Review of:

Diaz Martínez, E., Sanz Rubio, E. y Martinez Frias, J (2002): Sedimentary record of impact events in Spain. Geological Society of America. Special Paper 356: 551-562.

by F. Claudin and K. Ernstson

Summary and Introduction

In 2000, a meeting on Catastrophic Events and Mass Extinctions:
Impacts and Beyond was held in Vienna. In the Proceedings book (Koeberl, C., and MacLeod, K. G., eds., Geological Society of America Special Paper 356), the article Diaz Martínez, E., Sanz Rubio, E. y Martinez Frias, J (2002): Sedimentary record of impact events in Spain. Geological Society of America. Special Paper 356: 551-562 was printed. Obviously, the editors did not consider a sound peer review of this article necessary, although from the Acknowledgments, we learn that there was a review performed by B. Simonson and W. U. Reimold, complemented by comments of the editor C. Koeberl. However, with respect to a series of errors, omissions and dubious assertions we can't understand why such a paper could have been published. Therefore, our objective is to make up this necessary sound review now and here in the form of a comment on the paper.

In the article, the Spanish authors from the Astrobiological Center in Madrid, which have never before made any active impact research, refer to different impact deposits on the Iberian peninsula. They address each of them with respect to their authenticity and whether they should be considered or not for future investigations. In fact, the discussed deposits provide a backdrop only for their central objective, namely to bring discredit upon the Azuara impact structure and to postulate its elimination from the lists of established impact structures.

Concretely, in the case of the Azuara structure, the authors are putting forward quite a few assertions (in bold italics) which we would like to comment with regard to our observations and interpretations most of which have been published previously. Here, we compile the most important data thereby referring also to new articles, and we add a lot of new photographs that may be interesting also for visitors of the impact region in Spain.

Diaz Martínez et al.: Azuara is the only impact structure on the Iberian peninsula formally proposed as such.

With respect to this assertion, it is emphasized - although we actually see that there is nobody as blind as the one who is not willing to see - that Azuara and Rubielos de la Cérida had already been proposed as an impact structures as early as 1994 (Ernstson et al., 1994), and in the January, 2001, issue of Geology, Azuara and Rubielos de la Cérida are explicitly discussed as paired impact structures to be the cause of the shock deformation of the autochthonous Buntsandstein conglomerates (Ernstson et al., 2001c).

Diaz Martínez et al.: Since Azuara has in the eighties been proposed as an impact structure, the controversy has continued but with decreasing intensity because the arguments in favor of the impact hypothesis are progressively refused. This is why the majority of the evidences presented by the impact advocates are inconclusive or permit other explications.

The evidence of an impact origin for the Azuara structure was as such formally presented to the scientific community in 1985 (Ernstson et al., 1985). Later, and with respect to enhanced material accumulated in the following investigations, an impact origin was also strengthened for the Rubielos de la Cérida structure (Ernstson et al., 1994, 2001c). We add the host of publications performed by the impact advocates (Ernstson et al., 1987; Fiebag, 1988; Ernstson and Fiebag; 1992; Ernstson and Claudin, 1990, Claudin et al., 2001; Müller and Ernstson 1990; Ernstson et al., 2001 a, b; Hradil et al., 2001; Ernstson et al., 2002; Schüssler et al., 2002) having progressively blown-up the controversy by provoking a number of articles and Reply papers (Cortes et al., 1999, 2002 a, b, Stel et al., 2002; Diaz Martínez et al., 2002). Only recently, Azuara and Rubielos de la Cérida have been proposed to be part of a multiple impact comprising a much larger region on the Iberian Peninsula (Ernstson et al., 2003).

With respect to the assertion that the proofs presented by the impact advocates are inconclusive, the question arises why and on the base of what. Here, we point to the most recent papers in which the impact opponents demand the elimination of Azuara from the the lists of established impact structures (Cortes et al., 2002; Diaz Martínez et al., 2002). Every reader paying attention to the References of these papers, will be aware that they only contain arguments which, slightly varied, are identical to those of previously published articles (Aurell et al., 1993; Aurell, 1994). Thus, while the impact advocates provide new data, the arguments of the impact opponents amount to nothing more than pure repititions. Scientific progress reflected the whole time by the presentation of new evidences and proofs, is in this case evident in the articles of the impact advocates but not in those of the opponents.

