Sedimentary and metamorphic petrology

In short

Sedimentary petrology studies rocks formed at or near the Earth's surface by weathering, transport, deposition and diagenesis of sediment, or by chemical and biological precipitation; metamorphic petrology studies rocks recrystallised in the solid state when pre-existing rocks are subjected to changed pressure, temperature and fluid conditions. Together they cover more than three-quarters of the exposed crust and host India's coal, limestone, iron ore, bauxite, placer heavy minerals, oil and gas, graphite and dimension stone. The topic is examined heavily in B.Sc/M.Sc papers, GATE Geology (GG), UPSC Combined Geo-Scientist and state PSC geologist papers, and in practical sections on hand specimens, thin sections and sedimentary logs. It is applied daily by geologists in GSI, ONGC, Oil India, Coal India, NMDC, MECL, AMD, Hindustan Zinc, Vedanta, Tata Steel, cement majors and groundwater agencies.

Explained

Sedimentary processes: from weathering to a rock

• Source → sink chain: weathering (physical disintegration, chemical decomposition) → erosion → transport → deposition → diagenesis → lithification.
• Transport media and agents: rivers (traction, saltation, suspension), wind, glaciers, waves and tides, gravity flows (debris flow, turbidity current), and solution transport of ions.
• Hydraulic behaviour: Stokes' law governs fine-particle settling; Hjulström–Sundborg curve shows silt/clay need higher velocity to *erode* (cohesion) than sand, but settle at very low velocity.
• Textural maturity: grain size, sorting, rounding and sphericity improve with transport distance and reworking; mineralogical maturity = loss of unstable minerals (olivine, feldspar, lithics) and enrichment in quartz.
• Diagenesis: compaction (porosity loss, dewatering), cementation (silica, calcite, haematite, clay), dissolution, replacement (dolomitisation, silicification), recrystallisation, authigenesis (glauconite, pyrite, chlorite), and maturation of organic matter to oil/gas. Grades upward into very low-grade metamorphism at roughly 150–200 °C (zeolite facies).
• Grain-size scale: Udden–Wentworth — boulder >256 mm, cobble 64–256, pebble 4–64, granule 2–4, sand 1/16–2 mm, silt 1/256–1/16, clay <1/256 mm; expressed as phi, φ = −log₂(d in mm).

Classification of sedimentary rocks

• Primary split: (a) clastic/terrigenous (conglomerate, breccia, sandstone, siltstone, shale), (b) chemical/biochemical (limestone, dolomite, chert, evaporites, ironstone, phosphorite), (c) organic/carbonaceous (peat, lignite, coal, oil shale), (d) volcaniclastic (tuff, agglomerate).
• Sandstone — Dott (1964) / Pettijohn scheme: matrix <15% = arenite, >15% = wacke; QFL apices give quartz arenite, arkosic arenite, lithic arenite, quartz wacke, feldspathic wacke (greywacke), lithic wacke. Greywacke = poorly sorted, matrix-rich, typically turbidite.
• Folk's classification uses quartz–feldspar–rock fragments plus textural maturity (immature, submature, mature, supermature) and separates orthoquartzite, arkose, litharenite.
• Limestone — Folk (1959): allochems (ooids, peloids, bioclasts, intraclasts) + orthochem (micrite or sparry calcite) → oosparite, biomicrite, pelsparite, intrasparite, etc. Dunham (1962): mudstone, wackestone, packstone, grainstone (mud- vs grain-supported), boundstone (organically bound), crystalline carbonate. Embry & Klovan extends Dunham for reefs (framestone, bafflestone, bindstone, floatstone, rudstone).
• Shale/mudrock: fissile = shale, non-fissile = mudstone/claystone; dominated by illite, smectite, kaolinite, chlorite + quartz silt.
• Evaporites: gypsum–anhydrite–halite–potash, precipitated in order of increasing solubility as brine evaporates (bar theory / Ochsenius; sabkha and playa models — Rann of Kachchh).

