Stage (stratigraphy)
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chronostratigraphy Chronostratigraphy is the branch of stratigraphy that studies the ages of rock strata in relation to time. The ultimate aim of chronostratigraphy is to arrange the sequence of deposition and the time of deposition of all rocks within a geologica ...
, a stage is a succession of rock strata laid down in a single
age Age or AGE may refer to: Time and its effects * Age, the amount of time someone or something has been alive or has existed ** East Asian age reckoning, an Asian system of marking age starting at 1 * Ageing or aging, the process of becoming older ...
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 geochr ...
, 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 Series may refer to: People with the name * Caroline Series (born 1951), English mathematician, daughter of George Series * George Series (1920–1995), English physicist Arts, entertainment, and media Music * Series, the ordered sets used in ...
are divided into stages, just as geological epochs are divided into ages. Stages can be divided into smaller stratigraphic units called
chronozone A chronozone or chron is a unit in chronostratigraphy, defined by events such as geomagnetic reversals (magnetozones), or based on the presence of specific fossils (biozone or biochronozone). According to the International Commission on Stratig ...
s. (See chart at right for full terminology hierarchy.) Stages may also be divided into substages or indeed grouped as superstages. The term faunal stage is sometimes used, referring to the fact that the same
fauna Fauna is all of the animal life present in a particular region or time. The corresponding term for plants is ''flora'', and for fungi, it is '' funga''. Flora, fauna, funga and other forms of life are collectively referred to as '' biota''. Zoo ...
(animals) are found throughout the layer (by definition).


Definition

Stages are primarily defined by a consistent set of fossils (
biostratigraphy Biostratigraphy is the branch of stratigraphy which focuses on correlating and assigning relative ages of rock Stratum, strata by using the fossil assemblages contained within them.Hine, Robert. “Biostratigraphy.” ''Oxford Reference: Dictiona ...
) or a consistent magnetic polarity (see
paleomagnetism Paleomagnetism (or palaeomagnetismsee ), is the study of magnetic fields recorded in rocks, sediment, or archeological materials. Geophysicists who specialize in paleomagnetism are called ''paleomagnetists.'' Certain magnetic minerals in rock ...
) in the rock. Usually one or more
index fossil Biostratigraphy is the branch of stratigraphy which focuses on correlating and assigning relative ages of rock Stratum, strata by using the fossil assemblages contained within them.Hine, Robert. “Biostratigraphy.” ''Oxford Reference: Dictiona ...
s 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 Trilobites (; meaning "three lobes") are extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the At ...
''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 and the propagation of elastic waves through the Earth or through other ...
and
radioactive dating Radiometric dating, radioactive dating or radioisotope dating is a technique which is used to date materials such as rocks or carbon, in which trace radioactive impurities were selectively incorporated when they were formed. The method compares ...
in the second half of the 20th century. Microscopic analysis of the rock (
petrology Petrology () is the branch of geology that studies rocks and the conditions under which they form. Petrology has three subdivisions: igneous, metamorphic, and sedimentary petrology. Igneous and metamorphic petrology are commonly taught together ...
) is also sometimes useful in confirming that a given segment of rock is from a particular age. Originally, faunal stages were only defined regionally; however, as additional stratigraphic and geochronologic tools were developed, stages were defined over broader and 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 referred to unofficially as the "International Stratigraphic Commission", is a daughter or major subcommittee grade scientific daughter organization that concerns itself with stratigra ...
(ICS) of the
International Union of Geological Sciences The International Union of Geological Sciences (IUGS) is an international non-governmental organization devoted to international cooperation in the field of geology. About The IUGS was founded in 1961 and is a Scientific Union member of the Inte ...
. As of 2008, the ICS is nearly finished a task begun in 1974, subdividing the
Phanerozoic The Phanerozoic Eon is the current geologic eon in the geologic time scale, and the one during which abundant animal and plant life has existed. It covers 538.8 million years to the present, and it began with the Cambrian Period, when anima ...
eonothem into internationally accepted stages using two types of benchmark. For younger stages, a
Global Boundary Stratotype Section and Point A Global Boundary Stratotype Section and Point (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 b ...
(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. Features Outcrops do not cover the majority of the Earth's land surface because in most places the bedrock or superficial ...
clearly demonstrates the boundary. For older stages, a
Global Standard Stratigraphic Age In the stratigraphy sub-discipline of geology, a Global Standard Stratigraphic Age, abbreviated GSSA, is a chronological reference point and criterion in the geologic record used to define the boundaries (an internationally sanctioned benchmark po ...
(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 Lithostratigraphy is a sub-discipline of stratigraphy, the geology, geological science associated with the study of stratum, strata or rock layers. Major focuses include geochronology, comparative geology, and petrology. In general, strata are ...
units (for example formations,
beds A bed is an item of furniture that is used as a place to sleep, rest, and relax. Most modern beds consist of a soft, cushioned mattress on a bed frame. The mattress rests either on a solid base, often wood slats, or a sprung base. Many be ...
,
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 The European Land Mammal Mega Zones (abbreviation: ELMMZ, more commonly known as European land mammal ages or ELMA) are zones in rock layers that have a specific assemblage of fossils (biozones) based on occurrences of fossil assemblages of Europe ...
*
Geologic record The geologic record in stratigraphy, paleontology and other natural sciences refers to the entirety of the layers of rock strata. That is, deposits laid down by volcanism or by deposition of sediment derived from weathering detritus (clays, sand ...
*
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 geochrono ...
*
North American land mammal age The North American land mammal ages (NALMA) establishes a geologic timescale for North American fauna beginning during the Late Cretaceous and continuing through to the present. These periods are referred to as ages or intervals (or stages when re ...
*
Type locality (geology) Type locality, also called type area, is the locality where a particular rock type, stratigraphic unit or mineral species is first identified. If the stratigraphic unit in a locality is layered, it is called a stratotype, whereas the standard o ...
*
List of geochronologic names This is a list of official and unofficial names for time spans in the geologic timescale and units of chronostratigraphy. Since many of the smallest subdivisions of the geologic timescale were in the past defined on regional lithostratigraphic unit ...
*
List of Global Boundary Stratotype Sections and Points This is a list of Global Boundary Stratotype Sections and Points. Since 1977, Global Boundary Stratotype Sections and Points (abbreviated GSSPs) are internationally agreed upon reference points on stratigraphic sections of rock which define the ...


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 Chart
from the
International Commission on Stratigraphy The International Commission on Stratigraphy (ICS), sometimes referred to unofficially as the "International Stratigraphic Commission", is a daughter or major subcommittee grade scientific daughter organization that concerns itself with stratigra ...


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