EVOLUTION OF THE PARADOX BASIN SUBSURFACE FLUID-FLOW SYSTEM AND PALEOFLUID-ROCK REACTION PRODUCTS (Invited Presentation)

Sedimentary rocks of the Paradox Basin host a wide range of evidence for fluid-rock reaction reflecting the generation and flow of diverse paleofluids through the basin’s ~300-Ma history. These manifestations include extensive bleaching, changes in cementation, concentrated mineral deposits, and hydrothermal alteration, in many cases associated with faults, folds, and fractures at a variety of scales. We hypothesize that this geologic record of paleofluid flow, combined with modern hydrogeological observations, provide an exceptional opportunity for understanding the long-term evolution of a basin-scale subsurface fluid-rock system and the emergent compositional and hydrogeologic patterns, as well as resource-related consequences, of similar settings. We find that modern fluids are dominated by a dilute topography-driven groundwater shallow flow system, connate and evaporite-dissolution-derived brines in Penn-Perm units at depth, some with low Eh and high metal concentrations, and evidence for an haline convection cell in Cutler red beds at intermediate depths between these domains. In some Jurassic units, paleofluid flow and some Fe-Mn deposits appear to be practically syndepositional, whereas in others, bleaching, cementation changes, and in some cases Cu deposits emanate from faults and/or localized upflow zones and are much younger, with mid-Cretaceous, Paleocene, and late Pliocene ages most common. These episodes may relate to distinctive tectonic intervals including rapid burial by the thermally and hydraulically insulating mid-K Mancos Shale, minor late Laramide deformation, and rapid Neogene erosion. Cu deposits and related features require at least two distinct flow episodes involving fluids with contrasting compositions. U-V deposits reflect yet another type of flow system, likely involving an early Mesozoic mineralization followed by later reworking associated with strong redox gradients. Dominant fluid sources are likely compaction, clay dewatering, and hydrocarbon maturation from Paradox Fm. clastic units.