Geology 2021 Paper II 50 marks Describe

Q4

(a) What is provenance? How can we use clastic quartz, feldspars and lithic grains in provenance interpretation of sandstones? (20 marks) (b) Define a 'sedimentary facies model'. Illustrate with neat sketches the sedimentary facies and association facies likely to develop in a meandering fluvial depositional environment. (20 marks) (c) Describe the genesis of any four sedimentary structures which have significance for palaeocurrent analysis. (10 marks)

हिंदी में प्रश्न पढ़ें

(a) उद्गम क्षेत्र क्या होता है? खंडज क्वार्ट्ज, फेल्डस्पार और शैली कणों का उपयोग हम किस प्रकार से बालूकाश्म के उद्गम क्षेत्र निर्वचन में कर सकते हैं? (20 अंक) (b) 'अवसादी फेसीज मॉडल' को परिभाषित कीजिए। एक विसर्पी नदीय निक्षेपण पर्यावरण से बनने वाली अवसादी फेसीज व साहचर्य फेसीज को स्वच्छ चित्रों से समझाइए। (20 अंक) (c) किन्हीं चार अवसादी संरचनाओं की उत्पत्ति का वर्णन कीजिए, जिनका पुराप्रवाह विश्लेषण में महत्व है। (10 अंक)

Evaluate your answer to this question → See all 2021 Geology questions

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 clear illustrations. Allocate approximately 40% effort to part (a) on provenance given its 20 marks, 40% to part (b) on facies models requiring detailed sketches, and 20% to part (c) on sedimentary structures. Structure as: brief introduction defining key terms → detailed treatment of each sub-part with diagrams for (b) → integrated conclusion linking provenance-facies-palaeocurrent for basin analysis.

Key points expected

  • Part (a): Definition of provenance (source area characteristics) and genetic classification of quartz (monocrystalline vs. polycrystalline, undulatory vs. non-undulatory extinction for metamorphic vs. plutonic sources), feldspars (K-feldspar vs. plagioclase indicating acid vs. intermediate igneous sources; freshness vs. alteration for transport distance), and lithic fragments (sedimentary, metamorphic, volcanic lithics indicating recycled orogenic sources)
  • Part (a): Application of QFL and Qm-F-Lt ternary diagrams for provenance discrimination (continental block, magmatic arc, recycled orogen fields per Dickinson et al.)
  • Part (b): Definition of sedimentary facies model as a generalised summary of facies characteristics and associations in a specific depositional environment, emphasizing predictability and Walther's Law
  • Part (b): Meandering river facies succession: channel lag (coarsest, basal scours) → point bar (upward-fining, trough cross-beds, lateral accretion surfaces) → levee (fine sand, ripples) → crevasse splay (sheet sandstones) → floodplain (mudstone, paleosols, root traces); with neat labelled cross-section showing lateral migration
  • Part (c): Genesis and palaeocurrent significance of four structures: (i) trough cross-bedding (3D dunes, bipolar in tidal, unidirectional in fluvial), (ii) tabular cross-bedding (2D dunes, migration direction), (iii) ripple marks (current ripples vs. wave ripples, flow direction from steeper lee side), (iv) parting lineation (upper flow regime, parallel to flow), (v) flute casts (turbidity currents, flow direction from bulbous to tapered end)
  • Part (c): Indian examples: Siwalik molasse provenance (Himalayan uplift), Gondwana fluvial facies (Damodar valley), Bhander sandstone palaeocurrents (Vindhyan basin)

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness25%12.5Precise genetic definitions for all three grain types in (a) with correct optical properties; accurate facies model definition incorporating Walther's Law and process-sediment linkage; correct genesis mechanisms for all four structures in (c) with clear distinction between formative processesBasic definitions correct but confuses quartz types or feldspar varieties; facies model defined generically without process linkage; structures described but genesis confused (e.g., ripple marks vs. dunes)Misidentifies provenance indicators (e.g., polycrystalline quartz as igneous); confuses facies model with facies description; fundamental errors in structure genesis (e.g., flute casts formed by traction currents)
Diagram / cross-section20%10Neat, labelled sketch for (b) showing: channel geometry, point bar lateral accretion surfaces, fining-upward log, and vertical facies succession; optional QFL ternary diagram for (a); clear 3D block diagrams for two structures in (c)Sketch present for (b) but missing key elements (e.g., no lateral accretion surfaces, confused vertical succession); diagrams for (c) present but poorly labelled or 2D onlyNo diagram for (b) despite explicit instruction; diagrams unlabelled or conceptually wrong (e.g., meandering channel drawn as braided); no attempt at structure illustrations
Field evidence20%10Cites specific Indian formations: Siwalik Group for provenance studies (Kakroli/Khetasarai sections), Gondwana Supergroup (Barakar/Kamthi for meandering facies), Vindhyan Supergroup (Bhander/Semri for palaeocurrents); mentions actual field criteria (grain size trends, palaeocurrent vector analysis)Mentions Indian examples but generic (e.g., 'Gondwana' without specific formation); or uses international examples (Mississippi, Navajo) without Indian correlationNo field examples; purely theoretical treatment; invented formations or incorrect stratigraphic assignments (e.g., Siwalik as Gondwana)
Quantitative reasoning15%7.5References QFL percentages for provenance fields (e.g., recycled orogen >95% quartz); mentions palaeocurrent vector mean and consistency ratio; notes bed thickness/ grain size statistics for facies interpretationMentions ternary diagrams without percentage ranges; qualitative description of palaeocurrent patterns without statistical termsNo quantitative treatment; avoids all numerical aspects of provenance and facies analysis
Indian / economic relevance20%10Links provenance to Himalayan tectonics and Siwalik petroleum system; connects meandering facies to Gondwana coal deposits (Damodar, Son valleys) and aquifer potential; notes Vindhyan uranium/copper deposits in reduced floodplain facies; mentions Cauvery/Krishna-Godavari basin applicationsMentions coal or petroleum generally without specific basin linkage; notes India has sedimentary basins without economic applicationNo Indian relevance; or incorrect economic links (e.g., primary gold in Siwalik sandstones)

Practice this exact question

Write your answer, then get a detailed evaluation from our AI trained on UPSC's answer-writing standards. Free first evaluation — no signup needed to start.

Evaluate my answer →

More from Geology 2021 Paper II