Stratigraphic Guide

Chapter 7. Biostratigraphic Units

A. Nature of Biostratigraphic Units

Biostratigraphic units (biozones) are bodies of strata that are defined or characterized on the basis of their contained fossils.

Biostratigraphic units exist only where the particular diagnostic feature or attribute on which they are based has been identified. Biostratigraphic units, therefore, are descriptive units based on the identification of fossil taxa. Their recognition depends on the identification of either their defining or characterizing attributes. Biostratigraphic units may be enlarged to include more of the stratigraphic record, both vertically and geographically, when additional data are obtained. In addition, since they depend on taxonomic practice, changes in their taxonomic base may enlarge or reduce the body of strata included in a particular biostratigraphic unit.

A biostratigraphic unit may be based on a single taxon, on combinations of taxa, on relative abundances, on specified morphological features, or on variations in any of the many other features related to the content and distribution of fossils in strata. The same interval of strata may be zoned differently depending on the diagnostic criteria or fossil group chosen. Thus, there may be several kinds of biostratigraphic units in the same interval of strata that may have gaps between them or overlaps of their vertical and horizontal ranges.

Biostratigraphic units are distinct from all other kinds of stratigraphic units in that the organisms whose fossil remains define them show evolutionary changes through geologic time that are not repeated in the stratigraphic record.

This makes the fossil assemblages of any one age distinctive from any other.

The relationship of biostratigraphic units to other kinds of stratigraphic units is discussed in Chapter 10.

B. The Fossils

1. Value of fossils
Fossils were once living organisms and as such are sensitive indicators of past environments, sedimentation patterns, and their distributions.

In addition, because of the irreversibility of evolution, fossils are particularly useful in working out the relative times of origin of sedimentary strata.

2. Fossil assemblages
Not all sedimentary rocks contain fossils but when they are present they reflect assemblages which have either lived and were buried in the area (biocoenosis), or they lived somewhere else and were brought into the area after death (thanatocoenosis), or they contain organisms transported alive away from their normal environment.

These may be mixed or interbedded in any proportion. All categories of fossil-bearing strata may be the basis for biostratigraphic zonation. Intervals lacking identifiable fossils or entirely without fossils are not subject to biostratigraphic classification.

3. Reworked fossils
Fossils from rocks of one age that have been eroded, transported, and redeposited in sediments of a younger age. Because of the difference in their significance with respect to age and environment, they should be treated apart from those believed to be indigenous.

4. Introduced or infiltrated fossils
Fossils introduced into older or younger rocks by fluids, through animal burrows or root cavities, or by sedimentary dikes or diapirs.They should be distinguished from indigenous fossils in biostratigraphic zonation.

5. Effects of stratigraphic condensation
Extremely low rates of sedimentation may result in fossils of different ages and different environments being mingled or very intimately associated in a very thin stratigraphic interval, even in a single bed.

C. Definitions

1. Biostratigraphy
The element of stratigraphy that deals with the distribution of fossils in the stratigraphic record and the organization of strata into units on the basis of their contained fossils.

2. Biostratigraphic classification
The systematic subdivision and organization of the stratigraphic section into named units based on their fossil content.

3. Biostratigraphic zone (Biozone)
A general term for any kind of biostratigraphic unit regardless of thickness or geographic extent. See section 3.A.7. After initial usage of a formal term, such as the Globigerina brevis Taxon-range Biozone, a simplified version of the formal nomenclature may be used, e.g. Globerigina brevis Zone. Biozones vary greatly in thickness, geographic extent, and represented time span.

4. Biostratigraphic horizon (Biohorizon)
A stratigraphic boundary, surface, or interface across which there is a significant change in biostratigraphic character.

A biohorizon has no thickness and should not be used to describe very thin stratigraphic units that are especially distinctive.

5. Subbiozone (Subzone)
A subdivision of a biozone.

6. Superbiozone (Superzone)
A grouping of two or more biozones with related biostratigraphic attributes.

7. Zonule
The use of this term is discouraged. It has received different meanings and is now generally used as a subdivision of a biozone or subbiozone.

8. Barren intervals
Stratigraphic intervals with no fossils are common in the stratigraphic section, both betweeen successive biozones and within a biozone. They are not subject to biostratigraphic classification but may be referred to as barren intervalls.

