks (1957), 
Schenk (1971), Rast and others (1976), and Strong (1979). 
The most recent summary descriptions of rocks in the 
Avalon Peninsula of Newfoundland are by King (1980; 
O'Brien and King, 1982) and Anderson (1981). Equiva- 
lent sequences have been described for example in 
Anglesey, Wales, by Wood (1974), in the English Mid- 
lands by Rast and others (1976), and in Morocco by 
Affaton and others (1980). The relationship of eastern 
Massachusetts and Rhode Island to the above terranes is 
discussed by Skehan (1969, 1973), Skehan and others 
(1978), Rast (1980), and Skehan and Murray (1980a). 

Gross correlation of rocks of eastern Massachusetts 
with those of the Avalon Peninsula is fairly easily made. 
The Mattapan Volcanic Complex and Boston Bay Group 
are similar to the Proterozoic Z sequence of the Avalon, 
the Conception Group and Harbour Main Volcanics, as 
summarized by King (1980; O'Brien and King, 1982) 
except that there the fossil record is more abundant, the 
sequence is apparently thicker, and no older basement is 
exposed. In Nova Scotia and New Brunswick, however, 
as in Massachusetts, a basement (Basement Complex, 
Greenhead Group, Kelly Mountain and George River 
Group, and Brookville gneiss) is present beneath the 
Proterozoic Z (Hadrynian) rocks (Georgeville Group, 
Fourchu Group, Coldbrook Group). The basement rocks 
cannot be readily correlated with the pre-Dedham Gran- 
ite strata, however. According to Rast (1980, p. 63), 
these rocks, apparently older than 770 Ma, consist of 
quartzites, argillites, and carbonates in part converted to 
schists, paragneisses and marbles. J.W. Skehan (written 
commun., 1983) suggested that these rocks are equiva- 
lent to the Westboro Formation and Blackstone Group. 
None of these rocks clearly resembles the volcanic- 
plutonic complex in eastern Massachusetts. Possibly the 
Hadrynian(?) Bass River Complex and George River 
Group ( = Greenhead Group) of Nova Scotia (Keppie, 
1979; Keppie and Schenk, 1982) are equivalents of the 
complex. 

The eastern Massachusetts terrane has also been 
correlated with rocks of the slate belt in Virginia, North 
Carolina, and South Carolina (Glover and Sinha, 1973; 
Rast and others, 1976; Williams, 1978; Nelson, 1981). 
The strata in the Carolina slate belt are equivalent to 
some of the rocks of the Boston basin area in lithology 
and fossil fauna, but it is not generally realized that the 
adjacent Charlotte belt bears a remarkable resemblance 
to the volcanic-plutonic complex in eastern Massachu- 



setts. In North Carolina, a probably Proterozoic Z 
volcanic-plutonic complex, consisting largely of plutonic 
rocks ranging from gabbro to granodiorite and enclosing 
masses of mafic to felsic volcanic rocks and metasedi- 
ments, forms the core of the Charlotte belt (Goldsmith 
and others, 1982b). This complex is flanked by Protero- 
zoic Z to Cambrian volcanic rocks and sediments of the 
Carolina slate belt and is intruded by early to mid- 
Paleozoic plutons of gabbro (and syenite) to granite and 
by late Paleozoic plutons of granite. This terrane thus 
described is quite similar to that in eastern Massachu- 
setts, particularly west and north of the Boston basin. 
Although no late Paleozoic granites are present in the 
Boston area, they are present in southern Rhode Island, 
and late Paleozoic volcanic rocks are present in northern 
Rhode Island. 



MESOZOIC AND TERTIARY STRATIGRAPHY OF 
CAPE COD AND THE NEARBY ISLANDS 



By E.G.A. Weed 



INTRODUCTION 

Cape Cod and the nearby islands, the Elizabeth 
Islands, Martha's Vineyard, and Nantucket, are almost 
entirely covered by Quaternary surficial deposits. No 
pre-Pleistocene rocks are exposed southeast of the Cape 
Cod Canal, except the distorted beds in the cliffs at the 
western end of Martha's Vineyard. However, pebbles, 
fragments, and blocks of older rocks are found in the 
glacial drift and may be encountered in sand and gravel 
pits. 

Data on the pre-Pleistocene rocks are available only 
from drill holes and other manmade excavations and the 
natural exposures on Martha's Vineyard noted above. 
These few data sources are clearly inadequate to accu- 
rately portray the distribution of rocks in the subsurface, 
even at 1:250,000 scale. Their very paucity, however, 
makes the information obtained from them all the more 
critical. 

Early studies of the geology of Cape Cod and the 
nearby islands mostly addressed materials found in out- 
crop (Emerson, 1917). Cretaceous and Tertiary fossils in 
this area were found by Lyell (1843), Desor (1849), 
Shaler (1888, 1889, 1890), White (1890), Dall (1894), 
Woodworth (1897), Hollick (1906), and Berry (1915), and 
later by Woodworth and Wigglesworth (1934) and Kaye 
(1964a,b,c; 1983b). Offshore studies of the U.S. Atlantic 
margin (Emery and Uchupi, 1972; Weed and others, 
1974; Austin and others, 1980) and hydrologic studies on 
land (Maevsky and Drake, 1963; Kohout and others, 
1977; Delaney, 1980; Walker, 1980; Guswa and LeBlanc, 



STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS 



E47 



1985) have contributed to our understanding of the 
Coastal Plain geology. Geophysical investigations in the 
Cape Cod area have been made by Hoskins and Knott 
(1961), Oldale and Tuttle (1964), Oldale (1969), Oldale 
and others (1973), Ballard and Uchupi (1975), Grow and 
Schlee (1976), Bothner (1977), and Klitgord and 
Behrendt (1979, 1980). The mineralogy of the Coastal 
Plain materials, in particular the clays, has been studied 
by L.J. Poppe (written commun., 1976) and S.A. Wood- 
Needell (written commun., 1976). 

As part of the investigations made for the State 
bedrock map (Zen and others, 1983), 15 shallow bore 
holes (sites 13-17, 23-24 and 26-33, fig. 15; tables 7, 8) 
were drilled in Martha's Vineyard, Cape Cod, Marsh- 
field, and Scituate from 1977 through 1982. Unconsoli- 
dated Tertiary material was recovered in two bore holes 
(sites 26 and 29). Hard material (granodiorite, sand- 
stone, shale, hardpan (till?)) was reached in eight holes 
but recovered in only six holes (sites 27-32). 

The locations of the drill and auger holes and the 
natural exposures that serve as sources for the data for 
this report are shown on figure 15. Lithology, depth, 
thickness, age, and principal references are presented in 
table 8. Because the pre-Mesozoic rocks of the subsurface 
are described by Wones and Goldsmith in chapter I of 
this volume under the heading "Proterozoic Z batholithic 
rocks," only the Mesozoic and Tertiary strata are 
described in this section. Data on the pre-Mesozoic 
basement encountered in the holes are included, how- 
ever, in table 8. 



GEOLOGIC SETTING 

The rocks beneath the Quaternary cover on Cape Cod 
and the islands lie southeast of the exposed rocks of the 
Milford-Dedham zone (Zen and others, 1983; Wones and 
Goldsmith, this vol., chap. I), which is interpreted as 
extending to the east of Cape Cod (Klitgord, 1984). The 
rocks known to be present in the subsurface of Cape Cod 
and the islands include, from oldest to youngest, granite, 
gneiss, and schist of probable Proterozoic Z age (Zgg of 
the State bedrock map), Triassic and Jurassic basalt 
(J"fcb), Cretaceous sediments (K), and Tertiary sedi- 
ments (T). K-Ar radiometric analyses of biotite and 
hornblende obtained from samples of the core of the 
Tubman hole (site 3) gave ages as follows: biotite from 
granodiorite (depth unknown) 348 ±12 Ma, hornblende 
518±30 Ma; biotite from a mafic xenolith at 190.2 m (624 
ft) depth 400 ±14 Ma, hornblende 566 ±22 Ma; biotite 
from granodiorite at 294.5 m (966 ft) depth 380±19 Ma 
(Weston Geophysical Research, Inc., 1977; Zartman and 
Marvin, this vol., chap. J, table 1). As these are mini- 
mum ages, a Proterozoic Z age for at least part of the 
basement under Cape Cod is confirmed. Probable Prot- 



erozoic rock was encountered in a few other drill holes on 
Cape Cod, at the holes at Harwich, Brewster, and Woods 
Hole (sites 1-5, fig. 15). 

The northeastern continuation of the Atlantic Coastal 
Plain, which is well exposed on the surface as far 
northeast as northern New Jersey (Weed and others, 
1974), and is present in the subsurface in Long Island (de 
Laguna, 1963; Perlmutter and Todd, 1965; Soren, 1971, 
1977; Minard and others, 1974; Weed and others, 1974), 
lies beneath Cape Cod and the islands. Because the sites 
of pre-Pleistocene materials known in southeastern Mas- 
sachusetts are few, each has considerable importance for 
interpreting the geology of the northern Coastal Plain 
and the Appalachian region. 

DESCRIPTION OF POST-PALEOZOIC UNITS 

Triassic and Jurassic strata consisting of basalt (J"fib) 
and sedimentary and volcanic rock (J"fi) have been iden- 
tified only beneath Nantucket and Nantucket Sound. 
Cretaceous clay, sand, and silt (K) have been reported 
from wells and outcrops at many places in southeastern 
Massachusetts and the islands (table 8), but no rocks of 
Mesozoic age have been identified in Cape Cod proper. 
The northwestern limit of Cretaceous materials in the 
subsurface is shown on the State bedrock map southeast 
of the main part of the Cape (fig. 15). The boundary 
passes through Monomoy Point to the northeast and 
between the Elizabeth Islands and Martha's Vineyard to 
the southwest. Tertiary deposits are scattered and frag- 
mentary. 

TRIASSIC AND JURASSIC BASALT (Jib) 

A 514-m hole drilled on Nantucket (site 7, USGS 6001) 
encountered Triassic and Jurassic basalt (J"fcb) from 470 
m depth to the bottom of the hole. Interpretations of 
seismic profiles in the area, including USGS multichannel 
seismic line 5 (Grow and others, 1979; Austin and others, 
1980), indicate that a northeast-trending Triassic and 
Jurassic basin, here called the Nantucket basin, extends 
from the middle of Nantucket Sound southeast about 25 
km (fig. 15). It is estimated to be about 100 km long, and 
the seismic profile of line 5 indicates that it is about 8 km 
deep (Folger and others, 1978, fig. 2). The basalt encoun- 
tered on Nantucket underlies a sedimentary and volcanic 
section (J"fc) interpreted from seismic profiles to be 
present under Nantucket Sound. The profiles indicate 
that the beds dip northeastward toward a normal fault 
south of Cape Cod (cross section E-E' on the State 
bedrock map). 

The basalt, although highly altered, is similar in 
appearance to basalts of the Newark basin in New Jersey 
(Faust, 1975). The alteration is apparently the result of 



E48 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 

70°30' 



Plymouth 




BUZZARDS BAY 
Elizabeth Islands 



NANTUCKET < 



20 KILOMETERS 



Figure 15.— Outcrop and auger- and core-drilling sites that provide information on the pre-Mesozoic basement and Mesozoic and 
Tertiary deposits in the area of Cape Cod and the nearby islands, Massachusetts. 



STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS 



E49 



Table 7 .—Location and identification of outcrops and auger- and core-drilling sites in the area of Cape Cod and the nearby islands, 

Massachusetts 
[Locations of sites shown on figure 15; total depth measured from Kelly bushing (K.B.); WHOI, Woods Hole Oeeanographic Institution; — , not given. See table 8 for 

principal data sources] 



Site 


Site designation 


Location 


Latitude, 
longitude 


Site altitude, 
meters 
(feet) 


Total depth, 
meters 
(feet) 


Recovery method 


Year 


1 


HJW-106 (Hole A) 


Harwich, Cape Cod 


41°41'06"N., 
70°06'55'W. 


8.2 (26.8) 


304.8 (1,000) 


Drilled, core 


1961. 


2 


BMW-23 (Hole C) 


Brewster, Cape Cod 


41°44'35"N., 
70°06'22'W. 


21.3 (70) 


304.8 (1,000) 


Drilled, core 


1962. 


3 


BMW-24 (Hole D) 
"Tubman." 


Brewster, Cape Cod 


41°44'50"N., 
70°05'11'W. 


22.9 (75) 


304.8 (1,000) 


Drilled, core 


1962. 


4 


BMW-25 (Hole B) 


Brewster, Cape Cod 


41°45'03"N., 
70°05'01'W. 


30.5 (100) 


132.6 (435) 


Redrilled, core 


1961. 


5 


WB-1 (WHOI dock) 


Woods Hole, Cape 
Cod. 


40°31'30"N., 
70°40'20'W. 


-2.4 (-8) 


85.7 (281) 


Drilled, core 


1965. 


6 


Assorted outcrops in 
bay west of Marion. 


Buzzards Bay coast 
in Plymouth and 
Bristol Counties. 


41°30' N. to 
41°40'N., 
70°50'W. to 
71°02'W. 




0(0) 


Outcrops 


1978. 


7 


USGS 6001 


Nantucket 


41°15'55"N., 
70°02'17"W. 


11 (36) 


514 (1,686) 


Drilled, rotary, 
split-spoon, and 
punch or driven 
barrel coring. 


1975. 


8 


ENW-50 


Martha's Vineyard 


41°23'50"N., 
70°35'30"W. 


10 (33) 


262 (860) 


Drilled, split-spoon 
coring. 


1976. 


9 


Gay Head Cliffs 


Martha's Vineyard 


41°21'00"N., 
70°50'10"W. 


0(0) 


0(0) 


Outcrop 


1888-1984. 


10 


Zack's Cliff 


Martha's Vineyard 


41°19'30"N., 
70°48'30"W. 


0(0) 


0(0) 


Outcrop, sea cliffs 


1977. 


11 


Nonamesset 


Lackey's Bay, Non- 
amesset Island. 


41°30'35"N., 
70°41'25"W. 


0(0) 


0(0) 


Outcrop, not 
exposed at time 
of inspection 
(1977). 


1934. 


12 


Coskata 


Nantucket 


41°22'00"N., 
70°01'30"W. 


0(0) 


102.2 (335) 


Drilled, cable tool 


1933. 


13 


MV-1 


West Tisbury, Mar- 
tha's Vineyard. 


— 


— 


— 


Mobile 50 rig auger 


1977. 


14 


MV-2 


Katama Beach, 
Martha's Vine- 
yard. 








Mobile 50 rig auger 


1977. 


15 


OBW 33-35 (MV-3) 


Sengekontacket 
Pond, Martha's 
Vineyard. 








Mobile 50 rig auger 


1977. 


16 


MV-4 


Katama Beach, 
Martha's Vine- 
yard. 








Mobile 50 rig auger 


1977. 


17 


ENW 70-73 


Trapps Pond, Mar- 
tha's Vineyard. 


— 


— 


— 


Mobile 50 rig auger 


1977. 


18 


Holden's Pond 


Cape Cod 


42°03'30"N., 
70°06'25'W. 


3.1 (10) 


80.5 (264) 


Pipe driven as cas- 
ing, samples. 


1960. 


19 


Stark's Well 


Cape Cod 


42°03'50"N., 
70°09'00'W. 


4.3 (14) 


61.9 (203) 


Pipe driven as cas- 
ing, samples. 


1960. 


20 


Jim's II 


Cape Cod 


42°03'25"N., 
70°11'00"W. 


3.1 (10) 


66.5 (217) 


Pipe driven as cas- 
ing, samples. 


1960. 


21 


Race Point 


Cape Cod 


42°02'40"N., 
70°14'40"W. 


6.2 (20) 


69.8 (229) 


Pipe driven as cas- 
ing, samples. 


1960. 


22 


DGW-193-195 


Corporation Beach, 
Dennis, Cape 
Cod. 


41°45'04"N., 
70°11'21'W. 


1.6 (5) 


94.2 (309) 


Drilled, auger and 
split spoon. 


1977. 


23 


YAW 


Windmill Park, Yar- 
mouth, Cape Cod. 


41°39'16"N., 
70°11'48"W. 


1.6 (5) 


91.5 (300) 


Drilled, auger and 
split spoon. 


1977. 



E50 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



Table 7. — Location and 


identification of outcrops 


and auger- and 


core-drilling sites 


in the area 


of Cape Cod and the nearby islands, 








Massachusetts - 


-Continued 








Site 
no. 


Site designation 


Location 


Latitude, 
longitude 


Site altitude, 
meters 
(feet) 


Total depth, 
meters 
(feet) 


Recovery method 


Year 


24 


FA-8-82-1 


Maravista, 
Falmouth, Cape 
Cod. 


41°32'45"N., 
70°35'10"W. 


1.6 (5) 


83.3 (273) 


Drilled, wire line, 
core barrel. 


1982. 


25 


PH-77-1 


Pine Hill, Manomet, 
Plymouth County. 


— 


— 


33.6 (110) 


Drilled, auger 


1977. 


26 


MB-77-1 


Marshfield 


— 


— 


5.5 (18) 


Drilled, auger 


1977. 


27 


MB-77-2 


Marshfield 


— 


— 


11.6 (38) 


Drilled, auger 


1977. 


28 


MK-79-1 


Kent Park 


42°06'00"N., 
70°41'00"W. 


— 


16.2 (53) 


Drilled, auger 


1977. 


29 


MH-77-1 


Marshfield Hills 


- 


- 


39.3 (129) 


Drilled, 11 cores 


1977. 


30 


MH-78-1, Pine Street Marshfield Hills 


— 


— 


— 


Drilled, auger 


1978. 


31 


NR-78-1, Bridge 
Street. 


North River, Nor- 
well. 


— 


— 


~ 


Drilled, auger 


1978. 


32 


SC-79-1 


Scituate filtration 
plant. 


42°10'00"N., 
70°43'00"W. 


— 


30.2 (99) 


Drilled, auger 


1979. 


33 


TC-77-1 


Third Cliff, Scituate 






22.6 (74) 


Drilled, auger, 10 
cores. 


1977. 



hydrothermal activity not long after emplacement of the 
basalt (Folger and others, 1978). The maroon clay above 
the basalt (table 8) mimics the structure of the basalt and 
appears to be a saprolite. This clay is directly overlain by 
Cretaceous sediments. K-Ar whole-rock age determina- 
tions of the basalt of 183 ±8 Ma and 164 ±3 Ma (Zartman 
and Marvin, this vol., chap. J, table 1) suggest that the 
basalt is Early Jurassic or older. The chemistry of the 
basalt is similar to that of basalts from various Eastern 
U.S. Mesozoic basins (Gottfried and others, 1977) except 
that it contains anomalous amounts of titanium and 
phosphorus (Folger and others, 1978, table 1). The basalt 
is at least 44 m thick; its lateral extent is unknown. 



TRIASSIC AND JURASSIC SEDIMENTS AND VOLCANIC 
ROCKS (J"fi) 

The presence of the Triassic and Jurassic sediments 
and volcanic rocks (J"fi) is inferred from seismic data 
through comparison of the seismic record with records 
from down-faulted rift basins in the Gulf of Maine 
(Ballard and Uchupi, 1975) and the New England conti- 
nental margin (Grow and others, 1979). The sediments 
and volcanic rocks in the Nantucket basin are probably 
similar to Triassic sediments that crop out along the Bay 
of Fundy and the Gulf of Maine, which are continental 
red beds that interfinger with tholeiitic basalts. The red 
beds consist of graywacke, arkose, orthoquartzite, and, 
at the base in places, sharpstone conglomerate (Powers, 
1916; Klein, 1962). 



CRETACEOUS SEDIMENTS (K) 

Cretaceous sediments (K) are known from drill holes in 
Nantucket and Martha's Vineyard and from surface 
exposures on Martha's Vineyard and Nonamesset (sites 
7-11, table 8). Bore hole USGS 6001 in Nantucket (site 7) 
contains the most complete Cretaceous section in Massa- 
chusetts. This section, 329 m thick, and that at site 8, 140 
m thick, are probably representative of the Cretaceous 
section in the area. The Cretaceous section of clays, silts, 
and sands in USGS 6001 (site 7) was divided by Folger 
and others (1978) into a lower unit, 108 m thick, of 
unconsolidated clayey sand and an upper unit, 221 m 
thick, of silty clay containing a few beds of silt and sand. 
The unfossiliferous lower unit that rests on the saprolite 
is considered to be part of the Albian Stage, by correla- 
tion on the basis of lithology with the Cretaceous Poto- 
mac Group in the Coastal Plain of New Jersey (fig. 16). 
The lowest 40 m of the lower unit consists of gray to 
white, clayey sand composed of quartz, plagioclase, 
kaolinite, illite, and minor amounts of smectite or mixed- 
layer illite-smectite. The middle 40 m of the lower unit 
consists of dark-red, gray, olive, and white mottled silty 
sandy clay composed of quartz, plagioclase, orthoclase, 
kaolinite, illite, and sparse smectite and hematite. The 
upper 25 m of the lower unit consists of coarse, light-gray 
silty sand composed of quartz, plagioclase, orthoclase, 
kaolinite, illite, and sparse smectite. The basal contact 
appears to be a disconformity representing Jurassic and 
Early Cretaceous time during which clastic and carbon- 
ate sediments were being deposited in similar basins to 



STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS 



E51 



Table 8.— Description of materials encountered in outcrops and in drilled holes in the area of Cape Cod and the nearby islands, Massachusetts 
[Locations of sites shown on figure 15; datum for unit depths is mean sea level. USGS, U.S. Geological Survey; — , not given] 



Site 
designation 



Unit 
depth, 

meters 
(feet) 



Unit 

thickness, 

meters 

(feet) 



Description of units 



Principal data source 



1 HJW-106 
(Hole A). 



BMW-23 
(Hole C). 



BMW-24 
(Hole D) 
"Tubman. 



+ 7.3 (+24) 

-41.3 (-136) 

-87.9 (-289) 

-123.4 (-405) 
-125.3 (-411) 



-143.8 (-471) 



-299.3 (-981) 
+21.3 (+70) 



-15.2 (-50) 

-36.3 (-119) 
-62.5 (-205) 

-105.2 (-345) 
-110.7 (-363) 

-283.7 (-930) 

+22.9 (+75) 
-97.9 (-321) 
-117.4 (-385) 



48.6 (160) 

46.6 (153) 

35.4 (116) 

1.8 (6) 
18.3 (60) 



155.5 (510) 



36.5 (120) 

21.0 (69) 

26.2 (86) 
42.7 (140) 



5.5 (18) 
172.9 (567) 



120.8 (396) 
19.5 (64) 
160.1 (525) 



Fine sand and scattered layers 
of coarse sand. 

Bluish-gray, coarse to clayey 
silt. 

Boulder till— boulders, mainly 
granite, in silt matrix. 

Iron-stained till 

Fine-grained, bluish-gray phyl- 
litic schist and, in upper 20 m, 
30 percent greenish-gray to 
gray crystalline limestone in 
beds 1 cm thick. 

Phyllitic schist as above lacking 
carbonate but containing abun- 
dant quartz veins and chlorite 
alteration; thinly bedded, well 
foliated. 38 cm (15 in) recov- 
ered at -181 m (-173.5 m 
datum) shows good foliation 
dipping about 80°. 

Fine to very coarse yellowish- 
brown sand containing scat- 
tered layers of gravel. 

Fine to very coarse gray sand 
containing scattered layers of 
gravel. 

Greenish-gray silty clay contain- 
ing sandy layers. 

Very fine to very coarse gray 
sand containing layers of 
gravel. 

Till 

Quartz-biotite gneiss (three sam- 
ples from core examined by 
Gore appeared to be granu- 
lated granodiorite). 

Unconsolidated material 

Granodiorite 



Quaternary 

Quaternary 

Quaternary 

Quaternary 
Proterozoic(?) 



Proterozoic(?) 



Quaternary 

Quaternary 

Quaternary 
Quaternary 



Quaternary 
Proterozoic Z(?) 



Quaternary 



Proterozoic Z 



BMW-25 


+30.5 (+100) 


130.5 (428) 


Unconsolidated cobble gravel, 


(Hole B). 






sand, and silt. 




-100.0 (-328) 


2.1 (7) 


White, pink, and rusty granite 
(boulder?). 



Quartz monzonite; contains xeno- Proterozoic Z 
liths at 164.7 m (540 ft) and 
190.3 m (624 ft). 



Quaternary 
Proterozoic Z(?) 



Maevsky and Drake, 1963; 
Koteff and Cotton, 1962; 
unpublished well data, 
USGS, Boston, Mass.; 
Oldale, 1976. 



Maevsky and Drake, 1963; 
unpublished well data, 
USGS, Boston, Mass.; 
R.Z. Gore, written com- 
mun., 1978. 



Maevsky and Drake, 1963; 
unpub. well data, USGS, 
Boston, Mass.; Zartman 
and Naylor, 1984; Zart- 
man and Marvin, this 
vol., chap. J, table 1; 
R.Z. Gore, written com- 
mun., 1964, 1978. 

Unpub. well data, USGS, 
Boston, Mass. 



E52 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



Table 8. — Description of materials encountered in outcrops and in drilled holes in the area of Cape Cod and the nearby islands, 

Massachusetts — Continued 





Unit 


Unit 


Site Site 


depth. 


thickness, 


no. designation 


meters 


meters 




(feet) 


(feet) 




-102.2 (-335) 


28.4 (90.9) 


5 WB-1 


-2.4 (-8) 


83.0 (272) 


(WHOI 






dock). 


-85.4 (-280) 


3.1 (10) 


6 Assorted 


_ 


— 


outcrops 






near 






Marion. 






7 USGS 6001 


+ 11.0 (+36) 


128.1 (420) 



Description of units 



Principal data source 



-117.1 (-384) 



-338.2 (-1,109) 



-446.2 (-1,463) 
-459.3 (-1,506) 



-503.3 (-1,650) 
+ 10.1 (+33) 



-70.2 (-230) 
-118.0 (-387) 



221.1 (725) 



108.0 (354) 



13.1 (43) 
43.9 (144) 



80.2 (262) 



47.9 (157) 
92.1 (302) 



Late Cretaceous 



Unconsolidated material as Proterozoic Z(?) 

above. Refusal at 130.5 m (428 

ft) in till or granitic rock. 
Light-brown fine to coarse sand Quaternary 

Gray to pink granodiorite Proterozoic Z(?) 

Fine- to coarse-grained granitic Proterozoic Z 
rock and local layers and 
lenses of gneiss. 

Tan, gray, olive, green, medium- Pleistocene 
to coarse-grained quartz and 
plagioclase sand; dominant 
clay mineral is illite, minor 
kaolinite. Some glauconite 
below -82 m. Material con- 
tains sparse Tertiary sporo- 
morphs and dinoflagellates and 
Cretaceous palynomorphs. 

Variegated red, gray, and yel- 
low clay and silt interbedded 
with light-gray sand; lignite in 
darker layers, subbituminous 
coal at -317 m; dominant min- 
erals are kaolinite and quartz. 

Light-gray, fine- to coarse- 
grained quartz sand in kaolin- 
ite matrix; subordinate inter- 
bedded clay. 

Massive maroon clay containing 
white blebs and veins. 

Altered maroon to gray amygda- 
loidal basalt containing veins 
and amygdaloid fillings of cal- 
cite, zeolite, and saprolite. 

Medium- to coarse-grained white Pleistocene 
sand, lignite, and several lay- 
ers of clay. Material contains 
Tertiary sporomorphs and 
mierofossils. 

Medium to fine sand and layers Cretaceous 
of silty clay. 

Mottled micaceous silty clay; Cretaceous 

white sand at -119 m and lig- 
nite between -164 and -170 
m; layers partly indurated. 



Unpub. well data, Ameri- 
can Drilling and Boring 
Company, Inc., East 
Providence, R.I. 

Williams and Tasker, 1978. 



Folger and others, 1978; 
Kohout and others, 1977; 
unpub. data, USGS, 
Woods Hole; R.A. 
Christopher, written 
commun., 1982; Walter 
Barrett, written com- 
mun., 1936; E.G.A. 
Weed, unpub. data; L.J. 
Poppe, written 
commun., 1976; S.A. 
Wood-Needell, written 
commun., 1976. 



Late Cretaceous 



Jurassic and 
Triassic. 

Jurassic and 
Triassic. 



Hall and others, 1980; 
Kaye, 1964a, 1983b; 
Delaney, 1980. 



STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS 



E53 



Table 8. 



-Description of materials encountered in outcrops and in drilled holes in the area of Cape Cod and the nearby islands, 

Massachusetts — Continued 



Site Site 

no. designation 



Unit 
depth, 
meters 

(feet) 



Unit 

thickness, 

meters 

(feet) 



Description of units 



Principal data source 



Kaye, 1964a, 1983b; 
Shaler, 1888, 1890; 
Woodworth and Wig- 
glesworth, 1934; Freder- 
icksen, 1984; L.J. 
Poppe, written 
commun., 1978. 



-210.1 (-689) 42.1(138) Sand and sandstone; clay at 

-230 m, and red arkosic sand- 
-252.2 (-827) stone at bottom. 

9 Gay Head — — Shelly, light-yellow to light-gray 

CUffs. sand, thin beds of fine sand 

and silt; massive ferruginous 
clay and silt. 
Greensand, largely glauconite, 
minor quartz and feldspar; 
contains apatite nodules and 
chert. 
Kaolinite, red, white, and lig- 
nitic clay containing siderite 
concretions. Clays largely kao- 
linite, minor quartz and feld- 
spar. 

10 Zack's Cliff — — Greensand 

Greensand 

11 Nonamesset — — Greensand 

Lignitic clay, gray clay, quartz 
gravel, and yellow sand. Sid- 
eritic nodules on nearby beach 
characteristic of known Creta- 
ceous beds at Gay Head. 



12 


Coskata 


0(0) 


44.2 (145) 


Sand and gravel 






-44.2 (-145) 


11.0 (36) 


Silt and clay 






-55.2 (-181) 


19.8 (65) 


Silty clay 






-75.0 (-246) 


27.1 (89) 


Silty clay 






-102.2 (-335) 






13 


MV-1 


— 


— 


Brown sand 
Gray sand 


14 


MV-2 


- 


- 


No recovery 


15 


OBW 33-35 
(MV-3). 


-134 (-439) 




Gray sand 
Brown sand 
Olive sand 
Olive clay 


16 


MV-4 






Brown sand 
Gray sand 
White sand 
Clayey sand 
Orange sand 
Gray sand 


17 


ENW 70-73 






Brown sand 
Gray sand 
White sand 
Olive-gray sand 



Pliocene and 
Miocene. 



Miocene 



Cretaceous 



Tertiary 
Cretaceous 
Miocene 
Cretaceous 



Pleistocene 
Pliocene and 

Miocene. 
Eocene 
Paleocene 

Pleistocene 

Pleistocene(?) 
Pleistocene 
Pleistocene 
Pleistocene 
early Pleisto- 
cene. 
Pleistocene 



Pleistocene 



R.A. Christopher, written 

commun., 1977. 
Woodworth and 

Wigglesworth, 1934. 



Folger and others, 1978. 



Delaney, 1980. 

Delaney, 1980. 
Delaney, 1980. 



Delaney, 1980. 



Delaney, 1980. 



E54 






THE BEDROCK GEOLOGY OF MASSACHUSETTS 




Table 8. — Description 


of materials 


encountered in outcrops and in drilled holes in the 


area of Cape Cod and the nearby islands, 










Massachusetts — Continued 










Unit 


Unit 








Site 


Site 
designation 


depth, 
meters 
(feet) 


thickness, 
meters 

(feet) 


Description of units 


Age 


Principal data source 


18 


Holden's Pond 


3.1 (10) 


80.5 (264) 


Clean sand 

Silty sand containing Eocene 
pollen, possibly reworked. 


Pleistocene 
Eocene(?) 


Zeigler and others, 1960. 


19 


Stark's Well 


4.3 (14) 


61.9 (203) 


Clean sand 
Silty sand 


Pleistocene 
Eocene(?) 


Zeigler and others, 1960. 


20 


Jim's II 


3.1(10) 


66.5 (218) 


Clean sand 
Silty sand 


Eocene(?) 


Zeigler and others, 1960. 


21 


Race Point 


6.2 (20) 


69.8 (229) 


Clean sand 
Silty sand 


Eocene(?) 


Zeigler and others, 1960. 


22 


DGW-193-195 






Brown sand 
Gray sand and silt 
Brown sand 


Pleistocene 


Guswa and LeBlanc, 1985. 


23 


YAW 




97 (300) 


Gray silt and sand 
Gravel 
Gray clay 
Basal till 


Pleistocene 


Guswa and LeBlanc, 1985. 


24 


FA-8-82-1 




24.4 (80) 

6.1 (20) 

51.9 (170) 


Brown medium sand 
Fine yellow sand 
Gray silty clay 
Hard layer (ate bit) 


Pleistocene 


E.G.A. Weed, unpub. 
data, 1982. 


25 


PH-77-1 


— 


— 


Dry yellow sand, cobbles at 
35, 50, 60, and 75 ft. 


Pleistocene 


E.G.A. Weed, unpub. 
data, 1977. 


26 


MB-77-1 


- 


2.7 (9) 


Gray clayey silty sand 


Pleistocene 


E.G.A. Weed, unpub. 








1.5 (5) 


Yellow sand 


Pleistocene 


data, 1977. 








0.9 (3) 


Greensand 

Rottenstone (disaggregated 
granite). 


Miocene(?) 
Proterozoic Z(?) 




27 


MB-77-2 


- 


7.6 (25) 


Olive clay, varved 


Pleistocene 


E.G.A. Weed, unpub. 








3.1 (10) 


Yellow sand 


Pleistocene 


data, 1977. 






11.6 (38) 


1.2 (4) 


Granodiorite 


Proterozoic Z(?) 




28 


MK-79-1 


- 


- 


Sand 


Pleistocene 


E.G.A. Weed, unpub. 








15.6 (51) 


Granite 


Proterozoic Z(?) 


data, 1977. 


29 


MH-77-1 


- 


4.6 (15) 


Silt and sand 


Tertiary 


E.G.A. Weed, unpub. 








12.2 (40) 


Gray clay 


Tertiary 


data, 1977. 








25.9 (85) 


Yellow sand 


Pleistocene 










37.8 (124) 


Granodiorite 


Proterozoic Z(?) 


E.G.A. Weed, unpub. 


30 


MH-78-1 


- 


- 


Sand 


Pleistocene 


data, 1978. 






33.2 (109) 


3.1 (10) 


Black shale 


Carboniferous 


P.C. Lyons, written com- 
mun., 1979. 


31 


NR-78-1 


- 


— 


Sand 


Pleistocene 


E.G.A. Weed, unpub. 
data, 1978. 






18.3 (60) 


— 


Red sandstone 


Carboniferous 


P.C. Lyons, written com- 
mun., 1979. 


32 


SC-79-1 


- 


- 


Sand 


Pleistocene 


E.G.A. Weed, unpub. 






29.6 (97) 


- 


Granite 


Proterozoic Z(?) 


data, 1979. 


33 


TC-77-1 




4.6 (15) 
1.5 (5) 
1.5 (5) 


Dark-gray till 
Clayey sand 
Gray clay 


Pleistocene 


E.G.A. Weed, unpub. 
data, 1977. 



STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS 



E55 



Table 8.— Description of materials encountered in outcrops and in drilled holes in the area of Cape Cod and the nearby islands, 

Massachusetts — Continued 



Site 
designation 



Unit 
depth, 
meters 

(feet) 



Unit 

thickness, 
meters 
(feet) 



Description of units 



Principal data source 



22.6 (74) 



1.5 (5) 


Olive sand 


3.1 (10) 


Clay layers containing sand 


3.1 (10) 


Silty clay 


3.1 (10) 


Brown sand 


1.5 (5) 


Till 


4.9 (16) 


Yellow sand containing 




cobbles. 




Till(?), refusal 



the east (Cousminer and Manspeizer, written commun. , 
1984). The middle unit of Folger and others (1978) is 
considered by them to extend from the Cenomanian into 
the Campanian (middle Late Cretaceous). The upper 
(221-m) unit consists of variegated red, gray, and yellow 
clay interbedded with light-gray sand. Lignite and sub- 
bituminous coal about 20 m above the top of the lower 
unit indicate a period of subaerial deposition. Of 30 
samples from the Cretaceous part of the core, 13 con- 
tained spore and pollen assemblages. These assemblages 
are correlated with assemblages from the Cretaceous 
section of the New Jersey Coastal Plain (R.A. Christo- 
pher, written commun., 1976). Correlation of the strata 
in the bore hole with the New Jersey section is shown on 
figure 16. No formal names have been assigned to the 
Cretaceous strata in Massachusetts. 

Hall and others (1980) divided the Cretaceous section 
in hole ENW-50 (site 8) into 11 parts, correlated them 
with USGS 6001 (site 7), and noted the presence of 
greensand, sphalerite, and lignite, as well as the clays 
and sand. X-ray diffraction analyses by Hall and others 
(1980, table 1, p. 11) showed three distinct mineral 
assemblages with increasing depth: (1) quartz-feldspar to 
-50 m; (2) quartz-mica-chlorite-smectite, -50 to -70 m; 
(3) quartz-kaolinite, -70 m to bottom. These three 
assemblages have been used to subdivide the Cretaceous 
section into three units in table 8. 

Kaye (1983b) divided the Cretaceous section in the 
ENW-50 hole (site 8, table 8) into five informal zones: A, 
white beds, 71-137 m depth, consisting primarily of 
white kaolinitic clays and sands; B, gray and black beds, 
138-173 m, consisting of interbedded gray clay and silt 
and some light-gray silt and sandy silt; lignite is present 
at 161-164 m, 166-168 m, and 172-173 m depth; C, 
marine horizon, 174-189 m, consisting of greenish-gray 
to medium-gray, micaceous, glauconitic, silty clay and 
silty sand that contains foraminifers; D, white beds, 
190-224 m, containing white kaolinitic quartz sand and 



minor sandy silt, and beds of white clay containing thin 
zones of red and yellow mottling from 202-209 m; and E , 
dense white beds, 225-245 m, consisting of white clay 
and coarse sand, more compact and having a higher ratio 
of clay to sand than the beds just above. Kaye assigned 
the entire Cretaceous section in this hole to the Upper 
Cretaceous Raritan Formation. He correlated the 
marine horizon (C) with marginal marine intervals con- 
taining foraminifers and plant fossils in USGS 6001, on 
Nantucket (site 7) at 310, 320, and 321 m identified by 
Folger and others (1978, p. 18). 

The Cretaceous at Gay Head, Martha's Vineyard (site 
9), and at Nonamesset (site 11) is characterized by 
kaolinite; red, gray and white clays, in part containing 
siderite concretions; and lignitic clay. Pollen analyses 
(R.A. Christopher, written commun., 1977) confirm the 
Late Cretaceous age for the clays at Gay Head, and 
Christopher correlated them with the bottom part of the 
Upper Cretaceous section of New Jersey, either the 
South Amboy Fire Clay Member of the Raritan Forma- 
tion or the basal part of the Old Bridge Sand Member of 
the Magothy Formation. Greensand at Zack's Cliff, 
Martha's Vineyard (site 10), was identified as Cretaceous 
(R.A. Christopher, written commun., 1977), but most of 
the greensand in the region is of Tertiary age (Kaye, 
1983b). 

TERTIARY SEDIMENTS (T) 

Tertiary sediments (T) are scattered and fragmentary 
and no longer form a continuous stratum. They all appear 
to be of nearshore marine origin. Sediments containing 
Tertiary macrofossils and microfossils are incorporated 
as blocks or disaggregated material within Pleistocene 
deposits (sites 7, 8, 29, 33). The most complete section of 
Tertiary deposits is a 58-m sequence of silt and clay on 
Nantucket (site 12, table 8) that ranges from Paleocene 
to Miocene and Pliocene in age. Greensand of Tertiary 
age, identified in places by pollen analyses as Miocene or 



E56 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



System 

or 
Series 


European 
Stage 


New Jersey 

outcrop 
equivalent 
formation 


Units of 

Folger 

and others 

(1978) 


cu c 
a .2 
E to 
ro tj 
CO o 


Depth 
Feet Meters 


CD 

c 
i> 
o 
o 

'53 

0- 






upper 


- o - 

- o - 

- o - 

= 8 = 

! = 

- o - 

= 83 

- o - 

- o - 

- • - 

- o - 


- 100 

- 200 

- 300 

100 - 




Upper 
Campanian 


Basal Navesink Formation 
and Mt. Laurel Sand 




- 400 

- 500 




Wenonah Formation 


Lower 
Campanian 


- Englishtown Formation 

? \ and Woodbury Clay / ' 




CO 

■D 

o 
cu 
o 

6 


Santonian 


Magothy 
Formation 


middle 


200 - 

- 700 

- 800 


/ Coniacian? \ 


\ Turonian? / 




a 

Q. 

Z> 








- • - 

- • - 


- 900 




Cenomanian 


Raritan 
Formation 






300 - 
- 1000 










II 

- • - 

- O - 


- 1100 

- 1200 


















- o - 




S 2 

5 ° 

o 2 

o 


Albian 




lower 


- o - 

- o - 


-1300 4Q0 _ 

- 1400 

- 1500 








to c £ 

i- CD •- 

3 H 










- 1600 

500 - 



Figure 16. —Correlation of the Cretaceous section in bore hole USGS 6001, Nantucket, with the 
exposed Cretaceous section in New Jersey. Modified from R.A. Christopher (written commun., 
1976). Solid circles indicate samples containing microfossils. 



Miocene and Pliocene, is exposed on Martha's Vineyard 
at Gay Head and Zack's Cliff (sites 9 and 10) and on 
Nonamesset Island (site 11). Greensand has also been 
reported from Wequobsque Cliff about 9.5 km east of 
Gay Head (Kaye, 1983b). 

Tertiary sediments of possible Eocene age are 
reported from the outer tip of Cape Cod (sites 18, 19, 20, 



21). Eocene pollen was identified in the silty sand at 
Holden's Pond (site 18), but this material may be 
reworked, and there is some doubt that the sediments 
themselves, and possibly those at sites 19, 20, and 21 as 
well, are Eocene. Reworking of Tertiary material into 
Pleistocene sediments is evident at the top of the section 
at hole ENW-50 (site 8) on Martha's Vineyard. Here the 



STRATIGRAPHY OF THE MILFORD-DEDHAM ZONE, EASTERN MASSACHUSETTS 



E57 



inversion of normal stratigraphic position of fossils sug- 
gests a disordered structure like that visible at Gay Head 
as described by Kaye (1964c, 1983b). 

A similar disruption of stratigraphic order is evident 
on the mainland in the Marshfield Hills area. At Marsh- 
field Hills (site 29), the 17 m of Tertiary silt, sand, and 
clay overlying Pleistocene glacial deposits is allochtho- 
nous. Greensand, probably Miocene, at site 26 appears to 
be autochthonous, however. Kaye (1983b) described two 
places in Marshfield near sites 26 and 27 where presum- 
ably autochthonous greensand overlies bedrock. 

STRATIGRAPHIC HISTORY 

The Triassic and Jurassic basalt and sediments were 
deposited in the Nantucket basin during rifting of the 
Proterozoic and Paleozoic basement in the early Meso- 
zoic (Ballard and Uchupi, 1975; Klitgord and Behrendt, 
1979; Goldsmith, this vol., chap. H). The sediments were 
presumably nonmarine, similar to those in the onshore 
Mesozoic basins of the East Coast. Following an interval 
of subaerial weathering (see site 7, -446 m, table 8) and 
erosion, littoral and shallow offshore-marine strata were 
deposited as onlap facies in the Cretaceous as eastern 
North America became a passive trailing edge of the 
westward-moving North American plate. Shallow- 
marine deposition continued into and probably through 
the Tertiary until disrupted by the glaciation of the 
Pleistocene. The northwest marine limit of the Tertiary 
marine deposits may be marked by fragments found in 
glacial drift, as at Marshfield Hills (site 29, fig. 15). 

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Stratigraphy of the Nashoba 
Zone, Eastern Massachusetts: 
An Enigmatic Terrane 

By RICHARD GOLDSMITH 

With a section on MASSABESIC GNEISS COMPLEX (OZma) 
By PETER ROBINSON 

THE BEDROCK GEOLOGY OF MASSACHUSETTS 

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1366-F 



CONTENTS 



Abstract Fl 

Introduction 1 

Description of units 3 

Marlboro Formation (OZm, OZmg) ? 

Shawsheen Gneiss (OZsh) 5 

Fish Brook Gneiss (OZf) 6 

Nashoba Formation (OZn, OZnb) 6 

Tadmuck Brook Schist (SZtb) 7 



Stratigraphic considerations F7 

Orientation 7 

Thickness and sequence 8 

Age 11 

Regional correlations 12 

Summary and conclusions 17 

Massabesic Gneiss Complex (OZma), by Peter Robinson 18 

References cited 20 



ILLUSTRATIONS 



Figure 1. Map showing major rock units and structural features in the Nashoba zone, eastern Massachusetts F2 

2. Correlation diagram of stratified and major intrusive rock units in the Nashoba zone and some rock units in the 

Merrimack belt 3 

3. Lithostratigraphic columns of the formations in the Putnam terrane and Nashoba zone, Connecticut and eastern 

Massachusetts 4 

4. Diagram showing con-elation of units in the Nashoba zone using repetition by faulting: A, nomenclature as published; 

B, alternative nomenclature suggested in this paper 10 

5. Map showing terranes in New England possibly correlative with the Nashoba zone 13 

6. Correlation diagram and relative thicknesses of formations in the Nashoba zone and of some possibly correlative 

formations elsewhere in Massachusetts, Connecticut, New Hampshire, and Maine 14 

7. Lithostratigraphic columns of units in southern and east-central Maine and southeastern New Hampshire that are 

probably correlative with units of the Nashoba zone 15 

8. Correlation diagram of stratified lower Paleozoic rocks in eastern Connecticut, as shown by Rodgers (1982) 16 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



STRATIGRAPHY OF THE NASHOBA ZONE, 

EASTERN MASSACHUSETTS: 

AN ENIGMATIC TERRANE 



By Richard Goldsmith 



ABSTRACT 

The Nashoba zone is a fault-bounded block of high-grade, steeply 
dipping metamorphic rocks, largely metavolcanic to the east (Marlboro 
Formation) and largely metasedimentary to the west (Shawsheen 
Gneiss, Fish Brook Gneiss, Nashoba Formation, and Tadmuck Brook 
Schist). The Marlboro Formation, presumably stratigraphically the 
lowest formation, consists of amphibolite, feldspathic gneiss, and 
subordinate pelitic schist and calc-silicate rock. The Shawsheen Gneiss, 
above the Marlboro, is predominantly a pelitic schist and gneiss, 
commonly rusty weathering. The Fish Brook Gneiss, above, is a 
conspicuous mottled-appearing feldspathic gneiss believed to be of 
felsic igneous origin, probably volcanic. The Nashoba Formation, which 
occupies a large part of the zone, consists of pelitic and semipelitic 
(metawacke) gneiss and schist, commonly rusty weathering, and sub- 
ordinate calc-silicate rock, marble, and amphibolite. The Tadmuck 
Brook Schist, consisting of pelitic schist and phyllite and subordinate 
quartzite, may lie unconformably above the Nashoba. Protoliths of the 
Marlboro were volcanic, volcaniclastic, and intercalated epiclastic 
materials deposited in a marine environment close to a volcanic source, 
probably to the east. The volcanic materials were primarily basaltic but 
also included andesitic, dacitic, and rhyodacitic materials. The proto- 
liths of the Shawsheen, Fish Brook, and Nashoba were marine volcan- 
iclastic, epiclastic, and minor volcanic materials and carbonate sedi- 
ments deposited in a slope or basin off a volcanic center (volcanic arc). 

The source of the material seems to be from the east, although two 
source terranes are indicated — a deeply weathered terrane and a 
volcanic source. The age of the material is bracketed between a 750-Ma 
U-Pb age on zircons from the Fish Brook Gneiss and a 1,500-Ma U-Pb 
age on detrital zircon from the Shawsheen Gneiss. 

Rocks of the Nashoba zone correlate fairly well with the Tatnic Hill 
and Quinebaug Formations in Connecticut and with the Cushing 
Formation of southeastern Maine and the Rye Formation of southeast- 
ern New Hampshire. The Passagassawakeag Gneiss of eastern Maine 
and the rocks of the Gander Group in southeastern Newfoundland bear 
similarities to the Nashoba Formation. Correlation across strike with 
rocks of the Brimfield Group or rocks on the flanks of the Bronson Hill 
anticlinorium is less certain and involves complex structural interpre- 
tations. The Massabesic Gneiss Complex of south-central New Hamp- 
shire contains rocks resembling those of the Nashoba zone. The 



Manuscript approved for publication November 16. 1987. 



Nashoba zone is distinct from the Merrimack belt, adjacent to the west, 
and from the Milford-Dedham zone to the east. It is considered to be an 
accretionary terrane that was part of a volcanic-arc complex lying 
southeast of the North American plate and containing material proba- 
bly derived from an African source. 



INTRODUCTION 

The Nashoba zone of eastern Massachusetts is defined 
as that mass of rock lying between the Merrimack belt on 
the west and the Milford-Dedham zone on the east. The 
Clinton-Newbury fault system bounds the block on the 
west and the Bloody Bluff-Lake Char fault system 
bounds the zone on the east (fig. 1). The zone consists of 
high-grade metamorphic rocks and intrusive plutonic 
rocks that together form a distinctive metamorphic- 
plutonic terrane extending northeast across eastern 
Massachusetts from Oxford, Mass., to the Gulf of Maine, 
south of Newburyport. Near Newburyport, the zone 
narrows appreciably between the Clinton-Newbury fault 
and the faults bounding the Newbury basins. These 
basins are small tectonic wedges between the Nashoba 
and Milford-Dedham zones and are discussed in the 
chapter on the Milford-Dedham zone (Goldsmith, this 
vol., chap. E). To the southwest, the Nashoba zone 
narrows at the salient in the Milford-Dedham zone near 
Oxford, where it is in tenuous continuity with the terrane 
containing the Putnam Group (Putnam block) in eastern 
Connecticut. 

The rock units within the Nashoba zone are considered 
to be part of a single lithotectonic entity because common 
lithologies are interlayered and because the zone is 
flanked by terranes of different lithologic and structural 
character. Internally, however, the structure is com- 
plex. Early synmetamorphic folds are largely obscured 
by later folds and faults. Differential vertical and lateral 



F2 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 




10 



20 MILES 



I I I 

10 20 KILOMETERS 

Figure 1. —Major rock units and structural features in the Nashoba zone, eastern Massachusetts. Correlation diagram and list of units are shown 

on figure 2. 



translation along faults has cut the zone into lenses of 
rock, the sequence of which is not now clear. 

The five formations of stratified rock within the Nash- 
oba zone in Massachusetts shown on the State bedrock 
map (Zen and others, 1983; fig. 2)— the Marlboro For- 



mation (OZm, OZmg), Shawsheen Gneiss (OZsh), Fish 
Brook Gneiss (OZf), Nashoba Formation (OZn, OZnb), 
and Tadmuck Brook Schist (SZtb)— represent an assem- 
blage of sedimentary, volcaniclastic, and volcanic strata 
metamorphosed to nonsulfidic and sulfidic, pelitic and 



STRATIGRAPHY OF THE NASHOBA ZONE, EASTERN MASSACHUSETTS 



F3 



MERRIMACK BELT 



Sharpners Pond 
ite and related 
rocks 




Pennsylvanian 



OZnb 

OZf 

OZsh 



OZt 
OZq 
OZma 



Ssqd 
SOagr 



DSw 

Sp 



Pgr 
Dfgr 



NASHOBA ZONE 

Tadmuck Brook Schist 
Nashoba Formation 
Boxford Member 
Fish Brook Gneiss 
Shawsheen Gneiss 
Marlboro Formation 

Homogeneous light-gray feldspathic gneiss 
Tatnic Hill Formation 
Quinebaug Formation 
Massabesic Gneiss Complex 

Intrusive Rocks 

Sharpners Pond Diorite 
Andover Granite 

MERRIMACK BELT 

Littleton Formation 
Worcester Formation 
Paxton Formation 



Fine- to medium-grained biotite granite 
Fitchburg Complex 



Figure 2. — Stratified and major intrusive rock units in the Nashoba 
zone and some stratified and intrusive rock units in the Merrimack 
belt. 



semipelitic schist and gneiss, calc-silicate rock, amphib- 
olite, felsic gneiss, and subordinate marble and quartzite 
(fig. 3). In adjacent Connecticut, along the strike of the 



units, the Quinebaug Formation (OZq) is equivalent to 
the Marlboro, and the Tatnic Hill Formation (OZt) is 
equivalent to the Nashoba. In Massachusetts, metavol- 
canic rocks increase in abundance eastward and presum- 
ably downsection, from the Tadmuck Brook Schist to the 
Marlboro Formation. The Tadmuck Brook Schist and 
Nashoba Formation are primarily metasedimentary, but 
the latter has a significant volcaniclastic component 
(Abu-Moustafa and Skehan, 1976). The Fish Brook 
Gneiss has a volcanic or volcaniclastic protolith. The 
Shawsheen Gneiss consists primarily of pelitic and semi- 
pelitic metasediments. The Marlboro Formation is pri- 
marily metavolcanic but has a significant metasedimen- 
tary component. Sulfidic sillimanitic schist and gneiss, 
calc-silicate rock, amphibolite, and biotite-quartz- 
feldspar gneiss with and without garnet are present in 
varying proportions in all the formations except the Fish 
Brook. 

The stratified rocks are intruded by the syn- or 
late-tectonic Silurian and Ordovician(?) Andover Granite 
(SOagr), dated by Zartman and Marvin (this vol., chap. 
J, table 1), and associated but possibly in part younger 
aplite and pegmatite (Zartman and Naylor, 1984), which 
was shown on the State bedrock map as part of the 
Andover map unit, and by a post-tectonic suite of dioritic 
to granitic plutons that includes the Silurian Sharpners 
Pond Diorite (Ssqd) and related rocks. The plutonic rocks 
are particularly abundant near Lowell and Billerica and 
decrease in abundance to the southwest (fig. 1). The 
intrusive rocks in the Nashoba zone are described in 
Wones and Goldsmith (this vol., chap. I). 



DESCRIPTION OF UNITS 

MARLBORO FORMATION (OZm, OZmg) 

The Marlboro Formation consists primarily of inter- 
layered metavolcanic and metavolcaniclastic rocks and 
marine metasedimentary rocks (fig. 3). It was named by 
Emerson (1917) for ledges of biotite-hornblende schist in 
the town of Marlboro (in the Township of Marlborough). 
The Marlboro is bounded on the east by the Bloody Bluff 
fault zone. North of Concord the Marlboro is cut out by 
the Bloody Bluff and by plutonic rocks, so that no 
Marlboro has been identified with certainty north of 
the Concord area. The Marlboro is in contact east of the 
fault with different units of the Milford-Dedham zone: 
Proterozoic Z granitoids, the Proterozoic Z Westboro 
(Zw) and Plainfield (Zp) Formations, and the Proterozoic 
volcanic-plutonic complex exemplified by the metamor- 
phosed mafic and felsic volcanic rocks (Zv) and intrusive 
diorite and gabbro (Zdigb, Zdi, Zgb). The upper contact 
of the Marlboro is apparently conformable with the 
Nashoba Formation in the Shrewsbury-Marlborough 



F4 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



area, Middlesex and Worcester Counties (fig. 1) (Skehan 
and Abu-Moustafa, 1976), but Hepburn and DiNitto 
(1978) showed the contact as a fault. Thickness consider- 
ations imply that Hepburn and DiNitto were probably 
correct in this area. To the north, in northern Middlesex 
County, the contact of the Marlboro with the overlying 
Shawsheen Gneiss is mostly cut out by the Andover 
Granite, but Alvord and others (1976, p. 319) believed 
that the top of the Sandy Pond Member (fig. 3) of the 
Marlboro was conformable with the Shawsheen Gneiss. 
In eastern Connecticut, amphibolite increases in abun- 
dance downward in the basal member of the Tatnic Hill 
Formation (OZt) (Dixon, 1974, for example), suggesting 
conformity with the underlying Quinebaug Formation 
(OZq), which is equivalent to the Marlboro (fig. 3). 

Bell and Alvord (1976) divided the Marlboro into two 
members (fig. 3), mainly on the basis of mapping in 



Essex and northern Middlesex Counties (fig. 1). The 
lower, unnamed member consists of mica schist, calc- 
silicate rock, marble, and amphibolite. The upper mem- 
ber, the Sandy Pond Member, is predominantly amphib- 
olitic. Skehan and Abu-Moustafa (1976), mapping in the 
Wachusett-Marlborough tunnel in southern Middlesex 
County, divided the Marlboro into 31 members (shown as 
10 units on fig. 3). In contrast to Bell and Alvord's 
section, most of the amphibolitic rocks are at the east or 
bottom part of Skehan and Abu-Moustafa's section. 
Hepburn and DiNitto (1978) and Hepburn (1978), map- 
ping the surface rocks above the tunnel in the Marlbor- 
ough and the Shrewsbury areas respectively, recognized 
four divisions of the Marlboro, one of which, their Sandy 
Pond amphibolite member, coincides with the Sandy 
Pond Member of Bell and Alvord and with the amphib- 
olitic lower part of Skehan and Abu-Moustafa's section. 



Dixon (1965a) 
Eastern Connecticut 



Hebron Formation 



D 
O 
cc 
O 

< 

z 
h- 

D 

Q_ 


C 

o 

1 

o 

i 
c 


E 

E 
| 

1 


Yantic 
Member 


(Muscovite)-biotite-oligoclase-quartz schist 
containing sillimanite, kyanite, or staurolite at 
x appropriate grade; amphibolite at base 


Fly Pond 


\ Calc-silicate gneiss, local marble; minor mica 
\ schist and amphibolite 


Lower 
member 


(Sillimanite)-gamet-(muscovite)-biotite gneiss; 


/ Garnet-biotite gneiss, rusty-weathering sillimanite 
mica schist, calc-silicate gneiss 


Rusty-weathering graphitic garnet-sillimanite- 


c 
o 

1 

o 
en 

c 
'd 

a 


E 

E 
1 

i 


Upper 


\ mica schist and minor amphibolite 


Black Hill 
Member 


\ (Hornblende)-epidote-biotite-quartz-plagioclase 
\ gneiss, locally porphyroblastic; amphibolite, calc- 
\ silicate rock, rare marble 


(Garnet)-muscovite-biotite-quartz schist, 
(muscovite)-calcite-hornblende-quartz-biotite- 
\ oligoclase gneiss, calcite-muscovite-plagioclase- 
quartz rock and calc-silicate rock 


Lower 
member 


Garnet-biotite-epidote-quartz-plagioclase gneiss, 
(biotitet-epidote-quartz-hornblende-plagioclase 
gneiss, amphibolite, biotite-quartz-plagioclase 
gneiss, and calc-silicate gneiss 

Lake Char fault 1 




Sandy 

Pond 



Hepburn and DiNitto (1978)' 

Marlborough, Mass., area 

Assabet River fault (?) 



Biotite-muscovite-plagioclase-quartz granulite, 
locally containing small plagioclase megacrysts; 
minor sillimanite-mica schist and amphibolite 



Rusty-weathering (garnet)-(sillimanite)-biotite- 

/ite schist and gneiss; minor amphibolite 



Calc-silicate rock, coticule, amphibolite 



Amphibolite, locally containing knots and boudin 
rich in epidote; minor garnet-(muscovite)-biotite 
schist and gneiss, hornblende gneiss, and calc- 
silicate rock 



Biotite-rich gneiss and schist; thickne 
uncertain 

Bloody Bluff fault 



'Thickness computed from ; 
on map, corrected for dip 



irage width of unit 



Figure 3.— Lithostratigraphic columns of the formations in the Putnam terrane and Nashoba zone, Connecticut and eastern 
Massachusetts. Column of Bell and Alvord (1976) is at 2.5 times smaller scale than the other columns. 



STRATIGRAPHY OF THE NASHOBA ZONE, EASTERN MASSACHUSETTS 



F5 



On the basis of the sections shown on figure 3, it seems 
likely that Hepburn and DiNitto's Sandy Pond is equiv- 
alent to Skehan and Abu-Moustafa's units M5-M16. 

A lenticular mass of biotite granodioritic gneiss 
(OZmg) near Grafton, considered by Emerson (1917) to 
be a pluton of Milford Granite (Zmgr), does not resemble 
the Milford and was named informally the Grafton gneiss 
by Dixon (written commun., 1977). The Grafton is not 
layered, is fairly uniform in composition, and has a 
foliation formed by oriented flakes of biotite. The Grafton 



does, however, contain some inclusions of amphibolite 
and biotite schist of the Marlboro. Hepburn (1978) con- 
sidered the Grafton to be a plutonic rock, but it could be 
a metamorphosed felsic volcanic rock. 

SHAWSHEEN GNEISS (OZsh) 

The Shawsheen Gneiss (Bell and Alvord, 1976) con- 
sists primarily of sillimanitic muscovite-biotite schist and 
gneiss, sulfidic near the base, and it contains a few lenses 



Skehan and Abu-Moustafa (1976) 
Wachusett-Marlborough tunnel, Mass 



Bell and Alvord (1976) 
Middlesex County, Mass. 



Clinton-Newbury fault 



Zen and others (1983) 

Massachusetts State 

bedrock map 




Figure 3.— Continued. 



F6 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



of layered and massive amphibolite (fig. 3). The Shaw- 
sheen is lithologically similar to much of the Nashoba 
Formation and was considered by Hansen (1956) to be 
part of the Nashoba. Bell and Alvord (1976) separated 
the Shawsheen from the Nashoba because the Fish 
Brook Gneiss intervened between the Shawsheen and 
the rest of the Nashoba Formation. On the other hand, 
Barosh and others (1977) included both the Shawsheen 
and the Fish Brook in the Nashoba. Hansen (1956) did 
not recognize the rock later called the Fish Brook Gneiss 
(Castle, 1965) as a mappable unit and similarly included 
all the rocks in the Nashoba Formation. 



FISH BROOK GNEISS (OZf) 

The Fish Brook Gneiss is a fine- to medium-grained, 
"pearly white to very light gray, distinctly foliated but 
generally unlayered biotite-quartz-plagioclase rock" 
(Castle, 1965, p. C81). The foliation is marked by ori- 
ented biotite flakes, which are more abundant to the 
north than to the south. According to Castle, the Fish 
Brook contains inclusions ranging in size and shape from 
schlieren to rectangular zones. These inclusions consist 
of amphibolite, thinly layered biotite gneiss, and other 
rock types; the foliation in the gneiss generally passes 
through these inclusions, although it wraps around some 
zones. The Fish Brook, like other units in the Nashoba 
zone, is intruded by the Andover and related granites. 
Castle (1965) considered the Fish Brook to be either a 
premetamorphic intrusive rock or a core gneiss of 
intrusive or sedimentary ancestry. Bell and Alvord 
(1976), however, claimed a volcanic and volcaniclastic 
origin for it. Zircons from the Fish Brook are almost 
certainly of volcanic origin (Olszewski, 1980, p. 1411). 
Alvord and others (1976, p. 320) described an exposure of 
the contact of the Fish Brook Gneiss and the Nashoba 
Formation near Billerica, where very thin beds of fine- 
grained amphibolite of the Boxford Member of the Nash- 
oba are interlayered with very thin beds of light-gray 
Fish Brook Gneiss. 



NASHOBA FORMATION (OZn, OZnb) 

The Nashoba Formation (Hansen, 1956) consists of 
interlayered sillimanite-bearing, partly sulfidic schist 
and gneiss, (hornblende)-biotite-quartz-feldspar gneiss, 
calc-silicate gneiss, and subordinate quartzite and mar- 
ble (fig. 3). Protoliths are primarily volcanogenic sedi- 
ments, interlayered with limy marine sediments and 
volcanic rocks (Abu-Moustafa and Skehan, 1976; Bell and 
Alvord, 1976). Bell and Alvord divided the Nashoba into 



10 members on the basis of lithology. Sillimanite-bearing 
pelitic and semipelitic schist and gneiss are interlayered 
with other rock types throughout their section. Calc- 
silicate rocks and marble characterize some members. 
Amphibolite is most abundant near the presumed base of 
the Nashoba, primarily in the Boxford Member (OZnb) 
(Boxford Formation of Castle, 1965). Alvord and others 
(1976) identified localities where some of their subdivi- 
sions of the Nashoba can be seen. Skehan and Abu- 
Moustafa (1976) subdivided the Nashoba as seen in the 
Wachusett-Marlborough tunnel into 30 members, which 
I have condensed for economy to 13 on figure 3. Lithol- 
ogies are similar to those described by Bell and Alvord, 
and in a general way their sections are similar, although 
Bell and Alvord's section is much thicker (note scale 
change for their section on fig. 3). The Boxford Member 
is not readily identifiable in the tunnel section. Possibly 
it thins and pinches out or is faulted out before it reaches 
the tunnel section. The Boxford is, however, extensive 
in the Lowell-Billerica area, where Castle (1965) divided 
it into an upper and a lower member. His upper mem- 
ber consists mostly of amphibolite and hornblende- 
plagioclase gneisses. His lower member consists chiefly 
of mica schist and quartzofeldspathic gneisses and sub- 
ordinate amounts of amphibolite and calc-silicate gneiss. 
This lower member is to a certain extent lithologically 
like the undivided lower part of Bell and Alvord's Marl- 
boro Formation. 

Subdivision of the Nashoba is conjectural south of 
Marlborough and Shrewsbury where only reconnais- 
sance mapping has been done. Hepburn (1978) and H.R. 
Dixon (written commun., 1978) identified a rusty- 
weathering sulfidic schist at the base of the Nashoba, 
which extends from south of Shrewsbury to the Webster 
area and which is probably the same sulfidic schist that 
forms the basal member of the Tatnic Hill Formation in 
Connecticut. Dixon (written commun. , 1978) recognized 
rock in the Nashoba in the Oxford quadrangle that was 
similar to subdivisions of the Tatnic Hill that she has 
mapped in Connecticut. 

The Boxford Member (OZnb) has been separated out 
from the rest of the Nashoba on the State bedrock map 
because this unit is the only one that can be recognized 
clearly in several areas. I doubt that a definite sequence 
of units exists throughout the Nashoba because of the 
lenticularity of assemblages and repetition of rock types, 
both of which could be accounted for by either sedimen- 
tary or tectonic processes. Evidence for tectonism has 
been found by practically everyone who has mapped in 
the Nashoba zone (Hansen, 1956; Castle, 1964; Alvord, 
1975; Bell and Alvord, 1976; Skehan and Abu-Moustafa, 
1976; Barosh, 1978; Hepburn, 1978; Hepburn and 
DiNitto, 1978). 



STRATIGRAPHY OF THE NASHOBA ZONE, EASTERN MASSACHUSETTS 



F7 



TADMUCK BROOK SCHIST (SZtb) 

Bell and Alvord (1976) gave the name Tadmuck Brook 
Schist to a sequence of largely pelitic rocks lying west of 
and presumably above the Nashoba (figs. 1, 3). The 
Tadmuck Brook consists primarily of sillimanite schist, 
graphitic staurolite-andalusite phyllite, and chlorite- 
biotite-muscovite phyllite in decreasing metamorphic 
grade from east to west. Near its base to the east, the 
unit contains subordinate layers and lenses of amphibo- 
lite and discontinuous beds of quartzite. Where exposed 
on Route 2 near Littleton (fig. 1), the Tadmuck Brook is 
rusty weathering. According to Alvord and others 
(1976), the Tadmuck Brook intertongues locally with 
quartzofeldspathic layers of the Nashoba Formation, 
but, from the Littleton area to the vicinity of Lawrence, 
the Tadmuck Brook appears to truncate units in the 
Nashoba; those authors, therefore, suggested the possi- 
bility of an unconformity or disconformity. However, as 
they pointed out, this contact is a fault in most places 
north of Route 2 (Littleton area) and has been so 
interpreted by Castle and others (1976) on the basis of a 
truncation of aeromagnetic pattern along this contact. 
Skehan and Murray (1980, p. 295) considered the Tad- 
muck Brook to be unconformable on the Nashoba 
because of a small angular discordance coupled with 
absence of fault-related features at the contact and the 
sharp contrast in deformation and metamorphism 
between the two units. 

The Tadmuck Brook is probably represented in Ske- 
han and Abu-Moustafa's (1976) tunnel section by their 
unnamed units U1-U9. Ul consists largely of quartzite, 
and in their section it is shown as lying possibly uncon- 
formably on the Nashoba. Hepburn (1978) was unable to 
identify the Tadmuck Brook south of Shrewsbury where 
the Boylston Schist (SObo) of the Merrimack belt lies 
against the Nashoba. To the north, the Tadmuck Brook 
is truncated at the top, sliced throughout, and locally 
phyllonitized by the Clinton-Newbury fault system 
(Alvord and others, 1976). 



STRATIGRAPHIC CONSIDERATIONS 



ORIENTATION 

The formations in the Nashoba zone have customarily 
been considered to be a sequence topping to the west, 
presumably because they lie on the east flank of the 
Merrimack synclinorium. This assumption may not be 
valid, however, because the bounding faults of the zone 
appear to be major dislocations. Even so, Bell and 
Alvord (1976) considered the Marlboro-to-Nashoba sec- 
tion to be homoclinal and topping to the west on the basis 



of many features seen in outcrop that they considered to 
be relict primary sedimentary features. On the other 
hand, Castle (1965, p. C84) observed no unambiguous 
primary structures, at least in the Boxford Member 
(compare Bell and Alvord, 1976, fig. 4, p. 186). Skehan 
and Abu-Moustafa (1976) made no mention of primary 
structures except bedding and presented no topping 
evidence in their description of their Wachusett- 
Marlborough tunnel section. They followed convention, 
however, and described the section as homoclinal and 
presumably topping to the west. They cited no evidence 
in their section that units are repeated by folding or 
faulting except on a small scale. 

I do not believe we can say with certainty which way 
the units face, nor can we say that the sequence is 
complete, if it is homoclinal, because of the flanking 
faults. What evidence there is indicates that the 
sequence is west facing. We do not see a depositional 
bottom and probably not a depositional top. We do not 
know for sure whether the Tadmuck Brook Schist is part 
of a Nashoba-Marlboro package or if it is the base of an 
overlying unconformable sequence. However, we can see 
that a difference exists in sedimentary fades within the 
terrane. The fades on the east indicate deposition in a 
marine basin next to a volcanic terrane, as evidenced by 
the greater amount of amphibolite interpreted to be of 
volcanic origin. Facies to the west indicate deposition in 
a somewhat more distal part of the basin, as evidenced 
by the greater amount of aluminous pelitic schist and 
gneiss and thinly layered calc-silicate rocks, although 
these western facies still contain a noticeable volcanic 
and volcaniclastic component. 

Abu-Moustafa and Skehan (1976), who have studied 
the rocks intensively, described the sediments of the 
Nashoba and Marlboro as having been derived from two 
major sources: one a deeply saprolitized, even lateritic, 
terrane and the other a terrane containing unweathered 
volcanic and plutonic rocks, whose average composition 
is dacite, and volcanogenic sediments. The hornblende- 
rich rocks of the Marlboro and Nashoba were volcano- 
genic sediments and flows and pyroclastic rocks of 
predominantly basaltic composition and subordinate calc- 
alkalic andesitic dacitic and rhyodacitic compositions 
(Nockolds, 1954). Abu-Moustafa and Skehan (1976, p. 32) 
inferred that the paleotectonic environment of deposition 
of the Nashoba (and presumably also the Marlboro) was 
a "relatively shallow marine basin that received deeply 
weathered soils, unaltered volcanogenic sediments, and 
some volcanic rocks, occasionally interbedded with thin 
limey beds..., thin quartz and quartz-feldspar sand..., 
and distinctive manganiferous-iron chert...." This basin 
presumably flanked an island arc lying to the east. 
However, the original Nashoba protolith need not have 
formed where it is now. The Nashoba zone could have 



F8 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



been moved into place with or without rotation and could 
have moved a considerable distance. 



THICKNESS AND SEQUENCE 

The true thicknesses of the formations are difficult to 
determine because of the differences in thicknesses given 
by different authors or estimated by me from their maps. 
Differences in these thicknesses need to be considered 
with regard to folding and faulting. Bell and Alvord's 
(1976) section of the Nashoba Formation is almost five 
times thicker than Skehan and Abu-Moustafa's (1976) 
tunnel section. This difference may be a function of 
differences in allowances for internal folding and faulting 
in preparing the two sections, although it would appear 
from their descriptions that the thicknesses in both 
sections were derived from the width of the lithologic 
unit or subunit corrected for dip. On the other hand, the 
units may thin stratigraphically or tectonically south- 
ward. In contrast to the discrepancy in thickness of the 
Nashoba Formation between sections, thicknesses of the 
Marlboro in Massachusetts given in the two reports are 
in fairly good agreement and agree also with thicknesses 
of the equivalent Quinebaug Formation in Connecticut 
(fig. 3). In fact, the Marlboro is thicker in Skehan and 
Abu-Moustafa's section than in Bell and Alvord's section, 
although this might be accounted for by the much greater 
amount of plutonic intrusive rock in Bell and Alvord's 
area. 

Let us first consider folding. Some repetition of litho- 
logic assemblages is evident in the sections of the Na- 
shoba, as for example the alternations between calc- 
silicate-bearing assemblages and pelitic schist and gneiss 
assemblages (fig. 3). These repetitions could be 
explained by cyclic sedimentation or, alternatively, by 
folding. Skehan and Abu-Moustafa's cross section (1976, 
fig. 1, p. 220) shows two major synforms and a central 
antiform, although all are broken by faults. These large 
folds were apparently not taken into account in their 
compilation of aggregate thicknesses because they could 
recognize no repetitions of units except where layers are 
repeated by small-scale folding and by drag associated 
with minor faults. Hansen (1956) did not show a complete 
section across the Nashoba but showed clearly at least 
one synform within the Nashoba, an antiform beyond the 
west flank, and a broad antiform or dome in the Marlboro 
on the east flank. These relations indicate a synformal 
structure for the Marlboro-to-Nashoba sequence but 
with apparent truncation on the west side where there is 
no Marlboro, unless the Reubens Hill Formation (SOrh) 
(Skehan, 1967) in the Merrimack belt is the stratigraphic 
equivalent of the Marlboro. Foliation and layering sym- 
bols in the Nashoba on the maps of the Billerica and 
Westford quadrangles (Alvord, 1975) are steeply dip- 



ping, but these symbols on their maps form zones of 
alternate east dip and west dip, suggesting tight folding 
of layering and foliation and thus probable repetition of 
units and an exaggerated thickness of section. 

Faulting probably has had an appreciable effect on the 
thicknesses of sections. Skehan and Abu-Moustafa (1976) 
described numerous faults of different kinds and ages 
and of large and small magnitude in their tunnel section. 
Many of Bell and Alvord's (1976) units are totally or in 
part bounded by faults. Such faults not only make 
estimates of thickness of each individual unit a minimum, 
disregarding folding, but also raise the possibility that 
total thickness of the formations in the zone is exagger- 
ated because of unrecognized repetition of units. The 
latter would be particularly important if appreciable 
imbrication has occurred as a result of movement along 
larger faults such as the Spencer Brook and Assabet 
River faults (fig. 1). The Assabet River fault separates 
the Shawsheen from the Fish Brook Gneiss in most 
places, but this contact is viewed by Bell and Alvord as 
stratigraphically conformable. The Assabet River fault is 
not identified in the tunnel section but is either a fault 
zone in unit N6 (near Station 266+35 of Skehan and 
Abu-Moustafa, 1976) west of the Nashoba-Marlboro con- 
tact or more likely, a thrust fault in N4 (at Station 
285+10-15 of Skehan and Abu-Moustafa, 1976). Hep- 
burn and DiNitto (1978) mapped a fault along the 
Nashoba-Marlboro contact in the Marlborough quadran- 
gle that may be the Assabet River fault and may be a 
surface representation of the appreciable faulting near 
the base of unit Nl described by Skehan and Abu- 
Moustafa. To the north, the Assabet River fault cuts off 
the top of the Shawsheen (Bell and Alvord, 1976, Appen- 
dix 1). The Spencer Brook fault separates the Billerica 
Schist from the Bellows Hill Member of the Nashoba. 
Alvord and others (1976, p. 322) presumed that if the 
contact were not obscured by faulting, the two units 
would intertongue and be conformable. The Spencer 
Brook may be a fault zone described by Skehan and 
Abu-Moustafa (1976, table 6) between units N17 and N18 
(at Station 177+53), or it could be one of the faults nearer 
the Marlboro contact. Possibly two faults in the tunnel 
section, one in N1-N5 and the other at the top of N6, 
may be the Assabet River and Spencer Brook faults, 
respectively. The Shawsheen and Marlboro are in normal 
stratigraphic contact, according to Bell and Alvord, and 
the Nashoba and Marlboro are in normal stratigraphic 
contact, according to Skehan and Abu-Moustafa, despite 
evidence for appreciable faulting in the vicinity of the 
contact. From Bell and Alvord's map (1976, fig. 1), 
though, it is not clear that the Shawsheen is ever in 
contact with the Marlboro without intervening plutonic 
rocks. The Shawsheen-Fish Brook contact was consid- 
ered by Alvord and others (1976, p. 320) to be conform- 



STRATIGRAPHY OF THE NASHOBA ZONE, EASTERN MASSACHUSETTS 



F9 



able, as were the Fish Brook-Boxford and the Billerica 
Schist-Bellows Hill contacts. In their view, the numer- 
ous faults shown by Bell and Alvord (1976, fig. 4) have 
done little to disrupt the sections. Yet, looking at the 
sections as presented in figure 3, we see that a thick 
wedge of rock comprising the Shawsheen Gneiss, the 
Fish Brook Gneiss, and possibly the lower part of Bell 
and Alvord's Nashoba section, including the Boxford 
Member, is missing in the tunnel section. One explana- 
tion is that the Shawsheen and Fish Brook are strati- 
graphic lenses, although this seems unlikely because the 
truncation is extremely angular. Another explanation, 
despite the statement of conformity by Bell and Alvord 
(1976), is that these units constitute tectonic slices (Gold- 
smith, this vol., chap. H). Possibly the larger faults, 
although they do not everywhere determine the forma- 
tional boundaries, have sliced once-continuous sections of 
rock into disjointed lenses, so that although masses of 
like rock may be in contact with each other, they have 
been transported there from some other part of the 
strike belt. 

A closer look at possible correlations between the 
sections may resolve the problem of thickness and appar- 
ent loss of section. A number of correlation schemes are 
possible. The most obvious are shown on figure 3 as solid 
lines; tielines of the more questionable correlations are 
marked by queries. A reasonable correlation can be made 
for the upper part of the Nashoba sections of Bell and 
Alvord (1976) and of Skehan and Abu-Moustafa (1976). 
However, the Nashoba Brook, Tophet Swamp Gneiss, 
Spencer Brook, and Billerica Schist Members are not 
recognizable in the tunnel section. These units may be 
cut out by the Spencer Brook fault. Possibly the Nashoba 
Brook is a lens or fold nose, and the Tophet Swamp is 
equivalent to part of N7-N10. The Bellows Hill Member 
is correlated with units N1-N6 in the lower part of the 
Nashoba in the tunnel section. The Boxford Member, the 
Fish Brook Gneiss, and the Shawsheen Gneiss lens out or 
are cut out. Possibly the Shawsheen Gneiss is equivalent 
to Hepburn and DiNitto's rusty schist member of the 
Marlboro. Their granulite member may be a facies 
equivalent of the Fish Brook Gneiss. Neither of these 
units is clearly recognizable in the tunnel section. Bell 
and Alvord's Sandy Pond Member of the Marlboro 
Formation can be correlated (possibility 2 of fig. 3) with 
units M5-M16 (?) of the tunnel section, a correlation 
supported by lithologic similarity and by the mapping of 
Hepburn and DiNitto (1978) and Hepburn (1978); this 
correlation, however, requires accounting for lack of 
equivalents in Bell and Alvord's section to the interval 
M17-M31 of Skehan and Abu-Moustafa's section. The 
missing upper part of the Marlboro in Bell and Alvord's 
section may be replaced by granite (SOagr) as suggested 
by the State bedrock map. A third possibility (possibility 



3, fig. 3), is that Bell and Alvord's Marlboro sequence is 
inverted by folding, in which case a fault is required 
between the Shawsheen and Marlboro. As Hepburn and 
DiNitto's section approximately corresponds on the sur- 
face to the tunnel section below, the discrepancy in 
detailed description of the units in the two sections, 
particularly between the interval M25-M31 of the tunnel 
section and the granulite member, can be construed to 
mean that the kinds of rock seen in scattered exposures 
on the surface in the zone of weathering do not necessar- 
ily represent the relative abundances of lithologies that 
can be seen in a corresponding section of rocks under- 
ground. Both Hepburn and DiNitto's section and Bell 
and Alvord's section are based on surface exposures, so 
it is not surprising that these two correspond somewhat 
better than either does to the tunnel section. 

Other possible correlations that require complicated 
structure equate the Shawsheen Gneiss and Boxford 
Member with tunnel units N1-N5 and N6, or alterna- 
tively the Shawsheen and Sandy Pond with units 
N7-N10 and N6 in reverse order. The Fish Brook again 
would have to terminate in some way, but this is not 
difficult if it is intrusive. None of the possible correla- 
tions is wholly satisfactory, and a fault solution seems the 
most promising. 

A correlation suggested by the lithostratigraphic col- 
umns of figure 3 involves repetition by faulting (fig. 4A). 
In this scheme, the Boxford is equated with the Sandy 
Pond Member, the Shawsheen of slice A is repeated 
above the Assabet River fault in slice B as the Bellows 
Hill member and above the Spencer Brook fault in slice C 
as the Billerica Schist and Spencer Brook members. 
Fault slice C could consist of other fault slices as the 
numerous faults in the sections on figure 3 suggest. 

This scheme depends on the validity of the correlation 
of the Boxford with the Sandy Pond. I have pointed out 
above that the Boxford section of Castle (1965) resembles 
Bell and Alvord's (1976) Marlboro section. Castle (1965) 
pointed out the similarity of the Boxford to isolated 
amphibolitic rocks to the southwest that he mapped as 
Marlboro. These isolated rocks of Castle are apparently 
on strike with the type belt of Marlboro extending from 
Marlborough to Concord. Castle's upper and lower Box- 
ford Formation are similar to Bell and Alvord's upper 
and lower Marlboro, although seemingly in reverse 
sequence to the Marlboro in the tunnel section. The 
Boxford Member of the Nashoba of Bell and Alvord thus 
could be part of the Marlboro repeated by faulting but 
representing the opposite limb of an earlier fold. The 
Fish Brook becomes part of the Marlboro Formation, in 
a stratigraphic position somewhat similar to that of the 
Grafton gneiss to the south. Barosh (1976, p. 309) noted 
rock resembling Fish Brook Gneiss near the top of the 
Marlboro near the intersection of I-A95 and 1-290 west of 



F10 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 




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STRATIGRAPHY OF THE NASHOBA ZONE, EASTERN MASSACHUSETTS 



Fll 



Marlborough. Possibly Hepburn and DiNitto's granulite 
member of the Marlboro is a fades of the Fish Brook 
Gneiss and their rusty schist member is equivalent to the 
Shawsheen Gneiss. If so, the Assabet River fault must 
cut below the granulite member, as suggested on figure 
4A Hepburn and DiNitto's Marlboro section correlates 
fairly well with the Quinebaug section of Dixon (1965a; 
fig. 3). If this correlation is carried across to the sections 
of Skehan and Abu-Moustafa and Bell and Alvord (fig. 
4A), the Shawsheen Gneiss, units N1-N6, and the Bel- 
lows Hill, Spencer Brook, and Billerica Schist Members 
of the Nashoba would be assigned to the Marlboro rather 
than to the Nashoba. 

The correlation scheme presented in figure 4A is, then, 
probably only one of several possible. The simplest 
possible scheme would be based on the interpretation 
that the faults have done little to disrupt the Marlboro- 
Nashoba sequence, as was implied by Alvord and others 
(1976) and that the sequence is homoclinal. The map 
pattern of Bell and Alvord (1976) can then be interpreted 
as showing a cross-sectional view of a metamorphosed 
stack of lenticular and intertonguing sedimentary, vol- 
caniclastic, and volcanic rocks. Certainly intertonguing 
fades would be expected in the depositional environment 
of the protoliths described earlier; however, the indica- 
tions of complex structure mentioned above in the sec- 
tion on thickness and sequence seem to me to make this 
view oversimplified. 

It seems clear from figure 3 that no consistently 
applied boundary exists between the Marlboro and the 
Nashoba. Possibly Bell and Alvord (1976) are correct in 
implying that the Boxford is a tongue of amphibolite in 
the Nashoba lying well above the Sandy Pond of the 
Marlboro and that the Shawsheen is a tongue of 
Nashoba-like schist below the Sandy Pond. The bound- 
ary between the Marlboro and the Nashoba can only be 
placed, then, as it has been in the past, where sequences 
containing abundant amphibolite give way to sequences 
in which pelitic schist and gneiss predominate. My sug- 
gested assignment of the boundary, based on the inter- 
pretation of figure 45, is that the top of the Sandy Pond 
and the Boxford (of Bell and Alvord, 1976) are more 
clearly defined boundaries than any of those lying above, 
such as the top of the Spencer Brook Member or the top 
of N6 of the tunnel section, and thus should mark the top 
of the Marlboro Formation. 

The inconsistent thicknesses of major units within the 
Nashoba zone, the lenticular map patterns of units, the 
repetition of rock types, and the difficulties in correlation 
of map units along strike demonstrated in the above 
discussion suggest that, rather than subdividing the 
Nashoba Formation into many members or dividing the 
zone into many formations, it would be preferable to 
keep only units that are distinct or readily recognizable 



throughout the zone. This approach has been taken on 
the State bedrock map, where the column of Bell and 
Alvord has been used (fig. 3). On this map, the Nashoba 
and Marlboro are obvious units of formational rank, and 
the Fish Brook Gneiss and Shawsheen Gneiss are 
retained as discrete units also of formational rank. The 
Boxford Member of the Nashoba is the only one of Bell 
and Alvord's members considered distinct enough to be 
shown on the map. Actually, the Fish Brook and Shaw- 
sheen are limited in area and should not have formational 
rank. The Fish Brook and Shawsheen were given forma- 
tional rank by Bell and Alvord (1976) because the Fish 
Brook has a distinctive lithology and separates the 
Shawsheen from the Nashoba to the west. However, 
they saw no reason why the Shawsheen could not other- 
wise be considered part of the Nashoba. Barosh and 
others (1977), in their compilation of the Boston l°x2 c 
sheet, placed the Shawsheen in the Nashoba, as did 
Hansen (1956) originally. Accordingly, in figure 45, I 
have placed the Shawsheen in the Nashoba. Until unified 
detailed mapping is done in the Nashoba zone, and the 
scheme of figure 45 tested, the order of units probably 
should be kept as on the State bedrock map. 

The weight of evidence seems to show that the present 
distribution of units within the zone is in large part 
controlled by structure. Folding may be partly responsi- 
ble, but I would suggest that the pattern can best be 
explained by faults, along some of which appreciable 
differential lateral movement has occurred. One can 
easily visualize that formerly continuous masses of rock 
have been sliced and transported into disjointed lenses so 
that masses of rock from different parts of the section are 
now juxtaposed. These masses have subsequently been 
given separate formational or member names by people 
mapping in different parts of the strike belt. A uniform 
effort of detailed mapping in the Nashoba zone may 
clarify some of the uncertainties. 



AGE 

The stratified rocks of the Nashoba zone are unfossil- 
iferous; their age can only be approximated within limits 
determined by radiometric data on rocks that intrude 
them. Upper limits on their age are established by ages 
on the Sharpners Pond Diorite (Ssqd) and phases of the 
Andover Granite (SOagr). The Sharpners Pond is non- 
foliated and gives a concordant U-Pb age of 430 ±5 Ma 
(Zartman and Marvin, this vol., chap. J, table 1), or early 
Silurian. The Andover Granite is in part foliated and, 
from field relations, is believed to be older than the 
Sharpners Pond. The most reliable age from what 
appears to be the oldest part of the Andover places it in 
the Ordovician or possibly early Silurian, on the basis of 



F12 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



Rb-Sr whole-rock ages of 450 ±23 Ma (Zartman and 
Marvin, this vol., chap. J, table 1; Handford, 1965). Thus 
the youngest possible age for the rocks of the Nashoba 
zone is Ordovician. A minimum age of Ordovician for the 
Tatnic Hill Formation (equivalent to Nashoba on strike 
in southeastern Connecticut) was determined by Zart- 
man and others (1965, p. D6). 

The stratified rocks of the Nashoba zone are probably 
as old as Proterozoic. Olszewski (1980) obtained a 
730±26-Ma age on a concordia plot of apparently igneous 
zircons from the volcanic (or plutonic) Fish Brook Gneiss. 
If the Fish Brook Gneiss is a core gneiss, as Castle (1965) 
suggested, then the zircons date only that unit and not 
necessarily the surrounding strata. However, field rela- 
tions seen by Bell and Alvord (cited above in the section 
on descriptions of units) indicate that the Fish Brook is 
part of the Nashoba-Marlboro sequence. A concordia plot 
of probably detrital zircons in the Shawsheen Gneiss 
gave an age of more than 1,500 Ma, thus dating a source 
terrane, proto-Africa(?), not now present, to the east. 
Olszewski (1980, p. 1414) pointed out that the pattern of 
ages in the Nashoba and Milford-Dedham zones is similar 
to the pattern in northwest Africa as described by 
Schenk (1971) and Hurley and others (1974). He also 
pointed out that the detrital zircons could be derived 
from the Grenville terrane to the west as well as from a 
proto- African source to the east. 

