4.8 Deformational-diachronous units
4.8.1 General properties and rules
4.8.1.1 Definition and background -- In connection with studies of geological mountain chains and non-orogenic, but tectonically-disturbed stratal successions, many discussions have arisen about how "deformational phases" ought to be defined, delimited and correlated. Guidelines for this must be accommodated to the properties characterizing a deformational event.
A deformational event (deformasjonshendelse, deformasjonsbegivenhet) is a tectonic disturbance, accompanied or unaccompanied by metamorphism or magmatic activity. The deformation takes place during a specified period and over a large area.
The deformational event is a time-transgressive, dynamic geological process or combination of processes. It may be compressional or tensional, and a single event may be characterized by these stress processes within the same area, simultaneously or consecutively, or they may dominate in separate parts of the area during the same period. The deformation may be expressed by folding, faulting or thrusting, or by a combination of two, or all three of these types of deformation. Metamorphism, intrusion, volcanism, lack of deposition, erosion, and sedimentation of coarsely clastic material may be geological consequences of a deformation.
A deformational-diachronous unit is the diachronous period taken up by a deformational event. The non-synchronous time boundaries of a deformational-diachronous unit can be identified from the geological results of the deformation, as preserved in the stratal succession.
4.8.1.2 Boundaries and erection of formal units -- A deformational event is recognized in a stratal succession by an unconformity surface that represents the very last moment for the deformation at the place in question (see Section 3.7.2). The start of the deformational-diachronous period may be indicated by a boundary that is (a) a contact or transition between non-deformed and deformed beds; (b) an unconformity; (c) a marked vertical change in lithofacies; or (d) a tectonic boundary, e.g. a thrust fault.
In areas adjoining the area in which deformation itself has occurred, the deformational event may be reflected by the sudden appearance of coarsely clastic sediments in an otherwise fine-grained succession. Fault-controlled basin subsidence can also be revealed stratigraphically by, for example, marine or lacustrine argillaceous sediments overlying coarser, continental deposits. Such changes in sedimentation may indicate a deformational event, but cannot alone form the basis for defining it.
A deformational-diachronous unit is only valid within the area in which the physical reference basis for the deformational event is found. Deformational-diachronous units can be correlated.
Formal deformational-diachronous units are erected in accordance with the "general rules for naming and defining geological units" (Chap. 2). Considerable emphasis must be placed on documentation of the stratigraphical definition of the boundaries for the deformational diachron and for the area in which it is applicable. The age and duration of the deformational diachron can be determined by chronostratigraphical methods, numerical age determinations, or both ways.
4.8.1.3 Units, rank and hierarchy -- Deformational Diachron (deformasjonsdiakron) is the fundamental, formal deformational-diachronous unit. It is without rank in the hierarchical classification system. Formal deformational-diachronous units, in decreasing order of rank, are, Deformational Episode (deformasjonsepisode), Deformational Phase (deformasjonsfase), Deformational Span (deformasjonstrinn) and Deformational Cline (deformasjonsskifte).
A deformational-diachronous unit is given rank in the hierarchical classification system according to its relative duration and importance, in common with the principles for diachronous units (Section 4.7).
4.8.1.4 Naming -- Deformational-diachronous units are given formal names in accordance with the guidelines set down in Section 2.2. The proper name is a geographical name, or an alternative name in the case of the continental shelf, derived from the type area for the deformational event. The name should preferably not be the same as the proper name of an existing formal geological unit and certainly not of another time unit. In the formal name, the proper name is combined with the words "Deformational Diachron", or the appropriate rank designation (see Section 4.8.1.3) using initial capital letters (lower-case letters and one compound word (deformasjonsdiakron) in Norwegian). Provided misunderstanding cannot arise, the term "Deformational" may be omitted from the formal name.
The terms "deformational event", "deformation event", "deformation", "time of deformation", and the like, may be used as informal deformational-diachronous terms. The formal terms "Deformational Episode", "Deformational Phase", "Deformational Span" and "Deformational Cline", or their abbreviated forms, can also be used informally (with lower-case initials), but never in combination with a formal proper name.
Key reference: this Code, based on diachronic units in NACSN (1983).