Guidebook for field trips in Connecticut: New England Intercollegiate Geological Conference 60th annual meeting, Yale University, New Haven, Connecticut, October 25-27, 1968: Trip D-

Cassie, Robert M.

Gates, Robert M.

Martin, Charles W.

English

UNH Scholars' Repository

University of New Hampshire University of New Hampshire Scholars' Repository NEIGC Trips New England Intercollegiate Geological Excursion Collection 1-1-1968 The Bedrock Geology of the Waterbury and Thomaston Quadrangles Gates, Robert M. Martin, Charles W. Cassie, Robert M. Follow this and additional works at: https://scholars.unh.edu/neigc_trips Recommended Citation Gates, Robert M.; Martin, Charles W.; and Cassie, Robert M., "The Bedrock Geology of the Waterbury and Thomaston Quadrangles" (1968). NEIGC Trips. 103. https://scholars.unh.edu/neigc_trips/103 This Text is brought to you for free and open access by the New England Intercollegiate Geological Excursion Collection at University of New Hampshire Scholars' Repository. It has been accepted for inclusion in NEIGC Trips by an authorized administrator of University of New Hampshire Scholars' Repository. For more information, please contact nicole.hentz@unh.edu. D“51Trip D-5THE BEDROCK GEOLOGY OF THE WATERBURY AND THOMASTON QUADRANGLESbyRobert M. Gates Charles W. MartinUniversity of Wisconsin Earlham CollegeRobert M. Cassie State University of New YorkCollege at BrockportINTRODUCTIONThe Waterbury gneiss dome is the southernmost of a series of gneiss domes extending from Chester, Vermont, to Waterbury along the eastern flank of the Green Mountain anticlinorium. Although not strictly a dome, since it is unroofed on its western side, it is mantled on the north, east, and south by a series of three conformable metasedimentary units traceable from the northern border of Connecticut almost to Long Island Sound.Igneous rocks ranging in composition from granitic to ultrabasic are erratically distributed throughout the quadrangle but do not constitute large plutons.vThe Waterbury Formation is a metasedimentary gneiss complex that forms the core of the dome and will be seen at stops 1, 2, 3 and 4. The complexly folded metasediments are intermixed in migmatitic fashion with granitic to trondhjemitic rocks that are similar to the biotite-quartz-plagioclase granulitic gneiss of the complex and may merely be recrystallized or mobilized parts of the metasedimentary pile. The structural style of the metasediments and of the migmatitic gneisses clearly separates the core gneiss from the mantling rocks.The rocks mantling the core constitute the Hartland Formation which has been divided into three distinctive lithologic units. The lowest, seen at stops 5 and 11, is called Unit I (= part of the Waterbury gneiss of Fritts, 1963; = Taine Mountain Formation, Stanley, 1964) and is predom­inately a mica-plagioclase quartz granulite and granulitic gneiss. The second unit, seen at stops 6, 7 and 8, is the Hitchcock Lake Member (= Collinsville Formation, Bristol Member of Stanley, 1964; = Reynolds Bridge Formation, Cassie, 1965), a strikingly banded assemblage of quartzo- feldspathic granulites and micaceous feldspar-quartz gneisses and schists.The third unit, The Straits Schist Member, is a lustrous medium- to coarse-grained muscovite-plagioclase-quartz schist containing porphyro­bIasts of garnet and kyanite, and is well displayed at Stop 10. Amphibolites are associated with all members of the Hartland Formation. These may be syntectonic intrusives, in part, and metavolcanics, in part. Discontinuous pods of amphibolite characterize the boundary between the Hitchcock Lake Member and The Straits Schist. This contact will be seen at stops 8, 12, and 16. At Stop 16, small lenses of calcite marble also occur locally at this contact,suggesting a metasedimentary origin for some of the amphib­olites associated with these marbles.In