Remarks to the origin of the crater-shaped
around Gilf Kebir and Djebel Uweinat (Egypt)
and to the supposed impact craters
In the region were deposited
continental, nearshore and fluvial sediments during the Paleozoic period.
They lie on an evened plain. It is an old metamorphic mountain range (Basement).
In the time of the Upper Devonian period parts of the Basement with Paleozoic
sandstones were once more uplifted by tectonic movements (Bretonian event
?). The Paleozoic sandstones are to be seen in the Uweinat and Gilf Kebir.
Since the age of the Devonian period until recent was the uplifted region
an island without sedimentations (exclude transgressive shallow marine Carboniferous
strata in the northern Gilf Kebir and at the eastern foreland of the Djebel
Uweinat). In the Cretaceous period, far distributes, were deposited continental,
fluvial and shallow marine sediments at the flat shores of the Nubian sea
(Nubian sandstones). They cover Paleozoic sandstones or Basement at the eastern
edges of the Uplift. Between the Paleozoic und Nubian deposits can lie well-rounded
gravel and angular debris. This is a horizon of eroded Paleozoic material.
Last, during the late Eocene of the Tertiary period (about
46 - 42 Ma) in the underground of the Uplift, a chamber with acidic magma
became active (ring-complexes of Djebels Uweinat, Arkenu, Babain, Bahari and
Then, in the Oligocene (38 - 28 Ma) in the Djebel Uweinat and Gilf
Kebir region and surrounding was occur a culmination of subvolcanic-hydrovolcanic
activities. Various kinds of features are present, i.e. small-scaled cones,
dykes or plugs as well as large crater-shaped structures. Frequently no volcanic
rocks are exposed, or exist only as relics in the centre of the structures.
In these cases, the circular pattern is manifested only by brecciated and
hardened sediments with a higher resistance to erosion than the country rock.
On other hand, basalt cones can be easely confounded with small sandstone
hills covered with desert varnish.
The widespread plugs and dykes are either related to the Tertiary subvolcanic
or intrusive ring complexes. In any case, there is an obvious correlation
between igneous activity and the regional fault pattern. Very often subvolcanic
rocks are situated at points of intersection of faults.
In general, the extrusives consist of alkaline, SiO2-undersaturated to oversaturated
trachytes and related rocks, as well as of olivine-basalt. To a minor extent,
intermediate to rhyolitic varieties are present.
- Alkali olivine
basalt: Cones and plugs of fine-grained alkali olivine basalts and associated
rocks occur far beyond the boundaries of the Uweinat basement inlier. They
can also be found on top of the southern Gilf Kebir plateau and in
its eastern foreland.
- Trachyte and
related rocks: Alkali trachyte, saturated trachyte, trachy-phonolite,
and phonolite are considered within this rock group which is the most common
subvolcanic group in the entire region. Typical of all is their fine-grained
trachytic fabric. Some rocks are clearly fluidally developed, while others
are completely unoriented. When the trachytic rocks are considered together
with the olivine basalt und with the related basanite, tephrite and phonotephrite
outcrops from east of the Gillf Kebir plateau, this could mean that two
or more such alkaline cycles have to be assumed.
- Rhyolotic rocks:
Extrusive rhyolitic rocks are scare within the Djebel Uweinat and
From the many
widespread subvolcanic structures two crater - fields are considered
up to now mostly:
Crater Field - GKCF (Outside of the uplifted region)
The basaltic magma intruded into Basement, Paleozoic and Nubian layers
with high energy. Thereby rocks and quartz grains were shaped by shock
waves and destroys (breccia, planar lamelles) and in addition melted
or fused by heat.
The melted or fused breccia became out pressed by the ascent of the
magma and forms the rim of the craters. In the breccia are included
demolished Basement material, broken parts of Paleozoic sandstones,
debris from the layer between them (well-rounded quartz and angular
debris) and finally light Nubian sandstones. The breccia is tips over
at the rim of the crater to outside and is broken. The basaltic plug
often is not lifted out or is spilled.
Clayton Craters (Central Uplift)
In the Clayton Craters are been found basaltic plugs. The rims of the
craters have an other structure. The rims are composed of steeply erected
Paleozoic sandstones. Nubian sandstones normally are missing (central
Uplift !). Frequent are also dykes with Basalt between the edges. Breccias
were not found up to now. It is evident, that the dynamic conditions
were different. Presumably the basaltic magma had a slighter explosive
power and is climbed slow and along longer time.
There are further striking subvolcanic structures, which should be discussed
structures "Oasis" , "B.P." and Arkenu
Spot" (Djebel Uweinat)
pipes nearby "Regenfeld" .... etc.
Map with crater-shaped structures
and plugs in the Western Desert
1.0 The craters of the uplift-region
around Gilf Kebir and Djebel Uweinat
The "Clayton Craters"
The group of Clayton's craters
"In the region of the
Djebel Uweinat und Gilf Kebir, there are hundreds of intrusive plugs und dykes,
ranging in composition from Carbonatites to Olivine Mela-Nephelinites, Alkali-Basalts
and minor Phonolites and Trachytes. Olivine Mela-Nephelinites, Basanites and
Alkali Basalts are the most abundant rock types. These rocks are from ring
complexes and mafic plugs located between the Djebel Uweinat and the Gilf
Kebir. These intrusive rocks range from 28.2 to 26.7 Ma in age. Olivine Mela-Nephelinites
and Basanites are not genetically related to each other by fractionation but
they represent primitive magmas generated by different degrees of partial
melting of a metasomatised upper mantle source. Incompatible trace elements
in Olivine Mela-Nephelinites may be modelled with small degrees of partial
melting of such a source. Close relationships are also observed with the slightly
older intrusive rocks from Gilf Kebir suggesting that Uweinat magmatism is
possibly related to a northeastward migration of the African plate above an
intracontinental Hot Spot."
