HR: 15:25h
AN: U23B-07    [Abstracts]
TI: Boulder, Pavement, Pit; Sample Selection for Cosmogenic Nuclide Dating on Alluvial Fans
AU: * Perg, L A
EM: lperg@umn.edu
AF: University of Minnesota, Geology and Geophysics 310 Pillsbury Dr SE, Minneapolis, MN 55455 United States
AU: Oskin, M E
EM: oskin@email.unc.edu
AF: University of North Carolina, Geological Sciences Mitchell Hall, CB#3315, Chapel Hill, NC 27599 United States
AU: Blumentritt, D
EM: blum0123@umn.edu
AF: University of Minnesota, Geology and Geophysics 310 Pillsbury Dr SE, Minneapolis, MN 55455 United States
AU: Strane, M
EM: michael.strane@gmail.com
AF: University of North Carolina, Geological Sciences Mitchell Hall, CB#3315, Chapel Hill, NC 27599 United States
AU: Mukhopadhyay, S
EM: sujoy@eps.harvard.edu
AF: Harvard University, Earth and Planetary Sciences 20 Oxford St, Cambridge, MA 02138 United States
AB: Selecting sample targets and methods that minimize the effects of erosion, inheritance, and material movement is one of the largest issues facing cosmogenic isotope dating. Since sample material availability often sharply limits methods, establishing error estimates in less-than-ideal sampling situations is also important. Our study of alluvial fan offsets in the Eastern California Shear Zone (Mojave Desert) takes advantage of multiple sample targets to develop method inter-comparisons. Alluvial fans along the Calico fault had abundant large basalt and quartz monzonite boulders, targets for 3He and 10Be respectively. In addition to the boulders, the incipient desert pavement on the youngest alluvial fan surface was sampled. The basalt boulders had higher relative cosmogenic nuclide concentrations, and a much higher scatter, due to very high inheritance and also likely boulder recycling into younger terraces. The quartz monzonite had lower, consistent concentrations on the youngest terrace, but presented erosional concerns due to the spalling and grussification common to granitic material. The incipient desert pavement had a comparable 10Be concentration to the quartz monzonite boulders; the highest concentration in each was the same, and the average pavement concentration lagged the boulders only slightly. The Lenwood fault alluvial fans had two targets. Small clasts formed weakly developed to mature pavement on the surfaces. The fan deposits were also thick enough to dig pits (1.5m). On the younger fan surface, both sediment and clasts (gravel to cobble) were collected in the pit; concentrations were comparable between the two. The profiles indicated high inheritance (comparable to modern wash samples), and some mixing at the near surface. Due to high inheritance, the surface sample alone would far overestimate the age. Using wash samples to estimate inheritance slightly overcorrects, due to mixing in the near-surface. Two types of clasts were sampled in the older surface, mixed granitoids and hydrothermally altered, cryptocrystalline metavolcanics. Successive leach steps and comparable concentrations to the granitoids indicate that the metavolcanics provide a good sample target. Obvious migration of the clasts, due to ped formation and surface dust inflation, make age interpretations more challenging. Surface samples become a relatively better target, since the contribution from inheritance becomes proportionally less through time.
DE: 1824 Geomorphology: general (1625)
DE: 1865 Soils (0486)
DE: 9350 North America
DE: 9605 Neogene
SC: Union [U]
MN: Fall Meeting 2005