Date of Award

1989

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Geology and Geophysics

First Advisor

Dag Nummedal

Abstract

Many streams of the northern Gulf of Mexico coastal plain have similar geomorphic and stratigraphic character and comparable influences on their drainage basin evolution. Potential geomorphic influences on floodplain morphology vary temporally and spatially in their effect and include inherent geomorphic processes, eustatic sea level rise, evolution of a stream's receiving basin, adjustment to active tectonics, and local climate fluctuation. A 14.3-km segment of meander belt in the middle Amite River valley in southeastern Louisiana was selected to test the relative effect of geomorphic factors on Holocene floodplain evolution. Floodplain evolution in this valley responded to sea level rise and sedimentation in the Mississippi River delta plain; however, channel response to peak floods and variability in bank stratigraphy imprint over the more general floodplain influences. Three alloformations, identified as Magnolia Bridge (MAG), Denham Springs (DS), and Watson (WAT), can be differentiated by unconformable boundaries, landscape morphology, and relative pedogenic development. Sedimentary deposits consist of a lower sandy and upper silty facies. Lithofacies and sedimentary environments are dependent on fluvial process and independent of alloformation boundaries. Preserved lithofacies consist of channel deposits, point bar and scroll bar deposits, channel fill, and overbank deposits. Separation of flow during large floods into primary and secondary components controls lithofacies geometry. Surface horizons of all alluvial soils are similar, but the overlie stratified parent material (Entisols) on the MAG, cambic horizons (Inceptisols) on the DS, and minimally developed argillic horizons (Alfisols) on the WAT. Important pedogenic processes include worm bioturbations and water table fluctuations that control the distribution of color mottles, stains, and sesquioxide concretions. Particle-size distributions indicate that soils formed in the silty facies from stratified alluvium. Alluvial stratigraphy and chronology indicate an active meander belt (WAT) by at least 9 ka, when sea level was lower and the Amite River was tributary to a larger drainage basin that extended across an exposed continental shelf. The DS became an active meander belt by 3 ka, after the Mississippi River delta formed the Pontchartrain basin. The current meander belt (MAG) developed, in part, during the European settlement of the drainage basin.

Pages

187

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