Geoparque Unesco

Pyrenean Tectonics

Mediano Anticline Viewpoint

The anticline structure is highly important in order to understand the structure of the Pyrenees.
Municipality: La Fueva

The place chosen as a viewpoint to observe the Mediano Anticline is located in the area near the 11th century Samitier Castle and the Hermitage devoted to San Emeterio and San Celedonio. This place is east of Samitier and can be accessed by a forest track departing from a small Square at the entrance of the village.
Accessing this track by car is difficult, but there is no problem for four-wheel drive vehicles.

It takes 30 minutes on foot from the village.

View of the Mediano Anticline structure from the Samitier Castle.

Geological Context

The Mediano Anticline structure (previously known as Domo de Clamosa) is located at the western end of the south-central Pyrenean Unit (USC) within a complex ramp context, approximately N-S orientated.
These structures strongly affected the sedimentation in this sector of the basin.

Geological Significance

The Mediano Anticline is one of the largest tectonic structures in Sobrarbe. Besides its importance to the geological history of Sobrarbe and the Pyrenees and its fairly remarkable influence on the sedimentation of some deposits in the Hecho Group, it is currently eroded in a way that allows us to study its inner Stratigraphy. There, we can find Triassic gypsum and clay, limestone and dune rocks from the Cretaceous and materials from the Lutetian stage.

This point of geological interest allows us to observe what an anticline structure involves in an educational way..

Rocks in the Sorrosal Waterfall

Sedimentary rocks of marine origin folded during the Alpide orogeny..
Municipality: Broto

The folding traces in the rocks have been highlighted in this image of the outcrop.

To access the outcrop and the waterfall, take the N-260 to Broto. Once in the village, take the detour towards Oto; just before leaving Broto, on the right, there is a path that leads us to the waterfall information panel and up towards the outcrop (access to hiking routes and to via ferrata routes for mountain climbers). Finding a parking place in Broto is easy.

Geological Context

These rocks come from sediments in deep marine environments (turbidites) of the South-Central Pyrenean Eocene Basin (from the Lutetian to the Bartonian stage), specifically, they belong to the turbidic system in Broto, framed by the Torla and Cotefablo systems. These systems, among others, belong to the Hecho Group (Mutti et al., 1972). The Broto system was developed during the Upper Ypresian and it was the first stage of sedimentation rich in sand from the Eocene turbidic basin. .

In the Aínsa basin, this turbidic system is known as the Arró system and it is formed by facies of turbidic channels and its characteristic filling deposits. Here, in the Jaca sub-basin, the Broto system is characterized by lobe depositional facies.

The outcrop can be observed from the waterfall in the Sorrosal River, as a consequence of the uneven terrain between the Sorrosal Gorge and the ancient glacial valley of the Ara River.

Geological Significance

This point has been chosen because it combines several interesting geological features in one single place. Firstly, we find the outcrop of deep marine sediments of the Broto system, which can be observed from different levels if you access it from the path leading to the waterfall. Secondly, we can also observe a clear example of stepping folds which, on a large scale, can affect sediments and become a perfect spot in order to understand the regional tectonics.

Thirdly, the Sorrosal waterfall is the confluence point of the Sorrosal and Ara Rivers. This waterfall marks the drop between the two valleys formed during the last Glaciation, 35000 years ago. A glacier flowed down the Ara Valley eroding the valley dramatically; however, there was no glacier flowing down the Sorrosal River course connecting to the glacier down the Ara Valley, causing two different types of erosion. The river that flows down the hanging valley finds a suitable place here to make its way, eroding the terrain and allowing us to observe the outcrop.

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