The Aquitanian Basin, Southwest France1
The Aquitanian Basin in southwestern France is surrounded by Variscan massifs to the north, northeast, and east, and by a Tertiary chain, the Pyrenees, with a Variscan core, to the south. Toward the west, the extension of the basin beneath the Atlantic remains unknown. The basin is asymmetric, inasmuch as there is a very gentle regional dip of Tertiary and Mesozoic rocks into the basin along the massifs, whereas complicated structural conditions exist along the mobile belt in the south.
The denudation products of the Variscan mountains were laid down in the lowlands of the continent during upper Carboniferous and Permian time in the form of sandstones and shales, locally with some coal, covered by red conglomerates and sandstones. The Triassic sediments consist of detritic rocks and evaporites with a short marine invasion in the south; this is the facies of the Germanic or epicontinental Triassic. At the beginning of the Jurassic a marine invasion came from the east, but the facies remains neritic-epicontinental and no trace of a geosynclinal development along the Pyrenees can be seen. During Triassic and Jurassic time the region had already developed a basin-like appearance with the north and east margins rather similar to today's, while the Pyrenees probably formed an island chain.
In Lower Cretaceous time a zone of subsidence developed along the general area of the Pyrenees, widening toward the west, and marine sediments several thousand meters in thickness were laid down, while the northern and northeastern part of the basin became dry land. This is the first indication of geosynclinal development in the region of the Pyrenees.
During the Upper Cretaceous, two marine invasions took place at about the same time, both coming definitely from the west. The first formed a narrow, long graben-like channel continuing in a restricted way the Lower Cretaceous zone of subsidence; this is the Pyrenean Flysch zone with sediments several thousand meters thick. The second invasion formed a wide and shallow gulf with the base along the Atlantic coast, covering the northern half of the basin. Although contemporaneous with the Flysch zone, limestones of shallow water epicontinental facies, less than 1,000 meters thick, were deposited in this gulf. This sea extended farther northward than that of Lower Cretaceous time and transgressed over the Jurassic.
During the Tertiary the gulfs from the Atlantic became gradually smaller, and almost disappeared from the present basin in Pliocene time. Quaternary fluviatile deposits, derived from the Pyrenees and the old massifs, cover much of the older beds, and are in turn covered in the coastal region by aeolic sands.
The main orogenic phase in the Pyrenees took place in Eocene time. The structure of the Flysch is very complicated and the Flysch seems to be upthrust onto the foreland. The Jurassic of the basin forms folds with east-west as the predominant trend. The intensity of folding decreases in the Cretaceous and more so in the Lower Tertiary. The Upper Tertiary is almost horizontal. Faulting seems to have been particularly active during Tertiary time; on the margins of the basin a northwestern trend parallel to the grain of the old massifs is very conspicuous, but many other faults of different directions are indicated by seismics. Doubtlessly the movements of the basement have influenced the structure of the sedimentary cover.
Almost all surface seepages of oil or gas are located along the Pyrenees, within or close to the Flysch trough. Many are connected with Triassic diapirs and range in age from Miocene to Aptian.
Numerous shows have been encountered in wells in the same area, from Eocene to Triassic. Triassic oil shows have been found by recent drilling northwest and southeast of Bordeaux. Many structures in and close to the foothill belt have been drilled, but most of them are dry.
At present one gas field is producing from Cenomanian at St. Marcet in the Flysch zone. One oil field at Lacq, close to the Flysch zone, is producing heavy sulfurous oil from the Senonian; a gas deposit in the Lower Cretaceous is not yet producing. In 1954 what appears to be a major oil field with rather light sweet oil in Lower Cretaceous dolomite, was discovered at Parentis, 80 kilometers southwest of Bordeaux.