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Grain Size Fractionation of Sediment by Turbidity Currents Between Slope and Basin Floor: Insights From Physical Experiments

Abstract

The exact composition of natural turbidity currents is poorly constrained but there is general agreement that many carry a sediment load with a broad grain-size range containing clay, silt and sand. Generally, the flows on continental slopes are confined within a channel or canyon whereas the flows can spread freely on the basin floor to form lobes. The volume and grain size of the sediment carried in the suspension that reaches the end of the slope channels determine the characteristics of the lobes at the toe of slope and basin floor. The fractionation of grain size classes between channel-related deposits (levees, terrace and channel fill deposits) and basin floor deposits (lobes) is hard to constrain from outcrops or the subsurface as datasets generally do not allow correlation between the disparate environments. Here we present laboratory experiments in which both leveed slope channels and lobes are represented. The controlled laboratory environment allows us to constrain the fractionation of sediment between and within the different sub-environments. The confining morphology on the slope was created by sand-laden (D50=135μ) turbidity currents on an initially featureless slope. The confinement is primarily a result of the deposition of bounding ridges that have geometric similarity with natural levees. The self-formed confining morphology has a dynamic interaction with the turbidity currents: As the confinement relief grows, there is an increase in axial flow velocity and degree of bypass of sediment beyond the slope. The experimental slope and basin floor deposits were sampled along a number of vertical sections to reveal proximal to distal and vertical grain-size trends. Slope deposition commences with unconfined and coarse-grained deposition followed by a gradual upward decrease in grain size as the channel relief grows. Following earlier work, this trend is attributed to sampling of progressively higher parts of the flow that contain finer grained sediment. The composition of associated lobes is influenced by two factors: (1) The volume of sediment that reaches the lobe increases as the supply channel evolves and gets more efficient. (2) Lobes are cleaner (i.e. contain less fines) when they are sourced by a more mature channel. We propose that the insights from the experiments can be translated to natural systems and that this aids prediction of slope-to-basin-floor facies tracts at each stage of slope channel evolution.