Chronozone

A chronozone or chron is a unit in chronostratigraphy, defined by events such as 
geomagnetic reversals (magnetozones), or based on the presence of specific fossils (biozone or biochronozone).
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.
Identification and acceptance of chronozones as 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).
An early use "was" hierarchical in that Harland "et al." (1989) used "chronozone" for the slice of time smaller than a faunal stage defined in biostratigraphy.
 The ICS has superseded this usage.
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 chronozone Pliocene is a subset of the chronozone Neogene, and the chronozone Pleistocene is a subset of the chronozone Quaternary.