Unconformities—erosional or non-depositional gaps—are abound in the geologic record. These erosional surfaces separate a lower, older strata from a younger, upper one (unless the sequence has been overturned). An unconfomity will typically form when an older layer is subject to a period of erosion before the deposition of new sediments. For example, the “Great Unconformity” of the Grand Canyon represents a span of time of over one billion years that is missing from the rock record!
Here are the primary types of unconformities:
Disconformity | A disconformity is a unconformity surface between two units that have parallel bedding planes. The unit in purple is first deposited and then experiences a period of erosion. Afterwards, the younger blue unit is deposited on top of this erosional surface, parallel to the surface and unit below it.
Angular unconformity | An angular unconformity differs from a disconformity because the upper and lower units are not parallel to one another. In this case, the purple layer is deposited and then tilted away from horizontal, usually due to regional tectonic activity. These tilted purple bedding planes experience an episode of erosion, and then the younger blue unit is deposited parallel to the erosional surface and at an angle to the purple unit below.
Nonconformity | Unlike a disconformity or angular unconformity, a nonconformity exists between sedimentary rocks and metamorphic/igneous rocks. The red igneous/metamorphic unit would be emplaced first and then experience erosion. The blue sedimentary layer is then later deposited parallel to the erosional surface.
Paraconformity | A paraconformity is also known as a pseudoconformity or a nondepositional unconformity because there is no apparent erosional surface. In this case, there would be a period of non-deposition, but it resembles a simple bedding plane and thus can be difficult to detect.
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