LATE CENOZOIC DEPOSITIONS PALEOENVIRONMENTS OF SOUTHEAST FLORIDA INTERPRETED FROM CORE LOGS

Abstract

The use of core logs from water wells and engineering-geological site investigations permitted spatial refinement of known paleodepositional environments along the southeastern coast of Florida where the narrow continental shelf was influenced by changes in sea level and shoreline position during the late Cenozoic. Lithological materials have been formalized in terms of the Tamiami Formation (early Pliocene), Fort Thompson Formation (late Pliocene), Anastasia Formation (Pliocene-Pleistocene), Miami Limestone (late Pleistocene), and Pamlico sands (Pleistocene) in southeastern Florida. The lack of good exposures, scarcity of biostratigraphically important microfossils, similarity of sedimentary deposits, and pervasive diagenesis of fossiliferous sections have mitigated against a consensus of interpretation for late Cenozoic stratigraphic sections. Inspection of more than 18,300 m of unpublished core log data combined into cross-sections that crisscrossed the Broward County study area, permitted mapping of vertical and horizontal relationships for Miocene, Miocene-Pliocene, early Pleistocene, and late Pleistocene/Holocene time frames. These lithostratigraphic interpretations of changing paleoenvironments formed the basis of a preliminary model for the coastal geological framework of Broward County. This study confirmed general late Cenozoic trends in sedimentation from marine conditions (Tamiami Formation) to nearshore (Anastasia Formation, Fort Thompson Formation) settings to very shallow marine environments (oolitic facies of the Miami Limestone) and eolian materials in the upper Pamlico sands. Late Pleistocene to Recent trends showed a shore-parallel pattern of shallow paleobathymetries that included barrier bars and tidal passes (paleoinlets). Erosional unconformities (interpreted as irregular karstified groundsurfaces with low paleotopographic relief) in the sections were frequently capped by dense subaerial weathering crusts of calcite which today act as semi-impermeable barriers to groundwater flow. Knowledge of this coastal geological framework is relevant to increased understanding of submarine groundwater discharges, modes of saltwater intrusion, boundary conditions for groundwater models, and has practical implications for optimized water management along the southeast coast of Florida.