Unconformity, Disconformities and Nonconformities in Structural Geology

What is Unconformity, Disconformities and Nonconformities in Structural Geology?

Unconformity

An unconformity is a contact between two rock units in which the upper unit is usually much younger than the lower unit . Unconformities are typically buried erosional surfaces that can represent a break in the geologic record of hundreds of millions of years or more . 

For example, the contact between a 400-million-year-old sandstone that was deposited by a rising sea on a weathered bedrock surface that is 600 million years old is an unconformity that represents a time hiatus of 200 million years. 

The sediment and/or rock that was deposited directly on the bedrock during that 200-million-year span was eroded away, leaving the "basement" surface exposed. 

There are three kinds of unconformities: 

disconformities, nonconformities, and angular unconformities

• Disconformities 

Disconformities are usually erosional contacts that are parallel to the bedding planes of the upper and lower rock units. 

Since disconformities are hard to recognize in a layered sedimentary rock sequence, they are often discovered when the fossils in the upper and lower rock units are studied. 

A gap in the fossil record indicates a gap in the depositional record, and the length of time the disconformity represents can be calculated . 

Disconformities are usually a result of erosion but can occasionally represent periods of nondeposition

Figure 1 Disconformity 

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• Nonconformities

A nonconformity  is the contact that separates a younger sedimentary rock unit from an igneous intrusive rock or metamorphic rock unit. 

A nonconformity suggests that a period of long-term uplift, weathering, and erosion occurred to expose the older, deeper rock at the surface before it was finally buried by the younger rocks above it. 

A nonconformity is the old erosional surface on the underlying rock.

Figure 2 Nonconformity

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• Angular unconformities 

An angular unconformity  is the contact that separates a younger, gently dipping rock unit fro m older underlying rocks that are tilted or deformed layered rock. 

Figure 3 Angular unconformities 

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The contact is more obvious than a disconformity because the rock units are not parallel and at first appear cross-cutting. 

Angular unconformities generally represent a longer time hiatus than do disconformities because the underlying rock had usually been metamorphosed, uplifted, and eroded before the upper rock unit was deposited.

Ref-Cliffs Quick Review Physical Geology by Mark J. Crawford, MS.