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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 HAMPSHIRE
"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
EZpE2wEZ6q:
BUI
Mafic rocks
9s 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
:?vvfj 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 (An20_29) 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
0 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).
REFERENCES CITED
Affaton, Pascal, Sougy, Jean, and Trompette, Roland, 1980, The
tectono-stratigraphic relationships between the Upper Precam-
brian and Lower Paleozoic Volta Basin and the Pan-African
Dahomeyide orogenic belt (West Africa): American Journal of
Science, v. 280, p. 224-248.
Anderson, M.M., 1981, The Randon Formation of southeastern New-
foundland; a discussion aimed at establishing its age and relation-
ship to bounding formations: American Journal of Science, v. 281,
p. 807-830.
Anstey, R.L., 1979, Stratigraphy and depositional environment of the
early Cambrian Hoppin Slate of southeastern New England and its
Acado-Baltic fauna: Northeastern Geology, v. 1, p. 9-17.
Austin, J. A., Jr., Uchupi, Elazar, Shaughnessy, D.R., III, and Bal-
lard, R.D., 1980, Geology of the New England passive margin:
American Association of Petroleum Geologists Bulletin, v. 64, no.
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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
0 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°x2c
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
QUEBEC
CANADA
UNITED STATES
VERMONT
Passagassawakeag
terrane
ATLANTIC OCEAN
vp^* RHODE
;# f ISLAND
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)
0
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
schist and quartz-actinolite schist;
a E
^E
16001
biotite and hornblendic injection gneiss;
amphibolite and hornblende schist; mino
o
CD
quartzite
CD
3
F
o
u_
£*
03
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DC
E ^
(Garnet)-(sillimanite)-mica schist and
% E
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 Robinson1
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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F22
THE BEDROCK GEOLOGY OF MASSACHUSETTS
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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 Robinson1 and Richard Goldsmith2
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.
2U.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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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 (An65_80), 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 H2S, 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 H20
plus C02 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 H2S in an
open-system sedimentary environment. Sulfur-isotope
data (Tracy and Rye, 1981) on several outcrops show
very light sulfur values with 834S 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
H2S.
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 H2S.
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 Si02 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 0 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 0 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 0 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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Rodgers, John, 1970, The tectonics of the Appalachians: New York,
Interscience Publishers, 271 p.
1985, Bedrock geological map of Connecticut: Hartford, Conn.,
Connecticut Geological and Natural History Survey, scale
1:125,000.
Seiders, V.M., 1976, Bedrock geologic map of the Wales quadrangle,
Massachusetts and Connecticut: U.S. Geological Survey Geologic
Quadrangle Map GQ-1320, scale 1:24,000.
Shride, A.F., 1976, Preliminary maps of the bedrock geology of the
Newburyport East and Newburyport West quadrangles, Massa-
chusetts-New Hampshire: U.S. Geological Survey Open-File
Report 76-488, 4 pis., scale 1:24,000.
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, Mass., Massachusetts University Gradu-
ate School, p. 237-244.
1969, Tectonic framework of southern New England and eastern
New York, in Kay, Marshall, ed., North Atlantic— Geology and
continental drift, a symposium: American Association of Petro-
leum Geologists Memoir 12, p. 793-814.
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.
Skehan, J.W., and Murray, D.P., 1980, Geologic profile across south-
eastern New England: Tectonophysics, v. 69, p. 285-316.
Smith, P. V., and Barosh, P.J., 1981, Structural geology of the Nashua
trough, southern New Hampshire [abs.]: Geological Society of
America Abstracts with Programs, v. 13, no. 3, p. 178.
Sriramadas, Alaru, 1966, Geology of the Manchester quadrangle, New
Hampshire: New Hampshire Department of Resources and Eco-
nomic Development Bulletin 2, 92 p.
Stewart, D.B., and Wones, D.R., 1974, Bedrock geology of the
northern Penobscot Bay area, in New England Intercollegiate
Geological Conference, 66th Annual Meeting, Orono, Maine, Oct.
12-13, 1974, Guidebook for field trips in east-central and north-
central Maine: Orono, Maine, University of Maine, p. 223-239.
Sundeen, D.A., 1971, The bedrock geology of the Haverhill 15'
quadrangle, New Hampshire: New Hampshire Department of
Resources and Economic Development Bulletin 5, 125 p.
Thompson, J.B., Jr., and Norton, S.A., 1968, Paleozoic regional
metamorphism in New England and adjacent areas, in Zen, E-an,
White, W.S., Hadley, J.B., and Thompson, J.B., Jr., eds., Studies
of Appalachian geology— Northern and maritime: New York,
Interscience Publishers, p. 319-328.
Thompson, J.B., Jr., and Robinson, Peter, 1976, Geologic setting of the
Harvard Conglomerate, Harvard, Massachusetts, in New Eng-
land Intercollegiate Geological Conference, 68th Annual Meeting,
Boston, Mass., Oct. 8-10, 1976, Geology of southeastern New
England; a guidebook for field trips to the Boston area and vicinity:
Princeton, N.J., Science Press, p. 345-351.
Thompson, J.B., Jr., Robinson, Peter, Clifford, T.N., and Trask, N.J.,
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in Zen, E-an, White, W.S., Hadley, J.B., and Thompson, J.B.,
Jr., eds., Studies of Appalachian geology— Northern and mari-
time: New York, Interscience Publishers, p. 203-218.
