3D seismic analysis of buried tunnel valleys in the central North Sea: morphology, cross-cutting generations and glacial history

This work presents the results of morpho-stratigraphic analyses carried out on more than 180 buried tunnel valleys imaged in the central North Sea using 3D seismic data at a variety of resolutions. The buried tunnel valleys form complex networks of multiple cross-cutting generations. In the northern part of the study area, cross-cutting relationships within higher resolution 3D seismic data are used to extrapolate seven generations of tunnel valleys over an area of 14,400 km2. Three generations of cross-cutting tunnel valleys are also observed within higher resolution 3D seismic data in the southern part of the study area. Detailed morphological measurements of individual tunnel valleys reveal average widths between 300 m and 3000 m, lengths in the region of tens of kilometres, and relatively straight and simple planform geometries. The majority of the tunnel valleys display undulating basal profiles which provide evidence for the uphill movement of water over distances greater than 10 km and indicate formation by meltwater under pressure. Tunnel valley geometries vary between generations and defy a simple link between formation process and end form. Detailed orientation measurements for the tunnel valley generations reveal an overall NE-SW trend in the northern part of the study area, and a NW-SE directionality in the south. When considered in light of palaeoglaciological models for the British and Scandinavian ice sheets during the Pleistocene, tunnel valley orientation and distribution are consistent with the majority of tunnel valley generations forming perpendicular to a retreating British ice sheet. Changes in orientations between generations (in both northern and southern study areas) provide evidence for changes in the dynamics of the British and Scandinavian ice sheets between subsequent glaciations of the central North Sea.