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A chronozone or chron is a slice of time that begins at a given identifiable event and ends at another. In the fossil record such tracer events are usually keyed to disappearance (extinction) of a widely distributed and rapidly changing species or the appearance of such a species in the geological record. Chonozones or chrons are used especially in the various disciplines related to Geology, notably in stratigraphy where relative dating is employed. There are also events susceptible to identification and analysis by the physical sciences, such as Earth's magnetic field reversals or the location of a combination of chemical evidence in a layer corresponding to the meteor strike believed to have caused the extinction of the dinosaurs. Hence chronozones, and the international identification and acceptance of a widespread chronozone as an official useful marker or benchmark of time in the rock record, is non-hierarchical in that chronozones do not need to correspond across geographic or geologic boundaries, nor be equal in length (despite an early constraint that one be defined as smaller than a geological stage[2]). A chronozone is usually defined in geologic terms for a geographical area by fossil names (biozone or biochronozone) or in worldwide terms by geomagnetic reversal identifiers (polarity chronozone). According to the International Commission on Stratigraphy, chronozone is the term used to refer to the rocks formed during the period of time in question, while the word chron refers to that time period.[3] The key factor in designating an internationally acceptable chronozone is whether the overall fossil column is clear, unambiguous, and widespread. Hence, some accepted chronozones contain others, and certain larger chronozones have been designated which span whole define geological time units, both large and small. For example, the chronozone known as the Reign of Tiberius (14 to 37 AD) is a subset of the chronozone Imperial Rome. Similarly the chronozone Pleistocene is a subset of the chronozone Neogene.
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