Skehan and Murray (1980, fig. 3) suggested that the 
Tadmuck Brook Schist, which overlies the Nashoba 
Formation, is Ordovician(?) to Cambrian in age, on the 
basis of the unconformity at its base and an Ordovician to 
Silurian age on the Ayer Granite that intrudes the units 
above the Nashoba in and west of the Clinton-Newbury 
fault zone. This suggestion is discussed in the chapter on 
the Merrimack belt (Robinson and Goldsmith, this vol., 
chap. G). 

The rocks of the Nashoba zone are shown on the State 
bedrock map as Ordovician or Proterozoic Z. This dual 
age was given because at the time the map was prepared, 
I was uncertain about the actual rocks sampled by 
Olszewski (1978), and because I held a strong belief in a 
correlation of rocks of the Nashoba zone with similar 
rocks to the west considered to be Ordovician in age (see 
below). Now, because of the radiometric ages, I believe 
that the rocks of the Nashoba zone, except possibly for 
the Tadmuck Brook Schist, could be Proterozoic. If so, 
however, they are unlike the Proterozoic rocks in the 
Milford-Dedham zone to the east (Goldsmith, this vol., 
chap. E). 



REGIONAL CORRELATIONS 

The rocks of the Nashoba zone can with some confi- 
dence be correlated with units along strike in Connecti- 



cut and in southeastern Maine (figs. 5, 6). They can be 
correlated with less confidence with rocks across strike 
in the Merrimack belt, with those on the east flank of the 
Bronson Hill anticlinorium in east-central Massachu- 
setts, Connecticut, and southwestern New Hampshire, 
and with the Massabesic Gneiss Complex of northern- 
most central Massachusetts and adjacent New Hamp- 
shire. 

The Nashoba and Marlboro Formations are equivalent 
to the Tatnic Hill and Quinebaug Formations of eastern 
Connecticut, respectively (figs. 3, 5, 6). The Nashoba 
thins near Oxford but is continuous with the Tatnic Hill 
to the south, although the passage near Webster is along 
a number of closely spaced fault slices (Barosh, 1974; 
H.R. Dixon, written commun., 1978). The Marlboro and 
Quinebaug are not continuous across the salient between 
Webster and Oxford, but the two have similar lithologies 
and occupy the same position with respect to the Na- 
shoba and Tatnic Hill. Emerson (1917) and Dixon (1965a) 
initially correlated the two sequences, and the correla- 
tion is now generally accepted (Barosh, 1977; Barosh and 
others, 1977; H.R. Dixon, written commun., 1978). 
Detailed lithologic correlation of members within the 
formations cannot be made between the two terranes 
(Nashoba and Putnam). However, similar groups of 
lithologies occupy similar structural positions (fig. 3). 
Dixon (written commun., 1978) recognized units of the 
Tatnic Hill in the Nashoba in the Oxford and Grafton 
areas during reconnaissance mapping for the State bed- 
rock map. The Fly Pond Member of the Tatnic Hill might 
correspond to either unit N29 or units N 12-20 of the 
tunnel section (Skehan and Abu-Moustafa, 1976). The 
Quinebaug, on the whole, however, appears to contain 
less amphibolite than the Marlboro. In Connecticut, the 
Tatnic fault (Wintsch and Hudson, 1978) separates the 
Tatnic Hill and the Quinebaug Formation in many places 
(Dixon, 1965b, 1968, 1974). This fault appears to be a 
bedding-plane fault that lies close to the normal strati- 
graphic contact between the two formations. Interlayer- 
ing of rock types that does not appear to be tectonic is 
common in the contact zone, and a rusty schist that 
Dixon considered to be the basal part of the Tatnic Hill 
(Dixon, 1965a) can be recognized at many places along 
the contact zone in Connecticut and at the base of the 
Nashoba in Massachusetts from Webster to Shrewsbury. 
The Tatnic Hill is much thinner than the Nashoba 
Formation. This thinning could be stratigraphic but more 
likely is tectonic, attributable to faulting along the Lake 
Char, Honey Hill, and Tatnic fault systems in Connect- 
icut (Goldsmith, this vol., chap. H). 

Correlation of rocks of the Nashoba zone with rocks to 
the northeast in southern Maine and New Hampshire 
(figs. 5-7) must be made across a gap. The Nashoba 
Formation in the Lowell area is truncated against the 



STRATIGRAPHY OF THE NASHOBA ZONE, EASTERN MASSACHUSETTS 

72° 70° 



F13 



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CANADA 

UNITED STATES 



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terrane 



ATLANTIC OCEAN 



vp^* RHODE 
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30 40 50 60 MILES 



r~n — r — V^ 

40 50 60 70 B0 90 KILOMETERS 



Figure 5. — Terranes in New England possibly correlative with the Nashoba zone. Terrane boundaries dashed where uncertain. 



F14 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



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STRATIGRAPHY OF THE NASHOBA ZONE, EASTERN MASSACHUSETTS 



F15 



Jewell 
Formation 



Scarboro 
Formation 



Diamond 

Island 
Formation 



Spring 

Point 

Greenstone 



Cape 
Elizabeth 
Formation 



Cushing 
Formatiofi 



Hussey (1968, 1971) 
Southern Maine 



Sulfidic and nonsulfidic two-mica schist 
containing thin beds of mica schist and 
quartzite 



0-120 



>600 



Ribbon limestone containing thin beds of 

biotite, phyllite, and schist 

300- \\\ Same as Jewell Formation 

1500 \\ Laminated black, sulfidic quartz-schist 

and phyllite 

Chlorite schist, actinolite schist (or 
amphibolite); feldspathic quartzite 

and metafelsite at top 

Muscovite-biotite-plagioclase-quartz 
schist, micaceous quartzite and lenses 

of calc-silicate rock 

Biotite-quartz-feldspar gneiss, amphibolite, 
biotite-feldspar-quartz schist and 
gneiss, calc-silicate gneiss; minor marble 
and sulfidic two-mica quartz schist 

Hussey (1968) 
Southeastern New Hampshire 

Thickness 
(meters) 



Upper 
member 


300- 
600 


fault- 

Porphyroblastic biotite-quartz-feldspar 
gneiss containing thin beds of feldspathic 
metagraywacke; felsic metatuff in upper 
part; includes amphibolite, calcareous 
metagraywacke, marble, and graphitic 
phyllite 


Lower 
member 


>300 


(Staurolite)-(sillimanite)-muscovite- 
biotite-feldspar-quartz schist; feldspar 
content varies 



Osberg (1979) 
East-central Maine 



Thickness 
(meters) 



Cushing 
Formation 



SCALE. IN METERS 
Hussey (1968, 1971), Osberg (1979) 



100 
200 
300 
400 
500 
- 1 - 600 



SCALE, IN METERS 
Novotny (1969) 

-r o 

200 

400 

600 

800 
-- 1000 
— 1200 




Rusty-weathering quartz mica schist and 
phyllite, and sparse quartz-feldspar 
layers; contains garnet and sillimanite 
Lenticular marble and calc-silicate rock 
Slightly rusty-weathering garnet- 
(sillimanite)-quartz-mica schist 

Upper massive biotite-quartz-plagioclase 
gneiss; medial interlayered biotite-quartz- 
plagioclase gneiss and biotite amphibolite 



Novotny (1969) 
Southeastern New Hampshire 

Thickness 
(meters) 









fault 
















Quartz-biotite-plagioclase gneiss 




§ i 




containing laminae of quartz-biotite 




ro -2 




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1600 1 


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quartz-feldspar-mica schist; feldspathic 






and garnetiferous quartzite; (diopside)- 




|E 




(garnet) amphibolite 




I 








_j 




Thickness calculated from cross sections 
on map of Novotny (1969) 



Figure 7. 



-Lithostratigraphic columns of units in southern and east-central Maine and southeastern New Hampshire that are probably 
correlative with units of the Nashoba zone. 



Clinton-Newbury fault and is largely engulfed in granite 
of the Andover pluton, so that only the Boxford Member 
of the Nashoba and the Fish Brook Gneiss extend to the 
Newburyport area and the Gulf of Maine. The Marlboro 
is similarly truncated or cut out to the northeast. How- 
ever, a rock sequence similar to the Nashoba is present 
in southern Maine from south of Augusta at least as far 
as the Brunswick area (figs. 5-7). These rocks have been 



mapped by Osberg (1979) as the Cushing Formation, 
part of the Casco Bay Group of Katz (1918) redefined by 
Hussey (1962, 1968, 1971). The age of the Cushing is 
uncertain, but the unit is considered to lie disconform- 
ably below the Silurian turbidite section of eastern and 
central Maine (Osberg, 1979). The Mine Hill Formation 
of Osberg (1979) and a similar sulfidic schist mapped by 
Hussey (1971) in Casco Bay intervene between the 



F16 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



Cushing and the turbidite section. These sulfidic schists 
are similar in lithology and occupy the same stratigraphic 
position relative to the Cushing as the Tadmuck Brook 
Schist does to the Nashoba (fig. 6), as pointed out by 
Osberg (1979, p. 42). The Cushing terrane in Maine is, 
however, appreciably offset to the west from the trend of 
the Nashoba zone in Massachusetts (fig. 5). Farther 
north, in the Belfast area, Maine, the Passagassawakeag 
Formation as described by Bickel (1976) appears to 
resemble the Nashoba Formation. The Passagas- 
sawakeag is considered to be probably Precambrian in 
age. Far to the north, Kennedy's (1976) description of 
rocks in the Gander zone of Newfoundland is remarkably 
similar to the descriptions of the rocks in the Nashoba 
zone. A specific correlation might be made between the 
Nashoba Formation and the Little Passage Gneisses of 
the Gander zone (Colman-Sadd, 1980). 

Similarities in structural position and in lithology 
indicate that parts of the Marlboro Formation are quite 
likely equivalent to the Rye Formation of southeastern 
New Hampshire and Maine (figs. 3, 6, 7). Castle (1965) 
suggested correlation of his Boxford Formation with the 
Rye Formation. Novotny's (1969) Rye section (fig. 7) is 
similar to, if somewhat thicker than, that of Hussey 
(1968), although the thickness may be an artifact of my 
calculations of thickness from Novotny's map and sec- 
tions. Both Rye sections resemble in lithology the part of 
Hepburn and DiNitto's Marlboro section above their 
Sandy Pond amphibolite member (fig. 3). The upper 
member of the Rye resembles Hepburn and DiNitto's 
granulite member of the Marlboro and the upper member 
of the Quinebaug Formation of Dixon (1965a) and resem- 
bles the undivided part of the Marlboro Formation of Bell 
and Alvord (1976) that lies below their Sandy Pond 
Member. The lower member of the Rye resembles to 
some extent Hepburn and DiNitto's rusty schist member 
of the Marlboro and the Shawsheen Gneiss as described 
by Bell and Alvord (1976). Correlation of the Marlboro 
Formation with the Rye Formation is not clear cut. 

Correlation of the rock units of the Nashoba zone with 
units west of the Nashoba strike belt, such as those of 
Peper and Pease (1976) (fig. 6) and the units on the east 
flank of the Bronson Hill anticlinorium (Robinson, 1979), 
is less certain. Prior to the isotopic work of Olszewski 
(1978, 1980), the Nashoba was considered to be of 
Ordovician age, on the basis of a more or less continuous 
mapping of rusty-schist lithologies from the Ordovician 
Partridge Formation, on the west flank of the Bronson 
Hill anticlinorium in New Hampshire (fig. 5), southward 
through the Brimfield Schist terrane in Massachusetts, 
where the rusty schists on the east flank of the anticli- 
norium and in the Merrimack synclinorium have been 
called Brimfield Schist (Emerson's (1917) term; Brim- 
field Group of Peper and Pease (1976)) and, in the Middle 



BRONSON HILL 
ANTICLINORIUM 



MERRIMACK SYNCLINORIUM 
Hebron and equivalent formations 

(including equivalents of Paxton 

and Oakdale Formations of 

Massachusetts) 



SOs SOh 



ligelow Brook SOh: Hebron Formation 
Formation SOs: Southbridge 

Formation 



Brimfield Schist and equivalent formations 



Och 

Collins Hill Formation 

( = Partridge Formation 

of New Hampshire) 

1 Om | 

Middletown 
Formation 


Obr | 


1 0ta 1 


Brimfield Schist 
(including Hamilton 
Reservoir Formation) 


Tatnic Hill 
Formation 


Oq 

Quinebaug 
Formation 



| Omo | 



Monson Gneiss 



AVALONIAN TERRANE 

( Proterozoic Z ) 



Figure 8. —Stratified lower Paleozoic rocks in eastern Connecticut, 
shown by Rodgers (1982). 



Haddam area, the Collins Hill Schist (Snyder, 1970; 
Eaton and Rosenfeld, 1972; Collins Hill Formation of 
Rodgers, 1982, on fig. 8) around the Chester syncline 
(Dixon and Lundgren, 1968) above the Honey Hill fault 
into the Tatnic Hill Formation of eastern Connecticut 
and thence north into the Nashoba Formation. Emerson 
(1917) mapped rusty schists in the Nashoba as Brimfield. 
Peter Robinson (oral commun. , 1979; Hall and Robinson, 
1982, p. 27) pointed out that the Tatnic Hill and Nashoba, 
although consisting of lithologies similar to those in the 
Partridge Formation on the Bronson Hill, contain appre- 
ciable magnetite and represent a more oxidized meta- 
morphic fades than does the Partridge. The Quinebaug 
(Marlboro) underlying the Tatnic Hill (Nashoba) was 
equated with the Middletown Gneiss on the east flank of 
the Bronson Hill in Connecticut (Dixon and Lundgren, 
1968), which in turn was equated by them with the 
Ammonoosuc Volcanics underlying the Partridge of cen- 
tral Massachusetts and western New Hampshire. This 
interpretation of the relations was followed by Rodgers 
(1982) in the preliminary bedrock geologic map of Con- 
necticut from which figure 8 has been adapted. 

The continuity on which the above correlation is based 
has been questioned recently. Wintsch (1979a; Wintsch 
and Kodidek, 1981) presented evidence that the Tatnic 



STRATIGRAPHY OF THE NASHOBA ZONE, EASTERN MASSACHUSETTS 



F17 



Hill is not continuous with the rusty schists in the 
Chester syncline and with the Brimfield and Collins Hill 
Schists to the west on the Bronson Hill. Thrust faults 
beneath the Brimfield Group (Peper and others, 1975) 
project into a high-angle fault, the Bone Mill Brook fault 
(Peper and others, 1975; Pease and Fahey, 1978; Pease, 
1982) along the east side of the Bronson Hill anticlino- 
rium in Connecticut, which creates a fault wedge of 
Emerson's type area of Brimfield Schist that is separate 
from the Bronson Hill sequence. However, Robinson and 
Tucker (1982) saw no evidence farther north in Massa- 
chusetts for a major fault along the east flank of the 
Bronson Hill; they equated the rusty schists of the 
Brimfield Group, Peper and Pease's (1976) Hamilton 
Reservoir Formation, with the Partridge, but in a thick- 
ened, basinward section. The Hamilton Reservoir, from 
detailed descriptions by Peper and Pease (1976) and 
Peper and others (1975), does not correlate particularly 
well with the Tatnic Hill Formation, but in general it 
does somewhat resemble Bell and Alvord's (1976) 
description of the Nashoba. Regional considerations sug- 
gest to some (Snyder, 1970, for example) that the 
Brimfield Group is inverted, although this does not affect 
the correlation. In support of this inversion, the Bigelow 
Brook Formation of Peper and Pease (1976) passes along 
strike into the Paxton Formation in Massachusetts 
where it overlies the Partridge-type rocks on the east 
flank of the Bronson Hill anticlinorium (Peter Robinson, 
oral commun., 1978). A U-Pb radiometric age of 440 Ma 
on zircon from a granite gneiss intruding the Hamilton 
Reservoir Formation (Pease and Barosh, 1981, p. 23) 
indicates an Ordovician or older age for this part, at 
least, of the Brimfield Group. G.R. Robinson, Jr. (writ- 
ten commun., 1982), has suggested that possibly only the 
Tadmuck Brook Schist is correlative with the Partridge 
and is Ordovician in age. This suggestion is in accord 
with Skehan and Murray's (1980) interpretation of a 
Cambrian-Ordovician(?) age for the Tadmuck Brook. 

Rocks similar to parts of the Nashoba have been 
observed in the Massabesic Gneiss Complex of south- 
central New Hampshire and adjacent Massachusetts 
(Massabesic terrane of fig. 5) within what has previously 
been mapped as the Fitchburg pluton (Billings, 1956). 
The Massabesic Gneiss Complex also contains rocks 
resembling the Ordovician Monson Gneiss of the Bronson 
Hill anticlinorium and the Quinebaug Formation in the 
Willimantic Dome in east-central Connecticut. U-Pb 
isotopic data on zircons obtained from the Massabesic, 
however, indicate both Proterozoic Z (645-Ma) and Ordo- 
vician (480-Ma) ages, as well as Permian (275-Ma) ages in 
late granite (Besancon and others, 1977; Aleinikoff and 
others, 1979). The Massabesic lies in an anticlinal area 
within the Merrimack synclinorium, northwest of the 
Nashoba zone. Its structural position relative to the 



Nashoba zone has yet to be determined. The Massabesic 
Gneiss Complex is described by Peter Robinson in a 
separate section at the end of this chapter. 



SUMMARY AND CONCLUSIONS 

The Nashoba zone is a fault-bounded wedge consisting 
of high-grade, steeply west-dipping metamorphic rocks, 
largely metasedimentary to the west (Tadmuck Brook 
Schist, SZtb; Nashoba Formation, OZn; Fish Brook 
Gneiss, OZf; and Shawsheen Gneiss, OZsh) and largely 
metavolcanic to the east (Marlboro Formation, OZm, 
OZmg). These rocks occupy a terrane distinct from the 
Merrimack belt to the west and the Milford-Dedham zone 
to the east. Complex faulting and folding and much 
granite intrusion hinder determination of a stratigraphic 
sequence throughout the zone. However, two major 
formations, presumably topping to the west, are recog- 
nized: the Nashoba Formation, consisting of pelitic and 
semipelitic (metawacke) gneiss and schist and subordi- 
nate calc-silicate rock, marble, and amphibolite, some of 
which was formed from impure carbonates; and the 
Marlboro Formation, consisting of amphibolite, feld- 
spathic gneiss, and subordinate pelitic schist and calc- 
silicate rock. The Tadmuck Brook Schist, consisting of 
pelitic schist and phyllite and subordinate quartzite, lies 
above the Nashoba, probably unconformably. The Shaw- 
sheen Gneiss, consisting of pelitic and semipelitic schist 
and gneiss similar to parts of the Nashoba, lies above the 
Marlboro. Above the Shawsheen and below the Nashoba 
is the Fish Brook Gneiss, a lenticular, felsic metavolcanic 
or intrusive rock. Possible assignment of stratigraphic 
units that differs from that shown on the State bedrock 
map is based on the similarity in lithology of the Shaw- 
sheen Gneiss to units in the Nashoba and on the obser- 
vation that the Fish Brook Gneiss appears to be lentic- 
ular. According to this scheme, the Shawsheen and the 
Fish Brook would become members of the Nashoba 
Formation. A third scheme that can be deduced from the 
fault pattern within the zone (figs. 1, 4) may be worth 
consideration. Under this scheme, the Boxford Member 
of the Nashoba correlates with the Sandy Pond Amphib- 
olite Member of the Marlboro. If the Boxford is reas- 
signed to the Marlboro, the Fish Brook Gneiss, lying 
below the Boxford Member and above the Assabet River 
fault, becomes the lowest unit in the Marlboro. The 
Shawsheen, lying in a different fault block below the 
Assabet River fault, could be assigned to the Nashoba. 

The protoliths of the Marlboro Formation were vol- 
canic, volcaniclastic, and epiclastic materials deposited in 
a marine environment close to a volcanic source to the 
east. The protoliths of the Nashoba Formation were 
volcaniclastic, epiclastic, and minor volcanic materials 



F18 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



and carbonate rocks deposited in the basin farther from 
the volcanic center. The minor volcanic component was 
primarily basaltic but also included andesitic, dacitic, and 
rhyodacitic materials. The Fish Brook Gneiss is a prom- 
inent felsic, probably volcanic unit. The material in the 
zone seems to have been derived from both a deeply 
weathered terrane and a volcanic source, according to 
Abu-Moustafa and Skehan (1976). A 1,500-Ma age for 
detrital zircon in the Shawsheen Gneiss (Olszewski, 1980) 
indicates an old source area not now recognized. A source 
of zircons of this age can be found in northwest Africa. 
No rocks of this age are known in the Grenville terrane 
to the west. Nearer at hand is the allochthonous Chain 
Lakes massif, western Maine (fig. 5), from which a 1.5- to 
1.6-Ga age is reported on zircon (Naylor and others, 
1973; Boudette and Boone, 1982). The rocks of the 
Nashoba zone may have been derived from two sources, 
but if they came from one, the African source seems most 
likely, on the basis of Olszewski's work. 

Igneous zircons from the Fish Brook Gneiss indicate a 
Proterozoic Z age (at least 750 Ma (Olszewski, 1980)), 
older than those so far recorded in the batholithic rocks 
of the Milford-Dedham zone to the east. Unless the Fish 
Brook is intrusive, this is a possible age for the Nashoba 
Formation. The pattern of volcanic and detrital ages is 
similar to that in northwest Africa in both time of 
volcanism and source of detritus according to Olszewski. 
The age of the rocks of the Nashoba zone therefore 
appears to be Proterozoic Z. The rocks can be no younger 
than Ordovician because they are intruded by the 
Andover Granite, parts of which could be as old as 
Ordovician (Zartman and Marvin, this vol., chap. J, table 
1). A strongly argued correlation with some of the rocks 
of the Brimfield Group to the west on the flanks of the 
Bronson Hill anticlinorium suggests an Ordovician age, 
but the pattern of deposition and the zircon age would 
indicate that the Nashoba zone belongs to the eastern 
basement, as indicated by Osberg (1978), rather than to 
North American basement or to a lower Paleozoic vol- 
canic island arc. 

Rocks of the Nashoba zone occupy the same strike belt 
as the Putnam Group of eastern Connecticut. To the 
north, the Nashoba Formation is correlated with the 
Cushing Formation of the Casco Bay Group in Maine 
although it is offset to the southeast in strike from the 
Cushing terrane (fig. 5). The Marlboro Formation is 
correlated with the Rye Formation in southeastern New 
Hampshire and Maine. Correlation with rocks in other 
lithotectonic belts of the Appalachians to the west across 
strike is uncertain. The rocks may be equivalent to units 
in the Bronson Hill anticlinorium and to parts of the 
Massabesic Gneiss Complex of southern New Hampshire 
and adjacent Massachusetts. If so, considerable struc- 
tural complexity exists across this part of the orogen 



(Goldsmith, this vol., chap. H). If not, or perhaps even 
so, the Nashoba zone represents an exotic, probably 
accretionary terrane that was formerly a part of north- 
west Africa. 



MASSABESIC GNEISS COMPLEX (OZma) 

By Peter Robinson 1 

The Massabesic Gneiss Complex (OZma) of northern- 
most central Massachusetts and adjacent New Hamp- 
shire (figs. 1, 2) forms an isolated area of Proterozoic Z 
and Ordovician rocks within the broad belt of Silurian 
and Devonian strata of the Merrimack synclinorium (fig. 
5). The Massabesic Gneiss Complex consists of layered 
Proterozoic Z gneisses and Ordovician granitic gneisses. 
These rocks are abundantly intruded by fine-grained, 
pink biotite granite of Pennsylvanian-Permian age (Pgr). 
Practically none of the rocks in this region are related to 
the Fitchburg Complex (Dfgr), which is a complex of 
sheetlike plutons of Devonian age intruding Silurian and 
Devonian strata in a broadly synclinal region west and 
southwest of the area of exposed Massabesic Gneiss. The 
Massabesic is exposed in an anticline, produced during 
Acadian deformation, that plunges south, so that the 
Massabesic is exposed only at the north edge of the State 
bedrock map. On the State bedrock map, the Massabesic 
is assigned to the Nashoba zone of pre-Silurian strata to 
the east, rather than to the Bronson Hill zone to the 
west, for the following reasons: It is geographically 
closer to the Nashoba zone than to the Bronson Hill zone, 
and the lithology and tectonic position of the Massabesic 
vaguely resemble those of the Quinebaug Formation in 
the Willimantic dome of southeastern Connecticut; the 
Quinebaug is correlated with the Marlboro Formation of 
the Nashoba zone. The fault relationships described in 
the Willimantic dome (Wintsch, 1979b) have not, so far, 
been identified in the Massabesic, although, like the 
Willimantic, the Massabesic has the form of an anticline. 

Stratigraphic classification of the Massabesic has long 
been controversial and was undergoing very rapid 
change as the State bedrock map was being compiled. 
Billings (1956) classified the Massabesic as part of the 
Fitchburg pluton of Devonian age. Later investigations 
of some of the gneissic Massabesic rocks in the Manches- 
ter, N.H., area (Besancon and others, 1977) yielded 
Proterozoic Z ages. In 1976-77, mapping in the 
Townsend (Mass.-N.H.) area (Robinson, 1978) and 
reconnaissance mapping in the Ashby (Mass.-N.H.) area 
(Peper and Wilson, 1978) showed all the granitoid rocks 



'Department of Geology, University of Massachusetts, Amherst, MA 01003. 



STRATIGRAPHY OF THE NASHOBA ZONE, EASTERN MASSACHUSETTS 



F19 



as part of a Devonian plutonic complex (Fitchburg Com- 
plex). Such an interpretation seemed assured, on the 
basis of more detailed mapping, in the rest of Massachu- 
setts (Grew, 1970; Hepburn, written commun., 1976; 
Tucker, 1977; Peper and Wilson, 1978), where the Fitch- 
burg pluton was seen as a complex of granite and 
granodiorite-tonalite sills (Maczuga, 1981) intruding Silu- 
rian and Devonian strata (Tucker, 1977). A radiometric 
age of 390±15 Ma on zircon (Zartman and Marvin, this 
vol., chap. J, table 1) from the probably slightly younger, 
massive muscovite-biotite granite at Rollstone Hill, 
Fitchburg (Peper and Wilson, 1978; Maczuga, 1981), 
appears to support this view. 

The Massabesic Gneiss Complex shown on the State 
bedrock map is based on the detailed mapping and 
radiometric dating of Aleinikoff (1978) in the Milford, 
N.H., area, just north of the Massachusetts-New Hamp- 
shire border; on field excursions in 1978 with Aleinikoff 
and J.B. Lyons in the Milford area and the adjacent 
Townsend, Mass.-N.H., area; and on reconnaissance 
reinterpretation in 1978 by Peter Robinson of previous 
work by G.R. Robinson (1978) and Peper and Wilson 
(1978) in the Townsend and Ashby areas. 

The work of Aleinikoff (1978; Aleinikoff and others, 
1979) in the Milford area showed that the area mapped in 
the previous literature as Massabesic Gneiss contains no 
major amount of Devonian plutonic rocks but consists of 
three major components: (1) layered plagioclase-quartz 
gneisses believed to be metamorphosed felsic volcanic 
rocks, yielding zircon ages of around 645 Ma (Proterozoic 
Z), (2) massive coarse-grained pink granitic gneisses, 
yielding zircon ages of around 480 Ma (Ordovician), and 
(3) massive fine-grained pink biotite granite ("Milford, 
N.H., granite"), yielding zircon ages of around 275 Ma 
(Pennsylvanian-Permian). The first two are assigned on 
the State bedrock map to the Massabesic Gneiss Com- 
plex (OZma); the third is shown separately among the 
intrusive rocks of the Merrimack belt as pink and gray 
biotite granite (Pgr). Aleinikoff (1978) showed that a 
geochemical affinity exists between the Ordovician gra- 
nitic gneisses and the late Paleozoic granites, suggesting 
the latter might be melting products of the former. 

Reinterpretation of the mapping of G.R. Robinson 
(1978) in the Townsend area showed that almost none of 
the "Fitchburg Granite" as mapped resembles the Fitch- 
burg Complex farther southwest in Massachusetts, but it 
can be divided amongst the three rock types identified by 
Aleinikoff. Specifically, most areas mapped as "granite 
gneiss" (Dfbg) by G.R. Robinson can be equated with the 
Proterozoic Z and Ordovician gneisses of the Massabesic 
Gneiss Complex proper. Although Proterozoic Z and 
Ordovician parts of the Massabesic have not yet been 
separately mapped in the area of the State bedrock map, 



it is clear that both are present. Thus, unlike other areas 
labeled "OZ" on the State bedrock map, in the case of the 
Massabesic (OZma) the "OZ" does not denote uncer- 
tainty of age but, rather, indicates that the unit defi- 
nitely contains rocks of both Ordovician and Proterozoic 
Z age. Massabesic Gneiss Complex proper (OZma) has 
also been identified (Robinson, 1978) in the extreme 
eastern edge of the Ashby quadrangle. 

An interesting feature of the Massabesic of the 
Townsend area is its pronounced east-west mineral lin- 
eation, identical in orientation and character to the 
lineation in a wide area of Silurian and Devonian strati- 
fied and plutonic rocks to the west and southwest and 
clearly the product of an intermediate stage of the 
Acadian (Devonian) orogeny (Robinson, 1979). 

The fine-grained nonfoliated part of the "Fitchburg 
granite" (Dfg) mapped by Robinson (1978) is nearly all 
identified as the late Paleozoic "Milford, N.H., granite" 
of Aleinikoff (1978). Field areas mapped in detail by G.R. 
Robinson show how this granite has intruded the 
Oakdale Formation in the Merrimack belt (So of the 
State bedrock map, Oqfg of G.R. Robinson) parallel to 
bedding or foliation to produce a series of elongate, 
slablike inclusions (Robinson and Goldsmith, this vol., 
chap. G). Misidentification of this fine-grained granite for 
Proterozoic Z parts of the Massabesic might lead to an 
interpretation based on these contact relations that the 
Oakdale is Precambrian in age. 

In light of their geochemical affinities (Aleinikoff, 
1978), it is not surprising that the finer grained parts of 
the Ordovician granitic gneiss of the Massabesic are very 
difficult to tell in the field from the late Paleozoic granite 
(Pgr); hence contacts on the State bedrock map, modified 
from those of Robinson (1978), are highly tentative. Field 
separation of the Proterozoic and Ordovician parts of the 
Massabesic is even more difficult and was not accom- 
plished even by Aleinikoff. An excellent exposure in a 
small abandoned quarry on the southwest shore of Pot- 
anipo Pond, Mass., shows the late Paleozoic granite 
(Pgr) truncating the foliation of the Massabesic Gneiss 
Complex (OZma). 

A major problem yet to be solved is the nature of the 
contact between the Massabesic and surrounding strati- 
fied rocks, though the problem to the east could be less 
severe if the Oakdale Formation is Proterozoic (Barosh, 
1982; Lyons and others, 1982) or Ordovician(?)-Cambrian 
(Skehan and Murray, 1980, fig. 4). If the surrounding 
rocks are mainly Silurian and Devonian, as we think, 
then a search should be made for either an unconformity 
or possibly a deformed subhorizontal fault surface similar 
to others described in southeastern New England (Lun- 
dgren and Ebblin, 1972; Castle and others, 1976; 
Wintsch, 1979b; Goldstein, 1982). 



F20 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



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Geological Society of America Abstracts with Programs, v. 10, no. 
2, p. 79. 

Peper, J.D., and Pease, M.H., Jr., 1976, Summary of stratigraphy in 
the Brimfield area, Connecticut and Massachusetts, in Page, L.R., 
ed.. Contributions to the stratigraphy of New England: Geological 
Society of America Memoir 148, p. 253-270. 

Peper, J.D., Pease, M.H., Jr., and Seiders, V.M., 1975, Stratigraphic 
and structural relationships of the Brimfield Group in northeast- 
central Connecticut and adjacent Massachusetts: U.S. Geological 
Survey Bulletin 1389, 31 p. 

Peper, J.D., and Wilson, F.A., 1978, Reconnaissance bedrock geologic 
map of the Fitchburg quadrangle and part of the Ashby quadran- 
gle, north-central Massachusetts: U.S. Geological Survey Miscel- 
laneous Field Studies Map MF-959, scale 1:24,000. 

Robinson, G.R., Jr., 1978, Bedrock geology of the Pepperell, Shirley, 
Townsend quadrangles, and part of the Ayer quadrangle, Massa- 
chusetts and New Hampshire: U.S. Geological Survey Miscella- 
neous Field Studies Map MF-957, scale 1:24,000. 

Robinson, Peter, 1979, Bronson Hill anticlinorium and Merrimack 
synclinorium in central Massachusetts, in Skehan, J.W., and 
Osberg, P.H., eds., The Caledonides in the U.S.A., Geological 
excursions in the northeast Appalachians, Caledonide Orogen 
Project 27: Weston, Mass. , Weston Observatory, p. 126-174. 

Robinson, Peter, and Tucker, R.D., 1982, The Merrimack synclinorium 
in northeastern Connecticut, Discussion: American Journal of 
Science, v. 282, no. 10, p. 1735-1744. 

Rodgers, John, comp., 1982, Yet another preliminary geological map of 
Connecticut, in New England Intercollegiate Geological Confer- 
ence, 74th Annual Meeting, Storrs, Conn., Oct. 2-3, 1982, Guide- 
book for fieldtrips in Connecticut and south-central Massachusetts: 
Connecticut Geological and Natural History Survey Guidebook 5, 
p. 1-4, map in pocket, scale 1:250,000. 

Schenk, P.E., 1971, Southeastern Atlantic Canada, northwestern 
Africa, and continental drift: Canadian Journal of Earth Sciences, 
v. 8, p. 1218-1251. 

Skehan, J.W., 1967, Geology of the Wachusett-Marlborough tunnel, 
east-central Massachusetts, a preliminary report, in Farquhar, O., 
ed., Economic geology in Massachusetts— Conference Amherst, 
Proceedings: Amherst, Massachusetts University Graduate 
School, p. 237-244. 

Skehan, J.W., and Abu-Moustafa, A. A., 1976, Stratigraphic analysis of 
rocks exposed in the Wachusett-Marlborough tunnel, east-central 
Massachusetts, in Page, L.R., ed., Contributions to the stratig- 
raphy of New England: Geological Society of America Memoir 148, 
p. 217-240. 



F22 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



Skehan, J.W., and Murray, D.P., 1980, Geologic profile across south- 
eastern New England: Tectonophysics, v. 69, p. 285-319. 

Snyder, G.L., 1970, Bedrock geologic and magnetic maps of the 
Marlborough quadrangle, east-central Connecticut: U.S. Geologi- 
cal Survey Geologic Quadrangle Map GQ-791, scale 1:24,000. 

Tucker, R.D., 1977, Bedrock geology of the Barre area, central 
Massachusetts: University of Massachusetts, Geology and Geog- 
raphy Department, Contribution No. 30, 132 p. 

Wintsch, R.P., 1979a, Recent mapping in the Chester area, Connect- 
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Society of America Abstracts with Programs, v. 11, no. 1, p. 60. 

1979b, The Willimantic fault: a ductile fault in eastern Connect- 
icut: American Journal of Science, v. 279, p. 367-393. 

Wintsch, R.P., and Hudson, M.R., 1978, Southeastward thrusting in 
eastern Connecticut [abs.]: Geological Society of America 
Abstracts with Programs, v. 10, no. 2, p. 91. 



Wintsch, R.P., and Kodidek, K.L., 1981, Local and regional implica- 
tions of recent mapping in the Essex area, Connecticut [abs.]: 
Geological Society of America Abstracts with Programs, v. 13, no. 
3, p. 184. 

Zartman, R.E., and Naylor, R.S., 1984, Structural implications of 
some radiometric ages of igneous rocks in southeastern New 
England: Geological Society of America Bulletin, v. 95, no. 5, 
p. 522-539. 

Zartman, R.E., Snyder, G.L., Stern, T.W., Marvin, R.F., and Buck- 
man, R.C., 1965, Implications of new radiometric ages in eastern 
Connecticut and Massachusetts: U.S. Geological Survey Profes- 
sional Paper 525-D, p. D1-D10. 

Zen, E-an, editor, and Goldsmith, Richard, Ratcliffe, N.M., Robinson, 
Peter, and Stanley, R.S., compilers, 1983, Bedrock geologic map 
of Massachusetts: Reston, Va., U.S. Geological Survey, 3 sheets, 
scale 1:250,000. 



Stratigraphy of the Merrimack 
Belt, Central Massachusetts 

By PETER ROBINSON, University of Massachusetts, and RICHARD 
GOLDSMITH, U.S. Geological Survey 

THE BEDROCK GEOLOGY OF MASSACHUSETTS 

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1366-G 



CONTENTS 



Abstract Gl 

Introduction 2 

Ware, Gardner, Southbridge, and Wachusett Mountain 

subbelts 8 

Fitch Formation (Sfs, Sfss) S 

Paxton Formation 12 

Sulfidic schist and sillimanite quartzite (Spsq) 12 

Sulfidic quartzite and rusty schist (Spqr) 14 

Granofels member (Sp) 15 

Amphibolite (Spa) 18 

Bigelow Brook Member (Spbs, Spbc) 18 

Southbridge Member (Spso) 18 

Sulfidic mica schist (Spss) 18 

Littleton Formation (Dl, Dl+Ops, Dlf, Dlo, Dim) 19 

Ware subbelt 20 

Gardner subbelt 22 

Wachusett Mountain subbelt 22 

Harding Hill syncline 23 



Nashua and Rockingham subbelts G24 

Nashua subbelt 25 

Boylston Schist (SObo) 25 

Tower Hill Quartzite (St, Sts) 26 

Oakdale Formation (So) 27 

Worcester Formation (DSw) 27 

Coal Mine Brook Formation (Pcm) 28 

Age relations 29 

Rockingham subbelt 29 

Vaughn Hills Quartzite (SOvh) 30 

Reubens Hill Formation (SOrh) 30 

Kittery Formation (SOk) 31 

Eliot Formation (Se) 31 

Berwick Formation (Sb, Sbs) 31 

Harvard Conglomerate (Ph) 32 

Age relations 32 

Discussion 33 

References cited 34 



ILLUSTRATIONS 



Figure 1. Map showing major divisions and structural features of the Merrimack belt and localities of Pennsylvanian strata G4 

2. Correlation chart of sedimentary and volcanic rocks of the Merrimack belt and their metamorphic equivalents 5 

3-6. Geologic maps showing: 

3. The Merrimack belt: A, western part; B, eastern part 6 

4. Gardner-Athol area 9 

5. Ware-Southbridge area 10 

6. Area of the Fitchburg plutons 11 

7. Correlation charts of previous nomenclature of stratified units in the eastern part of the Merrimack belt: A, Nashua 

subbelt; B, Rockingham subbelt 16 

8. Columnar sections for the eastern part of the Merrimack belt from eastern Connecticut to southeastern 

New Hampshire and southwestern Maine 26 

9. Geologic map of the Worcester area 28 



TABLE 



Table 1. Distribution of Silurian-Devonian lithic units of the Massachusetts State bedrock map in western subbelts of the Merrimack 
belt 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



STRATIGRAPHY OF THE MERRIMACK BELT, 
CENTRAL MASSACHUSETTS 



By Peter Robinson 1 and Richard Goldsmith 2 



The Merrimack belt of central Massachusetts overlaps the junction 
between two zones of Ordovician and older rocks, the Bronson Hill zone 
on the west and the Nashoba zone on the east, and consists of Upper 
Ordovician, Silurian, Lower Devonian, and local Pennsylvanian strata. 
Distinctive groups of these strata form six subbelts. These are, from 
west to east, the Ware, Gardner, Southbridge, and Wachusett Moun- 
tain subbelts, in the western and central parts of the belt, and the 
Nashua and Rockingham subbelts, in the eastern part. 

The Fitch Formation, a well-bedded, rusty-weathering, graphite- 
bearing, calc-silicate granofels (Sfs) and subordinate rusty- weathering 
sillimanite-graphite-pyrrhotite-biotite schist, locally mapped sepa- 
rately (Sfss), is a thin unit confined to the Ware subbelt. The Fitch 
corresponds to the Fitch Formation in the Lovewell Mountain area, 
New Hampshire, and to the Francestown Formation (formerly a 
member of the Littleton Formation) in the Peterborough area and 
elsewhere in central and southern New Hampshire. 

The Paxton Formation (Sp) is a predominantly gray-weathering, 
slabby quartz-plagioclase-biotite granofels widely distributed in the 
Gardner, Southbridge, and Wachusett Mountain subbelts. The Paxton 
is subdivided into members containing distinctive rock types. The 
lowest unit is a rusty-weathering, white to bluish-gray sulfidic and 
graphitic, highly magnesian schist and interbedded slabby-weathering, 
feldspathic, micaceous, and sillimanitic quartzite (Spsq) 30-60 m thick 
that forms a long belt along the west margin of the Gardner subbelt. 
The protolith was deposited under reducing conditions, characterized 
by sulfur-reducing bacteria and slow accumulation of fine-grained 
Fe-bearing silicate. The magnesian schist differs from underlying 
schists of the Middle Ordovician Partridge Formation and the Lower 
Silurian Rangeley Formation in the high magnesium content of the 
biotite, the abundance of rutile, and the lack of ilmenite and garnet. 
The unit resembles the Middle Silurian Smalls Falls Formation of 
northwestern Maine. Sulfidic quartzite and rusty schist (Spqr) forms a 
unit more than 250 m thick in the axial zone of a major refolded 
recumbent anticline in the Wachusett Mountain subbelt. Although 
graphitic, sulfidic, and rusty-weathering, the unit lacks the Mg-rich 
biotite and cordierite of the sulfidic schist and quartzite unit (Spsq) but 
may be a facies equivalent of it. Quartzites in the unit resemble Clough 



Manuscript approved for publication November 16, 1987. 

'Department of Geology, University of Massachusetts, Amherst, MA 01003. 

2 U.S. Geological Survey. 



Quartzite of the Connecticut Valley belt. It may be broadly equivalent 
to the Smalls Falls Formation or the uppermost part of the Perry 
Mountain Formation in Maine. 

The granofels member (Sp) forms the bulk of the Paxton Formation. 
It is a well-layered, purple-gray quartz-calcic plagioclase-biotite gran- 
ofels commonly interbedded with beds of green to pink calc-silicate 
granofels and rare beds of diopside-bearing marble. Rusty-weathering, 
graphitic, sillimanitic mica schist similar to schist of the Partridge and 
Rangeley Formations is locally interbedded with the granofels except 
in the western part of the Wachusett Mountain subzone. Tourmaline- 
bearing pegmatites are common in the unit. The unit resembles the 
Warner Formation in New Hampshire and the Madrid Formation in 
Maine. It is lithically similar to the Vassalboro Formation in central 
Maine, the Eliot and Berwick Formations of southeastern New Hamp- 
shire, the Oakdale Formation of the Nashua subbelt, and the Hebron 
Gneiss of Connecticut. The Bigelow Brook Member (Spbs) consists of 
about equal proportions of thick-bedded granofels and sulfidic schist. It 
also contains a few marble beds and zones of calc-silicate granofels 
(Spbc). The Southbridge Member (Spso) consists of interbedded gran- 
ofels and calc-silicate rock. Schist beds are lacking. Mappable areas of 
sulfidic mica schist (Spss) occur throughout the Paxton, but the largest 
area lies in the Gardner anticline above the granofels and below gray 
schist assigned to the Littleton Formation. 

The Littleton Formation (Dl) occupies parallel narrow synclines in 
the Ware, Gardner, and Wachusett Mountain subbelts. It consists 
characteristically of gray-weathering schists that differ somewhat in 
composition in the different synclines. The Littleton schists typically 
contain sillimanite and garnet, and, at the highest grade of metamor- 
phism, cordierite. In the westernmost Ware subbelt, Littleton resem- 
bles Littleton in the Connecticut Valley belt. East of the Hardwick 
pluton, it is quartz rich and resembles part of Rangeley C of the Maine 
sequence. West of the Coys Hill pluton, a belt of schist interbedded 
locally with fine-grained granofels and flanked on either side by Fitch 
Formation and characterized by large garnets is correlated with the 
uppermost part of the Warner Formation of New Hampshire. In the 
Ware subbelt, east of the Coys Hill pluton, the isoclinal synclines of 
Littleton lie between belts assigned on the State bedrock map to the 
Partridge Formation (Dl+Ops), some of which actually should be 
assigned to the Rangeley Formation. Schist in one syncline coincides 
with the Mount Pisgah Formation. In another, the schist coincides with 
gray schist formerly mapped as the upper part of the Hamilton 
Reservoir Formation. Two synclines contain a white quartz-feldspar 
gneiss (Dlf); another contains an orthopyroxene-biotite gneiss (Dlo). 
These units are considered to have igneous protoliths. In the Gardner 



Gl 



G2 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



subbelt to the east, Littleton overlying Paxton Formation in synclinal 
folds contains subordinate beds of calc-silicate rock. In the Wachusett 
Mountain subbelt, the gray schist of the Littleton shows little variety 
but locally contains lenses of calc-silicate rock. East of the Fitchburg 
Complex, the Littleton is at slightly lower metamorphic grade and 
contains andalusite and locally staurolite. Schist in a syncline at the 
southern edge of the Wachusett Mountain subbelt contains marble 
(Dim). An extension of this belt along the boundary of the Southbridge 
and Gardner subbelts into Connecticut was formerly mapped as part of 
the Bigelow Brook Formation. 

The Boylston Schist (SObo), 1,000 m thick, at the base of the 
sequence in the Nashua subbelt, west of the Clinton-Newbury fault, is 
a rusty-weathering sillimanite schist containing subordinate calc- 
silicate rock. In fault contact with and above the Boylston, the Tower 
Hill Quartzite, 0-130 m thick, consists of thin-bedded quartzite (St) and 
schist and phyllite (Sts). The Tower Hill is considered to be the base of 
a turbidite sequence that includes the overlying Oakdale and Worcester 
Formations. The Oakdale Formation (So), possibly as much as 6,000 m 
thick, consists of ankeritic and actinolitic metamorphosed siltstone and 
interbedded calcareous phyllite and schist. In bulk composition it is 
similar to much of the Paxton Formation, but it contains more pelitic 
lenses and is finer grained and lower grade. The Oakdale may lie in an 
isoclinal syncline, but it is interpreted to lie in stratigraphic position 
between the Tower Hill and the Worcester Formation. The Worcester 
Formation (DSw), 2,000-4,000 m thick, consists of gray and dark-gray 
interbedded phyllite, fine-grained metamorphosed graywacke, and 
rare calc-silicate rock and marble. Because the Worcester is conform- 
able with the underlying Silurian Oakdale and yet lithologically resem- 
bles the Lower Devonian Littleton Formation, it is assigned a Silurian 
and Early Devonian age. Unconformably above the turbidite sequence 
is the Coal Mine Brook Formation (Pcm), 50-330 m thick, exposed at 
two places in down-faulted blocks near Worcester. At one, the Coal 
Mine Brook Formation consists of carbonaceous slate and phyllite and 
a 2-m-thick bed of metamorphosed anthracite. Plant fossils indicate a 
Middle Pennsylvanian age. At the other, conglomerate beds are 
interbedded with phyllite. 

At the bottom of the sequence in the Rockingham subbelt, west of 
the Clinton-Newbury fault, are the Vaughn Hills Quartzite (SOvh) and 
Reubens Hill Formation (SOrh). The Vaughn Hills, 0-200 m thick, 
consists of thin-bedded quartzite, interbedded locally rusty- weathering 
phyllite and schist, and minor beds of calc-silicate rock. The Vaughn 
Hills is possibly equivalent to the Tower Hill Quartzite of the Nashua 
subbelt. The Reubens Hill Formation, 600 m thick, a metamorphosed 
igneous unit consisting of amphibolite and homblende-plagioclase 
gneiss, has no counterpart in the Nashua subbelt. A fault separates the 
Vaughn Hills and Reubens Hill from the overlying turbiditic sequence 
that consists of the Kittery Formation (SOk), the Eliot Formation (Se), 
and the Berwick Formation (Sb, Sbs). These units, continuous with 
previously named units in southern Maine and New Hampshire, 
lithologically resemble the Oakdale and Paxton Formations in the 
subbelts to the west. The Kittery, 4,000 m thick, consists of thin- 
bedded calcareous, commonly actinolitic metamorphosed siltstone, 
phyllite, and schist, and minor quartzite. The Eliot Formation, 300 m 
thick, consists of thin-bedded dark-gray to green slate and phyllite, 
commonly dolomitic, and metamorphosed siltstone, partly actinolite- 
bearing. The Berwick Formation, 2,000 m thick, consists of thin- to 
thick-bedded calcareous, biotitic metamorphosed siltstone containing 
actinolite, or, at higher grade, diopside, minor garnet-mica schist, and 
feldspathic quartzite. Partly rusty-weathering schist and phyllite (Sbs) 
forms mappable lenses near East Pepperell and near Haverhill. Meta- 
morphic grade increases from greenschist facies in the Kittery and 
Eliot Formation in the east to amphibolite facies in the Berwick 
Formation in the west, but metamorphic grade drops to greenschist 
facies again in the Oakdale Formation in the adjacent Nashua subbelt 
to the west. Unconformably on the older rocks, and correlated with the 



Coal Mine Brook Formation, is the Harvard Conglomerate (Ph), 
estimated to be 100 m thick, consisting of polymict metamorphosed 
conglomerate interbedded with, and overlain by, gray, green, and 
purple chloritoid-bearing phyllite. 

The pre-Pennsylvanian units in the Nashua and Rockingham sub- 
belts were shown on the State bedrock map as Ordovician or Silurian, 
Silurian, and Early Devonian on the basis of correlation with litholog- 
ically similar rocks along strike in the Silurian fossiliferous sequence at 
Waterville, Maine, and the similarity of lithologies and sequences to 
those in the subbelts of the Merrimack belt to the west whose age 
assignments are based on continuity of lithologies to fossiliferous strata 
in New Hampshire and western Maine. Radiometric dating of the 
intrusive Newburyport Complex indicates that the Kittery Formation 
might be as old as Ordovician. Recent radiometric dating of intrusions 
into the Eliot and Berwick Formations in New Hampshire indicate that 
the stratified rocks in the sequences are Ordovician or older. Some 
workers in the region consider the sequences to be Late Proterozoic. 
The similarity of the sequences in the Nashua and Rockingham belts to 
sequences in the central and western part of the Merrimack belt raises 
questions that must be answered in future work. 

INTRODUCTION 

The Merrimack belt is here defined as the belt of 
Upper Ordovician, Silurian, Lower Devonian, and local 
Pennsylvanian strata east of the easternmost exposures 
of Lower Silurian Clough Quartzite and west of the 
Clinton-Newbury fault. The strata in the western part of 
the Merrimack belt closely resemble the Silurian and 
Lower Devonian strata of northwestern Maine (Moench 
and Boudette, 1970) and correlatives in central and 
southern New Hampshire (Hatch and others, 1983). 
Some firm correlations with these areas had been made 
at the time the Massachusetts bedrock geologic map (Zen 
and others, 1983; hereafter referred to as the State 
bedrock map) was compiled in 1980, but subsequent 
mapping has modified what is shown and requires fur- 
ther discussion. The strata in the eastern part of the belt 
are along strike from and resemble the fossil-bearing 
Silurian strata of the Waterville area, Maine (Osberg, 
1968), but specific correlations have not been estab- 
lished, partly because the stratigraphic position of the 
Waterville itself has been uncertain until recently 
(Osberg, 1980). In addition, some field interpretations of 
the contact relations of the Massabesic Gneiss Complex 
(Bothner and others, 1984) and some interpretations of 
the isotopic data on granitoid rocks in the Massabesic, as 
well as on plutons that intrude the sequence in the 
eastern part of the Merrimack belt (Zartman and Mar- 
vin, this vol., chap. J), suggest to some workers a 
pre-Silurian age for many of the strata of the belt (Lyons 
and others, 1982; Hatch and others, 1984; Bothner 
and others, 1984). Correlations within Massachusetts 
between the eastern and western parts of the belt are 
complicated by the fact that the western part is largely 
sillimanite grade or higher, whereas the eastern part is 
largely garnet grade or lower and also by the postmeta- 
morphic faults of the Wekepeke system. Nevertheless, 



STRATIGRAPHY OF THE MERRIMACK BELT, CENTRAL MASSACHUSETTS 



G3 



Table 1.— Distribution of Silurian-Devonian lithic units of the Massachusetts State bedrock map (Zen and others, 1983) in western subbelts 

of the Merrimack belt 



Age 



Ware subbelt 



Gardner subbelt 



South bridge subbelt 



Wachusett Mountain subbelt 



Devonian Littleton Formation (Dl) 
Feldspar gneiss member 

(Dlf) 
Orthopyroxene gneiss 
member (Dlo) 
Silurian Fitch Formation 

Sulfidic calc-silicate (Sfs) 
Sulfidic schist (Sfss) 



Littleton Formation (Dl) 



Paxton Formation 
Sulfidic schist (Spss) 
Granofels (Sp) 
Sulfidic schist and quartzite 
(Spsq) 



Littleton Formation (Dl) 



Paxton Formation 
Granofels (Sp) 
Southbridge Member (Spso) 
Bigelow Brook Member 

(Spbs) 
Calc-silicate (Spbc) 



Littleton Formation (Dl) 
Ribbon marble (Dim) 



Paxton Formation 
Sulfidic schist (Spss) 
Granofels (Sp) 
Quartzite and rusty 
schist (Spqr) 



we find general correlations to be clear across the belt 
and to the north, with fossil-bearing rocks of Maine and 
New Hampshire. We find no compelling evidence for an 
important tectonostratigraphic boundary between the 
eastern and western parts of the belt. 

The distribution of stratified rocks within the Merri- 
mack belt, particularly the western part, is controlled by 
a very complex structural history that we are just 
beginning to understand. In brief, the outcrop pattern is 
now (1984) thought to have been produced by four 
episodes of folding and faulting and one of postmetamor- 
phic faulting, as follows (Robinson, 1979): (1) Early 
nappes of 15- to 30-km amplitude, originally overfolded 
from east to west, produced numerous repetitions of 
stratigraphy but few hinges that can be recognized on a 
local scale. (2) Complex backfolding, in which the nappe 
axial surfaces were refolded in major nappelike folds 
directed from west to east, resulted generally in intense 
flattening of the rock units. During the later parts of the 
backfolding stage, prominent east-west mineral linea- 
tions formed parallel to minor fold axes and to the 
internal fabric of mylonites in west-dipping semiductile 
shear zones. (3) North-northeast-trending recumbent 
folds in foliation formed parallel to a strong mineral 
lineation that merges with the lineation pattern of the 
gneiss domes to the west. (4) A series of broad north- 
trending arches and depressions in foliation formed 
across the belt. The most important of these are the 
Gardner anticline, which makes a foliation anticline out of 
most of the western part of the Merrimack belt, and the 
Wachusett syncline, which runs near the center of the 
Fitchburg plutons and separates a broad area dominated 
by east dips from the dominant west dips in the eastern 
part of the belt. (5) Postmetamorphic normal faults 
formed as part of the Wekepeke fault system in the 
region east of the Fitchburg plutons. 

For convenience of description, the Merrimack belt 
has been divided intn stratigraphic-tectonic subbelts 
(Robinson, 1979), each with Silurian-Devonian sequences 



of slightly different character or with a particular 
arrangement of stratigraphic and tectonic features (table 
1, fig. 1). These subbelts are described very briefly here 
to assist the reader with the stratigraphic description 
that follows. The reader should here be aware that the 
horizontal arrangement of rock units in the correlation of 
map units (fig. 2) is generally based on their present 
surface distribution from west to east and not on their 
horizontal distribution at the time of deposition, because 
the original order may have been reversed by recumbent 
folding. 

The westernmost subbelt, here called the Ware sub- 
belt (figs. 1, 3A), is characterized by gray-weathering 
schists assigned to the Littleton Formation exposed in a 
series of isoclinal synclines. The Ware subbelt is defined 
to include all those Silurian-Devonian strata that lie east 
of the west margin of the Hardwick pluton, or a line 
extended southward from it, and west of the western- 
most exposures of the Paxton Formation, units Spsq and 
Sp (fig. 3A). Sulfidic calc-silicate rocks basal to the gray 
schist are here assigned to the Silurian Fitch Formation 
(Sfs). Since the final map compilation, several localities 
have been found that suggest that rocks assigned to the 
Littleton Formation (Dl) or the Partridge Formation 
(Op) might be better assigned to the Silurian Rangeley 
Formation, on the basis of similarity to rocks in the 
central New Hampshire sequence (Hatch and others, 
1983). The general scarcity of Silurian strata in the Ware 
subbelt may be ascribed to a pre-Devonian unconform- 
ity. In southern Massachusetts, the Ware subbelt 
includes strata mapped as the Mount Pisgah Formation 
and as gray-weathering schists in the upper schist mem- 
ber of the Hamilton Reservoir Formation by Seiders 
(1976) and Pomeroy (1977). 

The next subbelt to the east, the Gardner subbelt (figs. 
1, 3A), is marked by the rather abrupt appearance of 
thick Silurian strata in the form of members of the 
Paxton Formation (Sp), dominated by calcareous grano- 



G4 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 




20 KILOMETERS 



Figure 1.— Major divisions and structural features of the Merrimack belt and localities of Pennsylvanian strata. 



fels, below the Devonian schists. The west margin of the 
subbelt is defined to include exposures of all members of 
the Paxton Formation. The east margin of the subbelt in 
northern Massachusetts is taken along the trace of a 
probable Mesozoic fault and its southern extension and in 
southern Massachusetts along the west margin of a belt 
of gray schist that is exposed in the center of the Gardner 
anticline (figs. 1, 3A). In the northern part of Massachu- 
setts, the Gardner subbelt includes strata dipping both 
west and east on opposite limbs of the Gardner anticline. 
In southern Massachusetts, the Gardner subbelt includes 
some strata mapped here and by Emerson (1917) as part 
of the Paxton Formation but mapped as parts of the 
Bigelow Brook and Hamilton Reservoir Formations by 
Seiders (1976), Pomeroy (1975, 1977), Moore (1978), and 



Pease (1972). Our detailed and reconnaissance mapping 
has shown that calcareous granofels of the Paxton For- 
mation, previously mapped by Pease (1972) as part of the 
Bigelow Brook Formation, can be traced around the 
north end of the Oakham anticline directly into rocks 
previously mapped by Seiders as part of the upper gneiss 
member of the Hamilton Reservoir Formation. 

The next subbelt, the Southbridge subbelt (figs. 1, 
3A), is a broad expanse of rather gently dipping strata of 
the Paxton and Littleton Formations; it represents an 
eastern extension of the Gardner subbelt in southern 
Massachusetts and is bounded on the east by the west 
margin of the Oakdale Formation. The Littleton Forma- 
tion is confined to a single complexly refolded isocline 
that extends from the northeast corner of the subbelt 



STRATIGRAPHY OF THE MERRIMACK BELT, CENTRAL MASSACHUSETTS 



G5 



Worcester Basin 



Intrusive Rocks 



Pern 




Ph 








Sp ^ 

0^\ 


_Spss_^ 










Spbs 
I (^SpbcN 


Spso 


So 


Sfss 


Sfs 


^ 




St 


/ 






Spsq Spqr 


Sts 


SOvh 








SObo 


SOrh 





Dfgr 


Dht 




Dchgr 





SOn 




SOa 








' 


f 



PENNSYLVANIAN 



DEVONIAN 



SILURIAN 



ORDOVICIAN 



EXPLANATION 



Pern 
Ph 

Dl, Dlo, 
Dlf, Dim 
Dl + Ops 



DSw 

Sfs, Sfss 
Sp, Spss, 
Spa, Spsq, 
Spqr, Spbs, 
Spso, Spbc 
So 

Sb, Sbs 
Se 

St, Sts 
SOvh 



Stratified rocks 
Coal Mine Brook Formation (Pennsylvanian) 
Harvard Conglomerate (Pennsylvanian) 
Littleton Formation (Lower Devonian) 

Littleton Formation (Lower Devonian) and 
Partridge Formation (Middle Ordovician), 
undivided 

Worcester Formation (Lower Devonian 
and Silurian) 

Fitch Formation (Upper Silurian) 

Paxton Formation (Silurian) 



Oakdale Formation (Silurian) 
Berwick Formation (Silurian) 
Eliot Formation (Silurian) 
Tower Hill Quartzite (Silurian) 
Vaughn Hills Quartzite (Silurian or 
Ordovician) 



SOk 
SOrh 
SObo 

Pgr 
Dmgr 

Dfgr 

Dht 
Dchgr 

SOn 

SOa 



Kittery Formation (Silurian or Ordo- 
vician) 

Reubens Hill Formation (Silurian or 
Ordovician) 

Boylston Schist (Silurian or Ordo- 
vician) 

Intrusive rocks 
Biotite granite (Pennsylvanian) 
Muscovite-biotite granite at Millstone 

Hill (Lower Devonian) 
Fitchburg Complex (Lower Devonian or 

younger) 
Hardwick Tonalite (Lower Devonian) 
Coys Hill Porphyritic Granite Gneiss 

(Lower Devonian) 
Newburyport Complex (Silurian and 

Ordovician) 
Ayer Granite (Lower Silurian and 

Upper Ordovician?) 



Figure 2.— Correlation of sedimentary and volcanic rocks of the Merrimack belt and their metamorphic equivalents and rocks 
intrusive into them that are referred to in text (modified from Zen and others, 1983). 



G6 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



CONNECTICUT 

VALLEY 

BELT 




/ 72°15'W / 

/ WARE /GARDNER /S 



72°W 
SOUTHBRIDGE SUBBELT 



71°45'W 



NASHUA SUBBELT 



Figure 3 A. — Geology of the western part of the Merrimack belt. Pre-Silurian rocks and Paleozoic intrusive rocks are labeled only 

if discussed in text. 



STRATIGRAPHY OF THE MERRIMACK BELT, CENTRAL MASSACHUSETTS 



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G8 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



into the Gardner subbelt to the west. In southernmost 
Massachusetts, the Paxton Formation is divided into a 
western Bigelow Brook Member and an eastern South- 
bridge Member, on the basis of earlier work along the 
Connecticut State line (Barosh, 1974; Moore, 1978), 
following the usage of Pease (1972) in northern Connect- 
icut. 

The Wachusett Mountain subbelt (figs. 1, 3A), east of 
the Gardner subbelt and north of the Southbridge sub- 
belt, contains strata well out on the east limb of the 
Gardner anticline and close beneath or intruded by the 
Fitchburg plutons. It is bounded on the west by a 
probable Mesozoic fault or its southern extension and on 
the east by the Wekepeke normal fault system of prob- 
able Mesozoic age. In the eastern frontal regions of the 
backfolded nappes, the strata of this subbelt are believed 
to be structurally higher than most of the rocks of the 
Gardner and Southbridge subbelts. If the folding were 
removed, we believe that these strata would occupy an 
intermediate position between the strata of the Ware and 
Gardner subbelts (see cross section D-D' of the State 
bedrock map). 

East of the Wachusett Mountain and Southbridge 
subbelts is the narrow Nashua subbelt (fig. 3), which 
contains phyllite and metamorphosed calcareous meta- 
siltstone of the Worcester (DSw) and Oakdale (So) 
Formations overlying an apparently basal phyllite and 
quartzite assemblage consisting of the Tower Hill 
Quartzite (St, Sts) and possibly the Boylston Schist 
(SObo). These rocks lie in a north-trending structural 
and metamorphic trough called the Nashua synclinal by 
Crosby (1880). The subbelt narrows to the south between 
the Southbridge subbelt and the Nashoba zone and 
extends into Connecticut south of Webster. Although the 
bulk of the strata are considered to be Silurian to 
possibly Devonian in age, Pennsylvanian rocks of the 
Coal Mine Brook Formation (Pcm) (Goldsmith and oth- 
ers, 1982) lie in fault-bounded blocks with older rocks at 
Worcester. The rocks in the Nashua subbelt are believed 
to occupy the down-faulted trough of an east-facing 
recumbent syncline (G.R. Robinson, 1981, p. 59-62), a 
major element of the backfolded nappe system men- 
tioned above. This syncline may correspond structurally 
to the synclinal core of the postulated Colchester nappe 
(Dixon and Lundgren, 1968) in eastern Connecticut. The 
structural trough coincides with a metamorphic trough of 
lower grade than the surrounding subbelts (Thompson 
and Norton, 1968). 

The Rockingham subbelt (fig. SB), containing strata of 
the Merrimack Group, forms a northward-widening 
wedge between the Nashua subbelt and the Nashoba 
zone. Calcareous metasiltstone, phyllite, metasandstone, 
and quartzite of the Kittery (SOk), Eliot (Se), and 
Berwick (Sb, Sbs) Formations forming the Merrimack 



Group are shown as Ordovician to Silurian on the State 
bedrock map, but they may be entirely Ordovician or 
older (Bothner and others, 1984). The strata for the most 
part are similar in composition and bedding style to the 
Paxton and Oakdale Formations of the Nashua trough, 
although at slightly higher metamorphic grade than the 
Oakdale. The boundary between the Rockingham sub- 
belt and the Nashua subbelt is the contact between the 
Oakdale and the Berwick Formations. It is marked 
primarily by a contrast in metamorphic grade but also by 
a difference in expression of relict bedding. The bedding 
characteristics probably reflect premetamorphic differ- 
ences in grain size at the time of deposition. In addition, 
the Merrimack Group in the Rockingham subbelt con- 
tains mappable zones of rusty-weathering schist and 
phyllite much like the Paxton Formation to the west in 
the Wachusett Mountain subbelt. However, rusty 
schists and phyllites are not found in the rocks of the 
intervening Nashua subbelt. 



WARE, GARDNER, SOUTHBRIDGE, AND 
WACHUSETT MOUNTAIN SUBBELTS 

In this section, rocks presumed to be Silurian (the 
Fitch and Paxton Formations) are discussed first, in 
generally west to east order, followed by discussion of 
rocks presumed to be Devonian (the Littleton Forma- 
tion). Alternative interpretations and correlations are 
given, including some that appear to be more likely now 
than at the time the map was compiled. The stratigraphy 
of these rocks is impossible to discuss without at least a 
rudimentary description of the structural geology, but 
the emphasis in this chapter is on the stratigraphy. In 
discussing the distribution of units, repeated references 
are made to figures 4, 5, and 6. 



FITCH FORMATION (Sfs, Sfss) 

The dominant rock of the Fitch Formation (Sfs) in the 
Merrimack belt is in outcrop a slabby, rusty-weathering 
quartz-plagioclase-graphite rock. Biotite is scarce or 
absent, and broken foliation surfaces show abundant 
graphite platelets. The rusty- weathering character is 
due to the presence of pyrrhotite, which makes up 5-6 
percent of the rock in some beds. Mineralogically, the 
rock is a calc-silicate granofels (Field, 1975, table 4) 
usually containing 20-60 percent quartz, 35-55 percent 
calcic plagioclase (An 65 _8 ), 3-35 percent diopside, 1-2 
percent sphene, 0-2 percent graphite, and local actino- 
lite, calcite, scapolite, clinozoisite, and biotite. Careful 
examination shows this unit to be well bedded, but 
bedding is usually obscured by the rusty weathering and 
the way the rock breaks into rectangular slabs. A 



STRATIGRAPHY OF THE MERRIMACK BELT, CENTRAL MASSACHUSETTS 



G9 




■•'./ 

///EAST RINDGE 



72°15' W 



72° W 



Figure 4.— Geology of the Gardner- Athol area. Most Paleozoic intrusive rocks and pre-Silurian rocks are not 
labeled. Dl, Littleton Formation; Sfs, Sfss, Fitch Formation; Sp, Spsq, Spss, Paxton Formation. Circled 
letters and numbers are discussed in text. 



G10 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 




Figure 5. -Geology of the Ware-Southbridge area. Paleozoic intrusive rocks (except Coys Hill and Hardwick plutons) and pre-Silurian 
rocks are not labeled. Dl, Littleton Formation; Sfs, Fitch Formation; Sp, Spqr, Spsq, Spss, Paxton Formation. Circled letters and 
numbers are discussed in text. Cross section lines D-D' and F-F' are from State bedrock map (Zen and others, 1983). 



STRATIGRAPHY OF THE MERRIMACK BELT, CENTRAL MASSACHUSETTS 



Gil 




WORCESTER 



Figure 6. — Geology of the area of the Fitchburg plutons. Paleozoic intrusive rocks and pre-Silurian 
rocks are not labeled. Dl, Dl?, Littleton Formation; Dim, marble in Littleton Formation; Sp, Spqr, 
Spss, Paxton Formation. Circled letters are discussed in text. Cross section line D-D' is from State 
bedrock map (Zen and others, 1983). 



G12 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



subordinate rock type in the Fitch Formation is rusty- 
weathering sillimanite-graphite-pyrrhotite-biotite schist 
(Sfss), which is very similar to the schist of the sulfidic 
schist-quartzite unit (Spsq) of the Paxton Formation. 
The Fitch sulfidic schist is locally interbedded with the 
Fitch calc-silicate and dominates the two lenses (labeled 
Sfss on fig. 4) near the eastern edge of the Ware subbelt. 

Contacts of the Fitch Formation with adjacent units 
are poorly exposed but generally are sharp. However, 
in large exposures on the northeast side of Ragged 
Hill (Robinson and others, 1982a, Stop 5), slightly 
rusty interbedded schists and quartzites lie between 
Fitch calc-silicates and gray "big-garnet" schists of the 
Littleton. 

Field (1975) estimated the thickness of the Fitch 
Formation in the Ware area to be 15 m. The wider belt in 
the southern part of the area is believed to have been 
caused by repetition of the unit thrice by isoclinal folding. 

The mineralogy of the Fitch Formation suggests that 
its protoliths were calcareous and dolomitic shales and 
siltstones and locally interbedded aluminous shales. The 
abundance of graphite and pyrrhotite suggests deposi- 
tion in a reducing environment of poor circulation with 
preservation of organic matter and much activity by 
sulfur-reducing bacteria. 

The rocks assigned to the Fitch Formation in the 
Merrimack belt all lie within the Ware subbelt, and most 
are very close to the Coys Hill Pluton. As early as 1972 
Robinson realized that the most distinctive rock type, 
the rusty-weathering granofels, or quartzite, corre- 
sponds exactly to the distinctive lithology of both the 
"rusty quartzite member" (Dlr) of the Littleton Forma- 
tion in the Monadnock quadrangle, New Hampshire 
(Fowler-Billings, 1949), and the Francestown Member of 
the Littleton Formation in the Peterborough quadran- 
gle, New Hampshire (Greene, 1970). The rusty granofels 
is also identical to rocks assigned to the Fitch Formation 
at Gee Mill in the Lovewell Mountain quadrangle, New 
Hampshire, which overlie Clough Quartzite (Heald, 
1950; Thompson and others, 1968; Dean, 1976). On this 
basis, Field (1975) assigned the rocks in Massachusetts to 
the Fitch Formation with separate formational status, 
rather than to a member of the Littleton Formation. 
Now that the Francestown Formation has been assigned 
formation status in New Hampshire (Nielson, 1981; 
Hatch and others, 1983; Thompson, 1983), this rusty 
granofels in Massachusetts could as well be assigned to 
the Francestown as to the Fitch. However, the correla- 
tion with Fitch is also valid and is emphasized by the 
occurrence of similar sulfidic rocks in the Fitch of the 
Connecticut Valley belt in Massachusetts (Robinson, 
1963, p. 62; Hatch and others, 1988). 

The most continuous outcrops of the Fitch Formation 
are in two extremely narrow belts (fig. 5, area C) west of 



the Coys Hill pluton in the Ware quadrangle (Field, 
1975). These two belts merge southward into a single 
belt near Ragged Hill, suggesting that the Littleton 
Formation between them is an isoclinal syncline. The 
only other good exposures on the west side of the Coys 
Hill pluton are southwest of Barre (fig. 4, area C) and in 
a large railroad cut northwest of Baldwinville (Robinson, 
unpub. data). Three widely separated inclusions of Fitch 
Formation are present in the Hardwick pluton. Two 
lenses of Fitch Formation lie along the east contact of the 
Coys Hill pluton (fig. 5, area F), northwest of Warren, 
between it and the Partridge Formation, and two other 
lenses are along Littleton-Partridge contacts near the 
east edge of the Ware subbelt (fig. 4, area I). 

PAXTON FORMATION 

The name Paxton Formation is used here to describe 
all the stratified rocks of presumed Silurian age exposed 
in the Gardner, Southbridge, and Wachusett Mountain 
subbelts. The name Paxton Formation is a modification 
and expansion of the name Paxton Quartz Schist used by 
Emerson (1917) to describe all areas in central Massa- 
chusetts dominated by gray-weathering slabby quartz- 
plagioclase-biotite granofels. The formation is named for 
the town of Paxton, Mass., where hundreds of stone 
walls are built of this slabby rock. In December 1976, 
Robinson and Tucker (Robinson and others, 1982a, Stop 
12A) rediscovered the large cascade exposure on the 
brook draining Eames Pond in the western part of 
Paxton, which probably was Emerson's type locality 
(Paxton Falls, fig. 6). In the future, as correlations with 
fossil-bearing strata in Maine improve, we imagine that 
various members will achieve formational status, and the 
name Paxton will either be dropped or be restricted to 
granofels equivalent to that exposed in Paxton. 

In the following section, we have attempted to 
describe the various members in what we think is the 
correct stratigraphic order. Where rock units are consid- 
ered roughly correlative but of different facies, they are 
discussed in geographic order from west to east without 
regard to original arrangement before folding. In many 
examples, the true stratigraphic facing direction and 
order are unknown, and evidence of complex recumbent 
folding makes several choices possible. 

SULFIDIC SCHIST AND SILLIMANITE QUARTZITE (Spsq) 

This unit forms a nearly continuous, contorted belt 
along the west margin of the Gardner subbelt (figs. 4, 5, 
area J) from just south of Route 2, in the northern part 
of the State, to well south of the Massachusetts Turnpike 
in the southern part. Since publication of the State 
bedrock map, Spsq has been traced several kilometers 



STRATIGRAPHY OF THE MERRIMACK BELT, CENTRAL MASSACHUSETTS 



G13 



into Connecticut (Berry, 1985). The unit was first defined 
by Field (1975) as the white schist member of the Paxton 
Formation of the Ware area and was mapped thence 
northward through Barre (Tucker, 1977) and into Tem- 
pleton (H.B. Stoddart, written commun., 1978; D.E. 
Klepacki, written commun., 1978). In southern Massa- 
chusetts, Spsq was lumped with the upper schist mem- 
ber of the Hamilton Reservoir Formation (Seiders, 1976; 
Pomeroy, 1977) or mapped as schist within the upper 
gneiss member of the Hamilton Reservoir Formation. 
The unit changes very slightly in character from the 
north, where it contains some muscovite in the 
sillimanite-muscovite-K-feldspar zone, to the south, 
where the only mica is biotite in the sillimanite-garnet- 
cordierite zone. 

The sulfidic schist and quartzite unit tends to form 
large rounded to overhanging outcrops and holds up a 
series of relatively high ridges. The surface is extremely 
rusty, is commonly yellow orange, and generally has 
coatings of secondary limonite as much as 1 cm thick. 
Accumulations of secondary sulfates are common under 
overhangs. Soils in the vicinity commonly have an 
orange-red color, and sulfates in surface water have led 
to such names as "Alum Pond" and "Little Alum Pond." 
Smooth upper surfaces of outcrops commonly have 
widely spaced, rounded pits 3 to 8 cm across within which 
shiny pyrite is visible. Despite the robust appearance of 
the outcrops as a whole, the upper surfaces are generally 
friable, and collecting fresh specimens is extremely dif- 
ficult. Beneath the crust of secondary limonite, the 
weathered rock commonly appears white due to the 
abundance of quartz, feldspar, sillimanite, and white 
mica and the absence of any dark mineral except discrete 
flakes of graphite. This characteristic led to the name 
"white schist member." However, really fresh rock is 
bluish gray. Interbedded with the typical schist is feld- 
spathic, micaceous, and sillimanitic quartzite that forms 
hard, tough beds 5 to 8 cm thick, which weather into 
slabs. The quartzite is probably responsible for the fact 
that the unit forms prominent topographic ridges. 

First encounters with this unit were in the sillimanite- 
garnet-cordierite zone many miles from the nearest 
stable occurrence of muscovite (Field, 1975). The abun- 
dance of what appeared to be white mica in the unit, as 
well as the sulfides, suggested that it might be a zone of 
secondary hydrothermal alteration. However, optical 
observations of the white mica consistently showed a 2V 
close to 0°, rather than the 30° characteristic of musco- 
vite, and thus led to electron probe analyses and to 
determination that the white mica is pure Mg-biotite 
(Field, 1975; Tracy and others, 1976; Robinson and 
Tracy, 1977; Robinson and others, 1982b). 

Thin sections of the unit (Field, 1975, table 6; Tucker, 
1977, table 4) show that it consists of 40-75 percent 



quartz, 2^40 percent andesine, 6-30 percent orthoclase 
or microcline, 2-10 percent sillimanite, 1-8 percent mag- 
nesian biotite, 0-3 percent magnesian cordierite, tr 
(trace)-4 percent graphite, 0-3 percent pyrite, tr-1 per- 
cent pyrrhotite, and tr-1 percent rutile. Muscovite 
occurs near the northern end of the outcrop belt. Locally, 
the sillimanite has the form of pseudomorphs after 
andalusite. The biotites range from very pale reddish- 
brown iron-bearing varieties to colorless Mg end mem- 
bers and have Mg/(Mg+Fe) ratios between 0.75 and 
0.999 (0.04 weight percent FeO). Even the magnesian 
biotites have octahedral Al and as much as 0.074 Ti per 
11 oxygens, and thus they are not properly phlogopites. 
The cordierites, present only in the schist and not the 
quartzite, are charged with graphite and appear as black 
to bluish lumps. Some are essentially pure Mg end 
members with 0.00 weight percent FeO and only 0.08 
weight percent MnO. Even where charged with detrital 
zircons, these cordierites lack pleochroic halos, presum- 
ably because of lack of iron to be oxidized by alpha 
bombardment. The pure magnesian cordierites contain 
approximately 2 weight percent H 2 S, due apparently to 
the high sulfur fugacity of the pyrite-pyrrhotite assem- 
blage in which they formed. 

The key to understanding the mineralogy and ulti- 
mately the genesis of these rocks lies in the graphite- 
oxide-sulfide assemblages (for details see Robinson and 
Tracy, 1977; Robinson and others, 1982b). Specimens 
containing iron-bearing pale red-brown biotite and iron- 
bearing cordierite invariably contain the assemblage 
graphite-rutile-pyrrhotite. In these bulk compositions, 
original pyrite is believed to have reacted with the Fe 
component of the hydrous silicates and graphite to 
produce pyrrhotite plus Mg-richer silicates plus H 2 
plus C0 2 until the pyrite was exhausted. Specimens 
containing the nearly pure magnesian biotite and cordi- 
erite contain the assemblage graphite-rutile-pyrite- 
pyrrhotite. In these bulk compositions, originally richer 
in sulfide and poorer in iron-bearing silicate, the same 
reaction proceeded until the Fe component of the sili- 
cates was exhausted, with some pyrite still remaining. In 
one sample, from the lower grade north end of the belt, 
the sense of this reaction is shown by pyrrhotite rims 
growing around euhedral pyrite cubes. Robinson and 
others (1982b) showed that the primary deposition of 
such a unit requires not only reducing conditions and 
sulfur-reducing bacteria but slow deposition and a high 
proportion of fine detrital grains of Fe-bearing silicate 
capable of reacting with bacterially produced H 2 S in an 
open-system sedimentary environment. Sulfur-isotope 
data (Tracy and Rye, 1981) on several outcrops show 
very light sulfur values with 8 34 S ranging from —25 to 
-29, identical to values obtained from modern muds in 
highly reduced deep zones of the Black Sea. 



G14 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



In regions where the unit is relatively rich in pyrrho- 
tite, contacts have been mapped by using a hand-carried 
magnetometer. The unit is characterized by extremely 
large-amplitude variations, whereas adjacent units are 
magnetically flat. This method was particularly success- 
ful in mapping the northern part of the Barre quadran- 
gle, where magnetic data show that the unit forms 
several synclinal outliers capping the summits of hills 
(D.E. Klepacki, written commun., 1978). Elsewhere, 
where pyrite predominates over pyrrhotite, the unit is 
not detectable magnetically. As stated above, this unit 
was earlier mapped as part of the upper schist member of 
the Hamilton Reservoir Formation (Seiders, 1976; 
Pomeroy, 1977), which is now mainly included in the 
Partridge Formation. In the field the sulfidic schist and 
quartzite of the Paxton is distinguished from pyrrhotite 
schist of the Partridge Formation by the extremely pale 
Mg-rich biotite as compared to dark red-brown biotite in 
the Partridge, by the local presence of pyrite, by the 
abundance of rutile, and by the total absence of ilmenite 
and garnet, which characterize the Partridge. The min- 
eralogy of the Partridge suggests that it contained a 
much higher proportion of detrital Fe-bearing silicate 
grains that were too coarse or too rapidly deposited to 
permit thorough reaction with biologically produced 
H 2 S. 

The western contact of the sulfidic schist and quartzite 
unit with the Partridge Formation is nowhere exposed 
within an interval of 100 ft (30 m) but is presumed to be 
an unconformity on the basis of regional relations and the 
apparent lack of rock types correlative with the lower 
part of the Silurian section in Maine and central New 
Hampshire (Hatch and others, 1983). The maximum 
thickness of the unit is probably 30-60 m in south-central 
Massachusetts, where it lies between the Partridge 
Formation and the granofels member of the Paxton. The 
unit appears to pinch out beneath younger units to the 
north and to the south. 

Those who have seen the units agree that the sulfidic 
schist and quartzite unit of the Paxton Formation in 
Massachusetts is a perfect lithic correlative of the Smalls 
Falls Formation of northwestern Maine (Moench and 
Boudette, 1970). Similarities include the magnesian bio- 
tite and cordierite (Guidotti and others, 1975, 1977), the 
abundant rutile and pyrrhotite, the andalusite grains or 
pseudomorphs, the abundance of quartzite beds, and the 
position in the sequence below a major unit of gray 
granulites and calc-silicates, which is the Madrid Forma- 
tion in Maine (Moench, 1971) and the granofels member 
of the Paxton in Massachusetts. 

In summary, the sulfidic schist and quartzite unit 
appears to be locally the basal Silurian unit in central 
Massachusetts. Its lithic character suggests that it con- 
sisted of clean, fine quartz sand mixed with fine-grained 



detrital clay and organic matter, which was slowly 
deposited in a closed marine environment that permitted 
extensive reaction with biogenically produced H 2 S. 

SULFIDIC QUARTZITE AND RUSTY SCHIST (Spqr) 

This unit forms a single continuous belt of strata within 
the Wachusett Mountain subbelt (fig. 6, area S) and has 
not been specifically identified anywhere else. The belt is 
considered to form the axial zone of a major, presently 
northeast-directed, recumbent anticline, in which appar- 
ently younger rocks of the granofels member are both 
above (right side up) and below (upside down). The belt 
has been refolded about the Wachusett syncline and 
several other late folds to form a crude C-shaped outcrop 
pattern open to the northeast. The ends of the belt, at 
the points of the "C" northwest of Wachusett Mountain 
and southeast of the Fitchburg plutons, are interpreted 
as hinges of the postulated recumbent fold. The unit is 
generally not well exposed, but where thickest it does 
form the high ridges of Asnebumskit Hill (1,395 ft) (425 
m), and several hills to the north, as well as the top of a 
cliff on the west face of Stonehouse Hill. The unit occurs 
entirely within the sillimanite-muscovite zone of Acadian 
regional metamorphism. 

Although the schists of this unit are graphitic, sulfidic, 
and rusty weathering, they do not apparently contain 
the extremely Mg-rich biotite and cordierite typical of 
the sulfidic schist and quartzite unit (Spsq) described 
above. However, lack of detailed petrographic work and 
lower metamorphic grade leaves this question somewhat 
open. In small outcrops and manmade excavations, in 
particular in a large quarry 3 mi (5 km) north of Asne- 
bumskit Hill, mica schist is found on most broken sur- 
faces. Only in well-weathered, glacially smoothed out- 
crops and in slabby float is it apparent that the unit is 
dominated by fine-grained to locally grit-sized feld- 
spathic quartzite, with subordinate mica schist beds. No 
other rock in the western part of the Merrimack belt 
comes as close to resembling the Clough Quartzite of the 
Connecticut Valley belt. 

In the northernmost outcrop in the belt, contact rela- 
tions with the structurally underlying granofels member 
are well exposed. Here the typical quartzites of the unit 
are separated from the granofels by 3^4 m of sulfidic mica 
schist. 

On the basis of our present structural and strati- 
graphic interpretation, the quartzite and rusty schist 
unit (Spqr) is probably a facies equivalent of the sulfidic 
schist and quartzite unit (Spsq), and the nappe in which 
it lies may be rooted at the west edge of the Gardner 
subbelt. Farther afield, the unit is quite similar to an 
unnamed rusty quartzite exposed on the bank of the 
Kennebec River in Maine (Osberg, 1980, Stop 3). Osberg 



STRATIGRAPHY OF THE MERRIMACK BELT, CENTRAL MASSACHUSETTS 



G15 



suggested that this unnamed unit may be equivalent to 
the Perry Mountain Formation, which overlies the 
Sangerville Formation and is overlain successively by 
the sulfidic schists of the Parkman Hill Formation 
(Smalls Falls) and the calcareous granofels of the Fall 
Brook Formation (Madrid). Thus, indications are that 
the sulfidic quartzite and rusty schist unit may be 
broadly equivalent to the upper part of the Perry Moun- 
tain Formation and the Smalls Falls Formation in Maine. 
Because the base of the unit is not exposed, only an 
estimate of a minimum thickness of 250 m can be given. 
The depositional environment appears to have been 
similar to that of the sulfidic schist and quartzite unit 
(Spsq), except that conditions for fixation of organic 
sulfur were less ideal. 

GRANOFELS MEMBER (Sp) 

The granofels member (Sp) contains the bulk of the 
Paxton Formation and its most characteristic rock types. 
The granofels occurs extensively in the Gardner and 
Wachusett Mountain subbelts (fig. 4, area M; fig. 5, area 
Y; fig. 6, areas R, W) and dominates the Southbridge 
subbelt. The outcrop pattern is crucial to major struc- 
tural interpretations, and the unit appears to be lithically 
equivalent to the less metamorphosed Oakdale Forma- 
tion in the Nashua subbelt to the east (fig. 7). Although 
the granofels member is recognized to have some facies 
variations, the distribution of these facies has not gener- 
ally been mapped in detail. 

The most characteristic rock type of the granofels 
member is well-layered slabby-weathering purple-gray 
quartz-plagioclase-biotite granofels with bedding thick- 
ness ranging from 2 cm to about 30 cm (Field, 1975; 
Tucker, 1977). The plagioclase is usually labradorite or 
bytownite. Commonly interbedded with the biotite gran- 
ofels are beds 1-5 cm thick of green to pink calc-silicate 
granofels composed of quartz, plagioclase, diopside, 
actinolite, clinozoisite, and sphene, and locally scapolite, 
grossular garnet, graphite, and calcite. A few outcrops 
contain beds of diopside marble as much as 10 cm thick. 
Tourmaline-bearing pegmatites are extremely common 
in the Paxton granofels and dominate the outcrop in 
many areas. Commonly small pegmatites have been 
dismembered by shearing during metamorphism, so that 
boudins and individual feldspar fragments distributed 
through the outcrops form a sort of "popcorn rock" (D.R. 
Wones, oral commun., 1976). 

Another important rock type in the granofels member 
is quartz-feldspar-mica-sillimanite-graphite-pyrrhotite 
schist. This schist is hardly distinguishable from schist of 
the Partridge Formation, although in general it lacks 
delicate bedding and forms more resistant outcrops than 
does the Partridge. The similarity of the two units 



creates a serious mapping problem. Rusty schist inter- 
bedded with granofels is characteristic of the granofels 
member of the Paxton from the Barre area southward 
along the west limb of the Oakham anticline into Con- 
necticut and also east of that anticline into Connecticut. 
In the very narrow synclines close to Wachusett Moun- 
tain, Robert Tucker (written commun., 1978) mapped 
schist and granofels separately (at a scale of 1:24,000), 
but they are all lumped as granofels member on the State 
bedrock map. 

The granofels member is very poorly exposed in the 
center of the Gardner anticline (fig. 4, area M), but the 
rock that can be seen appears to be dominantly granofels. 
Emerson (1917) did not extend the Paxton very far north 
of Gardner, but abundant granofels and calc-silicate float 
in a gravel pit at Whitney Hill, Winchendon, led us to 
several localities including an excellent roadside expo- 
sure near East Rindge, N.H. 

The granofels member in the western part of the 
Wachusett Mountain subbelt (fig. 6, area R), including 
the type locality at Paxton, seems to be largely free of 
sulfidic schist. The granofels also seems to show some 
internal stratigraphy: lower parts are dominated by 
well-bedded calc-silicate rocks, and upper parts close to 
the overlying Littleton Formation are dominated by 
micaceous granofels layers and gray schists. Similar 
internal stratigraphy has been described in the Madrid 
Formation in Maine (Moench, 1971) and the Madrid and 
Warner Formations in central and southern New Hamp- 
shire (Hatch and others, 1983; Thompson, 1983). Since 
the State bedrock map was published, a similar sequence 
has been seen in new outcrops of the granofels member 
low on the west face of New Ipswich and Pratt Moun- 
tains, N.H., northwest of Ashburnham (Peterson, 1984). 

