REGOLITH FORMATION ON AN ALPINE HILLSLOPE

The rate of conversion of bedrock into loose, transportable regolith exerts control over rates of erosion, and consequently over landscape evolution, in weathering-limited landscapes. Models commonly frame regolith production rate as a function of regolith thickness, without addressing the specific weathering processes involved. This study explores whether chemical or physical weathering processes control regolith development from granitic bedrock on an alpine hillslope at an elevation of 3600 m at Osborn Mountain, Wind River Range, Wyoming. Cosmogenic nuclides constrain regolith production rates to ~14 m/My, yielding an average particle residence time in the 1m thick regolith of ~70 ky. Mineralogy of the <2mm fraction determined with quantitative XRD shows smectite + illite (13 wt%), chlorite (8 wt%) and kaolinite (<1 wt%) in the regolith. A mass balance approach is used to quantify chemical solid phase mass gains and losses. We calculate τ, the percent mass changes of an element in a sample relative to the parent material, using Zr as an immobile element. In 5 pits along a 150 m hillslope, the average mass losses are K (-0.54) > Si (-0.33) > Al (-0.10) > Na (-0.04), while mass gains are Mg (2.23) > Fe (0.92) > Ca (0.18). The gains of Mg and Fe, low Na losses, and the presence of smectite and chlorite could reflect inputs of volcanic ash, likely from Cascade Range volcanoes, or inputs of eolian dust. We infer that these inputs dominate the calculated τ, and therefore that chemical weathering plays a small role in regolith formation here. Physical processes at Osborn Mountain are dominated by frost cracking and frost creep. We saw no evidence of burrowing animals, and the tundra vegetation was sparse. The bedrock from which regolith is derived is highly fractured. From surface temperature measurements, we calculate that temperatures are suitable for frost-cracking 4.5% of the year under present conditions. The implication is that about 2 weeks of frost cracking per year is responsible for the regolith production rate.