Contouritic depositional systems influenced by complex seafloor topography : Late Cenozoic seismoacoustic reconstructions from the Galicia and Angola Continental Margins

Abstract

Contourites are sediment drift bodies that form under the persistent influence of bottom currents. As their morphology and composition records changes in the ambient current regime, they form excellent paleoceanographic archives. The shape and size of a contourite is determined by strength and variability of the bottom current as well as sediment supply, but also by the topographic framework with which the current interacts. Especially smaller scale topographic features such as seamounts are known to interact with and thereby amplify bottom currents, leading to the evolution of distinct contourite drifts. The interplay between topography and bottom current and the resulting depositional pattern are complex and not entirely understood. However, due to the associated amplification of currents the resulting deposits are often more sensitive to variations in the background hydrodynamic regime. This makes them especially valuable for paleoceanographic reconstruction. However, before that information can be utilized, the interaction mechanisms which ultimately shape the drift have to be understood in their entirety. Moreover, changes in topography, be it through burial by sediments, tectonic activity or even biogenic construction of topographic features, may cause similar changes in the interaction process as a variation in the current forcing. This project addresses interaction between currents and topography in two representative yet diverse study areas. At the Galicia Margin, a strongly dissected topography formed during the opening of the North Atlantic Ocean and was furthermore shaped by the Pyrenean and Betic orogenies. Various seafloor features, such as topographic obstacles and ridges have interacted with ocean density fronts passing through the transition zone between Mediterranean Outflow Water and Labrador Seawater, which was lowered during glacials and shoaling in deglacial periods. The second study area is located at the shallow Angola Margin, where the seafloor is actively deforming through gravity-driven salt tectonic processes. Extensional grabens open up in the process of salt rafting and are rapidly filled by sediments depositing under the influence of bottom currents. Furthermore Cold Water Corals grow in marked mound and ridge structures in the area, adding more variability to the already dynamic topography. Overall, the present thesis provides new insight into the interaction of bottom currents and small-scale topographic features and their role in shaping contourite depositional systems through numerous examples. Conceptual models of the influence of these interaction processes on sedimentary deposits were advanced and the influence of bottom current topography interaction in previously undisclosed settings was discovered. The detailed investigation of these processes is an important step to more holistic understanding to the role of bottom currents in shaping complex continental margins and also a step towards the deciphering of valuable paleoceanographic information archived in these systems.