RECONSTRUCTING THE DEPOSITIONAL ENVIRONMENT OF A LATE PLIOCENE SHELL BED IN FLORIDA: INSIGHTS FROM BENTHIC FORAMINIFERS

One of the most problematic aspects of the Plio-Pleistocene shell beds of Florida is determining their mode and environment of deposition. Many earlier studies relied heavily on the diverse and abundant molluscan fauna that forms the bulk of these beds, from which population assemblages have been put together to reconstruct their paleoenvironments. Due to the obvious taphonomic reworking of these units, it is unclear how autochthonous these units are; i.e., do the molluscan taxa reflect the depositional environment or only the habitat(s) in which they lived prior to transport? This study is focused on analyzing the benthic foraminifera from a 4.5 m section of the Pinecrest shell beds exposed in Phase 8 of the SMR Quarry (formerly Quality Aggregates) east of Sarasota, Florida. These shell beds are thought to represent reworked marine sea-grass communities in conjunction with storm deposition. Based on lithological differences, the sequence was subdivided into eight units, and this study focused on the sediment preserved within still articulated specimens of Mercenaria mercenaria collected throughout the sequence. External sediment samples were also collected adjacent to the Mercenaria for comparisons in grain size, population counts, and the degree to which the foraminifera tests had been taphonomically altered. The abundance of Ammonia sp. and Elphidium sp. within the section studied suggests that, the depositional environment was shallower and more brackish than traditionally assumed. An additional unexplained feature found in half of the units in the sequence was the presence of oblong calcareous, potentially algal ‘ooids', 200 to 100 µm in diameter, as well as perfectly round silica spheres, 200 µm diameter, the latter of which appear to have no modern analog.