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 pre- servation 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 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/147/4/615/4890507/gsjgs.147.4.0615.pdf by guest on 28 September 2021 616 S. G. MOLYNEUX 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