HR: 1340h
AN: OS23C-1328    [Abstracts]
TI: Autocyclic Behavior of Experimental Turbidity Currents
AU: * Gerber, T P
EM: tpg@duke.edu
AF: Nicholas School of the Environment and Earth Sciences, Duke University, Durham, NC 27708
AU: Pratson, L F
AF: Nicholas School of the Environment and Earth Sciences, Duke University, Durham, NC 27708
AU: Wolinsky, M A
AF: Nicholas School of the Environment and Earth Sciences, Duke University, Durham, NC 27708
AU: Mohr, J
AF: Department of Geological Sciences, University of Minnesota at Duluth, Duluth, MN 55812
AU: Swenson, J B
AF: Department of Geological Sciences, University of Minnesota at Duluth, Duluth, MN 55812
AU: Paola, C
AF: St. Anthony Falls Laboratory, University of Minnesota Twin Cities, Minneapolis, MN 55414
AB: Subaqueous turbidity currents are dilute sediment-water mixtures that are driven along the water bottom by their excess density relative to the ambient water. Turbidity currents continuously deposit and resuspend sediments as they move. If resuspension exceeds deposition, the excess density of the current grows and the current `ignites'. If deposition exceeds resuspension, the excess density is depleted and the current `dies'. The bulk flow discharge and bed slope largely control these two regimes. Deposition from a waning turbidity current falls off exponentially downstream. A continuous depositional current therefore steepens the sedimented slope it flows over. At a critical steepness, the sediment resuspension will balance deposition and the flow should shift to a bypassing regime. Laboratory experiments combined with a simple numerical model show that this behavior triggers an autocyclic mechanism that can both create and regulate deltaic slopes even while they prograde. Six flume experiments using turbidity currents generated with mixtures of sand ($D_{50}$ =110$\mu$m) and silt ($D_{50}$ =24$\mu$m) were directed at determining if turbidity currents would build a slope through deposition to a critical steepness at which the flows would bypass it. With water level fixed, deposition from a continuous experimental turbidity current repeatedly steepened a slope to a critical angle before bypassing it and depositing a sediment wedge at the slope base. Continued deposition then caused the wedge to grow back updip to the top of the slope lowering it once again below the critical angle and reinitiating a cycle of oversteepening. The critical slope was observed to vary with the bulk sediment discharge and the amount of sand in the flow. The observed autocyclic progradation is simulated with a model of a depositional turbidity current governed by a critical bypass slope that depends on the grain size and bulk discharge of the flow. Predicted critical slopes for a range of sediment discharges and grain sizes representative of field-scale turbidity currents are typically $< 8^o$. The model predicts the critical slopes measured in the experiments if the coarsest ($D_{95}$) fraction of the sand is used in the resuspension relation. Our results document a mechanism for grading deltaic slopes as they prograde by processes of sediment transport rather than mechanical failure. The process also reveals a mechanism for the delivery of bypassed sand to basin deeps, leaving deepwater turbidites typically interpreted to mark a fall in relative sea level. On an experimental scale, units in the flume deltas bounded by bypass surfaces conform to stratigraphic sequences but are not related to changes in base level.
DE: 3022 Marine sediments--processes and transport
DE: 1815 Erosion and sedimentation
DE: 1857 Reservoirs (surface)
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting