4 Geological units defined on the basis of time or age
4.1 General
Geological units that are defined on the basis of time or age relationships are (a) bodies of rock or superficial deposits formed during a certain period of time, and (b) time units (Fig. 29). The first category consists of chronostratigraphical (Section 4.2) and polarity-chronostratigraphical units (Section 4.5), whilst time units comprise geochronological (Section 4.3), geochronometrical (Section 4.4), polarity-chronological (Section 4.6), diachronous (Section 4.7) and deformational-diachronous units (Sections 4.8), (see Table 1).
Chronostratigraphical units are usually defined on the basis of selected biostratigraphical type sections, and named after these (Section 3.5), whereas polarity-chronostratigraphical units are erected with the polarity zone (Section 3.4.2) as the physical reference basis. Geochronological units have old-established names (e.g. Palaeozoic, Mesozoic, Tertiary, Quaternary), or are defined and named after chronostratigraphical material units. Geochronometrical units are time units that are defined by time boundaries denoted as a specific number of years before present, independent of physical type or reference sections. Polarity-chronological units are intervals of time defined on the basis of material polarity-chronostratigraphical units. All these units, no matter whether they are material units, time units defined on the basis of a reference succession or units defined purely chronometrically without any material reference unit, have synchronous lower and upper boundaries. This means that each individual boundary is defined as having the same age throughout the world. The individual unit between two such synchronous boundaries is isochronous. Hence, the period separated by the two boundary surfaces is of equal duration worldwide.
A diachronous unit is a time unit that is bounded by non-synchronous lower and upper boundaries. Such a period may be represented by the unequal lengths of time required for deposition of a lithostratigraphical or biostratigraphical unit. Deformational-diachronous units are similarly the unequal lengths of time taken up by deformational events. These again may be defined on the basis of unconformities (Section 3.7.2).
Names of time units and material units that are defined by time and age are to be looked upon as abstract terms or general designations. As these are collective names, initial letters are not capitalized in Norwegian unless other rules take supremacy. When the term is used in English, the initial letter is capitalized. Names such as Precambrian, Silurian, Jurassic, Oligocene and Holocene are therefore written in lower case in Norwegian (i.e. prekambrium, silur, jura, oligocen og holocen).
The descriptions and designations of the various units dealt with in this chapter are arranged in slightly different ways depending upon the distinctive characteristics of the units and the importance placed on them by NSK in this national Code.
