| Summary: | Synchronous deposition of carbonate and siliciclastic sediments occurred on the South Florida Platform during the late Oligocene to early Pliocene, produced a large number of complex mixed carbonate/siliciclastic lithologies, some perhaps unique to the region. All 14 defined subfacies contain a mix of carbonate and siliciclastic sediments along with quartz sand and phosphorite grains. Only a small percentage of the stratigraphic section contains sediments with a solely carbonate or solely siliciclastic composition. Transitions between subfacies are both transitional and abrupt. The hypothesis that carbonate and siliciclastic mixed sediment sequences show mostly abrupt boundaries (Mount, 1984) is not supported.Based on the interpretations of the depositional environments for the 14 subfacies found in the Hawthorn Group, the entire stratigraphic section was deposited on a ramp with a high percentage of the sediments containing a carbonate mud component. Homoclinal ramp deposits are characterized by low, rather uniform slopes from shallow water into the basin with continuous grading of sediment types from nearshore sands to deep water sands and muds. Many described ramp deposits contain little or no mud in the open inner or outer ramp subfacies, such as the eastern Florida ramp, the current west Florida ramp, and other wave-dominated ramps, such as southern Australia (James et al., 1994; Boreen and James, 1995). Modern ramp deposits bordering restricted water bodies, such as the Arabian Gulf do contain a belt of muddy open-water inner and outer ramp deposits. Ancient epeiric sea ramp deposits also produced wackestone and mudstone deposits in the open shelf area. Therefore, the southern Florida ramp deposited during the late Oligocene to early Pliocene was more similar to a restricted-sea ramp than a wave-dominated ramp.A new chronostratigraphy was developed for the upper Paleogene and Neogene sediments on the central part of the South Florida Platform. The ages of the sediments were determined by the combined use of calcareous nannofossils, planktonic foraminifera, diatoms, strontium-isotope stratigraphy, magnetostratigraphy, and carbon and oxygen isotope variations. Based on these integrated dating techniques, the following age constraints using the Berggren (1995) time scale were placed on the formations in this region: the Suwannee Limestone is constrained from between 33.7(?) to 28.5 Ma, the Arcadia Formation of the Hawthorn Group is constrained from between 26.5 to 12.4 Ma, the Peace River Formation of the Hawthorn Group is constrained to between 11(?) to 4.3 Ma, the Tamiami Formation is constrained between 4.29 to 2.15 Ma, and the Caloosahatchee Formation is constrained from 2.14 to 0.6 Ma.Eleven third-order sea-level events were recognized in the stratigraphic record between the late Oligocene and early Pliocene. With the exception of the early Miocene sea-level events, the remaining seven events corresponded closely in time with the global sea-level curve of Haq et al. (1988). However, the depth of flooding on the Florida Platform differed from the relative depths predicted by the Haq curve. During the late Aquitanian and Burdigalian, Haq observed three third-order sea-level events, but four events were recorded in the cores studied. It is hypothesized that two of the events correlate to event 2.1 of Haq et al. (1988), which is a revision of the global curve.
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