Rochechouart crater

45.8166666666670.78333333333333Koordinaten: 45 ° 49 '0 "N, 0 ° 47' 0" E

The crater Rochechouart - Chassenon is a nearly 200 million year old impact crater in southwestern France. The approximately 20 kilometers in diameter amount end impact structure was caused by an asteroid that struck towards the end of the Triassic to the basement of the western Massif Central's.

Description of the structure

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

Because of its Mesozoic age today of the structures of the original meteorite crater (Wall, central mountain or central ring ) is topographically preserved nothing more. The only evidence of the former depression is perhaps the Vienne, which is deflected to the crater center towards the north of the crater area of its western course quite strongly to the south. In addition to the Vienne drain the Charente ( upper reaches ), the Graine and the Gorre the crater area in northwestern direction. This brings the current topography expressed in comparison to the base level of flowing through the north Vienne (about 160 meters ) to the south and east of the crater structure is up to 150 meters higher.

Have been preserved, it left a crater in the deeper layers of the interior Impaktbrekzien ( suevite and debris masses) as well as the underlying, modified by shock metamorphism crystalline Variscan basement.

The suevite are not uniform, but can be divided into three very different facies, which also have a separate geographic distribution. The underlying debris masses ( Brèches de Rochechouart ) are continuous in their distribution and cover an elliptical area of about 14 × 11 km.

History

The impact rocks of Rochechouart crater Chassenon been known since the 1770s and have been described by Nicolas Desmarest for the first time, she was still indicated as " bands of granite ". Until the scientific recognition of Impaktnatur the Rocks ( by the work of François Kraut in 1969 ) there was under French geologist various attempts to explain the impact rocks, the majority as a volcanic origin ( explosive in nature ) were considered (even a connection with the volcanic fields in the eastward located Auvergne was considered). As early as 1966 F. were noticed cabbage in thin sections shock crystals, a little later, he discovered also Pseudotachylite and 1969 eventually along with American geologists cone of rays. Since shock Crystals and cone beam only under the extreme conditions of shock metamorphism can occur, as they are only achieved when the impacts of meteorites or nuclear explosions, came as an explanation for the puzzling breccias of Rochechouart therefore only an extraterrestrial cause in question.

Geology

The polymetamorphic crystalline basement rocks of the north-western Massif Central, in the struck the meteorite crater area consists of the following stones:

• Paragneiss of the Lower and Upper Gneiss ceiling.
• Leptynitgneise the Lower Gneiss ceiling.
• Foliierter leucogranite massif of Chéronnac ( Chéronnac - Leucogranit ).
• Granitoids (two- mica granites ) of Saint- Gervais ( Saint- Gervais- granite ) and Chirac Étagnac ( Chirac Étagnac - granite).
• Quartz diorite of Exideuil ( Exideuil - quartz diorite ).
• Micro Granite.
• Lamprophyres.

The crater touches in its northeast corner just in the overthrust of the superiors of the Lower Gneiss ceiling. In the Lower Gneiss ceiling appear even smaller scattered occurrences of amphibolites and serpentinites.

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

• Ballistic deposits: Polygenic breccias.

• Monogenic breccias.
• Cataclasites.
• Hydrothermal breccias.
• Pseudotachylite.
• Cone of rays.

Polygenic breccias

The polygenes breccias are clearly allochthonous nature and can be distinguished in the debris masses of Rochechouart breccia at the base and in the overlying suevite. The Rochechouart breccia contains virtually no glass residues, whereas the deposited from pyroclastic flow Suevite some very rich in glass ( and devitrification ) are.

Rochechouart breccia

The concrete-type Rochechouart breccia consists of a very fine-grained, pure clastic matrix are stored in the most angular fragments of the basement. The matrix represents the former explosion dust mingled with the ejected out of the basement rock fragments and then settling to the crater floor. There he compressed by the prevailing pressure, high temperature and the time factor. The size of rock fragments contained in the matrix is highly variable, it moves generally between the millimeter and meters range. The color of the rock is also very variable and depends on the predominant rock type among the fragments. Occasionally, graduation, a phantom -like layering, sometimes a preferred direction in the breccia are recognized. Also primary and secondary cavities are observed.

