IMPACT AND NON-IMPACT CRATERS IN EASTERN SAHARA

Mario Di MartinoINAF – Osservatorio Astronomico di Torino

“Tunguska 96” conferenceBologna July 15-17, 1996

EGYPT

LIBYA

SUDAN

Gilf Kebir Plateau

J. Arkenu

J. Uweinat

“Impact crater field”

Great Sand Sea

Kebira “impact crater”

Arkenu ”impact craters”

BP

Oasis

The Gilf Kebir region and surroundings

El-Baz volcanic crater

Libyan Desert Glass

The “trigger”

GKCF10

“The largest impact crater field on the Earth”

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 6 (20 m) GKCF 7 (40 m)

GKCF 13 (950 m)

GKCF 8 (75 m)

GKCF 12 (500 m)GKCF 11 (1.200 m)

November 2005 expedition

GKCF 1 (630 m)

GKCF 1

GKCF 1

El-Baz volcanic crater

GKCF 1GKCF 13

Pseudo-shatter cones (1)

GKCF 1

Sudbury

Gosses Bluff

Brookes, I.A. Geomorphology 56, 155, 2003.

Dashed lines are early Holocene W and NW flows, solid lines are late Holocene N and NE flows.

Pseudo-shatter cones (2)

No Planar Deformation Features (PDF)

Breccias found around most of the structures we visited could have been produced by fluidized sediments reaching the surface.

CONCLUSIONS (1)CONCLUSIONS (1)The crater-like structures in Gilf Kebir area we studied are notof impact origin, but likely related to endogenic processes typical ofhydrothermal vent complexes in volcanic areas, which may reflect the emplacement of subvolcanic intrusives.

Oasis (11.5 km)

BP (2.8 km)

Arkenu(10.3, 6.8 km)

1

2

Libyan “impact” structures

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.

CONCLUSIONS (2)CONCLUSIONS (2)

NON-IMPACT ORIGIN OF THE ARKENU CRATERS (LIBYA)M. Di Martino (1), C. Cigolini (2), L. Orti (3)

1) INAF-Osservatorio Astronomico di Torino, Italy2) Dipartimento di Scienze Mineralogiche e Petrologiche, Università di Torino, Italy3) Dipartimento di Scienze della Terra, Università di Firenze, Italy.

Results presented at the:

“Large Meteorite Impacts and Planetary Evolution IV” LPI conferenceVredeford, South Africa, 17-21 August 2008 – Extended abstract

Huge impact crater found in EgyptThe 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.)

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

Luna

Mars

Moon

Earth(Eastern Sahara)

The Kamil cratera “lunar” crater on the Earth

The crater in false color images

Radar image (X band) by Cosmo-SkyMed satellite

The Kamil crater

Chunks of cosmic iron

The tracks of our ancestorsCrater age ≤ 5,000 years

The expedition “Kamil 2010”

February

The “baby” (83 kg)

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 mDepth ≈ 12 m

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

The iron meteorite (1)

Macroscopic internal structure: Polished endcut etched with Nital o FeCl3

Brecciation

Schreibersite

Schreibersite

- No Widmannstätten pattern;

- Abundant inclusions of schreibersite[(Fe,Ni)3P, troilite (FeS) and daubreelite (FeCr2S4) with a large rim of kamacite;

- Evidences of shock effects (brecciation, shear zones);

The iron meteorite (2)

Chemical analysis

Ni = 198 mg/g

Co = 7.5 mg/g

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)

Iron meteorites with Ni contents between 15 and 25 wt%, Ir < 1g g-1, Ge 100 -150 g g-1 and Ga 30-60 g g-1 are very rare. The one most similar is Morradal (Norway), “ungrouped” iron.

An unusual iron meteorite

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.

An unusual iron projectile