LATE HOLOCENE BEHAVIOR OF SMALL DRAINAGE BASINS ON THE COLORADO PLATEAU: INFLUENCES OF LITHOLOGY, BASIN FORM AND CLIMATE CHANGE

With evidence mounting demonstrating the reality of anthropogenic greenhouse warming, geomorphologists have become especially interested in the impacts of century and decadal scale climate changes during the Holocene on geomorphic processes. Recent studies of the semiarid landscapes of the Colorado Plateau in northeast Arizona indicate that they are lithologically sensitive to climate changes on a millennial and shorter temporal scales. In the area of Blue Gap, Arizona, landscapes seem to have been sensitive to climate changes on an even shorter time scales. All small basins of this escarpment have been characterized by spatially widespread aggradation over the last millennium; older Holocene valley floor alluvial deposits are nowhere present. Detailed study of stratigraphy and soils in the upper reaches of the basin valley fill, supported by radiocarbon dating and dendrochronologic studies, show that this sediment (locally up to 3-4 meters thick) is only 500-1000 years old. The observed rapid aggradation is largely attributed to the highly erodible Jurassic bedrock (Bluff sandstone and the Salt Wash member of the Morrison Formation). Soil studies of hillslopes show that a combination of chemical and mechanical weathering, associated with hydration-dehydration cycles, enables extremely rapid weathering and sediment production on basin slopes. The mode of operation of the fluvial systems in a few sub-basins has notably changed since late 19th century, from net aggradation to channel incision (arroyos). The timing of this change (late 19th century) is unrelated to overgrazing by domestic livestock; instead, we suggest that it may reflect climatic changes at the end of the Little Ice Age (LIA ~1200-1900AD). The variable geomorphic expression of this recent change in climate may be due to the lag time between this climate change and the basin response, with such responses likely linked to variations in basin characteristics such as aspect, vegetation, slope weathering, soil development and sediment production. At least in this study area, the weakly clay cemented sandstones of the Blue Gap field area seem to be especially sensitive to minor climatic changes of the Holocene as compared to more resistant rocktypes elsewhere in the region.