STRATIGRAPHY - INTRODUCTION/HISTORY OF STRATIGRAPHY/ PRINCIPLES OF STRATIGRAPHY/DEFINATION OF STRATIGRAPHY.
1. INTRODUCTION :
DEFINATION:
" Stratigraphy is the branch of geology that deals with succession and correlation of rocks ".
Since when the subject started there where 2 groups of thoughts regarding origin of rocks.
NEPTUNIST:
- These were the scientist who believe that all rocks including igneous and metamorphic are of aqueous origin.
- They are in considered all the rock having layered structure that's why word Stratigraphy.
- (Strata - Layer , Graphy - Measuring ) . These scientist believed that as all the rocks are of aqueous origin the earth must have " onion like structure".
- In this group of scientist the name Werner (1750-1817) who calculated age of earth from sedimentary origin to 6000 years.
PLUTONIST:
- These were the scientist who believed that only sedimentary rocks are of aqueous origin and other igneous and metamorphic are not in aqueous origin .
- In these group of scientist name include James Hutton (1736-1796).
- James Hutton is best known for his contribution in Science of geology , uniformitarianism and the great age of earth.
- James Hutton works on unconformities in most of the part of his life . His famous quotation in geology is -
" Present is the key to the past " to which he termed as Principle of actualism.
- This principle was supported by Charles Lyell (1797- 1875) by giving concept of uniformitarianism which goes as follows-
" The past geological processes are explain in terms of the contemporary processes which can be observed and redefined at present ".
- It doesn't means that all processes which are seen today were operating in the past exactly in the same manner but the conditions are constant.
2. HISTORY OF STRATIGRAPHY:
- The first work in Stratigraphy was of Niles Stenson (1638- 1686) also called as Steno who was a medical doctor from Denmark.
- When he visited Italy on the sea coast he found 2 fossils namely - Bettmenite and Glossopteria (The teeth of Shark).
- The 2 Fossils were hollow and there was sedimentation of sand within the fossils . These were regarded as some heavenly material by the god for sinuous . But after examination Steno explained it as Fossils and published a paper in 1667 with Title " Occurrence of a Solid within a Solid".
- From here the subject stratigraphy began where he quoted 3 principles of stratigraphy:
2. Principle of Lateral continuity
3. Principle of order of superposition.
- After this James Hutton who are basically a civil engineer working as a canal in England . He work on unconformity Charles lyll , Adam Sedgewick and others.
3. PRINCIPLE OF STRATIGRAPHY:
INTRODUCTION:
- As we have seen that this branch of geology started to began in 17 th century . The pioneer workers in the field of stratigraphy where Niles Stenson (1638-1686) , Werner (1750-1817), Charles lyll, James Hutton (1726-1797) who worked for different domains of stratigraphy.
- Among these Niles Stenson was a maiden worker who proposed 3 initial principle of Stratigraphy:
(a) PRINCIPLE OF ORDER OF SUPERPOSITION:
- When steno published his first paper with the findings of fossils found on the sea coast of Italy like bettemenites and Glossopteria the research paper titled " Occurrence of a solid within a solid" .
- In which he profounded the principal of superposition which states that in a normal condition the layer which is below will be older than the layer of the above formation.
- Principle of stratigraphy or methods of correlation they formulated are actually based on a single understanding or correlation .
(b) PRINCIPLE OF ORIGINAL HORIZONTALITY:
- It states that regardless of altitude in which we found sedimentary strata today they were originally deposited in horizontal layer.
- Though it is a valid principle but sometimes it is not strictly true. e.g. In non- marine condition , coarse grained deposits, deposition can takes place on slopes upto 30 degree.
- This Steno's principle was not invalidate because of these exception and geologists can interpret the folding and tectonic history from study of layers.
(c) LAW OF FACIES:
- James Walther (1889- 1894) which states that " Those lithofacies that are found next to each other in a vertical section of rock formed adjacent to each other in space" .
- This simple concept or Walther's law or law of facies has only hedge that must be considered that it is applied only to uninterrupted sequences of genetically related sedimentary rocks.
