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

117

lapilli + bloc fraction is difficult to estimate, varying between
30 % (bed P3d) and 60 % (bed P3c). Sedimentary structures
typical of surge deposits are more frequent than in other lay-
ers. Some channels, filled by coarser ash and characterized by
diminishing grain-size towards the margin, are interpreted as

erosional features of the pyroclastic surges. Cross-

stratifications are observed in the P3b bed (Fig. 6.8, 2a and

2b) as well as a dune prograding to azimuth 70° overlapping

oblique stratifications in which coarser beds are found (bot-

tom left of the photography). This structure indicates that the

Fig. 6.8 The five main types of sedimentary structures of the Pavin
deposit. Each number is accounting for a type of structure. (a) is the
sedimentary structure as observed on the outcrop. (b) is the interpreta-

tion. (1) Anti-dunes and associated second order ripples. (2) Cross-

stratifications. (3) U-type erosional channels. (4) Normal grading. (5)

Impact-sags

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