Bellerophon formation

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The Bellerophon Formation is a lithostratigraphic formation of the Upper Permian in the Southern Alps . It is named after the snail genus Bellerophon . Their predominantly shallow marine sediments come from the lagoon and inner shelf area . The overall thickness of the formation is subject to fluctuations, it can reach a maximum of 600 meters.

Bellerophon snail from the Museum Val Gardena in Ortisei in Val Gardena

history

Rudolf Hoernes first used the term Bellerophonkalk in his work On the Geology of South Tyrol , published in the journal of the German Geological Society in Berlin. As a result, Guido Stache used the term Bellerophon limestone in his work on the Permian fossils in southern Tyrol , so in 1877 and 1878 in his articles Contributions to the fauna of the Bellerophon limestone in South Tyrol , each appeared in the yearbooks of the Geological Research Institute. The Italian geologist Bruno Accordi was the first to use the term Formazione a Bellerophon in 1958 and divided it into two members: a Facies fiamazza , named after the Val di Fiemme at the base, which is characterized by dolomitic lagoon sediments and evaporites , and a Facies badiota overlying it , named after the Gadertal , which is represented by fossil-bearing limestones .

stratigraphy

The Bellerophon Formation ( Italian Formazione a Bellerophon ), which is 250 meters thick on average, can be interlocked at the base with the Val Gardena Formation , from which it is generally also underlain. It is superimposed on a clearly recognizable border, but not discordantly, by the Tesero member of the Werfen formation or by the Seiser layers .

The Verrucano , for example the Gray Verrucano in the Engadine Dolomites , functions as a temporal equivalent to the Bellerophon Formation .

A track exemplary equivalent of the Bellerophon formation sees the Austrian geologist Alexander Tollmann in black, layered Dolomites, in gypsum mining Viennese as involvement in Haselgebirge on Grundlsee occur.

Lithology and facies

Heavily folded evaporitic facies of the Bellerophon Formation at Passo Rolle

Bosellini and Hardie (1973) divide the Bellerophon Formation into two facies: a lower dolomite-gypsum facies - the Fiamazza facies - and an upper micritic, holey carbonate facies - the Badiota facies .

The bulbous, gypsum-rich layers of the Fiamazza facies alternate with dark carbonate banks rich in organic matter , which were dolomitized during a very short diagenesis . The lower facies, which are usually less than 100 and exceptionally up to 500 meters thick, represent the edge of a marine basin on the inner shelf ( Salina ). Their sediments were deposited in a lagoon with only shallow water. Under the prevailing hot, dry climate, roughly comparable to today's Persian Gulf , the dissolved salts were cyclically evaporated. This facies is exposed in the northern, western and southern Dolomites.

The 200 to 300 meter thick upper Badiota facies are only present in the central and eastern Dolomites. Their sediments - thin, well-banked, fossil-rich limestones, partly in alternation with marls - have been deposited in the offshore area in the central part of a basin. They testify to the re-establishment of open ocean circulations or a rise in sea level and transgression in a westerly direction.

The Bletterbach Gorge near Aldein. The Permian Triassic border and thus the transition from the Bellerophon to the Werfen formation runs roughly in the center of the picture.

In the description of the Stratigraphic Commission of Italy, three lithological subunits of the formation are identified. The lower two correspond to the Facies fiamazza by Accordi, the upper one to the Facies badiota .

The lowest lithological unit, with a maximum thickness of several tens of meters, is made up of rocks that were created under the influence of repeated sea ​​floods over the underlying sandstones of the Val Gardena formation. These are light, microcritical dolomites that are layered in the centimeter to decimeter range and are interrupted by gray, millimeter to centimeter thick marl layers. In fact, it is a constricted stretch of coast with poor water circulation.

