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Biostratigraphy
 
 
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Biostratigraphy is the branch of stratigraphy
Stratigraphy

Stratigraphy, a branch of geology, studies rock layers and layering . It is primarily used in the study of sedimentary rock and layered volcanic rocks....
 which focuses on correlating and assigning relative ages of rock strata
Stratum

In geology and related fields, a stratum is a layer of rock or soil with internally consistent characteristics that distinguishes it from contiguous layers....
 by using the fossil
Fossil

Fossils are the preserved remains or trace fossil of animals, plants, and other organisms from the remote past. The totality of fossils, both discovered and undiscovered, and their placement in fossiliferous Rock formations and sedimentary rock layers is known as the fossil record....
 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
Sediment

Sediment is any particulate matter that can be sediment transport by fluid dynamics, and which eventually is deposited.Sediments are most often transported by water transported by wind and glaciers....
s of the same age can look completely different because of local variations in the sedimentary environment
Sedimentary depositional environment

In geology, sedimentary depositional environment describes the combination of physical, chemical and biological processes associated with the deposition of a particular type of sediment and, therefore, the rock types that will be formed after lithification, if the sediment is preserved in the rock record....
.






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Biostratigraphy is the branch of stratigraphy
Stratigraphy

Stratigraphy, a branch of geology, studies rock layers and layering . It is primarily used in the study of sedimentary rock and layered volcanic rocks....
 which focuses on correlating and assigning relative ages of rock strata
Stratum

In geology and related fields, a stratum is a layer of rock or soil with internally consistent characteristics that distinguishes it from contiguous layers....
 by using the fossil
Fossil

Fossils are the preserved remains or trace fossil of animals, plants, and other organisms from the remote past. The totality of fossils, both discovered and undiscovered, and their placement in fossiliferous Rock formations and sedimentary rock layers is known as the fossil record....
 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
Sediment

Sediment is any particulate matter that can be sediment transport by fluid dynamics, and which eventually is deposited.Sediments are most often transported by water transported by wind and glaciers....
s of the same age can look completely different because of local variations in the sedimentary environment
Sedimentary depositional environment

In geology, sedimentary depositional environment describes the combination of physical, chemical and biological processes associated with the deposition of a particular type of sediment and, therefore, the rock types that will be formed after lithification, if the sediment is preserved in the rock record....
. For example, one section might have been made up of clay
Clay

Clay is a naturally occurring material composed primarily of fine-grained minerals, which show plasticity through a variable range of water content, and which can be hardened when dried and/or fired....
s and marl
Marl

Marl or Marlstone is a calcium carbonate or lime-rich mud or mudstone which contains variable amounts of clays and aragonite. Marl is originally an old term loosely applied to a variety of materials, most of which occur as loose, earthy deposits consisting chiefly of an intimate mixture of clay and calcium carbonate, formed under...
s while another has more chalk
Chalk

Chalk is a soft, white, porous sedimentary rock, a form of limestone composed of the mineral calcite. It forms under relatively deep marine conditions from the gradual accumulation of minute calcite plates shed from micro-organisms called coccolithophores....
y limestone
Limestone

File:Limestone Formation In Waitomo.jpgLimestone is a sedimentary rock composed largely of the mineral calcite . The deposition of limestone strata is often a by-product and indicator of biological activity in the geology record....
s, but if the fossil species recorded are similar, the two sediments are likely to have been laid down at the same time.

Ammonite
Ammonite

Ammonites are an Extinction group of marine animals of the Subclass Ammonoidea in the class Cephalopoda, phylum Mollusca. They are excellent index fossils, and it is often possible to link the rock layer in which they are found to specific Geologic time scale....
s, graptolite
Graptolite

Graptolites are fossil colonial animals known chiefly from the Upper Cambrian through the Lower Carboniferous . A possible early graptolite, Chaunograptus, is known from the Middle Cambrian....
s and trilobite
Trilobite

Trilobites are extinction marine arthropods that form the class Trilobita. They appeared in the Early Cambrian period and flourished throughout the lower Paleozoic era before beginning a drawn-out decline to extinction when, during the Late Devonian extinction, all trilobite orders, with the sole exception of Proetida, died out....
s are index fossil
Index fossil

