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.

On the deformation sequence in the New Harbour Group of Holy Island, Anglesey, North Wales

“…The sequence of deformation events in the SSG has been described by Greenly (1930), Barber and Max (1979), Cosgrove (1980), Phillips (1991b) and Roper (1992). Lisle (1988) introduced a nomenclature scheme for deformation events in the SSG using letter subscripts (D XYY etc.)…”

“…Barber and Max (1979) and Barber et al (1981) have argued that the sequence of events was dierent in the SSG and NHG, concluding that the contact between the two groups was an important thrust. The most obvious major features of the deformation are two kilometrescale bedding folds known as the Penrhyn Mawr antiform (PMA) and the Rhoscolyn antiform (RA) (Cosgrove 1980;Phillips 1991b).…”

“…Whilst some workers have argued for late Precambrian deformation events (Thorpe et al 1984;Gibbons and Hora k 1990), others have argued that the deformation is of Lower Palaeozoic age (Barber and Max 1979). At least two deformation events are constrained to be pre-Arenig by the presence of clasts of deformed NHG containing a folded schistosity in Arenig conglomerates (Barber et al 1981).…”

“…If the Gwna Group is of Lower Cambrian age then at least two deformation events must be Cambrian or earliest Ordovician. However, if the SSG, NHG and Gwna Group are not part of a continuous sequence and do not share the same tectonic history (Barber and Max, 1979), then some events in the SSG and NHG could be Precambrian.…”

Geophysical framework and the Appalachian-Caledonide connection

“…The deep seismic reflection profile WINCH (Brewer et al 1983) shows a series of reflectors with a northwestward component of dip whose uppermost surface reaches the sea-floor at the southernmost of the two suggested boundaries. The Menai Straits line (Barber & Max 1979), which is regarded as the boundary between the stable craton margin and the imbricated marginal zone, may be the onshore continuation of the WINCH reflector.…”