Versuch einer pragmatischen Stratigraphie. Stellungnahme zu einzelnen Fragen der stratigraphischen Nomenklatur
published: Jul 25, 1975
A system of stratigraphic nomenclature and terminology is undoubtedly necessary, but it must be remembered that a too perfectionist system runs the risk of increasing rather than decreasing the confusion. If the number of formal terms becomes too great, their correct use by others than specialists is not assured. A limited number of basic terms placed in context will permit more gradations in expressions than a multitude of rigid classifications. In view of the importance of stratigraphy to Geology as a whole, it would seem particularly necessary not to bar access to this field by an overdeveloped and too dogmatic terminology. The number of these formal "classifications" may easily be limited to the two following: (1) lithostratigraphy s. l. (the "rock stratigraphy" of English authors), (2) geochronological standard scale of relative ages (radiometric dating is not discussed here). The formal units of lithostratigraphy s. 1. are the groups, the formations, etc. in general use, preferably defined by easily observed lithological characteristics. These units give us a system of spatial reference within which to situate all of our stratigraphic observations. Thus it is possible to do without other formal classifications based on the physical or chemical characteristics of the rocks. A scale of relative ages must be based on events in the history of the earth which have left their trace in the rocks. In order for these events to be capable of furnishing us with a means of dating, the traces must, obviously, be specific, that is, they must not be confused with those events which are not synchronous. In the Phanerozoic (with the exception of the Quaternary) only the irreversible evolution of the organic world meets these requirements. In practice, however, one is bothered by the biogeographical and ecological limitations of the fossils and will always be confronted with the problem of concurrent regional zonations. A worldwide standard to define the limits of the major units for the epoch in question, must be based on the most favorable zonation from the practical point of view. The "stratigraphic events" furnished by organic evolution are the appearance and extinction of species. Thanks to the phylogenetic control, the appearance is, in general, a more trustworthy, biochronological criterion. Once the succession of these "stratigraphic events" is well known, the locally belated appearance or exceptionally early disappearance of species, only affects the precision of dating but cannot place the validity of the biochronological scale in doubt. There is no justification of material "chronostratigraphic" units. It is sufficient to indicate the age of the lithostratigraphic units in terms of an immaterial (dating) geochronological scale. If one wishes to distinguish between rocks formed during a given period and an abstract time, this can be done within the context without recourse to two parallel hierarchal scales composed of chronologically congruent divisions bearing the same proper name. By using only the truly customary terms, the geochronological scale would correspond to the hierarchy of the following units: era, system, stage, zone (or chronozone). As to the basic unit, it is the stage for a world standard still to be created, whereas the basic unit of regional chronologies is more readily the zone (chronozone) which can be subdivided into sub-zones. Since the disappearance of one species practically never coincides with the appearance of another, a zone can hardly correspond to the lifetime of its index species. Taking this fact into account, there is no need for a complex terminology of "range zones", "concurrent range zones", etc. A zone is defined at the limits by its "stratigraphic events"; the description of the faunal content (diagnosis) indicates more clearly than a multitude of rigid terms, what it is. The choice of the limit of a zone is a question of practical considerations. Often a characteristic association is stressed but the stratigraphic limits of an association, defying a precise definition, the term "association zone" has no meaning. There is an important difference between lithostratigraphical and geochronological stratotypes. The stratotype of a formation corresponds broadly to a holotype of a species in Zoology. It is the official type-specimen which must remain constant and which does not limit the liberal interpretation of the lateral limits of a formation by another researcher. Except for practical purposes, a geochronological limit, being defined by an organic evolutionary event, does not need a material standard. A complete reference section and a type region must be chosen wherein the limit can be indicated by means of a reference point. The principal advantage of such a procedure lies in the fact that the reference section and its biostratigraphic context, can be studied again independently of the possibly debatable delimitation of a species. This standard (which it is better not to refer to as a stratotype) must, however, remain sufficiently flexible to enable its adaptation to the progress of science as are the standards of the meter and the second. The necessary stability is guaranteed if the modifications of the standard are contingent upon an international accord. The recommendations of the ISSC advocate the strictly invariable chronostratigraphic limitotype (the famous "golden spike") in order for once and for all to have stable limits which will not be affected even by a change in our methods of chronological correlation. In reality there are two aspects to the stability of the geochronological limits: absolute stability is an impossibility. The Silurian/Devonian limit recently defined by a limitotype corresponding to the appearance of Monograptus uniformis in the sequence at Klonk (Bohemia) is an example. Since that time, throughout the whole world, the Silurian/Devonian limit is precisely indicated by the appearance of M. uniformis (insofar as it is can be shown that it is not delayed because of ecological factors, etc.). If one day, however, M. uniformis is found at a lower level at Klonk, the limit must be modified in one way or another. Either one remains true to the "golden spike", then it is necessary to modify, along with all the dependent local correlations, the Silurian/Devonian limit throughout the whole world, or the biochronological definition is respected and then it is only necessary to correct the limit at Klonk. The latter solution seems to me to be the more rational because it guarantees a larger relative stability. On the other hand, the reference point can be useful. In the case of an index species being proved useless due to a premature definition, the reference point would permit the choice of a new index closer to the original limit. A purely descriptive biostratigraphy based on formal units is superfluous. The local stratigraphic distribution of a species is better expressed within the framework of the lithostratigraphic unit and by range charts. Chronostratigraphy, as method, is nothing other than dating of rocks which do not need a surname. The fact that the relative age of the Phanerozoic rocks is expressed in units of a biochronological scale, does not prevent the use of other means of dating them. Inasmuch as (according to the ISSC) no method provides the perfect isochrones, it is not easy to understand to what, in nature, the material chronostratigraphic units correspond. They are fiction, useful from the theoretical point of view, but from which a formal classification of rocks can hardly be derived. The essential is to bear in mind that all datings are only approximations (as are also the quantitative measurements of the exact sciences) where an attempt must be made to estimate the margin of error. Thus chronostratigraphy is reduced to an immaterial geochronological scale for which, here too, no surname is required.