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Late Quaternary history of the Chemehuevi Mountain piedmont, Mojave Desert, deciphered using ¹⁰Be and ²⁶Al

Kyle K. Nichols^*,, Paul R. Bierman^*, M. C. Eppes^**, Marc Caffee^***,, Robert Finkel^*** and Jennifer Larsen^****

^* School of Natural Resources and Department of Geology, University of Vermont, Burlington, Vermont 05405
^** Department of Geography and Earth Science, University of North Carolina Charlotte, Charlotte, North Carolina 28223
^*** Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, Livermore, California 94405
^**** Department of Geology, University of Vermont, Burlington, Vermont 05405

Present address: Department of Geosciences, Skidmore College, 815 N. Broadway, Saratoga Springs, New York 12866; email: knichols{at}skidmore.edu

Present address: PRIME Laboratory, Purdue University, West Lafayette, Indiana 47907

We used the cosmogenic nuclides, ¹⁰Be and ²⁶Al, as cosmic-ray dosimeters to track sediment movement across the incised Chemehuevi Mountain piedmont surface. The piedmont extends 12 km from highland source basins to an ephemeral wash at the toe of the piedmont. Nuclide activity in sediment from steep source basins and from the bedrock pediment allows us to estimate rates of sediment generation and thus the rate at which sediment is supplied to the piedmont from weathering rock. Analysis of amalgamated sediment samples, collected along 4-km-long transects spaced at 1 km intervals from the mountain front provide context for understanding nuclide activity in sediment samples collected from two soil pits and demonstrate that sediment steadily moves down the channel network. Samples from the two soil pits allow for estimates of sediment deposition rates and thus further illuminate piedmont history.

Model results suggest that the Chemehuevi Mountains are eroding about 40 mm ky^–1 whereas the mountain-proximal bedrock pediment and the interpiedmont Sawtooth Range are eroding more slowly, 10 to 21 mm ky^–1 and 9.5 mm ky^–1, respectively. Cosmogenic nuclide profiles in two soil pits suggest a complex history of deposition (at rates between 19 to 39 mm ky^–1) and stability over the past 60 ky. A change in process from sediment deposition to sediment transport occurred distally on the piedmont approximately 8 ky ago, while the mid-piedmont piedmont surface has been stable since the mid-Holocene.

The incised Chemehuevi Mountain piedmont is changing at rates similar to planar and uniformly active piedmonts such as those adjacent to the Iron and Granite Mountains. All three piedmonts have average sediment grain velocities of decimeters per year, long-term source basin erosion rates between 35 to 40 mm ky, depositional hiatuses around the Pleistocene-Holocene climatic transition from moister to drier conditions, and Pleistocene deposition rates of 18 to 39 mm ky. Such similarity in piedmont behavior, despite clear differences in morphology and appearance, might suggest large scale geologic and climatic controls on the generation, transport and deposition of sediment in the Mojave Desert.

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				RECENT PAPERS Geological Society of America Special Papers, January 1, 2006; 415(0): vii - xii. [Full Text] [PDF]