In-place barrier drowning on Louisiana Continental Shelf

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. Ship Shoal and Trinity Shoal provide the only convincing case of recent barrier island in-place drowning along the US 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 US east coast, are generally completely removed bymore » 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.« less