| dc.description.abstract | Sørlige Nordsjø II (SNII) is an offshore wind site situated in the Southern Norwegian North Sea and is part of the larger goal to decarbonise the energy industry. The shallow geology of the North Sea is highly heterogeneous, partly due to the complex networks of buried tunnel valleys that formed under the extensive glaciation of the middle to late Pleistocene epoch. The understanding of tunnel valleys has potential implications for North Sea glacial history and the determination of sub-seabed conditions for marine infrastructure such as offshore wind turbines and carbon capture and storage. The goal of this thesis is to divide the tunnel valleys at the SNII site into distinct generations based on, firstly, their geomorphological measurements for orientation, maximum incising depth, and the relief of the tunnel-valley thalweg, and secondly, their cross-cutting relationships. Based on the interpretation of 2000 km2 of 3D seismic data, ~50% (45 tunnel valleys) of the SNII site is covered by detectable tunnel valleys, with widths ranging from 0.5 to 2.8 km, maximum depths of ~400 m, maximum lengths of 41 km, and orientations ranging from NE-SW to NW-SE. All mapped tunnel valleys had undulating thalwegs and slightly sinuous longitudinal shapes, with abrupt start and end slopes when present inside the study area. K-means clustering, a machine learning algorithm, was used to divide the mapped tunnel valleys into five groups based on similarities in morphology. Five levels of relative age were discovered by analysing cross-cutting relationships. The generational analysis concluded that seven events of tunnel valley formation have occurred at the SNII site, including one generation that is composed of a paleo valley with seabed relief. The oldest generation (G1) is of proposed Elsterian age (MIS 12), and the youngest generation (G7) is of probable MIS 2 age, corresponding to the Last Glacial Maximum. Three main types (and two subtypes) of infill were inferred from seismic facies: diamicton/gravels/till deposited subglacially, glaciolacustrine/glaciomarine deposited in a proglacial environment, and dipping ‘clinoform’ packages either deposited created ice-marginally by glaciofluvial backfilling (subtype one) or in a pro-glacial environment (subtype two). This was found to support a time-transgressive formation of tunnel valleys. Potential geohazards and implications for seabed conditions have been reviewed, with the main challenges for the safety and stability of seabed infrastructure being partially filled paleo valleys, buried lake sediments, the potential activity of salt tectonics, and tunnel valleys. Two main geohazards were found related to tunnel valleys: Unstable buried valley shoulders close to the seabed, and the tunnel valleys tendency to act as migration paths for hydrocarbons. The findings of this thesis can be implemented in future offshore wind sites where tunnel valleys make up the dominant facies of the shallow subsurface. | |
