DATING ALLUVIAL DEPOSITS WITH OPTICALLLY-STIMULATED LUMINESCENCE, AMS 14C AND COSMOGENIC TECHNIQUES, WESTERN TRANSVERSE RANGES, CALIFORNIA, USA.
DeLong, Stephen B.Department of Geosciences, University of Arizona, 1040 E 4th Street, Tucson AZ 85721, USA
Arnold, Lee, J. Oxford Luminescence Research Group, School of Geography and the Environment, University of Oxford, Mansfield Rd, Oxford OX1 3TB, UK
Abstract
In an effort to better understand chronology of alluvial episodes in Cuyama Valley in the western Transverse Ranges of California, USA, we employed optically-stimulated luminescence, radiocarbon and cosmogenic radionuclide surface exposure dating methods. Twenty-one optical dates ranging from 0.01 to ~27 ka were obtained from exposures of late Holocene axial-fluvial deposits, Pleistocene-Holocene alluvial-fan deposits, and axial-fluvial sands interbedded within a late-Pleistocene alluvial-fan. These were cross-checked with thirty-seven AMS radiocarbon dates from charcoal and wood from within fluvial material and five 10Be surface exposure dates from boulders on alluvial-fan surfaces. The OSL results show generally good stratigraphic consistency, logical comparison with the radiocarbon and cosmogenic data, and appear to be the best method for accurate dating within deposits of this nature because suitable material is fairly easy to find in these environments. The radiocarbon data contained numerous “detrital ages”, but well-bedded lenses of apparently in situ or minimally-transported charcoal provide reliable age estimates for the fluvial material. Radiocarbon dating of detrital charcoal in the older alluvial-fan deposits was problematic. Our cosmogenic surface-exposure dating was consistent stratigraphically and with our other data, but we were unable to determine its accuracy due to the limited number of samples and the possibility of inherited radionuclides and post-depositional erosion. In light of our results, we suggest that OSL dating using the latest analytical techniques combined with rigorous methods for analyzing paleodose estimates is reliable and of increasing utility in otherwise difficult-to-date coarse alluvial environments in the southwestern United States and elsewhere.
Corresponding author: sdelong@geo.arizona.edu tel. 520-621-6003