Geomorphic constraints on the age of the western Grand Canyon

Hypotheses for the age of the western Grand Canyon (WGC) range from younger than 6 Ma to older than 70 Ma. We study the relationships among topography, geology, and available erosion rates in space and time to place constraints on plausible canyon incision histories. Evidence suggests that lateral retreat of the Shivwits Plateau escarpment left a lithologically controlled bench on the Sanup Plateau, but the Hualapai Plateau is beveled indiscriminately across rock types of the Paleozoic stratigraphic section. A period of accelerated base-level fall in the Tertiary is implicated by the canyon incised into the beveled Hualapai Plateau surface, consistent with higher erosion rates observed in canyons than on the surrounding plateau. Streams draining the Hualapai Plateau preserve relict headwater segments that were equilibrated with a slower base-level-fall rate before canyon incision. These relict segments are now separated from the Grand Canyon by knickpoints indicative of a transient landscape. Relief production since the beveling of the Hualapai Plateau is ∼1000 m in the WGC. Comparison of hillslope and channel morphologies between the Grand Wash Cliffs and the WGC provides a test to distinguish hypothesized ages of canyon incision. The data strongly suggest that carving of the WGC is younger than relief production due to slip on the Grand Wash fault ca. 18–12 Ma. Thus the geomorphic data are only fully consistent with the late Tertiary, transient incision model of canyon incision beginning at integration after 6 Ma.