14 8
communication), or the mixing of two separate Plinian-
derived eruptives within a short time range (days to
weeks).
8.2.2 Montchal Tephra
Bourdier ( 1980 ) described the products and the activity of
Montchal volcano. Short lava fl ows are present at the south-
ern foot of the volcano, but another one that is 16 km long
followed the Clamouze valley where Besse-en-Chandesse is
located. The tephra, composed essentially of scoriaceous
lapilli and ash, was not identifi ed in outcrops beyond 3 km
from the volcano (Fig. 8.3a ). Its phenocrysts are, in order of
frequency, clinopyroxenes, olivines, feldspars (plagioclases),
and opaque Fe-Ti oxides. Juvigné and Bastin ( 1995a ) identi-
fi ed Montchal tephra in three peat bogs situated west of the
fallout lobe as delimited by Bourdier ( 1980 ) (Fig. 8.3a, b ).
Since the macroscopic features of that tephra are similar to
those of an immediately older tephra deposit (see below), its
identifi cation was only possible after geochemical analyses
of bulk magma, which is (whole sample basis) a trachybasalt
(box S1 in Fig. 8.4 ).
8.2.3 Montcineyre Tephra
Bourdier ( 1980 ) described the products of Montcineyre
volcano. A 7-km-long lava fl ow followed southwards the
valley of Couze de Valbeleix where the village Compains is
located. The tephra, essentially composed of black scoria-
ceous lapilli and ash, was identifi ed in various outcrops as
far as 5 km away from the volcano towards the N-NE and
S-SW (Fig. 8.3a ). The tephra consists of an accumulation
of centimetre- thick beds without internal erosion features,
so that it corresponds to a series of fall deposits related to
as many pulses (Strombolian activity) during a very short
time span (weeks to months). The occurrence of
Montcineyre tephra in peat bogs and lakes enabled it to be
mapped as delimited by Bourdier ( 1980 ) (Fig. 8.3a, b ). By
naked eye, the Montcineyre tephra can be confused with
the Montchal tephra in the area where both lobes overlap
west of Pavin volcano. Otherwise, the tephra was not found
in peat-bogs south of La Godivelle and northwards in the
Sancy mountainous area, while the bed is still centimetre to
decimeter in thickness in the most western and eastern
investigated peat bogs. Unfortunately beyond those loca-
tions, the lack of peat bogs means it cannot be mapped
further.
In the peat bogs near the Pavin-Montchal-Estivadoux
group, the Montcineyre tephra is directly overlain by coarse
volcanic products that can be linked to ash-falls from
Montchal and/or Estivadoux , but these younger, coarse ash-
fall deposits were not investigated.8.3 Pollen Sequences
and TephrochronologyIn all pollen diagrams where both Pavin and Montcineyre
tephras were depicted, the tephras are shown to occur
between the beginning of the continuous curves of Fraxinus
(earlier) and those of Abies and Fagus (later) (see above,
Fig. 8.2 ), i.e. in the Atlantic palynozone (Reille et al. 1985 ).
Later on, referring to the age of the 2 %-point of the increase
of individual pollen curves, Juvigné et al. ( 1996 ) obtained
the following mean^14 C ages: for Fraxinus , 6702 ± 72 B.P.;
for Abies, 4994 ± 50 B.P.; and for Fagus 4994 ± 47 B.P. Hence,
the^14 C ages derived from pollen-bearing sediment and
bracketing the eruptive epoch of the MEMP volcanoes would
be 6702 ± 72 B.P. and 4994 ± 50 B.P. (see Juvigné et al.
1988a , p. 38), that is about 5620 B.C. and 3800 B.C. respec-
tively, after calibration. In this calculation, uncalibrated^14 C
data were processed for evaluating the age of the 2 % point of
the beginning of the continuous curve of both taxa in various
logs and fi nally averaged. Indeed, it would be more rigorous
to process initially calibrated data since the^14 C B.P. time
scale is not linear. This new calculation, which needs a
devoted development of the Bayesian model, has not yet
been achieved; however, preliminary results suggested that
the bracketing limits will not be signifi cantly modifi ed.
In each peat bog where both the Montcineyre and Pavin
tephras are present, they are separated by a few centimetres of
peat attesting that some time has elapsed between the relevant
eruptions. Based on the rate of peat formation of the interbed-
ded segment, the duration of the hiatus was estimated at
between 93 and 456 years; the large amplitude of the interval
can be explained by various factors including changing rate
of peat formation along the relevant segments and in the dif-
ferent peat bogs, disturbance of the original thickness of the
interbedded segment while coring, and deviation of^14 C dates
of the dated segments below and above the pair of tephra.
This hiatus between the eruption of the Montcineyre and
Pavin tephras is shown also by weak soil development in the
upper part of the Montcineyre tephra (Camus et al. 1973 ).8.4 Numerical Dating of the MEMP
VolcanoesCurrent numerical ages for the crater of Pavin and its three
companion volcanoes (the MEMP group) mainly rely on the
radiocarbon method and, to a lesser extent, on the thermolu-
minescence method (Table 8.1 ).E. Juvigné and D. Miallier