Unconformity

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, 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 1788.[1][2]

The rocks above an unconformityare younger than the rocks beneath (unless the sequence has been overturned). An unconformity represents time during which no sediments were preserved in a region. 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 ahiatus .

Hutton's Unconformity at Jedburgh, Scotland, Contents illustrated by John Clerk in 1787 and photographed in Types of unconformities 2003. Disconformity Nonconformity Angular unconformity Paraconformity Buttress unconformity Blended unconformity Gallery References Further reading

Types of unconformities

Disconformity 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] A paraconformity is a type of disconformity in which the separation is a simple bedding plane with no obvious buried erosional surface.[5] Disconformity

Nonconformity A nonconformity exists between sedimentary rocks andmetamorphic 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.[6]

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 furtherorogenic activity. A typical case history is presented by the paleotectonic evolution of the Briançonnais realm (Swiss and French Prealps) during the Jurassic.[7][8] Nonconformity

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.[9][10] Short paraconformities are calleddiastems .

Buttress unconformity Angular unconformity A buttress unconformity occurs when younger bedding is deposited against older strata thus influencing its bedding structure.[11]

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 Paraconformity rock.[12]

Gallery

There is a billion-year The Taconic Eemian disconformity in Hutton's angular gap in the geologic Unconformity near a fossil coral reef on unconformity at Siccar record where this 500- Catskill, New York. This Great Inagua, The Point where 345-million- million-year-old dolomite angular unconformity Bahamas. Foreground year-old Devonian Old nonconformably overlies separates the Austin shows corals truncated Red Sandstone overlies 1.5-billion-year-old Glen Formation by erosion; behind the 425-million-year-old rhyolite, near Taum Sauk (Ordovician) from the geologist is a post- Silurian greywacke.[13] Hydroelectric Power overlying Rondout erosion coral pillar which Station, Missouri. Formation (Silurian) and grew on the Manlius Formation disconformity after sea (Devonian). level rose again.

Disconformity with the Nonconformity between Disconformity (at the Angular unconformity of Lower Cretaceous the Pennsylvanian hammer) between Shawangunk Formation Edwards Formation Fountain Formation (left) underlying Mississippian overlying Martinsburg overlying a Lower and Precambrian gneiss Borden Formation and Formation at railroad cut Permian limestone; (right) at Red Rocks overlying Pennsylvanian near Otisville, New York hiatus is about 165 Park, near Golden, Sharon Conglomerate, million years; Texas. Colorado near Jackson, Ohio

References

1. Hutton's Unconformity (http://www.snh.org.uk/publications/on-line/geology/elothian_borders/hutton.asp) Archived (htt ps://web.archive.org/web/20150924104617/http://www.snh.org.uk/publications/on-line/geology/elothian_borders/hutt on.asp) 2015-09-24 at the Wayback Machine. 2. Keith Montgomery (2003)."Siccar Point and Teaching the History of Geology" (http://nagt.org/files/nagt/jge/abstracts/ Montgomery_v51n5.pdf) (pdf). University of Wisconsin. Retrieved 2015-03-16. 3. Monroe J.S., Wicander R. & Hazlett R.W. (2007). Physical geology: exploring the Earth (https://books.google.com/bo oks?id=iXfhFnoQBQ0C&pg=PA267). Cengage Learning. p. 267.ISBN 9780495011484. 4. Disconformity at EPGeology (http://www.epgeology.com/sedimentology-f19/difference-between-disconformity-and-un conformity-t65.html) 5. Dictionary of Geological Terms. New York: Dolphin Books, 1962. 6. Stokes, W. Lee (1982). Essentials of Earth History 4th Edition. Prentice Hall,Inc. p. 65. ISBN 0-13-285890-8. 7. 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.) 8. 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. 9. Catuneanu O. (2006). Principles of Sequence Stratigraphy (https://books.google.com/books?id=8j-6UTXHmDkC&pg =PA15). Elsevier. p. 15. ISBN 9780080473987. 10. Encyclopedia of Science and Technology: An International Reference Work, Volume 14 (https://books.google.com/b ooks?id=irgrAQAAMAAJ&dq=paraconformity+pseudoconformity&focus=searchwithinvolume&q=pseudoconformity). McGraw-Hill. 1966. p. 192. 11. Groshong R. H. (2013). 3-D Structural Geology: A Practical Guide to Surface and Subsurface Map (https://books.go ogle.com/books?id=pvG5BAAAQBAJ&pg=SA1-PA7&dq=%22buttress+unconformity%22&hl=en&sa=X&redir_esc=y #v=onepage&q=%22buttress%20unconformity%22&f=false). Springer Science & Business Media. ISBN 9783662039120. 12. Neuendorf K.K.E. (2005).Glossary of Geology (https://books.google.com/books?id=yD79FqfECCYC&pg=PA278). p. 73. ISBN 9780922152766. 13. Cliff Ford (2 September 2003)."Siccar Point" (http://www.geos.ed.ac.uk/undergraduate/field/siccarpoint/). Field Excursion Preview. University of Edinburgh School of GeoSciences. Retrieved 2008-10-20.