Biostratigraphic units (biozones) are bodies of strata that are defined or characterized on the basis of their contained 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. Four kinds of intervals are found in sedimentary rocks:strata without fossils; strata containing organisms that lived and were buried in the area (biocoenosis); strata containing organisms that lived somewhere else and were brought into the area after death (thanatocoenosis); and strata that 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.
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 common in the stratigraphic section.
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.
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 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 and highest stratigraphic occurrence 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.
i. Definition. The body of strata including the overlapping parts of the range zones of two specified taxa.
ii. Boundaries. The boundaries of a concurrent-range zone are defined in any particular stratigraphic section by the lowest stratigraphic occurrence of the higher-ranging of the two defining taxa and the highest stratigraphic occurrence 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 (see Figures 3 and 4).
4. Lineage Zone (see Figure 5).
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.
5. Assemblage Zone (see Figure 6).
6. Abundance zone (see Figure 7).
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.
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.
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 so long as it is not already employed.
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.