The geology of the upper Salat and Pallaresa Valleys, Central Pyrenees, France/Spain

A sequence of more than 4000 m of marine sediments, mainly unfossiliferous and apparently without any unconformities, range in age from probable Cambrian to pre-Hercynian Carboniferous. The lower formations are of neritic facies and there is no indication of a Pyrenean basin before the Devonian, the deposits of which are much thicker in the centre of the present axial zone than on the margins. A relatively thin band of black shales of Silurian age acted as a tectonic lubricant and thus its presence resulted in a marked disharmony between the infra- and supra-structures. The infra-structure is very complicated and consists of multiple composite anticlinoria and synclinoria in which the tectonic shortening is mainly accounted for by the smallest fold unit — the tightly isoclinal micro-folding. Fold axes and b-lineations of this cleavage microfolding plunge consistently in the same direction over sharply delimited areas of up to hundreds of square km. In the supra-structure the microfolding plays a much smaller role than in the infra-structure; the folding is less composite and high-amplitude folds of some 1000 times larger dimensions provide a real shortening of about 40—50 %. A thinning of roughly 20 % of the Devonian sediments by compression has been calculated from fracture phenomena in thin slate intercalations in limestone beds. This thinning thus gives an apparent shortening which is greater than is actually the case. The northern boundary of the main dome of Lower Palaeozoic is formed by a steep flexured zone with a throw of at least 2 km. Adjacent to this flexure on the northern side is a zone of steep isoclinally folded Upper Palaeozoic rocks cut by an E—W branch of the North-Pyrenean fault system, resulting in a tilting of both blocks towards the north. The main dome is flanked to the south by a deep Upper Palaeozoic syncline of which the southern flank in the Monseny area passes into recumbent folds directed towards the south. After the main folding arching caused a fanning out of the originally vertical structure elements. Genetically related to this fanning is a late fracture cleavage (knick-zones) which displaces the syn-tectonic cleavage in such a way as to indicate a dilatation in a N—S direction. A subsequent, yet pre-Triassic vertical jointing, visible on aerial photographs, shows a complicated picture with many strike maxima of poor regional consistency. These major lineaments greatly influence the drainage. Important remnants of pre-glacial denudation surfaces have been preserved and lie at 2400—2600 m and 1850—2350 m altitude. The lower altitudes of these ranges are found towards the west of the area. The snow line of the last glaciation — derived from the lowest level of nivation cirque excavation — lay at 1500—1600 m in the north rising to 2100—2200 m in the south. A purely petrographical description is given of granodiorite batholiths, dykes, sills and basic rock intrusions. The talc of Fonta probably originated from dolomite by metasomatic addition of large quantities of hydrothermal quartz which penetrated from the granodiorite intrusion along a fault plane. The galena and sphalerite occurrences of Carbauere are also connected with a fault.