The Late Quaternary Brazos and Colorado Deltas, Offshore Texas, U.S.A.—Their Evolution and the Factors that Controlled their Deposition
Kenneth C. Abdulah, John B. Anderson, Jennifer N. Snow, Lynette Holdford-Jack, 2004. "The Late Quaternary Brazos and Colorado Deltas, Offshore Texas, U.S.A.—Their Evolution and the Factors that Controlled their Deposition", Late Quaternary Stratigraphic Evolution of the Northern Gulf of Mexico Margin, John B. Anderson, Richard H. Fillon
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Over the last 150,000 years, both the Brazos and the Colorado rivers have constructed large, fluvial-dominated deltas that migrated across the Texas continental shelf in response to variations in global sea level, sediment supply, and climate. The drainage basin of the Brazos River encompasses dry subhumid and semiarid climatic belts, and large discharge and accompanying fine-grained, suspended sediment load are typical. The drainage basin of the Colorado River also straddles subhumid to semiarid climatic belts with sediment contributions from eroding crystallized basement and sandy Paleozoic bedrock. The resulting immature, coarse-grained sediment load together with lower discharge rates have resulted in a stronger component of bed-load transport in the case of the Colorado River than for the Brazos River.
A regional chronostratigraphic framework, based on biostratigraphic ages, existing tephrochronology, oxygen-isotope stratigraphy, and radiocarbon dates, and tied to regional seismic horizons, allows us to examine and compare depositional styles of these rivers and their deltas during periods of global sea-level highstand, lowstand, and transgression. Key bounding surfaces are (1) regionally extensive erosional surfaces (sequence boundaries) tied to eustatic lowstands (oxygen isotope Stages 6 and 2), and (2) regionally extensive coherent, high-amplitude reflectors (flooding surfaces) correlated to oxygen isotope Stage 5e and 3 highstands. The comparison of the depositional styles and the timing of the deposits allow inferences regarding the factors that controlled deposition.
The sedimentary record during the falling limb of sea level (oxygen isotope Stages 5e, 5c, 5a, and 3) was characterized by prograding, and laterally migrating, fluvial-dominated deltas. There is a marked absence of Stage 4 deposits on the shelf. During the last maximum (Stage 2) lowstand, the sandy Colorado shelf-margin delta directly sourced two slope fans, and the Brazos River abandoned its late-highstand channel and shifted to the east, merging with the incised Trinity–Sabine incised valley. These valleys supplied sediment to upper-slope minibasins during the lowstand. The shelf-margin delta associated with the Brazos incised-fluvial-valley system consists mainly of mud and interbedded silt layers with little evidence for shelf-margin collapse during the last lowstand.
During the early part of the Stage 2 to Stage 1 transgression, the Brazos and Colorado shelf-margin deltas backstepped onto the outer shelf. The rapid transgression and associated erosion removed their topset beds. Continued transgression led to transgressive erosion, decapitation of sandy fluvial and deltaic facies, reworking of these sands into shelf sand bodies, and further backstepping of these transgressive, shelf-phase deltas.
Owing to a higher, and potentially more consistent, sediment supply, the Brazos system was less affected by variations in climate; eustasy played a far greater role in the distribution of lithofacies for the Brazos system than was the case for the Colorado system.
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The northern Gulf of Mexico margin encompasses a variety of depositional settings characterized by different drainage basin size, physiography, fluvial morphology, climatic setting, and structural and diapiric activity. This, plus the abundance of long sediment cores and platform borings from oil industry activities, make it an unparalleled natural laboratory for sedimentological and stratigraphic studies and for testing sequence stratigraphic concepts. This volume contains twelve papers describing results from high-resolution stratigraphic studies of late Quaternary strata of the northern Gulf of Mexico, from the mouth of the Apalachicola River to the Rio Grande. These papers focus on fluvial response to climate and base-level change, variations in delta growth and evolution across the shelf, lowstand delta-fan evolution, the evolution of transgressive deposits on the shelf, the preservation of these deposits. The robust chronostratigraphic frameworks developed for the different study areas allows comparison of stratal geometries produced by contemporaneous depositional systems operating under identical eustatic conditions. This volume will appeal to sedimentologists and stratigraphers interested in source to sink issues, such as how various forcing mechanisms influence strata formation on continental margins.