Residence time of basinal fluids and paleo flow system in the Paradox Basin using radio-krypton isotopes

Abstract:

Residence time of basinal fluids is crucial 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 provides 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, formation waters in the basal Mississippian and Devonian formations contain younger waters (~0.8 Ma based on ⁸¹Kr), and evidence for meteoric recharge, and dissolution of overlying evaporites, and interaction with underlying crustal rocks. We hypothesize activation of the regional flow system underlying the Paradox Salt is a recent phenomenon related to incision of the Colorado River during the past 4-10 Ma. The increase in hydraulic gradients allowed for flushing of dense brines previously trapped by negative buoyancy.