Aquitaine basin

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The Aquitaine Basin is the second largest Mesozoic and Cenozoic sedimentary basin in France after the Paris Basin . The approximately 66,000 square kilometer basin lies above the Variscan basement , which was eroded during the Permian and began to sink gradually since the Triassic . In the Parentis Basin and the Sub-Pyrenees Basin , the basement is hidden at its deepest point under a sediment load of 11,000 meters.

Geographical breakdown

The geological provinces of France, the Aquitaine Basin at the bottom left

The Aquitaine Basin, named after the Aquitaine region of the same name , has roughly the shape of a funnel, the opening of which to the west represents the straight, approximately 330 kilometers long, north-south-running Atlantic coast . Its southern limit is the east-south-east-west-north-west trending Pyrenees for around 350 kilometers . In the southeast, the basin in the Détroit de Carcassonne extends between Montagne Noire in the north and Mouthoumet in the south to just before Narbonne , where it is crossed by Pyrenees. The north-east adjoining arched frame is formed by the Massif Central . In the threshold region of the maximum 100 kilometers wide Seuil du Poitou in the north there is a connection to the adjacent Paris Basin. In the extreme north, the basin is bounded for around 110 kilometers in an east-west direction by the Vendée , the southernmost branch of the Armorican massif .

Pelvic structure

The Aquitaine Basin is a very asymmetrical foreland basin . Its deepest point at 11,000 meters is in the immediate vicinity of the North Pyrenees crossing .

The 2000 meter isobath roughly follows the course of the Garonne and divides the basin into a shallow plateau area in the north (the so-called Aquitaine Plateau ) and a deep, narrowly folded area in the south. The tabular plateau in the north has only a strongly reduced sediment sequence with long-wave folds and occasional disturbances . The folded area in the south has shown subsidence since the Lower Triassic, the degree of deformation increases more and more towards the crossing of the North Pyrenees, and there is also a strong salt tectonics ( diapirism ).

The Parentis Basin on the edge of the Atlantic represents a deviation from this somewhat simplified scheme . The Parentis Basin also reaches the proud depth of 11,000 meters. It forms a symmetrical depression that runs from the Golfe de Gascogne towards Arcachon and is already partially underlain by 100 to 95 million year old ( Cenomanium ) oceanic crust seaward . It is possible that this is a pull-apart basin in a long-term transform disorder with dextral shear components that spans the continent .

Sedimentary development

The information given on the sediment sequence and its thickness is based on more than 70 exploratory boreholes, which occasionally only encountered the basement at a depth of over 6000 meters.

The sedimentary development begins in the Aquitaine Basin in the Lower Triassic in the immediate vicinity of the North Pyrenees crossing (Note: Permotriassian Basins such as the Brive Basin and the Grésigne Basin are still included in the basement). From here, the depression slowly spread north.

Triad

Sedimentation begins in the Lower Triassic ( red sandstone ) with colorful sandstones and claystones , followed by dolomitic limestones , salt layers and colorful claystones in the Middle Triassic ( shell limestone ) . In the Upper Triassic ( Keuper ) the salt deposition continues, ending with ophitic lavas ( dolerites and tholeiites , only in the south). The salt layers were later activated as diapirs during the Pyrenees orogeny , the colored clay stones served as shear horizons, on which Triassic sediments were later pressed up to the Arcachon - Toulouse line .

The triass sediments are characterized by their Germanotypic character. On the Aquitaine Plateau they are of continental origin, only the Keuper remains. In the south, however, they take on marine character and show their full training. The triastransgression took place from the south or south-east from the Tethys area across the Pyrenees, which did not exist at the time. The sediments suggest a restrictive marine deposit environment and shallow water area that occasionally fell dry; this also explains the strong presence of evaporites . The sediments can be up to 1000 meters thick. In the north they extend roughly as far as the line between Garonne and Brive.

law

In terms of sequence stratigraphy, the marine Jura cycle can be divided into seven second-order sequences, three in the Lias, two in the Dogger and two in the Malm:

Hettangic transgessional anesthesia from Nontron
  • Hettang-Sinemurian sequence.
  • Lotharingian-Carixian-Domeric sequence (Upper Sinemurian-Pliensbachian).
  • Toarcian-Eel sequence.
  • Bajoc subbathon sequence.
  • Middle Bathon-Callovic sequence.
  • Oxfordian-Sequanic Sequence (Oxfordian-Lower Kimmeridgian).
  • Kimmeridgian-Portland sequence (Upper Kimmeridgian-Tithonian).

