Génis Unit

The Génis unit is a Paleozoic metasedimentary sequence of the southern Limousin and geologically belongs to the Variscan basement of the französischens Massif Central. It has Cambrian / Ordovician to Devonian age.

Name and type locality

The Génis unit is named after its type locality, the place Génis, located in the north Dordogne.

Geographical distribution

The Génis unit extends approximately 26 km in WNW- ESE direction along the northeast edge of the Dordogne. Scenic it forms part of the Bas Limousin, a strongly leveled during the early Tertiary basement desk plaice, usually reach their altitudes only 300 to 400 meters. Their geological northern border is the Südlimousin disorder, a major ductile, dextral side shift, which separates the Génis unit from the north following Thiviers - Payzac Unit. In the south, the unit is overlain by the Liassic sediments of the Aquitaine basin. At the east end the Permian red beds of the Briver basin lie on. The maximum Ausstrichsbreite the unit in NNE -SSW direction is not much more than 5 kilometers. The sequence of Génis unit can be observed best along the Auvézère.

Comparable units with identical stratigraphic sequence can be found in the Vendée and in the Rouergue.

Stratigraphy

The Génis unit shows the following stratigraphic structure ( from the hanging wall to the footwall ):

• Génis - greenschist
• Génis - Serizitschiefer
• Puy -de- Cornut - arkose
• Génis - porphyroids

Génis - greenschist

The Génis - greenschist are the recent formation of Génis unit. It is here to mafic igneous rocks such as former gabbros and basaltic pillow lava, basic volcanoclastics and relatively rare inclusions of pebbles and slate claystone. The green schists, a unterdevonisches age can be assigned.

Can be found at the base of Génis - greenschist Kalklinsen that contain conodonts from the Silurian.

Génis - Serizitschiefer

The subjacent Génis - Serizitschiefer are very rich in the minerals quartz, chlorite and muscovite ( sericite variety ). You should Ordovician age and untersilurisches have (rare Ordovician Acritarchenfunde ).

Puy -de- Cornut - arkose

Among the Génis - Serizitschiefern is the Puy -de- Cornut - arkose. The arkoses are strongly silicified and form clingstones in the field. They are considered stratigraphically equivalent considered to Puy -des- AGES - quartzite from the neighboring Thiviers - Payzac Unit. Also, a relationship to the Grès Armoricain Brittany is contemplated. An Ordovician age for the arkoses is therefore very likely.

Génis - porphyroids

Under a clear angular unconformity formed are the Génis - porphyroids, former alkaline, rhyolitic ignimbrites ( Metaignimbrite ) with unterordovizischem age ( Tremadocian ). They contain phenocrysts of quartz, alkali feldspar and plagioclase ( albite ) in a very fine-grained matrix (5 microns ) of quartz, feldspar, sericite and rare chlorite. Original Fiamme are hardly recognizable, welded glassy layers but can be partly still identified as such. The Metaignimbrite are potassium- stressed and contain more than 70 % SiO2.

The deeper subsoil of Génis unit is formed by the Donzenac - schists and the Thiviers sandstones, the latter come to light in Fugeyrollas anticline. These two formations from the Thiviers - Payzac Unit is neoproterozoisches awarded to kambrisches age.

Structural design

The Génis unit is verfaltet throughout. This is a fairly engständig standing upright Faltenbau with a wavelength of about 150 meters. The fold axes strike ESE - WNW (N 110) and covered with a 10 ° slightly towards the east. The bedding planes (S0) are often well recognizable and steep dip (usually by 75 to 80 ° ) to the north or south. Parallel to the fold axis planes, a clear foliation has formed (S1). The closely spaced Faltenbau is overprinted by a second fold, which has ( two km wavelength ) of two synclines with an intervening anticline deformed into a long-wave sequence ( Cubas - syncline in the south, Fougeyrollas anticline, Génis - syncline in the north) the entire unit. On the layer faces clearly trained stretching Linear have been identified that are more or less parallel to the fold axes. Newly formed metamorphic minerals have arranged preferentially along this direction ( The Linear scatter between N 110 and N 135). Furthermore, the linear direction is accompanied by a Kleinfältelung in the millimeter range, the fold axes are also aligned to N 110.

Metamorphism and structural development

The former rock series of Génis unit was lowered during the Variscan orogeny and metamorphosed. The metamorphosis was retrograde (with formation of chlorite ) and registered the epizonal conditions of the greenschist facies. This fact is of importance for the Massif Central, the geology, as low metamorphic series are under-represented and only rarely occur. Usually are the metasedimentary rocks in the Massif Central strongly deformed and ( amphibolitfaziell ) before metamorphosed and therefore can only inaccurate statements about their parent rocks to.

This retrograde metamorphism is known in the Massif Central and otherwise, and is classified as a mid- Carboniferous time.

The entire unit was subject Génis well as the above already mentioned Südlimousin disorder a ductile, steeply dipping, dextral shear, it can therefore be viewed as a whole a relatively broad, OSO - WNW- trending shear zone. Dextral shear criteria can be found in all formations of the unit. Asymmetric quartz pebbles in conglomeratic layers of Thiviers sandstone indicate right-sided shear. In the Génis - Porphyroiden the conditions are even more pronounced, so here have formed around the phenocrysts of quartz and alkali feldspar pressure shadows, which also indicate a dextral sense of shear. Shear bands in the millimeter range of Génis - Serizitschiefers reveal the same shear direction.

The chronology of tectonic movements is based on comparisons with similar terrain in the Armorican Massif ( Chantonnay - Syncline in the Vendée ) and in the Rouergue. In the Armorican Massif made ​​the right side shear movements in Namur and Westphal ( Serpukhovian to Moscovian, before 325-305 million years ago). Analogously, may therefore be assumed for the medium-to Génis unit of Südlimousins ​​a spätkarbonisches age of deformation.

The continuous shearing motions are responsible for the folding structures of Génis unit, which can be interpreted as Zugfalten in a transpressive ductile shear zone thus.