Biostratigraphy 670765 220393676 2008-06-19T17:19:54Z Fabartus 281293 /* Concept of zone */ {{further|[[chronozone]]} {{unreferenced|date=April 2007}} '''Biostratigraphy''' is the branch of [[stratigraphy]] which focuses on correlating and assigning relative ages of rock [[Stratum|strata]] by using the [[fossil]] assemblages contained within them. Usually the aim is correlation, 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 [[sediment]]s 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 [[clay]]s and [[marl]]s while another has more [[chalk]]y [[limestone]]s, but if the fossil species recorded are similar, the two sediments are likely to have been laid down at the same time. [[Ammonite]]s, [[graptolite]]s and [[trilobite]]s are [[index fossil]]s that are widely used in biostratigraphy. [[micropaleontology|Microfossils]] such as [[acritarchs]], [[chitinozoa]]ns, [[conodonts]] [[dinoflagellate]] cysts, [[pollen]], [[spores]] and [[foraminifera]]ns are also frequently used. Different fossils work well for sediments of different ages; [[trilobite]]s, 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 [[nautiloid]]s. == Fossils as a basis for stratigraphic subdivision == ===Concept of stage=== <!-- thought of redirecting [[Faunal stage]] to this section title, as seems best fit here. But am too uneducated in the field to know if that would be best. That currently redirects to [[Stage (geology)]], which would appear to be a main article for the topic of this section --> {{see|stage (geology)|age (geology)}} A stage 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. French [[palaeontologist]] [[Alcide d'Orbigny]] is credited for the invention of this concept. He named stages after geographic localities with particularly good sections of rock strata that bear the characteristic fossils on which the stages are based. ===Concept of zone=== {{further|[[chronozone]]}} In 1856 German palaeontologist [[Albert Oppel]] introduced the concept of zone (also known as biozones or Oppel zone). A zone includes 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. The zone is the fundamental biostratigraphic unit. Its thickness range from a few to hundreds 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'' include the stata between two specific biostratigraphic surfaces. It can be based on lowest or highest occurrences. *''Lineage biozone'' are strata containing species representing a specific segment of an evolutionary lineage. *''Assemblage biozones'' are strata that contain a unique association of three or more taxa. *''Abundance biozone'' are strata in which the abundance of a particular taxon or group of taxa is significantly greater than in the adjacent part of the section. ====Index fossils==== To be useful in stratigraphic correlation index fossils should be: *Independent of their environment *Geographically widespread (provincialism/isolation of species should be avoided as much as possible) *Rapidly evolving *Abundant (easy to find in the rock record) *Easy to preserve (Easier in low-energy, non-oxidized environment) *Easy to identify ==See also== *[[Lithostratigraphy]] *[[Chronostratigraphy]] *[[Tectonostratigraphy]] ==External links== [http://www.bslsonline.com/ Biostratigraphic Lithostratigraphic Column Generator] {{geology-stub}} [[Category:Subfields of paleontology]] [[Category: Stratigraphy]] [[Category: Fossils]] [[de:Biostratigrafie]] [[es:BioestratigrafĂ­a]] [[fr:Biostratigraphie]] [[id:Biostratigrafi]] [[it:Biostratigrafia]] [[nl:Biostratigrafie]] [[no:Biostratigrafi]] [[pl:Biostratygrafia]] [[sv:Biostratigrafi]]