Ruhpolding formation

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The Ruhpolding Formation is a sedimentary formation that was deposited in the Northern Limestone Alps at the beginning of the Upper Jurassic . The high- sea formation is characterized by its abundance of pebbles .

designation

The Ruhpolding formation, named after its type locality , the Upper Bavarian municipality of Ruhpolding , is also known as the Ruhpolding radiolarite or the Ruhpolding layers . The type locality is located southwest of Ruhpolding am Gschwendlbach near Röthelmoos . However, it has been poorly chosen, as the red radiolarite, which is characteristic of the entire Austroalpine region, is exceptionally missing and only gray to red silica limestone is present, overlaid by the Ruhpolding marble . For this reason, Gawlik (2000) proposed a new type profile on the Mörtlbach northeast of Hallein with complete formation of the radiolarite.

In a broader sense, all the radiolarites of the Upper Bajoc , Callovian , Oxfordian , Kimmeridgian and Untertithons in the Northern Limestone Alps are now combined to form the Ruhpolding Radiolarite Group  (RRG).

Occurrence

The eponymous type locality forms part of the Lechtal blanket of the Upper Eastern Alpine Bajuvarikum . The formation occurs from the Allgäu Alps and the Lechtal Alps in the west to the Chiemgau Alps in the east. The Ruhpolding Formation in the narrow sense but also in the south adjoining the Bajuvarikum Tirolikum encountered and thus also extends to the eastern part of the Northern Limestone Alps.

The Ruhpoldinger-Radiolarit-Gruppe has a much wider range (spatially and temporally) and appears in the Karawanken and in the Southern Alps . In Austroalpine it is also to be found, but it is here already metamorphosed before (examples are the Austroalpine as part of the Tauern window and Grisons ). The group even performs in the Pennine area ( Piedmont zone ).

stratigraphy

In Bajuvarikum the Ruhpolding Formation follows concordant to the Chiemgauer layers - siliceous, chert leading spätige limes - or in the Allgäu Alps to the gray, marly younger Allgäu layers . It is in turn concordantly overlaid by aptych layers of the Ammergau Formation (dense, yellowish to greenish-gray marl limestone from the Tithonian to the Berriasian ). The transition to the aptych layers with Lamellaptychus lamellosus and Punctaptychus punctatus takes place gradually. In the middle section of the northern Limestone Alps, the Oberalm Formation of the Kimmeridgium to Berriasium follows the Ruhpolding Formation, in places with a base conglomerate; however, the transition usually takes place gradually and is mainly characterized by an increasing lime content.

In the northern Tirolikum , for example in the Tauglboden basin , the Ruhpolding formation lies with a clear bench joint on the underlying, bulbous (the tubers consist of manganese iron oxide) red limes of the Klaus formation . The bench joint is emphasized by a pitch several centimeters thick. About the Ruhpolding Formation then lays Tauglboden formation of Kimmeridgiums and Untertithons .

Occasionally the Ruhpolding Formation can reach down to the Adnet Formation or the Vilser Kalk .

In the Sillenkopf Basin of the southern Tyrolean region, the Ruhpolding Formation is overlaid by the Strubberg Formation and by the Sillenkopf Formation, which coincides with the Tauglboden Formation .

Lithology

Different radiolarians under the microscope

Lithologically , the Ruhpolding Formation, which is around 50 meters thick on average (variation width 5 to 100 meters), consists of black-green and red radiolarites, silica limes, pebbly marls and siliceous clays . It essentially emerged from radiolarian silt. The mud solidified into a uniformly finely layered and banded Chertformation consisting of chert - pebble lime and - chert layers is established. The individual pebbly layers are usually separated by wafer-thin layers of clay. The resulting cyclicality can possibly be associated with Milanković cycles . A possible diagnetically induced segregation as an explanation of the banding can be excluded by cross-cycle sliding structures.

The color of the fine-grained, very hard, splintery to shell-like breaking and weather-resistant rock is predominantly red, but greenish-light gray to black shades can also be found. The red color ( hematite ) is due to the complete oxidation of the iron compounds contained in oxygen-rich deep water (ratio Fe 3+ / Fe 2+ > 1). In the case of the greenish radiolarites, the ratio Fe 3+ / Fe 2+ <1, bound to the minerals sericite and chlorite . The FeO content is very high, and pyrite should also play a role in the coloring.

