Journal of the Geological Society, London, Vol. 147, 1990, pp. 607-610, 2 figs. Printed in Northern Ireland

SHORT PAPER which also yielded Microfossil Assemblage V of Molyneux (1987). The FennianStage, whosebase isdefined at Arenig biostratigraphy and correlation in Cwm-banau, north of Whitlandhas yielded the most di- the Welsh Basin verse faunas in theArenig of the Welsh Basin;it istyp- ifiedbysuch trinucleid trilobites as Stapeleyella and R.A. FORTEY’ & R.M. OWENS’ Bergamia, cyclopygids (e.g. Pricyclopygebinodosa Department of Palaeontology, Natural History eurycephala, Cyclopyge grandis brevirhachis) andthe Museum, London SW75BD, UK graptolites Didymograptus (Expansograptus) uniformis le- ‘Department of Geology, National Museum of pidus and Pseudotrigonograptus ensiformis. The earliest Wales, Cardiff CFl3NP, UK biserial graptolites (‘Glyptograptus’ species)occur inthe B. rushtoni Biozone, becoming abundant in the Lland- fallteg Formation, D. levigena Biozone. MicrofossilAss- emblages VIand VI1 of Molyneux(1987) arefrom the Recent work on the Arenig Seriesin the Welsh Basin is reviewed; it is now known to be far more complete and richly fossiliferous than Pontyfenni Formation. has been supposed. Three regional stages, Moridunain, Whitlandinn In the St David’s district, only the Whitlandian and the and Fennian, and Seven trilobite-basedbiozones have been defined. lower part of the Moridunian can be identified directly on Outstandingproblems are highlighted,among which remains the palaeontological grounds.Because of theirstratigraphic definition of a stratotype for a base for the series, for which there position, the cleaved shalesintervening between the Pen- appears to be only one feasible candidate in this area, in the Shelve maenDewi andAber Mawr Shale(i.e. ‘Llanvirn Quarry inlier. beds’) formations are presumably of Fennian age. Traynor (1988), in his study on the sedimentology of the Arenig in Until quite recently, the Arenig Series in the Welsh Basin SWWales implied alate Moridunian age for theAber- was thought to be typified by the succession in the type area bach Member of his LlanferranFormation but this was in North Wales atArennig Fawr, Gwynedd, where there not supported bypalaeontological evidence,and later occurs transgressive, a broadly arenaceous sequence Moridunian has yet to beproved in the St David’s area. overlain by an argillaceous one,both carrying arather The early Moridunian Ogof Hen Formation with limited fauna. Work by the authors (Fortey & Owens 1978, Neseuretus ramseyensis and Merlinia murchisoniae rests 1987) in South Wales over the past fifteen years has shown disconformably on presumedupper Cambrian (Merioneth thatin this area there is a much fuller andmore richly Series)Ogof Velvet Formation (‘LingulaFlags’) whose fossiliferous succession thanat Arenig. It represents the precise age is not known. fullest development of the Arenigin western Gondwana. The three stages and trilobite-based biozones proposed by Shelve inlier: column 4 There are a few ties into S Wales Fortey & Owens (1987) and their approximate equivalence in the thick Mytton Flagssuccession, exceptfor to established graptolitebiozones are shown on Fig. 1. Neseuretus murchisoni and Cyclopye grandis grandis which Selected Arenig successions are discussed below. suggestequivalence of parts at least of theMoridunian and Whitlandianstages respectively. Asaphidand trin- SW Wales (Dyfed): columns 1-3 Between them, the Car- ucleid trilobites are represented by different species. The marthenand Whitland districts (columns 3 and 2) afford presence in the so-calledShelve ChurchBeds of biserial the most complete Arenig succession in the Welsh Basin, graptolites(e.g. Glyptograptusshelvensis; seeStrachan and it is fromthese sequences that the stage and zonal 1986) and tribolites such as Placoparia cambriensis,Pricy- schemeshave beenderived. Thesequence is exposedin clopygebinodosa and Selenopeltis buchi macrophthalmus, several separateareas, and it is possible thatmore than common to the Fennian in South Wales, suggests strongly oneterrane is present.Bases forthe Whitlandianand thatthey are of late,not early Arenig age as theyhave Fennianstages are definedin this