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An extensive data set consisting of vibracores, seismic reflection profiles, surface sediment sampling, bathymetry, and historical maps and bathymetric charts strongly suggests that the two major shoals on the western Louisiana shelf, Ship Shoal and Trinity Shoal, originated through the in-place drowning of earlier barrier-island systems. The last barrier remnants were still visible in the early 1800s.
The island systems originated as delta-flank barriers during the abandonment of Holocene lobes of the Mississippi delta. The islands probably developed through rapid shoreface retreat and transgression of deltaic distributary-mouth bar sands along the central part of the abandoned "headland." Flanking these headlands, regressive barrier spits and islands developed in response to a high rate of sediment supply from the eroding headland. These barriers prograded into the progressively deeper water of old interdistributary bays. Rapid, local, relative sea level rise, due to delta lobe subsidence, prevented the destruction of these flanking barriers by the erosional effects of the retreating shoreface. Consequently, the barriers drowned in place and the shoreline was "instantaneously" dis laced approximately 15-20 km (10-12 mi) landward.
Ship Shoal and Trinity Shoal provide the only convincing case of recent barrier island in-place drowning along the United States coastline. An additional, possible example may exist in the western extension of the Maguire Island chain off Prudhoe Bay, Alaska.
Barriers subject to in-place drowning have the potential of preserving thick reservoir sands in the geologic record. These Louisiana shelf shoals, in particular, may be rapidly buried by the prodelta muds of a new phase of delta progradation. In contrast, barrier islands along coastlines with a slower relative rate of sea level rise, such as the United States east coast, are generally completely removed by the eroding shoreface. Their sand is transported seaward into an inner-shelf sand sheet. This sand sheet may subsequently be molded into linear sand ridges by contemporary storm-generated shelf currents. Such ridge sands are stratigraphically and sedimentologically distinct from the drowned barriers.
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