ANOMALOUS OSL APPARENT AGES OF THE BISKRA PALMS ALLUVIAL FAN

Recent efforts to constrain the geologic slip rate of the Coachella Valley segment of the San Andreas fault have employed several age-dating methods to determine the age of an offset alluvial fan near Indio, CA. Cosmogenic ¹⁰Be, soil development, and U-series on pedogenic carbonate methods all yield data consistent with a fan age of 45-54 ka (Behr et al., GSA Bulletin, in press; Fletcher et al., GSA Bulletin, in press). In contrast, this study reports Optically Stimulated Luminescence (OSL) analyses of quartz grains that yield anomalously young and out-of-sequence apparent ages for sand layers 1.4 to 30 m beneath the fan surface. The discrepancy between the OSL and other age-dating methods is probably due to a number of factors that are inherent to the Biskra Palms site. Chemical analyses reveal that the amount of Thorium present in some of the sampled sediment is higher than expected and can thus mean there is disequilibrium in the samples. This disequilibrium may result in a high dose-rate and consequently an underestimate in OSL age. Additionally, it is likely that a wetter late Pleistocene climate resulted in a higher water content of the sediment at this location. A 30 to 50% increase in water content can increase the age calculation by up to 25%. Lab analyses also indicate that the quartz luminescence signals are very weak and are dominated by a slow-medium component. This may result from the coarse and proximal setting of the sampled sediment and/or the sediment possibly being eroded directly from bedrock. The sediment may not have undergone enough transport and deposition cycles to sensitize the sediment and become fully bleached in this proximal setting. Anomalous lab results usually signal a white flag to researchers with regard to the validity of an age-dating method. Using more than one age-dating method where possible can help corroborate the data. In the case of Biskra Palms, cosmogenic and U-series methods seem to corroborate with one another, but there are problems with the OSL analysis. These results suggest that using the OSL method in proximal alluvial fan settings may yield results that do not represent the depositional age of the sediment.