Flysch (/flɪʃ/) is a sequence ofsedimentary rock layers that progress from deep-water andturbidity flow deposits to shallow-watershales andsandstones. It is deposited when a deep basin forms rapidly on the continental side of a mountain building episode. Examples are found near theNorth American Cordillera, theAlps, thePyrenees[1][2] and theCarpathians.
Flysch consists of repeated sedimentary cycles with upwardsfining of the sediments. There are sometimes coarseconglomerates orbreccias at the bottom of each cycle, which gradually evolve upwards intosandstone andshale/mudstone. Flysch typically consists of a sequence of shales rhythmically interbedded with thin, hard,graywacke-like sandstones. Typically the shales do not contain manyfossils, while the coarser sandstones often have fractions ofmicas andglauconite.
In acontinental collision, asubducting tectonic plate pushes on the plate above it, making the rockfold, often to the point wherethrust faults form, and a mountain chain rises. On the upper plate, the land between the mountains and the undeformed continent bends downward, forming aforeland basin. If the basin forms slowly, as in the northernAppalachians, it fills with shallow-water sediments.[3] If it forms rapidly, as in the east side of theNorth American Cordillera, then sea water may rush in, and the first sedimentary deposits are deep water deposits. If the mountain slope is steep enough at the edge of the basin, it will shed material in rapidly moving sedimentary flows calledturbidity currents, resulting inturbidite deposits. As the basin fills up, shallow-water sandstones and continental deposits form.[3][4] Most of the resulting rocks have little deformation, but near the edge of the mountain chain they can be subject to folding and thrusting.[3] After the basin fills up, continental sediments (molasse) are deposited on top of the flysch.[4]
The name flysch was introduced in geologic literature by theSwiss geologistBernhard 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 theGerman wordfliessen, which meansto flow, because Studer thought flysch was deposited by rivers. The insight that flysch is actually a deepmarine sediment typical for a particularplate tectonic setting came only much later.[5]
The name flysch is currently used in many mountain chains belonging to the Alpine belt. Well-known flysch deposits are found in theforelands of thePyrenees andCarpathians and in tectonically similar regions inItaly, theBalkans and onCyprus. In the northern Alps, the Flysch is also alithostratigraphic unit.
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