The sequences are separated from one another by discordances. The Jura succession is only completely preserved in the Quercy, in the south (Sub-Pyrenees Basin) it is partly very sketchy.

The basal hettangisch -sinemurische sequence has transgressive character and for the first time come now in the Aquitaine basin full navy, but generally relatively poor fossil sediments. From the Sinemurian onwards - characterized by calcareous-dolomitic, partly oolithic sedimentation - this Lias transgression gradually spreads to the whole of Aquitaine, and then despite minor regressions in the Pliensbachian towards the end of the Lias and in the Dogger, the basement of the western Massif Central, sometimes with more than 30 Kilometers to overlap - the same applies to the western Vendée. Up to the line La Rochelle - Angoulême - Périgueux - Figeac , an inner shelf is being built in the northern section. Here, the generally detritic transgression sediments of the basal hettangium usually consist of a basic conglomerate , arkoses and relatively thick-banked sandstones and claystones rich in plant material. Restrictive marine sediments of a lagoon- lacustric facies follow in the rest of the Hettangium (green claystones, colored marls, dolomitic limestone and plate limestone rich in dwarf faunas and evaporitic layers). The sediments of the Sinemurian have a fully marine character, they contain a pelagic fauna (soft band limestone and hard lithographic limestone). At the end of the Sinemurium there is a sudden regression with the formation of hard grounds.

The second Liasse sequence also has a marine-transgressive character and begins in the Lotharingium / Lower Carixian - well dated by a rich ammonite fauna ( Arietites , Oxynoticeras , Deroceras and Uptonia jamesoni ). The sediments are calcareous, rich in quartz grains and boulders of reclaimed Sinemurium. The Upper Carixium consists of very fossil-rich ( Aegoceras capricornu ) marl limestone banks with intermediate gray marl layers. This is followed by ammonite-bearing ( Amaltheus margaritatus ) and oyster-bearing ( Gryphaea cymbium ) marls, which reveal a shelf open to the Atlantic Ocean. In the lower domerium there is a sea connection to the Paris Basin for the first time via the Seuil du Poitou and to the Jurassic Sea in southeast France via the Détroit de Rodez and the Détroit de Carcassonne. In the course of the Upper Domerium there is a renewed regression with sandy limestone of the littoral, which are very fossil-rich ( Pleuroceras spinatum , Pecten aequivalvis ) and can occur in peripheral zones as iron-rich oolites. Hard grounds form again at the end of the sequence.

The third and last Liasse sequence in the Lower Toarcium changes immediately to black ammonite-bearing ( Harpoceras falciferum and Hildoceras bifrons ) marls without detritic deposits . Towards the end of the Toarcian / beginning of the Aalenian , the sediments with sandy limestone take on a regressive character. The limestones contain oyster beds, ironoolite and gypsum layers and usually end with an erosion discordance. When it comes to fossils, they have Pleydellia aalensis and Leioceras opalinum .

In the southern section of the Aquitaine Basin, salt deposition (with layers of anhydrite ) continues, which in the Lias can be up to 500 meters thick.

The Dogger reaches its maximum thickness of 300 meters along a north-south running line Angoulême - Tarbes . Reef complexes build up along this line, dividing the Aquitaine Basin in two. The reefs (east of Angoulême, northwest of Périgueux, east of Pau) are associated with Kalkoolithen, which indicate a high-energy area. On the flat shelf to the east of the reefs, neritic limestone and dolomite (in the south) are deposited and in the Quercy even supratidal lignite-bearing limestones. In the area open to the Atlantic, pelagic calcareous marl containing ammonites are deposited, which are very rich in microfossil thread formers ( bryozoa ).

The first Great Dane sequence begins east of the reef / oolite bar in the Bajocium, dolomitic transgressive, the environment is restrictive. Aalenium is being reconditioned in places. The Bathonium is calcareous in the northeast and dolomitic in the southeast. Towards the end of the cycle in the Lower Bathonium there is a regressive tendency (lignites, breccias, lacustric fossils in the Quercy). In the Pyrenees area, a long-lasting layer gap forms. Up to the Kimmeridgian there are no more ammonites in the eastern section - this of course makes exact dating difficult.

The second Dogger sequence begins in the Middle Bathonium with lacustric limestone and breccia-containing rubble. This is followed by mainly neritic limestone settled in the calm shallow water (in the south still dolomites). The end of the callovium is formed by littoral marginal facies.