Cyclical sequences of gray, more or less marly, micritic dolomites lie on top of this at the base, followed by black, carbon-rich marls and black shale clay (indicate anoxic conditions), as well as gypsum and anhydrite in the hanging wall. The individual cycles are a maximum of three meters thick and are each made up of four sequences. The first sequence consists of thin, gray, earthy dolomites. In places, thin, dark pitches are also interposed. Worm structures and traces are often found. The second sequence consists of massive, dark dolomites with isolated gypsum nodules. The third sequence consists of densely packed gypsum tubers, with the tubers embedded in a dark quartz-dolomite matrix. Solid chicken wire plaster of paris and irregularly laminated plaster of paris form the final sequence. These cycles can be interpreted as deposits of a progressive, shallow lagoon and represent a Sabcha cycle. The tropical lagoon was separated from the open sea by a tectonically caused ridge (structural high of Comelico ), which nevertheless allowed periodic flooding of the open sea. In this constricted lagoon, limestone sludge first sedimented, followed by clay-rich sediments. Due to the high rate of evaporation in the hot climate, gypsum and anhydrite were also deposited. The evaporites are becoming increasingly important to the east and are ultimately even dominant there. The second unit can be between 150 and 200 meters thick.

The third unit is made up of limestone , dark dolomites and marls. The upper part of the lithozone consists largely of micrites and bioclastic calcarenites ( packstones ) with rich fossil record (both micro and macro fossils ). Calcareous algae , benthic foraminifera , mussels, snails (including Bellerophon ), nautiloids, brachiopods and ostracods are very common . The predominantly carbonate sediments of the third lithozone were deposited in the open sea on a continental platform. The sea depth had probably increased due to a gradual rise in sea levels. The thickness of this unit is around 200 meters.

In the dark limestone and dolomites there are local layers of dolomitic breccias and cell dolomites , which can be identified as a separate unit.

Particularly noteworthy is the tongue-like interlocking with continental sediments of the Val Gardena formation at the base of the third unit. This alluvial ingression proves that the Bellerophon formation has temporarily dried up. But it is only noticeable in the western half of the deposit area.

Sequence stratigraphy

According to Noé and Buggisch (1994), the Bellerophon formation can be subdivided into three complete third-order sequences using sequence stratigraphy . The beginning of a fourth sequence then leads over to the Tesero member of the throwing formation. The first and third sequence also form cycles (parasequences) of the fourth order. The first and the second sequence are as transgressions-high stand tract ( English transgressive systems tract High systems tract ) formed (TST-HST), the third sequence as the depression-transgressions-high stand tract ( Lowstand Systems Tract-transgressive systems tract High systems tract LST-TST-HST) and the beginning of the fourth sequence only as a transgression tract ( Transgressive Systems Tract TST).

The lowest lithological unit represents a transgression stage (TST) - the transition from the continental Val Gardena formation to a restricted lagoon facies - and ends with a maximum flooding surface (MFS). This is followed by the high-level tract (HST) with its cyclic parasequences of the second lithological unit (coastal sabcha cycles with dolomite-gypsum alternation). The second sequence consists of the sequence TST-HST, which is divided into two by an MFS. It expresses itself lithologically as lagoon facies with limestone and dolomites for the TST and silty dolomites for the HST, which finally ends in a clear regression . The third sequence is marked by the ingression of the continental Val Gardena formation, which is an LST. After passing through a transgressive surface (TS), the lime-dolomite parasequences of the third lithological sequence begin in the subsequent TST. The alternate layers of lime and dolomite are sufficient after a new MFS even in the following HST, but show slightly regressive tendencies. They end in a 2 meter thick calcareous biosparite with stenohaline fauna and flora - indicators for an open, shallow marine deposit environment . The biosparite already opens the fourth sequence (TST) and leads over to the oosparite of the Tesero member.

Fossils

The red alga Solenopora

As already mentioned, fossils in the upper section of the Bellerophon Formation include the eponymous snail Bellerophon (preserved as a stone core ), various bivalves such as Aviculopecten , Permophorus , Schizodus and Towapteria , brachiopods such as Comelicania doriphora , C. haueri , Comelicothyris , Janiceps , Ombonia , Orthothetina and Nautiliden with Germanonautilus , Liroceras , Paratirolites , Tainoceras , Tirolonautilus crux and T. sebedinus . In addition, diverse join bryozoans , foraminifera ( Glomospiriden , Milioliden , Nodosariiden and Textulariiden ) as Agathammina , Colaniella , Dagmarita , Earlandia , Geinitzina , Globivalvulina , Glomospira , Hemigordius , Nankinella , Pachyphloia , Paraglobivalvulina gracilis , P. mira , P. septulifera and Stipulina , Calcareous algae such as Atractyliopsis , Gymnocodium , Macroporella , Mizzia , Permocalculus , Solenopora , Tauridium and Vermiporella , as well as corals , ophiurids and ostracods .

tectonics

De Bellerophon Formation forms a disharmonious boundary surface within the southern alpine layer package, as its plasters reacted like a diapir . Their abundance of evaporites also facilitated folding processes . At the same time, many thrusts are rooted in the gypsum-rich areas.