Thompson, P.J., 1983, Silurian-Devonian stratigraphy, Monadnock
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Abstracts with Programs, v. 15, no. 3, p. 186.
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natural pure magnesian cordierite and biotite from pyrite-
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of America Abstracts with Programs, v. 13, no. 7, p. 569.
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Contribution No. 30, 132 p.
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.
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 terranes2 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)
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10
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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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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
H19
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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.S1, MASS
CONNJI R 1
Cape Cod
Bay
J&
0 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
0 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
0 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
0 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
0 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 Ponkapoag3 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 Sx 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
S2 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
4Since 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°-65c 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
H49
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THE BEDROCK GEOLOGY OF 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
lithotectonic units. Explanation for figure 15 follows on p. H52.
STRUCTURAL AND METAMORPHIC HISTORY OF EASTERN MASSACHUSETTS
H51
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Figure 15.— Continued.
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.
Some pre-Triassic faults may be transcurrent
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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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+K20 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 Na20-K20-Al203 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. Wones1,2 and Richard Goldsmith3
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.
2Virginia Polytechnic Institute and State University, Blacksburg, Va.
3U.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 Gneiss3
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
0 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 0 0 .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 0 nr
Hornblende .... 23.1 33.6 26.8 .6 5.0(0.8-7.0)
Chlorite 2 .3 2.9 0 nr
Magnetite 2.0 .7 .4 .9 1.8(0.5-3.3)*
Titanite 3.5 .1 .7 0 nr
Apatite 3 .8 .2 .3 nr
Allanite 0 0 0 tr nr
Pyroxene 0 0 0 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-km2 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 Fe203; 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
Si02 51.7 52.5 58.8 51.52
A1203 16.5 16.3 15.4 17.06
Fe203 9.46 8.37 9.65 1.97
FeO nd nd nd 8.60
MnO 15 .15 .33 0
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
K20 1.44 .71 4.92 1.77
Ti02 1.32 .85 .84 2.45
P205 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 0
Crn 0 0 0 0
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 0 0 0 5.8
Fo 0 0 0 4.5
Fa 0 0 0 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 0
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
Th2 5.0 3.4 2.3
Th 4.6 3.2 2.6
U2 1.4 .9 .6
U 1.4 1.0 nd
^203 calculated as FeO, except sample 4.
2Delayed 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), H20 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 Si02
Figure 6.— CaO and NaaO+K^O plotted against Si02 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 km2 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 km2 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
Na20
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)]
Sample 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 0 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
0 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 km2 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 km2 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 0 .2 .1 1.3 .1
Muscovite 1.0* .7* 0 0 0 0 .9 .3
Epidote 0 0 .7 .4 .5 4.9 1.1 1.0
Hornblende 0 0 tr tr 0 tr 0 0
Chlorite 0 0 0 3.6 3.0 12.9 .5 .9
Magnetite 6 .1 .2 tr .2 .1 0 .1
Hematite 2* 0 0 0 0 0 0 0
Titanite 0 0 0 .1 tr .2 0 0
Apatite 0 0 0 tr 0 .10 0
Allanite 0 0 .4000 0 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 km2) 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
km2 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 km2 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 An10 to An30 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 Fe203. 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
Si02 7L34 73^42 7498 7EU1 76^89 7L48 65^94 12M 6&85 75/78 73T81 74^11
A1203 14.90 13.66 13.18 13.63 13.52 13.93 15.85 14.49 13.98 12.50 14.04 14.40
Fe203 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
Ti02 36 .26 .16 .10 .10 .29 .74 .30 .46 .04 .21 .16
P206 11 .08 .04 .04 .02 .08 .17 .10 .08 .02 .05 .04
(Na20 + K20)/Al203.. .54 .60 .62 .67 .61 .55 .37 .59 .29 .67 .59 .54
Normative-mineral composition, 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 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 0 0 0 0 0 0 0 0 0 .2 0 0
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 0 0 .4 .7 .2 0 .1 .1
DI 84 88 92 96 95 85 69 89 79 92 93 89
Trace-element abundances, in parts per 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
Th2 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
U2 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
'Fe20, calculated as FeO. zDelayed 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 0
Titanite 2
Apatite 0
Allanite 1
Garnet .1
Zircon 0
Calcite 0
"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
0
1.4
.8
.1
.8
4.5
4.3
3.1
1.1
0
1.6
1.3
0
.1
.4
7.9
2.2
4.2
1.8
0
0
0
.3
0
.8
1.0
.5
0
.1
tr
0
0
.1
.1
.2
.1
0
0
.2
.3
.1
.2
0
.3
0
.1
.3
.3
.6
0
0
0
.1
0
tr
.1
.1
.1
.1
.2
.1
.1
0
tr
tr
0
tr
0
.1
.1
0
0
0
0
.1
0
0
0
0
0
.7
.1
.1
0
0
0
0
0
0
0
0
0
.4
0
0
0
0
0
0
0
0
0
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 Fe203. 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
Si02 75.64 74.73 77.9 73.08 67.48
A1203 12.46 12.70 12.2 14.47 15.81
Fe203 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
Ti02 06 .14 .17 .23 .40
P205 01 .04 .01 .07 .13
(Na20+K20)/Al203 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 0 0 0 .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
Th2 24.8 17.3 19.0 14.4 8.3
Th 30.2 15.9 18.7 14.5 9.9
U2 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.
