TAPHONOMY OF SHELL BEDS FROM THE PLEISTOCENE BERMONT "FORMATION" AT LONGAN LAKES QUARRY, FLORIDA

Interpretation of the depositional history of fossil-rich beds provides a critical framework for further evolutionary and paleoecological research. The purpose of this project is to investigate taphonomic factors that affected the various taxa preserved and correlate those factors to the sedimentary history of two larger-scale depositional modes recorded in the Bermont “Formation” sampled and described at Longon Lakes Quarry in Collier County, Florida. The section was subdivided into six primary units defined on coarsening- or fining-upward sequences and delineated by exposure surfaces; the primary units were further divided into three sub-units each. These dense shell beds have no modern analogue and have been the subject of considerable research. Although a diverse assemblage is preserved, this study focuses on Chione elevata, because it is the most abundant taxon in each sample. The specimens sampled from each friable unit were assigned a taphonomic grade based on their degree of corrasion (the net effects of corrosion, both chemical and biological, as well as physical abrasion); size distribution, fragmentation, and encrustation were also examined. Shells could not be effectively sampled in the more lithifed units and required a more qualitative assessment. The compiled data reflect two taphonomic phases: 1) initial taphonomic alteration reflecting conditions in the original habitat; and 2) taphonomic alteration imprinted during transport to form the shell bed. Most samples have a small percentage of pristine chionids that indicate very little taphonomic alteration. Chione from units 6C and 6B, at the base of the exposure, show very little corrasion, which, in comparison to other the samples analyzed, represents a lower degree of time averaging. In contrast, unit 4C has a relatively high degree of corrasion, representing greater time averaging. All other basal sub-units had intermediate amounts of corrasion. In terms of shell bed formation, there are numerous virtually pristine, thin shelled, highly ornamented specimens suggestive of minimal corrosion during transport and the possibility that the final assemblage represents a sedimentary mixture of habitats.