Page 125 - Caldera Volcanism Analysis, Modelling and Response

100 A. Pittari et al.

2.1.2. Ignimbrites
Along with vesiculated juvenile clasts, free crystals and glass shards, lithic fragments
(fine lithic and vitric ash may be indistinguishable) are an important component of
ignimbrites, although they may vary from trace to greater than 50 vol.% in
abundance and occur in a diversity of associated facies types. Relatively high lithic
clast abundances may be widespread throughout massive to stratified ignimbrite
body facies (e.g. Calvache and Williams, 1992; Bryan et al., 1998b; Calder et al.,
2000; Pittari and Cas, 2004; Pittari et al., 2006; Edgar et al., 2007). Relatively larger
isolated ‘‘outsized’’ lithic clasts may be dispersed throughout massive ignimbrite
facies (e.g. Bryan et al., 1998a; Pittari et al., 2006). Lithic-rich breccias may occur as
stratigraphically confined or spatially localised facies or concentrations within
broader ignimbrite deposits. The different types are discussed below.
Ground breccias or ‘‘ground layers’’ (Walker et al., 1981; Druitt and Sparks, 1982;
Suzuki-Kamata, 1988), which are a type of Layer 1 deposit of Sparks et al. (1973),
or layer 1(H) of Wilson and Walker (1982), are sharply bounded layers enriched in
dense clasts and commonly depleted in fine ash relative to overlying pumiceous
ignimbrite facies. Ground breccias may frequently occur well away from proximal
areas and are interpreted to have formed at the head of a pyroclastic flow (Walker
et al., 1981).
Co-ignimbrite lag breccias (Wright and Walker, 1977; Wright, 1981; Druitt and
Sparks, 1982; Druitt, 1985; Bacon, 1983; Walker, 1985; Druitt and Bacon, 1986;
Suzuki-Kamata et al., 1993; Freundt and Schmincke, 1985; Self et al., 1986;
Fierstein and Hildreth, 1992; Cole et al., 1993; Rosi et al., 1996; Nairn et al., 2004;
Sottili et al., 2004; Pittari et al., 2006) are coarse (largest lithic clasts W2m
diameter), thick (20–30 m), often fines-depleted, clast-supported and stratified
lithic breccias which occur in vent-proximal locations. They are often deposited at
the peak of an eruption, particularly at the onset of caldera collapse, from expanded
pyroclastic flows near their point of initiation at the base of a collapsing eruption
column.
Outer slope fines-depleted lithic breccias (e.g. Roobol et al., 1987; Pittari et al., 2006)
are generally finer-grained than co-ignimbrite lag breccias, and were deposited
from pyroclastic flows that experienced enhanced flow-turbulence-induced fines
elutriation as a response to steep and/or highly vegetated slopes.
Widespread intra-ignimbrite matrix-supported lithic breccias or lithic concentration zones
can occur near the base (2bL of Sparks, 1976; see also Sparks et al., 1973;
Yokoyama, 1974; Freundt and Schmincke, 1985; Suzuki-Kamata and Kamata,
1990; Palladino and Valentine, 1995; Allen and Cas, 1998; Pittari et al., 2006), or at
higher stratigraphic levels (e.g. Bryan et al., 1998a; Pittari et al., 2006) within the
main body deposit of single ignimbrite depositional units. These types of breccias
may be locally discontinuous, but their occurrence at specific stratigraphic levels is
widespread. They can be single or multiple clast-thick horizons typically less than
1 m thick. Controls on their occurrence are generally related to variations in supply
from the vent and/or in response to specific topographic situations and local
substrate flow entrainment processes (Druitt and Sparks, 1982; Freundt and
Schmincke, 1985; Buesch, 1992; Allen and Cas, 1998; Bryan et al., 1998a; Pittari
et al., 2006).