Depositional Processes and Stratigraphy of Fluvially Dominated Lacustrine Deltas: Mississippi Delta Plain

Abstract

ABSTRACT A geomorphic and stratigraphic investigation of lacustrine delta deposits in the Atchafalaya Basin, Louisiana, revealed that lacustrine delta formation is rapid and cyclic in nature. During the Holocene, thin, coarse-grained, regionally extensive, fluvially dominated lacustrine deltas filled interdistributary basins in the transition zone between the alluvial valley and the marine delta plain of the Mississippi River. One delta, Lake Fausse Pointe delta, prograded 6.5 km, partially filling the lake and covering more than 29 km2 in area in twelve years. Depositional processes in Lake Fausse Pointe ranged from suspension-settling of mud and organic matter during low sediment input to traction deposition of sand during floods. Hyperpycnal flow conditions, set up by the introdu tion of sediment-laden river water into the freshwater lake, induced underflows that scoured the lake bottom and deposited upward-coarsening lobes. The low relief of the Mississippi basin, a constant large volume of fine-grained sediment supply, and prevalent fluvial processes have formed fluvially dominated lacustrine deltas that differ sedimentologically and stratigraphically from Gilbert-type and brackish-water lacustrine deltas. Parallel-laminated prodelta mud, rippled to cross-laminated delta-front silty sand, and very finegrained to medium-grained distributary-mouth-bar sand are characteristic of rapidly deposited sediment, whereas rooted and burrowed sediments signify periods of minimal deposition. In a fluvially dominated setting, delta geometry and sediment distribution patterns are controlled by river-mouth processes, basin shape, and bathymetry. Thickest sand accumulations occur in linear, dip-elongate distributary-m uth-bar and natural-levee lobes that are separated laterally by mud-filled channels and interdistributary troughs. Lacustrine-delta deposition is driven by basin subsidence (regional downwarping and sediment compaction) and the concomitant development of distributary channels in the interdistributary basins. Numerous lacustrine-deltaic wedges, each bounded by rooted backswamp clay, are preserved multilaterally in Atchafalaya Basin sediments. Although lacustrine-deltaic deposits constitute much of the basin's sedimentary fill, each individual delta sequence records depositional events of only 100 years duration. These deltaic deposits represent basin-filling (aggradational) episodes that preceded the formation of marine delta complexes such as the Maringouin, Teche, La Fourche, and Atchafalaya deltas of the Mississippi River.