Aquitaine Basin (geology)
Encyclopedia
The Aquitaine Basin is after the Paris Basin
the second largest Mesozoic
and Cenozoic
sedimentary basin
in France
, 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.
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 westnorthwest–eastsoutheast 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
.
. 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 fault
s) and probably represents a pull-apart basin
.
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.
s and mudstone
s, followed during the Middle Triassic by dolomitic
limestone
s, evaporite
layers and coloured mudstones. During the Upper Triassic evaporites continued being precipitated, crowned by ophitic lava
flows (dolerites and tholeiite
s). The evaporites were later activated as diapir
s 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.
, three in the Lias
, two in the Dogger
and two in the Malm
:
The complete Jurassic cycle is only preserved in the Quercy
; farther south, e.g. in the Subpyrenean Basin, the cycle has many gaps.
-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
, arkose
s, 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 (lagoon
al) 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 ammonite
s — (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.
attains a maximum thickness of about 300 m along a north-south-trending zone running from Angoulême to Tarbes
. Along this zone reef
s 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 dolomites. 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, breccia
s, 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.
. In the western domain, initially ammonite-bearing marl
s 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
. In the end, the sea withdrew south of the Garonne River.
In the Lower Oxfordian, the first sequence of the Malm seems to follow the Callovian without a distinctive break. Yet cellular limestones and breccias indicate sediment reworking (this was certainly the case in the Grands Causses farther east). During the Middle and the Upper Oxfordian, marine limestones are laid down which incorporate occasional reefs. The Lower Kimmeridgian sediments are sedimented close to the shore, they bear oysters, urchin
s, and ripple marks
.
The second sequence of the Malm starts in the Upper Kimmeridgian, only in places does it show regressive traits, nevertheless the sedimentary character changes. Laid down are breccias and the sediments also show synsedimentary reworkings; periodically interbedded limestones and marls carrying lignite horizons begin to form. The sediments can be dated by the ammonites Aulacostephanus
and Aspidoceras orthocera
. This strongly disturbed depositional environment with a coexistence of open marine facies and muds deposited under reducing conditions in a restrictive setting seems to coincide with a first sedimentary individualisation of the Pyrenean realm. The event has received its name Virgulian from the oyster Exogyra virgula
. During the Tithonian, the shrinkage of the basin became even more evident, only to end in a nearly complete withdrawal of the sea from the Aquitaine Plateau before the close of the Tithonian (the south is not affected by this). During the Tithonian, iron-bearing calcareous oolites interbedded with marls, as well as dolostones and border facies deposits develop — dated by Gravesia portlandicum.
has less pronounced sequences. The Lower Cretaceous sediments are restricted to close to the Pyrenees. Most likely the exchange of ocean water masses was better towards the Tethyan realm than towards the Atlantic.
Sedimentation increased again after a longer hiatus in the Lower Creataceous, but only in two locales — the Parentis Basin and the Adour Basin. Both sub-basins manifest a huge subsidence
. During the Lower Cretaceous the Parentis Basin received 2,000 m of sediment and the Adour Basin 4,000 m. The remainder of the Aquitaine Basin is meanwhile subjected to strong erosion.
The first deposits in the two sub-basins were littoral sediments in Wealden facies, mainly sandstones and shales. During the Barremian
, marine shallow-water carbonates were precipitated, changing to detritic sediments in the northern Parentis Basin. Near Lacq
, they change to lagoonal anhydrites. In the Upper Aptian
, the reef-forming Urgonian facies became established in both sub-basins — fossiliferous limestones composed of algae
, coralline polyps, and rudists. The Urgonian facies completely surrounds the Parentis Basin and persists into the Albian.
Since the onset of the Albian
, strong halokinetic movements affect the southern Aquitaine Basin and in turn profoundly influence sedimentation patterns. As a result breccias, thick conglomerates, and turbidite
s are shed. In the Parentis Basin, a distinct unconformity
develops. At the same time, the sediments on the Aquitaine Plateau farther north are folded into gentle wavetrains following the Hercynian strike (northwest-southeast). All these movements are correlated with the first tectonic stirrings in the Western Pyrenees. Towards the end of the Albian, the sea level is rising and the Urgonian calcareous reefs are consequently draped by muds.
. In the northern part of the Aquitanian Basin, the Cenomanian sea reclaimed nearly the same areas that had been occupied by the Jurassic sea; in the east, however, it only reached the line Brive-Cahors
-Agen-Muret
-Carcassonne. The region of the later formed North Pyrenean Thrust is a decisive facies boundary at this time: to the north, shelf sedimentation continued but to the south rapidly subsiding basins developed into which flysch
sediments (and partially also wildflysch breccias) from the Pyrenean realm were shed. Near Saint-Gaudens
, the flysch sediments are even accompanied by volcanic rocks — trachyte
s, and ultrabasic lavas. The sedimentation in the flysch basins during the Turonian
and during the Coniacian
is very unsettled. The flysch sedimentation then continues right through the Upper Cretaceous, mainly interbedded sandstones and shales with some carbonaceous layers were laid down. Towards the end of the Upper Cretaceous, there are signs of the start of a regression and the sea then actually retreats before the K/T boundary. In the Subpyrenean Basin near the Petits Pyrénées, the sea lingers on till the lowermost Paleocene
(Danian
).
In the remainder of the Aquitaine Basin, mainly pelagic limestones (chalk
facies) are sedimented during the Upper Cretaceous, including the type localities for the Coniacian
, Santonian
, and Campanian
in the Charente.
At the northern edge of the basin, more differentiated coastal facies develop. In the north, the Cenomanian is made up of three sedimentary cycles (from young to old):
The Turonian reflects a transgressive period with the sea spreading into the Lot. At this point, the Upper Cretaceous sea had reached its highstand. This also coincides with a climatic optimum with global average sea-water temperatures around 24°C compared to today's 13°C. The Turonian can be subdivided into two parts:
Towards the end of the Turonian, the Massif Central experienced uplift
which is reflected in the sediments of the northeastern Aquitaine Basin as a strong input of detritus, mainly sands in the upper part of the Angoumian.
The Coniacian and the Santonian are expressed as typical chalky limestones in the north, but both stages take on a more sandy character east of Périgueux.
The Campanian
follows after a pronounced unconformity. The southern flysch basins began to expand northward. Near Pau before the onset of the flysch sedimentation, a very strong erosion removed the entire Lower Cretaceous, the entire Jurassic and sometimes even cut right down to the basement. North of Pau, the Campanian is a marly facies called Aturian. In the northern Aquitaine Basin, the sediments become more homogenised and settle out as fully marine flint
-bearing calcareous micrite
s.
During the Maastrichtian
, a regression commences. After the initial deposition of bioclastic rudist-bearing limestones and the formation of some reef complexes composed of rudists and single corals, the sea level started dropping. Northern Aquitaine became emersed and the sea withdrew in stages southward to the line Arcachon-Toulouse. At the same time, the northern edge of the basin experienced another folding episode with low-amplitude folds striking northwest-southeast.
, the coastline roughly followed the line Arcachon-Toulouse. In the North Aquitaine Zone north of this line, the sediments possess continental character — red mudstones, sands, and lacustrine limestones. The sea made a short-lived advance into this domain and left echinid
-bearing limestones behind. In the Central Aquitaine Zone (northern half of the southern basin), a shelf built out to the line Audignon
-Carcassonne. Farther south in the South Aquitaine Zone, deep water conditions prevailed in the west, shallowing out towards the east. The sediments in the Aturian Gulf (Golfe Aturién) in the west are pelagic limestones containing globigerinids
, operculinids, and alveolinids
. Near the Petits Pyrénées, the sediments change into shallow-water facies rich in madreporians, echinids, and operculinids. Farther east in the Ariège
and in the Corbières
, the sediments become totally continental and lacustrine.
In the Lower Eocene
(Ypresian
), another transgressive period saw the sea advance north into the Médoc
and south of Oléron
; in the southeast it even reached the Montagne Noire. In the Aturian Gulf, Globorotalia-bearing marls were deposited, while farther east turritella
-rich marls and limestones were formed. The newly inundated areas receive sands and limestones rich in alveolinids and nummulite
s. Meanwhile iron-rich sands (in the Charente) and molasse
s (in the Libournais and in the Agenais
) were sedimented in the continental north and northeast. The provenance area of these continental deposits up to Middle Ypresian times was mainly the Massif Central.
The sea-level kept rising during the Middle Eocene (Lutetian
and Bartonian
). The area covered by alveolinid- and nummulite-bearing limestones increased, northward to Blaye
and Saint-Palais
and eastward into the Agenais. The Subpyrenean Basin deepened and was simultaneously being filled by conglomerates brought in from the east, the so-called Poudingues de Palassou. This marked the beginning of uplift in the Pyrenean orogen and a switch-over in detritus provenance from the Massif Central in the north to the Pyrenees in the south. Coalescing alluvial fan
s built out north into the Castrais. On the northern flank of the fans, lakes formed, precipitating lacustrine limestones. The detrital sediments with provenance from the meanwhile strongly eroded Massif Central (muds, sands, gravels) then affected only a small fringe zone in the northeast. In the Périgord and in the Quercy, the Sidérolithique accumulated — iron-rich sediments that resemble laterite
s indicating a subtropical climate.
During the Upper Eocene (Priabonian
), a regression set in. The Subpyrenean Basin became completely filled with the erosional debris of the rising Pyrenees. In the Médoc, nummulite-bearing marls and limestones were still being laid down, but east of Bordeaux already continental molasses appeared that change farther south into gypsum-bearing formations.
During the Lower Oligocene
(Rupelian
), a permanently marine environment persists in the south with marls and sands rich in nummulites, lamellibranchs, and echinids. The anomiid
-bearing limestones of the southern Médoc are lagoonal deposits. After a short-lived advance at the beginning of the Chattian
with seastar
-bearing limestones in the northern Médoc and in the Libournais and with mammal-bearing molasses in the Agenais, the sea made a big retreat at the end of the Oligocene. This retreat was accompanied by tectonic movements creating trains of deeper-seated anticlines in the central and northern Aquitaine Basin. The debris-carrying alluvial fans issuing from the rising Pyrenees reached into the Agenais
and attained their largest extent. They pushed the surrounding belt of lakes ahead of them (in northerly directions) thereby spreading lacustrine limestones well into the Quercy, onto the Causses
, and even onto the Massif Central.
(Aquitanian). Marine, littoral, and lacustrine facies interchange. During a minor regression, a huge lake formed near Condom
, the Lac de Saucats, in which grey lacustrine-limestones precipitated, the so-called Calcaire gris de l'Agenais. Shortly thereafter the sea attained its highstand. It was rimmed completely by continental deposits whose thickness increased towards the southeast. For the first time, the alluvial fans along the Pyrenean front receded, the reason being increased subsidence in front of the orogen; yet they still stretched as far north as the Agenais.
The retreat of the alluvial fans also continued during the Middle Miocene (Langhian
and Serravallian
). Consequently the lacustrine band reached as far south as the Armagnac
.
The Upper Miocene (Tortonian
and Messinian
) witnessed a drastic withdrawal of the sea to the west. This process started first in the Bordelais
and in the Bazadais
, ending with a nearly complete withdrawal from the basin. In areas left behind by the sea in the Armagnac, unfossiliferous sands and muds were deposited. At the same time in the north and in the east, today's river network draining the Massif Central was already being beginning to form.
During the Pliocene
(Zanclean
), the sea occupied merely a small strip near the Arcachon Basin south of Soustons
. Sandy shales very rich in a benthic microfauna were deposited. In the rest of the Aquitaine Basin, continental sands were laid down, the so-called Sables fauves. The alluvial fans restricted their activity to the immediate vicinity of the Pyrenean mountain front and created the alluvial fans of Ger, Orignac
-Cieutat
, and Lannemezan
. The drainage system of the Garonne already resembled more or less today's pattern, the river avoiding the Miocene gravel accumulations as much as possible and then following between Toulouse, Agen and Bordeaux a weekly subsiding graben.
The progressive landfall of the Aquitaine Basin proceeded from the northeast and was coupled with an important subaerial erosion. As a consequence several peneplanations were carved out from the detrital alluvial plains:
On the pliocene peneplain, today's drainage system was firmly established.
The three last Pleistocene
ice age
s — Mindel
, Riss, and Würm
— are also documented in the Aquitaine Basin, mainly by different levels of river terasses. Additionally amongst glacial phenomena the following can be cited:
The development of the Gironde
estuary goes back about 20,000 years into the late Würm.
Finally, the rich prehistoric
finds and their sites in the Aquitaine Basin merit mentioning, especially in the Département Dordogne
.
