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Flow and Disintegration of Sand-Rich Debris Flows and the Possible Origin of Deepwater Sandy Bodies

Anders Elverhoi1, Fabio V. De Blasio1, Johan Petter Nystuen1, Dieter Issler1, Peter Gauer1, and Jeff Marr2
1 International Center for Geohazards/University of Oslo, Oslo, Norway
2 University of Minnesota, Minneapolis, MN

Although deep sea sand bodies are commonly assumed to derive from turbidity currents, the mechanism of emplacement is in reality not fully understood. It has been suggested that part of the subaqueous sand bodies may possibly be the consequence of processes occurring at higher densities, such as subaqueous mudflows, debris flows, or landslides. To study the physical phenomena occurring during the emplacement of subaqueous sandy bodies, experimental debris flows with variable composition were created in a flume. After release, the flow spontaneously stratified into three layers: a turbidity current at the top, a nearly laminar layer of clay and fluidized sand in the middle, and a deposition sand layer at the bottom. Analytical estimates are presented to appreciate the dynamical behaviour of the material in the different layers. The transport capability of sandy debris flow and the application to large scale natural events are quantitatively investigated with analytical calculations and simple numerical estimates. Extrapolating our laboratory results, we speculate on the possible phenomena which might contribute to the emplacement of deep water sand bodies. We recognize three possible processes: i) hindered settling of sand grains suggests that sand can potentially flow in a fluidized state; ii) high velocity sand waves were observed in the experiments and we suggest that such waves could be a means of sediment delivery in natural setting; these waves might generate stacked graded sequences starting from a single debris flow; iii) a sufficient amount of clay in the slurry produces a cohesive mixture which can flow lubricated