A new look at the Mona Complex (Anglesey, North Wales)

Abstract

Re-examination of the Mona Complex (Anglesey, North Wales) has led to a radical re-interpretation of its stratigraphy, structure and geological history. The gneisses are regarded as representatives of an earlier continental basement, as Greenly (1919) originally proposed. In the northern part of Anglesey the Bedded Succession can be separated into 3 structural units. The South Stack Unit of Holy Island is overlain by the more highly deformed New Harbour Unit; the contact between these two units is interpreted as a thrust plane. A third unit, the Cemlyn Unit, outcropping in the NW, comprises the Church Bay Tuffs, Skerries and Gwna Groups of Greenly's succession, as well as greywackes and slates, correlated by Greenly with the New Harbour Group. However, the Cemlyn Unit is less deformed than the New Harbour Unit and their relationships may be unconformable. It is proposed that Greenly's Fydlyn Group does not form part of the Mona Complex but may be correlated with the Caradocian volcanics of Parys Mountain. No major structural discordance occurs between the Gwna Group and the overlying Ordovician rocks on the N coast. The time gap represented by this contact has also been reduced by recent fossil finds, which indicate that some members of the Mona Complex are of Cambrian Age. The implications of these discoveries are that there was no 'Irish Sea land mass' in the Anglesey area in the Lower Palaeozoic and that major structures in the complex previously attributed to a late Precambrian orogeny are of Caledonian age. Using the new structural and stratigraphic data, a synthesis of the sedimentary, tectonic and structural evolution of the complex is proposed, in the context of plate tectonics.