THE JACKSON GROUP (EOCENE) OF THE EASTERN GULF COASTAL PLAIN: BENTHIC FORAMINIFERAL PALEOECOLOGY AND SEQUENCE STRATIGRAPHY

The Jackson Group of the eastern Gulf Coastal Plain consists of the Moodys Branch Formation and the Yazoo Clay. The Jackson Group increases in thickness from 57 meters in the Chickasawhay River Valley of eastern Mississippi to more than 142 meters in western Mississippi. Additionally, the western Mississippi sections contain a rather uniform clay facies within the Yazoo while sections in eastern Mississippi and western Alabama are differentiated into well defined, lithologically distinct members. Correlation has been established between the two regions using plankton biostratigraphy but many questions remain, especially with regard to sequence stratigraphic interpretations. Previous paleobathymetric interpretations of the Jackson Group 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, Hanzawaia, and Nonionella (assemblages more common in eastern sections) have been considered to represent neritic conditions. Assemblages dominated by species of Uvigerina, Bulimina, Bolivina, and Spiroplectammina (assemblages more common in western sections) have been interpreted to represent outer neritic to upper bathyal conditions. The apparent deep-water conditions in western Mississippi are in conflict with evidence indicating clastic sediment infilling from the west. Values of the planktonic/benthic (P/B) foraminiferal ratio in both eastern and western sections 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-Red Bluff-Forest Hill contact. While the benthic foraminiferal assemblages indicate relatively deep water in the western sections, the P/B ratios suggest a rapid influx of sediment. A similar paleoecologic pattern has been recognized in the Quaternary of the Gulf of Mexico. It is suggested here that the bulk of the Yazoo Clay in western Mississippi was deposited during forced regressions and represents multiple falling stage systems tracts.