Chronostratigraphy: Beyond the GSSP
Schloss Seggau, Leibnitz, Austria
3–9 June 2006
- William A. Berggren
- Department of Geology, Rutgers University, Piscataway, New Jersey 08854, USA
- John A. Van Couvering
- The Micropaleontology Project, Inc., 256 Fifth Avenue, New York, New York 10001, USA
- Werner Piller
- Department of Geology, University of Graz, Heinrichstrasse 26, A8010 Graz, Austria
- Jan A. Zalesiewicz
- Geology Department, University of Leicester, University Road, Leicester LE1 7RH, UK
- Brian McGowran
- School of Earth and Environmental Sciences, University of Adelaide, Mawson Building DP 313, Adelaide SA 5005, Australia
Chronostratigraphy, the temporal organization and classification of (predominantly) sedimentary strata, provides the framework for deciphering earth history. Conceptually developed in the latter half of the nineteenth century, it has undergone successive metamorphoses, but at unprecedentedly accelerated rates in the last two decades. Given the size and scope of new databases, their ever-growing complexity and importance, and the multidisciplinary nature of modern studies, we urgently need to reexamine the bases upon which our classifications of rock, events, and time are assembled.
Thirty-one scientists from Africa, Australia, Europe, and North America and six part-time European observers attended this Penrose Conference at Schloss Seggau in Leibniz, Austria, from 3–9 June 2006. The conference was divided into several topics, prefaced by a keynote address by Gian Battista Vai (University of Bologna), who traced the history of the early bipartite, and latterly, tripartite chronostratigraphic subdivision as seen from the perspective of a century of international geological congresses.
The first topic of the conference reviewed the status quo of Cenozoic chronostratigraphy. William A. Berggren (Rutgers University) discussed some of the difficulties involved in constructing a satisfactory chronostratigraphy around some Cenozoic chronostratigraphic boundaries because of historic usage. He examined the heterogeneity in the procedures followed by various working groups in establishing Global Standard Stratotypes and Points (GSSPs), and questioned the use of the stage as the basic unit of the chronostratigraphic hierarchy. John Flynn (American Museum of Natural History) demonstrated how magnetostratigraphy and isotope stratigraphy have been instrumental in correlating terrestrial and marine stratigraphies, using examples from the South American Cenozoic record, the Neogene of Mexico, and the Paleocene-Eocene boundary interval in Asia and North America. Mike Woodburne (Museum of Northern Arizona, Flagstaff) discussed the concept of North American Land Mammal Ages and the need for renewed biostratigraphic studies in order to improve both boundary definitions and the potential development of continental stages. As reviewed by Dennis Kent (Rutgers University), magnetochronology now forms the backbone of the Late Jurassic through Cenozoic time scale, providing a resolution of <50 k.y. Whereas the Early and Middle Jurassic sequences of geomagnetic polarity reversals are as yet poorly known, Triassic sections have yielded a reliable magnetostratigraphy. Paleozoic magnetostratigraphy is promising. Carl Swisher (Rutgers University; as presented by D. Kent) pointed to the vast discrepancy between the analytical precision (<1%) of 40Ar/39Ar ages with the uncertainty (>1%) due to calibration and interlaboratory variations. His new age calibration on Paleogene tie-points brings the Paleogene time scale in synchrony with the new Neogene time scale (ATS04).
The second topic addressed problems in chronostratigraphy. Nick Christie-Blick (Columbia University) explained the rationale and procedures for placing in Australia the GSSP of the newly defined Ediacaran System of the Neoproterozoic erathem. Stan Finney (Long Beach State College) reviewed the status of the Paleozoic systems with emphasis on the Ordovician. He pointed to the major role of graptolite stratigraphy in guiding GSSP definitions. Jim Ogg (Purdue University) remarked on the slow progress in the definition of Mesozoic GSSPs. There is no GSSP for the base of the Cretaceous as yet, and the Berriasian Stage may have to be abandoned. Rick Fluegeman (Ball State University) reviewed the state of Cenozoic GSSPs. He pointed out the importance of erecting a Sparnacian Stage between the Thanetian and Ypresian s.s. stages. Brad Pillans (Australian National University, Canberra) reviewed the problem with equating the Pleistocene with the Quaternary and supported the recent suggestion to decouple the two and retain the Tertiary and Quaternary as suberathem/subera of the Cenozoic. Stan Finney (on behalf of Maria Cita, University of Milano) presented an overview of the history of Mediterranean Neogene stages. Werner Piller (Graz University) discussed the history of regional stages for the Paratethys and their correlation to Mediterranean stages. He indicated that the only appropriate resolution to these correlation problems is the application of an integrated stratigraphic approach within which sequence stratigraphy provides the basic framework for correlation supported by bio- and magnetostratigraphic tie points. He questioned the usefulness of regional stages in the light of a GSSP-based chronostratigraphy.
The third topic dealt with recent methodologies in chronostratigraphy. As discussed by Nick Christie-Blick, there is strong potential for miscorrelation of genetically (un)related surfaces, with significant implications for time-stratigraphy. Sequence stratigraphy is most useful at the basin scale and cannot serve for any global stratigraphic framework. It is not a convention or scheme for stratigraphic classification. Linda Hinnov (Johns Hopkins University) reviewed the principles of cyclostratigraphy and its relationship to the astronomical time scale (ATS). She distinguished the canonical (Cenozoic-Mesozoic, insolation-based) ATS and floating (pre-Mesozoic, orbital-like pattern-based) ATS, and reviewed the Latemar controversy. Joszef Palfy (Hungarian Natural History Museum, Budapest) showed how the methods of Unitary Association (UA) and Constrained Optimization (CONOP) assist in the definition of GSSPs and the evaluation of the reliability of correlations. Thierry Moorkens (Antwerp, Belgium) discussed the role of sequence- and cyclostratigraphy in studies of the Rupelian and Ypresian unit stratotypes and recommended (re)introduction of the Sparnacian Stage at the base of the Eocene. Based on Toarcian sections, Stephen Hesselbo (Oxford University) demonstrated that very high resolution carbon isotope stratigraphy has great potential for global (marine and terrestrial) correlations, with stability and resolution that far exceed ammonite-based stratigraphy. Andy Gale (British Museum, London) reiterated this point based on Upper Cretaceous successions in England and showed that carbon isotopic records yield a Milankovich cyclicity.
