Abstract

Examination of regional stratigraphic relationships and graphic correlation of biostratigraphic data from two important localities has resulted in a new interpretation of the sequence stratigraphy of the Eocene-Oligocene boundary interval in Mississippi and Alabama. The much-studied section at St. Stephens Quarry in southwestern Alabama does not contain all of the components of the depositional sequence. At St. Stephens, the highstand systems tract is missing and is represented by a marine hiatus (condensed section) at the top of the Upper Eocene Shubuta Clay. As a consequence, the surface of maximum starvation and the overlying sequence boundary are merged. Lowermost Oligocene sediments, which were deposited in the lowstand systems tract, rest on this maximum flooding surface / sequence boundary. These findings agree with those from other parts of the Gulf Coast where the Eocene-Oligocene boundary has been recognized as a sequence boundary associated with a subaerial unconformity and a downward shift in coastal onlap. Revision of the sequence stratigraphic interpretation at St. Stephens Quarry alleviates much of the confusion concerning the relationship between the Eocene-Oligocene boundary and the sequence boundary. This new interpretation shows that not all sections contain all components of a particular depositional sequence and that caution is warranted when sequence stratigraphic interpretations are used to evaluate observations. The sequence stratigraphic interpretation of the section at St. Stephens has undergone revision as new observations have become available.

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