The Northern Province or Aquitaine Plateau forms a typical continental shelf region with reduced sedimentation and several periods of emersion (during the entire Lower Cretaceous and during parts of the Upper Cretaceous and the Cenozoic). The basement is rarely encountered deeper than 2,000 m. Triassic and Jurassic together have an accumulated sediment thickness of 1,000–1,700 m. The Lower Cretaceous is completely missing and the Upper Cretaceous only reaches a thickness of several hundred metres. The Paleogene
is, if present, very thin in the north but increases in thickness towards the south where it is overlain by thin Neogene
.
In the eastern section, one can discern several low-amplitude structures that are parallel to the Pyrenees farther south and strike westnorthwest-eastsoutheast:
Generally the Northern Province is characterised by rather simple structures (syn- and antiforms, low-amplitude fold trains, faults) that follow hercynian, armorican and variscan strike directions. The structures were formed during several tectonic phases:
The Southern Province is characterised by the deep Parentis and Adour sub-basins with the Mimizan
high ground in between. Compared with the Northern Province its sediments show a pronounced increase in thickness (5,000–11,500 altogether). The Triassic and the Jurassic combined reach 2,000–3,000 , the Lower Cretaceous 500-1,500 m. The Upper Creataceous can vary between 500 and 3,000 m and even the Neogene still attains a thickness of nearly 1,000 m.
The tectonic movements were much more complicated in the Southern Province, having superimposed on them very strong halokinetic motions (salt diapirism). A large part of the formed structures is concealed under plio-quaternary detritus. The many exploration wells drilled for hydrocarbons and groundwater helped immensely in unravelling these structures. Similar to the Northern Province, the major structures are again trains of parallel anticlines whose wavelength steadily diminishes approaching the Pyrenean front. Inversely the effects of the salt movements become stronger towards the south. The anticlines were formed during the uplift of the Pyrenees during the Eocene/Oligocene. The structures were set in place by Miocene times. The following anticlines can be distinguished (from north to south):
Isostatic
movements during the Plio-Quaternary at the northeastern edge of the Aquitaine basin lead to an uplift and rejuvenation of the peneplained basement in the Massif Central. In the Aquitaine basin itself, these movements follow already existing basement structures and entrain the tilting of some of the pliocene peneplains. This in turn has a strong effect upon the hydrographic network, for example in the drainage basins of the Garonne and Adour, the rivers' courses were changed or entirely abandoned.
Tectonic movements are still at work in the Aquitaine Basin today — strong earthquakes in the Pyrenees (with destroyed villages and churches) and somewhat milder tremors near the island of Oléron keep reminding us of that fact.
explorations the Variscan basement hidden under the sediments of the Aquitaine Basin can be subdivided into several northwest-southeast-striking tectono-metamorphic zones (from north to south):
in the Aquitanian Basin is 36 km, following more or less the path of the Garonne River. Towards the Massif Central in the northeast, the discontinuity flattens out to 30 km. The same holds for approaching the northern edge of the Pyrenees, here the discontinuity is also at 30 km depth. In the oceanic part of the Parentis Basin, it is already encountered at 20 km depth. This implies a significant stretching of the continental crust
and the beginning of oceanisation. As a comparison, underneath the Central Pyrenees the continental crust is 50 km thick.
In the Upper Triassic (Carnian
) about 230 million years ago, the supercontinent Pangaea started slowly to break up. In the Atlantic domain, the disintegration began in the area of the Central Atlantic. Already in the Lower Jurassic, the initial rifting process had given way to the marine drifting stage. In the Toarcian
about 180 million years ago, the Central Atlantic was spreading and North America
, South America
and Africa
were separating. By Callovian
times, the Central Atlantic was fully marine. Spreading continued and gradually also began to affect the Northern Atlantic domain. During the Tithonian
about 150 million years ago, a rift arm infiltrated along today's continental margin of northwestern France. Consequently Iberia, so far being positioned right below the Armorican Massif (Brittany
), was wedged southward. This gave the Atlantic the chance to directly reach the Aquitaine Basin for the first time. In the wake of the southward drifting Iberia during the Lower Cretaceous, the Bay of Biscay opened up. The microcontinent Iberia underwent in addition to its southward drifting motion a counterclockwise rotational movement that eventually brought it in close contact with southern France (reflected in first tectonic movements in the Pyrenees during the Albian
; also documented by metamorphism in the Pyrenees dated between 108 and 93 million years ago and by the transgression of the Cenomanian
sea). The final collision happened during the Eocene
/Oligocene
uplifting the mountain chain and subjecting it to severe erosion at the same time. The main phase of uplift ended with the close of the Aquitanian, followed mainly by isostatic movements lasting to this day.
and gas
are without doubt of prime importance for the French economy. Major sources are found in the following sub-basins of Lower Cretaceous age:
Source/host rocks are Jurassic (Kimmeridgian) and Lower Cretaceous limestones and dolostones. Lower Aptian shales function as seals.
aquifer
s are found in Upper Cretaceous and Cenozoic rocks of the Bordelais
. Recently a gigantic aquifer has been discovered in Eocene sands near Lussagnet
— of great importance for the region Pau-Toulouse.
The strong northwest-southeast-striking tectono-metamorphic zonation in the basement has profoundly influenced the structural and the sedimentary evolution of the Aquitaine Basin. The same hercynian direction is also followed by the continental edge of northwestern France which formed during the evolution of the Bay of Biscay. The continental edge finds its prolongation in the superdeep Subpyrenean Basin. The system of anticlinal ridges affecting the sedimentary cover is also arranged in this direction. The prominent South Armorican Shear Zone farther north also strikes northwest-southeast, but additionally has a distinct dextral wrenching movement. Like the South Armorican Shear Zone, the anticlinal ridges too are affected by similar shearing movements and are not purely compressional in origin. Even the Parentis Basin is bounded by these transtensional dextral shear zones and can hence be interpreted as an east-west-oriented pull-apart basin. Within the context of the opening of the Bay of Biscay, the Parentis Basin can furthermore be regarded as a failed attempt of the Atlantic to wedge into the continent's interior. The reason for this is the counterclockwise rotational motion of Iberia blocked further rifting.
Since the Cenomanian, the Aquitaine Basin is under the influence of the Pyrenean orogeny with its westnorthwest-eastsoutheast-striking structural grain. The Pyrenean orogen likewise has not merely a compressional origin but also a strong, in this case, sinistral transtensional component. The Pyrenean orogen exerted a very profound influence upon the Aquitaine Basin up to this day, subjecting it not only to more or less north-south-directed compression but also to transtension. The effects were of a penetrative nature — tectonic repercussions of the Pyrenean orogeny can even be seen at the northeastern basin margin in the immediate vicinity of the Massif Central.
Paris Basin (geology)
The Paris Basin is one of the major geological regions of France having developed since the Triassic on a basement formed by the Variscan orogeny.-Extent:...
the second largest Mesozoic
Mesozoic
The Mesozoic era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the age of reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time...
and Cenozoic
Cenozoic
The Cenozoic era is the current and most recent of the three Phanerozoic geological eras and covers the period from 65.5 mya to the present. The era began in the wake of the Cretaceous–Tertiary extinction event at the end of the Cretaceous that saw the demise of the last non-avian dinosaurs and...
sedimentary basin
Sedimentary basin
The term sedimentary basin is used to refer to any geographical feature exhibiting subsidence and consequent infilling by sedimentation. As the sediments are buried, they are subjected to increasing pressure and begin the process of lithification...
in France
France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...
, 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
Permian
The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...
and then started subsiding in the early Triassic
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...
. 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 regionRégions of France
France is divided into 27 administrative regions , 22 of which are in Metropolitan France, and five of which are overseas. Corsica is a territorial collectivity , but is considered a region in mainstream usage, and is even shown as such on the INSEE website...
Aquitaine
Aquitaine
Aquitaine , archaic Guyenne/Guienne , is one of the 27 regions of France, in the south-western part of metropolitan France, along the Atlantic Ocean and the Pyrenees mountain range on the border with Spain. It comprises the 5 departments of Dordogne, :Lot et Garonne, :Pyrénées-Atlantiques, Landes...
, is roughly funnel-shaped with its opening pointing towards the Atlantic Ocean
Atlantic Ocean
The Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about , it covers approximately 20% of the Earth's surface and about 26% of its water surface area...
. 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 westnorthwest–eastsoutheast trending Pyrenees
Pyrenees
The Pyrenees is a range of mountains in southwest Europe that forms a natural border between France and Spain...
. In the southeast, the basin reaches the Seuil de Naurouze
Seuil de Naurouze
The Seuil de Naurouze, aka Col de Naurouze, is a mountain pass in southern France. It is the watershed point identified by Pierre-Paul Riquet when he designed and built the Canal du Midi. Water falling on the western side of this point flows to the Atlantic Ocean and on the eastern side to the...
(also called Seuil du Lauragais) between the Montagne Noire
Montagne Noire
* Not to be confused with the Montagnes Noires in Brittany.The Montagne Noire is a mountain range in central southern France. It is located at the southwestern end of the Massif Central in the border area of the Tarn, Hérault and Aude departments...
on its northern side and the Mouthoumet
Mouthoumet
Mouthoumet is a commune in the Aude department in southern France.-Population:...
range in the south. Just west of Narbonne
Narbonne
Narbonne is a commune in southern France in the Languedoc-Roussillon region. It lies from Paris in the Aude department, of which it is a sub-prefecture. Once a prosperous port, it is now located about from the shores of the Mediterranean Sea...
, the basin is overridden by Pyrenean thrusts
Thrust fault
A thrust fault is a type of fault, or break in the Earth's crust across which there has been relative movement, in which rocks of lower stratigraphic position are pushed up and over higher strata. They are often recognized because they place older rocks above younger...
. The northeastern boundary of the basin is formed by the arcuate basement outcrops of the Massif Central
Massif Central (geology)
The Massif Central forms together with the Armorican Massif one of the two big basement massifs in France. Its geological evolution started in the late Neoproterozoic and continues to this day. It has been shaped mainly by the Caledonian orogeny and the Variscan orogeny. The Alpine orogeny has...
. Via the 100 km wide Seuil du Poitou
Seuil du Poitou
The Seuil du Poitou is a geological denomination for an area in western central France where the Paris and Aquitaine sedimentary basins meet, and which also is a gap between the ancient mountain ranges Massif Armoricain and the Massif Central .Situated to the south of Poitiers, the area is the...
in the northeast, the basin is connected to the Paris Basin
Paris Basin (geology)
The Paris Basin is one of the major geological regions of France having developed since the Triassic on a basement formed by the Variscan orogeny.-Extent:...
. In the far north, the basin abuts the east-west oriented Variscan basement of the Vendée
Vendée
The Vendée is a department in the Pays-de-la-Loire region in west central France, on the Atlantic Ocean. The name Vendée is taken from the Vendée river which runs through the south-eastern part of the department.-History:...
, the southernmost part of the Armorican Massif
Armorican Massif
The Armorican Massif is a geologic massif that covers a large area in the northwest of France, including Brittany, the western part of Normandy and the Pays de la Loire. Its name comes from the old Armorica, a Gaul area between the Loire and the Seine rivers...
.
Basin structure
The Aquitaine Basin is a very asymmetric foreland basinForeland basin
A foreland basin is a depression that develops adjacent and parallel to a mountain belt. Foreland basins form because the immense mass created by crustal thickening associated with the evolution of a mountain belt causes the lithosphere to bend, by a process known as lithospheric flexure...
. 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
Garonne
The Garonne is a river in southwest France and northern Spain, with a length of .-Source:The Garonne's headwaters are to be found in the Aran Valley in the Pyrenees, though three different locations have been proposed as the true source: the Uelh deth Garona at Plan de Beret , the Ratera-Saboredo...
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
Arcachon
Arcachon is a commune in the Gironde department in southwestern France.It is a popular bathing location on the Atlantic coast southwest of Bordeaux in the Landes forest...
. This sub-basin is underlain on its far western side by oceanic crust
Oceanic crust
Oceanic crust is the part of Earth's lithosphere that surfaces in the ocean basins. Oceanic crust is primarily composed of mafic rocks, or sima, which is rich in iron and magnesium...
dated at 100-95 million years BP (Cenomanian
Cenomanian
The Cenomanian is, in the ICS' geological timescale the oldest or earliest age of the Late Cretaceous epoch or the lowest stage of the Upper Cretaceous series. An age is a unit of geochronology: it is a unit of time; the stage is a unit in the stratigraphic column deposited during the corresponding...
). It is bounded by dextral wrench faults (possible transform fault
Transform fault
A transform fault or transform boundary, also known as conservative plate boundary since these faults neither create nor destroy lithosphere, is a type of fault whose relative motion is predominantly horizontal in either sinistral or dextral direction. Furthermore, transform faults end abruptly...
s) and probably represents a pull-apart basin
Pull Apart Basin
250px|thumb|[[Cami Lake]] in [[Tierra del Fuego]] develops on a [[Patagonian Ice Sheet|glacially]] excavated pull apart basin along the [[Magallanes-Fagnano Fault]], hence its elongated form...
