Stratigraphy and palaeontology

In short

Stratigraphy is the study of layered rocks — their order, age, lateral relationships and the events recorded in them; palaeontology is the study of fossils preserved in those layers. Together they supply geology's time framework: without them a rock has no age and no correlation to any other rock. In India they underpin coal exploration in Gondwana basins, hydrocarbon prospecting in Cambay, Krishna–Godavari and Assam–Arakan basins, groundwater mapping and Geological Survey of India (GSI) mapping. The topic carries the largest single block of marks in GATE Geology (GG), UPSC Geologist/Combined Geo-Scientist, CSIR-NET Earth Sciences and state PSC papers, and is the staple of interviews at GSI, ONGC, Oil India, Coal India, MECL, Directorate General of Hydrocarbons and mineral exploration firms.

Explained

Fundamental principles and stratigraphic relations

• Steno's laws (1669): *superposition* — in an undisturbed sequence, each bed is younger than the one below; *original horizontality* — sediments settle in near-horizontal layers; *lateral continuity* — a bed extends laterally until it thins out or meets a basin margin.
• Law of faunal succession (Smith, 1796): fossil assemblages succeed one another in a definite, non-repeating order, so a bed can be dated by its fossils.
• Cross-cutting relationships: a dyke, fault or vein is younger than everything it cuts. Inclusions: a clast is older than the rock enclosing it.
• Uniformitarianism (Hutton, Lyell): present-day processes explain past records — "the present is the key to the past". Modern usage keeps the *method* but allows non-uniform rates and rare catastrophic events (neocatastrophism).
• Unconformity — a buried erosional or non-depositional surface representing missing time (a *hiatus*). Types:
  • *Disconformity* — parallel beds above and below, an irregular erosion surface between.
  • *Paraconformity* (an obscure or "hidden" unconformity) — parallel beds, no visible erosion; detected only by fossils or dating.
  • *Angular (non-)conformity or unconformity* — tilted/folded beds below, horizontal above, implying deformation then erosion.
  • *Nonconformity* — sediments resting on igneous or metamorphic basement (for example Vindhyan on Bundelkhand granite).
• Overlap (onlap) — successive younger beds extending beyond the margins of older ones; offlap — the reverse, during regression. Outlier — younger rock surrounded by older; inlier — older rock surrounded by younger.
• Facies — the character of a rock reflecting its environment. Walther's law of correlation of facies: facies that occur vertically in a conformable succession were once laterally adjacent environments — the basis of reading transgression and regression.
• Transgression (sea advancing) gives a fining-upward, deepening sequence; regression gives coarsening-upward. Both are governed by the balance of sediment supply, subsidence and sea level.
• Diastem — a very short break; lacuna — total missing section (hiatus + erosional vacuity).

Kinds of stratigraphic units (International Stratigraphic Guide / ISSC)

• Lithostratigraphy — units defined purely by rock character, mappable and diachronous. Hierarchy: *Supergroup → Group → Formation → Member → Bed*. The Formation is the fundamental mappable unit; it needs a designated type section (holostratotype). Igneous equivalent: *flow*, *complex*.
• Biostratigraphy — units defined by fossil content; the basic unit is the biozone. Types: *range zone* (total or concurrent/taxon-range), *Oppel zone* (overlapping ranges), *assemblage zone*, *acme/abundance zone*, *interval zone* (between two datums), *barren interzone*.
• Chronostratigraphy — units of rock representing an interval of time, with geochronologic (time) counterparts: | Chronostratigraphic (rock) | Geochronologic (time) | |---|---| | Eonothem | Eon | | Erathem | Era | | System | Period | | Series | Epoch | | Stage | Age | | Chronozone | Chron |
• Magnetostratigraphy — correlation using reversals of the geomagnetic field; the basic unit is the *polarity chron* (Brunhes normal, Matuyama reverse, Gauss, Gilbert). Vital for dating the Siwaliks and ocean-floor spreading.
• Chemostratigraphy — correlation by isotope curves, mainly δ¹³C and ⁸⁷Sr/⁸⁶Sr; used across the Vindhyan and Marwar Supergroups and at the Precambrian–Cambrian boundary.
• Seismic and sequence stratigraphy — a depositional sequence is a package bounded by unconformities and their correlative conformities. Key surfaces: *sequence boundary (SB)*, *transgressive surface (TS)*, *maximum flooding surface (MFS)*. Systems tracts: *lowstand (LST)*, *transgressive (TST)*, *highstand (HST)*, *falling-stage (FSST)*. Accommodation (the space available for sediment) = eustasy + subsidence; sediment supply then fills it, so water-depth change = accommodation − sediment accumulated. The standard framework in ONGC/OIL exploration.
• GSSP (Global Boundary Stratotype Section and Point) — a physical "golden spike" in one agreed section defining the base of a stage. India hosts one ratified GSSP — the base of the Meghalayan Stage/Age of the Holocene, defined in a stalagmite from Mawmluh Cave, Meghalaya (ratified 2018); the Spiti sections are reference/candidate sections for Palaeozoic and Permo-Triassic boundaries. The International Commission on Stratigraphy (ICS) chart is revised periodically — always quote the current edition.