Sedimentary structures, facies and provenance

• Pre-depositional/erosional: sole marks — flute casts, groove casts, scour marks (load casts also sit on bed soles but are post-depositional loading features) (useful as way-up and palaeocurrent indicators).
• Syn-depositional: planar lamination, cross-bedding (planar/tabular, trough, herringbone in tidal settings), graded bedding (Bouma sequence Ta–Te in turbidites), ripple and dune bedforms, hummocky cross-stratification (storm), imbrication, parallel and current lineation.
• Post-depositional: mud cracks, raindrop impressions, convolute lamination, flame structures, slumping, concretions, stylolites, trace fossils (ichnofacies: Skolithos, Cruziana, Zoophycos, Nereites).
• Way-up criteria: graded bedding (fining upward), cross-bed truncation (top), mud cracks (open upward), sole marks (base of bed), stromatolite convexity (upward).
• Facies and Walther's law: facies in vertical succession that are conformable were laterally adjacent — the basis of reading fluvial (fining-upward cycles), deltaic, aeolian (large-scale cross-beds, well-rounded frosted grains), tidal, shelf, reef, turbidite and glacial (tillite, dropstones, striated clasts) successions.
• Provenance tools: detrital modes (QFL plots of Dickinson — craton interior, recycled orogen, magmatic arc), heavy minerals (ZTR index = zircon + tourmaline + rutile as a maturity measure), detrital zircon U–Pb ages, clast petrography, palaeocurrents.
• Basin types: rift, passive margin, foreland, foreland-related (Himalayan foreland = Siwalik Group), intracratonic (Vindhyan, Cuddapah), pull-apart, trench/forearc (Andaman flysch).

Metamorphism: controls, types and grade

• Definition: mineralogical, chemical and structural adjustment of a solid rock to P–T–fluid conditions different from those of its formation, below melting (above which anatexis gives migmatite).
• Agents: temperature (≈150/200–850 °C typical range), pressure (lithostatic + directed/deviatoric stress), chemically active fluids (H₂O, CO₂), and time.
• Types: (1) contact/thermal (around intrusions — hornfels, aureoles, skarn where carbonate reacts), (2) regional/orogenic (dynamothermal) — Himalaya, Aravalli, Dharwar, (3) burial (zeolite facies), (4) dynamic/cataclastic (fault zones — mylonite, cataclasite, phyllonite), (5) hydrothermal/metasomatic (serpentinisation, Na/K metasomatism), (6) impact/shock (Lonar crater, Dhala structure — coesite, shatter cones), (7) ocean-floor metamorphism.
• Grade and zones: very low → low → medium → high grade. Barrovian sequence (Scottish type, medium-P): chlorite → biotite → garnet → staurolite → kyanite → sillimanite. Buchan/Abukuma (low-P): andalusite and cordierite instead of kyanite/staurolite.
• Index minerals mark the first appearance of a mineral; an isograd is the mapped line joining points of equal metamorphic grade (strictly, of a given reaction).
• Metamorphic facies (Eskola): zeolite, prehnite–pumpellyite, greenschist, amphibolite, granulite (high T), blueschist (high P/low T — subduction), eclogite (very high P), hornblende hornfels / pyroxene hornfels / sanidinite (contact), albite–epidote hornfels.
• Facies series (Miyashiro): low-P (contact/Buchan), medium-P Barrovian, high-P blueschist–eclogite; paired metamorphic belts mark subduction margins. Geothermal gradient in the crust controls which series forms.
• Prograde vs retrograde: prograde = increasing T (dehydration, devolatilisation); retrograde needs fluid influx, so is often incomplete — why high-grade assemblages survive.

Metamorphic textures, structures and classification

• Crystalloblastic textures: granoblastic (equigranular, hornfels, granulite), schistose (platy minerals aligned), gneissose (banded, coarse, >50% granular), slaty (very fine, perfect cleavage), phyllitic (silky sheen), porphyroblastic (large porphyroblasts of garnet/staurolite in finer matrix), poikiloblastic/sieve, decussate (random mica in hornfels), mylonitic (grain-size reduction, ribbon quartz).
• Relict/other: palimpsest textures (blastoporphyritic, blastophitic), augen structure, helicitic inclusion trails, pressure shadows, S–C fabrics.
• Foliation terms: slaty cleavage, crenulation cleavage, schistosity, gneissosity, lineation (mineral, intersection, rodding); relation to deformation phases (S₀ bedding, S₁, S₂; porphyroblast–matrix relations give pre-, syn-, post-tectonic growth).
• Naming: by texture + mineral (garnet–biotite schist, hornblende gneiss) or by protolith/composition — metapelite (slate–phyllite–schist–gneiss), metabasite (greenschist–amphibolite–granulite/eclogite), quartzite, marble, calc-silicate, serpentinite, charnockite (orthopyroxene-bearing granulite, type locality Pallavaram, Chennai), khondalite (garnet–sillimanite–graphite granulite of the Eastern Ghats), granulite, eclogite, amphibolite.
• Metamorphic differentiation and segregation produce banding; porphyroblast zoning records P–T paths.

