ICHNOSEDIMENTOLOGIC COMPARISON OF CARBONATE DEPOSITIONAL RHYTHMS IN CRETACEOUS SELMA GROUP CHALKS, EASTERN GULF COASTAL PLAIN

Like most other Cretaceous shelf-sea chalk-marl deposits, units of the Santonian-Maastrichtian Selma Group of the eastern Gulf coastal plain—i.e., in ascending order, Mooreville, Demopolis, and Prairie Bluff chalks—are characterized by decimeter-scale carbonate rhythms that may be linked to Milankovitch orbital cycles, particularly axial precession. However, variations in average carbonate contents, sediment texture and composition, and ichnofossil assemblages among these units reflect differences in depositional setting and paleoenvironmental responses to orbitally-induced climate and/or paleoceanographic change. The Santonian-Campanian Mooreville Chalk, excluding the upper Arcola Limestone Member, is characterized by alternating clay-rich marly chalks and marls within which ichnofabrics are comparatively poorly expressed. Chalkier intervals contain low-diversity trace fossil assemblages representing the Zoophycos ichnofacies, whereas more clastic-rich intervals are characterized by smaller, compressed burrows or burrow mottles reflecting soupier and more poorly oxygenated substrates. Mooreville depositional rhythms likely reflect combined clastic dilution-redox cycles operating in a middle-shelf setting. The Campanian Demopolis Chalk, excluding its upper Bluffport Marl Member, is characterized by alternating relatively pure chalks, chalky marls, and/or marls. Ichnofabrics therein are well-expressed throughout, particularly at bed transitions, and preserve moderately diverse trace fossil assemblages representative of the Zoophycos ichnofacies. Demopolis depositional rhythms likely reflect clastic dilution cycles in a well-oxygenated outer-shelf setting. The Prairie Bluff Chalk is characterized by alternating variably glauconitic, phosphatic, and quartzose sandy chalks and marls, both of which contain Cruziana ichnofacies trace fossil assemblages. Prairie Bluff carbonate rhythms are attributed to combined clastic dilution-scour cycles operating in a shallower inner-shelf setting. Variability among Selma Group units likely reflects relative sea-level changes and their impacts on absolute water depths and rates and modes of clastic sediment influx in the region.