La Tour-Blanche Anticline
Encyclopedia
The La Tour-Blanche Anticline, also called Chapdeuil Anticline or Chapdeuil-La Tour-Blanche Anticline, is a tectonically
caused, dome-like upwarp
in the sedimentary succession of the northeastern Aquitaine Basin in France
. The structure is oriented westnorthwest-eastsoutheast.
was named after La Tour-Blanche
, a small town in the northwestern Dordogne
. Yet the centre of the structure is situated farther southeast within the commune of Chapdeuil
, hence the names Chapdeuil Anticline or Chapdeuil-La Tour-Blanche Anticline. In plan view the structure has the shape of a nearly rectangular parallelogram with the base lines striking ESE-WNW (N 120) and the sides more or less N-S (reference horizon being the Ligérian
/Angoumian
boundary). In its long dimension, the anticline measures about 6 km, yet across strike merely 3 km. Similar in shape to the Mareuil Anticline
, the La Tour-Blanche Anticline has an asymmetric profile with a steeper northeastern flank (dipping 20° to the NNE) and a very gentle southwestern flank (dipping 5° to the SSW). The northeastern flank is accompanied by a reverse fault
with very little heave (10–15 m).
On its northern side, the La Tour-Blanche Anticline yields to the Villebois-Lavalette-La Chapelle-Montabourlet Syncline (also called Gout-Rossignol-Léguillac Syncline). After a pronounced structural flexure, traceable from Verteillac
to Grand-Brassac
, follows to the south another syncline (northeast of Ribérac
) entirely composed of Campanian
strata at the surface. The next anticlinal high traverses Montmoreau
in the Charente
department and ends just northwest of Ribérac.
The eastern end of the La Tour-Blanche Anticline is crossed by several normal faults trending mainly NE-SW. East of Saint-Just
the axis of the anticline slowly disappears simultaneously changing its direction to an easterly course. The structure then definitely ends on the east side of the Boulou
(near Paussac
).
The area of the anticline is drained in a southeasterly direction by the Euche river, a right-hand tributary of the Dronne
, and by the Buffebale, a small rivulet and left-hand tributary of the Euche. It demonstrates inverted topography, i.e. the core of the structure is a depression.
. In the centre of the anticline, the sedimentary cover already reaches a thickness of 1000 m.
Like the Mareuil Anticline, the La Tour-Blanche Anticline is a regional feature and can be traced northwestwards to Cognac
in the Charente
department. To the southeast, the structure links up via the Bussac
high with the Périgueux
anticline (Beauronne
valley); it then continues in the Saint-Cyprien
Anticline (an upfaulted anticline) and finally reaches Cahors
in the Lot
department via the Cazals
flexure.
. Lowest outcropping member is Uppermost Kimmeridgian
overlain by Lower Portlandian. The Uppermost Kimmeridgian is being cut by the Buffebale and can be observed in the slopes of the rivulet. It has a detrital (sand
y) base and then changes into calcareous sandstone
s and finally into bioclastic, oolithic
limestone
s. Two facies domains can be distinguished: a detrital domain in the east (Serie de la Marteille) and a reef
al domain with single coral
s, occasional oyster
s and nerineids
in the west (Serie de Cercles). This differentiation into two facies domains also persists through the Lower Portlandian, with the eastern domain showing detrital intercalations made of shelly debris, gravel
s, and breccia
s. The Lower Portlandian — generally composed of 35 m of well-bedded cryptocrystalline micrite
s — takes up the major part of the anticline. Its grey to yellowish, sometimes reddish beds are between 10 and 20 cm thick and are separated by thin marl
y or clay
ey layers.
