SALT SPRINGS AND QUATERNARY TRAVERTINE DEPOSITS IN THE RIO GRANDE RIFT: A CONNECTION BETWEEN PAST AND PRESENT HYDROLOGIC SYSTEMS

Modern salt springs of the Rio Grande rift basin, and associated travertine accumulations, offer insight into interactions among seismicity, magmatism, input of saline waters, and Rio Grande surface water chemistry and quality. Water chemistries reflect two subsets of ground water; "upper world" dilute waters of the Albuquerque Basin and flanking Paleozoic aquifers, and high salinity "lower world" water rich in CO₂. Travertine-depositing springs reflect the “lower world” input, and their contribution, though volumetrically small, can alter river water chemistry. Conventionally it has been assumed that the CO₂ issuing with the saline, calcium-carbonate waters is atmosphere-derived during aquifer recharge. However, elevated concentrations of Na, Cl, He and CO₂, and relatively low pH of the environment indicate that the controlling factor for supersaturation with carbonate is mixing of deep waters rich in CO₂ with “upper world” waters. Recent inspection of water quality of the Rio Grande through a combination of geochemical tracers indicates that “lower world” waters are important contributors to the fluvial salt load. Rapid degassing of CO₂-charged waters facilitates travertine deposition. Travertine deposits are amenable to U-series and paleomagnetic evaluation hence providing geochronologic control for ancient hydrologic conditions. Travertine-depositing springs occur mainly along basement penetrating Laramide faults most of which have Neogene reactivation. Extensive travertine deposits occur near the Socorro magma body, the intersection of the Jemez Lineament and the Rio Grande rift, and along the Jemez lineament. This distribution suggests that travertine accumulation is influenced by CO₂ from magmatic sources. U-series dates of 350 ka from upper layers of commercial travertine deposits near Belen, NM, and reversed polarity magnetizations at the base of the same quarry (thus probably > 778 ka) suggest a travertine record exceeding 1Ma. Further structural investigation, geochemical analysis of spring waters and gases, and dating of travertine deposits will improve the understanding of the recent history of the Rio Grande hydrologic and neotectonic system.