The document describes a crater field in the Eastern Sahara that was previously thought to be impact craters but was found through field work and analysis to actually be formed by endogenous processes related to volcanic activity. It also describes two circular structures in Libya called the Arkenu craters that were determined to likely be formed by subvolcanic intrusions and hydrothermal alteration, not meteorite impacts. Additionally, it summarizes the discovery of a new giant impact crater found in Egypt's western desert through satellite image analysis.

1. IMPACT AND NON-IMPACT CRATERS
IN EASTERN SAHARA
Mario Di Martino
INAF – Osservatorio Astronomico di Torino

2. “Tunguska 96” conference
Bologna July 15-17, 1996

4. The Gilf Kebir region and surroundings
Libyan Desert Glass
Great Sand Sea
BP
EGYPT
Kebira “impact crater”
Oasis
El-Baz volcanic crater
LIBYA Gilf Kebir Plateau
“Impact crater field”
Arkenu ”impact craters”
J. Arkenu
J. Uweinat
SUDAN

5. The “trigger”

6. “The largest impact crater field on the Earth”
GKCF10

7. November 2005 expedition
We carried out fieldwork on 7 of 13 craters identified as impact craters, namely GKCF 1, GKCF 6, GKCF 7,
GKCF 8, GKCF 11, GKCF 12, GKCF 13, and we collected rock samples from GKCF 1, GKCF 7, GKCF 11,
GKCF 13, on which petrographic studies has been in part performed. Also some other similar circular
structures in the surroundings have been examined.
GKCF 13 (950 m)
GKCF 1 (630 m)
GKCF 12 (500 m)
GKCF 11 (1.200 m)
GKCF 6 (20 m) GKCF 7 (40 m)
GKCF 8 (75 m)

8. Pseudo-shatter cones (1)
GKCF 1
GKCF 13
Sudbury
GKCF 1
Gosses Bluff
GKCF 1
GKCF 1
El-Baz volcanic crater

9. Pseudo-shatter cones (2)
Dashed lines are early Holocene W
and NW flows, solid lines are late
Holocene N and NE flows.
Brookes, I.A. Geomorphology 56, 155, 2003.

10. No Planar Deformation Features (PDF)
Breccias found around most of the structures we
visited could have been produced by fluidized
sediments reaching the surface.

11. CONCLUSIONS (1)
The crater-like structures in Gilf Kebir area we studied are not
of impact origin, but likely related to endogenic processes typical of
hydrothermal vent complexes in volcanic areas, which may reflect the
emplacement of subvolcanic intrusives.

12. BP (2.8 km)
1
Arkenu
2 (10.3, 6.8 km)
Libyan “impact” structures
Oasis (11.5 km)

13. CONCLUSIONS (2)
The Arkenu craters are not of impact origin, but very probably the result of
the intrusion of a paired nearly cylindrical subvolcanic stocks coupled with ring
dike injections in the surroundings, accompanied by hydrothermal alteration
and degassing.
This process was followed by local structural adjustments, likely due to
thermal contraction of the whole edifices along circular fractures.
Erosion did its cycle and finally revealed the present architecture.

14. NON-IMPACT ORIGIN OF THE ARKENU CRATERS (LIBYA)
M. Di Martino (1), C. Cigolini (2), L. Orti (3)
1) INAF-Osservatorio Astronomico di Torino, Italy
2) Dipartimento di Scienze Mineralogiche e Petrologiche, Università di Torino, Italy
3) Dipartimento di Scienze della Terra, Università di Firenze, Italy.
Results presented at the:
“Large Meteorite Impacts and Planetary Evolution IV” LPI conference
Vredeford, South Africa, 17-21 August 2008 – Extended abstract

15. Huge impact crater found in Egypt
The crater dwarfs the next largest known Saharan crater
A giant crater made by a meteorite impact millions of
years ago has been discovered in Egypt's western
desert. (F. El-Baz and E. Ghoneim, Boston Univ.)

16. Lesson
The analysis of satellite imagery is
fundamental for looking for impact
craters, but has to be considered only a
preliminary step
BUT
Before claiming that a circular structure
has been originated by a cosmic body
impact, accurate field investigations and
laboratory analyses are necessary

17. The Kamil crater Moon
a “lunar” crater on the Earth
Earth
(Eastern Sahara)
Luna
Mars

18. The crater in false color images

19. Radar image (X band) by Cosmo-SkyMed satellite

20. The Kamil crater

21. Chunks of cosmic iron

22. The tracks of our ancestors
Crater age ≤ 5,000 years

23. The expedition
“Kamil 2010”
February

24. The “baby” (83 kg)

25. Geophysical Data
a) Digital elevation model with superimposed
magnetic anomaly map detected after
systematic searches and collection of
meteorites >1 g. Maxima are localized along
the northern, southeastern and
southwestern ejecta rays where abundant
microscopic melt particles of the projectile
are interspersed within the ejecta blanket.
b) A representative section of the crater
based on digital elevation model and ground
penetrating radar survey of the crater floor
(lower panel).
c) Diameter ≈ 45 m
Depth ≈ 12 m

26. The iron meteorite (1)
Basic observations
- density = 7.90 g/cm3 (typical density of iron meteorites)
- Very fresh appearance, no fusion crust, no “heat alteration zone”, “twisted”
edges --> the examined specimens, except one, are fragments formed
during the meteorite impact event (“schrapnels”)
“twisted” edges
“twisted” edges

27. The iron meteorite (2)
Macroscopic internal structure: Polished endcut etched with Nital o FeCl3
- No Widmannstätten pattern;
- Abundant inclusions of schreibersite
Schreibersite
[(Fe,Ni)3P, troilite (FeS) and
daubreelite (FeCr2S4) with a large rim
of kamacite;
Schreibersite
- Evidences of shock effects
(brecciation, shear zones);
Brecciation

28. An unusual iron meteorite
Chemical analysis
Iron meteorites with Ni contents between 15 and 25
Ni = 198 mg/g wt%, Ir < 1g g-1, Ge 100 -150 g g-1 and Ga 30-60 g
Co = 7.5 mg/g g-1 are very rare. The one most similar is Morradal
(Norway), “ungrouped” iron.
Ga = 49.5 g g-1
Ge = 121 g g-1
Ir = 0.39 g g-1 (the following trace elements have been also determined:
V, Cr, Cu, Zn, As, Mo, Ru, Rh, Pd, Sn, Sb, W, Re, Pt, Au)

29. An unusual iron projectile
1) A very rare type of Ni-rich iron meteorite
2) A very fresh and moderately shocked iron meteorite,
structurally transitional between the ataxites and the
plessitic octahedrites
3) Only one individual meteorite (83 kg) has been
found. The other meteorite fragments (shrapnels) have
been formed during the impact event
4) The estimated total mass of the impactor is of the
order of 5-10 x 103 kg, corresponding to a
pre-atmospheric mass of ~20-40 x 103 kg.