The supergroup is composed of four
groups which in stratigraphic order i.e. youngest at top, are: •
Southern Highland Group •
Argyll Group •
Appin Group •
Grampian Group The upper three groups are applied to Ireland too; the Grampian Group is not recorded here nor in Shetland where the Dalradian is divided into a Clift Hills ‘Division’ which equates to the Southern Highland Group, a Whiteness ‘Division’ which equates to the Argyll Group and a Scatsta ‘Division’ which equates to the Appin Group. Though now
metamorphosed, the Dalradian sequence was originally deposited as marine
sands and
muds,
silt and
limestone. Metamorphism has been low to medium grade for the most part and resulted in the formation of
slates,
phyllites,
psammites,
pelites and semipelites. The Tayvallich Subgroup contains
volcanics within a
turbidite basin and there are
lavas within the overlying Southern Highland Group. An additional group is recognised by some workers, the Trossachs Group, forming the uppermost part of the succession, which outcrops close to the Highland Boundary Fault.
Trossachs Group The Trossachs Group as defined lies conformably above rocks of the Southern Highlands Group just northwest of the Highland Boundary Fault. Most of the outcrops of the Trossachs Group are fault-bounded, making overall correlation difficult. The oldest part of the group is interpreted to be the Keltie Water Formation, which includes the Leny Limestone and Slate Member from which lower Cambrian
trilobite fossils have been recovered. Apparently higher in the succession is the Margie Formation, which includes the Margie Limestone Member, which has yielded
conodont fossils of early Ordovician age. Rocks of this group are interpreted to be everywhere in tectonic contact structurally below the
Highland Border Ophiolite. The sedimentary rocks that lie unconformably above the ophiolite include the Dounans Limestone Formation that contains a fossil fauna of mid-Arenig age (near the boundary between the lower and middle Ordovician).
Southern Highland Group The Southern Highland Group is found along the entire southeastern margin of the Grampian Highlands from
Kintyre to
Stonehaven and also in the northeast along the coastal strip between
Fraserburgh and
Portsoy, extending south to the
Don valley. In Shetland, the Clift Hills Division extends from north of
Lerwick south to
Fitful Head. In Ireland the most extensive outcrop of the Southern Highland Group is to the north and south of
Lough Foyle and west to
Lough Swilly. Smaller exposures occur as far to the southwest as
Inishbofin, County Galway.
Argyll and Appin groups In mainland Scotland, the Appin and Argyll group sequences occupy the intermediate ground between the Southern Highland and Grampian groups. The Argyll Group is divided into four subgroups, thus: • Tayvallich Subgroup • Crinan Subgroup • Easdale Subgroup • Islay Subgroup whilst the Appin Group is divided into three subgroups: • Blair Atholl Subgroup • Ballachulish Subgroup • Lochaber Subgroup In Shetland the Whiteness ‘Division’ forms the core of
Mainland whilst the Scatsta ‘Division’ forms the western halves of
Unst and
Fetlar, all of
Whalsay and much of the southeastern part of Mainland, east of the
Nesting Fault. These rocks are also present in Ireland across
County Londonderry and
County Donegal and appear again in the
Ox Mountains, the
Nephin Beg Range and the
Twelve Pins of Connemara.
Grampian Group The Grampian Group rocks occupy the ground south west from
Elgin and extending down the Great Glen as far as
Corran on
Loch Linnhe and, further east, as far south as
Tyndrum. It is divided into three subgroups: • Glen Spean Subgroup • Corrieyairack Subgroup • Glenshirra Subgroup
Shetland sequence , on Unst, Shetland The stratigraphic position of the sequence identified as Dalradian in the Shetland Islands is uncertain, because the main marker within the sequence in Scotland and Ireland, the
Port Askaig Tillite Formation (of the Islay Subgroup), is not present. Carbon isotope data from four metamorphosed limestones within this mainly siliclastic sequence, suggest that the entire Shetland sequence probably lies stratigraphically above the tillite marker, explaining its absence. ==Structure==