Remarks to the origin of the craters around Gilf Kebir and Djebel Uweinat (Egypt)
and 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 age 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. To these uplifted region belong also Arkenu, Bahari, Babain, unnamed plateau etc. Since the age of the Devonian time until recent was the uplifted region an island without sedimentations (exclude transgressive Carboniferous strata in the northern Gilf Kebir and at the eastern edge of the Djebel Uweinat). In the Cretaceous age, 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 Djebel Uweinat, Arkenu etc.).
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-scale 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.

From the many widespread subvolcanic structures two crater - fields are considered up to now mostly:
 

Crater field Gilf Kebir (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.



Map with crater-shaped structures and plugs in the Western Desert


 

1.0  The craters of the uplift-region around Gilf Kebir and Djebel Uweinat

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 along and on top of the southern Gilf Kebir Plateau,

  • large crater field of the "Contour" in the eastern direction of Gilf Kebir,

  • tracks of small dykes in the northeastern direction from the crater field

All craters - here specially 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 (Andras Zboray, 2007).

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

   
                                      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 of the southern Gilf Kebir plateau


In the area of the southern Gilf Kebir plateau (Kemal-el-Din Plateau) we can see on pictures of "Landsat" numerous craters on the uppermost altitude of the plateau, which is difficult accessible with cars. It is described, that the top of the plateau is perfectly flat and featureless, interrupted by low basalt hills frequently. It is however sure, that many of these "craters" include such basaltic bodies. Up to now nobody has made photos evidently.



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. It is the eroded remain of a plug. Up to the plateau in this valley are to be found further in part essentially larger basaltic structures.

 

  Crater El-Baz  nearby the Gilf Kebir with basaltic material
 Longitude 26° 24' E; Latitude 24° 13' N



Pillar of brecciated rocks in the northern direction of crater El-Baz
Longitude 26° 28' 60"E; Latitude 27° 07' 60" N
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 is only a picture of a pillar of 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.



1.3   The crater "White Spot" on top of the Djebel Uweinat

(the so named crater by Andras Zboray, Hungary

          
The crater "White Spot" on Hassanein plateau                                          The basaltic plug is visible in the foreground

       
Rim of the crater

                  
Crater "White Spot" -- Position: Longitude 25°02'48'' E; Latitude 21°55' 01'' N

The crater "White Spot" on top of Djebel Uweinat (Hassanein plateau) is clearly a basaltic plug. At the bottom of the crater is basaltic material available. The rims of the crater are composed of Paleozoic sandstones. They are not erected in contrast to the Clayton Craters. Probably the sandstones were broken by the up-directed pressure in concentric steps. The originally horizontal stratification was therefore not changed in principle. Directly inside of the crater are to be found few up squeezed sandstones too. The crater is found on the top of Hassanein plateau. The whole plateau has been uplifted by an intrusive event.



By intrusiva uplifted Hassanein plateau



Basaltic columns in the Karkur Talh (nearby White Spot)

In the entire northern Uweinat are to be found further structures, which have emerged by subvolcanic activities in the underground.



Erected layers above Karkur Thal



Uplifted sandstone-block



A big dyke in the northeastern part of Djebel Uweinat



The dyke-line


                                                                                                                                                                                            

 

 

2.0  The craters of the "Contour" - outside of the uplifted region (GKCF craters)

(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 (GKCF01, GKCF02 and GKCF12).
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 knowledge, that the circular structures on the field being 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 remarks to it:

Andras Zboray told me, that the crater (meant is GKCF13) 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 (GKCF08-10). 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 GKCF02. 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 GKCF13 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 GKCF13 is presented in the sample below. Rate also the following pictures and comments.


View into the crater GKCF13 of diameter 950m. Position: Longitude 26°55'28'' E; Latitude 23°18'23'' N


Another view into GKCF13 from the northern position

                               
                  GKCF13 from the satellite view                                                                 Craters nearby GKCF13 in southern direction

                                
Crater GKCF13                                                                                                                     (Andras Zboray)           

 
 Views of the crater GKCK13 in 2004 by the author: Left: The rim with >90 deg erected brecciated layers. Right: Erected and broken sandstone layers

Structural samples for the origin of the Gilf Kebir craters


 

    
        Typical sections of the crater field "Contour-North" (~ 27°05'E / 24°05'N)


Typical landscape of the crater field
 

  
Crater GKCF02 Position: Longitude 27°19'18'' E; Latitude 23°27'10'' N   (Andras Zboray)

                         
    Brecciated and fused sandstone at a dyke structure                                                                            Block of fused conglomerate  (GKCF02)

