Isere.fr

The geography of the department of Isère is the geography of a catchment area: from the banks of the Rhone to the summits of the Ecrins, the department of Isère extends over the Western flank of the Alps. This characteristic is what gives the department its unity and shapes its development. It not only marks the form of the relief, drainage and vegetation but also conditions the history and pattern of settlement: it is the key to understanding the major landscapes and their current dynamics.

The geological structure of the department of Isère

Principal units from east to west

The uplifted and metamorphosed basement forms the external crystalline massifs (granites and gneisses) of Oisans, Pelvoux, Grandes Rousses and Belledonne. It includes some coal-bearing shale basins of Paleozoic age (La Mure, Grandes Rousses and Belledonne). The sedimentary cover is detached from the basement and folded, forming the subalpine chains of Chartreuse and Vercors. A depression (Liassic) hollowed out by successive phases of fluviatile and glacial erosion separates the basement from the detached cover: it corresponds to the Alpine trough (in Grésivaudan and the Drac Valley). The tectonic foreland of Dauphiné is made up of materials filling the Rhone basin (Miocene molasse, and Piocene deposits with quartzite pebbles in a red clay matrix).

Geological history

Paleozoic

At the end of the Paleozoic era, the Pre-Triassic peneplain* was formed on a surface of granites and metamorphic schists. Carboniferous deposits are present as veneers or pinches in the tectonic faults formed at the end of the Hercynian orogenic cycle*.

Mesozoic

At the beginning of the Mesozoic era (Triassic period), an overstretching of the Earth's crust affected the Euro-African continent ("Tethyan extension"). This created a depression that became flooded, where sediments were accumulated.

During the Jurassic, the extension continued and limestones and marls formed a succession several thousands of metres thick in the Dauphiné basin (in particular, the high rock ledges of Tithonian age at the top of the Jurassic and Urgonian in the Lower Cretaceous). A second depression, known as the Piedmont basin, is located farther east and corresponds to an oceanic domain (Mesogea, part of the Tethys ocean) where the sediments were injected by greenstones (or ophiolites*). Between the two depressions, there is a "ridge" or submarine rise: the Briançonnais geanticline. During the Mid-Cretaceous, a process of subduction started with the European plate being thrust under the African plate. During the Late Cretaceous, the thrusting of the plates was accentuated, leading to the uplift of the topography. At the end of the Cretaceous, there was widespread emergence of the land.

Tertiary

The Paleogene: the marine transgression and tectonic movements of this period primarily concern the Briançonnais and Piedmont zones: uplift, overthrusting, back-thrusting and metamorphism. In the Miocene, the Dauphiné zone was covered by water for the last time: molasse* deposits were laid down in a lacustrine or epicontinental depression that extended from the valley of the Rhone all the way through Bavaria and Switzerland to Austria.

At the Miocene-Pliocene boundary, a paroxysmal phase took place during which the Dauphiné zone attained an aspect close to its present-day physiography. There was the final uplift of the external crystalline massifs, as well as the decollement and folding of the sedimentary cover (subalpine chains). The shortening of the chain, which started in the Cretaceous, came to an end.

At the end of the Pliocene, the Rhone basin was filled by materials eroded off the Alps, which were then transported and spread in sheets by powerful rivers. Pebbles of Alpine quartzites coated in a decalcified reddish clay form the surface of the plateaus in the tectonic foreland of Dauphiné (Bonnevaux, Chambarans, etc.).

Quaternary

The Quaternary period is characterized by glaciations and the appearance of Man. Only the last two glaciations are known in the Alps: Riss (external moraines with a front situated at Lyon) and Würm (internal moraines with a front at Rives). The existing form of the relief has been marked significantly by glacial erosion.

Riss Glaciation (three successive advances: Riss I, II and III)

The glacier of the Isère divided into two lobes downstream from the Cluse de Voreppe. One probably followed the present-day valley of the Isère as far as St Marcellin, while the other went beyond the Rives sill and covered the plain of Bièvres up to the morainal rampart of Faramans. The glacier of the Rhone, which advanced as far as Lyon, coalesced with the Isère glacier at Banchet hill (between the Bièvres and Liers plains). Although the glacier of the Drac crossed the Croix-Haute col and flowed into the Buëch during Riss I, it did not completely cover Trièves and Beaumont during Riss II.

The Grenoble basin was greatly overdeepened by the Riss glacier, becoming a lake that was quickly filled in.

Würm Glaciation (two successive advances: Würm I and II)

Towards the West, the glacier of the Isère did not extend beyond the Rives sill or the Polienas narrows. In the south, the glacier extended up the valley of the lower Drac to the current site of the Monteynard dam, thus forming a natural barrage impounding the Trièves Lake, which was subsequently filled by clay and gravel. The glacier of the Romanche overflowed the Laffrey sill, and its successive terminal moraines now separate the present-day lakes in the Matheysine area.

Pre-Triassic peneplain: eroded surface, almost without relief, that existed just before the Mesozoic era (the earliest period of the Mesozoic is called Triassic).

Hecynian orogenic cycle: succession of phenomena leading to the formation and subsequent erosion of mountain chains during the latter half of the Paleozoic era.

Ophiolites : submarine igneous rocks thrust up onto the continental crust during the collision between two plates.

Molasse : detrital sedimentary formations deposited at the end of the Alpine folding and derived from the erosion of neighbouring young relief.