The Toarcien (of Toarcium , Latin name of Thouars in Two-Sevres) is a stage Jurassic lower (Lias). It extends from 183,0  ±  1,5 with 175,6  ±  2,0  million years. It succeeds the Pliensbachien and precedes the Aalénien.

Geology

This stage includes/understands a sedimentary formation of origin marinades very rich in fossils with in particular more 80  species of Ammonite S, the Foraminifère S, the Ostracode S, the Lamellibranche S, the Gastropodous S…

Toarcien is composed by the lithostratigraphic succession following:

• Calcareous sandstone,
• bioclastic with Oolite S ferruginous,
argillaceous
• calcareous,
• marl S.

History

The sedimentary rocks (sandstone, limestones) of the site thouarsais were exploited by small careers with open sky, which extracted some from the stones of size and the Moellon. In 1849, Alcide Dessalines d' Orbigny made use of one of these careers, close to the farm of Rigollier with Vrines, on the commune of Holy-Rod, to establish the toarcian. This stratotype was then refined by Etienne Eudes-Deslonchamps then by Jules Welsh. In 1962, Jean Gabilly made use of it to define new subdivisions of the stage, the " horizons" , based on the distribution of faunas of ammonites. The stratotype of the toarcian was classified " natural reserve géologique" by the ministerial decree n°87-950 of the 23  November 1987, inter alia thanks to the action of the municipality of the town of Thouars and of an association of amateurs in paleontology, the Company of Geology and Paleontology Thouarsaise Alcide d' Orbigny . The Pays Thouarsais is a zone standard used by all the Paléontologue S of the world.

Layers

The Swiss career of Hauenstein in , in the the Jura presents a layer of the end of Toarcien where one finds inter alia the Ammonite of reference, the Pleydellia aalensis .

The career of the Ciments Lafarge located at Belmont-with Azergues (the Rhone) is one of the most beautiful layers of ammonites of the Jurassic lower. Toarcien is strongly represented there with in particular an abundance d' Hildoceras will bifrons which is the Marqueur. One also finds number of Lytoceras cornucopia , of Grammoceras , Pseudogrammoceras , d' Harpoceras , d' Hammatoceras and d' Haugia . The geological museum of Saint-Jean-of-Vines created by the section geology-paleontology of Lafarge Cements shelters the most beautiful specimens within a very teaching framework.

See too

• geological Time scale

External bonds

• Geological Center of Interpretation of Thouarsais.
• Alcide d' Orbigny on the site of the town of Thouars.

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