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Abstract: Quaternary Alluvial Plains and Incised Valleys, Texas Gulf Coastal Plain: Chronology, Stratigraphic Architecture, and Facies Organization

BLUM, MICHAEL D., University of Nebraska - Lincoln; ANDRES ASLAN, Mary Washington College.

The Texas Coastal Plain is the updip component of the passive-margin western Gulf of Mexico Basin, and consists of a series of coalescing Quaternary alluvial plains constructed by a variety of medium to large fluvial systems.

Based on mapping from satellite imagery, identification of stratigraphic relations in the field, and a series of thermoluminescence ages, Quaternary alluvial plains can be subdivided into a series of incised valleys and valley fills that reflect the 100 ky glacio-eustatic cycles of the middle to late Pleistocene and Holocene. Individual incised valley axes become partitioned by glacio-eustatic lowering of base-level below interglacial highstand positions, when channels incise through highstand aggradational/progradational wedges and valley axes become fixed in place as they extend across the newly subaerial shelf. Individual valleys then undergo multiple episodes of net deposition and sediment storage, punctuated by incision and/or sediment bypass, during each glacio-eustatic cycle. Periods of channel aggradation, lateral migration, and/or flood plain construction within the valley represent large volumes of sediment delivered from upstream sources, or increased flood magnitudes that result in widespread vertical accretion. By contrast, unconformities within the overall valley fill represent periods of channel incision and/or sediment bypass due to decreased sediment supply, abandonment of flood plains due to decreased flood magnitudes, or meanderbelt avulsion and relocation within the incised valley. In each case, soils develop on abandoned flood plains and define the upper boundary to smaller-scale stratigraphic units within the overall valley fill.

Episodes of deposition, periods of soil development, and production of unconformities within incised valleys probably reflect changes in discharge regimes and sediment supply, as controlled by climate changes in the continental interior. However, stratigraphic- and facies-scale architecture within an individual valley fill closely correspnds to the trend of base-level change. Small-scale stratigraphic units deposited during the long and complex glacial cycle falling stage, when the shoreline was lower in elevation and farther basinward, are sand-dominated and occur deep within the incised valley as a flight of downward-stepping terraces with underlying sandbodies. These units represent the manifestation of the concept of forced regression and the “forced regressive systems tract” within incised valleys, and should correlate to the series of basinward- and downward-stepping falling stage shorelines discussed by other workers. After weathering and soil development, falling stage stratigraphic units may be partially cut out, but mostly onlapped and buried, by stratigraphic units deposited during transgression and highstand. Transgressive and highstand systems tracts consist mostly of ribbon sands produced by frequent avulsion of channels, as well as a variety of associated thin sheet sands that are interpreted to represent crevasse splays, all encased in floodbasin muds.

Since falling stage sand bodies occur deep within the valley fills, their preservation in the stratigraphic record is very likely, and, indeed, falling stage sandbodies may comprise the bulk of reservoir-quality sands within Texas Gulf Coastal Plain incised-valley fills. By contrast, channel and splay sands associated with transgression and highstand occur higher up in the valley fills, and may be removed by subsequent erosion before they have a chance to subside and be preserved in the stratigraphic record.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah