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.

The depositional environment and provenance of the Santa Clara Formation of the Upper Triassic Barranca Group in eastern Sonora were studied based on a detailed description of 875 m of section and study of 123 thin sections from both outcrops and cores. The Santa Clara Formation is a cyclic unit with typical fluvial cycles of 4 to 16 m. Highly altered, quartz-rich sandstone is interbedded with black and gray shale and minor beds of graphite and anthracite, which were formerly mined. Detrital chert and remobilized chert cement are significant features of the sandstones of the Barranca. Diagenesis is extensive, of high rank, and commonly blurs the distinction between primary detrital matrix and chemical cement to produce a predominance of quartz-rich wacke. A quiescent basin with tropical climate, subdued marginal relief, and no active volcanism is inferred for the Santa Clara, and limited crossbedding measurements indicate a source to the north. Basin type and its relation to other Upper Triassic deposits remain major problems.

Coal deposits in the state of Sonora are distributed mainly in two regions. One is the Cabullona region, in the northeast, where the coal is contained in the Cintura Formation of Early Cretaceous age; its physical and chemical characteristics place it in the hypo-bituminous rank. The other region is in La Barranca, east-central Sonora, where the coal is contained in the Santa Clara Formation of Triassic-Jurassic age; there the coal ranks as anthracite. This region has been divided into the San Marcial, San Javier, Santa Clara, and San Enrique areas. The Cabullona region is currently in the regional exploration stage, whereas in the La Barranca region, although there has been previous exploitation, exploration has not been completed. Therefore, the coal potential of Sonora has not yet been fully determined.