COMPARATIVE LATE JURASSIC-LATE CRETACEOUS SEDIMENTARY AND TECTONIC EVOLUTION OF NW SONORA, MEXICO AND SE CALIFORNIA-SW ARIZONA, USA

Recent studies indicate striking similarities, but also some differences, in the Late Jurassic to Late Cretaceous sedimentary and structural evolution of the Caborca area, NW Sonora, Mexico, and SE California-SW Arizona. In the Caborca area, stratigraphic relations are locally obscured by deformation and metamorphism, but the section may include Upper Jurassic marine clastic rocks of the Cucurpe Formation. Of certain presence is the Upper Jurassic(?) to upper Lower Cretaceous Bisbee Group, which includes both marine and nonmarine (alluvial, fluvial, lacustrine, and delta plain) facies. The Bisbee is succeeded by terrestrial foreland basin deposits of the Upper Cretaceous El Chanate Group, which is, in turn, overlain by andesitic volcanic and associated rocks of the Upper Cretaceous El Charro Formation. Deformed and metamorphosed equivalents of the above units, referred to as the Altar Schist, sit locally beneath Late Cretaceous, NE-directed thrust sheets composed largely of Proterozoic crystalline rocks and Neoproterozoic-Paleozoic miogeoclinal units of the Caborca block. In SE California-SW Arizona, Upper Jurassic-Upper Cretaceous sedimentary units are known collectively as the McCoy Mountains Formation. Units of marine origin have not been described, but the sedimentary facies are otherwise strikingly similar to those of the equivalent-aged units of the Caborca area. Late Cretaceous deformation of the McCoy Mountains Formation occurred during both the Maria (SW-directed) and Mule Mountains (NE-directed) thrust events, both of which involved transport of thrust sheets of Proterozoic crystalline rocks, Paleozoic cratonal sequences, and early Mesozoic igneous rocks. The above features all represent parallels between SE California-SW Arizona and the Caborca area. A notable difference, however, pertains to the presence south of the McCoy belt of the Orocopia Schist, which has been interpreted as a Late Cretaceous-early Cenozoic subduction complex. One possibility is that the Orocopia Schist extends into Sonora as the Altar Schist. More likely, however, the Orocopia Schist dies out between the two regions. It is of great importance to understand what happens in this transition zone.