The major facies boundary of the reef bar initially remains in the Malm . In the deeper west area ammonite-bearing marls and limes are deposited, in the east area, however, calcareous dolomites . The retreat of the Jurassic Sea is noticeable in the outgoing Tithonium , for example dolomites and breccias appear in the Adour Basin , evaporites in the Charente , extremely littoral sediments in the Quercy , lacustrine limestone in the Parentis Basin and anhydrite in the Gers department . The straits close again and one last reef is still in the Périgord (near La Tour-Blanche ). The sea finally retreats to the south behind the Garonne.

The first Malm sequence seems to follow the Callovian without interruption in the Lower Oxfordian. Cell limestone and breccia, however, indicate sediment rearrangement ( this is certain in the Malm der Grands Causses ). In the Middle and Upper Oxfordium, limestones are sedimented under open marine conditions, under which some reefs are embedded. The Lower Kimmeridgium is close to the beach again, indicated by oysters, sea ​​urchins and ripple marks .

The second Malm sequence begins in the Upper Kimmeridgian only occasionally regressively, but the sedimentary character changes. Brecciated facies, synsedimentary reprocessing and rhythmic alternation of limestone and clay with lignite horizons emerge. The ammonites Aulacostephanus and Aspidoceras orthocera appear on datable fossils . This very restless sedimentation with a juxtaposition of open marine facies and reducing silt sedimentation under restrictive conditions seems to correspond to a first sedimentary individualization of the Pyrenees. This phase bears the name Virgulien after the oyster Exogyra virgula . The narrowing of the basin becomes even clearer in the tithonium, and finally, before the onset of the Cretaceous, it changes into an almost complete sea retreat (the southern area is excluded from this). In the Tithonium, for example, iron-bearing calcoolites with marl layers, dolomites and marginal facial sediments are formed, dated by means of Gravesia portlandicum .

Lower Cretaceous

Compared to the Jura, the Cretaceous has less clear sequences, and the Lower Cretaceous is limited to sedimentation areas near the Pyrenees. The sea connection in the Lower Cretaceous was probably better towards the Mediterranean than to the Atlantic.

After a longer hiatus, sedimentation begins again in the Lower Cretaceous. The sedimentation remains limited to two areas - the Parentis Basin and the Adour Basin . Both sub-basins have an astonishing subsidence . For example, up to 2000 meters of sediment are deposited in the Parentis Basin in the Lower Cretaceous, and up to 4000 meters in the Adour Basin. The rest of the Aquitaine Basin continues to be subject to severe erosion during the same period.

The first deposits in the two sub-basins are littoral sediments in Wealden facies , mainly sandstones and claystones.

In the Barremium , marine shallow-water limestones are deposited, which are detached by detritic deposits in the northern Parentis Basin. At Lacq , the limestone even turns into lagoon facies (anhydrite).

In the Upper Aptium , the partly reef-forming Urgon facies - fossil limestone with algae , coral polyps and rudists - establish themselves in the two sub-basins . The Urgon Facies completely girdle the Parentis Basin; it persists into the Albium.

From the Albium onwards , strong salt tectonic movements take place in the southern Aquitaine Basin, which in turn exert a great influence on sedimentation - breccias , powerful conglomerates and turbidites develop . As a result, a remarkable discordance even forms in the Parentis Basin . In the northern Aquitaine Basin, on the other hand, the sediments lay in long-wave folds in the Hercynian direction at the same time. All of these events are related to the first tectonic movements in the western Pyrenees. The sedimentation in the outgoing Albium under rising sea levels is clayey, the limestone reliefs of the Urgon facies are completely covered by it.

Upper Chalk

The transgression, which already began in the outgoing Albium, spreads very quickly northwards in the Cenomanium . In the north, the sedimentation area ultimately assumes roughly the same dimensions as it did in the Jura. In an easterly direction, however, the sea only penetrates until shortly before Brive, Cahors , Agen, Muret and Carcassonne. At this point in time, the North Pyrenees crossing forms the facies boundary between shelf sedimentation in the north and sinking troughs in the south, which absorb flysch sediments (sometimes brecciated wild flysch ) from the Pyrenees region. At Saint-Gaudens , the flysch sediments are even accompanied by volcanic rocks - from trachytes to ultra-basic lavas. In Flyschbereich sedimentation during is Turoniums and Coniaciums very restless. The rest of the Upper Cretaceous also occurs in flysch sedimentation - sandstones and claystones in alternating layers with isolated layers of carbonate. Towards the end of the Upper Cretaceous, there are increasing signs of regression, which usually took place before the Cretaceous / Tertiary transition. In the Sub-Pyrenees Trench in the Petits Pyrénées, the sea lasts even into the lowest Paleocene ( Danium ).

