Directional fabric measurements: an investigative approach to transport and depositional mechanisms in pyroclastic flows

Citation
B. Capaccioni et al., Directional fabric measurements: an investigative approach to transport and depositional mechanisms in pyroclastic flows, J VOLCANOL, 107(4), 2001, pp. 275-292
Citations number
62
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Earth Sciences
Journal title
JOURNAL OF VOLCANOLOGY AND GEOTHERMAL RESEARCH
ISSN journal
0377-0273 → ACNP
Volume
107
Issue
4
Year of publication
2001
Pages
275 - 292
Database
ISI
SICI code
0377-0273(200107)107:4<275:DFMAIA>2.0.ZU;2-O
Abstract
Computer-assisted image analysis data of rock fabrics from two quaternary i gnimbrites in the Vulsini and Cimini Volcanic Districts of Central Italy ar e interpreted in terms of transport and depositional mechanisms. Samples we re collected vertically at in spaces up two sections through each deposit. The Orvieto-Bagnoregio ignimbrite (OBI) is a non-welded ignimbrite that sho ws both fluctuations in the mean particle orientation values of up to appro ximately +/- 60 degrees, and large variations in the strength of particle i so-orientation with height. The circular frequency distributions of particl e orientations are almost always anisotropic and unimodal, in line with a t heoretical Von Mises distribution (the circular equivalent of a unimodal, l og-normal distribution). In contrast, the welded Cimina ignimbrite (CI) sho ws vertical homogeneities in mean orientation values with height, and gener ally lower degrees of anisotropy. Such differences are interpreted as being the results of different depositional mechanisms: incremental deposition a t the base of a density-stratified, partially turbulent flow for the OBI; d eposition of a laminar mass flow for the CI. In the former case, during tra nsport particles under solidus temperature are subjected to a frictional re gime, particles gliding and dispersive pressures, which finally produce siz e-inverse grading and variable fabric development, depending on the residen ce time of particles at the basal shear conditions. In the latter case, elo ngated particles, supported in a laminar flowing viscous matrix, undergo pe riodic motions which tend to develop parallel-to-flow iso-orientation. Fabr ic data in the deposit suggest vertical constancy in the rheological proper ties of the flow, absence of rheological decoupling and (shearing pervasive ly during transport) a minor importance of plug horizons. (C) 2001 Elsevier Science B.V. All rights reserved.