Dating and correlation methods

• Relative dating: superposition, cross-cutting, fossil succession, degree of weathering, varve and tree-ring counting (strictly these last two are numerical).
• Absolute (radiometric) dating: parent decays to daughter with a fixed half-life.
  • U–Pb on zircon (t½ ²³⁸U = 4.47 Ga) — best for Precambrian and for ash beds; most precise.
  • K–Ar and Ar–Ar (⁴⁰K, t½ 1.25 Ga) — volcanics, micas; used on Deccan Trap basalts (~66 Ma, spanning the K–Pg boundary).
  • Rb–Sr (t½ ⁸⁷Rb ≈ 48.8 Ga) — whole-rock granites, metamorphic events.
  • Sm–Nd, Lu–Hf — mafic rocks, crustal residence ages.
  • ¹⁴C (t½ 5,730 yr) — organic material younger than about 50,000 years; Quaternary archaeology and palaeoclimate.
  • Fission-track and (U–Th)/He — exhumation and cooling histories, widely applied to Himalayan uplift.
• Correlation tools: marker beds (bentonite, coal seam, tuff), electrical/gamma log signatures (well-log correlation is the practical industry method), seismic reflectors, fossil datums, isotope excursions, magnetic reversals.
• Limitations: fossils give only relative age; radiometric ages date the *mineral's cooling*, not the sediment — hence sediments are dated by interbedded volcanics, detrital-zircon maximum ages, or biostratigraphy.

Palaeontology: fossilisation, major groups and uses

• Fossil — any evidence of past life preserved in rock. Body fossils (shell, bone, wood) versus trace fossils/ichnofossils (burrows, tracks, coprolites, borings).
• Modes of preservation: unaltered remains (amber, permafrost), *permineralisation/petrifaction* (silica, calcite in pore space), *carbonisation* (plant compressions, coal), *recrystallisation*, *replacement* (pyritisation, silicification), *moulds and casts*, *impressions*, *chemical fossils* (biomarkers).
• Conditions favouring preservation: rapid burial, hard parts, anoxic low-energy water, fine sediment; absence of scavengers. Taphonomy studies everything between death and discovery; Lagerstätten are exceptionally preserved deposits.
• Index (guide) fossil criteria: wide geographic spread, short vertical (time) range, abundance, distinctive and easily identified, preservation-resistant, environment-tolerant. Planktonic and nektonic forms make the best index fossils; benthic forms are better facies indicators.
• Important groups and their stratigraphic use:
  • Trilobites (Cambrian–Permian) — Cambrian zonation; India: Spiti, Kashmir, Salt Range successions.
  • Graptolites (Ordovician–Silurian) — pelagic, superb Lower Palaeozoic index fossils.
  • Brachiopods (Cambrian–Recent) — *Productus* in the Permian of the Salt Range and Spiti.
  • Ammonoids (Devonian–Cretaceous) — the finest Mesozoic zonal fossils; Kutch Jurassic ammonites are world-standard for correlation.
  • Belemnites, bivalves, gastropods — facies and palaeotemperature indicators (belemnite PDB is the oxygen-isotope standard).
  • Echinoids, corals, bryozoans, stromatoporoids — reef and shallow-marine facies.
  • Foraminifera — small, abundant, planktonic forms (globigerinids, globotruncanids) give high-resolution zonation; larger benthics (*Nummulites*, *Assilina*) date Eocene; indispensable to petroleum micropalaeontology.
  • Ostracods, radiolaria, diatoms, conodonts — conodonts (Cambrian–Triassic) are the premier Palaeozoic microfossil; radiolaria date deep-sea cherts and ophiolite mélanges (Indus–Tsangpo suture).
  • Palynomorphs (spores, pollen, dinoflagellates) — the standard dating tool for non-marine Gondwana coal measures and for Cenozoic sequences.
  • Stromatolites — cyanobacterial laminated structures; main Proterozoic biostratigraphic tool in India (*Collenia*, *Conophyton*, *Baicalia*) and oldest life evidence (about 3.5 Ga).
• Palaeoecology/palaeobiogeography: fossils establish past climates, salinity, depth and the former union of Gondwanaland — the *Glossopteris* flora and the reptile *Lystrosaurus* shared by India, South America, Africa, Antarctica and Australia.
• Evolutionary concepts examined: organic evolution, phyletic gradualism versus *punctuated equilibria* (Eldredge & Gould), adaptive radiation, convergence, homology versus analogy, Cope's rule, Dollo's law of irreversibility, living fossils (*Limulus*, *Nautilus*, *Ginkgo*).
• Mass extinctions (the "big five"): end-Ordovician, Late Devonian (Frasnian–Famennian), end-Permian (largest, ~90% marine species lost), end-Triassic, end-Cretaceous (K–Pg; Chicxulub impact plus Deccan volcanism). The Permian–Triassic boundary in India is best seen in Spiti and Kashmir.

