ORIGIN AND AGE OF PHOSPHORITE FROM THE SOUTH-CENTRAL FLORIDA PLATFORM - RELATION OF PHOSPHOGENESIS TO SEA-LEVEL FLUCTUATIONS AND DELTA-C-13 EXCURSIONS

Francolite (carbonate fluorapatite) from predominantly peloidal phosphorite grains in the Miocene Hawthorn Group from the Babcock Deep core (southwestern Florida) is highly substituted with an average composition Of (Ca4.61Na0.22Mg0.14K0.05Sr0.02) (PO4)2.23(CO3)0.62(SO4)0.12(F)1.04. deltaC-13 values of -1.0--7.8 parts per thousand PDB of the francolite CO3 indicate a substantial contribution of C from an organic source. The sulfur isotopic composition of francolite SO4 is depleted to slightly enriched relative to Miocene seawater and suggests that phosphogenesis occurred during early burial diagenesis in organic-rich sediments above or in the zone of sulfate reduction. The uniformly high Sr, Na, Mg, SO4, and CO3 contents of the francolite, the deltaO-18 values of the francolite CO3 (-0.2--4.2 parts per thousand PDB), and the inclusion of pyrite and organic matter in phosphorite grains suggest that the francolite has had minimal chemical alteration since its formation. Phosphorite from the Babcock Deep core has Sr(87/86) ratios that range from 0.708148-0.708880 and Sr-derived ages that range from late Oligocene (25.6 +/- 0.7 Ma) to early late Miocene (9.2 +/- 1.4 Ma). The Sr-derived ages of relatively fragile calcareous benthic foraminifers are consistently younger than associated hard, tightly cemented phosphorite and support textural evidence of extensive reworking. The age of the host sediment ranges from early Miocene to Pliocene and its depositional history is related to sea-level fluctuations. Approximately three-quarters of the total phosphorite in the Babcock Deep core formed during or within 2 my before the major early to mid-Miocene positive deltaC-13 shift as recorded at DSDP Site 588 (southwest Pacific). One-third of the total phosphorite formed just prior to the carbon shift in the short interval between 21.9 and 20.4 Ma. These results indicate that reworked phosphorite deposits may provide a proxy of organic carbon cycling on continental shelves that correspond to sea-level fluctuations and to variations in the deltaC-13 record.