boundary of a stage on the geologic time scale From Wikipedia, the free encyclopedia
Remove ads
A GSSP, more fully a Global Boundary Stratotype Section and Point, is an internationally agreed point which defines the start of a stage on the geologic time scale.
Many GSSPs are marked by golden spikes. This 'golden spike' marks the GSSP for the start of the Ediacaran.
The work is done by the International Commission on Stratigraphy, a part of the International Union of Geological Sciences. As of 2012, 64 of the 101 stages that need a GSSP have been decided.[1]
Remove ads
Rules for GSSP
A geologic section has to follow the rules to be adapted as a GSSP by the ICS. The list below are the rules:[2][3]
A GSSP has to decide the lower boundary (start of a geological period) of a geologic stage.
The lower boundary (start of a geological period) has to be decided using a primary (main) marker (usually first appearance datum of a fossil specie).
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
Sedimentation has to be constant without any changes in facies
The outcrop should not be affected 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.
Because defining a GSSP depends on finding well-preserved geologic sections and identifying key events, this task becomes more harder as one goes farther back in time. Before 640 million years ago, boundaries on the geologic timescale are defined simply by reference to fixed dates, known as "Global Standard Stratigraphic Ages". The oldest GSSP is the one that marks the start of the Ediacaran at 635 million years ago.
