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"Deweyville" Terraces and Depostits of the Texas Gulf Coastal Plain
Michael D. Blum (1), Robert A. Morton (2), James M. Durbin (1)
Bernard (1950) defined the Deweyville beds as underlying a terrace along Sabine River that is intermediate in elevation between Pleistocene Beaumont alluvial plain surfaces and Holocene floodplains, and which has abandoned meanders that are considerably larger than those of the Beaumont surface or modern highly sinuous Sabine channel. Subsequent workers identify 2 or 3 distinct terraces and/or suites of deposits that fit the original morphostratigraphic concept of the Deweyville along the Sabine and other rivers of the Texas Coastal Plain, but most commonly attribute oversized meanders to greater annual discharge and/or extreme high magnitude floods during the late Pleistocene glacial period. This paper builds on the idea of a broader stratigraphic concept for "Deweyville" terraces and deposits, and suggests a process model that emphasizes fluvial responses to interacting climatic and glacio-eustatic controls.
We suggest the multiple "Deweyville" terraces and underlying fills of the Texas Gulf Coast should be treated as a series of unconformity-bounded allostratigraphic units that record: (a) abandonment of Beaumont isotope stage 5 alluvial plains ca. 100 ka, which partitioned post-Beaumont incised valleys; and (b) multiple episodes of lateral migration, aggradation, and/or degradation within those valleys during the stage 4, 3, and 2 glacial cycle when channels were graded to shorelines at midshelf or farther basinward positions. "Deweyville" allostratigraphic units of the Sabine, Trinity, Guadalupe, and Nueces Rivers have steeper gradients than modern floodplains, and the youngest "Deweyville" surfaces are onlapped by Holocene strata at or near modern bay-head deltas. Similar units are present in the Colorado and Brazos(?) valleys, but onlap by Holocene strata occurs 100 km or more inland from the present highstand shoreline.
"Deweyville" allostratigraphic units may represent a glacial period process regime with more annual runoff, but smaller peak discharges than present The deep inland penetration of tropical moisture and/or tropical cyclones, responsible for most extreme floods on Coastal Plain rivers, was rare through the 80-90 ky of the glacial cycle when temperatures were cooler and the Gulf was smaller. "Deweyville" allostratigraphic units also lack clear evidence for high magnitude overbank floods, as they are sand-dominated, much like channel facies of late Holocene streams, but there is a paucity of vertical accretion floodplain facies which suggests most flood events remained within bankfull channel perimeters. The shorter wavelength, highly sinuous meanders typical of the present interglacial process regime may reflect adjustments to bank-stabilizing vertical accretion facies produced by deep overbank floods, and, in lowermost reaches of the Coastal Plain, to a forced flattening of gradients due to postglacial sea level rise.
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