The discussion only too well reminds of previous and recent controversies related with other impact structures (Ries, Vredefort, etc.). In these cases, opponents of an impact origin (basically geologists familiar with non-impact geological models of the region) always argued that the data suggesting an impact could better be explained by non-impact models of the regional geologic setting (see, e.g., Dehm, 1969, Gibson and Reimold, 2001; Zimmerman, 2003). In fact, an observation can always be explained by different models, but usually one tends to choose the model that explains all the observations in the easiest and most coherent manner.

Diaz Martínez et al.: Other impact structures of similar age and of comparable size (Haughton, Ries) present numerous impact features as are impact melt rocks and intense shock metamorphism. This is not observed in Azuara.

Impact features in Azuara hitherto studied, documented and referred to by the impact advocates are (Ernstson et al, 1985; Ernstson et al., 1987; Fiebag, 1988, Ernstson and Claudin, 1990; Ernstson and Fiebag, 1992, and many references therein; Müller and Ernstson 1990, Ernstson et al., 1994; Ernstson, 1994; Ernstson and Hilt, 2002; Claudin et al, 2001; Ernstson et al., 2001 (a, b, c); Ernstson et al., 2002 - most of the references having been printed when Diaz Martinez et al. edited their paper):

microscopic silicate melt, relics of carbonate melt, a large diversity of monomictic and polymictic breccias and breccia dikes, extensive zones of megabreccias and monomictic movement breccias, dislocated megablocks, extensive and continuous impact ejecta, inverted stratigraphy, gravity anomalies consistent with an impact origin, high-pressure/short-term deformations of clasts from the impact ejecta, spallation craters and spallation fractures in shocked quartzite cobbles from autochthonous conglomerates. The moderate to strong shock effects found in and reported for Azuara rocks are: schock-produced melt, diaplectic quartz and diaplectic glass, multiple sets of planar deformation features (PDFs; prevailing {10-13} and {10-12}orientations) in quartz, multiple sets of planar fractures (PFs) in quartz, extreme kink-banding in mica, and multiple sets of microtwins in calcite. We may discuss the interpretation of the observations, but they are not allowed to be said they do not exist or have not reported for Azuara. Here, it is emphasized that the results of the Azuara/Rubielos de la Cérida impact research have been published by well-known journals like Earth and Planetary Science Letters, Geology, Meteoritics, and Geologische Rundschau.

Diaz Martínez et al.: Originally, the impact nature was proposed on the base of inverted stratigraphy, the occurrence of megabreccias, megablock and breccia dikes, and features pointing to high P/T effects.
And: The inverted stratigraphy is related with Alpidic tectonics.

The assertion that the inverted stratigraphy is related with Alpidic tectonics reveals the methodical thinking and working of the impact opponents. Instead of asking for and discussing the possible causes of inverted stratigraphy, they conclude from the, in their mind, non-existing impact event that the inverted stratigraphy must be related with Alpidic tectonics. And because the inverted stratigraphy is Alpidic, its existence cannot be used as argument in favor of an impact. The impact advocates have never claimed an unambiguous impact origin for the inverted stratigraphy, but they have pointed to the possibility, because inverted startigraphy (the "overturned flap") can especially be expected in the rim zone of impact structures.

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Moreover, it is curious to note that in the zones affected by inverted stratigraphy (an observation agreed to by both groups), breccia dikes occur (polymictic and monomictic; near Santa Cruz de Nogueras and Olalla) that cut through the inverted Paleozoic and Mesozoic rocks. Clear and strong shock metamorphism has been observed in some of these breccia dikes, for which an origin from karst processes can clearly be excluded (see below and the photos 1, 2, 3, 4).

