(Upper Ordovician) graptolites from the Cardigan area ...

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undertook a detailed examination of the cliff exposures from sea, as part of its survey ..... Williams. 7. Facies distribution of graptolites in the Cardigan succession.

c 2003 Cambridge University Press Geol. Mag. 140 (5 ), 2003, pp. 549–571.  DOI: 10.1017/S0016756803008057 Printed in the United Kingdom

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Stratigraphical and palaeoecological importance of Caradoc (Upper Ordovician) graptolites from the Cardigan area, southwest Wales MARK WILLIAMS*†, JEREMY R. DAVIES*, RICHARD A. WATERS*, ADRIAN W. A. RUSHTON§ & PHILIP R. WILBY* *British Geological Survey, Keyworth, Nottingham NG12 5GG, UK §Department of Palaeontology, The Natural History Museum, South Kensington, London SW7 5BD, UK

(Received 28 October 2002; accepted 29 April 2003)

Abstract – Graptolites from more than 60 horizons in the basinal Caradoc succession of southwest Wales, between Fishguard and Cardigan, allow recognition of the multidens, clingani and linearis biozones. The biostratigraphy permits recognition of major differences in the sedimentary rocksequence north and south of structures associated with the Fishguard–Cardigan Fault Belt. The Penyraber Mudstone Formation, disconformably overlying the Fishguard Volcanic Group (Llanvirn), is partly of multidens Biozone age. It is succeeded south of the Newport Sands Fault by the Cwm yr Eglwys Mudstone Formation of clingani to linearis biozones age. North of the fault the Cwm yr Eglwys Mudstone Formation is replaced laterally by the northwards-thickening, sandstone turbidite-dominated Dinas Island Formation (clingani and linearis biozones). Graptolite stratigraphical distribution indicates that Dicranograptus clingani occurs only rarely within the caudatus Subzone of the clingani Biozone and that Climacograptus antiquus s.l. also does not range above the lower part of the clingani Biozone. The first occurrence of Dicellograptus morrisi, within the upper clingani Biozone, confirms its value as a marker for the morrisi Subzone, and is associated with the first occurrences of Diplacanthograptus dorotheus and Normalograptus minimus. Dicellograptus flexuosus, used to indicate the morrisi Subzone elsewhere, occurs throughout the clingani Biozone in the Cardigan area. The linearis Biozone is recognized by Climacograptus tubuliferus. Oxic bottom conditions in early and early mid-Caradoc times largely precluded the influx of, or preservation of, graptolite faunas in the Penyraber Mudstone Formation. Anoxic mudstones of the Cwm yr Eglwys Mudstone and Dinas Island formations preserve graptolite assemblages of 21 and 26 species, signalling strong open marine influences which persisted in this area until late Caradoc times. This contrasts with the shelfal faunas in the Whitland area (south Pembrokeshire), where the late Caradoc is dominated by low-diversity Normalograptus-dominated assemblages. Keywords: Graptolites, biostratigraphy, palaeoecology, Ordovician.

1. Introduction

This paper presents biostratigraphical aspects of recent British Geological Survey mapping of the Caradoc (Late Ordovician) succession which crops out in southwest Wales between Fishguard, in north Pembrokeshire, and Cardigan, in south Ceredigion (British Geological Survey, 2002; Davies et al. 2003), hereafter, for brevity, referred to as the Cardigan area. It aims specifically to present details of new faunal, mainly graptolitic, assemblages collected during the study, and to assess their biostratigraphical and palaeoecological significance in the context of recent syntheses of Caradoc graptolite assemblages in Wales and elsewhere (e.g. Zalasiewicz, Rushton & Owen, 1995; Goldman, Mitchell & Joy, 1999). † Author for correspondence: [email protected]

The complexly folded and faulted sedimentary succession is up to 1.3 km thick and forms part of the fill of the Lower Palaeozoic Welsh Basin. Sited on the ancient microcontinent of Avalonia, the thick basinal succession contrasts with the incomplete and attenuated contemporary successions formed on the Midland Platform to the south and east. Subsidence of the basin, during Caradoc times, may have been sustained by crustal extension (or transtension) in a back-arc setting associated with subduction of the Iapetus oceanic crust along a zone located to the north of the English Lake District (Kokelaar et al. 1984a,b; Rushton, 1999). The Caradoc sequence disconformably succeeds the Llanvirn Fishguard Volcanic Group (Bevins & Roach, 1979; Lowman & Bloxam, 1981; BGS, 2002). It comprises exclusively marine facies, including shelly sandstones, mega-debrites, and both mudstone- and

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Figure 1. Summary geological map for the Cardigan and Fishguard areas (from Davies et al. 2003), geographical location of the area studied and the principal sections collected (see Figs 3–7). Key localities mentioned in the text are marked. The faults named in the legend for the map collectively form the Fishguard–Cardigan Fault Belt.

sandstone-dominated turbidite units. Both the underlying volcanic group and the Caradoc succession exhibit abrupt changes in thickness and facies across the newly recognized Fishguard–Cardigan Fault Belt, a zone of major ENE–WSW-trending faults, thus demonstrating its existence and influence both during Llanvirn volcanism and Caradoc sedimentation (Kokelaar, 1988; Davies et al. 2003). However, the current outcrop pattern largely reflects later movements along the fault belt, principally associated with Acadian deformation. Rocks of early Ashgill age conformably succeed the Caradoc sequence. The Caradoc facies are spectacularly exposed in the coastal cliffs east of Fishguard, on Dinas Island, between Newport Sands and Cemaes Head, and along the Teifi estuary at Poppit Sands, St Dogmaels and around Cardigan (Fig. 1). Exposure inland is generally poor. Many of the precipitous sea-cliff sections, which locally rise to 160 m in height, are both inaccessible and hidden from view on land. As this has previously precluded a systematic analysis of the sequence, BGS undertook a detailed examination of the cliff exposures

from sea, as part of its survey of the Cardigan 1:50 000 Sheet 193. This permitted palaeontological sampling of many hitherto unvisited sites, with the result that more than 60 graptolite-bearing horizons have now been identified in the coastal sections (see Fig. 1 and Appendix 1). The lower parts of the sequence remain poorly constrained biostratigraphically, but probably are no younger than the multidens Biozone. The more fossiliferous upper parts of the succession, including all the coastal exposures between Dinas Island and Cardigan, lie almost entirely within the clingani Biozone, with assemblages of both the caudatus and morrisi subzones recognized. Faunas assigned to the latest Caradoc, linearis Biozone have been recovered from the uppermost part of the sequence, representing the first definite record of this biozone in Wales. The Caradoc facies associated with the Fishguard– Cardigan Fault Belt contrast markedly with those exposed in the Preseli Hills to the south. There, a richly graptolitic, late Llanvirn and Caradoc black mudstone sequence, identical to that of south Pembrokeshire and

Ordovician graptolites from Cardigan

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Carmarthenshire (Evans, 1945; see Fortey et al. 2000), yields faunas of the teretiusculus, gracilis, multidens and clingani biozones.

lower Bala) identical to that of south Pembrokeshire and Carmarthenshire, including the Hendre Shales and Mydrim Shales formations (see Fortey et al. 2000).

2. Previous work

3. Lithostratigraphy and facies architecture

Graptolites have rarely been reported from the Cardigan sequence, hampering its correlation with Ordovician successions elsewhere in Wales (see Fortey et al. 2000). The few localities that have previously yielded graptolites suggested a Caradoc age for the sediments (‘Bala’ of earlier papers) and a general northward younging towards Cardigan. Keeping (1882) reported the first graptolites in the area, collected from rocks in the cliff south of Pen Pistyll [SN 0538 4111], at the north end of Newport Sands. These were examined by Charles Lapworth, but were considered too poorly preserved for precise identification. Keeping also collected graptolites from an outcrop in Cardigan Town, from which he recovered specimens identified as Dicellograptus morrisi by Lapworth, suggesting a horizon in the upper clingani or linearis Biozone. Latter (1925) reported graptolites from several localities between Traeth-y-bal, north of Newport Sands, and the banks of the Afon Teifi in Cardigan. His material was identified mainly by Gertrude Elles, and includes a number of identifications which suggested the Caradoc gracilis and multidens biozones. Myers (1950) reported a small collection of graptolites from strata about 420 m south of Pen Pistyll, at the north end of Newport Sands. The material was identified for him by the Geological Survey as ‘somewhere about the Glyptograptus teretiusculus–Nemagraptus gracilis boundary or a little higher’. Lowman & Bloxam (1981) recorded graptolites of the murchisoni Biozone from mudstones intercalated with the Fishguard Volcanic Group at Castle Point [SM 962 377], and reported Nemagraptus gracilis (as yet unconfirmed) from the overlying Caradoc strata at Aber Howel [SM 990 385], between Fishguard and Dinas Island. The perception that the succession younged northwards, from Newport Sands to Cardigan, underpinned a lithostratigraphical scheme erected by McCann (1992; see also Fortey et al. 2000). He identified two further graptolite-bearing horizons. One to the south of Pen Pistyll, close to that identified by Keeping (see above), was interpreted as being indicative of the gracilis Biozone. A second collection, from the banks of the Afon Teifi, west of Cardigan, was assigned to the clingani Biozone. The results of limited sedimentological and structural investigations of Caradoc outcrops have been presented by McCann (1992) and James (1975, 1997). In the Preseli Hills, south of the Fishguard– Cardigan Fault Belt, Evans (1945) recognized a fossiliferous Caradoc succession (his Llandeilo and