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(d) LAW OF CROSS- CUT RELATIONSHIPS:
- This is very simple principle of stratigraphy it states "That only structure that cuts the sedimentary rocks will be younger than the sedimentary rock."
(e) PRINCIPLE OF BIOTIC SUCCESSION:
- The very simple question and thought comes to mind when we see geological time scale that's why Precambrian have been classified in less subdivision while post- Cambrian time is classified in much more detailed and the answers is in the fact that in post- Cambrian evolution of life was more intensified and diversified because of which we get large no. of fossils in rock of post - Cambrian period and these fossils enabled geologists to classify the rocks in much detail.
Why we use fossils for correlation of the rock preferably while we have radiometric dating in our hand which gives direct age :
1. This biotic succession become one of the strongest principle of stratigraphy because of the applicability in the correlation of rock.
2. This principle of floral and faunal succession was given by William Smith who first prepared geological map of England and was an engineer basically . He observed during the canal construction that beds of the similar lithology at different places have similar fossils assemblages.
3. This principle of biotic succession is useful in determination of relative age is based on the fact that fossil assemblages is never repeated and the organism continuously undergo change in morphology progressively .
This is called law of irreversibility of biological evolution states that -
"The organisms cannot evolve to a preceding form even if they find themselves in exactly similar condition to which this preceders lived".
Hence , each assemblage of past life preserved in the rocus reflects a specific stage of evolution of life.
(f) PRINCIPLE OF UNIFORMITARIANISM:
- Werner a Neptunist was a professor in Freiburg university , Germany his idea about aqueous origin were in fashion those days .
- But James Hutton came with the concept that granite and basalts were consolidated from magma and are not of aqueous origin .
- James Hutton the plutonist published a paper titled " Theory of earth " in 1788 and quoted that "All the geological processes operated in the past also operate at the present time ".
- John Playfair clarified Hutton's idea in his book which he published in 1802 titled " Illustration of Hutton's theory " . and Charles lyll used the principle of uniformitarianism in his textbook " Principles of geology' in 1830.
(g) COMPLETEINESS / INCOMPLETEINESS:
- Hutton recognized the importance of missing times known as unconformity . A sudden change in lithological characters or fossils assemblages with the erosional surface denotes to unconformities.
- These unconformities are of 3 types as we know that law of succession of flora and fauna, any stack of layered rocks of containing 2 distinct fossils assemblages. There must be a time gap which is important to be known for reconstruction and correlation with the rocks of other areas.
- Attempts have been made to correlate the rock types on the basic of these discontinuities named "Allostratigraphic units" .
(h) PRINCIPLE OF LATERAL CONTINUITY:
- This concept states that the strata where deposited in continuous layers across the depositional basins and may be correlated over the distance and across valleys that may cut them.
- Although this concept is crucial in understanding for regional stratigraphy it must be limited to given depositional basin indiscriminate application of this law give rise to early theory of " Onion skin " geology while we know the fact that sedimentary layers pinch out and at end the margins of depositional basins.
- This principle of lateral continuity helps in correlating rocks of 2 different areas on the basis of dissimilarities.
4. DEFINATION OF STRATIGRAPHY:
Stratigraphy is defined in different ways y more than one geo - scientists .
- According to ISSC (International Subcommision on Stratigraphic Classification). In 1976 , Stratigraphy as defined in " Basics of Physical Stratigraphy and Sedimentology " edited in 1989 , by William Fritz and J.N Moore as
" The branch of geology that deals with the origin , composition , description and correlation of stratified rocks through the understanding of recent sedimentary processes modern environments and principle of biology , ecology , physics and chemistry "
- While in the textbook of fundamentals of historical geology and stratigraphy of India by Ravindra Kumar , new age publications (2021) .
- Stratigraphy is defined as -
- In the textbook of sedimentology be Brenner (1998) it is said that in stratigraphy we must consider with more technical jargon the additional dimension of time with length , breadth and depth . and this is clear understanding of relationship of time with space and sedimentary rocks and it is defined as -
- Here the 4 th dimension is time.
