Geologic units in Middlesex county, Connecticut

Monson Gneiss (Middle or Lower Ordovician?) at surface, covers 23 % of this area

(May be equivalent to part of Waterford Group) - Interlayered light to dark, mostly medium to coarse-grained gneiss and amphibolite; gneiss composed of plagioclase, quartz, and biotite, with hornblende in some layers and microcline in others; traces of garnet, epidote, and magnetite.

Portland Arkose (Lower Jurassic) at surface, covers 14 % of this area

Reddish-brown to maroon micaceous arkose and siltstone and red to black fissile silty shale. Grades eastward into coarse conglomerate (fanglomerate).

Hebron Gneiss (Silurian and Ordovician) at surface, covers 13 % of this area

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.

Brimfield Schist (Includes Hamilton Resevoir Formation) (Upper? and Middle Ordovician) at surface, covers 10 % of this area

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.

Collins Hill Formation (Upper? and Middle Ordovician) at surface, covers 7 % of this area

( = Partridge Formation of New Hampshire) - Gray, rusty-weathering, medium- to coarse-grained, poorly layered schist, composed of quartz, oligoclase, muscovite, biotite, and garnet, and commonly staurolite, kyanite, or sillimanite, generally graphitic, interlayered with fine-grained two-mica gneiss, especially to the west, and with calc-silicate and amphibolite layers, also rare quartz-spessartine (coticule) layers.

Glastonbury Gneiss (Middle? Ordovician) at surface, covers 3 % of this area

Gray, medium- to coarse-grained, massive to well-foliated granitoid gneiss composed of oligoclase, quartz, microcline, and biotite (as patches), also epidote and hornblende in many areas, commonly associated with layers of amphibolite; elsewhere minor muscovite and garnet.

East Berlin Formation (Lower Jurassic) at surface, covers 3 % of this area

Maroon siltstone, silty and sandy shale, and fine-grained silty sandstone, generally well laminated and commonly well indurated, alternating with dark fissile shale; dolomitic carbonate common in cement, concretions, and thin argillaceous laminae. Local arkose; grades eastward into coarse conglomerate close to eastern border fault. The East Berlin Formation of the Hartford basin contains eight facies: trough cross-bedded sandstones, horizontally stratified sandstones, interbedded sandstones and mudrocks, ripple cross-laminated siltstones, black shales, stratified mudrocks, disrupted shales, and disrupted mudstones. These facies are interpreted as a continental depositional system and are divided into two assemblages. Sandflat/alluvial plain facies assemblage (sandstones and siltstones) is composed of sheet-flood deposits. The lacustrine assemblage (shales and mudrocks) represents a saline lake-playa system (Gierlowski-Kordesch and Rust, 1994).

Upper member [of Middletown Formation] (Middle Ordovician) at surface, covers 3 % of this area

Light-gray, generally rusty weathering, well-layered gneiss and granofels, composed of oligoclase, quartz, biotite, and amphibole (cummingtonite, anthophyllite, gedrite, or hornblende, or several of these), also garnet and chlorite. Many layers of amphibolite and biotite gneiss throughout.

Middletown Formation (Middle Ordovician) at surface, covers 3 % of this area

( = Ammonoosuc Volcanics of New Hampshire) - Heterogeneously interlayered dark- to light-gray, generally medium grained gneiss and granofels, ranging from quartz-biotite gneiss through felsic amphibole gneiss to amphibolite and characteristically containing anthophyllite or cummingtonite with or without hornblende. Also layers of calc-silicate rock and of biotite gneiss with quartz-sillimanite nodules.

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

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.

Lower member [of Middletown Formation] (Middle Ordovician) at surface, covers 3 % of this area

Dark- to medium-gray amphibolite and hornblende gneiss, commonly with garnet, diopside, or epidote, interlayered with light-gray gneiss composed of oligoclase, quartz, biotite, and generally one or more amphiboles, also garnet.

Holyoke Basalt (Lower Jurassic) at surface, covers 2 % of this area

Greenish-gray to black (weathers bright orange to brown), fine- to coarse-grained, grading from basalt near contacts to gabbro in the interior, composed of pyroxene and plagioclase with accessory opaques and locally olivine or devitrified glass.

