Mareuil Anticline

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The Mareuil Anticline, also called Mareuil-Meyssac Anticline, is a structural high within the sedimentary sequence of the northeastern Aquitaine Basin. The northwest-southeast trending anticline was caused by tectonic movements probably starting in the Upper Cretaceous.

Sediment Particulate solid matter that is deposited on the surface of land

Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand and silt can be carried in suspension in river water and on reaching the sea bed deposited by sedimentation and if buried, may eventually become sandstone and siltstone.

Aquitaine Basin Mesozoic and Cenozoic sedimentary basin in France

The Aquitaine Basin is, after the Paris Basin, the second largest Mesozoic and Cenozoic sedimentary basin in France, occupying a large part of the country's southwestern quadrant. Its surface area covers 66,000 km2 onshore. It formed on Variscan basement which was peneplained during the Permian and then started subsiding in the early Triassic. The basement is covered in the Parentis Basin and in the Subpyrenean Basin—both sub-basins of the main Aquitaine Basin—by 11,000 m of sediment.

Anticline geological term

In structural geology, an anticline is a type of fold that is an arch-like shape and has its oldest beds at its core. A typical anticline is convex up in which the hinge or crest is the location where the curvature is greatest, and the limbs are the sides of the fold that dip away from the hinge. Anticlines can be recognized and differentiated from antiforms by a sequence of rock layers that become progressively older toward the center of the fold. Therefore, if age relationships between various rock strata are unknown, the term antiform should be used.

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Description of the structure

A cross-section through the Mareuil Anticline with vertical exaggeration MareuilProfile.jpg
A cross-section through the Mareuil Anticline with vertical exaggeration

The anticline was named after Mareuil, a small town in the northwestern Dordogne, France. The centre of the structure is near Sainte-Croix-de-Mareuil, somewhat farther to the northwest. In plan view the anticline has the shape of an elongated pear with its long axis aligned NW-SE and reaching about 5 km in length (the Cenomanian/Turonian boundary serving as a reference horizon). The short axis is only 2 km in length. The structure is doubly asymmetric, with steeper dipping northeastern and northwestern limbs. It is accompanied on its northeastern side by the Mareuil Fault, which starts at La Rochebeaucourt and continues to the north of Brantôme. Along this fault the northeastern side of the anticline was pushed up by about 30 m.

Mareuil, Dordogne Part of Mareuil en Périgord in Nouvelle-Aquitaine, France

Mareuil, known locally as Mareuil-sur-Belle, is a former commune in the Dordogne department in Nouvelle-Aquitaine in southwestern France. On 1 January 2017, it was merged into the new commune Mareuil en Périgord. It was the birthplace of the troubadour Arnaut de Mareuil and of historian Michel Tardieu.

Dordogne Department of France in Nouvelle-Aquitaine

Dordogne is a department in Southwestern France, with its prefecture in Périgueux. The department is located in the region of Nouvelle-Aquitaine between the Loire Valley and the Pyrenees and is named after the river Dordogne that runs through it. It corresponds roughly with the ancient county of Périgord. It had a population of 416,909 in 2013.

France Republic with mainland in Europe and numerous oversea territories

France, officially the French Republic, is a country whose territory consists of metropolitan France in Western Europe and several overseas regions and territories. The metropolitan area of France extends from the Mediterranean Sea to the English Channel and the North Sea, and from the Rhine to the Atlantic Ocean. It is bordered by Belgium, Luxembourg and Germany to the northeast, Switzerland and Italy to the east, and Andorra and Spain to the south. The overseas territories include French Guiana in South America and several islands in the Atlantic, Pacific and Indian oceans. The country's 18 integral regions span a combined area of 643,801 square kilometres (248,573 sq mi) and a total population of 67.3 million. France, a sovereign state, is a unitary semi-presidential republic with its capital in Paris, the country's largest city and main cultural and commercial centre. Other major urban areas include Lyon, Marseille, Toulouse, Bordeaux, Lille and Nice.

On the southeastern side of the anticline, there are several smaller cross-cutting faults locally disturbing the angle of dip. These cross faults are all following Riedel directions, either the R- or the R'-position, hinting at an underlying shear zone.

Riedel is a German surname. Notable people with the surname include:

Shear zone structural discontinuity surface in the Earths crust and upper mantle

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 (crustal) blocks stay relatively unaffected by the deformation. Due to the shearing motion of the surrounding more rigid medium, a rotational, non co-axial component can be induced in the shear zone. Because the discontinuity surface usually passes through a wide depth-range, a great variety of different rock types with their characteristic structures are produced.

Regional geology

Seen from the edge of the northeastern Aquitaine Basin, the Mareuil Anticline forms the first structural high within the sedimentary cover. It runs more or less parallel with the edge of the Massif Central, at a distance of about 15 km. The sedimentary cover reaches a thickness of altogether 400 m in the anticline. In the syncline on the northeast (the Combiers-Saint-Crépin-de-Richemont Syncline), the sediments are 500 m thick. In the adjoining syncline on the southwestern side (the Gout-Rossignol-Léguillac Syncline), the sediments reach 700 m in thickness. This is followed 8 km to the southwest by another anticlinal ridge, the La Tour-Blanche Anticline. In this second anticlinal ridge, the sediments already show a thickness of 1000 m.

