criteria required of a stratotype shows that few of these are possessed by the Dob's Linn section. The ... St. John's, Newfoundland, Canada AlB3XS; William B. N. Berry, Department of Paleontology, .... of approximately 25Ma (Harland et ai.
The Ordovician-Silurian boundary stratotype: consequences of its approval by the IUGS PIERRE J. LESPfiRANCE, CHRISTOPHER R. BARNES, WILLIAM B. N. BERRY, ARTHUR J. BOUCOT AND MU EN-ZHI
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Lesperance, Pierre J., Barnes, Christopher R., Berry, William B. N., Boucot, Arthur J. & Mu En-zhi 1987 07 15: The Ordovician-Silurian boundary stratotype: consequences of its approval by the IUGS. Lethaia, Vol. 20, pp. 217-222. Oslo. ISSN 00241164. The Ordovician-Silurian stratotype at Dob’s Linn, Scotland is the second systemic boundary approved by the International Union of Geological Sciences (IUGS). A review of the internationally accepted criteria required of a stratotype shows that few of these are possessed by the Dob’s Linn section. The strongest attribute of the section is the presence of zonal graptolites, although the boundary is recognized primarily on a single biological event (base of acwninatw Zone) for which the evolutionary relationships of the taxa are not established. In approving this boundary proposal in 1985, which received only simple majority support within the Ordovician-Silurian Boundary Working Group (OSBWG) and which fails to meet most of the accepted prerequisites for a stratotype, the International Commission on Stratigraphy (ICS) and the IUGS have established an unfortunate precedent. It foUows that future systemic boundaries need not meet the accepted standards. It raises serious questions on the assessment and voting procedures of the International Commission on Stratigraphy and on the credence accorded the recoaunendations developed by the International Subcommission on Stratigraphic Classification of IUGS. 0 OrdovicianSilurian boundary, systemic boundary criteria, Dob’s Linn, Scotland, biologic events. Pierre 1. Lespkrance, Dkpartement de gkologie, Universitk de Montrkal, C.P. 6128, Montrkal, Canada H3C 3J7; Christopher R. Barnes, Department of Earth Sciences, Memorial University of Newfoundland, St. John’s, Newfoundland, Canada AlB3XS; WilliamB. N. Berry, Departmentof Paleontology, Universily of California at Berkeley, Berkeley, California, U.S.A., 94nO; Arthur J. Boucot, Deparrmcnt of Geology, Oregon State University, Corvallis, Oregon, U.S.A. 97331; Mu En-zhi, Nanjing Institute of Geology and Palaeontology, Academia Sinica, Chi-Ming-Ssu,Nanjing, People’s Republic of china; 2Sth August, 1986.
The recent approval of the Ordovician-Silurian first two authors. Additional co-authors listed boundary by the International Union of Geo- thereby approve in principle the ideas herein logical Sciences (IUGS) (Bassett 1985) is an presented. important step towards a Standard Global ChronostratigraphicScale as it is the second systemic boundary approved, after the Silurian- Lessons from the SilurianDevonian boundary in 1972. Devonian boundary The purpose of this contribution is to examine critically the criteria which were used for the McLaren (1977:25) stated that the Silurianchoice of the Ordovician-Silurian boundary and Devonian committee (established to study the to draw the necessary conclusions on the con- Silurian-Devonian boundary and the Lowersequences for international chronostratigraphy. Middle Devonian boundary) did not reach comIt will be shown that the criteria which were plete agreement on the principles involved in considered for the Silurian-Devonian boundary, systemic boundary selection. Nonetheless, this and even more extensive criteria suggested by the committee did propose a model for boundary Subcommission on StratigraphicClassification of definition (McLaren 1972:271), discussed it the IUGS Commission on Stratigraphy (ICS), (McLaren 1977:16) and chose between the four were largely ignored. Uncertainty is created when contending areas for the Silurian-Devonian guidelines or criteria are consciously disregarded boundary through a procedure in which: by the Commission and Subcommission auth- Each of these was reviewed in terms of certain criteria that the orized to uphold and apply them. Committee had already held to be important: faunal and floral The text of this contribution was written by the development, stratigraphic considerations [marine, continuous,
218 Pierre J. Lesperance and others
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etc.], structural considerations, facies and biodiversity, geographical accessibility, possibility of preservation of sections (McLaren 1977:1&17).
