Print this pageAutocyclic Behavior of Experimental Turbidity Currents
Thomas P. Gerber1, Lincoln F. Pratson1, Matthew A.
Wolinsky1, Jere Mohr2, John B. Swenson2, and Chris
Paola3
1 Duke University, Durham, NC
2 University of Minnesota Duluth, Duluth, MN
3 University of
Minnesota, Minneapolis, MN
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 slope it flows over. At a critical steepness, resuspension will balance deposition and the flow should bypass the slope.
Laboratory experiments show that this behavior can trigger an autocyclic mechanism that can both create and regulate deltaic slopes while they prograde. With water level fixed, deposition from a continuous turbidity current repeatedly steepened a slope to a critical angle before bypassing it and depositing a sediment wedge at the slope base. Continued deposition 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 depositional oversteepening. The observed autocyclic progradation is simulated with a depositional model governed by a critical bypass slope that depends on the grain size and bulk discharge of the flow. Predicted critical slopes for field-scale turbidity currents are typically < 8º.
Our results document: (1) a mechanism for creating high frequency stratigraphic cycles with no change in base level, (2) deltaic progradation by processes of sediment transport rather than mechanical failure, and (3) a process of autocyclic deposition that may apply to a variety of granular gravity flows.
AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005