2Delayed 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 0 6.8 4.5 2.1 7.4 5.7 0 1.9 9.9
Muscovite 1.2 0 .3 1.1 .1 2.0 2.7 .1 .2
Epidote 0 1.1 0 .6 1.0 .9 0 .1 3.5
Hornblende 0 0 0 0 .3 0 0 0 0
Chlorite 0 .10 .4 0 .2 0 tr .3
Magnetite .3 .10 tr 0 0 1.6 0 tr
Hematite 0000 00 1.5 0 0
Titanite 0 .1 .1 .2 .7 .1 tr tr .5
Apatite 0 0 .3 tr .2 .1 0 tr .2
Allanite 0 .2 tr 0 .2 tr 0 tr tr
Garnet 0 tr 0 tr 0 0 0 .1 0
Zircon tr tr tr 0 0 0 0 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 Fe203. 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
Si02 75.82 71.08 72.59 75.51 76.01
A1203 12.74 13.36 13.24 13.39 11.72
Fe203 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
K20 4.94 2.58 3.44 4.90 4.02
Ti02 09 .48 .36 .17 .07
P205 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 0 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 0 .8 0 0 0
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 0
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
Th2 7.1 8.5 9.9 20.3 9.0
Th 8.3 8.8 9.2 21.0 11.2
U2 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
'Fe203 calculated as FeO.
2Delayed 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 km2. 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#04
2°°3
^o Harwich
Cape Cod
41°30'
^--Marion
o5
Woods
Martha's
Vineyard
Nantucket Sound
Hole
Nantucket
! t
0 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 km2 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 0 5-15 0 31 24 0 28 3 26^7 27-31 23^1
Plagioclase 19.4a 55-65 5-20 25-40 50L 0 3 0 41 0 0 0
Microcline 36.7p 0-6c 0 25-40p 0 60p 63p 77-97 63p 43 51-71 64-67 53-72
Biotite 7.5 0-2 2-5 3-7 15 0 1 1-5 .3 2 1-5 0 0
Muscovite 4 tr 0 0 0
Epidote 4 tr - 3-5 tr - -
Hornblende tr 0-10 5-15 5-10 5 10* 5 1-7 6 9 0 3^r l-6r
Other amphibole — — — — — — — -3 1 — — —
Pyroxene 0 30-45g 25-40g - 20u 1-9 .3 0 0 0
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 km2. 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 Fe203; 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
Si02 75.22 76.7 43.73 74.9 70.24 77.6 72.24
A1203 15.42 12.3 20.17 11.6 9.80 11.9 13.18
Fe203 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 0 .02 .14 0 .10
MgO 0 0 3.91 .04 .06 0 .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
Ti02 08 .10 4.23 .20 .17 .25 .36
P206 02 0 .15 0 0 0 .07
Zr02 0 0 nd 0 .20 0 0
(Na20+K20)/Al203... .51 .63 .19 .77 .97 .74 .68
Normative-mineral composition, in weight percent,1
and differentiation index (DI)
Qtz 37.1 37.4 0 34.8 25.2
Crn 4.4 .8 0 0 0
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 0
Di 0 0 9.5 .2 0
Hd 0 0 1.9 .2 0
Ac 0 0 0 0 15.5
Wo 0 0 0 0 1.2
En 0 0 3.3 .(
Fs 2.1 2.0 .7 .01 3.1
Fo 0 0 1.4 0 2.6
Fa 0 0 .7 0 0
Ilm 2 .2 8.0 .4 .3
Mag 0 0 6.1 0 0
Ap 0 0 .3 0 0
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
0
0
29.5
29.6
32.1
33.7
.7
3.3
0
.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
Th2 37.3 17.3
Th 38.8 16.8
U2 9.7 3.3
U 9.0 3.2
Zr/Hf 28 nd
'Fe^j calculated as FeO.
2Delayed 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 km2. 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-km2 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 km2
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 km2 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+K20)/Al203 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-km2 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-km2 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 km2 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
K20 (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
Al203
50% Al203
Na20
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-K20-Al203 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 Na20-K20-Al203 diagram (fig. 22) in which the
amount of A1203 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 A1203
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 H20-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 H20 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).
2Data 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
Others1 . . .
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
0-0.6
tr
0-0.2
0-6.5
0-33
0.2-17
0-4
0
0-3
0
tr
0
0
0-13
19-38
0
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
km2 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 An55, averages An40-An45, 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 An30-An40.
The biotite tonalite (table 14, no. 6) is limited to about
10 km2 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 An15-An33. 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 km2 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 km2 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 Fe203. 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
Si02
A1203
Fe203
MnO
MgO
CaO
N^O
K20
Ti02
P205
(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
Th2 ..
. 16.7
U2 ...
. 2.5
Zr/Hf
. 44
'Fe203 calculated as FeO.
2Delayed 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 km2
in area. The Indian Head may be equivalent in age to
small (less than 1 km2) 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
/MASS
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 3-35 281 29m
Mafic minerals2 5 12-33 8 4
'20 percent as phenocrysts, 8.5 percent in groundmass.
2Mafic 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 km2 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 km2 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 Or80,
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 km2 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 Or95 (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 (An37-An71), 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 km2 (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 207Pb/206Pb 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 (An5-An13), 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 aspects of early Paleozoic
intrusive history. Terranes similar to the Nashoba are
present in the Maritime Provinces in Canada (Rast and
others, 1976a), and some observers (for example,
Williams and Hatcher, 1983) have placed the Nashoba
zone within the Gander zone as defined in Newfoundland.
