Late Miocene Colorado River arrival in the Bidahochi basin supports spillover origin of Grand Canyon.

The timing and mechanism of the integration of the Colorado River and incision of the Grand Canyon remain among geology's enduring controversies. A key question is the configuration of the upper Colorado River watershed between 11 and 6 million years ago. In this study, we present new evidence from zircon uranium-lead geochronology for the arrival of distinctive Colorado–Green River sediment in the Bidahochi basin by 6.6 million years ago derived from the Browns Park Formation. This is coeval with an order-of-magnitude increase in depositional rate, an increase in carbonate strontium isotope (87Sr/86Sr) ratios, the appearance of large fish species characteristic of fast-flowing waters, and other sedimentological changes. This evidence is consistent with the Colorado River supplying water and sediment to the Bidahochi basin before spillover integration of the river through the Grand Canyon. Editor's summary: The Colorado River has not always flowed through the Grand Canyon area, and it is not known when and how that relationship began. He et al. collected uranium-lead ages for thousands of zircon crystals in formations located upstream and downstream of the canyon. Age patterns revealed a "fingerprint" of Colorado River sediment suggesting its arrival into an upstream lake by 6.6 million years ago and its subsequent integration with the Grand Canyon and downstream catchment. The direct connection may have happened through a combination of processes, but the zircon data give support to a long-debated "lake spillover" hypothesis. —Angela Hessler [ABSTRACT FROM AUTHOR]