Linking climate, sea level, and sedimentary response on the Texas shelf and upper slope: Examples from the Brazos and Colorado fluvial-deltaic systems
AbstractA 2D high-resolution seismic grid was combined with cores to develop a stratigraphic model of the East Texas shelf and slope, which showed that fluvial deltaic systems do not significantly impact upper slope stratigraphic sequences until very late during a eustatic fall. Rather, prograding delta lobes on the shelf correlate to a condensed section on the slope that continued until the deposition of a shelf-edge delta during OI Stage 3. Soon after, the Brazos system shifted away from the study area and a thick, shale-dominated healing phase wedge developed. These wedge sediments originated from a shelf-edge delta to the east and were transported westward along the upper slope revealing that significant sediment volumes moved not only downdip, but also laterally, into and out of the expected area of influence.
During the transgression, the Brazos, Colorado, and Rio Grande fluvial systems each constructed a succession of deposits beginning with a fluvial-dominated delta (12--9 ka), followed by a wave-dominated delta (9--5 ka), and then retreated from the shelf. This unique succession correlates to climate changes occurring in their upper drainage basins. A separation in climate conditions between central Texas and northwestern Texas developed from 14--8 ka. Central Texas became arid, while northwest Texas experienced wet/dry fluctuations. Climate studies from other southwestern states indicate an intensification of the Arizona Monsoon was the cause, driven by a solar insolation maxima. Before this interval, climate and eustatic forcing functioned in tandem. Between 12--9 ka however, the two mechanisms diverged, with the insolation maximum occurring well before the sea level high. This divergence allowed the fluvial response to climate to be deconvolved from the response to eustacy.
The Modern Brazos Delta continues to reflect a strong climate influence. The delta plain is composed of preserved ridge/trough pairs that correlate to climate fluctuations. Each rige/trough pair corresponds to a La Nina induced drought that is broken by an El Nino induced flood. Other strong El Nino events not represented on the delta plain belong to a dominantly wet period in Texas so that, without the proceeding drought, sediment was not available to initiate ridge development