MAPPING THE STRATIGRAPHIC FACIES OF A ROCK GLACIER USING GROUND PENETRATING RADAR (GPR): YANKEE BOY BASIN, CO, USA

Rock glaciers are lobate or tongue-shaped landforms composed of mixtures of poorly-sorted, angular, blocky rock debris and ice. They serve as primary sinks for ice and water storage in mountainous areas and represent transitional forms in the debris transport system, accounting for approximately 60% of all mass transport in some alpine regions. Geomorphologists have had the luxury of idle speculation regarding the internal structure of rock glaciers because of the difficulty in obtaining detailed, internal physical data. With the application of ground-based remote sensing techniques, we now have the ability to identify in detail the subsurface features of these landforms. Two rock glaciers in the San Juan Mountains of southwest Colorado were surveyed using ground penetrating radar (GPR). Longitudinal profiles (ƒ=25 MHz and ƒ=50 MHz) oriented along the central axis of the rock glaciers show several moderate to strong coherent reflection boundaries that represent 10-15 m thick depositional facies. These horizons can be recognized clearly to a depth of 40 m and reflections of an underlying moraine are clearly visible. The facies units are believed to be distinct flow lobes, each comprised of material that was transported from different locations of the cirque headwall. Layers are interpreted to represent ice-supersaturated sediments and coarse blocky rockslide deposits, perhaps generated by seasonal snow pack buried by high-magnitude talus events. We interpret the rock glacier to be a composite feature formed by a process involving the development and subsequent overlap of discrete flow lobes that have overridden older features and protalus rampart materials. These materials have coalesced and are incorporated into the present flow structure of the rock glacier. We have correlated individual flow lobes with late Quaternary/Early Holocene glacial stands in the San Juan Mountains.