THE CHEMEHUEVI FORMATION – A GEOLOGIC EXAMPLE OF EXTRAORDINARY SEDIMENT LOADING IN THE COLORADO RIVER DURING THE LATE PLEISTOCENE

Widespread deposits along the Colorado River valley downstream from Grand Canyon record multiple fluvial aggradation episodes since the Miocene, some of which were caused by extraordinary sediment loading from upstream. The ubiquitous, well-exposed late Pleistocene Chemehuevi formation records the most recent major aggradation episode along the lower Colorado River valley. The Chemehuevi formation preserves a set of geomorphic features and sedimentary facies assemblages that illustrate the response of a sand-bed river to extraordinary sediment loading over geologic time (10³ – 10⁴ yrs). A basal gravel layer at least 15 m thick is dominated by imbricated, rounded exotic cobbles and pebbles, and is interpreted to represent a gravel-bed river at or near grade. The basal gravel is overlain by sand, silt, and clay-dominated sediment as thick as 150 m. The bulk of the formation consists of a sand-dominated facies, representing ancient sandy channel sediments, and a fine bedded, mud-rich facies, inferred to be the remnants of vertically accreting floodplains. In many locations where they are found together, the sand facies overlies the mud facies on an abrupt erosion surface. We interpret the sand-over-mud stratigraphy as showing an aggrading channel bed overtopping the floodplain surface; the prevalence of this sequence throughout the river valley suggests that the channel was aggrading faster than the floodplain. The top of the formation, as surveyed along >600 km of valley, indicates a depositional gradient as much as 50% steeper than the modern valley profile. This is consistent with aggradation driven by extraordinary sediment loading from upstream, and is a geologic example of the gradient of a large river adjusting to an increased sediment load. The predominantly fine-grained aggradation package is overlain unconformably by a series of gravel-rich fluvial deposits that mantle terraces beveled in the Chemehuevi formation. These terrace gravels were deposited during temporary halts in the period of incision following the aggradation, and show that the bed of the incising river was coarser than that of the aggrading river. The switch from aggradation to incision along the river was caused by decreased sediment supply, the crossing of a threshold in valley gradient, or both.