Reactions, phase rule and P–T–t paths

• Chemographic tools: ACF, AKF, A'KF diagrams (Eskola) for mafic and pelitic systems; AFM (Thompson) projection for metapelites; petrogenetic grid of reaction curves in P–T space.
• Goldschmidt's mineralogical phase rule: in a rock at equilibrium, number of phases ≤ number of components (P ≤ C); P = C is the normal (divariant) case. Gibbs phase rule F = C − P + 2.
• Key reactions: kaolinite → pyrophyllite + quartz; muscovite + quartz → K-feldspar + Al₂SiO₅ + H₂O (upper amphibolite); biotite + sillimanite + quartz → garnet + K-feldspar + melt (granulite); calcite + quartz → wollastonite + CO₂ (contact/skarn); Al₂SiO₅ polymorphs kyanite–andalusite–sillimanite meet at a triple point near 0.4–0.45 GPa and ~500–510 °C (values vary between calibrations).
• Geothermobarometry: exchange thermometers (garnet–biotite Fe–Mg, GARB), net-transfer barometers (GASP: garnet–plagioclase–Al₂SiO₅–quartz), two-feldspar, Ti-in-zircon, Zr-in-rutile; thermometers use reactions with steep dP/dT slopes (nearly pressure-independent), barometers ones with shallow slopes (nearly temperature-independent).
• P–T–t path: clockwise paths typify collisional thickening then exhumation (Himalaya); anticlockwise paths suggest magmatic heating. Dated by U–Pb (zircon, monazite), Ar–Ar (hornblende, mica), Sm–Nd (garnet).
• Metasomatism and skarn zoning: endoskarn/exoskarn, garnet–pyroxene–wollastonite zones — host to tungsten, tin and iron ores.

Key points

• Clastic rock - made of transported solid particles; chemical rock - precipitated from solution, in situ.
• Sorting and rounding - textural maturity indicators; aeolian sands are best sorted and rounded, tillite the worst.
• Phi scale - φ = −log₂(d in mm); coarse sediment has negative φ.
• Dott scheme - 15% matrix cut-off separates arenites from wackes; greywacke = matrix-rich turbidite sandstone.
• Dunham classification - mudstone/wackestone/packstone/grainstone by grain support and mud content.
• Folk limestone names - allochem prefix + micrite/sparite suffix (biomicrite, oosparite).
• Bouma sequence - Ta–Te divisions of a single turbidite bed; graded base, laminated top.
• Walther's law - vertically conformable facies were once laterally adjacent environments.
• ZTR index - zircon + tourmaline + rutile percentage of transparent heavy minerals; high = mature/recycled.
• Diagenesis–metamorphism boundary - roughly 150–200 °C, zeolite facies; no sharp line.
• Metamorphic grade - intensity of metamorphism; facies - the P–T field defined by a mafic mineral assemblage.
• Index mineral and isograd - first appearance of a mineral; line of equal grade on a map.
• Barrovian zones - chlorite, biotite, garnet, staurolite, kyanite, sillimanite with rising T at medium P.
• Blueschist–eclogite - high P, low T; diagnostic of subduction zones; glaucophane, lawsonite, jadeite, omphacite.
• Granulite facies - orthopyroxene + garnet, anhydrous, >750 °C; charnockite and khondalite in south India.
• Mylonite - ductile fault rock with grain-size reduction and strong lineation; not a hornfels.
• Goldschmidt's phase rule - P ≤ C at equilibrium; an assemblage with too many phases suggests disequilibrium or extra components.
• Migmatite - mixed rock of restite (melanosome) and leucosome, marking the onset of partial melting.
• Retrograde metamorphism - needs fluid; commonly incomplete, so peak assemblages are preserved.

Formulas and examples

Example 1 — grain size statistics and sorting. A sandstone's cumulative curve gives φ5 = 0.6, φ16 = 1.2, φ50 = 1.9, φ84 = 2.6, φ95 = 3.2.

• Mz = (1.2 + 1.9 + 2.6)/3 = 5.7/3 = 1.90 φ → d = 2^−1.9 = 0.268 mm, i.e. medium sand.
• σI = (2.6 − 1.2)/4 + (3.2 − 0.6)/6.6 = 0.350 + 0.394 = 0.744 φ → "moderately sorted" (Folk's verbal scale: 0.35–0.50 well sorted, 0.50–0.71 moderately well sorted, 0.71–1.00 moderately sorted — this value sits just inside moderately sorted).
• SkI numerator-1 = (1.2 + 2.6 − 3.8) = 0, so first term = 0; second term = (0.6 + 3.2 − 3.8)/[2(3.2 − 0.6)] = 0/5.2 = 0 → SkI = 0.00, near-symmetrical distribution.