.
Sedimentary evolution
(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
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...
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 sandstoneSandstone
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...
s and mudstone
Mudstone
Mudstone is a fine grained sedimentary rock whose original constituents were clays or muds. Grain size is up to 0.0625 mm with individual grains too small to be distinguished without a microscope. With increased pressure over time the platey clay minerals may become aligned, with the...
s, followed during the Middle Triassic by dolomitic
Dolomite
Dolomite is a carbonate mineral composed of calcium magnesium carbonate CaMg2. The term is also used to describe the sedimentary carbonate rock dolostone....
limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
s, evaporite
Evaporite
Evaporite is a name for a water-soluble mineral sediment that result from concentration and crystallization by evaporation from an aqueous solution. There are two types of evaporate deposits, marine which can also be described as ocean deposits, and non-marine which are found in standing bodies of...
layers and coloured mudstones. During the Upper Triassic evaporites continued being precipitated, crowned by ophitic lava
Lava
Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...
flows (dolerites and tholeiite
Tholeiite
The tholeiitic magma series is one of two main magma series in igneous rocks, the other magma series being the calc–alkaline. A magma series is a series of compositions that describes the evolution of a mafic magma, which is high in magnesium and iron and produces basalt or gabbro, as it...
s). The evaporites were later activated as diapir
Diapir
A diapir is a type of intrusion in which a more mobile and ductily-deformable material is forced into brittle overlying rocks. Depending on the tectonic environment, diapirs can range from idealized mushroom-shaped Rayleigh-Taylor instability-type structures in regions with low tectonic stress...
s during the Pyrenean orogeny and the mudstones served as decollement
Decollement
Décollement is a gliding plane between two rock masses. In French, "décoller" means "to detach from" or "to rip off" and was first used by geologists studying the structure of the Swiss Jura Mountains, but is also known as a detachment zone. This is a structure of strata owing to deformation,...
horizons along which Triassic sediments were squeezed northwards to the line Arcachon–Toulouse
Toulouse
Toulouse is a city in the Haute-Garonne department in southwestern FranceIt lies on the banks of the River Garonne, 590 km away from Paris and half-way between the Atlantic Ocean and the Mediterranean Sea...
.
The sediments are typically germanotype in character, i.e. very similar to the Triassic succession in Germany
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
. 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
Tethys Ocean
The Tethys Ocean was an ocean that existed between the continents of Gondwana and Laurasia during the Mesozoic era before the opening of the Indian Ocean.-Modern theory:...
) 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 unconformitiesUnconformity
An unconformity is a buried erosion surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval of time before deposition of the younger, but the term is used to describe...
, three in the Lias
Lias
Lias may refer to:*In geology:**The Lias Group, a group in the stratigraphy of Great Britain, formed during the Late Triassic to Early Jurassic epochs. It includes the Blue Lias and White Lias...
, two in the Dogger
Dogger
Dogger can mean:-*A sea area in the North Sea, noted in shipping forecasts, named after:**The Dogger Bank, a large shallow area in the North Sea between Britain and Denmark...
and two in the Malm
Malm
is a former municipality, a village, and the administrative centre of the municipality of Verran in Nord-Trøndelag county, Norway. Malm is located along the Trondheimsfjord, about a drive west from the town of Steinkjer. The village of Malm has a population of 1,572. The population density of...
:
- 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
Quercy
Quercy is a former province of France located in the country's southwest, bounded on the north by Limousin, on the west by Périgord and Agenais, on the south by Gascony and Languedoc, and on the east by Rouergue and Auvergne....
; farther south, e.g. in the Subpyrenean Basin, the cycle has many gaps.
Lias
The basal HettangianHettangian
The Hettangian is the earliest age or lowest stage of the Jurassic period of the geologic timescale. It spans the time between 199.6 ± 0.6 Ma and 196.5 ± 1 Ma . The Hettangian follows the Rhaetian and is followed by the Sinemurian.In Europe stratigraphy the Hettangian is a part of the time span in...
-Sinemurian sequence is fully transgressive
Transgression (geology)
A marine transgression is a geologic event during which sea level rises relative to the land and the shoreline moves toward higher ground, resulting in flooding. Transgressions can be caused either by the land sinking or the ocean basins filling with water...
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
Sinemurian
In the geologic timescale, the Sinemurian is an age or stage in the Early or Lower Jurassic epoch or series. It spans the time between 196.5 ± 2 Ma and 189.6 ± 1.5 Ma...
, characterised by calcareous-dolomitic, partially oolitic
Oolite
Oolite is a sedimentary rock formed from ooids, spherical grains composed of concentric layers. The name derives from the Hellenic word òoion for egg. Strictly, oolites consist of ooids of diameter 0.25–2 mm; rocks composed of ooids larger than 2 mm are called pisolites...
sediments. Despite smaller regressions during the Pliensbachian
Pliensbachian
The Pliensbachian is an age of the geologic timescale or stage in the stratigraphic column. It is part of the Early or Lower Jurassic epoch or series and spans the time between 189.6 ± 1.5 Ma and 183 ± 1.5 Ma . The Pliensbachian is preceded by the Sinemurian and followed by the Toarcian.The...
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
Shelf
Shelf may refer to:* Shelf , a flat horizontal surface used for diplay and storage* Shelf , a user interface feature in the NeXTSTEP operating system* Shelf, West Yorkshire, a village in England...
was constructed as far south as the line La Rochelle
La Rochelle
La Rochelle is a city in western France and a seaport on the Bay of Biscay, a part of the Atlantic Ocean. It is the capital of the Charente-Maritime department.The city is connected to the Île de Ré by a bridge completed on 19 May 1988...
-Angoulême
Angoulême
-Main sights:In place of its ancient fortifications, Angoulême is encircled by boulevards above the old city walls, known as the Remparts, from which fine views may be obtained in all directions. Within the town the streets are often narrow. Apart from the cathedral and the hôtel de ville, the...
-Périgueux
Périgueux
Périgueux is a commune in the Dordogne department in Aquitaine in southwestern France.Périgueux is the prefecture of the department and the capital of the region...
-Figeac
Figeac
Figeac is a commune in the Lot department in south-western France.Figeac is a sub-prefecture of the department.-History:Figeac is on the via Podiensis, a major hiking medieval pilgrimage trail which is part of the Way of St. James...
. On this shelf the generally detritic transgression sediments of the Hettangian normally comprise a base conglomerate
Conglomerate (geology)
A conglomerate is a rock consisting of individual clasts within a finer-grained matrix that have become cemented together. Conglomerates are sedimentary rocks consisting of rounded fragments and are thus differentiated from breccias, which consist of angular clasts...
, arkose
Arkose
Arkose is a detrital sedimentary rock, specifically a type of sandstone containing at least 25% feldspar. Arkosic sand is sand that is similarly rich in feldspar, and thus the potential precursor of arkose....
s, 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 (lagoon
Lagoon
A lagoon is a body of shallow sea water or brackish water separated from the sea by some form of barrier. The EU's habitat directive defines lagoons as "expanses of shallow coastal salt water, of varying salinity or water volume, wholly or partially separated from the sea by sand banks or shingle,...
al) 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
Carbonate hardgrounds
Carbonate hardgrounds are surfaces of synsedimentarily cemented carbonate layers that have been exposed on the seafloor . A hardground is essentially, then, a lithified seafloor. Ancient hardgrounds are found in limestone sequences and distinguished from later-lithified sediments by evidence of...
.
The second sequence of the Lias again is marine-transgressive and commences during the Lotharingian/Lower Carixian. The sediments can be well dated by ammonite
Ammonite
Ammonite, as a zoological or paleontological term, refers to any member of the Ammonoidea an extinct subclass within the Molluscan class Cephalopoda which are more closely related to living coleoids Ammonite, as a zoological or paleontological term, refers to any member of the Ammonoidea an extinct...
s — (Arietites
Arietites
Arietites is a genus of massive, giant evolute, psiloceratacean ammonites in the family Arietitidae in which whorls are subquadrate and transversely ribbed and low keels in tripicate, separated by a pair of longitudinal grooves, run along the venter...
, Oxynoticeras
Oxynoticeras
Oxynoticeras is an extinct genus of ammonite from the Early Jurassic of Europe and North America.-External links:* in the Paleobiology Database...
, Deroceras
Deroceras
Deroceras is a taxonomic genus of small to medium sized, air-breathing, land slugs, terrestrial pulmonate gastropod mollusks in the family Agriolimacidae.-Description:...
, and Uptonia jamesoni
Uptonia
Uptonia is an extinct ammonite from the Lower Jurassic that's included in the eoderoceratacean family Polymorphitidae.The shell of Uptonia is evolute with rounded simple ribs that form strong chevrons as they cross the venter on the outer rim, and which are free of tubercles. Some grew to be fairly...
). They are mainly calcareous and rich in quartz grains and pebbles of reworked Sinemurian. The Upper Carixian consists of very fossiliferous (Aegoceras capricornu
Aegoceras
Aegoceras is an evolutely wound ammmonite, with wide spaced ribs, from the Early Jurassic included in the Liparoceratidae and superfamily Eoderoceratidae. Related genera are Leparoceras and Beaniceras.-References:*...
) marly limestone layers interlayered with grey marls. These are followed by ammonite-bearing (Amaltheus margaritatus
Amaltheus
Amaltheus is an oxyconic ammonite with a fairly open umbilicus, serrated keel, and slightly sigmoidal ribs from the Lower Jurassic, many of which are strigate. Amaltheus, named by de Montfort ,1808, is indicative of the upper Pliensbachian stage in Europe, north Africa, Caucasus, Siberia, N...
) and oyster
Oyster
The word oyster is used as a common name for a number of distinct groups of bivalve molluscs which live in marine or brackish habitats. The valves are highly calcified....
-bearing (Gryphaea cymbium
Gryphaea
Gryphaea, common name Devil's toenails, is a genus of extinct oysters, marine bivalve mollusks in the family Gryphaeidae.These fossils range from the Jurassic to the Cretaceous periods. They are particularly common in many parts of Britain....
) 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
Seuil du Poitou
The Seuil du Poitou is a geological denomination for an area in western central France where the Paris and Aquitaine sedimentary basins meet, and which also is a gap between the ancient mountain ranges Massif Armoricain and the Massif Central .Situated to the south of Poitiers, the area is the...
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
Pleuroceras
Pleuroceras is a genus of fungi in the family Gnomoniaceae. The genus was first described by H. Riess in 1854. Several species in the genus are plant pathogens. The genus contains 12 species.- Species :*P. arollanum*P. bottnicum...
, Pecten aequivalvis
Pecten
Pecten is Latin for "honeycomb." Its genitive form is pectinis, which yields the root pectin-.Pecten may refer to:*Pecten , any honeycomb structure in animals...
). 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
Toarcian
The Toarcian is, in the ICS' geologic timescale, an age or stage in the Early or Lower Jurassic. It spans the time between 183.0 Ma and 175.6 Ma...
without any detrital deposits at its base, the sediments being black ammonite-bearing marls (with Harpoceras falciferum and Hildoceras bifrons
Hildoceras
Hildoceras is a genus of ammonite form the upper Lower Jurassic in the ammonitid family Hildoceratidae characterized by a narrow discoidal evolute shell, keeled venter, concave ribs along the outer flanks, and a shallow spiral goove running along smooth inner flanks. Whorls slightly overlap,...
). Towards the end of the Toarcian and the beginning of the Aalenian
Aalenian
The Aalenian is a subdivision of the Middle Jurassic epoch/series of the geologic timescale that extends from about 175.6 Ma to about 171.6 Ma . It was preceded by the Toarcian and succeeded by the Bajocian.-Stratigraphic definitions:...
, the sediments turn into sandy limestones indicating another regression. Interlayered with these sandy limestones are oyster beds, iron oolite and gypsum
Gypsum
Gypsum is a very soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O. It is found in alabaster, a decorative stone used in Ancient Egypt. It is the second softest mineral on the Mohs Hardness Scale...
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
Anhydrite
Anhydrite is a mineral – anhydrous calcium sulfate, CaSO4. It is in the orthorhombic crystal system, with three directions of perfect cleavage parallel to the three planes of symmetry. It is not isomorphous with the orthorhombic barium and strontium sulfates, as might be expected from the...
) begun in the Triassic carries on right through the Lias; it reaches a thickness of up to 500 m.
Dogger
The DoggerDogger
Dogger can mean:-*A sea area in the North Sea, noted in shipping forecasts, named after:**The Dogger Bank, a large shallow area in the North Sea between Britain and Denmark...
attains a maximum thickness of about 300 m along a north-south-trending zone running from Angoulême to Tarbes
Tarbes
Tarbes is a commune in the Hautes-Pyrénées department in south-western France.It is part of the historical region of Gascony. It is the second largest metropolitan area of Midi-Pyrénées, with 110,000 inhabitants....