The Ordovician-Silurian boundary stratotype: selection procedures
It is interesting to note that the Klonk section was chosen instead of KarlStejn, within the Barrandian of Czechoslovakia, because of limited outcrop and tectonic disruption of the latter section, despite extensive collecting at KarlStejn which had yielded richer faunas (McLaren 1977:19). Nonetheless, KarlStejn was designated an auxiliary type section.
The OrdovicianSilurian Boundary Working Group (OSBWG) was established in 1976 and over the next eight years compiled over 50 reports on important boundary sections around the world. In 1979, it visited some sections in the United Kingdom, including the Dob’s Linn section near Moffat, Scotland (Bassett et al. 1979; Williams 1983). This meeting was held jointly with the Subcommission on Silurian Stratigraphy and a majority of OSBWG voting members attended. It was agreed at that meeting also to visit Anticosti Island, QuCbec. This excursion was held in 1981 with appropriate documentation (LespCrance 1981; McCracken & Barnes 1981). Regrettably, only a minority (about one-third) of voting members attended. Other scientific field meetings with conferences in the Carnic Alps, Austria (Schonlaub 1980) in 1980 and in central China (Mu er al. 1984) in 1983 were not deemed to be official OSBWG meetings and only a minority of voting members attended. The only major scientific forum in which to debate the boundary stratotype question prior to submission to the Commission on Stratigraphy was that of the Fourth Ordovician System Symposium held at Sundvollen, near Oslo, Norway in 1982. Scientific papers were presented and long business meetings allowed debate. Straw votes, among those present, of both voting, corresponding and of non-voting members of OSBWG and other specialists, favoured Anticosti Island over Dob’s Linn by nearly 3 :1. Again, a minority of the voting members on the OSBWG attended these meetings. Following submission of a few more reports and in an attempt to reach a decision for the meeting of the Commission on Stratigraphy at the International Geological Congress in Moscow in 1984, a final mail vote was taken within the OSBWG. The vote for Dob’s . Linn over Anticosti was bare majority (12 for, 5 against, with 2 abstentions), as was a second vote to select the acuminatus Zone over the persculptus Zone as the faunal level for the boundary (10 for, 5 against, with 4 abstentions) (Cocks 1985). At the Moscow meeting of the Commission on Stratigraphy it was argued by some of the present authors and others that the Dob’s Linn section did not meet the criteria established for boundary stratotypes (the same arguments were presented at the IUGS council meetings). This extended
The International Stratigraphic Guide Principles, problems, and procedures with respect to systemic boundaries have long been a concern of the IUGS Subcommission on Stratigraphic Classification. The International Stratigraphic Guide (Hedberg 1976) refers specifically to the Silurian-Devonian boundary decision of 1972 and stated that: It is to be hoped that global standard boundary-stratotypes between other systems and their principal subdivisions will be established in a similar manner in the near future (Hedberg 197657).
Among the principal points recommended for the definition of systems (or other units) of the Standard Global Chronostratigraphic Scale, the following two, of the ten enumerated, are particularly pertinent to the present discussion: 5. Selection of specific sections for study and consideration based on probable continuity of sedimentation through the critical boundary interval; completenessof exposure; adequate thickness of sediments; abundance and variety of well-preserved fossils; favorable facies for development of widespread, reliable, and time-significant correlation horizons; close ties to other facies; freedom from structural complication, metamorphism, or other alteration; freedom from unconformities; amenability to isotopic age determination; historical appropriateness; and accessibility.
. . , . . , . _ _ . . . . . . . . . . 10. Marking of the boundary-stratotype in the field and establishment of arrangements for its preservation and accessibility to authorized study (Hedberg 1976:80-81).
Clearly, the ‘Guide’ is even more explicit than the criteria listed by McLaren (1977). The qualities that a boundary stratotype section must have are self-evident from the above, and need not be elaborated any further here. Our point is that most of these are lacking in the Dob’s Linn section.
