Pramollo group

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The Pramollo group is a lithostratigraphic group in the Carnic Alps that was deposited in the Upper Carboniferous . Their molasses deposits form part of the Pontebba supergroup , which, after the Variscan tectogenesis has been completed, initiates the Alpid geosynclinal cycle. The Paleozoic base had remained mobile and was flooded again immediately after the mountain formation - the alpine regeneration geosyncline emerged from the east .

designation

The Nassfeld pass with the Auernig (1825 m) and layers of the Pramollo group

The Pramollo group, formerly also known as the Auernig group or Auernig layers , is named after the Pramollo , the Italian name for the Nassfeld .

history

The Pramollo group was first scientifically described by E. Schellwien as Auernigschichten in 1892. Two years later followed a description by F. Frech and in 1896 by G. Geyer. The current name was only established in 1990 by Venturini.

geology

The Carnic Alps belonging to the Southern Alps are separated from the Eastern Alps by the Periadriatic Lineament . North and south of the lineament , the rocks differ considerably in terms of both facies and tectonics. For example, the Permian in the Southern Alps is almost exclusively marine developed, but terrestrial north of the lineament. The tectonic structure in the north is characterized by long-range cover transports , but fracture tectonics and short thrust distances predominate in the south .

The older Pre- Variscan sedimentation cycle began in the Carnic Alps in the Ordovician and ended with the Variscan orogeny in the middle Moskovian (Westfal D) around 308 million years BP . The more recent post-variscan sedimentation sequence started immediately afterwards shortly before the beginning of the Kasimovian (Upper Carboniferous, Pennsylvania ) and lasted into the Triassic ( Ladinian ).

After the conclusion of the Variscan orogeny, which did not lead to the final consolidation of the alpine area, as well as the resulting erosive phase, marine trench structures were sunk in the western Paleotethys , including the Pramollo basin . Today's remnants of this basin are exposed in a pull-apart, about 20 kilometers long and 5 kilometers wide, stretched to the east-south-east between the Kronalpe in the east and the Zollnersee in the west. The great thickness of the basin filling in the Upper Carboniferous and Lower Permian - the Pramollo Group and the Rattendorf Group are together around 1700 meters thick - came about through transtensive, synsedimentary subsidence. The conspicuously cyclical character of the deposited sediments was caused by tectonic movements in combination with glacio-eustatic sea level fluctuations, which in turn were controlled by the permocarbon glaciation of the southern continents ( Gondwana ).

Lithology

The Pramollo group can be divided into five formations . From hanging to lying are:

The group lies diachronically over the underlying Bombaso formation , which in turn can in places reach up to the Meledis and Pizzul formation and is occasionally added to the Pramollo group. The Rattendorf group follows above the Pramollo group.

The Pramollo group can reach up to 1090 meters in thickness. Lithologically, it consists of quartz-rich sandstones , quartz-rich component-supported conglomerates and shale clays ; the latter are particularly common in the Meledis, Corona and Carnizza Formations and lead to intermediate bioclastic limescale . Compared to the underlying Bombaso Formation, the Pramollo Group is considerably richer in quartz detritus, but the transition takes place gradually. The group is structured cyclically, with clastic sediments alternating with calcareous sediments.

The conglomerates have a high degree of maturity and consist almost exclusively of well-rounded milky quartz up to 10 centimeters in size. There are also less well rounded black Lydites . The sandstones are usually layered diagonally, often also graded. Fine sandstones are usually very rich in mica and often contain beautiful plant fossils - mainly ferns , calamites and lepidodendraces . The dark gray, mostly thin slate horizons occasionally contain well-preserved plant material. Small coal seams are also rare . The carbonate horizons and lenses are usually 10 to 15 meters thick and consist of fossil-rich, dark gray anthracoporella or bioclastic limestone. From time to time, selectively silicified organisms are also included.

Facies

Facially, the Pramollo group is primarily to be addressed as marginal marine , with its paleofacies extending from the river delta to the area near the beach. Nevertheless, open shelf conditions with a maximum water depth of 40 to 60 meters have also been implemented. The following types of sediments and facies can be distinguished:

  • Quartz-rich conglomerates in the area near the beach
  • Sandstones with trough sloping layers of the upper beach area
  • Sandstones with uneven oblique stratification of the lower beach area
  • Ransacked and partly fossil-containing siltstones and shale clays from the offshore area
  • Offshore fossil limestone
  • thin layers of coal and dark, plant-rich slate in the hanging walls of localized conglomerates - swamps near the coast.

