Geological period

Short description: Subdivision of geological time; shorter than an era and longer than an epoch

A geological period is one of the several subdivisions of geologic time enabling cross-referencing of rocks and geologic events from place to place. It is the fundamental unit in the hierarchy of divisions into which geologists have split the Earth's history. Periods are combined into eras and may themselves be subdivided into epochs.[1]

The rocks formed during a period belong to a chronostratigraphic unit called a system.[2]

## Contents

* 1 Structure
* 2 Correlation issues
* 3 See also
* 4 References

## Structure

The twelve currently recognised periods of the present eon – the Phanerozoic – are defined by the International Commission on Stratigraphy (ICS) by reference to the stratigraphy at particular locations around the world.[3] In 2004 the Ediacaran Period of the latest Precambrian was defined in similar fashion, and was the first such newly designated period in 130 years.[4]

A consequence of this approach to the Phanerozoic periods is that the ages of their beginnings and ends can change from time to time as the absolute age of the chosen rock sequences, which define them, is more precisely determined.[5]

The set of rocks (sedimentary, igneous or metamorphic) that formed during a geological period is known as a system; for example, the 'Jurassic System' of rocks was formed during the 'Jurassic Period' (between 201 and 145 million years ago).[2]

The following table includes all currently recognized periods. The table omits the time before 2500 million years ago, which is not divided into periods.

Eon | Era | Period  | Extent, Million
Years Ago  | Duration, Millions
of Years

|
|
|
|

Phanerozoic | Cenozoic | Quaternary (Pleistocene/Holocene) | 2.588–0  | 2.588+
Neogene (Miocene/Pliocene) | 23.03–2.588  | 20.4
Paleogene (Paleocene/Eocene/Oligocene) | 66.0–23.03  | 42.9
Mesozoic | Cretaceous | 145.5–66.0  | 79.5
Jurassic | 201.3–145.0  | 56.3
Triassic | 252.17–201.3  | 50.9
Paleozoic | Permian | 298.9–252.17  | 46.7
Carboniferous (Mississippian/Pennsylvanian) | 358.9–298.9  | 60
Devonian | 419.2–358.9  | 60.3
Silurian | 443.4–419.2  | 24.2
Ordovician | 485.4–443.4  | 42
Cambrian | 541.0–485.4  | 55.6
Proterozoic | Neoproterozoic | Ediacaran | 635.0–541.0  | 94
Cryogenian | 720–635  | 85
Tonian | 1000–720  | 280
Mesoproterozoic | Stenian | 1200–1000  | 200
Ectasian | 1400–1200  | 200
Calymmian | 1600–1400  | 200
Paleoproterozoic | Statherian | 1800–1600  | 200
Orosirian | 2050–1800  | 250
Rhyacian | 2300–2050  | 250
Siderian | 2500–2300  | 200

Units in geochronology and stratigraphy[6] | Segments of rock (strata) in chronostratigraphy | Time spans in geochronology | Notes to
geochronological units

|
|

Eonothem | Eon | 4 total, half a billion years or more
Erathem | Era | 10 defined, several hundred million years
System | Period | 22 defined, tens to ~one hundred million years
Series | Epoch | 34 defined, tens of millions of years
Stage | Age | 99 defined, millions of years
Chronozone | Chron | subdivision of an age, not used by the ICS timescale

## Correlation issues

In a steady effort ongoing since 1974, the International Commission on Stratigraphy has been working to correlate the world's local stratigraphic record into one uniform planet-wide benchmarked system.[7]

American geologists have long considered the Mississippian and Pennsylvanian to be periods in their own right though the ICS now recognises them both as 'subperiods' of the Carboniferous Period recognised by European geologists.[8] Cases like this in China, Russia and even New Zealand with other geological eras has slowed the uniform organization of the stratigraphic record.[9]

Notable changes

* Changes in recent years have included the abandonment of the former Tertiary Period in favour of the Paleogene and succeeding Neogene periods. This remains controversial.[10]
* The abandonment of the Quaternary period was also considered but it has been retained for continuity reasons.[11]
* Even earlier in the history of the science, the Tertiary was considered to be an 'era' and its subdivisions (Paleocene, Eocene, Oligocene, Miocene and Pliocene) were themselves referred to as 'periods'[12] but they now enjoy the status of 'epochs' within the more recently delineated Paleogene and Neogene periods.[3]

## See also

* Geologic time scale

## References

1. ↑ Jackson, Julia A., ed (1997). "period [geochron]". Glossary of geology. (Fourth ed.). Alexandria, Viriginia: American Geological Institute. ISBN 0922152349.
2. ↑ 2.0 2.1 Jackson 1997, "system [stratig]".
3. ↑ 3.0 3.1 "International Commission on Stratigraphy". 2021. https://stratigraphy.org/.
4. ↑ Knoll, A. H.; Walter, MR; Narbonne, G. M; Christie-Blick, N (30 July 2004). "A new period for the geologic time scale". Science 305 (5684): 621–622. doi:10.1126/science.1098803. PMID 15286353. http://www.stratigraphy.org/bak/ediacaran/Knoll_et_al_2004b.pdf.
5. ↑ Gradstein, Felix; Ogg, James; Schmitz, Mark et al., eds (2012). The Geologic Time Scale. Elsevier B.V.. ISBN 978-0-444-59425-9.
6. ↑ Cohen, K.M.; Finney, S.; Gibbard, P.L. (2015), International Chronostratigraphic Chart, International Commission on Stratigraphy, http://www.stratigraphy.org/ICSchart/ChronostratChart2015-01.pdf .
7. ↑ Martinsson, Anders; Bassett, Michael G. (1980). "International Commission on Stratigraphy". Lethaia 13 (1).
8. ↑ Davydov, V.I.; Korn, D.; Schmitz, M.D.; Gradstein, F.M.; Hammer, O. (2012), "The Carboniferous Period" (in en), The Geologic Time Scale (Elsevier): pp. 603–651, doi:10.1016/b978-0-444-59425-9.00023-8, ISBN 978-0-444-59425-9, https://linkinghub.elsevier.com/retrieve/pii/B9780444594259000238, retrieved 2021-06-17
9. ↑ Lucas, Spencer G. (6 November 2018). "The GSSP Method of Chronostratigraphy: A Critical Review". Frontiers in Earth Science 6: 191. doi:10.3389/feart.2018.00191.
10. ↑ Knox, R.W.O’B.; Pearson, P.N.; Barry, T.L.; Condon, D.J.; Cope, J.C.W.; Gale, A.S.; Gibbard, P.L.; Kerr, A.C. et al. (June 2012). "Examining the case for the use of the Tertiary as a formal period or informal unit". Proceedings of the Geologists' Association 123 (3): 390–393. doi:10.1016/j.pgeola.2012.05.004.
11. ↑ Gibbard, Philip L.; Smith, Alan G.; Zalasiewicz, Jan A.; Barry, Tiffany L.; Cantrill, David; Coe, Angela L.; Cope, John C. W.; Gale, Andrew S. et al. (28 June 2008). "What status for the Quaternary?". Boreas 34 (1): 1–6. doi:10.1111/j.1502-3885.2005.tb01000.x.
12. ↑ See, for example, Sahni, B. (1940). "Presidential Address: The Deccan Traps: An Episode of the Tertiary Era". Current Science 9 (1): 47–54.

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Chronology

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Related topics|

* Chronicle
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