The deposits of two volcanic debris avalanches (VDA I and II) that occur
in the upper Maronne valley, northwest sector of Cantal Volcano, France,
were studied to establish their mechanisms of formation, transport and
deposition. These two volcanic debris avalanches that clearly differ
with regard to their structures, textures and extensions, exemplify the
wide spectrum of events associated with large-scale sector collapse.
VDA I is voluminous (similar to1 km(3) in the upper Maronne valley) and
widespread. The deposits comprise two distinct facies: the block facies
that forms the intermediate and upper part of the unit and the mixed
facies that crops out essentially at the base of the unit. The block
facies consists of more or less brecciated lava, block-and-ash-flow
breccia and pumice-flow tuff megablocks set in breccias resulting from
block disaggregation. Mixing and differential movements are almost
absent in this part of the VDA. The mixed facies consists of breccias
rich in fine particles that originate from block disagregation, as well
as being picked up from the substratum during movement. Mixing and
differential movements are predominant in this zone. Analysis of
fractures on lava megablocks suggests that shear stress during the
initial sliding is the principal cause of fracture. These data strongly
indicate that VDA I is purely gravitational and argue for a model in
which the initial sliding mass transforms into a flow due to
differential in situ fragmentation caused by the shear stress.
VDA II is restricted to low-topography areas. Its volume, in the studied
area, is about 0.3 km(3). The deposits consist of brecciated, rounded
blocks and megablocks set in a fine-grained matrix composed essentially
of volcanic glass. This unit is stratified, with a massive layer that
contains all the megablocks at the base and in the intermediate part,
and in the upper part a normally graded layer that contains only blocks
<1 m in size. The different lithologies present are totally mixed. These
observations suggest that VDA II may be of the Bezymianny-type and that
it underwent a flow transformation from a turbulent to a stratified flow
consisting of a basal hyperconcentrated laminar body overlain by a
dilute layer. (C) 2000 Elsevier Science B.V. All rights reserved.