A. Nature of biostratigraphic unitsEdit
A biostratigraphic unit is a body of rock defined or characterized by its fossil content. They are descriptive units based on the identification of fossil taxa. Their recognition depends, therefore, on the identification of either their defining or characterizing attributes. The basic unit in biostratigraphic classification is the biozone, of which there are several kinds.
Note 1. Enclosing strata. Fossils that define or characterize a biostratigraphic unit are commonly contemporaneous with the body of rock that contains them. Some biostratigraphic units, however, may be represented only by their fossils, preserved in normal stratigraphic succession (e.g. on hardgrounds, in lag deposits, in certain types of reworked accumulations), which alone represent the rock of the biostratigraphic unit. In addition, some strata contain fossils derived from older or younger rocks or from essentially coeval materials of different facies; such fossils should not be used to define a biostratigraphic unit.
Note 2. Independence from lithostratigraphic units. Biostratigraphic units are based on criteria which differ fundamentally from those for lithostratigraphic units.
Note 3. Independence from chronostratigraphic units. The boundaries of most biostratigraphic units, unlike the boundaries of chronostratigraphic units, are both characteristically and conceptually diachronous. An exception is an abundance biozone boundary that reflects a mass-mortality event. The vertical and lateral limits of the rock body that constitutes the biostratigraphic unit represent the limits in distribution of the defining biotic elements. The lateral limits never represent, and the vertical limits rarely represent, regionally synchronous events.
B. Kinds of biostratigraphic unitsEdit
|Fig. 5-1. Types of biostratigraphic interval zones. (a) Taxon Range Zone – The lower, upper and lateral limits of this zone are determined by the range of occurrence of taxon a. (From ISSC 1994, Fig. 5, p. 58). (b) Concurrent Range Zone - The lower, upper and lateral limits of this zone are determined by the range of concurrent occurrence of taxon a and b. (From ISSC 1994, Fig. 6, p. 59). (c) Interval Zone of ISSC (1976, p. 53, 60); a and b can be related or unrelated taxa. (d) Interval Zone of ISSC (1976, p. 53, 60); c and d are unrelated taxa.|
1. Range ZoneEdit
A range zone is the body of strata representing a known range of occurrences of any selected element or elements of the assemblage of fossils present in a stratigraphic sequence. The word "range zone" means extent in both stratigraphic and geographic sense. There are two types of range zones defined herein, the taxon-range zone and the concurrent-range zone.
a. Taxon-range Zone (Fig. 5-1a)
i. Definition. A taxon-range zone is the body of strata representing the known range of occurrence (stratigraphic and geographic) of specimens of a particular taxon (species, genus, family, etc.). It is the sum of the documented occurrences in all individual sections in which the particular taxon has been identified. An example is the calcareous nannofossil taxon range zone of Discoaster quinqueramus whose highest and lowest occurrence represents Zone NN11.
ii. Boundaries. The boundaries of a taxon-range zone are surfaces marking the outermost limits of known occurrences in each and every local section of specimens of the taxon whose range is to be represented by the zone. The boundaries of the taxon-range zone in any one section are the horizons of lowest stratigraphic occurrence and highest stratigraphic occurrence of specified taxon in that section.
iii. Name. The taxon range-zone is named from the taxon whose range it expresses, for example, Globigerina brevis Taxon-range Zone.
iv. Local range of a taxon. Reference to the local range of a taxon should be indicated by the name of the taxon followed by the locality or area where it is found, e.g. " the range zone of taxon F at section Z".
b. Concurrent-Range Zone (Fig. 5-1b)
i. Definition<. A concurrent-range zone is the body of strata including the concurrent, coincident, or overlapping parts of the range zones of two specified taxa selected from among the total forms contained in a sequence of strata. Other taxa may be included as characterizing members of the zone, or beginning or terminating in the zone, but only two can be used for defining the boundaries.
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 of the taxa which characterize the biozone by their concurrence, e.g. Taxon A-Taxon B Concurrent-range Zone.
2. Interval Zone (Figs. 5-1c and 5-1d)Edit
a. Definition. An interval zone is a body of fossiliferous strata between two specified biostratigraphic horizons (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. Barren intervals between two distinctive biohorizons are not interval zones.
The base or the top of an interval zone may be marked by
- the horizon of the lowermost documented occurrence of a specified taxon in any particular section.
- the horizon of the uppermost documented occurrence of a specified taxon in any particular section.
