An unconformity is a buried erosional or non-depositional surface separating two rock masses or

strata In geology and related fields, a stratum (: strata) is a layer of Rock (geology), rock or sediment characterized by certain Lithology, lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by v ...

of different ages, indicating that

sediment Sediment is a solid material that is transported to a new location where it is deposited. It occurs naturally and, through the processes of weathering and erosion, is broken down and subsequently sediment transport, transported by the action of ...

deposition was not continuous. In general, the older layer was exposed to erosion for an interval of time before deposition of the younger layer, but the term is used to describe any break in the

sedimentary Sedimentary rocks are types of rock formed by the cementation of sediments—i.e. particles made of minerals (geological detritus) or organic matter (biological detritus)—that have been accumulated or deposited at Earth's surface. Sedime ...

geologic record. The significance of angular unconformity (see below) was shown by

James Hutton James Hutton (; 3 June Old Style and New Style dates, O.S. 1726 – 26 March 1797) was a Scottish geologist, Agricultural science, agriculturalist, chemist, chemical manufacturer, Natural history, naturalist and physician. Often referred to a ...

, who found examples of Hutton's Unconformity at

Jedburgh Jedburgh ( ; ; or ) is a town and former royal burgh in the Scottish Borders and the traditional county town of the Shires of Scotland, historic county of Roxburghshire. History Jedburgh began as ''Jedworð'', the "worth" or enclosed settlem ...

in 1787 and at Siccar Point in Berwickshire in 1788, both in Scotland. The rocks above an unconformity are younger than the rocks beneath (unless the sequence has been overturned). An unconformity represents

time Time is the continuous progression of existence that occurs in an apparently irreversible process, irreversible succession from the past, through the present, and into the future. It is a component quantity of various measurements used to sequ ...

during which no sediments were preserved in a region or were subsequently eroded before the next deposition. 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. It is a kind of

relative dating Relative dating is the science of determining the relative order of past events (i.e., the age of an object in comparison to another), without necessarily determining their absolute age (i.e., estimated age). In geology, rock or superficial d ...

Types

Disconformity

A 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.

Nonconformity

A nonconformity exists between sedimentary rocks and metamorphic or

igneous rocks Igneous rock ( ), or magmatic rock, is one of the three main Rock (geology)#Classification, rock types, the others being sedimentary rock, sedimentary and metamorphic rock, metamorphic. Igneous rocks are formed through the cooling and solidifi ...

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, then the plane of juncture is a nonconformity.

Angular unconformity

An angular unconformity is an unconformity where horizontally parallel

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 Briançonnais realm (Swiss and French Prealps) during the Jurassic. Angular unconformities can occur in

ash fall Volcanic ash consists of fragments of rock, mineral crystals, and volcanic glass, produced during volcanic eruptions and measuring less than 2 mm (0.079 inches) in diameter. The term volcanic ash is also often loosely used to re ...

layers of pyroclastic rock deposited by volcanoes during explosive eruptions. In these cases, the hiatus in deposition represented by the unconformity may be geologically very short – hours, days or weeks.

Paraconformity

A paraconformity is a type of unconformity in which the sedimentary layers above and below the unconformity are parallel, but there is no obvious erosional break between them. A break in sedimentation is indicated, for example, by fossil evidence. It is also called nondepositional unconformity or pseudoconformity. Short paraconformities are called diastems.

Buttress unconformity

A buttress unconformity, also known as onlap unconformity, occurs when younger bedding is deposited against older strata thus influencing its bedding structure.

Blended unconformity

A blended unconformity is a type of disconformity or nonconformity with no distinct separation plane or contact, sometimes consisting of soils,

paleosol In Earth science, geoscience, paleosol (''palaeosol'' in Great Britain and Australia) is an ancient soil that formed in the past. The definition of the term in geology and paleontology is slightly different from its use in soil science. In geo ...

s, or beds of pebbles derived from the underlying rock.

Gallery

File:Disconformity Horni Pocernice.jpg, ''Disconformity'' at Horni Pocernice, Czech Republic File:Borden-Sharon unconformity.jpg, ''Disconformity'' (at the hammer) between underlying Mississippian Borden Formation and overlying Pennsylvanian Sharon Conglomerate, near Jackson, Ohio File:Taum Sauk precambrian-cambrian unconformity.jpg, 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 Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture (geology), texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained matri ...

, near Taum Sauk Hydroelectric Power Station, Missouri. File:Ratssteinbruch Dresden-Doelzschen.jpg, ''Nonconformity'' at Ratssteinbruch near

Dresden Dresden (; ; Upper Saxon German, Upper Saxon: ''Dräsdn''; , ) is the capital city of the States of Germany, German state of Saxony and its second most populous city after Leipzig. It is the List of cities in Germany by population, 12th most p ...

, Germany File:Siccar Point red capstone closeup.jpg, Hutton's ''angular unconformity'' at Siccar Point where Famennian age (371–359 Ma) Devonian Old Red Sandstone overlies

Llandovery Llandovery (; ) is a market town and community (Wales), community in Carmarthenshire, Wales. It lies on the River Tywi and at the junction of the A40 road, A40 and A483 road, A483 roads, about north-east of Carmarthen, north of Swansea and w ...

age (444–433 Ma) Silurian

greywacke Greywacke or graywacke ( ) is a variety of sandstone generally characterized by its hardness (6–7 on Mohs scale), dark color, and Sorting (sediment), poorly sorted angular grains of quartz, feldspar, and small rock fragments or sand-size Lith ...

File:Angular Unconformity at Praia do Telheiro in Algarve in Portugal.png, ''Angular unconformity'' of

Triassic The Triassic ( ; sometimes symbolized 🝈) is a geologic period and system which spans 50.5 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.4 Mya. The Triassic is t ...

rocks overlying steeply-tilted

Carboniferous The Carboniferous ( ) is a Geologic time scale, geologic period and System (stratigraphy), system of the Paleozoic era (geology), era that spans 60 million years, from the end of the Devonian Period Ma (million years ago) to the beginning of the ...

rocks at Praia do Telheiro, Portugal File:Steamboat Butte New Mexico.jpg, ''Angular unconformity'' between the underlying Dockum Group and the overlying Exeter Sandstone at Steamboat Butte in the valley of the Dry Cimmarron, New Mexico File:Winkeldiskordanz.JPG, Angular unconformity in Jingtai County, China File:Tephra Layers at Chimborazo Volcano in Ecuador.jpg, Angular unconformity in pyroclastic rock layers erupted by

Chimborazo Chimborazo () is a stratovolcano situated in Ecuador in the Cordillera Occidental (Ecuador), Cordillera Occidental range of the Andes. Its last known Types of volcanic eruptions, eruption is believed to have occurred around AD 550. Although not ...

volcano, Ecuador Image:GP_0147_%282%29.JPG, Geological unconformity, Camelback mountain, Arizona, showing deposition of Chattian sandstone (right) on Precambrian granite (left).