An unconformity is a buried erosional or non-depositional surface that separates two different-age rock masses or strata, indicating that the deposition of sediments was not continuous. The older layer was generally exposed to erosion for an interval of time before the younger layer was deposed, but the term is used to describe any break in the sedimentary geological record.
How is an unconformity formed?
Unconformities are gaps in the geologic record that may indicate episodes of crustal deformation, erosion, and sea level variations. They are a characteristic of stratified rocks and are thus usually found in sediments (but can also be found in stratified volcanics). They are surfaces that form a substantial break (hiatus) in the geological record between two rock bodies (sometimes people say inaccurately that “time” is missing). Unconformities represent times when deposition stopped, some of the previously deposited rock was removed by an erosion interval and finally resumed deposition.
What are Types of Unconformity?
A disconformity is an unconformity between parallel layers of sedimentary rocks which is a period of erosion or non-deposition. Disconformities are characterized by subaerial erosion features. This type of erosion may leave in the rock record channels and paleosols. A paraconformity is a type of disconformity where separation is a simple bedding plane with no apparent buried erosional surface.
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. Namely, if the rock below the break is igneous or has lost its bedding due to metamorphism, the plane of juncture is a nonconformity.
An angular unconformity is an unconformity in which horizontally parallel strata of sedimentary rock are deposited on tilted and eroded layers, resulting in angular discordance with the horizontal layers above. Further orogenic activity can deform and tilt the entire sequence later.
A paraconformity is a type of unconformity in which strata are parallel ; no apparent erosion occurs and the surface of the unconformity resembles a simple bedding plane. It is also called pseudoconformity or nondepositional unconformity. Short paraconformities are called diastems.
When younger bedding is deposited against older strata, an unconformity of the buttress occurs, thus influencing its bedding structure.
A blended unconformity is a type of disconformity or nonconformity that has no distinct plane or contact separation, sometimes consisting of soils, paleosols, or pebble beds derived from the rock.
What is the difference between Disconformity and nonconformity?
A nonconformity is what its called when sedimentary rock strata are over crystalline (metamorphic or igneous) strata. A disconformity is when the sedimentary strata is over another sedimentary strata.