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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.

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

Hutton's Unconformity at Jedburgh, Scotland, illustrated by John Clerk in 1787 and photographed in 2003.

Hutton's angular unconformity at Siccar Point where 345 million year old Devonian Old Red Sandstone overlies 425 million year old Silurian greywacke.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.

Types of unconformities

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

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.

Angular unconformity

Angular unconformity at Vallis Vale.

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.

See also

• Hutton's Unconformity

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.
• Keith Montgomery (2003). "Siccar Point and Teaching the History of Geology" (pdf). University of Wisconsin. Retrieved on 2008-03-26.

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