geological map

Lewisian

Moine

Grampian Orogeny

Dalradian

Caledonian Orogeny

Devonian

Mesozoic

Tertiary

geological evolution

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-west 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.

Within Shetland's Dalradian lie four thick bands of calcite marble; Shetland's limestones. These (and in particular the Whiteness and Girlsta Limestones) record evidence of the end of episodes of 'Snowball Earth' global scale glaciations during the Neoproterozoic (1000-543 Ma). These glaciations were followed by short-lived ultra-greenhouse events during which the average global surface temperature may have been in the order of 50C.   Just above the Neoproterozoic glacial deposits world-wide there is a sharp transition into chemically precipitated limestones known as 'Cap Carbonates' because they lie directly on top of (they 'cap') the glacial deposits. Cap carbonates have unusual chemical composition interpreted as recording a global oceanic alkalinity 'dump' following the 'melting' of a Snowball Earth under the extremely elevated atmospheric CO2 of a ultra greenhouse state. Acid rain would weather exposed silicate and carbonate rock, especially glacial debris, releasing large amounts of calcium which, when washed into the ocean, would form distinctively textured layers of carbonate sedimentary rock.   Recent research measuring 13C/12C ratios has shown that the Whiteness and Girlsta Limestones are marked by C-isotopic values characteristic of two of the most significant perturbations of the global C cycle. These are associated with the formation of cap carbonates following the demise of the world-wide 635 Ma Marinoan glaciation (Whiteness) and the unique c.600 - 550 Ma Shuram-Wonoka warming event (Girlsta). This research is of major importance and correlates the Whiteness Limestone and the Girlsta Limestone with dated rocks in Africa and China but not so far with any in the British Isles.  

(Paper: A.R. Prave, R.A. Strachan, and A.E. Fallick. Global C cycle perturbations recorded in marbles: a record of Neoproterozoic Earth history within the Dalradian succession of the Shetland Islands, Scotland. Journal of the Geological Society, January 1, 2009; 166(1): 129 - 135.)