The principles of relative dating (superposition, original horizontality, cross-cutting relationships, included fragments and unconformities) and how they are combined to establish the sequence of geological events.

What this dot point is asking

WJEC wants you to know the principles of relative dating and to apply them, in combination, to reconstruct the order of geological events from a cross-section or field exposure. This is a heavily examined Component 2 skill: you are given a sequence with beds, intrusions, faults and unconformities and must put the events in order from oldest to youngest.

The answer

Relative versus absolute dating

Relative dating establishes the order of events (what came before what) without giving an age in years. Absolute dating gives a numerical age, usually by radiometric methods. Relative dating came first historically and is the everyday tool of the field geologist, because the principles can be applied directly to an outcrop.

The principles

Superposition

in an undisturbed pile, lower is older. Always check way-up first (using sedimentary structures), because overturning reverses the order.

Original horizontality

sediments are deposited in approximately horizontal layers, so beds that are now tilted or folded have been deformed since deposition.

Cross-cutting relationships

any feature that cuts across another (a fault, a dyke, an intrusion, an erosion surface) must be younger than the feature it cuts, because the rock had to exist before it could be cut.

Included fragments (inclusions)

fragments contained within a rock are older than the rock that encloses them. Pebbles in a conglomerate, or xenoliths caught up in a granite, predate the host.

Unconformities

a buried erosion surface marks a gap in the record. An angular unconformity has tilted older rocks beneath flatter younger rocks; a disconformity is a gap between parallel beds; a nonconformity has sedimentary rocks resting on eroded igneous or metamorphic rock.

Combining the principles

Real sequences need several principles together. A granite that contains fragments of the country rock (so the country rock is older), is cut by a later dyke (so the dyke is younger), and is overlain across an unconformity by sediments (so those sediments are younger still) can be fully ordered only by using inclusions, cross-cutting and the unconformity in turn.

Examples in context

Hutton's unconformity at Siccar Point. James Hutton recognised gently dipping Old Red Sandstone resting on near-vertical greywackes, reading deposition, tilting, erosion and renewed deposition from one outcrop, the founding example of deep time. Dating an intrusion against a fault. A mineralised vein cut by a fault is older than the fault, so a prospector knows the faulting post-dates and may offset the ore, a practical use of cross-cutting. Conglomerate provenance. Rounded pebbles of older rock within a conglomerate prove a source terrain was eroding before the conglomerate formed, using the inclusion principle to reconstruct landscape history.

Try this

Q1. State the principle of superposition and the condition under which it holds. [2 marks]

• Cue. In an undisturbed sequence each layer is younger than the one below; it holds only if the sequence has not been overturned (so check way-up first).

Q2. A dyke cuts through three sedimentary beds. Is the dyke older or younger than the beds, and which principle tells you? [2 marks]

• Cue. Younger; the principle of cross-cutting relationships, because the dyke must intrude rock that already exists.

Q3. Name the three types of unconformity. [3 marks]

• Cue. Angular unconformity (tilted older rocks below flatter younger rocks), disconformity (gap between parallel beds), nonconformity (sediments on eroded igneous or metamorphic rock).