Journal of rhe Geological Society, London, Vol. 147, 1990, pp. 611-614, 1 fig. Printed in Northern Ireland

SHORT PAPER rences and have proved more difficult to apply with preci- Ordovician graptolite biostratigraphy in sion in Wales. the Welsh Basin: a review Problemswith biozones. Many practical difficulties are encountered when applying the Ordovician graptolite bio- A.W. A. RUSHTON zones in the Welsh Basin. Many,perhaps most, species British Geological Survey, Keyworth, Nottingham are in need of critical taxonomic revision, and until this is NG12 5GG, UK accomplished determinations can only be approximate. Assignment of an assemblage of graptolites to a zone can- not always be accomplished using objective criteria; often Ordovician graptolite bmstratigraphy in Wales is crude compared abiostratigrapher has tobalance probabilities based on with that of the Siurian and is in need of relhement, especially by personalexperience of faunaldistributions and ranges of detailed collecting from suitabledons and consequent taxonomic species, or has to assess the significance of docbtfully de- work. This has been done in parts of the Arenig, kdeilo and terminedor poorly preservedmaterial. Through most of lower Caradoc successions. mere is particular potential for subdivi- theOrdovician succession thereare no detailedrange- sion of and improvement in the correlation of the Llpnvirn Series charts. The only comprehensive listing of British Ordovi- and the grucilis-linearis biozones, butthere is littleprospect of cian graptolitespecies is that of Elles & Wood (1901- much new and detailed information from the upper Tremadoc or 1918), where 15 zones were recognized andthe occur- Ash@ Series. rences of species in those zones were tabulated. Strachan (1971) consideredthis table and commented that it ‘has been accordeda precision whichis unmerited’.Hughes Graptolite biostratigraphy has proved of great value in the (1989, p. 13) expressedfurther criticisms of Elles & correlation of Ordovician and Silurian rocks, especiallyin Wood’s table,stressing that such tabulationobscured the basinalfacies. It providesan independent time-scale for lowest and highest appearances of species within azone. geological processes in basin development. In the Silurian, Rickards (1976) and Zalasiewicz (this volupe) have shown graptolites can achieve a biostratigraphicalaccuracy as good that recording the succession of lowest appearances (and, asany in thestratigraphical column (Zalasiewicz, this toa lesser extent, highest occurrences) of species yields volume).In comparison the existing Ordovician biostrati- much valuablebiostratigraphical information in the Silu- graphy is crude, each zone representing, on average, some rian.This approach should be applicable tothe Ordovi- 4Ma (or about five times as long asin the Silurian), but cian; it was used by Finney (1986) to resolve contentious there is potential for considerable refinement. problems in correlation of thelater Ordovician of North America, and R. A.Cooper & K. Lindholm (pers. Graptolitebiozones. Graptolitebiostratigraphy is com- comm.), by logging theorder of first occurrences of nu- monly discussed in terms of zones(biozones). Although merousearly Ordovician species world-wide, have suc- each zone has acharacteristic assemblage of species(as ceeded in making a gratifyingly detailed subdivision of the outlined by Elles 1925), they are not so much assemblage graptolitic succession. zones (Strachan 1971, p.4) as Oppel-zones,that is, The zonal sequence itself is composite, being based on they are moreor less complicatedforms of concurrent- stratigraphical successions in different parts of the country, range zones or partial-rangezones (Hughes 1989, p.4). in differentenvironmental settings, and even in different Ultimatelythe recognition of suchbiozones depends on faunal provinces (Fig.1). One of theadvantages of knowing theorder of appearance of species,their strati- graptolites is thatthey occur in arange of facies(Elles graphicalranges and, toa lesserextent, their relative 1940) and tendto cross provincial boundaries, but in the abundances;information that can be obtained only by earlierOrdovician the provincial differencesare sufficient thedetermination of speciesfrom carefully collected to make intercontinental comparisons difficult (Skevington successions. 1973;Dewey et al. 1970). For example, Stone & Strachan Inthe British Ordovician (Tremadoc to Ashgill) there (1981) had difficulty correlatingan Arenig fauna from are 16 to 21 graptolitebiozones nowin use;the exact Ballantrae, southern Scotland, with theArenig succession number recognized depends on the status accorded to the in the Skiddaw Group of northern England. divisionsin the Ashgill Series and on the number of bio- More puzzling are some of the much more local varia- zones recognized in theTremadoc and Arenig. The bio- tions in thecomposition of graptoliteassociations. The zones consideredhere are given inFig. 1. Those of the theoriesthat graptolite species were subjectto depth- lower Ordovician are based mainly on Welsh and English zonation or confined to particular water-masses have been successions, and,although developed in varioustypes of considered(Finney 1984), but neither model is applicable graptolitic biofacies (inouter-shelf, slope and basinal set- to all instances and probably each is valid to some extent. tings), they can be applied with some success in the Welsh Some of the correlation problems caused by geographical Lower Palaeozoic basin. The mid-Ordovician Nemagraptus and facies variation of faunas can be met by integrating gracilis Biozone is basedon the Scottish occurrences thegraptolite zones with conodontbiostratigraphy where, however, there are no underlying graptolitic rocks (Bergstrom 1986). to provide a lower limit; the zone is widely recognized in Wales andthe Welsh Borderland. With the exception of the overlying Diplograptus multidens Biozone and the top- Applications in theWelsh Basin. There follows a brief most Ordovician Glyptograptuspersculptus Biozone,the review of thepresent status of the Ordoviciangraptolite upperOrdovician biozones are basedon Scottish occur- zones, and of their recent use in the Welsh basin. 611

