[meteorite-list] Discovery of a Double Impact Crater in Libya

From: N Lehrman <nlehrman_at_meteoritecentral.com>
Date: Thu Apr 22 10:18:02 2004
Message-ID: <014101c3c6b6$df281780$d4b1fea9_at_homeportal.2wire.net>

Charles,

Exactly the right question! The source crater for LDG is still absolutely
unknown. Whether these new features might be the source was my INSTANT
question, and I was disappointed that the subject received no comment. I'm
sure it will in due time, as this is still one of the big missing crater
mysteries! The address of the new craters certainly sounds suspicious.

Cheers,

Norm (http://TektiteSource.com)


----- Original Message -----
From: "Charles Viau" <cviau_at_beld.net>
To: "'Meteorite Mailing List'" <meteorite-list_at_meteoritecentral.com>
Sent: Friday, December 19, 2003 8:45 PM
Subject: RE: [meteorite-list] Discovery of a Double Impact Crater in Libya


Fantastic!
The somewhat obvious question is that could these structures possibly be
a source for Libyan Desert Glass, or is the source for that material
already well known?

CharlyV

-----Original Message-----
From: meteorite-list-admin_at_meteoritecentral.com
[mailto:meteorite-list-admin_at_meteoritecentral.com] On Behalf Of Ron
Baalke
Sent: Thursday, December 18, 2003 11:52 AM
To: Meteorite Mailing List
Subject: [meteorite-list] Discovery of a Double Impact Crater in Libya


Observatoire aquitain des sciences de l'Univers
Floirac, France

For further information, please contact:

Philippe Paillou
UMR 5804, OASU (Observatoire aquitain des sciences de l'Univers)
paillou_at_obs.u-bordeaux1.fr
(33) (0)557 776 126

16 December 2003

Discovery of a double impact crater in Libya

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.


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Received on Sat 20 Dec 2003 12:05:20 AM PST