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

In the semiarid landscape of the Colorado Plateau in northeastern Arizona, small basins associated with the "Blue Gap" escarpment are rapidly aggrading over the last millennium. The mode of operation of the fluvial systems in a few sub-basins, however, has changed since late 19th century from net aggradation to net erosion as shown by formation of deeply incised, discontinuous arroyos. Detailed study of stratigraphy and soils in the upper reaches of the basin valley fill, along with radiocarbon dates and dendrochronology show that this sediment, sometimes as thick as 3-4 meters, is only 500-1000 years old. Additionally, the soil-stratigraphic record shows that during most of this period, aggradation has been the consistent mode of channel behavior in this area. The lack of paleochannels show that until the 19th century arroyos formed, there were no other previous periods of deep channel incision. The observed rapid aggradation is partly attributable to the highly erodible Jurassic sandstones that make up the cliffs of the escarpment, including the Bluff sandstone of the San Rafael Group, and the Salt Wash sandstone of the Morrison Formation. Soil studies of these cliff-forming sandstones show that a combination of chemical and mechanical weathering, associated with hydration-dehydration cycles, enables extremely rapid sediment production on basin slopes.

The recent switch from aggradation to incision in some of the basins could be attributed to the effects of late Holocene climate change. The timing of this switch (late 19th century) suggests that the change could be associated with the end of the Little Ice Age (LIA ~1200-1900AD) and high-amplitude, dry-wet climatic fluctuations. The geomorphic expressions we see across the basins are inconsistent, however, because of the lag time between the climate change and the basin response and because responses are likely subject to subtle variations in basin characteristics such as aspect and vegetation. This study also reinforces previous studies (McFadden and McAuliffe, 1997; Bull, 1991) recognizing lithology as a key factor in dictating rates and processes of hillslope sediment production and sediment deposition. In this case, the weakly cemented sandstones of the Blue Gap field area are likely more sensitive to minor climatic changes of the Holocene than more resistant lithologies.