A Geomorphic Process-Response Model for the Mississippi River Chenier Plain, USA: Did Higher-Than-Present Stands of Holocene Sea Level Play a Role?

Using LIDAR topographic profiles, air-photo interpretation, and historical shoreline-change data, coastal processes were evaluated along the Mississippi River Chenier Plain to explain the geomorphology of primary landforms. The Louisiana Chenier Plain is located west and downdrift of the Mississippi River. This late-Holocene, microtidal, storm-dominated coast 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 high. Most Chenier-Plain ridges represent open-Gulf paleoshorelines. Past shoreline morphodynamics allow ridges to be classified as transgressive, regressive, or laterally accreted.

Chenier-Plain development was commonly accepted to have been driven entirely by Mississippi-River autocyclicity as illustrated by Hoyt's (1969) model depicting transgressive cheniers and regressive mudflats. However, Hoyt's model oversimplifies Chenier-Plain evolution because it omits ridges created by non-transgressive processes. Thus, Chenier-Plain evolution is more complex than Mississippi River channel avulsions, and it involved not only chenier ridges (transgressive), but also beach ridges (regressive) and spits (laterally accreted). A six-stage geomorphic process-response model was developed to explain Chenier-Plain evolution 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, development of transgressive, regressive, and laterally-accreted ridges typically occurred contemporaneously along the same shoreline at different locations.

Possible geomorphic evidence for higher-than-present stands of Holocene sea level lies in the development of various ridges that have maximum elevations that exceed 2.5 m. Of the eight primary paleoshoreline trends on the Chenier Plain, maximum elevations tend to occur along two prominent ridge trends, the Little Chenier-Little Pecan Island trend and the Grand Chenier trend, which are the two most important paleoshorelines because of their lateral extent and relief.