v Stratigraphic 

 ^ Names in the

New London Area

, »

; Connecticut

By RICHARD GOLDSMITH

,- W

.^ CONTRIBUTIONS TO STRATIGRAPHY

^ 

'"V

GEOLOGICAL SURVEY BULLETIN 1224-J

A revision and redefinition 

of some Stratigraphic units 

in the New London area

- Y

v
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary

GEOLOGICAL SURVEY 

William T. Pecora, Director

U.S. GOVERNMENT PRINTING OFFICE, WASHINGTON : 1966

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington. D.C. 20402 - Price 10 cents
CONTENTS

Pago

Abstract__ ________-_---__---_________-__-_.-----_--___-------__ Jl

I ntroduction ____________________________________________ _ ________ 1

Brimfield Schist. .___..____._.._.__...__.__._ _.__.__.. ._ 3

Monson Gneiss..._________________________________________________ 4

New London Gneiss.___-____-_-__-_----_-_-_-_-----_.____-------__ 4

Joshua Rock Gneiss Member..___---______-__--_-__-____--______ 5

Mamacoke Formation.__________-_----______-___-_-_____-_-________ 5

Plainfield Formation.___._______.__---._____.________.____---___-__ 6

Sterling Plutonic Group..__--___-_-_-_______.__-----_-__.__---_--__ 7

Age of the units._______-__________--_-____-____----_-____-_-_---__ 8

References cited.____._--____--_-_-_--___-____--_-_______.-___--___ 9

ILLUSTRATION

Page 

FIGUEE 1. Generalized geologic map of the New London area, Connecticut. J2

TABLE

Page 

TABLE 1. Approximate thicknesses of units in the New London area,

Connecticut_.. ________________________________________ J3

in
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CONTRIBUTIONS TO STRATIGRAPHY

STRATIGRAPHIC NAMES IN THE NEW LONDON AREA

CONNECTICUT

BY RICHARD GOLDSMITH

ABSTRACT

W;

Detailed geologic mapping since 1955 in southeastern Connecticut has made

-k,1 necessary the revision and redefinition of older stratigraphic units. The geologic 

section in this area consists of sedimentary and volcanic rocks of middle and early

-* Paleozoic to possibly Precambrian age metamorphosed to gneiss, quartzite, and 

schist of sillimanite grade. Intrusive granite and quartz monzonite are inter-

- spersed throughout the section. The stratigrapbic column presented in this 

v, paper seems to be in accord with current interpretations of the stratigraphy in 

areas mapped elsewhere in southeastern Connecticut and with the general stratigraphic column being worked out in eastern Massachusetts. Units described, 

from youngest to oldest, are the Brimfield Schist, Monson Gneiss, New London

-< Gneiss and Joshua Rock Gneiss Member, Mamacoke Formation, and Plainfield 

Formation. Also described are granitic rocks of the Sterling Plutonic Group of

v pre-Pennsylvanian age.

INTRODUCTION

^

y f Detailed geologic mapping since 1955 in southeastern Connecticut 

by geologists of the U.S. Geological Survey and the Connecticut

V Geological and Natural History Survey has made necessary the revision and redefinition of older stratigraphic units. The units discussed 

here lie south of the Honey Hill fault (Lundgren and others, 1958;

-y Snyder, 1961,1964; Lundgren, 1962; fig. 1, this rept.) and are within

w the old New London 15-minute quadrangle which encompasses the

present Uncasville, Montville, Niantic, and New London T1/^-minute

-* quadrangles. Most of the units have appreciable continuity along 

the southern, and to a lesser extent, the eastern and western margins 

of the eastern highlands of Connecticut (Eodgers and others, 1959).

->- Some of the units are recognized in Ehode Island and Massachusetts.

