A restudy of the chitinozoans of the Ordovician/Silurian gssp at dob’s linn, scotland (uk) and correlation with other sections on avalonia in northern England and Belgium  

Jacques Verniers¹, Thijs Vandenbroucke¹, Jan Vanmeirhaeghe¹ and Man Yi-Mee^1.   With contributions from Euan Clarkson², Michael Melchin³ and S. Henry Williams⁴  

¹ Research Unit Palaeontology, Dept. Geology and Pedology, Ghent University, Krijgslaan 281 S8, B–9000 Gent, Belgium. E–mail: Jacques.Verniers@UGent.be

² Department of Geology and Geophysics, The University of Edinburgh, The King’s Buildings, West Mains Road, Edinburgh, EH9 3JG, Scotland, U.K. E–mail: Euan.Clarkson@glg.ed.ac.uk

³ Department of Geology, St. Francis Xavier University, P.O.Box 5000, Antigonish, Nova Scotia B2G 2W5, Canada. E–mail: Mmelchin@stfx.ca

Key words: Chitinozoans. Correlation. Ordovician. Silurian. Dob’s Linn.

The GSSP of the Ordovician/Silurian boundary at Dob’s Linn, Scotland (UK) was accepted in 1992 (see discussion in Holland and Bassett, 1989). For correlation of Ordovician, Silurian or Devonian strata chitinozoans have proved to be an effective (micro-)fossil group in addition to graptolites and conodonts (see Paris, et al., 1999; 2000; Verniers et al., 1995). A preliminary study of the Dob’s Linn section by Whelan (1988; see also Whelan and Burton, 1988) did not show a rich and well preserved assemblage. Hence chitinozoans have not been used to correlate from this GSSP to other localities world wide. However in other localities it was possible to calibrate chitinozoan biozonation with graptolite biozonation. The Silurian Spinachitina fragilis global biozone, defined by the first occurrence of the index fossil, appears at about the same level as the base of the A. ascensus graptolite biozone (Dufka, 1992; Dufka and Fatka, 1993; Verniers et al. 1995; Bourahrouh, 2002) which is defined as the base of the Silurian (see Holland and Bassett, 1989).

A new sampling for a chitinozoan study and a detailed logging of the Linn branch section at Dob’s Linn in the Southern Uplands of Scotland was undertaken in October 2002 in section 2, where the GGSP was defined. Graptolite biozonation as defined by Williams and Ingham (1989) is followed here. Seventeen samples were taken in the 4.23 m below the base of the Birkhill Formation: seven samples between that base and the highest dark grey shale bands, defined as anceps band E (a 1.97 m interval), ten samples in and around the five dark grey graptolite rich shale band called anceps bands A to E (another 2.25 m interval), twelve samples around the GSSP in a 2.10 m interval and four samples more spaced in a section 3.0 to 6.0 m above the base of the Birkhill Formation. For location of the samples and the detailed stratigraphical log of 10.5m the many thick to very fine bentonite beds were used but also the pyrite rich levels and some fine beds of siltstone.

In addition to sampling on the field, chitinozoans have also been dissolved from parts of slabs of the graptolite collection of H. Williams and restudied by M. Melchin; 13 samples from the persculptus biozone, eleven from the ascensus biozone, nine from the acuminatus biozone, and eight from the vesiculosus biozone.

The chitinozoans extracted from the new samples contain a moderately well preserved and diversified assemblage, allowing a biozonation, which can be accurately calibrated with the graptolite biozonation. The study shows a correlation with areas outside Laurentia: such as the ongoing chitinozoan studies in the Ashgill type section in northern England (see work by Vandenbroucke, this volume) and uppermost Ordovician, possibly glacial, sea level drop induced, conglomerate beds within a coarser grained formation, recently discovered in the Condroz Inlier in Belgium (Verniers et al., 2001; Herbosch et al., 2002; Vanmeirhaeghe and Verniers, 2002). Both areas are situated in Avalonia. Further correlation with northern Gondwana is presented with chitinozoans of the Ordovician to Silurian transition as recently described by Bourahrouh, from areas such as Algeria, Morocco, Czech Republic and France (Bourahrouh, 2002; Piçarra et al., 2002).

