It can be easily understood that it is important that a GSSP is selected in a continuous stratigraphic section, without obvious hiatusses. After a completed proposal is approved by a stratigraphic subcommision it is submitted to the ICS for approval and ratification by IUGS; the final step is formal publication of the ratified proposal, generally and preferably in the IUGS journal Episodes.
Overview of Global Boundary Stratotype Sections and Points (GSSP's)
Discussion
It is now 25 years ago that the first stratigraphic boundary was defined by a boundary stratotype or 'golden spike', inaugurating the concept of the Global Boundary Stratotype Section and Point (GSSP). This event of historic proportions for chronostratigraphy and geochronology involved the boundary between the Silurian and Devonian Periods, or rather the lower limit of the Devonian, at a locality called Klonk in Czechoslowakia.
The problem of the Silurian-Devonian boundary and its consensus settlement in the Klonk section, hinged on a century old debate, known as the 'Hercynian Question' that touched many outstanding geoscientists of the previous century. The issue came to the foreground after 1877, when Kaiser stated that the youngest stages (étages) of Barrande's 'Silurian System' in Bohemia, corresponds to the Devonian System in the Harz Mountains of Germany and other regions. Kaiser's findings contrasted with the conventional 19th century wisdom that graptolites became extinct at the end of the Silurian. Eventually, it became clear that socalled Silurian graptolites in some sections occur together with socalled Devonian fossils in other sections, leading to the modern consensus that graptolites are not limited to Silurian strata.
A bronze plaque in the Klonk outcrop shows the exact position of the modern Silurian-Devonian Boundary, that also represents the base of the Lochkovian Stage, which is the lowest stage in the Devonian. The base of the Lochkovian Stage is defined by the first occurrence of the Devonian graptolite Monograptus uniformis in bed # 20 of the Klonk Section, NE of the village of Suchomasty. The lower Lochkovian index trilobites with respresentatives of the Warburgella rugulosa group occur in the next younger limestone bed # 21 of that section.
At present over 40 boundary stratotypes have been defined (see table 1), but there are almost 60 more Phanerozoic stages without ratified base definition. As may be seen from the listing in charts, the stratigraphic communities studying the Silurian, Devonian and Neogene have embraced the GSSP concept much more rapidly than for example the Paleogene or the Mesozoic ones, where few stage boundaries at present have been ratified by IUGS. Many GSSP's may take considerable debate before a consensus decision is reached.
The reasons that not all stages already have boundary stratotypes are varied, but three main factors that play a role are that
- for each stage boundary many sections in the world may have to be studied in detail,
- many stratigraphers hope to find the criteria in the rock record for the perfect GSSP definition, rather an adequate and practical one, and
- selection of a GSSP in a working group may temporary bog down in historic dogma.
The latter point ignores that there is no rule of historical priority in stratigraphy, and that all stratigraphy ultimately is guided by subjective consensus. There is an abundance of occasions where a set of zones traditionally associated with one stage eventually were re-assigned to the next underlying or overlying stage. Hence, beds assigned to such zones were re-assigned a younger or older age. Preference for stratigraphic priority is desirable when selecting GSSP's, but subsidiary to scientific and practical merit. The search for perfection also ignores the fact that emendations of a GSSP definition are feasible under IUGS rules, using consensus voting on emended proposals.
The philosophy behind the boundary stratotype or GSSP is to define a level in an outcrop section where time and rock coincide, and thus define successive stages with precisely defined criteria for their lower and upper limits in relative time. In contrast, stage stratotypes define a body of rock typical for the unit stage. If in the next decade(s) all Phanerozoic stages have GSSP's for their lower limits, a stratigraphic continuum will have been defined that to the best of our knowledge is at the same time chronostratigraphic and geochronologic in nature.
The concept of the GSSP has gained acceptance among those stratigraphers that consider it a pragmatic and practical solution to the common problem that conventional stage type sections inevitably leave gaps, or lead to overlap between succesive stages. The boundary stratotype very much relies on the notion that it is possible to arrive at accuracy in correlation through the use of events, like a geomagnetic reversal, a global change in a stable isotope value, or the evolutionary appearance of one or more prominent and widespread pelagic fossil taxa. Thus, the limits of a stage can now be defined with multiple event criteria that to the best of our current knowledge have a narrow errorbar in rock and in time correlation, rather than with a (rock) unit stratotype. Delimiting successive stages in a clear and practical manner enhances their value as standard units in chronostratigraphy and ultimately in geochronology. Without standardized units chronostratigraphy cannot exist.
This is not to say that the classical concept of the stage stratotype has suddenly become obsolete, and should be abolished. Although the GSSP concept is not ruled by priority regulation, going back through the historic notions of how and where a stage was originally conceived, defined and correlated, sheds valuable light on the geologic meaning and correlative content of stages and the historic notion of their boundaries. However, it is clear that at a time where our scientific viewpoints are becoming increasingly more global, stage stratotypes have more regional rather than global significance. We may consider the stage stratotype as a body of rock with a typical lithology, cycle and fossil content in a particular basin.
An interesting exception with a regional historical precedent for a basal disconformity in a GSSP is the lower limit of the Zanclean Stage and Pliocene Epoch, ratified in 2000. Because the onset of the Pliocene in the Mediterranean traditionally is the marine transgression following the Messinian drying out of that region, the GSSP section for the Miocene/Pliocene boundary also reflects such.
Due to the fact that most of the Proterozoic lacks adequate fossils for correlation, a different type of boundary stratotype was needed for that interval of time on earth. The new boundary stratotype is called Global Standard Stratigraphic Age (GSSA). The definition of a boundary by its linear age is the consequence of the fact that the the Proterozoic now recognizes units of global stratigraphic subdivision, where the boundaries are defined in terms of the age in millions of years. However, although there appears to be consensus that the subdivision of the Proterozoic in three Eras - Paleoproterozoic, Mesoproterozoic and Neoproterozoic is excellent, the finer Period subdivisions often contain no datable rocks, which makes their use haphazard. It is our opinion that abstract Period definitions that cannot be sustained by the Precambrian rock record on Earth should be avoided.
It may be desirable to add some documentation to a GSSP proposal about the historical significance of a new stage GSSP in the hierarchy of stratigraphic units of higher rank. Careful consideration needs to be given to the fact that e.g. the GSSP for the base of the Quaternary Period in the Vrica Section in southern Italy, automatically defines the boundary of the Pliocene-Pleistocene, the base of the Pleistocene Epoch, and the base of the Calabrian Stage. In the same vein, the base of the Cambrian Period as defined in a GSSP in the Fortune Head section in southwest Newfoudland, also defines the base of the Nemakitian-Daldynian Stage, often considered to be the lowest stage in the Cambrian. Since the Cambrian by stratigraphic consensus is considered to be the lowest Period in the Paleozoic Era and the Paleozoic the lowest major unit in the Phanerozoic Eon, the Fortune Head GSSP also defines the lower boundary of these major chronostratigraphic unit. It is along these lines of logical stratigraphic reasoning that definition of a GSSP should entail some historical background research, to ensure that the criteria of accepted stratigraphic hierarchy are not violated, but subsidiary to scientific and practical merit.
[ Overview on the International Stratigraphic Chart ]
[ The Global Boundary Stratotype Section and Point (GSSP) ]
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