A LATE MIOCENE-EARLY PLIOCENE CHAIN OF LAKES FED BY THE COLORADO RIVER: EVIDENCE FROM SR ISOTOPES OF THE BOUSE FORMATION BETWEEN GRAND CANYON AND THE GULF OF CALIFORNIA

The initial development of the Colorado River is the subject of much controversy, and the origin of the Bouse Formation and similar deposits exposed along the lower Colorado River has important implications for this debate. The latest Miocene to early Pliocene Bouse Formation was originally interpreted as a Miocene embayment of the Gulf of California on the basis of paleontology; however, recent isotopic work supports a lacustrine origin for the Bouse Formation. Rather than a single-lake model, elevations of Bouse outcrops suggest a lake-overflow model involving a chain of three separate lake basins for integration of the lower Colorado River. The Hualapai Limestone of the Lake Mead area records other lakes that existed farther upstream. Past studies have established an estimate for the overall geochemical character of the Bouse Formation and related deposits. Strontium isotope data from Bouse Formation carbonates give an average ⁸⁷Sr/⁸⁶Sr ratio (0.7108) close to the Sr ratio of the present-day Colorado River (0.71075) and well above late Neogene marine strontium (0.7090). However, samples analyzed in these studies have no clear age relationships. We conducted more detailed and systematic sampling to obtain Bouse Formation samples with a discernable relationship to one another. By sampling the basal 10 cm of marl at various elevations in each of the three paleo-lake basins, a time evolution of lake waters is established. The up-elevation deposition of the basal marl layers tracks the filling of the basins through time. Comparisons are also made with the Miocene Hualapai Limestone, located in the Grand Wash Cliffs area. The Hualapai Limestone has a significantly higher ⁸⁷Sr/⁸⁶Sr ratio (0.7145), and the transition to the lower strontium values that characterize the Bouse Formation is under investigation. A sample taken from limestone near Frenchman Mountain, Nevada shows a Sr isotope ratio very similar to that of the Bouse Formation; Sr isotope results from the Bouse Formation in the Mohave basin are within the general range of previous values. Further analyses of Sr isotope data from Bouse carbonates and related lake deposits may illuminate the importance of lake-overflow and mixing processes in the evolution of the lower Colorado River drainage basin.