Geology 2024 Paper II 50 marks Describe

Q4

(a) Discuss the various factors that control the composition of sandstone. (15 marks) (b) What do you understand by facies model ? Describe the facies and facies association produced in a fluvial environment. (15 marks) (c) What are heavy minerals ? Describe methods of their separation and comment on the utility of heavy mineral suite in provenance interpretation. (20 marks)

हिंदी में प्रश्न पढ़ें

(a) बालुकाश्म के संघटन को नियंत्रित करने वाले विभिन्न कारकों पर चर्चा कीजिए। (15 अंक) (b) संलक्षणी मॉडल से आप क्या समझते हैं ? नदीय पर्यावरण में उत्पन्न संलक्षणी व संलक्षणी संयोजन का वर्णन कीजिए। (15 अंक) (c) भारी खनिज क्या होते हैं ? उनके पृथक्करण की विधियों का वर्णन कीजिए तथा उद्गम क्षेत्र की व्याख्या में भारी खनिज संजाती की उपयोगिता पर प्रकाश डालिए। (20 अंक)

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Directive word: Describe

This question asks you to describe. 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 'describe' demands systematic, detailed exposition with appropriate examples. Allocate approximately 30% time/words to part (a) on sandstone composition factors, 30% to part (b) on facies models and fluvial environments, and 40% to part (c) on heavy minerals given its higher mark weightage. Structure: brief introduction defining key terms → systematic treatment of each sub-part with diagrams → conclusion synthesizing sedimentary petrology's applied value.

Key points expected

  • Part (a): Factors controlling sandstone composition — source rock lithology, climate, relief, transport distance, depositional environment, diagenesis; reference to QFL ternary diagram and Pettijohn's classification
  • Part (a): Dott's (1964) or Folk's compositional maturity concepts with Indian examples like Vindhyan or Barakar sandstones
  • Part (b): Definition of facies model as a generalised summary of facies characteristics and their vertical/horizontal relationships; distinction from facies
  • Part (b): Fluvial facies — channel (conglomerate, trough cross-bedded sandstone), levee (fine sandstone, siltstone), floodplain (mudstone, paleosols); facies associations like fining-upward cycles in meandering systems
  • Part (c): Heavy minerals defined as accessory minerals with specific gravity >2.85; separation methods — panning, heavy liquid separation (bromoform, tetrabromoethane), magnetic separation, centrifuge
  • Part (c): Heavy mineral suite utility — provenance discrimination (ultramafic vs. acidic source), transport history, correlation; Indian examples like Sambhar Lake heavy minerals or Kerala beach placers

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness25%12.5Demonstrates precise understanding across all parts: for (a) correctly identifies tectonic setting as primary control with QFL framework; for (b) distinguishes facies model from facies and correctly identifies architectural elements (channel, point bar, floodplain); for (c) accurately describes heavy liquid densities and mineral stability series (zircon-tourmaline-rutile index)Covers basic factors in (a) and defines facies model in (b) but confuses facies with facies model; describes panning for (c) but omits heavy liquid specifics or misstates specific gravity thresholdFundamental errors: conflates sandstone composition with texture; treats facies model as synonym for lithofacies; defines heavy minerals by weight rather than specific gravity; omits diagenetic controls entirely
Diagram / cross-section20%10Includes three quality diagrams: QFL ternary plot for (a) with Indian sandstone fields marked; vertical facies succession diagram for (b) showing fining-upward meandering sequence with sedimentary structures; labeled flowchart or apparatus sketch for (c) heavy liquid separation; all properly annotated with scalesProvides two diagrams with basic labels; QFL triangle without plotted points; facies column without sedimentary structures; separation method described textually onlySingle generic diagram or none; poorly labeled sketches; diagrams contradict text description; missing essential components like grain size scale or legend
Field evidence20%10Cites specific Indian formations: Vindhyan Supergroup quartzarenites for compositional maturity; Siwalik molasse for lithic-rich sandstones; Son-Mahanadi or Ganga fluvial systems for facies analogues; Sambhar Lake heavy mineral concentrates or Manavalakurichi (Tamil Nadu) placer deposits for provenance studiesMentions general examples like 'Gondwana sandstones' or 'Himalayan rivers' without specific formation names; references 'beach sands of Kerala' without naming heavy mineral depositsNo Indian examples; relies entirely on foreign type localities (e.g., Navajo sandstone, Mississippi River) or provides no field evidence whatsoever
Quantitative reasoning15%7.5Provides quantitative benchmarks: specific gravity thresholds (>2.85 for heavy minerals, >2.9 for separation liquids); QFL percentages for Indian sandstones; heavy mineral abundance ranges (0.1-1% typical, >10% in placers); calculates or references ZTR index values for maturity assessmentStates 'high specific gravity' without numbers; mentions 'trace amounts' of heavy minerals; describes QFL triangle qualitatively without percentage rangesNo quantitative data; incorrect orders of magnitude; confuses weight percent with volume percent in compositional descriptions
Indian / economic relevance20%10Explicitly connects to economic geology: heavy mineral placers (ilmenite, rutile, zircon, monazite) in Kerala-Tamil Nadu coast as source of Ti, REE, Th; sandstone composition controls for groundwater aquifer quality (Vindhyan aquifers) and hydrocarbon reservoirs (Cambay Basin); facies models applied in Ganga basin groundwater exploration and coal exploration in Damodar valleyMentions 'economic importance' in passing; notes beach placers without specifying commodities; states sandstone is 'reservoir rock' without Indian examplesNo economic or applied relevance discussed; treats topic as purely academic; misses opportunity to mention atomic minerals (monazite) or groundwater significance entirely

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