Abstract

The Pecos country of this report includes most of the Pecos Valley drainage area from 60 miles north of Roswell to the New Mexico-Texas line. The area of some 12,500 square miles is dominantly surfaced by Permian carbonate and evaporite rocks of the San Andres Formation and the Artesia Group, both mostly of 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 to 100 miles 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 feet in several exposures to its full thickness of 1,300 to 2,000 feet.

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.

Chemical analyses of numerous samples of the San Andres show increase in dolomite content in the upper part, especially in Bonney Canyon and Fourmile Draw Members. Northward considerable increase in dolomite is also shown, especially in the Rio Bonito Member. Limestone makes up most of this member in the south as far as the northern end of the Guadalupe Mountains, but in the next dozen miles northward, dolomite increasingly replaces the limestone.

In the Pecos Valley much previously mapped Artesia is found to be Gatuna Formation. A number of field relationships suggest that the Gatuna may be late Tertiary rather than Pleistocene.

The structure of the area is dominated by a low eastward dip that is referred to as the Pecos slope. Much of the structural slope results from late Tertiary fault-block tilting of the Sacramento and Guadalupe uplifts that dominate the southwestern part of the area. In the northern half of the area, the Pecos slope is caused by the broad rise of the Mescalero arch which roughly coincides with the Permian buried Pedernal uplift. The western side of the Mescalero arch descends through faulted and igneous complications into the middle Tertiary Sierra Blanca basin.

The northwestern part of the area is the site of a large cluster of stock and laccolith centers known as the Lincoln County porphyry belt. Most of the intrusives lie to the west of the area, but the large Capitan intrusive, anomalous in its eastward trend, occupies an important position in the regional tectonics. Several stratigraphic and structural changes take place across the line of the intrusive. Fourmile Draw beds occur to the north, but not to the south of the intrusion, and the crest of the regional Mescalero arch is left offset about 10 miles.

Along the middle of the Pecos slope, the western part from the Capitan intrusion southward is separated from the eastern part by a north-south line of deformation, consisting of the Dunken uplift to the south and the Tinnie folds to the north. This belt of deformation in many respects separates the slope into two contrasting structural areas. The western part is dominated by the Sacramento uplift, the Mescalero arch, and the Sierra Blanca basin. East of the Dunken-Tinnie belt, the slope is relatively simple except for the long, narrow, northeastward-trending buckles. These buckles are right-wrench fold-faults which are undoubtedly Precambrian rooted and show evidence of activity at least as old as Permian.

Two faults, Barrera and Carlsbad, are mapped at the base of the Capitan reef escarpment. Offsets at the surface suggest that the basin side is downthrown, although not of a magnitude equal to that of the escarpment. Most of the relief of the escarpment is due to erosion of the relatively weak basin evaporitic facies. Little or no confirmation of these faults is found in subsurface vertical separations. Strike slip is considered possible, but the nature and origin of the faults are unknown.