LIMITATIONS ON LOWSTAND METEORIC DIAGENESIS IN THE PLIOCENE-PLEISTOCENE OF FLORIDA AND GREAT BAHAMA BANK - IMPLICATIONS FOR EUSTATIC SEA-LEVEL MODELS

Data from three Bahamian and South Floridan cores with deeper water fa cies show that, contrary to current models, there is no phreatic meteo ric diagenesis that can be attributed to large-scale sea-level lowstan ds, Models of meteoric diagenesis in carbonate sediments commonly assu me major phreatic alteration during glacio-eustatic sea-level lowstand s, However, the diagenesis previously attributed to large-scale (>80 m ) lowstands is found in shallow-water facies that have been repetitive ly exposed, In order to document diagenesis from large-scale lowstands , an interval is needed that was deposited in deeper water and not imm ediately exposed to meteoric fluids, Three cores with such deeper-wate r facies indicate that the majority of phreatic meteoric diagenesis in Florida and the Bahamas happened during glacio-eustatic highstands, n ot during lowstands, The data also suggest that diagenetically active meteoric lenses in Florida and the Bahamas are restricted to within 60 m, and perhaps less, of the land surface, The most likely reasons dee per lenses are chemically inactive are (1) the greater percolation dis tance allows the water to reach saturation prior to entering the lens, and (2) the large distance exceeds the reach of soil-derived organic matter, known to drive diagenesis in meteoric lenses, Models that curr ently assume a constant rate for alteration in meteoric fluids need to accommodate this variation with thickness of the vadose zone. This st udy also questions the use of paleophreatic lenses as records of eusta tic sea level, as the large-scale falls may not leave any record.