Editing Unconformity

{{Short description|Rock surface indicating a gap in the geological record}}
[[File:Hutton Unconformity , Jedburgh.jpg|thumb|''[[Hutton's Unconformity]]'' at [[Jedburgh]], [[Scotland]], illustrated by [[John Clerk of Eldin|John Clerk]] in 1787 and photographed in 2003.]]
An '''unconformity''' is a buried [[erosion surface|erosional]] or non-depositional surface separating two [[Rock (geology)|rock]] masses or [[Stratum|strata]] of different ages, indicating that [[sediment]] deposition was not continuous. In general, the older layer was exposed to erosion for an interval of time before deposition of the younger layer, but the term is used to describe any break in the [[Sedimentary rocks|sedimentary]] [[geologic record]].  The significance of angular unconformity (see below) was shown by [[James Hutton]], who found examples of [[Hutton's Unconformity]] at [[Jedburgh]] in 1787 and at [[Siccar Point]] in [[Berwickshire]] in 1788, both in Scotland.<ref>[http://www.snh.org.uk/publications/on-line/geology/elothian_borders/hutton.asp Hutton's Unconformity] {{Webarchive|url=https://web.archive.org/web/20150924104617/http://www.snh.org.uk/publications/on-line/geology/elothian_borders/hutton.asp |date=2015-09-24 }}</ref><ref name=monty>{{cite web |url=http://nagt.org/files/nagt/jge/abstracts/Montgomery_v51n5.pdf |title=Siccar Point and Teaching the History of Geology |access-date=2015-03-16 |author=Keith Montgomery |year=2003 |publisher=University of Wisconsin }}</ref>

The rocks above an unconformity [[Law of superposition|are younger than]] the rocks beneath (unless the sequence has been overturned). An unconformity represents [[Geologic time|time]] during which no sediments were preserved in a region or were subsequently eroded before the next deposition. The local record for that time interval is missing and geologists must use other clues to discover that part of the geologic history of that area.  The interval of geologic time not represented is called a '''hiatus'''. It is a kind of [[relative dating]].

==Types==

===Disconformity===
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[[File:Disconformity.jpg|thumb|120px|Disconformity]]
A '''disconformity''' is an unconformity between parallel [[stratum|layers]] of [[sedimentary rocks]] which represents a period of erosion or non-deposition.<ref name="Monroe">{{cite book | url=https://books.google.com/books?id=iXfhFnoQBQ0C&pg=PA267 | title=Physical geology: exploring the Earth | publisher=Cengage Learning | author=Monroe J.S., Wicander R. & Hazlett R.W. | year=2007 | pages=267 | isbn=9780495011484}}</ref> Disconformities are marked by features of [[subaerial]] erosion. This type of erosion can leave channels and [[paleosols]] in the rock record.<ref>{{cite book |last1=Boggs |first1=Sam |title=Principles of sedimentology and stratigraphy |date=2006 |publisher=Pearson Prentice Hall |location=Upper Saddle River, N.J. |isbn=0131547283 |edition=4th |page=405}}</ref>

===Nonconformity===
[[File:Nonconformity.jpg|thumb|120px|Nonconformity]]
A nonconformity exists between sedimentary rocks and [[metamorphic rocks|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, then the plane of juncture is a nonconformity.<ref>{{cite book|last=Stokes|first=W. Lee|title=Essentials of Earth History 4th Edition|publisher=Prentice Hall, Inc.|year=1982|isbn=0-13-285890-8|page=[https://archive.org/details/essentialsofeart0000stok/page/65 65]|url-access=registration|url=https://archive.org/details/essentialsofeart0000stok/page/65}}</ref>

===Angular unconformity===
[[File:Angular unconformity.jpg|thumb|120px|Angular unconformity]]
An angular unconformity is an unconformity where horizontally parallel [[stratum|strata]] of sedimentary rock are deposited on tilted and eroded layers, producing an angular discordance with the overlying horizontal layers.{{sfn|Boggs|2006|pp=404-405}}  The whole sequence may later be deformed and tilted by further [[orogeny|orogenic]] activity. A typical case history is presented by the Briançonnais realm (Swiss and French Prealps) during the Jurassic.<ref>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.)</ref><ref>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.</ref>
<!--[[File:Briançonnais modèles 7F.jpg|thumb|alt= modèles palinspastiques|Block diagrams and profiles of Palinspastic Alps (Briançon sl) Middle Jurassic]][[File:Discordances successives Jurassique Préalpes.jpg|thumb|Successive angular unconformities and fans on the margin N-Briançon, affecting 650m of sediment during 50 Ma; Dogger-Malm, Chablais Alps (Suisse, France).]]-->

Angular unconformities can occur in [[ash fall]] layers of [[pyroclastic rock]] deposited by volcanoes during [[explosive eruption]]s. In these cases, the hiatus in deposition represented by the unconformity may be geologically very short – hours, days or weeks.

