Cosmogenic He age estimates of Plio-Pleistocene alluvial-fan surfaces in the Lower Colorado River Corridor, Arizona, USA

a r t i c l e i n f o Keywords: Colorado River basalt cosmogenic 3 He surface exposure ages climate change desert pavement desert alluvial fans southwest USA geomorphology Plio-Pleistocene deposits of the Lower Colorado River (LCR) and tributary alluvial fans emanating from the Black Mountains near Golden Shores, Arizona record six cycles of Late Cenozoic aggradation and incision of the LCR and its adjacent alluvial fans. Cosmogenic 3 He (3 He c) ages of basalt boulders on fan terraces yield age ranges of: 3.3–2.2 Ma, 2.2–1.1 Ma, 1.1 Ma to 110 ka, b 350 ka, b 150 ka, and b 63 ka. T1 and Q1 fans are especially significant, because they overlie Bullhead Alluvium, i.e. the first alluvial deposit of the LCR since its inception ca. 4.2 Ma. 3 He c data suggest that the LCR began downcutting into the Bullhead Alluvium as early as 3.3 Ma and as late as 2.2 Ma. Younger Q2a to Q4 fans very broadly correlate in number and age with alluvial terraces elsewhere in the southwestern USA. Large uncertainties in 3 He c ages preclude a temporal link between the genesis of the Black Mountain fans and specific climate transitions. Fan-terrace morphology and the absence of significant Plio-Quaternary faulting in the area, however, indicate regional, episodic increases in sediment supply, and that climate change has possibly played a role in Late Cenozoic piedmont and valley-floor aggradation in the LCR valley. Introduction Fluvial systems in the southwestern USA often record multiple episodes of Plio-Quaternary aggradation and incision (Haynes, 1968; Bull, 1991). These events are recorded as fluvial deposits (emplaced during aggradation) and their bounding geomorphic surfaces (abandoned during incision) preserved along valley floors and on piedmonts. Studies of cycles of alluvial aggradation and incision in the southwestern USA go back to the earliest days of geomorphology (e.g. Bryan, 1922), but Haynes (1968) was the first to suggest that cycles of aggradation and incision are regionally correlative and provide geochronologic evidence for such a correlation in the Late Quaternary portion of the record. On piedmonts, cycles of alluvial aggradation and incision have been linked the internal dynamics of the fluvial system, including upstream drainage reorganization (Ritter, 1972) and coupled oscillations of the channel's bed and banks (Schumm and Parker, 1973). Knox (1983), working in Holocene alluvial valleys in Wisconsin, developed the biogeomorphic response model that became the basis for Bull's (1991) suggestion that …