Rope Ferry Gneiss (Proterozoic Z?) at surface, covers 2 % of this area

(May be equivalent in part to Monson Gneiss) - Interlayered (but layers commonly lenticular to indistinct) light- to dark-gray, fine- to medium-grained gneiss, composed of plagioclase, quartz, and biotite, with hornblende in some layers and microcline in others; local layers of amphibolite. Rope Ferry described as locally massive, gray-colored, lenticularly layered hornblende-biotite-quartz-plagioclase gneiss. Thickness varies; averages 1,130 m. U-Pb analysis of zircon and sphene from the Rope Ferry yields a 620+/-3 Ma age. Protolith consisted primarily of mafic metavolcanic rocks. Unconformably underlies Potter Hill Granite Gneiss of Sterling Plutonic Suite (Skehan and Rast, 1990).

Waterford Group (Proterozoic Z?) at surface, covers 2 % of this area

(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).

Metavolcanic member [of Collins Hill Formation] (Upper? and Middle Ordovician) at surface, covers 1 % of this area

Ranges from mafic to felsic, from dark layered amphibolite and hornblende schist, locally with garnet or epidote, to light-gray (in places purplish), laminated gneiss, composed of quartz, oligoclase, and biotite, in which some layers contain garnet (generally manganiferous) and hornblende or cummingtonite.

Hampden Basalt (Lower Jurassic) at surface, covers 1 % of this area

Greenish-gray to black (weathers bright orange to brown), fine- to medium-grained, grading from basalt near contacts to fine-grained gabbro in the interior, composed of pyroxene and plagioclase with accessory opaques and locally olivine or devitrified glass.

Maromas Granite Gneiss (Devonian?) at surface, covers 0.9 % of this area

Light-gray to buff, medium- to fine-grained granitic gneiss, composed of quartz and microcline with minor plagioclase and biotite. Central body is massive, but outlying strips are foliated and have accessory hornblende or garnet. Massive parts may be young anatectic intrusive rocks; foliated parts may include older felsic metavolcanic rocks belonging to unit Ochv. Pegmatite bodies are common in the vicinity.

Littleton Formation (Devonian) at surface, covers 0.8 % of this area

Gray to silvery, generally non-rusty, medium-grained, massive to well-layered alternating schist and micaceous quartzite, composed of quartz, muscovite, biotite, garnet, and oligoclase, also staurolite, graphite, and ilmenite, and in certain areas kyanite or sillimanite in schist.

Gneiss (metavolcanic) member [of Brimfield Schist] (Upper? and Middle Ordovician) at surface, covers 0.8 % of this area

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.

Potter Hill Granite Gneiss plus Narragansett Pier Granite (Proterozoic Z? and Permian) at surface, covers 0.7 % of this area

Potter Hill Granite Gneiss - Light-pink to gray, tan-weathering, fine- to medium-grained, rarely porphyritic, well-foliated (not lineated) granitic gneiss, composed of microcline, quartz, oligoclase (or albite), biotite, and magnetite, minor muscovite, and local garnet. Narragansett Pier Granite (Permian) - Pink to red, medium- to coarse-grained (commonly pegmatitic), generally massive (not gneissic) granite, composed of microcline, oligoclase, quartz, and biotite, and accessory muscovite and magnetite. Considerable associated pegmatite.

Brimfield Schist (Includes Hamilton Resevoir Formation) (uncertain) (Upper? and Middle Ordovician) at surface, covers 0.6 % of this area

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.

Canterbury Gneiss (Devonian) at surface, covers 0.4 % of this area

(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.

Buttress Dolerite (Middle? Jurassic) at surface, covers 0.3 % of this area

Dark-gray to greenish-gray (weathers brown or gray), medium- to fine-grained, commonly porphyritic, generally massive with well-developed columnar jointing, grading from basalt near contacts to fine-grained gabbro in the interior, composed of plagioclase and pyroxene with accessory opaques and locally devitrified glass, quartz, or olivine.

Talcott Basalt (Lower Jurassic) at surface, covers 0.3 % of this area

Greenish-gray to black (weathers bright orange to brown), fine- to medium-grained, grading from basalt near contacts to fine-grained gabbro in the interior, composed of pyroxene and plagioclase with accessory opaques and locally olivine or devitrified glass. Pillows in many places; volcanic breccia with fragmentary pillows in others.