. Along this zone reef
Reef
In nautical terminology, a reef is a rock, sandbar, or other feature lying beneath the surface of the water ....
s 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
Lignite
Lignite, often referred to as brown coal, or Rosebud coal by Northern Pacific Railroad,is a soft brown fuel with characteristics that put it somewhere between coal and peat...
-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
Bajocian
In the geologic timescale, the Bajocian is an age or stage in the Middle Jurassic. It lasted from approximately 171.6 Ma to around 167.7 Ma . The Bajocian age succeeds the Aalenian age and precedes the Bathonian age....
with dolomites. In places, Aalenian is reworked. The Bathonian
Bathonian
In the geologic timescale the Bathonian is an age or stage of the Middle Jurassic. It lasted from approximately 167.7 Ma to around 164.7 Ma...
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, breccia
Breccia
Breccia is a rock composed of broken fragments of minerals or rock cemented together by a fine-grained matrix, that can be either similar to or different from the composition of the fragments....
s, 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
Callovian
In the geologic timescale, the Callovian is an age or stage in the Middle Jurassic, lasting between 164.7 ± 4.0 Ma and 161.2 ± 4.0 Ma. It is the last stage of the Middle Jurassic, following the Bathonian and preceding the Oxfordian....
with littoral border-facies deposits.
Malm
The facies dividing reef-zone persists into the MalmMalm
is a former municipality, a village, and the administrative centre of the municipality of Verran in Nord-Trøndelag county, Norway. Malm is located along the Trondheimsfjord, about a drive west from the town of Steinkjer. The village of Malm has a population of 1,572. The population density of...
. In the western domain, initially ammonite-bearing marl
Marl
Marl or marlstone is a calcium carbonate or lime-rich mud or mudstone which contains variable amounts of clays and aragonite. Marl was originally an old term loosely applied to a variety of materials, most of which occur as loose, earthy deposits consisting chiefly of an intimate mixture of clay...
s 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
Tithonian
In the geologic timescale the Tithonian is the latest age of the Late Jurassic epoch or the uppermost stage of the Upper Jurassic series. It spans the time between 150.8 ± 4 Ma and 145.5 ± 4 Ma...
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
Périgord
The Périgord is a former province of France, which corresponds roughly to the current Dordogne département, now forming the northern part of the Aquitaine région. It is divided into four regions, the Périgord Noir , the Périgord Blanc , the Périgord Vert and the Périgord Pourpre...
at La Tour-Blanche
La Tour-Blanche
La Tour-Blanche is a commune in the Dordogne department in Aquitaine in southwestern France.-Population:-References:*...
. In the end, the sea withdrew south of the Garonne River.
In the Lower Oxfordian, the first sequence of the Malm seems to follow the Callovian without a distinctive break. Yet cellular limestones and breccias indicate sediment reworking (this was certainly the case in the Grands Causses farther east). During the Middle and the Upper Oxfordian, marine limestones are laid down which incorporate occasional reefs. The Lower Kimmeridgian sediments are sedimented close to the shore, they bear oysters, urchin
Urchin
Urchin or urcheon is the Middle English term for "hedgehog". As such, it is applied to many things that take a similar form to a hedgehog:* Street children, homeless children who live on the street...
s, and ripple marks
Ripple marks
In geology, ripple marks are sedimentary structures and indicate agitation by water or wind.- Defining ripple cross-laminae and asymmetric ripples :...
.
The second sequence of the Malm starts in the Upper Kimmeridgian, only in places does it show regressive traits, nevertheless the sedimentary character changes. Laid down are breccias and the sediments also show synsedimentary reworkings; periodically interbedded limestones and marls carrying lignite horizons begin to form. The sediments can be dated by the ammonites Aulacostephanus
Aulacostephanus
Aulacostephanus is an extinct ammonoid cephalopod genus from the Upper Jurassic Kimmeridgian belonging to the perisphinctacean family Aulacostephanidae....
and Aspidoceras orthocera
Aspidoceras
Aspidoceras is an extinct ammonoid cephalopod genus that lived during the Late Jurassic with a fairly broad distribution...
. This strongly disturbed depositional environment with a coexistence of open marine facies and muds deposited under reducing conditions in a restrictive setting seems to coincide with a first sedimentary individualisation of the Pyrenean realm. The event has received its name Virgulian from the oyster Exogyra virgula
Exogyra
Exogyra is an extinct genus of bivalve mollusks in the family Gryphaeidae, the foam oysters or honeycomb oysters . These bivalves grew cemented by the right valve, the left or upper valve is flatter and the beak is curved to one side. Exogyra lived on solid substrates in warm seas....
. During the Tithonian, the shrinkage of the basin became even more evident, only to end in a nearly complete withdrawal of the sea from the Aquitaine Plateau before the close of the Tithonian (the south is not affected by this). During the Tithonian, iron-bearing calcareous oolites interbedded with marls, as well as dolostones and border facies deposits develop — dated by Gravesia portlandicum.
Lower Cretaceous
In comparison with the Jurassic, the CretaceousCretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
has less pronounced sequences. The Lower Cretaceous sediments are restricted to close to the Pyrenees. Most likely the exchange of ocean water masses was better towards the Tethyan realm than towards the Atlantic.
Sedimentation increased again after a longer hiatus in the Lower Creataceous, but only in two locales — the Parentis Basin and the Adour Basin. Both sub-basins manifest a huge subsidence
Subsidence
Subsidence is the motion of a surface as it shifts downward relative to a datum such as sea-level. The opposite of subsidence is uplift, which results in an increase in elevation...
. During the Lower Cretaceous the Parentis Basin received 2,000 m of sediment and the Adour Basin 4,000 m. The remainder of the Aquitaine Basin is meanwhile subjected to strong erosion.
The first deposits in the two sub-basins were littoral sediments in Wealden facies, mainly sandstones and shales. During the Barremian
Barremian
The Barremian is an age in the geologic timescale between 130.0 ± 1.5 Ma and 125.0 ± 1.0 Ma). It is a subdivision of the Early Cretaceous epoch...
, marine shallow-water carbonates were precipitated, changing to detritic sediments in the northern Parentis Basin. Near Lacq
Lacq
Lacq is a commune in the Pyrénées-Atlantiques department in south-western France.It lies just northwest of the local capital of Pau.-Economy:...
, they change to lagoonal anhydrites. In the Upper Aptian
Aptian
The Aptian is an age in the geologic timescale or a stage in the stratigraphic column. It is a subdivision of the Early or Lower Cretaceous epoch or series and encompasses the time from 125.0 ± 1.0 Ma to 112.0 ± 1.0 Ma , approximately...
, the reef-forming Urgonian facies became established in both sub-basins — fossiliferous limestones composed of algae
Algae
Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...
, coralline polyps, and rudists. The Urgonian facies completely surrounds the Parentis Basin and persists into the Albian.
Since the onset of the Albian
Albian
The Albian is both an age of the geologic timescale and a stage in the stratigraphic column. It is the youngest or uppermost subdivision of the Early/Lower Cretaceous epoch/series. Its approximate time range is 112.0 ± 1.0 Ma to 99.6 ± 0.9 Ma...
, strong halokinetic movements affect the southern Aquitaine Basin and in turn profoundly influence sedimentation patterns. As a result breccias, thick conglomerates, and turbidite
Turbidite
Turbidite geological formations have their origins in turbidity current deposits, which are deposits from a form of underwater avalanche that are responsible for distributing vast amounts of clastic sediment into the deep ocean.-The ideal turbidite sequence:...
s are shed. In the Parentis Basin, a distinct unconformity
Unconformity
An unconformity is a buried erosion surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval of time before deposition of the younger, but the term is used to describe...
develops. At the same time, the sediments on the Aquitaine Plateau farther north are folded into gentle wavetrains following the Hercynian strike (northwest-southeast). All these movements are correlated with the first tectonic stirrings in the Western Pyrenees. Towards the end of the Albian, the sea level is rising and the Urgonian calcareous reefs are consequently draped by muds.
Upper Cretaceous
The transgression that began in the late Albian spread rapidly northward during the CenomanianCenomanian
The Cenomanian is, in the ICS' geological timescale the oldest or earliest age of the Late Cretaceous epoch or the lowest stage of the Upper Cretaceous series. An age is a unit of geochronology: it is a unit of time; the stage is a unit in the stratigraphic column deposited during the corresponding...
. In the northern part of the Aquitanian Basin, the Cenomanian sea reclaimed nearly the same areas that had been occupied by the Jurassic sea; in the east, however, it only reached the line Brive-Cahors
Cahors
Cahors is the capital of the Lot department in south-western France.Its site is dramatic being contained on three sides within an udder shaped twist in the river Lot known as a 'presqu'île' or peninsula...
-Agen-Muret
Muret
Muret is a commune in the Haute-Garonne department in southwestern France.It is an outer suburb of the city of Toulouse, even though it does not belong to Greater Toulouse, which it has declined to join...
-Carcassonne. The region of the later formed North Pyrenean Thrust is a decisive facies boundary at this time: to the north, shelf sedimentation continued but to the south rapidly subsiding basins developed into which flysch
Flysch
Flysch is a sequence of sedimentary rocks that is deposited in a deep marine facies in the foreland basin of a developing orogen. Flysch is typically deposited during an early stage of the orogenesis. When the orogen evolves the foreland basin becomes shallower and molasse is deposited on top of...
sediments (and partially also wildflysch breccias) from the Pyrenean realm were shed. Near Saint-Gaudens
Saint-Gaudens
Saint-Gaudens is a commune and a sub-prefecture of the Haute-Garonne department in southwestern France.Saint-Gaudens lies at an altitude of 405 m on a ledge overlooking the valley of the Garonne...
, the flysch sediments are even accompanied by volcanic rocks — trachyte
Trachyte
Trachyte is an igneous volcanic rock with an aphanitic to porphyritic texture. The mineral assemblage consists of essential alkali feldspar; relatively minor plagioclase and quartz or a feldspathoid such as nepheline may also be present....
s, and ultrabasic lavas. The sedimentation in the flysch basins during the Turonian
Turonian
The Turonian is, in the ICS' geologic timescale, the second age in the Late Cretaceous epoch, or a stage in the Upper Cretaceous series. It spans the time between 93.5 ± 0.8 Ma and 89.3 ± 1 Ma...
and during the Coniacian
Coniacian
The Coniacian is an age or stage in the geologic timescale. It is a subdivision of the Late Cretaceous epoch or Upper Cretaceous series and spans the time between 89.3 ± 1 Ma and 85.8 ± 0.7 Ma...
is very unsettled. The flysch sedimentation then continues right through the Upper Cretaceous, mainly interbedded sandstones and shales with some carbonaceous layers were laid down. Towards the end of the Upper Cretaceous, there are signs of the start of a regression and the sea then actually retreats before the K/T boundary. In the Subpyrenean Basin near the Petits Pyrénées, the sea lingers on till the lowermost Paleocene
Paleocene
The Paleocene or Palaeocene, the "early recent", is a geologic epoch that lasted from about . It is the first epoch of the Palaeogene Period in the modern Cenozoic Era...
(Danian
Danian
The Danian is the oldest age or lowermost stage of the Paleocene epoch or series, the Paleogene period or system and the Cenozoic era or erathem. The beginning of the Danian age is at the Cretaceous-Tertiary extinction event...
).
In the remainder of the Aquitaine Basin, mainly pelagic limestones (chalk
Chalk
Chalk is a soft, white, porous sedimentary rock, a form of limestone composed of the mineral calcite. Calcite is calcium carbonate or CaCO3. It forms under reasonably deep marine conditions from the gradual accumulation of minute calcite plates shed from micro-organisms called coccolithophores....
facies) are sedimented during the Upper Cretaceous, including the type localities for the Coniacian
Coniacian
The Coniacian is an age or stage in the geologic timescale. It is a subdivision of the Late Cretaceous epoch or Upper Cretaceous series and spans the time between 89.3 ± 1 Ma and 85.8 ± 0.7 Ma...
, Santonian
Santonian
The Santonian is an age in the geologic timescale or a chronostratigraphic stage. It is a subdivision of the Late Cretaceous epoch or Upper Cretaceous series. It spans the time between 85.8 ± 0.7 mya and 83.5 ± 0.7 mya...
, and Campanian
Campanian
The Campanian is, in the ICS' geologic timescale, the fifth of six ages of the Late Cretaceous epoch . The Campanian spans the time from 83.5 ± 0.7 Ma to 70.6 ± 0.6 Ma ...
in the Charente.
At the northern edge of the basin, more differentiated coastal facies develop. In the north, the Cenomanian is made up of three sedimentary cycles (from young to old):
- An upper cycle with regressive tendencies. In the northwest, sandy rudist-bearing limestones and oyster-bearing marly shales were deposited; in the northeast, very shallow marine gypsum-bearing shales and sands.
