Allgäu ceiling

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The Allgäu-ceiling is a tectonic ceiling unit in Bajuvarikum the Northern Limestone Alps .

designation

The Allgäu ceiling is named after its type locality - the Allgäu Alps .

definition

In the Northern Limestone Alps, which are part of the Oberostalpin , a number of tectonic nappes can be distinguished, some of which also have certain strata sequences. Three main ceilings are separated - the Bajuvarikum in the lying position in the north, followed by the Tirolikum and the Juvavikum of the hanging wall further south.

The Bajuvarikum, for its part, is further subdivided into two ceiling systems - the northern deep bayuvarikum in the lying wall and the southern high bayuvarikum in the hanging wall . The Tiefbajuvarikum includes both the Cenoman marginal scale (which is sometimes also referred to as the Randcenoman ) on its northern edge and the Allgäu ceiling. The Hochbajuvarikum consists of the Lechtal blanket .

In the west of the Northern Limestone Alps, the Bajuvarikum is followed further south by the Tirolikum with the Inntal Nappe and the Krabachjoch Nappe on top of it .

introduction

The Höfats (2259 m) belonging to the Allgäu ceiling is located immediately northwest of the Allgäu main hollow . It is built up at its core from resistant layers of the Ammergau formation .

The Allgäu ceiling is particularly strongly scaled or isoclinally folded tightly . Wrinkles and scales also occur side by side and often also merge into one another - for example in their western core section. The thrust of the scales can reach a considerable extent of 3 and more kilometers. Further to the east in the foothills of the Ammer Mountains , it appears in a very narrow volume with little internal structure. Between the Kochelsee and Schliersee there are again scales that merge into horizontal folds in the foreland of the Wendelstein .

The tectonic boundary surface of the Allgäu Nappe opposite the flysch zone is very steep north-verged and only turns into a flatter course at depth. It continues underground below the Lechtal ceiling in a south direction, which it largely covers.

Due to the fact that both ceiling units were severely deformed, folded and scaled internally in the course of their north-west to north-westerly transport, the contact conditions at the front edge of the Lechtal ceiling being driven over are quite complicated. This is expressed along the eastern border of the Allgäu Nappe in its finger-like east-north-east extension of the exposure area or in a strip-like advance of the Lech Valley Nappe towards the west.

Geographical description

The Allgäu Nappe forms the actual forehead of the Northern Limestone Alps and is pushed in a northerly direction onto the Cenoman rim scale of the Bajuvarikum northern rim and the flysch zone of the Penninic . Along its north-western edge, it overlays the Arosa zone - a tectonic mixed zone of eastern Alpine and Penninic rocks, which was carried along under the Lech Valley blanket as a sliding carpet, so to speak.

The Allgäu-ceiling sets with diagonal smear on Kalkalpenwestrand east of the Great Walsertal south-southwest of Oberstdorf (at Lechleiten , Schröcken and train ) and a relatively narrow band at ending Inzell 15 kilometers west of Salzburg , where the Staufen Hell mountain-ceiling of Tirolikum is superimposed. From Füssen to the Inn , it runs in the frontal area of ​​the Northern Limestone Alps as a relatively straight, 2 to a maximum of 5 kilometers wide strip free in front of the overlying Lech Valley blanket. Continuous tectonic contact with the Genoman marginal scale in the north and a significant, also uninterrupted thrust to the Lechtal cover, the front edge of which is divided by a series of half-windows and decking blocks, proves the tectonic independence of the Allgäu cover in this section. After interruptions with two smaller deposits near Salzburg, it reappears as Langbath plaice on the Traunsee and east of the Steyr as the Ternberger Nappe . From the Weyerer arches it stretches as the Frankenfelser blanket to the eastern edge of the Alpine Orogen north of Gießhübl .

In terms of surface area, the Allgäu Nappe reaches its greatest extent in the Allgäu northeast of Oberstdorf, where it can withstand a good 20 kilometers to Reutte and the Säuling in a north-easterly strike direction . At right angles to painting, the ceiling takes up about 13 kilometers here, but can swell between north of Bad Hindelang and the Hornbachtal up to a maximum width of 23 kilometers.

Between Füssen and Bad Hindelang in the north, Reutte and Hochvogel in the south, the simple construction scheme shown at the beginning changes. The Lechtal ceiling is divided into several sub-units (such as the Plattjoch-Schuppe , the Gehrenspitz-Halbklippe and the Lailach-Schuppe ) and the long, finger-like striped windows in between, such as the Vilstal , Benna- and Tannheimer Half-windows - repeatedly reveal a view of the Allgäu ceiling immediately below. A special case is the Falkensteinzug , which overlays the Allgäu Nappe in the contact area with the Cenomanian scale, isolated from its trunk cover from the Hornburg east of Füssen to shortly before Bad Hindelang as a narrow strip, a maximum of 2.5 kilometers wide.

At the western end of the Vilser and Tannheim mountains , the Lechtal blanket finally lifts all the way to the west. The northern edge of the parent unit of the Lech Valley ceiling jumps far south back and runs southeast from Schochen and Roßkopf into high bird area, which protrudes from the Allgäu-ceiling Bärgründele-half window , the saddle head half window and the two controversial half windows of Luitpold zone (Luitpold-half window ) and the Hornbach zone (Hornbach half-window) of the Hornbach valley . South of the Hornbachtal, the Ramstall-Schuppe , the Allgäuer Hauptkamm-Schuppe and the Zuger Schuppe with the Götzneralm half- window continue the ceiling boundary of the Lechtal ceiling to the southwest. Braunarlspitz-Schuppe and Wandfluh-Schuppe then completely cut off the Allgäu ceiling at the southwest end.

The north-western end of the Northern Limestone Alps is clearly dominated by the Allgäudecke south of the Falkenstein range, but here it is subdivided into a series of sheds of sometimes considerable dimensions: Jochschrofen-Voruppe , Jochschrofen-Schuppe with Hintersteiner window , Iseler-Schuppe , which runs through the western edge the Rubihorn scale is replaced, the short Geißhorn scale or its western equivalent, the thumb (pan-wood) scale and finally the foghorn scale (or Nebelhorn-Rauhhorn-scale) in front of the front of the Lechtal- Ceiling.

To the southeast of Oberstdorf, two windows of the Arosa zone appear within the Allgäu ceiling - the Gerstruben and the Birgsau window . There is also a small flysch window northwest of the Birgsau window. Further to the southwest, two deck floes then swim in the Arosa zone - the Kanzelwand and the Bärenkopf deck floe . The Kanzelwand cover clod is separated from the actual Allgäu ceiling at the Fiderepass by the Fidere half-window . To the south of the almost completely separated Bärenkopf-Deckscholle form the Große Widderstein-Schuppe and the Kleine Widderstein-Schuppe form the rear western edge of the Allgäu-Nappe. Finally, the southwest end in the Großer Walsertal contains the Hochkünzel-Schuppe and the Zitterklapfen-Schuppe . The latter is upstream to the north of the Graeshorn deck clod , which is also separated from the Arosa zone.

Structurally defining within the Allgäu Nappe is the Allgäu main hollow - a double, predominantly northeast but also east-striking synclinorium .

Mountain peaks

The main peaks within the Allgäu blanket are Bärenkopf (2083 m), Breitenberg (1893 m), Geißhorn (2366 m), Giebel (1949 m), Großer Thumb (2280 m), Großer Widderstein (2533 m), Himmelhorn (2111 m) , Höfats (2259 m), Iseler (1876 m), Jochschrofen (1625 m), Kanzelwand (2058 m), Kleiner Widderstein (2236 m), Kreuzeck (2376 m), Krinnenspitze (2000 m), Lachenkopf (2111 m), Laufbacher Eck (2194 m), Linkerskopf (2459 m), Nebelhorn (2224 m), Ponten (2045 m), Rauheck (2384 m), Rotspitze (2033 m), Rubihorn (1957 m), Schmalhorn (1952 m), Schneck (2268 m) and Schochen (2100 m).

stratigraphy

Red-colored, flat-lying Adnet formation at the summit of Schochen (2100 m)

Sequence of layers

The sequence of layers of the Allgäu Nappe, which is a little over 3,000 meters thick, is incomplete and sheared along the Raibler layers . It therefore begins with the main dolomite . In detail, it shows the following stratigraphic structure (from hanging wall to lying ):

Sedimentary development

During the Norium, up to 2,000 meters of main dolomite were deposited in a huge lagoon on a shelf platform on the southeastern continental margin of Alcapia (but only 900 meters in the area of ​​the Allgäu ceiling). The main dolomite appears both in the Allgäu Nappe and in the Lechtal Nappe as a main triassic rock creator. The plate limestone above it in the late Norian to Rhaetian marks the return to shallow marine limestone sedimentation . The overlying Kössen formation is characterized by an alternating layer of marl and basin limestone. The Oberrhätkalk represents an Upper Triassic carbonate platform facies and interlocks towards the basin with the Kössen formation.

