The "Gweni Fada" structure in Chad is NOT an Astrobleme

Norbert Brügge, Germany
Dipl.-Geol.

The Gweni Fada and the Aorounga structures in Chad (Africa) are classified as astroproblemes by certain dogmatic scientists due to dubious evidence, e.g. planar deformations in quartz grains. It should meanwhile be known that such "evidence" can be completely nonsensical in this context. It has been shown that such deformations can also be generated by volcanic shock waves of different power, this applies to both PFs and PDFs.
 PDFs are only genuine if certain features can be verified by a TEM examination. Even then, PDFs are not yet a feature of an impact, because the PDF-paradigm is based on findings in impact structures in question. Likewise, the necessary shock forces for the formation of PDFs cannot only be induced by an impact.
The same applies to so-called impact breccia, which are used again and again without precise examination. Even a centrale uplift is not suitable as proof, the so-called "target rocks" are not a "pudding" after all.The mostly of the structures defined as impact craters around the world are not anyway.

Thus, it is appropriate to fathom the true origin of at the Gweni-Fada crater, which has hardly been visited or examined. A comprehensive description of the geological and structural conditions found was published by Alain Beauvilain (1996). However, the conclusions must be viewed critically.
Andras Zboray (Hungary) and Ursula Steiner (Switzerland) are now providing new observations and a number of photos (also of breccias) from a trekking tour in 2021. They provide further good evidence of the actual origin of Gweni-Fada.

Alain Beauvilain (1996)
 Quote (translated)
"The Gweni Fada structure is hosted in paleozoic period Spirophyton-sandstones and is approximately 13 kilometers in diameter. The structure is characterized primarily by its asymmetry and a uplifted central zone. From a morphological point of view, the structure consists of a complex central zone with chaotic relief and a more regularly formed outer zone. There is a sandy depression between the outer and central zones. It is crescent-shaped and has a maximum width of 3 kilometers. This depression is about 150 m deeper compared to the ridgeline of the outer zone.

 At the bottom of the the structure, along north to north-west, runs a 5km slope of up to 150m in height of slumped sandstone. To the south (where a depression between the rim and the uplift is absent, the outer boundary is more confusing and irregular. The depression itself is occupied by a series of sloping terraces, formed by narrow, more or less inclined slabs that are close together.

The central complex contrasts with its tormented, chaotic relief with no discernible order. Its outline is very irregular. The central complex exceeds the height of the outer rim of the structure by up to 200m.
It consists of many adjacent blocks arranged in two roughly concentric sets: A heart with all vertices and a crown. There are only sandstone formations whose stratification remains recognizable. The core of the structure is the most distorted part. They consist of (more or less coarse) sandstone, sometimes with quartz dragees. The trace fossil Harlania was found in three locations there. These fossils are known at Ennedi beneath the sandstones with the trace fossil Spyrophyton, formations attributed to the Silurian or Ordovician.

The inner crown is formed by a series of large blocks surrounding the heart. The slopes are variable, but less than 45�, and are usually directed towards the outside of the structure. The heart of the structure is not in the center but shifted to the south. Despite its chaotic appearance and irregularity at first glance, this set represents a structural dome with the oldest formations in the center. Its breakdown into faulty blocks results from a large uplift.

Hardened breccias emerge as veins or dikes. It is remarkable that apart from these fracture zones, the sandstones have undergone macroscopically hardly visible changes. No impactites were found during exploration routes within the structure.
However, the sandstones in the heart of the central-complex have often acquired a quartzite-like appearance through compaction. A sample of shocked quartz was also found there.

Koeberl et al. (2005) -- Unfortunately, only a very limited number of samples were available for our (planar deformation) study. These samples are quartzite, �hematite-stained sandstone�, a pebbly sandstone, and a quartz conglomerate. These samples are derived from the central disturbed area, as well as from the outer margin of the ring depression (fluvial terrace ?).

New insights

Detailed descriptions in the publication from 1996 in connection with new photos and observations allow a completely different conclusion to the origin of the crater. As is so often the case with such structures, classification as an astroblem is without substance and is therefore wrong.

In the assessment are the following characteristics:

  • Undisturbed horizontally bedded sandstone strata in the outer walls of the structure. No signs of impact fragmentation.

  • No ejecta outside the structure.

  • A depression within the crater filled with fluvial deposits.

  • A complete and undisturbed sandstone complex has broken down from the northern edge of the structure and tilting strangely inward.

  • A central uplift that appears to be connected to the western edge of the structure.

  • Undulated or tilted sandstones in an outer zone, steeply erected and shattered sandstones in a chaotic inner zone of the uplift.

  • Gaps with sub-volcanic filled magnetite flow and brecciated quartz pieces within.

  • Sub-volcanic dissolution of the sandstone matrix with partial bleaching found at the foot of the Uplift.

  • Metamorphic overprint of sandstones in the uplift visible by pre-sunrise reflections.

  • No igneous rocks have been found to date. But there is an unexplored elevated fault line about 10 km long just SW of the structure that may be filled with igneous material.

At first it is obvious that the outer edge of the structure shows no traces of a catastrophic event.The sandstones are in their original condition and bedded horizontally. In contrast, the central uplifted elevation consists of a tangle of broken and partly changed sandstones indicating a destructive und thermal force. The fairy tale that has been widespread again and again that impact craters can be recognized by a central uplift can safely be ruled out when looking for the cause. A challenge for the analysis was the sickle-shaped sandy depression between the outer edge and the central uplift, but this can ultimately be explained as a break-in zone. This is documented by the inclined and seamless immersion of slipped sandstone formations on the northern wall.

