PLIOCENE SUBSURFACE FLUID FLOW DRIVEN BY RAPID EROSIONAL EXHUMATION OF THE COLORADO PLATEAU

Subsurface fluid flow systems evolve over time in response to uplift and erosion, leaving fingerprints in the rocks that can be identified as a series of superimposed physical and geochemical alterations. Geology and topography determine the configuration, chemistry, and fluxes of such flow systems. Topography is a major driver of groundwater flow, so the spatial pattern of erosion can influence the direction and extent of fluid transport, and magnitude of regional-scale subsurface flow systems. Erosion may also remove regional aquitards and induce gas exsolution. Here we examine the geologic record of subsurface flow in the sedimentary rocks of the Paradox Basin in the Colorado Plateau, including the distribution and ages of Fe-Mn-oxide deposits and bleached, former red bed sandstones. We compare our results to those of previous low-temperature thermochronology studies documenting up to 2 km of rapid erosional exhumation ~3-4 Ma in the region (Murray et al., 2016; 2019), as well as abundant Fe-and Mn-oxides precipitated close to the surface (<0.5 km) at 3.6 Ma along fault zones at Flat Iron Mesa (Garcia et al., 2018).

We used (U-Th)/He and K-Ar dating to document two new examples of distinctive subsurface flow of reduced fluids between 3 and 4 Ma in this region. The first is transport of Mn and precipitation of Mn-oxides, in contact with more oxic meteoric waters, along the Moab Fault zone at 3.9 ± 0.2 Ma. Second is a widespread clay mineralization event in the upper section of the Moab Member sandstone, which we dated using K-Ar illite-age-analysis (authigenic 1M_d-polytype) to 3.60 ± 0.03 Ma. We interpret this age to represent the timing of flow of buoyant reduced fluids and red bed bleaching. The coincidence of the timing of bleaching, formation of Fe- and Mn-oxides in multiple locations, and rapid erosional exhumation 3-4 Ma raises the question of how surface erosion can induce a phase of reductive fluid flow in the subsurface. We suggest that recent rapid erosional exhumation of the Colorado Plateau caused: (1) gas exsolution from hydrocarbon-bearing source rocks and formation waters; and (2) the formation of steep topography that enhanced regional groundwater flow, whereby meteoric water circulation flushed reduced fluids towards discharge zones.