ABSTRACT

Geomorphic evolution of the southwestern Louisiana chenier plain is examined by analyzing ridge distribution and orientation. The configuration of and inter-relationships among former outer gulf shorelines are used to examine depositional processes associated with ridges during long-term chenier-plain progradation. The chenier plain (4,900 km²) has been organized into a hierarchy of geomorphic features (1st, 2nd, 3rd, and 4th order) based on form, dominant vector direction of shoreline movement, and spatial scale. The chenier plain is the first order feature and is composed of second order features designated as complexes: chenier, spit, and beach ridge (35-300 km²). Individual cheniers (3rd order features) encompass beach ridges, spits, and washover beaches (4th order features), and all of these features comprise complexes. Within the geomorphic hierarchy thirteen individual shoreline trends have been identified to establish chronological evolution of the area. Net westerly sediment transport along the chenier plain is indicated by (1) an increase in the number of ridges on the eastern side of the Sabine, Calcasieu, and Mermentau Rivers, (2) westerly deflection of minor streams (including the Mermentau River), (3) curved spit deposits, and (4) decrease of `inter-ridge width to the west. However, anomalies in net transport direction do occur such as the curved eastern end of Little Chenier and west of the Sabine and Calcasieu Rivers.

The western section of the chenier plain between the Sabine and Calcasieu Rivers is dominated by a beach-ridge complex (170 km²) east of the Sabine River. Immediately west of the Calcasieu River, cheniers and beach ridges are scarce as sediment has accumulated in a beach-ridge complex (50 km²) on the updrift side of the Calcasieu River. Ridges diverge to the east (towards the Mermentau River) and are separated by mudflats (2-5 km wide) forming a classic chenier complex (300 km²). Grand Chenier, a regional transgressive shoreline, is composed of a curved-spit complex (35 km²) immediately east of the Mermentau River. The Grand Chenier trend (Front Ridge, Oak Grove Ridge, Long Island, Pecan Island, and Front Ridge East) truncates a small chenier complex (153 km²) east of the Mermentau River. Farther east in the Pecan Island area, a chenier complex (70 km²) is truncated by two transgressive shorelines: first by Back Ridge and then by Front Ridge East. South of the Pecan Island chenier complex, more recent chenier-complex development (60 km²) is evident at Mulberry Island, Beef, Bill, and Sand Ridges. More shore-normal orientations of Chenier au Tigre and Belle Isle likely represent ancient bay shorelines and oyster reefs. Existence of different ridge-complex types suggests multiple shoreline formation processes, sediment sources, and transport directions that are not always associated with major switches of the Mississippi River mouth. Shoreline evolution is determined by interaction of sediment transport with the dynamic diversion capability of entrances.