What we know about the large crater-shaped structure "El-Baz"
in the Western Desert (Egypt)
 
Norbert Brügge, Germany
Dipl.-Geol.

In 1981, Farouk El-Baz discovered via Landsat images a circular structure of about 4 km diameter among the linear dunes in the Great Sand Sea of the Egyptian Western Desert, at lat. 24° 12'45"N, and lang. 26° 24'05"E. Generaly is described that the crater-like structur "has a sharp and crenulated rim crest, a terraced wall, a discontinuous inner structure (approximately 1.6 kilometers in diameter), and a few rim blocks. Its morphological and morphometric characteristics are similar to other circular structures in the Western Desert".

With the cooperation of the Egyptian Geological Survey and Mining Authority (EGSMA), and the Natural History Museum Milano, Italy, a special expedition was set up to carry out geological observations on the El-Baz Crater at the first time, on May, 1991.
Field observations made during this expedition revealed that
El-Baz crater is a circular feature made up by basaltic intrusion into the "Nubian" sandstone country-rock in the form of a basaltic sill, generaly dipping NW, and basaltic plugs and ring dykes (Barakat, et al., 1991).

To ensure this view, Aly Barakat carried out studies with following results:

Barakat, A. A. (2001): El-Baz Crater: Basaltic intrusion versus meteorite impact crater. Annals Geol. Surv. Egypt, 24, 167-177
"Eight representative samples of basalt (4 from the sill and 4 from one dyke) were analysed for major elements by the normal wet chemical analysis.

 wt/%

Sill plug Dyke
1 2 3 4 5 6 7 8

SiO2

47.03 45.73 45.47 45.81 46.22 45.81 46.19 46.67

TiO2

3.06 3.00 2.88 3.31 3.18 3.44 3.26 2.88

Al2O3

11.03 9.57 9.35 10.97 11.00 10.76 11.19 11.09

Fe2O3

4.52 4.36 4.07 4.64 3.67 4.22 4.31 3.65

FeO

8.68 8.84 9.38 8.44 10.01 8.81 8.46 9.69

MgO

8.49 11.28 12.05 10.58 10.62 10.62 9.27 11.78

CaO

10.75 9.89 10.75 9.68 9.95 9.48 11.18 9.25

MnO

0.17 0.18 0.16 0.16 0.18 0.17 0.15 0.16

Na2O

3.14 3.48 3.54 3.54 3.08 3.77 2.92 2.81

K2O

0.66 0.88 0.82 0.88 0.71 0.76 0.66 0.66

H2O

1.31 1.97 0.97 1.53 1.02 1.83 1.49 1.15

Sum

98.74 99.18 99.44 99.54 99.64 99.67 99.08 99.79
1     -- Very fine-grained porphyritic rock from the chilled margin of the sill.
2 , 4 -- Medium-grained ophitic/subophitic rocks from the lower and upper parts of the sill.
3     -- Coarse-grained rock from the middle part of the sill.
5, 8  -- Fine-grained porphyritic rocks from the outer parts of the studied dyke.
6, 7 -- Medium-grained ophitic/subophitic texture from the middle part of the studied dyke.

"The major oxides of the first four rocks which represent different parts of the sill, it is clear that the central part of the sill shows lower contents of SiO2, Al2O3, Fe2O3 and TiO2, and shows higher contents of MgO, FeO, and Na2O than that of the chilled margin and the lower and upper parts. Otherwise, the studied rocks from the studied dyke show no regular variation in the major oxides. This can be attributed to the irregular exposure of this dyke which prevent systematic sampling. However, the chilled samples are enriched in MgO."

The basalt is a Si-undersaturated olivine basalt. The basalt is composed of plagioclase, pyroxene and olivine, with opaques and minor amounts of apatite and biotite. The average of the medal volume composition of samples is: plagioclase 40%, pyroxene 30%, olivine 18%, opaques 9-10% vol.%, and others unresolved phase 5%. Free quartz is absent from the studied rocks.

Olivine can be easily destinguished even within the fine-grained rocks as conspicous phenocrysts show marginal alterationn to reddish iron oxides. The color varies from dark-black to pale-gray-black, in the case of the fresh brocken surfaces. The old surfaces vary in color from brownish-black to grayish- black. Granularity varies from fine-grained  to coarse-grained. Texture varies from porphyritic (large phenocrysts of olivine, pyroxene and plagioclase set in aphanitic groundmass) to ophitic texture (characterized by spotted appearance in the hand specimens).
Field observations, hand specimens details and microscopic studies revealed the presence of clear variation in color, granularity, texture and the relative abundance of the mineral constituents from the middle to the lower and upper parts of the sill. The middle part is mainly pale-gray-black in color, coarse-grained and showing intergranular texture, whereas the lower and upper parts of the same sill are mainly black in color, medium-grained, and showing ophitic-subophitic texture. The middle part is enriched in olivine (20 vol.%) and it's plagioclase is more calcic (An50), compared with the lower and upper parts.
The lowermost part of the sill (chilled margin) is mainly dark-black in color fine-grained and showing porphyritic texture. This part contains less olivine (10 vol.%) and it's plagioclase is more sodic (An35).
But note that systematic variation in the studied sill-plug is not clear, because the irregular exposure on the sill. The contact zones of the studied dyke are mainly dark-black in color, fine-grained and show porphyritic texture, whereas the middle part is mainly black in color, fine to medium-grained (0.5-3mm) and showing ophitic/subophitic texture. The contact zones are relatively enriched in olivine (~ 20 vol.%) and the plagioclase is more calcic."

An analysis of modern satellite images now provides much more detailed insights into what is arguably the largest crater-shaped structure in the entire Western Desert.
The structure is basically a sill of uplifted sandstones. This semicircular sill is bounded by walls. The walls are occupied by dykes. In contrast to the surroundings the walls reach a maximum height of 100m in the southeast and east of the structure. The interior of the structure sinking to the northwest and is permeated by tectonic fissures and further small dykes.
A comparison with other structures in the Western Desert (Gilf Kebir Crater Field, Clayton Craters) signals for this structure a deviating mechanism in its formation. The dykes are cracks with basaltic intrusions. The enveloping sandstone in the walls was broken up and uplifted. The ring-shaped arrangement of the dykes speaks for a significant magmatic body in the underground.

     

1
Sill with long dyke

5
Northern rime of dykes

2
Sill

6
Group of dykes

3
Sill + dykes

  7
Isolated plug outside

4
Dyke-like rim

8
Center

Photogallery

So far, the author has only a few photos of the El-Baz structure made by trekking tourists. Thankfully, Aly Barakat will soon provide more insightful photos that will show more details.
 


  Black rocks on top of the sandstone wall (Source: Angela Petralia)


Stratification in the sandstone wall is to see


         
In parts, stratification and original color of the sandstones are still recognizable (Source: Zarzora expedition)


  
A isolated cone (map 7) of about 250 m diameter just west of the large structure is a plug-like dyke. On the top of this sandstone cone are to see black colored basaltic boulders.