Historical Geology, Stratigraphy and Palaeontology
This domain tests how geologists order and date events, and how fossils constrain both age and environment. Typical PAU tasks are a described cross-section with a fault, a dyke and an unconformity, or a fossil assemblage to be dated and interpreted environmentally.
Relative dating principles
- Superposition: in an undeformed sequence, older beds lie below younger ones.
- Original horizontality and lateral continuity: tilted beds were tilted after deposition.
- Cross-cutting relationships: a fault or intrusion is younger than everything it cuts.
- Inclusions: a clast inside a rock is older than the rock containing it. Faunal succession: fossil assemblages change in a reproducible order.
Unconformities
Angular unconformity (tilted beds truncated, then overlain by flat-lying beds); disconformity (erosion surface between parallel beds); nonconformity (sedimentary beds over igneous or metamorphic basement). Each implies a gap in the record, not missing time only in the section.
Absolute dating
Radiocarbon works only on organic carbon and only to about 50,000 years. K–Ar and Ar–Ar suit volcanic rocks; Rb–Sr suits metamorphic and igneous rocks; U–Pb on zircon is the reference method for very old rocks because zircon resists alteration and has a high closure temperature. Half-life logic: after n half-lives, the parent fraction is (1/2)ⁿ.
Time scale and major events
Hadean–Archaean–Proterozoic–Phanerozoic; Palaeozoic with Cambrian explosion (541 Ma), colonisation of land, Permian–Triassic extinction (252 Ma, about 90% of marine species); Mesozoic with Pangaea break-up, ammonites, rudists and dinosaurs, and the K–Pg impact extinction (66 Ma); Cenozoic with Alpine–Pyrenean compression, nummulitid limestones, Mediterranean desiccation (Messinian) and Pleistocene glaciations.
Fossils as tools
Index fossils need wide geographical range and short time range: ammonites (Jurassic–Cretaceous), trilobites (Palaeozoic), nummulites (Eocene), rudists (Cretaceous reefs). Taphonomy, facies and biozones matter: a benthic assemblage indicates substrate and depth, a planktic one indicates open water.
Worked example
A sequence of folded limestones with ammonites is cut by a granite intrusion, both are truncated by an erosion surface, and flat conglomerates with Eocene nummulites lie on top. Ordering: limestone (Jurassic–Cretaceous) → folding → granite intrusion (post-folding, so post-Cretaceous) → erosion → conglomerate (Eocene). The surface is an angular unconformity, and the granite can be dated by U–Pb or K–Ar.
Traps that cost marks
Applying radiocarbon to limestone; calling any erosion surface an angular unconformity; dating a fault by the rocks it cuts rather than by cross-cutting logic.