GEO 101 Lecture Notes - Lecture 19: Unconformity, Erosion Surface, Metamorphic Rock
23 Jun 2018
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Unconformities
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.
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
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Document Summary
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. 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 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.