In the rest of the Aquitaine Basin, predominantly pelagic limestones are excreted during this period (for example, the type localities for the Coniacium, the Santonium and the Campanium are in the Charente).

More differentiated facies occur on the northern edge due to the coastal location. The Cenomanium in the north has three sedimentation cycles:

  • In the upper cycle a slight regression with sandy rudist limestone and oyster marl in the northwest and very shallow marine, gypsum-bearing marls and sands in the northeast.
  • In the middle cycle, which extends into the Quercy , generally deeper marine marls (with the exception of isolated paleo reliefs in the Périgord with littoral facies and lignites).
  • In the lower cycle, shallow water facies with rudist reefs in the northwest and continental influence with lignites in the northeast.

The turonium has a marine transgressive character and spreads to the Lot . It can generally be divided into two parts:

  • In an angoumia consisting of massive, brecciated rudist limestone followed by limestone sand . The Angoumien sometimes forms quite distinctive terrain levels.
  • In a chalky , marly ligérien at the base.

Towards the end of the Turonian there is a rise in the Massif Central, which is reflected in the sediment as extensive sand heaps.

In the northern Aquitaine Basin, coniacium and santonium are generally made up of limestone, but east of Périgueux these are increasingly sandy in character.

The Campanium follows with a pronounced discordance. The Flyschtrog is now expanding to the north. In the vicinity of Pau , the entire Lower Cretaceous and the entire Jura were eroded away before the start of flysch sedimentation, and in places the erosion even reached down to the basement. North of Pau, the Campanium occurs in marl facies, the so-called aturia . In the northern Aquitaine Basin, on the other hand, the sediments become uniform and fully marine chert- bearing lime micrites are deposited.

In Maastrichtian regression begins. After the initial deposition of bioclastic rudist limestone and isolated reefs from rudists and individual corals, the sea level drops significantly and finally emersion occurs. The sea gradually retreats behind the Arcachon-Toulouse line. At the same time, isolated flat lines of folds with a Hercynian stroke direction appear on the northern edge.

Cenozoic

During the Paleocene , the coast roughly follows the Arcachon-Toulouse line. North of this line (in the North Aquitanian Zone ) the sedimentation is continental - red claystones, sands and lacustrine limestones. A short sea advance takes place, leaving Echinid limestone behind. In the southern basin area, a flat shelf forms up to the line Audignon- Carcassonne ( Central Aquitaine Zone ). The South Aquitanian Zone , further to the south, is deep water, but it flattens out towards the east. In the Gulf of Aturia in the west, pelagic limestones are sedimented which contain globigerines , operculins and alveolins . In the vicinity of the Petits Pyrénées the limestones are already shallow water facials, they lead here madreporia , echinids and operculines. On the Ariège and in the Corbières in the east, the sediments finally take on an entirely continental and lacustrine character.

In the Lower Eocene ( Ypresian ) a new transgression takes place, the sea advances into the Medoc and to the south of Oléron , in the southeast it reaches the Montagne Noire. Globorotalia- bearing marls are now being deposited in the Gulf of Aturia , further east turitellen-rich marls and limes. In the newly flooded area, sands and limestone sediment rich in alveolines and nummulites . In the continental northeast, iron-rich sands (in the Charente) and molasses (in the Libournais and Agenais ) are deposited at the same time . The continental sediments come mainly from the Massif Central up to the middle Ypresian.

In the Middle Eocene ( Lutetium and Bartonium ) the rise in sea levels continues. The alveoline and nummulite limestones continue to spread, north to Blaye and Saint-Palais and east to Agenais. The North Pyrenees Trench deepens, at the same time conglomerates are poured into its eastern part, the Poudingues de Palassou . This marks the beginning of the uplift of the Pyrenees and the dominance of detritic sediment input from the south. The coalescing fans of erosion rubble extend north into the Castrais . To the north of the rubble fan there are lakes in which lacustrine limescale is deposited. The detritic sediments originating from the meanwhile heavily leveled Central Massif (clays, sands, gravel) are now limited to a narrow edge zone on the northeastern edge. In the Périgord and the Quercy, the Sidérolithique spreads at this point - iron-rich sediments that emerged from laterite-like deposits.

