BASE HETTANGIAN/JURASSIC GSSP PROPOSAL
Nicol Morton, Chairman ISJS
Geoffrey Warrington, Convenor TJBWG
Gert Bloos, Secretary TJBWG
This document has been prepared to explain the background and procedures followed by members of the Triassic/Jurassic Boundary Working Group (TJBWG) for selection of the preferred GSSP for the base of the Hettangian Stage and the Jurassic System. It is intended to be a factual account of what happened and to give a reasonably objective account of why the voting went as it did, so that this proposal can be scrutinized objectively. It has some minor revisions from the document submitted to the Jurassic Subcommission.
The basal boundary of the Jurassic has been a long-lasting problem, illustrated for example by the status of the Rhaetian Stage, variously placed in the Triassic or Jurassic, until international agreement was reached to include it as the final stage of the Triassic. The Triassic/Jurassic boundary interval was the time of one of the major evolutionary crises in Earth history, and one that has proved to be complex. It was also a time of active plate tectonics during early stages of the breakup of Pangaea (and therefore rapidly changing palaeogeography), of a major volcanic phase (Central Atlantic Magmatic Province) and apparent major changes of sea-level in many areas, possibly global. This last event resulted in there being very few areas where a continuous sequence of strata across the boundary is preserved.
The Jurassic Subcommission established a Working Group, led by the late René MOUTERDE (France) succeeded by Geoffrey WARRINGTON (UK), to research and eventually propose definition of the boundary at a stratotype (GSSP). More recently, IGCP Project 458 on Triassic/Jurassic Boundary Events, now completed, was set up with leaders Stephen HESSELBO (UK), Christopher McROBERTS (USA) and Jozsef PALFY (Hungary). Both groups contributed a large amount of new data and ideas on the Triassic/Jurassic boundary interval.
By 2003 the Triassic/Jurassic Boundary Working Group (TJBWG) had identified and compared four sections with continuous sedimentation and marine faunas across the boundary interval – St. Audrie’s Bay (SW England), New York Canyon (Nevada), Kunga Island (western Canada) and Chilingote (Peru), but no favoured candidate emerged. Subsequently, two new sections were discovered and proposed as candidates – Kuhjoch (Austria) in 2005, and Waterloo Bay (Northern Ireland) in 2006. Also, a broader range of markers was proposed, including radiolarians and a carbon isotope excursion in addition to ammonites. In September 2006, the 7th International Congress on the Jurassic System, held in Krakow (Poland), enabled discussion of the various proposals; during this the candidature of Chilingote (Peru) was withdrawn.
In 2007 the following proposals were formally submitted to the members of the TJBWG for comparison, discussion and, eventually, selection:
(a) Ferguson Hill section, New York Canyon, Nevada, USA
By S.G. LUCAS, D.G. TAYLOR, J. GUEX, L.H. TANNER and K. KRAINER
with primary marker the ammonite Psiloceras spelae.
(b) Ferguson Hill section, New York Canyon, Nevada, USA
By C.A. McROBERTS, P.D. WARD and S. HESSELBO
with primary marker a carbon isotope excursion..
(c) Kuhjoch section, Karwendel Mountains, Northern Calcareous Alps, Tyrol, Austria
By A. von HILLEBRANDT, L. KRYSTYN, W.M. KUERSCHNER (with contributions by P.R. BOWN, C. McROBERTS, M. RUHL, M. SIMMS, A. TOMASOVYCH and M. URLICHS
with primary marker the ammonite Psiloceras cf. spelae.
(d) Kunga Island section, Queen Charlotte Islands, British Columbia, Canada
By L.M. LONGRIDGE, E.S. CARTER, J.W. HAGGART and P.L. SMITH
with primary marker a major evolutionary change of radiolarian faunas
(e) St. Audrie’s Bay section, Somerset, England, UK
By G. WARRINGTON, J.C.W. COPE and H.C. IVIMEY-COOK
with primary marker the ammonite Psiloceras planorbis.
