Restudying a global stratotype for the base of the Silurian: a progress report  

Michael J. Melchin¹  

¹Department of Earth Sciences, St. Francis Xavier University, Antigonish, Nova Scotia, B2G 2W5, Canada. E–mail: mmelchin@stfx.ca

Key words: Graptolites. Stratotype. Ordovician. Silurian. Dob’s Linn.

Since its ratification in 1985 the Global Stratotype Section and Point (GSSP) for the base of the Silurian has proven to be controversial. A widespread feeling that Dob’s Linn, southern Scotland, has been unsuitable as a GSSP for this level led to a call by the membership of the Subcommission on Silurian Stratigraphy in 2000, to formally re–examine this stratotype. I was asked to organize a new Ordovician–Silurian Boundary Working Group (OSBWG) to organize this restudy.

The current state of our understanding of stratigraphy and biostratigraphy of Dob’s Linn has been recently reviewed by Melchin (2001), including some new contributions to its biostratigraphy by Melchin and Williams (2000), based on new collections. These new collections have yielded a much more diverse graptolite fauna than has previously been recorded at Dob’s Linn. The newly discovered taxa include a number of biostratigraphically useful forms both below and above the GSSP. In addition, the fauna includes distinctive elements of both paleotropical and peri–Gondwanan regions, thus facilitating more precise global correlation between these regions. Also included in the collections immediately above the GSSP are specimens of A. ascensus that appear to be primitive, transitional forms, retaining features of the proximal end characteristic of their probable normalograpid ancestors (Melchin and Williams, 2000). This, together with the broader phylogenetic framework into which the akidograptines have recently been placed (Melchin, 1998), suggests a reasonable line of ancestry for these taxa, and that the Dob’s Linn succession captures their evolutionary origin. Thus, it appears that Dob’s Linn is not only adequate for providing the graptolite succession necessary for the precise definition of the O–S boundary, it may be the best section in the world for this purpose.

Dob’s Linn still possesses significant drawbacks as a global stratotype section. It is structurally complicated, relatively condensed (although not by comparison with many other graptolitic localities), and of limited extent in outcrop. Besides graptolites and palynomorphs, other fossil groups are very poorly represented.

The question before the new OSBWG is whether the stratigraphic community would be better served by retaining the current GSSP or replacing it with another. I believe that most workers would agree that stability is important and that, if a new GSSP is chosen, the stratigraphic level should at or close to the current biostratigraphic horizon. Two other regions that have been the principal contenders for the GSSP are Anticosti Island, Canada, and Wangjiawan, Hubei, China. New data have come to light in both of these areas that bear on the GSSP issue.

Correlations based on carbon isotope chemostratigraphy (Underwood et al., 1997; Brenchley et al., 2003) and biostratigraphic data (Melchin et al., 1991; Copper, 2001; Melchin, 2002) provide conflicting evidence regarding the stratigraphic continuity and correlation of the uppermost Ordovician succession on Anticosti Island (Melchin, Holmden and Williams, this volume). Correlations based on matching of C–isotope curves suggest that all or much of the upper Hirnantian is missing in the Anticosti succession. By contrast, brachiopod, conodont, and graptolite data suggest that the Hirnantian succession is essentially complete, although the possibility of a short hiatus at the systemic boundary cannot be ruled out. In addition, detailed lithological and biofacies studies (e.g., Zhang and Barnes, 2002) have shown no evidence of a significant break in the succession. If the succession is complete, then the timing of the positive C–isotope excursion on Anticosti Island is not synchronous with that seen in several other parts of the world. On the other hand, if the C–isotope excursions are synchronous between Anticosti Island and Baltica and Dob’s Linn, the faunal changes must be diachronous and the Anticosti sections incomplete. These questions must be resolved before any Anticosti localities could be reconsidered as potential GSSP candidates.

Chen and Rong (2002) proposed that the Wangjiawan section be considered as a possible GSSP by the new OSBWG. This section has been recently restudied in detail by Chen et al., (2000, in press). By comparison with Dob’s Linn, the section is much less structurally complex and thermally altered. However, the only fossils reported at and immediately above the boundary level are graptolites. Although shelly fossils occur in the underlying strata, they are not useful for defining the boundary level. Wangjiawan and Dob’s Linn have similar patterns of change in the carbon isotope chemostratigraphic record (Wang et al., 1997; Underwood et al., 1997). The patterns of graptolite faunal change through the boundary interval are also very similar to those seen at Dob’s Linn except that both the A. ascensus Biozone and the underlying N. persculptus Biozone are much more condensed at Wangjiawan. In addition, the primitive form of A. ascensus seen at Dob’s Linn has not been found at Wangjiawan, although it does appear to occur in at least one other section in South China (Fang et al., 1989). Therefore, although the Wangjiawan section contains a well–preserved graptolite succession and is structurally simple and relatively unaltered, it has thus far yielded no biostratigraphic or chemostratigraphic data that make it more useful from the point of view of international correlation than the current GSSP.

