Argyll Group

The lower boundary of the Islay Subgroup and hence the Argyll Group as a whole is defined at the base of the Port Askaig tillite, a diamictite which displays limestone clasts overlain by quartzite and granite clasts. The tillite is thickest in its type area and on the Garvellachs. The Schiehallion Boulder Bed of the eastern Grampians correlates with this tillite. The tillite is overlain by the Bonahaven Dolomite and then the Jura Quartzite which can reach up to 5km thick but which is more typically between 500 and 1000m thick. Psammites and pelites occur within the sequence in the Ladder Hills area.[2]

The Easdale Subgroup consists of a basal Scarba Conglomerate overlain by the Craignish Phyllite. It also includes pelites, semipelites, psammites and some volcaniclastic rocks. Extending between Jura, Knapdale and the Moray Firth coast, the unit can reach to 3km thick.[3]

The Crinan Subgroup is formed by the Crinan Grits Formation, a submarine fan deposit. These rocks which are between 1.5 and 3km thick, extend the width of Scotland from Kintyre to Fraserburgh. Their base is characterised by a coarse quartzite in Argyll, otherwise it is formed from pelites, semipelites and psammites.[4]

The lithology of this subgroup varies across Scotland but is dominantly limestone though this is replaced in the east by psammite and quartzite. Lavas and pelites are seen around Tayvallich in Knapdale, after which area the subgroup is named, where it consists of the Tayvallich Slate and Limestone Formation and the overlying Tayvallich Volcanic Formation.[5] The succession is between 100 and 250m thick but can reach up to 2.5km in the east where the psammites and semipelites occur.[6] The Loch Tay Limestone Formation extends from Campbeltown to Glen Isla.[7][8]