area. Thereare prob- for so long supposed to be. The Tankerville Flags likewise lems associated with definition of the base of the Moridu- have a late Arenig (Fennian) fauna with elements (includ- nian Stage (and thus of the Arenig), discussed below. The ing Asaphellus whittardi) common tothat stage in S Moridunianis characterized by transgressivesandstones Wales. There is, however, no known equivalent at Shelve containing Neseuretus ramseyensis and N. murchisoni in of the lastest Arenigfauna of the Llanfallteg Formation. theirupper part, overlain by mudstones withspecies of It is possible that fauna1differences of the Shelvesucces- the asaphid Merlinia, inparticular M. selwynii. A local sion aredue to the presencethere of persistently shallo- olenidbiofacies is developed in theCarmarthen district, wer waterconditions, breaks in thesequence, or a com- the ‘Peltura punctata Beds’ of olderliterature. Molyneux bination of both. (1987)defined four microfossil (acritarchand chitinozoa) assemblages in the typeMoridunian and within these the North Wales:columns 5-7. Work byZalasiewicz (1984) ‘Veryhachiun trispinosum’ group in Assemblage I1 and Beckly(1987) atArennig and by thelatter in LQn, (CwmfffidMember, Carmarthen Formation) may prove Bangor and Anglesey (Beckly 1987, 1988, 1989) has shown to be a useful biostratigraphic marker once the taxonomy that the three stages can be recognized in various degrees andbiostratigraphy of members of thegroup havebeen of completeness over North Wales. Thus at Arenig much refined. The base of the WhitlandianStage isdefined at of theFennian, all of the Whitlandianand thelater Cwm yr Abbey, east of Carmarthen. It is characterized in Mordinunian are absent. The Henllan Ash Member of the the lower part by trinucleid Furcalithus radix andthe Carnedd Iago Formationcontains Merlinia selwynii, M. asaphid Ogyginus hybridus, and in the upper part by the murchisoniae, Neseuretus murchisoni and Ampyx cetsarum, trinucleid Gymnostomix gibbsii, the shumardiid Shumardia suggesting equivalence tothe BolahaulMember of the gadwensis, andthe cyclopygid Cyclopyge grandisgrandis, Ogof Hen Formation and the earlier part of the Carmar- 607

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TRILOBITE AND GRAPTOLITE Doming 1989) typified by Stelliferidiurn trifidurn similar to SPFCIFS that of the Watch Hill Grits of theLake District (Moly- Neseuretus ramseyensis new & Rushton 1988)which probably post-dates the late Merlinra murchrsontae Tremadoc Angelina sedgwickii Biozoneand pre-dates the Neseuretus murchisonr earlyArenig Didyrnograptus defexus Biozone,and thus Ampyx cetsarum Myttonta cf. fearnstdesi equates with the late Lancefieldian La2,La3 or early Phyllograptus cf. angustifolius Bendigonian Be1 of Australia,and with theHunneberg Phyllograptus cf. densus Substage of Sweden. It mayalso include equivalents of Merlinra selwynii the Tetragraptus approxirnatus Biozone, which is often ac- Merlinia rhyakos Furcalithus radix cepted as the base of the Arenig in an international sense. Azygograptus eivfonfcus Unfortunately there does not seem to be an uninterrupted Ogyginus hybridus passage fromthese ‘Loginbeds’ intothe AlltCystanog Bohemopyge scutatrtx Cnemidopyge salter; Member. The trijidurn flora (Fortey et al. 1990)is widely Shurnardia gadwensis distributed, having been identified also in the Isle of Man Gymnostomtx gibbsti and possibly Spain,and is clearly of greatstratigraphical Azygograptus hicksii potential. Tetragraptus serra Cyclopyge grandis grandis In the Shelve inlier, the HabberleyShales were for Didymogr.(Expansogr) goldschmid longbelieved to bethe equivalents of and synonymous Stapeleyella abyhons with the Shineton Shales of the Sheinton-Wrekin district. Asaphellus whittardi They occurimmediately below the basal ArenigStiper- Cyclopyge grandis brevtrhachfs Pricyclopyge binodosa eurycephal, stonesQuartzite, andthrough misidentification of Bergamia rushtoni Shurnardia (Conophrys) saZopiensis (‘pusilla’ of earlier li- Ormathops ntcholsoni terature)from the top of the shales it wasassumed that Ampyx linleyoides thetop of theTremadoc wasmissing. Workin progress Pseudotrigonogr ensfformis Dtdymogr (Expansogr.) unrf lepidu by theauthors