An investigation into sequence stratigraphic controversy in the central Gulf Coastal Plain (tertiary)

Outcrop studies of Eocene-Oligocene depositional sequences exposed in the Coastal Plain of Alabama and Mississippi disagree on the sea level history recorded across several stratigraphic intervals. The placement of depositional sequence boundaries (unconformities caused by sea level fall) and maximum flooding surfaces relative to the lithostratigraphy by various authors implies that relative sea level change was diachronous across the central Gulf Coast. In addition, the number of depositional sequences (i.e., the number of third-order sea level fluctuations) interpreted for these units commonly varies. These discrepancies are puzzling, given the short distances among study areas and the fact that most workers have based their interpretations on observations from the same outcrops. Regional subsurface studies of these generally marine units integrate several data sets to constrain the sea level history as recorded across three controversial stratigraphic intervals. Lithologic and chronostratigraphic data from two test cores located in southern Alabama provide facies information and age constraints. Thin sections from the cores were point-counted to obtain quantitative petrographic trends for detrital and authigenic sedimentary components; these trends define transgressive-regressive (T-R) cycles that record relative sea level change. Oxygen and carbon isotopic data from the cores test the hypothesis that chemical stratigraphic changes record eustatic sea level fluctuations. Finally, regional subsurface mapping of geologic units locally reveals stratigraphic truncation, which is interpreted to reflect erosion following a sea level fall. The revised sequence stratigraphic interpretations proposed herein result in part from the application of depositional models tailored to fit the central Gulf. Previous depositional models applied to the Alabama section do not account for variability in the basic parameters that control stratal architecture in depositional sequences, such as sediment flux and basin physiography. Due primarily to a low sediment supply, the Eocene-Oligocene section of the central Gulf Coastal Plain has little in common with traditional Exxonian "slug" models for deposition. Instead, a new model is proposed, whereby the majority of sediment progradation takes place in response to sea level fall. This model reinterprets some units previously referred to as highstand deposits as lowstand progradational/aggradational deposits, thus requiring a sequence boundary at their base