Characterization of phreatomagmatic deposits from the eruption of the Pavin Maar (France)

Abstract

The crater-lake Pavin, located in Auvergne and dated 6740 years BP, is the youngest volcano and one of the two acidic maars in metropolitan France. Field missions to the Pavin area were conducted for the past 5 years, leading to a better understanding of the Pavin tephrostratigraphy and geological history. Based on field textures, componentry and SEM morphoscopy of juvenile ash particles, a new complete tephrostratigraphy of the Pavin volcanic deposit is defined with a new reference section named Clidères. The 26 tephra beds and bed sets correspond to 4 volcanic units. The deposits are composed of high energy basal surges, lapilli fall and mixed dynamisms. The vertical variations of the maar deposits provide a way to access the fluctuating eruptive conditions related to changing magma-water interactions of the 4 main phases. The changes are associated to simultaneous variations of three factors: the pulsating mass eruption rates, the depth of fragmentation and the aquifer yield. Based on the combination of two geophysical methods, ground penetrating radar and electrical resistivity surveys, the boundaries of the volcanoclastic deposits are visualized and the average thickness of the formation is followed from proximal to intermediate locations. Including the combination of field observations of near 50 trenches of 1-2 m depth, a core drilling and geophysical profiles, the total volume of deposits is now estimated at 5.2 × 107 m3 which is 31 % less than previously estimated.