THE AOUELLOUL CRATER, MAURITANIA - ON THE PROBLEM OF CONFIRMING THE IMPACT ORIGIN OF A SMALL CRATER

The impact origin of small craters in sedimentary rocks is often diffi cult to confirm because of the lack of characteristic shock metamorphi c features. A case in point is the 3.1 Ma Aouelloul crater (Mauritania ), 390 m in diameter, which is exposed in an area of Ordovician Oujeft and Zli sandstone. We studied several fractured sandstone samples fro m the crater rim for the possible presence of shock metamorphic effect s. In thin section, a large fraction of the quartz grains show abundan t subplanar and planar fractures. Many of the fractures are healed and are evident only as fluid inclusion trails. A few grains showed sets of narrow and densely spaced fluid inclusions trails in one (rarely tw o) orientations per grain, which could be possible remnants of planar deformation features (PDFs), although such an interpretation is not un ambiguous. In contrast, an impact origin of the crater is confirmed by Re-Os isotope studies of the target sandstone and glass found around the crater rim, which show the presence of a distinct extraterrestrial component in the glass.