Hasel Mountains (Oberostalpin)

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The Haselgebirge , occasionally also Haselgebirge formation or Alpine Haselgebirge formation , is an evaporitic tectonostratigraphic unit of the Upper Eastern Alps . The corresponding sediments were in the late Permian during the late Dehnphase in a grave rupture on the northern edge of the western Paleo-Tethys Ocean deposited. The Alpine Haselgebirge Formation is the type of occurrence of the tectonic facies, also known as the Haselgebirge .

Plate tectonic reconstruction of the Tethys space around 249 million years BP. The Hasel Mountains were deposited on the northwestern edge of the Meliata Ocean.

etymology

The term Haselgebirge comes from the language of mining and can be traced back to the 16th century. It is first mentioned in 1598. The etymological origin of hazel has not been clarified.

Initial description

The Haselgebirge, which has been mined for more than 3,500 years, was scientifically described by Leopold von Buch in 1802 , but the term was already in use by other geologists from the middle of the 18th century, such as Sterzinger in 1757.

Geological framework

After the continental collision between Gondwana and Laurussia during the Variscan orogeny , the supercontinent Pangea A was formed towards the end of the Carboniferous . In the course of the Middle Permian, this configuration was then modified to Pangea B by dextral shear along an approximately 3,000 kilometer long shear zone .

In the period of 300 to 250 million years, the Alpine region was subject to stretching forces and sedimentary depressions were created in the western, eastern and southern Alps, but also in the later Molasse region. Clastic formations such as the Präbichl Formation , the Val Gardena Formation and the Hasel Mountains accumulated in the basins with a total thickness of up to 1500 meters (the Val Gardena Formation becomes sulphate in the hanging wall).

These siliciclastic formations are interpreted as Synrift deposits on the Variscan basement , which were then replaced by a marine incursion with the evaporites of the Hasel Mountains . The deposit of the Haselgebirge is likely to be due to a progression of the rift process, which 15 million years later was to expand to the Meliata Ocean . At the same time as the Hasel Mountains, Verrucano and the shallow marine Bellerophon Formation were also sedimented in other parts of the Alps in addition to the uppermost Val Gardena formation (with plait stream and Playas sediments).

The Werfen Formation and the Reichenhall Formation usually follow above the Hasel Mountains, as well as the fluvial Alpine Buntsandstein in the far west . The Werfen formation is the result of a large-scale transgression of a flat sea in the central and eastern section of the Alps. In the Reichenhall Formation, due to a subsequent regression in the Tethys in the Anisium (between 245 and 243 million years BP), evaporites were formed again, but these should not be confused with the actual Hazel Mountains of the Upper Permian. In addition to fossils, the sulfur isotopes can be used as a distinguishing criterion. For example, the δ 34 S content of + 25.3 ‰ CDT in the Reichenhall Formation is significantly higher than the marine signal of + 11.6 ‰ CDT in the actual Hasel Mountains. The Reichenhall Formation can be interpreted as Sabcha , in which Rauwacken , collapse breccias and holey dolomites were formed.

Petrology

Inclusions of dolomite in the anhydrite salt mine Berchtesgaden

The Haselgebirge is a tectonically determined two-component mixture, the matrix of which is formed from red to white rock salt, usually 2 to 3 millimeters in size, halite . The salt content is very variable and can be between 10 and 70%. In addition to halite, there are also anhydrite (muriazite), gypsum , polyhalite and accessory magnesite as well as rare minerals such as blödite , glauberite and kieserite . Quartz and feldspar should be mentioned as autogenous mineral formations .

Adjacent rock inclusions, predominantly shale and clay stones, swim in the matrix . Siltstones , sandstones, dolomites , anhydrite fragments and even igneous rocks ( e.g. intraplate basalt , tuff , cushion basalt , fine-grain gabbro , syenite ) as well as very rare metamorphic rocks are also encountered.The matrix is ​​very strongly deformed in places (up to mylonite and ultramylonite ), but also the adjacent rock inclusions can be completely brecciated as cataclasites .

Sedimentology

The Hasel Mountains mark the emergence of a passive continental margin in the Northern Limestone Alps , on the edge of which it was sedimented during the Upper Permian in association with the sediments of the Hallstatt Group . At that time, the deposit area was located at around 10 ° north latitude and thus in the tropical, summer-wet area. Trenches facing east-west in the shelf edge were flooded by the Tethys from the south-east . Under subtidal to supratidal conditions, sediments settled cyclically, which, based on siliciclastic deposits at the base, show a typical evaporation sequence from anhydrite to rock salt. The trenches were surrounded in the north, south and west by alluvial cones , flood plains and playas , in which the sediments of the Präbichl Formation, the Mitterberg Formation and the Alpine Verrucano were deposited. The marine evaporation cycles were occasionally interrupted by continental groundwater influx with clastic inundation.

