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 geochronol ...
, 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 and is followed by the
Aalenian.
The Toarcian Age began with the
Toarcian turnover, the
extinction event
An extinction event (also known as a mass extinction or biotic crisis) is a widespread and rapid decrease in the biodiversity on Earth. Such an event is identified by a sharp change in the diversity and abundance of multicellular organisms. I ...
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.
Stratigraphic definitions
The Toarcian takes its name from the city of
Thouars, 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 statio ...
in the
Loire Valley of
France. The stage was introduced by French palaeontologist
Alcide d'Orbigny 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 ei ...
of this age in a
quarry 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 cuttlefish) ...
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 domain, the Toarcian contains the following ammonite
biozones:
*zone of ''
Pleydellia aalensis''
*zone of ''
Dumortieria pseudoradiosa''
*zone of ''
Phlyseogrammoceras dispansum''
*zone of ''
Grammoceras thouarcense''
*zone of ''
Haugia variabilis''
*zone of ''
Hildoceras bifrons
''Hildoceras bifrons'' is an extinct species of ammonite in the family Hildoceratidae. It dates from about 175 million years ago in the Early Jurassic when it was both widespread and common. Fossils have been found in North Africa and Europe, in ...
''
*zone of ''
Harpoceras serpentinum''
*zone of ''
Dactylioceras tenuicostatum''
References
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.
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