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GSA provides this and other forums for the presentation of diverse opinions and positions by scientists worldwide, regardless of their race, citizenship, gender, religion, or political viewpoint. Opinions presented in this publication do not reflect official positions of the Society. Geological Society of America Field Guide 6 2005 From cirques to canyon cutting: New Quaternary research in the Uinta Mountains Jeffrey S. Munroe Geology Department, Middlebury College, Middlebury, Vermont 05753, USA Benjamin J.C. Laabs Geology Department, Gustavus Adolphus College, St. Peter, Minnesota 56082, USA Joel L. Pederson Geology Department, Utah State University, Logan, Utah 84322, USA Eric C. Carson Geology Department, San Jacinto College, Houston, Texas 77049, USA ABSTRACT The Quaternary record of the Uinta Mountains of northeastern Utah has been studied extensively over the past decade, improving our understanding of the Pleisto- cene glacial record and fl uvial system evolution in a previously understudied part of the Rocky Mountains. Glacial geomorphology throughout the Uintas has been mapped in detail and interpreted with reference to other well-studied localities in the region. In addition, studies in Browns Park and Little Hole in the northeastern part of the range have provided information about paleofl ooding, canyon cutting, and integration of the Green River over the Uinta Mountain uplift. Notable contributions of these studies include (1) constraints on the timing of the local last glacial maximum in the southwest- ern Uintas based on cosmogenic surface exposure dating, (2) insight into the relationship between ice dynamics and bedrock structure on the northern side of the range, and (3) quantifi cation of Quaternary incision rates along the Green River. This guide describes a circumnavigation of the Uintas, visiting particularly well-documented sites on the north and south fl anks of the range and along the Green River at the eastern end. Keywords: Uinta Mountains, Colorado Plateau, Last Glacial Maximum, Quaternary. INTRODUCTION on aspects of these records in the range. Their work was unifi ed by recognition of the characteristics that make the Uinta Mountains For much of the past century, the Quaternary record of the important to our overall understanding of landscape evolution in Uinta Mountains escaped the attention focused on the neighbor- the interior western United States. First, the Uintas contain an ing Colorado Front Range, Wind River Range, and Yellowstone unusually complete record of post-Laramide tectonics, erosion, Plateau. That situation changed in the mid-1990s when research- and drainage integration, and they contained an extensive glacier ers interested in glacial and fl uvial geomorphology began working complex during the Pleistocene glaciations. Second, the unique Munroe, J.S., Laabs, B.J.C., Pederson, J.L., and Carson, E.C., 2005, From cirques to canyon cutting: New Quaternary research in the Uinta Mountains, in Pederson, J., and Dehler, C.M., eds., Interior Western United States: Geological Society of America Field Guide 6, p. 53–78, doi: 10.1130/2005.fl d006(03). For permission to copy, contact [email protected]. © 2005 Geological Society of America 53 54 J.S. Munroe et al. east-west orientation of the mountains allows study of paleopre- the present. Much of this work was conducted within the 456,704- cipitation gradients in a direction parallel to primary storm tracks acre High Uintas Wilderness Area, which is jointly administered and moisture transport. Third, the location and orientation of the by the Ashley and Wasatch-Cache National Forests. Depending range allow the Quaternary record in the Uintas to function as upon the season, it is often not possible to visit sites in the interior a link between the middle Rockies and Colorado Plateau to the of the Uintas. However, this trip takes full advantage of view- Great Basin and the Sierra Nevada. Finally, numerous famous points and exposures around the perimeter of the range in order to geologists worked in the Uintas in the late 1800s and early 1900s, summarize our work and to introduce lingering questions. including Hayden (1871), Powell (1876), King (1878), Atwood The route of this trip circumnavigates the Uinta Mountains (1909), and Bradley (1936). Many of their seminal ideas regard- in a counter-clockwise direction (Fig. 1). The fi rst day focuses ing tectonics and landscape evolution evolved through consider- on glacial deposits on the south slope of the range, where county ation of the evidence observed in the Uinta