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Mineral Resources > Online Spatial Data > Geology > by state > Connecticut Geology

Geologic units in Windham county, Connecticut

Hope Valley Alaskite Gneiss (Proterozoic Z?) at surface, covers 4 % of this area

Hope Valley Alaskite Gneiss - Light-pink to gray, medium- to coarse-grained, locally porphyritic, variably lineated and foliated alaskitic gneiss, composed of microcline, quartz, albite or oligoclase, and minor magnetite, and locally biotite and muscovite. Lineation formed by rods of quartz. Locally contains quartz-sillimanite nodules.

Lithology: granitic gneiss

Middle member [of Bigelow Brook Formation] (Silurian or perhaps Ordovician) at surface, covers < 0.1 % of this area

Middle member [of Bigelow Brook Formation] - Greenish-gray, medium-grained calc-silicate rock, composed of plagioclase, quartz, and diopside (locally hornblende and scapolite), interbedded with schist and granofels composed of plagioclase, quartz, biotite, and commonly garnet and sillimanite.

Lithology: calc-silicate rock; schist; granofels

Tatnic Hill Formation (Upper? and Middle Ordovician) at surface, covers 10 % of this area

Tatnic Hill Formation - Medium- to dark-gray, medium-grained gneiss or schist composed of quartz, andesine, biotite, garnet, and sillimanite, locally kyanite, muscovite, or K-feldspar, interlayered with locally mappable units and thinner layers of rusty-weathering graphitic pyrrhotitic two-mica schist, amphibolite, and calc-silicate rock.

Lithology: gneiss; schist; mica schist; amphibolite; calc-silicate rock

Plainfield Formation (Late Proterozoic? or older?) at surface, covers < 0.1 % of this area

Plainfield Formation - Pale-tan to gray, fine-grained quartzite, and pale- to medium-gray quartz-mica schist. Quartzite is massive to layered (bedded?). Includes Plainfield Formation and some rock mapped formerly as Blackstone Series.

Lithology: quartzite; mica schist

Yantic Member [of Tatnic Hill Formation] (Upper? and Middle Ordovician) at surface, covers 3 % of this area

Yantic Member [of Tatnic Hill Formation] - Medium- to dark-gray, fine- to medium-grained schist, composed of quartz, oligoclase, biotite, and muscovite, some layers with garnet, staurolite, and kyanite or garnet and sillimanite, local epidote, or K-feldspar; some layers of rusty-weathering graphitic, pyrrhotitic, two-mica schist.

Lithology: schist; mica schist

Foliated quartz diorite (Devonian in part, probably Ordovician in part) at surface, covers < 0.1 % of this area

Foliated quartz diorite - Mainly dark-gray, medium-grained, well-foliated gneiss (locally strongly sheared, especially near contacts), composed of plagioclase, quartz, biotite, and hornblende, locally also pyroxene.

Lithology: mafic gneiss

Dioritic phase [of Lebanon Gabbro] (Devonian) at surface, covers 0.1 % of this area

Dioritic phase [of Lebanon Gabbro] - White to black, streaked, medium-grained, foliated or sheared gneiss, composed of plagioclase, biotite, quartz, and generally hornblende.

Lithology: mafic gneiss

Scituate Granite Gneiss (Proterozoic Z) at surface, covers < 0.1 % of this area

Scituate Granite Gneiss - Gneissic granite containing biotite in small clots. Equivalent to part of former Northbridge Granite Gneiss (usage now abandoned). Gradational with Zhg.

Lithology: granite

Porphyritic member [of Southbridge Formation] (Silurian or Ordovician or both) at surface, covers 0.2 % of this area

Porphyritic member [of Southbridge Formation] - Light- to medium-gray, fine-grained porphyritic massive to layered gneiss, composed of quartz, oligoclase, microcline, and biotite, with megacrysts 1 to 2 cm long of microcline.

Lithology: gneiss

Hebron Gneiss (Silurian and Ordovician) at surface, covers 14 % of this area

Hebron Gneiss - Interlayered dark-gray, medium- to coarse-grained schist, composed of andesine, quartz, biotite, and local K-feldspar, and greenish-gray, fine- to medium-grained calc-silicate rock, composed of labradorite, quartz, biotite, actinolite, hornblende, and diopside, and locally scapolite. Local lenses of graphitic two-mica schist.

