--> Abstract: Current Thinking On Late Quaternary Alluvial Successions Of The Texas Gulf Coastal Plain, by M. D. Blum; #90928 (1999).

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BLUM, MICHAEL D.
University of Nebraska-Lincoln, Lincoln, NE

Abstract: Current Thinking on Late Quaternary Alluvial Successions of the Texas Gulf Coastal Plain

The Texas Coastal Plain consists of coalescing alluvial-deltaic plains constructed by medium to large fluvial systems. In the early 20th century when coastal plain deposits were first mapped and subdivided, three large-scale depositional units were assigned to the Pleistocene, and early genetic models were developed when the Pleistocene was thought to consist of 4 glacial-interglacial cycles. Valley entrenchment and sediment bypass was inferred for glacio-eustatic falling stage and lowstand, whereas depositional units represented alluvial terraces and delta plains constructed during transgression and interglacial highstand. Most workers assigned the youngest Pleistocene unit, the Beaumont Formation, to the last "Sangamon" interglacial, whereas post-Beaumont valleys represented entrenchment during the "Wisconsin" glacial and filling during post-glacial sea-level rise and highstand.

Research over the last 2-3 decades has taken place within the context of (a) more complex views on Quaternary glacial cycles and glacio-eustasy, (b) development of depositional systems and sequence stratigraphic paradigms, and (c) improved geochronological tools. Beaumont and post-Beaumont successions are now viewed as a series of unconformity- and paleosol-bounded cross-cutting 100 ky incised valley fills that span the last 400 ky or more, and record fluvial responses to interacting glacio-eustasy and climatic change. Valleys are partitioned by sea level fall below highstand depositional shoreline breaks, when channels incise and extend across the shelf, then during falling stage and lowstand multiple cycles of lateral migration and sand-rich aggradation, followed by incision, occur within the valleys in response to climatic change. With transgression and highstand valleys fill at rates set by hinterland controls on sediment delivery, and develop a series of anastamosing or distributary channelbelt and splay sands within overall mud-rich successions. Complete filling promotes avulsion and the largescale relocation of valley axes before the next sea level fall, such that successive 100 ky valley fills commonly show a distributary pattern.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas