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Sequence Stratigraphy and Chronostratigraphy of Upper Eocene and Lower Oligocene Strata, Eastern Gulf Coastal Plain
Upper Eocene (Jacksonian) strata of the Eastern Gulf Coastal Plain historically have included the Yazoo Clay of the Jackson Group and its members (North Twistwood Creek Clay, Cocoa Sand, Pachuta Marl, and Shubuta Clay Members). Lower Oligocene (Vicksburgian) strata have included the Red Bluff Clay, Bumpnose Limestone, Forest Hill Sand, and Marianna Limestone and its Mint Spring Marl Member of the Vicksburg Group. Based on the vertical distribution of planktonic foraminifera (last occurrences of Turborotalia cerroazulensis cocoaensis (Cushman) and Hantkenina alabamensis Cushman), the Jacksonian-Vicksburgian provincial stage boundary and the Eocene-Oligocene boundary have been placed at the top of the Yazoo Clay. Recently, some workers have placed the Eocene-Oligocene (Priabonian-Rupelian) boundary either below or above the Yazoo-Red Bluff contact. Also, the Eocene-Oligocene boundary has been described by most sequence stratigraphers as a surface of maximum sediment starvation and as a maximum marine flooding surface at the Yazoo-Red Bluff contact. In southwestern Alabama, these surfaces are part of a depositional sequence interpreted to consist of transgressive systems tract deposits (Cocoa and Pachuta), condensed section deposits (Shubuta and lower Red Bluff), and highstand systems tract deposits (upper Red Bluff/Bumpnose/Forest Hill). Recently, however, some workers have interpreted the Yazoo-Red Bluff/Forest Hill contact as a sequence boundary or have interpreted the maximum marine flooding surface as occurring in the middle Shubuta beds, with the upper Shubuta beds being assigned to the highstand systems tract of the Yazoo-Red Bluff/Forest Hill depositional sequence. By utilizing sequence stratigraphic analysis and chronostratigraphic correlation, these recent findings can be reconciled with the interpretation that the Jackson-Vicksburg Group contact corresponds to a surface of maximum sediment starvation and a maximum marine flooding surface in strata of the easternmost Gulf Coastal Plain.
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