In the stratigraphy sub-discipline of geology,
a Global Standard Stratigraphic Age, abbreviated
GSSA, is a chronological reference point and
criterion in the geologic record 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.
The 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 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, lacking a good fossil record.
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.
The geologic record becomes spotty prior to
about 542 million years ago.
This is because 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 that can act as a time calibration
are rare.
For more recent periods, a Global Boundary
Stratotype Section and Point (GSSP), largely
based on paleontology 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 ==
European Mammal Neogene
North American Land Mammal Age
Type locality
List of GSSPs