- A generally deeper marine middle-cycle with marls. These sediments spread into the Quercy. In the Périgord, littoral facies and lignites accumulated near paleohighs.
- A shallow marine lower-cycle with rudist reefs in the northwest and continent-derived lignites in the northeast.
The Turonian reflects a transgressive period with the sea spreading into the Lot. At this point, the Upper Cretaceous sea had reached its highstand. This also coincides with a climatic optimum with global average sea-water temperatures around 24°C compared to today's 13°C. The Turonian can be subdivided into two parts:
- The so-called Angoumian (named after AngoulêmeAngoulême-Main sights:In place of its ancient fortifications, Angoulême is encircled by boulevards above the old city walls, known as the Remparts, from which fine views may be obtained in all directions. Within the town the streets are often narrow. Apart from the cathedral and the hôtel de ville, the...
) on the top. It consists of massive, partially brecciated rudist limestones at its base followed by ocre calcareous sands. The resistant Angoumian formed extensive cliffs. - The so-called Ligerian (Latin name for the Massif Central) at the base — wavy chalky marls.
Towards the end of the Turonian, the Massif Central experienced uplift
Tectonic uplift
Tectonic uplift is a geological process most often caused by plate tectonics which increases elevation. The opposite of uplift is subsidence, which results in a decrease in elevation. Uplift may be orogenic or isostatic.-Orogenic uplift:...
which is reflected in the sediments of the northeastern Aquitaine Basin as a strong input of detritus, mainly sands in the upper part of the Angoumian.
The Coniacian and the Santonian are expressed as typical chalky limestones in the north, but both stages take on a more sandy character east of Périgueux.
The Campanian
Campanian
The Campanian is, in the ICS' geologic timescale, the fifth of six ages of the Late Cretaceous epoch . The Campanian spans the time from 83.5 ± 0.7 Ma to 70.6 ± 0.6 Ma ...
follows after a pronounced unconformity. The southern flysch basins began to expand northward. Near Pau before the onset of the flysch sedimentation, a very strong erosion removed the entire Lower Cretaceous, the entire Jurassic and sometimes even cut right down to the basement. North of Pau, the Campanian is a marly facies called Aturian. In the northern Aquitaine Basin, the sediments become more homogenised and settle out as fully marine flint
Flint
Flint is a hard, sedimentary cryptocrystalline form of the mineral quartz, categorized as a variety of chert. It occurs chiefly as nodules and masses in sedimentary rocks, such as chalks and limestones. Inside the nodule, flint is usually dark grey, black, green, white, or brown in colour, and...
-bearing calcareous micrite
Micrite
Micrite is a limestone constituent formed of calcareous particles ranging in diameter up to 4 μm formed by the recrystallization of lime mud.The term was coined in 1959 by Robert Folk for his carbonate rock classification system...
s.
During the Maastrichtian
Maastrichtian
The Maastrichtian is, in the ICS' geologic timescale, the latest age or upper stage of the Late Cretaceous epoch or Upper Cretaceous series, the Cretaceous period or system, and of the Mesozoic era or erathem. It spanned from 70.6 ± 0.6 Ma to 65.5 ± 0.3 Ma...
, a regression commences. After the initial deposition of bioclastic rudist-bearing limestones and the formation of some reef complexes composed of rudists and single corals, the sea level started dropping. Northern Aquitaine became emersed and the sea withdrew in stages southward to the line Arcachon-Toulouse. At the same time, the northern edge of the basin experienced another folding episode with low-amplitude folds striking northwest-southeast.
Paleogene
During the PaleocenePaleocene
The Paleocene or Palaeocene, the "early recent", is a geologic epoch that lasted from about . It is the first epoch of the Palaeogene Period in the modern Cenozoic Era...
, the coastline roughly followed the line Arcachon-Toulouse. In the North Aquitaine Zone north of this line, the sediments possess continental character — red mudstones, sands, and lacustrine limestones. The sea made a short-lived advance into this domain and left echinid
Echinidae
Echinidae is a family of sea urchins in the order Echinoida. Members of the family are found in the Atlantic Ocean and the Antarctic.-Characteristics:...
-bearing limestones behind. In the Central Aquitaine Zone (northern half of the southern basin), a shelf built out to the line Audignon
Audignon
Audignon is a commune of the Landes department in Aquitaine in south-western France.-References:*...
-Carcassonne. Farther south in the South Aquitaine Zone, deep water conditions prevailed in the west, shallowing out towards the east. The sediments in the Aturian Gulf (Golfe Aturién) in the west are pelagic limestones containing globigerinids
Globigerinida
The Globigerinida are a common group of foraminiferans that are found as marine plankton . They produce hyaline calcareous tests, and are known as fossils from the Jurassic period onwards. The group has included more than 100 genera and over 400 species, of which about 30 species are extant...
, operculinids, and alveolinids
Alveolinidae
The Alveolinidae is a family of spheroidal to fusiform milioline foraminfera with multiple apertures and complex interiors in which chambers are subdivided into chamberlets and subfloors interconnected by passageways. As with all Miliolina, the test wall in alveolinids is porecelaneous and...
. Near the Petits Pyrénées, the sediments change into shallow-water facies rich in madreporians, echinids, and operculinids. Farther east in the Ariège
Ariège
Ariège is a department in southwestern France named after the Ariège River.- History :Ariège is one of the original 83 departments created during the French Revolution on 4 March 1790. It was created from the counties of Foix and Couserans....
and in the Corbières
Corbières
Corbières is a municipality in the district of Gruyère in the canton of Fribourg in Switzerland. On 1 January 2011 the former municipality of Villarvolard merged into the municipality of Corbières.-History:...
, the sediments become totally continental and lacustrine.
In the Lower Eocene
Eocene
The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...
(Ypresian
Ypresian
In the geologic timescale the Ypresian is the oldest age or lowest stratigraphic stage of the Eocene. It spans the time between and , is preceded by the Thanetian age and is followed by the Eocene Lutetian age....
), another transgressive period saw the sea advance north into the Médoc
Médoc
The Médoc is a region of France, well known as a wine growing region, located in the département of Gironde, on the left bank of the Gironde estuary, north of Bordeaux. Its name comes from Medullicus, or "country of the Medulli", the local Celtic tribe...
and south of Oléron
Oléron
Île d'Oléron is an island off the Atlantic coast of France , on the southern side of the Pertuis d'Antioche strait....
; in the southeast it even reached the Montagne Noire. In the Aturian Gulf, Globorotalia-bearing marls were deposited, while farther east turritella
Turritella
Turritella is a genus of medium-sized sea snails with an operculum, marine gastropod mollusks in the family Turritellidae. They have tightly coiled shells, whose overall shape is basically that of an elongated cone....
-rich marls and limestones were formed. The newly inundated areas receive sands and limestones rich in alveolinids and nummulite
Nummulite
thumb|left|250px|Fossil Nummulites in [[Urbasa]], [[Basque Country |Basque Country]]A nummulite is a large lenticular fossil, characterized by its numerous coils, subdivided by septa into chambers. They are the shells of the fossil and present-day marine protozoan Nummulites, a type of foraminiferan...
s. Meanwhile iron-rich sands (in the Charente) and molasse
Molasse
The term "molasse" refers to the sandstones, shales and conglomerates formed as terrestrial or shallow marine deposits in front of rising mountain chains. The molasse is deposited in a foreland basin, especially on top of flysch, for example that left from the rising Alps, or erosion in the Himalaya...
s (in the Libournais and in the Agenais
Agenais
Agenais, or Agenois, was a province of France located in southwest France south of Périgord.In ancient Gaul the region was the country of the Nitiobroges with Aginnum for their capital, which in the fourth century was the Civitas Agennensium, which was a part of Aquitania Secunda and which formed...
) were sedimented in the continental north and northeast. The provenance area of these continental deposits up to Middle Ypresian times was mainly the Massif Central.
The sea-level kept rising during the Middle Eocene (Lutetian
Lutetian
The Lutetian is, in the geologic timescale, a stage or age in the Eocene. It spans the time between and . The Lutetian is preceded by the Ypresian and is followed by the Bartonian. Together with the Bartonian it is sometimes referred to as the Middle Eocene subepoch...
and Bartonian
Bartonian
The Bartonian is, in the ICS's geological timescale, a stage or age in the middle Eocene epoch or series. The Bartonian age spans the time between and . It is preceded by the Lutetian and is followed by the Priabonian age.-Stratigraphic definition:...
). The area covered by alveolinid- and nummulite-bearing limestones increased, northward to Blaye
Blaye
Blaye is a commune and subprefecture in the Gironde department in Aquitaine in southwestern France.-Population:Its inhabitants are called Blayais or the Blayaises.-Geography:...
and Saint-Palais
Saint-Palais
Saint-Palais is the name or part of the name of several communes in France:* Saint-Palais, Allier, in the Allier département* Saint-Palais, Cher, in the Cher département* Saint-Palais, Gironde, in the Gironde département...
and eastward into the Agenais. The Subpyrenean Basin deepened and was simultaneously being filled by conglomerates brought in from the east, the so-called Poudingues de Palassou. This marked the beginning of uplift in the Pyrenean orogen and a switch-over in detritus provenance from the Massif Central in the north to the Pyrenees in the south. Coalescing alluvial fan
Alluvial fan
An alluvial fan is a fan-shaped deposit formed where a fast flowing stream flattens, slows, and spreads typically at the exit of a canyon onto a flatter plain. A convergence of neighboring alluvial fans into a single apron of deposits against a slope is called a bajada, or compound alluvial...
s built out north into the Castrais. On the northern flank of the fans, lakes formed, precipitating lacustrine limestones. The detrital sediments with provenance from the meanwhile strongly eroded Massif Central (muds, sands, gravels) then affected only a small fringe zone in the northeast. In the Périgord and in the Quercy, the Sidérolithique accumulated — iron-rich sediments that resemble laterite
Laterite
Laterites are soil types rich in iron and aluminium, formed in hot and wet tropical areas. Nearly all laterites are rusty-red because of iron oxides. They develop by intensive and long-lasting weathering of the underlying parent rock...
s indicating a subtropical climate.
During the Upper Eocene (Priabonian
Priabonian
The Priabonian is, in the ICS's geologic timescale, the latest age or the upper stage of the Eocene epoch or series. It spans the time between and...
), a regression set in. The Subpyrenean Basin became completely filled with the erosional debris of the rising Pyrenees. In the Médoc, nummulite-bearing marls and limestones were still being laid down, but east of Bordeaux already continental molasses appeared that change farther south into gypsum-bearing formations.
During the Lower Oligocene
Oligocene
The Oligocene is a geologic epoch of the Paleogene Period and extends from about 34 million to 23 million years before the present . As with other older geologic periods, the rock beds that define the period are well identified but the exact dates of the start and end of the period are slightly...
(Rupelian
Rupelian
The Rupelian is, in the geologic timescale, the older of two ages or the lower of two stages of the Oligocene epoch/series. It spans the time between and . It is preceded by the Priabonian stage and is followed by the Chattian stage....
), a permanently marine environment persists in the south with marls and sands rich in nummulites, lamellibranchs, and echinids. The anomiid
Anomiidae
Anomiidae is a family of bivalve molluscs related to scallops and oysters, and known as anomiids. It contains seven genera.The family is known by several common names, including jingle shells, saddle oysters and mermaid's toenails....
-bearing limestones of the southern Médoc are lagoonal deposits. After a short-lived advance at the beginning of the Chattian
Chattian
The Chattian is, in the geologic timescale, the youngest of two ages or upper of two stages of the Oligocene epoch/series. It spans the time between and . The Chattian is preceded by the Rupelian and is followed by the Aquitanian .-Stratigraphic definition:The Chattian was introduced by Austrian...
with seastar
SeaStar
SeaStar may refer to any of the following aircraft designs .* the AAC SeaStar, a two-seat biplane from Canada* the Dornier Seawings Seastar, an amphibious aircraft with two engines in a push-pull configuration...
-bearing limestones in the northern Médoc and in the Libournais and with mammal-bearing molasses in the Agenais, the sea made a big retreat at the end of the Oligocene. This retreat was accompanied by tectonic movements creating trains of deeper-seated anticlines in the central and northern Aquitaine Basin. The debris-carrying alluvial fans issuing from the rising Pyrenees reached into the Agenais
Agenais
Agenais, or Agenois, was a province of France located in southwest France south of Périgord.In ancient Gaul the region was the country of the Nitiobroges with Aginnum for their capital, which in the fourth century was the Civitas Agennensium, which was a part of Aquitania Secunda and which formed...
and attained their largest extent. They pushed the surrounding belt of lakes ahead of them (in northerly directions) thereby spreading lacustrine limestones well into the Quercy, onto the Causses
Causses
The Causses are a group of limestone plateaus in the Massif Central. They are bordered to the north-west by the Limousin and the Périgord uplands, and to the east by the Aubrac and the Cévennes. Large river gorges cut through the plateaus, such as the Tarn, Dourbie, Jonte, Lot and Aveyron...
