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Unconformities
At this location you can see an angular unconformity. Unconformities are gaps in the geologic record that may indicate episodes of crustal deformation, erosion, and sea level variations. They are a feature of stratified rocks, and are therefore usually found in sediments (but may also occur in stratified volcanics). They are surfaces between two rock bodies that constitute a substantial break (hiatus) in the geologic record (sometimes people say inaccurately that "time" is missing). Unconformities represent times when deposition stopped, an interval of erosion removed some of the previously deposited rock, and finally deposition was resumed.
Commonly there are a few types of unconformities distinguished by geologists:
Disconformity
A disconformity is an unconformity between parallel layers of sedimentary rocks which represents a period of erosion or non-deposition. Disconformities are marked by features of subaerial erosion. This type of erosion can leave channels and paleosols in the rock record. A paraconformity is a type of disconformity in which the separation is a simple bedding plane with no obvious buried erosional surface.
Nonconformity
A nonconformity exists between sedimentary rocks and metamorphic or igneous rocks when the sedimentary rock lies above and was deposited on the pre-existing and eroded metamorphic or igneous rock. Namely, if the rock below the break is igneous or has lost its bedding due to metamorphism, the plane of juncture is a nonconformity.
Angular Unconformity
An angular unconformity is an unconformity where horizontally parallel strata of sedimentary rock are deposited on tilted and eroded layers, producing an angular discordance with the overlying horizontal layers. The whole sequence may later be deformed and tilted by further orogenicactivity. A typical case history is presented by the paleotectonic evolution of the Briançonnais realm (Swiss and French Prealps) during the Jurassic.
Paraconformity
A paraconformity is a type of unconformity in which strata are parallel; there is no apparent erosion and the unconformity surface resembles a simple bedding plane. It is also called nondepositional unconformity or pseudoconformity.
To earn credit for this Earthcache, please email me with the answers to these questions. Please do not include answers in your visit log.
- Are the layers parallel?No
- How many layers do you see?2 or more
- What type of unconformity are you looking at?Angular unconformity
References
- Monroe J.S., Wicander R. & Hazlett R.W. (2007). Physical geology: exploring the Earth. Cengage Learning. p. 267.
- Dictionary of Geological Terms. New York: Dolphin Books, 1962.
- University of Indiana. (2016). Retrieved May 30, 2017, from http://www.indiana.edu/~geol105b/images/gaia_chapter_6/unconformities.htm
- Neuendorf K.K.E. (2005). Glossary of Geology. p. 73. ISBN 9780922152766.