The Aorounga impact structure, situated in northern Chad approximately 110 km southeast of the Emi Koussi volcano in the Tibesti Massif, has undergone significant erosion resulting in deep incisions. The entire structure is dissected by longitudinally-oriented yardangs – parallel ridges with sharp crests carved by wind abrasion – as evidenced by remote sensing imagery.

Comprised of fine-grained, well-sorted sandstone of Upper Devonian age with a minor carbonate component, the Aorounga structure is characterized by an outer and an inner ring wall at distances of 7 and 11 km from the center, respectively, separated by an annular depression. The ring walls rise up to 100 m above the surrounding area, while the innermost part of the structure is a hilly terrain approximately 1.5 km wide. Breccia consisting of fine-grained clasts with a fluidal texture has been discovered atop the inner ring wall.

Recent remote sensing, petrographic, and geochemical investigations by Christian Koeberl and colleagues in 2005 revealed that Aorounga comprises a structurally complex central zone measuring 9 km wide, surrounded by a ring structure of rather low topography measuring 2-3 km wide, and then a second, outer ring of 3.5 km width. It was proposed that the latter could represent the crater rim zone, which would indicate a diameter of approximately 16 km, larger than the diameter of 13 km cited in earlier literature. Furthermore, new evidence of shock deformation in monomict breccia samples of quartzite and sandstone was discovered, with quartz grains displaying up to five sets of planar deformation features (PDFs) in one sample and abundant grains with planar fractures (PFs) in several other samples. These findings provided conclusive support for Aorounga’s status as an impact structure, a conclusion that had previously been proposed by Becq-Giraudon and his team but had not been deemed convincing at the time.

Initial aerial and satellite imagery of Aorounga in 1976 by Norbert Roland indicated that the structure might be a granite “diapir” or an impact crater, with Roland ultimately favoring the “diapir” hypothesis. However, 12 years later, Richard Grieve and colleagues supported an impact origin.

The age of the structure was initially estimated based on 14C dating during the French expedition of 1992 to be between 3,500-12,000 years, a range that seems unlikely in light of the structure’s eroded appearance. Unfortunately, no melt-bearing samples have been discovered in the structure to provide a basis for dating attempts with the more promising 40Ar-39Ar and U-Pb zircon chronometers. Cosmogenic nuclide exposure age dating, thermoluminescence dating, and electron spin resonance dating of quartz from Aorounga were used as alternative methods, yielding ages of 0.5-1 Myr. Wolf Uwe Reimold and C. Koeberl’s recent review of African impact structures regarded these ages as probable minimum values for the Aorounga impact event. Therefore, the assumed Upper Devonian age of the target rock remains the only – and maximum – age limit currently available for the Aorounga impact event.

While there is ample potential for further detailed ground analysis of this impact structure, political unrest in northern and western Africa over the past several decades has made any attempts to visit Aorounga highly discouraged. Test