Recently, sedimentary dikes in the centimeter range have also been identified, which indicate earthquake activity during the sedimentation.

Facies

The following sedimentary facies can be distinguished for the area of ​​the rift:

  • a marginal siliciclastic evaporitic facies separated before the actual salt deposition. It documents the transition from subaeric to shallow marine red bed facies to evaporitic milieu.
  • a sulphate facies. In it, sediment cycles up to 3 meters thick can be seen, which document a decrease in water depth towards the hanging wall. Transition from silt stones to tuber, mosaic and layer anhydrite, in the hanging wall occurrence of polyhalite layers.
  • a halitic facies. It usually overlays the sulphatic facies with tectonic contact.
  • a sulphate-carbonate facies during the Lower Triassic (Reichenhall Formation).

tectonics

In the course of the alpine orogeny (formation of a relatively thin-skinned fold and thrust belt and continental collision in the Priabonian period around 35 million years BP ) the Hasel Mountains were extremely severely deformed. This manifests itself in a clear foliation and lineation in the halite as well as on the basis of other structural elements such as isoclinal and pocket folds. The sense of shear can usually be reconstructed from these structures and other indicators. The plastic deformation was accompanied by grain boundary migration, undersized grain rotation and extensive recrystallization of the halite, which was also deposited in distorted areas of the fissured and broken host rock inclusions. White fibers made of halite in fractures as well as harness straps on faults document the younger movements. In general, it can be observed that the internal structures in the Hasel Mountains conform to the surrounding large structures, but the individual occurrences differ significantly from one another in their respective structural structure.

Evaporites are generally very incompetent rock bodies and so it is not surprising that the Hasel Mountains functioned as an easily deformable sliding horizon during the eoalpine nappes around 95 million years ago BP ( Cenomanium ) and the rigid, up to 3000 meters thick limestone nappes facilitated their advance northwards . Haselgebirge can be found, for example, on the bottom of the Juvavian (predominantly) but also the Tyrolean ceilings. It is assumed that the salt bodies rose like a diapir from the gliding horizons towards the surface and then received their present shape during the final Cenozoic deformation phase.

metamorphosis

With the beginning of the ceiling stacking and the associated closure of the Meliata Ocean, the Hasel Mountains suffered a very weak regional metamorphic overprint in the period 150 to 90 million years BP ( Malm to Turonium ) . Leitner and colleagues (2012) estimate that the aggregate was heated to 180 to 240 ° C. They indicate the pressures that have occurred as 2.5 to 4.5 megapascals . The deformation rate was very high at 10 −9 to 10 −10 s −1 .

Age

The Haselgebirge formation was deposited in the outgoing Upper Permian during the Lopingium . Their absolute age is 255 to 251 million years BP, with their lying age being uncertain. The stratigraphic upper limit to the Werfen formation could, however, be well dated to 251 million years BP. The secondary growth of polyhalite in the course of diagenesis was assigned an age of 234/233 to 210 million years BP ( Ladinium to Norium ) by Leitner and colleagues .

On the basis of palynological examinations, Klaus (1974) was able to confirm an upper Permian age using spore taxa such as Nuskoisporites , Gigantosporites , Lueckisporites and Klausipollenites schaubergi .

Occurrence

The Alpine Haselgebirge is limited to the Eastern Alps and occurs in a band almost 400 kilometers long in the Northern Limestone Alps - the known deposits extend from Hall in Tirol near Innsbruck to Hinterbrühl near Vienna , but are mainly concentrated in the center of the Salzkammergut .

