The Influence of Slope on Turbidity Currents and Their Deposits in a Process-Based Model

Abstract

Turbidity currents transport land-derived sediment to deep sea where theirdeposits form large geological structures termed submarine fans. Giventheir large areal extent and high sand content, submarine-fan deposits formsignificant hydrocarbon reservoirs. Their internal architecture is often poorlyresolved. With much of the three-dimensional architecture of the turbiditelobes below seismic resolution, there is commonly a significant level of un-certainty associated with respect to the reservoir geometry and quality.At the end of submarine channels, turbidity currents lose confinementand encounter a slope break, in response, the current decelerates and forms a deposit, a turbidite. Repeated passage of turbidity currents forms a stackof deposits that are generically termed lobes.Subsequent flow events modify the bathymetry by erosion and deposi-tion, thereby affecting the trajectory of subsequent flow events. Due to self-formed relief, beds start stacking laterally, in 'compensation'. A complex but ordered stratigraphy is created by repeated cycles of shifts in deposition, which results in variability in deposits that governs reservoir connectivity. In this study, multiple consecutive turbidity currents flowing over self-formed relief are modeled using a process-based numerical model (Delft3D-FLOW). The response of successive turbidity currents and their deposits to variations in channel slope was tested. Models with steeper channel slope were observed to result in more vigorous flows with deposits that cover a larger surface area. Following passages of turbidity currents were observed to erode bed sediment in the channel and thereby diminish the amount ofreadily erodible bed sediment for following turbidity currents. With the current model set-up, the deposit relief was insufficient to observe lateral stacking of deposits. These simulations provide insight into the depositional processes and the controls on the geometry and sedimentary trends of the deposits emplaced by successive turbidity currents flowing over a slope break and losing confinement.