Seismic stratigraphy of the Mississippi-Alabama shelf and upper continental slope

Abstract

The Mississippi-Alabama shelf and upper continental slope contain relatively thin Upper Pleistocene and Holocene deposits. Five stages of shelf evolution can be identified from the early Wisconsinan to present. The stages were controlled by glacioeustatic or relative sea-level changes and are defined by the stratigraphic position of depositional and erosional episodes. The stratigraphy was identified on seismic profiles by means of geomorphic pattern, high-angle clinoform progradational deposits, buried stream entrenchments, planar conformities, and erosional unconformities. The oldest stage (stage 1) of evolution occurred during the early Wisconsinan lowstand; the subaerially exposed shelf was eroded to a smooth seaward-sloping surface. This paleosurface is overlain by a thin (< 10 m) drape of transgressive deposits (stage 2). Stage 3 occurred in three phases as the late Wisconsinan sea retreated: (1) fluvial channel systems eroded across the shelf, (2) deposited a thick (90 m) shelf-margin delta, and (3) contemporaneously deposited sediments on the upper slope. Stage 4 included the rapid Holocene sea-level rise that deposited a relatively thin transgressive facies over parts of the shelf. The last major depositional episode (stage 5) was the progradation of the St. Bernard delta over the northwestern and central parts of the area. A depositional hiatus has occurred since the St. Bernard progradation. These Upper Quaternary shelf and slope deposits provide models for analogous deposits in the geologic record. Primarily, they are examples of cyclic sedimentation caused by changes in sea level and may be useful in describing short-term, sandy depositional episodes in prograding shelf and slope sequences.