Global Boundary Stratotype Section and Point

The 'golden spike' (bronze disk in the lower section of the image) or 'type section' of the Global Boundary Stratotype Section and Point (GSSP) for the base of Ediacaran period.

The 'golden spike' marking the Ediacaran GSSP

A Global Boundary Stratotype Section and Point, abbreviated GSSP, is an internationally agreed upon reference point on a stratigraphic section which defines the lower boundary of a stage on the geologic time scale. The effort to define GSSPs is conducted by the International Commission on Stratigraphy, a part of the International Union of Geological Sciences. Most, but not all, GSSPs are based on paleontological changes. Hence GSSPs are usually described in terms of transitions between different faunal stages, though far more faunal stages have been described than GSSPs. The GSSP definition effort commenced in 1977. As of 2012, 64 of the 101 stages that need a GSSP have been formally defined.

Rules

A geologic section has to fulfill a set of criteria to be adapted as a GSSP by the ICS. The following list summarizes the criteria:

• A GSSP has to define the lower boundary of a geologic stage.
• The lower boundary has to be defined using a primary marker (usually first appearance datum of a fossil species).
  • There should also be secondary markers (other fossils, chemical, geomagnetic reversal).
  • The horizon in which the marker appears should have minerals that can be radiometrically dated.
  • The marker has to have regional and global correlation in outcrops of the same age
  • The marker should be independent of facies.
• The outcrop has to have an adequate thickness
• Sedimentation has to be continuous without any changes in facies
• The outcrop should be unaffected by tectonic and sedimentary movements, and metamorphism
• The outcrop has to be accessible to research and free to access.
  • This includes that the outcrop has to be located where it can be visited quickly (International airport and good roads), has to be kept in good condition (Ideally a national reserve), in accessible terrain, extensive enough to allow repeated sampling and open to researchers of all nationalities.

Agreed-upon Global Boundary Stratotype Section and Points

Main article: List of Global Boundary Stratotype Sections and Points

World map of all ratified GSSPs.

The Precambrian-Cambrian boundary GSSP at Fortune Head, Newfoundland is a typical GSSP. It is accessible by paved road and is set aside as a nature preserve. A continuous section is available from beds that are clearly Precambrian into beds that are clearly Cambrian. The boundary is set at the first appearance of a complex trace fossil Treptichnus pedum that is found worldwide. The Fortune Head GSSP is unlikely to be washed away or built over. Nonetheless, Treptichnus pedum is less than ideal as a marker fossil as it is not found in every Cambrian sequence, and it is not assured that it is found at the same level in every exposure. In fact, further eroding its value as a boundary marker, it has since been identified in strata 4m below the GSSP! However, no other fossil is known that would be preferable. There is no radiometrically datable bed at the boundary at Fortune Head, but there is one slightly above the boundary in similar beds nearby. These factors have led some geologists to suggest that this GSSP is in need of reassigning.

Once a GSSP boundary has been agreed upon, a "golden spike" is driven into the geologic section to mark the precise boundary for future geologists (though in practice the "spike" need neither be golden nor an actual spike). The first stratigraphic boundary was defined in 1977 by identifying the Silurian-Devonian boundary with a bronze plaque at a locality called Klonk, northeast of the village of Suchomasty in the Czech Republic. GSSPs are also sometimes referred to as Golden Spikes.

Global Standard Stratigraphic Ages

Main article: Global Standard Stratigraphic Age

Because defining a GSSP depends on finding well-preserved geologic sections and identifying key events, this task becomes more difficult as one goes farther back in time. Before 630 million years ago, boundaries on the geologic timescale are defined simply by reference to fixed dates, known as "Global Standard Stratigraphic Ages".

See also

Units in geochronology and stratigraphy

Segments of rock (strata) in chronostratigraphy	Time spans in geochronology	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	34 defined, tens of millions of years
Stage	Age	99 defined, millions of years
Chronozone	Chron	subdivision of an age, not used by the ICS timescale

• Body form
• European Mammal Neogene
• Fauna (animals)
• Geologic time scale
  • New Zealand geologic time scale
• List of GSSPs
• North American Land Mammal Age
• Type locality

References

• Hedberg, H.D., (editor), International stratigraphic guide: A guide to stratigraphic classification, terminology, and procedure, New York, John Wiley and Sons, 1976
• International Stratigraphic Chart from the International Commission on Stratigraphy
• GSSP table with pages on each ratified GSSP from the ICS Subcommission for Stratigraphic Information
• USA National Park Service
• Washington State University
• Web Geological Time Machine
• Eon or Aeon, Math Words - An alphabetical index

External links

• The Global Boundary Stratotype Section and Point (GSSP): overview
• Chart of The Global Boundary Stratotype Sections and Points (GSSP): chart
• Table of Global Boundary Stratotype Sections and Points (GSSPs) with links to summary pages for each one: chart
• Geotime chart displaying geologic time periods compared to the fossil record - Deals with chronology and classifications for laymen (not GSSPs)