Marcory formation

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The Marcory Formation is a weakly metamorphic siliciclastic formation in the south of the French Massif Central, up to 1300 meters thick . It is of Lower Cambrian age and forms the base of the blankets in the south and north of the Montagne Noire .

Etymology and type locality

The opposite steep walls of the Falaises d'Olques near Castanet-le-Haut consist of the Pardailhan formation belonging to the Mélagues ceiling; in the lying position the Marcory formation.

The Marcory Formation, French Formation de Marcory or Grès de Marcory (Marcory sandstone), is named after the Signal de Marcory , an 800 meter high hilltop north-west of Pardailhan in the Hérault department , where it stands.

The type locality or the type profile of the Marcory formation is on the right side of the Orbiel valley in the immediate vicinity of the D 101. Other key profiles are in the Gorges de la Clamoux along the D 112 and at Rieussec on the D 907 from Narbonne to Saint- Pons-de-Thomières .

Geographical distribution

The Marcory Formation is found both in the submerged ceilings on the southern edge of the Montagne Noire and in the pushed-on ceiling pile on its northern edge.

In Fournes-ceiling is the gold mine of Salsigne pending to the southeast of Rieussec, also in the somewhat more northerly Minervois blanket east of Lastours . In the Pardailhan Nappe to the east , it is found in a band that strikes north from Félines-Minervois to Pardailhan, in the Ferrals-les-Montagnes sector to Saint-Pons-de-Thomières and in a thin band about 2 kilometers north of Poussarou .

North of the Montagne Noire, the Marcory Formation emerges in the Avène-Mendic Nappe , in which it girds the granitic core of the Roc Mendic between Truscas , Graissessac and Le Bousquet-d'Orb . It is also present between Mélagues and Albès in the lying area of ​​the Mélagues ceiling .

stratigraphy

Chancelloria - Natural History Museum Vienna

The Marcory Formation is a pelitic-sandy formation that reaches a total thickness of up to 1,300 meters. It consists of an alternation of green, sandy pelites and gray, violet-colored or white sandstones , in which conglomerate layers are occasionally inserted. Your basic contact is not open anywhere. It is concordantly overlaid by the Pardeilhan formation . The upper limit of the Pardeilhan Formation is determined by the first appearance of a more than 10 centimeter thick limestone bank, which can already contain archaeocyathids . This bench can also be designed as a calcareous lens.

The first 500 meters of the Marcory Formation are clearly dominated by pelites with only very few sandstone inclusions and calcareous lenses. Between 500 and 800 meters, the piles of sand, which are interpreted as delta sediments, increase very significantly, only to then decrease again completely. From 900 meters onwards, more sand is poured, but lenses and discontinuous layers of bioclastic limestone in the centimeter range also occur.

The calcareous lenses in question in the lower section are interpreted as tectonic cliffs or as allodapic landslides from the Pardailhan formation, in addition to an origin within the formation. These are 2 to 15 meters thick white to bluish limestone, which are embedded in a monotonous ensemble of green clays and fine sandstone layers.

Fossil content

Reconstruction of the hyolithid Haplophrentis

At the level of the limestone horizons of the upper stratigraphic section, Geyer (1984 and 1986) found hyolithids ( Biconulites sensu Berger et al. 1993) and sclerites of Chancelloriidae (Walcot, 1920) of the Allonia tripodophora type (Doré and Reid 1963) and Chancelloria sp . prove. The Marcory formation ends with the first appearance of archaeocyathids.

Age

The presence of Chancelloriidae in the upper section of the Marcory Formation allows the formation to be placed in the Lower Cambrian.

Individual evidence

  1. ↑ Álvaro, JJ et al: Nouveau découpage stratigraphique des séries cambriennes des nappes de Pardeilhan et du Minervois (versant sud de la Montagne Noire) . In: Géologie de la France . n ° 2, 1998, p. 3-12 .
  2. ↑ Berger, GM et al .: Notice explicative de la feuille de Carcassonne (n ° 1037) de la carte géologique de la France . Editions BRGM, 1993, p. 78 .
  3. ^ Lepine, J .: Le gisement sulfo-arsénifié aurifère du Cambrien inférieur de la mine traditional de Salsigne (Montagne noire, France). Contexte structural, métallogénique et sédimentologique. Thèse Université de Toulouse III (doctoral thesis), 1989, p. 243 .
  4. ^ Klein, C .: Les nappes du Minervois (sud du Massif Central, France) . In: Physio-Géo. tape 6 , 1983, pp. 71-99 .
  5. ^ Geyer, M .: Contributions to the geology of the western Minervois and Cabardès on the southwestern edge of the Montagne Noire (Dept. Aude, southwestern France) - diploma thesis . Würzburg 1984, p. 125 .
  6. ^ Geyer, M .: Découverte de microfaune dans le Cambrien inférieur du versant méridional de la Montagne noire (Cabardès et Minervois, Aude - France) . In: Bull. Soc. Hist. Nat. tape 122 . Toulouse 1986, p. 11-17 .