Permo-Triassic sedimentary and volcanic rocks in basins to the north and west of Scotland

Abstract

Abstract Red-bed sequences of known or inferred Permian and Triassic age are preserved in numerous offshore basins to the north and west of Scotland. These developed following the Late Carboniferous Variscan Orogeny, as compressive uplift was superseded by a tensional stress regime causing the fragmentation of the Variscan foldbelt and its northern foreland. Basin formation persisted through the Permian and Triassic periods, and the sediment infills reflect syn- and post-depositional fault activity. Syndepositional basins take the form of deep, fault-bounded half-grabens whose development was controlled by the extensional reactivation of earlier (Precambrian/Caledonian) thrusts and transfer faults. These basins accumulated wedge-shaped packages of sediment dipping into major basin-bounding faults. In contrast, post-depositional basins are characterized by a parallel-bedded infill which appears to reflect local preservation as a result of later faulting. These basins may be Late or post-Triassic in age. The Permo-Triassic rocks were deposited during a period of major regression, with fluviatile, lacustrine and aeolian environments predominating. Principal lithologies include sandstones, that are partly conglomeratic, intercalated with mudstones and claystones. The localized development of carbonates and evaporites indicates some marine influence. Palaeontological data are commonly sparse throughout the succession and generally cannot readily differentiate the two systems. There is limited evidence for both Early Permian and Late Triassic volcanism. The Early Permian volcanism was ‘within-plate’ and ‘continental’ in character, and was probably related to the first phase of post-Variscan continental disintegration. The Late Triassic volcanism indicates a subsequent episode of synrift igneous activity west of Britain and Ireland.