Briver basin

The Briver Basin is a tectonically caused basin on the western edge of the French Massif Central in the Nouvelle-Aquitaine region . It originated at the end of the Upper Carboniferous , developed mainly in the Permian and lasted into the Triassic . Today it represents a lowland. The Briver Basin is very closely connected to the Aquitaine Basin , the eastern end of which it forms.

geography

Location map of the Briver basin (in yellow) compared to the rest of the Limousin

The 600 square kilometer Briver Basin, French Bassin de Brive (Basin of Brive), has as its focus the eponymous city of Brive-la-Gaillarde , sub-prefecture and largest city of the Corrèze department . To the north it extends to Allassac and Donzenac , to the west to Terrasson-Lavilledieu , to the south to Noailles and to the east to just before Aubazines .

The Briver Basin is surrounded by the following natural spaces:

• the Pays d'Uzerche in the north
• the Pays de Tulle and the Xaintrie in the east
• the Limargue , the Causses du Quercy and the Périgord noir in the south
• the Périgord central in the west.

A total of 24 municipalities share the area of ​​the Briver basin, including 15 municipalities from the Communauté d'agglomération de Brive , 6 municipalities from the Communauté de communes Vézère - Causse and 3 municipalities from the Communauté de communes des Portes du Causse .

Topographically, the Briver Basin occupies very low altitudes compared to the rest of the Limousin , which rarely exceed 350 meters. The deepest points are still below 90 meters in altitude. The city center of Brive, for example, is 110 meters above sea level and Cublac is 100 meters above sea level.

Hydrography

The Briver basin is drained by the Vézère and its left tributary, the Corrèze . At the southernmost edge of the basin, the Dordogne is just touched. The general direction of discharge is to the southwest.

geology

The castle of Collonges-la-Rouge , built of red Triassic sandstones from the Briver basin.

Geologically, the Briver Basin is separated from the Variscan basement in the northeast by several significant southeast-trending faults . The transition can also take place without any discernible offsets. The east-south-east trending and right-shifting Meyssac Fault and its western, somewhat offset extension, the Condat Fault , then form the southern boundary of the Briver Basin. The sediments of the Aquitaine Basin are already in the south of the fault. In the north lie the Briver basin sediments of the Thiviers-Payzac unit and the Génis unit , which descend under the basin. At the southwest corner of the Briver Basin, the basement appears again as a boundary in the Horst von Châtres . The transition to the Aquitaine basin sediments north of this basement block takes place gradually.

In the late Upper Carboniferous, after the elevation of the Massif Central was completed, the newly formed Variscan orogen began to stretch , as a result of which the Briver Basin subsided due to disturbances. The initial, originating from the Massif sedimentation was predominantly coarse clastic the Upper Carboniferous, it formed the Grès de Donzenac - conglomerates and gray to black sandstones , in the relatively rare coal layers were interposed (seen in Saint-Antoine-Les-Plantades near from Brive). The latter were then also dismantled at Cublac and Le Lardin-Saint-Lazare .

In the Permian the sedimentation changed to continental bimodal red sediments with occasional gray layers. On the one hand, coarse, diagonally layered sandstones with only slightly rounded quartz fragments were deposited , on the other hand, fine-grained clay stones and pelite . The sedimentation environment consisted of alluvial fans extending from the interior of the Massif Central and partly overlapping . The thickness of the Permian layers deposited under subtropical conditions (savannah climate) is given as 600 to 800 meters. Except for traces of salamander as Ichnofossils and plant chaff like Walchia, the sediments are generally quite low in fossils . An exception to the purely clastic sedimentation is the Calcaire de Saint-Antoine , which consists of limestone and bituminous clay slate. It follows the Grès de Donzenac at the base of the Permian and is in turn overlaid by the Grès de Lanteuil . The Grès de Noailhac form the end of the Permian sedimentation .

In the 50 to 80 meter thick Triassic, which began with an angular discordance (observed at Cosnac ), the erosion of the hinterland continued. White and brightly colored continental sandstones of the formations Grès inférieurs de Brive and Grès supérieurs de Brive now formed . Towards the end of the Triassic, the Massif Central was almost completely leveled.

Due to the transgression of the Atlantic in the lowest Lias , the sedimentation in the Briver area gradually became similar to the rest of the Aquitaine basin or became part of it. The 50 to 80 meter thick hettangium with dolomitic limestone already shows lagunar sediments and the following 40 to 50 meter thick Sinemurium with mussel and oyster limestone already has a fully marine character. However, mainly marl was sold . In the Aalenium , the entire Massif Central would have been covered by the sea.

The Briver Basin reappeared for the first time in the Lower Cretaceous .

literature

• J. Boissonnas and others: Brive-la-Gaillarde . In: Carte géologique de la France at 1/50 000 . tape XXI-35 . BRGM.