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1. Chronostratigraphic Units
Purpose of choronostratigraphic classification
 The general purpose of chronostratigraphic classification is to
organize systematically the Earth’s sequence of rock strata into
named units (Chronostrtigraphic units), corresponding to
intervals of geologic time (geochronologic units), to serve as a
basis for time-correlation and a reference system for recording
events of geologic history.
 Specific objectives are
1. To determine local time relation; local time-correlation of
strata; determination of the relative age of strata in local
section; organization of strata into named chronostratigraphic
units of worldwide application
2. To establish a standard Global Chronostratigraphic Scale

2. Definitions
Chronostratigraphy
 The element of stratigraphy that deals with the age of strata
and their time relations.
Chronostratigraphic Classification
 The organization of rock strata into units on the basis of their
age or time of origin.
Chronostratigraphic Unit
 A body of rock strata that is unified by being the rocks formed
during a specific interval of geologic time.
Chronozone
 A zonal unit of all rocks formed anywhere during the time
range of some geologic feature or some specified interval of
rock strata.

3.  The chronozone based on the range of a certain taxon should
be clearly distinguished from the biozone based on the range
of that same taxon (taxon-range-zone).
 The term chronozone may be used formally as a
chonostratigraphic unit of minor rank.
Chronostratigraphic Horizon (Chronohorizon)
 A stratigraphic surface or interface that is isochronous-
everywhere of the same age.
 The term chronohorizon is commonly applied also to very thin
and distinctive intervals that are essentially isochronous over
their whole geographic extend.

4. Kinds of chornostratigraphic Units
Hierarchy of chronostratigraphic and Geochronologic
Unit-Terms
 Following terms and geochronologic equivalents to
express units of different rank or time scope.
Chronostratigraphic Geochronologic
Eonothem Eon
Erathem Era
System Period
Series Epoch
Stage Age
Chronozone Chron

5. Chronozone (and Chron)
 Chronozone is a formal term for the lowest ranking
division in the hierarchy of chronostratigraphic terms.
 Chron is the corresponding geochronologic term.
 The chronozone may be defined in either of two ways.
 First, it may be made to correspond to the time span of
the stratotype of the unit; in this case the time span of
the chronozone would be permanently fixed.
 Second, the time span of the chronozone may be made
to the total time span of the unit (which may be larger
than that of the stratotype).

6.  The chronozone takes its name from the stratigraphic
unit on which it is based, eg. Exus albus Chronozone
(derived from the Exus albus Range –zone), Barrett
Chronozone (derived from the Barrett Formation).
 A chron takes the same name as its chronozone.
 The time span of a chronozone is usually defined in
terms of the time span of a previously designated
stratigraphic unit such as a formation or a member or a
biozone.

7. Stage (and Age)
 The stage is a chronostratigraphic unit of relatively minor
rank in the conventional hierarchy of formal
chronostratigraphic terms, representing a relatively minor
interval of geologic time.
 Its geochronologic equivalents is known as an age.
 The stage has been called the basic working unit of
chronostratigraphy because it is suited in scope and rank
to the practical needs and purposes of intraregional
chronostratigraphic classification.
 It is one of the smallest units in the standard
chronostratigraphic hierarchy that is prospect may be
recognized worldwide.
 Stages may be subdivided into substages and
superstages.

8.  The lower and upper boundary-stratotypes of a stage
represent specific moments in geologic time, and the
time interval between them is the time span of the stage.
 On the average they range from 3 to 10 million years as
indicated by isotopic age determinations.
 The name of a stage preferably should be derived from a
geographic feature in the vicinity of its type section or
type area.
 Most stages have been given geographic name, but
many currently used stages have the name of the
lithostratigraphic units eg. Cenomanian stage

9. Substages and Superstages
 The substages is a subdivision of a stage.
 Some stages have been divided completely into formally
named substages; others have had only certain parts
designated as substage.
 The geochronologic equivalents of a substage might be
termed a subage.
 Preferabley it is called simply as age.
 A substage is defined by boundary-stratotypes.
 Several adjacent stages may be grouped into a
superstage.
 It is preferably simply to subdivide the original stage into
two or more new stages.

10. Series (and epoch)
 The series is a unit in the conventional
chronostratigraphic hierarchy, ranking above a stage and
below a system.
 The geochronologic equivalent of a series is an epoch.
 A series is always a subdivision of a system
 Most systems have been divided into three series, but
the number varies from two to six.
 Series commonly include from two to six stages.
 The terms superseries and subseries have been used
only infrequently
 Most series can be recognized worldwide.

11.  Currently accepted series vary in time span, but average
about 15 million years.
 The time span of a series, if divided fully into stages, is
the sum of the time span of its component stages.
 A new series name should preferably be derived from a
geographic features in the vicinity of its type section or
type area.

12. System (and Period)
 The system is a unit of major rank in the conventional
chronostratigraphic hierarry above a series and below an
erathem.
 The geochronologic equivalent of a system is a period.
 Occasional need it is divided into subsystem and
supersystem.
 Time span of a system can be defined as the time span
of the sum of its component series or component stages.

13.  The time spans of the Phanerzoic systems
range from 35 to 70 million years and average
about 50 to 60 million years.
 The names of currently recognized systems are
of diverse origin. Some are indicative of position
eg Tetiary and Quaternary); others have a
lithologic cannotation (eg. Carboniferous,
Cretaceous); and others are tribal (eg.
Ordovician, Silurian) and still others are
geographic (eg. Permian, Devoanian)

14. Erathem (and Era)
 An erathem ( from the Greek roots era and them “the
deposits of an era”).
 It is the largest formal unit commonly recognized in the
chronostratigraphic hierarchy and usually consists of
several adjacent systems.
 The interval of geologic time corresponsing to an
erathem is an era. It carries the same name as its
equivalent erathem.
 The Erathems have traditionally been named to reflect
major changes in the development of life on the Earth:
Paleozoic (old life), Mesozoic (Intermediate Life), and
Cenozoic (recent Life), Archeozoic (most ancient life)

15. Eonothem (and Eon)
 The term eon has been used for a geochronologic unit
greater than an era.
 The chronostratigraphic equivalents would be an
eonothen.
 Two eons are generally recognized.
 One is the Phenerozoic eon (time of evident life), which
encompasses the Paleozoic, Mesozoic and Cenozoic
eras.
 The other eon covers pre-phanerozoic (pre-Paleozoic)
time and has been known as the Cryptozoic Eon (time of
hidden life), and Archeozoic eon (time of most ancient
life), or simply the Precambrian Eon.