 
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: Longitude 27°38'47'' E; Latitude 23°07'58'' N

       
                   Hydrovolcanic pipes ?                                                                                                      Destroyed basalt columns on "Black Hill"

          
The "Beacon Hill" is a crater-like structure with fused sandstone on  the rim
Position: Longitude 27°16' E; Latitude 23°00' N


Walls of fused sandstones in the crater field


Sample of a other large basalt body in the crater field
Position: Longitude 27°34'53'' E; Latitude 23°18'23'' N

       
Two basaltic outflows nearby Gilf Kebir plateau
Positions: 26°35' 02'' E / 24°07'51'' N as well as 26°35'33'' E / 24°06'08'' N

In the northeastern direction from the crater field the author found in 2006 among others eroded tracks and breccias at dykes
Position: Longitude 28° 16' 28" E; Latitude 24° 16' 28" !!



Eroded doming structure with fused sandstone on top



Breccias


        
 

Now some "impact proofs" from the Internet-presentations by Philippe Paillou and Alain Carion:

 

                               
                                          Breccia  with included basement material  (GKCF01) and  (GKCF05)                        Broken sandstone with marks of hydrovolcanic metasomatism (GKCF04)

Source: www.impactika.com/PaillouCRAS04.pdf



Breccia of red sandstones                                                                            Breccia                                                                                                     Breccia                      

        
                  Planar fractures in quartz grains (GKCF09 & GKCF12)                                                        PF's (GKCF02)
Source: www.impactika.com/PaillouCRAS04.pdf                                                                 

 

Some further pictures from crater GKCF13 - presented by Mario Di Martino and team (2006):



Breccia



Marks of hydrovolcanic metasomatism



Wall of breccias



 Sandstone layers were tipped over to the outside


    
Erected layers were tipped over to the outside of crater GKCF13
 



An impressive example for a subvolcanic structure
 (1 km SSE of GKCF13)

Conclusion


The supposed "largest impact crater field on Earth in the Gilf Kebir region" is a legend. The craters of the "Contour" (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 "Contour" (GKCF13 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.

 

                                                                                                                                   
What's new since 2005:

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."
 


My interpretation of the published GPR-profile

 Remarks to the results of measurements with
Ground-Penetrating Radar (GPR) through small crater GKCF28
 (Position: 27°03'15''E and 24°05'46"N)

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.


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 1, GKCF 6, GKCF 7, KCF 8, GKCF 11, GKCF 12, GKCF 13, and we collected rock samples from GKCF 1, GKCF 7, 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 1, 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)."


Some important details from crater GKCF01 - presented by Mario Di Martino and team (2006):


View into the crater GKCF01 of diameter 630m. Position: Longitude 27°27'37'' E; Latitude 23°14'37'' N


      
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                            

 

                                                                                                                                                                                           

 

3.0  Further crater -fields at the Egyptian-Sudanese border
(Djebel Kamil and surrounding)

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 ("Contour").
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.



Many crater-shaped structures in northern Sudan



Basalts between the crater-shaped structures at the Djebel Kamil



Basalts between the crater-shaped structures



Further  large structures: 21°26'28'' N/ 26° 04'42'' E


Geological map

  

 

                                                                                                                                                                                           

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."



B.P.  3D-picture




Crater B.P.



Erected and broken sandstones in the foreground



Crater B.P.



Crater B.P.


                              



The uplifted sandstones from the central block were subjected intensely shear-movements and heat
                                                                                                 
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.


         
B.P. ring dyke structure with central uplift


4.2  Crater Oasis

 



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



Descent



Central uplift


           
Oasis ring dyke structure with central uplift





Oasis 3D-picture


 
  Crater field Oasis (prominent plugs in the north)



An unidentified plug in the Djebel Babein (borderline Libya/Egypt)



The prominent plugs in the northern direction of Oasis structure
are columns of  Trachyte (?)



Plugs of Trachyte (?) in the northwestern direction of  Oasis
Position: Longitude 23° 52' E; Latitude 24° 45' N



In the foreground a fused conglomerate
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)
                                                                                                         
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.



Paillou's craters Arkenu:  Ark 1 with  Longutide 23° 47' E; Latitude 22° 05' N



Sandstone with Pseudo Shatter Cones



Planar fractures in a quartz fragment



Hardened breccia with not rounded fragments of basement material



Centre of Paillou's crater Arkenu 2



Brecciated crater rim



Breccia (or white Trachyte ?) in the centre



Breccia at crater rim
                                                                                                                                                            

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.
 

Structural sample for the origin the Libyan craters Oasis and BP

Conclusion


The "impact" craters in Libya are comparably to the craters of the "Contour" 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.