Precambrian rocks of the Sierra Oscura and northern San Andres Mountains
— A. J. Budding and Kent C. Condi

The Sierra Oscura and San Andres Mountains provide an almost continuous exposure of Precambrian rocks in a north trending zone through south-central New Mexico. These rocks are part of a Precambrian crustal province ranging in age from about 1.0 to 1.8 b.y. which extends northeastward across the United States from southeastern Arizona to Ohio (Goldich and others, 1966). Whole-rock Rb-Sr ages and zircon ages from Precambrian rocks in northern New Mexico fall in the range of 1.5 to 1.8 b.y., while similar ages from the Franklin Mountains near El Paso, Texas and other locations in central and west Texas fall in the range of 0.95 to 1.2 b.y. (Wasserburg and others, 1962; Denison and Hetherington, 1969; Fullager and Shiver, 1973; Barker and others, 1974; Brookins, 1974).

 

Muehlberger and others (1966) report a single Rb-Sr whole rock age of 1.3 b.y. on a granite from the Sierra Oscura and an age of 1.36 b.y. on muscovite of the same granite, determined by the K-Ar method. The same authors determined ages of 1.38 and 1.4 b.y. (K-Ar) on biotite from gneiss of the San Andres Mountains..

 

Since these ages are not Rb-Sr isochron ages or zircon ages, their interpretation is uncertain although they probably represent minimal ages for the Precambrian rocks in the Sierra Oscura and San Andres Mountains. The exposures in these ranges are particularly important as they may include rocks that form a transition zone between 1.8 b.y. old rocks in northern New Mexico and 1.0 b.y. old rocks in the Franklin Mountains.

 

The Sierra Oscura and San Andres Mountains are north-trending, block-faulted ranges with Precambrian rocks exposed in the lower elevations. The Sierra Oscura block is tilted east and Phanerozoic rocks, which form the crest of the range, dip eastward at angles of 5 to 10 degrees. On the west, the range is bounded by a series of major high-angle faults. Although much of this normal faulting is obscured by alluvial deposits, normal faults (down on the west) are exposed in the bedrock along the steep western slope of the range (Fig. 1). The San Andres Mountains north of Rhodes Canyon expose Precambrian rocks along its eastern slope (Fig. 2). According to the geological mapping of Bachman (1965, 1968) and Bachman and Myers (1969), their structure is that of a complexly faulted north-trending range, tilted westward. Normal faulting is present along the steep eastern slope (Fig. 3). Low-angle gravity glide structures have emplaced tectonic outliers of the Phanerozoic sedimentary sequence at lower elevations along the mountain ront.

 

Outcrops along the steep slopes are plentiful and allow, in most places, collection of unweathered samples of Precambrian rock types. Arroyos have cut the thin veneer of pediment gravels, and extensive bedrock exposures show relationships between the various lithologic varieties. Precambrian-exposures of the Sierra Oscura and San Andres Mountains are within the boundaries of the White Sands Missile Range, a military reservation under the jurisdiction of the Department of Defense. The authors wish to express their gratitude to the personnel of W.S.M.R. and in particular to Felix Sedillo and Ismael Rel for the cooperation received in the course of the geological field work.