Provenance study of latest Paleozoic to Mesozoic El Antimonio Group, Sonora, Mexico

The Permian-Jurassic El Antimonio Group is interpreted to be a fragment of a forearc basin adjacent to the incipient Cordilleran magmatic arc on the coast of southwest Laurentia. Due to the scarcity of Mesozoic rock exposed in northwestern Sonora, the tectonic evolution and the source of detritus deposited in the El Antimonio Group are speculative. The section of El Antimonio Group exposed in Sierra del Alamo, Sonora, Mexico, is approximately 3.5 km thick and is composed of the Antimonio, Rio Asuncion, and the Sierra de Santa Rosa Formations, which together make up 14 unconformity-bound sequences. Most sequences consist of a basal sandstone/conglomerate layer ranging in thickness from 10's to 100's of meters, depositionally overlain by limestone and mudstone.;The El Antimonio Group is interpreted to be predominantly marine, with age controls provided by marine fossils. The current study focuses on basal sandstone/conglomerates of Triassic age that are interpreted to be fluvial in nature. Sandstone and conglomerate petrographic and paleocurrent data in conjunction with zircon geochronology are used to recreate Sonoran paleogeography during three key timespans: Lower Triassic (Griesbachian and Spathian), lower Middle Triassic (Anisian), and upper Middle to Upper Triassic (Ladinian, Carnian, and Norian) times.;During Lower Triassic time, flashy stream systems deposited detritus shed from nearby incised Permian volcanoes. Middle Triassic sedimentation is distinguished by the introduction of detritus sourced from plutonic rocks exposed by either continued incision of Permian volcanoes and/or exposure of Precambrian basement rock, as well as detritus sourced from exposed Paleozoic carbonate rocks. During upper Middle and Upper Triassic time stream systems with more constant discharge observed in Sequences V and VII shed detritus derived from exposed Precambrian basement rocks, Paleozoic sedimentary rocks, and incised Middle Triassic volcanoes.