The biostratigraphical results with the mapping of the coastal sections have shown earlier stratigraphical and architectural models for the north Pembrokeshire Caradoc succession (McCann, 1992; James, 1997) to be untenable and have required new lithostratigraphical nomenclature to be erected (Fig. 2a,b; BGS, 2002; Davies et al. 2003). The lowest part of the succession comprises the Penyraber Mudstone Formation (PeA). At the base, around 50 m of shelly sandstones and conglomerates, the Castle Point Member (CaP), rest on an irregular erosion surface incised into an ash flow tuff of the Fishguard Volcanic Group (Lowman & Bloxam, 1981). The succeeding Saddle Point Member (SaP) comprises a 200 m thick sequence of mega-debrites in which rafts of various volcanic lithologies, of thinly bedded sandstone, and fissile black mudstone, some ranging up to several metres across, are set in a mudstone matrix. An estimated 500 m of finely cleaved, pale grey, diffusely laminated mudstones with sporadic burrow-mottling, and common nodules and thin beds of black, phosphatized mudstone, make up the upper parts of the Penyraber Mudstone Formation. These oxic facies are succeeded by the Cwm-yr-Eglwys Mudstone Formation (CyE), a sequence of dark grey turbidite mudstones with abundant laminae and thin beds of siltstone and fine-grained sandstone, in which common thin beds of delicately laminated hemipelagic mudstone testify to deposition under anoxic bottom waters (see Cave, 1979; Davies et al. 1997). South of the Newport Sands Fault, the Cwm-yrEglwys Mudstone Formation ranges up to the top of the local Caradoc sequence, but to the north of the fault, it passes laterally into a thick, sandstone-dominated turbidite unit, the Dinas Island Formation (DI; Fig. 2a). Thick mudstone packets persist within this formation and can be traced for limited distances in the principal cliff sections. Some have been given informal names in an attempt to subdivide the formational sequence (Figs 3–6). However, only the Carreg Bica Mudstone (CBi), low in the observed sequence, and the Cwm Degwel Mudstone (CwD), everywhere present at the top of the formation, have been given formal member status (Fig. 2a, b). The presence of thin beds of laminated hemipelagic mudstone throughout the Dinas Island Formation confirms that bottom waters remained anoxic during deposition. The Cwm Degwel Mudstone Member is succeeded abruptly by the Nantmel Mudstones Formation (Ntm), thought to be of Ashgill age; this is an oxic burrow-mottled facies which is widely recognized throughout the Welsh Basin (e.g. Davies et al. 1997).

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Figure 2. For legend see facing page.

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Ordovician graptolites from Cardigan Detailed examination of the main cliff sections in the Dinas Island Formation has revealed a rapid northwards thickening away from the Newport Sands Fault. A little over 200 m of strata are assigned to the formation at Newport Sands, but to the north, on the northern limb of the Pen Cafnau Syncline, the formation is at least 550 m thick. A similar thickness is exposed on Dinas Island. In all these sections the formation overlies the Cwm-yrEglwys Mudstone Formation. To the north of the Ogof Cadno Fault, the thickness of the exposed Dinas Island Formation sequence increases to an estimated 1.3 km, but there its base is not seen. Similar pronounced thickness variations in the underlying Fishguard Volcanic Group suggested to Kokelaar (1988) that the thickest parts of the volcanic pile, on Strumble Head, were erupted and confined within a graben-like structure. Thus, it is possible that the lateral thickness and facies variations displayed by the Caradoc sequence record subsequent deposition within the same fault-controlled trough (Davies et al. 2003). The disconformity at the base of the undated Castle Point Member records post-volcanic emergence and erosion along the southern edge of the trough, prior to Caradoc marine transgression, inviting comparison with the sub-gracilis unconformity of North Wales (Howells & Smith, 1997). Subsequently, during the multidens and clingani biozones, renewed faulting, allied to marine transgressive deepening, rapidly established a deep-water setting which was supplied with resedimented detritus derived from the upfaulted margins of the trough. The geometry of the Dinas Island Formation shows that deposition of thick turbidite sands was initially confined to the north of the Ogof Cadno Fault (OCF), before expanding southwards during the upper part of the clingani Biozone (morrisi Subzone) (Fig. 2a; see also Davies et al. 2003). The contrast between the diverse and laterally variable sequence of Caradoc facies associated with the Fishguard–Cardigan Fault Belt and the black mudstone sequences to the south in the Preseli Hills (Evans, 1945) appears to confirm that the southern edge of the fault zone acted as an important Caradoc facies divide. However, this contrast was not necessarily achieved by normal faulting alone. The abrupt nature of the facies change could also record a juxtapositioning of once widely separated successions achieved by strike-slip displacement along the fault belt (Davies et al. 2003). 4. Biostratigraphy

The relationship of the principal sections sampled for graptolites, and other individual sample points, to the

553 new stratigraphical architecture is shown in Figure 2a and b. Details of the graptolite assemblages obtained from the main sections (A to E) are presented in Figures 3–7. These also indicate horizons where conodonts (mainly Amorphognathus) and linguliformean brachiopods occur. Appendix 1 lists localities and the registered numbers of the relevant graptolite collections at the British Geological Survey. In addition to the new BGS material, the earlier graptolite collections of Myers (1950) and McCann (1992) have been reexamined and re-identified. The collection made by Latter (1925), and identified by Gertrude Elles, has not been located. The Castle Point Member, at the base of the Penyraber Mudstone Formation, post-dates mudstones with murchisoni Biozone graptolites present in the upper part of the Fishguard Volcanic Group at Castle Point (Fig. 2a, locality 1; see also Lowman & Bloxam, 1981). Graptolites from a black mudstone raft in the overlying Saddle Point Member, at Aber Howel [SM 990 385], include Diplograptus foliaceus, Dicranograptus ziczac, Lasiograptus costatus, Corynoides calicularis and Orthograptus ex gr. calcaratus, and indicate a maximum age of multidens Biozone (Fig. 2a, locality 2). Though the presence of N. gracilis, previously reported from this locality by Lowman & Bloxam (1981), has not been confirmed, the lower part of the Penyraber Mudstone Formation may well range down into the gracilis Biozone in the underlying Castle Point Member (see above). No other graptolite faunas have been recovered from the Penyraber Mudstone Formation. Graptolites from the Cwm-yr-Eglwys Mudstone Formation south of the Newport Sands Fault include assemblages of both the clingani and linearis biozones (Fig. 2a, Section E; Fig. 7). North of the Newport Sands Fault, where upper levels of the formation are replaced by the Dinas Island Formation, the surviving Cwmyr-Eglwys Mudstone Formation sequence lies entirely within the clingani Biozone (Fig. 2a, sections B and D; Figs 4, 6). However, the mudstone sequence on Dinas Island has only yielded graptolites of the caudatus Subzone, whereas that at Newport Sands [SM 053 408] also includes assemblages of the higher, morrisi Subzone. To the north of the Ogof Cadno Fault the lowest exposed part of the Dinas Island Formation probably lies within the caudatus Subzone (Fig. 2a, section A; Fig. 3), graptolites of this age, including Diplacanthograptus spiniferus and Climacograptus antiquus s.l. being recovered from the lower part of the Carreg Bica Mudstone Member. The caudatus–morrisi subzonal boundary occurs in the middle of this member (Fig. 3).

Figure 2. (a) Revised lithostratigraphy and facies architecture for the Caradoc succession of the Cardigan area based on the recent BGS work (BGS, 2002; Davies et al. 2003) and showing the positions of the principal graptolitic sections and sample points. Localities 1 to 6 (right hand side of diagram) are mentioned in the text. (b) Chronostratigraphical version of (a). Abbreviations: ARF – Aber Richard Fault; CBF – Ceibwr Bay Fault; NSF – Newport Sands Fault; OCF – Ogof Cadno Fault.

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Figure 3. Graptolite biostratigraphy of Section A, Ogof Cadno to Cemaes Head (see Figs 1, 2a). The lithostratigraphical column is to scale, but the space between sample horizons within the various formations and members is arbitrary. The occurrence of Climacograptus antiquus s.l. in the Dinas Island Formation just north of Ogof Cadno (horizon 1) identifies the lower part of the clingani Biozone or an older horizon. Further to the north, at Traeth Bach, the Carreg Bica Mudstone Member yields assemblages which identify the lower part of the clingani Biozone, characterized by the overlapping ranges of Climacograptus cf. antiquus s.l. and Diplacanthograptus spiniferus, and the upper part of the Biozone, characterized by Dicellograptus morrisi, Diplacanthograptus dorotheus and Normalograptus minimus sensu Elles & Wood (horizons 4 and 5). The Craig yr Odyn, Pwllgranant and Pen-y-Craig mudstones are informal lithological units.

