Vias (volcano)

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Coordinates: 43 ° 19 ′ 0 ″  N , 3 ° 25 ′ 0 ″  E

The basaltic Vias volcano , along with Roque Haute and Saint-Thibéry, is one of the most recent volcanic phenomena in the Languedoc in southern France . It originated in the Middle Pleistocene around 670,000 years ago.

introduction

The arrangement of volcanic deposits in the Hérault department. Together with the volcanic center of Agde and Roque Haute , the Vias volcano is located at the southern end of the lineament in the immediate vicinity of the Mediterranean.

The underlying volcanic vias are located at the end of a north-south series of volcanic centers that reach the Mediterranean from the Cézallier via the Cantal , the Aubrac , the Grands Causses and the Escandorgue in the Hérault department near Agde . The deposits belonging to the Agde basin consist of basaltic tuffs , which are interpreted as hyaloclastites deposited in shallow marine waters .

description

The maximum extent of the tuffs of the main deposit at Vias is a little more than 3 kilometers in north-south and 2 kilometers in east-west direction. The approximately 10-meter-thick sediment package is stored on gravels of the villa franchise . The tuffs contained in it are in alternation with layers of clayey-sandy and calcareous, foraminifera -containing sediments of the lagoon or lacustrine area. Its extraction center cannot be clearly defined, but on the basis of the granulometry of the deposits - grain sizes and individual thicknesses of the tufa layers increase slightly to the west - should be looked for further west; it is possibly identical with the chimney of the Terres Noires de Preignes or even with the Roque Haute .

Occurrence

A total of 6 occurrences of the vias tuff are known:

  • Main occurrence of vias - 3000 × 2000 meters
  • Occurrence on the left bank of the Libron - 1000 × 300 meters
  • Occurrence at La Jourdane - tiny
  • Occurrence between Preignes (Terres Noires de Preignes) and Médeilhan (Médeilhan tuff) - 2000 × 700 meters
  • Occurrence at the northeast foot of the Roque Haute - tiny
  • Occurrence on the Petit Pioc'h of the volcanic center of Agde - tiny

The maximum spread of the tuff is therefore around 10 kilometers in east-west direction and almost 6 kilometers in north-south direction. It is possible that the eruptions also reached Castelnau-de-Guers, 18 kilometers away .

stratigraphy

In the 9.75 meter thick sediment package, seven significant tuff horizons can be seen from the lying to the hanging wall (tuff layers A to G). Compared to the surrounding sediment, the persistent dark tufa layers are hardened. Their undersides can be very irregular in shape and show indentations into the underlying sediment (density inversions) - especially beneath pyroclastic bombs , some of which have penetrated quite deeply and penetrated layers.

The detailed stratigraphy of the tuff horizons is presented as follows

  • Horizon G: sustained and calm sedimentation, altogether around 70 centimeters thick, in addition to peridotite bombs there are also several centimeters of limestone blocks .
  • Horizon F: persistent and calm sedimentation, up to 50 centimeters thick, with peridotite bombs up to 10 centimeters in size, which can sink several decimeters into the underlying sediment.
  • Horizon E: strongly undulating sedimentation with wavelengths up to 7 meters, 4 to 5 centimeters thick, lapilli up to 10 millimeters in diameter. The tuff horizon fixes sloping layers and undulations of the underlying clays. Deposition of calcareous sediments over wave troughs.
  • Horizon D: partly detaching, strongly undulating sedimentation with wavelengths of 2.5 to 5 meters, only 2 centimeters thick, irregular undersides of the bank with density inversion.
  • Horizon C: persistent and calming sedimentation, relatively thick with 20 centimeters in the east, but rising up to 40/50 centimeters in the west, glass lapilli in the west, ashes up to 2 millimeters in diameter in the east, rich in peridotite nodules as well as broken quartz pebbles and hard , red clay fragments.
  • Horizon B: persistent but restless, on average 12 centimeters thick, thickness variations, poorly sorted, irregular underside of the bench, only 10% augite microliths, but hornblende prepped. Onset of inclined stratification, rearrangement of ash and lapilli, formation of mega-ripples and incipient erosion phenomena .
  • Horizon A: persistent and undisturbed, 8 centimeters thick, ash particles up to 3 millimeters in diameter, predominance of augite microphenocrystals (around 80%), presence of picotite.

Foraminifera

The Foraminifere Textularia sp.

The following taxa on foraminifera could be identified in the sediments:

Age

Radiometric dating of the Vias volcano using the potassium-argon method found 0.67 ± 0.05 million years BP and is virtually identical to the Roque Haute volcano , which was dated 0.64 ± 0.09 million years BP. The Vias volcano was therefore active in the Middle Pleistocene shortly before the beginning of the Haslach glacial period .

Petrology

The Vias volcano was very explosive compared to the volcanic centers of Agde, Saint-Thibéry and Roque Haute and differs from them by its rather high content of bulbous ejections of peridotites ( dunite , lherzolite ), augitites and hornblendites, as well as the appearance of brown ones Hornblende in the heavy mineral fraction. The explosiveness of the volcano can be seen in the long and at the same time only slightly varying tuff horizons. It can be explained by a marine ingression into the Proto-Hérault river valley, which is reflected in the gradual establishment of marine foraminifera in the sediment package.

The heavy mineral spectrum in the vias tuffs includes various augites, basaltic hornblende, clinopyroxene ( diopside ), orthopyroxene ( enstatite , hypersthene ) and spinel ( picotite ). In the background sedimentation and also in the tuffs there are additional minerals such as staurolite , tourmaline , epidote , garnet , sillimanite , andalusite , rutile , titanite , brookite and, rarely, thistle , corundum and anatase .

Individual evidence

  1. ↑ geze, B .: Languedoc Méditerranéen Montagne Noire . In: Guides Géologiques Régionaux . Masson, Paris 1979, ISBN 2-225-64120-X .
  2. ↑ a b Kloosterman, JB: Le volcanisme de la région d'Agde, Hérault, France (doctoral thesis) . In: Geologica Ultraiectina . No. 6 . Mededelingen van het Mineralogical-Geological Institute of the Rijksuniversiteit te Utrecht, 1960.
  3. ↑ Frechen, J. von and Lippolt, HJ: Potassium-argon data on the age of Laacher volcanism, the Rhine terraces and the ice ages . In: Ice Age and the Present . tape 16 , 1965, pp. 5-30 .