In the

stratigraphy Stratigraphy is a branch of geology concerned with the study of rock layers (strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks. Stratigraphy has three related subfields: lithost ...

sub-discipline of

geology Geology (). is a branch of natural science concerned with the Earth and other astronomical objects, the rocks of which they are composed, and the processes by which they change over time. Modern geology significantly overlaps all other Earth ...

, 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,

epoch In chronology and periodization, an epoch or reference epoch is an instant in time chosen as the origin of a particular calendar era. The "epoch" serves as a reference point from which time is measured. The moment of epoch is usually decided b ...

s or ages on the overall

geologic time scale The geologic time scale or geological time scale (GTS) is a representation of time based on the rock record of Earth. It is a system of chronological dating that uses chronostratigraphy (the process of relating strata to time) and geochro ...

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 are defined by the

International Commission on Stratigraphy The International Commission on Stratigraphy (ICS), sometimes unofficially referred to as the International Stratigraphic Commission, is a daughter or major subcommittee grade scientific organization that concerns itself with stratigraphy, strati ...

(ICS) under the auspices of their parent organization, the

International Union of Geological Sciences The International Union of Geological Sciences (IUGS) is an international non-governmental organization devoted to global cooperation in the field of geology. As of 2023, it represents more than 1 million geoscientists around the world. About Fo ...

(IUGS), and are used primarily for time dating of rock layers older than 630 million years ago, lacking a good

fossil record A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

. The geologic 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 by

tectonic Tectonics ( via Latin ) are the processes that result in the structure and properties of the Earth's crust and its evolution through time. The field of ''planetary tectonics'' extends the concept to other planets and moons. These processes ...

and weathering forces, and older rocks and especially readily accessible exposed

strata In geology and related fields, a stratum (: strata) is a layer of Rock (geology), rock or sediment characterized by certain Lithology, lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by v ...

that can act as a time calibration are rare. For more recent periods, a

Global Boundary Stratotype Section and Point A Global Boundary Stratotype Section and Point (GSSP), sometimes referred to as a golden spike, is an internationally agreed upon reference point on a stratigraphic section which defines the lower boundary of a stage on the geologic time scale. ...

(GSSP), largely based on

paleontology Paleontology, also spelled as palaeontology or palæontology, is the scientific study of the life of the past, mainly but not exclusively through the study of fossils. Paleontologists use fossils as a means to classify organisms, measure ge ...

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 A stratigraphic section is a sequence of layers of rocks in the order they were deposited. It is based on the principle of original horizontality, which states that layers of sediment are originally deposited horizontally under the action of ...

. 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.

References

* * Hedberg, H.D., (editor), ''International stratigraphic guide: A guide to stratigraphic classification, terminology, and procedure'', New York, John Wiley and Sons, 1976 *

External links

The Global Boundary Stratotype Section and Point (GSSP)
overview

chart

- Deals with chronology and classifications for laymen (not GSSA/GSSPs) {{Chronology Geologic time scales of Earth Units of time Earth sciences Geochronology Stratigraphy

See also

References

External links