Rifting in the Northern North Sea commenced during the early Triassic, peaked during the late Jurassic, and terminated by the late Cretaceous. The Viking Graben can be seen as a failed arm of proto-Atlantic rift. Almost throughout this period, the Shetland area formed an area of positive relief.

Permo-Triassic rocks occur just offshore from Shetland in great thicknesses: over 3600m in the Unst Basin, 2700 m in the West Fair Isle Basin and 2000 m in the East Fair Isle Basin (Johnston et al., 1993). As in the Devonian, conditions were arid and continental and both sandstone- and mudstone-dominated red bed successions were laid down. These rocks do not thin towards the narrow landmass of Shetland, as would be expected if erosion of this terrain was a major source of sediment. Instead, it appears that Permo-Triassic sediments formerly covered Shetland and were stripped after younger (?early Tertiary) uplift. A similar picture occurs in the West Orkney Basin where Permo-Triassic sediments are up to 3 km thick (Evans, 1997).

Early to Middle Jurassic sediments are absent from much of the East Shetland Platform. Shetland lay above sea level and shed debris to large deltas at the edge of the Viking Graben. Subsidence of basins in the northern North Sea in the Late Jurassic was accompanied by the accumulation of deep sequences of grey to black siltstones and clays belonging to the Kimmeridge Clay. These carbon-rich sediment became the main hydrocarbon source rock of the North Sea oil field.

Late Cretaceous chalk at Mons Klint, Denmark. Similar rocks lie east of Orkney and supplied erratics of chalk and flint to the shelly till of East Mainland

The three-armed rift of the central and northern North Sea and the generalised oil bearing sequence Source: UKOOA

The three-armed rift of the central and northern North Sea and the generalised oil bearing sequence Source: UKOOA

Lower Cretaceous sediments are now only a few tens of metres thick on the East Shetland Platform. An unconformity is developed across the underlying Jurassic strata, suggesting uplift and planation of the Shetland land mass. Upper Cretaceous strata are also absent, perhaps lost to erosion in response to Early Tertiary uplift. In contrast to the rest of the North Sea and southern England, the Upper Cretaceous rocks above 59ºN are mudstones, probably sourced from Greenland. In the East Orkney basin, the remaining Upper Cretaceous are up to 300 m thick (Andrews et al., 1990). These rocks are but the remnants of a much thicker pile of Mesozoic sediments removed by Early Tertiary erosion. Evans (1997) estimates that ~2 km of Permian-Cretaceous cover rocks have been removed from over Orkney to supply the Palaeogene sands of the northern North Sea.

Geology of Orkney

  • Orkney Stratigraphy

    The Orkney Islands consist almost entirely of sedimentary rocks and subordinate lavas and tuffs of Middle and Upper Old Red Sandstone age. A Basement Complex composed of metamorphic rocks of Moinian type and Caledonian granites forms a number of small inliers near Yesnaby and Stromness in West Mainland and on the island of Graemsay.

  • Devonian environments

    During the Devonian Period, North America and Northern Europe were joined together forming Euramerica, one of the three major continental masses on the Devonian globe, with Orkney placed south of the equator. Most of Britain formed part of this landmass with mountains in the northwest and the open marine Devonian Sea covering southwest England.

  • Permian dykes

    The age of the Orkney dykes is 252+/-10 million years, placing the time of intrusion in the late Permian, a period of renewed tectonic movement in the sedimentary basins to the west of Orkney.

  • Hydrocarbon Potential

    The Petroleum System consists of four main Geologic components, source, reservoir, seal and trap plus additional processes necessary to generate and store hydrocarbons in the subsurface. Their presence is required to generate a viable EXPLORATION TARGET.

  • Tertiary of the northern North Sea

    At the opening of the Tertiary chalk sedimentation continued on the northern North Sea. Starting around 63 Ma, the East Shetland Platform was uplifted and tilted towards the south-east. ...

  • Quaternary of the northern North Sea

    The Quaternary deposits found on the East Shetland Platform are thin (Johnson et al., 1993). The Quaternary thickens above the Viking Graben to as much as 300m and provides a long and relatively detailed record of glaciation.

  • Mesozoic of the northern North Sea

    Rifting in the Northern North Sea commenced during the early Triassic, peaked during the late Jurassic, and terminated by the late Cretaceous. The Viking Graben can be seen as a failed arm of proto-Atlantic rift. Almost throughout this period, the Shetland area formed an area of positive relief.