Impact cratering is now recognized as a major geological process on Earth. In
particular, giant impacts had a fundamental influence on the geological and
biological evolution of our planet with possible climatic effects. There are
more than 160 confirmed impact craters on Earth, among which 17 are located in
Africa, but it is estimated that only 10% of impact craters larger than 10km and
younger than 100Ma are known.
The Sahara is a particularly favorable region to host young impact craters, but
according to cratering rate estimates, most of them still remain to be
discovered, hidden under dry sandy sediments. Only four confirmed impact craters
are currently known in eastern Sahara. Two are located in eastern Libya: B.P.
(British Petroleum) structure and Oasis crater, and two are located in northern
Chad: Aorounga and Gweni-Fada craters. While optical sensors can only image the
desert’s surface, it was shown twenty years ago that orbital Synthetic Aperture
Radar (SAR) could retrieve subsurface information hidden under a few meters of
dry sand. Within an international project — dubbed SAHARASAR — that aims at
mapping the near subsurface of the Sahara and Arabian regions using
satellite-borne radar, we realized a regional-scale radar mosaic at 100m
resolution over the eastern Sahara from existing JERS-1 archives (a Japaneese
satellite operated from 1990 to 1998).
This unique data set allowed us to discover a double circular structure in the
southeastern part of the Libyan desert. Fieldwork confirmed that this formation
is an unknown double impact crater with a diameter around 10 km.
The newly discovered structures are located 110km west of Djebel Arkenu and
250km south of Kufra oasis in Libya, at coordinates N22 deg 04′, E23 deg 45′. It
is a flat and hyperarid area, presenting a Cretaceous sandstone formation
covered by active aeolian deposits and Quaternary soils, located tens of
kilometers away from any track, in a hazardous zone due to the proximity of
Second World War minefields. The optical Landsat-7 image shows a sandy region
with large sand dunes trending SW-NE, while the corresponding L-band radar image
extracted from the JERS-1 radar mosaic reveals two circular structures partially
hidden by Quaternary deposits. The radar scene then clearly reveals a double
circular structure composed of a southwestern crater 10.3km in diameter and a
northeastern crater of diameter 6.8km. The NE crater is composed of concentric
inner and outer rings separated by a depression filled with sediments, also
observed in the optical scene. Its morphology is very similar to the Aorounga
crater in Chad, corresponding to a typical complex crater. The SW crater also
presents a circular shape with possibly three concentric annular ridges. The
host rock of the double circular structure is a cross-bedded coarse-grained to
conglomeratic sandstone of Lower Cretaceous age containing plant fossils and
thin shale interbeds, leading to an estimated impact age of less than 140Ma.
A field survey was carried out during April 2003 in order to obtain definitive
proof of the impact origin of the observed structures (i.e. shatter cones, high
shock pressure metamorphism, planar microstructures in quartz grains, high
pressure polymorphs such as coesite and stishovite, Iridium enrichment). We
observed quantities of shatter cone structures on the site, all located close to
the inner ridge of the NE crater. Large outcrops of allochthonous impact breccia
could also be observed in both craters. We could find several quartz grains
presenting planar fractures (PFs) in these breccia. Such planar microstructures
are diagnostic shock effects in a pressure range from 5 to 20GPa.
We can assert from these observations that the newly discovered circular
structures were produced by the impact of a 500m diameter pair of asteroids.
Because of the proximity of Djebel Arkenu, we proposed to name the two new
impact craters as follows: “Arkenu 1” for the NE crater and “Arkenu 2” for the
SW crater.
Peer reviewed publication and references
Ph. Paillou, A. Rosenqvist, J.-M. Malézieux, B. Reynard, T. Farr, E. Heggy,
“Discovery of a double impact crater in Libya: the astrobleme of Arkenu”, Acad.
Sci. Paris, C.R. Geoscience, 335 (2003), 1059-1069.
