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ABSTRACT

Micropaleontologists using planktonic foraminifera for biostratigraphic age correlation differ on the placement of the Eocene-Oligocene boundary from those workers utilizing calcareous nannoplankton. Employing the stratigraphic distribution of the planktonic foraminifera recovered from the upper Eocene Yazoo Clay and lower Oligocene Red Bluff Clay/Bumpnose Limestone, Forest Hill Sand and Mint Spring Marl Member of the Marianna Limestone in southeastern Mississippi and southwestern Alabama, the epoch boundary is recognized to be at or near the top of the Yazoo Clay. The extinctions of the key calcareous nannoplankton species occur at a lower stratigraphic level. The dilemma regarding the placement of the Eocene-Oligocene boundary exists probably because changes in faunal and floral assemblages were gradual during the late Eocene through the early Oligocene and because this epoch boundary represents a stratigraphically condensed section of a Type 2 depositional sequence. No dramatic faunal or floral changes should be expected at the Eocene-Oligocene boundary because of the absence of a major drop in sea level and because no sudden climatic changes occurred at this time.

In southeastern Mississippi and southwestern Alabama, the Cocoa Sand, Pachuta Marl and Shubuta Clay Members of the Yazoo Clay, the Red Bluff Clay, the Bumpnose Limestone, and the Forest Hill Sand comprise an unconformity-bounded, Type 2 depositional sequence that accumulated during the TE3.3 coastal onlap cycle of the Td supercycle. The contact of the Cocoa/Pachuta with the underlying North Twistwood Creek Clay Member of the Yazoo Clay (highstand regressive deposits of the underlying depositional sequence) is a Type 2 unconformity. The transgressive deposits of the TE3.3 sequence consist of the Cocoa and Pachuta. The condensed section of the sequence includes the Shubuta (lower condensed section deposits) and Red Bluff/Bumpnose (upper condensed section deposits). The Shubuta-Red Bluff/Bumpnose contact, which approximates the Eocene-Oligocene boundary, is a surface of maximum starvation associated with the greatest landward transgression of the coastline during a relative rise in sea level. The Forest Hill Sand overlies the Red Bluff/ Bumpnose and comprises the highstand regressive deposits of the sequence. The contact of the Forest Hill with the overlying Mint Spring is a Type 2 unconformity. The Mint Spring represents the transgressive deposits of the overlying sequence.

The use of stratigraphically condensed sections, sequence boundaries and genetic depositional sequences has the potential to be a useful correlation tool for resolving stratigraphic and depositional problems in the Gulf Coastal Plain. The age of a condensed section within a specific depositional sequence should be synchronous worldwide.

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