Geological time scale and the Indian stratigraphic record

• Time scale framework: Hadean, Archaean, Proterozoic (together Precambrian, about 88% of Earth history) → Phanerozoic: Palaeozoic (Cambrian 538.8 Ma onwards), Mesozoic (252–66 Ma), Cenozoic (66 Ma–present). Ages follow the current ICS chart and are revised; quote the edition used.
• Archaean cratons: Dharwar, Bastar, Singhbhum, Aravalli–Bundelkhand, and the Eastern Ghats/Southern Granulite terrains. Oldest Indian rocks reported from the Singhbhum craton and Odisha (Archaean tonalites/zircons older than 3.5 Ga). Greenstone belts (Sargur, Dharwar Supergroup: Bababudan and Chitradurga Groups) host BIF (Kudremukh, Sandur–Hospet; the Bailadila BIF belongs to the Bastar craton's Iron Ore Group) and gold (Kolar, Hutti).
• Proterozoic "Purana" basins: Cuddapah, Vindhyan, Chhattisgarh, Pranhita–Godavari, Bhima, Kaladgi, Marwar. The Vindhyan Supergroup (Semri, Kaimur, Rewa, Bhander Groups) rests nonconformably on Bundelkhand granite; famous for stromatolites, Chorhat Sandstone trace-fossil claims, diamond-bearing conglomerates (Panna) and the Majhgawan kimberlite.
• Aravalli and Delhi Supergroups (Rajasthan) — Palaeoproterozoic metasediments hosting Zawar zinc–lead and Khetri copper.
• Palaeozoic: well-developed only in the Tethyan (Spiti, Kashmir, Kumaon, Zanskar) and Salt Range successions — Cambrian Haimanta/ Parahio, Ordovician–Silurian, Devonian Muth Quartzite, Permo-Carboniferous Kuling/ Gungri, Panjal Traps (Permian) in Kashmir.
• Gondwana Supergroup (Upper Carboniferous–Lower Cretaceous): continental, coal-bearing, in Damodar, Son–Mahanadi, Pranhita–Godavari, Satpura and Rajmahal basins.
  • *Talchir* (glaciogenic — needle shales, tillite, boulder bed; evidence of Permo-Carboniferous glaciation) → *Karharbari* → *Barakar* (India's principal coal seams) → *Barren Measures/Ironstone Shales* → *Raniganj* (second major coal horizon) → *Panchet* → *Supra-Panchet/ Mahadeva*.
  • Floral succession: *Gangamopteris–Glossopteris* (Lower Gondwana) → *Dicroidium/Ptilophyllum* (Upper Gondwana). Rajmahal Traps mark the Early Cretaceous.
• Mesozoic marine: Kutch (Jhurio, Jumara, Jhuran, Bhuj — classic Jurassic ammonite zonation), Jaisalmer basin, Spiti Shales, Umia, Trichinopoly–Ariyalur (Cretaceous of Tamil Nadu, richly fossiliferous: Uttatur, Trichinopoly and Ariyalur Groups, with the Dalmiapuram Formation a reefal limestone inside the Uttatur).
• Deccan Traps (latest Cretaceous–earliest Palaeogene, ~66 Ma): flood basalts with intertrappean beds (freshwater fossils) and infratrappean (Lameta) beds yielding dinosaur bones and eggs (Jabalpur, Balasinor).
• Cenozoic: Palaeogene of Assam (Jaintia, Barail), Kutch and Cambay with *Nummulites*; Siwalik Group (Miocene–Pleistocene: Lower/Kamlial–Chinji, Middle/Nagri–Dhok Pathan, Upper/Tatrot–Pinjor) — spectacular vertebrate faunas (*Sivapithecus*, *Hipparion*, Proboscidea). The Main Boundary Thrust separates Siwaliks from Lesser Himalaya; the Himalayan Frontal Thrust separates them from the Indo-Gangetic alluvium.
• Quaternary: Indo-Gangetic alluvium (Bhangar older, Khadar newer), Thar dunes, coastal and laterite deposits, Narmada valley hominin find (*Narmada Man*).
• Boundary problems commonly asked: Precambrian–Cambrian (Spiti, Salt Range), Permo-Triassic (Spiti, Kashmir), K–Pg (Um Sohryngkew section in Meghalaya — India's best studied K–Pg boundary with an iridium anomaly).

