Global Standard Stratigraphic Age
Encyclopedia
In the stratigraphy
sub-discipline of geology
, a Global Standard Stratigraphic Age, officially abbreviated GSSA, is a chronological reference point and criterion in the world's rock records
used to define the boundaries (an internationally sanctioned benchmark
point) between different geological periods, epoch
s 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. But such work goes slowly despite a lot of effort, as one criterion is such 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 (below) 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 , before a good fossil record exists. The ICS first expends large effort 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 at about 542 , and the ICS may well have to resort to defining additional supplementary GSSAs between the two dates. This issue is bound up in the fact that 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
(see the section about selecting the ideal GSSP, in the next link) that can act as a time calibration
For more recent periods, a Global Boundary Stratotype Section and Point
(GSSP), largely based on research progress in geobiology
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.
Stratigraphy
Stratigraphy, a branch of geology, studies rock layers and layering . It is primarily used in the study of sedimentary and layered volcanic rocks....
sub-discipline of geology
Geology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...
, a Global Standard Stratigraphic Age, officially abbreviated GSSA, is a chronological reference point and criterion in the world's rock records
Geologic record
The geologic record in stratigraphy, paleontology and other natural sciences refers to the entirety of the layers of rock strata — deposits laid down in volcanism or by sediment deposition of weathering detritus including all its fossil content and the information it yields about the history...
used to define the boundaries (an internationally sanctioned benchmark
Benchmark
-Geology:*Benchmark , a point of reference for a measurement**Benchmarking , an activity involving finding benchmarks*Benchmark , used in pricing crude oil-Technology:...
point) between different geological periods, epoch
Epoch (geology)
An epoch is a subdivision of the geologic timescale based on rock layering. In order, the higher subdivisions are periods, eras and eons. We are currently living in the Holocene epoch...
s or ages on the overall geologic time scale
Geologic time scale
The geologic time scale provides a system of chronologic measurement relating stratigraphy to time that is used by geologists, paleontologists and other earth scientists to describe the timing and relationships between events that have occurred during the history of the Earth...
in a chronostratigraphically
Stratigraphy
Stratigraphy, a branch of geology, studies rock layers and layering . It is primarily used in the study of sedimentary and layered volcanic rocks....
useful rock layer. A worldwide multidisciplinary effort has been ongoing since 1974 to define such important metrics. But such work goes slowly despite a lot of effort, as one criterion is such 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 (below) are defined by the International Commission on Stratigraphy
International Commission on Stratigraphy
The International Commission on Stratigraphy , sometimes referred to by the unofficial "International Stratigraphic Commission" is a daughter or major subcommittee grade scientific daughter organization that concerns itself with stratigraphy, geological, and geochronological matters on a global...
(ICS) under the auspices of their parent organization, the International Union of Geological Sciences
International Union of Geological Sciences
The International Union of Geological Sciences is an international non-governmental organization devoted to international cooperation in the field of geology.-About:...
(IUGS), and are used primarily for time dating of rock layers older than 630 million years ago , before a good fossil record exists. The ICS first expends large effort 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
Geologic record
The geologic record in stratigraphy, paleontology and other natural sciences refers to the entirety of the layers of rock strata — deposits laid down in volcanism or by sediment deposition of weathering detritus including all its fossil content and the information it yields about the history...
becomes spotty at about 542 , and the ICS may well have to resort to defining additional supplementary GSSAs between the two dates. This issue is bound up in the fact that 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
Stratum
In geology and related fields, a stratum is a layer of sedimentary rock or soil with internally consistent characteristics that distinguish it from other layers...
(see the section about selecting the ideal GSSP, in the next link) that can act as a time calibration
For more recent periods, a Global Boundary Stratotype Section and Point
Global Boundary Stratotype Section and Point
A Global Boundary Stratotype Section and Point, abbreviated GSSP, is an internationally agreed upon stratigraphic section which serves as the reference section for a particular boundary on the geologic time scale. The effort to define GSSPs is conducted by the International Commission on...
(GSSP), largely based on research progress in geobiology
Geobiology
Broadly defined, geobiology is an interdisciplinary field of scientific research that explores interactions between the biosphere and the lithosphere and/or the atmosphere....
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
- Body form
- European Mammal Neogene
- Geologic time scaleGeologic time scaleThe geologic time scale provides a system of chronologic measurement relating stratigraphy to time that is used by geologists, paleontologists and other earth scientists to describe the timing and relationships between events that have occurred during the history of the Earth...
- New Zealand geologic time scaleNew Zealand geologic time scaleWhile also using the international Geologic time scale, many nations - especially those with isolated and therefore non-standard prehistories - use their own system of dividing geologic time into epochs and faunal stages....
- New Zealand geologic time scale
- North American Land Mammal Age
- Fauna (animals)
- Type localityType locality (geology)Type locality , also called type area or type locale, is the where a particular rock type, stratigraphic unit, fossil or mineral species is first identified....
- List of GSSPs
- List of GSSAs
External links
- The Global Boundary Stratotype Section and Point (GSSP): overview
- Chart of The Global Boundary Stratotype Sections and Points (GSSP): chart
- Geotime chart displaying geologic time periods compared to the fossil record - Deals with chronology and classifications for laymen (not GSSA/GSSPs)