Abstract The sequence stratigraphy of the lower Oligocene in the eastern Gulf Coastal Plain of Alabama and Mississippi has been the focus of research since the early 1980s. The Red Bluff Clay and its correlatives have long been recognized as the lowermost Oligocene lithostratigraphic units in the region. The contact of the Red Bluff Clay with the underlying Shubuta Clay Member of the Yazoo Clay is considered by most workers to be the Eocene–Oligocene boundary. Interpretations of the Vailian sequence stratigraphy of this interval vary, with the Yazoo–Red Bluff contact being viewed as a maximum flooding surface by some workers and as a depositional sequence boundary by others. If the uppermost Yazoo Clay is a condensed section within a sequence, the overlying Red Bluff Clay is a part of the highstand systems tract. If the Yazoo–Red Bluff contact is a depositional sequence boundary, the Red Bluff Clay is a part of the transgressive systems tract. Nearly two decades of studies of this interval have failed to resolve this question satisfactorily. Benthic foraminiferal assemblages and planktic:benthic (P:B) ratios can provide proxies for fluctuating sea level. Both the Yazoo Clay and the Red Bluff Clay are rich in foraminifera that provide additional data for the study of the sequence stratigraphy of the Yazoo–Red Bluff contact. Benthic foraminifera were collected from the upper Shubuta Clay Member of the Yazoo Clay and from the Red Bluff Clay in the Mobil–Mississippi cores in Wayne County, Mississippi, with the goal of understanding the detailed paleoecology of this interval. Samples were taken at 10 cm intervals through the upper Yazoo Clay and the Red Bluff Clay. Samples collected from outcrops of this interval at Little Stave Creek in Alabama and along the Chickasawhay River in Mississippi were also processed for foraminifera. The benthic foraminiferal assemblage in the Red Bluff Clay is characterized by Cibicidoides pippeni, Cibicidoides cookei, Hanzawaia mississippiensis, Lenticulina vicksburgensis, Siphonina advena, Spiroplectammina latior, Massilina cookei, and Uvigerina vicksburgensis. Subtle changes in species composition are present from sample to sample. This Cibicidoides-Hanzawaia benthic foraminiferal assemblage is characteristic of middle-neritic environments in the early Oligocene and contrasts with the outer-neritic Uvigerina-Bulimina assemblage collected from the upper Yazoo Clay. Planktonic:benthic (P:B) ratios are lower at the base of the Red Bluff than in the upper Yazoo but increase upwards through the Red Bluff before falling sharply in the upper half. The Yazoo–Red Bluff contact also marks the appearances of numerous species typical of the Vicksburg benthic foraminiferal fauna. The benthic foraminiferal assemblages collected in this study suggest that the Red Bluff Clay was formed as a part of a transgressive systems tract in a depositional sequence. It represents the earliest Oligocene transgression in the eastern Gulf Coastal Plain and is likely associated with the end of the Oi-1 glacial event. Geologic Problem Solving with Microfossils: A Volume in Honor of Garry D. Jones SEPM Special Publication No. 93, Copyright © 2009 SEPM (Society for Sedimentary Geology), ISBN 978-1-56576-137-7, p. 293-307.

Abstract: 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 Mississippi Office of Geology #1 Mossy Grove core in Hinds County and the Mobil–Mississippi #1 Young in Wayne County) was undertaken to identify benthic foraminiferal assemblages. The goal was to gain insight into paleoecologic conditions, especially relative paleobathymetry, 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). Foraminifera were picked from a total of 40 samples from the Young core and 55 from the Mossy Grove core. A census of the total population of benthic foraminifera represented was obtained for each sample. These values were analyzed by hierarchal cluster analysis with the goal of identifying meaningful assemblages of benthic foraminifera within the Moodys Branch Formation and Yazoo Clay. Four clusters of samples correlate with the benthic foraminiferal biofacies ( Nonion, Siphonina–Cibicidoides, Hanzawaia, Uvigerina ) previously recognized in the St. Stephens core in western Alabama. Two clusters of samples in the Young core define assemblages ( Textularia, Discorbis–Quinqueloculina ) that had no analog in the St. Stephens core. The results of this study show two different successions of benthic foraminiferal biofacies in the cores. The succession of assemblages in the Young core is characteristic of a shallowing of paleobathymetry, while the pattern in the Mossy Grove core represents increasing depth. It is suggested that the biofacies succession of the Young core was produced in a section dominated by sediment supply, while the succession in the Mossy Grove core was dominated by subsidence.

The Paleogene section of Alabama and Mississippi has been the focus of several sequence stratigraphy studies. A sequence model proposed by Baum and Vail (1988) focuses strongly on the southern Alabama marine section and has Late Eocene and Early Oligoccne sequence boundaries that are out of sync with the Gulf Coast stage/group boundaries and (in the opinion presented here) with the regional stratigraphy. The transgressive marine sands of the Middle Eocene Gosport Sand of the upper Claiborne Group are placed as lowstand deposits at the base of the Late Eocene Jackson Group sequence TE3. The Gulf Coast Eocene-Oligocene boundary, which coincides with the Jackson-Vicksburg Group boundary, is placed as a surface of maximum flooding (a condensed section) contrary to regional evidence to the opposite-that it was formed at a lowstand associated with a sequence boundary. Late Oligoccne channel sands, lignites, and shales of the basal Vicksburg Group’s Forest Hill Formation are placed as highstand regressive deposits of the Late Eocene sequence TE3 even though these sediments unconformably overlie a deeply channelled (type 1 unconformity) upper Jackson Group surface. At its type locality in central Mississippi, lignitic shales of the Forest Hill Formation unconformably overlie a paleosol developed on marine clays of the upper Jackson Group’s Yazoo Formation. Both biostratigraphy and unpublished radiometric dates on bentonites by Obradovich and Swisher that range between 34 and 35 Ma indicate the upper Yazoo Formation to be equivalent to the latest Late Eocene sediments of the Eocene-Oligocene boundary stratotype at Massignano, Italy, The youngest date of 34.3 Ma occurred in an unweathered cored section just below the upper contact with the Forest Hill Formation. Paleogene molluscan faunal turnovers also argue agaiast the Late Eocene - Early Oligoccne sequence stratigraphy of Baum and Vail (1988) . High extinction rates occur at the Claibome-Jackson and Jackson-Vicksburg Group boundaries. These mass extinctions of benthic invertebrates are more likely to have occurred during the loss of habitat associated with a lowstand rather than during a time of marine transgresion or maximum flooding.