Petrography and origin of the Silurian Rochester and McKenzie Shales, Morgan County, West Virginia

Abstract

Abstract The Rochester Formation consists mainly of black fissile clay shale; highly abraded biosparite limestone occurs at the base, with thin beds of finer-grained ostracod, pellet, or micrite limestone in the upper portions. All original micrite has recrystallized to microspar, with partial obliteration of fecal pellets. The lower limestones accumulated in a high-energy shell bank adjoining the Keefer barrier bar, whereas the upper dark gray shales and fine limestones formed in a brackish, stagnant lagoon protected by the Keefer bar. Occasional storms formed edgewise conglomerate beds within the shale. The McKenzie Formation is chiefly olive to gray clay shale with siltier laminae and burrow fillings. Ostracod biosparites are common, and fibrous sparry calcite overgrowths on the shells have greatly expanded the fabric in some specimens. All of the originally micritic rocks have recrystallized to microspar; some contain brachiopod shells with geopetal fecal pellets and syngenetic primary dolomite crystals filling the former body cavities, The dolomite is optically perpendicular to, and molded on top of, fibrous sparry calcite overgrowths. The McKenzie was deposited in an open marine area with variable salinity, affected by influx of muddy river water.