114
6.4.1.3 Minerals (Mnls)
Free minerals in the PD are diverse in nature. Two categories
of free minerals have to be distinguished: xenocrysts from
the basement (quartz, some feldspars and micas) and free
minerals from the juvenile magma (feldspar, amphibole and
pyroxene). Minerals are often found broken. The amounts
vary widely depending on the fractions’ size.
6.4.2 The Clidères Section
The Clidères section is located 1.5 km south-east from the
vent. By creating three study stations with different orienta-
tions, the Clidères outcrop allows an optimal characteriza-
tion of the sedimentary structures in the PD (Fig. 6.6). Station
n°1 shows the regional tephrostratigraphy of the Pavin group
whereas the two other stations include only the Pavin deposit
and a part of its direct basement layer as a reference level
(i.e. Montchal trachy-basaltic lapilli deposit, noted BML).
The Montcineyre basaltic lapilli are not present at this dis-
tance from the vent.
Stratigraphic subdivisions were performed according to
petrographical, sedimentological and grain size data. 26
tephra beds are grouped in four main units in the PD, which
has a total thickness of 2.53 m at the Clidères outcrop (Fig.
6.7). The nomenclature used to describe the geological for-
mations is as follows: P refers to the Pavin deposit, which is
then subdivided in 4 units numbered from the oldest to theFig. 6.6 The Clidères outcrop. (a) Location of the Pavin’s reference outcrops; C – Clidères, L – La Liste, The topographic map is from IGN, 2006.
(b) Photography of the Clidères outcrop and localization of the 3 study stations. (c) Station 1. (d) Station 2. (e) Station 3
H. Leyrit et al.