FOSSIL COMMUNITIES WITHIN TWO MARINE MEMBERS OF THE UPPER HINTON FORMATION, UPPER MISSISSIPPIAN, WEST VIRGINIA

The marine-dominated Fivemile and Eads Mill members of the upper Hinton Formation were defined and described by Beuthin and Blake (2004). These two distinct zones are separated by terrestrial deposits, which characterize the majority of the formation. Whereas the lithology of the upper Hinton Formation is well documented, no previous work has been undertaken to identify the fossil genera present within these two marine sequences above the middle Hinton Avis Limestone. In this study faunal assemblages for each member were first identified and then compared to each other. Differences between the fossil communities may have resulted from variation in paleoenvironmental factors such as salinity, water depth, and oxygen content. The older Fivemile Member represents a predominantly restricted marine environment. Evidence for this is the relatively low faunal diversity (8 genera to date). The genera most commonly identified (Modiolus, Lingula, Septimyalina), are indicative of euryhaline conditions. Fossils occur as thin bivalve pavements (crinkle beds), or on the surfaces of siderite concretions. Many show evidence of phosphatic preservation. In contrast, the younger Eads Mills Member contains a diverse fossil assemblage (25 genera to date) indicative of stenohaline conditions. Strong evidence for an open marine environment is the presence of diverse brachiopods, bryozoans (Archimedes), pelmatozoans, and numerous solitary rugose corals. Calcite preservation of the fossils typifies the unit. Differences between the paleoenvironments of the two members may be linked to glacio-eustatic sea level variations as noted by Smith and Read (2000) and Maynard et al. (2006). Such variations caused rapid marine inundation amidst characteristically non-marine sediments such as paleosols and incised valley fills. This study demonstrates that these periods of marine onlap are highly variable in their lateral extent and depth, thus creating differences in the paleoenvironment and noted invertebrate assemblages.