Lower Cretaceous strata of the Sonora Bisbee Basin: A record of the tectonomagmatic evolution of northwestern Mexico

Lower Cretaceous strata of the Sonora Bisbee Basin record four discrete Paleozoic and Mesozoic magmatic episodes and indicate a previously unrecognized Early Cretaceous regional tectonic event along the southwestern margin of North America. Strata of the Bisbee Group in the Sonora Bisbee Basin overlie Upper Jurassic strata on an angular unconformity. Cretaceous strata are predominantly continental fluvial deposits. Bisbee Group sandstones are sublitharenites and litharenites rich in paleovolcanic lithic fragments. Detrital-zircon age populations form five main groups-Proterozoic, Early Permian-Early Triassic, Late Triassic-Jurassic, earliest Cretaceous, and late Early Cretaceous (Aptian). U-Pb zircon ages from three types of conglomerate clasts in the Morita Formation match these age groups: (1) Quartzite clasts are dominated by Proterozoic ages; (2) rhyolite clasts are Middle Jurassic (ca. 172 Ma); (3) andesite clasts are earliest Cretaceous (ca. 140 Ma). Proterozoic zircons and quartzite clasts were recycled from strata of the Caborca block, whereas Permian and Mesozoic grains and andesite and rhyolite clasts were derived from volcanic arc rocks to the west and northwest. We identify two separate episodes of Early Cretaceous volcanism recorded in the basin, the first between 145 and 133 Ma and the second beginning ca. 125 Ma and continuing into the Albian. The presence in the Sonora Bisbee Basin of near-source fluvial detritus from the first of these episodes requires either the presence of a continental arc along the North American margin during the earliest Cretaceous or the accretion of similar-aged material to the margin prior to ca. 130 Ma. Neovolcanic zircon grains in the upper part of the Morita Formation record the initiation of volcanism in the Alisitos arc, an Early Cretaceous volcanic arc presently exposed in the Peninsular Ranges of Baja and southern California. Structural, stratigraphic, and detrital-zircon data suggest that a heretofore unrecognized episode of regional shortening, uplift, and incision occurred to the southwest of the study area in the interval 148-130 Ma.