Residence time of basinal fluids is key to interpreting the evolution of fluid flow systems in sedimentary basins over geologic time. Radio-krypton isotopes (81Kr), combined with other chemical and isotopic tracers, provide new insights into the timing and mechanisms of basinal fluid flushing events versus retention of saline fluids within the Paradox Basin. Preliminary results of formation waters associated with kilometer-thick Pennsylvanian evaporite deposits suggest they are beyond the 81Kr dating range (1.3 Ma), consistent with their chemical and isotopic composition reflecting their origin as highly evaporated paleo-seawater that was subsequently modified via diagenetic reactions. Formation waters in the Pennsylvanian Honaker Trail Formation, overlying the evaporites, have 81Kr ages of ~0.5 Ma and are composed of meteoric waters mixed with younger, less evaporated paleo-seawater. Surprisingly, the basal Mississippian and Devonian formations contain younger waters (~0.8 Ma based on 81Kr), likely from introduction of topographically-driven meteoric recharge, which interacted with radiogenic basement rocks or intervening shales, diluted residual brines, and dissolved evaporites at the base of the Paradox Formation. We hypothesize activation of the regional flow system underlying the Paradox Formation evaporites is a recent phenomenon related to incision of the Colorado Plateau during the past 4-10 Ma. The increase in hydraulic gradients and removal of shale confining units allowed for flushing of dense brines previously trapped by negative buoyancy.
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