The structure of the Lower Palaeozoic rocks of the Arklow district (Ireland) ¹

Abstract

Summary The Lower Palaeozoic succession consists mostly of slates but contains a large volcanic series, apparently of Bala age. This is composed essentially of rhyolitic pyroclasts but contains some basic lavas in parts. It reaches a thickness of more than 8000 feet of virtually uninterrupted volcanic rocks in the north-east, but fades out rapidly towards the west. The basic structure of the area is a fold of synclinal type. In the north-east this is sharply overturned towards the north-west, but it changes south-westwards to a shallow, symmetrical synclinorium. This fold has been cut through by numerous thrust-planes with displacements averaging two to three miles and by many high-angle faults, some of considerable dimensions. Evidence on the relative ages of intrusions and faults is abundant and allows the complex Caledonian history to be largely deciphered. The initial formation of the synclinorium was succeeded by numerous phases of faulting and a variety of igneous intrusions. Metasomatic events and periods of mineralization are also found in this history.

The Pre-Cambrian Rocks of Southern Co. Wicklow (Ireland)

“…The main fault structures are found to be in accordance with those of the Arklow district (Tremlett, 1959), and, as in that area, faults have a considerable influence on the surface distribution of the rocks. They belong to four different sets, which are, in order of formation, low-angle thrusts to the S.E.…”

“…It should be noted that the deformation shown by the Knockrath strata, the general dynamic metamorphism of the contained siltstone bands, and the occasional alteration of arenaceous beds of the Bray Series to thick quartzites in parts of the folds where the rocks suffered maximum stress, far exceed any folding or dynamic metamorphism of similar Lower Palaeozoic sediments achieved by the Caledonian orogenesis in this district or the adjacent Arklow district (Tremlett, 1959).…”

“…They have yielded no fossils from the north-western flanks of the synclinorium of the Arklow district, but in the south-eastern limb similar sediments at a similar distance beneath the thick volcanic series of the district, have yielded Arenig graptolites (Elles, 1910, Tremlett, 1959.…”

“…There are not sufficient distinctive horizons in the upper part of the Clara Series to allow definite determination of its relationship to the Dunganstown Sandstones, but in the Glenealy district the persistence for over a mile of mauve slates immediately beneath the sandstones suggests a conformable relationship. It should be noted, however, that along the south-eastern flanks of the synclinorium of the Arklow district, about 25 miles to the south, the sandstones appear to have been deposited directly on the Bray Series (Tremlett, 1959), having completely overlapped or overstepped the Clara Series, while still further south they lie unconformably on the Rosslare Migmatite Series (Baker, 1955, p. 65).…”

The Caledonian history of the north-eastern Irish Sea region and its relation to surrounding areas

“…In south-east Ireland, however, there is no record of any angular discordance between the Caradocian and Lower Ordovician strata (Tremlett 19596) suggesting that strong intra-Lower Ordovician folding is absent. Similarly in the Murrisk district of western Ireland, a conspicuous overstep at the base of the Upper Llandovery, within a thick Ordovician-Silurian sequence, is attributed by Dewey (1963, p. 339) to end-Ordovician (Taconic) concentric flexing and uplift.…”