2022 CSDMS meeting-081


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A new explanation for bedrock-pediment incisions under toe-moving conditions described as a diffusive system

Madeline Kelley, (she/her),Arizona State University Tempe Arizona, United States. mmkelle4@asu.edu
Christian Gonzalez, Arizona State University Tempe Arizona, United States. cgonzle1@asu.edu
Mark Schmeeckle, Arizona State University Tempe , United States.
Ron Dorn, Arizona State University Tempe , United States.

The Luke and Higley basins of Phoenix, AZ (USA) were once endorheic basins that gradually filled up with sediments (i.e., they aggraded). At the start of the Pleistocene ( ~2.5 Ma), the Salt and Gila rivers integrated into these basins, changing them to exoreic rivers. Aggradation remained after integration and persisted to the present day, producing a continuous local base-level rise. In the presence of aggradation, the expectation is to observe channel infilling on pediments and alluvial fans. However, we observed the exact opposite condition in some cases: increased incision. We hypothesize that a massive lateral shift in piedmont base-level produced by Salt and Gila rivers integration explains the increase in the local incision, despite the basin aggradation. We tested our hypothesis through a 1D diffusion model representing an idealized piedmont profile under different toe displacement conditions. The diffusion simulations support the hypothesis that base-level rise and lateral shifting can generate piedmont incisions. Indeed, incisions would only appear if u*tan( β)/v > 1, where u, v, and β are the rate of lateral shift, rate of base-level rise, and initial elevation angle of the piedmont, respectively. Our findings suggest that some past sedimentological records of pediments and alluvial-fan systems could have been misinterpreted (i.e., associated with base-level fall). However, additional research is necessary to confirm our initial findings.