Fluvial response to base level change: A case study of the Brazos River, east Texas, United States
Taha, Zaid Patrick
Anderson, John B.
Doctor of Philosophy
A study of the Holocene avulsion history of the Brazos alluvial valley revealed that two processes, aggradation and valley tilting, were responsible for generating two styles of avulsion, avulsion-by-progradation and avulsion-by-annexation. As aggradation migrated inland, prograding avulsions tracked the locus of aggradation. Concurrently, a nodal avulsion site developed at 55 km inland, creating avulsions by-annexation. Geomorphic evidence suggests movement along a normal listric fault occurred in proximity to the nodal avulsion site. Within the alluvium above the marine-Oxygen isotope Stage 2 onshore incised valley of the Brazos River, the pattern of stacked channels generated by avulsion was mapped to investigate the aggrading response of the Brazos River to sea level rise. The stacked channels within the valley decrease from eight, at 40 km from the coast, to four, at 65 km from the coast, which reflects the diminishing influence of eustacy inland. As aggradation decreased, while the avulsion frequency remained constant, the younger channels became more isolated, in contradiction to previous stacking models. Those models, however, neglected the influence of antecedent topography during aggradation. Vertically, the eight stacked channels within the lower valley are organized into four stratigraphic units that are attributed to changes in the alluvial valley gradient during aggradation, as calculated from the position of backstepping (retrograding) offshore deltas (paleoshorelines) and their correlative (aggradating) onshore floodplain deposits. The style of avulsion and the channel stacking pattern are both understood with respect to realizable subaerial accommodation. Previous subaerial accommodation models emphasized a proportional upward shift in an equilibrium profile during a sea level rise. Yet, an equilibrium profile must be anchored at both ends. The updip elevation of an alluvial valley is controlled by sediment yield and the cumulative aggradation from all earlier episodes of sea level rise, which should exert a limit on the downdip creation of subaerial accommodation. This study, therefore, quantified the differences in the long-term sediment yield of the Brazos and Trinity rivers of east Texas over past sea level cycles, and concludes that the lower sediment yield of the Trinity River has suppressed its equilibrium profile, thereby limiting the present creation of subaerial accommodation.
Physical geography; Geology; Hydrology