SPATIAL VARIATION IN 10BE EROSION RATES AND INCREASING RELIEF IN THE SOUTHERN ROCKY MOUNTAINS

Measurements of cosmogenic ¹⁰Be in alluvium imply erosion rates on a 10³-10⁴-year timescale for small (0.01-47 km²), unglaciated basins in northern Colorado, southern Wyoming and adjacent western Nebraska. Basins that formed in Proterozoic cores of Laramide ranges are eroding more slowly (23 ± 7 mm kyr^-1, n = 20) than adjacent basins draining weakly lithified Cenozoic sedimentary rocks (69 ± 31 mm kyr^-1, n = 20). Erosion rates are correlated with rock resistance and, for a given rock type, to basin slope, but not to mean annual precipitation. We also estimated longer-term (> 10⁵-year time scale) erosion rates for the granitic core of the Front Range by measuring the concentration of ¹⁰Be and ²⁶Al produced mainly by muon interactions at depths 1.7 to 10 m below the surface. Concentrations imply erosion rates of 10-40 mm kyr^-1, similar to shorter-term erosion rates inferred from surface sediment. The spatial distribution of erosion rates taken with stratigraphic evidence imply that relief in the southern Rocky Mountains increased in the late Cenozoic; modern relief probably dates from post-middle Miocene time.