The "Gweni Fada" structure in Chad is
Norbert Brügge, Germany
The Gweni Fada and the
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 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)
"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
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.
The presence of shock metamorphism
confirms that the Gweni-Fada structure is, as suggested by its morphological
appearance, a crater formed by the impact of a hypervelocity asteroid."
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
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
A complete and undisturbed sandstone complex has
broken down from the northern edge of the structure and tilting strangely
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, as well bleaching in sandstones in the
Metamorphic overprint of sandstones (quartzite)
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 volcanic 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
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 hematite-flow.
This contains angular quartz pieces from the Paleozoic 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
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
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
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 and was fed by a magma
chamber that must have been of considerable proportions.
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 without tectonically stressed layers
probably from a gap filling in the the central uplift, found on a gravel terrace
in the depression
Gravel terasse: Pulpy grind up mass with angular
Before Sunrise: We see the metamorphic quartzitization
Gravel terasse: Rounded solitary quartz in deposts
of fluvial origin
Hydro-volcanic bleached and rusted sandstone layers
Becq-Giraudon J-F., Rouzeau O., Goachet E & Solages S.
-- Impact hypervéloce d'une météorite géante à l'origine de la dépression
circulaire d'Aorounga au Tchad (Afrique).
C.R. Acad. Sci. Paris, t. 315, Série II,
pp. 83-88. (1992)
Vincent P.M. & Beauvilain A. -- Découverte d'un nouveau
cratère d'impact météoritique en Afrique : l'astroblème de Gweni-Fada (Ennedi,
Sahara du Tchad).
C.R. Académie des Sciences, Paris, t. 323, série IIa, pp. 987-997. (1996)
Beauvilain A. -- L' astrobleme de Dweni-Fada,
Ennedi quest. (1996)
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
Clayton Craters and other crater-shaped structures in Egypt and Libya
Archean Basement outcrops of East Sahara Ghost Craton in the Western Desert