Geology of France

(in construction)

The France is geologically very diverse. Its structure, rather complex, is due to the proximity of the French territory and Europe in general with the limit of the plates tectonics since at least 600 million years. The oldest rocks found in France go back to it-there-have 2 billion years. One can restore his geological history only starting from 640 million years, when the petrologic data make it possible to reconstitute the climatic conditions, physics and Tectonique S of the French territory, and, more generally, European. One can nevertheless distinguish, coarsely, three fields: the field Hercynien, the field of the basins céno - Mesozoic S and the alpine field. In all the cases, these great divisions are only diagrammatic and do not reflect completely, far from, natures of the rocks that one finds there.

Great geological fields

The paleozoic field

The Armorique

One can distinguish two areas in Armorique: the field cadomien in north and the field Hercynien in the south (more the Pays of Leon, which belongs geologically to the south of Brittany).

The cadomien field

They are the scraps of the old chain Cadomienne, consisted of plutons granitic dating from the Protérozoïque (Perros-Guirec, bay of Saint-Brieuc, area of Sartilly, Mayenne) accompanied by sediments for more the share resulting from this chain, therefore detrital, them also Précambrien S (Southern of the Cotentin, west of the Calvados, north of the Mayenne, area of the Rance), to which Sédiment S Paléozoique S were added, in continuity with the former deposits (center of the Brittany, of Douarnenez to Angers while passing by Rennes), or without continuity, and which was slightly folded during orogenesis hercynienne. The unit was injected, with the Carbonifère, of granitic Pluto S hercyniens (areas of Bourbriac, Plœuc-on-Dependant and Dinan), and cut out by a fault of the same time, running of the Rade of Brest to the the Sarthe (north-Armorican crushed zone). In north, one finds some plutons gneiss going back to 2200 my at 1800 my (orogenesis icartienne), the oldest rocks of France. One finds them in small outcrops around bay of Lannion, with the Cape of La Hague, in the Channel Islands, more particularly with Guernesey. One however did not find rocks of the period going of 1800 my to 620 my, therefore more than one billion years, is the younger rock icartienne with oldest known rock cadomienne.

The Armorican field hercynien

The south-Armorican grounds mainly consist of granites of origin hercynienne, injected along a series of faults running of the Pointe of the Strong currents to the the Loire, named south-Armorican crushed zone. Some sedimentary zones cambriennes, ordovicians or siluriennes (Belle-Ile-en-mer, west coast of the the Vendée, area in the south of Angers, Large Brière), folded or metamophized during orogenesis hercynienne, form the paleozoic sedimentary rocks of the area.

The Massif Central

Of complex composition, it is composed of a Marqueterie of rocks.

The the Vosges

The Corsica

Corsica hercynienne

The Black Mountain

The the Ardennes

Composed of rocks Paleozoic Sediment surfaces S, of the Ardennes solid mass at summer slightly affected by orogenesis Hercynien. One finds rocks Cambrien there born from with dimensions from Sedan, but the majority is of origin Dévonien.

The field of the basins céno - Mesozoic S

The Paris basin

The Paris basin is a vast basin Sédiment surface with the rocks of marine origin , Lac ustre and lagunaires, then fluviatile, accumulated, in the center of the basin (surroundings of Castle-Thierry), on 3000 meters of depth on a base hercynien, which is little compared to other sedimentary basins. This basin is delimited by old solid masses hercyniens (the Ardennes, Hunsrück, the Vosges, Morvan, Massif Central and Armorican Massif). It communicates with the Aquitanian basin by the Seuil of Poitou, with the valley of the Saone by the threshold of Burgundy and with the germano-Polish Plaine by the plain of Flanders. In a diagrammatic way, one can compare the basin with a series of the concentric aureoles, youngest in the center and oldest with the periphery, in a configuration similar to a pile of the plates, smallest encased in largest. These sedimentary aureoles are delimited from/to each other by slopes, the " Cuesta s", (Coasts of the Bar, Coast of Champagne,…)(the section of the plates).

The Aquitanian Basin

The basin of South-east

The alpine field

The the Alps and the the Jura

The the Pyrenees

Alpine Corsica

The Graben of the the Rhine

The Rhone-native Furrow

The grabens Languedoc iens and provençaux

Geological history

The Protérozoïque

The geologists do not know large thing of French geology to the Protérozoïque, that had with the obvious difficulties encountered during the identification of the origin of rocks largely metamorphized during later times. They have nevertheless could identify rocks dating from paléoprotéozoïque, known as rocks icartiennes, which one finds with Guernesey (Pointe of Icart), in bay of Lannion, in the Coast-with Armor and with the point of the Cotentin. It is about Gneiss, of volcanic origin. Nothing the situation is known in which they were created, share has that they is of volcanic origin. It-there-have then a " trou" in the geological history of France, of more than one billion years, that is to say between -1800 million years and -640 million years, period of which we did not find in France any rock.

In Néoprotérozoïque however, the trace returns. France is located in the Southern hemisphere, and is at the edge, or on the margin, of the Supercontinent Gondwana. At that time, towards 640 million years it seems that a Subduction starts on this margin, and that thus the Ocean floor is subducté under current France.

One can distinguish at that time, divided France. Indeed, the north of France (roughly Picardy, North, and the Ardennes) is certainly located on the same continental terrace but a few thousands of kilometers (this measurement is random) at the west of current Brittany. This northern part is attached to England, the Netherlands and the north of Germany, which one will call later Avalonia.

The ocean floor subducte thus under Brittany, and probably (because we let us not have the proof of it) under the Ile-de-France, then further in Germany and Tchéquie. This subduction creates what one calls the chain cadomienne, of the mountains which one sees the traces everywhere in the north of Brittany and Cotentin. It is initially about a insular Arc followed of a phase of tectonisation during which the former sediments are folded. Orogenesis ends towards 540 million years and the reliefs erode gradually.

The Cambrien

The Ordovicien

The Silurien

The Dévonien

The Carboniferous

Silesian

With Silesian Gondwana and Laurasia complete their joining. The ocean rheic is entirely closed.

Dinantien

The collision which forms the continuous Pangée. France is entirely emerged, and at its place the high chain of the mountains hercyniennes is located. Crossing all Europe of west in is, the Chaîne hercynienne is at its stage more raised, in term of altitude. It rises probably as high, to see more, as the the current Himalayas, of the tops bordering the 10.000 meters not being to exclude.

Climate in Europe equatorial east . To the north of the chain the Coal basin north-European extends. The Flanders, Artois, Picardy and the Ardennes are located in this basin, and are leant with the imposing reliefs which limit them to the South. This basin is made up lowlands, marshes, covered episodically by the sea. These zones are covered with vast forests of prémitives giant Fougères, of Prêle S, Lépidodendron, and which populate of the giant dragonflies, the invertebrates and the Amphibiens. They are these forests, covered by marine water overflowing periodically and covering them with sediments, which will form the carboniferous layers of Flanders, with Lorraine, England and Germany.

The mountains hercyniennes are very strongly eroded by abundant precipitations which imply an equatorial climate. The most tops are most probably covered with snow and of glaciers, while the valleys created by the faults which start to shear the solid masses are they also invaded by an exuberant vegetation. The basin asturien knows similar conditions with the basins of the north of the chain.

The Permian

The Sorted

The Jurassic

The Cretaceous

The the Paleogene

The Neocene

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