Background:the ancient igneous and metamorphic basement rocks of Orkney comprise three rock types,

  • neisses (with amphibolite)
  • foliated granites and
  • minor and late, undeformed granite and aplite veins

The oldest rocks in Orkney are the granite gneisses and schists exposed in West Mainland at Yesnaby, Stromness and on Graemsay (Strachan, 2003). The absolute age of the Orkney basement rocks is unknown. The rocks resemble migmatised Moine rocks found within the Kirtomy Nappe above the Swordly Thrust in eastern Sutherland.

Basement Moine Map

These rocks, and by inference the Orkney inliers,  with mica schists, psammites and subordinate semipelites belong to the Loch Eil Group of the Moine, with a sedimentary age of about 950 million years.

Recent work on the rims of Zircon crystals which grew during migmatisation has given ages of 461+/- 13 million years for the Kirtomy assemblage and 467+/-10 million years for the Naver assemblage establishing the  presence of a middle Ordovician (Taconic) tectonothermal event.

This is in contrast to the age of gneiss formation and migmatisation recorded in Moine rocks of the south-western part of the Northern Highland block and older rocks of the Central Highlands where the main tectonothermal event is dated about 840+/-11 million years. Detrital Zircons within the sedimentary precursors of these migmatites fall in the age range of 1,850 to 1,000 million years indicating that deposition of the Moine sediments in Sutherland probably occurred later than 1,000 million years ago.

After a long period of burial in the crust of the Earth, these rocks were thrust to the surface during the Caledonian Mountain building episode between 500 and 400 million years ago. By the close of this period deep erosion had exposed the gneisses as a series of low domes. a terrain that was then buried by the breccias, conglomerates and sandstones of the Early Devonian including in Orkney the Harra Ebb Formation.

Orkney Stratigraphy

Geology of Orkney

  • 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.