Geoscientist Online

Nicol Morton, Former Chair of ISJS, describes the mystic-awful process of defining the base of a system, and the location of its “Golden Spike”

Geoscientist 21.11 December 2011/January 2012

The Jurassic System is renowned for the high precision of relative dating and correlation that can be achieved, based on ammonite biochronology. In favourable circumstances horizons with estimated mean durations of less than 100ky can be identified1. So, it may seem strange that, uniquely for a Geological System, neither the base nor the top had been defined formally by reference to a Global Stratotype Section and Point (GSSP).

For the top of the Jurassic, the Jurassic/Cretaceous boundary (more precisely the base of the Cretaceous System) the problem is essentially one of extreme faunal provincialisation, severely restricting possibilities for inter-basinal correlation. This remains to be resolved.

The problems of the base of the Jurassic, the Triassic/Jurassic boundary, are different and complex:

• A major extinction event that almost included the ammonoids;
• A dramatic sea-level fall resulting in very few continuous marine sections across the boundary;
• Extensive tectonic events, especially rifting and resulting palaeogeographic changes;
• A major volcanic event in the Central Atlantic (the CAMP volcanism).

This boundary has only recently been defined by decisions within the International Subcommission on Jurassic Stratigraphy (ISJS) in June 2008, by the International Commission on Stratigraphy (ICS) in May 2009 and ratified by the Executive Committee of the International Union of Geosciences (IUGS) in April 2010. The site selected is in the Northern Calcareous Alps, the Kuhjoch section in the Karwendel National Park, Tyrol, Austria (Fig. 1).

Fig. 1. Above: The Kuhjoch section in the Karwendel National Park, Northern Calcareous Alps, Tyrol, Austria. The boundary strata form the col between the Upper Triassic carbonates (to the right) and the Lower Jurassic carbonates (to the left).

Historical Background

Fig. 2. The excavated section of the Triassic-Jurassic boundary on Kuhjoch Mountain (47O29’0”N 11O31’50”E).

For many years there was uncertainty about the Triassic/Jurassic boundary notably in northern Europe as to whether the “Rhaetic” Beds, between the non-marine Permo-Triassic New Red Sandstone and the marine Lias, should be placed in the Triassic or the Jurassic (compare, for example Arkell 1933 and 1956). The distinctly Triassic nature of the Rhaetian faunas was the key factor in deciding.

This meant that the basal Stage of the Jurassic is the Hettangian. For many years the oldest known Jurassic ammonites were Psiloceras planorbis and related species, best represented in Somerset, S.W. England. The base of the Stage and of the Jurassic System was placed here and proposed as GSSP2,3, but with confusion as to whether it should be at the first appearance of the ammonites or at the (lithostratigraphic) base of the Lower Lias. However, no Triassic ammonoids occur in this area and the evolutionary origins of Psiloceras planorbis remain unknown.

The Jurassic Subcommission was re-established under new leadership (Arnold Zeiss) and during the 1984 Jurassic Congress in Erlangen, Germany, Working Groups were set up for each of the Stages. For the Hettangian (and base Jurassic) Rene Mouterde (France) was appointed Convenor, later (2000) succeeded by Geoff Warrington (UK) on Mouterde’s retirement. The task was to document possible sections, select and propose one to ISJS as GSSP.

A parallel development that also yielded important data and discussions was IGCP Project 458 Triassic-Jurassic Boundary Events, led by Jozsef Palfy (Hungary), Stephen Hesselbo (UK) and Chris McRoberts (USA).

Fig. 3. Axel von Hillebrandt drives in the spike to mark the base of the Jurassic. Note one of the information panels in the background.

Sections with marine Upper Triassic overlain by marine Lower Jurassic became known in other areas, notably Alpine Europe and western America, but usually with an apparent gap at the boundary. However, in the 1990’s the occurrence of Psiloceras spelae, a species evidently older than P. planorbis and a short distance above Triassic Choristoceras was documented in Nevada, so this section was proposed as GSSP4. The same species and stratigraphy were then demonstrated in Peru, but a proposal as GSSP was subsequently withdrawn.

In the meantime, criteria other than ammonites were being researched and two were of sufficient merit to be proposed as GSSP marker event. The first is a spectacular evolutionary turnover of radiolarian faunas documented first in western Canada5,6 and confirmed in Japan. The second is a prominent, but brief negative carbon isotope excursion, first established in Nevada but also recognisable elsewhere7.

