Abstract: Carbonate Cycles in Upper Cretaceous (Campanian) Demopolis Chalk, West-Central Alabama

WARREN, JEFFREY D.
Phillips Petroleum Co., Houston, TX;
CHARLES E. SAVRDA
Dept of Geology, Auburn University, AL

The lower unnamed member of the Upper Cretaceous Demopolis Chalk is dominated by rhythmically-bedded chalks, marly chalks, and marls. Detailed carbonate analyses of two cores (MG, 42.76 m thick; and AP, 85.64 m thick) from west-central Alabama were used to generate high-resolution time series that express carbonate rhythmicity at decimeter, meter, and decameter scales. Palynofacies analyses of selected samples from core MC indicate that the Demopolis is uniformly characterized by marine-dominated organic assemblages with admixtures of subordinate terrestrial components (Type II kerogen). Non-parametric tests reveal no statistically significant differences in organic assemblages between carbonate-rich and carbonate-poor interbeds. Results of pyrolysis analyses of decalcifield samples confirm the presence of a Type II kerogen and indicate that organics are thermally immature. Whatever the paleoceanographic mechanisms responsible for depositional rhythms, they likely were controlled by climatic changes mediated by quasi-periodic fluctuations in the Earth's orbit (i.e., Milankovitch cycles). Stratofabric and spectral analyses of the carbonate time series in cores MC and AP, and grayscale time series, core MC only, indicate a control by axial precession, orbital eccentricity, and axial obliquity. Estimated cycle period indicate that net-sedimentation rates for Demopolis were on the order of 3.1 to 4.6 cm/ka.

AAPG Search and Discovery Article #90932©1998 GCAGS/GCS-SEPM Meeting, Corpus Christi, Texas