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ABSTRACT: Paleoenvironmental Analysis of the Lower Oligocene Mint Spring and Marianna Formations across Mississippi and Southwestern Alabama

Cecil W. Pettway, Dean A. Dunn

Examination of calcareous nannoplankton floras from the Mint Spring Formation and the Marianna Limestone indicates that these formations are diachronous along their outcrop belt in Mississippi and southwest Alabama. In east-central Mississippi and Alabama, these strata correlate to the Ericsonia subdisticha Zone (NP21), while in west-central Mississippi they contain nannofossils of the younger Helicopontosphaera reticulata Zone (NP22).

Quantitative species abundance data for samples from 12 outcrops were subjected to R-mode and Q-mode factor analysis, and to discriminant analysis. These multivariate techniques reveal that the Mint Spring Formation was deposited in a shallow-water, nearshore inner-shelf environment, while normal pelagic sedimentation resulted in the Marianna Limestone being deposited as a carbonate shelf across east-central Mississippi and southwest Alabama. In west-central Mississippi, these strata were less affected by the transgression of northern Gulf waters in the early Oligocene, as indicated by higher abundances of shallow-water species within these formations.

Based on discriminant analysis, the Mint Spring Formation and the Marianna Limestone reflect similar paleoenvironments in western Mississippi, with the uppermost Mint Spring samples near the formational contact containing nannofossil species assemblages that are indistinguishable from the nannofossil assemblages from the lowermost Marianna samples immediately above the contact.

Both these units were deposited during the first half of sea-level cycle TA4.4, and based on relatively constant species abundances throughout the extent of both formations, the Mint Spring Formation and the Marianna Limestone appear to have been deposited as part of a single continuous transgression.

AAPG Search and Discovery Article #90999©1990 GCAGS and Gulf Coast Section SEPM Meeting, Lafayette, Louisiana, October 17-19, 1990