The occurrences in detail (from west to east):

Individual evidence

  1. ^ Hirn, J .: Archduke Maximilian the German Master - Regent of Tyrol . tape 2 . Athesia, Bozen 1915.
  2. Stöllner, T. and Oeggl, K .: Bergauf Bergab. 10,000 years of mining in the Eastern Alps . Verlag Marie Leidorf, Rahden / Westfalen, Germany 2015, p. 143 .
  3. book, L. v .: Geognostic observations on trips through Germany and Italy . tape 1 . Haude, Berlin 1802, p. 320 .
  4. Sterzinger, N .: Origin and true properties of Hall-Innthalic common salt . Wagner, Innsbruck 1757, p. 26 .
  5. Stampfli, GM, Hochard, C., Vérard, C., Wilhem, C. and von Raumer, J .: The formation of Pangea . In: Tectonophysics . tape 593 , 2013, p. 1–19 , doi : 10.1016 / j.tecto.2013.02.037 .
  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. Hubmann, B., Ebner, F., Ferretti, A., Kido, E., Krainer, K., Neubauer, F., Schönlaub, HP and Suttner, TJ: The Paleozoic Era (them) . In: Treatises of the Federal Geological Institute . tape 66 , 2014, p. 135 .
  8. Krainer, K .: late- and post-Variscan sediments of the eastern and southern Alps . Ed .: Von Raumer, JF and Neubauer, F., Pre-Mesozoic Geology in the Alps. Springer, Berlin 1993, p. 537-564 , doi : 10.1007 / 978-3-642-84640-3_32 .
  9. Schmid, SM, Bernoulli, D., Fügenschuh, B., Matenco, L., Schefer, S., Schuster, R., Tischler, M. and Ustaszewski, K .: The Alpine-Carpathian-Dinaridic orogenic system: Correlation and evolution of tectonic units . In: Swiss Journal of Geosciences . tape 101 , 2008, p. 139-183 , doi : 10.1007 / s00015-008-1247-3 .
  10. Spötl, C .: Sedimentological-facial analysis of tectonized evaporite series - a case study using the example of the alpine Hasel Mountains (Permoskyth), Northern Limestone Alps . In: Geol. Paleoont. With. Innsbruck . tape 15 , 1988, pp. 59-69 .
  11. a b Spötl, C. and Pak, E .: A strontium and sulfur isotopic study of Permo-Triassic evaporites in the Northern Calcareous Alps, Austria . In: Chemical Geology . v. 131, 1996, pp. 219-234 , doi : 10.1016 / 0009-2541 (96) 00017-4 .
  12. ^ Ziegler, T .: The Haselgebirge South of Grundlsee: Geological Structure, Metabasaltic Rocks and Geodynamic Setting [MS thesis] . Paris-Lodron University Salzburg, Salzburg, Austria 2014, p. 174 .
  13. Mandl, GW: The Alpine sector of the Tethyan shelf — Example of Triassic to Jurassic sedimentation and deformation from the Northern Calcareous Alps . In: Communications from the Austrian Geological Society . tape 92 , 2000, pp. 61-77 .
  14. ^ Blakey, R .: Gondwana paleogeography from assembly to breakup - A 500 my odyssey . In: Fielding, CR, Frank, TD and Isbell, JL, Resolving the Late Paleozoic Ice Age in Time and Space (Eds.): Geological Society of America Special Paper . tape 441 , 2008, p. 1-28 , doi : 10.1130 / 2008.2441 (01) .
  15. ^ Sidor, CA et al.: Permian tetrapods from the Sahara show climate-controlled endemism in Pangea . In: Nature . v. 434, 2005, pp. 886-889 , doi : 10.1038 / nature03393 .
  16. Leitner, C. et al: Alpine halite-mudstone-polyhalite tectonite: Sedimentology and early diagenesis of evaporites in an ancient rift setting (Haselgebirge Formation, eastern Alps) . In: Geological Society of America Bulletin . 2017, doi : 10.1130 / B31747.1 .
  17. ^ Tollmann, A .: Geology of Austria. Extra-central alpine part . tape 2 . Deuticke, Vienna, Austria 1985, p. 710 .
  18. Leitner, C. and Neubauer, F .: Tectonic significance of structures within the salt deposits Altaussee and Berchtesgaden-Dürrnberg, Northern Calcareous Alps. In: Austrian Journal of Earth Sciences . tape 104/2 , 2011, pp. 2-21 .
  19. a b Leitner, C. et al .: Salt rock of the Alpine Haselgebirge Formation - ages, temperatures and structures . In: Geophysical Research Abstracts . Vol. 14, 2012.
  20. Piller, WE et al: The stratigraphic table of Austria 2004 (sedimentary sequences) . Austrian Geological Society, Vienna, Austria 2004.
  21. Mara Pakalnė, E. Pak, Wilhelm Klaus: New contributions to the dating of evaporites of the Upper Permian . In: Carinthia II . tape 164 . Klagenfurt 1974, p. 79–85 ( PDF on ZOBODAT ).