Rochechouart-Chassenon crater

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Coordinates: 45 ° 49 ′ 0 ″  N , 0 ° 47 ′ 0 ″  E

Map: France
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Rochechouart-Chassenon​
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France

The Rochechouart-Chassenon crater is an approximately 200 million year old impact crater in southwest France . The impact structure, which is around 20 kilometers in diameter, was caused by an asteroid that hit the basement of the western Massif Central towards the end of the Triassic .

Description of the structure

Map of Rochechouart-Chassenon crater. Dashed the presumed extent of the crater. Gravity anomalies in gray. The star marks the center of the impact structure.

The Rochechouart-Chassenon crater was named after the small town of Rochechouart in the Haute-Vienne department and the town of Chassenon in the Charente department . The actual center of the crater, determined by the Bouguer - gravity anomalies , but is 4 kilometers further west from Rochechouart in the hamlet of La Judie in the town of Pressignac (Charente).

Due to its Mesozoic age, nothing of the topographical structure of the original meteorite crater (wall, central mountain or central ring) has survived. The only sign of the previous depression is possibly the Vienne , which in the north of the crater area is diverted quite strongly from its west course to the south towards the crater center. In addition to the Vienne, the Charente (upper reaches), the Graine and the Gorre drain the crater area in a north-westerly direction. This expresses the current topography, which is up to 150 meters higher in the south and in the east of the crater structure compared to the base level of the Vienne flowing through in the north (approx. 160 meters).

The layers of impact breccias ( suevites and debris ) left behind in the deeper interior of the crater, as well as the underlying crystalline, Variscan basement, modified by shock wave metamorphosis , have been preserved .

The Suevites are not uniform, but can be divided into three very different facies, which also have a separate spatial distribution. The underlying debris ( Brèches de Rochechouart ) are more coherent in their distribution and cover an elliptical area of ​​about 14 × 11 kilometers.

history

The impact rocks of the Rochechouart-Chassenon crater have been known since the 1770s and were first described by Nicolas Desmarest , who at that time still interpreted them as "ribbon granite". Until the scientific recognition of the impact nature of the rocks (through the work of François Kraut in 1969), there were various attempts to explain the impact rocks among French geologists , the majority of which were considered to be of volcanic origin (explosive nature) (even a connection with the volcanic fields in the Auvergne further east had been considered). As early as 1966, F. Kraut noticed shock quartz in thin sections , a little later he discovered pseudotachylite and in 1969 finally together with American geologists cones of rays . Since shock crystals and cones of rays can only arise under the extreme conditions of shock wave metamorphosis, as can only be achieved with impacts by meteorites or nuclear explosions, the only possible explanation for the enigmatic Breccias of Rochechouart was an extraterrestrial cause.

geology

The polymetamorphic, crystalline basement of the northwestern Massif Central , into which the meteorite struck, consists of the following rocks in the crater area:

In its northeast corner, the crater just touches the thrust of the upper over the lower gneiss cover. In the lower gneiss cover there are even smaller, scattered occurrences of amphibolites and serpentinites .

The actual impact rocks inside the crater can be broken down as follows:

Polygenic breccias

The polygenic breccias are clearly of an allochthonous nature and can be differentiated into the debris of the Rochechouart breccia at the base and the suevites above. The Rochechouart breccia contains almost no glass residues, whereas the suevites deposited from the glowing cloud are sometimes very rich in glass (and devitrification).

Rochechouart breccia

Rochechouart breccia handpiece

The concrete-like Rochechouart breccia consists of a very fine-grained, purely clastic matrix in which mostly angular fragments of the basement are embedded. The matrix represents the former explosion dust that mixed with the rock fragments ejected from the basement and then settled on the crater floor. There it condensed due to the prevailing pressure , the high temperature and the time factor. The size of the rock fragments contained in the matrix is ​​very variable; it generally ranges between the millimeter and meter range. The rock color is also very variable and depends on the predominant type of rock under the fragments. Occasionally gradation , a phantom-like layering , sometimes also a preferred direction in the breccia can be recognized. Primary and secondary cavities can also be observed.

