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Mineral Resources > Online Spatial Data > Geology > by state > Rhode Island Geology

Geologic units in Washington county, Rhode Island

monchiquite (Jurassic) at surface, covers < 0.1 % of this area

monchiquite - Dark-gray to greenish-black, fine-grained lamprophyre dikes, with flow-banding parallel to contacts; contains microphenocrysts of titaniferous augite, olivine, kaersutite, phlogopite, apatite, and opaque minerals enclosed in a matrix of analcite and calcite; ocellar texture locally developed. Locally abundant megacrysts (up to 5 cm) of pyroxene, and rounded xenoliths (up to 8 cm) composed of olivine, enstatite, chrome-diopside, and chrome-spinel.

Lithology: lamprophyre

Other metasedimentary rocks - metaclastic rock, undivided (Pennsylvanian? or Late Proterozoic?) at surface, covers < 0.1 % of this area

Other metasedimentary rocks - metaclastic rock, undivided - Red to gray to green polymict conglomerate, sandstone, and shale. Variably metamorphosed. Includes rock mapped formerly as Bellingham Conglomerate.

Lithology: meta-conglomerate; metasedimentary rock

Sterling Igneous Suite - granite gneiss (Late Proterozoic) at surface, covers 4 % 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

Waterford Group - Mamacoke Formation (Late Proterozoic? or older?) at surface, covers 1 % of this area

Waterford Group - Mamacoke Formation - Light- to dark gray, medium-grained amphibolitic gneiss composed of plagioclase, microcline, quartz, biotite, hornblende, and garnet; locally contains quartz-sillimanite nodules, and thin layers of quartzite and calc-silicate rock.

Lithology: gneiss; quartzite; calc-silicate rock

Scituate Granite Gneiss (Proterozoic Z?) at surface, covers < 0.1 % 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 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

Scituate Igneous Suite - granodiorite (Devonian) at surface, covers < 0.1 % of this area

Scituate Igneous Suite - granodiorite - Dark-gray to black, fine- to medium-grained, massive to faintly foliated rock, containing plagioclase, microcline, quartz, and accessory hornblende, biotite, sphene, apatite, and opaque minerals. Includes some rock previously mapped as Cowesett Granite.

Lithology: granodiorite

Scituate Igneous Suite - alkali-feldspar granite (Devonian) at surface, covers 0.6 % of this area

Scituate Igneous Suite - alkali-feldspar granite - Gray to pink, medium- to coarse-grained, porphyritic to subporphyritic alkali-feldspar granite (phenocrysts of perthite). Contains perthite, quartz, and accessory biotite, hornblende, and lesser riebeckite, aegerine, zircon, allanite, sphene, fluorite, and opaque minerals. Plagioclase is sparse to absent. Secondary muscovite, chlorite, and calcite. Ovoid clots of mafic and accessory minerals are locally prominent. Rock is a hypersolvus, mildly peralkaline granite. Fabric ranges from massive to locally lineated and foliated. Includes rock mapped formerly in part as Scituate Granite Gneiss, Cowesett Granite, and perthitic Cowesett Granite.

Lithology: alkali-granite (alaskite)

unknown (unknown) at surface, covers 5 % of this area

unknown

Lithology: indeterminate

Narragansett Bay Group - Rhode Island Formation (Pennsylvanian) at surface, covers 11 % of this area

Narragansett Bay Group - Rhode Island Formation - In northern Rhode Island, consists of gray to black, fine- to coarse-grained quartz arenite, litharenite, shale, and conglomerate, with minor beds of anthracite and meta-anthracite. In southern Rhode Island, consists of meta-sandstone, meta-conglomerate, schist, carbonaceous schist, and graphite. Plant fossils are common.