D. Kinds of Biostratigraphic Units

1. General
Five kinds of biozones are in common use:range zones, interval zones, assemblage zones, abundance zones, and lineage zones. These types of biozones have no hierarchical significance, and are not based on mutually exclusive criteria. A single stratigraphic interval may, therefore, be divided independently into range zones, interval zones, etc., depending on the biostratigraphic features chosen.

2. Range Zone
The body of strata representing the known stratigraphic and geographic range of occurrence of a particular taxon or combination of two taxa of any rank.

There are two principal types of range zones: taxon-range zones and concurrent-range zones.

a. Taxon-range Zone

i. Definition
The body of strata representing the known range of stratigraphic and geographic occurrence of specimens of a particular taxon. It is the sum of the documented occurrences in all individual sections and localities from which the particular taxon has been identified.

ii. Boundaries
The boundaries of a taxon-range zone are biohorizons marking the outermost limits of known occurrence in every local section of specimens of the taxon whose range is to be represented by the zone. The boundaries of a taxon-range zone in any one section are the horizons of lowest stratigraphic occurrence (LO) and highest stratigraphic occurrence (HO) of the specified taxon in that section.

iii. Name
The taxon-range zone is named from the taxon whose range it expresses.

iv. Local Range of a Taxon
The local range of a taxon may be specified in some local section, area, or region as long as the context is clear.

b. Concurrent-range Zone

i. Definition
The body of strata including the overlapping parts of the range zones of two specified taxa.

This type of zone may include taxa additional to those specified as characterizing elements of the zone, but only the two specified taxa are used to define the boundaries of the zone.

ii. Boundaries
The boundaries of a concurrent-range zone are defined in any particular stratigraphic section by the lowest stratigraphic occurrence (LO) of the higher-ranging of the two defining taxa and the highest stratigraphic occurrence (HO) of the lower-ranging of the two defining taxa.

iii. Name
A concurrent-range zone is named from both the taxa that define and characterize the biozone by their concurrence.

3. Interval Zone

a. Definition
The body of fossiliferous strata between two specified biohorizons.

Such a zone is not itself necessarily the range zone of a taxon or concurrence of taxa; it is defined and identified only on the basis of its bounding biohorizons.

In subsurface stratigraphic work, where the section is penetrated from top to bottom and paleontological identification is generally made from drill cuttings, often contaminated by recirculation of previously drilled sediments and material sloughed from the walls of the drill hole, interval zones defined as the stratigraphic section comprised between the highest known occurrence (first occurrence downward) of two specified taxa are particularly useful.

Interval zones defined as the stratigraphic section comprised between the lowest occurrence of two specified taxa (“lowest-occurrence zone”) are also useful, preferably in surface work. Interval zones defined between the highest occurrence of an oldest taxon and the lowest occurrence of a younger taxon are also used, particular in plankton stratigraphy.

b. Boundaries
The boundaries of an interval zone are defined by the occurrence of the biohorizons selected for its definition.

c. Name. The names given to interval zones may be derived from the names of the boundary horizons, the name of the basal boundary preceding that of the upper boundary; e.g. Globigerinoides sicanus-Orbulina suturalis Interval Zone.

In the definition of an interval zone, it is desirable to specify the criteria for the selection of the bounding biohorizons, e.g. lowest occurrence, highest occurrence etc.

An alternative method of naming uses a single taxon name for the name of the zone. The taxon should be a usual component of the zone, although not necessarily confined to it.

4. Lineage Zone

Lineage zones are discussed as a separate category because they require for their definition and recognition not only the identification of specific taxa but the assurance that the taxa chosen for their definition represent successive segments of an evolutionary lineage.

a. Definition
The body of strata containing specimens representing a specific segment of an evolutionary lineage.

It may represent the entire range of a taxon within a lineage or only that part of the range of the taxon below the appearance of a descendant taxon.

The boundaries of lineage zones have strong time significance and approach the boundaries of chronostratigraphic units. However, a lineage zone differs from a chronostratigraphic unit in being restricted to the actual presence of the segment of the evolutionary lineage upon which it is based.

Lineage zones are the most reliable means of correlation of relative time by use of the biostratigraphic method.

b. Boundaries
The boundaries of a lineage zone are determined by the biohorizons representing the lowest occurrence of successive elements of the evolutionary lineage under consideration

c. Name
A lineage zone is named for the taxon in the lineage whose range or partial range it represents.

