Bündner slate
The Bündner schists are calcareous-clay sediments of the Alps that were deposited during the Jura and the Lower Cretaceous in the Pennine Walliser Trough , a sea basin of medium depth, and in the oceanic area of the Alpine Tethys . Due to later Alpine metamorphosis , it is now limestone , clay , mica or limestone slate. The name is derived from the Swiss canton of Graubünden , where they are particularly common. In France they are referred to as Schistes lustrés (' shiny slate'), in Italy they are part of the more broadly defined calcescisti , as they also occur in the Apennines . The Bündner slate was deposited in the Pennine Ocean , which was located between the southern edge of the Eurasian and Apulian plates and was closed during the alpine mountain formation .
rocks
The main mass of the Bündner schists consists of dark, fine-grained clay stones with varying sand and lime content, which often have a silky shine due to tectonic deformation and the mica formed during metamorphosis . Radiolarites and ophiolites are common with the Bündner schists, especially in the southern Penninic . This socialization suggests that the Bündnerschist was mostly deposited on oceanic crust . It is typical of the Penninic and was described by Gustav Steinmann , among others , after whom it was named Steinmann Trinity . Within the schist there are also chaotic slide masses containing large blocks and tangled masses of flysch sediments . The thickness of the layers is difficult to determine because of the intense deformation; in some areas it is several thousand meters.
Storage room
The slate depository was divided into two parts: in the south, the Bündner slate was deposited in the Piedmont-Ligurian Ocean , which is also known as the Alpine Tethys. Here the deposition took place almost entirely on oceanic crust, and ophiolites are common. In the north, the sedimentation took place in the Valais Ocean , which at least in its southern area also had oceanic crust. Between these oceanic areas was a high area with a continental crust, the Briançonnais , from which sediment material was repeatedly delivered into the oceanic areas.
tectonics
Since the Bündner schists consist of relatively easily deformable rocks, they were severely affected by the thrust of the alpine nappes . In some cases, they have experienced an intensive tectonic deformation, are mostly severely schisto , disturbed and wrinkled . In the vicinity of the ceiling sheeting, they are present as mylonites , in which boudins of other rocks are regulated, mostly ophiolites and marbles .
Within the Penninic units, the Bündner slate is essentially limited to the nappes of the North and South Penninic depending on their original deposit area. They often appear as a separating unit between the crystalline ceilings (so-called ceiling separators ).
The metamorphosis of the Bündner slate is always clear. Widespread evidence of a high pressure metamorphosis is the result of the subduction of the oceanic areas under the Apulian plate and the accretion wedge that precedes it.
Occurrence
As typical rocks of the Penninic, Bündner slate occurs in large deposits west of the Swiss-Austrian border in the entire western Alpine arc.
In Austria they are only known from the Rechnitz window , the Tauern window and the Engadine window , everywhere else they are overlaid there by the Eastern Alpine ceilings . The slate of the Rhenodanubian flysch , which accompanies the Alpine front almost all the way between Lake Constance and Vienna , is also referred to by some authors as Bündner slate.
In Switzerland, for example, the soft limestone and clay slates of the Bündner slate form the mountains between Brig and Nufenen Pass . They form the framework of the Bedretto Valley , the Safien Valley and the Hinterrheintal , and the Domleschg , Schanfigg and Prättigau valleys are also deepened into them. They are also found in the southern and eastern surroundings of the Gotthard massif , for example near Zermatt and Saas Fee . Here they also appear across the border in Italy. In France they continue as Schistes lustrés, which stretch south via Savoy and also appear in northern Corsica .
literature
- Toni P. Labhart: Geology of Switzerland . Ott Verlag, Thun 2001, ISBN 3-7225-6760-2 .
- Stefan M. Schmid , Bernhard Fügenschuh, Eduard Kissling, Ralf Schuster: Tectonic map and overall architecture of the Alpine orogen. In: Eclogae geologicae Helvetiae , Vol. 97, 2004, pp. 93–117 ( PDF )
- Pantić, N. & Isler, A. (1980): "Schistes-lustrés" deposits from the Tethys. Eclogae Geologicae Helvetiae 73, 799-822. ( PDF )
Web links
- Andreas Ebert: The geological development of the Central Alps - Regional geology in the area around Zermatt , 2001