Flysch Sequence Formation

Flysch Sequence 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.

Formation of Flysch Deposits

Flysch deposits are formed when a deep basin forms rapidly on the continental side of a mountain building episode. The basin is filled with sediment that is eroded from the mountains. The sediment is deposited by turbidity currents, which are dense flows of sediment that occur in deep water. 

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.

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

Sedimentology of the Flysch Sequence

Flysch sequences are typically composed of alternating layers of sandstone and mudstone, with the sandstone beds being thicker and coarser-grained than the mudstone beds. The sandstone beds are often graded, meaning that the grain size of the sand decreases from the bottom of the bed to the top. This grading is a result of the turbidity currents that deposited the sandstone beds.

Turbidity currents are dense, fluid mixtures of sediment and water that flow down submarine slopes. They are typically triggered by earthquakes or other events that disturb the seafloor. Turbidity currents can travel at great speeds and can carry large amounts of sediment. When a turbidity current reaches the base of a slope, it slows down and deposits its sediment load. The coarser-grained sediment is deposited first, followed by the finer-grained sediment. This is what creates the graded bedding that is characteristic of flysch sequences.   

layers of flysch on the coast at Zumaia, Spain
Layers of flysch on the coast at Zumaia, Spain
Photo by: Jean Michel Etchecolonea

The sedimentology of flysch sequences is complex and has been the subject of much study. The rhythmic bedding of flysch is thought to be caused by turbidity currents, which are dense, gravity-driven flows of sediment that can occur in deep-marine environments. Turbidity currents are often triggered by earthquakes or other events that cause the sediment on the seafloor to become unstable.

When a turbidity current flows down a slope, it carries with it a large amount of sediment. The sediment is eventually deposited on the seafloor, and the layers of sediment that are deposited become thicker and coarser-grained as the turbidity current slows down. The shale layers are thought to be deposited from finer-grained sediment that is suspended in the water column.

Flysch sequences are often very thick, and they can contain a wide variety of sedimentary structures. These structures can provide clues about the environment in which the flysch was deposited. For example, flute casts are grooves that are formed by the erosive action of turbidity currents, and they are a common feature of flysch sequences.

Tectonic of the Flysch Sequence

Flysch sequences are formed in a variety of tectonic settings, including foreland basins, back-arc basins, and accretionary wedges. Foreland basins are located in front of mountain ranges, and they are formed as the tectonic plates collide. Back-arc basins are located behind volcanic arcs, and they are formed as the tectonic plates pull apart. Accretionary wedges are located along convergent plate boundaries, and they are formed as material from one plate is scraped onto the other plate.

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 origin of the Flysch Sequence

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, Spain. 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
Transgression and Regression in a Sedimentary Outcrop?
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