RECONSTRUCTING WATER DEPTH AND PALEOENVIRONMENT OF THE PLIOCENE JACKSON BLUFF FORMATION, NORTHERN FLORIDA, USING BENTHIC FORAMINIFERS

The Pliocene Jackson Bluff Formation outcrops in high bluffs along the Ochlockonee and Apalachicola rivers in the panhandle of northern Florida and provides an opportunity to reconstruct paleoenvironments using fossil foraminifera. The unit is composed of very poorly sorted, fossiliferous sand where it outcrops along the Ochlockonee River, west of Tallahassee, and it occurs as a dark brown fossiliferous organic clayey sand where it outcrops along the Apalachicola River at Alum Bluff. Bulk sediment samples were taken for paleoenvironmental analysis from both sites. Samples were soaked in 3% H₂O₂, sieved, and dried at 60°C prior to picking.

Foraminifera recovered from the bulk sediments are largely of rotalids and miliolids, possibly because allogromids and textularids may have been damaged during sample processing. Because most species live within specific depth ranges, benthic foraminifera are useful as a tool for reconstructing paleobathymetry. Amphistegina lessonii, Elphidium spp., Cibicides lobatulus, Gyroidina spp., Heterostegina depressa, Rosalina spp., and Triloculina trigonula are several species that were found in the study samples that can be used as depth markers. Triloculina trigonula occurred in three samples collected from the top half of the outcrop at the Ochlockonee River site, which indicated a paleo-water-depth of 0-5 m. Although samples from the Apalachicola River – Alum Bluff site also contained a few planktic forms, suggesting that those facies of the Jackson Bluff Formation may have been deposited more seaward, initial interpretations from both sites indicate that the Jackson Bluff Formation was deposited in very shallow waters. However, sedimentological and macrofossil data suggest that environmental mixing may have occurred through post-depositional reworking. Further analysis of the microfauna and taphonomy will allow us to explore for more diagnostic benthic foraminifera that will enhance our initial paleoenvironmental interpretations of these significantly different facies of the Jackson Bluff Formation.