Shore-parallel sand shoals are common depositional features on the continental shelves of North America and are a significant component of the stratigraphic record generated by the Holocene transgression. Erosional shoreface retreat and in-place drowning of barrier shorelines have been incorporated in models to explain the deposition and evolution of these features. Sand shoals within the retreat path of the Holocene Mississippi River deltas were investigated using high-resolution seismic profiles and vibracores to determine their stratigraphic development and to test the models proposed for inner-shelf shoal formation.

Ship Shoal is a shore-parallel sand body 50 km long having a relief of 3-6 m. Historic records suggest that it is migrating landward 10-15 m/yr and is presently located 25 km from the shoreline in water depths of 7-15 m. The Ship Shoal sedimentary sequence is 4-6 m thick and coarsens upward through the shoal-base, shoal-front, and shoal-crest facies. The shoal lies disconformably over deposits of the Maringouin delta, which was transgressed approximately 4000 years BP. A laterally continuous unit of lagoonal muds at the base of the shoal sequence attests to the former existence of barrier shoreline environments. No in-situ barrier shoreline deposits were found within the shoal sand body.

Data indicate that Ship Shoal is a marine sand body sourced from the erosion of a former barrier shoreline through a process termed transgressive submergence, which integrates the mechanisms of shoreface erosion and relative sea level rise within the process of coastal submergence. The models of erosional shoreface retreat and in-place drowning do not adequately explain the morphology or stratigraphy of Mississippi Delta inner-shelf shoals. Ship Shoal can be used as a model to explain the occurrence of shelf sandstones observed tens of kilometers from the penecontemporaneous shoreline in interior seaways and foreland basins.