Achronozone orchron is a unit inchronostratigraphy, defined by events such asgeomagnetic reversals (magnetozones), or based on the presence of specific fossils (biozone or biochronozone).According to theInternational Commission on Stratigraphy, the term "chronozone" refers to the rocks formed during a particular time period, while "chron" refers to that time period.[1]
Although non-hierarchical, chronozones have been recognized as useful markers or benchmarks of time in therock record. Chronozones are non-hierarchical in that chronozones do not need to correspond across geographic or geologic boundaries, nor be equal in length. Although a former, early constraint required that a chronozone be defined as smaller than ageological stage. Another early usewas hierarchical in that Harlandet al. (1989) used "chronozone" for the slice of time smaller than afaunal stage defined inbiostratigraphy.[2] TheICS superseded these earlier usages in 1994.[3]
The key factor in designating an internationally acceptable chronozone is whether the overall fossil column is clear, unambiguous, and widespread. Some accepted chronozones contain others, and certain larger chronozones have been designated which span whole defined geological time units, both large and small.For example, the chronozonePliocene is a subset of the chronozoneNeogene, and the chronozonePleistocene is a subset of the chronozoneQuaternary.
| Segments of rock (strata) inchronostratigraphy | Time spans ingeochronology | Notes to geochronological units |
|---|---|---|
| Eonothem | Eon | 4 total, half a billion years or more |
| Erathem | Era | 10 defined, several hundred million years |
| System | Period | 22 defined, tens to ~one hundred million years |
| Series | Epoch | 38 defined, tens of millions of years |
| Stage | Age | 101 defined, millions of years |
| Chronozone | Chron | subdivision of an age, not used by the ICS timescale |