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

From: Charles Viau <cviau_at_meteoritecentral.com>
Date: Thu Apr 22 10:18:02 2004
Message-ID: <021901c3c6b4$1741ff80$1800a8c0_at_chupa>

Fantastic! =20
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=20
particular, giant impacts had a fundamental influence on the geological
and=20
biological evolution of our planet with possible climatic effects. There
are=20
more than 160 confirmed impact craters on Earth, among which 17 are
located in=20
Africa, but it is estimated that only 10% of impact craters larger than
10km and=20
younger than 100Ma are known.

The Sahara is a particularly favorable region to host young impact
craters, but=20
according to cratering rate estimates, most of them still remain to be=20
discovered, hidden under dry sandy sediments. Only four confirmed impact
craters=20
are currently known in eastern Sahara. Two are located in eastern Libya:
B.P.=20
(British Petroleum) structure and Oasis crater, and two are located in
northern=20
Chad: Aorounga and Gweni-Fada craters. While optical sensors can only
image the=20
desert's surface, it was shown twenty years ago that orbital Synthetic
Aperture=20
Radar (SAR) could retrieve subsurface information hidden under a few
meters of=20
dry sand. Within an international project -- dubbed SAHARASAR -- that
aims at=20
mapping the near subsurface of the Sahara and Arabian regions using=20
satellite-borne radar, we realized a regional-scale radar mosaic at 100m

resolution over the eastern Sahara from existing JERS-1 archives (a
Japaneese=20
satellite operated from 1990 to 1998).

This unique data set allowed us to discover a double circular structure
in the=20
southeastern part of the Libyan desert. Fieldwork confirmed that this
formation=20
is an unknown double impact crater with a diameter around 10 km.

The newly discovered structures are located 110km west of Djebel Arkenu
and=20
250km south of Kufra oasis in Libya, at coordinates N22 deg 04', E23 deg
45'. It=20
is a flat and hyperarid area, presenting a Cretaceous sandstone
formation=20
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=20
Second World War minefields. The optical Landsat-7 image shows a sandy
region=20
with large sand dunes trending SW-NE, while the corresponding L-band
radar image=20
extracted from the JERS-1 radar mosaic reveals two circular structures
partially=20
hidden by Quaternary deposits. The radar scene then clearly reveals a
double=20
circular structure composed of a southwestern crater 10.3km in diameter
and a=20
northeastern crater of diameter 6.8km. The NE crater is composed of
concentric=20
inner and outer rings separated by a depression filled with sediments,
also=20
observed in the optical scene. Its morphology is very similar to the
Aorounga=20
crater in Chad, corresponding to a typical complex crater. The SW crater
also=20
presents a circular shape with possibly three concentric annular ridges.
The=20
host rock of the double circular structure is a cross-bedded
coarse-grained to=20
conglomeratic sandstone of Lower Cretaceous age containing plant fossils
and=20
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=20
proof of the impact origin of the observed structures (i.e. shatter
cones, high=20
shock pressure metamorphism, planar microstructures in quartz grains,
high=20
pressure polymorphs such as coesite and stishovite, Iridium enrichment).
We=20
observed quantities of shatter cone structures on the site, all located
close to=20
the inner ridge of the NE crater. Large outcrops of allochthonous impact
breccia=20
could also be observed in both craters. We could find several quartz
grains=20
presenting planar fractures (PFs) in these breccia. Such planar
microstructures=20
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.=20
Because of the proximity of Djebel Arkenu, we proposed to name the two
new=20
impact craters as follows: "Arkenu 1" for the NE crater and "Arkenu 2"
for the=20
SW crater.

Peer reviewed publication and references

Ph. Paillou, A. Rosenqvist, J.-M. Mal=E9zieux, B. Reynard, T. Farr, E.
Heggy,=20
"Discovery of a double impact crater in Libya: the astrobleme of
Arkenu", Acad.=20
Sci. Paris, C.R. Geoscience, 335 (2003), 1059-1069.


______________________________________________
Meteorite-list mailing list
Meteorite-list_at_meteoritecentral.com
http://www.pairlist.net/mailman/listinfo/meteorite-list
Received on Fri 19 Dec 2003 11:45:22 PM PST