Practice, codes and institutions in India

• GSI maps on the Formation as basic unit and publishes *Lexicon of Indian Stratigraphy* and the *Geology and Mineral Resources* series; nomenclature follows the International Stratigraphic Guide, interpreted nationally.
• Palaeontology Division, GSI (Kolkata) maintains type collections; the Birbal Sahni Institute of Palaeosciences (BSIP), Lucknow is the national centre for palaeobotany and palynology; Wadia Institute of Himalayan Geology (WIHG), Dehradun for Himalayan stratigraphy.
• Statutory frame: the Mines and Minerals (Development and Regulation) Act, 1957 as amended (2015, 2021, 2023 amendments introduced auctions, exploration licences and critical-mineral provisions) governs exploration; GSI itself has no Act of its own — it is an attached office of the Ministry of Mines, working under the MMDR rules and notifications — while the National Mineral Policy, 2019 sets exploration priorities. Mineral resource reporting uses the UNFC framework and India's reporting template, with NI 43-101/ JORC used for international filings.
• Baseline geoscientific data (NGCM, NGPM) is published through GSI's *Bhukosh* portal; stratigraphic columns there are the standard reference for mapping work.
• Protected fossil sites include the National Fossil Wood Park, Tiruvakkarai (Tamil Nadu), Shivalik Fossil Park (Saketi, Himachal), Mandla and Ghughua fossil parks (Madhya Pradesh) and the Salkhan/Sonbhadra stromatolite site; these are notified as geoheritage/National Geological Monuments by GSI. A dedicated geoheritage statute has been under consideration — check current status rather than citing an Act number.

Key points

• Superposition - in undeformed strata the lower bed is older; the first premise of all stratigraphy.
• Faunal succession - fossil assemblages follow a unique, irreversible order, allowing worldwide dating.
• Walther's law - vertically stacked facies in a conformable sequence were once side-by-side environments.
• Formation - the fundamental mappable lithostratigraphic unit, defined by a type section, not by age.
• Biozone - basic biostratigraphic unit; range, concurrent-range, Oppel, assemblage, acme and interval types.
• Chronostratigraphy vs geochronology - System/Series/Stage are rock units; Period/Epoch/Age are the matching time units.
• Unconformity - a surface of missing time; angular unconformity implies tilting plus erosion, nonconformity means sediment on basement.
• Index fossil - widespread, short-ranged, abundant, distinctive; planktonic and nektonic forms (graptolites, planktonic foraminifera, ammonoids) are best.
• Trace fossil - records behaviour in situ, so it is never transported; a reliable environment (ichnofacies) indicator.
• Taphonomy - all post-mortem processes; explains why the fossil record is biased towards hard-shelled marine forms.
• Radiometric dating - uses parent-daughter ratios; U–Pb zircon for Precambrian, ¹⁴C below about 50 ka, Ar–Ar for Deccan basalts.
• Magnetostratigraphy - polarity chrons (Brunhes, Matuyama, Gauss, Gilbert) date the Siwaliks and ocean floor.
• Sequence stratigraphy - unconformity-bounded sequences with LST, TST, HST tracts; accommodation = eustasy + subsidence, which sediment supply then fills.
• Gondwana Supergroup - Talchir (glacial) → Barakar (main coal) → Barren Measures → Raniganj (coal) → Panchet; *Glossopteris* flora proves Gondwanaland.
• Kutch Jurassic - India's internationally used ammonite zonation; Spiti and Kashmir give the Palaeozoic and Permo-Triassic record.
• Siwaliks - Miocene–Pleistocene fluvial molasse with the richest Indian vertebrate fauna; bounded by MBT above and HFT below.
• K–Pg boundary - Deccan volcanism plus Chicxulub impact; Meghalaya's Um Sohryngkew section preserves the iridium anomaly.
• End-Permian extinction - the largest, removing roughly 90% of marine species; the Palaeozoic–Mesozoic divide.

