Thresholds of intrabed flow and other interactions of turbidity currents with soft muddy substrates

Controlled laboratory experiments reveal that the lower part of turbidity currents are able to enter fluid mud substrates, if the bed shear stress is higher than the yield stress of the fluid mud and the flow density is higher... [ view full abstract ]

Controlled laboratory experiments reveal that the lower part of turbidity currents are able to enter fluid mud substrates, if the bed shear stress is higher than the yield stress of the fluid mud and the flow density is higher than that of the substrate (Baas et al., 2014, Processes and products of turbidity currents entering soft muddy substrates. Geology, 42, 371-4). Upon entering the substrate, the turbidity current induces mixing between flow-derived sediment and substrate sediment or it forms a stable horizontal flow front inside the fluid mud. Such ‘intrabed’ flow is surrounded by plastically deformed mud; otherwise it resembles the front of a bottom-hugging turbidity current. The ‘suprabed’ portion of the turbidity current, i.e, the upper part of the flow that does not enter the substrate, is typically separated from the intrabed flow by a long horizontal layer of mud, which originates from the mud that is swept over the top of the intrabed flow and then incorporated into the flow. The intrabed flow and the mixing mechanism are specific types of interaction between turbidity currents and muddy substrates that are part of a larger group of interactions, which includes bypass, deposition, erosion, and soft sediment deformation. A classification scheme for these interaction types is proposed, based on an excess bed shear stress parameter and an excess density parameter. Based on this classification scheme and the sedimentological properties of the laboratory deposits, the facies model for intrabed turbidites of Baas et al. (2014) is extended to the other types of interaction. The physical threshold of flow-substrate mixing versus stable intrabed flow is defined using the gradient Richardson number, and this method is successfully validated with the laboratory data. The gradient Richardson number is also used to show that stable intrabed flow is likely only in natural turbidity currents with flow velocities well below c. 3.5 m s-1, despite the fact that a wider range of flows is capable of entering fluid muds. Below this threshold velocity, intrabed flow is stable only at high density gradients and low velocity gradients across the upper boundary of the turbidity current. Finally, the gradient Richardson number is used as a scaling parameter to set the flow velocity limits of a natural turbidity current that formed an inferred intrabed turbidite in the deep-marine Aberystwyth Grits Group (UK).