The Tonian (from Greek tonas, "stretch") is the first geologic period in the Neoproterozoic Era and lasted from 1000 Ma to 850 Ma (million years ago). Instead of being based on stratigraphy, these dates are defined by the ICS based on radiometric chronometry.
Events leading to the formation and breakup of supercontinent Rodinia started in this period. Rodinia began forming about 1,300 Ma from the amalgamation of three or four pre-existing continents, in an event known as the Grenville Orogeny, consolidating perhaps 1,100 to 1,000 Ma.
The Grenville Province is a terrane of metamorphism and deformation running from Labrador to Texas, once comprising the southern coast of Laurentia. Ar/Ar dating of gabbros and basalts from the Nova Floresta Formation in western Brazil, parts of the Amazon craton which collided with the region that is now Texas during the formation of Rodinia (Llano Uplift), have yielded dates around 1,200 Ma (Tohver).
The first radiation of acritarchs occurred during the Tonian. Acritarchs are small organic fossils, present from approximately 1,400 to 3,200 million years ago to the present.
- Tohver et al. 2002