GEOCHRONOLOGY AND GEOMORPHIC MAPPING TO ESTIMATE DEBRIS-FLOW DEPOSITIONAL AGES AND RECURRENCE INTERVALS ALONG STATE HIGHWAY 145 AND 62, SAN MIGUEL COUNTY, COLORADO

The Colorado Geological Survey (CGS) was tasked by the Colorado Department of Transportation (CDOT) to perform debris-flow fan mapping and geochronology along a segment of State Highway 145 near Telluride and Highway 62 near Ridgway. The goal of the study was to radiometric date debris-flow deposits with intent to establish debris-flow recurrence intervals, identify factors contributing to regional debris-flow initiation, and compare ¹⁴C and optically stimulated luminescence (OSL) age-dating techniques and results of debris-flow deposits. The CGS identified two sites with well-exposed debris-flow sequences and easy accessibility to dig soil pits. Site A is located on Highway 62 and Site B is located on State Highway 145. Both sites have sedimentary sequences including abundant shale and claystone in their source areas. At Site A, two distinct facies were sampled from one channel exposure and two hand-dug soil pits (six total samples for ¹⁴C and OSL analysis). At Site B, at least six distinct debris-flow facies were identified and four were sampled based on accessibility and potential to yield a wide range of ages. Ten samples were submitted for ¹⁴C analysis; age estimates range from 12,707 – 28,011 cal yrs BP at Site A and 1,270 – 5,468 cal yrs BP at Site B. At Site A, ¹⁴C age estimates indicate a mean recurrence interval of 3,766.3 years. The distribution of age results may suggest steady fan migration from south to north. At Site B, ¹⁴C age estimates indicate a recurrence interval of 1,021 years. The distribution of the age results may suggest lateral, back-and-forth fan migration across the fan surface. Three samples were submitted for OSL analysis; one from Site A and two from Site B. Site A yielded an OSL age estimate of 1,985 ± 210 yrs BP. The samples from Site B yielded age estimates of 12,312 ± 530 and 14,910 ± 720 yrs BP. The OSL age estimates did not overlap with ¹⁴C estimates, but overdispersion values ranging from 13 to 28% indicate OSL analysis may be a viable option for dating debris-flow deposits. Some ¹⁴C and OSL age estimates correlate with wetter climates and glaciations. Future work includes more OSL sampling and increased vertical and lateral sampling density.