Dalradian Shetland

Dalradian cover of the Grampian Terrane

Following the break up of Rodinia there was an amalgamation of East and West Gondwana to form the Vendian supercontinent. The c. 580 Ma break-up of the Vendian supercontinent (Fig. 3b) saw the separation of Laurentia and Baltica from Gondwana to eventually form the Iapetus Ocean. This was a prolonged period of continental rifting and ocean widening (late Precambrian to early Ordovician, 750 to 470 Ma) with extensive sedimentary sequences being laid down on the passive continental margin of eastern Laurentia.

This rifting history is recorded in two distinct rock sequences in Scotland and Ireland. The first of these, the Cambrian-Ordovician shelf succession of north-west Scotland, is not found in Shetland. This was a shallow marine-shelf sequence of rocks deposited on the landward side of the eastern margin of Laurentia.

The second sequence, the Dalradian Supergroup, was laid down further offshore in deeper water and is exposed from Shetland south-east through Scotland into Ireland and is similar in age to the Fleur de Lys Supergroup of Newfoundland and the Eleonore Bay Supergroup of East Greenland.

In Scotland the Dalradian Supergroup is seen as a thick conformable sequence of folded and metamorphosed shallow to deep-water sediments and volcanics comprising the Grampian Group, The Appin Group, the Argyll Group and the Southern Highland Group. The lower sequence, The Grampian Group, has not been recognised in Shetland nor has the glacial episode at the base of the Argyll Group, the c. 650 Ma Port Askaig Tillite. The Shetland equivalents are; the Scatsta Division (Appin Group), Whiteness Division (Argyll Group) and the Clift Hills Division (Southern Highland Group).

Overall the Shetland Dalradian has been folded on a regional scale so that the sequence lies on its side with almost vertical dip and a NNE strike. With a thickness of about 12km, they form the largest group of metamorphic rocks in Shetland.