Level (geology)
|
Corresponding units in chronostratigraphy and geochronology |
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| Chronostratigraphy | geochronology |
|---|---|
| Aeonothem | aeon |
| Arathem | era |
| system | period |
| series | epoch |
| step | Age |
The level is the lowest unit (basic unit) of chronostratigraphy in the history of the earth and is used globally. In geochronology it has an equivalent in the unit “age” , which represents the corresponding absolute time interval. The unit level (or age) usually covers periods of several hundred thousand years to several million years, rarely more than ten million years.
definition
The position of chronostratigraphic units within a hierarchy level is described by an above-below relationship. The base of a stage is determined in a GSSP ("Global Boundary Stratotype Section and Point") by the global first appearance or disappearance of an animal or plant species from the fossil record , and / or by the beginning or the end of a magnetic polarity chronozone, and / or a major extinction event and / or an anomaly in the element or isotope distribution and other time areas in the sedimentary rock. The upper limit of a level, on the other hand, is not explicitly defined, but is clearly defined by the base or the lower limit of the subsequent level. A level therefore extends from its "base point" to the "base point" of the next higher level. The chronozones , which can be based on different stratigraphic units ( biostratigraphy , magnetostratigraphy or even lithostratigraphy ), are completely outside the hierarchical levels of chronostratigraphy . The internationally used chronostratigraphic levels are defined and ratified by the ICS (International Commission on Stratigraphy) (see geological 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 The stage in its old meaning is an outdated biostratigraphic unit that was defined by the first appearance of a fossil genus .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 to the unit “age” in geochronology. Since the boundaries of the chronostratigraphic levels can change due to other or newer age determination methods, the absolute age information may only be regarded as approximate.
structure
The level represents the lowest level in the hierarchical system of chronostratigraphy; it is the basic unit. The next unit is the most several stages comprehensive series . A level can, but does not have to, be divided into several sub-levels. The subdivision of a level into lower levels has so far only been made in a few cases (e.g. in the Pliensbachian level of the Jura , where there were historical reasons).
Naming
The levels should be named after local names (e.g. names of villages, towns, regions). In German usage, the stage names are formed by adding the ending - "ium" (in English 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, the GSSP are not (any longer) in the vicinity of the original type locality indicated by the proper name of the unit . The reasons for this can lie in the lack of a suitable profile in the original type region that is accessible for scientific research and yet protected. Other reasons are the incompleteness of the profile or a lack of fossils. The term stage was used in the past e.g. Sometimes also used for other units of chronostratigraphy or stratigraphy in general (e.g. "Lias level": Lias = lithostratigraphic unit or "Jura level": Jura = system ). Individual chronostratigraphic units that were previously treated with the rank of a level have now been raised to the hierarchical rank of a series (e.g. Wenlock ).
Regional and historical levels
In addition to the now internationally recognized and globally correlable levels, numerous other chronostratigraphic levels were defined in the past. The reasons for the proposals were diverse, as were the reasons for the rejection as internationally customary levels or the downgrading to regional levels.
Historical stages
Many historical proposals for steps have not been able to assert themselves in the scientific community for various reasons. The most common reason was that a later investigation found that the type profiles were incomplete, i.e. had gaps in the stratum or did not contain any of the frequently used key fossil groups . However, they may have been used frequently in earlier literature and may also be found in older geological maps. It is therefore still necessary to correlate these older levels as precisely as possible with today's internationally recognized levels.
Regional levels
Regional levels were often introduced because a correlation with the international levels was not possible or only possible to a limited extent. That was z. B. always the case when index fossils, which were used to define an international chronostratigraphic level in the GSSP, do not occur in a particular region. This is basically the case, for example, in continental deposits. An alternative level structure was developed here, based, for example, on the remains of mammals in Neogen ( ELMMZ Neogen = "European Land Mammal Mega-Zones"). However, these levels are defined biostratigraphically and are therefore methodologically very difficult to correlate with the international chronostratigraphic levels. But also political-national reasons or simply a certain historical tradition can be the reason for maintaining a regional chronostratigraphically or biostratigraphically defined level. Frequently used and well-defined regional levels are the regional chronostratigraphic levels in the " Tertiary " of the Paratethys and the North German Tertiary Basin .
literature
- A. Salvador: International Stratigraphic Guide (A Guide to Stratigraphic Classification, Terminology and Procedures). 2nd ed., XIX + 214 pp., International Union of Geological Sciences, Geological Society of America, Washington / DC 1994 ISBN 0-8137-7401-2 .
- North American Commission on Stratigraphic Nomenclature (NACSN): North American stratigraphic code. American Association of Petroleum Geologists Bulletin, 89: 1547-1591, Tulsa, Oklahoma 1983 ISSN 0149-1423 PDF
- FF Steininger & WE Piller: Recommendations (guidelines) for handling the stratigraphic nomenclature. Courier Forschungsinstitut Senckenberg, 209: 1-19, Frankfurt am Main 1999 ZDB -ID 530500-7 .