Inner-Shelf Shoal Formation Through Transgressive Barrier Submergence

Shea Penland, John R. Suter, Ron Boyd

Shore-parallel sand shoals are common depositional features on the continental shelves of North America and make up a significant component of the stratigraphic record generated by the Holocene transgression. Erosional shoreface retreat and in-place drowning of barrier shorelines are models that have been proposed to explain the generation of these deposits. Sand shoals within the retreat path of the Holocene Mississippi River delta plain 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 with a relief of 3-5 m and is migrating landward 10-15 m/year in surrounding water depths of 7-15 m. The hoal lies disconformably over regressive deposits of the Maringouin delta, transgressed 6,000 years ago. Reworked beach rock, Crassostrea sp. shell, and Rangia sp. shell, along with lagoonal mud, attest to the former existence of barrier shoreline environments. No in-situ barrier shoreline deposits were found within the shoal sand body. These data indicate Ship Shoal is a shelf sand body sourced from the erosion of a former barrier shoreline. Trinity Shoal is a landward migrating, shore parallel, lunate sand body associated with the abandoned Teche delta, transgressed about 3,500 years ago. Lying in 7-10 m of water, with an inner shelf relief of 2-4 m, Trinity Shoal is a 5-7 thick sand body lying disconformably over regressive Teche delta deposits. Interpreted high-resolution seismic pro iles identified the existence of tidal inlet channels within the shoal sand body, indicating Trinity Shoal is cored by in-situ deposits representing the platform of a former barrier shoreline.

The current transgressive barrier models of erosional shoreface retreat and in-place drowning do not adequately explain the morphology or stratigraphy of Mississippi delta inner-shelf shoals. Formation of these shoals is a three-step process: (1) barrier island transgression, (2) barrier island submergence, and (3) inner-shelf shoal reworking. This process operates through multiple mechanisms that cannot be attributed to one particular model or catastrophic event. We propose Trinity Shoal and Ship Shoal formed through the process of transgressive barrier submergence, which integrates the mechanisms of shoreface erosion and relative sea level rise within the concept of coastal submergence.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.