Response of unconfined turbidity current to normal‐fault topography

Abstract

Turbidity currents descending the slopes of deep‐water extensional basins or passive continental margins commonly encounter normal‐fault escarpments, but such large‐magnitude phenomena are hydraulically difficult to replicate at small scale in the laboratory. This study uses advanced computational fluid dynamics numerical simulations to monitor the response of large, natural‐scale unconfined turbidity currents (100 m thick and 2000 m wide at the inlet gate) to normal‐fault topography with a maximum relief of nearly 300 m. For comparative purposes, the turbidity current is first released on a non‐faulted pristine slope of 1·5° (simulation model 1). The expanding and waxing flow bypasses the slope without recognizable deposition within the visibility limit of 8 vol.% sand grain packing. Similar flow is then released towards the tip (model 2) and towards the centre (model 3) of a normal‐fault escarpment. In both of these latter models, the sand carried by flow tends to be entrapped in four distinct depozones: an upslope near‐gate zone of flow abrupt expansion and self‐regulation; a flow‐transverse zone at the fault footwall edge; a flow‐transverse zone at the immediate hangingwall; and a similar transverse zone near the crest of the hangingwall counter‐slope, where some of the deposited sand also tends to be reshuffled to the previous zone by a secondary reverse underflow. The near‐bottom reverse flow appears to be generated on a counter‐slope of 1·1°, increased to 2·0° by deposition. The Kelvin–Helmholtz interface instability plays an important role by causing three‐dimensional fluctuations in the flow velocity magnitude and sediment concentration. The thick deposits of large single‐surge flows may thus show hydraulic fluctuations resembling those widely ascribed to hyperpycnal flows. The study indicates further that the turbiditic slope fans formed on such fault topographies are likely to be patchy and hence may differ considerably from the existing slope‐fan conceptual models when it comes to the spatial prediction of main sand depozones.

Mud Volcanism in a Canyon: Morphodynamic Evolution of the Active Venere Mud Volcano and Its Interplay With Squillace Canyon, Central Mediterranean

“…Detailed bathymetric maps and corresponding interpretations of seafloor structures to indicate the relative age relationships between bedforms, see index map in bottom right for locations. A hydraulic jump zone may develop at the toe, scouring sedimentary beds at the foot of the escarpment(Breda et al, 2007;Ge et al, 2017), a process similar to the Detailed bathymetric maps, corresponding interpretations, and bathymetric-topographic cross sections of seafloor structures in the study area, see index map on right side for locations. (b) Mudflow deposits with furrowed surfaces overlain by younger mudflow deposits without furrows and sediment waves on the caldera floor; note gryphon-cone in top right corner.…”

“…shown that the deposition of sand may not only occur at the toe of such a morphology but also along the upper edge(Ge et al, 2017). (a) Mudflow deposits with furrowed surfaces overlain by younger mudflow deposits without furrows and bathymetric-topographic cross section of sediment waves on the caldera floor.…”

Response of unconfined turbidity current to deep‐water fold and thrust belt topography: Orthogonal incidence on solitary and segmented folds

“…The flow upper surface concurrently developed a hydraulic instability in the form of backwards‐rolling large Kelvin–Helmholtz (K–H) waves, generated by the interface shear between the flow and the ambient seawater (Britter & Simpson, ; Ge et al ., ; Kostaschuk et al ., ). These internal K–H waves, by propagating concentrically from the flow U‐shaped front, came into interference within the flow (Fig.…”

“…The turbidity current flow was modelled using the deterministic process modelling CFD software MassFlow‐3D™, which is a customized version of Flow‐3D™ (FlowScience, ; http://www.flow3d.com). The reliability of MassFlow‐3D™ was verified by imitating laboratory and natural‐scale turbidity currents (Heimsund, ; Heimsund et al ., ) and the software has been used extensively to simulate such deep‐water sediment gravity currents (Aas et al ., ,, ; Janocko et al ., ; Basani et al ., ; Ge et al ., , ). The numerical code describes fluid motion by solving the system of Reynolds‐averaged Navier–Stokes (RANS) equations by a finite‐volume finite‐difference method for a 3D computational grid.…”

Morphodynamics and depositional signature of low‐aggradation cyclic steps: New insights from a depth‐resolved numerical model

“…() and Ge et al . () use the same model to simulate turbidity currents and can provide further detail.…”