Historical geology, stratigraphy, palaeontology and geological time
Domain 4 - Historical geology, stratigraphy, palaeontology and geological time. Questions ask you to order geological events from cross-sections, date them, and place them on the time scale using fossils and isotopes.
Principles of relative dating
- Superposition, original horizontality, lateral continuity, cross-cutting relationships, inclusions, and faunal succession.
- Unconformities: angular (tilted beds truncated by younger horizontal ones), disconformity (erosion surface between parallel beds), nonconformity (sediment on igneous or metamorphic basement).
- A dyke cutting a fold is younger than the fold but older than any bed that seals it.
Stratigraphy and correlation
- Lithostratigraphy (formations, members), biostratigraphy (zones based on index fossils), chronostratigraphy (systems and stages), magnetostratigraphy and chemostratigraphy (δ13C, δ18O).
- Sequence stratigraphy uses transgressive and regressive systems tracts driven by eustasy, subsidence and sediment supply.
- Index fossils must be abundant, widely distributed, short-ranging and facies-independent: ammonites, graptolites, trilobites, planktonic foraminifera, calcareous nannofossils.
Absolute dating
- Radioactive decay: N = N0·e^(−λt); half-life t½ = ln2/λ. Useful systems: U–Pb in zircon (closure ~900 °C, the gold standard), K–Ar and Ar–Ar (volcanics), Rb–Sr, and 14C for organic carbon younger than about 50,000 years.
- Closure temperature matters: a mineral date records cooling below that temperature, not crystallisation.
The time scale
Precambrian (Hadean, Archean, Proterozoic) is about 88 % of Earth history. Phanerozoic: Palaeozoic (Cambrian 541 Ma to Permian), Mesozoic (Triassic, Jurassic, Cretaceous), Cenozoic (Palaeogene, Neogene, Quaternary). Key markers: Great Oxidation Event ~2.4 Ga, Cambrian explosion 541 Ma, end-Permian extinction 252 Ma, K–Pg impact 66 Ma.
Worked example
A section shows folded limestone cut by a granite, both overlain by horizontal conglomerate and shale containing ammonites. Order: deposition of limestone → folding → intrusion and contact metamorphism of the granite → erosion (angular unconformity) → deposition of conglomerate and shale. The ammonites place the youngest beds in the Mesozoic, so the granite is older than that, and a U–Pb zircon age from the granite would bracket the folding.
Traps
- Using 14C on basalt or on a shell with old carbon.
- Assuming an unconformity represents no time rather than a gap in the record.
- Applying superposition to overturned strata without geopetal structures.
- Treating a fossil range as a single point rather than a zone.