Emmanuel Chapron

French Alpine Foreland Holocene Paleoseismicity Revealed by Coeval Mass Wasting Deposits in Glacial Lakes

The French alpine foreland area has been struck by several earthquakes with magnitudes above 5 on Richter scale in recent history. In this paper we document the regional impact of historical and Holocene earthquakes based on the identification of mass wasting deposits in glacial lakes at different settings. Lake Le Bourget and Lake Paladru are situated at low elevations (respectively 231 m–492 m) and Lake Blanc Huez is located at 2500 m altitude. Through the integration of high-resolution acoustic profiles and accurately dated sediment samples from cores, recent coeval mass wasting deposits in each lake were correlated with nearby historical earthquakes, whereas coeval mass wasting deposits around 5200 cal BP and 9550 cal BP in these three lakes were correlated to regional earthquakes events.

Lake Pavin Paleolimnology and Event Stratigraphy

In this chapter we present an up-to-date database of sedimentary sequences retrieved from Lake Pavin during the last 50 years in both oxic and anoxic waters. The detailed history of this mid Holocene crater lake can be reconstructed from the correlation of radiocarbon dated sedimentary sequences retrieved from the deep central basin, a subaquatic plateau and littoral environments. High-resolution measurements of sediment composition (diffuse spectral refl ectance, XRF core scanning) combined with the analysis of organic matter composition and preliminary pollen and diatom assemblages investigations on selected sediment cores are used to reconstruct (i) the evolution since ca. 7000 cal BP of Lake Pavin limnology together with its radiocarbon reservoir effect and (ii) the impact of a wide range of subaquatic slope failure events. Such a multidisciplinary approach of Lake Pavin basin fi ll revealed contrasted sedimentation patterns just after the volcanic eruption and following the development of a dense vegetation cover along the slopes of the crater. Pavin sedimenta-tion is rapidly and largely dominated by organic rich and fi nely laminated diatomite formation , but several short periods of enhanced mineral inputs might refl ect the infl uence of wetter periods, such as the Little Ice Age. Over the last millennium two large subaquatic mass wasting events are also identifi ed and may have signifi cantly impacted its limnology.

Lake Pavin Sedimentary Environments

In the recent years, Lake Pavin sedimentary basin has been intensively studied by several acoustic surveys (high resolution seismic reflection profiling, multibeam bathymetry) and gravity coring campaigns. This new data set combining acoustic images and multidisciplinary study of sediment cores allows characterizing contrasted subaquatic sedimentary environments along the littoral slopes, a subaquatic plateau (close to the lake outlet), steep slopes and its deep central basin. Two main types of lacustrine sediments are identified (i) between the lake shore and 26 m water depth (massive organic rich sandy silts), and (ii) below 26 m water depth when the lake floor slopes are less than 15° steep (organic rich laminated diatomites). A large and recent slide scar is in particular clearly identified at the edge of the plateau just above the deep central basin. Evidences of former gravity reworking phenomena within the crater ring draining into Lake Pavin also include a large subaquatic slump deposit accumulated on the subaquatic plateau and several small scale rock fall deposits originating from outcropping lavas within the crater ring. The identification of two recent outstanding erosive sandy layers in Pavin littoral environment also suggests that some of this gravity reworking phenomena have been associated with unusually violent waves and/or abrupt lake level drop. Lake Pavin geomorphology and sedimentary environments are in addition compared to the ones of the nearby Lake Chauvet based on a similar acoustic and sedimentary data base in order to highlight the influence of maar age and geomorphology on the development of sedimentary environments and Natural Hazards in this volcanic region of the French Massif Central. Lake Chauvet is comparatively to Lake Pavin characterized by a shallower central basin, less steep slopes and no subaquatic plateaus. A recent and relatively large mass wasting deposit is, however, clearly identified along the slopes of a small delta facing the only tributary of this maar lake. This work suggest that maar lakes from the study area are concerned by subaquatic slope stabilities, especially in Lake Pavin were slope failures may in addition impact the development or the stability of its meromicticity.