Formulas and examples

Example 1 — radiocarbon age of a peat bed in the Ganga alluvium. A peat layer shows ¹⁴C activity 25% of the modern standard. t½ = 5,730 yr.

• A/A₀ = 0.25 = (1/2)², i.e. exactly two half-lives.
• t = 2 × 5,730 = 11,460 years (about 11.5 ka, so late Pleistocene–Holocene transition).
• Check by formula: t = (5,730/0.693) × ln(4) = 8,268 × 1.3863 ≈ 11,460 yr. ✔

Example 2 — U–Pb age of a zircon from a Dharwar greenstone belt. Measurement: ²⁰⁶Pb/²³⁸U = 0.4. λ(²³⁸U) = 1.55125 × 10⁻¹⁰ yr⁻¹.

• t = (1/λ) ln(1 + D/P) = (1/1.55125 × 10⁻¹⁰) × ln(1.4)
• ln(1.4) = 0.33647 → t = 0.33647 / 1.55125 × 10⁻¹⁰ = 2.169 × 10⁹ yr
• Age ≈ 2.17 Ga — Palaeoproterozoic, i.e. younger than the 3.0–2.5 Ga Dharwar greenstone and granite record, so a post-Dharwar intrusive or metamorphic zircon.

Example 3 — sedimentation rate and compaction, Barakar Formation. A Barakar section is 620 m thick and palynology brackets it between 290 Ma and 272 Ma.

• Duration = 290 − 272 = 18 Ma.
• R = 620 m / 18 Ma = 34.4 m/Ma = 0.0344 mm/yr.
• If original porosity reduced the pile by 30% on compaction, decompacted thickness = 620 / 0.70 = 886 m → R ≈ 49.2 m/Ma, showing why raw rates understate subsidence.

Example 4 — true thickness from outcrop width. A Kaimur Sandstone bed dips 25° and crops out over a horizontal width of 180 m measured perpendicular to strike on flat ground.

• t = w × sin δ = 180 × sin 25° = 180 × 0.4226 = 76.1 m.
• If a student wrongly used the outcrop width as thickness, the error would be +136%.

Example 5 — relative dating from a field sketch. Observed: granite basement; above it, conglomerate, then shale, then limestone (all parallel); a basalt dyke cuts granite, conglomerate and shale but stops at the base of the limestone; limestone contains *Nummulites*.

• Order (oldest first): granite → (nonconformity) → conglomerate → shale → dyke intrusion → erosion surface → limestone.
• The limestone is Eocene (*Nummulites* range Palaeocene–Oligocene, acme Eocene), so the dyke is pre-Eocene; the granite is older still.

Where it is used

• Hydrocarbon exploration (ONGC, Oil India, Vedanta Cairn, DGH): biostratigraphy and sequence stratigraphy to build well correlations and identify reservoir–seal pairs in Cambay, Krishna–Godavari, Mumbai Offshore and Assam–Arakan basins; micropalaeontologist and well-site geologist roles.
• Coal and lignite (Coal India subsidiaries, SCCL, NLC India, CMPDI): Gondwana lithostratigraphy and palynology to trace Barakar and Raniganj seams, resolve faulted seam correlation and classify resources under UNFC.
• Mineral exploration (GSI, MECL, Hindustan Zinc, NMDC, Hutti Gold): Precambrian stratigraphy of greenstone belts and Purana basins to target BIF, base metals, diamonds (Panna) and limestone.
• Groundwater and engineering geology (CGWB, state departments, dam and tunnel projects): aquifer geometry from stratigraphic correlation; Quaternary stratigraphy for seismic microzonation and liquefaction studies.
• Academia, museums and geoheritage: BSIP Lucknow, WIHG Dehradun, university departments; curation at GSI's Palaeontology Division and fossil parks such as Tiruvakkarai and Ghughua.
• Cement, construction materials and climate research: limestone resource mapping by stratigraphic horizon; palaeoclimate reconstruction from pollen and isotope records used in IMD/monsoon studies.

