J. geol. Soc. London, Vol. 136, 1979, pp. 407432, 8 figs. Printed in Northern Ireland.

A new look at the Mona Complex (, 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 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 ‘ 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, centraland southern parts of Anglesey theBedded granite and its related hornfels. He regarded Successionpassed into the 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 TheBedded 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, . 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

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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 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

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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

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overthrust by theoverlying New ‘Harbour Unit. On p. 110; 1920) distinguished as the Penmynydd Zone of the other hand, the New Harbour Unit was already metamorphism. As Greenly (1919, p. 161) madeclear, foliated, lineated and folded before being emplacedin the Penmynydd Zone is a metamorphic and structural, its present position above the South Stack Unit. The rather than a lithological unit; he suggested that the uppermost unit, the CemlynUnit, composed of the rocks of thePenmynydd Zoneare a highergrade ChurchBay Tuffs, the Skerries Group,part of metamorphic modification of the Bedded Succession, Greenly’s New Harbour Group and the Gwna Group, and pointed to the association of quartzite-limestone- exposed in the NW part of the island, is considerably graphite schist amongthe rocks of boththe Pen- less deformed and probably rests unconformably on mynydd Zone and the Bedded Succession. Shackleton the New Harbour Unit. (1969, p. 12) extended the proposition by observing The SE margin of the Northern Region is marked that the so-called ‘triple group’ occurs also within the by amajor mylonite belt, extending from SW-NE gneisses(Greenly 1919, p. 327), andused this to across the island from to Point Lynas (Fig. support his contention that the Penmynydd Zone rep- 1). This mylonite belt is intimately related to the New resentsan intermediate stage in themetamorphic Harbour Group with which it was mapped by Greenly transition from unaltered rocks of the Bedded Succes- (1920),and shows the same foliar, linearand fold sion into gneisses. structures.Mylonites belonging to this zoneare ex- It is evidentthat Greenly did not make a clear posed on the foreshore at Rhosneigr (Nat. Grid Ref. distinctionbetween his ‘basal gneisses’ andgneisses 304735), and may be traced NE through Llantrisant which he includedwithin the Penmynydd Zone of (350840),where a substantial area depicted as New metamorphism. The Penmynydd Zone of the central Harbour Group on Greenly’s (1920) map is mylonite, region is regarded by us as a composite unit consisting to mylonites shown as Gwna greenschists in Mynydd partly of gneisses analogous to the basement gneisses Eilian (456919) S of . and partly of younger metasediments metamorphosed The mylonites appear to have been formed from the in the amphibiolite facies, but which do not show the gneisses of theCentral Region and attained their migmatization evident in the gneisses; at high states of present position prior to the deposition of the Ordovi- deformation this distinction is difficult. At our present cian rocks of the principal area which extend across state of understanding, the possibility that more highly the mylonite belt. On the other hand, in the Gynfor metamorphosed representatives of the Bedded Succes- outliers on the N coast of Anglesey, where the Or- sion are incorporated in the Penmynydd Zone of the dovician rests on rocks of theCemlyn Unit (e.g. central region cannot be excluded. 378948), there is no major structural discordance be- S of the Penmynydd Zone is an extensive outcrop of tween the rocks of the Mona Complex and the overly- rocks of the Gwna Group (Fig. l), the type area of the ing Ordovician. Both rock groups are found to have groupnamed after the valley of the river Gwna only one main cleavage and the Gwna melange matrix (Greenly1919, p. 63).As in thenorthern part of in this region has yielded Cambrian fossils (Muir et al. Anglesey, the Gwna Group contains blocks of a vari- 1979). ety of sedimentary materials including quartzite and The northern region is disrupted by a major E-W limestone in a fine-grained matrix. Extensive outcrops zone of faulting and thrusting which includes the Car- of pillow lavasalso occur in this areaas at Engan me1 Head Thrust (Greedy 1919,pp. 541-9). This fault (Engan Spilitic Formation, Shackleton 1975, table 5) zone brings to the surface the gneissose basement in and Llanddwyn Island (Llanddwyn Formation, Shack- the Gader and Mynachdy inliers, and represents the leton, op. cit.), S of Malldraeth Sands. reactivation of a zone of faulting and uplift which, as At Braich-Lwyd the matrix of the Gwna melange is Bates (1972, pp. 56-7) has demonstrated, operated in apurple slate in a low state of deformationand Caradocian times. Bates (op. cit.) estimated that hori- metamorphism. The contact with the rocks of the zontalmovement along the CarmelHead Thrust Penmynydd Zone is exposed just to the N of the point amounted to no more than a few kilometres. (338674). The rock type adjacent to the contact is a ‘flaser undulose mica-schist’ (Greenly 1919, p. 340), a Central Region cataclastic rock type familiar from the Moine Thrust This extends SE from the southern margin of the Zone of NW Scotland,where it is termed ‘frilled mylonite belt to the Berw Fault (Greenly 1920) be- schist’ or ‘Oyster-shellRock’ (Peach et al. 1907). In tween Llanddwyn and Red Wharf Bay. It includes the the Moine Thrust Zone this rock type is interpreted as main outcrop of the gneisses and also the Nebo inlier formed by themylonitization of Lewisian gneisses. in the NE. The gneisses are intruded by the Coedana The mylonites at Braich-Lwyd have also presumably Granite, and have been hornfelsed by the intrusion. been derived from gneisses. The mylonitic foliation is At Hen-shop(395745) hornfels has been produced complexlyfolded, in contrast to the steeplydipping from rocks which had previously been mylonitized. slaty cleavage of theGwna Group;the contactbe- S of the Coedana Granite and the main gneissbelt is tween the Penmynydd Zoneand the Gwna Group the zone of metamorphic rocks which Greenly (1919, must therefore be faulted at this locality.

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The relationships seen at Braich-Lwyd suggest that the rocks of the Penmynydd Zone werealready in theirpresent metamorphic state as mylonitized am- phibolite facies gneisses before the Gv:na Group was deposited and that the relationships between the two groups is essentially an unconformable one. In the N of the central region, Ordovician rocks of the principal area rest, with marked unconformity and a well-developed basal conglomerate, on the gneisses and the Coedana Granite and its hornfels. To the SE, in the area, the Ordovician rocks without a basal conglomerate rest, apparently unconformably, on rocks of the Penmynydd Zone. Exposure is poor in this area and most of the contacts appear to be thrusts or faults (Greenly 1919, p. 437). Direct relationships between the Ordovician and the Gwna Group are not seen in the central region as the two groups are not in contact, although they approach very closely in poorly exposed groundSE of Llangwyllog. However, both groups are distinguished from the rocks of the Pen- mynydd Zone by showing only one main cleavage and, as in the northern region, the discovery of Cambrian FIG. 4. Geological map of Penrhyn Nefyn, Lleyn, fossils in jasper cherts within the pillow lavas of to show the relationships between the gneiss and Llanddwyn Island (Muir e? al. 1979) means that the the Bedded Succession. time gap between the Gwna Group and thebasal beds of the Ordovician(Arenig, Greenly 1919, p. 437) is not as great as Greenly supposed. Zone,are severaloccurrences of amphibolite facies The main deformation of the Gwna Group and the gneisses, both hornhlendicand migmatitic quartzo- Ordovician took place before Lower Old Red Sand- feldspathic gneisses, well exposed in thequarry at stone times, as the unconformable base of the Lower Holland Arms (480728).Greenly (1919, p. 255) Old Red Sandstone oversteps both units in the NE suggested that these rocks were analogous to the basal part of the region (Fig. 1). gneisses seen elsewherein Anglesey. These occurr- Thesouthern margin of the central region is the ences of gneiss form pods several kilometres in length Berw Fault Zone, extending SE from Red Wharf Bay, which are separated from the rest of the region by nearPentraeth, S of MalldraethMarsh to New- zones of mylonite, seen in the fields to the S of the borough Warren (Fig. 1). Although the Berw Fault has quarry at Holland Arms (478726). clearly moved since Carboniferous times, the zone is The rest of the region consists of lowergrade also marked by intense mylonitization (e.g. at 533791 metamorphic rocks, mainly mica-schists, which include nearRed Wharf Bay andat 415653 near New- lenticular masses of amphibolite. Some of these am- borough). Further to the SW, across Caernarvon Bay, phibolites, in abelt which extendsnorthwards from the Berw Fault Zone is continued in Lleyn (Fig. 3), the Marquess of Anglesey’s Monument (535715), con- whereat Penrhyn Nevin (296410) tuffs and pillow tainglaucophane (Fig. 1). These glaucophane schists lavas of the Gwna Group are in faulted contact with are strongly foliated,lineated and folded, and were mylonitized gneisses (Fig. 4). A short distance to the interpreted by Thorpe (1972) on geochemical grounds W of this contact the classic pillow lavas of Dinllaen as having originated as ocean-floor basalts. (2741), with a steepdip and younging to theSE, Greenly (1919, 1920) correlated the eastern part of continuethe trend of the similarly oriented pillow the Aethwy Region, where the rocks contain blocks of lavas of Llanddwyn Island (Fig. 3). quartziteand limestone, with the Gwna Group. Greenly’s (1920) map suggests that there is a struc- Aethwy Region tural discordance, either a thrust or an unconformity, between the rocks of the Penmynydd Zone and the This lies between the BerwFault Zoneand the Gwna Group. On the otherhand, his synthetic map of MenaiStrait. The type locality of the Penmynydd foliation trends (Greenly 1919, fig. 101) suggests that Zone of metamorphism lies within this region (Fig. 1) there is no structural discordance at this boundary. so thatGreenly (1919, 1920) correlated the Nataraj (1967) divided the rocks of the Aethwy metamorphic rocks with the amphibolite facies rocks Region into a number of lithostratigraphic units, the of the central region. Indeed, along the northern mar- sequence being broken up by slides which resulted in gin of the AethG Region, adjacent to the Berw Fault northward attenuation and overstepping of some of

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the units. In the E (Greenly’s Gwna Group) the rocks Complex and the Ordovician in the Aethwy Region are practically unmetamorphosed, but the metamor- are not clear; all the boundaries appear to be faulted phic grade increases westwards through greenschists to (Greenly 1920; Bates 1972, fig. 11). The distribution epidote amphiboliteand muscovite-epidote-garnet of the Ordovician rocks,extending along the Berw schists. Glaucophane and lawsonite are developed Fault Zone,the S shore of Red Wharf Bay, south- over part of the area (Fig. l), showing that metamor- wards to and along the shores of the Menai phicconditions corresponding tothe glaucophane- Strait near Garth Ferry (578739), suggests that Or- lawsonite facies werereached. The area showing dovician rocks once overlay the whole region and cut glaucophane schist metamorphism extends across the across the structural and metamorphic trends of the boundaries of the lithostratigraphic units. According Mona Complex. The rocks of the Mona Complex in toNataraj the boundarybetween the Penmynydd the Aethwy Region, including the Gwna melange, Zone and Gwna Group on Greenly’s (1920) map does must have been deformed and metamorphosed before notcorrespond with any lithological, structural or Ordovician times. This conclusion conflicts with the metamorphic boundary. He regarded the Aethwy Re- evidence seen elsewhere in Anglesey that the rocks of gion as a single metamorphic unit in which the grade the Gwna Group weredeformed forthe first time increases progressively from E to W. together with the Ordovician in post-Ordovician Zones of mylonite are very common in the Aethwy times. Either the melange beds in the Aethwy Region Region. Acid and basic mylonites, closely resembling are of a different age to the melanges of the Gwna thosederived from Lewisian gneiss in the Moine Group elsewhere, or there is astructural break be- Thrust Zone, are exposed on the foreshore at Church tween the schists of the Penmynydd Zone and the Island (552717). The rocks are also much disrupted by Gwna Group which was not recognised by Nataraj late-stage low-angle faulting. (1967). In either case theserelationships require Metamorphic rocks analogous to those in the further investigation. Aethwy Region do not outcrop elsewhere in the vicin- ity of Anglesey. In Lleyn, along the strike to the SW, rocks of the Aethwy Region are replaced by the Gwna Group of the Central Region (Fig. 3). In the eastern Proposed revision of the stratigaphy part of the Aethwy Region the strike of the ‘Gwna and structure of the Mona Complex Group’ (Greenly 1919, fig. 101) suggests that these rocks should extend southwards into the Bangor area Gneisses to the SE of the Menai Strait; rocks exposed here belong tothe Bangor Volcanic Series, passing up Outcrops of gneiss were mapped by Greenly (1920) without a break into the overlying Cambrian (Wood in the northern, central and southern regions of Ang- 1969), followed by the Ordovician. It is possible that lesey (Fig. 1). Similar gneisses were described by Mat- metamorphic rocks continuous with those of the ley (1928) from the Lleyn peninsula to the SW (Fig. Aethwy Region underlie the Bangor Volcanic Series. 3). Greenly (1919, pp. 128-4-3) recognized both ortho- Onthe other hand, the extensive development of and para-gneiss types. Orthogneisses include basic mylonites in the neighbourhood of the Menai Strait amphibolites and hornblende gneiss, and acid suggests that transcurrent faulting may have been im- granodioritic and tonalitic gneiss. Paragneisses include portant in the tectonics of the region, so that there psammites, micaceous gneiss containing sillimanite, may be no continuity between the rocks on either side andforsterite marble. The gneisses frequently show of the Strait. concordant or cross-cutting pegmatiticsegregations In the Aethwy Region, Greenly (1919, pp. 389-99) and veins, which may be outlined by biotite-rich sel- described anumber of remnants of rocks which he vages (see Greenly 1923, figs. 4, 5 & 6). Quartzo- interpreted as possibly of Cambrianage, protruding feldspathic veining in basic gneisses may break up the from beneath the margin of the Ordovician outcrop. rock into angular blocks to form agmatites. In fact the The largest of these occurs at Baron Hill (5975) near gneisses show many of the characteristics of a typical Beaumaris. Volcaniclastic sediments here, and an ig- basement migmatite complex. nimbrite at Bwlch Gwyn (485732) near Holland Arms Wherever we have examined outcrops of the gneiss- (Wood 1969), have been correlated by both Greenly es they show evidence of a complex structuraland and Wood with the late Precambrian/Cambrian Arvo- metamorphic history (see also Baker in discussion of nian Volcanic Series of Bangor on the mainland on the Shackleton 1969, p. 20). The composite gneissose SE side of the Menai Strait. A further series of out- banding is generally folded by at least one later fold crops of presumedCambrian age, the Careg-Onen phase accompanied by a penetrative axial planar schis- Beds of Greenly (1919, pp. 393-8), follow the margin tosity. The banding may also be cut by a variety of of the Ordovician outcrop to the S of Red Wharf Bay intrusions, ranging in composition from acid through (581820, 545795). intermediate to basic. At Treferwyd (373793), an The relationshipsbetween the rocks of the Mona amphibolitedyke cuts across the gneissose banding

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/136/4/407/4885859/gsjgs.136.4.0407.pdf by guest on 28 September 2021 414 A. J. Barber & M. D. Max but contains a penetrative schistosity which passes out From our study of the stratigraphy, structureand into the surrounding gneisses. sedimentology of the northern part of Anglesey, we The presence of migmatitic structures with evidence have found that the Bedded Succession can be divided of an extensive structural and metamorphic history in into 3 tectono-sedimentary units, separated by 2 the gneisses, reminiscent of the Lewisian Gneiss of the major structural discordances (Fig. 26). NWHighlands of Scotland,led Greenlyand other earlyworkers onthe complex to proposethat the South Stack Tectonic Unit gneisses represented an earlier high grade metamor- Thistectono-sedimentary unit includes theSouth phic basement upon which the Bedded Succession had Stack Beds and HolyheadQuartzite of Holy Island been unconfonnablydeposited. Unfortunately, as (Greenly 1919, pp. 40-5) as well as the Rhoscolyn Beds Greenly (1919, p. 161)pointed out, no unconformable in thesouthern part of Holy Island (Shackleton contact is seen between the gneisses and the Bedded 1969). It excludes, however, theCoeden Beds of Succession anywhere on Anglesey. in central N Anglesey, correlated by The absence of an unconformable contact and the Greenly (1919, p. 45) with the South Stack Series. presence of an apparent metamorphic transition be- Bedded greywacke sandstones are dominant, alter- tween the Bedded Succession andthe gneisses in nating with siltstones, and with intercalated massive Lleyn,led Shackleton (1969) to propose thatthe quartzite units, such as that which formsHolyhead Bedded Succession passed, by an increase in Mountain. The bedding is folded into large scale up- metamorphic grade, through the schists of the Pen- right folds (main phase folds) on steeply dipping axial mynydd Zone into migmatitic gneisses. We have re- planes, dramatically exposed in the cliffs atSouth examined theshore section at Penrhyn Nefyn Stack. The fold axes are oriented SW-NE and gener- (296410) in Lleyn where the transition was described ally plunge at an angle of a few degrees to the NE. A (Fig. 4). Nearthe jetty are gneisses containing spaced cleavage is developed sub-parallel to the axial quartzo-feldspathic(granodioritic) segregations, planes of the folds and is refracted at the boundaries broken up by shear surfaces into lenticularpods. A of the beds. The rocks arenot, however, greatly few metres to the W the gneisses pass into mylonitic internallydeformed, and clastic sedimentarygrains schists with a strong horizontal rodding lineation. The still frequently show their original dimensions. mylonitescontain a few residualpods of quartzo- Sedimentaryway-up structures, such asgraded and feldspathicmaterial. These mylonitic rocks have cross-bedding and load-casts,provide evidence that clearly been derived from the gneisses. There is then the structures are upward facing with respect to the an abrupt change to slightly foliatedlaminated tuffs main cleavage. Cleavage is also the dominant structure and then spilitic lavas with jasper cherts, resembling in the massive quartzite of , where members of the Bedded Succession in Anglesey. Al- bedding and way-up structures are not clear. though the strike of the foliation in the mylonites and S of South Stack the folds become slightly over- the tuffs is roughly parallel, thedip is in opposite turnedand the cleavage dips steeply tothe NW. directions (Fig. 4), suggesting thatthe contact is a Inversion in the steep limbs of the folds can be easily fault. We interpret the transition at Penrhyn Nefyn as detected by way-upstructures and the relationships due to cataclasis and recrystallization of the gneisses in between bedding and cleavage. The area immediately a marginala shear zone. Similar conclusions were around South Stack itself appears to be the hinge zone reached by Baker (1969, pp. 254-5) from a study of all of a major fold, while the area to the S, towards the the schist/gneiss junctions in Lleyn. contact with the New Harbour Group, represents the In Anglesey as in Lleyn, all the exposedcontacts inverted limb of this major structure (Fig. 5). betweenthe gneisses andmembers of theBedded The main phase folds and associated structures are Succession are faulted, thrust or marked by mylonite modified by a second phase of reclined folding around zones.From this study we have concluded that horizontal fold axes and sub-horizontal axial planes. Greenly’s interpretation of the gneisses is correct. The second folds deform the earlier axial plane cleav- Nowhere is there a progressive metamorphic transition age and develop their own axial planarcrenulation fromthe Bedded Succession through schists into cleavage, with associated pressure solution effects. gneisses as Shackleton (1969) proposed. All outcrops Two sets of conjugate kink bands, oneither sub- of migmatitic gneiss in Anglesey represent fragments horizontal or sub-vertical axes, followed thesecond of gneissose continental basement. fold phase. At Rhoscolyn at the southern end of Holy Island, thesame assemblage and sequence of structures is Bedded Succession seen in the Rhoscolyn Bedsand their intercalated Inthe currentlyaccepted stratigraphic synthesis quartzite units (Shackleton 1969,p. 3). This region proposed by Greenly (1919) and amended by Shackle- also forms the hinge zone of a major main phase ton (1969), the Bedded Succession is regarded as an anticlinal fold, the Rhoscolyn anticline (Shackleton essentially continuous stratigraphic sequence (Fig. 2~). 1969, and Fig. 5).

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HDLYHEAD MTN. RHOSCOLYN BAY N. W. S. E.

FIG. 5. Interpretation of the structure of Holy Island. Arrows indicate direction of younging.

