Mixing of thermal and nonthermal waters in the margin of the Rio Grande Rift, Jemez Mountains, New Mexico
— Frank W. Trainer

The geology of the southwestern Jemez Mountains has been described by Wood and Northrop (1946), Ross, Smith, and Bailey (1961), and Smith, Bailey, and Ross (1970). Trainer (1974) summarized the geohydrology and presented representative chemical analyses of ground water. The Jemez Mountains (Fig. 1) comprise a complex pile of late Tertiary and Quaternary volcanic rocks that lies athwart the fault zone at the west margin of the Rio Grande rift. San Diego Canyon has been excavated, in part, in the marginal fault zone. The Jemez River, flowing through the canyon, drains Valles Caldera. Rocks exposed in San Diego Canyon include Precambrian granitic rock, Paleozoic and Mesozoic sedimentary rocks, and Tertiary and Quaternary volcanic rocks and alluvial fill. Water discharged by thermal and cold mineral springs in San Diego Canyon is believed to be derived in part from a hydrothermal reservoir in Valles Caldera and to have flowed out along the fault zone, largely in limestone that overlies the granitic rock. Other hydrothermal features include areas of solfataras and hydrothermal alteration, believed to result from the activity of deep thermal water; and several thermal springs, on the flanks of young volcanoes in the ring-fracture zone of the caldera, that are believed to discharge water which has circulated to only relatively shallow depths.