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J2 CONTRIBUTIONS TO STRATIGRAPHY

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STRATIGRAPHIC NAMES, NEW LONDON, CONNECTICUT J3

The geologic section consists of sedimentary and volcanic rocks of 

middle and early Paleozoic to possibly Precambrian age metamor-. 

pliosed to gneiss, quartzite, and schist of sillimanite grade. Sills, 

sheets, phacoliths, and lenticular plutons of granite and quartz monzonite are interspersed throughout the section (fig. 1). Most of the 

plutonic rocks are of pretectonic or syntectonic age, though posttectonic granite dikes are common toward the coast. The structure is 

complex; domes and open folds are superimposed on large nappe 

structures. Primary sedimentary features useful in determining tops 

of beds are rarely seen. The units are considerably thinned in some 

areas and thickened in others; probably this varied thickness is in 

part primary, but it also appears to have been caused by tectonic 

activity in some areas.

In such a terrain, establishment of a particular stratigraphic column 

cannot be precise; other interpretations of the structure may favor a 

different stratigraphic arrangement. Table 1 is in accord with current interpretations of the stratigraphy in southeastern Connecticut 

in areas mapped to the west by Lundgren (1962) and to the east by 

Feininger (1965a, b). Furthermore, it appears to be in accord with 

the general stratigraphic column being worked out in eastern Massachusetts by N. P. Cuppels and coworkers (oral commun., 1964). In 

the calculation of the thicknesses of the units, the granitic rocks of 

the Sterling Plutonic Group and the younger dikes have been excluded.

TABLE 1. Approximate thicknesses, in meters, of units in the New London area,

Connecticut 

Brimfield Schist.____-_-__-_-____._________________________ 450-± 1,350

Monson Gneiss____________________________________________ 75-1,270

New London Gneiss (excluding main massive body).___________ 0-715

Joshua Rock Gneiss Member____________________________ 0-485

Mamacoke Formation:

Upper part_____________________________________________ 0-410

Lower part_____________________________________________ 0-1,140

Plainfield Formation:

Upper part (northwestern Moritville)______________________ 330

Middle part (northwestern Montville, northwestern Uncasville) _ 480-575 

Lower part (near Uncasville village) _______________________ 1,250

Total in Montville and Uncasville----------------------- ±2,100

Total in Lyme Dome (Niantic)..________________________ ±2,100

BRIMFIELD SCHIST

The Brimfield Schist is the youngest of the metasedimentary and 

metavolcanic rocks in the New London area; its presence was not 

recognized by previous workers. The formation here consists of
J4 CONTRIBUTIONS TO STRATIGRAPHY ^

garnetiferous and sillimanitic biotite-quartz-feldspar gneisses 1 and -« 

quartz-biotite-feldspar gneisses that locally are rusty weathering. 

Also present are minor layers of calc-silicate rock, quartzite, and 

amphibolite. The Brimfield here is at a metamorphic grade in which ^ -« 

muscovite is no longer stable; instead, the rock contains coexisting 

sillimanite and potassium feldspar. Pegmatite pods, lenses, and 

stringers are abundant. The bottom of the Brimfield is locally Wmarked by a thin quartzite; but where the quartzite is absent, the 

garnetiferous schist and gneiss of the Brimfield can be clearly 

distinguished from the underlying more feldspathic hornblende- * 

bearing gneisses of the Monson Gneiss. ^

MONSON GNEISS

Usage of the name Monson Gneiss follows that of Lundgren (1963, >. -. 

p. 14r-15), and the unit, as mapped here, is continuous with his ^ 

Hadlyme belt of the Monson in the Deep River quadrangle. Monson 

Gneiss was not recognized in this area as a distinct unit by the earlier vworkers, but was lumped with either the Mamacoke Gneiss or the New 

London Granite Gneiss of Gregory (Rice and Gregory, 1906). The 

Monson Gneiss and the underlying New London Gneiss form a group y~ 

of related plagioclase-rich gneisses and amphibolites, and probably 

represent a series of metamorphosed volcanic and associated intrusive 

rocks.

