Präbichl formation

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The Präbichl Formation is a lithostratigraphic formation of predominantly continental, fluvial red sediments that was deposited in the Eastern Alps during the Middle and Upper Permian . It forms the basis of the Northern Limestone Alps and is primarily discordant on the Variscan folded ancient Paleozoic rocks of the Greywacke Zone .

etymology

The Präbichl Formation (sometimes also written Prebichl ), or the Präbichl layers or the Präbichl conglomerate , is named after the Präbichl Pass ( 1226  m ) in Styria .

Type locality

Coordinates: 47 ° 32 ′ 0 ″  N , 14 ° 58 ′ 14 ″  E

Map: Austria
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Präbichl formation​
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Austria

The type profile is located west of the Leobner Hütte between the Knappensteig to Polster and the Hirscheggsattel ( 1699  m ).

History

The concept of the Präbichl formation was first introduced in 1929 by R. Schwinner as a Präbichl conglomerate . Nevertheless, the conglomerates at the Leobner Hütte had already been described by Redlich in 1923 and by Spengler in 1926. In 1937, Cornelius introduced the term Präbichl layers in his description of the Rax . Detailed edits were made by Sommer in 1968 and 1972.

Geological framework

Plate tectonic reconstruction of the Tethys space around 249 million years BP. The Präbichl Formation was deposited on the northwestern edge of the Meliata Ocean.

After the continental collision between Gondwana and Laurussia during the Variscan orogeny , the supercontinent Pangea A was formed towards the end of the Carboniferous . It is assumed that already at the beginning of the Middle Permian the Variscan high mountains had been severely eroded and only in places had the character of a low mountain range. In the further course of the Middle Permian, the configuration of Pangea A was changed to Pangea B by dextral shearing along a shear zone of around 3,000 kilometers .

Due to the mega-ashing, the Alpine region was subject to stretching forces in the period from 300 to 250 million years BP and as a result sedimentary sinking basins formed in the area of ​​the western, eastern and southern Alps, but also in the later Molasse region. Synrift sediments accumulated in the basins created on the eroded Variscan ceiling pile - clastic formations such as the Präbichl Formation and the Val Gardena Formation with a total thickness of 1500 meters. At the same time, the Laas Formation in the Drauzug and the Werchzirm Formation in the Gurktal Nappe , both very similar formations.

The Werfen formation then follows over the Präbichl formation . The Haselgebirge acts as a lateral representative .

Sedimentology

The siliciclastic red sediments ( English Red Beds ) of the Präbichl Formation at the type locality are a good 160 meters thick and consist primarily of conglomerates , breccias , immature sandstones and shale clays . They contain abundant reclaimed clasts from the local basement and therefore form polymictic breccias, conglomerates, lithic arenites and lithic greywackes . The heavy mineral fraction is dominated by tourmaline to more than 70% , and zircon , rutile and apatite are also present .

The almost fossil-free sediments (with the exception of conodonts and radiolarians in the basal limestone pebbles) can be interpreted as proximal to distal deposits of alluvial cones ( rubble fans , English alluvial fans ) that merge distally into fine-grained playas sediments with caliche crusts and rare algae layers . In their formation, they are very similar to recent to fossil alluvial fan-shaped sediments, as found in semi-arid to arid climates. The sequence of alluvial rubble fans – playa sediments is arranged in megasequences, the grain size of which is reduced towards the hanging wall .

The deposit area of ​​the Präbichl Formation was then at about 10 ° north latitude, the climatic conditions were semi-arid to arid .

Lithology

The Präbichl Formation usually begins with a limestone conglomerate up to 50 meters thick (and, to a lesser extent, limestone breccia) that contains pebbles from the lying layers (e.g. limestone with siderite and ankerite ). This is followed by alternating quartz-rich conglomerates and violet siltstones , which merge into sandstones up to 50 meters thick, which in turn alternate with claystones and siltstones. In the quartz-rich conglomerate, Chert clasts often occur , which come from reclaimed Devonian and carbon . In the hanging wall , the Präbichl Formation gradually merges into the Werfen Formation with a decrease in grain size and the inclusion of clay-schistigen layers.

The 160-meter-thick sequence on the type profile is made up of 3 mega-sequences, the two lowest of which have been completely preserved. These are two overlapping rubble compartments. The lower, almost 50 meters thick rubble fan consists of a 5 meter thick base conglomerate, as well as a basal conglomeratic proximal facies with coarse embankment and an overlying distal stratified flood facies in fine embankment. The sediments are generally red. The approximately 100-meter-thick upper rubble fan is also made up of two facies, a pigtail flow system of the middle fan area with green-colored, cyclical sedimentation with a decrease in grain size and a stratified flood facies of the distal fan - red, clayey to sandy sediments with isolated channel fillings. The top 10 meters are also red, fluvial sediments, but it cannot be decided whether this is also a rubble fan.

