The Arkenu structures, also known as the Arkenu craters, are a pair of prominent circular geological structures in eastern Libya. The structures are approximately and in diameter, and lie about west of

Jabal Arkanu Mount Arkanu or Jabal Arkanu (also Jebel Arkenu or Gebel Árchenu) is a mountain in Libya. Geography The mountain is located in the Libyan Desert in the Kufra District of Libya, about 300 km southeast of El Tag.Bertarelli (1929), p. 515 and ab ...

on the eastern margin of the al-Kufrah Basin.Paillou P., A. Rosenqvist A., J.M. Malezieux, B. Reynard, T. Farr, and E. Heggy (2003) ''Discovery of a double impact crater in Libya: The astrobleme of Arkenu.'' Comptes Rendus Geoscience. vol. 335, no. 15, pp. 1059–1069.Cigolini, C, C Laiolo, and M Rossetti (2012) ''Endogenous and nonimpact origin of the Arkenu circular structures (al-Kufrah basin-SE Libya)'' Meteoritics & Planetary Science. 47(11):1772–1788. It has been argued that both structures were formed by simultaneous meteorite impacts. Field investigations by Dr. P. Paillou, Dr. A. Rosenqvist, and others reported the presence of impact breccias at the structures’ bottoms, shatter cones pointing toward the center of the structures, and microscopic planar deformation features (PDFs) found in quartz grains of sandstones outcropping in the structures. Based on these reports and other observations, it was proposed that both structures are extraterrestrial impact craters that were formed simultaneously as a double

impact event An impact event is a collision between astronomical objects causing measurable effects. Impact events have physical consequences and have been found to regularly occur in planetary systems, though the most frequent involve asteroids, comets or me ...

less than 140 million years ago ( Jurassic or younger). More recently, on the basis of field, petrographic, and textural observations, it has been argued that the Arkenu structures are not in fact extraterrestrial impact craters. Field studies found a lack of identifiable shatter cones at both structures. Instead, the striations which were previously reported as shatter cones were identified as ventifacts created by wind erosion in sandstones. These striations are surficial features that are unrelated to fracturing of the sandstone; are consistently oriented with the prevailing Holocene wind patterns; and occur within and outside of, even distal to, both of the Arkenu structures. In addition, detailed petrographic analyses of rock samples from both of the Arkenu structures found a lack of any microscopic effect of shock metamorphism including a lack of planar deformation features (PDFs) in quartz grains and evidence of impact melting, or presence of glass. A lack of any apparent differences between the sedimentary rocks outcropping inside and outside these circular structures was found. Finally, field observations found silicified sandstone dikes and igneous rocks, such as

syenite Syenite is a coarse-grained intrusive igneous rock with a general composition similar to that of granite, but deficient in quartz, which, if present at all, occurs in relatively small concentrations (< 5%). Some syenites contain larger proport ...

, porphyries, tephrites and

phonolite Phonolite is an uncommon extrusive rock, of intermediate chemical composition between felsic and mafic, with texture ranging from aphanitic (fine-grained) to porphyritic (mixed fine- and coarse-grained). Phonolite is a variation of the igneous ...

s, and lamprophyres ( monchiquites) directly associated with each circular feature. Based on these and other observations, it was concluded that the Arkenu structures are

stocks Stocks are feet restraining devices that were used as a form of corporal punishment and public humiliation. The use of stocks is seen as early as Ancient Greece, where they are described as being in use in Solon's law code. The law describing ...

of porphyritic syenitic that have intruded the Nubia Formation to form rather simple and eroded ring dike complexes. Hydrothermal activity that followed the intrusion of these ring dike complexes resulted in the formation of massive magnetite–

hematite Hematite (), also spelled as haematite, is a common iron oxide compound with the formula, Fe2O3 and is widely found in rocks and soils. Hematite crystals belong to the rhombohedral lattice system which is designated the alpha polymorph of . ...

deposits and dikes of silicified sandstone.Di Martino, M, C Cigolini, and L. Orti (2008) ''Non-impact origin of the Arkenu craters (Libya)'' Large Meteorite Impacts and Planetary Evolution IV, 17–21 August, Vredefort Dome South Africa. abstract no. 3012, Lunar and Planetary Institute, Houston, Texas. As a result of this research, the Arkenu structures were removed from and are currently not listed in the Earth Impact Database.Anonymous (nd
''Africa'' (Impact Craters)

Planetary and Space Science Centre
University of New Brunswick, New Brunswick, Canada.

References

External links

*Brügge, N. (2012
''Remarks to the origin of the craters around Gilf Kebir and Djebel Uweinat (Egypt)and the supposed impact craters of Libya''
*Di Martino, M. (2010
''Impact and non-impact craters in eastern Sahara''
INAF – Osservatorio Astronomico di Torino Talk of the "International Workshop on Paolo Farinella (1953-2000): the Scientists, the man", Pisa, 14–16 June 2010 *Willis, K. J. (2008
''Arkenu Craters, Libya : Image of the Day.''
Earth Observatory, EOS Project Science Office, NASA Goddard Space Flight Center, Greenbelt, Maryland. {{Impact cratering on Earth Impact craters of Libya Sahara