COSMOGENIC NUCLIDE DATING OF ARID REGION ALLUVIAL FANS

Cosmogenic nuclide exposure ages have been used to infer rates and styles of surface and tectonic processes responsible in evolving arid region landscapes. Precise Pliocene-Quaternary fan chronologies in arid regions are necessary for calculating strain rates and seismicity recurrence intervals. Soils geomorphology and cosmogenic 10Be in quartz on alluvium in the U.S. southwest (Providence Mtns., Fish Lake Valley, and Amargosa Valley) reveal that it is not always possible to attain high precision fan chronologies. Examination of alluvial fan sediments and surfaces from four lithologically distinct catchments in the Providence Mtns (eastern California) indicates that although fan aggradation is approximately synchronous around 75±10 ka (QF3 fans), older and younger fans correlated along the same range front do not show this synchronicity. Instead of invoking geomorphological explanations of the exposure age disparities among correlative fans, we turned to the assumptions of the dating method. For the Mojave region, the 75-kyr aggradation event may be the most optimally suited for exposure dating. Older fans have significantly and variably disturbed surfaces (erosion, mixing, and burial also vary with elevation), so exposure ages (from surface boulders and subsurface and pavement pebbles) of correlated fans will vary. Large changes in bulk densities during pedogenesis also need to be considered. For younger fans, inheritance is the dominating control on the ages. In the Providence Mtns. and Amargosa Valley (western Nevada), modern wash sediment has as much as 70-kyr worth of inherited 10Be. Even with concentration-depth profiles using amalgamated pebble samples it is not possible to estimate this amount of inheritance with sufficient accuracy to correct a Holocene fan exposure age. The influence of inheritance diminishes with depositional age of a fan. Furthermore, when the option exists, we suggest choosing fans from rapidly eroding or previously glaciated catchments for dating, and correlating other fans along a range front with soils. Ages on a Holocene fan emanating from a glaciated catchment in Fish Lake Valley show lower standard deviations and are closer to radiocarbon ages than ages on a correlated radiocarbon-dated fan from a non-glaciated adjacent catchment.