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.
Extent 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.
Tectonic setting
In common with the Devonian basins of Norway and East Greenland, the Orcadian Basin lies entirely within the area affected by crustal thickening during the Caledonian orogeny. The recognition of extensional faulting at various scales in these areas at the same time as deposition led to the suggestion that these basins reflect the gravitational collapse of this thickened zone. Other tectonic models have suggested that transtensional sinistral (left lateral) strike-slip movement on the Great Glen Fault, which passes through the centre of the basin, was the main cause of basin formation. The continuity of Middle Devonian facies belts across the trace of the fault zone, after accounting for subsequent reactivation of the Great Glen Fault in a dextral sense, has been used to argue against strike-slip activity during sedimentation. However, the uplift of a block containing late granitic veins within the basement Moine complex in Easter Ross, whose intrusion has been dated as earliest Eifelian, before deposition of the overlying Middle Devonian sediments, is evidence of some continuing activity along this structure into the middle Devonian period.
Development
Early Devonian Evidence from onshore in Easter Ross and offshore in the Inner Moray Firth shows that a series of isolated half-grabens were formed at this time filled with a mixture of coarse, often conglomeratic sediments, combined with organic-rich lacustrine shales, such as those exposed at Strathpeffer.
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