A chronozone or chron is a unit in chronostratigraphy, defined by events such as geomagnetic reversals (magnetozones), or based on the presence of specific fossils (biozone or biochronozone). According to the International Commission on Stratigraphy, the term "chronozone" refers to the rocks formed during a particular time period, while "chron" refers to that time period.[1]

Although non-hierarchical, chronozones have been recognized as useful markers or benchmarks of time in the rock record. Chronozones are non-hierarchical in that chronozones do not need to correspond across geographic or geologic boundaries, nor be equal in length. Although a former, early constraint required that a chronozone be defined as smaller than a geological stage. Another early use was hierarchical in that Harland et al. (1989) used "chronozone" for the slice of time smaller than a faunal stage defined in biostratigraphy. [2] The ICS superseded these earlier usages in 1994.[3]

The key factor in designating an internationally acceptable chronozone is whether the overall fossil column is clear, unambiguous, and widespread. Some accepted chronozones contain others, and certain larger chronozones have been designated which span whole defined geological time units, both large and small. For example, the chronozone Pliocene is a subset of the chronozone Neogene, and the chronozone Pleistocene is a subset of the chronozone Quaternary.

Units in geochronology and stratigraphy[4]
Segments of rock (strata) in chronostratigraphy Time spans in geochronology Notes to
geochronological units
EonothemEon4 total, half a billion years or more
ErathemEra10 defined, several hundred million years
SystemPeriod22 defined, tens to ~one hundred million years
SeriesEpoch34 defined, tens of millions of years
StageAge99 defined, millions of years
ChronozoneChronsubdivision of an age, not used by the ICS timescale

See also

References

  1. ↑ Salvador, Amos, ed. (1994). "Chapter 8. Magnetostratigraphic polarity units". Stratigraphic Guide (Second (abridged) ed.). International Commission on Stratigraphy. Archived from the original on 12 June 2020.
  2. ↑ 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
  3. ↑ Poulson, Niels (August 2004). "Book Announcement: The Jurassic rocks of Denmark and East Greenland" (PDF). International Subcommission on Jurassic Stratigraphy Newsletter. No. 31. pp. 27–30, page 29. Archived (PDF) from the original on 24 June 2021.
  4. ↑ Cohen, K.M.; Finney, S.; Gibbard, P.L. (2015), International Chronostratigraphic Chart (PDF), 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. Bibcode:2001GeoM..138..213G. doi:10.1017/S001675680100509X. S2CID 131211543. 1.
  • Hedberg, H.D., (editor), International stratigraphic guide: A guide to stratigraphic classification, terminology, and procedure, New York, John Wiley and Sons, 1976
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