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Post-Ordovician Units |
1 Geological Survey of Ireland, Beggars Bush, Haddington Road, Dublin 4, Ireland
2 Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK
3 Department of Geology, University of Leicester, University Road, Leicester LE1 7RH, UK
Interpretation of the stratigraphy and structure of the Lower Palaeozoic Manx Group has been strongly influenced by the assumed equivalence of distinct sandstone sequences, the Niarbyl and Lonan Flags, exposed along the west and east coasts of the Isle of Man, respectively. However, new palaeontological evidence confirms an Arenig age for the Lonan Flags and indicates a mid-Silurian age for the Niarbyl Flags, thereby necessitating a complete revision of previous interpretations. The Niarbyl Flags are formally defined here as the Niarbyl Formation and are assigned to a new group, the Dalby Group. A previously unrecognized lithofacies in the formation, laminated hemipelagite, is also distinguished and described and this indicator of anoxic deposition is compared and contrasted with the Manx Group and other Lower Palaeozoic sequences. Combined turbidite facies, palaeocurrent and petrofacies analysis of the turbidite suite further serves to reinforce the distinction with the Manx Group, in particular palaeocurrent and petrofacies data that indicates provenance from a magmatic-arc source to the west-northwest. The turbidite sequence is interpreted as a mid-lower fan, slope-apron sequence of interlobe and sandy lobe packets. Finally, the regional correlation and significance of the formation in its Iapetus Ocean setting, as well as the significance of the Niarbyl Shear Zone that defines its lower contact with the Manx Group, is considered and compared.
