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Arenig Biostratigraphy and Correlation in the Welsh Basin

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Arenig Biostratigraphy and Correlation in the Welsh Basin 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 the Arenig 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 fauna1 differences of the Shelve succes- the asaphid Merlinia, inparticular M. selwynii. A local sion are due 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 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/147/4/607/4890501/gsjgs.147.4.0607.pdf by guest on 26 September 2021 I I L *0 * f U W U$ cn dnOtl9 3A13HS 1 I I I I I T NVINN3j INVlaNVlllHMI NVlNnaltlOYU OIN3tlV Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/147/4/607/4890501/gsjgs.147.4.0607.pdf by guest on 26 September 2021 SHORT PAPER 609 TRILOBITE AND GRAPTOLITE Doming 1989) typified by Stelliferidiurn trifidurn similar to SPFCIFS that of the Watch Hill Grits of the Lake 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
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