Similar terranes have not been identified in the southern
Appalachians.
The intrusions into the eastern flank of the Merrimack
belt in Massachusetts are only in part similar to those in
the eastern part of the Merrimack belt of Maine. The
older Ordovician to Silurian intrusions are not found
there, and only Devonian granite forms plutons (Osberg
and others, 1984). The synkinematic fabric and sheetlike
nature of some of the Devonian intrusions in Massachu-
setts and Connecticut (Dixon and Pessl, 1966; Snyder,
1967; Pease, 1972; Tucker, 1977; Maczuga, 1981) corre-
spond to the style of emplacement of Devonian plutons in
the Merrimack belt farther north in New England (Hay-
ward and Gaudette, 1984), but the metamorphic evi-
dence indicates that the southern plutons were emplaced
at a slightly deeper structural level than the northern
ones.
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scale 1:250,000.
Radiometric Ages of Rocks in
Massachusetts
By ROBERT E. ZARTMAN and RICHARD F. MARVIN
THE BEDROCK GEOLOGY OF MASSACHUSETTS
U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1366-J
CONTENTS
Abstract Jl
Introduction 1
Published studies 1
Critical evaluation 2
Tables of abbreviated RADB records 3
References cited 4
Appendix: Radiometric Age Data Bank 18
ILLUSTRATION
Figure 1 . Map of Massachusetts showing locations of radiometrically dated samples J2
TABLES
[Tables follow References Cited]
Table 1. Published K-Ar, Rb-Sr, and zircon U-Pb ages of rocks from Massachusetts J6
2. Published zircon Pb-alpha and apatite fission-track ages of rocks from
Massachusetts ^
THE BEDROCK GEOLOGY OF MASSACHUSETTS
RADIOMETRIC AGES OF ROCKS IN MASSACHUSETTS
By Robert E. Zartman and Richard F. Marvin
ABSTRACT
In this chapter, K-Ar, Rb-Sr, U-Pb, Pb-alpha, and fission-track
analyses published through 1986 are tabulated and critically evaluated
for use in assigning geologic ages to the rock units of Massachusetts.
The compilation was facilitated by the use of the U.S. Geological
Survey's Radiometric Age Data Bank (RADB), a computer-based data
storage and retrieval system for geochronological information. Approx-
imately 400 individual radiometric ages are contained herein in a
synoptic table derived from the RADB. Each entry is identified
according to lithotectonic zone, town or village, latitude and longitude,
rock unit, analyzed mineral(s), method of dating, an interpretive
coding, and reference.
A complete RADB record is described in the appendix, which
illustrates how the various information fields are arranged and can be
interrogated. A printout of all RADB records for Massachusetts is
available as U.S. Geological Survey Open-File Report 87-170.
INTRODUCTION
Geologic interpretation requires that stratigraphic
units and structural elements be placed into a time
framework of sufficient accuracy to resolve the sequence
of events affecting the rocks. Age assignments are made
both in a relative sense, by determining the order of
interrelated features, and in an absolute sense, by pale-
ontologic and radiometric dating. In a State as geologi-
cally complex as Massachusetts, many problems arise
when we attempt to construct such a time framework.
Long-range correlation with fossil localities has some-
times been made over distances of hundreds of kilome-
ters, even where mapping is inadequate and strati-
graphic continuity is, at best, uncertain. The juxtaposing
of distinct tectonic blocks across major faults further
inhibits the extrapolation of meager stratigraphic con-
trol. Indeed, entire rock sequences are now known to
abruptly terminate at such boundaries.
The purpose of this chapter is to provide a general
documentation for the radiometric ages from Massachu-
Manuscript approved for publication November 16, 1987.
setts that bear on the time framework of primary strat-
igraphic units and superimposed metamorphic fabrics.
To this end, a tabulation (table 1) has been made of all
published ages that were used for compilation of the
State bedrock map (Zen and others, 1983). This table
thus serves as (1) an abbreviated summary of the radio-
metric ages and (2) a link to the original literature. The
specific application of these data in establishing a relative
and absolute chronology within each compilation area is
incorporated into the appropriate chapters elsewhere in
this professional paper.
Although modern isotopic geochronology spans barely
three decades, a vast body of analytical data has already
accumulated. To take proper advantage of the radiomet-
ric ages pertinent to this study, it was necessary to
catalogue the published literature bearing on the isotopic
dating in Massachusetts and to critically evaluate these
data. Particular attention was given to providing time
control on the primary stratigraphic ages of rock units
and to recognizing patterns of metamorphic overprinting
that are recorded by disturbed or recrystallized mineral
systems. A comprehensive treatment of the rock chro-
nology, of course, cannot be carried out independent of
firm paleontologic and stratigraphic correlations, and the
age assignments accompanying the geologic map repre-
sent an attempt to accommodate all evidence. Generally,
if the responses of the various dating methods, the
regional pattern of metamorphic overprinting, and other
geologic constraints are carefully examined, even very
complex isotopic systematics can contribute important
age control.
