Biozones or biostratigraphic zones are the fundamental units for naming and classifying stratigraphic sections based on their fossil content. There are five biozones: range (taxon-range, concurrent range), abundance, lineage, assemblage, and interval zones.
On the other hand, a biostratigraphic unit refers to a body of rock delineated by diagnostic fossil content. Its general description is a biozone.
Let us investigate the idea of biozones and biostratigraphic units, including their meaning and the various zones and definitions. We will also discuss their nature, naming, boundaries, establishment, etc.
What are biostratigraphic units?
Biostratigraphic units refer to bodies of rocks or stratigraphical intervals characterized by the fossils they have. These defined and discrete stratigraphic units represent bodies of rocks formed during a certain time interval when fossil organisms existed.
Generally described as biozones, biostratigraphic units are descriptive units based on identifying a characterizing or defining attribute that includes diagnostic taxon, combination taxa, defined morphology, or any other feature that affects the distribution or content of fossil in strata.
The vertical and lateral extent they define is modifiable with new evidence, including a change in taxonomic base. Furthermore, delineate the same stratigraphic section or interval using different 1) stratigraphic units, 2) criteria, or 3) fossil groups. How you define the units may result in one strata interval with gaps or overlaps.
Biostratigraphic units may be local or global. Also, their thickness varies, and their definition is independent of rock lithology/type or its time of origin. Neither does it have to correspond with lithostratigraphic units or formations or their boundaries. Therefore, a formation may have one or several biostratigraphic units, and several formations be one biozone.
Lastly, biostratigraphic units provide a more refined strata subdivision than lithology. Also, they allow further subdivision of even a stratum and rocks of the same lithology.
Index and fossil assemblage
Index fossils and fossil assemblage help determine the relative age or times of origin of rocks and form the basis of biostratigraphic zonation. Also, they indicate the distribution of paleoenvironments and sedimentation patterns.
When using fossils, identify and exclude reworked, introduced, or infiltrated ones. Why? Because they don’t represent their time of origin and sometimes environment.
Biostratigraphic classification
Biostratigraphic classification systematically subdivides and arranges strata sections into units defined by their diagnostic fossil content.
What is a biostratigraphic zone or biozone?
Biostratigraphic zone or biozone is a general term for any biostratigraphic unit. It is the fundamental unit for biostratigraphic nomenclature and classification.
Biozones are independent of thickness, geographic extent, and time. Also, you can divide them based on one or more taxonomic groups.
In the initial formal naming of any biostratigraphic unit, add it with the first letter capitalized, i.e., Globigerina brevis Taxon-range Biozone. Subsequent usage requires you to omit the kind of biostratigraphic unit and the word biozone and use the word Zone (capitalized), i.e., Globigerina brevis Zone.
Also, the term zone replaces biozone once it is clear it is a substitute to avoid confusion with other zones. This applies to subzones to mean subbiozones or superzones for superbiozones.
Lastly, biozones are important in paleogeography, palaeoecology, paleoclimatology, geology, and palaeoceanography, besides biostratigraphy.
More definitions
- Subbiozones or subzones – Refer to a partial or complete division of biozones following the rules used to define and characterize a biozone.
- Superbiozones or superzones – Two or more biozones sharing some biostratigraphic features or attributes grouped.
- Biostratigraphic horizons or biohorizons – Biohorizons are stratigraphic surfaces, boundaries, or interfaces characterized by a notable change in fossil content type or biostratigraphic character. This surface lacks thickness and cannot describe a unique biozone.
- Barren intervals – These stratigraphic intervals lack fossils in stratigraphic sections inside or between successive biozones. We don’t biostratigraphically classify these intervals.
What is the nature of biostratigraphic units
It concerns the idea of unfossiliferous rocks and the contemporaneity of rocks and fossils. Also, it looks at the independence of biostratigraphic units from 1) lithostratigraphic units and 2) chronostratigraphic units.
1. Unfossiliferous rock
A body of rock without named fossils doesn’t have biostratigraphic character. Therefore, they don’t comply with biostratigraphic classification.