Index fossils are fossils used to define and identify geologic columns . They work on the premise that, although different sediments may look different depending on the conditions under which they were laid down, they may include the remains of the same species of fossil....
s that are widely used in biostratigraphy. Microfossils
Micropaleontology

File:Microfossils.JPGMicropaleontology is that branch of paleontology which studies microfossils. Microfossils are fossils generally not larger than four millimeters, and commonly smaller than one millimeter, the study of which requires the use of light or electron microscopy....
 such as acritarchs, chitinozoa
Chitinozoa

Chitinozoa are a taxon of laboratory flask-shaped, organic matter walled marine biology microfossils produced by an as yet unknown animal. Common from the Ordovician to Devonian periods , the millimetre-scale organisms are abundant in almost all types of marine sediment across the globe....
ns, conodonts, dinoflagellate
Dinoflagellate

The dinoflagellates are a large group of flagellate protists. Most are marine plankton, but they are common in fresh water habitats as well. Their populations are distributed depending on sea surface temperature, salinity, or depth....
 cysts, pollen
Pollen

Pollen is a fine to coarse powder consisting of Gametophyte , which produce the male gametes of spermatophyta. A hard coat covering the pollen grain protects the sperm cells during the process of their movement between the stamens of the flower to the pistil of the next flower....
, spores and foraminifera
Foraminifera

The Foraminifera, or forams for short, are a large group of amoeboid protists with reticulating pseudopods, fine strands of cytoplasm that branch and merge to form a dynamic net....
ns are also frequently used. Different fossils work well for sediments of different ages; trilobite
Trilobite

Trilobites are extinction marine arthropods that form the class Trilobita. They appeared in the Early Cambrian period and flourished throughout the lower Paleozoic era before beginning a drawn-out decline to extinction when, during the Late Devonian extinction, all trilobite orders, with the sole exception of Proetida, died out....
s, for example, are particularly useful for sediments of Cambrian
Cambrian

The Cambrian is a geologic period that began about Mya at the end of the Proterozoic eon and ended about Ma with the beginning of the Ordovician period ....
 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 poorer the stratigraphic precision, so fossils that evolve rapidly, such as ammonites, are favoured over forms that evolve much more slowly, like nautiloid
Nautiloid

Nautiloids are a group of marine mollusks in the subclass Nautiloidea, which all possess an external shell, the best-known example being the modern nautiluses....
s. Often biostratigraphic correlations are based on a fauna, not an individual species, as this allows greater precision. Further, if only one species is present in a sample, it can mean that (1) the strata were formed in the known fossil range of that organism; (2) that the fossil range of the organism was incompletely known, and the strata extend the known fossil range. For instance, the presence of the fossil Trichophycus pedum
Trichophycus pedum

Trichophycus pedum is regarded as the earliest widespread complex trace fossil. Its earliest appearance, which was contemporaneous with the last of the Ediacaran biota, is used to define the dividing line between the Ediacaran and Cambrian Period ....
 was used to define the base of the Cambrian period, but it has since been found in older strata.

Fossil assemblages were traditionally used to designate the duration of periods. Since a large change in fauna was required to make early stragtigraphers create a new period, most of the periods we recognise today are terminated by a major extinction event or faunal turnover.

Fossils as a basis for stratigraphic subdivision


Concept of stage

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
Alcide d'Orbigny

Alcide Charles Victor Marie Dessalines d'Orbigny was a France Natural history. He made major contributions in many areas, including zoology, malacology, palaeontology, geology, archaeology and anthropology....
 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

In 1856 German palaeontologist Albert Oppel
Albert Oppel

Carl Albert Oppel was a Germany paleontologist....
 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
    Lithostratigraphy

    Lithostratigraphy is a sub-discipline of stratigraphy, the geology science associated with the study of stratum or rock layers. Major focuses include geochronology, comparative geology, and petrology....
  • Chronostratigraphy
    Chronostratigraphy

    Chronostratigraphy is the branch of stratigraphy that studies the age of rock stratum in relation to time.The ultimate aim of chronostratigraphy is to arrange the sequence of deposition and the time of deposition of all rocks within a geological region, and eventually, the entire geologic record of the Earth....
  • Tectonostratigraphy
    Tectonostratigraphy

    In geology tectonostratigraphy refers either to rock sequences in which large-scale layering is caused by the stacking of thrust sheets or nappes in areas of thrust tectonics or the effects of tectonics on lithostratigraphy....


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