Orcadian Basin

[3] 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.

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.

[9] 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.

[10][11][12] Sedimentation during the middle Devonian covered the full extent of the Orcadian Basin with only local exposed basement highs, such as near Stromness in Orkney.

At the deep permanent lake stage in a typical cycle there is normally a laminite, consisting of fine-scale alternations of clastic, carbonate and organic laminae, thought to represent annual varves.

As the lake shallows and becomes ephemeral the sequence consists of alternations of silts and fine sandstones often with mudcracks, showing the repeated periods of drying out.

This deep lake interval is dated as late Eifelian and is correlated to the global Kačák Event of marine anoxia, which was associated with significant extinctions.

Elsewhere, such as in Orkney in the Eday Group, the sequence shows more variety with major intercalations of marls and flagstones, marking the local return of lacustrine conditions.

This and other later marine incursions have been correlated with global sea level highstands previously recognised from the middle to upper Devonian sequences of New York State, including the Taghanic, Genundewa, Middlesex and Rhinestreet events.

These form isolated exposures of varying chemistry, indicating a change in magma source from one with a strong influence from earlier subduction to the alkali basalts more normally associated with extensional tectonics.

The later history of the Orcadian Basin is only known from offshore well data, as the youngest exposed Devonian known is Late Givetian in age.

The houses at Skara Brae, the tomb at Maes Howe, the Ring of Brodgar and Standing Stones of Stenness, were all built with flagstone.