On the active line following structures are found from the southwestern to
the northeastern direction:
Spot", dykes and basaltic plugs in the Djebel Uweinat,
and craters in the area between Djebel Uweinat and Gilf Kebir,
of Clayton's Craters,
crater-shaped structures along and on top of the southern Gilf Kebir
fields of the "Scarp-Contours" in the eastern direction of Gilf Kebir,
pipes nearby "Regenfeld",
structure "Jasper Mountain",
All of the craters -
meant are the Clayton Craters
- are not volcanic explosion craters (in sense K. SANDFORD, 1934). The best
argument against explosion-events are layers in the centre of a large crater,
which contain undisturbed plant - prints (Carboniferous
plant-remains in the Western Desert).
craters were first seen by an airplane during the 1932 Almasy - Clayton expedition
by Sir Robert Clayton. Therefore the craters were named to him.
The only serious study of the craters was done by K.S.
Sandford, who accompanied the 1934 Bagnold expedition. During his limited
time he determined, that the hills were ancient volcanic explosion craters.
An intrusive dyke in the foreground
An other dyke with erected layers
The rims of the craters
are of sandstone, tilted vertical by intrusive forces, and fused by heat on
their inner sides. There traces of intrusive rocks ("Basalt") within some
of the craters. The plugs are sometimes visible as small hexagonal columns.
Likely it was a slow upthrust of magma, that also explains the almost perfectly
vertical walls. In some craters the magma remained, while in others it fully
subsided, leaving empty craters only containing material from the erosion
of the rim and inblown sand of the desert.
central plug of intrusive white Trachyte
View of a crater with erected Paleozoic sandstones
Many craters are associated
with dykes formed by two parallel uptilted rows of sandstone, filled with
white Trachyte in between. Some craters are imbued by the dykes even within
their rims, and the only intrusive material was found in between the dyke
There are also narrow dykes, which cross rim and bottom of the crater, where
they have the same kind of sandwitched structure with two upright walls of
sandstone, and intrusive material in between (Andras
It is quite clear, that magma having welled here up along fissures. These
dykes are a little bit younger or at most contemporary with the main structure,
because they crossed the rims and the bottom of craters.
The magma-chamber in the underground was still activ. The magma in the plug
was not yet hardened. New magma has been transported up on fresh fissures.
The fissures can cross also craters and it plugs therein.
There are large and small dykes. The small dykes are only thin fissures. The
filling with basaltic magma in all kinds of dykes is visible or not (similar
like at the plugs). The erected walls at the dykes (large or small) within
the craters are really no rubble. That means, that the plug is hidden more
deeply. Over it lies a lifted stopper of original sandstone layers.
Structural samples for the origin
A big crater outside and inside
Group of craters
Erected and crushed layers
A large flat crater
A long dyke at the crater
Trachyte in the middle
A. Zboray: "Here we noted that the crater
floor is crossed by vertical sandstone dykes, and the only trace of volcanic
material is located
in a dyke flanked by parallel sandstone walls, running along the south edge
of the crater."
A big crater with a visible central plug of white Trachyte
Outside of the big crater we see dykes
Erected layers at the crater rims
Strongly heated and vertically erected
Erected layers of sandstone at the crater
A large crater
A half small crater
basaltic plugs on top and nearby the southern Gilf Kebir plateau
In the area of
the southern Gilf Kebir plateau (Kemal-el-Din Plateau) we can
see on pictures of "Landsat" numerous craters on the uppermost altitude
of the plateau, which is difficult accessible with cars. It is described,
that the top of the plateau is perfectly flat and featureless, interrupted
by low basalt hills frequently. It is however sure, that many of these
"craters" include such basaltic bodies. Up to now nobody has made photos
23° 07' 60'' N / 26° 02' 33''
Since short time
now exist the first remarkable photo of basalt columns in the Wadi Akhdar.
This hill has a base of 300 x 125 m and was discovered by
in 2008. Up to the plateau in this valley are to be found further basaltic
structures. More photos are now available of Ursula Steiner, Switzerland.
basalt hill. Wadi Mashi. 23° 38' 54'' N / 26° 22' 25''
Shallow circular structure with
a basalt plug in the centre
Eroded basalt plug
Basalt rock on ground
time is documented now a circular structure with a basaltic plug in
found by Andras
Zboray (2010) in the Wadi Mashi:
"Along the rim, we found numerous fragments of fused sandstone that
was obviously molten by some intense heat source. On reaching the centre
of the crater, the source was evident in the form of a low eroded basalt
Crater El-Baz nearby the Gilf
24° 13' N / 26° 24' E
Dyke shaped structur "Qaret el
Hanash"south of Silica field
Pillar of brecciated rocks
in the northern direction of crater El-Baz
27° 07' 60" N / 26° 28' 60"E
a subvolcanic gap with outflow
of quartzitic solutions (Jasper)
25° 04' 30" N / 25° 56' 12
Not far from the
southern Gilf Kebir plateau are to be found in the northeastern direction
further structures on pictures of "Landsat". Up to now there is however
no reconnaissance at the places. There only rare pictures of a pillar
with brecciated rocks. It is named "Sakhret-al-Amud". The
most known is the crater El-Baz at the beginning of the Great Sand Sea.
It is reported, that in this crater-like structure are to be found Basalt.
recent tour in 2010 the author found in the western foreland of the
Gilf Kebir, between Wadi Sura and Aqaba passage, surprising many dykes,
with erected layers therein.
On the satellite pictures is to see a network of such structures. This
means that the subvolcanism in a larger area was active as known before.
1.3 The crater
"White Spot" on top of the Djebel Uweinat and further structures
(the so named crater by
Andras Zboray, Hungary
The crater "White Spot" on Hassanein-Bey
The basaltic plug is visible in the foreground
Rim of the crater
Crater "White Spot" . Position: 21°55'
01'' N / 25°02'48'' E
The crater "White
Spot" on top of Djebel Uweinat (Hassanein plateau) is clearly a basaltic
plug. At the bottom of the crater is basaltic material available. The
rims of the crater are composed of Paleozoic sandstones. They are not
erected in contrast to the Clayton Craters. Probably the sandstones
were broken by the up-directed pressure in concentric steps. The originally
horizontal stratification was therefore not changed in principle. Directly
inside of the crater are to be found few up squeezed sandstones too.
