GEOLOGY OF THE SOUTHERN SACRAMENTO MOUNTAINS, OTERO AND CHAVEZ COUNTIES, NEW MEXICO

Between 2005 and 2009 the NMBGMR mapped 19 ¼ 7.5-minute quadrangles east of the Sacramento range crest. We compiled this new mapping with the excellent map of Pray (1961) covering the western escarpment to produce the first geologic map of this hydrologically significant area since the regional study of Kelley (1971). The dominant bedrock units in the newly mapped area are the Permian Yeso and San Andres Formations.

The Yeso Formation is composed of gray limestone and gray to tan dolomite, gray, buff, and reddish mudstone, and yellow to tan siltstone and fine sandstone. Gypsum, anhydrite, and halite are present in several deep wells several hundreds of feet below the surface but have not been observed in outcrop in this study. Intraformational dissolution-collapse features and chaotic bedding dips are common in road-cut exposures of the Yeso Formation and solution-enlarged fractures are abundant in carbonate bedrock pavements in stream channels. The unit is usually covered with alluvium and colluvium and the base is not exposed.

The San Andres Formation is composed of light to dark gray and limestone and dolomite. Subdivision of the San Andres into the lower thick-bedded Rio Bonito Member and middle medium- to thin-bedded Bonney Canyon member (Kelley, 1971) was based largely on interpretation of aerial photographs. We estimate the thickness of these units at 580 and 400 feet respectively. Thickness variations on the order of ± 100 feet are likely in both members. The Four Mile Draw member is present in the northeast corner of the mapped area. Here it is composed of thin bedded, locally brecciated dolomite but contains gypsum elsewhere. The exposed thickness is at least 100 feet.

Structure contours of the Yeso-San Andres and Rio Bonito – Bonney Canyon member contacts were derived from elevation points along mapped contacts. The Mayhill fault, Six-Mile and Y-O Buckles, and the Dunken-Tinnie anticlinorium are prominent structural features associated with faulting and folding. Southeast- and north-south-trending faults of the Sacramento River valley and the northern Guadalupe escarpment are associated with complex, though broad, folding.