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
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
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.
1 Fossils as a basis for stratigraphic subdivision
1.1 Concept of stage
1.2 Concept of zone
1.2.1 Index fossils
2 See also
4 External links
Fossils as a basis for stratigraphic subdivision
Concept of stage
Further information: Faunal 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
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: 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 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
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 strata between two specific
biostratigraphic surfaces. It can be based on lowest or highest
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.
Amplexograptus, a graptolite index fossil, from the
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)
Abundant (easy to find in the rock record)
Easy to preserve (Easier in low-energy, non-oxidized environment)
Easy to identify
^ Gluyas, J. & Swarbrick, R. (2004) Petroleum Geoscience. Publ.
Blackwell Publishing. pp. 80-82
^ Gehling, James; Jensen, Sören; Droser, Mary; Myrow, Paul; Narbonne,
Guy (March 2001). "Burrowing below the basal
Cambrian GSSP, Fortune
Head, Newfoundland". Geological Magazine. 138 (2): 213–218.