The Rochechouart breccia is found within a radius of 5-7.5 kilometers from the impact center. In terms of area, it occupies the largest proportion of the impact breccias. However, it does not appear as a coherent blanket, but is distributed over several sub-areas (main deposits near Bors , Mandat and Videix , deposits near Chassenon, deposits near Rochechouart and other smaller, scattered deposits).

The following phenomena can be observed in the matrix under the microscope :

  • Shock crystals. These had already been discovered by F. Kraut in 1966. The crystals have a very engständige (microscale) "Pseudospaltbarkeit", but in reality the finest Dislokationsebenen ( deformation lamella represented) at which the crystal lattice is sheared due to the explosion pressure (also known as planar elements or in English as a PDF - planar deformation feature called - ).
  • Kink bands in biotite and feldspars .
  • Discoloration of the biotite (ferrugenization). The iron was partially leached out and deposited as cryptocrystalline limonite .
  • Generally broken crystals.
  • Twisted Feldsparrow twins.

But the rock fragments also bear the signs of shock wave metamorphosis, so the following effects come into play in the feldspars:

  • Loss of birefringence
  • Collapse of the crystal lattice
  • Isotropization
  • Beginning of melting

Chassenon suevite

The Chassenon suevite

At Chassenon, the Rochechouart breccia gradually merges with the Chassenon suevite above. This can contain up to 15 percent by volume of glass. The color characteristic of this suevite is green due to its strong accumulation of nickel oxide (mainly in the glass portion), but it can also appear colorful in places due to reddened basement fragments. In contrast to the Rochechouart breccia, the fragments of rock it contains are much smaller in size. The cavities are also disappearing. The millimeter to centimeter-sized glass inclusions contained in the gray-green matrix in turn encase occasional rock fragments. The glass is mostly dark green in color and has undergone a strong transformation into clay minerals .

The Chassenon suevite remains limited to the immediate vicinity of Chassenon and takes on an elliptical area of ​​3.5 × 2 kilometers there. The Roman settlement Cassinomagus was supposedly built from it. In the former Roman quarry, a well-layered, horizontal, ash-like layer made of the finest basement fragments (cinerite) could be made out above the suevite.

Microscopically, the Chassenon suevite shows the same effects as the Rochechouart breccia. Under the microscope, the glass appears dark green to light yellow, rarely colorless. It has a fluid texture and contains vacuoles (bubbles). The vacuoles can be filled with tridymite . The refractive index is greater than 1.52, which is abnormally high for the rhyolitic to trachytic composition of this diaplectic glass compared to volcanic glasses of the same composition.

Montoume suevite

The Montoume suevite

The Montoume suevite occurs in three separate occurrences in the south of the crater. At Montoume (municipality of Chéronnac ), the southernmost and widely isolated occurrence, it is very likely directly on the crater floor, at Mandat (municipality of Saint-Gervais) and at Videix above the Rochechouart breccia. It contains a lot of glass inclusions. Its deep red color is due to a high content of iron (iron oxides or hydroxides), which most likely comes from the meteorite. The suevite occasionally also contains black inclusions of manganese oxide, which may also come from the meteorite or represent a hydrothermal conversion product.

In addition to its color, the Montoume suevite differs from the Chassenon suevite by its higher proportion of glass. The glass has a blood-red to violet-red color and is less transformed than in Chassenon suevite. It can occur both as isolated inclusions and mixed with the clastic matrix. Based on this criterion, the Montoume suevite can be divided into two lithological sub-types.

The rock fragments have a fairly uniform size in the centimeter range, but are very heterogeneously distributed. The rock types present among the fragments vary considerably due to the varied geology of the crater floor; in addition to the predominant gneisses, granites, granodiorites and / or micro- granites are also present.

The Montoume suevite has microscopically the same shock effects as the Chassenon suevite. Smaller deviations affect the glass, which has significantly fewer vacuoles, and the shock crystals, which are less affected by dislocations. The isotropization is very pronounced overall and the biotites are always ferruginized. The glass in the matrix is ​​mostly fluidized with streaks up to a decimeter in size. It also contains potassium feldspar microliths.

