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Relative article: How to read a chronostratigraphic chart?
First of all, there are important stratigraphy principles (figure 1):
The succession: we need to situate events and geological formations in relation to each other.
The duration: we need to evaluate the time interval between two events (Figure a)
The simultaneity: If two events are occurring, we need to determine if they are synchronous or not
The observation: prospection, drilling, pictures… , we need to understand the geometrical relationship between the observed elements
The superposition: any geological layer is more recent than the one it covers (only applicable if the deposits and their position is preserved (e.g. folds, tectonics..) (Figure b)
The original horizontality: states that the deposition of sediments occurs as essentially horizontal beds (Figure c)
The intersection: any structure that intersects with another is subsequent to it (e.g. faults, breaks, veins..) (Figure d)
The inclusion: any object contained in a stratigraphic unit or an object is older than it (figure e)
The palaeontological identity: if we observe the same fossil taxon within two stratigraphic units, they have the same age
Figure 1: Schematic representation of the important stratigraphy principles
Every natural phenomenon will leave marks in the fields, and some are used to give an age to geological units via specific methodology
Continuous and irreversible phenomena: two in the Earth history:
The biological evolution (depends on the species), allows biostratigraphy and biochronology
The radioactivity: radioactive disintegration of chemical isotopes (constant velocity), allows radiochronology
Discontinuous and irreversible phenomena: (brief time) (e.g. biological crisis), allows biostratigraphy
Continuous and reversible phenomena (e.g. weathering and erosion)
Discontinuous and reversible (or repetitive) phenomena (e.g. continental drift)
Instantaneous and repetitive phenomena (really short time at geological scale, <10 000 years)
Glaciations (with the presence of moraines)
Geysers
Magnetic polarity inversion, allows magnetostratigraphy and magnetochronology
Meteorite crashes and volcanic eruptions, allows tephrochronology (chemistry and age of volcanic ash)
Biostratigraphy and biochronology
Biostratigraphy: The use of fossils or any fossil trace of biologic activity contained in the geological layers to organise it in units which are ordered in relative terms over time.
Biochronology: Independent from the stratification, it is the work on fossil remains to determine the taxa and order them over time.
As it is based on biological evolution, it is of importance to chose relevant taxa to use.
A stratigraphic fossil should also present the following characteristics:
A short vertical repartition. It should be organisms that are evolving quite quickly to have to have real differences between organisms along time. Indeed, if there is no or very slow evolution, the organisms that you observe are the same for a huge period of time and it is not possible to use them to make a distinction between different geological units.
A wide special repartition (independent from the environment in which it evolves). The larger the area where you can find it the better. Indeed, if you can find the chosen fossil in a high number of places around the Earth, you will be able to identify and date a high number of geological layers.
A high abundance in the sediment. The chosen taxa should be easy to find, and represented by a high number of individuals to identify the taxa.
Usually, the first appearance of the taxa defines the bottom of the geological layer and the last appearance of the taxa, the top of the geological layer (figure2).
Figure 2: Schematic representation of fossils distribution
Radiochronology
This method is based on the use of radioactive isotopes. First of all, what are isotopes?
An atom is constituted of a nuclear and an electron cloud. In the nuclear, there are protons (positive charge) and neutrons (neutral charge). There are always the same number of protons than electrons, and each chemical element is defined by its protons (and so electron) number.
But for the same chemical elements, the number of neutrons can vary, changing the mass of the atom (the more neutrons in the atom core, the heavier the atom). The different atoms from the same chemical element (i.e. same number of protons) having different neutron number and different masses are called isotopes. Sometimes the isotopes are stable, i.e. they remains as they are, and sometimes they are not stable and disintegrate to form a new chemical element (that can be stable or not), they are called radioactive isotopes and undergo radioactive disintegration. The primary element is called the parent element and the element resulting from the radioactive decay is called the child element.
To evaluate the amount of atoms of each element, we principally use a mass spectrometer (figure 3).
Figure 3: schematic representation of a mass spectrometer
In the mass spectrometer, the atoms contained in the rock or in the mineral are ionised and sent in a vacuum environment at very high speed. They are diverted along a circular trajectory with an electromagnet and thanks to centrifugal force, the heavier atoms are not following the same trajectory than the lighter ones, and it becomes possible to separate the atoms depending on their mass. As mentioned above, the mass makes possible to make a distinction between the chemical elements. We obtain the ration between the child element and the parent element.
