Genesis and Architecture of Incised Valley Fill Sequences: A Late Quaternary Example from the Colorado River, Gulf Coastal Plain of Texas
Michael D. Blum, 1993. "Genesis and Architecture of Incised Valley Fill Sequences: A Late Quaternary Example from the Colorado River, Gulf Coastal Plain of Texas", Siliciclastic Sequence Stratigraphy: Recent Developments and Applications, Paul Weimer, Henry Posamentier
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Sequence stratigraphy provides a potential unifying framework for interpreting much of the rock record. However, because first-generation models emphasize subdivision and correlation of strata on the basis of surfaces produced by base level change and translation of the shoreline, their utility for interpreting alluvial (and other nonmarine) deposits remains uncertain. Alluvial successions in the stratigraphic record represent the direct coupling of upstream controls on sediment supply and basinal controls on accommodation and preservation, and it is important to consider how these upstream controls interact with base level changes to produce features and characteristics that should play a role in subdivision and correlation of alluvial successions within a sequence stratigraphic framework.
The Colorado River is a large extrabasinal fluvial system that drains the Southern High Plains and Edwards Plateau physiographic regions of west Texas, then transects the Gulf Coastal Plain until discharging into the Gulf of Mexico. Due to the physiographic setting of the Colorado system, the supply of sediments for the Quaternary alluvial-deltaic complex is predominantly from the upper Colorado drainage, and the lower Colorado channel serves as a conduit that transfers this material to the alluvial-deltaic plain and Gulf Coast Basin. This chapter outlines the alluvial history of the Colorado drainage, then examines the relative influence of climatically controlled changes in sediment supply versus glacio-eustatically driven base level changes on genesis and architecture of the late Quaternary incised valley fill sequence on the lower Colorado alluvial-deltaic plain.
Late Pleistocene through modern alluvial deposits of the Colorado drainage have been subdivided into four unconformity bounded allostratigraphic units. Radiocarbon ages provide chronological control on episodes of alluvial deposition, channel and/or valley incision, and flood plain abandonment with soil formation. Periods of net deposition occurred during the late Pleistocene full-glacial (approximately 20–14 Ka), early to middle Holocene (approximately 12–5 Ka), late Holocene (approximately 5–1 Ka), and through the last few hundred years. Unconformities representing valley incision and/or sediment bypass developed approximately 14–12 Ka, whereas unconformities representing flood plain abandonment and soil formation accompanied by continued lateral migration of channels developed approximately 5 and 1 Ka. Throughout the upper Colorado drainage, and within the lower Colorado valley upstream from the apex of the Quaternary alluvial-deltaic plain, younger allostratigraphic units occur at lower elevations than older units due to a protracted trend of bed-rock valley deepening through the late Cenozoic. Farther downstream, allostratigraphic units occur within a well-defined valley incised into the Quaternary alluvial-deltaic plain, and late Holocene units, deposited contemporaneously with the present highstand, onlap and bury late Pleistocene and early to middle Holocene units that were emplaced during the last glacio-eustatic lowstand and the transgression that followed.
The downstream continuity of allostratigraphic units, combined with downstream changes in stratigraphic architecture, provides the empirical framework necessary for discussing interactions between upstream climatic and downstream base level controls. Allostratigraphic units and bounding unconformities throughout the Colorado system correlate with independently identified climatic and environmental changes, and represent a series of adjustments to changes in the relationship between discharge regimes and the concentration of sediments along the valley axis, as driven by changes in climate. No apparent relationship exists between trends in base level change and periods of alluvial deposition, valley incision and/or sediment bypass, or flood plain abandonment and soil formation, hence the basic elements of the allostratigraphic framework were generated independently of the last glacio-eustatic cycle. However, downstream from the apex of the subsiding alluvial-deltaic plain, the last glacio-eustatic cycle controlled stratigraphic architecture, or stacking patterns, within the incised valley fill. In addition, glacio-eustatically driven base level changes controlled partitioning of the incised valley, but the basal incised valley fill unconformity actually represents a composite surface cut in part during sea level fall and lowstand conditions and, in part, during transgression as the channel was responding to high-frequency climatic changes that affected discharge regimes and sediment supply. As a result, there is no unique surface to which sequence boundaries, as classically defined in coastal and marine strata, can be correlated.