The sedimentary record of submarine channel morphodynamics

Abstract

Submarine channels are ubiquitous on the ocean floor and are considered to be the equivalent of rivers on land. These channels are created by turbidity currents, which originate from the continental margins and which can transport sediment for thousands of kilometres into the oceans. The aim of this thesis is to investigate the relation between the properties of turbidity currents and the formation of these channels. Direct measurements of turbidity currents in submarine channels are sparse because the processes occur at great water depths and turbidity current activity is only intermittent in most places. Flume experiments have the advantage that flow processes and deposits can both be measured which is essential for process interpretations. One challenge is that small-scale laboratory currents do not always have long runout distances similar to natural systems. In the thesis it is shown that experiments with appropriate boundary conditions can result in turbidity currents that completely bypass sediment inside a channel. For the first time, we can therefore simulate the main phase of channel activity and not only the channel filling and abandonment stage. Three questions related to submarine channel formation are addressed with the experiments. Firstly, how is a channel initiated on a slope with no initial channel? Secondly, can the characteristics of levees that form due to overspill from a channel be related to the vertical profile of turbidity currents? Thirdly, how much sediment is extracted from turbidity currents due to deposition around different types of channels and how does this affect deposition at the downstream end of a channel. Density and grain-size stratification in turbidity currents are identified in the experiments as main factors that influence deposit architecture and facies around submarine channels. A simple analytical model is developed that can predict stratification in turbidity currents with multiple sediment grain sizes. The model can reproduce grain-size and density profiles of turbidity currents that have been measured in experiments. A submarine channel fill at a field site in Chile is used as a case study where the model is applied. Stratification is implied from a change in deposit grain size with height above the channel thalweg. The model was used to relate the vertical change in grain size to a turbidity current structure and flow properties. It is concluded that submarine channels are highly dynamic features on the seafloor that constantly change in shape due to feedbacks between the currents and channel form. Stratification in turbidity currents determines the amount of sediment loss onto the levees due to overspill and the amount of sediment that is transferred inside a channel.