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Principes de subdivision stratigraphique de l’histoire de la terre ; le cas du Quaternaire, sa signification et son rang hiérarchique

Principles of subdivision of the earth history, the case of the Quaternary, significance and hierarchical ranking
Gilles S. Odin, Nicole Limondin-Lozouet and Jean-Philip Brugal
p. 9-22

Abstracts

Throughout the history of geology, stratigraphical units have been identified and named according to the observation that macrofossils evolved over time from one deposit to the next. For example, the Aptian Stage is named after the particular fossils observed in deposits near Apt, SE France (Orbigny, 1840 in Moullade et al., 2006) or the Campanian Stage named after the deposits of the Champagne Charentaise area, SW France (Coquand, 1857 in Neumann and Odin, 2001). The sections selected as typical for a stage are called stratotypes.

Thus, for deposits where fossils are common, the fundamental stratigraphical unit is the Stage which most commonly corresponds to a particular fauna found in a given three-dimensional-formation. The quite constant mechanism of evolution has led to the fact that those Stages have duration of the same order of magnitude (3 to 10 Ma) from the Cambrian System to the Neogene System, which lends to the concept of Stage an approximate time-significant value.

These Stages are grouped in higher ranking units according to the significance of the cuts in terms of changes in the biosphere. These changes are reported in figure 1 according to modern information gathered by Lethiers (1998). It is clear that the pioneer stratigraphers recognised most of the main cuts in the biological evolution and used them as major unit boundaries (Era or System).

So, from the practical point of view, historical Stages are defined as strata characterised by a particular fauna. This practice has led to some difficulties as far as the location of the boundaries is concerned. Often, the apparently “new fauna” results from transgressive deposits on the continental platform following a sedimentary break (and corresponding lack of record, see case 2, fig. 2). In a few other cases, the successive Stages defined in different basins include contemporaneous deposits with different faunas due to endemism or environmental differences (case 1, fig. 2).

In order to solve these problems, it was first accepted that boundaries would be defined at the base of the historical stratotypes; in this situation, all deposits, known or unknown, and older than those located at the base of the stratotype pertain to the previous Stage. The remaining problem is that deposits immediately older than the boundary were not necessarily documented in the stratotypes.

In order to solve the latter problem, stratigraphers (mainly represented by experts of the Phanerozoic interval of time) have decided to create a new kind of convention using the concept of Global Standard Stratotype Section and Point (GSSP or PSM –Point Stratotypic Global– in French, see fig. 3). According to this new convention, Stage boundaries (instead of Stage bodies) become the key for defining Stages. The GSSP defines a Stage by a point selected in a section where deposition is continuous and the Stage located above the point is defined by this point (Remane et al., 1996). According to this approach, the boundary becomes perfectly defined with deposits able to characterise the geological history above and below.

French speaking stratigraphers have discussed this kind of definition (see Odin et al., 2004, 2005) and suggested that a single GSSP is not enough for the full definition of a stratigraphical unit. A complete definition needs three conventions : 1- the GSSP for the lower (older) boundary, 2- the GSSP for the upper (younger) boundary and 3- the historical stratotype which gives its name to the unit and generally documents the major portion of the unit’s content.

This new approach using GSSP is interesting in that the distinct concepts of time on the one hand and strata deposited during (and documenting) this time on the other hand are fully coincident. This coincidence suggests that a single terminology is sufficient for designating time and rock units (Zalasiewicz et al., 2004) i.e., the differentiation previously made in some languages between rock units (Erathem, System, Series, Stage, Lower and Upper sub Stages), and the corresponding time units (Era, Period, Epoch, Age, Early and Late sub Stages, see fig. 5) is no longer necessary. The question is : which are the best words to be selected ? Zalasiewicz et al. (2004) would prefer the words shown in the right hand column (fig. 5) while Odin et al. (2004, 2005) would recommend the words shown in bold type in the same figure because they are distinct from the confusing words of the common language (such as epoch or period) and because they have long been used in many languages (upper, lower), even when geological time is concerned.

These conventions are worked out within the International Union of Geological Sciences Organisation (IUGS). Within it, the International Commission of Stratigraphy is the appropriate body in charge for these conventions regarding stratigraphical units. This Commission is organised in sub commissions which create Working Groups each in charge for a particular convention (fig. 6). When a 60 % majority vote is obtained for a proposal within a Working Group, it is voted by the parent sub Commission, and the proposal is submitted to the Commission which votes and the accepted convention is submitted to IUGS for ratification. Within this organisation, a comparatively small number of people is usually able to significantly influence the work and decision of a large number of experts. An example of practical GSSP is given in figure 7.

