TRANSGRESSIVE DEPOSITIONAL SYSTEMS OF THE MISSISSIPPI DELTA PLAIN: A MODEL FOR BARRIER SHORELINE AND SHELF SAND DEVELOPMENT I

Abstract

Depositional sequences generated in the Mississippi River delta plain consist of a regressive and a transgressive component. The transgressive component has been considerably less studied but accounts for the majority of the surface area on the lower Mississippi River delta plain and up to 50 percent of the total sequence thickness in shallow-water deltas. The development and preservation of transgressivedepositional systems in abandoned delta complexes follows the process oftransgressive submergence in which the horizontal component ofreworking occurs during shoreface retreat, combined with a vertical component of submergence acting to preserve the sequence. The evolution of transgressive depositional systems in each of the abandoned Holocene Mississippi River delta complexes can be summarized in a three-stage model beginning with stage 1, an erosionalheadlandandflanking barriers. In this stage, regressive sand deposits contained within abandoned deltaic headlands are reworked by the eroding shoreface and dispersed longshore into contiguous flanking barriers enclosing restricted interdistributary bays. Submergence of the delta plain during relative sea-level rise generates an intradeltaie lagoon separating the former stage I sand body from the shoreline, forming stage 2, a transgressive bamer island arc. The landward-migrating barrier island arc is unable to keep pace with relative sea-level rise and the retreating mainland shoreline, resulting in submergence and the formation of stage 3, an inner-shelfshoaL Following submergence, the former barrier island arc sand body continues to be reworked into a marine sand body on the inner continental shelfduring stage 3. This sequence of coastal evolution provides direct evidence of barrier island formation, with each stage producing a distinctive stratigraphic signature. The current sea-level-rise models of shoreface retreat and in-place drowningdeveloped for the U.S. Atlantic continental shelfdo not adequately explain either the morphology or the stratigraphy of transgressive Mississippi River delta sand bodies. Current models of Mississippi deltaic stratigraphy emphasize the deep-water, artificially maintained Balize delta, which differs considerably from the shallow-water, shelf-phase delta complexes that are the primary depositional constituents of the Holocene Mississippi River delta plain.