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Unconformities

© Geoff Habiger

The second of these obvious unconformities is called a nonconformity. In a nonconformity, the rocks below the erosional surface are either igneous or metamorphic rocks. The rocks above the erosional surface are sedimentary rocks laid down above the igneous and metamorphic rocks. It is important not to misinterpret an area where igneous rocks have intruded on existing sedimentary sequences. The nonconformity has a clear erosional break between the igneous or metamorphic rocks and the sedimentary rocks above them.

The other two types of unconformities are not so easy to see in the field. A disconformity occurs where an erosional surface separates sequences of rock that are parallel to each other. In this case, the unconformity represents erosion, but there were never any structural stresses to tilt or fold the rocks, and younger sediments later buried them.

The last type of unconformity is called a paraconformity. In a paraconformity, the sediments are parallel and there is no obvious erosional surface. The unconformity is only recognized by other evidence, such as fossils located in the upper layers that are different from the fossils in lower layers of rock. The fossils must be different enough (such as finding trilobites of Cambrian age in the lower layer, and trilobites of Devonian age in the successive layers) to show that a large gap in the geologic record occurred between the depositions of the sedimentary layers. There is some discussion among geologists to eliminate the term paraconformity since it is so similar to a disconformity. The term is still used for the time being.

Unconformities are an important part of the stratigraphic record. Even though they represent periods of non-deposition and erosion, this in itself is important evidence in the interpretation of the sequence of rocks for a particular area. Knowing how to interpret and identify unconformities can help a geologist piece together the history of the stratigraphic record, and how that sequence of rocks relates to other sequences. Next week I will cover how time and rock units are used in stratigraphy.

Glossary:

Angular unconformity: The sequences of rocks beneath the unconformity are tilted with respect to the rock sequences above the unconformity.
Disconformity: An unconformity where the erosional surface separates sequences of rock that are parallel to each other.
Nonconformity: An unconformity where the erosional surface truncates either igneous or metamorphic rocks and is covered by sediment.
Paraconformity: Unconformity where the sediments are parallel and there is no obvious erosional surface.