LATE CENOZOIC INCISION RATES OF THE UPPER COLORADO RIVER, WESTERN COLORADO, CONSTRAINED BY BURIAL OF GRAVELS BY BASALT DEBRIS-FLOWS

The upper Colorado River has incised up to 1 km during the late Cenozoic; yet the time of initiation of this incision and the ensuing rate and path of downcutting remain unclear. As part of a project to understand late Cenozoic incision of the Roan Plateau, Colorado, by tributaries to the Colorado River, we target the incision history of the upper Colorado River where it is bounded by the Roan Plateau to the north, and by Battlement Mesa to the south. We rely on the unique geologic setting, in which multiple basalt-rich debris-flows, presumably originating from the erosionally retreating Battlement Mesa, spilled out over the Colorado River floodplain, burying river gravels and thus preserving the former position of the river. The oldest of these buried gravels is preserved atop Mt. Callahan, beneath a debris flow containing basalt blocks up to 1-m across, at an elevation of 2623 m, and ~1100 m above and directly north of the modern Colorado River. Five of these blocks have an average ⁴⁰Ar/³⁹Ar age of 9.17 ± 0.06 Ma, which is consistent with two dated basalt flows on North Mamm Peak of Battlement Mesa, ~22 km to the southeast (elevation of 3390 m), the most likely source of debris-flow material. Since deposition of these basalt blocks at or after ~9.17 Ma, the Colorado River has downcut ~1100 m, at a minimum mean rate of 120 m/My. We are now using the cosmogenic burial age method to date lower, and hence younger, Colorado River gravels on Grass, Holms, and Flatiron Mesas directly south of the Colorado River. These gravels were deposited atop strath bedrock surfaces and subsequently buried by debris-flows from Battlement Mesa. We sampled gravels at a depth of 1 m. Relying on mesa geometry to constrain minimal post-burial production of cosmogenic radionuclides, we use the ²⁶Al/¹⁰Be ratio in the samples to date their burial. We interpret burial ages as minimum ages for occupation of the terrace by the Colorado River. A burial age of 1.77 +0.71/-0.51 Ma for river gravels on Grass Mesa, 225 m above the modern river, yields a mean incision rate of 127 m/My. We will use burial ages from Holms and Flatirons Mesas, at 621 m and 198 m above the modern Colorado River, respectively, to improve the resolution of our incision record and allow more meaningful interpretation of the geomorphic and tectonic history of this area.