THE LATE QUATERNARY BRAZOS AND COLORADO DELTAS, OFFSHORE TEXAS, U.S.A.—THEIR EVOLUTION AND THE FACTORS THAT CONTROLLED THEIR DEPOSITION

Abstract

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.