RECONSTRUCTING PALEO-DEPTH IN THE FLORIDA PLIO-PLEISTOCENE SHELL BEDS USING BENTHIC FORAMINIFERA: WAS IT A MID-SHELF ENVIRONMENT?

The Plio-Pleistocene shell beds of southwest Florida are highly fossiliferous, containing diverse and abundant macro- and microfaunas. The conclusion that molluscan species richness in Florida recovered completely from a Late Neogene extinction relies, in part, on the assumption that Pliocene units contain roughly equal numbers of species from 0 to 30 m in depth as today. In this study, we use benthic foraminifera to determine whether a mid-shelf paleoenvironment is a probable analog for that recorded in the shell beds. Benthic foraminifer genera are restricted to a limited set of environmental parameters and allow for precision in assessing paleodepths and salinities. Foraminifer samples were collected from 7 beds of the Late Pliocene Tamiami Formation and the Early Pleistocene Bermont Formation and compared to similar Recent samples using cluster analysis of standard 300 counts. Our Recent samples include habitats from south Florida representing shallow estuaries (1 m) to deep shelf (100 m) depths.

Pinecrest and Bermont shell beds sampled were dominated by an Ammonia-Elphidium assemblage indicative of brackish mangals or estuaries. This assemblage is not found in normal marine salinity environments which typically represent water depths greater than a few meters. Based on results of a cluster analysis, none of the modern normal marine or localities >3 m in depth had samples similar to the assemblages found in the shell beds. This mismatch is unlikely to result from preservational effects as marine benthic foraminifers are more robust than their brackish counterparts. Instead, the shell bed foraminifer assemblages cluster with those Recent samples collected from shallow (~2 m depth), representing restricted estuarine environments with freshwater inputs.