In the Corona, Auernig and Carnizza Formations, these lithofacies form high-frequency sediment cycles of 10 to 40 meters in thickness, the so-called Auernig cyclothemes . Each individual cycle begins with conglomerates at the base, followed by obliquely layered sandstone, siltstone, clay slate, fossil limestone, siltstone with clay slate and finally sandstone. The conglomerates had formed when the sea level was relatively low, but the fossil limestone when the sea level was high. According to Krainer (1992), these cycles were caused by changes in sea ​​level caused by the Ice Age .

Environmental conditions

The deposition area of ​​the Pramollo group was at that time at 5 ° north to 10 ° south latitude. The prevailing paleoclimate was rainy and tropical-humid . Fossil plant communities such as Equisetopsida , Filicotopsida , Lycopodiopsida and Pteridospermae are characteristic of a humid tropical climate. The fairly frequent occurrence of thin coal seams in conglomerates and sandstones in the pre-delta area also points to high-rainfall environmental conditions. In return, there are no signs (such as evaporites) of an arid climate. Paradoxically, the limestone layers were apparently deposited under cool water temperatures, recognizable for example by the lack of oolites . Elias Samankassou (2002) explains this contradiction in terms of the periodic surge of deep water . The overall rather low carbonate production in the Pramollo group is ultimately due to tectonic causes. The mobile, constantly sinking inner shelf area of ​​the Paleotethys, and the associated strong clastic supply with temporarily high sedimentation rates, inhibited the development of more powerful carbonates.

Fossils

Sphenophyllum miravallis
Cartilaginous fish tooth Petalodus ohioensis

Paleontologically, the Pramollo group is characterized by a rich and varied fossil content . - have been preserved - especially in Kalklagen and shales algae (Dasycladaleen-Algenmounds with Anthracoporella spectabilis the red alga, Archaeolithophyllum missouriense and Epimastopora ), brachiopods , very diversified bryozoans , crinoids , echinoderms , foraminifera , insects , cephalopods , corals , mussels , ostracods , including Entomazoa , sphinctozoa , sponge needles , spiders , trilobites , tubiphytes and a rich and well-preserved palaeoflora with Alethopteris , Annularia , Calamites , Cordaites , Neuropteris , Pecopteris , Pseudomariopteris busquetii and Sphenophyllum angustifolium .

The group is also home to a very important Ichno fauna and can therefore be referred to as an Ichno deposit. The following taxa are found: Ancorichnus , Archaeonassa , Beaconites , Curvolithus simplex , Cylindrichnus , Helminthoidichnites tenuis , Parataenidium , Planolites , Psammichnites plummeri , Skolithos and Zoophycos .

Among the arthropods , the finds of an arachnid ( flagellum scorpion Parageralinura marsiglioi from the Pizzul formation) and a Eurypterid ( adelophthalmus from the Meledis formation) are to be cited

Vertebrate remains are extremely rare, so far only tetrapod traces and the tooth of a Petalodus ohioensis belonging to the cartilaginous fish (Chondrichthyes) have been found.

metamorphosis

The Pramollo group was shaped by a very low to low regional metamorphosis in the course of the Alpid mountain formation . The physical conditions reached temperatures of about 270 ° C, this maximum was probably passed in the Cretaceous around 100 million years BP . The thermal overprinting can be explained well by the Permo-Mesozoic sediment overlay.

Age

The Pramollo group can be assigned to the period Upper Moskovian to Gzhelian , which corresponds to about 308 to 300 million years BP.

Occurrence

The occurrences of the Pramollo group are limited to the central Carnic Alps in the Austrian-Italian border area and the southern Karawanken . Your type locality is on Auernig, southeast of Nassfeld. Other occurrences are:

