3.2.5 Member (Ledd)
3.2.5.1 A member is a body of rock or superficial deposits that occurs in a stratified formation.
3.2.5.2 Member is a formal lithostratigraphical unit ranking next below formation. A member may be subdivided into beds or flows.
3.2.5.3 The extent of a member is usually less than the extent of the formation of which it forms a part. A member can pass laterally from one formation into another (Fig. 4).
3.2.5.4 A member need not be mappable on the same base map as is required for the portrayal of the formation of which it forms a part. It may be traceable in the subsurface using geological and geophysical methods.
3.2.5.5 A member is defined when it is desirable to distinguish a particular lithostratigraphical part of a heterogeneous formation. A formation need not be subdivided into members. The entire formation or parts of it may be erected as members. A member may consist of beds or flows, but cannot contain other members.
3.2.5.6 A member can be given a formal or an informal name in accordance with the "general rules for naming and defining geological units" (Chap. 2). The proper name is placed between the words "the ... Member" in formal names. A descriptive lithological designation can also be added between the proper name and the word "Member". A formally defined and named member should form part of a formal formation; otherwise, the unit is informal. It is often practical to use informal designations for members (Section 2.3).
3.2.5.7 A member may change character laterally making it most practical to call the unit a formation in an area away from its type area (Fig. 4). The proper name of an originally defined member can be retained even if its stratigraphical rank is changed to formation in such areas.
3.2.5.8 Examples: The Late Precambrian -- Cambrian Vangsås Formation near Mjøsa is formally divided into the Vardal Sandstone Member and the Ringsaker Quartzite Member (Bjørlykke et al. 1967). Føyn et al. (1983) divided the Late Precambrian Ifjord Formation in Finnmark into three formal members, the Gozavarre, Loavdajavarre and Elvevika Members.
3.2.5.9 Key references: ISSC (1976), NACSN (1983).
