The Upper Cretaceous (Santonian through Maastrichtian) stratigraphic section of the eastern Gulf Coastal Plain includes 270 m of nonmarine, marginal marine, and marine siliciclastic and carbonate sediments. The section to the northeast (southern Tennessee and northern Mississippi) and to the east (southeast Alabama and southwest Georgia) consists of a basin margin succession of siliciclastic shoreline to coastal shelf strata. Depositional systems in these areas include fluvio-deltaic, shoreline, and marginal marine paleoenvironments. The section in eastern Mississippi and southwest and south-central Alabama consists of mixed siliciclastic and carbonate shelf to basin strata, reflecting deposition in marine shelf environments. Microfossil assemblages indicate water depths of around 35 m for these marine shelf environments. The marine shelf calcareous siliciclastic units are rich in macrofossils, whereas these fossils are significantly less abundant in the pure limestones and chalks.

Sediment accumulation occurred along a passive continental margin with deposition being affected by siliciclastic sedimentation rates, carbonate productivity, subsidence, eustasy, and local paleoenvironmental conditions. During times of high siliciclastic sedimentation, carbonate productivity and accumulation were minimal, whereas during times of low siliciclastic sedimentation, carbonate productivity and accumulation were high. Limestone and/or chalk deposition were particularly prevalent during times of marine maximum flooding or optimal marine conditions. Carbonate particles primarily consist of marine microfossils. The siliciclastic-carbonate cycles observed in these strata reflect times of high/low siliciclastic sedimentation and of high/low carbonate productivity and accumulation. These cycles are interpreted to be a result of a combination of carbonate productivity and dilution due to siliciclastic sediment influx.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah