TOARCIAN STAGE

division representing all rocks deposited worldwide during the Toarcian Age (193187 million years ago). The Toarcian is the fourth of 11 divisions (in ascending order) that make up the Jurassic System and is the uppermost stage of the Early Jurassic. No global stratotype section and point (GSSP) for the base of the stage has been approved by the International Commission on Stratigraphy. Many different and conflicting stage names have been proposed for the Jurassic System, but the present scheme conforms closely to the order imposed by the French geologist Alcide d'Orbigny in the mid-19th century. The stage's name is derived from the village of Thouars (Latin: Toarcium), which lies 75 km (45 miles) north of Niort in the Deux-Svres region of western France. The standard succession is better known from the Lorraine region of northeastern France, where about 100 m (330 feet) of marls and shales with nodular limestones are represented. Four ammonite cephalopod biozones (shorter spans of time characterized by a type of fossil mollusk), beginning with Dactylioceras tenuicostatum and ending with Lytoceras jurense, are recognized in Toarcian strata in Europe. Some species of Dactylioceras have a worldwide distribution and are known from the circum-Pacific belt. Rocks of the Toarcian Stage overlie those of the Pliensbachian Stage and underlie rocks of the Aalenian Stage.