BGS classification of lithodemic units : proposals for classifying tectonometamorphic units and mixed-class units

Lithodemic units are those mappable geological units that are defined and delimited primarily by their lithological character, and which lack primary stratification. This report sets out proposals for classifying and naming all types of lithodemic unit other than intrusions. The units are organised in two hierarchical chains referred to as tectonometamorphic units and mixed-class units. These proposals complete a comprehensive BGS scheme for classifying and naming lithostratigraphical and lithodemic units, which is presented here in four hierarchical chains spanning up to seven ranks (Table 1). Within the scheme, the seven established types of lithostratigraphical unit (NACSN, 1983; Rawson et al., 2002), thirty-four types of intrusive unit (Gillespie et al., 2008), and twelve types of tectonometamorphic unit (this report) are placed in parallel parent–child chains that straddle the same six ranks (Rank 1 to Rank 6). Each of these three hierarchical chains represents a distinct class of mappable unit. Within each chain, parent units embrace children of the same class (i.e. at lower ranks in the same chain), so the units in each of these three hierarchies are referred to as ‘single-class units’. In the fourth chain (this report), parent units embrace children from two or more of the three hierarchies of single-class units, so the seven types (and four ‘special cases’) of lithodemic unit within it are referred to as mixed-class units. At present, the units in this chain straddle five ranks, one of which is not shared by any of the other chains; additional types of mixed-class unit may be inserted in other ranks of this chain if a need is demonstrated when the scheme is used to classify mapped units. Guidance is given for assigning formal names to tectonometamorphic and mixed-class units. Completion of the scheme means it is now possible to classify and name lithostratigraphical and lithodemic units, separately and in any combination, in any setting, in a systematic and