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Geological Society, London, Memoirs; 2003; v. 27; p. 51-85;
DOI: 10.1144/GSL.MEM.2003.027.01.05
© 2003 Geological Society of London

Chapter 5 Interpreting ignimbrites lithofacies

This chapter presents an approach for ignimbrite description and interpretation. It draws on field, granulometric and fabric data from published descriptions of ignimbrites. To describe ignimbrites, we adopt a non-genetic lithofacies scheme (Table 5.1). This avoids possible connotations of 'ideal' sequences or of specific emplacement models (as in the previous schemes of 'Layers 1, 2a, 2b', 'ignimbrite types 1-3', 'ground layer' and 'basal layer'). We describe some of the more common lithofacies in ignimbrites. Our list is not intended to be prescriptive, and it is to be expected that workers will in the future modify or subdivide our groupings. We then show how the lithofacies might be interpreted in terms of flow-boundary zone processes. Understanding is far from complete, and in some cases we give possible alternative interpretations that require testing (also see summary on Table 7.1, p. 120). Consideration of lithofacies that record sedimentary reworking (e.g. by wind or water) is beyond the scope of this work.

A lithofacies scheme for ignimbrites

A lithofacies refers to the character of a deposit, or part of a deposit, that is distinct according to some combination of stratification, grain size, grain shape, sorting, fabric and composition. It is non-genetic and non-stratigraphic. Some common ignimbrite lithofacies are listed in Table 5.1, with convenient abbreviations. Primary lithological descriptors include tuff (T), lapilli-tufT (LT), lapillistone (L) and breccia (Br), subdivided into lithofacies according to stratification type sorting, composition and fabrics, for example massive lapilli-tufT (mLT) or stratified lapilli-tufT (sLT). The list can be extended and further subdivisions made as desired.

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