Understanding cohesive, clay-laden subaqueous sediment gravity flows is vital, because clay is one of the most abundant sediment types on Earth and sediment gravity flows transport large volumes of this sediment into the ocean. Previous cohesive sediment gravity flow studies have overlooked the distinct cohesive strengths of different types of clay mineral. To isolate the effect of clay mineral type, lock-exchange experiments contrasted sediment gravity flows composed of weakly cohesive kaolinite clay, strongly cohesive bentonite clay, and non-cohesive silica flour at a wide range of volume concentrations. For high-density sediment gravity flows of the same concentration, kaolinite flows had a higher maximum head velocity and a longer runout distance than bentonite flows, because the bentonite flows were able to form a stronger network of particle bonds of greater rheological strength. Frictional forces reduced the mobility of the silica-flour flows at high concentrations. Dimensional analysis shows that the yield stress of the suspension can be used to predict the runout distance and the dimensionless head velocity of the sediment gravity flows, independent of the clay type. Clay mineral type within natural, cohesive, high-density sediment gravity flows is expected to control their runout distance and the geometry of their deposits. A metadata analysis was conducted to determine if the cohesive strength of different clay minerals left a signature in the geometry of modern, mud-rich submarine fans. For the fans studied, the normalised vertical-fan-growth-rate increased for fans dominated by kaolinite via illite to smectite, suggesting fans containing strongly cohesive clays may cover a smaller area and be thicker. The dominant clay mineral in the fans also appeared to have a latitudinal control, because the weathering intensity on adjacent continents controls clay mineral formation. These results suggest the relationships between latitude, clay mineral assemblage and the geometry of modern, mud-rich submarine fans should be further explored.
In the natural environment, cohesive sediment gravity flows are commonly composed of mixtures of clay minerals. Lock-exchange experiments produced sediment gravity flows carrying mixtures of bentonite and kaolinite at a fixed 20% volumetric concentration. Above bentonite proportions of 20%, further increasing the bentonite proportion increases the yield stress of the starting suspension and reduces the head velocity and runout distance of the flows. However, the mixture containing 10% bentonite had a lower yield stress and was more mobile than the pure kaolinite flow, suggesting that the small amount of bentonite reduced the cohesive strength of the suspension. In contrast, for pure-bentonite, high-density sediment gravity flows, increasing the volume concentration by adding 25% sand increases the yield stress of the suspension and reduces the runout distance and the velocity of the flows. This demonstrates that non-cohesive particles can contribute to the cohesive properties of a sediment gravity flow.
Geological fieldwork in the distal, mud-rich part of the submarine fan that makes up the Aberystwyth Grits Group and the Borth Mudstone Formation (Wales, U.K.) identified two novel mixed mud–sand bedforms: large current ripples and low-amplitude bed-waves (bedforms a few millimetres high and up to several meters long). The large ripples and low-amplitude bed-waves are interpreted to have formed under clay rich, transient-turbulent flows with enhanced and attenuated near-bed turbulence, respectively. These mixed sand-mud bedform types may be an important tool in interpreting fan fringe environments.