Thiviers sandstone

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The Thiviers sandstone is a Cambrian formation of the French Massif Central . The formation forms part of the Thiviers-Payzac unit as well as the Génis unit . It emerged from former rhyodacitic volcanic rocks .

etymology

The Thiviers sandstone, French Grès de Thiviers , is named after its type locality , the French commune Thiviers 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 term "sandstone" is a bit misleading, as the formation is clearly derived from rhyodacite tuffs and all other facies are only conversion products.

Stratigraphically, the Thiviers sandstone forms the deepest surface formation of the Thiviers-Payzac unit and takes up about two thirds of its surface area. It is a volcanic-detritic sequence of Cambrian (and possibly late Neoproterozoic ) ages. The deeper subsurface of the formation is nowhere exposed, but it is likely to be a neoproterozoic basement ( mica schist and gneiss).

The Thiviers sandstone appears both in the Thiviers-Payzac unit and in the Génis unit to the south. It also forms the main mass of the 12-kilometer-long and 5-kilometer wide Horst von Châtres (with Châtres sandstone ), set off to the south from the basement .

In the Thiviers-Payzac unit, the formation occurs in two anticlines - in the Saint-Cyr-les-Champagnes anticline in the north and in the Saint-Mesmin anticline (or Saint-Sulpice-d'Excideuil anticline in the western section ) further south. The exposure area in the Génis unit is the Fougeyrollas anticline . The northern outcropping belt is separated in the north by means of the Estivaux fault shifting to the left opposite the gneiss of the upper and lower gneiss cover . The southern digestion band and the occurrence of Fougeyrollas anticline are in the west of Liassedimenten of the Aquitaine basin face down and dive under the East Permian from red beds.

Overall, the external northern outcrop of the Thiviers sandstone follows a roughly 70-kilometer-long circular arc segment - starting from a little west of Thiviers in the north of the Dordogne via Lanouaille , Payzac, Orgnac-sur-Vézère , Donzenac to a little east of Brive in the Corrèze department . Its strike direction is initially WNW-ESE (N110), but it then turns east of the Loyre (north of Orgnac) in the NW-SE direction (N135). The southern outcropping belts are much shorter and strike - with the exception of the Allassac arm - east-southeast.

Magmatism

The Thiviers sandstone is home to three granitic intrusions - the Corgnac granite in its western section and, to the east of the Loyre, the Lower Carboniferous Estivaux granite and the Lower Ordovician Saut-du-Saumon orthogneiss .

Petrology

The Thiviers sandstone in its Payzac quartzite facies near Travassac near Donzenac . The vertical foliation is nice to see.

Petrologically , the following facies can be distinguished in Thiviers sandstone:

The Thiviers sandstone is also interspersed with countless dolerite tunnels in the meter range. Allassac slate , which is mined as roof slate , forms a special facies .

The rhyodacitic tuffs are now available as dark, massive or thick banked rocks in the meter range. In a fine-grained matrix of chlorite , light mica, quartz and albite , quartz, plagioclase (albite or oligoclase) and epidote stand out as millimeter-sized clasts. Detritic muscovite as well as microcline , titanite and calcite are added as accessories . The following phenomena underline the explosive character of the volcanic parent rock: broken, angular, pointed-pointed quartz, angular plagioclase and, above all, incorporated rock fragments rich in albite, leucocratic lavas .

The slates develop mainly in the hanging wall of the formation. They are fine-grained, sometimes velvety, shiny rocks of gray, greenish or yellowish color. Their structure is lepidoblastic . Their foliation runs almost parallel to the stratification. They carry the minerals quartz, chlorite, sericite and occasionally plagioclase.

The Grauwacken have a mineralogical structure similar to the tuff, but they are more quartz-accentuated and their matrix is ​​richer in phyllosilicates. They are likely to have emerged from the Rhyodacites. They also differ by a more restrictive grain size distribution and by a stronger degree of rounding of the clasts.

The siltstones are dark gray to black, very fine-grained rocks with banks in the decimeter to meter range. Some of the original, centimeter-thick layers can still be recognized. Even oblique stratification can be observed. Matted mica lamellae (chlorite, sericite, biotite ) surround 50 μ large even-grained clasts of quartz and plagioclase. Apart from the grain size, they hardly differ geochemically from the Grauwacken.

The conglomerates appear as meter-thick lenses in the hanging wall of the formation. They arose intraformationally and were of polygenetic origin. They prove the instability of the deposit area at that time and herald epirogenetic movements, which led to regional emergence towards the end of the Cambrian. The pebbles are often flattened and in the foliation stretched. Its main components are rhyodacite tuff and greywacke, but other rocks are also represented, such as microlithic volcanic rocks, slate rich in epidote and quartz, granophyres and even granitoids.

The chemical composition of quartzites is also almost identical to that of rhyodacites. Although they are separated as a separate formation ( Payzac quartzite ), they only represent a higher metamorphic, mesozonal facies of the rhyodacites. They appear either as very fine-grained, massive, extremely hard rocks of dark blue or black color or as silver-gray to yellowish sericite slate. Mineralogically, they differ from the rhyodacites by the appearance of granoblastic structures , by chloritized biotite, spongy almandine and accessory zircon .

The Dolerite form 1 to 15 meter thick corridors and sometimes also corridors within the Thiviers sandstone. They are dark green, fine-grained, massive and very hard rocks. Their originally magmatic structure of millimeter-sized plagioclase strips and pyroxenes was metamorphically impressed. The primary pyroxenes were completely replaced by amphibole and the plagioclase can sometimes only be seen in a phantom. New formations are chlorite, epidote, albite and accessory quartz.

