4.3 Geochronological units
4.3.1 General properties and rules
A geochronological unit is a division of time that is traditionally distinguished on the basis of a rock succession and the chronostratigraphical unit that can be deduced from it. A geochronological unit is isochronous, and its beginning and end correspond to the synchronous base and top of the physical reference unit.
The hierarchy of geochronological units, in decreasing order of rank, is eon, era, period, epoch and age. These are shown in Table 2 alongside the corresponding chronostratigraphical units. A chron is the non-hierarchical geochronological unit corresponding to the chronozone. The sum of several ages need not correspond to an epoch or form a continuum.
Names of periods and units of lower rank are identical with those of the corresponding chronostratigraphical units. They are also written in the same way, with a lower-case initial in Norwegian, and capitals in English when used formally. The relevant geochronological unit term replaces the chronostratigraphical term in the formal name. To distinguish chronostratigraphical and geochronological units which have the same proper name, the rank designation should be added (e.g. Jurassic System/Jurassic Period, Dogger Series/Dogger Epoch and Bathonian Stage/Bathonian Age).
In formal and informal geochronological units the terms Early (tidlig), Middle (mellom) and Late (sen) are to be used; these correspond to Lower, Middle and Upper in formal chronostratigraphical units (Section 4.2.1.5).
Key reference: NACSN (1983).
