Earth History, Palaeontology and Geological Time
This section covers Earth history and geological time. The exam tests whether you can order events, interpret fossils and date rocks, and distinguish relative chronology from numerical ages.
Stratigraphic principles
- Superposition: in undisturbed sequences, older strata lie below younger ones.
- Original horizontality: sediments are deposited near horizontal; tilting and folding postdate deposition.
- Cross-cutting relationships: a fault, dyke or intrusion is younger than the features it cuts.
- Inclusions: clasts inside a rock are older than the rock containing them.
- Faunal succession: fossil assemblages change in a consistent order and allow correlation.
Radiometric dating
- A parent isotope decays to a daughter at a rate described by its half-life. Closure temperature marks when the system stops exchanging daughter isotope.
- Isochron methods test whether daughter isotope was inherited; disturbed systems may give meaningless ages.
- Carbon-14 dates organic material only over tens of thousands of years; long geological ages require isotopes such as uranium-lead or potassium-argon.
Worked example
A dyke cuts a sandstone. Above an erosion surface, a conglomerate contains sandstone clasts and also truncates the dyke. Ordering: sandstone first, dyke second, erosion third, conglomerate last. Reasoning: the dyke is younger than the sandstone because it cuts it; the conglomerate is younger than the sandstone because it contains sandstone clasts; the conglomerate is younger than the dyke because it overlies and truncates it.
Traps
- Treating a radiometric age as the age of every event affecting the rock; it dates closure, not necessarily deposition.
- Correlating rocks by similar appearance rather than fossil or stratigraphic evidence.
- Forgetting that an unconformity represents missing time.
- Using a fossil that is long-ranging or geographically restricted as a precise index fossil.