PALEOWEATHERING AND PROVENANCE OF THE SILICICLASTIC ROCKS OF THE SIERRA DE SANTA ROSA FORMATION, SONORA, MEXICO: CONSTRAINTS FROM MAJOR OXIDES GEOCHEMISTRY

The upper Permian-Triassic and lower Jurassic sedimentary rocks of the El Antimonio Group are well exposed in the vicinity of Caborca area of Sonora, Mexico. The El Antimonio Group consists of three formations, from the base to the top, Antimonio, Rio Asuncion and Sierra de Santa Rosa formations. The whole-rock major oxides geochemistry was undertaken on the sandstones and shales of the Sierra de Santa Rosa Formation that crop out in the Sierra del Alamos section. The purposes of the present study are to determine the paleoweathering conditions and to deduce the provenance signatures. Geochemically these rocks are classified as arkose, wacke and shale. Both sandstones and shales show large variations in SiO₂ and Al₂O₃ contents (SiO₂: 52.6 to 81.2%, 56.6 to 74.5%, respectively; Al₂O₃: 7.8 to 13.4%, 12.1 to 15.5%, respectively). Many sandstones show high CaO content that may be due to the presence of calcite cement. The chemical index of alteration (CIA) values of sandstone (40 to 59) indicates a low intensity of chemical weathering, whereas CIA values of shale (62 to 76) suggest moderate chemical weathering in the source region. Many sandstone samples show lower CIA values (below 50) indicate the sorting effect. The major element based multidimensional tectonic discrimination diagram reveals the rift setting for the source rocks of the Sierra de Santa Rosa Formation. The SiO₂/Al₂O₃ and Al₂O₃/TiO₂ ratios and discriminant function diagram suggest that the sandstones and shales were derived mainly from felsic to intermediate source rocks.