In the New London area the Monson Gneiss is a lenticularly layered, 

locally massive gray hornblende-biotite-quartz-plagioclase gneiss and, 

in places, a biotite-quartz-microcline-plagioclase gneiss. Scattered ->- 

lenses of amphibolite occur throughout the unit. Mafic minerals tend A 

to be concentrated in clots or streaks. The Monson is fairly uniform 

throughout the area. On the east side of the Niantic River, however, 

the upper part of the Monson consists of thin layers as much as 3 inches ^ 

thick, ranging from rocks rich in hornblende to those poor in mafic 

minerals. Variations also can be seen in the northern part of the --« 

area, where the Monson Gneiss is finer grained than to the south and 

where layers of amphibolite 2 to 4 feet thick are fairly abundant immediately below the Honey Hill fault. The base of the Monson is v-i 

placed above the first conspicuously dark- and light-layered gneiss

that characterizes the upper part of the New London Gneiss.

**

NEW LONDON GNEISS ^"

The rocks mapped as New London Gneiss in general correspond 

to those described by Gregory (Rice and Gregory, 1906) as New

1 Mineral modifiers in rock names are given In order of increasing abundance.
STRATIGRAPHIC NAMES, NEW LONDON, CONNECTICUT J5

London Granite Gneiss. As mapped by Gregory, however, the New. 

London Granite Gneiss included some rocks nere mapped a& Mdnson 

Gneiss. The name is here changed to New London Gneiss, as the 

rock is "primarily granodiorite and it may consist of alternating 

amphibolite and light-colored granodiorite outside the immediate 

New London area. : ..

The New London Gneiss can be separated into'three facies: a layered 

facies, a massive facies, and an aegerine-augite granite gneiss called 

the Joshua Rock Gneiss. Member. " The layered facies forms the tipper 

and lower parts-of the unit, and consists of alternating layersVof liglltcolored biotite-quartz-plagioclase (granodiorite) gneiss and amphibolite. In some places the layering is sharp and uniform' in outline 

and texture; in others it is somewhat irregular. Likewise, in some 

places the layers are commonly less than 1 foot thick, and elsewhere 

either the amphibolite or the granodiorite gneiss may be tens of feet 

thick. In general, the bottom part of the New London is less distinctly layered than the upper part. ;

The massive facies is present where the unit is thickest and granodiorite gneiss is the dominant rock. The granodiorite gneiss is unifornily textured and streaked and has shiny black biotite plates and 

conspicuous magnetite grains. The gneiss contains elongate inclusions and locally rotated blocks of amphibolite, and shows evidence 

of flowage.

JOSHUA BOOK GNEISS MEMBEB :' ....'".

The Joshua Kock Gneiss was named by Lundgren (1963, p. .16). 

The rock as an : even-textured medium-grained gray granite gneiss 

commonly having scattered spots of cherry-red hematite stain, and 

characterized by aegerine augite, a coarse perthite, and a rare-earthbearing sphene. The unit is not separated on the map (fig.,1) as it 

is too thin to show at that scale. It is, however, a distinctive unit, 

and is here adopted for use by the U.S. Geological Survey as a member 

in the New London Gneiss. The fact. that iii occurs exclusively as 

phacolithic masses in the lower part of the New London Gneiss suggests that the Joshua Rock Gneiss is a .part of the metamorphosed 

layered sequence. The Joshua Kock Gneiss Member may, however, 

be younger than the rest of the New London Gneiss, and may. represent a facies of the Sterling Plutonic Group.

MAMACOKE FORMATION

The name Mamacoke Gneiss, as used by Gregory (in Rice and Gregory, 1906), is here changed to Mainacoke Formation, and is restricted 

to the metavolcanic and metasedimentary rocks and the gray biotitic

208-071 66 2
J6 CONTRIBUTIONS TO STRATIQRAPHY ,

gneisses below the New London Gneiss and above the highest thick 

quartzite sequence of the Plainfield Formation. The original type 

area, Mamacoke Island in the Thames River north of New London 

and the shore of the Thames River east of the island, is still within the 

area of the formation; however, exposures more typical of the formation may be seen in the hills east of New London.

