Geoparque Unesco

The South Pyrenean Basin in the Tertiary Period and the Formation of the Pyrenees

IIn the central area of the Sobrarbe Geopark, we can observe a complete succession of stratum, ranging from the Eocene to the Oligocene, and recording with incredible detail the most intense stage in the formation of the Pyrenees, reflected in the sedimentary filling of the Aínsa-Jaca Basin and the sea withdrawal during the Tertiary. Among the sedimentary rocks in this basin, those rocks formed during marine and coastal sedimentary processes stand out due to their scientific and didactic interest at international level. .

In eastern Sobrarbe, among the Foradada, Toscar and the Arro mountain ranges, near the western border of the Cotiella dipping fault, we find traces of submarine gorges from the Eocene, which are the main transfer sediment pipes from the shore to the foot of the slope of the continental platform. At the foot of the marine slope, gravitational flows lose strength forming turbidity currents that spread sediments on the bottom of the sea giving rise to turbidites. At the west of the Anticline in Boltaña, we can find a turbidite deposit 4500m thick.

Geological forces raised the Pyrenees and left several marks in our landscape (structural pattern). Anticline in Boltaña, in the Jánovas narrow pass.

The name turbidite was proposed by Kuenen in 1957. They are sedimentary rocks formed by the materials carried by gravitational flows. One example is found in the water masses laden with sediments: clay, lime, sand and pebbles of different sizes that are moved by gravity and travel at high-speed across the bottom of lakes, seas and oceans.

These flows can be formed during periods of heavy rain and river flooding, when sediments are pushed through the continental platform, gorges and submarine channels, until they reach the valleys at the bottom of the sea. Turbidic sedimentation can be triggered by earthquakes and coastal storms.

From a scientific point of view, the relation between the deep marine sedimentation and the dipping fault development allows us to find out details about the age of the Pyrenean formation process.

In the Sobrarbe Geopark we find many types of turbidites that attract geologists world-wide. These sedimentary deposits, formed by a superposition of sandstone, lime, clay or marlstone strata, are often related to crude oil systems. When sandstones are porous and permeable, they constitute good storage for crude oil and gas when clay and marlstone, which are impermeable, seal the storage space. Sobrarbe is both a natural laboratory and an excellent counterpart of crude oil systems.

Magnificent view from Castellazo of the rocky formation of the Sobrarbe Delta. Peña Montañesa in the background.

At the end of the Eocene era, while dipping faults emerged and the Aínsa Basin was refilled, the sea withdrew westwards. Over the marine materials, other coastal sediments were placed, especially those coming from the Sobrarbe Delta. Finally, the basin was filled to the brim by continental sediments both fluvial and alluvial.

View eastwards and northwards from Eripol, where we can observe the traces of the Tertiary sea withdrawal from Sobrarbe. The lower strata (on the left) are older and were formed at the bottom of the sea of the Eocene basin. Above, we can observe coastal materials from the Sobrarbe Delta and, finally, the reddish upper strata (on the right) which were deposited by rivers and torrents and are more modern and of continental origin.

25 million years ago the sea had definitely withdrawn. The first continental sediments were deposited by short rivers and torrents coming form the east and the south. However, sediments coming from the north, where new Pyrenean mountains were being eroded, prevailed early. Rivers dug valleys, alluvial systems were placed by the active margins of dipping faults, forming conglomerates such as the ones found in Canciás and Olsón. .

The Aínsa - Jaca basin was moved several tens of kilometres southwards, at the same time as the continental crust was shortened and dipping faults were formed. We could say that it was piggybacked by dipping faults. From then on, sedimentation does not prevail in this area. Pyrenean erosion still provides the Aquitania Basin to the north and the Ebro basin to the south with sediments.

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