Thermal mineral springs in Canon de San Diego as a window into Valles caldera, New Mexico
Frank W. Trainer
Two groups of mineral springs are among the best-known surficial thermal features in the Jemez Mountains. Detailedchemical data for the spring waters were first collected in 1912. Despite accumulation of considerable information since then, the springs remain puzzling features. The objective of this report is a re-examination of the springs as a basis for inference of geohydrologic conditions up-canyon, toward and into Valles caldera.
The complex pile of volcanic rock that forms the Jemez Mountains straddles the western marginal fault zone of the Rio Grande rift (Fig. 1). Pre-volcanic rocks—Precambrian crystalline rocks and Paleozoic and Mesozoic sedimentary rocks outside t m began in late Tertiary time and culminated, about 1.4 and 1.0 m.y. ago, in explosive eruptions that led to the successive formation of two calderas and two extensive tuff sheets. The subsequent history of the younger (Valles) caldera included formation of a resurgent structural dome and of a series of rhyolite domes in the ring-fracture zone (Smith and Bailey, 1968); fumarolic and hot-spring activity that has occurred during at least part of the post-caldera period continues at present.
Part of the main aquifer in Valles caldera is known to be a hot-water geothermal reservoir beneath the resurgent dome. Vapor-dominated conditions are present locally. The reservoir rocks are fractured tuff, sedimentary and crystalline rocks, and semi-consolidated valley-fill sediments. The geothermal water, dominated by Na and Cl, contains about 2,500 mg/L Cl. Temperatures are on the order of 260°C and higher (Dondanville, 1978; Kerr, 1982). Recharge of the reservoir is believed to be by infiltration of meteoric water along fractures. Subsurface discharge from the reservoir is also controlled by fractures, and is largely through limestone beneath Cation de San Diego, the canyon which carries surface drainage from the caldera.
The two groups of thermal mineral springs are at Soda Dam and Jemez Springs (Fig. I) in the canyon, about 5.8 and 7.5 km, respectively, outside the rim of the caldera. Summers (1976) described the history and the geographic setting of the springs and presented representative chemical data. Goff and Kron (1980) have recently mapped the geology near the springs and logged a geothermal well at Jemez Springs. At Soda Dam the springs flow from fractured rock where a fault contact between sedimentary and crystalline rocks is exposed in the canyon floor. Jemez Springs issue from alluvium, but alignment of the springs indicates control by faults in the underlying bedrock. Observations to date have shown large, consistent, and puzzling differences between the two spring waters.
- Trainer, Frank W., 1984, Thermal mineral springs in Canon de San Diego as a window into Valles caldera, New Mexico, in: Rio Grande rift--northern New Mexico, Baldridge, W. S.; Dickerson, P. W.; Riecker, R. E.; Zidek, J., New Mexico Geological Society, Guidebook, 35th Field Conference, pp. 249-255.