Luminescence dating of sand ramps in the Eastern Mojave Desert

Abstract

Until recently the timing of the movement of sand and accumulation of sand by aeolian processes in the Eastern Mojave Desert has remained the subject of speculation. The results of a luminescence dating program involving 78 samples of material from nine sand ramp complexes have enabled recognition of regional and local patterns of sand accumulation in the Eastern Mojave. The study area extends from Lake Manix, in the west, to the Colorado River, in the east. The periods of accumulation have been identified on the basis of quartz TL dating, and potassium feldspar TL and IRSL dating. At a regional scale, two major depositional phases can be identified: (1) Late Pleistocene: 20–30 ka, with sequences at Hank's Mountain, Balch, Dale Lake and the Big Marias; and (2) Late Pleistocene-Early Holocene: 15-7 ka with sequences at Cat Dune, Dale Lake, Iron Mountain, Big Marias (above palaeosol) and, at the western end of the study area, at Soldier Mountain (20-7 ka). Late Holocene sequences appear to be much more localized, and are confined to the West Cronese, Old Dad Mountain and Balch sand ramps. The earliest phases of sand accumulation on the ramps studied immediately predate the formation of pluvial Lake Mojave (ca. 24.5 ka). The later main phases of accumulation coincide with the existence of Lake Mojave and the end of accumulation on the majority of the sand ramps immediately post-dates the final end of Intermittent Lake Mojave III (9.7 ka). The continuity of sand supply to potential deflation areas is seen as a critical condition for accumulation of material on the sand ramps during the Late Quaternary in the Eastern Mojave.