New Harbour Tectonic Unit Greenly(1919, p. 46) defined the New Harbour Group from the New Harbourat Holyhead. The group is shown on his map covering much of Holy Island and the adjacentmainland, as well as occupying alarge tract in the N of Anglesey extendingfrom Carmel Head (Fig. 1) to Point Lynas. Over much of this areathe rocks are greenschists, containing thin siliceous bands, which from the presence of sedimen- tary grains are recognizably sandstones or grits. Other FIG. 6. Map of Holy Island (after Greenly 1920, rock types included by Greenly in the New Harbour I.G.S. 1972 [with permission of the Director, Insti- Groupare serpentinites,gabbros (Maltman 1975, tute of Geological Sciences]), showing the plunge 1977), jasper cherts, tuffs and spilitic lavas. The latter of the main phase folds and localities mentioned in may show deformed pillow shapes, e.g. S of Point the text. Lynas (483928). The characteristicstructural feature of the New Harbour Group is an intense planar schistosity which quartzitic greywackes of the Rhoscolyn Beds with is frequently mylonitic. Deformation has separated the right-way-up cross-bedding and load-cast structures gritbands into thin lenticles by transposition and are well exposed on the foreshore (272749). In a small pressure solution, giving a typical 'flaser' appearance knoll (277753) N oi Borthwen a steep cleavage dip- to the bedding. An intenselinear structure due to ping NW is axial planar to upright folds, slightly highly elongated clastic mineral grains and chloritic overturned to the SE, on axes plunging gently towards smears is developed within the schistosity. Tight or 060". These folds belong to the main phase of folding isoclinal intrafolial folds with axial planes parallel to in the South Stack Unit and are congruent with the the schistosity and axes oriented N-S parallel to the main Rhoscolyn anticline. lineation occur throughout. At Rhoscolyn asecond NE of Borthwen,between Pentre Cyndal and phase of tight minor folds has been recognized, folding Pentre Gwydell (279755), well-foliated and lineated the schistosity. greenschists of the New Harbour Group, overlying the These earlierstructures are folded by large scale Rhoscolyn Beds down the plunge of the Rhoscolyn folds, upright or slightly overturned, with amplitudes anticline, are also affected by folds plunging towards of a few metres on axes plunging gently to the NE. 070" at 26" and Overturned towards the SE. These are These main phase folds commonly have a chevron main phase folds in the New Harbour Unit, and in that form and crenulation cleavage may be developed unit are preceded by 2 earlier phases of deformation. parallel to their axial planes. It is clearfrom these relationships thatthe main The structural sequence observed in the New Har- phase folds in the Rhoscolyn Beds and in the New bour Group of Holy Island may also be recognized Harbour Beds represent the samephase of folding, over much of the area mapped by Greenly (1920) as which was also responsible forthe main Rhoscolyn New HarbourGroup. Greenlyplotted thenorth- anticline. In the Rhoscolyn Beds it is the bedding easterly plunge of the main phase fold axes from Holy which is folded, while in the New Harbour Beds it is Island (Fig. 6) all the way to Point Lynas. both bedding and schistosity (Fig. 5). The New Har- The relationships between the South Stack Unit and bour Beds were already deformed before they were the New Harbour Unit may be seen at Rhoscolyn at affected by the Rhoscolyn anticline, while the Rhosco- the southern end of Holy Island (Fig. 6). Here bedded lyn Beds show no earlier phase of folding.

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Similar relationships to those described from Rhos- Conversely, a traverse along the N coast from Hen colyn may be seen at Kingsland (239809), in expos- Borth (318930) to the boundary fence of the Nuclear ures described by Shackleton (1969, p. 6, fig. 3), and Power Station at Wylfa (347938) passes across rocks at Holyhead Golf Course (240806). Wherever South classified by Greenly in the New Harbour and Skerries Stack Beds arein contact with theNew Harbour Beds, Groups.The rocktypes include green and purple the internaldeformation state of the rockschanges slates, bedded siltstones, mudstones and grits. These abruptly across the boundary. This contrast in defor- rocksshow only a simple structural history with a mation state is implicit in Greenly’s (1920) map; he single cleavage and little internal deformation. Turbi- distinguished between ‘cleavage’ and ‘bedding’ dips in dite sandstones S of the Power Station fence contain theSouth StackUnit, while in the New Harbour wisps of red shale, mudflakesand rounded siliceous Group his mapshows only ‘cleavage’ or ‘foliation’ pebbles, including jasper, noneof which are deformed. dips. At Esgair Cemlyn (337933), there is a fold related to An important tectonicdiscordance, a thrust or slide, the cleavage with its axis plunging toward 050” at 17” separates the South Stack Beds from the New Har- corresponding with the main phase folds of the South bour Beds. When the contact between the two units is Stack Tectonic Unit. These rock units, with their low traced around the nose of the Rhoscolyn antiform it is deformationstate and simplestructural history, are seento cut across the bedding (e.g. 278757) in the analogous tothe South StackUnit of Holy Island. underlying Rhoscolyn Beds. The New Harbour Beds However, they are not contiguous with this unit, from were deformed before being thrust over the Rhoscolyn which they are separated by the New Harbour Unit, Beds, then both groups, together with the intervening nor do they relate either structurally or sedimentologi- thrust plane, were folded around the Rhoscolyn antic- cally to the New Harbour Group, to which they were line and the othermain phase foldsof the South Stack assigned by Greenly(1920). For these reasons we Unit.It is significant that all theway-up indicators have separated these rocks to form a distinct tectonic used by Shackleton (1969) to determine the order of unit. deposition of the Bedded Succession on Holy Island Thereare two localities where thedeformation were taken from the South Stack Unit. This is due to states of the New Harbour and Cemlyn Tectonic units the higher state of deformation in the New Harbour can be directly compared. Greenly (1920) showed the Unit, which has destroyed possible way-up structures; boundary of the New Harbour Group and the Church even where such structures have been preserved they Bay Tuffs at Porth Defaid (289858) (Greenly 1919, pp. are probably only of local significance. 157-8). Atthe contact betweenthe two unitshe showed a small sliver of lava belonging to the New Harbour Group. Thegreenschists of the New Harbour Cemlyn Tectonic Unit Group are well-foliated and lineated, both structures The contrast in structural state between the South beingfolded on a smallscale by foldsplunging to- Stack Beds and the New Harbour Beds in Holy Island wards050”. These rocks clearly belong to the New can be applied elsewhere in Anglesey. In the northern Harbour Tectonic Unit. The lava at the contact is not part, NW of a line from Porth Defaid (289858)to Bull highly deformed,and is includedwithin slightly de- Bay (426944), rocks with a simple structural history, formedpurple shales, which occurboth above and similar to that in the South Stack Tectonic Unit, can below the lava. Extending northwards along the shore be separated from those with a more complex struc- are shales, agglomeratesand tuffs,with occasional tural history (Fig. 1). Werefer to this unitas the more massive lavas associated with jasper, belonging CemlynTectonic Unit, well seenat CemlynBay to the Church Bay Tuffs, with a low state of deforma- (3393). Rocks with a more complex history are clas- tionand simple structure, and forming part of the sified with the New Harbour Tectonic Unit, as defined CemlynUnit. A faulted contact separates the two on HolyIsland, with which theserocks are in con- units. tinuity. S of CarmelHead is the well-known locality This structural classification does not coincide with (296930)where the Amlwch Beds rest on a thrust the lithological divisions described by Greenly (1919) plane above Ordovician Slates (Greenly 1919, pl. 31, in his Memoir, and shown on his map (Greenly 1920). pp. 54-9). The Amlwch Beds are here typical, highly A traverse from Llanddeusant (344851)to deformed,multiply folded, greenschists of the New (369913) crosses rocks shown asNew Harbour Group, HarbourTectonic Unit. Carmel Head itself is com- Church Bay Tuffs, Gwna greenschists, and to the Nof posed of Gwna melange in a low state of deformation the Ordovician outcrop, the Coeden Beds, correlated forming part of the Cemlyn Tectonic Unit. The narrow by Greenly (1920) with the South Stack ‘Series’. All inlet between the headlands marks the outcrop of a the pre-Ordovician stratigraphic units show the same faultor steep thrust which separatesthe two units structural history as the New Harbour Tectonic Unit, (Greenly 1919, p. 295). with a dominant schistosity and lineation, folded by Rock units included within the Cemlyn Unit were main phase structures. variously classified by Greenly (1919, 1920) as Church

