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Non-Cohesive, Mixed and Cohesive High-Density Turbidity Currents: Internal Flow Properties Inferred from Laboratory Experiments

 

Manica, Rafael1, Jaco H. Baas2, Ana Luiza de Oliveira Borges1, Rogério Dornelles Maestri1, Jeff Peakall2, William. D. McCaffrey2 (1) Universidade Federal do Rio Grande do Sul, Porto Alegre, Brazil (2) University of Leeds, Leeds, United Kingdom

 

Non-cohesive, mixed and cohesive high-density turbidity currents were generated in the laboratory to investigate their flow-dynamic (Previous HitvelocityNext Hit and concentration) and depositional properties. Each of the three types of turbidity current was studied at five different densities between 2.5% and 30% by volume. Head, body and tail velocities (at 10 different heights) and concentrations (at 4 different heights) were measured simultaneously, using Ultrasonic Doppler Velocimetry Profiling probes and Ultrasonic High Concentration Meters, respectively. The Previous HitvelocityNext Hit and concentration profiles disclosed predictable changes in flow character with progressive changes in suspended sediment concentration and cohesive character. These changes were expressed best in flow geometry, height of maximum Previous HitvelocityNext Hit, near-bed suspended distribution and settling character. Moreover, Kelvin-Helmholtz instabilities were observed to modify the internal properties of the flows well below their upper boundary. The results indicate that in non-cohesive and mixed flows the concentration and Previous HitvelocityTop data collapse well whereas at concentrations above 10% in cohesive currents the presence of kaolin modifies drastically the hydrodynamic behaviour (and possibly the rheology) of the flow. Three depositional models are distinguished. In the decelerating non-cohesive flows, abrupt deposition of sediment prevails, whilst in the mixed flows deposition is more gradual. In the cohesive flows, a high-concentrating basal layer forms just after the passage of the head, which seems to behave as a slowly moving fluid mud. Above the fluid mud, the flow is more dilute, faster moving and more turbulent. The implications for the spatial evolution of natural high-density turbidity currents and their deposits are discussed.

 

AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California