Le Drac, morphologie, stratigraphie et chronologie quaternaires d'un bassin alpin .

Abstract : Wholly submerged during the Mesozoic. the Drac basin area began to emerge by the end of the Cretaceous( Senonian ). An hydrographic system differing from the present one is formed at the Eocene ( Lutetian. Priabonian). It is only on the Oligocene that the streams settle in today's valleys. the general morphology thustakes shape by' steps. After various events.the main features of the system are competed by the end of the Miocene. Although nothing is know of its pliocene. villafranchian and. middle to lower quatemary history. it is sure that the valleys were deepened. then that the high moutains. built up during the" ponto'pliocène .. orogenic mouvements were covered by ice which flowed down, perhaps digging out the first under-deepings. The morphology of the Drac basin is typically glacial. especially in the profiles of the valleys. There are ten main transfiuences and diffluences, some umbilics and bars, numerous hanging valleys, glacial circuses and numberless minor forms. In this region, there are no nested valleys dating différent glacial periods. and no glacial ganging . Two sorts of transfluences are distinguised, according to their origin, either preglacial ( section of old fluvial valleys ) or only glacial ( erosion of passes) ; their genesis is explained by the dynamics of the glaciers and a relative chronology is propounded. The bars are systematically located on the relative by more resistant zones, the umbilics in the weaker ones. The evolution of these forms is governed by the regional structure and by ice dynamics, the holds true for the hangings valleys in relation with the power of their glaciers . We propound a more precise genetic definition of glacial circuses. In fact, glacial erosion is a highly developped kind of plain differencial erosion. The Quatemary story begins durIng the Mindel-Riss Interglacial with the deepening of the first Drac fossil river system. Then, the ice invaded ail the basin ( Riss 1 1 ). overf!owing through the Croix-Haute pass and establishing a large icestream from the Durance to the Isere glaciers. During the Riss. II, a continuous gravel terrace is deposited. It corresponds, upstream. to large torrential fans, under a peri glacial climate. The glaciers came again twice, ,on Riss III., over the previous deposits but to a lesser extent, as the ice did not overflow through the Croix -Haute pass. . During the Riss-Wurm Interglacial, the Drac river dug a second epigenic fossil valley system, as deep as the first. On the beginning of Wurm ( Wurm 1 ), another gravel terrace filled up the interglacial talwegs. Immediatly after, the glaciers arrived and reached their maximal extension of this period, but with little development in the Drac bassin ( Würm II). There are only valley glaciers, located in the high range, excepted in Champsaur. The main phenomenon was a big glacio-lacustrine obturation by the Isère glacier in the ice-free Trièves and Beaumont. Bedded clays, sands and deltaïc gravels were deposited, overlying all the earlier formations. The Würm II-III Intestadial lasted long, and the glaciers went up far away in the high vaIIeus. During the Wurm III, there was a new, barely less extensive advance on the previous deposits. No overflowing occured this time, but there was a last glacio-lacustrine obturation to the South of Grenoble. The glaciers then finally retreated and the Drac and its affluents river system, emplaced since the beginning of the Wurm II retreat, deepened epigenically down to their present-day level.
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Guy Monjuvent. Le Drac, morphologie, stratigraphie et chronologie quaternaires d'un bassin alpin .. Géomorphologie. Université Paris-Diderot - Paris VII, 1979. Français. ⟨tel-00441486⟩

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