3.2.3 Group (Gruppe)
3.2.3.1 A group is a stratified body of rock or superficial deposits comprising two or more formations, or a corresponding number of informal lithostratigraphical units.
3.2.3.2 Group is the formal lithostratigraphical unit next in rank above formation. Two or more groups can be defined as a supergroup.
3.2.3.3 The thickness and extent of a group is determined by the total thickness and regional extent of the formations or informal lithostratigraphical units constituting it. A group need not consist of the same formations throughout its area of distribution (Fig. 4).
3.2.3.4 A group is mappable at the surface and traceable in the subsurface with the help of geological and geophysical methods. It is usually a practical lithostratigraphical unit for portrayal on small-scale regional maps.
3.2.3.5 A group may consist of (a) formally defined and named formations, (b) formally defined, named formations along with at least one undefined and informal lithostratigraphical unit, and (c) informal lithostratigraphical units alone, with or without a name or designation, which it may be assumed can be ranked as formations at a later date. The formal or informal lithostratigraphical units in a group are closely related as regards geological evolution and mode of genesis. There may also be a certain similarity in lithology, but this is not essential. The group can be defined as an entity from one or more type section(s) (see Section 2.4.5), without any formations having previously been defined. When the group is erected as an assemblage of already defined formations, its lower and upper boundaries are determined by the lower and upper boundaries of its lowermost and uppermost formations, respectively.
3.2.3.6 Groups are only named formally, this being done in accordance with the "general rules for naming and defining geological units" (Chap. 2). The proper name (Section 2.2.2) is placed between the words "the ... Group", as in the Sassendalen Group, the Løkvikfjellet Group, the Bandak Group (in Norwegian, Sassendalgruppen, etc.). A lithic designation describing the main lithology can be placed between the proper name and the word "Group", but this is not customary. A group that is defined in an area where the formational stratigraphy is established retains its name in areas where formations have still not been formally erected.
3.2.3.7 A group may change character regionally, making it more practical to designate it as a formation in an area away from its type area. The proper name of the originally defined group can be retained even though the stratigraphical rank in such areas is changed to formation. In certain cases it may be desirable to divide the group into subgroups (undergrupper). These can be formal having their own names, or informal having designations such as "lower", "middle" and "upper" (Section 2.3.2).
3.2.3.8 Examples: Examples of modern, formally defined groups are to be found, for example, in Siedlecka & Siedlecki (1971), Mørk et al. (1982), Pharaoh et al. (1983) and Worsley et al. (1983).
3.2.3.9 Key references: ISSC (1976), NACSN (1983).
