Gour de Tazenat
| Gour de Tazenat | ||
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| Geographical location | Canton of Manzat , Puy-de-Dôme department , Auvergne-Rhône-Alpes region , France | |
| Tributaries | Ruisseau de Rochegude | |
| Drain | Ruisseau de Rochegude → Morge (Allier) | |
| Location close to the shore | Riom | |
| Data | ||
| Coordinates | 45 ° 58 '48 " N , 2 ° 59' 28" E | |
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| Altitude above sea level | 625 m | |
| surface | 30 ha | |
| width | 700 m | |
| Maximum depth | 66 m | |
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particularities |
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| Lakeshore with pending basement | ||
The Gour de Tazenat is a water-filled maar on the extreme northern edge of the Chaîne des Puys in the French department of Puy-de-Dôme .
etymology
The masculine noun gour , Occitan gorg , Latin gurges , originally means a sinter terrace in French . The term was later expanded to include all water points that do not dry out over the summer and retain their water. Gour is related to gouffre in the meaning of shaft cave . Tazenat is a small hamlet west of the maar from which it was named.
geography
The Gour de Tazenat belongs to the municipality of Charbonnières-les-Vieilles and is located 20 kilometers northwest of Riom and five kilometers northeast of Manzat . The almost circular lake with a diameter of 700 meters is located in the Combrailles landscape , 2.5 kilometers south of Charbonnières-les-Vieilles at 625 meters above sea level. It occupies a surface of 30 hectares and has a water depth of 66 meters. The lake is surrounded by a wooded rampart, some more than 50 meters high (free of forests in the north and northeast, reaching a height of 713 meters in the northeast), which slopes steeply towards the lake and over which a 2.8-kilometer circular hiking trail leads.
Soundings have shown that the lake has no conical or funnel-shaped, but a plate-shaped, flat subsoil with a diameter of 250 meters and is covered by about 14 meters of sediment.
Three kilometers to the southwest is the next volcanic center of the Chaîne des Puys, the ash cone of the Puy de Chalard with its northwestern lava flow made of trachy basalt.
geology
introduction
The Chaîne des Puys is the youngest volcanic district in France with typical alkaline intraplate volcanism. It can boast over 100 volcanic centers that were created over an arched eyrie of the Variscan basement , the Plateau des Dômes . This plateau consists of rhyolite tuffs of the Viséum in the north , but further south of peralkalic granitoids of the Upper Carboniferous and migmatic gneisses of the early Paleozoic. It is bounded to the east by the Limagne Trench and to the west by the Olby Trench (Fossé d'Olby). The volcanoes are lined up over a distance of 30 kilometers in a 3 to 4 kilometer wide band in north-south direction and thus run parallel to the edge fault of the Limagne Trench.
Volcanism began around 100,000 years BP and lasted up to 5,000 years BP, with the majority of the volcanic centers being formed between 13,500 and 7,000 years BP, with a maximum of 10,000 years BP for the basalts. It is questionable whether it is now completely extinguished. The extracted magmas form a series of magmas differentiated by fractional crystallization , which ranges from potassium-rich alkali basalts to mugearite and benmoreite (rarely) to trachytes . A total of around 8 cubic kilometers of basaltic lavas and 1 to 2 cubic kilometers of trachy derivatives as viscous lava domes were formed.
description
The Maar des Gour de Tazenat was created by a phreatomagmatic explosion . The magma responsible has a basaltic composition and rose along a fault in the basement. Sediment samples showed a minimum age of 29,000 years BP , the explosion therefore occurred towards the end of the Würm Ice Age , but before its coldest maximum .
The ejecta products, which are no more than 13 meters thick, are layered and distributed over the north, east and south-west sectors. At its base there are basaltic blocks, bombs, lapilli and pozzolans mixed with rock fragments ( granites , micro- granites , granulites and other metamorphic rocks such as the rhyolite tuffs of the Viseum ), which were torn from the crystalline basement by the vehemence of the explosion . The basement fragments can make up up to 60%. The volcanic bombs often show the flatbread or "cauliflower structure" typical of phreatomagmatic explosions, the cracks of which were caused by contact with water. The basal ejecta comes from the initial phase of the eruption, when the rising magma came into contact with groundwater or with the water of a lake or river. The resulting steam explosion was strong enough to shatter the crystalline basement and eject it as bombs. After the water that had penetrated had evaporated, the activities took on a more and more magmatic character and so the higher layers already contain far fewer basement fragments. When the eruption ended, the explosion crater gradually filled with water from the Ruisseau de Rochegude, which flows into it from the south . The brook built a small delta on the south side and is responsible for most of the sediment input. He evacuates the maar on the west side.
