Mareuil anticline
The Mareuil anticline , also known as Mareuil-Meyssac anticline , is a tectonically induced bulge in the sedimentary strata of the northeast Aquitaine basin . The structure follows a northwest-southeast direction. The first movements on it have probably already occurred in the turonium .
Description of the structure
The anticline was named after the small town of Mareuil in the north-west of the Dordogne department . The actual center of the structure is a little further northwest at Sainte-Croix-de-Mareuil . The anticline shows an elongated, pear-shaped shape with a NW-SE trending longitudinal axis reaching 5 kilometers ( Cenomanian / Turonian boundary as reference horizon). The breadth is only 2 kilometers. The structure is doubly asymmetrical, with a steeper northeast and northwest flank. It is also accompanied by a fault along the northeast flank , the Mareuil fault , which runs from La Rochebeaucourt to north of Brantôme . At this fault, the northeast flank of the anticline was dragged, the offset is about 30 meters.
At the southeast end of the anticline near Mareuil cross several transverse fractures, which led to local tilting of the strata. What is striking is their arrangement as so-called Riedels (in R and R 'positions) and which therefore possibly indicate a shear zone in the deeper subsurface.
Regional context
Seen from the edge of the basin, the Mareuil anticline forms the first high zone in the north-eastern Aquitaine Basin. It is about 15 kilometers from the edge of the pool. The sediment cover in the anticline is around 400 meters. In the syncline to the north-east ( Combiers-Saint-Crépin-de-Richemont-Syncline ), the sediment thickness already reaches 500 meters. It then follows to the southwest the Gout-Rossignol-Léguillac syncline with a sediment thickness of around 700 meters. The next, roughly parallel anticline, the La Tour Blanche anticline , follows the Mareuil anticline 8 kilometers further south-west. The sediment cover here has grown to a thickness of almost 1,000 meters.
The Mareuil anticline is a large-scale structure that can be traced to the northwest as a fault zone beyond Angoulême to the Île d'Yeu . To the southeast it also merges into a fault zone, which continues via Terrasson to Meyssac . It may also be associated with Lacassagne Disorder and Souillac Flexure .
Stratigraphy of the building layer package
In the core of the anticline near Sainte-Croix-de-Mareuil, the Upper Jura ( Kimmeridgian ) emerges - thin-banked, cryptocrystalline limestone ( micrite ) - of which only the top 20 meters are exposed . After a large gap in the layer there follows a 8-20 meter thick, transgressive cenomanium consisting of green marls , which are rich in oysters , and sandy, alveoline- bearing limestone. The concordant, 55 - 65 meter thick turonium is built up from the bulbous, chalky limestone of the Ligérien and the rudist limestone of the Angoumien . The 50-65 meter thick coniacium consists mainly of hard fossil limestone. Finally, 45 - 60 meters thick, chalky , partly glauconite-containing Santonium , which is occasionally rich in oyster shill , follows .
The layers of the Oxfordium and the Dogger ( Bajocium and Bathonium , with a total thickness of 210 meters) are not exposed in the anticline below the Kimmeridgium, which is 120 meters thick . It is not known whether there is any thin lias underneath .
Timeframe
The first movements in the area of the Mareuil anticline must have already taken place in the Lower Cretaceous, since the Cenomaniac follows the Kimmeridgian with an erosive discordance. These movements were not only tied to the anticline, but of regional importance. Synsedimentary Rutschungsvorgänge ( Engl. Slumps ) in layers of Turoniums suggest movements in the Mareuil Anticline during that time period. The main phase of movement, however, clearly took place after the Santonium, since the entire layer package was deformed. An end- Campanian - Maastrichtian phase at the end of the Chalk is assumed for the formation of the anticline ridges . This main phase may also be related to the movements in the Pyrenees during the Eocene / Oligocene ( Pyrenees main phase ). As in the Pyrenees, restrictive, if not transpressive, movements occurred at the Mareuil anticline . As is well known, the constricting movements during the Pyrenees orogenesis even covered the northern edge of the Aquitaine Basin (clearly visible in thrusts, for example in the Saint-Martial-de-Valette quarry ).
meaning
The Mareuil anticline forms part of the anticline ridges in the northern Aquitaine Basin, which essentially follow the Armorican direction NW-SE and thus run parallel to the South Armorican shear zone . It is very likely that they also share the same dextral sense of movement with the latter. Also noteworthy is the parallel staggering of the ridges with almost identical distances in the deca kilometer range (15 to 20 kilometers). An exception to this scheme is the La Tour Blanche anticline, which is a local narrowing (to 8 kilometers).
literature
- Floc'h, JP et al .: Carte Géologique de la France at 1:50,000. Feuille Nontron . BRGM, Orléans.
- Vigneaux, M .: Aquitaine Occidentale . Masson, Paris 1975, ISBN 2-225-41118-2 .