Lithology: arenite; shale; conglomerate; coal; metasedimentary rock; meta-conglomerate; schist

Scituate Igneous Suite - rhyolite (Devonian) at surface, covers < 0.1 % of this area

Scituate Igneous Suite - rhyolite - Pink to gray, fine-grained, locally porphyritic felsite (phenocrysts of quartz and Carlsbad-twinned orthoclase). Includes some hypabyssal intrusive rocks (feeder dikes and sills), and some probable lava flows. Contains accessory biotite, opaque minerals, zircon, monzonite, fluorite, sphene, apatite, and epidote. Quartz phenocrysts commonly are embayed and partly resorbed; matrix is fine-grained intergrowth of quartz and feldspar, probably formed from devitrified glass. Includes some rock mapped formerly as Spencer Hill Volcanics and Maskerchugg Granite.

Lithology: rhyolite

Raritan Formation (Cretaceous) at surface, covers < 0.1 % of this area

Raritan Formation - Red, white, gray, and black clay and poorly lithified white sand. Black clay contains lignite fragments and pyrite and siderite nodules. Exposed only in a few wave-cut bluffs on Block Island, but inferred to have originally onlapped both the Esmond-Dedham and Hope Valley subterranes. This unit may occur as large glacial erratic blocks enclosed in till, and may not represent in situ bedrock.

Lithology: clay or mud; sand

Sterling Igneous Suite - alaskite gneiss (Late Proterozoic) at surface, covers 17 % 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

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

Scituate Igneous Suite - volcaniclastic rock (Devonian) at surface, covers < 0.1 % of this area

Scituate Igneous Suite - volcaniclastic rock - Dark-gray, pink, greenish, and purplish tuff and epiclastic rock (sandstone, breccia, conglomerate) containing abundant volcanic detritus. Tuff may include ignimbrite and ash-flow tuff. Fiamme common in some rock mapped formerly as Spencer Hill Volcanics.

Lithology: tuff; ignimbrite; ash-flow tuff; volcanic breccia (agglomerate)

Scituate Igneous Suite - fine-grained granite (Devonian) at surface, covers 0.3 % of this area

Scituate Igneous Suite - fine-grained granite - Gray to pink, fine-grained, equigranular to locally porphyritic granite (phenocrysts of perthite and plagioclase). Contains microcline, perthite, plagioclase, quartz, and accessory biotite, zircon, allanite, sphene, monazite, fluorite, and opaque minerals; minor garnet is present in some varieties. Partly massive, but locally lineated and foliated. Includes rock mapped formerly as fine-grained Scituate Granite Gneiss, and some rock mapped as Hope Valley Alaskite Gneiss.

Lithology: granite

Narragansett Pier Plutonic Suite - porphyritic granite (Permian) at surface, covers 4 % of this area

Narragansett Pier Plutonic Suite - porphyritic granite - Similar to Png, except contains phenocrysts of microcline and plagioclase up to 1.5 cm in a medium-grained groundmass. Unit is a 5 km by 40 km batholith that underlies the southern coast of RI. Intrusion during the latter part of Alleghanian deformation of the Narragansett Basin is indicated by its Permian age, rapid cooling, and field relations. The age of the Narragansett Pier is well constrained by several factors. Brown and others (1978) report a Pennsylvanian fossil contained in an inclusion in the pluton. Radiometric ages of 276 Ma by Kocis and others (1978), 272+/-4 Ma by Hermes and others (1981), and 273+/-2 Ma by Zartman and Hermes (1987) are consistently Early Permian. Structural studies by Mosher (1983) and Reck and Mosther (1988) indicate that intrusion began during the third period of deformation in the Narragansett Basin. Argon release patterns (Dallmeyer, 1982) indicate that the granite and intruded sediments had cooled below argon retention temperatures of hornblende by the Late Permian and of biotite by the Early Triassic. Intruded by the Westerly Granite, which has been radiometrically dated at 276+/-7 Ma. The two granites are probably genetically related based on similarities in age, mineralogy, and geochemistry (Hozik, 1992).

Lithology: granite

Narragansett Pier Granite (Permian) at surface, covers < 0.1 % of this area

Narragansett Pier Granite - 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.

Lithology: granite; pegmatite

Plainfield Formation (Proterozoic Z?) at surface, covers < 0.1 % 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

Potter Hill Granite Gneiss (Proterozoic Z?) at surface, covers < 0.1 % 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. 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).

