Historical geology, stratigraphy and palaeontology
This domain tests relative and absolute dating, stratigraphic correlation and the use of fossils, with Andalusian examples. Questions usually ask you to order events or justify a correlation.
Relative dating
- Principles: superposition, original horizontality, lateral continuity, cross-cutting relationships, inclusions, faunal succession.
- Unconformities: angular (beds tilted then eroded), disconformity (erosion surface between parallel beds), nonconformity (sediments over igneous or metamorphic rock), paraconformity (no visible erosion).
Absolute dating
- Radioactive decay follows N = N0(1/2)^(t/t1/2); the parent/daughter ratio gives age, and isochrons test closed-system behaviour.
- Zircon U-Pb is the workhorse for old rocks; radiocarbon is limited to organic remains younger than about 50,000 years.
Stratigraphy and correlation
- Lithostratigraphic, biostratigraphic and chronostratigraphic units are different things; a biozone is not a chronozone.
- Index fossils combine wide geographic range with short time range; ammonites, graptolites and foraminifera are classic examples.
- Sequence stratigraphy recognises unconformity-bounded sequences and transgressive-regressive cycles.
Palaeontology
- Preservation modes: permineralisation, recrystallisation, moulds and casts, carbonisation, amber, and trace fossils.
- Andalusian record: Palaeozoic trilobites and graptolites in Sierra Morena; Mesozoic ammonites in Betic carbonates; Miocene reefs and mammals; early Pleistocene vertebrates and hominin evidence at Orce (Granada).
Worked example
A sequence shows tilted Jurassic limestones, an erosion surface, horizontal Miocene conglomerates, and a basalt dyke cutting everything. Order the events: deposition and tilting of the Jurassic; erosion producing an angular unconformity; deposition of the Miocene; intrusion of the dyke. The dyke is youngest because it cross-cuts all earlier units.
Traps that cost marks
A dyke is younger than the faults it cuts, not older. Unconformity types are easily confused. Reworked fossils give a maximum age for the host bed, not its actual age. Half-life arithmetic and percentage conversions cause silent errors. A biozone does not equal a time plane.
Key points
- Cross-cutting relationships date faults, dykes and folds.
- Angular unconformity requires tilting before erosion.
- Radiometric dating needs closed systems and suitable minerals.
- Index fossils are geographically wide and temporally short.
- Orce and Sierra Morena provide Andalusian palaeontological anchors.