New Mexico Geological Society Annual Spring Meeting — Abstracts

The NMBGMR has recently completed a new 1:100,000-scale geologic map of the southern Sacramento Mountains. The mapped area extends from Alamogordo on the west to Dunken on the east, and from the southern border of the Mescalero Apache Reservation to the northern border of McGregor Military Range. The compilation is comprised of new mapping from the Statemap program, mapping funded under the Sacramento Mountains Hydrogeology Study, and existing mapping by Pray (1961) along the western escarpment and Black (1973) near Piñon. It covers approximately twenty-four 7.5’quadrangles never before mapped at 1:24,000-scale.

The important units exposed over the majority of the map area are the Permian Yeso and San Andres formations. The Yeso formation consists of red and yellow siltstones and mudstones, and grey carbonates. It is exposed, generally poorly, along valley bottoms and lower valley walls throughout the north-central part of the map area, along the upper flanks of the western escarpment and in the Sacramento River drainage, and in isolated outcrops in the southeast. Springs are very common below the contact between the Yeso and San Andres Formations. Following Kelley (1971), three members of the San Andres formation were mapped, except in the highest elevations, where exposures are poor and vegetation precluded aerial photo identification.

Major structures are 1) the Tertiary Alamogordo Fault, along which the east-tilted mountain block was uplifted; 2) the Tertiary Sacramento River fault zone, composed of west-side-down, southeast-trending normal faults that define the Sacramento River drainage; 3) the southern termination of the Dunken-Tinnie anticlinorium, a north-south zone of tight folding and associated faults with variable dip-slip and possible strike-slip motion; and 4) the southwestern termination of the Six-Mile and Y-O buckles, right-lateral strike-slip faults that cross the Pecos Slope. The latter three structures are only constrained to be younger than Permian in age. Complex faults and folds in the southeast probably resulted from the interaction of Tertiary block-faulting with existing northeast- trending basement structures. We interpret the northeast– trending segment of the Rio Peñasco near Mayhill to be a fault, and it is a hydrologically significant boundary.