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Biostratigraphy is the science of dating rocks by using the fossils contained within them. Usually the aim is correlation. That is, demonstrating that a particular horizon in one geological section represents the same period of time as another horizon at some other section. The fossils are useful because sediments of the same age can look completely different because of local variations in the sedimentary environment. For example, one section might have been made up of clays and marls while another has more chalky limestones, but if the fossil species recorded are similar, the two sediments are likely to have been laid down at the same time.
Ammonites, graptolites and trilobites are index fossils that are widely used in biostratigraphy. Microfossils such as acritarchs, chitinozoans, conodonts dinoflagellate cysts, pollen, spores and foraminiferans are also frequently used. Different fossils work well for sediments of different ages; trilobites, for example, are particularly useful for sediments of Cambrian age. To work well, the fossils used must be widespread geographically, so that they can occur in many different places. They must also be short lived as a species, so that the period of time during which they could be incorporated in the sediment is relatively narrow. The longer lived the species, the less accurate the correlation, and so fossils that evolve rapidly, such as ammonites, are favoured over forms that evolve much more slowly, like nautiloids.
Fossils as a Basis for stratigraphic subdivision
Concept of Stage: It is a major subdivision of strata, each systematically following the other each bearing a unique assemblage of fossils. Therefore, stages can be defined as a group of strata containing the same major fossil assemblages. D'Orbigny, a French palaeontologist, is credited for the invention of this concept. He named these stages after geographic localities with particularly good section of rock that bear the characteristic fossils on which the stages are based.
Concept of Zone: Albert Oppel, in 1856, introduced the concept of zone (also known as biozones or Oppel zone). A zone is a strata characterised by the overlapping range of fossils. They represent the time between the appearance of species chosen at the base of the zone and the appearance of other species chosen at the base of the next succeeding zone. Oppel's zones are named after a particular distinctive fossil species, called an index fossil. Index fossils are one of the species from the assemblage of species that characterise the zone.
Index fossils: To be useful in correlation work, index fossils should be:
- Independent of their environment
- Geographically widespread (Provincialism/isolation of species should be avoided as much as possible)
- Fast evolving
- Abundant (Easy to find in the rock record)
- Easy to preserve (Easier in low-energy, non-oxidized environment)
- Easy to identify (Not too small for instance)
The Zone is the fundamental biostratigraphic unit. Its thickness range from a few to hundred's of metres, and its extant range from local to worldwide. Biostratigraphic units are divided into six principal kinds of biozones:
Taxon range biozone: Represent the known stratigraphic and geographic range of occurrence of a single taxon.
Concurrent range biozone: Include the concurrent, coincident, or overlapping part of the range of two specified taxa.
Interval biozone: The stata between two specific biostratigraphic surfaces. It can be based of lowest or highest occurrences.
Lineage biozone: Strata containing species representing a specific segment of an evolutionary lineage.
Assemblage biozones: Strata that contain a unique association of three or more taxa.
Abundance biozone: Strata in which the abundance of a particular taxon or group of taxa is significantly greater than in the adjacent part of the section.