TEPHROCHRONOLOGIC AND STRATIGRAPHIC CONSTRAINTS ON THE INCEPTION AND EARLY EVOLUTION OF THE LOWER COLORADO RIVER SUPPORT LACUSTRINE OVERFLOW AS THE PRINCIPAL FORMATIVE MECHANISM

A growing body of geologic evidence strongly supports the concept that the course of the Colorado River below Grand Canyon integrated through a step-like process of basin-to-basin lacustrine spillover. Compelling stratigraphic relationships between distinctive suites of deposits link downstream-directed fluvial and lacustrine processes that immediately pre-date the arrival of the through-going river in Cottonwood and Mohave valleys. Fortuitously located tephra beds provide excellent temporal constraints on major landscape changes that transpired in this area in the late Miocene and early Pliocene. A key bedrock highland separates Cottonwood and Mohave valleys at the present site of Davis Dam. North of the divide in Cottonwood Valley, a suite of lacustrine deposits containing the 5.5 Ma Connant Ck tephra bed is overlain by the basal limestone of the Bouse Formation. South of the divide in Mohave Valley, the same tephra bed occurs in fanglomerate deposits that grade to an axial channel complex near the modern valley axis. The axial channel complex is deeply gouged and overlain by a coarse fluvial conglomerate dominated by clasts derived from the bedrock highland between the valleys. The flood deposit is conformably overlain by the basal limestone of the Bouse Formation. These complementary sequences strongly suggest that the spillover of a lake in the northern valley corresponded with the occurrence of a large flood in the southern valley; subsequently, both valleys were inundated by a large body of standing water. Bouse deposits in Mohave Valley, including the thin basal limestone and locally thicker fine clastic deposits, indicate a deep valley much like that of today was inundated by about 400 m of water. Drainage of the lake by spillover to the south and erosion of Bouse and underlying deposits was soon followed by the arrival of the Colorado River, which filled the valley with hundreds of meters of alluvium. A key tephra bed near the top of this alluvial fill indicates that the thick aggradation culminated by 3.6-4.2 Ma. This timing of river inception and thick aggradation is very similar to constraints on river inception and deep canyon excavation known from the mouth of Grand Canyon and the arrival of Colorado River sediment in the Salton Trough, implying that accelerated upstream erosion led to massive downstream aggradation.