Lithology: granitic gneiss

Narragansett Pier Plutonic Suite - leucocratic granite (Permian) at surface, covers 1.0 % of this area

Narragansett Pier Plutonic Suite - leucocratic granite - White to gray leucocratic granite composed of microcline, oligoclase, and quartz. Up to 10% accessory muscovite and garnet, with lesser apatite, zircon, and monazite. Biotite and opaque minerals notably sparse or absent. Mainly massive, but locally exhibits flow foliation caused by alternating layers of pegmatite, aplite, and medium-grained equigranular granite; pegmatitic and aplitic material of mineralogy similar to the host granite. The granite contains widespread xenolithic inclusions and screens of metasedimentary rock, some of which are PAnbr; bedding and fabric in the screens generally is oriented parallel to that in the host metasedimentary country rock. Unit is a 5 km by 40 km batholith that underlies the southern coast of RI. Intrusion during the latter part of Alleghanian deformation of the Narragansett Basin is indicated by its Permian age, rapid cooling, and field relations. The age of the Narragansett Pier is well constrained by several factors. Brown and others (1978) report a Pennsylvanian fossil contained in an inclusion in the pluton. Radiometric ages of 276 Ma by Kocis and others (1978), 272+/-4 Ma by Hermes and others (1981), and 273+/-2 Ma by Zartman and Hermes (1987) are consistently Early Permian. Structural studies by Mosher (1983) and Reck and Mosther (1988) indicate that intrusion began during the third period of deformation in the Narragansett Basin. Argon release patterns (Dallmeyer, 1982) indicate that the granite and intruded sediments had cooled below argon retention temperatures of hornblende by the Late Permian and of biotite by the Early Triassic. Intruded by the Westerly Granite, which has been radiometrically dated at 276+/-7 Ma. The two granites are probably genetically related based on similarities in age, mineralogy, and geochemistry (Hozik, 1992).

Lithology: granite; pegmatite; aplite; metasedimentary rock

Narragansett Pier Plutonic Suite - granite (Permian) at surface, covers 9 % of this area

Narragansett Pier Plutonic Suite - granite - Dark-pink to pale-gray, medium-grained equigranular granite, with lesser granodiorite and quartz monzonite. Composed of microcline, oligoclase, quartz, and accessory biotite, magnetite, ilmenite, apatite, sphene, zircon, monazite, apatite, and allanite; muscovite and garnet locally present; secondary chlorite and calcite. Mainly massive, but locally exhibits faint flow foliation. Cut locally by abundant pegmatite, aplite, and composite aplite-pegmatite of mineralogy similar to the host granite. Unit is a 5 km by 40 km batholith that underlies the southern coast of RI. Intrusion during the latter part of Alleghanian deformation of the Narragansett Basin is indicated by its Permian age, rapid cooling, and field relations. The age of the Narragansett Pier is well constrained by several factors. Brown and others (1978) report a Pennsylvanian fossil contained in an inclusion in the pluton. Radiometric ages of 276 Ma by Kocis and others (1978), 272+/-4 Ma by Hermes and others (1981), and 273+/-2 Ma by Zartman and Hermes (1987) are consistently Early Permian. Structural studies by Mosher (1983) and Reck and Mosther (1988) indicate that intrusion began during the third period of deformation in the Narragansett Basin. Argon release patterns (Dallmeyer, 1982) indicate that the granite and intruded sediments had cooled below argon retention temperatures of hornblende by the Late Permian and of biotite by the Early Triassic. Intruded by the Westerly Granite, which has been radiometrically dated at 276+/-7 Ma. The two granites are probably genetically related based on similarities in age, mineralogy, and geochemistry (Hozik, 1992).

Lithology: granite; granodiorite; quartz monzonite; pegmatite; aplite

Waterford Group - Rope Ferry Gneiss (Late Proterozoic? or older?) at surface, covers 0.6 % of this area

Waterford Group - Rope Ferry Gneiss - Light- to dark gray, fine- to medium-grained tonalite gneiss composed of plagioclase, quartz biotite, and hornblende, commonly interlayered with granite gneiss composed of microcline, plagioclase, quartz, biotite, and hornblende; local layers of amphibolite.

