PROVENANCE STUDY OF LATEST PALEOZOIC TO MESOZOIC EL ANTIMONIO GROUP, SONORA, MEXICO, AND IMPLICATIONS FOR TECTONIC EVOLUTION OF SOUTHWEST LAURENTIA

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 ~3.5 km thick and is composed of the Antimonio (AF), Rio Asuncion (RAF), 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 -100 m depositionally overlain by limestone and mudstone. The El Antimonio Group is interpreted to be predominantly marine, with age controls provided by abundant marine fossils. The current study focuses on basal sandstone/conglomerates of Triassic age that are interpreted to be fluvial in nature, and may provide valuable provenance data.

Sandstones of the El Antimonio Group range from compositionally immature litharenites (AF) to relatively mature subarkose (RAF). Volcanic lithics dominate sanstones of the lower AF, with an increase in plutonic, sedimentary, and metamorphic lithics increasing in sandstones of the upper AF and RAF. Abundant volcanic and plutonic clasts in conglomerates in the lower AF slowly give way to increasing percentages of sedimentary and metamorphic clasts documented in the upper AF, with a slight increase in igneous clasts found in conglomerates of the RAF. Sparse paleocurrent measurements in the lower AF and the RAF and corrected for a moderate clockwise rotation suggest currents originating from the south-southwest and north-northeast, respectively.

Petrographic and clast-compositional data from the AF and RAF suggest a volcanic unroofing sequence. Compositionally immature, angular sandstones, combined with the presence of abundant carbonate and granitic conglomerate clasts suggest that Antimonio Group detritus was sourced from nearby uplifts of mixed arc and transitional continental block provenance. Significant differences in paleocurrent direction may be due to variations in local paleotopography.