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SCOTLAND ENGLAND & WALES

Parakidograptus acuminatus \* SlLURlAN

c 7 Glyptograptus persculptus Climacograptus? extraordinarius 1 Ashgill

Dicellograptus complanatus Pleurograptus lhearis -_ - -_- Dicranograptus clhgani Climacograptus wilsoni Caradoc Climacograptus peltifer 6 Diplograptus multidens - - Nemagraptus gracilis ------Llandeilo 4 Glyptograptus teretiusculus -_-- Didymograptus murchisoni Llanvirn Didymograptus artus Didymograptus hirundo lsograptus gibberukrs Scottish Didymograptus nitidus Arenig Southern Uplands Didymograptus deflexus Tetragraptus approximatus - --- (no gaptolites) Adelograptus tenellus Trernadoc Wales . ' ' '/I { flabelliformis Rhabdinopora flabelliformis 1 socialis / 4 l I Fii. 1. A table of Ordovician graptolite zones used in Britain, showing those based primarilyon Scottish and Anglo-Welsh localities. Correlation with the standard series is most precise at the base of the Tremadoc, the Llanvirn and the Silurian. The map indicates outcropsof Ordovician (black); the numbers show areas where the correspondingly numbered zones were definedor are typically developed.

Tremadoc. Bulman & Rushton (1973) discussed the use of where the sedimentary succession is very thick, Tremadoc zones in Britain; the graptolite zones were based superpositionalevidence scanty andmeasured sections onthe lower part of the ShinetonShales in the Welsh unknown. Current work in the Lake District by the British Borderland and have been applied with some success to the Geological Survey indicates that Jackson's (1962) restricted Tremadoc rocks of North Wales. The higherzones are version of Elles' zonal scheme for the Arenig (Elles 1933) is based on shelly faunas.Cooper (1979)recognized a applicable to the Skiddaw Group, but that thescheme needs world-widesuccession of graptolite assemblages and refinement andextending down to includebeds of about assigned the Shineton Shales faunas to his Assemblages 2 basal Arenig age(Molyneux & Rushton 1988).In Wales and 3. Allen et al. (1981, p. 319) recorded a twofold local studies by Zalasiewicz (1986) at Arenig, by Fortey & Owens subdivision of the jlabelliformis Biozone around the Harlech (1987) in southwest Wales and by Beckly in L19n (1988) Dome,and Cooper'sAssemblage 1 mayoccur there, at indicate that the upper zones of the Skiddaw Group can be Bryn-llin-fawr and Ogof-ddQ (Rushton 1982), but is yet to recognized in the Welsh Basin, but the lower zones generally be proved. cannot because of the scarcity of graptolites. Application of In the Welsh Basin the late Tremadoc is very sparingly these zones to the succession in the Shelve Inlier has proved graptolitic and Cooper's Assemblage 4 is unrecognized. The problematical (Strachan 1986). A fuller description of the only determinablematerial from horizons as high as the graptolite species of the Lake District is needed to enable a Shumardiapusilla Biozonecomprises species of bettercomparison with those of the WelshBasin and Rhabdinopora [formerly Dictyonema] fromthe 'Upper adjoining areas. Dictyonema Band' in Nant Gwynant, SW of Dolgellau (Cox & Wells 1921), which appears to overlie strata with a pusilla Llanvirn. The subdivision of the Llanvirn Seriesinto a Biozone shelly fauna. Rhabdinopora alsooccurs with a Didymograptus bifidus Biozone (below) and D. murchisoni pusilla Biozonefauna in theDeanshanger Borehole in Biozone (above) has been recognized in several areas: South Buckinghamshire (Bulman & Rushton 1973). Neither of andNorth Wales, the Shelve and Builth inliers, and the these is diagnostic of Cooper'sAssemblage 4, which is, Skiddaw Group in the Lake District. The nameof the lower however, represented in the Lake District by Araneograptus zone was changed to the D. artus Biozone (Fortey & Owens pulchellus (T. S. Hall); this is from strata low in the Skiddaw 1987), because it was found that British material had been Groupand is taken to indicatea horizon just below the wrongly assigned to D. bifidus (J. Hall) (summary in Fortey Tremadoc-Arenig boundary (Rushton 1985). ef al. 1990). In Scania, Sweden, the Upper Didymograptus Shale(strata equivalent to the Llanvirn Series)has been Arenig. The zonal scheme is imprecise, being based on the divided into five zones(Ekstrom 1937). Although such faunas of the Skiddaw Group in the English Lake District detailed results have notbeen achieved in Britain,the

