Date of Award

1989

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Geology and Geophysics

First Advisor

Dag Nummedal

Abstract

The Holocene flood basin along a meander belt margin of the Mississippi River includes two principal geomorphic features: a flat featureless natural levee, and a system of anastomozed, entrenched crevasses with intervening splay lobes. Levee and splay lobe deposits are identical and consist of thinly (cm) to minutely (mm) rhythmically layered sand, silt and clay which are pervasively disrupted by unlined meniscate burrows of the trace fossil, Muensteria. Pelletal fabrics develop where overlapping backfilled burrows completely obliterate primary stratification so that only traces of small ripple laminations and parallel laminations are preserved. Pelletal silt is the most extensive lithology present. A proximality trend in stratification is present relative to distance from sheet flood sources. With increasing distance from the trunk channel and crevasses, sandy rhythmites are successively replaced by finer-grained and more thinly interlayered deposits such as silty rhythmites and interlaminated silt and clay. As layering thins distally, burrowers more effectively disrupt the sediment and stratification grades into pelletal silt, pelletal mud and pelletal clay of the backswamp. Thus levee and splay lobe deposits are identical except that the same lithofacies extend further basinward on splay lobes. The crevasse fill includes mudball conglomerate, plant debris, complex clay-rich laminates, graded sands, mottled muddy sand and minor ripple laminated sand. The sedimentology reveals that both morphologic features formed in an analogous manner, by sheet flow sedimentation. This result differs from previous views that (1) levees form from overbank flooding and sheet flow and (2) crevasse splays form via the crevassing process which generates thick, sharp-based, fining-upward cross-stratified sands. The crevasse channels and splay lobes are not depositional features that formed as minor mouth bars, crevasse channels, lenticular channel sands, and bar forms migrated basinward. Rather, after crevasse entrenchment, the splay lobes accreted by overbank flooding and sheet flood sedimentation. After establishment of the meander belt (marked by a lithologic change from lake clays to subaerial pelletal silt) two episodes of levee/splay progradation (3-4 m thick) are recorded. Each represents the gradual migration of the trunk channel towards the site, and abruptly ends with a neck cut-off.

Pages

360

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