PRELIMINARY OBSERVATIONS OF CARBONATE DEPOSITIONAL RHYTHMS IN PART OF THE UPPER CRETACEOUS PRAIRIE BLUFF CHALK, MOSCOW LANDING, SUMTER COUNTY, WESTERN ALABAMA

The Upper Cretaceous (Maastrichtian) Prairie Bluff Chalk of the Selma Group, eastern Gulf coastal plain, is characterized by alternating sandy chalks and marlstones. The upper part of this unit exposed at Moscow Landing on the west bank of the Tombigbee River, Sumter County, Alabama, is being studied in attempt to characterize carbonate cyclicity and interpret depositional and paleoceanographic mechanisms responsible. The ~8.5-m-thick studied section is characterized by thoroughly bioturbated, variably sandy and glauconitic, light gray chalks and light to medium gray marlstones (CaCO₃ contents range from 56-87%) with ichnofabrics dominated by Thalassinoides, Cylindrichnus, and Chondrites (Cruziana ichnofacies). The chalk-marlstone monotony is broken near the top of the section by a thin but distinct fossil lag bed characterized by bored and encrusted Exyogyra and phosphatic internal molds of bivalves, gastropods, and cephalopods and previously interpreted as a condensed section. Otherwise, thorough bioturbation and poor color contrast between beds typically inhibits clear delineation of individual beds in the field. However, duplicate carbonate analyses of ~90 closely-spaced (~10 cm) samples reveal carbonate cycles of variable amplitude and period. Data indicate the presence of ~22 decimeter-scale, high-frequency, and generally low-amplitude carbonate cycles, bundles of which define 4 or 5 meter-scale, lower-frequency and generally higher-amplitude cycles. Character of carbonate curves, and comparison with that of the precession index, suggest that carbonate depositional cyclicity was mediated by the ~20-ka axial precession and ~100-ka orbital eccentricity cycles. While ongoing petrographic and ichnologic analyses of the study section will help better access the paleoenvironmental or paleoceanographic changes (e.g., clastic dilution cycles, scour cycles, etc.) that occurred in response to apparent Milankovitch orbital forcing, preliminary observations suggest cyclic variations in the flux of clastic sediments by storms (dilution cycles) and/or in bottom-current energy (scour cycles).