Biostratigraphy and its units
The purpose of biostratigraphic classification is to organize rock strata systematically into named units based on a wide range of fossil characteristics. Biostratigraphy provides the most reliable means for correlation between sedimentary successions around the world. Changes in fossil faunas and floras are the essential markers in biostratigraphy. The basic principle of biostratigraphy is that evolutionary changes in faunas and floras are irreversible. These changes form the basic biostratigraphic unit, which is called a biozone or a biostratigraphic zone.
Biostratigraphic Unit: a body of rock strata unified by its fossil content or paleontological character and thus differentiated from adjacent strata.
Biostratigraphic Zone (Biozone): a general term for any kind of biostratigraphic unit. Bio- should be used in front of the term zone to distinguish biostratigraphic zone from other kind of zones whenever there is danger of confusion.
Superzones and Subzones: Several biozones with common biostratigraphic features may be grouped into Superzones (superbiozones). Any kind of biozone may be divided into subzones (subbiozones) to express finer biostratigraphic details. A zone need not be completely subdivided into subzones. Subzones have also been divided into very small units called zonules.
Biohorizons; surfaces of biostratigraphic change or of distinctive biostratigraphic character; predominantly valuable for correlation (not necessarily time-correlation); commonly used as a biozone boundary, though often recognized as horizons within biozones.
Biohorizons have also been variously termed as surfaces, horizons, levels, limits, boundaries, bands, markers, indexes, datums, datum planes, datum levels, marker beds, key beds and so on.
Kinds of biostratigraphic units (Biozones):. The following general types of biozones are in common use:
Assemblage Zone – is a group of strata characterized by a distinctive natural assemblage of all the forms regardless of their ranges. (The Assemblage Zone is also known as Cenozone, from a Greek word meaning common. Although this term has the advantage of being derived from a classical language,
Range Zone – The range zone is a group of strata representing the stratigraphic range of some selected fossil element. Each species exhibits a restricted stratigraphic range. The word range implies extent in both horizontal and vertical directions.
Taxon-Range Zone – A taxon-range zone is the body of strata representing the total range of occurrence (horizontal and vertical) of specimens of a particular taxon. The taxon-range zone is particularly valuable as an indicator of geologic age because of the limited time range (life span) of specific taxon through geologic history.
Concurrent-Range Zone (Overlap Zone) – The overlap or concurrent range zone is marked by overlapping ranges of specified fossils. Definition of a concurrent range zone does not require that the ranges of all taxa present should concur or overlap, nor does it require that all those that do overlap should be considered for biozonation.
Lineage Zone (Phylozone) – A lineage zone is a type of range zone consisting of the body of strata containing specimens representing a segment of an evolutionary or developmental line or trend, defined above and below by changes in features of the line or trend.
Successive appearance and disappearance Zone – Continuous changes in fossil sequences can also lead to the recognition of successive appearance and successive disappearance zones. This is defined as body of strata between two successive appearances or disappearances of taxa.
Acme Zone (Abundance Zone) – The acme zone is based on the abundance of the certain form, independent of association or range. An acme zone is a body of rock strata representing the acme or maximum abundances or frequency of occurrence of some species, genus or other taxon, but not its total range.
Interval Zone – The interval zone or gap zone is not recognized by the range of any particular faunal element within the interval but only from its position relative to the underlying and overlying zones. Thus, an interval zones is an interval or gap between two distinctive biostratigraphic horizon.
Barren Interzones or Intrazones – Intervals lacking in fossils between successive biozones may be called as barren interzones and informally referred to the adjacent biozones.
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