In addition to this curious coincidence, the rocks affected by the inversion, in this case the Muschelkalk limestones near Olalla, are presenting a structural style much more compatible with impact shock deformation than with "normal tectonics". In these materials, we observe impressive scour marks and slickensides (photos 5, 6) on scales between tens of meters and millimeters similar to those described by Nicolaysen and Reimold for the Vredefort impact structure (Gibson and Reimold, 2001). The whole complex is characterized by a brecciation through and through over hundres of meters (monomictic movement breccia including grit brecciation and mortar texture; photos 7, 8, 9, 10).

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Diaz Martínez et al.: The major part of the breccias are diagenetic or pedogenetic (produced by the dissolution and collapse of evaporites, by karstification, and by calcrete formation).

Here, and although the impact opponents are cautious not to pretend that all breccias have a diagenetic and pedogenetic origin, some distinctions have to be made. Actually, when a breccia (monomictic or polymictic) is named an impact breccia, the impact advocates refer to the following features:

Polymictic and monomictic breccias and breccia dikes:

• Prominent breccia dikes are observed in carbonate as well as in silicate rocks. The polymictic breccia dikes cannot be confused with tectonic breccias. And karstification in silicate rocks of the Iberian System.... ?
  
• Karstification is a process related with carbonate dissolution. Impact breccia dikes in the Azuara structure in general show sharp-edged clasts in the matrix and fragmented walls lacking any dissolution features, and, therefore, cannot possibly be confused with karst structures.
  
• Breccias-within-breccias are abundant; multiple (up to four) generations are observed. Breccia-dike generations are common: Breccia dikes are running within other breccias dikes, or they cut them. These multiple breccia generations are hardly explained by karstification processes but are well known from impact structures. Photos 11, 12.
  

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• Frequently, fractured clasts in breccias dikes can be observed that are clearly coherent, shown by the fitting of the fragments, despite separation by the matrix (photo 13, 14). This texture is hardly explained by karstification and rather attributed to injection processes, all the more considering the reaction of the injected material with the fragments and the host rock which is also fractured.
  

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• Within the clasts incorporated in breccias and breccia dikes, shock metamorphism is observed, and we need not stress that this is not typical of karstification or soil formation. Concretely, silicate melt particles, diaplectic glass, and PDFs have been reported for the polymictic dike breccias of, e.g., Santa Cruz de Nogueras and Muniesa (Ernstson et al., 1985, Fiebag, 1988; Ernstson and Fiebag, 1992, Ernstson, 1994; Ernstson et al., 2002). Except for the 2002 paper, all publications were printed when Diaz-Martinez et al. edited their paper.
  
• Breccias frequently contain clasts which look like wrapped by matrix material of different color (photos 15, 16). This peculiar texture, far from being related with lacustrine or soil-formation processes as has been suggested by authors refusing the impact (Aurell, 1994; Cortes et al., 2001; Diaz et al., 2002), clearly points to accretion around a nucleus similar to accretionary lapilli (Lorenz, 1974; Reimer, 1983; Colombo y Martí, 1989). Accretionary lapilli has been reported to occur also in the suevite of the Ries impact structure (Graup 1981).
  

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The Belchite and Herrera de los Navarros megabreccias:

As has again and again been pointed out previously (Fiebag, 1988; Ernstson and Fiebag, 1992; Katschorek, 1990; Ernstson, 1994), the megabreccia must not be confused with the Cortés de Tajuña Fm. and the related collapse breccias. The confusion has been introduced by the impact opponents who suggest a brecciation by dissolution of evaporites. In fact, the megabreccia is polymictic, shows cataclastic flow texture, and the involved stratigraphy is different from the Cortés de Tajuña Fm. For more details see
http://www.impact-structures.com/spain/controversy/megabrecha.htm

Monomictic movement breccias (Reiff, 1978)

On a small scale, monomictic movement breccias displaying grit brecciation, mortar texture and cataclastic flow texture my in rare cases be attributed to tectonics (cataclasites). When these breccias, however, cover areas uninterruptedly over hundreds of meters, as is the case in the Azuara and Rubielos de la Cérida structures (Ernstson et al., 1992; Ernstson, 1994; Ernstson et al., 2002), then an impact deformation is the only reasonable explanation.

