Holocene Sand Shoals Offshore of Mississippi River Delta Plain, Louisiana

Shea Penland, John R. Suter, Randolph A. Mcbride, S. Jeffress Williams, Jack L. Kindinger, Ron Boyd

Offshore of the Mississippi River delta plain lies a series of Holocene sand shoals marking the position of ancient submerged shorelines. These ancient shorelines represent stillstand positions during which the Holocene transgression drove sea level across the former lowstand sub-aerial erosion surface of the Mississippi River delta plain. Short periods of rapid sea level rise led to the transgressive submergence of these sandy shorelines. Two shoreline trends can be recognized at the - 10-m and - 20-m isobaths on the continental shelf.

The -10-m shoreline trend is represented by Trinity Shoal and Ship Shoal, which are associated with the late Holocene Mississippi River delta plain, 3,000-7,000 Ma. Trinity Shoal is derived from the Bayou Cypremont and Bayou Sale delta complex. This sand shoal is located 20 km offshore of Marsh Island and is 35 km long, 8 km wide, and 5-6 m thick. The facies architecture indicates Trinity Shoal is a submerged barrier system in the initial stages of marine reworking. To the east lies Ship Shoal, which is associated with the Maringouin-Teche delta complex. This shoal is located 20 km offshore of the Isles Dernieres and is 50 km long, 8-10 km wide, and 4-6 m thick. The facies architecture indicates Ship Shoal is a marine sand body sourced from a reworked submerged barrier island complex.

The -20-m shoreline trend is represented by the Outer Shoal and St. Bernard Shoals which are associated with an earlier Mississippi River delta plain, 7,800 Ma. The Outer Shoal is a low-relief sand body that lies seaward of Ship Shoal immediately west of the Mississippi Canyon. The eastern continuation of the - 20-m shoreline trend is the St. Bernard Shoals, which lie 15 km offshore of Chandeleur Islands. In contrast to the other shoal systems, the St. Bernard Shoals form a shore-parallel zone of more than seven smaller sand shoals which in many respects are similar to a shore-oblique sand ridge field.

Collectively, these sand shoals represent a tremendous source of aggregate for shoreline erosion control and heavy minerals mining. Scientifically, these shoals provide insight into the processes that control coastal erosion and shelf sand development under the condition of sea level rise.

AAPG Search and Discovery Article #91029©1989 AAPG GCAGS and GC Section of SEPM Meeting, October 25-27, 1989, Corpus Christi, Texas.