Eonothem

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An eonothem is the total stratigraphic record deposited during a certain eon of the geologic timescale. It is therefore a chronostratigraphic unit.

Eonothems are not often used in practice. When they are they can be subdivided into erathems, which can be subdivided in turn in systems. Eonothems have the same names as their corresponding eons, which means during the history of the Earth only three eonothems were formed: the Archean, Proterozoic and Phanerozoic.