Cyclic aggradation and downcutting, fluvial response to volcanic activity,and calibration of soil-carbonate stages in the western Grand Canyon, Arizona

In the western Grand Canyon, fluvial terraces and pediment surfaces, both a ssociated with a Pleistocene basalt flow, document Quaternary aggradation a nd downcutting by the Colorado River, illuminate the river's response to ov erload and the end of overload, and allow calibration of soil-carbonate sta ges and determination of downcutting rates. Four downcutting-aggradation cy cles are present. Each begins with erosion of older deposits to form a new river channel in which a characteristic suite of deposits is laid dean. The current cycle (I) started similar to 700 yr B.P. The oldest (IV) includes the 603,000 +/- 8000 to 524,000 +/- 7000 yr Black Ledge basalt how, emplace d when the river channel was similar to 30 m higher than it is now. The flo w is overlain by basalt-cobble gravel and basalt sand. Soils reach the stag e V level of carbonate development. Calibrated ages for soil stages are Sta ge V, similar to 525,000 yr; stage IV, <525,000 yr, greater than or equal t o 250,000 yr; stage III, <250,000 yr, greater than or equal to 100,000 yr. The monolithologic basalt sand beds represent overloading by volcanic ash p roduced by an eruption 30-50 km upstream. The basalt-cobble beds signal bre aching and rapid destruction of lava dams and erosion of flows. These depos its show that the Colorado River responds to overload by aggrading vigorous ly during the overload and then downcutting equally vigorously when the ove rload ends. The overall downcutting rate for the interval studied is 1.6 cm /1000 yr, much lower than rates upstream. The current downcutting rate, 11- 14 m/1000 yr, likely is a response both to the end of late Pleistocene and early Holocene overload and to the reduction of sediment supply caused by G len Canyon Dam. (C) 2000 University of Washington.