HR: 16:30h
AN: H34B-03    [Abstracts]
TI: Coastal Evolution of the Mississippi River Chenier Plain: A Geomorphic Process-Response Model
AU: * McBride, R A
EM: rmcbride@gmu.edu
AF: Geology and Earth Science Program, Dept. of Environmental Science and Policy George Mason University, 4400 University Drive, Fairfax, VA 22030, United States
AU: Taylor, M J
EM: mtaylor7@du.edu
AF: Dept. of Geography, University of Denver, Boettcher West 113, 2050 E. Iliff Ave., Denver, CO 80208, United States
AU: Byrnes, M R
EM: mbyrnes@appliedcoastal.com
AF: Applied Coastal Research and Engineering, Inc., 766 Falmouth Road, Suite A-1, Mashpee, MA 02649, United States
AB: Using 28 topographic profiles, air-photo interpretation, and historical shoreline-change data, coastal processes were evaluated along the Mississippi River Chenier Plain to explain the occurrence, distribution, and geomorphic hierarchy of primary landforms. The Louisiana Chenier Plain, classified as a low-profile, microtidal, storm- dominated coast, is located west and downdrift of the Mississippi River deltaic plain. This late-Holocene, marginal-deltaic environment is 200 km long, less than 30 km wide, and composed of mud deposits capped by marsh interspersed with thin sand- and shell-rich ridges ("cheniers") that are less than 4 m in elevation. Most Chenier-Plain ridges represent open-Gulf paleoshorelines. Past shoreline morphodynamics allow ridges to be classified as transgressive (cheniers), regressive (beach ridges), or laterally accreted (spits). Geomorphic zones that contain two or more regressive, transgressive, or laterally accreted ridges are termed complexes. Consequently, we further refine the Chenier-Plain definition by Otvos and Price (1979, Marine Geology) and define Chenier Plain as containing at least two or more chenier complexes. As such, a geomorphic hierarchy of landforms is devised relative to dominant coastal process. The Chenier Plain is defined as a first-order feature (5000 km2) composed of three second-order features (30 to 300 km2): chenier complex, beach ridge complex, and spit complex. Individual ridges of each complex type were further separated into third-order features: chenier, beach ridge, and spit. To understand long-term evolution of the Chenier Plain, modern tidal-inlet processes operating at Sabine, Calcasieu, and Mermentau river entrances were also examined relative to the inlet-stability ratio. Prior to human modification and stabilization efforts, the Mermentau River entrance is classified as wave-dominated, Sabine Pass as tide-dominated, and Calcasieu Pass as tide-dominated to mixed. Hoyt (1969, American Association of Petroleum Geologists Bulletin) presented the first depositional model for chenier genesis and mudflat progradation. However, Hoyt's model oversimplifies Chenier-Plain evolution because it omits ridges created by non-transgressive processes. Thus, the geologic evolution of the Chenier Plain is more complicated than Mississippi River channel avulsions, and it involved not only chenier ridges (transgressive), but also beach ridges (regressive) and spits (lateral accreted). A six-stage geomorphic process-response model is presented to describe Chenier-Plain evolution primarily as a function of: 1) the balance between sediment supply and energy dissipation associated with Mississippi River channel avulsions, 2) local sediment reworking and lateral transport, 3) tidal-entrance dynamics and sediment trapping, and 4) possibly higher-than-present stands of Holocene sea level. Hence, the development of transgressive, regressive, and laterally-accreted ridges typically occurred contemporaneously along the same shoreline at different locations.
DE: 1641 Sea level change (1222, 1225, 4556)
DE: 1824 Geomorphology: general (1625)
DE: 1862 Sediment transport (4558)
DE: 3022 Marine sediments: processes and transport
DE: 4217 Coastal processes
SC: Hydrology [H]
MN: 2007 Fall Meeting