Foraminiferal Paleoecology and Sequence Stratigraphy of the Pachuta and Shubuta Members of the Yazoo Clay (EOCENE; PRIABONIAN), Wayne County, Mississippi

The Pachuta Marl Member and the Shubuta Clay Member of the Yazoo Formation in the Mobil #1 Young core, Wayne County, Mississippi, represent Gulf Coast depositional sequence TE 3.3 in eastern Mississippi. Foraminfera were picked from both the Pachuta and the Shubuta in the Young core in order to calculate the planktonic:benthic (P:B) ratio and to identify the benthic foraminiferal assemblages throughout the interval.

The P:B ratio increases upwards through the Pachuta Marl Member. In the lower half of the Shubuta Clay Member, P:B ratios fluctuate between very high (2.57) and very low (0.09) values. Ratio values in the upper half of the Shubuta are lower (around 0.5). The fluctuations of the P:B ratio in the lower Shubuta likely represents influxes of clastic sediment during early highstand while the more stable, lower values of the P:B ratio in the upper Shubuta may reflect a drop in eustatic sea level during the late Priabonian.

The benthic foraminiferal assemblages collected from the Pachuta Marl Member are dominated by, Siphonina jacksonensis, Cibicidoides cocoaensis, and Anomalinoides bilateralis. Through the Pachuta Marl interval and upwards through the Shubuta Clay, there is an increase in the abundance of Uvigerina, Bulimina, Cibicidoides, Hanzawaia, and Spiroplectammina.

The P:B ratios and the benthic foraminiferal assemblages collected from the Young #1 core are consistent with a stratigraphic progression through a transgressive systems tract, the maximum flooding surface, and the increased sedimentation rate of the highstand systems tract. These results differ somewhat from those obtained by others at the St. Stephens Quarry in southwestern Alabama. It is suggested that the section studied in the Young core is in a shallower setting more proximal to the source of clastic sediment than sections studied at St. Stephens or Little Stave Creek, Alabama.