Stratigraphy in an accretionary prism: the Ordovician rocks in North Down, Ireland

Abstract

ABSTRACTSediments, mainly sandstones, conglomerates and shales, accumulated in small turbidite fans along the northern arc–trench margin of the Iapetus Ocean from middle Ordovician to Silurian time. These fans, together with the underlying pelagic facies and part of the oceanic crust, were sliced and accreted northward resulting in the Lower Palaeozoic accretionary prism which forms the Scottish Southern Uplands and the Longford-Down inlier in Ireland. North Down is the continuation of the Northern belt of the Southern Uplands of Scotland into Ireland, bounded to the S by the Orlock Bridge fault. Lithological and petrographical comparison with the rest of the Northern belt indicates closer affinities with the Southern Uplands of Scotland than with the western end of the Longford-Down inlier. Major ENE—WSW-trending Caledonian strike faults define five blocks, in which new formations of Caradoc and ? Ashgill age are defined. Pillowed spilitic rock, interpreted as a fragment of the ocean-floor, is only recognised in the Ballygrot block. Pelagic and hemipelagic black shales and cherts are overlain by arenaceous sediments in all blocks.