Jean-François Buoncristiani

Hirnantian glacial and deglacial record in SW Djado Basin (NE Niger).

Pluridisciplinary fieldwork highlights features generated by an extended ice-sheet in the Djado Basin during the Hirnantian. Two glacial palaeovalley systems associated with glacial pavements and separated by thin glaciomarine interstadial series are revealed. Rigid glacial pavements characterised by abrasion erosion are differentiated from soft glacial pavements characterised by soft-bed deformation. Glacial pavements are associated with subglacial bedforms such as megaflutes, flutes and meltwater channels. They are also associated with clastic dykes and glaciotectonic structures such as deformed flutes, subglacial folds and duplex structures. This record demonstrates that ice was warm-based and flowed rapidly on the highfluid- pressure soft substrate, as for ice streams. The erosional glacial landscape is typical of areal scouring, and the depositional sediment-landform assemblage corresponds to subglacial processes. These data afford a reconstruction of glacial events which is consistent with the two polyphased low-frequency glacial cycles inferred in previous studies. During interstadial and postglacial stages, grabens, normal faults, radial extensional microfaults and extensional dihedrons were generated by extensional tectonics during glacio-isostatic rebound. In sectors highly affected by this tectonics, doleritic dykes reflect a basal crust fusion increase induced by adiabatic decompression.

The palaeogeography of the last two glacial episodes in France: the Alps and Jura

This chapter discusses the palaeogeography of the last two glacial episodes in Alps and Jura. Both the Jura and the Alps were covered by major glaciers during the past cold periods of the Quaternary. Nowhere in the Alps have Quaternary deposits been completely preserved. Therefore, the basic stratigraphic subdivision is combined from various regional stratigraphies, though in many cases the determination is difficult because age control is lacking. Indications of Early Quaternary cold stages have long since been reported from the Italian and French Alps. In both areas, an abrupt change from fine-grained sedimentation to the deposition of coarse gravel and conglomerates is manifested in what was regarded as the “Upper Pliocene.” However, the evidence is ambiguous. The change in sedimentary environment might either be explained by tectonic causes or by climatic changes.

Chapter 10 – Quaternary Glaciations in the French Alps and Jura

During last glacial maximum, the pre-existing relief of the Alps and the Jura Mountains controlled glacier development. The Jura massive morphology, only slightly dissected by valleys, resulted in a wide ice-sheet; and this situation contrasts with the Alps where numerous deep valleys were filled by big glaciers.

Palaeogeography of the last two glacial episodes in the Massif Central, France

This chapter discusses paleogeography of the last two glacial episodes in the Massif Central in France. As a result of the altitudinal gradient from the west to the east in the massif, different glacial systems developed. To the west the massifs of Mont-Dore, Cantal, Aubrac, and Margeride thus form a north–south orientated mountain barrier extending about 150 km, where the highest peaks reach 1400–1880 m. Glaciation was, therefore, most extensive in this region. The most widespread Pleistocene glacier system in the Massif Central is found on the Massif du Cantal. Based on well-marked frontal moraines and other traces of glaciation, paleogeographical reconstruction shows that there was an ice-cap with several outlet glaciers. The Mont-Dore Massif is only a quarter the size and is also much younger than the Cantal.