Stratigraphic Classification, nomenclature and descriptions

Stratigraphic Classification and nomenclature

Of the many possible kinds of stratigraphic units, three have been particularly useful. They are

  1. Lithographic (or Rock-stratigraphic) Units:
  2. Biostratigraphic units
  3. Chronostratigraphic (or Time-rock or Time-stratigraphic) units.

The above three types units are interrelated in many ways. However, a separate scheme of classification and terminology has developed for each type of unit. Considerable controversy exists about the relative importance of these three separate classification. However, each is important for particular purposes and relative importance varies with the circumstances. Lithostratigraphic studies constitute the first step in establishing the stratigraphy of any area/region and though lithostratigraphic and biostratigraphic studies should generally precede chronostratigraphic classification.

Some basic Definitions of Stratigraphic

  1. Stratum: A geologic stratum is a layer of rock with some unifying characters, properties or attributes, distinguishing it from adjacent layers. Adjacent strata may be separated by visible planes of bedding or parting or by less perceptible boundaries of changes in lithology, mineralogy, fossil content, chemical constitutions, physical properties, age or other properties of rocks.
  2. Stratigraphic units: A stratigraphic unit is a stratum or assemblage of adjacent strata, recognized as an unit (or distinct entity) in the classification of the Earth’s rock sequence, with respect to any of the many characters, properties or attributes which the rock may possess.
  3. Formal vs. Informal Stratigraphical terminology: Stratigraphic units and their names are classified as informal if they are not formally proposed and are used in a broad or a free sense without the precise connotation as required by the code of stratigraphic nomenclature. A formal unit is a name representative of an established or conventionally agreed scheme of classification.

Earlier established stratigraphic units and their nomenclature, if they do not conform to the code and satisfy the rules are considered as “informal”. They are placed in inverted comma or parentheses if they are used. The initial letter of a named formal unit/term should be in capital.

Correlation: To correlate in a stratigraphic sense is to show correspondence in character of the strata and their stratigraphic positions. Lithological correlation is a demonstration of correspondence in lithologic character and stratigraphic position. A correlation of two fossil beds is a demonstration of correspondence in their content and in their stratigraphic position. A chronostratigraphic correlation is a demonstration of correspondence in age and stratigraphic position.

Lithostratigraphic units:

The lithostratigraphic units are the subdivision of rocks in the Earth’s crust distinguished and delimited on the basis of lithologic characteristics. The units are recognized and defined by observable physical features rather than by inferred geological history; boundaries may be placed at sharp contacts or drawn arbitrarily within a zone of gradation. Both vertical and lateral lithostratigraphic boundaries are placed where the lithology changes. Rock-stratigraphic units are the practical units of general geological work that serve as a foundation for describing, mapping and studying lithology, local and regional structures, stratigraphy, economic resources and geologic history. The units are independent of any time concept. Their boundaries may coincide with or transgress time horizons. The ‘Formation’ is the fundamental unit in rock stratigraphic classifications. A formation is a body of rock characterized by lithologic homogeneity; it is mappable at the Earth’s surface or traceable in the subsurface.

Ranks of Lithostratigraphic Units

The following hierarchy of formal lithostratigraphic units is recognized







A member is next in rank below a formation it is not defined by specified shape, extent or thickness. A bed is the lowest rank in formal lithostratigraphic unit. A group consists of two or more successive and naturally related or associated formation and is higher in rank than a “Formation”. In certain areas stratigraphers have named and defined assemblages of formations, within already established useful groups of formations; these constitute supergroups eg., Vindhyan Supergroups

  1. Biostratigraphic Units: A biostratigraphic unit is a body of rock strata characterized by its content of fossils contemporaneous with the deposition of strata. All fossils contained in a biostratigraphic unit are remains of organisms that lived when the sediment surrounding them was deposited. The organisms might have been buried in situ or transported to their place of burial, but in either case, they belong to the deposit as contemporaneous original constituents. Some sedimentary rocks contain reworked fossils derived from other rocks; they can be distinguished from the fauna and flora indigenous to the deposits and are to be ignored in defining a biostratigraphic unit. Biostratigraphic units are fundamentally different from rock stratigraphic units. The boundaries of these two types of units may or may not coincide. The biostratigraphic evidence is the most useful means of determining time-stratigraphic boundaries. Fossils reflect irreversible evolutionary change and adoption to the environment and all biostratigraphic units are records of time and biofacies

Types of Biostratigraphic Zones:

Two types of formal biostratigraphic zones are recognized viz.,

  1. The assemblage zone
  2. The range zone

An assemblage zone is body of strata characterized by a certain natural assemblage or association of fossils, with regard to their ranges, eg. Meeker as assemblage zone of Spiti. Thus, it is a body of strata whose content of fossils of a certain kind taken in its entirety constitutes a natural assemblage or association from adjacent strata. An assemblage zone should have a specifically designated and decimated type section or reference section, eg., the otoceras – Ophiceras assemblage zone, Lilang, and Spiti, characteristics fossils of an assemblage zone are called guide fossils.

A range zone is a body of strata representing the total horizontal and vertical range of occurrence of one particular species or genus, e.g., Miscellanes miscella range zone. The principal reference of a range zone is the type specimen or reference specimen.

An epibole (acme zone/peak zone) is the body of strata representing the maximum development of a species/genus. It is a special type of range zone. Eg., Glossopteris acme zone in the flora of Raniganj Formation.

The fossil names are to be italicized. The initial letter of formal zone terms, except the name of species should be capitalized.

Biostratigraphic boundaries may be directly or indirectly applicable to establishing both rock stratigraphic and time-stratigraphic units. They contribute to a better understanding of the correlation, depositional and faunal history of strata sequence.

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

Categories of Classification Basis of Classification Units
Formal Informal
Lithostratigraphic (rock stratigraphic) Lithology

Lithological characters of attributes of the rocks

Hierarchy of Terms Supergroup, Group, Subgroup, Formation, Member beds Beds, zone


Biostratigraphic Paleontology contemporaneous fossil content of rock types

1)      Assemblage of all or specific kinds of fossil

2)      Range of specified taxon

3)      Overlapping range of specified taxon

4)      Peak of development of a specified taxon

1)      Assemblage Zone

2)      Range zone

3)      Concurrent-range zone

Epibole (acme/peak zone)
Chronostratigraphic (time-stratigraphic) Geochronology: Geologic age of the rocks System




Geologic time units. These are defined after chronostratigraphic units Geologic time:

These are not stratigraphic but abstract units.





Age, time

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