Molasses

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Rock formation from Alpine Molasse: " Les Pénitents " ("The Penitents"), consisting of a conglomerate that was deposited by rivers at the turn of the Miocene to the Pliocene in the foothills of the Western Alps. Plateau de Valensole , Alpes-de-Haute-Provence department , Southeast France.

Molasses is the name for sedimentary rock , which in the removal of wrinkles mountains (→  Orogen to its extensive planarization) from the last phase of its formation (raising phase) to a mountain hull occur. Molasse is differentiated from flysch , which is actually only deposited during ore formation.

Word origin and concept development

The term molasse was introduced into scientific literature in 1779 by Horace-Bénédict de Saussure . The term is possibly derived from the identical French-Swiss word for very soft (this was already used in the 14th century and in turn goes back to the Latin mollis ). The term was then applied in 1789 by Gregor de Razumowski to fine-grained, soft sandstones in the foothills of the Alps, which harden quickly when they dry in the open air. In western Switzerland, sandstones were also known as molasses , from which millstones ( Latin : mola ) could be made.

Later the name was extended to similar rocks in the entire northern foothills of the Alps. Today the term is used worldwide for sediments that are mainly deposited in the foreland of the mountains ( orogeny ) that are rising in the course of their mountain formation ( orogenesis ) and that come from the erosion material of the rising mountain body.

Formation and rocks

Miocene Nagelfluh of the lower freshwater molasse vom Speer in the Swiss Prealps
Outcrop with alpine molasses in the ravine of the Linzer Aach north of Überlingen , Baden-Württemberg. It is a sandstone of the Upper Sea Molasse (Miocene).

In contrast to the predominantly deep marine flysch , molasses sediments are mainly sedimented in terrestrial or shallow marine deposit environments, in the foreland of a mountain range (outer molasses) or in its interior (inner molasses). Scree , sand , silt and clays are transported by the rivers from the mountains to the fore and aft and deposited there. Sandstones and conglomerates predominate near the edge of the mountains (the latter also known in geology, in the case of river deposits in the Swabian Alb and especially in the Swiss Alps as the Nagelfluh). If the parent rocks contain lime , z. B. Deposits of Juranagelfluh or finer deposits such as marl rocks . At greater distances from mountains, finer deposits such as fine sand or silt predominate.

In the area of ​​an outer molasse, sea and land sediments alternate due to temporary sea advances. In the foreland of the Alps, the freshwater molasses with fluvial and limnic sediments and the sea molasses with more marine facies were formed , which, due to large-scale sea advances and retreats, are divided into a lower and upper sea molasse , which are each replaced by the lower and upper freshwater molasses . The deposits of the sea molasses are partly characterized by a wealth of fossils, as is the case, for example, with the Erminger Turritellenplatte near Ulm .

Rocks of an inner molasse are predominantly reddish in color and associated with volcanic deposits.

Molasse sediments often contain remains of plants and other fossils . This is why coal ( Ruhr area ) and biogenic natural gas (for example in the Alpine foothills of Upper Bavaria and Upper Austria ) formed in the sediments of an outer molasse . The lower - lying oil and natural gas deposits in southern Bavaria and Upper Austria, on the other hand, originate from the fish slate of the Oligocene , which lies below the Alps, and only reached the lowest sandstone layers of the Molasse Basin from the Upper Eocene through migration .

Deposits of the outer molasses are usually created by the sinking of the earth's crust in the foreland of a pile of roofs due to its heavy weight - the corresponding depression is therefore also called the foreland sink or foreland basin . As a result of constant subsidence, a foreland basin can hold sediment piles several thousand meters thick. Inner molasses are deposited in smaller-scale basins that sink directly into the ceiling stack. The formation of these pelvises usually results from stretching processes, which can have various causes. The expansion processes are often also the cause of the volcanism that accompanies the inner molasses sedimentation.

Occurrence

Miocene molasses, open the undercut slope of a river in the foothills of the Ukrainian Carpathians
This Upper Carboniferous conglomerate from the Aachen district, the so-called "Gedauer conglomerate", is one of the oldest molasse sediments in the Variscan Mountains .
A Central Devonian conglomerate exposed on Schunemunk Mountain in Orange County, New York State. The so-called Schunemunk conglomerate belongs to the molasses of the Acadian fold belt , which is now part of the Appalachian Mountains.

The tertiary molasse basin in the foothills of the Alps , an outer molasse , is known. The molasses sediments of the Alps are widespread in the French foothills of the Alps, in the Swiss plateau and in the German and Austrian foothills of the Alps . You continue in the Carpathian fore to the east and follow the Carpathian Arc. In the Alps, however, numerous deposits of inner molasses have been preserved, for example in the Plateau de Valensole , in the Plateau de Chambaran or in the Klagenfurt Basin . The Po Valley contains the inner molasse of the Alps, at the same time it is the outer molasse of the Apennines .

Although the Pannonian Plain is enclosed by Alpine mountain ranges, due to the size and the history of the development of the Pannonian Basin, its deposits are not to be regarded as internal but rather external molasses.

The mountains of the southern Balkans and the mountain islands in the Aegean Sea also have their molasses sediments.

Outer molasses of the Pyrenees is found in the Aquitaine Basin in southwest France and in the Ebro Basin in northern Spain. It also originated in the Tertiary (the sedimentation history of the Aquitaine Basin begins well before the formation of the Pyrenees).

An example of a molasse basin outside of Europe are the Muree and Siwaliks Formations of the Subhimalayas, which are already crossed by the Himalayas as outer molasses and pushed onto the Quaternary sediments, which are deposited by the Ganges , Indus and Brahmaputra as the youngest molasses of the Himalayas to date.

Examples of Molassesedimente older age are the removal remains of the Variscan Mountains from the younger Carboniferous and Permian ( Permian ) in France, Belgium and Germany, such as the Saar - Nahe basin (Innenmolasse) and across the lap of Namur , the Aachen Revier and the Ruhr area extending coal basin (external molasse). Similar sediments can also be found on and in the Appalachian Mountains in the USA .

See also

Web links

Individual evidence

  1. ^ Hans Murawski, Wilhelm Meyer: Geological dictionary . 11th edition. Elsevier / Spektrum, Heidelberg 2004, ISBN 3-8274-1445-8 , pp. 146 .
  2. ↑ Molasse , Lexique de géologie sédimentaire, Université de Liège (French)
  3. ↑ Reinhard Schönenberg, Joachim Neugebauer: Introduction to the geology of Europe. 4th edition, Verlag Rombach, Freiburg 1981, 340 pages, ISBN 3-7930-0914-9 . P. 259f
  4. ↑ A. Vamvaka, A. Kilias, D. Mountrakis and J. Papaoikonomou: Geometry and structural evolution of the Mesohellenic Trough (Greece): a new approach. Geological Society, London, Special Publications. Vol. 260, 2006, 521-538, DOI: 10.1144 / GSL.SP.2006.260.01.22 , limited preview in Google book search
  5. ↑ Adonis D. Photiadis: The Ophiolitic molasses Unit of Ikaria Iceland. Turkish Journal of Earth Sciences. Vol. 11, 2002, pp. 27-38 online (PDF; 408 kB)
  6. ^ P. Molnar, P. Tapponnier: Cenozoic tectonics of Asia - effects of a continental collision. Science. Vol. 189, 1975, pp. 419-426.