Concepts of Stratigraphy

A Simple Presentation depicting the concepts os Stratigraphy. The Stratigraphy is a branch of geology concerned with the study of rock layers and layering .

1. Concepts of Stratigraphy Sharik Shamsudhien 183104008

2. Stratigraphic concepts The field of stratigraphy is a branch of geology that deals with rock succession through time and space Stratigraphy is a complex and intriguing form of geology which tells the story of the Earth through time. In short it is the time frozen in rocks. Stratigraphy is largely based on the law of superposition, originally brought into vogue by Nicholas Steno in the 1600's

3. Types of Stratigraphy Lithostratigraphy Physical properties and stratigraphic position relative to other lithostratigraphic units. Organization of strata on the basis of their lithologic characteristics Chronostratigraphy Integrated approach to establishing the time relationships among geologic units It is the organisation of strata on the basis of Time relationships Biostratigraphy Study of the fossil record with emphasis on faunal succession to establish relative time relationships Organization of strata on the basis of the fossils they contain

4. Magnetostratigraph y Study of the magnetic properties of rock units for the purpose of correlation using magnetic polarity reversals Organization of strata on the basis of their magnetic characteristics Allostratigraphy Study of rock units defined by unconformities and other features generated by base level change Geochronology Various techniques, especially isotope geochemistry , to establish the absolute age of rock units Depositional sequences, packages of strata bounded by unconformities   Seismic Stratigraphy Organization of strata on the basis of their seismic characteristics

5. Methods of Stratigraphic sub division

6. Correlation

7. Principles (Laws) of Stratigraphy The principle of original horizontality It is based on the fact that sediment usually accumulates in horizontal layers. All sedimentary rocks are originally deposited horizontally If sedimentary rocks lie at an angle, we can infer that tectonic forces tilted them after they formed

8. The principle of superposition It states that sedimentary rocks become younger from bottom to top (as long as tectonic forces have not turned them upside down). This is because younger layers of sediment always accumulate on top of older layers. In the figure below the sedimentary layers become progressively younger in the order E, D, C, B, and A.

9. Principle of Lateral Continuity It states that if layers are deposited horizontally over the sea floor, then they would be expected to be laterally continuous over some distance.  Sedimentary rocks are laterally continuous over large areas

10. Principle of crosscutting relationships If one geologic feature cuts across another, the feature that has been cut is older. Younger features are truncate (cut across) older features. It states that a rock must first exist before anything can happen to it.

11. Principle of Inclusions The Law of Inclusions states that if a rock body (Rock B) contained fragments of another rock body (Rock A), it must be younger than the fragments of rock it contained.

12. Principle of Faunal Succession Law of Faunal Succession states that fossils occur in a definite, invariable sequence in the geologic record. Fossils represent living creatures that have evolved through time, so when we found a fossil of the same type in two different areas (sections) that are not laterally continuous, we are pretty sure that the rocks are about the same age.

13. Principle of Uniformitarianism Was postulated by James Hutton (1726-1797) The features like mud cracks, ripple marks, graded bedding and so on, where the same features that could be seen forming in modern environments, and that past geologic events can be explained by phenomena and forces observable today Hutton concluded that processes are currently operating on the Earth must be the same processes that operated in the past. Later on stated as "the present is the key to the past".

14. UNCONFORMITIES Layers of sedimentary rocks are conformable if they were deposited without interruption. An unconformity represents an interruption in deposition, usually of long duration. During the interval when no sediment was deposited, some rock layers may have been eroded Thus, an unconformity represents a long time interval for which no geologic record exists in that place. The lost record may involve hundreds of millions of years

15. Disconformity : A disconformity is an unconformity between parallel  layers of sedimentary rocks which represents a period of erosion or non-deposition. Disconformities are marked by features of  subaerial   erosion. Nonconformity : A nonconformity exists between sedimentary rocks and  metamorphic or  igneous rocks when the sedimentary rock lies above and was deposited on the pre-existing and eroded metamorphic or igneous rock. Angular unconformity : An angular unconformity is an unconformity where horizontally parallel strata of sedimentary rock are deposited on tilted and eroded layers, producing an angular discordance with the overlying horizontal layers. Paraconformity : A paraconformity is a type of unconformity in which strata are parallel; there is no apparent erosion and the unconformity surface resembles a simple bedding plane. It is also called nondepositional unconformity or pseudoconformity .

16. Types of unconformities

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