The Paxton Formation in the Wachusett Mountain 
subbelt, east of the Fitchburg plutons (fig. 6, area W) 
and west of the Wekepeke fault, as mapped by G.R. 
Robinson (1981) and Peck (1976), consists of thin-bedded 
feldspathic quartzite or metamorphosed siltstone inter- 
bedded with calc-silicate rock and subordinate beds and 
lenses of mica schist. The quartzite is a very fine grained, 
tan to brown, locally green-gray equigranular rock con- 
taining abundant reddish-brown biotite and some green 
chlorite. Greenish-gray calc-silicate beds are thin and 
commonly lenticular. Biotite schist layers, usually 
garnet and staurolite bearing, are only abundant near 
the Fitchburg pluton in the western part of the belt. 
According to G.R. Robinson (1981), bedding in the 
metasiltstone is expressed by differing modal propor- 
tions of quartz, plagioclase, biotite, and muscovite or 
actinolite. Less than 50 percent of the metasiltstone 
contains amphibole. Because of the immaturity of the 
sediments forming the metasiltstone, Peck (1976) sug- 
gested that they were derived in part from a volcanic 



G16 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



terrane. Thin, light-colored calcareous beds containing 
plagioclase, quartz, amphibole, and, in some beds, calcite 
compose less than 1 percent of the Paxton in this belt. 



Beds average 2-12 cm in thickness and are rarely as 
much as 1 m thick. Graded beds are rare; the rocks in the 
staurolite-kyanite zone in the eastern part have a fine 



NASHUA SUBBELT 



Age 


Emerson 
(1917) 


Hansen 
(1956) 


Skehan 
(1967) 


Grew 
(1970) 


Peck 
(1975, 1976) 


Zen and others 

(1983) 

State bedrock map 


Pennsylvanian 


Worcester 
Phyllite 


Worcester 
Formation 


No rocks 
assigned 


Worcester 
Formation 


No rocks 


Coal Mine Brook 
Formation 


Harvard 

Conglomerate 

Lentil of the 

Worcester 

Formation 


Harvard 
Conglomerate 


Early Devonian 
and Silurian 


Worcester 
Phyllite 


No rocks 
assigned 


Oakdale 

and 

Worcester 

Formations 


Holden 

Formation 

(part) 


Units 3 and 4 


Worcester 
Formation 


Silurian 


Oakdale 
Quartzite 

and 
Worcester 

Phyllite 


Oakdale 
Formation 


Unit 2 


Oakdale 
Formation 


Oakdale 
Quartzite 


Tower Hill 
Quartzite 
Member of the 
Boylston 
Formation 


Unit 1 


Tower Hill 
Quartzite 


Boylston 
Schist, 

Worcester 
Phyllite 


Worcester 
Formation 

(phyllite 
facies) 

Vaughn Hills 

Quartzite 

Member of the 

Worcester 

Formation 


Boylston 
Formation 


No rocks 
assigned 


Boylston 
Schist 


Silurian 

or 

Ordovician 


No rocks 
assigned 


Reubens Hill 
amphibolite 


not 
discussed 


Reubens Hill 
igneous complex 


Reubens Hill 
Formation 


Vaughn Hills 
Formation 


Vaughn Hills 

Member of the 

Tadmuck Brook 

Schist 


Vaughn Hills 
Quartzite 


Silurian (?). 
Ordovician, or 
Proterozoic Z 


Brimfield 
Schist 


Worcester 

Formation 

(mica schist 

facies) 


Nashoba 
Formation 


Tadmuck 
Brook 
Schist 


Tadmuck Brook 
Schist 

unconformity ? 

Nashoba 
Formation 


Ordovician or 
Proterozoic Z 


Gneisses and 

schists of 

undetermined 

age 


Nashoba 
Formation 



Figure 7. —Correlation of previous nomenclature of stratified units in the eastern part of the Merrimack belt: A, Nashua subbelt; B, Rockingham 

subbelt. 



STRATIGRAPHY OF THE MERRIMACK BELT, CENTRAL MASSACHUSETTS 



G17 



lamination, locally showing crossbedding. These features 
disappear as the rocks become coarser grained to the 
west in the sillimanite zone around the Fitchburg Com- 
plex. The rock adjacent to, and as inclusions in, the 
Fitchburg Complex is a coarse quartzofeldspathic gneiss. 
Rusty schists have not been found in this belt of Paxton. 
The Paxton is overlain conformably by mica schist here 
assumed to be the Littleton Formation (G.R. Robinson, 
1981). 

This belt of Paxton coincides in part with the original 
belt of Oakdale Quartzite of Emerson (1917) and corre- 
sponds to the western belt of Oakdale of Grew (1970) and 
the Oakdale Formation of G.R. Robinson (1978, 1981). 
Peck (1976) did not give the rocks a formal designation. 



We have not followed these authors' or Emerson's exact 
nomenclature for rocks in this belt because it became 
clear in the compilation for the State bedrock map (see 
cross sections D-D' , F-F') that the Paxton of area W 
(fig. 6) must be continuous around the Fitchburg plutons 
with the Paxton of area R (fig. 6), although this contin- 
uation is somewhat obscured by the Wekepeke fault 
south of Oakdale. In addition, this belt of Paxton contains 
less interbedded pelite than the rocks shown on the State 
bedrock map as Oakdale, east of the Wekepeke fault in 
the Nashua subbelt (fig. 7). 

The narrow belt of Paxton (fig. 5, area AA) east of the 
Southbridge Member (Spso) (area Z) consists of rock 
similar to the Southbridge (described below) but finer 



ROCKINGHAM SUBBELT 



Age 


Emerson 
(1917) 


Sundeen (1971), 
Sriramadas (1966) 


Novotny 
(1969) 


Zen and others 

(1983) 

State bedrock map 


Pennsylvanian 


No rocks 
assigned 


No rocks 
assigned 

Littleton 
Formation 


No rocks 
assigned 

Littleton 
Formation 


Harvard 
Conglomerate 


Early 
Devonian 


No rocks 
assigned 


Silurian 

Silurian 

or 

Ordovician 


Gneisses and 

schists of 

undetermined 

age 


Berwick 
Formation 


Berwick 
Formation 


Berwick 
Formation 


a 

O 

1 

I 


Eliot 
Formation 


Eliot 
Formation 


Merrimack 
Quartzite 


Eliot 
Formation 


not 
discussed 


Kittery 
Formation 


Kittery 
Formation 


No rocks 
assigned 


Rye 
Formation 


Reubens Hill 
Formation 


Vaughn Hills 
Quartzite 


Silurian (?), 
Ordovician, or 
Proterozoic Z 


Brimfield 
Schist 


Tadmuck 
Brook 
Schist 


Ordovician or 
Proterozoic Z 


Gneisses and 

schists of 

undetermined 

age 


Nashoba 

Formation 





Figure 7.— Continued. 



G18 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



grained and more thinly and uniformly bedded. It corre- 
sponds to the Hebron Formation as mapped by Pease 
(1972) in the Eastford quadrangle, northeastern Con- 
necticut, and was considered by Moore (1978) to be a 
formation beneath the Southbridge in his Paxton 
"group." On the geologic map, we have included it in the 
undifferentiated Paxton Formation (Sp). 

The base of the granofels member is generally poorly 
exposed, but it apparently rests conformably on the 
sulfidic schist and quartzite unit (Spsq) or the quartzite 
and rusty schist unit (Spqr) or unconformably on the 
Partridge Formation. At the top, it either is overlain by 
separately mapped sulfidic schist or appears to grade 
into the Littleton Formation. Because of structural com- 
plexity, we have no reliable thickness figures. Tucker 
(1977) suggested a minimum thickness of about 200 m for 
the unit in the Barre area. 

As hinted above, the granofels member (Sp) of the 
Paxton Formation, particularly where close to rocks 
correlated with the Smalls Falls Formation of Maine 
(Moench and Boudette, 1970) or the Littleton Formation 
of central New Hampshire (Hatch and others, 1983), is 
very similar in lithology and sequence to the Madrid and 
Fall Brook Formations of Maine and the Warner Forma- 
tion of central New Hampshire, as well as the Fitch 
Formation of the Connecticut Valley belt. All of these 
units may be reasonably placed at the top of the Silurian 
section in their regions. The granofels member of the 
Paxton is also similar lithically to the Vassalboro Forma- 
tion in central Maine (Osberg, 1980) and to the Eliot and 
Berwick Formations of southeastern New Hampshire, 
the Oakdale Formation in the Nashua subbelt in 
Massachusetts, and the Hebron Formation in eastern 
Connecticut. 

For the derivation of the protoliths for the granofels 
member, the model for the Madrid Formation (Moench, 
1971) is adequate. The member originally consisted of 
interbedded feldspathic calcareous siltstone and impure 
calcareous shale and dolomitic limestone, possibly with a 
contribution of feldspar from volcanic ash. Interbedded 
schists were sulfidic black shales deposited when the 
volcanic and silt contributions were less. 

AMPHIBOLITE (Spa) 

Amphibolite is not a common rock type in the Paxton 
Formation, but a number of small lenses have been 
mapped. Some of these may have been contemporaneous 
volcanic deposits; others are probably mafic sills. Their 
structural and stratigraphic setting needs to be more 
thoroughly investigated. 

BIGELOW BROOK MEMBER (Spbs, Spbc) 

The Bigelow Brook Member is a subdivision of the 
granofels member of the Paxton in southern Massachu- 



setts (fig. 5, area Y). The unit is defined as the eastern 
part of the rocks previously mapped as the Bigelow 
Brook Formation (Pomeroy, 1975; Seiders, 1976; Moore, 
1978), specifically excluding the gray-weathering alumi- 
nous schists and gneisses that we include in the Littleton 
Formation. This member consists of about equal propor- 
tions of granofels and sulfidic schist usually interbedded 
in thick beds, as can be seen by looking at a large 
Massachusetts Turnpike roadcut from the Route 149 
overpass. In addition, because of the high metamorphic 
grade, many aluminous beds within the granofels contain 
garnets as much as 3 cm in diameter. A few marble beds 
crop out, as at the spillway of Westville Dam in South- 
bridge. Within this interbedded unit a few zones domi- 
nated by calc-silicate granofels (Spbc) have been sepa- 
rately mapped. 

SOUTHBRIDGE MEMBER (Spso) 

The Southbridge Member (fig. 5, area Z) lies in sharp 
contact along the Black Pond fault (Peper and Pease, 
1976) with the Bigelow Brook Member (Spbs) to the 
west. In contrast to the Bigelow Brook Member, the 
Southbridge is free of schist beds and is almost pure 
interbedded granofels and calc-silicate. Because of either 
metamorphic grade or bulk composition, it does not have 
large garnets. The Black Pond fault becomes a bedding- 
plane fault in southern Massachusetts and loses its 
identity northward, so that in the current state of 
mapping the Southbridge is not readily distinguished 
from the main granofels member of the Paxton. The 
Southbridge Member is most similar to the belt of Paxton 
between the Fitchburg plutons and the Wekepeke fault. 

SULFIDIC MICA SCHIST (Spss) 

Sulfidic mica schist is the designation used for mappa- 
ble layers of sulfidic mica schist of the Paxton Formation 
wherever they have been separately mapped. Some of 
these layers have continuity and are stratigraphically 
significant, whereas others appear to be merely lenses in 
other members. 

The largest areas of sulfidic mica schist occur at the top 
of the formation, above the granofels member, in the 
Gardner subbelt of the northern part of the State, on 
both limbs of the Gardner anticline. On the west limb of 
the anticline southwest of Gardner, the granofels mem- 
ber is directly overlain by gray schists of the Littleton 
Formation (H.B. Stoddart, written commun., 1977), but 
in giant highway excavations due west of Gardner a 
broad belt of sulfidic schists reappears rather abruptly 
below the Littleton (fig. 4, area L). The contact with the 
Littleton to the west is exposed in these highway cuts, 
but the rocks are so folded and sheared that significant 
interpretation cannot be made. The contact with the 
granofels member to the east is not exposed. The domi- 



STRATIGRAPHY OF THE MERRIMACK BELT, CENTRAL MASSACHUSETTS 



G19 



nant rock type is schist containing quartz, plagioclase, 
orthoclase, sillimanite, muscovite, biotite, garnet, 
graphite, ilmenite, and pyrrhotite typical of the 
sillimanite-muscovite-orthoclase zone and identical in 
character to many schists of the Partridge Formation. 
Assignment of this rock to the Paxton is encouraged by 
its stratigraphic position and more particularly by sev- 
eral large boudins as much as 20 m across of rather 
typical Paxton purple biotite granofels with spectacular 
green and pink calc-silicate beds. Also present in this 
exposure are boudins of a dismembered metamorphosed 
gabbro dike that cuts the sulfidic schist. North of the 
highway exposures this sulfidic schist belt is poorly 
exposed, but it has been tentatively traced north to New 
Hampshire. 

On the east limb of the Gardner anticline is a much 
more extensive and wide belt (fig. 4, area N) of identical 
sulfidic schist that has only been studied in reconnais- 
sance. These two belts probably correlate across the 
crest of the anticline, thus overlying the granofels mem- 
ber (Sp) and underlying the gray schist of the Littleton 
Formation (Dl) to the east. If our interpretation of the 
stratigraphic position of these sulfidic rocks is correct, 
they are possibly equivalent to sulfidic rocks in central 
New Hampshire (Malinconico, 1982; Hatch and others, 
1983) that locally lie between the gray granofels of the 
Warner Formation and the gray schists of the Littleton 
Formation. 

As previously described, the narrow belts shown as 
Paxton granofels member (Sp) along the west side of the 
Fitchburg Complex actually consist of granofels and 
sulfidic schist that could be mapped separately at 
1:24,000. One of these belts has been tentatively traced 
north to the New Hampshire line. In this belt the 
granofels pinches out and the belt consists entirely of 
sulfidic mica schist locally containing exposures domi- 
nated by feldspathic quartzite. It is here labeled "Spss" 
(fig. 6, area T). Since the State bedrock map was 
compiled, Peterson (1984) has completely revised the 
map pattern near Ashburnham and has demonstrated 
that these sulfidic rocks are much more extensive than 
previously shown. At present they pose a stratigraphic 
dilemma. Their association with typical Paxton granofels 
at Wachusett Mountain suggests that they may be 
merely a more schist-rich facies of the granofels member 
(Sp). Their similarity to rusty schist at the top of the 
formation (Spss) near Gardner suggests a correlation 
with the uppermost Silurian. The similarity of the feld- 
spathic quartzite to the quartzite and rusty schist unit 
(Spqr) of the Wachusett Mountain subbelt suggests a 
correlation with the lower part of the Silurian. If our 
structural interpretation of the Wachusett Mountain 
subbelt is correct and the narrow belts of Silurian strata 
are in anticlinal east-directed nappes from a western 



region where the entire Silurian is thin, all three of the 
tentative correlations may be correct. Sulfidic schist 
inclusions within the Fitchburg Complex are identical to 
the sulfidic schists just discussed and are also labeled 
"Spss" (fig. 6). 

Within the Paxton Formation southeast of the Fitch- 
burg plutons, Grew (1970) mapped several isolated 
lenses of sulfidic schist, here shown as "Spss." Where 
exposed on the north side of Indian Lake (fig. 6), the rock 
is a very fine grained black mica schist, which is very 
magnetic due to abundant pyrrhotite and appears to be 
much lower in grade than any of the schists described 
above. 



LITTLETON FORMATION 
(Dl, Dl+Ops, Dlf, Dlo, Dim) 

The Littleton Formation in the Merrimack belt is an 
extension of gray-weathering schists of the Littleton in 
the eastern part of the Connecticut Valley belt. Unfor- 
tunately all the Littleton rocks of the Merrimack belt are 
exposed in narrow synclinal belts mostly representing 
folds of the nappe stage, so that physical tracing of 
stratigraphy across strike is impossible. That some of the 
belts are synclinal has been demonstrated by graded 
bedding, particularly in the syncline that runs through 
Mt. Pisgah west of Brimfield and Wales (Peper and 
Pease, 1976) and in two narrow synclines in the village of 
Barre and just east of it (Tucker, 1977). Many of the 
synclines in the Ware subbelt contain no recognized 
Silurian strata, and the gray schists assigned to the 
Littleton are in contact with rocks mapped as Middle 
Ordovician Partridge Formation. In some of these syn- 
clines, the gray- weathering rocks may not be Littleton 
Formation and perhaps should be assigned to some other 
gray-weathering unit in the central New Hampshire 
sequence. In the Gardner and Wachusett Mountain sub- 
belts, the gray-weathering schists are in sequences with 
underlying rocks assigned with confidence to the Silu- 
rian, which makes their assignment to the Littleton more 
certain. 

The different character of the rock in the different 
belts of Littleton Formation results from variations in 
the protoliths, as well as from metamorphic grade and 
degree of deformation during metamorphism. In the 
Ware and Gardner subbelts, the grade ranges from 
sillimanite-muscovite in the north to sillimanite- 
orthoclase-garnet-cordierite in the south. In the Wachu- 
sett Mountain subbelt, metamorphism seems to be in the 
sillimanite-muscovite zone over a broad area west of the 
Fitchburg plutons where sillimanite pseudomorphs after 
andalusite are abundant and assemblages of muscovite- 
biotite-garnet-cordierite are locally present (Peterson, 
1984). Inclusions in the Fitchburg plutons locally contain 



G20 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



sillimanite-muscovite-orthoclase assemblages again with 
andalusite pseudomorphs. In the narrow belt of Littleton 
along the east margin of the Fitchburg plutons, meta- 
morphic grade drops rather abruptly from sillimanite- 
muscovite assemblages with andalusite pseudomorphs at 
the western contact through andalusite-muscovite schist 
to garnet-mica schist locally containing kyanite at the 
eastern contact of the belt (Nelson, 1975; Peper and 
Wilson, 1978; Robinson and others, 1978). In the South- 
bridge subbelt, Littleton only occurs along the border 
with the adjacent Wachusett Mountain and Gardner 
subbelts. 

WARE SUBBELT 

The Littleton Formation in the western part of the 
Ware subbelt, west of the Hardwick pluton (fig. 4, area 
A), is identical in all aspects to the Littleton in areas 7 
(figs. 4, 5) and 8 (near Amherst in the Connecticut Valley 
belt (Hatch and others, 1988)). The unit is rather poorly 
exposed and commonly contains strongly foliated rock of 
mylonitic aspect in the sillimanite-muscovite-orthoclase 
and higher grade areas. The rock is commonly a gray- 
weathering, dark, biotite-rich schist with fine sillimanite 
and garnet grains and conspicuous augen of feldspar. 
Field (1975) identified narrow, poorly exposed belts of 
gray schist on either side of the Hardwick pluton in the 
Ware area (fig. 5, area A) and suggested that the pluton 
may be in the middle of an isoclinal syncline of Littleton 
Formation. However, the distribution of Partridge For- 
mation and Fitch Formation as inclusions and along the 
margins of the pluton near Petersham and Phillipston 
(C.K. Shearer, written commun., 1980) renders this 
suggestion improbable. 

The next belt of gray schist, entirely east of the 
Hardwick pluton (figs. 4, 5, area B), appears to hinge out 
to the south and has been called the Ragged Hill syncline 
(Field, 1975; Robinson and others, 1982a, Stop 4). These 
rocks are conspicuously well bedded quartz-rich schists 
with abundant 0.5- to 1-cm grains of garnet, sillimanite, 
and commonly cordierite. Robinson and others (1982a), 
on the basis of the quartz-rich character, suggested a 
correlation with the Perry Mountain Formation of cen- 
tral New Hampshire, but J.B. Lyons and N.L. Hatch, 
Jr. (oral commun., 1982), suggested that these schists 
much more closely resemble the lower part of the Range- 
ley Formation. Correlation with the Rangeley is greatly 
enhanced by the fact that the belt of red-rusty schists 
containing calc-silicate pods directly to the west, origi- 
nally mapped by Field (1975) with considerable uncer- 
tainty as the Lyon Road belt of Partridge Formation, is 
now considered by us to correlate with the upper part of 
member C of the Rangeley. Unfortunately both of these 



belts, exposed over relatively broad areas near Ware, 
virtually pinch out to the south and north, preventing 
significant direct correlations. 

The next belt of Littleton to the east, flanked on both 
sides by Fitch Formation north of Ware, has been called 
the Big Garnet syncline (figs. 4, 5, area C). The hinge of 
the syncline is located on the east face of Ragged Hill 
near Ware. Nearly the entire width of the syncline is 
exposed in a single set of cliffs on the northeast slope of 
Ragged Hill (Robinson and others, 1982a, Stop 5), and 
subsequent observation of this outcrop showed excellent 
graded bedding topping east on the west side and west 
on the east side. The characteristic rocks of the Big 
Garnet syncline are poorly exposed in Barre but superbly 
exposed in the southwest part of the town of Templeton. 

The characteristic rock of the Big Garnet syncline is 
rather homogeneous, poorly layered, gray, medium- to 
fine-grained, sillimanite-biotite-cordierite-garnet schist. 
This schist is cut by vein networks, commonly deformed, 
of quartz, K-feldspar, plagioclase, and cordierite enclos- 
ing subhedral to euhedral garnets as much as 4 cm in 
diameter. The size difference between garnets in the 
matrix (—1-3 mm) and in the feldspathic veins (l^i cm), 
as well as the character and physical arrangement of the 
veins, strongly suggests that the vein garnets grew in 
the presence of a felsic silicate melt, probably a product 
of local fluid-absent melting (Robinson and others, 1982a, 
p. 341-342). In some exposures southwest of Templeton, 
the large garnets are set in a more massive matrix of 
quartz, cordierite, and plagioclase, suggesting that their 
original melt host may have been transported away 
during metamorphism. Electron probe analyses of these 
garnets (Richardson, 1975; Tracy and others, 1976; Rob- 
inson and others, 1982a) show that they are completely 
homogeneous except where their outer edges touch an 
adjacent grain of cordierite or biotite, permitting local- 
ized retrograde ion exchange. Commonly the big garnets 
contain internal zones of crystallographically oriented 
ilmenite plates parallel to and several millimeters inside 
crystal faces. In addition to ilmenite, these rocks also 
contain traces of magnetite, a mineral not found in any of 
the other types of Littleton. Field (1975) described two 
samples of this rock containing sillimanite pseudomorphs 
after andalusite. 

Locally interbedded with the characteristic rock are 
beds as much as 2 m thick of fine-grained, light-purple- 
gray biotite-feldspar granofels. This rock type, as well as 
the big-garnet rock itself, led N.L. Hatch, Jr., and J.B. 
Lyons (oral commun. , 1982) to suggest a direct correla- 
tion with the uppermost part of the Warner Formation of 
central New Hampshire, essentially at its contact with 
the Littleton. Such a correlation, not shown on the State 
bedrock map, with the uppermost Warner, rather than 



STRATIGRAPHY OF THE MERRIMACK BELT, CENTRAL MASSACHUSETTS 



G21 



with the Littleton itself, seems eminently suitable for the 
rocks of the Big Garnet syncline and would help explain 
some of their peculiarities. 

East of the Big Garnet syncline, the same big-garnet 
rock also occurs fairly consistently as a selvage a few 
meters thick between the Fitch Formation and the west 
contact of the Coys Hill pluton (figs. 4, 5, area D). This 
relationship supports the contention of Field (1975) that 
the pluton occupies a position in the stratigraphy equiv- 
alent to that of the Littleton Formation. In the southern 
part of the Ware area, the Coys Hill pluton converges on 
the east margin of the Monson Gneiss, so that all of the 
synclinal belts just described are squeezed into a very 
narrow, poorly exposed zone known to local workers as 
"The Slot" (fig. 5, near letter E). 

East of the Coys Hill pluton, in the Ware subbelt, 
rocks assigned to the Littleton Formation occur mainly 
in four synclinal belts (figs. 4, 5): the Coys Hill syncline 
(area F), the Prouty Road syncline (area G), the Gilbert 
Road syncline (area H), and the Kruse Road syncline 
(area I). Along much of the east contact of the Coys Hill 
pluton, the granite is in direct contact with Partridge 
Formation or with lenses of Fitch Formation, but near 
Barre a thin strip of gray schist of the Littleton appears 
along the contact. Several miles farther north, in Barre, 
the Partridge of the intervening anticline hinges out so 
that the Coys Hill and Prouty Road synclines merge. In 
the northern part of the State, the exposure in this 
region east of the Coys Hill pluton is so poor that several 
areas have been designated on the State bedrock map 
only as "interfolded Littleton and Partridge" (Dl +Ops). 
From Barre south (fig. 5, area G), the Prouty Road 
syncline contains a large area of Littleton Formation 
including some unusual rock types interpreted as meta- 
morphosed volcanic rocks. Area G includes the type area 
of the Mount Pisgah Formation of Peper and others 
(1975; see also Peper and Pease, 1976; Seiders, 1976; 
Pomeroy, 1977) shown as Littleton Formation on the 
State bedrock map. Graded bedding demonstrates the 
synclinal nature of the belt near Mt. Pisgah and also in 
Barre (Tucker, 1977). The Gilbert Road and Kruse Road 
synclines (figs. 4, 5, areas H and I) are the most 
continuous and consistent belts in the Ware subbelt. 
They have been traced tentatively from southernmost 
Massachusetts into New Hampshire. The Kruse Road 
syncline, though well exposed elsewhere, is hardly 
exposed at all in the Ware area, where Field (1975) did 
not show it. In southern Massachusetts the gray schists 
of these synclines were mapped as part of the upper 
schist member of the Hamilton Reservoir Formation. 
The synclinal nature of the Gilbert Road belt is shown by 
graded bedding in the Barre area (Tucker, 1977). 

The gray schists of the Prouty Road syncline (area G) 
are conspicuously rich in garnet and sillimanite and also I 



commonly contain cordierite or muscovite at appropriate 
metamorphic grade. Graphite and ilmenite are the 
opaque minerals. In the Ware and Barre areas, quartz- 
ose beds are subordinate, and some are graded (Tucker, 
1977). Quartzose beds are conspicuous or even dominant 
near Mt. Pisgah in the southern part of the State. The 
schists commonly contain layers and concretionary 
lenses of gray biotite-plagioclase granofels or green to 
pink diopside-grossular calc-silicate granofels (Tucker, 
1977). 

An abundant and locally predominant rock type in the 
Littleton of the Prouty Road and Coys Hill synclines 
(figs. 4, 5, areas G and F, respectively) of the Ware and 
Barre areas is white quartz-feldspar gneiss (Dlf), 
mapped as the feldspathic gneiss member by Field (1975) 
and Tucker (1977) and interpreted as metamorphosed 
felsic volcanic rocks. The gneiss commonly contains 
minor garnet and biotite, and locally sillimanite and 
secondary muscovite, and bears some resemblance to 
highly metamorphosed and deformed pegmatite and to 
the felsic upper part of the Ammonoosuc Volcanics 
(Robinson, 1963). The structural position of the gneiss 
and its locally intimate interlayering with gray schist 
favor its interpretation as volcanic rocks within the 
Littleton. 

Also present in the Prouty Road syncline of the Ware 
area southwest of New Brain tree (fig. 5), and commonly 
associated with the feldspar gneisses, are several lenses 
of coarse-grained, weakly foliated, brown-weathering 
orthopyroxene-biotite gneiss (Dlo), mapped as the ortho- 
pyroxene gneiss member by Field (1975). The rock 
consists of andesine or labradorite, intermediate ortho- 
pyroxene, cummingtonite, and biotite, with or without 
hornblende, quartz, and magnetite (Robinson and oth- 
ers, 1982a). Emerson (1917) published an analysis of 
this rock, which he mistakenly described as "wehrlite," 
showing 50 percent Si0 2 and the composition of a 
hypersthene-olivine andesite. Because of its strati- 
graphic position, the orthopyroxene gneiss is considered 
most probably to have been flows of andesite of unusual 
composition, although an intrusive origin cannot be ruled 
out. 

The Littleton schists of the Gilbert Road syncline 
(Field, 1975; figs. 4, 5, area H) are gray-weathering, 
well-bedded sillimanite-garnet-biotite schists with 
quartzose beds 5-10 cm thick, showing excellent graded 
bedding in the Barre area. Garnets are typically 1-2 cm 
in diameter. Gray to white beds, 5-8 cm thick, of 
equigranular calc-silicate granofels are common in this 
belt and consist predominantly of quartz and calcic 
plagioclase with subordinate grossular garnet, diopside, 
clinozoisite, and sphene. Several zones of feldspathic 
gneiss (Dlf) were mapped separately in this belt. 
Pomeroy (1977) mapped these rocks as part ("husg") of 



G22 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



the upper schist member of the Hamilton Reservoir 
Formation, and they are also mapped with certainty into 
outcrops of Littleton feldspathic gneiss north of Route 2 
in the Templeton area. The tracing of these rocks north- 
ward to New Hampshire and into slightly lower meta- 
morphic grades is based on reconnaissance only. J.B. 
Lyons and N.L. Hatch, Jr. (oral commun., 1982), sug- 
gested that these rocks look more like the lower part of 
Rangeley member C of central New Hampshire than like 
typical Littleton, and the geology of central Massachu- 
setts provides no solid evidence against this correlation. 
The Littleton schist of the Kruse Road syncline (fig. 4, 
area I) east of Barre (Tucker, 1977) typically is well- 
foliated, gray-weathering, poorly layered garnet- 
sillimanite-plagioclase schist, commonly with a streaked 
appearance due to the abundance of pegmatite veins. 
Garnets are 0.5 cm in diameter or smaller. This belt was 
mapped separately by Pomeroy (1977) and Seiders (1976) 
as the East Hill belt ("husn") of gray sillimanite schist 
within the upper schist member of the Hamilton Reser- 
voir Formation. It can be found in two or three outcrops 
near New Braintree and West Brookfield. South of 
Barre, garnets tend to be larger, and cordierite has been 
locally identified. North of Barre, muscovite is abundant. 
The homogeneous, poorly bedded character and the local 
occurrence of schist of the Fitch Formation (Sfss) on one 
flank of this belt southeast of Templeton make correla- 
tion of these rocks with the Littleton Formation more 
probable than it was for the rocks of the Gilbert Road 
syncline. 

GARDNER SUBBELT 

The Littleton Formation of the Gardner subbelt occurs 
in three bands, the first in a complex syncline along the 
west limb of the Gardner anticline (figs. 4, 5, area K), the 
second in a deep, complexly refolded syncline that 
appears to trace westward from near Worcester across 
several anticlinal features to the vicinity of the Oakham 
anticline (figs. 4, 5, 6, area P), and the third in the 
Ashburnham area (figs. 4, 6, western part of area Q). 
The second band, here called the Harding Hill syncline 
("the Worm of central Massachusetts") after Tucker 
(1977), is discussed separately below. The third band, 
included in the Gardner subbelt on structural grounds, is 
better discussed in the stratigraphic context of the 
Wachusett Mountain subbelt. 

The belt on the west limb of the Gardner anticline 
(area K) was described in detail by Tucker (1977), who 
identified it as a gray graphitic schist member of the 
Paxton Formation. In 1978 R.D. Tucker and Peter 
Robinson reinterpreted the belt as an isoclinal syncline of 
Littleton Formation, here called the Natty Pond syn- 
cline, on the basis of comparisons of the Barre and 



Wachusett Mountain areas. The syncline appears to 
hinge out southward in a poorly exposed area northwest 
of Oakham between the sulfidic schist and quartzite unit 
(Spsq) and the granofels member (Sp) of the Paxton 
Formation. The belt is best exposed in the Hubbardston 
area and east of Templeton and has been traced in 
reconnaissance to New Hampshire. 

Typical schists of the Natty Pond syncline (Tucker, 
1977, p. 36; Robinson, 1979, p. 156) are in the sillimanite- 
K-feldspar-muscovite zone. They are gray- to slightly 
brown-weathering schists and subordinate beds of calc- 
silicate granofels, a few with carbonate cores, and typi- 
cally have quartzofeldspathic segregations 15-20 cm long 
strung out throughout the rock. The dominant minerals 
are quartz, plagioclase, biotite, muscovite, and garnet 
(crystals usually 1-3 mm) and minor orthoclase, silliman- 
ite, graphite, and ilmenite. The schist is medium grained 
and evenly foliated and in places has slight compositional 
layering, but bedding is poorly displayed. Pegmatite and 
medium- to fine-grained garnetiferous granitic gneiss 
sills are generally more abundant than in adjacent units. 
These schists are quite similar to the Littleton of the 
Kruse Road syncline and in the Wachusett Mountain 
subbelt. 

Contacts of the Littleton of the Natty Pond syncline 
with the sulfidic schist and quartzite (Spsq) of the Paxton 
to the west appear to be sharp and are easily mapped 
because of the dramatically different character of the 
rocks. Contacts with the granofels member (Sp) of the 
Paxton to the east are also generally distinct, but, in 
areas of particularly good outcrop, as at the top of the 
cliffs west of Riverside Cemetery east of Barre (Robin- 
son and others, 1982a, Stop 14), biotite granofels and 
sulfidic schist typical of the Paxton granofels member 
appear to be interbedded with gray schist typical of the 
Littleton over several tens of meters. The contact with 
the sulfidic schist unit (Spss) of the Paxton is exposed in 
cuts on Route 2 in East Templeton, but pegmatites and 
complex folding make contact relations cryptic. 

WACHUSETT MOUNTAIN SUBBELT 

Gray-weathering schists mapped as Littleton Forma- 
tion are the dominant stratified rocks of the Wachusett 
Mountain subbelt and the extreme eastern edge of the 
Gardner subbelt (figs. 3, 6). They form the envelope 
surrounding the belts of Paxton Formation that are 
interpreted as isoclinal anticlines and form inclusions in 
the Fitchburg plutons. These schists are exposed at the 
summits, or on high subsidiary peaks, and on the steep 
slopes of New Ipswich Mountain, Pratt Mountain, Mt. 
Watatic, Little Watatic, Wachusett Mountain, and Little 
Wachusett. West of the Fitchburg plutons the schists are 
in the sillimanite-muscovite zone of regional metamor- 



STRATIGRAPHY OF THE MERRIMACK BELT, CENTRAL MASSACHUSETTS 



G23 



phism; east of the plutons they appear in a rather steep 
contact gradient away from the plutons and may be as 
low as garnet zone. 

West of the plutons (fig. 6, area Q), the gray schists 
show local variety but an overall monotony of aspect. 
Aluminous beds are fairly abundant and may contain 
sillimanite pseudomorphs (or retrograded pseudo- 
morphs) after andalusite, which appear as projections on 
weathered surfaces and are referred to by local workers 
as "andalumps." Irregular quartzose beds, which do not 
display graded bedding, and poorly layered, gray quartz- 
plagioclase-biotite-muscovite schists and gneisses are 
common. Minerals include quartz, plagioclase, silliman- 
ite, biotite, muscovite, garnet (usually 1 to 3 mm or less 
in diameter), graphite, and ilmenite. Some outcrops are 
pitted, due to weathering of secondary chlorite that has 
replaced garnet. Structural and stratigraphic interpreta- 
tion is hindered by several generations of quartz veins 
and pegmatites and by several generations of folding not 
easily deciphered. Another common feature of these 
rocks is "footballs" or zoned lenses of gray to green or 
pink calc-silicate granofels as much as 1 m in diameter. 
The lenses usually have an outer zone of gray biotite- 
calcic plagioclase granofels and an inner zone of spotted 
plagioclase granofels with diopside and grossular. Cores 
of the lenses locally contain preserved calcite. 

A nearly continuous belt of gray mica schist (fig. 6, 
area U) occurs along the east margin of the Fitchburg 
plutons from Fitchburg south, and similar schist occurs 
as large (mappable) and small (unmappable) screens 
within the intrusions. In the Sterling area (northwest of 
Oakdale in fig. 6), Hepburn (written commun., 1976) 
called these schists the Bee Hill Formation, and similar 
rocks south of Maiden Hill (fig. 6, areas U and P) were 
called Holden Formation by Grew (1970). The southern 
part of Grew's Holden Formation is the Littleton of the 
Harding Hill syncline at the southern edge of the Wachu- 
sett Mountain subbelt. South of Fitchburg the gray 
schist along the east margin of the Fitchburg plutons is 
highly varied, depending on metamorphic grade, but 
locally contains pink or white andalusite. Some expo- 
sures show excellent bedding. 

Near Fitchburg the gray schist unit consists of gray 
graded-bedded pelite and rare thin layers of white 
quartzite (Peper and Wilson, 1978). The schist is com- 
posed primarily of muscovite, quartz, biotite, and plagi- 
oclase. It commonly contains staurolite and garnet por- 
phyroblasts and rarely kyanite porphyroblasts locally 
converted to fibrolite. Fibrolitic sillimanite is found 
primarily adjacent to granite of the Fitchburg plutons. 
The schist is apparently interbedded at the base with 
granofels of the underlying Paxton Formation (Peper 
and Wilson, 1978). 



A thin section of an inclusion of gray schist in Fitch- 
burg granite from the Quabbin Tunnel shows andalusite 
porphyroblasts that have been completely replaced by 
coarse sillimanite. What was apparently once a rim of 
muscovite around the andalusite has been completely 
replaced by fibrous sillimanite and orthoclase. The tex- 
ture gives insight into possible complexities in the met- 
amorphic history of this region. 

The belt of gray schist along the west edge of the 
Massabesic Gneiss Complex is labeled "Dl?" (fig. 6, area 
V) but is not like the Littleton described elsewhere. The 
rock is rather massive, poorly layered schist containing 
abundant quartz and plagioclase and very coarse biotite 
and muscovite. It may correspond to rocks mapped as 
the lower part of Rangeley member C in central New 
Hampshire. 

The Littleton in the narrow bands interspersed in the 
Paxton Formation in the Townsend area (fig. 6, area X) 
consists of lustrous mica schist containing few quartzo- 
feldspathic layers (G.R. Robinson, 1981). Graded beds 
indicate that the mica schist overlies the Paxton (Robin- 
son, 1981). The thickness of the schist in the Townsend 
area is from to 235 m. 

HARDING HILL SYNCLINE 

The sinuous band of Littleton mapped along the axial 
surface of the Harding Hill syncline (figs. 4-6, area P) is 
discussed last because it is poorly exposed, because its 
connections are highly uncertain, and because it passes 
through or along the boundaries of several subbelts. The 
band is presently interpreted as the core of a refolded 
recumbent syncline overfolded to the east, with an 
inverted section of Paxton Formation structurally above 
and to the north and a right-side-up section of Paxton 
structurally below and to the south. 

The schist in the syncline is traced westward from a 
postmetamorphic normal fault east of Holden and north 
of Worcester (fig. 6, area P) in a sinuous path along the 
southern edge of the Wachusett Mountain subbelt. The 
schist is well exposed where the band crosses the axis of 
the Gardner anticline west of Rutland in a cut on the 
abandoned Central Massachusetts Railroad known as 
Shannock's Folly. The schist in this eastern part of the 
band is typically gray, medium-grained biotite- 
muscovite-sillimanite schist, similar to the Littleton else- 
where in the Wachusett Mountain subbelt. Southeast of 
Holden, a narrow, poorly exposed strip of laminated 
siliceous marble (Dim) locally contains idocrase (Grew, 
1970; J.C. Hepburn, written commun., 1977). 

From Rutland the band is traced southward (fig. 5, 
area P) through a zone of synmetamorphic faults and 
diorite intrusions into Connecticut. The band passes 
across the highly attenuated southern extension of the 



G24 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



Hubbardston syncline and thence northward along the 
east limb of the Oakham anticline. For some distance 
along this limb, the Paxton Formation is missing, and the 
Littleton is in direct contact with the Partridge Forma- 
tion of the Oakham anticline, presumably along a syn- 
metamorphic fault. From Rutland southward there is a 
progressive change in the character of the Littleton from 
medium-grained sillimanite-biotite-muscovite schist to 
coarse-grained sillimanite-garnet-cordierite-K-feldspar 
schist and gneiss within about 15 km of the Connecticut 
line. Within the higher grade parts, strongly foliated 
sillimanite-garnet pegmatites, locally containing graph- 
ite, are abundant. Granitic layers contain 1- to 2-cm 
aggregates of coarse sillimanite and biotite apparently 
pseudomorphous after cordierite. Within the schists are 
more feldspathic granulitic layers, several meters thick, 
that contain diopside-grossular calc-silicate layers 5-10 
cm thick. Sillimanite pseudomorphs of andalusite have 
been identified at several localities. In overall aspect 
these higher grade schists look very similar to the 
Littleton in the southern part of the Ware subbelt. 
Spectacular exposures in this area include the cut on the 
north side of the Massachusetts Turnpike where it is 
crossed by New Boston Road (Robinson and others, 
1982a, Stop 11) and outcrops near the southeast end of 
the pond in Bigelow Hollow State Park in Connecticut, 
where Seiders (1976) identified several mylonites. Over- 
all these rocks (Dl on the State bedrock map) are the ones 
considered typical by Peper and Pease (1976; Peper and 
others, 1975) of the western part of their Bigelow Brook 
Formation. 

Fairly extensive exposures of Littleton are present 
east of the northern oval of Partridge Formation in the 
core of the Oakham anticline (figs. 4, 5, area P), where 
Paxton Formation again appears between the Littleton 
and Partridge. These rocks are again lower grade 
muscovite-bearing schists. They appear to project north- 
ward into an area of no exposure along the line of 
the Quabbin Aqueduct Tunnel, in which Fahlquist 
(1935) identified them as the "Middle Member of the 
Paxton Formation. " He characterized the "Middle Mem- 
ber" as mica schist overlain and underlain by typical 
purple granofels with calc-silicate of his "Upper" and 
"Lower Members." In the present context the "Middle 
Member" is interpreted as a recumbent syncline of 
Littleton structurally overlain and underlain by the 
granofels member of the Paxton Formation (Sp). Tracing 
of the recumbent syncline of Littleton around the north 
end of the Oakham anticline is based entirely on inter- 
pretation of Fahlquist's tunnel data and collection of 
specimens, because there are virtually no surface out- 
crops. The position of the synclinal hinge west of the 
Oakham anticline is conjectural. 



NASHUA AND ROCKINGHAM SUBBELTS 

The eastern part of the Merrimack belt (and the 
Merrimack synclinorium) consists of two subbelts: the 
Nashua subbelt, which lies east of the Southbridge and 
Wachusett Mountain subbelts and west of the Nashoba 
zone, and the Rockingham subbelt, which forms a wedge 
between the Nashoba zone and the Nashua subbelt in the 
northeast part of the Merrimack belt in Massachusetts 
(figs. 1, 35). The subbelts are distinguished partly on the 
basis of minor differences in rock type, metamorphism, 
and structure but mostly on historical differences in 
mapping of units and in terminology. The Nashua sub- 
belt, named for the Nashua River, which drains the area, 
coincides with a metamorphic and structural(?) trough 
called the Nashua synclinal by Crosby (1880) and the 
Nashua trough by Smith and Barosh (1981). The Rock- 
ingham subbelt gets its name from the Rockingham 
anticlinorium, which projects into this area from New 
Hampshire and southern Maine (Billings, 1956; Hussey, 
1968). The western margin of the Nashua subbelt is 
essentially the Wekepeke and Pine Hill faults of the 
Mesozoic(?) Flint Hill fault system (Rodgers, 1970, p. 
107). The eastern margin of the Merrimack belt and the 
subbelts is defined by the Clinton-Newbury (also known 
as Essex) fault system and coincides with the western 
boundary of the Nashoba Formation of the Nashoba 
zone. 

Appreciable mapping has been done in the Nashua and 
Rockingham subbelts since the early work of Burbank 
(1876), Crosby (1880), Emerson (1898), and Perry and 
Emerson (1903): Grew (1970, 1973, 1976) and Hepburn 
(written commun., 1976; 1978) in the Shirley and in the 
Worcester areas, Barosh (1974, 1976, 1977) and Dixon 
(1974; written commun., 1977 and 1978) in the Oxford 
and Webster area, and Peck (1975, 1976) in the Clinton 
area; Skehan (1967) and Skehan and Abu-Moustafa 
(1976) presented detailed data from the Wachusett- 
Marlborough tunnel at the east margin of the Nashua 
subbelt near Clinton. To the north, G.R. Robinson (1978, 
1981) mapped and described the rocks in the Lancaster 
and Pepperell areas, overlapping the subbelts; Gore 
(1976) studied the plutonic rocks near Ayer; Sriramadas 
(1966), Novotny (1969), Sundeen (1971), and more 
recently Shride (1976) mapped and described the rocks in 
the Rockingham subbelt. Correlation of previous nomen- 
clature with that used on the State bedrock map is shown 
in figure 7. Barosh and others (1977) compiled the 
existing mapping for their map of the geology of the 
Boston l°x2° quadrangle. Their divisions are similar to 
those on the State bedrock map, but their unit designa- 
tions are based largely on lithology. 



STRATIGRAPHY OF THE MERRIMACK BELT, CENTRAL MASSACHUSETTS 



G25 



NASHUA SUBBELT 

On Emerson's (1917) geologic map of Massachusetts, 
the east part of the Merrimack belt coincides with the 
boundary between his "gneisses and schists of undeter- 
mined age," which he informally called "Bolton Gneiss" 
and is now called the Nashoba Formation (Hansen, 1956) 
to the east and his Worcester Phyllite, Oakdale Quartz- 
ite, Boylston Schist, Paxton Schist, and Merrimack 
Quartzite to the west (fig. 7), all of which he assigned to 
the Carboniferous on the basis of plant fossils found at 
Worcester. Grew (1970, 1973, 1976) showed that the 
fossiliferous strata at Worcester are isolated in a fault- 
bounded block and that the Carboniferous age assign- 
ment is not applicable to the other rocks mapped as 
Carboniferous by Emerson and Hansen. Emerson's gen- 
eral arrangement of rock units has persisted, however, 
and for the most part only new age assignments and 
minor changes in nomenclature have been made during 
subsequent years (Goldsmith and others, 1982). How- 
ever, the detailed stratigraphy of the rocks in the 
subbelts is still a matter of controversy because of 
differing interpretations of primary sedimentary fea- 
tures and minor structural features in the rocks and 
because of difficulty in correlating rock units across 
faulted boundaries. 

Most of the rocks in the Nashua and Rockingham 
subbelts are interlayered calcareous metasiltstone, meta- 
pelite, and subordinate quartzite and marble that repre- 
sent the eastern limit of outcrop of the largely Silurian 
and Devonian turbidite and fan basin-fill of the Merri- 
mack synclinorium. In the Nashua subbelt, these strata 
consist of the Boylston Schist (SObo), Tower Hill Quartz- 
ite (St, Sts), Oakdale Formation (So), and Worcester 
Formation (DSw). Unconformably overlying the turbid- 
ite sequence is the Pennsylvanian Coal Mine Brook 
Formation (Pcm) in the Worcester area. 

Primary sedimentary features are preserved in 
the less metamorphosed rocks. Graded bedding in the 
coarser beds and thin to cyclic laminar bedding in 
the finer grained beds, as well as local current bedding 
and scour, suggest turbidite deposition. Orthoquartz- 
ite and metaconglomerate in the lower part of the 
sequence are interpreted as proximal or shoal deposits, 
whereas the calcareous metasiltstone and pelite higher in 
the sequence are interpreted as proximal to distal depos- 
its. Noncalcareous carbonaceous phyllite and schist, 
characteristic of the upper part of the sequence in the 
axial area of the Merrimack synclinorium, are less abun- 
dant in this eastern margin. On the basis of current- 
direction studies, Peck (1976, p. 248) suggested that the 
sediment came from the west. Sulfidic schists are mostly 
lacking in the Nashua subbelt, except in the basal part of 
the sequence. 



The Ayer Granite (Sagr) and its phases and granite at 
Millstone Hill (Dmgr) intrude or appear to intrude the 
sequence on the east, and the Fitchburg Complex (Dfgr) 
and its phases intrude the sequence on the west. The 
metamorphic trough lies between these two areas. 

Metamorphism ranges from greenschist to amphibolite 
fades. Rocks are in the chlorite zone in the northern part 
of the Nashua subbelt but increase in grade to the south. 
However, the low-grade trough persists southward into 
eastern Connecticut (Thompson and Norton, 1968). 
Rocks on the flanks of the Nashua subbelt may reach 
andalusite-staurolite or locally sillimanite zone. Minerals 
typical of regional metamorphism in the metasiltstones in 
the chlorite and biotite zones are chlorite, biotite, anker- 
ite, and locally calcite or dolomite. Actinolite and actin- 
olitic hornblende, garnet, and locally diopside are found 
in rocks at higher grades. Epidote is present throughout. 
The mineralogy tends to give many rocks a gray-green 
color; however, biotite is characteristically brown or 
red brown and where abundant tends to give these rocks 
a purplish or brownish tinge on a fresh surface. Noncal- 
careous pelites tend to be gray to dark gray due to 
contained carbonaceous material, but at the higher 
grades of metamorphism they may be silvery and span- 
gled with prominent muscovite. Pelitic layers in the 
middle grades of metamorphism may contain andalusite 
and staurolite. Metasiltstone appears to be thicker bed- 
ded and coarser grained (sand size rather than silt size) 
at higher grades than it is at the lower grades (G.R. 
Robinson, 1981, p. 27-36), and much of it at higher 
grades has a salt-and-pepper appearance. The coarse 
grain size in these areas could be in part a primary 
sedimentary feature as well as a result of recrystalliza- 
tion. A steep metamorphic gradient exists westward 
from the Nashoba Formation in the sillimanite zone 
through andalusite in the Tadmuck Brook Schist (fig. 7) 
into the rocks of the Nashua subbelt, which are in the 
chlorite and biotite zones. 

BOYLSTON SCHIST (SObo) 

The Boylston Schist (Emerson, 1917; Grew, 1970; 
Hepburn, 1978) consists of gray to dark-gray sillimanite- 
bearing quartz-muscovite schist and gneiss, containing 
locally prominent quartzofeldspathic layers as much 
as 0.5 m thick, and subordinate rusty- weathering 
sillimanite-mica schist and tan to light-greenish-gray 
calc-silicate rock. Some beds of schist contain thin lenses 
of coticule. The Boylston is estimated to be 1,000 m thick 
(Grew, 1973; Hepburn, 1978). The distribution of the 
Boylston is essentially that shown by Emerson (1917, p. 
67-68). Grew (1970) included phyllite in his Boylston 
Formation (Unit B of Grew, 1973) that we now assign to 
the Worcester Formation. We now assign Grew's medial 



G26 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



Nashua subbelt 



Rockingham subbelt 



Eastern 
Connecticut 



Oxford-Webster 
area 



Worcester- 
Clinton area 



Northeastern 
Massachusetts 



Southeastern 
New Hampshire and 
southwestern Maine 




Figure 8. — Columnar sections for the eastern part of the Merrimack belt from eastern Connecticut to southeastern New Hampshire and 
southwestern Maine. Dashed tielines indicate alternate interpretation. Queried tielines are conjectural. 



calc-silicate unit to the Oakdale Formation and have only 
kept as Boylston his lowest unit, which coincides with 
part of the Science Park unit of Hepburn (1976). On the 
State bedrock map, the Boylston forms the base of the 
sequence from Worcester to Oxford (figs. 3A, 8). The 
Boylston lies in a stratigraphic position similar to that of 
the Tadmuck Brook Schist of the Nashoba zone and 
possibly part of the Vaughn Hills Quartzite of the 
Rockingham subbelt (fig. 8). The higher metamorphic 
grade of the Boylston, in contrast to the typically lower 
grade of the overlying Oakdale and Worcester Forma- 
tions, suggests that it could actually be part of the 
Nashoba zone caught between strands of the Clinton- 
Newbury fault system. 

TOWER HILL QUARTZITE (St, Sts) 

Quartzite (St) and schist and phyllite (Sts) between the 
Boylston Schist and the Oakdale Formation in the 
Worcester and Clinton areas (fig. 35) was named the 
Tower Hill Quartzite by Grew (1970; Unit C of Grew, 
1973). The Tower Hill Quartzite (St) consists of light- 
gray, finely crystalline quartzite in tabular beds 5 cm to 
1.5 m thick and minor interbedded dark-gray, carbona- 
ceous laminated phyllite and mica schist that locally 
contain garnet and chloritoid. The phyllite (Sts) also 
forms sequences as much as 65 m thick above and below 
the main mass of quartzite. Beds of granule and pebble 
metaconglomerate are found in the Boylston area (Hep- 
burn, 1976) on Green Hill, Tower Hill, and the hill 
crossed by Pierce St., West Boylston. The thickness of 



the Tower Hill Quartzite ranges from to 91 m (Peck, 
1976) or 130 m (Grew, 1973). 

Some assignments of previously mapped units have 
been followed on the State bedrock map; others have not 
(fig. 7). The Tower Hill is Unit 1 of Peck (1976), which he 
considered to be the base of the turbidite sequence. The 
upper part of Grew's (1973) Unit B is shown on the State 
bedrock map as phyllite (Sts) of the Tower Hill in the 
Worcester area but is shown as Worcester Formation in 
the belt from Worcester to Webster (fig. 3A). In support 
of this assignment in the Worcester area, Grew (1973) 
described lenses of quartzite in Unit B that are as much 
as 25 m thick. The nature of the basal contact of the 
Tower Hill is not known. As mapped by Peck (1975), it is 
everywhere a fault except in Clinton (fig. 35) and on 
Green Hill to the south, east of Wachusett Reservoir (W, 
fig. 3fi), where the quartzite overlies Ayer Granite 
(Sagr). Emerson (1917, p. 224-226) noted the lack of 
evidence for intrusion of the Ayer into the Merrimack 
and Oakdale quartzites. Because of the extensive fault- 
ing in the Clinton-Worcester area, it is not certain 
whether the Tower Hill is equivalent to the Vaughn Hills 
Quartzite of the Rockingham subbelt or is a quartzite 
higher in the section. The Tower Hill lies above the 
Boylston according to the cross section drawn by Grew 
(1970, p. 117). Hepburn (oral commun., 1978) believed 
that the Tower Hill lies higher in the section and is 
equivalent to the quartzite at Franklin in eastern Con- 
necticut, which lies beneath the pelitic Silurian and (or) 
Devonian Scotland Schist and overlies calcareous and 



STRATIGRAPHY OF THE MERRIMACK BELT, CENTRAL MASSACHUSETTS 



G27 



dolomitic metasiltstone and schist of the Ordovician and 
Silurian Hebron Formation. 

OAKDALE FORMATION (So) 

The Oakdale Formation (Emerson, 1917; Goldsmith 
and others, 1982), which conformably overlies the Tower 
Hill Quartzite (Peck, 1975), consists of interlayered 
brownish-gray to light-gray ankeritic metasiltstone and 
greenish-gray, gray, and dark-brown calcareous phyllite 
in beds 3 cm to 2 m thick. Beds are internally laminated 
and contain cyclic silt-clay couplets. In the actinolite zone 
of metamorphism, metasiltstone beds contain actinolite. 
The Oakdale in the Clinton area is 1,220-2,130 m thick 
(Peck, 1976); in the Townsend area, it is 2,000-6,000 m 
thick (G.R. Robinson, 1978, 1981). 

The Oakdale forms a large part of the turbidite 
sequence in the Nashua subbelt and conforms to the 
eastern part of Emerson's Oakdale Quartzite. The 
Oakdale extends from Connecticut, where it has been 
mapped as part of the Ordovician and Silurian Hebron 
Formation, to the Hollis area, New Hampshire, where it 
has been mapped as Berwick Formation of the Merri- 
mack Group (Sriramadas, 1966). The Oakdale is shown as 
stratigraphically underlying the Worcester Formation 
(DSw) on the State bedrock map. Pease (1972) and 
Barosh (1976) considered the sequence to be homoclinal 
and the Worcester to be a large pelitic lens in a west- 
dipping and west-topping Oakdale-Paxton sequence. 
However, Grew (1973), G.R. Robinson (1981), and Hep- 
burn (1976) gave evidence suggesting that the Oakdale 
lies in an isoclinal syncline. 

The western belt of Emerson's (1917) Oakdale Quartz- 
ite from Oakdale north, which we have shown as Paxton 
Formation on the State bedrock map, was mapped as 
Oakdale by Grew (1970) and G.R. Robinson (1981) and as 
gray and brown granofels by Peper and Wilson (1978). 
However, Peck (1976) distinguished the rock west of the 
Wekepeke fault (his Unit 5; our Paxton) from the rock 
east of it (his Unit 2; our Oakdale) on the basis of 
differences in bedding character. He thought that they 
might be time-equivalent units but of different deposi- 
tional regimes (Peck, 1976, p. 244-245). Peper and 
Wilson (1978) and G.R. Robinson (1981) drew the bound- 
ary with the Paxton where bed thickness and grain size 
increase. This boundary coincides approximately with 
the sillimanite-andalusite isograd (Peper and Wilson, 
1978) and supports the suggestion by Billings (1956) that 
the difference between Emerson's Paxton and Oakdale is 
one of metamorphic grade rather than bedding style or 
original grain size. Emerson (1917, p. 60) noted that the 
Oakdale and Paxton grade into one another west of 
Worcester. Crosby (1880) depicted the Oakdale and 
Paxton as equivalents on either side of his Nashua 



"synclinal." We have elected to show the metasiltstone 
west of the Wekepeke in the belt north of Oakdale as 
Paxton for the reasons mentioned in the section on 
Paxton Formation. We have shown Oakdale west of the 
Wekepeke fault, however, in the area south of Worces- 
ter, following mapping by Barosh (1974, 1977) in which 
the Oakdale is distinguished from the Paxton by the 
greater amount of interbedded phyllite in the Oakdale. 

WORCESTER FORMATION (DSw) 

The Worcester Formation (Emerson, 1889, 1898), a 
nonsulfidic pelitic and arenaceous unit (figs. 35, 1A, 8), 
forms an extensive, overall gently west dipping belt 
north and west of Clinton and a narrower, moderately 
west dipping belt extending south from Worcester to 
Oxford. The Worcester Formation comprises most of the 
rocks mapped by Perry and Emerson (1903) and Emer- 
son (1917) as Worcester Phyllite, except that the fossil- 
iferous rocks of definitive Pennsylvanian age have all 
been found to be in a separate fault block from the rest of 
the rock Emerson called the Worcester. The fossiliferous 
rocks have been renamed the Coal Mine Brook Forma- 
tion (Pern) (Goldsmith and others, 1982). 

The Worcester consists mostly of gray and dark-gray 
carbonaceous slate and phyllite, in part containing anda- 
lusite and chiastolite, and interbedded silt-sized meta- 
graywacke, which is most abundant in the upper part. 
Beds of gray to brown impure marble, calc-silicate rock, 
and marble breccia are interbedded sparsely in the slate 
and phyllite. Beds are tabular and 0.6 cm to 1.25 m thick; 
the thicker beds are in the upper part of the formation. 
Slate and graywacke beds are internally laminated, and 
metagraywacke beds are locally crossbedded. The 
Worcester ranges in thickness from 3,050 to 4,270 m in 
the Clinton area (Peck, 1976) and from 2,000 to 3,000 m 
in the Townsend area (G.R. Robinson, 1978, 1981). 

The exact stratigraphic position of the Worcester 
Formation is uncertain. On the State bedrock map, it is 
shown as overlying the Oakdale, with which it is con- 
formable (Peck, 1975, 1976). The Worcester Formation is 
truncated on the west by the Wekepeke fault, and no 
other formation is known to lie above it. If we assume an 
upward-younging sequence, the Worcester could be cor- 
relative with the Lower Devonian Littleton Formation 
(Dl) in the Merrimack synclinorium to the west, on the 
basis of similarity of lithology, or with all or part of the 
Lower Devonian Gonic Formation or Shapleigh Group of 
Hussey (1968) along strike in southern Maine. The 
stratigraphic position of the Worcester at the top of the 
sequence as indicated by Peck (1975) was questioned by 
Grew (1973) and G.R. Robinson (1981), who gave evi- 
dence from graded beds to suggest that the Worcester 
underlies the Oakdale. In support of this contention, 



G28 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



G.R. Robinson (1981) and Hepburn (1976), as mentioned 
above, have described features suggesting that the 
Oakdale lies in an east-facing overturned syncline. This 
interpretation shifts Crosby's synclinal axis from the 
Worcester to the Oakdale Formation and places the 
Worcester stratigraphically lower in the section and 
equivalent to the Tower Hill Quartzite. G.R. Robinson 
(1981) suggested that the absence of quartzite in the 
Worcester is because the Worcester is a basinward facies 
in the overturned limb of this syncline, whereas the 
quartzite of Tower Hill represents a shoal facies in the 
normal limb. We have kept this correlation in mind but 
have shown the Worcester on the State bedrock map as 
lying above the Oakdale and as partly equivalent to the 
Littleton (fig. 8). An element of confusion may exist 
through the similarity of phyllite of the Worcester to 
phyllite of the Tower Hill (Sts). The Worcester Forma- 
tion, the phyllite of the Tower Hill Quartzite, and even 
the Littleton Formation of the Wachusett Mountain 
subbelt may constitute three or two units or may be all 
one unit. Grew (1973) and Hepburn (1976) described thin 
beds of marble alternating with phyllite on the south side 
of the Wachusett Reservoir, an association recalling the 
Waterville Formation of Maine (Osberg, 1968). Peck 
(1976) and Robinson (1981), however, mapping to the 
north, did not mention beds of marble in their descrip- 
tions of the Worcester. 

Uncertainty exists about the structural and strati- 
graphic position of thick pelite units in the turbidite 
section of the eastern part of the Merrimack synclino- 
rium (see Osberg, 1978). Pelitic units such as the Worces- 
ter Formation, the Waterville Formation of Maine, and 
the Scotland Schist of Connecticut cannot be demon- 
strated everywhere as infolds of blanket pelites similar 
to the Devonian Littleton and Seboomook Formations of 
New Hampshire and Maine. These units may be major 
lenses of pelitic facies in the turbidite sequence, as 
suggested by Pease (1972) and Barosh (1976). 

COAL MINE BROOK FORMATION (Pcm) 

The Coal Mine Brook Formation (Pcm) (Goldsmith and 
others, 1982) crops out in two localities near Worcester 
(fig. 9). One locality, containing an abandoned coal mine 
(Grew, 1976), is on the property of the Notre Dame 
Institute, Worcester, and the other is within the City of 
Worcester. Contact relations of the Pennsylvanian strata 
with the adjacent Oakdale and Worcester Formations 
are not visible at the coal mine locality, which is bounded 
by faults (Grew, 1970, 1973, 1976). Fossil flora found at 
the coal mine were identified as Middle to Late Pennsyl- 
vanian in age (Grew and others, 1970; Lyons and others, 
1976). The phyllite and conglomerate beds of the Coal 
Mine Brook at the other locality are poorly exposed and 





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EXPLANATION 






^Sagr ^ 



Coal Mine Brook Formation 
Granite at Millstone Hill 
Worcester Formation 
Ayer Granite 
Oakdale Formation 
Tower Hill Quartzite (phyllite) 
Boylston Schist 
Nashoba Formation 

Contact — Dotted where concealed 



Fault — Dashed where approximately 

located, dotted where concealed. 
D, downthrown block, U, upthrown 
block 



Figure 9.— Geology of the Worcester area. Adapted from Grew 
(1970). 



STRATIGRAPHY OF THE MERRIMACK BELT, CENTRAL MASSACHUSETTS 



G29 



unfossiliferous. The conglomerate, however, contains 
clasts of the adjacent granite at Millstone Hill (Dmgr), 
which has been dated at 372±7 Ma and 365±15 Ma 
(Zartman and Marvin, this vol., chap. J, table 1). These 
strata must therefore be of Carboniferous age. 

The Coal Mine Brook Formation at the coal mine 
consists of gray to dark-gray, very carbonaceous slate 
and phyllite containing a 2-m-thick bed of meta- 
anthracite. The beds at the coal mine, some of which are 
graded, are 3 cm to 1 m thick and total 50 m in thickness. 
In Worcester, the Coal Mine Brook consists of interbed- 
ded carbonaceous phyllite, white arkose, and polymict 
conglomerate containing granules and pebbles of phyl- 
lite, mica schist, quartzite, granite, and vein quartz. 
Beds are from 2 mm to 2 m thick, and total thickness of 
the unit is 330 m. 

These rocks have undergone low-grade metamor- 
phism. Phyllites contain manganiferous garnet porphy- 
roblasts and have a secondary slip cleavage, but they are 
not folded as complexly as the Silurian and Devonian 
strata. All the evidence indicates that the Coal Mine 
Brook lies unconformably on the older formations in spite 
of the lack of exposures of the basal contact. 

AGE RELATIONS 

The Boylston Schist has been assigned an Ordovician 
or Silurian age because of its position adjacent to the 
Proterozoic Z or Ordovician Nashoba Formation to its 
southeast, because of its high metamorphic grade, like 
that of the adjacent Nashoba, and because it seems to be 
part of the overlying turbidite sequence to its northwest. 
Emerson considered the Boylston to be contact- 
metamorphosed Worcester Phyllite. From the descrip- 
tion of the Boylston, it is more likely correlative with the 
Tadmuck Brook Schist of the Nashoba zone (fig. 8) or the 
probably somewhat younger Vaughn Hills Quartzite of 
the Rockingham subbelt. The Tower Hill Quartzite (St, 
Sts) and the Oakdale Formation (So) are in contact with 
the Ayer Granite (Grew, 1973, p. 124; Peck, 1976, p. 242) 
of Late Ordovician and Early Silurian age (433 ±5 Ma; 
Zartman and Marvin, this vol., chap. J, table 1). If the 
Ayer is intrusive into the Tower Hill and Oakdale 
sequence, as we believe, and if the age of the Ayer is 
accepted as valid, the sequence is probably Ordovician. 
The sequence is shown as Silurian on the State bedrock 
map on the basis of regional correlations mentioned 
earlier. The Worcester Formation is intruded by small 
stocks of granite or granodiorite of the Early Devonian 
Fitchburg Complex, which place an upper limit on its 
age. The Fitchburg Complex has a U-Pb zircon age of 
390 ±15 Ma and an Rb-Sr age of 402 ±11 Ma (Zartman 
and Marvin, this vol., chap. J, table 1). Because the 
Worcester is conformable with and physically overlies 



the Tower Hill and Oakdale, which have been assigned a 
Silurian age, and because of possible correlation with the 
Lower Devonian Littleton Formation of New Hamp- 
shire, the Worcester has been assigned a Silurian and 
Early Devonian age on the State bedrock map. 



ROCKINGHAM SUBBELT 

Partly calcareous metasiltstone, metasandstone, phyl- 
lite, and schist north and west of the Clinton-Newbury 
fault and east of the northern part of the Nashua subbelt 
(fig. 3S) are assigned to the Kittery Formation (SOk), 
the Eliot Formation (Se), and the Berwick Formation 
(Sb) of the Merrimack Group (Hitchcock, 1877; Katz, 
1917; Billings, 1956; Hussey, 1968) of southeastern Maine 
and New Hampshire. These formations (fig. IB) project 
into Massachusetts from southeastern New Hampshire 
along the southerly plunging Rockingham anticlinorium 
(Billings, 1956; Sriramadas, 1966; Novotny, 1969; Sun- 
deen, 1971). The distribution of the Kittery, Eliot, and 
Berwick Formations as mapped by Sriramadas (1966), 
Novotny (1969), and Sundeen (1971) has been modified 
extensively by the mapping of G.R. Robinson (1981) in 
the Pepperell area and of Shride (1976; written com- 
muns., 1977, 1979) in the Newburyport and Haverhill 
areas. The lithology of these units is in many respects 
similar to that of the Oakdale and Paxton Formations. 
Emerson (1917) called most of these rocks the Merrimack 
Quartzite, which he equated with his Oakdale Quartzite 
of the Worcester area. He identified the garnet-grade 
rocks of the Berwick as gneisses and schists of undeter- 
mined age. The Merrimack Group forms the bulk of the 
strata in the Rockingham subbelt with the addition of 
the Vaughn Hills Quartzite (SOvh) and its overlying 
Reubens Hill Formation (SOrh) and the Pennsylva- 
nia^?) Harvard Conglomerate (Ph), which unconform- 
ably overlies the sequence. The Vaughn Hills and 
Reubens Hill are included in the Rockingham subbelt 
because they appear to project beneath the Merrimack 
Group northeast of Clinton in the Clinton-Newbury fault 
zone. However, the two formations do not clearly form a 
sequence with the Merrimack Group because of the 
intervening Ayer Granite and faults. The Kittery For- 
mation lies on the west flank of the Rye anticline, which 
has the Silurian, Ordovician, or older Rye Formation in 
its core, and thus apparently forms the base of the 
Merrimack Group. Structural studies indicate that the 
Eliot and Berwick successively overlie the Kittery to the 
west (Billings, 1956, p. 42; Hussey, 1968, p. 293). 

Metamorphic grade increases progressively through 
chlorite and biotite zones in the east to garnet zone in the 
west, but a sharp gradient back to chlorite zone coincides 
with the contact of the Berwick Formation and the 
Oakdale Formation to the west in the adjacent Nashua 



G30 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



subbelt. The boundary marks a change in bedding style 
and grain size from thin bedded and fine grained 
(Oakdale) to thick bedded and coarse grained (Berwick). 
The Tadmuck Brook Schist (SZtb) was assigned to the 
Nashoba zone on the State bedrock map because of its 
composition, a sulfidic pelite, and its high metamorphic 
grade, although the grade decreases westward within 
the unit. Because the metamorphism of the Nashoba 
Formation is considered to be no younger than Late 
Ordovician, the Tadmuck Brook is most likely Ordovician 
or older. However, as discussed in the chapter on the 
stratigraphy of the Nashoba zone (Goldsmith, this vol., 
chap. F), arguments exist for an unconformable relation 
between the Nashoba Formation and the Tadmuck 
Brook Schist (for example, Skehan and Murray, 1980, p. 
295). It is possible, then, that the metamorphism of the 
Tadmuck Brook is younger than Ordovician and the same 
age as the metamorphism in the adjacent Merrimack 
belt, which is considered to be Acadian (Late Devonian). 
If so, the Tadmuck Brook could be part of the turbidite 
sequence of the east side of the Merrimack belt, and 
some or all of the unit could be as young as Silurian. 

VAUGHN HILLS QUARTZITE (SOvh) 

The Vaughn Hills Quartzite (Hansen, 1956; Grew, 
1970; Peck, 1975; Goldsmith and others, 1982) forms the 
base of the sequence between Shrewsbury and West 
Chelmsford. Near Shrewsbury the Vaughn Hills and 
Tadmuck Brook Schist (SZtb) of the Nashoba zone 
project beneath a slice of Nashoba-zone rocks east of the 
Rattlesnake Hill fault of the Clinton-Newbury fault 
system (Hepburn, 1978). 

The Vaughn Hills consists of cream- to flesh-colored, 
fine-grained, thin-bedded quartzite in beds 3 cm to 1.5 m 
thick, interbedded pale-green, locally rusty-weathering, 
laminated phyllite and mica schist, minor beds of calc- 
silicate-bearing metawacke, and, in the upper part, 
partly calcareous chlorite schist. Conglomerate beds are 
present between Chelmsford and Ayer (Jahns and oth- 
ers, 1959). The unit ranges in thickness from to 100 m. 

The identity and stratigraphic position of the Vaughn 
Hills Quartzite have been subject to several interpreta- 
tions. Peck (1975) considered the Vaughn Hills to be the 
upper part of the Tadmuck Brook Schist; similarly, Bell 
and Alvord (1976) believed that the Tadmuck Brook 
grades up into the Vaughn Hills. Hansen (1956, p. 26) 
described gradation of his mica schist facies of the 
Worcester Formation (now mapped as Tadmuck Brook) 
into quartzite of the Vaughn Hills. The Vaughn Hills 
Quartzite is equivalent to part of Skehan and Abu- 
Moustafa's (1976) unnamed units 1 through 12 east of the 
Rattlesnake Hill fault. Their unit 12, a quartzitic unit, is 
probably the Vaughn Hills, although Skehan (1969, p. 



802) indicated that their unit 1, which is also a quartzite 
unit, might be the Vaughn Hills. Unit 1 lies below a 
sequence consisting largely of schist, units 2-9, that is 
the probable equivalent of the Tadmuck Brook Schist. 
Their quartzite unit is probably equivalent to the lens of 
quartzite near the base of the Tadmuck Brook shown by 
Bell and Alvord (1976, fig. 4). 

A quartzite, phyllite, and pebble metaconglomerate 
unit lying west of the Tadmuck Brook Schist and below 
the Berwick Formation between West Chelmsford and 
Ayer was mapped by Jahns and others (1959) as Harvard 
Conglomerate. This unit has been shown as Vaughn Hills 
Quartzite on the State bedrock map because of its 
position adjacent to the Tadmuck Brook Schist. Alvord 
(1975) considered the conglomerate a breccia and 
mapped the unit as cataclastic rock of the Clinton- 
Newbury fault. 

The location of the top of the Vaughn Hills is somewhat 
uncertain. In the Clinton area, the Vaughn Hills is 
overlain by the Reubens Hill Formation. The chlorite 
schist described by Hansen at the top of the Vaughn Hills 
southeast of Bare Hill Pond (SOvh south of Harvard, fig. 
35) may be equivalent to part of the overlying Reubens 
Hill. Nearly everywhere, however, the Vaughn Hills is 
in fault contact with phases of the Ayer Granite (Sagr, 
Sacgr) so that it is difficult to establish a sequential 
relationship with the turbidite sequence west of the 
Clinton-Newbury fault zone. The Vaughn Hills was 
apparently included in the Brimfield Schist and Oakdale 
Quartzite by Emerson (1917). 

REUBENS HILL FORMATION (SOrh) 

The Reubens Hill Formation (Skehan, 1967; Goldsmith 
and others, 1982) lies between the Vaughn Hills Quartz- 
ite and the Ayer Granite and lies east of the Clinton fault 
of Skehan and Abu-Moustafa (1976) of the Clinton- 
Newbury fault system. The Reubens Hill, about 590 m 
thick, consists of layered to massive metavolcanic and 
hypabyssal intrusive rocks. These are, in part, interlay- 
ered dark-greenish-gray chlorite-hornblende schist and 
amphibolite and gray to brownish-gray plagioclase- 
biotite-quartz schist and biotite-hornblende-plagioclase 
gneiss and, in part, massive greenish-gray metadiorite 
and dark-gray hornblende and actinolite rock poor in 
feldspar. Some amphibolite contains features that are 
possibly relic pillow structure. 

The Reubens Hill has always been readily identifiable 
because its composition is different from that of adjacent 
units. It is equivalent to the Diorite of Crosby (1880), the 
Straw Hollow Diorite of Emerson (1917), the Reubens 
Hill amphibolite of Skehan (1967), units 13-15 of Skehan 
and Abu-Moustafa (1976), and the Reubens Hill igneous 
complex of Peck (1975). Units 13-15 of Skehan and 



STRATIGRAPHY OF THE MERRIMACK BELT, CENTRAL MASSACHUSETTS 



G31 



Abu-Moustafa contain massive phases principally com- 
posed of hornblende and actinolite that suggest a mafic or 
ultramafic protolith. Peck (1975) mentioned that some of 
the rock appears to have been derived from pillow lavas 
and that part of the formation is a massive metadiorite. 
Layered schist and gneiss in the complex are interpreted 
by both Peck (1976) and Skehan and Abu-Moustafa (1976) 
as volcanogenic deposits. 

The Reubens Hill occupies only a small area near 
Clinton and the Wachusett Reservoir within the Clinton- 
Newbury fault zone. The unit is apparently faulted out 
elsewhere along the east flank of the Merrimack belt, 
except possibly near the Vaughn Hills (see comments by 
Thompson and Robinson, 1976, p. 348). Possibly the 
Reubens Hill appears again in southeastern New Hamp- 
shire as the volcanic part of the Ordovician, Silurian, or 
older Rye Formation (see also Goldsmith, this vol., chap. 
F, figs. 5, 6). 

KITTERY FORMATION (SOk) 

The Kittery Formation (Kittery Quartzite of Katz, 
1917; Billings, 1956; Hussey, 1968) is primarily a thin- 
bedded calcareous biotite metasiltstone. In detail, it 
consists of gray- to greenish-gray feldspathic, calcare- 
ous, biotitic metasiltstone; biotite phyllite and schist, 
which is commonly actinolitic; calc-silicate gneiss; and 
minor fine-grained quartzite and feldspathic quartzite. A 
few beds of fine-granule conglomerate lie at the base of 
thicker graded beds (Novotny, 1969). The finer grained 
rocks are thinly bedded to laminated; thicker beds show 
cross lamination, and some are graded. Small-scale 
scour-and-fill structures and ovoid calcareous concre- 
tions are common. The Kittery is 2,350-2,450 m thick 
(Hussey, 1968). 

The Kittery appears to be intruded by the Newbury- 
port Complex and is truncated to the south along the 
Clinton-Newbury fault. The Kittery probably rests dis- 
conformably (Hussey, 1968, p. 294) on the Ordovician or 
Silurian, or older, Rye Formation of southeastern New 
Hampshire, although this contact was shown by Novotny 
(1969) as a fault (Portsmouth fault) along the west side of 
the Rye. The Kittery appears to pass upwards or later- 
ally (Billings, 1956, p. 39) into the more pelitic Eliot 
Formation, although Novotny (1969, p. 11) noted a 
disconformity at one place between the Eliot and the 
Kittery. 

ELIOT FORMATION (Se) 

The Eliot is a dominantly thinly layered pelitic unit as 
described by Hussey (1968) in Maine, but as mapped by 
Sundeen (1971) in the Haverhill area, New Hampshire, it 
includes metasiltstone sequences. The Eliot, 300 m thick, 
consists of thinly bedded, dark-gray to dark-green slate 



and phyllite, commonly dolomitic, and light-gray to light- 
gray-green to brown, partly calcareous and actinolite- 
bearing metasiltstone (Hussey, 1968; Novotny, 1969). 

On the State bedrock map, the wide band of Eliot 
mapped by Sundeen in the Haverhill area has been 
narrowed to include only the primarily pelitic facies. The 
thicker bedded metasiltstone facies mapped by Sundeen 
has been assigned to the Berwick Formation. A.F. 
Shride (written commun., 1979) placed the Eliot- 
Berwick contact along the Merrimack River east of 
Haverhill. 

BERWICK FORMATION (Sb, Sbs) 

The Berwick in Massachusetts and New Hampshire 
consists primarily of thin to thick tabular and lenticular 
beds of calcareous metasiltstone, biotitic metasiltstone, 
and fine-grained metasandstone. Some layers contain 
quartz, biotite, and actinolite, others contain diopside, 
hornblende, and plagioclase. Interbedded with these 
rocks are small amounts of quartz-muscovite-garnet 
schist and feldspathic quartzite. Actinolite increases 
westward at the expense of biotite. In the sillimanite 
zone, the metasiltstones have a salt-and-pepper appear- 
ance on weathered surfaces. Two thick sequences of 
partly rusty-weathering pyrrhotite- or pyrite-bearing 
mica schist and phyllite (Sbs) have been shown sepa- 
rately on the State bedrock map (fig. 3B). The eastern 
sequence (west of Newburyport) is a black to green 
pyritiferous phyllite (Sundeen, 1971). The western 
sequence, located midway between North Chelmsford 
and Townsend, is a massively bedded, locally cataclastic 
quartz-rich pyrrhotitic schist containing aggregates of 
biotite (G.R. Robinson, 1978, 1981). The Berwick is 
mostly in the garnet zone of metamorphism but in the 
western part of its outcrop area is in the sillimanite zone. 
Metamorphism has largely obscured primary sedimen- 
tary structures. The unit ranges in thickness from 1,850 
to 2,450 m (Hussey, 1968). 

The pyritiferous phyllite as mapped by Sundeen 
(1971), forming the eastern sequence of Sbs, is a contin- 
uation of the Calef Member of the Eliot Formation of 
Billings (1956). A.F. Shride (written commun., 1976) 
reported that this phyllite occupies a narrower belt than 
that shown by Sundeen. Because the enclosing rock is 
now mapped as Berwick rather than Eliot, the pyritifer- 
ous phyllite is assigned to the Berwick Formation. 
Sundeen considered this phyllite to lie in the east limb of 
a north-trending anticlinal structure, whose axis would 
pass west of Haverhill. Shride traced Sundeen's phyllite 
unit into Massachusetts at Haverhill. The western 
sequence of Sbs in the east Pepperell area was mapped 
by G.R. Robinson (1981) as part of his Merrimack 



G32 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



Formation. These rocks have been assigned to the Ber- 
wick Formation on the State bedrock map. 

HARVARD CONGLOMERATE (Ph) 

The Harvard Conglomerate (Burbank, 1876) is nonfos- 
siliferous; it rests unconformably on Ayer Granite 
(Thompson and Robinson, 1976) of Late Ordovician(?) 
and Early Silurian age (Zartman and Marvin, this vol., 
chap. J, table 1). The Harvard consists of polymict 
conglomerate containing mostly flattened and stretched 
pebbles of green and gray quartzite less than 10 cm in 
length but also pebbles of slate, phyllite, mica schist, and 
milky quartz, in an argillitic matrix. The conglomerate is 
in lenticular beds a few centimeters to 1 m thick. 
Interbedded with and overlying the conglomerate is 
gray, green, and pale-purple phyllite containing quartz, 
chloritoid, chlorite, and white mica. The thickness of the 
unit is indeterminate because of faulting. 

The Harvard has traditionally been correlated with the 
conglomerate beds of the Pennsylvanian Coal Mine 
Brook Formation in Worcester because of its apparent 
unconformity on the older rocks. The Harvard crops out 
in two localities, both bounded by faults: the classic 
locality at Pin Hill near Harvard, described by Emerson 
(1917), Thompson and Robinson (1976), and Gore (1976), 
and at the northern of the two hills known as Vaughn 
Hills near Bolton, described by Hansen (1956). At Pin 
Hill (fig. 1), the conglomerate is clearly unconformable 
upon Ayer Granite, but at Vaughn Hills to the south, 
Hansen, who did not recognize the Clinton-Newbury 
fault, described the conglomerate as part of a sequence 
from Vaughn Hills Quartzite into phyllite and into Har- 
vard Conglomerate. Reconnaissance by G.R. Robinson, 
Jr. , and Goldsmith in the area of Vaughn Hills indicated 
not only that quartzite of the Vaughn Hills tongues out 
into phyllite similar to that of the Tower Hill Quartzite 
but that the polymict conglomerate mapped by Hansen 
as Harvard Conglomerate tongues out into the same 
phyllite. At that time, we interpreted the conglomerate 
in this area to be a submarine-channel slide deposit near 
or at the base of the turbidite sequence. The angularity 
of some rock fragments in the conglomerate was noted by 
Emerson (1917, p. 66, pi. VIIIB), Hansen (1956), and 
Thompson and Robinson (1976). However, in most places 
the quartzite pebbles are flattened, as is clearly seen in 
the roadside quarry south of Pin Hill. The greenish 
quartzite beds in the phyllite that is gradational with 
Harvard Conglomerate were compared by Hansen (1956, 
p. 25) with the quartzite beds of the Vaughn Hills. The 
quartzite clasts in the conglomerate beds are generally 
believed to have been derived from quartzites of the 
turbidite sequence (Currier and Jahns, 1952). Possibly 
some of the quartzite clasts in the conglomerate are 



dismembered thin quartzite beds. Emerson (1917, p. 66), 
however, described the clasts as consisting of several 
kinds of quartzite (Oakdale quartzite, Westboro quartz- 
ite, and vein quartz) and several kinds of slate. The 
relations of rocks mapped as Harvard Conglomerate in 
the past, such as at Vaughn Hills and those mapped by 
Jahns and others (1959) southwest of West Chelmsford, 
need to be reexamined to see the differences between the 
Vaughn Hills and the Harvard and to determine if two 
conglomerates are present, one Ordovician and Silurian 
and the other Pennsylvanian. 

AGE RELATIONS 

The age of the rocks in the Rockingham subbelt is 
Ordovician and (or) Silurian and older. A Silurian age can 
be inferred on the basis of correlation with the Silurian 
Vassalboro Formation and associated units in central 
Maine (Hussey, 1968; Osberg, 1968; Ludman, 1976). The 
age assignments on the State bedrock map are based 
partly on this correlation and partly on the similarity of 
the strata with the Silurian formations, such as the 
Paxton, in central Massachusetts. An Ordovician or older 
age, however, has been determined for at least part of 
the Merrimack Group largely on the basis of the age of 
intrusions into the sequence (Zartman and Marvin, this 
vol., chap. J; Gaudette and others, 1984). Zartman and 
Marvin (this vol., chap. J, table 1) reported a U-Pb age of 
455±15 Ma on zircon from granodiorite of the Newbury- 
port Complex. The Newburyport is believed to have 
intruded the Kittery and Eliot Formations (Novotny, 
1969), and the map pattern in the Newburyport East and 
West quadrangles (Shride, 1976) seems to support this 
interpretation. A search of the literature reveals no 
observation of an actual intrusive contact or of inclusions 
of Kittery or Eliot in the Newburyport Complex, nor 
does the pattern of metamorphic isograds in that area 
seem to conform to the shape of the complex. However, 
the U-Pb age of the Newburyport Complex and similar 
Rb-Sr isotopic ages on intrusions into the Kittery and 
Eliot Formations of the Merrimack Group farther north 
in New Hampshire (Gaudette and others, 1984) show 
that part of the Merrimack Group, and probably all of it, 
is Ordovician or older. D.R. Wones (oral commun., 1979) 
correlated the Merrimack Group with pre-Silurian rocks 
of similar lithology that he studied in the Penobscot Bay 
area of Maine (Stewart and Wones, 1974). 

On the State bedrock map, the Kittery was assigned 
an Ordovician or Silurian age because of the possible 
Ordovician age of the Newburyport Complex. At the 
time the map was prepared, the isotopic age of the 
Newburyport Complex was less certainly Ordovician 
(445 ±15 Ma, Zartman and Naylor, written commun., 
1978), and the dating of intrusions farther north in New 



STRATIGRAPHY OF THE MERRIMACK BELT, CENTRAL MASSACHUSETTS 



G33 



Hampshire had not begun. The Eliot, even though it is in 
contact with the Newburyport Complex, was assigned a 
Silurian age through correlation with the Silurian Vas- 
salboro Formation in central Maine. The Berwick was 
similarly assigned a Silurian age. 

Both the Vaughn Hills Quartzite and the Reubens Hill 
Formation are assigned an Ordovician or Silurian age 
because of their position at the base of the section 
or below the turbidite sequence and because the 
Vaughn Hills seems to be in a sequence with the Tad- 
muck Brook Schist of Proterozoic Z, Ordovician, or 
Silurian age. Their relation to the Ayer Granite could not 
be established. 

The radiometric ages discussed above suggest that the 
strata in the Nashua and Rockingham subbelts are 
Ordovician or older, which is older than expected from 
lithologic correlation with the Silurian strata to the north 
in Maine. No stratigraphic break is discernible between 
the rocks of the Nashua and Rockingham subbelts and 
the Silurian and Devonian rocks in the synclinorium to 
the west, however. Probable age range of the sequence 
at present is from Cambrian and (or) Ordovician to Early 
Devonian although Cambrian and (or) Ordovician to 
Silurian is most likely. J.B. Lyons and others (1982, p. 
54) suggested a Proterozoic age for the sequence on the 
basis of 1,188-Ma ages of detrital zircons from the 
Berwick Formation (Aleinikoff, 1978). Olszewski and 
others (1984) and Bothner and others (1984) supported a 
Proterozoic Z age for the sequence because of its appar- 
ent conformity with the Massabesic Gneiss Complex, 
which contains rock dated at 646 Ma (Aleinikoff and 
others, 1979; Lyons and others, 1982). The Massabesic 
Gneiss Complex is similar in part to rock in the Nashoba 
zone and has produced similar isotopic ages; however, we 
feel that the rocks of the Merrimack Group are a 
sequence that differs in lithology and depositional style 
from the rocks of the Nashoba zone and the Massabesic 
Gneiss Complex. The problem remains concerning the 
similarity in lithology and apparent continuity, in places, 
of the Merrimack Group sequence, if Proterozoic Z, with 
the Oakdale-Worcester and Paxton-Littleton sequences 
in the rest of the Merrimack belt, which are mapped as 
Silurian to Lower Devonian. 