A sea retreat occurs during the Upper Eocene ( Priabonian ). The North Pyrenees Trench is now completely backfilled with the rubble of the Pyrenees chain. Nummulite limestone and marl can still be found in the Medoc, but to the east of Bordeaux , continental molasses are already appearing, which change into gypsum-bearing formations south of the Gironde.

A permanent marine environment remains during the Lower Oligocene ( Rupelian ) in the south with nummuliten-, lamellibranchien- and echinidenführenden marls and sands persist. In contrast, the anomiid limestone of the southern Medoc is lagoon. After a short-lived sea advance at the beginning of the Chattian (with starfish-bearing limestone in the northern Medoc and in the Libournais; with mammal- bearing molasses in the Agenais), the sea retreated very far towards the end of the Oligocene. This general retreat is accompanied by tectonic movements that create anticlinical ridges in the north and in the center. The rubble fans from the Pyrenees now reach into the Albigeois and thus achieve their greatest extent. They push the lakes that surround them, whose lacustrine limestone sedimentation then spreads over the Quercy and encompasses the Causses and even parts of the Massif Central.

In the Lower Miocene ( Aquitanium ) the sea transgresses again, starting from its retreat in the south-western Landes department , to the north and east. Marine, littoral and lacustrine facies alternate with one another. During a slight temporary retreat, a huge lake emerges at Condom , the Lac de Saucats , in which a gray lacustrine lime, the Calcaire gris de l'Agenais, is deposited. After that, the sea reaches its peak. It is framed by continental sediments, the thickness of which increases towards the southeast. The Pyrenees rubble fans show for the first time a regressive tendency due to increased subsidence in the immediate vicinity of the orogen, but they still extend into the Agenais.

In the Middle Miocene ( Langhian and Serravallian ) the rubble fans retreat further. The lacustrine zone consequently extends south into Armagnac .

The Upper Miocene ( Tortonian and Messinian ) is characterized by a drastic sea retreat to the west. The sea retreats first from the northeastern Bordelais and from the Bazadais , to finally leave most of the basin. In the Armagnac areas abandoned by the sea, fossil-free sands and clays sediment. In the dry areas in the north and east, the river network that drains from the Massif Central is already establishing itself today.

In the Pliocene ( Zancleum ) the sea is limited to a narrow strip near the Arcachon Basin south of Soustons . It leaves sandy marls that are very rich in benthic microfauna. In the rest of the Aquitaine Basin, continental sands, the Sables fauves, are deposited . The rubble fans from the Pyrenees now only sediment in the immediate vicinity of the mountain foot (rubble fans from Ger , Orignac - Cieutat , Lannemezan ). The drainage network of the Garonne with its tributaries is already taking on the shape that still exists today - the Garonne largely avoids the Miocene accumulations of rubble and then follows a slightly subsidiary ditch between Toulouse, Agen and Bordeaux.

The progressive silting of the Aquitaine Basin, starting from its northeastern edge, was accompanied by significant subaeric erosion, which carved out several levels of flattening in the detritic alluvial fans , including:

  • An Eocene leveled surface.
  • An Aquitaine leveled area that is mostly heavily silicified - clearly visible in the Agenais, Périgord and Quercy.
  • A Pliocene (Zancleum) leveling area, characterized by pebbly clays in the Bordelais and the Landes.

Today's drainage network was installed on the Pliocene leveled area.

quaternary

The Venus de Brassempouy from the Upper Paleolithic

The last three Pleistocene ice ages Mindel , Riss and Würm could also be detected in the Aquitaine Basin, mainly through the different river terraces in the drainage network. Other ice age phenomena can be cited:

  • Cave fillings. These are of great importance when dating archaeological finds.
  • Deposits of Aeolian origin. These cover more than a third of Aquitaine, mainly in the Médoc and the Landes. They were deposited during the last two stages of the Würm Ice Age. The dune belt along today's littoral comes from the Holocene .
  • Colluvium covering slopes and ridges.
  • Cryoclastic debris.

The history of the development of the Gironde estuary goes back around 20,000 years to the outgoing Würm.

Finally, the rich prehistoric finds and sites in the Aquitaine Basin should be mentioned, especially in the Dordogne department .

