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that addresses also the formation of very small hypervelocity impact craters (Carancas, Peru, type).

Here we remind of an interesting abstract article presented at the 2011 Lunar and Planetary Science Conference (LPSC):

SEM and TEM analyses of minerals xifengite, gupeiite, Fe2Si (hapkeite?), titanium carbide (TiC) and cubic moissanite (SiC) from the subsoil in the Alpine Foreland: Are they cosmochemical? 

Authors: M. Hiltl 1, F. Bauer 2, K. Ernstson 3, W. Mayer 4, A. Neumair 4, and M.A.  Rappenglück 4 – 1 Carl Zeiss Nano Technology Systems GmbH, Oberkochen, Germany (mhiltl@online.de), 2 Oxford Instruments GmbH NanoScience, Wiesbaden, Germany (frank.bauer@oxinst.com), 3 University of Würzburg,Germany (kernstson@ernstson.de), 4 Institute for Interdisciplinary Studies, Gilching, Germany (info@mayer-chiemgau.de, agneumair@arcor.de, mr@infis.org).

The abstract may be downloaded HERE.

In addition to the images shown in the abstract paper we display here a photograph of one of the most highlighting iron silicide particles so far found in the area of the Chiemgau impact meteorite crater strewn field:

Fig. 1. Iron silicide particle (maximum size 18 mm) with cubic moissanite crystals (details see Fig. 2) sticking out from the matrix that is mostly composed of xifengite, Fe5Si3, and gupeiite, Fe3Si. An iron silicide, stoichiometrically Fe2Si, has also been analyzed, which may represent the rare mineral hapkeite. Hapkeite has so far been found on Earth only in the Dhofar 280 meteorite assumed to originate from the Moon.

Fig. 2. SEM image of moissanite crystals from the iron silicide specimen shown in Fig. 1. Source Carl Zeiss Nano Technology Systems GmbH.

An EBSD image of moissanite crystals together with titanium carbide TiC crystals in iron silicide matrix is shown in Fig. 3:

Fig. 3. Titanium carbide (TiC, dark gray), and silicon carbide (moissanite, SiC, black) crystals in a matrix of intergrowth of various iron silicides. Iron silicide particle from the Chiemgau impact meteorite crater strewn field. EBSD image; the field is c. 500 µm wide.

It is interesting to note that the silicon carbide moissanite (SiC) has been reported to occur together with the lonsdaleite diamond variety in impact melt rock from the Ries impact crater (Nördlinger Ries):

R. M. HOUGH, I. GILMOUR, C. T. PILLINGER, J. W. ARDEN, K. W. R. GILKESS, J. YUAN & H. J. MILLEDGE (1995): Diamond and silicon carbide in impact melt rock from the Ries impact crater. – Nature, 378, 41-44.

The authors suggest that the minerals formed by chemical vapour deposition from the ejecta plume in the impact event and hence may be used as a reliable diagnostic tool for hypervelocity impact on Earth.

In the case of the Chiemgau impact, the extreme purity of the moissanite and TiC crystals and their association with the xifengite and gupeiite iron silicides are rather speaking in favor of a primary cosmic origin however.

Kord Ernstson (2012)

Since a few years there is evidence of a doublet meteorite crater at the bottom of Lake Chiemsee (Fig. 3) in the Chiemgau meteorite crater strewn field. The search for a suspected impact into the lake was originally based on reports of fishermen about unusual sharp-edged large stones at the lake bottom that had damaged their fishnets. Such stones are in fact foreign matter in the lake. A general echo sounder campaign, followed by a detailed survey veritably revealed a peculiar structure, likewise a foreign element in the lake (Fig.1), with all evidence of a rimmed doublet crater. The similarity to meteoritic dual craters on Mars that formed synchronously by twin projectiles is striking (Fig. 2).

Fig. 1. The proposed meteorite doublet crater at the bottom of Lake Chiemsee from detailed SONAR echo sounder measurements. Meter scale indicates water depth.

Fig. 2. Meteoritic dual craters on Mars (image source NASA) and a counterpart at the bottom of Lake Chiemsee: A remarkable similarity. The more diffuse contours of the Lake Chiemsee craters is not surprising because of the impact into water and a loosely bound and water-saturated sedimentary target below.