DISCUSSION 

At the present time, uncertainties connected with the 
stratigraphy along the east flank of the Merrimack 
synclinorium are so numerous that one can only speculate 
as to the actual stratigraphic relations. On the basis of 
the structure and topping evidence available, several 
correlation schemes could be prepared. One is based on 
G.R. Robinson's (1978) evidence that the Worcester 
underlies the Oakdale and is equivalent to the Tower 



Hill; another on Pease and Barosh's (1981) contention 
that the Scotland Schist (equals Worcester) is a pelitic 
lens in the Hebron Formation (equals Oakdale and 
Paxton). Our preferred scheme and that shown on the 
State bedrock map is shown in figure 8. We believe that 
the Tadmuck Brook, Vaughn Hills, Reubens Hill, and 
Boylston are the base of the section and are probably 
Ordovician or older in age. We believe that the Boylston- 
Oakdale-Worcester sequence in the narrow belt from 
Worcester to Webster is a greatly compressed section of 
that north of Worcester. It is possible, however, that 
reexamination of this sequence might show that it is 
equivalent to the upper units of the Tatnic Hill Forma- 
tion, so that the Oakdale is equivalent to the calc- 
silicate-bearing Fly Pond Member of the Tatnic Hill and 
the Worcester as mapped is equivalent to the two- 
mica-schist-bearing Yantic Member. The Boylston would 
be equivalent to part of the lower member of the Tatnic 
Hill. The Boylston, as described, contains several differ- 
ent lithologies including calc-silicate-bearing beds (Grew, 
1973; Hepburn, 1976), as does the Vaughn Hills (Hep- 
burn, 1976). Thus, lithologies seem to interfmger near 
the base of the section. The adjacent Nashoba Formation 
also contains calc-silicate-bearing zones (Bell and Alvord, 
1976). The Boylston Schist lithologically most resembles 
the Tadmuck Brook Schist, and the Tower Hill Quartzite 
resembles the Vaughn Hills Quartzite. As mapped by 
Peck (1975), the Tower Hill lies west of the Clinton- 
Newbury fault zone and the belt of Ayer Granite, 
whereas the Vaughn Hills lies within and east of the 
Clinton-Newbury fault zone and the belt of Ayer Gran- 
ite, so that the Tower Hill and the Vaughn Hills never 
meet. J.C. Hepburn (oral commun., 1979) suggested that 
the Tower Hill occupies a position similar to that of the 
Franklin Quartzite in Connecticut. The latter lies in 
upright position beneath graded metagraywacke beds of 
the Scotland Schist and above calcareous metasiltstone 
and phyllite assigned to the Hebron Formation. The 
Hebron Formation also lies above the Scotland, either in 
normal stratigraphic order (Pease and Barosh, 1981) or 
in an overturned section (Dixon and Lundgren, 1968). In 
the Clinton area, the Tower Hill lies in upright position 
beneath the calcareous metasiltstone and phyllite of the 
Oakdale Formation, and the Worcester Formation over- 
lies the Oakdale in normal stratigraphic order (Peck, 
1976) or in an overturned section (G.R. Robinson, 1981). 
If the Scotland Schist is merely a pelitic lens in the 
Hebron Formation, as Pease and Barosh (1981) stated, 
the Scotland Schist could be anywhere in the Hebron 
section and the correlation of the Tower Hill with the 
Franklin Quartzite could be more reasonably made. The 
Reubens Hill Formation of the Rockingham subbelt must 
wedge out between the Tower Hill and the Oakdale, an 
interpretation for which there is no evidence, or, if the 



G34 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



Tower Hill does not correlate with the Vaughn Hills but 
lies higher in the section, as Hepburn believed, the 
Reubens Hill and the Vaughn Hills wedge out above the 
Boylston Schist. The Tower Hill, then, might correlate 
across into the Rockingham subbelt with the Kittery 
Formation (part of Emerson's (1917) Oakdale Quartzite 
with part of Katz's (1917) Kittery Quartzite). A correla- 
tion, not strongly held by us, of the Reubens Hill 
Formation, Vaughn Hills Quartzite, and Tadmuck Brook 
Schist of the Rockingham subbelt with the Rye Forma- 
tion of southeastern New Hampshire and southwestern 
Maine (fig. 8) results from the position of the Rye with 
respect to the Kittery Formation. An alternative corre- 
lation is presented by Goldsmith (this vol., chap. F), in 
which the Rye is correlated with the Marlboro Formation 
of the Nashoba zone. The Reubens Hill Formation is a 
key unit, worth more study, in making a correlation 
between the Rye, the Marlboro, and the lower part of the 
sequence in the Rockingham subbelt. 

On the basis of lithology and what little we know of the 
sequence, the rocks of the Nashua and Rockingham 
subbelts should not have different nomenclatures. The 
Paxton is much like the Berwick in lithology, and the 
Eliot and Kittery are somewhat like the Oakdale. The 
Eliot and Oakdale represent pelitic fades, rarely at high 
grade however, so that contained micas would not have 
been converted to less hydrous phases to produce a less 
schistose rock, like much of the Berwick. 

The formations are largely distinguished on the basis 
of bedding style, grain size, and mica content, which 
probably represent differences in sedimentary facies, as 
well as in degree of metamorphism. A single formational 
name could very well apply to the whole sequence, in the 
same way as the Hebron Formation is used in Connect- 
icut (Rodgers, 1985). Ideally, as the Merrimack termi- 
nology has historical precedence (Hitchcock, 1877; Katz, 
1917; Billings, 1956; Hussey, 1968) over Oakdale and 
Paxton (Perry and Emerson, 1903; Emerson, 1917) in 
Massachusetts and over Hebron Formation (Rice and 
Gregory, 1906; Gregory and Robinson, 1907) in Connect- 
icut, it should be applied throughout the region. Unfor- 
tunately, the subunits of the Merrimack Group have not 
been clearly defined in southeastern New Hampshire in 
the zone between southeastern Maine and Massachu- 
setts. Formation boundaries are drawn in different 
places by different people. Rather than use formational 
names for these units, the best approach might be to call 
the sequence Merrimack Formation, rather than Group, 
as G.R. Robinson (1981) has done. This formation could 
then be subdivided into members on the basis of differ- 
ences in sedimentary facies characteristics recognized 
beneath the metamorphic overprint. A somewhat arbi- 
trary boundary has been established on the State bed- 
rock map between units certainly of the Merrimack 



Group and units like the Oakdale and Paxton, which are 
well established by Emerson in Massachusetts. We have 
chosen to carry Emerson's terminology as far as possible, 
except that Emerson's western belt of Oakdale is now 
assigned to the Paxton. 

We have not here suggested a revised nomenclature. 
A unified study of the stratigraphy and structure along 
the east side of the Merrimack belt in Massachusetts is 
needed in order to better understand the relations before 
the nomenclature can be revised. This study should be 
tied in closely with mapping in the same belt in south- 
eastern New Hampshire and southern Maine. 

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G36 



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STRATIGRAPHY OF THE MERRIMACK BELT, CENTRAL MASSACHUSETTS 



G37 



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scale 1:250,000. 



Structural and Metamorphic 
History of Eastern Massachusetts 

By RICHARD GOLDSMITH 

THE BEDROCK GEOLOGY OF MASSACHUSETTS 

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1366-H 



CONTENTS 



Abstract HI 

Introduction 2 

Nashoba zone east of the Merrimack belt 4 

Internal structure and metamorphism 4 

Folds and minor structural features 4 

Metamorphism o 

Faults 8 

Spencer Brook and Assabet River faults 9 

Inclination of and movement on fault surfaces 9 

Structural position of the Nashoba zone 9 

Clinton-Newbury fault 11 

Description 11 

Age of the Clinton-Newbury fault zone 16 

Structural relations west of the Clinton-Newbury fault 16 

Nashua trough 16 

Rockingham anticlinorium 18 

Summary 20 

Milford-Dedham zone 20 

Structure and metamorphism in the basement blocks 21 

Milford antiform 21 

Salem block 24 

Melrose subblock 24 

Fault pattern in the Salem block 26 

Mystic fault 26 

Dedham and Foxborough blocks 27 

Fall River block 27 

Metamorphism of the metavolcanic and plutonic 

rocks 27 

Structural features in the New Bedford area 29 

Assawompset Pond graben and related 

structures 32 



Milford-Dedham zone — Continued 

Structure and metamorphism in the basement blocks- 
Continued 
Fall River block— Continued 

Joints H32 

Summary and discussion 32 

Proterozoic metamorphism 33 

Structure and metamorphism of the basins in the Milford- 
Dedham zone 33 

Boston basin 33 

Newbury basins 36 

Bellingham basin 36 

Narragansett basin 37 

Norfolk basin 40 

Middleton basin 41 

Nantucket basin 41 

Mafic dikes 41 

Summary of blocks and basins of the Milford-Dedham zone.. 42 

Bloody Bluff fault zone 43 

Burlington mylonite zone 45 

Wolfpen lens 46 

Attitudes of fault surfaces and sense of movement in 

the Bloody Bluff fault zone 46 

Age of the Bloody Bluff fault zone 47 

Tectonic events in eastern Massachusetts 47 

Milford-Dedham zone 47 

Nashoba zone 53 

Newbury basins 53 

East flank of the Merrimack belt 54 

Zone boundaries 55 

Accretion 56 

References cited 58 



ILLUSTRATIONS 



1. Index map showing major structural features of eastern Massachusetts H3 

2. Map showing structural features of eastern Massachusetts 6 

3. Map showing major structural features of southeastern New England 10 

4. Geologic map of the Wachusett Reservoir area 12 

5. Geologic map of the east flank of the Merrimack belt, Massachusetts and New Hampshire, showing locations of cross 

sections (A-A'-A", B-B', C-C of figure 7) 17 

6. Schematic section near cross section C-C across the Nashua "synclinal" as drawn by Crosby (1880, pi. Ill) 18 

7. Schematic cross sections across the east flank of the Merrimack belt, Massachusetts and New Hampshire 19 

8-10. Maps showing: 

8. Basins of the Milford-Dedham zone, eastern Massachusetts 20 

9. Form lines on foliation and direction of plunge of lineation in the Milford antiform, eastern Massachusetts 22 

10. Structural features in and adjacent to the Salem block of the Milford-Dedham zone, eastern Massachusetts 25 



IV CONTENTS 

Page 
FIGURE 11. Maps of the Fall River-New Bedford area, southeastern Massachusetts, showing: A, Structural features; 

B, Interpretive cross sections; and C, Joints H28 

12-15. Maps showing: 

12. Structural features of the Boston basin, eastern Massachusetts 34 

13. Structural features of the Norfolk and Narragansett basins, eastern Massachusetts 38 

14. Structural features along part of the Bloody Bluff fault zone, northeastern Massachusetts 44 

15. Tectonic events in eastern Massachusetts corresponding approximately to time intervals indicated on 

table 3 50 



TABLES 



Table 1. Metamorphism, plutonism, and faulting in basement rocks of the Milford-Dedham zone, eastern Massachusetts H21 

2. Metamorphism and structure of cover rocks in basins of the Milford-Dedham zone, eastern Massachusetts 42 

3. Tectonic events in eastern Massachusetts 48 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



STRUCTURAL AND METAMORPHIC HISTORY OF 
EASTERN MASSACHUSETTS 



By Richard Goldsmith 



Two terranes differing in stratigraphy, plutonism, and metamor- 
phism compose most of eastern Massachusetts. On the bedrock geologic 
map of Massachusetts (hereafter called the State bedrock map), these 
are the Nashoba zone east of the Merrimack belt and the Milford- 
Dedham zone. The Nashoba zone, bounded on the west by the 
Clinton-Newbury fault and on the east by the Bloody Bluff fault 
system, is a high-grade metamorphic terrane of northeast-trending, 
steeply dipping Proterozoic Z or Ordovician schist, paragneiss, and 
metavolcanic rocks intruded by synkinematic Ordovician S-type and 
postkinematic Silurian Ttype plutons. The Nashoba zone may be 
synformal; only a few large-scale folds have been identified. The zone 
contains many small-scale, mostly easterly verging folds. The zone is 
cut by many northeast-trending longitudinal faults, many of which are 
younger than the Silurian plutons. Early faults, both within the zone 
and in the Clinton-Newbury fault system, are west-dipping thrusts and 
reverse faults characterized by mylonite in which movement sense is to 
the east and southeast. Later faults tend to be steep and characterized 
by brecciation and gouge. The Science Park block near Worcester on 
the east flank of the Merrimack belt is interpreted to be a detached slice 
of Nashoba-zone rock within the Clinton-Newbury fault zone. Struc- 
tural features within the eastern flank of the Merrimack belt are 
believed to be primarily Acadian and can be related only in part to 
structures in the adjacent Nashoba zone. 

The Clinton-Newbury fault zone extends from the Massachusetts- 
Connecticut boundary to the Gulf of Maine. It dips steeply and 
truncates the rock units on either side of the zone at a low angle to the 
south and a high angle to the north. Within the fault zone are early, 
possibly Acadian, low-angle eastward-directed thrusts and reverse 
faults; later faults are younger than the Pennsylvanian strata in the 
Worcester area and may be in part Mesozoic. 

The Milford-Dedham zone is that part of eastern Massachusetts east 
and southeast of the Bloody Bluff fault system. It is probably bounded 
on the east, off shore, by an extension of the Meguma terrane of Nova 
Scotia. The zone consists largely of a mostly crystalline Proterozoic Z 
plutonic-metamorphic basement in which older Proterozoic Z metased- 
iments and mafic volcanic and plutonic rocks have been intruded by 
younger Proterozoic Z granite and granodiorite batholiths. In the lower 
and middle Paleozoic, the zone was intruded by plutons of Ordovician to 
Devonian gabbro and alkalic granite. The terrane has been broken by 
normal and reverse faults into upthrown blocks of basement rock and 
downthrown blocks containing sedimentary and volcanic cover rocks 



Manuscript approved for publication November 16, 1987. 



ranging in age from Proterozoic Z to Triassic-Jurassic. The Milford 
antiform, to the west, a northeast-plunging foliation arch, and the 
southern part of the Fall River block, which lies southeast of 
the Narragansett basin, contain variably gneissic granitoid rocks and 
amphibolite-facies metasediments in which less deformed areas are 
bounded by more intensely deformed shear zones. The Milford antiform 
is truncated to the north by the northeast-trending Bloody Bluff fault. 
Proterozoic Z plutonic rocks in the Salem, Dedham, and Foxborough 
blocks, in northeastern Massachusetts, and the northern part of the 
Fall River block are at most brittlely deformed in the greenschist 
facies. The Salem block is appreciably sheared near the Bloody Bluff 
fault zone, as for example in the 8-km-wide Burlington mylonite zone. 
A prebatholithic metamorphism in the Proterozoic Z metasedimentary 
and metavolcanic and plutonic rocks is greenschist facies to the east and 
amphibolite facies to the west. 

The major basins between basement blocks of the Milford-Dedham 
zone contain strata of Proterozoic Z and Cambrian, Silurian and 
Devonian, Pennsylvanian, and Triassic-Jurassic age. Fossils in strata 
of Cambrian, Silurian, and Devonian age are of Acado-Baltic affinity. 
The unmetamorphosed Proterozoic Z marine strata in the Boston basin 
that lie unconformably on the batholithic rocks, and the coal-bearing 
alluvial Pennsylvanian strata in the Norfolk and Narragansett basins, 
have both been folded and cut by reverse faults that trend east- 
northeast, indicating a synchronous compressional event probably 
Alleghenian in age. This event has been suggested as having been 
produced by a left-lateral shear system striking northeast. North-south 
faults and diabase dikes that cut the east-northeast-trending structures 
indicate a succeeding tensional regime. 

The two narrow, fault-bounded Newbury basins between the 
Milford-Dedham and Nashoba zones in northeastern Massachusetts 
probably lie in a graben of Mesozoic age in the same fracture zone as the 
Triassic and Jurassic Middleton basin to the south. The Bellingham 
basin (also known as the Woonsocket basin) contains greenschist-facies, 
unfossiliferous, metamorphosed Pennsylvanian or possibly Proterozoic 
Z(?) rocks that lithologically resemble those of the Boston basin. The 
bounding faults of the basin are continuous with faults forming the 
southwest end of the Boston basin. 

The Bloody Bluff fault system has a long history of movement. 
Mylonites in the system formed by ductile deformation at amphibolite- 
to greenschist-facies metamorphism have had superimposed brittle 
deformation. Shearing in the Burlington mylonite zone, part of the 
Bloody Bluff fault system in northeastern Massachusetts, is older than 



H2 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



the Early Devonian Peabody Granite, 1 but radiometric data from a 
continuation of the fault zone in Connecticut indicate an Alleghanian 
age for the latest movement in the Bloody Bluff system. At its north 
end, the Bloody Bluff system may be offset to the north along 
north-trending faults flanking the Middleton and Newbury basins. The 
Wolfpen lens, near Framingham, is a splinter of basement caught 
between branches of the fault zone. Fault surfaces in the Bloody Bluff 
system dip steeply westward in most places, and movement has been 
generally considered to be toward the east and southeast. There seems 
to be little evidence that the Bloody Bluff is a major strike-slip zone. 

In the Milford-Dedham zone, a Proterozoic Z volcanic-plutonic arc 
was intruded by Proterozoic Z granite batholiths and was metamor- 
phosed and deformed to at least greenschist facies in the east and 
possibly amphibolite facies to the west. A subsequent dilational event 
also in Proterozoic Z time led to development of a rift system 
accompanied by felsic volcanism and deposition of turbidite. Deposition 
of Cambrian shelf sediments ushered in a period of stability and 
subsidence. Throughout early and middle Paleozoic time, the Milford- 
Dedham zone was in an extensional regime during which alkalic and 
gabbroic plutons were intruded. During Devonian to Pennsylvanian 
time, compressive movements tied to collision of the plate containing 
the Milford-Dedham zone with the North American plate resulted in 
east- to southeast-directed thrusting, uplift, erosion, and deposition of 
fluvial Pennsylvanian strata in a marginal basin. About this time 
deep-seated deformation under amphibolite-facies conditions along the 
margins of the zone produced orthogneisses in the Milford antiform and 
the New Bedford area and produced alteration and fracturing in the 
batholithic rocks in the center of the zone. Recent investigations in 
Rhode Island have led to the useful proposition that the Milford- 
Dedham zone consists of two terranes of similar age but of dissimilar 
history. The gneissic terrane on the west appears to have underlain the 
Merrimack belt during the Acadian orogeny. The nongneissic terrane 
to the east, which contains features most like those on the Avalon 
peninsula of Newfoundland, was juxtaposed to the gneissic terrane 
later, in the Alleghanian. This proposition remains to be tested, 
however. Continued compression deformed the Pennsylvanian and 
older rocks in the basins. Renewed movement occurred along such 
preexisting zones of weakness as the Bloody Bluff and Clinton- 
Newbury faults, during which the Newbury basins probably formed. 
Subsequent rapid uplift followed by east-west extension led to forma- 
tion of the Triassic and Jurassic basins and widespread north-south 
faulting of basement and basin rocks. A period of subsidence resulted in 
Cretaceous overlap of coastal-plain deposits. 

The Nashoba zone had a different history until the Pennsylvanian. 
The high-grade dynamothermal metamorphism of these rocks is pene- 
contemporaneous with emplacement of the Silurian or Ordovician 
Andover Granite. Intrusive Silurian calc-alkaline granite and granodi- 
orite, differing chemically from Paleozoic alkalic intrusives of the 
Milford-Dedham zone and from Ordovician to Devonian calc-alkaline 
and peralkaline intrusive rocks in the east flank of the Merrimack zone, 
were not metamorphosed. The rocks of the Nashoba zone project 
beneath the rocks of the Merrimack belt, as must those of the 
Milford-Dedham zone, and could be the basement for the strata of the 
Merrimack belt. The difference in nature of the plutonic rocks in the 
Nashoba and Milford-Dedham terranes indicates, however, that the 
two terranes were not connected until late in, or after, Devonian time. 
Both the origin and the placement of the Nashoba terrane are enig- 
matic. Features in the bounding fault zones suggest a long, complex 



'The Peabody Granite is shown as Middle Devonian on the State bedrock map 
on the basis of Rb-Sr and K-Ar age determinations by Zartman and Marvin 
(1971). Refinement of isotopic age determinations since the map was compiled 
indicates an Early Devonian U-Pb age for the Peabody (Zartman and Marvin, this 
vol., chap. J, table 1). 



history of movement in which pre-Devonian and post-Devonian ductile 
deformation preceded later brittle deformation. Evidence for Taconic 
or Acadian compressive deformation in the Milford-Dedham zone is 
lacking, but Acadian deformation is recognized on the east flank of the 
Merrimack belt and possibly the west flank of the Nashoba zone. The 
Ordovician dynamothermal event that affected the Nashoba zone is not 
necessarily in the same tectonic framework as the Taconic event of 
western Massachusetts. The original times of accretion of these ter- 
ranes to each other and to North America is uncertain, but the present 
configuration of the terranes in eastern Massachusetts is a result 
largely of Alleghanian movements on which has been superimposed 
early to middle Mesozoic faulting. 



INTRODUCTION 

Eastern Massachusetts east of the Merrimack belt 
consists of two terranes 2 of rock, the Milford-Dedham 
zone and that part of the Nashoba zone exposed at the 
surface east of the Merrimack belt (fig. 1). The term 
"Nashoba zone" is used in this restricted sense in this 
chapter. These terranes differ in stratigraphy and in 
metamorphic and plutonic history from each other and 
from rocks to the west (Goldsmith, this vol., chaps. E 
and F; Wones and Goldsmith, this vol., chap. I). The two 
terranes are bounded by major fault systems, and these 
fault systems also have differing metamorphic and tec- 
tonic histories. Thus, the terranes constitute distinct 
blocks of the Earth's crust. The Clinton-Newbury fault 
system on the west separates the Nashoba zone from the 
Merrimack belt. The Nashoba zone contains sedimentary 
and volcanic rocks of Proterozoic Z or possibly early 
Paleozoic age metamorphosed to high grade and intruded 
by Ordovician syn- or late-metamorphic S-type granite 
(Andover Granite) and by Silurian I-type postkinematic 
plutons of granodiorite and tonalite that are unique in 
composition in New England (Wones and Goldsmith, this 
vol., chap. I). The Bloody Bluff fault system separates 
the Nashoba zone from the Milford-Dedham zone to the 
east. The Milford-Dedham zone contains 590- to 630-Ma 
calc-alkaline granite batholiths that intrude medium- to 
low-grade metasedimentary, metavolcanic, and mafic 
plutonic and metaplutonic rocks of probable Proterozoic 
Z age. Overlying this basement, preserved in basins, are 
slightly metamorphosed to unmetamorphosed remnants 
of sedimentary and volcanic sequences of Proterozoic Z, 
Cambrian, Silurian and Devonian, and Pennsylvanian 
age. Plutons of alkalic granite of Ordovician and Devo- 
nian age and gabbro of Ordovician age intrude the 



^The term "terrane" is used in a broad sense for an area of rock having a 
different lithostratigraphic, metamorphic, and deformational history from adja- 
cent areas. It is equivalent to a zone or belt as these terms are used on the 
bedrock geologic map of Massachusetts (Zen and others, 1983) but is also used for 
a distinctive area within a larger unit; for instance, Putnam terrane in the 
Nashoba zone, ductilely deformed terrane in the Milford-Dedham zone. The 
rationale for use of the terms "zone" and "belt" on the State bedrock map is 
explained in Hatch and others (1984). 



STRUCTURAL AND METAMORPHIC HISTORY OF EASTERN MASSACHUSETTS 



H3 




EXPLANATION 



Pennsylvanian to Ordovician 
metasedimentary rocks 



Ordovician to Proterozoic Z 
metamorphic rocks 

Paleozoic pi u tons 

Paleozoic and Protero2oic Z 
sedimentary volcanic 
basins 

Proterozoic piutonic, 
metapiutonic, metavolcanic, 
and metasedimentary rocks 
including areas of the 
Rhode island anticlinorium 
and southeastern Mass- 
achusetts batholith 

Contact 

Fault — Dashed where inferred; 
dotted where concealed 

Limit of Rhode Island anti- 
clinorium not otherwise 
defined 



ATLANTIC OCEAN 



Cape Cod 



?\v?-- 



Nantucket 



Figure 1.— Index map and major structural features of eastern Massachusetts. 



basement and the pre-Ordovician cover rocks. The east- 
ern boundary of the Milford-Dedham zone may lie east of 
Cape Cod where a different terrane, possibly an exten- 
sion of the Meguma terrane of Nova Scotia, is reflected in 
magnetic signatures (Klitgord, 1984). 



The arrangement of this chapter is based largely on 
geographic distribution of the zones, which may reflect 
sequential docking onto the eastern edge of North Amer- 
ica. Within each of the two terranes, the arrangement 
follows the relative age of recognizable tectonic events. 



H4 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



A description of the structure and metamorphism of the 
Nashoba zone and of its western boundary, the Clinton- 
Newbury fault system, begins the chapter. Because the 
oldest events in the Milford-Dedham zone are recorded in 
the Proterozoic basement rocks, the structure and met- 
amorphism of each basement block within the Milford- 
Dedham zone are described next, followed by the 
description of the overlying sedimentary basins in the 
zone and the nature of their boundaries. A description of 
the Bloody Bluff fault system concludes the section on 
the structure and metamorphism of the Milford-Dedham 
zone. A summary section attempts to place the tectonic 
events into a sequential development of accreted eastern 
North America, a concept implied on the State bedrock 
map (Zen and others, 1983). 

The material described and discussed in this chapter is 
derived almost entirely from the work of others, and in 
only a few areas are the descriptions based on original 
observations of mine. The sources of data are many and 
include those listed on the State bedrock map as well as 
those referred to specifically in this chapter. The selec- 
tion of structural features shown on the map was my 
responsibility. The selection process was aided by ideas 
resulting from discussions with colleagues involved in 
compilation of the State bedrock map and others involved 
in the geology of eastern Massachusetts, in particular 
Marland P. Billings, L. Peter Gromet, J.C. Hepburn, 0. 
Don Hermes, Richard S. Naylor, and James W. Skehan. 



NASHOBA ZONE EAST OF THE MERRIMACK 
BELT 

The Nashoba zone east of the Merrimack belt, between 
the Clinton-Newbury fault system on the west and the 
Bloody Bluff fault system and faults bounding the New- 
bury basins on the east (figs. 1, 2), is distinct from the 
Milford-Dedham zone to the east and the Merrimack belt 
to the west. The Nashoba zone consists of gneisses and 
schists of high metamorphic grade, primarily the Na- 
shoba and Marlboro Formations, that are derived from 
sedimentary and volcanic protoliths (Goldsmith, this 
vol., chap. F), synkinematic plutonic rocks of Ordovician 
age, and distinctive postkinematic plutonic rocks of Silu- 
rian age (Zartman and Marvin, this vol., chap. J; Wones 
and Goldsmith, this vol., chap. I). The zone has a 
magnetic signature that contrasts with that of adjoining 
zones (Alvord and others, 1976; Castle and others, 1976; 
Harwood and Zietz, 1976). Foliation and layering dip 
steeply throughout the zone, and the rocks apparently 
have been folded and extensively cut and imbricated by 
faults, most of the traces of which are aligned parallel 
and subparallel to the trend of the rock units (Bell and 



Alvord, 1976, fig. 4; Skehan and Abu-Moustafa, 1976). 
Metamorphism is at the upper limit of the amphibolite 
fades (sillimanite metamorphic zone) throughout most 
of the zone, dropping somewhat lower (andalusite- 
staurolite metamorphic zone) on the northwest flank. 
Intrusion of the Ordovician Andover Granite provides a 
younger (upper) limit for the age of the formations, but 
these formations may be as old as Proterozoic Z. Olszew- 
ski (1980) determined a U-Pb isotopic age of 730 ±26 Ma 
on zircon from the Fish Brook Gneiss. The main period of 
metamorphism is believed to have occurred close to the 
time of intrusion of the S-type Andover Granite. The 
complex internal folding and faulting within the zone 
make it difficult to determine the stratigraphic order of 
the units. The Nashoba and Marlboro Formations have 
been interpreted to be an upright homoclinal sequence 
facing west (Bell and Alvord, 1976; Skehan and Abu- 
Moustafa, 1976; Skehan and Murray, 1980b); however, 
the map pattern of the lithologies (Bell and Alvord, 1976) 
and the magnetic pattern (Alvord and others, 1976) 
mentioned above suggest that this is not a homoclinal 
sequence. Some aspects of the folding and faulting within 
the Nashoba zone have been discussed in chapter F of 
this volume and will only be summarized here. 

INTERNAL STRUCTURE AND METAMORPHISM 

FOLDS AND MINOR STRUCTURAL FEATURES 

Major folds in the Nashoba zone have been identified in 
only a few places, whereas most mappers have recog- 
nized widespread small-scale folding. Hansen (1956) 
mapped major folds on the west flank of the Nashoba 
zone and indicated an overall synformal structure for the 
Nashoba-Marlboro sequence; however, the west flank of 
this synform has been truncated, eliminating the Marl- 
boro Formation. If the Reubens Hill Formation is equiv- 
alent to the Marlboro Formation, small remnants of the 
truncated west limb are preserved in small fault-bounded 
blocks in the Merrimack belt. Skehan and Abu-Moustafa 
(1976, fig. 1) showed a somewhat schematic set of two 
synforms and a central antiform in their cross section of 
the Wachusett-Marlborough tunnel, which extends from 
the Wachusett Reservoir, southwest of Bolton, to Marl- 
borough. Foliation symbols on the maps of the Billerica 
and Westford quadrangles (Alvord, 1975) indicate zones 
of alternating steep east and west dips. As this foliation 
is parallel to compositional layering that represents 
modified bedding, one might deduce that the original 
bedding has been flattened into tight folds about steep 
axial surfaces. Such folds are suggested by the map 
pattern, by repetition of lithologic units, and by the 
aeromagnetic pattern (Alvord and others, 1976; Castle 
and others, 1976; Harwood and Zietz, 1976; Barosh, 



STRUCTURAL AND METAMORPHIC HISTORY OF EASTERN MASSACHUSETTS 



H5 



1977), but they cannot be unequivocally mapped. Hinges 
of such folds would be difficult to identify in this highly 
deformed terrane. Traces of Hansen's large-scale folds 
are parallel to the general northeast trend of the gneiss- 
osity and schistosity in the zone. A synform at Rattle- 
snake Hill in Bolton (fig. 2) mapped by Hansen (1956) 
plunges gently northeastward and has a wavelength of 
about 0.8 km. Another synformal fold of his near Box- 
borough also plunges northeast, but steeply. 

Most minor folds in the Nashoba Formation fold both 
bedding and a parallel foliation. Folds of layering, to 
which the foliation is axial planar, have not been 
recorded. Younger minor folds have locally been super- 
posed on the earlier generation. Axial planes of the older 
folds described by Hansen (1956, p. 52) strike parallel to 
the trend of the rock units, dip to the west, and plunge 
moderately to the north, northeast, and southwest. 
Hansen has deduced from lengths of fold limbs that the 
major structure is synformal. Skehan and Abu-Moustafa 
(1976) described the same sense of folding in minor folds 
in their tunnel section. Locally in the tunnel, axial planes 
have been rotated into the horizontal. Many of the 
small-scale folds described by Skehan and Abu-Moustafa 
are related to faulting. On the southeast side of the 
Nashoba zone in the Marlboro Formation, Hansen (1956) 
observed that the minor folds trend obliquely to the 
trend of the units. Minor structures have a similar trend 
in the Marlborough area to the south (Hepburn and 
DiNitto, 1978). These minor folds range in size from 
minute wrinkles related to slip cleavage to broad undu- 
lations as much as a meter in wavelength. Their axial 
surfaces dip west or southwest, and their axes plunge 
steeply north to northwest. This geometry suggests 
either a significant lateral component of movement or 
rotation into the plane of a major thrust zone. Many folds 
here are broken or displaced by small thrust faults. 
Skehan and Abu-Moustafa confirmed these observations 
in the east end of the tunnel section near the Marlboro- 
Nashoba contact. These relationships suggest a struc- 
tural discordance between the Nashoba and Marlboro 
Formations. Indeed, Hepburn and DiNitto (1978) 
mapped a fault along this contact. Hansen suggested that 
the Marlboro underwent a second period of folding, 
which is obscured in the Nashoba Formation by a differ- 
ence in competency. Hansen (1956, p. 55) suggested, on 
the basis of orientation of superposed obliquely trending 
minor folds, that the Nashoba moved northeastward 
relative to the flanking formations. He noted (p. 57-58) 
that, at a few localities on the west side of the Nashoba 
zone, flow cleavage and schistosity in his mica schist 
facies of the Worcester Formation, now the Tadmuck 
Brook Schist, are axial planar to small isoclinal fold 
hinges in relatively competent quartzite layers. 



In summary, the rocks of the Nashoba zone may be 
tightly folded on a large scale, but such folds have not 
been recognized in the map pattern of rock units. Ver- 
gence of the early minor folds is generally to the east and 
southeast, but the orientation of later folds, particularly 
on the eastern side of the zone, indicates subsequent 
lateral transport in a northeast to east direction. 

The primary schistosity predates the emplacement 
of the Silurian Sharpners Pond Diorite and related 
plutons and also predates, but possibly not by much, 
the emplacement of the Ordovician Andover Granite. 
Hansen (1956, p. 55) observed that most of the major and 
minor folds were formed before feldspathization (that is, 
development of granitic and pegmatitic stringers and 
lenses, or partial melting). Most of these stringers and 
lenses lie in the plane of the foliation and are presumed to 
have formed while the regional stress field that produced 
the primary foliation in the Nashoba Formation still 
prevailed. This I interpret to mean that the peak of 
deformation preceded the thermal peak of metamor- 
phism at sillimanite grade. The thermal peak was pre- 
sumably close to the period of generation of the Andover 
Granite. The only deformation later than emplacement of 
the Silurian Sharpners Pond Diorite and the Ordovician 
Andover Granite appears to be related to faulting during 
movement on the regional Bloody Bluff and Clinton- 
Newbury systems. In this regard, it is worthwhile to 
point out here that the Ordovician Andover Granite is 
considered to have been emplaced as a wet granite at 
considerable depth, whereas the younger, Silurian plu- 
tonic rocks such as the Sharpners Pond Diorite that also 
intrude the rocks of the Nashoba block are dry and 
presumably were intruded at a higher level in the crust 
(Wones and Goldsmith, this vol., chap. I). 

METAMORPHISM 

The Nashoba zone lies in a narrow north-south belt of 
high-grade metamorphism in southern New England 
(Thompson and Norton, 1968). Pelitic rocks of the 
Nashoba Formation, the Shawsheen Gneiss, and the 
eastern part of the Tadmuck Brook Schist contain 
sillimanite-muscovite and sillimanite-orthoclase mineral 
assemblages characteristic of the upper amphibolite 
facies. Pelitic rocks of the Marlboro Formation and the 
western part of the Tadmuck Brook Schist have assem- 
blages containing staurolite and andalusite. These 
assemblages are truncated abruptly on the east by the 
Bloody Bluff fault and on the west by the Clinton- 
Newbury fault. Formations to the west, in the eastern 
part of the Merrimack belt, lie in a trough (Nashua 
Trough) and contain garnet-, biotite-, and chlorite-zone 
assemblages. To the east, the metamorphic rocks of the 
Milford-Dedham zone are low to middle amphibolite 



H6 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 







STRUCTURAL AND METAMORPHIC HISTORY OF EASTERN MASSACHUSETTS 



H7 




H8 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



facies near the boundary and are greenschist facies 
farther east. The Silurian and Devonian Newbury Vol- 
canic Complex is at subgreenschist facies, judging from 
the descriptions by Shride (1976b). 

Abu-Moustafa and Skehan (1976) gave the most thor- 
ough description of the metamorphism in the Nashoba 
zone in their study of the Wachusett-Marlborough tun- 
nel. They described pelitic assemblages containing 
sillimanite+orthoclase and sillimanite+muscovite. The 
orthoclase is commonly porphyroblastic. Garnet and cli- 
nozoisite are common. They placed the assemblages in 
the sillimanite-almandine-orthoclase subfacies of the 
almandine-amphibolite facies. The mineral associations 
suggested to them that the rocks of the Nashoba zone 
recrystallized at about 625-650 °C and at about 6 kbar, 
indicating a depth of cover of more than 23 km, assuming 
■f'ioad = -f > H o- Thompson and Norton (1968) placed the 
Nashoba zone east of and on the lower pressure side of 
the isobaric triple-point line for New England, on the 
basis of the assemblages. No one knows how much of the 
muscovite present is prograde dynamothermal and how 
much is later static hydrothermal. Examination of the 
rocks in some places suggests that some muscovite has 
replaced aluminum silicate. Presumably some muscovite 
was crystallized during late emplacement of the rela- 
tively hydrous Andover Granite (Castle, 1965a). Olszew- 
ski (1980) has shown from Rb-Sr data that a massive 
resetting of the Rb-Sr systems and a widespread fluid 
activity occurred about 450 Ma, about the time of 
emplacement of the Andover Granite (Handford, 1965); 
this then was apparently close to the peak of thermal 
metamorphism of the rocks. Emplacement of the Silurian 
Sharpners Pond Diorite and related rocks apparently 
had little metamorphic effect. Olszewski found no evi- 
dence for an Acadian metamorphic event, but he did 
recognize in his isotopic work the Carboniferous and 
Permian thermal event so widespread in southern New 
England (Zartman and others, 1970). The metamorphism 
we see in the Nashoba zone is, in part at least, penecon- 
temporaneous with an early Paleozoic thermal-plutonic 
event. We do not know if an earlier (Proterozoic?), lower 
grade metamorphism was overprinted by the early Pale- 
ozoic metamorphism. The truncation of isograds by the 
bounding faults on the flanks of the zone indicates that 
the block was metamorphosed before moving to its 
present position. 



Faulting within the zone has been fairly well docu- 
mented and also widely inferred. A complex fracture 
pattern is shown on the bedrock geologic map of the 
Boston l°x2° quadrangle by Barosh and others (1977), a 
pattern that probably includes joints, faults, and other 



lineaments (Barosh and others, 1974; Alvord and others, 
1976). Most of these features are not shown on the State 
bedrock map of Zen and others (1983), on which only 
faults having significant displacement are shown. Care- 
ful study by Skehan (1968) and Skehan and Abu- 
Moustafa (1976) of the many major and minor faults 
cutting the Nashoba and Marlboro Formations in the 
Wachusett-Marlborough tunnel showed that faulting in 
the zone is indeed complex. Faults are most abundant on 
the flanks of the zone in the vicinity of the Clinton- 
Newbury and Bloody Bluff faults, but some that Skehan 
and Abu-Moustafa considered significant are located in 
the tunnel section between the bounding faults. Most 
faults described by them are reverse and thrust faults 
having a sense of movement to the east. A few are high- 
or low-angle normal faults. They recognized different 
ages of faults in the tunnel section: some control emplace- 
ment of pegmatite and granite; others cut and displace 
these masses. Some faults are characterized by mylonite 
and ultramylonite, and these, in places, are cut by later 
faults. Later faults are usually characterized by breccia- 
tion, alteration, silicification, and zones of gouge. The 
Marlboro Formation is particularly broken by faults of 
both types, and the number of faults increases toward 
the Bloody Bluff fault on the east side of the Marlboro. 
The faults generally dip to the west, and movement 
sense is generally to the east and southeast, in the same 
general direction as the vergence of the minor folds. A 
few faults show a vertical sense of movement, and one 
fault in the lower part of the Nashoba, unit N9 of Skehan 
and Abu-Moustafa (1976), has a right-lateral sense of 
movement. Basalt dikes in the lower part of the Marlboro 
noted by Skehan and Abu-Moustafa cut granite and 
minor folds and in places appear to be controlled by 
preexisting faults and fractures. 

The faults observed by Skehan and Abu-Moustafa in 
the Wachusett-Marlborough tunnel have not been 
directly related to faults mapped on the surface in the 
Nashoba zone by Alvord (1975), Barosh (1976, 1978), and 
Hepburn and DiNitto (1978) nor to faults deduced from 
the aeromagnetic pattern by Alvord and others (1976) 
and Castle and others (1976). Alvord and others (1976) 
showed unnamed thrust faults at the top and bottom of 
the Fort Pond Member of the Nashoba Formation; the 
upper thrust would pass through the middle of the 
Nashoba in the tunnel section. Alvord and others showed 
this fault as a continuation of a northeast-trending fault 
near Shrewsbury (fig. 2) that splays off the Clinton- 
Newbury fault zone and truncates the upper part of the 
Nashoba. Castle and others (1976) showed the northeast- 
trending fault but did not extend it northward. Instead, 
they continued it to the northeast to end near Marlbor- 
ough. I have shown the interpretation of Castle and 
others on the State bedrock map. This fault could coin- 



STRUCTURAL AND METAMORPHIC HISTORY OF EASTERN MASSACHUSETTS 



H9 



cide with a fault zone that lies between units N18 and 
N17 of Skehan and Abu-Moustafa (1976), which they 
suggested may be regionally important. 

Spencer Brook and Assabet River Faults 

The Spencer Brook and Assabet River faults (Alvord 
and others, 1976; Bell and Alvord, 1976) (fig. 2) are 
shown on the State bedrock map because as mapped the 
faults truncate the Shawsheen Gneiss and Fish Brook 
Gneiss. Castle and others (1976) did not show the Spen- 
cer Brook fault but did show a fault coinciding with the 
Assabet River fault. Skehan and Abu-Moustafa (1976) 
did not identify the Spencer Brook and Assabet River 
faults of Alvord and others as such in the tunnel section. 
North of the Concord area, I have shown the Spencer 
Brook fault as curving to the north following a continuous 
foliation pattern in the Andover Granite shown on Cas- 
tle's (1964) maps of the Wilmington and Salem Depot 
quadrangles and connecting two areas of sheared rock 
shown by Castle and others (1976, pi. 1). This inter- 
preted fault intersects the Clinton-Newbury fault south 
of Lawrence. I have shown the Assabet River fault as 
similarly following the curvilinear contact of the east 
edge of the main mass of Andover Granite. Castle (1964) 
showed no features crossing the foliation pattern that 
might suggest that the faults trend east-northeasterly to 
the Newbury area, as is shown by Barosh and others 
(1977), to meet the east-trending faults forming the 
boundaries of the northern part of the Newbury basins 
(fig. 2). Nor do any features on the aeromagnetic maps 
indicate that such a connection exists. 

The continuation of the Spencer Brook and Assabet 
River faults south of the Marlborough area is not clear. 
Barosh (1977) showed the faults coming together in an 
anastomosing pattern west of Marlborough near the 
intersection of 1-290 and 1^95 and continuing as one 
fault south to the Bloody Bluff-Lake Char fault zone. 
Castle and others (1976) showed an inferred fault coin- 
ciding with the Assabet River fault and merging south of 
Shrewsbury into an inferred fault near the top of the 
Sandy Pond Amphibolite Member of the Marlboro For- 
mation, a fault not shown by Alvord and others (1976). 
Skehan and Abu-Moustafa (1976) showed a fault zone of 
probable significant displacement west of the contact of 
the Nashoba and the Marlboro, which may be the junc- 
tion of the Spencer Brook and Assabet River faults. The 
mapping of Hepburn and DiNitto (1978) and Hepburn 
(1978) suggests that the Assabet River fault in the 
Marlborough and Shrewsbury areas cuts downward 
below the basal schist of the Nashoba and truncates 
higher units of the Marlboro. The Tatnic fault (Dixon and 
Lundgren, 1968, p. 227; Wintsch and Hudson, 1978) in 
eastern Connecticut between the Quinebaug and the 



Tatnic Hill Formations occupies a similar position in 
places between the basal rusty schist of the Tatnic Hill 
and the Quinebaug. Dixon (written commun., 1979), 
however, recognized no fault between the rusty schist of 
the Nashoba and the Marlboro in the Grafton quadrangle 
in her reconnaissance. Accordingly, on the State bedrock 
map I have shown the continuation of the Assabet River 
fault (1, fig. 2) as lying above the rusty schist of the 
Nashoba rather than below it. However, exposures are 
insufficient to locate the fault at either stratigraphic 
level, if a fault exists. Possibly it cuts down below the 
Nashoba as suggested by the mapping of Hepburn and 
DiNitto (1978) and cuts down to the Bloody Bluff-Lake 
Char fault as shown by Barosh (1977). If it does not do so 
south of Marlborough, it certainly must do so to the south 
near Oxford. Without detailed mapping in the Grafton 
and Worcester South quadrangles, the location of the 
southern continuation of the Spencer Pond and Assabet 
River faults can only be speculated upon. 

Inclination of and Movement on Fault Surfaces 

Little information is available concerning the amount 
of dip of the fault surfaces within the Nashoba zone. The 
inferred faults of Castle and others (1976) dip 60°-70° to 
the west, as computed from aeromagnetic data. This dip 
coincides with the dips of many of the younger faults in 
the Wachusett-Marlborough tunnel section. Some of the 
apparently older faults characterized by mylonite dip 
30°-45° west. Observations in the tunnel are not sufficient 
to say which faults control the map pattern of the units. 
Several ages of faulting have clearly been superposed. 

Barosh (1977) showed numerous faults in the Nashoba 
Formation in the area between Marlborough and Oxford 
and referred to the whole zone as the Nashoba thrust 
belt. The numerous faults observed in the Wachusett- 
Marlborough tunnel indeed show the zone as highly 
faulted. However, most of the faults in the tunnel appear 
to have insignificant displacement and accordingly are 
not shown on the State bedrock map. Major displacement 
is confined primarily to the faults at and near the flanks 
of the zone. Discussion of the stratigraphy in the zone 
(Goldsmith, this vol., chap. F) led to an interpretation 
that the lithologic units within the block are imbricated 
to some extent on such faults as the Spencer Brook and 
Assabet River faults. Movement on most faults, as 
described above, is east directed, west side up, and right 
lateral, at least at a late stage. 

STRUCTURAL POSITION OF THE NASHOBA ZONE 

The Nashoba zone (figs. 2, 3) forms a lens-shaped block 
thinning to both north and south and tapering down dip 
to the west. If the trends of the two major fault zones 



H10 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



EXPLANATION 



Cover rocks of the Milford-Dedham zone 
(Mesozoic to Proterozoic Z) 



///K\\\\\1//// /////// /'////piAVvV 

v<\Ay/// ///////// <$&//// /Jv?v 

/ \\\\ 7 / ////////// '<$&/// /y-/<77>-^^ — 
'A^-$?-\y/i// /////// '<c&/////// /////// 
/K&y?// /////// /jxfr/ //// /f /' ' 



Gulf 
Maine 




10 



20 MILES 



10 20 KILOMETERS 

Figure 3. — Major structural features of southeastern New England. 



STRUCTURAL AND METAMORPHIC HISTORY OF EASTERN MASSACHUSETTS 



Hll 



remain the same, the Nashoba zone lenses out some- 
where in the Gulf of Maine. The Nashoba zone narrows 
considerably in southern Massachusetts near Webster 
but widens in outcrop width in Connecticut where it is 
represented by the Putnam terrane. The Nashoba For- 
mation and the Fish Brook and Shawsheen Gneisses 
decrease in thickness southward from 8,765 m (I arbi- 
trarily use one-half Bell and Alvord's (1976) thickness 
because of suspicion of repetition by faulting and folding) 
in the Westford-Billerica area to 3,740 m in the Wachu- 
sett tunnel section and to 1,701 m in the Tatnic Hill 
Formation in Connecticut (Dixon, 1965a; see Goldsmith, 
this vol., chap. F). Eastward along the Honey Hill fault, 
in southeastern Connecticut, the Tatnic Hill thins even 
more. Near Chester, Conn. , less than 155 m of Tatnic 
Hill is present (Lundgren, 1963). The Marlboro Forma- 
tion and the equivalent Quinebaug Formation in Con- 
necticut are fairly uniform in thickness from north to 
south: 2,140, 2,702, and 2,215 m (Bell and Alvord's 
section has not been halved in these measurements 
because there appears to be no internal duplication of 
units). In southeastern Connecticut, the Quinebaug thins 
and is eventually cut out by eastward convergence of the 
Tatnic and Honey Hill faults (Rodgers, 1982). The Tatnic 
Hill Formation thins between the Tatnic fault and a fault 
at a still higher level, possibly the inferred Clinton- 
Newbury, at the base of the Canterbury Gneiss, so that 
only the Yantic Member of the Tatnic Hill Formation can 
still be recognized in the vicinity of the Chester syncline 
(C, fig. 3) (Wintsch, 1979) in the keel of the Merrimack 
synclinorium. However, rocks lithologically similar to 
parts of the Tatnic Hill can be recognized in the Chester 
and Hunts Brook synclines (H, fig. 3) in the Proterozoic 
Z terrane of southeastern Connecticut (Lundgren, 1967; 
Goldsmith, 1967a,b). The Proterozoic Z basement is 
exposed (Snyder, 1964) in the Willimantic window (W, 
fig. 3) in central-eastern Connecticut beneath the Willi- 
mantic fault, which is equivalent to the Honey Hill fault 
(Wintsch, 1979). Here the Quinebaug equivalent is very 
much thinned and lies between the Willimantic-Honey 
Hill fault and a fault that is either the Tatnic fault or a 
higher level fault. 

These observations indicate that the Nashoba zone is a 
downward-thinning wedge between the Milford-Dedham 
zone and the Merrimack belt. The shallowness of the 
discontinuity beneath the Nashoba zone is indicated by 
the trace of the Bloody Bluff, Lake Char, and Honey Hill 
faults in southeastern Connecticut and the presence of 
Milford-Dedham basement in the Willimantic window. 
As the Merrimack belt plunges north from southern 
Connecticut, sections across the Nashoba zone and Put- 
nam terrane can only show the two terranes by projec- 
tion as shallow westward-dipping wedges (cross section 
F-F' on the State bedrock map) beneath the rocks of the 



Merrimack belt. The Science Park block of Nashoba 
(figs. 2, 4) mapped by Hepburn (1978) near Worcester is 
apparently an upthrust sliver of the Nashoba wedge 
caught in the Clinton-Newbury fault zone. The depth to 
the Nashoba zone or its equivalent beneath the Paxton 
Formation to the west is highly speculative (cross sec- 
tions D-D' and F-F' on the State bedrock map). The 
Massabesic Gneiss Complex may be a larger upthrust 
slice of Nashoba-zone rocks than the Science Park block. 
The argument for low dips for the Clinton-Newbury and 
Bloody Bluff faults at depth, which is based on observed 
distribution of Proterozoic Z basement in Massachusetts 
and Connecticut, is in opposition to the argument for 
appreciable transcurrent movement for the faults postu- 
lated by Zen and Palmer (1981) and Zen (1983). It is 
possible that transcurrent movement has occurred, an 
idea more easily accommodated in the steeply dipping 
segment north of Ayer and perhaps the Rattlesnake Hill 
fault than to the south. Any transcurrent movement 
must be late. We do not yet have a clear picture of the 
three-dimensional disposition of the major blocks of rock 
in southeastern New England. 

CLINTON-NEWBURY FAULT 

DESCRIPTION 

The Clinton-Newbury fault (fig. 2) forms the boundary 
between the Nashoba zone and the Merrimack belt. 
Northeast of Ayer, it truncates units of both the Merri- 
mack belt and the Nashoba zone, but south of Ayer its 
trace coincides with the trend of units in both terranes, 
and it lies below and locally within what are apparently 
the lowermost stratigraphic units of the Merrimack belt. 
The fault forms the southern boundary of the Newbury- 
port Complex in the Newburyport area, and the Ayer 
Granite lies west of the main trace of the fault through- 
out its extent. 

Segments of the Clinton-Newbury fault were identi- 
fied as early as 1880 (Crosby, 1880, p. 95-96; Clapp, 1921, 
pi. 1). Other workers suspected but could not identify a 
convincing break between the less metamorphosed rocks 
of the Merrimack belt on the west and the more meta- 
morphosed rocks of the Nashoba zone on the east (Emer- 
son, 1917, p. 77-78; Hansen, 1956, p. 20). The fault was 
first recognized as a major dislocation by Castle (1964; 
1965a,b) in the Lawrence-Groveland area (fig. 2) but was 
named by Skehan (1968, p. 281-283; Skehan and Murray, 
1980b) for the northeast-trending fault zone extending 
from the Wachusett Reservoir at Clinton, Mass. , north- 
east to the vicinity of Newbury and Newburyport. Castle 
and others (1976) reviewed the history of identification of 
the zone and named the fault the Essex fault; however, 



H12 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 




STRUCTURAL AND METAMORPHIC HISTORY OF EASTERN MASSACHUSETTS 



H13 



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H14 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



by this time the term "Clinton-Newbury" had come into 
common use. 

The Clinton-Newbury fault has fairly clear linear 
expression between Newburyport and Lowell where it 
consists of a single strand (Castle, 1964; Shride, 1976a). 
To the south, however, it is depicted as an anastomosing 
system of faults (Alvord, 1975; Peck, 1975; Gore, 1976b). 
The trace of the Clinton-Newbury fault on the branch 
southeast of the Tadmuck Brook Schist, southeast of 
Ayer, is based in large part on truncation of aeromag- 
netic pattern and lithic units in the Nashoba Formation 
along this line. Alternatively this truncation has been 
suggested to be an unconformity beneath the Tadmuck 
Brook Schist (Bell and Alvord, 1976). 

Skehan (1968) identified a group of fault zones at the 
west end of the Wachusett-Marlborough tunnel in the 
Clinton area, which he later included in the Clinton- 
Newbury fault zone. These are the Rattlesnake Hill fault 
zone, the Boylston fault zone, the Clinton fault zone, and 
the Wachusett Reservoir fault zone. Between the Rat- 
tlesnake Hill fault zone and the Boylston fault zone is the 
Science Park block, an apparently upthrust mass of 
Nashoba Formation (the Science Park unit of Hepburn, 
1976) and muscovite granite forming a lens that extends 
as far south as the Worcester area. South of Worcester 
the main trace is not clearly identified and is somewhat 
arbitrarily shown as separating the Nashoba Formation 
from the Boylston Schist of the Merrimack belt. 

Castle and others (1976) argued that the Clinton- 
Newbury fault is misnamed and should be called the 
Essex fault for its clear expression in Essex County 
where it dips steeply, because the Clinton fault, as 
identified, is a thrust. They felt that the main trace of the 
Essex fault is the steep Rattlesnake Hill fault of Skehan 
that bounds the allochthonous block of Nashoba near 
Shrewsbury on the east side (fig. 4) rather than the more 
gently dipping Boylston and Clinton faults that lie west 
of this block. However, the Rattlesnake Hill fault has not 
been traced as such south of Shrewsbury, and, as men- 
tioned above, the name "Clinton-Newbury" is now well 
established in the literature for the whole zone. The 
Clinton fault of Skehan (1968) is not necessarily the main 
trace of the Clinton-Newbury zone even though it forms 
the lower limit of the masses of Ayer Granite. No Ayer 
Granite is presently identified in the Nashoba zone, and 
the Ayer appears to be entirely confined to the rocks of 
the Merrimack belt. The Tadmuck Brook Schist, the 
Vaughn Hills Quartzite, the Boylston Schist, and the 
Tower Hill Quartzite of the Merrimack belt lie west of 
the Rattlesnake Hill fault. Skehan and Murray (1980b, p. 
288-289) showed these units as part of the Nashoba zone. 
Hepburn (1978), however, showed a fault bounding these 
units on the east side (3, fig. 4) as part of the Clinton- 



Newbury fault zone. This fault is shown on the State 
bedrock map. 

The main trace of the Clinton-Newbury fault as 
defined above dips steeply in its northern part. A.F. 
Shride (oral commun., 1979) stated that the dip is steep 
in the Newburyport area, where the Clinton-Newbury 
coincides with Shride's (1971) Scotland Road fault. Mylo- 
nites close to the fault west of Groveland dip 60° to the 
west (Castle and others, 1976). Near Harvard the fault 
dips steeply and is characterized by zones of mylonite 
and mylonite gneiss, most conspicuous in Ayer Granite. 
Near Harvard the fault splits into two strands in the 
Tadmuck Brook Schist and forms a lens containing Ayer 
Granite and Harvard Conglomerate at Pin Hill in Har- 
vard. Dips are steep in sheared Ayer Granite on both the 
west side of Pin Hill (Barosh, 1976, p. 311; Gore, 1976a, 
p. 106, and written commun., 1978) and the east side 
(Thompson and Robinson, 1976, p. 348). Farther south, 
the Rattlesnake Hill fault of Skehan (1968) dips 60°-65° to 
the west and was shown by Skehan and Abu-Moustafa 
(1976) as a reverse fault. The Clinton and Boylston faults 
to the west and those in the Carville basin of the 
Wachusett Reservoir have dips of 30°^5° (Skehan, 
1968). Sense of movement on these faults is northwest 
over southeast. In the Wachusett-Marlborough tunnel 
section, steeper faults, such as the Rattlesnake Hill fault, 
locally fold and truncate the more shallowly dipping 
faults (Skehan and Murray, 1980b). The Boylston fault, 
however, according to Skehan and Abu-Moustafa (1976), 
is characterized by hydrothermal alteration, the pres- 
ence of pyrite, and development of chlorite. This associ- 
ation suggests that it is a younger, shallower fault than 
the fault associated with ductile mylonites observed in 
the Harvard area to the north. G.R. Robinson, Jr. 
(written commun., 1984), observed, in the steeply dip- 
ping part of the Clinton-Newbury fault near Ayer, 
evidence for an earlier right-lateral displacement fol- 
lowed by left-lateral displacement. 

The location of the principal strand of the Clinton- 
Newbury fault system south of Clinton is arguable. The 
structural position of the block of rock (fig. 4) containing 
Hepburn's (1976) Science Park unit and the Rattlesnake 
Hill pluton that lies between the Rattlesnake Hill and the 
Clinton and Boylston faults is obscure (see Castle and 
others, 1976). I have interpreted the Science Park block 
as a slice of Nashoba Formation thrust up eastward along 
the Rattlesnake Hill fault to override a thin sequence of 
units that may lie unconformably on the Nashoba (Ske- 
han and Murray, 1980b). To have been brought up from 
the west, the rocks of the Nashoba zone must extend at 
shallow depth an appreciable distance westward under 
the east flank of the Merrimack belt. Such a configura- 
tion is shown on cross section F-F' of the State bedrock 
map. 



STRUCTURAL AND METAMORPHIC HISTORY OF EASTERN MASSACHUSETTS 



H15 



The location of the Rattlesnake Hill fault south of 
Shrewsbury is uncertain. Hepburn's (1978) fault (3, fig. 
4) that diverges from the southeast margin of the Rat- 
tlesnake Hill pluton bounds a narrow wedge of Tadmuck 
Brook Schist and Vaughn Hills Quartzite (Skehan and 
Abu-Moustafa's (1976) units Ul through U10). Whether 
this fault rejoins the Rattlesnake Hill fault is not known. 
South of Shrewsbury, Castle and others (1976) showed 
the Rattlesnake Hill fault as offset to the west along a 
northeast-trending lineament and inferred fault (1, fig. 
4), coinciding with an abrupt cessation of north-trending 
magnetic anomalies. South of this offset they showed the 
Essex (Rattlesnake Hill) fault as continuing within the 
Nashoba Formation (4, fig. 4) to join the trace of the 
Clinton-Newbury near Oxford. They also raised the 
possibility that the fault trends directly south to join the 
Lake Char fault east of Oxford. 

On the State bedrock map, I have extended the Rat- 
tlesnake Hill fault southwestward to the northeast- 
trending lineament (1, fig. 4), which incidentally is shown 
fairly well by a discordance in structural data on Hep- 
burn's Shrewsbury map (unpub. data, 1979), and thence 
offset it along the lineament so that it eventually joins, 
near Lake Quinsigamond, the faults lying west of the 
Science Park block (fig. 4). On figure 4, I have shown the 
fault as offset again to the north along the Pine Hill fault 
at Lake Quinsigamond. South of Worcester I have placed 
the main trace of the Clinton-Newbury fault at the 
boundary between the Boylston Schist and the Nashoba 
Formation. This location coincides with an unnamed fault 
of Castle and others (1976), lying between their Essex 
(Rattlesnake Hill) fault and their westernmost fault, the 
Clinton. In the Oxford- Webster area, I have placed the 
main trace at the base of the Ayer Granite (Sagr) 
following Barosh (1978) and Dixon (unpub. data, 1978). I 
have interpreted Castle and others' (1976) Clinton fault 
south of Worcester as a continuation of the high-angle 
Wekepeke fault (Peck, 1975) because it bounds the 
low-grade Worcester Formation on the west as the 
Wekepeke does farther north. The different generations 
of faults in the Oxford-Webster area have not been 
identified to my satisfaction. Somewhere in this area the 
north-south Flint Hill fault system (Rodgers, 1970; Ske- 
han and Murray, 1980b, p. 294), characterized by brec- 
ciation, silicification, and hydrothermal alteration of 
probable Triassic and Jurassic age, of which the Weke- 
peke fault (Pine Hill fault of Castle and others) is a part, 
must connect with the similar fault system typified by 
the Lantern Hill fault in southeastern Connecticut (Gold- 
smith, 1985) and to the north-trending silicified and 
brecciated rocks that roughly coincide with the Lake 
Char ductile fault zone in eastern Connecticut (Dixon, 
1965b, 1968). The trace of the Clinton-Newbury fault 
southeast of Webster is speculative. Possibly it curves 



southwestward to lie at the base of the Canterbury 
Gneiss (Ayer equivalent) in eastern Connecticut, as 
shown by Pease (1982, fig. 1). There it merges with the 
Honey Hill fault zone south of Colchester, Conn. In 
support of this idea, the base of the Canterbury Gneiss 
exposed along Connecticut Route 11 west of Salem Four 
Corners, Conn., has a cataclastic foliation dipping about 
20° to the northwest. On the other hand, Castle and 
others (1976) projected the Clinton-Newbury fault 
(Essex and Clinton faults) into faults within the Tatnic 
Hill Formation shown by Dixon (1974, 1982) in northeast 
Connecticut. The west-dipping faults like the Clinton of 
Skehan (1968) must be truncated by the steeply dipping 
Wekepeke fault, and their trace should theoretically be 
offset to the north by uplift on the west side of the 
Wekepeke. No such fault or faults have been mapped. 
Numerous thrusts involving Ayer Granite, however, are 
shown south of Worcester by Barosh (1974, 1977). 

The Clinton-Newbury fault projects eastward into the 
Gulf of Maine where Simpson and others (1980) sug- 
gested that it is aligned with the trend of the South Atlas 
fault, or it may in some way connect with the Cobequid- 
Chedebucto fault of Nova Scotia. It may connect with 
the Isleboro fault of eastern Maine, although the lat- 
ter seems more identifiable with the Bloody Bluff fault 
from the nature of the rocks on either side of it. It 
does not appear to project into the Norumbega fault 
(Wones and Thompson, 1979) of eastern and southeast- 
ern Maine. The Norumbega seems rather to project 
toward the Massabesic terrane to the west, as suggested 
by Lyons and others (1982). Structural features of east- 
ern Maine and northeastern Massachusetts are not 
directly correlatable. 

The north-northeast-trending Portsmouth fault of 
Novotny (1969), which lies between the Rye Formation 
of southern New Hampshire and the Kittery Formation 
of the Merrimack Group, does not project on the surface 
into the Clinton-Newbury as might be expected from its 
strike. Carrigan (1984a) found that the mylonites of the 
Portsmouth fault (P, fig. 3) are folded eastward around 
the south-plunging nose of the Rye anticline. This folded 
mylonite suggests that the Portsmouth fault may mark 
the reappearance to the north of the ductile portion of the 
Clinton-Newbury fault. The lithology and metamor- 
phism of the Rye as described by Carrigan (1984b) 
resemble those of the Marlboro Formation in the Na- 
shoba zone. Such an interpretation changes the position 
of the trend of the Clinton-Newbury and brings the fault 
closer to projected connections with the faults in Maine. 
An important fact to be emphasized is that the Clinton- 
Newbury fault, although it appears to be a boundary 
between major basement blocks, coincides on the surface 
in Massachusetts with an apparent unconformity 
between the rocks of the Merrimack synclinorium and 



H16 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



the rocks of the subjacent Nashoba zone (Robinson and 
Goldsmith, this vol., chap. G). Typical Nashoba rocks, 
although disrupted by the Clinton-Newbury fault, 
project below the rocks of the Merrimack belt, as dis- 
cussed above, and form part of the basement on which 
the Merrimack-belt strata are deposited. An alternative 
explanation is that part or all of the overlying Merrimack 
strata have moved into their present position on an early 
detachment surface. 

AGE OF THE CLINTON-NEWBURY FAULT ZONE 

The Clinton-Newbury is a composite fault zone, includ- 
ing both thrusts and reverse faults that have formed over 
a period of time. Some of the movement may have been 
as old as Acadian, particularly the low-angle thrusts, 
which may be related to the east-directed back-thrusting 
recorded by Robinson and Hall (1980) in the Bronson 
Hill zone to the west. Some of these thrusts contain 
sillimanite-grade mylonites, according to G.R. Robinson, 
Jr. (written commun., 1984). The deformation of the 
Devonian plutons suggests, however, that the bulk of the 
movement, particularly on the steeply dipping segments, 
is Late Devonian or younger. Faults in the Clinton- 
Newbury zone are clearly younger than the metamor- 
phism of the rocks in both the Nashoba zone and the 
Merrimack belt, and faults in the zone clearly cut the 
Devonian muscovite granite of the Rattlesnake Hill 
pluton. It is not certain how faults that enclose the 
Middle Pennsylvanian Coal Mine Brook Formation at 
Worcester are related to the Clinton-Newbury or to the 
probably Triassic and Jurassic faults of the Flint Hill 
fault system. 

In summary, at least three different styles and ages of 
faulting are present in the Clinton-Newbury fault zone. 
The oldest is low-angle, east-directed thrusts and 
reverse faults. They are possibly of Acadian age but 
could be late Paleozoic. The next group is high-angle 
faults that truncate the former and that in places contain 
evidence of strike-slip movement. These are probably 
late Paleozoic. Both of these groups are ductile. The 
third style of faults is high-angle, apparently normal 
faults characterized by brecciation and silicification. 
They are probably Mesozoic in age. 



STRUCTURAL RELATIONS WEST OF THE CLINTON- 
NEWBURY FAULT 

The Clinton-Newbury fault now forms much of the 
presently exposed east margin of the Merrimack belt. 
The east flank of the Merrimack belt, as here defined, is 
that area between the Wekepeke fault and the Eastford 
fault in Connecticut (figs. 3, 5), on the west, and the 
Nashoba zone, on the east. It is a structurally complex 



area containing an assemblage of rocks whose sequence 
is uncertain but which are younger than the rocks of the 
Nashoba zone (Robinson and Goldsmith, this vol., chap. 
G). The area can be divided structurally into the Nashua 
trough (Crosby, 1880; Smith and Barosh, 1981), to the 
west and northwest, and the Rockingham anticlinorium 
(Billings, 1956), to the north and northeast. The bound- 
ary between the Nashua trough and the Rockingham 
anticlinorium in Massachusetts is in part a fault (F, fig. 3) 
coinciding with a metamorphic gradient, but near the 
New Hampshire border the boundary is primarily a 
metamorphic gradient and coincides with the contact 
between the Oakdale Formation and the Berwick For- 
mation (or the equivalent units mapped by G.R. Robin- 
son, 1978). The Rockingham anticlinorium is truncated 
diagonally by the Clinton-Newbury fault so that to the 
south, near Worcester, rocks of the Nashua trough lie 
adjacent to rocks of the Nashoba zone across the Clinton- 
Newbury fault. The Nashua trough persists to the south 
into Connecticut through a complex system of faults near 
the Massachusetts-Connecticut border and appears to 
coincide with a low-grade metamorphic trough extending 
southward through eastern Connecticut (Thompson and 
Norton, 1968) that contains units such as the Scotland 
Schist (Dixon and Lundgren, 1968). The Rockingham 
anticlinorium and the Nashua trough contain similar 
lithologies, but differences in metamorphic grade be- 
tween the two structural belts and the complex style of 
deformation make it difficult to establish stratigraphic 
sequences that could aid in determining the structure. 

NASHUA TROUGH 

The structure within the Nashua trough remains to be 
convincingly worked out. Crosby (1880) considered the 
Nashua trough to be synclinal (fig. 6) so that the Oakdale 
and Paxton Formations were equivalent units on oppo- 
site limbs and the Worcester Formation lay in the 
trough. Peck (1975) showed overturned beds in the 
Worcester Formation, but he also showed upright beds a 
short distance across strike so that the folding in these 
rocks may be primarily megascopic; his section, like that 
of Grew (1970), on the whole is upright and west facing. 
On the other hand, Grew (1973), Hepburn (1976), and 
G.R. Robinson (1981) presented evidence that suggests 
that the Oakdale Formation lies in an isoclinal syncline so 
that the Tower Hill Quartzite and the Worcester Forma- 
tion are on opposite limbs. Crosby's Nashua "synclinal" 
possibly exists but in more complicated fashion than he 
figured. The whole section may be recumbently folded 
(G.R. Robinson, 1981, p. 59-63; fig. IB, C). Robinson 
(1981) and Peper and Wilson (1978) both noted, in the 
area east of the Fitchburg Complex, a flat cleavage that 
deforms an earlier, relatively flat regional schistosity 



STRUCTURAL AND METAMORPHIC HISTORY OF EASTERN MASSACHUSETTS 



H17 



tvF ■ Exeter. 




Coal Mine Brook Formation and Harvard 
Conglomerate 

Chelmsford Granite and Fitchburg Complex 

Ayer Granite, Exeter Diorite, and 
Newburyport Complex 

Littleton Formation 

Worcester Formation 

Oakdale Formation 

Paxton Formation and Merrimack Group 

Tower Hill Quartzite, Reubens Hill Formation, 
Vaughn Hills Quartzite, and Boylston 
Schist 

Contact — Dashed where inferred 

Fault — Dashed where inferred 

A' 

Line of section — Shown on 

figure 7 



^lASS 
R 1 



20 KILOMETERS 



Figure 5. —Geology of the east flank of the Merrimack belt, Massachusetts and New Hampshire, showing locations of cross sections (A-A'-A" 

B-B' y C-C of figure 7). 



H18 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 
WEST 



Nashua "synclinal" through Sterling and Hudson 



EAST 



''J>'->\1 



EXPLANATION 

Fitchburg Complex 
Ayer Granite 



m 



. fl Harvard Conglomerate 



m 






Worcester Formation (and 
phyllite fades of the 
Tower Hill Quartzite?) 

Oakdale Formation and 
Paxton Formation 

Tadmuck Brook Schist and 
Vaughn Hills Quartzite 

Nashoba Formation 



WEST 



Contact — Dashed where 

projected 




EAST 



Nashua "synclinal" through Shirley and Harvard 

Figure 6. — Schematic section near cross section C-C (see fig. 5 for location) across the Nashua "synclinal" as drawn by Crosby (1880, pi. III). 
Unit names are those used on the State bedrock map (Zen and others, 1983). 



accompanied by small-scale isoclinal folds. This deforma- 
tion pattern suggests at least two stages of horizontally 
directed stress. Tucker (1978) described early stages of 
recumbent folding followed by open, upright folding to 
the west in the Wachusett Mountain area. These flat 
folds are indicated on cross section D-D' of the State 
bedrock map. The configurations shown in figure 7 are 
largely the result of the latest folding. Such inferences 
can be made to account for the discrepancies in topping 
evidence between Peck (1975) and Robinson (1981), 
presuming a regional northward plunge. However, no 
really satisfactory interpretation accommodates all the 
mapping. It is possible that submarine slides have dis- 
rupted the original depositional sequence. Furthermore, 
lateral stratigraphic facies changes can obscure equiva- 
lencies of units and thus lead to false inferences as to 
structure. 

The formations in the Nashua trough are at lower 
metamorphic grade than are those in adjacent belts, and 
they coincide with a metamorphic trough (Thompson and 
Norton, 1968). The Nashua trough is bounded on its west 
side by a high-angle fault, the Wekepeke fault, that 
brings up higher grade metamorphic rocks on the west in 
and adjacent to the Fitchburg Complex and to the 
Massabesic Gneiss Complex in the Massabesic anticlino- 
rium. The Worcester Formation contains andalusite in 
the western part of its outcrop area near the Wekepeke 



fault and the Fitchburg Complex. The Wekepeke fault is 
part of the Flint Hill-Silver Hill fault system of probable 
Permian age (Lyons and others, 1982) and (or) Mesozoic 
age (Rodgers, 1970). 

ROCKINGHAM ANTICLINORIUM 

The Rockingham anticlinorium consists of folded but 
overall gently dipping low- to medium-grade metamor- 
phic rocks of the Merrimack Group (figs. 5, 7). In New 
Hampshire, the Rye Formation of Proterozoic Z or early 
Paleozoic age is exposed in the easternmost anticline, the 
Rye anticline, but the Rye Formation does not reach 
Massachusetts on the surface. The formations are in open 
folds about steep axial surfaces on which are superim- 
posed local zones of tight folding. The folding is tighter 
and more discordant to the west, and early isoclinal folds 
on a mesoscopic scale (not indicated in the section, fig. 
1A) are locally present. Existing sections imply that the 
strata are not severely deformed. G.R. Robinson's study 
(1978) suggested that the structure within the Merri- 
mack Group in the Pepperell-Ayer area, at least, is not 
one of simple anticlines and synclines but that these are 
at least second-generation folds and that more than one 
earlier generation of folds is present. The cross sections 
of Sriramadas (1966) in the Nashua area of the Nashua 
trough indicate fairly tight folding, but in the Rocking- 
ham anticlinorium to the east the rocks of the Merrimack 



STRUCTURAL AND METAMORPHIC HISTORY OF EASTERN MASSACHUSETTS 



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H20 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



Group appear to be less deformed and in more open folds 
(fig. 1A). However, an earlier generation of small-scale 
isoclinal folds seen in outcrops (Sundeen, 1971) suggests 
that the structure here too may be complex. Robinson 
(1978, section B"-B'") showed a unit equivalent to the 
Berwick Formation overlying a unit equivalent to the 
Oakdale Formation in the Pepperell area. The section is 
probably a combination of intertonguing facies and soft- 
sediment deformation that has subsequently been poly- 
deformed. 

Metamorphic grade increases across the Rockingham 
anticlinorium. The formations are in the chlorite and 
biotite zones in the east and reach garnet grade to the 
west. The garnet zone coincides with an area containing 
many plutons of granite and diorite of Silurian and 
Devonian age. A sharp gradient back to chlorite zone 
forms the boundary between the Rockingham anticlino- 
rium and the Nashua trough in the Pepperell area. This 
boundary coincides with the contact of the Berwick and 
Oakdale Formations; it marks a change in bedding style 
and grain size from thin bedded and fine grained 
(Oakdale) to thick bedded and coarse grained (Berwick). 
The metamorphic zones, like the rock units, are trun- 
cated to the south by the Clinton-Newbury fault. 



In summary, the Nashoba zone is a lens of internally 
folded and faulted high-grade paragneiss and metavol- 
canic rock bounded by major fault systems. These faults 
separate the zone from terranes different in lithology, 
metamorphism, and plutonism. Regional relations indi- 
cate that the Nashoba zone projects beneath the Merri- 
mack belt to the west. The paragneiss is intruded by 
late-metamorphic granite and quartz diorite of Ordovi- 
cian age and postmetamorphic granite and granodiorite 
of Silurian and Devonian age. The main period of thermal 
metamorphism and folding is Ordovician or older. Faults 
range in age from Late Devonian (postgranite) to prob- 
ably Triassic and Jurassic. East-directed thrusts and 
reverse faults are older than high-angle faults. The 
folding and thrusting on the east flank of the Merrimack 
belt are thought to be Acadian because the Upper 
Ordovician(?) and Lower Silurian Ayer Granite is 
involved. 



MILFORD-DEDHAM ZONE 

The Milford-Dedham zone has been broken by normal 
and reverse faults into upthrown blocks of Proterozoic 
plutonic and metamorphic basement rock and down- 
thrown blocks containing sedimentary and volcanic cover 
rocks of Proterozoic Z, Cambrian, Silurian-Devonian, 
Pennsylvanian, and Triassic-Jurassic age (figs. 1, 8). The 



N H 
MASS 




EXPLANATION 



Jurassic and 
, Triassic 

I \] Pennsylvanian 

Devonian and 

I , Silurian 

1 Cambrian 

, , and Proter- 

Massacnusetts 

Bay 



ozoic Z 



Bellingham 
basi 
MA . S 1, MASS 
CONNJI R 1 



Cape Cod 
Bay 



J& 




10 20 30 KILOMETERS 



Figure 8.— Basins of the Milford-Dedham zone, eastern Massachu- 
setts. 

major blocks of basement are the gneissic Milford anti- 
form at the north end of the Rhode Island anticlinorium, 
the brittlely deformed Salem block to the northeast, and 
the southeastern Massachusetts batholith, subdivided 
into the Dedham, Foxborough, and Fall River blocks. 
Intervening basins are, from north to south, the New- 
bury, Middleton, Boston, Norfolk, Bellingham (also 
locally known as Woonsocket), Narragansett, and Nan- 
tucket basins. The Milford-Dedham zone east of the 
Bloody Bluff fault is at least 200 km wide. U.S. Geolog- 
ical Survey seismic line 5 (cited by Grow and others, 
1979) places the edge of the continental crust at about the 
northern edge of the East Coast magnetic anomaly 
(Klitgord and Behrendt, 1979) about 190 km south of 
Nantucket. Klitgord (1984) suggested that the boundary 
between the Avalon terrane (Milford-Dedham zone) and 
the Meguma terrane exposed in southern Nova Scotia is 
either within the Gulf of Maine platform or at the 
boundary of the Georges Bank rift basin. In the latter 
case, the edge of the Milford-Dedham zone would lie east 
of Nantucket Island and Cape Cod. In the former case, 
the boundary could not lie west of Cape Cod Bay and 
probably lies east of Cape Cod judging from the materi- 
alin cores drilled at Harwich, Mass. (Goldsmith, this vol., 
chap. E; Wones and Goldsmith, this vol., chap. I). 



Table 



STRUCTURAL AND METAMORPHIC HISTORY OF EASTERN MASSACHUSETTS H21 

1.— Metamorphism, plutonism, and faulting in basement rocks of the Milford-Dedham zone, eastern Massachusetts 





Milford antiform 


Salem block 


Dedham block 


Foxborough block 


Fall River block 




Northwest 


Southeast 


Paleozoic intrusive rocks 


Rock type 


(In Rhode Island 
Devonian alkalic 
granite, partly 
gneissic). 


Devonian to Ordo- 
vician alkalic 
granite and gab- 
bro. 


Devonian to Ordo- 
vician alkalic 
granite. 


Devonian alkalic 
granite. 


Not present 


Not present. 


Cambrian strata 


Rock type 
Metamorphism 


Not present 


Argillite and mar- 
ble. 

Contact metamor- 
phosed. 


Slate, argillite, 
sandstone, and 
quartzite. 

Subgreenschist 


Slate, argillite, and 
quartzite. 

Subgreenschist 


Not present 


Not present. 


Proterozoic Z plutonic rocks 


Rock type 


Orthogneiss and 
granite to dio- 
rite. 


Granite to gabbro, 
partly cataclastic 
and altered. 


Granite to gabbro, 
partly cataclastic 
and altered. 


Granite to gabbro, 
partly cataclastic 
and altered. 


Granite, partly cat- 
aclastic and 
altered. 


Orthogneiss, 
granite, 
and dio- 


Metamorphism 


Amphibolite 


Greenschist to sub- 
greenschist. 


Greenschist to sub- 
greenschist. 


Greenschist to sub- 
greenschist. 


Greenschist to sub- 
greenschist. 


rite; 

locally cat- 
aclastic. 
Amphibolite. 


Proterozoic Z stratified rocks 


Rock type 
Metamorphism 


Gneiss, schist, 
quartzite, and 
amphibolite. 

Amphibolite 


Gneiss, schist, 
quartzite, 
amphibolite, fine- 
grained para- 
gneiss and green- 
stone. 

Amphibolite to 
greenschist. 


Phyllite, quartzite, 
and greenstone. 

Greenschist 


Phyllite and green- 
stone. 

Greenschist 


Phyllite and green- 
stone. 

Greenschist 


Gneiss and 
schist. 

Amphibolite. 


Faults 




Largely ductile 


Ductile and brittle 


Brittle 


Brittle 


Brittle 


Brittle and 
ductile(?). 



STRUCTURE AND METAMORPHISM IN THE 
BASEMENT BLOCKS 

MILFORD ANTIFORM 

The largely batholithic rocks of the Rhode Island 
anticlinorium project into Massachusetts as the Milford 
antiform (Milford anticline of Hall and Robinson, 1982, 
fig. 1). The Milford antiform consists of gneissic plutonic 
rocks of the Proterozoic Z Sterling Plutonic Suite (Gold- 
smith, 1966; Goldsmith and others, 1982) of Connecticut, 
Rhode Island, and Massachusetts, the Proterozoic Z 
Milford Granite and adjacent unnamed granite (Zgr, on 
the State bedrock map) in Massachusetts, and the meta- 
sedimentary and metavolcanic rocks of the Plainfield and 
Westboro Formations and the Blackstone Group. A 



large area of Devonian granite has been identified in 
central Rhode Island in what was formerly considered to 
be part of the Sterling Plutonic Suite (Hermes and 
others, 1981), but this rock does not extend into Massa- 
chusetts. Metasedimentary and metavolcanic rocks of 
the Plainfield and Westboro Formations and the Black- 
stone Group flank the antiform on the west, north, and 
east, respectively, and form septa within it. These rocks 
are mostly in the amphibolite fades of metamorphism 
(table 1). Rocks of the Proterozoic Z mafic metamorphic- 
plutonic complex (Zv and Zdigb) of eastern Massachu- 
setts are most abundant north of the antiform in the 
Salem block. 

The Milford antiform is bounded on the west by the 
Lake Char fault. The antiform plunges northeastward in 



H22 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 




5 10 KILOMETERS 

Figure 9. — Form lines on foliation and direction of plunge of lineation in the Milford antiform, eastern Massachusetts. Data from McKniff (1964), 
Shaw (1967), Barosh (1974, 1978), Dixon (1974, written commun., 1977 and 1978), Nelson (1975a,b), Volckmann (1977), and Hepburn and 
DiNitto (1978). 



the Marlborough-Framingham area where it is truncated 
obliquely by the Bloody Bluff fault. The trend of gneiss- 
osity and rock units in the Milford antiform forms a 
partial arc around a core of Milford Granite (figs. 2, 9). 
Internally, the antiform shows an interference pattern of 
fold structures evident in the East Douglas area. Low 
plunges of lineations combined with the complex map 



pattern of units on the east side of the antiform near the 
Massachusetts-Rhode Island border suggest the pres- 
ence of refolded nappe structures or of low-angle ductile 
deformation, which is reminiscent of features character- 
istic of metamorphic core complexes in the Western 
United States (see Coney, 1980). The west-projecting 
node at Oxford and Webster, called by Barosh (1982) the 



STRUCTURAL AND METAMORPHIC HISTORY OF EASTERN MASSACHUSETTS 



H23 



Oxford anticline, is apparently younger than the Lake 
Char fault because it folds the fault. 

The Proterozoic metasedimentary and metavolcanic 
rocks (Zp, Zw, Zb) within the Milford antiform are in the 
amphibolite facies and contain minor folds older than the 
gneissic fabric in the plutonic rocks. These folds are less 
noticeable in the west, where the plutonic rocks have a 
marked gneissic fabric, than to the east, where the 
plutonic rocks are less deformed, for example in and east 
of Hopedale. In most places, however, the foliation in the 
metamorphic rocks tends to parallel the foliation in the 
plutonic rocks, indicating that the two were deformed 
together. 

In general the intensity of deformation increases from 
east to west across the Milford antiform toward the 
Bloody Bluff fault. However, within the antiform are 
zones of rock more gneissose than adjacent rock. Dis- 
crete zones of gneissic rock can be seen north and south 
of the Milford Granite; O'Hara and Gromet (1984) iden- 
tified in northern Rhode Island a discrete zone of gneiss- 
osity (fig. 3), separating the Hope Valley Alaskite Gneiss 
(Zhg) to the west from the Ponaganset Gneiss (Zpg) and 
Devonian Scituate Granite to the east, that they believed 
to be the southward continuation of the Bloody Bluff 
fault. The gneissosity is more apparent in biotitic rocks, 
such as the Ponaganset Gneiss and the biotitic phase of 
the Milford Granite (Zmgd), than in the biotite-poor 
Hope Valley Alaskite Gneiss (Wones and Goldsmith, this 
vol., chap. I). The gneissosity is marked by preferred 
orientation of biotite, where present, and by flat lenses 
and laminae of quartz and of feldspar. This foliation is 
parallel to that in the metavolcanic and metasedimentary 
rocks, although the plutonic rocks can be seen in places to 
truncate layering and an earlier foliation in the metased- 
imentary rocks at a low angle. In places, the Hope Valley 
clearly cuts the other plutonic rocks. The fabric in the 
Milford Granite (Zmgr), in the core of the Milford 
antiform, is a lineation rather than a foliation (parallel 
planar metamorphic fabric), although the more biotitic 
phase (Zmgd) as mentioned above does have a foliation in 
places, particularly north of the central core. The Milford 
Granite and the unnamed granite (Zgr) to the north and 
east of the Milford are less deformed than are the 
orthogneisses to the west. They are characterized by 
rounded aggregates of bluish quartz and more equant 
feldspars, rather than flat aggregates of quartz and of 
feldspar characterizing the rocks to the west. To the 
northeast, the Hope Valley Alaskite Gneiss exposed in 
many roadcuts along the Massachusetts Turnpike in the 
Framingham area and within the Bloody Bluff fault zone 
is fine grained (which I believe is due to tectonic commi- 
nution of grain size) and has a marked foliation. East of 
the Milford Granite, in the Ashland-Holliston area, the 
foliation is less distinct. Southeast of the Milford Granite, 



the unnamed granite (Zgr) and the Esmond Granite 
(Zegr) are not conspicuously deformed except for a 
north-northeast-trending zone of shear and cataclasis as 
much as 0.5 km wide in Zgr (2, fig. 2) along the northwest 
side of the Bellingham basin in Franklin and Blackstone. 
This cataclasis is little recrystallized and forms a mylo- 
nite gneiss rather than a blastomylonite like that present 
in the Bloody Bluff fault zone to the north. A thin 
east-trending strand of well-foliated Scituate Granite 
Gneiss (Zsgr) flanks the Blackstone Group in the Black- 
stone River valley on the south side of the less deformed 
Milford Granite (3, fig. 2). The structural significance of 
this narrow zone of penetrative deformation is not 
readily apparent. On the State bedrock map I have 
indicated that the gneissic terrane includes the Milford 
and the unnamed granite and that the division between 
the gneissic and nongneissic terranes falls about on a line 
from the southwest end of the Boston basin through the 
Bellingham basin. This line may be offset to the west 
along the Blackstone River valley and may lie at the west 
side of the Esmond Granite in Rhode Island (E, fig. 2). 
The Milford and the unnamed granite are included in the 
gneissic terrane because of their blastocataclastic fabric 
(blue quartz aggregates) and local zones of lineation 
and marked gneissosity. West of the line south- 
west from the Boston basin are variably penetra- 
tively deformed plutonic rocks (orthogneisses) and 
amphibolite-facies quartzite and paragneisses. East of 
the line are fractured, brittlely deformed, and partly 
hydrothermally altered plutonic rocks and greenschist- 
facies metasedimentary and metavolcanic rocks. The line 
coincides with faults that are a continuation of the 
northern boundary fault of the Boston basin and that 
project in the area of the Bellingham basin. The cataclas- 
tic zone in the unnamed granite (Zgr) west of the 
boundary is considered to be the result of deformation 
along this boundary. Sheared granite crops out sporadi- 
cally elsewhere along the bounding fault and along the 
trace of the fault splaying southwestward off the bound- 
ary fault from west of Med way toward Milford. Gromet 
and O'Hara (1984) and O'Hara and Gromet (1984) 
believed that a discrete boundary separates the terrane 
characterized by the less foliated Milford Granite from 
the terrane characterized by the more foliated Hope 
Valley to the west. They placed this boundary west of 
the outcrop area of the Ponaganset Gneiss and thence 
north to intersect the Bloody Bluff fault (figs. 2, 3, 9) and 
thus would exclude the Milford Granite from the gneissic 
suite of rocks. 

In summary, the Milford antiform is the northeast- 
ward-plunging nose of the polydeformed Rhode Island 
anticlinorium in which the foliation arcs around a more or 
less lineated and less deformed core and eastern flank. 
The Proterozoic Z plutonic rocks within the antiform 



H24 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



are markedly gneissose on the west and north and 
decrease in gneissosity eastward. The decrease is not 
systematic, however, for the width of the zone and the 
intensity of deformation vary, and narrow zones of 
gneissic rock, like that described by O'Hara and Gromet 
(1984), flanked by less gneissic rock extend both east- 
ward and northeastward. The zones of gneissosity 
decrease in prominence eastward to about a line extend- 
ing from the Boston basin to the Bellingham basin, along 
which there is increased shearing and cataclasis. 

The relations described above indicate a greater 
degree of pervasive deformation and recrystallization to 
the west than to the east. This distinction indicates not 
only a concentration of deformation but also a higher 
temperature regime to the west. The age of the defor- 
mation that produced the gneissosity is discussed below 
in the section on Proterozoic metamorphism. In the 
Milford antiform, the metasedimentary and metavolcanic 
formations are in the amphibolite fades of metamor- 
phism (table 1). There is some evidence to indicate that 
they were metamorphosed before the intrusion of the 
plutonic rocks, but a subsequent differentially pervasive 
dynamic and thermal metamorphism involving the plu- 
tonic rocks has rendered such relationships obscure, 
particularly in the more highly deformed areas. 

SALEM BLOCK 

The basement north of the Milford antiform between 
Framingham and Newbury is here called the Salem block 
because of the extensive distribution of diorite and 
gabbro (Zdigb) (Salem Gabbro-Diorite, in part; Wones 
and Goldsmith, this vol., chap. I) within it (figs. 1, 2). It 
is bounded on the west by the Bloody Bluff fault, on the 
southeast by the northern border fault of the Boston 
basin, and on the south by faults and the northwest- 
trending rocks at the north end of the Milford antiform 
(4, fig. 2; fig. 10). The block consists primarily of mafic 
plutonic rocks (Zdi, Zgb, Zrdi, Zdigb), mafic to felsic 
metavolcanic rocks (Zv), and minor metasedimentary 
rocks (Zw). The diorite at Rowley (Zrdi) has a K-Ar age 
of 656±16 Ma (Zartman and Marvin, this vol., chap. J, 
table 1). These rocks have been intruded by the some- 
what younger Proterozoic Z Dedham Granite and Tops- 
field Granodiorite and by early and middle Paleozoic 
alkalic granite and gabbro. The Lynn Volcanic Complex 
lies in the Melrose subblock of the Salem block. The 
Newbury basins containing Upper Silurian-Lower Devo- 
nian strata (Newbury Volcanic Complex), and the Mid- 
dleton basin containing unmetamorphosed Triassic 
strata ("Re), flank the Salem block on the northwest side. 
The Boston basin lies to the southeast. 

