Biostratigraphy is a part of stratigraphy that studies the distribution of fossils in a stratigraphic record. It uses these fossils to subdivide, organize, and classify strata into units based on their diagnostic fossil content before correlating and assigning relative age to strata.
Willian Smith was the biostratigraphy pioneer, with other notable contributors being Georges Baron Cuvier, Alexandre Brongniart, Gerard Deshayes, Heinrich Georg Bronn, and Charles Lyell. However, d’Orbigny and Albert Oppel contributed most to modern biostratigraphy.
Let us look at the meaning of biostratigraphy, its historical background, and its significance. Also, we will discuss something small on sequence stratigraphic analysis, biochronology, and land mammal ages.
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What is biostratigraphy?
Biostratigraphy is a branch of stratigraphy that correlates and assigns strata relative ages based on the fossils and fossil assemblage they have. It deals with fossil distribution in rock layers and uses the fossils present to classify bodies into biostratigraphic units (biozones), give relative ages, and match strata.
Also, it deals with determining the paleoenvironment where the various fossils lived. Otherwise, facies fossils would result in wrong correlations of rock layers.
During a biostratigraphy study, one needs common fossils or an assemblage from good taxa and an excellent location that 1) allows noting vertical fossil changes and selecting good boundaries and 2) whose changes and boundaries remain consistent in other places.
Fossils and biostratigraphy
Biostratigraphy defines and recognizes the bodies of rocks based on their fossil content, i.e., preserved past remains of organisms (plants and animals). Most of these organisms are extinct.
What influence fossil distribution are evolution and paleoecology. Evolution tells time, and the irreversibility of evolving organisms makes stratigraphers partition strata with boundaries. Boundaries could be when they first emerged, went mass extinct, or by other criteria.
On the other hand, paleoecology is about how past ecosystems were composed and distributed through geological timelines. It helps determine the sedimentary environment.
The fossils we will encounter in biostratigraphy include index, facies, and fossil assemblage. Let us look at each briefly.
1. Index fossils
Index fossils (zone, guide, key, or indicator fossils) are preserved remains of 1) abundant, 2) geographically widespread, 3) easy to recognize, 4) distinctive, and 5) fast-evolving organisms that help identify and define geological periods or faunal stages.
Most index fossils were pelagic, evolved, went extinct fast, and were not influenced by local conditions. Examples include Ammonites, Scaphites, Brachiopods, Graptolites, or Trilobites like Bathyuriscus rotundatus.
Besides giving a narrower interval biozone (not an entire formation), a single index fossil can determine rock relative age alone. This is one thing that methods like lithology, magnetic polarity, isotopic compositions, and seismic velocity can not do alone.
2. Fossil assemblage
A fossil assemblage is a collection of fossils found in the same layered rock or a sedimentary sequence. Their occurrence together indicates they all lived at the time the sedimentary rock was forming.
You can have a death (transported) or life (lived, interacted, and died in situ) fossil assemblage, but you should be careful to exclude any derived ones. Derived fossils are those obtained from existing rocks, then transported and deposited like clasts before lithification.
Life index or zone fossils can help assign relative ages and stratigraphic correlations.
3. Facies fossils
We don’t use facies fossils for stratigraphic correlations because they are not time significant. Instead, they help give paleoenvironments like depositional conditions control them. Why? Because organisms controlled by facies often migrate with changes in the depositional environment and tend to evolve slowly.
To know facies, determining fossil distribution is comparing the biostratigraphy of the suspected group with others in the same locality. Also, you can use any physical stratigraphic methods like lithostratigraphy to identify facies.
Biostratigraphy historical background and development
1. Early conception
William Smith (1769–1839), an English geologist and surveyor, while studying outcrop in Bath, noticed that 1) rock units have characteristic distinctive fossils, 2) these fossils succeeded each vertically in a definite, reliable, and predictable order, and 3) the fossils were identifiable over horizontal distances. This observation is what he called the principle of faunal succession.
Smith didn’t know why fossils succeeded each other in that manner, i.e., his observation predated Darwin’s Origin of Species (1859), which later explained that organic evolution was responsible for the successive change throughout geologic time. However, he used the fossils in rock layers to correlate rocks at different locations and arrange them in the right stratigraphic position, laying a foundation for biostratigraphy.
Also, Georges Baron Cuvier (1769–1832), leader of catastrophism, and Alexandre Brongniart (1770–1847) had their contribution, producing Paris Basin’s stratigraphical sequence. They also added the concept of using fossils to indicate palaeoecological conditions, not just stratigraphic positions. Although he recognized, Cuvier didn’t subdivide strata using fossils alone.
Other notable contributors were Gerard Deshayes, Heinrich Georg Bronn, and Charles Lyell, who, in 1830, 1831, and 1833, respectively, especially Lyell, applied Smith’s concepts to subdivide Tertiary strata using fossils alone.
2. Alcide Dessalines d’Orbigny stage concept
Later, in 1942, French geologist Alcide Dessalines d’Orbigny (1802–1857) showed fossil assemblages are the keys to correlation, further refining biostratigraphy. These assemblages were independent of lithology/formations or location. He called a stratum, defined by the fossil assemblage as a stage.
He then divided the Jurassic system into 10 stages and 7 in Cretaceous rocks, characterized only by fauna fossils. In his division, he based his boundaries on the appearance and disappearance of the distinctive assemblages of fossils/lifeforms or their replacement in rock layers or records by another assemblage.
