Smith Lecture: The Pliocene Bouse Formation and Initiation of the Modern Colorado River
About 99% of the water in the Colorado River is derived from the Colorado Plateau and southern Rocky Mountains. The river flows through the Grand Canyon and across the Basin and Range Province to the Gulf of California. Before about 5 Ma, all of this water flowed elsewhere on some as yet unidentified path. Earlier studies concluded that tectonic rifting and opening of the Gulf of California lowered base level and triggered headward erosion into the Colorado Plateau, leading to river capture and diversion of voluminous flow down what became the Colorado River course. Some more recent studies consider lake spillover and top-down drainage integration to have initiated the modern Colorado River. The lower Pliocene Bouse Formation in the lower Colorado River Valley has been at the center of debate regarding these alternatives for genesis of the modern Colorado River.
The ~5 Ma Bouse Formation consists of basal marl and dense tufa overlain by siltstone and fine sandstone grading upward into Colorado River gravels. The Bouse Formation in Blythe basin contains marine, estuarine, and fresh-water fossils and shells, whereas farther north it contains only sparse fresh-water fossils. The Bouse Formation had been initially interpreted as an estuarine deposit related to early opening of the Gulf of California. At least three types of evidence, supported by a growing database, support a lacustrine origin. These are as follows: (1) Strontium-isotopic analyses of Bouse carbonates and shells indicates isotopic similarity to modern Colorado River water but dissimilarity to early Pliocene river water. This is especially significant because seawater contains substantially more Sr than river water (7x for the modern Colorado), and even a small amount of seawater influx into an estuary should be detectable. (2) Maximum elevations of Bouse Formation strata represent two "bathtub rings" in basins separated by a bedrock gorge, as would be expected for basin filling followed by spillover. The estuarine interpretation requires uplift of the lower Colorado River valley from below sea level. Uplift had been linked to uplift of the Colorado Plateau, with greater uplift of northern areas, but is not consistent with the step-like character of maximum Bouse elevations. (3) Mohave-Cottonwood Valley, which includes the southern tip of Nevada, contains an outburst flood deposit with a northern source, overlain by Bouse Formation. This is consistent with abrupt inundation by river water from a northern source, not estuarine inundation from a southern source. The presence of marine species in the Bouse Formation continues to raise doubts concerning a lacustrine origin for the Bouse Formation, however, even though there are numerous examples of birds inadvertently carrying viable fauna to colonize lakes and springs, sometimes over astonishingly long distances.