The Significance of the Boundary Between the Rhoscolyn and New

Home , Coedana, Geology of Anglesey, Holyhead Breakwater, Holyhead Mountain, Porth Dafarch, Rhoscolyn

Geol. Mag. 150 (3), 2013, pp. 519–535. c Cambridge University Press 2012 519 doi:10.1017/S0016756812000775 The signiﬁcance of the boundary between the Rhoscolyn and New Harbour formations on Holy Island, North Wales, to the deformation history of Anglesey

∗ ∗ JACK E. TREAGUS , SUSAN H. TREAGUS † & NIGEL H. WOODCOCK‡ ∗ School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester M13 9PL, England ‡Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, England

(Received 3 July 2012; accepted 4 September 2012; ﬁrst published online 20 December 2012)

Abstract – The boundary between the Rhoscolyn and New Harbour formations on Holy Island, Anglesey, has been described as a high strain zone or as a thrust. The boundary is here described at four localities, with reference to the contrasting sedimentary and deformational character of the two formations. At one of these localities, Borth Wen, sandstones and conglomerates at the top of the Rhoscolyn Formation are followed, without any break, by tuffs and then mudstones of the New Harbour Formation. It is concluded that there is clear evidence of depositional continuity across the boundary here, and that both formations subsequently shared a common two-phase deformation. The first (D1) was manifestly different in intensity and scale in the two formations, whereas the second (D2) produced very similar structures in both. The other three localities provide continuity of sedimentary and tectonic features at this boundary in a traverse along the length of Holy Island, leading us to identify two previously unrecognized major D1 folds in addition to the Rhoscolyn Anticline. At one of these localities (Holyhead), we confirm the presence of Skolithos just below the boundary, supporting radiometric evidence for a lower Cambrian or later age for the Rhoscolyn Formation. A turbidite interpretation for both the Rhoscolyn and New Harbour formations best fits the available evidence. A deep-water depositional environment is still compatible with the sporadic presence of Skolithos burrows, but less so with reported observations of hummocky and swaley cross-stratification lower down the South Stack Group. Keywords: North Wales, Monian Supergroup, Rhoscolyn Formation, New Harbour Formation, structure, folds.

1. Introduction with blueschists of the Aethwy Terrane suggested ﬁrst to Wood (1974) and then to later authors (e.g. Gibbons, The island of Anglesey, North Wales, exposes the 1983; Gibbons & Horák, 1996) that the Monian most varied area of Neoproterozoic and related Composite Terrane represents an accretionary complex Cambrian rocks in southern Britain. Anglesey lies at a subduction zone. This possibility has established south of the Iapetus Suture that marks Silurian ocean Anglesey as an area of international importance closure (Fig. 1a), and therefore probably originated (e.g. Kawai et al. 2006, 2007). However, there are in Proterozoic time as fragments of the Gondwana differing views on whether the accretion happened supercontinent. Anglesey forms the main part of the predominantly by dip-slip thrusting (Kawai et al. 2007) Monian Composite Terrane, separated from the larger or by strike-slip faulting (Gibbons & Horák, 1996), Avalon Composite Terrane to the southeast by the and whether the Monian Supergroup is part of the Menai Strait Fault System (Fig. 1b). The Monian accretionary complex (Kawai et al. 2007) or post-dates Composite Terrane comprises late Proterozoic gneisses assembly of the Coedana and Aethwy terranes (McIlroy and granite (Coedana Terrane), Cambrian to possibly & Horák, 2006). Crucial to testing these hypotheses is a early Ordovician metasediments (Monian Supergroup better understanding of the stratigraphic and structural Terrane), and related shear zones, including a blueschist sequence within the component terranes on Anglesey. belt (Aethwy Terrane) (McIlroy & Horák, 2006). It The sequence in the Monian Supergroup (Fig. 2) has continues southwestward to the Rosslare Complex of been particularly problematic since Shackleton (1954, southeastern Ireland. Northwest of the Monian Terrane 1969) showed that the long-established sequence of is the Leinster–Lakesman Terrane, which exposes no Greenly (1919) must be incorrect. The debate was Proterozoic basement. fuelled by Barber & Max (1979), who divided the The association in the Monian Supergroup of deep- supergroup into three tectonostratigraphic units (struc- water turbidites, maﬁc igneous rocks and a major turally upwards, the South Stack, New Harbour and olistostrome (Gwna Group), and their juxtaposition Cemlyn units) separated by tectonic contacts. Gibbons & Ball (1991) proposed that the Cemlyn unit in fact † Author for correspondence: [email protected] equates to the Gwna melange, and has a stratigraphic

Downloaded from https:/www.cambridge.org/core. Open University Library, on 19 Jan 2017 at 23:18:34, subject to the Cambridge Core terms of use, available at https:/www.cambridge.org/core/terms. https://doi.org/10.1017/S0016756812000775 520 J. TREAGUS AND OTHERS

Figure 1. (a) Terrane map of the southern British Isles showing location of Anglesey. (b) Geological map of Anglesey showing location of Holy Island. (c) Geological map of Holy Island showing D1 syncline and anticline axial traces and the line of cross-section X–X (Fig. 3). Terrane map modiﬁed from Holdsworth, Woodcock & Strachan (2012) after British Geological Survey (1996).

Figure 2. Schematic tectonostratigraphic columns for the Monian Supergroup of Anglesey, together with the gneiss below and the Ordovician and Silurian sequences above, showing the evolution of proposals since that of Greenly (1923).

contact with the underlying New Harbour Group. the southeast, in a NE–SW-trending Monian basin Phillips (1991a), however, maintained the view that (Phillips, 1991a; McIlroy & Horák, 2006). the South Stack/New Harbour contact was tectonized, The New Harbour Formation on Holy Island mostly at least on Holy Island. The Monian sequence has originated as mudstones, with quartzose silt–mud more recently been questioned (a) by detrital zircons couplets defining a thin lamination. The mudstones in the South Stack Group (Collins & Buchan, 2004), are now metamorphosed to quartz-chlorite-mica schists implying that it is no older than 522 ± 6 Ma (early at greenschist facies, and polydeformed on a smaller Cambrian), and (b) by strontium isotope stratigraphy scale than the Rhoscolyn Formation. The New Harbour from the Gwna Group (Horák & Evans, 2011) showing Group as defined by Phillips (1991a), which includes that some of its component melange clasts are of the overlying Church Bay Tuffs and Skerries Formation Early Neoproterozoic age. If the accumulation of the (not exposed on Holy Island), was also interpreted Gwna Group was anywhere near the age of these by him as turbidites. The group, as broadly defined, clasts, it cannot then form the top unit of a mostly includes volcaniclastic sandstones derived from an Cambrian Monian Supergroup. As Horák & Evans andesitic arc (Phillips, 1991a; McIlroy & Horák, (2011) concluded, the stratigraphy of the supergroup 2006), with sediment probably derived from the north requires re-examination. or northwest. Thus, Phillips (1991a) concluded that the Monian Supergroup was derived from more than one source area, consistent with an active continental margin model. 2. The boundary between the Rhoscolyn and New It should be emphasized that Phillips’s (1991a) geo- Harbour formations: previous work chemical and sedimentological data for the provenance The present paper reports a re-examination of the of the New Harbour Group do not come from the boundary between the South Stack and New Harbour New Harbour Formation as defined by Greenly (1919), groups on Holy Island, Anglesey (Fig. 1c). On the nor from exposures on Holy Island as discussed in island, the South Stack Group comprises the South this paper. Instead, his data come from the Lynas and Stack Formation, the Holyhead (Quartzite) Formation Bodelwyn formations from northernmost Anglesey, and the Rhoscolyn Formation, overlain by the New otherwise known as the Amlwch Beds (Greenly, 1919), Harbour Group, here comprising only the New Harbour whose direct correlation to the New Harbour Formation Formation (Greenly, 1919; Shackleton, 1969; Phillips, on western Anglesey is not proven. Phillips (1991a, 1991a). The Rhoscolyn Formation is an assemblage p. 1086) stated specifically that the New Harbour of polydeformed and metamorphosed (greenschist metasediments in western Anglesey were too deformed facies) sandstones and subordinate mudstones. These for their depositional environment (or presumably their rocks have been interpreted as turbidites, deposited geochemistry) to be evaluated. The implication in later in a submarine fan with continental provenance from work (e.g. Hudson & Stowell, 1997; McIlroy & Horák,

