Linking Grain Size and Sedimentary Structure to Autogenic and Allogenic Processes Associated with Holocene Valley Infill and Evolution, Brazos River, TX

Abstract:

The Brazos River exhibits extraordinarily well-constrained allogenic processes including sea level and climate change, and our understanding of these variables prime the Brazos River to be an excellent natural laboratory for the examination of fluvial morphodynamics under the influence of allogenic and autogenic processes. This research seeks to elucidate an understanding of autogenic and allogenic signatures on stratigraphy through morphodynamic modeling of aggradation and avulsions of the Brazos River in conjunction with climate and sea level changes throughout Holocene time. Backwater length scales are hypothesized to propel autogenic responses of the Brazos system by dictating the loci of aggradation, which back-step and fill the valley as sea level rises. The backwater length scale and sea level covary, thus inducing an intrinsic, but historically complex, relationship between autogenic and allogenic processes that is sought to be discerned using the morphodynamic model presented herein. We simulate Brazos fluvial evolution via changes in fluid flow, sediment transport, and bed topography under the influence of allogenic perturbations. The results are used to characterize and quantify the stratigraphic evolution of the Brazos incised valley fill and are readily comparable to numerous previous studies of the system. The infill model is informed by grain size data collected from modern deposits and by modern channel dimensions. This research attempts to link rising Holocene sea level with a backwater length scale which coincides with the extent of back-stepping aggradation within the Brazos incised valley. Modeling responses to perturbations of the Brazos fluvial system can be applied, more holistically, to predict future coastal dynamics and to inform interpretations of paleo-fluvial systems and hydrocarbon reservoirs.