This report presents the geologic framework critical in understanding spring discharge and the hydrogeology in Black Canyon directly south of Lake Mead below Hoover Dam, Nevada and Arizona. Most of the springs are thermal 2 Geologic Framework of Thermal Springs, Black Canyon, Nevada and Arizona with temperatures as much as 45 degrees C. This study is part of a hydrogeologic and geochemical study of the Black Canyon thermal springs by the U.S. Geological Survey, funded by the National Park Service and National Cooperative Geologic Mapping Program of the U.S. Geological Survey. The study consisted of (1) compilation of existing geologic mapping, augmented by new field geologic mapping and geochronology (Felger and others, 2014), (2) collection and analysis of structural data adjacent to the springs of interest (appendix 1; Anderson and Beard, 2011; Beard and others, 2011a), and (3) construction of regional cross sections (pl. 1). The most significant results identify faults, fracture zones, and rock characteristics that influence the hydrogeology of Black Canyon. Additional results include refinement of the volcanic stratigraphy based on field mapping and new geochronology. This report will be integrated into a companion hydrogeologic report that includes new geochemical and spring flow data that describes groundwater components of Black Canyon thermal springs (M. Moran, written commun, 2013).