ABSTRACT

The coastal zone of southwestern Louisiana is the product of regressions and transgressions in response to complex interactions among marine, riverine, aeolian, and storm processes. The two primary components of the chenier plain, a marginal deltaic environment, are sand/shell ridges, or relict beaches, and extensive inter-ridge marshes. Previous studies have interpreted fine-grained prograding deposits of the chenier plain to be the result of high sediment loads from the Mississippi River when the primary discharge pathway is to the west. Conversely, cheniers are thought to have formed as the result of delta lobe switching of the Mississippi River from west to east, causing decreased sediment supply to the chenier plain and enabling wave activity to erode deposits along the shoreline and concentrate sand/shells into ridges. Although information exists regarding the general sedimentary framework of the study area, questions remain regarding the timing of deposition and its relationship to switching of the Mississippi River and distributaries, sediment source, and spatial variations in sedimentary facies as a function of sediment source and environment of deposition.

Approximately 30 vibracores (up to 9 m long) were obtained along the central chenier plain that extend from the upper continental shelf (5 m water depth) north to the Holocene-Pleistocene margin of the coastal plain. Most cores contain the entire Holocene record of sediment deposition and penetrate the pre-Holocene surface as a basal unit. At least six distinct sedimentary facies characterize subsurface deposits, ranging from clean sandy/shelly units (1 to 4 m thick) to interbedded sandy muds (up to 4 m thick) to fine laminated clay (1.5 to 3 m thick). Depth to the pre-Holocene surface shallows northward and varies between 5 and 7 m near the present coastline. Spatial variability in the thickness and extent of specific units varies depending on sediment source and antecedent topographic control. A transition from predominantly estuarine to marine macrofossil assemblages (north to south) is accompanied by an increase in the extent of relict beach deposits, some ranging up to 4 m thick and 200 m wide. Although the Mississippi River has provided important input of fine-grained material to the chenier plan, local sediment sources, including the concentration of sand and shell from eroding marsh/mudflat deposits during transgressions, have contributed substantially to chenier evolution.

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