Bristol Gneiss

Light, medium-grained, massive to well-layered gneiss, composed of plagioclase, quartz, and biotite, also muscovite and garnet in many layers, interlayered in places with dark amphibolite. Bristol Member of Collinsville Formation (Stanley, 1964) is here raised in rank and revised as Bristol Gneiss in the report area. Garnet- and epidote-bearing amphibolite and hornblende gneiss assigned by Stanley to the upper part of his Bristol Member is here assigned to base of the overlying Collinsville Formation and composes the unnamed hornblende gneiss member. As defined here, the Bristol Gneiss consists of plagioclase-quartz gneiss characterized by foliae of muscovite or biotite, or locally chlorite, which in places give the unit a striking pinstriping; contains scattered small garnets and large pods of amphibolite. Average thickness may be about 2,000 ft; thickness varies due to local intense folding. Probably correlates with the Moretown Formation of central MA, in which case it would constitute a fourth member of Stanley's Taine Mountain Formation. Formational status assigned in this report based on its unique textural and lithological character. Inferred age is Middle Ordovician (Simpson, 1990).
State Connecticut
Name Bristol Gneiss
Geologic age Middle? Ordovician
Lithologic constituents
Major
Metamorphic > GneissLight, medium-grained, massive to well-layered gneiss, composed of plagioclase, quartz, and biotite, also muscovite and garnet in many layers.
Minor
Metamorphic > Amphiboliteinterlayered in places with dark amphibolite
Comments Part of Central Lowlands; Iapetus (Oceanic) Terrane - Connecticut Valley Synclinorium; Gneiss Dome Belt. Secondary unit description per CT008.
References

Rodgers, John, compiler, 1985, Bedrock Geological Map of Connecticut: Connecticut Geological and Natural History Survey, Hartford, Connecticut, 2 sheets, scale 1:125,000.

Simpson, H.E., 1990, Bedrock geology of the Bristol quadrangle, Hartford, Litchfield, and New Haven Counties, Connecticut: U.S. Geological Survey Bulletin, 1573, 13 p.

https://pubs.er.usgs.gov/publication/b1573

NGMDB product
Counties Hartford - Litchfield