Quick questions on Unconformities: angular unconformity, disconformity and nonconformity and the gaps in the record - Eduqas A-Level Geology

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.

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.

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.

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

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.

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

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

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