Preprints
https://doi.org/10.5194/egusphere-2024-71
https://doi.org/10.5194/egusphere-2024-71
07 Feb 2024
 | 07 Feb 2024
Status: this preprint is open for discussion.

The impact of bedrock meander cutoffs on 50 ka-year-scale incision rates, San Juan River, Utah

Aaron T. Steelquist, Gustav B. Seixas, Mary L. Gillam, Sourav Saha, Seulgi Moon, and George E. Hilley

Abstract. Incision rates of major rivers may reflect the effects of drainage reorganization, hillslope processes, tectonic uplift, climate, the properties of rocks into which rivers incise, and other autogenic processes. On the Colorado Plateau, incision rates along the Colorado River have been interpreted as resulting from abrupt base-level changes produced by the integration of the Colorado River system. Specifically, the integration of the Colorado River in the location of Grand Canyon is thought to have created a knickpoint, enhanced by lithologic contrasts, which is retreating upstream. While evidence exists for a <1 Ma acceleration of incision on parts of the Colorado River, uncertainty about the processes reflected in shorter-term incision rates muddies comparison with longer-term averages. In this work, we combine a cosmogenic radionuclide depth profile exposure age and post-Infrared Infrared Stimulated Luminescence (p-IR IRSL) to date fluvial deposits adjacent to the San Juan River, a major tributary of the Colorado River, near Mexican Hat, Utah. The deposits, resting on a 32 m strath surface, are constrained to be ∼28–40 ka, suggesting an incision rate of 804–1151 m Myr-1, nearly an order of magnitude higher than the long-term rate of ∼140 m Myr-1 over the past ∼1.2 Ma. We observe fluvial deposits that were abandoned due to a bedrock meander cutoff, which partially explains our accelerated incision rate. We use a simple geometric model, informed by our field data, to demonstrate how planform river evolution may, in some circumstances, increase short-term incision rates, relative to long-term incision rates. These short-term rates may also reflect a combination of autocyclic and climatic processes, which limits their ability to resolve longer-term changes in incision rate that may be related to changes in base-level or tectonics.

Aaron T. Steelquist, Gustav B. Seixas, Mary L. Gillam, Sourav Saha, Seulgi Moon, and George E. Hilley

Status: open (until 23 Mar 2024)

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Aaron T. Steelquist, Gustav B. Seixas, Mary L. Gillam, Sourav Saha, Seulgi Moon, and George E. Hilley
Aaron T. Steelquist, Gustav B. Seixas, Mary L. Gillam, Sourav Saha, Seulgi Moon, and George E. Hilley

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Short summary
The rates at which rivers erode their bed can be used to interpret the geologic history of a region. However, these rates depend significantly on the time window over which you measure. We use multiple dating methods to determine an incision rate on the San Juan River and compare it to regional rates with longer timescales. We demonstrate how specific geologic events, such as cutoffs of bedrock meander bends, are likely to preserve material we can date but also bias the rates we measure.