HIGH-ORDER, PLEISTOCENE SEA-LEVEL FLUCTUATIONS IN SOUTHWEST FLORIDA

Florida's Plio-Pleistocene strata record episodes of sea-level highstands. The age of the strata is often ambiguous as there are no consistently reliable dating techniques that can be unequivocally applied to many of the units. The lack of preservation of continuous Plio-Pleistocene sedimentary sequences is a consequence of Pleistocene mean sea-levels not flooding peninsular Florida, the low volume of sedimentary supply, and the lack of new accommodation space. This study investigates a 6 m cyclothem-type set of six shallow-marine shell beds separated by five subaerially exposed packstone beds. These strata are part of the biostratigraphically-defined early Pleistocene (1.1 – 1.6 Ma) Bermont formation and were likely deposited during a 160 kyr interval between 1.3 and 1.1 Ma. The shell beds are mollusk-rich and contain moderately well-sorted fine sands. The packstones contain sparry calcite cements and show evidence of subaerial weathering, such as an irregular upper solution surface, root molds, and sparry freshwater calcite cements. The upper surfaces of the packstones are unconformities that separate five episodes of highstand deposition. A grain-size analysis shows an upward-coarsening trend between depositional episodes, which most likely indicates a progressively decreasing water depth. The bivalve assemblages suggest a mean paleodepth during the deposition of the shell beds of approximately 7.5 m. A subsidence rate of 6 m/Ma is inferred from the thickness of deposits near the locality. Based on a comparison of the height of the strata with existing eustatic curves, the early Pleistocene age of the formation, and the 6 m/Ma subsidence rate, the most parsimonious duration for the interval between the cyclothems is 41 kyr, dominantly forced by obliquity orbital variability. Combining the data indicates that the early Pleistocene sea level was between 11.2 and 14.4 m above sea level (asl); previous estimates of early Pleistocene highstands have shown an elevation approximately 15 m asl.