Glacial Geology of the Southern Uinta Mountains

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 scientifi c attention. Results of recent 1:24,000-scale fi eld mapping of surfi cial deposits in the southern Uinta Mountains indicate that glaciers in the southwestern and southeastern valleys were confi ned to deep canyons during the Last Glacial Maximum, whereas large glaciers in the south-central drainage basins extended beyond the mountain front. In contrast to the abundance of small valley glaciers on the north slope of the range, the south slope was dominated by six larger glaciers that attained areas in excess of 150 km in the North Fork Duchesne, Rock Creek, Lake Fork, Yellowstone, Uinta River, and Whiterocks drainage basins. During the Last Glacial Maximum, these glaciers had maximum ice thicknesses of ~500 m. In addition, seven smaller valley glaciers (3.5 to 79.3 km) occupied minor catchments in the southern Uinta Mountains. Latero-frontal moraines marking the maximum advance of glaciers are best preserved below the mouths of Lake Fork, Yellowstone and Uinta canyons. These landforms provide evidence of multiple Pleistocene advances. The youngest are the Smiths Fork and Blacks Fork Glaciations, which, on the basis of cosmogenic dating and morphology of moraines, occurred during marine oxygen-isotope stages 2 and 6, respectively. An earlier (stage 16?) glacial episode, herein termed the Altonah Glaciation, is indicated 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 on valley fl oors in the southern Uinta Mountains were deposited during the last deglaciation (~17.6 to 12 ka); these include moraines that may indicate a minor ice advance at ~13 ka in a south-central valley (Carson, 2003). In contrast, ice had disappeared from at least one valley in the eastern Uintas by ~14 ka (Munroe, 2002), indicating spatial variability in the responses of glaciers to latest Pleistocene climate change. Additional research aimed at identifying the time of the local Last Glacial Maximum and subsequent deglaciation in the southern Uinta Mountains is underway. *Geology Department, Gustavus Adolphus College, St. Peter, MN 56082 [email protected] †Geology Department, San Jacinto College, Houston, TX 77049 Laabs, B.J.C., Carson, E.C., 2005, Glacial geology of the southern Uinta Mountains, in Dehler, C.M., Pederson, J.L., Sprinkel, D.A., and Kowallis, B.J., editors, Uinta Mountain geology: Utah Geological Association Publication 33, p. 235-253. were numerous in the Uinta Mountains during the Last Glacial Maximum (LGM), as documented by Atwood (1909), Bradley (1936), Bryant (1992), Munroe (2001), and surficial geologic mapping described herein. Thirteen drainage basins on the south side of the Uinta range contained alpine valley glaciers that left a well-preserved record of ice extent. However, the extent of past glaciers in these valleys has been only broadly delimited in previous mapping efforts: accordingly, the primary of objectives of this paper are to describe the glacial geology of the southern Uinta Mountains, to provide a reconstruction of glacial extents during the LGM, and to summarize the Quaternary glacial history of the Uinta Mountains based on the mapped record. A detailed understanding of the glacial record in the Uinta Mountains will provide the framework for studying glacial chronology and paleoclimate in this unique physiographic setting. The last two glaciations in the Uinta Mountains were termed the Blacks Fork and Smiths Fork Glaciations by Bradley (1936), and were later correlated respectively to the Bull Lake and Pinedale Glaciations in the Wind River Mountains by Richmond (1965) (table 1). Results of recent work in the Wind River Mountains and elsewhere 236 Glacial Geology of the Southern Uinta Mountains B.J.C. Laabs and E.C. Carson Figure 1. A) Inset location map of the Uinta Mountains in Utah. B) Shaded-relief map of the southern Uinta Mountains. Glaciated valleys are labeled; NFD = North Fork Duchesne, BS = Blind Stream, LH = Log Hollow, RC = Rock Creek, LF = Lake Fork, YS = Yellowstone, CC = Crow Canyon, DG = Dry Gulch, UR = Uinta River, PC = Pole Creek, WR = Whiterocks, DF = Dry Fork, SC = Split Creek. Table 1. Pleistocene glaciations in the Uinta Mountains