Glacial Geology of the Southern Uinta Mountains Benjamin J.C. Laabs* and Eric C. Carson† 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 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 km2 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 km2) 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. INTRODUCTION Licciardi and others, 2001; and the Colorado Rockies, Benson and others, 2004a) and adjacent The Uinta Mountains form an east-west to the eastern edge of the Lake Bonneville basin. trending range in the central Rocky Mountains, Their orientation and location provides a unique with primary drainage to the north and south in opportunity to study what may represent glacial formerly glaciated valleys (figure 1). The range advance under cold, dry conditions in the eastern is located centrally among well-dated alpine Uintas and cold, wet, pluvial conditions in glacial localities (e.g., the Wind River Mountains, western valleys that were closer to Lake Bonneville Gosse and others, 1995; the Yellowstone Plateau, (Munroe and Mickelson, 2002). Alpine glaciers *Geology Department, Gustavus Adolphus College, St. Peter, MN 56082 Laabs, B.J.C., Carson, E.C., 2005, Glacial geology of the southern Uinta [email protected] Mountains, in Dehler, C.M., Pederson, J.L., Sprinkel, D.A., and Kowallis, †Geology Department, San Jacinto College, Houston, TX 77049 B.J., editors, Uinta Mountain geology: Utah Geological Association Publication 33, p. 235-253. 235 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. were numerous in the Uinta Mountains during the were termed the Blacks Fork and Smiths Fork Last Glacial Maximum (LGM), as documented by Glaciations by Bradley (1936), and were later Atwood (1909), Bradley (1936), Bryant (1992), correlated respectively to the Bull Lake and Munroe (2001), and surficial geologic mapping Pinedale Glaciations in the Wind River Mountains described herein. Thirteen drainage basins on by Richmond (1965) (table 1). Results of recent the south side of the Uinta range contained alpine work in the Wind River Mountains and elsewhere valley glaciers that left a well-preserved record of ice extent. However, the extent of past glaciers in Table 1. Pleistocene glaciations in the Uinta Mountains these valleys has been only broadly delimited in previous mapping efforts: accordingly, the primary Uinta Rocky Mountain Marine Approximate of objectives of this paper are to describe the Mountain Correlative Oxygen Age (ka)2 glacial geology of the southern Uinta Mountains, Glaciation Isotope Stage to provide a reconstruction of glacial extents (MIS) during the LGM, and to summarize the Quaternary Smiths Fork1 Pinedale 2 24 - 12 glacial history of the Uinta Mountains based on the Blacks Fork1 Bull Lake 6 186 - 128 mapped record. A detailed understanding of the Altonah Sacagawea Ridge3 16 659 - 620 glacial record in the Uinta Mountains will provide 1From Bradley (1936). the framework for studying glacial chronology 2From Imbrie and others (1984). See text for approximate ages and paleoclimate in this unique physiographic of glaciation. setting. 3Temporary correlation and age assignment (see Chadwick and The last two glaciations in the Uinta Mountains others, 1997). 236 Dehler, Pederson, Sprinkel, and Kowallis, editors 2005 Utah Geological Association Publication 33 in the Rocky Mountains demonstrates that these RESULTS OF GLACIAL MAPPING glacial cycles occurred during marine oxygen- isotope stage (MIS) 6 (186 to 128 ka; Imbrie and Glacial Erosional Landforms others, 1984) and MIS 2 (24 to 12 ka; Imbrie and others, 1984), respectively. Glacial maxima during U-shaped valleys resulting from broadening of MIS 6 occurred at ~160 ka in the nearby Wind River preexisting valleys by alpine glaciers are ubiquitous Mountains (Sharp and others, 2003) and at 23 to in the glaciated part of the southern Uinta Mountains 16 ka in the Wind Rivers, the Yellowstone Plateau, (fi gure 1). Glaciers in tributary valleys of major and the Colorado Rockies during MIS 2 (Gosse drainage basins coalesced and advanced into U- and others, 1995; Licciardi and others, 2004; shaped, trunk valleys. The widest U-shaped valley Benson and others, 2004b). Based on cosmogenic in the southern Uintas is in the Rock Creek drainage 10Be surface-exposure dating of moraine boulders, basin, where the horizontal distance between Laabs (2004) found that the Smiths Fork maximum trimlines on valleys sides exceeds 3 km. Whereas occurred prior to 17.6 ± 1.1 ka in the south-central streams in many glacial valleys have subsequently Uinta Mountains, which confirms the correlation smoothed their long profi les by downcutting into of the Smiths Fork and Pinedale moraines. For bedrock, some tributary streams still exhibit a the purpose of this paper, we use the correlations glacial staircase or step-down topography common of Richmond (1965); at least until direct dating of to alpine glacial valleys (fi gure 2). Blacks Fork-age deposits is established. Hanging valleys are common in the southern Uinta Mountains where tributary valleys intersect METHODS The geomorphology of the south flank of the Uinta Mountains was mapped at the 1:24,000 scale on 25 U.S.G.S topographic maps and digital elevation models. Surficial deposits and landforms were first identified and delineated on maps and air photos. Then, glacial deposits and landforms were field checked in most accessible areas, but some features were identified only on maps and aerial photographs. Ice extents were interpreted from end moraines at the mouths of glacial valleys, by lateral moraines and trimlines on valley sides, and by breaks in slope on cirque headwalls (i.e., the uppermost ice extent was considered to have been below the points where the slope angle on the cirque headwall exceeds 60°; Meierding, 1982). Areal extent of ice during past glaciations was calculated in a geographic information system. Maximum ice thicknesses were estimated using the differences in elevation of the lateral moraine surface and the valley floor at locations of paleo-equilibrium lines; the Figure 2. Step-down topography near the head of the latter were estimated by Shakun (2003) based North Fork Duchesne canyon. Dashed line indicates on the accumulation area ratio, toe-headwall ice extent during the last glaciation. Arrows indicate altitude ratio, lateral-moraine elevation, and former ice flow directions. Topographic map is a portion cirque-floor elevation methods. of the U.S. Geological Survey Mirror Lake 7.5-minute quadrangle. 237 Glacial Geology of the Southern Uinta Mountains B.J.C. Laabs and E.C. Carson more deeply eroded trunk valleys. Several glaciated accumulation areas of glaciers were largest and the hanging valleys in this area are very broad; for development of ice caps that drained into multiple example, Brown Duck basin is more than 12 km valleys was common. In the south-central Uintas, the wide above Lake Fork canyon (fi gure 3). Such most prominent arêtes have more than 450 m of relief valleys must have contained many small glaciers at and are more than 10 km in length. In the southeastern the onset of past glaciations that ultimately coalesced Uintas, rounded unglaciated divides locally termed before joining larger glaciers during ice advance. The “bollies” are more common than narrow arêtes. mouths of some hanging valleys are characterized by Examples of these features include broad divides that waterfalls in tributaries, whereas others have deep, separate glacial valleys in the headwaters of Dry Fork narrow, bedrock gorges probably cut by streams and Ashley Creeks (fi gure 4).