Cenozoic nonmarine sedimentary and volcanic rocks of the southern Colorado Plateau and adjacent Verde Basin are divided into five tectonostratigraphic packages that document stages in the evolution of the transition zone between the Colorado Plateau and the Basin and Range provinces in central Arizona. These two units are differentiated on the basis of, 1) geomorphic position, 2) clast composition and provenance variations, 3) tectonic setting and structural history, and 4) radiometric ages of intercalcated volcanics. The areal extent, lithology, provenance, depositional environments and age of these stratigraphic units are poorly constrained, and differences of opinion exist as to their interpretation. One school of thought is that the Colorado Plateau escarpment was formed by erosion during the Oligocene, and the other is that it was formed by extensional faulting beginning during the Miocene, less than 10 million years ago. Until these contradictory interpretations of the geologic history of the Plateau margin are resolved, the tectonic and geomorphic history of the Colorado Plateau and Basin and Range provinces remains speculative. This study will establish the age relationships, and environmental and tectonic history of the Cenozoic rocks in the southern Colorado Plateau-Mogollon Rim area. These interpretations will be based on petrologic and facies analyses of five tectonostratigraphic packages and more precise definition of their ages based on paleontology, K-Ar and fission track dating.
