Abstract

The present knowledge of cohesive clay-laden sediment gravity flows (SGFs) and their deposits is limited, despite clay being one of the most abundant sediment types on earth and subaqueous SGFs transporting large volumes of sediment into the ocean. Lock-exchange experiments were conducted to contrast SGFs laden with noncohesive silica flour, weakly cohesive kaolinite, and strongly cohesive bentonite in terms of flow behavior, head velocity, runout distance, and deposit geometry across a wide range of suspended-sediment concentrations.

The three sediment types shared similar trends in the types of flows they developed, the maximum head velocity of these flows, and the deposit shape. As suspended-sediment concentration was increased, the flow type changed from low-density turbidity current (LDTC) via high-density turbidity current (HDTC) and mud flow to slide. As a function of increasing flow density, the maximum head velocity of LDTCs and relatively dilute HDTCs increased, whereas the maximum head velocity of the mud flows, slides, and relatively dense HDTCs decreased. The increase in maximum head velocity was driven by turbulent support of the suspended sediment and the density difference between the flow and the ambient fluid. The decrease in maximum head velocity comprised attenuation of turbulence by frictional interaction between grains in the silica-flour flows and by pervasive cohesive forces in the kaolinite and bentonite flows. The silica-flour flows changed from turbulence-driven to friction-driven at a volumetric concentration of 47% and a maximum head velocity of 0.75 m s⁻¹; the thresholds between turbulence-driven to cohesion-driven flow for kaolinite and bentonite were 22% and 0.50 m s⁻¹, and 16% and 0.37 m s⁻¹, respectively. The HDTCs produced deposits that were wedge-shaped with a block-shaped downflow extension, the mud flows produced wedge-shaped deposits with partly or fully detached outrunner blocks, and the slides produced wedge-shaped deposits without extension. For the mud flows, slides, and most HDTCs, an increasingly higher concentration was needed to produce similar maximum head velocities and runout distances for flows carrying bentonite, kaolinite, and silica flour, respectively. The strongly cohesive bentonite flows were able to create a stronger network of particle bonds than the weakly cohesive kaolinite flows of similar concentration. The silica-flour flows remained mobile up to an extremely high concentration of 52%, and frictional forces were able to counteract the excess density of the flows and attenuate the turbulence in these flows only at concentrations above 47%.

Dimensional analysis of the experimental data shows that the yield stress of the pre-failure suspension can be used to predict the runout distance and the dimensionless head velocity of the SGFs, independent of clay type. Extrapolation to the natural environment suggests that high-density SGFs laden with weakly cohesive clay reach a greater distance from their origin than flows that carry strongly cohesive clay at a similar suspended-sediment concentration, whilst equivalent fine-grained, noncohesive SGFs travel the farthest. The contrasting behavior of fine-grained SGFs laden with different clay minerals may extend to differences in the architecture of large-scale sediment bodies in deep marine systems.