Unconformities and the geological record: the angular unconformity (tilted or folded beds overlain at a different angle), the disconformity (parallel beds separated by an erosion surface) and the nonconformity (sediments on eroded igneous or metamorphic basement); the ordered sequence of events each records (deposition, uplift, tilting, erosion, renewed deposition); the gap (hiatus) in the record; and the use of unconformities to reconstruct geological history on maps and cross-sections.

What this dot point is asking

Eduqas wants you to recognise an unconformity as a buried surface that represents a gap in the geological record (a missing interval of time), to classify the three types (angular unconformity, disconformity, nonconformity), and to read the ordered sequence of events that each one records. The skill is examined in Components 1 and 3, where you reconstruct the history of an area from a cross-section or map and have to place the unconformity correctly in the sequence.

The answer

What an unconformity is

The key idea is that an unconformity is not just a line on a diagram: it is missing time. Whole formations, and the fossils and events they would have contained, can be absent across it.

Angular unconformity

In an angular unconformity, the older beds below are tilted or folded and then truncated (cut off) by erosion, and the younger beds above are deposited across them at a different angle. The contrast in dip between the two sets of beds is the diagnostic feature. It records the fullest history of the three types.

Disconformity

In a disconformity, the beds above and below are parallel (both roughly horizontal), but an erosion surface and a time gap separate them. Because there is no angular contrast, a disconformity is harder to spot: you recognise it from a buried, irregular erosion surface, a fossil soil or hardground, reworked pebbles at the base of the upper unit, or missing fossil zones that show the lost time.

Nonconformity

In a nonconformity, sedimentary rocks rest on eroded igneous or metamorphic basement (crystalline rock). The contact puts bedded sediment directly on coarse granite or foliated schist, usually with a weathered top to the basement and a basal conglomerate containing fragments of it.

The sequence of events an unconformity records

An angular unconformity records a complete cycle, which you must be able to put in order (oldest first):

1. Deposition of the lower beds (originally horizontal).
2. Folding or tilting by tectonic stress (compression or extension).
3. Uplift above sea level.
4. Erosion of the tilted beds to a flat surface (the unconformity surface), removing an unknown thickness.
5. Subsidence (or a rise in sea level).
6. Renewed deposition of the younger beds across the eroded surface.

A disconformity records the same cycle without the tilting step; a nonconformity records the formation, uplift and erosion of the basement before sediments were laid on top. This ordered narrative is exactly what extended-answer questions reward.

Using unconformities to reconstruct history

On a map or cross-section an unconformity is the most powerful single structure for reconstructing history, because it brackets a whole episode of deformation and erosion between two dated rock sets. You use it together with the principle of superposition (younger on top), cross-cutting relationships (a structure is younger than what it cuts) and the principle of included fragments (a clast is older than the rock containing it) to build the full order of events. The unconformity marks where part of that history is simply missing.

Examples in context

Example 1. Hutton's unconformity at Siccar Point. Steeply tilted older greywackes are truncated and overlain by gently dipping younger red sandstones, a textbook angular unconformity that records deposition, folding, uplift, erosion and renewed deposition, and was historic evidence for the immense length of geological time.

Example 2. A nonconformity beneath a sedimentary basin. Across much of a continent, flat-lying sandstones rest on a deeply eroded granite and gneiss basement; the nonconformity records the long interval in which the basement formed, was uplifted and was planed off before the cover sediments were deposited, a major missing chapter of the record.

Try this

Q1. Define an unconformity and state what the rocks below it are relative to the rocks above. [2 marks]

• Cue. A surface representing a gap (hiatus) in the geological record (erosion or non-deposition); the rocks below are older than those above.

Q2. Name the type of unconformity where horizontal sandstones rest on eroded granite, and give one recognition feature. [2 marks]

• Cue. A nonconformity; recognised by bedded sediment on crystalline basement, often with a weathered top and a basal conglomerate containing fragments of the basement.

Q3. State, in order, the sequence of events recorded by an angular unconformity. [3 marks]

• Cue. Deposition of the lower beds, folding or tilting, uplift, erosion to a flat surface, subsidence, then renewed deposition of the younger beds.