(f) Waterloo Bay section, Larne, Northern Ireland, UK
By M.J. SIMMS and A.J. JERAM
with primary marker the ammonite Psiloceras planorbis.
All except the second last were published electronically in the International Subcommission on Jurassic Stratigraphy Newsletter no. 34, part 1. It is planned to publish the St. Audrie’s Bay proposal in a forthcoming Newsletter. They can be downloaded freely from the Jurassic Subcommission website at: http://www.es.ucl.ac.uk/people/bown/ISJSwebsite.htm
The quality of the science in all these proposals is very impressive and all the authors are to be congratulated on their work.
Selection and voting
In view of the large number of proposed sections and primary markers, the membership of the TJBWG was enlarged by the Convenor and Secretary in consultation with the Subcommission Chairman. All who were known to have an interest and input were invited to participate, to make the Group as representative as possible so that any decisions reached would carry authority. (For a list of those who accepted the invitation see ISJS Newsletter 34,1). To enable discussions, a special password-protected website was established by Christopher McROBERTS, containing the full proposal documents and with facilities to enable TJBWG members to post discussions on the various topics. In the final weeks of the selection processes there was also a huge amount of constructive discussion and exchanges by emails circulated to all WG members.
The selection procedures were organised by the Secretary of the TJBWG in consultation with the Subcommission Chairman. The Convenor 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
Voting form issued 14th February 2008; response deadline 29 February 2008.
Four different markers were proposed:
(a) A carbon isotope negative excursion identified in several sections; this is the lowest stratigraphically and would have placed some strata traditionally regarded as Triassic in the Jurassic;
(b) A major evolutionary turnover in the radiolarian faunas, slightly predating the first appearance of ammonites traditionally regarded as Jurassic;
(c) The first appearance of the ammonite genus Psiloceras, traditionally regarded as the oldest “Jurassic” ammonite, represented by the species Psiloceras spelae;
(d) The first appearance of the ammonite Psiloceras planorbis, for many decades regarded as the oldest (then known) Jurassic ammonite and index of the Planorbis Zone as basal zone of the Hettangian; this would have placed the boundary at the highest of the levels proposed.
Number of TJBWG members = 75; returned voting forms (all valid) = 67 (89.3%)
(i) for Psiloceras spelae/cf.spelae
[Jean Guex and Axel von Hillebrandt agreed that the Ferguson Hill and Kuhjoch ammonites represent the same species, with minor population differences at subspecies level, and concluded that they were contemporaneous within acceptable limits of resolution.]
votes for = 36 (53.7%)
Guex, Lucas, Pienkowski, Kozur, Krystyn, Hillebrandt, Michalik, Meister, Blau, Aberhan, Tanner, Bucher, Hallam, Korte, Yin, Siblik, Urlichs, Haas, Furrer, Bartolini, Bloos, Lathuillere, Tomasovych, Kürschner, Feist-Burkhardt, Vörös, Götz, Page, Vuks, Lindström, Sha, Polubotko, Repin, Hesselbo, Enay, Menning.
(ii) for Psiloceras planorbis
votes for = 13 (19.4%)
Cope, Simms, Hounslow, Stevens, Prinz-Grimm, Warrington, Hall, Ivimey-Cook, Donovan, Jeram, Riccardi, Mancenido, Damborenea.
(iii) for Carbon Isotope Excursion
votes for = 11 (16.4%)
Ward, Olsen, Lord, McRoberts, Cohen, Coe, Bown, Ciarapica, Smith, Longridge, Stanley.
(iv) for Radiolarian faunal turnover
votes for = 7 (10.5%)
Ruckwied, Hirsch, Matsuoka, Hori, Palfy, Carter, Herngreen.
2. Selection of the preferred section
Voting form issued 6 March 2008; response deadline 17 March 2008.
Following the voting, the primary marker chosen by a majority of the TJBWG members was the ammonite Psiloceras spelae, which had been proposed and could be recognised in only two candidate sections – Kuhjoch section (Austria) and Ferguson Hill section (USA). The second stage ballot offered a choice between these two candidate sections.