2. Rock and fossils contemporaneity
Most fossilized organisms lived at the same time as when the enclosed body of rock forms, i.e., most fossils are contemporaneous with enclosing rock bodies. This includes those derived from different depositional environments with the same time of origin, i.e., coeval.
However, some bodies or rocks may have fossils derived from older or younger rocks. Such fossils are not contemporaneous with the rock body that encloses them and shouldn’t help define, identify, or characterize a biostratigraphic unit.
3. Independence from lithostratigraphic units
The fundamental criteria for biostratigraphic units differ from those used in lithostratigraphic units. Their boundaries may or may not coincide, but they have no inherent relationship or interdependence.
4. Independence from chronostratigraphic units
Unlike chronostratigraphic units’ boundaries, biostratigraphic units’ boundaries are diachronous conceptually and characteristically. Their vertical and lateral limits define recorded distribution limits for characterizing and defining fossil elements.
Regionally, a biostratigraphic unit’s upper and lower boundaries rarely have synchronous surfaces, and the lateral boundaries never have synchronous surfaces. However, biostratigraphic units work effectively in interpreting chronostratigraphic relations.
What are the kinds of biostratigraphic units
There are five recognized biozones, namely range, interval, lineage, assemblage, and abundance biozones. These biozones are not hierarchically interrelated with their names, only descriptive terms.
Also, biozones are not based on exclusive mutual criteria. Therefore, a single stratigraphic unit may have interval, lineage, or range zones. The approach for setting up and recognizing biozones depends on 1) the scientists’ approach, 2) the biostratigraphic features chosen, 3) the biota, and 4) the problem under investigation.
Most commonly, it recognizes upper and lower boundaries based on the first and last occurrence of the taxon or taxa. These taxa may or may not have a direct phylogenetic link.
Using the lowest/highest occurrence or maximum abundance to define boundaries is not necessarily restricted to any biozone or doesn’t necessarily have to range through the entire biozone.
1. Range biozones
A range biozone refers to a body of rock representing a known range of stratigraphic (vertical) and geographic (lateral) occurrences of a certain chosen taxon or combination of two taxa of any taxonomic rank.
Put differently, a range biozone is a body of rock representing an entire geological lifespan of a unique group of organisms marked by the first occurrence (lowest) and highest occurrence (extinction).
You may notice gaps in range zones. They occur due to variable preservation, little abundance, a small sample size, and structural complications.
There are two types of biozones, namely:
a). Taxon-range biozones
A taxon-range biozone refers to a body of rock representing a known stratigraphic and geographic range of a chosen taxon, such as the same family, genus, or species, and is named after the defining taxon. When in a certain locality, it will be a local range zone, teilzone, topozone, or local zone.
Delineation is based on the taxon’s entire occurrence range (lowest to highest stratigraphic occurrence – LO to HO) and will have inherently diachronous boundaries. Why? Because the first appearance and extinction will occur at different times in different locations.
b). Concurrent range biozone
A concurrent range zone (range overlap or overlap zone) is a body of rock representing an overlapping section of two taxa. It helps recognize strata representing a narrower rock record or time increment provided by range zones and occurs where both taxa are present together. Such will have a similar tolerance, geographic scope, short space, and time (temporal) duration.
The two defining taxa determine the boundaries, and this zone is named after the two taxa. The lowest stratigraphic occurrence (LO) of the taxa whose range is higher stratigraphically and the highest stratigraphic occurrence (HO) of the lower-ranging taxa form the lower and upper boundaries.
They tend to be less diachronous when properly defined than the taxon-range zone. Also, facies have less controlling effects. However, unlike assemblage zones, you don’t include all elements of fossil assemblages present.
c). Oppel zones vs. concurrence zones
An Oppel zone is a body of rock representing a group of fossils’ overlapping range and range limits. The first appearance of a selected species serves as the lower boundary, while the upper will be the appearance of a chosen species for the next successive zone.