Dating
Until 1991 the Gour de Tazenat was placed in the Holocene because of its good morphological preservation . However, sediment samples from the lake floor at a depth of 10 meters already showed an age of around 14,000 years BP ( oldest dryas period ). Since the sediments are up to 14 meters thick, Juvigné and Stach-Czerniak (1998) estimate that the eruption will be around 29,000 years old. Thermoluminescence ages were also determined, which are much higher at 45,000 years BP (blue light) and 125,000 years BP (red light), but are in doubt.
Sampling of the lake sediments
A palynological and sedimentological examination of 10 meters of lake sediment produced the following results:
- The sediments go back at least to the oldest Dryas period.
- They contain three tephra horizons, all of which are attributed to the eruptions of Lac Pavin ( Mugearit - 10,280 BP - Alleröd , Trachyte - 8220 BP - Boreal and 6770 ± 80 BP - Atlantic ).
- The pollen profiles clearly document the reforestation at the beginning of the Holocene ( preboreal ), the decline of the forest due to the clearing of Neolithic farmers in the subboreal with the simultaneous first appearance of grain seeds , the natural recovery of the forest starting from 2910 ± 30 BP and finally its renewed drastic decline in Subatlantic due to settlement by Celts and Romans .
- The transition from the Atlantic to the subboreal is located at a sediment depth of 6.3 meters and brings about a significant change in the tree flora: the hazel is declining very strongly, slightly less elm , ash and linden ; but beeches are expanding enormously and alders are also on the rise.
fauna and Flora
The lake, which is quite rich in fish, is closed for nautical activities and bathing. It mainly contains perch , pike , carp , sunfish , roach , trout and other white fish, and occasionally eels , crabs , freshwater jellyfish ( Craspedacusta sowerbyi ) and arctic char .
In the literature
In his novella Mes vingt-cinq jours , published in Contes divers in 1885 and in Mont-Oriol in 1887 , Guy de Maupassant brings the Gour de Tazenat to mind.
Individual evidence
- ↑ a b c d e Pierre Boivin and others: Volcanologie de la Chaîne des Puys . 4e éd. Parc naturel régional des Volcans d'Auvergne, 2004, p. 106-108 .
- ↑ a b Juvigné, Etienne and Stach Czerniak, Alicja: Étude et sédimentologique palynologique of dépots Lacustres tardiglaciaires et holocènes du Gour de Tazenat (Massif Central, France) . In: Quaternaire . Volume 9 - Numéro 1, 1998, p. 15-23 , doi : 10.3406 / quate.1998.2102 .
- ↑ Villemant, B. et al .: Cristallisation fractionnée d'un magma basaltique alcalin: la série de la Chaîne des Puys (Massif Central, France). II. Geochimie. In: Bulletin de Minéralogie . tape 103 , 1980, pp. 267-286 .
- ↑ Boivin, P. et al.: Volcanologie de la Chaîne des Puys, Massif Central Français (58 edition) . Parc Naturel Régional des Volcans d'Auvergne, Clermont-Ferrand 2009, p. 196 .
- ↑ Camus, G .: La Chaîne des Puys (Massif Central français). Etude structurale et volcanologique. Thèse (doctoral thesis) . In: Ann. Université de Clermont-Ferrand, Série Géologie et Minéralogie . tape 56/28 , 1975, pp. 1-322 .
- ↑ Maurice Krafft and François-Dominique de Larouzière: Guide des volcans d'Europe et des Canaries . Paris 1991, ISBN 2-603-00813-7 , pp. 455 .
- ↑ Goer de Hervé, A. et al .: Volcanologie de la Chaîne des Puys . 1 card à 1/25 000 and one notice. Parc naturel régional des volcans d'Auvergne, 1991, p. 128 .
- ↑ Pilleyre T .: Data Transportation par thermoluminescence. Application a la chronologie des retombées volcaniques. Thèse (doctoral thesis) . Université de Clermont II, DU 345, Clermont-Ferrand 1991, p. 191 .