Biostratigraphy

 

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.

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

Biostratigraphic zone (Biozone). A general term for any kind of biostratigraphic unit regardless of thickness or geographic extent.  Biozones vary greatly in thickness, geographic extent, and represented time span.

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

 Kinds of Biostratigraphic Units

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.

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.

1.     Taxon-range Zone

·       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.

·       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.

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

2.     Concurrent-range Zone.

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

·       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.

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

Interval Zone 

·       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.

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

·       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;

 

Lineage Zone

·       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 (Figure 5A) or only that part of the range of the taxon below the appearance of a descendant taxon (Figure 5B).

·       The boundaries of lineage zones approach the boundaries of chronostratigraphic units.

·       However, a lineage zone differs from a chronostratigraphic unit in being restricted, as all biostratigraphic units are, to the actual spatial distribution of the fossils.

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

b

Assemblage Zone 

·       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.

·       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.

·       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 

·       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.

·       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.

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