|2050 – 1800 Ma|
|Regional usage||Global (ICS)|
|Time scale(s) used||ICS Time Scale|
|Time span formality||Formal|
|Lower boundary definition||Defined chronometrically|
|Lower boundary GSSP||N/A|
|Upper boundary definition||Defined chronometrically|
|Upper boundary GSSP||N/A|
The Orosirian Period ( //; Greek: ὀροσειρά, romanized: oroseirá, meaning "mountain range") is the third geologic period in the Paleoproterozoic Era and lasted from 2050 Mya to 1800 Mya (million years ago). Instead of being based on stratigraphy, these dates are defined chronometrically.
Two of the largest known impact events on Earth occurred during the Orosirian. Early in the period, 2023 Mya, 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 Mya.
For the time period from about 2060 to 1780 Mya, an alternative period based on stratigraphy rather than chronometry, named the Columbian, was suggested in the geological timescale review 2012 edited by Gradstein et al., but as of February 2017[update], this has not yet been officially adopted by the IUGS.