After the regression in the Uppermost Jurassic and the complete withdrawal during the entire Lower Cretaceous, the sea transgressed again during the Cenomanian
. The usually fairly thin, littoral Cenomanian deposits are very variable in thickness and can reach 40 m in certain places. They can be divided into three sequences: a detrital sequence at the base (sands with shelly debris and lignite
intercalations) followed by a calcareous sequence (reddish-brownish to greyish limestones) and another detrital sequence at the top (greyish-blackish to greenish claystone
s that are very rich in oyster
s). Concordantly on top of the Cenomanian follow the Turonian
deposits which can be subdivided into 15–40 m of Ligérian
(chalky, nodular limestones) and 35–65 m of Angoumian
(rudist limestones). The overlying Coniacian
is slightly discordant. It consists of hard fossiliferous limestones reaching a thickness of 50–80 m. The sedimentary sequence is topped by the typical chalk
s of the Santonian
(60–80 m) and the Campanian
(100–180 m); it can reach all in all a total thickness of 535 m.
The deeper parts of the La Tour-Blanche Anticline have been studied by drilling
, reaching the variscan
basement
(dark grey metamorphic schist
s) at 1085 m depth. From top to bottom, there is a very thick Upper Jurassic (670 m) followed by 158 m of Middle Jurassic
, 182 m of Lias
, and 75 m of Triassic
.
s and slickolites. The following structures can be recognized:
The (most likely) polycyclic movements in the anticline clearly were not just due to compression, but were also caused by shearing (transpression
or transtension
).
All these observations invoke a shear zone
underlying the anticline, probably with a dextral sense of shear.
phase at the end of the Creataceous. Certainly the pyrenean orogeny
with its strong north-south compression must have had an influence in the shaping of the structures. The main deformation in the Pyrenees happened during the Eocene
(Ypresian
to Lutetian
), peaking during the Lutetian.
The southern Armorican Massif
shows a rather similar spatial organisation in the eastern Vendée
; here dextral shear zones in dekakilometric spacing and the intervening far less deformed synclinoria follow the same trend. It therefore can be assumed that the variscan spatial organisation of the southern Armorican Massif also continued into the northern Aquitaine shelf.
Furthermore this seems to imply that the same or very similar deformation patterns continued right into the Cenozoic
. The fractures/shear zones affecting the variscan basement must have been reactivated and consequently left their imprint on the overlying Mesozoic
sedimentary cover.
Tectonics
Tectonics is a field of study within geology concerned generally with the structures within the lithosphere of the Earth and particularly with the forces and movements that have operated in a region to create these structures.Tectonics is concerned with the orogenies and tectonic development of...
caused, dome-like upwarp
Anticline
In structural geology, an anticline is a fold that is convex up and has its oldest beds at its core. The term is not to be confused with antiform, which is a purely descriptive term for any fold that is convex up. Therefore if age relationships In structural geology, an anticline is a fold that is...
in the sedimentary succession of the northeastern Aquitaine Basin in France
France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...
. The structure is oriented westnorthwest-eastsoutheast.
Description of the structure
The anticlineAnticline
In structural geology, an anticline is a fold that is convex up and has its oldest beds at its core. The term is not to be confused with antiform, which is a purely descriptive term for any fold that is convex up. Therefore if age relationships In structural geology, an anticline is a fold that is...
was named after La Tour-Blanche
La Tour-Blanche
La Tour-Blanche is a commune in the Dordogne department in Aquitaine in southwestern France.-Population:-References:*...
, a small town in the northwestern Dordogne
Dordogne
Dordogne is a départment in south-west France. The départment is located in the region of Aquitaine, between the Loire valley and the High Pyrénées named after the great river Dordogne that runs through it...
. Yet the centre of the structure is situated farther southeast within the commune of Chapdeuil
Chapdeuil
Chapdeuil is a commune in the Dordogne department in Aquitaine in southwestern France.-Population:...
, hence the names Chapdeuil Anticline or Chapdeuil-La Tour-Blanche Anticline. In plan view the structure has the shape of a nearly rectangular parallelogram with the base lines striking ESE-WNW (N 120) and the sides more or less N-S (reference horizon being the Ligérian
Ligérian
The Ligérian is a regional geological substage of the Turonian. It designates sediments of the Lower Turonian in western France.- Etymology :The name Ligérian, in French Ligérien, is derived from the Latin Ligera, which stands for the Loire River....
