The Toarcian is, in the

ICS ICS may refer to: Computing * Image Cytometry Standard, a digital multidimensional image file format used in life sciences microscopy * Industrial control system, computer systems and networks used to control industrial plants and infrastructu ...

geologic timescale The geologic time scale, or geological time scale, (GTS) is a representation of time based on the rock record of Earth. It is a system of chronological dating that uses chronostratigraphy (the process of relating strata to time) and geoch ...

, an age and stage in the Early or Lower Jurassic. It spans the time between 182.7 Ma (million years ago) and 174.1 Ma. It follows the

Pliensbachian The Pliensbachian is an age of the geologic timescale and stage in the stratigraphic column. It is part of the Early or Lower Jurassic Epoch or Series and spans the time between 190.8 ± 1.5 Ma and 182.7 ± 1.5 Ma (million years ago). The ...

and is followed by the

Aalenian The Aalenian () is a subdivision of the Middle Jurassic Epoch/ Series of the geologic timescale that extends from about 174.1 Ma to about 170.3 Ma (million years ago). It was preceded by the Toarcian and succeeded by the Bajocian. Stratigraphic ...

. The Toarcian Age began with the

Toarcian turnover The Jurassic ( ) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Jurassic constitutes the middle period of ...

, the extinction event that sets its fossil faunas apart from the previous

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.

Stratigraphic definitions

The Toarcian takes its name from the city of

Thouars Thouars () is a commune in the Deux-Sèvres department in western France. On 1 January 2019, the former communes Mauzé-Thouarsais, Missé and Sainte-Radegonde were merged into Thouars. It is on the River Thouet. Its inhabitants are known ...

, just south of

Saumur Saumur () is a commune in the Maine-et-Loire department in western France. The town is located between the Loire and Thouet rivers, and is surrounded by the vineyards of Saumur itself, Chinon, Bourgueil, Coteaux du Layon, etc.. Saumur ...

in the Loire Valley of

France France (), officially the French Republic ( ), is a country primarily located in Western Europe. It also comprises of overseas regions and territories in the Americas and the Atlantic, Pacific and Indian Oceans. Its metropolitan ar ...

. The stage was introduced by French palaeontologist

Alcide d'Orbigny Alcide Charles Victor Marie Dessalines d'Orbigny (6 September 1802 – 30 June 1857) was a French naturalist who made major contributions in many areas, including zoology (including malacology), palaeontology, geology, archaeology and anthrop ...

in 1842, after examining

rock strata In geology and related fields, a stratum ( : strata) is a layer of rock or sediment characterized by certain lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as e ...

of this age in a

quarry A quarry is a type of open-pit mining, open-pit mine in which dimension stone, rock (geology), rock, construction aggregate, riprap, sand, gravel, or slate is excavated from the ground. The operation of quarries is regulated in some juri ...

near Thouars. In Europe this period is represented by the upper part of the Lias. The base of the Toarcian is defined as the place in the stratigraphic record where the

ammonite Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid and cuttle ...

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial n ...

'' Eodactylites'' first appears. A global reference profile (a

GSSP A Global Boundary Stratotype Section and Point (GSSP) is an internationally agreed upon reference point on a stratigraphic section which defines the lower boundary of a stage on the geologic time scale. The effort to define GSSPs is conducted ...

) for the base is located at

Peniche, Portugal Peniche () is a seaside municipality and a city located in the Leiria District of Central Portugal. It has 26,431 inhabitants, in an area of 77.55 km2. The city itself has a population of about 15,600 inhabitants. The present mayor is Henriq ...

. The top of the stage is at the first appearance of ammonite genus '' Leioceras''. In the Tethys domain, the Toarcian contains the following ammonite

biozone In biostratigraphy, biostratigraphic units or biozones are intervals of geological strata that are defined on the basis of their characteristic fossil taxa, as opposed to a lithostratigraphic unit which is defined by the lithological properties ...

s: *zone of '' Pleydellia aalensis'' *zone of '' Dumortieria pseudoradiosa'' *zone of '' Phlyseogrammoceras dispansum'' *zone of '' Grammoceras thouarcense'' *zone of '' Haugia variabilis'' *zone of '' Hildoceras bifrons'' *zone of '' Harpoceras serpentinum'' *zone of '' Dactylioceras tenuicostatum''

References

Sources

*; 2004: ''A Geologic Time Scale 2004'',

Cambridge University Press Cambridge University Press is the university press of the University of Cambridge. Granted letters patent by Henry VIII of England, King Henry VIII in 1534, it is the oldest university press in the world. It is also the King's Printer. Cambr ...

. *; 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.

External links

GeoWhen Database - ToarcianLower 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 {{Geological history, p, m *04 Geological ages