4.2 Chronostratigraphical units
4.2.1 General properties and rules
4.2.1.1 Chronostratigraphy is concerned with clarifying the relative or absolute age of stratified bodies of rock or superficial deposits. The aims of chronostratigraphical classification are (a) to provide a framework for temporal correlation, (b) to place rocks and superficial deposits in a systematic age sequence in relation to Earth history, and (c) to construct a Standard Global Chronostratigraphical Scale.
4.2.1.2 Definition -- A chronostratigraphical unit is a stratified body of rock or superficial deposits formed during a specific interval of time. Both lower and upper boundaries are distinct synchronous surfaces. The unit constitutes the material reference for all rocks and superficial deposits formed during the same period of time. A chronostratigraphical unit can be defined on the basis of a biostratigraphical or lithostratigraphical unit.
4.2.1.3 Type section and boundaries -- A chronostratigraphical unit should be erected with reference to a type section that includes the entire unit (unit stratotype, see Section 2.4.5). In such a section, the whole unit should preferably be completely exposed, without any stratigraphical or tectonic break, and containing a continuous fossil record. Because such ideal sections are rarely found, a chronostratigraphical unit can be defined by its lower boundary in a type section for this feature (a basal boundary type - typesnitt for undre grense). The upper boundary of a chronostratigraphical unit should not be defined before the lower boundary for the next younger chronostratigraphical unit has been determined. This will avoid overlaps or gaps in the total chronostratigraphical succession.
When erecting or revising chronostratigraphical units the "general rules for naming and defining geological units" (Chap. 2) are to be followed. Revision or redefinition of a unit of system or higher rank can only be carried out by international agreement in the IUGS Commission on Stratigraphy.
4.2.1.4 Correlation -- Chronostratigraphical correlations are chiefly based on palaeontological techniques. Other, preferably supplementary, correlation methods are the use of remanent magnetism, relative-age criteria and indirect criteria such as climatic changes, changes in eustatic sea level, degree of weathering, and relations to unconformities (Section 3.7.2). It should be emphasized that despite the use of methods such as these, which have limited precision for chronostratigraphical correlations, the boundaries of chronostratigraphical units are to be looked upon as synchronous and in their individuality independent of fossils, lithology, magnetism and other physical properties in the reference succession.
4.2.1.5 Naming -- When chronostratigraphical units are being named the "general rules for naming and defining geological units" (Chap. 2) are to be followed. Names of chronostratigraphical units are collective and are therefore written with lower case initials in Norwegian (Section 4.1). In English, each component of compound names is written as a separate word and is capitalized in formal names except when names are based on trivial taxonomical terms (Section 4.1). Except for chronozones (Section 4.2.7.2), names proposed for new chronostratigraphical units should not duplicate those already used for other geological units.
Adjectives such as Lower (undre), Middle (midtre) and Upper (øvre) may be used for formal and informal chronostratigraphical units. Note that the adjectives Early (tidlig) and Late (sen) should only be used for time units (see Section 4.3.1).
4.2.1.6 Rank and hierarchy -- The hierarchy of chronostratigraphical units, in decreasing order of rank, is eonothem, erathem, system, series and stage (Table 2).
System is the fundamental chronostratigraphical unit. This and units of higher rank have worldwide validity. Systems, erathems and eonothems are always divided into units of next lower rank.
The chronozone is a non-hierarchical, usually lower rank, formal or informal chronostratigraphical unit. The corresponding time division is the chron.
More than one defined stage or chronozone need not together form the next higher ranking unit. Nor need stages and chronozones be defined so that they together form a continuous succession.
The position of the individual unit within the hierarchy is decided by the time interval represented by the unit, not by the thickness or areal extent of the biostratigraphical or lithostratigraphical unit forming the reference basis.
