Re-evaluating Shallow Geology in the Central North Sea

Seismic data from Quad 30 in the Central North Sea is revealing hitherto unreported aspects of the shallow sequence. In the south and west of the study area a stratigraphic unit outcropping at seabed (MLP4) forms a more or less continuous cover overlying a surface (SR4) characterised by a mega-scale glacial lineation. The MLP4 unit contains two main seismic facies, the lowermost of which comprises a bedded sequence which loses its identity towards the middle of the unit where the seismic texture becomes highly chaotic with few coherent reflections. Geotechnical boreholes show the chaotic facies to be a disordered assemblage of sands and slickensided clays. Towards the northeast the bedded facies within MLP4 disappears, the SR4 reflection loses its identity and the chaotic facies becomes more pronounced. In the furthest northeast the entire shallow sequence displays evidence of disruption and tectonisation down to a detachment surface at 90m – 140m below seabed. Base of the tectonised and un-tectonised sequences throughout the study area is taken as the erosian surface defined by the oldest generation of tunnel valleys recognised in the area. This surface is commonly dated to the Elsterian glaciation and marks the top of the Early-Middle Pleistocene Aberdeen Ground Fm.