In contrast, to the south of the Ogof Cadno Fault, the base of the formation appears to lie within the morrisi Subzone (Fig. 2a, section D; Fig. 6). The assemblage from Pen Pistyll, considered to be of gracilis Biozone age by McCann (1992), is now identified as of the

upper clingani Biozone. The Cwm Degwel Mudstone Member, in immediately underlying the Nantmel Mudstones Formation, is the lateral correlative of the uppermost part of the Cwm yr Eglwys Mudstone Formation, which is of linearis Biozone age. The

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Figure 4. Graptolite biostratigraphy of Section B, Dinas Island (see Figs 1, 2a). The lithostratigraphic column is to scale, but the space between sample horizons within the various formations and members is arbitrary. The Cwm yr Eglwys Mudstone Formation north of the Newport Sands Fault is in the caudatus Subzone (lower clingani Biozone). For the overlying Dinas Island Formation, cliff top exposures at Aber Pensidian yield poorly preserved Diplacanthograptus dorotheus? and ‘Glyptograptus’ sp. 1? (horizon 11) suggesting a level within the morrisi Subzone (upper clingani Biozone). Horizon 1∗ combines the graptolite faunas from two along-strike localities (see Appendix 1). Conodonts at horizon 5 are Amorphognathus. The Pwll Glas, Aber Pensidian and Trwyn Pendalfa mudstones are informal lithological units.

presence of a possible Climacograptus tubuliferus and Orthograptus pauperatus in an assemblage obtained from the Cwm Degwel Mudstone Member (Fig. 6, horizon 18), north of Pen-y-bal [SM 048 417], confirms this correlation. Though lower parts of the overlying Nantmel Mudstones Formation are typically unfossiliferous, the change from anoxic to oxic bottom conditions at the base of this division is widely taken as a proxy for the Caradoc–Ashgill boundary within the Welsh Basin (e.g. Davies et al. 1997). It is therefore inferred that the top of the Cwm Degwel Mudstone Member further north is also of linearis Biozone age. South of the Fishguard–Cardigan Fault Belt, the black mudstone sequence of the Preseli Hills has been sampled by the BGS at several localities and is graptolitic from the late Llanvirn onwards (see also Evans, 1945; Zalasiewicz, Rushton & Owen, 1995).

Mudstones at Pen-cnwc Bach (Fig. 2a, locality 3 [SM 124 375]) yield graptolites including Amplexograptus coelatus, Hustedograptus cf. teretiusculus, Dicranograptus cf. furcatus and Cryptograptus ex gr. tricornis which indicate the late Llanvirn teretiusculus Biozone. Exposures at Felindre Farchog (Fig. 2a, locality 4 [SM 0975 3870]) yield D. cf. ziczac, L. costatus, Pseudoclimacograptus cf. scharenbergi and possible Nemagraptus fragments which suggest the gracilis Biozone, whilst at Crymych (Fig. 2a, localities 5 and 6 [SM 183 338]) graptolites identify the multidens (see Evans 1945, p. 100) and clingani biozones (BGS collections), the latter interval yielding numerous diagnostic species such as Diplacanthograptus spiniferus, D. dorotheus, Normalograptus pollex and the first British record of Diplacanthograptus lanceolatus (Fig. 8) from the railway cutting between SM 1845 3390 and SM 1843 3393.

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Figure 5. Graptolite biostratigraphy of Section C, Traeth Cell-Howel to Cardigan (see Figs 1, 2a). The lithostratigraphic column is to scale, but the space between sample horizons within the various formations and members is arbitrary. Graptolites occur at several horizons particularly Diplacanthograptus dorotheus, Leptograptus flaccidus s.l. and Normalograptus cf. daviesi which indicate the upper part of the clingani Biozone. From a higher horizon, near the top of the Dinas Island Formation, Keeping’s (1882) Cardigan Town locality, re-identified by the survey (horizon 6), yields Dicellograptus morrisi, D. cf. pumilus and Normalograptus cf. minimus, also identifying the upper clingani Biozone (morrisi Subzone). Horizon ‘a’ is situated on the coast at Traeth Cell-Howel. The other horizons (1 to 6) are from the banks of the Afon Teifi and Cardigan town. The Net Pool and Godir Rhyg mudstones are informal lithological units.

5. Biostratigraphical utility of Caradoc graptolites

The Caradoc rock succession between Fishguard and Cardigan yields over 30 graptolite species, many of which are biozonal or subzonal index species. Comparison of the biostratigraphical distribution of these graptolites with other parts of the Welsh Basin, Scotland, Scandinavia and North America indicates differences in the recorded ranges of several species (Fig. 9). Thus, Finney (1986) gives a range for D. spiniferus which is equivalent to the clingani and part of the linearis Biozone of the British graptolite biozonation. Most other authors record this species only from the clingani Biozone or its equivalents. Similarly, there are marked differences for the recorded ranges of D. flexuosus, Neurograptus margaritatus, Ensigraptus caudatus and D. clingani. Some of this variation may be a consequence of different author

concepts of species, or of differences in the definition of graptolite biozones which heuristically have been considered homologous (see Bettley, Fortey & Siveter, 2001, fig. 11), or of collection bias. The different ranges may also reflect palaeoenvironmental conditions operating in different marine basins (see Sections 7 and 8) or may be influenced by the migration of faunas from one palaeocontinent to another, facilitated by plate tectonics or marine transgressive events (see Bettley, Fortey & Siveter, 2001). For this reason, in our analysis of the graptolite biostratigraphy of the Cardigan area, we have assessed the biostratigraphical significance of species based on the compilation of data presented in Figure 9. Species which appear to have the most consistent recorded ranges include C. antiquus s.l., thought to range no higher than the lower part of the clingani Biozone. In the Welsh Basin the zonal fossil D. clingani appears

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Figure 6. Graptolite biostratigraphy of section D, Newport Sands (see Figs 1, 2a). The lithostratigraphic column is to scale, but the space between sample horizons within the various formations and members is arbitrary. The sequence yields 18 graptolite-bearing horizons. Material collected by Myers (1950) from Cesig duon, near the base of the exposed sequence (horizon 2∗ ) was originally identified as about the teretiusculus–gracilis boundary or slightly higher, but his collection contains ‘Glyptograptus’ sp. 1, a species known to occur at horizons referable to the upper clingani Biozone in Section C (see Fig. 5). Faunas collected from the succeeding sequence mainly indicate the upper part of the clingani Biozone (morrisi Subzone), yielding Normalograptus minimus sensu Elles & Wood. Near the top of the Dinas Island Formation, a collection from the Cwm Degwel Mudstone Member at Traeth y Bal (horizon 18) yields possible Orthograptus pauperatus and Climacograptus tubuliferus, suggesting the linearis Biozone. Conodonts at horizon 10 are Amorphognathus. The Godir Tudur mudstone is an informal lithological unit.

to be a useful marker for the lower part of the clingani Biozone (as locally defined; see Zalasiewicz, Rushton & Owen, 1995), though it is recorded at a much earlier horizon in North America (Finney, Grubb & Hatcher, 1996). The appearance of D. morrisi is indicative of the base of the morrisi Subzone of the clingani Biozone and it ranges to younger horizons (Zalasiewicz, Rushton & Owen, 1995). Diplacanthograptus dorotheus, which appears to form a lineage from the earlier D. spiniferus, also arises close to the caudatus–morrisi subzonal boundary (see Zalasiewicz, Rushton & Owen, 1995). Normalograptus minimus sensu Elles & Wood appears to characterize the morrisi interval in southern Scotland and Wales (Fig. 9). Although P. linearis is not known from Wales, C. tubuliferus can be taken as diagnostic of the linearis Biozone (see Fig. 9). Ensigraptus caudatus, regarded as a subzonal fossil for the lower part of the clingani Biozone by Zalasiewicz, Rushton & Owen (1995), is recorded only tentatively in the Cardigan sequence, but is known from horizons equivalent to both the lower and upper part of the clingani Biozone in North America (Fig. 9). Its range appears to be strongly

influenced by factors associated with sea-level change (Goldman, Mitchell & Joy, 1999). Dicellograptus flexuosus, considered indicative of the morrisi Subzone at Hartfell and Whitland (Zalasiewicz, Rushton & Owen, 1995), occurs throughout the clingani Biozone of the Cardigan area. 6. Stratigraphical distribution of graptolites in the Cardigan succession

Based on the ranges of graptolites given in Figures 3–7, an amalgamated graptolite range chart for the Caradoc succession of the Cardigan area can be produced. Figure 10 depicts the ranges of 24 key taxa for the interval of the clingani and linearis biozones. Lithostratigraphical data used in the construction of this chart are: the base of the Nantmel Mudstones Formation (= proxy for the base of the Ashgill Series); the base of the Cwm Degwel Mudstone Member; and the relative position of the various thick mudstone units of the Dinas Island Formation. The range chart uses the key graptolite-bearing horizons depicted in Figures 3–7.

