Remarks to the origin
of the crater-shaped structures
around Gilf Kebir and Jebel 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. 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 Jebel
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 Jebels Uweinat, Arkenu, Babain, Bahari and
Then, in the Oligocene (38 - 28 Ma) in the Jebel Uweinat and Gilf Kebir
region and surrounding was occur a culmination of subvolcanic 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 Jebel Uweinat and
From the many widespread
subvolcanic structures two crater - fields are considered up to now mostly:
Gilf Kebir 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
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" (Jebel 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 Jebel Uweinat
The "Clayton Craters"
The main group of the Clayton
"In the region of
the Jebel 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 Jebel 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:
Crater "White Spot",
dykes and basaltic plugs in the Jebel Uweinat,
many dykes and
craters in the area between Jebel Uweinat and Gilf Kebir,
all groups of Clayton's
basalts and crater-shaped
structures along and on top of the southern Gilf Kebir Plateau,
large crater fields
of the "Scarp-Contours" in the eastern direction of Gilf Kebir,
crater like structure
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
Carboniferous layers in the centre of a large crater (with bright trachyte
therein), which contain plant imprints (Carboniferous
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 first serious study of the
craters was done by K.S. Sandford, who accompanied the 1934 Bagnold expedition.
SANDFORDS's study is the most comprehensive in the
region between Gebel Uweinat and the Gilf Kebir he described:
1) open craters, probably the result of single explosions.
2) craters with subidiary plugs of trachyte.
3) trachyte necks with boundary walls of indurated sandstone.
4) isolated hills capped with trachytic lava.
The crater walls he described as "almost vertical, of intensely indurated
and silicified sandstones, brecciated on a large scale as weII as in the hand-specimen,
locally with columnar structure imposed upon them, the prisms being more or
less horizontal (at right angles to the source of heat). With the exception
of the columnar joints, this general crater morphology was present even in
those features that lacked central igneous masses.
The contact between sandstone and the various trachytic plugs was typically
fused. From thin sections of these fused zones, Sandford described "heat or
explosion-shattered grains of well-rounded quartz, with an isotropic black,
brown, and glassy material between them".
An intrusive dyke in the foreground of the crater
An other dyke with erected layers
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
or Trachyte) within some of the craters. The basaltic 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
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 walls.
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.
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."
Structural samples for the origin the
A large significant crater
with Carboniferous plant imprints and Trachyte inside.
Mountains "Peter & Paul"
The area with the striking
mountains "Peter & Paul" is located between Clayton Craters and the granitic
"unknown" plateau in the north. The both mountains consist of trachyte intrusions.
A further hill east of Peter and Paul typifies the locality. It stands approximately
25 m above the plain. The upper half is composed of trachyte with characteristic
jointing that gives it a columnar, steep-sided appearance. The lower half
is composed of a mixture of trachyte and basal conglomerate with a distinct
apron of debris.
"Peter & Paul"
Granite in the foreground
The basaltic plugs on top and nearby the southern Gilf Kebir plateau
Detected augite ferrobasalt at northeastern
site auf Kemal-el-Din plateau
It was found a porphyritic, fine-grained igneous rock dominated by phenocrysts
of olivine (30 vol% pheno) and augite in a groundmass of augite, abundant
magnetite, minor plagioclase and nepheline. Olivine phenocrysts form colourless,
euhedral crystals up to 2 mm in size and are partly altered to red brown
iddingsite. Augite phenocrysts are euhedral crystals up to 1 mm in size
and often contain magnetite inclusions. The groundmass consists of subhedral
brown augite, small laths of plagioclase and euhedral magnetite. Nepheline
occurs as areas of nepheline.
Specimen is a primitive basalt dominated by olivine and pyroxene with
very little plagioclase. Minor amounts of nepheline indicate this is a
silica undersaturated alkali basalt. Enough magnetite is present in the
sample to make it highly magnetic.
Rock Library, Imperial College London, Author: Matt Genge
1.4 Structures in the Great Sand Sea nearby the
Gilf Kebir plateau
Crater El-Baz nearby the Gilf
24° 13' N / 26° 24' E
Dyke shaped structure"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
Not far from the
Gilf Kebir plateau are to be found in the northeastern direction further
structures on pictures of "Landsat". 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
In the near past,
remarkable finds were made of dark silica that are included in sediments
Crater "El-Baz" (4)
"Qaret el Hanash" near
Silica field: Basaltic intrusion contain debris of basement (?)
and infiltrated silica solutions (Jasper)
Basaltic boulders with
Structures at the western and northern foreland of Gilf Kebir
During the 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.
