3.5 Biostratigraphical units
3.5.1 General properties and rules
Biostratigraphy is concerned with describing and classifying sediments and sedimentary rocks on the basis of fossil content. A biostratigraphical unit is a body of sediment or sedimentary rock which is defined solely on the basis of its fossil content.
Fossils that define a formal biostratigraphical unit must be of the same age as the horizon or sequence they occur in. Reworked fossils that were originally deposited in older beds must not be used in formal biostratigraphical classification. Great care must be taken when studying well material, where risk of contamination is high.*
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*Reworked fossils may be difficult to distinguish from indigenous ones. This is particularly true in the case of micro- and nannofossils where an individual fossil specimen may behave like a single grain of sediment and pass through one or more cycles of sedimentation with little evidence of wear. ISSC (1976, p. 47) also opens for the possibility of using reworked fossils as a basis for biostratigraphical zonation by stating "however, because of the difference in their significance with respect to age and environment, fossils that can be identified as reworked should be treated apart from those believed to be indigenous". The use of reworked fossils is mostly practised in Quaternary biostratigraphy for assemblage zones or remanie zones.
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Biostratigraphical units are defined on the basis of criteria that are essentially different from those used for
lithostratigraphical classification. The boundaries of these two
categories of stratigraphical unit may or may not coincide. Biostratigraphical and lithostratigraphical units are completely independent of one another.
The stratigraphical and geographical limitations of a biostratigraphical unit also represent the boundaries for the distribution of the fossils defining the unit, based on the observed evolutional appearance or extinction of the fossil taxa. The boundaries for most biostratigraphical units will therefore be diachronous, in contrast to those of chronostratigraphical units. Biostratigraphical units are therefore also in principle independent of chronostratigraphical classification. Biostratigraphical units will nevertheless usually be the most effective aids for interpreting chronostratigraphical relationships between different successions.
The fundamental biostratigraphical unit is the biozone (abbreviation for biostratigraphical zone). There are three main types of biozone: range zone, assemblage zone and abundance zone.