A GEOMORPHIC PROCESS-RESPONSE MODEL FOR THE MISSISSIPPI RIVER CHENIER PLAIN, USA

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, <30 km wide, and composed of mud deposits capped by marsh interspersed with thin sand- and shell-rich ridges (“cheniers”) that are <4 m in elevation.

Most Chenier-Plain ridges represent open-Gulf paleoshorelines. Past shoreline morphodynamics allow ridges to be classified as transgressive, regressive, or laterally accreted. As such, a geomorphic hierarchy of landforms is devised relative to dominant coastal process. To understand long-term evolution of the Chenier Plain, modern tidal-inlet processes operating at Sabine, Calcasieu, and Mermentau Passes were 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) 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 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.