Many volcanic Fissures with erected
layers between Wadi Sura and Aqaba (6)
Hydrovolcano (Um Ras)
plugs in the "Crater Kebira" area
Pushed zone of white erratic blocks.
Loc 24°32'25"N / 25°05'30"E
1.6 The crater
"White Spot" on top of the Jebel Uweinat and further structures
(the so named
Andras Zboray, Hungary
The crater "White Spot" on top of Jebel 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.
Crater "White Spot" (21°55'
01'' N / 25°02'48'' E)
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) several 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).
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
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.
The Shatter Cones are not valid.
These structures are
Pseudo Shatter Cones
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
The supposed "largest impact crater field on Earth in the Gilf Kebir
region" is a legend therefore !
the following pictures and comments.
Gilf Kebir Crater Fields
The red points are craters which were identified by the satellite SPOT.
Typical landscape of the crater
Images from the prominent Crater GKCF-13
View into GKCF-13 from
the northern position
View into the crater
GKCF-13 with a diameter of 950m. Position: 23°18'23'' N / 26°55'28'' E
Some "impact proofs" from fieldwork of Philippe Paillou and team
Breccia with included basement
material (GKCF-01and GKCF-05)
Broken sandstone with marks
Planar fractures in quartz grains
Basalts hills and fused sediments of GKCF
A remarkable Basalt plug inside
of a crater
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 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
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
Pseudo Shatter 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 (2005).
What's new since 2004:
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
Remarks to 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.
Non-impact origin 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
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 been examined.
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
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 volcanic crater.
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
Don’t show any macroscopic or microscopic shock effects, in particular no
PDF’s, no evidence of melting, no glass.
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.
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
with sizes ranging from some tens of meters to kilometers (and some of these
are associated with basalts).
These evidences indicates that all these features have a common non impact
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."
View into the crater GKCF-01 of diameter 630m.
Position: 23°14' 37'' N / 27° 27' 37'' E
Further proofs for the hydrovolcanic origin of the GKCF
New images from structures of the GKCF
Small Crater nearby GKCF-13
Tilted Layers (Author
Small Crater (Author 210)
Hill with uplifted
red layers (2)
Crater Field nearby Jasper
Hydrovolcanic Structures (2)
Hydrovolcanic pipes nearby Abu Ballas and "Regenfeld"
Jasper Mountain nearby Abu Ballas
Photos by Ursula
Further crater-fields at the Egyptian-Sudanese border
(Jebel Kamil and surrounding)
In 2009, the author
have discoverd further crater - fields by Google - Earth, which are located
in east of Jebel Uweinat (Jebel 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
crater-field in the Jebel 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° 00' E.
4.0 The supposed impact craters of Libya
(The "impact" craters B.P., Oasis
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."
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."
25° 19' N / 24° 20' E
Erected and broken sandstone walls
The uplifted sandstones from the
central block were subjected intense shear-movements
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 central
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 observed."
A dyke nearby the Oasis structure
Crater field Oasis (prominent plugs
in the north)
Oasis ring dyke structure with central
Prominent plugs of Trachyte
in the northern direction with erected layers at outside
Plug in the Jebel Babein
Plugs of Trachyte with fused
layers 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.
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
Ch. Koeberl 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 uplift.
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 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.
fieldworks were found silicified sandstone dykes and
igneous rocks, such as syenite, porphyries, tephrites and phonolites,
and lamprophyres (monchiquites) directly associated with each circular feature.
Based on these and other observations, it was concluded that the Arkenu structures
are stocks of porphyritic syenitic that have intruded the sandstone formation
to form rather simple and eroded ring dike complexes. Hydrothermal activity
that followed the intrusion of these ring dike complexes resulted in the formation
of massive magnetite–hematite deposits and dikes of silicified sandstone.
Paillou's craters Arkenu:
Ark 1 position 22° 05' N / 23° 47' E
Centre of Paillou's crater Arkenu-2
Subvolcanic structures at the western
edge of Arkenu-2
Western edge of the structure Arkenu-2
Breccia (or white Trachyte) in the
Breccia at crater rim
Arkenu-1: Yellow Jasper in silicified
sandstone - formed by hydrovolcanic solutions
Hardened breccia with
not rounded fragments of basement material
Two small craters; 40 km WNW from
the Arkenu structures
Crater southeast of Arkenu
(21° 57' 50' N/ 23° 46' 39'' 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 also.
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
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 subvolcanic region.
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.