Timescales of Bedrock Valley Widening: Using Optically Stimulated Luminescence and Field Measurements to Explore the Roles of Talus Pile Age, Talus Volume, and Valley Wall Height on the Genesis of Wide Bedrock Valleys

Bedrock valleys play a vital role in the evolution of landscapes as they are largely responsible for the rate a landscape will evolve and the shape it will take. If vertical incision outpaces lateral erosion, the result will be a deep narrow valley, while if lateral erosion outpaces vertical incision, wide bedrock valleys can form. Of the two processes, bedrock valley widening has received relatively little attention. For a valley to widen, the river must first laterally erode or undercut the valley wall until the overlying bedrock collapses into a talus pile. Once the material has collapsed, it then must be transported away from the bedrock wall so that the river can continue to erode the valley wall. This two-step mechanisms for valley widening suggests that the rate at which a valley widens is dependent not only on the ability of the river to erode the bedrock wall, but also the capability of the river to remove the collapsed material. The volume of the talus pile, the size of the individual talus blocks, the transport capacity of the river, and the height of the valley wall likely influence this process. The Buffalo National River in northwest Arkansas, with its wide and narrow valleys, dormant tectonism, and uniform climate is an ideal natural laboratory for evaluating the factors that influence the bedrock valley widening process. Data collected from 30 sites in both wide and narrow valleys along the Buffalo River includes volume and grain size distributions of talus piles, channel geometry, and the height of the valley walls. Using this data, we calculate timescales of talus pile evacuation based on the transport capacity of the river. The greater the volume of the talus pile, the longer it will take the river to remove the pile and the larger the individual talus blocks, the less often the river will exceed the transport threshold of the blocks. To better understand the residence times of collapsed bedrock material, we collected OSL samples from both talus blocks and underlying sediment to explore the link between talus pile age, talus pile characteristics, and valley width. This novel dating method and ideal study site offer a unique opportunity to shed light on the processes required to create wide bedrock valleys, a common geographical feature whose genesis is not yet understood.