Paper No. 4
Presentation Time: 1:55 PM

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


DETHIER, D.P., Dept. Geosciences, Williams College, Williamstown, MA 01267, OUIMET, William B., Geography, University of Connecticut, Beach Hall, Unit 2045, Storrs, CT 06269-4148, BIERMAN, Paul, Department of Geology and Rubenstein School of Environment and Natural Resources, University of Vermont, Delehanty Hall, 180 Colchester Ave, Burlington, VT 05405, ROOD, Dylan H., AMS Laboratory, Scottish Universities Environmental Research Centre (SUERC), East Kilbride, G75 0QF, United Kingdom and BALCO, Greg, Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709, ddethier@williams.edu

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.