The crater is found on the top of Hassanein plateau. The whole plateau
has been uplifted by an intrusive event.
By intrusiva uplifted Hassanein
Basalt or only
entire northern Uweinat are to be found further structures,
which have emerged by subvolcanic activities in the underground.
Basaltic plug (Karkur Talh)
A long dyke-line in the
northeastern part of Djebel Uweinat
The long dyke-line
in northeastern part of
2.0 The craters in the
"Scarp-Contours" -- outside of the uplifted region (craters GKCF)
(the so named "impact
craters field" by Philippe Paillou, Observatoire Aquitain des Sciences
de l'Univers in Floirac, France)
In the year 2004 explored Philippe Paillou
and a team by two fieldworks (February and December) a large number of circular
structures in the east of the Gilf Kebir plateau. In an area between
26°30' - 27°30' E and 23° 00' - 24°40' N (~21.000 km2) were detected many
hundred crater-shaped structures in satellite-images. The fieldwork should
confirm that is a large impact crater field. The impact origin was derived
by the observation of shock-related structures, such as Shatter Cones (SC)
and planar fractures (PF's) in quartz grains of breccias. In the first time
13 craters were studied. Shatter Cones were found on the border of three craters
(GKCF-01, GKCF-02 and GKCF-12).
are however not valid Shatter Cones.
These structures are
Pseudo Shatter Cones
(PSC). The enormous
Pseudo Shatter Cone structures were modelled by wind erosion. This forms were
found also outside of the crater field in the Gilf Kebir region.
There are however meanwhile further
important knowledges, that the circular structures in the large field, are
of subvolcanic origin. Some structures contain basalts. Also dykes are frequent.
Breccias on the crater rims contain marks of subvolcanic activities. The
supposed "largest impact crater field on Earth in the Gilf Kebir region" is
a legend therefore !
Some controversial observations
Hungary, told me, that the crater (meant is GKCF-13) superficially looks
almost the same as the Clayton Craters, with a major difference. The crater
walls are very clear of horizontally bedded sandstone, whereas at Clayton
Craters the walls are all rotated 90 degrees. The layers seem to dip slightly
inwards (?). There became found a layer of fused sandstone breccia, with purple
sandstone fragments embedded into a white sandstone matrix. However the purple
fragments appear rounded and weathered (a brown cortex visible where rock
fractured around the embedded fragments), not sharp angular.
It was visited also the central field (GKCF-08,-10). They were all much smaller
bowl shaped depressions, all with inward dipping but clear horizontally bedded
sandstone around the rim. It was seen some breccia, but again the fragments
seemed more rounded than sharp angular.
Andras Zboray visited once more in 2004 specially GKCF-02. He noted on his
homepage: "It was a near circular bowl shaped depression with steep outer
walls, superficially similar to some of Clayton's Craters, however some differences
were readily evident. The sandstones around the rim were all horizontally
bedded, dipping slightly inwards, unlike the vertically upturned and heat-fused
walls of craters of volcanic origin. Inside the walls we also noted a layer
of brecciated sandstone, however the purplish sandstone fragments all appeared
rounded and having a brownish crust, unlike sharp angular pieces expected
from an impact breccia. From what we could establish, the crater was certainly
not of volcanic origin."
Paillou has also described: "Abundant polymict breccias were observed
along the rim of all craters, forming pluri-decimetric to metric beds, sometimes
interbedded with sandstones. They consist of centimeter- to decimeter-sized
irregular fragments, embedded in a fine-grained quartz matrix. Beds are systematically
dipping inwards the crater, with a steep dip dose to vertical on the highest
rims down to about 30° on the lowest observable rims."
Are these observations possible? I have the crater GKCF-13 and others
seen also. In the craters the beds dipping not inwards, in contrary
the beds climb upwards, were tipped over on the rim and broken outside. Consider
the pictures below. My opinion for the origin of the crater GKCF-13 is presented
in the sample below. Rate also the following pictures and comments.
Gilf Kebir Crater Fields (GKCF)
The red points are craters which were identified by the satellite SPOT.
Typical sections of the crater field "Scarp-Contours-North" (~ 24° 05'
N / 27° 05' E )
Typical landscape of the crater field
View into the crater GKCF-13 with a
diameter of 950m. Position: 23°18'23'' N / 26°55'28'' E
Another view into GKCF-13 from the northern
GKCF-13 from the satellite view
Craters nearby GKCF-13 in southern direction
Views of the crater GKCK13 (the
author 2004): Left: The rim with tilted and brecciated layers.
Right: Erected and broken sandstone layers
Structural samples for the origin
of the Gilf Kebir craters
Crater GKCF-02 Position: 23° 27' 10''
N / 27° 19' 18'' E
New pictures from a small crater; the
An other crater nearby GKCF-13; the author
Brecciated and fused sandstone at a dyke structure nearby
Block of fused conglomerate (GKCF-02)
Dykes with erected or fused sandstone
Hills of basalt are features within the area eastern
of Gilf Kebir --- here in the south of "impact crater field" the so-called
Position: 23° 07' 58'' N / 27° 38' 47''
Hydrovolcanic pipes nearby Abu Ballas
Destroyed basalt columns on "Black Hill"
Hydrovolcanic pipes on the rim of a crater-shaped structure;
The "Beacon Hill" is a crater-like structure with fused
sandstone on the rim
Position: 23° 00' N / 27° 16' E
Sample of a other large basalt body in the crater field
Position: 23° 18' 23'' N / 27° 34' 53''
Two samples of basaltic outflows nearby Gilf Kebir plateau
Positions: 24° 07' 51'' N / 26° 35'
02'' E as well as 24° 06 '08'' N / 26° 35' 33'' E
In the northeastern direction from the
crater field the author found in 2006 among others eroded tracks and
breccias at dykes
Position: 24° 16' 28" N / 28° 16' 28" E !!
