4.7 Diachronous units
4.7.1 General properties and rules
4.7.1.1 Definition and use -- A diachronous geological time unit comprises the unequal spans of time represented by the deposition of a lithostratigraphical, biostratigraphical, pedostratigraphical or morphostratigraphical unit, or an assemblage of two or more units within one of these categories. A diachronous time unit represents the time span of a geological event (hendelse). In a geological sense an event is therefore a period of time characterized by certain geological processes (e.g. sedimentation, volcanism, folding, glacier advance, etc.) within a specified area.
Diachronous classification of geological time units provides a basis for (a) comparing the spans of time represented by stratigraphical units with non-synchronous boundaries at different localities; (b) determining the beginning and end of deposition of a stratigraphical unit at different localities; (c) assessing how quickly a unit was deposited over a given area; (d) determining and comparing the speed of deposition of a unit at different localities; and (e) comparing various temporal and spatial relationships in diachronous stratigraphical units.
A diachronous time unit is only valid within the area in which the physical reference unit occurs, or in which a stratigraphical unit can be found that is correlatable with the reference unit.
The relationship between diachronous, chronostratigraphical and geochronological units is shown in figure 30.
4.7.1.2 Boundaries and erection of formal units -- The boundaries of a diachronous unit are the times recorded by the beginning and end of deposition of the reference unit at a given place (Figs. 30, 31). These times may be determined using chronostratigraphical methods or numerical age determinations. One or both of the boundaries are time-transgressive. They are defined by a number of boundary reference sections that enable the age of the boundary to be determined (Fig. 31).
Formal diachronous units are erected in accordance with the "general rules for naming and defining geological units" (Chap. 2). In addition to the requirements stated in Section 2.4.6, several reference sections must be specifically identified and described to document the temporal and spatial variations.
Formal diachronous units should only be defined and named if this serves a useful purpose. Informal designations can be used to refer to diachronous intervals encompassing the deposition of a stratigraphical unit.
4.7.1.3 Units, rank and hierarchy -- The diachron (diakronen) is the fundamental, non-hierarchical, formal diachronous unit. If a hierarchy of formal diachronous units is needed, the following terms are recommended, in decreasing order of rank, Episode (episode), Phase (fase), Span (trinn) and Cline (skifte) (Fig. 31).
A diachronous unit is given rank in the hierarchical classification system according to its relative duration and importance (Fig. 31). Diachronous units may therefore vary greatly in duration -- even units having the same rank. One and the same diachronous unit may also have the same duration at different localities, even though it started and ended at different points in time at the individual localities.
4.7.1.4 Naming -- Diachronous units are given formal and informal names in accordance with the "general rules for naming and defining geological units" (Chap. 2). The proper name (Section 2.2.2) is a geographical name (or an alternative name in the case of the continental shelf). If the diachronous unit covers the period spanning the deposition of a single stratigraphical unit, the proper name of this depositional unit can also be used as the proper name of the derived diachronous unit. If the diachronous unit encompasses several formal stratigraphical units, the proper name of the diachron must not repeat any of the proper names of the reference units covered by the diachron. In such a case, the superior diachronous unit is given a new proper name.
In English, the proper name is combined with the word "Diachron", or the relevant rank designation, using capitals for the initial letters in formal names. In Norwegian, the term is written in one word without a capital letter (see Section 4.1). To refer to diachrons informally, the designations "event" ("hendelse" or "begivenhet") and "time" ("tid") may be used. The formal terms "episode", "phase", "span" and "cline" may also be used informally, but are never then combined with a formal proper name. An informal designation of a diachronous period of time might, for example, be "Rjukan time" (rjukantid). This means the variable period of time during which the Rjukan Group formed.