The time necessary for the radioactive decay is specific to each radioactive chemical element. Each radioactive element has his own disintegration period, or half-life, which corresponds to the time necessary to disintegrate half of the quantity of considered atoms present at the very beginning (the closure of the system). For example, when minerals are crystallizing, the elements they contain are trapped inside. Then, they undergo radioactive decay. Knowing the disintegration period of the analysed element and the quantity of son element measured in the rock or mineral sample, we can calculate the time spend since the closure of the system (i.e. the formation of the rock or mineral). This is the age of the rock or the age of the mineral.
This method has been used to estimate the age of the Earth, but also the age of meteorites.
Magnetostratigraphy
The Earth's magnetic field has two poles (bipolar) and these two poles are defining the magnetic axis (figure 4), offset from few degrees compared to the Earth’s rotation axis. This method is based on the use of the Earth's magnetic field and its magnetic pole reversals that have taken place in the past.
Figure 4: Schematic representation of the Earth magnetic field
Today, the magnetic North pole is at the geographic South pole of the Earth (this is the normal polarity configuration, usually represented in black on schema), but it has not been always the case and it has changed several times in the past. On long time scales (million years) we do observe magnetic field inversion, instantaneous at the geological scale. There are time intervals during the ones the magnetic North pole was in the geographic South pole (this is the inverse polarity configuration, usually represented in white on schemas).
The Earth magnetic field is defined using different parameters:
Its intensity (F). The unit is nT (nanotesla) and its value is changing depending on the position on Earth and time. The intensity is higher at high latitude (on the poles) and lower at low latitude (on the equator).
Its declination (D). This is the angle between the horizontal component of the magnetic field and the geographical north. It varies from 0 to 360 ° along time.
The inclination (I). This is the angle between the horizontal component of the magnetic field and the direction of the magnetic field. I depends on the latitude. If I>0 (going in the ground direction), it is the northern hemisphere, If I<0 (going in the sky direction), it is the southern hemisphere, and if I=0, we are at the equator.
The origin of the Earth magnetic field is not 100 % understood yet but we know that it is due around 90% to internal processes (e.g. convection currents in the liquid core, generating electric currents) and around 10% to external activity (e.g. solar activity).
The rocks and minerals are recording the different parameters of the Earth magnetic field along time and to use if for datation, we look at the palaeomagnetism, i.e. what were these different parameters in the past.
To do so, we look at two specific types of rocks: volcanic and metamorphic rocks. Those rocks are issued from magma or very ductile and very high temperature solution containing all the chemical elements to make minerals. When it gets cooler, the minerals crystallise. Some of them are containing elements sensitive to the Earth magnetic field, and crystalise in a certain structure aligned with the Earth magnetic field, freezing the values of the parameters defining it at the time of crystallisation. It is the natural (or primary) magnetisation. This is what is studied to record the past Earth magnetic field oscillation along time.
There are two major places where we look at the palaeomagnetism: dated volcanic flows and ocean ridges. The magnetic inversions have been dated and recorded all around the earth. With these data, we defined the time series with all the inversions and their age (figure 5). Knowing this, when we find a new record of palaeomagnetic data without an age, we compare it to the world record and we identify the time interval it is corresponding to, and this allow us to give an age to the interval.
This has been used for many applications, as for example:
The determination of the position of a continent at a specific time;
The determination of the oceanic expansion and its speed.
Figure 5: chronostratigraphic chart with the geological stages and the magnetic inversion record with corresponding names (Raffi et al., 2020). “r” means reverse configuration and “n” means normal configuration.
Tephrochronology
When there is a (huge) volcanic eruption, a layer of volcanic ash (tephra) gets deposited over a large area. Each single eruption created a tephra horizon with a unique chemical signature that allows the deposit to be identified across the area affected by fallout. The tephrochronology is based on the use of these discrete layers of tephra, which are identified and independently dated. Because the volcanic ash layers are easily identified in many sediments and deposited relatively instantaneously over a wide spatial area, they provide accurate temporal marker layers that can be use to identify geological stages.
Each of these methods have its own limits, margins of error, advantages and disadvantages, and should be used and interpreted carefully.
Bibliography
Raffi, I., Wade, B. S., Pälike, H., Beu, A. G., Cooper, R., Crundwell, M. P., Krijgsman, W., Moore, T., Raine, I., Sardella, R., and Vernyhorova, Y. V.: The Neogene Period, in: Geologic Time Scale 2020, Elsevier, 1141–1215, https://doi.org/10.1016/B978-0-12-824360-2.00029-2, 2020.
February 2025
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