Suevite beccias:

As early as in their 1992 paper, Ernstson and Fiebag show that the originally so-called basal breccia has the characteristics of a polymictic suevite breccia, that is shocked fragments and impact melt components in a clastic matrix.

We add: Suevite breccias have also been reported for the Rubielos de la Cérida structure. They occur in the megabreccia zone of Barrachina - Torre los Negros together with impact melt rocks (by the impact opponents without any analysis said to be volcanic ash), diamictic megablocks and megablocks of limestone monomictic movement breccias (Hradil et al., 2001, Ernstson et al., 2001 a, Ernstson et al., 2002) (photos 17, 18, 19). The impact opponents interpret this impact-typical occurrence of a megabreccia containing suevites and impact melt rocks as a phenomenon related with the dissolution of gypsum.

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Diaz Martínez et al.: The gravity negative anomaly results from incomplete data measured in the interior of the sedimentary Azuara basin.

The questions arise: What is the definition of completeness of data in geophysics? And: What actually is the kind of geophysical considerations that led the impact opponents to the assertion that the data are incomplete? In other words: The curious argumentation by the impact opponents lacks any objectivity and is based on impressions only. The Azuara gravity campaign has been performed by K.Ernstson, who, since 30 years, is familiar with gravity measurements and with geophysics of impact structures (Ries, Rochechouart, Steinheim) in general (see, e.g., Ernstson, 1972, 1974, 1976; Pohl et al., 1977, 1978; Ernstson, 1982, 1984 a, b, Ernstson and Pohl, 1974, 1977; Ernstson and Diele, 2003).

Assertions like these, without presenting any data and observations, reveal the style of argumentation of the impact opponents, which has already been denounced by F. Claudin in his comment on the article of Cortés et al. (2002) (see http://www.estructuras-de-impacto.impact-structures.com/spain/controversy/newpubli.htm, comentario sobre el artículo publicado en MAPS por Cortés et al., 2002).

Diaz Martínez. et al.: Cortés and Casas (1996) consider Azuara consistent with a regional N-S shortening in the Tertiary. For these authors, the structure is a syncline.

So what? The impact advocates have never doubted the action of Alpidic tectonics in the Azuara zone. But according to their observations (Ernstson et al., 2002; and others), Azuara is an impact structure. A regional shortening, in whatever direction, is incapable to explain the moderate to strong shock effects in the Pelarda Fm., the breccia dikes containing moderately to strongly shocked clasts, the megabreccias, the dislocated megablocks, the monomictic movement breccias extensive over hundreds of meters, the suevite breccias, and more.

Diaz Martínez et al.: The proposed ejecta layer (Pelarda Fm.) of the Azuara structure is overlaying the deposits of the adjacent Calatayud-Monatlbán basin. Traditionally, the Pelarda Fm. has been interpreted as an alluvial Plio-/Pleistocene sedimentary cover. Diaz et al (2002) interpret the Pelarda Fm. as a local Plio-/Pleistocene alluvial deposit similar to others located along zones of large relief in Spain.

The Pelarda Fm. ejecta are in a clear manner interstratified between Eocene and Miocene materials. This can be verified in the environs of Salcedillo (coordinates 30667259E/4535565N, at an altitude of ~1250 m; fotos 20, 21, 22). Therefore, considering the Pelarda Fm. as a local Plio-/Pleistocene deposit does not make any sense. As the term "traditionally" obviously refers to previous age determination, it is worth mentioning that the Pelarda Fm. has so far been dated between Eocene and Oligocene (Adrover, 1982; Carls & Monninger, 1974; Ernstson & Claudin, 1990; Ernstson and Fiebag, 1992; Ernstson et al; 2002) and Quaternary (Lendínez et al., 1989; Pérez, 1989; Ferreiro et al., 1991; Aurel et al., 1993; Aurell, 1994; Cortés y Martínez, 1999; Cortés et al., 2002; Diaz et al., 2002). Moreover, the Pelarda Fm. has been interpreted either als fluvial (Carls & Monninger, 1976; Adrover, 1982; Smit, 2000), or impact ejecta (Ernstson & Claudin, 1990; Ernstson et al., 2002), or alluvial (Lendínez et al., 1989; Pérez, 1989; Ferreiro et al., 1991; Aurel et al., 1993; Aurell, 1994; Cortés y Martínez, 1999; Cortés et al., 2001; Diaz et al., 2002).

