The Colorado River system and neogene sedimentary formations along its course: Apparent Sr isotopic connections

Water samples from the Colorado River and its tributaries provide a present-day Sr-87/Sr-86 profile of the Colorado River system from Moab, Utah, down to the Gulf of California. Sr-87/Sr-86 values around 0.7097 are prevalent above the Grand Canyon, and values of 0.7103 or higher are observed below the Grand Canyon. Sr-87/Sr-86 values close to the seawater ratio of 0.70917 are seen near the Colorado River delta. All tributaries except the Green River contribute Sr that is lower in Sr-87/Sr-86 than the main Colorado. The cause of the increase in Sr-87/Sr-86 through the Grand Canyon and Lake Mead area has not been found. Samples of carbonate and gypsum from the lower lacustrine member of the mid-Miocene to Pliocene Bidahochi Formation in northern Arizona yield average Sr-87/Sr-86 of 0.70972. This agrees quite well with weighted averages of present-day Colorado system rivers that flow into the area, which can be computed at 0.70972 or 0.70984, depending on assumptions. This correspondence argues that the present-day rivers serve as a model for similar rivers that contributed to Bidahochi Formation deposition. Samples from middle and upper members of the Bidahochi Formation, younger than similar to6 Ma, do not fit this pattern so well. The Sr isotopic ratios of the Miocene Bidahochi Formation, the mainly Pliocene Bouse Formation, and the Pliocene Imperial Formation generally correspond well with the present-day Colorado River system and the marine influence at its delta. This suggests a strong link between the ancestral Colorado drainage and all these Neogene deposits.