How to read a chronostratigraphic chart?

First of all, what is a chronostratigraphic chart?

It is a table presenting all the geological stratigraphic sections, from the beginning of the Earth history to nowadays, with the dates of beginning and ending of these sections (defined using different methodologies (Figure).

You can download the most recent version here: https://stratigraphy.org/chart

Now, how to read it?

A chronostratigraphic chart must be read from bottom right to top left. The different section you can identify are (from the largest to the smallest subdivisions): Eons, Eras, Periods, Epochs and Stages/ages. The ages corresponding to the beginning and the end of these geological sections are given on the right in Ma (mega-annum): Million years ago. Gradstein and Ogg (2020) and Raffi et al., (2020) describe in details how it is done. There are two types of ages: the ones given by absolute dating of the rocks (the little clock) and the ones given by biostratigraphic markers, i.e. little fossils (the little pins).

Each section (or geological unit) is defined by continuous characteristics (e.g. the nature of the rocks, the fossils it contains, it physical and chemical properties): a stratotype, different from the section they follow and the section they precede.

The very first section you can identify while reading is the Hadean, within the Precambrian. This is the very beginning of the Earth history, 4567 million years ago (Ma). The scale is not linear, the first seven eras (from Hadean to Meso-proterozoic) are covering the first 3567 Ma of the Earth history. The era right after, the Neo-proterozoic is covering 462 Ma, ending by the Ediacaran period, where the first pluricellular forms of life have been found (Droser et al., 2017, Droser and Gehling 2014, Buatois and Mángano 2016, Narbonne et al., 2005, Narbonne et al., 2012), and followed by the life expansion during the Cambrian (to go further, you can read Wonderful Life: The Burgess Shale and the Nature of History, from Stephen Jay Gould, see in the bibliography for the full reference).

This means that the remaining 3 columns are covering 538 Ma, with much more details than the preceding ones. The chart ends with the Neogene and Quaternary at the top left, the Holocene being the most recent epoch in which we still are.

With the current climate changes, the ongoing massive biological extinction and all the environment shifts due to anthropogenic activities, some scientists are in favour of creating a new epoch: the Anthropocene, coming after the Holocene (Ruddiman, 2013). This is still under debate in the scientific community.

This chart is used all over the world by geoscientists as a reference for the ages of each Eons, Eras, Periods, Epochs and Stages/ages.

Knowing this, there are two ways to use it:

The first situation is that you have an age and you would like to know the geological period it corresponds to. In this situation, you look for the age you have on the right and you observe the stratigraphic unit it corresponds to.

The second situation is that you have a rock or a fossil you know belongs to a specific geological unit but you do not know the approximate age, then you look for the stratigraphic unit in the table and find the corresponding age. This also works if you are reading about one period and you want to know at what age it does correspond.

Bibliography

Buatois, L. A. and Mángano, M. G.: Ediacaran Ecosystems and the Dawn of Animals, in: The Trace-Fossil Record of Major Evolutionary Events, vol. 39, edited by: Mángano, M. G. and Buatois, L. A., Springer Netherlands, Dordrecht, 27–72, https://doi.org/10.1007/978-94-017-9600-2_2, 2016.

Droser, M. L. and Gehling, J. G.: The advent of animals: The view from the Ediacaran, Proc. Natl. Acad. Sci. U.S.A., 112, 4865–4870, https://doi.org/10.1073/pnas.1403669112, 2015.

Droser, M. L., Tarhan, L. G., and Gehling, J. G.: The Rise of Animals in a Changing Environment: Global Ecological Innovation in the Late Ediacaran, Annu. Rev. Earth Planet. Sci., 45, 593–617, https://doi.org/10.1146/annurev-earth-063016-015645, 2017.

Gradstein, F. M. and Ogg, J. G.: The Chronostratigraphic Scale, in: Geologic Time Scale 2020, Elsevier, 21–32, https://doi.org/10.1016/B978-0-12-824360-2.00002-4, 2020.

Narbonne, G. M.: THE EDIACARA BIOTA: Neoproterozoic Origin of Animals and Their Ecosystems, Annu. Rev. Earth Planet. Sci., 33, 421–442, https://doi.org/10.1146/annurev.earth.33.092203.122519, 2005.

Narbonne, G. M., Xiao, S., Shields, G. A., and Gehling, J. G.: The Ediacaran Period, in: The Geologic Time Scale, Elsevier, 413–435, https://doi.org/10.1016/B978-0-444-59425-9.00018-4, 2012.

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.

Ruddiman, W. F.: The Anthropocene, Annu. Rev. Earth Planet. Sci., 41, 45–68, https://doi.org/10.1146/annurev-earth-050212-123944, 2013.

Also read: Stephen Jay Gould, 1989. Wonderful Life: The Burgess Shale and the Nature of History, New York: W. W. Norton, 1989, ISBN 978-0-393-02705-1. 347 pp.

French traduction: Stephen Jay Gould, 2004. La vie est belle, Les surprises de l’évolution, Seuil, 2004, ISBN-10 ‏ : ‎ 2020352397, ISBN-13 ‏ : ‎ 978-2020352390, 480 pp. 

January 2025

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