The geology of the Upper Ribagorzana and Baliera valleys, Central Pyrenees, Spain

In the mapped area there is a well-exposed low-grade metamorphic marine sequence from Ordovician to Lower Carboniferous, unconformably overlain by Permo-Triassic continental deposits. Determinable fossils are rare. The Ordovician consists of a quartzite/shale sequence with one marly limestone intercalation in the upper part. The Silurian is developed as a classical graptolite-bearing black shale facies with an Orthoceras limestone near the top. The Devonian rock-sequence in the north differs from that in the south. The northern or Sierra Negra facies area consists of a thin (120—250 m) alternation of mainly limestone and slate; the southern, Baliera facies area, is thicker (340—780 m), shows more individual limestone-slate units, and is moreover characterized by a conspicuous quartzite member (0—50 m) in the middle part of the Devonian. The Devonian sequence in both areas is subdivided into four or five separate formations which have been mapped individually. The Carboniferous consists in both areas of micaceous slates with a low sand content. The continental Permo-Triassic is developed in the Germanic facies of red mud- and silstones, sandstones, and conglomerates at the base, followed by a non-fossiliferous limestone/dolomite (Muschelkalk) and gypsum-bearing, vividly coloured marls of the Keuper. The major structural movements, which probably began already in early Carboniferous times, increased in strength towards the Westphalian B. Several phases of deformation have been recognized. The first deformation produced concentric, open to tight asymmetric folds without cleavage development. Their axial planes have a general E-W to ESE-WNW trend in the north (Sierra Negra Unit), a constant NE trend in the centre (Baliera Unit), and an E-W and NW-SE trend in the south (Ribagorzana Unit). The second deformation, representing the main phase, was caused by a N-S compression and is characterized by tight to isoclinal folds with a steep northward-dipping axial plane cleavage in the north, the dip becoming more moderate in the centre and south. Fold axes and \u03b4-lineations show a girdle distribution. A third deformational phase bent the entire structure around a NNE-trending axis coinciding with the bed of the Ribagorzana River. The Maladeta granodiorite and accompanying dykes intruded parallel to the general cleavage trend and caused a metamorphic aureole of moderate width. Near its southern border, gravity folds were formed locally. The economic occurrence of galena, exploited near the village of Bono, is related to this igneous activity. A fourth deformational phase was produced by a renewed N-S compression, causing local folding of the first or main-phase cleavage, also showing a weak secondary axial plane cleavage (fracture or crenulation cleavage), and local thrust movements along the earlier cleavage plane. This deformation might be a late Hercynian or an Alpine phase. The Alpine orogeny initiated or rejuvenated important northward-dipping overthrusts and minor thrust movements along the main phase cleavage in the Palaeozoic, which at the same time caused asymmetric folding of the Permo-Triassic strata above the unconformity. The post-Miocene erosion following the Alpine uplift led to various aplanation levels, some dubious remnants of which are preserved in our area. Glacial forms developed during the Pleistocene, and subsequent river erosion modified the glacial morphology.