HOW VISCOSITY AND DENSITY STRATIFICATION CHANGES SUBMARINE MASS-FLOW BEHAVIOUR

Vertical stratification of sediment concentration is ubiquitous within submarine mass-flows, particularly those containing a wide range of grain-sizes with highly-variable settling velocities. Concentration stratification leads to stratification of density, viscosity and grain-size. Preliminary experiments with two layers of variable density show that density-stratified currents and homogeneous density currents behave in fundamentally different ways. For instance, if the density difference is sufficient, the basal high-density layer can out-run the overlying less-dense layer. Ongoing experiments are investigating flow behaviour when the lower-layer is BOTH denser and more viscous. This is likely to be the case for many submarine mass-flows. If the particle concentration and viscosity is sufficiently high in the lower layer, it travels more slowly that the upper layer. The head is thus fed primarily by low density and viscosity fluid from the upper layer. In certain situations the head can then accelerate away from the high-density and viscosity body of the flow. This leads to longitudinal stratification of the flow into distinct components. Debris-flow deposits are commonly seen sandwiched within clean sand of high-density turbidity current deposits in northern Italy. These 'sandwich beds' are very similar to lahar deposits formed in a transition zone, within which a debris flow progressively mixes with river water to produce an associated hyperconcentrated stream flow. This hyperconcentrated stream flow out-runs the debris flow and deposits the basal clean sand. The Italian intra-bed debris flow deposits represent significant baffles, as they can extend for over 20 km. Somewhat similar 'sandwich beds' are observed in other classic deep-marine sequences (e.g. Piera Cava basin) and in the deposits of other particulate gravity currents (e.g. lahars and pyroclastic flows). Stratified-flow experiments and field observations will investigate whether these 'sandwich beds' result from longitudinal stratification due to the variable forward speed of flow-components, or a variety of other mechanisms.