HR: 0800h
AN: H41D-0765    [Abstracts]
TI: Lithologic feedbacks between valley and channel form in canyons of southeast Utah
AU: * Johnson, J P
EM: joelj@alum.mit.edu
AF: MIT EAPS, 77 Massachusetts Ave, Cambridge, MA 02139, United States
AU: Whipple, K X
EM: kxw@asu.edu
AF: School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287,
AU: Sklar, L S
EM: leonard@sfsu.edu
AF: Department of Geosciences, San Francisco State University, San Francisco, CA 94132,
AB: Does the slope of an incising channel decrease, increase or stay the same when cutting across a lithologic contact from a stronger to a weaker rock unit? It depends--all of these patterns of incision occur at a particular lithologic contact along bedrock channels in southeast Utah, near the Henry Mountains and Navajo Mountain. We interpret that changes in longitudinal slope at the contact result from (1) valley-channel coupling and local sediment supply, in which coarse sediment from the more resistant unit above mantles the channels below, and (2) variations in the amount of bedload transported from upstream, which influences the incision of both rock units. We compare channels near the lithologic contact between the Wingate sandstone above (a well cemented aeolian cliff-former that is resistant to weathering and is jointed at the scale of meters to several meters) and the Chinle formation below (a mudstone which contains occasional resistant sandy and carbonate-rich lenses but overall is a slope-former that weathers easily). Along many channels (e.g. Fourmile Canyon), longitudinal slope increases through the weaker Chinle mudstone: once the lower unit is exposed, the overlying Wingate sandstone is undermined by Chinle slope weathering which in turn dumps resistant colluvial boulders of Wingate into the channel and armors the bed. A second key variable that influences slope at the lithologic contact is the amount of bedload supplied from upstream (i.e. transportable sediment that is not locally derived). Channels with high coarse sediment supplies from upstream are incisional but effectively alluvial and show little to no break in slope associated with the lithologic contact (e.g. Trachyte Creek, Swett Creek). In contrast, channels with negligible bedload supply appear to have difficulty incising through the Wingate sandstone, leaving very steep channels or cliffs at the Wingate-Chinle contact (e.g. Trail Canyon at Navajo Mountain). Channels with moderate bedload supply cut relatively narrow slot canyons through the Wingate sandstone and then steepen at the Chinle contact, as described above. Together, these field observations demonstrate ways that bedrock lithology can influence patterns of channel incision.
DE: 1815 Erosion
DE: 1825 Geomorphology: fluvial (1625)
DE: 1826 Geomorphology: hillslope (1625)
SC: Hydrology [H]
MN: 2007 Fall Meeting