Chronozone

A chronozone or chron is a slice of time that begins at a given identifiable event and ends at another. Such tracer events are usually keyed to the disappearance (extinction) of fossils of a widely distributed and rapidly changing species or the appearance of such a species in the geological record. Researchers use chronozones or chrons especially frequently in the various disciplines related to geology, notably in stratigraphy where they contribute to relative dating.

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) can also help define chronozones. Hence chronozones, and the international identification and acceptance of widespread chronozones as official useful markers or benchmarks of time in the rock record, are 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). 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, the term "chronozone" refers to the rocks formed during a particular time period, while "chron" refers to that time period. 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.

...
Wikipedia