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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 67 (1983)

Issue: 3. (March)

First Page: 506

Last Page: 506

Title: Eocene-Oligocene Sea Level Changes as Reflected in Alabama Outcrop Sections: ABSTRACT

Author(s): T. S. Loutit, G. R. Baum, R. C. Wright

Article Type: Meeting abstract


Outcrops at Little Stave Creek and St. Stephens Quarry in southwestern Alabama contain continuous sections across the Eocene-Oligocene boundary. The sequence of lithologic and biologic changes recorded across the boundary in Alabama can be best explained by a rising sea level. From the base to top, the sequence consists of: silty glauconitic marl (Pachuta); glauconite marl (Shubuta); thin glauconitic clay (unnamed); glauconitic clay and marl (Red Bluff) interbedded with silty limestone (Bumpnose); and grading upward into a carbonaceous clay (Forest Hill). The last occurrence of the planktonic foraminifera Globorotalia cerroazuleisis cocoaensis occurs just below the top of the Shubuta marl. The last occurrence of the calcareous nannofossil Discoaster saipanensis is within he Pachuta. Lithologic and paleontologic studies indicate that the Pachuta-Shubuta units represent a deepening-upward sequence. As water depths increased, the locus of terrigenous deposition moved updip or shoreward of the sections at Little Stave Creek and St. Stephens Quarry, resulting in the production of a compressed marine sequence capped by a nondepositional marine hiatus. During and after the period of deepest water, renewed terrigenous deposition resulted in a shallowing upward sequence (Red Bluff, Bumpnose, and Forest Hill).

We suggest that: (1) the changes in water depth and sedimentation in the Alabama sections occurred as a result of a rapid rise in relative and eustatic sea level (Pachuta, Shubuta), reaching a maximum during the time of deposition of the unnamed blue clay, followed by a period of less rapid relative rise and slow eustatic fall (Red Bluff, Bumpnose, Forest Hill); and (2) the decrease in sedimentation rates (and hence a decrease in stratigraphic resolution) caused by a rapid rise of eustatic sea level may also account for the apparently synchronous first and last appearances of a number of microfossil lineages at the Eocene-Oligocene boundary in Alabama and elsewhere.

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