Abstract

The Annecy lacustrine depression lies in a tectonic transverse valley of structural origin, linking the present day towns ofAnnecy and Ugine and bounded by the Bauges and Bornes massifs. Only the last two major glacial periods are represented in the stratigraphie and geomorphic evidence from the area, but its successive glaciations that have been largely responsible for the form of the valley within which the present day lake lies. Of particular importance were the lobes of the Arve glacier, one shaping the depression from the southeast; the other from the north. The retreat of the glaciers left behind a more extensive precursor of the present day Lac d'Annecy, which latter constitutes the residual body of open water after lake level lowering, partial infilling and drainage diversions within the catchment. The drainage basin of the extended lake formed shortly after the last glacial maximum (LGM) in the region. Its extent can be determined from field sampling and coring and its elevation can bc established at 460 m above present day sea level. The present day lake is more isolated, since the Fier river, to the north, no longer drains into it. To the southeast, the course of the Chaise river has also been diverted, leaving the lake's drainage basin reduced to half its post LGM size. The major paleogeographical events in the LateQuaternary evolution of the basin are described in chronological sequence.

Introduction

This paper aims to reconstruct the history of the lacustrine depression situated in the transverse valley which stretches from Annecy to Ugine. The alignment of the valley is structurally determined and reflects tectonic and neotectonic extension of a fault system. The present day form of the val ley is largely an expression of the erosive action of latePleistocene glaciers. Within it are features linked to each of the last two major glacial stages, the "Riss" and "Würm". The over­deepening of the valley reftects the combined action of two separate arms of the ArveMont Blanc glacier, together with the sometimes reinforcing, sometimes opposing effects of the local Bauges and Val d'Arly glaciers, as well as the major Rhône glacier to the north.

There is clear stratigraphie evidence for the formation of a lake and for its partial infilling subsequent to each of the last two major glacial stages (Figure 1). The extent of the lake formed after the penultimate (`Riss') glaciation is difficult to establish since so little material, either morainic or lacustrine, survives from this period. The `postWürm' lake appears to have formed within the overall limits of the earlier one, but to have been slightly narrower. By now, it is partially infilled, both to the north and south of the present day stretch of open water.

One of the initial aims of the CLIMASILAC Project was to ascertain the environmental history of the lake and especially the pattern and sequence of late­Pleistocene sedimentation within the basin.

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Fig. 5 - Localisation of the sediments cores for the recent Lac d'Annecy (pdf 204ko)

Conclusions

The Annecy cross-valley between Bauges and Bornes is the result of structural and tectonic circumstances favourable to river erosion. Its geographical position has facilitated the advance of the Arve glacial flows to the south-east and north, which the local glaciers joined.

Two glacial periods have been identified: - the "Rissian" and the "Würmian". After each one, a Lac d'Annecy was established, due to deep glacial erosion of the terrain.

The earliest of the pre-existing lakes was established subsequent to the `Rissian' glaciation. It left traces only at the southern end of the present day lake. These are presently being studied, using recent cores. The fine lacustrine sediments have been covered by the layered interglacial lignite s.l., occurring as a sequence of discrete bands between 510 m and 450 m.

On the retreat of the glaciers during the late "Würm", a lake was re-established at a water level of around 460 m. Its 440 km2 drainage basin encompassed that of the Fier River which drained into it, thereby contributing to the infilling at the north end. When the Fier left the lacustrine depression to head west with all its sediments, the Lac d'Annecy remained isolated behind the deltaic infill from the Fier. The lake's drainage basin was thereby reduced to 280 km2, its present size.

Several factors remain unclear: the extent of the lacustrine infilling; the recent lake's southern limit; the degree of involvement and the duration of the Chaise River and the Arve-Arly fluvio-glacial flows during the post-glacial infilling. As for the precise dating of these events, from the "Würmian" glaciation to the Fier's rerouting, this remains enigmatic for field geologists. It is for this reason that the findings of the lacustrine cores ANI -2-3 are important.