Cosmogenic Nuclide Surficial Age Comparison between Boulders and Desert Pavement in the Mojave Desert, California

Cosmogenic nuclides can be an effective way to analyze surface ages when proper sampling strategies are employed given the depositional environment, erosion history, and approximate age of the surface. Recent cosmogenic dating work was performed on the Sheep Springs Wash alluvial terraces in the Rodman Mountains, Mojave Desert, California. Clast types on the terraces are predominantly quartz monzonite and basalt. Sampling strategies primarily included the tops of boulders (40-120 cm in diameter), which are thought to be the best representation of surface exposure ages. Complimentary surface samples were also collected from the incipient desert pavement for comparison to boulder ages. Desert pavement clasts were slightly angular, some with flat faces, and some with stream-rounded edges. 10Be concentration ages measured from quartz monzonite boulders and amalgamated desert pavement clasts have similar values where the average ages are 58.7 ka and 55.4 ka, respectively, and the maximum ages are nearly identical (63.4 and 63.2 ka), with the greatest discrepancy in the minimum ages of 56.0 ka (boulder) and 48.5 ka (pavement). These ages indicate two possible pavement clast sources, (1) they are from the original stream channel deposit, or (2) they are the result of boulder spalling. Given that many of the pavement surfaces are located in low boulder density areas, contain a wide variety of clast types, including spallation-resistant basalt, and pavement ages are not consistently higher than boulder ages, our contention is that the desert pavement is primarily derived from small clasts deposited on the surface just prior to its abandonment. Contamination of desert pavement samples from clasts exhumed from deeper in the terrace does not appear to have significantly altered its age. Desert pavements may be more attractive to sample because they are far less strenuous and time consuming to collect.