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higher part of the artus Biozone has been distinguished in British successions (in either Wales or Scotland) must await the Builth Inlieras a separateSubzone of D. speciosus detailed work graptoliteon distribution in Britain. (Elles 1940), and likewise in the Skiddaw Groupas a Incidentally, Hughes (1989)also showedthat the species Subzone of Nicholsonograptusfasciculatus (Skevington Diplograptus multidens Elles & Wood is a junior synonym 1970). It is not yet known how closely these two subzones of D. foliaceus (Murchison), so when the biozone is revised may becorrelated with each other. Work inthe Shelve a change of nomenclature can be expected. Inlier did not yield results allowing further subdivision of the Llanvirn Series,partly because of local facies changes UpperCaradoc-basal Ashgill. The succeeding Ordovician (Strachan 1986),such asthe incoming of shallow-water zones of Elles & Wood‘s tableare basedon Scottish sedimentationin the WestonBeds. Probably a more successions, and, working onthe section at Dob’s Linn, detailed zonalsubdivision of the LlanvirnSeries will be Williams (1982a, b, 1983, 1987) has reviewed the graptolite provided by a critical study of suitable collections from the distributions in all the zones from the top of the clingani Builth Inlier. Biozone to the base of the Silurian. In Scotland the faunas arecharacterized not onlyby diplograptidsbut alsoby Llandeilo and basalCaradoc (teretiusculus and gracilis various dicranograptids and leptograptids; the latter groups biozones). Skevington (1971)discussed the difficulties in include species that are useful stratigraphically and tend to recognizing the teretiusculus Biozone; he advocated that it be less difficult to determine than the diplograptids. In the should beabandoned and that the base of the gracilis WelshBasin thelater Ordovician graptolite faunas are Biozone should be taken at the appearance of the genera mainly composed of diplograptids and it can prove difficult Dicellograptus and Nemagraptus. However, Hughes (1989), to assign a fauna to a particular biozone. However, Mitchell having studied the graptolite successions in the Shelve and (1987) has shown thatthere are subtlevariants in Builth inliers from the topof the Didymograptus murchisoni diplograptidproximal developmentand if thesecan be Biozone into the Diplograptus multidens Biozone, was able recognized in flattened specimens it should be possible to to recognize a teretiusculus Biozone, its base being taken at separate the various diplograptid lineages, and hence delimit the disappearance of Didymograptus murchisoni. The base the species more exactly. The clingani Biozoneis known of the overlying gracilk Biozone is taken at the appearance with certainty from the Mydrim Shales of southwest Wales of N. gracilis itself and the extent of the zone is determined and the Carmel Group (Roberts 1929) of the Tywi Anticline by thetotal range of this species. Hughesnot only gave (recently provedduring BGS mapping). It hasbeen general range-charts for the various species, but also gave recordedfrom North Wales (Howells et al. 1978). Cave the ranges of each species in individual measured sections. (1965) reported a significant occurrence of the clingani His results show how, by detailed collecting and attention to Biozone at Welshpool, where it is associated with an Onnian taxonomy,biostratigraphical refinement may be achieved; shelly fauna; but Ingham & Wright(1970) suggested that but they have yet to be applied to other areas,such as South this fauna might be referable to the Linearis Biozone. The Wales. lineark Biozone may be represented in the Carmel Group, in which Orthograptusquadrimucronatus cf. spinigerus LowerCaradoc (post-gracilis Biozone). Strachan (1971, p. (Lapworth)has been collected, and appears also tobe 5) commented on the unsatisfactory way in which the Zone present in theupper beds of the MydrimShales near of Climacograptus peltifer was proposed. The name of the Whitland,Carmarthen (J. Zalasiewicz,pers. comm.). zone is unsuitable because Riva (1976) has shown that C. Harper (1956) recorded linearis Biozone faunas from black peltifer Lapworth is a junior synonym of C. bicornis (J. shales below the Ashgill Series near Llanystwmdwy, North Hall), but it is pointless to alter the name of the zone until Wales. the status of the zone itself has been reviewed. The fauna of the peltifer Biozone, as listed by Elles (1925), is so similar to Ashgill. In Scotland the lower Ashgill is largely barren but that of gracilis that assemblages lacking Nemagraptus gracilis the higherAshgill anceps Biozone hasbeen divided into itself can prove difficult to distinguish. The overlying wilsoni subzones (Williams 1982b), and a very thin representation Biozone is a relatively impoverishedzone between the of the extraordinarius Biozone. In the Welsh Basin records better-known gracilis plus peltifer and the clingani biozones; of Ashgill graptolites below the persculptus Biozone are it is based on the Scottish succession and it has not been scanty. Thus detailed correlation of the graptolite zones with distinguished in the WelshBasin. Insteadthe multidens the late Ordovician limestones and the shelly stages of the Biozone has been used for the faunas overlying those of the Ashgill Series remains uncertain. In Wales the complanatus gracilis Biozone. In the Shelve InlierHughes (1989) Biozone has not been recognized. There are records of the discussed criteria for recognizing the base of the multidens anceps Biozone (Cave & Hains 1986), based mainly on Biozone,but these criteria have yet to betested, for occurrences of Orthograptusabbreviatus Elles & Wood; example in the Mydrim (or Dicranograptus) Shales of South Dicellograptus anceps itself is uncommon in Wales. In the Wales.Geological Survey officers (Strahan et al. 1914) northern part of the Llandovery area a doubtful record of subdivided this graptoliticsequence into the arctus, Climacograptussupernus Elles & Wood,referred to the Mesograptus, vulgatus and truncatus Beds, and Elles (1940, supernus Zone in Cocks et al. (1984, p. 149), presumably p. 445) said that she found it easy to map these divisions. represents the anceps Biozone. The persculptus Biozone was Current work is attempting to assess their correlative value defined in the Welsh basin and has proved easy to recognize in the light of more recent studies. there (Cave & Hains 1986), but ismuch more difficult to In North America the faunas above the gracilis Biozone recognize in Scotland. and below equivalents of the clingani Biozonehave been The base of the Silurian is defined to coincide with the variously assigned to a bicornis or multidens Zone (Finney base of the acuminatus Biozone at Dob’s Linn in Scotland 1986) but the applicability of the Americanzones to the (Cocks & Rickards 1988). The acuminatus Biozone is widely

