Advances and Problems in Ordovician (E.G.Molyneux 1979), Despite Inadequate Documentation Palynology of England and Wales of Their Biostratigraphy

Home , Caradoc Series, Coryphidium, Watch Hill (Cockermouth)

Journal of the Geological Sociery, London, Vol. 147, 1990, pp. 615-618, 2 figs. Printed in Northern Ireland

SHORT PAPER Acritarchs. Acritarchs have often been used todate otherwise unfossiliferous Ordovician successions in Britain Advances and problems in Ordovician (e.g.Molyneux 1979), despite inadequate documentation palynology of England and Wales of their biostratigraphy. Their affinities are uncertain, but most are thought to be marine planktonic algalcysts S. G.MOLYNEUX (Downie 1984). They occur widely in the Ordovician rocks British Geological Survey, Keyworth, Nottingham of Wales andnorthern England, and in the Ordovician NG12 5GG, UK subcrop of the Midlands and eastern England. Their preservation varies from good in the Welsh Borderland to poor in northern England and Wales. Acritarchsand chitinozoa are wed to date Ordovicinn rocks in Work by Rasul(1979 and references therein),Booth Englandand Wales, but thedata-base forboth groups is poor. (1979) and Turner (1984, 1985) has provided much of the Study of chitinozoa has been neglected in the UK, in contrast to available information. Recent work in Wales and the Lake North Americaand southwest Europe where biozonations have District, combined with mapping by the British Geological been established. Foddefinition of acritarch biozones may. soon Survey (BGS), has furnished additional data. So far, none be possible in the Tremadoc-Arenig, but acritarch biostratigraphy of thiswork has resulted in the publication of formally in the Lhvirn-Ashgill is insuKiciently documented. named biozones.

Besides other microfossils, Ordovician rocks yield acritarchs Advances: Tremadoc. Rasul (1979)analysed 52 samples and chitinozoa. Although these are valuable in correlation, from the Shineton Shales, establishingeight informal there have been relatively few systematic investigations of biozones. The wider occurence of these biozones has still to their biostratigraphy in the UK, so their potential is far from be demonstrated, but Rasul’s work makes it possible to date realized. Tremadoc rocks proved in boreholes on the Midlands Platform. Rasul (in Bulman & Rushton 1973,p. 8), for Chitinozoa. Chitinozoa are flask-shaped bodies, present example, recognizedassemblages in the Deanshanger inrocks of Ordovician to Devonian age. Their biological Borehole that closely resemble those from thethe lower part of the Shumardia pusilla beds in the Shineton Shales. affinities are unknown; some authors consider them to be Arenig. Arenig acritarch biostratigraphy has benefitted metazoan eggs or egg-cases,while otherstake the view from extensivesampling in the Lake District and Wales that they have protist or fungalaffinities (Paris 1981, pp. 78-81). Few studies (Rhodes 1961; Jenkins 1967; Atkin- (Fig. 1). Adistinctive assemblage, characterized by (i)a combination Tremadoc and post-Tremadoc taxa, and (ii) son & Moy1971; Molyneux 1987) have documented their of occurrence in the British Ordovician but this belies their certain diagnostic species (1-9 in Fig. l), occurs in the Bitter of theLake District widespread distribution. They commonly occur in samples Beck and WatchHill formations (Molyneux & Rushton 1988) and in the ‘Loginbeds’ of from North Wales (Atkinson & Moy1971, author’s un- South Wales (Molyneux & Dorning 1989). The age of this published data), northern England (Lister in Arthurton & assemblage is not known precisely, but it is probably older Wadge1981), and the subsurface of eastern England than the deflexus Biozone and younger than some part of (Rushton & Hughes 1981, p. 624). the sedgwickii Biozone (Molyneux & Rushton 1988, p. 52). Advances. Almost all therecent advances have occurred It occurs elsewhere in beds close to the Tremadoc-Arenig boundary, for example in the Barriga Formation of outside Britain. In Quebec and western Newfoundland, 22 southwest Spain (Mette 1989). biozones were defined by Achab (1989) in an incomplete Assemblages from the deflexus Biozone of the Lake late Canadian-Cincinnatian (Arenig-Ashgill) sequence. Over the same interval in southwest Europe, Paris (1981) District and the Moridunian Stage of South Wales are less diverse(Fig. l), but contain potentially diagnostictaxa. defined a different sequence of 18 biozones, also in an incomplete succession. It may yet prove possible to apply Notable appearances are those of Coryphidium (17, Fig. 1) and Micrhystridium aff. acuminosum (18) (Molyneux 1987; one or other of these biozonations to at least part of the Molyneux & Rushton 1988; author’s unpublished data). British Ordovician. A late Arenig assemblage, characterized by taxa 20-30 (Fig. l), is present in the Lake District and Wales. Certain Problems. Detailed taxonomic studies of British material, elements (26, 28, 30) appear in beds of probable early or not yet undertaken, are an essential prerequisite tothe middle Arenig age in Wales, but the assemblageis fully establishment of an Anglo-Welsh chitinozoan biozonation. represented in the gibberulus Biozone of the Menai Straits Application of eitherthe Quebec-Newfoundland or Inlier and the hirundo Biozone of the Afon Seiont section at southwest European scheme to England and Wales is Caernarfon. In the Lake District, the assemblage occurs in complicated by provincialism (Achab 1988). Siphonochitina, beds of probable hirundo Biozone age in the Black Combe for example, occurs in Llanvirn faunasfrom Shropshire, Inlier (Rushton & Molyneux 1989), and in the Kirkstile and northern England and southwest Europe, but not in faunas Buttermere (sensu Cooper & Molyneux1990, fig. 5) fromBaltica orLaurentia; in contrast, Hercochitina is formations of the Skiddaw Inlier. The latter occurrences are present in the Caradoc type section and in late Ordovician not correlated preciselywith the Lake District graptolite rocks of Baltica and Laurentia, butis absent from southwest biozones, but almost certainly include beds of gibberulus Europeand north Africa. The increasing importance of Biozone age. Baltic andLaurentian chitinozoa through the Ordovician No acritarch biozones are defined in the Arenig. In means thatneither of the available biozonations can be South Wales,a succession of eight acritarch assemblages, applied wholly to Anglo-Welsh successions. which may form the basis of assemblage biozones, has been 615