- any other distinctive biostratigraphic feature (biohorizon).
An example of an interval zone is the calcareous nannofossil Zone NN18 which is marked by the highest occurrence of Discoaster brouweri on top and the highest occurrence of Discoaster pentaradiatus at the bottom.
An interval zone defined as the stratigraphic section between the lowest known occurrence of two specified taxa (lowest occurrence zone) is also a useful type of biozone.
Interval zones established to partition the range of a taxon on the basis of the occurrence of two other taxa whose ranges do not overlap, in other words, the part of the range of the taxon above the uppermost documented occurrence of one taxon and below the lowermost documented occurrence of the other taxon, have been called partial-range zones (Report of the Stratigraphical Code Sub-Committee, Geological Society of London, 1967, p. 85). An example is the Globigerina ciperoensis Partial-range Zone which, by definition, is the interval zone between the uppermost occurrence of Paragloborotalia opima opima and the lowermost occurrence of Globorotalia kugleri and occupies only a part of the range of Globigerina ciperoensis.
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 with the name of the basal boundary preceding that of the upper boundary; for example, Globigerinoides sicanus-Orbulina suturalis Interval Zone. A name of this type, however, does not tell whether the named taxa appear or disappear at the zone boundaries, or if some other criterion is involved (e.g., abundance, dwarfing or gigantism, coiling direction).
Alternatively, the name of a single taxon well represented in an interval zone, though not necessarily confined to it, may be used to name the zone, even though the boundaries of the zone may have been selected on the basis of the occurrence of other taxa.
|Fig. 5-2. Examples of lineage zones (from ISSC, 1994, Fig. 9, p. 61). In (A) the lineage zone represents the entire range of taxon b, from the highest occurrence of its ancestor, taxon a, to the lowermost occurrence of its descendant, taxon c. In (B) the lineage zone represents that part of the range of taxon y between its lowest occurrence and the lowest occurrence of its descendant, taxon z.|
3. Lineage Zone (Fig. 5-2)Edit
a. Definition. A lineage zone is a 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 (Fig. 5-2a) or only that part of the range of the taxon below the appearance of a descendant taxon (Fig. 5-2b).
b. Boundaries. The boundaries of a lineage zone are determined by the biohorizons representing the lowest occurrence of successive elements in 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; for example, the Miogypsina intermedia Lineage Zone or the Globorotalia fohsi fohsi Lineage Zone. This type of biozone has also been called an evolutionary zone, a morphogenetic zone, or a phylogenetic zone.
The term phylozone, though having etymological affinity, has been applied to a kind of stratigraphic unit different from a lineage zone. As originally defined by van Hinte (1969, p. 271), a phylozone is the "belt of rock formed during a biochron". Since a biochron is a unit of geologic time, the total span of time of existence of a taxon, the phylozone, as defined by van Hinte, represents all the rocks formed anywhere during such a time span, whether or not the taxon is actually present. It is, therefore, a chronostratigraphic unit, not a biostratigraphic unit.
|Fig. 5-3. Abundance zones (from ISSC, 1994, Fig. 11, p. 64).|
4. Abundance Zone (Fig. 5-3)Edit
a. Definition. An abundance zone is a stratum or 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 part of the section, regardless of either association or range. The abundance zone has been called acme zone (first edition of ISG), peak zone and flood zone. An example of abundance zone is the calcareous nannofossil Zone NN10 to NN11 which is marked by the abundance of Reticulofenestra pseudoumbilicus.
b. Boundaries. The boundaries of an abundance zone are defined by the biohorizon of notable change in abundance of the taxon or group of taxa chosen to characterize the zone.
c. Name. The abundance zone takes its name from the taxon or taxa whose significantly greater abundance it represents.
Note 1. Unusual abundance of a taxon or group in the stratigraphic record may be the result of a number of processes, many of them influenced by local environmental, ecological or post depositional conditions and may therefore vary in stratigraphic position between geographically separated sections or occur at several levels anywhere within the stratigraphic range of the taxon or group of taxa under consideration (Fig. 5-3). The only sure way to identify a particular abundance zone is to trace it laterally. More than contents of taxa alone must be used, or unwarranted assumption of continuity may mistakenly be made. Abundance zones, therefore, are generally of local utility only.
|Fig. 5-4. Assemblage zone. In this example, the assemblage diagnostic of the zone includes nine taxa with diverse stratigraphic ranges. for this assemblage zone to be useful, it may be necessary to provide some explicit description of its boundaries; for example, the lower boundary can be said to be placed at the lowermost occurrence of taxa a and g and the upper boundary at the highest occurrence of taxon e. Most of the taxa of the assemblage characteristic of the zone should, however, be present.|
5. Assemblage Zone (Fig. 5-4)Edit
a. Definition. An assemblage zone is a stratum or body of strata characterized by a distinctive assemblage or association of three or more fossil taxa that, taken together, distinguishes it in biostratigraphic character from adjacent strata.