More evidence of a meteorite impact into the lake came up with the frequent finds of pumice at the shore of Lake Chiemsee (more can be read HERE) and the observation of very young tsunami deposits uncovered in the environs of the lake (see HERE and HERE).

 Fig. 3. Location map for the Lake Chiemsee in southeast Bavaria hiding the probable meteorite doublet crater.

Unfortunately, because of the deep water the crater is not accessible directly.

ESA’s Mars Express has returned new images of an elongated impact crater in the southern hemisphere of Mars. Probably, the crater was caused by a train of projctiles and thus may be considered a model for the formation of the Rubielos de la Cérida elongated impact basin in Spain.

Rubielos de la Cérida impact basin. For comprehensive information click here.

PDFs are planar shock deformations in minerals (especially quartz) in the form of closely spaced isotropic lamellae following crystallographic planes (Fig. 1). They may be decorated by fluid and/or mineral inclusions as the result of annealing.

Fig. 1. Planar deformation features in quartz; Ries impact structure. Photomicrograph, crossed polarizers; the field is 460 µm wide.

According to current knowledge, PDFs cannot form in endogenetic geologic processes. Therefore, the occurrence of PDFs plays an important role in the establishment of authentic impact structures. And here, a problem turns out. In the past, curved PDFs have repeatedly been claimed to be of non-impact origin, and we mention for example the paper written by Reimold, W.U. & Koeberl, C. (2000): Critical Comment on: A.J. Mory et al. ‘Woodleigh, Carnavon Basin, Western Australia: A New 120 km Diameter Impact Structure EPSL v. 184, pp. 353-357). In this paper we read:“However, ‘planar’ and ‘locally curved’ represent a contradiction. There are several papers in the literature (e.g. [2,3]) that demonstrate that non-planar lamellae are not shock-diagnostic evidence.”

What are the facts? In general, PDFs in quartz are indeed straight and parallel (Fig. 1) due to crystallographic planes in an undeformed crystal lattice. However, as is well known, quartz crystals may undergo plastic deformations resulting in a deformation of the lattice. In thin section, these deformations are easily seen in the form of undulatory extinction on rotation of the stage of the polarization microscope. Obviously, the crystallographic “planes” in the crystal are no longer plane.

What happens when a shock wave impinges on an already plastically deformed quartz grain to produce PDFs in it? We think that according to definition, the PDFs will develop with respect to the deformed lattice, and, consequently, they must be bent.

What happens when a shock wave impinges on an undeformed quartz crystal to produce straight and parallel PDFs, and the crystal experiences a post-shock plastic deformation? According to the critics of bent PDFs, it’s true the lattice becomes deformed displaying undulatory extinction, but the PDFs must remain straight. As, however, they are no longer following the crystallographic orientation, these PDFs are no PDFs. Everything all right? We to the contrary see things much simpler and predict bent PDFs reflecting the deformation of the crystal lattice.

As an impact may affect a tectonically overprinted target, bent PDFs in pre-shock plastically deformed quartz must not be surprising. On the other hand, bent PDFs from post-shock plastic deformation are expected to develop in the impact cratering process itself (in the excavation and modification stages) not excluding a later tectonic overprint and further bending of PDFs.

Fig. 2. Two sets of slightly bent PDFs in quartz. Photomicrograph, crossed polarizers; the field is about 1.5 mm  wide. – Shocked quartzite cobble from crater 004 of the Chiemgau impact strewnfield (see. http://www.chiemgau-impact.com). 

We conclude and express (what we have been doing since many years) that curved PDFs (Fig. 2) must belong to the shock inventory of impact structures and that they may even be diagnostic if the bending is correlated with undulatory extinction proving crystallographic orientation. In Fig. 3 we show photomicrographs obtained from the rotation of the stage of a polarization microscope (xx polarizers), and we can clearly observe that the bent PDFs are intimately associated with the undulatory extinction.

Fig.3. Bent PDFs closely corresponding with undulatory extinction of the quartz grain. Photomicrographs taken by stepwise rotation of the stage of the polarization microscope. Same quartzite cobble as in Fig. 2.