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Figure 7. Graptolite biostratigraphy of Section E, Frongoch (see Figs 1, 2a). The lithostratigraphic column is not to scale and the space between sample horizons is arbitrary. Occurrence of Climacograptus tubuliferus at locality 4 is used to identify the linearis Biozone.

It does not include horizons characterized by one or a combination of graptolites which are indeterminate, long-ranging, or lie at a level intermediate within their total range in the Cardigan area. In Figure 10, certain horizons may overlap, for example the caudatus Subzone levels of the Cwm yr Eglwys Mudstone Formation at Dinas Island and those of the Carreg Bica Mudstone north of Ogof Cadno, but the overall superposition is correct. The range-chart focuses on the criteria used to define the biostratigraphy in this sequence. The boundary between the multidens and clingani biozones remains poorly constrained; it may lie somewhere within the oxic mudstones of the upper part of the Penyraber Mudstone Formation between Aber Howel and the first diagnostic clingani Biozone faunas of the Cwm yr Eglwys Mudstone Formation. The lower part of the clingani Biozone (caudatus Subzone as defined by Zalasiewicz, Rushton & Owen, 1995) is identified by D. clingani and also by the overlapping ranges of C. antiquus s.l. and D. spiniferus. Given the early occurrence of D. clingani in North America (Finney, Grubb & Hatcher, 1996) and the fact that the base of the clingani Biozone as defined at Whitland appears to lie at a higher level than the

base of the biozone as defined at Hartfell in Scotland (see Bettley, Fortey & Siveter, 2001), the occurrence of D. clingani in the Cwm yr Eglwys Mudstone Formation at Dinas Island probably lies towards the top of its total range. The subzone index E. caudatus is identified only tentatively. The lowest occurrence of E. caudatus-like graptolites appears just to precede that of D. clingani (Fig. 10), though E. caudatus-like graptolites range throughout the clingani Biozone (including the morrisi Subzone) in the Cardigan area. This situation is similar to the range of E. caudatus recorded in North America (see Fig. 9). The upper part of the clingani Biozone, the morrisi Subzone, is identified chiefly by the incoming of D. morrisi. The incoming of D. dorotheus, N. minimus sensu Elles & Wood and ‘Glyptograptus’ sp. 1 probably occur at about the same level as D. morrisi. Dicellograptus cf. pumilus and Leptograptus flaccidus s.l. occur at levels high within this subzone, but appear much earlier in the Caradoc sequence elsewhere (see Bettley, Fortey & Siveter, 2001). At the top of the Caradoc sequence, the linearis Biozone is identified by C. tubuliferus, though P. linearis itself is not present.

Ordovician graptolites from Cardigan

Figure 8. Diplacanthograptus lanceolatus VandenBerg, BGS MWL3193. Mydrim Shales Formation, clingani graptolite Biozone, Crymych railway cutting [National Grid Reference SN 1843 3393], southwest Wales. Magnification ×6. This species is previously reported from Australasia (see VandenBerg & Cooper, 1992). At Crymych it co-occurs with Diplacanthograptus dorotheus (Riva), Lasiograptus cf. costatus Lapworth, Normalograptus miserablis (Elles & Wood) and N. cf. daviesi Williams.

7. Facies distribution of graptolites in the Cardigan succession

The distribution of graptolites in the Caradoc sequence of the Cardigan area was strongly influenced by the character of the marine facies. These, in turn, were a product of the interaction of global rise in sea-level during Caradoc times (see Barnes, Fortey & Williams, 1995) and the local physiography of the fault-controlled trough in which the sequence was deposited. The early and early mid-Caradoc interval is represented by the predominantly oxic marine facies of the Penyraber Mudstone Formation (Fig. 2a,b). This may have been deposited along the flank of the local trough, in a setting above the level of the oxygenminimum zone, allowing colonization of the substrate by a scavenging and bioturbating fauna. However, it is far from certain that this implies a shallower environment than black mudstone settings in the Preseli Hills to the south. Circulation patterns along the flanks of the north Pembrokeshire fault trough may have

559 sustained oxygenated bottom conditions at greater depths than prevailed elsewhere. Graptolites occur at only one horizon in the Penyraber Mudstone Formation (Saddle Point Member), more than 100 m above its base, within a raft of black mudstone. The graptolites include D. ziczac, L. costatus, D. foliaceus and O. ex gr. calcaratus, forms that are typical of Caradoc black mudstones elsewhere in the Welsh Basin. This suggests that graptolites were probably present in the overlying water column throughout deposition of the Penyraber Mudstone Formation, but were preserved only when and where anoxic conditions were present on the seafloor. The change from the predominantly oxic facies of the Penyraber Mudstone Formation to the anoxic facies of the Cwm yr Eglwys Mudstone Formation may have been caused by deepening associated with progression of the Caradoc marine transgression, or by changes in oceanic, or basinal or local trough related circulation. Local deepening was also emphasized by contemporary movements on the Fishguard–Cardigan Fault Belt. The oldest graptolites to be preserved in the anoxic mudstones of the Cwm yr Eglwys Mudstone Formation are of low diversity (one to two species) Lasiograptus and Normalograptus faunas (Fig. 4, horizons 1∗ , 4). The exact timing of this oxic (Penyraber Mudstone) to anoxic (Cwm yr Eglwys Mudstone) shift is difficult to pin-down, because of the absence of diagnostic graptolites in the transition, though the lowest age diagnostic faunas of the Cwm yr Eglwys Mudstone Formation suggest a level in the lower clingani Biozone (Fig. 4, horizon 6). The influx of a diverse graptolite fauna into the Cardigan area, or at least the development of anoxic marine facies that allowed its preservation, was well developed by early clingani Biozone times. The relatively high-diversity faunas preserved in the Cwm yr Eglwys Mudstone Formation (c. 21 species) and Dinas Island Formation (c. 26 species) are taxonomically similar, and include a range of graptolites typical of deep-water black mudstones known elsewhere in the Welsh Basin and Scotland. Both formations include Dicellograptus (D. flexuosus, D. aff. moffatensis), climacograptids (D. spiniferus, C. antiquus s.l.), Normalograptus (N. minimus, N. miserablis, N. brevis), Orthograptus (O. ex gr. calcaratus), Corynoides, Lasiograptus, Phormograptus and possible E. caudatus. The slightly higher diversity of the Dinas Island Formation is reflected in additional species such as D. dorotheus, N. margaritatus, L. flaccidus s.l., D. morrisi, D. pumilus and N. daviesi, many of which occur in the upper part of the formation. The graptolites are preserved within black anoxic mudstones; no graptolites have been recovered from the turbidite sandstones. Nevertheless, within some of the anoxic black mudstone intervals, such as the Aber Pensidian Mudstone of the Dinas Island Formation on Dinas Island (Fig. 4), and parts of the

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Figure 9. For legend see facing page.

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Ordovician graptolites from Cardigan

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Figure 10. Composite range diagram for key graptolite taxa recorded in the Cardigan succession. The reconstruction uses lithostratigraphical correlations, particularly the position in the sequence below the Nantmel Mudstones Formation and Cwm Degwel Mudstone Member, and relative correlation of the various thick mudstones. In the reconstruction there may be some overlap in the superposition of horizons from Dinas Island and Ogof Cadno at the level of the caudatus Subzone. Arrowed dotted lines indicate that a taxon has been collected from lower in the sequence at Aber Howel, where the overall fauna is of probable multidens Biozone age. Dotted lines without an arrow indicate that a taxon has been recorded with a different range elsewhere (see Fig. 9). The range of Normalograptus cf. daviesi is probably greater than depicted in the diagram, but the precise position of its lowest occurrence is difficult to ascertain. ‘Glyptograptus’ sp. 1 appears to be a useful index for the upper part of the clingani Biozone.

Cwm-yr-Eglwys Mudstone Formation at Newport Sands, graptolites occur only intermittently and many laminated hemipelagic mudstone beds are barren. Unlike the oxic, burrowed mudstones of the Penyraber Mudstone or Nantmel Mudstones formations, these deep-marine anoxic mudstones should provide a taphonomic window for the preservation of graptolites. The absence of fauna suggests a dearth of graptolites in the overlying water column caused by adverse environmental conditions. Finney & Berry (1997, p. 921) have noted similar barren intervals in the black mudstone sequences of the Vinini Formation in Nevada. Graptolites are not preserved in the overlying Nantmel Mudstones Formation, which marked a return to oxic burrowed facies in this area, and which may

have been the result of changes in ocean circulation during Ashgill times (see Armstrong & Coe, 1997). 8. Palaeoecological associations of graptolites

Although deep-water conditions were maintained in the Cardigan area throughout mid- and late Caradoc times, there are marked differences in the graptolite assemblages preserved at individual horizons. These suggest different ecological associations of graptolites in the overlying water column corresponding to local environmental changes. Low-diversity assemblages present in the sequence, characterized by only one or two identifiable species, generally of Normalograptus (see Fig. 11), may represent periods of adverse

Figure 9. Combined total stratigraphical ranges for key graptolite species relevant to the Cardigan succession for the interval of the clingani and linearis biozones as recognized in Britain, and North American stratigraphical equivalents. The overall ranges of species are taken from the following references: Z, Zalasiewicz, Rushton & Owen (1995); W, Williams (1982); W2, Williams (1994); WB, Williams & Bruton (1983); G, Goldman, Mitchell & Joy (1999); F, Finney (1986). T stands for total calculated range and includes information from Elles & Wood (1901–18), Williams (1995), Finney, Grubb & Hatcher (1996), VandenBerg & Cooper (1992) and Bettley, Fortey & Siveter (2001). Dotted lines indicate that records of species from the particular interval have not been figured and require taxonomic confirmation. Some of the differences in given ranges for species are probably due to taxonomic inconsistency. The morrisi–miserablis to Normalograptus proliferation interval is after Zalasiewicz, Rushton & Owen (1995).

