Evidence for the overflow origin of the Grand Canyon

Abstract The Colorado River's course through Grand Canyon and across the topographic high imposed by the Kaibab Plateau represents one of the most well-known transverse drainages. Only four processes allow a river to flow across a topographic obstruction: antecedence, superimposition, piracy, and overflow. Since Powell first described the canyon in 1875, various authors have repeatedly proposed variations of all four mechanisms to explain the formation of the Grand Canyon, and this paper does not review or try to refute this prior scholarship. This paper's purpose rests in providing a detailed evaluation of evidence associated with overflow upstream, across, and downstream of the East Kaibab monocline paleodivide. Upstream of the paleodivide, we re-interpreted the Bidahochi Formation. The older units of this formation represent a fluctuating system between playa and lacustrine deposition fed by multiple localized tributaries entering a topographically barred basin. The younger Bidahochi units signify the abrupt arrival of an additional drainage with a distinctive geochemical signature into the basin roughly 6.8 Ma, supported by three lines of evidence: (1) an order of magnitude increase in depositional rates from 3 m/Ma in the older units to 50 m/Ma in the younger units that buried the paleo-landscape, at least locally, up to an elevation of at least 2250 m (7380 ft); (2) new evidence of an elevated 87Sr/86Sr ratio in the younger units when compared to the older units and the modern Colorado River; and (3) fossil evidence of the existence of large, freshwater lakes and rivers in the Bidahochi Basin during deposition of the younger units. Newly re-evaluated evidence at the East Kaibab paleodivide focuses on a southern bend of the Colorado River's path across the Kaibab Plateau, as well as the confluence of the Colorado and the Little Colorado rivers at the junction of the Kaibab Plateau and Cedar Ridge. Both lines of evidence at the paleodivide focus on the impact of topography on Grand Canyon incision, and both lines of evidence are consistent only with an overflow process. Evidence downstream of the paleodivide includes the abrupt arrival of river water with an elevated 87Sr/86Sr ratio followed by Colorado River sediment into a series of successively lower downstream basins that integrated via overflow. New 10Be/9Be geochemistry data support the relative timing of deposition of the Bidahochi Formation followed soon after by deposition of the Bouse Formation downstream of the Grand Canyon. We conclude that newly presented and re-evaluated available evidence supports an overflow process for the Colorado River's transverse section across the Kaibab Plateau, ultimately resulting in incision of the Grand Canyon.