NEW CONSTRAINTS ON THE PLEISTOCENE INCISION AND AGGRADATION HISTORY OF THE LOWER COLORADO RIVER FROM U-TH SERIES DATING OF PEDOGENIC CARBONATES AND LUMINESCENCE DATING OF THE CHEMEHUEVI FORMATION AND BRACKETING UNITS, COTTONWOOD VALLEY, ARIZONA AND NEVADA, USA

The Lower Colorado River corridor contains a suite of conspicuous fluvial sediments consisting of mud, sand, and minor gravels representative of floodplain and channel environments. These units locally interfinger with alluvial fans emanating from the surrounding mountain ranges, and together they record the history of aggradation and incision of the Colorado River during Pleistocene time. This geomorphic evolution reflects the fluvial response of the North American continent to global climate change and other controls. The light-colored, fine-grained sediments of the Lower Colorado River, called the Chemehuevi Formation, are the most recent and best-preserved example of aggradation events that demonstrate fluvial changes in the Colorado River during Pleistocene time. Previous studies place the best age estimate for deposition of the Chemehuevi Formation at ~70 ka, but this age is poorly constrained which limits our ability to make interpretations about the controls on deposition of the Chemehuevi.

To better constrain the depositional history and to reveal past drivers of the evolution of the Colorado River, we present new age constraints on the fluvial aggradation of the Lower Colorado River. U-Th series dating of pedogenic carbonates developed in capping alluvial fan and river terrace gravels provide minimum and maximum depositional ages for the Chemehuevi. This bracketing age data is paired with infrared stimulated luminescence (IRSL) dating of Chemehuevi Formation sediments.

We found alluvial fan and river terrace gravels capping the Chemehuevi express soils that developed ~30-70 ka. At one site, soils developed ~150-220 ka in a fluvial unit underlying the Chemehuevi, providing maximum constraint on the timing of deposition. IRSL results from the Chemehuevi indicate deposition occurred at ~100 ka. Additional U-Th dating of soils developed in fluvial terraces that cut into the Chemehuevi suggest the latest period of river incision was underway by at least 75 ka. These data indicate periods of net incision and aggradation in the Lower Colorado River region which generally follow late Pleistocene glacial and interglacial climate variations, respectively.