5. Assemblage Zone

a. Definition
The body of strata characterized by an assemblage of three or more fossil taxa that, taken together, distinguishes it in biostratigraphic character from adjacent strata.

b. Boundaries
The boundaries of an assemblage zone are drawn at biohorizons marking the limits of occurrence of the specified assemblage that is characteristic of the unit. Not all members of the assemblage need to occur in order for a section to be assigned to an assemblage zone, and the total range of any of its constituents may extend beyond the boundaries of the zone.

c. Name
The name of an assemblage zone is derived from the name of one of the prominent and diagnostic constituents of the fossil assemblage.

6. Abundance zone (or acme zone)

a. Definition
The body of strata in which the abundance of a particular taxon or specified group of taxa is significantly greater than is usual in the adjacent parts of the section.

Unusual abundance of a taxon or taxa in the stratigraphic record may result from a number of processes that are of local extent, but may be repeated in different places at different times.For this reason, the only sure way to identify an abundance zone is to trace it laterally.

b. Boundaries
The boundaries of an abundance zone are defined by the biohorizons across which there is notable change in the abundance of the specified taxon or taxa that characterize the zone.

c. Name
The abundance zone takes its name from the taxon or taxa whose significantly greater abundance it represents.

E. Hierarchy of Biostratigraphic Units

The different kinds of biostratigraphic units described above do not represent different ranks of a biostratigraphic hierarchy, except in the case of subzones and superzones, where the prefix indicates the position in a hierarchy.

F. Procedures for establishing Biostratigraphic Units

It is recommended that the definition or characterization of a biostratigraphic unit include the designation of one or more specific reference sections that demonstrate the stratigraphic context of the taxon or taxa diagnostic of the unit.

G. Procedures for extending Biostratigraphic Units - Biostratigraphic Correlation

Biostratigraphic units are extended away from the areas where they were defined or from their reference sections by biostratigraphic correlation, which is the establishment of correspondence in biostratigraphic character and position between geographically separated sections or outcrops based on their fossil content. Biostratigraphic correlation is not necessarily time-correlation. It may approximate time correlation, or it may be the identification of the same biofacies, which may be diachronous; unconformities may also truncate the ranges of taxa, reducing considerably their normal ranges.

H. Naming Biostratigraphic Units

The formal name of a biostratigraphic unit should be formed from the names of one, or no more than two, appropriate fossils combined with the appropriate term for the kind of unit in question. The function of a name is to provide a unique designation for the biozone. Thus, any taxon in the characteristic assemblage of a biozone may serve as name-bearer unless it is already part of the name of a biozone in a different zonal scheme.

The printing of fossil names for stratigraphic units should be guided by the rules laid down in the International Code of Zoological Nomenclature or the International Code of Botanical Nomenclature. The initial letter of the unit-term (Biozone, Zone, Assemblage Zone) should be capitalized as well as that of the generic names; the initial letter of the specific epithets should be in lowercase; taxonomic names of genera and species should be in italics, for example Exus albus Range Zone.

The name of the taxon chosen to designate a biozone should include the entire name of the taxon. Thus, Exus albus is correct. After the first mention, the name may be abbreviated in any way consistent with clarity.

Codification of biostratigraphic zones by letters or numbers or a combination of both is common practice. If used consistently and judiciously such code designations can be extremely useful. They are brief, generally indicate the sequence and relative positions of the zones, and they facilitate communication between biostratigraphers, geologists, and other professionals. However, they do not lend themselves to insertions, combinations, deletions, or other modifications once the zonation has been published. Also, they may be a source of confusion if more than one zonation of a particular sequence of strata employs the same designations but in different ways. Code designations of biostratigraphic units should be considered informal nomenclature.

I. Revision of Biostratigraphic Units

Revision of biostratigraphic units honors priority for the sake of stability and precision in communication.

However, the first biostratigraphic zonation to be described is not necessarily the most useful. Revision or new biozonations should be clearly defined and/or characterized, be more widely applicable, offer greater precision, and be more easily identified.

Changes in nomenclature of biostratigraphic units conform with changes in the names of taxa as required by the International Codes of Zoological and Botanical Nomenclature.

Named biostratigraphic units will automatically change scope to accord with changes in the scope of taxa defining or characterizing them. A fossil name once used for a biozone is not available for use in a different zonal sense by a later author. If it is desirable to continue the use of a taxonomic term that is no longer valid, the term should be placed in quotation marks, e.g. “Rotalia” beccari Zone.