Geometrical restoration of a late Neoproterozoic depositional framework and an intrabasinal unconformity in the Laurentian margin Dalradian Supergroup, Grampian Highlands, Scotland

Abstract

Restoring primary depositional frameworks from orogenic settings is challenging. To demonstrate a robust determination of original, but now highly deformed, depositional frameworks and their first-order sequence-stratigraphy, we analyse the Dalradian succession of Tyndrum–Glen Lyon (Breadalbane) in the southwestern Grampian Highlands of Scotland. In Breadalbane, several distinctive Appin and Argyll group Dalradian formations are absent. Omission has been attributed to ductile shearing on the Boundary Slide structure, during the Grampian Orogeny ( c. 470 Ma). Alternatively, we restore and describe a primary depositional framework and widely developed intra-Dalradian basin unconformity in Breadalbane, preserved in the relatively low-strain lower limb of the Grampian D 2 Ben Lui Syncline. On this unconformity, locally distinctive strata of the Easdale Subgroup, and more regionally typical strata of the Crinan Subgroup, were deposited directly on strata of the Lochaber Subgroup. Northeastward loss of strata of the Ballachulish, Blair Atholl and Islay subgroups, observed SW of Tyndrum, contrasts with gradual reappearance of correlative units northeastwards from Glen Lyon. Onlap and/or overstep relationships are well preserved; although strain is enhanced locally along pronounced stratal or rheological contrasts, the stratigraphical framework remains essentially intact. Our Scotland-wide analysis of the Dalradian depositional framework recognizes other probable basin-scale unconformities that locally influenced patterns of superimposed orogenic deformation.