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 of Sonora contains one of the most complete stratigraphic successions for the Upper Permian to Lower Jurassic in any Cordilleran terrane. This Group is herein proposed as a new lithostratigraphic unit that encompasses the Antimonio, Rio Asunción and Sierra de Santa Rosa Formations. The type section for the Antimonio, Rio Asunción and the lower part of the Sierra de Santa Rosa Formations is located in the Sierra del Alamo whereas the representative upper part of the Sierra de Santa Rosa Formation is located in the mountains of same in northwestern Sonora. The 4.5 km thick sedimentary succession of this Group is abundantly fossiliferous 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, tens to hundred meters thick each, and that from base upwards grade from a fluvial to shallow marine conglomerate to open marine shale. Invertebrate fossils from the high diversity, Upper Triassic succession of the Antimonio Formation reveal closest affinities with coeval taxa from west-central Nevada and also reveal affinities with the Eastern Klamath terrane and to a lesser extent with inboard island arc terranes (Eastern Klamath, Stikinia, and Quesnellia). The El Antimonio succession is correlated with several other incomplete sections that are known in Sonora and Baja California, and a more regional correlation is proposed with sections in southern Nevada and southeastern California. New data on petrography, detrital zircon U/Pb geochronology and Nd/Sm isotopic geochemistry provenance are presented in this work. Our data and interpretation intend to better understand the original paleogeographic position of the El Antimonio succession and the role that the Mojave-Sonora megashear may have played in the Mesozoic tectonic evolution of northwestern Sonora.