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The Azuara impact structure

image by courtesy of Manuel Cabedo

From stratigraphic considerations, its age is estimated to be Upper Eocene or Oligocene. Except for very small magmatic intrusions, the target was purely sedimentary, which is a rare case with very large impact structures. The target was composed of the Paleozoic core of the Iberian chain and the Mesozoic cover overlain by Tertiary molasse sediments of probably 1000 - 2000 m thickness.

This prominent thickness of more or less loosely bound sediments and the carbonate rocks prevailing in the sedimentary sequence are related to many peculiarities of the Azuara structure such as relics of carbonate melt, a special facies of the impact ejecta, and distinct breccia dikes.

Young post-impact sediments widely hamper the investigation of the central part of the Azuara structure. Therefore, most outcrops concentrate on the rim region where they are very abundant enabling excellent insight into the geology of the impact process.

The impact hypothesis for the Azuara structure was first established by Ernstson et al. (see list of references) in 1985. Until then, the structure attracted practically no interest except for a paper by Carls (1983), wherein a tectonic Azuara block has been mentioned. Later, the Azuara structure is considered by Spanish geologists a basin which formed by syn-tectonic sedimentation. More recently, the Azuara structure has also been suggested as to originate from a granitic intrusion.

The model of a giant meteorite impact is based on a very detailed geological mapping, geophysical measurements, photo-lineation studies, and petrographic analyses of about 1,000 thin sections. From these investigations (see list of references and below), the impact is documented by gravity negative anomalies, abundant monomict and polymict breccias and breccia dikes (frequently displaying shock-metamorphic effects such as diaplectic glass, melt particles, all kinds of planar deformation features [PDFs] , extended and thick megabreccias, a large quantity of dislocated megablocks, hydrothermal signature (Siegert 1997), and extended and thick impact ejecta (see figures below).

Among these observations, especially the shock-metamorphic effects indicating extremely high pressures ( several 100 kilobars = several 10 GPa) and temperatures (> 1000 °C) are basically incompatible with models of an endogenous origin (by tectonics, syn-tectonic sedimentation, granitic intrusion) of the Azuara structure.

Besides the papers in the reference list, we would like to point to the following articles and theses about the Azuara impact:

FIEBAG, J. (1988): Contributions to the geology of the Azuara structure: Geological mapping in the area between Herrera de los Navarros and Aladrén and SE of Almonacid de la Cuba, and special investigations of the breccias and breccia dikes in the light of the impact origin of the Azuara structure. - Doctoral thesis, Univ. Würzburg, 271 pp., Würzburg (in German).

Johannes Fiebag († 1999) has markedly pioneered the geologic work in the Azuara impact structure. Several pictures of this homepage have either been taken by himself or have been taken from rocks of his personal collection.

Bärle, F. (1988): Geological investigations at the SW rim of the Azuara structure (NE Spain): Geological mapping (1 : 10,000 scale) and special tectonic investigations. - Unpublished Diploma Thesis, Univ. Würzburg, 94 pp., Würzburg (in German).

GWOSDEK, C. (1988): Geological investigations at the SE rim of the Azuara structure (NE Spain): Geological mapping (1 : 10,000 scale) in the environs of Moneva with special reference to the tectonic inventory. - Unpublished Diploma Thesis, Univ. Würzburg, 108 pp., Würzburg (in German).

HUNOLTSTEIN-BUNJEVAC, D. v. (1989): Geological investigations at the SE rim of the Azuara structure (NE Spain): Geological mapping (1 : 10,000 scale) in the environs of Moyuela with special reference to structural features. - Unpublished Diploma Thesis, Univ. Würzburg, 111 pp., Würzburg (in German).

KATSCHOREK, T. (1990): Geological investigations at the N rim of the Azuara structure (NE Spain): Geological mapping (1 : 20,000 scale) of the area between Fuendetodos and Belchite with special reference to structural features. - Unpublished Diploma Thesis, Univ. Würzburg, 198 pp., Würzburg (in German).

König, R. (1988): Geological investigations at the SW rim of the Azuara structure (NE Spain): Geological mapping (1 : 10,000 scale) and special tectonic investigations. - Unpublished Diploma Thesis, Univ. Würzburg, 142 pp., Würzburg (in German).

LINNEWEBER, 5. (1988): Geological investigations at the SE rim of the Azuara structure (NE Spain): Geological mapping (1 : 10,000 scale) of the area between Blesa and Muniesa with special reference to tectonic features. - Unpublished Diploma Thesis, Univ. Würzburg, 103 pp., Würzburg (in German).

Mayer, G. (1991): Geological investigations at the SE rim of the Azuara structure (NE Spain): Geological mapping (1 : 20,000 scale) with special reference to tectonic features and in the light of the impact origin of the structure. - Unpublished Diploma Thesis, Univ. Würzburg, 216 pp., Würzburg (in German).

Müller, H. (1989): Geological investigations at the SE rim of the Azuara structure (NE Spain): Geological mapping (1 : 20,000 scale) between Ventas de Muniesa and Moneva with special reference to facies and structural features. - Unpublished Diploma Thesis, Univ. Würzburg, 167 pp., Würzburg (in German).

Waasmaier, E. (1988): Geological investigations at the S rim of the Azuara structure (NE Spain): Geological mapping (1 : 10,000 scale) west of Blesa with special reference to tectonic features. - Unpublished Diploma Thesis, Univ. Würzburg, 108 pp., Würzburg (in German).

Siegert. I. (1997): Thermomicrometrical and petrographical investigations of quartzites from the Azuara - Rubielos de la Cérida doublet structure (Spain). - Unpublished Diploma Thesis, Univ. Bremen, 72 pp., Bremen (in German).

Rampino, M.R.: El impacto de Azuara y sus effectos. - Universo, 29, 48 - 50, 1997.

An excellent and very detailed geological mapping on a 1 : 10,000 scale of the N rim of the Azuara structure between Aguilón and Fuendetodos has been performed by H.Feld within the frame of a DFG (German Science Foundation) research program.

In a recently published paper,

Cortés Gracia, A.L. & Martínez Peña, M.B.: Controversia científica para el aula: ¿Tiene la cubeta de Azuara un origen extraterrestre? (Scientific controversy for teaching: Is the origin of the Azuara Basin extraterrestrial?) - Enseñanza de las Ciencias de la Tierra, 7.2, 143-157, 1999.,

the origin of the Azuara structure and the related scientific controversy are discussed from a didactic point of view. Such discussions were more up-to-date some 20 - 30 years ago. Then, different from today, knowledge about large meteorite impacts as a normal geological process (see the textbook: H.J. Melosh, Impact cratering. A geologic process, 245 pp., Oxford University Press, New York, 1989) was very limited among geologists, which mostly refused ideas about impact models.

The biassed character of the article (against the impact model) has provoked a reply paper:

Anguita, F.; Márquez, A. y Claudin, Ferran (2000): Réplica al artículo “Controversia científica para el aula: ¿ Tiene la cubeta de Azuara un origen extraterrestre?” - Enseñanza de las Ciencias de la Tierra, 8.2, 148-153.

You will read more about the co-author Fernando Claudin HERE

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