St. Petersburg Coastal and Marine Science Center
The modern Mississippi River and delta overlie a deep incised valley that was excavated during the last Pleistocene fall and lowstand in sea level. The valley was subsequently backfilled when sea level rose during the late Pleistocene and Holocene. The extant delta plain was constructed by sediment transported through a series of distributary channels that began prograding about 7,000 years ago and deposited sediment in overlapping delta lobes. Thick aggradational peats that accumulated in interdistributary areas underlie much of the emergent delta-plain wetlands, and the Holocene sedimentary section within the incised Pleistocene alluvial valley may be more than 60 meters thick. In general, the direction of sediment supply was perpendicular or at a high angle to the gulf shoreline. This depositional style left remnant active and abandoned distributary channels and promoted the development of lakes (areas of incomplete filling) and a network of tidal channels in the wetlands.
The 10 delta-plain study areas encompassed several different physiographic and geologic settings, including: (1) an upper delta-plain levee flank of the Mississippi River (Ironton), (2) a pre-existing upper delta-plain interior channel (Bayou Perot), (3) four upper delta-plain interdistributary areas (Bully Camp, Pointe au Chien, DeLarge, and Madison Bay), (4) three lower delta-plain interdistributary areas (Bay St. Elaine, Leeville, and Fourchon), and (5) a lower delta-plain beach-ridge margin area (Caminada).