The metavolcanic rocks in the Salem block contain 
assemblages typical of the amphibolite facies (table 1). 
Northwest of Salem the rocks are primarily epidote- and 



andesine-bearing amphibolite and feldspathic augen 
gneiss (Toulmin, 1964; Bell and Alvord, 1976). Nelson 
(1974, p. 10) noted the presence of sillimanite in his 
Claypit Hill Formation (included in Zv on the State 
bedrock map) in the Framingham area, and his units are 
within the garnet-amphibolite facies. Chlorite-bearing 
assemblages are probably partly retrogressive. The 
Proterozoic Z Salem Gabbro-Diorite intruded these 
mafic metavolcanic rocks (Toulmin, 1964, p. 69), and all 
these rocks are sheared in the Burlington mylonite zone 
(fig. 2; Castle and others, 1976, p. 33). 

The Proterozoic Z plutonic rocks of the Salem block are 
not pervasively gneissic like those on the west side of the 
Milford antiform. They have, however, been subjected to 
a variably distributed cataclasis that moderately frac- 
tured quartz and feldspar grains and was accompanied by 
crystallization of epidote and chlorite-group minerals 
along fractures and shear planes. These assemblages 
indicate metamorphism no higher than greenschist 
facies. The deformation and retrogression are more 
pronounced toward the Bloody Bluff fault and within the 
Burlington mylonite zone and less pronounced to the 
east, as, for example, near Salem. The Topsfield Grano- 
diorite is much more extensively altered than the Ded- 
ham Granite to the south. 

In the Salem block then, we see evidence of an 
amphibolite-facies metamorphism in the stratified meta- 
volcanic rocks before or during intrusion of the mafic 
plutonic rocks. A retrogressive greenschist-facies meta- 
morphism was superposed on the mafic metavolcanic- 
plutonic complex as well as on the younger Dedham 
Granite and Topsfield Granodiorite and may have accom- 
panied the shearing along the major faults in the area. 

Melrose Subblock 

Bell and Alvord (1976) and Kaye (1980) have outlined 
an area herein called the Melrose subblock bounded on 
the south by the northern boundary fault of the Boston 
basin, on the west by the Mystic fault (M, fig. 10), and on 
the north and east by the curvilinear Walden Pond fault 
(figs. 2, 10). This subblock, much of it in the town of 
Melrose, contains the outcrop area of the Lynn Volcanic 
Complex and the type area of the Middlesex Fells 
Volcanic Complex. I have applied the name Melrose to 
this subblock rather than using one of the rock unit 
names. Bell and Alvord (1976) showed the Walden Pond 
fault as intersecting the northward-trending Mystic fault 
at a sharp angle. Kaye (1980) modified this arrangement 
by dropping the Mystic fault north of the Melrose block 
and considered the remaining southern part of the Mystic 
fault to be a continuation of the curvilinear Walden Pond 
fault. This is approximately the configuration shown on 
the State bedrock map. 



STRUCTURAL AND METAMORPHIC HISTORY OF EASTERN MASSACHUSETTS 



H25 




EXPLANATION 

Contact of Peabody pluton 

Fault — Dotted where concealed 

Reverse fault 

Mystic fault of Bell and Alvord (1976) 

- — Possible fault interpretations dis- 
cussed in the text; dashed where 
inferred, dotted where concealed 
MD Medford Diabase 
S Serpenrinite 



5 10 KILOMETERS 

Figure 10. —Structural features in and adjacent to the Salem block of the Milford-Dedham zone, eastern Massachusetts. Basins are indicated 

by pattern. 



The subblock contains an assemblage of rocks some- 
what different from that in the Salem block to the north. 
The most abundant unit is the Lynn Volcanic Complex, 
which does not crop out outside the Melrose subblock, at 
least to the north and west, except for outcrops on 
Marblehead Neck. The Lynn on Marblehead Neck and 
adjacent islands may actually be part of the Melrose 



subblock offset to the north along a north-northeast 
trending fault through Marblehead harbor (C, fig. 10). 
Other units in the subblock are older and include the 
Proterozoic Z Dedham Granite, the Westboro Forma- 
tion, and mafic metavolcanic rocks (Zv) of the Middlesex 
Fells Volcanic Complex. The older metavolcanic rocks 
are metamorphosed to amphibolite facies and contain a 



H26 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



foliation parallel to layering. A common mineral assem- 
blage, similar to that in the rest of the Salem block, 
includes hornblende, oligoclase and andesine, and epi- 
dote, the latter forming pods and lenses in some places 
(Bell and Alvord, 1976). These rocks are locally chlori- 
tized. Because the grade of metamorphism in the over- 
lying Lynn Volcanic Complex is at most greenschist 
facies, a Proterozoic episode of amphibolite-facies meta- 
morphism must have occurred before deposition of the 
Lynn Volcanic Complex. The rock assemblage in the 
Melrose subblock suggests that this subblock represents 
a shallower stratigraphic level than the rest of the Salem 
block; it is equivalent to the assemblage of rocks imme- 
diately beneath the Boston Bay Group in the Boston 
basin in the south. Significantly, C.A. Kaye (oral com- 
mun., 1979) identified a small area of Roxbury Conglom- 
erate within the subblock, supporting the suggestion 
that the subblock contains shallow-level rocks. A sliver of 
the Melrose subblock may lie south of the northern 
border fault of the Boston basin in the Nahant area (see 
discussion of Boston basin below). 

Fault Pattern in the Salem Block 

The Salem block has been appreciably broken by 
faulting. Individual faults are difficult to locate because 
of intermittent exposures and similarity of rock types. 
Several different interpretations of the fault pattern 
exist (Bell and Alvord, 1976; Castle and others, 1976; 
Nelson, 1976; Barosh and others, 1977). Faults shown on 
the State bedrock map were taken from original quad- 
rangle maps modified by recent observations by Kaye 
(1980) and A.F. Shride (written commun., 1979). Only 
the more significant faults have been shown. Shride (as 
reported in U.S. Geological Survey, 1980, p. 62) 
described the faults and fault zones in northeastern 
Massachusetts as varying greatly in their characteris- 
tics: 

Some of the principal faults (that is, strike-slip faults of regional 
extent with displacement measurable in kilometers to tens of kilome- 
ters) [Bloody Bluff fault system and Burlington mylonite zone] are 
marked by zones that are narrow along lengths of many kilometers and 
are composed of cohesive mylonitic materials; these faults give way 
abruptly to bordering rocks that exhibit little cataclasis. Other regional 
faults are characterized by central zones whose widths are variable but 
approach 1 km, bordered by zones of pervasively shattered rock 
measuring hundreds of meters wide. Some central zones are domi- 
nantly mylonite or ultramylonite, whereas, in others, breccias make up 
considerable parts. Some of the weak and, therefore, rarely exposed 
fault zones are made up of thinly sheeted highly friable rock parted 
along innumerable slickensided shear surfaces; incoherent gouge, 
apparently, is a rare constituent. The cataclastic zones of secondary 
faults, those with displacements of no more than 1 to 2 km, are mostly 
only a few meters wide and sharply defined; a few exceptional zones are 
more than 250 m in width. Breccias seemingly are more characteristic 
of secondary faults than faults of regional extent. 



The Walden Pond fault (fig. 10) is a steeply dipping 
fault, most likely a north-dipping reverse fault like the 
northern border fault to the south. Its curvilinear trace 
may actually mark the locus of a series of straight short 
faults that intersect at obtuse angles. The Melrose 
subblock has dropped down on these faults relative to the 
rest of the Salem block and moved up relative to the 
Boston basin. Although mapped, the Walden Pond fault 
itself has not been described by anyone. 



Mystic Fault 

Bell and Alvord (1976, fig. 1) showed the Mystic fault 
(M, fig. 10) extending northeastward from the Boston 
basin at Arlington to the structurally complex area near 
Lynnfield, where the Burlington mylonite zone and the 
Bloody Bluff fault meet and lose their identities. The 
southern segment of the Mystic fault joins the Walden 
Pond fault on the State bedrock map. Bell and Alvord's 
Mystic fault, if projected to the northeast, would come 
close to or truncate the northwest margin of the main 
pluton of Peabody granite at Peabody. From there the 
fault could reasonably be drawn on the basis of the 
aeromagnetic pattern as the fault shown by Castle and 
others (1976) (E, fig. 10) striking north to the Bloody 
Bluff fault zone near Lynnfield. However, neither Castle 
and others (1976) nor A.F. Shride (oral commun., 1979) 
found evidence in surface exposures for a connection 
between the Mystic fault at the edge of the Melrose 
subblock and a fault north of the main pluton of the 
Peabody Granite. I have chosen arbitrarily to shift 
Castle and others' (1976) fault to the west where it passes 
through the serpentinite mass at Lynnfield (S, fig. 10) 
and thence continues southward west of the Peabody 
pluton to intersect the Walden Pond fault. The north- 
south fault through the serpentinite is connected on the 
State bedrock map with a fault that extends across the 
Melrose subblock from the Boston basin. Other north- 
south faults of small displacement (A,B, fig. 10) that 
extend from the Melrose subblock into the Boston basin 
have been mapped by Kaye (1980, 1983). The significance 
of an east-west fault (F, fig. 10) shown by Castle and 
others (1976) cutting across the serpentinite mass from 
the Burlington mylonite zone to the pluton at Peabody is 
not apparent. 

The north-south faults shown on the State bedrock 
map that extend from the Boston basin into the Melrose 
subblock (Billings, 1976a,b; Kaye, 1980, 1983) are appar- 
ently the youngest faults in northeastern Massachusetts; 
they probably are the same age as the fault bounding the 
Triassic Middleton basin and perhaps the faults bounding 
the Silurian-Devonian Newbury basins. 



STRUCTURAL AND METAMORPHIC HISTORY OF EASTERN MASSACHUSETTS 



H27 



DEDHAM AND FOXBOROUGH BLOCKS 

Southeast of the Salem block and the Milford antiform 
in the Milford-Dedham zone is a terrane containing the 
Proterozoic Z southeastern Massachusetts batholith. 
This terrane is divided into the Dedham block, the 
Foxborough block, and the Fall River block (figs. 1, 2). 
The first two blocks are discussed here, and the third in 
the next section. Metasedimentary and metavolcanic 
rocks of Proterozoic age are not abundant in the Dedham 
and Foxborough blocks and primarily form inclusions and 
screens within the Proterozoic Z plutonic rocks. The 
Dedham block can be construed as continuing southwest- 
ward to northeastern Rhode Island, where it contains 
the Blackstone Group, Esmond Granite, and associated 
rocks east of the Bellingham basin. To the north it is 
interpreted to abut the Salem block beneath the Boston 
basin (fig. 2). 

The metasedimentary and metavolcanic rocks (Zb, 
Zw) in the Dedham and Foxborough blocks are primarily 
amphibolite, phyllite, and quartzite, in the upper 
greenschist- and epiclote-amphibolite facies of metamor- 
phism (table 1). In the Norwood area, metabasalt con- 
tains green amphibole, and metarhyolite contains saus- 
suritized plagioclase and green biotite variably altered to 
chlorite (Chute, 1966). Scattered exposures of quartz- 
sericite phyllite locally containing pyrite cubes are 
present in the Dedham block to the southwest. In 
northern Rhode Island there is evidence for a period of 
regional dynamothermal metamorphism in the Black- 
stone Group before intrusion of the Esmond and Dedham 
Granites (Coyle and others, 1984). North of Woonsocket, 
the Ordovician-Silurian granite (SOqgr) clearly intrudes 
folded and metamorphosed (epidote-amphibolite facies) 
Blackstone Group. Lack of evidence for early Paleozoic 
metamorphism in the zone indicates that the metamor- 
phism is probably Proterozoic. 

Textures in the plutonic rocks of the Dedham and 
Foxborough blocks are similar to those in the less 
deformed parts of the Salem block, but the rocks on the 
whole are less fractured and altered. Most of the cata- 
clasis and alteration is concentrated along north- to 
north-northwest-trending faults that transect the bound- 
aries of the blocks. The rocks of the Foxborough block 
are appreciably shattered and altered at its southwest- 
ern end where the Norfolk and Narragansett basins 
merge. The diorite and gabbro (Zdigb) retains its igneous 
texture, but some feldspars are saussuritized and mafic 
minerals altered to chlorite and epidote. Where diorite or 
gabbro has been intruded by Dedham Granite, horn- 
blende has been altered to biotite, and in places a hybrid 
rock has been produced. The alteration to chlorite and 
sericite in the Dedham is primarily along small shear 
fractures most numerous near faults. Small veinlets 
containing epidote and quartz are common in these 



altered areas. The batholithic rocks in the eastern part of 
the Foxborough block are little altered. 

Numerous north-trending faults chop the rocks into 
slices, as shown by the crenulate contact of the south side 
of the Foxborough block. The principal fault of this sort 
is the Stony Brook fault, which projects into the Boston 
basin (fig. 2) and is further described in the section on 
that basin. A similar north-south fault set is present in 
the basement beneath the Pennsylvanian strata in the 
eastern part of the Narragansett basin (Williams and 
Willey, 1973) and in the exposed basement in the north- 
ern part of the Fall River block (Koteff, 1964). Vein 
quartz is associated with these north-trending faults 
(Lyons, 1977). The border faults along the edges of many 
of the blocks are discussed in sections describing the 
structure of these basins. 

FALL RIVER BLOCK 

Structural features and metamorphism in the Fall 
River block have not hitherto been described and have 
been mapped only in two places: the Assawompset Pond 
area (Koteff, 1964) and the Tiverton area (Pollock, 1964). 
Accordingly, features in this block are described more 
completely in the following pages than has been done for 
the other areas. 

The structural and metamorphic features in the north- 
ern part of the Fall River block (fig. 11) are in general 
similar to those in the Foxborough block north of the 
Narragansett basin. The batholithic rocks closely resem- 
ble those north of the basin, not only in composition and 
texture but also in degree of deformation and alteration. 
Like them, they are not foliated and lack gneissosity. 
The Proterozoic metavolcanic-metasedimentary septa 
are in the greenschist facies of metamorphism. In the 
southern part of the Fall River block, however, the 
plutonic rocks tend to be gneissose, and the metavolcanic 
rocks are in the amphibolite facies (table 1). 

Metamorphism of the Metavolcanic and Plutonic Rocks 

Metamorphism of the Proterozoic metaigneous rocks 
(Zgs, Zgn) in the Fall River block appears to be rela- 
tively simple. The gneiss and schist (Zgs) have a princi- 
pal foliation parallel to the layering, which is locally 
crenulated by a later slip cleavage. Foliation in both 
gneiss and schist is marked by a preferred mineral 
orientation, primarily of the micas. Lineation is promi- 
nent only in the noses of the major folds in biotite gneiss. 
It appears to be an intersection lineation marked by 
streaks of biotite and rodding of quartz and feldspar. 
Splintery, lineated rock in other areas may mark noses of 
isoclinal or tight folds, but no minor folds of this sort 
were seen in outcrop. The slip cleavage strikes to the 
northwest and dips steeply. 



H28 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 




Figure HA.— Structural features of the Fall River-New Bedford area, southeastern Massachusetts. Pr, Rhode Island 
Formation; Pp, Pondville Conglomerate; Zfgr, granite of the Fall River pluton; Zpgr, porphyritic granite; Zagr, 
alaskite; Zdi, diorite; Zgg, granite, gneiss, and schist, undivided; Zv, metamorphosed mafic to felsic flow, 
volcanielastic, and hypabyssal intrusive rocks; Zgs, gneiss and schist near New Bedford; Zgn, biotite gneiss near 
New Bedford. 



STRUCTURAL AND METAMORPHIC HISTORY OF EASTERN MASSACHUSETTS 



H29 



EXPLANATION 

Contact — Dashed where approximately located; 

dotted where concealed 
Fault — Dashed where inferred; dotted where 

concealed 

Antiform — Showing plunge 

Synform — Showing plunge 
Strike and dip of bedding 
Strike and dip of foliation 

Inclined 

Vertical 

Strike and dip of foliation and compositional 
layering 

Strike and dip of flow foliation in plutonic 
rock 
Inclined 
Vertical 

Strike and dip (vertical) of axial surface 
of minor fold of foliation 

Strike and dip of cleavage 

Strike and dip of shear fracture in 
plutonic rock 
Inclined 
Vertical 

Strike and dip of mylonite 
Inclined 
Vertical 

Lineation — Showing bearing and plunge 
Mostly elongation or streaks of min- 
erals such as biotite, quartz, feld- 
spar, and hornblende 

Minor fold axis — Showing bearing and 
plunge 

Figure 11A. — Continued. 



The textures in the gneiss and schist indicate that a 
static thermal phase succeeded an earlier dynamother- 
mal stage. The thermal phase could be attributed to the 
emplacement of the plutonic rocks. However, because 
the plutonic rocks described below also seem to be 
involved in the thermal progression, the thermal phase 
may have been superposed much later, in the same way 
as the late Paleozoic (Alleghanian) thermal event was 
superposed on the Proterozoic rocks of southeastern 
Connecticut and southern Rhode Island (Lundgren, 
1966; Zartman and others, 1983). Mineral assemblages in 
the metaigneous rocks of southeastern Massachusetts 
indicate greenschist-facies metamorphism to the north 
and amphibolite-facies metamorphism to the south. To 
the north, around Long Pond and Assawompset Pond 
(fig. 11A), inclusions and septa in the granites are in the 
greenschist facies. Koteff (1964) mapped a sericite- 
quartz-feldspar schist on the west side of Long Pond and 
noted a fine-grained inclusion in granite to the west that 
contains quartz, sericite, and subordinate epidote, 
opaque minerals, and apatite. A metadacite porphyry 
near Rock (fig. 11 A) contains relic igneous texture but 
greenschist-facies mineral assemblages: saussuritized 
plagioclase, interstitial epidote, and biotite altered to 
chlorite. Primary indicators of the amphibolite-facies 



metamorphism to the south are the coexistence of epi- 
dote, calcic oligoclase, and blue-green hornblende. No 
truly pelitic assemblages are known here. The biotite 
gneiss along the shores of Buzzards Bay contains abun- 
dant discontinuous pegmatite patches and veins, locally 
exhibiting a swirled pattern indicating plasticity and 
local melting. 

The plutonic rocks in the northern and western parts 
of the Fall River block are little deformed except by 
faulting. They locally contain an indistinct foliation. 
From about Long Pond southward and southwestward, 
the plutonic rocks show widely spaced, discrete shear 
zones containing feldspar augen and reoriented streaks 
of mafic minerals. These zones become more numerous 
southward, so that the rocks develop a pervasive gneissic 
fabric and become augen and flaser gneiss. This transi- 
tion is best seen in the porphyritic granite (Zpgr) that 
extends from Long Pond to Acushnet near New Bedford. 
Granite of the Fall River pluton (Zfgr) to the east is 
equigranular and poor in mafic minerals, and in it the 
transition is not well demonstrated, although zones of 
shear can be recognized locally. South of Fall River in the 
Tiverton area, the local phase, the Bulgarmarsh Granite, 
is relatively unfoliated, but, to the southeast near West- 
port Point, Zfgr is gneissic. Foliation symbols in the Long 
Pond area shown on figure 11A represent recognizable 
zones of gneissosity in the granite of the Fall River 
pluton and in the porphyritic granite. These zones have a 
general west to west-northwest trend. 

The gneissic terrane is primarily east of a prominent 
topographic lineament (the inferred North Dartmouth 
fault, fig. 11 A) that marks the contact between the 
granite of the Fall River pluton and the belt of gneiss and 
schist extending from the south end of Long Pond 
south-southwest to Westport Factory, North Dart- 
mouth. From there the belt of gneiss and schist extends 
southwestward in a vaguely defined zone toward the 
Rhode Island State line west of Center Village, West- 
port. The east-west change is more abrupt than the 
north-south change and is in part attributed to a differ- 
ence in competency between the relatively mafie- 
mineral-poor granite of the Fall River pluton and the 
porphyritic granite to the west. Mafic-mineral-poor 
rocks such as the alaskitic granite (Zagr) are gneissic, 
however, in the Westport-New Bedford area. The 
increase in gneissosity southward and southeastward is 
somewhat similar, but in reverse direction, to the 
increase in gneissosity on the west side of the Rhode 
Island anticlinorium in western Rhode Island and east- 
ern Connecticut. 

Structural Features in the New Bedford Area 

All the units in the New Bedford area are complexly 
folded, as shown by the trend of gneissosity and schis- 



H30 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



South Dartmouth 
antiform 




c 



Zfgr 
-Zfgr 




10 KILOMETERS 



EXPLANATION 

Contact — Dashed where inferred; dotted where projected 

Fault — Dotted where projected 

Figure US. — Interpretive cross sections of the Fall River-New Bedford area, southeastern Massachusetts. 



tosity as well as by contacts between units. Lineation is 
prominent in fold hinges, particularly in the alaskitic 
granite in the South Dartmouth antiform. Foliation in 
the plutonic rocks is formed by parallel orientation of 
biotite and flattened feldspar megacrysts. Lineation is 
formed primarily by elongated biotite clots and by quartz 
and feldspar rods. 

The orientation of these major folds is not consistent. 
Axial surfaces strike east-northeast, and vergence seems 
to be to the south (fig. 115). The South Dartmouth 
antiform (fig. 11A,B) extending from Sconticut Neck 
through South Dartmouth to South Westport plunges to 
the west. However, the diorite at Acushnet (Zdi) to the 
north forms a phacolithic body in the variably northeast- 
plunging Acushnet synform (fig. 11A,B); to the south- 
west, in the vicinity of Westport Point, lineation in 
gneissic granite plunges to the southeast. A zone of 
interfmgering units and probably of tight folding and 
rotation, possibly associated with east-trending strike- 
slip faults, lies between the Acushnet synform and the 



South Dartmouth antiform. A shear zone may separate 
the South Dartmouth antiform from the area of southeast 
plunges near Westport Point, but exposures are poor in 
this area. Two mylonite seams are exposed on the shores 
of Buzzards Bay in the Slocum River area. The more 
prominent zone, striking N. 65° W. and dipping 65° S., is 
well exposed on Potomska Point, Dartmouth. Asymme- 
try of feldspar augen tails and folding of sheared pegma- 
tite indicate right-lateral displacement along the zone. A 
thin seam is exposed in ledges on a road to the east in 
rock containing a lineation S. 65° E. plunging 25°. The 
relatively steep plunges of lineation in the New Bedford 
region suggest a significant component of lateral move- 
ment between structural domains. 

The systematic increase in gneissosity from the Long 
Pond area to the New Bedford area may be more 
apparent than real because of lack of continuity of 
exposures. The west- to west-northwest-trending shears 
in the porphyritic granite in the Long Pond area have the 
same trend as the cleavage in the metamorphic rocks. 



STRUCTURAL AND METAMORPHIC HISTORY OF EASTERN MASSACHUSETTS 



H31 




Figure 11C. — Joints in the Fall River-New Bedford area, southeastern Massachusetts. 



H32 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



Neither is parallel to the axial surfaces of the major folds 
in the New Bedford area. These shears may be younger 
than the gneissosity, and movement on them may have 
caused the discrepancy in direction of plunge in the 
different fold domains. The gneissic terrane may have a 
relatively sharp northeast-southwest boundary, but in 
reconnaissance in the area no sharp boundary was 
observed. 

Assawompset Pond Graben and Related Structures 

A downdropped block containing Rhode Island Forma- 
tion was mapped by Koteff (1964) south of Middleboro in 
the Assawompset Pond area (figs. 2, 1LA). Slickensides 
in the northeast corner of the quadrangle plunge steeply 
down dip, and the contact of the porphyritic granite 
(Zpgr) with granite of the Fall River pluton (Zfgr) is not 
greatly offset. Koteff cited seismic studies that indicate 
that two buried preglacial channels across Mason Road in 
Freetown and North Avenue in Rochester coincide with 
the position of the faults. Koteff believed that the faults 
do not extend far south of the quadrangle; however, 
some evidence indicates that the western bounding fault 
continues south to the New Bedford area. Here, east- 
trending units appear to be slightly offset across the 
Acushnet River, and, at the Route 140 interchange with 
Phillips Street north of New Bedford, an outcrop of 
crushed and broken granite is laced by quartz veins 
trending both N. 15° E. and N. 50° W. and dipping 85° 
and 35° N., respectively. The veins are cut by steeply 
dipping fractures N. 40° E. and N. 10° W. This outcrop is 
about on the probable trend of the western bounding 
fault. The continuation of the eastern bounding fault is 
less clear. It either dies out or, as shown on figures 2 and 
11, turns southeast and merges with an inferred fault 
along a pronounced topographic break and an abrupt 
cessation of outcrop east of a line from Rock through 
Snipatuit Pond to Mattapoisett Harbour. I have inter- 
preted the eastern bounding fault to project northward 
east of the inlier at Middleboro as Lyons (1977) showed 
it, rather than to the west as Williams and Willey (1973) 
suggested. This interpretation eliminates what would be 
a long, narrow horst east of the graben that at its north 
end would encompass the inlier. 

The North Dartmouth fault trending south-southwest 
from Long Pond to North Dartmouth is largely inferred 
from topography, although the linear topographic low 
may be entirely or in part due to differential erosion 
along a belt of gneiss and schist (Zgs). This valley serves 
to separate altered but not pervasively gneissic plutonic 
rocks from mostly gneissic plutonic rocks. This fault is 
inferred to continue south down the east branch of the 
Westport River, although a more southwesterly trend 
would align it with a northeast-trending magnetic linea- 



ment east of Kirby Corner and Central Village, West- 
port (U.S. Geological Survey, 1971a,b). A northwest- 
trending fault through Hixville that splays off the North 
Dartmouth fault is inferred because of apparent offset of 
the gneiss and schist and granite of the Fall River pluton. 
Granite exposed in a gravel pit south of Hixville is partly 
crushed and sheared and has an anastomosing shear 
fabric varying from N. 60° W. to N. 70° E. Other small 
northwest-trending faults are shown at Fall River, 
where some rock is sheared in a northwest direction; 
along the west branch of the Westport River, for which 
there is little evidence except topography and an aero- 
magnetic lineament; and at the north end of the Fall 
River pluton at Lakeville. The fault at the west end of 
Assawompset Pond is based primarily on the basement 
configuration shown by Williams and Willey (1973) and 
by the observation that much of the granite exposed on 
Route 140 to the south is veined with quartz. 

Joints 

Joint sets in the plutonic rocks (fig. 11C) reflect shear 
and fault directions. One set of joints strikes west- 
northwest to east-northeast generally parallel to shear 
directions in the rocks. This set does not seem to be 
reflected in the Pennsylvanian rocks to the west nor to 
noticeably affect the boundary between the plutonic 
rocks and the Pennsylvanian strata. Another set strikes 
northeast, about parallel to the east boundary of the 
Narragansett basin. Still another strikes north in the 
same direction as faults that cut the Narragansett basin. 
The north- and northeast-striking joints in many places 
contain vein quartz. Quartz-filled fractures are abundant 
in exposures along State Route 140 southwest of 
Lakeville. Northeast-trending joints along the west side 
of the Fall River pluton near Assonet are parallel to 
crushed and altered zones. The generally east-trending 
sets are considered to be older than the northeast- and 
north-trending sets and to have formed in a more com- 
pressive environment. They are possibly related to the 
stresses producing the east-west or west-northwest 
shears in the basement rock. 

Summary and Discussion 

The metasedimentary and metavolcanic rocks in the 
Fall River block range in metamorphic grade from 
greenschist facies in the north to amphibolite facies in the 
south. As well as can be determined, the metamorphism 
preceded or was contemporaneous with the intrusion of 
the batholithic rocks. The foliation is folded and is cut 
locally by a northwest-striking slip cleavage. The plu- 
tonic rocks become gneissic to the south and are folded 
along with the metasedimentary and metavolcanic rocks 
into a complex pattern. This deformation apparently took 



STRUCTURAL AND METAMORPHIC HISTORY OF EASTERN MASSACHUSETTS 



H33 



place at relatively high temperature, because there is 
little apparent retrogressive metamorphism in the plu- 
tonic rocks of the New Bedford area, in contrast to the 
situation to the north. Therefore I infer a late source of 
heat to the south and stresses producing structures 
oriented in an east-northeast to east direction superim- 
posed on structures trending northerly. The relations are 
similar to those west of the Narragansett basin in 
southern Rhode Island and southeastern Connecticut. 

The age of the metamorphism of the metavolcanic 
rocks is not clear. To the west in the Newport, R.I., area 
(fig. 2), metavolcanic rocks probably of the same age as 
those in the Fall River block were metamorphosed to low 
grade before intrusion of the Newport Granite (Kay and 
Chappie, 1976; Rast and Skehan, 1981). The metavol- 
canic rocks in the Fall River block were presumably 
metamorphosed at about the same time, before intrusion 
of the granite of the Fall River pluton; however, later 
deformation has obscured the relations. 



PROTEROZOIC METAMORPHISM 

The metamorphism and structure in basement rocks of 
the Milford-Dedham zone are summarized in table 1. 
Evidence exists in several places that the basement 
rocks were involved in a Proterozoic Z episode of meta- 
morphism. The Cambrian strata have undergone no 
greater metamorphism than nearby Carboniferous 
strata, or, in the Boston area, than Proterozoic Z basin 
fill. I have cited evidence in my discussion of the Milford 
antiform and Salem blocks that there was at least a 
low-grade greenschist-facies metamorphism to the east 
and a middle-grade amphibolite-facies metamorphism to 
the west in the Proterozoic metavolcanic and metasedi- 
mentary rocks before intrusion of the 630-Ma Dedham 
and related granites. However, we do not know to what 
extent the amphibolite-facies metamorphism in the west- 
ern and the extreme southeastern parts of the zone was 
produced during the Proterozoic metamorphic event and 
how much was superimposed during a late Paleozoic 
event known to be more intense to the south and less 
intense to the north. The Proterozoic rocks described by 
Nelson (1974), Volckmann (1977), Bell and Alvord (1976), 
and Goldsmith (this vol., chap. E, table 3) in the less 
intensely metamorphosed area of the Salem and Dedham 
blocks are all in the amphibolite facies, although in many 
places they contain retrogressive assemblages that 
include epidote, chlorite, and sericite (Nelson, 1974; Bell 
and Alvord, 1976). As the Dedham and related granites 
are also involved in this retrogressive metamorphism, it 
must be post-Dedham. The overprinted amphibolite- 
facies metamorphism in these blocks is pre-Dedham. In 
the Milford antiform, where the batholithic rocks tend to 
be gneissic, the metasedimentary and metavolcanic 



rocks are also in the amphibolite facies. There the 
post-Milford Granite deformation is more intense and is 
amphibolite rather than greenschist facies, so no retro- 
gressive assemblages are found in this area. Devonian 
alkalic granite in Rhode Island is partly gneissic (O'Hara 
and Gromet, 1984), as are the metamorphic rocks in the 
New Bedford area. In parts of the Dedham and Foxbor- 
ough blocks, and in the northern part of the Fall River 
block, the metamorphosed Proterozoic rocks are in the 
greenschist facies, although some uncertainty exists as 
to the proper age assignment for the phyllites (Zb, Zbs) 
and volcanic rocks (Pwv, Zv) in the Wrentham, Dedham, 
Medfield, and Plympton areas. The interpreted pattern 
of Proterozoic metamorphism is shown on the State 
bedrock map. The western part of the Salem block could 
have been shown in the amphibolite facies, but the 
superimposed Paleozoic greenschist-facies metamor- 
phism is pronounced in this area and masks the older 
metamorphic mineral assemblages. 



STRUCTURE AND METAMORPHISM OF THE BASINS IN 
THE MILFORD-DEDHAM ZONE 

Analysis of the structures affecting basins containing 
cover rocks that overlie the Proterozoic basement helps 
us interpret the tectonic events that affected both base- 
ment and cover in the Milford-Dedham zone. The major 
basins (fig. 8) are the Boston basin, occupied by Proter- 
ozoic Z sedimentary and volcanic rocks, the Newbury 
basins, occupied by Silurian and Devonian sedimentary 
and volcanic rocks, the Bellingham basin, occupied by 
Pennsylvanian (or Proterozoic Z) sedimentary and volca- 
nic rocks, the Norfolk basin, occupied by Pennsylvanian 
sedimentary rocks, and the Narragansett basin, occu- 
pied by Cambrian and Pennsylvanian sedimentary rocks. 
A small basin containing Triassic sedimentary rocks, the 
Middleton basin, has been identified in the Lynnfield- 
Middleton area (Kaye, 1983). A larger basin containing 
Triassic and Jurassic sedimentary and volcanic rocks, the 
Nantucket basin, is located in the subsurface beneath 
Nantucket and Nantucket Sound. These basins, and the 
structures within and bounding them, will be described 
in order of decreasing age of contained rocks. Discussion 
of the ages of the fold and fault systems in the basins is 
given in the section on blocks and basins of the Milford- 
Dedham zone. 

BOSTON BASIN 

The structure of the Boston basin has been described 
and summarized by M.P. Billings in a series of papers 
(Billings, 1929; 1976a,b; 1979) based on engineering 
studies by him and others in tunnels in the Boston area, 
on surface mapping by him and his students, and on 



H34 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 




5 10 KILOMETERS 

Figure 12.— Structural features of the Boston basin, eastern Massachusetts. A, B, C, Wl, and W2 are alternative interpretations of faults and 

fault blocks referred to in text. M, Medford Diabase. 



earlier mapping by W.O. Crosby (cited by Billings) and 
LaForge (1932). Billings' studies have been recently 
supplemented by Bell (written commun., 1976), who 
mapped primarily on the periphery of the basin, and by 
Kaye (1980; written commun., 1978, 1979). 

The Boston basin contains unmetamorphosed Proter- 
ozoic Z sedimentary and volcanic rocks that are in gentle 
to tight open folds and are cut by mostly reverse faults. 
Both faults and folds trend east-northeast. The basin is 
bounded on the north and northwest by the northern 
border fault (figs. 2, 12) and on the south by the Mount 
Hope and Blue Hills faults. West of Hingham, in the 
Hingham anticline (fig. 12), Roxbury Conglomerate rests 
nonconformably on Dedham Granite. The southwest end 
of the basin is terminated by north- to northwest- 
trending high-angle normal faults that are younger than 
the reverse faults. Folds are open to the north but 



become tighter and higher in amplitude to the south. 
Plunge of the major folds averages 8°-15° east-northeast 
to east, steepening to as much as 18° toward the southern 
part of the basin. However, folds between the Mattapan 
anticline and the Blue Hills fault plunge gently to the 
southwest (Billings, 1976a, p. 42). The Hingham anti- 
cline, which contains Dedham Granite in its core, plunges 
steeply to the west. High-angle faults cut off the limbs of 
some of the folds in the southern half of the basin so that 
they are almost coincident with synformal axes named by 
Billings (1976a) (not shown on fig. 12, but see cross 
section C-C" on the State bedrock map). Drag along 
these faults has produced locally steep dips. Axial sur- 
faces of the folds dip steeply north. Cleavage is devel- 
oped locally in the Mattapan Volcanic Complex and 
Boston Bay Group and also dips steeply north. However, 
the cleavage is not axial planar to the folds in the basin 



STRUCTURAL AND METAMORPHIC HISTORY OF EASTERN MASSACHUSETTS 



H35 



(Billings, 1929, p. 101-102) and does not seem to be 
present in the older rocks (Nelson, 1976, p. 1379). 

The northern border fault is a reverse fault that 
thrusts the Melrose subblock and Salem block southward 
and southeastward over the Boston Bay Group. It is at 
least 38 km long, extending from south of Natick into 
Massachusetts Bay near Nahant. As exposed in the 
Maiden tunnel (Billings and Rahm, 1966), the fault dips 
about 55° north and is knife sharp. Billings (1976a, p. 41) 
suggested that the stratigraphic throw is not great. My 
reconstruction of the geology in cross section C-C on the 
State bedrock map suggests a throw of about 1.3 km. The 
border fault has customarily been drawn at the base of 
the topographic scarp near Medford and Saugus. How- 
ever, south of this scarp, the Nahant peninsula and the 
adjacent shore at Revere Beach contain Cambrian 
strata, rather than strata of the older Boston Bay Group, 
and contain the Ordovician Nahant Gabbro, the only 
Paleozoic intrusive rock in the basin. These observations 
suggest that the Nahant Gabbro and adjoining rocks are 
in a displaced segment (A, fig. 12) of the Melrose 
subblock from north of the northern border fault as 
suggested by Billings (1979, fig. 7; written commun., 
1979). C.A. Kaye (written commun., 1978) suggested 
that the fault shown on the State bedrock map in the 
Boston basin in Watertown (Wl, fig. 12) continues east 
to pass south of Nahant (W2, fig. 12). These interpreta- 
tions are not shown on the State bedrock map because of 
their speculative nature. 

The location of a projection of the border fault into 
Massachusetts Bay is uncertain. The fault appears to be 
offset along the northeast-trending fault from Nahant 
Bay through Marblehead Harbor (C, fig. 12). To the west 
and southwest, the northern border fault loses its iden- 
tity near Watertown and may splay off to join the 
Weston fault. South of this juncture, Nelson (1975a) 
showed the border fault as a normal fault having throw to 
the south (fig. 12). On the other hand, it may be 
represented in the system of faults trending southwest- 
erly and forming the west side of the Boston basin 
through Wellesley and Natick. The possibility that it is 
part of the system of faults extending toward Woon- 
socket that separates the Milford block from the Dedham 
block has been mentioned above. The northern border 
fault is cut by the Mesozoic Medford Diabase (J"6d), 
which places an upper limit on its age. 

A series of east-northeast-trending faults, the Mount 
Hope, Neponset, and Blue Hills faults, within the basin 
parallel the northern border fault (fig. 12). They dip 
steeply, and downthrow is to the north. Billings 
(1976a,b) believed that these faults were originally 
northward-directed thrusts dipping less than 45°, which 
have since been rotated to near vertical. He calculated 
throw on the Mount Hope fault from two sites to be 183 



m and 350 m, the greater throw to the east. Throw on the 
Neponset fault is about 610 m. The Mount Hope fault 
extends southwest to join with northerly trending faults 
enclosing the southwest end of the Boston basin. To the 
east, the Mount Hope fault extends into Massachusetts 
Bay along the north limb of the Mattapan anticline. The 
Neponset fault cuts off the southeast limb of the Hyde 
Park syncline and the north limb of the narrow Milton 
anticline. The Neponset fault becomes indistinct to the 
west where it cuts into the crystalline rocks. To the east, 
the Neponset fault extends into Quincy Bay, where it 
appears to merge with the Mount Hope fault. Billings 
(1982) presented convincing arguments for the existence 
of the Blue Hills thrust fault, despite contrary strong 
arguments by Kaye and Zartman (1980, p. 258). North of 
Hingham the Blue Hills fault juxtaposes members of the 
Dedham Granite and Roxbury Conglomerate on the 
south with the Cambridge Argillite on the north. West of 
Milton the Blue Hills fault swings to the southwest, 
where it is disrupted by north-trending faults and cannot 
be recognized in the crystalline Dedham Granite. The 
fact that Cambrian and Ordovician rocks to the south are 
thrust over the older Proterozoic Z Boston Bay Group to 
the north on the Blue Hills fault was explained by 
Billings (1982) as due to an earlier Ordovician cauldron 
subsidence of the complex in the Blue Hills consisting of 
the Quincy Granite and Blue Hills Granite Porphyry. 
These rocks and their overlying Carboniferous cover on 
the south have since been thrust northward, rotated, and 
tilted to the south (Billings, 1982, p. 919). Billings 
credited W.O. Crosby (1900) with first suggesting the 
southward tilting of the Blue Hills block. The amount of 
cauldron subsidence was postulated by Billings to be on 
the order of 5,000 m. He suggested that some of the 
north-south faults in the Blue Hills may have been 
originally the bounding faults of the Ordovician cauldron 
subsidence. These north-south faults are clearly post- 
Pennsylvanian and are possibly Mesozoic but could have 
followed the loci of the Ordovician faults. He also sug- 
gested that the southwest-trending part of the Blue Hills 
fault west of Milton may coincide with a former ring 
fault. 

The fault shown on the State bedrock map near 
Watertown (Wl, fig. 12) brings up conglomerate and 
tuffaceous beds on its south side. Billings (1929) formerly 
placed a fault here but in a later paper (1976a, p. 43) 
indicated that this conglomerate is a tongue of Roxbury 
projecting into the Cambridge Argillite at a fairly low 
horizon and that a fault is not needed. I have preferred to 
keep the fault, because the exposure is close to the axis 
of the Charles River syncline where lower stratigraphic 
units would not be expected to appear on the surface. 
Kaye's (written commun., 1978) fault W2 (fig. 12), 
deduced from review of the tunnel data of Billings, may 



H36 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



be a continuation of this fault, as mentioned above. A 
parallel fault deduced by Kaye is shown as B on figure 12. 
The orientation of the folds in the Boston Bay Group 
indicates to me that they are related to the east- 
northeast-trending faults in the basin and that the 
stresses were compressional. 

North- and northwest-trending faults shown by Bill- 
ings (1976a,b) and Kaye (1980) truncate the east- 
northeast-trending faults and folds. Some of these 
younger faults offset the Pennsylvanian strata in the 
Norfolk basin and are therefore post-Pennsylvanian in 
age. The largest of these is the Stony Brook fault, which 
Billings (1976a) considered a normal fault along which the 
west side has been downthrown 640 m. It appears to be 
part of an en echelon system extending from the Nar- 
ragansett basin to north of the northern border fault. 

NEWBURY BASINS 

The Newbury Volcanic Complex lies in two fault- 
bounded, wedge-shaped and lenticular basins between 
the Nashoba zone and the Milford-Dedham zone in 
northeastern Massachusetts. A very small area near 
Lynnfield shown as Newbury strata by Castle and others 
(1976) is probably the Triassic and Jurassic Middleton 
basin. The exact nature and attitudes of the faults 
bounding the Newbury basins are uncertain. The east- 
northeast-trending faults that flank the east-trending 
segment of the northern basin (fig. 10) were shown by 
Shride (1976a) to continue southwest to the Andover 
area. The northernmost, called the Parker River fault by 
Shride (1976b, fig. 1), has more recently been shown by 
Shride (written commun., 1979) to merge with the 
Clinton-Newbury fault near Lawrence and is shown thus 
on the State bedrock map. The southern fault cannot be 
carried far with confidence into the Nashoba zone to the 
west. Movement sense appears to be right lateral on both 
faults (Shride, 1976a). Barosh and others (1974, 1977) 
and Bell and Alvord (1976) showed these two faults as 
continuous with the Spencer Brook and Assabet River 
faults, respectively, of the Nashoba zone. However, 
there seems to be little evidence for such a connection in 
the maps of Castle (1964). Castle and others (1976) limit 
the faults to the arcuate borders of the basins and show 
them to be offset by north-south faults. The northern 
basin contains steeply dipping strata overturned to the 
east and southeast and topping into the basement rocks, 
thus requiring a fault on the southeast side. The eastern 
fault forming the boundary of the southern, wedge- 
shaped part of the northern basin truncates units of the 
Newbury Volcanic Complex more clearly than the north- 
trending fault on the west. 

The smaller basin southwest of Topsfield (fig. 10) 
mapped by Toulmin (1964) is probably also fault bounded 



on both sides. The strata strike northeast about parallel 
to the contacts and dip 40°-75° to the northwest. It is not 
known whether or not the strata are overturned. If they 
are right side up, a fault is required on the west side, as 
Toulmin (1964, p. A71) hypothesized. Shride (1976b, p. 
151) argued that the eastern boundaries of both basins 
are faults. Castle and others (1976) and Shride (1976b, 
fig. 1) showed faults on both sides of the basin at 
Topsfield. 

Because of the attitude of the Silurian-Devonian strata 
in the basins, the beds must have been appreciably 
rotated since their deposition, yet the rocks of the basins 
are little metamorphosed. Shride (1976b, p. 151) noted 
the lack of cataclasis in the Newbury as opposed to the 
relatively pervasive cataclasis and alteration of the adja- 
cent Proterozoic Z Topsfield Granodiorite. In the adja- 
cent Nashoba zone to the west, the paragneisses and 
schists are in the upper amphibolite fades of metamor- 
phism. Because the contacts are tectonic, it is difficult to 
say where the rocks of the Newbury Volcanic Complex 
were in relation to the adjacent Nashoba-zone rocks at 
the time of their metamorphism or to the Proterozoic 
rocks to the east at the time of their alteration and 
cataclasis. As the Newbury is subgreenschist facies at 
best and is not sheared, it most likely lies in a graben 
possibly of Mesozoic age. The location of the bounding 
faults of the basins could well be predetermined by the 
presence of the older Bloody Bluff fault system. 

BELLINGHAM BASIN 

The Bellingham basin contains poorly exposed meta- 
sedimentary and metavolcanic rocks, which have been 
considered to be of Pennsylvanian age but in which no 
fossils have been found. It could possibly be all, or in 
part, of Proterozoic Z age, like the rocks in the Boston 
basin (Goldsmith, this vol., chap. E). The basin is 
bounded by faults that are an extension to the southwest 
of the northern and western bounding faults of the 
Boston basin and the Mount Hope fault. The attitude of 
the faults bounding the Bellingham basin is not certain, 
but the linearity of the basin boundaries suggests that it 
is probably steep. However, the granite west of the basin 
is pervasively sheared in a zone about 0.5 km wide, as 
noted in the section on the Milford antiform above. 
Foliation in the granite dips moderately to shallowly to 
the west. A shear zone this wide is not characteristic of 
faults in and flanking the Boston basin. I believe that the 
shallowly dipping shear zone is older than the apparently 
steep faults now forming the west side of the basin. A 
narrow, steeply dipping shear zone in granite is exposed 
on the east side of the basin near Woonsocket, but the 
few exposures of alkalic granite on the flanks of the basin 
to the north are not noticeably sheared. 



STRUCTURAL AND METAMORPHIC HISTORY OF EASTERN MASSACHUSETTS 



H37 



The strata in the Bellingham basin are metamorphosed 
in the greenschist fades. Pebbles and cobbles in meta- 
conglomerate are extremely to moderately flattened in 
the plane of the foliation, which in turn is locally folded 
and crenulated. Dips of bedding and schistosity range 
from moderately flat to steep. According to Rose and 
Murray (1984), the earliest deformation produced a 
northwest to west-northwest schistosity dipping north. 
Axes of later folds and crenulations trend northeast to 
east-northeast. The western boundary of the Bellingham 
basin effectively forms the boundary between the largely 
brittlely deformed terrane to the east and the variably 
gneissic, more ductilely deformed terrane to the west. 
Phyllite assigned to the Blackstone Group east of the 
basin is at lower metamorphic grade than rocks of the 
Blackstone in the Milford antiform to the west. 

NARRAGANSETT BASIN 

The structure of the Narragansett basin (figs. 2, 13) 
has received considerable attention because of interest in 
the coal-bearing Rhode Island Formation. Early obser- 
vations were made principally by Woodworth (in Shaler 
and others, 1899); Skehan and others (1979) and Skehan 
and Murray (1980a,b) summarized the structure more 
recently. In the central and eastern parts of the northern 
Narragansett basin, Massachusetts, the Pennsylvanian 
strata lie in broad, east-trending, open folds and are not 
metamorphosed (Hepburn and Rehmer, 1981). In the 
northwest corner of the basin, the structure is compli- 
cated by reverse faults and thrusts and north- to 
northeast-trending folds. Farther south, in Rhode 
Island, the structure again becomes complex (Mosher, 
1983), and the rocks reach sillimanite grade. The follow- 
ing description deals only with that part of the basin in 
Massachusetts. 

The strata within the northern part of the Narragan- 
sett basin in Massachusetts east of Attleboro and 
between Mansfield and the Taunton River lie in broad, 
open, east-northeast-trending folds (Lyons, 1977) and 
are cut by a single cleavage. The Dighton Conglomerate, 
the highest unit in the Narragansett Bay Group (Skehan 
and others, 1979), occupies the synclines. There is no 
convincing evidence for more than one period of defor- 
mation in this area (Woodworth, in Shaler and others, 
1899, p. 157), in contrast to the more complex history 
found in the southern Narragansett basin (Murray and 
Skehan, 1979; Burks and others, 1981; Mosher, 1983, for 
example). In the Attleboro area, at the northwest corner 
of the basin, the Pennsylvanian strata are folded about 
north-northeast axes and have been cut by north- 
trending, but curvilinear, reverse faults as well as by the 
ubiquitous north-south normal faults (Lyons and Chase, 
1976; Lyons, 1977). The Blake Hill fault block (Wood- 



worth, in Shaler and others, 1899, p. 183) near Plainville 
(A, fig. 13) is a gently to moderately dipping block of rock 
that appears to have been thrust northward over steeply 
dipping strata. To the south, a large north-trending 
synclinal fold passes south and east of the inlier of 
Hoppin Hill (Lyons, 1977). East-directed thrusts lie on 
the east side of the syncline. The Hoppin Hill inlier 
contains Dedham Granite and its fossiliferous Cambrian 
cover. The shallowness of the basin in this area is 
affirmed not only by the exposures at Hoppin Hill but 
also by the presence of a small inlier of granite exposed 
at the southwest end of the Manchester Pond Reservoir 
(J. P. Schafer, oral commun., 1979) southeast of Hoppin 
Hill. The complexity of deformation in the northwest 
corner of the basin is probably due to adjustments of 
basement blocks near their junctions during compressive 
post-Pennsylvanian deformation of the Narragansett and 
Norfolk basins. 

The shape of the Narragansett basin is only partly 
determined by observed faults. Depositional contacts of 
the Pennsylvanian strata on the Proterozoic Z basement 
are present on the north and southeast sides. Predepo- 
sitional or syndepositional faults are inferred to be 
present around much of the basin, however. On the north 
margin of the basin, basal Pennsylvanian strata can be 
seen to rest nonconformably on the Dedham Granite in 
several places. North of Mansfield, weathered granite 
regolith passes upward into bedded arkose. At another 
site near Plainville, moderately north-dipping red arko- 
sic sandstone fills a fracture in the granite. Dips in the 
Pennsylvanian strata near the contact are moderately 
steep to the south, but basinward dips are flatter, 
indicating that the basement block moved during or after 
deposition. Woodworth (in Shaler, 1899, p. 128) noted 
that from Mansfield to Brockton, the dip of the basal 
beds becomes steeper than that of the higher beds as the 
contact is approached. To explain this discrepancy, he 
suggested that an unconformity exists between the basal 
Pennsylvanian red beds and the overlying gray carbona- 
ceous beds typical of the Rhode Island Formation. How- 
ever, he also conceded that the steeper dips along the 
border could be attributed to drag upward along the 
edges of the basin due to downfaulting of rocks in the 
basin. Woodworth described several places where small 
northeast-trending faults can be seen near the edge of 
the basin. The attitudes of the Pennsylvanian strata are 
the opposite of what would be expected in listric faulting. 
An inferred fault concealed beneath the strata at the 
surface is shown along this contact in cross section C-C" 
on the State bedrock map. 

A north-to-south gravity traverse by Peter Sherman 
(Weston Observatory, Boston College, 1976, fig. 6B) 
from Mansfield to Assonet that crosses the northern part 
of the basin indicates what might be the general config- 



H38 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 




5 10 KILOMETERS 

Figure 13.— Structural features of the Norfolk and Narragansett basins (stippled) in eastern Massachusetts. 



STRUCTURAL AND METAMORPHIC HISTORY OF EASTERN MASSACHUSETTS 



H39 



EXPLANATION 

Contact 

High-angle fault — Dashed where 

inferred; dotted where concealed. 

D, downthrown side; U, upthrown 

side (where known) 
Reverse fault — Hachures on upthrown 

side 
Thrust fault — Teeth on upthrown 

side 
Anticline 

Syncline 
Geologic features 

Blake Hill fault block 

Diamond Hill 

Hoppin Formation at West Wrentham 

Sheared gabbro 

Basement inlier at Pondville 

Mesozoic mafic dike at 
North Middleboro 



Figure 13. — Continued. 

uration of the floor of the basin and the possible slope of 
the margins. The profile indicates a little more than 2 km 
of downthrow on the northern and southern margins 
along fault planes that dip moderately into the basin. 
Skehan and Murray (1980b, fig. 4) showed a normal fault 
downthrown to the south within the basin and south of 
the northern contact with the crystalline rocks to account 
for an abrupt thickening of the Pennsylvanian strata. On 
the basis of the abrupt thickening and lack of observed 
faulting at the predepositional to syndepositional sur- 
face, a fault that does not reach the surface is shown 
along the northern margin of the basin on cross section 
C-C" on the State bedrock map. Between Rehoboth and 
Dighton, the floor of the basin is about 1 km shallower 
than it is to the north, but the basin deepens toward the 
south edge. A moderately dipping normal fault is in- 
ferred in the profile on the north side of this block. The 
gravity profile indicates that a normal fault is present on 
the south side of the basin near Assonet (figs. 11A,B; 13). 
Yet at the contact here, as on the north side of the basin, 
the Pennsylvanian strata seem to rest nonconformably 
on the granitic basement because, near Assonet, shallow 
drill holes basinward from the contact indicate that 
reworked regolith lies at the base of the Carboniferous 
beds (J. A. Sinnott, oral commun., 1979). Thus, if a fault 
of the amount of displacement indicated by the gravity 
profile exists, it must be farther out in the basin. Even 
so, the granite of the Fall River pluton just east and 
south of Assonet shows fracturing and alteration. The 
existence of a northeast-trending fault here is supported 
by the abrupt increase in depth to top of basement shown 
(Williams and Willey, 1973) in the hydrologic map of the 
Taunton River drainage area west of the inlier of base- 
ment located north of the town of Middleboro (Wood- 



worth, in Shaler and others, 1899; Hartshorn, 1960; 
Lyons, 1977). On the basis of the above data, an inferred 
fault has been drawn along the southeast side of the 
Narragansett basin from the Fall River area to the 
North Plympton area. It may be farther away from the 
contact of the Pennsylvanian strata with the basement 
than shown on cross section C-C" and may be in part or 
all pre- or syn-Carboniferous, like the fault inferred at 
the north side of the basin. From Fall River south, 
Pennsylvanian pebble conglomerate rests nonconform- 
ably on the Proterozoic Z basement. The fault, if present, 
presumably lies to the west. The existing scarp at the 
boundary of the basin may be due solely to difference in 
rock resistance to weathering and erosion. This inferred 
northeast-trending fault is along the same trend as the 
Beaverhead fault (Murray and Skehan, 1979) of southern 
Rhode Island and may represent a continuation of it or 
one of a parallel zone of faults (Barosh and Hermes, 1981; 
Burks and others, 1981, p. 268). This fault may be offset 
by faults bounding a small horst of basement shown by 
Quinn (1971) on the north end of Aquidneck Island west 
of Tiverton. Skehan and Murray (1980b, fig. 6) showed a 
series of west-directed thrust faults south of this locality, 
which have moved the crystalline rocks westward over 
the basin strata. In this case, the inclination of the 
Assonet fault may actually be to the southeast. 

The shape of the east end of the Narragansett basin is 
largely controlled by north-trending faults in part deter- 
mined by mapping and in part inferred from the hydro- 
logic maps of Williams and Willey (1973) and Williams 
and others (1975). The most notable feature is the 
Assawompset Pond graben mapped by Koteff (1964) and 
described above (figs. 2, 11). Similar faults of lesser 
magnitude offset the contact of the basin to the north- 
east. The north-trending faults that offset the 
Pennsylvanian-basement rock boundary in the Hanover 
area are based on basement configuration, well informa- 
tion, surface topography, and alignment of boulders of 
vein quartz observed by Lyons (1977). A northeast- 
trending fault extending from Silver Lake to Marshfield 
is inferred from topography and outcrops of shattered 
and silicified rock and veined rock near Marshfield 
(Chute, 1965). 

The extension of the Narragansett basin to the area 
north of the Marshfield Hills is based on drilling in 1979 
by the U.S. Geological Survey (E.G.A. Weed, unpub. 
data, 1979). Two shallow drill holes along the North 
River, one north of North Marshfield and the other near 
the coast south of Scituate, passed through red sand- 
stone of either the Wamsutta Formation or the Rhode 
Island Formation. The Marshfield Hills and adjoining 
areas to the south proved to be underlain by granite. The 
boundaries of the basin shown on the State bedrock map 



H40 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



in this area are only approximate, but the east-west 
reach of the North River south of Scituate surely lies 
within the basin. 

The west side of the Narragansett basin lies primarily 
in Rhode Island, where the boundary is considered to be 
a fault (fig. 13). This fault, the Diamond Hill fault, 
projects northward north of the Massachusetts-Rhode 
Island State line and the mass of vein quartz at Diamond 
Hill (B, fig. 13) (Quinn, 1971, p. 45). The area north of 
Diamond Hill coincides with the faulted southern termi- 
nation of the Norfolk basin and its junction with the 
Narragansett basin. The small outcrop of Cambrian 
Hoppin Formation south of West Wrentham (C, fig. 13) 
appears to lie in a fault wedge at this juncture. A splay of 
the western boundary fault of the basin appears to 
project to the north into a fault shown by Volckmann 
(1977) that extends from Franklin through the Dedham 
area toward the Boston basin. A sliver of sheared gabbro 
is exposed along this trace northwest of West Wrentham 
(D, fig. 13). 

NORFOLK BASIN 

The Norfolk basin is a partly fault-bounded, east- 
northeast-trending synclinal basin between the Dedham 
and Foxborough blocks (fig. 13). Basin fill consists of the 
alluvial Pondville Conglomerate and the Wamsutta For- 
mation. These Pennsylvanian strata are folded and 
cleaved but are unmetamorphosed (Hepburn and Reh- 
mer, 1981). Cazier (1984) mapped two sets of minor folds 
in the Pennsylvanian strata, both having horizontal axes 
that strike roughly east-northeast parallel to the trend of 
the basin margins. The first is isoclinal and has a related 
pressure-solution cleavage dipping about 40° N.; the 
second is open and has an axial planar cleavage dipping 
about 50° N. Cazier noted that thrust faults along the 
southeast margin of the basin deform both cleavages. 
The north side of the basin appears to be an unconformity 
that now dips steeply. The south side of the basin is the 
clearly defined Ponkapoag 3 fault. The basin is offset 
along its length by the faults of the late north- to 
north-northwest-trending system that offset the bound- 
ing faults of the Narragansett and Boston basins. 

The Ponkapoag fault separates the Pennsylvanian 
strata from the Proterozoic Z Dedham Granite of the 
southeastern Massachusetts batholith. East of Brain- 
tree, the Pennsylvanian rocks are cut out by the fault so 
that the Ordovician and Silurian Quincy Granite and the 
Cambrian Braintree Formation lie against the Dedham. 
Near Hingham, the fault cuts into the batholithic rocks, 



3 Although the town near this fault is labeled Ponkapog on the State bedrock 
map, the name of the fault was spelled Ponkapoag by Billings (1976a, 1982), and 
this usage is followed here. 



isolating a patch of Dedham Granite north of the fault 
and nonconformably overlying Roxbury Conglomerate in 
the Hingham anticline. The Ponkapoag fault appears to 
dip steeply throughout its extent. Near Weymouth the 
fault dips about 80° N.; farther west it dips about 60° N., 
although Cazier (1984) described thrust faults along the 
southeast margin of the basin that dip 25°. Billings 
(1976a,b) computed the stratigraphic throw near Hing- 
ham to be about 410 m. He believed then that the fault 
was originally a northward-directed thrust, which had 
been rotated back to the south, like some of the faults in 
the Boston basin. Billings (1982, p. 919) more recently 
proposed that the fault is normal and that the down- 
thrown block is to the north. 

The north side of the Norfolk basin in the vicinity of 
the Blue Hills is not a fault but an unconformity (Chute, 
1969). Here the basal Pondville Conglomerate dips about 
60° S., slightly less steeply than the underlying Blue 
Hills Granite Porphyry (SObgr) in the Blue Hills (Bill- 
ings, 1982, p. 917). These relations led Billings to con- 
clude that the block between the Blue Hills fault and the 
Norfolk basin has been tilted about 60°, up to the north 
and down to the south, since deposition of the Pennsyl- 
vanian strata. 

Elsewhere along the north side of the Norfolk basin, 
the relations are less clear. West of the Stony Brook fault 
(fig. 13), Chute (1964, p. 41-42) cited truncation of units 
as evidence for a southeastward-directed thrust. I sug- 
gest that the discontinuous nature of Pondville Conglom- 
erate along the flanks of the basin need not indicate 
faulting but could be due to nondeposition. No basal 
conglomerate is present to the south, on the north side of 
the Narragansett basin, but lenses of conglomerate 
indicating channels are present within the Rhode Island 
Formation above the base of the section (Lyons, 1977). 
However, on the State bedrock map I have shown 
Chute's (1964) fault on the north side of the basin as 
continuing south into the Franklin area, because the 
geometry there seems to require it. 

The southwestern end of the Norfolk basin is appre- 
ciably broken by faults and is flanked in part by felsic 
volcanic rocks of uncertain age. The Sharon Syenite just 
east of the Ponkapoag fault exposed on 1^95 south of 
Franklin is appreciably altered and fractured, but else- 
where faults themselves are not exposed. A horst or 
inlier (E, fig. 13) near Pondville containing Dedham 
Granite and fine-grained granite flanked by steeply 
dipping Pennsylvanian strata splits the southwestern 
end of the basin into two parts. The Wamsutta Forma- 
tion in the southern part is continuous with Wamsutta in 
the Narragansett basin. The Wamsutta in the northern 
part, near Weymouth, is cut off against the Ponkapoag 
fault. 



STRUCTURAL AND METAMORPHIC HISTORY OF EASTERN MASSACHUSETTS 



H41 



MIDDLETON BASIN 

The Middleton basin, containing Upper Triassic and 
Lower Jurassic red beds (Kaye, 1983), lies in the same 
north-northeast-trending structural zone as the New- 
bury basins and is close to the southern Newbury basin 
(figs. 2, 10). The Middleton basin is estimated to be about 
5.7 km long and no greater than 0.5 km wide; however, 
the strata are actually exposed in only one place. A 
normal fault striking northeast, exposed on the southeast 
margin of the basin, separates the basin from sheared 
rock of the Proterozoic basement. Kaye (1983) estimated 
the throw on this fault to be greater than 500 m. The 
western contact of the basin is not exposed. Beds of 
arkosic conglomerate contain cobbles of crystalline rocks 
of the area, and the beds are clearly unconformable on 
the crystalline rocks. The Triassic and Jurassic red beds 
are not sheared but dip to the west, more steeply at the 
bounding fault than away from it. If the dips do not 
reverse, a fault should lie at the west side of the basin 
also, but Kaye indicated that the beds probably do dip to 
the southeast on the west side. This contact, then, which 
is shown on the State bedrock map as a fault, could 
instead be an unconformity. 

The basin lies at a location where the Bloody Bluff fault 
and the Burlington mylonite zone cease to be readily 
identified and where the contact between the Nashoba 
zone and the Milford-Dedham zone turns northerly from 
its general northeast trend. The fault at the southeast 
side of the basin is clearly post-Late Triassic and indi- 
cates that Mesozoic faulting has occurred in the area. The 
faults bounding the Newbury basins in this same zone 
are quite possibly, in part at least, Mesozoic. This 
younger faulting would account for the lack of pervasive 
mylonitic texture in the Newbury rocks such as charac- 
terizes the basement rocks in the Bloody Bluff and 
Burlington mylonite zones, which have had a longer 
history of deformation (see section below on the Bloody 
Bluff fault zone). 

NANTUCKET BASIN 

A northeast-trending basin containing sandstone and 
basalt of Triassic and Jurassic age is buried beneath the 
Coastal Plain sediments on Nantucket Island and under 
Nantucket Sound (Weed, in Goldsmith, this vol., chap. 
E; cross section E-E' of the State bedrock map). This 
basin trends northeast and is inferred to be about 25 km 
wide and 100 km long (Austin and others, 1980). The 
northern boundary, beneath the middle of Nantucket 
Sound, is interpreted as a southerly dipping normal fault, 
on the basis of seismic-reflection profiles (Ballard and 
Uchupi, 1975). The strata within the basin appear to dip 
north. The shape of the floor of the basin is not known, 
nor is the nature of the southern boundary. The deposits 



are at least 1 km thick. A smaller northeast-trending 
basin extending from the lower arm of Cape Cod across 
Cape Cod Bay has been postulated by Ballard and 
Uchupi (1975), on the basis of seismic-reflection profiles 
and magnetic data. 

The Nantucket basin, like the Middleton basin, is part 
of the system of Mesozoic rift basins of the Eastern 
United States and maritime Canada (Klitgord and 
Behrendt, 1979; Grow and others, 1979), a swarm of 
which are present to the north in the Gulf of Maine and 
the Bay of Fundy (Ballard and Uchupi, 1975; Klitgord, 
1984). 



MAFIC DIKES 

The orientations and ages of mafic dikes in eastern 
Massachusetts provide clues to the pattern of regional 
stress through time. The larger dikes such as the Med- 
ford Diabase and the dike in the Bellingham basin are 
shown on the State bedrock map. A few smaller dikes are 
shown elsewhere, as on islands in Boston Harbor and in 
the Marlborough area. One dike noted by Lyons (1977) is 
shown cutting the Rhode Island Formation at North 
Middleboro (F, fig. 13). Most dikes trend north-south and 
are considered to be of Mesozoic age. A few thin north- 
south dikes not shown on the map can be seen in the 
crystalline basement southwest and west of Boston in 
highway cuts along 1-495 and 1-90. Dikes are more 
numerous in the Milford-Dedham zone than is shown on 
the State bedrock map, and they are not all of Triassic or 
Jurassic age. LaForge (1932) and Ross (1981) described 
dikes of diabase and lamprophyre of several ages in the 
Boston area. Ross, who studied the dikes most recently, 
found from isotopic work that most are probably Triassic 
and Jurassic in age, but they range in age from Devonian 
to Jurassic. The orientation of the dikes in relation to age 
has not been measured systematically. LaForge (1932), 
however, divided the dikes into an older group, striking 
west to northwest and dipping variably, and a younger 
group, striking north and dipping steeply. Kaye (oral 
commun., 1981) noted that diabase dikes present on some 
of the islands in Boston Harbor are sill-like. The older 
west-striking dikes are presumably of late Paleozoic age, 
whereas the younger north-striking dikes are Mesozoic. 
Some of LaForge's older dikes are probably related to 
the Brighton Melaphyre and therefore would be Proter- 
ozoic Z in age rather than post-Pennsylvanian. No data 
are available on these dikes. Some lamprophyre dikes 
could be lower and middle Paleozoic and related to the 
alkalic plutons in the area; others may be younger. 

Dikes and faults of similar ages have similar orienta- 
tions in the Boston basin area. The Triassic and Jurassic 
dikes are aligned in the general north- to north- 



H42 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



Table 2.— Metamorphism and structure of cover rocks in basins of the Milford-Dedham zone, eastern Massachusetts 



Newburv basins 



Miildletmi basin 



Boston basin 



Bellingham basin 



Norfolk basin 



Narragansett basin 



Rocks 



Metamorphism 



Oldest bounding 
faults. 



Major deforma- 
tion and trend 
of stress. 



Age of major 
deformation. 



Mudstone, silt- 
stone, rhyolite, 
and basalt. 



Arkose, conglom- 
erate, and 
shale. 



Silurian and Triassic and 

Devonian. Jurassic. 

Low greenschist None 



East-northeast 
and north- 
northeast; high- 
angle, east- 
northeast faults 
could be 
reverse faults 
(rotation 
toward basin). 

North-northwest 
compression; 
rotation of 
strata. 

Post-Early Devo- 
nian. 



Northeast, high- 
angle, normal. 



West-northwest 
tension; tilting 
of strata. 



Jurassic(?) 



Slate, argillite, 
conglomerate, 
and volcanic 
rocks. 

Proterozoic Z to 
Cambrian. 

Subgreenschist 



East-northeast 
reverse faults, 
movement 
toward basin. 



North-northwest 
compression; 
open to tight 
folds. 



Alleghanian 



Phyllite, metacon- 
glomerate, and 
volcanic rocks. 

Pennsylvanian or 
Proterozoic Z. 
Greenschist 



North-northeast 
to northeast, 
high-angle, may 
be reverse 
faults toward 
basin. 



West-northwest 
to northwest 
compression; 
fold flattening. 

Alleghanian 



Metasandstone, 
conglomerate, 
and volcaniclas- 
tic rocks. 

Pennsylvanian 

Subgreenschist 



East-northeast 
reverse fault 
toward basin on 
south side. Part 
of north side 
high-angle, pos- 
sibly reverse 
fault toward 
basin. 

West-northwest 
compression; 
rotation of 
strata. 

Alleghanian 



Sandstone, conglomer- 
ate, meta-anthracite, 
and volcanic rocks. 

Pennsylvanian. 

Subgreenschist to 
greenschist (in 
Massachusetts). 

North-south, high- 
angle, normal on 
west side; none 
known on north side 
unless buried. Possi- 
ble northeast- 
trending buried fault 
on southeast side. 
North-south, high- 
angle on east side. 

West-northwest com- 
pression, open folds; 
reverse faults and 
thrusts in northwest 
corner. 

Alleghanian. 



northwest trend of the young faults that cut the basin 
margins. The older east- to northeast-trending dikes are 
parallel to the east-northeast-trending faults bounding 
and within the Boston basin. 



SUMMARY OF BLOCKS AND BASINS OF THE MILFORD- 
DEDHAM ZONE 

Rocks in the basins of the Milford-Dedham zone are 
folded and faulted but, except for those in the Bellingham 
basin, are little metamorphosed (table 2). The Protero- 
zoic Z strata in the Boston basin and the Cambrian strata 
there and at Hoppin Hill are no more metamorphosed 
than are the strata in the Silurian and Devonian New- 
bury basins and the Pennsylvanian strata in the Norfolk 
basin and northern part of the Narragansett basin. The 
Triassic-Jurassic rocks of the Middleton basin have only 
been faulted and are not metamorphosed at all. Some of 
the strata in the Bellingham basin are no more deformed 
and metamorphosed than strata in the Boston basin. The 
other greenschist-facies rocks mapped within the Bell- 
ingham basin may actually be equivalent to the Proter- 
ozoic Z Blackstone Group rather than the Pennsylvanian 
Bellingham Conglomerate. The evidence thus indicates 
that little deformation or metamorphism occurred within 
the zone between the Proterozoic Z time and the late 
Paleozoic. The folding and metamorphism seem to be 



related to compressive crustal movements in the late 
Paleozoic Alleghanian orogeny. Stresses at that time 
consisted primarily of west-northwest-east-southeast 
compression that broke up the crystalline basement and 
preserved covering strata in differentially down-dropped 
blocks flanked by reverse faults. Farther south in Rhode 
Island, Alleghanian events described by Mosher (1983) 
and Murray and Mosher (1984) involved early east-west 
compression producing westward-directed folds and 
thrusts, followed by a regional strike-slip component 
producing folds oriented slightly more northeasterly and 
verging eastward. Their third phase of folding is local- 
ized along an east-northeast-trending shear related to a 
regional east-west megashear system to the south. It is 
not clear how these structures relate to the deformation 
observed to the north in Massachusetts. Probably the 
major control in eastern Massachusetts was preexisting 
zones of weakness in the Proterozoic Z basement, which 
were reactivated during crustal movements in the late 
Paleozoic. Such preexisting zones of weakness may have 
controlled the original shapes and locations of the basins 
and may have controlled the formation of the Boston 
basin during a Proterozoic Z rifting event. 

The Pennsylvanian strata were deposited in an alluvial 
plain possibly in a broad intermontane or rift basin that 
formed following the Devonian extensional volcanic- 
plutonic events. McMaster and others (1980) postulated 
sinistral shearing on northeast-trending faults to form 



STRUCTURAL AND METAMORPHIC HISTORY OF EASTERN MASSACHUSETTS 



H43 



the basins, followed by dextral shearing on the same set 
of faults in the Permian to cause the post-Pennsylvanian 
compressional events and north-south extension, permit- 
ting intrusion of the east-trending Narragansett Pier 
Granite. The abundant north- to north-northwest- and 
locally north-northeast-trending faults are clearly later 
than the faults and folds of Alleghanian age and are 
ascribed to early Mesozoic rifting. 

The north-south faults are clearly younger than depo- 
sition of the Pennsylvanian deposits in the Narragansett 
basin, as they offset the contact with the crystalline 
rocks at the northeastern and northern edges of the 
basin. Some of these faults, such as those at Diamond 
Hill and in the Hanover area, are associated with vein 
quartz. Brecciation and veining by quartz is typical of 
faults of Triassic and Jurassic age in eastern New 
England (Rodgers, 1970), and I consider the generally 
north-south faults in eastern Massachusetts to be Meso- 
zoic in age. The Assawompset Pond graben is probably a 
Mesozoic structure. The west- and northwest-trending 
faults are possibly of similar age or older. The northeast- 
trending Beaverhead fault of southern Rhode Island 
must be post-Pennsylvanian because it cuts out Pondville 
Conglomerate on southern Conanicut Island. The 
inferred Assonet fault, however, may be an older fault 
in the basement that partly determined the location of 
the margin of the Narragansett basin before deposi- 
tion of the Pennsylvanian strata. Apparently, post- 
Pennsylvanian movement here has not been great. The 
steeply dipping, west-trending shear zones in the crys- 
talline rocks of the Fall River block are possibly pre- 
Pennsylvanian because no such shearing has been 
reported in the Pennsylvanian strata in the basin. The 
age of the northwest-striking thin mylonite zones in 
South Dartmouth is not known. No clear relationship is 
seen between structures in the crystalline complex of the 
Fall River block and polydeformation recorded in the 
pre-Pennsylvanian rocks in the southern Narragansett 
basin to the east (Kay and Chappie, 1976; Rast and 
Skehan, 1981). The gneissic plutonic rocks in the New 
Bedford area were deformed at higher temperatures 
than were the rocks at Newport, R.I., but these are in a 
different block. It is most likely that the Fall River block 
was relatively rigid during Alleghanian deformation. 
Possibly the primary foliation formed earlier, at the same 
time as the S x cleavage in the greenschist-facies Proter- 
ozoic Z metasedimentary and metavolcanic rocks at 
Newport, R.I., described by Kay and Chappie (1976) and 
Rast and Skehan (1981). The east-northeast-trending, 
southward-verging fold pattern in the crystalline rocks in 
the New Bedford area could be related to development of 
S 2 cleavage in the Newport rocks, but the supposition is 
weak. In the Pennsylvanian rocks of the southern Nar- 
ragansett basin, the oldest deformation is visualized as 



east-west compression in which the Fall River block 
moved westward (Burks and others, 1981, p. 272). 
Possibly the east-west shears in the block are tears that 
developed during this process. 

In the Boston, Norfolk, and northern Narragansett 
basins, vergence of folding and thrusting has been to the 
south and generally oriented north-northeast. The strata 
in the Newbury basins are tilted into orientations 
approximating the trends of the bounding faults, which 
are east-northeast in the northern part of the northern 
basin and north-northeast in the southern part and in the 
southern basin. The east-northeast trends in the Boston, 
Norfolk, and northern Narragansett basins seem to me 
to form a pattern of deformation that, because it affects 
Pennsylvanian rocks, indicates the deformation is a 
reflection of Alleghanian crustal movements. This defor- 
mation increases in intensity from north to south. The 
greater complexity of structures at the northwest end of 
the Narragansett basin near its junction with the Norfolk 
basin is probably due to complications arising from the 
adjustments in cover rocks as basement subblocks were 
jostled during Alleghanian deformation. The controlling 
stress could be the left-lateral shear system striking 
northeast mentioned above. 



BLOODY BLUFF FAULT ZONE 

The Bloody Bluff fault (Cuppels, 1961, p. D46; Skehan, 
1968, p. 282) forms the boundary between the Milford- 
Dedham zone and the Nashoba zone in eastern Massa- 
chusetts from near Westborough to Lynnfield (figs. 1, 
14). The history of recognition of the fault and its 
configuration are described by Castle and others (1976). 
The boundary between the Milford-Dedham zone and the 
Nashoba zone from south of Marlborough to Oxford is the 
Lake Char fault, which on the State bedrock map is 
shown as coinciding with the trace of the Bloody Bluff. 4 
The fault is not well exposed in this interval. In Connect- 
icut the Lake Char fault forms the boundary between the 
gneissic part of the Milford-Dedham zone in the Rhode 
Island anticlinorium and the Putnam terrane as far south 
as the Preston Gabbro (PG, fig. 3) in North Stonington, 
Conn., where the boundary turns west as the Honey Hill 
fault. 

The main zone of the Bloody Bluff fault is a polyde- 
formed zone of ductile deformation and cataclasis as 
much as 3.2 km wide near Framingham (Nelson, 1976) 
and 5 km wide in the Burlington area where it is the 
Burlington mylonite zone of Castle and others (1976). 
However, at the type locality of the fault at Bloody 



4 Since the State bedrock map was prepared, O'Hara and Gromet (1984) have 
identified a shear zone east of the Lake Char fault, which they consider to be the 
southward continuation of the Bloody Bluff fault (see figs. 3, 9). 



H44 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 




EXPLANATION 

Fault — Dashed where inferred. 
Alternative fault interpretations 

not shown on the State bedrock 

map 
Topsfield Granodiorite 
Serpentinite at Lynnfield 
Anticline in Burlington mylonite zone 



Figure 14.— Structural features along part of the Bloody Bluff fault zone, northeastern Massachusetts. 



Bluff (fig. 14) (Cuppels, 1961) and other places, brittle 
deformation structures are common. This complexity 
indicates that the Bloody Bluff zone has had a long 
history of deformation. Within the zone of mylonitic 
rocks are markedly layered blastomylonites of felsic and 
mafic rock and mylonites of quartzite. Similar rocks can 
be seen in the Proterozoic Z Waterford Group in the 
lower plate of the Honey Hill fault zone southeast of the 
Preston Gabbro in southeastern Connecticut (Goldsmith, 
1980, 1985). 

Southwest of Westborough, mylonitic rocks form a 
relatively narrow zone. The fault is exposed on 1^95 
east of Westborough. From Westborough northeast, the 
Bloody Bluff-Lake Char fault system widens and splits 
into several east- trending branches. The central splay is 
the main Bloody Bluff fault of Cuppels (1961, p. D46; 
Skehan, 1968, p. 282). A southern splay, trending east 
toward the Boston basin, was called the Weston fault by 
Nelson (1975a,b; 1976). The northern splay is arcuate and 
less well documented. Castle and others (1976) ended the 
Bloody Bluff fault against (A, fig. 14) a fault (B, fig. 14) 
extending from Westborough to Framingham, which 
they called the Lake Char fault. The splay off the Bloody 



Bluff fault from near Millbury to the Framingham area 
shown on the State bedrock map is largely interpreted 
from discordance of foliation pattern. O'Hara and 
Gromet (1984) placed their split between the Lake Char 
and Bloody Bluff faults south of Millbury (5, fig. 2). The 
location of some of these faults is interpretive, and other 
fault patterns could be drawn in northeastern Massa- 
chusetts. Castle and others (1976) and Nelson (1976) 
based much of their interpretations of the location of 
major faults on aeromagnetic patterns. Because the 
faults are zones of crushed rock rather than discrete 
surfaces, they could have been shown more accurately on 
the State bedrock map as patterned areas, rather than as 
discrete fault traces. On the State bedrock map the 
significant fault in the Bloody Bluff system is considered 
to be the westernmost fault, the one that separates 
recognizable Milford-Dedham rock assemblages from 
Nashoba assemblages. 

The Bloody Bluff fault ends as a discrete entity in the 
Danvers- Lynnfield area of Essex County, at the south- 
ern end of the southern Newbury basin and the Middle- 
ton basin. This locality appears to be a structural nexus, 
for it is about here that the Burlington mylonite zone is 



STRUCTURAL AND METAMORPHIC HISTORY OF EASTERN MASSACHUSETTS 



H45 



no longer recognized, and the postulated Mystic fault of 
Bell and Alvord (1976, fig. 1; fig. 10) intersects the 
Bloody Bluff fault. This nexus probably represents the 
intersection of fault systems of somewhat different 
trends and ages. The identity of the Bloody Bluff fault 
northeast of the Lynnfield-Danvers area is questionable, 
and the fault may change character. The Proterozoic Z 
plutonic rocks in this area have a nonsystematically 
oriented crushed fabric and a retrogressive mineral 
assemblage, unlike the directed, well-recrystallized fab- 
ric of the mylonite zones to the southeast. The western 
boundary of the Milford-Dedham zone from Lynnfield 
north is marked by narrow fault zones, such as those 
forming the boundaries of the Newbury basins and at 
least one side of the Middleton basin (Kaye, 1983). The 
Bloody Bluff and the Burlington mylonite zone have 
northeasterly trends, whereas the faults bounding the 
Newbury basins have more northerly trends. The north- 
trending faults bounding the basins are later than, and 
have truncated and offset, the zones of more pervasive 
northeasterly deformation in the Bloody Bluff and Burl- 
ington zones. Possibly a splay of the Bloody Bluff con- 
tinues northeast along the southern boundary of the 
Topsfield Granodiorite (T, fig. 14), to the Gulf of Maine 
where it lies under the Quaternary sands of Plum Island. 

The Weston fault of Nelson (1975a, b) splays easterly 
from the Bloody Bluff fault near Westborough toward 
the northern border fault of the Boston basin in the 
vicinity of Weston (fig. 14). This area is so broken by 
faults that it is difficult to determine the actual relation- 
ship of the Weston fault to the border fault; however, the 
north-northeast-trending Burlington mylonite zone 
projects southwestward into the trace of the Weston 
fault, and I have accordingly shown the eastern bound- 
ary of the zone as a fault branching off the Weston fault 
near Weston. The Burlington mylonite zone is described 
below. 

The fault shown on the State bedrock map splaying off 
the Lake Char fault in the vicinity of Millbury is drawn to 
connect with an east-northeast-trending fault mapped by 
Nelson (1975a) south of Framingham. This fault offsets 
rock units as much as 6 km in a right-lateral sense. 
Southwest of Framingham near Marlborough, Nelson's 
fault was not recognized by Barosh (1978) or by Hepburn 
and DiNitto (1978). Although its existence southwest of 
the Framingham quadrangle is questionable, it is drawn 
on the State bedrock map along contacts of units aligned 
parallel to its possible projected trace and parallel to the 
gneissic foliation in the plutonic rocks in this area. 

BURLINGTON MYLONITE ZONE 

The Burlington mylonite zone of Castle and others 
(1976) extends from Weston to Lynnfield and Danvers 



(figs. 2, 14). They visualized the northwest side of the 
zone as the primary locus of dislocation on the Bloody 
Bluff fault. As described by Castle and others (1976), the 
mylonite zone is about 1.5 km thick although it has been 
folded and faulted to produce an outcrop width of as 
much as 5 km. Castle (1964) showed a north-plunging fold 
of foliation in the zone southwest of Lynnfield (C, fig. 14). 
The rocks range from ultra-fine-grained laminated mylo- 
nites to coarse augen and flaser gneisses. Mylonitic 
quartz schist and blastomylonitic, thinly layered, leuco- 
cratic, siliceous rocks are seen in thin section to have a 
ribbon-like fluxion structure. The augen and flaser 
gneisses tend to be near the center of the zone, and the 
more laminated rocks are on the flanks. This division 
may reflect original differences in rock composition as 
well as degree of deformation. Within the zone are areas 
of layered mafic nonmylonitic rock and areas of massive 
mafic rock sheared only around the margins. The latter 
form large-scale augen of competent rock around which 
the strain was distributed. Bands and seams of chlorite 
schist are common in sheared mafic rock. The zone of 
serpentine mapped by Castle (1964) near Lynnfield (S, 
fig. 14) is mostly antigorite. Kaye (1983, p. 1076) sug- 
gested that it was ultramafic material squeezed up in a 
shear zone. The ductile mylonites are cut by later brittle 
faults (Castle and others, 1976). 

The Burlington mylonite zone does not seem to persist 
as a discrete feature northeast of Lynnfield nor south- 
west of Weston. To the northeast, its last good expres- 
sion seems to be at the south end of the Newbury and 
Middleton basins. Castle and others (1976, p. 17) con- 
cluded that the Burlington mylonite zone does not extend 
seaward toward the Gulf of Maine but turns northward 
and is cut out against, or dips beneath, weakly magnetic 
plutonic rocks (Sharpners Pond Diorite and related 
rocks) and the Newbury Volcanic Complex. Kaye (1983) 
did not show the mylonite zone extending beyond the 
Lynnfield and Danvers area but did suggest (oral com- 
mun., 1980) that it swings to the east following the 
southern edge of the Topsfield Granodiorite (T, fig. 14). 
However, as the Topsfield is part of the Milford-Dedham 
assemblage, this zone cannot be the main boundary 
between the Nashoba and Milford-Dedham blocks, which 
must lie beneath the Newbury basins. To the south, the 
Burlington mylonite zone ends near the Weston fault of 
Nelson (1976). On the State bedrock map, I have shown 
the eastern boundary of the mylonite zone as a fault 
continuing south to merge into the Weston fault near 
Weston. This fault line does not represent a discrete fault 
surface but a boundary between mostly sheared and 
mostly unsheared rock. 

It is quite likely that the Burlington mylonite zone is of 
composite age. However, Castle and others (1976) stated 
that the Burlington mylonite zone is intruded by the 



H46 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



Paleozoic alkalic granites, presumably in this case the 
Early Devonian Peabody Granite (Dpgr). This provides 
an upper (young) age limit for some of the deformation, 
at least in this part of the zone. 

Mylonitic and cataclastic rocks of the kind described by 
Castle and others (1976) in the Burlington zone and by 
Nelson (1976) along the Bloody Bluff fault zone are 
repeated to the south, although often in a narrower zone, 
along the Lake Char and Honey Hill faults in eastern and 
southeastern Connecticut (Lundgren and Ebblin, 1972; 
Goldstein, 1982; Dixon, written commun., 1983; Gold- 
smith, 1985) and in the zone east of the Lake Char fault 
identified by O'Hara and Gromet (1984). The wide, 
nonuniform distribution of faults and shear zones, such as 
the Burlington mylonite zone in northeastern Massachu- 
setts, may be the culmination at a shallower level of the 
more pervasive, ductile strain evident in the western 
side of the Rhode Island anticlinorium to the south. The 
irregular distribution of the deformation is probably in 
large part caused by the distribution of rocks of different 
competencies; in addition, this area is a major boundary 
between different terranes in which movement is distrib- 
uted over a fairly wide zone forming a typical anastomos- 
ing cataclastic pattern, albeit on a regional scale. The 
gabbroic rocks of the diorite and gabbro suite act as 
resistant knots around which the strain was distributed, 
as is the case with the Preston Gabbro in Connecticut. 
The major strain appears to have been accommodated by 
the quartzitic and quartzofeldspathic rocks of the West- 
boro Formation and the overlying volcanic units to 
produce the quartzitic mylonites noted by Castle and 
others (1976). 

WOLFPEN LENS 

A lens of rock called the Wolfpen lens (figs. 2, 14) lies 
between the northern branch and the main trace of the 
Bloody Bluff north of Framingham and east of Marlbor- 
ough. It consists of sheared amphibolite and metaplu- 
tonic rock that was called the Wolfpen Tonalite by 
Emerson (1917) and "altered and sheared rocks" by 
Hepburn and DiNitto (1978). A question exists as to 
whether these rocks belong to the Milford-Dedham zone 
or to the Nashoba zone. Nelson (1975b) showed the 
Wolfpen lens as much smaller than is shown on the State 
bedrock map. To the west of the boundary of Nelson's 
lens, he mapped a poorly exposed unnamed granite, 
which in its eastern part contains an inclusion of his 
Proterozoic Z Claypit Hill Formation. The western part 
of this granite, which Nelson showed as intruding the 
Marlboro Formation, is now mapped as granodiorite of 
the Indian Head pluton (igd on the State bedrock map) 
and assigned to the Nashoba zone. If, as mapped by 
Nelson, there is only one granite, then this is the only 



place in southeastern New England where plutonic rock 
straddles the boundary between the Milford-Dedham 
zone and the Nashoba zone. Because the Claypit Hill is a 
formation in the Milford-Dedham zone, the granite that 
encloses it must also belong to the Milford-Dedham zone. 
Either the rocks mapped as Claypit Hill Formation are 
misidentified and are actually Marlboro Formation, or 
two different granites exist here. Mapping by Barosh 
(1978) and Hepburn and DiNitto (1978) in the Marlbor- 
ough area indicates that Nelson's granite may be two 
different plutonic rocks. Hence I have drawn the north- 
west boundary of the lens along a topographic lineament 
aligned with Barosh's and Hepburn and DiNitto's faults 
bounding the zones so that the Wolfpen inclusion of 
Claypit Hill is within the Milford-Dedham zone. This lens 
ends near Sudbury where its northern bounding fault 
rejoins the main strand of the Bloody Bluff fault. The 
sharp bend in the trace of the fault north of the junction 
was shown by Castle and others (1976) as an offset on a 
transecting north-south fault. Barosh and others (1977) 
also showed a north-south fault here (D, fig. 14) that 
offsets the Bloody Bluff fault to the south. They contin- 
ued the Bloody Bluff to intersect the main trace near 
Lincoln. Shride (written commun., 1979), however, did 
not recognize a fault here and believed the trace of the 
Bloody Bluff fault is only folded. Following Shride, the 
north-south fault is not shown on the State bedrock map. 

