|<span style="float:right;"><span class="plainlinksneverexpand">e  h</span></span><center><big>Units in geochronology and stratigraphy</big><ref>Cohen, K.M., Finney, S., Gibbard, P.L. (2013), International Chronostratigraphic Chart, International Commission on Stratigraphy.</ref></center>|
|Segments of rock (strata) in chronostratigraphy||Time spans in geochronology||Notes to<br/>geochronological units|
|<center>Eonothem</center>||<center>Eon</center>||4 total, half a billion years or more|
|<center>Erathem</center>||<center>Era</center>||10 total, several hundred million years|
|<center>Series</center>||<center>Epoch</center>||tens of millions of years|
|<center>Stage</center>||<center>Age</center>||millions of years|
|<center>Chronozone</center>||<center>Chron</center>||subdivision of an age, not used by the ICS timescale|
</noinclude> A chronozone or chron is a slice of time that begins at a given identifiable event and ends at another. In the fossil record such tracer events are usually keyed to disappearance (extinction) of a widely distributed and rapidly changing species or the appearance of such a species in the geological record. Chronozones or chrons are used especially in the various disciplines related to geology, notably in stratigraphy where relative dating is employed.
There are also events susceptible to identification and analysis by the physical sciences, such as Earth's magnetic field reversals or the location of a combination of chemical evidence in a layer corresponding to the meteor strike believed to have caused the extinction of the dinosaurs. Hence chronozones, and the international identification and acceptance of a widespread chronozone as an official useful marker or benchmark of time in the rock record, is non-hierarchical in that chronozones do not need to correspond across geographic or geologic boundaries, nor be equal in length (despite an early constraint that one be defined as smaller than a geological stage). A chronozone is usually defined in geologic terms for a geographical area by fossil names (biozone or biochronozone) or in worldwide terms by geomagnetic reversal identifiers (polarity chronozone).
According to the International Commission on Stratigraphy, chronozone is the term used to refer to the rocks formed during the period of time in question, while the word chron refers to that time period. The key factor in designating an internationally acceptable chronozone is whether the overall fossil column is clear, unambiguous, and widespread. Hence, some accepted chronozones contain others, and certain larger chronozones have been designated which span whole define geological time units, both large and small.
For example, the chronozone known as the Reign of Tiberius (14 to 37 AD) is a subset of the chronozone Imperial Rome. Similarly the chronozone Pliocene is a subset of the chronozone Neogene, and the chronozone Pleistocene is a subset of the chronozone Quaternary.
- Body form
- European Mammal Neogene
- Geologic time scale
- North American Land Mammal Age
- Fauna (animals)
- Type locality
- List of GSSAa
- List of GSSPs
- An early use in Harland, W.B., Armstrong, R.L., Cox, A.V., Craig, L.E., Smith, A.G., and Smith, D.G. (1989) A Geologic Time Scale Cambridge Univ. Press, Cambridge, was hierarchical in that chronozone was used for the slice of time smaller than a Faunal stage defined in Biostratigraphy. This usage has been superseded by the ICS.
- "Magnetostratigraphic polarity units" International Commission on Stratigraphy
- Gehling, James; Jensen, Sören; Droser, Mary; Myrow, Paul; Narbonne, Guy (March 2001). "Burrowing below the basal Cambrian GSSP, Fortune Head, Newfoundland". Geological Magazine 138 (2): 213–218. doi:10.1017/S001675680100509X. 1.
- Hedberg, H.D., (editor), International stratigraphic guide: A guide to stratigraphic classification, terminology, and procedure, New York, John Wiley and Sons, 1976
- International Stratigraphic Chart from the International Commission on Stratigraphy
- USA National Park Service
- Washington State University
- Web Geological Time Machine
- Eon or Aeon, Math Words - An alphabetical index
- The Global Boundary Stratotype Section and Point (GSSP): overview
- Chart of The Global Boundary Stratotype Sections and Points (GSSP): chart
- Geotime chart displaying geologic time periods compared to the fossil record