Puy-des-Âges quartzite

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The Puy-des-Âges quartzite is an Ordovician formation of the French Massif Central . The formation forms part of the Thiviers-Payzac unit .

etymology

The Puy-des-Âges quartzite, French Quartzite du Puy des Âges , is named after its type locality - the 415 meter high ridge Puy-des-Âges southeast of Payzac in the northeast of the Dordogne department .

Geography and geology

Geological overview map for the location of the Thiviers-Payzac unit (in green) and the Génis unit (in light green)

The maximum 100 meters thick Puy-des-Âges quartzite is only exposed in the Thiviers-Payzac unit as a thin band, which follows the trough axis of the Puy-des-Âges syncline . This band, about 8 kilometers long, begins in the middle of the Donzenac slate at the hamlet of Mazat 3 kilometers southwest of Payzac and then disappears into the slates at the hamlet of Chignac about 4 kilometers north-northwest of Juillac .

Stratigraphically , the Puy-des-Âges quartzite is the highest formation in the Thiviers-Payzac unit. It overlays the Donzenac slate, but also the Engastine Mafit in places .

The quartzite forms hardened backs in the area, especially at its type locality. Its very sparse vegetation stands out clearly from the meadows and forests of the surrounding slate rocks .

Petrology

The Puy-des-Âges quartzite from the hamlet of Chacord near Saint-Mesmin in the Dordogne department.

When fresh, the Puy-des-Âges quartzite is a pure white quartzite that weathered slightly yellowish, whereby the rock disintegrates into centimeter-thick flakes. Its internal structure is granoblastic to lepidoblastic . Milky white quartz veins or veins mostly follow the foliation or run at a flat angle to it.

The essential mineral components are quartz and muscovite . Quartz consists of tabular crystals flattened in the cleavage plane or more or less polygonal crystals of the same diameter. Muscovite forms small, hypidiomorphic lamellae.

Chemical composition

The formation is very rich in silicon with an SiO 2 content of 90 to 95 percent by weight. It emerged from a former, very SiO 2 -rich sandstone . It is likely to have been epicontinental sediments that were washed into narrow, recessed channels. The subsurface, consisting of underlying formations from the Thiviers-Payzac unit, was likely to have emerged at that time.

tectonics

The formation is steeply standing, verschiefert, internally in Hektometerbereich verfaltet and sweeps ENE (N 110). The internal layers are also folded and reveal a right-shifting drag fold. The quartzite follows the hollow core of the Puy-des-Âges syncline . In the brittle area it is interspersed with several steep transverse fractures . The fractures displace the quartzite train and strike mainly northeast, occasionally also north-northeast and north. The system of transverse breaks can also be observed very nicely on the handpiece. The kinematic deformation of the rock can best be explained by a right-shifting, transpressive shear zone. The transverse fractures indicate a brittle stretch parallel to the shear direction.

Age position

The Puy-des-Âges quartzite is considered to be the equivalent of the Puy-de-Cornut-Arkose in the Génis unit and is itself still regarded as the equivalent of the Grès armoricain in Brittany. He is therefore assigned a sub-ordovician age ( Arenigium or Floium ).

See also

literature

  • Pierre-Louis Guillot and others: Feuille Juillac . In: Carte géologique de la France at 1/50 000 . BRGM, 1978.
  • Pierre-Louis Guillot and others: Feuille Thiviers XIX-33 . In: Carte géologique de la France at 1/50 000 . BRGM.
  • Pierre-Louis Guillot: La série métamorphique du Bas Limousin de la vallée de l'Isle à la vallée de la Vézère, le socle en bordure du bassin aquitain. (Doctoral thesis) . Univ. d'Orléans, France, 1981, pp. 391 .
  • JM Peterlongo: Massif Central . In: Guides Géologiques Régionaux . Masson, 1978, ISBN 2-225-49753-2 .

Individual evidence

  1. ↑ Roig, J.-Y., Faure, M. and 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 .
  2. ↑ Pierre-Louis Guillot and others: Feuille Juillac . In: Carte géologique de la France at 1/50 000 . BRGM, 1978.