Eroded doming structure with fused
sandstone on top
Now some "impact proofs" from the Internet-presentations
by Philippe Paillou and Alain Carion:
Breccia with included basement material (GKCF-01and GKCF-05)
Broken sandstone with marks of hydrovolcanic metasomatism (GKCF-04)
Planar fractures in quartz grains (GKCF-09 & GKCF-12)
An impressive example for a subvolcanic
(1 km SSE of GKCF-13)
The supposed "largest impact crater field on Earth in the Gilf Kebir
region" is a legend. The craters of the "Scarp-Contours" (and crater
El-Baz) can be distinguished by a geological particularity from the
Clayton Craters. The rims the of Clayton Craters contain only Paleozoic
sandstones (Nubian sandstones were not deposited in the area of the
Uplift). The rims of the craters in the "Scarp-Contours" (GKCF-13 and
others) contain Nubian sandstones with embedded breccia of Paleozoic
debris or breccia with pure Devonian/Basement material. The erected
Nubian sandstones and the breccia were tipped over on the crater rim
and broken outside. The breccia was changed by heat and pressure. Is
it possible, that planar fractures in quartz grains emerge by effect
of this intrusive magmatism ? I think it is possible.
Planar fractures (PF's) in quartz grains are an indicator
for low shock pressure ( e.g. 3-5 GPa). Clear proofs for impact structures
does not give it up to now here. PF's in quartz grains are no clear
proofs. Possible proofs would be the occurrence of Coesite and Stishovite.
Both are high pressure modifications of quartz with high density. In
the supposed "impact" craters in the Gilf Kebir region was found none.
The evidence of orthomagmatic-hydrovolcanic metasomatism prove subvolcanic
processes. The enormous
Cone structures were modelled by wind erosion.
These important counterevidences are considered too slight up to now.
All other structural geological and tectonic conditions are ignored
up to now. It is also remarkable, that in the crater field were found
basalts and many dykes. Some structures are cut by dykes. First investigations
of the geological aspects were made now by Di Martino and team (2006).
A similar crater field was found recently in the central Libya nearby
the basalt shield of
Al-Haruj al Aswad. There some craters are filled visibly with basaltic
No "largest impact crater field on Earth
in the Gilf Kebir region" ?
"We (Philippe Paillou and team)
visited 62 structures during two expeditions in February and December 2004.
Their diameters range from 10 to 2120 m. Except for a couple of small structures
covered by the Quaternary sand sheet, most of them present well-defined rims,
with heights ranging from a couple of meters to more than 80 m. Most structures
are more or less filled with Quaternary aeolian deposits, their centre being
in general higher than the surroundings. Rims are made of tilted sandstone
layers of the Sabaya Formation (Albian age, around 110 Ma) covered by breccia,
some-times also covered by paleo-soils. Some structures are cross-cut by
basalt dykes, indicating that volcanism took place after their formation.
Since basalts in the region are of Lutetian age (46 Ma), we can conclude that
the structures certainly formed before this time. Shatter-cone-like features
were found along the rim of several crater structures. However, wind erosion
of exposed rocks can produce such features, and we could not clearly observe
the typical striation patterns of shatter cones. Abundant occurrences
of breccias were observed along the rim of numerous structures, forming pluri-decimetric
to metric beds, sometimes interbedded with breccia formations can be produced
by classical geological processes such as tectonics and rock falls, but they
do also occur in and around impact structures. Optical microscopic analysis
of thin sections of breccia and sandstone samples collected on the rims of
several structures have shown that quartz is the predominant mineral component
of all samples; minor components include phyllosilicates, iron oxides, and
some accessory minerals such as zircon. Many quartz grain in these samples
contain planar and sub-planar micro-deformations, strongly reminiscent of
planar fractures (PFs), known from weakly shocked quartz of many impact structures,
but also from tectonic settings. GPR soundings were performed on 10 of the
visited structures and on some areas between these structures. The collected
data showed the occurrence of faulting, fractures and chaotic buried terrains
in the quasi totality of the radar transects. All GPR profiles reveal the
same subsurface morphology: a perturbed paraboloid structure buried under
sediments. In terms of lack of stratigraphy and scattering phenomena, they
are quite different from typical profiles observed for volcanic craters for
"The typical size (about 150 m) and number (more than 1300) of the structures
in the Gilf Kebir region are compatible with the hydrothermal vent hypothesis
and the brecciated sediments found around most of the structures that were
visited could have been produced by fluidized sediments reaching the surface.
However, southwestern Egypt is not known as part of a large igneous province,
it is thus required to discover a major (and still unknown) hydrothermal event
there that could have produced such vent complexes. GPR sounding performed
on several structures revealed a flat floor covered by sedimentary deposits:
hydrothermal vents should show tracks of a vertical structure, the conduit
zone connecting to the tip of a sill intrusion. Also, we could not find evidence
of sediment dykes and pipes in the 62 structures we visited, even though they
should be abundant in the case of hydrothermal vents."
My interpretation of the published
the results of measurements with
Ground-Penetrating Radar (GPR) through small crater GKCF-28
(Position: 24° 05' 46"N / 27° 03' 15''E)
profile is atypically for an impact event. It refutes the impact theory.
A typical deep crater is not developed. In the curvature is to be recognized,
that probably a basaltic plug is found only few meters under a aeolian
covering (a dig makes a sense). At the edges are to be recognized the
pushed up breccia and/or sandstones.
of the crater field in the Gilf Kebir region (SW Egypt)
M. Di Martino 1, L. Orti
2,3, L. Matassoni 2, M. Morelli 2,3, R. Serra 4, A. Buzzigoli 5
1) INAF-Osservatorio Astronomico di Torino, Pino Torinese, Italy
2) Museo di Scienze Planetarie, Prato, Italy
3) Dipartimento di Scienze della Terra, Università di Firenze, Italy
4) Dipartimento di Fisica, Università di Bologna, Italy
5) Laboratorio di Geofisica Applicata, Dipartimento di Ingegneria Civile,
Università di Firenze, Italy
expedition we carried out fieldwork on 7 of 13 craters identified as
impact craters, namely GKCF-01, GKCF-06, GKCF-07, GKCF-08, GKCF-11,
GKCF-12, GKCF-13, and we collected rock samples from GKCF-01, GKCF-07,
GKCF-11, GKCF-13, on which petrographic studies has been in part performed.
