Unconformity Types and Examples

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Diagram of Unconformity Types

Simple diagrams and explanations
Unconformity Types and Examples Symbols on the left are for Pennsylvanian age (bottom) and Triassic age (top), separated by at least 50 million years. Diagram (c) 2011 Andrew Alden, licensed to About.com ( fair use policy)

Unconformities are breaks or gaps in the geologic record, as shown by the arrangement of sedimentary (stratigraphic) features in the rock. This gallery shows the basic unconformity types recognized by U.S. geologists plus photos of examples from outcrops. This article gives more details about unconformities.

Here are the four main unconformity types. British geologists classify the disconformity and paraconformity as nonsequences because the rock beds are conformable, that is, parallel. Learn more in this article.

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Angular Unconformity, Pebble Beach, California

Covered and exhumed
Unconformity Types and Examples. Photo (c) 2010 Andrew Alden, licensed to About.com ( fair use policy

Strongly tilted sedimentary rocks have been eroded and covered with much younger flat-lying sediments. Wave erosion of the young layers has exhumed the old erosion surface.

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Angular Unconformity, Carlin Canyon, Nevada

Tilt, tilt, tilt
Unconformity Types and Examples From the Nevada Geological Attractions Gallery. Photo courtesy Ron Schott, all rights reserved

This famous unconformity involves two rock units of Mississippian (left) and Pennsylvanian (right) ages, both of which are now tilted.

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Angular Unconformity in Conglomerate

Closeup of an unconformity
Unconformity Types and Examples. Photo (c) 2011 Andrew Alden, licensed to About.com ( fair use policy

The tilted pebbles in the lower half mark the bedding plane in this conglomerate. The erosion surface is covered by finer material laid down parallel to the photo frame. The time gap represented here may be very short.

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Nonconformity, Red Rocks, Colorado

Misnamed but still great
Unconformity Types and Examples From the Red Rocks of Red Rocks Gallery. Photo (c) 2007 Andrew Alden, licensed to About.com ( fair use policy

This widespread feature is known as the Great Unconformity, but the Precambrian rock on the right is gneiss overlain by Permian sandstone, making it a nonconformity. It dramatically represents a billion-year time gap.

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Alden, Andrew. "Unconformity Types and Examples." ThoughtCo, Jan. 13, 2017, thoughtco.com/unconformity-types-and-examples-4123229. Alden, Andrew. (2017, January 13). Unconformity Types and Examples. Retrieved from https://www.thoughtco.com/unconformity-types-and-examples-4123229 Alden, Andrew. "Unconformity Types and Examples." ThoughtCo. https://www.thoughtco.com/unconformity-types-and-examples-4123229 (accessed December 12, 2017).