flysch

Sequence of shales interbedded with thin, hard, graywacke-like sandstones. Such sequences are usually thousands of yards thick, but the individual beds are only a few inches to a few yards thick. The occasional presence of fossils indicates marine deposition. The term originally was applied to a formation of the Tertiary Period in the northern Alpine region, but it now denotes similar deposits of other ages and places.

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Flysch is a sequence of sedimentary rocks that is deposited in a deep marine facies in the foreland basin of a developing orogen. Flysch is typically deposited during an early stage of the orogenesis, when the orogen evolves the foreland basin will become shallower and molasse will be deposited on top of the flysch. It is therefore called a syn-orogenic sediment (deposited contemporaneously with mountain building).

Sedimentological properties

Flysch consists of repeated sedimentary cycles with upwards fining of the sediments. At the bottom of each cycle are sometimes coarse conglomerates or breccias, which gradually evolve upwards into sandstone and shale/claystone. Typically the shales don't contain many fossils, the coarser sandstones often have fractions of micas ans glauconite.

Flysch is formed under deep marine circumstances, in a quite and low-energetic depostional environment. The coarser layers (which require higher energy) are disruptions in these circumstances, caused by pulsewise flows of mass transport from the forming orogenic wedge. In many cases the mass transports are represented in the record by turbidites.

Tectonics

Flysch deposits form at convergent plate boundaries at the stage of continental collision, often in remnant ocean basins that exist along the same boundary. The sedimental material in the flysch is derived from the forming mountains and deposited along the axis of the new mountain chain into remnant ocean basin. The same ocean basin is in the process of subducting under the orogenic wedge. As subduction continues, the flysch sediments are scraped off the down-going oceanic plate and are accreted onto the orogenic wedge. As a result, flysch deposits are often highly deformed by thrust faulting and folding.

Name and use

The name flysch was introduced in geologic literature by Swiss geologist Bernhard Studer in 1827. Studer used the term for the typical alternations of sandstone and shale in the foreland of the Alps. The name comes from the German word fliessen, which means to flow, because Studer thought flysch was deposited by rivers. The insight that flysch is actually a deep marine sediment typical for a particular plate tectonic setting came only much later.

The name flysch is currently used in many mountain chains belonging to the Alpine belt. Well-known flysch deposits are found in the forelands of the Pyrenees and Carpathians and in tectonically similar regions in Italy, the Balkans and on Cyprus. In the northern Alps, the Flysch is also a lithostratigraphic unit.

References

Literature

• ; 1999: Earth System History, W.H. Freeman and Company, New York, ISBN 0-7167-2882-6. See p. 243.
• ; 2000 (2^nd ed.): Sedimentary Basins, Evolution, Facies, and Sediment Budget, Springer, ISBN 3-540-66193-X. See p. 606-610.

External links

• Flysch at 1911encyclopedia.org