Late Permian to early Mesozoic rifting and sedimentation offshore NW Ireland

Abstract

A late Permian, Triassic and early Lias rift succession up to 1 km thick is revealed from offshore NW Ireland. Sediments are preserved in deep, narrow grabens from the Donegal Basin in the north, southwestwards through the Erris and Slyne Troughs, to a series of newly defined basins collectively forming the North Porcupine Basin, with a further structure, the ‘Galway Graben’ on the Porcupine Bank. This line of basins, superimposed on a Caledonoid fractured basement, underwent a complex structural evolution, but since the late Permian they developed primarily as a wrench-modified rift system. During the latest Lias, crustal doming west of Britain may have produced a regional angular unconformity and created a source for thick Middle Jurassic alluvial clastics deposited in the Main Porcupine Basin to the south. Early Tertiary transpression of the North Porcupine area caused late-stage fragmentation of the basin. The rift succession consists of continental red beds bounded by two dateable transgressive intervals. These intervals provide valuable stratigraphic control for constraining the timing and scale of seaways propagating across the fracturing Pangean supercontinent, from the late Permian to the early Jurassic. Evidence is also presented for an episode of syn-rift volcanism temporally unique to western Britain and Ireland.