Global Standard Stratigraphic Age
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Global_Standard_Stratigraphic_Age".

In the Stratigraphy sub-discipline of Geology, a Global Standard Stratigraphic Age, officially abbreviated GSSA, is a chronological reference point and criteria in the world's rock records used to define the boundaries (a internationally sanctioned benchmark point) between different geological periods, epochs or ages on the overall geologic time scale in a chronostratigraphically useful rock layer. A world wide multidisciplinary effort has been ongoing since 1974 to define such important metrics. But such work goes slowly despite a lot of effort, as one criteria is such points and strata need be widespread and contain a identifiable sequence of layers or other unambiguous marker (identifiable or quantifiable) attributes.

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GSSAs, and the generally more recent and preferred benchmark GSSPs (below) are defined by the International Commission on Stratigraphy (ICS) under the auspices of their parent organization, the International Union of Geological Sciences (IUGS), and are used primarily for time dating rock layers older than 630 million years ago (Ma), before a good fossil record exists. The ICS first expends large effort 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.

The geologic record becomes spotty at about 542 Ma ([[mya (unit)|Millions-of-years-ago), and the ICS may well have resort to defining additional supplementary GSSA's between the two dates. This issue is bound up in the fact that the Earth's crust in geological time scales is constantly being recycled by tectonic and weathering forces, and older rocks and especially readily accessible exposed strata (See the section about selecting the Ideal GSSP, in the next link) that can act as a time calibration

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 and selected criteria.

See also

Multidiscipline comparison

e  h
Units in geochronology and stratigraphy
Chronostratigraphic
strata units of the
geologic record
Geochronological
units of the
geological time system
 Main article: geologic time scale

Notes
Eonothem
Eon
 4 total, largest spans of years
Erathem
Era
  12 total, roughly span multiples
of a hundred million years
System
Period
  Some familiar names, defined in
21 major divisions and 2 minor
Series
Epoch
 48 units & each spanning
tens of millions of years
Stage
Age
 100+ units & most all
spanning millions of years
Outside the hierarchy, units cross stage and age boundaries as needed
if the strata has good dating characteristics.
Chronozone Chron  only in more recent strata*
* Most and the most time specific time divisions[1]


Related other topics

External links

Notes and references

  1. ^ International Commission on Stratigraphy. "International Stratigraphic Chart". Retrieved on 2008-06-17.

References
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