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As mentioned earlier (Ernstson & Claudin, 1990; Ernstson et al., 2002), the occurrence of striated clasts (the striae oriented NE-SW, that is pointing to the centers of the Azuara and Rubielos de la Cérida structures), rotated fractures, clear evidence of moderate to strong shock metamorphism (PDFs) in cobbles and boulders of Bámbola and Armorican quartzite, the exposition at the top of one of the highest mountain chain in the region, and more, exclude a formation by "normal" sedimentation as proposed in the paper under discussion (Diaz Martínez et al., 2002).

Diaz Martínez et al.: The ejecta from an impact structure are for the most part deposited within about one crater radius beyond the rim (a non-oblique trajectory provided).

Here, we simply remind of the Belize ejecta from the Chicxulub impact structure. The diamictites interpreted as basal mass flow occur up to a quadruple (!) crater radius (Pope et al, 1999; Ocampo et al., 2000), and then disappear. In the case of the Azuara impact, the Pelarda Fm. ejecta occur up to a distance of no more than twice the crater radius. Hence, this argument, also used by Cortés et al. (2001) to refuse the impact origin for the Pelarda Fm., is absolutely out of place. At that time (see the reply (by F. Claudin) to the paper of Cortés et al. on www.estructuras-de-impacto.impact-structures.com), this discrepancy was perfectly obvious, and we didn't further pay attention to it. Now being again quoted, we point to this untenable argument.

Diaz Martínez et al.: According to Langenhorst and Deutsch (1996) and a personal communication by Langenhorst (2000), the deformation structures proposed as evidence of shock metamorphism are not related with impact metamorphism.

Back to the same old story (see comment on the article of Cortés et al. (2002), published in MAPS, written by F. Claudin on http://www.estructuras-de-impacto.impact-structures.com/spain/controversy/newpubli.htm): An opinion lacking any support by solid and checkable evidence, is a waste of time. This holds true also, if the opinion is given by a person of some scientific reputation.

What is the base of Langenhorst's (2000) assertion that the microscopic deformations are not related with impact metamorphism? In case it is the only sample they describe in more detail in their article (Langenhorst & Deutsch; 1996), a direct confrontation is given:

In a letter to A. Deutsch, K.Ernstson described the sample to be from autochthonous rocks outside the Azuara structure and suggested the planar features in this sample to be probably basal deformation lamellae (and not PDFs!). Ignoring these hints, Langenhorst and Deutsch nevertheless submitted the paper with the misleading results and the assertion that there is no shock in Azuara, and categorically refused to withdraw it, even when K. Ernstson had insistently protested. Moreover, in the paper of Langenhorst and Deutsch, absolutely no reference is given to the report (Ernstson et al., (1985), Ernstson & Fiebag (1992), and Ernstson (1994)) of strong shock (PDFs, diaplectic glass) in polymictic dike breccias from within the Azuara structure. This "nice loyalty to a colleague" was the reason that K.Ernstson reproached Langenhorst and Deutsch with scientific dishonesty. Diaz-Martínez et al.., and other impact opponents (e.g., Cortes et al., 2002), continue to adopt the misleading results and the scientific dishonesty

Among the three specimens, Deutsch and Langenhorst had in hand, only one had originated from the Pelarda Fm. ejecta (which displayed PFs and no PDFs). It is well known that PDFs are in general rather discontinuously distributed in shocked rocks and that an adequate number of samples must be analyzed. How is that possible, without having ever put their foot on Azuara terrain, to infer from only three samples from outside the impact structure that there is no shock in Azuara?