/Angoumian
Angoumian
The Angoumian is a geological group restricted to the northern Aquitaine Basin. The group consists of two fossiliferous limestone formations deposited during the Turonian.- Etymology :...
boundary). In its long dimension, the anticline measures about 6 km, yet across strike merely 3 km. Similar in shape to the Mareuil Anticline
Mareuil Anticline
The Mareuil Anticline, also called Mareuil-Meyssac Anticline, is a structural high within the sedimentary sequence of the northeastern Aquitaine Basin...
, the La Tour-Blanche Anticline has an asymmetric profile with a steeper northeastern flank (dipping 20° to the NNE) and a very gentle southwestern flank (dipping 5° to the SSW). The northeastern flank is accompanied by a reverse fault
Fault
Fault may refer to:*Fault , planar rock fractures which show evidence of relative movement*Fault , in dog breeding, is an undesirable aspect of structure or appearance that indicates the dog should not be bred...
with very little heave (10–15 m).
On its northern side, the La Tour-Blanche Anticline yields to the Villebois-Lavalette-La Chapelle-Montabourlet Syncline (also called Gout-Rossignol-Léguillac Syncline). After a pronounced structural flexure, traceable from Verteillac
Verteillac
Verteillac is a commune in the Dordogne department in Aquitaine in south-western France.-Demography:-Sights:* Château du Breuil, 16th-17th century* Château de la Grénerie, 19th century* Château de la Meyfrenie, 18th-19th century* Verteillac church...
to Grand-Brassac
Grand-Brassac
Grand-Brassac is a commune in the Dordogne department in Aquitaine in southwestern France.-Population:-References:*...
, follows to the south another syncline (northeast of Ribérac
Ribérac
Ribérac is a commune in the Dordogne department in Aquitaine in southwestern France. The commune is situated by the Dronne River.-History:In 1793, the commune of Faye joined with Ribérac. In 1851, a part of the commune was dismembered for the creation of the new commune of Saint-Martin-de-Ribérac...
) entirely composed of Campanian
Campanian
The Campanian is, in the ICS' geologic timescale, the fifth of six ages of the Late Cretaceous epoch . The Campanian spans the time from 83.5 ± 0.7 Ma to 70.6 ± 0.6 Ma ...
strata at the surface. The next anticlinal high traverses Montmoreau
La Chapelle-Montmoreau
La Chapelle-Montmoreau is a commune in the Dordogne department in Aquitaine in southwestern France.-Population:-References:*...
in the Charente
Charente
Charente is a department in southwestern France, in the Poitou-Charentes region, named after the Charente River, the most important river in the department, and also the river beside which the department's two largest towns, Angoulême and Cognac, are sited.-History:Charente is one of the original...
department and ends just northwest of Ribérac.
The eastern end of the La Tour-Blanche Anticline is crossed by several normal faults trending mainly NE-SW. East of Saint-Just
Saint-Just, Dordogne
Saint-Just is a commune in the Dordogne department in Aquitaine in southwestern France.-Population=:References*...
the axis of the anticline slowly disappears simultaneously changing its direction to an easterly course. The structure then definitely ends on the east side of the Boulou
Boulou
Boulou is a village in the Bassar Prefecture in the Kara Region of north-western Togo.-External links:**...
(near Paussac
Paussac-et-Saint-Vivien
Paussac-et-Saint-Vivien is a commune in the Dordogne department in Aquitaine in southwestern France.-Population:-References:*...
).
The area of the anticline is drained in a southeasterly direction by the Euche river, a right-hand tributary of the Dronne
Dronne
The Dronne is a 201 km long river in southwestern France, right tributary of the Isle River. Its source is in the north-western Massif Central, east of the town of Châlus at an elevation of 510 m...
, and by the Buffebale, a small rivulet and left-hand tributary of the Euche. It demonstrates inverted topography, i.e. the core of the structure is a depression.
Regional extent
Seen from the edge of the Aquitaine Basin, the La Tour-Blanche Anticline forms the second anticlinal ridge. At a distance of 25 km, it runs more or less parallel to the edge of the Massif CentralMassif Central (geology)
The Massif Central forms together with the Armorican Massif one of the two big basement massifs in France. Its geological evolution started in the late Neoproterozoic and continues to this day. It has been shaped mainly by the Caledonian orogeny and the Variscan orogeny. The Alpine orogeny has...