Chemical composition

Oxide
wt.%		 Slate 1 Slate 2 Tuff 1 Tuff 2 Tuff 3 Tuff 4 Greywacke
SiO 2 57.50 60.90 64.40 66.30 69.60 70.40 71.75
TiO 2 0.90 0.84 0.72 0.73 0.66 0.86 0.83
Al 2 O 3 18.10 18.10 15.30 14.00 13.50 13.26 15.58
Fe 2 O 3 3.70 2.35 0.70 1.85 0.85 0.20 5.07 dead
FeO 3.90 5.10 4.80 3.60 3.80 4.75
MnO 0.10 0.15 0.12 0.09 0.09 0.07 0.06
MgO 3.65 2.90 2.60 2.70 2.65 2.25 1.83
CaO 1.10 1.35 2.45 3.30 1.50 1.40 0.21
Na 2 O 2.00 1.85 4.70 3.95 3.95 3.50 3.50
K 2 O 3.70 3.80 1.75 1.45 1.25 1.60 1.66
P 2 O 5 0.17 0.16 0.14 0.16 0.14 0.13
H 2 O - 0.10 0.10 0.05 0.05
H 2 O + 4.70 2.00 2.20 2.10 2.25 1.45

The former volcanic rocks generally have a Dacite to rhyolite composition - the K 2 O-enriched andesitic slates (Shoshonite Banakites ) are an exception . The sub-alkaline rhyodacites are calcareous rocks with a medium K content. Your Na content is quite high.

tectonics

The Thiviers sandstone is embedded in extensive folds with a wavelength of around 5 kilometers. This rather wide fold structure is in turn folded tightly and steeply, whereby the wavelength of the narrow fold structure is only 150 meters. Parallel to the OSO or SO-trending folds axis planes was a regional foliation . Continuous shear movements are responsible for the fold structures in the Thiviers sandstone, which can thus be interpreted as tensile folds rotating in the maximum stretching direction in a transpressive , ductile shear zone . But the sense of shear is not uniform. It is on the left in the northern outcropping band, but on the right further south after crossing a mixed zone. It is assumed that the right-sided shear sense overprinted the left-sided shear sense.

However, the tectonic stresses did not cease in the Thiviers sandstone when the ductile deformations ended . In the brittle area , for example, the unit was offset on the left by numerous smaller, mostly NE-SW-oriented lateral shifts, with offset amounts of 500 meters. An exception is the Dussac fault north of Lanouaille with a left-hand displacement amount of almost 6 kilometers.

metamorphosis

The Thiviers sandstone was not only folded, but also regionally metamorphically embossed, with the degree of metamorphism generally increasing from the south to its northern boundary with the gneiss blankets. While epizonal conditions with biotite and chlorite still prevail in the south, mesozonal conditions are already encountered in the north when the almandine and staurolite isograde are exceeded. The normal facies are therefore in the form of Payzac quartzite .

Age

There is no absolute age for the Thiviers sandstone. However, it is undoubtedly older than the Saut-du-Saumon orthogneiss that penetrated it, which Bernard-Griffith and colleagues (1977) dated to a sub-ordovician age of 477 ± 22 million years. Mostly the Middle to Upper Cambrian is assumed to be the origin of the Thiviers sandstone.

The regional metamorphosis of the Barrow type took place in the course of the Upper Devonian and was completed by 350 million years. The Estivaux granite is likely to represent a late anatectic phase .

The temporal classification of the tectonic movements is based on comparisons with lithologically and structurally similar terrains in the Armorican Massif ( Chantonnay Synclinorium in the Vendée) and in the Rouergue. In the southern Armorican massif, the right-hand shear movements occurred in the Namur and Westphal ( Serpukhovian to Moskowian , 325 to 305 million years ago). Analogous to this, a mid to late Carboniferous deformation can be assumed for the Thiviers sandstone of the Bas-Limousin (which is viewed as the southern extension of the Vendée). This is further supported by the comparable age of the synkinematic leukogranites in northern and central Limousin.

In contrast to this, however, the age information obtained from the Tournaisium using the argon method stands for the intrusion age of the Estivaux granite and for the mylonitic movements on the Saut-du-Saumon-Orthogneiss. They imply a tectonic process already in the early Lower Carboniferous for the southern limousine ( Breton phase ) around 360 million years.

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.
  • Gilbert Le Pochat and others: Feuille Périgueux (Est) XIX-34 . In: Carte géologique de la France at 1/50 000 . BRGM.
  • JM Peterlongo: Massif Central . In: Guides Géologiques Régionaux . Masson, 1978, ISBN 2-225-49753-2 .

Individual evidence

  1. ↑ Pierre-Louis Guillot and others: Feuille Juillac . In: Carte géologique de la France at 1/50 000 . BRGM, 1978.
  2. ^ Berthé, D. and Brun, JP: Evolution of folds in the South Armorican Shear Zone . In: J. Struct. Geol. Band 2 , 1980, p. 127-133 .
  3. ↑ 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 .
  4. ↑ Bernard-Griffith, J., Cantagrel, JM and Duthou, JL: Radiometric evidence for an Acadian tectonometamorphic event in Western Massif Central français . In: Contrib. Miner. Pet. Band 61 , 1977, pp. 199-212 .
  5. ^ Gilbert Le Pochat et al.: Feuille Périgueux (Est) XIX-34 . In: Carte géologique de la France at 1/50 000 . BRGM.
  6. ↑ Duthou, JL et al .: Paleozoic granitoids from the French Massif Central: age and origin studied by 87 Rb / 87 Sr system . In: Phys Earth Planet Interiors . tape 35 , 1984, pp. 131-144 .