Unconfined gravity current interactions with orthogonal topography: Implications for combined-flow processes and the depositional record.

Turbidity current behaviour is affected by interactions with seafloor topography. Changes in flow dynamics will depend on the physiographic configuration of the topography (orientation and gradient), and the character of the incoming flow (magnitude and rheology). A better understanding of how unconfined turbidity currents interact with topography will improve interpretations of the stratigraphic record; we address this using 3D flume tank experiments with unconfined saline density currents interacting with a ramp orientated perpendicular to flow direction. The incoming flow parameters remained constant, whilst the slope angle was independently varied. On a 20 slope, super-elevation of the flow and flow stripping of the upper, dilute region of the flow occurred high on the slope surface. This resulted in a strongly divergent flow and the generation of complex multidirectional flows (i.e., combined flows). The super-elevation and extent of flow stripping decreased as the slope angle increased. At 30 and 40, flow reflection and deflection, respectively, are the dominant flow process at the base of slope, with the reflected or deflected flow interacting with the parental flow, and generating combined flows. Thus, complicated patterns of flow direction and behaviour are documented even on encountering simple topographies; a planar slope orientated perpendicular to flow direction. Combined flows in deep-water settings have been linked to the interaction of turbidity currents with topography and the formation of internal waves with a dominant oscillatory flow component. Here, combined flow occurs in the absence of an oscillatory component. A new process model for the formation and distribution of hummock-like bedforms in deep-marine systems is introduced. This bedform model is coupled to a new understanding of the mechanics of onlap styles (draping versus abrupt pinchout) and triggers for soft-sediment deformation processes to produce a spatial model of gravity-current interaction, and deposition, on slopes to support palaeogeographic reconstructions.