The Pliocene Bouse Formation and initiation of the modern Colorado River Jon Spencer, Arizona Geological Survey Jonathan Patchett, University of Arizona Jennifer Roskowski, University of Arizona Philip Pearthree, Arizona Geological Survey Kyle House, U.S. Geological Survey David Dettman, University of Arizona

Grand Wash trough

Colorado River did not arrive to the Lake Mead area until after 6 Ma. We know this because sedimentary rocks in Grand Wash trough, deposited over 6-8 million years, indicate that the area was not influenced by voluminous water and sediments (sand and gravel) from the Colorado River. Hualapai Limestone

Colorado River sand By 5 Ma, the Colorado River was delivering sand, derived from the Colorado Plateau, to the ancestral Gulf of California mudstone MODERN COLORADO RIVER WAS BORN AT 5 TO 6 Ma

Bouse Formation consists of marl (silty limestone), siltstone, and where it rests on bedrock, hard limestone called “travertine” or “tufa”. It is about 5 Ma as indicated by the age of an interbedded volcanic ash bed.

Fossils indicate fresh water, brackish water, and marine salt water environments during deposition of the Bouse Formation. Such conditions are typical of estuaries. Marine inundation has been attributed to a “failed rift arm” extending northward from early tectonic extension in the Gulf of California – Salton Trough. Marine fossils at elevations of up to 125 meters, and Bouse Formation sediments at elevations of up to 555 meters, indicate tectonic uplift since 5 Ma

87 Rb 87 Sr + e - + v

Sr isotopes indicate that Bouse carbonates were derived from water like that in the Colorado River. Maximum Bouse elevations are consistent with deposition in lakes produced by first arrival of Colorado River water to closed basins inherited from Miocene tectonic extension.

Sr isotopes indicate that Bouse carbonates were derived from water like that in the Colorado River. Maximum Bouse elevations are consistent with deposition in lakes produced by first arrival of Colorado River water to closed basins inherited from Miocene tectonic extension. But what about the marine fossils?

Sr isotopes indicate that Bouse carbonates were derived from water like that in the Colorado River. Maximum Bouse elevations are consistent with deposition in lakes produced by first arrival of Colorado River water to closed basins inherited from Miocene tectonic extension. But what about the marine fossils? Evaporation can readily elevate salinity to sea-water levels

Conclusion Bouse Formation is lacustrine and was deposited at about 5 Ma from first-arriving Colorado River water. Salinity levels increased to near sea-water levels due to evaporative concentration of lake water in the southernmost Bouse lake (lake Blythe). Marine fauna were delivered to salty lake Blythe inadvertently by birds. The Bouse Formation has no implications for the timing of uplift of the Colorado Plateau.