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A geochronologic framework for the Ziegler Reservoir fossil site, Snowmass Village, Colorado

Published online by Cambridge University Press:  20 January 2017

Shannon A. Mahan*
U.S. Geological Survey, Denver Federal Center, Box 25046, MS-974, Denver, CO 80225, USA
Harrison J. Gray
U.S. Geological Survey, Denver Federal Center, Box 25046, MS-974, Denver, CO 80225, USA
Jeffrey S. Pigati
U.S. Geological Survey, Denver Federal Center, Box 25046, MS-980, Denver, CO 80225, USA
Jim Wilson
Aeon Laboratories, LLC, 5835 N Genematas Dr, Tucson, AZ 85704-5919, USA
Nathaniel A. Lifton
Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN 47907, USA
James B. Paces
U.S. Geological Survey, Denver Federal Center, Box 25046, MS-963, Denver, CO 80225, USA
Maarten Blaauw
School of Geography, Archaeology and Palaeoecology, Queen's University Belfast, BT7 1NN Belfast, UK
Corresponding author.E-mail (S.A. Mahan).


The Ziegler Reservoir fossil site near Snowmass Village, Colorado (USA), provides a unique opportunity to reconstruct high-altitude paleoenvironmental conditions in the Rocky Mountains during the Last Interglacial Period. We used four different techniques to establish a chronological framework for the site. Radiocarbon dating of lake organics, bone collagen, and shell carbonate, and in situ cosmogenic 10Be and 26Al ages on a boulder on the crest of a moraine that impounded the lake suggest that the ages of the sediments that hosted the fossils are between ~140 ka and >45 ka. Uranium-series ages of vertebrate remains generally fall within these bounds, but extremely low uranium concentrations and evidence of open-system behavior limit their utility. Optically stimulated luminescence (OSL) ages (n = 18) obtained from fine-grained quartz maintain stratigraphic order, were replicable, and provide reliable ages for the lake sediments. Analysis of the equivalent dose (DE) dispersion of the OSL samples showed that the sediments were fully bleached prior to deposition and low scatter suggests that eolian processes were likely the dominant transport mechanism for fine-grained sediments into the lake. The resulting ages show that the fossil-bearing sediments span the latest part of Marine Oxygen Isotope Stage (MIS) 6, all of MIS 5 and MIS 4, and the earliest part of MIS 3.

University of Washington

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