| Summary: | Optically stimulated luminescence dates from the last glacial period fallingstage and lowstand Deweyville deposits along the Trinity River valley in Texas provided insight to their timing of deposition and allogenic controls on fluvial processes. Three distinguishable periods of incision and lateral channelbelt migration were the effect of both lowered sea levels and climate controlling factors within the drainage basin. Valley widening and deposition of the High, Middle, and Low Deweyville units were constructed and subsequently preserved as fluvial terraces during oxygen isotope stages (OIS) 3 through 2, from 6532.5 ka, 32.523 ka, and 2316 ka respectively.
Although numerous workers using discharge retrodiction equations have inferred much greater discharges as the primary cause of the larger paleochannels of the glacial age Deweyville units, the reconstructed paleohydrology of the Trinity River using measurable parameters such as channel width, sediment caliber, and other geomorphic planform properties yields contrasting results to earlier studies.
Sediment mass stored and exported during valley creation was assessed and calculated using a mass balance approach. Results clearly show that falling stage and lowstand fluvial deposits account for a large volumetric portion of the Trinity valleyfill sediments. Results also show that excavated sediment from the Coastal Plain represents only ca. 13% of the total hinterland derived sediment flux delivered by the Trinity system to downstream point sources over the last glacial period: periods of channel incision and valley deepening contribute little additional export of sediment, rather the process of lateral migration and channelbelt formation is contemporaneous with enhanced sediment flux to downslope systems. These results contrast long held concepts of incision and bypass during valley creation.
An alternative hypothesis using a process based framework is presented as the primary cause of the larger Deweyville paleochannels. The resultant alluvial morphology of the Deweyville units was the result of floodplain longitudinal profile adjustment to sealevel change and the preexisting boundary conditions of the drainage basin and the emerging topography of the shelf, and an attempt of the fluvial system to attain the minimum channel slope required to transport upstream controls on water and sediment loads.
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