The conventions discussed above are mainly valid and of easy application for the fossil-bearing deposits, those which are the subject of interest of a majority of stratigraphers. However, the stratigraphical tools (the techniques of investigation documented in the deposits) applicable in the rocks are not the same in the deposits of different geological age (fig. 8). From that point of view, there are three distinct stratigraphies (Odin, 1994) : 1- the one where Stages are applicable for subdividing the geological history using the presently accepted and fully relevant concept of GSSP ; there, there is an abundant documentation by fossils (biostratigraphy is the key unequivocal dating tool) ; 2- for older time, the key unequivocal dating tool is geochronology ; accordingly, the appropriate sub commission on Precambrian Stratigraphy decided to select numerical ages for definition of conventional unit boundaries (Plumb, 1991) ; 3- for the Quaternary history, there is a large variety of particular tools often applicable in particular deposits or more or less local areas each one being able to generate its own scale (fig. 10). In this situation, the best use of the information suggests that each particular tool may be accepted for subdivision of the geological history. The use of a single series of integrated boundaries would not be easily applicable because all tools do not necessarily locate cuts at the same place while each kind of cut is an interesting piece of information. The correlation between the diversified columns may be achieved with more or less precise connection, using the known age (in years) of the key events. As a result, the Quaternary interval of time does not need unique conventions but is better based on evolving knowledge. According to us, conventions which are constraining and fixed concepts should thus be replaced by knowledge which gives a wider freedom for adapting to evolving information.

The Quaternary is not similar to the older stratigraphical units. It is short but its hierarchical ranking must first depend on the significance one wants to give to it. Taking into account the major role of the biosphere in the previous subdivision of the geological history, the ranking of the Quaternary unit should reflect the considerable change in the process of evolution of life brought when the genus Homo appears. From that point of view, the history of the biosphere may be subdivided as shown in figure 9. Looking at this scheme, it is clear that there is a single taxon (man) which is able to influence the whole biosphere to its benefit (?) for the first time on Earth. This is not a detail of the history. The key role of this species is an observation but it is also necessary to point out this key role for becoming conscious of its responsibility with regard to its environment for the future. Taking this into account, the stratigraphical unit where man becomes a major actor of evolution must have the highest ranking, at least at the level of an Era (see fig. 5) distinct from the Cenozoic (or Tertiary) or possibly a sub Era if this kind of unit is useful in order to conform to the proposal of Pillans and Naish (2004). This means that the base of the Quaternary must cut all previous units of lower ranking in the hierarchy and cannot be part of a previous System as suggested in the proposal of inclusion in the Neogene System considered by Clague (2006).

The question of the location of the lower boundary of the Quaternary is different from that of its ranking. As far as the nature of the key character of the unit (influence of man on the biosphere) is concerned, there are plenty of units of the time scale for which the key factor is NOT perceptible immediately above the base of the unit (trilobites do not appear at the base of the Palaeozoic, large dinosaurs do not appear at the base of the Mesozoic; mammals do not dominate at the base of the Tertiary). However, it would be advisable to locate the base of the Quaternary near the “guide event” related to the funding of the influence of man on biosphere which is the appearance of the conceptual thinking documented in the deposits by the first man artefacts about 2,5 Ma ago (Semaw et al., 1997, 2003).

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References

Bibliographical reference

Gilles S. Odin, Nicole Limondin-Lozouet and Jean-Philip Brugal, « Principes de subdivision stratigraphique de l’histoire de la terre ; le cas du Quaternaire, sa signification et son rang hiérarchique », Quaternaire, vol. 18/1 | 2007, 9-22.

Electronic reference

Gilles S. Odin, Nicole Limondin-Lozouet and Jean-Philip Brugal, « Principes de subdivision stratigraphique de l’histoire de la terre ; le cas du Quaternaire, sa signification et son rang hiérarchique », Quaternaire [Online], vol. 18/1 | 2007, Online since 01 March 2010, connection on 04 March 2021. URL : http://journals.openedition.org/quaternaire/937 ; DOI : https://doi.org/10.4000/quaternaire.937

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About the authors

Gilles S. Odin

Géochronologie et Sédimentologie Océanique, Université Pierre & Marie Curie, 4 Place Jussieu, 75252 Paris Cedex 05. Courriel : gilodin@ccr.jussieu.fr

Nicole Limondin-Lozouet

Laboratoire de Géographie physique CNRS, 1 Place A. Briand, 92195 Meudon Cedex. Courriel : Nicole.Limondin@cnrs-bellevue.fr

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Jean-Philip Brugal

UMR 6636 du CNRS, Maison Méditerranéenne des Sciences de l’Homme, 5 rue du Château de l’Horloge, BP 647,13094 Aix-en-Provence cedex 2. Courriel : brugal@mmsh.univ-aix.fr

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