Laabs, B.J.C., 2004. Late Quaternary Glacial and Paleoclimate History of the Southern Uinta Mountains, Utah: [Unpublished Ph.D. dissertation] University of Wisconsin, Madison, 162 pp. (04-18)

ABSTRACT

It has been known for over a century that the Uinta Mountains contained numerous alpine glaciers during parts of the Quaternary Period, yet until recently, the glacial record on the south side of the range had received little scientific attention. Results of this 1:24,000-scale field mapping of surficial deposits in the southern Uinta Mountains indicate that glaciers in the southwestern and southeastern valleys were confined to deep canyons during the Last Glacial Maximum, whereas large glaciers in the south-central drainage basins extended beyond canyon mouths. In contrast to the abundance of small valley glaciers on the north slope of the range, glaciers in the North Fork Duchesne, Rock Creek, Lake Fork, Yellowstone, Uinta River, and Whiterocks drainage basins attained areas in excess of 150 km². During the Last Glacial Maximum, these glaciers had maximum ice thicknesses of ~500 m. In addition, six smaller valley glaciers (with areas of 3.5 – 79.3 km²) occupied minor catchments in the southern Uinta Mountains.

Latero-frontal moraines marking the maximum advance of glaciers are best preserved beyond the mouths of the Lake Fork, Yellowstone and Uinta River Canyons. These landforms provide evidence of multiple Pleistocene advances. The youngest are the Smiths Fork and Blacks Fork Glaciations, which occurred during marine oxygen-isotope stages 2 and 6, respectively. An earlier (stage 16?) glacial episode, herein termed the Altonah Glaciation, is represented by an extensive lateral moraine beyond the mouth of Yellowstone Canyon as well as moraines in Lake Fork and Uinta River Canyons. At higher elevations, alpine glacial landforms, including cirques, rock glaciers, arêtes, and hanging valleys are ubiquitous.

Most glacial sediments in the southern Uinta Mountains were deposited during the last deglaciation (~18 – 12 cal. ka); these include moraines that may indicate a minor ice advance at ~13 cal. ka in one valley in the central Uintas (Carson, 2003). In contrast, ice had disappeared from at least one valley in the eastern Uintas by ~14 cal. ka (Munroe, 2002), indicating variable responses of glaciers to latest Pleistocene climate change.

            Recent improvements in our understanding of glacial extents and chronologies in the Wasatch and Uinta Mountains and other mountain ranges in the western U.S. call for a more rigorous approach to using glacier reconstructions to infer paleoclimates. A coupled 2-D mass balance and ice-flow numerical modeling approach developed by Plummer and Phillips (2003) allows exploration of the combined effects of temperature, precipitation, shortwave radiation and many secondary parameters on past ice extents in alpine settings. I applied this approach to the Little Cottonwood Canyon in the Wasatch Mountains and the Lake Fork and Yellowstone Canyons in the south-central Uinta Mountains. Results of modeling experiments indicate that the Little Cottonwood glacier required more precipitation during the Last Glacial Maximum (LGM) than glaciers in the Uinta Mountains, assuming lapse rates were similar to modern. Model results suggest that, if temperatures in the Wasatch Mountains and Uinta Mountains were ~8 – 9̊C colder than modern, corresponding precipitation changes were 0.7 – 1.5x modern in Little Cottonwood Canyon, 0.5 – 1.2x modern in Lake Fork Canyon, and 0.4 – 0.7x modern in Yellowstone Canyon. Greater amounts of precipitation in the Little Cottonwood Canyon likely reflect moisture derived from the surface of Lake Bonneville, but the mass balance of glaciers in the south-central Uinta Mountains was likely unaffected by the lake.

            Although the glacial record in the Uinta Mountains is well documented and has been used to infer climate during the local LGM, the timing of this event has not been previously determined. Cosmogenic ¹⁰Be-exposure ages of moraine boulders suggest the LGM in the south-central Uinta Mountains occurred at 17.6 + 1.1 cal. ka and, therefore, was approximately synchronous with the maximum extent of Lake Bonneville. This age also indicates that the Lake Fork and Yellowstone Canyon glaciers reached their maximum extents after the global LGM (21 + 2 cal. ka).