Génis unit
The Génis unit is a paleozoic metasedimentary series of the southern Limousin and belongs geologically to the Variscan basement of the French massif Central . It was deposited between 490 and 400 million years ago and is of Ordovician to Upper Devonian age. In the limousine's ceiling pile , it is viewed as the structurally highest, only slightly metamorphosed unit.
Name and type locality
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The Génis unit was named after its type locality , the place Génis , located in the northern Dordogne department .
Geographical distribution
The Génis unit extends about 26 kilometers in WNW-OSE direction on the northeast edge of the Dordogne department. In terms of landscape, it forms part of the Bas Limousin - a high-level slab of the basement that was heavily leveled during the Old Tertiary , and whose altitudes usually only reach 300 to 400 meters. Its northern geological limit is the South Limousin Fault , a significant ductile, dextral side shift that separates the Génis unit from the Thiviers-Payzac unit to the north . In the south, the unit is covered by the Liass sediments of the Aquitaine Basin . The Permian red sediments of the Briver Basin lie on the east end . The maximum smear width of the unit in NNE-SSW direction is not much more than 5 kilometers. The sequence of the Génis unit can best be observed along the Auvézère .
Comparable units with an identical stratigraphic sequence can be found in the Département Vendée and in the Rouergue .
stratigraphy
The Génis unit shows the following stratigraphic structure (from hanging wall to lying ):
- Génis Greenschist - 420 to 400 million years old
- Génis serite slate - 470 to 420 million years old
- Puy-de-Cornut-Arkose - 480 to 470 million years
- Génis porphyroids - 490 to 480 million years old
Génis Greenschist
The Génis Greenschist are the youngest formation in the Génis unit. It is here to former mafic igneous rocks such as gabbro and basaltic pillow lava , basic volcanoclastics and relatively rare inclusions of pebble shale and mudstone . The Greenschist can be assigned a Lower Devonian age of 420 to 400 million years.
Génis sericite slate
The underlying Génis sericite slates are very rich in the minerals quartz , chlorite and muscovite (sericite variety). They are likely to have Ordovician and Upper Silurian ages (rare Ordovician acritic finds ). In the hanging wall of the Génis serizite schist there are lime lenses containing about 420 million year old conodonts from the Obersilur .
Puy-de-Cornut-Arkose
The Puy-de-Cornut-Arkose is located under the Génis sericite slates . The arkose is heavily silicified and forms hardness in the terrain. It is considered to be stratigraphically equivalent to the Puy-des-Âges quartzite from the neighboring Thiviers-Payzac unit . A relationship to the Grès armoricain of Brittany is also being considered. A Middle-Ordovician age (470 to 460 million years) for the arkosis is therefore very likely.
Génis porphyroid
The Génis porphyroids , formerly alkaline, rhyolite ignimbrites (Metaignimbrites) with a sub-ordovician age ( Tremadocium ) lie beneath a clearly developed angle discordance . The phenocrystals contain quartz, alkali feldspar and plagioclase ( albite ) in a very fine-grained matrix (5 μm) of quartz, feldspar, sericite and the rare chlorite. Original flames can hardly be seen anymore, but welded glassy layers can still be identified as such. The metaignimbrites are potassium-accentuated and contain more than 70% SiO 2 .
The deeper subsoil of the Génis unit is formed by the Donzenac shale and the Thiviers sandstone , the latter emerging in the Fugeyrollas anticline . These two formations, however, already belong to the Thiviers-Payzac unit, they are said to have been neoproterozoic to Cambrian ages .
Structural structure
The Génis unit is continuously folded . It is a fairly close standing, upright fold structure with a wavelength of about 150 meters. The fold axes strike OSE-WNW (N 110) and dip slightly to the east at 10 °. The strata (S 0 ) are often clearly recognizable and show a steep dip (usually around 75 to 80 °) to the north-north-east or south-south-west. Parallel to the folding axis planes a significant has foliation formed (S 1 ). The close-fitting fold structure is dominated by a second fold, which has deformed the entire unit into a long-wave sequence (wavelength two kilometers) of two synclines with an anticline in between ( Cubas syncline in the south, Fougeyrollas anticline , Génis syncline in the north). On the layer surfaces clearly developed elongation lines can be seen , which run more or less parallel to the fold axes. Newly formed metamorphic minerals have preferably arranged themselves along this direction (the linear ones scatter between N 110 and N 135). Furthermore, the linear direction of a Kleinfältelung accompanied in the millimeter range, the fold axes are also aligned to N 110th
Metamorphosis and structural development
The former rock series of the Génis unit was lowered and metamorphosed during the Variscan orogeny . The metamorphosis was retrograde (with the formation of chlorite ) and registered the epizonal conditions of the green schist facies . This fact is important for the geology of the Massif Central, since low metamorphic series are underrepresented and only occur very rarely. The metasediments in the Massif Central are usually very strongly deformed and ( amphibolite facially ) metamorphosed and therefore only allow imprecise statements about their parent rocks.
This retrograde metamorphosis is also known elsewhere in the Massif Central and is classified as the Middle Carboniferous.
The entire Génis unit was subject to a ductile, steep, dextral shear , as was the Südlimousin fault already mentioned above ; it can therefore be viewed as a relatively broad, OSE-WNW-trending shear zone . Dextral shear criteria are found in all formations of the unit. Asymmetrical quartz pebbles in conglomerate layers of the Thiviers sandstone indicate right-hand shear. In the Génis porphyroids, the conditions are even clearer; pressure shadows have formed around the phenocrystals of quartz and alkali feldspar , which also indicate a dextral shear sense. Shear bands in the millimeter range of the Génis sericite slate show the same shear direction.
The temporal classification of the tectonic movements is based on comparisons with similar terrains in the Armorican Massif ( Chantonnay Synclinorium in the Vendée) and in the Rouergue. In the Armorican massif, the right-hand shear movements occurred in the Namur and Westphal ( Serpukhovian to Moskovian , 325 to 305 million years ago). In analogy to this, a mid to late Carboniferous age of deformation can be assumed for the Génis unit of the southern limousine.
The continuous shearing movements are responsible for the fold structures of génis unit, thus serving as Zugfalten ( english tear folds ) in a transpressional can be interpreted, ductile shear zone.
Individual evidence
- ↑ Collomb, P .: Étude géologique du Rouergue cristallin . In: Mem. Serv. Carte Géol. 1970, p. 419 .
- ↑ Guillot, PL & Lefebvre, J .: Découverte de conodontes dans le calcaire à entroques de Génis en Dordogne (série métamorphique du Bas Limousin) . In: CR Acad. Sci. tape 280 , 1975, pp. 1529-1530 .
- ↑ Faure, M. & Pons, J .: Crustal thinning recorded by the shape of the Namurian-Westphalian leucogranites in the Variscan belt of the Northwest Massif Central, France . In: Geology . tape 19 , 1991, p. 730-733 .
- ↑ Roig, J.-Y., Faure, M. & Ledru, P .: Polyphase wrench tectonics in the southern French Massif Central: kinematic inferences from pre- and syntectonic granitoids . In: Geologische Rundschau . tape 85 , 1996, pp. 138-153 .
swell
- BRGM: Feuille Juillac . In: Carte géologique de la France at 1/50 000 . 1978.
- Peterlongo, JM: Massif Central . In: Guides Géologiques Régionaux . Masson, 1978, ISBN 2-225-49753-2 .
- Roig, J.-Y., Faure, M. & Ledru, P .: Polyphase wrench tectonics in the southern French Massif Central: kinematic inferences from pre- and syntectonic granitoids . In: Geologische Rundschau . tape 85 , 1996, pp. 138-153 .