restudy of the chitinozoans of the Ordovician/Silurian
gssp at dob’s
(uk) and correlation with other
sections on avalonia in northern
England and Belgium
Verniers1, Thijs Vandenbroucke1, Jan Vanmeirhaeghe1 and Man Yi-Mee1. 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).
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Received: February 15, 2003
Accepted: June 15, 2003