GEOLOGIC CONSTRAINTS THAT PRECLUDE THE EXISTENCE OF A PALEOGENE WESTERN GRAND CANYON ON THE HUALAPAI PLATEAU: AN UPDATED ILLUSTRATED REVIEW

There are several geologic relationships associated with the >300-meter-thick Tertiary fluvial and volcanic fill on the Hualapai Plateau of western Grand Canyon that preclude the presence of significant canyon relief following the disruption of the northeast-flowing Late Cretaceous-Early Paleogene paleocanyons that predate the emergence of the modern Colorado River gorge (ca. 6 Ma). 1) The remains of the paleocanyon system contain a sequence of fluvial and lacustrine sediments that were largely emplaced by late Oligocene time as documented by a 24 Ma ash bed high in the preserved Tertiary section at the head of Peach Springs Wash. There are no significant erosional unconformities within this Paleogene aggradational sequence. 2) The basal Music Mountain Formation gravels (formerly “Rim gravels”) are clearly older than Middle Eocene from paleontological specimens collected in lacustrine and/or paludal carbonates beneath the Miocene basalts at Long Point, 40 km southwest of Grand Canyon Village. 3) Debris flow and alluvial fan deposits of Music Mt. age, sourced from upper Paleozoic outcrops on the adjacent Shivwits Plateau, and clearly associated with the early Tertiary northeast-flowing drainage, are currently located on the opposite (south) rim of the modern canyon, a relationship that is incompatible with the concurrent existence of the Colorado gorge. 4) The source of relatively thin basalts capping Oligocene gravels at the southern branch of Separation Canyon is on the opposite (southwest) rim of the 900-meter-deep Spencer Canyon, the largest Hualapai Plateau tributary to the modern Colorado River. Fluvial aggradation on the Plateau continued following widespread middle Miocene volcanism, depositing up to 90 meters of additional sediment on the central Hualapai Plateau that resulted in a surface of low-relief. 5) The modern Shivwits Plateau scarp is too close to the adjacent canyon to be consistent with a Paleocene position. 6) Had the Colorado gorge been in existence prior to Miocene time there would have had to have been two canyon systems on the Hualapai Plateau that would have had to merge (or cross) in the vicinity of the Hurricane fault, flowing in opposite directions. All these relations are incompatible with an early west-flowing predecessor to the modern Grand Canyon west of the Hurricane fault.