has shown thatthe Habberley Shales Pseudisograptus stel1u.s overlie the Shineton Shales, have a different lithology and Didymogr. (Expansogr ) hirundo alsocontain a distinctive, if sparse,fauna dominated by Selenopeltis buchi macroph. the trilobite Asaphellus cf. grafi and lingulacean brachiop- ‘Glyptograptus’ dentatus Dionide levigena ods. Rare olenid trilobites also occur. The trilobites are Pricyclopyge binodosa binodosa specifically different fromthose of the ShinetonShales, Barrandra homfrayf and also from those in the late Tremadoc in north Wales. Stapeleyella inconstans Ormathops llanvunensis Angelina sedgwickii, characteristic of the youngest Trema- ‘Glyptograptus’ austrodentatus doc in north Wales hasbeen identified below the Acrograptus acutidens Asaphellus cf. grafi-bearingbeds at Shelve. There seems Selenopeltis buchi buchf to beno obvious break in sedimentationbetween the Dionide turnbull; Didymograptus artus Habberley Shales andStiperstones Quartzite, although very shallow water conditions, equating with the late Tre- madocregression, certainlyobtained. With their very 2. Ranges of selected trilobite and graptolite species in the Fig. sparse fauna, the base of the Stiperstones Quartzite is not Arenig of the Welsh Basin. an appropriate position for definition of the base of the Arenig;instead, this mightoccur within the Habberley thenFormation in South Wales. At Bangorthe earliest Shales, although the paucity of thefauna so far known transgressive sandstones are of probable Whitlandian age, hardly makes this anappropriate stratotype, and the im- andthose in Anglesey of Fennianage; they contain portant trijidurn flora hasnot yet beenfound. A more different Neseuretus species, N. caerhunensis and N. rnon- promising sequence is that of the Lake District, where the ensis respectively.Beckly (1987) demonstratedthat Skiddaw Group contains later Tremadoc and early Arenig different fault-boundedregions of N Waleshad different faunas from a more exterior site, on the margins of Gon- sedimentaryand structural histories throughoutthe Are- dwana. nig, and in thissense they areterranes. Only in the Aberdaronarea the strata representative of all three Base of the Llanvirn. Thismarks thetop of theArenig stages present.Problems remain in precise dating of the by definition. The Llanvirnwas recognized originally in bases of the Bangor and Angleseysuccessions. However, theSt David’s district, principally fromthe famous Llan- it is considered likely that Angleseywas notgreatly dis- virn Quarry fauna and the overlying Didymograptusmur- tant from the rest of Gondwana (it wasunlikely to have chisoni shales. Thedark shales (Tetragraptus Shales of beena mid-oceanicisland asNeuman & Bates (1978) earlierliterature) underlying the beds at Llanvirn Quarry suggested). are cleaved and virtually unfossiliferous, andan intrusion intervenes at Llanvirn itself. A farbetter sectionis Base of the Arenig. Inboth south and north Wales the afforded by an oldrailway cutting at Llanfallteg, NW of oldest demonstrableMoridunian sediments lie with dis- Whitland. Here, the mudstones of the Llanfallteg Forma- conformity onthe underlyingsediments: presumed upper tion containan abundant, well-preservedmixed trilobite- Cambrian in SW Wales,Tremadoc in Gwynedd. In the graptolitefauna, and the base of the Llanvirnis recog- Carmarthen district sediments underlying the early Morid- nizedby the influx of pendentdidymograptids, and al- unian Allt Cystanog Member of the Ogof H&n Formation thoughmost of the trilobite speciescross the boundary near Loginhave yielded anacritarch flora (Molyneux & some, including Cyclopyge kossleri,Novakella copei,