In March and April 2000, Dr. Ann Therriault (Geological Survey of Canada, and co-worker of Dr. R.A.F. Grieve) made a very thorough PDF analysis of sedimentary samples from the Azuara impact structure in Spain. She analyzed quartz from a strongly shocked dike-breccia clast and from the Pelarda Fm. ejecta and found among 48 measured PDF sets 9.3 % basal, 40.7 % ω, 12.9 % π, 12.9 % ξ, 7.4 % r,z and subordinate s, x  and m PDFs. And Dr. Therriault explicitly stated in her report “… many curved PDFs!!“ (see Fig. 4, and for the PDFs histogram:https://www.impact-structures.com/impact-spain/the-azuara-impact-structure/shock-effects-shock-metamorphism-in-rocks-from-the-azuara-impact-structure/).

Fig. 4. Sets of bent PDFs in quartz; quartzite cobbles from the Pelarda Fm. ejecta of the Azuara impact structure (Spain).

We want to add that meanwhile there are other impact researchers having become aware of peculiar post-shock processes in impact target rocks, and we especially point to an LPSC XXXV (2004) abstract article (http://www.lpi.usra.edu/meetings/lpsc2004/pdf/1730.pdf) on the Charlevoix impact structure.  In Fig. 1 of that article nicely bent PDFs in quartz parallel to {0001} are shown (copied to our Fig. 5), and we cite from the text: “Bent planar deformation features (PDFs) can reflect the continuous bending of the crystal lattice (Fig. 1).”

Fig. 5. Bent planar deformation features (PDFs) in quartz from the Charlevoix impact structure. (from Trepmann and Spray 2004).

In fact and strictly speaking, “bent planar deformation features” is a contradiction in terms, but as long as we understand that PDFs refer to the crystal lattice and not to the mathematical definition of a plane, a renaming seems to be dispensable.

An Impact Crater Chain in Northern Spain

by K. Ernstson1, U. Schüssler2, F. Claudin3 & T. Ernstson4

Fig. 1. Location map for the Azuara – Rubielos de la Cérida impact crater chain (frame in Fig. 2) and suspected impact locations (A, B, C).

The term impact crater chain is commonly used when there are three or more lined up impact structures that have formed in a single impact event. Apart from a few established paired craters (e.g., Clearwater West and East in Canada, Ries and Steinheim Basin in Germany), three terrestrial crater chains with well-separated structures have so far been proposed. The alignment of eight structures in Missouri, USA, is peculiar, but age considerations do not speak in favor of a single event, and only two of them are established impact craters. The well-known Aorounga imapact structure in the Chad may be accompanied by one or possibly two additional craters, but respective field evidence is outstanding. In 1998, a single impact event thought to have formed five individual structures, though spread over two continents, has been suggested. On the other hand, multiple impact structures are well known from the Moon, Mars, Venus, and Jupiter’s satellites Ganymede and Callisto. Especially the detection of the impressive crater chains on Ganymede and Callisto, mysterious until the 1994 crash of the torn Shoemaker-Levy 9 comet with Jupiter, raised new interest and initiated investigations about multiple impacts by disrupted asteroids and comets. Here, we report on the previously established Mid-Tertiary Azuara impact structure in Spain and its nearby Rubielos de la Cérida companion crater as parts of a more or less continuous crater chain, the first of this kind known on Earth, of more than 100 km length (Fig. 1, 2).

Fig. 2. The topography of the Azuara/Rubielos de la Cérida crater chain. (from the digital map of Spain, 1 : 250,000; provided by Manuel Cabedo).

The detection of the suggested impact crater chain has been a long process. It begins in the eighties and early nineties with the establishment (by K. Ernstson and co-workers) of the 35 – 40 km-diameter Azuara impact structure at the margin between the Alpidic fold belt of the Iberian Chain and the Tertiary Ebro Basin (Fig.2). The structure is affected by advanced erosion and partly covered by post-impact sediments, and a very soft sedimentary target may have contributed to an altogether weak morphological signature. However, in the rim region, there is abundant and impressive evidence of the giant impact event macroscopically documented by the occurrence of various kinds of typical impact rocks, dislocated megablocks and extended impact ejecta. On a microscopic scale, strong shock metamorphism in the form of melt glass, diaplectic glass (a glass produced by shock damage and not by melting) and all kinds of planar deformation features in minerals is found. According to these occurrences, Azuara figures in the international Earth Impact Database of currently about 165 authentic impact structures worldwide.

