New palynological data from the Leinster Lower Palaeozoic massif, southeastern Ireland

In southeastern Ireland, the tectonic evolution of the northwestern Avaloni an margin is rather poorly understood and only partially constrained. This is because the stratigraphy comprises in the main unfossiliferous turbidite s. Nevertheless, some authors have attempted to define 'terranes' and 'trac ts' in the region, aided by several significant Caledonoid-trending structu ral breaks that can be determined in the field. Palynological work was carr ied out in the early 1970s and established a range of ages from approximate ly mid Cambrian to late Silurian, although much of the succession proved ba rren. In the current study, the Lower Palaeozoic sequences W of the Leinste r Granite and in the Slievenamon Inlier to the S have been palynologically re-investigated. Previous work proposed an unbroken succession from lower O rdovician Ribband Group turbidites and volcanics younging westwards conform ably into upper Silurian Kilcullen Group sediments. The new study clearly s hows that the Ordovician palynomorphs in the older part of the Kilcullen Gr oup are reworked. In fact, there exists a major stratigraphical break betwe en the Ribband Group dated as early Ordovician, Arenig, and the Kilcullen G roup which is entirely Silurian, late Llandovery to early Wenlock in age. T his major break has a minimum strike length of 150 km and is most likely mu ch longer, extending some hundreds of kilometres SW to Dingle and possibly equating with a similar discontinuity in the Isle of Man to the NE. This br eak would thus appear to be a major feature within the succession of the no rthwestern Avalonian margin.