2006) that the New Harbour Formation is turbiditic structure and stratigraphy of the Monian using ‘way-up therefore needs confirmation. evidence’. Although considering the boundary of the This part of the Monian Supergroup was traditionally Rhoscolyn Formation and New Harbour Formation to described as Precambrian (Greenly, 1919; Shackleton, be originally sedimentary, Phillips (1991a, pp. 1080– 1969). However, a radiometric date of 522 ± 6Ma 1) described the boundary as a ‘tectonized sedimentary from a detrital zircon in the South Stack Formation contact’ marked by a ‘high strain zone’ that included a (Collins & Buchan, 2004), together with the presence ‘highly sheared metabasalt’. of Skolithos burrows in the Rhoscolyn Formation To explain the differences in deformation between (Greenly, 1919; McIlroy & Horák, 2006), which we the Rhoscolyn Formation and the apparently more confirm later in this paper, now suggest an early highly deformed New Harbour Formation, Barber & Cambrian or later age. Arenig (lower Ordovician) rocks Max (1979) proposed that the boundary marked a unconformably overlie the Monian Supergroup and structural discordance, and that the New Harbour provide a minimum age constraint. Formation was an older and more strongly deformed The deformation history of this part of the Monian unit, thrust over the Rhoscolyn Formation during has been the subject of much debate and several subduction. This proposal was vigorously debated in conflicting interpretations (Shackleton, 1969; Barber the published discussion of Barber & Max (1979, & Max, 1979; Cosgrove, 1980; Roper, 1992; Phillips, pp. 424–31), with four separate discussions (Wood, 1991b; Hudson & Stowell, 1997; Treagus, Treagus Powell, Maltman, Gibbons) variously suggesting that & Droop, 2003; Hassani, Covey-Crump & Rutter, the deformations could be ‘matched’ across the two 2004). Areas for debate have been (1) the ‘age’ of formations, and that the contrast in style and intensity the Rhoscolyn Anticline (whether D1,D2, etc.), (2) was the result of their lithological differences, and correlations between structures and fabrics in the the strong bedding-parallel first schistosity/foliation contrasting Rhoscolyn and New Harbour formations, developed in the more pelitic New Harbour Formation. and (3) the nature of the boundary between the two Our observations, in the present paper, tend to support formations. We provided evidence (Treagus, Treagus & this conclusion. Droop, 2003) to quantify two significant deformations In this paper, we describe the geology and structures in South Stack Group rocks at Rhoscolyn: D1, creating at four localities at or close to the boundary between the Rhoscolyn Anticline as an initially upright NE– the Rhoscolyn Formation and New Harbour Formation SW-trending fold; and D2, modifying this anticline to on Holy Island, Anglesey, to demonstrate continuity of a SE-vergence, and creating crenulation fabrics and sedimentation and structures across the boundary, in the minor folds. However, these two clear-cut deformation field and on the microscopic scale. We will show that phases in the Rhoscolyn Formation are less obvious there are no mesoscopic or microstructural structures in the New Harbour Formation. Major and mesoscale indicative of a tectonic shear zone at this boundary. Our folds of sandstone beds or packets of beds are the observations will also contribute to the discussion of dominant D1 structures in all the South Stack Group the sedimentary environment in this part of the Monian on Holy Island, but no structures of this scale have of Anglesey. ever been described in the New Harbour Formation. The New Harbour Formation is sedimentologically 3. The boundary between the Rhoscolyn and New far more uniform, lacking prominent sandstone beds, Harbour formations: new observations and is characterized by a strong bedding-parallel foliation/schistosity and lineation, and a variety of We describe the boundary at four localities, A–D discontinuous small-scale folds and bands. Three or (Figs 1c, 3). The most completely exposed section is four phases of deformation have been proposed for at A, on the steep SE-dipping limb of the regional the New Harbour Formation based on their small- Rhoscolyn Anticline, where the deformational history scale structures (Cosgrove, 1980; Phillips, 1991b; has been much debated, as cited in Section 2. Hudson & Stowell, 1997). One of the purposes of the present study is to resolve these apparent differences 3.a. Borth Wen, locality A in deformation between the Rhoscolyn Formation and the New Harbour Formation, specifically by analysing The boundary between the Rhoscolyn Formation and the geology and structures at the boundary in the field the New Harbour Formation is seen most completely and in thin-section. here, at Borth Wen (also known as Rhoscolyn Beach) Greenly (1919, p. 156) discussed several exposures (Fig. 4). The best-exposed outcrop of the Rhoscolyn of the boundary of the Rhoscolyn Formation and New Formation, nearest to the boundary between the two Harbour Formation, stating: ‘at all these sections there formations, is seen at (i) [SH 2714 7501], in a near- is a rapid but unbroken change from one type of horizontal 100 ×10 m beach exposure of coarse to sedimentation to another’. In his description for Borth fine sandstones interbedded with subordinate mud- Wen (Rhoscolyn Bay; locality A of this paper), Greenly stones. The sandstones contain about 80 % quartz, (1919, p. 157) described basic tuffs and spilitic lavas the remainder comprising plagioclase, microcline and close to the boundary. Sedimentary continuity is im- heavy minerals. Each sandstone bed typically grades plied by Shackleton (1969) in his reinterpretation of the up into mudstone, now with metamorphic muscovite.

Figure 3. Cross-section X–X shown in Figure 1, showing position of localities A–D discussed in text. The boundary of the Rhoscolyn/New Harbour formations (RF – blank, NHF – shaded) is shown, interrupted where uncertain, with notional minor D1 folds and cartoons of minor D2 fold geometry and S2 axial-planar cleavage. Three major D1 folds to which S1 is axial-planar are shown: RA – Rhoscolyn Anticline, KA – Kirkland Anticline and HS – Holyhead Syncline. The latter two are newly described and discussed in this paper.

Sandstone bed thicknesses vary from approximately have been observed, and no clasts show evidence of an 15 to 150 cm. Sandstones contain scattered laminated inherited deformational fabric. mudstone clasts and some differentially weathering To the southwest, in exposures of the Rhosco- ellipsoidal structures interpreted as post-depositional lyn Formation closer to the contact (Fig. 4 (ii)) nodules. As well as graded bedding, some sandstone [SH 2708 7495], the thickness of the lateral continu- beds show ripple cross-lamination. Both depositional ation of the conglomerate horizon is at least 8 m in structures show that the steep SE-dipping succession plan view. Here it is less matrix-rich, and the strongly youngs to the southeast and faces upwards on the steep elliptical clasts have maximum long to short axial ratios NW-dipping S1 cleavage (Fig. 3). of between 5 and 10:1 perpendicular to S1. In thin- These beach exposures (Fig. 4 (i)) reveal lenses section there are seen to be lenses of granule and of matrix-supported conglomerate (Fig. 5), previously fine pebble conglomerate here with clasts from 1 to unrecorded, that vary in structural thickness from 10 mm in length that locally act as a matrix to the one to several metres (sub-perpendicular to bedding larger clasts. and cleavage). The conglomerate clasts include a The boundary of the Rhoscolyn Formation with the range from coarse sandstone through to mudstone, New Harbour Formation is exposed in a 5 m long cliff all now aligned in the first cleavage (S1) (Fig. 5). on the west side of Porth-y Corwgl [SH 2702 7478], Clast long dimensions range from 1 to 20 cm. In just to the southwest of Borth Wen (Fig. 4 (iii)a), shown two-dimensional views perpendicular to S1, all the in Figure 6a. Below we describe the continuously clasts appear strongly ‘flattened’ (elliptical). From exposed, near vertically dipping sequence here, from the limited three-dimensional exposure available, the the top of the Rhoscolyn Formation through the deformed clasts appear oblate. Some mudstone clasts 40 m of the succeeding thickness of New Harbour have a more angular outline and exhibit bedding Formation. In this description we refer to thin-sections, at a high angle to the clast length. Both the clast 15 of which have been made, five from the top 2 m lithologies and their matrix are very similar to the of the Rhoscolyn Formation, two from the lowest neighbouring lithologies in the Rhoscolyn Formation New Harbour Formation, four across the tuff member here, suggesting an intrabasinal origin. In particular, no described in below and the remainder at intervals exotic clasts, such as vein quartz or igneous material, through the stratigraphically higher New Harbour