The red tuber limestones of the Adnet Formation represent a condensed deep sill facies that are very thin and often absent. In contrast, the Allgäu Formation represents an average 550-meter-thick basin facies , which is widespread in the Allgäu Nappe and can be extremely powerful up to 1,500 meters (Jurassic main rock former of the Allgäu Nappe). The Lower Allgäu Formation ( Hettangium to Pliensbachium ) consists of pelagic, bioturbatic marl limestone, in which sandstones , calcarenites and micro- breccias can be incorporated. The magnesium-rich black marl and slate of the Middle Allgäu Formation of the Toarcium correlate well with the global anoxic event. The sedimentation of the silica limes of the Upper Allgäu Formation then continued into the Middle Jurassic ( Upper Bajocian ). In the early Upper Jurassic the red to green cherts ( radiolarites ) and marls of the Ruhpolding formation accumulated . In the Kimmeridgian, carbonate production recovered again, so that the pelagic limestones of the Ammergau Formation were now deposited. Their sedimentation lasted into the Middle Aptium without a hiatus or an increasing marl content being recognizable. The deposit area of ​​the Schrambach Formation of the Aptium was located in a basin system dominated by carbonate. Episodically recurring terrigenous suspension flows result in fine-clastic interventions.

The Tannheim Formation of the late Aptian marks the beginning of synorogenic sedimentation, which brought about a change from purely carbonate to mixed carbonate / siliciclastic sediments. It merges into the conglomeratic Losenstein Formation of the Albium. The siliciclastics collected in a basin parallel to the orogen front. The increase in grain size towards the hanging wall and the component spectrum with an increasing fraction of exotic and mafic basement pebbles in the hanging wall clearly show the approaching of the ceiling front and the eventual closure of the basin in the course of the Albium.

tectonics

The razor-sharp thrust orbit of the Lechtal Nappe over the Allgäu Nappe lies below the two Höllhorns on the right edge of the picture

General

The tectonics of the Allgäu Nappe is very complex due to the folds and scaling already mentioned. The axis directions of the articulated and dump trucks are very variable. In the northern third of the ceiling, east-west directions predominate (with local deviations to east-north-east and east-south-east). In the middle part, north-east directions clearly shape the fold structure. In the south-west third, in addition to north-east, eastern strike directions appear again. The spatial distribution of the folds is also very irregular, especially with regard to their wavelength . In areas with very tight folds such as For example, in the central area around the Höfats or at the southwest end near the Rothorn, wavelengths of up to 500 meters are observed (in the area of ​​the Höfats there are 8 articulated trucks at a distance of only 5 kilometers), whereas in normal areas wavelengths of 2,000 meters and more are the order of the day.

The structure of the folds was changed in a later stage by fracture tectonics due to faults and leaf displacements. The predominant directions are north-north-east (offset to the left), north-south, north-north-west (offset to the right), north-west (right, but also offset to the left), north-east and east-south-east.

Thrusts

Steep crossing of the heavily folded main dolomite of the Lechtal Nappe on the inverted Upper Rhätkalk of the Allgäu Nappe at the Wiedemer Kopf . The contact follows the gap above the Prinz-Luitpold-Haus . In the background Höfats and Schneck.

The thrust of the Lechtal Nappe onto the Allgäu Nappe is exposed below the Höllhörner , at the Jochspitze on the Hornbachjoch and north of the Großer Wildden to the southeast of the Schönberghütte and west of the Kreuzkopf . It is razor-sharp and slopes mostly flat to the south to south-southeast. Directly below the ceiling boundary, the Upper Triassic main dolomite miners of the Lechtal Nappe appear in the driven, completely sheared, Lower Jurassic Allgäu Formation of the Allgäu Nappe, with hard benches often being intensively broken down into phacoids . However, the Allgäu formation is not everywhere parallel to the ceiling, but can also meet the ceiling boundary from below at obtuse angles to almost vertically.

However, the flat nappes are only in the least of the cases straight, but mostly show a jagged course. A good example is the crossing of the Lechtal ceiling north of the Litnisschrofens , which shows strong changes in direction and also very variable incidence values . This can be explained both by the structural geological structure of the underlying Algäu Nappe and by the shifting of leaves perpendicular to the thrust direction.

