An unconformity is a buried erosional or non-depositional surface separating two rock masses or 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 sedimentarygeologic 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.[1][2]
The rocks above an unconformity are younger than the rocks beneath (unless the sequence has been overturned). An unconformity represents 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.
A disconformity is an unconformity between parallel layers of sedimentary rocks which represents a period of erosion or non-deposition.[3] Disconformities are marked by features of subaerial erosion. This type of erosion can leave channels and paleosols in the rock record.[4]
NonconformityEdit
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, then the plane of juncture is a nonconformity.[5]
Angular unconformityEdit
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 ash fall layers of pyroclastic rock deposited by volcanoes during explosive eruptions. In these cases, the hiatus in deposition represented by the unconformity may be geologically very short – hours, days or weeks.
ParaconformityEdit
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 unconformityEdit
A buttress unconformity also known as onlap unconformity, occurs when younger bedding is deposited against older strata thus influencing its bedding structure.[12]
Blended unconformityEdit
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]
Angular unconformity between the underlying Dockum Group and the overlying Exeter Sandstone at Steamboat Butte in the valley of the Dry Cimmarron, New Mexico
^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.
^Catuneanu O. (2006). Principles of Sequence Stratigraphy. Elsevier. p. 15. ISBN9780080473987.
^Encyclopedia of Science and Technology: An International Reference Work, Volume 14. McGraw-Hill. 1966. p. 192.
^Groshong R. H. (2013). 3-D Structural Geology: A Practical Guide to Surface and Subsurface Map. Springer Science & Business Media. ISBN9783662039120.
^Neuendorf K.K.E. (2005). Glossary of Geology. Springer. p. 73. ISBN9780922152766.
^"Redheugh Sandstone Formation". BGS Lexicon of Names Rock Units. British Geological Survey. Retrieved 5 February 2023.
The Wikibook Historical Geology has a page on the topic of: Unconformities
Wikimedia Commons has media related to Unconformities.
Further readingEdit
U.S. Bureau of Mines Dictionary of Mining, Mineral, and Related Terms published on CD-ROM in 1996.
October 23, 2023
unconformity, unconformity, buried, erosional, depositional, surface, separating, rock, masses, strata, different, ages, indicating, that, sediment, deposition, continuous, general, older, layer, exposed, erosion, interval, time, before, deposition, younger, l. An unconformity is a buried erosional or non depositional surface separating two rock masses or 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 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 1 2 Hutton s Unconformity at Jedburgh Scotland illustrated by John Clerk in 1787 and photographed in 2003 The rocks above an unconformity are younger than the rocks beneath unless the sequence has been overturned An unconformity represents 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 Contents 1 Types 1 1 Disconformity 1 2 Nonconformity 1 3 Angular unconformity 1 4 Paraconformity 1 5 Buttress unconformity 1 6 Blended unconformity 2 Gallery 3 See also 4 References 5 Further readingTypes EditDisconformity Edit nbsp DisconformityA disconformity is an unconformity between parallel layers of sedimentary rocks which represents a period of erosion or non deposition 3 Disconformities are marked by features of subaerial erosion This type of erosion can leave channels and paleosols in the rock record 4 Nonconformity Edit nbsp NonconformityA 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 then the plane of juncture is a nonconformity 5 Angular unconformity Edit nbsp Angular unconformityAn 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 Brianconnais realm Swiss and French Prealps during the Jurassic 7 8 Angular unconformities can occur in ash fall layers of pyroclastic rock deposited by volcanoes during explosive eruptions In these cases the hiatus in deposition represented by the unconformity may be geologically very short hours days or weeks Paraconformity Edit nbsp ParaconformityA 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 Edit 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 Edit 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 Edit nbsp Disconformity at Horni Pocernice Czech Republic nbsp Disconformity at the hammer between underlying Mississippian Borden Formation and overlying Pennsylvanian Sharon Conglomerate near Jackson Ohio nbsp 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 nbsp Nonconfirmity at Ratssteinbruch near Dresden Germany nbsp 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 nbsp Angular unconformity of Triassic rocks overlying steeply tilted Carboniferous rocks at Praia do Telheiro Portugal nbsp Angular unconformity between the underlying Dockum Group and the overlying Exeter Sandstone at Steamboat Butte in the valley of the Dry Cimmarron New Mexico nbsp Angular unconformity in Jingtai County China nbsp Angular unconformity in pyroclastic rock layers erupted by Chimborazo volcano Ecuador nbsp Geological unconformity Camelback mountain Arizona showing deposition of Chattian sandstone right on Precambrian granite left See also EditGreat UnconformityReferences Edit 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 Monroe J S Wicander R amp Hazlett R W 2007 Physical geology exploring the Earth Cengage Learning p 267 ISBN 9780495011484 Boggs Sam 2006 Principles of sedimentology and stratigraphy 4th ed Upper Saddle River N J Pearson Prentice Hall p 405 ISBN 0131547283 Stokes W Lee 1982 Essentials of Earth History 4th Edition Prentice Hall Inc p 65 ISBN 0 13 285890 8 Boggs 2006 pp 404 405 Septfontaine M 1984 Le Dogger des Prealpes medianes suisses et francaises stratigraphie evolution paleogeographique et paleotectonique Mem Soc Helv Sci Nat vol 97 121 p Birkhauser ed 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 Catuneanu O 2006 Principles of Sequence Stratigraphy Elsevier p 15 ISBN 9780080473987 Encyclopedia of Science and Technology An International Reference Work Volume 14 McGraw Hill 1966 p 192 Boggs 2006 p 401 Groshong R H 2013 3 D Structural Geology A Practical Guide to Surface and Subsurface Map Springer Science amp Business Media ISBN 9783662039120 Neuendorf K K E 2005 Glossary of Geology Springer p 73 ISBN 9780922152766 Redheugh Sandstone Formation BGS Lexicon of Names Rock Units British Geological Survey Retrieved 5 February 2023 nbsp The Wikibook Historical Geology has a page on the topic of Unconformities nbsp Wikimedia Commons has media related to Unconformities Further reading EditU S Bureau of Mines Dictionary of Mining Mineral and Related Terms published on CD ROM in 1996 Retrieved from https en wikipedia org w index php title Unconformity amp oldid 1174000612, wikipedia, wiki, book, books, library,