Also some other similar circular structures in the surroundings have
Macroscopic and petrographic observations
The outcropping lithologies in the investigated area mainly consists
of quartz-arenites and minor micro-conglomerates. There is also the
presence of two kinds of breccias, one of which is an intraformational
(sedimentary) the other one possibly of hydrothermal origin.
Quartz-arenites, micro-conglomerates. Medium- to coarse-grained, rarely
fine-grained, sandstones with sub-rounded/sub-angular elements mainly
composed of quartz and minor sedimentary lithic grains (fine-grained
quartz-arenites). Matrix, when not altered, is composed of very fine
quartz grains and phyllosilicates; more commonly it’s partially or totally
replaced by iron oxides/hydroxides cement and minor ferriferous carbonates.
Accessory minerals: zircon, tourmaline, muscovite. Quartz grains often
show micro-fracturing. Polycrystalline quartz is present. In some places
the arenites grades to micro-conglomerates with rounded/sub-rounded
quartz grains and sedimentary lithic clasts (fine-grained quartz-arenites)
mm- to cm-sized.
They outcrop in the inner parts of the circular structures but also
in the surroundings external parts. Intraformational breccias are constituted
by quartz-arenitic clasts, angular/sub-angular in shape, cm- to dm-sized,
rarely up to 20-40 cm. Their stratigraphic position is not always clear,
but they seems intercalated with quartz-arenites. Breccias of possible
hydrothermal origin are constituted by sub-angular/sub-rounded quartz
grains, mm- to cm-sized and by rock fragments (fine- to coarse-grained
quartz-arenites and minor siltites), cm- to dm-sized and varying in
shape from angular/sub-angular to sub-rounded. In this second kind of
breccia rare boulders constituted by sedimentary breccia are included.
When not altered, matrix is composed of fine-grained quartz grains and
phyllosilicates; often it’s totally replaced by iron oxides/hydroxides
cement and minor ferriferous carbonates. Accessory minerals: zircon,
tourmaline (<1%). Quartz grains often show micro-fracturing. Polycrystalline
quartz is rarely present.
Morphology and Structural Setting
In some cases the rim of these roughly circular features is made by
tilted layers of sandstones. In some other cases there is the presence
of arc-shaped (and concentric) nearly vertical fracture planes, which
characterize not only the supposed “impact craters”, but are also present
around their periphery and in isolated structures in the surroundings.
These fracture planes are often associated with Fe-oxide/hydroxide mineralizations
(situation observable not only at the “crater-scale” but also at smaller
scale) and with breccia bodies of probable hydrothermal origin.
The surfaces of the outcropping rocks in the craters area are characterized
by striations that, in some cases, resemble very much to shatter cones.
From the data collected in the field it’s possible to say that:
- the striations observed are superficial and not pervasive;
- these features are characterized by directions always varying from
N 20° to N 340°, rather consistent with the main winds direction (from
N and N-E at present, from NW in the early Holocene, and no more present);
- the same striations are visible also on the breccia surfaces and on
the surfaces of rocks out of the craters area;
- they are not fracture surfaces but occur on outcrops surfaces.
From these observations we conclude that this “pseudo-shatter cones
features” are likely due to wind abrasion. GKCF-01, GKCF-13, El-Baz
It has been suggested that
the Gilf Kebir crater field could be the result of a meteorite impacts
(Paillou et al., 2004; Paillou et al., 2006). The present study has
been carried out in order to confirm or not the impact origin of these
structures. From the fieldwork and from the preliminary analyses, the
following results can be summarized.
“Target rocks”: Don’t show any macroscopic or microscopic shock effects,
in particular no PDF’s, no evidence of melting, no glass.
Breccias: Don’t show any macroscopic or microscopic shock effects.
They are of intraformational and possibly hydrothermal origins.
Pseudo-shatter cones: The striations which characterize some rocks
near the “craters”and identified as shatter cones are due, very likely,
to the wind erosion.
Morphology: The same morphology, characteristic of the supposed “impact
craters”, is identifiable (both in the field and in satellite images)
in many others structures in their surroundings with sizes ranging from
some tens of meters to kilometers (and some of these are associated
These evidences indicates that all these features have a common non
On the basis of this preliminary investigation, we can say that there
are no clear and unequivocal evidence for an impact origin of the circular
structures in Gilf Kebir region:
the origin of the craters is very probably associated to endogenic geological
processes. The hydrothermal venting could account for the origin of
such an extended field of circular structures. In this case the brecciation
could be fluid-induced, probably for the fluctuations in pore fluids
pressure. These hydrothermal fluids could also have been enriched with
iron oxide during their way to the surface, crossing oxidized sediments
or paleo-soils. The pre-existing sets of fractures planes could have
driven partly the fluids circulation.
The presence of such an extended field of circular structures, linked
to a widespread volcanic activity in the surroundings, leads to take
in consideration a hydrothermal origin for these structures (as also
suggested by Paillou et al., 2006)."
Some pictures from crater GKCF-13 - presented
by Mario Di Martino and team (2006):
Marks of hydrovolcanic metasomatism
Sandstone layers were tipped over to the outside
Erected layers were tipped over
to the outside of crater GKCF-13
Some further important details from crater GKCF-01 - presented by
Mario Di Martino and team (2006):
View into the crater GKCF-01 of
diameter 630m. Position: 23°14' 37'' N / 27° 27' 37'' E
Erected layers and breccias on
Macrobreccias of Paleozoic sandstones
and basement material (right)
Partially with melted material and with marks of hydrovolcanic metasomatism
of a crater-like mountain in GKCF,
consisting of tilted sandstone (?) layers with strongly siliceous
and ferruginous impregnations.