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debate was barely reported in the Minutes (i.e. for absent voting members of the ICS, and for the IUGS council). A later mail ballot within the Commission resulted in the formal approval of Dob’s Linn as the boundary stratotype (with 15 in favour, 4 against, and 6 abstentions: Bassett 1985). Curiously, the Commission has subsequently stated (Cowie 1986:81) that a 60% majority vote (‘genuine declared abstentions’ excluded from the calculation) should normally be obtained within a Boundary Working Group prior to formal h a 1 ratification. On this basis, the acuminatus Zone won by 66% of the votes within the OSBWG. When the Commission’s procedures were established it was considered that the normal process of careful committee selection and work would result in a series of truly final decisions in establishing a Global Chronostratigraphical Scale. It is becoming clear that difficult, biased, or hasty decisions may be possible within the current process and that decisions by the Commission can be subject to scientific reexamination after a period of time (e.g. eight years is the minimum time stated in Cowie 1986:82). We argue that the decision taken in selecting Dob’s Linn as the Ordovician-Silurian boundary stratotype was inappropriate and judge that it will require reconsideration at a future stage. We recognize that continental glaciation in North Africa during the Late Ordovician and earliest Silurian produced widespread stratigraphic breaks, climatic effects, and that this glaciation broadly correlates with faunal extinctions. It is not our purpose here to discuss these faunal extinctions, but nonetheless they now appear to be significantly older than the base of the Silurian, with perhaps many of them concentrated near the base of the Hirnantian Stage (Latest Ordovician). As a result, it proved difficult for the OSBWG to find many sections in the world that met the prerequisites of a stratotype or that even could be shown to be continuous across the boundary. It is also a matter of public record that some of us preferred Anticosti for the boundary stratotype, and that some of us considered that a decision on the boundary stratotype was premature. It is not our intent to renew here the debate over various potential stratotypes (a subject best left aside in this contribution; Berry 1987 expands on a number of pertinent considerations), but to protest the disregard of stratigraphic principles and to warn of the consequences. We specifically
Ordovician-Silurian boundary stratotype 219
note below the many inadequacies of the Dob’s Linn section and its failure to meet international standards for a boundary stratotype.
Dob’s Linn, Scotland Dob’s Linn is in the central part of the Southern Uplands of Scotland, within a belt of distinct deepwater stratigraphic sequences consisting of basalt and chert and/or graptolitic shale, below thick greywackes. The fine-grained siliciclastic strata are considered pelagic (Leggett 1978). The overall tectonics of the Southern Uplands has been compared to an accretionary prism, and at least ten fault-bounded sequences have been recognized (Leggett, McKerrow & Eales 1979); if this is the case, the Southern Uplands are the result of subduction on the western margin of the Iapetus Ocean. On the other hand, Holland (1986) has stated that the accretionary prism model is not applicable to the Southern Uplands. Clearly, the chosen boundary stratotype lies within a structurally complex terrane whose exact nature is still uncertain. Dob’s Linn is the type locality for the Moffat Shales, divided into the Glenkiln (6 m; Caradoc), Hartfell (48 m; Caradoc, Ashgill), and Birkhill Shales (43 m; predominantly Llandovery) (Cocks 1985). This 97 m thick condensed sequence, ranging from the Nemagraptus gracilis Zone to the Rastrites maximus Zone, represents a time span of approximately 25Ma (Harland et ai. 1982; similar time spans are found in other current time scales). It is therefore a highly condensed sequence. The Moffat Shales are highly folded and faulted at Dob’s Linn (Leggett, McKerrow & Eales 1979:763;Bassett et al. 1979:4246; map as Fig. 2 in Cocks 1985). The acuminatus boundary can be recognized in several separate faultbounded blocks but is of limited lateral extent and demands careful structural-stratigraphic study at each section. The base of the Silurian System is defined at a point 1.6m above the base of the Birkhill Shale, in a narrow trench in the cliff on the north side of the Linn Branch stream at Dob’s Linn (Bassett 1985;Cocks 1985; for current physical state, see Berry 1987). The section is therefore structurally complex with limited lateral extent at the boundary level. The section has been recently restudied (Williams 1983; Williams & Rickards 1984), including a description of the lithologies. It is essentially a