the vicinity of Reynolds Bridge, the position normally occupied by the Hitchcock Lake Member below The Straits is occupied by a large mass of texturally and compositionally heterogeneous quartz-oligoclase-microcline- biotite gneiss with locally abundant amphibolite layers. Small concordantD-52HARTLANDG r a n u l i t e :AMDS C H I S T  CH,BRISTOL DOMElytiolds  BRIDGE g )| e S s  (R.B.)g r a n i t e  (&)HITCHCOCK LAKE MEMBER (HhOHARTLAND UN)T\ I M M6ERAMPHIBOLITE (A)WATERBURY FORMATIONT R O N D H J E M I T I CROCKS CT) \ M ILES3D-5Fig. 1. Index map of the essential parts of the Waterbury Gneiss Dome and the Reynolds Bridge Gneiss.Granite (G)Reynolds Bridge Gneiss (R.B.)Hartland FormationSouthington Mountain Member The Straits Schist Member (TS)Hitchcock Lake Member (Hhl)Unit I MemberUnit I and Hitchcock Lake Member undivided (Hgs)Amphibolites (A)Granite gneiss (GN)Waterbury Formation(Paragneisses, migmatites, and granites)Trondhjemite (T) (large masses only)lenses of this granodioritic gneiss also occur elsewhere in the area, always within the Hitchcock Lake at or near the Straits contact and gradational into the Hitchcock Lake in some exposures. The gneiss, designated the Reynolds Bridge Formation, will be seen at its type locality (Stop 15) and its contact relationships with other rock units can be studied at stops 13, 14, 16 and 17. The Reynolds Bridge Gneiss is most abundant at the crest of a broad anticline which plunges 35 at N 80° W, a crossfold superimposed upon the previously isoclinally folded and foliated Hartland rocks. The development of abundant folia­tion crenulations and minor folds accompanied the formation of this and}other broad superimposed folds in the region; this late tectonic activity occurred at or near the peak of regional metamorphism. The structural localization of the Reynolds Bridge Formation along the axial region of the crossfold, coupled with petrographic data, points to a metasomatic origin for these rocks. The introduction of potash-bearing fluids into the Hitchcock Lake appears to have followed dilatant fold hinge particularly that of the broad anticlinal crassfold, and the contact with the overlying relatively impervious Straits Schist; this meta­somatism may have accompanied the emplacement of the stock of Nonewaug Granite less than a mile to the southwest.Granitic to granodioritic sills and dikes are common in the Hartland Formation,, particularly in Unit I and in the Hitchcock Lake Member. Post- tectonic Nonewaug-type granite and pegmatite are common although rarely of mappable extent; one such dike will be seen at Stop 13. Pegmatites are most common in The Straits Schist.mStructurally, the Hartland Formation is an isoclinally folded series of metasediments which have been refolded late in their tectonic and meta­morphic history (probably in Acadian time). The late refolding was con­trolled or influenced by the resistant, buttressing pre-Hartland crystal­line Waterbury Formation or by rising, partially melted "domes" in the formation.Stop 1 (25.25 N - 51.90 E; Waterbury quadrangle) West of new Highway 8, 2000 ft north of Naugatuck River Bridge. Outcrops extend over knob west of highway and north of power line.Rock: Waterbury Formation. This area represents the many variations in the Waterbury Formation. The paragneisses are thick to thinly inter­layered (1) biotite streaked plagioclase-quartz granulitic gneiss,(2) kyanite bearing biotite-plagioclase quartz gneiss and schist,(3) quartz-plagioclase granulites, with large and small migmatitic streaks and pods of quartz and feldspar. (See Gates and Martin, 1967, Table 1 for modes of these rock types.) Several sill-like and dike-like massesof fine-grained gray trondhjemite are also present. The layering in this area is rather uniform compared to other places where intricate folding and convolutions are common.