New data on the lithostratigraphy, detrital zircon and Nd isotope provenance, and paleogeographic setting of the El Antimonio Group, Sonora, Mexico

The El Antimonio Group is herein proposed as a new lithostratigraphic unit that encompasses the Antimonio, Río Asunción, and Sierra de Santa Rosa Formations in a revised nomenclature from Lucas and Estep (1999b). The type section for the Antimonio, Río Asunción, and the lower part of the Sierra de Santa Rosa Formations is located in the Sierra del Álamo, whereas the representative upper part of the Sierra de Santa Rosa Formation is located in the mountains of same name in northwestern Sonora. The ∼4.5-km-thick sedimentary succession of this group is abundantly fos- siliferous, and its biostratigraphic age is constrained between the Late Permian and Early Jurassic. The 3.4-km-thick section that crops out in the Sierra del Alamo is divided into 14 unconformity-bounded sequences that are tens to hundreds of meters thick and grade from the base upward from a fluvial to shallow marine conglomerate to open marine shale. The El Antimonio succession is correlated with several other Triassic and Jurassic sections that are known in Sonora, all of which are located south of the proposed trace of the Mojave-Sonora megashear. The closest Triassic and Lower Jurassic sections that are located north of the Mojave-Sonora megashear that we correlate with the El Antimonio are known in southern Nevada and southeastern California and include the Moenkopi, Virgin Limestone, Union Wash, Silverlake, and Fairview Valley Formations and the Kings sequence. On the basis of these proposed correlations, we suggest that the El Antimonio Group was deposited in an evolving shallow shelf (Upper Permian-Triassic) to fore-arc basin (Lower Jurassic) that was originally positioned adjacent to southern California and later translated to its present position, along with the Caborca block, by left-lateral Jurassic displacement of the Mojave-Sonora megashear. In this proposed paleogeography, a lower Mesozoic magmatic arc that accumulated volcanic, volcaniclastic, and shallow marine sedimentation in the Mojave Desert and along the California-Nevada border separated the El Antimonio basin from a shallow shelf that developed to the north. New U-Pb geochronology on detrital zircon and Sm/Nd isotope and petrographic data from terrigenous samples of the El Antimonio Group may help to elucidate its provenance and to support this paleogeography. Zircon grains from samples of the lower, middle, and upper parts of the El Antimonio Group yielded ages that cluster around 1.8, 1.6-1.7, 1.4, and 1.00-1.18 Ga and 340, 270-240, and 190 Ma. The Pro- terozoic zircons are interpreted to indicate provenance from the basement provinces of the southwestern United States, although a reworked source for these grains is also possible as they are present in the Cordilleran miogeocline and off-shelf assemblages of Nevada and California and in Proterozoic and Paleozoic strata in Sonora. The closest known sources for the Permian and Lower Triassic zircons are plutons and volcanic rocks that formed a lower Mesozoic magmatic arc extending from southeastern to northern California and western Nevada. Probable sources for the single zircon grain dated at 340 Ma are the Sierra-Klamath terranes, according to interpretation by other authors of grains of similar age in rocks of Nevada. Grains dated around 190 Ma in the youngest sample most probably reveal provenance from the Lower Jurassic magmatic arc of southeastern California or southern Arizona. The Sm/Nd isotopic data from three samples of the lower, middle, and upper parts of the El Antimonio Group indicate a progressive decrease in model ages, from the base upward (T _DM = 1.9-1.8 to T _DM = 1.13 Ga) of this succession, indicating a most probable derivation from the Yavapai and Grenville provinces in the southwestern United States. Sandstone and conglomerate clast composition in the El Antimonio Group indicate mixed sources of provenance from sedimentary and vulcanoplutonic origin. These most probably correspond to the Proterozoic and Paleozoic sedimentary successions of southwestern North America and to the Triassic-Jurassic magmatic arc of this same region, respectively.