Flysch

Flysch is a sequence of sedimentary rock layers that grades from deep-water turbidity current deposits upward into shallower-water shale and sandstone, forming when a deep basin develops rapidly on the continental side of a mountain-building episode. A flysch sequence repeats in rhythmic cycles: each typically begins with a coarse conglomerate or breccia at its base that fines upward into sandstone and then shale or mudstone, producing rhythmically interbedded shale and graywacke-like sandstone; the shale layers are usually barren of fossils while the coarser sandstone beds carry fractions of mica and glauconite. Classic flysch sequences are exposed near major mountain ranges including the North American Cordillera, the Alps, the Pyrenees and the Carpathians, marking the foreland basins that formed as those ranges rose. Swiss geologist Bernhard Studer introduced the term in 1827, deriving it from the German word fliessen, meaning to flow, because he initially believed flysch sediment was laid down by rivers; that view was later corrected once geologists recognized flysch as a deep marine deposit characteristic of specific plate tectonic settings. This entity documents flysch as a general depositional facies and rock-sequence type, distinct from the Zumaia Flysch, a single named outcrop exposing this kind of sequence on Spain's Basque coast.