Arkenu structures

Landsat image of the Arkenu structures; screen capture from NASA World Wind

The Arkenu structures, also known as the Arkenu craters, are a pair of prominent circular geological structures. These structures are 10 km and 6.8 km in diameter, and lie about 70 km west of Jabal Arkanu on the eastern margin of the al-Kufrah Basin.[1][2]

It has been argued that both structures were formed by two simultaneous meteorite impacts. Field investigations by Dr. P. Paillou, Dr. A. Rosenqvist, and others reported the presence of impact breccias at the structures’ bottom, shatter cones pointing toward the center of the structures, and microscopic planar deformation features (PDFs) found in quartz grains of sandstones outcropping in these structures. Based on these reports and other observations, it was proposed that both structure are extraterrestrial impact craters that were formed simultaneously as a double impact event less than 140 million years ago (Jurassic or younger).[1]

More recently, on the basis of field, petrographic, and textural observations, it is argued that these geological structures are not extraterrestrial impact craters. Field studies of these geological structures found a lack of identifiable shatter cones at both structures. Instead, the striations, which were previously reported shatter cones, are 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, fieldwork found silicified sandstone dikes and igneous rocks, such as syenite, porphyries, tephrites and phonolites, and lamprophyres (monchiquites) directly associated with each circular feature. Based on these and other observations, it was concluded that the Arkenu structures are stocks 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 deposits and dikes of silicified sandstone.[2][3] As a result of this research, the Arkenu structures were removed from and are currently not listed in the Earth Impact Database.[4]

References

  1. 1 2 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.
  2. 1 2 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.
  3. ↑ 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.
  4. ↑ Anonymous (nd) Africa (Impact Craters), Earth Impact Database, Planetary and Space Science Centre, University of New Brunswick, New Brunswick, Canada.

External links

Coordinates: 22°6′5″N 23°47′54″E / 22.10139°N 23.79833°E / 22.10139; 23.79833

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