Define biostratigraphic dating
Biostratigraphy is a sub-discipline of sedimentary geology that relies on the physical zonation of biota, both in time and space, in order to establish the relative stratigraphic position (i.e.older, younger, same age) of sedimentary rocks between different geographic localities.If it is desirable to continue the use of a taxonomic term that is no longer valid, the term should be placed in quotation marks, e.g. plants that take in carbon through C3 photosynthesis, which changes carbon dioxide into a compound having three carbon atoms.The same interval of strata may be zoned differently depending on the diagnostic criteria or fossil group chosen. 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. Biozones vary greatly in thickness, geographic extent, and represented time span. A biohorizon has no thickness and should not be used to describe very thin stratigraphic units that are especially distinctive. 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.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. A general term for any kind of biostratigraphic unit regardless of thickness or geographic extent. The name of the taxon chosen to designate a biozone should include the entire name of the taxon. After the first mention, the name may be abbreviated in any way consistent with clarity.
Biostratigraphic units, therefore, are objective units based on the identification of fossil taxa. 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.Although the concept is generally straightforward, i.e.the presence of a specific fossil species in two geographic localities indicates the rocks containing the fossil specimens were deposited at about the same time, in practice biostratigraphic studies tend to be complex. Stratigraphic intervals with no fossils common in the stratigraphic section. These types of biozones have no hierarchical significance, and are not based on mutually exclusive criteria. 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. 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 in any one section are the horizons of lowest stratigraphic occurrence and highest stratigraphic occurrence of the specified taxon in that section. With respect to taxon-range zones, there is no need for a hierarchy of biozone terms because the hierarchical system of biological taxonomy extends also to these biostratigraphic units in the sense that the range zone of a species is subsidiary to the range zone of the genus to which it belongs, and so on. Five kinds of biozones are in common use:range zones, interval zones, assemblage zones, abundance zones, and lineage zones. The body of strata representing the known range of stratigraphic and geographic occurrence of specimens of a particular taxon. 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 taxon-range zone is named from the taxon whose range it expresses. 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. 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.