Stage (stratigraphy)

The stage is the lowermost unit (base unit) of the chronostratigraphy, which is used globally in the Earth's history. It has an equivalent in the unit "age", which is the corresponding absolute time interval in geochronology. The unit level (or age) usually includes periods of several hundred thousand years to several million years, rarely more than ten million years.

Definition

The position chronostratigraphical units within a hierarchy level is described by a top - down relationship. The base of a stage in a GSSP ("Global Boundary Stratotype Section and Point" ), by the global first appearance or disappearance of a plant or animal species from the fossil record, and / or by the beginning or end of a magnetic polarity chronozone and / or a larger extinction event, and / or an anomaly in the element or isotope distribution and other time areas defined in the sedimentary rocks. The ceiling of a stage is, however, not explicitly defined, but is uniquely determined by the base station or the lower limit of the next stage. That is, a stage ranging from its " foot " to the " foot " of the next higher level. Completely outside of the hierarchy levels of chronostratigraphy are the Chrono zones, which may be based on different stratigraphic units ( biostratigraphy, magnetostratigraphy, or even the lithostratigraphy ). The chronostratigraphic levels internationally used are defined by the ICS ( International Commission on Stratigraphy ) and ratified (see Geologic time scale). The stage in its old meaning is an outdated biostratigraphic unit that was defined by the first appearance of a fossil genus. The level corresponds in geochronology the unit " age ". Since the boundaries of chronostratigraphic stages may still change by other or more recent age determination methods, the absolute age information should be treated as only approximately.

Structure

The stage represents the hierarchical system of Chronostratigraphy represents the lowest level; it is the basic unit. The next unit is the most comprehensive series several stages. A step can, but must not be divided into several sub-stages. The subdivision of a stage in the lower grades has been made ​​only in a few cases ( eg in the Pliensbachian stage of the Jura, where historical reasons existed ).

Naming

The naming of the stages is (eg names of villages, towns, regions) carried out by local names. In the German language the stages name by suffixing to be - " ium " formed ( in Engl by the ending - "ian ", French - " ien ", etc.). The oldest stage names date back to the first half of the 19th century. In many cases, however, are the GSSP (no longer) in the vicinity of the given by the proper name of the unit, original type locality. The reasons may lie in the absence of a suitable, accessible to scientific investigation and still protected profile in the original type region. Other reasons are the incompleteness of the profile or fossil poverty. The term stage was partly commonly used in the past for other units of chronostratigraphy and stratigraphy (eg " Lias - level": Lias = lithostratigraphic unit or " Jurassic stage ": Jura = System). Individual stratigraphic units that were previously treated with the rank of a step today have been raised in the hierarchical rank of a series (eg, the Wenlock ).

Regional and historical levels

In addition to the now internationally recognized and globally correlatable levels many more, of their definition, chronostratigraphic stages have been defined in the past. The reasons for the proposals were diverse, as are the reasons for rejection as common international stages or the downgrade to regional levels.

Historical levels

Many historical proposals for stages have the scientific operation can not enforce a variety of reasons. The most common reason was that it turned out in a later study that the type profiles were incomplete, so had hiatuses or did not contain the Leitfossilgruppen frequently used. Maybe they were but is often used in the earlier literature and are found under certain circumstances in older, geological maps. The most accurate correlation of these older levels with today's internationally recognized levels is therefore still necessary.

Regional levels

Regional levels were often introduced because a correlation with the international levels was not or only partially possible. This was, for example, always the case when index fossils that have been used to define an international chronostratigraphic stage in the GSSP, do not occur in a particular region. This is, for example, in continental deposits generally the case. Here is an alternative steps outline was developed, based on the example of the Neogene to mammalian residues ( ELMMZ Neogene = " European Land Mammal Mega - Zones". These levels, however, are biostratigraphically defined and therefore methodologically related very difficult to correlate with the international chronostratigraphic stages. But political and national reasons, or simply a specific historical tradition may be the cause for the maintenance of a regional chronostratigraphic or biostratigraphically defined level. frequently used and well- defined regional levels are the regional chronostratigraphic stages in the " tertiary " the Paratethys and the North German tertiary basin.