Definition:

Flysch is a sequence of sedimentary rock layers that progress from deep-water and turbidity flow deposits to shallow-water shales and sandstones. Flysch, sequence of shales rhythmically interbedded with thin, hard, graywacke-like sandstones. The total thickness of such sequences is commonly many thousands of metres, but the individual beds are thin, only a few centimetres to a few metres thick.

It is deposited when a deep basin forms rapidly on the continental side of a mountain building episode. Examples are found near the North American Cordillera, the Alps, the Pyrenees and the Carpathians.

Sedimentology:

Flysch and Molasse are terms that describe different sets of sedimentary facies associated with orogenic belts. Flysch a thick sequence of marine sedimentary rocks, primarily detrital in origin, characterized by a cyclic alternation of several lithologic varieties of graded beds with finer grains toward the top, regardless of mineralogical composition. Flysch consists of many thin cycles that are only several centimeters to several decimeters deep.

Flysch Sequence Formation
Flysch Rock Formation Zumaia, Photo: © Elvira Oliver



The cycles are also called cyclothems, cyclosomes, or multiple beds and are sometimes inaccurately referred to as rhythms.

The cycles are formed by two to four beds. The underlying beds consist of granulites, such as sandstone, aleurolite, and cal-carenite, and often display cross-bedded and irregular sediments; the overlying beds are composed of pelites. Hieroglyphs of various shapes and origins are typical on the lower surface of the cycles. Some of the hieroglyphs are the impressions of irregularities on the floor of the body of water; the irregularities resulted from the erosional action of short-lived turbidity currents and from the vital activity of various animals, mostly worms, that crawled along the bottom. Fucoids, which resemble algae in shape and are at least as characteristic of flysch as are hieroglyphs, owe their origin to worms.

The cyclic nature of flysch is caused by specific features of the accumulation of sediment, that is, by more or less periodic turbidity currents.

The presence of rare fossils indicates marine deposition. Flysch facies are now generally believed to have accumulated in moderate to deep (up to 2,000 m [6,500 feet]) marine waters. Coarse angular sands probably were deposited from turbidity currents (subaqueous sediment-laden flows); the extraordinary coarse conglomeratic mudstones in some flysch may be a product of submarine mudflows.

Tectonic:

In a continental collision, a subducting tectonic plate pushes on the plate above it, making the rock fold, often to the point where thrust faults form, and a mountain chain rises. On the upper plate, the land between the mountains and the undeformed continent bends downward, forming a foreland basin. If the basin forms slowly, as in the northern Appalachians, it fills with shallow-water sediments.

Flysch is a typical geosynclinal formation that marks the pre-orogenic phase of the development of eugeosynclines, miogeo-synclines, or both. In the preorogenic phase, a cordillera emerged along a flysch trough. A cordillera is a long chain of islands on the slope of which wildflysch and coarse-grained wildflysch formed. Wildflysch is associated with submarine landslide formations, and coarse-grained wildflysch is enriched with conglomerates and sandstone.



Away from the source, the wildflysch is typically replaced by flysch, which in turn is replaced by immature flysch and semiflysch, or subflysch. In a vertical sequence of geologic formations, flysch occupies an intermediate position between slate formation and molasses. In folded regions that arose at the sites of geosynclines, strongly dislocated flysch deposits constitute the outer part, or the externides.

Name

The term originally was applied to a formation of the Tertiary Period (later subdivided into the Paleogene and Neogene; 65.5 to 2.6 million years ago) occurring in the northern Alpine region but now denotes similar deposits of other ages and other places.

Flysch Sequence Formation
Flysch Rock Formation Zumaia, Photo: © Gerhard Huber,

The name flysch was introduced in geologic literature by the 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.

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.

See also: 
Types of Unconformities
Spectacular Outcrop of Submarine Landslide Deposits
How to Identify Transgression and Regression in a Sedimentary Outcrop?
 
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