Relative dating: the principles used to order geological events (superposition, original horizontality, cross-cutting relationships, included fragments and faunal succession); the recognition of way-up evidence; the application of these principles to construct the geological history of a cross-section, including faults, intrusions and unconformities.

What this dot point is asking

OCR wants you to state and apply the principles of relative dating (superposition, original horizontality, cross-cutting relationships, included fragments and faunal succession), to recognise way-up evidence, and to use these principles to reconstruct the geological history of a cross-section that includes faults, intrusions and unconformities.

The answer

The principles of relative dating

Relative dating puts geological events in order (which came first) without giving an actual age in years. The principles are:

• Superposition. In an undeformed sequence, each bed is younger than the one below it and older than the one above. So the oldest rocks are at the bottom.
• Original horizontality. Sediments are deposited in horizontal (or near-horizontal) layers, so any tilting or folding happened after deposition.
• Cross-cutting relationships. Any feature (a fault, a dyke, an unconformity) that cuts across a rock is younger than the rock it cuts, because the rock had to exist first.
• Included fragments (inclusions). A fragment enclosed in a rock is older than the rock containing it (for example xenoliths in a granite, or pebbles in a conglomerate).
• Faunal succession. Fossil species succeed one another in a definite, recognisable order, so a bed can be dated relatively by the fossils it contains.

Way-up evidence

In folded or overturned rocks the beds may be upside-down, so you need way-up (younging) evidence to tell which way is the original top. Useful indicators include graded bedding (coarse at the base, fine at the top), cross-bedding, ripple marks (crests point up) and desiccation cracks (wider at the top). These tell you the original top of the bed and so the correct order.

Reconstructing a geological history

To read a cross-section, work in a disciplined order:

1. Apply superposition to the undisturbed beds (oldest at the base).
2. Add deformation events (folding, tilting) using original horizontality.
3. Place unconformities as gaps (uplift, erosion, non-deposition).
4. Add cross-cutting features (faults, intrusions) as younger than everything they cut.

Examples in context

Example 1. Xenoliths dating an intrusion. Angular fragments of an older country rock caught inside a granite (xenoliths) must predate the granite, so by the inclusion principle they help bracket the intrusion's age relative to the surrounding rocks.

Example 2. Reading an unconformity in the field. Where folded, eroded older beds are overlain by flat younger beds, the angular unconformity records a full cycle of deposition, deformation, uplift, erosion and renewed deposition, slotting neatly into the relative history.

Try this

Q1. State the principle of superposition. [1 mark]

• Cue. In an undeformed sequence, each bed is younger than the bed below it and older than the bed above it.

Q2. A dyke cuts a series of sandstone beds. State whether the dyke is older or younger than the sandstones and why. [2 marks]

• Cue. Younger; by cross-cutting relationships the cutting feature must be younger than the rock it cuts, which had to exist first.

Q3. Explain how graded bedding can be used as way-up evidence. [2 marks]

• Cue. Graded bedding is coarse at the base and fines upwards, so the fine end marks the original top; this tells you the way up in folded or overturned rocks.