Lithology: schist; calc-silicate rock; mica schist

Eastford gneiss phase [of Canterbury Gneiss] (Devonian) at surface, covers 5 % of this area

Eastford gneiss phase [of Canterbury Gneiss] - Mainly light-gray, medium-grained, foliated to strongly lineated gneiss, composed of quartz, microcline, oligoclase, or albite, biotite, and muscovite.

Lithology: gneiss

Lower member [of Bigelow Brook Formation] (Silurian or Ordovician or both) at surface, covers 5 % of this area

Lower member [of Bigelow Brook Formation] - Chiefly gray, medium-grained, well-layered granofels, composed of quartz, oligoclase, and biotite, commonly with garnet and sillimanite, interlayered with thinly fissile sillimanitic, graphitic, pyrrhotite schist.

Lithology: granofels; schist

Tatnic Hill Formation (Ordovician or Proterozoic Z) at surface, covers < 0.1 % of this area

Tatnic Hill Formation - Sulfidic sillimanite schist, sillimanite schist and gneiss, biotite gneiss; minor amphibolite, calc-silicate gneiss and marble.

Lithology: schist; gneiss; biotite gneiss; amphibolite; marble

Biotite-hornblende diorite and quartz-bearing diorite (Devonian) at surface, covers < 0.1 % of this area

Biotite-hornblende diorite and quartz-bearing diorite - Mostly foliated; intrudes Dl.

Lithology: diorite; quartz diorite

Mylonite along Paleozoic faults (Upper or Middle Paleozoic) at surface, covers 0.6 % of this area

Mylonite along Paleozoic faults - Mylonite, blastomylonite, and blastomylonitic gneiss, composed of intensely granulated quartz, plagioclase, biotite, and epidote, in places with hornblende or microcline and commonly with secondary minerals. In places has later been silicified (compare unit Jsi)..

Lithology: mylonite; gneiss

Fly Pond (calc-silicate) Member [of Tatnic Hill Formation] (Upper? and Middle Ordovician) at surface, covers 2 % of this area

Fly Pond (calc-silicate) Member [of Tatnic Hill Formation] - Light-gray, medium-grained, layered to massive calc-silicate gneiss, composed of andesine, quartz, hornblende or actinolite, epidote, and commonly diopside, biotite, and scapolite; some layers are calcitic.

Lithology: gneiss

Canterbury Gneiss (Devonian) at surface, covers 8 % of this area

Canterbury Gneiss (may be equivalent to Ayer Granite of Massachusetts) - Light-gray, medium-grained, variably foliated, locally strongly lineated gneiss, composed of quartz, oligoclase, microcline, and biotite, locally also muscovite, or epidote, and generally with megacrysts 1 to 2 cm long of either or both feldspars.

Lithology: gneiss

Scotland Schist (Devonian or Silurian or both) at surface, covers 7 % of this area

Scotland Schist - Gray to silvery, locally rusty, fine- to medium-grained schist, composed of quartz, muscovite, biotite, staurolite, and oligoclase, locally with kyanite or sillimanite; interlayered, especially below and to the west, with quartz-oligoclase-biotite schist and granofels and locally with quartzite.

Lithology: schist; granofels; quartzite

Sterling Igneous Suite - alaskite gneiss (Late Proterozoic) at surface, covers < 0.1 % of this area

Sterling Igneous Suite - alaskite gneiss - Pale pink, orange, or gray, fine- to medium-grained granite gneiss, rarely with porphyroclasts of microcline/orthoclase. Composition is granite with generally less than 3% dark minerals. Composed of sodic plagioclase, quartz, microcline/orthoclase, minor biotite, and opaque minerals; minor muscovite (in part secondary), and rare garnet and sphene in some rocks; accessory apatite and zircon; secondary chlorite. Varies from massive to layered. Strongly foliated and locally well lineated. Includes most rock mapped formerly as Hope Valley Alaskite Gneiss.

Lithology: granitic gneiss

Preston Gabbro (Middle Ordovician or older) at surface, covers < 0.1 % of this area

Preston Gabbro - Dark, medium- to coarse-grained, mainly massive gabbro, composed of labradorite, augite, and opaques, generally with hornblende, locally hypersthene, or olivine or both.

Lithology: gabbro

Southbridge Formation (Silurian or Ordovician or both) at surface, covers 7 % of this area

Southbridge Formation - Dark- to light-gray, locally rusty, fine- to medium-grained interlayered granofels and schist, composed of quartz, plagioclase, and biotite, with muscovite in schist, and amphibole, calc-silicate minerals, K-feldspar in certain layers; also locally mappable units and thinner layers of calc-silicate rock, amphibolite, and sillimanite-garnet and sillimanite-graphite-pyrrhotite schist.

