Current interpretation divides the Orkney rocks into:

  • Basement Metamorphic Complex;
  • Lower Devonian – Breccias and Sands;
  • Middle Devonian to Upper Devonian – Orkney Flagstone Group and Eday Group (including the Hoy sandstones).

The original investigations into the sedimentary rocks of Orkney and Caithness by Roderick Murchison (1859) led to a simple three-fold classification of the Old Red Sandstone into:

  • Lower Old Red Sandstone (sandstones and conglomerates of the south-west Mainland);
  • Middle Old Red Sandstone (grey flagstones of the Mainland and islands);
  • Upper Old Red Sandstone (red and yellow sandstones of Hoy and Eday).

This early interpretation was modified when angular unconformity was recognized between the sands and conglomerates at Yesnaby and the overlying flagstones, and between the flagstones and sandstones in north Hoy. This appeared to fit the tectonic picture from other parts of Scotland and led to their classification into Lower, Middle and Upper Devonian epochs.

Since no unconformity existed between the Eday sandstones and the Middle Old Red Sandstone flagstones, the Eday Sands were initially assigned to a Mid-Devonian age and thus deemed older than the Hoy sandstones, which were given a late Devonian date. This stratigraphic interpretation was adopted by Peach and Horne (1880) and is  used in many  publications.

However, detailed stratigraphic sections, sedimentology and palaeogeographic reconstructions of the sandstones in the South Isles have shown that the Eday Group sandstones are equivalent to the Hoy Group (Astin, T pers comm).

Recognition of contemporaneous fault movements on the North Scapa and Bring Fault systems and volcanic eruption at the base of both the Eday and Hoy sandstones further enhances this correlation.

The North Hoy Unconformity is now considered a local rather than a regional feature.

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.