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Gastropolusobtusicaudatus, Dionide turnbulli and Weare grateful to P. Knapman, G. Evansand L. Norton for Selenopeltis buchi buchi have not yet been found below it assistance. (Fortey & Owens 1987; Kennedy 1989). The name of the lower Llanvirngraptolite biozone was changed by Fortey References & Owens (1987) from bifidus to artus, sincethe former BECKLY,A. J. 1987. Basin development in North Wales during the Arenig. occurs in theArenig of Laurentia,where independently Geological Journal, 22, 19-30. derivedpendent didymograptids appeared earlier than in - 1988. The stratigraphy of the Arenig Series in the Aberdaron to Sam Gondwana. D. bifidus haslong been misidentified in Bri- areas, western Lljk, north Wales. Geological Journal, 23, 321-337. -1989. A new Arenig trilobite fauna from the Bangor area, North Wales. tain, where D. artus is a common and characteristic form Bulletin of the British Museum (Natural History) (Geology), 45, 1-20. of the lower Llanvirn. FORTEY,R. A. & OWNS,R. M.1978. Early Ordovician (Arenig) stratigraphy and faunas of the Carmarthen district, south-westWales. Bulletin of the British Museum (Natural History) (Geology), 30,225-294, Graptolitebiozones and correlation. It isdifficult to re- PIS 1-11. -&- 1987. The Arenig Series in South Wales. Bulletin of the British cognise the classical biozones in much of the Welsh Basin Museum (Natural History) (Geology), 41, 69-307. succession, firstly because of facies, and secondly because -, BASEIT,M. G., HARPER, D.A. T., HUGHES,R. A., INGHAM,J. K., of thestate of thegraptolite taxonomy. Widespread MOLYNEUX,S. G., OWEN,A. W., OWENS,R. M,, RUSHTON,A. W. A. & species such as Pseudotrigonograptuserniforrnis, Didyrn- SHELDON.P. R. 1990 Progress and problems in the selection of stratotypes for the bases of series in the OrdovicianSystem of the ograptus (Exparnograptus) uniforrnislepidus and historicaltype area; In: WILLIAMS,H. (ed.), Fifth Ordovician Glyptograptus dentatus occur in deeper water sediments in Symposium volume, (in press) the Fennian, and isograptids, typical of more exterior sites KENNEDY, R. J. 1989. Ordovician (Llanvirn) trilobites from SW Wales. havebeen found on Ramsey Island, Menai Straits and Monograph of thePalaeontographical Society, London, 1-55, 14pls (Publication no. 576, part of volume 141 for 1987). Lljk Together these allow correlation of the Fennian with MOLYNEUX,S. G. 1987. Acritarchs and chitinozoa from the Arenig Series of the Lake District sequence, and with those in Baltica and south-westWales. Bulletin of theBritish Museum (Natural History) the Pacific Province.Correlation of theWhitlandian and (Geology), 41, 309-364. Moridunianstages cannot be done other than by more - & DORNING,K. J. 1989. Arcitarch dating of latest Tremadoc-earliest indirect means at present. Arenig (early Ordovician) sediments in the Carmarthen district, south Wales. Geological Magazine, 126, 707-714. - & RUSHTON,A. W. A. 1988. The age of the Watch Hill Grits (Ordovician), English Lake District: structural and palaeogeographical Conclusions. (1). The Arenig of the Welsh Basin forms a implications. Transactions of theRoyal Society of Edinburgh: Earth much morecomplete and biostratigraphically rich se- Sciences, 79, 43-69. NEUMAN,R. B. & BATES,D. E. B.1978. Reassessment of Arenig and quencethan hitherto supposed; (2) the fullestsuccessions Llanvirn age (early Ordovician) barchiopods from Anglesey, north-west occur in theCarmarthen-Whitland District; (3) the type Wales. Palaeontology, 21, 571-613, pls 63-8. Arenig in N Wales is incomplete; (4) thetransgressive STRACHAN,I. 1986. The Ordovician graptolites of the Shelve district, arenaceousbase of theArenig is of differentages in Shropshire. Bulletin of the British Museum (Natural History) (Geology), differentareas, with distinct Neseuretus species; the 40, 1-58. (5) TRAYNOR, J. -J. 1988. The Arenig in South Wales: sedimentary and volcanic localbase of theArenig may bea non-sequence or dis- processes during the initiation of a marginal basin. Geological JOUrMl, conformity,coinciding with worldwideregression at this 23,275-92. time; (6) the base of the overlying Llanvirn can be recog- WILLIAMS,A., STRACHAN,I., BASSE~,D. A., DEAN,W. T., INGHAM.J. K., nized in a continous fossiliferous succession NWof Whit- WRIGHT,A. D. & WHITIINGTON,H. B. 1972. A correlation of Ordovician rocks in the British Isles. Geological Society Special Report no. 3. land in theregressive Llanfallteg Formation, making this ZALASIEWICZ,J. 1984. A re-examination of the type Arenig Series. Geological an appropriate stratotype. Journal, 19, 105-24.

Received 20 December 1989; revised typsecript accepted 6 March 1990.

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