AN OSL GEOCHRONOLOGIC FRAMEWORK FOR THE ZIEGLER RESERVOIR FOSSIL SITE, SNOWMASS VILLAGE, COLORADO

High-altitude discoveries of diverse remains of Late Pleistocene fauna and flora are relatively uncommon. The Zeigler Reservoir site, near Snowmass, Colorado provides a unique opportunity to test the application of optically stimulated luminescence (OSL) to date glacio-lacustrine sediment within a small alpine area basin. We used four techniques for dating; radiocarbon, U-series, and cosmogenic nuclides, but these three met with less success than the luminescence application. We date a series of 19 samples taken from the lake sediment using a combination of single-aliquot OSL on quartz and IRSL on feldspar. Analysis of the sample equivalent dose distribution characteristics reveals that these lacustrine sediments were subjected to an almost complete bleaching prior to deposition, contrary to expectations. Where the sample distributions are not as well dispersed, the sediments incorporate more debris flow component than lacustrine sediment. The use of appropriate burial dose rates and moisture content is also deemed vital to ensure that accurate age estimates are produced for this site. The resultant geochronologies are examined and discussed. These formal decision procedures yield sample ages that are stratigraphically consistent for 100% of the 15 lacustrine samples examined. The resulting OSL ages were used to produce a Bacon age-depth model that aims to model flexible accumulation rates (modified after Blaauw and Christen 2011). The Bacon model typically uses uncalibrated radiocarbon ages as inputs, but was modified to accept luminescence ages for this project.