PUBLISHED STUDIES
Approximately 400 individual radiometric ages have
been reported in the literature for rocks of Massachu-
setts. As of the end of 1986, these data included 140
K-Ar, 174 Rb-Sr, 49 U-Pb, 32 Pb-alpha, and 4 fission-
track analyses determined on 300 separate samples. A
J2
THE BEDROCK GEOLOGY OF MASSACHUSETTS
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Ficure 1. — Locations of radiometrically dated samples, Massachusetts.
similar number of ages from adjacent States also bear
directly on a regional time framework; they were evalu-
ated during compilation of the map, but only Massachu-
setts localities are contained in the tabulation of this
chapter. Taken at face value, the radiometric ages are a
complex mixture of chronometric systems, some yielding
the time of primary deposition of strata or emplacement
of plutons, others recording various episodes of meta-
morphic overprinting, and a substantial number provid-
ing ambiguous or minimally useful information. They
represent the measurements of many different rock
types, tectonic environments, and grades of metamor-
phism found within this statewide transect of the Appa-
lachian orogen (fig. 1). Only after careful evaluation of
interrelated sets of data, often containing apparent con-
tradictions, could we hope to construct a significantly
precise chronology of events. Although this goal has
certainly not yet everywhere been reached, many
revised or confirmed age assignments incorporated into
the new State map arise either entirely or in large part
from modern radiometric dating.
Some years ago it was realized that the manual recov-
ery of all data bearing on a specific age assignment was
becoming increasingly difficult because of the rapidly
growing quantity of radiometric age information. The
ever-expanding literature on the subject, which includes
easily overlooked articles in obscure publications and
geochronologic data buried in broader studies, demanded
that abstracted records of radiometric ages be system-
atically stored by a computer for more ready identifica-
tion and retrieval. The Radiometric Age Data Bank
(RADB) was established by the U.S. Geological Survey
(Zartman and others, 1976) to fulfill this objective. Its
application in compiling the pertinent information for the
geologic map of Massachusetts represents the first
attempt to use the RADB on a comprehensive statewide
basis. Additional information about the RADB is given in
an appendix to this chapter, in which a record, the basic
unit of the data storage and retrieval system, is
described in some detail. Tables 1 and 2 have been
abstracted and compiled from the inventory of all Mas-
sachusetts records. A printout of all RADB records for
Massachusetts was prepared by Zartman and Marvin
(1987).
CRITICAL EVALUATION
If the observed parent and daughter isotopic contents
of a whole-rock or mineral sample were determined
RADIOMETRIC AGES OF ROCKS IN MASSACHUSETTS
J3
solely by radioactive transformation subsequent to its
formation, a calculated "age" would always relate
straightforwardly to the origin of that rock or mineral.
However, because physicochemical conditions also influ-
ence the behavior of these isotopic species, some amount
of interpretation is usually necessary in order to trans-
late calculated "ages" into a meaningful time framework.
Although for a rock having a simple history, such as a
rapidly cooled granitic rock that encounters no subse-
quent metamorphism after its initial crystallization, all
the radiogenic systems would ideally yield the correct
crystallization age, this situation rarely prevails for the
rocks of a polymetamorphic terrane. In fact, many of the
geochronologic studies in southern New England make
sense only after one recognizes the different responses to
subsequent metamorphism of isotopes used in the vari-
ous dating methods. Success in deciphering the complex
age patterns found even within a single rock unit relies
heavily on an understanding of both the relative stability
of the radiogenic systems during later thermal and
hydrothermal events and the constraints placed on each
sample by the local and regional geologic setting.
What, then, are the distorting factors that can cause a
calculated "age" of, say, an igneous rock to deviate from
the true age of crystallization? Basically, two types of
problem may beset any radiometric system: (1) the initial
presence of daughter isotopes in the crystallizing rock or
mineral and (2) movement of parent or daughter isotopes
into or out of the rock or mineral after its original
crystallization. Examples of the former are radiogenic
argon trapped into a newly forming pyroxene and an
overgrowth of zircon nucleating on an inherited core of
the same mineral. Examples of the latter are the diffu-
sive loss of radiogenic argon from a biotite during
thermal metamorphism and the metasomatic removal of
radiogenic strontium from a retrogressively altered
rock. All of these complicating processes were operative,
at least locally, during the geologic development of
Massachusetts and have affected the radiometric sys-
tems to varying degrees. It is not surprising, therefore,
that early attempts to establish the ages of rocks were
sometimes unsuccessful because neither the strati-
graphic and structural complexities nor the vagaries of
the chronometers were fully appreciated. Only as mod-
ern field mapping revealed the true nature of the geology
and as the stability ranges of the various dating methods
were determined have studies been designed to produce
a time framework of necessary accuracy.
Particularly valuable for deciphering the temporal
relations within orogenic terranes, such as the Appala-
chians, is the intercomparison of "ages" obtained from
several radiometric systems with quite different
responses to superimposed physicochemical conditions.
Although it has proven difficult to quantify precisely the
temperature, pressure, and chemical environment nec-
essary to reset a given radiometric system, a relative
ranking and semiquantitative calibration for each system
have evolved from field and laboratory attempts to
determine the activation energies associated with diffu-
sion and recrystallization. In practice, however, there
are rarely enough geochronologic data to permit a thor-
ough intercomparison of radiometric systems involving
numerous dating methods and many sample localities for
a rock unit. Rather, the recognition of isotopic age
patterns has often been haphazard. The synthesis of a
number of separate studies may be required to sort out
the effects of primary crystallization and later metamor-
phic overprintings. Eventually, the accumulating body of
data allows one to identify and delineate areas in which,
say, all K-Ar analyses of biotite give a Permian thermal
event age even for host rocks formed at a much earlier
time. Likewise, certain granitic rocks may contain
ancient inherited zircon in addition to newly crystallizing
zircon where magmas have penetrated a considerably
older sialic basement. When brought to light, these
patterns not only contribute to an increased understand-
ing of the time dimension of geologic processes but also
provide information about the physical and chemical
conditions surrounding these processes.
