The Interpretation of Vertical Sequences in Turbidite Beds: The Influence of Longitudinal Flow Structure

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Because turbidite beds aggrade progressively beneath a moving current, the vertical grain-size profile of a bed is generally an indication of the longitudinal velocity structure of the flow, and longitudinal gradients in suspended sediment concentration (density). A current is more likely to show a simple waning flow history farther from its source; this is because faster-moving parts of the flow overtake slower moving parts, and the flow organizes itself over time so that the fastest parts are at the front. Thus distal (e.g., basin plain) turbidites commonly show simple, normally graded profiles, whereas more proximal turbidites often show complex vertical sequences within a bed, related to unsteadiness. A turbidity current may deposit a structureless, poorly sorted bed where the capacity of the current is exceeded, i.e., where there is insufficient turbulent kinetic energy to maintain the entire suspended mass. Capacity-driven deposition may occur where the flow decelerates. Where flow nonuniformity is the cause of capacity-driven deposition, a massive interval will form the lowest part of the bed, and will have a flat base. Where flow unsteadiness is the cause, a normally graded massive interval may overlie erosional features or traction structures at the base of the bed. Based on the assumption of longitudinal gradients in velocity, density, and grain-size distribution, the longitudinal density structure of a current may induce a switch, at any given point, from capacity-driven deposition to either (1) bypass and resuspension, (2) bypass with traction, or (3) competence-driven deposition, each resulting in a characteristic upward change in deposit character. The temporal evolution of the flow at a point varies systematically in a streamwise sense. Taking account of these longitudinal variations permits predictions of complex vertical sequences within beds, and of their downstream relations.