A LONG, COMPLEX HISTORY IN THE DALE LAKE SAND RAMP, SAN BERNARDINO COUNTY, CALIFORNIA

Luminescence dating of eolian deposits in the eastern Mojave Desert during the 1990s suggested that sand ramps were principally constructed during two depositional phases over the past 40 ka. At the massive Dale Lake sand ramp, situated on the west flank of the Sheep Hole Mountains, a large arroyo exposes a minimum of 12 buried soils, a large buried channel, and an alluvial fan at the base of the ramp. Only stratigraphic units in the lower part of the sand ramp can be traced across the arroyo, which indicates a spatially complex history of deposition and erosion. A prominent Stage III carbonate-rich soil near the top of the ramp north of the arroyo is 70-120 ka by U-series dating. Two carbonate-rich soils located in the upper third of the ramp base are dated at greater than 300 k. Thus, the bulk of the deposits in upper sand ramp are of Middle Pleistocene age. This is confirmed by green-light-stimulated (GSL) ages on quartz grains of >115 ka and >152 ka for a cross-bedded sand fill in a deep buried channel exposed on the south wall of the arroyo. The channel was probably incised between 200-160 ka (OIS VI) by runoff from adjacent bedrock slopes. The uppermost sand on the south arroyo wall is 30-35 ka, dated by GSL, and the soil carbonate adjacent to and overlying it is about 30 ka, dated by U-series. The next lower eolian unit has a GSL age of about 48 ka. Arroyo cutting was also active during the last glacial episode: sand was eroded from the upper ramp and re-deposited across its base. Two GSL ages of 15-17 ka were obtained near the top of the fan.

GSL and U-series dating indicates that a third, if not more, of the Dale Lake sand ramp was constructed during the Middle Pleistocene. The Late Pleistocene is represented by thin wedges of eolian sand, scattered channel gravels, and buried soils on the lower two thirds of the sand ramp. The youngest ramp deposits have been reworked by wind from alluvial fans constructed at the toe of the sand ramp during the Last Glacial Maximum.