ABSTRACT

Deposits in interdistributary basins are largely controlled by the relative rates of subsidence and the amount of sediment transported into the basin. Marine delta progradation and subsequent abandonment on approximately a 1000 to 1500 year cycle isolate extensive backswamp and marshy areas between major distributary channels. The resulting interdistributary basins areally occupy a large portion of the lower alluvial valley and upper delta plain. Extensive backswamps occur at the apex of these basins and grade seaward into fresh, brackish, and saline marshes. Following delta-lobe abandonment, high subsidence rates combined with low sediment input lead to flooding of the basin, both by marine incursion and the enlargement of lakes. Analyses of sedimentary processes in the Barataria and Atchafalaya Basins of the Louisiana delta plain reveal two contrasting styles of sediment accumulation.

Thick sequences of peat, interfingering with fine-grained lacustrine, marsh, and backswamp deposits are characteristic of a basin starved of terrigenous clastic sediment (Barataria Basin). Formation of a distributary channel in the basin (Atchafalaya Basin) results in the introduction of large quantities of coarser-grained detrital clastics to the lowland. Coarsening-upward lacustrine delta sequences, which average several meters in thickness, prograde tens of kilometers into the interdistributary basin, ultimately coalescing with adjacent deltaic, alluvial and backswamp deposits.

In the Atchafalaya Basin, deposition of backswamp deposits with coarser-grained lacustrine deltaic and alluvial sediments aggrades the interdistributary basin and delta plain in the process of prograding a distributary channel and marine delta. Once this marine delta complex is abandoned, the site of active deposition shifts to another area. The lack of detrital influx increases the relative subsidence rate, and peat accumulation becomes the dominant process in the basin. Continual subsidence leads to complete inundation of the backswamp and termination of distributary channel in the basin. An upstream channel avulsion, or distributary formation, may again initiate siliciclastic deposition in the basin.