3. Friedrich Quenstedt and Albert Oppel
Friedrich Quenstedt found that d’Orbigny stages were limiting in his study of Jurassic rocks in Germany. He began dividing sections based on stratigraphic ranges using individual taxa.
Quenstedt’s student, Albert Oppel (1831-1865), later amplified his work and realized narrower horizons or stratigraphic ranges. He did this by exploring the vertical range of various species. Oppel then defined his zone based on the joint occurrence of species that didn’t occur together beneath and above the zone.
Therefore, Oppel’s stratigraphic ranges were characterized by overlapping fossil ranges only, i.e., not based on the lithology of the fossil bearding beds. Also, they were independent of paleontological succession. These are similar to concurrent-range biozones and are known as Oppel zones. However, he still used the name stages.
His lower boundaries were where certain species first appeared and upper where others ended (last appeared). However, some species ranged through or even beyond the zone. He named each zone after an index fossil – just one with an assemblage.
Creating a narrow range meant it was possible to delineate strata into clearly defined or cut, smaller-scale time units. Thus, Oppel’s zonation was an accurate local, regional, and possibly global way of reconstructing strata that would measure some geological time.
However, relating biozones and absolute time remained elusive until the discovery of radiometric dating methods. Also, the same biozone boundary isn’t evidence of contemporaneous sedimentation.
Lastly, Oppel’s zonation didn’t work worldwide because the ammonites he used were unavailable worldwide. This happened because he didn’t show the role of evolution and paleontology in influencing biostratigraphic distribution. Also, modern ammonite studies lump the species Oppel considered distinct.
4. Amanz Gressly
Through his work on facies, Amanz Gressly (1814–1865) recognized facies fossils influenced by the local environment. These fossils had no time-significant event compared to the index or fossil assemblage.
Significance of biostratigraphy
Biostratigraphic helps in time-stratigraphic correlation, relative dating, and piecing history of various lifeforms and geological events. All these make it a vital instrument for creating the geological time scale by constructing Earth’s history.
1. It is used in relative dating
The unidirectional nature organic evolution process means fossil taxa don’t repeat in the whole stratigraphic record and are independent of paleontological and lithological observations. Also, fossils occur in a predictable and recognizable sequence in time and space. Thus, you can infer the strata’s relative age by comparing characteristic fossils within any sequence.
To date, biostratigraphy remains one of the relative dating methods applied to know which rock layer/body of rock or event is older than the other. Other ways are paleomagnetism and magnetostratigraphy. However, stratigraphic analysis has seen improvement in methodology, resulting in accurate results, especially using quantitative stratigraphy.
2. It helps in biostratigraphic correlation (bicorrelation)
Fossil assemblage and index fossils are the same age globally. Thus, these fossils can form a basis for matching rock layers of the same age and placing these layers in the correct stratigraphic position at different locations. It is then possible to use fossils and assemblage of fossils to create mappable 3-D space-like rock units.
This mapping and biostratigraphic correlation help create a picture of how the planet Earth looked at any interval of time throughout geologic history. However, as you interpret Earth’s history, don’t forget to consider temporal (changes with time) and environment.
3. Helps us understand past events and lifeforms
Biostratigraphy helps us understand past events and the evolution of 1) lifeforms, 2) geologic structures, 3) ancient climates, and 4) the ancient depositional environment on Earth and the sequence of other geological events.
4. Helps us understand sedimentary environments
Biostratigraphic principles can help understand sedimentary rocks and the depositional environment. For instance, using fossil records, you can know depositional boundaries or cycles by looking at shoaling or deepening events. This is possible where involved fossils are sensitive to environmental changes.
Also, these principles and biographic data may guide interpreting deposition rate and history.
5. May help in determining absolute dates
Biostratigraphy, especially that of marine sedimentary rock records, connects with relative and precise ages for most periods and boundaries of stages.
5. Itis a high-resolution stratigraphic tools
The significance of biostratigraphy has not diminished even with the discovery of modern ways to measure absolute relative age. Fossils remain tools for high-resolution stratigraphy tools. They allow recognition of a short time, i.e., 10000 to 100000 years, vital in constructing a detailed picture of various events over time.
Biostratigraphic resolution depends on the frequency of the evolution of species. i.e., how soon new species replace the old ones within the same lineage.
What is sequence stratigraphic analysis
Sequence stratigraphy analyses genetically related depositional units (sedimentary facies) within a chronostratigraphic (rock-time) context. It is a relatively newer approach that correlates depositional sequence. However, biostratigraphy is still important in providing a broad temporal framework in sequence stratigraphic analysis.
Also, using graphic schemes provides evidence for changes in sedimentation rates and hiatuses, both vital for sequence stratigraphic analysis.
Understanding biochronology and land mammal ages
Fossils of land mammals are important Cenozoic stratigraphic markers. However, they occur isolated in pockets and are rarely deposited in a thick superposed stratigraphic section, as with marine invertebrates. Thus, biostratigraphers use a different approach to work with mammalian fauna sequence, i.e., biochronology, not the usual Oppelian method.
Biochronology uses fossils (an assemblage of fossils, to be exact) to define intervals in geologic time. However, unlike biostratigraphy, it doesn’t tie fossils to stratigraphic sections.
The idea borrows from Cuvier’s conception that different organisms lived during certain geological time intervals. Therefore, fossils of a certain organism, i.e., index, represent a specific geologic time when it lived.
Biochronology uses biochronologic units or biochron Land Mammal Ages (LMA). Various regions have their own LMA units, with those of North America being NALMA.
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
- 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