DECIPHERING LANDSCAPE EVOLUTION AND DEVELOPMENT OF THE COLORADO RIVER IN THE UPPER LAKE MEAD AREA—THE ROLE OF GEOLOGIC MAPPING

Geologic mapping of the Hiller Mountains and northern White Hills provide evidence for how the landscape in the Gold Butte–White Hills area evolved from interior extensional basins to the through-going Colorado River. Upper Miocene extensional basin fills accumulated on either side of the tilted Gold Butte-White Hills fault block. Angular unconformities within upper Miocene sections record that the master W-dipping South Virgin-White Hills detachment fault, which unroofed the block, locked and died when it had tilted to about a 30° dip. Younger basin fill on either side of the block becomes finer upward, marking diminished stream power, and culminates in the Hualapai Limestone. The limestone interconnects, through passes, the basins on either side of the block. The deposits of Hualapai Wash that accumulated on top of the limestone east of the block in Greggs Basin contain far-traveled material from the Colorado Plateau and have been interpreted to record the initial entry and deltaic spreading of the incipient Colorado River on a basin floored by Hualapai Limestone. Perched river gravels mark multiple paths of the incipient Colorado River escaping westward from Greggs Basin following and incised into the limestone floor where it interconnects the basins across low passes.

Arrival of the Colorado River signaled a major change in geomorphic regimen from Miocene interior basin filling to a Pliocene regimen governed by fluctuating base levels set by the river as it cycled between deep entrenchment and aggradation. The river’s early Pliocene deep incision and basin exhumation was followed by refilling of the basins by aggrading fluvial braidplains consisting of the Bullhead Alluvium. Upland patches of soil calcrete and of old tributary alluviums mark remnants of subsequent landscapes graded to levels below the top of the Bullhead Alluvium. Those landscapes are since dissected due to deep river re-entrenchment and associated degradation.