Biostratigraphy is the branch of stratigraphy that deals with the distribution of fossils in rock layers and uses fossils to divide rock formations into biostratigraphic units. The key types of biostratigraphic units are biozones, which are defined by characteristic fossil assemblages. Common biozones include range biozones defined by the first and last appearances of an index fossil, and interval biozones defined between two significant fossil horizons. Together, biozones allow rock formations to be correlated between areas and their relative ages to be determined based on principles of evolution and succession of fossil taxa.

1. Biostratigraphy
Stratigraphy based on the fossil
occurrence

2. Introduction
• "Stratigraphy based on the paleontological aspects of
rocks".
• As early as the 1700’s William Smith realized that fossils
could be used to recognize strata of different ages.
• Alcide d’Orbigny, in roughly 1842 came up with the idea of
creating major subdivisions of strata based on a unique
assemblage of fossils. D’Orbigny name the fossil
assemblage strata groups as STAGES.
• Albert Oppel (1856) conceived the idea of small-scale units
defined by the stratigraphic ranges of fossil
species. Species stratigraphic range zones are referred to
as range zones. The species which marks a range zone
is known as an index fossil.
• A paleontological record is the position of a fossil taxon in a
rock sequence
•

3. Types of fossil assemblages
Two types of fossil assemblages
– Biocoenosis
– Thanatocoenosis
Biocoenosis is an assemblage representing the living
assemblage of any particular environment. This is a
natural assemblage which is reliable for
biostratigraphy.
Thanatocoenosis is an assemblage which undergone
transportation or reworked. This assemblage cannot
be used for age determination.

4. Fundamentals of biostratigraphy
Biostratigraphy is the branch of stratigraphy that deals with
the distribution of fossils in the stratigraphic record and
the classification of bodies of rock or rock material into
biostratigraphic units based on their contained fossils.
• Biostratigraphic units are distinct from all other kinds of
stratigraphic units because their contained fossils record
the unidirectional process of organic evolution.
• The stratigraphic record as a whole contains an
unrepeated sequence of fossil taxa that may be used to
determine the relative age of their enclosing strata.
• The biostratigraphy is based on the biostratigraphic
events which includes the first appearance and
disappearance from the geologic record.

5. Biostratigraphic events
• Fossil events are the result of the continuing evolutionary
trends of life on earth; they differ from physical events in
that they are unique, non-recurrent, and that their order
is irreversible.
• Events that usually utilised in biostratigraphy are
– First appearance
– Last occurrence
– Rapid increase in population
– Rapis decrease in population

6. Boundaries
• The boundaries of a biozone are drawn at surfaces that
mark the lowest occurrence, highest occurrence, limit,
increase in abundance, or decrease in abundance of one
or more components of the fauna or flora.
• Furthermore, the base or top of one kind of biozone may
not, or need not, coincide with the base or top of another
kind of biozone.

7. Index fossils
• Characteristic of Index fossils
– Well preserved and easily retreived, particularly
microfossils.
– Short stratigraphic range
– wide geographic distribution, free from facies. can be
found in many rock types e.g. planktonic forms
– abundant
– Distinctive & easily recognized forms

8. Nature of Biostratigraphic Units
• A biostratigraphic unit is a body of rock that is defined or
characterized by its fossil content.
• Most fossils are contemporaneous with the body of rock
that contains them, including those derived from different
but coeval sedimentary environments.
• Fossils not contemporaneous with the enclosing body of
rock should not be used to define, characterize, or
identify a biostratigraphic unit.
• Biostratigraphic units are based on criteria that differ
fundamentally from those used for lithostratigraphic units
• The boundaries of most biostratigraphic units, are both
characteristically and conceptually diachronous.

9. Fundamental Unit
• The biozone is the fundamental unit of
biostratigraphic classification.
• Biozone is a condensed expression for
‘‘biostratigraphic zone.’’

10. Kinds of Biostratigraphic Units
• The biozone is the fundamental biostratigraphic unit.
Five specific kinds of biozones are recognized herein:
- range biozone,
– interval biozone,
– lineage biozone,
– assemblage biozone, and
– abundance biozone
• These five kinds of biozones are not hierarchically
interrelated.
• The most common choice of biozone is one in which
both the lower boundary and the upper boundary are
based on the lowest occurrences of individual taxa; the
two taxa may or may not have a direct phylogenetic link.

12. Range biozone
• A range biozone is a body of rock representing the
known stratigraphic and geographic range of occurrence
of any selected element or elements of the chosen fossil
taxon, or taxa, present in the rock record
• Three types of range biozones
– Taxon range biozone
– Concurrent range biozone
– Partial-range biozone

13. Taxon range biozone
• A taxon-range biozone is a body of rock representing the
known stratigraphic and geographic range of a chosen
taxon
• The lower boundary is marked by first appearance
datum (FAD) and the upper boundary is marked by last
appearance datum (LAD) of the same species.
LAD
FAD

14. • For example
• Globorotalia
pseudomenardii zone

15. Concurrent-range biozone
• A concurrent-range biozone is a body of rock including
the concurrent, coincident, or overlapping part of the
ranges of two specified taxa.
• Lower boundary is marked by first occurrence of a taxon
and the top is marked by last occurrence of ather taxon

16. Partial range biozone
• Partial range biozone is a body of rock located between
the last appearance of a taxon and the first appearance
of another taxon.
• The lower boundary is marked by last appearance and
the upper boundary is marked by first appearance.

17. Interval biozone
• An interval biozone is a body of rock between
two specified biostratigraphic biohorizons
(surfaces)
• Two types of interval biozones
– The boundaries are based on
first appearance
– The boudaries based on last
appearance

18. Interval biozones
• Based on last appearances
• eg.
• Globorotalia velascoensis
• Based on first appearances
• eg.
• Globorotalia psuedobulloides

19. Lineage biozone
• A lineage biozone is a body of rock
containing species representing a specific
segment of an evolutionary lineage.

20. Assemblage biozone
• An assemblage biozone is a body of rock characterized
by a unique association of three or more taxa, the
association of which distinguishes it in biostratigraphic
character from adjacent strata.
• An assemblage biozone may be based on a single
taxonomic group, for example, trilobites, or on
more than one group, such as acritarchs and
chitinozoans

21. Abundance (acme) biozone
• An abundance biozone is a body of rock in which the
abundance of a particular taxon or specified group of taxa is
significantly greater than in adjacent parts of the section.
• Abundance zones may be of limited, local utility because
abundances of taxa in the geologic record are largely
controlled by paleoecology, taphonomy, and diagenesis
Lower boundary is marked by the
increase of population of taxa
The upper boundary is marked by the
decrease of population of taxa.
e.g. Emiliania huxleyi biozone

22. Name
• The name of a biozone consists of the name of one or
more distinctive taxa or parataxa (for trace fossils) found
in the biozone, followed by the word ‘‘Biozone.’’ (e.g.,
Turborotalia cerrozaulensis Biozone or Cyrtograptus
lundgreni-Testograptus testis Biozone).
• The name of the species whose lowest occurrence
defines the base of the zone is the most common choice
for the biozone name.