My proposal to the new OSBWG is that the current GSSP be maintained, despite the physical shortcomings of the locality. However, the biostratigraphic definition of the boundary must be changed to reflect our new understanding of the distribution of taxa. In particular, the GSSP is marked by the base of the A. ascensus Biozone, defined by the first appearance of A. ascensus and P. praematurus. Several other biostratigraphically useful taxa make their first appearance low in the A. ascensus Biozone, including Normalograptus lubricus, Akidograptus cuneatus, and Atavograptus ceryx (=A. primitivus).

A number of research areas still require a significant amount of work. Additional stratigraphically well–constrained radiometric dates are needed to more reliably calibrate the absolute age of the O–S boundary. There remain significant uncertainties related to the timing of the conodont and chitinozoan biozonal boundaries and the end of the C–isotope excursion relative to the graptolite biozonation. In addition, various possible sections and stratigraphic horizons should be tested for levels of uncertainty in the degree to which they can be regionally and globally correlated using constrained optimization (Sadler, 2001) or other quantitative methods. It may be possible to weigh the relative merits of possible GSSP candidates as measured by the precision with which they can be placed in a global composite succession.

Brenchley, P.J., Carden, G.A., Hints, L. Kaljo, D., Marshall, J.D., Martma, T., Meidla, T., and Nõlvak, J. 2003. High–resolution stable isotope stratigraphy of Upper Ordovician sequences: constraints on timing of bioevents and environmental changes associated with mass extinction and glaciation. Geological Society of America Bulletin, 115: 89–104.

Chen Xu and Rong Jia–yu 2002. A Proposal of the Candidate Section of the Base of the Silurian. Silurian Times, 10: 14–16.

Chen Xu, Rong Jia–yu, Mitchell, C.E., Harper, D.A.T., Fan Jun–xuan, Zhan Ren–bin, Zhang Yuan–dong, Li Rong–yu, and Wang Yi. 2000. Late Ordovician to earliest Silurian graptolite and brachiopod biozonation from the Yangtze region, South China with a global correlation. Geological Magazine, 137: 623–650.

Chen, Xu, Fan Jun–xuan, Melchin, M.J. and Mitchell, C.E. In press. Graptolites of the Hirnantian Substage (latest Ordovician) from the Upper Yangtze Region, China. Palaeontology.

Copper, P. 2001. Reefs during the multiple crises towards the Ordovician–Silurian boundary: Anticosti Island, eastern Canada, and worldwide. Canadian Journal of Earth Sciences, 38: 153–171.

Fang, Yiting, Liang Shijing, Zhang Daliang, and Yu Linlong 1989. Stratigraphy and Graptolite Fauna of Lishuwo Formation from Wuning, Jiangxi. Nanjing University Publishing House, 155 p. {In Chinese with English summary.]

Melchin, M.J. 1998. Morphology and phylogeny of some Early Silurian "diplograptid" genera from Cornwallis Island, Arctic Canada. Palaeontology, 41: 263–326.

Melchin, M.J. 2001. The GSSP for the Base of the Silurian System. Silurian Times, 9: 36–41.

Melchin, M.J. 2002. Restudy of some Ordovician–Silurian boundary graptolites from Anticosti Island: implications for the age and correlation of the Ellis Bay Formation. Canadian Paleontology Conference, Program and Abstracts No. 12, Ottawa, September, 2002, 31–33.

Melchin, M. J. and Williams, S. H. 2000. A restudy of the akidograptine graptolites from Dob’s Linn and a proposed redefined zonation of the Silurian Stratotype. Palaeontology Down Under 2000, Geological Society of Australia, Abstracts 61, 63.

Melchin, M.J., McCracken, A.D. and Oliff, F.J. 1991. The Ordovician–Silurian boundary on Cornwallis and Truro islands, Arctic Canada: preliminary data. Canadian Journal of Earth Sciences, 28, 1854–1862.

Sadler, P. M., 2001. Constrained optimization approaches to the stratigraphic correlation and seriation problems: a user’s guide and reference manual to the CONOP program family, copyright 1998–1999 Peter M. Sadler, University of California, Riverside, 142 p.

Underwood, C. J., Crowley, S. F., Marshall, J. D., and Brenchley, P. J. 1997. High–resolution carbon isotope stratigraphy of the basal Silurian Stratotype (Dob’s Linn, Scotland) and its global correlation. Journals of the Geological Society, London, 154, 709–718.

Wang Kun, Chatterton, B.D.E., and Wang Y. 1997: An organic carbon isotope record of Late Ordovician to Early Silurian marine sedimentary rocks, Yangtze Sea, South China: implications for CO2 changes during the Hirnantian glaciation. Palaeogeography, Palaeoclimatology, Palaeoecology, 132, 147–158.

Zhang, Shunxin and Barnes, C.R. 2002. Late Ordovician–Early Silurian (Ashgilllian–Llandovery) sea level curve derived from conodont community analysis, Anticosti Island, Québec. Palaeogeography, Palaeoclimatology, Palaeoecology, 180:5–32.

 

 

Received: February 15, 2003

Accepted: June 15, 2003