220 Pierre J . Lesptrance and others monofacial sequence involving barren grey mudstones, black shales that are commonly rich in graptolites, and some metabentonite bands found mainly in the anceps Zone. The intervening barren beds do not allow proof of continuous sedimentation and may suggest periods of deep sea erosion by oxygenated bottom currents as partly documented by Williams & Rickards (1984). The shales at Dob’s Linn are famous for their graptolite faunas, initially studied by Charles Lapworth. These graptolites have been reexamined recently (Williams 1983). Graptolite faunas in the uppermost Hartfell Shales are poor, and in fact it is only recently that new finds have been recorded (Williams 1983). Most of the graptolite zones in the section cannot be precisely defined because the thin graptolitic shales are separated by thicker intervals of barren mudstones (e.g. in the anceps, extraordinarius and persculptus zones; the extraordinarius Zone, for example, is only represented by a l c m thick bed, now deeply excavated). Williams (1983) placed zonal boundaries at undefined levels somewhere within the intervening barren strata. Early preference by many specialists was to use the persculptus Zone as the boundary level. This level cannot be defined at Dob’s Linn. Consequently, the only zone that could be precisely defined within a probably continuous, but condensed, graptolitic sequence was the acurninatus Zone, lying 1.6 m above the base of the Birkhill Shale: The boundary therefore coincideswith the appearance of Parakidograptus acurninatus (Nicholson 1867) and Akidograptus ascensus (Davies 1929), which are rather widely distributed taxa in the graptolitic facies and are easily recognized. Some questions remain about their familial assignment, and their ancestordescendent relationships are unknown. There is some difficulty in using their fist appearances in such condensed sequences. In China, P. acurninatus and A . ascensus do not appear at the same level (the latter is older) and, furthermore, the pre-acurninatus strata (persculptus Zone) bear a monograptid fauna (Muet al. 1984). One is thus led to different zonal concepts of the persculptus and the acurninatus zones between these two areas, with consequent uncertainty in correlation between Dob’s Linn and China, mainly due to the condensed nature of strata at Dob’s Linn. Fossils other than graptolites are extremely rare at Dob’s Linn. A specialized dalmanitacean trilobite, Dalrnanitina (Songxites) sp. (nov.), occurs below the base of the Silurian (10 cm below the
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extraordinarius Zone) ,but the species is restricted to Dob’s Linn; the subgenus, however, appears to be restricted to the Himantian (Lesp6rance in press). In scanning graptolitic shale surfaces, S. H. Williams discovered rare, poorly preserved conodonts at a few levels. Further collecting and processing have provided some additional material. A low diversity fauna includes zonal species of the conodont Arnorphognathus in the Hartfell Shales, but nothing diagnostic from Silurian strata (Barnes & Williams in press). The conodont ‘color alteration index’ (CAI) of these specimens agrees with other CAI data from the Southern Uplands of Scotland as reported by Bergstrom (1980, Figs. 5, 8). These values lie in the range of CAI 5-7 indicating temperatures over 300°C. Hence, the strata at Dob’s Linn are thermally complex, limiting their value in studies of palynology (e.g. acritarchs, chitinozoans), magnetostratigraphy, chemostratigraphy, and some aspects of geochronology. As an example of this limited value, initial magneto-stratigraphic signatures are reset when CAI values exceed 24 to 3. Thus Dob’s Linn is a monofacial and