Interpreting ignimbrite 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.
Figures & Tables
Pyroclastic Density Currents and the Sedimentation of Ignimbrites
Pyoclastic density currents are awesome volcanic phenomena that can wreak destruction on a regional scale and can impact global climate. They deposit ignimbrites, which include vast impact lansdscape-modifying sheets with volumes exceeding 1000 km3.This book takes stock of our understanding of pyroclastic density currents and presents a new conceptual framework for investigating how ignimbrites are deposited. It integrates the results of field-based studies, laboratory experiments and numerical modelling, including work on clastic sedimentologym and industrial particle transport. Topics covered include the behaviour or particulate currents, mechanisms of clast support and segregation, interpreting ignimbrite lithofacies and architectures, and future research directions. The new approach focuses on processes and conditions within the lower flow-boundary zone of currents. Superb diagrams explain many new concepts, while the 95 photographs make an explanatiry atlas of deposit types. This is essential reading for workers investigating volcanic hazards, and for anyone wishing to interpret modern or ancient ignimbrites, as well as other catastrophically emplaced sediments.
“Given the depth of scholarship that they have brought to the subject, the power of their arguments, and the degree of synthesis with other fields, this would seemto qualify as a seminal work… I think that this will be the paper on the topic that others will have to contend with for many years to come.” Marcus Bursik, State University of New York