THE IMPACT OF CEMENTATION AND PRESERVATION BIAS ON THE MIOCENE MOLLUSCAN RECORD IN SOUTHEASTERN VIRGINIA

Unconsolidated sediments occur throughout the Cenozoic fossil record, but are rare in Paleozoic and early Mesozoic units, complicating attempts to quantify Phanerozoic biodiversity. Local variation in the extent of lithification is rare within a single unit but may provide useful estimates of lithification bias on diversity and paleoecological metrics. The purpose of this study is to determine how cementation influences estimates of taxonomic composition, richness, and evenness in a marine fossil assemblage from the Miocene of southeastern Virginia. One site located on the south side of the James River in Surry County (Virginia) exhibits local instances of cementation within fossiliferous beds of the Eastover Formation. Six bulk samples of cemented and uncemented material were collected from the upper Cobham Bay Member (Eastover Formation, Miocene). Fully and partially cemented samples were processed by examining material in three dimensions, breaking samples into progressively smaller units, and unconsolidated material was sieved to 4mm and sorted. 900 specimens spanning 26 genera were identified to the lowest taxonomic level possible and used to quantify genus composition, richness, abundance, and evenness. Maximum shell dimension was measured for whole specimens and taxonomically identifiable fragments. Information on shell mineralogy and life habit were compiled from the literature and relevant entries from The Paleobiology Database. Preliminary data suggest a significant depletion in alpha-level genus diversity and abundance in cemented samples, primarily the result of preservational bias against aragonitic taxa.