In addition to the early analysis by Ernstson et al. (1985), PDF universal-stage analyses have been performed by Eugenio Guerrero (University of Madrid) and by the reputed Ann Therriault (Geological Survey of Canada). All three analyses more or less revealed the same results especially with respect to the dominating {10-13} and {10-12} PDFs. A general confusion?

Today and here, we remark: The authors of the article under review, Diaz Martínez, E. et al., participated in the informal field trip on occasion of the 2001 Granada IMPACT workshop. They could have taken samples, but where are the analyses?

Diaz Martínez et al.: Neither impact melts nor suevites have been reported for the Azuara structure (that is why no ages are available ....)

Reading this, we are flabbergasted. Impact melts in the Azuara structure have been described in detail by Fiebag (1988), and again in Ernstson and Fiebag (1992; p. 418: 4. High-pressure and high-temperature features), Ernstson and Fiebag (1993, p. 758 ) and Ernstson (1994; pp. 38-41). As already mentioned above, the basal breccia has been described to reveal the typical aspects of a suevite breccia. This has been pointed out in Ernstson and Fiebag (1992, pp.413-414) and Ernstson (1994, p. 45). For the interested reader, we add that meanwhile impact melt rocks and suevites have been reported also for the Rubielos de la Cérida twin impact structure (Hradil et al., 2001, Ernstson et al., 2001 a, Ernstson et al., 2002).

Diaz-Martínez et al.: Ernstson and Fiebag (1992) suggest an impact age of Late Eocene - Oligocene, because Miocene sediments are not affected by tectonics and because Eocene sediments contribute to the material of some breccia dikes.

Ernstson and Fiebag (1992) suggest this age with respect to a palaeontological dating of gastropodes in sediments unaffected by the impact, and to Eocene sediments incorporated not only into breccia dikes but also into the megabreccia and the continuous ejecta deposits (Pelarda Fm.).

Conclusion

It is an enigma, and we shall never understand why the article under discussion could have been printed in the Special Paper series of the Geological Society of America. It is the same mystery surrounding the publication of the paper: Cortés, A.L., Díaz-Martínez, E., Sanz-Rubio, E., Martínez-Frías, J & Fernández, C. (2002): Cosmic impact versus terrestrial origin of the Azuara structure (Spain): A review. Meteoritics Planet. Sci., 37: 875-894. (see http://www.estructuras-de-impacto.impact-structures.com/spain/controversy/newpubli.htm)

Obviously, the criteria of publication used by the "arbiters" are different for both groups. While, of the impact advocates, they request more precision for the location of the outcrops, various inspections and examinations of the microscopic data, references to the articles published by the impact opponents, and more .... it seems that, of the impact opponents, the "arbiters" request, if at all, not to make too many editing mistakes only.

A few years ago, a long article arouse attention, written by Brian Martin and entitled "The Suppression of Dissent in Science" (also see http://www.uow.edu.au/arts/sts/bmartin/dissent/). With respect to the Azuara/Rubielos de la Cérida "affair", we tend to slightly vary the title to "The Suppression of Science by Personal Dissent".

References

Adrover, R., Feist, M., Hugueney, M., Mein, P. and Moissenet, E. (1982): L’âge et la mise en relief de la formation detritique de la Sierra Pelarda (Prov. Teruel, Espagne). C.R. Acad. Sc. Paris, 295: 231-236.

Aurell, M., González, A., Pérez, A., Guimerá, J., Casas, A. & Salas, R. (1993): Discusión of “The Azuara impact structure (Spain): New insights from geophysical and geological investigations” by K. Ernstson & J. Fiebag. Geologische Rundschau, 82: 750-755.

Aurell, M. (1994): Discusión sobre algunas evidencias presentadas a favor del impacto meteorítico de Azuara. In: Extinción y registro fósil (E. Molina, ed.), Cuadernos interdisciplinares, 5: 59-74, Seminario interdisciplinar de la Universidad de Zaragoza, Zaragoza.

Carls, P. & Monninger, W. (1974): Ein Block-Konglomerat im Tertiär der östlichen Iberischen Ketten (NE-Spanien).- Doctoral Thesis, Univ. Würzburg. 155 pp.

