An unconformity is a buried erosion surface or non-depositional surface separating two rock masses or Stratum 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 layer, but the term is used to describe any break in the sedimentary geologic record. The significance of angular unconformity (see below) was shown by James Hutton, who found examples of Hutton's Unconformity at Jedburgh in 1787 and at Siccar Point in Berwickshire in 1788, both in Scotland.[ Hutton's Unconformity ]
The rocks above an unconformity are younger than the rocks beneath (unless the sequence has been overturned). An unconformity represents Geologic time 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.
Types
Disconformity
A
disconformity is an unconformity between parallel
stratum 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 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
stratum 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
orogeny activity. A typical case history is presented by the Briançonnais realm (Swiss and French Prealps) during the Jurassic.
[Septfontaine M. (1984): Le Dogger des Préalpes médianes suisses et françaises - stratigraphie, évolution paléogéographique et paléotectonique.- Mém. Soc. Helv. Sci. Nat., vol. 97, 121 p. (Birkhäuser éd.)][Septfontaine M. (1995): Large scale progressive unconformities in Jurassic strata of the Prealps South of lake Geneva: interpretation as synsedimentary inversion structures. Paleotectonic implications. Eclogae geol. Helv., 88:3 553–576.]
Angular unconformities can occur in ash fall 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
.
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,
, or beds of pebbles derived from the underlying rock.
==Gallery==
, Czech Republic]]
and overlying Pennsylvanian Sharon Conglomerate, near Jackson, Ohio]]
, near Taum Sauk Hydroelectric Power Station, Missouri.]]
, Germany]]
rocks overlying steeply-tilted
Carboniferous rocks at Praia do Telheiro, Portugal]]
and the overlying
Exeter Sandstone at Steamboat Butte in the valley of the Dry Cimmarron, New Mexico]]
, China]]
See also
Further reading
-
U.S. Bureau of Mines Dictionary of Mining, Mineral, and Related Terms published on CD-ROM in 1996.
