The Dakhleh Glass: Product of an impact airburst or cratering event in the Western Desert of Egypt?

Abstract

Abstract-Impact cratering is a ubiquitous geological process on the terrestrial planets. Meteorite impact craters are the most visible product of impact events, but there is a growing recognition that large aerial bursts or airbursts should occur relatively frequently throughout geological time. In this contribution, we report on an unusual impact glass-the Dakhleh Glass (DG)-which is distributed over an area of ~400 km 2 of the Dakhleh Oasis, Egypt. This region preserves a rich history of habitation stretching back to over 400,000 years before the emergence of Homo sapiens. We report on observations made during recent fieldwork and subsequent analytical analyses that strengthen previous suggestions that the DG formed during an impact event. The wide distribution and large size of DG specimens (up to ~50 cm across), the chemistry (e.g., CaO and Al 2 O 3 contents up to ~25 and 18 wt%, respectively), the presence of lechatelierite and burnt sediments, and the inclusion of clasts and spherules in the DG is inconsistent with known terrestrial processes of glass formation. The age and other textural characteristics rule out a human origin. Instead, we draw upon recent numerical modeling of airbursts to suggest that the properties of DG, coupled with the absence of a confirmed crater, can best be explained by melting of surficial sediments as a result of a large airburst event. We suggest that glass produced by such events should, therefore, be more common in the rock record than impact craters, assuming that the glass formed in a suitable preserving environment.

Very high-temperature impact melt products as evidence for cosmic airbursts and impacts 12,900 years ago

“…5), suggesting an origin by one of those causes. Lechatelierite is found in material from Meteor Crater (16), Haughton Crater, the Australasian tektite field (17), Dakhleh Oasis (18), and the Libyan Desert Glass Field (17), having been produced from whole-rock melting of quartzite, sandstones, quartz-rich igneous and metamorphic rocks, and/or loess-like materials. The consensus is that melting begins above 1,700°C and proceeds to temperatures >2;200°C, the boiling point of quartz, within a time span of a few seconds depending on the magnitude of the event (26,27).…”

“…Lechatelierite cannot be produced volcanically, but can form during lightning strikes as distinctive melt products called fulgurites that typically have unique tubular morphologies (15). It is also common in cratering events, such as Meteor Crater, AZ (16), and Haughton Crater, Canada § , as well as in probable high-temperature aerial bursts that produced melt rocks, such as Australasian tektites (17), Libyan Desert Glass (LDG) (17), Dakhleh Glass (18), and potential, but unconfirmed, melt glass from Tunguska, Siberia (19). Similar lechatelierite-rich material formed in the Trinity nuclear detonation, in which surface materials were drawn up and melted within the plume (20).…”

Impact Cratering: Processes and Products

“…The documentation of spherule beds and tektites is a topic that is discussed in chapters on impact ejecta (Chapter 4) and impact melting (Chapter 9). While many of these Figure 1.12 Field images of Dakhleh Glass, the potential product of an airburst event (Osinski et al, 2007;Osinski et al, 2008c). (see Chapter 5) and the production of vapour plumes and ejecta deposits (see Chapters 3 and 4) are still not fully understood.…”

Microspherules-Bearing White Sandstone: Implication of Cosmic Impact Event near Jurassic-Cretaceous Boundary in West Central Sinai, Egypt

“…Alternatively, penetration craters form when the smaller diameter projectile is slowed down by passage through the Earth's atmosphere (e.g., the Sikhote-Alin crater field in Russia formed from a meteorite shower in 1947). Aerial burst is taken as a significant category of impact episodes that either doesn't form craters, or which forms very shallow structures that are simply erased [11]. However, visible impact craters are less common because they become eroded, hidden or altered by tectonic over time.…”

“…The factors affecting the form of impact craters and the resultant ejecta are the size and velocity of the impactor and the geology of the target surface. Impact craters reveal hypervelocity impact event and they form when a projectile is huge and cohesive enough to penetrate the Earth's atmosphere with little or no slowing and to hit the earth at nearly its original cosmic speed (>11 km/s) [11]. Alternatively, penetration craters form when the smaller diameter projectile is slowed down by passage through the Earth's atmosphere (e.g., the Sikhote-Alin crater field in Russia formed from a meteorite shower in 1947).…”