HOLOCENE GEOLOGY, SEDIMENTARY FRAMEWORK AND SHORELINE CHANGE OF THE LOUISIANA DELTA PLAIN COAST AND INNER SHELF

The Mississippi River delta plain represents one of America’s most important coastal ecosystems in terms of natural resources, human infrastructure and cultural heritage. At the same time, it has the highest rates of coastal erosion and wetland loss in the Nation due to natural and man-made causes. Regional-scale restoration plans are being developed by federal and state agencies for the delta plain that have the objectives of maintaining the barrier islands, reducing wetland loss, and enhancing natural processes. Much of the planning and decision making is driven by models; critical to model reliability is scientific understanding of the geologic history and processes of the region, including the rivers, wetlands, coast, and inner shelf.

A variety of recent geophysical studies of Late Quaternary sedimentary framework and coastal processes by scientists at LSU, USGS, and UNO demonstrate that the Louisiana delta plain is the product of a complex history of cyclic delta switching by the Mississippi River and its distributaries over the past ~7k yrs resulting in laterally migrating depocenters. The interactions among riverine, coastal, inner shelf, and storm processes have been superimposed on the Holocene transgression resulting in distinctive landforms and sedimentary sequences. Four shelf-phase delta complexes have been identified using seismic reflection records and vibracores;delta complexes are bounded by transgressive surfaces. Following each cycle of deposition, the delta complexes undergo regional subsidence and marine reworking that results in transgressive barrier systems. The distal end of each of the abandoned delta complexes is marked by marine sand bodies representing drowned barriers. These shoals offer the highest quality sand for beach nourishment.

Comparison of historical maps since 1855 and recent aerial photography show the Louisiana coast undergoing net erosion at highly variable rates. Rates have increased significantly during the past several decades. Earlier determined statewide average shoreline erosion rates of 6.6m/yr have increased to 10.3m/yr in collective response to storms, rapid subsidence, and pervasive man-made alterations. The sustainability of coastal Louisiana's natural resources and human infrastructure depends on the successful restoration of natural geologic processes.