2.4.5 Type section, type locality, type area, reference section
The definition of a formal geological unit should be based on its occurrence in one or more type sections, a type locality or a type area (its stratotype). The occurrence of the unit here has to be as far as possible representative for that found throughout its area of distribution.
The type section or stratotype (typesnitt) comprises the unit stratotype (typesnitt for enheten) and boundary stratotype (typesnitt for grensen). The stratotype contains the entire unit, including its lower and upper boundaries. Stratigraphical units of limited vertical extent can be readily defined on the basis of such stratotypes. In the case of thicker stratigraphical units, e.g. a group or supergroup, it will often be difficult to locate a representative section covering the whole unit. The unit can then be defined using a composite stratotype (sammensatt typesnitt) which consists of a boundary stratotype and one or more reference sections (which may also include a unit stratotype). A type section may be a mountain slope, stream section, roadcut or some similar section which it may be assumed will remain generally accessible far into the future. Drill cores, drill logs and excavated sections can form stratotypes for subsurface units (see Section 2.4.12).
The type locality (typelokalitet) is the geographical area containing the boundary stratotype and one or more sections through all or part of the unit. Many well-established units have been given names from type localities or type areas without a stratotype having been defined and described. A reference section (see below) can be erected for such units.
The type area (typeområde) is the more wide-ranging geographical area within which a geological unit has been defined and which may contain the type section and/or type locality. A type area will often be more suitable for defining lithodemic, geomorphological, structural and morphostratigraphical units than one or more type sections. The type area for subsurface units can be defined by a set of drill cores, drill logs and seismic profiles (Section 2.4.12).
A reference section or hypostratotype (referansesnitt) may constitute the unit stratotype for lower-ranking stratigraphical units which, together with a boundary stratotype, defines a higher-ranking stratigraphical unit in a composite stratotype. Reference sections may also be sections that are suitable for (a) demonstrating variation and heterogeneity in a unit, beyond that shown by the type section, (b) replacing a type section that has been destroyed, and (c) describing old-established and named, formal units which lack a type section. A reference section need not be located within the type area of the unit.
ISSC (1976, p. 26) gives specific designations for various kinds of type section.