3.2.2 Formation (Formasjon)
3.2.2.1 A formation is a body of rock or superficial deposits occurring as part of a succession. It is characterized by its stratigraphical position in the succession and by a set of characteristic lithological properties that distinguishes it from adjacent units of rock or superficial deposits. Formations often have a three-dimensional shape such as a sheet, slice, lens, wedge or tongue (Figs. 2, 3 and 4).
3.2.2.2 Formation is the fundamental formal unit for lithostratigraphical classification and nomenclature. Formations have a practical use when geological maps are being made and when the geological, geophysical and geotechnical properties of an area and its geological history are being described. A formation can be subdivided into members, and two or more formations can be defined as a group.
3.2.2.3 No limitations are placed on the dimensions of a formation, except that it must be mappable or be traceable in the subsurface (Section 3.2.2.4). Its thickness can vary from less than one metre to several kilometres.
3.2.2.4 A formation must be mappable at the surface or traceable in the subsurface. A formation exposed at the surface has to be mappable on ordinarily available base maps (official topographical maps, land-use maps, or privately produced, but openly available maps). A formation must also be capable of being portrayed on specially constructed maps, or sections on the same scale, intended to show its extent and three-dimensional form in the subsurface. A particularly thin formation can be portrayed as a single, thin line on maps and sections. A formation is mapped on the surface and recorded in the subsurface by ordinary field geological methods, excavation of sections, drilling and geophysical measurements.
3.2.2.5 A formation may consist of (a) a single type of rock or superficial deposit, (b) repetitions of two or more types of rock or superficial deposit, or (c) an inhomogeneous lithology which in itself constitutes a unit relative to adjacent units of rock or superficial deposit. A formation is recognized by at least one lithological property, such as mineral composition, chemical composition, fossil content, structures, grain size and other textural features. The boundaries of a formation may be lithologically sharp, or they may need defining by a change in at least one lithological property if there is a gradual transition between two adjacent lithostratigraphical units (Fig. 2). A formation may be characterized by electrical, thermic, magnetic, radiometric, hydraulic, seismic and other physical characteristics which derive from lithological properties. It may contain two or more unconformity surfaces (Section 3.7.2), which may, for example, be reflected by a lack of continuity in fossil content. The fossil content of a formation may define one or more biozones (Section 3.5.2).
3.2.2.6 A formation is given a formal or an informal name 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 ... Formation" in formal names, e.g. the Kistefjellet Formation, the Frigg Formation, the Båtsfjord Formation (in Norwegian, Kistefjellformasjonen, etc.). Instead of "Formation", the second part of the name can be a brief descriptive designation of the dominant lithology in the formation, for example, "Sandstone", "Clay", "Basalt", "Tuff", "Phyllite", as in, for example, the Moelv Tillite, the Stokkvola Conglomerate (in Norwegian, Moelvtillitten, etc.). These two forms of nomenclature can be used for one and the same formation as formal designations of equal standing. The combination using "Formation" is recommended when it is desirable to emphasize the formal status of the unit.
3.2.2.7 A formation can change character regionally to such a degree that in an area beyond its type area it may be most practical to designate the unit as a member or group (Fig. 4). The proper name of the originally defined formation can be retained there even though its stratigraphical rank is changed. It can even be retained if the lithic component of the name is changed as a result of the rock changing from a non-metamorphosed to a metamorphosed state, e.g. from shale to phyllite (see Section 2.5).
3.2.2.8 Examples: Many formation names are in use in Norwegian geological literature. Many of these are traditionally-used names for units that are more or less well defined on the basis of a type area. Examples of modern, formally defined formations are found in Siedlecka & Siedlecki (1971), Mørk et al. (1982), Nystuen (1982), Pharaoh et al. (1983) and Worsley et al. (1983).
3.2.2.9 Key references: ISSC (1976), NACSN (1983).