Babaudus suevite

The yellow impact melt of the Babaudus suevite
The violet-colored, blistered facies of Babaudus suevite
The gray-red facies of Babaudus suevite with few blisters

The Babaudus suevite is an impact melt (English impact melt breccia ) and occurs in the crater center. It is only several meters thick and lies above the Rochechouart breccia. It remains limited to several smaller, isolated occurrences, for example at Fonceverane and La Valette (municipality of Pressignac) and at Babaudus , Petits-Ajaux and Recoudert (municipality of Rochechouart).

The frequency of the rock fragments is much lower, their size is between 2–3 centimeters. Their outlines are indistinct and they are mostly interspersed with vacuoles. Due to the high degree of melting, the original mineralogy of the fragments can usually no longer be recognized. Occasionally, however, granite-granodiorite, porphyry microgranite, paragneiss and leptynite gneiss can be identified in harmony with the basement of the crater floor. The matrix generally occupies a very high percentage, it is very rich in vacuoles and amygdals; in the amygdals (tonsils) elongated shapes (up to several centimeters) predominate over spherical shapes (millimeter range). The vacuoles are filled in the central area by phyllosilicates (iron-rich chlorite , smectite , illite , chromium-containing phengite ), which in turn are surrounded by a hematite or orthoclase aureole or a sequence of these two minerals.

Microscopically, the glass fraction appears in two varieties: on the one hand as almost colorless glass rich in bubbles or as yellowish to greenish glass of high fluidity, recognizable by iron hydroxide streaks. The latter variety of glass is very rich in orthoclase microliths in the sub-millimeter range (0.01-0.01 millimeters).

Rock analyzes of the Babaudus suevite are characterized by an extremely high K 2 O content (mean value at 10.2%), i.e. H. the glass phase must be enriched in potassium. The high potassium content is possibly due to hydrothermal phenomena during the cooling phase. The nickel and chromium values are also greatly increased (up to 600 ppm, mean value at 150 ppm for nickel; up to 310 ppm for chromium). The nickel is enriched up to a factor of 40 compared to the basement and undoubtedly comes from the meteorite.

Several facies can be distinguished in Babaudus suevite:

  • Yellowish-beige facies. Predominant facies with extremely high melting, the fragment size is one centimeter. Occurrence near La Valette and near Fonceverane.
  • Violet facies rich in blisters. This facies is very poor in rock fragments. The vacuoles are stretched and regulated. They are lined with iron hydroxides or an amorphous greenish mineral. Occurs southwest of La Valette.
  • Gray-red facies with few blisters. The matrix is ​​gray to purple in color. The rock fragments consist of paragneiss, granite and micro-granite, their size ranges between 1–50 centimeters. They are often capriciously surrounded by the fluidized matrix. Occurrence west of La Valette.
  • Greenish to colorful facies. Only found in the form of reading stones in the center of the crater.

Monogenic breccias

The monogenic breccias, also known as dislocation or fragmentation breccias, are found in the crater floor. They usually come to lie above the cataclasites and generally consist of only one type of rock. They are usually (sub) autochthonous in nature, i. H. more or less remained at their original place of formation (in-situ breccias). In their structure, they show fragments of the basement in the centimeter to meter range, which have only been shifted relatively slightly against each other. In between, cavities can arise which remain empty or are filled with a clastic matrix created by mutual abrasion. In terms of its volume, however, this matrix remains of subordinate importance.

Immediately after the impact, the crater floor experienced significant relief movements. This meant that the breccias not always remained in their native position and as Brekziengänge (English breccia dykes injected) to higher altitudes or "sucked" could be. The monogenic character of the breccias is also no longer guaranteed in the border area between two different types of rock.

The fact that the internal stratigraphy of the dislocation breccias can be very complex was proven by a research borehole near Chéronnac, which itself found a position of the Rochechouart breccia sandwiched between several alternating layers of dislocation breccias and cataclasites (with associated pseudotachylites).