However, the thrusts can also be very steep, as can be seen in the south of the Luitpold Zone and generally along the entire northern edge of the Allgäu Nappe.

Age

The Allgäu Nappe was formed about 97 million years ago during the Cenomanium as the deepest and last moving unit of the Bajuvian nappes. It then laid over the Cenomanian marginal scale to the north by 87 million years in the course of the Coniacium . The Lechtal blanket had started its north-west to north-facing movements much earlier in the Albium and was preparing to drive over the Allgäu blanket. The Lechtal ceiling in turn was pushed over by the Inntal ceiling in the Coniacium .

literature

  • Alexander Tollmann: The construction of the Northern Limestone Alps . Deuticke, Vienna 1976, p. 449 .
  • Volker Jacobshagen, Klaus Schwerd and Thomas Hornung: Explanations for sheet 8628 Hochvogel . In: Bavarian State Office for the Environment (ed.): Geological map of Bavaria 1: 25 000 . 2014, p. 89 .

Individual evidence

  1. ↑ SM Schmid, B. Fügenschuh, E. Kissling and R. Schuster: Tectonic map and overall architecture of the Alpine orogen . In: Eclogae Geologicae Helvetiae . tape 97 (1) , 2004, p. 93-117 .
  2. ↑ a b Alexander Tollmann: Tectonic map of the Northern Limestone Alps 3rd part: The western section . In: Communications from the Geological Society in Vienna . tape 62 , 1969, p. 78-170 .
  3. ↑ Manfred P. Gwinner: Geology of the Alps . E. Schweizerbart'sche Verlagbuchhandlung (Nägele and Obermiller), Stuttgart 1971, ISBN 3-510-65015-8 .
  4. ^ I. Fruth and R. Scherreiks: Hauptdolomit - Sedimentary and paleogeographic models (Norian, Northern Calcareous Alps) . In: Geologische Rundschau . tape 73 (1) , 1984, pp. 305-319 .
  5. ↑ A. Tollmann: Analysis of the classical North Alpine Mesozoic Era: Stratigraphy, fauna and facies of the Northern Limestone Alps . Vol. 2. Deuticke, Vienna 1976, p. 580 .
  6. ↑ R. Golebiowski: pools and reefs of the Alpine Triassic - Lithostratigraphy and biofacies the Kössener formation . Ed .: D. Nagel and G. Rabeder, excursions in the Upper Palaeozoic and Mesozoic periods in Austria. Austrian Palaeontological Society, Vienna 1991, p. 79-119 .
  7. ↑ R. Huckriede: The Lech formation near Kaisers and Holzgau in the Lechtal Alps (Apt - Lower Cenoman) . In: Negotiations of the Federal Geological Institute . Vienna 1958, p. 71-86 .
  8. ↑ Volker Jacobshagen: The Allgäu layers (Jura spotted marl) between the Wetterstein Mountains and the Rhine . In: Yearbook of the Federal Geological Institute Vienna . tape 108 , 1965, pp. 1-114 .
  9. ^ S. Neumeister et al .: Redox conditions and depositional environment of the Lower Jurassic Bächental bituminous marls (Tyrol, Austria) . In: Austrian Journal of Earth Sciences . tape 109 (2) , 2016, pp. 142-159 .
  10. ^ A. Bartolini, PO Baumgartner and JC Hunziker: Middle and Late Jurassic carbon stable-isotope stratigraphy and radiolarite sedimentation of the Umbria-Marche basin (Central Italy) . In: Eclogae Geologicae Helvetiae . tape 89 , 1996, pp. 811-844 .
  11. ^ J. Schweigl and F. Neubauer: Structural evolution of the central Northern Calcareous Alps . In: Eclogae Geol. Helv. Volume 90 . Basel 1997, p. 303-323 .
  12. ^ R. Gaupp: Sediment history and palaeotectonics of the calcareous Middle Cretaceous (Allgäu, Tyrol, Vorarlberg) . In: Zitteliana . tape 8 , 1982, pp. 33-72 .
  13. ↑ a b H. Ortner: Cretaceous thrusting in the western part of the Northern Calcareous Alps (Austria) - evidences from synorogenic sedimentation and structural data . In: Communications from the Austrian Geological Society . tape 94 , 2003, p. 63-77 .