Stratigraphy, also known as ‘Historical Geology’ is the study of the stratified rocks that aims at unravelling the geological history of the earth.
These studies are based on the following few principles:
1. Law of superposition:
It states that “an overlying bed or lamina is younger than that underlying it; under normal conditions”.
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2. Doctrine of uniformitarianism:
“The study of the present is a key to the study of the past”.
3.The geological events of the past are mostly indicated by the rock units from their lithological and palaeontological characteristics. Rock units are disiinguished from each other by their colour, texture, composition, fossil contents etc. The smallest rock unit is a bed; a number of individual beds together form a formation; a number of formations together constitute what is known as a group ; linaily an assemblage of associated groups constitute a Super-group.
4. Facies:
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When a particular system (with reference to the geological time-scale) in a country is represented by different kinds of rocks in its different localities, it is said to possess different facies.
For example, the cretaceous system of India is represented by geo-synclinal deposits in the type area of Spiti, marine transgressional deposits in the Coramondal coastal areas, fluviatile and estuarine deposit in Madhya Pradesh and igneous rocks of volcanic nature in the extrapeninsular region.
5. Homotaxis:
The similarity of position of strata or system in a sequence not implying similarity of age is termed homotaxis.
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When geological formations are equivalent in age but situated wide apart in different districts, countries or continents, they are said to be homotaxial.
6.Rock units formed at different places exactly at the same time are known as synchronous-beds. In these beds the same species of the fossils are found.
7. Contemporaneous:
Those rock units which are formed near about the same time, marked by the presence of the same genera of fossils, they are said to be contemporaneous.
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8. Geological time:
‘Era’ is the largest grouping of periods of geologic time and each era covers many millions of years. The geologic periods, are formed of epochs and the epochs are in turn subdivided into ages.
The geologic time is well correlated with the stratigraphic units. For example, A geologic period represents a system; rock units formed during an epoch is known as series; and the rock units which are formed during an age are known as stages.
9. Stratigraphic correlation:
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It is of three types like-lithostratigraphic, bio-stratigraphic and chronostratigraphic correlation.
The lithostratigraphic correlations are made by the following: methods:
(i) Continuity of contacts between units,
(ii) Lithologic similarity.
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(iii) Stratigraphic position of a unit in a sequence of strata,
(iv) Well logs,
(v) Structural characteristics.
The Bio-stratigraphic correlation involves the following methods:
(i) Stage of evolution of fauna. (ii) Guide fossils, (iii) Faunal resemblance, (iv) Position in a bio-stratigraphic sequence.
Chrono-stratigraphic correlations are made with the help of the following methods:
(i) Quantitative chronology-radioactive dating methods..
(ii) Eustatic changes in sea level.
(iii) Palaeontology.