Number of TJBWG members = 75; returned voting forms (all valid) = 57 (76.0%)
(i) for Kuhjoch section, Austria
votes for = 32 (56.1%)
Hillebrandt, Pienkowski, Aberhan, Feist-Burkhardt, Vörös, Blau, Hounslow, Hallam, Menning, Yin, Meister, Bown, Page, Siblik, Cope, Boomer, Bloos, Michalik, Olsen, Stevens, Haas, Vuks, Repin, Polubotko, Hesselbo, Tomasovych, Longridge, Hall, Furrer, Kürschner, Urlichs, Krystyn.
(ii) for Ferguson Hill section, USA
votes for = 18 (31.6%)
Guex, Lucas, Hirsch, Lord, Matsuoka, McRoberts, Carter, Ruckwied, Korte, Kozur, Götz, Taylor, Hori, Ciarapica, Sha, Mancenido, Enay, Lindström.
votes for = 7 (12.3%)
Warrington, Palfy, Riccardi, Damborenea, Ivimey-Cook, Jeram, Smith.
3. Confirmation of majority vote
Voting form issued 19 March 2008; response deadline 7 April 2008.
The majority vote for the Kuhjoch section as preferred GSSP candidate did not achieve the required 60% majority of the votes cast. Therefore, a further ballot was held to seek the approval of TJBWG members for this section to be proposed to the Jurassic Subcommission.
Number of TJBWG members = 75; returned voting forms (all valid) = 61 (81.3%)
(i) YES votes = 48 (78.7%)
Guex, Vuks, Polubotko, Repin, Kürschner, Prinz-Grimm, Stanley, Menning, Hounslow, Bloos, Simms, Pienkowski, Yin, Hallam, McRoberts, Hillebrandt, Korte, Bartolini, Donovan, Tomasovych, Hesselbo, Enay, Stevens, Meister, Haas, Cope, Aberhan, Michalik, Krystyn, Smith, Hall, Herngreen, Boomer, Olsen, Vörös, Longridge, Tanner, Lindström, Furrer, Lathuilière, Feist-Burkhardt, Urlichs, Blau, Götz, Siblik, Gazdzicki, Ruckwied, Bown.
(ii) NO votes = 6 ( 9.8%)
Damborenea, Mancenido, Warrington, Hori, Ward, Palfy.
(iii) ABSTAIN votes = 7 (11.5%)
Kozur, Jeram, Matsuoka, Hirsch, Carter, Ivimey-Cook, Riccardi.
At the same time a ballot was held to ask TJBWG members opinions of a suggestion to propose the Ferguson Hill section as auxiliary stratotype (ASSP).
Number of TJBWG members = 75; returned voting forms (all valid) = 57 (76.0%)
(iv) YES votes = 35 (61.4%)
Guex, Vuks, Polubotko, Repin, Stanley, Menning, Bloos, Pienkowski, Hallam, Hillebrandt, Bartolini, Donovan, Tomasovych, Enay, Kozur, Stevens, Haas, Cope, Aberhan, Michalik, Smith, Hall, Korte, Hori, Herngren, Boomer, Longridge, Tanner, Lindström, Furrer, Hirsch, Carter, Blau, Götz, Ruckwied
(v) NO votes = 9 ( 15.8%)
Kürschner, Hounslow, Simms, McRoberts, Hesselbo, Warrington, Olsen, Lathuilière, Palfy
(vi) ABSTAIN votes = 13 (22.8%)
Damborenea, Prinz-Grimm, Yin, Mancenido, Meister, Krystyn, Jeram, Matsuoka, Vörös, Feist-Burkhardt, Gazdzicki, Riccardi, Bown
The Kuhjoch section, Karwendel Mts., Northern Calcareous Alps, Austria, has been accepted by vote of the members of the TJBWG to be proposed as GSSP for the base of the Hettangian Stage and the Jurassic System. The Ferguson Hill section, New York Canyon, Nevada, USA has been accepted by vote to be proposed as ASSP; an integrated proposal must be prepared for later submission to the Jurassic Subcommission.and to the International Commission on Stratigraphy.