Mountains. roads (CR) and Forest Service roads (FR) allow access to glacial deposits in the Lake Fork and Yellowstone River valleys. The Trip Overview second day of the trip explores the post-Laramide tectonic and drainage evolution of the eastern end of the range, including the This fi eld trip provides an overview of recent work illuminat- Quaternary stratigraphy of the Green River and recognition of a ing aspects of the glacial and fl uvial history of the Uinta Moun- signifi cant paleofl ood event. The fi nal day continues westward tains from the Last Glacial Maximum (MIS-2, ca. 22–18 ka) to along the north slope of the Uintas, visiting late Quaternary gla- Figure 1. Route of the fi eld trip through northeastern Utah and southwestern Wyoming. Inset shows the state of Utah, the extent of Lake Bonnev- ille at the Last Glacial Maximum (LGM), the Uinta Mountains (stippled pattern on inset), and the fi eld trip route. Larger map shows the recon- structed outlines of LGM glaciers (dark gray), the route of the trip (solid white line), alternate roads in case of early season snow (dotted white lines), and numbered stops (corresponding to text). BP—Browns Park; RCL—Red Canyon Lodge. New Quaternary research in the Uinta Mountains 55 cial and fl uvial deposits in several localities before returning to topography at ~3000 m in the headwaters of the Rock Creek Salt Lake City. and Duchesne River. Stratigraphically above this unit are the As a note to guidebook users, mechanized equipment of any Mississippian Humbug (limestone and dolomite) and Doughnut kind is prohibited within federally designated wilderness areas. (shale, limestone, and dolomite) Formations, which are 10–50 m Thus, access to fi eld sites within the High Uintas Wilderness is by thick. Large landslides are common near the contact of these two foot or by horseback only. Limitations also exist on the number units, one of which can be observed at Stop 1.2. Upper Paleozoic of people and heads of stock allowed per party. Please check with strata include the Permian Weber Sandstone, which forms local either the Ashley National Forest (+1-435-789-1181, Vernal, hogbacks and which is overlain by upper Permian and Mesozoic Utah) or the Wasatch-Cache National Forest (+1-307-789-3194, strata of varying thickness at the mouths of glacial valleys. Evanston, Wyoming) for details if you are interested in visiting Flat-lying, weakly cemented Tertiary gravel deposited the Uinta backcountry. during uplift of the Uinta Mountains is generally found above angular unconformities with tilted, pre-Cenozoic bedrock. On Physical Setting the north slope, these deposits are mapped as the extensive Wasatch Formation (Bryant, 1992). On the south fl ank, these The Uinta Mountains are the longest east-west–trending gravels include fl uvial sandstone, conglomerate, and colluvial mountain range in the conterminous United States, extending or mudfl ow diamictite of the Duchesne River Formation, which ~200 km eastward from the Wasatch Mountains at Kamas, Utah, are most common on unglaciated uplands between the Duchesne into northwestern Colorado. Physiographically, the range can River and Whiterocks drainage basins (Bryant, 1992). Sandstone be divided into two sections: the western glaciated Uintas and and conglomerate beds in this unit are poorly indurated and are a the eastern nonglaciated Uintas (Hansen, 1986). The boundary source for mass-wasting and large alluvial fan deposits in tribu- between the two subprovinces is located ~40 km west of the taries of glacial valleys. The age of this unit is considered to be Utah-Colorado border, along a line extending north from Vernal early Tertiary; however, recent work by D. Sprinkel (2004, per- (Fig. 1). The core of the western Uintas, centered on Kings Peak, sonal commun.) suggests that several gravels currently mapped the highest mountain in Utah at 4136 m, is referred to as the as part of this unit may be signifi cantly younger (late Tertiary or “High Uintas.” early Pleistocene). The drainage systems in the Uinta Mountains fl ow primarily Igneous rocks are almost entirely absent from the Uintas. north and south away from the crest of the range. On the north Several small mafi c dikes exist along the crest of the uplift fl ank, short tributaries originating in alpine cirques drain into (Ritzma, 1983). Dates from these dikes range from 552 ± 17 to main streams in the glaciated valleys. Bear River, at the western 453 ± 29 Ma on the basis of Rb/Sr wholerock and K/Ar methods end of the range, drains into Great Salt Lake; all other streams (Hansen et al., 1982; Ritzma, 1983; Rowley et al., 1985).