Q) Discuss how the depth of a fold can be calculated. Enlist the assumptions in the calculation. (Marks = 15)

This question has left students wondering – what is the ‘Depth of a Fold?’ Is it related to the amplitude or is it something entirely different? Here we provide the explanation of this term, how to calculate it and what are the assumptions involved.

The depth of a fold is not to be confused with amplitude.

The amplitude is measured by taking half the distance along the axial plane from one anticlinal hinge to the surface enveloping the two adjoining synclinal hinges (or vice versa), namely, the distance along the axial plane from the median surface to the hinge. A train of folds may fold a surface periodically or non-periodically.

DEPTH OF A FOLD:

The depth of a fold is the distance between a local datum (eg. sea level) and the detachment at the base of the folded sequence. Detachment is a faulting process in which a large rock mass, usually sedimentary, becomes detached from the rocks beneath and independently moves laterally a great distance, which is typically measured in miles.

Depth of the fold may be known from seismic reflection profiles or borehole litholog data. In the absence of such data, a commonly used method is depth-to-detachment calculation

Excess area method:

• Classically, the excess-area method derived by Chamberlin has been used to estimate the depth of detachment beneath a fold.
• This method is based on the area-conservation principle.
• It predicts that the depth of the fold equals the excess area beneath a particular horizon uplifted above the revional divide by the shortening undergone by this horizon.

• Shortening displacement, D, on the lower detachment produces deformation an uplift of an excess area, S, above the regional elevation (the datum).
• Assuming constant area, the excess area is:
  • S = Dh
• If we also assume that the bed length remained constant during deformation, then
  • D = L₀-L₁
• Where
  • L₀ = curved-bed (original) length,
  • L₁ = straight-line length
• This gives:
  • h = S/(L₀-L₁)

Assumptions in excess area method

1. It is assumed that no material enters or leaves the ends of the cross-section.
2. The bed length is assumed to be constant.
3. The area is assumed to be constant, neither gained (such as by extension fracturing) nor lost (such as by pressure solution)
4. The original regional elevation of a horizon is assumed to be known, because it may be shifted vertically from its original level by deformation.
5. The reference horizon and the lower detachment are assumed to be parallel,

If the aforementioned assumptions are not met the depth of the fold estimated by the excess area method might not be correct.

Further modifications/improvements

Some modifications can be applied to the original method to find depth to detachment if L0 and L1 are not known. For example, we could calculate excess area of multiple beds rather than a single bed and use the slope to estimate the depth of the fold.

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General Geology

1. Discuss the different types of meteorites. (5)
2. Explain the internal structure of the earth. Add a note on discontinuities with neat sketches. (20)
3. Explain the magnitude and intensity as an expression of the size of an earthquake. Add a note on the damage caused by an earthquake. (15)
4. Discuss the mechanism of seafloor spreading and give four evidences indicateing the expansion in oceanic crust. (15)

Geomorphology

1. Briefly describe two diagnostic landforms each of hot-arid and cold-arid climate. (5)
2. Discuss the different in geomorphology of the Eastern Coast and Western Coast of India. What is the main reason of the difference? (15)
3. Describe three main controlling factors of soil formation. How does the modern soil differ from the palaeosoil? (15)

Remote Sensing

1. Discuss various fields in which IRS data can be used for the development work and management of resources. (5)
2. Discuss the salient features of Indian satellites Cartosat-3 and RISAT-2B. How are these superior to their predecessors? (20)

Structural Geology

1. How do we determine the top of beds from primary features? (5)
2. Briefly describe the classification of fold on the basis of fold attitude (dip and plunge) (5)
3. What is an unconformity and what are its structural and stratigraphic importance? Discuss four different types of unconformities  with neat sketches. (15)
4. What is shear zone and what are the conditions of its formation? Discuss its general characteristics and types based on deformation type. (15)
5. Discuss Mohr’s stress diagram and its significance. What is its relevance in interpreting different stress conditions in rocks? (20)

Paleontology

1. Describe three processes by which plant remains and invertebrate shells can be preserved as fossils. (5)
2. Discuss the morphological trends in the evolution of Homo sapiens from Proto-hominins. (20)
3. Give an account of the Lower Gondwana flora from peninsular and extra-peninsular India. What environment conditions do they indicate? (15)
4. Name two microfossils and discuss how these are used in reconstructing the palaeoclimatic conditions. (15)

