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

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ABSTRACT 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; Martini, 1971), while in west-central Mississippi they contain nannofossils of the younger Helicosphaera reticulata Zone (NP22). Quantitative species abundance data for samples from twelve outcrops were subjected to R-mode and Q-mode factor analysis, and to discriminant analysis. Factor analysis indicates that the Mint Spring Formation was deposited in a shallow nearshore inner-shelf environment, while normal pelagic sedimentation resulted in the Marianna Limestone Formation being deposited as a deeper-water carbonate shelf across east-central Mississippi and southwest Alabama. In western Mississippi, these strata were less affected by the early Oligocene transgression, as indicated by higher abundances of shallow-water species Based on discriminant analysis, the Mint Spring and Marianna formations 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 (Haq et al., 1987), and based on relatively constant species abundances throughout the extent of both formations, they appear to have been deposited as part of a single continuous transgression.