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 west-northwest-east-southeast.

The Mareuil Anticline is a large-scale structure. To the northwest it changes into a fault zone that can be traced from Angoulême to Isle de Yeu. To the southeast it also changes into a fault zone running from Terrasson to Meyssac. It possibly continues into the Lacassagne Fault or even into the Souillac flexure.

Angoulême Prefecture and commune in Nouvelle-Aquitaine, France

Angoulême is a commune, the capital of the Charente department, in the Nouvelle-Aquitaine region of southwestern France.

Terrasson-Lavilledieu Commune in Nouvelle-Aquitaine, France

Terrasson-Lavilledieu is a commune in the Dordogne department in Nouvelle-Aquitaine in southwestern France.

Meyssac Commune in Nouvelle-Aquitaine, France

Meyssac is a commune in the Corrèze department in central France. Its inhabitants are called the Meyssacois(es).

Stratigraphy of the sedimentary sequence

In the core of the anticline the top 20 m of Upper Jurassic (Kimmeridgian) strata are exposed — thinly bedded, cryptocrystalline, fossiliferous limestones (micrites). After a long hiatus follow 8–20 m of transgressive Cenomanian consisting of green, glauconitic marls rich in oysters and sandy, alveoline-bearing limestones. The Cenomanian is concordantly overlain by 55–65 m of Turonian strata divided into the nodular, chalky limestones of the Ligérian and the rudist-bearing limestones of the Angoumian. This is followed by 50–65 m of Coniacian — mainly hard, fossiliferous limestones. The sequence finishes with 45–60 m of chalky, glauconitic Santonian, in places rich in oyster debris.

The Jurassic period was a geologic period and system that spanned 56 million years from the end of the Triassic Period 201.3 million years ago (Mya) to the beginning of the Cretaceous Period 145 Mya. The Jurassic constitutes the middle period of the Mesozoic Era, also known as the Age of Reptiles. The start of the period was marked by the major Triassic–Jurassic extinction event. Two other extinction events occurred during the period: the Pliensbachian-Toarcian extinction in the Early Jurassic, and the Tithonian event at the end; however, neither event ranks among the "Big Five" mass extinctions.

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 157.3 ± 1.0 Ma and 152.1 ± 0.9 Ma. The Kimmeridgian follows the Oxfordian and precedes the Tithonian.

Limestone Sedimentary rocks made of calcium carbonate

Limestone is a carbonate sedimentary rock that is often composed of the skeletal fragments of marine organisms such as coral, foraminifera, and molluscs. Its major materials are the minerals calcite and aragonite, which are different crystal forms of calcium carbonate (CaCO3). A closely related rock is dolostone, which contains a high percentage of the mineral dolomite, CaMg(CO3)2. In fact, in old USGS publications, dolostone was referred to as magnesian limestone, a term now reserved for magnesium-deficient dolostones or magnesium-rich limestones.

The strata below the 120 m thick Kimmeridgian do not outcrop, but the presence of 210 m of underlying Oxfordian, Bajocian and Bathonian strata is known from drilling results in the vicinity. Whether thin Lias underlies the Dogger cannot be ascertained.

Age of movements

The northwestern edge of the Mareuil Anticline. The Turonian (Angoumian) cliff is dipping 35deg to the NNE. MareuilAnticline.jpg
The northwestern edge of the Mareuil Anticline. The Turonian (Angoumian) cliff is dipping 35° to the NNE.

The first movements in the region of the Mareuil Anticline must have occurred in the Lower Cretaceous because of the transgressive nature of the Cenomanian. But these movements were not specific to the anticline; they affected the entire northeastern Aquitaine Basin. Synsedimentary slumps in the Turonian strata near the anticline are a good indicator that the first tectonic stirrings had happened in the anticline. Yet the major event responsible for the creation of the structure clearly must have taken place after the Santonian, because the entire sedimentary sequence has been deformed by it. This major event is thought to have occurred at the end of the Campanian and during the Maastrichtian (end-Campanian-Maastrichtian event). But there is also a good possibility that the main orogenic event during the Eocene/Oligocene in the Pyrenees is somehow involved in the formation of the anticline. Like the Pyrenees, the Mareuil Anticline was created under compressive forces, maybe even under transpression. It is known that compressive forces responsible for the Pyrenean orogeny likewise affected the northeastern edge of the Massif Central (i.e. upthrusts within the Dogger can be seen in the quarry of Saint-Martial-de-Valette).

Structural observations

The Mareuil Anticline forms part of the anticlinal ridges within the northern Aquitaine Basin. These ridges follow a more or less northwesterly-southeasterly direction and run parallel to the South Armorican Shear Zone. Their age of formation is thought to be end-Campanian/Maastrichtian. Possibly they have a transpressive component and share with the South Armorican Shear Zone (SASZ) the same dextral sense of movement. Importantly they have a parallel alignment and a rather uniform spacing of 15–20 km. A local exception in this scheme is the La Tour-Blanche Anticline narrowing to a spacing of 8 km and following a WNW-ESE direction.

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References

Coordinates: 45°28′N0°25′E / 45.467°N 0.417°E / 45.467; 0.417