TABLES OF ABBREVIATED RADB RECORDS
All pertinent radiometric age data for rock units of
Massachusetts and relevant parts of adjacent States
were reviewed in conjunction with the preparation of the
State bedrock map. A judgment based on this informa-
tion together with paleontologic and stratigraphic evi-
dence has resulted in the age assignments appearing on
the map and in this report. Documenting this effort are
two tables giving the abbreviated RADB records of
K-Ar, Rb-Sr, and U-Pb ages (table 1) and Pb-alpha
and fission-track ages (table 2) for the Massachusetts
localities.
The records in each table are arranged by increasing
geologic age within lithotectonic zones (Hatch and oth-
ers, 1984) in a west-to-east progression across the State.
Record locality is identified geographically by town or
village, latitude, and longitude. Rock units are named
according to the terminology of the State bedrock map,
which may be revised from the original author's usage.
Map symbols of rock units are also included except where
the dated rock is not given map status (for example,
pegmatite, dike, and minor intrusion). Each record con-
tains one or more radiometric ages listed under the
appropriate dating method by mineral analyzed, numer-
ical value and uncertainty of calculated or interpreted
age, and an explanatory symbol. This latter explanatory
symbol serves the very important function of providing
J4
THE BEDROCK GEOLOGY OF MASSACHUSETTS
some critical evaluation of the radiometric age data.
Generally, the critical evaluation was made by the orig-
inal authors, but sometimes subsequent information
available at the time that the RADB record was being
compiled has resulted in a reevaluated interpretation.
Accordingly, one can recognize at a glance those ages
that closely constrain the primary stratigraphic time
framework or that record some superimposed disturbing
event.
The Pb-alpha ages in table 2 are not well defined
analytically or geologically, and they are presented here
mainly for historical purposes and completeness of the
compilation. The four fission-track ages in table 2 record
times of final uplift and cooling— when the terrane expe-
rienced broad upwarping long after igneous and meta-
morphic activity of the Paleozoic orogenies had ceased.
The last column of tables 1 and 2 gives reference to the
publication from which the RADB record was compiled.
The reader should go to this primary literature for more
information about the dated sample and for a detailed
explanation of its geologic interpretation.
REFERENCES CITED
Hatch, N.L., Jr., Zen, E-an, Goldsmith, Richard, Ratcliffe, N.M.,
Robinson, Peter, Stanley, R.S., and Wones, D.R., 1984, Lithotec-
tonic assemblages as portrayed on the new bedrock geologic map
of Massachusetts: American Journal of Science, v. 284, p.
1026-1034.
Steiger, R.H., and Jager, E., 1977, Convention on the use of decay
constants in geo- and cosmochronology: Earth and Planetary
Science Letters, v. 36, p. 359-362.
Zartman, R.E., Cole, J.C., and Marvin, R.F., 1976, User's guide to the
Radiometric Age Data Bank (RADB): U.S. Geological Survey
Open-File Report 76-674, 77 p.
Zartman, R.E., and Marvin, R.F., 1987, Radiometric ages on file in the
Radiometric Age Data Bank (RADB) of rocks from Massachusetts:
U.S. Geological Survey Open-File Report 87-170, 421 p.
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.
TABLES 1 AND 2 AND APPENDIX
J6
THE BEDROCK GEOLOGY OF MASSACHUSETTS
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THE BEDROCK GEOLOGY OF MASSACHUSETTS
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RADIOMETRIC AGES OF ROCKS IN MASSACHUSETTS
J17
Table 2. — Published zircon Pb-alpha and apatite fission-track ages of rocks from Massachusetts
[Lead-alpha ages calculated by the equation, t (m.y.) = K x (p/a), where p is lead content in ppm, a is alpha activity in alphas/mg-hr, and K = 2580. Fission-track
ages are given in terms of the decay constant, \f = 8.46 x 10~17 yr"1. The explanatory symbol (x) indicates that the radiometric system records an age interpreted
to be time of uplift and cooling below 110 °C; (xx) indicates that the age is of questionable analytical or geological meaning. Ref., reference from list below. — ,
not determined; Do. , do. , ditto]
Towno
r village
Lat, N.
Long, W.
Rock unit
Pb-alpha
Fission-track
No.
. Explanatory
Age , ,
symbo
Age
Explanatory
symbol
Ref.
Bronson Hill zone
1
2
3
Erving
Orange
Erving
42°37'18"
42°33'30"
42°36'00"
72°22'06"
72°18'12"
72°22'00"
Pauchaug Gneiss (Ope)
Monson Gneiss (OZmo)
Fourmile Gneiss (OZfm)
-
97±5
82+3
94±5
X
X
X
4
4
4
Merrimack synclinorium
1
Leominster
42°31'45"
2
West Chelmsford
42°37'57"
3
Bolton
42°25'00"
4
Clinton
42°23'50"
71°48'00" Fitchburg Complex (Dfgd) 230±25
71°25'05" Chelmsford Granite (Dcgr) 480±51
71°39'00" Ayer Granite (Sacgr) 410±50
71°41'15" do. 420±50
Nashoba zone
42°23'25"
71°40'40" Muscovite granite (mgr)
Milford-Dedham zone
1 Peabody
2 Do.
3 Rockport
4 Do.
5 Beverly
6 Norfolk
7 Foxborough
8 Do.
9 Wrentham
10 Milford
11 Do.
12 Whitinsville
13 Uxbridge
42°31'00" 70°58'00" Peabody Granite (Dpgr)
42°30'05" 70°57'55" do.