With these possibilities the stage was set for an important meeting of the Triassic-Jurassic Boundary Working Group during the 7th International Congress on the Jurassic System in Krakow, Poland in September 2006. However, two surprises turned up in advance of the Congress.

The first, in 2005, was the discovery of Psiloceras cf. spelae (subsequently named as a new subspecies, but from the same ammonite horizon) in sections in part of the Northern Calcareous Alps in Austria, an area where previously there was thought to be a hiatus8.

The second was the discovery, announced in 2006, of a section in Northern Ireland that contained a more expanded section and a more complete sequence of species of Psiloceras, including P. planorbis, than those in S.W. England9. Neither of the authors of this proposal was able to be present at the Congress, but the poster showing preliminary results was a great surprise to Congress participants – how could such an important section have remained unknown for so long in a “well-explored” country like Britain?

Selecting the GSSP

Fig. 4. The GSSP “golden spike” for the base of the Jurassic System, the Lower Jurassic Series and the Hettangian Stage receives the touch of approval from (left to right) Jozsef Palfy (Budapest, Hungary, Chair of ISJS), Stan Finney (Long Beach, USA, Chair of ICS), Axel von Hillebrandt (Berlin, Germany, lead author of the GSSP proposal), Leopold Krystyn (Vienna, Austria, co-author of the GSSP proposal), Nicol Morton (Vogüé, France, former Chair of ISJS).

1. Working Group

The members of the ISJS Working Group, augmented beyond those involved in proposals to ensure an authoritative recommendation, were faced with a choice of six proposals:

• Ferguson Hill section, New York Canyon, Nevada, USA by S.G. Lucas, D.G. Taylor, J. Guex, L.H. Tanner & K. Krainer, with primary marker the ammonite Psiloceras spelae;
• Ferguson Hill section, New York Canyon, Nevada, USA by C.A. McRoberts, P.D. Ward & S.P. Hesselbo, with primary marker a carbon isotope excursion;
• Kuhjoch section, Karwendel Mountains, Northern Calcareous Alps, Tyrol, Austria by A. von Hillebrandt, L. Krystyn & W.M. Kuerschner, with primary marker Psiloceras cf. spelae;
• Kunga Island section, Queen Charlotte Islands, British Columbia, Canada by L.M. Longridge, E.M. Carter, J.W. Haggart & P.L. Smith, with primary marker a major evolutionary change of radiolarian faunas;
• St. Audrie’s Bay section, Somerset, England, UK by G. Warrington, J.C.W. Cope & H.C. Ivimey-Cook with primary marker the ammonite Psiloceras planorbis;
• Waterloo Bay section, Larne, Northern Ireland by M.J. Simms & A.J. Jeram with primary marker Psiloceras planorbis.

The proposals, updated where appropriate, were published in the ISJS Newsletter, circulated, or available to download from the ISJS website, to all Voting and Corresponding Members of the Jurassic Subcommission and to all members of the Working Group. It was generally agreed that none of the proposed sections was ideal, but that these were the best available at the time and for the foreseeable future. To enable full discussions a password-protected website was set up by Chris McRoberts and access made available to all Working Group members. In addition, hundreds of emails were exchanged.

The selection procedures were organised by the Secretary of the Working Group, Gert Bloos in consultation with the ISJS Chairman Nicol Morton. The W.G. Convenor, Geoff Warrington, withdrew from these in view of his involvement with one of the proposals.

Voting was carried out in three stages:

1. Selection of the primary marker; returned votes 67/75 = 89.3%; Ps. spelae/cf. spelae 53.7%; Ps. planorbis 19.4%; carbon isotope excursion 16.4%; radiolarian turnover 10.5%.
2. Selection of preferred section (of two with sekected primary marker); returned votes 57/75 = 76.0% Kuhjoch section 56.1%; Ferguson Hill section 31.6%; abstain 12.3%.
3. Confirmation of majority vote; returned votes 61/75 = 81.3% YES 78.7% NO 9.8% ABSTAIN 11.5%

Therefore, the proposal for the Kuhjoch section with primary marker Psiloceras cf. spelae (now Psiloceras spelae tyrolicum) was forwarded.

2. Jurassic Subcommission

Only the Executive (Chair, Vice-Chair, Secretary) and Voting Members of the Subcommission have a vote; returned votes 21/22 95.5% YES 66.7% NO 19.0% ABSTAIN 14.3%

3. International Commission on Stratigraphy

The proposal, updated in the light of comments and suggestions made by Subcommission members was forwarded to the Commission in August 2008. Unfortunately changes of personnel in the Commission, plus other urgent business, resulted in the vote being delayed.