Comments on the result of the votes
This comment does not repeat or evaluate the arguments on the different candidate GSSPs, their boundary definitions and correlation potential, but considers only some general aspects.
The focus of investigation and of discussion was on the boundary itself. Only those sections that have continuous sedimentation and succession of sufficiently preserved fossils through the boundary interval could be considered as candidate GSSPs. Our search for such sections revealed that they are rather rare. Most sections shows deficiencies of one or other kind so that the boundary cannot be recognized.
It is important that even if the boundary itself is not recognizable itself, the ages of the rocks above and below the boundary can be recognized unequivocally. This was the case with traditional boundary levels used in the past, although they have turned out to differ in age, more or less. However, one common characteristic was that the ranges of Choristoceras marshi, Ch. crickmayi and the conodont Misikella posthernsteini, were regarded as latest Triassic, while Psiloceras was regarded as earliest Jurassic wherever found in the world. In complete sections these ranges are separated by an intermediate interval where time-diagnostic fossils are essentially lacking. Thus, an overlap of index fossils of latest Triassic and earliest Jurassic is excluded. Rare exceptions, such as Choristoceras minutum or the late conodont “Neohindeodella” detrei offer no difficulty because these species are known only from sediments above the ranges of the accepted latest Triassic index species. A boundary level at the appearance of Psiloceras spelae is situated between the ranges of the two groups of age-diagnostic fossils referred to above and in this respect continues historical stratigraphical usage.
On the other hand, if the boundary had been placed at a lower level, this would separate the stratigraphic range of, for example, Misikella posthernsteini into a Triassic part and a Jurassic part. Consequently, this important index fossil would lose its unequivocal age-diagnostic character distinguishing latest Triassic from earliest Jurassic.
Similarly, a boundary at a higher level defined by a species within the psiloceratid succession could be correlated only within the faunal province where the GSSP is situated; not all psiloceratids would be Jurassic. In other faunal provinces with different ammonite sequences, only tentative correlations would be possible and stratigraphic allocation of new, hitherto unknown, psiloceratids would remain uncertain.
The investigations of the TJBWG have yielded results that are certainly significant beyond the primary aim, the selection of a GSSP for the base of the Jurassic. Each candidate section will remain an important international stratigraphical reference in the future contributing to the elucidation of a very special important time-span in the history of Earth. Some open questions remain and new ones arise; therefore, investigations must be continued, as indicated below.
Remarks on the Triassic/Jurassic transitional boundary interval
This is essentially an attempt to integrate the results from the six candidate GSSP proposals into an idea on what happened at the end of the Triassic as now understood and where in this history the T-J boundary is defined. The primary causes of the environmental changes across the T-J boundary are not discussed here because they are not yet sufficiently known.
For a long time the late Rhaetian was known as a period of decreasing diversity in important fossil groups, such as ammonoids, bivalves, brachiopods, conodonts, ostracodes, and foraminifers. The immediate transition into the Jurassic, however, was generally obscure, for various reasons. The end-Triassic sea level changes caused regionally widespread gaps in sedimentation and breaks of facies sequence. Moreover, unfavourable facies caused poor records of fossils with regard to frequency, diversity and preservation. Additionally, subsequent influences such as diagenetic processes and thermal metamorphism changed or even deleted biotic and non-biotic signals.
The time of decline in the late Rhaetian ended in a major crisis for the fossil groups referred to above, with a final interval characterized by a minimum of frequency and diversity. It is remarkable that not only the groups mentioned above as being in decline were affected but so also were radiolarians, whose diversity (against the general negative trend) still increased up to the beginning of the final crisis.
Most probably the beginning of the final crisis is indicated by the strikingly rapid extinction of about 70 Triassic species of radiolarians (which were still present in bed 9 of Kunga Island; 57 of them are missing already in bed 10, while 13 short-lived holdovers are absent in bed 13; Longridge et al. 2007 ISJS Newsletter 34(1): 21-33, fig. 4). Only three Triassic species persisted above the extinction level. From bed 10 upwards 20 new species appeared. Thus there was minimal overlap of the earlier and later groups.