In 1994, K. Ernstson and co-workers made a first reference to a suspected companion crater, the Rubielos de la Cérida structure, of similar size, located some 50 km south-southwest of the Azuara structure, and of the same Mid-Tertiary age. Meanwhile, Rubielos de Cérida presents all evidence of an impact structure. It shows the complete impact inventory as are a distinct morphology including a prominent central uplift, peculiar structural features, extended impact ejecta, suevite breccias, impact melt rocks, and strong shock metamorphism (Fig.3). As especially peculiar finds in and around the Rubielos de la Cérida structure, we mention

• melt rocks composed of nearly pure silicate glass.
• a carbonate-phosphate melt rock composed of calcite globules in a matrix of phosphate glass. This melt rock is assumed to reflect a liquid immiscibility of a carbonate and a phosphate melt similar to a liquid immiscibility of carbonate and silica melt found in impact rocks from, e.g., the Ries and Haughton craters.
• glassy amorphous carbon particles in a hitherto unknown bond with oxygen, possibly fullerenes. The amorphous carbon may have originated from extremely shocked limestones or as a quench product from shock-melted coal in the target rocks.
• large deposits of in situ shocked conglomerates which are composed of quartzite cobbles displaying prominent fractures and craters formed by collision and impact spallation.

Fig. 3. Photomicrograph of strongly shocked quartz from the Rubielos de la Cérida basin.

In a paper with more than 40 photographs and photomicrographs recently published by the Geological Museum in Barcelona (Spain) (see References), a comprehensive progress report on Azuara and Rubielos de la Cérida as the currently most prominent terrestrial doublet impact structure is given.

Despite the clear impact evidence, some aspects of the morphological signature of the Rubielos de la Cérida structure remained a worry. In the north and in the west, the structure is morphologically well defined to display a rim, a depression and a central uplift as is expected for a complex impact structure (Fig. 2). The morphological contours, however, become diffuse and leave a circular symmetry towards the east and the south. (The hilly terrain adjoining the central uplift in the northeast must not confuse, because it is built up of post-impact younger sediments.) In the beginning of a more detailed investigation of the Rubielos de la Cérida structure, we considered this morphology the probable result of a later tectonic overprint. This interpretation became questionable when we extended our field studies to the eastern and southern poorly defined rim regions of the Rubielos de la Cérida structure. Unexpectedly, we found more impact shock features in autochthonous rocks too strong as to have originated from the Azuara and/or Rubielos de la Cérida impacts. In the east, these finds focused on a nearly perfectly circular ring structure formed by steeply dipping beds. The diameter of this structure amounts to about 12 km, and it is called the Torrecilla ring or the Torrecilla impact structure (Fig. 2). The ring is suggested to have formed in the Azuara/Rubielos de la Cérida impact event by a separate projectile resulting in a morphological overlap east of the Rubielos de la Cérida central uplift.

Fig. 4. Part of the central uplift chain emerging from the Rubielos de la Cérida impact basin.

In the south, there is another explanation for the untypical morphology. Actually, Rubielos de la Cérida is not a circular impact structure but forms the northern end of an elongated basin (Fig. 2). Its rims are running roughly in the north-south direction, and also the Rubielos de la Cérida central uplift extends to the south in the form of a mountain chain more or less symmetrical to the rims of the elongated basin (Fig. 2, Fig. 4). This peculiar continuation of both the Rubielos de la Cérida central uplift and basin is closely connected with the omnipresent occurrence of well-known impact features in rocks already documented in the original Rubielos de la Cérida impact structure. The impact evidence comprises structural features, abundant monomictic and polymictic breccias and breccia dikes, extensive megabreccias and petrographical significance in the form of impact glass and shock metamorphism.

