TAPHONOMY, BIOEROSION, ENCRUSTATION, AND PALEOENVIRONMENTAL RECONSTRUCTION OF MOLLUSCAN ASSEMBLAGES FROM THE PLIO-PLEISTOCENE NASHUA FORMATION OF NORTHERN FLORIDA

Three samples of the Plio-Pleistocene Nashua Formation were collected from spoils at a quarry in northern Florida. Bulk samples were sieved and specimens over 5mm retained for paleoecological analyses of molluscan assemblages as part of a paleontology class research project at Georgia State University. Non-molluscan faunas were also examined as related to reconstructing paleoenvironments. Taxa were identified, and evidence of bioerosion and encrustation documented on the interior and exterior of shell specimens in an effort to examine post-mortem impacts. Molluscan genera recorded across multiple samples were further assessed for quality of preservation using a graded scale for shells. In addition, small sub-samples were removed from unprocessed replicates for sedimentological examination (e.g., grain size, angularity, sorting, and composition). The goals here are to evaluate these deposits of the Nashua Formation to investigate any differences therein that may be representative of lower vs. upper parts of the unit and in preparation for regional comparisons to correlative strata. This fossiliferous unit represents a biogeographic gap in knowledge that has yet to be thoroughly studied with respect to extinction events across the Plio-Pleistocene compared to other fossil-rich marine strata in the region.

Non-molluscan specimens encompass barnacles, bryozoans, coral, crab claws, sand dollars, sea urchin spines, bone fragments, and shark teeth. Traces of bioerosion and encrustration (nearly 10% of specimens studied in each of the samples) additionally revealed the presence of boring sponges and worms. Sedimentological analyses demonstrated that the samples are variably calcareous and composed primarily of poorly-sorted, angular, medium-fine quartz sands with shell fragments. Silts and clays are documented as well. Small phosphate nodules were noted and larger pieces of cemented shell matrix were observed in varying frequencies. Taphonomic grading of the shells yielded similar patterns across all samples. Future work will continue to examine characteristics related to depositional environments of the Nashua Formation once ongoing specimen processing is fully completed. The results so far are consistent with previous interpretations of open marine, shallow shelf paleoenvironments.