. In the centre of the anticline, the sedimentary cover already reaches a thickness of 1000 m.
Like the Mareuil Anticline, the La Tour-Blanche Anticline is a regional feature and can be traced northwestwards to Cognac
Cognac
Cognac is a commune in the Charente department in southwestern France. It is a sub-prefecture of the department.-Geography:Cognac is situated on the river Charente between the towns of Angoulême and Saintes. The majority of the town has been built on the river's left bank, with the smaller right...
in the Charente
Charente
Charente is a department in southwestern France, in the Poitou-Charentes region, named after the Charente River, the most important river in the department, and also the river beside which the department's two largest towns, Angoulême and Cognac, are sited.-History:Charente is one of the original...
department. To the southeast, the structure links up via the Bussac
Bussac
Bussac may refer to the following places in France:*Bussac-Forêt, in the Charente-Maritime département*Bussac, in the Dordogne département...
high with the Périgueux
Périgueux
Périgueux is a commune in the Dordogne department in Aquitaine in southwestern France.Périgueux is the prefecture of the department and the capital of the region...
anticline (Beauronne
Beauronne
Beauronne may refer to:*Beauronne, Dordogne, a commune in southwestern France*three small rivers in southwestern France, all tributaries of the Isle River:**Beauronne , flowing through Chancelade...
valley); it then continues in the Saint-Cyprien
Saint-Cyprien
Saint-Cyprien is the name or part of the name of several places. Most of them are named after Cyprian:-Canada:* Saint-Cyprien, Bas-Saint-Laurent, Quebec, municipality in Bas-Saint-Laurent, Quebec...
Anticline (an upfaulted anticline) and finally reaches Cahors
Cahors
Cahors is the capital of the Lot department in south-western France.Its site is dramatic being contained on three sides within an udder shaped twist in the river Lot known as a 'presqu'île' or peninsula...
in the Lot
Lot
Lot or lots may refer to:*Lot , a unit of weight used in many European countries since Middle Ages until the beginning of the 20th century*Lot, a set of goods, together for sale in an auction; or a quantity of a financial instrument...
department via the Cazals
Cazals
-Geographical:Cazals is the name or part of the name of several communes in France:* Cazals, in the Lot department* Cazals, in the Tarn-et-Garonne department* Cazals-des-Baylès, in the Ariège department...
flexure.
Stratigraphy
The La Tour-Blanche anticline is cored by Upper JurassicJurassic
The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
. Lowest outcropping member is Uppermost Kimmeridgian
Kimmeridgian
In the geologic timescale, the Kimmeridgian is an age or stage in the Late or Upper Jurassic epoch or series. It spans the time between 155.7 ± 4 Ma and 150.8 ± 4 Ma . The Kimmeridgian follows the Oxfordian and precedes the Tithonian....
overlain by Lower Portlandian. The Uppermost Kimmeridgian is being cut by the Buffebale and can be observed in the slopes of the rivulet. It has a detrital (sand
Sand
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
y) base and then changes into calcareous sandstone
Sandstone
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...
s and finally into bioclastic, oolithic
Oolite
Oolite is a sedimentary rock formed from ooids, spherical grains composed of concentric layers. The name derives from the Hellenic word òoion for egg. Strictly, oolites consist of ooids of diameter 0.25–2 mm; rocks composed of ooids larger than 2 mm are called pisolites...
limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
s. Two facies domains can be distinguished: a detrital domain in the east (Serie de la Marteille) and a reef
Reef
In nautical terminology, a reef is a rock, sandbar, or other feature lying beneath the surface of the water ....
al domain with single coral
Coral
Corals are marine animals in class Anthozoa of phylum Cnidaria typically living in compact colonies of many identical individual "polyps". The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.A coral "head" is a colony of...
s, occasional oyster
Oyster
The word oyster is used as a common name for a number of distinct groups of bivalve molluscs which live in marine or brackish habitats. The valves are highly calcified....
