Devonian stratigraphy of the San Andres Mountains, Dona Ana, Sierra, and Socorro Counties, New Mexico

Devonian stratigraphic relationships are described from 16 localities in the San Andres Mountains, Mud Springs Mountains, and at Hermosa, New Mexico. Devonian strata in the San Andres Mountains comprise a thin (25-50 m or 60-170 ft), unconformity-bounded sequence, commonly with diastemic formation or member boundaries. The formations recognized throughout most of the area are the Onate, Sly Gap, and Contadero Formations of Givetian, Frasnian, and Famennian ages, respectively. The setting during Devonian time was a shelf area characterized by calcareous, argillaceous, and sandy sediments, grading into dark basinal shales at the south end of the range. Over most of the area, the Onate Formation is characterized by brown, silty, dolomitic rocks overlain by shales and argillaceous limestones of the Sly Gap Formation. The overlying Contadero Formation, mostly consisting of shales and fine sandstones, can be subdivided into members in the northernmost quarter of the area. These units are the basal Salinas Peak member, the middle Thurgood Sandstone Member, and the upper Rhodes Canyon Member of interbedded tan shale and sandstone. These units cannot be identified south of Cottonwood Canyon, and the Sly Gap and Contadero Formations can be differentiated only as far south as San Andres Canyon. South of San Andres Canyon, dark shales comprise the Devonian rocks above much-thinned Onate Formation dolomites. All units are fossiliferous, with excellent brachiopod faunas providing ages of each. Corals that are present throughout the Frasnian part of the section are extremely rare in older rocks and absent in the Famennian rocks. Faunal elements are recognizable to the west in outcrops examined at the Mud Springs Mountains and at Hermosa, New Mexico. Both the Onate and Slay Gap beds are demonstrably present to the west, and at Hermosa they occur beneath typical Percha Shale.