Structural structure and tectonics

Sub- Portland micrite from the La Tour Blanche anticline . Lateral shift in east-southeast-west-northwest direction with a calcite-filled pull-apart structure. The anticline was consequently also subject to distensive shear forces

Structurally, the Aquitaine Basin can be divided into two areas, which are separated from each other by a major disturbance . This is the North Aquitanian flexure that runs from Arcachon in the direction of Carcassonne. It represents the extension of the north Aquitaine continental slope and structurally divides the Aquitaine Basin into a northern province and a southern province.

The Northern Province or Aquitaine Plateau is a typical continental shelf with greatly reduced sedimentation and several phases of emergence (throughout the Lower Cretaceous, in phases in the Upper Cretaceous and in the Cenozoic). The basement is rarely deeper than 2000 meters. Triassic and Jurassic together reach a thickness of 1000 to 1700 meters. The lower chalk is missing, the upper chalk only reaches a few hundred meters. The paleogene is only very thin in the north, if present, but gains in thickness towards the south and is then overlaid by thin neogene .

In the eastern part, parallel to the edge of the Pyrenees, several large-scale structures extending from west-north-west-east-south-east can be seen:

  • Depression of the Quercy.
  • Tarn-et-Garonne highlands .
  • Castres Trench.
  • Toulouse plateau.

In general, the northern province is characterized by uncomplicated tectonic structures (regional depressions and high areas, long-wave folds, faults) that follow Hercynian, Armorican and Variscan strokes. These structures were created in the course of several phases:

  • Jurassic phase. The structures created are mostly of synsedimentary origin and follow variskish strike directions. During the later chalk sedimentation, they have a lasting effect on the facies and the transgression mode.
  • Final Campanian-Maastricht phase. This phase reinforces the structures already drawn up in the Jurassic. The following anticlinal ridges are created, which run more or less parallel to the north-eastern edge of the basin and can be followed for over 200 kilometers:
    • The Mareuil-Meyssac anticline . This structure is an asymmetrical anticline at Mareuil, but between Terrasson and Meyssac a fault with a significant jump height.
    • The Périgueux anticline. This structure runs from Cognac via La Tour Blanche to Périgueux and Saint-Cyprien; it is formed as a typical anticline at La Tour Blanche and Saint-Cyprien.
    • The Oléron- Jonzac - Ribérac - Sauveterre-la-Lémance anticline. This structure is a clear anticline in Jonzac and Sauveterre.

These anticlines are interrupted by the northwest-southeast trending synclinal depressions of Sarlat and Saintes .

  • Eocene-Oligocene phase. There are further anticlines, mostly on a lower floor and not recognizable on the surface:

The southern province is characterized by the deep sub-basins Parentis and Adour, as well as the threshold area in between by Mimizan . Compared to the northern province, the sediment thickness (5000 - 11,500 meters) increases considerably. Triassic and Jurassic reach 2000-3000 meters, the Lower Cretaceous 500-1500 meters. The Upper Cretaceous varies between 500 and 3000 meters, the Palaeogen between 1000 and 3000 meters, and even the Neogen can still show almost 1000 meters.

The tectonic movements in the southern province were of a much more complicated nature and are also superimposed by significant salt tectonics ( diapirism ). Unfortunately, a large part of the structures formed is covered by plio-quaternary detritus. Thanks to the numerous exploratory drillings for oil and aquifers, however, they are widely known. As in the northern province, these are mainly parallel anticline ridges, the wavelength of which decreases steadily towards the south. Conversely, the intensity of the halokinesis increases towards the south. The anticline ridges formed during the elevation of the Pyrenees in the Eocene / Oligocene. Their structuring was completed in the Miocene.

The following anticline trains can be seen (from north to south):

During the Plio Quaternary, tectonic compensatory movements on the north-eastern edge of the Aquitaine Basin lead to an elevation and rejuvenation of the clod of the Massif Central ; in the basin interior they mostly follow preliminary drawings in the basement and this causes some Pliocene leveling surfaces to be tilted. This, in turn, has a very large influence on the hydrographic network, resulting in river shifts and river tasks in the basins of the Garonne and the Adour .

The tectonic movements in the Aquitaine Basin are by no means complete even today, as strong earthquakes on the northern edge of the Pyrenees and medium-strength earthquakes on Oléron show.