s and nerineids
Nerineidae
The Nerineidae is an extinct taxonomic family of fossil sea snails, marine gastropod mollusks in the informal group Lower Heterobranchia.-Genera:Genera within the family Nerineidae include:* Nerinea, the type genus* Bactroptyxis...
in the west (Serie de Cercles). This differentiation into two facies domains also persists through the Lower Portlandian, with the eastern domain showing detrital intercalations made of shelly debris, gravel
Gravel
Gravel is composed of unconsolidated rock fragments that have a general particle size range and include size classes from granule- to boulder-sized fragments. Gravel can be sub-categorized into granule and cobble...
s, and breccia
Breccia
Breccia is a rock composed of broken fragments of minerals or rock cemented together by a fine-grained matrix, that can be either similar to or different from the composition of the fragments....
s. The Lower Portlandian — generally composed of 35 m of well-bedded cryptocrystalline micrite
Micrite
Micrite is a limestone constituent formed of calcareous particles ranging in diameter up to 4 μm formed by the recrystallization of lime mud.The term was coined in 1959 by Robert Folk for his carbonate rock classification system...
s — takes up the major part of the anticline. Its grey to yellowish, sometimes reddish beds are between 10 and 20 cm thick and are separated by thin marl
Marl
Marl or marlstone is a calcium carbonate or lime-rich mud or mudstone which contains variable amounts of clays and aragonite. Marl was originally an old term loosely applied to a variety of materials, most of which occur as loose, earthy deposits consisting chiefly of an intimate mixture of clay...
y or clay
Clay
Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...
ey layers.
After the regression in the Uppermost Jurassic and the complete withdrawal during the entire Lower Cretaceous, the sea transgressed again during the Cenomanian
Cenomanian
The Cenomanian is, in the ICS' geological timescale the oldest or earliest age of the Late Cretaceous epoch or the lowest stage of the Upper Cretaceous series. An age is a unit of geochronology: it is a unit of time; the stage is a unit in the stratigraphic column deposited during the corresponding...
. The usually fairly thin, littoral Cenomanian deposits are very variable in thickness and can reach 40 m in certain places. They can be divided into three sequences: a detrital sequence at the base (sands with shelly debris and lignite
Lignite
Lignite, often referred to as brown coal, or Rosebud coal by Northern Pacific Railroad,is a soft brown fuel with characteristics that put it somewhere between coal and peat...
intercalations) followed by a calcareous sequence (reddish-brownish to greyish limestones) and another detrital sequence at the top (greyish-blackish to greenish claystone
Claystone
Claystone is a geological term used to describe a clastic sedimentary rock that is composed primarily of clay-sized particles ....
s that are very rich in oyster
Oyster
The word oyster is used as a common name for a number of distinct groups of bivalve molluscs which live in marine or brackish habitats. The valves are highly calcified....
s). Concordantly on top of the Cenomanian follow the Turonian
Turonian
The Turonian is, in the ICS' geologic timescale, the second age in the Late Cretaceous epoch, or a stage in the Upper Cretaceous series. It spans the time between 93.5 ± 0.8 Ma and 89.3 ± 1 Ma...
deposits which can be subdivided into 15–40 m of Ligérian
Ligérian
The Ligérian is a regional geological substage of the Turonian. It designates sediments of the Lower Turonian in western France.- Etymology :The name Ligérian, in French Ligérien, is derived from the Latin Ligera, which stands for the Loire River....
(chalky, nodular limestones) and 35–65 m of Angoumian
Angoumian
The Angoumian is a geological group restricted to the northern Aquitaine Basin. The group consists of two fossiliferous limestone formations deposited during the Turonian.- Etymology :...
