GEOMORPHIC INFERENCES FROM REGOLITH THICKNESS NEAR THE GLACIAL LIMIT, BOULDER CREEK CATCHMENT, COLORADO

The weathered zone records the balance between rock resistance, weathering and erosion near the late Pleistocene glacial limit in the Front Range, Colorado. We used drillers’ logs from ~1000 wells and field measurements to map patterns of regolith thickness in six adjoining 7.5’ quadrangles in the Boulder Creek catchment. Alluvial and glaciofluvial deposits generally are thin (< 3 m) except along Boulder Creek upstream from the glacial limit, where gravel in limited areas is as thick as 15 m. Most regolith formed from weathering of fractured and faulted Precambrian granitic gneiss and metasedimentary rocks that are locally rich in biotite. Saprolite thickness is not a simple function of bedrock type, faulting, or modern topography. Areas glaciated by Pinedale and Bull Lake valley glaciers expose fresh rock and zones of fractured, slightly weathered rock as thick as 5 m. Ridges separating glaciated valleys expose a relatively thick weathered mantle at elevations as high as about 2800 m. East of the glacial limit; the mean depth to bedrock is between 5 and 10 m and exceeds 10 m in extensive areas. The sharp boundary between glacially stripped bedrock and thick weathered zones implies that the zone of periglacial processes did not extend more than a few km beyond ice margins. Where the weathered zone is exposed in roadcuts, the upper 1 to 4 m consists of grus, saprolite or weathered saprolite with density values between 1.2 and 1.7 g cm-3. Geomorphic evidence suggests that the weathered mantle could not have formed under present conditions. Soils developed on Bull Lake-age moraines expose Bt horizons on the most stable sites, but in many areas soils are oxidized to depths <150 cm and contain > 50 percent fresh to slightly weathered clasts. These observations indicate that in climate typical of late Pleistocene time, the weathered bedrock zone would have taken >> 100 kyr to form. Concentrations of cosmogenic nuclides (CRNs) in quartz extracted from stream sediment from several local catchments suggest that erosion rates over the past 30 kyr have averaged about 2.5 cm/kyr. Erosion rates derived from CRNs thus are 2 to 3 X soil formation rates and by inference, regolith formation rates. Deeply weathered bedrock may reflect a pre-Pleistocene landscape mainly removed by glacial activity and thinning rapidly in areas beyond the glacial limit.