CONTROLS ON REEF FACIES DEVELOPMENT IN THE PLEISTOCENE (MIS 5e) KEY LARGO LIMESTONE

Several different and contrasting explanations exist that describe the environment of formation of the exposed, coral-rich unit of the Pleistocene Key Largo Limestone. The most important aspect of the various explanations is reconciliation of the absence of Acropora palmata in the Pleistocene with an abundance of A. palmata in the Recent (the last 10 ka). Because coral reefs of south Florida are at the latitudinal extreme of reef development in the western Atlantic, the absence of A. palmata in the Key Largo Limestone is not surprising. This is due to contraction and expansion of species ranges in response to changing environmental conditions through time and space. At the apex of the last major interglacial, the entire south Florida Peninsula was flooded, creating a broad, shallow Bahamas Bank-type carbonate platform. This would have allowed unimpeded flow from a much-enlarged Gulf of Mexico over a broad, shallow carbonate bank in the west to the Straits of Florida and open Atlantic in the east. Reef communities forming at the interface between the two would have been bathed by clear, warm, oceanic waters from the Gulf Stream on incoming tides and waters of variable quality, especially in regard to temperature, salinity, and sediment on outgoing tides. With no exposed islands (Keys) to protect the reefs, during the winter months, chilled and sediment-laden bank waters would have periodically poured onto these shelf-margin reefs. Under this scenario the resulting coral community would have been essentially devoid of the shallow, thermophilic reef-crest species A. palmata. At slightly deeper depths on the fore reef, ephemeral coppices of A. cervicornis would have filled in sheltered pockets in a head-coral assemblage dominated by Montastraea–Diploria–Porites.