The upper part of the Mamacoke Formation consists of a layered 

sequence with a thin white quartzite locally at the top. Below the 

quartzite, the first fairly consistent unit is a coarse-grained inequigranular,' swirled, somewhat blocky amphibolite, which in turn is 

underlain by a series of dark even-textured biotite-plagioclase gneisses, 

some layers of which contain abundant garnet with or without sillimanite. Below the dark gneisses is a light-colored sugary-textured 

biotite-quartz-feldspar gneiss with quartz-sillimanite nodules, either 

interf olded or interlay ered with greenish calc-silicate gneiss and schist. 

Layers in this sequence are not everywhere of the same thickness and 

may be missing locally. Lenses of nonlayered amphibolite are rare.

The lower and dominant part of the Mamacoke Formation consists of 

gray biotite-quartz-feldspar gneiss, which is mostly plagioclase rich 

but locally contains appreciable potassium feldspar forming migmatite and granite. The gneiss is indistinctly to distinctly layered; 

the layers contain different amounts of biotite. The biotite within a 

given layer is characteristically but not always present as evenly distributed individual flakes. Hornblende-bearing layers and' layers 

of amphibolite are subordinate.

Unfortunately, biotite-quartz-feldspar gneiss is a common minor 

rock type throughout the section from the Monson into the Plainfield, 

and this leads to difficulties in stratigraphic identification in areas of 

poor exposures. Northwest and west of New London, the Mamacoke 

contains thin quartzites, but these are lacking in the immediate New 

London area. The base of the Mamacoke is placed above the first 

thick section of quartzite and associated pelitic gneiss that characterize 

the underlying Plainfield Formation.

The name Plainfield Formation is here used by Lundgren (1962, 

1963) for rocks formerly mapped as Plainfield Quartz Schist (Rice 

and Gregory, 1906) because the unit contains many rock types other 

than quartz schist. The distinguishing characteristic of the unit, 

however, is its quartzites. Although exposures of the Plainfield Formation are perhaps better at other places in eastern Connecticut than 

at Plainfield, the term Plainfield is retained because of common usage.

The Plainfield Formation consists of a sequence of quartzite, quartzmica schist, micaceous and garnetiferous gneisses, and calc-silicate-
^ STRATIGRAPHIC NAMES, NEW LONDON, CONNECTICUT J7

bearing schist, gneiss, and quartzite. The rocks are abundantly interleaved with and swollen by granite so that the true thickness and

*" sequence of the unit is uncertain. Rapid facies changes and struc-

*' tural complications make correlation of sections observed in different 

areas extremely difficult except in a general manner. In much of the 

^ New London area the Plainfield Formation can be divided into three 

w parts, but there is no clear boundary between them.

The upper part of the Plainfield consists of an upper thin-bedded 

gray quartzite which shows thin micaceous partings. Interlayered is

* subordinate mica schist and mica gneiss, which may locally appear 

above the highest quartzite beds. The beds are mostly i/2 to 3 inches 

thick, but may be as much as 1 foot thick. The schists of the uppermost part are typically dark and biotitic and contain little muscovite 

or sillimanite. In the lower part of the upper quartzite section are 

thin-bedded impure metasandstones, 2- to 3-foot beds of white quart-

** zite, and beds of white to light-green diopside-bearing lime silicate 

gneiss.

The middle part of the Plainfield consists of a gray biotite-quartzfeldspar gneiss containing hornblende and diopside, a distinctive calcsilicate quartzite, and associated calc-silicate gneiss, amphibolite, garnetiferous schist, sugary sillimanite-bearing biotite-feldspar-quartz

* y gneiss, and thin beds of white to gray quartzite. Quartzite is subordinate in this part of the formation. In the area northeast of New 

London, the upper and middle parts of the formation are not clearly 

separable.

