QUATERNARY STRATIGRAPHY AND LANDSCAPE EVOLUTION OF EASTERN GRAND CANYON: INTERACTION OF DIFFERENT SCALE LANDSCAPE FEATURES DURING CLIMATE CHANGE

The tributary drainages and Colorado River corridor of Eastern Grand Canyon contain a suite of well-exposed Pleistocene hillslope and stream deposits that hold the key to longer-term landscape evolution of this unique area. Deposits in the tributary drainages consist of debris flows reworked to varying degrees by streams, and colluvial remnants, whereas Colorado River corridor deposits consist exclusively of alluvium. Landscape position, surface characteristics, soil development, deposit morphology, and absolute dating have been used to divide the deposits into five discrete fills, representing five aggradational episodes that are correlative between the tributary drainages and the main river corridor. Up to eleven terraces are associated with these five fills. Along the main river corridor, tributary terraces partly interfinger with and overly correlative Colorado River terraces. Data indicate the long profiles of the tributary terrace treads diverge up-drainage and that the fills may thicken near stream junctions where the trunk stream of a tributary drainage received large sediment inputs. Tributary drainage terraces can be observed that grade into large, thick hillslope colluvial remnants.

Results indicate tectonics is the longer-term mechanism controlling incision of the landscape, but climate change is the mechanism controlling the shorter-term formation and morphology of fluvial terraces in eastern Grand Canyon. Additional conclusions are that smaller-scale landscape features prograde over correlative larger-scale features and the amplitude of individual aggradation episodes of the Colorado River was greater than the amplitude of the associated aggradation by tributary streams. Ongoing geochronologic work should reveal whether the deposits formed specifically during glacial-interglacial transitions, as predicted by the existing working model for hillslope/stream responses to climate change in the Southwest.