The Brock Canyon Formation was named for exposures overlying the Ordovician Vinini Formation in Brock and Cottonwood Canyons in the Cortez Mountains, Sec. 34, T. 28 N., R. 49 E. (Gilluly and Gates, 1965; Muffler, 1964).

Age determinations are uncertain, ranging from Pennsylvanian and/or Permian to Early Mesozoic. Fossils are rare with only a few plant and mollusk fragments recovered (Roberts and others, 1967). The Brock Canyon is commonly considered Late Pennsylvanian to Early Permian in age.

At Brock Canyon in the southern Cortez Mountains, the Brock Canyon Formation consists of dolomite, conglomerate, sandstone, limestone, and carbonaceous shale. Quartzite and fine-grained siltstone to sandstone dominate the poorly exposed Brock Canyon Formation in the Dry Hills (Muffler, 1964; Gilluly and Masursky, 1965). The basal contact of the Brock Canyon is everywhere a fault or unconformity.

At the type section, the Brock Canyon contains four mappable units (Muffler, 1964; Gilluly and Masursky, 1965). In ascending order these are: 1) 300 to 750 feet of black to grey, fine to medium-grained quartzite; black chert-pebble and cobble conglomerate with well rounded to angular clasts of black and gray chert, sandstone, quartzite, and quartz up to 5 inches in diameter in a silty to sandy matrix; black, well-sorted and clean lithic sandstone; and minor coarse-grained light gray dolomite in beds 2 to 12 feet thick, which are sheared and brecciated near the basal contact; 2) 750 to 1,000 feet of dark-gray, thinly laminated to thickly-bedded, cherty dolomite, and white to grey quartzite; 3) 1,881 feet of reddish-grey claystone and yellowish siltstone; and 4) 1,630 feet of gray and light brown arkosic sandstone.

In the Cortez Quadrangle, Gilluly and Gates (1965) describe a section of the Brock Canyon Formation which although thinner, is lithologically comparable with the type section. Of interest however, is the dominance in the upper member of shaly gray limestone with abundant interbedded sandstone in beds from a few inches to 2 feet in thickness. They also note the presence of at least one low-angle, nearly bedding plane parallel fault, between dolomites and conglomerate and sandstone in the upper members of the formation.

The Brock Canyon Formation is about 4,900 feet thick at the type section in the Cortez Mountains (Muffler, 1964), and 1,500 feet thick in the Crescent Valley Quadrangle (Gilluly and Gates, 1965).

The Brock Canyon Formation is present in the Dry Hills, Cortez and Simpson Park Mountains.

The lithic clasts contained in the Brock Canyon were derived from the underlying Valmy Formation. Gilluly and Masursky (1965) suggest that the Brock Canyon may have moved eastward a small amount since its base is a thrust plane, but that the formation is at least para-autochthonous. Gilluly and Gates (1965) report that the formation is autochthonous to the south in the Simpson Park Mountains, suggesting any movement of the formation is indeed local and of small magnitude. The Brock Canyon formed during erosion of the uplifted upper plate of the Roberts Mountains thrust and was deposited as fan delta sediments within a local and relatively shallow depositional basin.