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Grain Size Fractionation Within Self-Channelized Turbidity Current Deposits


Dilute, particulate-laden, density-driven flows - turbidity currents - are thought to be the predominant mechanism for transporting sediment from source to sink in deep marine environments. The porous, coarse-grained fraction of their deposits (turbidites) can form hydrocarbon reservoirs. A key question therefore is: how does turbidite porosity vary, from source to sink? Of particular interest here are self-channelized turbidity currents, which transport sedimentary material for thousands of kilometers and form the largest sedimentary landforms on the planet. Exisiting theory concludes that for turbidity currents to persist, sediment must be self-maintained in suspension, i.e., be in a state of autosuspension. It has been shown that such self-maintained sediment suspensions can only occur whilst inertial forces are greater than gravitational forces, entailing supercritical flow. This conclusion is paradoxical, as inertia dominated flows rapidly entrain fluid, thereby thickening and slowing to become subcritical. However, current theory only applies to the proximal upper slope regions of seafloor channels where flows are fully confined. This is in contrast to the distal reaches of long run out turbidity current systems, where the flow is only partially confined through self-channelization. Here it is shown that overspill of partially confined flow has a significant effect on the hydro- and morphodynamics of long run out turbidity current systems. The new model indicates that channel overspill acts to negate the effects of ambient fluid entrainment: a dynamic balance limits increases in flow depth. In established (1D-depth average) autosuspension theory the flow mass balance, as controlled by depositional or erosional processes, is a significant control on flow dynamics and thus run out. In the modelled scenario this effect is complicated by flow stratification and by mass loss through overspill onto channel banks. By incorporating both erosional/depositional processes, and mass losses arising from stratified flow channel overspill, a complete description of the progressive fractionation of sediment grain size distribution of sediment deposits along flow pathways can be achieved. This approach allows changes in bed porosity, from source to sink, to be specified, both in relative terms, and, given calibration data, in absolute terms.