unconformity
Information about unconformity
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 phenomenon of angular unconformities was discovered by James Hutton, who found examples at Jedburgh in 1787 and at Siccar Point in 1788.[1]
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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 deposited and 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. There are four types of unconformities: disconformity, nonconformity, angular unconformity and paraconformity.
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Hutton Unconformity at Jedburgh, Scotland, illustrated by John Clerk in 1787 and photographed in 2003.
Disconformity
An unconformity between parallel layers of sedimentary rocks which represents a period of erosion or non-deposition. A paraconformity is a type of disconformity in which the separation is a simple bedding plane; i.e., there is no obvious buried erosional surface.(AGI, 366) 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
An unconformity 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.Angular Unconformity
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Angular unconformity at Vallis Vale.
Paraconformity
An unconformity where beds above and below are parallel and no erosional surface is evident. Paraconformity can only be recognized based on the gap in the rock record when rocks of a particular age are absent from the sequence of rocks.See also
References:
- American Geological Institute. Dictionary of Geological Terms. New York: Dolphin Books, 1962.
- U.S. Bureau of Mines Dictionary of Mining, Mineral, and Related Terms published on CD-ROM in 1996.
Erosion is displacement of solids (soil, mud, rock and other particles) usually by the agents of currents such as, wind, water, or ice by downward or down-slope movement in response to gravity or by living organisms (in the case of bioerosion).
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Balanced Rock stands in Garden of the Gods park in Colorado Springs, CO]] A rock is a naturally occurring aggregate of minerals and/or mineraloids. The Earth's lithosphere is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic.
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stratum (plural: strata) is a layer of rock or soil with internally consistent characteristics that distinguishes it from contiguous layers. Each layer is generally one of a number of parallel layers that lie one upon another, laid down by natural forces.
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Sediment is any particulate matter that can be transported by fluid flow and which eventually is deposited as a layer of solid particles on the bed or bottom of a body of water or other liquid. Sedimentation is the deposition by settling of a suspended material.
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Sedimentary rock is one of the three main rock groups (the others being igneous and metamorphic rock). Rock formed from sediments covers 75-80% of the Earth's land area, and includes common types such as chalk, limestone, dolomite, sandstone, conglomerate and shale.
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James Hutton (3 June 1726 OS (14 June 1726 NS) Edinburgh, Scotland — 26 March 1797) was a Scottish geologist, naturalist, chemist and experimental farmer. He is considered the father of modern geology.
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Jedburgh
Gaelic - Deadard
Scots - Jeddart, Jethart,
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Gaelic - Deadard
Scots - Jeddart, Jethart,
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Siccar Point is a rocky promontory in the county of Berwickshire on the east coast of Scotland. It is famous in the history of geology as a result of a boat trip in 1788 in which James Hutton, with James Hall and John Playfair, observed the angular unconformity which Hutton
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The geological time scale is used by geologists and other scientists to describe the timing and relationships between events that have occurred during the history of Earth.
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stratum (plural: strata) is a layer of rock or soil with internally consistent characteristics that distinguishes it from contiguous layers. Each layer is generally one of a number of parallel layers that lie one upon another, laid down by natural forces.
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Sedimentary rock is one of the three main rock groups (the others being igneous and metamorphic rock). Rock formed from sediments covers 75-80% of the Earth's land area, and includes common types such as chalk, limestone, dolomite, sandstone, conglomerate and shale.
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Metamorphic rock is the result of the transformation of a pre-existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat (greater than 150 degrees Celsius) and extreme pressure causing profound
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Igneous rocks (etymology from latin ignis, fire) are rocks formed by solidification of cooled magma (molten rock), with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks.
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stratum (plural: strata) is a layer of rock or soil with internally consistent characteristics that distinguishes it from contiguous layers. Each layer is generally one of a number of parallel layers that lie one upon another, laid down by natural forces.
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Orogeny (Greek for "mountain generating") is the process of mountain building, and may be studied as a tectonic structural event, as a geographical event and a chronological event, in that orogenic events cause distinctive structural phenomena and related tectonic activity,
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The Great Unconformity is a geologic feature where two adjacent layers of rock were not formed one after another in the earth's development. The lower layer was always at a high point, undergoing erosion, and in this case for an extended time, even in geological terms.
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