Permian sedimentary facies, central Guadalupe Mountains, New Mexico

The Permian rocks of the Guadalupe Mountains undergo striking changes in short distances between basin, basin-margin, and shelf facies. The upper Guadalupian Capitan reef, and equivalent shelf and basin deposits, have been studied intensively in the southern Guadalupe Mountains by many workers. This report is the first detailed study of the equally important facies changes within the Leonardian and lower Guadalupian strata north of the TX-NM border. Field work included the geologic mapping of El Paso Gap quadrangle.The strata in the northern part of the area are shelf sediments, largely carbonates, and include the Yeso, San Andres, Grayburg, and Queen Formations. In the southern part of the area the Grayburg Formation, Queen Formation, and Carlsbad Group of the shelf phase overlie units of the basin and basin-margin phases. The Queen Formation and Carlsbad Group pass into the Goat Seep reef and the Capitan reef, respectively, in this area. Basin deposits encroached some 9 mi shoreward of the present Capitan reef front during deposition of the Cherry Canyon Sandstone tongue and the Cutoff member of the Bone Spring Formation. The upper pat of the Cherry Canyon tongue grades into the lower Grayburg Formation, but the remainder, together with the underlying Cutoff member, passes into a mile-wide transition facies of the San Andres Formation. The southern part of this facies is characterized by patch reefs. The Victorio Peak member of the Bone Spring Formation occurs further north than was previously supposed. In the central part of the area studied, it underlies some 600 ft of San Andres strata and presumably passes into the lower San Andres in the subsurface.The San Andres Formation is one of the most persistent and uniform of the shelf units, extending over an area of hundreds of mi in TX and NM. As the formation is only sparsely fossiliferous, its exact age with reference to the highly fossiliferous sequence of marginal and basin formations has always been debatable, even though the San Andres is a well-known stratigraphic datum in the subsurface. Some geologists believe that it is entirely Leonardian in age, but inconclusive available evidence indicates that the Leonardian-Guadalupian boundary lies within the San Andres Formation.Some of the dense dolomite which characterizes much of the Guadalupian shelf phase possesses a microcrystalline texture resulting from recrystallization, and some exhibits a fine primary detrital texture indicative of calcilutites. Many dolomite layers in the Guadalupian shelf deposits exhibit laminations similar to those in algal-controlled sediments now forming on tidal flats and in shallow water off the coasts of Florida and the Bahama Islands.Large and diverse fossil collections were obtained from the Cherry Canyon Sandstone tongue and the Cutoff member of the Bone Spring Formation where the units pass into the shelf phase. Thus the basin-margin environment favored abundant invertebrate life in the late Leonardian and early Guadalupian time. Although some patch reefs were formed along the basin margin at this time, a barrier reef was not established before middle Guadalupian time. A diverse invertebrate fauna was obtained from a thin bed in the Queen Formation, but in general the Guadalupian shelf deposits were formed in an environment inhospitable to invertebrate life.