ATTITUDES OF FAULT SURFACES AND SENSE OF 
MOVEMENT IN THE BLOODY BLUFF FAULT ZONE 

The attitudes of fault surfaces in the Bloody Bluff fault 
system have been measured directly in several places. 
The sense of movement is less well determined. The 
Lake Char fault in Connecticut dips about 25° to the west 
in northeastern Connecticut, as determined by measure- 
ments of mylonitic fabric. Dips steepen to about 45° as 
this fault is traced into Massachusetts and steepen to 60° 
and more where the fault becomes the Bloody Bluff and 
starts to curve easterly in the Westborough area. Castle 
and others (1976) computed dips of 60° on the basis of 
magnetic data in the Bloody Bluff zone near Marlbor- 
ough. Mylonite on the west edge of the Burlington zone 
dips 60°-85° to the west. Nelson (1975a) showed dips of 
60°-65 c on branches of the Bloody Bluff fault in the 
Framingham area. Castle and others (1976) stated that 
the Bloody Bluff dips more gently at about 20° northwest 
where the fault turns from northeasterly to northerly 
near Lynnfield. 

The sense of movement on the main Bloody Bluff fault 
system has been generally considered to be a thrust or 
high-angle fault with movement toward the southeast 
and east (Skehan, 1969; Nelson, 1976). The opposite 
sense of movement has been proposed at least for the 
latest movements on the Lake Char fault (Goldstein, 



STRUCTURAL AND METAMORPHIC HISTORY OF EASTERN MASSACHUSETTS 



H47 



1982) and the related Honey Hill fault (Lundgren and 
Ebblin, 1972) in Connecticut. A strike-slip component of 
movement on the Bloody Bluff was proposed by Bell (as 
cited in U.S. Geological Survey, 1969, p. A21-A22) at 
least from Framingham north. At the type locality, 
Smith and Barosh (1983) recognized dextral displace- 
ment that deformed an earlier foliation aligned parallel to 
the fault trend. O'Hara and Gromet (1984) stated that 
movement sense in the shear zone east of the Lake Char 
fault is dextral. However, Castle and others (1976) 
pointed out that "b" lineations in minor folds associated 
with the mylonites have gentle plunges. Any transla- 
tional movements like those noted by Smith and Barosh 
(1983) may have been relatively late. Nelson (1976) saw 
little evidence for lateral displacement in the ductilely 
deformed rocks. 

Some transcurrent movement has been reported for 
minor and probably young faults in the zone. Northeast 
of Lynnfield, where the major fault zones are not 
exposed, sense of movement on northeast- and north- 
northeast-trending faults flanking the basins (fig. 10) is 
right lateral, according to Bell and others (1977). Other 
and younger minor faults (D, fig. 10) strike northwest 
across the trend of the rocks and dip at high angles. In 
the Cape Ann and Ipswich area, Dennen (1975) showed 
the northeast-trending faults as having right-lateral 
transport and also a vertical component, up on the 
northwest. Dips are not indicated but are presumably 
steep to the northwest. 

AGE OF THE BLOODY BLUFF FAULT ZONE 

The metamorphic mineral assemblages and textures of 
the ductilely deformed rocks within the Bloody Bluff 
zone indicate that they were formed initially at pressures 
and temperatures prevailing at medium metamorphic 
grade. According to Nelson (1976), movement may have 
commenced during or after the regional metamorphism 
of the metasedimentary and metavolcanic rocks. This 
metamorphism probably occurred before intrusion of the 
Proterozoic Z Dedham Granite. Retrogressive mineral 
assemblages, cataclasis, and hydrothermal alteration 
indicate that deformation continued, or occurred later, 
within the fault zone at lower temperature and pressure. 
The Ordovician Andover Granite in the adjacent Na- 
shoba block is mylonitized (Nelson, 1976, p. 1383). An 
upper age limit on at least some of the movement is 
provided by Castle and others (1976), who, as mentioned 
above, observed that the Early Devonian Peabody Gran- 
ite intrudes and is not deformed by the Burlington 
mylonite zone. Nelson (1976, p. 1382) mapped unde- 
formed pegmatite cutting cataclastic rock of the Bloody 
Bluff zone, but its age is unknown. The contrast in 
deformation between the Newbury Volcanic Complex 



and the adjacent Topsfield Granodiorite suggests a post- 
Proterozoic, pre-Late Silurian movement on the Bloody 
Bluff in that area. However, a growing weight of evi- 
dence indicates that extensive crustal movements 
occurred in the late Paleozoic during the Alleghanian 
orogeny in eastern New England (Rast and Skehan, 
1983; Gromet and O'Hara, 1984; Hermes and Zartman, 
written commun., 1984; Murray and Mosher, 1984). 
From this evidence, some of the deformation along the 
Bloody Bluff fault zone might have occurred in the late 
Paleozoic. Because the fault that bounds the Triassic 
Middleton basin on its south side cuts the mylonitic 
rocks, one can conclude that Mesozoic movement has 
occurred in other places also along the Bloody Bluff zone. 
Therefore, I conclude that the Bloody Bluff fault zone 
had a long history starting with postmetamorphic perva- 
sive ductile deformation, before the Late Silurian and 
Early Devonian, and concluding with shallow, brittle 
deformation probably ending in the Mesozoic. 



TECTONIC EVENTS IN EASTERN 
MASSACHUSETTS 

Eastern Massachusetts consists of all or parts of three 
lithotectonic zones or belts distinguished by differences 
in stratigraphy and in metamorphic, plutonic, and defor- 
mational history (Goldsmith, this vol., chaps. E, F; 
Robinson and Goldsmith, this vol., chap. G; Wones and 
Goldsmith, this vol., chap. I). These three are the 
Milford-Dedham zone, the Nashoba zone east of the 
Merrimack belt, and the east flank of the Merrimack 
belt. The Newbury basins, described as part of the 
Milford-Dedham zone (Goldsmith, this vol., chap. E), are 
not readily assignable to either of the adjacent zones and 
are treated as a separate terrane for purpose of discus- 
sion in the following section. The present positions of the 
zones are largely determined by post-Pennsylvanian 
tectonic events. Before that time the terranes had dif- 
fering geologic histories (table 3, fig. 15). The sources of 
these terranes and their times of mutual accretion are 
only partly understood. Questions and uncertainties 
about the tectonic events summarized in table 3 and 
figure 15 are discussed below, as are speculations on the 
history of accretion of the terranes. 



MILFORD-DEDHAM ZONE 

The Milford-Dedham zone is fragmented by faults into 
blocks of exposed basement and blocks containing Prot- 
erozoic Z to Mesozoic cover rocks. This fragmentation 
took place primarily in the Permian and Triassic- 
Jurassic, but earlier events probably predetermined loci 
of later deformation. Earlier events include the forma- 



H48 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



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STRUCTURAL AND METAMORPHIC HISTORY OF EASTERN MASSACHUSETTS 



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Figure 15.— Tectonic events in eastern Massachusetts corresponding approximately to intervals of time indicated on table 3. Nashoba and 
Milford-Dedham zones are exotic terranes accreted to the North American craton during the Paleozoic. Base is present-day arrangement of 
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STRUCTURAL AND METAMORPHIC HISTORY OF EASTERN MASSACHUSETTS 



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H52 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



EXPLANATION 

Deposition and igneous activity 
Sedimentation 



i.' *'■ L \ Sedimentation and volcanism 

-X ■ . • .:? Sedimentation, volcanism, and plutonism 

Metamorphic grade 

Subgreenschist fades or lower 

Greenschist facies 



Amphibolite facies 
Upper amphibolite facies 
Plutonism (may be combined) 
S-type 

1-type 

Calc-alkaline 
Alkaline 
Faults — Dashed where inferred; queried where uncertain 

High-angle; U, upthrown block: D, downthrown block. 
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Reverse; teeth on upthrown block 

Low-angle, teeth on upper block. Movement sense may 
be thrust or normal 

Antiform 
Synform 

FIGURE 15. — Continued. 



tion of a Proterozoic Z magmatic arc, probably on a 
continental margin, eventual rifting of this margin, and a 
poorly documented post-Devonian, pre-Pennsylvanian 
orogenic (collisional?) event, which led in the Pennsylva- 
nian to uplift, erosion, and subaerial sedimentation from 
an eastern source no longer present. Compressive defor- 
mation and dynamothermal metamorphism probably 
related to subduction, recorded in the stratified rocks of 
the Proterozoic basement, appear not to have occurred 
again until the Late Pennsylvanian and Permian. In the 
interim, through much of the Paleozoic, a static, essen- 
tially extensional, intracratonic plutonic-volcanic regime 
prevailed. 

The actual timing of the Proterozoic metamorphism 
and the subsequent events that led to development of the 
present map pattern are not entirely clear. The stratified 
rocks were metamorphosed before intrusion of the bath- 
oliths, but how much earlier is not certain. Because the 



metamorphosed mafic and felsic volcanic rocks (Zv) are 
considered to be precursors to, or penecontemporaneous 
with, the mafic plutonic rocks (Zgb, Zdi, Zdigb), the time 
of metamorphism could lie between the times of their 
emplacement, as indicated by the 656-Ma date on the 
diorite at Rowley, and the emplacement of the Dedham 
Granite at 630 Ma (table 3). However, it is possible that 
the Westboro Formation and its equivalents and the 
overlying mafic volcanic rocks (Zv) were metamorphosed 
earlier, before emplacement of the mafic plutonic rocks. 
The dated diorite at Rowley is little deformed and may 
represent a fairly young phase of the mafic plutonism. 

The metamorphic rocks are at greenschist facies 
throughout much of the zone. They are, however, in the 
amphibolite facies along the west side of the Rhode 
Island anticlinorium and the Milford antiform and in the 
New Bedford area. In these areas, the Proterozoic Z 
plutonic rocks have also been deformed in the amphibo- 
lite facies (O'Hara and Gromet, 1984). Therefore, the 
time of amphibolite-facies metamorphism on the west 
and southeast sides of the Milford-Dedham zone is open 
to question. I have shown this metamorphism on the 
State bedrock map as Proterozoic in age. However, other 
possibilities are that the greenschist-facies Proterozoic 
metamorphism has been overprinted by an amphibolite 
facies at the time of metamorphism of the Nashoba zone, 
at the time of the widespread Permian dynamothermal 
event of southern New England, or in the Late Devo- 
nian. The first seems rather unlikely because the steep 
metamorphic gradient between the two zones and their 
distinctly different Paleozoic plutonic signatures pre- 
clude the two zones being in juxtaposition before and 
during the early Paleozoic. The Permian event is more 
likely in view of the mounting evidence that an intense 
thermal metamorphism affected basement and cover 
alike in southern Rhode Island and southeastern Con- 
necticut (see, for example, Skehan and Murray, 
1980a,b). This event extended northward into central 
Rhode Island where Day and others (1980) implied fairly 
shallow northerly dipping isotherms. However, there is 
no record of a Permian amphibolite-facies metamorphism 
affecting the Nashoba zone. The somewhat higher grade 
of metamorphism on the west and southeast flanks of the 
Milford-Dedham zone as opposed to the generally lower 
grade of metamorphism in the center could be inter- 
preted primarily as resulting from greater uplift along 
the western and southeastern parts of the zone during 
the Permian, which exposed deeper levels of the crust, in 
similar fashion to the greater uplift inferred along coastal 
Rhode Island and Connecticut (Lundgren and Ebblin, 
1972). There is a good possibility, though, that the 
deformation that transformed the Proterozoic plutonic 
rocks into orthogneisses may have begun earlier than the 
Permian. Castle and others (1976) noted that the Early 



STRUCTURAL AND METAMORPHIC HISTORY OF EASTERN MASSACHUSETTS 



H53 



Devonian Peabody Granite intrudes but is not deformed 
by the Burlington mylonite zone. Nelson's (1976) obser- 
vation that pegmatite cuts the mylonite in the Bloody 
Bluff fault zone suggests a pre-Devonian age for the 
mylonite. To the south along the zone boundary, Pignolet 
and others (1980) have dated cataclasis in the Honey Hill 
fault zone (fig. 3) and deformation in the Silurian and 
Devonian Canterbury Gneiss of the upper plate as Devo- 
nian, although O'Hara and Gromet (1983) questioned 
their interpretation of the data. Losh and Bradbury 
(1984) recognized both pre-424-Ma and Acadian move- 
ments in the Honey Hill fault zone. The Proterozoic 
rock units in southeastern Connecticut are multiply 
deformed, producing an interference pattern (Goldsmith, 
1985). This deformation involves a Carboniferous alkalic 
granitoid (R.E. Zartman, oral commun., 1981) formerly 
assigned to the New London Gneiss (the Joshua Rock 
Member). It seems likely that a post-mid-Devonian met- 
amorphism, probably Carboniferous, ranging from 
amphibolite to greenschist fades has been superimposed 
on the earlier Proterozoic greenschist- or possibly lower 
amphibolite-facies metamorphism. Although the age of 
earlier movements is not well documented, all agree that 
latest major movements on the Lake Char and Honey 
Hill faults in this region are at greenschist facies and are 
of Permian age. Because the same age and kind of rocks 
having similar styles of metamorphism and cataclasis 
exist along the Milford-Dedham zone boundary at the 
north end of the Rhode Island anticlinorium as in south- 
eastern Connecticut, I feel somewhat confident in using 
the conclusions reached in southeastern Connecticut 
to support the conclusions reached from observations 
made on the boundary of the Milford-Dedham zone in 
Massachusetts. 

NASHOBA ZONE 

The Nashoba zone is a relatively homogeneous terrane 
containing high-grade metamorphic rocks derived from 
volcanic and sedimentary rocks of Proterozoic Z or early 
Paleozoic age (table 3; shown as Proterozoic on fig. 15A). 
Compositional layering, metamorphic foliation, and unit 
contacts dip steeply throughout the zone. The metamor- 
phic rocks are intruded by foliated peraluminous granite 
of Ordovician age and nonfoliated diorite, granodiorite, 
and granite of Silurian and perhaps also Devonian age. 
The zone is cut by numerous faults of late Paleozoic and 
probably Mesozoic age. The timing of the high-grade 
dynamothermal metamorphism in the Nashoba zone 
(table 3) is not clearly defined. The parallelism of the 
foliation in the Ordovician Andover Granite to the met- 
amorphic foliation in the host Nashoba and Marlboro 
Formations indicates that intrusion of the Andover was 
paracontemporaneous with the regional metamorphism. 
Accordingly the metamorphism is shown as Ordovician 



on figure 15C. However, there could also have been an 
earlier metamorphism of Proterozoic Z age. As men- 
tioned above, the relatively steep metamorphic gradient 
between the Nashoba zone and the Milford-Dedham zone 
indicates that the two zones were not juxtaposed at the 
time of high-grade metamorphism in the Nashoba zone. 
The fact that the Nashoba contains a suite of Paleozoic 
plutonic rocks distinct from those in the Milford-Dedham 
zone (Wones and Goldsmith, this vol., chap. I) indicates 
that the two were not juxtaposed through the Silurian 
and perhaps the Devonian. The Silurian intrusive rocks 
of the zone (fig. 15D) are of a type associated with 
continental volcanic arcs (Wones and Goldsmith, this 
vol., chap. I). Some aplite and pegmatite in the Andover 
Granite are younger than the bulk of the Andover 
Granite (Wones and Goldsmith, this vol., chap. I) and are 
possibly of Silurian and (or) Devonian age. Evidence for 
Alleghanian metamorphism and plutonism is lacking in 
the Nashoba zone. Retrogressive assemblages have not 
been developed to any great extent (see Abu-Moustafa 
and Skehan, 1976) except for an episode of hydration, 
which is probably associated with Andover plutonism. 
There is, however, a considerable amount of late Paleo- 
zoic faulting. Within the Nashoba zone, the older dyna- 
mothermal metamorphic fabric has been cut in more or 
less imbricate fashion by later shearing and faulting that 
strikes subparallel to the bounding faults (Skehan, 1967, 
1968) so that the whole block could be considered a zone 
of shear, the Nashoba thrust belt of Barosh (1982), 
between the Bloody Bluff fault zone and the Clinton- 
Newbury fault zone. However, we can only speculate as 
to whether the movement sense was dominantly lateral 
or dominantly inclined. The Clinton-Newbury fault zone 
itself contains both moderately dipping and steeply dip- 
ping fault surfaces, which seem to be of different ages 
and textural fabrics and which have not been sorted out 
satisfactorily. The Bloody Bluff fault zone likewise con- 
tains differently dipping fault surfaces and both ductile 
and brittle fabrics. Most of the faults within and on the 
flanks of the Nashoba zone appear to be fairly steep, 
judging from surface observations and the data of Castle 
and others (1976). The movements could be largely 
translational; however, the regional relations require 
that the faults flatten with depth (cross sections D-D' 
and F-F' of the State bedrock map) and that earlier 
movements were thrusts. Any strike-slip or normal 
movement is probably late. 

NEWBURY BASINS 

The Newbury basins are fault bounded, so that their 
relationship to the adjacent Nashoba and Milford- 
Dedham zones is uncertain. We do not see the basement 
for the Newbury Volcanic Complex. The rocks of the 



H54 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



Newbury Volcanic Complex are at a much lower meta- 
morphic grade than the rocks of the adjacent Nashoba 
zone and are much less deformed and altered than the 
adjacent Proterozoic rocks of the Milford-Dedham zone. 
The Newbury basins are apparently wedges of higher 
level material emplaced between the two larger bound- 
ing blocks. Presumably the Newbury lies unconformably 
on one or the other of them, most likely on the Milford- 
Dedham Proterozoic Z basement, as seems to be indi- 
cated by correlation with similar units in the Coastal 
Volcanic belt of eastern Maine. The Newbury Volcanic 
Complex is correlated on the basis of similar Acado- 
Baltic faunal assemblages with the Leighton Formation 
of the Pembroke Group in the Eastport area, Maine; the 
Leighton, however, contains no volcanic rocks (Shride, 
1976b). Volcanic rocks of similar age are present in the 
Milford-Dedham zone in the East Greenwich Group of 
central Rhode Island (Quinn, 1971; Hermes and others, 
1981). These rocks are alkalic, like the lower and middle 
Paleozoic plutonic rocks of the Milford-Dedham zone and 
like those in the Gulf of Maine (Hermes and others, 
1978). The Newbury Volcanic Complex is also compara- 
ble to the Silurian and Devonian Castine Volcanics of the 
Penobscot Bay area, Maine, described and dated by 
Brookins and others (1973). The Castine consists of 
bimodal volcanic rocks and sedimentary rocks that rest 
unconformably on metamorphosed Ellsworth Schist 
(Stewart and Wones, 1974). The Castine, like the New- 
bury, lies in an area of appreciable faulting that has 
produced many blocks containing different sequences of 
rock. The Castine is intruded by the mildly alkalic 
Bays-of-Maine Igneous Complex of Chapman (1962) of 
roughly comparable age. If the Castine and Newbury are 
equivalents, then the Newbury could be related to the 
alkalic and peralkalic rocks of the plutons of the Milford- 
Dedham zone by correlating Chapman's Bays-of-Maine 
with the alkalic rocks from the Gulf of Maine. It would be 
helpful in placing the Newbury in context if one could 
correlate the lower Paleozoic Ellsworth Schist on which 
the Castine rests with a rock unit in eastern Massachu- 
setts. This correlation has not been made to date. The 
Silurian plutonic rocks of the Nashoba zone are unique to 
New England (Wones and Goldsmith, this vol., chap. I) 
and are probably not genetically related to the Newbury; 
comparisons of trace elements may help prove or dis- 
prove this relationship. The Newbury Volcanic Complex 
most likely belongs to the Milford-Dedham zone rather 
than the Nashoba zone but may be the remnant of an 
entirely separate terrane. The present position of the 
Newbury basins and the Newbury Volcanic Complex can 
only be attributed to post-Early Devonian faulting. 
There has, of course, also been post-Triassic faulting in 
this area (Kaye, 1983). 



EAST FLANK OF THE MERRIMACK BELT 

The east flank of the Merrimack belt, consisting of the 
Nashua trough and the Rockingham anticlinorium (Rob- 
inson and Goldsmith, this vol., chap. G), has a somewhat 
uncertain history primarily because the age of the strat- 
ified rocks is uncertain. However, the arguments pre- 
sented below indicate that the rocks of the Nashoba zone 
underlie, at least in part, those of the Merrimack belt and 
in fact may be the basement upon which the Merrimack- 
belt rocks were deposited. The Nashoba zone projects 
beneath the Merrimack belt west of the Clinton- 
Newbury zone as indicated by the "cross-sectional" map 
view of the Merrimack synclinorium exposed in south- 
eastern Connecticut (fig. 3; see Rodgers, 1970). There 
the Tatnic Hill and Quinebaug Formations, which are the 
equivalents of the Nashoba and Marlboro Formations in 
the Putnam terrane, extend westward beneath the tur- 
bidite section of the Merrimack belt and above the 
Proterozoic Z basement. The Science Park block of the 
Nashoba zone (figs. 2, 4) must have been thrust up from 
beneath the Merrimack-belt rocks to the west. Whether 
the Putnam terrane extends to the Bronson Hill anticli- 
norium is a matter of controversy (Rodgers, 1981; Pease, 
1982; Robinson and Tucker, 1982; London, 1984). In 
eastern Connecticut, the rocks of the Putnam terrane 
pinch out westward between the Honey Hill fault and the 
probable extension of the Clinton-Newbury fault. A 
terrane similar to the Nashoba, containing the Massabe- 
sic Gneiss Complex, forms a structural high within the 
Merrimack synclinorium. To the north in Maine, a ter- 
rane similar to the Nashoba, the Passagassawakeag, 
containing the Casco Bay Group, is shown on the latest 
bedrock geologic map of Maine (Osberg and others, 1984) 
as resting upon, rather than underlying, the turbidite 
sequence of the Kearsarge-central Maine synclinorium, a 
division of the Merrimack synclinorium. However, a 
preliminary seismic profile across this terrane (Stewart 
and others, 1986) suggests that the Passagassawakeag 
terrane structurally underlies the turbidite sequence. 

The rock units of the Merrimack belt exposed in the 
Clinton-Newbury fault zone near the contact with the 
Nashoba zone are quartzites and conglomerates and are 
probably basal to the sequence in the Merrimack belt. A 
strong possibility exists that the Vaughn Hills Quartzite, 
the Reubens Hill Formation, the Kittery Formation, and 
possibly the Tower Hill Quartzite are Ordovician (or 
older) in age and lie unconformably on the Ordovician or 
Proterozoic Z Nashoba Formation (Robinson and Gold- 
smith, this vol., chap. G). The Clinton-Newbury fault 
zone thus appears to coincide with a hinge line because 
littoral facies of the Merrimack sequence appear within 
the zone. The Merrimack belt and the Nashoba zone have 
been parts of a single terrane from at least early Paleo- 



STRUCTURAL AND METAMORPHIC HISTORY OF EASTERN MASSACHUSETTS 



H55 



zoic time if the apparent unconformity is correctly inter- 
preted. However, subsequent deformation and meta- 
morphism have clearly affected both terranes. Arguing 
against an early docking of the Nashoba zone and the 
east flank of the Merrimack belt is the steep metamor- 
phic gradient between them (Thompson and Norton, 
1968) and the apparent difference in type of early and 
middle Paleozoic plutonism (Wones and Goldsmith, this 
vol., chap. I), which suggests that the two terranes were 
not together until after the Devonian. The metamorphic 
and structural style of the Nashoba zone differs from that 
of the Merrimack belt. This difference is in part because 
the Nashoba zone appears to have undergone an earlier 
period of early Paleozoic or older metamorphism. Stau- 
rolite and andalusite porphyroblasts in the Merrimack- 
belt rocks and in the Tadmuck Brook Schist of the 
Nashoba zone were formed in a primarily static thermal 
environment, either at the time of intrusion of the 
Silurian and Devonian granites of the Merrimack belt or 
during the Permian. However, some of the Ayer Granite 
and rocks of the Fitchburg Complex, as well as the 
Canterbury Gneiss of Connecticut, have a fabric suggest- 
ing that they were emplaced during a period of dynamo- 
thermal metamorphism that is the Late Devonian meta- 
morphism indicated on the State bedrock map. This 
event (fig. 15E) probably culminated in the orogenic 
events that preceded and accompanied the deposition of 
the Pennsylvanian strata. The Middle Pennsylvanian 
Coal Mine Brook Formation at Worcester is metamor- 
phosed to garnet grade, so clearly a post-Pennsylvanian 
metamorphism affected the rocks of the eastern flank of 
the belt also (fig. 15G). Successive metamorphic over- 
prints may have smoothed out the steep gradient 
between the high-grade Nashoba zone and the low-grade 
sequence in the east flank of the Merrimack belt. Post- 
Pennsylvanian faulting (fig. 15G,H), however, has upset 
the preexisting pattern of isograds, bringing together 
rocks earlier metamorphosed at different levels. 

ZONE BOUNDARIES 

The two major zones, the Milford-Dedham and the 
Nashoba, were separate at least through the end of the 
Devonian and did not reach their present relative posi- 
tions until after the Permian events. The earliest possi- 
ble time for docking of the Milford-Dedham zone to the 
Nashoba zone would be after intrusion of the unique 
Silurian granites and diorites. The first bridge is the 
probably synchronous deposition of the Middle Pennsyl- 
vanian Coal Mine Brook Formation in the Merrimack 
belt to the west and the stratigraphically equivalent 
Rhode Island Formation in the Milford-Dedham zone to 
the east (fig. 15F). Subsequent late Paleozoic and early 



Mesozoic faulting and erosion have disrupted the Penn- 
sylvanian depositional blanket (fig. 15G,H). 

The major zones of dislocation have been active over 
an appreciable period of time and under different 
regional stress systems. Castle and others (1976) and 
Nelson (1976) indicated evidence for at least pre- 
Devonian deformation along the Bloody Bluff fault zone, 
and there is clear evidence for Permian movement from 
both field and isotopic work (O'Hara and Gromet, 1983). 
In addition, Mesozoic faulting is present in the Bloody 
Bluff fault zone in the Middleton area and apparently also 
in the Worcester-Clinton area on the east flank of the 
Merrimack belt. In places, low-angle thrusting and later 
high-angle faulting under ductile conditions have been 
succeeded by high-angle tensional normal faulting. We 
have then a reactivation of zones of crustal weakness 
over a period from possibly early Paleozoic to Mesozoic 
under differing conditions of regional stress. 

In the Penobscot Bay region of Maine, terranes similar 
to those in eastern Massachusetts have been separated 
by faults, one of which is dated as pre-Middle Devonian 
because it was cut by a pluton of that age (Stewart and 
Wones, 1974). Other faults in this region are younger 
(Wones and Thompson, 1979). Farther north in New 
Brunswick and Newfoundland, the major faults range 
from early Carboniferous to post-Pennsylvanian (Zen, 
1983, and references therein). In Massachusetts we lack 
the evidence of plutonic intrusion across faults to delimit 
the time of faulting. 

The major fault zones of eastern Massachusetts have 
been interpreted as thrust and reverse faults (Harwood 
and Zietz, 1976; Barosh and others, 1977; Skehan and 
Murray, 1980a) and as possible parts of a strike-slip fault 
system of regional extent on the basis of paleomagnetic 
data (Irving, 1977; Kent and Opdyke, 1978, 1979, 1980; 
Brown, 1980). The descriptions of the fault zones indicate 
that the ductile zones tend to be earlier and flatter and 
that the brittle faults tend to be later and steeper, so that 
possibly both views are correct. The attitudes of the 
ductile fault surfaces vary from place to place, however, 
although the zones appear to be relatively linear. The 
Bloody Bluff fault has both steeply and shallowly dipping 
segments. The Lake Char fault is typically shallow 
dipping. Such variations are most likely due to subse- 
quent folding. The Rattlesnake Hill fault of Skehan 
(1968), which seems to be the principal fault separating 
the Nashoba zone from the Merrimack belt (Castle and 
others, 1976; Gore, 1976b; Skehan and Murray, 1980b), is 
relatively steep and is younger than the flatter ductile 
mylonite zones like the thrusts in the Clinton-Wachusett 
Reservoir area or parts of the Bloody Bluff. Several 
proposals have been made for the direction of movement 
on the flatter faults. The Lake Char fault (fig. 3) of 
Connecticut has been proposed to be a thrust bringing 



H56 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



the higher grade Putnam Group over the Proterozoic Z 
basement (Dixon and Lundgren, 1968; Wintsch, 1979) 
and a decollement or "normal" fault in which the Putnam 
Group has slid off the Proterozoic Z basement (Lundgren 
and Ebblin, 1972; Goldstein, 1982; Danforth and Owens, 
1984). The decollement concept might be likened to the 
movement within the metamorphic core complexes of the 
Great Basin in the Western United States (Coney, 1980). 
By analogy, one would expect the ductile part of the 
Bloody Bluff fault to have a sense of movement similar to 
that of the Lake Char, although O'Hara and Gromet 
(1984) believed these to be two separate faults in north- 
ern Rhode Island. However, the normal fault movement 
conflicts with the southeast vergence of folds and thrusts 
seen in the Nashoba and Putnam terranes and in the 
New London anticlinorium (fig. 3) of southeastern Con- 
necticut (Goldsmith, 1985). The normal sense of move- 
ment thus is probably only a late phenomenon. 

The Permian pattern of deformation (fig. 15G) can 
probably be tied to a single stress field. It could be 
argued that the east-northeast pattern of compressive 
Permian deformation in the Milford-Dedham zone is the 
result of major northeast-trending right-lateral move- 
ment along the Bloody Bluff and Clinton-Newbury 
zones. Mosher (1983), Murray and Mosher (1984), and 
McMaster and others (1980) postulated large-scale right- 
lateral transcurrent movement in the Alleghanian to 
account for the structural configuration in the southern 
Narragansett basin. However, no clear evidence exists 
to prove significant strike-slip motion along the major 
faults of eastern Massachusetts, although the steep dips 
in places of some of the major fault surfaces are sugges- 
tive. The steep lineations in the Nashoba zone could be 
indicators of strike-slip movement, but they also could 
have been produced by intersection of later structural 
features of slightly different orientation on already 
steeply dipping rocks. Eberly (1984) commented on the 
paucity of faults showing strike-slip movement in a 
traverse across eastern Connecticut. Nevertheless, the 
paleomagnetic data of Kent and Opdyke (1978, 1979, 
1980) and of others indicate considerable lateral displace- 
ment of terranes in New England in the Late Pennsyl- 
vanian to Permian, and a number of reconstructions have 
been made to accommodate the data (Van der Voo, 1983; 
Le Fort, 1983, for example). On the other hand, recent 
articles such as that of Irving and Strong (1985) suggest 
caution in reconstructing plate positions from paleomag- 
netic data. 



ACCRETION 

The Nashoba and Milford-Dedham zones are exotic 
terranes that have been accreted to the North American 
craton during the Paleozoic. It is generally agreed that 



the Taconian suture marking the closing of the Iapetus 
Ocean and accretion of an island arc lies west of the 
Merrimack synclinorium (Stanley and Hatch, 1988) and 
that further collisional events occurred in Acadian time. 
The nature and time of amalgamation of the terranes east 
of the Taconian suture are less certain. Accreted ter- 
ranes in the Appalachian region, with specific reference 
to eastern Massachusetts, have been discussed recently 
by Williams and Hatcher (1983) and Zen (1983). From the 
evidence presented in this chapter, and in chapter E 
(Goldsmith, this vol.), it is possible that Nashoba-type 
rocks were basement for the Merrimack strata by at 
least Ordovician time and that the Milford-Dedham zone, 
or at least the gneissic part of it, was joined to the 
Nashoba zone by Carboniferous time; this join thus may 
mark the site of an Acadian suture. Crustal adjustments 
in the Permian and later in the Mesozoic disrupted this 
amalgamation and produced the arrangement we see 
now. Gneissic rocks similar to those in the Milford 
antiform reappear in the Willimantic dome in eastern 
Connecticut and the Pelham dome in central Massachu- 
setts. These gneissic rocks have apparently been 
involved in the Acadian orogeny that affected central 
Massachusetts, whereas there is little or no evidence of 
an Acadian orogeny in the eastern, nongneissic part of 
the Milford-Dedham zone. The gneissic terrane on the 
west may be considered to have underlain the Merrimack 
and Nashoba rocks at an early stage, certainly before the 
Pennsylvanian and apparently before th^ Late Devo- 
nian. The eastern, nongneissic part i the Milford- 
Dedham zone is most like the rocks of the Avalon 
Peninsula in Newfoundland. O'Hara and Gromet (1984) 
suggested from evidence in Rhode Island that the 
gneissic, western part of the Milford antiform is actually 
a separate block from that containing the Dedham and 
Esmond batholiths to the east, even though the two 
contain rocks of the same age. They suggested that the 
two parts were joined during the Alleghanian, whereas 
the gneissic rocks were deformed primarily earlier in the 
Acadian orogeny. This suggestion seems to resolve sev- 
eral problems in treatment of the Milford-Dedham zone 
as a single entity on the State bedrock map, particularly 
the discrepancy in metamorphic grade and style of 
deformation between the ductilely and brittlely de- 
formed terranes and the uncertainty of the timing of the 
amphibolite-facies metamorphism in the gneissic ter- 
rane. It permits the gneissic terrane, or at least that part 
beneath the Merrimack synclinorium, to have been 
involved in Acadian or earlier deformation without 
requiring Acadian deformation in the rest of the Milford- 
Dedham zone. However, if two terranes are present, the 
significance of the Clinton-Newbury fault is called into 
question. 



STRUCTURAL AND METAMORPHIC HISTORY OF EASTERN MASSACHUSETTS 



H57 



The two distinct boundaries in eastern Massachusetts 
on the surface are those between the Milford-Dedham 
and Nashoba zones and between the Nashoba zone and 
the Merrimack belt. The first boundary, the Bloody Bluff 
fault system in Massachusetts and the Lake Char and 
Honey Hill fault systems in Connecticut, may mark the 
site of a cryptic suture. Osberg (1978) suggested that the 
Paleozoic volcanic-plutonic assemblage (Newbury Volca- 
nic Complex and alkalic plutons) of eastern Massachu- 
setts and coastal Maine is part of an arc formed adjacent 
to a suture; this may be the cryptic Acadian suture of Le 
Fort (1983) between an African plate, in which he 
included eastern Massachusetts but not the Avalon Pen- 
insula of Newfoundland, and an Avalon prong to the 
northwest. Le Fort's Silurian and Devonian volcanic- 
plutonic arc represents an Andean- or Cordilleran-type 
margin developed on the African plate during the closing 
of the Theic Ocean on an east-dipping subduction zone. 
How the Nashoba zone fits into this scheme is not clear. 
I consider the Nashoba to be equivalent to part of the 
Gander zone of Newfoundland (Goldsmith, this vol., 
chap. F). There is no evidence of ocean-floor material in 
or near this suture zone unless one considers the serpen- 
tinite at Lynnfield or the Reubens Hill Formation on the 
east flank of the Merrimack belt as described by Peck 
(1975) to be such. 

The Milford-Dedham zone of eastern Massachusetts 
has been suggested to be formerly part of a microconti- 
nent lying between an African plate and the North 
American plate (Schenk, 1971; Skehan and others, 1978; 
Strong, 1979; Rast, 1980) and to have close affinity to 
rocks in northwest Africa (Hurley and others, 1974; 
Skehan and others, 1978; Olszewski, 1980; Simpson and 
others, 1980). The 730±26-Ma zircon age on the Fish 
Brook Gneiss and the 1,500-Ma age on detrital zircon 
from the Shawsheen Gneiss (Olszewski, 1980), both of 
the Nashoba zone, form a pattern that has a counterpart 
in northwest Africa. However, the original spatial rela- 
tion of the Nashoba to the Milford-Dedham zone is 
unknown. The two may have been originally from differ- 
ent environments on the same plate, as suggested by 
Skehan and Murray (1980b, p. 313), equivalent to the 
northern apron of the Avalon platform of Rast (1980), or 
they may have been parts of two different plates. On the 
other hand, following the model of O'Hara and Gromet 
(1984), perhaps the gneissic terrane of the Milford- 
Dedham zone has been attached to the Nashoba terrane 
longer than has the nongneissic terrane. 

If the Nashoba and Milford-Dedham zones were joined 
together and accreted to North America before the 
Pennsylvanian, then the Permian deformation and met- 
amorphism were produced not by collision of the Milford- 
Dedham zone with the terranes to the west but by 
movement and rotation of these terranes along preexist- 



ing sites of juncture or other zones of weakness as a 
result of intraplate adjustments or impact of another 
plate or microplate that lay south or east of the already 
accreted terranes. This zone of Permian movement is the 
Variscan orogenic belt of Rast and Skehan (1984). The 
rotation in a clockwise direction of Africa (the Milford- 
Dedham zone) against the Nashoba (and Putnam) zone to 
the west in the Permian proposed by Wintsch and Le 
Fort (1983) does not necessarily mean the closing of a 
suture but could reflect continued or renewed movement 
along a zone in which the suture had already formed. The 
Clinton-Newbury fault zone can easily be construed as an 
intraplate movement zone of this sort. The collision and 
slip hypothesis developed to explain the features in the 
southern Narragansett basin (McMaster and others, 
1980; Mosher, 1983; Murray and Mosher, 1984) seems 
reasonable. Broad-wavelength magnetic anomalies 
(Zietz and others, 1980) and regional gravity patterns 
(Haworth and others, 1980) suggest an east-northeast- 
trending zone of discordance between crustal domains 
south of the New England coast, which may be a 
complementary part of the regional northeast strain 
pattern. Rast and Skehan (1984) using other arguments 
also speculated on a dislocation in Long Island Sound. 
Whether this is an interplate boundary or intraplate 
boundary is unknown. 

Although the sequence of events in eastern Massachu- 
setts is not entirely clear, it appears that the accretion- 
ary sequence is Nashoba-type basement beneath the 
Merrimack belt, Nashoba zone east of the Merrimack 
belt, and Milford-Dedham zone. The Nashoba-type rocks 
and the gneissic Milford-Dedham rocks appear to have 
been present beneath the Merrimack belt at the time of 
Acadian deformation and thus correspond perhaps to the 
craton X basement of Zen (1983). Most of the exposed 
Nashoba, east of the Merrimack belt, and the non- 
gneissic Milford-Dedham rocks show no evidence of 
Acadian deformation, either because they were in place 
but far removed from the center of Acadian orogeny, as 
Zen suggested, or because they were moved into place 
later. The rotation and subgreenschist-facies metamor- 
phism of the Newbury Volcanic Complex might be 
construed as Acadian, but there is no evidence for 
metamorphism of this age in the adjacent structural 
blocks, whereas there is for Alleghanian deformation. 
The sequence of events discussed above does fit in a 
general way the west-to-east, oldest-to-youngest accre- 
tionary model of Williams and Hatcher (1983). 

The arrangement of terranes resembling the Nashoba 
and Milford-Dedham zones along the coast of New Eng- 
land and the Maritime Provinces of Canada indicates that 
they are now wedges or splinters of once more continu- 
ous terranes extending to Europe and Africa (see, for 
example, Rast and others, 1976). Reconstructing the 



H58 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



whole requires careful consideration of the details of 
stratigraphy and structure in each area and careful 
correlation of these details between areas. 



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76-0337, 32 p. 

Williams, Harold, and Hatcher, R.D., Jr., 1983, Appalachian suspect 
terranes, in Hatcher, R.D., Jr., Williams, Harold, and Zietz, 
Isidore, eds., Contributions to the tectonics and geophysics of 
mountain chains: Geological Society of America Memoir 158, p. 
33-53. 

Williams, J.R., and Willey, R.E., 1973, Bedrock topography and 
texture of unconsolidated deposits, Taunton River Basin, south- 
eastern Massachusetts: U.S. Geological Survey Miscellaneous 
Investigations Map 1-742, scale 1:48,000. 

Williams, J.R., Willey, R.E., and Tasker, G.D., 1975, Hydrologic data 
of the coastal drainage basins of southeastern Massachusetts Weir 
River, Kingston: U.S. Geological Survey Massachusetts 
Hydrologic-Data Report 16, 63 p. 

Wintsch, R.P., 1979, The Willimantic fault: a ductile fault in eastern 
Connecticut: American Journal of Science, v. 279, p. 367-393. 

Wintsch, R.P., and Hudson, M.R., 1978, Southeastward thrusting in 
eastern Connecticut [abs.]: Geological Society of America 
Abstracts with Programs, v. 10, no. 2, p. 91. 

Wintsch, R.P., and Le Fort, Jean-Pierre, 1983, A temperature- 
time-strain path as a memory of late Hercynian intraplate defor- 
mation [abs.]: Geological Society of America Abstracts with Pro- 
grams, v. 15, no. 3, p. 196. 

Wones, D.R., and Thompson, Woodrow, 1979, The Norumbega fault 
zone: a major regional structure in central eastern Maine [abs.]: 
Geological Society of America Abstracts with Programs, v. 11, no. 
1, p. 60. 

Zartman, R.E., Hurley, P.M., Krueger, H.W., and Giletti, B.J., 1970, 
A Permian disturbance of K-Ar radiometric ages in New Eng- 
land—its occurrence and cause: Geological Society of America 
Bulletin, v. 81, p. 3359-3373. 

Zartman, R.E., and Marvin, R.F., 1971, Radiometric age (Late 
Ordovician) of the Quincy, Cape Ann, and Peabody granites from 
eastern Massachusetts: Geological Society of America Bulletin, v. 
82, p. 937-958. 

Zartman, R.E., Pease, M.H., Jr., and Hermes, O.D., 1983, Encroach- 
ment of the late Paleozoic Variscan Front in southern New 
England [abs.]: Geological Society of America Abstracts with 
Programs, v. 15, p. 147. 

Zen, E-an, 1983, Exotic terranes in the New England Appalachians- 
Limits, candidates, and ages: A speculative essay, in Hatcher, 
R.D., Jr., Williams, Harold, and Zietz, Isidore, eds., Contribu- 
tions to the tectonics and geophysics of mountain chains: Geological 
Society of America Memoir 158, p. 55-81. 

Zen, E-an, editor, and Goldsmith, Richard, Ratcliffe, N.M., Robinson, 
Peter, and Stanley, R.S., compilers, 1983, Bedrock geologic map 
of Massachusetts: Reston, Va., U.S. Geological Survey, 3 sheets, 
scale 1:250,000. 

Zen, E-an, and Palmer, A.R., 1981, Did Avalonia form the eastern 
shore of Iapetus Ocean? [abs.]: Geological Society of America 
Abstracts with Programs, v. 13, no. 7, p. 587. 

Zietz, Isidore, Haworth, R.T., Williams, Harold, and Daniels, D.L., 
1980, Magnetic anomaly map of the Appalachian orogen: Memorial 
University of Newfoundland Map 2a, scale 1:1,000,000. 



Intrusive Rocks of 
Eastern Massachusetts 



By DAVID R. WONES, Virginia Polytfchnic Institute and State University, 
and RICHARD GOLDSMITH, U.S. Geological Survey 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1366-1 



CONTENTS 



Abstract II 

Introduction 1 

Intrusive rocks of the Milford-Dedham zone 2 

Proterozoic Z mafic plutonic rocks 2 

Diorite and gabbro (Zdigb) 8 

Gabbro (Zgb) 10 

Diorite (Zdi) 10 

Diorite at Rowley (Zrdi) 10 

Sharon Syenite (Zssy) 11 

Cumberlandite (cu) 12 

Serpentinite (u) 12 

Discussion of the mafic plutonic rocks 12 

Proterozoic Z batholithic rocks 12 

Dedham batholith 12 

Dedham Granite (Zdgr, Zdngr) 12 

Topsfield Granodiorite (Ztgd) 15 

Esmond Granite (Zegr) 17 

Grant Mills Granodiorite (Zgmgd) 17 

Westwood Granite (Zwgr) 22 

Fine-grained granite (fgr) 23 

Granite of the Fall River pluton (Zfgr) 23 

Porphyritic granite (Zpgr) 23 

Alaskite (Zagr) 25 

Granite, gneiss, and schist, undivided (Zgg) 26 

Plutonic rocks of the Milford antiform 29 

Hope Valley Alaskite Gneiss (Zhg) 29 

Scituate Granite Gneiss (Zsg) 29 

Ponaganset Gneiss (Zpg) 30 

Milford Granite (Zmgr, Zmgd) 31 

Biotite granite (Zgr) 33 

Paleozoic intrusi ve rocks 33 

Nahant Gabbro (Ongb) 33 

Quiney Granite (SOqgr) 36 

Blue Hills Granite Porphyry (SObgr) 38 

Cape Ann Complex 38 

Alkalic granite and quartz syenite (SOcgr) 38 

Beverly Syenite (SOcb) 40 

Squam Granite (SOcsm) 40 

Micrographic rhyolite of the Newbury Volcanic 

Complex (DSnr) 40 



Intrusive rocks of the Milford-Dedham zone— Continued 
Paleozoic intrusive rocks— Continued 

Alkalic granite in Franklin (DOgr) 140 

Peabody Granite (Dpgr) 40 

Wenham Monzonite (Dwm) 41 

Cherry Hill Granite (Dcygr) 41 

Granite of the Rattlesnake Hill pluton (Drgr) 41 

Mesozoic intrusive and silicified rocks 41 

Diabase dikes (Jd) 41 

Massive quartz and silicified rock (q) 42 

Discussion of the intrusive rocks of the Milford-Dedham 

zone 42 

Relation to extrusive rocks 44 

Interpretation of igneous events 45 

Intrusive rocks of the Nashoba zone 47 

Andover Granite (SOagr) 47 

Sharpners Pond Diorite (Ssqd) 48 

Straw Hollow Diorite and Assabet Quartz Diorite (Ssaqd)... 48 

Granodiorite of the Indian Head pluton (igd) 48 

Orange-pink rusty- weathering granite (Sgr) 49 

Light-gray muscovite granite (mgr) 49 

Discussion of the intrusi ve rocks of the Nashoba zone 49 

Intrusive rocks of the eastern part of the Merrimack belt 50 

Newburyport Complex 52 

Tonalite and granodiorite (SOngd) 52 

Granite (Sngr) 52 

Ayer Granite (Sagr) 52 

Clinton fades (Sacgr) 52 

Devens-Long Pond facies (SOad) 53 

Diorite and tonalite (DSdi) 53 

Chelmsford Granite (Dcgr) 53 

Muscovite-biotite granite at Millstone Hill (Dmgr) 53 

Fitchburg Complex (Dfgr) 54 

Discussion of the intrusive rocks of the eastern part of the 

Merrimack belt 54 

Intrusive rocks of eastern Massachusetts and plate-tectonic 

models 55 

Regional relations 56 

References cited 57 



ILLUSTRATIONS 



Figure 1. Map showing distribution of intrusive rocks of eastern Massachusetts 13 

2. Con-elation diagram of intrusive rocks and selected sedimentary and volcanic rocks and their metamorphic equivalents in 

eastern Massachusetts 4 

3. Map showing distribution of Proterozoic Z batholithic rocks in the Milford-Dedham zone 8 



IV 



CONTENTS 



I 4. Map showing distribution of Proterozoic Z mafic plutonic and volcanic rocks in the Milford-Dedham zone 19 

5. Quartz-plagioclase-K-feldspar-mafic minerals diagram of modes of Proterozoic Z intrusive rocks of the Milford-Dedham 

zone 13 

6. Plot showing CaO and Na,0+K 2 against SiO, for Proterozoic Z intrusive rocks of the Milford-Dedham zone 14 

7. Ternary plot of normative albite, anorthite, and orthoclase for Proterozoic Z intrusive rocks of the Milford-Dedham 

zone 15 

8. Ternary alkalis-FeO-MgO plot of Proterozoic Z intrusive rocks of the Milford-Dedham zone 15 

9. Chondrite-normalized plot of rare-earth elements in Proterozoic Z mafic plutonic rocks of the Milford-Dedham zone 17 

10. Plot of thorium against uranium for Proterozoic Z intrusive rocks of the Milford-Dedham zone 17 

11. Map showing distribution of Proterozoic Z batholithic rocks in the Milford-Dedham zone 18 

12-14. Quartz-plagioclase-K-feldspar-mafie minerals diagram of modes of: 

12. Dedham Granite and Topsfield Granodiorite 19 

13. Westwood Granite; Westwood Granite in Plymouth quarry , Weymouth; and Milford Granite 20 

14. Fall River pluton including the porphyritic granite 21 

15. Chondrite-normalized plot of rare-earth elements in Proterozoic Z batholithic rocks, northeastern Massachusetts: 

A, Dedham Granite; B, Dedham Granite north of Boston, Westwood and Esmond Granites, and Topsfield 

Granodiorite 30 

16. Chondrite-normalized plot of rare-earth elements of Proterozoic Z batholithic rocks in the Milford-Dedham zone, 

southeastern Massachusetts 32 

17. Map showing locations of drill holes encountering bedrock in Cape Cod and the adjacent mainland 33 

18. Chondrite-normalized plot of rare-earth elements of Proterozoic Z batholithic rocks, Milford antiform 34 

19. Map showing distribution of Paleozoic intrusive rocks in the Milford-Dedham zone 35 

20. Geologic map and cross section for Andrews Point, Cape Ann 39 

21. Ternary quartz-plagioclase-K-feldspar plot of modal composition of Dedham Granite as compared with fields of 

composition of Dedham Granite of Dowse (1949) and Lyons (1969), Milford Granite, and Westwood Granite 42 

22. Plot of a part of the ternary system Na 2 0-K 2 0-Al 2 03 in molecular percent for intrusive rocks of the Milford-Dedham 

zone 43 

23. Ternary plot of normative albite, anorthite, and orthoclase for Paleozoic intrusive rocks of the Milford-Dedham zone 43 

24. Ternary alkalis-FeO-MgO plot of Paleozoic intrusive rocks of the Milford-Dedham zone showing field of Proterozoic Z 

intrusive rocks of the Milford-Dedham zone 43 

25. Chondrite-normalized plot of rare-earth elements of Paleozoic intrusive rocks 44 

26. Map showing distribution of intrusive rocks in the Nashoba zone 46 

27. Map showing distribution of intrusive rocks in the eastern part of the Merrimack belt 51 



TABLES 



Table 1. Ages and age relations of intrusive rocks of the Milford-Dedham zone 17 

2. Modes of representative plutonic rocks of the Proterozoic Z mafic volcanic-plutonic complex, Milford-Dedham zone 10 

3. Major-oxide, normative-mineral, and trace-element compositions of plutonic rocks of the Proterozoic Z mafic volcanic-plutonic 

complex 11 

4. Major constituents determined by point-count of stained slabs of intrusive rocks of the Milford-Dedham zone 16 

5. Modes of plutonic rocks of the Dedham batholith in northeastern Massachusetts 22 

6. Major-oxide, normative-mineral, and trace-element compositions of Proterozoic Z intrusive rocks, Milford-Dedham zone, 

northeastern Massachusetts 24 

7. Modes of plutonic rocks of the Dedham batholith, southeastern Massachusetts 25 

8. Major-oxide, normative-mineral, and trace-element compositions of Proterozoic Z intrusive rocks, southeastern 

Massachusetts 26 

9. Modes of plutonic rocks of the Milford antiform 27 

10. Major-oxide, normative-mineral, and trace-element compositions of Proterozoic Z intrusive rocks in the Milford antiform 28 

11. Modes of some Paleozoic intrusive rocks of the Milford-Dedham zone 36 

12. Major-oxide, normative-mineral, and trace-element compositions of some Paleozoic plutonic rocks, Milford-Dedham zone 37 

13. Ages and age relations of intrusive rocks of the Nashoba zone and the eastern part of the Merrimack belt 47 

14. Modes of intrusive rocks of the Nashoba zone 47 

15. Major-oxide, normative-mineral, and trace-element compositions of granodiorite of the Indian Head pluton 49 

16. Modes of intrusive rocks of the Merrimack belt east of the Wekepeke fault 52 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 



By David R. Wones 1,2 and Richard Goldsmith 3 



ABSTRACT 

The intrusive rocks of eastern Massachusetts east of the Wekepeke 
fault form three distinctly different assemblages coinciding with the 
three lithotectonic terranes that form eastern Massachusetts: the 
Milford-Dedham zone, the Nashoba zone, and the eastern part of the 
Merrimack belt. In the Milford-Dedham zone, a largely mafic volcanie- 
plutonic complex of Proterozoic Z age is intruded by 630-Ma batholiths 
of calc-alkaline granite to granodiorite. These batholithic rocks occupy 
two regions. To the east, the Dedham-Fall River batholith consists of 
partly altered but mostly nongneissic granites and granodiorites; to 
the southwest, the Milford antiform consists of gneissic to partly 
gneissic granites and granodiorites. The batholithic rocks in the two 
regions are similar in age and chemistry. Slightly younger granite is 
associated with a Proterozoic Z rifting(?) event that produced a 
bimodal volcanic suite in the area of the Boston basin. During the early 
and middle Paleozoic, the Proterozoic Z rocks were intruded by 
discrete alkaline and peralkaline plutons ranging from gabbro to 
granite, which lie primarily in a northeast-trending belt northwest of 
the Narragansett basin. No late Paleozoic intrusive rocks are known in 
the Milford-Dedham zone in Massachusetts, although they are present 
in southern Rhode Island. Mesozoic intrusive rocks are confined to 
mafic dikes associated with early Mesozoic rifting. 

Intrusive rocks of the Nashoba zone are all Paleozoic; they range 
from the peraluminous, S-type, partly gneissic Andover Granite of 
Late Ordovician to Silurian age but including some rock of Devonian 
age, to postmetamorphic I-type quartz-diorite and to granite of Silurian 
age. Since the bedrock map of Massachusetts (Zen and others, 1983) 
was prepared, the latter granite has been found to include rock of 
Devonian age. The Silurian plutonic rocks contrast in composition with 
the Andover Granite and with the Paleozoic rocks of the Milford- 
Dedham zone and are I-type granite associated with volcanic arcs. Data 
on the shallow intrusions of the nearby Late Silurian and Early 
Devonian Newbury Volcanic Complex are insufficient to determine 
whether they are chemically related to the Silurian plutons of the 
Nashoba zone. Some of the granite that has recently been determined 
to be Devonian in age is peraluminous, like the older Late Ordovician 
and Silurian Andover Granite. These changes in composition with time 
imply a changing crustal source for the material. 

The intrusive rocks of the eastern part of the Merrimack belt 
constitute a different assemblage. These rocks are primarily calc- 



Manuscript approved for publication November 16, 1987. 

'Deceased. 

2 Virginia Polytechnic Institute and State University, Blacksburg, Va. 

3 U.S. Geological Survey. 



alkaline granodiorite and granite of Silurian to Ordovician age and 
include small masses of diorite and, in one pluton, norite. Some of these 
rocks, such as parts of the Ayer Granite, are gneissic and appear to 
have been deformed during Acadian metamorphism and intrusion. 
Younger muscovite-bearing granite of Devonian age forms small 
aligned plutons that are probably satellitic to the larger Fitchburg 
Complex west of the area but that extend into Connecticut as the 
Canterbury Gneiss. The Ordovician and Silurian Newburyport Com- 
plex bears mineralogical resemblance to some of the Silurian intrusions 
of the Nashoba zone. Recently determined ages of the intrusions in the 
eastern part of the Merrimack belt indicate that some or all of the 
strata in this part of the Merrimack belt are somewhat older than 
shown on the State bedrock map. 

The differences in the compositions, styles of emplacement, and 
times of intrusion of the intrusive rocks of the three lithotectonic belts 
of eastern Massachusetts are a basis for attempting to establish a 
history of accretion of crustal blocks during the Paleozoic. The Milford- 
Dedham zone was a cratonic element during the Paleozoic and was not 
joined to the Nashoba zone until after intrusion of the Devonian 
granites. The Nashoba zone could have been close to the east flank of 
the Merrimack belt somewhat earlier. The two seem to share a 
Devonian magmatic event and possibly an earlier one as well. The three 
belts clearly were assembled by the Pennsylvanian, although their 
present boundaries involve Permian events. 

The Proterozoic Z rocks of the Milford-Dedham zone consist of many 
more intrusive rocks and many fewer supracrustal rocks than the 
Avalon terrane of Newfoundland. The Proterozoic Z rocks of the zone 
bear some resemblance to rocks in the Charlotte belt of North and 
South Carolina in both age and volume of intrusive rocks, although the 
intrusive rocks in the Charlotte belt are more mafic. The Ordovician 
and Silurian intrusive rocks of the Nashoba zone resemble intrusions in 
the lithotectonically equivalent Passagassawakeag terrane in south- 
eastern Maine. The intrusive rocks of the east flank of the Merrimack 
belt in Massachusetts are in part older and are more deformed and 
metamorphosed than the intrusive rocks in the Merrimack belt to the 
north in Maine. The Ordovician to Silurian rocks of the east flank of the 
Merrimack belt in Massachusetts do not have obvious counterparts 
along strike to the north beyond New Hampshire. The Devonian 
intrusions of the Merrimack belt in Massachusetts are compositionally 
similar to those in eastern Maine but were generally more affected by 
Acadian dynamothermal metamorphism than were the mostly high- 
level, postmetamorphic plutons of northeastern New England. 

INTRODUCTION 

Eastern Massachusetts is divided into three lithotec- 
tonic terranes: a part of the Merrimack belt east of the 



ii 



12 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



Wekepeke fault and the Fitchburg Complex and north- 
west of the Clinton-Newbury fault; the Nashoba zone, 
between the Clinton-Newbury fault and the Bloody Bluff 
fault; and the Milford-Dedham zone, southeast of the 
Bloody Bluff fault (fig. 1). These three terranes have 
distinctive strata and distinct intrusive, deformational, 
and metamorphic histories (Zen and others, 1983; Hatch 
and others, 1984). Fossils are present in the supracrustal 
rocks of the Milford-Dedham zone, but they are found 
only in the Pennsylvanian strata at Worcester in the 
eastern part of the Merrimack belt and are lacking in the 
Nashoba zone. Therefore, lithologic correlations within 
each terrane have been made in varying degrees on the 
basis of comparisons of the strata alone (Goldsmith, this 
vol., chaps. E and F; Robinson and Goldsmith, this vol., 
chap. G). The oldest known rocks are in the Milford- 
Dedham terrane. The stratified rocks of this zone consist 
of metasedimentary, metavolcanic, and metaplutonic 
rocks of probable Proterozoic Z age (fig. 2). These are 
intruded by calc-alkaline batholithic rocks of Proterozoic 
Z age, which form a basement for weakly metamor- 
phosed to unmetamorphosed sedimentary and volcanic 
rocks ranging in age from latest Proterozoic Z and 
Cambrian to Pennsylvanian. 

The strata of the Nashoba zone are high-metamorphic- 
grade pelitic schists and gneisses, calc-silicate rocks, 
minor marble, feldspathic gneisses, and amphibolite. The 
protoliths of some of these rocks contained a large 
proportion of mafic volcanic or volcaniclastic material. 
Their high grade of metamorphism contrasts sharply 
with the lower grade strata in the Milford-Dedham zone 
on the east and in the east part of the Merrimack belt on 
the west, indicating that the faults bounding these 
terranes are of significant magnitude. The age of these 
rocks is somewhat uncertain. On the geologic bedrock 
map of Massachusetts (Zen and others, 1983; hereafter 
referred to as the State bedrock map), they are shown as 
Ordovician or Proterozoic Z. 

The strata in the eastern part of the Merrimack belt 
are primarily a low- to middle-metamorphic-grade tur- 
bidite sequence of calcareous metasiltstone and phyllite 
containing minor quartzite and rare conglomerate beds 
near the base. These rocks, equivalent to the Berwick, 
Eliot, and Kittery Formations of the Merrimack Group of 
southeastern New Hampshire and the Oakdale Forma- 
tion of Massachusetts, are of uncertain age. On the State 
bedrock map, they were considered to be of Silurian age 
and were correlated with the Paxton Formation of 
central Massachusetts, but radiometric dating of intru- 
sive rocks in Massachusetts (Zartman and Marvin, this 
vol. , chap. J) and recent radiometric dating and mapping 
in southern New Hampshire (Lyons and others, 1982; 
Bothner and others, 1984) have indicated that these 



strata may be much older, and they may belong to a 
separate terrane. 

The intrusive rocks of the three terranes (fig. 1) also 
have histories and compositions that are quite distinct 
from each other. In this report, we describe the intrusive 
rocks of each terrane and explain the reasons behind the 
particular groupings and splits that were used on the 
State bedrock map. We also offer an interpretation of the 
thermal history of eastern Massachusetts. This analysis 
depends greatly on the radiometric determinations dis- 
cussed and summarized by Zartman and Naylor (1984), 
Zartman and Marvin (this vol., chap. J), and Hermes and 
Zartman (1985). We have been greatly aided in the 
preparation of this chapter by reviews by 0. Don Her- 
mes, J. Christopher Hepburn, G. William Leo, and 
David B. Stewart. Because of the death of the senior 
author during preparation of this manuscript, material is 
included in it that was intended for a future paper 
developing more fully some ideas and concepts regarding 
the intrusive rocks of eastern Massachusetts. Thus, 
these ideas and concepts as expressed in the present 
chapter may be somewhat fragmentary, but the data are 
presented for others to use as they see fit. 

INTRUSIVE ROCKS OF THE MILFORD-DEDHAM 
ZONE 

The intrusive rocks of the Milford-Dedham zone (table 
1) range in age from Proterozoic Z to Jurassic, but they 
form four major groups. The first is a suite of Proterozoic 
Z rocks consisting of gabbro, diorite, and syenite (Sharon 
Syenite) that form the plutonic part of a largely mafic 
volcanic-plutonic complex, the volcanic part of which is 
described in chapter E of this volume (Goldsmith, this 
vol.). The second consists of calc-alkaline granitic to 
granodioritic rocks of Proterozoic Z age that form two 
batholithic masses intruding the mafic plutonic rocks and 
older metasedimentary and metavolcanic rocks. The two 
batholithic masses are the Dedham batholith, in which 
we include the granite of the Fall River pluton and 
associated rocks in southeastern Massachusetts, and the 
plutonic rocks of the Milford antiform (fig. 3). The third 
major group consists of plutons of alkalic granite and 
gabbro of Ordovician to Devonian age. The fourth group 
consists of dikes of Mesozoic age. 

PROTEROZOIC Z MAFIC PLUTONIC ROCKS 

The Proterozoic Z mafic plutonic rocks occupy a large 
area in northeastern Massachusetts (fig. 4) where there 
are also numerous masses of metavolcanic rocks. South 
and west of Boston, the mafic plutonic rocks occupy 
scattered roof pendants and enclaves in the granitic 



INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 



13 




Figure 1. — Distribution of intrusive rocks of eastern Massachusetts. 



14 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



MERRIMACK BELT 



Sedimentary and 

volcanic rocks and 

their metamorphic 

equivalents 

Worcester 
basin 



Intrusive rocks 



■H 



SOngd 



Sp 


So 


Sb 




Se 


St 


SOvh 


SOk 


SObo 


SOrh 






1 



NASHOBA ZONE 



-■Sacgr 
SOad 



Sedimentary and 

volcanic rocks and 

their metamorphic 

equivalents 




SZtb 



* Fossil-bearing unit 

Figure 2. —Correlation of intrusive rocks and selected sedimentary and volcanic rocks and their 
metamorphosed equivalents in eastern Massachusetts; adapted from Zen and others (1983). 



INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 



[5 



MILFORD-DEDHAM ZONE 



Sedimentary and 

volcanic rocks and 

their metamorphic 

equivalents 



Narragansett 
basin 



Newbury 
basin 



pSni 
DSn 



Boston 
basin 



H S 



Dpgr 


Drgr 


Dwm 


Dcygr 



SOcsm 
SOcgr 



- — ' o o o § 



H H 




Lower Jurassic 



Pennsylvanian 



Cambrian 



Cambrian to 
Proterozoic Z 



Proterozoic Z 



Figure 2.— Continued. 



[6 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



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INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 



17 



Table 1 . — Ages and age relations of intrusive rocks of the Milford-Dedham zone 

[Radiometric age data from Zartman and Marvin, this vol., chap. J, table 1. Ages are interpreted to be within 5 percent of primary age of the intrusion except as noted. 

bio, biotite; hbl, hornblende; rieb, riebeckite; wr, whole rock. — , no data] 



Map unit 


Name 


Intrudes 


Age (Ma), method 


Mesozoic and younger rocks 


Jd 
u 


Medford Diabase 
Serpentinite 


- 


194±6, K-Ar/bio. 


Paleozoic rocks 



Dpgr 


Peabody Granite 


Zdigb; Zv, metamorphosed mafic to 
felsic volcanic rocks. 


Dwm 


Wenham Monzonite 


Zdigb, Zv 


Drgr 


Granite of the Rattlesnake Hill pluton 


Zdgr 


DOgr 


Alkalic granite in Franklin 


— 


SOcgr, 


Cape Ann Complex 


Zdigb, Zv 


SOcb, 






SOcsm. 






SOqgr 


Quincy Granite 


Cbw, Braintree and Weymouth 
Formations. 


SObgr 


Blue Hills Granite Porphyry 


- 


Ongb 


Nahant Gabbro and gabbro at Salem 


€bw 



Neck. 



359±24, Rb-Sr/wr, 395±20, U-Pb/ 

zircon. 
395±20, U-Pb/zircon. 
382±14, K-Ar/rieb. 

426±6, Rb-Sr/wr, 450±25, U- 
Pb/zircon. 

450±25, U-Pb/zircon, 466±14, K- 
AiVrieb. 

450±13, Rb-Sr/bio, 483±21, K-Ar/bio. 



Batholithic plutonic rocks of Proterozoic Z age 



Zdgr 


Dedham Granite 


Zdngr 


Dedham Granite north of Boston 


Ztgd 


Topsfield Granodiorite 


Zwgr 


Westwood Granite 


Zegr 


Esmond Granite 


Zgmgd 


Grant Mills Granodiorite 


fgr 


Fine-grained granite 


Zfgr 


Granite of the Fall River pluton 


Zpgr 


Porphyritic granite 


Zagr 


Alaskite 



Zgg Granite, gneiss, and schist, undivided 

Zgr Biotite granite 

Zmgr Milford Granite 

Zmgd Milford Granite, mafic phase 

Zhg Hope Valley Alaskite Gneiss 

Zsg Scituate Granite Gneiss 3 

Zpg Ponaganset Gneiss 



Zdigb; Zv; Zdi; Zb, Blackstone Group; 

Zw, Westboro Formation. 
Zdi, Zgb, Zdigb, Zv, Zw 
Zdigb, Zrdi, Zv 
Zdgr 
Zb 
Zb 

Zgs, gneiss and schist near New Bed- 
ford. 

Zgs 

Zgs; Zgn, biotite gneiss near New Bed- 
ford. 

Zdi; Zgb; Zw; Zvf, metamorphosed fel- 
sic volcanic rocks. 
Zb 
Zb 
Zsg; Zp, Plainfield Formation 

Zb 



630±15, U-Pb/zircon. 

595±17, Rb-Sr/wr, 640±14,K-AWhbl. 

579+28, Rb-Sr/wr. 1 
621±8, U-Pb/zircon. 2 

630±15, U-Pb/zircon. 



630 ±15, U-Pb/zircon. 

630±15, U-Pb/zircon, 601+5, U-Pb/ 

zircon. 2 
630±15, U-Pb/zircon. 



Proterozoic Z mafic plutonic rocks 



Zrdi 
Zdi 
Zdigb 
Zgb 

Zssy 



Diorite at Rowley 

Diorite 

Diorite and gabbro 

Gabbro 

Sharon Syenite 

Cumberlandite 



Zv 



656±16, K-Ar/hbl. 



interpreted to be within 10 percent of primary age of intrusion. 

Radiometric data from Hermes and Zartman (1985). 

"The Scituate Granite Gneiss of the State bedrock map, not the Devonian Scituate Granite. 



batholithic rocks. It is possible that not all the rocks 
included in this category on the State bedrock map 
(Zdigb, Zgb, Zdi, Zssy) are of Proterozoic Z age. Some 



gabbros, for example, may be found to be of Ordovician 
or Devonian age and may be associated with the alkalic 
plutons. However, contact relations for many bodies 



18 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 




10 



20 



30 MILES 
_J 



i 1 — — i n 

10 20 30 KILOMETERS 
EXPLANATION 
f '. - - '- '.'■ Pennsylvanian to Proterozoic Z cover rocks 

■;■:■;■;•:•] Mattapan and Lynn Volcanic Complexes 

Proterozoic Z batholitic rocks 
t;N\>V;i Nongneissic rocks 



,/ *77\ Gneissic rocks 



Proterozoic Z metamorphic and mafic plutonic 
rocks and granites of Paleozoic plutons 

- Contact 

- Indefinite boundary of New Bedford 

gneissic terrane 



Figure 3. — Distribution of Proterozoic Z batholithic rocks in the 
Milford-Dedham zone. 



indicate that the mafic rocks are older, possibly only 
slightly older, than the granite to granodiorite batho- 
liths. Modes for some of these rocks are given on table 2. 



DIORITE AND GABBRO (Zdigb) 

The "diorite and gabbro" (Zdigb) designation was used 
on the State bedrock map for masses of diorite and 
gabbro, separated by many screens of stratified, largely 
mafic, metavolcanic rocks, that cannot be distinguished 
separately on the scale of the map. These masses are 
mostly northwest and north of Boston. In addition, 
small-scale intrusions of granitic rock of Proterozoic Z 
age are scattered through the unit. All of the rocks 
formerly mapped as Newburyport Quartz Diorite and all 
of the rocks that earlier workers (Emerson, 1917; Toul- 
min, 1964; Dennen, 1981) assigned to the Salem Gabbro- 
Diorite, except the gabbros at Salem Neck, have been 
included in Zdigb. Also included are intrusive mafic dikes 
and sills and light-colored dikes that may be contempo- 
rary or younger. Probably most of the rock mapped as 
Zdigb is gabbro rather than diorite, as for example the 
rock labeled diorite (table 3, no. 4) by Emerson. The two 
diorites (table 2, nos. 1-2) have a quartz content in the 
gabbro range. The Sharon Syenite (table 3, no. 3) is 
actually closer to a monzonite than a syenite. 

The suite of mafic plutonic and metavolcanic rocks is 
well exposed in roadcuts along Route 2 in Arlington and 
Lexington (fig. 4). In Forest River Park, Salem, volcanic 
fabrics are preserved, and several ages of dikes are 
evident. These dikes may be equivalent to the Ordovician 
Nahant Gabbro, the Proterozoic Z Brighton Melaphyre, 
the diorite at Rowley, or the metavolcanic rocks. 

The urbanization of the region and the superposed 
Pleistocene glacial deposits, coupled with several periods 
of fracturing and alteration, make distinguishing these 
mafic rocks a formidable task. Dennen (1975, 1981) 
suggested that much of the diorite and gabbro in the 
Cape Ann area is coeval with the Cape Ann Complex, the 
gabbro at Salem Neck, and the Nahant Gabbro. How- 
ever, because much of the gabbro and diorite west of 
Cape Ann is intruded by the Dedham Granite and its 
equivalents in age, we have chosen to classify most of 
these rocks as Proterozoic Z. The 656-Ma age of the 
diorite at Rowley (Zrdi) (table 1) and the 630-Ma age 
established by Zartman and Nay lor (1984) for the gra- 
nitic rocks at Lexington and Saugus (Zdngr, Zdgr) that 
in this area intrude gabbro and diorite and amphibolite 
(Zdigb, Zv) indicate that much of the material called 
Salem Gabbro-Diorite by earlier workers is Proterozoic 
Z in age. Careful geologic mapping of individual outcrops 
at large scales, coupled with geochronologic studies, 
will be required to sort out the mafic rocks within this 
map designation. The wide variations in reported geo- 
chronologic ages may be true, even the suspicious K-Ar 
age of 886±22 Ma (Zartman and Marvin, this vol., chap. 
J, table 1) for hornblende from a gabbroic pegmatite in 
Lexington. 



INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 
30' 15' 71"00' 



I!) 




Figure 4. —Distribution of Proterozoic Z mafic plutonic and volcanic rocks in the Milford-Dedham zone. Zrdi, Diorite at Rowley; Zv, 
metamorphosed mafic to felsic volcanic rocks; Zdigb, diorite and gabbro; Zdi, diorite; Zgb, gabbro; Zssy, Sharon Syenite; Zbv, greenstone 
and amphibolite of the Blackstone Group. 



no 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



Table 2.— Modes, in percent, of representative plutonic rocks of the 
Proterozoic Z mafic volcanic-plutonic complex, Milford-Dedham 
zone 

[Samples 1-4 are single thin sections of representative samples; sample 5 is an 
average of modes from 6 thin sections (Lyons, 1977); figures in parentheses 
give range of modes. Zdi, diorite; Zssy, Sharon Syenite. Sample localities 
shown by field number in Wones and others (1986). n, andesine; nr, not 
reported; tr, trace] 

Sample no 1 2 3 4 5 

Unit Zdi Zdi Zdi Zssy Zssy 

Points 

counted 1223 1210 1185 1528 

Field no F-32 We-9* NBN-21a' W-40* 

Quartz 2.2 4.4 11.0 2.9 1.4(0.2-2.8) 

Plagioclase 7.4** 42.8n 36.0 22.0 nr 

Microcline .2 64.7 87.3(82.8-94.4) 

Biotite 26.8 13.7 14.0 .7 nr 

Epidote 34.4** 3.6 7.8 nr 

Hornblende .... 23.1 33.6 26.8 .6 5.0(0.8-7.0) 

Chlorite 2 .3 2.9 nr 

Magnetite 2.0 .7 .4 .9 1.8(0.5-3.3)* 

Titanite 3.5 .1 .7 nr 

Apatite 3 .8 .2 .3 nr 

Allanite tr nr 

Pyroxene 7.9 3.6 (1.0-6.6) 

Other nr nr nr nr .8 (0-1.3) 

* Chemical analysis in table 3. 

** Most plagioclase converted to epidote. 

* Reported as opaque minerals. 



GABBRO (Zgb) 

Gabbro (Zgb) of the State bedrock map comprises 
masses of gabbro that intrude older stratified rocks and 
have been intruded by younger Proterozoic rocks. Some 
masses are equivocal in their age limits and could be 
Paleozoic in age. Age relations are especially well pre- 
served in the Holliston area (fig. 4; Volckmann, 1977), 
where the older stratified rocks, the intrusive gabbro, 
and the intruding Milford Granite can all be found. In the 
large roof pendant in this area, layering is well preserved 
in a gabbro that has been partly uralitized. A small mass 
of anorthositic gabbro is spatially associated with cum- 
berlandite (cu) east of Woonsocket, R.I., but the contact 
relations are not known (Rutherford and Carroll, 1981; 
Rutherford and Hermes, 1984). 

DIORITE (Zdi) 

The diorite (Zdi) consists of undifferentiated masses of 
plagioclase-hornblende rock (tables 2, 3; fig. 4) that 
formed intrusive masses of intermediate composition; 
some recrystallized older volcanic rocks may be included 
in the category. The modal variations in the diorite are 
large. Some are quite rich in plagioclase, whereas others 
are enriched in amphibole or biotite. Much of the rock 
northeast of Boston that earlier workers assigned to the 
Newburyport Quartz Diorite has been included in this 
unit. 



Many of the diorite masses are intruded by the Prot- 
erozoic Z batholithic assemblage of granite and granodi- 
orite. In the large roof pendant near Randolph, and west 
of Spot Pond, Stoneham, recrystallization of the older 
volcanic rocks to dioritic- and quartz dioritic-appearing 
rocks in contact zones of the Dedham Granite is well 
exhibited. At Canton, between Randolph and Sharon, 
intrusive dikes of diorite can be found in the older 
volcanic rocks. We are unaware of any localities where 
diorite (Zdi) intrudes the Dedham Granite or its equiva- 
lents. A mass of altered diorite (Zdi) (table 2, no. 1; fig. 
4) straddling the Massachusetts-Rhode Island State line 
southwest of Wrentham has septa of metavolcanic rocks 
of the Blackstone Group and apparently has intruded 
these rocks. Several masses of diorite crop out in the 
New Bedford area of southeastern Massachusetts (fig. 
4). One of these forms a phacolithic mass well exposed in 
a local crushed-rock quarry (Warren Brothers quarry) at 
Acushnet. Two other partly lenticular masses crop out in 
scattered exposures near the mouth of Slocums River 
and along the shore at Horseneck Beach, Westport. The 
rock in the plutons ranges from gabbroic diorite to quartz 
diorite (table 2, nos. 2-3; table 3, nos. 1-2). The diorite is 
dark gray, medium grained, and mostly nongneissic to 
faintly gneissic but is markedly foliated in and near shear 
zones. The rock consists mostly of hornblende, some of 
which contains relic pyroxene cores, and plagioclase and 
subordinate biotite and quartz. The plagioclase is a 
zoned, lathlike calcic andesine to labradorite in the 
Horseneck Beach pluton but is saussuritized in the 
Acushnet pluton. Accessory minerals are pyrite, tita- 
nite, apatite, and secondary chlorite and epidote. 

Relations with the adjacent granitic rocks are not 
everywhere clear, and more than one age of diorite may 
be present. Most of the diorite masses of southeastern 
Massachusetts appear to be older than the granitic rocks. 
At Slocum's Neck, in the Slocums River area, diorite 
containing dark greenstone inclusions is cut by dikes of 
granite, and in one place an intrusion breccia consisting 
of fragments of diorite in a granitic matrix is exposed. On 
a point across the river to the east, however, a fine- 
grained dike of diorite sharply cuts coarse-grained 
inequigranular granite. This dike is interpreted to be of 
Paleozoic or Mesozoic age. At Horseneck Beach, diorite 
is cut by alaskite and biotite granite. In the Warren 
Brothers quarry at Acushnet, diorite is intruded by 
even-grained granite or granodiorite that has a contact 
zone of spotted rock containing clots of biotite and 
hornblende. A few aplite and pegmatite dikes intrude all 
the rocks in this quarry. 

DIORITE AT ROWLEY (Zrdi) 

The diorite at Rowley (Zrdi) is a 5-km 2 circular stock of 
hornblende diorite that intrudes the metavolcanic rocks 



INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 



111 



Table 3. — Major-oxide, normative-mineral, and trace-element com- 
positions, volatiles omitted, of plutonic rocks of the Proterozoic Z 
mafic volcanic-plutonic complex 

[Major-oxide compositions for samples 1-3 from X-ray spectroscopy by Paul 
Hearn and Susan Wargo; all Fe reported as Fe 2 3 ; sample 4, whole-rock 
analysis by H.S. Washington (in Emerson, 1917). Trace-element abundances 
from instrumental neutron activation analyses by L.J. Schwartz except Rb and 
Sr determined by X-ray spectroscopy by G.A. Sellars and B. McCall. Trace 
elements not determined for sample 4. nd, not determined] 

Sample no 1 2 3 4 

Unit Zdi Zdi Zssy Zdigb 

Major-oxide composition, in weight percent, 
and alkali-alumina ratio 

Si0 2 51.7 52.5 58.8 51.52 

A1 2 3 16.5 16.3 15.4 17.06 

Fe 2 3 9.46 8.37 9.65 1.97 

FeO nd nd nd 8.60 

MnO 15 .15 .33 

MgO 6.2 7.2 .1 4.87 

CaO 8.1 7.4 3.7 8.59 

NajO 2.6 2.9 3.6 3.44 

K 2 1.44 .71 4.92 1.77 

Ti0 2 1.32 .85 .84 2.45 

P 2 5 35 .16 .18 nd 

(NajjO+KjjOyAljjOa 24 .22 .55 .30 

Normative-mineral composition, in weight percent, 1 
and differentiation index (DI) 

Qtz 2.2 3.8 5.1 

Crn 

Or 8.5 4.2 29.1 10.4 

Ab 21.9 24.5 30.4 29.1 

An 29.1 29.4 11.3 25.8 

Di 7.3 5.3 3.3 8.0 

Hd 5.8 

Fo 4.5 

Fa 4.2 

En 13.7 16.6 .2 5.1 

Fs 13.6 13.0 16.9 4.3 

Ilm 2.5 1.6 1.6 4.6 

Ap 8 .4 .4 

DI 33 32 65 39 

Trace-element abundances, in parts per million, 
and selected ratios 

Rb 52 26 51 

Cs 1.8 1.2 3.7 

Sr 469 395 113 

Ba 477 216 2603 

Rb/Cs 29 22 14 

Rb/Sr 1 .06 .4 

Sc 22.6 25.8 20.3 

Cr 187 371 nd 

Co 38.8 33.4 1.2 

Zn 115 122 132 

La 24 21 31 

Ce 50 65 41 

Nd 26 21 42 

Sm 6 4 8 

Eu 1.60 1.33 7.33 

Gd 4.2 3.8 5.7 

Ho 6 .6 .7 



(Zv) in the town of Rowley. Contact relations with the 
adjacent Topsfield Granodiorite are not known. The only 
radiometric age from this group of rocks, 656 Ma (table 
1), comes from this stock. The diorite consists of plagio- 
clase, hornblende, and biotite; it has been fractured and 
silicified, and the minerals have been altered to epidote 
and chlorite (Dennen, 1981). 

SHARON SYENITE (Zssy) 

The Sharon Syenite forms a long narrow mass along 
the southeast side of the Norfolk basin (fig. 4) of gray to 
dark-gray syenite and minor ferrogabbro. Both rock 
types are intruded by the Dedham Granite (Zdgr), but 
their relationship to other Proterozoic Z gabbros is 
unknown. Near Sharon, the map pattern suggests that 
the syenite is intruded by diorite (Zdi). 

The syenite is gray and relatively unaltered; it is 
marked by ubiquitous minor hornblende (table 2, nos. 
4-5; table 3, no. 3). Its modal composition is closer to that 
of a monzonite than of a syenite because of the amount of 
plagioclase. Pyrite is common and contributes to the 



Table 3. — Major-oxide, normative-mineral, and trace-element com- 
positions, volatiles omitted, of plutonic rocks of the Proterozoic Z 
mafic volcanic-plutonic complex— Continued 

Sample no 1 2 3 4 

Unit Zdi Zdi Zssy Zdigb 

Trace-element abundances, in parts per million, 
and selected ratios— Continued 

Tb 81 .70 1.07 

Tm 32 .29 .47 

Yb 2.1 2.3 2.7 

Lu 29 .34 .45 

La/Yb 11 9 11 

Hf. 3.7 3.4 2.4 

Zr nd nd nd 

Th 2 5.0 3.4 2.3 

Th 4.6 3.2 2.6 

U 2 1.4 .9 .6 

U 1.4 1.0 nd 

^203 calculated as FeO, except sample 4. 

2 Delayed neutron reactivation determination by H.T. Millard, Jr., and C. McFee. 



Description of samples 
Sample localities shown in Wones and others (1986) 

1. We-9. Diorite; biotite hornblende diorite cut by alaskdte and granite; 
ledges east end of East Horseneck Beach, 200 m east of Horseneck 
Road. UTM grid: N45968-E3312. 

2. NBN-21a. Diorite; biotite hornblende quartz diorite; Warren Broth- 
ers quarry, Main St., 1 km south of Acushnet center. UTM grid: 
N46151-E3414. 

3. W^IO. Sharon Syenite; loose blocks at water tank at top of Knuckup 
Hill, 1.1 km south of Wrentham center. UTM grid: N46585-E3069. 

4. Diorite and gabbro; diorite; Peaches Neck, Salem; from Emerson 
(1917, p. 180, no. 1 in table), H 2 also reported. 



112 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



characteristic deep rusty weathering of the Sharon Sye- 
nite (Chute, 1966) and its high iron content. Lyons (1969) 
described the syenite and found the ranges in modal 
composition shown in table 2. The mafic-mineral content 
of these rocks ranges between 7 and 16 percent. 

The Dedham Granite intrudes the Sharon Syenite in 
outcrops along Route 1-95 southwest of Sharon, where 
small dikes of Dedham lithology have become desilicated 
and crystallized as quartz syenite. These syenites are 
distinctly different from the dark, coarse-grained, 
sulfide-bearing rocks typical of the Sharon Syenite. 

CUMBERLANDITE (cu) 

Cumberlandite (cu, fig. 4), actually a mela-troctolite, is 
an unusual cumulate rock that crops out in northeastern 
Rhode Island (Rutherford and Carroll, 1981; Rutherford 
and Hermes, 1984); it is presumably genetically related 
to the adjacent anorthositic gabbro (Zgb), which intrudes 
the Blackstone Group (Zb). The contact relations of the 
cumberlandite itself with the Blackstone Group or with 
the adjacent Esmond Granite (Zegr) of Rhode Island are 
not known (O.D. Hermes, written commun., 1984). Her- 
mes has found chemical evidence to suggest that the 
gabbro could be related to the Paleozoic alkalic magmatic 
rocks rather than to the Proterozoic Z suite. The cum- 
berlandite is a black, dense rock that contains tabular 
plagioclase in a preferred orientation. Rutherford and 
Carroll (1981) reported a mode of 49 percent olivine, 32 
percent titaniferous magnetite, 15 percent plagioclase, 
and accessory ilmenite and Al-rich spinel. 

SERPENTINITE (u) 

Serpentinite forms a poorly exposed, ill-defined mass 
(u, fig. 4) within the undifferentiated diorite and gabbro 
(Zdigb) in the town of Lynnfield. It may be part of the 
Proterozoic Z plutonic and volcanic complex, but, as it 
lies along a highly faulted zone (Goldsmith, this vol., 
chap. H), assigning an age is difficult. Kaye (1983, p. 
1076), for example, suggested that it is ultramafic mate- 
rial injected into a shear zone. If so, it is more likely to be 
middle to late Paleozoic in age, or possibly Mesozoic. We 
feel, however, that it is more likely a highly sheared and 
altered ultramafic rock of the Proterozoic Z mafic com- 
plex. 

DISCUSSION OF THE MAFIC PLUTONIC ROCKS 

The diorites (Zdi, Zdigb, tables 2 and 3) appear to be 
normal mafic rocks of a calc-alkaline suite (figs. 5-8) and 
to have normal rare-earth-element (REE) patterns (fig. 
9). The Sharon Syenite, on the other hand, differs from 
others of the suite and from the Proterozoic Z batholithic 
rocks. The Sharon is off the calc-alkaline trend in the 



Peacock diagram (fig. 6) and has a positive rather than a 
negative europium anomaly (fig. 9), suggesting cumulate 
plagioclase or K-feldspar. It does, however, fall on trend 
with the diorites in thorium-uranium ratio (fig. 10). 



PROTEROZOIC Z BATHOLITHIC ROCKS 

The Proterozoic Z batholithic rocks fall into two major 
groups, those in the Dedham batholith and those in the 
Milford antiform (figs. 3, 11). The former tend to be 
nongneissic, primarily brittlely deformed, whereas the 
latter tend to be gneissic or partly gneissic and to have 
been ductilely deformed. An exception to this grouping is 
the plutonic rocks at the south end of the Dedham 
batholith, southeast of the Fall River pluton, which tend 
also to be gneissic. The Topsfield Granodiorite is consid- 
ered to be an outlier of the Dedham batholith. All these 
rocks are approximately the same age, about 630-580 Ma 
(table 1). These batholithic rocks have been sampled 
more extensively (Wones and others, 1986) than the 
Proterozoic Z mafic plutonic rocks, and so more miner- 
alogical and chemical data are available. Major constitu- 
ents of these rocks (derived from point-counts of stained 
slabs) are listed in table 4, and their relative amounts are 
plotted in figures 5 and 12-14. Modes from point-counts 
of thin sections from some of these rocks and chemical 
compositions of some of these rocks are listed in tables 
5-10. 

DEDHAM BATHOLITH 
Dedham Granite (Zdgr, Zdngr) 

The Dedham Granite (Zdgr) occupies a large area 
within several regions in the Milford-Dedham zone (fig. 
11). The Dedham north and west of the Boston basin 
consists of both granite and granodiorite (Zdngr; table 5, 
no. 6; table 6, nos. 7-8). The Dedham intrudes rocks of 
the Blackstone Group (Zb), the Westboro Formation 
(Zw), and the mafic volcanic-plutonic complex (Zdigb, 
Zdi, Zgb, Zv). 

Contacts with older rocks are particularly well 
exposed in Saugus, Stoneham, Canton (C, fig. 11), and 
Cohasset, although dikes of granitic rocks similar to the 
Dedham are found throughout the Proterozoic Z terrane 
southeast of the Bloody Bluff fault zone. Igneous breccias 
in which the Dedham encloses blocks of mafic rock are 
well exposed west of Spot Pond in Stoneham and in 
Cohasset. Near Arlington, granite dikes grade into sye- 
nite where they terminate within the older masses of 
amphibolite and gabbro. Small masses of pink granite 
intruding diorite and gabbro north of the Boston basin 
are considered to belong to the northern, more mafic 
phase of the Dedham (Zdngr); this pink granite is the 
most mafic of the rocks mapped as Dedham. 



INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 
Quartz 



113 



K-feldspar 




"A~ 



TT 



A 



OA 



A 



~7V 






X- 



j AO 

O CA ^ A 

° A O ° A 
O 



A^ 



V 




Plagioclase 



Mafic minerals 
EXPLANATION 

Dedham Granite, Esmond Granite, and 

Westwood Granite (table 5) 
Granites of southeastern Massachusetts 

(table 7) 
Plutonic rocks of the Milford 

antiform (table 9) 
Proterozoic Z mafic plutonic 

rocks (table 2) 
Sharon Syenite (table 2) 



FIGURE 5.-Q-P-K-M (quartz-plagioclase-K-feldspar-mafic minerals) diagram of modes of Proterozoic Z intrusive rocks of the Milford-Dedham 
zone from tables 2, 5, 7, and 9. Mafic minerals used are biotite, hornblende, pyroxene, iron oxides, and chlorite. 