(25.30 N - 50.72 E) Road cut on Country Club Road, east of Highway 63, and southeast of Johnson School.Rock: A major portion of the road cut is small intrusive of trondhjem­ite in the paragneisses of the Waterbury Formation (fig. 2). The trond­hjemite ranges from fine-grained to coarse-grained and from massive to gneissic. Fine-grained cognate inclusions of fine-grained trondhjemite are in the coarser grained varieties (fig. 3). See Gates and Martin5D-5Fig. 2. Trondhjemite in Waterbury Formation paragneiss. 10" hammer shows scale.. Location: north side of Highway 64, 500 ft east of intersection with Highway 63..T *Fig. 3. Trondhjemite "inclusion" in granite. 6" pencil shows scale. Stop 2, 1000 ft east of Highway 63 on Country Club Road.Stop 3 (25.65 N - 50.67 E; Waterbury quadrangle) 500 feet south ofInterstate 84 on Highway 63, west of highway.This is an area where rocks excavated in the building of Interstate 84 were dumped. These are blocks representative of the road cuts in Highway 84 east and west of this point.Rock: The rocks in this dump area are representative of essentiallyall of the rock types of the Waterbury Formation. Of particular interest are the many varieties of migmatites which clearly reflect the mineralogy and textures of the paragneisses (fig. 4 and Gates and Martin, 1967, fig. 3). These same rock types can be seen in the road cuts on Interstate 84 east and west of Highway 63 and on the east-bound access road from Highway 6A.Stop 4 (26.00 N - 52.30 E; Waterbury quadrangle) Fine Hill, south ofInterstate 84 east of Naugatuck River. This hill is recognized by numerous religious symbols on north side.Rock: This is a type area for the Waterbury Formation. All of the various paragneisses, migmatites, granites, amphibolites, and trondhjemites of the Waterbury Formation can be seen on this hill.Stop 5 (27.85 N - 51.70 E; Waterbury quadrangle) East of old Highway 8near Chase Brass Factory.Rock: The Straits Schist, Hitchcock Lake Member. Hartland Unit I,and granite gneiss. The south-facing cliff is The Straits Schist Member and the contact between it and the underlying Hitchcock Lake Member can be seen near the base. The Hitchcock Lake Member is very thin here as it is cut off by the granite gneiss which underlies the relatively flat area between the cliff and the knobs of Hartland Unit I a thousand feet south­east.Stop 6 (28.25 N - 51.42 E; Waterbury quadrangle) 1000 to 1500 feet southof Frost Bridge Road underpass on new Highway 8.Rock: Hitchcock Lake Member. In the road cuts on both sides of thehighway are exposed the major rock types of the Hitchcock Lake Member.The two major rock types to observe here are (1) the hard, finely streaked mica oligoclase granulite (fig. 5) and (2) coarsely streaked mica oligoclase quartz gneiss (fig. 6). These two rock types are interlayered on a scale ranging from layers an inch thick to several feet. The small amplitude folding is readily apparent. (See Gates and Martin, 1967, Table 5 for modes of the principal rock types of the Hitchcock Lake Member.)Stop 7 (28.20 N - 51.57 E; Waterbury quadrangle) Intersection of old High­way 8 and Waterbury Road one-half mile north of Chase Brass Factory.Rock: South of intersection is The Straits Schist Member, north ofintersection is the Hitchcock Lake Member and 1200 feet north is Hartland Unit I.The heterogeneity of the Hitchcock Lake Member is well exposed in this road cut. Amphibolites, calc-silicates, and pegmatites are seen here as well as the granulite and gneiss seen at Stop 6.