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A high-resolution seismic-reflection survey was conducted in an area east of the Mississippi River delta along the Louisiana-Mississippi continental shelf and slope. Seismic data showed that the topography and subsurface sediment characteristics of the shelf and upper slope are the result of depositional sequences. These depositional sequences are delta out-building over transgressive sediments with intervening periods of erosion during low sea level stands. On the shelf, little evidence of structural deformation caused by faults, diapirs, and shallow gas is present. In contrast, the upper slope has occasional diapirs with associated faulting. The upper slope also has a few faults and scarps resulting from down-to-basin sediment movement. Surface sediments over the entire area relate to several depositional periods.
Minisparker (400-joule) profiles reveal at least 7 regressive and transgressive sequences. The oldest recognizable surface is an erosional surface. This erosional surface is overlain by transgressive sediments that downlap onto it near mid-shelf. The late Wisconsin is represented by 3 regressive stages. These stages are the result of fluctuations in sea level. The first two stages are characterized by deposition on the shelf, and the subsequent erosion of these deposits by stream channeling as sea level lowered. This fluvial system constructed a relatively large delta which prograded beyond the shelf-break. The delta construction is a complex set of prograding and onlapping sequences. The third stage of this regressive sequence occurred during the farthest retreat of sea level. This s age was the deposition of sediments on the upper slope, which onlap the previously prograded delta. The regression was followed by a rise in sea level which deposited transgressive sediments on the inner shelf. A sea level fluctuation during this sequence exposed part of the shelf. This paleosurface is possibly the Pleistocene-Holocene boundary. The most recent sequence was the deposition of the St. Bernard delta on the inner shelf.
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