Biostratigraphy - opengeology.in

Biostratigraphy deals with the classification and correlation of rock strata based on their fossil contents. Fossils help in assigning relative ages to stratigraphic units can be correlated with different parts of the world.

The biostratigraphy is based on the concept that there is evolutionary changes in the flora and fauna which if unidirectional. These animals have their first appearance and they become extinct with their last appearance. Some species have very short life but they appared over the large geographical area, we call them as index fossils.

A biostratigraphic zone is defined as the unique set of fossils that is called as fossils assemblage.

Components of a fossil assemblage may have three different derivations.

Some species of assemblage inhibited the area of deposition known as biocenose community and the area inhibited by the community adapted to its environment is known as biotope.
- Study of these fossils help in reconstruction of ecological condition of deposition.
Some species may have been derived from neighboring or distant biotopes. The total species consisting the inhibited area and the remains of species that lived elsewhere is referred to as thenatocoenose.
- such species may come after their death.
Some species may be derived from the older sequence by the surficial and sedimentary processes.
- These fossils can be easily identified and separated from the indigenous material.

Biozones

Biozones are intervals of geological strata that are defined on the basis of their characteristic fossil taxa.

Assemblage biozones : It is a biozone defined by three or more different taxa which may or may not be related. The boundaries of an assemblage zone are defined by the typical specified fossil assemblage’s occurrence. Assemblage zones are named for the most characteristic or diagnostic fossils in its assemblage.
Taxon-range biozone : A taxon-range zone is simply the biozone defined by the first and last occurrence of a simple taxon. The boundaries are defined by the lowest and highest stratigraphic occurrence of that particular taxon.
Concurrent-range zone : A concurrent-range zone uses the overlapping range of two taxa, with low boundary defined by the appearance of one taxon and high boundary defined by the another taxon. Concurrent-range zones are named after both of the taxa in it.
Lineage biozone : A lineage zone, also known as consecutive range zone. These biozones are defined by being a specific segment of an evolutionary lineage. Lineage zones are different from most other biozones because they need that the segments are bounded by successive segment of an evolutionary lineage. Lineage zones are named for the specific taxon they represent.

Abundance (acme) biozone : An abundance zone or acme zone is a biozone that is defined by the range in which the abundance of a particular taxon is highest. The only way to define them is to trace the abundance of the taxon through time. As local environmental factors influence abundance, this can be an unreliable way of defining a biozone.
Interval zones : An interval zone is defined as the body of strata between two horizons, which are arbitrary chosen. A highest-occurrence zone is a biozone with the upper boundary being the appearance of one taxon, and the lower boundary the appearance of another taxon.

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