July 30. Flysch

On July 21, when I talked about the Marathon Orogeny in West Texas, I mentioned in passing the relatively deep, narrow seaway between the colliding continents. The Pennsylvanian sediments that were dumped into that trough, and eventually got caught up in the collision, made a sedimentary sequence called flysch.

Flysch sediments are called syn-tectonic or syn-orogenic, meaning that they are deposited contemporaneously with the mountain building. Often the basin in which they are laid down may be a foreland basin, the result of loading the crust and depressing it so more and more sediment can go into the basin, or sometimes flysch may be deposited in a remnant bit of ocean basin or something more complex.   

Zumaiako Flysch, Spain. Photo by Torpe via Wikipedia

Flysch deposits are cyclic, reflecting pulses of mountain building. When mountains are higher, coarser sediments are often dumped into the adjacent basin, and as the mountainous relief reduces through erosion, sediments become finer and finer. So a typical flysch sequence is one where the grain size of the rocks becomes finer upward, as the rocks get younger. The whole package can be repeated many times as the mountainous sources for the sediments are uplifted and eroded down. The cycles are usually clastic rocks – clastic means broken, and the broken fragments can be as coarse as pebbles to make conglomerate, but often it’s just various grain sizes in sandstones, siltstones, and muds.

Because the flysch deposits are often dumped into narrowing ocean basins during collisional tectonics, and because the oceanic crust underlying the flysch is often being subducted beneath one of the colliding continents, flysch deposits are often highly deformed, tightly folded and faulted. There are spectacular examples of tilted and folded flysch rocks in the Alps and other collision zones, made all the more evident because the alternating layers are often quite thin, so the structures are pretty obvious.

The word is something of a misnomer, since it was applied to rocks that were thought to be deposited by rivers. It comes from German meaning to flow, as in flowing rivers. But flysch deposits were generally laid down in deep oceanic water – sometimes, by flowing subsea currents called turbidite flows, which took sediments great distances out into oceanic basins.

Eventually, a basin begins to fill up, or the tectonism reduces in intensity, and shallow-water sediments typically overlie deep-water flysch deposits. Those shallow-water or even terrestrial deposits are called molasse. That’s a French word meaning “soft,” since the molasse sediments are often less well cemented than the underlying flysch deposits.

—Richard I. Gibson

Zumaiako Flysch, Spain. Photo by Torpe via Wikipedia under Creative Commons license