Principle of Stratigraphy
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History of Principle of Stratigraphy
In his book published in 1913 entitle ‘Principle of stratigraphy’ Amadeus William Grabau defined stratigraphy as “ it is inorganic side of historical geology, or the development through the successive geological ages of the earth in rocky framework or lithosphere”, Krumbein and Sloss (1951) stated that stratigraphy may now be considered as the integrating science which combines data from almost all other branches of earth science, resulting in historical geology. Waller (1960) defines stratigraphy as the branch of geology that deals with the study of an interpretation of stratified and sedimentary rocks and with the identification, description, sequences (both horizontal and vertical), mapping and correlation of stratigraphic rock units. The word ‘Stratified” include layered metamorphic as well as volcanic rocks.
In the past over six decades, a large volume of data pertaining to subsurface geology, mainly from the exploration and exploitation of petroleum, coal, and water and oceanographic survey, contributed significantly to the understanding of the stratigraphy of many inland and offshore regions of the world.
Present day Study of Stratigraphy
Sedimentary rocks and the fossils they containing the fundamental materials for understanding stratigraphic concepts. The study of the present-day sediments, their formation and distribution is essential for understanding the principle of stratigraphy, the interpretation of ancient sedimentary rocks and for establishing the stratigraphic record.
Components of Stratigraphy:
The subject of stratigraphy can be divided into two major parts viz.,
- Physical stratigraphy and
the components of which are shown in Table 1.
|Stratigraphy Column||Biostratigraphic column|
|Classification of Sediments||Classification of Organisms|
|Sedimentary process||Biological processes|
|Lithologic correlation||Biostratigraphic correlation|
|Sedimentary tectonics||Organic evolution|
Most stratigraphic studies help in solving problems in palaeogeography, historical geology, and economic geology. All the data collected are organized, analyzed, interpreted or synthesized to give the required results.
The stratigraphic column for the Earth was built up over a few hundred years, mainly since the 18th century and was based on many principles
- Earlier catastrophic theories of Earth history
- The principle of superposition of strata
- Principle of uniformitarianism
- Walther’s law concerning sedimentary facies
- Principle of faunal succession
The catastrophic concept: In the earlier part of the 19th century, the opinions or the explanations of time distribution of organisms were greatly influenced by Cuvier, a French vertebrate paleontologist, and his followers. According to Cuvier, each major period of geologic time was terminated by a world-wide catastrophe which wiped out all life. He believed that animals and plants were then reestablished in modified form by an act of Creation at the beginning of the succeeding period, that within each period of time, the various animals and plants were unchanged, that new forms were introduced at widely separate times.
Herodotus, a Greek philosopher, deduced correctly in the 4th century BC that fossils were remains of ancient sea creatures and that the rocks containing them were formed beneath the sea. His ideas became lost for over 2000 years, suppressed by religious beliefs. As late as in the 17th century, some scientists in Britain believed that Earth was 6000 years old, that fossils are relics on Noah’s flood or the work of the devil and that geological history in a series of deluges which killed all life, and followed by the special creation of new fauna.
The principle of Superposition of strata:
Nicolas Steno (1638-87) recognized a sequence of lithologic events in the mountains of western Italy. He recognized that older rocks were overlain by younger rocks, he drew up stratigraphic laws to explain the organic origin of fossils and tried to explain unconformities, folding and faulting. His principle of superposition of strata states that “If one series of rock lies above another then the upper series was formed after the lower series, unless it can be shown that the beds were inverted as a result of the tectonic action.”
The Principle of Uniformitarianism:
This principle, established by James Hutton (1726-97), is the fundamental concept in geology. It propounds that process which operates at present also operated in the past and produced similar results. Put in a simpler way it states that, “ the present is the key to the past”, the various process need not have operated at the same rate nor with the same intensity. It is possible that a process which operated in the past is not seen today or all the process now operating did not operate in an exactly similar way in the past. Organic materials may have changed progressively in form and character through the influence of organic evolution, with advancing geological time. The degree of correlation between ancient and present-day process decreases as the time interval increases.
There is a very close relationship between sedimentation and stratigraphy. The study of recent sediments their process of formation and distribution helps in establishing the principles of the interpretations of ancient sedimentary rocks. The stratigraphic record is the result of the continuity of sedimentary processes through geological time.
Walther’s Law concerning Sedimentary Facies Johannes Walter (1894) stated that- “the various deposits of the same facies, area and similarity the sum of rocks different facies area were formed beside each other in space, but in crustal profile, we see them on top of other. It is a basic statement of far-reaching significance that only those facies and facies areas can be superimposed, primarily, that can be observed beside each other at the present time.
Middleton (1973) summoned up this concept as follows: “A comfortable vertical sequence of facies was generated by a lateral sequence of environment”. This law is one of the basic assumptions of facies analysis and was demonstrated in many modern sedimentary environments in which sub-environments prograde across one another.
The Principle of Faunal Succession:
In the last years of the 18th century, William Smith, an English engineer, recognized distinctive leads within rocks like the chalk and the coal-bearing strata in widely separated areas. He observed that each group contained a particular assemblage of fossils quite distinctive from above of the strata above and below.
Smith was able correlate apparently the similar sedimentary rocks on the basis of the similarity of the fossils. He also noticed that there was the same succession of fossil assemblages from older to younger beds in all parts of Englands. He scrutinized that each of the succession of fossils belonged to a specific spin of geological history. That rocks formed during that time are likely a contain the same fossils. This is called The principle of faunal succession and has proved very useful in the correlation of rocks occurring in widely separated by the fossils they contain.
The above principles combined with the mapping of the rocks in the field, head to an understanding of the gradually evolving Earth and its history. Attempts to work out the actual geological dates with the help of radioactive isotopes of certain elements led to the calculation of the ages of rocks. These methods are referred as radiometric dating methods and have proved specifically useful in fixing the stratigraphic positions of unfossiliferous rocks.
By a detailed study of the lithological characteristics, the palaeontological characteristics (fossil contents), order of superposition and the structural characteristics of strata, supported by absolute dating of rocks and it is possible to establish the local stratigraphy and the regional stratigraphy of an area and correlate the stratigraphic sequences even between widely separated areas.
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