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LAKEDISTRICT NORTHWALES SOUTHWALES (NORTHERN BELT) (CAERNARFON-BANGOR) CARMARTHEN-WHITLAND)

W W 12,15?,22,29?,30,33, 36,37. - -LNF 12,15,20,21,22,23,24, 4 12,15?,17,20,23,28,29?, n 26,27?,28,30,31.* NFF 30,31,34,35. I m. _____--_------PF 20?,25,30,32?,35, z 38,39. -z 12,15,20,21?,22,23, D ? 24,25?,26,27,28, 12,15?,17,20,23,26,28, 20?,22?,28?,29, 30,35. Z 29,30. 29,30,34. ______--__--* -CBF COF B4 12,17,26,28,30,32,33. AFF AFF:AFON FFINNANT FM., BBF: BITTER - BECKFM., CAF: CARMARTHEN FM., CAF 4 12,13,17?,30. CBF: CWMFELINBOETH FM., 4 g?, 17,18. OHF COF:COLOMENDY FM., EVG: EYCOTT VOLCANICGP, HBF: HOPE BECKFM.. LB KF:KIRKSTILE FM., LB: LOGIN BEDS, LNF:LLANFALLTEG FM., LWF: LOWESWATEI ? FM.,MGM: MAES Y GEIRCHENSST. MER, W NFF:NANT FFRANCON FM., OHF: OGOF HEN &INCLUDES DATA FROM BEDS FM., PF: PONTYFENNIFM., WHF: WATCH TOTHE SOUTH OFTHECAUSEY HILLFM. PIKE FAULT).

Fig. 1. The occurrence of selected acritarch species in Arenig sequences of theLake District and Wales. Lithostratigraphy.Lake District: BGS (cf. Cooper & Molyneux 1990); North Wales: Beckly1987, 1989; South Wales: Fortey & Owens 1987. Acritarch data. Lake District: Molyneux & Rushton 1988, Rushton & Molyneux 1989, author's unpublished data; North Wales:Booth 1979; South Wales: Molyneux1987, Molyneux & Doming 1989. Key to acritarch taxa: 1, Acanthodiacrodium?dilatum; 2, Caldariola glabra; 3, Cymatiogalea deunfii;4, C. messaoudi; 5, Stellechinatum sicaforme; 6, Stelliferidiurn trifidum; 7, Striatotheca prolixa; 8, Tetraniveum arenigum SI.; 9, Vogtlandia coalita;10, Cristallinium aff. cambriense; 11, C. velifera; 12, Micrhystridium spp.; 13, Striatotheca spp.; 14, Timofeevia phosphoritica; 15, Veryhachium lairdii; 16, Vulcanisphaera spp.; 17, Coryphidium, spp.; 18, Micrhystridium aff. acuminosum; 19, Polygonium?sp. A of Molyneux & Rushton 1988; 20, Coryphidium aff. bohemicum sensu Rushton & Molyneux 1989; 21, Frankea breviuscula; 22, F. hamata; 23, F. sartbernardensb; 24, Marrocanium simplex; 25, Stellechinatum papulessum; 26, Striatotheca principalis parva; 27, S. principalis principalis; 28, S. rarirrugulata; 29, Uncinisphaera?sp. A of Molyneux 1987; 30, Veryhachium trispinosum SS.; 31, Micrhystridium sp. A of Rushton & Molyneux 1989; 32, Stelliferidium fimbrium; 33, S. striatulum; 34, Peteinosphaeridium trifurcatum; 35, Stellechinatum uncinatum; 36, S. celestum; 37, Striatotheca quieta; 38, Orthosphaeridium ternatum;39, Dasydorus cirritus?. Numbers in italics signify the first appearance of species in each section.

