3.3.2 Lithodeme (Litodem)
3.3.2.1 A lithodeme is a body of intrusive, volcanic or highly metamorphosed and/or thoroughly deformed rock that lacks primary structures. It is characterized by having a set of lithological properties that distinguishes it from adjacent geological units.
3.3.2.2 Lithodeme is the fundamental formal unit in lithodemic classification and nomenclature. Two or more lithodemes of the same class can be defined as a suite.
3.3.2.3 No limitations are placed on the dimensions of a lithodeme except that it must be mappable at the surface or traceable in the subsurface.
3.3.2.4 A lithodeme should be mappable at the surface or traceable in the subsurface using geological and geophysical methods. It should be mappable on ordinarily available base maps (official topographical maps, land-use maps, and privately-produced, but openly available maps).
3.3.2.5 A lithodeme consists of a rock that can be identified by its lithological properties using field geological methods. It may consist of (a) a single rock type, (b) two or more rock types showing a characteristic repetition or pattern, or (c) a heterogeneous lithology which in itself constitutes a unit that is distinct from adjoining rock units. A lithodeme may be an intrusive rock occurring, for example, as a pluton, diapir, stock, laccolith, sheet or plug, a body of volcanic rocks which have mutually complex boundary relationships, or a body of strongly metamorphosed rock. The boundaries of a lithodeme may be lithologically sharp, or may need to be defined by a change in at least one lithological property if there is a gradual transition to one or more adjoining geological unit(s). A lithodeme may be characterized by electrical, thermic, magnetic, radiometric, hydraulic, seismic and other physical characteristics derived from its lithological properties (see Figs. 1, 5).
3.3.2.6 A lithodeme is given a formal or an informal name in accordance with the "general rules for naming and defining geological units" (Chap. 2) and according to the provisions in Section 3.3.1. The most common formal type of name is a geographical name (or an alternative name on the continental shelf) combined with a relevant lithological designation ("the ... Granite", "the ... Syenite", "the ... Breccia", "the ... Gneiss", etc.), or with a structural/genetical designation ("the ... Plug", "the ... Sheet", etc.). Such types of name may also be informal, e.g. "the Leka ophiolite", "the Gardnos breccia", "the Lofoten intrusives", "the Nesodden dyke", etc.
3.3.2.7.a Lithodeme corresponds in rank to formation in the lithostratigraphical classification system, also when used informally on maps or in descriptions.
3.3.2.7.b A lithodeme may change its character regionally. It may therefore be more practical to designate the unit as a suite beyond its type area; the original proper name may then be retained (see Section 2.5.1).
3.3.2.7.c A lithodeme that has been defined in a metamorphic succession can be redefined as a formation if it can be proved that the unit has a definite stratigraphical position within a succession of originally sedimentary and/or volcanic rocks. When this kind of redefinition takes place the original proper name is retained (see Section 2.5.1).
3.3.2.8 Examples: Many named bodies of intrusive rock have lithodemic-type names, even though the bodies have not been formally defined as lithodemes. Several examples of this are to be found among names of plutonic rocks from the Oslo Region in Sæther (1962) and Gaut (1981).
3.3.2.9 Key reference: NACSN (1983).
