Chronostratigraphic units and Geological Time units:-

A chronostratigraphic unit is a body of rock strata unified by representing the rocks formed during a specific interval of geological time. The geologic time units are abstract units and are defined after the chronostratigraphic unit.

A chronostratigraphic unit is defined after a specifically designate and delimited type section or reference section. Each unit is the record of an interval of time that extended from the part to the end of its deposition. These units are used for

1.      Correlation of rocks of different areas on the basis of age equivalence or contemporaneity of origin

2.      Establishing the systematic geologic time sequence of rocks for indicating their relative position and age with respect to Earth’s history.

Both the upper and lower limit of a chronostratigraphic unit should be defined in the type section for providing a standard for the unit. In the type area, the boundary should preferably be based on stratigraphically useful objective criteria representing some significant geological episodes. In the type area, the boundaries are usually made to coincide with one of the observable stratigraphic units (either biostratigraphic or lithostratigraphic). Paleontological parameters provide the most successful way of local and worldwide correlation of all rank of Phanerozoic chronostratigraphic units. Relationship to adjacent strata, unconformities, and intrusions, lithologic similarity, radioactive ages, et., are also useful in local time correlation.

The isotopic age determination technique can be applied to sedimentary rock by dating authigenic minerals and to igneous rocks by dating suitable primary minerals. Radiometric age determination is of great importance for interregional and intraregional chronostratigraphic correlation of Precambrian rocks.

Ranks of Chronostratigraphic and Geologic Time Units: The hierarchy of these units is as under:

A system is chronostratigraphic unit of major representing a major division or grouping of rock strata corresponding to a specific major interval of geological time

The Phanerozoic rocks are formerly classified into the following system:

Cambrian, Ordovirian, Silurian, Devonian, Carboniferous, Permian, Triassic, Jurassic, Cretaceous, Tertiary and Quartrary.

Most of the formal systems have their region in Europe. Subdivisions of some of the standard systems may be used locally, eg., Paleogene and Neogene subsystems of the Tertiary system.

A series is a chronostratigraphic unit next in rank, below a system. The rocks of a series represent the specific interval of geological time of the type section of strata for the series. Both the boundaries of the types section or reference section should be defined with precise stratigraphic boundaries. Upper, Middle, and Lower may be used for series divisions of a system.

A stage is a chronostratigraphic unit next in rank below a series. It is defined after a type section or reference section. It is an important unit for chronostratigraphic classification and correlation. Generally, it is based on a succession of zone of biostratigraphy. A stage can be subdivided into sub-stages, if necessary, ‘Chronozone’ may be used in informally to cover a body of strata formed during any minor interval of geological time. The time span is represented after a lithostratigraphic or biostratigraphic unit. ‘Age’ and ‘Time’ may be used informally to cover any geological time.

A formal chronostratigraphic unit is usually given a binomial name. The initial letters should be capitalized e.g. Lower Cretaceous series: Devonian System. It should be named after a geographic feature in the type area. When fossil name is used, the fossil name (genus and species) should be capitalized and printed in Roman type to distinguish it from a biostratigraphic unit. Standard name approved by the International Subcommission on Stratigraphic Classification should be followed for the systems.

The term for the geologic time corresponding to a system is a period and bears the same name as the system. Eg. Cambrian period, Triassic Period. A period can be subdivided and referred to as Late, Middle, and Early. An era is geological time unit of higher rank, than the period and consists of several periods, eg., Paleozoic Era. A series is named after its position within a system qualified by capitalized adjective – Upper, Middle, and Lower, eg., Upper Triassic Series. A stage should preferably be named after the type area, eg., Camic Stage. The term for the geologic time corresponding to a stage is ‘age’ and bears the same name as the ‘stage’. The name of a sub stage should be derived from its type area.

Geologic-Climatic units:

A geologic Climatic unit is an inferred widespread climatic episode from a subdivision of Quaternary rocks. It is an abstract unit. A geologic-climatic unit is defined from the records which include rock bodies, soil and organic material. Such units can be extended geographically as far as the geologic climate record can be recognized. These units are used for correlating episodes of Quaternary deposition in different areas and in establishing the historical sequence of climatic events in the Quaternary period. Glaciation and interglaciation are the two units of geologic-climatic classification. These are named after a lithostratigraphic and soil stratigraphic units.

South Africa and the erstwhile USSR differed from stratigraphers of the other parts of the world and did not approve of the three tier, Litho, Bio and Chronostratigraphic classification. South African geologists use only one set of stratigraphic terms- Group system, Series, stage., and consider the use of these terms, prefixed where necessary, by the qualifications Litho- or Chronostratigraphic, is to be preferred to separate series of terms for such system of classification. Soviet geologists believe that the system of stratigraphic subdivisions should represent natural stages of geological change of Earth as a whole or of individual large regions, and correspondingly should be based on the combination of all manifestations of deposition which objectively indicate these stages, namely by overall analysis of the complex data of evolution of the organic and non-organic world. They are of the view that each of the subdivisions of the Earth and in the evolution of the organic world. Thus, the Soviet geologist feels that in stratigraphy, there can only be one scale which unifies the stratigraphic subdivisions. They do not believe in the concept of differentiation into several stratigraphic divisions (litho-, bio- and chrono-) with non-coinciding boundaries.

The Soviet stratigraphic commission uses, in brief, the following criteria

1.      Correlation on the basis of a single geochronological scale based on “natural” stages and evolution organic life;

2.      Stratigraphic scale should be evolved taking into account all the evidence together

3.      Irreversibility of geological phenomena, alternation of large transgressions and regression of the sea and corresponding changes in the course of organic evolution

4.      The presently recognized system are natural divisions their boundaries are characterized by unconformities and/or stratigraphic breaks, abrupt changes of species and evidence of igneous activity. The system is paleontology distinct and is successively marked by the development of new life groups.

A few Russian Stratigraphers have pleaded for the modification of Russian Code to fall in line with the International Code of stratigraphic nomenclature. The important points touch upon this text, are tabulated below