recognized (Molyneux 1987). However,stratigraphical England,but they are inadequately documented, with coverage has to be improved to ascertain the range of each neither descriptions nor illustrations. Lister & Holliday assemblage and determineto whether additional (1970) described and figured acritarchsfrom the nearby assemblages are present. The Skiddaw Group of the Lake Teesdale inlier,from an assemblage for which they District may also yield an acritarch biostratigraphy. suggested a possible early Llanvirn age. Booth (1979) TheAfon Seiont section indicates some of the recorded acritarchs in nine samples from widely separate differences between Arenig and Llanvirn assemblages, with areas, including the Afon Seiont section, the Shelve Inlier theappearance of Stellechinatum celestum (36) and and the Lake District. Though providing some insight into Striatotheca quieta (37) in the artus Biozone (Fig. 1). Other the composition of Llanvirn assemblages, stratigraphical taxa recorded from rocks of Llanvirn but not Arenig age coverage is insufficient for the definition of biozones. include Arkonia tenuata, A. virgata, Dicrodiacrodium spp., Llandeilo. Coverage of the Llandeilo is equally poor. Frankea hamulata, F. longiuscula and Striatotheca frequens The only good data are from nine samples from the type (Booth 1979; Downie 1984).Precisely wherethese taxa area(Turner 1985).Many of therecorded species range appear in relation to the base of the Llanvirn is not known through all or most of the succession; 33% of species are because critical sections, such as that at Llanfallteg in South recorded from either overlying or underlying series, and a Wales (Fortey & Owens 1987), have not been sampled. further 27% from both. Some taxa, e.g. Leiofusa sp. A of Llanvirn. Data from the Llanvirn arepoor. Lister (in Turner (1985), Stellechinatum llandeilium and Veryhachium Arthurton & Wadge 1981) recordedacritarchs from the sp. A of Turner (1985), are of potential biostratigraphical Kirkland Formation of the Cross Fell Inlier,northern value although their relatively short ranges need to be

