Geoscientist Online

Tertiary: survival of the fittest?

by Robert Knox, Phil Gibbard, John Cope, Andy Gale, John Powell, Peter Rawson, Alan Smith, Colin Waters and Jan Zalasiewicz

Although used for nearly two centuries as a standard, universal stratigraphic term, the Tertiary has been absent from the International Union of Geological Sciences (IUGS) approved timescales since 1989, with the interval between the Cretaceous and the Quaternary being represented solely by the Paleogene and Neogene periods.

More recently, the Quaternary was similarly omitted from an International Commission of Stratigraphy (ICS)-sponsored time chart; although this move was not sanctioned by IUGS. Strong objections to this apparent suppression were immediately raised and led to extensive discussion between those for and against its reinstatement. The matter was finally settled by the IUGS Executive Committee on 29 June 2009, with the formal ratification of the Quaternary as a period/system within the geological timescale. The Quaternary is thus here to stay as a formal unit.

The ratification of the Quaternary puts a new perspective on the discussion regarding the status of the Tertiary. The Tertiary has never been explicitly eliminated by IUGS, and has continued to be used in an informal sense, alongside the Paleogene and Neogene. One of the arguments put forward against retention of both the Tertiary and Quaternary is that they are relics of a redundant 18th Century system that originally included the Primary and Secondary. To equate the terms Tertiary and Quaternary with Primary and Secondary is quite unjustified, however, since Primary and Secondary were abandoned long ago. Clearly, the recent IUGS ruling on the retention of the Quaternary counters any argument for eliminating the Tertiary on the basis of its 18th Century roots.

The other main argument for abandoning the Tertiary is that the term has fallen out of use. Thus in 2005 the ICS recommended that the Tertiary not be considered as a formal division of the geological timescale, “because it is nearly redundant with the entire Cenozoic Era”. This statement was refuted by the late Amos Salvador, who pointed out that in 2006 the term Tertiary was being used more often than the terms Paleogene and Neogene.

Continued usage of “Tertiary” is not confined to individual articles and books. It has also been retained by some national stratigraphic commissions, (eg., the German Stratigraphic Commission) by some national geological mapping organisations, [eg. the German Bundesanstalt für Geowissenschaften und Rohstoffe (BGR) and the Bureau de Recherches Géologiques et Minières (BRGM), and in some national geological survey time-charts (eg, USGS)]. Clearly, the Tertiary has by no means fallen out of use, despite its omission from the ICS charts.
Recent arguments for the reinstatement of the Tertiary as an officially sanctioned unit have been based primarily on its role as a practical time unit, referring to the post-Mesozoic– pre-Quaternary interval.

The continued use of the term Tertiary clearly reflects the need by many geoscientists for a single term that encompasses both the Paleogene and the Neogene. The reason for this may lie in the origin of the Paleogene/Neogene division. The term Neogene originated in Europe, where it was applied to younger Tertiary (Miocene and Pliocene) strata that rested unconformably on Eocene strata (and thus post-dated the climax of the Alpine orogeny. The term Paleogene was introduced initially to equate with the Eocene and was subsequently expanded include the Paleocene and Oligocene. In its original concept, therefore, the division of the Tertiary into the Paleogene and Neogene wastectonostratigraphic as well as biostratigraphic in nature. Although the tectonostratigraphic element of the definition soon became sidelined, the application of the terms Neogene and Paleogene for many decades applied only to those regions affected by Alpine tectonism.

While the Paleogene and Neogene have subsequently acquired international recognition, it is open to question whether they represent appropriate divisions of time at the period/system level. In recent decades it has become increasingly apparent that the most fundamental and permanent change in post-Cretaceous, pre-Quaternary global climate and environments between the Cretaceous and the Quaternary took place at the Eocene–Oligocene transition, marking the change from a greenhouse to an icehouse world (Figure 1). The Paleogene/Neogene boundary does not reflect this, and this may be one reason why many stratigraphers prefer to retain the Tertiary as the fundamental unit of time (ie. period) between the Cretaceous and the Quaternary.

It should also be pointed out that, at the time the decision was made to give formal period rank to the Paleogene and Neogene, no compromise seemed possible with the Tertiary/Quaternary scheme. This is because the Neogene extended beyond the Tertiary/Quaternary boundary. Following the recent decision to terminate the Neogene at the base of the Quaternary, this bar to integrating the two schemes no longer exists, and we have a unique opportunity to give due recognition to both historical schemes for subdivision of Cenozoic time at the Period level.

Whatever the motivation, the continued use of the term “Tertiary” speaks for itself. In our opinion, therefore, the time interval between the end of the Cretaceous and the beginning of the Quaternary is best represented by a single period: the Tertiary (Figure 2). The GSSP for the base of the Tertiary Period/System would be that already established for the Paleogene, at El Kef, Tunisia. The Tertiary would thus have the same status as the Quaternary. One significant effect of the assignment of period status to the Tertiary would be the downgrading of the terms Paleogene and Neogene. It is clearly in the interests of stability of nomenclature that these terms be maintained, and we recommend that the Paleogene and Neogene be given sub-period/sub-system status, with their meaning remaining unchanged. It may be noted that units of sub-period/sub-system level already exist in the ICS/IUGS nomenclature, i.e. the Mississippian and Pennsylvanian divisions of the Carboniferous Period.