The central uplift consists of debris from sandston bedrock that was uplifted up by an igneous intrusion. Since no igneous rocks have been found so far, it can be assumed that the intrusion has not reached the surface or is hidden under the debris pushed in front of it. The asymmetry in the structure indicates that the intrusion had an angled rise. The sickle-shaped depression is a collapse structure in an emptying magma chamber, and with the sandstones from above filled again with terrace-like slipped blocks from the today's edges.
There is no depression on the western side of the asymmetrical structure because the plug collided with the sandstone there and caused probably a crushing zone there.

The found breccias come from the central structure are an important indication of volcanic activity in the subsurface. The are originate from gaps partially filled with a magnetite-flow. This contains angular quartz pieces (cherts) from the Precambrian basement. This composition suggests that it is a sub-volcanic processed Banded Iron Formation (BIF).

The planar features in quartz pieces (found by V. & B.) from the inner central zone of the uplift are only simple PFs and an indication of shock waves that were induced by a raised magma.

Adjacent sandstones have changed by a methamorhose. On site we can also see alternately bleached or rusted areas in the sandstones, which can only arise by a hydrothermal (better hydro-volcanic) process and happens slowly.

From an analytical point of view, the Gweni-Fada structure is nothing other as a collapsed dome with a hidden magmatic intrusion (plug), and the breccias come from gaps with hydro-volcanic infill.

Conclusion

The Gweni-Fada structure is originally a dome-like elevation in the sandstone strata caused by a subsurface magma chamber that did not reach the surface. The central uplift is a plug within the sandstone dome was formed by partially rising magma. The occurrence of gaps with magnetite flow and breccias with shocked components from the basement (BIF) are proof that we are dealing here exclusively with volcanic forces.
The current state of the structure can be explained by the fact that after the magma chamber was emptied, the structure collapsed and finally the central depression arose. The sharp outer edges of the structure and above all the layers that slided down at the northern edge and sloped inwards also speak in favor of such scenario. In a slow process, these have tilted inwards, broken off from the edge and partially sunk into the inner structure..
The central uplift seems to be little affected by the secondary sinking. This means that it is anchored in a solidified volcanic plug.

Finally, it is also very likely that both the Gweni-Fada and the very similar Aorounga structure have something to do with volcanism in the Tibesti of northern Chad, the most prominent feature of which is the Emi Koussi stratovolcano. The Emi Koussi was active 2.4 and 1.3 million years ago.
Emi Koussi was the pulsating valve in a far-reaching magna chamber. Aorounga and Gweni-Fada are products of temporary overpressure but have not reached an eruptive stage.


Photo Gallery (1)
Source: Alain Beauvilain (1996)


  


 


 


Inward tilted less disturbed layers at the outer edge of the central uplift

  
    
 Uplift from south

Photo Gallery (2)
Source: Andras Zborya & Ursula Steiner (2021)

Outer edge


Outer edge without tectonically stressed layers (Eastern side)

Central Uplift


Uplift from north-east: Strata bent upwards by shock waves
 

     
View of the outer zone of the uplift with tilted layers


View on the uplift from east

     
Sections with tormented and extremely tormented relief

        

   
Mixture of tormented steep and horizontal strata at the beginning of the central uplift

Northern edge: Inwards dipping layers



Northern edge of the structure



Large area of sandstone blocks that have slid off from the edge and tilted inwards



Tilted and sunken sandstone beds inside the structure at the northern edge


Inward (south) tilted strata along the eastern side at the entrance to the structure



The abraded surface of the inclined strata



Inward (south) tilted strata along the western side at the entrance to the structure

 

Sub-volcanic breccia, probably from a gap filling in the the central uplift, found on a gravel terrace in the depression
 


   
Dark magnetite flow with angular quartz pieses from basement therin (the grains are magnetite and quartz crystals, not sand grains).



 


   
Magnetite flow in quartz pieces (components of BIF-cherts
Anyone who claims that such angular pieces of quartz come from Paleozoic sandstone is not a geologist.


  
Planar features (PFs) in such quartz pieces (V.& B.)
 They are identical to those in debris found in the Gilf Kebir Crater Field (GKCF) in SW-Egypt
 which is undoubtedly of sub-volcanic origin 


Addition
 


Gravel terrasse: Pulpy dissolved mass with angular basement pieces


Before Sunrise: We see the metamorphic quartzitization of Sandstones



Pulpy dissolved sandstone hydro-volcanic bleached


        
Hydro-volcanic bleached and rusted sandstone layers

References:

Becq-Giraudon J-F., Rouzeau O., Goachet E & Solages S. -- Impact hyperv�loce d'une m�t�orite g�ante � l'origine de la d�pression circulaire d'Aorounga au Tchad (Afrique).
C.R. Acad. Sci. Paris, t. 315, S�rie II, pp. 83-88. (1992)

Vincent P.M. & Beauvilain A. -- D�couverte d'un nouveau crat�re d'impact m�t�oritique en Afrique : l'astrobl�me de Gweni-Fada (Ennedi, Sahara du Tchad).
C.R. Acad�mie des Sciences, Paris, t. 323, s�rie IIa, pp. 987-997. (1996)

 Beauvilain A. -- L' astrobleme de Dweni-Fada, Ennedi quest. (1996)
https://meteoritestchad.monsite-orange.fr/page-55ca0eb105df4.html

Koeberl Ch., Reimold W.U., Cooper G., Cowan D. & Vincent P.M. -- Aorounga and Gweni Fada impact structures, Chad : remote sensing, petrography, and geochemistry of target rocks. Meteoritics and planetary Science 40, Nr 9/10, pp 1455-1471. (2005)

Zboray, A. --Ennedi Expedition, Chad, 19th November - 8th December, 2021
 https://fjexpeditions.com/frameset/past.htm

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