The Dedham Granite is typically medium to coarse 
grained, is commonly fractured, and usually has a salmon 
color mottled by green clots of epidote and chlorite 
formed by alteration. Fresh rocks that do not contain 
closely spaced (less than 1 m) joints are gray. Weathered 



surfaces commonly have prominent knobs of quartz, the 
so-called "hob-nailed boot" texture. Modes of Dedham 
Granite (tables 4-6) and ternary plots (figs. 5, 12) 
show that the composition ranges mostly from granite 
to granodiorite. A few are quartz monzodiorite to 



114 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



1 1 

EXPLANATION 


1 
















• Proterozoic Z batholithic rocks (tables 6, 8, and 10) 


















■ Proterozoic Z mafic plutonic rocks (table 3) 
D Sharon Syenite (table 3) 






















• 




^v 




• 


t^ 


• 




J^*^T 






• 


• 










i 




• 
• 


• 


• 


v 




-m 



PERCENT Si0 2 

Figure 6.— CaO and NaaO+K^O plotted against Si0 2 for Proterozoic Z intrusive rocks of the Milford-Dedham zone. Lines are approximate fit 

to data. 



monzonite. The K-feldspar crystals are perthitic and 
subhedral, and they appear to have formed with plagio- 
clase and quartz early in the crystallization sequence. 
The plagioclase is partly saussuritized; chemical analyses 
(table 6) indicate that the plagioclase probably ranged 
from albite to andesine but was mostly oligoclase. Biotite 
is the dominant mafic mineral, although there are trace 
amounts of hornblende in some rocks. Titanite is ubiqui- 
tous as euhedral primary crystals and as anhedral prod- 
ucts of biotite alteration. Apatite, zircon, magnetite, and 
other opaque minerals are common accessories. Plagio- 
clase has been altered to the assemblage albite-sericite- 
epidote, and biotite and hornblende have altered to 
chlorite, epidote, and titanite. A hematite dust in the 
alkali feldspars gives the rock its characteristic salmon 
color. 

REE analyses show a general enrichment in light 
REE and a negative europium anomaly (fig. 15), 
although two samples (2 and 3, table 6) show a lower and 
flatter pattern of heavy REE with respect to chondrite 
than the others, and sample 9 is less enriched in light 
REE. The profile of the mafic sample of Dedham Granite 
north of Boston is similar to the other profiles but is 
elevated above them, suggesting an overall enrichment 
of some sort. 

The Dedham Granite is enriched in anorthite, horn- 
blende, and biotite where it is close to the older mafic 
rocks, confirming Crosby's (1913) ideas on assimilation of 
wall rocks. Most of the modes that plotted in the grano- 
diorite and tonalite fields (figs. 5, 12) were collected from 



rocks close to the older rocks. The K-feldspar content 
increases from north to south within the Dedham Granite 
and its related rocks. This increase may be an actual 
gradient, or perhaps a deeper erosional level is exposed 
in the south and assimilated roof pendant material has 
contaminated the shallower portions of the batholith 
exposed toward the north. 

The Dedham Granite also contains porphyritic variet- 
ies, not distinguished in table 4, that contain alkali 
feldspar megacrysts 1^1 cm long and 0.5-2 cm wide. 
Lyons (1969) identified a separate pluton of this variety 
in the area north of Mansfield that he named the 
Barefoot Hill quartz monzonite (no. 1, table 6). The 
Barefoot Hill quartz monzonite of Lyons is included in 
the Dedham Granite on the State bedrock map because it 
is in gradational contact with the Dedham in the region of 
Brockton (Chute, 1950) and is herewith referred to as the 
porphyritic variety of Dedham. The REE pattern for the 
porphyritic granite is only slightly richer in heavy REE 
than samples 2 and 3 (table 6). Porphyritic rocks occupy 
large areas near Brockton and Assawompset Pond. 
According to Lyons, the same rock appears in the inlier 
of basement near Middleboro (fig. 11) and is probably the 
porphyritic granite (Zpgr) of the Fall River-New Bed- 
ford area. There is an apparent concentration of this 
variety of the Dedham Granite to the southeast in the 
Milford-Dedham zone. 

The Dedham Granite as mapped on the State bedrock 
map may include small masses of younger granite, such 
as the Westwood Granite, whose boundaries have not 



INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 



115 




EXPLANATION 

• Plutonic rocks of northeastern 

Massachusetts (table 6) 
o Plutonic rocks of southeastern 

Massachusetts (table 8) 
^ Plutonic rocks of the Milford 

antiform (table 10) 
■ Proterozoic Z mafic plutonic 
rocks (table 3) 
Figure 7. —Ternary plot of normative albite (Ab), anorthite (An), and 
orthoclase (Or) for Proterozoic Z intrusive rocks of the Milford- 
Dedham zone. Fields of silica-saturated rocks from O'Connor (1965) 
as modified by Barker (1979). 



been determined. The rock that Chute (1965b) mapped as 
Westwood Granite in the Scituate and Marshfield areas 
was believed by Wones to be a phase of the Dedham and 
not the typical Westwood. It is possible, but not certain, 
that the terrane underlain by the Dedham Granite is a 
composite of many smaller plutons. Bateman and others 
(1963) demonstrated that the Sierra Nevada batholith is 
a composite batholith but that individual plutons within it 
are commonly larger than the 1,400 km 2 estimated for the 
Dedham Granite. The cover of glacial deposits, urban 
development, and extensive faulting in eastern Massa- 
chusetts have made it difficult to resolve this question. 

Topsfield Granodiorite (Ztgd) 

The Topsfield Granodiorite (Ztgd), first described by 
Toulmin (1964), occupies an area of 80 km 2 between 
Middleton and Newbury (fig. 11). It intrudes the diorite 
and gabbro (Zdigb) and mafic and felsic metavolcanic 
rocks (Zv). It is bounded on the northwest by faults 
bordering the Newbury and Middleton basins and north- 
ward extensions of the Bloody Bluff (and Mystic?) fault 
zones (Goldsmith, this vol., chap. H), and on the south- 



east by a splay(?) off the Bloody Bluff fault. Although the 
Topsfield Granodiorite has not been dated, we consider it 
to be part of the Proterozoic Z batholithic assemblage 
because it is similar in mineralogy and alteration to the 
Dedham Granite and because it intrudes the Proterozoic 
Z mafic complex. It is probably overlain by the Silurian 
and Devonian Newbury Volcanic Complex (Dennen, 
1975). The contact is now a fault. 

The composition of the Topsfield ranges from granite 
to tonalite. The only sample shown in figure 12 happens 
to be a tonalite. The Topsfield is typically altered to red 
and green. The plagioclase is altered to sericite and 
epidote, and it is strongly dusted with hematite. In 
places, blue quartz forms large anhedral grains or mosa- 
ics of grains. Much of the rock is transected by recrys- 
tallized mylonite bands that truncate and incorporate 
altered grains, implying that some of the deformation 
postdates the alteration. The one sample described here 
(table 4; table 6, no. 9) is a typical tonalite. The REE 
profiles (fig. 15) are flat and are approximately three 
times the values for chondrites, quite unlike rocks of 
similar bulk composition (Frey and others, 1978). The 
thorium content is unusually low relative to uranium, 
which may reflect the relatively high degree of alteration 
of the rock. 



FeO 



Na 2 




MgO 



EXPLANATION 



• Batholithic rocks of northeastern 

Massachusetts (table 6) 
° Batholithic rocks of southeastern 

Massachusetts (table 8) 
a Batholithic rocks of the Milford 

antiform (table 10) 
■ Mafic plutonic rocks (table 3) 
-Ternary AFM (alkalis-FeO-MgO) plot of Proterozoic Z 
intrusive rocks of the Milford-Dedham zone. 



116 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



Table 4.— Major constituents, in percent, determined by point-count 

of stained slabs of intrusive rocks of the Milford-Dedham zone 
[1,200-2,000 points counted per slab. Totals may not sum to 100 because of 

rounding. Sample localities shown by field number in Wones and others (1986)] 

Samp le UTM grid Mafic 

"""•F" Quartz K-feldspar Plagioclase , 

no. N E minerals 

Dedham Granite south of the Boston basin (Zdgr) 

DMA-30 46770 3220 33^0 28ii 33.6 5.0 

DMA-32 .... 46781 3196 29.4 23.8 45.6 1.2 

DMA-33 .... 46779 3197 29.6 35.8 32.6 2.1 

DMA-34 46767 3196 10.2 15.3 62.5 12.0 

DMA-34A... 46766 3190 29.7 29.4 37.5 3.4 

DMA-41 .... 46481 3050 46.7 30.5 22.5 .3 

DMA-93 .... 46813 3242 31.0 38.1 24.3 6.6 

DMA-94 .... 46603 3158 34.4 36.7 24.7 4.5 

DMA-98 46729 3360 30.9 38.4 29.1 1.5 

DMA-99 .... 46729 3360 25.7 39.7 31.9 2.7 

DMA-100 . . . 46717 3361 20.0 42.1 30.9 7.0 

DMA-101 . . . 46701 3369 31.2 20.0 43.2 5.6 

DMA-102 . . . 46677 3358 25.0 29.8 39.3 5.9 

DMA-104 . . . 46718 3387 22.4 44.4 24.6 8.5 

DMA-105 . . . 46723 3388 19.6 10.8 64.5 5.1 

DMA-108 . . . 46621 3429 30.0 35.8 32.5 1.6 

DMA-109B . . 46720 3425 30.9 29.3 35.2 4.5 

DMA-116 . . . 40787 3421 15.4 30.8 44.0 9.8 

DMA-117A . . 46798 3462 21.0 18.4 51.2 9.2 

DMA-119 . . . 46799 3475 20.3 33.5 39.6 6.7 

DMA-120 . . . 46804 3485 29.4 35.4 29.4 5.8 

DMA-121 . . . 46807 3494 19.4 22.4 45.0 13.2 

DMA-123 . . . 46773 3524 29.7 36.8 28.8 4.7 

DMA-129 . . . 46597 3568 30.2 47.6 19.9 2.3 

DMA-132 . . . 46566 3571 30.1 45.0 22.3 2.6 

DMA-138 . . . 46601 3552 38.5 32.8 23.4 5.3 

DMA-140 . . . 46588 3536 35.0 11.1 49.8 4.1 

DMA-143 . . . 46618 3456 37.0 28.3 33.3 1.4 

DMA-146 . . . 46644 3392 30.0 28.0 38.9 3.1 

DMA-147 . . . 46646 3390 24.4 15.0 48.9 11.6 

DMA-150 . . . 46632 3353 26.1 28.5 41.4 4.0 

DMA-151 . . . 46649 3346 29.2 37.9 30.9 2.0 

DMA-153B . . 46653 3316 30.8 39.2 27.9 2.0 

DMA-157 . . . 46614 3248 26.9 20.4 46.3 6.3 

DMA-158 . . . 46590 3231 32.1 29.8 33.9 4.2 

DMA-159 . . . 46587 3205 29.7 24.9 34.1 6.3 

DMA-160 . . . 46613 3182 34.7 41.6 19.3 4.4 

DMA-166 . . . 46609 3295 26.7 24.4 43.6 5.2 

DMA-196 . . . 46558 3009 27.9 35.0 34.0 3.0 

DMA-201 . . . 46695 3018 33.9 39.1 23.5 3.6 

DMA-203 . . . 46663 3048 34.1 39.4 24.8 1.7 

DMA-204 . . . 46654 3018 29.5 33.2 35.4 1.9 

DMA-208 . . . 46805 3150 26.7 39.0 30.0 4.3 

F-l 46588 3000 39.1 36.1 24.2 .6 

W-3 46576 3042 2.6 51.4 34.0 11.9 

W-4 46568 3059 24.9 28.4 40.0 6.7 

W-6 46567 3061 35.3 28.3 28.3 8.1 

Dedham Granite north of the Boston basin (Zdngr) 

DMA-65 . . . . 47038 3328 2SS> lil 41.9 17.1 

DMA-66 .... 47040 3329 26.4 32.5 36.9 4.2 

DMA-67 .... 47052 3321 26.1 2.8 59.1 17.0 

DMA-127 . . . 46995 3148 27.6 29.9 35.9 6.6 
Topsfield Granodiorite (Ztgd) 

DMA-60 47334 3480 30.6 47.5 21.9 

Dedham Granite west of the Boston basin (Zdgr) 

DMA-1 46925 3134 36^6 3L7 27.5 4.2 

DMA-3 46907 3128 37.0 20.2 37.1 5.7 

DMA-5 46882 3089 31.8 30.0 34.1 3.8 

DMA-3 46883 3089 33.4 17.9 43.3 5.5 

DMA-7 46851 3105 34.2 39.5 25.9 .4 



Table 4. — Major constituents, in percent, determined by point-count 
of stained slabs of intrusive rocks of the Milford-Dedham zone— 
Continued 



Sample 


UTM grid 


Quartz 


K-feldspar 


Plagioclase 


Mafic 


N. 


E. 


minerals 


Dedham Granite west of the Boston basin (Zdgr) 


— Continue! 




DMA-S . . . 


46839 


3089 


28.3 


50.4 


18.4 


2.8 


DMA-9 . . . 


46817 


3060 


34.8 


7.2 


48.9 


8.8 


DMA-10 . . 


46827 


3064 


4.7 


30.4 


53.0 


10.9 


DMA-13 . . 


. 46813 


3022 


28.8 


35.9 


28.2 


7.1 


DMA-19 . . 


46891 


2984 


39.9 


26.1 


25.9 


8.1 


DMA-20 . . 


46885 


2987 


28.8 


23.5 


40.8 


7.0 


DMA-174 . 


46844 


3038 


38.2 


39.1 


18.8 


3.9 


Westwood Granite (Zwgr) 


DMA-29 . . 


46772 


3218 


26.2 


35.9 


33.6 


4.3 


DMA-31 . . 


46790 


3184 


31.5 


36.5 


28.4 


3.6 


DMA-36 . . 


46744 


3182 


35.0 


29.3 


32.5 


3.2 


DMA-44 . . 


46687 


3315 


28.5 


35.7 


30.9 


3.9 


DMA-70 . . 


46688 


3318 


31.9 


34.2 


28.9 


5.0 


DMA-206 . 


46794 


3182 


30.7 


33.4 


32.4 


3.6 


DMA-209 . 


46774 


3124 


28.5 


^7.3 


32.1 


2.1 


DMA-109 . 


46720 


3425 


28.8 


35.1; 


32.9 


3.3 


DMA-110 . 


. 46725 


3427 


19.6 


35.0 


41.6 


3.7 


DMA-117B 


46790 


3462 


26.3 


47.5 


25.1 


1.0 


DMA-137 . 


46715 


3414 


24.5 


33.4 


38.4 


3.8 


Milford Granite (Zmgr; Zmgd where noted) 


DMA-14 . . 


. 46837 


2970 


38.8 


36.3 


18.7 


6.2 


DMA-22 . . 


46709 


2926 


32.3 


38.3 


26.3 


3.1 


DMA-27 . . 


46729 


3072 


28.1 


34.8 


33.4 


3.2 


DMA-28 . . 


46703 


3108 


28.4 


29.5 


38.7 


2.9 


DMA-179 . 


46726 


2957 


28.6 


43.7 


25.6 


2.2 


DMA-183 . 


46775 


2960 


32.0 


26.1 


32.5 


9.3 


DMA-194 . 


46865 


2972 


17.9 


55.3 


24.1 


2.6 


Mi 3 


46702 
46718 


2931 
2918 


36.1 
37.0 


33.4 
38.4 


23.2 
17.3 


7.3 


MM (Zmgd 


7.3 


Mi 5 


46727 


2910 


38.4 


4.9 


42.1 


14.2 








Granite of the Fall River pluton (Zfgr) 


A 2 


46313 
46310 


3338 
3315 


30.5 
30.9 


34.0 
31.4 


32.2 
36.2 


3.2 


A-3 


1.5 


A-5b 46290 


3309 


38.2 


34.2 


24.7 


2.8 


A 7 


46300 
46285 


3306 
3291 


33.9 
24.8 


35.2 
41.4 


28.7 
28.1 


2.2 


A-8 


5.7 


A 9 


46283 
46242 


3294 

3276 


36.5 
32.5 


33.0 
39.4 


28.9 
24.2 


1.7 


A-10 


3.9 


A-ll 46253 


3263 


37.1 


38.4 


21.7 


2.7 


AP-la 46269 


3371 


32.6 


28.9 


33.7 


4.8 


AP-4 46319 


3362 


25.3 


9.1 


51.7 


13.9 


AP-6 46331 


3341 


33.8 


34.9 


25.6 


2.7 


AP 7 46293 


3374 
3224 

3187 


31.5 
19.5 
32.3 


35.3 
42.8 
21.4 


29.4 
31.7 
39.5 


3.7 


FR 2 46165 


6.0 


FR-3 46163 


6.8 


FR-8 46187 


3221 


35.9 


35.9 


20.2 


7.9 


FR-12 46184 


3202 


42.0 


40.1 


15.6 


2.3 


FR-15 46162 


3223 


29.2 


25.4 


34.7 


10.8 


FRE-3 46189 


3237 


33.9 


30.8 


28.3 


4.4 


FRE-4 46231 


3234 


36.3 


33.8 


19.8 


10.1 


FRE-11 46234 


3252 


33.2 


38.1 


23.1 


5.6 


FRE-12 46211 


3261 


37.2 


33.9 


25.2 


3.7 


T 1 46099 


3178 


40.0 


28.1 


27.2 


5.0 






Porphyritic granite (Zpgr) 


NBN-9 46146 


3411 


30.6 


28.0 


34.1 


7.3 


We-2 45968 


3240 

3270 


16.2 
25.4 


55.1 
45.3 


21.8 
20.7 


5.9 


We-4 45987 


8.5 


NBN-22 46179 


3422 


30.0 


25.4 


27.4 


14.0* 



' Plus 3.1 percent muscovite. 



INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 



117 



100 
90 
80 
70 
60 



CD DU 
< 

DC 

w 40 

< 



O 

O 20 
•2 
(J 
O 



A 

\ 
- \ 


Ill 


I 

A 

/ \ 
' \ 
/ \ 

/ \ 

\ 

\ 


I I I I I i 


i 


" 




\ 


\ 

\ 


- 


" 


\^> \ 




\ 
\ 


- 


- 


EXPLANATION 


\x 




-^^^ 


" 










" 


Sharon Syenite (sample 3) 










I I I I I 


i 


i i i i i i 


i 



La Ce 



Nd 



Sm Eu Gd Tb 



Tm 



Figure 9. — Chondrite-normalized plot of rare-earth elements in Proterozoic Z mafic plutonic rocks of the Milford-Dedham zone. 

from table 3. 



Yb Lu 

Sample numbers 



40 
30 

20 - 



EXPLANATION 

• Proterozoic Z batholithic rocks 
■ Proterozoic Z mafic plutonic rocks 
a Sharon Syenite 



1 2 3 4 5 6 7 
U (ppm) 



9 10 11 12 



Figure 10.— Plot of Th (thorium) against U (uranium) for Proterozoic 
Z intrusive rocks of the Milford-Dedham zone. 



Esmond Granite (Zegr) 

The Esmond Granite (Zegr) occupies nearly 100 km 2 of 
Rhode Island. It forms an elongate pluton southeast of 
Woonsocket of which only the northern part is in Massa- 
chusetts (fig. 11). A similar pluton lies west of Woon- 



socket, and phases of the biotite granite (Zgr) and the 
Milford Granite (Zmgr) resemble it. In fact, Quinn (1971) 
thought the Esmond and the Milford might be the same 
rock. A coarse-grained facies of the Esmond intrudes the 
Blackstone Group and a related tonalite. A fine-grained 
facies intrudes both the tonalite and the coarse-grained 
facies (Hermes and Zartman, 1985). Contact relations 
with the Milford Granite are not known. The age of the 
Esmond (621 ±8 Ma) appears to be close to that of the 
Milford (table 1). The Esmond Granite is mottled red and 
green, like the Dedham Granite, and is massive except 
where foliated by local deformation. The representative 
modes (Quinn, 1971; table 5, no. 7) and chemical content 
(table 6, no. 10) indicate that the dominant rock type is 
biotite granite poor in mafic minerals. The plagioclase is 
altered to muscovite and epidote, and the biotite is 
usually chloritized. The REE pattern (fig. 15) is similar 
to that of the Dedham Granite. 

Grant Mills Granodiorite (Zgmgd) 

The Grant Mills Granodiorite (Zgmgd) occupies an 
area of 25 km 2 in northeastern Rhode Island (fig. 11). 
According to Quinn (1971), it intrudes older quartz 
diorite and is gradational with the Esmond Granite. O. 
Don Hermes (written commun., 1985; Hermes and Zart- 
man, 1985) considered it to be a porphyritic variety of 



118 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 




41°30' 



Figure 11. — Distribution of Proterozoic Z batholithic rocks in the 
Milford-Dedham zone. W, Wrentham; C, Canton; A, Acushnet; LP, 
Long Pond; AP, Assawompset Pond; H, Holliston; M, Milford. Ztgd, 
Topsfield Granodiorite; Zdngr, Dedham Granite north of Boston; 
Zdgr, Dedham Granite; Zwgr, Westwood Granite; Zfgr, granite of 
the Fall River pluton; Zpgr, porphyritic granite; Zagr, alaskite; Zgg, 



granite, gneiss, and schist, undivided; Zgmgd, Grant Mills Granodio- 
rite; Zegr, Esmond Granite; Zsg, Scituate Granite Gneiss; Zpg, Pona- 
ganset Gneiss; Zhg, Hope Valley Alaskite Gneiss; Zmgr, biotite granite 
of the Milford Granite; Zmgd, mafic phase of the Milford Granite; Zgr, 
biotite granite; Zm, Mattapan Volcanic Complex; DZ1, Lynn Volcanic 
Complex. 



INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 



119 



Quartz 



EXPLANATION 

• Dedham Granite south of Boston 

o Dedham Granite west of Boston 

+ Dedham Granite north of Boston 

□ Topsfield Granodiorite 



K-feldspar 




Mafic minerals 
Figure 12. — Q-P-K-M (quartz-plagioclase-K-feldspar-mafic minerals) diagram of modes of Dedham Granite and Topsfield Granodiorite, from 

table 4. Fields of igneous rocks from Streckeisen (1973). 



120 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 
Quartz 



EXPLANATION 

Westwood Granite 

Westwood Granite in Plymouth 

quarry, Weymouth 
Milford Granite 



K-feldspar 




Mafic minerals 

Figure 13.— Q-P-K-M (quartz-plagioclase-K-feldspar-mafic minerals) diagram of modes of Westwood Granite; Westwood Granite in Plymouth 
quarry, Weymouth; and Milford Granite, from table 4. Fields of igneous rocks from Streckeisen (1973). 



INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 



121 



Quartz 



K-feldspar 



EXPLANATION 

o Granite of the Fall River pluton 
• Porphyritic granite 




Mafic minerals 



Figure 14.-Q-P-K-M (quartz-plagioclase-K-feldspar-mafic minerals) diagram of modes from the Fall River pluton including the porphyritic 
granite, from table 4. Fields of igneous rocks from Streckeisen (1973). 



122 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



Table 5.— Modes, in percent, of plutonic rocks of the Dedham batholith in northeastern Massachusetts 
[Zdgr, Dedham Granite; Zdngr, Dedham Granite north of Boston; Zegr, Esmond Granite; Zwgr, Westwood Granite. Sample localities shown by field number in Wones 

and others (1986). a, albite; n, andesine; p, perthitic; tr, trace] 

Sample no I 2 3 4 5 6 7 8 

Unit Zdgr Zdgr Zdgr Zdgr Zdgr Zdngr Zegr Zwgr 

Points counted 2009 1485 1451 1586 1811 1761 2101 1728 

Field no N-3 F-40 F-29 W-6* W-37 BN-1 P-5' BH-3 

Quartz 32.2 31.5 25.3 37.2 21.6 32.8 38.8 29.3 

Plagioclase 27.6a 36.6a 28.9a 41.6a 33.0a 33.8n 25.9 30.4a 

Microcline 37.8p 30.2p 40.0p 16.9p 44.5p 14.8 35.4 37.9p 

Biotite 1.0* .8* 3.9 .2 .1 1.3 .1 

Muscovite 1.0* .7* .9 .3 

Epidote .7 .4 .5 4.9 1.1 1.0 

Hornblende tr tr tr 

Chlorite 3.6 3.0 12.9 .5 .9 

Magnetite 6 .1 .2 tr .2 .1 .1 

Hematite 2* 

Titanite .1 tr .2 

Apatite tr .10 

Allanite .4000 tr 

Garnet 00 .6 000 00 

Zircon OOtrOOO 00 

Calcite 000000 .10 

* Chemical analysis in table 6. 

$ Biotite altered to muscovite and hematite. 



the Esmond. The dominant rock type is granodiorite 
(Quinn, 1971). It has the pink and green coloration due to 
alteration that is characteristic of the Dedham Granite 
and related rocks. Earlier workers (Warren and Powers, 
1914; Emerson, 1917; Quinn, 1971) considered the Grant 
Mills to be equivalent to the Dedham Granite in both age 
and texture. 



Westwood Granite (Zwgr) 

The Westwood Granite (Zwgr) forms small (less than 
10 km 2 ) masses of light-colored granite (fig. 11) that 
intrude the Dedham Granite and older rocks (Dowse, 
1949; Chute, 1950, 1966). Some masses that resemble 
Westwood Granite, such as those exposed in the Ply- 
mouth quarries in Weymouth and in scattered outcrops 
near Scituate, have been included within the Dedham 
Granite on the State bedrock map. The Westwood Gran- 
ite crops out in the areas dominantly of Dedham Granite, 
and parts of it were included by Kovach and others ( 1977) 
in an investigation of the age of the Dedham by Rb-Sr 
dating methods. Extensive intrusion breccias occur at 
the contacts of the Westwood with older mafic rocks. The 
contacts with the Dedham Granite commonly are abrupt, 
with dikes of Westwood cutting the Dedham and rare 
inclusions of Dedham within the Westwood (Chute, 
1966). There have been no reports of Westwood cobbles 
within the Roxbury Conglomerate, so it is possible that 
the Westwood is an intrusive equivalent of the extrusive 
Mattapan Volcanic Complex that underlies the Roxbury. 



The Westwood was therefore not exposed to erosion at 
the time of deposition of the Roxbury. 

The Westwood Granite is fine grained and low in mafic 
minerals (tables 4-6). Much of the plagioclase is altered 
to sericite and the biotite to chlorite. It is usually lighter 
in color than the Dedham Granite but does contain 
microscopic hematite, which gives the rock a pink cast. It 
does not appear to be as deformed as the Dedham 
Granite. The Westwood Granite occupies a very compact 
field when projected onto the quartz-plagioclase- 
K-feldspar plane (fig. 13). The ranges of modes from this 
study (table 5) and from Chayes (1952) and Chute (1966) 
are 26-35 percent quartz, 29^15 percent K-feldspar, 
17-34 percent plagioclase, and 1-5 percent micas. Com- 
mon accessories are biotite, magnetite, titanite, and 
apatite. 

The mineral assemblage of the Westwood Granite is 
more like that of the Dedham Granite than that of any 
other intrusive unit in the Milford-Dedham zone. The 
Westwood differs from the Dedham Granite in having a 
finer grain size, lower An content of the plagioclase, lack 
of hornblende, and lower color index. It has a slightly 
lower heavy-REE content (fig. 15). It may represent a 
late-stage aplitic differentiate of the Dedham magma or a 
later, separate magma from a similar source material. 
Part of a small elliptical pluton of the Westwood Granite 
in Weymouth (not shown separately on the State bedrock 
map) partly exposed in the Plymouth quarries differs 
from the type locality in having a lower quartz content, 
slightly lower REE content, and higher U and Th 
contents. As mentioned above, the Westwood may be 



INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 



123 



the intrusive equivalent of the Mattapan and Lynn 
Volcanic Complexes (Zm and DZ1, fig. 11). A somewhat 
questionable 579±28-Ma age (table 1) obtained from the 
Westwood indicates that it might be younger than the 
Dedham. It may be about the same age as the Mattapan, 
from which a U-Pb age on zircon of 602 ±3 Ma has been 
obtained (Kaye and Zartman, 1980). The granitic intru- 
sion at Pine Hill, Medford, which intrudes the Lynn 
Volcanic Complex (Zarrow, 1978), is probably the West- 
wood Granite. 

Fine-Grained Granite (fgr) 

Two small bodies of fine-grained biotite granite (fgr) 
lie in fault blocks near the southwest end of the Norfolk 
basin near Wrentham (W, fig. 11). Each is less than 5 
km 2 in area. They appear to intrude the Dedham Gran- 
ite, although they may be close to it in age. 

Granite of the Fall River Pluton (Zfgr) 

The granite of the Fall River pluton (Zfgr) occupies an 
area of 300 km 2 southeast of the Narragansett basin 
between Middleboro and Buzzards Bay (fig. 11). The 
radiometric age of the granite (table 1) shows the rock to 
be the same age as the Dedham Granite, 630±15 Ma, and 
the mineralogy and texture of the granite of the Fall 
River pluton make it most probable that this body is 
equivalent to the Dedham Granite. A minimum age of 
516 Ma has been obtained by Rb-Sr whole-rock methods 
(Galloway, 1973) on rock in the southern part of the 
pluton, south of Fall River, called the Bulgarmarsh 
Granite (Pollock, 1964). We were unable to distinguish 
the Bulgarmarsh from the rest of the Fall River pluton in 
our reconnaissance for the State bedrock map. Skehan 
and others (1978) and Skehan and Murray (1980) sug- 
gested that the granite of the Fall River pluton might be 
equivalent to a porphyritic granite (Kay and Chappie, 
1976) on Aquidneck Island, R.I., that was dated at 
592 ±12 Ma by Rb-Sr methods (Smith, 1978). We think 
this unlikely because the granite of the Fall River pluton 
is not typically porphyritic. The Fall River pluton may 
contain younger intrusions not to date distinguished. 

The granite of the Fall River pluton is light gray to 
gray, locally light reddish orange, medium grained, 
equigranular to slightly seriate, and rarely porphyritic. 
Most of the rock is fresh, and it is only locally fractured 
and altered. In the Assonet area, along the edge of the 
Narragansett basin, the granite is fractured and more 
severely altered than elsewhere. Modes and chemical 
compositions of the granite (tables 4, 7, 8) lie within the 
field of the Dedham Granite (figs. 5, 14), and the REE 
patterns (fig. 16) are similar to those of the Dedham 
Granite. Some quartz forms mosaic aggregates. Much of 
the microcline is perthitic. Plagioclase ranges in anor- 



thite content from An 10 to An 30 but is partly saussurit- 
ized. Biotite is less than 5 percent. The muscovite is 
usually next to plagioclase, and we suggest that some, if 
not all, of the muscovite (table 7) may be sericite 
recrystallized to muscovite during a Permian thermal 
high. Accessory minerals are opaque minerals, zircon, 
apatite, sparse titanite, garnet, and allanite. Sericite, 
epidote, and titanite are common alteration phases. 

The rock is not gneissic in the north, although local 
cataclastic textures are present, but south of Fall River 
it becomes gneissic, particularly in the Westport area. 
The rock is more leucocratic at Freeport and is coarse 
grained east of Assawompset Pond. No distinct contact 
between the main mass of the Fall River pluton and the 
Bulgarmarsh Granite of Pollock (1964) was observed by 
us. At a roadcut on Route 24 north of Tiverton, in the 
area of the Bulgarmarsh Granite, the rock contains 
granular quartz aggregates similar to those in the 
Milford Granite (table 7, no. 4; table 8, no. 4). Emerson 
(1917), on the earlier State map, showed an area of Quincy 
Granite northeast of Assonet. Although the granite in 
this region is more varied in texture than elsewhere, we 
were unable to locate any Quincy Granite. 

Porphyritic Granite (Zpgr) 

Porphyritic granite (Zpgr) is a distinct rock type in the 
southeastern part of the Dedham batholith (fig. 11). It 
lies mostly east of the Fall River pluton and extends from 
Long Pond (LP, fig. 11) south to Buzzards Bay and east 
and north to Middleboro, where it is exposed in an outlier 
surrounded by Pennsylvanian rocks. Lyons (1969) con- 
sidered the porphyritic granite to be equivalent to his 
Barefoot Hill quartz monzonite of the Mansfield area (see 
description of Dedham Granite above); its analysis in 
table 4 (DMA-166) shows that it fits within the spread of 
modes of the porphyritic granite (table 7). Chemically, 
however, if no. 1, table 6, and no. 5, table 8, are truly 
representative, the porphyritic granite is somewhat 
more mafic than Lyons' Barefoot Hill. 

The porphyritic granite is gray to greenish gray, 
seriate to porphyritic, with microcline phenocrysts 
(locally 2 cm long) set in a finer grained matrix of quartz, 
plagioclase, microcline, and biotite aggregates. It is 
retrogressively sheared or foliated in varying degrees in 
the northern part of its area; near New Bedford it is 
pervasively gneissic but much less altered. Gneissic 
varieties may be seen in gravel pits north of Acushnet, 
and in the Westport shore area (for example, at Goose- 
berry Neck). An outcrop of gneissic rock north of Marion 
included in the area of granite, gneiss, and schist, 
undivided (Zgg), may be equivalent. Inclusions of dark 
schist and porphyritic mafic rock are oriented parallel to 
the feldspar orientation. In the more foliated rocks, gray 



124 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



Table 6.— Major-oxide, normative-mineral, and trace-element compositions, volatiles omitted, of Proterozoic Z intrusive rocks, Milford- 

Dedham zone, northeastern Massachusetts 

[Major oxides determined by X-ray spectroscopy by Paul Hearn and Susan Wargo; all Fe reported as Fe 2 3 . Trace-element abundances determined by instrumental 

neutron activation analyses by L.J. Schwartz except Rb and Sr by X-ray spectroscopy by J. Lindsay, B. McCall, and G.A. Sellars. nd, not determined] 

Sample no I 2 3 4 5 6 7 8 9 10 Li i2~~ 

Unit Zdgr Zdgr Zdgr Zdgr Zdgr Zdgr Zdngr Zdngr Ztgd Zegr Zwgr Zwgr 

Major-oxide composition, in weight percent, and alkali-alumina ratio 

Si0 2 7L34 73^42 7498 7EU1 76^89 7L48 65^94 12M 6&85 75/78 73T81 74^11 

A1 2 3 14.90 13.66 13.18 13.63 13.52 13.93 15.85 14.49 13.98 12.50 14.04 14.40 

Fe 2 3 2.21 1.54 1.43 .86 .95 2.37 4.63 2.54 5.31 1.19 1.00 1.10 

MnO 08 .04 .05 .02 .03 .09 .10 .05 .12 .05 .05 .05 

MgO 92 1.05 .41 .22 .04 .54 1.78 .23 1.74 .61 .17 .26 

CaO 1.72 1.28 .93 .25 .24 1.66 2.84 1.04 3.16 .83 .69 1.29 

NaaO 4.06 4.11 3.17 4.25 3.70 3.79 2.66 3.88 3.53 3.92 4.15 3.76 

K;,0 3.94 4.08 4.99 4.92 4.60 3.88 3.15 4.71 .58 4.42 4.21 3.98 

Ti0 2 36 .26 .16 .10 .10 .29 .74 .30 .46 .04 .21 .16 

P 2 6 11 .08 .04 .04 .02 .08 .17 .10 .08 .02 .05 .04 

(Na 2 + K 2 0)/Al 2 3 .. .54 .60 .62 .67 .61 .55 .37 .59 .29 .67 .59 .54 

Normative-mineral comp osition, in weight percent, 1 and differentiation index (PI) 

Qtz 20 2&8 ill 30^5 36^5 206 27!i 2&0 33J5 32/7 iL~8 33.4 

Crn 1.1 .3 1.0 1.0 2.1 .4 3.4 1.4 2.0 1.5 1.7 

Or 23.4 24.2 30.7 29.2 27.2 23.0 19.0 27.8 3.5 26.3 25.3 23.7 

Ab 34.5 34.9 27.0 36.1 31.3 32.0 23.0 32.8 30.5 33.4 35.7 32.1 

An 7.8 5.9 4.4 1.0 1.1 8.2 13.3 4.5 15.5 3.5 3.1 6.2 

Wo .2 

En 2.3 2.6 1.0 .6 .1 1.3 4.5 .6 4.4 1.5 .4 .7 

Fs 3.6 2.5 2.5 1.5 1.6 4.0 7.6 4.3 9.4 2.2 1.6 1.9 

Hm 7 .5 .3 .2 .2 .5 1.4 .6 .9 .1 .4 .3 

Ap 3 .2 .1 .1 .4 .7 .2 .1 .1 

DI 84 88 92 96 95 85 69 89 79 92 93 89 

Trace-element abundances, in parts pe r million, and selected ratios 

Rb 130 94 144 174 145 159 114 119 20 130 131 112 

Cs 2.6 0.7 1.9 1.4 1.2 3.9 2.1 1.5 1.6 1.1 1.4 1.1 

Sr 291 314 118 58 54 155 284 142 nd 94 133 335 

Ba 1217 1292 628 430 386 522 580 773 137 727 1084 1451 

Rb/Cs 50 134 76 124 121 41 54 79 12 118 94 109 

Rb/Sr 4 .3 1.2 3.0 2.5 1.0 .4 .8 nd 1.4 1.0 .3 

Sc 4.2 2.5 2.3 4.2 4.2 5.9 13.1 4.1 23.6 2.7 2.1 11.5 

Cr 3.2 2.3 2.4 5.3 5.6 nd 7.6 2.6 5.4 6.0 4.9 4.2 

Co 3.7 3.3 1.6 .3 .3 2.2 12.0 2.6 11.2 .2 .7 1.2 

Zn 44 22 27 17 18 49 79 63 85 31 42 12 

La 39 26 39 27 35 35 35 75 6 21 38 36 

Ce 68 50 73 55 72 67 64 133 14 43 71 57 

Nd 27 18 28 25 34 33 28 64 7 19 24 18 

Sm 4 2 4 6 7 7 6 14 3 5 4 2 

Eu 85 .54 .75 .44 .45 .77 1.18 1.55 .65 .79 .63 .46 

Gd 3.7 1.4 2.0 4.0 5.0 5.0 5.3 10.0 3.0 3.5 3.0 3.3 

Tb 51 .24 .50 .84 .97 1.08 .72 2.13 .77 .64 .43 .18 

Ho 7 .6 .7 .9 1 1 .7 2.2 .8 .5 .3 .2 

Tm 33 .24 .26 .63 .67 .51 .33 1.0 .47 .46 .28 .28 

Yb 2.0 .8 1.8 3.4 4.0 4.0 2.3 6.3 3.3 2.1 1.5 .6 

Lu 28 .13 .28 .50 .60 .55 .34 .86 .48 .31 .23 .12 

La/Yb 19 32 22 8 9 9 15 12 2 10 25 60 

Hf 4.5 3.4 3.7 3.6 3.9 5.6 5.2 11.6 1.9 2.7 4.1 2.8 

Zr 202 157 178 210 186 277 273 383 510 54 153 118 

Th 2 11.0 14.3 17.9 7.2 11.2 20.2 11.6 13.2 .7 10.2 10.9 14.2 

Th 11.1 15.6 16.3 12.6 18.5 15.9 14.2 13.2 .9 11.2 12.7 16.7 

U 2 2.4 4.4 4.1 2.5 3.7 3.3 2.1 2.5 .5 2.5 2.1 4.3 

U 2.4 4.4 3.5 3.5 4.1 3.8 2.0 2.5 1.0 2.1 2.3 4.2 

Zr/Hf 45 46 48 58 48 49 52 33 nd 20 37 42 

'Fe 2 0, calculated as FeO. z Delayed neutron reactivation analyses by H.T. Millard, Jr., CM. Ellis, and V.C. Smith. 



INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 



125 



Table 6. — Major-oxide, normative-mineral, and trace-element com- 
positions, volatiles omitted, of Proterozoic Z intrusive rocks, 
Milford-Dedham zone, northeastern Massachusetts— Continued 

Description of samples 
Sample localities shown in Wones and others (1986) 

1. DMA-166. Inequigranular biotite granite, Dedham Granite; Bare- 
foot Hill quartz monzonite of Lyons (1969). Rockland and Rockland 
Circle, Brockton. UTM grid: N46609-E3295. 

2. DMA-123. Dedham Granite; Government Island, Cohasset. UTM 
grid: N46773-E3524. 

3. DMA-94. Dedham quartz monzonite of Lyons (1969), Dedham 
Granite; Roadcut, 1-95, 906 m south of Cocasset St., Foxborough. 
UTM grid: N46603-E3158. 

4. DMA-105. Aplite dike (35 cm wide), Dedham Granite, intruding 
hornblende-bearing granodiorite; west end of rest area, south side of 
Rte. 3, Weymouth. UTM grid: N46723-E3388. 

5. DMA-73. Biotite granite, Dedham Granite; Roadcut, northwest side 
of intersection of Rte. 128 and U.S. Rte. 1A. UTM grid: N46776-E3197. 

6. W-6. Slightly altered biotite granite, Dedham Granite; roadcut, 
1^95, 360 m east of U.S. Rte. 1A intersection. UTM grid: 
N46567-E3061. 

7. DMA-65. Granodiorite intruding quartzite, Dedham Granite north of 
Boston; SW. corner intersection of Main St. and Middlesex Fells 
Parkway, Saugus. UTM grid: N47038-E3328. 

8. DMA-127. Granite intruding amphibolite and diorite, Dedham 
Granite north of Boston; Spring St. , Lexington, 100 m north of Rte. 2. 
UTM grid: N46995-E3148. 

9. DMA-60. Granodiorite, Topsfield Granodiorite; Topsfield granite of 
Toulmin (1964), Granodiorite of Oxpasture Brook of Bell and others 
(1977). UTM grid: N47334-E3480. 

10. P-5. Esmond Granite, slightly altered, locally contains garnet; road- 
cut, 1-295, 300 m east of Woonsocket Reservoir, Smithfield-Lincoln 
town line, Rhode Island. UTM grid: N46458-E2938. 

11. DMA-206. Granite, Westwood Granite (Chute, 1966); roadcut, 
Rtes. 128 and 109, Norwood. UTM grid: N46794-E3182. 

12. DMA-110. Granite, Westwood Granite; Plymouth quarries, Wey- 
mouth. UTM grid: N46725-E3427. 



or bluish quartz occurs in aggregates or granular 
streaks, plagioclase is saussuritized, microcline is white 
to pink, and biotite, titanite, and epidote occur in clus- 
ters. Accessory minerals are apatite, opaque minerals, 
allanite, epidote, sericite, and chlorite. Plagioclase and 
microcline show recrystallization at their margins. 

Porphyritic granite is well exposed along Route 140 
west of Long Pond and in Fall River (near Quarry Street 
Church and at the Route 24-Brayton Street inter- 
change). Finer grained porphyritic varieties crop out to 
the northeast near Snipatuit Pond. O.D. Hermes and 
D.P. Murray (written commun., 1985) have identified a 
mass of undeformed alkalic granite within the area 
mapped as porphyritic granite on the State bedrock map 
near the small mass of diorite (Zdi) on the southwest side 
of Slocums River. 

Alaskite (Zagr) 

Light-colored gneissic alaskite (Zagr) forms pha- 
colithic masses near New Bedford that extend east 
toward Buzzards Bay and Cape Cod (fig. 11). The rock is 
resistant to weathering and is well exposed southwest of 
New Bedford. It has a remarkable resemblance to the 
Hope Valley Alaskite Gneiss (Zhg) of south-central Mas- 
sachusetts, western Rhode Island, and adjacent Con- 
necticut. 

The rock is pale orange to cream colored, fine to 
medium grained, and equigranular and has a weak 
foliation and lineation imparted by preferred orientation 
of micas and flattened quartz aggregates. Coarse- 
grained phases contain lenses of gray quartz and pinkish- 
orange K-feldspar. 



Table 7. — Modes, in percent, of plutonic rocks of the Dedham batholith, southeastern Massachusetts 
[Zfgr, granite of the Fall River pluton; Zpgr, porphyritic granite; Zagr, alaskite. Sample localities shown by field number in Wones and others (1986). a, albite; 

o,oligoclase; p, perthite; tr, trace] 



Sample no 1 

Unit Zfgr 

Points counted 1728 

Field no FRE-3* 

Quartz 33.9 

Plagioclase 28.3a 

Microcline 30. 8p 

Biotite 3.9 

Muscovite 1.0 

Epidote 1.5 

Chlorite 1 

Magnetite 

Titanite 2 

Apatite 

Allanite 1 

Garnet .1 

Zircon 

Calcite 

"Chemical analysis in table 8. 



Zfgr 

1559 

FRE-10 



Zfgr 
1613 
Mn-10 



Zfgr 
1870 
T-l 



Zpgr Zpgr 

1725 1967 

Ap-2 FR-16 



Zpgr 
1632 

NBN-22 



Zpgr 
1587 
We-2 



9 
Zagr 
1543 

We-14 



Zagr Zagr 

1680 1500 

FRE-6 NBN-3 



32.5 


36.1 


40.0 


37.7 


39.7 


30.0 


25.8 


33.0 


38.0 


36.7 


26.5a 


30.1 


27.5a 


24.8 


16.2 


27.4 


20.5a 


34.0a 


34.0a 


33.7o 


39.3p 


26.3 


28. lp 


21.1 


34.2 


25.4 


44.3 


31.2 


25.5 


26.7 


.1 


6.9 


2.0 


2.7 


2.5 


9.4 


5.5 


1.2 





1.4 


.8 


.1 


.8 


4.5 


4.3 


3.1 


1.1 





1.6 


1.3 





.1 


.4 


7.9 


2.2 


4.2 


1.8 











.3 





.8 


1.0 


.5 





.1 


tr 








.1 


.1 


.2 


.1 








.2 


.3 


.1 


.2 





.3 





.1 


.3 


.3 


.6 











.1 





tr 


.1 


.1 


.1 


.1 


.2 


.1 


.1 





tr 


tr 





tr 





.1 


.1 














.1 

















.7 


.1 


.1 





























.4 






























126 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



Table 8.— Major-oxide, normative-mineral, and trace-element com- 
positions, volatiles omitted, of Proterozoic Z intrusive rocks, south- 
eastern Massachusetts 

[Major-oxide compositions from X-ray spectroscopy by Paul Heam and Susan 
Wargo; all Fe reported as Fe 2 3 . Trace-element abundances from instrumental 
neutron activation analyses by L.J. Schwartz except Rb and Sr determined 

by X-ray spectroscopy by G. Sellars, B. McCall, and R. Johnson, nd, not 
determined] 

Sample no 1 2 3 4 5 

Unit Zfgr Zfgr Zfgr Zfgr Zpgr 

Major-oxide composition, in weight percent, 
and alkali-alumina ratio 

Si0 2 75.64 74.73 77.9 73.08 67.48 

A1 2 3 12.46 12.70 12.2 14.47 15.81 

Fe 2 3 87 1.20 1.4 2.13 2.71 

MnO 10 .05 .04 .07 .08 

MgO 26 .20 .20 .21 .94 

CaO 13 .70 .08 1.08 2.78 

NaaO 3.71 3.34 2.6 3.35 3.42 

KaO 4.68 4.60 4.58 4.55 3.76 

Ti0 2 06 .14 .17 .23 .40 

P 2 5 01 .04 .01 .07 .13 

(Na20+K 2 0)/Al 2 3 67 .62 .59 .55 .45 

Normative-mineral composition, in weight percent, 1 
and differentiation index (DI) 

Qtz 35.5 35.0 42.2 31.1 24.8 

Cm 1.1 .9 1.5 1.9 1.4 

Or 28.2 27.2 27.1 27.0 22.8 

Ab 32.1 28.3 21.9 32.2 29.7 

An 6 3.3 4.0 4.9 13.3 

En 7 .5 .5 .5 2.4 

Fs 1.7 2.0 2.3 3.7 4.6 

Ilm 1 .2 .3 .4 .8 

Ap .2 .3 

DI 96 90 91 90 77 

Trace-element abundances, in parts per million, 
and selected ratios 

Rb 290 143 152 142 135 

Cs 3.0 4.0 1.9 1.7 3.0 

Sr 28 48 55 80 302 

Ba 93 274 286 465 546 

Rb/Cs 97 36 80 84 45 

Rb/Sr 10.3 3.0 2.8 1.8 .4 

Sc 4.5 5.9 5.2 7.0 6.9 

Cr 3.0 nd nd 4.2 3.0 



The alaskite consists of quartz, microcline and micro- 
cline perthite, albite to sodic oligoclase, and typically less 
than 2 percent biotite. It contains accessory muscovite, 
garnet, magnetite, apatite, zircon, and rare allanite and 
monazite. Secondary minerals are calcite, chlorite, and 
epidote. Chemically the alaskite is inferred to be similar 
in composition to the Hope Valley Alaskite Gneiss. If the 
albite were plotted as alkali feldspar rather than plagio- 
clase, the location of the alaskite on the ternary plot (fig. 



Table 8. — Major-oxide, normative-mineral, and trace-element com- 
positions, volatiles omitted, of Proterozoic Z intrusive rocks, south- 
eastern Massachusetts — Continued 

Sample no 1 2 3 4 5 

Unit Zfgr Zfgr Zfgr Zfgr Zpgr 

Trace-element abundances, in parts per million, 
and selected ratios— Continued 

Co 3 2.2 1.2 1.5 5.7 

Zn 32 30 36 46 41 

La 22 24 31 70 29 

Ce 51 59 95 137 54 

Nd 28 23 44 57 22 

Sm 9.0 4.6 7.9 11 4.0 

Eu 40 .50 .75 .97 .85 

Gd 7.8 3.3 5.8 7.4 3.2 

Tb 1.58 .55 .96 1.32 .49 

Ho 2.2 .8 .8 .9 .7 

Tm 67 .25 .41 .80 .41 

Yb 7.2 1.7 3.3 4.4 2.0 

Lu 1.0 .25 .47 .62 .30 

La/Yb 3 14 9 16 14 

Hf 3.8 4.3 4.6 6.1 3.0 

Zr 145 145 nd 255 138 

Th 2 24.8 17.3 19.0 14.4 8.3 

Th 30.2 15.9 18.7 14.5 9.9 

U 2 5.6 2.0 4.2 4.0 5.0 

U 6.6 2.4 4.0 4.0 4.9 

Zr/Hf 38 34 nd 42 46 

'FeA calculated as FeO. 

2 Delayed neutron reactivation analyses by H.T. Millard, Jr., CM. Ellis, and V.C. Smith. 

Description of samples 
Sample localities shown in Wones and others (1986) 

1. AP-5. Granite of the Fall River pluton; leucocratic granite; roadcut, 
Rte. 140, just west of Pickens St. overpass, Lakeville. UTM grid: 
N46326-E3352. 

2. FRE-3. Granite of the Fall River pluton; biotite granite; roadcut, 
Rte. 24, 1.1 km south of interchange 40, Fall River. UTM grid: 
N46189-E3237. 

3. FR-17. Granite of the Fall River pluton; biotite granite, Bulgar- 
marsh Granite of Pollock (1964); roadcut, Rte. 24, 500 m east of 
interchange with Rte. 138, Tiverton. UTM grid: N46123-E3168. 

4. BG-72. Granite of the Fall River pluton; biotite granite, Bulgar- 
marsh Granite; sampled by Galloway (1973); same location as sample 3. 

5. DMA-172. Porphyritic granite; inequigranular gneissic biotite gran- 
ite, roadcut, Rte. 140, 1.6 km south of exit 9, on the Lakeville- 
Freetown town line. UTM grid: N46277-E3369. 



5) would be shifted to the quartz-K-feldspar side of the 
diagram and would identify it as an alkali-feldspar 
granite. 

Granite, Gneiss, and Schist, Undivided (Zgg) 

Cape Cod and the adjacent mainland south of Ply- 
mouth are covered by Pleistocene glacial materials, 
which conceal the bedrock to such an extent that only 



INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 



127 



Table 9. — Modes, in percent, of plutonic rocks of the Milford antiform 

[Zhg, Hope Valley Alaskite Gneiss; Zsg, Scituate Granite Gneiss; Zpg, Ponaganset Gneiss; Zgr, biotite granite; Zmgr, Milford Granite; Zmgd, mafic phase of the 

Milford Granite. Sample localities shown by field number in Wones and others (1986). a, albite; o, oligoclase; p, perthite; tr, trace] 

Sample no 1 3 4 5 6 7 8 9 10 

Unit Zhg Zsg Zsg Zsg Zpg Zgr Zgr Zmgr Zmgd 

Points counted 1390 1817 1619 1819 1525 1568 1493 1634 1621 

Field no M-10* Ge-8 U-6* B-31 C-l B-20 H-6b MM Mi-5' 

Quartz 40.1 38.7 40.3 33.0 39.0 33.8 33.8 46.6 38.4 

Plagioclase 28.5a 20.7a 26.9a 29.2o 30.7 33.5 5.0 24.4o 42.1 

Microcline 29.9p 32.3 27.6p 33.1 20.4 23.8 55. 4p 26.7 4.9 

Biotite 6.8 4.5 2.1 7.4 5.7 1.9 9.9 

Muscovite 1.2 .3 1.1 .1 2.0 2.7 .1 .2 

Epidote 1.1 .6 1.0 .9 .1 3.5 

Hornblende .3 

Chlorite .10 .4 .2 tr .3 

Magnetite .3 .10 tr 1.6 tr 

Hematite 0000 00 1.5 

Titanite .1 .1 .2 .7 .1 tr tr .5 

Apatite .3 tr .2 .1 tr .2 

Allanite .2 tr .2 tr tr tr 

Garnet tr tr .1 

Zircon tr tr tr tr tr 

Calcite 000 .200000 

"Chemical analysis in table 10. 



rare scattered outcrops and a few deep drill holes give 
clues as to the nature of the bedrock (fig. 17). Therefore, 
the units mapped in the Fall River-New Bedford area 
cannot be mapped in this region. The diverse rock types 
encountered in the few exposures and in the drill holes 
are assigned to an undivided unit of granite, gneiss, and 
schist (Zgg). Presumably most of the plutonic and met- 
amorphic rock units in the Fall River-New Bedford area 
project into the concealed area to the east. A marked 
north-south topographic lineament, parallel to faults 
bounding the Assawompset Pond graben, that passes 
through Snipatuit Pond (fig. 11) forms a convenient 
boundary for separating the region on the west, in which 
recognizable rock units can be mapped, from the region 
to the east, in which units cannot be mapped. 

The bedrock is known in this region in a few places 
west of Cape Cod. Both gneissic biotite granite and 
gneissic alaskite crop out on Front Street north of 
Marion center. North of Marion, no outcrops are known. 
Goldsmith was unable to locate outcrops indicated by 
Williams and Willey (1973) in the Plympton area south- 
west of Plymouth. Deep coreholes drilled by Bechtel 
Corporation for the Boston Edison Company at Rocky 
Point near Manomet (fig. 17) encountered a gray, 
medium-grained granodiorite (sample DMA-167) and 
gray aplite. In the Duxbury area, bedrock exposures 
were numerous enough for Chute (1965a) to assign a 
light-colored pinkish-gray granite to the Westwood 
Granite and one outcrop of coarse-grained granite to the 
Dedham Granite and to identify as a separate unit a 
medium-gray foliated biotite granite that intrudes the 



Westwood. Most of the outcrops in the Duxbury area are 
light-colored granite. It is not certain that the light- 
colored granite in the Duxbury area is actually the same 
Westwood Granite mapped north of the Narragansett 
basin. None of the rocks in the Duxbury area have been 
shown separately on the State bedrock map; all are 
included in the undivided granite, gneiss, and schist unit. 
Chute (1965a) also identified an exposure of light-pink to 
lavender-gray porphyritic rhyolite at Cripple Rocks 
(CR, fig. 17) in Kingston Bay and another of greenish 
rhyolite in Green Harbor cut by aplite dikes. The age of 
these rocks is unknown. Larger areas of rhyolite could 
quite likely be present under the glacial cover. 

Even less information is available for Cape Cod (Weed, 
in Goldsmith, this vol., chap. E, table 8, fig. 15). A 300-m 
drill hole in Harwich, Mass. (1, fig. 17), reported by 
Koteff and Cotton (1962) penetrated 171 m of steeply 
dipping, medium-gray, fine-grained, phyllitic schist con- 
taining layers of greenish-gray to gray crystalline lime- 
stone in the upper 15.2-18.3 m. Two 300-m holes in 
Brewster reported by R.Z. Gore (written commun., 
1978; 2 and 3, fig. 17) penetrated plutonic rocks. One of 
these holes went through 131 m of granite containing 
diorite and quartz-diorite inclusions. The granite is 
flanked above and below by granodiorite. A shear zone or 
foliated zone separates the upper granodiorite from the 
granite, and an alteration zone passes through the lower 
granodiorite near the bottom of the hole. The second hole 
penetrated sheared granodiorite. Another hole near 
Brewster (4, fig. 17) ended in till or granite. Unidentified 
bedrock has been reported from other wells on Cape Cod 



128 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



Table 10. — Major-oxide, normative-mineral, and trace-element 
compositions, volatiles omitted, of Proterozoic Z intrusive rocks in 
the Milford antiform 
[Major oxides determined by X-ray spectroscopy by Paul Hearn and Susan 
Wargo; all Fe reported as Fe 2 3 . Trace-element abundances determined by 

instrumental neutron activation analyses by L.J. Schwartz except for Rb and 
Sr by X-ray fluorescence analyses by G. Sellars and B. McCall] 

Sample no 1 2 3 4 5 

Unit Zmgr Zmgd Zgr Zsg Zhg 

Major-oxide composition, in weight percent, 
and alkali-alumina ratio 

Si0 2 75.82 71.08 72.59 75.51 76.01 

A1 2 3 12.74 13.36 13.24 13.39 11.72 

Fe 2 3 1.18 3.27 2.64 1.42 1.20 

MnO 05 .08 .07 .05 .02 

MgO 31 1.18 .45 .21 .18 

CaO 62 3.13 1.88 .97 .22 

NaaO 3.58 3.61 3.24 3.35 4.11 

K 2 4.94 2.58 3.44 4.90 4.02 

Ti0 2 09 .48 .36 .17 .07 

P 2 5 04 .11 .10 .04 .02 

(NaaO + rLXO/AljAs 67 .46 .50 .62 .69 

Normative-mineral composition, in weight percent, 1 
and differentiation index (DI) 

Qtz 33.7 30.5 34.9 33.9 35.9 

Cm 5 1.0 .9 .3 

Or 29.4 15.4 20.7 29.0 24.3 

Ab 30.5 30.9 28.0 28.3 35.6 

An 2.8 12.8 8.8 4.6 1.0 

Wo .8 

En 8 3.0 1.1 .5 .5 

Fs 2.1 5.4 4.5 2.4 2.2 

Ilm 2 .9 .7 .3 .1 

Ap 1 .3 .2 .1 

DI 94 77 84 91 96 

Trace-element abundances, in parts per million, 
and selected ratios 

Rb 116 73 77 172 137 

Cs 1.1 2.2 .7 2.3 .6 

Sr 66 261 208 63 51 

Ba 540 927 1038 297 593 

Rb/Cs 105 33 110 75 228 

Rb/Sr 1.8 .3 .4 2.7 2.7 

Sc 1.2 11.6 4.1 2.8 5.6 

Cr 2.0 3.0 6.8 .2 5.5 



(J. Givens and D. LeBlanc, written commun., 1977). A 
hole near Woods Hole (5, fig. 17; Weed, in Goldsmith, 
this vol., chap. E, table 8, fig. 15) encountered gray to 
pink granodiorite at 83 m below sea level. Thus, the 
concealed basement under Cape Cod appears to consist 
of rocks similar to those exposed on the mainland to the 
northwest. As confirmation of this conclusion, seismic 
compressional-wave velocities in the vicinity of the 
Brewster and Woods Hole holes were 18,000-20,000 feet 



Table 10. — Major-oxide, normative-mineral, and trace-element 
compositions, volatiles omitted, of Proterozoic Z intrusive rocks in 
the Milford antiform— Continued 

Sample no 1 2 3 4 5 

Unit Zmgr Zmgd Zgr Zsg Zhg 

Trace-element abundances, in parts per million, 
and selected ratios— Continued 

Co 4 3.6 3.5 1.1 .1 

Zn 30 64 59 34 28 

La 32 40 50 36 21 

Ce 65 84 92 73 72 

Nd 29 43 47 34 26 

Sm 5.6 8.8 9.7 7.1 6.0 

Eu 1.06 2.01 1.93 .62 .71 

Gd 2.8 7.1 6.8 5.8 4.6 

Tb 49 1.15 1.21 1.06 .92 

Ho 5 1.1 1.7 1.1 1.0 

Tm 23 .46 .63 .74 .76 

Yb 1.1 3.5 3.5 4.8 4.6 

Lu 16 .48 .48 .69 .62 

La/Yb 29 11 14 7 5 

Hf 3.3 5.8 6.0 4.0 6.9 

Zr 109 288 230 175 228 

Th 2 7.1 8.5 9.9 20.3 9.0 

Th 8.3 8.8 9.2 21.0 11.2 

U 2 1.3 2.1 1.4 3.7 2.6 

U 1.2 1.9 1.5 3.5 2.5 

Zr/Hf 33 50 38 44 33 

'Fe 2 3 calculated as FeO. 

2 Delayed neutron determination analysis by H.T. Millard, Jr., and B. McCall. 

Description of samples 
Sample localities shown in Wones and others (1986) 

1. DMA-22. Milford Granite; massive granite; roadcut, southeast side 
of intersection of Rtes. 85 and 1-495, Milford. UTM grid: 
N46710-E2927. 

2. Mi-5. Milford Granite, mafic phase; gray, seriate granite; roadcut, 
northbound lane 1^95 at Haven St. overpass, Milford. UTM grid: 
N46727-E2911. 

3. DMA-19. Biotite granite; cataclastic granite; Winch Road, 200 m 
southwest of Edmands Rd., Framingham. UTM grid: N46886-E2986. 

4. U-6. Scituate Granite Gneiss; splotchy, gneissic, biotite granite; 
quarry, Quarry Hill, on Hartford Ave. West, 1.5 km west of North 
Uxbridge, Uxbridge. UTM grid: N46623-E2801. 

5. M-10. Hope Valley Alaskite Gneiss; leucocratic gneissic granite; 
roadcut, 1-495, northbound lane, 1 km south of interchange 10 (Mas- 
sachusetts Turnpike), Hopkinton. UTM grid: N46812-E2889. 



per second (fps), similar to velocities in the areas of 
Dedham Granite on the mainland (Oldale and Tuttle, 
1965). Seismic velocities in the vicinity of the Harwich 
hole had a similar but wider range of 17,800-21,500 fps 
(Oldale and Tuttle, 1964), indicating the greater variabil- 
ity of rock types in this area. The granite, gneiss, and 
schist unit is inferred to underlie upper Cape Cod, the 
Elizabeth Islands, and the sedimentary and volcanic 
rocks of Mesozoic age beneath Nantucket Sound. 



INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 



129 



PLUTONIC ROCKS OF THE MILFORD ANTIFORM 

The plutonic rocks of the Milford antiform (fig. 11) are 
flanked by, and contain belts of, older metamorphic rocks 
of the Plainfield and Westboro Formations and the 
Blackstone Group (included in other rocks of fig. 11). The 
plutonic rocks are granitic in composition, although the 
Milford Granite (Zmgr) has a granodioritic phase 
(Zmgd). The plutonic rocks are similar in composition to 
rocks in the Dedham batholith, ranging from alkali 
feldspar granite to granodiorite (fig. 5); however, most 
tend to have a well-developed to poorly developed 
gneissic fabric, unlike the granitic rocks of the Dedham 
batholith except those in the New Bedford area. The 
Hope Valley Alaskite Gneiss (Zhg) and associated gra- 
nitic gneisses in the Milford antiform are the northern 
continuation of a belt of Proterozoic Z plutonic rocks that 
extends from the New London anticlinorium in south- 
eastern Connecticut (Goldsmith, 1985; Rodgers, 1985) 
and adjacent Rhode Island along the Connecticut-Rhode 
Island border into southern Massachusetts, where they 
bend around the north side of the Milford antiform in the 
Framingham area. Age relations seen in southeastern 
Connecticut and in the Milford antiform indicate that the 
Hope Valley is the youngest of the gneissic suite. This 
age is also suggested by the 601 ±5-Ma age obtained from 
the Hope Valley in southern Rhode Island by Hermes 
and Zartman (1985). Its age relative to the Milford 
Granite in the core of the antiform is not known; the two 
may lie in separate terranes (O'Hara and Gromet, 1985). 
The plutonic rocks of the Milford antiform are less 
gneissic to the east than to the west; within the antiform 
are zones of shearing bounding areas of less sheared to 
unsheared rock. O'Hara and Gromet (1985; Gromet and 
O'Hara, 1984) have placed their major terrane boundary 
along one of these. 

Hope Valley Alaskite Gneiss (Zhg) 

The Hope Valley Alaskite Gneiss (Zhg) forms tabular 
masses along the west side of the Rhode Island anticli- 
norium from southern Rhode Island and eastern Con- 
necticut to northwestern Rhode Island; it flanks the west 
side of the Milford antiform and terminates at the north 
end of the anticlinorium in Massachusetts (fig. 11). The 
Hope Valley intrudes the Plainfield Formation in Con- 
necticut and rocks mapped as Blackstone Group in Rhode 
Island (Hermes and others, 1981). The 630-Ma radiomet- 
ric age (table 1) of this suite of rocks comes from this unit 
in Massachusetts; the 601±5-Ma age comes from the 
Hope Valley in southern Rhode Island. 

The rock is light pink to tan. It contains little biotite 
and sparse magnetite (table 9); in some places, magnetite 
is more abundant than biotite. Rodded aggregates of 
quartz give the rock a pronounced lineation in some 



places. A foliation is produced in other places by a 
preferred orientation of flat lenses of quartz and flat 
lenses of feldspar as well as by parallel orientation of 
biotite where present. Quinn (1971) gave modal values of 
24-43 percent quartz, 18-40 percent K-feldspar, 20-40 
percent plagioclase, and l-i percent biotite and magne- 
tite for the Hope Valley in Rhode Island. Muscovite is a 
typical accessory mineral, and some rock contains small 
garnets. Chemically the rock is poor in mafic constituents 
(table 10, no. 5; figs. 7, 8). It shows enrichment in 
rare-earth elements and a negative europium anomaly 
(fig. 18), like most of the other Proterozoic Z rocks. 

Scituate Granite Gneiss (Zsg) 

The Scituate Granite Gneiss (Zsg) on the State bed- 
rock map forms elongate masses in northwestern Rhode 
Island and Massachusetts (fig. 11). The name Scituate 
Granite Gneiss is no longer appropriate because the type 
Scituate Granite Gneiss of Quinn (1971) at Scituate, R.I., 
has been shown by Hermes and others (1981) and Her- 
mes and Zartman (1985) to be Middle Devonian (373 ±7 
Ma), on the basis of a radiometric determination of 
zircons from the type Scituate, and is part of a Devonian 
alkalic suite. The Scituate Granite Gneiss of the type 
area in central Rhode Island described by Quinn (1971) is 
here renamed Scituate Granite and assigned a Middle 
Devonian age. The area of the Scituate Granite coincides 
with that shown as the main mass of Scituate by Quinn 
(1971) but with minor adjustments of boundaries (O.D. 
Hermes and Peter Gromet, oral commun., 1984). The 
type Scituate Granite Gneiss of Quinn is henceforth 
referred to as the Scituate Granite. The Scituate Granite 
Gneiss shown on the Massachusetts bedrock map con- 
tains roof pendants of Plainfield Formation and is Prot- 
erozoic Z in age (table 1). We suggest reviving Emer- 
son's old name Northbridge Granite Gneiss for this unit, 
although detailed mapping in this part of Massachusetts 
may reveal that several distinct types of granite are 
present. The Middle Devonian Scituate Granite is at 
present not known in Massachusetts. 

The Scituate Granite Gneiss of the State bedrock map 
is medium to coarse grained, weathers to a pink or tan, 
and has a foliation and pronounced lineation defined by 
aggregates of biotite. Of the modes given in table 9, no. 
4 is probably the most representative. The Scituate 
Granite Gneiss, like the Hope Valley Alaskite Gneiss, 
contains accessory muscovite and garnet; it is grada- 
tional with the Hope Valley Alaskite Gneiss. These two 
units may represent products of a large zoned magma 
chamber that has undergone deformation after primary 
crystallization. In chemical composition, the Scituate is 
similar to the Hope Valley in many respects except that 
it contains more mafic constituents (table 10). The REE 
pattern is similar to that of the Hope Valley (fig. 18). 



130 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



100 








I I I 


I I 


I 






i 






90 










EXPLANATION 










- 


80 










Dedham 


Gran 


te 


(Zdgr) 




- 


70 






















- 


60 






\ff 
















- 


50 






sX 


















40 




\r> 




\ x ^\\\\ 
















30 














b_ 










20 








T 
























_6 > 

1 




10 






















~^^ 


9 


















3_ 




^^^^ - 


8 


















%^ 




- 


7 






















- 


6 






















- 


5 






















V 


4 


- 






















3 
? 


A 

i 


I I 




I I I 


I I 


I 






i 




I 



La 



Ce 



Nd 



Sm 



Eu 



Gd 



Tb 



Tm 



Yb 



Lu 



Figure 15. — Chondrite-normalized plot of rare-earth elements in Proterozoic Z batholithic rocks, northeastern Massachusetts: A, Dedham 
Granite; B, Dedham Granite north of Boston, Westwood and Esmond Granites, and Topsfield Granodiorite. Numbers are sample numbers 
from table 6. 



PONAGANSET GNEISS (Zpg) 

The Ponaganset Gneiss (Zpg) is largely confined to 
Rhode Island, but masses of porphyritic biotite gneiss 
assigned to the Ponaganset project into Massachusetts 
(fig. 11). The Ponaganset Gneiss intrudes the Blackstone 
Group in Rhode Island (Quinn, 1971). In southern Rhode 
Island, Feininger (1963) demonstrated that the Ponagan- 
set Gneiss is older than the Hope Valley Alaskite Gneiss 
and appears to be the oldest granitic intrusive in the 
Milford-Dedham terrane. 

The rock is highly variable in color index, quartz 
content, and ratio of plagioclase to total feldspar, but 
most of it, as mapped in Massachusetts, is a gray, 



biotitic, inequigranular, granitoid rock (table 9, no. 6) 
with a gneissic foliation produced by parallel orientation 
of the biotite and elongation of megacrysts. The trace of 
hornblende in sample 6 (table 9) indicates that this rock 
is more mafic than the other plutonic rocks of the 
antiform. In eastern Connecticut, the unit typically 
contains large round to ellipsoidal megacrysts of K- 
feldspar and aggregates of K-feldspar and plagioclase 
(Harwood and Goldsmith, 1971). In places, particularly 
in Massachusetts, the megacrysts are flattened, or less 
commonly rodded, to produce a markedly lineated 
gneiss, as in outcrops at the gaging station on the West 
River, north of Route 16 in the town of Uxbridge. The 



INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 



131 







i i i i i i 




i i i i 

EXPLANATION 


1 


200 









Dedham Granite north of Boston 

(Zdngr) (samples 7 and 8) 

Topsfield Granodiorite (Ztgd) 

(sample 9) 

Esmond Granite (Zegr) (sample 10) 








Westwood Granite (Zwgr) 










(samples 1 1 and 12) 




100 




^\# 






- 


90 










- 


80 










- 


70 










- 


60 


v ^ 








~ 


50 












40 








^~~~~— — -__S 




30 




\ ^"N-^L \ \ / 

\ \ \ / 








20 




'•■■£ "'••■ v \ \ 

■ \ \ s 






- 






-~- '• \ \ \ / 

\ -. \ 

w 




9 


^~-~- 




-^is \ 

\ 




\ 


10 


- 




11 


~ ~— r 


9 






8 


_ 








11 


7 


_ 


6 


" 






\V'"' 


- 


5 


- 








- 


4 


- 




'•-''' 




- 


3 

9 


B 


I i I ! I I 


i i 


1 1 i 1 


i 



La 



Ce 



Nd 



Sm Eu Gd Tb 

Figure 15. — Continued. 



Tm 



Yb 



Lu 



inequigranular texture persists in Massachusetts and is 
one of the bases for mapping the unit there. In places in 
northern Rhode Island, some rock that has been mapped 
as Ponaganset Gneiss is equigranular. The gneissic fabric 
may be due to Paleozoic deformation. As mapped, the 
Ponaganset Gneiss may actually consist of several rock 
types. 



Milford Granite (Zmgr, Zmgd) 

The Milford Granite (Zmgr, Zmgd) occupies an area of 
about 100 km 2 . Its central mass near Milford (fig. 11) is 
elliptical and has been divided into two phases: a light- 
colored phase (Zmgr) and a dark-colored phase (Zmgd) 
that defines an irregular border for the largest of the 
light-colored plutons. The Milford intrudes the Black- 



132 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 




Figure 16.— Chondrite-normalized plot of rare-earth elements of Proterozoic Z batholithic rocks in the Milford-Dedham zone, southeastern 

Massachusetts. Numbers refer to sample numbers in table 8. 



stone Group (Zb) and the Ponaganset Gneiss and has 
been deformed with them at a later undefined time. 

The Milford Granite is characterized by a distinctive 
salmon-pink color, bluish quartz on weathered surfaces, 
and a lineation defined by lenticular mosaics of quartz 
and oriented patches of biotite. This lineation contrasts 
distinctly with the brittle fractures and massive texture 
of the Dedham Granite and with the cataclastic fabrics 
commonly found in the biotite granite (Zgr) of the 
Framingham area. 

The light-colored phase is granitic in composition, 
whereas the dark-colored phase is granodioritic (tables 
4, 9). Biotite is the most abundant ferromagnesian 



mineral. In the dark-colored phase, clots of epidote and 
biotite are suggestive of hornblende as a primary mag- 
matic phase, although no hornblende has been observed 
in these rocks. The light-colored phase of the Milford 
contains garnet, entrained epidote grains, and muscovite 
as accessory minerals, whereas the mafic phase contains 
titanite and opaque minerals. Both phases contain apa- 
tite, allanite, and zircon. 

The limited chemical data of table 10 (plotted in figs. 7, 
8) illustrate the difference in composition between the 
two phases. Both the granite and granodiorite phases 
have marked light-REE enrichment, but the granite has 
heavy-REE depletion relative to the granodiorite, which 



INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 



133 







70°30' 70W 




Scituate # 




I I 




Green Harbor # 






Duxbury m 




Provincetown 


42W 


^^"piym 


CR 

outh DMA-167 




s Manomet. 
/ ^Approximate southward 


Cape Cod Bay 




limit of outcrops 










O^ 


Brewster #0 4 
2°°3 

^o Harwich 
Cape Cod 


41°30' 


^--Marion 


o 5 

Woods 

Martha's 
Vineyard 


Nantucket Sound 
Hole 








Nantucket 

! t 



10 20 KILOMETERS 

Figure 17. — Locations of drill holes encountering bedrock in Cape Cod 
and the adjacent mainland and limit of mappable rock units. CR, 
Cripple Rocks. Numbers refer to drill holes discussed in text. 
DMA— 167, granodiorite from a hole near Manomet. 

has a fairly flat profile (fig. 18). The granodiorite contains 
more scandium (11.6 ppm) than does the granite (1.2 
ppm). These results are typical, but not definitive, of 
plutons found in compressive tectonic regimes, where 
zoned plutons are common (Bateman and Chappell, 1979; 
Noyes and others, 1983) and where hornblende fraction- 
ation may cause the development of peraluminous mag- 
mas (Cawthorn and O'Hara, 1976; Guy, 1980). 

Biotite Granite (Zgr) 

The biotite granite (Zgr) crops out over 180 km 2 south 
of the Bloody Bluff fault zone in the Framingham, 
Holliston, and Franklin areas (fig. 11). It intrudes earlier 
gabbro (Zgb), diorite (Zdi), and Proterozoic Z stratified 
rocks (Zv, Zvf, and Zw) (Nelson, 1975a,b; Volckmann, 
1977). The granite as mapped has different textures in 
different places, and later detailed mapping may show 
that it can be separated into several units or assigned to 
other existing units. Much of the biotite granite in its 
northern part was mapped previously as Milford Granite 
(Nelson, 1975a,b; Volckmann, 1977), and further map- 



ping may delineate areas of rock that should be reas- 
signed to that unit. The biotite granite is deformed by 
mylonitization and cataclasis near fault zones; in places it 
has the brittle fractures characteristic of the Dedham 
terrane, and elsewhere it has a gneissic fabric like the 
other granite gneisses of the Milford terrane. 

The biotite granite is gray to light gray and weathers 
to pink or buff. It lacks the conspicuous red and green 
coloration characteristic of the Dedham Granite. The unit 
is mainly granite but is in part granodiorite and tonalite. 
Some of the rock is quite poor in mafic minerals. The 
muscovite reported in the modes (table 9) is probably 
secondary, as are the epidote and chlorite. Some of the 
granite contains phenocrysts of plagioclase and perthitic 
alkali feldspar, although the superposed deformation 
makes the identification of primary igneous textures 
difficult. Biotite is the dominant mafic phase. Adjacent to 
the Bloody Bluff fault zone, allanite is a conspicuous 
accessory that distinguishes the biotite granite from the 
Dedham and Milford Granites, which contain only minor 
amounts of allanite. The biotite granite lacks the euhe- 
dral titanites characteristic of the Dedham Granite and 
the garnet characteristic of the Milford Granite and the 
Hope Valley Alaskite Gneiss. The sample analyzed (table 
10, no. 3) comes from near the Bloody Bluff fault zone 
and may not be representative of the larger mass of 
granite south and southwest of Framingham. The biotite 
granite is assigned a Proterozoic Z age because of its 
compositional and textural similarities to the other Prot- 
erozoic Z intrusive rocks of the Milford antiform. 



PALEOZOIC INTRUSIVE ROCKS 

The Paleozoic intrusive rocks of the Milford-Dedham 
zone (fig. 19) comprise a group of relatively discrete 
plutons ranging in age from Ordovician to Devonian 
(table 1) and in composition from gabbro to granite. The 
granitic plutons are alkalic in composition and include the 
quartz-poor phases syenite and monzonite. The dated 
gabbroic and dioritic plutons are Ordovician in age. Some 
might be younger and related to the Ordovician and 
Silurian to Devonian alkalic granites. In addition to the 
gabbro and alkalic granite plutons, micrographic rhyolite 
(DSnr) intrudes the Silurian and Devonian Newbury 
Volcanic Complex, and dikes of diabase and basalt of 
Mesozoic age cut the older rocks of the zone. All these 
rocks in Massachusetts appear to be unmetamorphosed 
but are locally cut by faults of late Paleozoic and Mesozoic 
age. 

NAHANT GABBRO (Ongb) 

The Nahant Gabbro (Ongb) intrudes the Lower Cam- 
brian Weymouth Formation at Nahant (fig. 19) and thus 
is younger than the gabbroic or dioritic rocks that are 



134 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



200 




Figure 18.— Chondrite-normalized plot of rare-earth elements of Proterozoic Z batholithic rocks, Milford antiform. Numbers refer to sample 

numbers in table 10. 



intruded by the Dedham Granite and Topsfield Granodi- 
orite. It appears from its aeromagnetic pattern (Har- 
wood and Zietz, 1976) to be a shallow cylindrical plug 
about 0.3 km in diameter. The Nahant Gabbro is pro- 
jected to lie at depth beneath the Cape Ann Granite, as 
shown in cross section B-B' of the State bedrock map, on 
the basis of the magnetic signature over the pluton. 
Unrecognized equivalents to the Nahant Gabbro at Na- 
hant and at Salem Neck (see p. 136) may be present 
within the gabbros and diorites (Zdigb, Zgb) shown on 
the State bedrock map (see Nelson, 1975b). 

Bell (1977) described three facies at Nahant, which are 
included within the Nahant Gabbro on the State bedrock 
map: pyroxene gabbro, olivine gabbro, and quartz dio- 
rite. The pyroxene gabbro (table 11, no. 2), which forms 
the main mass at Nahant, is described by Bell (1977) as 
a massive equigranular rock that locally has subophitic 



texture. The plagioclase is tabular. In addition to the 
minerals shown in table 11, the rock contains accessory 
zircon. Alteration products include epidote, chlorite, and 
calcite. 

The olivine gabbro (table 11, no. 3) crops out on the 
north shore of Nahant and is compositionally banded. It 
has a mottled appearance with white or gray flecks 
within a black matrix and contains accessory sulfide 
minerals. Rb-Sr and K-Ar analyses of biotite from this 
rock were made for the Ordovician age determination 
(table 1). 

The quartz diorite crops out on Little Nahant west of 
Nahant proper. The quartz diorite (table 11, no. 4) is fine 
to medium grained. Some samples contain accessory 
apatite and titanite; chlorite, epidote, and calcite are 
alteration products. The quartz diorite has undergone 
more brittle deformation and alteration than have the 



INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 



135 




Figure 19. -Distribution of Paleozoic intrusive rocks in the Nahant Gabbro; Dpgr, Peabody Granite; Dwm, Wenham Mon- 

Milford-Dedham zone. DH, Diamond Hill; AP, Andrews Point. zonite; Jd, Jurassic dike; SOqgr, Quincy Granite; DOgr, alkalic 

SOcgr, alkalic granite and quartz syenite of the Cape Ann granite in Franklin; Drgr, granite of the Rattlesnake Hill'pluton 
Complex; SOcb, Beverly Syenite; SOcsm, Squam Granite; Ongb, 



136 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



Table 1 1.— Modes, in percent, of some Paleozoic intrusive rocks of the Milford-Dedham zone 
[DOgr, alkalic granite in Franklin; Ongb, Nahant Gabbro; SOqgr, Quincy Granite; SOcgr, granite of the Cape Ann Complex; SOcb, Beverly Syenite; Dpgr, Peabody 
Granite; Dwm, Wenham Monzonite; Drgr, granite of the Rattlesnake Hill pluton. a, albite; L, labradorite; p, perthite; c, orthoclase; u, augite; g, pigeonite; 
r, riebeckite; — , not reported] 

Sample no I 2 3 4 5 6 7 8 9 10 11 12 13 

Unit DOgr Ongb Ongb Ongb Ongb SOqgr SOcgr SOcb Dpgr Dwm Drgr Drgr Drgr 

Quartz 34.6 0-5 5-15 31 24 28 3 26^7 27-31 23^1 

Plagioclase 19.4a 55-65 5-20 25-40 50L 3 41 

Microcline 36.7p 0-6c 25-40p 60p 63p 77-97 63p 43 51-71 64-67 53-72 

Biotite 7.5 0-2 2-5 3-7 15 1 1-5 .3 2 1-5 

Muscovite 4 tr 

Epidote 4 tr - 3-5 tr - - 

Hornblende tr 0-10 5-15 5-10 5 10* 5 1-7 6 9 3^r l-6r 

Other amphibole — — — — — — — -3 1 — — — 

Pyroxene 30-45g 25-40g - 20u 1-9 .3 

Olivine - 0-2 20-50 - 4 - 

Chlorite 4 tr tr tr - - - - - - - 

Opaque minerals tr 3-10 tr 2-5 5 - 0.2-8.1 0-2 .9 1 

Titanite 2 tr - - - - tr - - - 

Apatite 1 0-1 - 0.2-0.7 tr .3 - - 

Other 2* tr** 2-5* 0.5-1** tr** - - Mt - - - - 

"Total amphibole and pyroxene. *Allanite; also includes trace amounts of zircon and fluorite. **Calcite. 
tAntigorite? ttNonopaque accessory minerals. 

Description of samples 
1. Alkalic granite in Franklin, F^9 (Wones and others, 1986); 1,924 points counted. 
2-4. Pyroxene gabbro from Nahant, olivine gabbro from Nahant, and quartz diorite from Little Nahant (Bell, 1977, p. 20e-25e). 

5. Gabbro at Salem Neck (Toulmin, 1964, p. A58). 

6. Quincy Granite from Quincy (Dale, 1923; Warren, 1913). 

7, 8. Alkalic granite and quartz syenite of Cape Ann Complex and Beverly Syenite (Dennen, 1981). 

9, 10. Peabody Granite (average of five samples) and Wenham Monzonite (average of two samples) (Toulmin, 1964, p. A31, A44). 
11-13. Biotite granite, coarse-grained granite, and fine-grained granite (Lyons and Krueger, 1976). 



other phases, and quite possibly it lies close to a fault 
zone (Goldsmith, this vol., chap. H). Wones suggested 
that the rock might be a more mafic phase of the Dedham 
Granite. 

The gabbro at Salem Neck (Toulmin, 1964; Ongb near 
Salem in fig. 19) is the same general age as the Nahant 
Gabbro (table 1). It is a pyroxene gabbro similar in 
composition to the Nahant Gabbro, but it contains more 
biotite (table 11, no. 5). It is less altered than the diorite 
and gabbro (Zdigb) surrounding it and is intimately 
mixed with syenitic material. 

QUINCY GRANITE (SOqgr) 

The Quincy Granite (SOqgr) intrudes the Middle Cam- 
brian Braintree Argillite at several localities along its 
eastern margin within the towns of Quincy and Braintree 
(Nellis and Hellier, 1976; fig. 19). The southern contact of 
the body is with the chemically and mineralogically 
similar Blue Hills Granite Porphyry (Chute, 1969). The 
Quincy Granite is bounded on the northwest by a reverse 
fault, the Blue Hills thrust (Billings, 1976; Goldsmith, 
this vol., chap. H), and on the west by the Neponset 
fault. No clasts of the Quincy Granite have been 
observed in the Lower Pennsylvanian Pondville Con- 



glomerate, but clasts of the overlying and probably 
related Blue Hills Granite Porphyry (see below) are 
common in the Pondville Conglomerate (Chute, 1969). 
The combined area of the Quincy Granite and the Blue 
Hills Granite Porphyry is 55 km 2 . Billings (1982) sug- 
gested that the Quincy Granite formed as a product of 
Ordovician cauldron subsidence. 

The fresh rock is dark gray to gray green but weathers 
to buff brown or salmon; its texture is hypidiomorphic 
granular. Joint and slickenside surfaces are typically 
coated with riebeckite. The amphiboles in the granite 
(table 11, no. 6) are riebeckitic, and the pyroxene is 
acmitic. The averaged analyses from the type Quincy 
Granite (table 12, no. 4) indicate the peralkalinity of the 
rock, as does the high alkali-to-alumina ratio. The mafic 
minerals are interstitial to subhedral quartz and perthite 
and clearly were late in the order of crystallization. 
Accessory minerals are astrophyllite, aenigmatite, 
ilmenite, magnetite, titanite, fluorite, and, uncommonly, 
biotite. The late crystallization of mafic minerals sug- 
gests significant subsolidus reactions. Late-stage quartz 
and calcite have been observed in vugs and fractures, 
along with chlorite, hematite, and limonite. 

Textural variants within the Quincy Granite include a 
fine-grained phase, especially abundant near the con- 



INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 



137 



Table 12. — Major-oxide, normative-mineral, and trace-element 
compositions, volatiles omitted, of some Paleozoic platonic rocks, 
Milford-Dedham zone 

[Major-oxide compositions for samples 1-2 from X-ray spectroscopy by Paul 
Hearn and Susan Wargo, all Fe reported as Fe 2 3 ; sample 3, whole-rock 
analysis by H.S. Washington (in Emerson, 1917); samples 4 and 6, whole-rock 
analyses by C.H. Warren and H.S. Washington (in Emerson, 1917); sample 5 
analyses by O.D. Hermes, L. Kwak, C. Mandeville, and C. Olson; sample 7 by 
O.F. Tuttle and N.L. Bowen. Trace-element abundances from instrumental 
neutron activation analyses by L.J. Schwartz except Rb and Sr determined by 
X-ray spectroscopy by G. Sellars and B. McCall. No trace-element analyses 
available for samples 3-7. nd, not determined] 

Sample no 1 2 3 4 5 6 7 

Unit DOgr DSnr Ongb SOqgr SOqgr SOcgr Dpgr 

Major-oxide composition, in weight percent, 
and alkali-alumina ratio 

Si0 2 75.22 76.7 43.73 74.9 70.24 77.6 72.24 

A1 2 3 15.42 12.3 20.17 11.6 9.80 11.9 13.18 

Fe 2 3 1.10 1.15 4.23 2.29 7.17 .55 .24 

FeO nd nd 6.93 1.25 2.50 .87 2.77 

MnO 02 .03 .02 .14 .10 

MgO 3.91 .04 .06 .20 

CaO 42 .5 10.99 .41 .58 .31 1.10 

NaaO 3.24 3.9 2.42 4.30 5.26 3.80 3.99 

KaO 4.56 3.83 1.45 4.64 4.24 4.98 5.01 

Ti0 2 08 .10 4.23 .20 .17 .25 .36 

P 2 6 02 .15 .07 

Zr0 2 nd .20 

(Na20+K 2 0)/Al 2 3 ... .51 .63 .19 .77 .97 .74 .68 

Normative-mineral composition, in weight percent, 1 
and differentiation index (DI) 



Qtz 37.1 37.4 34.8 25.2 

Crn 4.4 .8 

Or 26.9 22.6 8.6 27.5 24.9 

Ab 27.4 33.0 20.4 36.3 26.6 

An 2.0 2.5 39.8 1.6 

Di 9.5 .2 

Hd 1.9 .2 

Ac 15.5 

Wo 1.2 

En 3.3 .( 

Fs 2.1 2.0 .7 .01 3.1 

Fo 1.4 2.6 

Fa .7 

Ilm 2 .2 8.0 .4 .3 

Mag 6.1 

Ap .3 

DI 91 93 29 98 77 

Trace-element abundances, in parts per mill 
and selected ratios 

Rb 358 101 

Cs 6.0 .9 

Sr 24 48 



35.5 


25.7 








29.5 


29.6 


32.1 


33.7 


.7 


3.3 





.2 



15 



tacts, that has micrographic textures of quartz and 
perthite. The latter mineral is seriate in this phase of the 
Quincy and, when coarse, gives a porphyritic appearance 
to hand specimens. In other places, pegmatitic segre- 
gations contain very coarse crystals of acmitic 
pyroxene and amphibole. There were probably several 
pulses of magma, closely spaced in time and of similar 
composition. 



