The Arkenu structures, also known as the Arkenu craters, are a pair of prominent circular geological structures in eastern
Libya
Libya, officially the State of Libya, is a country in the Maghreb region of North Africa. It borders the Mediterranean Sea to the north, Egypt to Egypt–Libya border, the east, Sudan to Libya–Sudan border, the southeast, Chad to Chad–L ...
. The structures are approximately and in diameter, and lie about west of Jabal Arkanu 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 impact
An impact event is a collision between astronomical objects causing measurable effects. Impact events have been found to regularly occur in planetary systems, though the most frequent involve asteroids, comets or meteoroids and have minimal eff ...
s. Field investigations by Dr. P. Paillou, Dr. A. Rosenqvist, and others reported the presence of impact
breccia
Breccia ( , ; ) is a rock composed of large angular broken fragments of minerals or Rock (geology), rocks cementation (geology), cemented together by a fine-grained matrix (geology), matrix.
The word has its origins in the Italian language ...
quartz
Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...
grains of
sandstone
Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...
s outcropping in the structures. Based on these reports and other observations, it was proposed that both structures are extraterrestrial
impact crater
An impact crater is a depression (geology), depression in the surface of a solid astronomical body formed by the hypervelocity impact event, impact of a smaller object. In contrast to volcanic craters, which result from explosion or internal c ...
s that were formed simultaneously as a double
impact event
An impact event is a collision between astronomical objects causing measurable effects. Impact events have been found to regularly occur in planetary systems, though the most frequent involve asteroids, comets or meteoroids and have minimal effe ...
less than 140 million years ago (
Jurassic
The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately 143.1 Mya. ...
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
ventifact
A ventifact (also wind-faceted stone, windkanter) is a rock (geology), rock that has been abraded, pitted, etched, grooved, or polished by wind-driven sand or ice crystals. The word "Ventifact" is derived from the Latin word "Ventus" meaning 'wind ...
s 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
The Holocene () is the current geologic time scale, geological epoch, beginning approximately 11,700 years ago. It follows the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene to ...
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
Shock metamorphism or impact metamorphism describes the effects of shock-wave related deformation and heating during impact events.
The formation of similar features during explosive volcanism is generally discounted due to the lack of metamorp ...
including a lack of planar deformation features in quartz grains and evidence of impact melting, or presence of glass. A lack of any apparent differences between the
sedimentary rock
Sedimentary rocks are types of rock (geology), rock formed by the cementation (geology), cementation of sediments—i.e. particles made of minerals (geological detritus) or organic matter (biological detritus)—that have been accumulated or de ...
s outcropping inside and outside these circular structures was found. Finally, field observations found silicified sandstone dikes and
igneous rock
Igneous rock ( ), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rocks are formed through the cooling and solidification of magma or lava.
The magma can be derived from partial ...
s, 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%). It is considered a porphyries, tephrites,
phonolite
Phonolite is an uncommon shallow intrusive or 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 var ...
s, and
lamprophyre
Lamprophyres () are uncommon, small-volume ultrapotassic igneous rocks primarily occurring as dikes, lopoliths, laccoliths, stocks, and small intrusions. They are alkaline silica- undersaturated mafic or ultramafic rocks with high magnesium o ...
s ( 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 and hand 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 de ...
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
Magnetite is a mineral and one of the main iron ores, with the chemical formula . It is one of the iron oxide, oxides of iron, and is ferrimagnetism, ferrimagnetic; it is attracted to a magnet and can be magnetization, magnetized to become a ...
–
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
The Earth Impact Database is a database of confirmed impact structures or impact crater, craters on Earth. It was initiated in 1955 by the Dominion Observatory, Ottawa, under the direction of Carlyle Smith Beals, Carlyle S. Beals. Since 2001, it h ...
University of New Brunswick
The University of New Brunswick (UNB) is a public university with two primary campuses in Fredericton and Saint John, New Brunswick. It is the oldest English language, English-language university in Canada, and among the oldest public universiti ...