ANOMALOUS HUMMOCKY LATERAL MORAINES IN THE EASTERN UINTA MOUNTAINS, NORTHEASTERN UTAH: EVIDENCE FOR EXTENSIVE MASS-WASTING-DERIVED SUPRAGLACIAL DEBRIS?
     MUNROE, Jeffrey S. Munroe, and DOUGLASS, Daniel C., Department of Geology &
Geophysics, University of Wisconsin, Madison, 1215 West Dayton Street, Madison
WI, 53706-1692, jmunroe@geology.wisc.edu and dcdougla@students.wisc.edu
Anomalous lateral moraines, strikingly dissimilar to those produced by other Pinedale glaciers in the region, are present along South Fork Ashley Creek at the eastern end of the Uinta Mountains, northeastern Utah.  In the lower part of Lakeshore basin (~2800 m asl) massive hummocky lateral moraines merge with a bulky end moraine that the stream breeches in a narrow boulder-choked gorge.  The lateral moraines are complexly hummocky, with local relief in excess of 20 m.  Locally in excess of 1.5 km wide, the moraines tower at least 70 m above the valley bottom, and are laterally continuous for 8-10 km. Outwash associated with these moraines forms a sparsely vegetated valley train with a remnant braided channel network that extends downstream for over 2 km. Despite this evidence for large-scale ice-marginal deposition of debris and stagnant ice, there is relatively little evidence of ice stagnation along the valley axis. Instead, ice appears to have actively backwasted, pausing long enough to form at least one major end moraine and associated outwash system.
    Reconstruction of the glacier system responsible for such massive features is a major part of this on-going investigation.  The upper part of the drainage is underlain locally by shaley facies of the PreCambrian Uinta Mountain Group, in contrast to the monotonous sequence of poorly indurated flat-lying orthoquartzites and metaquartzites that characterizes lower elevations.  Shale outcrops were noted to be particularly abundant above 3200 m.  We suggest that the presence of these shale layers may have facilitated large-scale mass wasting onto the accumulation zone of the Pinedale glacier.  This debris would have subsequently been transported downice and incorporated into the lateral moraines.