The Rochechouart breccia is found in a radius of 5 to 7.5 km from the impact center. It takes a moderately shallow for the largest share among the Impaktbrekzien. But they do not occur as contiguous ceiling, but is distributed over several sections (main occurrence in Bors, mandate and Videix, occurrence at Chassenon, deposits at Rochechouart and other minor scattered occurrence).

Under the microscope can be observed in the matrix following phenomena:

• Shock Crystals. These were discovered in 1966 by F. herb. The crystals have a very closely spaced ( in the micron range ) " Pseudospaltbarkeit ", but in reality fine Dislokationsebenen (deformation lamella ) represents in which the crystal lattice has been sheared due to the explosion pressure (also known as planar elements or in English as PDF - called - planar deformation feature ).
• Kink bands in biotite and feldspars.
• Discoloration of biotite ( Ferrugenisierung ). The iron was partially dissolved out and deposited as a crypto-crystalline limonite.
• Generally broken crystals.
• Twisted field sparrow Willinge.

But the rock fragments bear the signs of shock metamorphism, so come in the feldspars the following effects to the fore:

• Loss of birefringence.
• Collapse of the crystal lattice.
• Isotropisation.
• Incipient melting.

Chassenon - Suevite

In Chassenon the Rochechouart breccia gradually changes into the overlying Chassenon - Suevite. It may contain up to 15 volume percent of glass. The characteristic of this Suevite color is green because of its strong enrichment of nickel oxide ( mainly in the glass content ), but can also at times appear colorful because reddened basement fragments. In contrast to the Rochechouart breccia clasts contained in it in centimeter size are much smaller. The cavities are increasingly disappearing. The glass inclusions contained in the gray - green matrix, millimeter - to centimeter-sized envelop themselves occasional rock fragments. The glass is usually of dark green color and has experienced a strong transformation in clay minerals.

The Chassenon - Suevite remains restricted to the immediate vicinity of Chassenon, where it holds an elliptical area of 3.5 × 2 km to. The Roman settlement Cassinomagus (including spas ) was built with it. In the former Roman quarry still a well- layered, horizontal, ash -like situation could previously be made out of the finest basement fragments ( Cinerit ) above the suevite.

Microscopically, the Chassenon - Suevite the same effects as the Rochechouart breccia. The glass appears dark green to light yellow, rarely colorless under the microscope. It has Fluidaltextur and contains vacuoles ( bubbles). The vacuoles can be filled with tridymite. The refractive index is greater than 1.52, which to this composition trachytic diaplektischen glass as compared to the same composition volcanic glasses is abnormally high for the rhyolite.

Montoume - Suevite

The Montoume - suevite occurs in three separate deposits in the south of the crater. In Montoume (municipality Chéronnac ), the southern and far secreted occurrence, it is very likely directly on the crater floor, in mandate ( municipality of Saint -Gervais ) and Videix on the Rochechouart breccia. It contains many glass inclusions. Its deep red color is due to a high content of iron ( iron oxides or hydroxides ), which most likely originates from the meteorites. The suevite contains occasional black inclusions of manganese oxide which probably also comes from the meteorite or represents a hydrothermal alteration product.

In addition to its color, the Montoume - suevite from Chassenon - Suevite distinguished by its higher glass content. The glass has a blood-red to violet-red color and has been less transformed than in Chassenon - Suevite. There may be mixed both as isolated inclusions occur as well as with the clastic matrix. Based on this criterion, the Montoume - Suevite can therefore differ in two lithological subtypes.

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

The Montoume - Suevite has the same microscopic shock effects such as the Chassenon - Suevite. Minor discrepancies relate to the glass, which is much poorer in vacuoles, and the shock crystals which are less affected by dislocations. The Isotropisation is overall very strong and biotite are always ferruginisiert. The glass in the matrix is usually fluidalisiert with up to dezimetergroßen streaks. It also contains K-feldspar microliths.

