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Remarks to the origin of the crater-shaped
structures
around Gilf Kebir and Djebel Uweinat (Egypt)
and to the supposed impact craters of
Libya
Geological History
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 unnamed
plateau).
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
Djebel Kamil.
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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 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
likewise here:
-
Libyan crater-shaped structures "Oasis" , "B.P."
and Arkenu
-
Crater "White Spot" (Djebel Uweinat)
-
Hydrovolcanic pipes nearby "Regenfeld" .... etc.
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Map with crater-shaped structures and
plugs in the Western Desert
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1.0 The craters of the uplift-region around Gilf Kebir
and Djebel Uweinat
1.1
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." (Source: www.geoscience.org.za).
On the active line following structures are found from the southwestern to the
northeastern direction:
-
Crater "White Spot",
dykes and basaltic plugs in the Djebel Uweinat,
-
many dykes and craters
in the area between Djebel Uweinat and Gilf Kebir,
-
all groups of Clayton's
Craters,
-
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,
-
hydrovolcanic pipes
nearby "Regenfeld",
-
crater like structure
"Jasper Mountain",
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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).
The 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.
Visible
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 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
Zboray).
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 the
craters
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."
Basaltic plug without crater (22° 18'
01'' N / 25°14' 15'' E)
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
sandstone layers
Erected layers of sandstone at the crater
rims
 
A big crater
A half small crater
1.2 The basaltic
plugs on top and nearby the southern Gilf Kebir plateau
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| 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 evidently. |
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23° 07' 60'' N / 26° 02' 33'' E
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| 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 Andras Zboray
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. |
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Conspicious basalt
hill. Wadi Mashi. 23° 38' 54'' N / 26° 22' 25'' E
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Shallow circular structure with a basalt
plug in the centre
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Eroded basalt plug
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Basalt rock on ground
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| The first time
is documented now a circular structure with a basaltic plug in the centre, 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 dome." |
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Crater El-Baz nearby the Gilf
Kebir
24° 13' N / 26° 24' E
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Dyke shaped structur "Qaret el Hanash"south
of Silica field
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Pillar of brecciated rocks
in the northern direction of crater El-Baz
27° 07' 60" N / 26° 28' 60"E
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a subvolcanic gap with outflow of quartzitic solutions (Jasper)
25° 04' 30" N / 25° 56' 12 " E |
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| 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. |
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During the 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
plateau
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-Bey 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-Bey plateau. The whole plateau
has been uplifted by an intrusive event. |
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By intrusiva uplifted Hassanein-Bey
plateau
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Basalt or only columns of splitted sandstone
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In the entire
northern Uweinat are to be found further structures,
which have emerged by subvolcanic activities in the underground.
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Basaltic columns in the Karkur Talh
(upper part)
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A long dyke-line in the northeastern
part of Djebel Uweinat
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The long dyke-line
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in northeastern part of Uweinat
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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).
These structures
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
Andras
Zboray, 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."
Philippe
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.
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Gilf Kebir Crater Fields (GKCF)
The red points are craters which were identified by the satellite SPOT.
Sources: Paillou/CNES
Right: Typical
sections of the crater field "Scarp-Contours-North" (~ 24° 05' N / 27° 05'
E )
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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
position
GKCF-13 from the satellite view
Craters nearby GKCF-13 in southern direction
Crater GKCF-13
 
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
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New pictures from a small crater; the author
2010
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An other crater nearby GKCF-13; the author
2010 |
Hydrovolcanic structure |
Erected Layers |
Brecciated and fused sandstone at a dyke structure nearby
Clayton Craters
Block of fused conglomerate (GKCF-02)
 
Dykes with erected or fused sandstone layers
 
Hills of basalt are features within the area eastern of
Gilf Kebir --- here in the south of "impact crater field" the so-called "Black
Hill"
Position: 23° 07' 58'' N / 27° 38' 47''
E
 
Hydrovolcanic pipes nearby Abu Ballas
Destroyed basalt columns on "Black Hill"
Hydrovolcanic pipes on the rim of a crater-shaped structure;
location "Regenfeld"
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''
E
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
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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 !!
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Eroded doming structure with fused sandstone
on top
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Breccias
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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)
Source:
www.impactika.com/PaillouCRAS04.pdf
 
