Biofacies analysis of late Mississippian ostracodes and their use as paleoenvironmental indicators in the Bluefield Formation, Mercer County, West Virginia

The Bluefield Formation (Chesterian, Upper Mississippian) of the Appalachian Basin is a lithologically variable unit, composed of mudstones lithofacies with subordinate amounts of sandstone and limestone. By analyzing sedimentology and the prominence of ostracodes, an ostracode biofacies model was generated that provides a large-scale paleoenvironmental framework for the Bluefield Formation.;Three ostracode assemblages were defined in an effort to identify environmental conditions during deposition of the Bluefield: Assemblage I - Polytylites Assemblage; Assemblage II - Sansabella Assemblage; and Assemblage III - Whipplella Assemblage. The assemblage distributions indicate that nearshore transitional marine environments were common and normal open marine conditions were very rare. The variable units and upward-shallowing sequences in the Bluefield reflect changes in the influx of freshwater that caused salinity fluctuations. Within the depositional environments of the Bluefield Formation, salinity was the most important controlling factor in the ostracode distribution.