Calculating hydraulic conditions of turbidity currents from surficial and subsurface deepwater channel architecture

Turbidity currents, a form of particulate gravity current, are an important agent of sediment, nutrient and pollutant transport into deep marine settings. They commonly form submarine channels, whose geometry can be used to condition hydraulic models that reconstruct the dynamics of formative flows. This approach has here been extended to integrate theoretical models of turbidity current dynamics with channel geometry to address three key challenges in deep-water sedimentology. Firstly, a new modelling approach is applied to two surficial channels of the Hikurangi Margin of New Zealand to predict whether turbidity currents can bypass sediment, depending upon flow height, the sorting of the sediment in suspension and the slope. Thick flows with well-sorted suspensions are shown to be more efficient in bypassing sediment than shallow flows transporting poorly-sorted sediment in suspension; these latter flows need steeper gradients to transport an equivalent median particle size of suspended material. Model results for the spatial distribution of sand from poorly-sorted flows are in good agreement with seismic amplitude maps and drop core data. Secondly, using high-resolution 3D seismic data, the new modelling approach is applied to mapped subsurface channel-forms within the Omakere Channel Complex of the Hikurangi Margin to determine if they represent palaeohydraulic conduits. The derivation of disequilibrium flow conditions at some channel bends suggests that these channel-forms either represent compound bodies or that they were not in equilibrium with traversing flows, possibly due to the presence of a mass-transport-deposit substrate. These factors constitute important sources of modelling uncertainty in the calculation of palaeo-hydraulic conditions from subsurface channel architecture. Finally, changes in the tilt of the flow-ambient fluid interface are modelled to assess whether a strong latitudinal control exists in the development of channel sinuosity. The effects of the slope (also proposed as a dominant control), channel size and flow properties are also evaluated. The outcomes suggest that the tendency of channels to become sinuous cannot be predicted by single end-members like latitude or slope. At any latitude, large, low-gradient channels predominantly traversed by dilute, shallow flows may be characterised by low sinuosities, whereas small, high gradient channels traversed by dense, deep flows may be sinuous.