Tectono-metamorphic zones of the basement

The Variscan basement, hidden under the sediments of the Aquitaine Basin, can be divided into several northwest-southeast-trending tectono-metamorphic zones (from north to south) using geophysical soundings:

  • Ligero-arvernian zone . It is bounded by the line Niort -Angoulême- Fumel - Montauban , which runs parallel to the dextral South Armorican shear zone in the north . Polymetamorphic core zone of the Variscan orogen.
  • South Armorican Zone . Its southern limit is the La Rochelle-Saintes- Chalais line . The zone ends at Bergerac . Devonian / Carboniferous crystal ceilings pushed southwards.
  • North Aquitanian Zone . Its southern border is identical to the southern Variscan thrust front or the northern Aquitan flexure. It follows the Arcachon-Agen-Toulouse line. In Pennsylvania, ceilings of the external zone thrust south.
  • Aquitaine Block , also known as the microcontinent of Aquitania . This zone extends to the North Pyrenees crossing and is to be equated with the southern province. Foreland block already belonging to Gondwana.

Moho lows

The maximum depth of the Mohorovičić discontinuity of 36 kilometers roughly follows the course of the Garonne. It flattens out in the direction of the Massif Central and only has around 30 kilometers here. The same applies to the approach to the Pyrenees, on the northern edge of the Pyrenees the Moho is also only 30 kilometers deep. In the oceanic part of the Parentis Basin, the Moho can already be found at a depth of 20 kilometers. This indicates an extreme thinning of the continental crust or its beginning oceanization. For comparison: under the Central Pyrenees the crust is 50 kilometers thick!

Geodynamic relationships

For a better understanding of the geological processes in the Aquitaine Basin, it is important to focus on larger geodynamic relationships. Two major developments are likely to be decisive here:

In the course of the Upper Triassic around 230 million years ago ( Carnian ), the supercontinent Pangea slowly began to break up. In the Atlantic area, this gradual rifting process began in the Central Atlantic area . Already during the Lower Jurassic, the initial continental rift valley breaks into their marine stage, the Central Atlantic began to spread (in the Toarcian , 180 million years ago) and North America , South America and Africa separated from each other. The Central Atlantic was already fully marine in the Callovium . However, the spreading process continued and gradually spread to the North Atlantic area. During the Tithonian 150 million years ago, in the course of the later opening of the North Atlantic, a rift arm penetrated along today's north-west French continental margin and began to push Iberia, which until then was still connected to the Armorican Massif, away in a southerly direction. The Atlantic was thus able to penetrate directly into the Aquitaine Basin for the first time. As a result, the Bay of Biscay opened up in the north of the retreating Iberia. In addition to its southern drift movement, Iberia carried out an additional anti-clockwise rotational movement throughout the Lower Cretaceous, which brought the north-eastern part of Iberia closer to southern France (first tectonic movements in the Pyrenees in Albium; Pyrenees metamorphosis 108 to 93 million years ago; cenomant transgression) and finally the Collision in the Eocene / Oligocene (elevation of the Pyrenees with simultaneous erosion) caused. The main phase of the elevation of the Pyrenees came to an end with the Aquitanium , which was essentially followed by isostatic compensatory movements, which continue to this day.

Mega-sequences

The geodynamic development of the Aquitaine Basin can be summarized somewhat simplistically in four megasequences, the starting point being the beginning of the rifting in the Biscay in the outgoing Jura (Tithonian):

  • Prairie mega-sequence. Triassic to Upper Jura. Mainly clastic sediments and carbonates, followed by mighty evaporites in the Triassic; Shelf carbonates in the Jura.
  • Synrift mega-sequence. Lower Cretaceous (Tithonium to Albium). Formation of the Bay of Biscay with partial oceanization. Non- and shallow marine clastic sediments and limestone in the Neocom, followed by mighty shelf carbonates in the Aptium and Albium. The mega-sequence ends in the Cenomanium due to the onset of inversion tectonics at former expansion fractures.
  • Postrift mega-sequence. Upper Cretaceous (Cenomanian to Paleocene). The sinistral movement of Iberia relative to France leads to the formation of several sub-basins, which are caused by shear movements with simultaneous expansion (pull-apart structures). Turbidite in the south.
  • Foreland basin mega-sequence. Cenozoic (Eocene to present). The collision of Iberia with France in the Eocene ended the stretch tectonics. In the foreland basin, the Pyrenean orogen carries flysch in the Eocene and molasses in the Miocene.