New London Gneiss (Proterozoic Z?) at surface, covers 0.3 % of this area

New London consists of a layered facies and a massive facies. Layered facies described as alternating layers of light-colored biotite-quartz-plagioclase gneiss and amphibolite. Massive facies described as a granodiorite gneiss with a uniform texture, grain size, and color. Contains shiny black biotite plates and distinctive magnetite grains. New London consists of a layered facies and a massive facies. Layered facies described as alternating layers of light-colored biotite-quartz-plagioclase gneiss and amphibolite. Massive facies described as a granodiorite gneiss with a uniform texture, grain size, and color. Contains shiny black biotite plates and distinctive magnetite grains (Skehan and Rast, 1990).

Shuttle Meadow Formation (Lower Jurassic) at surface, covers 0.3 % of this area

Maroon to dark-gray, silty shale, siltstone, and fine-grained silty sandstone, generally well and thinly laminated. In the southern part of the State includes a layer, up to 5 m thick, of blue, commonly sandy, fine-grained limestone or dolomitic limestone, grading laterally into calcareous siltstone. Coarser and more arkosic to east and south, grading into conglomerate near the eastern border fault.

Clough Quartzite (Silurian) at surface, covers 0.3 % of this area

White, medium-grained, glassy to granular, well-layered quartzite and muscovitic quartzite, locally with garnet; conglomeratic (commonly with tourmaline) in lower part.

Potter Hill Granite Gneiss (Proterozoic Z?) at surface, covers 0.3 % of this area

Light-pink to gray, tan-weathering, fine- to medium-grained, rarely porphyritic, well-foliated (not lineated) granitic gneiss, composed of microcline, quartz, oligoclase (or albite), biotite, and magnetite, minor muscovite, and local garnet. 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. The Ponaganset is interpreted as an intensely deformed phase of the Esmond Plutonic Suite. The Sterling, as defined here, includes only the Hope Valley Alaskite Gneiss and the Potter Hill Granite Gneiss. These rocks intrude the Plainfield Formation and the Waterford Group in southeastern CT and western Rhode Island and are Late Proterozoic in age. The gneiss of the Potter Hill is mainly weathered, deeply stained, somewhat crumbly, and generally slabby. Contains xenoliths of the Plainfield Formation. Is distinguished from Hope Valley by its higher biotite content (Skehan and Rast, 1990).

Fitch Formation (Silurian) at surface, covers 0.2 % of this area

Gray, fine- to medium-grained calc-silicate rock, composed of quartz, biotite, calcite, actinolite, diopside, microcline, and locally garnet, scapolite, or epidote, interlayered with two-mica schist.

New Haven Arkose (Upper Triassic; possibly Lower Jurassic at top) at surface, covers < 0.1 % of this area

Red, pink, and gray coarse-grained, locally conglomeratic, poorly sorted and indurated arkose, interbedded with brick-red micaceous, locally shaly siltstone and fine-grained feldspathic clayey sandstone.

Mamacoke Formation (Proterozoic Z?) at surface, covers < 0.1 % of this area

Interlayered (but layers locally indistinct) light- to dark-gray, medium-grained gneiss, composed of plagioclase, quartz, and biotite; sillimanite, garnet, hornblende, or microcline in certain layers; in upper part locally contains quartz-sillimanite nodules or thin layers of quartzite, amphibolite, or calc-silicate rock.

Quartzite unit [in Plainfield Formation] (Proterozoic Z?) at surface, covers < 0.1 % of this area

Light-gray, glassy, generally thin bedded quartzite, also feldspathic and micaceous quartzite containing quartz-sillimanite nodules.

Middletown Formation (Middle Ordovician) at surface, covers < 0.1 % of this area

( = Ammonoosuc Volcanics of New Hampshire) - Heterogeneously interlayered dark- to light-gray, generally medium grained gneiss and granofels, ranging from quartz-biotite gneiss through felsic amphibole gneiss to amphibolite and characteristically containing anthophyllite or cummingtonite with or without hornblende. Also layers of calc-silicate rock and of biotite gneiss with quartz-sillimanite nodules.

Ultramafic rock (Ordovician or older) at surface, covers < 0.1 % of this area

Ultramafic rock, originally composed of olivine and pyroxene, now generally altered to tremolite, talc, chlorite, or serpentine.

Joshua Rock Member [of New London Gneiss] (Proterozoic Z?) at surface, covers < 0.1 % of this area

Medium-gray (weathers with red spots of hematite), medium-grained, foliated gneiss composed of microperthite, quartz, albite, aegerine-augite, and magnetite; rare riebeckite.