monofaunal sequence, condensed, metamorphosed to over 30O0C,and structurally highly disturbed. It is easily accessible, but its limited extent in outcrop leaves us perplexed as to how much collectingthis locality can sustain. Strictly enforced limits to collecting will presumably have to be enacted, and collecting trips by pre-university students (as witnessed by some of us) banned. The lower boundary of the Silurian, as defined at Dob’s Linn, can be correlated elsewhere only by a single biologic event, the appearance of one (or two) graptolite species (Williams 1983). This is in sharp contrast to the Silurian-Devonian boundary where many fossil groups can define (although more approximately), and allow correlation of, the chosen boundary. The ordered succession of appearances of Monograptus uniforrnis, Icriodus woschrnidti, and Warburgella rugulosa within and outside the Barrandian has reinforced the correlatable nature of the chosen boundary. No such confirmation exists for the Ordovician-Silurian boundary. Lesptrance (1985) has attempted to correlate the acuminatus boundary with carbonate platformal sequences; he noted that the appearance of the trilobite Acernaspis apparently coincides with an acurninatus boundary, but this has not as yet been significantly tested. The boundary at Dob’s Linn
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Ordovician-Silurian boundary stratotype 221
is only useful for strictly graptolitic, typically simple majority vote has priority for the ICS. We deep-water facies. suspect that a long-standing policy, reaffirmed in 1983, that all bodies within ICS were encouraged by the Chairman to speed
Conclusions We have tried to document the inadequate nature of the Dob’s Linn section for a boundary stratotype. It does not meet international standards in that it is a condensed monofacial sequence, structurally and thermally complex, yielding a low-diversity fauna consisting almost exclusively of graptolites. The limited lateral extent and the condensed nature of the sequence raise concern whether the section can sustain the increased long-term collecting as a stratotype, particularly since alternative reference sections are not conveniently available. The positive aspects of the section are limited only to having a boundary definable on graptolites, in being accessible, and in having historical appropriateness. The sequence is probably continuous at the boundary level, but barren intervals are common in the immediately underlying strata (Hartfell Shales), rendering the definition of graptolite zonal boundaries in that part of the section very subjective. We do not regard this balance of negative and positive attributes in terms of the internationally acceptable criteria as adequate for accepting Dob’s Linn as the boundary stratotype. The Ordovician-Silurian Boundary Working Group voted marginally in favour of the chosen boundary, while the International Commission on Stratigraphy voted but slightly more in favour (see above). Certainly we can conclude that a systemic boundary to be approved need only obtain from the working group a bare majority vote. We feel that previously suggested criteria for the selection of boundary stratotypes have been largely ignored, if not totally discarded, in the recent choice of the OrdovicianSilurian boundary. If this is the case, one wonders how useful the Subcommissionon StratigraphicClassification is to the ICS, or for that matter, how pertinent its recommendations are to the Commission’s decisions. In fact, the Hedberg (1976) ‘Guide’ ‘. . . was never adopted by ICS as a statutory policy document’ (Cowie 1986:78). Furthermore, if suggested criteria have indeed been de fact0 discarded, we feel that this gives precedence for many abuses, not to mention ad hoc criteria. Apparently, a working group’s
up their work on establishingstandards, and where possible to submit formal proposals for consideration at the 27th Session of the International Geological Congress in Moscow (Bassett 19858748)
was largely responsible for a hasty illogical decision, which must be quickly reappraised in the best interests of global chronostratigraphy. Acknowledgemenrs. - P. J . LesMrance and C. R. Barnes are thankful to NSERC (Natural Sciences and Engineering Research Council of Canada) for continuing grants.
