Geology of the Pyrenees

At their far eastern end in Provence, typical Pyrenean fold trends are superimposed by Alpine structures to be finally cut off by the arc of the Western Alps.

The Campanian and Maastrichtian flysches comprise 2000 to 3000 m of periodically interlayered fines (marls, calcareous shales, and mudstones) and coarser sediments (conglomerates, sandstones, and greywackes).

Within the northern foreland east of the river Aude, the Paleozoic basement uplift of the Mouthoumet appears, a horst tilted to the south and covered by continental Eocene strata.

From the beginning of the Jurassic till the end of the Lower Cretaceous, a shallow-water carbonate platform developed during tectonic quiescence with mainly limestones being sedimented.

This changeover marks the inception of the North Pyrenean Basin, a 400 km long trough of pull-apart origin filled with unconformable, turbiditic flysch sediments during the Upper Cretaceous.

Altogether the Coniacian-Maastrichtian series reaches a thickness of 3000 m. The Paleozoic basement pierces the sedimentary cover in several almond-shaped, horst-like uplifts, their size ranging from 1 to 300 km2.

They are intercalated in sediments of the Lias and the Upper Cretaceous (Aptian till Campanian) and are found mainly in the west (near Tarbes, Orthez, and in the Basque country).

In the eastern part of the axial zone, the fractures are generally upright, a good example being the mylonitic Merens Fault at Pic del Port Vell near Mérens-les-Vals.

In this thrust sheet west of the Gállego River, the structures simplify: in the Basque and in the Cantabrian Mountains, the sedimentary cover is affected only by long and relatively open fold trains, which are occasionally pierced by doming Keuper salt.

During Ediacaran to Ordovician times, Pyrenees were located at the Northwest margin of Gondwana, where they formed a lateral continuity of neighbouring areas, such as the Montagne Noire and the Mouthoumet massifs and Southwestern territory of Sardinia.

The included volcanic rocks and the conglomerates hint at unsettled tectonic conditions, which are probably connected with an early stage of the Caledonian orogeny (Taconian Phase).

Another important consequence of the orogeny was late-orogenic magmatism emplacing granitoids (granodiorites and biotite granites) of mainly acid but occasionally also of basic composition.

[7] Detrital formations of lacustrine affinity with coal measures during the Stephanian (Kasimovian and Gzhelian) followed by red sandstones with plant remains during the Permian are typical erosional products of a chain not having reached stability.

The Grey Unit of the Kasimovian is a sequence of decreasing grain-size, starting with breccias and conglomerates and changing into sandstones and coal-bearing shales (anthracite is mined near Campo de la Troya).

It reaches 400 to 500 m in thickness and is made up of coarse conglomerates, sandstones, psammites with plant remains (Equisetites, Coniferomyelon) as well as green and red to purple claystones.

At the limit, Upper Triassic/Hettangian doleritic tholeiites (ophites) formed in the Pyrenees and in the southern Aquitaine Basin, indicating further movements along the fracture zones (submarine fissure eruptions and sills in unsolidified Keuper sediments).

The basin was under tension during this period and as a result long horsts and graben structures of different subsidence rates were created following more or less the trend of the Variscan fractures.

Its total thickness varies between 150 and 400 m. The sea level kept rising during the Hettangian and fossiliferous limestones were deposited; this trend reversed later on into a regression leaving evaporites (rock salt and anhydrite with some calcareous interlayers).

During the Upper Lias (Toarcian), the sea reached a high stand, continuing with the fine-grained detrital sedimentation and depositing black pelagic marls (marnes noires and schistes esquilleux).

Iberia started to rift off the Armorican Massif in a southerly direction and in its wake the Bay of Biscay slowly began to spread (with formation of oceanic crust from the Middle Albian till the end of the Coniacian).

The 100 to 150 m thick Upper Oxfordian is represented west of the oolite barrier by intratidal platform sediments (argillaceous to sandy, pyrite-bearing limestones), whereas, in the east, dolomitization continues.

After a southeasterly re-advance of the sea in the Berriasian via a small strait east of Pau, which deposited 100 m of inter– to sub–tidal limestones and a sandy to clayey detrital border facies, emersion set in during the Neocomian.

During Valanginian and Hauterivian times, clayey marls on top of the emerged horsts were transformed under ferralitic climatic conditions into bauxites, which were fossilised by later transgressions.

On the southern side of the Pyrenees in Catalonia, folded conglomeratic formations have been dated as Upper Lutetian to Bartonian, representing the interval 44 to 37 million years ago.

At the northern margin of the Ebro Basin close to the Sierras Marginales, for example, folded Oligocene is covered unconformably by flat-lying, detrital Miocene of continental origin.

Several peneplanation levels have been found on very different heights (3000 to 2000 m in the axial zone, close to a 1000 m in the Pays de Sault, near 400 m in the Agly massif and at 100 m in the Corbières).

Due to the rotational motion, the northeastern edge of Iberia started to interfere with Aquitania, first creating transtensional pull-aparts along the North Pyrenean Zone in the Middle Albian.

In Ilerdian and Cuisian times (Paleocene/Eocene boundary, Thanetian/Ypresian, about 55 million years ago), the Pyrenees underwent very strong compression in the upper crust, bringing about the orogen's actual zonation and structural organisation.

This Pyrenean main phase lasted till about 47 million years ago (beginning of the Lutetian), showing high shortening rates of 4.0 to 4.4 mm/year and emplacing for example the Lower Pedraforca and the Montsec thrust sheets.

Since the Neogene, the orogen exhibits post-kinematic collapse (graben structures at its eastern end, volcanism near Olot) associated with the extension of the Golfe de Lion and the opening of the Valencia Trough.