La Tour-Blanche Anticline

The La Tour- Blanche anticline, often referred to as Chapdeuil anticline or Chapdeuil - La Tour- Blanche Anticline, is a tectonic, dome-like bulge in the sedimentary layers of the Northeastern Association of Aquitaine basin. The structure follows a west-north- west- ostsüdöstlichen direction.

Description of the structure

The anticline was named in the northwest of the department of Dordogne to La Tour Blanche. The actual center of the structure is but a little farther to the southeast in the municipality of Chapdeuil, hence the name Chapdeuil anticline or Chapdeuil -La Tour Blanche anticline. In plan, the structure shows a rectangular to parallelogram shape with the two base lines underline N 120 and the two longitudinal sides roughly north-south (with border Ligérien / Angoumien as a reference horizon). The longitudinal dimension is scarce 6 km in width, the anticlinal measures 3 kilometers. Similar to the Mareuil- anticline the La Tour- Blanche anticline shows in profile an asymmetrical structure - here is the north face steep (up to 20 ° ) and is also a disorder with low Aufschiebungskomponente ( 10-15 meters) accompanied tions Northeast. The southern flank is very flat with only 5 ° one to the south.

To the north in the direction of Mareuil- anticline, the Villebois -Lavalette -La Chapelle- Montabourlet - syncline connects (or Gout - Rossignol - Léguillac - syncline ). To the south, followed by the pulling of Verteillac to Grand Brassac flexure of Synklinalbereich northeast of Riberac which is completely occupied by strata of Campanian.

Comparable to the Mareuil- anticline, the eastern end of the structure of multiple faults is crossed ( deportations ), followed by the most of the northeast-southwest direction.

East of Saint -Just done a slight descent as well as a tightening of the longitudinal axis in the east-west direction; the structure can then be further pursue ( municipality of Paussac ) shortly after the East Boulou Valley.

The anticline is called by the disease, a right tributary of the River Dronne, and her little northward side branch, Julie, or Buffebale, drained towards the Dronne by (East ) South-East. In the area of the anticline there is relief inversion, ie, it represents a topographic depression

Regional context

From the edge of the pool is the La Tour- Blanche Anticline behind the Mareuil- anticline the second high zone in the northeastern Aquitanian 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 sedimentary cover is in the center of the structure already 1000 meters.

Similar to the Mareuil- anticline and the La Tour- Blanche anticline is a large-scale structure, up to Cognac in the Charente can be pursued to the northwest. After Southeast is a connection over the high structure at Bussac the anticline of Périgueux ( Beauronne Valley), then further to the anticline of Saint- Cyprien (combination of bulge and disorder) and by the flexure of Cazals even to Cahors in the Lot département.

Stratigraphy of the constituent layer packet

At the core of the anticline Upper Jurassic is open. Deepest upcoming layer member is 10-15 m thick Supreme Kimmeridgian, which in turn is overlaid conformably by the Lower Portlandium. The Supreme Kimmeridgian is still cut by Buffebale and stands on the slopes of the river to. It starts detritisch ( sandy) and then passes into calcareous sandstones and finally into bioclastic Oolithkalke. There are two distinct facies: a detritisch stressed area in the East ( Series de la Marteille ) and a riffnaher area with single coral ( polyps), occasional oysters and Nerineen in the West ( series de Cercle ). This Faziesdifferentiation then remains in the Lower Portlandium continue to exist, the eastern section may also have gröberklastische activations as Schill, boulders and Brekzienlagen during the Lower Portlandiums. 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 pitches - then fills the main part of the anticline.

After the Sea retreat in the High Jura and siltation throughout the Lower Cretaceous deposits of the transgressive Cenomaniums follow. His widths are highly variable and relatively thin, but can at times still achieve up to 40 m. The littoral Cenomanian is composed of three series: a detrital series at the base (sands with Schill and Lignitlagen ), followed by a calcareous series (reddish - brown to gray limestones ) and a detrital series in the hanging wall ( gray-black to greenish mudstones, which very rich in oysters ). About the Cenomanian encamped concordant from the fossilkalkreiche Turonian, composed of 15-40 m at Ligérien ( chalky nodular ) and 35-65 m at Angoumien ( Rudistenkalke ). Unconformably above the Turonian follow the hard Fossilkalke of Coniaciums which can reach a thickness of 50-80 m. The conclusion then form typical of Kreidekalke Santoniums (60-80 m ) and the Campanian ( 100-180 m). The disrupted layers in the area of ​​La Tour- Blanche anticline can therefore reach a total thickness of up to 535 m.

The deeper subsoil of Antiklinals is known by reconnaissance drilling. This reached at a depth of 1085 m the Variscan basement rocks were encountered dark gray metamorphic slate. The hole went through 670 m of very powerful trained Upper Jurassic, 158 m Dogger, Lias 182 m and 75 m Triassic.

Structural observations

Numerous structures are evidence of the tectonic stress of the sediments in the vicinity of the anticline, in particular the Lower Portlandium was sometimes very strongly deformed, immediately recognizable by the many stylolites and Slickolithen. Specifically, the following structures can be identified:

• Page shifts in OSO - WNW- direction (N 115) with significant Slickolithen;
• Rechtsverschiebende transverse faults in NNE -SSW direction (N 020) with stylolites;
• With calcite filled Zerrspalten (N 165 to N 175, N 020 and subordinate ), they can also be interpreted as a pull -apart structures at the lateral displacements;
• NW-SE fractures (N 130 - N 140 ) with Stylolithe.

The movements in La Tour- Blanche anticline were therefore not only a gender nature, but also had a significant shear component, suggesting transpression or transtension. They should also be done multiphase.

The listed tectonic observations speak for the whole, a former shear zone in the area of ​​La Tour- Blanche anticline, very likely with dextral sense of movement.

Timeframe

The compressive narrowing of the layer package has to be done after deposit of the Campanian. For the emergence of Antiklinalzüge in the Aquitaine basin is therefore usually assumed by a endcampanischen - maastrichtischen phase at the end of the Cretaceous. Undoubtedly, but also the Pyrenäenorogenese with its north-south directed narrowing likely to have had a major role in the eventual formation of the structures; time the Pyrenees deformations occur in the Eocene ( Ypresium to lutetium ) to lie ( the maximum of the Pyrenees narrowing was achieved in lutetium ).

Importance

The La Tour- Blanche anticline joins in the system of Antiklinalzüge in the northeastern Aquitanian basin. In addition to compression, a significant shear component was involved in its creation in the period Cretaceous to Eocene Supreme. Noteworthy is the spatial organization of these large-scale structures that follow in OSO NWN or SE-NW direction in decadent kilometric (15-20 km ) intervals to the edge of the Massif Central.

The southern Armorican Massif shows in the eastern Vendée a very similar organization ( rechtsverschiebende shear zones in the Deka km distance with intervening borrowed end, less deformed Synklinalbereichen ). It may therefore be assumed that these Variscan pre-drawn spatial organization of the southern Armorican Massif was continued also in the northeastern Aquitanian shelf area.

This seems to suggest that the same or similar kind deformation pattern continued lasted until the Paleogene into it. The Variscan already mapped out fracture zones of the basement were reactivated and then consequently pausten through in the sedimentary cover.