SPRING AND GAS CHEMISTRY OF TRAVERTINE-DEPOSITING SYSTEMS IN GRAND CANYON AND THE COLORADO PLATEAU: A PALEOHYDROLOGIC RECORD

A genetic model for the formation of travertine has emerged from the deeply-dissected hydrologic system of Grand Canyon of the southwestern U.S. This model has implications for paleohydrology, neotectonics and water quality in the Southwest. The role of CO2 degassing in the depositional phase of travertine formation is well-known, but the amount and source of CO2 needed to dissolve carbonates in limestone aquifers has been less clear. Spring waters associated with active travertine accumulations are rich in CO2, saline (Na-Cl and Na-HCO3), elevated in 87/86-Sr (from 0.710 to 0.735), and have 3/4-He ratios up to 0.15 Ra, suggesting a mantle origin of the gases and deep circulation of waters. Our model for solute acquisition focuses on the role of magmatism and extensional faulting in transporting mantle-derived CO2 and views extensive travertine accumulations as a geologic record of extreme fluxes of CO2. Travertine Grotto in western Grand Canyon provides a case study of both the modern hydrologic and paleohydrologic systems extending back 350,000 years. Travertine deposition results from the demonstrated mixing of deeply-circulated fluids with the shallower Colorado Plateau aquifer system. The Quaternary travertines (dated using high-precision U-series dating) can be used to provide age control on the erosional landscape, and they contain other geochemical signals that record paleohydrologic conditions. Major ion compositions define four compositionally distinct “Lower World” (deeply-circulated) inputs. Each mixes with the narrowly-defined “Upper World” (Ca-HCO3 aquifer) end member and is associated with a fault-bounded tectonic region (San Francisco Peaks, Hurricane/Toroweap/Uinkaret (including Travertine Grotto), Cataract fault zone, and Grand Wash trends). The combined gas and water characterization of spring waters may thus serve as an indicator for compositionally distinct tectonic/magmatic provinces. The extraction of crustal radiogenic Sr through deep circulation is manifested in a downstream increase in 87/86-Sr in the Colorado River. Even low volumes of the deeply-circulated waters can measurably impact the shallower aquifer system. Other chemical characteristics, such as elevated As and U contents, may also have a significant impact on water quality in the southwestern U.S.