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recognized in southernScotland, in Ireland, and in the Stratigraphy, 22, 119-142. WelshBasin. With regard to wider correlation this makes HARPER,J. C.1956. The Ordoviciansuccession near Llanystwmdwy, the acuminatus Biozonea practical base tothe Silurian Caernarvonshire. Liverpool & Manchester Geological Journal, 1, 385-393. System. HOWELLS, M.F., FRANCIS,E. H., LEVERIDGE, B.E. & EVANS,C. D. R. 1978. Classical Areas in British geology: Cape1 Curig and Betws-y-Coed. Thanks to many colleagues for discussion and comments on this Institute of Geological Sciences. HMSO. paper, whichis publishedwith the permission of the Director, HUGHES,R. A. 1989. Llandeilo and Caradoc graptolites of the Builth and British Geological Survey (NERC). Shelve inliers. Monographs of the PalaeontographicalSociety, No. 577. INGHAM,J. K. & WRIGHT,A. D. 1970. A revised classification of the Ashgill Series. Lethain, 3, 233-242. References JACKSON,D. E. 1962. Graptolite zones in the Skiddaw Group in Cumberland, ALLEN,P. M,, JACKSON, A. & A. RUSHTON, A. W. A. 1981. The stratigraphy England. Journal of Paleontology, 36, 300-313. of the Mawddach Group in the Cambrian succession of North Wales. MITCHELL,C. E. 1987.Evolution and phylogenetic classification of the Proceedings of the Yorkshire Geological Society, 43, 295-329. Diplograptacea. Palaeontology, M,353-405. BECKLY,A. J. 1988. The stratigraphy of the Arenig Series in the Aberdaron MOLYNEUX,S. G. & RUSHTON,A. W. A. 1988. The age of the Watch Hill to Sarn area, western Llyn, North Wales. Geological Journal, 23, Grits (Ordovician), EnglishLake District: structural andpalaeogeo- 321-337. graphical implications. Transactions of the Royal Soceity of Edinburgh: BERGSTROM,S. M. 1986. Biostratigraphic integration of Ordovician graptolite Earth Sciences, 79, 43-69. and conodont zone-a regional review. In: HUGHES,C. P. & RICKARDS, RICKAROS,R.B. 1976. 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Received 7 December 1989; revised typescript accepted 6 February 1990.

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