Cataclasites

The cataclasites produced by the impact differ significantly from their namesakes produced by faults . They have no structural regulation whatsoever, but instead have open gaps up to 1 centimeter wide. The columns can be arranged in a star shape; mostly they are lined with iron hydroxides. Direction of stretching or planes of flattening cannot be seen.

The cataclasites reach a thickness in the tens of meters range. They are usually found below the dislocation breccias.

Hydrothermal breccias

The hydrothermal breccias only occur in the Champagnac quarry (Rochechouart municipality), where the former crater floor is exposed. The crater floor is represented by a dark blue layer of enamel (micro-breccia) in the centimeter to decimeter range; this may also be a flat fault (shear).

The hydrothermal breccias are on or directly below this enamel layer. They were able to develop primarily in the competent rocks of the crater floor, such as tonalites , granodiorites, leukogranites and lamprophyren . The hydrothermal transformation affected the rock fragments of the breccia through the following processes:

The white to gray colored quartz can be solid, banded or geode . The carbonation was carried out by means of calcite (mainly), anchorite , siderite and dolomite . The sulfide minerals are scattered or found in small agglomerations.

North-south facing fissures and other fractures in the adjacent basement are also lined with the same minerals.

Pseudotachylite

Pseudotachylite from the Champagnac quarry

The pseudotachylites are glassy mylonites that were created by intensive, mechanical friction at fault zones. They also occur in the Champagnac quarry and were found in the Chéronnac well. In Champagnac they are mostly associated with the hydrothermal breccias. The pseudotachylites form dark to gray-green veins in the millimeter to decimeter range and, like the hydrothermal breccias, are limited to competent rocks of the crater floor. They too have undergone partial to total silicification and chloritization, caused by the same hydrothermal fluids ( paragenesis of quartz-pyrite-carbonates).

Cone of rays

30 centimeter high beam from the granite of Saint-Gervais

The discovery of cones of rays in 1969 confirmed the impact nature of Rochechouart-Chassenon crater. The cones can reach a height of up to 30 centimeters, whereby cones larger than 5 centimeters are often composed of several smaller sub-cones (proportional to the main cone). They usually deviate from the ideal geometric shape, often showing curved sides and sometimes even resembling ponytails.

The cones of rays are also mainly found in competent rocks of the crater floor, especially in isotropic gangue rocks such as micro-granites or lamprophyren. Very beautiful cones of rays can be seen in the Champonger quarry (Chassenon municipality). The cones are usually more or less upright in the rock ( angle of incidence 75 ° to 90 °), with the cone tip pointing upwards or towards the center of the crater. In less competent rocks such as paragneiss, leptynite gneiss or amphibolites, the cones are hardly recognizable as such, they appear here like a fan and strongly flattened.

Microscopic examinations of host rocks show micro-fractures, dislocations and loss of birefringence in feldspars and rare shock quartz crystals.

Age dating

The impact must have occurred after the Variscan orogeny, whose youngest rocks (lamprophyres) in the Rochechouart area are 295 million years old. Paleomagnetic dating methods found the period 200 to 180 million years BP . Several radiometric examinations were also carried out, which provided the following results:

  • K / Ar: 271 to 149 million years BP
  • Ar / Ar: 214 million years BP
  • Rb / Sr: 186 ± 8 million years BP
  • Fissure traces: 206 to 198 million years BP

The latest research by the University of Heidelberg is evidently leveling off at the period 201 to 200 million years BP, i.e. H. at the immediate end of the Triassic ( Rhaetium ). The Rochechouart impact event may have contributed to the mass extinction at the end of the Triassic. End Triassic tsunamites on the Anglo-Norman islands in the English Channel could also be explained.

The age of 214 million years BP of the early Norium , determined in 1997 by SP Kelley and JG Spray using the argon method , was generally supported for a long time; It gave rise to a multiple impact theory, since other impact craters such as the Manicouagan and the Saint-Martin crater in Canada fall in the same time period and are similar to the impact event of the Comet Shoemaker-Levy 9 on Jupiter, paleogeographically reconstructed with the crater of Rochechouart -Chassenon can be connected to one behind the other, one behind the other.