Babaudus - Suevite

The Babaudus - Suevite is a Impaktschmelze ( English impact melt breccia ) and occurs in the crater center. It is only several meters thick and overlies the Rochechouart breccia. He remains on several smaller, isolated from each occurrence is limited, for example in Fonceverane and La Valette (municipality Pressignac ) and in Babaudus, Petits - Ajaux and Recoudert (municipality Rochechouart ).

The rock fragments occur in their frequency back very, their size is between 2-3 inches. Their outlines are indistinct and they are usually interspersed with vacuoles. Due to the high melting the original mineralogy of the fragments is usually no longer visible. However, occasionally porphyritic micro- granite, paragneiss and Leptynitgneis can be identified in line with the upcoming basement rocks of the crater floor granite - granodiorite. The matrix generally takes a very high percentage, it is very rich in vacuoles and Amygdalen; at the Amygdalen (almonds ) outweigh ausgelängte forms (up to several centimeters) compared to spherical shapes ( millimeter range ). The vacuoles are in the central area of phyllosilicates filled ( iron-rich chlorite, smectite, illite, chromium-bearing phengite ), which in turn are clothed with a hematite or Orthoklasaureole or a sequence of these two minerals.

Microscopically takes the glass fraction in two varieties on: once as a nearly colorless and bubble- rich glass or as a yellowish to greenish glass of high Fluidalität, recognizable Eisenhydroxidschlieren. The latter Glasvarität is very rich in orthoclase microlites in the submillimeter range ( 0.01 to 0.01 mm).

Rock analyzes of Babaudus - Suevits are characterized by an extremely high K2O content of (mean 10.2% ), ie the glass phase must be enriched in potassium. The high potassium content may be due to hydrothermal phenomena during the cooling phase. Also the nickel and chromium values ​​are greatly increased (up to 600 ppm, 150 ppm in average of nickel, up to 310 ppm of chromium ). The nickel is enriched up to a factor 40 compared to the basement and comes undoubtedly from the meteorites.

In Babaudus - Suevite several facies can be distinguished:

• Yellowish -beige facies. The dominant facies with extremely high melting, the fragment size is one centimeter. Occurrence in La Valette and near Fonceverane.
• Purple -colored, bubble- rich facies. This facies is very poor in rock fragments. The vacuoles are extended and adjusted. They are lined by iron hydroxides or an amorphous greenish mineral. Occurs southwest of La Valette.
• Gray - red, bubble- poor facies. The matrix is gray to purple in color. The rock fragments consist of gneiss, granite and micro- granite, their size ranges between 1-50 inches. They are often encircled by the capricious fluidalisierten matrix. Deposits west of La Valette.
• Greenish to colorful facies. Was found only in the form of rock fragments in the crater center.

Monogenic breccias

The monogenic breccias, also called dislocation or Fragmentationsbrekzien, can be found in the crater floor. They come here usually to lie over the cataclasites and generally consist only of a rock. They are usually ( sub) autochthonous nature, ie more or less remained at their original education center ( in - situ breccias ). In their structure they show fragments of the basement in the centimeter to meter range, which were only slightly shifted relative to each other. In between cavities can arise that remain empty or filled with an incurred by the mutual abrasion clastic matrix. This matrix remains in its volume but generally of minor importance.

Immediately after the impact the crater floor has experienced significant relief movements. This had the consequence that the breccias are not always remained in their native position and as Brekziengänge ( English breccia dykes ) was injected into higher altitudes and could be " sucked ". The monogenic nature of the breccias is also no longer guaranteed the borderline between two different rock types.

The fact that the internal stratigraphy of the Dislokationsbrekzien can be very complex, proved a research drilling at Chéronnac, the exchange between several layers of Dislokationsbrekzien and cataclasites ( with associated pseudotachylites ) was interposed itself a position of Rochechouart breccia.

Cataclasites

The cataclasites generated by the impact differ significantly from the interference generated at namesakes. So they have no structure control, but have it open columns up to 1 centimeter in width. The columns can be arranged in a star -shaped; they are usually lined by iron hydroxides. Stretching directions or Plättungsebenen can not be identified.

