Swiss plateau: Encyclopedia II - Swiss plateau - Geology

Swiss plateau - Geology

Swiss plateau - Geological layers

Thanks to many deep drillings for oil and gas, the geological layers of the Swiss plateau are relatively well known. The base level is cristalline basement which outcrops in the central cristalline Alps as well as in the Black Forest and in the Vosges mountain range but forms a deep geosynclinal in the Swiss plateau and in the Jura. Around 2500 – 3000 meters below the ground, but considerably deeper near the Alps, the drillings have hit the cristalline basement. It is covered by unfolded layers of Mesozoic sediments from the Triassic, the Jurassic and the Cretaceous. Its gauge gradually decreases from about 2.5 kilometers in the west to 0.8 kilometers in the east. These layers, like the ones of the Jura mountains, have sedimented in a relatively flat sea, the Tethys Ocean. Above the Mesozoic layers, there is Molasse, consisting of conglomerates, sands, marl and clay and compressed to stone by the weight of the sediments that have covered it. The uppermost layer consists of gravel and loose stones that have been transported by the glaciers of the Ice ages.

Swiss plateau - Molasse

Geologically the most important layer of the Swiss plateau is the mighty molasse that has been sedimented at the border of the Alps due to mountain erosion. The gauge of the molasse increases from west to east (at the same distance from the Alps). The former Alpine rivers, which mostly did not correspond to the contemporary rivers, built huge fans of sediments at the foot of the mountains. The most important examples are the Napf fan and the Hörnli fan; other sedimental fans exist in the Rigi region, in the Schwarzenburg region and in the region between the eastern lake Geneva and the middle reaches of the Saane/Sarine.

The eroded material has been sorted by grain size. The coarse material has predominantly sedimented near the Alps, as soon as the water current which carried it became too feeble. In the middle of the plateau, there are finer sandstones and near the Jura, clays and marl.

history of the Swiss plateau molasse

In the uprise of the Tertiary, around 60 – 40 millions of years ago, the area of today's Swiss platea was a Karst plateau somewhat inclined southwards and drained in that direction, too. Through processes of rising and lowering that were brought by the folding of the Alps, the area was twice flooded by a sea. The corresponding sediments are distinguished as sea molasse and freshwater molasse, even though the latter is consists rather of fluvial sediments and aerosols (so it is rather a kind of mainland molasse).

• Lower sea molasse (around 37 - 30 millions of years ago): The lime plateau lowered gradually, and a shallow sea broke in, spreading east until the Carpathian Mountains. The sediments consisted of fine-grained sands, clay and marl. There were no conglomerate fans yet since the proper Alpine folding began only at the end of that period.
• Lower sweetwater molasse (around 30 - 22 millions of years ago): The sea receded because of enhancements, but also because of a worldwide decreasement of the mean sea level. Along with the folding of the Alps, their erosion started and the first conglomerate fans accrued.
• Upper sea molasse (around 22 - 16 millions of years ago): For a second time, a shallow sea broke in. The formation of the conglomerate fans of the Napf and of the Hörnli began.
• Upper sweetwater molasse (about 16 - 2 millions of years ago): The sea receded definitively. The formation and of the Napf and Hörnli fans continued (alongside other minor fans). At the end of this period, the gauge reached about 1500 meters.

In the following time, especially the western part of the plateau was again significantly risen, so that in this area, the sediments of the upper sweetwater molasse and the upper sea molasse have been largely eroded.

A characteristic of the sea molasses ar petrified snails, shells and shark teeth, whereas in the sweetwater molasses, petrifications of typical land mammals can be found or of the former subtropical vegetation (for instance palm leaves).

Swiss plateau - The Ice ages

The contemporary landscape of the Swiss plateau has been shaped by the Ice age glaciers. During all the known alpine glaciations (Günz glaciation, Mindel glaciation, Riss glaciation and Würm glaciation), huge glaciers penetrated the Swiss plateau. During the warm interglacials, the glaciers receded to the high alps (sometimes more than today) and subtropical vegetation spread in the plateau.

During the Ice ages, the Rhône glacier ramified into two branches when leaving the Alps, covering the whole western Swiss plateau until today's regions of Solothurn and Aarau. In the region of Bern it merged with the Aar glacier. The glaciers of the Reuss, the Limmat and the Rhine advanced sometimes as well until the Jura. The glaciers formed the land by erosion, but also by base moraines (very fine stone meal) that measure often several meters, and by the front end rivers depositioning gravel.

Traces of the older Günz and Mindel glaciation are only left in few places, because most has been removed or transferred by the later glaciations. The greatest extension was reached by the glaciers of the Riss glaciation, when the entire Swiss plateau was covered with ice except for the Napf and Töss regions. Most notable are the traces of the Würm glaciation about 115 000 years ago. The end moraines of different retraction phases have been conservated.

Swiss plateau - Landscapes

A look at a map still reveals the directions where the ice age glaciers runned. The farthest expansion of the Rhône glacier to the northeast is indicated by way the western Swiss plateau valleys run: The valleys of the Broye and the Glâne as well as lake Neuchâtel, lake Biel and lake Murten run all northeast, parallel to the Jura and to the Alps. The glaciers of the Reuss and the Limmat have carved the valleys of the central Swiss plateau that run northwest (among others the valleys of the Wigger, the Suhre, the Seetal, the Reuss and the Limmat). The Rhine glacier has mostly left traces that run west: The eastern Swiss plateau Thur valley and lake Constance. In certain places, there are characteristic drumlins of pressed base moraine, often clustered, especially in the highlands of Zürich, in the Hirzel region, in the lake Constance region and between the Reuss valley and the lake Baldegg.

Another reminiscence of the glaciation are glacial erratics which are found all over the Swiss plateau. These rocks, sometimes of an enormous size, are of alien stones, mostly Granite and Gneiss from the central cristalline Alps. They were one of the clues that led to the substantiation of the glaciation theory in the 19th century since a transport by water or by vulcanism was physically impossible.

Gravel deopositions in the bottoms of the valleys are another testimonial of the glaciation. During the pushes and withdrawels of the glaciers, gravel layers were deposited in the valleys, sometimes mighty ones, though most of it eroded in the subsequent interglacials except for a few rests. Therefore, many valleys have characteristic terraces, the lower terraces consisting of Würm glaciation gravel, the higher terraces of Riss glaciation terraces. Sometimes, there is also gravel from older glaciations.

Adapted from the Wikipedia article "Geology", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki