Page 109 - Caldera Volcanism Analysis, Modelling and Response

84 Roberto Sulpizio and Pierfrancesco Dellino

5. Field Evidences of Stepwise Aggradation in Pulsating
PDCs

Following the previous discussion about recent developments in depositional
models for PDCs and their interaction with topography, this section will illustrate
some examples of real deposits interpreted using the model of stepwise
aggradation of discrete pulses. The range of the chosen examples is testament to
the applicability of the model to a wide spectrum of PDCs. The examples
comprise various typologies of flow-boundary zones developed within each
aggrading pulse, varying from traction- to granular flow- or fluid escape-
dominated.
Lithofacies analysis and association are the main tools used in reconstructing
the architecture of chosen PDC deposits and in supporting their sedimentological
interpretation. Lithofacies analysis is commonly used in sedimentological studies
of marine (e.g. Lowe, 1982), fluvial (Miall, 1978, 1985; Mathisen and Vondra,
1983; Smith, 1986, 1987) and alluvial fan environments (Waresback and
Turbeville, 1990; Zanchetta et al., 2004b), and has also been applied to the
study of complex sequences of pyroclastic deposits (e.g. Walker et al., 1980;
Walker, 1985; Sohn and Chough, 1989; Chough and Sohn, 1990; Colella and
Hiscott, 1997; Gurioli et al., 2002; Sulpizio, 2005; Sulpizio et al., 2007).
Lithofacies analysis was also successfully applied in describing the lateral and
vertical variations of sedimentary structures in single flow units within widespread
ignimbrites (e.g. Freundt and Schmincke, 1986; Druitt, 1992; Cole et al., 1993;
Allen and Cas, 1998). Lithofacies analysis allows us to gain insight into some
physical conditions that characterise the flow-boundary zone of a PDC at the
time of deposition, and provides some constraints on transport mechanisms and
interactions between solid and fluid phases.
The time-space association of different lithofacies represents the lithofacies
architecture. The different lithofacies record the physical conditions at the
flow-boundary at the time of deposition, while changes in lithofacies record the
non-uniformity of the current in time and space. Therefore, the study of lithofacies
architecture allows for inferences about changes in depositional regime that
occurred in time and space for a given PDC or for a set of PDCs.

5.1. Case studies

The repetitive occurrence of stacked, massive bodies with lithofacies mLA and
mLB, sometimes with reverse grading of lithic blocks (lithofacies mLA (il) and
mLB (il) ; Figure 15) is common in PDC successions. These successions have been
interpreted in the past as having been deposited en masse from different PDCs when
the driving gravitational force fell below the resistance force due to friction
between grains and between the flowing mass and the topographic surface (en masse
freezing; e.g. Sparks, 1976). An alternative model considers these deposits as
aggraded from sustained currents that changed their transportability with time
(aggrading model; e.g. Branney and Kokelaar, 1992).