Aquitaine Basin - Structural Organisation and Tectonics

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 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:

• Quercy synform.
• Tarn-et-Garonne antiform.
• Castres 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 Terrasson and Meyssac.
  • The Périgueux anticline. This structure can be followed from Cognac via La Tour-Blanche to Périgueux and Saint-Cyprien. It forms a typical anticline near La Tour-Blanche. Near Saint-Cyprien, it is mainly a normal fault.
  • The Oléron-Jonzac-Ribérac-Sauveterre-la-Lemance anticline. This structure is an anticline near Jonzac and Sauveterre.
    Between the anticlines are synforms near Saintes and Sarlat.

• 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 Couquèques.
  • The northwest-southeast-striking upwarp of Sainte-Hélène-Carcans.
  • The Bordeaux syncline.
  • The east-west-striking anticline of La Teste-Villagrains-Landiras-Miramont in the Guyenne.

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):

• Parentis-Bouglon-Agen.
• Mimizan-Roquefort-Créon-Cezans-Lavardens.
• Boos-Audignan-Nogaro.
• Saubrigues-Biarotte-Bastennes-Garlin.
• Peyrehorade-Sainte-Suzanne-Lacq-Pau-Meilhon.

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.