A WELL-PRESERVED SEQUENCE OF HIGH-ELEVATION ICE AGE ECOSYSTEMS: CAN THE ZIEGLER RESERVOIR SITE, SNOWMASS, CO BE DATED USING OPTICALLY STIMULATED LUMINESCENCE?

The discovery of mammoth fossils during the excavation of Ziegler Reservoir, Snowmass Village, CO in October 2010 gradually revealed one of the most prolific Pleistocene paleontological sequences in North America. The fossils are found in a high altitude (2730 m) basin that may have formed when a Bull Lake glacier dammed a small catchment ~130-150 ka ago. Attempts to date wood and shells from the uppermost sediments revealed the lake was filled by at least 45 ka ago, which presents the possibility that as much as 100 ka may be represented by the lake sediments. To date the lake basin sediment, and, by inference, the fossils, we collected 11 samples retrieved from various clayey silt units across the lake basin for optically and infrared stimulated luminescence dating (OSL/IRSL). As the reservoir stratigraphy is not continuously exposed, we focused sampling along critical layers; in particular, Unit 10, a yellow oxidized layer that is exposed from lake margin to lake margin and used as a major stratigraphic marker.

Preliminary data shows that the sediments underwent a complex history of saturation that can only be accurately addressed through multiple systems of dosimetry measurements. In-situ gamma spectrometry readings reveal that luminescence dose rates were likely dampened by exposure to ground water but did not show any significant disequilibrium in the U:Th ratios. Sediments are being dated using both infrared stimulated luminescence (IRSL) on polymineral grains of fine silt size (4 to 11 microns) and quartz optically-stimulated luminescence (OSL) on fine grained sand (180-63 microns). Preliminary responses to both OSL and IRSL dating suggest that the possibility exists that the sediments are much younger than the Bull Lake Glaciation (130-150 Ka) and may have formed during OIS 4.