Fault of El Guix (Sallent)

The Guix fault is a large tectonic accident that runs in a straight line of about 10 km from southwest to northeast, from the north of Santpedor to Horta de Avinyó, crossing the Llobregat river and the southern slopes of the Montcogul mountain range. It has its center, its maximum development and its greatest complexity in the La Botjosa and El Cogulló area, south of Sallent. It is called the Guix fault in a simplified way, although the structure is much more complex.

In top views, the structure is an anticline with a reverse fault in the axis, where the north block has slipped off above the south block as a result from pushing tectonic forces. The salt that is found roughly 400 meters in depth and up to 300 meters thick has strongly contributed to origin the anticline. Salt rock is plastic in comparison to the stiffness of the strata of lake limestones, sandstones and other rocks that it has on top. Hence, salt slid to the core of the anticline after the push of tectonic force and the weight of the rocks that it holds. The Guix fault is the southernmost manifestation in Catalonia of the Pyrenean orogeny in its later phase. The height of the fault gap and the distance of slip vary along the fault, both being bigger in its central stretch, in Sallent.

In detail, the arrangement of the strata in the fault zone is much more complex, with consecutive folds and, at least in the central section, the splitting into two parallel faults.

The passage of the railway cuts a long trench on the east side at the foot of the Cogulló mountain, between the miner village of La Botjosa and the valley of Soldevila, opening a beautiful window to the Guix fault that crosses perpendicular. The upper image gallery shows three zones of the Guix fault as seen in this railway trench and arranged from south to north. The 1st and 2nd photos correspond to the two fault planes that run parallel, the 3rd to a comparatively smaller fold located further north.

Some 1,200 meters to the west, a stream going down from the dumpfill of Cogulló towards the valley of Soldevila discovers a small section of the Guix fault (4th photo, lower gallery) that likely matches the 3rd image seen from the trench of the railway that we have called north fold.

Some 750 meters to the northeast, the river Llobregat, as the railway trench, crosses the Guix fault line in the industrial area of Illa (photo 5, lower gallery). In this case, the image corresponds to the main fault.

All photos are taken facing west, so all the images show the south on the left and the north on the right.

The Guix fault toponym refers to the existence of layers of primary plaster (guix in Catalan) in the raised, northern block of the fault.

The mining of potash in Sallent and Balsareny stopped in July 2020. The mine digged into the of northern block of the Guix fault. After the misfortune and the disregard for the environment, the two dumpfills of saline waste in Sallent, these are La Botjosa and Cogulló, are located above the Guix fault without waterproofing the soil before dumping saline waste and underestimating the risk of overall saline contamination of the waters. Indeed, salty water flows through the fault plane south of Guix in Sallent everywhere it can be seen to the right of the river Llobregat, in the trench of the railway coming from the Cogulló dump that is located above and on the shore of the Llobregat with increased flow after adding brine from the dumpfill of La Botjosa.

[photos Florenci Vallès and Jordi Badia]