Principles of Biostratigraphy: The ‘Biostratigraphy Party’
Biostratigraphy is the process of assigning an age to a rock based on the fossils it contains. To do this, a biostratigrapher needs to identify a particular type of fossil called an ‘index fossil’. Index fossils need to be (relatively) easily identifiable, geographically widespread and have a rapid evolutionary turnover. This is so that rocks of the same age can be easily correlated across the globe and that particular species are diagnostic of a particular time interval. Types of fossils that are useful in biostratigraphy include, but are not limited to: foraminifera, nannofossils, spores and pollen, conodonts, graptolites and ammonites.
Biostratigraphers call the length of time between a species’ first appearance (speciation) and its last appearance (extinction) in the fossil record its ‘range’. Often the range of stratigraphically useful fossils are in the region of several million years, however with high resolution dating some species’ range can be calculated as several hundred thousand years. Ideally the range of an organism is determined by its occurrence between two horizons of known age. These horizons may contain minerals that are suitable for radiometric dating such as those within volcanic ash beds (bentonites) or contain rocks that record a diagnostic isotopic signature (typically oxygen, carbon or strontium). Where this succession of rocks occurs, we may call this location the type section or stratotype for a particular species. Once the range of an organism is established from its type locality we can interpret layers of rocks that contain the same organism in different locations around the globe (e.g. where dateable bentonites are absent) to be of the same, or similar, age.
Biostratigraphers may further subdivide successions of rocks into ‘biozones’. Taken from the International Stratigraphic Guide biozones are defined as bodies of rock strata that are characterised on the basis of their contained fossils. They exist only where the particular diagnostic biostratigraphic feature or attribute on which they are based has been identified. Biozones are, therefore, descriptive units based on the identification of fossil taxa.
A biozone may be based on:
- The appearance of a single, easily identifiable or abundant taxon
- A combination of taxa
- Relative abundances
- Specified morphological features
- Variations in features related to content and distribution of fossils in strata
There are several different types of biozone that are best described by means of analogy. Imagine geological time as a 12 hour clock and that organisms such as foraminifera are gregarious creatures that like to party. In this analogy, the foraminifera have arranged a ‘biostratigraphy party’ that begins at 6pm and ends at midnight.
Disco and Yab are the life and soul of any party, and when they turn up the other guests have a memorable time. Disco arrives at the party at 6pm (his first appearance or lowermost occurrence) and leaves at 8pm (his last appearance or uppermost occurrence). Guests remember his attendance as the ‘Disco Zone’. Yab turns up to the party between 10pm and midnight, guests remember this part of the party as the ‘Yab Zone’. This scenario represents the ‘taxon-range zone’ where the zonal boundaries are defined by the lowest and highest stratigraphic occurrence of a specified taxon in a particular stratigraphic section.
As all biostratigraphers will tell you, foraminifera are huge fans of early 90’s American thrash metal. At the party, the foraminifera are divided into two groups – Megadeth fans and Pantera fans. At 7pm the DJ decides to play ‘Symphony of Destruction’ by Megadeth, at which point all the Megadeth fans come together rock out. The guests remember this part of the evening as the ‘Megadeth Zone’. Later on, at 11pm, the DJ plays ‘Cemetery Gates’ by Pantera. As you can imagine, the Pantera fans cannot resist rocking out again to 7.03 minutes of melodic guitar riffs and soaring vocals. This period of the evening is remembered fondly as the ‘Pantera Zone’. This scenario is referred to as the ‘abundance zone’ where boundaries are defined by a notable change in abundance of the taxon or group of taxa chosen to characterise the zone.
Concurrent range zone
Al arrives at the party at 6pm and stays until 10pm. Al is waiting for her friend Wilf who arrives at the party at 8pm and leaves at midnight. The only time both Al and Wilf are both at the party together is between 8pm and 10pm and guests remember this as the ‘Al and Wilf Zone’. This represents the ‘concurrent range zone’ which is defined by the lowest stratigraphic occurrence of the higher ranging of the two taxa (Wilf, 8pm) and the highest stratigraphic occurrence of the lower ranging of the two taxa (Al, 10pm).
There are many other types of biostratigraphic zones (such as lineage and interval zones) and variations of all the types mentioned. I hope this analogy gives you an insight into the different types of methods employed by biostratigraphers to determine the age of rock units.