3.2.6 Bed (Lag), Flow (Strøm)
3.2.6.1 A bed is the smallest formal lithostratigraphical unit in sedimentary sequences. A flow is the correspondingly smallest unit in volcanic rocks and deposits formed by flowing lava or ash.
3.2.6.2 Beds and flows have lithostratigraphical rank next beneath members, but may also be formally erected in formations lacking formally erected members.
3.2.6.3 Beds and flows usually have limited lateral persistence within the distribution area of the higher lithostratigraphical unit of which they form a part.
3.2.6.4 Beds and flows are usually not mappable other than on special, particularly large-scale maps. They can be portrayed on sections of suitable scale. They may be traceable in the subsurface using geological and geophysical methods.
3.2.6.5 A bed usually represents a single depositional event in a sedimentary sequence and is characterized by composition, structure and texture. A flow is a volcanic extrusive rock formed during a single eruption. It is characterized by composition, structure, texture, palaeomagnetism and other properties, e.g. such as the various rhomb porphyry flows of Permian age in the Oslo Region. Beds and flows may contain laminae (singular: lamina); these are not named.
3.2.6.6 Beds and flows are given formal or informal names in accordance with the "general rules for naming and defining geological units" (Chap. 2). The proper name is placed between the words "the ... Bed" or "the ... Flow" in formal names. A lithic designation may be added between the proper name and the term "Bed" or "Flow". Formal names for beds and flows should only be erected when this will have particular stratigraphical or practical value. It may often be practical to apply informal designations to beds and flows (Section 2.3).
3.2.6.7 A key bed (nøkkellag) or marker bed (ledelag) is a particularly prominent and easily recognizable bed or member of large lateral extent (Fig. 4). Such units are of practical importance when structural patterns are being correlated and resolved. Key beds may be given formal or informal names.
3.2.6.8 Examples: Formally defined beds from Svalbard are the Skilis Bed, the Brevassfjellet Bed, the Blanknut Bed, the Verdande Bed (Mørk et al. 1982) (in Norwegian, e.g. Skilislaget) and the Marhøgda Bed (Bäckström & Nagy 1985). The Brentskardhaugen Bed is an important marker bed in the Jurassic succession on Svalbard. The name was first used by Parker (1967) who did not define the unit formally. Since a description which satisfies requirements for formal definition has subsequently been given by Bäckström and Nagy (1985), the Brentskardhaugen Bed is now a formal name. The Vedde Ash Bed (Veddeaskelaget) is a formally defined marker bed from the Quaternary succession in West Norway (Mangerud et al. 1984). The subdivision of the Permian lava sequence in the Oslo Region using the alphanumerical system B1, RP1, RP2, etc., is an example of informal classification of flows (Oftedahl 1960).
3.2.6.9 Key references: ISSC (1976), NACSN (1983).
