Ordovician Graptolite Biostratigraphy in Sion in Wales
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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 little prospect 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 tend to 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 In the 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 the Arenig 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 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/147/4/611/4890409/gsjgs.147.4.0611.pdf by guest on 24 September 2021 612 A. W. A. RUSHTON 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