ABSTRACT

The Mississippi River has established six delta complexes in the last 9000 years. Sediments are currently supplied only to two of these complexes, neither of which is more than 500 years old. Deltaic sedimentation during most of the Holocene has occurred in shoal-water deltas unlike the modern Balize lobe which is located at the shelf break. Mississippi delta sedimentation is cyclic and consists of a regressive and a transgressive phase. Each delta complex first experiences progradation until over-extension leads to abandonment and reworking. Mississippi deltas undergoing transgression are dominated by subsidence and marine basin processes leading to the formation of sandy barriers and lagoons.

Each of the six delta complexes followed a common pattern of development after-abandonment. Current delta geomorphology reflects the variation in age of each delta complex and can be summarized in a three stage model beginning with Stage 1, Erosional Headland and Flanking Barriers. Here, distributary sands are reworked by the retreating shoreface and dispersed laterally by longshore transport into flanking barriers which enclose interdistributary bays. In Stage 2, Transgressive Barrier Island Arc, submergence of the erosional headland generates an intradeltaic lagoon which separates the barrier island arc from the retreating mainland. The model ends with Stage 3, Inner Shelf Shoals, where the retreating barrier island arc is unable to keep pace with relative sea level rise or the more rapidly retreating mainland. This results in submergence of the barrier island arc, which continues to be reworked as a sandy shoal on the inner continental shelf. This model of delta evolution illustrates mechanisms for generation of both barrier islands (in Stages 1 and 2) and continental shelf sand bodies (in Stage 3).