Geology, geomorphology and slope instability of the Maar Lake Pavin (Auvergne, French Massif Central)

Abstract

Maars are craters created by violent phreatomagmatic eruptions. The crater shape involves roughly circular rims whose asymmetric slopes may be unstable long after the initial eruption. Lakes that occupy many maars are natural receptacles that enclose geologic archives such as slope deposits. Here we describe the geologic and geomorphic characteristics of the maar and Lake Pavin in Auvergne with emphasis on recent and current slope instability. This is due to its geometry (the 800 × 92 m lake occupies a wide depression cut deep in pre-existing lava flows and Montchal cone), loose surficial formations on steep subaerial slopes and fractured lava scarps, and loose and gas-rich sediment on sub-lacustrine steep slopes. Our study of the outer rim slopes (<20°) of the maar shows that current geomorphic processes apparently act slowly, but mapping of the steepest (>31°), inner rim slopes suggests that instability is now related to runoff, solifluction and perhaps rockslides or deep-seated landslides. The slow and often small-sized mass movements occur on steep slopes >31° and fractured lava flow scarps while solifluction is favored by loose and thick, surficial maar deposits and a 4-5 month-long snow cover. Geomorphic anomalies on top of the north and NE maar rims suggest deep-seated, (slow moving?) rotational landslides that may record a long-lasting post-eruptive response to maar collapse. One of the large, recent rock fall scree on the NNE edge of the lake is apparently connected to the submerged platform capping a syn-eruptive collapse mass. The quasi-vertical edge of this platform may act as a source of debris transfer towards the deep lake bottom. Long subaerial slopes south and SE of the maar point out to the most unstable sector: fractured, thick lava flow scarps topple and produce scree, and permanent springs feed runoff and streams above the underlying clay-rich pyroclastic deposit. The south slope overhangs subaquatic lava cliffs which can transfer debris directly to the lake bottom 90 m below. In contrast, mapping of the recent fan of the lake outlet and the adjacent Gelat valley to the north, in which the outlet stream is incised, show no evidence for debris-flow deposits that were claimed to be emplaced by a historical catastrophic event triggered by a lake breakout.