HR: 16:00h
AN: H54B-01    [Abstracts]
TI: The Pulse of Calm Deltas
AU: * Kim, W
EM: geowskim@uiuc.edu
AF: Department of Civil and Environmental Engineering, University of Illinois, Urbana- Champaign, Urbana, IL 61801, United States
AU: * Kim, W
EM: geowskim@uiuc.edu
AF: National Center for Earth-surface Dynamics, University of Minnesota, Minneapolis, Minneapolis, MN 55414, United States
AU: Jerolmack, D J
EM: sediment@sas.upenn.edu
AF: Department of Earth and Environmental Sciences, University of Pennsylvania, Philadelphia, Philadelphia, PA 19104, United States
AU: Jerolmack, D J
EM: sediment@sas.upenn.edu
AF: National Center for Earth-surface Dynamics, University of Minnesota, Minneapolis, Minneapolis, MN 55414, United States
AB: At the heart of interpreting the history of Earth-surface evolution preserved in the rock record is distinguishing environmental (allogenic) forcing from internally-generated (autogenic) ¢®¡Ænoise¢®¡¾. Allogenic deposits have classically been recognized by their cyclic nature, apparently resulting from periodic changes in base level, sediment supply, or tectonics. Autogenic deposits, although quite variable in their origin and scale, are caused by the nonlinearity of sediment transport and expected to have a random or scale-free (fractal) signature. Here we describe a robust mechanism that generates cyclic deposits by an autogenic process in experimental fan-deltas. Sheet flow over the fan surface induces deposition and an increase in fluvial slope, until a critical slope is exceeded leading to a channelization instability. Channelized flow results in incision and degrading of the fan surface to a lower slope, releasing a pulse of sediment that pushes the shoreline forward. Sheet flow resumes once the surface is regraded, and the cycle repeats in a surprisingly periodic fashion to produce cyclic foreset parasequences. We use a one-dimensional fan evolution model to (1) demonstrate how time-varying flow width can cause pulses in sediment discharge at the shoreline in agreement with the experiments, and (2) scale our results up to field conditions. Alternating sheet- and channelized-flow is known to operate on non-cohesive fans in nature. Our results suggest that, rather than reflecting variation in environmental forcing, many observed cyclic sedimentation may be a signature of the autogenic ¢®¡Æpulse¢®¡¾ of deltas under calm environmental conditions.
DE: 1825 Geomorphology: fluvial (1625)
DE: 1847 Modeling
DE: 1862 Sediment transport (4558)
SC: Hydrology [H]
MN: 2007 Fall Meeting