FRACTURE DISTRIBUTION AND CHARACTERIZATION AMONG ROCK TYPES IN BETASSO GULCH, COLORADO

The purpose of this study is to evaluate the relationships between Proterozoic rock types and fractures that formed during the Colorado and Laramide orogenies in Betasso Gulch, Colorado. Betasso Gulch is located in the Front Range of the Colorado Rocky Mountains, just west of Boulder. The bedrock found in the gulch is primarily comprised of the Boulder Creek Granodiorite, and is characterized by a long geologic history dating back to the Proterozoic. Detailed geologic and fracture maps of the study area were made to analyze the fracture data. SEM/EDS analysis allowed for characterization of each of the various rock types found in the area. The formation of the Front Range of Colorado began about 1.8 Ga. Both the Colorado orogeny (1.8 Ga) and the Laramide orogeny (80-60 Ma) are thought to have contributed to the current fracture patterns seen in Betasso Gulch. Initial faults in the Proterozoic contributed a suite of fracture sets in the Front Range region. Uplift of the Front Range during the Laramide orogeny was accommodated by several reactivated faults. This deformation resulted in additional fractures that cut the Proterozoic fractures. In addition to the Boulder Creek Granodiorite (XgdB), a biotite-rich variation of the granodiorite also occurs in Betasso Gulch. The XgdB is a medium-grained igneous rock made up of quartz, plagioclase, biotite and hornblende. The biotite variation is similar but has a greater quantity of biotite. These share major joint sets of 335°/64°E and 350°/35°W. The granodiorite, however, has additional joint orientations of 10°/40°E and 80°/75°N, while the biotite-rich unit has unique orientations of 45°/75°S and 75°/70°S. The fracture map uses stereonets representing fracture orientations split up by rock type to define structural domains. The lowest elevation region is dominated by biotite-rich granodiorite with major fracture orientations of about 350°/35°W, 340°/70°E and 45°/75°S. There is also a band of biotite-rich granodiorite at the top of the gulch with orientations of 45°/80°S, 70°/85°N and 70°/70°S. The remaining area, dominated by XgdB, exhibits fewer discernable fracture patterns by domain. Further analysis will conclude what fracture patterns and distributions exist within Betasso Gulch, as well as how they relate to the bedrock and preexisting structural formations.