The rim of the crater is deeply silicified and rich in jaspers and chalcedony.
Very small quartz crystals have been observed in small cavities.
Richard de Nul, Belgium; Location
is called "Jaspis (Jasper) Mountain"; about 24° N /27°E .
Jasper with Chalcedony crystals
in small cavities
Further crater -fields at the Egyptian-Sudanese border
(Djebel Kamil and surrounding)
In 2009, the author
have discoverd further crater - fields by Google - Earth, which are located
in east of Djebel Uweinat (Djebel Kamil) and southwest of this in northern
Sudan. Most of the structures are similarly of them in the large crater -
field "GKCF" in the east of Gilf Kebir ("Scarp-Contours").
Amid the crater - shaped structures are to be recognized basaltic cones. The
structures in the Djebel Kamil have great similarity to the Clayton Craters
at the Gilf Kebir. The crater-field in the Djebel Kamil has the coordinates
between 21° 58' to 22° 03' N and 26° 13' to 26° 20' E. The crater-field in
the northern Sudan has the coordinates 21°50' to 21°58' N and 25°51' to 26°
Basalts between the crater-shaped
Basalts between the crater-shaped
structures at the Djebel Kamil
Many crater-shaped structures
in northern Sudan
Large structures: 21°26' 28''
N / 26° 04' 42'' E
4.0 The supposed impact craters of Libya
(The "impact" craters B.P., Oasis
"The B.P. impact structure
is located in Libya at 25° 19' N and 24° 20' E. The structure consists of
two eroded and discontinuous rings of hills surrounding a central block, the
southern half of which is deeply eroded. The inner ring is about 2 km in diameter
with an average relief of 30 m, while the outer ring has a diameter of about
2.8 km and a maximum relief of about 20 m.
The inner ring is more structurally deformed than the outer ring. The central
block has a diameter of 0.6 km, and a maximum relief of 38 m. Bedding within
this central block is intensely jointed, so that true bedding is extremely
difficult to identify.
Rocks exposed are the Cretaceous Nubia Group (?) and include quartz sandstone,
siltstone, and conglomerate.
Shock metamorphism is abundant in the sandstones at the B.P. structure. The
most diagnostic features are open fractures and planar deformation features,
which are quite abundant. Planar deformation fractures are present but poorly
developed, which probably suggests low peak shock pressures. Shock lamellae
are extremely well developed, with as many as nine distinct sets occurring
in one quartz grain."
"French el al. (1974)
and Underwood and Fisk (1980) described BP as consisting of two discontinuous
rings that surrounded a central block; the outer ring of hills is 2.8 km in
diameter with about 20 m of maximum relief, with sandstone beds dipping inward
at 3°-15°. The inner ring of hills is more deformed, has a diameter of about
2 km, and an average relief of 30 m. Most of these beds dip outward at 20°-40°.
These two rings form an asymmetric (wider on the eastern side) ring syncline.
According to Underwood and Fisk (1980), the inner ring also shows numerous
gently plunging folds with axes tangent to the structure and dipping outwards
by as much as 70°. The central block is 0.6 km in diameter and has about 38
m of relief. Beds arc intensely jointed, and the eroded southern half of the
block exposes the oldest rock in the area, a light-coloured (purplish to whitish)
sandstone that has been complexly folded.
Within the structure itself, Underwood and Fisk (1980) reported only Nubian
Sandstone. These authors noted that the structure consisted of three near-circular
concentric rock outcrops. The innermost ring had high-angle and chaotic dips
forming a mass of craggy outcrop. The middle ring consisted of uniformly outward-dipping
(30°-50°) strata. The third ring dipped inward at 5°-15° and formed a low
scarp, for most of its extent barely protruding above the surface. Underwood
and Fisk (1980) interpreted these outer two rings to define a ring syncline."
N / 24° 20' E
Erected and broken sandstones
in the foreground
The uplifted sandstones from the
central block were subjected intense shear-movements
and Oasis Impact Structures, Libya: Remote Sensing and Field Studies
1, W.U. Reimold 2, J.Plescia 3
1 Department of Geological Sciences of Vienna
2 Impact Cratering Research Group, University of Witwatersrand
3 Applied Physics Laboratory, John Hopkins University
at BP - First Observations
"Upon approach in the field, the structure shows - in comparison to
the mesa and butte terrane of the region — limited, but clearly discernible,
topography. The "inner ring" is a central block of ca. 500 m width,
with steeply upturned rock units that are intensely folded, faulted,
and – in places - brecciated. A variety of microdeformation has been
noted in quartz from these breccias and from other sandstone samples,
including a few bona fide planar deformation features.
What has previously been referred to as the "middle ring" is in fact
the actual crater rim with a diameter of about 2 km. This ring is characterized
by a distinct series of hills of up to 30 m elevation above the surrounding
desert, with rocks dipping at 30 to 50° outwards. Some parts of the
rim, in particular the southern and eastern part of this zone, are characterized
by intense folding and faulting, whereby up to 100 m wide blocks have
been rotated and displaced inwards out of the generally circular trend
within this ring. The "outer ring" is a discontinuous ring of subdued,
generally less than 1-2 m, but locally up to 10 m high, topography.
Here, the sandstones dip inwards, at values of ± 10°. No geological
or geomorphological evidence that might support a rim diameter of 3.2
km was found in the field. This interpretation of such a large crater
diameter is based on the Interpretation of radar satellite imagery,
whereby it is now clear whether the radar data indicate only structure
that occurs underneath sand cover. It is possible that the shallowly
inwards dipping structure at 1.6 km from the centre represents a fault
zone, along which strata could have moved inwards towards the centre
of the crater, in response to central uplift formation.
B.P. ring dyke structure with
4.2 Crater Oasis
"The Oasis impact structure is centred at 24° 35' N and 24° 24' E. This
eroded structure has a diameter originally determined to be about 11.5
km, but the most prominent part is a central ring of hills, about 5.1
km in diameter and 100 m high.
