Group (stratigraphy)

The term group is used in geology for the formation parent lithostratigraphic unit. Crucial for the classification of a series of formations as a group is a close spatial relationship and a certain commonality in terms of the general conditions of deposition. Hence, often all information of a sediment basin or at least a certain stage of basin development are combined into one group. The group is the second largest unit in the hierarchy of lithostratigraphic units and can be summarized together with other groups to form a supergroup (complete sequences of several adjacent sedimentary basins, or a very large basin ). Your definition is independent of the other branches of geoscience stratigraphy as the chronostratigraphy, biostratigraphy, or sequence stratigraphy.


A group is defined by the type profiles of the individual associated formations in a given region (type region). The limits are set only by changes in lithology, not geochronological or chrono- or biostratigraphically. The lower limit of a group is at the same time, the lower limit of the bottom formation, which is counted as a group. It represents a very marked change in lithology (for example, the change from the red sandstone group to the limestone group; switch from terrestrial to marine sedimentation). How powerful a rock sequence must be so that it is referred to as a group, is not defined. It is indirectly through the premises of the unit " formation" as defined in that it includes several rock units that need to be represented on a geological map 1:10,000. In general, a group of several hundred to over a thousand meters thick.


Several groups can be combined to form a supergroup. A group can be ( but need not ) be further divided into subgroups. It always consists of at least one, usually consist of several formations, which in turn builds up from individual banks.


The name of a group should be in two parts and is usually (separated by a hyphen in German ) from the name of the type region or a historical concept with the addition of the term group be formed (eg limestone ) group. The telling a group name must be unique, for example, a particular locality name (or a historical name ) can be used only once.

Groups welcome as any other lithostratigraphic units in an inventory of existing lithostratigraphic names that are guided by the geological surveys of the respective federal states. In Austria, new lithostratigraphic units of the Geological Survey in Vienna and will be published in the database " Lithstrat " of the Austrian Academy of Sciences, received the Working Group " stratigraphy " of the Austrian Geological Society and the Geological Survey. In Germany, the Federal Institute for Geosciences and Natural Resources ( BGR) in Hannover maintains an online database of the lithostratigraphic units of Germany ( LithoLex ).

Historic designations

Previously lithostratigraphic units, which are now considered as a group (or super- and sub-groups ), referred to as formations, systems, periods, and other terms. These terms are used either for other ranks of the lithostratigraphic hierarchy level (for example, for the formation ) or there are terms from Chronostratigraphy or geochronology. The term 'group' has previously been used in the geology as defined in part a collection of "layers" (e.g., " Mesozoic layers group "). This is no longer permitted as defined in the official stratigraphic guidelines to avoid confusion with the lithostratigraphic term " group ".