Lithology: mafic gneiss; granitic gneiss; amphibolite

Scituate Igneous Suite - granite (Devonian) at surface, covers 15 % of this area

Scituate Igneous Suite - granite - Gray to pink, coarse-grained, porphyritic to subporphyritic subsolvus granite (phenocrysts of perthite, microcline, and plagioclase). Contains microcline, perthite, plagioclase, quartz, and accessory biotite, hornblende, allanite, zircon, apatite, sphene, fluorite, and opaque minerals; minor garnet is present in some varieties; some secondary muscovite, calcite, and epidote. Ovoid clots of mafic and accessory minerals are locally prominent. Some rock is massive, but most is commonly lineated and foliated, especially in the west and south near the Hope Valley shear zone. Includes most rock mapped formerly as Scituate Granite Gneiss, and some rock mapped as Hope Valley Alaskite Gneiss.

Lithology: granite

Esmond Igneous Suite - augen granite gneiss (Late Proterozoic) at surface, covers 19 % of this area

Esmond Igneous Suite - augen granite gneiss - Pale- to dark-gray, medium- to coarse-grained, granite gneiss characterized by large feldspar porphyroclasts (> 1 cm). Composition ranges from tonalite to granodiorite to quartz monzonite. Composed of sodic plagioclase, quartz, microcline/orthoclase, biotite, opaque minerals; hornblende and epidote common in more mafic types; accessory apatite, sphene, allanite, and zircon; secondary chlorite. Typically massive, but with strong penetrative foliation and lineation defined by major minerals. Commonly contains structurally conformable layers of amphibolite. Includes most rock mapped formerly as Ponaganset Granite Gneiss and Ten Rod Granite Gneiss.

Lithology: augen gneiss; amphibolite

Hope Valley Alaskite Gneiss (Proterozoic Z?) at surface, covers < 0.1 % 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

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

Mamacoke Formation - 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.

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

Rope Ferry Gneiss (Proterozoic Z?) at surface, covers < 0.1 % of this area

Rope Ferry Gneiss (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).

Lithology: gneiss; amphibolite

Narragansett Pier Plutonic Suite - fine-grained granite (Permian) at surface, covers < 0.1 % of this area

Narragansett Pier Plutonic Suite - fine-grained granite - Gray to pink, fine-grained equigranular, massive to faintly foliated granite composed of microcline, oligoclase, quartz, biotite, magnetite, zircon, monazite, allanite, and apatite; secondary chlorite, muscovite, calcite. Occurs mainly as elongate (up to 2.5 km) dikes, up to 80 m wide. Mapped formerly as Westerly Granite.

Lithology: granite

Plainfield Formation (Late Proterozoic? or older?) at surface, covers 2 % 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

Blackstone Group - epidote and biotite schist (Late Proterozoic? or older?) at surface, covers < 0.1 % of this area

Blackstone Group - epidote and biotite schist - Green to gray, fine-grained, massive to thinly-bedded mica schist, quartzite, and marble. Schist consists of quartz plus chlorite, muscovite, and/or biotite. Includes rock mapped formerly as Sneech Pond Schist, Mussey Brook Schist, and marble.

Lithology: mica schist; quartzite; marble

Sterling Igneous Suite - mafic/intermediate gneiss (Late Proterozoic) at surface, covers 1.0 % of this area

Sterling Igneous Suite - mafic/intermediate gneiss - Gray medium-grained biotite gneiss, commonly with porphyroclasts of microcline/orthoclase, and amphibolite gneiss. Compositions range from quartz diorite to granodiorite. Composed of plagioclase, quartz, microcline/orthoclase, biotite, opaque minerals (commonly as conspicuous magnetite octahedra); accessory apatite, zircon, sphene; secondary chlorite. Varies from massive to layered, with strong penetrative foliation and lineation defined by major minerals. Includes some rock mapped formerly as Blackstone Series in southwestern and westernmost Rhode Island.

Lithology: biotite gneiss; gneiss