Formation (geology)
The formation in geology is the terrain easily identifiable and in a geological map easily visualized rock unit that the detailed description and breakdown of the rock strata in a region used. The formation is the basic unit in lithostratigraphy . It can be further subdivided as well as combined with further formations to form larger units. Its definition is independent of chronostratigraphy , biostratigraphy and geochronology .
Requirements for a formation
A formation should differ lithologically clearly from spatially or temporally neighboring lithostratigraphic units. Furthermore, a formation in a geological map must be able to be represented as an independent rock body or mapping unit on a scale of at least 1: 10,000 . This requires certain minimum values with regard to the spatial extent of the corresponding rock body.
definition
A formation is defined by a type profile at a very specific location ( type locality ). The boundaries to the neighboring rock units are only determined by changing the lithology, not geochronologically, chrono- or biostratigraphically. To define a formation, morphological , sedimentological , petrological , mineralogical , paleontological as well as chemical or physical features can be used. First of all, fossils are viewed as a lithological feature. The name of a formation is in two parts and is usually formed from the name of the type locality or a historical term with the addition of the term "formation" (separated by a hyphen) (e.g. Mergelstetten formation after Mergelstetten or Opalinus Clay formation = old historical term; both examples from the southern German Jura ).
The descriptive name of a formation must be unique, which is why a locality name (or historical name) can only be used once. Like all other lithostratigraphic units, formations are included in name directories that are kept by the geological services of the respective federal states.
In Austria, new lithostratigraphic units are published by the Federal Geological Institute in Vienna and included in the “Lithstrat” database of the Austrian Academy of Sciences , the “Stratigraphy” working group of the Austrian Geological Society and the Federal Geological Institute. In Germany, the Federal Institute for Geosciences and Natural Resources (BGR) in Hanover maintains an online database of Germany's lithostratigraphic units ( LithoLex ).
structure
A formation can - but does not have to - be subdivided into sub-formations ( members ), which in turn can be subdivided into individual banks . Several formations can be combined into groups .
Historical names
In the older literature, e.g. Even in today's popular scientific literature, the lithostratigraphically determined term “formation” is often used blurred and / or for completely different units in geology. Formation was often synonymous with the chronostratigraphic terms system , series or z. T. also level , used (“Jura Formation”, “Lias Formation” or “Tithon Formation”). The term “formation” was also used to designate larger lithostratigraphic units (e.g. “Subapennine Formation”), which are now referred to as being much higher in the hierarchy of lithostratigraphic units.
Conversely, lithostratigraphic units, which are now referred to as formation or subformation, were often referred to as “ series ” or “layer”. In particular, the term “series” should no longer be used in the sense of the current meaning of formation (or subformation), since it is reserved for a chronostratigraphic unit.
literature
- Fritz F. Steininger , Werner E. Piller: Recommendations (guidelines) for handling the stratigraphic nomenclature. In: Courier Research Institute Senckenberg 209, 1999, ZDB -ID 530500-7 , pp. 1-19.
