DECONVOLUTION OF PYROCLASTIC GRAIN-SIZE SPECTRA FOR INTERPRETATION OFTRANSPORT MECHANISMS - AN APPLICATION TO THE AD 79 VESUVIO DEPOSITS

A computer code using sequential fragmentation/transport theory was us ed to deconvolute and characterize a large grain-size data set taken f rom the AD 79 Vesuvio deposits. The results allow us to interpret tran sport and deposition processes. Four principal morphological classes o f grain-size spectra were recognized in the AD 79 deposits: 1 unimodal distributions with coarse modes and very good sorting; 2 polymodal di stributions in which relative fractions of each subpopulation are cons iderably variable; 3 polymodal distributions, but with one mode greatl y prevailing over the other ones; 4 flat spectra in which a large numb er of size classes show the same loading. Because different eruptive, transport and deposition conditions may have operated on pyroclasts wh ich occur in the same bed, we have assigned grain-size subpopulations, with different modes to specific mechanisms of particle movement and sedimentation depending on the size range of the particles and the tex tures of the beds. The fragmentation/transport processes considered he re occur either within dilute flows (as fall, traction, saltation and suspension loads) or in high-concentration flows (as a fluidized syste m or one with an extremely high sedimentation rate). Variation in stre ngth and position of modes throughout the entire vertical section of A D 79 products illustrates changes in transport and deposition processe s with time. Size spectra from Vesuvio quantitatively demonstrate cont emporaneous deposition from fall and surge mechanisms as well as contr ibutions from different levels of hydrovolcanic products. In contrast, vertical variations in size spectra within individual pyroclastic flo w deposits suggest variation from high particle concentration near the base of the bed to more dilute depositional conditions towards the to p. Lateral variations in size spectra for one marker horizon show how a local pyroclastic flow in a channel grades into a surge on the margi ns. This study supports the model of continuous modification in loadin gs of several discrete subpopulations during deposition from a single explosive cloud.