References Barnes, C. R. & Williams, S. H. in press: Conodonts from the Dob’s Linn boundary stratotype, southern Scotland. I n Cocks, L. R. M. & Rickards, R. B. (eds.): A global analysis of the Ordovician-Silurian boundary. Brihh Museum (Natural History), Bulletin. Bassett, M. G. 1985: Towards a ‘ c o r n o n language’ in stratigraphy. Epirodes 8, 87-92. Bassett, M. G., Cocks, L. R. M., Holland, C. H., Ingham, J. K., Lawson, J. D., Rickards, R. B. & Temple, J. T. 1979 Field meeting, Great Britain, March %April 11, 1979. Subcommhsion on SilurianStratigraphy and OrdoviciaAilurian Boundary Working Group (IUGS) 48 pp. National Muscum of Wales, Cardiff. BergstrSm, S. M. 1980: Conodonts as paleotemperature tools in Ordovician rocks of the Caledonides and adjacent areas in Scandinavia and the British Isles. G e o l o g i h F6reningenF i Stockholm F6rhandlingar 102, 377-392. Berry, W. B. N. 1987: The OrdoviciawSilurian boundary: new data, new concerns. Lethaia 20, 209-216. Cocks, L. R. M. 1985: The Ordovician-Silurian boundary. Episodes 8, 9b100. Cowie, J. W. 1986. Guidelines for boundary stratotypes. EpC SO& 9, ~ 8 2 . Harland, W.B., Cox, A. V.,Llewellyn, P. G., Pickton, C. A. G., Smith, A. G. & Walters, R. 1982 A Geologic Time Scak. 131 pp. Cambridge University Press, Cambridge. Hedberg, H. D. (ed.) 1976 Internati’onalStratigraphic Guide. A Guide to Stratigraphic Classification, Terminology, and Procedure by International Subcommirswn on Stratigraphic Classification of IUGS Commirsion on Slllrrigraphy. 200 pp. Wiley, New York. Holland, C. H. 1986: Does the golden spike still glitter?l o u d of the Geological Society 143, 3-21. Leggett, J. K. 1978: Eustacy and pelagic regimes in the Iapetus Ocean during the Ordovician and Silurian. Earth and Planetary Science Letters 41, 163-169. Leggett, J. K., McKerrow, W. S. & Eales, M. H.1979: The Southern Uplands of Scotland a Lower Palaeozoic accretionary prism. Journal of the Geological Society 136,755-770. LesMrance, P. J. (ed.) 1981: Field Meeting, AnticosMaspL, Qdbec, 1981. International Umon of Geologd Sciences
222 Pierre J . Lesptrance and others (IUGS), Subcommission on Silurian Stratigraphy and Ordov i c i a n - S i l h Boundary Working Group. Vol. I: Guidebook 56 pp. Vol. 11: Stratigraphy and Paleontology. 321 pp. D6partement de geologic, Universit6 de Montreal, Montreal. Les@rance, P. J. 1985: Faunal distributions across the Ordovician-Silurian boundary, Anticosti Island and Per&, Quebec, Canada. Canadian Journal of Earth Sciences 22,
838-849. Les@rance, P. J. in press: Trilobites. In Cocks, L. R. M. & Rickards, R. B. (eds.): A global analysis of the OrdovicianSilurian boundary. British Museum (Natural History), Bulletin. McCracken, A. D. & Barnes, C. R. 1981: Conodont biostratigraphy and paleoecology of the Ellis Bay Formation, Anticosti Island, Quebec, with special reference to Late Ordovician-Early Silurian chronostratigraphy and the systemic boundary. Geological Survey of Canada, Bulletin 329, Part2, 51-134. McLaren, D. J. 1972: Report from the Committee on the Silurian-Devonian boundary and stratigraphy to the President of the Commission on Stratigraphy. IUGS Geological Newsletter 1972, 268-288.
LETHAIA 20 (1987) McLaren, D. J. 1977:The Silurian-Devonian boundary Committee. A final report. In Martinsson, A. (ed.): The SilurianDevonian Boundary, 1-34,International Union of Geological Sciences, Series A , 5. E. Schweizerbart’sche Verlagsbuchhandlung, Stuttgart. Mu En-zhi, Zhu Zhao-Ling, Lin Yao-kun & Wu Hong-ji 1984 The Ordovician-Silurian boundary in Yichang, Hubei. In Nanjing Institute of Geology and Palaeontology (compilor): Stratigraphy and Palaeontology of Systemic Boundaries in China: OrdoviciarGWurian Boundary, I , 15-44. Anhui Science and Technology Publishing House. Schenlaub, H. P. (ed.) 1980: Second European conodont symposium (ECOS 11). Guidebook, abstracts. Geologische Bundesanstalt, Abhandlungen 35. 213 pp. Williams, S. H. 1983:The Ordovician-Silurianboundary graptolite fauna of Dob’s Linn, southern Scotland. Palaeontology 26, 605-639. Williams, S. H. & Rickards, R. B. 1984: Palaeoecology of graptolitic black shales. In Bruton, D. L. (ed.): Aspects of the Ordovician System. Palaeontological Confribm’onsfrom the Uniuerisity of Oslo 295, 159-166. Universitetsforlaget, Oslo.
