Flysch Formation: Definition, Characteristics

The Flysch Formation refers to a sequence of sedimentary rocks that typically form in deep marine environments along convergent tectonic plate boundaries, often in foreland basins. Flysch deposits are characterized by an alternating series of thin layers of sandstone, shale, siltstone, and sometimes limestone. These rocks are usually formed from rapid sedimentation as tectonic activity uplifts nearby mountains, leading to the erosion of materials that are then transported into the basin.

Flysch Formation Definition

The Flysch Formation is defined as a specific type of sedimentary rock sequence characterized by interbedded layers of sandstone, shales (or mudstone), and occasionally conglomerate.

Formation of Flysch Sediments

Flysch sediments form in deep marine environments along convergent tectonic plate boundaries, especially in foreland basins at the edges of mountain ranges. The formation process begins with intense tectonic uplift, which causes erosion of nearby mountains. This erosion generates large volumes of sediment, which rivers and other gravity-driven processes transport into adjacent basins.

Flysch typically accumulates through underwater landslides or sediment gravity flows, known as turbidity currents. These flows deposit layers of sand, silt, and clay, creating a pattern of alternating, graded bedding over time. The resulting Flysch sequence records cyclical deposition influenced by tectonic and climatic changes, preserving a history of mountain-building events and basin evolution.

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 Rock Formation Zumaia.
Photo: © Elvira Oliver

Key Sedimentological Characteristics of Flysch

Grain Size and Sorting: Flysch sequences show distinct grain size variations. The sandstones are generally coarse to medium-grained and poorly sorted, reflecting rapid sediment deposition. Finer-grained layers, such as shale and mudstone, often represent slower settling particles from suspension after the turbidity currents slow down.

Sedimentary Structures: Flysch deposits commonly feature graded bedding and rhythmic layering. Graded bedding is typical, with coarser grains at the bottom and finer grains on top, a direct result of settling within turbidity currents. Ripple marks, cross-bedding, and laminations are also common within the sandstone layers.

Alternating Lithologies: Flysch is characterized by repeated layers of sandstone and shale. This alternation is linked to episodic turbidity currents, with each event depositing a sequence of coarser and finer materials. These recurring layers can vary in thickness depending on sediment supply and transport energy.

Turbidite Sequences A diagram illustrating the typical sequence of sedimentary layers deposited by turbidity currents, known as the Bouma Sequence.

Turbidite Sequences (Bouma Sequence): A hallmark of flysch sedimentology is the Bouma sequence, which describes the typical layering in turbidites (sediments deposited by turbidity currents). The sequence begins with a massive or graded sandstone layer (Ta) at the base, followed by parallel-laminated sand or silt (Tb), rippled sand or silt (Tc), parallel-laminated silt (Td), and finally mud (Te). Flysch often displays incomplete Bouma sequences due to variability in current strength and sediment load.

A complete Bouma Sequence consists of five divisions, labeled A through E, representing a waning of energy in the turbidity current:

A: Massive, normally graded sandstone.
B: Parallel-laminated sandstone.
C: Cross-laminated sandstone.
D: Parallel-laminated siltstone.
E: Massive mudstone.

Source Material: The sandstones and shales in flysch sequences often have a mixed composition, reflecting the erosion of continental crust and older sedimentary formations. Quartz, feldspar, and lithic fragments are common in the sandstones, while the shales contain clay minerals, organic matter, and fine-grained silts.

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

Tectonic of the Flysch Formation

Flysch Formation 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.

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.

Sedimentation