Glacial Geology of the Northern Uinta Mountains

Glacial deposits on the north slope of the Uinta Mountains were investigated through map and air photo interpretation combined with fi eld mapping. Deposits representing the Smiths Fork (local Pinedale equivalent), Blacks Fork (Bull Lake equivalent) and pre-Blacks Fork Glaciations are present in the form of terminal and lateral moraines, ground moraine, and outwash valley trains. Nineteen separate valley glaciers covering ~940 km were present in the northern Uinta Mountains during the peak of the Smiths Fork Glaciation. Ice cover was even more extensive (~1100 km) during the penultimate Blacks Fork Glaciation. Distinct patterns of glaciation are evident from east to west along the north slope. Smiths Fork-age deposits in the Ashley Creek/Carter Creek/Sheep Creek region at the eastern end of the north slope indicate extensive ice stagnation. In contrast, Smiths Fork-age glaciers at the western end did not stagnate, and retreated actively after formation of their terminal moraines. In the central part of the north slope, bedrock structure apparently exerted a strong control on glacier dynamics in the Burnt Fork/Beaver Creek/Henrys Fork region. Reconstructed glacier equilibrium line altitudes for the Smiths Fork Glaciation drop dramatically at the western end of the range, indicating that glaciers in the western Uintas received considerably more precipitation than those farther east, probably due to lake effect snow derived from pluvial Lake Bonneville. Many drainages also contain small cirque-fl oor moraines indicating glacier advances after the Smiths Fork Glaciation, likely during the latest Pleistocene/early Holocene. Two drainages also contain evidence for Neoglaciation in the late Holocene, but before the classic Little Ice Age. Geology Department, Middlebury College, Middlebury, VT 05753 [email protected] Munroe, J.S., 2005, Glacial geology of the northern 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. 215-234. History of Glacial Investigations W.W. Atwood of the U.S. Geological Survey undertook the fi rst study of the glacial record in the Uintas, producing a map and a report detailing the distribution of glacial deposits on the north and south slopes of the range (Atwood, 1909). Atwood divided end moraines, ground moraine and outwash deposits into two groups representing an older and younger glaciation. He also noted evidence for an earlier glaciation in some valleys. Despite diffi culties including a primitive topographic base and a lack of aerial photos, Atwood’s fi nal map was a substantial contribution to our understanding of the Uinta Mountains and remained the most comprehensive investigation of the glacial record in the Uintas for almost a century. Most subsequent mapping of glacial deposits in the Uintas focused on the north slope of the range. Bradley (1936) recognized three separate ice advances in his report on the geomorphology of the north slope. From oldest to youngest, he named these the Little Dry, Blacks Fork, and Smiths Fork Glaciations, after localities where particular deposits are well preserved. Later, in his summary report on glaciation of the Rocky Mountains, Richmond (1965) correlated the Little Dry deposits with the Illinoian and pre-Illinoian glacial stages of the U. S. Midwest, the Blacks Fork with the early Wisconsin, and the Smiths Fork with the late Wisconsin Glaciation. Richmond also correlated the Blacks Fork advance with the Bull Lake Glaciation in the Wind River Range, and the Smiths Fork with the Pinedale. More recent work has also considered the Blacks Fork to be correlative with the Bull Lake Glaciation (Richmond, 1986; Bryant, 1992), which has been shown to pre-date the early Wisconsin (Sharp and others, 2003). Several unpublished graduate theses have also reported aspects of the glacial geology of the northern Uintas. Schoenfeld (1969) investigated the Quaternary geology of the Burnt Fork drainage, part of the area studied by Bradley (1936). Barnhardt (1973) reported on the glacial and periglacial geomorphology of the Bald Mountain area along the Mirror Lake Highway. He employed both lichenometry and radiocarbon dating to develop a chronology for the late stages of the Pinedale Glaciation and local manifestations of the Neoglaciation. Grogger (1974) divided the Blacks Fork Glaciation into two stades, the Smith Fork into four stades, and the Neoglaciation into four stades on the basis of till weathering, vegetation, soil development, stratigraphic relations, and erosional modifi cation. Gilmer (1986) focused on aspects of the geomorphology of a section of the north slope just north of the Utah-Wyoming border. Schlenker (1988) studied the geomorphology of the Blacks Fork area and corroborated the mapping of Atwood (1909). Zimmer (1996) revisited the glacial geology of the Smiths Fork drainage in his study of soil development on glacial landforms. Finally, Douglass (2000) investigated the complex of end moraines located where the West Fork Beaver Creek passes through the Madison Limestone hogback south of Lonetree, Wyoming. On the basis of geomorphic and soils evidence he was able to subdivide the Smiths Fork-age glacial deposits into an early and late advance. Scope and Methods of This Study Glacial deposits across the entire north slope of the Uinta Mountains were investigated through fi eld mapping between 1998 and 2000 (Munroe, 2001). This work was part of a geomorphic mapping initiative undertaken by the U.S. Forest Service as part of the land systems inventory for the Wasatch-Cache National Forest. Lateral moraines, end moraines, ice-marginal drainages, and heads of outwash were identifi ed and mapped at 1:24,000 scale on parts of twenty 7.5-minute quadrangles. Glacial limits were correlated between adjacent valleys on the basis of position, elevation, crosscutting relationships of outwash deposits confi ned to valley fl oors (valley trains), and the extent of weathering. All fi eld mapping and air photo interpretations were compiled in a GIS, and Smiths Fork-age glaciers were reconstructed from the distribution of ice-marginal features (fi gure 1). A map illustrating the surfi cial geology of the glaciated valleys of the northern Uintas appears as a plate in the digital archive of this volume (MunroePlate in CD). A variety of metrics were calculated for the reconstructed glaciers (table 1), including length, area, thickness (a minimum estimate from the difference in elevation between lateral moraines and 216 Glacial Geology of the Northern Uinta Mountains J.S. Munroe 2005 Utah Geological Association Publication 33 217 Dehler, Pederson, Sprinkel, and Kowallis, editors Figure 1. Reconstructed Smiths Fork-age glaciers of the Uinta Mountains. North slope glaciers considered in this report (from Munroe, 2001) are shown in darker gray. Glaciers shaded in lighter gray are from Shakun (2003)--south slope, and Oviatt (1994)--west of the Mirror Lake Highway. North slope glacier numbers are keyed to the list below. The UtahWyoming stateline and the Mirror Lake Highway are shown for reference. Inset shows the Smiths Fork-age glaciers of the Uinta Mountains (UM) in relation to the state of Utah and the extent of the Lake Bonneville highstand (LB, from the Utah Automated Geographic Reference Center, http://agrc.its.state.ut.us/) ca. 15 ka BP. SLC is Salt Lake City; V is Vernal; M is Moab, and CR is the Colorado/Green River system.