BIOFACIES DISTRIBUTION ALONG AN ENVIRONMENTAL GRADIENT IN THE AMES MEMBER (UPPER PENNSYLVANIAN, GLENSHAW FORMATION): NORTHERN APPALACHIAN BASIN

The Ames Sea supported a variety of benthic faunas in the most extensive marine event of the Pennsylvanian and the final marine event in the Paleozoic of eastern North America. Seven outcrops in the northern Appalachian Basin were sampled for paleoecological analysis. Direct gradient analysis of the proportional abundance of taxa produced an environmental continuum along which five biofacies were distributed. The environmental factors controlling the distribution of the five biofacies are salinity and clastic influx related to sea level change, along with an overlapping oxygen gradient related to the foreland geometry of the Ames Basin. Marine waters that filled the Ames Basin extended from the Midcontinent Basin during sea level highstand. Low clastic influx and marine salinities persisted during transgression and led to the establishment of Biofacies 4, dominated by the opportunistic, suspension-feeding brachiopod Neochonetes. At maximum transgression, freshwater runoff driven by offshore trade winds resulted in a stratified water column and a localized salinity-driven pycnocline in the foredeep trough in the eastern part of the Ames Basin in West Virginia. In the deepest parts of the basin, the initial Neochonetes-dominated Biofacies 4 was replaced by small, deposit-feeding microgastropods of Biofacies 3 in a dysoxic environment with low sedimentation and marine salinity. In parts of the basin near local source areas, Biofacies 4 was replaced by Biofacies 2, which is dominated by very small (< 5 mm at greatest width) nuculid bivalves in a dysoxic environment with high sedimentation and low salinity. During regression, distributary sources on the eastern margin became active and delivered clastics to the Ames Sea. Biofacies 1 and 2 occur exclusively in the eastern part of the Ames Basin and are composed primarily of small (< 5 mm at greatest width) eurytopic molluscs and a few species of brachiopods that were tolerant of high clastic influx and lowered salinity associated with these sources. On the western margin in Ohio, Ames deposition took place on a carbonate shelf in a shallow-water environment far from eastern source areas. Reduced clastic influx and marine salinity supported Biofacies 5 composed of sessile, suspension-feeding crinoids, brachiopods, and bryozoans.