An assemblage zone may be based on all kinds of fossil present, or it may be restricted to only certain specified kinds. Thus 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; an assemblage zone of planktonic forms or an assemblage of benthic forms; and so on.
Assemblage zones are usually linked to local areas or regions, as they are closely associated with life environments that vary greatly geographically. However, marine planktonic fossil assemblages may approach worldwide extent within restricted latitude ranges and under conditions where variation in temperature is low. Assemblage zones, therefore, may be particularly significant as indicators of environment. They also may be indicators of geologic age.
Note 1. Nature. The bases for recognizing assemblage zones include variations in the fossil taxa, in abundance of specimens, or in both. Such variations are usually in response to environment though evolutionary change may be a factor. The assemblage zone may indicate ecologic facies or age or both. It is, however, primarily a grouping of strata according to directly observable fossil content. Assemblage zones may be based on all the fossils or only on specific kinds. The assemblage on which a specific unit is based should be defined in a specified section.
Note 2. Naming. The assemblage zone is usually named from one or more taxa particularly prominent or diagnostic of the assemblage, although name-givers need not be confined to the zone or found in every part of it.
- Example. The Heterostegina Assemblage Zone of the Gulf Coast and the Lepidocyclina Assemblage Zone of the Visayan Basin are examples.
- Guide fossils. The fossil or fossils most characteristic of an assemblage zone, and those chosen to name it, as well as other characteristic fossils in the assemblage, are termed guide fossils. Neither the name-givers nor the other guide fossils are necessarily restricted to the zone, nor are they found in every part of it.
b. Boundaries. The boundaries of an assemblage zone are drawn at surfaces (biohorizons) marking the limits of occurrence of the assemblage characteristic of the unit. Not all members of the assemblage selected for its characterization need occur in order for a section of strata to be assigned to an assemblage zone, and the total range of any of its constituents may extend beyond the boundaries of the assemblage zone. Identification of the zone and its limits depends on identification of the characteristic assemblage. Whenever an assemblage zone is established, the fossil assemblage that characterizes it should be explicitly given. However, in cases when an assemblage zone is proposed on the basis of many fossil taxa with diverse stratigraphic ranges, the recognition of its limits is difficult.
c. Name. The name of an assemblage zone should be derived from the name of one or preferably no more than two, of the prominent and diagnostic constituents of the fossil assemblage; for example, Eponides Assemblage Zone.
C. Hierarchy of biostratigraphic unitsEdit
The different kinds of biostratigraphic units described above do not represent different ranks of a biostratigraphic hierarchy. Range zones, for example, are not subdivided into assemblage zones or vice versa. Some kinds of biozones, however, may be usefully subdivided into subbiozones (subzones) and/or grouped into superbiozones (superzones)
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.
D. Procedures for establishing biostratigraphic unitsEdit
General procedure for establishing stratigraphic units is discussed in Chapter 1 and the procedure for biostratigraphic units accords closely with that for other stratigraphic units in most respects. Special mention is made again, however, of the need to specify the kind of biostratigraphic unit being proposed and the basis used for defining its limits. Figures and descriptions of the taxa diagnostic of a unit should also be provided, or references to the literature in which they can be found should be given.
In setting up new biozones or in selecting for use biozones that already have been proposed, practicability in identification and correlation should be considered. Other things being equal, units based on abundant, widespread, stratigraphically restricted and recognized taxa should be preferred. Esoteric zonal criteria lose value because of difficulty of application.
Biostratigraphic units are often based on concepts which cannot be readily tied in advance to a specific interval of the stratigraphic section because the stratigraphic scope of the unit may vary widely with increasing information. The scope and character of a biostratigraphic unit should, therefore, be defined by carefully specifying the kind of zone under discussion and the diagnostic taxon or taxa on which it is based. It is desirable, however, that the definition and description of a biostratigraphic unit and of its boundaries include the designation of one or more specific reference sections that demonstrate the occurrence of the taxon or taxa diagnostic of the unit and that permit its recognition elsewhere. The designation of one or more such reference sections serves as protection against the inadequacies of language, fossil recovery, and the uncertainties of taxonomic identification.