Table 12.— Major-oxide, normative-mineral, and trace-element 
compositions, volatiles omitted, of some Paleozoic plutonic rocks, 
Milford-Dedham zone— Continued 

Sample no 1 2 3 4 5 6 7 

Unit DOgr DSnr Ongb SOqgr SOqgr SOcgr Dpgr 

Trace-element abundances, in parts per million, 
and selected ratios— Continued 

Ba 144 298 

Rb/Cs 60 112 

Rb/Sr 15.0 2.1 

Sc 1.1 3.5 

Cr 4.7 nd 

Co 2 .4 

Zn 22 22 

La 46 39 

Ce 98 83 

Nd 41 42 

Sm 9.7 9.3 

Eu 28 .42 

Gd 6.9 8.6 

Tb 1.60 1.78 

Ho 1.5 1.8 

Tm 1.98 1.23 

Yb 7.3 8.7 

Lu 98 1.16 

La/Yb 6 4 

Hf 6.8 6.8 

Zr 189 nd 

Th 2 37.3 17.3 

Th 38.8 16.8 

U 2 9.7 3.3 

U 9.0 3.2 

Zr/Hf 28 nd 

'Fe^j calculated as FeO. 

2 Delayed neutron determination analyses by H.T. Millard, Jr., C. McFee, and C. Bliss. 

Description of samples 

1. Alkalic granite in Franklin; massive granite but in this place cut by quartz 
veins and mylonite seams; Ledgewood, Franklin Apartments, Unionville, Frank- 
lin. Field number DMA-197, locality shown in Wones and others (1986). UTM 
grid: N46624-E2999. 

2. Micrographic rhyolite from Newbury Volcanic Complex; collected by A.F. 
Shride. Field number 957-C. 

3. Nahant Gabbro, Nahant; from Emerson (1917, p. 182). 

4. Quincy Granite; average of three samples from Quincy, Mass.; from Emerson 
(1917, p. 191, no. 4 in table). 

5. Quincy Granite, Cumberland, Rhode Island, from Hermes and others (1981, p. 
319, table 1, no. 1). 

6. Alkalic granite to quartz syenite of Cape Ann Complex; hornblende granite, old 
Rockport Granite Co. quarry, Rockport; from Emerson (1917, p. 191, no. 6 in 
table). 

7. Peabody Granite; old quarry, South Lynnfield; from Tuttle and Bowen (1958, 
table 11; cited by Toulmin, 1964). 



The small mass of "Quincy Granite" (SOqgr) east of 
Woonsocket (fig. 19) near the Massachusetts-Rhode 
Island border is similar in texture to the Quincy Granite 
at Quincy but is more peralkaline (table 12, no. 5). The 
mass at Woonsocket was thought at the time of map 
compilation to be the same general age as the granite at 
Quincy. Its age has more recently been determined to be 
Devonian, or possibly Carboniferous, rather than Late 



138 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



Ordovician and Early Silurian (Hermes and Zartman, 
1985). It has been most recently described by Rutherford 
and Carroll (1981), who distinguished an equigranular 
variety and a porphyritic variety. The latter contains 
amphibole with less riebeckite component. 

BLUE HILLS GRANITE PORPHYRY (SObgr) 

Naylor and Sayer (1976) considered the Blue Hills 
Granite Porphyry (SObgr; called Blue Hill Granite Por- 
phyry on the State bedrock map) to be equivalent in 
composition and age to the Quincy Granite. Chute (1969) 
recognized inclusions of Quincy Granite within the Blue 
Hills Granite Porphyry in Quincy and considered it 
comagmatic with, but slightly younger than, the Quincy 
Granite. Warren (1913) observed dikes of Quincy Granite 
in the Blue Hills and suggested that the Quincy Granite 
was the younger unit. Rb-Sr work by Bottino and others 
(1970) suggested a late Paleozoic age for the Blue Hills, 
but Naylor and Sayer (1976) argued that this age was 
reset. The Blue Hills was assigned a Late Ordovician and 
Early Silurian age because of its close similarities, in 
both mineralogy and chemistry, to the Quincy Granite. 
Cobbles of the Blue Hills Granite Porphyry have been 
found in the Pondville Conglomerate of Early Pennsyl- 
vanian age (Chute, 1969). The Blue Hills is bounded by 
the Quincy Granite on the north and east and the 
Neponset fault on the west and is overlain unconform- 
ably by the Pondville Conglomerate on the south. 

The Blue Hills is dark gray to blue gray and weathers 
to buff brown and salmon. Quartz (12 percent) and 
microperthite (40 percent) phenocrysts are set in a 
matrix of quartz, perthite, amphibole, and acmitic pyrox- 
ene. Some of the pyroxene occurs as inclusions in the 
perthite, indicating early crystallization of this pyrox- 
ene. Accessory minerals are aenigmatite, astrophyllite, 
magnetite, ilmenite, hematite, fluorite, zircon, and 
calcite. 

Chemically, the Blue Hills is similar to the Quincy 
Granite and shows some similarities to the Peabody 
Granite and the alkalic granite and quartz syenite of the 
Cape Ann Complex (Buma and others, 1971; Naylor and 
Sayer, 1976). However, both the Quincy and the Blue 
Hills Granite Porphyry are more peralkaline than the 
Peabody Granite and the granite and quartz syenite of 
the Cape Ann Complex. The latter are chemically similar 
to the granite of the Rattlesnake Hill pluton (Lyons and 
Krueger, 1976). 

CAPE ANN COMPUEX 

Most of the Cape Ann Complex forms the Cape Ann 
peninsula of northeastern Massachusetts (fig. 19), but 
two small stocks lie south of Salem. The Cape Ann 
consists of three rock units: alkalic granite to quartz 
syenite (SOcgr), forming the main phase, the Beverly 



Syenite (SOcb), and the Squam Granite (SOcsm). The 
whole forms a pluton covering 385 km 2 . The Cape Ann 
intrudes greenschist, diorite, and gabbro (Zv, Zdigb) 
that earlier workers (Toulmin, 1964; Dennen, 1981) 
assigned to the Marlboro Formation, Salem Gabbro- 
Diorite, or Middlesex Fells Volcanic Complex. Radio- 
metric ages for the unit straddle the Ordovician-Silurian 
boundary (table 1). 

Dennen (1975, 1981) considered masses of diorite and 
gabbro mapped as Salem Gabbro-Diorite within and 
adjacent to the Cape Ann Complex to be cogenetic with 
it and to be equivalent in age to the Nahant Gabbro and 
the gabbro at Salem Neck described above. We recom- 
mend that the term "Salem Gabbro-Diorite" be 
restricted, in future usage, to these masses of diorite and 
gabbro in and around the Cape Ann pluton that are 
younger than the Dedham Granite and cogenetic with the 
Late Ordovician to Early Silurian Cape Ann Complex. 
The gabbro at Salem Neck is probably representative. 

Alkalic Granite and Quartz Syenite (SOcgr) 

The alkalic granite to quartz syenite (table 11, no. 7; 
table 12, no. 6) is medium to coarse grained; it is grayish 
green and weathers to tan and salmon. Quartz and 
feldspar content vary widely; rock lacking quartz is 
mapped as Beverly Syenite (Toulmin, 1964; Dennen, 
1981). Numerous inclusions of anorthositic, dioritic, and 
granitic rocks, as well as segmented mafic dikes, are 
interpreted to have been emplaced contemporaneously 
with the cooling granitic magma. Late-stage aplites and 
pegmatites are common. 

Dennen (1981) has studied the Cape Ann Complex 
extensively and has contoured the variations in its quartz 
content. Mineral contents vary on a scale of meters to 
kilometers. In addition to the major constituents, clino- 
pyroxene, fayalite, titanite, zircon, fluorite, allanite, 
magnetite, and ilmenite have been found among the 
accessory minerals. Toulmin (1964) suggested that alkali 
feldspar cumulates are the main cause of the variations. 
Dennen (1981) ascribed the abrupt changes in areal 
extent to later faulting, which displaced the contacts of 
these cumulate masses. 

An outcrop at Andrews Point (fig. 20) illustrates the 
complicated relationships among the textural variations 
(Martin, 1977). Here a foundered block of fine-grained 
granite contains disjointed diabase dikes. The diabase 
contains miarolitic cavities with terminated quartz and 
alkali feldspar. This block is intruded on the north side by 
the main phase of the Cape Ann Complex. On the south 
side, the block and the main phase of the Cape Ann are 
intruded by an aplitic dike that contains pegmatite pods 
with coarse crystals of quartz, feldspar, amphibole, and 
fayalite. Much of the fayalite has been altered to gruner- 
ite and magnetite and is rimmed by a titaniferous annite. 



INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 



139 





Figure 20.— Geology of Andrews Point, Cape Ann (see fig. 19 for location). Mapped by D.R. Wones and P.L. Pelke in 1971. 



140 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



The youngest rock at Andrews Point is a metasomatic 
mass of solvsbergite, formed in a late fracture where 
solutions replaced the host rocks (Martin, 1977). 

Numerous light-colored dikes southwest of the Cape 
Ann Complex have been labeled Quincy or Cape Ann. 
Such dikes may be related not only to the Cape Ann 
Complex but also to the Peabody Granite or even to 
Dedham Granite. Ross (1984) has suggested a wide range 
of ages for lamprophyre and dolerite dikes in the Boston 
area. 

Beverly Syenite (SOcb) 

The Beverly Syenite (SOcb) phase of the Cape Ann 
Complex was described by Toulmin (1964) and Dennen 
(1981). It forms subordinate elongate masses within the 
main phase of the Cape Ann (fig. 19), suggesting that it 
is most probably a cumulate. However, dikes of syenite 
do intrude the gabbro at Salem Neck, and other dikes of 
syenite are common in the Salem area (Toulmin, 1964), 
suggesting that the Beverly is in part, at least, a 
differentiate. 

The syenite is medium to coarse grained, cream col- 
ored, and rich in alkali feldspar (table 11, no. 8). Acces- 
sory minerals are apatite, zircon, titanite, magnetite, 
sulfides, allanite, and astrophyllite. 

Squam Granite (SOcsm) 

The Squam Granite (SOcsm) forms a modally variable 
5-km 2 mass within the Cape Ann Complex (fig. 19). 
Dennen (1981) recognized smaller masses of this rock 
elsewhere in the Cape Ann. Contact relations suggest 
that the Squam is an inclusion of perhaps an earlier 
cognate phase; however, it could be a synchronous 
textural variant of the alkalic granite. 

The Squam Granite is fine to medium grained, is gray, 
and weathers to brown. The texture ranges from aplitic 
to porphyritic. The plagioclase content is highly variable 
(5-40 percent), and the anorthite content lies between 30 
and 55 percent. The rock contains both microcline and 
orthoclase microperthite. Quartz ranges from 15 to 30 
percent; hornblende and biotite, from 5 to 20 percent. 
Rare pigeonite remains as unreacted cores in horn- 
blende. Accessory minerals include opaque minerals, 
apatite, zircon, monazite, allanite, and titanite. 

MICROGRAPHIC RHYOLITE OF THE NEWBURY VOLCANIC 
COMPLEX (DSnr) 

Lenticular sills of micrographic rhyolite (DSnr) 
100-600 m thick intrude the Newbury Volcanic Complex 
of the northern Newbury basin. These hypabyssal intru- 
sions are contemporaneous with deposition of the volcan- 
iclastic strata in the basin. They are brownish gray to 
orange pink and aphanitic to sugary-textured massive 
felsite characterized by micrographic and spherulitic 



intergrowths (Shride, 1976). The composition of one 
sample of rhyolite is shown in table 12. The sample is 
included in the ternary diagram for chemically analyzed 
Paleozoic intrusive rocks in the Milford-Dedham zone 
(see fig. 23). 

ALKALIC GRANITE IN FRANKLIN (DOgr) 

The alkalic granite in Franklin (DOgr), mislabeled 
SOqgr on the State bedrock map (DOgr in the explana- 
tion), forms a north-trending pluton of about 70 km 2 
north of Franklin (fig. 19). Volckmann (1977) mapped 
this rock as Milford Granite. The pluton is fault bounded 
on the north, south, and west. The eastern contact with 
the Dedham Granite may be intrusive, but mylonitization 
seen in places along this contact suggests that it too 
might be a fault. The rock is considerably more alumi- 
nous than the other alkalic units (table 12, no. 1), but the 
traces of hornblende, the ubiquitous presence of fluorite 
(table 11, no. 1), and hypersolvus texture suggested to 
Wones that the rock belongs with the Ordovician to 
Devonian alkalic granite suite. 

The light-gray rock weathers to buff or brown. The 
hypidiomorphic granular texture is dominated by subhe- 
dral to anhedral perthite and quartz. Plagioclase, biotite, 
and fluorite are all anhedral. The zircons are euhedral 
and are confined to regions of recrystallized biotite. The 
biotite crystals may be pseudomorphs after amphibole, a 
trace of which is present in sample 1, table 11. The 
zircons are all within biotite aggregates, which them- 
selves are interstitial to the perthite and quartz. The lack 
of inclusions of mafic minerals or zircon within the quartz 
or the perthite, implying a late-stage crystallization of 
the mafic minerals and zircon, is characteristic of peral- 
kaline rocks (Watson, 1979). 

Mylonitic fabrics are common and tend to concentrate 
quartz, sericite, and fluorite. Mylonitic zones contain 
lenticles of equigranular (0.1-mm) quartz and also, in 
places, biotite. Quartz on the margins of the lenticles 
shows undulose extinction, and the enclosing perthite 
and plagioclase are highly fractured. A partly crushed 
rock, not listed in table 11, consists of 41 percent 
perthite, 35 percent quartz, 14 percent plagioclase, 5 
percent biotite, and 5 percent sericite. Sericite is con- 
fined to plagioclase and zones of mylonite. 

PEABODY GRANITE (Dpgr) 
The Peabody Granite (Dpgr) occurs in two plutons (fig. 
19) that total approximately 50 km 2 in area. The large 
mass at Peabody (Peabody pluton) is the main body 
(Toulmin, 1964). The smaller mass near Reading (Read- 
ing pluton) is lithologically similar to the Peabody Gran- 
ite in the Peabody pluton. Both masses intrude the rocks 
of the Proterozoic Z mafic volcanic-plutonic complex (Zv, 
Zdigb). 



INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 



141 



The Peabody is a creamy- to tan-weathering gray- 
green rock (Toulmin, 1964), consisting of alkali feldspar, 
quartz, ferrohornblende (table 11, no. 9), and accessory 
pyroxene, biotite, magnetite, ilmenite, zircon, titanite, 
allanite, and sulfide minerals. Xenoliths are locally abun- 
dant. The paucity of aplite and pegmatite is in striking 
contrast to the Cape Ann Complex. 

Chemical analyses of the Peabody (Toulmin, 1964; 
table 12, no. 7) show values of (Na20+K 2 0)/Al 2 3 less 
than 1, indicating a subalkaline magma. Buma and others 
(1971) observed an early saturation of zircon in the 
Peabody magma in contrast to the Quincy Granite in 
which zircon crystallized late in the sequence. However, 
the rock still tends toward the alkaline field, as do the 
Quincy and Cape Ann. These observations fit well with 
the experimental work of Watson (1979), who suggested 
that peralkaline magmas have higher Zr solubilities than 
subalkaline magmas. Although the major minerals and 
bulk compositions of the Quincy Granite, the Cape Ann 
Complex, and the Peabody Granite are similar, their 
petrography, fabrics, and ages of intrusion indicate that 
they resulted from distinct magmatic events. 

WENHAM MONZONITE (Dwm) 

The Wenham Monzonite (Dwm) (Toulmin, 1964) is a 
body of 10-km 2 area north of Peabody (fig. 19) that 
intrudes the Proterozoic Z mafic volcanic-plutonic com- 
plex (Zv, Zdigb). The rock is gray, weathering to a cream 
color, is medium grained, and is made up of alkali 
feldspar, plagioclase, amphibole (table 11, no. 10), and 
accessory biotite, quartz, opaque minerals, apatite, zir- 
con, and titanite (Toulmin, 1964). The presence of plagi- 
oclase and biotite and the interstitial nature of the quartz 
distinguish this unit from the rocks of the Cape Ann 
Complex. 

CHERRY HILL GRANITE (Dcygr) 

The Cherry Hill Granite (Dcygr) is a 3-km 2 body of 
coarse-grained leucocratic granite that intrudes the 
Wenham Monzonite. It contains 1 percent or less mag- 
netite and sparse zircon (Toulmin, 1964). The feldspar is 
microperthitic microcline that weathers to pink. 

GRANITE OF THE RATTLESNAKE HILL PLUTON (Drgr) 

The granite of the Rattlesnake Hill pluton (Drgr) 
(Lyons and Krueger, 1976) has an area of 22 km 2 east of 
Wrentham (fig. 19) and intrudes the Dedham Granite. 
The granite is made up of three separate phases. The 
coarse biotite granite (table 11, no. 11) is light to medium 
gray, weathers yellowish brown, and consists of quartz, 
alkali feldspar, biotite, and accessory magnetite, zircon, 
apatite and fluorite. The coarse riebeckite granite (table 
11, no. 12) is a gray, tan- to orange-weathering rock that 
consists of quartz, alkali feldspar, riebeckite, and acces- 



sory biotite, magnetite, zircon, and fluorite. The fine- 
grained riebeckite granite (table 11, no. 13) is light gray, 
weathers to pink to orange, and consists of quartz, alkali 
feldspar, riebeckite, and accessory magnetite, zircon, 
and apatite. The western margin of the fine-grained 
granite contains irregular masses of pegmatite contain- 
ing riebeckite (Lyons and Krueger, 1976). 

MESOZOIC INTRUSIVE AND SILICIFIED ROCKS 

DIABASE DIKES (Jd) 

Many mafic dikes cut the Proterozoic Z and Paleozoic 
rocks of the Milford-Dedham zone, but only those that 
can be traced any distance are shown on the State 
bedrock map. Most are considered, or are known, to be 
Triassic or Jurassic in age, but some in northeastern 
Massachusetts range in age from Devonian to Carbonif- 
erous (Ross, 1984). Some dikes seem to be lamprophyres 
related to the Paleozoic alkalic granites. LaForge (1932) 
distinguished at least four sets of diabase dikes in the 
Milford-Dedham zone. He recognized that an east-west 
set that was highly altered intruded the Dedham Granite 
but was cut by the Quincy Granite. A set of northwest- 
southeast dikes may be related to these older east-west 
dikes, because they are also highly altered and their age 
relationships are obscure. A younger east-west set is not 
as severely altered. A set of north-trending unaltered 
dikes may be correlative with the Medford Diabase dike 
(see below). These observations agree with those made 
by Ross (1984). 

Ross (1981) observed that dikes of Triassic or Jurassic 
age in this region tend to strike north or close to north. 
The large north-trending Medford Diabase dike (fig. 19) 
of Jurassic age (table 1) cuts the Boston Bay Group and 
crosses the northern border fault of the Boston basin into 
the Melrose block containing the Lynn Volcanic Com- 
plex. An undated north-trending dike cuts rocks of the 
Bellingham basin at Woonsocket. Lyons (1977) found a 
small north-trending trap dike cutting Pennsylvanian 
strata at North Middleboro. Basalt dikes and sills cut the 
Boston Bay Group on islands in the Boston Harbor (C.A. 
Kaye, written commun., 1979). All these dikes have been 
assigned to the Jurassic on the State bedrock map on the 
basis of the age of the Medford Diabase dike. Other 
narrow north-trending dikes not shown on the State 
bedrock map have been observed in roadcuts in the area, 
such as at several places on 1-90 between Natick and 
Marlborough and on 1-495 in Wrentham, Franklin, and 
Milford. 

The Medford dike is the largest diabase dike in the 
Milford-Dedham zone. This particular dike trends north- 
northeast and, as mentioned above, crosses without 
offset the northern border fault of the Boston basin 
(Goldsmith, this vol., chap. H), but centimeter-scale 



142 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



left-lateral displacements along east-west faults are 
present west of Spot Pond, Stoneham. The Medford dike 
weathers deeply, and immature soil profiles have devel- 
oped on the glaciated surfaces. The fresh diabase is made 
up of plagioclase and augite with accessory orthoclase, 
biotite, magnetite, and ilmenite. It contains 1.7 percent 
K 2 (Emerson, 1917, p. 77) and is an alkaline rock. 
McHone (1981) and Hermes and others (1978) have 
shown that alkaline mafic dikes of Mesozoic age are 
common in New England, and the Medford dike appears 
to be part of that group. 

MASSIVE QUARTZ AND SILICIFIED ROCK (q) 

Masses of quartz and silicified rock (q) are common 
near many of the faults in eastern Massachusetts (Gold- 
smith, this vol., chap. H). The only one large enough to 
show on the State bedrock map is the large mass at 
Diamond Hill (DH, fig. 19) east of Woonsocket (Quinn, 
1971). The host rock that was silicified was a felsic 
volcanic rock of presumed Pennsylvanian age. The silici- 
fication lies along a north-trending fault, which cuts the 
Lower and Middle Pennsylvanian Wamsutta Forma- 
tion. Terminated quartz crystals suggest open voids and 
a brittle fracture regime. The faulting is post- 
Pennsylvanian and from its style is presumed to be 
Mesozoic in age. 



DISCUSSION OF THE INTRUSIVE ROCKS OF THE 
MILFORD-DEDHAM ZONE 

The oldest dated intrusive rock in this zone is the 
diorite at Rowley (Zrdi) (table 1). The diorite at Rowley 
and gabbroic dikes intrude the metavolcanic rocks (Zv) 
northwest of the Boston basin. These rocks and the 
extensive diorite and gabbro (Zdigb) may be penecon- 
temporaneous with the volcanic material and form part of 
a volcanic-plutonic complex in which the mafic plutons 
have intruded the volcanic cover. The diorite at Rowley 
appears to be undeformed relative to the older rocks and 
is perhaps the youngest of the intrusive suite. On the 
other hand, it may be the earliest recognized intrusion 
that is comagmatic with the Milford and Dedham Gran- 
ites, as is suggested by the apparent fit of the other 
Proterozoic Z diorites (Zdi) of table 3 to the lines through 
the batholithic rocks shown on the Peacock diagram (fig. 
6) and by the Th/U ratios plotted on figure 10. If so, the 
sequence is similar to mafic to felsic intrusive sequences 
recognized from the circum-Pacific Mesozoic batholiths 
(Silver and others, 1979; Bateman, 1983). The polarity of 
such a sequence in eastern Massachusetts is not discern- 
ible because the exposed zone is narrow and disrupted. 
The greater abundance of mafic rocks north of Boston is 
attributed more to the level of erosion across a north- 




EXPLANATION 



Dedham Granite as used in this report 
Dedham Granite of Dowse (1949) 
Dedham Granite of Lyons (1969) 
Milford Granite 
Westwood Granite 



Figure 21.— Ternary quartz-plagioclase-K-feldspar plot of modal 
composition of Dedham Granite as compared with fields of composi- 
tion of Dedham Granite of Dowse (1949), Dedham Granite of Lyons 
(1969), Milford Granite, and Westwood Granite. 



plunging structure than to a lateral change in composi- 
tion of intrusions across a belt. 

Most of the batholithic rocks fall in the granite field 
(figs. 5, 12-14). A few of the batholithic rocks, like some 
of the Dedham Granite north of Boston (Zdngr) and the 
Topsfield Granodiorite (Ztgd), fall in the granodiorite 
field (fig. 12). The mafic phase of the Milford Granite 
approaches tonalite (fig. 13). The Westwood Granite 
samples (fig. 13) cluster closely in the granite field. 
Porphyritic phases, such as the porphyritic granite of the 
New Bedford area and the Ponaganset Gneiss, range 
from granite to granodiorite (fig. 14), the same range as 
the other batholithic rocks. The field of Dedham Granite 
established in our study for the State bedrock map is 
broader than the fields established in the more areally 
limited studies of Dowse (1949) and Lyons (1969) (fig. 
21). The REE patterns of the Dedham Granite and 
granite of the Fall River pluton are closer to the REE 
patterns of the Hope Valley Alaskite Gneiss and the 
Scituate Granite Gneiss than to those of the Milford 
Granite and the biotite granite (Zgr) (figs. 15, 16, 18), 
although the small number of samples of all these rocks 
diminishes the significance of this. 

The peraluminous Milford Granite and the metalumi- 
nous Dedham Granite are contemporary, according to 
the available radiometric ages, but they differ in their 
mineralogy, composition, and deformational history. The 



INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 



143 



Al 2 3 




50% Al 2 3 



Na 2 



EXPLANATION 

o Paleozoic intrusive rocks 

(table 12) 
• Proterozoic Z batholithic 

rocks (tables 6, 8, and 10) 
■ Proterozoic Z mafic plutonic 

rocks (table 3) 

Figure 22.— Plot of a part of the ternary system Na20-K 2 0-Al 2 3 in 
molecular percent for intrusive rocks of the Milford-Dedham zone. 
Sample 1 is alkalic granite in Franklin; see table 12 for further data. 



Milford contains garnet, muscovite, and lensoid quartz 
that has been annealed. The Dedham contains titanite 
and magnetite, and hornblende is commonly found adja- 




Figure 23.— Ternary plot of normative albite (Ab), anorthite (An), 
and orthoclase (Or) for Paleozoic intrusive rocks of the Milford- 
Dedham zone, from table 12. Fields of silica-saturated rocks from 
O'Connor (1965) as modified by Barker (1979). 



cent to the contacts with older mafic rocks. The Dedham 
is brittlely deformed and altered by hydrolysis and 
oxidation to a characteristic red color with green veins 
and patches of epidote and chlorite. The Dedham is 
characteristic of plutonic rocks that form in subduction 
regimes, with its great volume relative to enclosed and 
adjacent metasedimentary and metavolcanic material, 
its intrinsically high oxidation state, and its chemistry. 
The high but variable percentage of K-feldspar in the 
Dedham and Milford (tables 4, 5, 7, 9; figs. 5, 12-14) is 
similar to that found in subduction zone batholiths far- 
thest "inboard" from the continental margin (Brown and 
others, 1984). 

Gromet and O'Hara (1984; O'Hara and Gromet, 1984, 
1985) have proposed a major tectonic boundary within 
the Milford-Dedham zone between the terrane contain- 
ing the Hope Valley Alaskite Gneiss and the terrane 
containing the Milford Granite in the Milford antiform. 
Such a boundary could have formed before intrusion of 
the Middle Devonian Scituate Granite and the granite in 
Franklin, but these rocks must have been emplaced 
before the major motion on the boundary because they 
are deformed along it. 

Our data on the Paleozoic rocks are limited, but it is 
clear that they tend to be alkalic. They plot in the part of 
the Na 2 0-K 2 0-Al 2 3 diagram (fig. 22) in which the 
amount of A1 2 3 is only slightly greater than the sum of 
the alkalis and towards the alkalic side of figures 23 and 
24. The granite in Franklin (DOgr) has a higher A1 2 3 
content than the other alkalic granites of table 12. An 




MgO 



Figure 24.— Ternary AFM (alkalis-FeO-MgO) plot of Paleozoic intru- 
sive rocks of the Milford-Dedham zone, from table 12, showing field 
of Proterozoic Z intrusive rocks of the Milford-Dedham zone (dashed 
line) from figure 8. 



144 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



200 




Figure 25. — Chondrite-normalized plot of rare-earth elements of Paleozoic intrusive rocks. Numbers refer to sample numbers in table 12; data 

for granodiorite of the Indian Head pluton from table 15. 



REE pattern of a sample from this unit (fig. 25, no. 1) 
shows marked REE enrichment and a pronounced neg- 
ative europium anomaly, remarkably similar to the pat- 
tern of the rhyolite (DSnr) from the Newbury Volcanic 
Complex (fig. 25, no. 2). Possibly they are cogenetic. 

RELATION TO EXTRUSIVE ROCKS 

Some or all of the Proterozoic Z volcanic rocks around 
the Boston basin and at a few places elsewhere in the 
Milford-Dedham zone may be related petrogenetically to 
the batholithic rocks. These rocks are described in the 
chapter in this volume on the stratigraphy of the Milford- 
Dedham zone (Goldsmith, this vol., chap. E). In this 
chapter, the principal extrusive units discussed that are 



equivalent to or younger than the southeastern Massa- 
chusetts batholith are the Lynn and Mattapan Volcanic 
Complexes (DZ1, Zm) and the Brighton Melaphyre 
(ftZrb) of the Boston Bay Group. Felsic metavolcanic 
rocks (Zvf) in the Framingham area are probably coge- 
netic with the batholithic rocks. 

The Mattapan and Lynn Volcanic Complexes consist 
largely of rhyolite and rhyodacite flows, in part porphy- 
ritic, but andesitic rocks have been reported (Nelson, 
1974). Chute (1966) reported that the rocks tend to be 
sodic rather than potassic. The volcanic rocks in the Blue 
Hills, the aporhyolite of Emerson (1917), are devitrified 
rhyolitic flows assigned to the Mattapan Volcanic Com- 
plex (Zm). The evidence for the stratigraphic positions of 



INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 



145 



the Mattapan and Lynn is conflicting. LaForge (1932) 
and Bell (1976) cited evidence for nonconformable rela- 
tions between the Lynn and the Dedham Granite. On the 
other hand, Zarrow (1978) described inclusions of Lynn 
in the Dedham and noted that the Lynn and the Dedham 
have similar trace-element contents. Kaye and Zartman 
(1980) described places where the Dedham Granite 
grades upwards texturally into fine-grained rhyolitic 
rock. Chute (1966) noted that fine-grained phases of the 
Westwood Granite resemble the Mattapan. The U-Th-Pb 
isotopic age of 602 ±3 Ma for the Mattapan (Kaye and 
Zartman, 1980) indicates that the Mattapan is younger 
than the Dedham, although possibly close in age to the 
Westwood. Volcanic rocks cogenetic with the Dedham 
and Westwood would differ in time of extrusion but 
might be difficult to sort out in the field. The felsic 
metavolcanic rocks (Zvf) in the Framingham area are 
considered to be part of the prebatholithic, largely mafic 
volcanic complex (Zv). Felsic metavolcanic rocks in this 
suite have been described and mapped by Nelson 
(1975a,b) and Volckmann (1977). The affiliation with the 
batholithic rocks is suggested by the fact that some of the 
felsic rocks were mapped as Milford Granite. In outcrop 
these are not unlike fine-grained phases of the Hope 
Valley Alaskite Gneiss of southeastern and eastern Con- 
necticut and adjacent Rhode Island. Both felsic and mafic 
metavolcanic rocks of pre-Dedham age have been iden- 
tified in the Middlesex Fells area. The Brighton Mela- 
phyre in the Brookline Member of the Roxbury Conglom- 
erate interfingers with and overlies the Mattapan, and 
dikes of Brighton cut the Mattapan. The Brighton con- 
sists primarily of quartz keratophyre, keratophyre, and 
spilite. Nelson (1975a) described basaltic and andesitic 
flows and tuffs. Bouchard (1979) described basaltic 
extrusive rocks and subordinate andesitic to rhyolitic 
lavas and tuffs. These volcanic rocks are a suite clearly 
younger than the batholithic rocks; they record a sepa- 
rate igneous event that is probably related to the depo- 
sition of the Proterozoic Z turbiditic material forming the 
Boston Bay Group in the Boston basin. 

Little volcanic rock that can be associated with the 
suite of Paleozoic alkalic granite intrusions is preserved 
in Massachusetts. The Blue Hills Granite Porphyry is 
clearly associated with the Quincy Granite, and volcanic 
rocks of the Newbury Volcanic Complex may be coge- 
netic with the alkalic plutons. In Rhode Island, the 
Spencer Hill Volcanics (Quinn, 1971) represent extrusive 
equivalents of mid-Paleozoic intrusions. 

INTERPRETATION OF IGNEOUS EVENTS 

The age relations and the composition and character of 
the plutonic rocks indicate the following history of events 
in the Milford-Dedham zone. A volcanic-plutonic arc 
developed probably in Proterozoic Z time. The early 



plutonic rocks in this arc were mafic; the later were 
primarily granitoid batholithic rocks such as the Dedham 
Granite. These later rocks may have been consanguine- 
ous with the mafic rocks. The sparseness of tonalite and 
granodiorite in the batholithic rocks suggests that they 
were intruded within a continental craton, which in turn 
suggests that the volcanic-plutonic arc formed at the 
edge of a continental mass. After the intrusion of the 
Dedham batholith, a small graben (rift basin?), the 
Boston basin, formed, siliceous lavas and pyroclastic 
rocks (Mattapan and Lynn Volcanic Complexes) were 
extruded, and the basin was filled by a sedimentary 
sequence (Boston Bay Group) that included basaltic lavas 
(Brighton Melaphyre) in Proterozoic Z and Early Cam- 
brian time (Kaye and Zartman, 1980; Goldsmith, this 
vol., chap. E). The Westwood Granite may be the 
intrusive equivalent of the Mattapan and Lynn, whereas 
the swarms of older east-west-trending mafic dikes (par- 
allel to the faults defining the basin) may be the intrusive 
equivalent of the basaltic lava flows of the Brighton. 

The next period of activity was during the Ordovician, 
when the Nahant Gabbro was intruded into the Boston 
basin at Nahant and into the Proterozoic Z basement at 
Salem Neck. This gabbro is 30 m.y. older than alkalic 
feldspar granites like those of the Cape Ann Complex 
and the Quincy Granite that followed it (table 1). The 
compositions of these alkalic granites indicate that the 
crust had matured sufficiently that no H 2 0-rich material 
melted to make more typical granites. The Cape Ann 
Complex and Quincy Granite are similar to rocks usually 
associated with intracratonic plutonism or a continental 
rift zone. Similar rocks extend well into the Gulf of Maine 
to the north (Hermes and others, 1978) and into Rhode 
Island to the south (Hermes and Zartman, 1985). 

A period of apparent quiescence, between about 450 
and 400 Ma, was followed during the Devonian by 
intrusion into stable crust of another group of alkaline 
rocks such as the Wenham Monzonite, Peabody Granite, 
granite of the Rattlesnake Hill pluton, and the Scituate 
Granite of central Rhode Island. Zartman (1977) 
believed, however, that the intrusions may be sequential 
rather than falling into two distinct groups. The compo- 
sition of these Devonian granites is distinctly different 
from that of the widespread Devonian plutons west of the 
Milford-Dedham and Nashoba zones. 

The final intrusions in the Milford-Dedham zone were 
of Permian age, the Westerly and Narragansett Pier 
Granites in Rhode Island. These granites are H 2 rich, 
are meta- to peraluminous, and from their composition 
and contact effects (Grew and Day, 1972; Day and others, 
1980; Hermes and others, 1981) must have intruded 
Pennsylvanian-age sediments from depths of about 12-15 
km. Their mineralogy suggests that they were derived 
from a crustal source different from the source of the 



146 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 




Figure 26.— Distribution of intrusive rocks in the Nashoba zone. OZmg, feldspathic gneiss in the Marlboro Formation; mgr, light-gray 

muscovite granite. 



earlier Paleozoic intrusions. This new source or modifi- 
cation of an older source for the siliceous magma formed 
between the Devonian and Permian probably is best 
explained by thrusting of the Milford-Dedham zone over 
a thick sequence of sediments before Permian time, or 
possibly by underplating of entirely different material as 
indicated by Zartman and Hermes (1984). Thus, the 



plutonic record documents a period of Precambrian sub- 
duction, a long period through the early and middle 
Paleozoic during which the crust had matured so that 
alkaline magmas were generated as a result of lower 
crustal heating, and finally the transport of the Milford- 
Dedham rocks over sedimentary rocks during the late 
Paleozoic. 



INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 



147 



Table 13. — Ages and age relations of intrusive rocks of the Nashoba zone and the eastern part of the Merrimack belt 
[Radiometric data from Zartman and Marvin, this vol., chap. J, table 1, except where noted. Ages are interpreted to be within 5 percent of the primary age of intrusion. 

wr, whole rock; — , no data] 



Map unit 



Intrudes 



Age (Ma), method 



Dfgr 



Dmgr 
Dcgr 

igd 
Sgr 



Sacgr 
SOad 

SOagr 



Sngr 
SOngd 



Fitchburg Complex 

Granite at Millstone Hill 
Chelmsford Granite 

Granodiorite of Indian Head pluton 
Orange-pink granite 
Sharpners Pond Diorite 

Straw Hollow Diorite and Assabet 

Quartz Diorite. 
Ayer Granite, Clinton facies 
Ayer Granite, Devens-Long Pond 

facies. 
Andover Granite: 

younger phase 

older, gneissic phase 

Newburyport Complex granite 
Newburyport Complex granodiorite 



Sp, Paxton Formation; Dl, Littleton 
Formation; DSw, Worcester 
Formation. 

So, Oakdale Formation 

Sb, Berwick Formation; Sagr; DSdi, 
diorite and tonalite. 

OZm, Marlboro Formation 

Ssqd 

OZn, Nashoba Formation; OZm, Marl- 
boro Formation; SOagr, older phase. 

OZn 

So, SOrh, Reubens Hill Formation 
Sb, Berwick Formation 



OZn, OZm, SOagr, older phase 
OZn, OZm 

Se, Eliot Formation; SOk, Kittery 
Formation. 



390±15, U-Pb/zircon, 402±11, Rb- 
Sr/wr. 

372±7, Rb-Sr/wr. 
389±5, Pb-Pb/zircon. 1 

402±5, Rb-Sr/wr. 2 

430±5, U-Pb/zircon. 



433 ±5, U-Pb/zircon. 
433 ±5, U-Pb/zircon. 



408±22, Rb-Sr/wr, 1 415, Rb-Sr/wr. 2 
446±32, Rb-Sr/wr, 1 450±23, Rb-Sr/wr, 
460, Rb-Sr/wr. 



455 ±15, U-Pb/zircon. 



'Data from Zartman and Naylor (1984). 
2 Data from Hill and others (1984). 

INTRUSIVE ROCKS OF THE NASHOBA ZONE 

The intrusive rocks of the Nashoba zone (fig. 26) range 
in age from Ordovician to Devonian (table 13) and are 
dominated by the Ordovician and Silurian, peraluminous, 
gneissic Andover Granite and a Silurian calc-alkaline 
suite consisting of quartz diorite, granodiorite, and gran- 
ite. The largest masses of intrusive rocks are in the 
northern part of the Nashoba zone. The area of the 
Andover Granite shown on the State bedrock map 
includes granite of several ages. Most of it is Ordovician 
in age, but it contains some Devonian granite and 
pegmatite. A feldspathic gneiss (OZmg) near Grafton in 
the southern part may be an intrusive rock (Hepburn, 
1978) or may be a metavolcanic unit in the Nashoba 
Formation. This gneiss is discussed in the description of 
the stratigraphic units in the Nashoba zone (Goldsmith, 
this vol., chap. F). 



ANDOVER GRANITE (SOagr) 

The Andover Granite (SOagr) is a light-colored 
muscovite- and garnet-bearing, mostly gneissic granite 
(table 14, no. 1) that intrudes the Nashoba Formation 
(OZn) at all scales. Within the Andover are nongneissic 
granitic rocks that are apparently younger than the main 
mass of the unit (Hansen, 1956; Castle, 1964). Dikes and 
apophyses of aplite and pegmatite infiltrate much of the 
Nashoba terrane and contribute to the migmatitic aspect 



Table 14. — Modes, inpercent, of intrusive rocks of the Nashoba zone 
[Modes 1-6 from Castle (1964); a, albite; o, oligoclase; n, andesine; tr, trace] 



Sample no. 
Unit 



Quartz. . . . 
Plagioclase 
Microcline . 
Muscovite . 
Biotite. . . . 
Hornblende 
Garnet . . . 
Titanite. . . 
Opaque 

minerals 
Others 1 . . . 



12 3 4 

SOagr SOagr SOagr Ssqd 



5 
Ssqd 



Ssqd igd 



24^9 27-38 
20-30a,o 26-46 
18-41 10-44 



25-39 0-7 13-18 25-40 32.5 

32-49 35-52n 44-49n 28-57o,n 35.9o 



2-14 
0-6 



0-11 
0-11 


0-0.6 
tr 

0-0.2 
0-6.5 



0-33 

0.2-17 
0-4 


0-3 


tr 






0-13 
19-38 

1 

2-7 
1-3 



10-23 
11-22 



2-24 
(M 



.4 
0-10 



'Others = sphene, zircon, and alteration minerals sericite. chlorite, epidote, and carbonate 
minerals. 

Description of samples 

1. Andover Granite; biotite granite gneiss facies, range of 14 samples. 

2. Andover Granite; binary granite facies, range of 24 samples. 

3. Andover Granite; pegmatitic granite facies, range of 9 samples. 

4. Sharpners Pond Diorite; hornblende-diorite facies, range of 5 sam- 
ples. 

5. Sharpners Pond Diorite; biotite-hornblende tonalite facies, range of 
4 samples. 

6. Sharpners Pond Diorite; biotite-tonalite facies, range of 9 samples. 

7. Granodiorite of the Indian Head pluton; roadcut on 1-495 at 
Southboro-Marlborough town line. Field no. M-6; sample locality given 
in Wones and others (1986); 1,152 points counted. 

of much of the Andover. The Acton Granite of Hansen 
(1956) probably belongs to the older material. The pre- 
ferred orientation of muscovite defines a pronounced 
foliation conformable to the regional trend. Mortal - tex- 



148 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



tures and recrystallized quartz are common. The gneissic 
Andover Granite is about 445-450 Ma, and the younger 
phase of granite and pegmatite is 41CM15 Ma (table 13). 

A crosscutting muscovite granite of the younger phase 
(table 14, no. 2), termed "binary granite" by Castle 
(1964), is white to gray and weathers to chalk white. The 
texture is seriate at most localities. Alteration minerals 
are chlorite (0-3 percent), epidote (0-2 percent), and 
carbonate minerals (0-3 percent). Myrmekite is well 
developed in this younger facies. 

Pegmatitic granite of the younger phase (table 14, no. 
3) is abundant in the southern parts of the Lawrence and 
South Groveland areas (fig. 26). It intrudes the gneissic 
facies of the Andover and the Sharpners Pond Diorite 
and is therefore younger than the rest of the Andover. It 
is gradational with the younger muscovite granite. Crys- 
tal shape and grain size vary randomly among the 
outcrops of this unit. Rutile and chlorite are alteration 
minerals from biotite. All three facies contain traces of 
zircon and apatite, in addition to the ubiquitous small 
amounts of garnet. 

SHARPNERS POND DIORITE (Ssqd) 

The Sharpners Pond Diorite (Ssqd) covers about 150 
km 2 and includes diorite, tonalite, and small amounts of 
granodiorite. It intrudes the Nashoba (OZn) and Marl- 
boro (OZm) Formations and the older gneissic phase of 
the Andover Granite. It is intruded by pegmatites, the 
younger phase of the Andover, and the orange-pink, 
rusty-weathering biotite granite (Sgr). The Sharpners 
Pond has a reliable age of 430 Ma (table 13). 

Castle (1964) recognized three separate facies within 
the Sharpners Pond: hornblende diorite, biotite- 
hornblende tonalite, and biotite tonalite. These three 
facies are gradational in composition and were intruded 
penecontemporaneously. Some of the hornblende diorite, 
where gneissic, may be recrystallized Marlboro Forma- 
tion, although much of the gneissic diorite clearly is 
intrusive. 

The hornblende diorite (table 14, no. 4) is equigranu- 
lar, medium grained, and hypidiomorphic to allotriomor- 
phic. Some rocks are quite altered and contain apprecia- 
ble chlorite (2-10 percent), white mica (0-20 percent), 
epidote (0.5-3 percent), and carbonate minerals (less 
than 0.6 percent). Plagioclase ranges in composition from 
An^ to An 55 , averages An 40 -An 45 , and displays weak 
zoning. Some hornblende encloses clinopyroxene. 

The biotite-hornblende tonalite (table 14, no. 5) is 
slightly younger than the hornblende diorite. The higher 
biotite content gives this facies a more foliate appear- 
ance. Alteration minerals include chlorite (1-2 percent), 
white mica (less than 3 percent), epidote (0.3-2 percent), 



and traces of carbonate minerals. The plagioclase com- 
positions are An 30 -An 40 . 

The biotite tonalite (table 14, no. 6) is limited to about 
10 km 2 in the Reading area, and its boundaries with both 
the Andover Granite and the other facies of the Sharp- 
ners Pond Diorite are ill defined (Castle, 1964). The rocks 
are fine to medium grained and variably foliated. Rocks 
with little K-feldspar are hypidiomorphic, whereas those 
with more K-feldspar are allotriomorphic. Alteration 
minerals are chlorite (0-5 percent), epidote (0.1-2.4 
percent), white mica (0-0.3 percent), and carbonate 
minerals. Plagioclase compositions are An 15 -An 33 . Sec- 
ondary K-feldspar has formed along early fractures. 



STRAW HOLLOW DIORITE AND ASSABET QUARTZ 
DIORITE (Ssaqd) 

The Straw Hollow Diorite and Assabet Quartz Diorite 
(fig. 26) are shown as one unit (Ssaqd) on the State 
bedrock map because of their similar occurrence and 
general lithology and their small size. The Straw Hollow 
Diorite, only 3 km 2 in area, intrudes the Nashoba For- 
mation and is in fault contact with the Marlboro Forma- 
tion. The rock is light gray to dark green, medium 
grained, and weakly foliated and contains hornblende, 
biotite, and accessory magnetite, titanite, garnet, and 
pyrite. It is considered to be Silurian in age because of its 
lithologic similarity to the Sharpners Pond Diorite. 

The Assabet Quartz Diorite, 23 km 2 in area, is believed 
to intrude the Marlboro Formation (although Hansen 
(1956) did not observe any intrusive contact) and the 
Andover Granite. The rock is weakly foliated and is 
composed of andesine, hornblende, quartz, and biotite 
with accessory apatite, titanite, and hematite. The 
gneissosity is defined by parallel orientation of horn- 
blende. Hansen (1956) reported that the Assabet is cut 
by dikes of aplite and pegmatite, which may be equiva- 
lent to the granite (Sgr) that intrudes the Sharpners 
Pond Diorite. 



GRANODIORITE OF THE INDIAN HEAD PLUTON (igd) 

The granodiorite of the Indian Head pluton (igd) (fig. 
26) consists of a normal and a coarse-grained facies 
(Hepburn and DiNitto, 1978), which intrude the Marl- 
boro Formation. It ranges from light gray where fresh to 
pink on weathering, is massive, and ranges in composi- 
tion from granodiorite to monzogranite. A mode given 
in table 14 (no. 7) indicates a granitic composition. A 
chemical composition of another sample given in table 15 
also indicates a granitic composition verging towards 
granodiorite. Accessory minerals are muscovite, epi- 
dote, and hornblende. This granitic sample may corre- 
spond to a two-mica granite phase of the Indian Head 



INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 



149 



Table 15.— Major-oxide, normative-mineral, and trace-element 
compositions of granodiorite of the Indian Head pluton 

[Major-oxide composition from X-ray spectroscopy by Paul Heam and Susan 
Wargo; all Fe reported as Fe 2 3 . Trace-element abundances from instrumental 
neutron activation analyses by L.J. Schwartz except Rb and Sr determined by 
X-ray spectroscopy by G. Sellars and B. McCall. Sample locality (field no. 
DMA-191) given in Wones and others (1986)] 



Major-oxide composition, 
in weight percent, and 
alkali-alumina ratio 



Si0 2 

A1 2 3 

Fe 2 3 

MnO 

MgO 

CaO 

N^O 

K 2 

Ti0 2 

P 2 5 

(NajjO + ICjOj/AloOs 



69.34 

16.02 

1.10 

.06 

.92 

2.60 

3.58 

3.81 

.39 

.11 

.46 



Normative-mineral composition, 
in weight percent, 1 and differen- 
tiation index (DI) 



Qtz. 
Crn 
Or.. 
Ab. 
An . 
En. 
Fs. . 
Ilm. 
Ap. 
DI . 



25.4 

1.5 

22.6 

30.4 
12.2 
2.3 

4.5 
.7 
.3 
78 



Trace-clement abundances, 

in parts per million, and 

selected ratios 


Rb ... 




. 108 


Cs ... 




2.6 


Sr ... 




. 404 


Ba ... 




.1078 


Rb/Cs 




. 41 


Rb/Sr 




.3 


Sc ... 




6.7 


Cr ... 




7.4 


Co ... 




5.3 


Zn ... 




. 57 


La ... 




. 52 


Ce ... 




. 93 


Nd .. 




. 35 


Sm .. 




. 5.5 


Eu ... 




. 1.01 


Gd .. 




. 4.8 


Tb ... 




.66 


Ho ... 




.80 


Tm .. 




.44 


Yb ... 




. 1.7 


Lu ... 




.25 


La/Yb 




. 31 


Hf ... 




. 5.2 


Zr ... 




.230 


Th 2 .. 




. 16.7 


U 2 ... 




. 2.5 


Zr/Hf 




. 44 



'Fe 2 3 calculated as FeO. 

2 Delayed neutron reactivation analysis (by H.T. Millard, Jr., C. McFee, and C. Bliss) gave 
same result as did instrumental neutron activation analysis. 

Description of sample 
Granodiorite, Farm Road, 300 m southwest of Cook Lane, Marlbor- 
ough. UTM grid: N46902-E2918. 

pluton that cuts an older, more mafic gneissic phase 
described by Hill and others (1984). They dated the older 
phase at 402 Ma. Our sample has an REE pattern (fig. 
25) that falls between the patterns of the two phases of 
Hill and others. This granite has a relatively high Th/U 
ratio compared with those of the Proterozoic rocks shown 
on figure 10. The Indian Head is the granodiorite 
described by Nelson (1975a) in the Framingham area. 
The entire mass in the Framingham area is about 70 km 2 
in area. The Indian Head may be equivalent in age to 
small (less than 1 km 2 ) masses of monzogranite near 
South Groveland and Georgetown reported by Castle 
(1964) and to the orange-pink rusty- weathering granite 
(Sgr) described next. 



ORANGE-PINK RUSTY-WEATHERING GRANITE (Sgr) 

The orange-pink rusty-weathering granite (Sgr) forms 
three masses, near Byfield, near Reading, and near 
Sudbury (fig. 26). The distribution of the granite shown 
on the State bedrock map is derived from reconnaissance 
mapping by Shride (written commun., 1979). The 
orange-pink granite consists of both medium-grained and 
coarse-grained facies. The granite at Byfield is a 
medium-grained, equigranular to porphyritic biotite 
granite that contains about equal amounts of white 
oligoclase and gray quartz, lesser amounts of perthitic 
microcline, and about 5 percent biotite (Shride, 1971). 
The rock is only altered near contacts and where faulted. 
It contains many inclusions and is contaminated by wall- 
rock material in contact zones; in these places it is 
difficult to tell from Sharpners Pond Diorite. It does, 
however, intrude the Sharpners Pond Diorite (Shride, 
1976). Hansen (1956) mapped the mass at Sudbury as 
Dedham granodiorite, but part of the rock Hansen 
mapped as Dedham is granodioritic to tonalitic in com- 
position and was subsequently mapped by Hepburn and 
DiNitto (1978) as Indian Head Hill granodiorite. 

The orange-pink granite may be equivalent in age to 
the granodiorite of the Indian Head pluton or to the 
younger muscovite granite and pegmatite phase of the 
Andover Granite. 



LIGHT-GRAY MUSCOVITE GRANITE (mgr) 

The light-gray muscovite granite (mgr) forms a pluton 
in the isolated fault block of the Nashoba Formation in 
the Clinton-Newbury fault zone near Shrewsbury (fig. 
26). This mass was called the Rattlesnake Hill pluton by 
Skehan and Abu-Moustafa (1976) (not the Rattlesnake 
Hill pluton of the Milford-Dedham zone). It is uncertain 
whether this muscovite granite is equivalent to the 
Devonian muscovite granite at Millstone Hill (Dmgr) or 
the Chelmsford Granite (Dcgr) of the Merrimack belt or 
is equivalent to the older gneissic phase of the Andover 
Granite. 



DISCUSSION OF THE INTRUSIVE ROCKS OF THE 
NASHOBA ZONE 

The sequence of intrusion in the Nashoba zone began 
with pre- or synkinematic intrusion of muscovite-bearing 
granite. This was followed by postkinematic intrusion of 
diorite to tonalite, succeeded by less mafic tonalite, 
granodiorite, and monzogranite, and finally intrusion of 
muscovite-bearing pegmatite and aplite at a late stage to 
produce migmatite in many areas. 

Confusion exists concerning the age of these 
sequences, as both the oldest and the youngest intrusive 



150 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



rocks have been included in the Andover Granite map 
unit and both are peraluminous. The early Andover 
Granite is foliate but also has the fabric of a migmatite in 
places. Resolving the age of this material will require 
precise geochronology after careful mapping of local 
areas. Zartman and Marvin (this vol., chap. J) consider 
the 450-Ma age reliable for the gneissic Andover. The 
415-Ma age of Hill and others (1984) can be considered to 
represent the age of the younger pegmatite and granite 
in the Andover. Both the older gneissic and the younger 
nongneissic Andover have the muscovite and garnet 
(table 14) typical of S-type granites derived from deep- 
seated sedimentary material in a high-temperature 
regime. This compositional similarity suggests regener- 
ation of magma derived from the original source or 
generation of magma from a newly arrived but similar 
source. The age of metamorphism and intrusion of the 
gneissic phase is considered to be 450 Ma (Zartman and 
Naylor, 1984). 

The Sharpners Pond Diorite and related intrusions 
form a suite that has mineralogies characteristic of lower 
pressure I-type intrusives. We have chemical analyses 
from only the granodiorite of the Indian Head pluton 
from this suite (table 15). This rock is relatively granitic 
and is probably the younger phase of the Indian Head 
Hill pluton of Hill and others (1984). Their older gneissic 
mafic phase dated at 402 Ma (table 13) would fit into a 
Late Silurian and Early Devonian plutonic-volcanic 
sequence of which the Newbury Volcanic Complex may 
be a part. The younger phase of the Indian Head 
granodiorite and the orange-pink granite could be a late 
granitic phase of a plutonic-volcanic suite having a calc- 
alkaline trend, as illustrated by Hill and others (1984); 
the dioritic intrusions, the Sharpners Pond Diorite and 
the Assabet Quartz Diorite, were an earlier mafic phase 
of this plutonic-volcanic suite. The Assabet Quartz Dio- 
rite and the Sharpners Pond Diorite intrusions are very 
suggestive of the formation of a short-lived continental 
arc. Hill and others (1984) suggested that the suite of 
rocks is the product of processes at convergent plate 
boundaries and contains assimilated crustal material of 
Proterozoic Z to early Paleozoic age, a suggestion with 
which we concur. 

The exposed Nashoba zone is too narrow to develop a 
sense of polarity for the plutons within it, so that 
determining the direction of subduction is difficult, if not 
impossible. Zartman and Naylor (1984) have dated the 
younger intrusions at 408±22 Ma (table 13). If this age is 
correct, then the precursor event at 446 ±32 Ma may 
represent the initial melting of trench sediments and may 
be analogous to the event that produced the Cretaceous 
granitoids of the Ryoke-Sanba region in Japan (Czaman- 
ske and others, 1981). Thus, we may be observing a 
record of about 30 m.y. of subduction. At a rate of 



1 cm/yr, 300 km of oceanic crust could have been sub- 
ducted during this time. However, uncertainty in field 
identification of the different phases of the Andover and 
the scatter of the radiometric ages make such an assess- 
ment speculative. 

The intrusion of the younger muscovitic granites, the 
young phases of the Andover Granite and the Indian 
Head pluton, and possibly the orange-pink granite may 
represent another major event rather than part of a 
continuum. Wones (1974, 1976, 1985) identified a similar 
sequence of intrusive events in the Orrington-Liberty 
anticlinorium, Passagassawakeag terrane, eastern 
Maine. Zircon dates on material collected by Wones 
yielded an age of 410 Ma for the formation of muscovite- 
and garnet-rich migmatite in that terrane (Marvin and 
Dobson, 1979, p. 18-19). Rb-Sr whole-rock ages from the 
same terrane include an older age of 1,360 ±68 Ma and 
younger ages of 494±25 Ma and 426±27 Ma, the latter 
ages being not unlike those from the Andover. The 
similarities between the Passagassawakeag (including 
the Casco Bay Group) and Nashoba terranes have been 
recognized earlier (Hussey, 1968). If the ages of 408 to 
415 Ma for the Andover (table 13) are correct, then the 
younger Andover Granite is analogous to the Manaslu 
granite of Nepal (Le Fort, 1981) and may reflect thrust- 
ing followed by decoupling of the Avalon terrane as it 
moved westward under the eastern part of the Merri- 
mack belt. These ages roughly correspond to the age of 
nappe formation in central Massachusetts and New 
Hampshire. It is important to note that the apparently 
younger diorites of the western and eastern Merrimack 
terrane have no counterparts in the Nashoba terrane. 

INTRUSIVE ROCKS OF THE EASTERN PART OF 
THE MERRIMACK BELT 

The intrusive rocks of the Merrimack belt east of the 
Wekepeke fault are dioritic to granitic intrusions of Late 
Ordovician to Early Devonian age (table 13) that lie 
primarily along the east side of the belt west of the 
Clinton-Newbury fault (fig. 27). Most of these intrusions 
are clustered in a zone from Clinton to the Pepperell 
area, where they consist primarily of the Ayer Granite 
and its facies. The Ayer Granite continues south of 
Worcester into Connecticut, where it is called the Can- 
terbury (and formerly Eastford) Gneiss. In northeastern 
Massachusetts, the Newburyport Complex forms an 
isolated pluton. The Ordovician and Silurian granites 
tend to be calc-alkaline, and the Devonian granites tend 
to be at least in part peraluminous. We have undertaken 
no chemical studies of the rocks in the eastern part of the 
Merrimack belt, and our descriptions are all from the 
observations of others. Masses of Ayer Granite and its 
equivalents in Connecticut lie on both sides of the 



INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 



151 



AVA'iWsohgd.li of 




Devonian and Silurian granite — Chelmsford Granite (Dcgr), Fitchburg 
Complex (Dfgr), and muscovite-biotite granite (Dmgr) 

Devonian and Silurian diorite (DSdi) 

Clinton fades of Ayer Granite (Sacgr) and granite of the 
Newburyport Complex (Sngr) 

Devens-Long Pond facies of Ayer Granite (SOad) 

Ayer Granite (Sagr) and granodiorite of the Newburyport 
Complex (SOngd) 

Metamorphic and sedimentary rocks 

Contact 

Fault 



20 MILES 



/MAS S 
CONN ~ I^IASJSACHUSETJS. 
h- RHODE ISLAND 



20 KILOMETERS 



Figure 27. — Distribution of intrusive rocks in the eastern part of the Merrimack belt. 



Wekepeke fault as it is projected south from the Worces- 
ter area, but all the Ayer and Canterbury granitic 
intrusions lie west of the Clinton-Newbury fault and its 
possible projection into Connecticut (see Pease, 1982, p. 
264, for example), indicating that the Clinton-Newbury 
fault is a more significant terrane boundary than the 



Wekepeke. East of the Wekepeke fault, west of Clinton, 
in the part of the Merrimack belt we are describing, are 
a few small granite stocks of the Devonian Fitchburg 
Complex, whose main mass lies west of the Wekepeke 
fault. These stocks are the only intrusions in Massachu- 
setts in the western part of the belt we are describing. 



152 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



Table 16.— Modes, in percent, of intrusive rocks of the Merrimack 

belt east of the Wehepeke fault 

[o, oligoclase; m, microcline] 

Sample no 1 2 3 4 

Unit Sngr SOngd Sacgr SOad 

Quartz 33 6-33 27 26 

Plagioclase 35 30-48 36o 40 

K-feldspar 27 3-35 28 1 29m 

Mafic minerals 2 5 12-33 8 4 

'20 percent as phenocrysts, 8.5 percent in groundmass. 

2 Mafic minerals unidentified in samples 1-2; includes biotite, chlorite, and opaque minerals in 
samples 3-4. 

Description of samples 

1. Granite of Newburyport Complex. Average of two point-counts of 
stained slabs. 

2. Tonalite and granodiorite of the Newburyport Complex. Range of 
modes from point-counts of stained slabs. 

3. Clinton facies of Ayer Granite. Average modal composition from 
Gore (1976). 

4. Porphyroblastic variety of Devens-Long Pond facies of Ayer Gran- 
ite. From Gore (1976). 



NEWBURYPORT COMPLEX 

Shride (1971) divided the Newburyport Complex into 
two facies: tonalitic granodiorite and granite. The tona- 
litic facies was originally termed the Newburyport 
Quartz Diorite, which also included dioritic rocks south of 
the Clinton-Newbury fault zone that are now called 
Sharpners Pond Diorite in the Nashoba zone and Tops- 
field Granodiorite in the Milford-Dedham zone. These 
correlations are not tenable on grounds of both age and 
composition. For this reason, the name Newburyport is 
restricted to the two facies in the Newburyport area. 
The complex forms a large mass near Newburyport (fig. 
27) and a small mass to the west of it. Both are truncated 
by the Clinton-Newbury fault. 

TONALITE AND GRANODIORITE (SOngd) 

The tonalite and granodiorite facies (SOngd) occupies 
the core of the Newburyport Complex at Newburyport 
and is intruded on the northwest by the granite facies. 
Medium- to dark-gray in fresh rock, the tonalite weath- 
ers to both green and red. Rock along joints has been 
altered to brick red. An age of 455 ±15 Ma was deter- 
mined on zircon of the tonalite (table 13). 

The rock is fine to medium grained and is highly 
variable in mineralogy (table 16, no. 2). The mafic 
minerals include hornblende, biotite, titanite, pyrite, and 
other opaque minerals. The euhedral biotite is usually 
completely chloritized. Ovoid inclusions are common. 

GRANITE (Sngr) 

The granite of the Newburyport Complex (Sngr) 
intrudes both the Kittery Formation and the tonalite and 



granodiorite facies; it is at least 45 km 2 in area. The 
granite (table 16, no. 1) is porphyritic and is light gray to 
dark gray, weathering to buff. Phenocrysts of K-feldspar 
(0.5-0.9 cm) are set in a medium-grained matrix. K- 
feldspar is uncommon in the groundmass (Shride, 1976). 
Dioritic inclusions are common and, where oriented, 
define a foliation. No radiometric ages are available for 
this facies, so it is conceivable that the tonalite and 
granodiorite and the granite facies are distinctly differ- 
ent in age. 



AYER GRANITE (Sagr) 

Gore (1976) divided the Ayer Granite (Sagr) in its type 
locality into two facies, the Clinton facies (Sacgr) and the 
Devens-Long Pond facies (SOad). In addition, there are 
masses of granite to tonalite (Sagr), not assigned to 
either of these facies, that intrude the Berwick Forma- 
tion (Sb) west and northwest of Lawrence (fig. 27) and 
masses that intrude the Paxton and Oakdale Formations 
(Sp, So) south of Worcester and west of the probable 
southern continuation of the Wekepeke fault. Radiomet- 
ric ages obtained on the facies of the Ayer pose a problem 
in assigning ages to the unfossiliferous metasedimentary 
rocks that they intrude. The Clinton facies has a 
well-defined Early Silurian age and the Devens-Long 
Pond facies a similar age (table 13), which greatly 
compresses the time available for the deposition, burial, 
deformation, and metamorphism of the Berwick and 
Paxton Formations, if these units are truly Silurian. 
Some of the bodies labeled Sagr on the map might have 
been more properly correlated with the Lower Devonian 
Chelmsford Granite and muscovite-biotite granite at 
Millstone Hill. The bodies south of Worcester that 
intrude the Paxton Formation might more properly be 
correlated with the Canterbury Gneiss of Connecticut, 
which lies on strike and has an Early Devonian age of 
392 ±9 Ma (Zartman and Naylor, 1984). Zartman and 
Naylor (1984) believed that the Ayer Granite is of the 
same age range as the Newburyport Complex. 

CLINTON FACIES (Sacgr) 

The Clinton facies of the Ayer Granite (Sacgr) occupies 
35 km 2 northwest of, and within, the Clinton-Newbury 
fault zone (fig. 27). The Clinton intrudes the Oakdale and 
Berwick Formations and, from Gore's (1976) map pat- 
tern, may intrude the Devens-Long Pond facies. 

The Clinton facies of the Ayer Granite is a foliated, 
porphyritic, coarse-grained, light- to dark-gray granite 
(table 16, no. 3) that weathers to a buff color. Apatite, 
allanite, zircon, opaque minerals, and muscovite are 
present as primary accessory minerals. Biotite is partly 
altered to chlorite. Epidote, sericite, chlorite, titanite, 



INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 



153 



and carbonate minerals are alteration products. The 
mean composition of the K-feldspar megacrysts is Or 80 , 
and the Or-rich lamellae are maximum microclines (Gore, 
1976). The Clinton facies is different in texture and 
mineralogy from the Devens-Long Pond facies and could 
represent a distinct and different magmatie event. 

DEVENS-LONG POND FACIES (SOad) 
The Devens-Long Pond facies of the Ayer Granite 
(SOad) forms an elongate pluton of 45 km 2 southeast of 
Pepperell (fig. 27), entirely west of bodies of the Clinton 
facies. The facies consists of both equigranular and 
porphyroblastic varieties. Both varieties are gneissic, 
whereas the Clinton facies is gneissic only in the Clinton- 
Newbury fault zone. The contact relations between the 
Devens-Long Pond facies and the Oakdale and Berwick 
Formations are indeterminate. Gore (1976) thought the 
contact could be an unconformity. Goldsmith, following 
mapping by Robinson (1978, and written commun., 1978) 
in the Pepperell-Ayer area, made the contact with the 
Oakdale a fault on the State bedrock map. However, the 
contact relations of the Devens-Long Pond facies with 
the Berwick Formation are not determined. The map 
pattern suggests that the Devens-Long Pond facies is 
intruded by the Clinton facies, but no diagnostic outcrops 
have been observed. The Chelmsford Granite clearly 
intrudes the Devens-Long Pond facies (Gore, 1976). 

The porphyroblastic variety (table 16, no. 4) typically 
is light to medium gray and exhibits granoblastic tex- 
tures. Quartz and plagioclase grains are 3-8 mm long and 
porphyroblasts of microcline are 1-2 cm long. Accessory 
minerals are allanite, tourmaline, zircon, apatite, opaque 
minerals, and muscovite. Alteration minerals are chlo- 
rite, epidote, sericite, titanite, and carbonate minerals. 
The alkali-feldspar porphyroblasts have the bulk compo- 
sition of Or 95 (Gore, 1976). The equigranular variety has 
a similar mineralogy but ranges in composition from 
quartz monzonite (granite) to quartz diorite and in tex- 
ture from homogeneous to inhomogeneous. Some of the 
rock contains schlierenlike bands (Gore, 1976). The 
gneissic texture of the rock gives it the appearance of 
being older than the radiometric age (433 ±5 Ma; see 
table 13) would indicate. Wones thought it resembles the 
Passagassawakeag Gneiss of Bickel (1976) in Maine. 
Gore's studies of the megacrysts of the Clinton facies and 
the porphyroblasts of the Devens-Long Pond facies 
demonstrated that the rocks have undergone different 
histories. Possibly the Devens-Long Pond facies should 
be a separate unit from the rest of the Ayer Granite. 

DIORITE AND TONALITE (DSdi) 

The diorite and tonalite designation (DSdi) includes all 
dioritic or gabbroic rocks that intrude the Berwick 



Formation and the Ayer Granite and are intruded by 
dikes of two-mica granite that may be correlative with 
the Chelmsford. Two main masses— one in Dracut 
(Dracut Diorite) and one east of Pepperell (fig. 27)— are 
assumed to be correlative with the Exeter Diorite in 
New Hampshire that intrudes the Eliot Formation. 
Other smaller masses intrude the Berwick Formation 
(SOb) in northern Massachusetts and southeastern New 
Hampshire. The Exeter had been dated at 408 Ma 
(Bothner, 1974), but since the State bedrock map was 
prepared the Exeter has been found to be 473±37 Ma 
(Bothner and others, 1984; Gaudette and others, 1984). 
Dennen (1943) described the stock at Dracut as a 
norite, as it contains hyper sthene, augite, hornblende, 
olivine, plagioclase (An 37 -An 71 ), and opaque minerals. 
Plagioclase is interstitial or poikilitic to hypersthene and 
augite. Hornblende occurs as a late-stage magmatie 
mineral and as an alteration product (uralite) with bio- 
tite, sericite, chlorite, talc, serpentine, kaolin, and hem- 
atite. Opaque minerals include ilmenite, magnetite, pyr- 
rhotite, chalcopyrite, and pentlandite. The Dracut 
Diorite has been mined for nickel from a pyrrhotite- 
pentlandite-chalcopyrite assemblage. 

CHELMSFORD GRANITE (Dcgr) 

The Chelmsford Granite (Dcgr) intrudes the Berwick 
Formation, the Ayer Granite, and the Dracut Diorite 
northwest of the Clinton-Newbury fault. It occurs in two 
elongate bodies that strike parallel to the regional north- 
east trend and occupy an area of 60 km 2 (fig. 27). 
Zartman and Naylor (1984) could not obtain a definitive 
age on the Chelmsford but believed it to be in the range 
of the Ayer Granite because the age would lie within the 
margin of error of their age determinations. The Devo- 
nian designation for the age of the Chelmsford given on 
the State bedrock map is based on a 207 Pb/ 206 Pb age of 
389±5 Ma (Zartman and Naylor, 1984). The difficulties in 
obtaining acceptable age determinations from the 
Chelmsford may be due to the pervasive ductile defor- 
mation in this granite. 

The Chelmsford is light colored and gneissic; it consists 
of quartz, microcline, plagioclase (An 5 -An 13 ), muscovite, 
and biotite. Zircon and garnet are accessory minerals, 
and epidote, chlorite, and titanite are alteration prod- 
ucts. Much of the foliation in parts of the Chelmsford is 
produced by elongate quartz grains aligned parallel to 
the preferred orientation of muscovite and biotite; this 
foliation indicates the pervasive ductile deformation of 
the unit. 

MUSCOVITE-BIOTITE GRANITE AT MILLSTONE HILL 
(Dmgr) 

A small stock of muscovite biotite granite (Dmgr) 
intrudes the Oakdale Formation at Millstone Hill in 



154 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



Worcester (fig. 27). An Rb-Sr age of 372 ±7 Ma has been 
obtained from this rock (table 13). 



FITCHBURG COMPLEX (Dfgr) 

Three small stocks of muscovite granite that intrude 
the Worcester Formation (DSw) east of the Wekepeke 
fault and west of Clinton were mapped as Fitchburg 
granite by Peck (1975). The granite in these stocks 
contains muscovite, and most samples contain biotite. 
Tourmaline is a characteristic accessory mineral, accord- 
ing to Peck, and garnet, magnetite, apatite, and zircon 
are other accessory minerals. These stocks are shown as 
Fitchburg Complex (Dfgr) on the State bedrock map. 



DISCUSSION OF THE INTRUSIVE ROCKS OF THE EASTERN 
PART OF THE MERRIMACK BELT 

The oldest intrusion recognized in the eastern part of 
the Merrimack belt is the tonalite and granodiorite of the 
Newburyport Complex at 455±15 Ma. This calc-alkaline 
unit is older and richer in K-feldspar than the tonalite of 
the Sharpners Pond pluton in the Nashoba zone. The two 
facies of the Ayer Granite have an age similar to that of 
the Sharpners Pond Diorite in the Nashoba zone (table 
13). In New England, these early Silurian ages are 
unique to the eastern Merrimack belt. The diorites, 
gabbros, and norites of the Merrimack belt in Massachu- 
setts are undated but may be equivalent to the Exeter 
pluton, which has now been dated as Early Ordovician. 
The youngest unit in this belt of plutons is the granite at 
Millstone Hill at 372 Ma (table 13). The Chelmsford 
Granite is a muscovitic granite thought to be equivalent 
in age to the granite at Millstone Hill because of its 
muscovite content, but the radiometric data are equivo- 
cal (Zartman and Marvin, this vol., chap. J). Zircons in 
the Chelmsford are dated at 430 Ma, but an Rb-Sr 
whole-rock age is 356 ±71 Ma. The margin of error of the 
Rb-Sr age permits the Chelmsford to be the same age as 
the Ayer. On the other hand, each of the ages of the 
Chelmsford may be approximately correct, reflecting 
both the original intrusion and the later deformation. The 
data of Zartman and Marvin make the equivalence in age 
of the Chelmsford to the granite at Millstone Hill uncer- 
tain but possible. 

The igneous rocks of the eastern part of the Merrimack 
belt do not appear to correspond to those of the western 
part of the belt, nor to those in any other part of New 
England. The nearest possible equivalent group is in the 
Nashoba zone, where the Newburyport tonalite may be 
approximately equivalent to the Sharpners Pond tonal- 
ite, and the younger phase of the Andover Granite to the 
granite at Millstone Hill. However, the metamorphic 



grade and lithologies of the host rocks make such a 
correlation highly unlikely. The host-rock lithologies 
(Merrimack Group, SOk, Se, Sb, and Oakdale Forma- 
tion, So) of the Ayer and Newburyport intrusions in the 
eastern part of the Merrimack belt were considered by 
Goldsmith and Robinson to be similar enough to the 
Paxton Formation (Sp) and associated rocks in the region 
to the west to combine the two regions into a single 
Merrimack belt on the State bedrock map. Wones felt 
that the ages and nature of the plutonic rocks argue 
against a straightforward correlation, no matter how 
similar the lithologies of the nonfossiliferous host rocks. 
The ages of the Merrimack Group have been recently 
suggested to be Proterozoic Z (Lyons and others, 1982; 
Bothner and others, 1984) on the basis of conformity of 
the Berwick Formation of the Merrimack Group with the 
Proterozoic Z to Ordovician Massabesic Gneiss Complex 
(OZma) of southern New Hampshire and adjacent Mas- 
sachusetts. If these relations are true, an older age for 
the Merrimack Group solves the problem of Ordovician 
rocks like the Newburyport Complex intruding a suppos- 
edly Silurian section. It may be that Gore's (1976) 
suggestion of an unconformity between the Oakdale 
Formation and the Devens-Long Pond facies of the Ayer 
Granite is correct and that the Merrimack belt east of the 
Wekepeke fault lies unconformably above the Nashoba 
terrane as Skehan and Murray (1980) suggested (see 
Robinson and Goldsmith, this vol., chap. G) and is 
somewhat older than Silurian. The relationship is com- 
plicated because east-directed thrusts in the Clinton- 
Newbury fault system place Merrimack-belt rocks over 
the Nashoba-zone rocks (Skehan and Murray, 1980). 
Goldsmith believes that the Wekepeke fault is not a 
major terrane boundary but juxtaposes similar regions of 
a single terrane that were at different crustal levels. 
Both the Ayer Granite and the Fitchburg Complex lie on 
both sides of the Wekepeke fault. 

The mineral compositions of the Ordovician and Silu- 
rian intrusions in the eastern part of the Merrimack belt 
resemble those derived from processes along convergent 
plate margins (Hepburn and others, 1987); these rocks 
may represent part of a volcanic-plutonic arc developed 
near or on a continental margin. The sequence of intru- 
sion appears to be from mafic rocks and granodiorite to 
muscovite granite. The Devonian plutons, because they 
tend to be two-mica granites, were derived from more 
sedimentary source material than the older rocks, and 
their ages indicate that they were at least in part 
emplaced during Acadian deformation and metamor- 
phism (Robinson and Hall, 1980). The source for the 
Devonian plutons, like that for the intrusions of the 
Nashoba zone, was probably a westward-subducting 
Avalonian plate (Wones, 1984). 



INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 



155 



INTRUSIVE ROCKS OF EASTERN 

MASSACHUSETTS AND PLATE-TECTONIC 

MODELS 

The contrast in both the age and the style of the 
intrusive rocks of the Milford-Dedham and Nashoba 
zones and eastern part of the Merrimack belt that form 
eastern Massachusetts indicates that eastern Massachu- 
setts was consolidated over a period of time from at least 
three different terranes (Hatch and others, 1984; Hill and 
others, 1984). The ages of the intrusive rocks permit 
reconstruction of events in time, and the compositions of 
the rocks permit an estimation of the tectonic setting. 
Some of these matters have been discussed elsewhere in 
this chapter; here we consider them in reference to 
plate-tectonic models. A number of analyses of the 
structural framework of New England with reference to 
lithotectonic belts and accretionary terranes have been 
published in recent years (Osberg, 1978; Robinson and 
Hall, 1980; Hall and Robinson, 1982; Williams and 
Hatcher, 1983; Zen, 1983). We do not present an exhaus- 
tive discussion of these matters here but only point out 
how the intrusive rocks of eastern Massachusetts con- 
strain the formation of eastern Massachusetts. 

The Proterozoic Z mafic volcanic-plutonic rocks repre- 
sented by the diorite and gabbro (Zdigb, Zdi, Zgb) and 
the mafic and felsic volcanic rocks (Zv, Zvf) in the 
Milford-Dedham zone represent part of a volcanic- 
plutonic arc that was subsequently intruded by calc- 
alkaline granitic rocks of batholithic dimensions, the 
Dedham Granite (Zdgr) and similar rocks. Some evi- 
dence exists that the rocks of the mafic complex were 
metamorphosed before intrusion by the 630-Ma granites 
(Goldsmith, this vol., chap. H). Mosher (1983) suggested 
a back-arc setting for the Blackstone Group of northern 
Rhode Island, which we consider to be part of the 
prebatholithic complex. The sparsity of tonalite and 
granodiorite in the Proterozoic Z batholithic rocks sug- 
gests that they were intruded into a sialic continental 
craton. 

After the intrusion of the batholiths, a graben devel- 
oped in the Boston area accompanied by bimodal volcan- 
ism, which formed the Mattapan and Lynn Volcanic 
Complexes (Zm, DZ1) and Brighton Melaphyre (ftZrb), 
and further intrusion of granite, the Westwood (Zwgr). 
During the early and middle Paleozoic, the Milford- 
Dedham terrane acted as a stable platform intruded by 
alkaline and peralkaline granite and gabbro under condi- 
tions of crustal extension. These intrusions correspond to 
continental intraplate activity and could be construed as 
being associated with a failed rift. Following intrusion of 
Devonian plutons, intrusive activity ceased in eastern 
Massachusetts although some bimodal volcanism (Pwv) 
accompanied deposition in the Narragansett basin in the 
Pennsylvanian. However, Permian granite was intruded 



in southern Rhode Island and in the Massabesic Gneiss 
Complex of southern New Hampshire and adjacent Mas- 
sachusetts following compressive deformation of the 
Pennsylvanian strata (Mosher, 1983). A high thermal 
regime (Zartman and others, 1970) and compressive 
deformation during the Permian may have produced the 
gneissosity in the Proterozoic Z rocks of the New Bed- 
ford area. 

The Nashoba zone shares none of this plutonic history. 
The peraluminous Andover Granite (SOagr) intruded 
metamorphosed slope-facies sediments and off-arc volca- 
nic rocks of the Marlboro and Nashoba Formations that 
are for the most part unlike the rocks of the Milford- 
Dedham zone. The Ordovician, gneissic phase of the 
Andover, an S-type granite (Chappell and White, 1974), 
was emplaced at considerable depth, and the Nashoba 
Formation is at high metamorphic grade. No Proterozoic 
intrusive rocks are known in the Nashoba zone in eastern 
Massachusetts. The Silurian intrusive rocks of the Na- 
shoba zone are I-type quartz-diorite to granite and are 
quite different from the middle Paleozoic alkaline gran- 
ites of the Milford-Dedham zone. Hill and others (1984) 
have shown that the Silurian intrusive rocks of the 
Nashoba zone are isotopically heterogeneous; they inter- 
preted this as indicating that the intrusions developed at 
a convergent plate boundary and assimilated varying 
proportions of Proterozoic Z to early Paleozoic crustal 
rock. There is some evidence, discussed above in this 
chapter, that the granite and pegmatite that intruded the 
rocks of the Nashoba zone in Devonian time were pera- 
luminous like the Ordovician gneissic phase of the 
Andover Granite, suggesting a recurrence of the crustal 
conditions that produced the earlier granite. The differ- 
ence in the Paleozoic intrusive rocks in the two zones 
indicates that they were derived from different crust 
under different conditions at least until after most of the 
Devonian and accordingly must have become joined after 
that time. 

The intrusions in the eastern part of the Merrimack 
belt bear a greater similarity to the intrusions in the 
Nashoba zone than the intrusions of the Nashoba zone do 
to those of the Milford-Dedham zone, except that there is 
no equivalent to the Ordovician peraluminous Andover 
Granite. The early intrusions, the Newburyport Com- 
plex and part of the Ayer Granite, are calc-alkaline. The 
peraluminous muscovitic granites in the Merrimack belt 
are Devonian in Massachusetts, although there is some 
question as to the age of the Chelmsford Granite. Car- 
boniferous peraluminous granite does form plutons in 
southern Maine and New Hampshire, however (Gaud- 
ette and others, 1982; Hayward and Gaudette, 1984). The 
Devonian granites of the eastern part of the Merrimack 
belt, such as the Ayer and its equivalent the Canterbury 
Gneiss of Connecticut, tend to be sheetlike, gneissic, 



156 



THE BEDROCK GEOLOGY OF MASSACHUSETTS 



and involved in the regional dynamothermal metamor- 
phism; they did not produce recognizable hornfels aure- 
oles in the host rocks. The rocks of the eastern part of the 
Merrimack belt in Massachusetts were affected by the 
Acadian metamorphism that predominates in central 
Massachusetts (Hall and Robinson, 1982). In contrast, 
the Milford-Dedham zone and most of the Nashoba zone, 
except possibly the west flank, show no indication of an 
Acadian dynamothermal metamorphic event. The 
recently recognized Devonian peraluminous granite and 
pegmatite associated with the Late Ordovician and Silu- 
rian Andover Granite may reflect an Acadian event, 
however, and suggest that at this time a source for the 
magmas of the eastern part of the Merrimack belt may 
have been Nashoba-zone material at depth and at high 
grade during the Silurian. Later faulting along the 
Clinton-Newbury fault has separated the Nashoba-type 
rocks beneath the Merrimack-belt rocks from the rocks 
now forming the Nashoba zone at the surface. 

The age and nature of the intrusive rocks, then, 
indicate that the eastern part of the Merrimack belt and 
the Nashoba belt may have been joined together by the 
end of the Devonian but that the Milford-Dedham zone 
may not have been accreted to the Nashoba until after 
the Devonian. Continental crust was the major source for 
the Paleozoic intrusive rocks of the Milford-Dedham 
zone, the Nashoba zone, and the east part of the Merri- 
mack belt, but the crust differed in kind and level at 
different times within and between each zone. The 
Nashoba and Milford-Dedham zones represented sepa- 
rate crustal blocks, if not microcontinents, early in the 
Paleozoic— although at one time, elsewhere, they might 
have been part of a single plate before being split and 
migrating differentially to arrive at their present posi- 
tions. We suggest that rocks similar to those in the 
Nashoba zone are the basement on which the Merrimack- 
belt rocks were deposited. The Massabesic Gneiss Com- 
plex within this belt resembles the rocks of the Nashoba 
zone. However, if the strata in the east flank of the 
Merrimack belt are as old as the intrusions indicate, they 
may be a more basinward facies of Nashoba-zone rock 
and may represent the dislocated basin-fill of a progeni- 
tor basin to the subsequent Silurian and Devonian trough 
of the Merrimack synclinorium. 

The intrusive rocks of Massachusetts have a complex 
history because they occupy different lithotectonic belts, 
whose relations have changed through time in response 
to plate-tectonic processes. The earlier Proterozoic Z 
mafic intrusive rocks and the later Proterozoic Z granitic 
batholithic rocks of the Milford-Dedham zone are the 
products of a subduction-related cycle of magma gener- 
ation. The latest Proterozoic Z volcanic rocks, such as the 
Mattapan and Lynn, could be related to a rifting center. 
The anhydrous peralkaline magmas that formed the 



Paleozoic plutons of the Milford-Dedham zone are the 
result of subsequent heating of crust under the influence 
of underplating of crust in the subduction process during 
the Paleozoic. 

The Nashoba zone has undergone a more repetitious 
history, during which there are renewed cycles of activ- 
ity. The older and younger phases of the Andover 
Granite were probably formed from magmas generated 
from added new sedimentary crustal material during the 
subduction process, whereas the intermediate-in-time 
Assabet Quartz Diorite, the Sharpners Pond Diorite, and 
the orange-weathering granite represent a cycle of mag- 
mas derived from mantle material. 

The origin and source for the calc-alkaline Paleozoic 
plutons of the eastern part of the Merrimack belt are 
somewhat more enigmatic, but there seems to be a mafic 
to felsic trend with time. This is interpreted as meaning 
that early, predominantly mantle-derived plutons were 
succeeded by plutons derived from continental crust. The 
sequence of plutons probably formed near a convergent 
plate boundary during subduction, although Wones 
(1984) ascribed the Devonian plutons farther west and 
north in Vermont, New Hampshire, Massachusetts, cen- 
tral Maine, and coastal Maine to underplating of crustal 
material. The Carboniferous and late Paleozoic peralu- 
minous intrusions of adjacent New Hampshire may be 
derived from overridden crust during westward thrust- 
ing of material during late Paleozoic plate collision. 



REGIONAL RELATIONS 

The Milford-Dedham zone is one of the Appalachian 
terranes considered to be Avalonian by Williams and 
Hatcher (1983). It does, however contain much more 
intrusive rock and much less extensive, and thinner, 
Proterozoic Z and Paleozoic supracrustal strata than the 
Avalonian terrane in Newfoundland. Other Avalonian 
terranes in the Appalachians have been recognized in 
New Brunswick and Nova Scotia (Rast and others, 
1976b; Skehan and others, 1978; Rast, 1980; Rast and 
Skehan, 1981) and in the southeastern Piedmont of the 
United States (St. Jean, 1973; Seiders, 1978). 

The intrusive rocks of the Milford-Dedham zone are in 
some respects similar in lithology, age relations, and 
setting to those of the Charlotte belt of North Carolina 
and South Carolina (Goldsmith and others, 1988). In the 
Charlotte belt of North Carolina, Proterozoic Z calc- 
alkaline plutonic rocks of batholithic dimensions intrude 
metavolcanic and metasedimentary rocks, which are 
present as roof pendants, screens, and mantling 
sequences. These granitoids consist primarily of quartz 
diorite to granodiorite, with lesser amounts of gabbro 
and diorite, an assemblage less mafic than the prebath- 



INTRUSIVE ROCKS OF EASTERN MASSACHUSETTS 



157 



olithic, Proterozoic Z mafic plutonic rocks of the Milford- 
Dedham zone but more mafic than the granitoids of the 
Dedham batholith and Milford antiform. In the Charlotte 
belt, discrete plutons of early to middle Paleozoic age 
include syenite and gabbro, an assemblage found also in 
the Milford-Dedham zone in Massachusetts. However, 
other early to mid-Paleozoic granites in the Charlotte 
belt are not particularly alkaline (Butler and Ragland, 
1969; Butler and Fullagar, 1978), in contrast to the early 
to mid-Paleozoic granites of the Milford-Dedham zone. In 
addition, the Charlotte belt contains large masses of late 
Paleozoic muscovite-bearing granite (Speer and others, 
1980) not found in the Milford-Dedham zone except in 
southern Rhode Island. The Paleozoic intrusions in the 
Charlotte belt of North Carolina are types that are 
present over a wide area of central and eastern New 
England and not specifically confined to the Milford- 
Dedham zone. Although the Charlotte belt does not have 
the Proterozoic Z and Paleozoic sedimentary basins that 
are found in the Milford-Dedham zone, Proterozoic Z to 
Cambrian rocks, both sedimentary and volcanic, which in 
many respects are similar to those in and around the 
Boston basin and which truly correlate with rocks of the 
Avalon Peninsula, Newfoundland, are present in the 
adjacent Carolina slate belt. 

The intrusive rocks of the Nashoba zone can less 
clearly be correlated with the intrusive rocks of similar 
lithotectonic terranes in the Appalachians. As pointed 
out above, the younger peraluminous intrusive rocks of 
the Nashoba zone are similar in composition and field 
relations to intrusive rocks in the Passagassawakeag 
block in southeastern Maine (Bickel, 1976; Osberg and 
others, 1984) as described by Stewart and Wones (1974, 
p. 231) and Kaszuba and Wones (1985); an older synmet- 
amorphic phase, the Winterport Granite (Stewart and 
Wones, 1974), similar to the gneissic phase of the 
Andover Granite, is also present. Rocks like the Silurian 
Sharpners Pond Diorite, however, seem to be absent in 
this block in Maine. The host rocks are similar, however, 
in lithology and metamorphic grade to those in the 
Nashoba zone. The two terranes are not on strike with 
each other and are inferred to be separate slices or blocks 
of what once was a single lithotectonic terrane. They 
apparently did not share all as