Cantabrian Zone
The Cantabrian Zone consists of Carboniferous and older Paleozoic unmetamorphosed rocks.
It is bounded on the west and south-west sides by a concave arc of Precambrian rocks called the Narcea window, and the Villandin window in the Narcea antiform.
The Herreria Formation from the Lower Cambrian consists of shale and feldspathic sandstone alternating, with some conglomerate. These have a thickness of 1 to 1.5 km.
The Lancara Formation consists of a couple of hundred metres of limestone. The lower part was formed in peritidal zones in the Lower Cambrian, and the upper member from the Middle Cambrian contains fossils and is red or green glauconictic and nodular limestone.
The Oville Formation from Middle to Upper Cambrian contains alternating shale and sandstone. Trilobite fossils are common in the shale.
The Barrios Formation is Arenigian and up to 500 metres (1,600 ft) thick. It consists of a white massive quartzite.
The Penas and Vidrias area, close to the western boundary of the Cantabrian zone has a complete succession of Ordovician deposits. Black shales from Llanvirnian times are found in the Central Coal Basin eastern side. But mostly in the Ordovician Period, this zone was above water and eroding.
The Formigoso Formation dates from Middle Llandovery time in the Silurian. It consists of Monograptus black shales and is up to 150 m thick.
The San Pedro and Furada Formations are up to 300 metres thick and consists of shale and iron bearing sandstone interbedded, These are from Wenlock Ludlow and Lower Gedinian times.
In the Devonian Period deposition occurred on the western side, with dolomite, argillaceous limestone, marl and shale from the Raneces Complex or La Vid Formation. It is 600 metres (2,000 ft) thick and Gedinian to Emsian in age.
The Santa Lucia Formation is of limestone. It contains coral near the Narcia Antiform in the west and has peritidal facies in the east near the Central Coal Basin. The Huergas Formation alternates between red sandstone and shale and is of Couvinian to Givetian age. The Portilla formation is of coralline limestone of Givetian to Frasnian age. This is topped off by sandstone layers up to 500 m thick from the Frasnian to Fammenian age. Devonian sediments are not found to the east of the central coal basin, and are thickest in the west.
A pelagic facies comes from the Pisuerga-Carrion province.
In Carboniferous times deposition started with black shales and cherts from the Tournaisian age, and then red limestone, red shale and radiolarites were formed in the Visean age. Mountain Limestone is a thick black lifeless limestone of Serpukhovian age. Turbidites with olistoliths also appear in the Serpukhovian, indicating the first sign of the Hercynian tectonic events. These first events happened in the Pisuerga-Carrion province.
Variscan compression lifted the west side, turning a sedimentary basin into a mountain range. Over time the compressed zone moved towards the east. In the Namurian A stage, the Olleros formation was byukt from turbidites in a trough in front of the orgen, and the Barcallente formation was a carbonate platform further off shore. In the Namurian B stage the trough was forming San Emillano Formation, and the Valdeteja Formation was offshore, but in deeper marine conditions. During Westphalian A time the trough was filled and deposits of terrestrial material formed the San Emiliano Formation and Sama Group and the Lena group being thickest in the Central Coal Basin Unit. Further east in the Picos de Europa it remained covered in shallow water with continuous formation of a carbonate platform.
The Westphalian age is represented by 5000 m of the Central Coal Basin, which as the name suggests contains coal. To the east this grades into the marine carbonates of the Picos de Europa. In the Pisuerga-Carrion province, there are conglomerates composed of quartzite, turbidites with slump deposits from deeper sea water. There are also some limestone layers with fossils.
The source of the Westphalian sediments was from the west and south. These were the mountains of the Hercynian chain, formed at the same time as these deposits. During the Westphalian the rocks in the Caqntabrian zone were folded and overthrust. The Paleozoic rocks broke off at the level of the Lancara Formation and were thrust over the top the upper layers forming nappes and thrust sheets. The Ponga Nappe Province is to the east of the Central Coal Basin,
Stephanian age molasse deposits overlie the other Carboniferous rocks and are unrelated to the Hercynian orogeny. Some final folding occurred at right angles to the Westphalian structures.
Further uplift occurred, and in the Stephanian period there were some land locked basins in the mountains over the west and south nappes. But the Picos de Europa Unit was still a marine area.
In the Permian and into the Mesozoic there was extension tectonics. The Permian Autunian succession - Viñon Formation formed when basins were created by downwards normal faulting. It is mostly limestone with layers of conglomerate, shale, gypsum and alkaline volcanics. The Villaviciosa Formation from the Saxonian formed on an arid continent with sandstone and conglomerate. Conditions in the Triassic were very arid, and lagoons evaporated depositing gypsum and marl. During the Jurassic and Cretaceous Periods the zone was underwater, but most deposits from these time have been eroded away.
Another way to look at the zone is by its structure: It consists of several thrust units: Somiedo-Correcilla, Sobia-Bodón, Aramo, (the first to move in Early Westphalian) Central Coal Basin, Ponga (second to move) and Picos de Europa (last to move in Early Stephanian) and Pisuerga-Carrión Unit (or Palantine) (which went nowhere).
In the Late Stephanian the zone was bent around a vertical axis to make the current crescent shape. This kind of bending is called an orocline.
Two theories explain the Permian basin formation due to crustal extension, lithosphere delamination as solid mantle sinks from the bottom of the lithosphere, being replaced by hot asthenosphere; or a continental rift.
Read more about this topic: Geology Of The Iberian Peninsula
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