Interview questions

• Q: Why is a formation defined by lithology and not by age? A: Because a formation must be mappable in the field on observable rock character; the same lithology may be deposited at different times in different places, making formations diachronous.
• Q: What makes a good index fossil? A: Wide geographic distribution, a short vertical range, abundance, distinctive easily recognised morphology and independence of facies — which is why planktonic and nektonic forms such as graptolites and ammonoids qualify.
• Q: How do you recognise a paraconformity in the field? A: You usually cannot see it — beds are parallel with no erosion surface; it is detected by a missing biozone, an abrupt fossil change, or a gap in radiometric or isotopic data.
• Q: State Walther's law and its use. A: Facies succeeding each other vertically in a conformable sequence represent environments that were once laterally adjacent; it lets you reconstruct transgression and regression from a single borehole.
• Q: Why can't sandstone be dated directly by radiometric methods? A: Its grains are older than the deposit, so radiometric ages give the source rock's age; the sediment is dated by interbedded volcanics, fossils, or a youngest detrital-zircon maximum age.
• Q: What is the Talchir Formation and why is it important? A: The basal Gondwana unit with tillite, boulder beds and varve-like needle shales, recording Permo-Carboniferous glaciation — key evidence for Gondwanaland and the base of India's coal-bearing sequence.
• Q: Which Indian succession is used internationally for Jurassic correlation? A: The Kutch basin, whose ammonite assemblages (Jhurio–Jumara–Jhuran) provide standard Middle to Upper Jurassic zonation.
• Q: Differentiate between lithostratigraphic, biostratigraphic and chronostratigraphic boundaries. A: Lithological boundaries follow rock change and cut across time; biostratigraphic boundaries follow fossil appearances or disappearances and approximate time; chronostratigraphic boundaries are defined isochronous surfaces fixed by a GSSP.
• Q: How does magnetostratigraphy help in Siwalik studies? A: The Siwaliks lack good marine fossils, so the recorded sequence of magnetic polarity reversals is matched to the global polarity time scale to give a numerical age framework.
• Q: What causes the end-Permian mass extinction, and how is it recognised in India? A: Most likely Siberian Trap volcanism with ocean anoxia and warming; in India it is seen in the Spiti and Kashmir Tethyan sections as a faunal turnover with a negative carbon-isotope excursion.
• Q: What is a GSSP and why was it introduced? A: A Global Boundary Stratotype Section and Point is a physical marker in one agreed outcrop defining the base of a chronostratigraphic unit, replacing vague correlation with a single reproducible reference.
• Q: In sequence stratigraphy, what does the maximum flooding surface represent? A: The horizon of greatest landward shift of the shoreline, separating transgressive from highstand deposits; it is usually a condensed, fossil-rich, low-sedimentation layer and an excellent correlation datum.
• Q: Why are trace fossils valuable even without identifiable body remains? A: They are made in place, so they cannot be transported, and they record organism behaviour, making them robust indicators of substrate, oxygen level and water energy.

Common mistakes

• Treating a formation boundary as a time line, and so correlating formations across basins as if they were coeval.
• Confusing chronostratigraphic and geochronologic terms — writing "Upper Jurassic time" or "Late Jurassic rocks" instead of Upper Jurassic rocks / Late Jurassic time.
• Mistaking an unconformity type: calling sediment on basement an "angular unconformity" instead of a nonconformity, or ignoring paraconformities altogether.
• Reporting outcrop width as bed thickness, forgetting the sin δ (or topographic) correction.
• Using benthic, facies-controlled fossils as index fossils, or dating a bed by a long-ranging form.
• Assuming a radiometric age dates deposition rather than crystallisation or cooling, and quoting ages without errors or the decay constant used.
• Reversing the Gondwana order (placing Raniganj below Barakar) or confusing *Glossopteris* (Lower Gondwana) with *Ptilophyllum* (Upper Gondwana).
• Quoting fixed boundary ages without noting that the ICS time scale is periodically revised.

Related topics

Structural geology · Geomorphology · Geochemistry · Sedimentology and sedimentary petrology · Economic geology and mineral exploration · Petroleum geology and well logging