MAPPING THE STRATIGRAPHIC FACIES OF A ROCK GLACIER AT YANKEE BOY BASIN, CO, USA: NEW RESULTS FROM A GROUND PENETRATING RADAR STUDY

Rock glaciers are lobate or tongue-shaped landforms that serve as primary sinks for ice and water storage in mountainous areas and represent prominent transitional forms in periglacial debris transport systems. Within alpine environments, these landforms are the visible expression of mountain permafrost and may serve as principal climatological indicators at latitudes and elevations below those of glaciers. Although they are common in cirques and valleys of many mountain ranges throughout the world, rock glaciers remain poorly understood in terms composition and spatial distribution of structures within them. A better knowledge of rock glacier structure and dynamics is required to improve estimates of debris transport rates and to justify the use of rock glaciers as proxies for areas where climate data are lacking. Using ground penetrating radar (GPR), an advancing rock glacier in the San Juan Mountains of southwest Colorado was surveyed using a center frequency of 25 MHz. The survey was conducted along a NW-SE transect oriented approximately normal to the longitudinal axis of the rock glacier. The survey crosses several depositional lobes that comprise the lower facies of the rock glacier, providing a cross-sectional perspective. Depth of penetration was ~ 37.5 m based on a velocity value of 0.12 m/ns. Strongly coherent reflections are continuous to discontinuous, with the longest extending 75 m. The contact between rock glacier and underlying glacial moraine is distinct and bottom morphology reflects the cirque floor geometry. Apparent thickness ranges from 15 m at the outer edge of the rock glacier to ~ 36 m at maximum. Reflections of materials comprising the rock glacier are distinct from those of the underlying moraine in that they are more discontinuous and conform to the reflection contours of the bedrock below. This view is consistent with the ‘scoop-shape’ of a glacial valley where the perimeter of the cirque is marked by upward sloping walls.