The mid-Tertiary Azuara and Rubielos de la Cérida paired impact structures (Spain)*

Kord ERNSTSON**, Fernando CLAUDIN***, Ulrich SCHÜSSLER**** and Klaudia HRADIL****

ABSTRACT

We report on the Azuara impact structure and its Rubielos de la Cérida companion crater, which establish the largest terrestrial doublet impact structure presently known. Both structures have diameters of roughly 35 - 40 km and they have been formed in a purely sedimentary target. From stratigraphic considerations and palaeontologic dating, an Upper Eocene or Oligocene age is very probable. Geological mapping has established abundant geologic impact evidence in the form of monomictic and polymictic breccias and breccia dikes, megabreccias, dislocated megablocks, remarkable structural features, extensive impact ejecta and impact signatures even in distant autochthonous deposits. The most striking impact evidence for both structures is given by strong shock metamorphism, including melt and diaplectic glass, planar deformation features (PDFs), different kinds of impact melt rocks (from former silicate melt, carbonate melt, carbonate-phosphate melt) and suevite breccias. Glassy amorphous carbon particles in a solid C-O compound may be related with fullerenes and may originate from a quenched melt of extremely shocked coal or from extremely shocked limestones. It is assumed that the impact had considerable influence on the Mid-Tertiary regional geology of the Iberian System, and we suggest that respective geologic models which have so far not considered this peculiar and far-reaching event, need considerable revision.

Key words: Azuara impact structure, Rubielos de la Cérida impact structure, Iberian chain (Spain), shock metamorphism, impact melt rocks, impact breccias, ejecta, Tertiary.

* Dr. Francisco Anguita, Associate Professor (Planetary Geology), Universidad Complutense de Madrid (Spain), referee of this work, sent us the following remarks:

"In my opinion, the present paper means a quantic jump towards the confirmation of Azuara and Rubielos de la Cérida structures as a doublet impact crater. Specifically, the analyses performed at centimeter to microscopic scales, reveal a body of evidence difficult to contradict. Any future research on this area will have to discuss necessarily the interpretations now offered by Ernstson el al. Lacking this discussion, any hypothesis defending that Azuara and Rubielos structures are the result of "normal" sedimentary and tectonic processes will be utterly untenable."

** Fakultät für Geowissenschaften der Universität Würzburg, Pleicherwall 1, D-97070 Würzburg, Germany. www.impact-structures.com

*** IES Giola, Llinars del Vallès. Barcelona-08450, Spain. www.impact-structures.com

**** Institut für Mineralogie der Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany.