The sedimentology and diagenesis of the late Pleistocene carbonates of the Florida Keys have been studied in a series of shallow cores from Key Largo and Big Pine Key. On Key Largo three stratigraphic units—defined by paleo-exposure surfaces—were wholly or partly penetrated. Skeletal grainstones and packstones are the dominant lithofacies and enclose abundant massive corals, especially in the upper parts of the two youngest stratigraphic units. This lithology, typical of the Key Largo Limestone of the northern keys, is interrupted by a single thick horizon of mollusk-rich quartz sand derived from paleo-highlands to the north. On Big Pine Key, the youngest stratigraphic unit grades laterally from an ooid grainstone (Miami Limestone) in the northwest to a skeletal-peloid packstone/grainstone (Key Largo Limestone) to the southeast. The latter facies also characterizes older underlying units which are again defined by paleo-exposure surfaces.
Vadose diagenesis during sea-level lowstands has largely resulted in alteration of the sediments to low-Mg calcite, but minor amounts of aragonite remain in the youngest stratigraphic unit. Pervasive development of secondary moldic and irregular dissolution porosity has accompanied this mineralogic transformation. Submarine cementation is insignificant, and freshwater phreatic lenses, if and where present, have failed to leave a distinctive petrographic imprint.
Overall, the Florida Keys (as manifest by the youngest stratigraphic unit) represent relatively high-energy deposits which accumulated on a slight but significant break in slope. Water depths were not great, and it is unlikely that a major contemporaneous reef barrier lay seaward at that time.