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Geomorphic response to late quaternary climate and sea -level change, Lower Nueces River, Texas

James Michael Durbin, University of Nebraska - Lincoln

Abstract

This research links the response of the Nueces River to climate and sea-level variations during the last glacial cycle (Oxygen Isotope Stages {OIS} 5 through 1). Three terraces and correlative unconformities, often with paleosols developed in fining-upward, cross-bedded gravelly sand, are the contacts for informal Deweyville allostratigraphic units. Deweyville unit terraces exhibit large meander scars and have steeper gradient profiles than the modern floodplain. Thermoluminescence (TL) and Optically Stimulated Luminescence (OSL) dates constrain deposition of the High (HD), Middle (MD), and Low (LD) Deweyville units to OIS3 through early OIS2 (53 to 27 ka). ^ The river subsequently eroded and buried the LD unit, depositing three informally-defined Post-Deweyville allostratigraphic units (PD1, PD2, and PD3). Paleosols mark the contacts between units that consist of rare channel facies and predominantly fine-grained sediments in thin, horizontal beds that thicken near the modern bayline. Channels in PD units exhibit small meanders and floodplain gradients become less steep with age. OSL dates constrain PD units to OIS2 and OIS1 (24 ka to present). ^ The episodic sea-level fall during OIS3 controlled the elevation to which the river graded, but had limited upstream impact. Deweyville meander dimensions, clast sizes, and terrace gradient profiles out of phase with sea level indicate climatic influences on river behavior. OIS3 climates were cool, with pervasive mid-latitude cyclonic storms that increased effective moisture, changed the vegetative cover, and produced thick soils. The aforementioned environmental factors produced bankfull-stage floods which caused lateral accretion, concentrated coarse sediments in large point bars, and allowed discharge-driven incision and terrace formation. ^ A glacio-eustatic rise flooded the lower valley and causing a progressive decrease in floodplain gradients during deposition of the PD units. The warming climate of the interglacial transition favored cloudbursts and infrequent tropical storms, decreased effective moisture, and resulted in vegetation changes, soil erosion, and increased suspended sediment loads. Overbank-floods deposited, fine-grained vertically accreted alluvium, ultimately contributing to smaller meanders and less channel migration. When combined with base-level influences, flood regime changes caused aggradation and channel avulsion in downstream reaches of the stream. ^

Subject Area

Physical Geography|Geology|Environmental Sciences

Recommended Citation

Durbin, James Michael, "Geomorphic response to late quaternary climate and sea -level change, Lower Nueces River, Texas" (1999). ETD collection for University of Nebraska - Lincoln. AAI9952676.
http://digitalcommons.unl.edu/dissertations/AAI9952676

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