Eonothem

Eonothem

In stratigraphy and geology, an eonothem is the totality of rock strata laid down in the stratigraphic record deposited during a certain eon of the continuous geologic timescale. The Eonothem is not to be confused with the eon itself, which is a corresponding division of geologic time spanning a specific amount of (millions of) years, during which rocks were formed that are classified within the eonothem. In practice, the rock column is discontinuous:

cquote|Technically, a complete geologic record doesn't occur anywhere. For such a record to develop would require the area to have been receiving sedimentary deposits continually ever since the origin of the earth. Nowhere is such a situation known to exist. If it did exist, we could not effectively look at the strata because they would still be buried, and modern strata would continue to be deposited on top of them.I2The earth's surface has been far too dynamic to allow that to occur anywhere. No area has been in such a static condition throughout the earth's long history. Areas that have had sediment deposited on them at one time are later uplifted and eroded. In some places this has occurred many times. There is ample evidence to prove such a sequence of events. [cite web|accessdate=2008-06-21
title=An Overview of the Geologic Record|author=Richard Burky, ©1990 by the Worldwide Church of God.
url=http://www.wcg.org/lit/booklets/science/burky2.htm
]

Eonothems, despite discontinuities (locally missing strata or unconformities), can be compared to others where the rock record is more complete and by correlation of points of correspondence be fixed appropriately within the eon. Eonothems are therefore useful as a broad chronostratigraphic unit, specifying approximate age within the timelines within the rock column.

Eonothems are subdivided into erathems and their smaller subdivisions within geology and paleobiology and their sub-fields, and a whole system of cross-disciplinary classification by strata is in place with oversight by the International Commission on Stratigraphy. Since oldest rocks are deposited first and lowest in a stratigraphic section, whether one is discussing the rock record is usually clear in context (A fossil or feature present in the "upper Phanerzoic" eonothem (strata) would be discussed as being dated within the "later Phanerzoic" eon, whereas something found in the middle Phanerzoic could be discussing a layer, stage or the relative time).

Eonothems are not often used in practice as expert dating estimates can be and usually are specified into the more refined timelines of smaller chronostratigraphic units, which can be subdivided in turn down to the many defined stages, the smallest units used in dating. (see the hierarchy of comparative units, five each for time division types and five for the rock record types.)

Eonothems have the same names as their corresponding eons, which means during the history of the Earth only three eonothems were formed. Oldest to youngest these are: the Archean, Proterozoic and Phanerozoic.

Dating standards

GSSAs are defined by the International Commission on Stratigraphy and are used primarily for time dating rock layers older than 630 million years ago (mya), before a good fossil record exists. The record becomes spotty at about 542 mya, and the ICS may well have resort to defining additional GSSA's between the two dates.

For more recent periods, a Global Boundary Stratotype Section and Point (GSSP), largely based on research progress in geobiology and improved methods of fossil dating is used to define such boundaries. In contrast to GSSAs, GSSPs are based on important events and transitions within a particular stratigraphic section. In older sections, there is insufficient fossil record or well preserved sections to identify the key events necessary for a GSSP so GSSAs are defined based on fixed dates.

ee also

Multidiscipline comparison

*Chronostratigraphy
*Lithostratigraphy
*Geologic record

Related other topics

* Body form
* Fauna (animals)
* Type locality

External links

* [http://www.stratigraphy.org/over.htm The Global Boundary Stratotype Section and Point (GSSP)] : overview
* [http://www.stratigraphy.org/gssp.htm Chart of The Global Boundary Stratotype Sections and Points (GSSP)] : chart
* [http://www.rocksandminerals.com/geotime/geotime.htm Geotime chart displaying geologic time periods compared to the fossil record.]

Notes and references

References

* Hedberg, H.D., (editor), "International stratigraphic guide: A guide to stratigraphic classification, terminology, and procedure", New York, John Wiley and Sons, 1976
* [http://www.stratigraphy.org/cheu.pdf International Stratigraphic Chart] from the International Commission on Stratigraphy
* [http://www2.nature.nps.gov/geology/usgsnps/gtime/gtime1.html USA National Park Service]
* [http://astro.wsu.edu/worthey/earth/html/md08.html Washington State University]
* [http://www.ucmp.berkeley.edu/help/timeform.html Web Geological Time Machine]
* [http://www.pballew.net/arithm16.html#aeon Eon or Aeon] , [http://www.pballew.net/etyindex.html Math Words - An alphabetical index]


Wikimedia Foundation. 2010.

Игры ⚽ Поможем решить контрольную работу

Look at other dictionaries:

  • Erathem — In stratigraphy, paleontology, geology, and geobiology an erathem is the total stratigraphic record deposited during a certain corresponding span of time, an era in the geologic timescale. NOTOC It can therefore be used as a chronostratigraphic… …   Wikipedia

  • Geologic time scale — This clock representation shows some of the major units of geological time and definitive events of Earth history. The Hadean eon represents the time before fossil record of life on Earth; its upper boundary is now regarded as 4.0 Ga.[1] Other… …   Wikipedia

  • System (stratigraphy) — NOTOC A system in the natural sciences and stratigraphy is an idealized composite unit of the geologic record made up of a succession of rock layers that were laid down together within a certain corresponding geological time span, and are used in …   Wikipedia

  • Geologic record — The layer cake like appearance of concordant strata laid down over hundreds of thousands of years …   Wikipedia

  • Stage (stratigraphy) — In chronostratigraphy, a stage is a succession of rock strata laid down in an single age on the geologic timescale, which usually represents millions of years of deposition. A given stage of rock and the corresponding age of time will by… …   Wikipedia

  • Geochronology — A schematic depiction of the major events in the history of our planet. Geochronology is the science of determining the age of rocks, fossils, and sediments, within a certain degree of uncertainty inherent to the method used. A variety of dating… …   Wikipedia

  • Chronostratigraphy — is the branch of stratigraphy that studies the age of rock strata in relation to time. The ultimate aim of chronostratigraphy is to arrange the sequence of deposition and the time of deposition of all rocks within a geological region, and… …   Wikipedia

  • Chronozone — For articles dealing with the fundamentals of geologic chronology, see: Biostratigraphy#Concept of zone, chronology (geology), and geologic time scale e  h Units in geochronology and stratigraphy …   Wikipedia

  • Series (stratigraphy) — Series are subdivisions of rock layers made based on the age of the rock. A series is therefore a chronostratigraphic unit, unrelated to lithostratigraphy, which divides rock layers on their lithology. Series are subdivisions of systems and are… …   Wikipedia

  • Äon (Geologie) — Korrespondierende Einheiten in Chronostratigraphie und Geochronologie Chronostratigraphie Geochronologie Äonothem Äon Ärathem Ära System Periode Serie …   Deutsch Wikipedia

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”