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ABSTRACT

A high-resolution single-channel seismic survey of the Mississippi-Alabama continental shelf and upper slope revealed relatively thin upper Pleistocene and Holocene deposits. The stratigraphy was defined by identifying depositional and erosional episodes recognized on seismic profiles by geomorphic patterns, high-angle clinoform progradational deposits, buried stream entrenchments, planar conformities, and erosional unconformities.

The regional bathymetry shows a gently sloping shelf with a relatively steeper continental slope. Local tectonics have had minimal effect on the shelf, with surface and subsurface faults exhibiting a random pattern. Diapirs on the shelf break and upper slope, in contrast, have created several topographic highs with associated faults.

The upper Pleistocene shelf and slope deposits are examples of two depositional environments, including four differentiated areas: inner shelf, middle shelf, outer shelf, and upper slope. The presence of sandy sediments in the shelf deposits is inferred to be the result of reworking by marine processes.

A five-stage model of sedimentation and shelf evolution is presented. The earliest stage (stage 1) is the early Wisconsinan transgression across a seaward-sloping erosional horizon. During the transgression, this paleosurface was overlain by a thin (< 10 m) layer of sediments (stage 2). The third stage was the late Wisconsinan regression, during which fluvial channels eroded the shelf, forming a thick (> 90 m) shelf margin delta. The Holocene transgression deposited a thin layer of sediment over parts of the shelf (stage 4). The last depositional episode occurred during the Holocene highstand and was the progradation of the St. Bernard delta (stage 5).

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