TAPHONOMIC, TAXONOMIC, AND SEDIMENTOLOGIC ANALYSIS OF FLORIDA'S PLIO-PLEISTOCENE SHELL BEDS: A COMPARISON OF TWO DEPOSITIONAL END MEMBERS

A detailed examination of the mode and depositional environment of late Neogene fossil concentrations was undertaken at two south Florida quarries, Dickerson Indrio Pit (DP; east coast; Pleistocene) and SMR Aggregates Phase 8 (west coast; Pliocene), focused on sedimentologic, taxonomic and taphonomic attributes. Taxonomically, molluscan species richness is relatively low at DP as compared to SMR, and the fauna is numerically dominated by two shallow-water taxa, Mulinia lateralis and Donax variabilis. Based on the more diverse faunal composition and the habitats of modern analogs, SMR contains mixed communities with species indicative of shelfal habitats. From a sedimentologic perspective, focused on shell orientation and packing, the SMR units are characterized by basal lags dominated by single, concave-up, Mercenaria valves, whereas in the DP units, the single Mercenaria valves are oriented in a current stable position. Furthermore, grain-size analysis reveals that there is a consistent difference in mud percentages with values of 10% and 5% for SMR and DP, respectively, suggesting a lower-energy setting for the former. Taphonomic examination also suggests separate depositional environments. At SMR, a high percentage of fragmentation is attributed to short-lived depositional events due to the absence of edge rounding. Furthermore, at SMR fine shell ornamentation was pristine, very small shells were preserved, and a relatively high percentage of articulated shells are present. Within the DP units, shells exhibit high degrees of abrasion and bioerosion, and articulated specimens are lacking. These sedimentologic, taxonomic, and taphonomic data suggest that the depositional modes displayed in SMR and DP shell units were different. The former is dominated by tempestite-type sedimentation whereas the latter points to a much higher degree of reworking, winnowing, and longer periods of exposure on the sea floor. The data suggest that the SMR units represent ‘snapshots' of shell accumulation, whereas those exposed at DP suggest much more pronounced time averaging. These differences in depositional mode have important implications for reconstructing various aspects of the paleoenvironments based on these and other shell beds.