===Paraconformity===
[[File:Paraconformity.jpg|thumb|120px|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.<ref name="Catuneanu">{{cite book | url=https://books.google.com/books?id=8j-6UTXHmDkC&pg=PA15 | title=Principles of Sequence Stratigraphy | publisher=Elsevier | author=Catuneanu O. | year=2006 | pages=15 | isbn=9780080473987}}</ref><ref>{{cite book | url=https://books.google.com/books?id=irgrAQAAMAAJ&q=pseudoconformity | title=Encyclopedia of Science and Technology: An International Reference Work, Volume 14 | publisher=McGraw-Hill | year=1966 | pages=192}}</ref> Short paraconformities are called [[diastem]]s.{{sfn|Boggs|2006|p=401}}

===Buttress unconformity===
A buttress unconformity, also known as onlap unconformity, occurs when younger bedding is deposited against older strata thus influencing its bedding structure.<ref name="Groshong">{{cite book | url=https://books.google.com/books?id=pvG5BAAAQBAJ&q=%22buttress+unconformity%22&pg=SA1-PA7 | title=3-D Structural Geology: A Practical Guide to Surface and Subsurface Map | publisher=Springer Science & Business Media | author=Groshong R. H. | year=2013 | isbn=9783662039120}}</ref>

===Blended unconformity===
A blended unconformity is a type of disconformity or nonconformity with no distinct separation plane or contact, sometimes consisting of soils, [[paleosol]]s, or beds of pebbles derived from the underlying rock.<ref name="Neuendorf">{{cite book | url=https://books.google.com/books?id=yD79FqfECCYC&pg=PA278 | title=Glossary of Geology | author=Neuendorf K.K.E. | year=2005 | pages=73 | publisher=Springer | isbn=9780922152766}}</ref>

==Gallery==
<gallery>
File:Disconformity Horni Pocernice.jpg|''Disconformity'' at [[Horni Pocernice]], Czech Republic
File:Borden-Sharon unconformity.jpg|''Disconformity'' (at the hammer) between underlying [[Mississippian (geology)|Mississippian]] [[Borden Formation]] and overlying Pennsylvanian Sharon Conglomerate, near [[Jackson, Ohio]]
File:Taum Sauk precambrian-cambrian unconformity.jpg|There is a billion-year gap in the geologic record where this 500-million-year-old [[dolomite (rock)|dolomite]] ''nonconformably'' overlies 1.5-billion-year-old [[rhyolite]], near [[Taum Sauk Hydroelectric Power Station]], Missouri.
File:Ratssteinbruch Dresden-Doelzschen.jpg|''Nonconformity'' at Ratssteinbruch near [[Dresden]], Germany
File:Siccar Point red capstone closeup.jpg|Hutton's ''angular unconformity'' at [[Siccar Point]] where [[Famennian]] age (371–359 Ma) Devonian [[Old Red Sandstone]] overlies [[Llandovery Epoch|Llandovery]] age (444–433 Ma) Silurian [[greywacke]]<ref>{{cite web |title=Redheugh Sandstone Formation |url=https://webapps.bgs.ac.uk/lexicon/lexicon.cfm?pub=REMU |website=BGS Lexicon of Names Rock Units |publisher=British Geological Survey |access-date=5 February 2023}}</ref>
File:Angular Unconformity at Praia do Telheiro in Algarve in Portugal.png|''Angular unconformity'' of [[Triassic]] rocks overlying steeply-tilted [[Carboniferous]] rocks at [[Vila do Bispo|Praia do Telheiro]], Portugal
File:Steamboat Butte New Mexico.jpg|''Angular unconformity'' between the underlying [[Dockum Group]] and the overlying [[Exeter Sandstone]] at Steamboat Butte in the valley of the Dry Cimmarron, New Mexico
File:Winkeldiskordanz.JPG|Angular unconformity in [[Jingtai County]], China
File:Tephra Layers at Chimborazo Volcano in Ecuador.jpg|Angular unconformity in [[pyroclastic rock]] layers erupted by [[Chimborazo]] volcano, Ecuador
Image:GP_0147_%282%29.JPG|Geological unconformity, Camelback mountain, Arizona, showing deposition of Chattian sandstone (right) on Precambrian granite (left).
</gallery>

==See also==
* [[Great Unconformity]]

==References==
{{Reflist}}
{{Wikibooks|Historical Geology|Unconformities}}
{{Commons category|Unconformities}}

==Further reading==
* U.S. Bureau of Mines Dictionary of Mining, Mineral, and Related Terms published on CD-ROM in 1996.

{{Authority control}}

[[Category:Historical geology]]
[[Category:Stratigraphy]]
[[Category:Unconformities|+]]