Example 2 — depth of burial from pressure. A kyanite-bearing schist equilibrated at 0.8 GPa. Average crustal density = 2,750 kg/m³, g = 9.81 m/s².

• h = P/(ρg) = 0.8 × 10⁹ / (2,750 × 9.81) = 0.8 × 10⁹ / 26,978 = 29,654 m ≈ 29.7 km.
• If peak T = 620 °C, apparent gradient = 620/29.7 = 20.9 °C/km — a Barrovian, collisional gradient (normal continental crust is about 25–30 °C/km; arc and contact settings exceed 40 °C/km).

Example 3 — porosity, density and compaction. A quartz sandstone core: bulk volume 100 cm³, dry mass 212 g, grain density 2.65 g/cm³.

• Grain volume = 212/2.65 = 80.0 cm³ → pore volume = 20.0 cm³ → n = 20.0%; e = 20/80 = 0.25.
• Water-saturated bulk density = 2.65 × 0.80 + 1.00 × 0.20 = 2.12 + 0.20 = 2.32 g/cm³.
• If compaction and quartz cement reduce n to 8%, saturated density = 2.65 × 0.92 + 1.00 × 0.08 = 2.438 + 0.08 = 2.52 g/cm³; reservoir quality falls sharply.

Example 4 — Stokes' law settling. A clay particle of d = 4 μm (4 × 10⁻⁶ m), ρs = 2,650, ρf = 1,000 kg/m³, μ = 1.0 × 10⁻³ Pa·s.

• v = (1,650 × 9.81 × (4 × 10⁻⁶)²)/(18 × 1.0 × 10⁻³) = (1,650 × 9.81 × 1.6 × 10⁻¹¹)/0.018.
• Numerator = 16,186.5 × 1.6 × 10⁻¹¹ = 2.590 × 10⁻⁷ → v = 2.590 × 10⁻⁷/0.018 = 1.44 × 10⁻⁵ m/s ≈ 1.24 m/day. Settling through 50 m of still water takes ≈ 40 days — hence clay travels far offshore.

Example 5 — Goldschmidt's phase rule check. A metapelite contains quartz, muscovite, biotite, garnet, plagioclase, ilmenite (6 phases). Components taken as SiO₂, Al₂O₃, FeO, MgO, K₂O, CaO, Na₂O, TiO₂ (8).

• P = 6 ≤ C = 8 → consistent with equilibrium; the assemblage is not over-determined. Adding both kyanite and andalusite in the same rock would imply the univariant polymorph boundary or, more likely, disequilibrium/relict growth.

Where it is used

• Hydrocarbons: ONGC, Oil India and private operators use reservoir petrography, porosity–permeability and diagenesis studies in the Mumbai Offshore, Krishna–Godavari, Cauvery, Assam–Arakan and Rajasthan (Barmer) basins; source-rock maturity from shale petrology and vitrinite reflectance.
• Coal and lignite: Coal India subsidiaries, SCCL and NLC on Gondwana coal measures (Raniganj, Jharia, Korba, Singrauli, Talcher) and Neyveli lignite — coal petrography (macerals, rank) for coking and washability.
• Cement, steel and industrial minerals: limestone and dolomite exploration (Cuddapah, Vindhyan, Bilaspur, Chandrapur) for UltraTech, ACC-Ambuja, Shree and Dalmia; marble and granite dimension stone in Rajasthan and Andhra; graphite and sillimanite from Eastern Ghats granulites; IS 1122, IS 1124 and IS 14223 govern dimension-stone testing and IS 1514/IS 1760 the chemical testing of limestone and dolomite (confirm current editions before quoting in a report).
• Metallic ores: BIF-hosted iron ore (Bailadila, Bellary–Hospet, Noamundi) in Dharwar/Iron Ore Group supracrustals; metamorphosed SEDEX zinc–lead at Rampura Agucha and Zawar (Aravalli); placer ilmenite–monazite–zircon of Kerala and Odisha beach sands worked by IREL; manganese in gondite (Nagpur–Bhandara).
• Engineering, groundwater and construction materials: aggregate suitability, alkali–silica reactivity of strained quartz in quartzite aggregate, tunnel behaviour of schistosity-controlled anisotropy in Himalayan projects, sandstone aquifer characterisation (CGWB).
• Job roles and recruiters: Geologist/Geoscientist in GSI, ONGC, CMPDI, MECL, AMD, NMDC, SAIL, Hindustan Zinc, Vedanta, Tata Steel; wellsite and mud-logging geologists; core loggers; QA geologists in cement plants; academia and GSI's National Geoscience Data Repository for mapping and standardised rock nomenclature.

