Turbidity currents and their deposits in abrupt morphological transition zones

Abstract

The principle transport agent in deep ocean environments are turbidity currents, avalanches of sediment and water that travel down the continental slope. Turbidity currents usually flow within deep-marine channels, comparable to terrestrial rivers on land, which can extend for 1000s kilometers across the ocean floor. At the downstream end of these channels are lobe shaped sandy deposits called submarine fans that represent potential reservoirs for hydrocarbons but also a sink for any material transported by the turbidity currents such as microplastic. The internal structure of submarine fans and their location depends on how sediment is deposited by the turbidity currents. Sediment deposition is controlled by the turbidity current dynamics that are strongly affected by changes in the ocean-floor topography across which the turbidity current is flowing. This thesis investigates these effects and links the turbidity current dynamics to the resulting deposition pattern. For this, turbidity currents are physically modeled in the laboratory and exhumed ‘real-world’ deep-marine deposits are investigated in outcrops. Experiments focused on turbidity currents going across a decrease in ocean-floor gradient and explains how these topographic change triggers deposition. A second experiment series dealt with turbidity currents leaving the confinement of a channel and revealed a novel flow mechanism we called ‘flow relaxation’. Flow relaxation describes the lateral spreading and thinning of the flows resulting in erosion and sediment bypass. The experiment results are used to explain sedimentary structures observed in the outcrop and to reconstruct and predict changes of the ancient ocean-floor topography.