Planar fractures in quartz grains (GKCF-09 & GKCF-12)
PF's (GKCF-02)
Source: www.impactika.com/PaillouCRAS04.pdf
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An impressive example for a subvolcanic
structure
(1 km SSE of GKCF-13)
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Conclusion
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 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 (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 outflows.
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What's new:
No "largest impact crater field on Earth
in the Gilf Kebir region" ?
Source:
www.lpi.usra.edu/meetings/lpsc2006/pdf/1286.pdf
"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 instance."
"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."
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My interpretation of the published GPR-profile
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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)Sources:
www.lpi.usra.edu/meetings/lpsc2005/pdf/2375.pdf
www.lpi.usra.edu/meetings/lpsc2006/pdf/1264.pdf
The published 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.
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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
Source: www.rssd.esa.int/SYS/docs/ll_transfers/296053_Di2DMartino_pres.pdf
http://sci.esa.int/science-e/www/object/doc.cfm?fobjectid=40214
"November 2005 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 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
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.
Pseudo-shatter cones
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.
Conclusion
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 with basalts).
These evidences indicates that all these features have a common non impact
origin.
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)."
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Some
pictures from crater GKCF-13 - presented by Mario Di Martino and team (2006):
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Breccia
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Marks of hydrovolcanic metasomatism
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Wall of
breccias
Sandstone layers were tipped over to the outside
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Erected layers were tipped over to the
outside of crater GKCF-13
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| Some
further important details from crater GKCF-01 - presented by Mario Di Martino
and team (2006): |
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View into the crater GKCF-01 of diameter
630m. Position: 23°14' 37'' N / 27° 27' 37'' E
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Erected layers and breccias on crater
rim
 
Macrobreccias of Paleozoic sandstones
and basement material (right)
Various macrobreccias:
Partially with melted material and with marks of hydrovolcanic metasomatism
Fused conglomerate
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New pictures 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.
Source:
Richard de Nul,
Belgium; Location is called "Jaspis
(Jasper) Mountain"; about 24° N /27°E .
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Jasper with Chalcedony crystals in small
cavities
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3.0 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° 00' E.
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Basalts between the crater-shaped structures
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Basalts between the crater-shaped structures
at the Djebel Kamil
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Many crater-shaped structures in northern
Sudan
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Large structures: 21°26' 28'' N / 26°
04' 42'' E
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4.0 The supposed impact craters of Libya
(The "impact" craters B.P., Oasis and Arkenu)
4.1 Crater
B.P.
"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."
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B.P. 3D-picture
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25° 19' N / 24°
20' E
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Crater B.P.
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Erected and broken sandstones in the
foreground
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Crater B.P.
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Crater B.P.
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The uplifted sandstones from the central
block were subjected intense shear-movements
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What's new:
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
Source:
www.univie.ac.at/geochemistry/koeberl/publication_list/246-BP Oasis-Impact
Tectonics-2005.pdf
Field Work 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. |
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B.P. ring dyke structure with central
uplift
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4.2 Crater Oasis
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Crater Oasis
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"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."
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A dyke nearby the Oasis structure
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Oasis ring dyke structure with central
uplift
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Oasis 3D-picture
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Crater field Oasis (prominent plugs
in the north)
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An unidentified plug in the Djebel
Babein (borderline Libya/Egypt)
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The prominent plugs in the northern
direction of Oasis structure
are columns of Basalt with fused layers on top
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Plugs of Basalt with fused layers
on top in the northwestern direction of Oasis structure
Position: 24°
45' N / 23° 52' E
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In the foreground a fused conglomerate
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| Source:
home.tiscali.be/afrique/libye04-photos-suite2.html |
|
"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." ( source:
www.lpi.usra.edu/meetings/lpsc97/pdf/1620.PDF)
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What's new:
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
Source:
www.univie.ac.at/geochemistry/koeberl/publication_list/246-BP Oasis-Impact
Tectonics-2005.pdf
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 basalts (Trachyte).
Trachyte harden without development of the distinctive column-like structure. |
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.
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Paillou's craters Arkenu:
Ark 1 position 22° 05' N / 23° 47' E
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Sandstone with Pseudo Shatter Cones
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Planar fractures in a quartz fragment
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Hardened breccia with not rounded
fragments of basement material
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Centre of Paillou's crater Arkenu-2
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Western edge of the structure Arkenu-2
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Subvolcanic structures at the western
edge of Arkenu-2
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Western edge of the structure Arkenu-2
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Breccia (or white Trachyte) in the centre
?
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Breccia at crater rim
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| Arkenu-1: Yellow Jasper in silicified sandstone - formed
by hydrovolcanic solutions |
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Two small craters; 40 km WNW from the
Arkenu structures
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Crater southeast of Arkenu 2
(21° 57' 50' N/ 23° 46' 29'' E)
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4.4 Hadid
Structure
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.
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Conclusion
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Structural sample for the origin
the Libyan craters Oasis and BP
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The "impact" 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.
The question 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.
All intrusive 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 Desert Glass.
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What's new:
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 Torino
3 Dipartimento di Scienze della Terra, Università di Firenze
Source: www.lpi.usra.edu/meetings/lmi2008/pdf/3012.pdf
“The interpretation 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 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.
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