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

Geologic units in Providence county, Rhode Island

Harmony Group - Woonasquatucket Formation (Late Proterozoic? or older?) at surface, covers 2 % of this area

Harmony Group - Woonasquatucket Formation - Gray, medium-grained felsic gneiss, composed of microcline-microperthite, quartz, albite, muscovite, and biotite. Commonly phyllonitic.

Lithology: felsic gneiss

Ponaganset Gneiss (Proterozoic Z) at surface, covers 0.2 % of this area

Ponaganset Gneiss - Gneissic biotite granite containing megacrysts of microcline; biotite in coarse streaks and patches. Equivalent to part of former Northbridge Granite Gneiss (usage now abandoned). 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: granite

Blackstone Group - undifferentiated rock (Late Proterozoic? or older?) at surface, covers 4 % of this area

Blackstone Group - undifferentiated rock - Consists of associations of the above rock types. Locally cut by granitic rocks of the Esmond Igneous Suite, resulting in migmatitic appearance. Includes rock mapped formerly as undifferentiated Blackstone Series and migmatite.

Lithology: migmatite

Other metasedimentary rocks - metaclastic rock, undivided (Pennsylvanian? or Late Proterozoic?) at surface, covers 3 % 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 2 % 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

Scituate Igneous Suite - alkali-feldspar granite (Devonian) at surface, covers 5 % 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)

Hope Valley Alaskite Gneiss (Proterozoic Z) at surface, covers < 0.1 % of this area

Hope Valley Alaskite Gneiss - Mafic-poor gneissic granite, locally muscovitic. Gradational with Zsg. Late Proterozoic Hope Valley Alaskite Gneiss occurs as one of several plutonic rocks in Milford antiform. Forms tabular masses along west side of Rhode Island anticlinorium from southern RI and eastern CT to northwestern RI; flanks west side of Milford anticlinorium and terminates at north end of anticlinorium in MA. Color is light pink to tan. Intrudes Plainfield Formation in CT and Blackstone Group rocks in RI. Isotopic age of 630 Ma by U/Pb methods on zircon is reported by Zartman and Naylor (1984) from a sample in MA. Age of 601 +/-5 Ma by U/Pb methods on zircon is reported by Hermes and Zartman (1985) from a sample in RI (Wones and Goldsmith, 1991).

Lithology: alkali-granite (alaskite)

Harmony Group - Absalona Formation (Late Proterozoic? or older?) at surface, covers 8 % of this area

Harmony Group - Absalona Formation - Gray, medium- to coarse-grained biotite granite gneiss characterized by alkali feldspar porphyroblasts. Consists of biotite, hornblende, quartz, microcline, microperthite, albite, epidote, garnet, and chlorite. Subordinate amounts of quartz-biotite schist, amphibolite, and quartzite occur as layers and isolated bodies.

Lithology: granitic gneiss; schist; amphibolite; quartzite

Hoppin Formation (Cambrian) at surface, covers < 0.1 % of this area

Hoppin Formation - Quartz arenite, argillite, and minor limestone; contains fossils of Lower Cambrian age.

Lithology: argillite; arenite; limestone

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

Narragansett Bay Group - Rhode Island Formation (Pennsylvanian) at surface, covers 14 % 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 - diorite/gabbro (Devonian) at surface, covers 0.2 % of this area

Scituate Igneous Suite - diorite/gabbro - Fine- to medium-grained, equigranular, massive to slightly foliated rock, composed of plagioclase, hornblende, clinopyroxene, and quartz, with accessory biotite, sphene, apatite, and opaque minerals; secondary chlorite, epidote, and saussurite. Varieties include quartz diorite, diorite, and gabbro.

Lithology: quartz diorite; diorite; gabbro

Narragansett Bay Group - Wamsutta Formation (Pennsylvanian) at surface, covers 0.4 % of this area

Narragansett Bay Group - Wamsutta Formation - Red sandstone, shale, and conglomerate, locally containing abundant volcanic detritus as clasts and matrix. Plant fossil localities occur in adjacent Massachusetts. Minor, but significant amounts of interstratified bimodal-composition volcanic lava flows consisting of alkalic basalt (locally pillowed) and rhyolite are present in adjacent Massachusetts.

Lithology: conglomerate; shale; graywacke; sandstone

cumberlandite (Late Proterozoic? to Devonian?) at surface, covers < 0.1 % of this area

cumberlandite - The designated state rock of Rhode Island. Black, medium-grained, porphyritic melatroctolite (phenocrysts of plagioclase), composed of magnetite, ilmenite, olivine, plagioclase, and accessory hercynite spinel; secondary chlorite and saussurite. Abundance of magnetite and ilmenite (up to 70% of rock) results in characteristic high specific gravity. The rock is massive, but locally exhibits a flow lamination caused by the planar orientation of plagioclase laths.