Lithology: granofels; schist; calc-silicate rock; amphibolite

Quartzite unit [in Plainfield Formation] (Proterozoic Z?) at surface, covers 4 % of this area

Quartzite unit [in Plainfield Formation] - Light-gray, glassy, generally thin bedded quartzite, also feldspathic and micaceous quartzite containing quartz-sillimanite nodules.

Lithology: quartzite

Brimfield Schist (Upper? and Middle Ordovician) at surface, covers 1 % of this area

Brimfield Schist (includes Hamilton Resevoir Formation) - Gray, rusty-weathering, medium- to coarse-grained, interlayered schist and gneiss, composed of oligoclase, quartz, K-feldspar, and biotite, and commonly garnet, sillimanite, graphite, and pyrrhotite. K-feldspar partly as augen 1 to 3 cm across. Minor layers and lenses of hornblende- and pyroxene-bearing gneiss, amphibolite, and calc-silicate rock.

Lithology: schist; gneiss; amphibolite; calc-silicate rock

Gneiss (metavolcanic) member [of Brimfield Schist] (Upper? and Middle Ordovician) at surface, covers 0.2 % of this area

Gneiss (metavolcanic) member [of Brimfield Schist] - Medium-gray, medium-grained, layered gneiss and schist, composed of oligoclase, quartz, and biotite; some gneiss and most schist layers contain garnet and sillimanite; some gneiss layers contain garnet, hornblende or pyroxene or grade into amphibolite or calc-silicate rock. Probably includes metavolcanic rocks.

Lithology: gneiss; schist; amphibolite; calc-silicate rock; metavolcanic rock

Ponaganset Gneiss (Proterozoic Z?) at surface, covers 3 % of this area

Ponaganset Gneiss - Dark-gray, coarse-grained, porphyritic, well-foliated gneiss, composed of oligoclase, quartz, microcline (mostly as megacrysts up to 8 cm long), biotite, magnetite, and generally hornblende; also garnet and muscovite where hornblende is absent. Sterling Plutonic Suite is here restricted to the Hope Valley terrane. (The Hope Valley together with the Esmond-Dedham terrane make up the Avalon superterrane of this report.) The Ponaganset Gneiss and the Ten Rod Granite Gneiss lie within the Esmond-Dedham terrane and are therefore removed from the Sterling. Ponaganset lies east of the Hope Valley shear zone and extends from RI into MA. [Apparently does not occur in CT.] Predominantly granite, ranges to tonalite. Gray to light gray, some pink, generally medium- to coarse-grained, but ranges from fine-grained to porphyritic. Alkali K-feldspar, plagioclase, quartz, and biotite are major minerals. The Ponaganset is interpreted as an intensely deformed phase of the Esmond Plutonic Suite. Strong lineation is defined by elongate aggregates of quartz and feldspar and trains of biotite and hornblende. Age is Late Proterozoic based on correlation with Northbridge Gneiss (MA), which has been dated at 557+/-4 Ma by Zartman and Naylor (1984) (Skehan and Rast, 1990).

Lithology: gneiss

Silicified rock and mylonite along Mesozoic faults (probably mainly Jurassic) at surface, covers < 0.1 % of this area

Silicified rock and mylonite along Mesozoic faults - Close network of quartz veins and veinlets cutting each other and older rock, which is mostly replaced by very fine grained quartz. In places, incompletely replaced rock shows strongly mylonitic texture.

Lithology: mylonite

Plainfield Formation (Proterozoic Z?) at surface, covers 3 % of this area

Plainfield Formation - Interlayered light-gray, thin-bedded quartzite, in places with feldspar, mica, graphite, or pyrite, light- to medium-gray gneiss composed of quartz, oligoclase, and biotite (rarely microcline), medium- to dark-gray schist composed of quartz, oligoclase, biotite, sillimanite, and garnet, dark-gray or green gneiss composed of plagioclase, quartz, biotite, and hornblende (commonly with diopside), amphibolite, diopsite-bearing quartzite, and calc-silicate rock. In places contains quartz-sillimanite nodules.

Lithology: quartzite; mica schist; gneiss; amphibolite; calc-silicate rock

Felsic gneiss member [of Quinebaug Formation] (Middle Ordovician or older) at surface, covers 0.4 % of this area

Felsic gneiss member [of Quinebaug Formation] - Light- to medium-gray, fine- to medium-grained gneiss, composed of plagioclase, quartz, biotite, and muscovite, commonly with K-feldspar.

