GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 70-11
Presentation Time: 5:00 PM


NOYES, Chandler1, PERSON, Mark2, FERGUSON, Grant3 and MCINTOSH, Jennifer1, (1)Department of Hydrology and Atmospheric Sciences, University of Arizona, Tucson, AZ 85721, (2)Department of Earth & Environmental Science, New Mexico Tech, 801 Leroy Place, Socorro, NM 87801, (3)Department of Civil, Geological and Environmental Engineering, University of Saskatchewan, Saskatoon, SK S7N 5A9, Canada

We collected and analyzed geochemical and isotopic samples from a series of wells within the metal-rich Lisbon Valley of the Paradox Basin in southeastern Utah, in order to assess groundwater flow pathways and residence times. The principal aquifers of interest are the Cretaceous Burro Canyon Aquifer (BCA), composed of stream-lain sandstones and carbonates, and the Jurassic Navajo Aquifer (NA), an eolian sandstone. All geochemical and isotopic results show that these distinct aquifers are not strongly hydrologically connected under current natural hydrologic conditions. The upper BCA has a calcium-sulfate signature with high TDS and higher concentrations of important ore-forming trace elements, such as Cu, Fe, Co, Mn, and U. Near-zero to negative values of δ34S-SO4 and δ18O-SO4 are consistent with sulfide oxidation in the mineralized aquifer. The BCA has a less radiogenic 87Sr/86Sr ratio, but higher concentrations of Sr2+, reflective of the carbonate lithology of the aquifer. Corrected radiocarbon ages in the BCA of 3,300-11,000 BP coupled with δ18O and δD values in the range of modern precipitation are indicative of recharge occurring during the Holocene. The lower NA contains sodium-bicarbonate groundwater with low-to-moderate TDS, while positive values of δ34S-SO4 and δ18O-SO4 and lower concentrations of SO42- are characteristic of eolian sandstones; alternatively, low DO and low concentrations of SO42- indicate an anoxic environment, which would be conducive to bacterial sulfate reduction, which can also produce positive values of δ34S-SO4 and δ18O-SO4. Groundwater in the NA is generally more radiogenic (higher 87Sr/86Sr) than the BCA, but has lower concentrations of Sr2+, characteristic of the higher silicate content of the NA. Corrected radiocarbon ages in the NA of 15,000-36,000 BP, coupled with δ18O and δD values lower than modern precipitation including snow, are indicative of Late Pleistocene recharge when the climate of the Four Corners area of the U.S. southwest was 5 to 7°C cooler than today. Corrected radiocarbon ages and stable water isotope values suggest recharge to both aquifers occurs at the La Sal Mountains, flowing southeast along a long lateral flow path to the Lisbon valley; distinct hydrogeochemistry in each aquifer suggests vertical flow is minimal.