Lake Pavin History, geology, biogeochemistry, and sedimentology of a deep meromictic maar lake

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reference section, named “La Liste” is compared and corre-
lated. These two reference sections were chosen for the good
quality of the outcrops, their very interesting geographical
locations and the possibility of studying the deposit in
different orientations with a 7–10 m extended side. Clidères
and La Liste outcrops are located respectively 1.5 km south-
east and 2.4 km south from the vent (Fig. 6.4) in the interme-
diate area of PD. They are located at the appropriate distance
from the vent to observe the bulk of sedimentary structures,
that is to say neither too proximal (thus the deposit is unstrat-
ified and too thick to reach the basement) or too distal (where
energy is too low to observe any sedimentary structure). By
describing structural, petrological and geometrical features
of each tephra bed, the two sections units can be correlated
and the main evolution of eruptive dynamics defined. We
estimate the roundness and sphericity of grains using the

visual chart of Krumbein and Sloss ( 1956 ). These outcrops,

associated with the 1979 Pavin drilling, are used to calibrate

the geophysical data: Electrical Resistivity Tomography

(ERT) and Ground Penetrating Radar (GPR).

6.3.2 Electrical Resistivity Tomography

The ERT is an electrical method based on the measurement

of the apparent resistivity ρa (Ω.m) in the ground after the

injection of a direct current IAB using two electrodes (A for

the injection and B for the reception). The voltage difference

ΔVMN is measured with two others electrodes (M and N)

along a line. The apparent resistivity is then calculated with

the following equation: ρa MN

AB

k V

I

= ∆

Fig. 6.3 Isopach map of the
trachy-andesitic pumiceous
flow from the Pavin
(thickness in meters)
(Bourdier 1980 )

6 Characterization of Phreatomagmatic Deposits from the Eruption of the Pavin Maar (France)