PRELIMINARY GEOMORPHIC ASSESSMENT OF THE BUSTED BOILER FAULT AND THE LOG HILL MESA FAULT ZONE, SOUTHWESTERN COLORADO

A preliminary geomorphic assessment of the Busted Boiler fault and the Log Hill Mesa fault zone, two north-striking faults on the southern Uncompahgre Plateau, was performed to evaluate their surficial expression and evidence for potential Quaternary fault activity. The study included mapping of possible tectonic features and Quaternary deposits near the faults using large-scale stereo photographs, topographic maps, and a brief aerial and field reconnaissance.

The study shows that the Busted Boiler fault and Log Hill Mesa fault zone are structurally complex and may have a history of repeated activity. Fault-line scarps on Cretaceous Dakota Sandstone along both faults are discontinuous and sinuous. The Log Hill Mesa fault zone consists of at least three strands that form a zone about 0.7 km wide and 3 to 5 km long. The scarp along the western strand of the Log Hill Mesa fault zone is more pronounced and higher (up to 75 m) than the scarp along the Busted Boiler fault (7.5 to 8 m), but the total observed length of the Busted Boiler fault is much longer, about 18 km. Because the Log Hill Mesa fault zone is covered by thick oak brush, the size and marked geomorphic expression of the scarp had not been recognized previously. Geomorphic expression suggests that the Log Hill Mesa fault zone may be the dominant geologic structure.

Our preliminary geomorphic study did not reveal any definitive evidence for fault scarps across Quaternary deposits along either fault. Because the fault scarps are primarily in rock, their tectonic interpretation is ambiguous. The scarps could be the result of geologically recent surface faulting (a tectonic fault scarp) or differential erosion along an older fault trace (fault-line scarp). The presence of possible tectonic fault scarps on fluvial terraces suggests late Quaternary activity on the Log Hill Mesa fault zone. However, further work (e.g., detailed geomorphic mapping, geophysics, and trenching) is needed to document the presence, location, size, geometry, and age of the scarps. If the scarps along the Log Hill Mesa fault zone and the Busted Boiler fault are geologically recent tectonic features, then the source faults for earthquakes may be either blind or partially emergent, and earthquake magnitude estimates based solely on surface fault length may underestimate magnitude.