La Tour Blanche Anticline
The La Tour Blanche anticline is a tectonically determined, dome-like bulge in the sedimentary strata of the northeastern Aquitaine basin . The structure follows a west-northwest-east-southeast direction.
Description of the structure
The La Tour Blanche anticline, also known as the Chapdeuil anticline or Chapdeuil-La-Tour-Blanche anticline , was named after La Tour-Blanche in the north-west of the Dordogne department . The actual center of the dome-like structure is a little further south-east in the municipal area of Chapdeuil . The first scientific description of the structure was made by P. Glangeaud in 1898. A doctoral thesis was completed by C. Famechon in 1961.
In plan, the structure has a rectangular to parallelogram shape, with the two base lines N 120 being deleted and the two long sides roughly north-south (with the border between Ligérien and Angoumien as the reference horizon). The longitudinal extension is just under 6 kilometers, the width of the anticline is 3 kilometers. Similar to the Mareuil anticline , the profile of the La Tour Blanche anticline has an asymmetrical structure - here, too, the north side is steeper (dip up to 20 ° to the north) and is additionally affected by a fault with low displacement components (10 - 15 meters ) accompanied to the northeast. The southern flank falls very flat with only 5 ° to the south.
The Villebois-Lavalette-La-Chapelle-Montabourlet-Syncline (or Gout-Rossignol-Léguillac-Syncline ) connects to the north in the direction of the Mareuil anticline . To the south, the flexure stretching from Distributionlac to Grand-Brassac is followed by the synclinal area northeast of Ribérac , which is completely occupied by layers of campanium.
Similar to the Mareuil anticline, the eastern end of the structure is traversed by several faults (faults), which mostly follow the northeast-southwest direction.
To the east of Saint-Just, there is a slight dip and the longitudinal axis is turned in the west-east direction; the structure can then be followed shortly to the east in the Boulou valley (municipality of Paussac ).
The anticline is drained by the Euche , a right tributary of the Dronne , and its small northward branch, the Julie , or Buffebale , in an (east) southeast direction towards the Dronne. In the area of the anticline there is a reversal of relief, i.e. topographically it represents a depression.
Regional context
Seen from the edge of the basin, the La Tour Blanche anticline behind the Mareuil anticline forms the second high zone in the north-eastern Aquitaine basin. It is about 25 kilometers from the edge of the pool and runs more or less parallel to the edge of the Massif Central . The sediment cover in the center of the structure is already 1000 meters.
Similar to the Mareuil anticline, the La Tour Blanche anticline is a large-scale structure that can be traced north-west to Cognac in the Charente department . To the south-east there is a connection via the high structure near Bussac to the anticline of Périgueux ( Beauronne Valley), then further to the anticline of Saint-Cyprien (combination of bulge and fault) and by means of the flexure from Cazals even to Cahors in the Lot department .
Stratigraphy of the building layer package
At the core of the anticline, the Upper Jura is open. The deepest pending layer member is the 10–15 m thick Upper Kimmeridgium , which in turn is concordantly overlaid by the Lower Portlandium . The uppermost Kimmeridgium is cut by the Buffebale and is located on the slopes of the river. It begins detritic ( sandy ) and then turns into calcareous sandstones and finally into bioclastic oolite limestone . Two facies areas can be distinguished: a detritic area in the east ( Serie de la Marteille ) and an area near the reef with individual corals (polyps), occasional oysters and nerinees in the west ( Serie de Cercles ). This facies differentiation then continues to exist in the Lower Portlandium, so the eastern section during the Lower Portlandium can also have coarser clastic inclusions such as Schill , rubble and breccia layers . The lower portlandium - generally up to 35 m thick, banked (bank thickness 10–20 cm), cryptocrystalline micrites of gray, yellow to slightly reddish color, which are separated by thin marl or clay layers - then fills the main part of the anticline.
