Wonsuck Kim, PhD, University of Texas (Austin)
Fluviodeltaic systems often exist on Earth under complex tectonic conditions, which create a myriad of motivations to understand the deltaic landscape evolution associated with tectonic activity. During the 1st year of the project we conducted a series of six experiments in the Sediment Transport and Earth-surface Processes (STEP) basin facility at the University of Texas at Austin. The STEP basin’s dimensions are 4-m long, 5-m wide, and 1.5-m deep, and contains a hinged table that acts as a subsiding basin basement, which can be raised or lowered to create many different subsidence patterns in combination with placement of the sediment source. We utilized the table to impose lateral basement tilting to examine the effects of spatially varying rates of subsidence on an evolving fluviodeltaic system. Time lapse images were taken every 20 seconds and topographic scans were collected every 30 minutes to capture the delta evolution by means of shoreline planform pattern associated with channel migration in response to the tectonic tilting throughout the experiments.
The hinge axis at the center of the basin maintained a constant base level and thus created a bisection of the evolving delta. The relative base level fall occurred on one side of the delta (uplift), while a relative base level rise occurred on the other side of the delta (subsidence). The differential relative base-level changes on both sides of the rotation axis were applied to the experiments with a different rate but kept constant for each individual experiment. This series of experiments showed variations in the overall asymmetrical shoreline planform pattern. The slow-tilting runs resulted in stronger shoreline progradation in the relatively uplifted side of the basin due to the shallow water depth in front of the delta, and caused asymmetrical shoreline pattern. However during the fast-tilting runs, the dominant section of prograding shoreline shifted to the subsiding side of the basin. The rapid tilting prevented progradation on the uplifted side and instead steered the channels in the direction of subsidence. The stratal architecture in the final deposit portrays the aforementioned processes at work in the modeled delta system. In areas where uplift is a dominant process, as the shoreline prograded, delta developed laterally discontinuous topset deposits with fairly deep scours and amalgamated channel sands, while on the other hand, areas of subsidence had aggradational stacking of well-preserved channel belt and overbank deposits.
The tectonically influenced fluvial processes organized themselves into a unique deltaic coastal pattern, providing insight into the integration of small-scale processes and large-scale landform. This link between the understanding of fluvial processes and landform changes further aids to couple the surface process with sedimentary record.