Lithology: troctolite

gabbro/diorite (Late Proterozoic? to Devonian?) at surface, covers 0.2 % of this area

gabbro/diorite - Dark-gray to black, fine- to medium-grained, equigranular, faintly foliated rock, composed of plagioclase, clinopyroxene, hornblende, biotite, and accessory sphene, apatite, and opaque minerals. Includes quartz diorite, diorite, and gabbro.

Lithology: quartz diorite; diorite; gabbro

Sterling Igneous Suite - alaskite gneiss (Late Proterozoic) at surface, covers 0.4 % 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

Scituate Igneous Suite - fine-grained granite (Devonian) at surface, covers 0.6 % 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

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

Esmond Igneous Suite - granite (Late Proterozoic) at surface, covers 9 % of this area

Esmond Igneous Suite - granite - Gray, tan, greenish, or pale-pink, medium- to coarse-grained, mainly equigranular rock. Contains microcline, perthite, plagioclase, quartz, and accessory biotite, epidote, zircon, allanite, monazite, apatite, sphene, and opaque minerals; secondary muscovite, chlorite, and calcite. Mainly massive, but locally foliated and lineated. Includes rock mapped formerly as Esmond Granite.

Lithology: granite

Blackstone Group - quartzite (Late Proterozoic? or older?) at surface, covers 2 % of this area

Blackstone Group - quartzite - Gray, medium-grained, massive to thinly bedded quartzite; commonly tectonically interleaved with other units of the Blackstone Group. Interpreted as olistostromal deposits. Includes rock mapped formerly as Quinnville Quartzite.

Lithology: quartzite

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

Wamsutta Formation (Middle and Lower Pennsylvanian) at surface, covers < 0.1 % of this area

Wamsutta Formation - Red to pink, well-sorted conglomerate, graywacke, sandstone, and shale; fossil plants. Wamsutta Formation occurs in Narragansett and Norfolk basins. Consists of conglomerate, lithic graywacke, sandstone, and shale. Also contains rhyolite and basalt horizons near Attleboro. Northwest of Attleboro, Wamsutta overlies Diamond Hill Felsite as used by Skehan and Murray (in Skehan and others, 1979). Volcanic rocks similar to Diamond Hill Felsite crop out west of Lake Pearl, between Franklin and Wrentham, on west flank of Norfolk basin. These are shown within Wamsutta Formation on MA State bedrock map of Zen and others (1983) because of their proximity to Diamond Hill. They also resemble Proterozoic Z Mattapan Volcanic Complex. Chute (1966) described lenses of carbonate rock in red and green shale in Wamsutta in Norwood quad. Limestone also observed in rocks mapped as Wamsutta adjacent to exposed Dedham Granite at Manchester Pond Reservoir (J.P. Schafer, 1982, oral commun.). Red and green shales may actually be Cambrian. Upper member of Pondville Conglomerate grades into and interfingers with Wamsutta; in turn, Wamsutta interfingers with Rhode Island Formation in northwest part of Narragansett basin. Nonconformably overlies Dedham Granite. Partly equivalent to Rhode Island Formation. Age is Early and Middle Pennsylvanian. Contains a few plant fossils (Goldsmith, 1991).

Lithology: conglomerate; graywacke; sandstone; shale

Esmond Igneous Suite - fine-grained granite (Late Proterozoic) at surface, covers 0.6 % of this area

Esmond Igneous Suite - fine-grained granite - Gray to pale-pink, fine-grained, equigranular rock, containing microcline, perthite, plagioclase, quartz, and accessory biotite, epidote, sphene, zircon, monazite, apatite, and opaque minerals. Secondary muscovite, chlorite, and calcite. Mainly massive. Includes rock mapped formerly as fine-grained Esmond Granite.

Lithology: granite

diabase (Triassic?) at surface, covers < 0.1 % of this area

diabase - Greenish-gray to gray-black, fine-grained, commonly porphyritic, generally massive dike rock composed of plagioclase, augite, opaque minerals, olivine pseudomorphed by serpentine and chlorite minerals, and sparse quartz.

Lithology: diabase

alkali-feldspar granite of Cumberland (Devonian? or Mississippian?) at surface, covers 0.3 % of this area

alkali-feldspar granite of Cumberland - Gray to blue-gray. medium- to coarse-grained, equigranular to porphyritic alkali-feldspar granite (phenocrysts of perthite). Massive to locally foliated. Contains perthite, quartz, abundant accessory riebeckite and aegerine, and lesser astrophyllite, anegmatite, zircon, allanite, fluorite, and opaque minerals. Plagioclase is notably absent. Hypersolvus, strongly peralkaline granite. Includes rock mapped formerly as Rhode Island Quincy Granite.

Lithology: alkali-granite (alaskite)

Narragansett Bay Group - Pondville Conglomerate (Pennsylvanian) at surface, covers < 0.1 % of this area

Narragansett Bay Group - Pondville Conglomerate - Gray to greenish conglomerate with sand-sized matrix, interbedded with quartz arenite and litharenite; typically lenticular and discontinuous. At type locality (Pondville Station, Massachusetts), unit consists of interbedded red and green slate, siltstone, arkose, and quartzite-pebble conglomerate.