The lower part of the Plainfield consists of thick sequences of white 

quartzite in beds y2 to 3 feet thick, which show sillimanitic partings, 

and interbedded mica schist and mica gneiss. These rocks grade 

downward into a fairly thick section of sillimanitic and nonsillimanitic micaceous schist, gneiss, and micaceous f eldspathic quartzite which, 

in the highest metamorphic grade, contain quartz-sillimanite nodules. 

Minor calcareous layers are present as well as some gray feldspathic 

and quartzose gneisses. Many of the rocks in this lower part are 

feldspathic. The lower part of the Plainfield appears to be thicker 

in the Lyme Dome than in the Montville-Uncasville area.

The base of the Plainfield Formation has not been recognized in the 

New London area. Lundgren (written commun., 1965) has suggested 

that some of the schist in the core of the Lyme Dome should be separated out as a formation beneath the Plainfield.

STERLING PLUTONIC GROUP

The pink and gray granite and quartz monzonite gneisses formerly 

mapped as Lyme Granite Gneiss and Sterling Granite Gneiss (Greg-
J8 CONTRIBUTIONS TO STRATIGRAPHY

ory, in Rice and Gregory^ 1906; see also Rodgers and others, 1959, 

p. 52, 57-58) are hereby combined under the name Sterling Plutonic 

Group. Rocks formerly mapped as Sterling Granite Gneiss in Rhode 

Island and Connecticut are now being subdivided according to lithology. Units within the Sterling Plutonic Group include the Scituate 

Granite Gneiss (Qiiinn, 1951), the Hope Valley Alaskite'Gneiss, and 

the Ten Rod Granite Gneiss (Moore, 1958). These units have not 

been separated on the map (fig. 1). The name Lyme Granite Gneiss 

is here abandoned as a useful name in this area.

Rocks of the Sterling Plutonic Group are gneissic in varying 

degrees. They form concordant to semiconcordant sheets, phacoliths, 

and dome-shaped plutons (fig. 1), and their arrangement and internal 

structure indicate that they are granites of pretectonic or syntectonic 

to late tectonic age. Some of the rocks clearly show intrusive relationships; others appear to have formed in place. In the New London area the Sterling granites do not occur higher in the section than 

the Monson Gneiss.

AGE OF THE UNITS

According to Lundgren (1962) the Brimfield Schist can be correlated across the Monson anticline with a belt of Brimfield in the 

Middle Haddam area (called Collins Hill Formation by Lundgren). 

Eaton «and Rosenfeld (I960, p. 171) have shown, that this belt of 

Brimfield on the west side of the Monson anticline lies unconforniably 

under a quartzite that can be traced into the Lower Silurian Clough 

Quartzite of New Hampshire. On the basis of a similar lithology 

in a similar stratigraphic position, they correlate the Middle Haddam 

belt of Brimfield Schist with the Partridge Formation, which underlies the Clough Quartzite in New Hampshire and central Massachusetts (Robinson, written commun., 1964). Thus, the Brimfield Schist 

is certainly pre-Middle Silurian and, if correlative with the Partridge^ 

may be Middle Ordovician. The Monson Gneiss and New London 

Gneiss must then be Ordovician or older. Brookins and Hurley (1965) 

discuss the ages of the Middle Haddam belt of Brimfield Schist and of 

the Monson Gneiss in connection with their work on the radiometric 

ages of the rocks in that area. Their work supports the regional 

stratigraphic interpretation that the Monson Gneiss is older than 

the Brimfield Schist and is Ordovician or older.

Quartzite boulders that resemble the quartzite of the Plainfield 

Formation occur in the conglomerates of the Narragansett basin in 

Rhode Island. These boulders contain Cambrian fossils. Gross stratigraphic relationships elsewhere in New England similarly suggest 

a Cambrian age for at least part of the formation in the New London 

area.
STRATIGRAPHIC NAMES, NEW LONDON, CONNECTICUT J9

The age of the Mamacoke Formation would presumably fall within 

the range of ages of the Monson and New London Gneisses and the 

Plainfield Formation.

