Flyschzone - Flyschzone

The flysch zone as part of the Penninic (purple) as a narrow band on the northern edge of the Alps

In geology, the flysch zone or Rhenodanubian flysch is a comparatively narrow rock unit stretching north of the Eastern Alps and the Carpathian Mountains . Geographically, it is part of the transition area between the Alps and the northern Alpine foothills . It consists predominantly of clay and sandstone , which is why it was formerly known as the sandstone zone.

Occurrence

Geological map of Germany, the German Geological Society, 1869
The flysch zone on the northern edge of the Alps and Carpathians (center right in orange)

As a unit that is particularly widespread in the Eastern Alps , the flysch rocks of the flysch zone on the northern edge of the Alps form gentle hill shapes . In the Swiss area, their rocks can only be detected in isolated isolated occurrences on the northern edge of the Alps.

The flysch zone is most pronounced at the western end of its main distribution area, where it forms the Bregenzerwald east of Lake Constance . Further to the east it no longer forms any noteworthy mountain formations - just like the underlying Helvetian nappes , which are still peaks in the Allgäu . On long stretches vanishes here among them about donating rocks of the Northern Limestone Alps .

Only east of Salzburg is the flysch zone several kilometers wide again ( Salzburg Pre-Alps , hills of the Salzburg Lake District ). It also forms the northern zone of the Upper Austrian Prealps and the Eisenwurzen and on the northeastern edge of the Alps the northern or Flysch- Wienerwald .

With the Bisamberg and the Waschbergzug, it continues beyond the Danube, runs over the cliff zone ( Steinitz Forest ) far beyond the Alps and accompanies the Carpathians on their north and east sides in a zone up to 100 km wide , especially as a Carpathian flysch . The last foothills of the flysch zone disappear north of Bucharest under the molasses sediments of the Carpathian pre-depth .

Surface shape

View to the north of the narrow zone of the Flysch Mountains around the Mondsee and the Alpine foothills behind it
In the opposite direction, the directly towering limestone wall of the Drachenwand

The mountains of the flysch zone are strongly overprinted by the glaciers and periglacial processes of the ice ages , as the flysch rocks that build them up are easily weathered, often do not have great internal cohesion and still tend to flow today ( Swiss German flysch ). There are gentle hill formations and rounded mountain tops .

The peaks of the northern limestone Alps adjoining to the south, as well as those of the Helvetic nappes, sometimes set off quite suddenly as a stick , karst plateau or wall against the significantly lower mountains of the flysch zone.

Formation and rocks

Geologically, the rhenodanubian flysch was formed during the folding of the Alps from the deposits in the northern half of the Pennine area of ​​the ancient Mediterranean, the Tethys , to the south of Europe . Both temporally and in the geographical sequence of the ceiling systems, it stands between the Helvetic nappes , which originate from the original European continental shelf, and the northern limestone Alps (the tectonically deepest layer of which is the Tirolean ), those from the northern edge of the Adriatic plate belonging to Africacome. In the Alpine region, the rocks of the flysch zone are associated with the other flysch rocks of the western Alpine Penninic such as the Bündner schist , which were also formed as flysch rocks in the Penninic Ocean. [1]

The Flysche formed from the Lower Cretaceous to the Eocene , ie in the period roughly 150–50 million years ago, and are thus younger than the typical Triassic (around 250–200 million years old) Alpine Limes. At this time the Alps began to unfold in depth.

The rocks the Flyschzone are predominantly in deep sea below the carbonate compensation depth (3000-5000 m) deposited Turbidite sequences - so sediments submarine landslides - of sandstones , siltstones , marls and shales , partly calcareous and sometimes kieselig that at their base can also contain scree layers , and are closed at the top by the finest-grained settlement layers. Only in the lower part of each layer sequence are fossils washed in with the turbiditesfound more often. These mass movements from the continental shelf at that time are likely to have been triggered by earthquakes, for example, and have been repeated many times, and the flysch zone has built up up to 2000 m thick. [2] [3]

The strata of the Limestone Alps and the strata of the Pennine Ocean that were originally in front of them to the north were pushed up and over a large area in the course of the mountain formation on the Helvetian shelf belonging to the European continent. The Rhenodanubian flysch forms the lowest layer in this nappe pile lying on the layers of the Helveticum. When it was pushed onto the European continental margin, it was pushed in front of the ceiling stack and was the first part of the stack to be pushed onto the European shelf. Later in the course of the further collision, the Limestone Alps slowly slid northwards from their position further south and higher in the pile of nappies when the pile of nappes bulged and slid over the flysch.

As a result, they form a comparatively narrow zone of evacuated layer sequences on the northern edge of the Alps, which quickly plunge into the depths towards the south. During deep drilling, the continuations of the now exposed flysch layer under the limestone Alps were found at around 5500 m ( Berndorf , Molln / Breitenau - only a dozen kilometers south of the edge of the Alps). In some places the flysch is still exposed in the middle of the Kalkalpen ( flysch window , for example in Windischgarsten in the Kalkalpen National Park). [2]

The subalpine molasse (molasse zone), which, like the flysch zone, also forms domes and is morphologically similar , which today adjoins the flysch zone to the north, arose in its main mass under very different geological conditions, and mainly erosion sediment from the already rising Alps into the Paratethys flat sea, the final phase of the Thetys north and east of the Alps, which lasted from the Eocene to the Miocene only about 15 million years ago.

Literature and maps

  • Johann Egger: On the paleogeographical position of the Rhenodanubian Flysch (Neocom-Eocene) of the Eastern Alps. In: Jahrbuch der Geologische Bundesanstalt Volume 133 / Issue 2, 1990, pp. 147–155 ( pdf , geologie.ac.at).
  • Geological map of Bavaria 1: 500000, explanations . Bavarian Geological State Office, Munich, 4th edition 1996.
  • S. M. Schmid, B. Fügenschuh, E. Kissling, R. Schuster: Tectonic map and overall architecture of the Alpine orogen. In: Eclogae geologicae Helvetiae 97 (2004), S. 93–117, PDF

Individual evidence

  1. Schmidt et al. 2004, p. 109.
  2. a b The Flysch Zone: Mudslides in the Deep Sea ( Memento of the original from January 30, 2013 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. . @1@2Template: Webachiv / IABot / www.geologie.ac.atIn the Federal Geological Institute:RockyAustria(geologie.ac.at, link no longer available)
  3. Sandstone balls - a specialty in the flysch zone. Granzer mineral and fossil collection (granzer.at).