Returned votes: 18/18 100% YES 17 94% NO 0 ABSTAIN 1 6%

Further suggestions for improving the proposal were received from members of ICS and were taken into account before submission to IUGS.

Executive Committee of International Union of GeosciencesProposal ratified on 16 April 2010.

The Golden Spike

The importance of GSSPs is emphasised by the placing of a physical marker in the point in the section, colloquially referred to as the “golden spike”. The one marking the base of the Jurassic System, the Lower Jurassic Series and the Hettangian Stage was inserted on the morning of Saturday 20th August 2011, during a ceremony arranged by Sylvain Richoz on behalf of the Austrian Commission for Stratigraphy, the Austrian National Committee for Earth Sciences and the IGC programme at the Austrian Academy of Science. It is Austria’s first GSSP.

The Karwendel Park authorities gave exceptional permission for excavation and protection of the site, with access for equipment and materials by helicopter. However, participants in the inaugural ceremony had to go on foot!

A group of geologists and others met at Fall (in Germany) and, with special permission from The Karwendel Natural Park authorities, drove up forestry tracks to the nearest accessible point to the section. From there it is a “short” (1.5–2 hrs) walk, about 1.5km horizontally but with a steep climb of over 500m vertically, to the section at 1780m altitude (Figs. 1 and 2). The section itself is somewhat uninspiring but it yields a range of data significant for correlation of the Triassic-Jurassic boundary.

Two of the three principal authors, Axel von Hillebrandt (Berlin) and Leopold Krystyn (Vienna), explained the stratigraphy of the section [Wolfram Kuerschner (Utrecht) was unable to be present] before the 1m long spike was driven into the appropriate point (Fig. 3) and the “golden” marker was put in place (fig. 4). Two large explanatory panels (see Fig. 3), with texts in German and English, were carried up the hill (and then back down until their permanent site has been is chosen).

This inauguration was the culmination of many years of hard work, sometimes in difficult places, producing much high quality science. The authors of and contrributors to ALL the GSSP proposals, not just the successful one, merit congratulations and thanks from the geological community.

Details of all the proposals and description of the selection and voting procedures can be found in the Newsletters of the Jurassic Subcommission, especially No. 34 (Part1) July 2007 and No. 35 (Part 1) December 2008. These can be downloaded freely from the ISJS website at http://jurassic.earth.ox.ac.uk/.

References

1. Callomon, J.H. 1995. Time from fossils: S.S. Buckman and Jurassic high resolution geochronology. In Milestones in Geology. Geological Society Memoir 16, 127-150.
2. Morton, N. (ed.) 1974. The definition of Standard Jurassic Stages. Mémoires du Bureau de Recherches Géologiques et Minières, 75, 83-93.
3. Warrington, G., Ivimey-Cook, H.C. & Cope, J.C.W. 1994. St Audrie’s Bay, Somerset, England: a candidate Global Stratotype Section and Point for the base of the Jurassic System. Geological Magazine, 131, 191-200.
4. Guex et al GSSP proposal in ISJS Newsl 24
5. Carter, E.S. & Tipper, H.W. 1999. Proposal of Kunga Island, Queen Charlotte Islands, British Columbia, Canada as Triassic/Jurassic global boundary stratotype. ISJS Newsletter 27, 27, 20.
6. Longridge, L.M., Carter, E.S., Haggart, J.W. & Smith, P.L. 2007. The Triassic-Jurassic transition at Kunga Island, british Columbia, Canada. ISJS Newsletter 34 (1), 21-33.
7. McRoberts, C.A., Ward, P.D. & Hesselbo, S. 2007. A proposal for the base Hettangian Stage (=base Jurassic System) GSSP at New York Canyon (Nevada, USA) using carbon isotopes. ISJS Newsletter 34 (1), 43-49.
8. Hillebrandt, A. von, Krystyn, L. & Kuerschner, W.M. 2007. A candidate GSSP for the base of the Jurassic Syatem in the Northern Calcareous Alps (Kuhjoch section, Karwendel Mountains, Tyrol, Austria. ISJS Newsletter 34 (1), 2-20.
9. Simms, M.J. & Jeram, A.J. 2007. Waterloo Bay, Larne, Northern Ireland: a candidate Global Stratotype Section and Point for the base of the Hettangian Stage and Jurassic System. ISJS Newsletter 34 (1), 50-68.

* Author address: 07200 Vogüé, France