GEOMORPHIC ANALYSIS OF REGOLITH THICKNESS IN THE BOULDER CREEK CATCHMENT, FRONT RANGE, COLORADO

Regolith thickness represents a balance between weathering and erosion and should reflect the influence of lithology, proximity to channels, and local slope. We assembled a GIS dataset and mapped bedrock weathering and regolith thickness in six adjacent 7.5' quadrangles using drillers' logs from 1872 wells, a digital geologic map of Colorado (Green, 1992), a high-resolution DEM, and field measurements from the Boulder Creek catchment. We measured outcrop weathering using the scheme of Clayton et al. (1979), which categorizes the decomposition of fresh granitic rocks to grus. Weathered profiles consisting of saprolite and grus are thickest beyond the area eroded by Pleistocene valley glaciers. Precambrian granitic rocks are most deeply weathered. Metasedimentary rocks are pervasively jointed and oxidized, but neither fresh nor deeply weathered material is common. Tertiary intrusive rocks show only minor evidence of chemical weathering but are extensively jointed. Rock units within the Colorado Mineral Belt are highly altered, but oxidation does not decrease with depth below the surface. Where highly weathered rocks are exposed in roadcuts or noted in drillers' logs, profiles are 1 to 7 m thick. Outcrops show variable weathering patterns that laterally juxtapose deeply weathered material against oxidized but otherwise unaltered rock. Outcrops near the Continental Divide are mainly unweathered (Clayton 1 and 2), whereas areas beyond the glacial limit give Clayton values as high as 6 and 7 (denoting grus). Density values range from 1.4 to 2.2 g cm-3. Boulder Creek granodiorite tends to weather to saprolite (1.9 g cm-3) and grus (1.7 g cm-3) overlying or surrounding denser core stones (2.5 g cm-3). Geochemical analyses of weathering profiles record expansion in the weathered substrate and progressive loss of Si and Ca. Spatial interpolations of regolith thickness show no simple correlation between lithology and depth of weathering, but suggest complex relationships among several variables. Multiple-regression analyses using local slope, proximity to channels and glacial limit, and lithology help explain the distribution of weathering depths and styles within the Boulder Creek catchment and provide a context for erosion models.