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

Supergroup

Group

Subgroup

Formation

Member

Bed

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