Stratigraphy / Indian Stratigraphy

1. Give an account of the mineral wealth associated with the Cuddapah Supergroup of rocks. (5)
2. Briefly describe the economic mineral deposits associated with the Tertiary rocks of India. (5)
3. Draw the tectonic sub division map of India and discuss the salient features of each sub-division. (15)
4. Describe the standard stratigraphic time scale of the earth, beginning from the oldest to youngest. Discuss the principal events that took place during the time units. (20)
5. Discuss about the Cretaceous- Tertiary boundary (K-T boundary) with examples from India. (15)

Hydrogeology and Engineering Geology

1. Explain the factors influencing the groundwater movement and storage. (5)
2. Discuss five engineering properties of rocks. (5)
3. Explain the process of salt water intrusion with neat sketches. Give examples form India. (15)
4. What is landslide? Explain the different types and their causes. Give two examples from India. (15)
5. Explain the natural and artificial recharge processes to enhance the ground water potential in hard and soft rock terrains. (20)

Mineralogy

1. How do you measure refractive index of minerals and distinguish high and low relief minerals under microscope? (10)
2. Write about the symmetry elements of a crystal. (10)
3. Briefly write about the crystal forms of hexagonal system using projection diagrams.
4. Define pleochroism of minerals and write about the utility of pleochroism to distinguish minerals and provide examples.

Igneous and Metamorphic Petrology

1. Explain the salient features exhibited by rocks due to thermal metamorphism. (10)
2. Explain with suitable examples the implications of albite-anorthite solid solutions in the understanding of crystallisation of magma. (10)
3. What do you understand by the term texture of a rock? How do you relate the textures of igneous rocks with the process of magmatic crystallisation? (20)
4. Critically discuss the petrogenesis of anorthosites. Comment on the tectonic significance on the distribution of anorthosites. (20)
5. Explain the effects of prograde metamorphism on impure carbonate rocks. (10)

Sedimentary Petrology

1. How are sedimentary rocks classified in general based on their process of formation. (10)
2. Illustrate with neat sketches the sedimentary facies and facies associations that are likely to develop in a progradational deltaic environment. (20)
3. ‘Classification of carbonate rocks based on textural components may be useful for interpretation of depositional environment’. Justify the statement with reason. (20)
4. How would you distinguish burial diagenetic cements in carbonate rocks from petrographic studies? Draw neat sketches in support of your answer. (10)

Economic Geology

1. Explain the processes of formation of syngenetic and epigenetic ore deposits with suitable examples. (10)
2. Describe the geology of the East Coast Bauxite deposits of India. (10)
3. Discuss the salient features of various types of lead-zinc ore deposits. Illustrate your answer with schematic diagrams to explain the process of ore formation. (20)
4. Describe the geological characteristic of important chromite deposits from different parts of the Indian shield. (20)
5. What is coal bed methane? How is it exploited? (10)

Mining Geology

1. Discuss various methods of conservation of minerals resources for achieving sustainable mineral development (10)
2. What are the factors which influence the mobility of elements in near surface environment? How does understanding the mobility of elements help in choosing indicators elements for a geochemical exploration programme? (20)
3. A vein type copper ore body trending E-W with 90^o dip and having strike length of 150 m is exposed on the surface. Four trenches are made across the ore body at constant interval, including one trench each at E and W end of the ore body. Thickness of the ore body and assay value measured from the trenches are given in the table.
   Assuming that the ore body extends up to a depth of 50 m, and the specific gravity of the ore is 2.8, calculate the tonnage, grade and total copper content of the ore body. (20)

4. Explain the seismic methods of mineral prospecting. (10)

Geochemistry and Environmental Geology

1. Illustrate with examples the role of chemical bonding in minerals in determining certain properties. (10)
2. Write about the causes of Sea level changes. (10)
3. What do you mean by ‘primitive mantle’? Illustrate the Cosmo chemical and observational constraints on mantle composition. (20)
4. Explain about the impact of volcanic eruptions on the environment. (20)
5. Briefly mention measures to mitigate lanslides. (10)

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