DIGITAL ANALYSIS OF SURFICIAL MATERIALS, COLORADO FRONT RANGE USING FIELD, GIS AND ASTER TECHNIQUES

We report data from a GIS-based map of surficial deposits in the Boulder Creek catchment (294 km2) based on field measurements and an image-to-image comparison using a 14-band, high-resolution Aster image collected by the Earth Observing System's satellite, Terra. The catchment heads in the glaciated, alpine Indian Peaks area and extends past the late Pleistocene glacial limit to the lightly forested zone at an elevation of about 2800 m. Surficial deposits reflect local bedrock geology, weathering, and erosion and transport by glaciers, mass movements and other surface processes. Calculations of long-term weathering and erosion rates for the Front Range require measurements for area and thickness of these surface deposits.

The digitized map is dominated by weathered bedrock and colluvium covered with forest (~54%), mainly east of the glacial limit. Till covers 18% of the mapped area and extends in tongues from the larger alpine catchments down U-shaped valleys between about 3400 and 2700 m. This elevation range separates glaciated valleys from weathered bedrock, talus, and periglacial deposits preserved above late Pleistocene glacial limits. Bedrock and talus comprise 14% of the map area and crop out on steep flanks of the valleys and upper basins, whereas alpine colluvium (8% of mapped area) and patterned ground (1.75%) are common on gently sloping ridges at elevations as high as 3900 m. Nearly 78% of the measured bedrock area is near the basin head; bedrock also crops out along steep canyon walls and tors east of the glacial limit. Alpine colluvial deposits and patterned ground occur exclusively above 3000 m.

From an imaging perspective, spectral distinctions between some surficial materials are not clearly defined because classifications cannot separate patterns easily perceived by the human eye. Therefore, we also used DEM-based slope and aspect measurements. Using a decision-tree method in conjunction with DEM data, for instance, enables classification of north and south-facing slopes separately because the former has insufficient spectral signature. Most surficial units are best defined using band ratioing. Overall, the classified Aster image suggests that in this environment, field-based mapping helps improve classification of surficial deposits over satellite imagery alone.