E. Procedures for extending biostratigraphic units – biostratigraphic correlationEdit
Biostratigraphic units are extended away from the areas where they were defined or from their reference sections by biostratigraphic correlation (biocorrelation), 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 and potentially diachronous because homotaxy does not always imply synchronicity.
F. Naming of biostratigraphic unitsEdit
The formal name of a biostratigraphic unit should be formed from the names of one, or preferably no more than two, appropriate fossils combined with the appropriate term for the kind of unit in question. A disadvantage of names formed from more than two taxa is their often cumbersome length. This difficulty has commonly been circumvented by naming the biozone for a single taxon that occurs in the interval although not otherwise specially diagnostic of the biozone. The function of the taxon thus selected is name-giving. It may or may not be common and may or may not be a particularly good guide-fossil among the members of the assemblage. Single-taxon names of this type may be considered formal provided their introduction is duly proposed and is accompanied by an unequivocal designation of the zonal limits.
The same name should not be used for different biostratigraphic units of the same kind, even if of different rank.
The writing and printing of fossil names for stratigraphic units should be guided by the rules laid down in the International Code of Zoological Nomenclature and in the International Code of Botanical Nomenclature. The initial letter of generic names should be capitalized; the initial letter of the specific epithets should be in lower case; taxonomic names of genera and species should be in italics. The initial letter of the unit-term (Biozone, Zone, Assemblage Zone) should be capitalized; for example, Exus albus Assemblage Zone.
The name of the fossil or fossils chosen to designate a biozone should include the genus name plus the specific epithet and also the subspecies name, if there is one. Thus Exus albus Assemblage Zone is correct. After the first mention, the genus name may be abbreviated to its initial letter if there is no danger of confusion with some other genus beginning with the same letter; for example, Exus albus may be shortened to E. albus. On the other hand, the use of the specific epithet alone, in lowercase or capitalized, in italics or not (albus Assemblage Zone, Albus Assemblage zone, albus Assemblage zone, or Albus Assemblage zone), is inadvisable because it can lead to confusion in the case of frequently used species name. However, once the complete name has been cited, and if the use of the specific epithet alone does not cause ambiguous communication, it may be used, in italics and lowercase, in the designation of a biozone; for example, uniformis Zone.
Codification of biostratigraphic zones by letters or numbers or a combination of both is becoming a common practice. If used consistently and judiciously such code designations can be extremely useful. They are brief and they avoid repetition of the lengthy formal names of zones (an advantage in both written and oral presentations); also, letter and/or number sequences automatically indicate the sequence and relative positions of the zones (not true of their formal names); and they facilitate liaison between biostratigraphers, geologists and other professionals such as engineers. On the other hand, code designations do not lend themselves readily to insertions, combinations, eliminations, or revisions within a zonal sequence once it has been published. Letter/number designations have no intrinsic meaning , and confusion can arise if two or more stratigraphers have applied them in different senses within the same general area. Code designations of biostratigraphic units, if used for reasons of brevity, should be considered informal nomenclature. They should be explained in each published work in which they are used or references to the literature in which they are set forth should be given.
G. Revision of biostratigraphic unitsEdit
Revision of biostratigraphic units in general is discussed in Chapter 1B-5 and the basic rules of priority are discussed in Chapter 1B-4. Priority should be preserved for the sake of stability and ease of communication, but in the case of biostratigraphic units, it must be kept in mind that out of the almost limitless number and variety of overlapping biozones that could be proposed, the first to be described and named is not necessarily the most useful. This means that workers must continually be free to propose new zones or improve previous proposals in both scope and nomenclature. Among critical considerations affecting the adoption of any newly proposed biozone or revision of an already existing biozone should be adequacy of description, freedom from ambiguity, extent of applicability, and, of course, that the new biozone is not a synonym.
Names of biostratigraphic units should be changed to conform with changes in names of taxa required by the International Code of Zoological Nomenclature and by the International Code of Botanical Nomenclature. Also, named biostratigraphic units will automatically change to accord with changes in the scope of taxa that may have been recognized subsequent to the naming of the unit. A fossil name once used for a biozone should not be available for use in a different zonal sense by a later author. If it is desirable to continue use of a taxonomic term which is no longer valid, the term should be in quotation marks, for example, "Rotalia" beccarii Zone.