Abstract: "Deweyville" Terraces and Deposits of the Texas Gulf Coast: A Reevaluation

Michael D. Blum, James M. Durbin, Robert A. Morton

The Deweyville terrace was defined by Bernard (1950) as a former floodplain level along Sabine River, intermediate in elevation between Pleistocene Beaumont surfaces and Holocene floodplains, and with meander scars considerably larger than the Beaumont surface or modern Sabine channel. Later workers identified 2 or 3 terraces that fit the original morphostratigraphic concept of the Deweyville, but most commonly attributed larger meanders to higher 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 an alternative process model.

Multiple "Deweyville" terraces and underlying fills of the Texas Gulf Coast should be treated as a series of 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 mid-shelf 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 allostratigraphic units are present in the Colo ado and Brazos(?) valleys, but onlap by Holocene strata occurs some 75-100 km farther upstream.

"Deweyville" allostratigraphic units 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 storms, responsible for most high magnitude 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" units also lack clear evidence for high magnitude overbank floods. Allostratigraphic units 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 small, high sinuosity meanders of the present interglacial process regime may ref ect 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 post-glacial sea level rise.

AAPG Search and Discovery Article #90955©1995 GCAGS 45th Annual Meeting and Gulf Section SEPM, Baton Rouge, Louisiana