At an age of 200 million years BP, the Red Wing Crater and Wells Creek Crater in the United States would be coincident with Rochechouart-Chassenon Crater.

Nature of the impactor

Opinions still differ about the nature of the impactor. There are three hypotheses:

  • Iron meteorite of igneous origin. Type IIA or IIAB. Is supported by the work of Janssens 1976–1977 and Schmid, Palme & Kratz 1998.
  • Iron meteorite of non-igneous origin. Type IIE or IA or IIC. Is endorsed by Tagle & Stöffler 2003 and Tagle, Schmitt & Erzinger 2009.
  • Stone meteorite ( chondrite ). Endorsed by Horn & El Goresy 1980, Shukolyukov & Lugmair 2000 and Koeberl, Shukolyukov & Lugmair 2007.

The impactor is likely to originate from the asteroid belt , despite the still unresolved question of type .

There is also uncertainty about the impact body itself. Was it a single, homogeneous asteroid, or was it a composite body? The latter assumption could better explain the very different types of Suevites with their specific spatial arrangement.

Physical considerations and effects of the impact event

Gravity measurements carried out in the Rochechouart-Chassenon crater (see the map above) revealed an almost circular, negative anomaly in the center of the crater, which reaches a value of −10 mGal in its center  . Their diameter is around 20 kilometers. This value is likely to represent the original size of the crater, at least 15 kilometers have been secured as a minimum value based on the spatial distribution of the Rochechouart breccia.

The missing mass is due to the impact, which, according to model calculations, blasted out a transit crater 2 kilometers deep. The impactor evaporated completely. Only a little later, the compensatory movements in the interior of the crater (English rebound ) started, which in large, complex craters usually lead to the formation of a central mountain (or central ring, in craters over 20 kilometers in diameter). Whether the Rochechouart-Chassenon crater ever had such a central mountain / central ring remains to be seen. In any case, there is no evidence of this.

With Baldwin's simple empirical formula, the kinetic energy of the meteorite can be estimated approximately:

Assuming a diameter of D = 20 km results in a kinetic energy of 1.2 × 10 28 erg or 1.2 x 10 21 J . By equating the meteorite with E kin = 1/2 · m · v 2 , its mass can then be estimated for given velocities . So at a speed of v = 20 km / s a ​​mass of 6 billion tons follows, at v = 50 km / s still a mass of 1 billion tons. Using the formula for the volume of a sphere and an assumed density of the impactor of 3.4 g / cm³ (density of chondrites), values ​​of 400 to 750 meters or a diameter of 0.8 to 1 can be calculated for its radius. 5 kilometers.

In order to get an idea of ​​this enormous amount of energy released during an impact, the energy released during an earthquake may be used as a comparison . For example, the strongest known earthquakes with a magnitude of 9 on the Richter scale release around 10 25 ergs. The Rochechouart meteorite therefore had a more than a thousand times higher energy than z. B. the Valdivia earthquake in 1960 or the earthquake in the Indian Ocean in 2004 !

Model calculations (for example with the Earth Impact Effects Program ) also show that all life within a radius of 100 kilometers was destroyed in less than 5 minutes. Animals and plants suffered severe to severe burns up to a radius of 300 kilometers. The ejecta of the crater covered a radius of more than 450 kilometers and the basement was permanently changed at the point of impact to a depth of 5 kilometers.

It is very likely that the impact did not occur not far from the coast at that time (the Lias transgression in the Aquitaine Basin , which penetrated into the Rochechouart area, took place only slightly later) and thus triggered a gigantic tsunami (see below Tsunamites of the Anglo-Norman islands already mentioned above ).

meaning

The Rochechouart-Chassenon crater is the seventh largest impact crater in Europe . The well-preserved middle and deeper areas (crater floor) are of importance for crater research. In particular, it shows a very broad spectrum of the structures typical of shock wave metamorphosis: from broken crystals to planar elements in quartz, kink bands in mica , diaplectic glasses, real fused glasses to the evaporation of silicates, which are expressed in the bubble-containing suevites of the Babaudus type finds. Temperatures of up to 10,000 ° C and pressures between 100 and 1000 GPa are therefore likely to have been achieved. Strangely enough, neither the quartz high pressure modification stishovite nor coesite have been found so far .

literature

  • P. Chèvremont et al .: Rochechouart . In: Carte géologique de la France at 1/50 000 . BRGM, 1996, ISBN 2-7159-1687-6 .

