EVIDENCE FOR A LOCAL IMPACT EVENT DURING MIDDLE STONE AGE OCCUPATION, DAKHLEH OASIS, WESTERN DESERT, EGYPT

Middle Pleistocene-aged lacustrine and paludal sediments of Dakhleh Oasis, which record humid conditions in the currently hyperarid Western Desert of Egypt, also preserve lithic artifacts indicating Middle Stone Age occupation of the area. A natural glass (Dakhleh Glass; DG), which occurs both within the lacustrine silts and lagged on the desert surface within the reconstructed extent of the Dakhleh paleolakes, is evidence of a significant disruption to the Dakhleh region during the period of lacustrine sedimentation. DG localities are spread over at least 35 km. The heating event required to form the Ca-Al-silica DG is substantial enough (to >1700°C) to rule out heating sources such as grass fires. Based both on the high temperatures required and the textural similarity between DG and impact glasses derived from volatile-rich sedimentary targets, a meteorite impact is the most likely cause of DG formation. The chemical composition of DG is compatible with formation from materials exposed in the Dakhleh Oasis, although no source crater has yet been discovered.

The scale of the event required to produce the observed volume of glass is such that plants and animals within several hundred square km of the impact would have been obliterated. The stratigraphic distribution of DG within the lacustrine sediments suggests that lake bottom materials were destabilized by the event; flowing into deeper portions of the lake basin as turbidites. While DG may have been reworked and distributed over a wider area than that in which it was originally formed, an outcrop of fused carbonate ~ 8 km S of the closest glass locality suggests the DG-forming event was far-reaching. Thus, this event would have significantly affected the inhabitants of all of Dakhleh Oasis region, both in the immediate aftermath of the impact, and as the lacustrine system and the local ecology recovered. Lake water would likely not have been potable for some time, though some spring sources along the escarpment may have flowed undisturbed if the impact occurred during a particularly wet climatic interval. Chronological control on both the impact event and the occupational history of the oasis is not currently fine enough to determine whether the impact resulted in lengthy depopulation of the area.