Torridon Group

The Torridon Group is a series of Tonian (lower Neoproterozoic) arenaceous and argillaceous sedimentary rocks, which occur extensively in the Northwest Highlands of Scotland.

[1] Stratigraphically, they lie unconformably on gneisses of the Lewisian complex and sandstones of the lithologically similar Mesoproterozoic Stoer Group and their outcrop extent is restricted to the Hebridean terrane.

[2] The dominant lithology of the Torridon Group is red and brown sandstone, often arkosic, with subsidiary amounts of shale, particularly towards the top of the sequence, with coarse conglomerates and breccias locally at the base.

They attain their maximum development in the Applecross, Gairloch and Torridon districts, form the greater part of Scalpay, and occur also in Rùm, Raasay, Soay and the Crowlin Islands.

[4] The lowest part of this formation consists of a basal breccia containing clasts derived from the underlying Lewisian complex with the thickest developments in the paleovalleys.

Most of the sandstone beds are affected by soft-sediment deformation structures suggesting liquefaction, possibly as a result of seismic activity.

Only the outcrops at Cape Wrath described above have a consistent radial pattern suggesting that the sequence was deposited in a bajada environment, by a series of smaller fans merging to form a braided river system.

This implies an age gap of at least 200 Ma between the deposition of the Stoer and Torridon groups, consistent with the paleomagnetic evidence of a major break.

The lower part of the Sleat Group show ages consistent with derivation from Scourian and to a lesser extent Laxfordian rocks, with no dates after 1700 Ma.

This is consistent with the generally westerly derived pebbly material throughout the thickness of the Applecross Formation, suggesting a constantly rejuvenated sediment source in that direction.