Stratigraphy and OSL geochronology of the Biskra Palms alluvial fan, Indio, CA

The Biskra Palms alluvial fan is displaced by two splays of the Mission Creek fault of the southern San Andreas fault near Indio, CA. The lower-most portion of the Biskra Fan, also referred to as the T2 fan, has been quarried by Granite Construction Co. exposing sediment down to 80 m depth. Stratigraphic descriptions and measurements of these sediments within the quarry reveal stacked alluvial fan systems that have been displaced from source canyons farther to the southeast than the Biskra site. Age dating of buried sediments through the Optically Stimulated Luminescence (OSL) technique reveals anomalously young apparent ages for sediment from 1.4 m down to 30 m depth. The young ages are attributed to Thorium disequilibrium. When Thorium values are modified to a factor of 4 times lower than the analyzed amount, the age of the sediment is increased by ~30%. Future studies to determine the offset and source canyon of the ancient fan systems can produce additional slip rate estimates for the SAF at this location. Additional analysis is recommended to determine how Thorium values affect the dose rate and ultimately the calculation of the OSL age of sediment.