GEOMORPHIC AND STRATIGRAPHIC EVIDENCE PROHIBITS ANCESTRAL PALEOGENE WESTERN GRAND CANYON

The western Grand Canyon on the Hualapai Plateau in Arizona was incised into a northeast-dipping, step-bench cuesta platform following an earlier, separate episode of Laramide scarp recession and Laramide drainage entrenchment. The older Laramide strike-controlled drainage through Milkweed and Hindu Canyons was deflected eastward at the base of the Shivwits cliffs at an intermediate scarp position during Laramide time. Significant scarp recession was required to allow the modern Grand Canyon to form 8 km further to the northeast under similar structural controls at a later time. Given the estimated range of worldwide scarp recession rates of between 125 and 500 m/Ma, it would require tens of millions of years for the Shivwits Plateau scarp to retreat the necessary 8 km in order for the modern Grand Canyon to have sufficient space to form at the base of these receding cliffs, but flowing in the opposite direction. The geomorphic relationships imply that there would have been no space for an ancestral Grand Canyon to have existed prior to the formation of Hindu Canyon, which is Late Cretaceous or Paleocene in age. In addition, Paleocene-Eocene fanglomerate deposits that contain clasts from the Coconino, Toroweap, and Kaibab Formations were spread southwestward from the Shivwits scarp while Hindu Canyon was a functioning drainage and are preserved beneath Eocene(?)-Oligocene conglomerates on the southwestern side of the modern gorge. These light-colored Permian formations that cap the Shivwits Plateau crop out only on the north side of the Colorado River, and their clasts could not have reached the preserved locations had any significant canyon relief existed in the location of the modern gorge. Capture of exhumed, southward-flowing Hindu Canyon tributaries by current Grand Canyon headward erosion is a further testament to the relative youthfulness of Grand Canyon. The existing field relationships do not permit an older ancestral canyon to have existed in early Tertiary time. Furthermore, had such an ancestral Grand Canyon formed so long ago, the modern Shivwits cliffs would have to have receded much further northeastward from their current location in the 50 to 70 Ma implied by alternative theories. All evidence is consistent with a youthful western Grand Canyon, with an origin no older than late Miocene time.