Late Quaternary Glacial History of the Southern Uinta Mountains

Benjamin J.C. Laabs

Summary
Alpine glaciers were numerous in the Uinta Mountains during the Last Glacial Maximum, as documented by W. W. Atwood (1909), G. Osborne (1973), J. Munroe (2001), E. Carson (2003), J. Shakun (2003) and myself (maps of Uintas and paleo-glacier extents). The timing of the Last Glacial Maximum in the Uinta Range has not yet been determined, however, and ascertaining this crucial information will add to a growing understanding of late-Quaternary glacial history and paleoclimate in the western U.S.

A major objective of this research project is to determine the extent and timing of late-Quaternary glacier advances and retreats on the south side of the Uinta Range and the timing of the Last Glacial Maximum for the entire Uinta Range. The results of my study, in collaboration with ongoing research in the Uinta Mountains (e.g., Munroe, 2003; Carson 2003), will include a paleoclimate reconstruction approximately spanning the end of the Last Glacial Maximum to historical time. Procedures utilized in this study include surficial mapping in the field and on air photos, radiocarbon dating of alpine lake sediments, and surface-exposure dating of moraines using cosmogenic 10Be and 26Al.

Fieldwork
In summer 2002, I began mapping the surficial geology of the southern Uinta Mountains in the Ashley National Forest, working with Dr. Jeffrey Munroe, Jeremy Shakun (Middlebury College, now at UMass), and Darlene Koerner (Ashley National Forest – Vernal Ranger District). I focused my mapping efforts on the western half of the south slope, and will move eastward next summer. Mapping glacial deposits will aid in developing preliminary paleo-glacier ELA reconstructions that will compliment those done by Munroe on the north slope; these reconstructions were done by Shakun et al. (2003; view abstract here).

I also began sampling quartzite boulders on moraines that formed during the Last Glacial Maximum (LGM) on the south slope to eventually obtain cosmogenic exposure ages. With these ages, I hope to constrain the timing of the LGM for the entire Uinta range. The PRIME Lab at Purdue University recently accepted a proposal to analyze 8 samples of LGM moraine boulders for 10Be and 26Al. We now have a first set of exposure ages for this moraine; these were presented at the GSA meeting in Seattle this November (view abstract here). The spread of the 10Be boulder-exposure ages verses boulder height on an LGM lateral moraine in Yellowstone canyon are shown here (from Laabs et al., 2003). The rest of the analyses, including analyses of cosmogenic nuclides on moraines in the eastern and western Uintas, will be done in spring 2004.

Other chronological control on the timing of glacial events in the Uintas will hopefully come from radiocarbon in lake-sediment cores retrieved from lateral moraine-dammed lakes. Jeff Munroe, Jeremy Shakun, and I are planning to core Heller Lake in March 2004 to build upon a radiocarbon chronology of surprisingly late glacier retreat in Dry Gulch canyon (Carson, 2003).

Laboratory work
I recently began work to extract cosmogenic nuclides of 10Be and 26Al from samples of quartzite moraine boulders. This work is being done in the UW cosmogenic nuclide extraction laboratory, supervised by Brad Singer and Danny Douglass. We have slightly modified methods developed by Dr. Michael Kaplan to accommodate specific physical and chemical properties of Uinta Mountain quartzite. Samples are being prepared into targets for AMS analyses at the PRIME Lab at Purdue University.

I am also beginning work to apply two 2-D numerical models developed by Plummer and Phillips (2003, QSR) to three formerly-glaciated valleys in the southern Uinta Mo

untains. The first model is used to calculate energy balance, which determines accumulation and ablation rates for cells within the valleys. The output of this model is read into the second model, an ice-flow model, which simulates glacier growth. Thus, glacier growth in the flow model is dependent on the paleoclimate parameters in the energy-balance model. The goal of this work is to determine a range of paleoclimate scenarios that may have been responsible for the glaciation during the LGM in the southern Uinta Mountains. Preliminary model results will be available soon. . .

Acknowledgments
This project is generously supported by the Geological Society of America, the Desert Research Institute, the UW-Madison Vilas Fellowship, the Purdue Rare Isotope Measurement Laboratory (PRIME Lab), and the Ashley National Forest.

Last updated by Ben Laabs, 12/12/03