CHARACTERIZATION OF FORMATION WATERS IN THE PARADOX BASIN TO CONSTRAIN PALEOFLUID FLOW AND WATER-ROCK-MICROBIAL INTERACTIONS

The Paradox Basin in the Colorado Plateau has diverse manifestations of paleofluid flow, including widespread sandstone bleaching and ore mineralization, salt tectonics, and abundant hydrocarbon, CO₂ and He reservoirs. Forty-six fluid samples were collected in 2018 to evaluate the hydrochemical distribution and sources of remnant fluids as an indicator of the long-term evolution of paleofluids in the Paradox Basin. Surface waters and shallow groundwater in Paradox and Sinbad salt valleys contain Na-Cl type brines from meteoric water dissolution of halite and anhydrite. Strontium and S isotopic signatures of the Na-Cl type brines confirm that dissolution of evaporites is the dominant source of solutes. Holocene to Pleistocene age shallow groundwater from Cretaceous Burro Canyon and Jurassic Navajo formations in the Paradox Valley are Na-Ca-HCO₃ type dilute meteoric waters with elevated U concentrations and S and Sr isotope values indicative of interaction with sulfides and carbonates. Stable isotope values of δ¹⁸O and δD and Cl/Br ratios of Ca-Cl brines in the Pennsylvanian Honaker Trail Formation indicate the formation waters likely originated from partially-evaporated seawater, which interacted with radiogenic minerals in Permian-age arkosic redbeds and sulfides, based on Sr and S isotopes, respectively. Desert Creek and Cane Creek members of the Pennsylvanian Paradox Formation contain the most saline Ca-Cl type formation waters with chemical and isotopic signatures indicative of paleo-evaporated seawater associated with deposition of the potash evaporite minerals. Basinal brines in underlying Mississippian and Devonian formations are derived from paleo-evaporated seawater diluted by meteoric water and influenced by dissolution of halite from the Paradox Formation. Radio-krypton (⁸¹Kr) and noble gases are being analyzed to further constrain the residence time of basinal fluids.