After the sea retreat in the Upper Jurassic and siltation during the entire Lower Cretaceous , the transgressive deposits of the Cenomanium follow . Its thicknesses are very variable and relatively thin, but can still reach up to 40 m in places. The littoral cenomanium is made up of three series: a detritic series at the base (sands with shingle and lignite layers ), followed by a calcareous series (reddish-brown to gray limestone ) and a detritic series in the hanging wall (gray-black to greenish claystones , the are very rich in oysters). The fossil-calcareous turonium , composed of 15–40 m of Ligérien (chalky tuberous limestone) and 35–65 m of Angoumia ( Rudist limestone ) , was concordantly deposited over the Cenomanium . The hard fossil limestone of the Coniacium follows discordant above the Turonian, and can reach a thickness of 50–80 m. Typical chalk limestone of the Santonium (60–80 m) and the Campanium (100–180 m) then form the end. The exposed layers in the area of the La Tour Blanche anticline can thus reach a total thickness of up to 535 m.
The deeper subsurface of the anticline has been known from an exploratory drilling since 1958 . This reached the Variscan basement at a depth of 1085 m ; Dark gray metamorphic slates were encountered . The borehole ran through 670 m of the very powerful Upper Jura, 158 m Dogger , 182 m Lias and 75 m Trias .
Structural observations
Numerous structures testify to the tectonic stress on the sediments in the area of the anticline, in particular the lower portlandium was partly very badly deformed, immediately recognizable by the many stylolites and slickolites . The following structures can be identified:
- Lateral shifts in ESE-WNW direction (N 115) with clear slickolites;
- right-shifting transverse fractures in NNE-SSW direction (N 020) with stylolites;
- with calcite filled Zerrspalten (N 165 and N 175, as well as subordinate N 020), these can also serve as pull-apart be interpreted structures on the side shifts;
- NW-SE breaks (N 130 - N 140) with stylolites.
The movements in the La Tour Blanche anticline were consequently not only of a purely restrictive nature, but also had a clear shear component , which indicates transpression or transtension . They are also likely to have occurred in several phases.
Overall, the tectonic observations mentioned speak for a former shear zone in the area of the La Tour Blanche anticline, very likely with a dextral sense of movement.
Timeframe
The compressive narrowing of the layer package must have taken place after the campanium has been deposited. For the formation of the anticline lines in the Aquitaine Basin, an end- Campanian- Maastrichtian phase at the end of the Cretaceous is usually assumed. However, the Pyrenees orogenesis, with its north-south narrowing, undoubtedly also played a major role in the ultimate development of the structures; temporally the Pyrenees deformations come to lie in the Eocene ( Ypresian to Lutetian ) (the maximum of the Pyrenees narrowing was reached in the Lutetian).
meaning
The La Tour Blanche anticline is part of the system of anticline trains in the northeast Aquitaine Basin. In addition to compression, a significant shear component was involved in its formation between the Upper Cretaceous and the Eocene. What is remarkable is the spatial organization of these large-scale structures, which follow the edge of the Massif Central in a decakilometric (15–20 km) distance in the ESE-NWN or SE-NW direction.
The southern Armorican Massif shows a very similar organization in the eastern part of the Département Vendée (right-shifting shear zones at a decakilometer distance with intervening, less strongly deformed synclinal areas). It can therefore be assumed that this Variscan spatial organization of the southern Armorican massif was also continued in the northeastern Aquitaine shelf area.
This seems to suggest that the same or similar deformation patterns persisted into the Paleogene . The rupture zones of the basement, which had already been marked in the Variscan way, were consequently reactivated and then paused into the sediment shell.
literature
- Platel, J.-P. et al .: Carte Géologique de la France at 1:50 000. Feuille Périgueux (Ouest) . BRGM, Orléans 1989.
- Vigneaux, M .: Aquitaine Occidentale . Masson, Paris 1975, ISBN 2-225-41118-2 .
- Campagne de sismique-réflexion du permis Gaz de France de La Tour-Blanche, CGG 1977. ( Seismic reflection)
Individual evidence
- ↑ P. Glangeaud: Les dômes de Mareuil et de Chapdeuil (Dordogne) . In: Assoc.fr. Av. Sciences . n ° 1, 1898, p. 147-148 .
- ^ C. Famechon: Contribution à l'étude geologique des formations du Mésozoïque de l'anticlinal de Chapdeuil — La Tour-Blanche . In: Thèse 3e cycle . univ. Bordeaux, 1961, p. 65 .
- ↑ Report de fin de sondage pétrolier: La Tour-Blanche I-SAPCO-1958 . 1958.
- ↑ Jean-Pierre Platel and others: Carte Géologique de la France at 1:50 000. Feuille Périgueux (Ouest) . BRGM, Orléans 1989.