Circular 125—Structural geology of northern part of Animas Mountains, Hidalgo County, New Mexico

By J. M. Soule, 1972, 15 pp., 8 figs., 1 sheet.

Physiography, climate, economy, rock units, structure, and tectonic history. The Animas Mountains are a north-south-trending range in south-central Hidalgo County, New Mexico. Precambrian basement rock is granite dated at 1,200±40 m.y. Approximately 3,500 ft of Paleozoic marine and 10,000 to 15,000 ft of Cretaceous clastic sedimentary rocks are exposed in the area. Cretaceous rocks are dominantly synorogenic with respect to latest Cretaceous-earliest Tertiary Laramide orogeny. Tertiary rocks are chiefly post-orogenic intrusives and volcanics. Laramide structure consists of thin thrust plates of Paleozoic and Lower Cretaceous rocks thrust to the north and northeast over Upper Cretaceous synorogenic rocks. An upfaulted block of basement and lower Paleozoic rocks at the north end of the area has resulted in local detachment of the lowest thrust plate outside the area studied. Post-Laramide structure involves normal faults and acidic intrusives injected along fractures, accompanied by volcanism. Normal faulting has continued to the present.

The area covered by this report consists of the northernmost part of the linear north-south-trending Animas Mountains in south-central Hidalgo County, New Mexico. The settlement closest to the area is the hamlet of Animas reached via U.S. Interstate Highway 10 and New Mexico State Highway 338. A passenger car could be driven on graded gravel roads to within a mile and a half of any part of the area. However, torrential rains during local summer storms can make roads impassable.

The objective of this work was to produce an accurate bedrock geologic map of the northern Animas Mountains, and interpret the structure of the area. Field work was undertaken for 39 days during the spring of 1971. Field data were collected and plotted directly on 1:20,000 U.S. Soil Conservation Service aerial photographs. Accurate or large-scale or planimetric base maps of the area suitable for geologic mapping were not available. The 15-min Playas topographic quadrangle covers nearly all the area, but the topography shown is highly generalized and too inaccurate for a geologic map base. The base map used was compiled as overlays on the aerial photographs. Thus, the map is distorted somewhat, especially in the north-central part. This distortion has little effect on the outcrop pattern or interpretation of the structure.

In addition to field work in the northern Animas Mountains, several days were spent reviewing the work of Zeller and Gillerman in the Big Hatchet, Animas, and Peloncillo Mountains, respectively. Type and reference stratigraphic sections were studies for better control of the stratigraphy in the northern Animas Mountains. Several geologic problems of the area remain unsolved. Although the complex structure of the area would make such work difficult, most details of the stratigraphy need additional study. Detailed analysis of fossils from Cretaceous rocks will aid greatly in understanding Laramide events. The sequence of igneous rocks presented in this paper are determined. The active faulting in the area may prove important in further understanding of deformation in the region.