Kamil Crater (Egypt) a natural laboratory to study shock metamorphism and impact melting

Kamil Crater is one of the most recent discovered impact craters. It was incidentally identified in 2008 by Vincenzo De Michele (Italy) during a Google Earth survey while he was searching for ruins of prehistoric settlements in a rocky desert area of the southwestern part of Egypt (22°01’06’’N, 26°05’16’’E; Figure 1). In 2010, an Italian-Egyptian geophysical campaign was organized with the aims to carry out the meteorite systematic sampling and geophysical surveys (e.g., radar and geomagnetic surveys).

Fig. 1. Enhanced true color QuickBird satellite scene (22 October 2005; courtesy of Telespazio) of the Kamil area (Egypt; see inset).

Kamil is a simple crater of only 45 m in diameter. It was generated by the hypervelocity impact of the Gebel Kamil iron meteorite into sandstone rocks of the Cretaceous Gilf Kebir Formation. On the basis of archeological evidence the impact occurred likely < 5000 yr ago.

Kamil can be considered a natural laboratory to study the cratering process of small impactors (about 1-m-in diameter) on Earth and the consequences produced by them for three main reasons:

  1.  Geological setting. The cratering involved horizontal bedded quartz-dominated sedimentary rocks, i.e. sandstones. Shock effects of quartz are the best studied because of its mineral structure and its abundance on Earth’s surface (e.g., Langenhorst and Deutsch, 2012, Elementshttp://elements.geoscienceworld.org/content/8/1/31.abstract).
  2.  Small crater. The Kamil Crater has a diameter of 45 m. On Earth, small impact craters (< 300 m in diameter) are rare, only 17 out of 184 impact craters already known (Earth impact Database – http://www.passc.net/AboutUs/index.html). Statistics estimate that bodies able to form small impact craters (< 300 m in diameter) occur on Earth with a decadal to secular time scale (Bland and Artemieva, 2006, MAPS, – http://onlinelibrary.wiley.com/doi/10.1111/j.1945-5100.2006.tb00485.x/abstract). This discrepancy is due to the fact that small impact structures are more subject to weathering and geological processes such as tectonic, erosion, sedimentation.
  3.  State of preservation. Contrary to the great majority of Earth impact craters Kamil is very well preserved, as mainly indicated by the rayed ejecta deposit (Figure 1). The good state of preservation allows the possibility to report and to study shock effects never associated at small impact craters.

In the framework of my PhD I am working on the shock metamorphism and impact melting of Kamil. At last Annual Meeting of the Meteoritical Society in Casablanca (8th-12th September 2014) I presented two abstracts (one as oral presentation and another as poster). Thanks to the abstract entitled “Shock metamorphism and impact melting in small impact craters on Earth: Evidence from Kamil Crater, Egypt”, authored by A. Fazio, L. Folco, M. D’Orazio, M. Frezzotti, and C. Cordier, I was selected by the program committee for the meeting to win the travel grant Brian Mason Award funded by the International Meteorite Collectors Association (IMCA). The paper related to this abstract was published in the December issue of the journal Meteoritics & Planetary Science (http://onlinelibrary.wiley.com/doi/10.1111/maps.12385/abstract).

This work is a detailed report of the petrography and some chemical observations of samples from the crater wall and ejecta deposits. Samples from the crater wall do not show any evidence of shock; hence, their features reflect those of the target rocks. They are mainly made of quartz (up to 99 vol%). The accessory phases constitute ~ 2 vol% and the most common are Fe-Ti oxides, zircon, and tourmaline. The matrix is composed by kaoline and minor by iron oxides. The porosity is generally < 4 vol%.

Shock features were found only in sandstone fragments from the ejecta. Sandstone fragments show an almost complete set of shock metamorphic features including fracturing  (Figure 2), planar deformation features1 (PDFs) in quartz (Figure 3) and tourmaline, zircon decomposition (Figure 3), SiO2 polymorphs (coesite (Figure 4) and stishovite), diamond, melt veins (Figure 5) and melt films in shatter cones2 (Figure 6).

Figure 2. Backscattered Scanning Electron Microscope (BSE-SEM) image showing concussion fractures in an ejected sandstone fragment.

