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1. Biostratigraphic Units
A. Purpose of Biostratigraphic Unit
• The purpose of biostratigraphic classification is to
organize rock strata systematically into named units
based on content and distribution of fossils.
B. Nature of Biostratigraphic Units
• Rock strata are classified biostratigraphically by dividing
them into units distinguished by differences in their fossil
content.
• A biostratigraphic unit may be based simply on the
presence of fossils as contrasted with their absence

2. • There are many different kinds of biostratigraphic units
depending on the paleontological feature considered.
• A biostratigraphic unit may be considered to be present
only within the observed limit of occurrence of the
paleontologic feature on which it is based.
Distinctive Nature of Units
• Biostratigraphic units are distinct from many other kinds
of stratigraphic units.
• All parts of all rock strata have lithostratigraphic
character, and all parts of all rock strata have
chronostratigraphic character, but many parts of the
Earth’s stratigraphic sequence lack significant fossil
remains and thus lack biostratigaphic character and are
not amenable to biostratigraphic classification.

3. Continuity of Fossils in Relation to Outer Limits of
Units
• Fossils usually constitute only a minor, disseminated,
fractional part of a rock stratum.
• Even within fossiliferous sequences fossils are rarely
found in every bed or formation, nor are they found
everywehre along a bed or formation.

4. Definitions
1. Biostratigraphy
• The element of stratigraphy that deals with the remains
or evidences of former life in strata and with the
organization of strata into units based on their fossil
content.
2. Biostratigraphic Classification
• The organization of strata into units based on their
fossil content.
3. Biostratigraphic Unit
• A body of rock strata unified by its fossil content or
paleontological character and thus differentiated from
adjacent strata.
• A biostratigraphic unit is present only within the limits
of observed occurrence of the particular biostratigrphic
features on which it is based.

5. 4. Biostratigraphic Zone (Biozone)
• A general term for any kind of biostratigraphic
unit
• Biozone is a short alternative term for
biostratigraphic zone.
• Biozones vary greatly in thickness and
geographic extend.
• They may range from a local bed to a unit
thousands of meters thick or extending
worldwide
• The total time represented by a biozone may be
referred to simply as its time or time-value, or its
biochron.

6. 5. Superzone and subzone
• Several biozones with common biostrtigraphic features
may be grouped into superzones (superbiozones)
• Any kind of biozones may be divided into subzones
(subbiozones) to express finer biostratigraphic detail.
• A zone need not be completely subdivided into
subzones.
• Subzones have also been divided into very small units
called zonules.

7. 6.Barren interzones and intrazones
• Intervals lacking in fossils between successive biozones
may be called barren interzones.
• The barren intervals of substantial thickness within
biozones may be referred to as barren intrazones.
7. Biohorizons
• Surfaces of biostratigraphic change or of distinctive
biostratigraphic character, valuable for correlation,
commonly used as a biozone boundary though often
recognized as horizons within biozones.
• In theory, a biohorizon is strictly a surface or interface
• In practice, the term may refer to a thin but
biostratigraphically distinctive bed.
•

8. • Features on which biohorizons are commonly based
include “first appeareances”, “ last coccurence”,
distinctive occurrences, changes in the character of
individuals taxons (Eg. Changes in direction of coiling in
foraminifera, or in number of septa in corals).
• Biohorizons have been called surfaces, horizons, levels,
limits, boundaries, bands, markers, indexes, datums,
datum planes, datum levels, key horizons, key beds,
marker beds and so on.

9. Kinds of Biostratigraphic Units
1. General
• Biozone is the general term for any kind of
biostratigraphic unit.
• There are different kinds of biozones
• The following four general types of biozones are in
common use
a) Assemblage-Zone
• A groups of strata characterized by a distinctive natural
assemblage of all forms present or of the forms
present of a certain kind or kinds.

10. b) Range-Zone
• A group of strata representing the stratigraphic range of
some selected element of the total assemblage of fossil
forms present.
c) Acme-Zone
• A group of strata based on the abundance or
development of certain forms, regardless of either
association or range.
d) Interval-Zone (Interbiohorizon Zone)
• The stratigraphic interval between two biohorizons.

