NEW CONSTRAINTS ON THE PLEISTOCENE INCISION HISTORY OF THE LOWER COLORADO RIVER FROM U-TH SERIES DATING OF PEDOGENIC CARBONATES, COTTONWOOD VALLEY, ARIZONA, USA

The Lower Colorado River region of Arizona and California contains generally conspicuous fluvial sediments lining the valleys of the Colorado River and several of its tributaries. These deposits consist of mud, sand, and gravel representative of floodplain and channel environments and locally interfingering with alluvial fans emanating from surrounding mountain ranges. Together they record the history of aggradation and incision of the Colorado River during Pleistocene time. To better constrain the depositional history recorded by fluvial sedimentary units, and to make inferences about past drivers of the evolution of the Colorado River, more precise ages on these units are needed. Here we present age constraints on the fluvial/alluvial sedimentation history of the Lower Colorado River based on a preliminary campaign of U-Th series dating of pedogenic carbonates developed in capping alluvial fan and river terrace gravels. Fine-grained aggradational sediments of the Lower Colorado are typified by the Chemehuevi Formation, the most recent and best-preserved example of aggradation events that demonstrate river responses to Pleistocene climate variation. Previous studies place the best age estimate for the Chemehuevi Formation at ~70 ka, but with significant uncertainty. Previous mapping has identified some Chemehuevi-like sediments that appear to be older based on assumption about the age of overlying fan gravels. New preliminary results U-Th series dating results suggest they are >140 ka. This confirms that Chemehuevi-like floodplain deposits record multiple aggradation events in the Lower Colorado River Corridor, with at least one depositional sequence occurring during the middle Pleistocene. Stratigraphic relationships based on geologic mapping indicate a period of incision likely followed, after which the Chemehuevi Formation was deposited in late Pleistocene time. U-Th dating of fluvial terraces cut into the Chemehuevi suggests the latest period of river incision was underway by at least 60 ka.