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Manx Group Stratigraphy and Lithofacies |
1 Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, UK
2 Geological Survey of Ireland, Beggars Bush, Haddington Road, Dublin 4, Ireland
3 Department of Geology, Oxford Brookes University, Gipsy Lane, Oxford OX3 0BP, UK
4 British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA, UK
5 Robertson Research International, Llanrhos, Llandudno, North Wales, LL30 1SA, UK
6 Department of Geology, University College, Belfield, Dublin 4, Ireland
7 Department of Geology, University of Portsmouth, Portsmouth PO1 3QL, UK
The existing lithostratigraphy of the Manx Group (lower Ordovician) of the Isle of Man is formalized as far as is possible given a paucity of biostratigraphical and structural control. Coherent packages of strata can be recognized within northeast striking structural tracts. Some tract boundaries are faulted but poor exposure leaves doubt about the nature of others. Some structural repetition of successions is likely. However, the long-held equivalence of the sandstone units on either side of the island can no longer be maintained, because the Niarbyl Formation in the west is now known to be of Silurian age. Repetition of Manx Group units across a syncline along the axis of the island is therefore in doubt. Different outcrops of superficially similar units are accordingly named separately on each flank of the island. The stratigraphy of part of the northern Manx Group is left unrevised awaiting further work.
The southeastern, sandstone-dominated units of the Manx Group comprise three tracts divided into the Lonan and Santon (lower Arenig) Formations, the Port Erin and Mull Hill Formations, and the Ny Garvain and Creg Agneash Formations. The Creg Agneash Formation is stratigraphically overlain by the mudstone-dominated Maughold Formation. The next two tracts to the northwest both contain a black mudstone unit, the possibly correlative Barrule and Glen Rushen (middle Arenig) Formations, each structurally overlain by laminated silty mudstone of the Injebreck Formation. A further unit of laminated siltstone and mudstone, the Creggan Mooar Formation, is defined in tract 6, west of which an enigmatic unit contains lowest Arenig fragmental volcanic rocks. In the most northwesterly tract, the Lady Port Formation is a varied assemblage of black mudstone, turbidites, pebbly mudstone and quartz arenite. Lithological correlation of these different Manx Group successions is attempted, but is constrained by only four biostratigraphical control points.
The Manx Group stratigraphy has long been correlated with that of the Skiddaw Group of the Lake District. A tentative correlation of events on this part of the early Ordovician margin of Gondwana can be made assuming some synchroneity of Early Arenig lowstand turbidite fans, of a mid-Arenig transgression, and of a late Arenig mass-wasting event.
