- Unconformity
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Hutton's Unconformity at Jedburgh, Scotland, illustrated by John Clerk in 1787 and photographed in 2003.
An unconformity is a buried erosion 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 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. 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.
Contents
Types of unconformities
Disconformity
DisconformityA 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.[3] A paraconformity is a type of disconformity in which the separation is a simple bedding plane with no obvious buried erosional surface.[4] 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.
Nonconformity
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 by metamorphism, the plane of juncture is a nonconformity.[5]
Angular unconformity
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. The whole sequence may later be deformed and tilted by further orogenic activity. A typical case history is presented by the paleotectonic evolution of the Briançonnais realm (Swiss and French Prealps) during the Jurassic.
Blocs diagrammes et profils palinspastiques des Préalpes (Briançonnais s.l.) au Jurassique moyen
Discordances successives angulaires et en éventails sur la marge N-briançonnaise, affectant 650m de sédiment durant 50 Ma; Dogger-Malm, Préalpes du Chablais (Suisse, France).Paraconformity
ParaconformityA paraconformity is a type of unconformity in which strata are parallel; there is little apparent erosion and the unconformity surface resembles a simple bedding plane. It is also known as nondepositional unconformity or pseudoconformity.
Buttress unconformity
A buttress unconformity is when younger bedding is deposited against older strata thus influencing its bedding structure.
Gallery
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The "Taconic Unconformity" near Catskill, NY. This angular unconformity separates the Austin Glen Formation (Ordovician) from the overlying Rondout Formation (Silurian) and Manlius Formation (Devonian)
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Eemian disconformity in a fossil coral reef on Great Inagua, The Bahamas. Foreground shows corals truncated by erosion; behind the geologist is a post-erosion coral pillar which grew on the disconformity after sea level rose again.
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Hutton's angular unconformity at Siccar Point where 345 million year old Devonian Old Red Sandstone overlies 425 million year old Silurian greywacke.[6]
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Disconformity with the Lower Cretaceous Edwards Formation overlying a Lower Permian limestone; hiatus is about 165 million years; Texas.
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Nonconformity between the Pennsylvanian Fountain Formation (left) and Precambrian Gneiss (right) at Red Rocks Park, near Golden, Colorado
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Unconformity (at the hammer) between underlying Mississippian Borden Formation and overlying Pennsylvanian Sharon Conglomerate, on US Rt. 35, near Jackson, Ohio
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Angular unconformity on the west coast of Point Loma, California, facing north
See also
References
- ^ Hutton's Unconformity
- ^ Keith Montgomery (2003). "Siccar Point and Teaching the History of Geology" (pdf). University of Wisconsin. http://nagt.org/files/nagt/jge/abstracts/Montgomery_v51n5.pdf. Retrieved 2008-03-26.
- ^ Disconformity at EPGeology
- ^ American Geological Institute. Dictionary of Geological Terms. New York: Dolphin Books, 1962.
- ^ Stokes, W. Lee (1982). Essentials of Earth History 4th Edition. Prentice Hall,Inc.. p. 65. ISBN 0132858908.
- ^ Cliff Ford (2 September, 2003). "Siccar Point". Field Excursion Preview. University of Edinburgh School of GeoSciences. http://www.geos.ed.ac.uk/undergraduate/field/siccarpoint/. Retrieved 2008-10-20.
- U.S. Bureau of Mines Dictionary of Mining, Mineral, and Related Terms published on CD-ROM in 1996.
- Keith Montgomery (2003). "Siccar Point and Teaching the History of Geology" (pdf). University of Wisconsin. http://nagt.org/files/nagt/jge/abstracts/Montgomery_v51n5.pdf. Retrieved 2008-03-26.
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