unusually feldspathic and, significantly, locally contain thin (millimetre-scale) stripes of dark opaque minerals (mainly haematite) similar to those seen in the New Harbour Formation above. In the quartzites nearest to the contact (X on Fig. 6a), open NE-plunging folds affect bedding and the S1 schistosity, together with its quartz elongation lineation that plunges steeply at right angles to the fold axes; thus, we attribute these folds to the second deformation (D2). The folds and lineation are equivalent in style and attitude to the main folds and lineation described below in the New Harbour Formation. Laterally (Fig. 4 (iii)b), ∼6 m southeast of the exposed boundary, and ∼2 m below it stratigraphically, the sandstones of the Rhoscolyn Formation are replaced by matrix-supported conglomerates, in a lens ∼10 m long, with similar constituent clasts to the conglomerates described previously. The mudstone matrix to the conglomerate clasts is now schistose. The boundary of the Rhoscolyn Formation with the New Harbour Formation is perfectly exposed (Fig. 6a, X), with the topmost sandstone of the Rhoscolyn Formation succeeded to its southwest by a unit, about 2 m thick, of finely laminated grey mudstones, of typical New Harbour Formation lithology, other than containing distributed fine haematite. The main schistosity (S1) parallels bedding, emphasizing the pervasive lamination so characteristic of the New Harbour Formation. Thin-sections show this lamination Figure 4. Map of Borth Wen (Rhoscolyn Bay) and Porth y to comprise alternations of quartz/feldspar-rich and Corwgl, locality A of Figures 1 and 3, showing locations (i)– chlorite/muscovite/haematite-rich laminae, 1 or 2 mm (iv), with (iii) subdivided into a–d, as discussed in the text. The thick. These dark New Harbour Formation mudstones line of the boundary between the Rhoscolyn and New Harbour contain several features that are seen in the Rhoscolyn formations (RF/NHF) is shown, displaced by a fault. Formation immediately below the boundary. Locally they contain stripes of dark opaque minerals (mainly haematite) identical to those in the Rhoscolyn Form- ation; they are affected by minor folds, which are of the same size, style and attitude as the D2 folds seen in the Rhoscolyn Formation; and the folded S1 schistosity contains a quartz elongation lineation identical in its attitude to that in the Rhoscolyn Formation below. Up section, to the southwest of these schists, there is a sharp boundary (Y on Fig. 6a) with a unit, about 4–5 m wide, of uniformly steep SE-dipping foliated rock of a darker blackish colour and locally pock- marked texture. In thin-section this consists of chlorite, muscovite, quartz, albite and ore minerals; minor heavy minerals include zircon and apatite, and brown oxidized ankerite is seen as an interstitial mineral. The rock is finely laminated, with laminae of relatively coarse quartz and albite alternating with laminae of finer- Figure 5. The conglomerate at Borth Wen, at locality (i) of Figure 4, viewed in sub-horizontal exposure. Pen is 15 cm long grained chlorite, muscovite and albite. Chlorite usually dominates over muscovite and the ratio of quartz to and oriented with top pointing SW, aligned with the S1 trace. albite is variable, but more commonly albite dominates. In one instance it can be seen that randomly oriented Formation. A summary log through the succession is albite clasts are pseudomorphed by metamorphic given in Figure 7, Log A. albite. The dark colour of the unit is owing to the The Rhoscolyn Formation here is steeply N-dipping presence of haematite, generally as a fine-grained dust and overturned (076◦/80◦ N), and comprises beds but also locally as randomly oriented laths up to a of sandstone and subordinate mudstone, 20–50 cm millimetre long. The opaques are most concentrated thick. In thin-section the sandstones are seen to be in chlorite-rich seams, sometimes as distinct layers

Figure 6. The Rhoscolyn/New Harbour Formation boundary at Porth y Corwgl (Fig. 4), at locality (iii)a. (a) Looking SW, X marks the boundary between the Rhoscolyn Formation (right) and New Harbour Formation, and Y is the boundary of the New Harbour Formation with the tuff horizon, as discussed in the text; person on left for scale. (b) Looking NE, hammer marks upper transitional boundary of tuff with New Harbour Formation (right). Sedimentary layering in the tuff appears as subvertical traces in the pale area, top left. themselves. This unit also locally contains lenses, 5– At the SW margin of the tuffs (Fig. 4 (iii)c), there 10 mm long, of pink ankerite. Sedimentary layering, as is a transition into the stratigraphically younger part well as schistosity, is clear in this unit (Fig. 6). of the New Harbour Formation (Fig. 6b). The New Greenly (1919, pp. 261–2) refers to these rocks as Harbour Formation schists were originally laminated tuffs (or tuff-schists). We concur with this term because mudstones, comprising alternations of quartz/feldspar- the rock is clearly laminated, and we conclude a basaltic and chlorite/muscovite-rich millimetre-thick laminae. origin from the mineralogy. Our observations do not Thin-sections (Fig. 8b) and polished slabs show support Phillips’s (1991a) description of this rock as critically that each original quartzose silt lamina is a metabasalt. As we shall see below, this tuff unit is sharp based and tends to grade up into the overlying remarkably persistent in other exposures of the New mudstone lamina. However, immediately above the Harbour Formation at this stratigraphical level across tuff, the lithology differs from the typical New Holy Island. Harbour Formation in that it include stripes of dark

Figure 7. Schematic lithological logs at localities A–D shown in Figures 1 and 3. Lithological ornaments show relative not absolute bed thicknesses. Stratigraphic thicknesses are approximate in these strongly deformed sections. Thicknesses and further details are described in the text.

opaque minerals, seen in thin-section to be dominantly Barber & Max, 1979) or a tectonized movement zone haematite, as seen in the New Harbour Formation (c.f. Phillips, 1991a). In particular, we emphasize that lithology below the tuffs. These mudstones, with a dark there is no evidence either in the field or in thin-section sheen, give way after ∼8 m to the typical New Harbour for mylonitic textures, shear bands or C-S fabrics. Formation schistose mudstones. The mudstones are The position of the Rhoscolyn Formation/New commonly affected by NW-verging, NE-plunging D2 Harbour Formation boundary can be identified in three metre-scale folds, as seen in Figure 6b, with a gently other localities on Holy Island (Figs 1, 3; B, C and D NW-dipping axial-planar crenulation cleavage. Folds Section 3.b–d), although none supplies as complete a of this scale and geometry are seen in all the New section as A at Borth Wen. The successions at these Harbour Formation schists on Holy Island and are best localities are also shown in simplified form in Figure 7. illustrated at Twr,ˆ locality C (see Fig. 9a). Above these haematite-rich New Harbour Formation mudstones, 3.b. Porth Dafarch Road, locality B/B ∼25 m from the boundary, exposures and thin-sections reveal D2 folds refolding small isoclinal folds we Here, components of the transition between the consider to be D1 folds. Figure 8a shows tight folds Rhoscolyn Formation and the New Harbour Formation of original sedimentary lamination with a fine axial- are seen in outcrop by the Porth Dafarch to Holyhead planar S1 fabric developed in the hinges. This becomes road (Fig. 7, Log B/B and Fig. 10a). The exposures are pronounced and indistinguishable from bedding on on the inverted limbs of two D1 minor anticlines that isoclinal D1 fold limbs, rendering bedding and S1 sub- are subsidiary to a major D1 anticline (Fig. 3), which parallel, and creating the pronounced foliation and trends E–W and is overturned to the south. The strikes mineral stretching lineation characteristic throughout of bedding and the cleavages are here ESE–WNW, in the New Harbour Formation that is seen folded in later contrast to the NE or ENE strike in localities A, C and D2 folds (Fig. 8b). D; this is considered to be the result of rotation in this Younger New Harbour Formation mudstones are area of unusually intense faulting. seen immediately across Porth y Corwgl (Fig. 4 (iii)d), about 30 m distant from the latter exposures. These 3.b.1. Locality B are now the typical finely banded schistose mudstones of the formation, as in Figure 8, lacking the opaque This locality was described by Shackleton (1969 pp. mineral stripes seen in the New Harbour Formation de- 6–7), but his data were mostly obtained from a quarry scribed above. The Rhoscolyn Formation/New Harbour immediately north of the roadside contact we describe; Formation boundary can also be identified in seaweed- the overgrown nature of this quarry now makes covered rocks (Fig. 4 (iv)), evidently displaced by a it inaccessible. However, the information we report fault on the east side of Porth y Corwgl. agrees precisely with Shackleton’s in the geometry We conclude from this best-exposed locality (Fig. 7, of the folded succession. Our only disagreements are Log A) that sedimentation is normal and continuous, firstly, that we have observed a few metres of New and that there is no evidence of a tectonic break (c.f. Harbour Formation mudstones between the Rhoscolyn

