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Hoosac Formation, Plymouth Member

Hoosac Formation, Plymouth Member - Quartzite, schistose quartzite, dolomitic quartzite; carbonaceous phyllite; buff to dark gray dolomite with partings locally of carbonaceous phyllite; quartz-sericite-chlorite-albite schist; carbonaceous albite schist. (Southern and Central Vermont). Revised the Plymouth Member of the Hoosac Formation of Doll and others (1961) to the Plymouth Formation. Consists of a series of feldspathic and dolomitic quartzites, dolostones and black phyllites that overlie probable Middle Proterozoic gneisses. The Plymouth Formation can be divided into several informal members. The lower contact of the formation is below a sequence of dolomitic quartzites or thin bedded quartzite. Dark laminated silty phyllites laterally replace the more feldspathic quartzites and dark-gray schistose quartzites, massive vitreous quartzites, and dolomitic quartzites pass upward to the east into well bedded cream-weathered light-gray dolostone breccia; these lithologies make up the dolostone member of the Plymouth Formation. The upper member of the Plymouth Formation is a black graphitic and siliceous phyllite that contains 1 to 3 cm thick layers of dark-gray ferruginous quartzite, dolomitic quartzite, and ribbony beds of dolostone. The upper contact of the Plymouth Formation is placed at the first occurrence of light-silvery-green magnetite-muscovite-quartz knotted phyllites of the Pinney Hollow Formation (Ratcliffe, 1994).
StateVermont
NameHoosac Formation, Plymouth Member
Geologic ageCambrian
Original map labelChop
CommentsPart of Green Mountains, Camels Hump Group Secondary unit description per VT012: Revised the Plymouth Member of the Hoosac Formation of Doll and others (1961) to the Plymouth Formation. Consists of a series of feldspathic and dolomitic quartzites, dolostones and black phyllites that overlie probable Middle Proterozoic gneisses. The Plymouth Formation can be divided into several informal members. The lower contact of the formation is below a sequence of dolomitic quartzites or thin bedded quartzite. Dark laminated silty phyllites laterally replace the more feldspathic quartzites and dark-gray schistose quartzites, massive vitreous quartzites, and dolomitic quartzites pass upward to the east into well bedded cream-weathered light-gray dolostone breccia; these lithologies make up the dolostone member of the Plymouth Formation. The upper member of the Plymouth Formation is a black graphitic and siliceous phyllite that contains 1 to 3 cm thick layers of dark-gray ferruginous quartzite, dolomitic quartzite, and ribbony beds of dolostone. The upper contact of the Plymouth Formation is placed at the first occurrence of light-silvery-green magnetite-muscovite-quartz knotted phyllites of the Pinney Hollow Formation. Age changed to Late Proterozoic and Early Cambrian (Ratcliffe and others, 1993) per VT004.
Primary rock typequartzite
Secondary rock typedolostone (dolomite)
Other rock typesphyllite; schist
Lithologic constituents
Major
Sedimentary > Carbonate > Dolostone
Metamorphic > Metasedimentary > Metaclastic > Quartzitequartzite, schistose quartzite, dolomitic quartzite
Minor
Metamorphic > Schistschistose quartzite, quartz-sericite-chlorite-albite schist; carbonaceous albite schist
Metamorphic > Metasedimentary > Metaclastic > Phyllitecarbonaceous phyllite
Map references
Nicholson, S.W., Dicken, C.L., Horton, J.D., Foose, M.P., Mueller, J.A.L., and Hon, Rudi, 2006, Preliminary integrated geologic map databases for the United States: Connecticut, Maine, Massachusetts, New Hampshire, New Jersey, Rhode Island, and Vermont: U.S. Geological Survey Open-File Report 2006-1272.
Unit references
Doll, C.G., Cady, W.M., Thompson, J.B., Jr., and Billings, M.P., 1961, Centennial Geologic Map of Vermont: Vermont Geological Survey, Miscellaneous Map MISCMAP-01, scale 1:250,000.
Ratcliffe, N.M., Potter, D.B. and Stanley, R.S., 1993, Bedrock geologic map of the Williamstown and North Adams quadrangles, Massachusetts and Vermont, and part of the Cheshire quadrangle, Massachusetts: U.S. Geological Survey Miscellaneous Investigations Series Map, I-2369, 13 p., 2 sheets, scale 1:24,000.
Ratcliffe, N.M., 1994, Changes in stratigraphic nomenclature in the eastern cover sequence in the Green Mountain massif from Ludlow to West Bridgewater, Vermont, IN Stratigraphic notes, 1992: U.S. Geological Survey Bulletin, 2060, p. 1-10.

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