Bulletin 78—Geology of the north half of the Pelona Quadrangle, Catron County, New Mexico

By C. E. Stearns, 1962, 46 pp, 1 plate, 1 appendix, 1 index.

The north half of the Pelona quadrangle lies in the Datil–Mogollon volcanic plateau of west-central NM. The area includes the western third of the San Augustin Plains, a broad intermontane basin of the continental divide. Pre-tertiary rocks, including Yeso, Glorieta, and San Andres Formations, and Triassic Sandstone crop out in one small area. They are overlain unconformably by about 3,000 ft of volcanic and associated sedimentary rocks, assigned to the mid-Tertiary Datil Formation. In the mapped area, the lower third of the formation is volcanic conglomerate, sandstone, and siltstone. Rhyolite, latite, and andesite flows, breccias, and tuffs, together with sedimentary beds, comprise the upper two-thirds of the formation. Variations in the nature of materials erupted from various centers at various times have led to complex intertonguing of the several facies. Individual lithologic units can be mapped readily in an area of this size, but they cannot be expected to maintain their identity in areas of much greater extent.

The Datil Formation is overlain with mild unconformity by a younger volcanic formation, given no formal name here, which attains a maximum thickness of 2,500 ft locally. Basalt and andesite-basalt comprise by far the greatest volume of the younger volcanic formation, but rhyolite, latite, andesite, and alluvial materials are intercalated locally. The formation is probably of late Tertiary and Quaternary age.

The San Agustin Plains occupy a graben, with inferred structural relief of at least 4,000 ft. The graben is younger than the younger volcanic formation. More than half the structural relief has been reduced by sedimentation. Most of the sediments antedate extinct Lake San Agustin, which occupied the graben in Wisconsin time. There is no evidence of tectonic activity in the graben younger than extinct Lake San Agustin.

Volcanic rocks in the uplands peripheral to the graben are gently warped and broken by numerous normal faults. A broad dome, the crest of which lies southwest of Horse Mountain, is older than the younger volcanic formation, as are some normal faults near the south boundary of the mapped area. A broad basin in Tularosa Mountains, the rim of which is an essentially continuous horst, and a series of arcuate, tilted fault blocks between the Tularosa Mountains and the Mangas Mountains are younger that the younger volcanic formation. A structural high between the area of arcuate fault blocks and the San Agustin Plains is probably continuous across the mouth of Patterson Canyon, between the Tularosa Mountains and the Structural dome southwest of Horse Mountain.