LIBYAN DESERT GLASS AREA IN WESTERN EGYPT: SHOCKED QUARTZ IN BEDROCK POINTS TO POSSIBLE PRESENCE OF A DEEPLY ERODED IMPACT STRUCTURE IN THE REGION

Libyan Desert Glass (LDG) is an enigmatic natural glass, about 28.5 million years old, which is found in an area of about 2,500 to 6,000 km² (the exact number varies between different publications), similar to a lag deposit, on the floor of corridors between sand dunes of the southwestern corner of the Great Sand Sea in western Egypt, near the Libyan border. The glass occurs as centimeter to decimeter-sized, irregularly shaped, and strongly wind-eroded pieces. The origin of the LDG has been the subject of much debate since its discovery, and a variety of exotic processes were suggested, including a hydrothermal sol-gel process or a lunar volcanic source. However, evidence of an impact origin of this glass included the presence of schlieren and partly or completely digested minerals, such as lechatelierite, baddeleyite (a high-T break-down product of zircon), and the presence of a meteoritic component in some of the glass samples. The exact source material of the glass remains an open question. Geochemical data indicate that neither the local sands nor sandstones from various sources in the region are good candidates to be the sole precursors of the LDG. No detailed studies of all local rocks exist, though. There are some chemical and isotopic similarity to rocks from the BP and Oasis impact structures in Libya, but no further evidence for a link between these structures and LDG was found so far. These complications and the lack of a crater structure in the area of the LDG strewn field have rendered an origin by airburst-induced melting of surface rocks as a much-discussed alternative. About 20 years ago, a few shocked quartz-bearing breccias (float samples) were found in the LDG strewn field. To study this question further, several basement rock outcrops in the LDG area were sampled during two expeditions in the area. Here we report on the discovery of shock-produced planar microdeformation features, namely planar fractures (PFs), planar deformation features (PDFs), and feather features (FFs), in quartz grains from bedrock samples. Our observations show that the investigated samples were shock to moderate pressure, likely not more than about 20 GPa. We interpret these observations to indicate that there was a physical impact event, not just an airburst, and that the crater has been almost completely eroded since its formation.