(1967, Table 2) for modes of trondhjemitic rock types.7D-5Fig. 4. Waterbury Formation migmatites of thinly layered kyanite-biotite- oligoclase-quartz paragneiss and micjrocline-quartz-plagioclase material from Interstate Highway 84 near Highway 63 interchange. Scale is shown by 10" hammer. Stop 3.Fig. 5. Finely streaked, mica-oligoclase-quartz granulite of the Hitchcock Lake Member (Hartland Formation). Location: east side of new Highway 8 1/4 mile8D-5Fig. 6. Coarsely streaked mica-oligoclase-quartz gneiss of the Hitchcock Lake Member (Hartland Formation). Scale is given by 6" pencil. Location: east side of new Highway 8, 1/4 mile south of Frost Bridge Road underpass. Stop 6.9D-59Stop 8 (28.50 N - 52.45 E; Waterbury quadrangle) In the stream banksbelow the dam at the intersection of Greystone Road and the railroad bridge.Rock: Thin layers of amphibolite are intercalated with the gneissesand granulites of the Hitchcock Lake Member. Amphibolitic rocks are found in discontinuous lenses and pods close to the contact between The Straits Schist and Hitchcock Lake Member.(27.50 N - 53.11 E; Waterbury quadrangle) 750 feet south of an unnamed road 3000 feet southeast of Chestnut Hill Reservoir.Rock: Amphibolite and garnet amphibolite. This rock type is charac­teristic of the amphibolites in the Hartland Unit I Member. The average mode of three garnet amphibolite samples collected near this stop is (in volume percent): 5.6 quartz; 12.2 plagioclase; 67.7 hornblende; 10.1 gar­net; 1.6 sphene; 1.8 opaques; 0.9 other.(28.60 N - 52.90 E; Waterbury quadrangle) Crest of hill on Andrews Road one-half mile south of northern boundary of Waterbury quadrangle.Rock: The Straits Schist. The rocks in an east-west belt here areexceptionally graphitic. This is a variant of the more common lustrous schist. (See Gates and Martin, 1967, Table 6 for an average mode of The Straits Schist.)Stop 11 (27.45 N - 52.70 E; Waterbury quadrangle) Intersection of ChestnutHill Road and power line 3500 ft southwest of Chestnut Hill Reservoir.Rock: Hartland Unit I. The gray muscovite-biotite-plagioclase-quartzgranulite or granulitic gneiss typical of Hartland Unit I is seen in the south-facing cliff on the north side of the power line. In the small knobs 1000 ft northeast is a small body of amphibolite. (See Gates and Martin, 1967, Table 4 for modes of gneisses of Unit I of the Hartland Formation.)(30.40 N - 52.93 E; Thomaston quadrangle) South of Terryville, intersection of South Main Street and Washington Road. Outcrops along South Main Street extend 1600 ft north from intersection.Rock: Contact between The Straits Schist and Hitchcock Lake membersof the Hartland Formation. Exposures on all four sides of the intersection are Straits Schist, and outcrops 400 ft northwest on the east side of South Main Street show two varieties of the Hitchcock Lake, the well-banded and slabby mica-quartz-oligoclase gneiss and the more schistose mica-oligoclase- quartz granulite. A small exposure of amphibolite, common at this contact, is present on the north side of Washington Road 400 ft east-northeast of the South Main Street intersection.Stop 13 (30.28 N - 49.86 E; Thomaston quadrangle) North side of new Highway 109, 2200 ft west of Northfield Road intersection. Outcrops extend westward for 500 ft on north side of road.Rock: Reynolds Bridge Gneiss, Hitchcock Lake Member, and NonewaugGranite-Pegmatite. This exposure shows one of the "satellite" bodies of the Reynolds Bridge Gneiss which occur conformably within the Hitchcock Lake. The Hitchcock Lake here is a heterogeneous interlayered assemblage of mica-oligoclase-quartz gneiss and schist. A fifty-foot thick body of Nonewaug Granite-Pegmatite displays locally crosscutting contacts and10D-5inclusions; these tabular bodies are generally sill-like overall, being elongate parallel to the country rock foliation in three dimensions.Stop 14 (30.11 N - 50.36 E; Thomaston quadrangle) New Highway 109 athigh voltage power line. Outcrops extend 2000 ft westward on north side of road. See Stop 6 of Trip B-4 for engineering aspects of this roadcut.Rock: Reynolds Bridge Gneiss and Straits Schist Member. The con­formable contact between the gneiss and schist is exposed at the east end of the road cut. The lustrous