Individual evidence

  1. ^ Nicolas Desmarest: Encyclopédie Méthodique, geographie physique . tome III. H. Agasse, Paris 1809.
  2. ^ Kraut, F .: Sur l'origine des clivages du quartz dans les brèches "volcaniques" de la région de Rochechouart . In: CR Acad. Sci . tape 264 , sér. D, no. 23 , 1967, p. 2609-2612 .
  3. ^ F. Kraut, N. Short, BM French: Preliminary report on a probable meteorite impact structure near Chassenon, France . In: Meteoritics . tape 4 , no. 3 , 1969, p. 190 .
  4. ^ Kraut, F .: Sur la présence de cônes de percussion ("shatter cones") dans les brèches et roches éruptives de la région de Rochechouart . In: CR Acad. Sci., Sér. D . tape 269 , no. 16 . Paris 1969, p. 1486-1488 .
  5. ↑ J. Pohl, H. Soffel: Paleomagnetic age determination of the Rochechouart impact structure (France) . In: Journal of Geophysics . tape 37 , 1971, p. 857-866 .
  6. ^ WU Reimold et al .: Rb-Sr-dating of the Rochechouart impact event and geochemical implications for the formation of impact breccia dikes . In: Meteoritics . tape 18 , 1983, p. 385-386 .
  7. ^ GA Wagner, D. Storzer: The age of the Rochechouart impact structure . In: Meteoritics . tape 10 , 1975, p. 503-504 .
  8. ^ M. Schmieder et al .: A Triassic / Jurassic boundary age for the Rochechouart impact structure (France) . In: 72nd Annual Meteoritical Society Meeting, abstract # 5138 . 2009.
  9. ^ M. Schmieder et al .: Did the Rochechouart impact (France) trigger an end-Triassic tsunami? In: 72nd Annual Meteoritical Society Meeting, abstract # 5140 . 2009.
  10. ^ MJ Janssens, J. Hertogen, H. Takahasti, E. Anders, P. Lambert: Lunar Science Institute, contribution 259 . 1976, p. 62 .
  11. ^ R. Tagle, D. Stöffler, P. Claeys, J. Erzinger: 34th Annual Lunar and Planetary Science Conference, March 17-21, 2003, abstract no. 1835 . League City, Texas 2003.
  12. ^ R. Tagle, RT Schmitt, J. Erzinger: Identification of the projectile component in the impact structures Rochechouart, France and Sääksjärvi, Finland: Implications for the impactor population for the earth . In: Geochimica et Cosmochimica Acta . tape 73 , no. 16 , August 15, 2009, p. 4891-4906 .
  13. ↑ W. Horn, AG Eloresy: Lunar and planetary science . tape XI , 1980, p. 468-470 .
  14. ↑ C. Koeberl, A. Shukolyukov, GW Lugmair: Chromium isotopic studies of terrestrial impact craters: Identification of meteoritic components at Bosumtwi, Clearwater East, Lappajärvi, and Rochechouart ' . In: Earth and Planetary Science Letters . tape 256 , no. 3–4 , 2007, pp. 534-546 .
  15. ↑ J. Pohl, K. Ernstson, P. Lambert: Gravity measurements in the Rochechouart structure . In: Meteoritics . tape 13 , 1978, p. 601-604 .
  16. ^ RB Baldwin: The measure of the moon . University of Chicago Press, 1963.
  17. ^ Earth Impact Effects Program

Web links

Commons : Rochechouart-Chassenon Crater  - Collection of images, videos and audio files