Stratigraphic evidence for the role of lake spillover in the inception of the lower Colorado River in southern Nevada and western Arizona

摘要: Late Miocene and early Pliocene sediments exposed along the lower Colorado River near Laughlin, Nevada, contain evidence that establishment of this reach river after 5.6 Ma involved flooding from lake spillover through a bedrock divide between Cottonwood Valley to north Mohave south. Lacustrine marls interfingered with conformably overlying sequence post–5.6 finegrained valley-fill deposits record an phase intermittent lacustrine inundation restricted Valley. Limestone, mud, sand, minor gravel Bouse Formation were subsequently deposited above unconformity. At end Valley, coarse-grained, lithologically distinct fluvial conglomerate separates subaerial, locally derived fan subaqueous Formation. We interpret key unit as for overtopping catastrophic breaching paleodivide immediately before deep both valleys. Exposures in valleys reveal substantial erosional unconformity records drainage predates arrival sediment through-going River. Subsequent aggradation culminated 4.1 3.3 Ma. The stratigraphic associations timing transition are consistent geochemical linking conditions River, timings integration canyon incision on Plateau, sand at its terminus Salton Trough, downstreamdirected mode common areas crustal extension.