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Depositional Environment of Limestones and Intercalated Marls, Arcola Interval (Campanian) of Mooreville Chalk, Upper Cretaceous, Central Alabama

Stephen P. Castleman, D. T. King

The Upper Cretaceous Arcola interval (Campanian) of the Mooreville Chalk, central Alabama Gulf Coastal Plain consists of 5 to 7 thin limestone beds (rather than the 2 and 3 beds reported previously), with intervening marl units up to 1.5 m thick. The limestones are algal-calcispheric packstones, whereas the intercalated marls are composed primarily of clay, mica, quartz, and a pelagic microfauna that provides a significant carbonate component. The limestones are hard Thalassinoides-bearing beds up to 40 cm thick, which can be traced laterally into limestone rubble zones. The rubble zones contain blocks 10-18 cm across that show conchoidal fracturing. Evidence of comminution means the limestone beds formed well above storm wave base and supports the hypotheses that the lim stones were cemented on the sea floor and owe their lateral continuity in part to syndepositional brecciation and scattering of debris.

The contact between the overlying marl unit (OMU) and the limestone is sharp, whereas gradational contacts exist between the underlying marl unit (UMU) and each limestone bed. Analysis of bulk composition using x-ray diffraction and thermogravimetric analysis shows that the UMUs contain, on the average, 69.8% of calcite and 13.3% of quartz and mica. These numbers differ significantly from the OMUs, which average 50.3% of calcite and 23.9% of quartz and mica. The limestone beds average 86.9% of calcite and 9.3% of quartz and mica. The increase of quartz and mica and the decrease in percentage of calcite in the OMUs result from increased terrigenous influx. These increases may coincide with regressive pulses in sea level.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.