LATE EOCENE AND EARLY OLIGOCENE BENTHIC FORAMINIFERALPALEOECOLOGY AND SEQUENCE STRATIGRAPHYIN THE EASTERN GULF COASTAL PLAIN, U.S.A.

摘要

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 0i-1 glacial event.