ALLUVIAL AND LACUSTRINE STRATIGRAPHIC EVIDENCE FOR THE LATE NEOGENE INCEPTION AND EARLY EVOLUTION OF THE LOWER COLORADO RIVER IN THE VICNITY OF PYRAMID CANYON, NEVADA / ARIZONA

A series of previously unstudied exposures of late Neogene alluvial and lacustrine stratigraphy in the vicinity of Pyramid Canyon (NV and AZ) provide unequivocal evidence for the timing, mode, and consequences of the inception of the Lower Colorado River (LCR). A key section in Laughlin, Nevada, indicates a pre-LCR linkage between a catastrophic, divide-breaching flood through Pyramid Canyon and immediately subsequent deposition of calcareous marl (Bouse fm) in a large body of standing water. Tephra below the Bouse Fm in Mohave Valley fixes the maximum age of this event at 5.6 Ma. Upstream of Pyramid Canyon, a section in Lost Cabin Wash, AZ, indicates a pre-Bouse transition from fanglomerate deposition to fine-grained axial valley and lacustrine deposition (Muddy Ck fm?) followed by subaerial exposure and, finally, deposition of calcareous marl, mud, and sand of the Bouse fm. This sequence is a record of two lakes: one in Cottonwood Valley that failed through Pyramid Canyon, and a subsequent, deeper lake that resulted from damming at the southern end of Mohave Valley. Lacustrine beach gravels and tufa encrustations on local mountain front embayments in Mohave Valley suggest that the Bouse lake rose rapidly, peaked, stabilized 100 m below its initial peak, and then drained through Topock Gorge. The arrival of the LCR in Mohave Valley is indicated by a distinctive fluvial deposit cut in pre-Bouse basin fill approx. 20 m above the modern river. Once in Mohave Valley, the LCR aggraded nearly 250 m. Tephra near the top of the fill places its culmination at approximately 3.9 Ma. The resulting age range of this major transition between 5.6 and 3.9 Ma closely reflects the timing of through-going drainage development on the Colorado Plateau and significant enlargement of the Grand Canyon. Thus supporting the logical process linkage between canyon excavation upstream and valley aggradation downstream. The late Neogene stratigraphic record near Laughlin indicates a downstream-directed process of river inception via an integrating chain of lakes. Models of river evolution that invoke abrupt marine incursion followed by rapid uplift and spectacularly efficient headward erosion cannot be reconciled with the physical evidence in this part of the LCR system.