The Orosirian (pronounced /ˌɒrɵˈsɪəriən/, Greek: oroseira - ὀροσειρά, meaning "mountain range") is the third geologic period in the Paleoproterozoic Era and lasted from 2050 Ma to 1800 Ma (million years ago). Instead of being based on stratigraphy, these dates are defined chronometrically.
Latter half of the period was an episode of intensive orogeny on virtually all continents.
Probably during this period Earth's atmosphere changed to oxygen-rich due to photosynthesis of cyanobacteria.
Two of the largest known impact events on Earth occurred during the Orosirian. At the very beginning of the period, 2023 Ma, a large asteroid collision created the Vredefort impact structure. The event that created the Sudbury Basin structure occurred near the end of the period, 1850 Ma.
* "Orosirian Period". GeoWhen Database. http://www.stratigraphy.org/geowhen/stages/Orosirian.html. Retrieved January 5, 2006. [dead link]