here. The beds descend steeply (085◦/62◦ N) towards the road. At the roadside, New Harbour Formation schistose mudstones, that have a characteristic black sheen, are seen in contact with and structurally beneath thinly bedded (1–2 cm thick) graded sandstones of the Rhoscolyn Formation. In thin-section the New Harbour Formation lithology has the same characteristics as those seen at locality A (Borth Wen), immediately above the Rhoscolyn Formation. The boundary trends ◦ ◦ 080 /48 N. D2 minor folds and bedding/S2 relations in the schists show northerly vergence. The New Harbour Formation schistose mudstones on this N- dipping limb of the anticline, still inverted from the grading in the Rhoscolyn Formation, can be seen in a series of discontinuous exposures to the south, on the northwest of the road. Exposures about 10 m south of the boundary, are typical of the tuff, as already described for locality A: feldspathic, calcitic laminated tuffs with delicate laminae of opaque minerals, mostly haematite. The rocks are affected by minor D2 folds, trending ESE (e.g. 10◦/110◦) and verging north. The New Harbour Formation mudstones are seen again (Fig. 10a (ii)) another 100 m further south (S1 095◦/26◦ N), probably separated from the previous exposures by a fault, one of several in the area that trend 020◦. The schistose mudstones are seen to be affected by D2 minor folds, folding a stretching lineation trending 110◦. These rocks are typical of the ﬁnely laminated lithology of the New Harbour Formation,

Figure 8. Two adjacent images, 15 mm wide, of a D2 fold in the except that they have a black sheen, owing to the New Harbour Formation about 25 m above the boundary at Porth concentration of opaque minerals as seen in locality A. y Corwgl (Fig. 4) at [SH 2702 7475]. (a) Upper part shows tight Typical New Harbour Formation schistose mudstones to isoclinal folds of ﬁne sedimentary layers that we interpret as without this sheen, and with only a minor opaque D1 folds, with axial-planar S1 fabric in the hinges, passing into a stronger S fabric on fold limbs. Lower part shows outer arc of mineral content, are seen a further 80 m to the south 1 (Fig. 10a (iii)) [SH 2360 8055] and from thereon to the the D2 foldshownin(b).(b)D2 fold of compositional S1/bedding foliation. Original sedimentary lamination comprises quartz south. silt grading into chlorite/mica laminae; lamination youngs

downwards in this view. S2 crenulation cleavage is weakly developed in the chlorite/mica laminae. 3.b.2. Locality B The boundary of the Rhoscolyn Formation with the New Harbour Formation is also seen in crags Formation and the tuffs (Fig. 7), and secondly that we [SH 242 805] 300 m to the southeast, at B (Figs 1c, identify the folds as being of a second generation, based 3, 10a (iv)). This locality again embraces exposures on the observation of an early cleavage folded by the named by Shackleton (1969, pp 6–7). Here the folds, supported by thin-section evidence. tuffs, comprising alternating laminae rich in opaque The boundary of the New Harbour Formation minerals and carbonate, are seen structurally be- mudstones with the Rhoscolyn Formation sandstones neath Rhoscolyn Formation sandstones, the boundary ◦ ◦ is seen in an exposure at the foot of the crag above dipping 080 /44 N. D2 folds in the New Harbour the road (Fig. 10a (i)) [SH 2385 8085], immediately Formation schistose laminated mudstones and tuffs opposite the boundary with a minor road. At the top of trend between 080◦ and 120◦ with N vergence. They the crag above the roadside, the Rhoscolyn Formation fold a strongly developed quartz stretching lineation comprises beds of sandstone containing scattered (typically 20◦/145◦) on the ﬂatter limbs of the folds. stretched 1.5 mm long quartz grains, alternating with Most importantly, a penetrative S1 cleavage is seen thin beds of mudstone. In thin-section, the sandstones in exposures of the Rhoscolyn Formation above the contain ∼5 % feldspar and the mudstones are locally boundary [SH 2425 8065]; the cleavage dips at a rich in ﬁne-grained haematite. The rocks are affected lower angle (e.g. 056◦/45◦ NW) than bedding (e.g. ◦ ◦ by 10 cm scale D2 folds that trend ESE and verge N 056 /52 NW), consistent with the position of these with axial-planar S2 cleavage in alternating mudstones; exposures on the overturned limb of a major D1 no certain bedding/S1 relations have been observed anticline, facing up to the southeast (Fig. 3). The

Figure 9. (a) D2 folds in the New Harbour Formation schists at Twrˆ (locality C; Fig. 10b (iii)). View looking NE; cliff is ∼8 m high. (b) Close-up of isoclinal D1 fold-pair indicated by pencil (15 cm long) and arrow, as located in (a).

boundary is seen again (Fig. 10a (v)), displaced by (i)) of the Rhoscolyn Formation, the sandstones begin a fault trending NNE. The stratigraphy for B and B is to show a progressively stronger schistosity as their summarized in Figure 7. original mud content increases. A penetrative S1 cleavage dips more steeply southeast than bedding (060◦/40◦ SE), and a strong stretching quartz lineation 3.c. Twr,ˆ locality C pitches down dip. In the scattered outcrops of these These outcrops occur southeast of the South Stack schistose sandstones there are outcrops showing to Holyhead road, in open land [SH 231 821] near a ﬂattened (20 cm long) chloritic, haematite-rich lenses, footpath leading southeast from the farm called Twr,ˆ at strongly elongated down dip. Within 50 m of the main the foot of Holyhead Mountain. Here the main outcrop cliff of New Harbour Formation schistose mudstones of the Rhoscolyn Formation (Fig. 10b (i)) is seen as (Fig. 9a), scattered outcrops of the tuff are seen, a ridge of prominent outcrops of a white sandstone, and in one outcrop (Fig. 10b (ii)) the tuff is seen vertical-dipping, NE-striking (058◦), while 150 m to beneath, and grading into the New Harbour Formation their southeast is a continuous cliff-like exposure of mudstones above, dipping 30◦ SE. The tuff is the the schistose mudstones of the New Harbour Formation typical tough, black, feldspathic, calcitic laminated (Fig. 10b (iii)). Between these two areas of continuous lithology, as seen in localities A and B, described outcrop there are only scattered outcrops (Fig. 10b (ii)) in Sections 3.a and 3.b. of the lithologies that characterize the upper Rhoscolyn The principal exposures of the New Harbour Formation and the New Harbour Formation near the Formation mudstones are seen 20 m to the southeast boundary, as seen at Borth Wen and the Porth Dafarch of this outcrop in a continuously exposed cliff- Road localities (A, B). At around [SH 2307 8205], section (Figs 9a, 10b (iii)), running NE to SW 30 m southeast of the principal outcrop ridge (Fig. 10b from [SH 2305 8190]. These typical ﬁnely laminated

