An unconformity typically results when earlier sediments are eroded before being buried under sediments deposited during a later episode of sedimentation. In general, the term unconformity describes any break in the sedimentary geologic record. There are three forms of unconformities: disconformity, nonconformity, and angular unconformity.
(ASTER image at left, courtesy of NASA - a large scale geological angular unconformity in northern Chile involving (right) Cretaceous sediments (146 to 65 million years old) that were tilted upward to an angle of about 50 degrees, then eroded. Subsequent volcanic pyroclastic deposits were laid down as a flat sheet, then eroded from the east, exposing the tilted Cretaceous and flat rock layers.)
A disconformity lies between parallel layers of sedimentary rocks, and the contact between younger and older beds is marked by a visible, irregular or uneven erosional surface.
The separation in a paraconformity is a simple bedding plane with no obvious buried erosional surface. Because the beds above and below a paraconformity are parallel and no erosional surface is evident, this unconformity can only be recognized because of the temporal gap (hiatus) in the rock record.
A blended unconformity has no distinct separation plane or contact, and sometimes consists of soils, paleosols, or beds of pebbles derived from the underlying rock.
A nonconformity between sedimentary rocks and metamorphic or igneous rocks occurs when sedimentary rock has been deposited above pre-existing (eroded) metamorphic or igneous rock, indicating an environmental alteration in mode of formation of strata.
(image at right - click to enlarge - 1 billion year hiatus between angled layer grey dolomite (~500 Ma) resting on reddish rhyolite (1.5 Ga) near Taum Sauk powerplant.)
In an angular unconformity, younger sediments rest upon the eroded surface of tilted or folded older rocks. Angular unconformities result when horizontally parallel strata of sedimentary rock are deposited on tilted and eroded layers, which may be vertical or at an angle to the overlying horizontal layers. The entire sequence may later be deformed and tilted by later orogenic activity.
(image at right - click to go to enlargement - angular unconformity at Vallis Vale in Somerset, courtesy of Patrick Mackie.)
Provided that the sequences have not been overturned, younger sediments lie above an unconformity. Because either no sediments were deposited or sediments were eroded during the hiatus, the local record for that time interval is missing and geologists must employ other evidence to deduce the region's geologic history.
[links: images: formations: The Great Unconformity, in the Grand Canyon, Colorado River, Tapeats Red Wall and The Great Unconformity; in Roaring Brook location, Southington and The Great Unconformity, Southington with contact between the base of the Triassic New Haven arkose and Ordovician Harrison gneiss ("Pumpkin Ground" spotted gneiss), in geological area near Las Vegas; unconformity, Shepard Point, Utah; unconformity ; unconformity between 3.22 Ga granitic basement overlain by 2.6 Ga polymictic conglomerate of the Keskarrah formation, Point Lake, N.W.T., Canada (Note the m-scale paleotopography on the erosional surface); Cretaceous unconformity surface, with underlying Turonian section of pillow basalt, argillite, and diabase from an overlying late Cretaceous section of basaltic lapilli tuff and conglomerate, Aruba; angular unconformities: spectacular angular unconformity; Hutton's angular unconformity, Siccar Pt., Scotland, and Hutton's Unconformity, and Hutton's Unconformity with gently dipping Carboniferous or Old Red Sandstone overlaying steeply dipping Dalradian schist; Devonian angular unconformity in which red and yellowish Devonian sandstones overlie inverted, Early Palaeozoic limestones (Arkfjellet unit) with a dark weathering horizon, Kneikfjellet, Svalbard; angular unconformity between the Precambrian basement and Early Carboniferous sediments of the Billefjorden Group, Ditlovtoppen, Spitsbergen; angular unconformity, I-70, New Castle; angular unconformity, Rooney Rd, west Denver; Taconic-angular unconformity, Catskill, NY; near-vertical angular unconformtiy, French Alps; labelled aerial view of angular unconformity; disconformities: Devonian-Mississippian Boundary, Southern Canadian Rockies, outcrop, and close-up; disconformity in cliffed bank, San Juan River, Paradox Basin, Southeastern Utah; disconformity between Red Wall limestone and Muav limestone; paraconformity: paraconformity, Hanging Lake area, Glenwood Canyon, in which the 25 million year-long Silurian period is missing between lower Ordovician Manitou Formation and the upper Devonian Chaffee Group, although no evidence is found here for major erosion in the lower layer; paraconformity, Whirlpool Canyon, Dinosaur N.M., in which the Ordovician, Silurian, and Devonian Periods are missing between the Cambrian Lodore Formation and the Mississippian Madison Limestone (forming cliff about a third of the way up the canyon walls to the right of the Green River); nonconformities: Late Cretaceous granitic rocks of the Salinian basement are nonconformably overlain by middle to upper Miocene sedimentary rocks (transgressive basal sandstone with overlying deepwater siliceous mudstone with some porcelanite and shale) and close-up; non-conformity of Triassic dolostone over crystalline basement, French Alps; Precambrian nonconformity; nonconformity in Grand Canyon, and Shinamo Crack; 3D nonconformity between Precambrian Orocopa Gneiss and Miocene sedimentary rocks in in upper Painted Canyon; nonconformity between older granitic rocks and younger basalt flows at Steamboat Rock; view from Hout Bay towards Chapmans Peak, showing the nonconformity between the Cape Granite and strata of the Cape Supergroup (Graafwater Formation). Cutting the granite and unconformity is a dolerite dyke, wp; nonconformity in Bou Azzer region, Morocco; nonconformity of Cambrian sedimentary rocks overlying Precambrian granite (MO); close-up: unconformity; slightly angular unconformity between the underlying mudstone of the Paleocene Santa Susana Formation and the basal conglomerate of the Eocene Llajas Formation, Rambla Pacifico, Santa Monica Mountains (and ancient granites/younger mudflow nonconformity in San Gabriel Mtns.); disconformity (ancient buried surface of erosion) in dolostones, southeastern California; disconformity between Shinarump Conglomerate and the De Chelly Sandstone, Canyon de Chelly; The Graafwater FormationCape Granite Nonconformity; diagrams: comparison of geologic column in Grand Canyon compared to Geologic Time Scale – each gaps is a hiatus; types of unconformity; types; development of an angular unconformity; disconformity; paper model of unconformity for students; unconformity on geological map; webpages: Scenery