Bourahrouh, A. 2002. Chitinozoaires et palynomorphes de l’Ordovicien supérieur nord-gondwanien: impact de la glaciation ashgillienne. PhD thesis Rennes University I: 1-300.

Dufka, P. 1992. Lower Silurian Chitinozoans of the Prague Basin (Barrandian, Czechoslovakia) – Preliminary results. Revue de Micropaléontologie. 25(2): 117-126.

Dufka, P. and Fatka, O. 1993. Chitinozoans and acritarchs from the Ordovician-Silurian boundary of the Prague Basin, Czech Republic. Special Papers in Palaeontology. 48: 17-28.

Herbosch, A., Verniers, J., Debacker, T., Billiaert, B., De Schepper, S., Belmans, M. 2002. The Lower Palaeozoic stratigraphy and sedimentology of the Brabant Massif in the Dyle and Orneau valleys and of the Condroz Inlier at Fosses: an excursion guidebook. Geologica Belgica, 5 (3-4): 71–143.

Holland, C. H., Bassett, M. G., 1989 (eds). A global standard for the Silurian System. National Museum of Wales, Geological Series, 9, 325 p. Cardiff.

Paris, F., Verniers, J., Achab, A., Albani, R., Ancilletta, A., Asselin, E., Chen Xiaohong, Fatka, O., Grahn, Y., Molyneux, S.G., Nolvak, J., Samuelsson, J., Sennikov, N.V., Soufiane, A., Wang Xiaofeng, Winchester-Seeto, T. 1999. Correlation of Ordovician regional chitinozoan biozonations. Acta Universitatis Carolinae Geologica. 43 (1-2): 291-294.

Paris, F., Winchester-Seeto, T., Boumendjel, K. and Grahn, Y. 2000. Toward a global biozonation of Devonian chitinozoans. Courier Forschungs Institut Senckenberg. 220: 39-55.

Piçarra, J.M., Robardet, M., Bourahrouh, A., Paris, F., Pereira, Z., Le Menn, J., Gourvenec, R., Oliveira, T., Lardeux, H. 2002. Le passage Ordovicien-Silurien et la partie inférieure du Silurien (Sud-est du Massif armoricain, France). C.R. Geoscience, 334: 1177-1183.

Vanmeirhaeghe, J. and Verniers, J. 2002. Chitinozoans of the Fosses and Génicot Formations (western Condroz Inlier) and further proofs of the Hirnantian glaciation in Belgium. Abstracts of the International C.I.M.P. Symposium «Palaeozoic Palynology in the third Millenium», Lille 5-7 September 2002: 66.

Verniers, J., Nestor, V., Paris, F., Dufka, P., Sutherland, S., Van Grootel, G. (1995). A global Chitinozoa biozonation for the Silurian. Geological Magazine 132 (6), 651-666.

Verniers, J., Herbosch, A., Van Guestaine, M., Geukens, F., Delcambre, B., Pingot, J.L., Bellanger, I., Hennebert, M., Debacker, T., Sintubin, M. and De Vos, W. 2001. Cambrian-Ordovician-Silurian lithostratigraphical units (Belgium) in P. Bultynck and L. Dejonghe (Eds.) Lithostratigraphical Scale of Belgium. Geologica Belgica, 4 (1-2): 5-38.

Whelan, G.M., 1988. Preliminary Acritarch and Chitinozoan distributions across the Ordovician-Silurian boundary stratotype at Dob’s Linn, Scotland. Bulletin of the British Museum of Natural History (Geology), 43: 41-44.

Whelan, G.M. and Burton, C.J. 1988. Acritarchs and Chitinozoans from the Ordovician-Silurian boundary stratotype sediments. Lethaia, 21: 38.

Williams S.H. and Ingham J.K. 1989. The Ordovician-Silurian boundary stratotype section at Dob’s Linn, southern Scotland. in: Holland, C. H., Bassett, M. G., 1989 (eds). A global standard for the Silurian System. National Museum of Wales, Geological Series, 9, Cardiff: 27-35.

 

 

Received: February 15, 2003

Accepted: June 15, 2003