Figures 10. Maps of the Rhoscolyn Formation (blank) and New Harbour Formation (shaded) at localities B/B, C and D of Figures 1c and 3. Interrupted lines are roads and footpaths; faults are shown as dash–dot lines. (a) Map of localities B and B, the area around the Porth Dafarch (PD)–Holyhead (H) road, showing locations (i) to (v) discussed in the text. (b) Map of locality C, the area southeast of the South Stack–Holyhead road SE of Twrˆ farm. The arrowed areas (i) and (iii) are prominent areas of outcrop of the Rhoscolyn Formation quartzites and the New Harbour Formation schists, respectively. The area between (i) and the dotted line is of scattered outcrop of quartzite; the area between the dotted line and the New Harbour Formation boundary is of scattered outcrop of tuff and schist, and (ii) is the tuff boundary with the New Harbour Formation, discussed in the text. (c) Map of locality D, the area west of the Holyhead Breakwater, showing locations (i) to (iv) discussed in the text.

mudstones comprise alternations of quartz/feldspar- with a 40 m gap between them. On the cliff-top to the and chlorite/muscovite-rich millimetre-thick laminae, west of the bay (Fig. 10c (i)), the Rhoscolyn Formation but are unusually haematite-rich compared with the greyish-white sandstones dip SE (050◦/65◦ SE), with ◦ ◦ normal New Harbour Formation lithology, giving a weak anastamosing S1 cleavage (e.g. 052 /80 SE); them a characteristic black sheen in the field. They they are well exposed, becoming more schistose exhibit SE-verging D2 folds (Fig. 9a), with a sheet- southeastwards towards the cliff edge [SH 2355 8380]. dip estimated to be 30◦ SE, similar to that of the tuff In these exposures Skolithos burrows can locally be contact described above, and taken as representative detected. On the cliff face below (Fig. 10c (ii)), of the boundary between the Rhoscolyn Formation and the honey-coloured rocks again dip steeply SE but the New Harbour Formation here. These folds deform a here with a well-developed S1 cleavage now dipping strongly developed quartz stretching lineation, trending steeply NW (065◦/80◦ NW) (Fig. 11a). In these cliff ◦ 130 on their flat limbs. Isoclinal D1 folds are seen exposures the Skolithos burrows are well exposed rarely (Fig. 9b), but exhibit no clear vergence, and in an interval some 20 m thick. The burrows, 5– neither ‘way-up’ nor bedding/S1 relations have been 10 mm across and up to 10 cm long, are prominent, determined. The typical New Harbour Formation sub-perpendicular to bedding and S1, and are well mudstones are seen in prominent ridges further to seen on the cleavage surfaces (Fig. 11b). The tubes the southeast (Fig. 10b (iv)), but the precise position consist of a finer-grained whiter sandstone than of the boundary with the haematite-rich lithology has the surrounding cream sandstone. The predominant not been established. The lithologies are represented, orientation in sections perpendicular to bedding and stratigraphically, in Figure 7, Log C. cleavage is sub-parallel to S1, consistent with an original orientation perpendicular to bedding (see Treagus, 1999). However, a more variable orientation is 3.d. Holyhead Breakwater, locality D seen on some S1 surfaces, suggesting slight variations in original burrow orientations. Their preservation The Rhoscolyn Formation and the New Harbour during the D1 cleavage-forming deformation might Formation, respectively, are exposed on the two sides indicate a somewhat flattening (oblate) D1 strain of a bay on the west side of Holyhead Breakwater, ellipsoid.

Figure 11. Rhoscolyn Formation quartzites northwest of the Holyhead Breakwater, at location (ii) of Figure 10c. (a) View looking ◦ approximately W at cliff face; trace of S1 cleavage is vertical, parallel to pen (15 cm long), bedding trace is approximately 45 down to left. Skolithos shows in relief, with an orientation dominantly sub-parallel to the S1 trace. (b) Skolithos seen on S1 cleavage surface, coin 2 cm diameter.

The most southeasterly exposures on the shore, and 4. Discussion: tectonic structure the uppermost stratigraphically, are white sandstone Summarizing from our introduction (see Fig. 2), the that locally contains white quartzite pebbles 5 mm– boundary between the Rhoscolyn Formation and the 2 cm in diameter. To their southeast, across an New Harbour Formation in the Monian Supergroup unexposed 40 m wide beach, New Harbour Formation of Anglesey, North Wales, has been interpreted schistose mudstones are seen in reefs on the shore previously in three different ways. (1) Greenly (1919, (Fig. 10c (iii)) and on the cliff at their back, as far 1923) considered it a true sedimentary boundary, as the breakwater [SH 236 837]. These mudstones with unbroken sedimentation from the Rhoscolyn are typical of the New Harbour Formation, with the Formation (his South Stack Series) into the New lustrous sheen, attributed to the presence of haematite, Harbour Formation; as did Shackleton (1969) in as described in localities A, B and C immediately above his revision of the stratigraphy. (2) Barber & Max the tuffs, which here are unexposed. The schistose (1979) proposed a major structural discordance at this mudstones are affected by D2 minor folds, with varying ◦ ◦ boundary, with the more highly deformed New Harbour axial trend between 040 and 090 and verging SE to Formation (which they considered older and already S, which fold a quartz stretching lineation trending ◦ deformed) thrust over the Rhoscolyn Formation before 130 . Some 10 m to the northeast, the typical regional the South Stack Group began its deformation history. New Harbour Formation lithology, without the lustrous (3) Phillips (1991a,b) proposed that the boundary was a sheen, is exposed on the side of the breakwater (Fig. 10c tectonized sedimentary contact, describing it as a high (iv)) [SH 2364 8380]; this unit must be displaced by strain/highly sheared zone. a NW-trending fault as shown in Figure 10c. They Our descriptions of the Rhoscolyn Formation/New are strongly affected by D2 folds, here up to 3 m in Harbour Formation boundary at four key localities on wavelength, verging SE, as described and illustrated Holy Island, Anglesey, demonstrate that there is indeed for Borth Wen (Figs 6, 8) and Twrˆ (Fig. 9). Isoclinal D1 sedimentary continuity at the boundary, supporting (1) closures are rarely seen in the New Harbour Formation above. We provide particular detail for locality A at here, but offer no indication of their overall vergence. Borth Wen, Rhoscolyn, because this is the best place to The lithological and stratigraphic information for this observe continuity in sedimentation and in structures. locality is simpliﬁed in Figure 7, Log D. Here, we found no evidence of unusually high strain