In the thin section it can be seen that the pebbly ground mass is composed of diagenetically converted, maximally 0.1 mm large radiolarian skeletons. The chert rock, which chemically consists of almost 100% SiO 2 , is traversed by fissure and fissure systems, some of which are organized in a network, which crystallized out with calcite . These fracture systems were caused by later tectonic stresses. Mafish tuff layers are quite common at the base of the Ruhpolding Formation. These are likely to be associated with Ehrwaldite magmatism .

Fossils

The Ruhpolding formation consists mainly of microfossils (radiolarians), with the exception of poorly preserved aptychs, crinoids such as saccocoma , needle remains and filaments , macrofossils are extremely rare. Benthonic foraminifera are rare and planktonic foraminifera are completely absent. A few taxa should be singled out from the enormously diverse radiolarians:

Deposit conditions

The Ruhpolding Formation is clearly of high marine origin and has been deposited in the pelagic area, recognizable by the very rare ammonite finds such as Hibolites semisulcatus Münster . At what depth the sedimentation ultimately took place is still a matter of dispute. Actuogeologically , radiolarian sludge forms below the compensation depth for calcite ( English calcite compensation depth or CCD ) in a water depth of 4,000 to 5,000 meters. However, it must be taken into account that the CCD in the Upper Jurassic was much higher (probably between 2000 and 3000 meters).

The blooming of the radiolarians may have been triggered by volcanism and / or changes in the water circulation. In addition to volcanic input, the required silica was possibly also supplied with cold buoyant water. Indirectly, far-reaching intra-oceanic changes in the spatial conditions of the Tethys area at that time can be concluded.

Ruhpolding turn

The sedimentation of the radiolarite of the Ruhpolding Formation means a sudden and drastic break in the sedimentation process of the Northern Limestone Alps, which is called the Ruhpolding Wende . This change in sedimentation character was not of a temporary nature. With the appearance of the radiolarites (and later the aptych layers), sediments spread that differed clearly and permanently from the lower and middle Jurassic.

This was preceded by a rapid decrease in sedimentation rates and thicknesses in the Dogger. In the upper Klaus formation the omission areas increased and the sedimentation became generally slower and more patchy. The starvation of the sedimentation was probably due to a continuous sinking movement.

During the Ruhpolding Wende, the existing sea floor relief was tectonically accentuated and individual threshold areas rose into the shallow water zone. Incidentally, the Ruhpolding Formation, which was limited to deeper basin areas, did not deposit in these raised thresholds, but red limestone continued to sediment here, for example the Agatha limestone (a cephalopod limestone of the deep-sea threshold facies from the Oxfordian to the Kimmeridgian) or the Hasselberg limestone (a clayey flasher limestone from the tithonium). Only later, with the onset of sedimentation of the aptych layers, were the tectonically induced contrasts slowly balanced again.

In addition to tectonic movements, the Ruhpolding Wende is also characterized by volcaniclastic deposits (tuffites), which are part of the basic, Upper Jurassic magma pulse .

With the Ruhpolding Wende, the frequency of autokinetic sediments such as turbidites , fluxoturbidites, mud flow breccias, grain flows, mud folds and sliding packages increased. Also Olistolithen are found. Here include light brown, allodapischen limes of Barmsteins (with Eingleitungen of Hallstatter Faziesbereichs) which Sonnwendbrekzie and Tauglboden- and Strub mountain formation. Most of the mass movements caused by tectonic movements occur either shortly before or shortly after the Ruhpolding transition. A good example during the turn of mass movements took place is the Grubhörndlbrekzie a Megabrekzie (with a 300-foot block of limestone!), The trending north-south on a fault slipped off to the west and toothed in the basin with the Ruhpolding lineup.

The gradual displacement of the radiolarian mud of the Ruhpolding Formation by coccolith mud of the overlying aptych layers can either be traced back to a further deepening and / or can be explained by a blooming of the calcareous nannoplankton .

Age

The Ruhpolding Formation in the narrower sense was deposited in the Upper Oxfordium, ie around 157 to 155 million years ago BP . This biostratigraphic age (maximum age) is based on ammonite finds in the uppermost Klaus formation.

