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The Subsidence and Wetland Loss Related to Fluid Energy Production, Gulf Coast Basin project included 10 delta-plain study areas 6 western chenier-plain study areas.
This reasearch is part of the Subsidence and Wetland Loss Related to Fluid Energy Production, Gulf Coast Basin project.
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).
The chenier plain consists of a thin wedge of Holocene sediments overlying stiff over-compacted Pleistocene sediments that were subaerially exposed during the late Pleistocene sea-level lowstand. Sabine and Calcasieu Lakes are unfilled remnants of incised valleys excavated by the Sabine-Neches and Calcasieu Rivers, respectively, during the same lowstand. Beginning about 3,000 years ago, the chenier plain was constructed by primarily alongshore processes resulting in the progradation of broad mudflats capped by wetland vegetation with intervening narrow, sandy beach ridges (cheniers). Below the chenier-plain surface, muddy sediments above the Pleistocene-Holocene unconformity range in thickness from less than a meter to about 6 meters, with thickness generally increasing toward the Gulf of Mexico. In general, the direction of sediment supply for the chenier plain was parallel to the gulf shoreline because sediment transport by coastal-plain rivers was trapped in Sabine and Calcasieu Lakes. As a result, only a few tidal channels formed within the chenier-plain wetlands.
The 6 western chenier-plain study areas were located in Sabine National Wildlife Refuge, which encompasses about 500 km2 of coastal wetlands between Sabine and Calcasieu Lakes in the western chenier plain. The refuge occupies a broad, shore-parallel, topographically low area that formed between the topographically higher beach ridges along the gulf shoreline to the south and Pleistocene upland areas to the north.