HR: 0800h
AN: H31A-1283    [Abstracts]
TI: Climatic, Tectonic, and Lithologic Sensitivity of Pediment, Piedmont Junction, and Tor Development
AU: * Strudley, M W
EM: mws10@duke.edu
AF: Division of Earth and Ocean Sciences/Center for Nonlinear and Complex Systems, Nicholas School of the Environment and Earth Sciences, Duke University, Box 90230, Durham, NC 27708 United States
AU: Murray, A B
EM: abmurray@duke.edu
AF: Division of Earth and Ocean Sciences/Center for Nonlinear and Complex Systems, Nicholas School of the Environment and Earth Sciences, Duke University, Box 90230, Durham, NC 27708 United States
AU: Haff, P K
EM: haff@duke.edu
AF: Division of Earth and Ocean Sciences/Center for Nonlinear and Complex Systems, Nicholas School of the Environment and Earth Sciences, Duke University, Box 90230, Durham, NC 27708 United States
AB: We previously developed a numerical model to explain the origin and maintenance of a) laterally extensive (km`s), planar bedrock pediments of low slope covered by a nearly uniformly thin regolith blanket, b) bare-bedrock or boulder pile edifices (tors and `inselbergs'), and c) the piedmont junction, a sharp slope discontinuity that often demarcates the transition of smooth, gently sloping pediments to steeply sloped, bare-bedrock tors and upland mountains. Our model produces this autogenic suite of landforms through a dynamic feedback between bedrock weathering and physical erosion (stream flow and diffusive processes) despite the absence of spatially variable mineralogy, lithology, or tectonic activity. In the model, physical erosion modifies regolith thickness, which, in turn, regulates bedrock-weathering rates. This relationship combined with `diffusive' smoothing by episodic streams in an environment that promotes high infiltration capacities and channel bank instability yields coupled, smooth bedrock and alluvial surfaces that lower together through time. The piedmont junction reflects an emergent threshold in erosion styles; mountain slopes are weathering-limited while pediments are transport-limited. Tors and larger, more heavily jointed and morphologically complex exposures, `inselbergs', may arise as a consequence of fluctuations in rainfall and sediment transport conditions, combined with a bedrock weathering mechanism that depends on regolith thickness and that exhibits a maximum for a finite thickness of cover. Specifically, periods of higher effective moisture, resulting in local base level incision and regolith thinning on pediments, may invoke a transition in which mantled surfaces lower at rates exceeding the bare-bedrock weathering rate. This condition favors the emergence and growth of tors in areas of bare bedrock and in areas covered by regolith thickness less than a threshold value. Subsequent shifts in climate or local base level that cause the sediment surface to lower at a rate less than the bare-bedrock weathering rate will lead to a progressive decrease in tor height, ultimately leading to their disappearance. Tors in these environments thus represent possibly transient features related to fluctuations in climate or local transport conditions. Here we present results describing how temporally variable effective moisture (invoking variable base level incision rates, regolith thinning, or piedmont deposition), transient and continuous uplift, and different lithologic and climatic environments (different styles of weathering and sediment transport) may alter the development of pediments, piedmont junctions, and tors. We present results suggesting that the development of tor fields is robust against large variations in annual rainfall (80-2400 mm/y), the degree of nonlinearity in fluvial sediment transport as a function of discharge, the thickness of the initial regolith cover (0.0-5.0 m), and the base level incision rate (>= 0.2 mm/y). We also present results illustrating the lack of sensitivity of pediment development to spatially variable lithology, including layered sedimentary rock, and how pediment development, incision, and exhumation may occur in disparate climatic and tectonic environments.
DE: 1826 Geomorphology: hillslope (1625)
DE: 1862 Sediment transport (4558)
DE: 1886 Weathering (0790, 1625)
DE: 4435 Emergent phenomena
DE: 4485 Self-organization
SC: Hydrology [H]
MN: Fall Meeting 2005