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Biostratigraphy is the branch of stratigraphy that involves the characterization, correlation, and assignment of relative ages of rock strata through the study of the fossil assemblages contained within them.(Boggs 2014i p. 416). This science is based on the principle of faunal succession.(Boggs 2014i p. 417)

Biostratigraphy is the branch of stratigraphy which focuses on correlating and assigning relative ages of rock 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 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.

Biostratigraphy originated in the early 19th century, where geologists recognised that the correlation of fossil assemblages between rocks of similar type but different age decreased as the difference in age increased. The method was well-established before Charles Darwin explained the mechanism behind it—evolution.

The first reef builder is a worldwide index fossil for the Lower Cambrian

Ammonites, graptolites, archeocyathids, and trilobites are index fossils that are widely used in biostratigraphy. Microfossils such as acritarchs, chitinozoans, conodonts, dinoflagellate cysts, ostracods, 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 poorer the stratigraphic precision, so fossils that evolve rapidly, such as ammonites, are favoured over forms that evolve much more slowly, like nautiloids. 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 trace fossil Treptichnus pedum 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 stratigraphers create a new period, most of the periods we recognise today are terminated by a major extinction event or faunal turnover.

The principle of biostratigraphy

The science of biostratigraphy is underpinned by the principle that organisms go through successive evolutionary changes as geological time progresses. As a result of this principle, the fossils contained within of any unit of sedimentary rock strata can be used to characterize and date it.(Boggs 2014i p. 416)

Fossils as a basis for stratigraphic subdivision

Fossils have very important uses in the practice of stratigraphy.(Boggs 2014i p. 416)

The first important use of fossils in stratigraphy is that they can be used as a further way to partition sedimentary rocks into stratigraphic units -- the first way being using sedimentary rock's physical properties to partition sedimentary rocks into stratigraphic units. Stratigraphic units that are determined from fossils are called biostratigraphic units.(Boggs 2014i p. 416)

The second important use of fossils in stratigraphy is that biostratigraphy can be used to subdivide the strata determined from lithographic studies into smaller subunits known as zones. These zones can be ordered and relative-age dated on large geographic scales up to continental or even global.(Boggs 2014i p. 416).

The boundaries of the stratigraphic units determined from the study of the fossil assemblages contained in them may or may not concur with the boundaries of the stratigraphic units that have been determined from the study of the lithology of the same sedimentary rocks. It is thus not unusual for biostratigraphic units to incorporate parts of or all of two or more geologic members or formations.(Boggs 2014i p. 416).

Using biostratigraphy to correlate stratigraphic units

A very important thing that biostratigraphy is used for is the correlation of stratigraphic units.(Boggs 2014i p. 416)

Biostratigraphic concepts

Concept of stage

Further information: Faunal stage

A stage is a major subdivision of strata with a unique collection of fossils in it. Stages follow each other in a systematic order. The same stage can have different formations (i.e. lithostratigraphic units) in different locations. Stages are named after places with good examples of the fossils characteristic of the stage in question.(Boggs 2014i p. 418)

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 information: Biozone and Chronozone

Zones are of a smaller scale than stages. Zones are units with clear boundaries and are defined by the stratigraphic ranges of fossil species with no regard for the lithology of the rock that they are in.(Boggs 2014i p. 418)

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 biozones 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 strata 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 biozones 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

Amplexograptus, a graptolite index fossil, from the Ordovician near Caney Springs, Tennessee.

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

• Easy to preserve (Easier in low-energy, non-oxidized environment)
• Easy to identify

Faunal succession

Fossil organisms succeed one another in a definite and determinable order and therefore any time period can be recognized by its fossil content.

Biostratigraphy as a science

Biostratigraphy is closely related to paleontology. A biostratigrapher should be well-versed in paleontology.(Boggs 2014i p. 416).

History of biostratigraphy

It was already perceived that fossils were the remains of organisms that were once alive, and a few observers had posed the idea that some species of these fossils were now extinct. William Smith, an English civil engineer and surveyor, is considered the person who discovered biostratigraphy's fundamental principle of faunal succession in the late eighteenth century. Smith first outlined and named the strata he observed by their lithological characteristics. He then used the fossils he discovered in those strata to characterize, subdivide, and correlate strata in different locations. He was the first person to do this. He discovered that the strata that contained fossils overlaid each other in a specific order that could be determined. Smith was thus able to use the fossils, in combination with lithological characteristics, to determine a stratigraphic succession.(Boggs 2014i p. 417)

Smith did not use fossils alone to subdivide rock successions. This was first done by various scientists in the 1830s on Tertiary sediments in Europe. For example, the Scottish geologist Charles Lyell divided Tertiary strata into four units based on the proportions of extant to extinct species in its rocks.(Boggs 2014i p. 417)

The French paleontologist Alcide d'Orbigny conceived of the idea of stages in the 1840s.(Boggs 2014i p. 417) d'Orbigny was mistaken in his idea that stages were global in their extent and were caused by the cataclysmic destruction of life on Earth, followed by the creation of new life. Nevertheless, d'Orbigny's basic concept of stages has remained a valid and important contribution to the science of biostratigraphy.(Boggs 2014i p. 417, 418)

The German Friedrich Quenstedt was critical of d'Orbigny's method of delimiting zones because he thought that it was a study of fossils on too large and crude a scale. Quentstedt's ideas were developed into the concept of the zone by his student Albert Oppel mid eighteenth century. Oppel introduced the idea of the zone in 1856 and this idea greatly altered the science of biostratigraphy.(Boggs 2014i p. 418)

See also

• Biochronology
• Chronostratigraphy
• Conodont biostratigraphy
• Lithostratigraphy
• Tectonostratigraphy
• Topostratigraphy

References

External links

• Biostratigraphic Lithostratigraphic Column Generator