As at B.P. a thin cover of sand obscures the outermost disturbed beds.
The structure exposes the same rocks as the B.P. structure.As at B.P.,
multiple sets of planar elements were detected in quartz grains. A few
samples of microbreccia were found at the crater, containing fragments
of brownish, partly devitrified glass with sandstone fragments and shocked
quartz grains. As at the B.P. structure contain the Oasis structure
sandstones of the Nubian Group."
"The Oasis impact
structure was reported by Underwood and Fisk (1980) to have a "disturbed
zone" of 11.5 km diameter, but the discontinuous hills that compose
the topographically prominent part of the structure form a circular
array of 5.1 km diameter. Most of the rocks in this prominent ring dip
outward, and locally they are intensely folded, Hills reach a maximum
height of about 100 m. Outside of this ring, the disturbed rocks have
only a meter of relief. No central block, like that at BP, had been
A dyke nearby the Oasis structure
Oasis ring dyke structure with
Crater field Oasis (prominent plugs
in the north)
An unidentified plug in
the Djebel Babein (borderline Libya/Egypt)
The prominent plugs in the northern
direction of Oasis structure
are columns of Basalt with fused layers on top
Plugs of Basalt with fused
layers on top in the northwestern direction of Oasis structure
24° 45' N / 23° 52' E
In the foreground a fused conglomerate
"The Oasis crater
is extremely eroded, and appears as a set of concentric ridges of deformed
rocks that rise above the local plain. The rocks are Nubian sandstones
that are commonly crushed and brecciated. Other rocks present in the
area include quartz conglomerates and siltstones, also members of the
Nubian formation. The most visible part of the crater is a central ring
with a diameter of 5.1 km. This central ring is made up of discontinuous
hills as tall as 100 m above the surface floor. The strata within this
ring are intensely crushed, and in some places strata are overturned
or vertical. There is no observed central uplift-structure, impact glass
or breccia associated with Oasis crater.
Shock metamorphism is abundant in the Nubian sandstones at Oasis. The
most diagnostic features are open fractures and planar deformation features
(PDF's), which are quite abundant. Planar deformation fractures are
present but poorly developed, which probably suggests low peak shock
pressures. Shock lamellae are extremely well developed, with as many
as nine distinct sets occurring in one quartz grain. These features
are extremely similar, if not identical, to the shock metamorphic features
found at the B.P. structure.
Although there have not been any megabreccias found at Oasis, a team
of scientists in the 1970s identified a glass-bearing microbreccia in
the rocks at Oasis. This microbreccia consists of shock quartz grains
and pieces of sandstone, intermixed with partially devitrified glass,
all within a recrystallized matrix.
It was studied the petrographical characteristics of 29 samples from
the B.P. and Oasis sites. The studies on thin sections of the samples
showed that they represent mostly submature, moderately to poorly sorted,
medium- to fine-grained quartzite sandstone, or quartzitic breccia.
Most of the studied samples do not show evidence of shock, but in a
few sections some quartz grains with up to 3 sets of shock-characteristic
planar deformation features (PDF's) were found."
BP and Oasis Impact Structures,
Libya: Remote Sensing and Field Studies
1, W.U. Reimold 2, J.Plescia 3
1 Department of Geological Sciences of Vienna
2 Impact Cratering Research Group, University of Witwatersrand
3 Applied Physics Laboratory, John Hopkins University
Field Work at
Oasis - First Observations
"For Oasis, to which we could only pay a cursory visit, a diameter
of 11.5 km has been quoted. However, the structure is characterized
by a topographically prominent - in comparison to all other topographic
features in the area - and intensely faulted ring of hills (the "middle
ring") of about 5-6 km diameter. The rocks in this prominent ring mainly
- but not always - dip outward. Much of the segment of this ring that
we traversed is intensely folded, on a macro- to meso-scale (i.e., hundreds
of meters to decimeters, even centimeters). There are two possibilities
to interpret this ring structure: (1) it could represent - in analogy
to the structure of Gosses Bluff - the eroded central uplift of a much
larger impact structure, whereby the actual crater rim would have been
entirely eroded, and (2) alternatively, this prominent ring could be
the remnant of the crater rim, whereby one can refer to the Landsat
imagery that indicates a central area that is distinct from the ring
feature and, itself, could represent a remnant of an eroded central
In the environs of this prominent ring of hills, no further prominent
topographic features have been identified, despite the locally strong
indications on the Landsat Images. However, in a few places to the north,
at a distance of about 3 km from the prominent ring feature, a shallowly
inwards dipping, low (< 1 m) ridge was observed, which was very reminiscent
of the outer ring feature around BP. It is clear that further detailed
structural work is required to resolve these open questions.
According to UNDERWOOD & FISK (1980), about 6-8 km north and northeast
of Oasis, a field of circular or rounded knobs of sandstone occur. These
are 10-100 m in diameter and 10-50 m high. These authors suggested that
the knobs might be linked to the crater-forming impact event. They described
them as clastic plugs, apparently composed of Nubian sandstone, which
had been emplaced by vertical flow of impact-fluidized sand. Our investigation
indicated that in some cases they have a distinct chimney-like form
with sandstone strata in the central areas having near-vertical bedding
orientations. However, no evidence for fluidization of sand - as speculated
by UNDERWOOD & FISK (1980) – was found. In contrast, several distinct
ridges were recognized as erosional remnants of complex fold structures.
The question whether or not this folding is related to the impact event,
or reflects regional tectonic deformation, still remains open. It appears
reasonable to interpret these plug-like structures as remnants of folds,
of which the fold hinges have been completely eroded, leaving the vertically
standing strata of the fold cores behind."