The fourth topic was concerned with the future of chronostratigraphy. Stan Finney observed that there are competing definitions of chronostratigraphy and reviewed the use of biostratigraphy in assisting the definition of GSSPs. He described the current status of the global stratigraphic correlation program and demonstrated the home page of the International Commission on Stratigraphy and its subcommissions. Bob Carter (James Cook University, Townsville) questioned whether the stratigraphic tools exist to satisfy future societal needs. He proposed broadening lithostratigraphy to include synthems as the highest hierarchical unit, abandoning the dual concept of chronostratigraphy, merging global chronostratigraphic units into global chronologic units down to the level of ages, abandoning the (local) stage, and retaining local biostratigraphies, including the oppelzones.
The fifth topic was devoted to examining potential improvements in current concepts and practices. Marie-Pierre Aubry (Rutgers University) traced the concept of GSSP to Harland’s approach to chronostratigraphy. She pointed to significant conceptual ambiguities in the GSSP approach, illustrated by the complex architecture of the Cenozoic stratigraphic record with its extended gaps and the resulting potential for miscorrelation based on event stratigraphy. Among other items, she proposed that boundary definition be based on horizons rather than points and that reference sections in terrestrial and marginal marine stratigraphies complement the marine-based GSSP definition. Lucy Edwards (U.S. Geological Survey, Reston) attempted to reconcile the desirability of a stable means of communication among stratigraphers with the need to revise stratigraphic codes and guides to reflect advances in the field of stratigraphy. Yuri Gladenkov (Geological Institute, Moscow) represented the view of the Russian school of stratigraphy. He noted that, whether regional or global, “natural” boundaries (i.e., based on major changes in earth history) should prevail in stratigraphic classification. He questioned the validity of the GSSP concept and suggested that unit-stratotypes be reconsidered. Finally, he recommended that the revision of the International Stratigraphic Guide involve the international stratigraphic community. Brian McGowran (University of Adelaide) noted that cultural diversity in stratigraphy was not removed by the Guide, that our perceptions of earth and life history have changed greatly (e.g., a resurgence of punctuationism driven by “revolutions” in plate tectonics, cyclo- and sequence stratigraphy, and impact theory), and that we have become much more unifying and integrating in recent decades. He cited assertions that stratigraphy has marginalized itself by way of the triad of litho/bio/chronostratigraphy, and he used examples from each of the three facets to show how stratigraphy must reassert its rightful place at the integrating and synthesizing center of earth and life history.
In the form of a debate between Jan Zalesiewicz (Leicester University) and Marie-Pierre Aubry, the conference provided the opportunity to discuss the proposal of the British Stratigraphic Commission to transform the dualistic hierarchy of current chronostratigraphy into a unitary system.
A proposal to develop cyclostratigraphy tools for the Mesozoic by a task force within the CHRONOS project was elucidated by Linda Hinnov and Jim Ogg. The idea of a Web-based community time scale was also discussed by Ogg and John Van Couvering (Micropaleontology Project). This proposal was strongly questioned because of inherent instability, risking the paper trail on which clarity of citation and communication must be based.
The conference ended with an open discussion that revolved around two main topics: the current status of stratigraphy and the content of the International Stratigraphic Guide. The audience expressed its concern at current levels of recognition and growth of stratigraphy in the earth sciences. Stratigraphy is central to geology in academia and industry and must remain there as the earth sciences contribute to society welfare. The training of experts in basic stratigraphic disciplines is an important step in meeting this challenge. Another matter of concern is the revision of the Guide, in which members of the stratigraphic community wish to participate. The consensus was that the Guide should not be simplified at this time, and it should be more explicit in some categories, in particular with regard to the description of the GSSP. The majority was for preserving the dual hierarchy of chronostratigraphic and geochronologic subdivision.
As to the science of stratigraphy itself, Nick Christie-Blick captured the spirit of the meeting thus: our great advances in precision and accuracy in correlation and age determination embolden us to ask those questions of the stratigraphic record that we have hitherto been too insecure to ask.
- In the interests of integration, synergy and synthesis, the stratigraphic community must be brought further together. Multidisciplinary advances notwithstanding, the community is still too divided by methodologies and expertise and according to different precepts in subdividing the Proterozoic and Phanerozoic eonothems.
- We need to unify concepts and protocols across different geocultural traditions.
- Stratigraphy plays a strong central role in discovering and elucidating earth and life history. We have to promote that role vigorously, beginning with a revival of historicist thinking in education by way of historical biology and historical geology.
- Regulating all stratigraphic tools and procedures is unnecessary (e.g., in sequence stratigraphy).
- There was, on balance, a preference for retaining the dual nomenclature of stratigraphy (from eon/eonothem to age/stage).
- Neither mammal ages nor biochronology are recognized explicitly in current stratigraphic codes or guides. This deserves further consideration.
Marie-Pierre J. Aubry