LATE PLEISTOCENE MONO BASIN BEACH BERMS, CALIFORNIA: PRELIMINARY OSL AGES

A series of 35 prominent beach berms, up to 30 km in length, line the NE shorelands of Mono Lake at elevations of between 1989 m and 2149 m (45 m to 218 m above the present-day lake surface). These depositional landforms record centennial-scale shoreline transgressions that interrupted overall lake recession from its late Pleistocene highstand (2155 m at 16 ka based on U-series dating- G. Ali, Ph.D. in progress). None of the 35 berms has heretofore been directly dated.

To determine whether optically stimulated luminescence dating is appropriate for sediments that compose the berms, we collected six samples from mostly back-berm positions in three landforms. The sand in the berms is derived from first-cycle weathering of local granitic and volcanic bedrock. Transport distances along the beaches were likely less than 30 km. We used 180-250 µm quartz grains and pulsed optically stimulated luminescence analysis of single aliquots because the quartz apparently contained feldspar inclusions that we were unable to remove with two 40% HF treatments. In-situ water contents were interpreted to be representative of this arid basin. Infrared stimulated luminescence (IRSL) from K-rich feldspar fractions was used to test the accuracy of quartz ages; smaller IRSL estimated doses (D_e) values from each sample suggests that the quartz fractions were likely well bleached. Acceptable statistics were obtained with about 18 aliqouts.

Analyses are not yet complete, but preliminary data (D_e , some dose-rate values, and few ages) from six samples collected from three of the berms give reasonable results based on four tests: First, the individual berms progressively young (within the margin of error) in the lakeward direction, a relationship mandated by the geomorphological processes that deposited them. Second, the berms all date apparently younger than the 16 ka highstand. Third, one of the berms, in three samples, produced values that are within the margin of error of one another. And fourth, the bulkiest and most highstanding of the berms (and by inference the one that represents the greatest amount of lake-level rise) yielded an age consistent with the Younger Dryas chron--a conclusion in keeping with independently dated (by U-series and ¹⁴C) near-shore depositional features at similar elevations elsewhere in the basin (Ali, Ph.D. in progress).