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:
|“||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.
The 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.
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 Phanerozoic" eonothem (strata) would be discussed as being dated within the "later Phanerozoic" eon, whereas something found in the middle Phanerozoic 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 four eonothems were formed. Oldest to newest these are the Hadean, Archean, Proterozoic, and Phanerozoic.
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.
|<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|
Related other topics
- 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.
Notes and references
- 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