Workers in Sonora recognize two distinct provinces of Upper Proterozoic and Paleozoic strata; one of shallow-water origin and one of deeper-water origin. The shallow-water miogeoclinal strata are best known in areas west and southwest of Caborca, and range in age from late Proterozoic to Permian. These correlate with strata in the Sierra Agua Verde-Cerro Cobachi area east of Hermosillo. Strong stratigraphic similarities between these rocks and those of Nevada have fueled hypotheses that they were adjacent in Paleozoic time and have been separated 800 km by left-lateral strike-slip displacement of mid-Jurassic age. Deeper-water strata occur farther to the south and...