The geology of the Iberian Peninsula consists of the study of the rock formations on the Iberian Peninsula, connected to the rest of the European landmass by the Pyrenees. The peninsula contains rocks from every geological period from the Ediacaran to the Quaternary, and many types of rock are represented. World-class mineral deposits are also found there.
The core of the Iberian Peninsula consists of a Hercynian cratonic block known as the Iberian Massif. On the northeast, this is bounded by the Pyrenean fold belt, and on the southeast, it is bounded by the Betic fold mountain chain. These two mountain chains are part of the Alpine belt. To the west, the peninsula is delimited by the continental boundary formed by the opening of the Atlantic Ocean. The Hercynian fold belt is mostly buried by Mesozoic and Cenozoic cover rocks to the east but nevertheless outcrops through the Iberian Chain and the Catalan Coastal Ranges.
Iberian Massif
The Iberian Massif consists of rocks from the Paleozoic Era. It was assembled about 310 Ma. Several zones occur in the Iberian Massif. These were the pieces that were assembled to form the block. On the north coast of Spain occurs the Cantabrian zone. Then to the west and also in the Iberian Chain and Catalan Coastal Ranges is the West Asturian-Leonese zone. Then the Central Iberian zone appears near A Coruña, through the north of Portugal, and through the middle of Spain, including the Montes de Toledo. The Ossa-Morena zone outcrops out to the east of Lisbon. This includes some Precambrian rocks. The furthest south part is the South-Portuguese zone.
The Variscan Orogeny occurred as the European Hunic Terrane (split off from Gondwana) and Laurentia-Baltica continents collided. In Iberia this occurred in the pre-Stephanian Carboniferous (354–305 Ma). The external part of the orogeny was the Cantabrian zone. This was deformed in the upper crustal layers. The West Asturian Leonese zone and Central Iberian zone are the external parts of the orogeny and are more deeply deformed and metamorphosed, and intruded. These three zones are part of one terrane. The Ossa-Morena zone and South Portuguese zone are two different terranes that have become attached. In the Mesozoic this was mostly covered with other sediments, which have since eroded.
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 Villabandin 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 and dolostone. 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 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 thick and Gedinian to Emsian in age.
The Santa Lucia Formation is of limestone. It contains coral near the Narcea 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 (Variscan) 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 built 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 Cantabrian 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 (Variscan) 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.
West Asturian Leonese zone
The West Asturian Leonese zone lies to the west and south-west of the Precambrian rocks of the Narcea antiform, and extend east to the Precambrian rocks of the Ollo de Sapo antiform. Rocks in this zone are mostly from Cambrian and Ordovician, with few from Silurian to Carboniferous. The Cambrian and Ordovician rocks were formed in shallow water in a subsiding trough. Later deposits were formed in deeper water. They have been metamorphosed to greenschist or low grade amphibolite. Also they mostly have a slaty cleavage. Folds face the centre of the arc. In the west the folds are recumbent and large: Mondoñedo and Courel folds. In the east the folds are asymmetrical. The base of the Mondoñedo fold is an overthrust with the same name. Another overthrust forms the edge of this zone where it meets the Narcean Antiform. Crenulation cleavage occurs near these overthrusts. All these structures formed between Lower Devonian and Stephanian B-C.
From the Cambrian Period, the Candana Quartzite is equivalent to the Herreria Formation and is 1 to 2 km thick. The Vegadeo Limestone is equivalent to the Lancara Formation and is 0.1 to 0.2 km thick. The Cabos Series is equivalent to the Oville and Barrios Formations and is 4 km thick.
Black shales, called the Luarca Slates are Llanvirnian to Llandeilian age (middle to upper Ordovician) and are 0.5 to 1 km thick. The Agüeira Formation consists of turbidites of Caradocian age and is 3 km thick. Following a disconformity the Silurian black slates are laid down 0.4 km thick.
There are few outcrops of lower Devonian rocks in the San Clodio area. And in the Carboniferous Period, this was an erosional zone forming the source of material for the Cantabrian Carboniferous deposits.
Central Iberian zone
The Central Iberian zone covers the middle part of the west side of the peninsula, including north and central Portugal. The top north west corner has been replaced with the Galicia-Tras-Os-Montes zone. The constituent rocks are metamorphosed sediments.
The oldest rocks are Proterozoic, metamorphosed sediments. They have been deformed by the Cadomian Orogeny. There are volcanics and further sediments from the end of the Ediacaran and Cambrian periods.
Before the Carboniferous this was deformed in a north east direction with thrusts and folds.
The oldest rocks are from the Cambrian, possibly the Precambrian, and are orthogneiss and paragneiss. These are found near Foz do Douro, and Miranda do Douro. Above this are schists or shales with beds of turbidites or limestone interleaved. The stratigraphic sequence can be observed south west of Salamanca in the Tamames Syncline, and in the Montes de Toledo. These are followed by an unconformity. Above the unconformity can be found reddish sandstone, shale and conglomerate of Tremadocian age up to 1 km thick. An Arenigian age quartzite formation is equivalent to the Amorican Quartzite. Then there is black shale or slate matching the Luarca Slate of Llanvirn to Llandeilo Age. On top of this is the Botella or Cantera Quartzite, 0.1 km thick of Llandeilian to Caradocian Age.
Above this is a lenticular limestone called Urbana Limestone and shale and sandstone of Caradocian to Asghilian Age. Then comes the Criadero Quartzite in the Almaden area at the base of the Silurian Period. Black graptolytic shale and basic volcanic rocks overlay this.
Granite appeared with the Variscan Orogeny.