Q4
(a) Discuss the genesis of any four sedimentary structures which are helpful in palaeocurrent analysis. How are the palaeocurrent patterns helpful in establishing the depositional environment? (20 marks) (b) Discuss the concept of facies model and give a brief account of deltaic facies model. (15 marks) (c) What are conglomerates? Describe their fabrics, classification and geological significance. (15 marks)
हिंदी में प्रश्न पढ़ें
(a) पुरातरंग विश्लेषण में उपयोगी किन्हीं चार अवसादी संरचनाओं की उत्पत्ति का वर्णन कीजिए। निक्षेपण पर्यावरण स्थापित करने में पुरातरंग प्रतिरूप किस प्रकार सहायक होते हैं? (20 अंक) (b) संलक्षणी (फेसीज) मॉडल की संकल्पना की विवेचना कीजिए तथा डेल्टा संलक्षणी मॉडल का संक्षिप्त वर्णन कीजिए। (15 अंक) (c) संगुटिकाश्म क्या होते हैं? उनके संविन्यास, वर्गीकरण एवं भूवैज्ञानिक महत्व का वर्णन कीजिए। (15 अंक)
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' demands a comprehensive, analytical treatment with logical flow. Allocate approximately 40% of time/words to part (a) given its 20 marks, and 30% each to parts (b) and (c). Structure: brief introduction defining sedimentary structures, facies models and conglomerates; systematic body addressing each sub-part with genesis mechanisms, depositional environments, and geological significance; conclusion synthesizing how these tools reconstruct basin evolution and hydrocarbon exploration.
Key points expected
- Part (a): Genesis of four sedimentary structures (e.g., ripple marks, cross-bedding, flute casts, groove casts, parting lineation, imbrication) with clear current-flow indicators; explanation of vector analysis, rose diagrams, and how palaeocurrent patterns distinguish fluvial (unimodal), tidal (bimodal), and deep marine (unimodal turbidite) environments
- Part (b): Definition of facies model as a generalised summary of sedimentary attributes characterising a particular depositional system; deltaic facies model covering river-dominated (Gilbert-type), wave-dominated, and tide-dominated deltas with their diagnostic sedimentary sequences and vertical profiles
- Part (c): Definition of conglomerates as coarse-grained, matrix-supported or clast-supported sedimentary rocks; fabrics including clast orientation (imbrication), packing, and matrix composition; classification by genesis (residual, lag, till, fluvial, alluvial fan, turbiditic) and by composition (petromict vs oligomict); geological significance as provenance indicators, palaeogeographic markers, and economic importance for placer deposits and aquifers
- Integration of Walther's Law in facies analysis and its application to predict vertical facies succession in deltaic systems
- Recognition of Indian examples: Siwalik conglomerates (fluvial), Kutch deltaic sequences, or Karewa conglomerates for palaeocurrent and provenance studies
Evaluation rubric
| Dimension | Weight | Max marks | Excellent | Average | Poor |
|---|---|---|---|---|---|
| Concept correctness | 25% | 12.5 | Precise genetic mechanisms for all four structures in (a) with correct terminology (e.g., Reynolds number for bedforms); accurate Walther's Law application in (b); technically sound fabric terminology (a-axis, imbrication angle) and genetic classification in (c); no confusion between matrix-supported and clast-supported conglomerates | Broadly correct concepts with minor errors in genesis explanation; acceptable but imprecise facies model description; basic conglomerate classification without fabric details or genetic nuance | Fundamental misconceptions (e.g., confusing cross-bedding with cross-lamination; treating facies model as lithostratigraphic unit; defining conglomerate simply as 'coarse sandstone'); omission of critical genetic processes |
| Diagram / cross-section | 20% | 10 | Clear, labelled diagrams for at least three of: (a) current rose diagram with vector mean, (b) deltaic facies model with topset-foreset-bottomset geometry, (c) imbrication fabric in 2D/3D, (d) vertical log of deltaic sequence; accurate scale and orientation indicators | Sketchy diagrams with partial labelling; missing scale or orientation; one adequate diagram covering either facies model or fabric but not both | No diagrams, or diagrams that misrepresent key relationships (e.g., showing delta foresets dipping landward; imbration dipping upstream); illegible or irrelevant sketches |
| Field evidence | 20% | 10 | Specific Indian field examples: Siwalik foreland basin conglomerates with unimodal palaeocurrents indicating Himalayan provenance; Kutch Cretaceous deltaic sequences; Karewa Group lacustrine-fluvial transition; mention of actual outcrop observations (e.g., clast composition, palaeoslope indicators) from published geological surveys | Generic references to 'Himalayan rivers' or 'Gondwana basins' without specific formation names; standard textbook examples without Indian context | No field evidence cited; invented or incorrect localities; confusion between modern and ancient examples without stratigraphic context |
| Quantitative reasoning | 15% | 7.5 | Numerical treatment where applicable: palaeocurrent vector statistics (resultant vector, consistency ratio), imbration angle ranges (15-30°), clast size ranges for conglomerate definition (>2mm), bed thickness ratios in deltaic sequences; quantitative distinction between sedimentary structures based on Reynolds/Froude numbers | Mention of 'statistical analysis' or 'measurements' without actual numbers; qualitative description of orientation patterns | No quantitative content; incorrect numerical values (e.g., imbration angles >90°); confusion between dimensional parameters |
| Indian / economic relevance | 20% | 10 | Explicit economic significance: Siwalik and Karewa conglomerates as aquifers and placer gold/tin deposits; palaeocurrent analysis for hydrocarbon reservoir geometry prediction in Cambay Basin or KG Basin; deltaic facies models for lignite exploration in Rajasthan; provenance studies for diamond exploration in Wajrakarur kimberlite fields | General mention of 'groundwater' or 'mineral deposits' without specific Indian examples; standard economic importance without linking to the specific structures or facies discussed | No economic or applied relevance discussed; irrelevant economic examples (e.g., coal in granite); missed opportunity to connect basin analysis to hydrocarbon exploration in Indian sedimentary basins |
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