INSIGHTS INTO THE ORIGIN AND DEVELOPMENT OF THE LOWER COLORADO RIVER FROM THE LATEST MIOCENE-EARLY PLIOCENE DEPOSITS AND PALEOGEOMORPHOLOGY OF COTTONWOOD AND MOHAVE VALLEYS

Geologic mapping and reconnaissance investigations of late Miocene - early Pliocene deposits in Mohave (MV) and Cottonwood (CV) valleys provide unique insights into the early evolution of the lower Colorado River (CR) system. Prior to arrival of CR water, the relatively low Pyramid bedrock divide separated these valleys. In southern CV, alluvial fans sloped west and east toward an axial valley fluvial system that drained north to a depocenter in central CV at >300 m above sea level (asl). The valley axis in northern MV sloped south, and alluvial fans all around the valley sloped inward toward the valley axis and the basin depocenter in southern/central MV at ~100 m asl. The depocenter of Chemehuevi Valley (ChV), the next valley to the south, was <145 m asl. When CR water arrived from the north after 5.6 Ma, CV filled to a shallow depth before spilling over the Pyramid divide down into MV, eroding the divide and depositing coarse sediment. As CR water continued to flow in, the divide was submerged and both valleys became part of a ~150 km long, 450 m deep lake, indicated by widespread outcrops of Bouse Formation carbonate deposits draped on alluvial fan surfaces and bedrock hillslopes, with local shoreline deposits, up to 555 m asl. This lake then overtopped the divide near Topock and spilled S into ChV. The divide was lowered by erosion as CV and MV filled with fine clastic Bouse deposits supplied primarily by the CR. The lake volume decreased as the divide lowered and sediment filled the valley, ceasing to be a lake when the level of sedimentation at the S end of MV reached that of the lowering divide, probably at ~440 m asl. Continued erosion of the divide resulted in recycling of stored clastic sediment into ChV and the much larger Parker-Blythe-Cibola (PBC) basin farther downstream. Either as a large lake or an estuary, the PBC basin was base level for this early CR, so none of the upstream basins would have been eroded below ~330 m asl, the maximum water level in the PBC basin, at ~4.8 Ma. Subaerial CR deposits of the early Pliocene Bullhead alluvium filled broad swaths of both CV and MV from <150-400 m asl, culminating at 4-3.5 Ma. This implies that the PBC basin must have drained and been eroded to a level far below 330 m asl fairly soon after 4.8 Ma, allowing for deep erosion of large volumes of clastic Bouse sediment prior to massive CR aggradation.