Endogenous and nonimpact origin of the Arkenu circular structures (al‐Kufrah basin—SE Libya)

Abstract

Abstract-The twin Arkenu circular structures (ACS), located in the al-Kufrah basin in southeastern Libya, were previously considered as double impact craters (the ''Arkenu craters''). The ACS consist of a NE (Arkenu 1) and a SW structure (Arkenu 2), with approximate diameters of about 10 km. They are characterized by two shallow depressions surrounded by concentric circular ridges and silica-impregnated sedimentary dikes cut by local faults. Our field, petrographic, and textural observations exclude that the ACS have an impact origin. In fact, we did not observe any evidence of shock metamorphism, such as planar deformation features in the quartz grains of the collected samples, and the previously reported ''shatter cones'' are wind-erosion features in sandstones (ventifacts). Conversely, the ACS should be regarded as a ''paired'' intrusion of porphyritic stocks of syenitic composition that inject the Nubia Formation and form a rather simple and eroded ring dike complex. Stock emplacement was followed by hydrothermal activity that involved the deposition of massive magnetite-hematite horizons (typical of iron oxide copper-gold deposits). Their origin was nearly coeval with the development of silicified dikes in the surroundings. Plugs of tephritic-phonolitic rocks and lamprophyres (monchiquites) inject the Nubian sandstone along conjugate fracture zones, trending NNW-SSE and NE-SW, that crosscut the structural axis of the basin.

Impact structures in Africa: A review

“…Most recently, Cigolini et al (2012) reported further evidence in support of a volcanic genesis of the Arkenu bodies from field and petrographic work. They did not observe shock metamorphic evidence in samples of sandstone from the Arkenu circular structures and also state clearly that the alleged shatter cones have an origin as wind-ablation features.…”

The catalog of the meteorite collection of the Italian Museum of Planetary Sciences in Prato (Italy)

“…MISP opened to the public on March 19, 2005. Since its early years of activities, the museum staff operated not only in preserving and enhancing the meteorite collection through the development of cultural and educational activities but also in its enrichment through the organization of research expeditions in hot deserts—that is, Adrar and Erg Chech (Algeria), Hammadah al Hamra (Libya), Hadramawt (Yemen), Libyan desert in Egypt, al‐Kufra (Libya), Acfer (Algeria), Atacama (Chile)—to recover new specimens (Cigolini et al., 2012; Orti et al., 2010; Pratesi et al., 2005). Regarding the path choice in the exhibition area, MISP organized the meteorite collection on display, combining educational functions and aesthetic concerns with a perspective centered on the needs and interests of the visiting public (Figure 2).…”

Comment on “Some observations facing the interpretation of Waqf as Suwwan structure, SE desert of Jordan, as an impact crater”, by A. M. Abed et al. (2014), Arab J Geosci

“…To mention only a few examples in the Arab world, the proposed impact structures of the two circular Arkenu structures in Libya (Paillou et al 2003), as well as a large field of multiple proposed impact craters in the Gilf Kebir region of southwestern Egypt (Paillou et al 2004), were proven as of non-impact but magmatic-endogenic origin during field work (Orti et al 2008;Cigolini et al 2012). The Lake Isli structure in the High Atlas Mountains of Morocco, recently interpreted by Ibhi et al (2013) as an impact crater because of the occurrence of lithic breccias, was shown to be related to tectonism and karst dissolution (Ibouh et al 2014).…”