Orcadian Basin

The Orcadian Basin is a sedimentary basin of Devonian age that formed mainly as a result of extensional tectonics in northeastern Scotland after the end of the Caledonian orogeny. During part of its history, the basin was filled by a lake now known as Lake Orcadie. In that lacustrine environment, a sequence of finely bedded sedimentary rocks was deposited, containing well-preserved fish fossils, with alternating layers of mudstone and coarse siltstone to very fine sandstone. These flagstones split easily along the bedding and have been used as building material for thousands of years. The deposits of the Orcadian Basin form part of the Old Red Sandstone (ORS). The lithostratigraphic terms lower, middle and upper ORS, however, do not necessarily match exactly with sediments of lower, middle and upper Devonian age, as the base of the ORS is now known to be in the Silurian and the top in the Carboniferous.

The exact extent of the Orcadian Basin is uncertain due to later tectonic effects and burial beneath younger sediments, but it is known to have reached from the south coast of the Moray Firth to the Shetland Islands in the north and from Strathy on the Caithness coast in the west, to the Outer Moray Firth and East Shetland Platform in the east, where it is proven by hydrocarbon exploration wells. Continental sediments of the same age are also known from the Clair oilfield west of the Shetland Islands and have been tentatively identified in the West Orkney Basin. The connection of the Orcadian Basin to the Devonian basins of western Norway and eastern Greenland is not known in any detail. To the south the basin may continue almost as far south as the Highland Boundary Fault, including the half-grabens at Turriff and Rhynie.

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