Quaternary Alluvial Plain Depositional Sequences, Texas Gulf Coast

M. D. Blum

The Texas Gulf Coastal Plain consists of a series of low-gradient, fan-shaped alluvial plains emanating from each river valley. Alluvial plain surfaces have been mapped as Pleistocene Beaumont Formation or younger unnamed strata, and interpreted to represent deposition during the penultimate and Holocene interglacial highstands. Reexamination of Beaumont and younger strata of the Colorado River suggests alluvial plain construction is more complex, and the chronostratigraphic framework needs substantial revision. Beaumont and younger strata also provide an opportunity to examine large-scale alluvial plain successions within a sequence stratigraphic framework, define some key characteristics of alluvial plain depositional sequences, as well as their heirarchal nature, and a dress the relative importance of glacio-eustatic versus climatic controls.

Mapping from satellite imagery, documentation of stratigraphic relationships, consideration of the stratigraphic significance of surface and buried soils, and a number of thermoluminescence ages suggests that Beaumont and younger alluvial plains consist of multiple cross-cutting and/or superimposed valley fill complexes of widely varying age, and may represent the last 400 ky or more. Individual valley fill complexes represent the non-marine equivalent to a 100 ky depositional sequence, and become partitioned by initial lowering of sea level below interglacial highstand positions, when channels rapidly incise and valley axes become fixed in place as they extend across the newly subaerial shelf. While shorelines remain basinward of highstand positions the remainder of the alluvial plai is characterized by non-deposition and soil development During this time, multiple episodes of lateral migration, aggradation, degradation, and/or floodplain abandonment with soil formation occur within incised and extended valleys in response to climatic controls on discharge and sediment supply. This creates a composite basal valley fill unconformity, at least partially correlative and perhaps traceable to the sequence boundary as defined farther basinward, as well as minor unconformity-bounded stratigraphic units within the valley fill complex. With late stages of transgression and highstand valleys fill at paces set by upstream controls on sediment delivery. As valley filling nears completion veneers of floodbasin sediments spread laterally, which buries soils developed on subsided owndip margins of the alluvial plain. Complete valley filling during highstand is one of several processes that promotes avulsion, with possible relocation of valley axes and refocussing of sediment input before the next sea level fall.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995