Abstract

One hypothesis for the origin of the Grand Canyon is that a broad Hopi Lake, of which lakebeds of the Miocene Bidahochi Formation are a vestigial record, ponded to a depth great enough near the Miocene-Pliocene time boundary to spill over the topographic barrier of the Kaibab-Coconino Plateau to initiate incision of the Grand Canyon below the lake outlet. Bidahochi paleogeography indicates that Hopi Lake was a playa system that never achieved appreciable depth. Topographic relations in northern Arizona show that the maximum elevation of Bidahochi lakebeds is not compatible with lake spillover through the Grand Canyon unless post-Bidahochi deformation or pre-Bidahochi canyon-cutting altered the landscape in ways unsupported by geologic evidence, or the surface of Hopi Lake rose transiently to elevations unrecorded by any sediment. The implications of erosional episodes affecting the Colorado Plateau, the timing of drainage reversal across the central Colorado Plateau, the spatial pattern of the Colorado River drainage system, and the analogous configurations of multiple river canyons cut into Precambrian basement within the river basin also challenge the Hopi Lake spillover model. A viable alternate scenario for incision of the Grand Canyon is the concept of an ancestral Miocene Colorado River that transited the Kaibab uplift on the site of the eastern Grand Canyon, but exited the Colorado Plateau into an ancestral Virgin River drainage before capture near the site of the present central Grand Canyon by a stream working headward through the western Grand Canyon from the Grand Wash Cliffs.

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