Lithology: felsic gneiss

Tatnic Hill Formation (Ordovician or Proterozoic Z) at surface, covers < 0.1 % of this area

Tatnic Hill Formation - Yantic Member - Gray mica schist.

Lithology: mica schist

Waterford Group (Proterozoic Z?) at surface, covers 1 % of this area

Waterford Group (may be equivalent in part to Monson Gneiss) - Interlayered part (but layers locally distinct) of Waterford Group, light to dark, generally medium grained gneiss, composed of plagioclase, quartz, and biotite, with hornblende in some layers and microcline in others. Some layers of amphibolite. Usage of Waterford Group follows Goldsmith (1980; 1985). Described as a sequence of metavolcanic and metaplutonic plagioclase gneisses and amphibolites that unconformably overlies the Plainfield Formation in the Hope Valley terrane. (Hope Valley and Esmond-Dedham terranes compose the Avalon superterrane of this report.) Thickness is variable; ranges to 3,100 m. Subdivided (ascending) into Mamacoke Formation with its upper Cohanzie Member (first used?), New London Gneiss, and Rope Ferry Gneiss. Age is Late Proterozoic based on U-Pb analyses of zircon and sphene in the Rope Ferry Gneiss (620+/-3 Ma, Wintsch and Aleinikoff, 1987) (Skehan and Rast, 1990).

Lithology: gneiss; amphibolite

Ayer Granite (Lower Silurian) at surface, covers < 0.1 % of this area

Ayer Granite - Granite to tonalite, partly porphyritic; locally gneissic, locally muscovitic; may include rocks older than Silurian; intrudes Sb and So. Ayer Granite is divided into the Clinton facies and the Devens-Long Pond facies (Gore, 1976). In addition, there are some masses not assigned to either facies that intrude Berwick Formation west and northwest of Lawrence, and that intrude Paxton and Oakdale Formations south of Worcester and west of probable southern continuation of Wekepeke fault. Radiometric ages obtained for facies of Ayer pose problems in assigning ages to unfossiliferous sedimentary rocks they intrude. Clinton facies has a well-defined U-Pb zircon age of 433 +/-5 Ma (Zartman and Naylor, 1984) that authors cite as Early Silurian; Devens-Long Pond facies has a similar age. This age greatly compresses the time available for deposition, burial, deformation, and metamorphism of Berwick and Paxton if these units are truly Silurian. Some of the Ayer not assigned to a facies may have been more properly correlated with Early Devonian Chelmsford Granite and muscovite-biotite granite at Millstone Hill. Bodies south of Worcester may be more properly correlated with Canterbury Gneiss of CT, which lies on strike with Ayer and has Early Devonian age of 329 +/-9 Ma (Zartman and Naylor, 1984). Zartman and Naylor (1984) believe Ayer Granite has same age range as Newburyport Complex. It is quite possible, based on textural and mineralogical differences that the two facies should be separate units, representing different magmatic events (Wones and Goldsmith, 1991).

Lithology: granite; tonalite

Scituate Granite Gneiss (Proterozoic Z?) at surface, covers 5 % of this area

Scituate Granite Gneiss - Light-pink to gray., medium- to coarse-grained, generally porphyritic, well-lineated and locally foliated granitic gneiss, composed of microcline, quartz, albite or orthoclase, biotite, hornblende, and magnetite. Megacrysts of microcline up to 3 cm long; lineation formed by splotches of biotite or by rods or quartz.

Lithology: granitic gneiss

Esmond Igneous Suite - granite gneiss (Late Proterozoic) at surface, covers < 0.1 % of this area

Esmond Igneous Suite - granite gneiss - Pale-gray to pale-pink, fine- to medium-grained granite gneiss, rarely with porphyroclasts of microcline/orthoclase. Composition ranges from quartz monzonite to granite. Composed of sodic plagioclase, quartz, microcline, biotite, opaque minerals; minor muscovite common, garnet more rare; accessory apatite and zircon; sphene and hornblende present in some rocks; secondary chlorite. Typically massive, but with strong penetrative foliation and lineation defined by major minerals. Commonly associated with, and in part gradational into, augen gneiss. Includes rock mapped formerly in part as Ponaganset Granite Gneiss, Scituate Granite Gneiss, Ten Rod Granite Gneiss, and Hope Valley Alaskite Gneiss.

