VICKSBURGIAN (RUPELIAN) MOLLUSCAN FAUNAS FROM THE BRIDGEBORO LIMESTONE AND OVERLYING OLIGOCENE RESIDUUM EXPOSED IN THE VICINITY OF BRIDGEBORO, GEORGIA

Two Oligocene Vicksburgian molluscan faunas are documented from the Bridgeboro Limestone and Oligocene residuum exposed by mining operations south of Bridgeboro, Georgia. The Bridgeboro Limestone, formally described by Huddleston (1993), is a rhodolithic limestone that occurs along the flanks of the Gulf Trough in southwestern Georgia, southeastern Alabama, and the Florida panhandle. While rhodoliths dominate the assemblage other macrofossils have been reported including molds and casts of mollusks, molds of large coral heads, calcitic-shelled body fossils including pectens (Chlamys anatipes and C. duncanensis), sand dollars (Clypeaster cotteaui) and sea biscuits (Rhyncholampas gouldii), bryozoans, and large foraminifera (Lepidocyclina). Previous paleoecological studies have suggested the low diversity of this shallow water fauna is due to the instability of the substrate that would have favored organisms adapted to boring (Lithophaga sp.), nestling (Limidae), or encrusting. Other bivalve taxa noted here for the first time are from the families Lucinidae, Chamidae, Cardiidae, and Veneridae. There is also a suite of cerithiform gastropods (families Batillariidae and Cerithiidae) and representatives of the families Turritellidae, Naticidae, Volutidae, and Conidae. Overlying the Bridgeboro Limestone in its type area is an Oligocene residuum that has tentatively been assigned to the Bucatunna Clay that contains a fauna of silicified mollusks, similar in composition as it also contains numerous cerithiform gastropods but also has members of the families Marginellidae, Olividae, Teredinidae, and Glycymerididae.