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Encyclopedia > Unconformity
There is a billion year gap in the geologic record where this 500 million year old dolomite unconformably overlays 1.5 billion year old rhyolite.
There is a billion year gap in the geologic record where this 500 million year old dolomite unconformably overlays 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. Image File history File links Taum_Sauk_precambrian-cambrian_unconformity. ... Image File history File links Taum_Sauk_precambrian-cambrian_unconformity. ... Dolomite crystals from Touissite, Morocco Dolomite is the name of both a carbonate rock and a mineral consisting of calcium magnesium carbonate (formula: CaMg(CO3)2) found in crystals. ... Rhyolite This page is about a volcanic rock. ... Severe soil erosion in a wheat field near Washington State University, USA. For erosion as understood by materials science, see Erosion (materials science) For erosion as an English analogy, see Erosion (figurative) Erosion is the displacement of solids (soil, mud, rock and other particles) by the agents of wind, water... The rocky side of a mountain creek near Orosí, Costa Rica. ... Goldenville Strata exposed at a quarry in Bedford, Canada. ... 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. ... Two types of sedimentary rock: limey shale overlaid by limestone. ...


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 three types of unconformities: disconformity, nonconformity, and angular unconformity. The table and timeline of geologic periods presented here is in accordance with the dates and nomenclature proposed by the International Commission on Stratigraphy. ...

Contents

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. Goldenville Strata exposed at a quarry in Bedford, Canada. ... Two types of sedimentary rock: limey shale overlaid by limestone. ...


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. 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, derived from the Greek words meta, change, and morphe, form. The protolith is subjected to extreme heat (>150 degrees Celsius) and pressure causing profound... Igneous rocks are formed when molten rock (magma) cools and solidifies, with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks. ...


Angular Unconformity

Angular unconformity at Vallis Vale.
Angular unconformity at Vallis Vale.

An unconformity where horizontally parallel strata of sedimentary rock are deposited on tilted and eroded layers that may be either vertical or at an angle to the overlying horizontal layers. The whole sequence may later be deformed and tilted by further orogenic activity. Image File history File links Vallisvale. ... Image File history File links Vallisvale. ... Vallis Vale (grid reference ST755490) is a 23. ... Goldenville Strata exposed at a quarry in Bedford, Canada. ... Orogeny is the process of mountain building, and as such is both a tectonic structural event, a geographical event and a chronological event, in that orogenic events happen within a time frame, affect certain regions of rocks and crust, and cause distinctive structural phenomena and related tectonic activity. ...


Reference:

  • American Geological Institute. Dictionary of Geological Terms. New York: Dolphin Books, 1962.

  Results from FactBites:
 
Unconformity - Wikipedia, the free encyclopedia (280 words)
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.
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.
An unconformity where horizontally parallel strata of sedimentary rock is deposited on tilted and eroded layers that may be either vertical or at an angle to the overlying horizontal layers.
Unconformities Summary - Unconformities Information (807 words)
Unconformities of large scale (i.e., cartographic unconformities) have been used by some geologists to delineate large groups of sedimentary formations that are bounded by unconformities.
In study of unconformities, it is preferred that evidence from at least two of the three categories be used to recognize an unconformity and criteria from all three categories, if possible, be used to assess the scope of an unconformity.
In other words, an unconformity may be thought of as a stratigraphic break that is significant in terms of lost record and a diastem as an insignificant break by the same measure.
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