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Bay Tuffs, GwnaGroup, New HarbourGroup and CemlynTectonic Unit. On HolyIsland theSouth SkerriesGroup. Owing to frequent tectonic breaks Stack Unit lies structurally beneath the New Harbour andthe scattered nature of inlandoutcrops, the Unit,as Shackleton(1969) demonstrated. On struc- stratigraphy is not entirely clear. A provisional revised tural grounds we suggest that the contact between the sequence is proposed in Fig. 2e. two units is a major tectonic discordance. The rela- Ina general wayall the rocks classifiedin the tionships between the Cemlyn Unit andthe other units Cemlyn Unit‘dip N or NE from PorthDefaid is not clear. In its structural history it resembles the (289858) towards the N coast (Fig. 1). Way-up indi- SouthStack Unit but their lithological featuresare cators show that all the beds are the right way up. The quite distinct and the two units areseparated both Church Bay Tuffs are therefore the oldest membersof geographically and structurally by the New Harbour the succession. At Port Swtan (300893) just above the Unit. All the contacts between the Cemlyn and New slipway, cleaved tuffaceous rocks of the Church Bay HarbourUnits are faults, butthe contrast between Tuffs pass upwards, by the appearance of large blocks their deformational histories suggests that the contact of quartziteand limestone in the tuffaceousmatrix, is either another overthrust or possibly a stratigraphic into melange beds (Fig. 28). break. No unconformable relationships are seen, but, To the N, acid volcanics of Greenly’s Fydlyn Group as will bedescribed in the following section, the are closely associated with Ordovician slates in tight, Cemlyn Unit passes up into the overlying Ordovician faulted infolds of Ordovician rocks. It is possible that without a major tectonic break and appears to form the Fydlyn Group does not form part of the Mona the base of the same sedimentological unit (Fig. 7). Complex. It only outcrops in this locality, and the acid tuffs of the Fydlyn Group are indistinguishable in the Relationships between the Ordovicianand field from acid ‘intrusions’ in the Ordovician around the Mona Complex Mynachdy (302921). Itmay also be significant that the outcrops of the Fydlyn Group occur in a correspond- Throughout most of Anglesey, Ordovician rocksrest ing structural position to the Caradocian volcanics of with a clear unconformity on the rocks of the Mona Parys Mountain in eastern Anglesey, and the volcanics Complex(Greenly 1919, pp.403-83; Bates 1972, of Lambay Island on the oppositeside of the Irish Sea. 1974).Unconformable relationships are less obvious Rocks which Greenly(1919, 1920) classified with along the N coast between Bay and Bull Bay, the New Harbour Beds, to the E of Carmel Head, are wherethe Ordovician is preservedas a number of here provisionally termed the Greywacke Group (Fig. fault-bounded outliers, the Gynfor outliers of Greenly 2~).Greenly’s Skerries Group occurs within this unit (1919,pp. 468-77). Here Ordovicianconglomerates onthe coastnear Llandrhwdrys Church (N of andsandstone, which Greenlyattributed tothe 320934). Way-up indicators in this area are scarce, but Caradocian Nernagraptws gracilis Zone, rest on Gwna such evidence as is seen (see Shackleton 1969, p. 7) melange.For Greenly, evidence of unconformity at shows that the sequence is the right-way-up and that the contactbetween the two units was clear;the the Skerries Group is interbedded with the Greywacke melange was the product of intense tectonic deforma- Group (Fig. 2~). tion in pre-Ordovician times. Shackleton’s (1969) rec- At Wylfa Headthe Greywacke Group passesup ognition of the melange as a sedimentary slide deposit intomelange beds by the loss of beddedimpure removesthe argument fora major unconformity at sandstonesand the incoming of enormousquartzite this junction.In fact theOrdovician and the Gwna blocks (Greenly 1919, pl. 22). We entirely accept the melange show the same degreeof deformation, as may explanation put forward by Shackleton (1969) for the beseen at the base of the cliff at Ogof Gynfor origin of the Gwna melange as a subaqueous olisto- (378948),where both units contain a single simple strome deposit, which continues a historyof instability steeply-dipping cleavage and show no substantial re- shown in the underlying sediments of the Greywacke crystallization (Fig. 7). The samerelationship is also Group by slumped beds, slump folds, pull-apart struc- seen to the E of Carmel Head (300928), where Gwna tures and thin-bedded debris flows. It appears that the melange is thrust over the Ordovician. A single cleav- formation of theGwna melange was nota unique age is the onlyplanar structure in either unit. Evi- event, as melange beds occur twice in the succession, dently the Gwna Groupand the Ordovicianrocks once after the depositionof the Church Bay Tuffs and weredeformed together for the first time in post- againafter the deposition of the Greywacke Group Ordovician (late Caledonian) times. (Fig. 2~). Since Greenly wrote his account, the time gap be- The evidence detailed above shows that Greenly’s tween the deposition of the Gwna Groupand the BeddedSuccession does not constitute a continuous Ordovicianhas been considerably reduced by fossil stratigraphicsequence. We have demonstrated that finds. Bates (1968) identified an Arenig shelly faunain the Bedded Succession in northern Anglesey may be the basalOrdovician rocks, and Wood & Nicholls divided into 3 tectonic units (Fig. 2~):1, South Stack (1973) found limestone blocks in the Gwna melange Tectonic Unit; 2, New Harbour Tectonic Unit;3, to containstromatolites, identified as of upper