Claudin, F., Ernstson, K., Rampino, M.R. & Anguita, F. (2001): Striae, polish, imprints, rotated fractures, and related features from clasts in the Puerto Minguez impact ejecta in NE Spain, in: Impact markers in the stratigraphic record, 6th ESF-IMPACT workshop Granada, abstract book: 15-16.

Colombo, F y Martí, J. (1989): Depósitos volcano-sedimentarios. In: Sedimentologia. Vol I (Alfredo Arche, coord..), Nuevas tendencias 11: 271-345, Consejo Superior de Investigaciones Científicas: Madrid.

Cortés, A.L. y Martínez, M.B. (1999): Controversia científica para el aula: ¿ tiene la cubeta de Azuara un origen extraterrestre? Enseñanza de las Ciencias de la Tierra, 7.2: 143-157.

Cortés, A.L., Díaz-Martínez, E., Sanz-Rubio, E., Martínez-Frías, J & Fernández, C. (2002): Cosmic impact versus terrestrial origin of the Azuara structure (Spain): A review. Meteoritics Planet. Sci., 37: 875-894.

Dehm, R. (1969): Geschichte der Riesforschung. - Geologica Bavarica, 61, 25-35.

Diaz Martínez, E., Sanz Rubio, E. y Martinez Frias, J (2002): Sedimentary record of impact events in Spain. Geological Society of America. Special Paper 356: 551-562.

Ernstson, K.(1974): The structure of the Ries crater from geoelectric depth soundings. - J. Geophys., 40, 639-659.

Ernstson, K. (1982): Ergebnisse von Schweremessungen auf drei Profilen über die Frankenwald-Randstörung. - N.Jb.Geol.Paläont.Mh., 1982/2, 65-72.

Ernstson, K. (1984): A gravity-derived model for the Steinheim impact structure. - Geol. Rundschau, 73/2, 483-498.

Ernstson, K. (1984): Ergebnisse neuer Magnetfeld-Messungen im Gebiet der Impakt-Struktur Steinheimer Becken. - Jber.Mitt.oberrh.geol.Ver., N.F. 66, 153-160.

Ernstson, K. & Pohl, J. (1974): Einige Kommentare zu den bohrloch- geophysikalischen Messungen in der Forschungsbohrung Nördlingen 1973. - Geologica Bavarica, 72, 81-90.

Ernstson, K. (1976): Magnetometrische und gravimetrische Untersuchungen über dem Basalt von Dürrfeld (Heldburger Gangschar). - Geol.Bl.NO-Bayern, 26, 135-143.

Ernstson, K. & Pohl, J.(1977): Neue Modelle zur Verteilung der Dichte und Geschwindigkeit im Ries-Krater. - Geologica Bavarica, 75, 355-371.

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

Ernstson, K., Feld, H., & Fiebag, J. (1987): Impact hipotesis for the Azuara structure (Spain) strengthened. Meteoritics, 22: 373.

Ernstson, K. & Claudin, F. (1990): Pelarda Formation (Eastern Iberian Chains, NE Spain): Ejecta of the Azuara impact structure. N.Jb. Geol. Palaönt. Mh: 581-599.

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

Ernstson, K., Anguita, F. & Claudin, F. (1994): Shock cratering of conglomeratic quartzite pebbles abd the search for an identification of an Azuara (Spain) probable companion impact structure. European Science Foundation Network: Impact cratering and evolution of Planet Earth. Third International Workshop on Shock wave behaviour of solids in nature an experiments, Limoges, France. Abstract book, p.25.

Ernstson, K. (1994): Looking for geological catastrophes: The Azuara impact case. In : Extinción y registro fósil (E. Molina, ed.), Cuadernos Interdisciplinares, 5: 31-57, Seminario Interdisciplinar de la Universidad de Zaragoza, Zaragoza.

Ernstson, K. & Hilt, M. (2002): Cratered cobbles in Triassic Buntsandstein conglomerates in northeastern Spain: An indicator of shock deformation in the vicinity of impact craters: Reply. Geology, 30: 1051-1052.

