Geologic units in Bristol county, Rhode Island

Narragansett Bay Group - Rhode Island Formation (Pennsylvanian) at surface, covers 66 % of this area

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.

Granites of southeastern Rhode Island - granite (Late Proterozoic) at surface, covers 16 % of this area

Pink to gray, coarse-grained, equigranular rock composed of microcline, perthite, plagioclase, quartz, and accessory biotite, sphene, zircon, and opaque minerals; secondary chlorite and muscovite. Generally massive, but locally foliated and lineated. Includes some rock mapped formerly as Bulgarmarsh Granite and Metacom Granite Gneiss.

mica schist (Late Proterozoic? or older?) at surface, covers 14 % of this area

Gray to green, fine-grained, thinly bedded schist consisting of muscovite, biotite, chlorite, and quartz. Locally contains think beds of quartzite, marble, and amphibolite. Includes rock mapped formerly as mica schist of Bristol, chlorite-biotite schist of Tiverton, and mica-chlorite schist of Sakonnet.

Narragansett Bay Group - Dighton Conglomerate (Pennsylvanian) at surface, covers 5 % of this area

Gray conglomerate consisting predominantly of quartz clasts set in a sand-sized matrix. Minor lenses of litharenite and arkosic sandstone.

vein quartz (Triassic?) at surface, covers 0.2 % of this area

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

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

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).

Dighton Conglomerate (Upper Pennsylvanian) at surface, covers < 0.1 % of this area

Coarse conglomerate having sandy matrix; minor sandstone. Dighton Conglomerate occurs in Narragansett basin. Consists of gray conglomerate composed mainly of rounded quartzite cobbles to boulders containing subordinate rounded granite cobbles and slate pebbles; very little sand matrix; lenses of medium-grained sandstone form less than 20 percent of unit. Age is Pennsylvanian (Goldsmith, 1991).