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Riphaean age (Downie 1975). These blocks and the up into the overlying Ordovician without any signific- enclosing matrix were examined by Muir ef al. (1979); ant sedimentological, tectonic or metamorphic break, in the limestone blocks, which weredeposited and that both the Cemlyn Unit and the Ordovician elsewhere and lithified before being incorporated in were deformed and metamorphosed for the first time the melange, theyfound stromatolites with arange during the Caledonianorogeny. Elsewhere in Ang- from Vendian to Cambrian. Graphitic material from lesey the Ordovician rests on rocks of the Mona the melange matrix yielded palynomorphs of probable Complex. The unconformablerelationship is clear lower Cambrian age. from the transgressive nature of the base of the Or- In the Ordovician rocks of the northern part of the dovician, and from the recorded occurrence of frag- island, including the principal areaand the Gynfor ments of most of the underlying groups in Ordovician outliers, the major structures are large scale simple basal conglomerates. The major part of the deforma- synclinal and anticlinal folds, much broken by faults tion and metamorphism of the New Harbour Unit, the and thrusts, one of the latter being the Carmel Head Penmynydd Zone and the gneisses, together with the Thrust Plane. The axes of the folds trend SW-NE and intrusion of the Coedana granite, took placebefore most of the folds plunge to the E or NE. Fold style is the deposition of the Ordovician. characteristically asymmetrical with the SE limbs of the synclines more gently inclined, while the NW limbs Regional implications aresteep and often overturned (Greenly1919, fig. 220; Bates1974, fig. 5). A dominant axial plane Re-interpretation of the Mona Complex of Anglesey cleavage is developed with relation to the majorfolds, on the lines proposed in the foregoing account de- being very well developed in argillaceous units. Cleav- mandsa re-appraisal of the evolution of the Mona age is less well developed in grits and conglomerates, Complex and of the place of Anglesey in the Irish Sea where it is usually restricted tothe matrix, rarely Region during the Lower Palaeozoic. affecting the pebbles (Bates 1974, p. 47). Minor folds are comparatively rare. This may be Proterozoic attributed to the general absence of thin-bedded units in the Ordovician succession. Where thin-bedded units As Wood (1974) pointed out, the Mona Complex do occur, as on the foreshore at Rhosneigr (317735), contains all the characteristiccomponents of asub- where thin sandstonebeds occur among the slates, ductedplate margin, including ophiolites, melanges, minor folds are found(Greenly 1919, figs.261-2). blueschists and ignimbrites; we would addancient These minor folds then show the same characteristics, continentalbasement material andmajor mylonite with overturning towards the SE, as the major struc- belts to this list. All these components, apart from the tures. Greenly (1920) showed the direction of plunge melanges, were already present in the Anglesey area of a few of the minor folds in the Ordovician on his by the beginning of Cambrian time, so that subduction map, but the general SW-NE trend of the fold axes is took place during the Proterozoic. The problem is to indicated by the strike of the cleavage. Only in the fit thesecomponents together to forma convincing NW of the principal area between Llanbabo (378868) plate tectonic model for the development of the com- and Llanfairynghornwy (318913) arethere any sig- plex. nificant variationsfrom this trend.This is a region 4 alternative plate tectonic models, including those much affected by thrusting, and similar variations in put forward by Dewey (1969) and Baker (1973) have trend are seen in the Cemlyn Unit in the same area. been considered by Thorpe (1974, pp. 131-3, fig. 6). The characteristics of the dominant folds in the Somevariant of models which propose that during Ordovician rocks are almost identical to those of the Proterozoic times Anglesey formed part of a micro- main phase folds previously described from the South continent onthe southern margin of theIapetus Stack TectonicUnit on Holy Island.It is therefore (Proto-Atlantic) Ocean,separated from the Welsh probable that the last major phase of folding to affect Continentto the S by a marginal basin which was the Mona Complex also affected the overlying Ordovi- undergoing subduction (cf. Thorpe 1974, figs. 68, c & cian rocks. These folds are therefore of post- D), is required to explain the major features of the Ordovician, i.e.Caledonian age. BothGreenly and Mona Complex. Bates were inhibited from making this obvious corre- The ancient continental basement is represented by lation because of their assumption that the deforma- basal gneisses which outcrop in all 3 of the structural tion in the Mona Complex was all of Precambrian age. units into which Anglesey is divided. 'Notall the The possibility was carefully considered by Greenly gneisses necessarily belonged to the same continent. (1919, pp. 557-9) but firmly rejected on grounds which The basement of thenorthern micro-continentpre- can now be seen to be fallacious. sumably lies beneath the Bedded Succession in the The evidence presented above shows that on the N northern part of Anglesey. It is not exposed beneath coast of Anglesey the deposition of the Gwna Group, the South Stack Unit of Holy Island, which structur- forming part of the Cemlyn Tectonic Unit, continued ally is the lowest tectonic unit in northern Anglesey.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/136/4/407/4885859/gsjgs.136.4.0407.pdf by guest on 28 September 2021 420 A. J. Barber & M. D. Max Gneisses appear in the Gader and Mynachdy inliers rows in the South Stack Beds (Greenly 1919, p.150) within the zone of disturbance related to the Carmel suggests that these rocks are of Upper Proterozoic or Head Thrust, where theyhave been brought to the even Cambrianage, unless these burrows are dis- surface by upthrusting of the underlying basement. counted (Muir et al. 1979). The largest outcrop of gneisses occurs in the central The South Stack Beds may be linked sedimentologi- region, separated from the Bedded Succession of the cally to the overlying New Harbour Tectonic Unit by northern region by a major mylonite belt (Fig.1). The way of the Coeden Beds. The Coeden Beds occur in mylonites have been derived partly from the gneisses the northern part of Anglesey, N of the Carmel Head and partly from the rocks of the New Harbour Group Thrust, and consist of graded sandstones and shales; to the N. Whether they are formed from gneisses or for this reason they were correlatedby Greenly (1919, sediments, they show the same structural featureswith p.45) with the South Stack Beds; however, they do not intrafolial folds, awell-developed schistosity and a contain quartzite intercalations andare relatively strong N-S stretching lineation. highly deformed. It may be that the Coeden Beds are The mylonitezone can betaken asa shear zone the more distal equivalents of the South Stack Beds, relatedto the subduction process, down which the originally deposited further to the S, but subsequently New Harbour Unit, with distal turbidites, manganifer- deformed in the subductionzone, and later thrust ous shales, jasper cherts and spilitic tuffs and pillow northwards across the South Stack Unit. lavas, together with gabbro and serpentinite (dunite, All the direction indicators in the Bedded Succes- harzburgite and lherzolite, Maltman 1975) represent- sion of northern Anglesey show that the source of the ing an ocean floor assemblage, was being subducted. sediments lay towards the N (Shackleton 1969, p.9). In thismodel the gneisses of the central region In their original relationships the South Stack Group would representthe leading edge of thesouthern passed southwards into the Coeden Beds and into the continent, beneath which the ocean floor was being more distal turbidites and ocean floor materials of the subducted. Movement of the ocean floor down the New Harbour Group. Quartzite intercalations in the subduction zone was responsible for the development South Stack Unit suggest the close proximity of a of the mylonitic foliation in the New Harbour Group, continental shelf on which mature quartz sands were andat the same time caused mylonitization of the accumulating. Thesequartz sands may now berep- overlying gneisses. Further extension of the margin of resented by large blocks of quartzite in theGwna thesouthern continental margin towards the SW is melange. Thequartzite blocks are associated in the indicated in the Rosslare Complex of SE Ireland (Max melange with limestones containing algal stromatolites & Dhonau 1974). Here basement gneisses, with an which indicate anUpper Proterozoic age fortheir age noolder than 2400 Ma (Max 1975)are also original deposition on the continental shelf (Downie margined to the NW by a major mylonite zone. The 1975). The lower part of the Bedded Succession may Cullenstown Formation occupies the same structural therefore be chronostratigraphically equivalent to the position and shows the same well-developed schistos- Dalradian. ity and N-S lineationas seen in the New Harbour This whole series of sedimentary facies moved Group of Anglesey. southwards towards the subduction zone. At the time It is possible that the N-S linear structure indicates that subduction ceased, the Coeden Beds and the New the direction of movement, as Ramsay & Graham Harbour Beds had been subducted and deformed, but (1970) demonstrated for small scale shear zones. In the South Stack Group had not yet reached the sub- thiscase the NE-SW trend of the mylonitebelt, duction zone. With cessation of subduction, the sub- combined with the N-S trending lineation within it, ductionzone was uplifted andthe deformedrocks suggests that in Proterozoic times subductiontook thrustnorthwards over the undeformedSouth Stack place obliquely to the margin of the southern conti- Group. nent, as Lambert & McKerrow (1976) suggested for All theseevents occurred in late Precambrian or the northern margin of the Iapetus Ocean during the earliestCambrian times as they do not affect the Lower Palaeozoic. Cambrian Gwna Group nor the Ordovician. The Or- Innorthern Anglesey the highly deformed New dovician rocksrest unconformably onthe mylonite Harbour Unit rests on the South Stack Unit. Reasons zone (Fig.1) and boulders of deformed rocks occur in have been adduced for proposing that the South Stack Ordovician basal conglomerates. Thesame relation- Unit was undeformed prior to theemplacement above ships are also seen in SE Ireland, as the deformation it of the New Harbour Unit by overthrusting. of the Cullenstown Group and the formation of mylo- The South Stack Unit consists of graded grits and nites in the Rosslare Complex also occurred before the shales with intercalatedslumped , trans- deposition of theadjacent Ordovician rocks (Max ported by turbidity current flows derived froman 1975). adjacent continental shelf and deposited in a continen- Greenly (1919), followed by Shackleton (1954a, b; tal slopeor deep-sea fan environment(Shackleton 1956) proposed that the amphibolite facies rocks of 1969, p.9). The presence of abundant Skolithos bur- the Penmynydd Zone in the Central Region were the

