STRATIGRAPHY: Stratigraphic classification, Lithostratigraphy and its Units

STRATIGRAPHY:
Stratigraphic classification, Nomenclature, Lithostratigraphy and its Units

Stratigraphic Elements classification

Stratigraphic is a study of the sequential arrangement of stratified rocks which are stacked one above the other, according to the their age. Is systematically arranges and separates the rock bodies of the earth into units based on their inherent properties and attributes. Hence, stratigraphic classification help to understand the geometry and sequence of the rock bodies .
Any rock sequence can be termed as ‘ conformable’ only if its strata were formed without any ‘time gap’ or deposition interruption. However, in nature, there are long gaps in the formation of rock strata due to erosion or non-depositonal phase. Such a surface of non- deposition is termed as an ‘unconformity’ Unconformities may envelope small or large time spans depending on the geological process and could be local or regional in extern
Stratigraphic elements
A rock sequence at any place can be described as a part of earth’ history, either with the help of
1)its physical criteria, must as lithological properties of the rocks ( lithostratigraphic criteria
2)with the help of its position in the geological time span ( geochronological criteria )
3)with the help of the development of life on the earth ( biostratigraphic criteria )
Further, the time units can be as simple time units ( e.g, Eras, period or Epochs ) or as an assemblage of stratigraphic sequence formed in a specific time interval ( chronostratigrsphic¬ units)
Hence, there are three criteria of the main elements of stratigraphy:
Chronostatigraphic lithostratigraphic and biostratigraphic.
The earliest geological studies were not particular about the importance of ‘geological time ‘and great emphasis was laid on the stratigraphic succession and the recognition of the rock systems as the fundamental groupings of strata (lithostatigraphy ) when radioactive dating methods for rocks became available, it was possible to date the exact time of formation of a rock. Thus, the chronostragraphic and geochronologic units were chosen as, the scale formation for standard stratigraphic classification.
1)Lithostratigraphy ;
Lithostratigraphy is a sub – branch of stratigraphy which deals with the study of strata or rock layers. Its thrust areas include geochronology , comparative geology and petrology. It mainly use the principle of superposition
Which states that a sedimentary rock layer in the tectonically undisturbed stratum is younger than the one beneath and older than the one above it.
Lithostratigraphic units : when lithological criteria are solely considered for classification of a rock succession,
Various terms used in litholostratigraphic classification are !) Group, !!) formation, !!! ) Member and !v ) bed
brief description of lithostratigraphic units is given below;
i)A group ; it is a broad units and comprises of a thick succession of rock formations covering a regional surface of the earth, e.g.; sausar group . two or more formations together maybe included in a unit to form a group. When different groups can be categorized as a mutually related groups then it is called as a Supergroup.
ii) A formation ; A formation is a fundamental unit in the local classification of the rocks. It is a rock succession deposited during a particular Epoch under identical condition of geological environment. It is recognized on the basis of its typical lithological characteristics and stratigraphic position. Hence, it is a genetic unit and is traceable in the local stratigraphic column. The boundaries of the formation are easily recognized in the field and shown on the map, e.g. Trichinopoly formation and uttature formation
iii)A Member : A member is a distinctive unit within a formation. As these are locally present, their names may not be recognized on a regional or ‘lower mudstone member’ are open used. A member may extend laterally from one formation to another.

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