42°40'50" 70°38'04" Cape Ann Complex (SOcgr)
42°40'41"
70°39'00"
do.
42°33'18"
70°52'12"
do.
42°07'00"
71°20'00"
Dedham Granite (Zdgr)
42°03'18"
71°17'32"
do.
42°02'30"
71°17'30"
do.
42°02'30"
71°20'00"
do.
42°09'45"
71°30'00"
Milford Granite (Zmgr)
42°09 '45"
71°30'15"
do.
71°40'00" Ponaganset Gneiss (Zpg)
7r:«'54" do.
275 ±28
273 ±27
260±21
290 ±23
260±21
290 ±33
230 ±18
265 ±25
235 ±15
265±22
225±25
390 ±30
335±31
380 ±33
365 ±35
220 ±22
385 ±38
365 ±33
310±28
360 ±33
355 ±32
270 ±30
270±29
265±31
215±17
245±26
620 ±53
1. Quinn, A.W., Jaffe, H.W., Smith, W.L., and Waring, C.L., 1957, 3. Zartman, R.E., Snyder, G.L., Stern, T.W., Marvin, R.F., and
Lead-alpha ages of Rhode Island granitic rocks compared to
their geologic ages: American Journal of Science, v. 255, p.
547-560.
2. Webber, G.R., Hurley, P.M., and Fairbairn, H.W., 1956, Relative
ages of eastern Massachusetts granites by total lead ratios in
zircon: American Journal of Science, v. 254, p. 574-583.
Bucknam, R.C., 1965, Implications of new radiometric ages in
eastern Connecticut and Massachusetts: U.S. Geological Sur-
vey Professional Paper 525-D, p. D1-D10.
4. Zimmerman, R.A., Reimer, G.M., Foland, K.A., and Faul, H.,
1975, Cretaceous fission-track dates of apatites from northern
New England: Earth and Planetary Science Letters, v. 28, p.
181-188.
J18
THE BEDROCK GEOLOGY OF MASSACHUSETTS
APPENDIX: RADIOMETRIC AGE DATA BANK
The Radiometric Age Data Bank (RADB) is a means
for collecting and organizing the approximately 100,000
radiometric ages presently published for the United
States. Although the goal of providing complete cover-
age for the entire country has not yet been reached, a
concentrated effort directed toward the six New Eng-
land States (Maine, New Hampshire, Vermont, Massa-
chusetts, Rhode Island, and Connecticut) has resulted in
the incorporation of at least 95 percent of the ages
published through 1986 for this area into our data file.
The RADB is constructed such that sample location,
petrography, analytical data, calculated age(s) together
with interpretive remarks, and literature citation are
linked to form an independent record for each sample.
The record can accommodate, singly or in combination,
all results on a sample pertinent to the K-Ar, Rb-Sr,
U-Th-Pb, Pb-alpha, and fission-track methods.
Data are processed by using the General Information
Processing System (GIPSY) developed at the University
of Oklahoma, which maintains the data file and builds,
updates, searches, and prints the records through simple
yet versatile command statements. The retrieval of
records is accomplished by specifying the presence,
absence, or numeric or alphabetic value of any element of
information in the data bank. For example, searches can
be made for all records relating to the Quincy Granite, all
records containing Rb-Sr biotite ages between 400 Ma
and 600 Ma, or all records derived from a particular
literature citation. Output is available in the form of
complete or abbreviated records, listings of specified
record elements, columnar tabulations, or input data for
certain ancillary programs, such as map or histogram
plots. A typical RADB record is discussed below to
illustrate the sort of information available from the data
bank. The reader with further interest in the operational
aspects of RADB is referred to the user's guide to the
Radiometric Age Data Bank (Zartman and others, 1976)
and additional references contained therein.
As an example, a slightly abridged RADB record (a
few coded elements, which, for our purpose, would be
redundant, have been deleted from the original record)
for a sample of granite from the Cape Ann Complex from
Rockport, Mass., is shown in figure Al. Many of the
individual elements of the record are self-explanatory,
but certain entries that are given in code or may be
otherwise enigmatic require further definition. The ital-
icized letters next to some of the elements are keyed to
the following comments. A, General information about
each sample, including a unique record number, a refer-
ence code (a complete reference is given at the end of the
record), and a code for the laboratory or laboratories that
performed the analytical determinations. B, Name of
person who compiled this record and the date (month and
year) of compilation. C, A one-digit code indicating
accuracy of sample location given by latitude and longi-
tude coordinates. D, A one-digit code indicating the
source of the sample; that is, outcrop, core, quarry,
mine, and so forth. E, A four-letter code of a formalized
geologic name conforming to the Standard Stratigraphic
Code of the U.S. Geological Survey. F, The known or
most probable stratigraphic age of the unit (which may or
may not coincide with the measured radiometric age)
given in three-digit code conforming to the Standard
Stratigraphic Code of the U.S. Geological Survey. G, A
four-character petrographic code for the rock from which
the sample was taken; this code is used in searches for
general rock groupings. H, The laboratory sample num-
ber as given by the author(s) of the published article. /,
A three-character mineralogic code for the analyzed
mineral; this code is used in searches for general mineral
family groupings. /, A two-digit code identifying the
method of analysis. K, Interpretive comments about the
radiometric age and its geologic significance. L, A suite
number. If a calculated age is related to the analyses of
other samples, as for a Rb-Sr whole-rock isochron or
U-Pb zircon discordia array, it is designated as being
part of a suite. A unique number ties together all
analyses belonging to the suite and can be used to
retrieve the other members of the suite.