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higher graderepresentatives of the Bedded Succes- the Harlech Dome, and northwards into eastern Ire- sion. It is apparent from the field relationships that land and the Isle of Man to form the Bray Group and these rocks were already in the condition of amphibo- the Manx Slates. Unfortunately for this hypothesis, lite facies metasediments when the mylonites related current direction measurements in the Harlech Dome to the subduction process were formed. Together with (Kopstein 1954) show a derivation from the S and E, the gneisses the metasedimentsformed part of the andthe generalcoarsening of the succession in an crystalline basement of the southerncontinent, de- easterly direction also shows that the source region for formed during subduction and locally retrogressed to the Cambrian sediments lay to the E (George 1961, greenschist facies mylonites. On the otherhand, intru- p.18). Similarly in Ireland current direction indicators sion of the Coedana Granite took place after subduc- show anortherly and north-westerly source for the tion had ceased, as mylonitized rocks occur within its Bray Group in Wicklow (Briick & Reeves 1976) and metamorphic aureole. Howth (van Lunsen & Max 1975). In the northern part of Anglesey metamorphism of The Cambrian age of the Gwna olistostrome means the subductedmaterial did not rise above the that Anglesey could not have formed a sedimentary greenschist facies. Further to the S, in the Aethwy sourcein Cambrian times, since it was anarea of Region, are rocks metamorphosed in the glaucophane- deposition. Furthermore, the olistostrome was trans- lawsonite facies (Nataraj 1967). If this metamorphism ported down a palaeoslope and was deposited at the is related to the subduction seen in the northern part foot of this slope. Evidently after subduction ceased in of the island, then these rocks represent a deeper level late Proterozoic times, Anglesey was uplifted and in the subduction zone, and correspondingly greater erodedbut had subsided sufficiently by Cambrian uplift was required to bring them up to the present times for the deposition of the olistostrome. level of erosion. The Aethwy Region, with fragments Components of the melange include ocean-floor of gneissose basement,glaucophane schists and materials such as spilitic lavas and tuffs, jasper cherts greenschists broken up by mylonite belts and thrusts, and manganiferous limestones and shales. These com- is tectonically extremely complex and may correspond ponents increase in importancesouthwards, atthe to zone a of imbrication (Fig.3). Again the expense of shelf-derived olistolites, and become do- metamorphism, and the major part of the deforma- minant at Llanddwyn Island. These ocean-floor mater- tion, followed by uplift and erosion, took place before ials may have originally accumulated in the subduction Ordovician times. trench and were then uplifted and became available to formthe melange. They have never beendeeply buried,as they are virtually undeformed andun- Lower Palaeozoic metamorphosed. Depositionrecommenced in thenorthern part of The source of thesematerials must have lain en- Anglesey with rocks of the Cemlyn TectonicUnit, tirely to the N of Anglesey. The extent of a possible including the Church Bay Tuffs, theGreywacke Group land mass in this direction is limited by the Isle of and the Gwna melange. The melange deposits, which Man,composed largely of Cambriansediments. On extend at least as far S as Llanddwyn Island and SW the western side of the Irish Sea the landmass must into Lleyn, were formed by the uplift of the margin of have lain to the N of the Howth near Dublin (Fig.8) the northern continent, accompanied by seismic activ- whereCambrian rocks, resembling the South Stack ity dislodging large blocks of lithified rock from the Beds (Censure Group) and the Gwna melange (Nose older continental shelf, which thenslumped down of Howth Group), show northerlya and north- south-eastwards as catastrophic rock slides to form the westerly provenance (van Lunsen & Max 1975). The Gwna melange. Lower Palaeozoic land mass must have been located in The dating of the Gwnamelange as Cambrian(Muir the area of the present Longford-Down massif (Fig.8) et al. 1979) necessitates a major revision of Lower where Silurian rocks rest directly on Lewisianoid Palaeozoicpalaeogeography in the IrishSea region granulites, brought up in Carboniferous volcanic vents (Fig. 8). Greenly interpreted the whole of the Mona (Strogen 1974). Complex as of Precambrianage, deformed, In Greenly’s (1919) original synthesis the Anglesey metamorphosed and uplifted in the late Precambrian land mass persisted into the Lower Ordovician, as he to providea source of sediments forthe Cambrian thought that Lower Ordovician rocks wereabsent deposits of North Wales. Thisinterpretation found from northern Anglesey, whereCaradocian rocks expression in the concept of the Irish Sea land mass rested directly onthe Mona Complex. Bates (1972) (geanticline, horst). In its fullest development (e.g. showed that Arenigsediments occur in this part of Bennison & Wright 1969, fig.4.5; Owen 1976, fig.24) Anglesey, so that again in Lower Ordovician times the this source region became an elongated ridge extend- main source of sediment must have lain entirely to the ing SW from Anglesey to include the Rosslare Com- N of Anglesey. plex of SE Ireland (Fig.8). Sediments were shed from Depositionin Ordovician times appearsto have this ridge southwards into the Welsh Slate Belt and taken place under active tectonic conditions. In some

