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ABSTRACT: Genesis of Upper Cretaceous Marl-Limestone Bedding, Alabama

David T. King, Jr.

In Alabama, marl-limestone bedding couplets and correlative coeval prograding elastic barrier-shoreline facies occur in the upper 20 m of a high-stand systems tract within the lower Campanian Mooreville-Demopolis depositional sequence. The Mooreville-Demopolis depositional sequence is terminated by a type-2 unconformity biostratigraphically dated at about 80 million years before present. Based on estimates using long-term sedimentation rate, the average periodicity of the mad-limestone couplets is approximately 100,000 years. Taken at face value, the apparent 100,000-year periodicity would suggest a climate-forced (or Milankovitch) origin of the marl-limestone couplets owing to cycles of productivity and dilution. However, significant variations in marl-limestone couplet hickness (ranging from less than 1 m to several meters) both within and between couplets suggests that average periodicity is not a meaningful figure with respect to these couplets. Further, climate-forcing is not favored because coarse clastic material fines upward in the marl beds and the petrography of the beds shows that the primary mode of biochemical deposition switched from pelagic nannofossils (in the marl) to benthic algal calcispheres (in the limestone). In order to adequately explain the petrographic and thickness observations, changing relative sea level is suggested as a viable hypothesis. Tectonic effects related directly to early stages of the Laramide orogeny (about 80 million years before present) may have affected relative sea level as far east as the Gulf Coastal Plain thus producing the marl-limestone sequences. Marl-limestone sequences may reflect either short-term buildup and relaxation of intraplate stresses or short-term eustatic changes that resulted from slight variations in Farallon spreading rates.

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