Mineral resources and natural raw materials

Hydrocarbons

Oil pumping station on the Étang de Biscarosse near Parentis-en-Born

The most economically important natural resource in the Aquitaine Basin is undoubtedly the hydrocarbons ( oil and gas ). The deposits are located in the Lower Chalk Basin of the Southern Province:

Mother and storage rocks are limestone and dolomite from the Jura (Kimmeridgian) and the Lower Cretaceous. Mudstones from the Lower Aptium act as a seal.

Groundwater

Classic groundwater reservoirs are located in the Upper Cretaceous and Tertiary of the Bordelais . The huge occurrence in the Eocene sands near Lussagnet has recently been added, an enormously important discovery for the Pau-Toulouse area.

Other resources

The following must also be stated:

  • Clays and mudstones. Raw material for countless brick factories (roof tiles, clinker, terracotta etc.). Deposits are mainly found in the Toarcian, Eocene (Lutetian), Oligocene and Miocene (Aquitanium, Burdigalium, Langhium and Tortonium ).
  • Kaolin . Raw material for porcelain manufacturers . Lenticular Eocene residual deposits, often in the Upper Cretaceous Karst (e.g. near Les Eyzies ).
  • Peat . Pleistocene and Holocene horizons in the Médoc (Gironde estuary).
  • Lignite . In the Cenoman des Sarladais , Upper Miocene / Pliocene deposits of the Landes , mined in the open pit near Arjuzanx .
  • Bauxite . In the Jura karst pockets between Pech and Lavelanet . Unprofitable.
  • Iron . In the Eocene Sidérolithique. Occurrence in Périgord and Quercy. No longer economically viable.
  • Non-ferrous metals. Mainly lead - zinc mineralization in the basal Sinemurium. In the Charente and Figeac, but no longer economically viable.
  • Wood . Extensive forest areas in the Landes and in the Dordogne provide the raw material for a diversified wood processing industry (production of firewood for thermal power stations and for private consumption, charcoal , activated carbon, pallets, construction timber, furniture industry)
  • Fruit growing . For example the plums of the Agenais ( Pruneau d'Agen ).
  • Wine . Famous wine-growing regions near Cognac, Bordeaux, Armagnac , Chalosse and Béarn .

Final consideration

The structure of the Aquitaine Basin is shaped by two essential factors:

  • The Variscan basement.
  • The Pyrenees orogen.

The northwest-southeast trending tectono-metamorphic zoning of the basement has also had a lasting effect on the sedimentary evolution and structuring in the Aquitaine Basin. The northwestern French continental margin, which was formed in the course of the opening of the Biscay, follows the same Hercynian direction . Its continuation towards the continent is found in the deep Sub-Pyrenees basin. The anticline ridges in the basin sediments are also essentially arranged along this direction. Similar to the South Armorican shear zone, which also follows the northwest-southeast direction and has a pronounced dextral shear component, the anticline ridges also show this dextral shear. Their creation is therefore not only of a purely compressive nature. The Parentis Basin is also bounded by Armorican, dextral shear zones, which were simultaneously subject to distension and ultimately created an east-west- oriented pull-apart basin. In the course of the Biscay expansion, the Parentis Basin represents the attempt of the Atlantic to penetrate further into the continental Aquitaine Basin. However, this was ultimately prevented by Iberia's counterclockwise rotation. From the Cenomanium onwards, the Aquitaine Basin is then subject to the influence of the resulting Pyrenees with its west-north-west-east-south-east structures. The Pyrenees, too, are not only of purely compressive origin, but in their case also show significant sinistral shear movement. The structure of the Pyrenees then influenced the Aquitaine Basin to this day and was of a pervasive nature - for example, the tectonic effects of the Pyrenees Orogen can still be observed on the northeastern edge of the Aquitaine Basin in the immediate vicinity of the Massif Central.

literature

  • Chantraine, J., Autran, A., Cavelier, C. et al. Carte géologique de la France à l'échelle du millionème. (1996). BRGM. ISBN 2-7159-2128-4
  • Gèze, B. & Cavaillé, A. Aquitaine orientale. (1977). Guides geologiques régionaux. Masson. ISBN 2-225-44935-X
  • Vigneaux, M. (1975). Aquitaine occidentale. Guides geologiques régionaux. Masson. ISBN 2-225-41118-2
  • Winnock, E. (1996). Basin d'Aquitaine. Contribution to the Encyclopaedia Universalis. ISBN 2-85229-290-4