In the meantime, however, the diachronicity of the Ruhpolding formation is generally recognized. A more recent study by Wegener, Suzuki & Gawlick (2003) found an age from the Middle Oxfordian to the Lower Kimmeridgian for the upper red radiolarite, ie 159 to 154 million years BP using the radiolarian stratigraphy.

For the Ruhpolding radiolarite group, Suzuki & Gawlick (2003a) give an age from Bajocium to Untertithon, i.e. H. the period from 171 to 147 million years BP.

literature

  • HR Grünau: Radiolarian Cherts and Associated Rocks in Space and Time . In: Eclogae Geol. Helv. Volume 58 . Basel 1965, p. 157-208 .
  • M. Gwinner: Geology of the Alps . Schweizerbarth, Stuttgart 1971.
  • Reading, HG: Sedimentary Environments and Facies . Blackwell Scientific Publications Ltd, Oxford 1978, ISBN 0-632-01223-4 .

Individual evidence

  1. ^ RE Garrison, AG Fischer: Deep-water limestones and radiolarites of the Alpine Jurassic . In: Soc. Econ. Paleontol. Mineral, Spec. Publ. Volume 14 . Tulsa 1969, p. 20-56 .
  2. ^ F. Trauth: The Facial Formation and Structure of the Upper Jurassic in the Northern Eastern Alps . In: Verh. Geol. Bundesanst. (Born 1948). Vienna 1950, p. 145-218 .
  3. ^ E. Steiger, T. Steiger: New radiolaria from the “Ruhpoldinger Marmor” of Urschlau (Late Jurassic, Chiemgau Alps, Bavaria) . In: Abh. Geol. Bundesanst. tape 50 . Vienna 1994, p. 453-466 .
  4. ↑ HJ Gawlick: The radiolarite basins in the Northern Limestone Alps (high Middle Jura, Upper Jura) . In: Communications Society Geology Mining Students Austria . tape 44 . Vienna 2000, p. 97-156 .
  5. ^ L. Krystyn: Stratigraphy, fauna and facies of the Klaus strata (Dogger / Oxford) in the eastern northern Alps (Austria) . In: Verh. Geol. Bundesanst. Vienna 1971, p. 486-509 .
  6. ^ HJ Gawlick, W. Frisch: The Middle to Late Jurassic carbonate clastic radiolarite flysch sediments in the Northern Calcareous Alps: sedimentology, basin evolution and tectonics - an overview . In: Neues Jahrb. Geol. Paläontol. Depending on the band 230 . Stuttgart 2003, p. 163-213 .
  7. ↑ on Ehrwaldite see C. Wolkersdorfer: Ehrwaldite discovered 125 years ago. In: Ausserferner Nachrichten. 4, Reutte June 7, 1990.
  8. ^ V. Diersche: The Radiolarite of the Upper Jura in the central section of the Northern Limestone Alps. In: Geotekt. Forsch. E. Schweizerbart, Stuttgart 1980, p. 1-217 , 3 pl., 45 fig., 1 tab. .
  9. ^ V. Diersche: The Radiolarite of the Upper Jura in the Northern Limestone Alps between Salzach and Tiroler Ache . In: Diss. Techn. Univ. Berlin .
  10. ^ W. Schlager, M. Schlager: Clastic Sediments associated with radiolarites (Tauglboden layers, Upper Jurassic, Eastern Alps) . In: Sedimentology . tape 20 . Amsterdam 1973, p. 65-89 .
  11. ^ J. Wendt: Stratigraphy and palaeogeography of the red Jurassic limestone in the Sonnwendgebirge (Tyrol, Austria) . In: N. Jb. Geol. Pal. Stuttgart 1969, p. 132, 219-238 .
  12. ↑ E. Wegener, H. Suzuki, H.-J. Gawlick: For the stratigraphic classification of silica sediments southeast of the Plassen (Northern Limestone Alps, Austria) . In: . Jb Geol B.-A. . tape 143 , no. 2 , 2003, p. 323-335 .
  13. ↑ Suzuki, H. & Gawlick, HJ: The Jurassic Radiolaria Zones of the Northern Limestone Alps . In: JT Weidinger, H. Lobitzer, I. Spitzbart (Ed.): Contributions to the geology of the Salzkammergut . 2003.