in the vicinity of the boundary, nor at any of the other controlled by competent sandstone beds, which have localities we describe. Importantly, there is no evidence different thicknesses and abundances in the South of shear-related structures, such as might be seen in Stack Formation, Holyhead Quartzite Formation and a brittle or ductile shear zone: no asymmetric shear Rhoscolyn Formation. In contrast, the New Harbour criteria, either in outcrop or thin-section, no significant Formation lacks thick sandstone beds and therefore the fractures, and no S-C fabrics or mylonitic textures that competence contrasts that might lead to the large-scale would indicate high strain or shear. Instead, there is a D1 structures seen in the underlying stratigraphy. Most simple upward progression of depositional units. The of the folds we have recorded in the New Harbour top of the Rhoscolyn Formation sandstones have a clear Formation are less than 20 cm in wavelength, and sedimentary boundary with haematite-rich mudstones many are on a millimetre scale (Fig. 8a). However, (1–2 m), overlain by a tuff unit (2–5 m), and then by because the D1 deformation was associated with the the thinly laminated mudstones of the New Harbour development of a penetrative S1 foliation throughout Formation. This formation is haematite-rich for several the South Stack Group and New Harbour Formation, metres, before taking on its more usual characteristics the later D2 deformation that folds this S1 foliation (quartz-chlorite-muscovite schist). (as well as bedding), and produces an S2 crenulation Our thin-section studies of the New Harbour Form- cleavage, can easily be correlated from the South Stack ation, from its stratigraphically lowermost localities Group into the New Harbour Formation. in this paper, and from exposures of New Harbour The appearance of several orientations and styles Formation throughout its succession in W Anglesey, do of early folds in the New Harbour Formation, and not support the description in Barber & Max (1979, p. their variable relationship to the main foliation and 415) that these rocks have an ‘intense planar schistosity lineation, might at first suggest more than one ‘phase’ of which is frequently mylonitic’. Their interpretation was deformation before the later (our D2) deformation. This that ‘deformation has separated the grit bands into thin was one reason that Barber & Max (1979) considered lenticles by transposition and pressure solution, giving the New Harbour Formation older, having suffered a typical “flaser” appearance to the bedding’. Instead, a prior deformation before being emplaced over the we conclude that the New Harbour Formation generally Rhoscolyn Formation. However, four contributors to preserves a thin sedimentary lamination originally the discussion of this paper (Wood, Powell, Maltman comprising graded silt–mud couplets (Fig. 8b). In & Gibbons in Barber & Max, 1979) disagreed. They places this lamination defines millimetric to centimetric suggested how and why the South Stack Group and the isoclinal folds to which the main schistosity/foliation New Harbour Formation could have shared the same (S1) can be seen to be axial-planar in thin-section D1 deformation, manifested differently in the different (Fig. 8a). It is this original sedimentary lamination units because of their rheological contrasts. Maltman that has been interpreted by others as an early tectonic specifically suggested that the fine sedimentary lay- fabric, leading them to interpret the main foliation ering of the New Harbour Formation caused smaller as S2 and the later folds as D3 (Cosgrove, 1980; D1 folds to develop (e.g. small-scale chevron folds Hudson & Stowell, 1997; Passchier, 2007). However, and conjugate pairs) than those in the underlying thick this is not our interpretation. We consider the main bedded units of the Rhoscolyn Formation. He argued foliation is S1, as shown in Figure 8, and the pervasive that every different fold and cleavage orientation on lineation on these surfaces to be L1, so that the the small scale should not be interpreted as a different later folds are D2. These conclusions thus conform tectonic event. We follow the same reasoning, and with our two-phase interpretation of structures in consider that the New Harbour Formation sediment was the South Stack Group in the Rhoscolyn Anticline initially thinly laminated silt–mud couplets, possessing (Treagus, Treagus & Droop, 2003). We conclude that sufficient rheological anisotropy to develop a range of the Rhoscolyn Formation and New Harbour Formation microscopic to mesoscopic D1 folds and banded struc- were stratigraphically conformable and that they share tures, as predicted for anisotropic materials (Cobbold, a two-phase deformation. The second deformation Cosgrove & Summers, 1971; Treagus, 2003). Because (D2) shows far more obvious continuity in folds and of the significant D2 deformation imposed on these fabrics across the boundary than the first deformation rocks (estimated from the Rhoscolyn Anticline as a (D1), a point that is highly relevant to previous plane strain ellipse in the anticline profile section, with structural interpretations, as acknowledged in our axial ratio ≈ 3; Treagus, Treagus & Droop, 2003), introduction. the present appearance of all these D1 structures in the This is not the place to present a full interpretation New Harbour Formation will have been much modified. of the structures in the New Harbour Formation on They are seen now as isoclinal folds of millimetre-scale Anglesey. However, it is relevant to acknowledge the bedding (Figs 8a, 9b), or microlithons that are remnant complexity and possible ambiguity of early structures hinge zones or kink bands. in the New Harbour Formation that pre-date the SE- This investigation into the nature of the New Harbour verging folds that we label D2. In the South Stack Formation/Rhoscolyn Formation boundary has enabled Group, there are clear large-scale D1 folds, such as the us to construct a new cross-section (Fig. 3) of the much-studied Rhoscolyn Anticline introduced earlier. Monian succession along the length of Holy Island. The size and geometry of the D1 folds is presumably There is a clear contrast between the steeply upward

NW-facing D1 structures at localities D and C, and and the upward gradation into capping mudstones. the steeply upward SE-facing structures at localities Observations of mudstone rafts within some sandstone B and A. This contrast reveals the presence of an beds are interpreted as rip-up clasts of partially intervening major D1 syncline, which we propose to consolidated mudstone. The turbidity flows concerned call the Holyhead Syncline, of the same generation were therefore energetic enough to erode along their as the well-known D1 Rhoscolyn Anticline in the depositional pathway, and had grain concentrations southeast of Holy Island. The geometry of the syncline high enough for mudstone clasts to be suspended has no doubt been modified by the D2 deformation, within the flow (Lowe, 1979) rather than being low- in the manner described for the Rhoscolyn Anticline concentration flows (Bouma, 1962). (Treagus, Treagus & Droop, 2003). To the southeast of The lenticular bodies of matrix-supported con- the Holyhead Syncline we have identified the presence glomerate observed at the Borth Wen locality are of a further D1 anticline (at locality B, B ), verging here interpreted as debris flows. The bodies lack to the SE, that we propose to call the Kingsland erosional channelized bases, so their lenticular form Anticline, after that district of Holyhead through is interpreted instead as the cross-sectional geometry which the axial trace passes. Other major D1 fold- of lobate debris flows with positive relief. The clasts traces presumably pass through the broadly synclinal are entirely of Rhoscolyn Formation lithologies, and intervening ground occupied by the stratigraphically the matrix is, as far as can be deduced in its intractable New Harbour Formation (Fig. 3), where metamorphosed state, compositionally similar to the the deformation is dominated by the later (D2) turbidite mudstones capping the graded sandstone folding. beds. The debris flows need not, therefore, be far- travelled and could even be the locally remobilized tops of recently deposited turbidites. 5. Discussion: depositional transition and Association of turbidites and debrites is well environments described in the literature. Debrites are classically 5.a. The Rhoscolyn/New Harbour Formation boundary assigned to an inner fan setting (Walker, 1976) such as the slope apron examples in Wales from the Yr Allt Although there is a significant sedimentary contrast Formation (upper Ordovician) (Davies et al. 1997), between the thickly bedded Rhoscolyn Formation and but they are now known from mid-fan settings too. the thinly laminated New Harbour Formation, we The best such examples in Wales come from the conclude that there is continuity of sedimentation Aberystwyth Grits Group of Silurian age (Talling et al. across the boundary. One subtle indicator is the 2004), where the debrites are considered to be deposited presence of fine-grained haematite as widely scattered along with the turbidites as hybrid flows. Another flakes within the top 3 m of the Rhoscolyn Formation example, geographically and stratigraphically closer to quartzite and in the lowermost few metres of the the Rhoscolyn Formation, is in the lower Ordovician New Harbour Formation schist. It is sufficiently Lady Port Formation of the Manx Group, Isle of Man concentrated to give an unusually dark colour to the (Woodcock & Morris, 1999). Given such examples, New Harbour Formation schistose mudstones above there is certainly no need to invoke a shallow-water the tuff horizon, for up to ∼5 m above the boundary. origin for the Rhoscolyn Formation conglomerates: in The boundary is also consistently marked by the tuff a shallow-water setting, they would be more obviously horizon of Greenly (1919), just within the base of the clast-supported. New Harbour Formation. The tuff, whose mineralogy Apparently in conflict with a deep-water origin for has been described in detail at Borth Wen (locality the Rhoscolyn Formation are the supposedly shallow- A), contains albite > quartz, plus haematite, chlorite, marine Skolithos burrows observed at the Holyhead ankerite and heavy minerals, and has the texture, Breakwater (locality D). Greenly (1919) and Barber mineralogy and composition of a fine tuff derived from & Max (1979) have recorded rare Skolithos also from a basic igneous melt. Greenly (1919, p.157) noted that the South Stack Formation. McIlroy & Horák (2006) the tuff can be observed at this stratigraphic level at record hummocky and swaley cross-stratification from more than 20 places on Holy Island, precluding the the Holyhead Formation and therefore suggest a possibility that the base of the New Harbour Formation shelfal storm sands origin for this and the South is a thrust. Stack Formation. However, although vertical burrows of the Skolithos ichnofacies are most common in nearshore marine shelf environments and in mid- 5.b. The Rhoscolyn Formation shelfal storm sands, they occur, exceptionally, in deep- Our new observations are consistent with the in- water turbidites (e.g. Frey, Pemberton & Saunders, terpretation of Phillips (1991a) that the Rhoscolyn 1990; Pemberton, MacEachern & Frey, 1992). They Formation comprises turbidites deposited in a mid- can be made in the tops of turbidites by ‘doomed fan setting. Supporting evidence in the sandstone beds pioneers’ (Föllmi & Grimm, 1990): shallow-water for a turbidite origin includes their lateral continuity, organisms carried by turbidity flows into deep and their sharp bases, the upward grading from coarser even anoxic water. Therefore, we continue to interpret to finer sand, the sporadic ripple cross-lamination the Rhoscolyn Formation as deposited in deep water,

