The Antimonio ramp in Sonora, Mexico

The Upper Permian to Lower Jurassic El Antimonio Group has been considered as part of the NW–SE-trending tectonostratigraphic Antimonio terrane in Sonora, Mexico. The Upper Triassic Rio Asuncion Formation of the El Antimonio Group comprises a shallow-marine detrital-carbonate succession at three major localities near Caborca and Hermosillo. Previous reconstructions have proposed differing depositional environments for the Rio Asuncion Formation (e.g., carbonate platform, carbonate ramp) and remained therefore nonconsensual. The present study has its focus on the Rio Asuncion Formation and includes the analyses of 129 thin-sections of carbonate and mixed siliciclastic–carbonate rocks from three localities (Barra los Tanques, Sierra del Alamo, and Sierra Santa Teresa). In Sierra del Alamo, ammonite findings allowed us to confirm the position of the Triassic–Jurassic boundary. Considering the lack of biostratigraphic markers and the clastic nature of the samples, a statistical approach was used to propose a depositional model that can provide more detailed insights into this sedimentary succession. Hierarchical cluster analyses were performed on the gathered abundance data to identify microfacies and to compare the results for each outcrop. Through these analyses, 22 microfacies were defined, which describe the depositional environments of the two main localities. Furthermore, it could be demonstrated through this approach that not only the localities of Barra los Tanques and Sierra del Alamo but also the outcrops near Sierra Santa Teresa belong to the Antimonio depositional system. To gain insights into the relative depositional conditions among the microfacies, non-metric multidimensional scaling was performed. The resulting trends of water energy and proximity to the shoreline of Laurentia were then used to propose a depositional model for the mixed siliciclastic–carbonate Antimonio ramp system.