MAPPING CANDIDATE DEEP PENETRATIVE FRACTURES AT GRAND CANYON NATIONAL PARK

Some aquifers of the southwestern Colorado Plateaus Province are deeply buried and overlain by several impermeable shale layers. Recharge to these aquifers probably is by seepage down penetrative fracture systems. This purpose of this 2 year U.S. Geological Survey study, sponsored by the U.S. National Park Service, was to map candidate deep penetrative fractures over a 120,000-km2 area, using gravity- and aeromagnetic-anomaly data together with surficial fracture data. The study area included the Grand Canyon, Williams, Prescott, Marble Canyon, Flagstaff, and Holbrook 1:250,000-scale quadrangles. Additional work was done in an area equal to one 1:250,000-scale sheet approximately centered on Grand Canyon Village on the south rim of the Grand Canyon and this was designated as the focus area.. The resulting database is a spatially registered estimate of deep fracture locations. Candidate penetrative fractures were located by spatial correlation of horizontal gradient and analytic signal maximums of gravity and magnetic anomalies with major surficial lineaments obtained from geologic and topographic data, side-looking airborne radar and satellite imagery. The product maps display a subset of candidate penetrative fractures because of limitations in the data coverage and the analytical technique. The data and analytical technique used cannot predict whether the fractures are open or closed. Correlations were carried out by using image-processing software such that every pixel on the resulting images was coded to uniquely identify which datasets are correlated. The technique correctly identified known deep fracture systems and delineated many new candidate deep fracture systems. The resulting penetrative fracture distribution maps constitute an objectively obtained, reproducible dataset and a benchmark from which additional studies can begin. The maps also define in detail the tectonic fabric of the southwestern Colorado Plateau.