Toarcian
Fourth and last age of the Early Jurassic
From Wikipedia, the free encyclopedia
The Toarcian is, in the ICS' geologic timescale, an age and stage in the Early or Lower Jurassic. It spans the time between 184.2 Ma (million years ago) and 174.7 ±0.8 Ma.[4] It follows the Pliensbachian and is followed by the Aalenian.[5]
| Toarcian | |||||||||
|---|---|---|---|---|---|---|---|---|---|
Map of Earth at 180 million years ago; the boundaries of present-day landmasses, countries, and states are superimposed as black outlines | |||||||||
| Chronology | |||||||||
| |||||||||
| Etymology | |||||||||
| Name formality | Formal | ||||||||
| Usage information | |||||||||
| Celestial body | Earth | ||||||||
| Regional usage | Global (ICS) | ||||||||
| Time scale(s) used | ICS Time Scale | ||||||||
| Definition | |||||||||
| Chronological unit | Age | ||||||||
| Stratigraphic unit | Stage | ||||||||
| Time span formality | Formal | ||||||||
| Lower boundary definition | Peniche, Portugal | ||||||||
| Lower boundary GSSP | FAD of the Ammonite D. (E.) simplex 39.3708°N 9.3853°W | ||||||||
| Lower GSSP ratified | 2014[2] | ||||||||
| Upper boundary definition | FAD of the Ammonites Leioceras opalinum and Leioceras lineatum | ||||||||
| Upper boundary GSSP | Fuentelsaz, Spain 41.1708°N 1.8333°W | ||||||||
| Upper GSSP ratified | 2000[3] | ||||||||
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.[6]
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 rock strata 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 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
- zone of Harpoceras serpentinum
- zone of Dactylioceras tenuicostatum