Impact and non-impact craters in eastern Sahara

M. Di Martino1 ; C. Cigolini2

1 INAF – Osservatorio Astronomico di Torino, Pino Torinese, Italy ; 2 Dipartimento di Scienze Mineralogiche e Petrologiche, Università di Torino, Torino, Italy

ABSTRACT

Recognizing true impact structures on the Earth is usually a very difficult task. Several circular crater-like structures are present in the eastern Sahara (SW Egypt, SE Libya). It has been previously suggested that some of them could be originated by cosmic body impacts, but the results of our geological and geophysical surveys in the area exclude this hypothesis. The first target of our investigations has been the Gilf Kebir region (SW Egypt), where a lot of small crater-like structures are present. On 2004 Paillou et al. [1] concluded that some of them are of meteoritic origin. This hypothesis is not supported by our observations and analyses, and we suggest an alternative interpretation [2]. The crater-like structures in Gilf Kebir area are likely related to endogenic processes typical of hydrothermal vent complexes in volcanic areas which may reflect the emplacement of subvolcanic intrusives. Moreover, a double circular structure, located in SE Libya, about 250 km south of Kufra oasis, was recognized as a double impact crater (the Arkenu craters) by [3], first in satellite imagery and then in the fieldwork, and was thus included in the terrestrial impact crater list. The Arkenu craters consist of a NE (Arkenu 1) and a SW structure (Arkenu 2), 10.3 km and 6.8 km in diameter, respectively, whose centers are located about 10 km apart. The interpretation of Arkenu structures as impact-related is based on the observation of shatter cones and impact breccias with planar fractures in quartz grains. We visited the area on November 2007 and we carried out a preliminary geological and structural survey. Our field, petrographic and textural observations do not support the idea that the Arkenu “impact craters” have been originated by the collision of two cosmic bodies. In fact, we did not observe PDF in the collected quartz grains and, in addition, the so called “shatter cones” (brought by Paillou and co-workers as an evidence of an impact event) have been likely produced by the erosion of sandstones (see [2]): all of them, in fact, are oriented in the same direction of the dominant winds (we prefer to name these structures as “pseudo-shatter cones”). Even in this case, the proposed impact origin for these structures is not supported by our observations and analyses. We propose that most of the crater-like structures in the eastern Sahara area are in general likely related to endogenic processes typical of volcanic areas, such as geothermal fields which in the past were sites of efficient hydrothermal processes and/or the result of intrusion of cylindrical subvolcanic stocks (coupled with ring dike injection in the surroundings) accompanied by hydrothermal degassing. Anyway, during a very recent scientific expedition, we have discovered in the eastern Sahara a true, small, and very fresh impact crater. We plan to organize a scientific expedition in the area in the very near future to accurately study this structure, which characteristics are unique among the impact structures presently known on the Earth. List of references 1) Paillou Ph., El-Barkooky A., Barakat A., Malezieux J. M., Reynard B., Dejax J., & Heggy E. Discovery of the largest impact crater field on Earth in the Gilf Kebir region, Egypt. Comptes Rendus Géoscience de l’Académie des Sciences. 2004. V. 336. P. 1491-1500. 2) Orti L., Di Martino M., Morelli M., Cigolini C., Pandeli E., & Buzzigoli A. Non-impact origin of the crater-like structures in the Gilf Kebir area (Egypt): implications for the geology of eastern Sahara. Meteoritics & Plan. Sci. 2008. V. 43. P. 1629-1639 3) Paillou Ph., Rosenqvist A., Malezieux J. M., Reynard B., Farr T., & Heggy E. Discovery of a double impact crater in Libya: the astrobleme of Arkenu. Comptes Rendus Géoscience de l’Académie des Sciences. 2003. V. 335. P. 1059-1069.

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