Description and classification of folds
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This document provides an overview of structural geology and folds. It defines structural geology as the study of geological structures like folds, faults, unconformities and joints. Folds occur when rock layers bend due to compressive forces, and come in different forms like anticlines and synclines. The key parts of a fold are identified as the crest, trough, limbs, hinge and axial plane. Folds can be classified based on their symmetry, plunge, bed thickness changes and other characteristics. Examples of different fold types include symmetrical, asymmetrical, isoclinal, overturned and plunging folds. Considerations for engineering projects involving folded rock formations are also outlined.
Description and classification of folds
- 1. Structural Geology Dr. P. Sarathbabu M.Sc. B.Ed. Ph.D. Department of Geology Acharya Nagarjuna University
- 2. The major structures are 1. FOLDS 2. FAULTS 3. UNCONFORMITIES 4. JOINTS
- 4. Structural geology is a branch of geology dealing with the study of structures found in rocks. It is also known as “Tectonic Geology” or simply Tectonics. Structure is a term used to describe the overall relationship of rock masses, eg: folding, faulting, jointing and unconformities.
- 5. Folds: Folds are one of the most common geological structures found in rocks. When a set of horizontal layers are subjected to compressive forces, they bend either upward or downwards. The bends noticed in rocks are called folds. Folds are described variously as wavy or arch- like or curved or undulatory or wraping appearances found in rocks.
- 6. Crest, Trough, Limbs, Anticline, Syncline, Hinge, Axis of fold, Axial plane, Plunge, Dome, Basin. Anatomy of Folds
- 7. Crest and Trough :The curved portions of the fold at the top and bottom are called crest and trough. Limbs or Flanks : These are the sides of a fold. There two limbs for every fold and one lime common to the adjacent folds. Anticline and Syncline : When the beds are bent upwards, the resulting fold is called anticline (anti-opposite, cline-inclination). Syncline is just opposite to anticline in its nature.
- 9. Hinge :The axial plane divides a fold into its two limbs The surface trace of an axial plane is called the hinge line (or axis) of the fold. Axis of Fold : An axis of fold is defined as the line of intersection between the axial plane and the surface of any of the constituent rock bed. Axial Plane : It is an imaginary plane or surface which divides a fold into two equal halves.
- 10. Plunge of Fold: Folds having inclined axes are called plunging fold.
- 11. Dome: Three-dimensonal anticline resembling inverted cereal bowl Basin : Three-dimensional syncline resembling upright cereal bowl
- 12. Classification and types of folds Based on different principles, the folds are variously classified. Usually, folds are classified on the basis of i) Symmetrical character ii) Upward or downward bend iii) Occurrence of plunge iv) Uniformity of bed thickness v) Behaviour of the fold pattern with depth
- 13. Types of folds : 1. Symmetrical fold 2. Asymmetrical fold 3. Isoclinal fold 4. Overturned fold 5. Recumbent fold 6. Chevron fold 7. Plunging and Non plunging fold 8. Similar and Parallel fold 9. Drag fold 10. Fan fold
- 16. Symmetrical and Asymmetrical folds when the axial plane divides a fold into two equal halves in such a way that one half is the mirror image of another, then such a fold is called symmetrical fold. If the two halves are not mirror images, then the fold is called as asymmetrical fold
- 17. Isoclinal fold :Folds that have parallel limbs are called Isoclinal folds. Overturned fold :In this fold the two limbs dip in the same direction but at different angles. The axial plane is inclined. Recumbent fold : A fold in which the axial plane is absolutely horizontal and the limbs are also more or less horizontal is called Recumbent Fold. Chevron fold : The fold, where the crest and trough are sharp and angular are called Chevron fold.
- 18. Plunging and Non-plunging folds: Folds having inclined axes are called the ‘plunging fold. Folds having horizontal axes are called non-plunging folds. Similar and Parallel fold : In similar folding the bedding planes are similar having the same shape downwards or upwards so the beds near the crest are thicker and the beds at the limbs are thinner Drag fold : These are may be defined as minor folds developed within the body of incompetent beds during the process of major folding. Fan fold : It is an upright fold in which both the limbs are overturned. In anticlines limbs dip towards the axial plane and synclines they dip away from it.
- 20. Folds; Engineering Considerations 1. For major project like a dam, tunnel, railway stations, etc., a site which is highly folded should be avoided because the engineer may have to face much troubles sooner or later as folds are easily fractured even due to a slight disturbance. 2. If the project is of a scattered nature like electric or telephonic poles the work can be carried out without much of a risk. 3. Synclinal folded rocks may yield hard and tough quality stones; where as anticlinal folded rocks will yield weaker stones. 4. The anticlinal folds provide good prospects for stored petroleum and hence in oil exploration, folds must not be overlooked.