Historical geology, paleontology and geological time
Historical geology reconstructs the order and duration of events. The PAU combines stratigraphic principles with radiometric dating and palaeontology, and expects you to place events on the geological time scale and to use fossils as chronological tools.
Relative dating principles
- Superposition, original horizontality, lateral continuity, cross-cutting relationships and inclusions.
- Faunal succession: fossil assemblages change in a consistent order, allowing correlation of distant outcrops.
- Unconformities record gaps: angular (tilted beds truncated below horizontal ones), disconformity (parallel beds with an erosional gap) and nonconformity (sediments over igneous or metamorphic basement).
- Uniformitarianism: present processes explain the past, without excluding catastrophic events.
Absolute dating
Radiometric methods use the decay equation N = N₀e^(−λt) and the half-life T½ = ln2/λ. U–Pb suits old zircons, K–Ar suits volcanic rocks, Rb–Sr requires closed systems, and ¹⁴C works only on organic remains younger than about 50,000 years.
The time scale and life
Eons: Hadean, Archean, Proterozoic, Phanerozoic; eras: Palaeozoic, Mesozoic, Cenozoic. Milestones include the Great Oxidation Event, the Cambrian explosion, the end-Permian extinction (the largest) and the K–Pg impact. Index fossils - trilobites, graptolites, ammonites, nummulites - are short-ranging, abundant and widespread.
Worked example
A granite intrudes folded limestones but is cut by an unconformity below horizontal sandstones. Reasoning: folding predates the granite, the granite predates erosion and the unconformity, and the sandstones are the youngest unit. If the granite yields a K–Ar age of 300 Ma, the folding is older than 300 Ma and the sandstones younger.
Number work
A sample retains 12.5 % of its parent isotope, so three half-lives have passed; with a half-life of 40 Ma the rock is 120 Ma old.
Traps that cost marks
- Dating a sedimentary rock from a clast inside it.
- Treating ¹⁴C as usable on limestone or on material older than about 50 ka.
- Reading an angular unconformity as a fault.
- Mixing the order of eras, or forgetting that the Precambrian covers roughly 88 % of Earth history.