LATE PLEISTOCENE AGGRADATION AND INCISION OF THE LOWER COLORADO RIVER DOWNSTREAM OF THE GRAND CANYON

Where the lower Colorado River and its tributaries traverse part of the Basin and Range province below the Grand Canyon, multiple cycles of sediment deposition, terrace formation, and intermittent erosion have been recognized (e.g. by Newberry, Longwell, Luchitta, Metzger, Bell, Williams, Howard, Faulds, Blair, among others). We report characteristics and age constraints from a subset of this complex stratigraphy that document significant late Pleistocene aggradation and subsequent incision of the river through this region. At sites near Lake Mead, Lake Mojave, and Yuma, we applied luminescence dating to fine-grained sediments in aggradational sequences and mass-spectrometric U-series dating to pedogenic and other secondary calcareous cements on fluvial gravels on terraces. A prominent aggradational sequence, locally more than 30 m thick, commonly consists of a distinctive upward succession of cross-bedded to plane-bedded fine sand, bedded mud and silt, fine sand, and a rounded poorly-exposed cap consisting of coarse sand and rounded to angular gravel. Luminescence dates on the fine sediments range from 40 ka to 70 ka, considering analytical uncertainty. A single area of the fine-grained sediments sampled for paleomagnetism at 4 beds over a vertical range of 15 m showed normal polarity magnetization with little apparent secular variation beyond dispersion that can be explained by compaction. Many exposures show that sedimentation from local tributaries was coeval with the aggradational fluvial sequence derived from the Colorado Plateau. The upper limits of discontinuous erosional remnants of the sequence define a smooth and steeper grade above that of the historical river, from greater than 100 m above modern grade north of 35o N to less than 25 m above modern grade south of Yuma. Terrace gravels inset at various grades below the upper limit of the aggradational sequence yield 230Th dates ranging from about 32 ka to 60 ka. Potential driving mechanisms for the aggradational sequence and subsequent incision include sediment-yield response of the large, diverse, and complex river basin to climate change, drought, fire, vegetation-ecosystem dynamics, glaciation, paleofloods, groundwater discharge, and building and destruction of natural dams produced by volcanism and landslides.