Ernstson, K. , Claudin, F., Schüssler, U., Anguita, F. & Ernstson, T. (2001a): Impact melt rocks, shock metamorphism and structural features in Rubielos de la Cérida structure, Spain: Evidence for a companion to the Azuara impact structure, in: Impact markers in the stratigraphic record, 6th ESF-IMPACT workshop Granada, abstract book: 23-24.

Ernstson, K., Rampino, M.R. & Hilt, M. (2001b) : Shock induced spallation in Triassic Buntsandstein conglomerates (Spain): an impact marker in the vicinity of large impacts, in: Impact markers in the stratigraphic record, 6th ESF-IMPACT workshop Granada, abstract book: 25-26.

Ernstson, K., Rampino, M.R. & Hilt, M. (2001c) : 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 del Museu de Geologia de Barcelona, nº 11: 5-65.

Ernstson, K., Schüssler, U., Claudin, F. & Ernstson, T. (2003): An impact crater chain in Northern Spain. Meteorite, August: 1-5.

Ernstson, K. & Diele, L.(2003): Zur Rotliegend-Verbreitung in Süddeutschland: Schweremessungen bei Bad Windsheim, Mittelfranken. - Geologica Bavarica (im Druck/in press).

Fiebag, J. (1988): Zur Geologie der Azuara-Struktur - Kartierung im Gebiet zwischen Herrera de los Navarros und Aladrén und süd-östlich von Almonacid de la Cuba sowie spezielle Untersuchungen der Breccien und Breccien-Gänge vor dem Hintergrund einer Impaktgenese der AzuaraStruktur. - Dissertation, Univ. Würzburg, 271 pp., Würzburg.

Gibson, R.L. & Reimold, W.U. (2001): The Vredefort impact structure, south Africa. The Scientific evidence and a two-day excursion guide. Council for Geoscience: South Africa.

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 Granada, abstract book: 49-50.

Lendínez, A., Ruiz, V. & Carls, P. (1989): Mapa y memoria explicativa de la hoja 439 (Azuara) del Mapa Geológico de España a escala 1:50000, ITGE. Madrid, 42 pp.

Lorenz, V. (1974): Vesiculated tuffs and associated features. Sedimentology, 21: 273-291.

Ocampo, A.C.; Pope, K.O.; Vega, F. & Fouke, B.W. (2000): Depositional facies of the Chicxulub ejecta blanket in the southern Yucatan Península, EOS, Trans. Amer. Geophys. Union, v. 81, nº 48, F798.

Pérez, A. (1989): Estratigrafia y sedimentologia del Terciario del borde meridional de la Depresión del Ebro (sector riojano-aragonés) y cubetas de Muniesa y Montalbán. Tesis Doctoral, Universidad de Zaragoza, 525 pp.

Pohl, J., Ernstson, K., & Lambert, P. (1978): Gravity measurements in the Rochechouart impact structure (France). Meteoritics, 13: 601-604.

Pope, K.O. et al. (1999): Chicxulub impact ejecta from Albion Island, Belize, Earth Planet. Sci. Let, v. 170: 351-364

Reimer, T.O. (1983): Rotation of Volcanic smoke column around a vertical axis. Bull. Volcanol.., 28: 321-332.

Reiff, W. (1978): Monomict movement breccias; an indicator of meteorite impact. Meteoritics, 13: 605-609.

Schüssler, U., Hradil, K., Ernstson, K.(2002): Impact-related melting of sedimentary target rocks of the Rubielos de la Cérida structure in Spain. Berichte der Deutschen Mineralogischen Gesellschaft, Beiheft 1 zum European Journal of Mineralogy, Vol. 14, S. 149.

Smit, J. (2000), written communication.

Zimmerman, A (2003): Rhytmic Layering in the suevite, and reworked “Bunte breccia” of the Otting quarry, Otting, Bavaria, Germany: Evidence for a diatremic origin for the Ries Basin, Bavaria, Germany. Seattle Annual meeting (November 2-5), Paper nº 208-14.