Age

Absolute radiometric dates are not available for the Bellerophon formation. It is now known, however, that the Permian Triassic boundary runs only a few meters above the Bellerophon Formation in the Tesero member of the Werfen Formation. Thus the Bellerophon Formation ends shortly before the end of the Changhsingian at 251 million years BP . The presence of the index fossil Paratirolites in the upper section of the formation also points to Changhsingium. The formation presumably already started in the Wuchiapingium , but this assumption is not confirmed.

Occurrence

The Bellerophon lineup is in their presence on the Southern Limestone Alps of eastern Südalpins limited their range extends from eastern Adige Valley in the west to Carinthia ( Carnic Alps and northern Karawanken ) and Slovenia (Southern Karavanke) to the east. Selected occurrences in detail are:

use

Miraculous image of the Madonna of Absam in Bellerophon stone on a house facade in Ortisei in Val Gardena

The Bellerophon stone, which occurs frequently in Val Gardena , is used by some artists there for stone sculptures.

Individual evidence

  1. ↑ Hoernes, R .: For the geology of South Tyrol. In: Journal d. German. Geol. Society . Berlin 1876.
  2. ^ Stache, G .: Contributions to the fauna of the Bellerphon limestone in South Tyrol. P.1, cephalopods and gastropods . In: Yearbook KK Geol. Reichsanst. tape 27 (3) . Vienna 1877, p. 272-318 .
  3. ^ Stache, G .: Contributions to the fauna of the Bellerphon limestone in South Tyrol. P. 2: Pelecypods and Brachiopods. In: Yearbook KK Geol. Reichsanst. tape 28 (1) . Vienna 1878, p. 93-168 .
  4. ↑ Bruno Accordi: Contributo alla del conoscenza Permiano mid-superiore della zona di Redagno (Bolzano). In: Ann. Univ. Ferrara . tape 3 , 1958, pp. 37-47 .
  5. ↑ Renato Posenato, Herwig Prinoth: Orizzonti a Nautiloidi ea Brachiopodi della Formazione a Bellerophon (Permiano Superiore) in Val Gardena (Dolomiti) . Geo. Alp., Vol. 1, pp. 71-85, Innsbruck 2004, page 74. PDF file
  6. ↑ Alexander Tollmann: Analysis of the classical North Alpine Mesozoic. Stratigraphy, fauna and facies of the Northern Limestone Alps . Part II of the monograph of the Northern Limestone Alps, Verlag Deuticke, Vienna 1976, ISBN 3-7005-4412-X , pages 43 and 47.
  7. ↑ Bosellini, A. and Hardie, LA: Depositional theme of a marginal marine evaporite. In: Sedimentology . tape 20 . Blackwell, Oxford 1973, p. 5-27 .
  8. ^ Commissione Italiana di Stratigrafia: Formazione a Bellerophon . In: Carta geologico d'Italia 1: 50000 . Catalogo delle formazioni, p. 64-73 .
  9. ^ Sibylle Noé and Werner Buggisch: Sequence Stratigraphy in Late Permian and Lowest Triassic of the Southern Alps (Dolomites; Northern Italy) with Special Regard to the Permian / Triassic Boundary . In: Yearbook of the Federal Geological Institute . 137 issue 2. Vienna 194, p. 297-318 .
  10. ↑ Farabegoli, E. and Perri, MC: Permian / Triassic boundary and Early Triassic of the Bulla section (Southern Alps, Italy): lithostratigraphy, facies and conodont biostratigraphy . In: Giornale di Geologia . Ser. 3ª, 60 (Spec. Issue ECOS VII, Southern Alps Field Trip). Bologna 1998, p. 292-311 .

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