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. published the occurrence of shock metamorphism, and Azuara was established (Grieve & Shoemaker 1994, Hodge 1994, Norton 2002 ^[2][3][4] 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).

• http://en.wikipedia.org/wiki/Azuara impact structure