3.7.2 Unconformity (inkonformitet)
3.7.2.1 An unconformity, or surface of unconformity, is a surface representing a substantial break in the geological record. It is the boundary surface between two stratigraphically adjacent units the younger of which was not deposited as part of a continuous geological process extending from the deposition of the older, underlying unit. An unconformity may reflect (a) an interruption in the depositional process leading to non-deposition of part of the sequence, or (b) uplift and erosion of bedrock (Figs. 11, 12 and 13).
3.7.7.2 An unconformity often reflects a structural event (see deformational-diachronous units, Section 4.8).
3.7.7.3 An unconformity may be of local or regional extent.
3.7.7.4 An unconformity is mappable with the help of geomorphological, geological and/or geophysical methods.
3.7.2.5 A single unconformity can be expressed in different ways within the area in which it is developed. A surface of unconformity can be defined by a time-transgressive or time-regressive stratigraphical succession.
3.7.2.6 An unconformity is given a formal or an informal name in accordance with the "general rules for naming and defining geological units" (Chap. 2) and the rules for naming geological form units (Section 3.7.1). The name consists of a geographical name (or an alternative name in the case of the continental shelf) from the type section, type locality or type area (Section 2.4.5), placed between the words "the ... Unconformity" (formal). The term distinguishing the type of unconformity (Section 3.7.2.7) may be used instead of "the ... Unconformity". Informal names may consist of the prefix "sub" used with the overlying lithostratigraphical unit or succeeding chronostratigraphical unit (e.g. the sub-Cambrian peneplain). Informal names may also contain the name of a deformational event that gave rise to the unconformity (e.g. the Late Kimmerian unconformity).
3.7.2.7 In British terminology the term unconformity (inkonformitet) is used for a structural discordance rather than a time gap. American terminology differentiates four types of unconformity: nonconformity, angular unconformity, disconformity and paraconformity. Apart from the last-mentioned term, which signifies an uncertain or doubtful unconformity between parallel beds (Dunbar & Rodgers 1957), it is recommended that these are used in Norwegian geological terminology.
3.7.2.7.a A nonconformity (ikke-konformitet) is a surface of unconformity separating an older lithodemic unit (plutonic or massive metamorphic rock) from an overlying, younger sedimentary or volcanic sequence. The older lithodemic basement (underlag) was exposed to erosion prior to deposition of the younger, superposed sequence (Fig. 11) (Dunbar & Rodgers 1957).
3.7.2.7.b A disconformity (diskonformitet) is a surface of unconformity where the rocks beneath and above the stratigraphical break are essentially parallel. A disconformity surface reflects a prolonged break in deposition and perhaps considerable erosion during which time the older beds remained horizontal (Fig. 12). The term disconformity should preferably be used for stratigraphical breaks separating units ranking at least as high as a formation (Stokes & Varnes 1955).
3.7.2.7.c An angular unconformity (vinkeldiskordans) is a surface of unconformity where the rocks beneath and above a stratigraphical break are not parallel; the underlying unit has been tilted or folded prior to erosion and deposition of the younger, superposed unit (Fig. 13).
3.7.2.7.d Terms used for unconformities must not be confused with terms for periods of time reflected by a stratigraphical break. Whereas hiatus is the period of time represented by an unconformity, diastem is a still shorter period during which sedimentation was interrupted.
3.7.2.8 Examples: In Norwegian literature it has not been customary to formally name surfaces of unconformity. In several cases they have, nevertheless, been used as the basis for defining deformational-diachronous units, even though the unconformities themselves have not been named (see Section 4.8), e.g. the "Trysil uplift" and the "Ekne orogeny" (Vogt 1928). The "sub-Cambrian peneplain" and the "sub-Permian peneplain" are informal terms for unconformities. In the North Sea area the Late Kimmerian unconformity has been used as a term for unconformities of both regional and local extent (Rawson & Riley 1982). The term has been ascribed varying content and meaning, and should be used informally.
3.7.2.9 Key references: Stokes & Varnes (1955), Dunbar & Rodgers (1957), Tomkeieff (1962), Bates & Jackson (1980).