Each sample locality is represented by a record similar
to the one shown in figure Al, and each record contains
1-6 individual ages (our example gives a K-Ar horn-
blende, an Rb-Sr whole-rock, and a U-Th-Pb zircon age).
This pool of information provides the data base from
which tables 1 and 2 have been compiled. The entire set
of unabridged records is not reproduced here, but a copy
of it is available as U.S. Geological Survey Open-File
Report 87-170 (Zartman and Marvin, 1987). Of course,
the data bank must be recognized as only a convenient
method of processing information originating elsewhere,
and primary references remain those of the published
articles. The list of primary references from which the
Massachusetts RADB records were compiled is included
in tables 1 and 2.
RADIOMETRIC AGES OF ROCKS IN MASSACHUSETTS J19
RADIOMETRIC AGE DATA BANK - U.S.G.S. BRANCH OF ISOTOPE GEOLOGY
LOCATION SAMPLE IDENTIFICATION A
COUNTRY UNITED STATES RECORD NUMBER 0001708
STATE MASSACHUSETTS REFERENCE NUMBER 71-00005
COUNTY ESSEX LABORATORY ( I ES> SD
QUAD SCALE QUAD NO. / NAME COMPILED BY B
1 1 24000 GLOUCESTER NAME: WILLIAMS, B. R.
LATITUDE 42-39-52 N DATE: 77 06
LONGITUDE 70-38-25 W
COMMENT ROCKPORT (FORMERLY LEONARD JOHNSON) QUARRY. 4200'
SOUTHWEST OF PIGEON HILL. ROCKPORT
PRECISION OF LOCATION: 2 C
SOURCE OF SAMPLE: 3 D
SAMPLE DESCRIPTION
GEOLOGIC NAME: CAPE ANN GRANITE GEOLOGIC NAME CODE: CPAN £
ROCK TYPE: ALKALIC GRANITE LEXICON AGE(S): 361 357 340 f
PETROGRAPHIC CODE: B240 G
DESCRIPTION: LIGHT BLUISH-GRAY. MEDIUM-GRAINED GRANITE
POTASSIUM- ARGON
LABORATORY SAMPLE NUMBER: 260 H
ROCK/MINERAL CODE: PC5 / HORNBLENDE
ANALYTICAL DATA:
K20 (%) 1.240
40AR-RAD (MOLES/GM) 7.93 X E- 1 0
% RADIOGENIC 97
CALCULATED AGE 390 +/- 12 MILLION YEARS
TYPE OF ANALYSIS 10 u
COMMENTS: MINIMUM AGE OF INTRUSION; DISTURBED BY LATER METAMORPHISM: FOR «
ADDITIONAL INFORMATION. SEE REFERENCE (S) 77-00005
RUB I D I UM-STRONT I UM
LABORATORY SAMPLE NUMBER: 260 H
ROCK/MINERAL CODE: AAO / WHOLE-ROCK
ANALYTICAL DATA:
RB (PPM) 154.4
SR (PPM) 16-3 N
87RB/86SR 27.36
87SR/86SR 0.871
CALCULATED AGE 435 +/- 6 MILLION YEARS
INITIAL 87SR/86SR 0.703
TYPE OF ANALYSIS 22 •»
ANALYTICAL COMMENT: 1 1-POINT WHOLE-ROCK ISOCHRON
SAMPLE SUITE: 00088 L „
COMMENTS: AGE OF INTRUSION! FOR ADDITIONAL INFORMATION. SEE REFERENCE K
77-00005
URAN I UM-THOR I UM-LE AD
LABORATORY SAMPLE NUMBER: 260 H
ROCK/MINERAL COOE : MCI / ZIRCON
ANALYTICAL DATA:
U (PPM) 1330
TH (PPM) 599.7
PB (PPM) 92.9
ISOTOPIC COMPOSITION OF LEAD (ATOM PERCENT)
204PB 206PB 207PB 208PB
0.042 81.83 5.168 12.96
206PB/238U AGE 417 +/- 8 MILLION YEARS
207PB/235U AGE 421 +/- 9 MILLION YEARS
207PB/206PB AGE 452 +/- 10 MILLION YEARS
208PB/232TH AGE 450 +/" 8 MILLION YEARS
CONCORDIA INTERCEPT AGE— 450 +/- 25 MILLION YEARS
ANALYTICAL COMMENT: 4-POINT ZIRCON DISCORD I A LINE
COMMENTS: AGE OF INTRUSION; FOR ADDITIONAL INFORMATION. SEE REFERENCE K
77-00005
REFERENCE: ZARTMAN. R.E.. AND MARVIN. R.F.. 1971. RADIOMETRIC AGE (LATE
ORDOVICIAN) OF THE QUINCY, CAPE ANN. AND PEABODY GRANITES FROM EASTERN
MASSACHUSETTS: GEOL . SOC. AMER. BULL.. V. 82. P. 937-958
Figure Al.-A Radiometric Age Data Bank (RADB) record for a sample of granite from the Cape Ann
Complex from Rockport, Mass. For explanation of abbreviations and italicized letters, see facing page.
gmmin
3 9999 05903 626
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