Chronostratigraphy (‘time-rock stratigraphy’) is the branch of stratigraphy concerned with the application of time to rock successions, the ultimatel goal of which is the establishment of a globally applicable standard time-scale. The chronostratigraphical scale originated as a relative one which was been built up, like a jig-saw puzzle, over the last two centuries, primarily through the application of biostratigraphy. Reliable numerical ages have been added gradually to form a parallel, numerical time-scale based on years.
The chronostratigraphical scale and the corresponding divisions of geological time are:
Chronostratigraphical units – Geochronological units
eonothem – eon
erathem – era
system – period
series – epoch
stage – age
chronozone – chron
Chronostratigraphical divisions are ‘time/rock’ units, i.e. they refer to the sequence of rocks deposited during a particular interval of time. Geochronological divisions are the corresponding (abstract) intervals of (continuous) geological time. Thus, it can be said that rocks of the Quaternary System were deposited during an interval of time called the Quaternary Period. This distinction dates back to the early days of geoscience, but the present-day need for it has been challenged (e.g. Harland et al. 1990) and has been reviewed by the Geological Society’s Stratigraphy Commission. However, the International Union of Geological Science’s (IUGS) International Commission on Stratigraphy has ruled that the distiction is important and must stand.
Many of the famiiar chronostratigraphical units were originally defined rather loosely. Hence modern work focuses on the rigorous definition of each component of the time-scale. For the Phanerozoic, it is based on the principle that the base of each chronostratigraphical division should be defined at a specific level in a type section, the Global Stratotype Section and Point (GSSP). The top of a division is automatically defined by the base of the overlying division. The base of a system equates with the lower boundary of the appropriate series or stage. Procedures for defining a GSSP are described by Remané et al. (1996). A ‘golden spike’ is placed at the GSSP, marking the unique place where a specified point in time is indicated. Sections elsewhere are then correlated with this using all possible methods. Hence, the GSSP must be at a level and locality that has maximum possible applicability for global correlation – though in practice no single GSSP is likely to be directly correlatable over the whole world, so that supplementary reference points may be necessary.
The IUGS’s International Commission on Stratigraphy, through its various subcommissions and working groups (for the Quaternary this is the Subcommission on Quaternary Stratigraphy or SQS), is responsible for the international collaboration necessary to achieve global agreement on the choice of individual GSSPs. Once formal proposals are made by the appropriate subcommission they are voted upon by members of the International Commission on Stratigraphy. The results are then published in Episodes, the official journal of the IUGS. So far only a few system, series and stage boundaries have been agreed; although the base of the Quaternary System/Period, and therefore the Pleistocene Series/Epoch is defined at the base of the Gelasian Stage.
Where an existing chronostratigraphical name is formalised, historical priority may be considered in defining a GSSP but will not take precedence over the needs for the GSSP to provide the best possible potential for correlation. It should be noted that the level of the GSSP would not change if further study should show that a particular index fossil appeared earlier in the sequence that originally thought.
The definition of the earlier Precambrian chronostratigraphical divisions follows a different route. Instead, geochronology is applied; for example, the Proterozoic/Archean eon boundary is commonly defined as being at 2500 Ma. Rather than being a GSSP, boundaries defined by an agreed age are termed a Global Standard Stratigraphic Age (GSSA). This dichotomy can potentially raise problems of definition. In particular, the top and base of the Proterozoic Eon will be defined in different ways; the top as a GSSP marking the base of the Cambrian, and the base (probably) at 2500 Ma.
Whichever system of time division is adopted, it is a central precept in chronostratigraphy that the boundaries defined are always isochronous surfaces, i.e. they are the same time everywhere. In other words, all divisions begin and end at precisely the same point in time throughout the World.
Chronostratigraphical terminology in practice
- At any level in the chronostratigraphical hierarchy, an initial capital letter is used for each formal component of the name: e.g. Quaternary System, Wisconsinan Stage.
- Chronostratigraphical units may be divided formally into Lower, Middle and Upper, the corresponding geochronological units into Early, Mid- and Late. Use of lower case initial letters (lower, mid-, etc.) implies an informal usage, either because it is being used more loosely or because a unit may not have been formally divided yet.
Note Some guides recommend the usage of ‘Mid’ for geochronological units, but some authors, particularly those from North America normally use ‘Middle’ (following Hedberg 1976), which thus fails to distinguish geochronological from chronostratigraphical units.
- ‘Series’ should be used only as a chronostratigraphical division and not as a lithological term. Series names should not normally end in the adjectival ‘-ian’, unlike stages.
- Stage names should preferably be based on a geographical name and end in ‘-ian’, e.g. Vistulian, Holsteinian, Rissian, etc.. A stage is the smallest chronostratigraphical unit that normally can be recognised globally, and in the Pleistocene typically has a duration of some 10-100 ky. Currently larger-scale stages, comparable to those defined for the Neogene, are in the process of being identified for the Quaternary. These stages are considerably longer in time that those conventionally recognised at regional scale, and for this reason, these ‘standard stages‘ are sometimes referred to as ‘super-stages‘. Four such stages are being formally defined for the Pleistocene: the Gelasian and Calabrian stages are already ratified, whilst two further stages, preliminarily named ‘Ionian’ (‘Chiban’) and ‘Tarentian’, are in the process of being considered for definition.
- Sub-divisions of stages (and ages; i.e. sub-stages or sub-ages) are widely used in the conventional terrestrial, as well as the marine isotope stratigraphy. Examples of the former include terms such as Early, Middle or Late Weichselian (Wisconsinan/Vistulian/Valdaian), etc.. These divisions are intermediate in scale between the short-duration chronozones and the longer term stages or ages. Further divisions, intermediate between sub-stages (or ages) and chronozones (or chrons), are also in common use, e.g. Upper, Lower Pleniglacial, etc., especially in the last cold stage, i.e. the Weichselian, in Europe. In the marine isotope stratigraphy substages are recognised especially within the predominantly warm-climate events, e.g. Marine Isotope Stage 5, substages a-e, etc.
- Chronozone names are beginning to be formally defined for the Quaternary, and as more high-precision numerical dates become available, it is to be expected that more will be proposed. The most widely identified unit at present is the ‘Younger Dryas Chronozone’. This should not be confused with climatostratigraphical divisions such as the ‘Younger Dryas Stadial’, etc., or biostratigraphical divisions, such as the ‘Younger Dryas biozone’. Whilst such units might coincide in time on occasions, in some instances they might not always do so.
Note All formally defined units must be defined from a type or reference section (or locality). Following standard practice, this is, in effect, definition of the base of the unit concerned, the top of the unit being defined by the base of the subsequent subdivision.
*This guide is based on that produced by Rawson et al. (2002) for the Stratigraphy Commission of the Geological Society of London.