GEOCHEMISTRY OF SPRINGS, TRAVERTINES AND LACUSTRINE CARBONATES OF THE GRAND CANYON REGION OVER THE PAST 12 MILLION YEARS: THE IMPORTANCE OF GROUNDWATER IN INITIATING DOWNWARD INTEGRATION OF THE COLORADO RIVER SYSTEM

We apply multiple geochemical tracers (⁸⁷Sr/⁸⁶Sr, δ¹³C, and δ¹⁸O) to waters and carbonates to evaluate the hydrology of the Colorado River system over the last 12 Ma. Modern springs in Grand Canyon show mixing of deeply derived (endogenic) fluids with meteoric (epigenic) recharge. Western springs show higher ⁸⁷Sr/⁸⁶Sr (0.710-0.735) relative to eastern springs (0.707-0.715). Travertine (< 0.5 Ma) and speleothems (2-4 Ma) show ⁸⁷Sr/⁸⁶Sr and C-O isotope variations that overlap with water values, thus carbonates are a proxy for the water that deposited them. Hualapai Limestone (12-6 Ma) and Bouse Formation (5.8-4.8 Ma) record paleohydrology leading to earliest Colorado River integration. Hualapai Limestone was deposited over the interval from 12 Ma (new ash age) to 6 Ma; carbonates thicken to ~ 210 m toward the Grand Wash fault suggesting deposition synchronous with fault slip. Detrital zircons indicate that no Colorado Plateau detritus was reaching this basin from 13-6 Ma. Grand Wash basin has ⁸⁷Sr/⁸⁶Sr, δ¹³C, and ?18O values and ranges similar to Grand Canyon springs and travertines suggesting a long-lived spring-fed lake/marsh system sourced by Colorado Plateau groundwater. A lack of evaporites (after ~ 12 Ma) indicates continuous groundwater flow through this sub-basin. Up-section decrease in ⁸⁷Sr/⁸⁶Sr and δ¹³C, and increase in δ¹⁸O, indicate a gradual increase in epigenic relative to endogenic inputs. Gregg Basin and Temple Basin were hydrologically distinct basins before ~ 6 Ma compatible with downward integration of basins involving groundwater sapping and lake spill-over. New field evidence shows m-scale rimstone dams at a spill-over divide between Grand Wash and Greggs Basins. Bouse Limestone (5.8-4.8 Ma) was also deposited in several hydrologically distinct basins separated by bedrock divides. Bouse carbonate chemistry is non-marine and ⁸⁷Sr/⁸⁶Sr data suggest it was not derived from Colorado River water, but from mixing of high ⁸⁷Sr/⁸⁶Sr Lake Hualapai waters with lower ⁸⁷Sr/⁸⁶Sr water. Bouse sub-basins exhibit distinct hydrochemistry suggesting incomplete mixing. There is a general southward trend toward less radiogenic ⁸⁷Sr/⁸⁶Sr values and less negative δ¹⁸O. Lower Blythe basin could have had intermittent marine mixing based on C-O isotope mixing trends, sedimentology, and paleontology.