As impressive structural signature, we show in Fig.5 a small part of the outcrop only recently produced by a new road cut through the rim fairly near the southern end of the elongated basin c. 20 km northeast of the town of Teruel. The outcrop of several 100 m length provides insight in voluminous drastically fractured and brecciated Buntsandstein and Muschelkalk rock units. As shown in Fig. 5, large-scale flow texture can be observed in the normally hard, now completely shattered rocks, Muschelkalk lens-shaped megablocks are floating within Buntsandstein material, and a prominent terraced faulting occurs. Within the heavily destroyed Triassic layers, large friction planes show intense slickensides and mirror polish (not seen in Fig. 5), and interleaved vesicular and foamy white material probably is crystallized carbonate melt from strong frictional heating of Muschelkalk carbonate. These features covering an area of uninterruptedly at least several hundreds of meters and similarly met in many places of the central-uplift chain and in the basin rims (Fig.6), cannot be explained by normal geologic, tectonic processes. In the cratering process of a very large impact, however, exactly such a scenario is to be expected and easily understood considering the giant rock movements during the excavation of the primary crater and the subsequent collapse of the transient crater in the modification stage. These movements under very high pressures may act so rapidly that the resulting friction temperatures are enough to transform the involved rocks into pure melt glass. We made such a peculiar find roughly 10 km northwest of the town of Teruel in the most southerly part of the central-uplift chain. In part, the glass seems to be completely homogenized suggesting temperatures in excess of 2,000°C (D. Griscom, pers. comm.).

Fig. 5. The crater rim in the southern part of the impact chain.

Impact structures showing deviation from a circular symmetry may result from a very low-angle oblique impact or from a post-impact tectonic deformation (as is the case with, e.g., the large Sudbury impact structure). The latter can in this case be excluded, and a low-angle trajectory of the projectile conflicts with the circular shape of the Azuara structure and the Torrecilla ring as obvious companions to Rubielos de la Cérida. Therefore, the formation of the 80 km long basin by a pile of projectiles is rather probable. They caused several individual central-uplift craters more or less strung together morphologically and structurally. Such an uninterrupted crater chain, additionally blown up by the Azuara and Torrecilla structures, is so far unique among terrestrial impact structures.

Because of advanced weathering, the impact melt rocks found so far do not allow for a reliable absolute radiometric dating. Therefore, the most striking evidence for a synchronous multiple impact of Azuara, Torrecilla, and the Rubielos de la Cérida crater chain is provided by the stratigraphic layering of an impact suevite breccia. Extending from the northern part of the Azuara structure up to the southern part of the crater chain near the town of Teruel, this peculiar breccia displaying shock effects and melt clasts (Fig.7) is regularly found at the base of the post-impact (and post-Alpidic) undisturbed Upper Tertiary rocks. The up to 20 m thick horizontally bedded breccia layer always contacts the older Mesozoic folded rocks and is more or less meshed with them without any soil formation. The uniformity of this polymict breccia and its fixed stratigraphic position found in a region of the size of roughly 120 km strongly suggests a single phase of the brecciation and deposition, which is typical for a large impact but not at all expected for “normal” geologic processes.

Fig. 6. Megabreccia and polished friction plane in the southern part of the central-uplift chain.

The impact event on the Iberian Peninsula may have been even more spectacular. Three enigmatic deposits (A, B, C in Fig.1) located more than 200 km northwest and northeast of the Azuara structure attract attention, because they show the typical features of impact ejecta deposits, and because their stratigraphic age may be related with the age of the Azuara/Rubielos de la Cérida impact event. All three deposits are composed of strongly plastically deformed clasts in an unconsolidated sandy matrix (Fig. 8). The clasts show heavy striations, imprints, penetration marks and rotated fractures very similar to deformations known from the Azuara/Rubielos de la Cérida ejecta and described also for the Ries crater (Germany) impact ejecta and the Chicxulub impact ejecta in Belize. Abundant moderate shock effects are observed to occur in silicate clasts that contribute to the diamictite deposits B and C (Fig.1). The age is Upper Eocene or younger for deposit A, post-Cretaceous for deposit B, and Upper Eocene or Oligocene for deposit C. Because the diamictites are in each case composed of local material, an emplacement as distant ejecta from the Azuara/Rubielos de la Cérida structures can be excluded. However, local candidates for possible impact craters are not known so far. This may be explained by a tectonic overprint, or they may be buried beneath post-impact young sediments. A systematic search is outstanding. However, the obvious impact signature of the three deposits and the conspicuous age relations with the Azuara/Rubielos de la Cérida impacts suggest that the multiple impact affected an even larger area sized of the order of at least 300 km. Thus, Azuara – Rubielos de la Cérida may in the future turn out to be important for further studies on the disruption of asteroids and multiple (cometary?) impacts on Earth and other planets.

