Ordovician and Silurian acritarch assemblages from the west Leinster and Slievenamon areas of southeast Ireland

The Lower Palaeozoic sequences west of the Leinster Granite and in the Slie venamon Inlier of southeast Ireland have been palynologically re-investigat ed. Most of the productive samples yielded sufficient identifiable acritarc hs for positive stratigraphical age determinations for several of the forma tions, The samples also include rare cryptospores, scolecodonts and tubular structures, Previous work in the area west of the Leinster Granite propose d an unbroken succession from Early Ordovician Ribband Group turbidites and volcanics passing up conformably to Early Ordovician to Late Silurian Kilc ullen Group. The new palynological data clearly show that the Kilcullen Gro up in this area is entirely Silurian (Llandovery-early Wenlock) in age, als o results obtained from the same group at Slievenamon confirm the previousl y reported Silurian age. Ordovician acritarchs found in the Kilcullen Group of both study areas are reworked and range in age from late Tremadoc to Ll anvirn. The new data reveal a major stratigraphic break between the Ribband Group dated as Early and Middle Ordovician and the Silurian Kilcullen Grou p. This major break extends some hundreds of kms southwest to the Dingle Pe ninsula and possibly equates with a similar discontinuity in the Isle of Ma n to the northeast. This break would thus appear to be a major feature with in the northwestern Avalonian margin sequence. (C) 2000 Elsevier Science B. V. All rights reserved.