(rudist limestones). The overlying Coniacian
Coniacian
The Coniacian is an age or stage in the geologic timescale. It is a subdivision of the Late Cretaceous epoch or Upper Cretaceous series and spans the time between 89.3 ± 1 Ma and 85.8 ± 0.7 Ma...
is slightly discordant. It consists of hard fossiliferous limestones reaching a thickness of 50–80 m. The sedimentary sequence is topped by the typical chalk
Chalk
Chalk is a soft, white, porous sedimentary rock, a form of limestone composed of the mineral calcite. Calcite is calcium carbonate or CaCO3. It forms under reasonably deep marine conditions from the gradual accumulation of minute calcite plates shed from micro-organisms called coccolithophores....
s of the Santonian
Santonian
The Santonian is an age in the geologic timescale or a chronostratigraphic stage. It is a subdivision of the Late Cretaceous epoch or Upper Cretaceous series. It spans the time between 85.8 ± 0.7 mya and 83.5 ± 0.7 mya...
(60–80 m) and the Campanian
Campanian
The Campanian is, in the ICS' geologic timescale, the fifth of six ages of the Late Cretaceous epoch . The Campanian spans the time from 83.5 ± 0.7 Ma to 70.6 ± 0.6 Ma ...
(100–180 m); it can reach all in all a total thickness of 535 m.
The deeper parts of the La Tour-Blanche Anticline have been studied by drilling
Drilling
Drilling is a cutting process that uses a drill bit to cut or enlarge a hole in solid materials. The drill bit is a multipoint, end cutting tool...
, reaching the variscan
Variscan orogeny
The Variscan orogeny is a geologic mountain-building event caused by Late Paleozoic continental collision between Euramerica and Gondwana to form the supercontinent of Pangaea.-Naming:...
basement
Basement
__FORCETOC__A basement is one or more floors of a building that are either completely or partially below the ground floor. Basements are typically used as a utility space for a building where such items as the furnace, water heater, breaker panel or fuse box, car park, and air-conditioning system...
(dark grey metamorphic schist
Schist
The schists constitute a group of medium-grade metamorphic rocks, chiefly notable for the preponderance of lamellar minerals such as micas, chlorite, talc, hornblende, graphite, and others. Quartz often occurs in drawn-out grains to such an extent that a particular form called quartz schist is...
s) at 1085 m depth. From top to bottom, there is a very thick Upper Jurassic (670 m) followed by 158 m of Middle Jurassic
Middle Jurassic
The Middle Jurassic is the second epoch of the Jurassic Period. It lasted from 176-161 million years ago. In European lithostratigraphy, rocks of this Middle Jurassic age are called the Dogger....
, 182 m of Lias
Lias
Lias may refer to:*In geology:**The Lias Group, a group in the stratigraphy of Great Britain, formed during the Late Triassic to Early Jurassic epochs. It includes the Blue Lias and White Lias...
, and 75 m of Triassic
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...
.
Structural observations
The tectonic deformations registered by the sediments in the La Tour-Blanche Anticline find their expression in many structural features. Especially the Lower Portlandian was quite strongly deformed and exhibits lots of styloliteStylolite
Stylolites are serrated surfaces at which mineral material has been removed by pressure dissolution, in a process that decreases the total volume of rock. Insoluble minerals like clays, pyrite, oxides remain within the stylolites and make them visible...
s and slickolites. The following structures can be recognized:
- strike-slip faults trending ESE-WNW (N 115) with pronounced slickolites.
- fractureFractureA fracture is the separation of an object or material into two, or more, pieces under the action of stress.The word fracture is often applied to bones of living creatures , or to crystals or crystalline materials, such as gemstones or metal...
s oriented NNE-SSW (N 020) with a right-lateral offset and stylolites pointing in the same direction. - tension gashes filled with calciteCalciteCalcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...
trending mainly N 165 to N 175, sometimes also N 020. They can be interpreted as pull-apartPull Apart Basin250px|thumb|[[Cami Lake]] in [[Tierra del Fuego]] develops on a [[Patagonian Ice Sheet|glacially]] excavated pull apart basin along the [[Magallanes-Fagnano Fault]], hence its elongated form...
s along releasing bends of the strike-slip faults. - NW-SE trending fractures (N 130 to N 140) with stylolites pointing in the same direction.