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Figure 11. Taxonomic composition, grouped at generic level, of graptolite faunas from individual horizons in the Cwm yr Eglwys Mudstone and Dinas Island formations calculated for single species and multiple species horizons. The category ‘other diplograptids’ includes specimens which lack proximal ends or are poorly preserved, though most of these are probably Normalograptus species. Horizons yielding low diversity faunas (of one to two species) are typically composed of Normalograptus species (either N. minimus sensu Elles & Wood, N. miserablis or N. cf. daviesi), Dicellograptus flexuosus and ‘other diplograptids’. Higher diversity faunas also include Normalograptus species, but are characterized by graptolites which may suggest deeper water or more offshore facies (oceanic influences), for example, Dicellograptus morrisi and Diplacanthograptus spiniferus. The group referred to as ‘others’ includes Dicranograptus, Corynoides, Lasiograptus and Orthograptus ex gr. calcaratus, as well as rarely occurring species such as Neurograptus margaritatus and Phormograptus sp. Because of poor preservation, ‘other diplograptids’ might sometimes include more than one species.

Ordovician graptolites from Cardigan environmental conditions. Higher diversity assemblages of more than five species may suggest optimal environmental conditions in which complex ecological associations of Dicranograptus, Phormograptus, Diplacanthograptus, Climacograptus and other graptolite taxa could thrive. Normalograptus occur at more than half of the graptolite-bearing horizons collected from the succession and are the most abundant element of the graptolite fauna. N. daviesi, N. miserablis and N. minimus are often the most common elements of the low-diversity assemblages (Fig. 11). Normalograptus also occur in the more diverse assemblages: N. minimus occurs at 11 horizons, in assemblages with up to seven species, whilst N. miserablis occurs at 14 horizons, in assemblages with up to eight species. Both of these graptolites occur in the Dinas Island and Cwm yr Eglwys Mudstone formations with a variety of other graptolites including L. harknessi, D. dorotheus, D. morrisi, D. flexuosus, D. pumilus, O. ex gr. calcaratus and other Normalograptus. Their overall distribution suggests wide ecological tolerance, and their occurrence in low-diversity assemblages suggests the ability to cope with stressed environmental conditions that may have excluded other graptolites (see also Zalasiewicz, Rushton & Owen, 1995; Goldman, Mitchell & Joy, 1999). Normalograptus brevis has a similar distribution in low- and higher-diversity assemblages (faunas of between two and six species), but is limited to the mudstone dominated intervals of the sequence (e.g. Carreg Bica Mudstone, Pwll Glas Mudstone). Goldman, Mitchell & Joy (1999) regarded N. brevis as a deep marine form, which is consistent with its distribution in the Cardigan sequence. Low-diversity assemblages also include D. flexuosus and O. ex gr. calcaratus. As with the Normalograptus, both of these species also occur in higher-diversity assemblages, and they may have been similarly tolerant of a range of environmental conditions. Certain species occur exclusively within the higherdiversity assemblages, particularly D. clingani, N. margaritatus, Phormograptus sp. and D. morrisi. These graptolite assemblages are preserved in mudstones from both the mud-rich and sandier parts of the sequence and do not cluster at any particular level. The high-diversity assemblages may preserve complex ecological associations of graptolites that existed in the water column during ‘graptolite-friendly’ environmental conditions. In the Cardigan sequence these represent a small proportion of the preserved assemblages (Fig. 11). Diplacanthograptus dorotheus also occurs only in these high diversity assemblages. Like N. brevis, it is restricted to the mudstone-dominated sequences of the Carreg Bica Mudstone, Net Pool Mudstone and Aber Pensidian Mudstone of the Dinas Island Formation (see Figs 3–5). Its possible evolutionary precursor, D. spiniferus, is also restricted to the mudstone-

563 dominated units, though it occurs in both low- and highdiversity assemblages. Both of these climacograptids are associated with a range of graptolites such as L. harknessi, Corynoides, Normalograptus and other climacograptids (C. antiquus s.l.). Some of their associates, such as O. ex gr. calcaratus, N. margaritatus, L. flaccidus s.l., D. morrisi and N. brevis suggest deepmarine influences. 9. A model for graptolite distribution in the Caradoc of southwest Wales

The Caradoc sequence to the south of the Fishguard– Cardigan Fault Belt, in the Preseli Hills and in south Pembrokeshire, is characterized by anoxic graptolitebearing mudstones of the Hendre Shales and Mydrim Shales formations (Evans, 1945; Zalasiewicz, Rushton & Owen, 1995; Fortey et al. 2000, fig. 7), and is graptolitic throughout. However, in the upper Caradoc, graptolite-diversity becomes much reduced and the faunas are dominated by Normalograptus. Locally these changes are equated with shallowing or physical changes in the water column, perhaps cooling or increased oxygenation (Zalasiewicz, Rushton & Owen, 1995). The sequence in the Cardigan area contrasts with that of the Preseli Hills in two key respects. Firstly, the lower Caradoc comprises mostly non-graptolitic oxic marine facies; secondly, diverse, deep-water graptolite assemblages persist through the sequence from the midto late Caradoc, indicating that links with more oceanic influences were sustained during this time. The gross differences in marine facies between the Caradoc sequences of Cardigan and south Pembrokeshire (e.g. Whitland) permit the distinction of different graptolite biotopes in the Welsh Basin (Fig. 12). The marine environments in which the Cwm yr Eglwys Mudstone and Dinas Island formations were deposited, and those of the Mydrim Shales Formation up to the level of the mid-clingani Biozone, represent a ‘graptolite-friendly’ ‘open marine biotope’. These faunas are relatively high-diversity, with a total of 21– 26 species, and contain a mixture of Climacograptus, Diplacanthograptus, Orthograptus, Normalograptus Dicellograptus, Dicranograptus and rarer species (Fig. 12). In the Cardigan area they also occur in rocksequences with Amorphognathus conodonts (e.g. BGS specimens MWL898, MWL904, MWL905, MWL976), thought by Armstrong & Owen (2002) to be adapted to an oceanic biofacies of cold, oxygenpoor, nutrient-rich upwelling waters. By contrast, the upper part of the Mydrim Shales Formation in the Whitland area yields a low-diversity Normalograptusdominated fauna, which may represent a more ‘inshore marine biotope’ or one associated with more elevated oxygen conditions that were adverse to all but the most opportunistic graptolites (see Zalasiewicz, Rushton & Owen, 1995).

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Figure 12. Graptolite distribution during the mid- and late Caradoc for the southern Welsh Basin. High global sea-level during the early to mid-Caradoc flooded the marine shelf establishing a high-diversity ‘offshore graptolite biotope’ in the Cardigan and Whitland areas. Reduced global sea-level during late Caradoc times produced shallowing on the shelf with the development in the Whitland area of an ‘inshore graptolite biotope’ of cratonic invaders, particularly Normalograptus. However, in the Cardigan area the ‘offshore graptolite biotope’ persisted as a result of the locally developed marine trough. Graptolite distributions appear to support the palaeoecological model suggested by Finney & Berry (1997), though in the studied sequence it is not possible to differentiate mesopelagic (mid-water faunas) from offshore epipelagic (surface water faunas). Letters denote: O, Orthograptus; N, Normalograptus; C, climacograptids (Diplacanthograptus and Climacograptus); P, Phormograptus; Nr, Neurograptus; Dn, Dicranograptus; D, Dicellograptus. That Normalograptus species may have been most common in surface waters, is suggested by their shelf occurrence.

Within the ‘open marine biotope’, differences in the associations of graptolites preserved at individual horizons in the rock sequence suggest different graptolite ecologies in the overlying water column. High-diversity assemblages, preserved intermittently, include graptolites such as N. brevis, D. clingani, D. dorotheus, N. margaritatus and Phormograptus sp. that may have been restricted to deep-water

(mesopelagic?) settings with ecological conditions that enabled the development of the local graptolite ‘climax community’. Low-diversity assemblages, which occur more commonly, contain opportunistic Normalograptus species that also occur in the upper Mydrim Shales Formation at Whitland, and which may signal intervals of less ‘graptolite-friendly’ marine conditions.