Remark: It is not sure, that the northern plugs in the centre contain
vertical oriented sandstone layers. There also could be white basalts
(Trachyte). Trachyte harden without development of the distinctive column-like
4.3 Craters Arkenu
"The JERS-1 radar mosaic reveals two
circular structures partially hidden by Quaternary deposits. The radar scene
then clearly reveals a double circular structure composed of a southwestern
crater 10.3 km in diameter and a northeastern crater of diameter 6.8 km. The
NE crater is composed of concentric inner and outer rings separated by a depression
filled with sediments, also observed in the optical scene. The SW crater also
presents a circular shape with possibly three concentric annular ridges. The
host rock of the double circular structure is a cross-bedded coarse-grained
to conglomeratic sandstone of Lower Cretaceous age containing plant fossils
and thin shale interbeds."
The team Philippe Paillou observed quantities of Pseudo Shatter Cone
structures on the site, all located close to the inner ridge of the NE crater.
Large outcrops of allochthonous impact (?) breccia could also be observed
in both craters. He could find several quartz grains presenting planar fractures
(PF's) in these breccia.
Paillou's craters Arkenu:
Ark 1 position 22° 05' N / 23° 47' E
Sandstone with Pseudo Shatter
Planar fractures in a quartz
Hardened breccia with not
rounded fragments of basement material
Centre of Paillou's crater
Western edge of the structure
Subvolcanic structures at the
western edge of Arkenu-2
Western edge of the structure
Breccia (or white Trachyte) in
the centre ?
Breccia at crater rim
Arkenu-1: Yellow Jasper in silicified
sandstone - formed by hydrovolcanic solutions
Two small craters; 40 km WNW from
the Arkenu structures
Crater southeast of Arkenu 2
(21° 57' 50' N/ 23° 46' 29'' E)
The Jebel Hadid Structure is located
in southeast Libya (20°52'N / 22°42'E). It is made up of five concentric
rings, the outermost of which is approximately 4.7 km across.
In 2009 a paper appeared in the journal
Marine and Petroleum Geology in which M. Schmieder and E. Buchner of
the Institut für Planetologie at Universität Stuttgart and D.P. Le Heron
of Royal Holloway, University of London, propose that the Jebel Hadid
Structure is an impact structure.
This is an error.
The structure fits in with the other subvolcanic structures in the region.
Structural sample for the origin
the Libyan craters Oasis and BP
craters in Libya are comparably to the craters of the "Scarp-Contours"
in Egypt (GKCF craters). We can see erected and tipped over sandstones
and crushed breccia. The breccia contains differently material. Planar
lamellae in quartz fragments are frequently. Clear basaltic material
was not found up to now, because it has climbed probably rarely to the
surface. There is ultimately no imperative proofs, that the craters
have emerged by an impact. The kind of the planar lamelles point to
a low pressure.
is, whether planar lamelles in quartz grains can emerge by volcanic
pressure too. The breccia is formed by heat and contains also material
from Paleozoic underground and the deeper basement. Microbreccia with
shock - quartz - fragments and pieces sandstone, mixes with partially
devitrified glass, all within are crystallized matrix, are no convincing
proofs for an impact. Possible proofs would be the occurrence of Coesite
and Stishovite. Both are two high pressure modifications of quartz with
high density. In the supposed "impact" craters here was found none.
A further proof are valid Shatter Cones, which are missing however here
In the north direction of the
crater Oasis are to be seen further structures, which can contain basaltic
material. In the northwest of the crater a clear evidence for intrusive
material exist (see pictures above). The ring-shaped structures outside
of the crater are dykes, on them are placed plugs.
plugs, dykes and to it belong craters in the region have emerged probably
at the same time in the Tertiary period. Them all become found at the
Howar-Uweinat Uplift nearby of tectonic fields and lineaments. The strikingly
directed series of the craters is possibly related to a northeastward
migration of the African plate above an intracontinental Hot Spot. These
magmatic activities evidently were joined with subvolcanic and orthomagmatic-hydrovolcanic
processes, which could have led also to the origin of the unique Libyan
NON-IMPACT ORIGIN OF
THE ARKENU CRATERS (LIBYA)
M. Di Martino 1,
C. Cigolini 2, L. Orti 3
1 INAF-Osservatorio Astronomico di Torino
2 Dipartimento di Scienze Mineralogiche e Petrologiche, Università di
3 Dipartimento di Scienze della Terra, Università di Firenze
of Arkenu structures as impact-related is based on the observation of
shatter cones and impact breccias with planar fractures in quartz grains."
"We visited the area on November 2007 and we carried out a preliminary
geological and structural survey.”
“In fact, we did not observe PDF in the collected quartz grains and,
in addition, the so called “shatter cones” (brought by Paillou and co-workers
as an evidence of an impact event) have been likely produced by the
erosion of sandstones. All of them, in fact, are oriented in the same
direction of the dominant winds (we prefer to name these structures
as “pseudo-shatter cones)".
Geological Survey: The rocks outcropping in the craters’ area
are a variety of sandstones and subordinated siltstones. Sandstones
on the top of the sequence that also outcrop within the craters, are
coarser and carry abundant concretions of diagenetic hematite (millimeters
to tens of centimeters in size). These sandstones are well preserved
within the Arkenu 2. Conversely, within Arkenu 1 the floor consists
essentially of disaggregated portions of these rocks coexisting with
massive magnetite deposits. In this case, field evidence and mineral
distribution (diagenetic hematite recrystallized as magnetite), suggests
digestion of the above sandstones by a subvolcanic intrusive body (now
partially outcropping within the crater area). The intrusion is
characterized by a mafic precursor, followed by granite locally preserved
in the northern sector of the crater. It is suggested that the mafic
precursor contributed to the melting of the original sandstone sequence
and was then followed by the injection of a granitic magma within the
Discussion: We, therefore, suggest the craters forming episode
is the result of intrusion of a paired, nearly cylindrical subvolcanic
stocks (coupled with ring dike injection in the surroundings) accompanied
by hydrothermal degassing. This process was then followed by local
structural adjustments, likely due to thermal contraction of the whole
edifices along circular fractures. This produced moderate folding and
subsidence of the “crater sectors” (currently delimited by the crater
rims) as well as the origin of the outer circular structures. Erosion
did its cycle and finally revealed the architecture we are now observing.