Flysch

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Carpathian flysch

A flysch is a sandstone formation, the word being borrowed from Swiss German.

"Flysch" is a relatively archaic term describing syn-orogenic (occurring contemporaneously with mountain building) clastic sedimentation within marine depositional facies. It is contrasted with the term molasse, which also describes syn-orogenic sediments, but are deposited in a terrestrial realm. These terms originated from descriptions of rocks within the European Alps. These terms were created before a true understanding of their tectonic significance was realized.

Flysch deposits occur at collisional plate boundaries, often where remnant ocean basins exist along the same boundary. They can form in two ways:

Sediment is derived from the uplifted mountains and deposited along its axis into remnant ocean basin. The same ocean basin is in the process of subducting along the same convergent margin. As subduction occurs, the sandstones and shales are scraped off the down-going oceanic plate and are accreted onto the overriding plate. As a result, flysch deposits are often highly deformed.

The flysch is deposited on the landward side of a mountain chain in a foreland basin, in a downwarped (depressed) area of the crust. The foreland basin forms quickly and is flooded by the sea; the first sediments to form are deep-water muds and turbidites, forming interbedded shale and turbidites. Concurrently, the flysch sediments are deformed by the neighboring orogeny, until the foreland basin is completely filled with sediment, and topped with terrestrial molasse deposits.(Stanley, 243)