Individual evidence

  1. ↑ Schellwien, E .: The fauna of the Carnic fusuline limestone. I. In: Palaeontographica . tape 39 . Stuttgart 1892, p. 1-56 .
  2. ↑ Frech, F .: The Carnic Alps. A contribution to comparative mountain tectonics. In: Dep. Naturforsch. Ges. Band 18 . Hall 1894, p. 1-514 .
  3. ^ Geyer, G .: About the geological conditions in the Pontafeler section of the Carnic Alps . In: Jb. KK Geol. R.-A. tape 46 . Vienna 1896, p. 127-233 .
  4. ^ Corrado Venturini: Geologia delle Alpi Carniche centro orientali . In: Commune di Udine, Edizioni del Museo Friulano di Storia Naturale . Pubblicazione n.36 Udine 1990, p. 1-220 .
  5. ^ Fohrer, Beate: Ostracodes from the Upper Carboniferous and Lower Permian of the Carnic Alps (Austria): Systematics, Biostratigraphy and Palecology . In: Jb. Geol. B.-A. 140 issue 2. Vienna 1997, p. 99-191 .
  6. ^ Selli, R .: Schema geologico delle Alpi Carniche e Giulie occidentali . In: Giorn. Geologia . tape 30 . Bologna 1963, p. 136 .
  7. ^ A b Corrado Venturini: Introduction to the geology of the Pramollo Basin (Carnic Alps) and its surroundings - Workshop Proceedings on tectonics and stratigraphy of the Pramollo Basin (Carnic Alps) . In: Giorn. Geologia . ser. 3a, 53 (1), 1991, pp. 13-47 .
  8. ↑ Karl Krainer: Facies, sedimentation processes and paleogeography in the carboniferous region of the Eastern and Southern Alps . In: Jabrbuch of the geological federal institute . tape 135 . Vienna 1992, p. 99–193 ( PDF on ZOBODAT ).
  9. ↑ Golonka, J., Ross, MI and Scotese, CR: Phanerozoic paleogeographic and paleoclimatic modeling maps . In: Embry, AF et al., Pangea: Global environments and resources (Ed.): Canadian Society of Petroleum Geologists Memoir . tape 17 , 1994, p. 1-47 .
  10. ↑ LePain, DL, Crowder, RK and Wallace, WK: Early Carboniferous transgression on a passive continental margin: Deposition of the Kekiktuk Conglomerate, northeastern Brooks Range, Alaska . In: American Association of Petroleum Geologists Bulletin . v. 78, 1994, p. 679-699 .
  11. ↑ Elias Samankassou: Cool-water carbonates in a paleoequatorial shallow-water environment: The paradox of the Auernig cyclic sediments (Upper Pennsylvanian, Carnic Alps, Austria-Italy) and its implications . In: Geology . v. 30; no. 7, 2002, p. 655-658 .
  12. ↑ Klaus Boeckelmann: Microfaces of the Auernig layers and borderland banks west of the Rudnig saddle (Karbon-Perm; Carnic Alps) . In: facies . Volume 13, Issue 1, 1985, pp. 155-173 .
  13. ↑ Elias Samankassou: Skeletal framework mounds of dasycladalean alga Anthracoporella, Upper Paleozoic, Carnic Alps, Austria . In: Palaios . V. 13, 1998, p. 297-300 .
  14. ^ Corrado Venturini: Evoluzione geologica delle Alpi Carniche . In: Pubblicazione 48 . edizioni del Museo Friulano di Storia naturale, Udine 2006, p. 207 .
  15. ↑ Adolf Fritz, Miente Boersma: Fund reports of plant fossils from Carinthia, Posts 9: Krone (Stefan), Carnic Alps . In: Carinthia II . tape 174/94 , 1984, pp. 145–175 ( PDF on ZOBODAT ).
  16. ↑ Andrea Baucon and Carlos Neto de Carvalho: From the river to the sea: Pramollo, a new ichnolagerstätte from the Carnic Alps (Carboniferous, Italy-Austria) . In: Studi Trent. Sci. Nat. Acta Geol. Band 83 , 2008, p. 87-114 .
  17. ^ Paul A. Selden, Jason A. Dunlop and Luca Simonetto: A fossil whip-scorpion (Arachnida: Thelyphonida) from The Upper Carboniferous of the Carnic Alps (Friuli, NE ItAly) . In: Rivista Italiana di Paleontologia e Stratigrafia . vol. 122 (1), 2016, pp. 7-12 .
  18. ↑ James C. Lansdell, Luca Simonetto and Paul A. Selden: First eurypterid from Italy: a new species of Adelophthalmus (Chelicerata: Eurypterida) from the Upper Carboniferous of the Carnic Alps (Friuli, NE Italy) . In: Rivista Italiana di Paleontologia e Stratigrafia . v. 119 no. 2, 2013, p. 147-151 .
  19. ↑ Paolo Mietto, Giuseppe Muscio, Corrado Venturini: Impronte di tetrapodi nei terreni carboniferi delle Alpi Carniche . In: Gortania - Atti del Museo Friul. Storia Nat. tape 7 . Udine 1985, p. 60-73 .
  20. ^ Dalla Vecchia, FM: First record of a petalodont (Petalodus ohioensis SAFFORD, 1853) from the Alps . In: Gortania - Atti del Museo Friul. Storia Nat. tape 9 (87) . Udine 1987, p. 47-56 .
  21. ↑ Gerd Rantitsch, Thomas Rainer, Barbara Russegger: Lowest- grade metamorphosis in the carbon of the southern Alps (Carinthia, Austria) . In: Carinthia II . 190./110. Vintage. Klagenfurt 2000, p. 537-542 ( PDF on ZOBODAT ).
  22. ^ Corrado Venturini: Carta geologica delle Alpi Carniche. (Geological Map of the Carnic Alps) . Museo Friulano di Storia naturale, Udine 2002.