, and even onto the Massif Central.
Neogene
Following its retreat in the southwestern Landes, the sea began transgressing towards the north and the east during the Lower MioceneMiocene
The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...
(Aquitanian). Marine, littoral, and lacustrine facies interchange. During a minor regression, a huge lake formed near Condom
Condom
A condom is a barrier device most commonly used during sexual intercourse to reduce the probability of pregnancy and spreading sexually transmitted diseases . It is put on a man's erect penis and physically blocks ejaculated semen from entering the body of a sexual partner...
, the Lac de Saucats, in which grey lacustrine-limestones precipitated, the so-called Calcaire gris de l'Agenais. Shortly thereafter the sea attained its highstand. It was rimmed completely by continental deposits whose thickness increased towards the southeast. For the first time, the alluvial fans along the Pyrenean front receded, the reason being increased subsidence in front of the orogen; yet they still stretched as far north as the Agenais.
The retreat of the alluvial fans also continued during the Middle Miocene (Langhian
Langhian
The Langhian is, in the ICS geologic timescale, an age or stage in the middle Miocene epoch/series. It spans the time between 15.97 ± 0.05 Ma and 13.65 ± 0.05 Ma . The Langhian was a continuing warming period defined by Lorenzo Pareto in 1864, it was originally established in the Langhe area north...
and Serravallian
Serravallian
The Serravallian is in the geologic timescale an age or a stage in the middle Miocene epoch/series, that spans the time between 13.65 ± 0.05 Ma and 11.608 ± 0.005 Ma...
). Consequently the lacustrine band reached as far south as the Armagnac
Armagnac (region)
The hilly countship of Armagnac , in the foothills of the Pyrenées between the Adour and Garonne rivers, is a historic countship of the Duchy of Gascony, established in 601 in Aquitaine...
.
The Upper Miocene (Tortonian
Tortonian
The Tortonian is in the geologic timescale an age or stage of the late Miocene that spans the time between 11.608 ± 0.005 Ma and 7.246 ± 0.005 Ma . It follows the Serravallian and is followed by the Messinian....
and Messinian
Messinian
The Messinian is in the geologic timescale the last age or uppermost stage of the Miocene. It spans the time between 7.246 ± 0.005 Ma and 5.332 ± 0.005 Ma...
) witnessed a drastic withdrawal of the sea to the west. This process started first in the Bordelais
Bordelais
Bordelais is a French term meaning "of Bordeaux" and can refer to* an inhabitant of the city Bordeaux* the area surrounding the city Bordeaux* an inhabitant of the commune Les Bordes-sur-Lez* Bordeaux wine, or the Bordeaux wine region...
and in the Bazadais
Bazadais
The Bazadais is a French breed of beef cattle.-History:This breed comes from ancient cattle bred in Bazas, a little town in the south of Bordeaux. It is probably related to Limousin or Blonde d'Aquitaine.-Morphology:...
, ending with a nearly complete withdrawal from the basin. In areas left behind by the sea in the Armagnac, unfossiliferous sands and muds were deposited. At the same time in the north and in the east, today's river network draining the Massif Central was already being beginning to form.
During the Pliocene
Pliocene
The Pliocene Epoch is the period in the geologic timescale that extends from 5.332 million to 2.588 million years before present. It is the second and youngest epoch of the Neogene Period in the Cenozoic Era. The Pliocene follows the Miocene Epoch and is followed by the Pleistocene Epoch...
(Zanclean
Zanclean
The Zanclean is the lowest stage or earliest age on the geologic time scale of the Pliocene. It spans the time between 5.332 and 3.6 Ma ± 0.005 Ma . It is preceded by the Messinian age of the Miocene epoch, and followed by the Piacenzian age....
), the sea occupied merely a small strip near the Arcachon Basin south of Soustons
Soustons
Soustons is a commune in the Landes department in Aquitaine in south-western France.Soustons hosts one of the LORAN-C transmitters....
. Sandy shales very rich in a benthic microfauna were deposited. In the rest of the Aquitaine Basin, continental sands were laid down, the so-called Sables fauves. The alluvial fans restricted their activity to the immediate vicinity of the Pyrenean mountain front and created the alluvial fans of Ger, Orignac
Orignac
Orignac is a commune in the Hautes-Pyrénées department in south-western France.-References:*...
-Cieutat
Cieutat
Cieutat is a commune in the Hautes-Pyrénées department in south-western France.Its name comes from the Latin word civitas because in the Early Middle Ages it was the capital of the former French province of Bigorre.-References:*...
, and Lannemezan
Lannemezan
Lannemezan is a commune in the Hautes-Pyrénées department in south-western France.-Geography:The Petite Baïse has its source in the commune.Lannemezan is around 30 km east of Tarbes, and around 100 km south-west of Toulouse.-References:*...
. The drainage system of the Garonne already resembled more or less today's pattern, the river avoiding the Miocene gravel accumulations as much as possible and then following between Toulouse, Agen and Bordeaux a weekly subsiding graben.
The progressive landfall of the Aquitaine Basin proceeded from the northeast and was coupled with an important subaerial erosion. As a consequence several peneplanations were carved out from the detrital alluvial plains:
- an eocene peneplain.
- an aquitanian, strongly silicified peneplain. This is very well developed in the Périgord, in the Agenais, and in the Quercy.
- a pliocene (zanclean) peneplain, characterised by gravel-bearing clays in the Bordelais and in the Landes.
On the pliocene peneplain, today's drainage system was firmly established.
Quaternary
Pleistocene
The Pleistocene is the epoch from 2,588,000 to 11,700 years BP that spans the world's recent period of repeated glaciations. The name pleistocene is derived from the Greek and ....
ice age
Ice age
An ice age or, more precisely, glacial age, is a generic geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental ice sheets, polar ice sheets and alpine glaciers...
s — Mindel
Mindel
The Mindel is a river in Bavaria, southern Germany. The Mindel originates west of Kaufbeuren, in the Allgäu region, and flows generally north. It flows into the Danube in Gundremmingen, east of Günzburg. The towns Mindelheim, Burgau and Thannhausen lie along the Mindel.The Mindel gave its name to...
, Riss, and Würm
Würm
The Würm is a river in Bavaria, Germany, right tributary of the Amper. It drains the overflow from Lake Starnberg and flows swiftly through the villages of Gauting, Krailling, Planegg, Gräfelfing and Lochham as well as part of Munich before joining, near Dachau, the Amper, which soon afterwards...
— are also documented in the Aquitaine Basin, mainly by different levels of river terasses. Additionally amongst glacial phenomena the following can be cited:
- cave infills. These are very important for dating archeological finds.
- aeolian deposits. They cover more than a third of the Aquitaine region and can be found mainly in the Médoc and in the Landes. They were deposited during the last two cold stages of the Würm glaciation. The dune beltDuneIn physical geography, a dune is a hill of sand built by wind. Dunes occur in different forms and sizes, formed by interaction with the wind. Most kinds of dunes are longer on the windward side where the sand is pushed up the dune and have a shorter "slip face" in the lee of the wind...
parallel to the Atlantic shoreline formed during the HoloceneHoloceneThe Holocene is a geological epoch which began at the end of the Pleistocene and continues to the present. The Holocene is part of the Quaternary period. Its name comes from the Greek words and , meaning "entirely recent"...
. It contains Europe'sEuropeEurope is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
largest dune, The Great Dune of PylaThe Great Dune of PylaThe Great Dune of Pyla is the tallest sand dune in Europe. It is located in La Teste-de-Buch in the Arcachon Bay area, France, 60 km from Bordeaux....
. - colluvium masking hillsides and hilltops.
- creeping cryoclastic debris.
The development of the Gironde
Gironde
For the Revolutionary party, see Girondists.Gironde is a common name for the Gironde estuary, where the mouths of the Garonne and Dordogne rivers merge, and for a department in the Aquitaine region situated in southwest France.-History:...
estuary goes back about 20,000 years into the late Würm.
Finally, the rich prehistoric
Prehistory
Prehistory is the span of time before recorded history. Prehistory can refer to the period of human existence before the availability of those written records with which recorded history begins. More broadly, it refers to all the time preceding human existence and the invention of writing...
finds and their sites in the Aquitaine Basin merit mentioning, especially in the Département Dordogne
Dordogne
Dordogne is a départment in south-west France. The départment is located in the region of Aquitaine, between the Loire valley and the High Pyrénées named after the great river Dordogne that runs through it...
.
Structural organisation and tectonics
Structurally the Aquitaine Basin can be divided into two provinces separated by a prominent fault zone, the so-called North Aquitaine Flexure. This fault zone extends from Arcachon to Carcassonne and represents the continuation of the continental slope onshore.The Northern Province or Aquitaine Plateau forms a typical continental shelf region with reduced sedimentation and several periods of emersion (during the entire Lower Cretaceous and during parts of the Upper Cretaceous and the Cenozoic). The basement is rarely encountered deeper than 2,000 m. Triassic and Jurassic together have an accumulated sediment thickness of 1,000–1,700 m. The Lower Cretaceous is completely missing and the Upper Cretaceous only reaches a thickness of several hundred metres. The Paleogene
Paleogene
The Paleogene is a geologic period and system that began 65.5 ± 0.3 and ended 23.03 ± 0.05 million years ago and comprises the first part of the Cenozoic Era...
is, if present, very thin in the north but increases in thickness towards the south where it is overlain by thin Neogene
Neogene
The Neogene is a geologic period and system in the International Commission on Stratigraphy Geologic Timescale starting 23.03 ± 0.05 million years ago and ending 2.588 million years ago...
.
In the eastern section, one can discern several low-amplitude structures that are parallel to the Pyrenees farther south and strike westnorthwest-eastsoutheast:
- Quercy synform.
- Tarn-et-GaronneTarn-et-GaronneTarn-et-Garonne is a French department in the southwest of France. It is traversed by the Rivers Tarn and Garonne, from which it takes its name.-History:...
antiform. - CastresCastresCastres is a commune, and arrondissement capital in the Tarn department and Midi-Pyrénées region in southern France. It lies in the former French province of Languedoc....
graben structure. - Toulouse antiform.
Generally the Northern Province is characterised by rather simple structures (syn- and antiforms, low-amplitude fold trains, faults) that follow hercynian, armorican and variscan strike directions. The structures were formed during several tectonic phases:
- Jurassic phase. The resulting structures are mainly of synsedimentary origin and follow variscan strike directions. They profoundly influenced facies distributions and transgressive style during the Upper Cretaceous.
- Late Campanian-Maastrichtian phase. This phase enhanced the structures already formed in the Jurassic phase. The following anticlinal ridges, which run more or less parallel to the northeastern basin margin and can be traced for more than 200 km, were being generated (from north to south):
- The Mareuil-Meyssac Anticline. This structure is an asymmetric anticline near Mareuil, becoming a high-offset normal fault between TerrassonTerrasson-LavilledieuTerrasson-Lavilledieu is a commune in the Dordogne department in Aquitaine in southwestern France.The Gardens of the Imagination —classified as a remarkable garden by the French Ministry of Culture—are situated in Terrasson...
and Meyssac. - The Périgueux anticline. This structure can be followed from CognacCognacCognac is a commune in the Charente department in southwestern France. It is a sub-prefecture of the department.-Geography:Cognac is situated on the river Charente between the towns of Angoulême and Saintes. The majority of the town has been built on the river's left bank, with the smaller right...
via La Tour-Blanche to Périgueux and Saint-CyprienSaint-CyprienSaint-Cyprien is the name or part of the name of several places. Most of them are named after Cyprian:-Canada:* Saint-Cyprien, Bas-Saint-Laurent, Quebec, municipality in Bas-Saint-Laurent, Quebec...
. It forms a typical anticline near La Tour-Blanche. Near Saint-Cyprien, it is mainly a normal fault. - The Oléron-JonzacJonzacJonzac is a commune of the Charente-Maritime department in southwestern France.-Population:-References:*...
-RibéracRibéracRibérac is a commune in the Dordogne department in Aquitaine in southwestern France. The commune is situated by the Dronne River.-History:In 1793, the commune of Faye joined with Ribérac. In 1851, a part of the commune was dismembered for the creation of the new commune of Saint-Martin-de-Ribérac...
-Sauveterre-la-LemanceSauveterre-la-LémanceSauveterre-la-Lémance is a commune in the Lot-et-Garonne department in south-western France.-See also:*Communes of the Lot-et-Garonne department...
anticline. This structure is an anticline near Jonzac and Sauveterre.
Between the anticlines are synformsSynclineIn structural geology, a syncline is a fold, with younger layers closer to the center of the structure. A synclinorium is a large syncline with superimposed smaller folds. Synclines are typically a downward fold, termed a synformal syncline In structural geology, a syncline is a fold, with younger...
near SaintesSaintesSaintes is a French commune located in Poitou-Charentes, in the southwestern Charente-Maritime department of which it is a sub-prefecture. Its inhabitants are called Saintaises and Saintais....
and Sarlat.