Ordovician graptolites from Cardigan

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Figure 13. Number of species and taxonomic composition for graptolite faunas from the clingani Biozone and equivalent intervals in the southern Welsh Basin (Cardigan and Whitland), the Southern Uplands of Scotland (Hartfell) and New York State (Mohawk Valley). Data for Whitland and Hartfell are from Zalasiewicz, Rushton & Owen (1995), and those for New York State are from Goldman, Mitchell & Joy (1999). The column for ‘number of horizons of occurrence at Cardigan’ emphasizes how common Normalograptus species are in the sequence.

10. Comparative diversity of the Cardigan graptolite assemblages

Comparison of the overall diversity for the graptolite assemblages from the Cardigan area, with those from Caradoc sequences elsewhere, is useful for determining

which marine loci were the most ‘graptolite-friendly’ in the Late Ordovician seas. The total fauna for the main graptolite-bearing interval of the Cardigan sequence, the clingani Biozone, comprises some 25 species (Fig. 13; see also Fig. 14). It is of similar diversity to that recorded from

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Figure 14. Caradoc graptolites of the Cardigan area, southwest Wales. Bibliographical details of the following named species can be found in Strachan (1997). Specimens referred to Climacograptus tubuliferus (Lapworth) and Normalograptus minimus sensu Elles & Wood have been compared to key material of these species at Birmingham University (rock slab BU1193 with C. tubuliferus) and Cambridge University (N. minimus sensu Elles & Wood specimens SM A19575 and SM A19576). (a, x) Orthograptus calcaratus aff. vulgatus Lapworth MS. (a) BGS MWL395. x, BGS MWL418. (b) Diplacanthograptus dorotheus (Riva), BGS MWL5209. (c) Neurograptus margaritatus (Lapworth), BGS MWL747. (d) Lasiograptus cf. harknessi (Nicholson), BGS MWL2130. (e, f, g) Climacograptus tubuliferus (Lapworth). (e) BGS MWL2262. (f ) BGS MWL2251. (g) BGS MWL2266.

Ordovician graptolites from Cardigan comparable sequences elsewhere in the Welsh Basin and to that of the Utica Shale in the Taconic Basin of the Mohawk Valley of New York State (Goldman, Mitchell & Joy, 1999), which biogeographically was part of the Laurentia palaeocontinent to the north of the Iapetus Ocean. By contrast, the same stratigraphical interval from the Hartfell Shales Formation in the Southern Uplands of Scotland, an area that was also biogeographically Laurentian, yields more than 40 species. Greater abundance for the Scottish faunas is also suggested by diversity at any one horizon, which may be as high as 18 species in the Hartfell Shales: even relatively ‘low’-diversity horizons in the Hartfell Shales typically yield more than five species (Zalasiewicz, Rushton & Owen, 1995). By comparison, of the more than 60 graptolite-bearing horizons identified in the Cardigan sequence, about half bear fewer than three species, while only a small proportion yield five or more taxa (Fig. 11). The maximum diversity recorded at any one horizon in the sequence is eight taxa for the fauna at Aber Howel, whilst a few faunas from the Cwm yr Eglwys Mudstone and Dinas Island formations yield seven taxa. The taxonomic composition of the faunas in the Welsh Basin, Taconic Basin and Southern Uplands are all broadly similar (Fig. 13; see also Fig. 14). Thus, Normalograptus, Dicellograptus and Orthograptus account for between 40 to 50 % of species. However, in the Scottish sequence, Orthograptus, particularly of the calcaratus group, are very common, whilst in the Welsh Basin Dicranograptus are uncommon (Fig. 13), both in abundance and diversity. Higher diversity and the greater abundance of the Scottish fauna may reflect stronger oceanographic influences on the Southern Uplands sequence, which, according to Late Ordovician palaeogeographical reconstructions, was situated on the ocean-facing seaboard of the Laurentia palaeocontinent, and may

567 have been adjacent to a zone of nutrient upwelling. Finney & Berry (1997, 1999) suggest that graptolites, rather than being trans-oceanic, flourished in relatively narrow bands of upwelling waters along and extending oceanward from certain continental margins. They termed this zone the ‘margin-dweller biotope’. The geographical extent of this biotope varied, dependent upon the efficiency of local upwelling conditions to provide nutrients for phytoplankton. Oceanward and landward of this zone, taxonomic diversity reduced. The Cardigan sequence, and perhaps that of the Taconic Basin, were deposited in more restricted marine settings, and the ‘open marine biotope’ of the Welsh Basin might represent the landward fringe of the ‘margindweller-biotope’, more distal from the maximum nutrient supply, which may account for its relatively reduced diversity. The Normalograptus faunas of the ‘inshore marine biotope’ of the upper Mydrim Shales Formation in the Whitland area might, in turn, represent graptolites which Finney & Berry (1997) styled as opportunistic ‘cratonic invaders’. Indeed, the predominance of Normalograptus in low-diversity assemblages (stressed environments?) of the Cardigan sequence supports this suggestion.

11. Conclusions

During the British Geological Survey mapping in the Cardigan area more than 60 graptolite-bearing horizons have been identified within the Caradoc sequence. In terms of local stratigraphy, these graptolite assemblages: (1) allow, for the first time, the erection of a stable biostratigraphical framework, identifying the multidens and clingani (caudatus and morrisi subzones) biozones, and for the first time in Wales, the linearis Biozone;

(h, i) Leptograptus flaccidus (Hall) s.l. (h) BGS MWL879. (i) BGS MWL825. ( j) Phormograptus sp., clathrum incompletely preserved, BGS MWL577. (k, l) ‘Glyptograptus’ sp. 1. (k) close-up of proximal end morphology and (l) full specimen, BGS MWL434. (m) Dicranograptus clingani Carruthers, BGS MWL5228. (n) Normalograptus brevis (Elles & Wood), BGS MWL392. (o) ‘Climacograptus’ sp., distal part of rhabdosome not shown, BGS MWL577. (p, q) Normalograptus miserablis (Elles & Wood). (p) BGS MWL610. (q) BGS MWL494. (r) Climacograptus antiquus Lapworth s.l., BGS MWL739. (s) Normalograptus cf. daviesi Williams, mid-part of rhabdosome incomplete, BGS MWL841. (t) Ensigraptus caudatus (Lapworth)?, scalariform specimen, BGS MWL518. (u, v, w, ll) Dicellograptus morrisi Hopkinson. (u, v) BGS MWL702. (w) MWL996. ll, BGS MWL652. (y–bb) Dicellograptus flexuosus Lapworth. (y) BGS MWL449. (z) BGS MWL474. (aa) proximal part of more complete rhabdosome, BGS MWL443. (bb) BGS MWL452. (cc) Normalograptus minimus sensu Elles & Wood, BGS MWL504. (dd) Diplacanthograptus spiniferus (Ruedemann), BGS MWL631. (ee) Orthograptus truncatus (Lapworth) s.l., BGS MWL2786. (ff ) Orthograptus ex gr. calcaratus (Lapworth), BGS MWL677. (gg) Orthograptus apiculatus Elles & Wood?, narrow specimen, BGS MWL515. (hh) Climacograptus cf. antiquus s.l., distal part of rhabdosome not shown, BGS MWL1046. (ii) Dicranograptid stipe fragment, BGS MWL475. ( jj, kk) Dicellograptus cf. pumilus Lapworth, BGS MWL995. (mm) Dicellograptus aff. moffatensis Carruthers, BGS MWL577A. (nn) Corynoides ultimus Ruedemann?, rhabdosome incomplete, BGS MWL690. Localities: (a, n, x) Section D, horizon 3; (b) Section C, horizon 1; (c, r) Section A, horizon 4; (d) Section B, horizon 1; (e, f, g) Section E, horizon 4; (h, i) Section C, horizon 3; ( j) Section D, horizon 14; (k, l) Section D, horizon 4; (m) Section B, horizon 6; (o, mm) Section D, horizon 14; (p) Section D, horizon 16; (q, t, cc, gg) Section D, horizon 11; (s) Section C, horizon 2; (u, v, ff, ll, nn) Section A, horizon 5; (w, jj, kk) Section C, horizon 6; (y, aa, bb) Section D, horizon 5; (z, ii) Section D, horizon 7; (dd) Section A, horizon 3; (ee) Section B, horizon 9; (hh) Section A, horizon 1. All magnifications × 4, except (c, d, k, n, u, kk) × 8.

