ABSTRACT

Studies of the surface features and underlying Recent sediments of the chenier plain of southwestern Louisiana have made it possible to establish the major events in its development. A chronological framework for these events has been determined through radiocarbon dating of more than 100 surface and subsurface samples.

The wedge of Recent sediments records the final stage in postglacial rise of sea level and the subsequent stillstand at the present elevation. The basal part of the wedge consists of transgressive brackish-water and marine deposits laid down on the underlying Pleistocene surface as the sea rose from -17 feet 5600 years ago to its present level about 3000 years ago. Coincident with or shortly before reaching this level, the longshore influx of sediment, chiefly from the Mississippi River, brought about a general outbuilding of the coast; in the western part of the area where a number of borings have been made and studied in detail, it has been shown that the nearsurface coastal deposits rest upon a seaward-thickening accumulation of gulf-bottom sediments that form the upper part of the sedimentary wedge. At times of abundant sediment supply, the shore moved rapidly seaward through the accumulation of marsh-capped mudflat deposits; during periods of slight sediment influx, wave attack slowed or halted the advance and locally brought about shore retreat. It was during these latter periods that the beach ridges or cheniers, which now stand as relict "islands" in the marsh, were developed. Ranging in age from 2800 to less than 300 years, these ridges record progressive changes in the configuration of the shoreline as it advanced seaward more than 10 miles to its present position.