Aquitaine Basin

The Aquitaine Basin is the second largest Mesozoic and Cenozoic sedimentary basin in France after the Paris Basin, occupying a large part of the country's southwestern quadrant. Its surface area covers 66,000 km² onshore. It formed on Variscan basement which was peneplained during the Permian and then started subsiding in the early Triassic. The basement is covered in the Parentis Basin and in the Subpyrenean Basin—both sub-basins of the main Aquitaine Basin—by 11,000 m of sediment.

Geography

The Aquitaine Basin, named after the French region Aquitaine, is roughly funnel-shaped with its opening pointing towards the Atlantic Ocean. Here it meets for 330 km the straight, more or less north–south-trending Atlantic coastline but continues offshore to the continental slope. To the south, it is delimitated for 350 km by the west-northwest–east-southeast trending Pyrenees. In the southeast, the basin reaches the Seuil de Naurouze (also called ''Seuil du Lauragais'') between the Montagne Noire on its northern side and the Mouthoumet range in the south. Just west of Narbonne, the basin is overridden by Pyrenean thrusts. The northeastern boundary of the basin is formed by the arcuate basement outcrops of the Massif Central. Via the 100 km wide Seuil du Poitou in the northeast, the basin is connected to the Paris Basin. In the far north, the basin abuts the east–west-oriented Variscan basement of the Vendée, the southernmost part of the Armorican Massif.

Basin structure

The Aquitaine Basin is a very asymmetric foreland basin. It reaches its deepest part of 11 km just in front of the North Pyrenean Thrust.

The 2,000 m isobath follows more or less the course of the Garonne River and divides the basin into a relatively shallow northern platform, the so-called ''Aquitaine Plateau'', and into a much deeper, tightly folded, southern region. The tabular platform in the north contains only a much reduced sedimentary succession that is gently undulating and occasionally faulted. The folding intensity in the southern region increases steadily towards the south, the structures being further complicated by superimposed salt diapirism.

This somewhat simplified structural subdivision gets complicated by the Parentis Basin which extends out into the Atlantic. The Parentis Basin is situated in the Golfe de Gascogne and also reaches 11 km depth; it is a symmetrical basin oriented east–west and comes ashore near Arcachon. This sub-basin is underlain on its far western side by oceanic crust dated at 100–95 million years BP (Cenomanian). It is bounded by dextral wrench faults (possible transform faults) and probably represents a pull-apart basin.

Stratigraphy

(Note: Permo-Triassic basins like the Brive Basin and the Grésigne Basin are considered to belong to the basement of the Massif Central.)

Structural and sedimentological investigations of the basin have been carried out in over 70 drilled wells that encountered the Variscan basement sometimes below 6,000 m of sedimentary cover.

The sedimentary evolution in the Aquitaine Basin begins in the Lower Triassic close to the North Pyrenean Thrust. From here, it slowly started spreading farther north.

Triassic

Sedimentation started in the very south of the Aquitaine Basin during the Lower Triassic with coloured sandstones and mudstones, followed during the Middle Triassic by dolomitic limestones, evaporite strata and coloured mudstones. During the Upper Triassic evaporites continued being precipitated, crowned by ophitic lava flows (dolerites and tholeiites). The evaporites were later activated as diapirs during the Pyrenean orogeny and the mudstones served as decollement horizons along which Triassic sediments were squeezed northwards to the line Arcachon–Toulouse.

The sediments are typically ''germanotype'' in character, i.e. very similar to the Triassic succession in Germany. In the north of the Aquitanian plateau, only a continental Upper Triassic is preserved. In the south, the sediments are marine and show their full development. The Triassic marine transgression probably invaded the southern Aquitaine Basin from the southeast or from the south (from the Tethys) via the then still immersed Pyrenean region. The sediments indicate a restricted shallow marine environment with drying-up periods that created evaporites. The Triassic sediments can attain a maximum thickness of 1,000 m and reach as far north as the line Garonne estuary – Brive.

Jurassic

The entirely marine Jurassic cycle can be subdivided into seven second-order sequences bounded by unconformities, three in the Lias, two in the Dogger and two in the Malm:
* Hettangian-Sinemurian sequence.
* Lotharingian-Carixian-Domerian sequence (Upper Sinemurian–Pliensbachian).
* Toarcian-Aalenian sequence.
* Bajocian–Lower Bathonian sequence.
* Middle Bathonian–Callovian sequence.
* Oxfordian–Sequanian sequence (Oxfordian–Lower Kimmeridgian).
* Kimmeridgian–Portlandian sequence (Kimmeridgian-Tithonian).
The complete Jurassic cycle is only preserved in the Quercy; farther south, e.g. in the Subpyrenean Basin, the cycle has many gaps.

