Date of Award

1986

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Abstract

Progradation of lacustrine deltas alluviated the interdistributary lowlands which occupy the transition zone between the alluvial valley and marine delta in the Atchafalaya Basin. Sediment introduced into the basin during the last 700 years by a major Mississippi River distributary has accumulated in the form of thin, regionally extensive deltas. Deltaic sedimentation is cyclic in nature. Rapid subsidence (basin downwarping and sediment compaction), in conjunction with lateral shifts in the site of deposition, creates vertically stacked deltaic wedges separated by backswamp deposits. One such delta prograded 6.5 km into Lake Fausse Pointe, grew to an area in excess of 29 km('2) in twelve years and is comprised of five depositional environments. A typical vertical sequence consists of coarsening-upward prodelta, delta front, and distributary mouth bar deposits which overlie lacustrine and backswamp sediments. Depositional processes in the lake ranged from suspension-settling of mud and organic matter during periods of low sediment input, to traction deposition of sand during floods. Hyperpycnal flow conditions, set up by sediment-laden water introduced into the freshwater lake, periodically induced underflows which were capable of scouring the lake bottom and depositing coarsening-upward lobes at their downdip extent. Parallel-laminated prodelta mud and rippled to cross-laminated delta front and distributary mouth bar silty sand are characteristic of rapidly deposited sediment intervals, whereas rooting and burrowing signify periods of relatively low sediment input and lake quiescence. Delta growth progressed through aggradation and fusion of lobes into large deltaic wedges separated by distributary channels. Within the delta lobes, significant volumes of sand were deposited as dip-elongate linear ridges. Progradation of laterally shifting lobes continued to fill Lake Fausse Pointe until artificial abandonment in the 1930's. Lake delta formation is a rapidly occurring and continuously repeated process in the Atchafalaya Basin, and, with a significant sediment source, lakes may fill within 100 years. Subsidence accelerates delta abandonment and the development of an overlying backswamp environment. Backswamp transgression creates a new lake to ultimately be filled by another lacustrine delta.

Pages

437

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