Group (geology)
The term group is used in geology for a lithostratigraphic unit superordinate to the formation . The key to classifying a series of formations as a group is a close spatial relationship and some commonality in terms of general deposition conditions. Therefore, often all formations of a sedimentary basin or at least a certain phase of the basin development are grouped together. The group is the second highest unit in the hierarchy of lithostratigraphic units and can be combined with other groups to form a supergroup (complete sequence of several neighboring sedimentary basins or a very large basin). Its definition is independent of the other branches of geoscientific stratigraphy , such as chronostratigraphy , biostratigraphy or sequence stratigraphy .
definition
A group is defined by the type profiles of the individual associated formations in a certain region (type region). The limits are only set by changing the lithology , not geochronologically or chrono- or biostratigraphically. The lower limit of a group is also the lower limit of the lowest formation that is included in a group. It represents a very marked change in lithology (for example the change from the red sandstone group to the shell limestone group; change from terrestrial to marine sedimentation ). How powerful a series of rocks must be in order to be called a group is not defined. It is indirectly defined by the premises of the unit “formation” in such a way that it comprises several rock units that must be represented on a geological map 1: 10,000. As a rule, a group is several hundred to over a thousand meters thick.
structure
Several groups can be combined to form a supergroup. A group can (but does not have to) be further subdivided into subgroups. It always consists of at least one, usually of several formations, which in turn can be made up of several sub-formations and banks .
designation
The name of a group should be in two parts and is usually formed from the name of the type region or a historical term with the addition of the term group (in German separated by a hyphen) (e.g. Muschelkalk group). The descriptive name of a group must be unique, for example a specific locality name (or a historical name) can only be used once.
Like all other lithostratigraphic units, groups are included in a register of existing lithostratigraphic names 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).
Historical names
In the past, lithostratigraphic units, which are now understood as groups (or super- and sub-groups), were called formations, systems, periods and other terms. These terms are either used for other ranks of the lithostratigraphic hierarchical level (for example for formation) or they are terms from chronostratigraphy or geochronology. The term “group” was previously used in geology to mean a collection of “layers” (for example “Mesozoic layer group”). This is no longer permissible in the sense of the official stratigraphic guidelines in order to exclude confusion with the lithostratigraphic term “group”.
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.
Individual evidence
- ↑ Guidelines for the handling of the stratigraphic nomenclature Chronostratigraphy and Lithostratigraphy , Lithstrat database ( Memento of the original from January 12, 2008 in the Internet Archive ) Info: The archive link has been inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. . Retrieved April 26, 2008
