Palaeontology and Stratigraphy
Palaeontology is the study of ancient life through fossils; stratigraphy is the study of rock strata and their sequence. Together they reconstruct Earth's 4.6-billion-year history and provide the relative time scale that underpins all geological dating.
Any preserved remains, impression or trace of a once-living organism. Fossils may be body fossils (bones, shells, leaves) or trace fossils / ichnofossils (footprints, burrows, faecal pellets, root traces). Conventionally, fossils older than about 10,000 years are called "true" fossils, with younger ones termed sub-fossils.
Modes of fossilisation
- Permineralisation — minerals fill the pore spaces of original tissue (most bones, petrified wood).
- Replacement — original material is replaced atom-by-atom by another mineral (silicified wood, pyritised ammonites).
- Carbonisation — volatile components driven off, leaving a carbon film (leaves, soft-bodied animals at Burgess Shale).
- Mould and cast — original tissue dissolves, leaving an impression (mould) which may be filled later (cast).
- Original preservation — rare; insects in amber, mammoths in permafrost, shells in young sediments.
- Trace fossils — direct preservation of behaviour rather than body.
Rapid burial in fine sediment with low oxygen levels gives the best preservation; soft-bodied animals are seldom fossilised.
Principles of stratigraphy
Five key principles, articulated chiefly by Nicolas Steno (1669) and William Smith (1796):
- Original horizontality — sediments are deposited as nearly horizontal layers.
- Superposition — in undisturbed sequences, older layers lie below younger ones.
- Lateral continuity — layers extend continuously until they thin, change character or hit a basin margin.
- Cross-cutting relationships — a feature that cuts another (e.g., a dike crossing strata) is younger.
- Faunal succession (Smith) — fossil assemblages succeed each other in a definite and recognisable order, allowing strata to be correlated worldwide.
These principles enabled the construction of the geological column long before radiometric dating provided absolute ages.
The geological time scale
The standard time scale is divided into eons, eras, periods, epochs and ages. Major divisions:
| Eon | Era | Period | Start (Ma) | Highlights |
|---|---|---|---|---|
| Phanerozoic | Cenozoic | Quaternary | 2.58 | Ice ages, Homo sapiens |
| Neogene | 23.0 | Mammals diversify, grasslands | ||
| Palaeogene | 66.0 | After K-Pg extinction; mammals radiate | ||
| Mesozoic | Cretaceous | 145 | Dinosaurs peak, flowering plants | |
| Jurassic | 201 | Pangea breaks up, large sauropods | ||
| Triassic | 252 | Mammals first appear | ||
| Palaeozoic | Permian | 299 | Largest mass extinction (~252 Ma) | |
| Carboniferous | 359 | Coal swamps, reptiles emerge | ||
| Devonian | 419 | Age of fishes, first tetrapods | ||
| Silurian | 444 | Vascular plants colonise land | ||
| Ordovician | 485 | Marine invertebrates flourish | ||
| Cambrian | 541 | "Cambrian explosion" of life | ||
| Proterozoic | – | – | 2500 | Eukaryotes, first multicellular life |
| Archean | – | – | 4000 | Early prokaryotes, oxygen-free atmosphere |
| Hadean | – | – | 4567 | Earth's formation, no rocks preserved |
- Phanerozoic = "visible life" (~last 541 Ma); Precambrian is everything older.
- Five great mass extinctions: End-Ordovician (~445 Ma), Late Devonian (~375 Ma), End-Permian (~252 Ma; ~95 % marine species lost), End-Triassic (~201 Ma), End-Cretaceous (~66 Ma; non-avian dinosaurs).
- Continents have repeatedly assembled and dispersed: Rodinia (~1100 Ma), Pangea (~335 Ma), Pangea's break-up begins ~200 Ma.
- The Anthropocene (informal) marks human-driven changes in the Quaternary.
Biostratigraphy
Strata are correlated by their fossil content. Index (zone) fossils are species that:
- Were geographically widespread,
- Existed for a short time interval,
- Are abundant and easily recognised.
Examples: graptolites (Ordovician–Silurian), ammonites (Mesozoic), trilobites (Palaeozoic), fusulinids (Carboniferous–Permian), foraminifera (Mesozoic-Cenozoic — especially planktonic forams, the workhorse of oil-industry biostratigraphy), conodonts (Cambrian–Triassic).
A biozone is the stratigraphic interval characterised by a particular index taxon or assemblage.
Lithostratigraphy and chronostratigraphy
Rock units are formally named in a hierarchy: bed → member → formation → group → supergroup. Formations are mapped by their lithology and are diachronous (their tops and bases need not be everywhere the same age).
Chronostratigraphic units (system, series, stage) correspond to time units (period, epoch, age); their boundaries are agreed by international stratigraphic commissions and are anchored at Global Boundary Stratotype Sections and Points (GSSPs).
Unconformities
An unconformity is a buried erosion surface representing missing time. Three main types:
| Type | Description |
|---|---|
| Disconformity | Parallel layers, with an erosion surface between |
| Angular unconformity | Younger horizontal strata over tilted older strata (Hutton's Siccar Point) |
| Nonconformity | Stratified rocks resting on crystalline igneous or metamorphic basement |
Unconformities mark major tectonic, climatic or sea-level events.
Radiometric dating
Radiometric methods provide absolute ages via radioactive decay:
| Isotope pair | Half-life | Typical use |
|---|---|---|
| U-238 → Pb-206 | 4.47 Ga | Zircons in igneous and metamorphic rocks |
| K-40 → Ar-40 | 1.25 Ga | Volcanic ash, micas |
| Rb-87 → Sr-87 | 49 Ga | Igneous rocks > 100 Ma |
| C-14 → N-14 | 5,730 yr | Organic remains < 50,000 yr |
| Sm-Nd | 106 Ga | Mantle samples, very old rocks |
Combining biostratigraphy (relative) and radiometric dates (absolute) underpins the modern time scale.
Mnemonic for the Phanerozoic periods: Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian, Triassic, Jurassic, Cretaceous, Paleogene, Neogene, Quaternary → Camels Often Sit Down Carefully; Perhaps Their Joints Creak; Persistent Noisy Quibbles.
Major events in Earth's history
- ~4.6 Ga: solar nebula collapses; Earth accretes.
- ~4.5 Ga: Theia collision forms the Moon.
- ~3.5 Ga: oldest definitive microfossils (stromatolites).
- ~2.4 Ga: Great Oxygenation Event — atmospheric O₂ rises.
- ~750–635 Ma: "Snowball Earth" glaciations.
- ~541 Ma: Cambrian explosion — most major animal phyla appear.
- ~252 Ma: End-Permian mass extinction.
- ~66 Ma: Chicxulub impact; non-avian dinosaurs extinct.
- ~5–7 Ma: divergence of human and chimpanzee lineages.
- ~300 ka: anatomically modern Homo sapiens in Africa.
Palaeontology and stratigraphy thus constitute the historical sciences within geology — providing the timeline on which all other geological processes are arranged.