The Variscan fold belt in Ireland

Abstract

The major geological features of Ireland south of Latitude 52°30' are described. Palaeofacies and isopachyte maps, combined with stratigraphic cross-sections, are used to trace the Late Palaeozoic development of the region. A thick red-bed sequence of Middle-Late Devonian age accumulated within an east-west trough, the Munster Basin, which was fault-controlled at its northern margin. The northward marine incursion across the region in late Devonian - early Carboniferous times is described. South of the Cork-Kenmare line (in the South Munster Basin) the dominantly Cork Beds sequence was developed and reflected a gradually deepening marine facies through Early Carboniferous time. Isopachyte data show the influence of an important intra-basinal positive element, the Glandre High, which effectively separated east and west depositional sub-basins. North of the Cork-Kenmare line the shelf area was dominated by carbonate deposition until the end of Early Carboniferous time. Turbidite deposition was a feature of Namurian deposition across the region, whilst evidence of the Westphalian is limited to the coal-bearing measures of Westphalian A in the small Kanturk Coalfield. The considerable control of structural styles exercised by bulk lithologies is demonstrated with the aid of structural cross-sections. Basement controls on both structural style and sedimentation are discussed, with particular reference to the Glandore High and the northern margin of the Munster Basin. Finally, the concept of a clearly-defined northern thrust front to the Variscan fold belt is examined and the conclusion reached that the supposed 'front' is better considered as a complex zone within which the northward diminution of tectonic intensity is affected by basement configuration, the thickness of sedimentary cover and the presence of older structural features.