Fig.7. Impact breccia (suevite) exposed in the southern part of the central-uplift chain.

The here proposed large multiple impact event and its age direct attention also to the end-Eocene (38 Ma) mass extinction and the debated possible relation with large impacts, similar to the Cretaceous/Tertiary (K/T) boundary extinction related with the giant Chicxulub impact. As for the end-Eocene mass extinction, the large Popigai impact structure in Russia and the Chesapeake crater in the United States have so far been proposed as possible “smoking guns” of the event. The northern Spain multiple impact could be a further candidate because of its size and the more than 10 km thick sedimentary target. At the time of the impact, a high proportion of this target consisted of carbonate rocks and probably also evaporite (gypsum) layers. Such target rocks in a giant impact cratering process have been suggested to be important in triggering global climate changes.

Fig. 8. Probable impact ejecta near Peñacerrada (location B in Fig. 1).

1. Fakultät für Geowissenschaften der Universität Würzburg, Pleicherwall 1, D-97070 Würzburg, Germany
2. Institut für Mineralogie der Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany
3. IES Giola, Llinars del Vallès. Barcelona-08450, Spain
4. Am Judengarten 23,  97204 Höchberg, Germany

References

Because of the limited space, only a few references are given here. A comprehensive list may be ordered from the corresponding author (kord@ernstson.de).

Ernstson, K., Hammann, W., Fiebag, J., Graup, G. (1985): Evidence of an impact origin for the Azuara structure (Spain). Earth Planet. Sci. Lett., 74, 361-370.

Ernstson, K. & Claudin, F. (1990): Perlada Formation (Eastern Iberian

Chains, NE Spain): ejecta of the Azuara impact structure. N. Jb. Geol. Paläont. Mh., 1990, 581-599.

Ernstson, K. & Fiebag, J. (1992): The Azuara impact structure (Spain): new insights from geophysical and geological investigations. Geol. Rundschau, 81, 403-427.

Melosh,H.J. and Schenk, P. (1993): Split comets and the origin of crater chains on Ganymede and Calisto, Nature 365, 731-733.

Bottke, W.F., Richardson, D.C. and Love, S.G. (1997) Note: Can tidal disruption of asteroids make crater chains on the Earth and Moon? Icarus 126, 470-474.

Richardson, D.C., Bottke, W.F.and Love, S.G. (1998): Tidal distortion and disruption of Earth-crossing asteroids. Icarus 134, 47-76.

Spray, J.G., Kelley, S.P. and Rowley, D.B. (1998): Evidence for a late

Triassic multiple impact event on Earth. Nature 392, 171-173.

Ocampo, A.C., Pope, K.O. and Fischer, A.G. (1997): Carbonate ejecta from the Chixculub crater: evidence for ablation and particle interactions under high temperatures and pressures. Lunar Planet. Sci. Conf., XXVIII, 1035 – 1036.

Hradil, K., Schüssler, U., Ernstson, K. (2001): Silicate, phosphate and carbonate melts as indicators for an impact-related high-temperature influence on sedimentary rocks of the Rubielos de la Cérida structure, Spain. in „Impact markers in the stratigraphic record“, 6th ESF-IMPACT workshop, abstract book, Granada, 49-50.

Ernstson, K., Rampino, M.R. and Hiltl, M. (2001) Cratered cobbles in Triassic Buntsandstein conglomerates in northeastern Spain: An indicator of shock deformation in the vicinity of large impacts. Geology 29, 11-14.

Ernstson, K., Claudin, F., Schüssler, U., Hradil, K. (2002): The mid-Tertiary Azuara and Rubielos de la Cérida paired impact structures (Spain). Treballs Museu de Geologia de Barcelona 11, 1-62.