Having all three terms available as formal chronostratigraphic/geochronological units is considered to provide the most pragmatic solution to this long-standing controversy. The proposed scheme provides geoscientists with the most versatile vocabulary for expressing post-Cretaceous, pre-Quaternary time.

Recommended reading

• Finney, S.C., 2010 in press, Formal definition of the Quaternary System/Period and redefinition of Pleistocene Series/Epoch: Episodes.
• Gibbard, P.L., Head, M.J, Walker, J.C. and the Subcommission on Quaternary Stratigraphy, 2009, Formal ratification of the Quaternary System/Period and the Pleistocene Series/Epoch with a base at 2.58 Ma: Journal of Quaternary Science, Published Online: 22 Sep 2009, DOI 10.1002/jqs.1338
• Head M. J, Gibbard P. L, and Salvador A., 2008b, The Tertiary: a proposal for its formal definition: Episodes, v. 31, pp. 248–250.
• Menning, M., 2010, Quartär bewahrt - Tertiär aufgeben? Geowissenschaftliche Mitteilungen, v. 39, 16–17.
• Salvador, A., 2006. The Tertiary and the Quaternary are here to stay: AAPG Bulletin, v. 90, pp. 21–30.
• Walker, J. D. & Geissman, J. W., 2009, 2009 GSA Geologic Time Scale: GSA Today, v. 19, no. 4, pp. 60–61. doi:10.1130/1052-5173-19.4-5.60
• Walsh, S. L., 2006, Hierarchical subdivision of the Cenozoic Era: A venerable solution, and a critique of current proposals: Earth-Science Reviews, v.78, pp.207–237.

Retain “Paleogene” and “Neogene” as periods

by Paul Pearson, and Mark Hounslow*

In response to Knox et al., and as an opposing viewpoint, we note that re-instating the Tertiary on the ICS / IUGS standard timescale and removing the ‘period’ status of Paleogene and Neogene would promote inconsistency. Moreover as a formal stratigraphic unit, the Tertiary has little intrinsically to recommend it.

As Knox et al. point out, the term ‘Tertiary’ remains in widespread use in some parts of the geological community. Equally, personal experience shows that it is obsolete in others - for example, among deep-sea Cenozoic stratigraphers and palaeoclimatologists. Meanwhile Paleogene and Neogene have achieved widespread acceptance as period-level divisions, and have been recognised as such for decades. We have no objection to informal use of the Tertiary; the issue is whether the ICS / IUGS should revert to using it on the official timescale, having previously omitted it, and at the same time downgrade Paleogene and Neogene to sub-period status. This move would force considerable change on stratigraphers. Such a decision should only be taken if there is an overwhelming case and solid support; but the community is evidently divided following the recent decision to recognise the Quaternary as a period. Aside from the politics of the situation, there are several shortcomings inherent in the concept of the Tertiary as presented by Knox et al.:

1. As there is no intention to abolish Paleogene and Neogene but rather to down-grade them to sub-periods, the proposal adds a level of hierarchy to the formal timescale. Sub-periods are not recognised for most of the Phanerozoic. The proposal would lock in this unnecessary complexity for future generations of geologists to learn.
2. Knox et al. question whether the Paleogene/Neogene periods represent appropriate divisions of time at the period/system level. We note that their durations (42.5 and 20.4 million years respectively) are in fact reasonable in the context of the Phanerozoic as a whole, being similar to the Ordovician [44.6 m. yr] and Silurian [27.7 m. yr] periods, for example). In contrast the Tertiary and Quaternary have a very large discrepancy in their respective durations (63 vs 2.6 million years). In the Knox et al. scheme the Paleogene sub-period is over 16 times longer than the Quaternary period! This hardly seems a rational way of parcelling geological time, particularly when a fine-scale orbital chronology is now available for most of the Cenozoic.
3. We have the ‘Cenozoic era’ to express the biologically important interval of time from the Cretaceous/Paleogene boundary to the Recent. It is not evident that “we also need a single term that encompasses both the Paleogene and the Neogene” as Knox et al. suggest. Because the Tertiary is essentially the Cenozoic minus the Quaternary, with the boundary set at an arbitrary level, it has almost no intrinsic coherence either in a palaeoclimatic or biological sense. It is, in effect, a stratigraphic 'wastebasket' left over from recognising the Quaternary as a period.
4. Significant biotic and climatic events mark the Paleogene-Neogene transition, including a supposed glacial episode known as ‘Mi-1’ and the first radiation of some important Neogene fossil groups. However we do acknowledge that, as Knox et al. point out, a more natural place for a mid-Cenozoic ‘break’ might be found in the complex series of events known as the Eocene–Oligocene transition. This, however, seems more of an argument for lowering the Paleogene/Neogene period boundary rather than downgrading them to sub-periods.

In summary, stability is best served by retaining the Paleogene and Neogene as periods and the Tertiary should be left as an informal unit of only historical relevance.

* School of Earth and Ocean Sciences, Cardiff University, Cardiff CF10 3YE. [email protected]; Centre for Environmental Magnetism and Palaeomagnetism, Lancaster Environment Centre, Farrer Avenue, Gordon Manley Building (A26), Lancaster University, Lancaster, UK., LA1 4YQ. [email protected]

• What do you think? Write to Andy Gale [email protected] Chair, Stratigraphy Commission and/or to Ted Nield [email protected] for publication.