HYDROGEOCHEMICAL EVOLUTION OF BASINAL FLUIDS IN THE PARADOX BASIN: IMPLICATIONS FOR SOURCES, PALEOFLUID FLOW, AND WATER-ROCK INTERATIONS

The Paradox Basin in the Colorado Plateau has abundant manifestations of multiple episodes of paleofluid flow, including sandstone bleaching and ore mineralization (Cu; U; Fe; Mn), salt tectonics, and hydrocarbon, CO₂ and He reservoirs. Major ion and isotopic (δ¹⁸O_water; δ²H_water; δ³⁴S_SO4; δ¹⁸O_SO4; ⁸⁷Sr/⁸⁶Sr) signatures of formation waters were used to evaluate the hydrochemical distribution and sources of remnant fluids and water-rock interactions by comparison with the sedimentary rock record. There are three distinct sources of salinity in basinal fluids: 1) highly evaporated paleo-seawater associated with the Pennsylvanian Paradox Salt Formation that has been diagenetically altered, 2) brine derived by the dissolution of evaporites around salt anticlines and at the base of the Paradox Salt Formation with the influx of topographically-driven meteoric water, and 3) more recent evaporated seawater with different extents of diagenetic reaction in the Pennsylvanian Honaker Trail Formation. Fresh to brackish groundwater in the Cretaceous Burro Canyon Formation contains low Cu and high SO₄ concentrations due to oxidation of Cu sulfides by the influx of meteoric water, while U concentrations are higher than other formation waters. High ⁸⁷Sr/⁸⁶Sr ratios in the Honaker Trail Formation water indicate the interaction with radiogenic minerals in siliciclastic formations above the salt. Elevated ⁸⁷Sr/⁸⁶Sr ratios in the formation waters from the Mississippian and Devonian formations suggest the interaction of basinal brines with underlying radiogenic crustal rocks. Understanding the various types and distribution of remnant fluids in the basin provides constraints on the paleofluids responsible for widespread sandstone bleaching, ore mineralization, and hydrocarbon migration.