Evolution of Atchafalaya lacustrine deltas, south-central Louisiana

Abstract

The entrenched Pleistocene/Holocene Mississippi alluvial valley was cut during the last low sea-level stand, and alluviation of the entrenched valley occurred during the Holocene sea-level rise. Regional Gulf Coast structures (salt domes and faults) have strongly influenced the morphology of the valley. Valley paleotopography has controlled depositional patterns and locations of major river courses, as well as partially isolated large interdistributary basins within the alluvial/deltaic plain. Interdistributary basin formation is further enhanced by distributary channel progradation, switching, and abandonment, in conjunction with compactional subsidence of the alluvial plain surface. Geomorphology and sediment cores from the Atchafalaya Basin, Louisiana, revealed that this basin has alternately accumulated fine-grained organic-rich sediments and terrigenous clastic-dominated meander-belt and lacustrine deposits. Basin subsidence and the presence of an active distributary channel in the basin are major determinants of the lithology of the sedimentary-fill. Prograding lacustrine deltas of the Atchafalaya River are the primary mechanism for filling the Atchafalaya Basin. These upward-coarsening deltas are relatively thin (3 m) but are areally extensive (tens to hundreds of kilometers), and because they occur so frequently during the infilling process, they account for a significant volume of the basin sequence. The Atchafalaya Basin sequence documents lacustrine deltaic episodes dating from the early Holocene (Maringouin and Teche deltas) to the present (Atchafalaya delta). Continued development of the Atchafalaya River will rework portions of the backswamp and lacustrine deposits into meander-belt deposits. Upon complete filling of the Atchafalaya Basin, sediment will mostly bypass the basin and become deposited in the marine delta in Atchafalaya Bay.