Christian Exner (2000) was able to divide the Präbichl formation in the Filzmoos area into three series (or members). From hanging to lying are:

  • Plate series (plate quartzite)
  • Filzmoos series
  • Wexler series

metamorphosis

The illite crystallinity of the rocks shows that the Präbichl Formation suffered a very weak to weak regional metamorphosis in the course of the alpine mountain formation .

Age

The Präbichl Formation was deposited between 270 and 251 million years BP ( Roadium to Changhsingium ).

Occurrence

The cushion (1910 m) built from Lower Devonian limestone with the Leobner Hut; the type profile of the Präbichl formation is on the right above the hut in the direction of the Hirscheggsattel.

The Präbichl Formation occurs in the eastern Grauwackenzone of the Upper Eastern Alps ( Eisenerzer Alps ). In the Noric Nappe (also Norisch-Tirolische Nappe) it overlays transgressive Palaeozoic deposits. It can reach down into the Paleozoic to different degrees - on the Styrian Erzberg, for example, into the carbon of the iron ore formation , at the type locality in Lower Devonian limestone and dolomite or elsewhere even in the Upper- Ordovician Blasseneck porphyry .

Whether there is a correlation with very similar occurrences in the western Grauwackenzone in the Wörgl - Leogang area cannot be decided due to a lack of age dating and lithological marker horizons. There may even be a connection to the Kristberg layers in the Montafon , although these have a marine to lacstrine character and are likely to be somewhat older.

Occurrences in detail are:

Individual evidence

  1. ↑ Schwinner, R .: Scree-bearing slate and other debris from the central zone of the Eastern Alps . In: Geologische Rundschau . tape 20 . Berlin 1929, p. 211-244, 343-370 .
  2. ↑ Redlich, KA: The ore train Vordernberg-Johnsbachtal. I. Iron ore. In: Mitt. Geol. Ges. Vienna . tape 15 . Vienna 1923, p. 207-262 .
  3. ↑ Spengler, E .: About the tectonics of the Grauwackenzone south of the Hochschwab group . In: Verh. Geol. B.-A. tape 6, 7 . Vienna 1926, p. 127-143 .
  4. ↑ Cornelius, HP: Sequence of layers and tectonics of the Limestone Alps in the Rax area . In: Jb. Geol. B.-A. tape 87 . Vienna 1937, p. 133-194 .
  5. ↑ Sommer, D .: The Prebichl layers as the Permotriadic base of the Northern Limestone Alps in the eastern Grauwackenzone (Styria, Austria) . In: Verh. Geol. B.-A. Born in 1972. Vienna 1972, p. 119-122 .
  6. ↑ Muttoni, G. et al .: Opening of the Neo-Tethys Ocean and the Pangea B to Pangea A transformation during the Permian . In: GeoArabia . v. 14, 2009, p. 17-48 .
  7. ^ Neubauer, F., Friedl, G., Genser, J., Handler, R., Mader, D. and Schneider, D .: The origin and tectonic evolution of the Eastern Alps deduced from dating of detrital mica: A review. In: Austrian Journal of Earth Sciences . tape 100 , 2007, pp. 8-25 .
  8. ↑ Krainer, K .: late- and post-Variscan sediments of the Eastern and Southern Alps. Ed .: Raumer, J. von and Neubauer, F., Pre Mesozoic Geology in the Alps. Springer, Berlin Heidelberg New York, 1993, p. 537-564 .
  9. ^ Hooke, Roger L .: Processes on arid-region alluvial fans . In: Joural of Geology . tape 75 , 1967, p. 438-460 .
  10. ↑ a b Krainer, K. and Stingl, V .: Alluvial rubble fan sediments in the Eastern Alpine Permian using the example of the Präbichl layers at the type locality near Eisenerz / Styria (Austria) . In: Communications from the Austrian Geological Society . tape 78 , 1985, pp. 231-249 .
  11. ↑ Kralik, M., Krumm, H. and Schramm, JH: Low grade and very low grade metamorphism in the northern Calcareous Alps and in the Greywacke Zone: illite-crystallinity data and isotopic ages . In: Flügel, HW and Faupl, P. (Eds.): Geodynamics of the Eastern Alps . Deuticke, Vienna 1987, p. 164-178 .
  12. ^ Neubauer, F .: Late Variscan structures of the Eastern Greywacke Zone (Eastern Alps). In: New Yearbook Geological Paleontological Messages . 1989, p. 425-432 .
  13. ^ Büchner, Karl-Heinz: The Prebichl layers on Klosterkogel near Admont (Styria, Austria) . In: Verh. Geol. B.-A. Born 1977, Issue 3, 1977, p. 299-302 .