PALEOBATHYMETRY OF THE JACKSON GROUP (EOCENE; BARTONIAN-PRIABONIAN) OF WESTERN MISSISSIPPI: DATA FROM FORAMINIFERAL ASSEMBLAGES

The Jackson Group of the eastern Gulf Coastal Plain consists of the Moodys Branch Formation and the Yazoo Clay. Previous paleobathymetric interpretations of these late Eocene-earliest Oligocene sediments have been based primarily on benthic foraminiferal assemblages and have provided estimates of depth ranging from 10 meters to greater than 200 meters. Assemblages characterized by foraminifera such as Siphonina, Discorbis, Cibicidoides, and Nonionella have been considered to represent neritic conditions while those assemblages dominated by species of Uvigerina, Bulimina, and Spiroplectammina have been interpreted as representing outer neritic to upper bathyal conditions. The apparent deep-water conditions are in conflict with evidence indicating clastic sediment infilling from the west. In order to resolve this discrepancy, the planktonic/benthic (P/B) foraminiferal ratio was studied in a continuous core through the Jackson Group.

Values of the P/B ratio are low in the lower part of the Jackson Group (Moodys Branch Formation) and increase into the lower Yazoo Clay. Following a return to low values, the middle Yazoo Clay contains some of the highest P/B ratios (0.4) in the core. The upper Yazoo records a gradual decrease in the P/B ratios through the Yazoo-Forest Hill contact. The middle and upper Yazoo interval in the core is interpreted as representing a gradual rise in sea-level followed by sediment infilling from the west.

While the benthic foraminiferal assemblages indicate relatively deep water, the P/B ratio records the influence and proximity of the shoreline. Used together, these two tools can paint a more accurate picture of paleobathymetric history of a depositional basin.