Unconformity
An unconformity is a buried
The rocks above an unconformity
Types
Disconformity
A disconformity is an unconformity between parallel
Nonconformity
A nonconformity exists between sedimentary rocks and
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.[6] The whole sequence may later be deformed and tilted by further orogenic activity. A typical case history is presented by the Briançonnais realm (Swiss and French Prealps) during the Jurassic.[7][8]
Angular unconformities can occur in
Paraconformity
A paraconformity is a type of unconformity in which the sedimentary layers above and below the unconformity are parallel, but there is no obvious erosional break between them. A break in sedimentation is indicated, for example, by fossil evidence. It is also called nondepositional unconformity or pseudoconformity.[9][10] Short paraconformities are called diastems.[11]
Buttress unconformity
A buttress unconformity also known as onlap unconformity, occurs when younger bedding is deposited against older strata thus influencing its bedding structure.[12]
Blended unconformity
A blended unconformity is a type of disconformity or nonconformity with no distinct separation plane or contact, sometimes consisting of soils, paleosols, or beds of pebbles derived from the underlying rock.[13]
Gallery
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Disconformity atHorni Pocernice, Czech Republic
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Disconformity (at the hammer) between underlying Mississippian Borden Formation and overlying Pennsylvanian Sharon Conglomerate, near Jackson, Ohio
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There is a billion-year gap in the geologic record where this 500-million-year-old dolomite nonconformably overlies 1.5-billion-year-old rhyolite, near Taum Sauk Hydroelectric Power Station, Missouri.
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Nonconfirmity at Ratssteinbruch near Dresden, Germany
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Hutton's angular unconformity at Siccar Point where Famennian age (371–359 Ma) Devonian Old Red Sandstone overlies Llandovery age (444–433 Ma) Silurian greywacke[14]
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Angular unconformity of Triassic rocks overlying steeply-tilted Carboniferous rocks at Praia do Telheiro, Portugal
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Angular unconformity between the underlying Dockum Group and the overlying Exeter Sandstone at Steamboat Butte in the valley of the Dry Cimmarron, New Mexico
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Angular unconformity in Jingtai County, China
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Angular unconformity in pyroclastic rock layers erupted by Chimborazo volcano, Ecuador
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Geological unconformity, Camelback mountain, Arizona, showing deposition of Chattian sandstone (right) on Precambrian granite (left).
See also
References
- ^ Hutton's Unconformity Archived 2015-09-24 at the Wayback Machine
- ^ Keith Montgomery (2003). "Siccar Point and Teaching the History of Geology" (PDF). University of Wisconsin. Retrieved 2015-03-16.
- ISBN 9780495011484.
- ISBN 0131547283.
- ISBN 0-13-285890-8.
- ^ Boggs 2006, pp. 404–405.
- ^ Septfontaine M. (1984): Le Dogger des Préalpes médianes suisses et françaises - stratigraphie, évolution paléogéographique et paléotectonique.- Mém. Soc. Helv. Sci. Nat., vol. 97, 121 p. (Birkhäuser éd.)
- ^ Septfontaine M. (1995): Large scale progressive unconformities in Jurassic strata of the Prealps South of lake Geneva: interpretation as synsedimentary inversion structures. Paleotectonic implications. Eclogae geol. Helv., 88:3 553–576.
- ISBN 9780080473987.
- ^ Encyclopedia of Science and Technology: An International Reference Work, Volume 14. McGraw-Hill. 1966. p. 192.
- ^ Boggs 2006, p. 401.
- ISBN 9783662039120.
- ISBN 9780922152766.
- ^ "Redheugh Sandstone Formation". BGS Lexicon of Names Rock Units. British Geological Survey. Retrieved 5 February 2023.
Further reading
- U.S. Bureau of Mines Dictionary of Mining, Mineral, and Related Terms published on CD-ROM in 1996.