Abstract: Pleistocene Beaumont Alluvial Plain Construction in Response to Interacting Glacio-Eustatic and Climatic Controls, Texas Gulf Coastal Plain

Michael D. Blum

The Beaumont Formation consists of a series of coast-parallel, coalescing, alluvial plains that comprise the youngest large-scale Quaternary depositional unit of the Texas Gulf Coast. Previous work has shown the Beaumont surface consists of a series of cross-cutting meander belts and intervening flood basins, with distal deltaic and littoral facies, and suggested that alluvial plains were constructed by autocyclic meander-belt avulsions and occupations during the last interglacial highstand. Reexamination of Beaumont strata suggests the stratigraphic framework is more complex, and that Beaumont deposition reflects interactions between glacio-eustatic and climatic controls operating over multiple glacial-interglacial cycles. As a result, Beaumont strata provide an opportunity to examin the relative scale and hierarchical nature of glacio-eustatically controlled vs. climatically controlled components of alluvial plain stratigraphic sequences.

Mapping from satellite imagery, field documentation of geomorphic and stratigraphic relationships, and consideration of the stratigraphic significance of surface and buried soils suggest that Beaumont alluvial plains consist of a series of cross-cutting and superimposed meander-belt/incised valley axes of widely ranging age. Major meander-belt/incised valley axes are partitioned by initial glacio-eustatic lowering of sea level below interglacial highstand positions and the elevation of aggradational-progradational highstand wedges. At this time, valley axes become fixed in place and extend across the shelf, incising into preexisting strata, and the remainder of the alluvial plain is characterized by nondeposition and soil development. Although shorelines are basinward of highstand pos tions, multiple episodes of lateral migration, aggradation, degradation, and/or floodplain abandonment with soil formation occur within incised valleys in response to climatic controls on discharge and sediment supply. This creates a composite valley-fill unconformity, as well as minor unconformity-bounded allostratigraphic units within the valley-fill sequence that record fluvial responses to climatic change. With transgression and highstand, incised valleys fill and deltas prograde at paces set by upstream controls on sediment delivery. As valley filling nears completion, thin veneers of flood-plain facies spread laterally, filling accommodation space created by subsidence and burying soils developed elsewhere on the alluvial plain. Complete or near-complete valley filling during highs and is one of several processes that promote avulsion, with relocation of meander-belt/incised valley axes and refocusing of sediment input before the next glacio-eustatic sea level fall.

AAPG Search and Discovery Article #90983©1994 GCAGS and Gulf Coast SEPM 44th Annual Meeting, Austin, Texas, October 6-7, 1994