that is, below storm wave base, until more persuasive ited as deep-water turbidites and possible associated evidence accumulates. hemipelagic fallout, the boundary represents a major shut-down or diversion of sand supply in the Monian depositional basin. In general, such a change could 5.c. New Harbour Formation either represent a major sea-level rise, or the tectonic- Our new observations of sporadic siltstone–mudstone ally induced isolation of the former quartz-rich source couplets in the New Harbour Formation have shown area. Phillips (1991a) deduced that the South Stack that some couplets are sharp based and graded from Group had been supplied with quartz sand from the silt up to mud, but the polyphase structural complexity southeast, whereas the New Harbour Group had been obscures their usefulness in determining regional way- sourced from a volcanic arc. This provenance for the up in these rocks. Weinterpret each couplet as a discrete New Harbour Formation awaits confirmation from the depositional event from a low-concentration density type New Harbour Formation on Holy Island. It is flow with velocities that waned during deposition. tempting, but too simplistic, to take the basaltic tuff A fine-grained component from vertical fallout of near the base of the New Harbour Formation to indicate suspended hemipelagic sediment cannot be excluded, the onset of arc volcanism, and to blame associated but the diagnostic very fine lamination would be tectonics for diverting the quartz-rich source for the obliterated by recrystallization. In the absence of other sand turbidites of the Rhoscolyn Formation. sedimentary structures, it is impossible to deduce palaeo-water depth from such a facies. However, no vertical burrows are preserved, implying that the 6. Conclusions sediment–water interface was either too deep or too low in oxygen to support an infauna. (1) New observations from key locations on Holy Laminated facies like that of the New Harbour Island, Anglesey, of the boundary between the Rhosco- Formation are common in turbidite basins. They lyn Formation (South Stack Group) and the overlying are particularly widespread in the belt of Cambro- New Harbour Formation demonstrate stratigraphic Ordovician rocks from the Skiddaw Group of NW conformity and continuity in sedimentation. England through the Manx Group of the Isle of Man to (2) The Rhoscolyn Formation preserves medium to the Ribband Group of SE Ireland. Examples of the thickly bedded graded sandstone–mudstone beds with facies are the Kirk Stile Formation in the Skiddaw sporadic lenticular matrix-supported conglomerates. Group (Cooper et al. 1995), the Maughold Forma- These were probably deposited as turbidites and tion and fine-grained components of the Lady Port debrites below storm wave base, though occasional Formation in the Manx Group (Woodcock et al. 1999; vertical burrows need special explanation. Woodcock & Morris, 1999), and the Riverchapel or (3) The New Harbour Formation mostly preserves Maulin formations in the Ribband Group (McConnell, laminated mudstones, comprising graded silt–mud Morris & Kennan, 1999). couplets. These record intermittent deposition from The origin of the haematite-rich variant of the New low-concentration sediment flows, almost certainly Harbour Formation is unclear. One possibility is that turbidites, in uncertain water depths. the iron was originally fixed by diagenesis as dispersed (4) The basal 10 m of the New Harbour Formation pyrite rather than haematite, only later to be oxidized tends to be rich in haematite and to include a laminated during metamorphism. This hypothesis is consistent tuff unit. with enhanced dysaerobic conditions in the basinal (5) No evidence is found for a tectonic boundary bottom waters at the time of the Rhoscolyn Form- between the Rhoscolyn Formation and New Harbour ation/New Harbour Formation boundary. A similar Formation, such as a thrust or shear zone, as described interpretation could apply also to the haematite-rich in some previous studies. part of the Rhoscolyn Formation below the boundary. (6) Changes in scale and style of first deformation The tuff near the base of the New Harbour Formation (D1) structures in the Rhoscolyn Formation and New was presumably deposited in the same deep, quiescent Harbour Formation across the boundary are explained and possibly anoxic bottom waters as the New Harbour by differences in bed thickness between the two Formation. The lack of any cross-lamination suggests formations, leading to differences in their rheology and that any lateral flow velocities during deposition were anisotropy during D1 and its related metamorphism. below the threshold for forming ripples. However, the (7) The later (D2) deformation shows continuity well-developed depositional lamination implies that in style of folds and fabrics across the transitional deposition either took place in a succession of low- boundary, confirming a shared tectonic history. volume events or possibly as one high-volume event (8) Mapping around the key localities of the bound- with a pulsed sediment delivery through time. ary of the Rhoscolyn Formation and New Harbour Formation leads to a new cross-section of major D1 folds on Holy Island. We identify two new major folds, 5.d. The Rhoscolyn/New Harbour transition the Kingsland Anticline and the Holyhead Syncline, On our favoured interpretation that both the Rhoscolyn and a marked change in facing direction along the line Formation and New Harbour Formation were depos- of section.