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ISLE OF MAN

0 Km

ANGLESEY

’ CULLENSTOWN

ST DAVIDS~

FIG.8. F’recambrian (black) and Cambrian (dotted) rocks in the Irish Sea Region. A-A’, northern limit of Cambrian rocks inthe non-metamorphic Caledonides; B-B’-”’, northernlimit oflow grademetamorphic rocks ofNew Harbour Group type, overthrust in Anglesey; C-C‘, SE limit of Cambrianrocks with ocean floor and trench affinities;C-D, Menai Strait Line (MSL) separating the subduction zone complex to the NW fromthe Welsh platform to the SE; M-M-M, mylonite belt.

places the basal beds of the Ordovician reston interveningelevated areas from which the melange melangedeposits, without any important structural had been stripped. discordance, while in other areas basal conglomerates Materials of the Ordovician basal conglomerates with boulders of the underlying rocks rest directly on appears to be of very local origin, and the lithological older members of the Mona Complex. The distribu- units of the Ordovician are also very localized in their tion of the Gwna melange suggests that by Ordovician distribution. Bates (1972, fig. 11) showed the Ordovi- times it had been localized into tectonic troughs with cian lithological units outcropping as a series of bands

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 MonaComplex (Anglesey,North Wales) 423

cutting across the general trend of the outcrop in the occurred after the deposition of the Ordovician and principal area. Each unit consists of coarse basal beds, the Silurian deposits of ParysMountain, when the often conglomeratic, followed by an upwards fining rocks were folded into large scale upright folds, asym- sequence. Each area of deposition has its own sequ- metrical towards the SE. The folding affected all the enceand no obviouscorrelations can be madebe- tectonic units of the Bedded Succession, including the tweenthem (Bates 1972). This distribution suggests Cemlyn Tectonic Unit and the overlying Ordovician. that a pattern of horsts and graben controlled Ordovi- Development of folds and cleavage continued at least ciandeposition. Eachgraben was filledby material until Lower Old Red Sandstone times, as rocks of this largely derived from the adjacent horsts. The occurr- age in Lligwy Bay are folded and cleaved, although ence of olistostrome deposits higher in the Ordovician notto the same extent as neighbouringOrdovician sequence, particularly in the Caradocianrocks adja- rocks. cent to the Carmel Head Thrust (Bates 1972), shows This last major phase of deformation constitutes the that the region was still tectonically active at this time, Caledonianorogeny in the Anglesey area. Subse- with high relief and active erosion inducing collapse of quently the MonaComplex has formed partof a stable material, including earlier Ordovician deposits (Bates cratonic block. 1972, p.50), from thehorsts into the adjacenttroughs. The fining upward sequences evidently record the decline of tectonic activity with the reduction of relief ACKNOWLEDGMENS.The authors would like to acknowledge and the loss locally derived materials as the Ordovi- the value of discussions with H. van Lunsen, S. K. Hanmer, of Laurie Richards, Michael Earle, Nick Shaw and Wes Gib- ciandeposits spread outfrom the graben onto the bons during the course of this study. Special thanks are due adjacent horsts. Volcanic activity developed latein to Prof R. M.Shackleton who stimulated the authors by Llanvirn or earlier Caradoc times at Parys Mountain penetrating criticism of our proposals. The presentation has (Bates 1972) and perhaps at Fydlyn. alsobenefited from comments by DenisBates and the The last major deformation of the Mona Complex Geological Society’s referees and editors.

References

BAKER,J. W. 1969. Correlation problems of unmetamorph- - 1923.Further researches on thesuccession and osed Precambrian rocks in Wales and S.E. Ireland.Geol. metamorphism in the Mona Complex of Anglesey. Q. J. Mag. 106, 246-59. geol. Soc. London, 79, 344-51. -1973. A marginal Upper Proterozoic ocean basin in the INSTKUTEOF GEOLOGICAL SCIENCES, 1972. 1:50,000 geologi- Welsh region. Geol. Mag. 110, 447-55. cal map of Anglesey. BATES, D.E. B. 1968. The Lower Palaeozoic brachiopod and Hsu, K. J. 1968. Principles of melange and their bearing on trilobite faunas of Anglesey. Bull. Brit. Mus. (Nat. His?.) the Franciscan-Knoxville Paradox. Bull. geol. Soc. Am. geol. 16, 127-99. 79, 1063-74. - 1972.The stratigraphy of the Ordovicianrocks of KOPSTEIN,F. P, H. W. 1954. Graded bedding of the Harlech Anglesey. Geol. J. 8, 29-58. Dome. Publ. Geol. Inst. R. Uniu. Groningen, 97 pp.

~ 1974. The structure of the Lower Palaeozoic rocks of LAMBERT,R. St. J. & MCKERROW,W. S. 1976. The Gram- Anglesey, with special reference to faulting. Geol. J. 9, pian orogeny. .Scoff.J. Geol. 12, 271-92. 39-60. LUNSEN, H. A. VAN & MAX,M. D.1975. The geology of BENNISON,G. M. & WRIGHT,A. E. 1969. The geological Howth and Ireland’s Eye, Co. Dublin. Geol. J. 10,35-58. history of the British Isles. Edward Arnold, London. m-, A. J. 1975. Ultramafic rocks in Anglesey-their BRUCK,P. M. & REEVES,T. J. 1976. Stratigraphy, sedimen- non-tectonic emplacement.J. geol. Soc. London, 131,593- tology and structureof the Bray Group in County Wicklow 603. and South County Dublin.Proc. R. Ir. Acad. 76B, 53-77. -1977. Serpentinites and related rocksof Anglesey. Geol. DEWY, .I. 1969.F. Evolution of theAppalachian/ J. 12, 113-28. Caledonian orogen. Nature, Lond. 222, 124-9. MATLEY,C. A. 1928. The Precambrian complex and as- DOWNIE, C. 1975. Precambrianof the British Isles: Palaeon- sociated rocks of south-western Lleyn. Q. J. geol. Soc. tology. In: HAWS A. L. et al. (eds) A correlation of the London, 84, 440-504. Precambrian rocks in the British Isles. Geol. Soc. Lond. Mm, M. D. 1975. Precambrian rocks of south-east Ireland. Spec. Rep. 6, 113-5. In HARRIS,A. L. et al. (eds). A correlation of the GEORGE,T. N. 1961. British Regional Geology: North Wales. Recambrim rocksin the British Isles. Geol. Soc. Lond. 3rd ed., H.M.S.O. London. Spec. Rep. 6, 97-101. GREENLY,E. 1919. The geology of Anglesey (2 vols). Mem. - & DHONAU,N. B. 1974. The Cullenstown Formation: geol. SUN. G. B. Late Precambrian sediments in south-east Ireland.Bull. - 1920.One inch geological map of Anglesey(sheets geol. SUN. Ireland, 1, 447-58. 92+93) Geol. SUN.G.B. MOORBATH,S. & SHACKLETON,R. M.1966. Isotopic ages

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