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Turbidite Grading as a Proxy for Flow Generation and Proximality: Applications and Limitations

Abstract

Submarine turbidity currents are a key mechanism in the transportation of clastic sediments from continents to deep seas. The vertical grading pattern of their deposits, turbidites, can act as a proxy for the dynamics of the overpassing flows. It is not uncommon for a turbidite to have multiple intervals of inverse grading instead of exhibiting a continuously upward fining grading structure. Such ‘pulsed’ turbidites can be deposited by turbidity currents that developed a multi-pulsed structure due to their triggering mechanism (e.g., retrogressive submarine landslides and/or complex patterns of ground shaking forced by very high magnitude earthquakes) or due to flow combination at channel confluences. Such mechanisms enable the interaction between individual flow pulses generated in series. Questions arise as to how much the deposits tell us about initiation mechanisms of multi-pulsed turbidity currents and over what length scale pulsed turbidites are expected to be found in the deposits. Experiments were conducted to model denser than ambient saline flows as turbidity current proxies. Two lock boxes were set up in series at one end of a flume, enabling the generation of multi-pulsed flows. Research methodologies include direct flow visualization, sample siphoning to construct vertical density profiles and velocity profiling using Acoustic Doppler Velocimetry to capture the variation in horizontal flow velocity. Results show that i) pulses always merged within the studied experimental configuration and ii) the signal of flow initiation was preserved proximally, progressively distorted and eventually shredded beyond the point of merging. The dependence of dimensionless parameters characterizing the merging length scale upon other initial flow conditions was studied by systematically varying experimental boundary conditions. This analysis permits estimation of the spatial persistence of pulsing signatures with natural scale turbidites. For instance, the interaction between adjacent individual pulses each of 2.0 km initial length and 100m initial height generated 10 minutes apart by a bi-pulsed earthquake could potentially generate a multi-pulsed turbidite no further than 20km from its initiation point. Beyond that point, a normally-graded deposit profile would be seen, that would permit no inference to be made regarding the flow initiation mechanism.