CONNECTING PALEOHYDROLOGY AND MODERN HYDROGEOLOGY TO CONSTRAIN THE EVOLUTION OF FLUID FLOW IN THE NAVAJO SANDSTONE

Sandstones of the Colorado Plateau host diverse examples of bleaching and mineralization that reflect the long-term geochemical and hydrogeologic evolution of now exhumed paleo-aquifers. The Navajo Sandstone, east of Escalante, UT hosts a distinct variety of mineralization characterized by iron oxide coated joint surfaces and pipe-like concretions extending consistently SSE towards the Colorado River, implying that mineralization occurred in a regional southerly flowing paleo-aquifer. We hypothesize that joints served as a conduit for deeper, more reduced, iron-bearing groundwater to flow upwards into the Navajo aquifer, resulting in mixing with more oxic meteoric water and iron oxide mineralization. Trace element abundances within concretions and joint coatings, sampled across a N-S transect, inform hydrogeochemical gradients and fluid-rock reactions within the now exhumed portions of the Navajo aquifer. Arsenic concentrations up to 1.3 wt% suggest that iron oxides efficiently scavenged As from groundwater, and decreasing rare earth element concentrations from N to S may represent progressive cation removal through adsorption onto iron oxides or clays. Prior (U-Th)/He dating suggests that most of this mineralization may have occurred between 200 ka and 1 Ma, raising the possibility that this process began in response to shifting hydraulic conditions driven by downcutting of the Colorado River around this time.

Within deeply incised canyons in the northern end of this study area, modern spring waters issuing from joints in the Navajo sandstone precipitate abundant iron oxides and exhibit chemical and isotopic characteristics distinct from regional Navajo groundwaters, suggesting upward flow from deeper stratigraphic levels. Geochemical modeling supports a scenario in which deeply circulated groundwaters interact with known natural gas accumulations in Permian strata, becoming more reducing, acidic, and capable of mobilizing iron. We hypothesize that these observations may represent modern analogues of hydrogeologic processes responsible for the iron oxide mineralization observed in the rock record. If so, this study highlights how investigations of paleo- and modern hydrogeology can be used in tandem to study ancient groundwater flow and aquifer evolution of the region and its connection to the broader geologic evolution of the Colorado Plateau.