Figure 3. BSE-SEM images showing (a) a portion of a quartz grain with four sets of PDFs and (b) a quartz grain with two sets of enlarged PDFs in an ejected sandstone fragments. The bright aggregate on the left-side of the image is a fine intergrowth of baddeleyite (ZrO2) and a SiO2 phase, resulting from the decomposition of a zircon crystal.

Figure 4. Coesite occurring in an ejected sandstone fragments. a) Raman spectra for coesite and for coesite + diaplectic glass/ SiO2 melt. b) Photomicrograph of intergranular colorless SiO2 melt surrounded by brownish cryptocrystalline and amorphous material (optical microscope plane polarized light image). c) BSE-SEM image of the area of photomicrograph (b). The arrows in (b) and (c) indicate the same vesicles within the colorless SiO2 melt. d) Detail of the outer zone (white rectangle in (c)) made up of sub-micrometric coesite grains (C) embedded in a glassy matrix (G). Similar structures were described in shocked sandstone from Barringer Crater and they are also known as symplectic regions (Kieffer et al., 1976, CMP –http://link.springer.com/article/10.1007/BF00375110).

Figure 5. BSE-SEM images of a melt vein occurring in an ejected sandstone fragments. a) Finely vesicular melt vein. The bright material is enriched in Fe and Ti. Note the straight contact with the undeformed host rock, and the arrowed injection vein on the right side of the vein. b) Close-up view of the white rectangular area in (a) showing a finely vesicular portion with schlieren and relict quartz grains planar amorphous lamellae (similar to PDFs).

Figure 6. Mesoscopic and microscopic features of a sample showing shatter cones. a) Shatter cone structures with striae arranged in a horse-tail patterns. b) Close-up view of the rectangular area in image (a). Striations on the shatter cone surface radiate from a common apex. They are discontinuously coated with by a white film (100s of µm thick) of silica-rich glass (black arrows). The white arrow indicates where images (c) were taken. c) A cross sectional view of the shatter cone surface coated by silica rich glass. d) BSE-SEM image of the silica-rich glass film on shatter cone striae. Vesicles in the melt are coherently stretched and oriented indicating that frictional melting contributed, at least in part, to shatter cones formation.

In addition to sandstone fragments, impact melt lapilli and bombs also occur in the ejecta deposit. Two types of glasses constitute the impact melt lapilli and bombs: a white glass and a dark glass (Figure 7). The white glass is highly vesicular and almost exclusively made of SiO2 (lechatelierite). The dark glass is a silicate melt with variable content of Al2O3 (0.84-18.7 wt%), FeO (1.83-61.5 wt%) and NiO (<0.01-10.2 wt%). The dark glass typically includes fragments (from few μm to several mm in size) of shocked sandstone, lechatelierite, and diaplectic glass3 and Ni-Fe metal spherules. The occurrence of two type of glass indicates that the white glass experienced a negligible interaction with the projectile and, conversely, the dark glass experienced an extensive interaction with the projectile.

Figure 7. Impact melt bombs (cut surfaces). a) White glass. b) Dark glass with inclusions of sandstone clasts, lechatelierite clasts, and meteorite fragments. Abbreviations: MF = meteorite fragment; SC = shocked sandstone clast; LG = lechatelierite.

Shock features found at Kamil are classified into two categories: 1- pervasive shock features and 2- localized shock features. Pervasive shock features include fracturing, PDFs, and impact melt lapilli and bombs and occupy ~100 vol% of sample. They reflect the shock pressure suffered by the target rock: fracturing, PDFs, and impact melting indicate a maximum shock pressure of 5 GPa, 20-25 GPa, and 30-60 GPa, respectively. Localized shock features include high-pressure phases and localized impact melts occurring as intergranular melt, melt veins and melt films enveloping shatter cones. They occupy less than 1 vol% of the sample. They are a consequence of a local enhancement of shock pressure and temperature corresponding to heterogeneities of the target rock.

The maximum shock pressures recorded at Kamil can be achieved through face-on impact velocities between 5.0 km s-1 (30 GPa) and 7.5 km s-1 (60 GPa), assuming an impact angle of 45°.

