Thus, chronostratigraphy deals with rock bodies, in opposition to geochronology which deals with the dating and subdivision of geologic time. There is a historical reason for this distinction. Before radioactive decay of natural elements was discovered, stratigraphers had no means to determine the true age of rocks. However, correlation allowed to order geologic events in time basing mostly on biostratigraphy and the superposition principle. A chronostratigraphic scale was thus soon established, where geologic time was subdivided into a hierarchy of chronostratigraphic units of unknown duration.
With the discovery of radioactive decay, radiometric dating techniques became available and the ages of boundaries between chronostratigraphic units could be estimated. A chronostratigraphic scale that integrates absolute ages is called a geologic time scale.
A chronostratigraphic unit includes all rocks deposited during a given interval of geologic time. Thus, a chronostratigraphic unit is a body of rocks, in opposition to geochronologic units which are time intervals.
The fundamental chronostratigraphic unit is the stage. Stages are then grouped into a hierarchical structure of higher rank units, or may be subdivided into substages. Each chronostratigraphic unit has a corresponding geochronologic unit, that can be seen as the time interval during which that chronostratigraphic unit formed.
|Chronostratigraphic units||Geochronologic units|
A sub- or super- prefix may be added to systems/periods or series/epochs if additional ranks are needed, and adjacent stages/ages may be grouped into superstages/superages.
Some units may take their name from the unit of immediately higher order, preceded by "lower", "middle" or "upper". For example, the three series of the Triassic are called Lower Triassic, middle Triassic and upper Triassic (note that only formally defined units are capitalized). Corresponding geochronologic units are called instead early-, mid- and late Triassic.
Formal chronostratigraphic units outside the standard hierarchical scheme may also be defined, and take the name of chronozones. For example: all rocks deposited during the Gauss magnetic polarity interval may be referred as the "Gauss (magneto)chronozone". Note that this example is realistic only because geomagnetic reversals are globally synchronous. A similar example using, e.g., a lithostratigraphic unit would be erroneous.
Definition of chronostratigraphic units
Chronostratigraphic units must be formally defined under the guidelines of the  (International Commission on Stratigraphy), as given in the International Stratigraphic Guide. The base of a stage is defined by a GSSP, and the top of it is defined by the GSSP of the overlying stage. The top of the Holocene series is currently under formation and thus it is not defined. The bases of higher-order chronostratigraphic units are defined by the GSSPs of their lowermost stages. Thus, for example, the bases of the Triassic system and Lower Triassic series are automatically defined by the GSSP of the Induan stage.
The chronostratigraphic scale
The Phanerozoic includes rocks where fossils are locally abundant. Thus, this eon is historically composed of many fine subdivisions, based mostly on biostratigraphic criteria. On the contrary, Precambrian fossils are rare, thus the Precambrian includes only broad subdivisions that are not defined by GSSPs.
The Ediacaran period is indicated in bold because it is defined by a GSSP. The base of the Eoarchean is not defined. All other subdivisions are defined by a GSSA, thus, their base is fixed at an arbitrary age.
Should the distinction between chronostratigraphy and geochronology be maintained?
The distinction between chronostratigraphic (time-rock) and geochronologic (time) units has often been a source of confusion, as stratigraphers sometimes used chronostratigraphic names indifferently for time or rocks. Harland et al. (1990) suggested that the use of GSSPs overcomes the need to maintain this distinction, because a GSSP fixes an event in time. Thus, for example, once the Early Triassic (geochronologic unit) is defined by GSSPs at its base and top, "Early Triassic rocks" precisely refer to all rocks deposited during the Early Triassic (i.e., between the events in time defined by the two GSSPs), and is equivalent to "Lower Triassic" (chronostratigraphic unit). This suggestion later reappeared in the literature, most noticeably in the latest Geologic time scale. An extended discussion of this proposal was given by Zalasiewicz et al. (2004). Basically, the advantages of the abandonment of the distinction between chronostratigraphy and geochronology are that stratigraphic nomenclature would be simplified, and the term "age" could be left for general use without confusion with the name of a geochronologic unit. Mainstream stratigraphers, however, still tend to maintain the distinction.
- Earth science
- Stage (geology)
- Age (geology)
- Geologic time scale
- Geologic ages of earth history
References and notes
- Salvador A.E., 1994 - International Stratigraphic Guide - A guide to stratigraphic classification, Terminology and procedure. Geological Society of America. ISBN 0-8137-7401-2
- The Cenozoic (or Cainozoic) is subdivided into two sub-eras: Tertiary and Quaternary. In the 2004 Geologic time scale, the base of the Quaternary corresponds to the base of the Gelasian (i.e., within the Pliocene). This convention is at odds with traditional use of the term Quaternary, which is usually intended as the Pleistocene+Holocene, and is currently hotly debated (cf. GRADSTEIN et al., 2004)
- The Pennsylvanian and Mississippian have the rank of subsystems. However, all but the last Carboniferous series comprise only one stage and have been omitted from this scheme
- Harland W.B., Armstrong R.L., Cox A.V., Craig L.E., Smith A.G., Smith D.G., 1990 - A Geologic Time Scale, Cambridge University Press, Cambridge, UK.
- Gradstein F.M., Ogg J.G. and Smith A.G., 2004, A Geologic Time Scale 2004, Cambridge University Press, Cambridge, UK.
- Zalasiewicz J., Smith A., Brenchley P., Evans J., Knox R., Riley N., Gale A., Gregory F.J., Rushton A., Gibbard P., Hesselbo S., Marshall J., Oates M., Rawson P., Trewin N., 2004 – Simplifying the stratigraphy of time. Geology, v. 32, pp.1-4.