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Unconformities

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What is Unconformities ?

An unconformity is contact between two rock units. Unconformities are typically buried erosional surfaces that can represent a break in the geologic record of hundreds of millions of years or more. It it called an unconformity because the ages of the layers of rock that are abutting each other are discontinuous at the unconformity. An expected age of layer or layers of rock is/are missing due to the erosion; and, some period in geologic time is not represented.

Unconformity Types

There are three types of 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.

Disconformity between massive Coconino Sandstone and thinner bedded Hermit Shale, Grand Canyon
Disconformity between massive Coconino Sandstone and thinner bedded Hermit Shale, Grand Canyon

Nonconformity

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.

Nonconformity in the Grand Canyon
Nonconformity in the Grand Canyon

Angular Unconformity

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

Angular Unconformity at Telheiro Beach, Portugal
Angular Unconformity at Telheiro Beach, Portugal

Buttress Unconformity

A buttress unconformity (also called onlap unconformity) occurs where beds of the younger sequence were deposited in a region of significant predepositional topography. Imagine a shallow sea in which there are islands composed of older bedrock. When sedimentation occurs in this sea, the new horizontal layers of strata terminate at the margins of the island. Eventually, as the sea rises, the islands are buried by sediment. But along the margins of the island, the sedimentary layers appear to be truncated by the unconformity. Rocks below the unconformity may or may not parallel the unconformity, depending on the pre-unconformity structure. Note that a buttress unconformity differs from an angular unconformity in that the younger layers are truncated at the unconformity surface

A buttress unconformity (contact at red arrow) is one in which the younger, overlying rocks are cut by the contact. This relationship occurs because the younger sediments are deposited against the older rocks as they stood out in topographic relief.
Buttress Unconformity, Northern Arizona.

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