In conclusion, from Kamil we learnt that the hypervelocity impact of meter-sized iron meteorite projectiles can produce shock effects similar to those observed in high velocity, larger impacts. The young age of the crater (most likely < 5000 yr), the mechanical strength of target rocks and the low erosion rates of the hot-desert area played a crucial role in the preservation of all these shock features. Moreover, Kamil is the smallest impact structure where shatter cones, coesite, stishovite, diamond, and impact melt (target and projectile) have been reported.

1Planar deformation features = Submicroscopic amorphous lamellae occurring in shocked minerals as multiple sets of planar lamellae (optical discontinuities under the petrographic microscope) parallel to rational crystallographic planes; they are indicative of shock metamorphism. (From Glossary of Geology – American Geological Institute).

2Shatter Cones = A distinctively striated conical structure in rocks, ranging in length from less than a centimeter to several meters, along which fracturing has occurred. It is generally found in nested or composite groups in the rocks of impact structures and formed by shock waves generated by impact (Dietz, 1959). Shatter cones superficially resemble cone-in-cone structure in sedimentary rocks. They are most common in fine-grained homogeneous rocks such as limestone and dolomite, but are also known in shale, sandstone, quartzite, and granite. The striated surfaces radiate outward from the apex in horsetail fashion; the apical angle varies but is close to 90°. (From Glossary of Geology – AGI).

3Diaplectic Glass = Amorphous form of crystals, “solid state glass”, resulting from shock wave compression and subsequent pressure release of single crystals or polycrystalline rocks; most commonly observed for tectosilicates. (From Glossary of Geology – AGI).

Kamil Bibliography

– D’Orazio M., Folco L., Zeoli A. and Cordier C. (2011) Gebel Kamil: The iron meteorite that formed the Kamil crater (Egypt). Meteoritics & Planetary Science vol. 46, pp. 1179–1196. – http://onlinelibrary.wiley.com/doi/10.1111/j.1945-5100.2011.01222.x/abstract

– Fazio A., Folco L., D’Orazio M., Frezzotti M. L. and Cordier C. (2014) Shock metamorphism and impact melting in small impact craters on Earth: Evidence from Kamil Crater, Egypt. Meteoritics & Planetary Science vol. 49, pp. 2175-2200.

– Folco L., Di Martino M., El Barkooky A., D’Orazio M., Lethy A., Urbini S., Nicolosi I., Hafez M., Cordier C., van Ginneken M., Zeoli A., Radwan A. M., El Khrepy S., El Gabry M., Gomaa M., Barakat A. A., Serra R. and El Sharkawi M. (2010) The Kamil Crater in Egypt. Science vol. 329, pp. 804. – http://www.sciencemag.org/content/329/5993/804.full

– Folco L., Di Martino M., El Barkooky A., D’Orazio M., Lethy A., Urbini S., Nicolosi I., Hafez M., Cordier C., van Ginneken M., Zeoli A., Radwan A. M., El Khrepy S., El Gabry M., Gomaa M., Barakat A. A., Serra R. and El Sharkawi M. (2011) Kamil Crater (Egypt): Ground truth for small-scale meteorite impacts on Earth. Geology vol. 39, pp. 179–182. – http://geology.gsapubs.org/content/39/2/179.abstract

– Urbini S., Nicolosi I., Zeoli A., El Khrepy S., Lethy A., Hafez M., El Gabry M., El Barkooky A., Barakat A., Gomaa M., Randwan A. M., El Sharkawi M., D’Orazio M. and Folco L. (2012) Geological and geophysical investigation of Kamil Crater, Egypt. Meteoritics & Planetary Science vol. 47, pp. 1842–1868. – http://onlinelibrary.wiley.com/doi/10.1111/maps.12023/abstract

About the Author

Agnese Fazio
Agnese Fazio is a final-year PhD student of the Dipartimento di Scienze della Terra (Earth Science Department) of Pisa University. Her thesis title is Shock Metamorphism and Impact Melting at Kamil Crater, Egypt. In 2014, she won the “Barringer Family Fund for Meteorite Impact Research” for a project on the study of the high pressure minerals occurring in Kamil sandstones and the Brian Mason Travel Award for the 77th Annual Meeting of the Meteoritical Society supported by the International Meteorite Collectors Association.