11. Assemblage-Zone (Cenozones)
Definition and significance
• A biostratigraphic assemblage-zone is a body of strata
whose content of fossils or of fossils of a certain kind
constitutes a natural assemblage.
• An assemblage-zone may be based on all kinds of
fossil forms present or it may be restricted to forms of
only certain kinds.
• We may have an assemblage-zone based only on
fossil fauna or one based only on fossil flora (an
assemblage-zone of corals or of foraminifers, or of
mollusks or of dasyclad algae, an assemblage-zone of
planktic forms or an assemblage-zone of benthic forms
and so on.

12. • Whatever its basis, an assemblage-zone is
characterized by an assemblage or association of fossil
forms that are assumed to have lived together or to
have died together or to have accumulated together, or
in any case , to have been entombed together.
• The assemblage-zone is particularly significant as an
indicator of environment.
• It is also a general indicator of geologic age
• These biostratigraphic units have high value of local
correlation.

13. Range-Zones
• A biostratigraphic range-zone is the body of strata
representing the total range of occurrences of any
selected elements of the total assemblage of fossil forms
in a stratigraphic sequence.
• The word “range” implies extent in both horizontal and
vertical directions.
• A biostratigraphic range-zone may represent the
stratigraphic range of some one taxonomic unit or of a
grouping of taxons or of a lineage or segment of a
lineage or of any of particular paeontological feature

14. • There are many kinds of range-zones. Some of the
principal kinds are
a) Taxon-Range-Zone
1.Definition and significance
• A taxon-range-zone is the body of strata representing
the total range of occurrence of specimens of
particular taxon (species, genus, family etc.,).
• The range-zone of a taxon means its maximum
geographic and stratigraphic extent.
• The terms genus-zone and species-zone have
occasionally been used in the same sense as taxon-
range-zone.

15. • The taxon-range-zone is particularly valuable as an
indicator or geologic age because of the limited time
range (life spans).
• The range-zone of a taxon may also be significant of
environment.
b) Concurrent-Range-Zone
Definition and significance
• A concurrent –range-zone is defined as the concurrent or
coincident parts of the range-zones of two or more
specified taxons selected among the total forms
contained in a sequence of strata.
• The principles of the concurrent-range-zone has long
been used in time-correlation.

16. c) Oppel-Zone
Definition and significance
• The Oppel-zone, named after the German stratigrapher,
Albert Oppel (1831-1865),.
• The term Oppel-zone has not been widely used.
• The Oppel-zone may be defined as a zone characterized
by an association or aggregation of selected taxons of
restricted and largely concurrent range.
• The lower part of the zone is commonly marked largely
by first appearance and its upper part by last
appearance of certain taxons

17. d. Lineage-Zone (Phylozone)
Definition and significance
• A lineage-zone is a type of range-zone consisting of the
body of strata containing specimens representing a
segment of an evolutionary or developmental line or
trend, defined above and below by changes in features
of the line or trend.
• This type of zone has also been termed as evolutionary
zone, a morphogenetic zone, a phylogenetic zone or a
phylozone.
• Best for time-correlation
• Identification of morphologic differentiation
• Variations in rates of evolution

18. 4. Acme-Zones
• An acme-zone is a body of strata representing the acme
or maximum development- usually maximum abundance
or frequency of occurrence – of some species, genus, or
other taxon, but not its total range.
• The acme-zone takes its name from the taxon whose
zone of maximum development it delimits, for eg.
Didymograptus Acme-zone.

19. 5. Interval –Zone
• A biostratigraphic interval-zone is an interval between
two distinctive biostratigraphic horizons.
• The of such a zone might be marked by the horizon of
first appearance of taxon-a or the last appearance of
taxon-b, and the top might be marked by the horizon of
first appearance of taxon-c or the last appearnce of
taxon-d
• The interal-zone is in common use for correlation
purposes.