Lias

The basal Hettangian-Sinemurian sequence is fully transgressive over basement rocks or Permo-Triassic sediments. At that time, the first open-marine sediments (yet rather poor in fossils) were being deposited in the Aquitaine Basin. The ''Lias Transgression'', as it is also called, started to encroach on the entire Aquitaine during the Sinemurian, characterised by calcareous-dolomitic, partially oolitic sediments. Despite smaller regressions during the Pliensbachian towards the end of the Lias and at the beginning of the Dogger the sea had onlapped the basement rocks of the Massif Central and the western Vendée (reaching today's limits) by 30 km. On the Aquitaine Plateau in the north, an interior shelf was constructed as far south as the line La Rochelle-Angoulême-Périgueux-Figeac. On this shelf the generally detritic transgression sediments of the Hettangian normally comprise a base conglomerate, arkoses, and fairly thick layers of sand- and mud-stones rich in plant material. The rest of the Hettangian is made up of marine sediments deposited in a restricted environment (lagoonal) evolving towards a lacustrine facies (green shales, coloured marls, dolomitic limestones and platy limestones rich in dwarf fossils, and evaporitic interlayers). The sediments of the Sinemurian are again fully marine and carry a pelagic fauna (soft banded limestones and hard lithographic limestones). At the end of the Sinemurian, a sudden regression occurred, forming hardgrounds.

The second sequence of the Lias again is marine-transgressive and commences during the ''Lotharingian''/Lower ''Carixian''. The sediments can be well dated by ammonites—(Arietites, Oxynoticeras, Deroceras, and '' Uptonia jamesoni''). They are mainly calcareous and rich in quartz grains and pebbles of reworked Sinemurian. The Upper Carixian consists of very fossiliferous ('' Aegoceras capricornu'') marly limestone layers interlayered with grey marls. These are followed by ammonite-bearing ('' Amaltheus margaritatus'') and oyster-bearing ('' Gryphaea cymbium'') marls indicating a shelf environment open to the spreading Atlantic Ocean. During the Lower Domerian, a connection to the Paris Basin is breached for the first time via the Seuil du Poitou and also to the Jurassic sea of southeastern France via the Détroit de Rodez and the Détroit de Carcassonne. During the Upper Domerian, another regression sets in leaving sandy limestones very rich in fossils (''Pleuroceras spinatum'', '' Pecten aequivalvis''). These littoral facies rocks can change into iron-rich oolites along their margins. The sequence finishes again with hardgrounds.

The third and last sequence of the Lias sets in during the Lower Toarcian without any detrital deposits at its base, the sediments being black ammonite-bearing marls (with '' Harpoceras falciferum'' and '' Hildoceras bifrons''). Towards the end of the Toarcian and the beginning of the Aalenian, the sediments turn into sandy limestones indicating another regression. Interlayered with these sandy limestones are oyster beds, iron oolite and gypsum layers; they contain ammonites like '' Pleydellia aalensis'' and '' Leioceras opalinum''. The sequence ends with an erosional unconformity.

In the southern part of the Aquitanian basin, the evaporite deposition (including layers of anhydrite) begun in the Triassic carries on right through the Lias; it reaches a thickness of up to 500 m.

Dogger

The Dogger attains a maximum thickness of about 300 m along a north–south-trending zone running from Angoulême to Tarbes. Along this zone reefs began to grow, splitting the Aquitaine Basin into two major facies domains. Prominent reef complexes are situated east of Angoulême, northwest of Périgueux and east of Pau. The reefs are associated with calcareous oolites and mark a high-energy zone. On the shallow shelf-domain east of the reefs, neritic limestones were deposited in the north and dolomites in the south; in the Quercy, even supratidal lignite-bearing limestones were formed. In the western domain open towards the Atlantic, the pelagic sediments comprise ammonite-bearing limy marls very rich in filamentous microfossils ( bryozoans).

The first sequence in the Dogger (note: sequences are only distinguished in the eastern shelf-domain) starts transgressing in a restricted environment during the Bajocian with dolomite. In places, Aalenian is reworked. The Bathonian is calcareous in the northeast, whereas in the southeast it keeps its dolomitic character. The end of the sequence in the Lower Bathonian shows regressive tendencies with lignites, breccias, and lacustrine fossils in the Quercy. No ammonites are found in the eastern domain right up to the Kimmeridgian—a great handicap for correct dating purposes.

The Pyrenean realm meanwhile is characterised by a long hiatus.

The second sequence in the Dogger begins in the Middle Bathonian with lacustrine limestones and in places with breccia-bearing detritus. This is followed by neritic limestones precipitated in calm conditions. Yet in the south, dolomites continue to be deposited. The sequence finishes in the Callovian with littoral border-facies deposits.

Malm

The facies dividing reef-zone persists into the Malm. In the western domain, initially ammonite-bearing marls and limestones were deposited, whereas in the eastern domain the sediments are calcareous dolomites. The retreat of the Jurassic sea became noticeable during the late Tithonian with dolomites and breccias in the Adour Basin, evaporites in the Charente, extremely littoral sediments in the Quercy, lacustrine limestones in the Parentis Basin, and anhydrites in the Gers. The seaways that had opened in the Lias closed again and a single reef persisted in the Périgord at La Tour-Blanche

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