Geology 2023 Paper II 50 marks Discuss

Q4

(a) Classify the conglomerate rocks on the basis of clast composition and grain-matrix ratio and discuss their genetic importance. (20 marks) (b) Briefly describe the mechanisms of gravity-controlled sediment flows and write about their characteristic features in the rocks. (15 marks) (c) Explain mineral-based techniques to decipher the source terrains and transport history of sediments. Give a list of minerals diagnostic of igneous and metamorphic provenances. (15 marks)

हिंदी में प्रश्न पढ़ें

(a) क्लास्ट संघटन और ग्रेन-मैट्रिक्स अनुपात के आधार पर संगुटिका (कांग्लोमरेट) चट्टानों का वर्गीकरण कीजिए और उनके आनुवंशिक महत्व पर चर्चा कीजिए। (20 अंक) (b) गुरुत्व नियंत्रित अवसाद प्रवाह की क्रियाविधियों का संक्षेप में वर्णन कीजिए और चट्टानों में उनकी विशिष्ट विशेषताओं के बारे में लिखिए। (15 अंक) (c) तलछटों के स्रोत भूभाग (टेरेन) और परिवहन इतिहास को समझने के लिए खनिज-आधारित तकनीकों की व्याख्या कीजिए। आग्नेय और कायांतरित स्रोतों के निदानात्मक खनिजों की एक सूची दीजिए। (15 अंक)

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Directive word: Discuss

This question asks you to discuss. The directive word signals the depth of analysis expected, the structure of your answer, and the weight of evidence you must bring.

See our UPSC directive words guide for a full breakdown of how to respond to each command word.

How this answer will be evaluated

Approach

The directive 'discuss' in part (a) demands critical examination with genetic interpretation, while parts (b) and (c) require descriptive and explanatory treatment respectively. Allocate approximately 40% of time and content to part (a) given its 20 marks, with ~30% each to parts (b) and (c). Structure with a brief integrated introduction on sedimentary petrology, followed by three distinct sections addressing each sub-part, and conclude with the significance of integrated provenance studies for basin analysis.

Key points expected

  • Part (a): Classification of conglomerates by clast composition (petromict vs. oligomict, polymict vs. monomict) and grain-matrix ratio (orthoconglomerate vs. paraconglomerate); genetic significance linking matrix-rich paraconglomerates to debris flows and matrix-poor orthoconglomerates to traction currents
  • Part (a): Discussion of genetic importance including depositional environment interpretation (alluvial fan, braided river, beach, glacial till) and the significance of clast lithology in revealing source rock types and tectonic setting
  • Part (b): Mechanisms of gravity-controlled sediment flows including turbidity currents (Newtonian, turbulent), debris flows (non-Newtonian, plastic), grain flows, and liquefied flows; rheological distinctions and flow transformations
  • Part (b): Characteristic sedimentary features including Bouma sequences (Ta-e divisions), massive/graded bedding, inverse grading in grain flows, clast-supported vs. matrix-supported textures, and sole structures (flute casts, groove casts)
  • Part (c): Mineral-based provenance techniques including heavy mineral analysis (assemblage studies, ZTR index), garnet geochemistry, zircon U-Pb dating and Hf isotopes, rutile thermometry, and bulk geochemical proxies (CIA, Th/Sc, La/Th ratios)
  • Part (c): Diagnostic mineral lists—igneous provenance: zircon, apatite, sphene, hornblende, pyroxene, olivine; metamorphic provenance: garnet, staurolite, kyanite, sillimanite, epidote, glaucophane, lawsonite, with stability ranges indicating metamorphic grade

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness25%12.5Precise distinction between orthoconglomerate and paraconglomerate based on matrix content (>15% vs. <15%); accurate rheological classification of sediment flows (Newtonian vs. Bingham plastic); correct application of heavy mineral stability series and index minerals for metamorphic grade; no confusion between turbidity current and debris flow mechanismsBasic correct definitions but some confusion between conglomerate types or flow mechanisms; incomplete understanding of why matrix content indicates depositional process; generic mention of provenance techniques without specific mineral stability conceptsFundamental errors such as confusing grain-matrix ratio with sorting, misidentifying debris flows as turbulent, or listing minerals without provenance significance; omission of genetic links between classification parameters and depositional processes
Diagram / cross-section15%7.5Clear labeled diagrams showing: (a) orthoconglomerate vs. paraconglomerate texture in hand specimen view; (b) Bouma sequence with all five divisions or debris flow rheology diagram with yield stress; (c) heavy mineral separation procedure or ternary provenance discrimination diagrams; neat, properly labeled with scaleRough sketches without proper labels or missing key features (e.g., Bouma sequence without basal contact); diagrams present but not directly referenced in text; confused or inaccurate representation of flow regimesNo diagrams despite clear need for visual representation; messy unlabeled sketches; diagrams that contradict written description; omission of diagrams for part (b) which particularly requires visual explanation of flow mechanisms
Field evidence20%10Specific Indian examples: Krol conglomerate (Himalayan foreland basin) for orthoconglomerate; Siwalik Group debris flows; Tirupati sandstone heavy mineral provenance from Eastern Ghats; Karewa Group glacial diamictite; Dhading orthoconglomerate; mention of GSI or ONGC provenance studies from Indian basinsGeneric field descriptions without specific localities; Western examples (Molasse, Alpine) instead of Indian analogues; mention of 'Himalayan foreland basin' without specific formation names; correct concepts but missing Indian contextNo field examples; invented or incorrect localities; complete absence of Indian geological context despite extensive Indian literature on Siwalik, Ganga Basin, and Cuddalore sandstone provenance studies
Quantitative reasoning20%10Numerical thresholds: matrix 15% boundary for ortho/paraconglomerate; ZTR index calculation for mineral maturity; QFL and Qm-F-Lt ternary diagrams with percentage ranges; Th/Sc >0.8 for felsic, <0.5 for mafic sources; CIA values for weathering intensity; mention of settling velocity equations for turbidity currentsQualitative mention of indices without numerical values; 'high' or 'low' without thresholds; correct concepts of quantification but no actual numbers; generic reference to 'ratios' without specificationNo quantitative treatment where clearly required; confusion between weight percent and volume percent; incorrect numerical values; complete absence of the 15% matrix threshold which is fundamental to part (a)
Indian / economic relevance20%10Economic significance: placer deposits (monazite in Kerala-Tamil Nadu coast, ilmenite-rutile-zircon in Chatrapur-Gopalpur); uranium in Siwalik conglomerates; diamond in Bunder kimberlite and Wajrakarur; petroleum reservoir quality linked to provenance in KG Basin, Cambay Basin; groundwater aquifer potential of conglomerate horizonsGeneric mention of placer deposits without Indian localities; economic importance stated but not linked to specific Indian resources; correct but superficial treatment of economic aspectsNo economic relevance discussed; missed opportunity to link heavy mineral provenance to beach placer deposits which are major Indian economic resources; no mention of reservoir quality or groundwater significance

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