3.5.2 Biozone (Biosone)
3.5.2.1 A biozone is a body of sediment or sedimentary rock that is characterized and defined by a specified fossil content.
3.5.2.2 Biozone is the fundamental formal unit for biostratigraphical classification and nomenclature.
3.5.2.3 The vertical extent of a biozone can include anything from a single bed to units several thousands of metres thick. The lateral extent may be local, regional or worldwide.
3.5.2.4 A biozone is mappable at the surface and traceable in the subsurface with the help of geological and palaeontological methods. Excavated sections or drillings are required to obtain samples for biostratigraphical classification of successions that only occur beneath the surface.
3.5.2.5 A biozone can be defined on the basis of various criteria for the occurrence of fossils, resulting in three main types of biozone: range zone (Section 3.5.2.5.1), assemblage zone (Section 3.5.2.5.2) and abundance zone (Section 3.5.2.5.3).
3.5.2.5.1 Range zone (forekomstsone) -- A range zone is the body of strata between two specified, documented lowest and/or highest occurrences of one or two fossil taxa (Fig. 6). The use of "interval zone" is not recommended since interval zone is defined in different ways in ISSC (1976) and NACSN (1983). There are three main types of range zone:
3.5.2.5.1.a The interval between the documented lowest and highest occurrences of a single taxon -- the taxon range zone (Fig. 6A).
3.5.2.5.1.b The interval between the lowest occurrence of one taxon and the highest occurrence of another taxon. If the two taxa partially overlap one another in their stratigraphical occurrence, the zone can be looked upon as a special type of Oppel zone or concurrent range zone (Section 3.5.2.5.2.b) involving only two taxa (Fig. 6B). If the occurrences of the two taxa do not partially overlap one another, but are used to divide the range of a third taxon, the interval is a partial range zone (Fig. 6C).
3.5.2.5.1.c The interval between documented successive lowest occurrences or successive highest occurrences of two taxa. When this interval is between two successive lowest occurrences in an evolutionary lineage it forms a lineage zone (Fig. 6D).
3.5.2.5.2 Assemblage zone or cenozone (samlingssone) (ISSC 1976, p. 50). An assemblage zone is a sequence that is defined by a natural assemblage of three or more taxa, without strict attention being paid to the distribution limits of each single taxon. The zone can be based on all the fossils present, or on only certain taxa (Fig. 7). Types of assemblage zones are:
3.5.2.5.2.a An assemblage zone may either contain one stratigraphically limited assemblage or two or more contemporaneous assemblages with shared characterizing taxa. The latter is a composite assemblage zone (sammensatt samlingssone) (Fig. 7A).
3.5.2.5.2.b An Oppel zone or concurrent range zone (ISSC 1976, pp. 55-57) (fellesforekomstsone) is characterized by more than two taxa. The zone boundaries are defined by two or more documented first and/or last occurrences of characterizing taxa (Fig. 7B).
3.5.2.5.2.c A cenozone (kenosone), as defined by ISSC (1976, p. 50), is characterized by a specified quantitative relationship between several taxa without any regard to the total stratigraphical distribution of the individual taxon (Fig. 7C). A definition of this nature will usually call for a precise quantitative specification of the characterizing taxa in the zone.
3.5.2.5.3 An abundance zone (maksimumssone) -- acme zone of ISSC (1976, p. 59) -- is a biozone defined by a quantitatively marked peak in the frequency of one or more taxa. The boundaries of the abundance zone are defined by significant changes in the frequency of the taxa present.
3.5.2.6 Formal erection of a biozone follows the "general rules for naming and defining geological units" (Chap. 2).
3.5.2.6.1 The name, which must also denote the type of zone, may be based on one or two characteristic and/or common fossils which either (a) are limited to the biozone, (b) reach their maximum frequency within it, or (c) have concurrent stratigraphical ranges within it. Such names will normally be based on genus or subgenus names, or full species or subspecies names (e.g. "the Exus (Protexus) taxon distribution zone" or "the Exus (Protexus) albus taxon distribution zone"). The genus and subgenus names can be abbreviated, but trivial species and subspecies names cannot be used alone (e.g. "the E. (P.) albus taxon distribution zone", but not "the albus taxon distribution zone").
3.5.2.6.2 The name can also be based on a combination of letters derived from the names of taxa which characterize the biozone (e.g. "the EPA taxon distribution zone").
3.5.2.6.3 The character of the biozone is to be clearly stated in the first formal definition of a biostratigraphical unit. When this has been done, it can be referred to more simply afterwards, e.g. "the Exus albus zone" instead of the more cumbersome "the Exus albus taxon distribution zone".
3.5.2.6.4 Alphanumerical divisions constructed on the basis of numbers and letters (e.g. as in the stage classification of the Oslo Region), or divisions based only on simple series of numbers or letters (1,2,3, or A,B,C) may be used for informal biostratigraphical units.
3.5.2.6.5 Changes of name. The biozone can be modified on the basis of new information. Its boundaries can be redefined or described in greater detail, and new characteristic taxa can be accepted, or existing taxa replaced. If substantial changes in the definition of the biozone are undertaken a new zone name should be proposed to avoid misunderstandings in later references. The zone name should also be changed if the taxa used in the name are given new names (see Section 2.5).
3.5.2.7.a A biozone can be completely or partly divided into formally defined sub-biozones or subzones (underbiosoner/undersoner) if such divisions serve a useful purpose.
3.5.2.7.b Unfossiliferous intervals between or within biozones are barren interzones (fossiltomme intersoner) and barren intrazones (fossiltomme intrasoner), respectively.
3.5.2.7.c Fossil taxa in certain assemblage and abundance zones may reflect dominant local ecological control. Biozones defined by such assemblages are to be considered as informal ecozones.
3.5.2.7.d Biohorizon (biohorisont). A biohorizon is a surface where a biostratigraphical change takes place, or that has a distinctive biostratigraphical character. It is therefore useful for correlation. Biohorizons are often used to define biozone boundaries, but can also occur within biozones. The term biohorizon is also sometimes used for thin, distinctive biostratigraphical zones. Examples of biohorizons are the first appearances and last occurrences of a taxon, changes in its frequency and abundance, evolutionary changes, and changes in its character.
3.5.2.8 Examples: On Norwegian territory there are few biozones where the zone type has been clearly defined. The zones in the Cambrian, based on trilobites (Henningsmoen 1957), can largely be looked upon as assemblage zones. The well-known evolutionary lineage in the genus Stricklandia, recently described in detail by Baarli (1986), may form a basis for defining lineage zones in the Llandovery. Many abundance zones that have been introduced into biostratigraphy reflect ecological conditions rather than processes controlled by evolution.
3.5.2.9 Key references: ISSC (1976), NACSN (1983).
