The controversy about the Azuara and Rubielos de la Cérida structures is for the main part linked with the debate that regional geology features are not compatible with an impact event. After 50 years of this barren discussion, geologists from the Zaragoza university, the Madrid center of astrobiology and the geological survey of Spain are continuing this neverending story. In the following, the major points for discussion are shortly outlined.
Polymictic megabreccia in the Carniolas (Rhaetian – Liassic) and Liassic layers
Fig. 1. Megabreccias near Almonacid de la Cuba – interpreted in the frame of regional geology as a dissolution/collapse brecciation in the Carniolas (Cortes de Tajuña Fm.)
Impact dike breccias/breccia dikes and other impact breccias
Fig. 2. Breccia dikes in Cambrian siltstones (near Olalla) …
Fig. 3. … and in Jurassic limestones (near Fuendetodos) – interpreted in the frame of regional geology as karstification and paleosol (caliche, calcrete) features.
Megabreccia on the crater floor of the Rubielos de la Cérida impact structure
Fig. 4. Megabreccia near Barrachina – interpreted in the frame of regional geology as to have originated from gypsum dissolution.
The impact melt rock in the Rubielos de la Cérida structure composed of nearly 100 % silicate glass
Fig. 5. Silicate glass of the impact melt rock (field is 15 mm wide) hosting strongly shocked minerals like ….
Fig. 6. … diaplectic feldspar (the elongated grain in the middle). Photomicrograph, left: xx nicols – interpreted in the frame of regional geology as a volcanic ash.
Suevitic basal breccia
Fig. 7. Polymictic suevitic basal breccia (near Cucalón) – interpreted in the frame of regional geology as a lacustrine limestone.
Pelarda Fm. impact ejecta and the ejecta at the Puerto Mínguez
Fig. 8. Impact ejecta (Pelarda Fm.) and an intercalated Buntsandstein megablock, ….
Fig. 9. … impact ejecta at the Puerto Mínguez, Mesozoic limestone clasts in a matrix of Tertiary and Paleozoic material, …
Fig. 10. … rotated fractures in a quartzite clast from the Pelarda Fm. ejecta, and …
Fig. 11. … strong plastic deformation of a limestone clast from the Puerto Mínguez impact ejecta – interpreted in the frame of regional geology as – alternatively – fluvial conglomerates, alluvial fans from syn-tectonic sedimentation, or Quaternary raña deposits.
Shock-metamorphic effects (e.g., PDFs, diaplectic and melt glass) in rocks from the Azuara and Rubielos de la Cérida structures
Fig. 12. Sandstone clast composed of isotropic quartz grains (diaplectic glass); photomicrograph, right: xx nicols; the field is 600 µm wide, and …
Fig. 13. … three sets of PDFs and isotropic spots in a diaplectic quartz; photomicrograph, xx nicols; the field is 400 µm wide – interpreted in the frame of regional geology as to have originated from hercynic and alpidic tectonic overprint.
Shock deformation of quartzite cobbles in the basal Buntsandstein conglomerates
Fig. 14. Open, subparallel tensile fractures and collision marks in quartzite pebbles, Buntsandstein conglomerate and …
Fig. 15. … spallation craters in a quartzite pebble from Buntsandstein conglomerate – interpreted in the frame of regional geology as to result from tectonic deformation and pressure dissolution.
The probably more than several 100 m thick Upper Tertiary, post-impact sediments in the center of the Azuara structure
Fig. 16. In the center of the Azuara impact structure. Northern ring anticline in the background – interpreted in the frame of regional geology as Devonian (!) evidently only little affected by an impact (written comm. by Jan Smit, sedimentologist from Amsterdam).