Interview questions

• Q: How do you distinguish a conglomerate from a breccia? A: Conglomerate has rounded gravel-size clasts implying significant transport; breccia has angular clasts indicating little transport or a fault/collapse origin.
• Q: What does greywacke tell you about its depositional setting? A: Poor sorting, angular grains, abundant matrix and lithic fragments point to rapid deposition from turbidity currents in a deep marine basin close to an active tectonic source.
• Q: Difference between Folk's and Dunham's limestone classifications? A: Folk names rocks from the allochem type plus micrite or sparite cement; Dunham uses depositional texture — whether the fabric is mud-supported or grain-supported — which is easier in core and hand specimen.
• Q: How do you tell way-up in a deformed sedimentary succession? A: Use graded bedding (fining upward), truncated cross-bed tops, mud cracks opening upward, sole marks on bed bases and upward-convex stromatolites.
• Q: What is the difference between metamorphic grade and metamorphic facies? A: Grade is the relative intensity (mainly temperature) of metamorphism; facies is a specific P–T field recognised from a mineral assemblage, classically in a basaltic composition.
• Q: Why are blueschists important tectonically? A: They record high pressure at low temperature, only achievable where cold crust is dragged down quickly — so they mark subduction zones and, with a paired high-T belt, an ancient convergent margin.
• Q: What is a charnockite and why is it significant in India? A: An orthopyroxene-bearing granulite-facies rock of granitic to intermediate composition, type locality Pallavaram near Chennai; it typifies the deep-crustal Southern Granulite Terrain.
• Q: How would you prove the kyanite in a rock is not detrital? A: Look for textural equilibrium — kyanite in equilibrium contact with garnet, staurolite and matrix mica, aligned with the foliation and with no rounding or detrital overgrowth relationships.
• Q: Why is retrograde metamorphism usually incomplete? A: Prograde reactions release fluids that escape; on cooling there is little water left, reaction rates fall with temperature, so peak high-grade assemblages are commonly preserved.
• Q: What does the ZTR index measure, and its limitation? A: The proportion of ultra-stable heavy minerals, used as a maturity/recycling index; it is misleading when the source rock itself was zircon-poor or when diagenetic dissolution has stripped unstable heavies.
• Q: A sandstone has 25% porosity at surface but 8% at 3 km. What happened? A: Mechanical compaction, grain rearrangement and pressure solution plus quartz and carbonate cementation during diagenesis destroyed pore space; porosity loss is largely irreversible.
• Q: Migmatite — sedimentary, igneous or metamorphic? A: Metamorphic, at the upper limit: a mixed rock where partial melting produced a leucosome within a refractory metamorphic melanosome, so it straddles metamorphic and igneous processes.
• Q: How does contact metamorphism differ from regional metamorphism in texture? A: Contact aureoles are driven by heat without directed stress, giving non-foliated granoblastic/decussate hornfels; regional metamorphism under differential stress produces foliated slate, schist and gneiss.

Common mistakes

• Confusing grade with facies, or quoting a facies name for a pelite when facies are defined on basaltic assemblages.
• Writing "quartzite" for both the sedimentary orthoquartzite (a quartz arenite) and the metamorphic quartzite without saying which — always state cement versus recrystallised interlocking fabric.
• Reversing the phi scale: forgetting that larger φ means finer grains, and reporting negative φ for clay.
• Using the Hjulström curve's erosion limb as a settling velocity, or applying Stokes' law to sand and gravel (turbulent, not laminar settling).
• Calling every fine-grained dark rock a hornfels or every banded rock a gneiss — mylonite, phyllonite and migmatite are distinct and diagnostic of process.
• Treating the Barrovian sequence as universal; low-pressure Buchan terrains give andalusite and cordierite, not kyanite and staurolite.
• Reciting Gibbs' phase rule F = C − P + 2 where Goldschmidt's mineralogical rule (P ≤ C) is asked, and ignoring that minor accessory phases need extra components.
• Assuming all limestone becomes marble at low grade while forgetting calc-silicate reactions need silica plus a CO₂-escaping environment.

Related topics

Mineralogy · Igneous petrology · Structural geology and tectonics · Stratigraphy of India and palaeontology · Economic geology and ore deposits · Geochemistry and isotope dating