Chronostratigraphic (time-stratigraphic) units are rock bodies formed and/or deposited within a corresponding geochronologic (or geologic time) interval. The magnitude of a time-stratigraphic unit is measured not by its thickness in feet or meters but by the length of geologic time interval to which the particular rock(s) correspond. Ideally, time-stratigraphic units should be of regional scope. The equivalence of chronostratigraphic and geochronologic units according to decreasing rank is illustrated below:
''''''Note 1. The terms "Early" and "Late" are applied to geochronologic units (e.g. Late Miocene), while "Lower" and "Upper", designating relative positions, should be used when referring to rock bodies (for example, Upper Miocene Limestone).'
''''''Note 2.. A formal time-stratigraphic unit is given a binomial name. The initial letter of both terms should be capitalized.
''''''Note 3.. Doubt in the assignment of rocks to time-stratigraphic units should be made explicit if criteria of time equivalence are inconclusive or lacking.
Doubt can be expressed in several ways: (a) if the total evidence available seem to favor one age assignment, the rock may be assigned to a specific time-stratigraphic unit with the doubt expressed by a question mark or by the words "probably" or "possibly"; (b) if the evidences suggest a position athwart a time-rock boundary, the doubt may be expressed by coupling the names of the two time-rock units with "or", "and" or a hyphen; (c) if the evidences indicate only an upper or lower limit, the assignment should be indicated by the prefix "pre" or "post", for example pre-Eocene, post-Miocene. It is not necessary to make formal time-stratigraphic assignments if evidence of age equivalence with established units is lacking.
The Standard Global Chronostratigraphic Scale (Geologic Time Scale 2004) as updated by the International Commission on Stratigraphy (Gradstein and others, 2004) is shown in Fig. 6-1.
|''''Fig. 6-1'. Geologic Time Scale 2004 (Gradstein and others, 2004)|