Coastal morphodynamics and Chenier-Plain evolution in southwestern Louisiana, USA: A geomorphic model
Introduction
The Chenier Plain of southwestern (SW) Louisiana extends from Sabine Pass at the Texas/Louisiana border, eastward almost 200 km to Southwest Pass at Vermilion Bay (Fig. 1). This Late-Holocene, marginal-deltaic environment is up to 30 km wide and is composed primarily of mud deposits that are capped by marsh and interspersed with thin sand- and shell-rich ridges known as cheniers. The term “chenier” comes from the French Cajun word chêne, which means “oak tree.” In the Chenier Plain, oak trees line these ridges, which are better drained and topographically higher than the surrounding marsh.
Shifts in the course of the Mississippi River and the geomorphic development of the Chenier Plain were discussed first by Russell and Howe (1935) and Howe et al. (1935). Fisk (1948) discussed the geology of Cameron and Vermilion Parishes and provided a general description of Chenier-Plain morphology and geology. Furthermore, Fisk emphasized the importance of shoreline orientation and relict river courses for interpreting shifts in shoreline position. Using geomorphology, stratigraphy, and extensive radiocarbon dating, Gould and McFarlan (1959) and Byrne et al. (1959) documented the geologic framework of the Chenier Plain and described seven major shorelines.
Hoyt (1969) presented the first detailed depositional model for chenier genesis and mudflat progradation (Fig. 2a). According to Hoyt's (1969) model, Chenier-Plain development reflects changes in Mississippi River flow direction that are caused by channel avulsion (see Aslan et al., 2005), which results in downstream delta switching along the coast, as illustrated in Fig. 2b and c (see Scruton, 1960, Coleman, 1988, Roberts, 1997, Roberts, 1998 for details regarding the delta-switching process). The Hoyt model correlates mudflat progradation with periods of Mississippi River discharge to the west (Fig. 2a, b). When the river becomes hydraulically unstable and avulses to a shorter, more eastward course to achieve hydraulic efficiency, fine-grained sediment supply to the Chenier Plain is reduced significantly, enabling wave reworking and erosion of mudflats, thus forming cheniers (Fig. 2a, b). Hoyt's hypothesis that the changing position of the Mississippi River is important to overall Chenier-Plain development is valid, but because it incorporates only regressive mudflats and transgressive cheniers, omitting ridges created by other means, his model oversimplifies Chenier-Plain evolution in SW Louisiana. This oversimplification has been challenged by subsequent researchers (Gosselink et al., 1979, Kaczorowski, 1979, Kaczorowski, 1980, Kaczorowski and Gernant, 1980), who recognized that the geologic evolution of the Chenier Plain was more complicated than channel avulsions of the Mississippi River, involving not only chenier ridges (i.e., transgressive), but also ridges that are genetically tied to beach ridges, recurved spits, eolian deposits, storm berms, natural levees, and ancient oyster reefs. Subsequent reviews (e.g., Walker and Coleman, 1987, Penland and Suter, 1989) supported the original scientific findings of Gosselink et al. (1979) and Kaczorowski and Gernant (1980) that the Chenier Plain contains more than true cheniers (i.e., transgressive ridges); however, a comprehensive understanding of Chenier-Plain evolution and a more-advanced geomorphic model that explains the genesis and distribution of different ridge types still are lacking.
The goal of this paper is to explain the geomorphic evolution of the SW Louisiana Chenier Plain. Specifically, the objectives are to (i) refine the definition of the term Chenier Plain; (ii) clarify what types of ridges are found in the Chenier Plain and map their spatial distribution; (iii) use the combination of shoreline-change data and tidal-inlet morphodynamics along the present Chenier-Plain coast as a modern analog by which to understand coastal processes and landforms along relict shorelines (ridges); (iv) identify global, regional, and local processes that are responsible for ridge formation; and (v) present a six-stage geomorphic process-response model that synthesizes the geologic evolution of the Chenier Plain.
Section snippets
Regional setting
The Louisiana coast, which consists entirely of Holocene sediments deposited directly or indirectly by the Mississippi River, is naturally divided into two primary geomorphic zones: the deltaic plain of southeastern Louisiana and the Chenier Plain of SW Louisiana (Fisk, 1944, Gould and McFarlan, 1959, Roberts, 1997). Except for local progradational areas around the active mouths of the Mississippi and Atchafalaya Rivers, as well as short segments east and west of Sabine and Calcasieu Passes and
Methods, landform definitions, and geomorphic hierarchy
A detailed geomorphic base map for the entire Louisiana Chenier Plain (Fig. 1) was needed because regional, geologic field and mapping studies of the area have been lacking since the Byrne et al. (1959) study. Also, high-quality, color infrared imagery datasets were collected in subsequent years, thus providing additional resolution and contrast for air-photo interpretation of form/process relationships. Specific datasets used to construct the geomorphic map include (i) NASA color infrared
Relative chronology of ridge trends (relict shorelines)
Fifteen shoreline trends (including the modern shoreline) are identified and presented in chronological order in Table 2. Although shorelines S1, S2, and S3 are the oldest ridges in the Chenier Plain, they are limited spatially and are considered minor shorelines (Fig. 1; Table 2). Because of its regional extent, the Little Chenier–Little Pecan Island trend (S4) is the oldest prominent shoreline trend in the area. Cypress Point and Fire Island may be eastward extensions of S4, as they are the
Coastal morphodynamics and Chenier-Plain evolution
To understand the long-term evolution of a coastal depositional system, primary process–response mechanisms and patterns found along the modern coast must first be identified. As such, present-day coastal processes operating along the outer shoreline of the Chenier Plain are synthesized. These processes play a central role in providing new insight in the construction of a geomorphic model for Chenier-Plain evolution.
Discussion
The geomorphic process-response model presented in this paper explains Chenier-Plain evolution at province scale and clarifies several important processes that are responsible for the genesis of different ridge types, such as local updrift erosion and downdrift sediment trapping at tidal entrances. Remaining critical issues entail the exact sources of sand and shell that compose ridge deposits and the forcing mechanisms responsible for Chenier-Plain evolution.
Summary and conclusions
- (i)
The SW Louisiana Chenier Plain is a complex geomorphic feature that contains not only transgressive ridges (traditional cheniers), but also regressive ridges (beach ridges) and laterally accreted ridges (spits or recurved spits). Thus, a geomorphic hierarchy of primary landforms relative to dominant coastal process has been devised. The term Chenier Plain is defined as a first-order feature and is composed of three types of smaller, second-order features: chenier complex, beach-ridge complex,
Acknowledgements
Research was funded by the U.S. Geological Survey Coastal and Marine Geology Program (cooperative agreement 14-08-0001-A0917, Byrnes and McBride co-principal investigators); the Office of Naval Research (Grants N00014-99-1-0817 and N00014-00-1-0247, McBride principal investigator); the Office of the Provost at George Mason University (Faculty Summer Research Grant to McBride); and the Louisiana Department of Natural Resources, Coastal Restoration Division. The R.J. Russell Foundation
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