The Aouelloul Crater, Mauritania: on the problem of confirming the impact origin of a small crater

Abstract

The impact origin of small craters in sedimentary rocks is often difficult to confirm because of the lack of characteristic shock metamorphic features. A case in point is the 3.1 Ma Aoulelloul crater (Mauritania), 390 m in diameter, which is exposed in an area of Ordovician Oujeft and Zli sandstone. We studied several fractured sandstone samples from the crater rim for the possible presence of shock metamorphic effects. In this section, a large fraction of the quartz grains show abundant subplanar and planar fracture. Many of the fractures are healed and are evident only as fluid inclusion trials. A few grains showed sets of narrow and densely spaced fluid inclusions trails in one (rarely two) orientations per grain, which could be possible remnants of planar deformation features (PDFs), although such an interpretation is not unambiguous. 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.