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The initial analysis of a complex ignimbrite sheet should be purely descriptive. An ignimbrite sheet may be divided into 'divisions'. A division is a basic architectural unit, and may comprise a layer, lens or any specified part of the deposit that has some common characteristics and/or bounding surface(s). A division may be characterized by a lithofacies or a group of lithofacies.
Conceptualizing architecture in a time-geometry framework
To determine how a pyroclastic density current evolved in time and space requires correlation of the internal lithofacies and divisions of an ignimbrite within a time-geometry framework. The deposition of layers, lithofacies and compositional zones in ignimbrites (Fig. 6.1) may be diachronous (see below). It is useful, therefore, notionally to subdivide an ignimbrite sheet with time surfaces, which form time-lines in cross-section. One type of time-surface is a depositional isochron, or depochron (new term), which joins pyroclasts deposited at the same instant in time (Fig. 6.2). A depochron represents an instantaneous aggradation surface. Ignimbrite lithofacies along a depochron record the depositional processes at the flow boundary of an entire pyroclastic density
(A) Gradational chemical zonation within massive lapilli-tuff (mLT). The gradual colour variation with height reflects a gradually increasing abundance of dark andesite pumice clasts, reflecting temporal changes
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