FLUVIAL INCISION RATES OF THE SAN JUAN RIVER USING IN-SITU 10-BE, MEXICAN HAT, UTAH

Understanding fluvial incision rates of tributaries of the Colorado River helps constrain the pace and tempo of processes that have shaped this largely arid landscape. Along the San Juan River, previous cosmogenic-radionuclide-based estimates of Quaternary incision have ranged from 700 m/Myr (Hanks et al., 2001) to 110 m/Myr (Wolkowinsky and Granger, 2003). To resolve these differences in rates, we studied fluvial terrace deposits along the San Juan River in the vicinity of Mexican Hat, UT. We collected sand over two meters depth and measured in-situ 10-Be in the quartz fraction. Additionally, structure-from-motion techniques were used to create a detailed DEM of geomorphic surfaces in the surrounding area, with depositional features of older and younger relative ages present. Best-fit depth-profile ages suggest that this surface, which is 36 meters above the active channel, was abandoned at ~40ka, resulting in an order of magnitude higher incision rate compared to a similar study conducted upstream near Bluff, Utah (Wolkowinsky and Granger, 2003). These results suggest that a Quaternary-averaged incision rate like that calculated in the Bluff study may not reflect modern incision rates along the San Juan River. Alternatively, assumptions of low terrace erosion rates could also explain the seemingly low concentrations present. In an attempt to test these hypotheses, we have collected granitic cobbles from below 3m in an attempt to estimate a 10Be/36Cl burial isochron. These samples will potentially resolve whether the incision rates calculated using the depth profile are accurate or if terrace erosion may be contributing significantly to the discrepancies between this study and previous studies conducted in this region.