The (most likely) polycyclic movements in the anticline clearly were not just due to compression, but were also caused by shearing (transpression
Transpression
Transpression is a geological term used to describe a region of the Earth's crust that experiences strike-slip shear and a component of shortening, resulting in oblique shear. Transpression typically occurs at a regional scale, such as plate boundaries that have an oblique convergence. More...
or transtension
Transtension
Transtension is the term used to describe a rock mass or area of the Earth's crust that experiences both extensive and transtensive shear. As such, transtensional regions are characterised by both extensional structures and wrench structures ....
).
All these observations invoke a shear zone
Shear zone
A shear zone is a very important structural discontinuity surface in the Earth's crust and upper mantle. It forms as a response to inhomogeneous deformation partitioning strain into planar or curviplanar high-strain zones. Intervening blocks stay relatively unaffected by the deformation...
underlying the anticline, probably with a dextral sense of shear.
Time frame
The compressional shortening affecting the sedimentary cover clearly must have happened after the deposition of the Campanian strata. The genesis of the anticlinal ridges in the northern Aquitaine Basin is generally attributed to an endcampanian-maastrichtianMaastrichtian
The Maastrichtian is, in the ICS' geologic timescale, the latest age or upper stage of the Late Cretaceous epoch or Upper Cretaceous series, the Cretaceous period or system, and of the Mesozoic era or erathem. It spanned from 70.6 ± 0.6 Ma to 65.5 ± 0.3 Ma...
phase at the end of the Creataceous. Certainly the pyrenean orogeny
Geology of the Pyrenees
The Pyrenees form part of the huge alpine orogenic system. This 430 kilometre long, roughly east-west striking, intracontinental mountain chain divides France, Spain, and Andorra. It has an extended, polycyclic geological evolution dating back to the Precambrian...
with its strong north-south compression must have had an influence in the shaping of the structures. The main deformation in the Pyrenees happened during the Eocene
Eocene
The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...
(Ypresian
Ypresian
In the geologic timescale the Ypresian is the oldest age or lowest stratigraphic stage of the Eocene. It spans the time between and , is preceded by the Thanetian age and is followed by the Eocene Lutetian age....
to Lutetian
Lutetian
The Lutetian is, in the geologic timescale, a stage or age in the Eocene. It spans the time between and . The Lutetian is preceded by the Ypresian and is followed by the Bartonian. Together with the Bartonian it is sometimes referred to as the Middle Eocene subepoch...
), peaking during the Lutetian.
Conclusions
The La Tour-Blanche Anticline forms part of a system of anticlinal ridges in the northeastern Aquitaine Basin. During the time interval Uppermost Creataceous till Eocene compressional movements coupled with considerable shearing motions were responsible in their genesis. Important to note is the rather regular spatial organisation of these structures, which extend ESE-WNW or SE-NW and follow in a dekakilometric spacing (15–20 km) the outline of the Massif Central.The southern Armorican Massif
Armorican Massif
The Armorican Massif is a geologic massif that covers a large area in the northwest of France, including Brittany, the western part of Normandy and the Pays de la Loire. Its name comes from the old Armorica, a Gaul area between the Loire and the Seine rivers...
shows a rather similar spatial organisation in the eastern Vendée
Vendée
The Vendée is a department in the Pays-de-la-Loire region in west central France, on the Atlantic Ocean. The name Vendée is taken from the Vendée river which runs through the south-eastern part of the department.-History:...
; here dextral shear zones in dekakilometric spacing and the intervening far less deformed synclinoria follow the same trend. It therefore can be assumed that the variscan spatial organisation of the southern Armorican Massif also continued into the northern Aquitaine shelf.
Furthermore this seems to imply that the same or very similar deformation patterns continued right into the Cenozoic
Cenozoic
The Cenozoic era is the current and most recent of the three Phanerozoic geological eras and covers the period from 65.5 mya to the present. The era began in the wake of the Cretaceous–Tertiary extinction event at the end of the Cretaceous that saw the demise of the last non-avian dinosaurs and...
. The fractures/shear zones affecting the variscan basement must have been reactivated and consequently left their imprint on the overlying Mesozoic
Mesozoic
The Mesozoic era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the age of reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time...
sedimentary cover.