The rocks of the Sterling Plutonic Group can be correlated with, 

and in part traced to, rocks in Rhode Island that lie unconformably 

beneath the Pennsylvanian rocks of the Narragansett basin. These 

rocks can thus be assigned a pre-Pennsylvanian age, but as yet there is 

little evidence on which to assign a more definite age. Radioisotope 

work on the Sterling gneisses has so far produced equivocal results 

(Quinn and others, 1957; Snyder, 1961, 1964; Zartman and others, 

1965).

REFERENCES CITED

Brookins, D. G., and Hurley, P. M., 1965, Rb-Sr geochronological investigations

in the Middle Haddam and Glastonbury quadrangles, eastern Connecticut:

Am. Jour. Sci., v. 263, p. 1-16. 

Eaton, G. P., and Rosenfeld, J. L., 1960, Gravimetric and structural investigations

in central Connecticut: Internat. Geol. Cong., 21st, Copenhagen 1960, Kept.,

pt. 2, p. 168-178. 

Feininger, Tomas, 1965a, Bedrock geologic map of the Ashaway quadrangle,

New London County, Connecticut, and Washington County, Rhode Island:

U.S. Geol. Survey Geol. Quad. Map GQ-403.

 1965b, Bedrock geologic map of the Voluntown quadrangle, New London 

County, Connecticut, and Washington and Kent Counties, Rhode Island: 

U.S. Geol. Survey Geol. Quad. Map GQ-436.

Lundgren, Lawrence, Jr., 1962, Deep River area, Connecticut; Stratigraphy and 

structure: Am. Jour. Sci., v. 260, p. 1-23.

 1963, The bedrock geology of the Deep River quadrangle: Connecticut

Geol. and Nat. History Survey Quad. Rept. 13, 40 p. 

Lundgren, Lawrence, Jr., Goldsmith, Richard, and Snyder, G. L., 1958, Major

thrust fault in southeastern Connecticut [abs]: Geol. Soc. America Bull., v.

69, No. 12, pt. 2, p. 1606. 

Moore, G. E., Jr., 1958, Bedrock geology of the Hope Valley quadrangle, Rhode

Island: U.S. Geol. Survey Geol. Quad. Map GQ-105. 

Quinn, A. W., 1951, Bedrock geology of the North Scituate quadrangle, Rhode

Island: U.S. Geol. Survey Geol. Quad. Map GQ-13. 

Quinn, A. W., Jaffe, H. W., Smith, W. L., and Waring, C. L., 1957, Lead-alpha

ages of Rhode Island granitic rocks compared to their geologic ages: Am.

Jour. Sci., v. 255, p. 547-560. 

Rice, W. N., and Gregory, H. E., 1906, Manual of the geology of Connecticut:

Connecticut Geol. and Nat. History Survey Bull. 6, 273 p. 

Rodgers, John, Gates, R. M., and Rosenfeld, J. L., 1959, Explanatory text for

preliminary geologic map of Connecticut, 1956: Connecticut Geol. and Nat.

History Survey Bull. 84,64 p. 

Snyder, G. L., 1961, Bedrock geology of the Norwich quadrangle, Connecticut:

U.S. Geol. Survey Geol. Quad. Map GQ-144.

 1964, Petrochemistry and bedrock geology of the Fitchville quadrangle,

Connecticut: U.S. Geol. Survey Bull. 1161-1, 63 p. 

Zartman, Robert, Snyder, G. L., Stern, T. W., Marvin, R. F., and Bucknam, R. C.,

1965, Implications of new radiometric ages in eastern Connecticut and

Massachusetts, in Geological Survey research 1965: U.S. Geol. Survey Prof.

Paper 525-D, p. D1-D10.