The"Aorounga" structure in Chad is NOT an Astrobleme

Norbert Brügge, Germany
Dipl.-Geol.

The Aorounga and Gweni-Fada structures in Chad (Africa) are classified as astroproblemes by certain 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 the 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.

The Aorounga structure has received little on-site investigation. A first description was published in Becq-Graudon et al. (1992). The expedition Vincent & Beauvilain (1994) provided the latest increase in knowledge. Breccias were also collected to microscopically examine possible planar features in quartz.The results are published in Koeberl et al. (2005). Some impressive photos from 1994 with commens were published by Alain Beauvilain (1996) and are essential to the evaluation of the Aorounga structure, in addition to other descriptions.

Alain Beauvilain 1966
Quotes (translated)
"The Aorounga structure have a diameter of ~13 km. This crater-like structure is well defined, with a broad, near-circular central area of an obvious hilly terrain, of ~9 km in diameter. It is surrounded by a flat annulus (maximum 3 km wide), which shows at its outer edge disturbance of circular geometry. Especially in the western and southwestern sectors, a distinctly hummocky (mountainous) ring encloses the flat annulus. It is, however, difficult to identify how far out this ring structure extends, especially in the northeastern and southwestern sectors where the crater structure is surrounded by complex terrain."
"The crater interior shows a small, irregularly shaped central peak area surrounded by a somewhat flatter ring, in turn surrounded by a broad and topographically strong ring. Aorounga structure is recognized as a multi-ring structure, which comprises a central, somewhat hummocky area, surrounded by a near-circular and 2 to 3 km wide ring feature of strong elevation, followed by the flat annulus and then by an outer ring as a feature that surrounds the entire crater structure, presumably the actual crater rim zone. The depression (a flat terrain around the central complex) is about 2 km wide and of even elevation in all four sectors of the crater structure. This feature is surrounded by a 3 to 3.5 km wide zone (outer ring) of variable but always significant relief. This ring feature probably represents the apparent crater rim. "
"The structure is characterized by an outer and an inner ring (diameters of ~11 and 7 km, respectively), which both rise about 100 m above the mean level of the surrounding plain The two rings are separated from each other by a depression of uniform width. A hill, or possible uplift structure, of ~1.5 km in diameter is located near the center of the central depression. The ring walls were described to consist of steeply outward dipping sandstone layers, with dips of 40–50° seen at the outer wall and 80° at the inner wall. Some breccia as consisting of a coarse-grained siliceous matrix with fine-grained, beige clasts (cm- to dm-size) with fluidal texture, was apparently found on top of the inner rim wall."
"No melt-bearing samples that might provide a key for dating of the impact event have been identified among the limited sample suite available "
"The outer wall is higher in the west than in the east. The outer annular depression has a regular width of 2 kilometers. To the west it is up to 150 meters deeper than the outer wall. Structurally, it is a fault-bounded "Syncline-Graben". The sandstone formations tumble down on each side with dips that can reach vertical to the foot of the edge."

"The peripheral crown has an unequal width of 3 to 4 kilometers and forms a western boundary with an elevation of 680 to 690 meters. The dips are variable, generally outwards. The presence of reverse faults, folds, and nonconforming streaks, likely shear, indicate compression with outward thrust. It is a multi-lobed tectonic boundary that must conform to more or less curved, inward-dipping normal faults. But mostly, the sandstones remained sub-horizontal."
 "The internal area has an average diameter of 7 kilometers and looks like a curved and raised sandstone cylinder, covered in its upper parts with a layer of breccias. The heights of the various elements are generally constant. The central plateau has a maximum elevation of 678 meters, a few meters lower than the ridgeline of the inner crown. In the inner crown they begin with a fraction with decimetric elements. On the other hand, on the middle plateau, a fine-grained suevite with small stone fragments and glass rests directly on the sandstone."

Thesis

Regardless previous conclusions about the origin of the Aorounga structure, a sub-volcanic origin is postulated here. The structure is basically a tectonic up-bulge of the subsurface with a chaotic open breach in the center. The causative factor is suspected to be the rising dome from a magma chamber in the deep subsoil, which in part also caused wavy deformations in the sandstone layers in the outer rim. The sandy depressions between the rings were formed by secondary subsidence as the magma dome emptied. These slow bending and lowering processes can be seen on the inside of the outer wall. There the sandstone layers are partly strongly inclined inwards, but mostly already broken off sharply and sunkne. The Basenent breccias occurring in the central part and the absence of igneous rocks suggest that only the "hat" on the rising magma dome has reached the surface. Nonetheless there is plenty of broken sandstone on the top. Regarding the suspected magma chamber, a connection with the volcano Emi Koussi is suspected.

In any case, a classification of the Aorounga structure as an astroblem is wrong. There are no large-scale fragmentations of the sandstone layers or ejecta outside. The magnetite-flow bearing breccias at the center of the structure are consistently filled with shocked debris from the Basement.


Photo Gallery
(Source: Alain Beauvilain


Horizontally bedded strata in the outer ring wall

   
Outer side of the inner ring structure with strongly disturbed layers

       
Depression within the internal structure. In the foreground the central elevation

   
View of the central elevation

      



On top of the central zone with disrupted sandstones and brecciated sub-volcanic ejecta


Breccia with wind abrasion marks and covered with desert varnish (?)


         
 
This sample of collected brecias consists of an accumulation of fragments from the Basement.

Fifteen breccias were collected from the central elevation. From microscopic examination. Six thin section photos are published (Koeberl et al, 2005) showing fractures in angular splinters of BIF-quartz from the Basement (!).
 Not only the fractures in angular pieses, that are siumple PFs, are interesting, but the breccias themselves. This breccias, come from the central piek, are an important indication of volcanic activity in the subsurface. The are originate from gaps partially filled with a probably hematite flow. This contains angular and subrounded quartz pieces. This composition suggests that it is hydro-volcanically processed Banded Iron Formation (BIF).These breccias are comparable to the well photographed breccias from the Gweni-Fada structure in Chad.
The planar deformations are identical to those in debris found in the Gilf Kebir Crater Field (GKCF) in SW-Egypt which is undoubtedly of sub-volcanic origin !



Locations


OR-3


AO-7


              

               

 

 

References:
 

Becq-Giraudon J. F., Rouzeau O., Goachet E. & Solages S. -- Impact hyperveloce d’une meteorite geante a l’origine de la depression circulaire d’Aorounga au Tchad (Afrique).
Comptes Rendus Academie Science Paris 315 (II):83–88 (1992).

 

Vincent P.M. & Beauvilain A. -- Decouverte d’un nouveau cratère d’impact meteoritique en Afrique: l’astrobleme de Gweni Fada (Ennedi, Sahara du Tchad).
 Comptes Rendus de l’Academie des Sciences 323 (II):987–997. (1996)

 

Beauvilain A.- -- L'Astrobleme d' Aorounga, Ennedi quest. (1996)
https://meteoritestchad.monsite-orange.fr/page-55c9c6ee736a0.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 & Planetary Science 40, Nr 9/10, 1455–1471 (2005)

Brügge, Norbert Clayton Craters and other crater-shaped structures in Egypt and Libya

Brügge, Norbert Archean Basement outcrops of East Sahara Ghost Craton in the Western Desert (Uweinat-Howar Uplift)