Summary of Pennsylvanian section in southwestern New Mexico and southeastern Arizona
Authors: Kottlowski Frank E. 
Published: March 2023

Pennsylvanian strata in southwestern NM and southeastern AZ range from Morrowan to Virgilian in age, are disconformable to angularly unconformable above Precambrian to Upper Mississippian rocks, and are as much as 4,000 ft thick, although in most localities they range from 1,000-2,000 ft in thickness. The Pennsylvanian-Permian contact in many areas appears to be gradational, and the boundary is drawn within a zone of indeterminate age between Virgilian and Wolfcampian fossil-bearing beds. Permian rocks at some localities, as well as Cretaceous and Tertiary rocks in other places, are erosionally unconformable on the Pennsylvanian strata. In the southern Hueco Mountains and the Florida Mountains areas, all the Pennsylvanian beds apparently were removed by erosion during early Permian time.

Pennsylvanian rocks in southwestern AZ are mapped as the Horquilla Formation and lower part of the Earp Formation of the Naco Group, and in south-central AZ as the Naco Formation and lower part of the Supai red beds. In southwesternmost NM, the upper part of the Horquilla Formation is younger than in AZ and includes Wolfcampian beds. Pennsylvanian strata in central and southwestern NM generally have been referred to the Sandia and Madera Formations of the Magdalena Group by the U.S. geological Survey, or to the faunal equivalents of the Morrow, Derry, Des Moines, Missouri, and Virgil series, which were subdivided by Thompson into groups and formations.

Lithic units have been named from outcrops in isolated mountain ranges, such as the Gobbler, Beeman, and Holder Formations in the Sacramento Mountains, the red House, Nakaye, and Bar B formations in the Caballo Mountains, and the Panther Seep Formation in the San Andres Mountains.

Clastic sediments were derived chiefly from the Pennsylvanian-age pedernal Mountains to the east, and locally from the Florida, Joyita, Zuni-Defiance, and Kaibab positive-tending areas, which at times during the Pennsylvanian were emergent and during other epochs were covered by shallow seas. Five troughs or basinal downwarps, where thick sections were deposited, stand out on the isopach map: (1) the Estancia trough, in which Pennsylvanian sediments approach 4,000 ft in thickness; (2) the Orogrande Basin, containing as much as 3,000 ft of Pennsylvanian rocks, of which more than two-thirds was deposited in Upper Pennsylvanian time; (3) the Pedregosa Basin, in which Pennsylvanian strata are almost 2,500 ft thick; (4) the Lucero Basin, with as much as 2,700 ft, and (5) the San Mateo Basin, with almost 2,700 ft of Pennsylvanian strata. The Orogrande, San Mateo, and Lucero Basins are aligned along a north-south trend that probably marks a channelway northward across the Cabezon sag to the Paradox Basin; the Pedregosa and Orogrande Basins probably connected eastward, in northern Chihuahua and westernmost Texas, with the Marfa and Delaware Basins. Over a large area from Clifton, AZ, southeastward to the Florida Mountains in NM, Pennsylvanian rocks are absent, and Permian or Cretaceous strata rest on pre-Pennsylvanian rocks.

The Pennsylvanian sequence is a limestone lithofacies throughout most of the southern part of the area. Northward it grades into lime-shale and then into shale-lime lithofacies of interbedded red beds and nodular limestone on the Mogollon Rim, but of interbedded grayish calcareous shale and fossiliferous limestone west of the south and sand-lime lithofacies on the north, whereas the Pennsylvanian beds in the Estancia trough are of shale-lime lithofacies that intertongue eastward, toward the Pedernal Mountains, with a sand-shale lithofacies. In small areas near the Joyita Hills and Florida Mountains, lime-sand and sand-lime lithofacies dominate; the Joyita Hills are on the east side of the Lucero Basin, which shows a westward gradation from sand-lime to lime-sand to lime-shale, and toward the Zuni positive area, to shale-lime. The thick San Mateo sections are of lime-shale lithofacies.

Pennsylvanian strata are potential sources of oil and gas at least in the northern and eastern parts of the region, on the southern edge of the Colorado Plateau, and in the Estancia Valley, Acoma embayment, Chupadera Mesa, Jornada del Muerto, and the Tularosa Valley. Even the basin-and-range country and Datil-Mogollon volcanic plateau are underlain by possibly productive Pennsylvanian beds. Large-scale use of the various Pennsylvanian rocks as industrial minerals and rocks is hampered by the long distances to populous areas, the limestones, shale, and gypsum having been used only locally for building stone and crushed rock, in agriculture, and to make bricks, tile, cement, and lime products.