INSIGHT INTO THE ORIGIN OF DENSE SHELL BEDS IN THE PLEISTOCENE CALOOSAHATCHEE FORMATION IN FLORIDA FROM GRAIN SIZE ANALYSIS OF SEDIMENT IN MATRIX AND WITHIN ARTICULATED MOLLUSKS

The highly fossiliferous shell deposits in Florida are famous for the density and diversity of macrofossils. We studied shell beds exposed in the Florida Shell quarry (west of Punta Gorda) of early Pleistocene age (~2.0 – 1.6 million years ago). These quarry exposures are less well-studied compared to the famous Pinecrest beds and may soon be no longer accessible. We are particularly interested in the processes that resulted in the highly fossiliferous sediments. Is the shell density the result of biological factors like high densities of living organisms? Or are the shell beds the result of physical processes like winnowing and removal of sediment, producing a high concentration of shells? Could low sedimentation rates or diagenesis also be a factor? We hope that analyses of the grain size and composition of sediment may shed some light on these questions.

Our samples were collected in-situ, from a vertical outcrop face of the Caloosahatchee Formation, within a 1.5-meter interval dominated by abundant small Chione mollusks. Five complete larger mollusks (3 Mercenaria, 1 Goeduck, and 1 Dinocardium robustum) were collected, along with a sample of the matrix adjacent to each shell. Two additional bulk samples of the matrix in this interval were collected for grain size analysis and to assess the degree of winnowing and removal of fine particles that might have occurred. We also examine sediment within the closed shells because they likely were not impacted by post depositional processes, and so might be more representative of existing conditions than the surrounding matrix.

Our preliminary results indicate that the interval studied is composed mostly of large shells and shell fragments (with 40% greater than 4mm on average), with the sand sized fraction in the matrix making up 40% of the sediment by weight. The composition of the sand sized matrix is about 80% quartz. Our research to date shows that sediment within the shells is dominated by sand size particles that are mostly quartz in composition. Interestingly, the mean grain size for the matrix sand component (140 um) and the internal shell sediment (154 um) is very similar, and both sediments consist mostly of fine and very fine sand. The fine-grained texture would seem to suggest winnowing is not an important factor in shell accumulation at our location.