Plainfield Formation

Quartzite, pelitic schist, minor calc-silicate rock and amphibolite. Plainfield Formation extend into MA in Webster-Oxford area from eastern CT and western RI along west flank of Rhode Island batholith, along same strike as elongate lenses of Westboro Formation (as shown on State bedrock map of Zen and others, 1983). Lithology of Plainfield in New London area of southeastern CT (Goldsmith, 1966, 1976) and in eastern CT (Harwood and Goldsmith, 1971), and its structural and stratigraphic position indicate Plainfield and Westboro are equivalent. Plainfield is truncated along CT-RI border by Lake Char fault; however, in New London area, Plainfield is overlain by suite of largely mafic metavolcanic rocks (Waterford Group of Goldsmith, 1980), a relationship similar to that of the Westboro Formation and overlying metavolcanic rocks. Base of Plainfield is unknown, but gneiss and schist in center of Lyme dome may lie below it. Probably equivalent to quartzite and schist of Blackstone Group on basis of similar lithology and structural relations with Rhode Island batholith rocks (Goldsmith, 1991).
State Massachusetts
Name Plainfield Formation
Geologic age Proterozoic Z
Lithologic constituents
Major
Metamorphic > Metasedimentary > Metaclastic > Quartzite
Metamorphic > Schist > Pelitic-schist
Minor
Metamorphic > Metasedimentary > Calc-silicate-rock
Metamorphic > Amphibolite
Comments Part of Milford-Dedham Zone (Tertiary and Older Rocks). Secondary unit description per MA023.
References
NGMDB product
Counties Worcester