Stratigraphy is the study of layered rocks, especially their sequence, correlation from place to place, relative ages, and interpretation. Several important stratigraphic principles emerged from the study of stratigraphy centuries ago by the early founders of the science of geology.
Principles of Stratigraphy
Following are some of the major principles of stratigraphy:
Principle of Superposition:
According to the principle of superposition: In any sequence of undisturbed strata, the oldest layer is at the bottom and higher layers are successively younger.
Principle of Original Horizontality:
According to the principle of original horizontality: most sedimentary rocks formed originally in close-to-horizontal layers (although many have since been moved from their original position).
Principle of Original Lateral Continuity
According to the principle of original lateral continuity: Originally sedimentary strata extended in all directions until they either; thinned out, ended abruptly at some kind of barrier, or graded into a different kind of sedimentary rock. This principle is important for correlating sedimentary rocks from place to place i.e. across a valley. Stratigraphic correlation is the practice of “matching up” equivalent bodies of rock from different locations. The equivalence may be in terms of lithology, age, or fossil content.
Principle of Uniformitarianism
According to the principle of uniformitarianism “The present is the key to the past” i.e. geologic processes operating today also operated in the past. e.g. river deposits forming today have a similar composition and character as their ancient counterparts; glacial erosional and depositional features are basically the same today as in the past…etc
Principle of Biological Succession
According to the principle of biological succession: Different kinds of plants and animals succeed one another in time because life has evolved continuously; therefore only rocks formed during the same age can contain similar assemblages of fossils. Since these fossil assemblages are unique for particular periods of the past, they can be used to:
- Correlate rocks from around the world,
- Order rock layers into a sequence of relative age (i.e. older …. newer).
Principle of Cross-cutting Relationship:
According to the principle of cross-cutting relationship: Any geologic feature which cuts across or penetrates another body of rock must be younger than the rock mass penetrated.
Principle of Inclusions
According to the principle of inclusions: Any rock that contains fragments of an adjacent rock must be younger than the adjacent rock.
Walther’s law of facies correlation:
According to Walther’s law of facies correlation: the succession of facies and lateral variation of deposition may be taken place at the same time in a different environment.
Law of palaeogeography:
This is the distribution and relationship of ancient seas and landmasses. Ancient geography is reconstructed through an interpretation of the sedimentary rocks and fossils of a certain age. E.g. the fossiliferous sediment of Mt. Everest shows once it was the bottom of an ocean floor in historic time.
Law of Unconformities
According to the law of unconformities: At any place of the geologic record, there is evidence of crustal upliftment followed by a long period of erosion. Such breaks or gaps in the record are called unconformities.
Principle of Correlation
According to the principle of correlation: correlation is the matching of rock strata of the relative same age. Similar stratigraphic units can be correlated to each other. The few most important evidence of correlation are:
A. Palaeontological evidence:
- Similarities of fossils
- Index fossils
- Palaeontological sequences.
- Palaeontologic similarity
- Evolutionary development.
B. Physical evidence:
- Continuity of strata
- Lithologic similarity
- Position of stratigraphic sequence
- Radioactivity etc.