Plastic Deformation, Erosion and Acceleration of Turbidity Currents Overriding Soft, Cohesive Substrates

Many aquatic environments have soft, muddy substrates, but this important property has largely been ignored in process-based models of Earth-surface flow. A soft, cohesive substrate with fluid-mud properties is believed to interact differently with a passing particulate density current (turbidity current) than a sandy substrate in terms of erosion and deformation.

Changes in flow and substrate properties are closely related to changes in bed shear stress, which controls flow turbulence properties, substrate erodibility, flow density and rates of deposition. This study aims to quantify the interaction of turbidity currents with soft, muddy beds in terms of changing flow properties, bed erosion and plastic deformation.

Novel laboratory experiments were carried out in the Hydrodynamics Laboratory at Bangor University (Wales, UK). Kaolin-clay laden turbidity currents were created with different initial concentrations (between 0.4 and 12.5 vol%) that moved over soft, fluid-mud like, kaolin-clay deposits (initial concentrations between 6 and 13 vol%). Ultrasonic Doppler Velocity Profilers (UDVP), Ultra High Concentration Meters (UHCM) and an HD video recorder were used to observe the interaction between flow and substrate at various contrasts in clay concentration.

Four types of flow-bed interaction were identified: 1) no interaction, 2) formation of a solitary bed surface wave travelling in front of the turbidity current ('leading wave’), 3) shear waves at the flow-bed interface, and 4) mixing and erosion. The type of interaction is inferred to be controlled by the difference between flow and bed concentration.

The most notable result is acceleration of turbidity currents over a plastically deformable, cohesive substrate in the absence of a slope gradient. The boundary conditions at which this acceleration takes place are under investigation, but the data suggest that erosion and changes in the geometry of the flow, associated with plastic deformation (i.e., shear waves), are key to explaining changes in turbidity current velocity.

The results of this study may: (1) explain the large continuity of some turbidite deposits in modern oceans and the geological record; (2) provide insights into the facies characteristics and architectural properties of these turbidite deposits, and; (3) therefore have important implications for the hydrocarbon industry.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California