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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.
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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 |
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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."
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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.
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Photogallery
So far, the author has only a few
photos of the El-Baz structure made by trekking tourists.
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Black rocks on top of the
sandstone wall (Source: Angela Petralia)
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Stratification in the sandstone
wall is to see
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In parts, stratification and original
color of the sandstones are still recognizable (Source: Zarzora
expedition)
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Send by Aly Barakat:
In the foreground a dyke inside the crater ...............
and fused sandstone in the crater
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Fused sanstone with "Pseudo-Shatter
Cones"
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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.
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