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Abstract: The Eastern Chenier Plain: An Update on Downdrift Coastal Progradation Associated with the Building of a New Holocene Delta Lobe in the Mississippi Delta Complex

Harry H. Roberts, O. K. Huh

Diversion of Mississippi River water and sediment into Atchafalaya Basin via the Atchafalaya River has been taking place for at least five centuries. Prior to the middle of this century, these sediments were deposited as lacustrine deltas and in swamps. By the early 1950s, the basin-filling process was nearly complete so that significant suspended sediments reached the coast to start a new delta-building event in Atchafalaya Bay and initiate progradation of the eastern chenier plain where coastal erosion (3-7m/yr.) had been active for many centuries. Fine-grained sediments flushed through Atchafalaya, Cote Blanche, and Vermilion Bays to the coastal waters of the chenier plain have resulted in mudflats that prograded the eastern 30 km of this coast. Progradation rates duri g the late 1980s and early 1990s averaged 70m/yr in the most actively accreting coastal sectors. Studies confirmed that both winter cold front passages (20-30/year) and occasional tropical storms result in water level set-up along the coast and shore normal transport of fluid mud from the nearshore shelf onto the shoreface where it is stranded. In the winter cold front case, water level set-down plus dry air and cloud-free post-frontal conditions promote rapid water loss in stranded muds resulting in dessication. Sheets of fluid mud composed of clay-sized particles are mudcracked into resistant polygonal clasts (centimeters in diameters) which armor the coast against erosion. Cores through new accretion units along the coast reflect a vertical accretion related to repeated deposition of -10 cm thick units of fluid mud over a thin layer of silt to sand. Each one of these sedimentary couplets is interpreted as a depositional response to a cold front passage or tropical storm. Time-series remote sensing data suggest that mudflat progradation is shifting westward with time.

AAPG Search and Discovery Article #90955©1995 GCAGS 45th Annual Meeting and Gulf Section SEPM, Baton Rouge, Louisiana