Geological Magazine

Original Article

Integrated Upper Ordovician graptolite–chitinozoan biostratigraphy of the Cardigan and Whitland areas, southwest Wales

THIJS R. A. VANDENBROUCKEa1 c1, MARK WILLIAMSa2, JAN A. ZALASIEWICZa2, JEREMY R. DAVIESa3 and RICHARD A. WATERSa4

a1 Research Unit Palaeontology, Ghent University, Krijgslaan 281/S8, 9000 Ghent, Belgium

a2 Department of Geology, University of Leicester, University Road, Leicester LE1 7RH, UK

a3 British Geological Survey, Kingsley Dunham Centre, Keyworth, Nottingham NG12 5GG, UK

a4 Department of Geology, National Museum of Wales, Cathays Park, Cardiff CF10 3NP, UK

Abstract

To help calibrate the emerging Upper Ordovician chitinozoan biozonation with the graptolite biozonation in the Anglo-Welsh, historical type basin, the graptolite-bearing Caradoc–Ashgill successions between Fishguard and Cardigan, and at Whitland, SW Wales, have been collected for chitinozoans. In the Cardigan district, finds of Armoricochitina reticulifera within strata referred to the clingani graptolite Biozone (morrisi Subzone), together with accessory species, indicate the Fungochitina spinifera chitinozoan Biozone, known from several Ordovician sections in northern England that span the base of the Ashgill Series. Tanuchitina ?bergstroemi, eponymous of the succeeding chitinozoan biozone, has tentatively been recovered from strata of Pleurograptus linearis graptolite Biozone age in the Cardigan area. The T. ?bergstroemi Biozone can also be correlated with the type Ashgill Series of northern England. Chitinozoans suggest that the widespread Welsh Basin anoxic–oxic transition at the base of the Nantmel Mudstones Formation in Wales, traditionally equated with the Caradoc–Ashgill boundary, is of Cautleyan (or younger Ashgill) age in the Cardigan area. In the broadly time-equivalent, graptolite-rich Whitland section, also in SW Wales, two Baltoscandian chitinozoan biozones and a subzone have been recognized (again using accessory species), namely the Spinachitina cervicornis Biozone?, the Fungochitina spinifera Biozone and the Armoricochitina reticulifera Subzone. The new chitinozoan data provide a more precise means of correlation between the Whitland and Cardigan successions and suggest that the Normalograptus proliferation interval of the Whitland section is at least partly attributable to the Dicellograptus morrisi Subzone of the Dicranograptus clingani Biozone, rather than equating with the overlying Pleurograptus linearis Biozone.

(Received January 26 2007)

(Accepted June 26 2007)

Keywords:

  • chitinozoans;
  • graptolites;
  • Ordovician;
  • biostratigraphy;
  • Wales

Correspondence:

c1 Author for correspondence: Thijs.vandenbroucke@UGent.be

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