graphitic Straits here contains ubiquitous garnets and some staurolite and kyanite. The crenulations of the mica folia are typical of the belt of Straits Schist extending south from here.Stop 15 (29.88 N - 51.05 E; Thomaston quadrangle) Reynolds Bridge Quarryand road cuts on Jackson Street at east end of bridge, new road cuts 200 ft west of bridge on new Highway 8.Rock: This is the type locality for the Reynolds Bridge Gneiss. Theconsiderable compositional and textural variation within the granitic gneiss is obvious. The composition average and range for 22 specimens of Reynolds Bridge Gneiss in volume percent is: quartz 32.2 (20-56); plagio­clase 38.2 (22-51); microcline 16.1 (nil-48); biotite 10.0 (trace-26); muscovite 2.5 (nil-9); garnet tr. (nil-2). Interlayered amphibolitic material (fig. 7) is more abundant here than in the rest of the exposed gneiss mass. The spectacular and extremely attenuated flow folds exposed here have caused this road cut to be branded "obscene" by one scholar of the Connecticut eugeosyncline. Two small dikes of the massive medium- grained Thomaston Granite are exposed at this locality, one in the quarry and one in the road cut. The steep southeastward dip of the 22-foot-thick dike in the quarry parallels the attitude of one locally prominent joint system. The inclusions of Reynolds Bridge Gneiss and amphibolite exposed in the quarry dike are noteworthy.Stop 16 (29.32 N - 50.97 E; Thomaston quadrangle) New Highway 8, 1.5 miles south of Reynolds Bridge. Extensive new road cuts on west side of highway.Rock: Straits Schist, Reynolds Bridge Gneiss, marble. The road cutoutcrops show The Straits Schist dipping under the Reynolds Bridge Gneiss.A discontinuous 3-foot bed of marble occurs at the contact here and can be seen at the same contact in a cliff 1800 ft due west of the north end of this road cut. That extensive cliff outcrop can be reached by following the old dirt road uphill from the south end of this road cut. In that cliff outcrop, The Straits Schist forms the top of the cliff and is under­lain by the Reynolds Bridge Gneiss and amphibolite. The mixture of Reynolds Bridge Gneiss and amphibolite and Hitchcock Lake rocks separating the two Straits belts can be seen along the old road between the new road cut and the cliff to the west. Intense minor folding is readily apparent in the road cut outcrops.Stop 17 (29.07 N - 51.00 E; Thomaston quadrangle) New Highway 8, 1.4 milessouth of Reynolds Bridge. Extensive road cuts on the west side of the high­way. See Stop 6 of Trip B-4 for engineering aspects of this cut.Rock: Straits Schist, Reynolds Bridge Gneiss, Hitchcock Lake. A layerof Reynolds Bridge Gneiss 50 to 100 ft thick is exposed here in contact with and structurally below The Straits. The southern half of the road cut shows Hitchcock Lake rocks structurally below the gneiss and schist.11D-5v .* -is* KFig. 7. Amphibolite in Reynolds Bridge Gneiss. Scale: approximately 40 ft from top to bottom. Location: west side of new Highway 8 at Reynolds Bridge.Stop 15.D-512REFERENCESCassie, R. M., 1965, The evolution of a domaI gneiss (granitic) and itsrelations to the geology of the Thomaston quadrangle, Connecticut: Unpub. Ph.D. dissertation, Univ. of Wisconsin, 109 p.Fritts, C. E., 1963, Bedrock geology of the Southington quadrangle,Connecticut: U. S. Geol. Survey Geol. Quad. Map GQ 200.Gates, R. M., and Martin, C. W., 1967, The bedrock geology of the Water­bury quadrangle: Connecticut Geol. Nat. History Survey Quad. Rept.22, 36 p.Stanley, R. S., 1964, The bedrock geology of the Collinsville quadrangleConnecticut Geol. Nat. History Survey Quad. Rept. 16, 99 p.\

The Bedrock Geology of the Waterbury and Thomaston Quadrangles

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The Bedrock Geology of the Waterbury and Thomaston Quadrangles

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