Bombaso formation

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The Bombaso Formation is a lithostratigraphic formation that was sedimented in the Carnic Alps in the course of the Upper Carboniferous ( Pennsylvanian , Moskovian ) . It is the oldest formation deposited on the Variscan basement of the Southern Alps and initiates the alpine cycle.

Geological framework

Coordinates: 46 ° 36 ′ 13 ″  N , 13 ° 4 ′ 39 ″  E

Map: Austria
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Bombaso formation​
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Austria

The substructure of the Carnic Alps was shaped by the Variscan orogeny in the Upper Carboniferous, but at the same time fell victim to erosion. The first marine incursion occurred as early as the Moscovian and so deck sediments were deposited over the folded and only slightly metamorphic Paleozoic. The orogenesis manifests itself in the terrain as a local angular discordance that separates the pre-variscan from the post-variscan sediment cycle. Its peak (Asturian phase) is in the Carnic Alps in the Bashkirian , ie around 318 to 310 million years BP . This was followed by trench fracture tectonics , with the erosion debris from the Variscan basement being transported into the trenches and lower valleys. The deposits consist of fluvial and shallow marine molasse sediments , more precisely an alternation of conglomerates , sandstones and fossil-rich limestone - the Bombaso formation. Siliciclastic cyclothemes dominate with marked lateral thickness and facies fluctuations, which can be attributed either to synsedimentary tectonic movements and / or to the filling of a former erosion relief.

The deposit area of ​​the Bombaso Formation was then at 5 to 10 ° north latitude. The prevailing paleoclimate was rainy and tropical-humid .

Lithology

The Bombaso Formation, also known as the Waidegger Conglomerate , Waidegger Group , Waidegg Formation or Collendiaul Formation , opens the Pontebba Super Group and is around 100 to 200 meters thick. It usually transgresses via the Hochwipfel Formation or the Dimon Formation , both from the Serpukhovian . These are poorly sorted, immature breccias , conglomerates and sandstones, rubble-bearing siltstones and sandy to silty shale clays with calcareous intermediate layers . The clasts in the Malinfier horizon originated mainly from Devonian to Silurian limestone, but in the Pramollo member from radiolarian chert or volcanic rocks. In the Malinfier horizon, meter-sized blocks of limestone can even be found lying flat. Overall, the Bombaso Formation is very rich in radiolarian, arenitic, volcanic and calcareous clasts. It therefore differs significantly from the superimposed quartz-accentuated Pramollo group . However, the transition is gradual.

Facies

The sediments are facially interpreted as mass flows in a delta fan , ie as of marine origin. Probably there is also a subaeric alluvial cone facies.

Fossils

Pecopteris arborescens

In addition to rich plant remains of cordaites , kalamites and pecopterids , sedimented in dark siltstones and shale, fossils also include brachiopods , crinoids and fusulinids , which in turn indicate a marine deposit environment. Breccias within the Bombaso Formation contain the oldest fusulinid fauna in the Carnic Alps. The findings come from the Quasifusilinoides quasifusilinoides / Protriticites ovatus zone and document the uppermost Moscovium. Other fusulinides are Fusiella lancetiformis , Ozawainella , Pseudotriticites , Staffella and Schubertella . As Ichnofossile are Curvolithus and Dictyodora to mention.

From the very rich flora, which represents a swamp forest community with fern-leafy seed plants and tree ferns , a few taxa should be selected:

Age

Based on plant finds such as Neuropteris scheuchzeri , the Bombaso formation can be assigned to the Odontopteris cantabrica zone . This corresponds to the regional level Cantabrian or upper Moscovian (around 308 to 307 million years BP). Venturini (2002) places the Bombaso formation in the upper Moskovium and in the Kasimovium .

Occurrence

The Zollnersee. The type profile is located between the lake and the hut.

The occurrences of the Bombaso Formation are limited to the Carnic Alps. The following sites are known:

Individual evidence

  1. ↑ Gerd Rantitsch: Organic metamorphosis on the Auernig (Naßfeld, Carnic Alps) . In: Yearbook of the Federal Geological Institute . tape 147 , 2007, pp. 331–334 ( PDF on ZOBODAT ).
  2. ^ Francesco Massari et al .: The Permian-Carboniferous cyclothems of the Pramollo Basin sequence (Carnic Alps) . In: Giornale di Geologia . ser. 3, 53, 1991, pp. 171-185 .
  3. ↑ 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 . tape 30 (7) , 2002, pp. 655-658 .
  4. ^ Corrado Venturini, Adriano Ferrari, Claudia Spalletta, Gian Battista Vai: La discordanza ercinica, il tardorogeno e il postorogeno nella geologia del Passo di Pramollo . In: Alberto Castellarin, Gian Battista Vai, Guida alla geologia del Sudalpino centro-orientale (ed.): Guide Geol. Reg. Soc. Geol. It. 1982, p. 305-319 .
  5. ↑ Alois Fenninger , Hans-Peter Schönlaub, Hans Ludwig Holzer, Gerd Flajs: To the basic formations of the Auernig layers in the Carnic Alps (Austria) . In: Verh. Geol. B.-A. tape 1976 , 1976, pp. 243-255 ( PDF on ZOBODAT ).
  6. ↑ a b Andrea Baucon, 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 .
  7. ↑ 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 ).
  8. ^ Corrado Venturini: Geologia delle Alpi Carniche centro-orientali . In: Pubblicazione 36 . Museo Friulano di Storia naturale, Udine 1990, p. 220 .
  9. ↑ Vladimir Davydov, Karl Krainer: Fusulinid assemblages and facies of the Bombaso Fm. And basal Meledis Fm. (Moscovian-Kasimovian) in the Central Carnic Alps (Austria / Italy) . In: Facies . tape 40 . Erlangen 1999, p. 157-196 .
  10. ↑ Adolf Fritz, Karl Krainer: Vegetation-historical and flora stratigraphic studies in the Upper Carboniferous and Lower Permian of the Eastern and Southern Alps (Part 1) . In: Carinthia II . 196/116 Vintage. Klagenfurt 2006, p. 93–120 ( PDF on ZOBODAT ).
  11. ^ Corrado Venturini: Carta geologica delle Alpi Carniche. (Geological Map of the Carnic Alps) . Museo Friulano di Storia naturale, Udine 2002.