568 (2) show earlier structural and stratigraphical interpretations to be untenable, requiring new nomenclature and architectural models to be erected (see Davies et al. 2003); (3) demonstrate major Caradoc facies and thickness variations associated with the newly recognized Fishguard–Cardigan Fault Belt. In terms of Caradoc graptolite biostratigraphical ranges, the sequence indicates: (1) an early, caudatus Subzone, first occurrence for Dicellograptus flexuosus; (2) the importance of Dicellograptus morrisi as a subzone index for the upper clingani Biozone; its incoming is closely associated with the first occurrences of Diplacanthograptus dorotheus and Normalograptus minimus sensu Elles & Wood, which may also be useful indices of this interval. In terms of palaeoecology, the graptolites suggest: (1) locally developed oxic conditions, along the flanks of the inherited Fishguard Volcanic Group graben, largely precluded the influx of, or preservation of, graptolite faunas in early and early mid-Caradoc times; (2) a moderately diverse graptolite assemblage of 21–26 species preserved in the anoxic mudstones of the Cwm yr Eglwys Mudstone and Dinas Island formations (clingani and linearis biozones), considered to signal an open marine graptolite biotope; (3) persistence of this open marine graptolite biotope from mid- to late Caradoc times, indicating continued oceanic influences in the Cardigan area, in contrast to the shallower marine facies in the Whitland area to the southeast, where the late Caradoc is associated with low-diversity (environmentally stressed?) Normalograptus-dominated assemblages. Acknowledgements. We thank Steve Tunnicliff (formerly curator, British Geological Survey) and Stewart Molyneux (BGS) for assistance in the field, Howard Armstrong (Durham) for identifying the conodonts, the Ludlow Research Group for interesting debate in the field, and Jan Zalasiewicz (Leicester University) and Stewart Molyneux and David Schofield (BGS) for commenting on an earlier version of this paper. Constructive reviews from Charles Mitchell (Buffalo) and Alan Owen (Glasgow) are gratefully acknowledged. Pauline Taylor, Paul Shepherd and Sue Wheeler helped with the curation of material. The authors publish with the permission of the Director, British Geological Survey.

References ARMSTRONG, H. A. & COE, A. L. 1997. Deep sea sediments record the geophysiology of the end Ordovician glaciation. Journal of the Geological Society, London 154, 929–34. ARMSTRONG, H. A. & OWEN, A. W. 2002. Euconodont diversity changes in a cooling and closing Iapetus Ocean. In Palaeobiogeography and biodiversity change: the Ordovician and Mesozoic–Cenozoic Radiations (eds J. A. Crame and A. W. Owen), pp. 85–

M. WILLIAMS AND OTHERS

98. Geological Society of London, Special Publication no. 194. BARNES, C. R., FORTEY, R. A. & WILLIAMS, S. H. 1995. The pattern of global bio-events during the Ordovician period. In Global events and event stratigraphy in the Phanerozoic (ed. O. H. Walliser), pp. 141–72. Springer. BETTLEY, R. M., FORTEY, R. A. & SIVETER, D. J. 2001. Highresolution correlation of Anglo-Welsh Middle to Upper Ordovician sequences and its relevance to international chronostratigraphy. Journal of the Geological Society, London 158, 937–52. BEVINS, R. E. & ROACH, R. A. 1979. Early Ordovician volcanism in Dyfed, SW Wales. In The Caledonides of the British Isles – reviewed (eds A. L. Harris, C. H. Holland and B. E. Leake), pp. 603–9. Geological Society of London, Special Publication no. 8. BRITISH GEOLOGICAL SURVEY. 2002. Cardigan, England and Wales Sheet 193. Solid and Drift. 1:50 000. Keyworth, Nottingham: British Geological Survey. CAVE, R. 1979. Sedimentary environments of the basinal Llandovery of mid-Wales. In The Caledonides of the British Isles – reviewed (eds A. L. Harris, C. H. Holland and B. E. Leake), pp. 517–26. Geological Society of London, Special Publication no. 8. DAVIES, J. R., FLETCHER, C. J. N., WATERS, R. A., WILSON, D., WOODHALL, D. G. & ZALASIEWICZ, J. A. 1997. Geology of the country around Llanilar and Rhayader. Memoir of the British Geological Survey, Sheets 178 and 179 (England and Wales). DAVIES, J. R., WATERS, R. A., WILBY, P. R., WILLIAMS, M. & WILSON, D. 2003. The Cardigan and Dinas Island district – a brief explanation of the geology. 1 : 50 000 Series England and Wales Sheet 193 (including part of Sheet 210). Keyworth: British Geological Survey. ELLES, G. L. & WOOD, E. M. R. 1901–1918. A Monograph of British Graptolites. Parts 1–52. Monograph of the Palaeontographical Society, London. EVANS, W. D. 1945. The Geology of the Prescelly Hills, North Pembrokeshire. Quarterly Journal of the Geological Society of London 101, 89–110. FINNEY, S. C. 1986. Graptolite biofacies and correlation of eustatic, subsidence, and tectonic events in the middle to upper Ordovician of North America. Palaios 1, 435–61. FINNEY, S. C. & BERRY, W. B. N. 1997. New perspectives on graptolite distributions and their use as indicators of platform margin dynamics. Geology 25, 919–22. FINNEY, S. C. & BERRY, W. B. N. 1999. Late Ordovician graptolite extinction: the record from continental margin sections in central Nevada, USA. Acta Universitatis Carolinae – Geologica 43, 195–8. FINNEY, S. C., GRUBB, B. J. & HATCHER, R. J. JR. 1996. Graphic correlation of middle Ordovician graptolite shale, southern Appalachians: an approach for examining the subsidence and migration of a Taconic foreland basin. Geological Society of America Bulletin 108, 355– 71. FORTEY, R. A., HARPER, D. A. T., INGHAM, J. K., OWEN, A. W., PARKES, M. A., RUSHTON, A. W. A. & WOODCOCK, N. H. 2000. A revised correlation of Ordovician rocks in the British Isles. Geological Society of London, Special Report No. 24. GOLDMAN, D., MITCHELL, D. E. & JOY, M. P. 1999. The stratigraphic distribution of graptolites in the classic upper Middle Ordovician Utica Shale of New York State: an evolutionary succession or a response to relative sealevel change? Paleobiology 25, 273–94.

Ordovician graptolites from Cardigan

569

HOWELLS, M. F. & SMITH, M. 1997. The geology of the country around Snowdon. Memoir of the British Geological Survey. Sheet 119 (England and Wales). JAMES, D. M. D. 1975. Caradoc turbidites at Poppit Sands (Pembrokeshire), Wales. Geological Magazine 112, 295–304. JAMES, D. M. D. 1997. Llanvirn–Llandovery activity on the Llangrannog Lineament in southwest Ceredigion. Mercian Geologist 14, 68–78. KEEPING, W. 1882. The Geology of Cardigan Town. Geological Magazine 19, 519–22. KOKELAAR, P. 1988. Tectonic controls of Ordovician arc and marginal basin volcanism in Wales. Journal of the Geological Society, London 145, 759–75. KOKELAAR, P., HOWELLS, M. F., BEVINS, R. E. & DUNKELEY, P. N. 1984a. The Ordovician marginal basin of Wales. In Marginal Basin Geology: volcanic associated sedimentary and tectonic processes in modern and ancient marginal basins (eds P. Kokelaar and M. F. Howells), pp. 245–69. Geological Society of London, Special Publication no. 16. KOKELAAR, P., HOWELLS, M. F., BEVINS, R. E. & ROACH, R. A. 1984b. Volcanic associated sedimentary and tectonic processes in the Ordovician marginal basin of Wales. A field guide. In Marginal Basin Geology: volcanic associated sedimentary and tectonic processes in modern and ancient marginal basins (eds P. Kokelaar and M. F. Howells), pp. 291–322. Geological Society of London, Special Publication no. 16. LATTER, M. P. 1925. Note on the age of the rocks around the Teifi estuary. Geological Magazine 62,187–8. LOWMAN, R. D. W. & BLOXAM, T. W. 1981. The petrology of the Lower Palaeozoic Fishguard Volcanic Group and associated rocks E of Fishguard, N Pembrokeshire (Dyfed), South Wales. Journal of the Geological Society, London 138, 47–68. MCCANN, T. 1992. The stratigraphy of the Ordovician rocks

around Cardigan, Wales. Proceedings of the Yorkshire Geological Society 49, 57–65. MYERS, J. 1950. Note on the age of the rocks on the east side of Newport Bay, Pembrokeshire. Geological Magazine 50, 263–4. RUSHTON, A. W. A. 1999. General Introduction. In British Cambrian to Ordovician stratigraphy (eds A. W. A. Rushton, A. W. Owen, R. M. Owens and J. K. Prigmore), pp. 3–12. Geological Conservation Review Series no. 18. Peterborough: Joint Nature Conservation Committee. STRACHAN, I. 1997. A bibliographic index of British graptolites (Graptoloidea). Part 2. Monograph of the Palaeontographical Society, London 151, 41–155. (Publ. no. 603). VANDENBERG, A. H. M. & COOPER, R. A. 1992. The Ordovician graptolite sequence of Australasia. Alcheringa 16, 33–85. WILLIAMS, S. H. 1982. Upper Ordovician graptolites from the top Lower Hartfell Shale Formation (D. clingani and P. linearis zones) near Moffat, southern Scotland. Transactions of the Royal Society of Edinburgh: Earth Sciences 72 (for 1981), 229–55. WILLIAMS, S. H. 1994. Revision and definition of the C. wilsoni graptolite Zone (middle Ordovician) of southern Scotland. Transactions of the Royal Society of Edinburgh: Earth Sciences 85, 143–57. WILLIAMS, S. H. 1995. Middle Ordovician graptolites from the Lawrence Harbour Formation, central Newfoundland, Canada. Palaeontographica A235, 21–77. WILLIAMS, S. H. & BRUTON, D. L. 1983. The Caradoc– Ashgill boundary in the central Oslo Region and associated graptolite faunas. Norsk Geologisk Tidsskrift 63, 147–91. ZALASIEWICZ, J. A., RUSHTON, A. W. A. & OWEN, A. W. 1995. Late Caradoc graptolitic faunal gradients across the Iapetus Ocean. Geological Magazine 135, 611–17.

