Upper Triassic Barranca Group; Nonmarine and shallow-marine rift-basin deposits of northwestern Mexico

The 3,000-m-thick Upper Triassic Barranca Group in the Sierra de San Javier in east-central Sonora, Mexico, is composed, in ascending order, of the Arrayanes, Santa Clara, and Coyotes Formations. The Arrayanes and Santa Clara Formations are composed of fluvial and marine-delta deposits of quartzose and arkosic sandstone, conglomerate, shale, and siltstone; the Santa Clara Formation includes minor amounts of coal and tuff. A sharp contact (perhaps an unconformity) separates the Santa Clara Formation from the overlying Coyotes Formation. The Coyotes consists of alluvial-fan deposits of pebble-to-boulder conglomerate. Paleocurrents were southward during deposition of the Arrayanes and Santa Clara Formations and southwestward during deposition of the Coyotes Formation, assuming that no major post-deposition tectonic rotation has occurred. The Santa Clara Formation has been dated paleontologically as Late Triassic; the age of the entire group is unknown, but is commonly assumed to also be Late Triassic. The Barranca Group in the Sierra de San Javier rests unconformably on a sequence of eugeosynclinal chert, argillite, quartzite, and carbonate rock of Paleozoic age, and is unconformably overlain by the Tarahumara Volcanics, which have been dated no more precisely than latest Triassic to earliest Cenozoic.

The thick, coarse, and laterally variable deposits of the Barranca Group indicate deposition in a basin, or basins, flanked by areas of high relief. Much of the Barranca in Sonora appears to have been deposited in a single basin, which is delineated by the occurrence of major outcrops of the Barranca Group in an east-west-trending belt about 110 km long and 40 km wide. The elongate shape of this basin and the interpretation of flanking areas of high relief suggests a basin of rift origin. If so, the Barranca Group is part of a broad zone of rift-related Upper Triassic sequences in northern Mexico that apparently formed by transtensional and/or extensional faulting.