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

R. H. Fluegeman, S. R. Robinson, M. L. Banser, M. L. Foye, R. E. Grande, C. T. Horner, and K. D. McKinney
Ball State University

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. This interval is of special interest as recent work at St Stephens Quarry in southwestern Alabama has identified two distinct depositional sequences in this interval placing a sequence boundary within the Pachuta. Foraminfera were picked from both the Pachuta Marl Member and the Shubuta Clay Member 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 and the benthic foraminiferal assemblages collected enable a direct comparison of the Pachuta and Shubuta Members of eastern Mississippi and southwestern Alabama.

The P:B ratio increases upwards through the Pachuta Marl Member. This pattern is characteristic of a transgressive unit. 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 still fluctuate but 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. Other important species in this assemblage includes Siphonina advena. Quinqueloculina mauricensis, and Cibicidina walli. 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 differ somewhat from those obtained by others at the St. Stephens Quarry in southwestern Alabama. It is suggested that the section obtained 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.

AAPG Search and Discover Article #90087 © 2008 AAPG/SEG Student Expo, Houston, Texas