Editing Geologic time scale (section)

=== Terminology ===

{{em|[[Chronostratigraphy]]}} is the element of [[stratigraphy]] that deals with the relation between rock bodies and the relative measurement of geological time.<ref name="ICS_chronostrat">{{Cite web |title=Chapter 9. Chronostratigraphic Units |url=https://stratigraphy.org/guide/chron |access-date=2022-04-02 |website=stratigraphy.org |publisher=International Commission on Stratigraphy}}</ref> It is the process where distinct strata between defined stratigraphic horizons are assigned to represent a relative interval of geologic time.

A '''{{em|chronostratigraphic unit}}'''{{Anchor|Chronostratigraphic unit}} is a body of rock, layered or unlayered, that is defined between specified stratigraphic horizons which represent specified intervals of geologic time. They include all rocks representative of a specific interval of geologic time, and only this time span. Eonothem, erathem, system, series, subseries, stage, and substage are the hierarchical chronostratigraphic units.<ref name="ICS_chronostrat"/><!-- this citation cites two previous sentences -->

A '''{{em|geochronologic unit}}'''{{Anchor|Geochronologic unit}} is a subdivision of geologic time. It is a numeric representation of an intangible property (time).<ref name="ICS_definitions">{{Cite web |title=Chapter 3. Definitions and Procedures |url=https://stratigraphy.org/guide/defs |access-date=2022-04-02 |website=stratigraphy.org |publisher=International Commission on Stratigraphy}}</ref> These units are arranged in a hierarchy: eon, era, period, epoch, subepoch, age, and subage.<ref name="ICS_chronostrat" /> 
{{em|Geochronology}} is the scientific branch of geology that aims to determine the age of rocks, fossils, and sediments either through absolute (e.g., [[radiometric dating]]) or relative means (e.g., [[Law of superposition|stratigraphic position]], [[paleomagnetism]], [[stable isotope ratio]]s). 
{{em|[[Geochronometry]]}} is the field of geochronology that numerically quantifies geologic time.<ref name="ICS_definitions"/> <!-- this citation cites two previous sentences -->

A {{em|[[Global Boundary Stratotype Section and Point]]}} (GSSP) is an internationally agreed-upon reference point on a [[stratigraphic section]] that defines the lower boundaries of stages on the geologic time scale.<ref name="ICS_GSSP">{{Cite web |title=Global Boundary Stratotype Section and Points |url=https://stratigraphy.org/gssps/ |access-date=2022-04-02 |website=stratigraphy.org |publisher=International Commission on Stratigraphy}}</ref> (Recently this has been used to define the base of a system)<ref name="Knoll_2006_Ediacaran">{{Cite journal |last1=Knoll |first1=Andrew |last2=Walter |first2=Malcolm |last3=Narbonne |first3=Guy |last4=Christie-Blick |first4=Nicholas |date=2006 |title=The Ediacaran Period: a new addition to the geologic time scale |url=http://doi.wiley.com/10.1080/00241160500409223 |journal=Lethaia |language=en |volume=39 |issue=1 |pages=13–30 |doi=10.1080/00241160500409223|bibcode=2006Letha..39...13K }}</ref>

A {{em|[[Global Standard Stratigraphic Age]]}} (GSSA)<ref name="Remane_1996_GSSP">{{Cite journal |last1=Remane |first1=Jürgen |last2=Bassett |first2=Michael G |last3=Cowie |first3=John W |last4=Gohrbandt |first4=Klaus H |last5=Lane |first5=H Richard |last6=Michelsen |first6=Olaf |last7=Naiwen |first7=Wang |last8=the cooperation of members of ICS |date=1996-09-01 |title=Revised guidelines for the establishment of global chronostratigraphic standards by the International Commission on Stratigraphy (ICS) |journal=Episodes |language=en |volume=19 |issue=3 |pages=77–81 |doi=10.18814/epiiugs/1996/v19i3/007 |issn=0705-3797|doi-access=free }}</ref> is a numeric-only, chronologic reference point used to define the base of geochronologic units prior to the Cryogenian. These points are arbitrarily defined.<ref name="ICS_chronostrat"/> They are used where GSSPs have not yet been established. Research is ongoing to define GSSPs for the base of all units that are currently defined by GSSAs.

The standard international units of the geologic time scale are published by the International Commission on Stratigraphy on the International Chronostratigraphic Chart; however, regional terms are still in use in some areas. The numeric values on the International Chronostratigrahpic Chart are represented by the unit [[Megaannum|Ma]] (megaannum, for 'million [[year]]s'). For example, {{Period start|Jurassic}} {{Period start error|Jurassic}} Ma, the lower boundary of the [[Jurassic]] Period, is defined as 201,400,000 years old with an uncertainty of 200,000 years. Other [[Si prefix|SI prefix]] units commonly used by geologists are [[Gigaannum|Ga]] (gigaannum, billion years), and [[Kiloannums|ka]] (kiloannum, thousand years), with the latter often represented in calibrated units ([[Before Present|before present]]).