Sinkhole of Pla de Reguant (Súria)

In 05.06.2022 a sinkhole appeared in El Pla de Reguant, in Súria. In the following days, the sinkhole enlarged to about 18 meters in diameter by 3 meters deep. The collapse caused some white poplars to fall from the riparian forest. Subsequently, the sinkhole has been partially filled with water from the subsoil, from the Cardener river‘s water table, becoming a pond. Immediately the surface of the pond water has been completely covered with duckweed (Lemna sp.) (1st photo). One and a half year later, in autumn 2023, the sinkhole keeps the same size and the water, though the duckweeds are not there in cold season (2nd photo).

A careful look at El Pla de Reguant discovers a more irregular surface than it might be expected in an alluvial plain. This new sinkhole is not the first one in El Pla de Reguant. Previous sinkholes from historical reports that are nowadays filled explain the uneven surface of this plain.

The Cardona geological formation, the great package of salts, is close to the surface in El Pla de Reguant, it is covered just by the sediments that the river Cardener has contributed and deposited. Here is the center of the Súria salt dome, raised on the southern flank of the Tordell reverse fault, in a geological structure that compares to the Cardona salt diapir, though in Súria it is beheaded and covered by the action of the river. In the area of ​​El Pla de Reguant named El Salí, salt was exploited in mining wells. Here, in 1912, the presence of also potash in the Cardona geological formation was discovered.

The circulation of underground water must have dissolved a part of the salt, opening a progressively larger cavity until its roof has finally collapsed and sank. The result is a sinkhole on the surface that becomes a pond. After its origin, its size and its recent formation, the pond of El Pla de Reguant is similar to those on the west shore of Lake Banyoles -the Cisó, Nou, Castellnou stream and others. However, while beneath the ponds in Banyoles the soluble salt is gypsum, beneath El Pla de Reguant there is common salt or sodium chloride, much more soluble and, therefore, much more prone to these geological phenomena.

[photos Jordi Badia]