A tour through the Basque Geopark flysch

My previous post provided background information, including location map, about the Basque Coastal Geopark (https://landscapes-revealed.net/the-basque-geopark-60-million-years-in-20-km/). This post provides more details about this extraordinary sequence of sedimentary rocks called “flysch” (pronounced “flish”). When you are in the Geopark, you will constantly see this word.

The fancifully-decorated FLYSCH sign is advertising “Guided activities during all of the year”. The boats for guided tours (routes shown by dotted blue lines) all have this insignia, as do the guides’ t-shirts. The red dots and lighter green color indicate the limits of the Basque Geopark. The yellow dots indicate guided tours by foot.

Other sightings of “flysch”: on street signs showing where to go to see flysch (upper right), as the name of a surf shop (lower right), and even as the name of a local wine (left). Txakoli (cha-ko-li) is the name for a varietal (hondarrabi zuri) that I think is unique to this area. It’s bright and tasty!

What in the world is flysch?

Flysch is an archaic word that was coined in 1827 by a Swiss geologist who used the term for layers of alternating thin layers of sandstone and shale in the Alps. The name comes from the German word fliessen, which means to flow. Geologists later realized that these were deep marine sediments and then applied the term to layers deposited in a “geosyncline [basin] shortly before a major orogeny [mountain-building event]”. This was before plate tectonics had been discovered and today the term “geosyncline” is meaningless.

We now know these alternating layers were deposited by turbidity currents and that they can form in a variety of marine environments. Sure, the layers in the Geopark were deposited in a basin that was later uplifted during the Alpine orogeny, but there was a long time in between these two events that occurred in two very different tectonic environments.

This photo shows “flysch” layers along the coast of Zumaia. This old term continues to be applied to thin layers of alternating sandstone and mudstone, plus or minus limestone, and that is unlikely to change. I have to admit that it’s a fun word, everyone in the region knows what it refers to, and most don’t realize it has an outdated (and erroneous) meaning. But now YOU know the truth. Nevertheless, from here on, I’ll give in to the reality of local usage!

Sedimentology 101: turbidity currents and pelagic sediments

The “flysch” layers were deposited by two processes: (1) sediments that were carried from shallow water to deep water by turbidity currents, and (2) sediments that originated in the ocean and then sank to the sea floor.

I found this small-scale “model” of a turbidity current on Deba Beach—really close to ancient layers deposited by these currents! Imagine that this flat piece of sand is the continental shelf, with land on the right and a slope to the deep ocean on the left. The water flow is laminar (sheet like) on the shelf, but when it reaches the steep slope it turns turbulent. These flows can continue for long distances in the deep ocean. As they slow down, the sediments settle out to create what we call a “turbidite” layer.
Here is a sequence of turbidites (the more resistant sand layers) alternating with mud. The turbidity flows happen when sediment from land gets carried seaward. In between flows, mud settles very slowly from suspension. This sequence is at Saturraran Beach at the western end of the Geopark (just west of Deba on the map below). See my previous post (link at top of page) to see what the ocean basin looked like at that time. As described in my previous post, the layers were uplifted and titled during the Alpine-Pyrenean Orogeny (collision between the Iberian and Eurasian plates).

Touring the flysch

The “flysch” can be viewed by car, boat, train, bus, or foot. The most complete views are via boat or foot. As described in my previous post (see link above), the best viewing is during low tide, particularly during times with large tidal ranges. So some pre-planning is useful.

This map shows most of the Basque Coastal Geopark—the part that extends between Deba and Zumaia (black letters on left and right sides of map). There is a fabulous 14-km (8.4 mile) trail that enables visitors to hike the entire distance. The numbers refer to points of interest along the way that are referred to in the park’s downloadable guides (https://geoparkea.eus/en/what-see/geo-routes). What’s great is that you can take the train or bus one direction and hike the other direction. We choose to take a 15-minute train ride from Zumaia (where we were staying) to Deba, and then hike the trail back to Zumaia. We had to drive to visit the westernmost part of the Geopark at Mutriku and Saturraran Beach (see map in previous post and “flysch” at Saturraran above).
Deba Beach, at the western end of the Geopark trail, is a good place to see the “flysch” and the surf. We enjoyed seeing these boys getting their first surf lesson. Impressively, several detailed signs along the promenade described the “science of surfing” and included detailed information about how waves and tides work.
From Deba, we continued eastward along the coastal path (see The Flysch Route map above), with incredible views along the way. This view is of Sakoneta, one of the few places where the trail goes down to beach level. At the bottom of the photo, you may notice two people descending the trail, which is part of El Camino del Norte that connects San Sebastián and Santiago de Compostela. We passed some groups of “pilgrims” along this trail.

The most famous part of the flysch at Zumaia

Because the continuous layers at Zumaia were deposited during major events in Earth’s history, it is one of the most studied and referenced sedimentary sequences in the world. The cross section below shows the geologic ages of the sequence and the events that are recorded there. Although the lower (older) part of Geopark “flysch” is mostly sequences of turbidites, the upper (younger) part at Zumaia is mostly pelagic sediments, with only occasional turbidite layers. This implies that the area was farther from land. As a reminder, the Geopark sequence was deposited during ~60 m.y., from ~110–50 Ma.

This image shows the Zumaia sequence from west (oldest layers) to east (youngest layers). The ages of the 10 m..y. part from 65.5 Ma to 55.8 Ma (K/T to P/E) are certain. But the layers extending east and west of this part include at least another 10 m.y. The upper ~15 m.y. of this sequence is shown in the photo at the top of this post.

The top line are the names of the Periods: Cretaceous, the end of the Mesozoic Era, and Paleogene (formerly the Tertiary), the beginning of the Cenozoic Era. The Paleogene is divided into two Epochs: Paleocene and Eocene. The Paleocene is divided into three Stages (D/S/T).

From oldest to youngest, the labelled events are: (1) K/T—Cretaceous/Tertiary boundary, although it should be labelled K/Pg since the Tertiary is now the Paleogene. (2) D/S—the boundary between the Danian and Selandian stages of the Paleocene. (3) S/T—The boundary between the Selandian and Thenatian stages of the Paleocene. (4) P/E—the boundary between the Paleocene and Eocene Epochs. Boundaries are based mainly on fossil content and typically mark times when there was a significant change in organism populations.

The next photos show the boundaries listed above.

The P/E boundary is not very scenic because there is only red clay layers that are recessive and covered with grass. This boundary is also called the PETM (Paleocene Eocene Thermal Maximum). This was a time of extreme temperature increase that lasted about 100,000 years. The layers at Zumaia, and other PETM sites around the world, are being carefully studied to learn information applicable to today’s climate changes.

Here is another photo of Zumaia—looking westward along the coast, with the San Telmo hermitage on the cliff top. The layers are tilted toward us, and we are just seeing their planar tops. Zumaia is an incredible spot—scientifically important but also very beautiful.
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  1. Lynn on October 12, 2023 at 1:33 am

    From our personal lesson, I now think I’ll (mostly) be able to ID which direction a turbidite layer is moving! All of this is fascinating, especially while I’m going through the Green Spain portion of the Spanish Wine Scholar course. It talks about soils and their formations in each of the four regions and your recents posts take that discussion much further. Now I’d love to get back to País Vasco and see more formations and soil IRL!

    • Landscapes Revealed on October 12, 2023 at 9:02 am

      I already want to go back too – maybe we can meet near there again next year!

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