Toarcian

 ( Geological Ages ) 

The Toarcian is, in the ICS' geologic timescale, an age and stage in the Early Jurassic. It spans the time between 184.2 Megaannum and 174.7 ±0.8 Ma.

It follows the Pliensbachian and is followed by the Aalenian.For a detailed geologic timescale see Gradstein et al. (2004)

The Toarcian Age began with the Toarcian Oceanic Anoxic Event, a major anoxic event associated with marine extinctions and increased global temperatures that sets its fossil faunas apart from the previous Pliensbachian age. It is believed to have ended with a global cooling event known as the Comptum Cooling Event, although whether it represented a worldwide event is controversial.

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Stratigraphic definitions The Toarcian takes its name from the city of Thouars, just south of Saumur in the Loire Valley of France. The stage was introduced by French palaeontologist Alcide d'Orbigny in 1842, after examining stratum of this age in a quarry near Thouars.

In Europe this period is represented by the upper part of the Lias Group.

The base of the Toarcian is defined as the place in the stratigraphic record where the ammonite genus Eodactylites first appears. A global reference profile (a GSSP) for the base is located at Peniche, Portugal. The top of the stage is at the first appearance of ammonite genus Leioceras.

In the Tethys Ocean, the Toarcian contains the following ammonite :

• zone of Pleydellia
• zone of Dumortieria
• zone of Phlyseogrammoceras dispansum
• zone of Grammoceras
• zone of Haugia
• zone of Hildoceras
• zone of Harpoceras
• zone of Dactylioceras

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Sources

• ; 2004: A Geologic Time Scale 2004, Cambridge University Press.
• ; 1842: Paléontologie française. 1. Terrains oolitiques ou jurassiques, Bertrand, Paris.
• Elmi, S., Rulleau, L., Gabilly, J. & Mouterde, R. 1997: Toarcien. In: Cariou, E. & Hantzpergue, P. (eds): Biostratigraphie du Jurassique ouest-européen et méditerranéen. Bulletin du Centre des Recherches, Elf Explor. Prod. Mém., 17.

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External links

• GeoWhen Database - Toarcian
• Lower Jurassic timescale, at the website of the subcommission for stratigraphic information of the ICS
• Stratigraphic chart of the Lower Jurassic, at the website of Norges Network of offshore records of geology and stratigraphy

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