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Azuara impact structure
The 35–40 km-diameter Azuara impact structure is located in northeast Spain roughly 50 km south of Zaragoza. The name is attributed to the small town of Azuara located near the center of the structure. The first hint to a possible impact origin was given by Wolfgang Hammann as early as 1980, and the first field evidence was provided by Johannes Fiebag in the early eighties. In 1985, Ernstson et al.〔Ernstson K., Hammann W., Fiebag J. and Graup G. (1985) Evidence of an impact origin for the Azuara structure (Spain). Earth and Planetary Science Letters 74:361-370〕 published the occurrence of shock metamorphism, and Azuara was established (Grieve & Shoemaker 1994, Hodge 1994, Norton 2002 〔Grieve R. A. F. and Shoemaker E. M. (1994) The record of past impacts on Earth. In Hazards Due to Comets and Asteroids, edited by T. Gehrels. Space Science Series Tucson, Arizona, USA: Univ. Arizona Press. pp. 417-462〕〔Hodge P. (1994) Meteorite craters and impact structures of the Earth. Cambridge, UK: Cambridge University Press. 124 p.〕〔"Norton, O.R. (2002) The Cambridge Encyclopedia of Meteorites"〕 as an authentic impact structure. From stratigraphic considerations and paleontological dating, its age is estimated to be Upper Eocene or Oligocene (roughly 30 - 40 Mill. years old).
The Azuara structure exhibits a roughly circular morphology implying a partly pronounced outer ring. The inner part is covered by post-impact Upper Cenozoic sediments, however in the outer rim zone, the outcrop conditions are excellent.
== Impact features==
The impact nature is documented by abundant polymictic and monomictic breccias, breccia dikes, extensive megabreccias and impact ejecta, dislocated megablocks, geophysical anomalies and shock metamorphism. Shock effects like melt glass, diaplectic glass, planar deformation features (PDFs) are found in breccias and breccia dikes, and PDFs are abundant also in quartzite clasts contributing to the ejecta (Pelarda Formation).〔〔Ernstson K. and Claudin F. (1990) Pelarda Formation (Eastern Iberian Chains, NE Spain): Ejecta of the Azuara impact structure. N. Jb. Geol. Paläont. Mh.:581-599.〕〔Ernstson K. and Fiebag J. (1992) The Azuara impact structure (Spain): new insights from geophysical and geological investigations. Int. J. Earth Sci. (Geol. Rundschau) 81(2):403-427 ()〕〔Ernstson, K., Claudin, F., Schüssler, U. & Hradil, K. (2002): The mid-Tertiary Azuara and Rubielos de la Cérida paired impact structures (Spain). - Treb. Mus. Geol. Barcelona, 11, 5 - 65.()〕
Shock metamorphism in the Azuara impact structure include planar deformation features in quartz. The histogram displays frequencies of crystallographically controlled planes of microdeformation. The and occurrences are especially diagnostic and are generally considered as in proof of impact shock.〔Stöffler, D. & Langenhorst, F. (1994). Shock metamorphism of quartz in nature and experiment: I. Basic observation and theory. Meteoritics. 29: 155-181.〕
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