Geology of the Valle de Arán (Central Pyrenees)

The results are given of field work undertaken in the central Pyrenean axial zone on low-grade metamorphic, highly folded Palaeozoic rocks. Absence of determinable fossils is chiefly due to severe tectonisation, but occasionally also of non-deposition. Dating is entirely based on the presence of Silurian developed in a very persistent black shale facies. The sandstones and shales of the Cambro-Ordovician show conglomerate horizons at different levels and a thickly developed limestone in the north and east. The Silurian ampelitic slates are characterised by their high content of organic matter. Their rusty appearance in the field resulted from oxidation of the abundant pyrite. The Devonian is developed in limestones and slates, but in the central part of the area sedimentation differs from the northern and southern parts of the axial zone; sandy deposits constitute the upper part of the Devonian sequence. Characteristic sedimentary structures together with the grading of part of the fine grained sandy deposits are considered evidence for residimentation by turbidity currents. Current directions measured from cross-laminations and convolutions indicate eastward directed transport of sediment. Continuing emergence in the west during Carboniferous times resulted in erosion of part of the Devonian sequence, followed by rapid sedimentation of greywackes in a paralic environment. Remnants of Triassic and late Miocene deposits are preserved north of the Maladeta granodiorite, probably as result of longitudinal faulting. Cleavage-type folding took place during the Hercynian orogeny. A marked disharmony in folding between Devonian and Cambro-Ordovician resulted from the plastic properties of the carbonaceous Silurian slates. Late Hercynian faults, mostly longitudinal, are in some proved instances reactivated during following orogenies. Observations on cleavage characteristics points to close relationship with and dependence on the lithology. Lineations measured from intersection of bedding and cleavage run roughly parallel to fold axis and plunge. Knicked cleavage, resulting from delatation, originated at the end of a late Hercynian uplift. The dykes of the eastern part of the mapped area show coarse fracture cleavage, which developed after the normal cleavage. Some structural features are associated with the intrusion of large granitic masses. Remnants of pre-glacial planation surfaces, presumably of post late-Miocene (Pliocene?) origin, were identified at three different levels. Karst phenomena are frequent in the metamorphic Cambro-Ordovician and Devonian limestones. The large sink-holes are situated on or above the main planation surface, springs related to these sink-holes are found near the present river levels. A geological map is provided showing lithostratigraphic subdivisions and five cross-sections.