LOWSTAND SEDIMENT PRESERVATION AND THE ROLE OF SYN-DEPOSITIONAL NORMAL FAULTS, UPPER PALEOGENE, ALABAMA COASTAL PLAIN

Lowstand deposits in the Alabama Coastal Plain commonly are thought to consist of either sandy incised valley fill or shelf margin marine and/or deltaic wedges, following the classic Exxon models. Consequently, many Eocene-Oligocene third-order sequences in Alabama are interpreted to lack lowstand shelf deposits, with workers depicting the lower surface as a sequence boundary amalgamated with a transgressive surface. This interpretation implies wholesale sediment bypass of the lowstand shelf due to a lack of accommodation space.

Core and wireline log analyses of Eocene-Oligocene sequences, coupled with outcrop observations and thin section analyses, strongly suggest that the shoreline rarely prograded basinward of the present outcrop belt in Alabama during this time interval. Thus for most sequences, accommodation space was available during all parts of the sea level curve. In addition, down-to-the-southwest normal faults created syn-depositional lows in which lowstand sediments were preserved from subsequent transgressive reworking. These lowstand sediments appear coarse-grained only within the half grabens, fining and thinning considerably toward the highs. Muddy and/or calcareous, easily reworked lowstand deposits can be reconciled within a generally sediment-starved system in which sequence boundaries exist as correlative conformities; disconformable erosion surfaces comprise the stratigraphically higher transgressive surfaces of ravinement. Once such a system is admitted, some previously interpreted highstand deposits can be better placed within late falling stage or lowstand systems tracts. This explanation has the potential to assuage controversies in the literature concerning the placement of stratigraphically significant surfaces, such as the basal Vicksburg sequence boundary and the “middle” Oligocene unconformity.