Geology of the Luna-Sil region, Cantabrian Mountains (NW Spain)

After a late-Precambrian folding, clastic deposits, partly continental, spread out over the region during the Lower Cambrian; later, marine intercalations became more abundant when upper Lower Cambrian marine sediments were deposited. This sedimentation continued until the Upper Carboniferous and occurred on a generally stable shelf between a rise in the NNE (the Asturian Geanticline) and a curved geosyncline (present at least until the Devonian) in the SSW. Unstable conditions prevailed during the Upper Cambrian and the Couvinian-Givetian. The Asturian Geanticline extended several times towards the SSW, which resulted in emergence, accompanied by erosion, of the Luna-Sil region during the Llanvirn-Llandovery, during the Gedinnian (only the NNE part) and at the end of the Devonian. The last epeirogenic uplifts resulted in strong erosion, especially in the NNE part of the region: Famennian-Tournaisian deposits unconformably overlie rocks of a Gedinnian age. These uplifts occurred along fundamental faults. Slight epeirogenic uplifts along the same faults during the Lower Viséan resulted in the erosion of the very thin black shales of a Tournaisian age. With the beginning of the greywacke deposition during the Upper Namurian B, initial to the folding and thrusting of the region, the palaeogeographic pattern changed: the source area was now situated in the SSW, while a rapidly subsiding basin lay in the place of the former shelf. The line of maximum sedimentation, just as the folding front, was displaced during the Namurian and Westfalian from SSW to NNE. After the orogenesis, followed by strong erosion, oblong basins developed along normal faults approximately parallel to the strike of the orogene, in which thick coal-bearing sediments accumulated during the Stephanian B and C; these sediments were folded during the Lower Permian. During the orogenesis thrusts were formed, generally parallel to the strike of the former curved basin, which developed from the breaking through of anticlines. These folds and thrusts have their vergence to the NNE. The shape and amplitude of the folds were mainly determined by a thick Ordovician quartzite formation. During the folding and thrusting a tectonic ‘Stockwerk’ was formed in the Palaeozoic rocks, each of the 11 levels in this ‘Stockwerk’ possessing its own tectonic style. During continued compression, movement along the thrust faults ceased and these faults were involved in the folding. This last stage of the folding has its vergence to the SSW. One important and a number of less important WSW trending faults cross the folds and thrust faults which are generally parallel to the chiefly WNW strike of the former curved basin. Due to a reorientation of the stress field in a direction perpendicular to the WSW trending faults, deviation of the generally WNW trending structures took place in a direction parallel to these faults. This deviation is strongest near the ends and nil in the central parts of the WSW trending faults.