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

Hydrothermal breccia

The hydrothermal breccias occur only in the quarry of Champagnac (municipality Rochechouart ), in which the former crater floor is open. The crater floor is represented by a dark blue enamel layer ( Mikrobrekzie ) in the centimeter to decimeter, maybe it is also a flat-lying disorder ( shearing ).

The hydrothermal breccias are at or just below the melting layer. You have priority to develop into competent rocks of the crater floor, such as in tonalite, granodiorite, Leukograniten and lamprophyres. The hydrothermal alteration has affected through the following processes, the clasts of the breccia:

• Silicification ( silicification ) and impregnated with pyrite, arsenopyrite and fluorite ( rare).
• Chloritization and carbonation of the ferromagnetic Saxon minerals ( biotite, amphibole ).
• Serizitisierung the primary Alkalifeldspats.
• Pseudomorphisierung of the plagioclase ( by white mica, carbonates, secondary alkali feldspar ).

The white to gray- colored quartz can be massive, banded, or act as geodes. The carbonation was carried out by calcite (mainly ), ankerite, siderite and dolomite. The sulfide minerals are scattered or found in small clusters.

North-south oriented Fiederspalten and other breaks in the upcoming basement are also lined with the same minerals.

Pseudotachylite

The Pseudotachylite are glassy mylonites that emerged through intensive, mechanical friction of fault zones. They also occur in the quarry of Champagnac, also they were encountered in the bore of Chéronnac. In Champagnac they are usually associated with the hydrothermal breccias. The Pseudotachylite form dark to gray-green veins in the millimeter to decimeter and like the hydrothermal breccias limited to competent rocks of the crater floor. They, too, have experienced a partial to total silicification and chloritization, caused by the same hydrothermal fluids ( mineral assemblage quartz - pyrite - carbonate ).

Cone of rays

The discovery of radiation cones in 1969 confirmed the Impaktnatur the crater Rochechouart - Chassenon. The cones can reach a height up to 30 inches, with taper of several smaller sub- cones are larger than 5 centimeters often assembled ( proportional to the master cone). They usually deviates from the geometric ideal shape from, often show curved sides and sometimes even resemble ponytails.

The cone of rays are predominantly found in competent rocks of the crater floor, especially in isotropic transition rocks such as granites or micro lamprophyres. Very nice cone of rays can be observed in the quarry of Champonger (municipality Chassenon ). The cones are usually more or less upright in the rock ( incidence angle 75 ° - 90 ° ), with the apex pointing upwards or towards the crater center. In less competent rocks such as paragneisses, amphibolites Leptynitgneisen or the cones are barely recognizable as such, they appear here like a fan and strongly flattened.

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

Age dating

The impact must have been made after the Variscan orogeny, have the youngest rocks ( lamprophyres ) in space Rochechouart an age of 295 million years ago. Paleomagnetic dating methods yielded the period 200 - 180 million years BP. Several radiometric investigations were carried out, provided the following results:

• K / Ar: 271 - 149 million years BP.
• Ar / Ar: 214 million years BP.
• Rb / Sr: 186 ± 8 million years BP.
• Fission tracks: 206 - 198 million years BP.

Recent studies at the University of Heidelberg commute obviously with the time section 201 - 200 million years BP a, ie at the very end of the Triassic ( Rhaetian ). May thus have contributed to the mass extinction at the end of the Triassic with the impact event of Rochechouart. Endtriassische Tsunamite to the Anglo-Norman islands in the English Channel would thus be explained as well.

Built in 1997 by S. P. Kelley and JG Spray determined by the argon method the age of 214 million years BP of the early Noriums was extended period of time generally supported; it provided the occasion for a multiple impact theory, as other impact crater as the Manicougan and the crater of Saint -Martin fall in Canada in the same period and is similar to the impact event of the comet Shoemaker- Levy 9 on Jupiter palaeogeographic reconstruction of the crater Rochechouart can be connected to a consecutive Einschlagskatena - Chassenon.

In an age of 200 million years BP, the Red Wing crater and the Wells Creek crater in the United States would be the same time as the crater of Rochechouart Chassenon.

