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