Lithology: conglomerate; arenite; slate; siltstone; arkose

Rhode Island Formation (Upper and Middle Pennsylvanian) at surface, covers 0.1 % of this area

Rhode Island Formation - Sandstone, graywacke, shale and conglomerate; minor beds of meta-anthracite. Fossil plants. Rhode Island Formation is thickest and most extensive formation in Narragansett basin. Does not extend to Norfolk basin. Consists of gray sandstone and siltstone and lesser amounts of gray to black shale, gray conglomerate, and coal beds 10 m thick. Interfingers with Wamsutta Formation in Narragansett basin. In places overlies Dedham Granite. Age is Middle and Late Pennsylvanian (Goldsmith, 1991).

Lithology: sandstone; graywacke; shale; conglomerate

Scituate Igneous Suite - granite (Devonian) at surface, covers 11 % 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

Blackstone Group - greenstone, amphibolite, serpentinite (Late Proterozoic? or older?) at surface, covers 2 % of this area

Blackstone Group - greenstone, amphibolite, serpentinite - Dark-green, massive to foliated metamorphosed gabbroic and basaltic rock interlayered with epiclastic rock; local pillow-structures preserved. Consists primarily of epidote, actinolite, chlorite, and plagioclase. Includes rock mapped formerly as Hunting Hill Greenstone.

Lithology: greenstone; amphibolite; serpentinite

Harmony Group - Nipsachuck Formation (Late Proterozoic? or older?) at surface, covers < 0.1 % of this area

Harmony Group - Nipsachuck Formation - Gray to tan, fine- to medium-grained granite gneiss, characterized by biotite streaks. Consists of biotite, muscovite, quartz, microperthite, and sodic plagioclase.

Lithology: granitic gneiss

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

Scituate Igneous Suite - monzonite/monzodiorite - Dark-gray to pink, fine- to medium-grained, equigranular rock, containing plagioclase, microcline, quartz, and accessory biotite, garnet, zircon, apatite, allanite, and opaque minerals; secondary muscovite and epidote. Mainly foliated. Varieties include quartz monzonite, monzonite, and monzodiorite. Includes some rock mapped formerly as Cowesett Granite.

Lithology: quartz monzonite; monzonite; monzodiorite

Esmond Igneous Suite - felsic volcaniclastic rock (Late Proterozoic) at surface, covers 0.3 % of this area

Esmond Igneous Suite - felsic volcaniclastic rock - Light-colored to dark-gray ash flow tuff, ignimbrite, and felsite; may include some shallow-level intrusive rock. Phenocrysts of quartz, orthoclase, and plagioclase in a fine-grained matrix of quartz, feldspar, biotite, and opaque minerals. Some varieties contain fiamme and lithic fragments. Includes rock mapped formerly as Pennsylvanian felsite at Diamond Hill.

Lithology: ash-flow tuff; ignimbrite; felsic volcanic rock

Esmond Igneous Suite - augen granite gneiss (Late Proterozoic) at surface, covers 20 % 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

Esmond Igneous Suite - granodiorite (Late Proterozoic) at surface, covers 1 % of this area

Esmond Igneous Suite - granodiorite - Gray, tan, greenish, or pale-pink, medium- to coarse-grained, mainly porphyritic rock (phenocrysts of microcline). Contains microcline, perthite, plagioclase, quartz, and accessory biotite, epidote, zircon, allanite, monazite, apatite, sphene, and opaque minerals; secondary muscovite, chlorite, and calcite. Mainly massive. Includes rock mapped formerly as Grant Mills Granodiorite.

Lithology: granodiorite

vein quartz (Triassic?) at surface, covers < 0.1 % of this area

vein quartz - En echelon and crosscutting veins (millimeter to meter widths) of massive quartz; crystal terminations common in vugs and open cavities. Contains local concentrations of cryptocrystalline jasper and quartz, and irregular distributions of hematite and goethite, some in botryoidal masses. Locally present are abundant rectangular crystals of barite (up to 2 cm), or hollow molds after dissolved barite. Age uncertain, but rocks lack deformation features present in nearby Pennsylvanian rocks of the Narragansett Basin; emplacement most likely postdates Alleghanian deformation

Lithology: plutonic rock (phaneritic)

Esmond Igneous Suite - mafic/intermediate rock (Late Proterozoic) at surface, covers 3 % of this area

Esmond Igneous Suite - mafic/intermediate rock - Dark-gray, purple, or black, medium- to coarse-grained rock that may contain plagioclase, quartz, clinopyroxene, hornblende, olivine, and accessory biotite, epidote, sphene, zircon, apatite, and opaque minerals; secondary chlorite, sericite, and saussurite. Massive to variably foliated. Composition includes tonalite, quartz diorite, diorite, and gabbro. Includes rock mapped formerly as quartz diorite.

Lithology: tonalite; quartz diorite; diorite; gabbro

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

Blackstone Group - epidote and biotite schist (Late Proterozoic? or older?) at surface, covers 4 % 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