Eonothem

In the scientific disciplines of stratigraphy and geology, an eonothem constitutes the entirety of rock strata—distinct layers of sediment, rock, or soil exhibiting consistent physical characteristics—that were formed during a specific geological time interval referred to as an eon. While an eon designates an extensive duration within the geologic timescale, often spanning hundreds of millions of years, an eonothem represents the tangible, stratified rock records deposited throughout that chronological span in the geological record. In essence, an eon delineates the time interval, whereas an eonothem embodies the corresponding geological deposits from that era.

Eonothems are systematically named to correspond with their respective eons. Earth's geological history is partitioned into four primary eonothems, arranged chronologically from the most ancient to the most recent:

• Hadean
• Archean
• Proterozoic
• Phanerozoic

Collectively, these eonothems encapsulate the principal stages of Earth's geological evolution. They serve as critical markers within the geologic timescale, reflecting profound geological and biological transformations that have occurred over the planet’s 4.6-billion-year history. A rock stratum, fossil or feature present in the "upper Phanerozoic" eonothem would therefore have originated within the "later Phanerozoic" eon. In practice, the rock column is discontinuous:

Technically, a complete geologic record doesn't occur anywhere. For such a record to develop would require the area to have been receiving sedimentary deposits continually ever since the origin of the earth. Nowhere is such a situation known to exist. If it did exist, we could not effectively look at the strata because they would still be buried, and modern strata would continue to be deposited on top of them. The earth's surface has been far too dynamic to allow that to occur anywhere. No area has been in such a static condition throughout the earth's long history. Areas that have had sediment deposited on them at one time are later uplifted and eroded. In some places this has occurred many times. There is ample evidence to prove such a sequence of events.^[1]

Eonothems, despite discontinuities (locally missing strata or unconformities), can be compared to others where the rock record is more complete and, by correlation of points of correspondence, be fixed appropriately within the eon. They are therefore useful as broad chronostratigraphic units, specifying approximate age within the timelines within the rock column.

Eonothems are subdivided into erathems and their smaller subdivisions within geology and paleobiology and their sub-fields, and a whole system of cross-disciplinary classification by strata is in place with oversight by the International Commission on Stratigraphy.

Eonothems are not often used in practice as expert dating estimates can be and usually are specified into the more refined timelines of smaller chronostratigraphic units, which can be subdivided in turn down to the many defined stages, the smallest formally recognised units used in dating. (see the hierarchy of comparative units, five each for time division types and five for the rock record types.)