Grampian Orogeny


Caledonian Orogeny





By 750 Ma Rodinia was splitting apart and the first of the cover rocks we see in Shetland, the sediments that became the Moine Supergroup, had been laid down (Fig. 3a). There is an ongoing debate as to the tectonic history leading to deposition of the Moine Supergroup and to the nature of its basement gneisses. The ‘traditional’ view is that basement gneisses are Lewisian and sedimentation was in rift basins along the margin of Laurentia during the separation of Baltica from Rodinia. It is also argued however, that the Moine rocks and their basement are an exotic terrane accreted onto the Laurentian margin, having being originally part of either Baltica or ancient South America.

Whatever their origin, Moine sediments were laid down in marine rift basins between 1000 Ma and 873Ma as fine grained sands, silts and muds on current swept off-shore deltas. The sandstones were later metamorphosed into psammites and the silts and muds into pelites.