In the stratigraphy sub-discipline of geology, a Global Standard Stratigraphic Age, officially abbreviated GSSA, is a chronological reference point and criterion in the world's rock records used to define the boundaries (an internationally sanctioned benchmark point) between different geological periods, epochs or ages on the overall geologic time scale in a chronostratigraphically useful rock layer. A worldwide multidisciplinary effort has been ongoing since 1974 to define such important metrics. But such work goes slowly despite a lot of effort, as one criterion is such points and strata need be widespread and contain an identifiable sequence of layers or other unambiguous marker (identifiable or quantifiable) attributes.
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 of rock layers older than 630 million years ago, 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, and the ICS may well have to resort to defining additional supplementary GSSAs 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.
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