Memoir 24—Geology of the Pecos country, southeastern New Mexico

By V. C. Kelley, 1971 reprinted 1979, 75 pp., 1 table, 26 figs., 7 plates, 7 appendices, 1 index.

The Pecos country of this report includes most of the Pecos Valley drainage area from 60 mi north of Roswell to the New Mexico-Texas line. The area of some 12,500 mi² is dominantly surfaced by Permian carbonate and evaporite rocks of the San Andres Formation and the Artesia Group, both mostly of the Guadalupian age. A wide swath of alluvium and terrace gravel covers the broad lowland of the Pecos River and its tributaries. Tributary drainage to the valley is quite asymmetrical as the result of the broad high uplifts to the west and the low Llano Estacado to the east. From the west nine major tributary systems 50-100 mi long contrast with the few short washes from the east.

Aside from one small inlier of Precambrian crystalline rocks at Pajarito Mountain, Leonardian siltstone, sandstone, gypsum, and dolomite of the Yeso Formation are the oldest rocks exposed in the area. At the Pajarito Precambrian area the Yeso is as little as a few feet thick, but in surrounding areas it ranges from about 1,000 ft in several exposures to its full thickness of 1,300-2,000 ft.

The San Andres Formation has been divided into three new members named, in ascending order; Rio Bonito, Bonney Canyon, and Fourmile Draw. Glorieta Sandstone tongues extend into the area from the north, and northward, not far beyond the mapped area, the Rio Bonito Member becomes the Glorieta Sandstone. The Bonney Canyon Member is thickest in the central part of the area. In the north it thins, and at least locally, grades into evaporite beds. To the south, the Bonney Canyon thins and is lost approaching the shelf margin in thick or massive beds. The Fourmile Draw Member is essentially the upper, noncherty member of Hayes in the back-reef country.

Formations of the Artesia Group are mapped individually as exposed east of the Pecos River and north of Roswell. Grayburg and Queen are mostly covered in the broad alluvial valley from the Seven Rivers area north to Roswell, where they are mapped together in an undivided sequence. Yates, Seven Rivers, and Tansill are mapped individually, but north of Hagerman, the Tansill is lapped out by Triassic beds or covered by alluvium. In the Capitan reef area, the Artesia formations are individually mapped close to the structureless reef or bank at the shelf edge. It is suggested the Lamar tongue near the top of the Bell Canyon Formation may project to near the base of the Yates Formation rather than high in the Tansill Formation.

The Castile Formation is mapped into lower and upper members, and it is suggested that both might be basin facies of the part of the Yates and Tansill Formations. Known Mesozoic units are mapped where present, but not given special attention. Triassic Santa Rosa Sandstone is mapped east of the Pecos, stepping down northward across the area from Dewey Lake Formation in the south to Yates in the north. In the Capitan-Ruidoso country, Dakota Sandstone is mapped southward, stepping down from Triassic Chinle Shale to the San Andres Formation.