BENTHIC FORAMINIFERAL PALEOECOLOGY OF RELATIVE SEA LEVEL CYCLE TE 3.2 (EOCENE; BARTONIAN) IN THE EASTERN GULF COASTAL PLAIN: A TWO SITE STUDY

The Moodys Branch Formation and North Twistwood Creek Clay Member of the Yazoo Clay represent relative sea level Cycle TE 3.2 in the eastern Gulf Coastal Plain. Examination of Cycle TE 3.2 in two cores in Mississippi separated by 185 km in an East-West direction (the Mossy Grove core in Hinds County and the Mobil-Mississippi #1 Young in Wayne County) was undertaken to identify benthic foraminiferal biofacies. The goal was to gain insight into paleoecologic conditions, especially bathymetry, in two distinct locations within the sedimentary record of the same relative sea level cycle. In both cores, the Moodys Branch Formation represents the Transgressive Systems Tract (TST) and the North Twistwood Creek Clay Member of the Yazoo Clay represents the Highstand Systems Tract (HST).

In both sections, the Moodys Branch Formation (TST) is dominated by the Siphonina-Cibicidoides biofacies. This biofacies represents middle shelf environments with paleodepth estimates of between 75 m to 100 m. Above the maximum flooding surface (mfs) at the top of the Moodys Branch, the benthic foraminiferal biofacies record diverges between the two cores. In the Mobil-Mississippi #1 Young core, the HST is dominated by the Nonion biofacies. This biofacies is considered to represent paleodepths of less than 30 m and is associated with sediments containing high percentages of quartz sand. Samples from the same interval in the Mossy Grove core still contain the Siphonina-Cibicidoides facies but show an increasing abundance of both the Hanzawaia and Uvigerina biofacies. The Hanzawaia biofacies is dominated by species of Hanzawaia, Cibicidina, and Spiroplectammina and is considered to represent open shelf conditions with estimated paleodepths of 30-75 m. The Uvigerina biofacies is dominated by species of Uvigerina and Bulimina and represents outer shelf environments with estimated paleodepths of 100 m-200 m.

The results suggest that the sedimentary record of Cycle TE 3.2 in the Young core is dominated by sediment supply as indicated by the decrease in paleodepth through the sequence. By contrast, the upper part of Cycle TE 3.2 in the Mossy Grove core contains benthic foraminiferal biofacies indicating an increase of paleodepth. This suggests that the western portion of the study area was undergoing subsidence during the latter part of Cycle TE 3.2.