ABSTRACT

Historically, the Porters Creek Formation in southern Alabama typically has been described as several hundred feet of black massive clay. Recent detailed field investigations indicate that the Porters Creek Formation consists of numerous diverse lithologies and exhibits complex lithofacies relationships. In southwestern Alabama, the basal and lower portions of the formation include marls, limestones and calcareous clays. The middle portion of this unit primarily is comprised of black massive clays with minor marl and limestone, and the upper portion of the formation (Matthews Landing Marl Member) in this area consists of glauconitic sands and marls. In south-central Alabama, the lower portion of the Porters Creek Formation includes interbedded limestones, marls, and calcareous clays. The middle portion of the unit is composed of marl and calcareous silty clay, and the upper portion (Matthews Landing Marl Member, in part) consists of cross-bedded sands and glauconitic marls.

The lower portion of the Porters Creek Formation in southern Alabama is assigned to the Danian planktonic foraminiferal Subbotina trinidadensis Interval Zone. The middle portion of the Porters Creek Formation rests within the Danian Morozovella uncinata Interval Zone. The upper portion (including the Matthews Landing Marl Member) is contained within the Selandian Morozovella angulata Interval Zone.

The lateral lithofacies changes in the Porters Creek Formation from west to east in southern Alabama suggest that depositional conditions in southwestern Alabama during the Danian were influenced by marginal marine sedimentation, while depositional conditions in south-central Alabama were dominated by the presence of a marine carbonate shelf. Initially, Porters Creek deposition was a continuation of Danian carbonate sedimentation throughout southern Alabama. Carbonate sedimentation in south-central Alabama prevailed throughout much of the Danian, while in southwestern Alabama carbonate deposition ceased in the early Danian as a result of an influx of terrigenous elastic sediments from the northwest. Marginal marine sedimentation did not encroach on the south-central Alabama area until the late Danian. The early Selandian marked a return to marine shelf elastic terrigenous deposition throughout southern Alabama.

In southwestern Alabama, the basal marls, limestones and calcareous clays of the Porters Creek are interpreted as progradational, regressive highstand deposits of a Type 1 depositional sequence. The lowstand shelf, transgressive, and condensed section deposits of this sequence are assigned to the underlying Clayton Formation. This sequence is disconformably overlain by glauconitic sandy marl and calcareous clays of the lower portion of the formation, which represents a Type 2 depositional sequence. In south-central Alabama, this depositional sequence is represented by the McBryde Limestone Member of the Clayton Formation (transgressive and condensed section deposits), and the lower interbedded limestones, marls, and calcareous clays of the Porters Creek (progradational, regressive highstand deposits). The sequence is disconformably overlain by marl and massive clays of the middle portion of the Porters Creek, which comprise a Type 2 depositional sequence. Cross-bedded sands, which are present locally, and glauconitic marl and sand (Matthews Landing Marl Member) comprise shallow-water shelf margin and transgressive to condensed section deposits of an overlying Type 2 depositional sequence that includes Porters Creek Formation and Naheola Formation strata.