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verified. The apparently short ranges of other taxa may be 54“N. misleading; Barakellafortunata occurs only in the Lower Llandeilo ‘Lloydolithus lloydiiFlags’, but as itis represented IRISH SEA by only one specimen, its stratigraphical significanceis doubtful. No acritarch biozones are defined. Caradoc. Acritarch occurrences in theCaradoc are documented more fully. Turner (1984) recorded acritarchs in 21 samples from the Caradoc type section. Their ranges (Turner 1984, fig. 6) indicatea possible biostratigraphy, although no acritarch biozones are defined. Ashgill. Few advances have been made in the study of Anglo-WelshAshgill acritarchs. Unlike theother Ordovi- 0 cian series, no PhD thesis has been undertaken. However, information from reconnaissance studies in NorthWales indicates a biostratigraphical potentialthat is worth developing (S. G. Molyneux & H. F. Barron, unpublished data). Facies control. The distribution of acritarchs in relation o Dicrodiacrodium to sedimentary environments has been documented by Gray Ordovicianoutcrop 0 & Boucot (1972), Jacobson (1979) and Dorning (1981). The general trends are for abundance and taxonomic diversity Fig. 2. Occurrence of selected acritarch genera in post-Llanvim to increase fromnearshore environments to shelf envir- rocks. 1, Caradoc type section (Turner 1982); 2, Castle Hill, onments,and then todecrease in slope and basinal Montgomery (Cave et al. 1988): early Caradoc trilobites; 3, environments. Llandegai (Trythall et al. 1987): possible teretiusculus or gracilis The relationship of acritarchsto facies in the Biozone graptolites;4, Pen y Gaer (Trythall et al. 1987): possible Anglo-Welsh Ordovician is poorly documented,and the gracilis Biozone graptolites;5, Brickworks Quarry Shale Mmbr, Knockerk Fm., Slane (Smith 1981): early Caradoc trilobites and effects of metamorphism, whichmay influence acritarch graptolites. recovery, must also be considered. A possible example of facies control occurs in the Dolwyddelan syncline of North Wales. Black graptolitic slates in the core of the syncline have failed to yield acritarchs, in contrast to their common Caradoc type section (Turner 1982), but were thought to be occurrence lower in the sequence, in the marine mudstones, recycled from rocks of Arenig orLlanvirn age. However, all siltstones and sandstones of the Llewelyn Volcanic Group or some of these genera have been recorded from Caradoc and Nant Ffrancon Formation (author’s unpublished data). rocks of easternIreland and Montgomery, and from It is also probable that the variable recovery of acritarch possible teretiusculus or gracilis Biozone rocks of North assemblages from the Skiddaw Group of the Lake District Wales (Fig. 2). Should all these occurrences be attributed to (author’s data) may be explained in part by facies control. recycling, or should the ranges of these genera be extended to embrace part or all of the Caradoc? Problems. Some of the problems have been alluded to in Recycling. Acritarchs areprone to recycling, but this the preceding discussion. For most Ordovician series, data may not be readily detectable unless it can be shown that are available for few sections, and stratigraphicalcoverage is theyoccur in recognizable sedimentary clasts (cf. incomplete. Ranges of taxa have not been tested, and only Richardson & Rasul 1978, p. 37). However, Turner (1982, the Tremadoc has a well defined biozonation. The problems pp. 137-140) has suggested that acritarchs can be recycled are particularly acute in the Llanvirn, Llandeilo and Ashgill. asindividual grains following erosion fromtheir original Inadequate documentation of acritarch occurrences. Lack host sediment. Insuch instances, reworked and in situ forms of data from the Llanvirn and Llandeilo creates difficulties wouldoccur together in the sedimentary matrix, and in dating middle Ordovician samples. It also undermines the although preservational differences can sometimes indicate reliability of Caradoc datesbased on acritarchs, because it is the presence of recycled specimens (Richardson & Rasul possible that apparently diagnostic species range down into 1978), this may not always be the case. Llandeilo orolder rocks. This point is illustrated by A further problem raised by recycling is the recognition attemptsto date the Borrowdale Volcanic Group in the of possible Lazarus taxa among acritarchs. Ten of the Lake District (Molyneux 1988). Veryhachium irroratum was species recorded by Turner (1982) from the Caradoc type recorded by Turner (1984) from the Caradoc but not from section were identified as ‘Tremadoc forms’ and a furthersix the Llandeilo (Turner 1985); nor is it known from Llanvirn as ‘probable Tremadoc forms’, though their preservation is or older strata in Fnglandand Wales (Booth 1979). Its asgood as that of the in situ Caradoc forms. Their presence in theBorrowdale Volcanic Grouptherefore reappearance so far above the Tremadocposes the question: seemed to be a good indication of Caradoc or younger age. are they recycled, or are they Lazarus taxa recurring after a However, evidence fromBohemia and Quebec, cited by long absence from the fossil record? Given that a major, Molyneux (1988), suggests that the species may appear there widespread and well-documented change in the composition in pre-Caradoc strata, so a Llandeilo age cannot be of acritarch floras occurs between the Tremadoc andArenig discounted. Series (Martin 1982), andthat many Tremadoc acritarchs Theuncertainties arising frominadequate docu- are not known from post-Tremadoc strata,it is more mentation in the middle Ordovician are furtherexem- probable that their reappearanceis due to recycling. If these plified by the genera in Fig. 2. Each of these occurs in the problems are to be resolved, the acritarch data-base will

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have to be improvedthrough systematic anddetailed smallOrdovician inlier in Teesdale (County Durham), England. investigations of relevant sections. Proceedings of the Yorkshire Geological Society,37, 449-460. MARTIN, F. 1982.Some aspects of late Cambrianand early Ordovician acritarchs. In: BASSETT,M. G. & DEAN, W. T. (eds) The This paperis published by permission of theDirector, British Cambrian-Ordouician boundary: sections,fossil dktribuions,and Geological Survey, (NERC). correlations. National Museum of Wales,Geological Series No. 3, Cardiff, 29-40. METE, W.1989. Acritarchs fromLower Palaeozoic rocks of the western Sierra Morena, SW-Spain and biostratigraphic results.Geologica et References Palaeontologica, 23, 1-19. MOLYNEUX,S. G. 1979. New evidence for the age of the Manx Group, Isle of ACHAB,A. 1988. Mise en evidence d'un provincialisme chez les chitinozoaires Man. In: HAWS, A. L., HOLLAND,C. H. & LEAKE,B. E. (eds) The ordoviciens. Canadian Journal of Earth Sciences, 25,635-638. 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Received 7 December 1989; revised typescript accepted 6 March 1990.

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