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4.8 Ma Age for Inception of the Modern Colorado River

Article Author(s): 

Jon Spencer

Age of the Colorado River

Bouse Formation. Fine, unconsolidated volcanic ash is present in the Bouse Formation at one small outcrop in the Chocolate Mountains in California. A similar volcanic ash bed is interbedded with the Bouse-like marl in Bristol basin (Fig. 2). Unaltered (glass-like) ash grains from both localities were analyzed by the U.S. Geological Survey to determine their primary chemical constituents. Comparison of six major elements (Mg, Al, Si, Ca, Ti, Fe) in volcanic glass (tephra) from these two ash beds, and comparison with ~5600 tephra analyses in the USGS tephra geochemistry database, indicate that these two ash beds are more similar to each other than to any of the other samples in the database (Spencer et al., 2011). This means that Blythe basin was flooded and Bouse carbonate was being deposited at the same time that Bristol basin was inundated and essentially identical travertine and marl were being deposited. Simultaneous inundation of both basins is consistent with flooding of both closed basins during first arrival of Colorado River water.

Statistical analysis also indicates that the ash beds are correlative with the Lawlor tuff that originated from the Sonoma volcanic field in the northern San Francisco Bay region. Near its source, the Lawlor tuff contains plagioclase grains of sufficient size to allow application of laboratory age-determination methods. A 40Ar/39Ar incremental-release isochron age of 4.834 ± 0.022 (2σ) Ma was derived from plagioclase from a sample of the Lawlor tuff in the northeastern San Francisco Bay region (Sarna-Wojcicki et al., 2011). This radiometric date and application to the ash beds in the eastern Mojave Desert through geochemical correlation indicates that the Colorado River water had arrived in the eastern Mojave Desert by 4.8 Ma.

Imperial Group. In late Miocene time, before deposition of the Bouse Formation, plate tectonic interactions along the southwestern margin of North America led to transfer of the Baja California from the North American plate to the Pacific plate. The new boundary between the two plates is partly extensional. Plate movement caused gradual opening of the Gulf of California and its northwestern extension, the Salton Trough. Voluminous sands accumulated in the Salton Trough beginning at a point in the sedimentary succession dated by analysis of magnetic polarity reversals in the sedimentary succession at about 5.3 Ma (Dorsey et al., 2011). These sands have been interpreted as derived from the Colorado River, but the 5.3 Ma age determination conflicts with evidence from the Bouse Formation for first river-water influx at 4.8 Ma.

A new study of the sands in the Imperial Group evaluated the sources of sands by isotopic analysis of uranium and lead in the trace mineral zircon. Zircon is as durable as quartz, tends to contain a small but significant amount of uranium, and retains the lead that is produced by the radioactive decay of the included uranium. Hence, zircon grains record the age of the igneous rocks from which they were ultimately derived. Thousands of analyses of zircon sand grains from Colorado Plateau sandstone units revealed that large fractions of the zircon grains in these sands were derived from eastern North America (Dickinson and Gehrels, 2010). Colorado River sands contain a high fraction of zircon grains from Plateau sandstone units, and so can be distinguished from sands derived primarily from western North America igneous sources, for example from the flanks of the Salton Trough.

This new study (Kimbrough et al., 2011) characterized the zircon sand-grain ages of modern Colorado and Gila River sands and of three samples of sandstone from the Imperial Group on the southwest side of the Salton Trough. The ~5.3 Ma sample from near the base of the Imperial Group yielded a spectrum of zircon U-Pb dates intermediate between those of modern Colorado and Gila River sands. The two younger samples, the oldest of which is dated at 4.7 Ma, have zircon age characteristics more like modern Colorado River sands. The detrital zircon age spectrum of the oldest sandstone from the Imperial Group may reflect an ancestral Gila River source rather than a Colorado River. While the origin of this hypothetical ancestral Gila River source is presently not understood, the age spectrum of the 5.3 Ma sandstone is problematic as a marker of first arrival of the Colorado River whereas the 4.7 Ma sandstone is a good marker.

Senior Geologist
Arizona Geological Survey


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