Nature of the impactor

On the nature of the impactor, opinions are still apart. There are three hypotheses:

• Iron meteorite magmatic origin. Type IIA or IIAB. This is supported by the work of Janssens 1976-1977 and Schmid, Palm & Scratch 1998.
• Iron meteorite non- magmatic origin. Type II or IA or IIC. Advocated by Tagle & Stöffler, 2003, and of Tagle, Schmitt & Erzinger 2009.
• Stony meteorite ( chondrite ). Endorsed by Horn & El Goresy 1980 Shukolyukov & Lugmair 2000 and Koeberl, Shukolyukov & Lugmair 2007.

The impactor is expected to come but despite the still unresolved question type is almost certainly out of the asteroid belt.

Uncertainty also exists about the Impaktkörper itself. Was it a single, homogeneous asteroid or was it a composite body? The latter assumption could explain the wide variety of settings Suevite better with their specific spatial arrangement.

Physical considerations and implications of the impact event

In the crater of Rochechouart Chassenon carried out gravity measurements (see map above ) resulted in a nearly circular crater center, negative anomaly, the value of at its center - reaches 10 mGal. Its diameter is about 20 kilometers. This value may well represent the original size of the crater roughly 15 kilometers as the minimum value are certainly secured from the spatial distribution of Rochechouart breccia.

The missing mass is due to the impact, according to the model calculations blew out a 2 km deep crater transit. In this case, the impactor vaporized completely. Just a little later then translated already the compensating movements in the crater inside a ( English rebound ), which result in large, complex craters mostly to the emergence of a central mountain ( or central ring, with craters larger than 20 km diameter). Whether the crater Rochechouart Chassenon ever had such a central mountain / central ring remains to be seen. In any case there is no evidence for it.

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

• Log E = 3.045 * log D 24.129

Assuming a diameter of D = 20 km results in a kinetic energy of 1.2 * 1028 erg and 1.2 * 1021 joules. About equating with Ekin = 1/ 2 * m * v2 can be estimated for given velocities of the meteorite then its mass. So at a speed of v = 20 km / sec followed by a mass of 6 billion tons, at v = 50 km / sec nevertheless still a mass of 1 billion tons. Above, the formula for the volume of a sphere and an assumed density of the impactor of 3.4 g/cm3 ( density of chondrites ) finally calculated values ​​for the radius of 400-750 meters or a diameter of 0.8 to 1, 5 km.

To get an idea of the enormous amount of energy released during the impact, may as a comparison the energy released during an earthquake energy be used. For example, setting the strongest known earthquake of magnitude 9 on the Richter scale around 1025 erg free. The meteorite of Rochechouart had therefore more than a thousand times higher energy than, say, the earthquake of Valdivia in 1960 and the earthquake in the Indian Ocean in 2004!

Model calculations ( for example, with the Earth Impact Effects Program) further show that in less than 5 minutes all life had been destroyed in a radius of 100 kilometers. Animals and plants suffered severe to severe burns for up to a radius of 300 kilometers. The ejecta of the crater covered an area of more than 450 kilometers, the basement has been permanently altered at the felling site to a depth of 5 kilometers.

It is very likely that the impact is not likely to have occurred near the former Coast ( the Liassic transgression in the Aquitaine Basin, penetrated up to the room Rochechouart, was only slightly later) and thus triggered a giant tsunami (see the next above- mentioned Tsunamite the Anglo-Norman islands).

Importance

The crater Rochechouart - Chassenon is the seventh largest impact craters in Europe. For the crater research of importance is the well-preserved him in the middle and lower ranges ( crater floor ). It shows in particular a very wide range of typical for shock metamorphism structures of broken crystals on planar elements in quartz, kink bands in micas, diaplectic glasses, real melting glass to the evaporation of silicates, which expressed in the bubble-containing suevites of Babaudus type place. There are therefore likely to temperatures up to 10 000 ° C and pressures between 100 and 1000 GPa have been realized. Curiously, neither the quartz - stishovite high-pressure modification nor coesite were found so far.