Génis sericite slate
The Génis Serizitschist is a geological formation from the Ordovician and Silurian of the French Massif Central . The formation forms part of the Génis unit .
etymology
The Génis sericite slate is named after its type locality , the French commune of Génis in the northeast of the Dordogne department .
Geography and geology
The Génis sericite slate is only available in the Génis unit. It appears here on the north and south legs of the Génis syncline and in the Cubas syncline . Stratigraphically, the formation overlays the Puy-de-Cornut-Arkose and is in turn covered by the Génis green schist. It can be followed in the direction of strike on the north leg of the Génis Synklinale with a strike width of a maximum of 2 kilometers over 12 kilometers. In the Cubas Syncline it is open in a triangle measuring 5 × 3 kilometers. The maximum thickness of the sericite slate is 150 meters. The sericite slates are submerged in the southeast under the Permian red sediments of the Briver basin . In the southwest they are covered by lias .
Petrology
The Génis sericite slate is a fine-grain, matt, silky, shiny slate , the color of which varies from gray-black to green to purple. The foliation is very distinct and usually only forms a very acute angle with the stratification - which is why the rock falls apart in thin sheets and splintery forms. The still recognizable stratification consists of alternating layers in the millimeter to centimeter range of white or greenish fine sand layers with darker, clay-rich layers.
The clayey layers contain quartz and, as mica, detritic muscovite and diagenetically formed chlorite (prochlorite). In addition, there is a recrystallized clay mineral fraction consisting of a mixture of very small crystals of chlorite and sericite.
The fine sand layers essentially consist of a mosaic of small quartz crystals, which are associated with the rarer albite as well as chlorite and sericite.
The formation is clearly shaped by the pitches, which in places swell up to several meters in thickness. Nevertheless, in some places the fine sand layers can form banks several decimetres thick but discontinuous.
Chemical composition
| Oxide wt.% |
Serite slate 1 | Serite slate 2 | Serite slate 3 |
|---|---|---|---|
| SiO 2 | 61.80 | 60.50 | 60.20 |
| TiO 2 | 0.92 | 1.07 | 0.97 |
| Al 2 O 3 | 19.50 | 19.60 | 20.10 |
| Fe 2 O 3 | 4.75 | 2.65 | 2.85 |
| FeO | 2.00 | 3.50 | 3.70 |
| MnO | 0.06 | 0.05 | 0.05 |
| MgO | 1.40 | 2.45 | 2.05 |
| CaO | 0.55 | 0.30 | 0.20 |
| Na 2 O | 0.70 | 0.85 | 0.85 |
| K 2 O | 4.45 | 4.25 | 4.10 |
| P 2 O 5 | 0.12 | 0.21 | 0.14 |
| H 2 O - | 0.10 | 0.05 | 0.15 |
| H 2 O + | 3.90 | 4.20 | 4.45 |
The chemical composition of the serite slate is quite uniform and corresponds to typical shale clay . Their Al 2 O 3 content is 20 percent by weight and is therefore extremely high. TiO 2 and total iron also show relatively high contents . The concentration of Na 2 O is below 1 percent by weight and is therefore very low. The alkalis are potash-based.
Fossil content
The slates have at various sites, such as the Moulin de Guimalet or on the road to the hamlets Puybelly (municipality Cherveix-Cubas) and Fontavril (municipality Génis), microfossils brought to light. Mention should be made of planktonic acritars , which, despite their poor state of preservation , reveal the taxa Veryhachium reductum (Deunff), Veryhachium lairdi-valiente (Cramer) and Priscogalea striatula (Vadr.). The two Veryhachia are characteristic of the beginning of the Ordovician, whereas Priscogalea is generally to be found in the Ordovician.
About 500 meters north of the Moulin du Pont , a crystalline calcareous lens was found in the hanging wall of the formation , which, in addition to round sea lily stems, housed a rich conodont fauna. Despite the poor state of preservation of the broken fossils, the following taxa could be identified: Spathognathodus sp. , Spathognathodus streinhornensis , Ligonodina sp. , Neoprioniodus sp. , Trichonodella sp. and the Upper Silurian key fossil Ozarkodina sp. .
Age
The fossil finds allow an age-related classification of the Génis sericite slate. The Acritars clearly speak for the Ordovician. The Conodonts point to Ludlow to Pridolium (Obersilur), ie to a period of 427 to 420 million years. Overall, the formation is therefore approximately 470 to 420 million years old.
See also
literature
- P. - L. Guillot et al.: Feuille Juillac . In: Carte géologique de la France at 1/50 000 . BRGM, 1978.
- JM Peterlongo: Massif Central . In: Guides Géologiques Régionaux . Masson, 1978, ISBN 2-225-49753-2 .
Individual evidence
- ↑ P. - L. Guillot and others: Feuille Juillac . In: Carte géologique de la France at 1/50 000 . BRGM, 1978.
- ↑ Guillot, PL and 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 .