Lithology: granitic gneiss; augen gneiss

Paxton Formation (Silurian) at surface, covers 0.1 % of this area

Paxton Formation - Undifferentiated biotite granofels, calc-silicate granofels, and sulfidic schist. The Paxton, here of group rank, includes strata formerly mapped in CT as the Hebron Formation and in MA as the Paxton Formation. It conformably overlies the Oakdale Formation and structurally and conformably underlies the Brimfield Group. It is undivided in central MA; in northeast CT and adjacent MA it is divided into the Dudley and Southbridge Formations. Age is Late Proterozoic(?) based on the intrusion of 440 m.y. Hedgehog Hill gneiss into the overlying Brimfield Group and an age of 1188 m.y. for detrital zircons from the Paxton (Pease, 1989).

Lithology: granofels; schist

Quinebaug Formation (Middle Ordovician or older) at surface, covers 13 % of this area

Quinebaug Formation - Medium- to dark-gray, commonly greenish, medium-grained, well-layered gneiss, composed of hornblende, andesine, biotite, and epidote, commonly with quartz or garnet, interlayered with amphibolite.

Lithology: mafic gneiss

Sterling Igneous Suite - granite gneiss (Late Proterozoic) at surface, covers < 0.1 % of this area

Sterling Igneous Suite - granite gneiss - Pale pink to gray, medium-grained granite gneiss, commonly with small porphyroclasts of microcline/orthoclase. Similar to alaskite gneiss, but with more than 3% dark minerals. Composition is granite with generally less than 3% dark minerals. Compositions range from quartz monzonite to granite. Composed of sodic plagioclase, quartz, microcline, biotite, opaque minerals; minor muscovite common, garnet more rare; accessory apatite and zircon; sphene present in some rocks; secondary chlorite. Typically is compositionally homogeneous, with strong foliation and locally well-developed lineation defined by major minerals. Includes some rock mapped formerly as Ten Rod Granite Gneiss, Hope Valley Alaskite Gneiss, Potter Hill Granite Gneiss, and Scituate Granite Gneiss.

Lithology: granitic gneiss

Upper member [of Bigelow Brook Formation] (Silurian and perhaps Ordovician) at surface, covers 1 % of this area

Upper member [of Bigelow Brook Formation] - Chiefly gray, rusty-weathering, medium-grained, generally well layered and locally fissile schist, composed of plagioclase, quartz, biotite, garnet, and sillimanite, locally with K-feldspar or cordierite, fissile layers commonly with graphite and pyrrhotite, interlayered with quartzose granofels with less biotite but with calc-silicate minerals.

Lithology: schist; granofels

Lebanon Gabbro (Devonian) at surface, covers 0.6 % of this area

Lebanon Gabbro - Dark, speckled, coarse-grained, massive but locally sheared gabbro, composed of hornblende, labradorite, and opaques. Some bodies contain biotite, and quartz; some smaller ones are nearly pure hornblende with local augite.

Lithology: gabbro

Littleton Formation (Lower Devonian) at surface, covers < 0.1 % of this area

Littleton Formation - Black to gray aluminous mica schist, quartzose schist, and aluminous phyllite.

Lithology: schist; phyllite

Black Hill Member [of Quinebaug Formation] (Middle Ordovician or older) at surface, covers 2 % of this area

Black Hill Member [of Quinebaug Formation] - Gray, medium- to fine-grained, well-layered schist and granofels, composed of oligoclase, quartz, and biotite, commonly with hornblende or muscovite, and locally with calcite, garnet, or epidote.

Lithology: schist; granofels

Quartzite unit [in Scotland Schist] (Devonian or Silurian or both) at surface, covers < 0.1 % of this area

Quartzite unit [in Scotland Schist] - Quartzite, generally micaceous, interlayered with mica schist.

Lithology: quartzite; mica schist

Tatnic Hill Formation (Ordovician or Proterozoic Z) at surface, covers < 0.1 % of this area

Tatnic Hill Formation - Fly Pond Member - Calc-silicate gneiss and marble.

Lithology: gneiss; marble

Brimfield Schist (uncertain) (Upper? and Middle Ordovician) at surface, covers 0.5 % of this area

Brimfield Schist (uncertain) (includes Hamilton Resevoir Formation) - Gray, rusty-weathering, medium- to coarse-grained, interlayered schist and gneiss, composed of oligoclase, quartz, K-feldspar, and biotite, and commonly garnet, sillimanite, graphite, and pyrrhotite. K-feldspar partly as augen 1 to 3 cm across. Minor layers and lenses of hornblende- and pyroxene-bearing gneiss, amphibolite, and calc-silicate rock.

Lithology: schist; gneiss; amphibolite; calc-silicate rock