AGE, ENVIRONMENT AND MODE OF DEPOSITION OF THE DENSELY FOSSILIFEROUS PINECREST SAND (PLIOCENE OF FLORIDA) - IMPLICATIONS FOR THE ROLE OF BIOLOGICAL PRODUCTIVITY IN SHELL BED FORMATION

The ''Pinecrest Sand,'' an abundantly fossiliferous Late Pliocene deposit best exposed near Sarasota, Florida, is a poorly understood stratigraphic unit of considerable paleobiological interest because of its geographic and temporal position and extremely abundant and diverse mollusk fauna. The absolute age of this unit has been a subject of some debate, and has considerable implications for its taphonomic history. Evaluation of published and unpublished data indicates that the Pinecrest at Sarasota spans more than one million years, the lower beds having been deposited between 3.5 and 3.0 Ma and the upper beds between 2.5 and 2.0 Ma. Paleoenvironmental data, some previously published and some presented here for the first time, suggest that water depths during Pinecrest deposition varied between 30-50 m and 1-2 m, and that water temperatures were somewhat cooler than those existing at these depths in the region today, possibly as a result of upwelling. Preliminary taphonomic data, most presented here for the first time, are consistent with an important role for time-averaging and stratigraphic condensation by sediment winnowing, together with at least occasionally high levels of biological productivity. These results support a role for biological productivity change in affecting both the taxonomic composition and taphonomic pattern of low-latitude fossil biotas in the Western Atlantic in the Late Neogene.