A New Look at the Mona Complex (Anglesey, North Wales)
Total Page：16
File Type：pdf, Size：1020Kb
J. geol. Soc. London, Vol. 136, 1979, pp. 407432, 8 figs. Printed in Northern Ireland. A new look at the Mona Complex (Anglesey, North Wales) A. J. Barber & M. D. Max SUMMARY: 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 Unitof 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 Tu&, Skerries and Gwna Groups of Greenly’s succession, as wellas greywackes and slates, correlatedby Greenly withthe 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 FydlynGroup does not form part of the MonaComplex but may be correlated with the Caradocian volcanics of Parys Mountain. No major structural discordance occurs between the Gwna Group and the overlying Ordovician rockson the N coast. The time gap represented by this contact has also been reduced by recent fossil finds, which indicate that some membersof 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 areof 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. Greenly (1919, 1920; Institute of Geological Sciences (Hsii 1968). In Greenly’s interpretation, the degree of 1972) divided the Mona Complex of Anglesey into 3 metamorphism increased south-eastwards, until in the major units: 1, Gneisses; 2,Bedded Succession; 3, central and southern parts of Anglesey the Bedded Coedana granite and its related hornfels. He regarded Successionpassed into the Penmynydd Zone of the gneisses as the oldest unit, representing an early metamorphism. The final stage in the development of Precambrianbasement complex. The basement is the Mona Complex was the intrusion of the Coedana overlainunconformably by theBedded Succession, granite, hornfelsingthe gneisses and rocks of the although no unconformable contacts are to be seen. Bedded Succession within the Penmynydd Zone into The Bedded Succession wasclassified by Greenly which it was intruded. into 6 units (Figs. 1, 2~):1, Fydlyn Group; 2, Gwna Greenlyproposed that all theseevents occurred Group; 3, Skerries Group (including Church Bay Tuffs during the Precambrian, on the grounds that on the and the Tyfry Beds); 4, New Harbour Group; 5, South adjacent mainland Cambrian rocks rest on the unde- Stack Series; 6, Holyhead Quartzite. Of these units, formed late Precambrian Bangor Volcanic Series, and the Fydlyn Group (1) was regarded by Greenly as the are overlapped by the Ordovician, which everywhere oldestunit and the Holyhead Quartzite (6) as the in Anglesey rests unconformably on the Mona Com- youngest. Greenly was concerned to demonstrate that plex. According to Greenly the Cambrian Period was theBedded Succession constituted continuousa not represented in Anglesey, except perhaps by a few stratigraphic sequence, quoting frequent examples of scattered remnants emerging from beneath the edgeof interbedding of the units adjacent to their contacts. the Ordovician outcrop in the SE. The order of age proposed by Greenly was based on Shackleton (1954a, b, 1956)modified Greenly’s t,he occurrence of the Fydlyn Group adjacent to the synthesis in severalimportant respects. Shackleton gneisses, regarded as the underlying basement, and the (1956)interpreted Greenly’s ‘basal gneisses’as the presence of pebbles of lithological types characteristic product of the metamorphism of the Bedded Succes- of oldermembers of thesequence in progressively sion, which passed with increasing metamorphic grade younger members. through the Penmynydd Zoneinto the migmatized Following the deposition of the Bedded Succession, gneisses. He also proposed that the generation of the Greenly proposed that the complex was recumbently Coedana granite, and the analogous Sarn granite on foldedand metamorphosed. A peculiar and charac- Lleyn, represented the culmination of this metamor- teristic feature of the deformation was the formation phic process. These granites have been dated isotopi- of an ‘autoclastic melange’,a form of large scale cally at 615-633 Ma (Moorbath & Shackleton 1966), boudinage in some members of the Bedded Succes- confirmingthe Precambrian age of graniteintrusion sion. Greenly’sdescription of theserocks has been and of all the earlier events in the complex. takenas the classic example of atectonic melange Shackleton(1969), using way-up criteria, inverted 0016-7649/79/0700-0407$02.00 @ The Geological Society Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/136/4/407/4885859/gsjgs.136.4.0407.pdf by guest on 28 September 2021 408 A. J. Barber & M. D. Max FIG. 1. Generalized geology and location map of Anglesey, after Greenly (1920) & I.G.S. (1972), reproduced by permission of the Director, Institute of Geological Sciences. In the key: 1, South Stack Tectonic Unit; 2,New Harbour Tectonic Unit; 3, Cemlyn Tectonic Unit. On the map: m, area of mylonite; h, hornfels related to the Coedana granite; g, glaucophane schists (distribution from Nataraj 1967). The figures around the margin relate to the National Grid at 10 km intervals. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/136/4/407/4885859/gsjgs.136.4.0407.pdf by guest on 28 September 2021 A new look at the Mona Complex (Anglesey, North Wales) 409 B TECTONIC UNITS OROGENY AN - SlLURlAN A SHALES(LLANDOVERY) VOLCANICS(CARADOCIAN) CALEDONIANOROGENY ORDOVICIAN SHALES(LLANVIRN) SlLURlAN GRITSIARENIG) SHALES.GRITS UNCONFORMITY ORDOVICIAN CEMLYN GWNAMELANGE (2) CAMBRIAN UNIT '-:.: i ;.'-.'..:..: :. SKERRIES GREYWACKE F].. .'; .:.,' GRITS S 1 . .. '. .. .. .. .. SHALES GROUP .:...zi.r...;:.:.:. GWNAGROUP ? OlSCOROANCE - THRUST OR UNCONFORMITY CHURCHBAY TUFFS 1 SKERRIES GROUP TYFRYBEDS LATE I PRECAMBRIAN NEW HARBOURGROUP I STRUCTURALDISCORDANCE - THRUST - ? I n RHOSCOLYN FM. Y SOUTH SOUTH HOLY D I STACK STACK HOLYHEAD ISLAND W QUARTZITEFM. m GROUP UNIT I GROUP SOUTHSTACK I FM AN OROGENIESAN ? 11 OROGENY GNEISSES - EVERY!WHERE GNEISSES IN FAULT OR MYLONITE CONTACT FIG 2. Stratigraphical and structural sequences in the Mona Complex. A, Current synthesis as proposed by Greenly, with modifications by Shackleton, Baker, etc. B, Revised sequence proposed in the present paper. Greenly's stratigraphic sequence in the Bedded Suc- The structure of Anglesey cession (Fig. 2~).He proposed that the South Stack Beds, with which the Holyhead Quartzite was inter- Anglesey is divided into 3 structural regions by major bedded, was the oldest member of the succession and tectonic lineaments; these units and the relationships the Fydlyn Group was the youngest. Shackleton used of Anglesey to the adjacent mainland are illustrated in the revised succession to support his concept that the Fig. 3. gneisses had been formed by the metamorphism of the Bedded Succession, pointing outthat the gneisses Northern Reon occurnext to the youngest member of the Bedded Succession, rather than the oldest, as should be the This occupies the NW part of the island including case if the gneisses represented the crystalline base- HolyIsland, incorporates outcrops of gneiss in the ment. Gader and Mynachdy inliers S of Carmel Head, but is Both Shackleton and Greenly recognized that the composed mainly of Greenly's Bedded Succession. It Mona Complex had been severely modified during the contains most of the type localities for the members of Caledonianorogeny, as Ordovician and Lower the succession as defined by Greenly (Fig. 1). Silurian rocks overlying the complex are folded, The structureof the northern region was interpreted cleaved, highly faulted and in places overthrust. They by Greenly as made up of 2 major recumbent folds, suggested that deformation and low grade the Rhoscolyn Fold and the Holyhead Fold, gaping to metamorphismduring the Caledonianorogeny was the NW and closing to the SE (Greenly 1919, fig. 35). likely to have had a retrogressive effect on the higher The lowermost of these folds, the Rhoscolyn Fold, is grade metamorphic rocks of the complex. exposed in thesouthern part of Holy Island; the 2 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/136/4/407/4885859/gsjgs.136.4.0407.pdf by guest on 28 September 2021 410 A. J. Barber & M. D. Max I I I I I I 1c 20 40 60 Northern Region , POINT .,&' -80 - -60 - - 40 ORDOVlClAN BANGOR VOLCANIC IMBRICATE ZONE SERIES FIG. 3. The Mona Complex in Anglesey and Lleyn and its relationships to the Precambrian and Cambrian rocks of North Wales. overlying Holyhead Fold is brought down to the NE are upward facing anticlines with no major inversions by the major NW-SSE faultsseen in HolyIsland compatible with the large scale recumbent folds post- (Figs. 1, 6). The main outcrop of the New Harbour ulated by Greenly. Group, with the othermembers of the Bedded Succes- We have found that the structure of the northern sion in northern Anglesey, forms the upper limb of the part of Angleseycan beinterpreted in terms of 3 Holyhead Fold (Greenly 1919, fig. 100). superimposed tectonic units (Fig. 2~).The lowermost Usingway-up criteria, Shackleton (1969) demon- of these units, the South Stack Unit, was apparently strated that both the Rhoscolyn and Holyhead Folds undeformed and unmetamorphosedbefore it was Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/136/4/407/4885859/gsjgs.136.4.0407.pdf by guest on 28 September 2021 A new look at the Mona Complex (Anglesey, North Wales) 411 overthrust by the overlying New ‘Harbour Unit. On p.