Appendix 1. Graptolite-bearing localities in the Cardigan area

are inaccessible, even by means of boat. Shoreline localities at Pwllgwaelod [SN 004 398 and Cwm yr Eglwys [SN 015 400], at the north end of Newport Sands [SN 054 410], and at Traeth Bach [SN 101 450], Ceibwr Bay [SN 110 457], Pwllgranant [SN 122 477] and Poppit Sands [SN 145 490] can be readily accessed from the cliff top. Bays at Godir Tudur [SN 052 423], Traeth Cell-Howel [SN 085 436], Craig yr Odyn [SN 130 499] and parts of Dinas Island can be accessed from the shoreline by boat. Some outcrops can be accessed at the top of cliffs, for example, at Aber Pensidian [SN 001 407], Pwll Glas [SN 012 411] and Carreg Bica [SN 093 443]. In the coastal sections the number of samples per unit thickness of rock is highest in the most accessible part of the sequence to the north of Cesig duon at the north end of Newport Sands. North of Ogof Cadno, the majority of graptolites are strongly deformed by cleavagebedding intersections. At some localities, for example Poppit Sands, these relationships are so detrimental that black anoxic mudstones yield no recognizable graptolites. Inland exposures, for example at Frongoch, are subject to exposure gaps caused by thick superficial deposits and vegetation cover. ]

Some 62 graptolite-bearing horizons have been identified in the Cardigan area, yielding several thousand graptolites, the majority from the coastal sections between Dinas Island and Cemaes Head (Fig. 1). Several inland localities have also yielded graptolites, particularly the sections along the banks of the Afon Teifi and tributaries west of Cardigan Town, from Frongoch just to the north of Newport, and from Pantsaeson and Moylgrove to the SE of Cardigan. Material from these collections is housed at the British Geological Survey, Keyworth (BGS MWL386–857, MWL858–1066, MWL1096–1119, MWL1985, MWL1986, MWL2081– 2299, MWL2469–2483, MWL2757–2831, MWL2971– 2975, MWL5200–5226, MWL5227–5304, JZB3712–3739). We have also studied material reported by Myers (1950) and housed in the BGS collections (BGS Zl635–642; see also Zx299–310), and those of McCann (1992) housed at the National Museum of Wales, Cardiff (NMW 89.12G.52–65). The resolution of the graptolite data depends on factors of outcrop accessibilty, tectonic structure, exposure gaps, and intervals of non graptolite-bearing strata. Parts of the coast

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Section A, Ogof Cadno to Cemaes Head Locality 1 2 3 4 5 6 7 8 9 10 11 12 13

scree, base of cliff, fault zone S of Ogof Cadno base of cliff on N side of small bay called Cyfrwy top of cliff at Carreg Bica Traeth Bach, S side of bay, foot of cliff Traeth Bach, near end of headland, S side of bay roadside crag at Moylgrove roadside crag at Moylgrove N bank of the Afon Teifi N bank of the Afon Teifi Pwllgrannant, S of waterfall, at base of cliff N bank, bend in stream 44 paces above waterfall Traeth y Rhedyn Craig yr Odyn, S side of bay

BGS no.

Grid reference

Boat3 Boat4 C2 C3 C1 Moylgrove1 Moylgrove2 PT4 PT5 SPT/MW/97/6 SPT/MW/97/1 no number Boat5

SN 0917 4399 SN 0920 4417 SN 0926 4420 SN 1014 4506 SN 1012 4508 SN 1186 4478 SN 1185 4482 SN 1717 4615 SN 1715 4616 SN 1212 4775 SN 1220 4780 SN 1230 4820 SN 1300 4990

Section B, Dinas Island Locality

BGS no.

Grid reference

1∗ 1∗ 2 3 4 5 6 7 8 9 10 11 12 13 14 15

99/1 99/2 2001/7 2001/6 99/7 99/15 2001/1 99/14 99/13 99/16 99/17 99/18 2001/2 2001/5 2001/4 2001/3

SN 0155 4004 SN 0165 4003 SN 0140 3996 SN 0135 3995 SN 0033 3997 SN 0026 4022 SN 0024 4025 SN 0013 4030 SN 0014 4030 SN 0001 4052 SN 0002 4054 SN 0011 4075 SN 0043 4089 SN 0112 4107 SN 0112 4108 SN 0112 4109

BGS no.

Grid reference

SPT/MW/97/4 PT10 PT6 PT7 PT8 PT9 Keeping

SN 0905 4380 SN 1712 4602 SN 1706 4602 SN 1696 4600 SN 1693 4599 SN 1687 4598 SN 1755 4605

S side of bay at Cwm-yr-Eglwys outcrop E of rocky promontory, Cwm-yr-Eglwys N side of small embayment at Pwllgwaelod N side of small embayment at Pwllgwaelod N side of small embayment at Pwllgwaelod centre of small bay just SE of Catch y Mitsiwr crags at top of cliff to the E of Catch y Mitsiwr N of Catch y Mitsiwr, N side of bay N of Catch y Mitsiwr, N side of bay SE corner of bay at Aber Careg y Fran near centre of the bay Aber Careg y Fran scree from near centre of bay Aber Pensidan old quarry just below top of cliff W of Pen-clawdd 5 m above sandstones, base of upper cliff, Pwll Glas scree near centre of exposure, upper cliff, Pwll Glas scree at centre of exposure, upper cliff, Pwll Glas

Section C, Traeth Cell-Howel to Cardigan Locality ‘a’ S of Ogof Cadno, scar 2/3 from the top of cliff 1 S bank just east of river bend 2 S bank opposite red marker of effluent outflow 3 S bank, 60 paces downstream from effluent outflow 4 S bank opposite rocky river cliff on N bank 5 S bank, 60 paces downstream from 4 6 at small bridge over tributary of the Afon Teifi

Ordovician graptolites from Cardigan

571

Section D, Newport Sands Locality 1 shoreline exposure at high water mark 2∗ Myers’ collections, near Cesig duon 3 cave at high water mark on shoreline 4 shoreline exposure at high water mark 5 shoreline exposure on large reef 6 shoreline exposure at high water mark 7 shoreline exposure, at base of cliff 8 N end of Newport Sands 9 cliff exposure 10 c. 4 m S of Pen Pistyll on shoreline 11 shoreline exposure, c. 3 m S of Pen Pistyll 12 shoreline exposure, 17 paces N of Pen Pistyll 13 shoreline exposure, c. 1.5 m S of Pen Pistyll 14 39 paces N of Pen Pistyll 15 shoreline exposure, 12 paces N of Pen Pistyll 16 49 paces N of Pen Pistyll 17 scree at base of cliff, opposite Carregedrywy 18 Traeth y Bˆal, at base of cliff on N side of bay no number, Godir Tudur no number, Pen y Bal

BGS no. P4 no number P1 P2 P3 N8 N7 99/9 N6 N1B N1 N3 N1A N4 N2 N5 Boat6 Boat1

Grid reference

SPT/MW/97/5

SN 0543 4067 SN 0535 4072 SN 0539 4080 SN 0540 4084 SN 0540 4088 SN 0543 4094 SN 0542 4099 SN 0522 4121 SN 0518 4125 SN 0538 4111 SN 0538 4111 SN 0535 4118 SN 0538 4111 SN 0535 4118 SN 0535 4118 SN 0535 4118 SN 0494 4190 SN 0490 4166 SN 0525 4225 SN 0480 4150

BGS no.

Grid reference

99/4 99/5 99/6 99/8

SN 0758 4098 SN 0759 4098 SN 0760 4102 SN 0749 4108

Locality 10 is about equivalent to McCann’s (1992, p. 64) macrofossil locality A

Section E, Frongoch Locality 1 2 3 4

outcrop near W end of quarry outcrop c. 15 paces E of W end of quarry nine paces upstream from E end of quarry stream bed, 52 paces upstream from old stone bridge

Other localities Locality Pantsaeson Farm, ridge, 78 paces W of field boundary Pantsaeson Farm, ridge Aber Howel

BGS no.

Grid reference

SPT/MW/97/2 SPT/MW/97/3 99/12

SN 1390 4505 SN 1370 4502 SM 9905 3852

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