The objective minimizes the diachrony of boundaries while maximizing geographic extent. Since they are difficult to define empirically, they were not included in the second edition of IUG.
2. Interval biozone (the zone between two biozones)
An interval biozone (gap, partial-range, or interbiohorizon) refers to a fossiliferous rock body bound by two specified biohorizons. This zone isn’t necessarily a taxon or concurrent taxa range zone.
For the subsurface (drilled downward), an interval zone is a stratigraphic section bound by the highest known occurrences of two specified taxa going downwards, making it a highest-occurrence zone.
On surface work, an interval zone is bound by the specified highest occurrence oldest taxon and lowest occurrence of the youngest taxon. For this reason, it is at times known as the lowest-occurrence zone.
Besides unique organisms, a change in taxon character may be a time marker that defines a boundary. For instance, the variation in the coiling direction of foraminifera from left to right represents a character change. It occurs with changes in ocean water temperatures due to global climate changes.
Lastly, naming an interval zone follows the taxa names of two defining taxa joined by a dash.
3. Lineage biozones
A lineage zone is a body of rock with a specific segment of a certain taxon’s successive evolutional lineage. Also known as phylozone, evolutionary, morphogenic, or morphoseries zones, its basis is the species’ first and last occurrence within the evolutional line under study and is named relevant taxon.
It is an accurate zone for biocorrelating strata, and its boundaries have time significance that nears chronostratigraphic units. However, unlike chronostratigraphic units, its basis is on an actual evolutionary lineage segment.
4. Assemblage biozones
An assemblage biozone (ecological or biofacies zone, ecozone, or faunizone) refers to a body of rock defined or characterized by three or more unique taxa associations that differentiate its biostratigraphic character from adjacent strata. These taxa associations represent organisms that lived together simultaneously (and preferably interacted).
Boundaries are the limits of occurrence of a specified assemblage (where they occurred at the same time), not necessarily the range of all the members of an assemblage. The lowest and highest may be of one or more members.
Usually, assemblage biozones have many taxonomic groups but can be one. However, you need at least two or more characteristic fossils with the first and last occurrence to define boundaries. These boundaries will be between where the fossils overlap.
Without at least two characteristic fossils, these zones will be unrecognized, or you will have the Oppel Zone, according to the North American Stratigraphic Code and the International Stratigraphic Guide.
Lastly, naming follows the taxa of easy-to-recognize and common member(s) of the fossil assemblage.
5. Abundance biozone (peak, flood, epibole, or acme zones)
An abundance biozone is a body of rock in which a specified taxon or a group of taxa are plentiful or significantly greater than in adjacent sections or parts. Change in the relative abundance of taxon or taxa marks and defines boundaries but doesn’t cover the full range of taxon/taxa.
Paleoecology, diagenesis, and taphonomy largely control abundance zones. Thus, most are a local utility, limited, and will have no time significance. However, some peak changes may be regional or global due to climatic changes. Such will be important time markers.
Lateral tracing is the unmistakable way to recognize abundance zones, and their significance is seen more in commercial biozones.
Lastly, naming follows the abundant taxon or taxa that characterize the zone.
6. Hybrid or new kinds of biozones
These represent other forms of biozone if useful, especially with new technologies and the progression of biostratigraphic analysis to address specific problems.
Understanding biozones boundaries
Biozone boundaries are surfaces drawn to mark the lowest or highest occurrence, limit, or increase/decrease in abundance of one or more fauna and flora components. The tops and bases of one biozone kind may or may not coincide with the base or top of another type. Neither do biozones of a taxonomic group need to coincide with another.
Successive biostratigraphic units that don’t overlap or are in different locations require a third as a fossiliferous and continuous reference section. It will help document the transition necessary for comparison and should have several taxonomic groups to use.
There are various approaches to deciding on boundaries of any biostratigraphic zone, i.e., mass extinct or faunal break, first occurrence, last occurrence, or a combination of first and final occurrences. Also, a change in abundance may be a basis.
