Ostracode Distribution in Late Pennsylvanian Finis Shale Cyclothem: ABSTRACT

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Four successive intergrading ostracode biofacies characterize the Finis Shale cyclothem (Cisco Group, Virgilian) near Jackboro, Texas. Composition of the ostracode biofacies parallels the distinct megafaunal assemblages ascribed by Boardman et al (1983) to changing oxygen concentrations, water depths, and salinity. Black, organic-rich, noncalcareous, fissile shales occur near the base of the Finis. This lithofacies is barren of ostracodes and megafauna, and is interpreted to represent a deeper water, anoxic environment. The overlying dark-gray, phosphatic shales contain a diminutive pyritic molluscan fauna with abundant ammonoids and a low-diversity ostracode assemblage dominated by Healdia, indicating a dysaerobic environment. The overlying lithofacies is a medium-gray, clay-rich, calcareous shale. The megafauna is dominated by a brachiopod-bryozoan assemblage, and a diverse Amphissites-Kirkbya ostracode association occurs. This fauna represents a shallow depth, aerobic environment. The Amphissites-Kirkbya biofacies is overlain by a dark-gray calcareous shale with abundant megafossils of the fusulind-bryozoan facies of Boardman et al. This interval is dominated by a Bairdia-Amphissites-Kelletina ostracode assemblage, indicating a shallower aerobic environment. The shale is interrupted by the Jacksboro Limestone Member, an algal limestone deposited at or near wave base, which is covered by a thin intertidal sandstone. The cyclothem is capped by noncalcareous, brown silty shale. It contains a low-diversity ostracode assemblage dominated by Cavellina and a megafaunal assemblage c aracterized by Myalina, which represents a shallow, brackish water environment. End_of_Article - Last_Page 239------------