- The Mareuil-Meyssac Anticline. This structure is an asymmetric anticline near Mareuil, becoming a high-offset normal fault between Terrasson
- Eocene-Oligocene phase. More anticlines at a deeper level that are not detectable at the surface were created:
- Northeast-southwest-striking domal upwarps near Listrac, near Blaye and near CouquèquesCouquèquesCouquèques is a commune in the Gironde department in Aquitaine in southwestern France.It is located next to Demott Pond.-Population:-References:*...
. - The northwest-southeast-striking upwarp of Sainte-HélèneSainte-Hélène-France:Sainte-Hélène is the name or part of the name of several communes in France:*Sainte-Hélène, in the Gironde department*Sainte-Hélène, in the Lozère department*Sainte-Hélène, in the Morbihan department*Sainte-Hélène, in the Saône-et-Loire department...
-CarcansCarcansCarcans is a commune in the Gironde department in Aquitaine in southwestern France.Carcans is a member of the Community of Municipalities Médoc's Lakes which includes the municipalities of Lacanau, Carcans and Hourtin : the territory Médoc Océan : http://www.medococean.co.uk/-Population:-External...
. - The Bordeaux syncline.
- The east-west-striking anticline of La Teste-Villagrains-LandirasLandirasLandiras is a commune in the Gironde department in Aquitaine in southwestern France.-Population:-References:*...
-Miramont in the GuyenneGuyenneGuyenne or Guienne , , ; Occitan Guiana ) is a vaguely defined historic region of south-western France. The Province of Guyenne, sometimes called the Province of Guyenne and Gascony, was a large province of pre-revolutionary France....
.
- Northeast-southwest-striking domal upwarps near Listrac, near Blaye and near Couquèques
The Southern Province is characterised by the deep Parentis and Adour sub-basins with the Mimizan
Mimizan
Mimizan is a commune in the Landes department in Aquitaine in south-western France. There are two separate districts of the town: Mimizan-Bourg and Mimizan-Plage .-History:...
high ground in between. Compared with the Northern Province its sediments show a pronounced increase in thickness (5,000–11,500 altogether). The Triassic and the Jurassic combined reach 2,000–3,000 , the Lower Cretaceous 500-1,500 m. The Upper Creataceous can vary between 500 and 3,000 m and even the Neogene still attains a thickness of nearly 1,000 m.
The tectonic movements were much more complicated in the Southern Province, having superimposed on them very strong halokinetic motions (salt diapirism). A large part of the formed structures is concealed under plio-quaternary detritus. The many exploration wells drilled for hydrocarbons and groundwater helped immensely in unravelling these structures. Similar to the Northern Province, the major structures are again trains of parallel anticlines whose wavelength steadily diminishes approaching the Pyrenean front. Inversely the effects of the salt movements become stronger towards the south. The anticlines were formed during the uplift of the Pyrenees during the Eocene/Oligocene. The structures were set in place by Miocene times. The following anticlines can be distinguished (from north to south):
- Parentis-BouglonBouglonBouglon is a commune in the Lot-et-Garonne department in south-western France....
-Agen. - Mimizan-Roquefort-CréonCréonCréon is a commune in the Gironde department in Aquitaine in southwestern France.-Population:-References:*...
-Cezans-LavardensLavardensLavardens is a commune in the Gers department in southwestern France.-Population:-References:*...
. - BoosBoos, Seine-MaritimeBoos is a commune in the Seine-Maritime department in the Haute-Normandie region in northern France.-Geography:A farming and light industrial town situated some south of Rouen at the junction of the D6014, D491 and the D138 roads...
-Audignan-NogaroNogaroNogaro is a commune in the Gers department in southwestern France.-Population:-References:*...
. - SaubriguesSaubriguesSaubrigues is a commune in the Landes department in Aquitaine in south-western France.The closest airport to Saubrigues is Biarritz Airport slightly further afield are Pau Airport , Lourdes Airport , or Bordeaux Airport ....
-Biarotte-BastennesBastennesBastennes is a commune in the Landes department in Aquitaine in south-western France....
-GarlinGarlinGarlin is a commune in the Pyrénées-Atlantiques department in south-western France.-References:*...
. - PeyrehoradePeyrehoradePeyrehorade is a commune in the Landes department in Aquitaine in south-western France.-See also:*Communes of the Landes department...
-Sainte-SuzanneSainte-SuzanneSainte-Suzanne may refer to a number of people* Susanna * Susanna * Saint Suzanne, a number of saints.* Saints Tiburtius and Susanna, a saint whose feast day is August 11....
-LacqLacqLacq is a commune in the Pyrénées-Atlantiques department in south-western France.It lies just northwest of the local capital of Pau.-Economy:...
-Pau-Meilhon.
Isostatic
Isostasy
Isostasy is a term used in geology to refer to the state of gravitational equilibrium between the earth's lithosphere and asthenosphere such that the tectonic plates "float" at an elevation which depends on their thickness and density. This concept is invoked to explain how different topographic...
movements during the Plio-Quaternary at the northeastern edge of the Aquitaine basin lead to an uplift and rejuvenation of the peneplained basement in the Massif Central. In the Aquitaine basin itself, these movements follow already existing basement structures and entrain the tilting of some of the pliocene peneplains. This in turn has a strong effect upon the hydrographic network, for example in the drainage basins of the Garonne and Adour, the rivers' courses were changed or entirely abandoned.
Tectonic movements are still at work in the Aquitaine Basin today — strong earthquakes in the Pyrenees (with destroyed villages and churches) and somewhat milder tremors near the island of Oléron keep reminding us of that fact.
Tectono-metamorphic organisation of the basement
According to geophysicalGeophysics
Geophysics is the physics of the Earth and its environment in space; also the study of the Earth using quantitative physical methods. The term geophysics sometimes refers only to the geological applications: Earth's shape; its gravitational and magnetic fields; its internal structure and...
explorations the Variscan basement hidden under the sediments of the Aquitaine Basin can be subdivided into several northwest-southeast-striking tectono-metamorphic zones (from north to south):
- Ligero-arvernian Zone. The zone is limited on the south by the line NiortNiortNiort is a commune in the Deux-Sèvres department in western France.The Latin name of the city was Novioritum.The population of Niort is 60,486 and more than 137,000 people live in the urban area....
-Angoulême-FumelFumelFumel is a commune in the Lot-et-Garonne department in south-western France.It is twinned with Uttoxeter, United Kingdom.In 1438 it was pillaged by Rodrigo de Villandrando....
-MontaubanMontaubanMontauban is a commune in the Tarn-et-Garonne department in the Midi-Pyrénées region in southern France. It is the capital of the department and lies north of Toulouse....
running parallel to the South Armorican Shear Zone farther north. It forms the polymetamorphic core region of the Variscan orogen in Europe. - South Armorican Zone. The zone is bounded on the south by the line La Rochelle-Saintes-ChalaisChalaisChalais may refer to:* Chalais, Switzerland* Chalais, Charente, a commune in France* Chalais, Dordogne, formerly Chaleix, a commune in France* Chalais, Indre, a commune in France* Chalais, Vienne, a commune in France...
and pinches out near BergeracBergerac, DordogneBergerac is a commune and a sub-prefecture of the Dordogne department in southwestern France.-Population:-Economy:The region is primarily known for wine and tobacco...
. It is composed of southward-thrust basement nappes of DevonianDevonianThe Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
/CarboniferousCarboniferousThe Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...
age. - North Aquitaine Zone. Its southern boundary is identical with the South Variscan Thrust Front (and also the North Aquitaine Flexure) and follows the line Arcachon-Agen-Toulouse. It is made up of nappes of the orogen's external zone which were thrust southward during the PennsylvanianPennsylvanianThe Pennsylvanian is, in the ICS geologic timescale, the younger of two subperiods of the Carboniferous Period. It lasted from roughly . As with most other geochronologic units, the rock beds that define the Pennsylvanian are well identified, but the exact date of the start and end are uncertain...
. - Aquitaine block, also called microcontinent Aquitania. It is bounded in the south by the North Pyrenean Thrust Front and is equivalent to the Southern Province. This continental foreland block already belonged to GondwanaGondwanaIn paleogeography, Gondwana , originally Gondwanaland, was the southernmost of two supercontinents that later became parts of the Pangaea supercontinent. It existed from approximately 510 to 180 million years ago . Gondwana is believed to have sutured between ca. 570 and 510 Mya,...
's northern edge.
Depths of the Mohorovicic Discontinuity
The maximum depth of the Mohorovicic DiscontinuityMohorovicic discontinuity
The Mohorovičić discontinuity , usually referred to as the Moho, is the boundary between the Earth's crust and the mantle. Named after the pioneering Croatian seismologist Andrija Mohorovičić, the Moho separates both the oceanic crust and continental crust from underlying mantle...
in the Aquitanian Basin is 36 km, following more or less the path of the Garonne River. Towards the Massif Central in the northeast, the discontinuity flattens out to 30 km. The same holds for approaching the northern edge of the Pyrenees, here the discontinuity is also at 30 km depth. In the oceanic part of the Parentis Basin, it is already encountered at 20 km depth. This implies a significant stretching of the continental crust
Continental crust
The continental crust is the layer of igneous, sedimentary, and metamorphic rocks which form the continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called sial due to more felsic, or granitic, bulk composition, which lies in...
and the beginning of oceanisation. As a comparison, underneath the Central Pyrenees the continental crust is 50 km thick.
Geodynamic setting
For a better understanding of the geological successions and structures in the Aquitaine Basin, it is important to consider the greater geodynamic setting. Two geodynamic developments are of primordial importance for the basin:- The break-up of PangaeaPangaeaPangaea, Pangæa, or Pangea is hypothesized as a supercontinent that existed during the Paleozoic and Mesozoic eras about 250 million years ago, before the component continents were separated into their current configuration....
and the opening history of the Northern Atlantic (and hence the Bay of BiscayBay of BiscayThe Bay of Biscay is a gulf of the northeast Atlantic Ocean located south of the Celtic Sea. It lies along the western coast of France from Brest south to the Spanish border, and the northern coast of Spain west to Cape Ortegal, and is named in English after the province of Biscay, in the Spanish...
). - The movements of the microcontinent Iberia.
In the Upper Triassic (Carnian
Carnian
The Carnian is the lowermost stage of the Upper Triassic series . It lasted from about 228.7 till 216.5 million years ago . The Carnian is preceded by the Ladinian and is followed by the Norian...
) about 230 million years ago, the supercontinent Pangaea started slowly to break up. In the Atlantic domain, the disintegration began in the area of the Central Atlantic. Already in the Lower Jurassic, the initial rifting process had given way to the marine drifting stage. In the Toarcian
Toarcian
The Toarcian is, in the ICS' geologic timescale, an age or stage in the Early or Lower Jurassic. It spans the time between 183.0 Ma and 175.6 Ma...
about 180 million years ago, the Central Atlantic was spreading and North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
, South America
South America
South America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...
and Africa
Africa
Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...
were separating. By Callovian
Callovian
In the geologic timescale, the Callovian is an age or stage in the Middle Jurassic, lasting between 164.7 ± 4.0 Ma and 161.2 ± 4.0 Ma. It is the last stage of the Middle Jurassic, following the Bathonian and preceding the Oxfordian....
times, the Central Atlantic was fully marine. Spreading continued and gradually also began to affect the Northern Atlantic domain. During the Tithonian
Tithonian
In the geologic timescale the Tithonian is the latest age of the Late Jurassic epoch or the uppermost stage of the Upper Jurassic series. It spans the time between 150.8 ± 4 Ma and 145.5 ± 4 Ma...
about 150 million years ago, a rift arm infiltrated along today's continental margin of northwestern France. Consequently Iberia, so far being positioned right below the Armorican Massif (Brittany
Brittany
Brittany is a cultural and administrative region in the north-west of France. Previously a kingdom and then a duchy, Brittany was united to the Kingdom of France in 1532 as a province. Brittany has also been referred to as Less, Lesser or Little Britain...
), was wedged southward. This gave the Atlantic the chance to directly reach the Aquitaine Basin for the first time. In the wake of the southward drifting Iberia during the Lower Cretaceous, the Bay of Biscay opened up. The microcontinent Iberia underwent in addition to its southward drifting motion a counterclockwise rotational movement that eventually brought it in close contact with southern France (reflected in first tectonic movements in the Pyrenees during the Albian
Albian
The Albian is both an age of the geologic timescale and a stage in the stratigraphic column. It is the youngest or uppermost subdivision of the Early/Lower Cretaceous epoch/series. Its approximate time range is 112.0 ± 1.0 Ma to 99.6 ± 0.9 Ma...
