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| Segments of rock (strata) inchronostratigraphy | Time spans ingeochronology | 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 | 38 defined, tens of millions of years |
| Stage | Age | 101 defined, millions of years |
| Chronozone | Chron | subdivision of an age, not used by the ICS timescale |
In thestratigraphy sub-discipline ofgeology, aGlobal Standard Stratigraphic Age, abbreviatedGSSA, is a chronological reference point and criterion in thegeologic record used to define the boundaries (an internationally sanctionedbenchmark point) between different geologicalperiods,epochs orages on the overallgeologic time scale in achronostratigraphically useful rock layer. A worldwide multidisciplinary effort has been ongoing since 1974 to define such important metrics. The points and strata need be widespread and contain an identifiable sequence of layers or other unambiguous marker (identifiable or quantifiable) attributes.
GSSAs are defined by theInternational Commission on Stratigraphy (ICS) under the auspices of their parent organization, theInternational Union of Geological Sciences (IUGS), and are used primarily for time dating of rock layers older than 630million years ago, lacking a goodfossil record.
Thegeologic record is more restricted before about 539 million years ago. This is because the Earth's crust in geological time scales is constantly being recycled bytectonic and weathering forces, and older rocks and especially readily accessible exposedstrata that can act as a time calibration are rare.
For more recent periods, aGlobal Boundary Stratotype Section and Point (GSSP), largely based onpaleontology 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 particularstratigraphic 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 and selected criteria.The ICS first attempts to meet the standards of the GSSPs (see below) and if those fail, usually have enough information to make a preliminary selection of several competing GSSA prospects or proposals.