Acknowledgements. We acknowledge use of facilities in GREENLY, E. 1923. The succession and metatmorphism in SEAES, University of Manchester, over many years. We the Mona Complex. Quarterly Journal of the Geological particularly thank Giles Droop and Richard Pattrick for their Society 79, 334–51. advice on the mineralogy in the tuff and New Harbour HASSANI,H.,COVEY-CRUMP,S.J.&RUTTER, E. H. 2004. Formation schists. Thanks also go to Richard Lisle for On the structural age of the Rhoscolyn antiform, pointing out newly exposed conglomerate at Borthwen, in Anglesey, North Wales. Geological Journal 39, 141– 2002. We appreciate the comments of referees on earlier 56. versions of this work and particularly thank David Schofield HOLDSWORTH,R.E.,WOODCOCK,N.H.&STRACHAN, for his careful review of the present version. R. A. 2012. Geological framework of Britain and Ireland. In Geological History of Britain and Ireland (2nd ed.) (eds N. H. Woodcock & R. A. Strachan), References pp. 19–39. Chichester: Wiley-Blackwell. HORÁK,J.M.&EVANS, J. A. 2011. Early Neoproterozoic BARBER,A.J.&MAX, M. D. 1979. A new look at the limestones from the Gwna Group, Anglesey. Geological Mona Complex (Anglesey, North Wales). Journal of Magazine 148, 78–88. the Geological Society, London 136, 407–32. HUDSON,N.F.C.&STOWELL, J. F. W. 1997. On the BOUMA, A. H. 1962. Sedimentology of Some Flysch Deposits. deformation sequence in the New Harbour Group of A Graphic Approach to Facies Analysis. Amsterdam: Holy Island, Anglesey, North Wales. Geological Journal Elsevier. 32, 119–29. BRITISH GEOLOGICAL SURVEY. 1996. Tectonic Map of KAWAI,T.,WINDLEY,B.F.,TERABAYASHI, M., YAMAMOTO, Britain, Ireland and Adjacent Areas.Keyworth,Not- H., MARUYAMA,S.&ISOZAKI, Y. 2006. Mineral tingham: British Geological Survey. isograds and metamorphic zones of the Anglesey COBBOLD,P.R.,COSGROVE,J.W.&SUMMERS,J.M. blueschist belt, UK: implications for the metamorphic 1971. Development of internal structures in deformed development of a Neoproterozoic subduction–accretion anisotropic rocks. Tectonophysics 12, 23–53. complex. Journal of Metamorphic Geology 24, 591– COLLINS,A.S.&BUCHAN, C. 2004. Provenance and age 602. constraints of the South Stack Group, Anglesey, UK: U- KAWAI,T.,WINDLEY,B.F.,TERABAYASHI, M., YAMAMOTO, Pb SIMS detrital zircon data. Journal of the Geological H., MARUYAMA,S.,OMORI,S.,SHIBUYA,T.,SAWAKI, Society, London 161, 743–6. Y. & I SOZAKI, Y. 2007. Geotectonic framework of the COOPER,A.H.,RUSHTON,A.W.A.,MOLYNEUX,S.G., Blueschist Unit on Anglesey-Lleyn, UK, and its role HUGHES,R.A.,MOORE,R.M.&WEBB,B.C. in the development of a Neoproterozoic accretionary 1995. The stratigraphy, correlation, provenance and orogen. Precambrian Research 153, 11–28. palaeogeography of the Skiddaw Group (Ordovician) LOWE, D. R. 1979. Sediment gravity flows: their classification in the English Lake District. Geological Magazine 132, and some problems of application to natural flows and 185–211. deposits. Special Publication of the Society of Economic COSGROVE, J. W. 1980. The tectonic implications of some Paleontologists and Mineralogists 27, 75–82. small-scale structures in the Mona Complex of Holy Isle, MCCONNELL,B.J.,MORRIS,J.H.&KENNAN, P. S. 1999. A North Wales. Journal of Structural Geology 2, 383–96. comparison of the Ribband Group (southeastern Ireland) DAV I E S ,J.R.,FLETCHER,C.J.N.,WATERS,R.A.,WILSON, to the Manx Group (Isle of Man) and Skiddaw Group D., WOODHALL,D.G.&ZALASIEWICZ, J. A. 1997. (northwestern England). In In Sight of the Suture: The Geology of the Country around Llanilar and Rhayader. Palaeozoic Geology of the Isle of Man in its Iapetus British Geological Survey Memoir. Keyworth, Notting- Ocean Context (eds N. H. Woodcock, D. G. Quirk, W.R. ham: Memoir, British Geological Survey, Sheets 178 & Fitches & R. P.Barnes), pp. 337–43. Geological Society 179 (England & Wales). of London, Special Publication no. 160. FÖLLMI,K.B.&GRIMM, K. A. 1990. Doomed pioneers: MCILROY,D.&HORÁK, J. 2006. Neoproterozoic: the late gravity-flow deposition and bioturbation in marine Precambrian terranes that formed Eastern Avalonia. In oxygen-deficient environments. Geology 18, 1069–72. The Geology of England and Wales (eds P. J. Brenchley FREY,R.W.,PEMBERTON,S.G.&SAUNDERS, T. D. A. 1990. & P. F. Rawson), pp. 9–23. London: The Geological Ichnofacies and bathymetry: a passive relationship. Society. Journal of Paleontology 64, 155–8. PASSCHIER, C. W.2007. Photograph of the month. Journal of GIBBONS, W. 1983. Stratigraphy, subduction and strike-slip Structural Geology 29, 1281. faulting in the Mona Complex of North Wales – a review. PEMBERTON,S.G.,MACEACHERN,J.A.&FREY,R.W. Proceedings of the Geologists’ Association 94, 147–63. 1992. Trace fossil facies models: environmental and GIBBONS,W.&BALL, M. J. 1991. A discussion of Monian allostratigraphic significance. In Facies Models: Re- Supergroup stratigraphy in northwest Wales. Journal of sponse to Sea Level Change (eds R. G. Walker & N. P. the Geological Society, London 148, 5–8. James), pp. 47–72. St Johns, Newfoundland: Geological GIBBONS,W.&HORAK, J. M. 1996. The evolution of the Association of Canada. Neoproterozoic Avalonian subduction system: evidence PHILLIPS, E. 1991a. The lithostratigraphy, sedimentology from the British Isles. Geological Society of America and tectonic setting of the Monian Supergroup, western Special Papers 304, 269–80. Anglesey, North Wales. Journal of the Geological GIBBONS,W.,TIETZSCH-TYLER,D.,HORÁK,J.&MURPHY, Society, London 148, 1079–90. F. C. 1994. Precambrian rocks in Anglesey, southwest PHILLIPS, E. 1991b. Progressive deformation of the South Llynˆ and southeast Ireland. In A Revised Correlation of Stack and New Harbour Groups, Holy Island, western Precambrian Rocks in the British Isles: Special Report Anglesey, North Wales. Journal of the Geological 25 (eds W.Gibbons & A. L. Harris), pp.75–84. London: Society, London 148, 1091–100. Geological Society. ROPER, H. 1992. Superposed structures in the Mona Complex GREENLY, E. 1919. The Geology of Anglesey. Geological at Rhoscolyn, Ynys Gybi, North Wales. Geological Survey of Great Britain Memoir. London: HMSO. Magazine 129, 475–90.

SHACKLETON, R. M. 1954. The structure and succession WALKER, R. G. 1976. Facies models. 2, Turbidites and of Anglesey and the Lleyn Peninsula. Advancement of associated coarse clastic deposits. Geosciences Canada Science 11, 106–8. 3, 25–36. SHACKLETON, R. M. 1969. The Pre-Cambrian of North WOOD, D. S. 1974. Ophiolites, melanges, blueschists and Wales. In The Pre-Cambrian and Lower Palaeozoic ignimbrites: early Caledonian subduction in Wales. In Rocks of Wales (ed. A. Wood), pp. 1–22. Cardiff: Modern and Ancient Geosynclinal Sedimentation (eds University of Wales Press. R. H. Dott & R. H. Shaver), pp. 334–44. TALLING,P.J.,AMY,L.A.,WYNN,R.B.,PEAKALL, WOODCOCK,N.H.&MORRIS, J. H. 1999. Debris ﬂows on the J. & ROBINSON, M. 2004. Beds comprising debrite Ordovician margin of Avalonia: Lady Port Formation, sandwiched within co-genetic turbidite: origin and wide- Manx Group, Isle of Man. In In Sight of the Suture: spread occurrence in distal depositional environments. The Palaeozoic Geology of the Isle of Man in its Iapetus Sedimentology 51, 163–94. Ocean Context (eds N. H. Woodcock, D. G. Quirk, W.R. TREAGUS, S. H. 1999. Are viscosity ratios of rocks meas- Fitches & R. P.Barnes), pp. 121–38. Geological Society urable from cleavage refraction? Journal of Structural of London, Special Publication no. 160. Geology 21, 895–901. WOODCOCK,N.H.,MORRIS,J.H.,QUIRK,D.G.,BARNES, TREAGUS, S. H. 2003. Viscous anisotropy of two-phase R. P., BURNETT,D.,FITCHES,W.R.,KENNAN,P.S.& composites, and applications to rocks and structures. POWER, G. M. 1999. Revised lithostratigraphy of the Tectonophysics 372, 121–33. Manx Group, Isle of Man. In In Sight of the Suture: TREAGUS,S.H.,TREAGUS,J.E.&DROOP,G.T.R. The Palaeozoic Geology of the Isle of Man in its Iapetus 2003. Superposed deformations and their hybrid effects: Ocean Context (eds N. H. Woodcock, D. G. Quirk, W.R. the Rhoscolyn Anticline unravelled. Journal of the Fitches & R. P. Barnes), pp. 45–68. Geological Society Geological Society, London 160, 117–36. of London, Special Publication no. 160.