Type section

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.[1]

Rules for GSSP

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:[2][3]

  • 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 GSSPs

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World map of all ratified GSSPs.[4]