GEOCHRONOLOGY REVEALS EOCENE FLUID-ROCK INTERACTION AND FAULT-CONTROLLED FLUID FLOW IN THE PARADOX BASIN

Understanding how the sources and migratory pathways of fluids in sedimentary basins evolve through time requires geochronologic constraints on fluid-rock reaction products. The Paradox Basin of the Colorado Plateau has a long history of subsurface fluid flow and hosts a wide variety of fluid types and fluid flow manifestations such as bleached sandstones, carbonate and silica metasomatism, Cu, U-V, Fe-Mn mineral deposits, and oil and gas fields. Here we show a widespread 40-50 Ma phase of fault activity contemporaneous with fluid flow and mineralization in the Paradox Basin, suggesting a close relationship between fluid movement and deformation in an enigmatic time interval. To date fault activity, we applied K-Ar and Rb-Sr geochronology to multiple size fractions of XRD-characterized clay-rich fault gouge from six faults. To date mineralization and fluid-rock reaction we also applied K-Ar geochronology to separated size fractions of clay from bleached sandstones, and Re-Os isochrons on bornite and chalcocite.

We quantified 2M₁ (detrital) and 1M_d (authigenic) illite polytypes for each clay fraction then applied illite-age-analysis to determine authigenic endmember ages. We measured an age of 62.9 ± 9.4 Ma (±1σ) for fault gouge on the Moab Fault, and an age of 42.9 ± 1.1 Ma for the Moab Fault Splay to the north. The nearby Lisbon Valley and GTO Faults yielded ages of 45.6 ± 0.72 and 44.2 ± 1.2 Ma; the latter also yielded an illite Rb-Sr age of 50.9 ± 3.5 Ma. Authigenic clay from a paleo-oil reservoir near the Moab Fault Splay gave an age indistinguishable from the fault, at 41.1 ± 2.6 Ma. This clay formed from the interaction of reduced fluids and oxidized red beds, suggesting the fault acted as a conduit for hydrocarbons. We also found a Re-Os age of 47.5 ± 1.5 Ma on Lisbon Valley bornite, reinforcing contemporaneous fault activity, fluid flow, and mineralization. However, an older Re-Os age of 112 ± 3.8 Ma for chalcocite and high initial ¹⁸⁷Os/¹⁸⁸Os from the younger bornite suggests multiple phases of fluid flow and mineralization at Lisbon Valley. Our results fall in a time where little is known about the exhumation history of the Paradox Basin, raising questions about fluid flow driving forces. Extension and/or local removal of Mancos Shale following the Laramide Orogeny may have influenced hydrogeologic flow patterns.