Abstract
A new scheme is introduced for classifying and naming mappable rock bodies that lack primary stratification. In recognition of their distinctive geological characteristics, these ‘nonstratiform’ bodies are defined and classified according to their 3D form, spatial distribution and genetic relationships, in two hierarchical (parent–child) chains: one for intrusions and one for tectonometamorphic units. Geologically complex units, encompassing bodies of different genetic classes, are classified in a third chain reserved specifically for ‘mixed-class’ units. The new classification scheme is offered as an alternative to existing recommendations in the International Stratigraphic Guide and North American Stratigraphic Code, in which nonstratiform bodies are recognized and defined primarily by their lithological character. BRUCS (the BGS Rock Unit Classification System) combines the three new parent–child chains for nonstratiform units with the well-established chain for stratiform units (bed–member–formation–group–supergroup) to create a flexible, practical and effective solution for classifying and naming all mappable rock bodies. The taxonomic rigour of BRUCS means that the considerable capabilities of modern digital systems for managing and communicating mapping data can be exploited fully.
Highlights
A new scheme is introduced for classifying and naming mappable rock bodies that lack primary stratification
We describe a new scheme for classifying and naming nonstratiform bodies that takes into account all of those key requirements and that experience, and we explain how that new scheme for nonstratiform bodies has been combined with the well-established scheme for stratiform bodies to create BGS Rock Unit Classification System (BRUCS), a unified system for classifying and naming all rock bodies at all normal mapping scales
Existing recommendations for classifying and naming nonstratiform bodies For several decades, most efforts to classify and name mappable rock bodies have drawn on the recommendations presented in two authoritative schemes (Fig. 1): the International Stratigraphic Guide (ISG; International Subcommission on Stratigraphic Classification, ISSC 1976, 1994; Murphy and Salvador 1999), and the North American Stratigraphic Code (NASC; North American Commission on Stratigraphic Nomenclature, NACSN 1983, 2005)
Summary
A lithological term is preferred (e.g. Someplace Granite, Someplace Schist), but in recognizing that ‘many bodies of intrusive rock ... are difficult to characterize with a single lithic term’ the important exception is made that a term to ‘denote form (e.g. dike, sill)’, or a term that is ‘neutral (e.g. intrusion, pluton)’ can be used instead, if necessary; names like Someplace Pluton and Someplace Intrusion are permissible. Requirements of a new scheme for classifying and naming nonstratiform bodies In the 1990s, the British Geological Survey (BGS) began converting analogue maps and records into digital datasets describing the geology of the UK; these include a publicly accessible database of all the named rock units of the UK (The BGS Lexicon of Named Rock Units (BGS 2020a)), and a range of digital geological maps (e.g. DiGMapGB-50 (BGS 2020b) and the latest 1:625 000 scale map of the UK (BGS 2008a, b)). In the course of reassessing UK geology for that purpose, it became clear that neither the ISG nor NASC provided an adequate solution for classifying and naming the nonstratiform rock bodies of the UK (Gillespie et al 2008; Leslie et al 2012), and that a new scheme was needed. The key requirements of that new scheme, as deduced from the reassessment of UK bodies and subsequent attempts to create hierarchical classifications of nonstratiform units for use in BGS databases and digital products, are summarized below
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