Palaeozoic sedimentary sequence as a reflection of deep structure in southwest Ireland

Abstract

In County Cork, southwest Ireland, the Upper Devonian-Tournaisian succession is spectacularly thick (maximum approximately 8 km). A thick Viséan carbonate succession exists in the ground around Cork City and northward from there, but that apart, there is only a meagre representation of Carboniferous rocks younger than Tournaisian. Namurian and Westphalian are better developed in areas to the north and to the south of the locus of maximum thickness of Upper Devonian. Tuffisitic intrusions occur close to the trace of a large fracture in the Bantry Bay area. They have been suspected to include material derived from the mantle. The whole association is interpreted as a Palaeozoic example of an inversion structure. The inversion is attributed to heating arising from a lithospheric delamination occasioned by the collision which closed the Iapetus Ocean early in Devonian time. The possibly mantle-derived intrusions are set within the dome produced during the course of the inversion. Variscan folding took place during the development of the dome and certain irregularities in the pattern of outcrop of Variscan folds can be regarded as interference effects imposed on the folds by late continuation of the doming. Brief comments are appended on the means of dating the progress of the process of delamination, on the outworn notion of a “Variscan Front” and on the open question of the state of Palaeozoic rocks, whether metamorphic or non-metamorphic, in the Celtic Sea region. A strong causal relationship is demonstrated between the sequence of tectonic events and the regional facies associations. The patterns of megafacies from Upper Devonian to Westphalian are described and discussed in this context.