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

Abstract: Fourteen overlapping 3D seismic reflection data sets, originally acquired by the oil and gas industry, were used to image and investigate complex networks of buried tunnel valleys in the UK sector of the Central North Sea. More than 180 individual tunnel valleys were observed and mapped to horizontal resolutions of 12–50 m and vertical resolutions of 8–10 m. The results presented here identify three recurring seismic facies that locally define stratigraphic organiza-tion from base to top of the buried tunnel valley fills: (i) high-amplitude disrupted units, (ii) a unit containing gently dipping disrupted reflections or transparent facies and (iii) well-layered continuous reflections. A comparison to similar works in the region is used to infer tunnel valley infill during changing depositional conditions from high-energy subglacial meltwater to quiet glaciolacustrine or glaciomarine environments. Tunnel valleys are channel-like landforms associ-ated with ancient lowland glaciations. Tunnel val-leys of Quaternary and older age are documented onshore and offshore, buried and at surface, in for-merly glaciated regions (O’Cofaigh 1996; Huuse & Lykke-Andersen 2000). Tunnel valleys may be found in isolation or as networks, and are recog-nized as important conduits for meltwater under-neath continental ice sheets. Debate regarding their method of formation has, in many cases, domi-nated the literature, with opposing theories based on fundamental differences related not only to tunnel valley genesis but also to differing models of glacial hydrology and relationship to substrate (see summary in O’Cofaigh 1996). In NW Europe, more recent work on tunnel val-leys ismoving away from a steady state versus catas-trophic view on tunnel valley formation and instead considers polygenetic origins (Huuse & Lykke