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Unconformities

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An unconformity are 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 called an unconformity because the ages of the layers of rock that are abutting each other are discontinuous. 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.

Unconformities

Disconformity

 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 unconformities. Rocks below the unconformities may or may not parallel the unconformities, 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 unconformities 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.

Unconformities Form

Nonconformities are due to relative changes in sea level over time. Wave wear corrodes the materials exposed on the coastline and smoothes surfaces. At thousands to million years of scale, the coastline can move in all regions. Removes materials exposed to erosion, wave and current. New (younger) materials may be deposited on the engraved surface.Shallow seas may flood in and then withdrawal repeatedly.Long-lasting transgressions can erode away entire mountain ranges with enough time.

A transition occurs when a coastline migrates towards land as the sea level (or lake level) rises.

A regression  occurs when a coastline migrates towards the sea when the coast falls to sea level (or lake level).

Sea-level changes may result from regional uplifts or global sea-level changes, such as the formation or melting of continental glaciers. Regardless of the reason for the change of sea level, when the sea level falls, sediments erode from exposed soils. When the sea level rises, sediments are typically deposited in shallow continental shelves or coastal plains, such as in low, swampy areas, in quiet water environments.

Cite this article as: Mahmut Mat, "Unconformities", in Geology Science, [online] Accessed 18th July 2019, Available at: http://geologyscience.com/methods-of-geology/unconformities/
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