Ordovician Dry Wall Volcanic rocks

Ordovician Dry Wall Volcanic rocks
State Maine
Name Ordovician Dry Wall Volcanic rocks
Geologic age Ordovician
Lithologic constituents
Igneous > Volcanic > Mafic-volcanic > BasaltW - Weakly metamorphosed; contains local occurrences of prehnite and pumpellyite; Protolith V - Basaltic volcanic rocks
Comments Depot Mountain Formation recently divided into unnamed lower and upper members in the belt between Seven Islands and Riviere Bleue. The conglomerate subunit of Boudette et al (1976) is not mappable. Rocks similar to those of the type locality are here assigned to the lower member, which consists of thick-bedded, dark gray, fine-to-coarse grained, feldspathic graywacke interbedded with dark gray and black slate or phyllite. Sequences of lithic and crystal tuff and roundstone conglomerate occur locally within lower member. Upper member consists of dark gray, foliated, lithic graywacke interbedded with laminated gray slate. Thickness of the lower member is unknown; upper member may exceed 1000 m. Graptolites from lower member yield Late Ordovician (Caradocian) age. Upper member younger than overlying Fivemile Brook Formation. Therefore, rocks assigned to Depot Mountain appear to extend in age from at least Caradocian to the middle of the Llandoverian (Early Silurian). Along Dead Brook fault, unit overlies St. Daniel Formation, extended from CAN to replace the name Estcourt Road Formation of earlier reports. (ME064) Unit description from USGS GEOLEX website (ME078). W - Weakly metamorphosed; contains local occurrences of prehnite and pumpellyite

Osberg, P.H., Hussey, A.M., and Boone, G.M., 1985, Bedrock geologic map of Maine: Maine Geological Survey, Dept. of Conserv., scale 1:500,000.

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Counties Piscataquis