O. R. 2007. A morphometric analysis of tunnel valleys in the eastern North Sea based on 3D seismic data.ABSTRACT: A 1250 km 2 3D seismic volume is used to provide a detailed spatial and geometrical analysis of fifteen Pleistocene tunnel valleys in the Danish North Sea. All the valleys are buried; they are up to 39 km long, 3-4 km wide and up to 350 m deep. The valleys are part of a vast tunnel valley province covering an area of some 0.5 million km 2 of the formerly glaciated lowland areas of North West Europe. The valleys consist of non-branching, non-anastomosing troughs; they exhibit strongly undulating bottom profiles with numerous sub-basins and thresholds, and are characterised by adverse end slopes. Cross-cutting relationships and theoretical considerations suggest the occurrence of seven major episodes of valley incision attributed to ice marginal oscillations within a few glacials. Calculations considering the valley end gradients and theoretical ice-surface profiles suggest that the valleys were formed by pressurised subglacial meltwater erosion. Given a range of theoretical ice-surface profiles, the adverse end slopes are well beyond the supercooling threshold, which suggests that the water was not in thermal equilibrium with the basal ice and that flow was concentrated in substantial conduits with sufficient mass and flux to maintain water temperature well above the freezing point.

Structure and lithology of the infill sediments from 16 subglacial buried tunnel valleys of Pleistocene age in the North Sea were analyzed using 3D seismic data and geophysical log data from five hydrocarbon exploration wells. The infill sediments are characterized by three seismic facies: Facies I, the volumetrically most important and structurally most distinct, is composed of clinoform reflections downlapping axially up-valley (up-ice), Facies II is composed of near-horizontal, continuous and well layered reflections onlapping the clinoform reflections and the valley walls and Facies III is composed of clinoform reflections downlapping axially down-valley (down-ice). A model of formation of this sediment architecture is proposed, in which valley incision and infill are causally linked. It is proposed thatFacies I is related to glaciohydraulic supercooling of subglacial meltwater in the distal parts of tunnel valleys. The valleys formed time-transgressively during ice retreat, whereby sediment eroded further up-ice was re-deposited along adverse subglacial slopes of the valleys close to the ice margin. The formation of Facies II and III is related to the deposition in proglacial basins during final deglaciation. Figure 2. (A) TWT-structure map of the 16 buried valleys displayed without infill sediments. The depth scale in TWT is in ms b.s.l. (B) Perspective view of (A) projected onto a horizon map of the Near Base Quaternary (NBQ) surface. The seafloor is at 80 ms TWT (60 m b.s.l.). Modified from the work of Kristensen et al. (2007).Figure 3. Seismic time slice at 340 s TWT (290 m b.s.l.) illustrating the structure of the clinoform reflections inside the southern end of Tunnel Valley V02. The reflections extend across the full width of the valley.Figure 6. Six vertical seismic profiles along the valley axes and TWT-structure maps of the valleys. Seismic infill facies is marked. From crosscutting pattern between the valleys, shown in Figure 2, it can be seen how the valleys in this figure represent several different episodes of valley generation. This figure is available in colour online at www.interscience.wiley.com/journal/espl T. B. Kristensen et al.Figure 8. Two vertical seismic profiles between wells (A) Siri-5, Siri-3 and Sandra-1 and (B) Sandra-1, Sofie-1 and Nolde-1. The positions of profiles are given in Figure 2.The difference between the seismic infill facies in Valleys V01 and V02 correlates very clearly with the lithology logs from the two valleys ( Figure 8). Furthermore, the infill stratigraphy from lithology logs in the two valleys does not correlate with that of the surrounding strata, but the overall lithology of the infill from Valley V02 is quite similar to that of the uppermost 300 m in the reference well. Interpretation: Infill ProcessesThe variation in lithology and seismic structure of the three seismic infill facies found in the buried valleys indicates that they represent different depositional processes. The recognition of clinoform reflections dipping axially up-valley Time-transgress...

This study provides results on buried tunnel valleys from the greatest number of released three-dimensional (3D) seismic surveys in the Danish North Sea sector to date. The valley infill sediments and the fluid content from buried Pleistocene tunnel valleys beneath the North Sea floor cause significant seismic velocity effects. Analysis of conventional 3D seismic data, high-resolution two-dimensional (2D) seismic data and wire-line log data from exploration wells reveal a multistage cut-and-fill history of tunnel valleys in the Danish North Sea. The seismic infill facies of the valleys is analysed and the associated velocity effects quantified. The valley infill generates both velocity pull up and push down of underlying seismic horizons. In contrast to expectations, our analysis shows no distinct correlation between particular seismic facies and a particular travel-time effect. Infill units characterized by the same seismic facies within the same valley can thus provide different travel-time effects. The anomalous velocities can be used as indicators of shallow gas and till content within the valleys leading to low and high velocities, respectively. Detailed investigations suggest that tunnel valley formation and sediment infill occurred over several episodes, with partial reuse of the kilometre-scale valleys and strong indications of ice-free periods between some of the formation episodes.

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