; also documented by metamorphism in the Pyrenees dated between 108 and 93 million years ago and by the transgression of the Cenomanian
Cenomanian
The Cenomanian is, in the ICS' geological timescale the oldest or earliest age of the Late Cretaceous epoch or the lowest stage of the Upper Cretaceous series. An age is a unit of geochronology: it is a unit of time; the stage is a unit in the stratigraphic column deposited during the corresponding...
sea). The final collision happened during the Eocene
Eocene
The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...
/Oligocene
Oligocene
The Oligocene is a geologic epoch of the Paleogene Period and extends from about 34 million to 23 million years before the present . As with other older geologic periods, the rock beds that define the period are well identified but the exact dates of the start and end of the period are slightly...
uplifting the mountain chain and subjecting it to severe erosion at the same time. The main phase of uplift ended with the close of the Aquitanian, followed mainly by isostatic movements lasting to this day.
Megasequences
By taking as a reference point the onset of rifting in the Bay of Biscay during the Tithonian the geodynamic evolution of the Aquitaine Basin can be subdivided into four megasequences (somewhat simplified):- Prerift megasequence. Triassic till Upper Jurassic. Mainly clastic sediments and carbonates followed by thick evaporites during the Triassic; shelf carbonates during the Jurassic.
- Synrift megasequence. Lower Cretaceous (Tithonian till Albian). The Bay of Biscay opened and became partially underlain by oceanic crustOceanic crustOceanic crust is the part of Earth's lithosphere that surfaces in the ocean basins. Oceanic crust is primarily composed of mafic rocks, or sima, which is rich in iron and magnesium...
. During the Neocomian nonmarine and shallow-marine clastics and carbonates formed, followed by thick shelf carbonates during the Aptian and Albian. The megasequence ends in the Cenomanian with inversion tectonics along preexisting tensional faults. - Postrift megasequence. Cenomanian till Paleocene. Sinistral motions of Iberia relative to France created several transtensional sub-basins (pull-apart structures). Turbidite sedimentation in the south.
- Foreland basin megasequence. Eocene till recent. The collision of Iberia with France in the Eocene terminated the transtensional tectonics. The rising orogen of the Pyrenees shed flysch sediments during the Eocene and molasses during the Miocene into the foreland basin.
Resources
Hydrocarbons
Amongst the many resources in the Aquitaine Basin, the hydrocarbons oilOil
An oil is any substance that is liquid at ambient temperatures and does not mix with water but may mix with other oils and organic solvents. This general definition includes vegetable oils, volatile essential oils, petrochemical oils, and synthetic oils....
and gas
Gas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...
are without doubt of prime importance for the French economy. Major sources are found in the following sub-basins of Lower Cretaceous age:
- In the Parentis Basin. Oil is found in ParentisParentis-en-BornParentis-en-Born is a commune in the Landes department in Aquitaine in south-western France.It is one of the biggest oil reserves of continental western Europe.-See also:*Communes of the Landes department*Lac de Biscarrosse et de Parentis...
, Cazaux and LavergneLavergneLavergne may refer to the following places:* Lavergne, Lot, a commune in the Lot department in France* Lavergne, Lot-et-Garonne, a commune in the Lot-et-Garonne department in France...
. The Parentis Basin contains most oil reserves of continental France. - In the Adour Basin. Gas is found in LacqLacqLacq is a commune in the Pyrénées-Atlantiques department in south-western France.It lies just northwest of the local capital of Pau.-Economy:...
, MeillonMeillonMeillon is a commune in the Pyrénées-Atlantiques department in south-western France.-References:*...
, and Saint-MarcetSaint-MarcetSaint-Marcet is a commune in the Haute-Garonne department in southwestern France.-Population:-References:*...
. With its 220 billion cubic meters of gas, the Adour Basin stocks practically all the gas reserves of France.
Source/host rocks are Jurassic (Kimmeridgian) and Lower Cretaceous limestones and dolostones. Lower Aptian shales function as seals.
Groundwater aquifers
Classic groundwaterGroundwater
Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of rock formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock...
aquifer
Aquifer
An aquifer is a wet underground layer of water-bearing permeable rock or unconsolidated materials from which groundwater can be usefully extracted using a water well. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology...
s are found in Upper Cretaceous and Cenozoic rocks of the Bordelais
Bordelais
Bordelais is a French term meaning "of Bordeaux" and can refer to* an inhabitant of the city Bordeaux* the area surrounding the city Bordeaux* an inhabitant of the commune Les Bordes-sur-Lez* Bordeaux wine, or the Bordeaux wine region...
. Recently a gigantic aquifer has been discovered in Eocene sands near Lussagnet
Lussagnet
Lussagnet is a commune in the Landes department in Aquitaine in south-western France....
— of great importance for the region Pau-Toulouse.
Other resources
Further notable resources include:- ClayClayClay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...
s and mudstoneMudstoneMudstone is a fine grained sedimentary rock whose original constituents were clays or muds. Grain size is up to 0.0625 mm with individual grains too small to be distinguished without a microscope. With increased pressure over time the platey clay minerals may become aligned, with the...
s. They are the raw material for innumerable tileTileA tile is a manufactured piece of hard-wearing material such as ceramic, stone, metal, or even glass. Tiles are generally used for covering roofs, floors, walls, showers, or other objects such as tabletops...
and brick factoriesBrickA brick is a block of ceramic material used in masonry construction, usually laid using various kinds of mortar. It has been regarded as one of the longest lasting and strongest building materials used throughout history.-History:...
(manufacture of tiles, bricks, terracotta, etc.). Horizons are found mainly in the Toarcian, in the Eocene (Lutetian), in the Oligocene, and in the Miocene (Aquitanian, Burdigalian, LanghianLanghianThe Langhian is, in the ICS geologic timescale, an age or stage in the middle Miocene epoch/series. It spans the time between 15.97 ± 0.05 Ma and 13.65 ± 0.05 Ma . The Langhian was a continuing warming period defined by Lorenzo Pareto in 1864, it was originally established in the Langhe area north...
, and TortonianTortonianThe Tortonian is in the geologic timescale an age or stage of the late Miocene that spans the time between 11.608 ± 0.005 Ma and 7.246 ± 0.005 Ma . It follows the Serravallian and is followed by the Messinian....
). - KaoliniteKaoliniteKaolinite is a clay mineral, part of the group of industrial minerals, with the chemical composition Al2Si2O54. It is a layered silicate mineral, with one tetrahedral sheet linked through oxygen atoms to one octahedral sheet of alumina octahedra...
. This is the raw material for the manufacture of chinaPorcelaineThe Porcelaine is a breed of dog originating from France. It is believed to be the oldest of the French scent hounds. Its alternate name is the Chien de Franche-Comté, named after a French region bordering Switzerland...
. Mainly found in residual pockets of Eocene age filling depressions and caves in Upper Cretaceous karstKARSTKilometer-square Area Radio Synthesis Telescope is a Chinese telescope project to which FAST is a forerunner. KARST is a set of large spherical reflectors on karst landforms, which are bowlshaped limestone sinkholes named after the Kras region in Slovenia and Northern Italy. It will consist of...
developments. For example, near Les EyziesLes Eyzies-de-Tayac-SireuilLes Eyzies-de-Tayac-Sireuil is a commune in the Dordogne department in Aquitaine in southwestern France.Les Eyzies-de-Tayac-Sireuil lies in the Périgord Noir area. It is served by the Gare des Eyzies railroad station...
. - PeatPeatPeat is an accumulation of partially decayed vegetation matter or histosol. Peat forms in wetland bogs, moors, muskegs, pocosins, mires, and peat swamp forests. Peat is harvested as an important source of fuel in certain parts of the world...
. PleistocenePleistoceneThe Pleistocene is the epoch from 2,588,000 to 11,700 years BP that spans the world's recent period of repeated glaciations. The name pleistocene is derived from the Greek and ....
and HoloceneHoloceneThe Holocene is a geological epoch which began at the end of the Pleistocene and continues to the present. The Holocene is part of the Quaternary period. Its name comes from the Greek words and , meaning "entirely recent"...
horizons in the Médoc (Gironde estuary). - LigniteLigniteLignite, often referred to as brown coal, or Rosebud coal by Northern Pacific Railroad,is a soft brown fuel with characteristics that put it somewhere between coal and peat...
. In the Cenomanian of the Sarladais; Upper Miocene/Pliocene deposits in the Landes are strip-mined near ArjuzanxArjuzanxArjuzanx is a commune of the Landes department in Aquitaine in south-western France....
. - BauxiteBauxiteBauxite is an aluminium ore and is the main source of aluminium. This form of rock consists mostly of the minerals gibbsite Al3, boehmite γ-AlO, and diaspore α-AlO, in a mixture with the two iron oxides goethite and hematite, the clay mineral kaolinite, and small amounts of anatase TiO2...
. In Jurassic karst pockets between PechPech, AriègePech is a commune in the Ariège department of southwestern France.-Population:Inhabitants of Pech are called Péchois.-References:*...
and LavelanetLavelanetLavelanet is a commune in the Ariège department in the Midi-Pyrénées region in southwestern France.-Population:Its inhabitants are called Lavelanetiens.-Tour de France:...
. Economically not viable. - IronIronIron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
. Contained within the Eocene age Sidérolithique, mainly in the Périgord and in the Quercy. Economically no longer viable. - MetalMetalA metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
s. Mainly leadLeadLead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
-zincZincZinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...
mineralisations occurring in the basal Sinemurian. Found in the Charente and near Figeac, yet economically no longer viable. - WoodWoodWood is a hard, fibrous tissue found in many trees. It has been used for hundreds of thousands of years for both fuel and as a construction material. It is an organic material, a natural composite of cellulose fibers embedded in a matrix of lignin which resists compression...
. Large forests in the Landes and in the Dordogne form the basis for a diversified industry (firewood for power stations and private use, charcoalCharcoalCharcoal is the dark grey residue consisting of carbon, and any remaining ash, obtained by removing water and other volatile constituents from animal and vegetation substances. Charcoal is usually produced by slow pyrolysis, the heating of wood or other substances in the absence of oxygen...
, palettes, timber, furniture industry, etc.). - Fruits. PlumPlumA plum or gage is a stone fruit tree in the genus Prunus, subgenus Prunus. The subgenus is distinguished from other subgenera in the shoots having a terminal bud and solitary side buds , the flowers in groups of one to five together on short stems, and the fruit having a groove running down one...
s in the Agenais (Pruneau d'Agen). - WineWineWine is an alcoholic beverage, made of fermented fruit juice, usually from grapes. The natural chemical balance of grapes lets them ferment without the addition of sugars, acids, enzymes, or other nutrients. Grape wine is produced by fermenting crushed grapes using various types of yeast. Yeast...
. World-famous wines like the Cognac, Bordeaux, Armagnac, Chalosse, and BéarnBéarnBéarn is one of the traditional provinces of France, located in the Pyrenees mountains and in the plain at their feet, in southwest France. Along with the three Basque provinces of Soule, Lower Navarre, and Labourd, the principality of Bidache, as well as small parts of Gascony, it forms in the...
are grown in the Aquitaine Basin.
Conclusions
The structural and hence sedimentary organisation in the Aquitaine Basin was ultimately influenced by two principal factors:- By structural inheritance from the Variscan basement.
- By the evolution of the Pyrenean orogen.
The strong northwest-southeast-striking tectono-metamorphic zonation in the basement has profoundly influenced the structural and the sedimentary evolution of the Aquitaine Basin. The same hercynian direction is also followed by the continental edge of northwestern France which formed during the evolution of the Bay of Biscay. The continental edge finds its prolongation in the superdeep Subpyrenean Basin. The system of anticlinal ridges affecting the sedimentary cover is also arranged in this direction. The prominent South Armorican Shear Zone farther north also strikes northwest-southeast, but additionally has a distinct dextral wrenching movement. Like the South Armorican Shear Zone, the anticlinal ridges too are affected by similar shearing movements and are not purely compressional in origin. Even the Parentis Basin is bounded by these transtensional dextral shear zones and can hence be interpreted as an east-west-oriented pull-apart basin. Within the context of the opening of the Bay of Biscay, the Parentis Basin can furthermore be regarded as a failed attempt of the Atlantic to wedge into the continent's interior. The reason for this is the counterclockwise rotational motion of Iberia blocked further rifting.
Since the Cenomanian, the Aquitaine Basin is under the influence of the Pyrenean orogeny with its westnorthwest-eastsoutheast-striking structural grain. The Pyrenean orogen likewise has not merely a compressional origin but also a strong, in this case, sinistral transtensional component. The Pyrenean orogen exerted a very profound influence upon the Aquitaine Basin up to this day, subjecting it not only to more or less north-south-directed compression but also to transtension. The effects were of a penetrative nature — tectonic repercussions of the Pyrenean orogeny can even be seen at the northeastern basin margin in the immediate vicinity of the Massif Central.