Biostratigraphic correlation (biocorrelation) involves subdividing larger fossiliferous stratigraphic units into small-scale units or biozones based on fossil content and using these zones to match local, regional, or global strata. It is independent of lithology, rock type, or lithostratigraphic correlation.
Some biocorrelations, i.e., those based on first and last appearance, can be time significant, while others will show more paleoenvironments.
Learn about biostratigraphic correlation using taxon range, assemblage, abundant, and interval biozones. We will also talk about the graphic biocorrelation of all fossil species in two or more sections and some of the challenges of biostratigraphic correlation challenges.
What is biostratigraphic correlation?
Biostratigraphic correlation or biocorrelation involves subdividing rock strata into smaller biostratigraphic units or biozones based on their fossil content and then using these units to match strata and arrange rock layers in their correct stratigraphic position in a sequence in different locations.
It means you can match and laterally trace biozones as you do with lithostratigraphic units, and it intends to demonstrate the age-equivalence and stratigraphic position of layered rocks. You will start correlating local flora or fauna, which can be matched globally by finding a succession elsewhere where local and global taxa schemes are present.
The ability to correlate and arrange rock layers in their correct stratigraphic position within a sequence owes to the principle of biological evolution. It states that the same fossils found in different places are the same age, and thus rock layers containing these fossils must have formed when the respective organisms were extant (surviving or existing).
Faunal succession is another important law that makes correlation possible. It is all about rock strata having diagnostic fossils that appear in the same predictable succession over a wide geographical area.
Lastly, it is worth noting that organism variation (speciation or evolution) doesn’t repeat and occurs at a unique place and time, i.e., non-reversible and unidirectional. Therefore, a certain organism goes extinct and doesn’t reappear in fossil records.
Index, assemblage, and facies fossils
Fossils, i.e., index fossils and fossil assemblage that occur regionally or worldwide, are important for regional or worldwide correlation. Such make good zonal organisms and tolerate various depositional environments. However, they may not help show ancient depositional settings.
On the other hand, fossils depend on the depositional environment, i.e., sensitive ones like coral. Such gives insight into the ancient depositional environment. However, they have no time significance and cannot correlate with sedimentary strata.
Biozones as chronostratigraphic units
Biozones may act as chronostratigraphic units if you have punctuated speciation (occurring quickly, representing an instant in geological time). This also means that the species must disperse quickly geologically speaking so that you consider the base boundary isochronous and representing a point in geologic time.
Therefore, if you consider punctuated equilibria and define a biozone as the first occurrence of rapidly distributing organisms such as floating or free swimming, a biozone can be a chronostratigraphic unit with the available resolution.
Extinct within a short geological time, the top boundary defining the extinction can even be an isochronous surface (representing the same time). However, most don’t take a short time.
Kinds of biostratigraphic correlations
Some biostratigraphic units are time significant, others diachronous (occur in different geologic times) when traced laterally. For instance, taxon-range and interval biozones, especially those defined by the taxa’s first appearance of the taxa, will have correlation lines that generally coincide with timelines.
On the other hand, abundance and assemblage biozones are diachronous. Therefore, when traced laterally, they will cross timelines. However, remember that whether the time is significant or not, correlation deals with identifying and matching the same rock units in the strata using fossils.
1. Correlation using assemblage biozones
Correlation by assemblage zones involves matching various bodies of rocks defined by at least three diagnostic fossil assemblages. The different fauna and flora successions represent these biozones, and how they succeed each other on strata shouldn’t have gaps or overlap.
Assemblage zones are a good indicator of the environment and will work best for local correlation. However, marine assemblage zones, especially planktonic, can help correlate a wider area.
Unfortunately, there are inherently fuzzy boundaries between the assemblage zone since above and below the limits has a transition zone where the diagnostic fossil assemblage is missing (has vanished or not appeared). This limits the achievable limits.
Lastly, since you deal with many fossil taxa, it becomes hard to incorporate data visually and have meaningful boundaries unless you use multivariate statistical analysis. It will help recognize and define these zones.
2. Abundance zone
Correlation by abundance biozone matches bodies of rocks based on the relative plentiful of one or more taxa (species, genus, or other taxa). The abundance time is when the respective taxon or taxa is at its development peak.
Abundance zone correlation is good for biogeographic provinces, and it isn’t a time-stratigraphic correlation as the peak may not occur everywhere simultaneously. Reasons include that not all taxa achieve maximum abundance, and rocks may not record the abundance. Also, favorable ecological conditions that favor abundance may occur at different times or persist longer in some areas.
Lastly, favorable followed by sudden unfavorable that cause massive death can cause abundance zones. Also, the mechanical concentration of fossils may be a cause.
3. Correlation by taxon-range biozones
Correlation by taxon-range biozone correlates the body of rocks defined by the highest and lowest occurrence of the same fossil taxon, usually an index fossil, whose range is known.
If the taxon has a short vertical stratigraphic range, then the taxon-range biozone correlation is time-significant. However, the correlation will not have a time significance taxa range that covers long geologic periods.
4. Interval biozone correlation
Correlating by interval zones entails matching fossiliferous stratigraphic sections defined by bounding biohorizons (stratigraphic boundaries). An interval zone can either be between two specified biohorizons or characterized by the lowest lowest-occurrence of two selected taxa. Such a correlation is vital where individual taxon-range biozone is long.
If you use the lowest occurrence of two specified taxa, you will have a time-stratigraphic correlation. They are time-significant because phyletic lineage evolutionary changes that define biohorizons happen rapidly. Therefore, the interval between the occurrence of two specified taxa will take a short stratigraphic interval, age, or time, making them nearly synchronous.
On the other hand, interval zones defined by the last occurrence (upper boundary) that represent extinction are not time significant as extinction doesn’t happen rapidly.
Graphic biostratigraphic correlation using taxon-range biozone
Interval biozones do define strata units deposited in relatively short geologic durations. However, they often have intrinsically fuzzy boundaries because of organisms migrating and appearing in another place or migrating then going extinct in another place.
Therefore, empirical determination of interval zone boundaries is difficult. You may get a section extending the known range of taxa already defined (in a reference section) as they may have persisted earlier or elsewhere.
To minimize the fuzzy zone boundaries, you can treat a range statistically using a graphic or Shaw’s method proposed by Alan Bosworth Shaw in 1964 and explained by F. X. Miller in 1977.
The graphic or Shaw’s method is a time-stratigraphic correlation. It establishes strata time equivalence in two separate stratigraphic sections by plotting the first and last appearance of all fossil species (not just a few) in one stratigraphic section against the first and last appearance of the same fossil species in another section.
Miller introduced using a single stratigraphic section as a reference section plotted against others. The reference section is the most complete and thickest. Also, it should be fossiliferous (with numerous and various fossils), free of faults or structural issues, and measured and sampled as completely as possible. The local range isn’t necessarily true but can help in correlations.
This method is popular with many stratigraphers who want detailed time-stratigraphic equivalence between sections, especially on a local scale.
1. How to go about the graphic correlation
To take measurements, you must choose an arbitrary reference point, some distance above the respective section you are measuring. Then, you will record distances from reference to the first and last appearance of each fossil you have for both sections.
Each fossil’s first and last appearance in the two sections will be the coordinates for plotting your graph. They represent a precise time-stratigraphic correlation.
Once you plot all the first and last appearances, you will draw a line joining the plotted points. In most instances, you will not have a nice line correlation, i.e., plotted points will scatter. In such a case, you will pick lines of best fit or use other statistical regression methods. If gradient or steepness varies, you will have the best-fit lines in each portion.
Time-stratigraphic significance events like stable isotopes and ash fall (which occurs over a large area almost instantaneously) may help verify the line of best fit. Why? Because their presence in both sections can mark time precisely and should fall on the best-fit line
2. Deductions from graphic correlations
From the bivariate plot, you can have various deductions. For instance, if the same fossils in two sections have the same range, you will get a 45° correlation line on your graph, i.e., a bivariate plot.
On the other hand, if the correlation lines are steeper or gentler, it show constant but different deposition rates. With this, it is possible to evaluate rates of sedimentation.
Also, you can have a bent line or dogleg, i.e., having segments with varying steepness, which indicates a change in relative accumulation rates. In contrast, the horizontal portion shows erosion or no deposit accumulation, such as a diastem or unconformity.
Point plotting well off the best-fit line indicates species in the two sections disappeared at different times. Such shows facies dependence (environmentally controlled) or a migration barrier caused them to appear at varying times.
3. Composite standard or section
Besides the correlation between the two sections, you can compile a composite standard or section by correlating more sections, one after another.
After making a database of composited fossil ranges, the database is scalable to chronostratigraphic/time units and used to correlate a specific stratigraphic section against the composite standard, just like one stratigraphic section correlates to another. Comparing individual ranges with the composite standard can help detect any breakages and if it need amendment.
Correlating the composite standard scaled in time units, you can determine the age of any stratigraphic section.
4. Why is graphic correlation better?
It is better than standard biostratigraphic methods because:
- This method can help evaluate sedimentation rates between sections (slope), and reveal a hiatus.
- It trains people to measure and accurately describe sections, including where the fossils are and their respective positions in strata.
- Graphic correlation is the most practical way to establish biostratigraphic standards where limitations and errors are not easy to notice.
- You can easily visualize the accuracy and precision of the various taxa ranges used, i.e., you can see all potential points of correlation and evaluate their usefulness.
- You can extend Shaw’s method to other relative dating methods.
5. Challenges of graphic correlations
One limitation of the graphic correlation is that it only applies within a certain biogeographic region. Others need their separate standards.
Subsurface correlation
Biostratigraphy. may help in subsurface analysis but using fossils only (radiolarians, forams, or nanofossils) obtained from core and drill cutting. These fossils may help determine the relative age of rock strata at different levels, and you can use the data to correlate the age of strata from other boreholes or exposed surfaces.
Biogeographical provincialism and strata correlation
A biogeographic province represents an area where groups or groups of organisms have lived and evolved in isolation for a long duration, separated by barriers (climatic or physical). Barriers include land, open marine, warm/cold water, deep/shallow water, fresh/saline water, plate movement, sea level changes, ocean currents, etc.
Biocorrelation within a biostratigraphy province is possible but will only represent an ancient sedimentary environment if no evolution happens. However, it is impossible to correlate between different regions.
Nonetheless, it helps show different paleoclimates and barriers that isolated an evolutionary lineage.
Fossil time correlations challenges
The thickness of a biostratigraphic unit depends on how quickly or slowly sediments accumulate during a given geologic time. Therefore, the sedimentation rate and frequency of speciation may present a challenge in biostratigraphic correlation.
For instance, rapid sedimentation with infrequent speciation may result in thick strata within the same biozone. Such a case means no further subdivision or correlation can happen.
Secondly, the local vertical range may be incomplete compared to the global one due to events such as migration or organisms breaking a biogeographical province. Also, unconformities, fossil preservation issues, and sampling may worsen it.
A range difference will result in miscorrelation as an organism deemed extinct in one area may have lived longer in another.
References
- 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.
- Boggs, S. (2014). Principles of Sedimentology and stratigraphy (5th ed.). Pearson Education.
- Brookfield, M. E. (2004). Principles of stratigraphy. Blackwell Pub.
- 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