There can be little doubt that this turnover reflects a major environmental change. To find out the reason, it seems important that the radiolarians are a pelagic group. Important also is that genera with highly specialised morphology were most concerned. Such forms obviously were adapted to a specialised mode of life that could not be continued after the environmental change. Since calcareous nannoplankton were also concerned it may be that the symbiotic algae were the particularly sensitive element in the radiolarians.
According to Williford et al. (2007 PPP 244: 290-296. fig. 1) the radiolarian turnover is situated within the initial negative Carbon Isotope Excursion (CIE) at Kennecott Point which has a range of about 4 m (110-114 m). Therefore it seems rather probable that the extinction of Triassic radiolarians and the CIE reflect the same environmental change and that this change was the reason for at least the beginning of the final crisis.
Whereas the reaction of the radiolarians to the environmental change was immediate and strong, other latest Triassic fossil groups survived a short time after the CIE. Holdovers into the critical interval are known from the radiolarians (Kunga Island), conodonts (UK), ammonoids (Ferguson Hill: Choristoceras crickmayi), foraminifers, ostracodes, palynomorphs (all Kuhjoch). This means that minimum diversity was reached a short time after the beginning of the crisis.
Of the later part of the critical interval little is known from most of the candidate GSSPs. Since the radiolarian change is completely situated within the CIE according to Williford et al. (2007) it seems that the recovery of the radiolarians began earlier than recoveries of other fossil groups. The reappearance of other fossil groups was clearly later than the CIE.
Most information on the upper part of the critical interval is provided at Kuhjoch and other sections in the same basin (Eiberg Basin). The excellent preservation in these sections (aragonite preserved) indicates that the sections offer the original fossil content without later alteration. The sections show that low diversity persisted almost up to the first appearance of Psiloceras spelae. Near this level also in other fossil groups new forms appeared: ostracodes, aragonitic forams, and palynomorphs.
The increase of diversity around the level of Psiloceras spelae indicates the end of the final crisis. The appearance of Psiloceras spelae at this level is not random. It is at the beginning of a general recovery. The T-J boundary is now proposed at this level. With this definition the final crisis is Triassic. The Jurassic begins with the appearance of new forms that persisted into later Jurassic.
It is very evident that in a complex situation such as that summarized above, a single reference section, the GSSP, is inadequate. Of course, the GSSP will remain as the ultimate international point of reference for the base of the Jurassic System. However, further thought must be given as to how best to complement the GSSP with other international points of reference. The following are some topics that require further consideration by the TJBWG and the Jurassic Subcommission:
1. The Ferguson Hill section in New York Canyon (Nevada, USA) provides important supporting information to that of the Kuhjoch section, and its proposal as Auxiliary Stratotype Section and Point (ASSP) has been supported by a vote in the TJBWG. A documented case for this must now be prepared.
2. Current research by Jean Guex and colleagues (see Schaltegger et al. 2008 EPSL 267,266-275) in the Utcubamba Valley, Peru, following up earlier research by Axel von Hillebrandt, has confirmed a similar ammonite succession to that in Nevada and Austria and provided U-Pb ages. Consideration should be given as to whether a selected section should be proposed as an additional Auxiliary Stratotype Section and Point (ASSP).
3. The evolutionary faunal turnover of the radiolaria documented especially in the Kunga Island section provides an important “proxy” for recognition of the base Hettangian/Jurassic, and for interpretation of the biological history of the Earth during this time interval. It provides an important international secondary reference point that merits official recognition.
4. The reference sections in western Europe (St. Audrie’s Bay, S.W. England, and Waterloo Bay, Northern Ireland) document the earliest Psiloceras faunas in a different faunal province. These are also the only sections proposed as candidates where a magnetostratigraphic record appears to be preserved that could enable correlation with the terrestrial record such as that in the Newark Basin and elsewhere. Provided correlation with the GSSP at Kuhjoch, for example using the Carbon Isotope Excursion, can be refined then these sections will also be useful international secondary reference points.