Historical Geology, Unconformities

In this article we shall look at the various types of unconformity in the geological record, and discuss why they exist and how they can be recognized.

What are unconformities, and why do they exist?

As we mentioned in the article on the geological column, we do not expect the fossil sequence to be complete in any given location, because there will be times when no sediment is deposited in that location, and/or times when it is destroyed by erosion after deposition.

Such an episode will show up in the geological record as an unconformity: a surface between successive strata representing a period of erosion or of no deposition. These come in several varieties, listed below.

What do unconformities look like?

Origin of an angular unconformity. (Image: Actualist, CC BY-SA 3.0)

In an angular unconformity the underlying beds meet the overlying beds at an angle. The diagram to the right shows the stages of the process.

First, sediment is laid down. Then tectonic events destroy its original horizontality, in this case by creating an upward fold. Then an erosional surface is produced, truncating the beds. Then more sediment is laid down in horizontal beds on top of the erosional surface. In the example in the diagram this has produced two sets of angular unconformities, one at each side of the diagram.

In a disconformity, the same thing happens, except that the underlying beds are not distorted by tectonic events, so that the first and second collection of beds lie parallel to one another rather than meeting at an angle, but they are still separated by an erosional surface.

A nonconformity is like an disconformity, except that the underlying rock is igneous or metamorphic rather than sedimentary. The presence of an erosional surface indicates a time when this basement rock was exposed to weathering and erosion, and so cannot have been protected by a blanket of sediment.

Finally, consider what would happen in a location where sediment is deposited, then it ceases, and then erosion does not take place, and then deposition starts again. This would produce what is known as a paraconformity. What would that look like? There would be no meeting of beds at an angle, there would be no erosional surface, there would just be beds of sediment lying on top of more beds of sediment.

Except that if the period of non-deposition lasted for any significant amount of time we would see a sudden jump in the faunal succession: where the geological column shows fossils in order (for example) P, Q, R, S, T, U, (counting from the bottom upwards) then at the location of the paraconformity we would see P, Q, T, U, where P and Q correspond to the first episode of deposition, T and U correspond to the second episode of deposition, and the missing fossils R and S correspond to the time at which no deposition was taking place. Note that if this jump forward in the faunal succession was just because there was no fauna around to be deposited, then we would see non-fossil-bearing sediment between Q and T; but instead we see nothing at all.

Obviously we will see a similar jump in the faunal succession in the case of angular unconformities and disconformities, but in the case of a paraconformity this is all that there is to see. This makes the identification of paraconformities less blatantly obvious than the identification of other unconformities, but since they look just like what we should see in locations where deposition stopped for a while and then restarted, it is reasonable to conclude that that's what they are.

Note that this only serves to identify sufficiently long periods of non-deposition. In cases in which the interruption of deposition only lasted for a small period of time then all we would see would at most be a bedding plane, and perhaps not even that.

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