Hoosac Formation, Turkey Mountain Member (Cambrian)
Hoosac Formation, Turkey Mountain Member - Amphibolite and actinolitic greenstone characterized by oval, 1/8 to 3/8 inch spots, chiefly of epidote. (Southern and Central Vermont). Discontinuous lenses of metabasalt, informally referred to as Turkey Mountain metabasalt member of Hoosac Formation, actually occur at different stratigraphic positions extending through a stratigraphic distance of 100 to 400 m above base of Hoosac Formation. From type area on Turkey Mountain in Saxtons River quad south to Massachusetts State line, basalts form at least three relatively persistent units (Ratcliffe, 1991). Basalt mapped in northeast corner of this map, above Wilmington thrust system, correlates with type Turkey Mountain. As used here, Turkey Mountain metabasalt member consists of several laterally and vertically discontinuous, nonidentical flows and volcaniclastic deposits including, but not restricted to, type Turkey Mountain Member of Hoosac Formation as used by Doll and others (1961) and by Skehan (1961). They mapped the lower basalts as unnamed greenstones in Hoosac Formation. Turkey Mountain metabasalt member consists of light-green to dark-green epidote-amphibole greenstones and amphibolite metabasalts. Metabasalt varies from massive to very well layered. Finely laminated, quartzose and epidotitic volcaniclastic beds several centimeters thick are interlayered with more massive, strongly foliated, black amphibolite. Where in contact with surrounding metasedimentary rocks, layering within metabasalt and volcaniclastic beds is concordant and gradational with enclosing metasediment. Light-gray or yellowish-greenish-gray, well-laminated quartzite or, less commonly, gritty, pebbly conglomerate 0.5 to 5 m thick marks upper contact with unnamed granofels member of Hoosac Formation. Base of Turkey Mountain metabasalt member in contact with rusty muscovite-albite-biotite schist. Metabasalts probably originated as thin composite basalt lava flows that contained intercalated basaltic volcaniclastic rocks. Age is Late Proterozoic and Early Cambrian (Ratcliffe, 1993).