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 depo sit best exposed near Sarasota, Florida, is a poorly understood strati graphic unit of considerable paleobiological interest because of its g eographic and temporal position and extremely abundant and diverse mol lusk fauna. The absolute age of this unit has been a subject of some d ebate, and has considerable implications for its taphonomic history. E valuation of published and unpublished data indicates that the Pinecre st at Sarasota spans more than one million years, the lower beds havin g been deposited between 3.5 and 3.0 Ma and the upper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 so me 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 t aphonomic data, most presented here for the first time, are consistent with an important role for time-averaging and stratigraphic condensat ion by sediment winnowing, together with at least occasionally high le vels of biological productivity. These results support a role for biol ogical productivity change in affecting both the taxonomic composition and taphonomic pattern of low-latitude fossil biotas in the Western A tlantic in the Late Neogene.