The evolution of the Brazos and Colorado fluvial/deltaic systems during the Late Quarternary: An integrated study, offshore Texas
Abdulah, Kenneth Christopher
Anderson, John B.
Doctor of Philosophy
The evolution of the Brazos and Colorado fluvial/deltaic systems over the last 190,000 years is examined. The data set includes high-resolution seismic data, lithostratigraphic data from offshore platform borings, cores and gamma-ray logs, biostratigraphic data, oxygen isotope data, and radiocarbon dates. Major periods of fluvial/deltaic deposition for the Brazos and Colorado fluvial/deltaic systems have been mapped through time, and across 20,000 square kilometers of the Texas continental shelf and upper slope. Chronologic control has allowed an independent assessment of the timing and nature of fluvial/deltaic deposition relative to the SPECMAP oxygen isotope curve. The Brazos and Colorado fluvial/deltaic systems have responded in phase with eustasy and facies distribution is related to fourth-order and fifth-order eustatic cycles. The Brazos fluvial/deltaic system developed deltas with highstand geometries during each of the oxygen isotope stages 5e, 5c, 5a, and 3. Fluvial incision and sediment bypass characterize the lowest sea-level stands. The Stage 2 to Stage 1 transgression is marked by a planar, time-transgressive ravinement surface. During the transgression, the Brazos deltas backstepped across the shelf. An oxygen isotope record, generated from benthic foraminifera, records the Mississippi meltwater pulse during the Stage 2 to Stage 1 transgression. Identification of isotope Stage 5 and Stage 3 is made possible by correlating the isotope data with paleoenvironmental data and regional downlap surfaces from seismic data. The interpretation of sea-level history and its role in fluvial/deltaic sedimentation has, therefore, been arrived at using three branches of stratigraphy (i.e., oxygen-isotope stratigraphy, biostratigraphy, and seismic stratigraphy). Lithologic data provide the ground truth for the seismic facies analyses. An additional aspect of this study was the acquisition of a low-cost, high-resolution 3-dimensional seismic survey. The experimental 3-D survey successfully imaged an incised valley and its sedimentary fill. The survey covered a 750 m x 2000 m area over the Stage 2 Brazos incised valley.