Relative dating and stratigraphic principles: the principles of superposition, original horizontality, lateral continuity, cross-cutting relationships and included fragments; way-up (younging) indicators; and the use of these principles to reconstruct the sequence of geological events from a section or map.

What this dot point is asking

Eduqas wants you to know the principles of relative dating (superposition, original horizontality, lateral continuity, cross-cutting relationships and included fragments), to recognise way-up (younging) indicators, and to use them to reconstruct the sequence of geological events from a cross-section or map. This is the core skill of Components 1 and 3, where you read the geological history out of a section without knowing any ages in years.

The answer

The principles of relative dating

Relative dating places events in order (older or younger) rather than giving ages in years. Five principles do almost all the work.

Cross-cutting relationships and inclusions

These two principles let you date intrusions, faults and erosion surfaces relative to the layers around them.

• A dyke or fault that cuts through beds is younger than every bed it cuts. If a later bed sits across the top of the dyke without being cut, that bed is younger than the dyke.
• Inclusions (xenoliths in an igneous rock, or pebbles of an older rock in a conglomerate) are older than the rock enclosing them. A granite containing fragments of the surrounding country rock must be younger than that country rock.

Together with superposition, these principles let you order an entire section.

Way-up (younging) indicators

When beds are steeply dipping, folded or overturned, superposition alone can mislead, so you need way-up indicators that record the original top and bottom:

• Graded bedding: fines upwards (coarse at the base, fine at the top), so the fine end was the top.
• Desiccation (mud) cracks: V-shaped, narrowing downwards and filled from above, so the points face the original bottom.
• Cross-bedding: the curved foresets are truncated at the top of each set, so the truncated surface is the top.
• Ripple marks: symmetrical wave ripples and asymmetrical current ripples have crests that point up; load casts bulge downwards.
• Geopetal structures and the position of fossils (for example shells convex-up) also indicate the way up.

Reconstructing the sequence

To read a geological history from a section, work through it systematically:

1. Use superposition to order the sedimentary beds (oldest at the base).
2. Check way-up indicators in case the beds are overturned.
3. Use cross-cutting to place faults, dykes and erosion surfaces relative to the beds.
4. Use inclusions to date intrusions and conglomerates relative to their fragments.
5. Identify unconformities as periods of uplift, erosion and non-deposition.

The result is a relative timeline of every event, from oldest to youngest.

Examples in context

Example 1. Dating a granite by its xenoliths. A granite containing angular fragments of the surrounding slate must be younger than that slate, because the fragments (inclusions) were torn off and engulfed as the magma rose.

Example 2. An overturned limb in the field. On the inverted limb of a recumbent fold, graded beds that appear to coarsen upwards are actually upside down; the way-up evidence corrects the order that superposition alone would get wrong.

Try this

Q1. State which is older, a dyke or the bed it cuts through, and name the principle. [2 marks]

• Cue. The bed is older; the dyke is younger by cross-cutting relationships.

Q2. Explain how graded bedding shows the way up. [2 marks]

• Cue. A graded bed fines upwards (coarse grains settle first at the base, fine grains last at the top), so the fine end is the original top.

Q3. State the principle that lets you correlate a bed across a valley where it is missing in the valley floor. [2 marks]

• Cue. Lateral continuity: the bed was originally continuous and has been removed by later erosion of the valley.