Late Quaternary Stratigraphic Evolution of the Alabama and West Florida Outer Continental Shelf

Abstract Approximately 3000 km of single-channel seismic data from the Alabama and west Florida outer continental shelf and upper slope were analyzed to characterize the late Quaternary regional stratigraphic framework of this ramp margin. Seismic analysis shows that thick delta lobes are located at several near-surface stratigraphic levels on the outer continental shelf. On the basis of the depocenter locations, we infer that sediments delivered to the deltaic wedges on the Alabama shelf were derived from the confluence of the ancestral Mobile–Tombigbee rivers. The smaller deltaic wedges on the west Florida outer continental shelf probably received sediment from the confluence of the Perdido, Escambia, Blackwater, and Yellow rivers. Deep, incised, cross-shelf fluvial valleys do not exist offshore of west Florida. There, outer-shelf depocenters probably received sediment by broad and shallow braided rivers. This situation is in stark contrast to the Texas, Louisiana, and Mississippi continental shelves, where large incised fluvial valleys occupied the shelf during eustatic lowstands. Subsurface mapping shows that the sinuous trend of the Alabama and west Florida shelf edge is a result of the primary delta morphology. We interpret the shelf-margin deltas as lowstand systems, and on the basis of our seismic correlation to chronologic control at a drill site in Main Pass lease area 303, we conclude that the youngest shelf-margin deltas were deposited during the last glacial maximum. The lack of slope canyons indicates that bypass was minimal in this area during the last glacial maximum. In a basinward direction, the clinoform toes and/or aggrading bottomsets of the lowstand deltaic units interfinger with thin but regionally extensive slope wedges. The seismic evidence of extensive erosion of the shelf-margin deltas suggests that slope wedges in this area may correspond to deposition during the last transgression and present highstand. The youngest seismic-stratigraphic unit is shelf perched and has regional extent and great thickness. We surmise that this shelf-perched unit represents a drowned coastal-plain system that was well established early during the last sea-level transgression.