Geological history of the North Sea

North Sea map The northern North Sea is situated between the Shetland Islands and Norway, and forms the northern arm of a trilete Late Jurassic rift system stretching from offshore eastern England to northern Norway (left, picture redrawn from Fraser et al., 2003). The most recent reviews of the tectonic history of the North Sea are given in Coward et al. (2003) and Zanella & Coward (2003), and they are summarised here. The pre-Devonian basement in northwest Europe formed by crustal accretion during the Caledonian collision between the ancient continents of Laurentia, Baltica and Avalonia. The location of the North Sea approximately coincides with the location of the closing of the Tornquist Sea and the Iapetus Ocean during this phase of deformation. By the Late Silurian to Early Devonian, convergence between Laurentia and Baltica in Europe had ceased. During Devonian times, the North Sea region was dominated by strike-slip tectonics as Europe was extruded laterally due to continuing collision between Laurentia and Baltica in the Appalachians. As a consequence, a large pull-apart basin formed in the same location as what was to become the Viking Graben in the Late Jurassic. Extensional half-graben also formed in the region at this time throughout the North Sea, development of which continued into the Early Carboniferous. In the Late Carboniferous, fold structures in Scotland and the North Sea indicate that structural inversion occurred. These were possibly related to orogenic Variscan tectonics in northwest Europe. In the North Sea, the Permian was characterised by basin subsidence due to extension and thermal cooling. However, tectonic activity was subdued during the Permian compared to the rifting that would follow in the Triassic. Triassic extension in an east-west direction occurred in the northern and central North Sea, and in the Moray Firth basin, although these regions were not yet connected to form a single rift system.

The most significant phase of deformation in the North Sea commenced in the Late Jurassic. Extension across the northern and central North Sea, and the Moray Firth basin, produced a triple junction of three distinct, but linked, rifts: the Viking Graben, the Central Graben and the Moray Firth. Rifting in the North Sea at this time was related to the linkage of the propagating Arctic and Central Atlantic rift systems, along which oceanic spreading and the separation of Europe and North America would occur. This rift event is the most well known in the North Sea due to good imaging in seismic data and dense sampling by boreholes, and because it gives rise to the most economically significant hydrocarbon reservoirs. The direction of extension during the Late Jurassic across the North Sea basin was generally east-west, but there is disagreement as to whether extension was orthogonal to each rift arm (e.g. Roberts et al., 1990), or whether it occurred in two phases, with a switch from east-west to southwest-northeast extension during the Early Cretaceous (e.g. Erratt et al., 1999). In either case, extension ceased during the Early Cretaceous. The remainder of the Cretaceous and the Cenozoic have seen little tectonic activity in the North Sea. Some authors have interpreted, on the basis of subsidence modelling, a minor phase of extension during the Late Cretaceous and Palaeocene (e.g. Bellingham, 2000; Hanne, 2002), although there is little evidence for faulting during this time and any extension must have been minor. The sediments deposited within the North Sea since the Early Cretaceous are more usually interpreted to be post-rift infill as the basin cooled after the Late Jurassic rift episode.

This dissertation is focussed exclusively on the Late Jurassic rifting in the northern North Sea. Earlier tectonic events are not considered because they are poorly imaged on seismic data, and few wells penetrate rocks older than Lower Jurassic. Late Jurassic extension is believed to have initiated well into the post-rift phase of any Triassic rifting, and thus there is little interference between Triassic and Late Jurassic fault-generated structures. Historically, for these reasons, it is the Late Jurassic rift episode which has been compared to rifting in central Greece, and the same approach is taken here.

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