Some scientists consider mass extinction or faunal breaks a sign of unconformity, as faunal breaks shouldn’t be abrupt. Thus, it is not a good basis for a boundary. However, it could be if the mass extinction is due to some catastrophe or climatic change.
The first occurrence is the biozone’s lower or base boundary and is good for people working on outcrops or exposed strata. The upper limit will be the base of the successive upper zone. This approach is a better way to help prevent blurred boundaries if the reference section has an unconformity. You can add another biozone on top.
Since we drill downwards, the last appearance works as a better top boundary for subsurface cases. Upper fossils will first come out during borehole drilling. Thus, this approach will eliminate picking upper fossils from the borehole wall. Also, you will avoid the issue of downward caving and reworking.
Some people propose the use of the first and last appearance.
Please note that the first appearance doesn’t represent the first evolution. It may have started elsewhere and taken time to spread. Also, incomplete sampling and poor preservation of some facies can result in a lack of some fossils, yet they were present. Similarly, the last appearance doesn’t represent extinction or evolution for the same reasons.
What is a reference section?
Unlike lithostratigraphic units, biostratigraphic units don’t use a type section or stratotype. However, a reference section is necessary for developing biozones. Also, a geographical extent is required.
Your reference section should be well-studied, have good exposure, and be easily accessed by geologists worldwide, including in the future for study and fossil collection. Also, it must be as complete as possible and have abundant, well-studied, and documented fossils.
For instance, a section with a complete evolutionary sequence of taxa or a group of taxa means no section is missing. Those with gaps will result in blurred boundaries should another more complete reference section exist.
Setting a biozone boundary
On agreeing upon a boundary, geologists place a physical marker (the Golden Spike), and no arguments will be necessary afterward. If the boundary decision is via an international agreement, it gets a GSSP (Global Standard Stratotype Section and Point) designation.
Naming biostratigraphic units
Establishing a formal biostratigraphic unit requires 1) a unique name, fossil content description (including characterizing or defining taxa), and their boundaries 3) spatial extent discussion.
Biozone name = Name of one or a combination of no more than two characteristic fossils or for trace fossils, a parataxa (full taxon name but don’t capitalize epithet name) + Unit-term (capitalize the first letter) + biozone (capitalize the first letter). Subsequence use, omit Unit-term and Biozone and use zone (capitalized).
The fossil name should conform to the International Code of Zoological Nomenclature or the International Code of Botanical Nomenclature guidelines.
You can use any name of the characteristic fossil(s) unless already used as part of a biozone name in another zone scheme. A common convention is to use the taxa or species name whose lowest occurrence defines the zone base, or you can use the abundant or those that define upper and lower boundaries.
Lastly, you can shorten the Name with alpha-numeric or abbreviations after a formal name to represent a biozone or subzone.
Revising biostratigraphic units
Biozone creation uses an empirical approach. Thus, new prevailing evidence, including taxonomy refinement, may warrant a revision of stratigraphic boundaries or removal or recognition of a new biozone. Also, you can pick new characterizing taxa.
These may increase or decrease biozone extent and require a formal description or amendment of the biozone. This process involves the designation/redesignation of characterizing or defining taxa and boundaries.
References
- North American Commission on Stratigraphic Nomenclature. (2021). North American stratigraphic code. Stratigraphy, 18(3), 153-204. https://ngmdb.usgs.gov/Geolex/resources/docs/NACSN_Code_2021.pdf
- Murphy, M. A., & Salvador, A. (Eds.). (2000). International Subcommission on stratigraphic classification of IUGS International Commission on stratigraphy. GeoArabia, 5(2), 231–266. https://doi.org/10.2113/geoarabia0502231
- Prothero, D. R., & Schwab, F. (2014). Sedimentary geology: An introduction to sedimentary rocks and Stratigraphy (3rd ed.). W.H. Freeman and Company.
- MacLeod, N. (2005). Biozones. In Selley, R. C., Morrison, C. L. R., & Plimer, I. R. (Eds.). Encyclopedia of geology (Vols. 1-5). Elsevier Academic.