In
chronostratigraphy, a stage is a
succession of rock strata laid down in a single
age on the
geologic timescale
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 geochron ...
, which usually represents millions of years of deposition. A given stage of rock and the corresponding age of time will by convention have the same name, and the same boundaries.
Rock
series are divided into stages, just as geological epochs are divided into ages. Stages are divided into smaller stratigraphic units called
chronozones or substages, and added together into superstages.
The term faunal stage is sometimes used, referring to the fact that the same
fauna
Fauna (: faunae or faunas) is all of the animal life present in a particular region or time. The corresponding terms for plants and fungi are ''flora'' and '' funga'', respectively. Flora, fauna, funga and other forms of life are collectively ...
(animals) are found throughout the layer (by definition).
Definition
Stages are primarily defined by a consistent set of fossils (
biostratigraphy) or a consistent magnetic polarity (see
paleomagnetism) in the rock. Usually one or more
index fossils that are common, found worldwide, easily recognized, and limited to a single, or at most a few, stages are used to define the stage's bottom.
Thus, for example in the local North American subdivision, a paleontologist finding fragments of the
trilobite ''Olenellus'' would identify the beds as being from the
Waucoban Stage whereas fragments of a later trilobite such as ''Elrathia'' would identify the stage as
Albertan.
Stages were important in the 19th and early 20th centuries as they were the major tool available for dating and correlating rock units prior to the development of
seismology
Seismology (; from Ancient Greek σεισμός (''seismós'') meaning "earthquake" and -λογία (''-logía'') meaning "study of") is the scientific study of earthquakes (or generally, quakes) and the generation and propagation of elastic ...
and
radioactive dating in the second half of the 20th century. Microscopic analysis of the rock (
petrology
Petrology () is the branch of geology that studies rocks, their mineralogy, composition, texture, structure and the conditions under which they form. Petrology has three subdivisions: igneous, metamorphic, and sedimentary petrology. Igneous ...
) is also sometimes useful in confirming that a given segment of rock is from a particular age.
Originally, faunal stages were only defined regionally. As additional stratigraphic and geochronologic tools were developed, they were defined over ever broader areas. More recently, the adjective "faunal" has been dropped as regional and global correlations of rock sequences have become relatively certain and there is less need for faunal labels to define the age of formations. A tendency developed to use European and, to a lesser extent, Asian stage names for the same time period worldwide, even though the faunas in other regions often had little in common with the stage as originally defined.
International standardization
Boundaries and names are established 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) of 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 ...
. As of 2008, the ICS is nearly finished with a task begun in 1974, subdividing the
Phanerozoic eonothem into internationally accepted stages using two types of benchmark. For younger stages, a
Global Boundary Stratotype Section and Point (GSSP), a physical
outcrop
An outcrop or rocky outcrop is a visible exposure of bedrock or ancient superficial deposits on the surface of the Earth and other terrestrial planets.
Features
Outcrops do not cover the majority of the Earth's land surface because in most p ...
clearly demonstrates the boundary. For older stages, a
Global Standard Stratigraphic Age (GSSA) is an absolute date. The benchmarks will give a much greater certainty that results can be compared with confidence in the date determinations, and such results will have farther scope than any evaluation based solely on local knowledge and conditions.
In many regions local subdivisions and classification criteria are still used along with the newer internationally coordinated uniform system, but once the research establishes a more complete international system, it is expected that local systems will be abandoned.
Stages and lithostratigraphy
Stages can include many
lithostratigraphic units (for example
formations,
beds,
members
Member may refer to:
* Military jury, referred to as "Members" in military jargon
* Element (mathematics), an object that belongs to a mathematical set
* In object-oriented programming, a member of a class
** Field (computer science), entries in ...
, etc.) of differing rock types that were being laid down in different environments at the same time. In the same way, a lithostratigraphic unit can include a number of stages or parts of them.
See also
*
European land mammal age
*
Geologic record
*
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 ...
*
North American land mammal age
*
Type locality (geology)
*
List of geochronologic names
*
List of Global Boundary Stratotype Sections and Points
Notes
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 Chartfrom 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 ...
External links
The Global Boundary Stratotype Section and Point (GSSP) overview
chart
deals with chronology and classifications for laymen (not GSSPs)
{{Chronology
Chronostratigraphy
.
Geochronology
Geologic time scales
Geology terminology
Geological units
Paleogeography
Paleobiology
Units of time